design of artificial microenvironment for cord blood cd34
TRANSCRIPT
Design of Artificial Microenvironment for Cord Blood
CD34+ Cells Expansion ex vivo
Yue Zhang
(M.Sc. Bioengineering, NUS)
A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF SCIENCE
GRADUATE PROGRAM OF BIOENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2005
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Acknowledgements
This thesis is the result of one and half years of work whereby I have been
accompanied and supported by many people. It is a pleasant aspect that I have now
the opportunity to express my gratitude for all of them.
The first people I would like to thank are my supervisors Professor Teoh Swee
Hin and Professor Kam W. Leong, who always kept an eye on the progress of my
work and always were available when I needed advice. Their enthusiasm on research
and mission for providing high-quality work has made a deep impression on me. I
owe them lots of gratitude for having me shown this way of research. I also would
like to thank the helpful discussion of Professor Ian McNiece, Professor Hai-Quan
Mao, and Dr Chai Chou.
My colleagues of Tissue Therapeutic Engineering lab in Division of Johns
Hopkins in Singapore gave me a lot of help in research and life. I would like to thank
Dr Xue-Song Jiang, for many discussions and collaboration in animal work. He also
provided me advice and tips that helped me a lot in staying at the right track. I thank
Mr Kian-Ngiap Chua, for the valuable technical advice in SEM imaging. I thank Mr
Roy Lee and Ms Peggy Tang for being good colleagues during the last one and half
years. They helped me a lot in animal experiment. I also want to specially thank Mr
Thong-Beng Lu and Mr Yan-Nan Du from National University of Singapore, XPS
staff from Institute of Materials Research & Engineering, for their valuable technical
support in AFM and Confocal imaging and XPS test.
In the end, I would like to thank the members of the school council for their
valuable input. I also want to thank Agency for Science, Technology and Research
(A*STAR) of Singapore and the Division of Johns Hopkins in Singapore who funded
the thesis project.
i
Table of Contents
Acknowledgements……………...…………………………………..……..………..... i
Table of Contents……………………………………………………………………...ii
Summary.......................................................................................................................vi
List of Tables..............................................................................................................viii
List of Figures...............................................................................................................ix
List of Symbols.............................................................................................................xi
Chapter 1 Introduction
1.1 Purpose.....................................................................................................................1
1.2 Motivation................................................................................................................2
1.3 Objectives and Scope...............................................................................................3
Chapter 2 Literature Survey
2.1 Review of HSC and clinic application.....................................................................5
2.1.1 Hematopoiesis and HSC..................................................................................5
2.1.2 HSC clinic application.....................................................................................6
2.1.3 Different sources of HSC in transplantation...................................................8
2.2 Review of HSC expansion.....................................................................................10
2.2.1 Selection of candidate cells for expansion....................................................10
2.2.2 Evaluation of ex vivo expansion...................................................................11
2.2.3 Expansion systems........................................................................................12
2.2.3.1 Stroma dependent expansion................................................................12
2.2.3.2 Stroma independent expansion.............................................................14
2.3 Review of HSC microenvironment........................................................................15
2.3.1 Cytokine microenvironment..........................................................................15
2.3.2 Stroma microenvironment.............................................................................18
ii
2.3.3 ECM microenvironment................................................................................19
2.3.4 3D topography...............................................................................................19
2.4 Protein immobilization and bioactivity study........................................................21
Chapter 3 Materials and methods
3.1 Materials:....................................................................................................................23
3.1.1 Biological materials...........................................................................................23
3.1.2 Chemical materials............................................................................................23
3.2 MSC and CB CD34+ cells co-culture ......................................................................24
3.2.1 MSC culture......................................................................................................24
3.2.2 Ex vivo expansion of CB CD34+ cells in co-culture systems.........................24
3.2.3 CFC....................................................................................................................26
3.2.4 Animal engraftment...........................................................................................26
3.3 MSC and CB CD34+ cells 3D co-culture..................................................................27
3.3.1 MSC culture in 3D system and assays..............................................................27
3.3.2 Ex vivo expansion of CB CD34+ cells in 3D co-culture systems and
functional assays...............................................................................................28
3.4 Study of FN immobilization bioactivity....................................................................28
3.4.1 FN conjugation and adsorption.........................................................................28
3.4.2 Chemical and physical characterization of the surface.................................29
3.4.3 Biological characterization of the surfaces...................................................30
3.5 Statistical analysis..................................................................................................32
Chapter 4 MSC and CB CD34+ cells co-culture
4.1 Screening of co-culture conditions.............................................................................33
4.1.1 MSC metabolic level in StemSpan medium..................................................33
4.1.2 Optimize expansion scale..............................................................................33
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4.1.3 Co-culture over MSC feeder layer................................................................35
4.1.3.1 Irradiated and non irradiated feeder layer............................................35
4.1.3.2 Feeder layer exchanged and non exchanged co-culture ......................36
4.1.4 Co-culture over MSC pellet..........................................................................36
4.2 Study of the contact and non-contact co-culture...................................................38
4. 3 Function assays of contact and non-contact co-culture.........................................40
4.3.1 Phenotype screening of the expanded cells I.................................................40
4.3.2 CFC I.............................................................................................................42
4.3.3 Animal engraftment I........................................................................................42
4.4 Discussion..............................................................................................................44
Chapter 5 MSC and CB CD34+ cells 3D co-culture
5.1 Screening of 3D scaffolds......................................................................................47
5.2 Study of 3D and 2D co-culture..............................................................................48
5.3 Functional assays of 3D and 2D co-culture...........................................................49
5.3.1 Phenotype screening of the expanded cells II..................................................49
5.3.2 CFC II...........................................................................................................51
5.3.3 Animal engraftment II...................................................................................52
5.4 3D culture of MSC.....................................................................................................53
5.5 Discussion..............................................................................................................54
Chapter 6 Study of FN immobilization bioactivity
6.1 Physical and chemical characterization of conjugated and adsorbed FN..............58
6.1.1 FN immobilization efficiency.......................................................................58
6.1.2 XPS assay......................................................................................................59
6.1.3 Surface contact angle measurements.............................................................60
6.1.4 AFM assay.....................................................................................................60
iv
6.2 Biological characterization of conjugated and adsorbed FN..................................62
6.2.1 ELISA test for RGD domain.........................................................................62
6.2.2 Ex vivo cell experiment.................................................................................63
6.3 Discussion..............................................................................................................63
6.3.1 FN conformation and its bioactivity........................................................63
6.3.2 FN adsorption..........................................................................................64
6.3.3 FN conjugation and fibrillogenesis.........................................................64
Chapter 7 Conclusions
7.1 Conclusions……………………………..…………………………………………68
7.2 Future prospect……………………………………………………………….……71
References........................................................................................................................73
v
Summary
Allogeneic transplantation of CB is limited by insufficient number of HSC as
well as PHPC. An efficient ex vivo expansion of CB can overcome this limitation. An
ideal ex vivo expansion condition should mimic the in vivo hematopoietic
microenvironment, which can efficiently expand the cell number while still
maintaining the “stemness” of the cells.
MSC, as non hematopoietic, well-characterized skeletal and connective tissue
progenitor cells within the bone marrow stroma, have been investigated as support
cells for the culture of HSC/PHPC. They are attractive for the rich environmental
signals that they provide and their immunologic compatibility and supporting effect in
transplantation. In this study we clearly demonstrated the benefit of MSC inclusion in
HSC expansion ex vivo. We also compared the contact and non-contact cultures of
the two cell types; the cell-cell direct interaction was necessary for the efficient
expansion of HSC.
HSC-MSC co-cultures have only been performed in 2D configuration. We
postulate that a 3D culture environment that resembles the natural in vivo
hematopoietic compartment would be more conducive for regulating HSC expansion.
The result showed that the direct contact between MSC and HSC in 3D cultures led to
statistically-significant higher expansion of CB CD34+ cells compared to 2D co-
cultures: 891- versus 545-fold increase in total cells, 96- versus 48-fold increase of
CD34+ cells, and 230- versus 150-fold increase in CFC. NOD/SCID mouse
engraftment assays also indicated a high success rate of hematopoiesis reconstruction
with these expanded cells. This enhanced expansion result was probably due to the
ECM secreted by MSC, especially FN, which facilitated the interaction with the
seeded CD34+ cells.
vi
Based on this finding, we hypothesize that the immobilization of ECM protein
FN onto the surface of culture systems might represent another stroma free expansion
strategy. A key problem in FN immobilization is the change of bioactivity during the
immobilization process. By comparing the FN bioactivity after two common
immobilization pathways: conjugation and adsorption, we found that the adsorption
method maintained a normal conformation of FN, leading to less change of its
bioactivity in the access of RGD domains and cell adhesion ability. The covalent
conjugation process, on the other hand, induced FN unfolding and fibrillogenesis ex
vivo, which blocked the access of active RGD domains and suppressed fibroblast
cells adhesion. This bioactivity variance on the level of immobilization methods may
provide us a way to control the function of the immobilized protein, which can be
helpful for the construction of an artificial niche.
vii
List of Tables
Number Page
1. Surface markers expression of expanded cells I ...................................................41
2. The engraftment and lineage expression by human cells in the bone marrow of NOD/SCID mice I..................................................................................................43
3. Surface markers expression of expanded cells II ..................................................50
4. The engraftment and lineage expression by human cells in the bone marrow of NOD/SCID mice II.................................................................................................53
5. Element proportion on FN immobilized surface....................................................60
viii
List of Figures
Number Page
1. Hematopoietic system hierarchy..................................................................................6
2. Scheme of FN immobilization on aminated PET surface.........................................29
3. MSC metabolic level in MSCGM medium or Stemspan serum free medium with cytokines.................................................................................................................34
4. CB CD34+ cells expansion in 96-well or 24-well tissue culture plates..................34
5. CB CD34+ cells expansion on irradiated or non irradiated MSC feeder layers... 35
6. CB CD34+ cells expansion with or without exchange of MSC feeder layer in the middle of co-culture...................................................................................................36
7. CB CD34+ cells expansion on MSC feeder layer or adherent MSC pellet..............37
8. MSC pellet morphology under phase contrast microscope.......................................37
9. CB CD34+ cells expansion in contact co-culture or non-contact co-culture.........39
10. Cell-cell contact study by CMFDA and CD34 expression.......................................39
11. CFC properties of expanded cells I............................................................................42
12. SEM image of PET meshes and co-culture in meshes...........................................47
13. CB CD34+ cells expansion in PET woven or unwoven mesh...............................48
14. CB CD34+ cells expansion in 2D or 3D systems..................................................49
15. CFC properties of expanded cells II...........................................................................51
16. MSC attachment, proliferation and protein production in 2D or 3D systems...........54
17. SEM and Confocol image of MSC culture and co-culture in 2D or 3D system.......55
18. FN immobilization efficiency....................................................................................59
19. XPS survey of FN immobilized PET surface............................................................59
20. Surface contact angle on clean, modified or FN immobilized PET surface.............61
21. AFM image of FN immobilized PET surface............................................................61
22. ELISA measurement of active RGD domains in immobilized FN compared with Micro-BCA result.......................................................................................................62
23. Cell adhesion on FN immobilized PET surface.........................................................63
ix
24. Hypothetical schemes of FN fibrillogenesis in conjugation process ........................66
x
List of Symbols
BM: Bone marrow
BMP: Bone morphogenetic protein
CB: Cord blood
CFC: Cloning forming cell
CFU-E: Colony forming unit erythrocyte colony-forming units
CFU-GEMM: Colony forming unit granulocyte, erythrocyte, monocyte,
megakaryocyte
CFU-GM: Colony forming unit granulocyte/macrophage colony-forming units
CLP: Common lymphocyte progenitor
CMP: Common megacytes progenitor
ECM: Extra cellular matrix
EPO: Erythropoietin
FBS: Fetal bovine serum
FGF: Fibroblasts growth factor
FL: Flt-3 ligand
FN: Fibronectin
G-CSF: Granulocyte-colony stimulating factor
GM-CSF: Granulocyte/macrophage colony-stimulating factor
GMP: Granulocyte / monocyte progenitor
GVHD: Graft-versus-host disease
HGF: Hematopoietic growth factor
HLA: Human leukocyte antigen
HSC: Hematopoietic stem cells
IL: Interleukin
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LIF: Leukemia inhibitory factor
LTC-IC: Long-term culture initiating cell
LTR: Long term repopulating
M-CSF: Macrophage-colony stimulating factor
MkEP: Megakaryocytes, erythrocytes progenitor
MNC: Mono-nucleus cells
MSC: Mesenchymal stem cells
NOD/ SCID: Non-obese diabetic/ severe combined immunodeficiency
PB: Peripheral Blood
PET: Polyethylene terephthalate
PHPC: Primitive hematopoietic progenitor cell
SCF: Stem cell factor
STR: Short term repopulating
TCPS: Tissue culture plates
TGF: Transforming growth factor
TNF: Tumor necrosis factor
TPO: Thrombopoietin
VLA-4/5: Very late antigen-4/5
2D: Two dimensional
3D: Three dimensional
xii
CHAPTER 1
INTRODUCTION
1.1 Purpose
All mature cells in the blood and immune system are derived from HSC and
all of these cell types can be generated from a single HSC [Osawa et al (1996)]. This
tremendous potential for reconstituting the hematopoietic system has allowed the
development of transplantation of HSC as a clinical strategy. HSC obtained directly
from the patient (autologous HSC) are used for rescuing the patient from the effects of
high doses of chemotherapy or used as a target for gene therapy vectors. HSC
obtained from another person (allogeneic HSC) are used to treat hematological
malignancies by replacing the malignant hematopoietic system with normal cells.
Allogeneic HSC can be obtained from siblings (matched sibling transplants), parents
or unrelated donors (mismatched unrelated donor transplants). Currently, about
45,000 patients each year are treated by HSC transplantation, a number that has been
increasing during the past decade.
There are several different sources for obtaining HSC for transplantation. One
important new source of HSC is CB that is collected during newborn deliveries. In
addition to their widespread availability, these HSC have several useful properties,
including their decreased ability to induce immunological reactivity against the
patient because of increased levels of immune tolerance in the fetus. Interest in this
approach has increased since the first successful transplantation of cord-blood HSC in
1988 [Gluckman et al (1989)], and there are now an estimated 70,000 units of CB that
are stored and available for transplantation [Gluckman (2001)]. However, their use is
limited by the number of HSC that can be collected, and it is clear that engraftment is
1
closely correlated with the number of cells that are infused [Wagner et al (2002)]. For
optimum engraftment, current transplantation requires at least 2,500,000 CD34+ cells
per kilogram of patient body weight [Koller et al (1993)]. An optimally collected CB
donation generates approximately 10,000,000 CD34+ cells, which is just 10-15% of
the optimal dose for an adult. For this reason, CB transplantation is limited to
pediatric patients right now. Furthermore, because the stem cells in CB are more
primitive than those in marrow, the hematopoietic recovery using CB stem cells is
approximately 25 days for neutrophils and 43 days for platelets [Leary et al (1988)],
which is significantly slower than that obtained in BM transplantation. And it leaves
the patient vulnerable to a fatal infection for a longer period of time.
To address these limitations, an effective strategy to expand CB cells ex vivo,
increasing stem cells for transplantation would considerably improve the therapeutic
potentials. What’s more, the ex vivo expansion produces more mature precursors,
supplementing stem cells graft, which help to shorten and potentially prevent
chemotherapy induced pancytopenia. The purpose of the research presented here is to
develop a more compatible and efficient system of increasing CB ex vivo expansion,
while still keeping the stem cells properties.
1.2 Motivation
The ability of environmental cues to affect stem cell fate is a concept from
embryology that has been extended to the adult organism with the notion that a highly
specialized set of conditions exists to regulate the ultimate fate of anatomically
restricted primitive cells. The physical interaction of stem cells with stroma cells,
matrix components and supportive factors is thought to define a niche critical for the
homeostatic maintenance of stem cell populations. For example the HSC changes the
2
location during development moving from extra embryonic regions to the aorto-
gonadal-mesonephros in the embryo and then to the liver in the first trimester. By the
second trimester, BM, spleen and thymus become populated and eventually become
the exclusive sites of hematopoiesis. With each change in location is an associated
change in the production emphasis of the stem cell population, with stem cell
expansion and erythropoiesis in great dominance early and a more balanced, full
immune repertoire of expansion later. Inspired by the research in hematopoiesis
microenvironment, we planned to reconstruct the hematopoiesis niche ex vivo for
more reliable expansion of CB cells.
1.3 Objectives and Scope
The objective of reconstructing hematopoiesis microenvironment ex vivo for
more efficient HSC expansion is achieved by the following specific approaches:
• The incorporation of MSC as stroma in supporting CB expansion (Chapter 4).
MSC are well characterized skeletal and connective tissue progenitor cells
within bone marrow. They can provide rich environment signals and show good
immunological compatibility in transplantation. For these reasons, we incorporated
MSC into expansion system. First of all, we screened and optimized the co-culture
conditions, and then we studied on the direct cell-cell contact effect during the co-
culture. The expansion result was evaluated by functional assays ex vivo and in vivo.
• The incorporation of 3D scaffold in co-culture of CB and MSC (Chapter 5).
We postulated that 3D culture environment that resembles the natural niche
would be more conductive for CB expansion, thus we incorporated the 3D scaffold
into MSC and CB co-culture system. We first screened and optimized the 3D scaffold
for co-culture, and then we compared the 2D co-culture with 3D co-culture through
3
expansion and functional assays. Finally we studied MSC culture and its ECM
secretion in 3D scaffold, which probably explained the higher expansion result in 3D.
• The bioactivity study of immobilized FN, as a supportive research for its
further application in CB expansion (Chapter 6)
Inspired by the co-culture result that FN was specially secreted by MSCs in
supporting CB’s expansion, we hypothesized that the immobilization of FN onto the
surface of culture system may represent another stroma free expansion strategy. A key
problem in this strategy is how to control the bioactivity of immobilized FN. In this
study, we aimed to study the bioactivity of FN after different immobilization
pathways: conjugation and adsorption. The chemical and physical properties of the
immobilized surfaces were studied by Micro-BCA, surface contact angle with water
and AFM. The biological properties of the immobilized surfaces were evaluated by
fibroblast adhesion assay and ELISA test for active RGD domain.
4
CHAPTER 2
LITERATURE SURVEY
2.1 Review of HSC and its clinic application
2.1.1 Hematopoiesis and HSC
Blood cells are responsible for constant maintenance and immune protection
of every cell type of the body, which can be classified into two main classes,
lymphoid cells (T, B, and natural killer cells) and myeloid cells (granulocytes,
monocytes, erythrocytes, and megakaryocytes). All blood cells have a limited lifespan:
several hours for granulocytes, several weeks for red blood cells, and up to several
years for memory T cells. Thus, a vast amount of blood cells must be permanently
produced. In normal adults, the body produces about 2.5 billion red blood cells, 2.5
billion platelets, and 10 billion granulocytes per kilogram of body weight per day
[Williams et al (1990)]. Hematopoiesis: the regulated production of eight different
blood cells is a continual process that is the result of proliferation and differentiation
of a small common pool of pluripotent cells called HSC, committed progenitor cells,
and differentiated cells. Within these three stages, extensive expansion of cell
numbers occurs through cell division. A single stem cell has been proposed to be
capable of more than 50 cell divisions or doublings and has the capacity to generate
up to 1015 cells, or sufficient cells for up to 60 years [Kay (1965)].
Alexander Maximow, working in St. Petersburg as a military doctor in 1909,
was the first to suggest that there is a HSC with the morphological appearance of a
‘lymphocyte’ capable of migrating through the blood to micro-ecological niches that
would allow them to proliferate and differentiate along lineage specific pathways
[Maximow (1909]]. The first evidence and definition of blood-forming stem cells
5
came from studies of people exposed to lethal doses of radiation in 1945. Basic
research soon followed. After duplicating radiation sickness in mice, scientists found
they could rescue the mice from death with bone marrow transplants from healthy
donor animals. In the early 1960s, Till and McCulloch began analyzing the bone
marrow to find out which components were responsible for regenerating blood [Till
and McCullough (1961)]. They defined what the two hallmarks of an HSC remain: it
can renew itself and it can produce cells that give rise to all the different types of
blood cells. For these characters, HSC has huge application in clinics.
Figure 1 Hematopoietic system hierarchy. Dividing pluripotent stem cells (LTR and STR) may undergo self-renewal to form daughter cells without loss of potential, and also experienced a concomitant differentiation to form oligopotent daughter cells (CLP, CMP, and CMP lead to GMP and MkEP) with more restricted potential. Continuous proliferation and differentiation result in the production of many mature cells. Macrophages are obtained from monocytes and similarly platelets are derived from megakaryocytes.
2.1.2 HSC clinic application
A: Leukemia and Lymphoma
6
Among the first clinical uses of HSC were the treatment of cancers of the
blood—leukemia and lymphoma, which result from the uncontrolled proliferation of
white blood cells. In these applications, the patient’s own cancerous hematopoietic
cells were destroyed via radiation or chemotherapy, and then replaced with a bone
marrow transplant, or with a transplant of HSC collected from the peripheral
circulation or the CB of matched donors.
B: Inherited Blood Disorders
Another use of allogeneic bone marrow transplants is in the treatment of
hereditary blood disorders, such as different types of inherited anemia (failure to
produce blood cells), and inborn errors of metabolism (genetic disorders characterized
by defects in key enzymes needed to produce essential body components or degrade
chemical byproducts). The blood disorders include aplastic anemia, beta-thalassemia,
Blackfan-Diamond syndrome, globoid cell leukodystrophy, sickle-cell anemia, severe
combined immunodeficiency, X-linked lympho proliferative syndrome, and Wiskott-
Aldrich syndrome.
C: Cancer Chemotherapy
Chemotherapy aimed at rapidly dividing cancer cells inevitably hits another
target—rapidly dividing hematopoietic cells. Doctors may give cancer patients an
autologous stem cell transplant to replace the cells destroyed by chemotherapy. They
do this by mobilizing HSC and collecting them from peripheral blood. The cells are
stored while the patient undergoes intensive chemotherapy or radiotherapy to destroy
the cancer cells. Once the drugs have washed out of a patient’s body, the patient
receives a transfusion of his or her stored HSC.
D: Other Applications
7
For example: One of the most exciting new uses of HSC transplantation puts
the cells to work attacking otherwise untreatable tumors.
Most of the therapies above require the use of allogeneic HSC Transplantation,
as the patients’ own HSC were damaged or dysfunctional. However, there was
significant risk of patient death soon after the transplantation either from infection or
from GVHD. Clinics usually try to find a matched donor who is typically a sister or
brother of the patient who has inherited similar HLA on the surface of their cells to
reduce the risk. In a study where all patients received an HLA-matched transplant
from a sibling, and the results were controlled for age, the relative risk of CB versus
bone marrow was 0.41 for acute GVHD and 0.35 for chronic GVHD[Kondo et al,
2003].
2.1.3 Different sources of HSC in transplantation
A: BM
The classic source of HSC is bone marrow. For more than 40 years, doctors
performed bone marrow transplants by anesthetizing the stem cell donor, puncturing a
bone—typically ilium or iliac bone of pelvis —and drawing out the bone marrow
cells with a syringe. About 1 in every 100,000 cells in the marrow is a long-term,
blood-forming stem cell; other cells present include stroma cells, blood progenitor
cells, and mature and maturing white and red blood cells.
B: PB
It has been known for decades that a small number of stem and progenitor
cells circulate in the bloodstream, but in the past 10 years, researchers have found that
they can coax the cells to migrate from marrow to blood in greater numbers by
injecting the donor with a cytokine, such as GCSF. The donor is injected with GCSF a
8
few days before the cell harvest. To collect the cells, doctors insert an intravenous
tube into the donor’s vein and pass his blood through a filtering system that pulls out
CD34+ white blood cells and returns the red blood cells to the donor. For clinical
transplantation of human HSC, doctors now prefer to harvest donor cells from
peripheral, circulating blood which is easier on the donor—with minimal pain, no
anesthesia, and no hospital stay. The peripherally harvested cells contain twice as
many HSC as stem cells taken from bone marrow and engraft more quickly. This
means patients may recover white blood cells, platelets, and their immune and clotting
protection several days faster than they would with a bone marrow graft.
C: CB
In the late 1980s and early 1990s, physicians began to recognize that blood
from the human umbilical cord and placenta was a rich source of HSC. This tissue
supports the developing fetus during pregnancy, is delivered along with the baby, and,
is usually discarded. Since the first successful umbilical CB transplants in children
with Fanconi anemia, the collection and therapeutic use of these cells has grown
quickly.
Scientists in the laboratory and clinic are beginning to measure the differences
among HSC from different sources. In general, they find that HSC taken from tissues
at earlier developmental stages like CB may hold an advantage in telomere length for
a greater ability to self-replicate, and are less likely to be rejected by the immune
system—making them potentially more useful for therapeutic transplantation. The
accepted explanation is that CB carries much less GVHD than bone marrow because
the newborn baby is "immunologically immature" – for example, the immune system
has not had time to be exposed to various foreign bodies and develop reactions against
them [Rocha (2000)].
9
2.2 Review of HSC expansion
There are three major aspects of stem cell expansion that impact clinical
feasibility.
• First is the ability to isolate and characterize an optimal candidate cell
population for expansion.
• Second is to define the desired characteristics of the population to be used for
engraftment including ways to measure those characteristics.
• Third is the development of reagents and reproducible procedures to support
the culture of a highly defined cell product for transplantation.
2.2.1 Selection of candidate cells for expansion
There are a number of issues related to the selection of a candidate stem
population for ex vivo expansion. Before determining if a protocol results in the
expansion of a stem cell, one must define what a stem cell is. Based on transplantation
of marked stem cells in murine model system, a “stem cell” is defined as a cell with
multi-lineage in vivo hematopoietic reconstituting potential, but has not yet been
proven in humans. Instead, monoclonal antibodies that react with cell-surface proteins
on hematopoietic cells have been used to define and isolate populations of candidate
stem cells phenotypically for further analysis. It is reasonable, though, to assume that
under ideal conditions one would try to expand the most primitive population that one
could identify. However, because of the close timing between preparative regimen
and re-infusion, the standard long-term biological assays for stem cells are impractical
for quantitating the level of expansion. Therefore, the only practical criteria for
infusion would likely be based on phenotypic analysis of the expanded product with
retrospective determination of hematopoietic potential ex vivo. Consequently, it is
10
important to choose a population for expansion that displays a strong correlation
between phenotype and functional activity.
It is widely held that the cell-surface glycoprotein CD34 is expressed on
almost all cells with discernable hematopoietic potential [Krause et al (1996)], as
determined by ex vivo models of progenitor cell proliferation such as CFC and
LTCIC assays [Sutherland et al (1990)]. In more recent years, though, additional
markers have been used to fractionate the CD34+ cells further into lineage-committed
progenitors and more primitive “stem” cells [Terstappen et al (1991)]. Data from
evaluation of sub fractions of CD34+ cells indicated that those cells expressing a
CD34+/CD38- phenotype are highly enriched for very primitive hematopoietic cells
[Baum et al (1992)]. In general, there is a strong correlation between these cell-
surface phenotypes and biological activity in primary cells isolated from different
tissue sources (fetal liver and BM, CB, PB).
2.2.2 Evaluation of ex vivo expansion
Ultimately, preservation of cell function is the determining criteria in the
success of ex vivo human HSC culture systems. Cell function can be queried ex vivo
using the CFC and LTC-IC assays, techniques that detect the presence of committed
and multi potent cells on the basis of the formation of morphologically distinguishable
colonies either directly in methylcellulose cultures (CFC), or after greater than 5
weeks on stroma feeder layers (LTC-IC) [Hogge et al (1996)]. In vivo functional
assays for HSC have been developed on the basis of the ability of these cells to
reconstitute hematopoiesis in the BM of SCID/NOD mice following intravenous
injection [Vormoor et al (1994)]. These so-called NOD/SCID-repopulating cells are
considered human HSC that home to and engraft the murine bone marrow, where they
11
subsequently proliferate and differentiate into multiple lineages [Cashman et al
(1997)].
2.2.3 Expansion systems
HSC must maintain a balance between self-renewing cell divisions (needed to
ensure their existence throughout the life of an organism) and differentiation (needed
to replace the loss of mature cell populations).The successful maintenance and
expansion of transplantable HSC ex vivo requires triggering stem cells to proliferate
without undergoing differentiation. Cultured HSC must additionally avoid apoptosis
and maintain the ability to home, engraft, and differentiate into appropriate receptive
tissues. Because the regulation of these processes involves interactions between cell-
surface receptors and a large group of soluble and matrix bound modulators of cell
function, many investigators have focused on developing culture systems
supplemented with stroma, appropriate combinations of stimulatory factors to
maintain and expand HSC ex vivo. Primarily the ex vivo expansion processes can be
divided into two groups: stroma dependent and stroma independent.
2.2.3.1 Stroma dependent expansion
Before the development of recombinant human cytokines, in the traditional
cultivation process, a stroma layer was known to produce ECM components and
growth factors to support PHPC expansion. The first successful hematopoietic culture
was performed by Dexter’s group, who used murine BM to develop a confluent
stroma layer [Dexter et al (1973)]. The addition of hydrocortisone to the medium
allowed the development of human BM stroma layers, which were found to support
HSC expansion from a variety of sources. [Quesenberry et al (1991)]. A second
12
source of BM or CB cells could be used to initiate this two-step stroma culture.
Gartner [Gartner and Kaplan (1980)] first identified conditions for the simultaneous
development of a stroma layer and stroma dependent hematopoiesis, a system which
is now termed one-step stroma culture. Both types of stroma cultures support HSC
expansion for a prolonged time period, but after approximately 6-8 weeks, the stroma
layer begins to lose integrity and hematopoiesis stopped. Recently the use of FGF-4
has been found to enable faster development and longer maintenance of healthy
stroma layer [Quito et al (1996)].
The finding that more frequent medium exchange increase the productivity of
stroma cultures [Schwartz et al (1991)] led to the development of perfusion bioreactor
systems. The first two perfusion systems consisted of flat bed chambers containing
stroma co-cultures that were perfused with media. They successfully increased the
expansion of total cells, CFC, and LTC-IC compared with static controls [Koller et al
(1993)]. A number of other reactor systems have been used with less success.
Sardonini and Wu investigated the use of stirred micro carrier, airlift and hollow fiber
systems for the culture of BM MNC and CD34+ cells, in all cases they found poor
expansion compared with static cultures [Sardonini and Wu (1993)]. An airlift
packed-bed system was used for murine BM cultures, but this required the use of
exogenous stroma-conditioned medium, and the cells were difficult to harvest from
the packed bed [Highfill et al (1996)]. A micro porous collagen-sphere perfusion
system has been developed for BM cultures, but an unidentified factor has been
shown inhibit the production of CFU-GM colonies [Wang et al (1995)].
Though use of stroma cells in ex vivo culture is advantageous for maturation
and attainment of functional properties in progenitor cells, stroma may pose several
disadvantages in clinical applications: (1) the use of allogeneic source of stroma cells
13
could be potentially harmful due to additional loading of miss-matched tissue, for
example GVHD and donor-mediated infection, (2) ex vivo expansion of PB/CB stem
cells for autologous transplantation requires an additional invasive harvest of patient
bone marrow to produce a preformed stroma layer, and (3) due to the adherent nature
of progenitor stem cells harvesting of cells may require harsh treatment that could be
detrimental to the desired cells.
2.2.3.2 Stroma independent expansion
With the advent of recombinant cytokines and CD34+ selection technologies,
it became possible to develop stroma-free culture systems with significant expansion
of hematopoietic cells. Most of these cultures have been performed in well-plates and
T-flasks utilizing CD34+ cells. For the scale-up of these cultures for clinical trials,
most investigators have chosen to use large culture bags, in which cells grow
undisturbed until harvest. It has been demonstrated that CD34+ cells or MNC cultured
in these environments produce qualitatively indistinguishable cells [Koller et al
(1995)].
A modification of a flat-bed culture chamber has allowed the development of
stroma-free hematopoietic perfusion culture [sandstorm et al (1996)]. The chamber
contains multiple microgrooves (perpendicular to the direction of the flow of medium)
that allow the diffusion of nutrition but retain cells in the chamber. Considerable
success has also been demonstrated in a stirred spinner flask culture at low agitation
rates using a high seeding density of BM MNC. Although there is no stroma layer in
this stirred system, it was demonstrated that the culture maintained and expanded
stroma cells precursors in suspension along with hematopoietic progenitors. Stirred
14
suspension systems have also been successfully used for both serum containing and
serum free culture of PB cells and CB cells.
Despite many of the positive effects of stroma cells can be partly substituted
by either stroma conditioned medium or recombinant growth factors, in the absence
of stroma support, the expansion of CB based on HGF-supplemented culture has been
suboptimal. Phase I clinical trials conducted on UCB expanded in cytokine-
supplemented culture have failed to demonstrate more rapid hematopoietic recovery
in CB recipients [Pecora et al (2000); Kogler et al (1999); Koller et al (1998)]. This
suggests a loss of stem cell function in the cytokine-based expansion in spite of an
increase in CD34+ cell dose. These observations corroborate recent advances in the
cellular and molecular mechanisms underlying the regulation of stem cell
differentiation, indicating that microenvironment or stem cell “niche” is crucial for
maintenance of self-renewal capacity of stem cells [Gupta et al (2000); Kiger et al
(2000); Tran et al (2000)].
2.3 Review of HSC microenvironment
In mammals, hematopoiesis occurs in the extra vascular spaces between
marrow sinuses, in a specialized area, known as niche. The stem cells niche is yet to
be described in the ultra structure level, but it is believed that niches provide
microenvironment to meet certain functional requirements of hematopoiesis. The
microenvironment is characterized by the local geometry and cellular arrangement, by
stroma cells, by the chemical nature of the microenvironment in terms of cytokines
and ECM components.
2.3.1 Cytokine microenvironment
15
The cytokine microenvironment in these cultures is dynamic [Zandstra et al
(1997)], with multiple cell types competing for cytokines and nutrients, many of
which are capable of influencing stem cell fate directly or indirectly. These complex
interactions make the cytokine composition of HSC expansion medium particularly
challenging to optimize. Furthermore, as culture time progresses, the composition of
the culture system can change dramatically and the endogenous production of factors
that have a suppressing effect on HSC self-renewal may significantly influence
culture output. Designing successful HSC expansion cultures thus requires strategies
to maintain an appropriate balance between stimulatory and inhibitory modulators of
HSC function.
Positive modulators of HSC function
Among the different stimulatory cytokines identified in the above mentioned
and other studies, combinations of SCF, FL, and TPO have been shown to be
particularly effective in eliciting self-renewal divisions of human HSC.
SCF, the ligand for the tyrosine kinase receptor c-Kit, promotes the survival
and growth of primitive hematopoietic progenitors when used in combination with
other stimulatory cytokines [Broudy (1997)]; little expansion is observed in cultures
supplemented with SCF alone [Ohmizono et al (1997)].
FL, a factor secreted by stroma cells and which has documented similarities to
SCF, has also been extensively investigated as a potential mediator of HSC expansion
and maintenance [Lyman and Jacobsen (1998)]. FL is believed to act by recruiting
HSC into the cell cycle [Haylock et al (1997)] and by promoting survival,
presumably through inhibition of apoptosis [Meyer and Drexler (1999)].
TPO supports the proliferation and long term maintenance of primitive
hematopoietic progenitors and repopulating HSC when used in combination with
16
other stimulatory cytokines [Won et al (2000)]. When used alone, TPO has little
effect on the expansion of HSC in suspension cultures. However, in the presence of
stroma cells, TPO may promote sustained HSC expansion. Thus, TPO may play a
key role in the long-term maintenance and development of primitive HSC in vivo, and
may be a useful additive for successful HSC expansion ex vivo [Luens et al (1998)].
Although most studies have focused on “hematopoietic” cytokines, other
groups have identified novel factors, not originally identified as a result of their
effects on hematopoietic cells that may regulate HSC responses. For example, Bhatia
et al. [Bhatia et al (1999)] reported that BMP can modulate the proliferation and
differentiation of primitive human hematopoietic cells and that BMP type-1 receptors
were expressed, along with downstream transducers of the BMP signaling pathway, in
subpopulations of hematopoietic progenitors. Recently, the addition of the Notch
ligand, Jagged-1, to cultures of primitive progenitor cells induced the survival and
expansion of repopulating HSC [Karanu et al (2000)] implicating Jagged-1 as another
novel growth factor for the modulation of human HSC function [Varnum et al (2000)].
Negative modulators of HSC function
In spite of extensive investigations, ex vivo expansion systems supplemented
with combinations of stimulatory cytokines have not, to date, yielded sustained HSC
growth, suggesting that HSC expansion may be limited by other factors. Ex vivo
production of inhibitory proteins is one potential reason for the transient expansion
observed in many cultures. Protein factors, such as TGF-b, TNF-a, and, more recently,
IL-3 have been shown to decrease repopulating potential by inducing differentiation
or apoptosis, as well as by decreasing the ability of repopulating stem cells to migrate
to the bone marrow microenvironment. For these inhibitory cytokines to affect HSC
growth ex vivo, they must be generated in culture. Evidence showing that this occurs
17
has been obtained from the analysis of hematopoietic progenitor cultures, where both
mRNA and protein expression of inhibitory and stimulatory factors have been
measured from stroma and progenitor cell populations [Majka et al (2001)].
2.3.2 Stroma microenvironment
The stroma cells include endothelial cell that line the marrow sinuses,
adventitial reticular cells, macrophages, and adipocytes. These stroma cells synthesize
growth and differentiating factors, and ECM components. The stroma cells and the
ECM form 3D scaffolding upon which the HSC lodge. The stroma cells through their
intimate physical contacts with the hematopoietic cells, through the ECM components
and the growth factors they secrete create an intricate hematopoietic inductive
microenvironment, which promotes and regulates hematopoiesis [Dexter (1989)]. It is
this type of inductive microenvironment is termed hematopoietic niche.
Endothelial cells have been observed to produce constitutively G-CSF and
GM-CSF [Brockbank et al (1986)]. They are also capable of transporting plasma
derived substances through their cytoplasm into the hematopoietic space. Adventitial
reticular cells are fibroblastoid cells that ectopically synthesize a number of growth
factors, e.g., G-CSF, M-CSF, GM-CSF, IL-1, IL-6, SCF, FL, LIF [Bianco et al
(2001)].Adventitial reticular cells are capable of taking up fat in the presence of
corticosteroids and converted into adipocyte [Greenberger (1978)]. In young age
marrow is red in color and cellular, with the age hematopoietic activity is reduced and
the marrow is composed of large proportion of fat cells (yellow marrow) [Tavassoli et
al (1974)]. Macrophages are critical in controlling erythropoiesis, which proceeds in
isolated islets surrounding an associated macrophage nurse cell in the BM [Crocker
and Gordon (1987)].Macrophages are considered to be a major source of cytokines
18
under normal conditions; these are, GM-CSF, IL-1 EPO [Rich (1988)]. They also
produce hematopoiesis inhibitory cytokine like, TNFa and IFNg.
2.3.3 ECM microenvironment
In BM, ECM components are mainly produced by endothelial and adventitial
reticular cells. The ECM of BM consists of collagen types I, II, IV [Zuckerman and
Wicha (1983)], laminin, FN [Bentley and Tralka (1983)], vitronectin [Coulombel et al
(1988)], thrombospondin [Long and Dixit (1990)], hemonectin [Campbell et al
(1987)], and proteoglycans with glucosaminoglycan side chains, such as heparan
sulfate and hyaluronic acid [Gordon et al (1988)]. ECM provides support and
cohesiveness for the marrow structure. There is a growing body of evidence
indicating that ECM is important for the regulation of hematopoiesis. For example
glucosaminoglycan side chains of heparan sulfate can sequester and compartmentalize
certain cytokines in local areas and present them to HSC [Roberts et al (1988)]. This
is the mechanism for presenting high, localized concentrations of growth factors that
are protected from proteolysis. Another important function of ECM is to provide
anchorage for immature hematopoietic cells.
2.3.4 3D topology
The successful reconstitution of long-term ex vivo hematopoiesis can perhaps
be best simulated by mimicking in vivo conditions. As mentioned earlier, the
hematopoietic microenvironment is composed of different stroma cells that secrete
ECM components and various soluble growth factors distributed in three-dimensional
space. Thus it was hypothesized that growth of stem cells on a three-dimensional
culture system will be a close replica of hematopoietic microenvironment.
19
Following the concept of the three-dimensional culture system, in 1998,
Mantalaris et al. reported culture of human bone marrow cells on porous collagen
sphere in a packed-bed bioreactor [Mantalaris et al (1998)]. Mononuclear cells of
healthy adult donors were cultured on collagen macro porous spheres in the presence
of SCF, IL-3, GM-CSF, IGF-I, and EPO for four weeks. A few significant
observations extracted from this report are: (a) microscopic examination of the thin
section reveals that the marrow cells populated the pores of the micro spheres in a 3D
fashion with cell-to-cell contact and the resulting interactions resembled the bone
marrow tissue structure in vivo; (b) morphological and staining techniques ensured
the presence of erythroblastic islands of matured red cells and monocytes; (c) no
adipocytes and blanket cells were observed in this culture. In this culture system, cells
of all lineages were present in significant levels, as compared to the 2D Dexter’s
culture.
In the recent past, two different three-dimensional culture systems were
reported for hematopoietic stem cells. The first was tantalum coated porous carbon
matrix, known as Cellfoam [Banu et al (2001)], and the other was non woven PET
fabric [Li et al (2001)]. It was shown that low concentrations of early acting cytokines
SCF and FL result in higher yields of cells that were enriched with primitive
progenitor and multipotent hematopoietic progenitor cells in a 3D Cellfoam bioreactor
as compared to plastic culture flasks. Similarly, 3D non woven PET matrix was
reported to produce two- to three fold more CD34+ cell when compared with the 2D
system in seven to nine weeks culture in the presence of TPO and FL. It was also
shown that both stroma and hematopoietic cells were distributed spatially within the
scaffold and facilitated cell-cell and cell-matrix interactions closure to the in vivo
bone marrow microenvironment. Despite the feasibility of culturing progenitor cells
20
on 3D supports, this system was limited by the problems such as insufficient
expansion of mononuclear cells and difficulties in harvesting cells from the scaffold
matrix.
2.4 Protein immobilization and bioactivity study
In the past twenty years different strategies were utilized to immobilize
functional protein like FN onto varied biomaterials surface. Adsorption was the most
commonly adopted method. Although less stable and less controllable, adsorption is
partially reversible leading to a relatively slow change of conformation of the
adsorbed molecules and a slow release of adsorbed molecules through exchange
reactions. The pioneering work of Grinnell and Feld in the study of adsorption
characteristics of plasma FN reported the higher adsorption and lower bioactivity of
FN on the hydrophobic substrate than on the hydrophilic surfaces [Grinnell and Feld
(1982)]. And it was confirmed by the following studies [George et al (2004);
Kowalczynska et al (2005)].
Recently there has been interest to immobilize FN to surfaces using covalent
coupling chemistries via the amine, thiol, and aldehyde groups [Staffan et al (1997);
Biran et al (2001)]. Covalent coupling presumably would be more efficient and stable
against detachment. A critical issue of the covalent strategy is the conservation of the
desired bioactivity of the immobilized FN. The degree of activity depends on the
binding procedure and the substrate. A combination of conjugation and adsorption can
sometimes produce the optimal results. By conjugating FN to the terminals of
PluronicTM F108 adsorbed on polystyrene, Biran et al. concluded that the FN was
more bioactive than the FN adsorbed directly onto polystyrene as revealed by the
neurite adhesion and outgrowth assay [Biran et al (2001)].
21
However, comparing the adsorption method with the covalent conjugation
approach in the literature is difficult because of the different chemistries and surfaces
used. In addition, the presence of serum and trypsin in the cell adhesion study, which
may alter the composition of the immobilized protein [Andrade and Hlady (1986)] or
degrade the cells surface integrins, can also confound the comparison.
22
CHAPTER 3
MATERIALS AND METHODS
3.1 Materials
3.1.1 Biological materials
CB CD34+ cells were purchased from AllCell Inc. (San Mateo, CA, USA) and
thawed according to reference [Rubinstein et al (1995)]. The purity of CD34+ cell was
91% with a viability of 95%. Recombinant human SCF, FL-3, TPO and IL-3, the
media used in hematopoietic stem cell expansion (StemSpan SFEM Medium) and
CFC assay (MethoCult GF+ H4435) were all purchased from StemCell Technologies
(Vancouver, BC, Canada). Purified human bone marrow MSC and MSC Growth
Medium - MSCGM PoieticsTM were purchased from Cambrex Bio Science
(Walkersville, MD, USA). BHK21 cell line was obtained from Cambrex Walkersville
Inc. (Maryland, USA). RPMI 1640 medium, FBS, penicillin and streptomycin were
purchased from Sigma Aldrich (St. Louis, USA).
3.1.2 Chemical materials
Bi-axially oriented PET films about 100um thickness were purchased from
Goodfellow Inc. (Cambridge, UK). Sterilized SEFAR PETEX 07-1/2 PET woven
mesh was purchased from Sefar Ltd. (Singapore) or Non woven PET Fibra-Cel® was
purchased from New Brunswick Scientific Co. Inc. Human plasma lyophilizated and
sterile FN, MTT were purchased from Roche Ltd. (Indiana, USA). Ethylenediamine,
Glutaradehyde, Ethanolamine, cyanoborohydride, were obtained from Sigma-Aldrich
Co. (St. Louis, USA). Micro BCA, BCA, Goat anti-rabbit IgG peroxidase conjugated
were obtained from PIERCE Biotechnology Inc. (Rockford, USA). 24/96 wells tissue
23
culture plate was from NUNC. Anti-VCIP, RGD domain, Human (rabbit) was
purchased from Oncogene Science Inc. (Cambridge, USA). Monoclonal anti-collagen
type I and IV, monoclonal anti-laminin, monoclonal anti-FN, and secondary antibody
were purchased from Sigma (Saint Louis, Missouri USA). CellTracker Green
CMFDA was purchased from Molecular Probes, Inc (Eugene, OP, USA)
3.2 MSC and CB CD34+ cells co-culture
3.2.1 MSC culture
Cryopreserved cells were thawed and plated in tissue culture flask (Nunc, New
York, USA). Adherent MSCs were maintained in MSCGM, and were harvested when
they reached 80-90% confluence with 1X trypsin/EDTA solution PBS for 5 min, and
then were diluted and inoculated into a fresh tissue culture flask. Cells of passage
around 4 were used for co-culture.
3.2.2 Ex vivo expansion of CB CD34+ cells in co-culture systems
For the 2D (contact) co-culture, MSCs (2.5 x 104 cells) were seeded into each
well of a 24 well plates (Nunc, New York, USA) and grown to confluence. After
washing 3 times with PBS, 600 CB CD34+ enriched cells suspended in 600µl
StemSpan SFM containing 40ug/ml low density lipoprotein (from Academy
Biomedical Company; Houston, USA), 100ng/ml SCF, 100ng/ml FL-3, 50ng/ml TPO,
and 20ng/ml IL-3 were seeded onto the MSC layer in each well. The cells were
cultured for 10 days without medium change at 37oC and 5% CO2. Plain 24-well plate
without the MSC layer was used as a control (2D or TCPS control).
For non-contact co-cultures (Transwell co-culture), MSC feeder layer was
prepared in 24-well plates as described above and incubated in 600ul of supplemented
24
StemSpan medium. A Nunc CC Insert (sterile Polycarbonate membrane, 0.4um pore
sized, pore density 1.5 x 108 pores per cm2, film thickness 20 microns, height above
the base of well 1mm, culturing area 0.5cm2), was placed into each well and seeded
with 600 CB CD34+ enriched cells in an additional 600 ul of supplemented StemSpan
medium. 10 days expansion at 37oC and 5% CO2. Plain 24-well plate with cell insert
was used as a control (Transwell control).
For irradiated MSC co-culture: MSCs were harvested and given irradiation at
15 Gy from a 60Co source. 1 x 105 irradiated cells were seeded into each well of 24-
well plates and formed confluent layer. After 24 hours adhesion, the adherent layers
were washed 3 times with PBS. 600 CB CD34+ enriched cells were added to the
feeder layer, with 600µl supplemented StemSpan medium as described above.
For MSC pellet co-culture: harvested MSCs were incubated in high density at
37ºC over night. After the formation of stable pellets, the MSC pellets were
transferred to the tissue culture plates. After 24 hours adhesion, the adherent pellets
were washed 3 times with PBS. 600 CB CD34+ cells were added to each well of 24-
well plates, suspended in 600µl supplemented StemSpan medium as described above.
At the end of 10-day expansion, cells were harvested by repeated pipetting and
counted using a hemacytometer. Antibodies against CD34, CD45 and other mature
cell surface markers (CD38, Glycophorin A [GlyA], CD15, CD3, CD19 from Becton-
Dickinson and CD41 from Dako) were incubated with cells on ice for 30 min and
analyzed using the FACSCalibur (Becton-Dickinson). At least 20,000 events were
acquired for each analysis. Analysis of flow cytometry data was carried out using
CellQuest Pro. CD34+, CD34+CD38- percentages were calculated as percentages of
blood cells gate (set on CD45+, low side scatter cells of the expanded cells, and
maintained throughout analysis).
25
3.2.3 CFC
After a 10-day culture, 1-3×103 expanded cells or 300-500 unexpanded cells
were harvested and transferred to 1.1 ml H4435 medium and plated 35-mm sterile
Petri dishes (StemCell Technologies). After incubation for 14 days at 37°C in a
humidified atmosphere at 5% CO2, granulocyte-macrophage, erythroid and multi-
lineage colony-forming cells (CFC-GM, CFC-E and CFC-GEMM) were counted
under an inverted microscope.
3.2.4 Animal engraftment
NOD/ SCID mice were purchased from Jackson Laboratories (Animal
Resource Centre in Perth, Australia) and maintained at the animal holding unit of
National University of Singapore. All animals were handled according to Institutional
regulations, under sterile conditions and maintained in micro-isolator cages. Mice to
be transplanted were given total body irradiation of 350 cGy from a 60Co source at
age of 6 to 8 weeks. Within 24 hours, expanded cells were harvested and injected into
the tail vein of the irradiated mice. 2×105 human bone marrow CD34 depleted cells
were irradiated with 1500cGy and injected together as carrier cells.
To assess the engraftment efficiency of the expanded human cells, mice were
sacrificed 6 weeks after transplantation. Bone marrow cells were flushed from both
femurs and tibias with HBSS containing 2% FBS and 5% human serum using a
syringe and a 26-gauge needle. Red blood cells were lysed with 0.16M ammonium
chloride and the remaining cells were washed with HBSS supplemented with 2% FBS.
To facilitate blocking of Fc receptors and prevent nonspecific binding of antibodies,
cells were suspended in HBSS with 2% FBS and 5% human serum and incubated
with mouse 2.4G2 MAb (Becton-Dickinson) for 10 min at 4°C. Cells were then
26
incubated with PerCP-Cy5.5-labeled monoclonal antibodies specific for human CD45
for 30 min at 4°C. Analysis was performed using a FACScalibur and more than
40,000 events were acquired. Successful engraftment of human hematopoietic stem
cell was defined by the presence of at least 0.2% of human CD45+ cells. To verify the
multi-lineage re-constitution of human cells, additional staining was performed with
anti-human CD19-PE /CD41-FITC / CD13-PE / GlyA-PE in combination with anti-
human CD45-PerCP-Cy5.5.
3.3 MSC and CB CD34+ cells 3D co-culture
3.3.1 MSC culture in 3D system and assays
Sterilized SEFAR PETEX 07-1/2 poly PET woven mesh (Sefar Singapore Pte.
Ltd., Singapore) or Non woven PET Fibra-Cel® (New Brunswick Scientific Co. INC)
were used as scaffolds for cell culture. The PET mesh was cut and fitted into the wells
of a 24-well plate. MSCs (2.5 x 104 cells) were seeded into the mesh, grown to
confluence.
SEM:
The cultured samples were fixed with 3% glutaraldehyde in PBS for 24 hours,
washed with PBS, followed by osmification in 1% OsO4 for 30 minutes. After
dehydration in a graded ethanol series, the samples were dried in a critical-point dryer.
After sputter coating with gold, the samples were examined using a SEM (JSM-
5660LV).
Confocol imaging:
For confocal microscopy (Zeiss 510 meta), the cultured samples were first
stained with CMFDA, then fixed with 1% PFA. After that they were stained with
27
antibody for 30 minutes (repeat staining for the use of secondary antibody).After
extensively washing, the samples were examined.
Cell attachment, proliferation and protein production assay for MSC
MSCs (25,000 per well) were seeded into 24-well tissue culture plate with or
without PET mesh. After 24-hr incubation, cells were harvested and counted in
hemacytometer. The result was expressed as attachment percentage of the seeded cells.
Cell proliferation was determined after 4 days of incubation, when the cells reached
~90% confluence. Cell protein production was determined by the BCA™ protein
assay (Pierce, IL, USA). After 4 days incubation, the confluent MSC layer was
washed by PBS and lysed in 300ul of 4% SDS. After centrifugation, 50ul supernatant
was mixed with 200ul BCA working solution and incubated at 37ºC for 30 min. The
total protein level was then measured by UV absorbance at 562nm. The result was
expressed as equivalent albumin production (ug) per 10000 cells.
3.3.2 Ex vivo expansion of CB CD34+ cells in 3D systems and functional assays
MSCs (2.5 x 104 cells) were seeded into the mesh, grown to confluence,
washed 3 times with PBS, and then 600 CD34+ enriched cells were seeded in the
same medium described above. Plain PET woven mesh without MSC layer was used
as a control (3D control). The other functional assays followed the above procedures.
3.4 Study of FN immobilization bioactivity
3.4.1 FN conjugation and adsorption
Figure 2 showed the scheme of FN immobilization. In conjugation pathway,
clean PET surface aminated by ethylenediamine solution (ethylenediamine:
ethanol=1:1) at 50ºC for 40 minutes, was immersed in freshly prepared 2%
28
glutaraldehyde solution with 0.05M sodium cyanoborohydride, reacted for 12-20
hours at room temperature. After washing, the glutaradehyde conjugated surface was
transferred to FN solution in sodium carbonate buffer with 0.05M sodium
cyanoborohydride, reacted for 24 hours at room temperature. To block unreacted
aldehyde sites, 0.06M ethanolamine was added and reacted for another 12 hours at
room temperature. Finally the surface was washed with 5M sodium chloride solution
for 24 hours, and then washed with the desired buffer. In FN adsorption pathway, the
same procedures as above were used, except that no FN was added in the protein
immobilization steps. After surface was blocked by the ethanolamine, FN was added
into the system, reacted for 24 hours, and then washed with 5M sodium chloride for
24 hours, and then washed with desired buffer.
Figure 2 Scheme of FN immobilization on aminated PET surface. (a) FN conjugation pathway (b) FN adsorption pathway
3.4.2 Chemical and physical characterization of the surface
Immobilized FN quantification:
2b) Wash
3b) Block
4b) + FN
5b) Wash
H2C
CH2
CH2
CH2
H2CN
H NH
H2C
CH2
OH
H2C
CH2
CH2
CH2
H2CN
H NH
H2C
CH2
OH
H2C
CH2
CH2
CH2
H2CN
H NH
H2C
CH2
OH
H2C
CH2
CH2
CH2
H2CN
H NH
H2C
CH2
OH
FN
H2C
CH2
CH2
CH2
CH
O
NH2a) Wash
3a) + FN
4a) Block
5a) Wash H2C
CH2
CH2
CH2
H2CN
H NH
H2C
CH2
CH2
CH2
H2CN
H NH
H2C
CH2
OH
H 2C
C H 2 C H 2
C H 2 H2CN
H NH
FN
FN
FN == Protein: FN
NH2HC
CH2
CH2
CH2
CH
OO
H2C
CH2
CH2
CH2
CH
O
NH
1) + Glutaraldehyde (GA)
29
The prepared surfaces were added to micro-BCA working solution (A: B: C:
sodium carbonate buffer=25:24:1: 50), incubated at 37ºC for 2 hours. The total
protein level was then measured by absorbance at 562nm.
Surface contact angle measurements:
Clean PET surface and FN immobilized PET surfaces were measured at room
temperature and 60% relative humidity, using sessile drop method on a telescope
goniometer. More than five measurements were carried out and the resulting values
were averaged.
XPS measurements:
FN immobilized PET surfaces were measured by VG ESCALAB MkII
spectrometer with a Mg Kα X-ray source (1253.6Ev photons) at a constant retard ratio
of 40Ev. The PET films were mounted on the standard sample studs by means of
double-sided adhesive tapes. The core-level signals were obtained at the
photoelectron takeoff angle of 90º. A survey scan spectrum was taken and surface
elemental stoichiometrics were determined from peak area rations.
AFM measurements:
The surface topography of FN conjugated PET, FN adsorbed PET was
examined on a Nanoscope IIIa AFM (Digital Instruments Inc. USA.) in air. Atomic
force microscope images were obtained by scanning surface in the tapping mode. The
driven frequency was 330 ± 50 KHz. The applied voltage was between 3.0 and 4.0 V,
and the drive amplitude was about 300Mv, scanning rate was 1.0 Hz. An arithmetic
mean of the surface roughness profile was determined.
3.4.3 Biological characterization of the surfaces
ELISA
30
FN-immobilized PET surfaces were first captured by primary antibody, rabbit
anti-human VCIP/RGD domain, (1:5000 dilution in BSA solution for 2 hours at 37ºC).
After washing, the surfaces were bound by secondary antibody, goat anti-rabbit IgG
conjugated with peroxidase (1:5000 dilutions in BSA solution for 2 hours at 37ºC).
After washing off the unbound secondary antibody, Slow-TMB-solution was added to
each well incubated for 5-30 min at room temperature in darkness, which was
oxidized by peroxidase, showing the absorbance value at wavelength of 450nm. The
ELISA results for active RGD domains were expressed as equivalent FN according to
the linear FN adsorption isotherm on ethanolamine blocked PET surface as a function
of total FN surface density (by micro-BCA).
In vitro cell culture experiment
The BHK21 cell line was maintained in RPMI 1640 medium supplemented
with 10% fetal calf serum, penicillin (100U/ml), and streptomycin (10mg/ml). The
cell studies were carried below passage 20. Before the cell assay on the surface, the
BHK 21 cells were pretreated with serum-free medium for 2 hours, and then detached
by incubation with non- enzyme cell dissociation solution, in order to avoid the serum
effect and protect cell surface receptor from degradation.
In cell viability assay, cells (seeding density 10,000 per well [96-well plate])
were cultured for 24 hours, and then measured by MTT test. In cell adhesion assay,
cells (seeding density 200,000 per well [24-well plate]) were incubated for 2 hours.
The unattached cells were washed by warmed PBS several times. The adherent cells
were lysed by 4% SDS and centrifuged. The supernatant was subjected to BCA assay.
Based on the assumption that the cells had equal amount of proteins, the total protein
content quantified by BCA was correlated with cell numbers. As the FN surface
density was very low, it would not affect the cell number determination.
31
3.5 Statistical analysis
Data were presented as the mean ± SD. The statistical significance of the data
was determined by student’s t test. The significant level was set at 0.05.
32
CHAPTER 4
MSC AND CB34+ CELLS CO-CULTURE
4.1 Screening of co-culture conditions
The expansion of CB CD34+ cells for clinic application requires serum free
condition. And as the CB cells are non adherent cells, the exchange of medium is not
allowed during the expansion process, which will lead to the loss of cells and the
increase the consumed cost of cytokines. In this project, we chose StemSpan serum
free medium combined with 40ug/ml low density lipoprotein, 100ng/ml SCF,
100ng/ml FL, 50ng/ml TPO, and 20ng/ml IL-3. Then we studied MSC metabolic level
in this serum free medium, and optimized the co-culture conditions.
4.1.1 MSC metabolic level in StemSpan medium
Alamarblue was used as an assay to continuously test MSC metabolic level in
MSCGM and StemSpan serum free medium, which were presented as Alamarblue
reduction percentages. Figure 3 showed that after 10 days incubation in StemSpan
serum free medium, MSC metabolic level (Alamarblue reduction percentages)
decreased from 60.8±0.95% to 55.4±2.81%. It suggested that MSC could survive in
this cord blood expansion condition, and maintain its basic metabolic level. Based on
this result, we concluded that the CB expansion condition was applicable to MSC.
4.1.2 Optimize expansion scale
To optimized the expansion scale of CB CD34+ cells, first of all, we
compared the CB CD34+ cells expansion in different scales: 600 cells expansion in
600ul medium per well of 24-well tissue culture plate and 100 cells expansion in
33
100ul medium per well of 96-well tissue culture plate. Figure 4 showed that the
expansion in 24 well tissue culture plate resulted higher total cells and CD34+ cells
expansion fold ( 342.2±22.3 fold increase of total cells, 9±0.7 fold increase of CD34+
cells) than the expansion in 96 well tissue culture plate (155.2±11.6 fold increase of
total cells, 3.3±0.5 fold increase of CD34+ cells). In the following experiment, we
followed larger expansion scale in 24-well tissue culture plates
0
10
20
30
40
50
60
70
0 3 6 9 12 15
Culturing days
Alam
arbl
ue re
duct
ion
%
Figure 3 MSC metabolic level in MSCGM medium or StemSpan serum free medium with cytokines. 25000 MSCs were seeded into each well of 24-well plate at day 0. After 4 days culture with MSCGM medium, MSCs reached the confluence, subjected to StemSpan serum free medium for another 10 days incubation. Cell metabolic level were evaluated by Alamarblue test at day 4 and day 10, expressed as Alamarblue reduction percentages (n>5).
0
50
100
150
200
250
300
350
400
96 w ell TCPS 24 w ell TCPS
Tota
l cel
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xpan
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fold
0
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1.5
2
2.5
3
96 w ell TCPS 24 w ell TCPS
CD
34+
cells
per
cent
age
%
0
2
4
6
8
10
12
96 w ell TCPS 24 w ell TCPS
CD
34+
cells
exp
ansi
on fo
ld
Figure 4 CB CD34+ cells expansion in 96-well or 24-well tissue culture plates. 96-well TCPS: each well 100 CB 34+ cells expanded in 100 ul medium. 24-
34
well TCPS: each well 600 CB34+ cells expanded in 600 ul medium. (n=3, p<0.05 for total cells and CD34+ cells expansion fold).
4.1.3 Co-culture over MSC feeder layer
4.1.3.1 Irradiated and non irradiated feeder layer
Usually feeder layers were irradiated before co-culture, in order to suppress
feeder layer’s overgrowth and its competition with expanding cells. Considering the
relatively low metabolic level of MSC in StemSpan medium, we hypothesized that
irradiation was unnecessary in this experiment. We compared CD34+ cells expansion
over the irradiated feeder layer and non irradiated feeder layer. The result (Figure 5)
showed that no difference existed between these two kinds of feeder layers in total
cells expansion fold, the non irradiated feeder layer even lead to higher CD34+
percentages (8.99±1.50 % to 5.97±0.31%) and higher CD34+ cells expansion fold
(47.4±4.8 to 31.9±1.6) than irradiated feeder layer. So MSC feeder layers could be
directly used for co-culture, and supported CB CD34+ cells expansion better than
irradiated feeder layer.
0
100
200
300
400
500
600
Non irradiatedfeeder layer
Irradiatedfeeder layer
Tota
l cel
ls e
xpan
sion
fold
0
2
4
6
8
10
12
Non irradiatedfeeder layer
Irradiatedfeeder layer
CD
34+
cells
per
cent
age
%
0
10
20
30
40
50
60
Non irradiatedfeeder layer
Irradiatedfeeder layer
CD
34+
cells
exp
ansi
on fo
ld
Figure 5 CB CD34+ cells expansion on irradiated or non irradiated MSC feeder
layers. (n=3, p<0.05 for CD34+% and CD34+ cells expansion fold)
35
4.1.3.2 Feeder layer exchanged and non exchanged co-culture
Inspired by the above result that non irradiated MSC feeder layers supported
higher expansion of CB CD34+ cells, we hypothesized that the higher metabolic level
of MSC may support CB expansion better. We designed an experiment that at the 5th
day of co-culture, the culture medium and expanded cells were all transferred to a
freshly prepared MSC feeder layer, then continued the left 5 days of co-culture. The
presence of freshly prepared MSC feeder layer in the mid of co-culture, could
enhanced the average MSC metabolic level. But the expansion result (Figure 6 )was
negative. The exchange of freshly prepared feeder layer lead to the decrease of total
cells expansion fold and CD34+ cells expansion fold. The possible explanation might
be the loss of the cells during transferring and metabolic competition from the freshly
prepared MSC.
0
100
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300
400
500
600
MSCcoculture
MSC addedcoculture
Tota
l cel
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1
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7
8
9
10
MSC coculture MSC addedcoculture
CD
34+
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per
cent
age
%
0
10
20
30
40
50
60
70
MSC coculture MSC addedcoculture
CD
34+
cells
exp
ansi
on fo
ld
Figure 6 CB CD34+ cells expansion with or without exchange of MSC feeder layer in the middle of co-culture. (n=3, p<0.05 for total cells expansion fold and CD34+ cells expansion fold).
4.1.4 Co-culture over MSC pellet
During the high density incubation, MSCs can aggregate, forming stable pellet.
Compared with the monolayer, pellet can accommodate more cells in the same
36
culturing surface. Here I tested if MSC pellets can support CB CD34+ cells expansion
better than MSC feeder layer. The result showed that despite of twice amount of the
MSCs in pellet co-culture, MSC pellet co-culture failed to produce significant
difference compared with MSC feeder layer co-culture. We found that once the pellet
adhered to the surface, MSCs tended to migrate from pellet to surface, forming new
monolayer. This kind of unstable morphology probably affect the expansion result,
which may explained the big variance of the data.
0
100
200
300
400
500
600
MSC feederlayer
MSC pellet
Tota
l cel
ls e
xpan
sion
fold
0
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8
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12
MSC feederlayer
MSC pellet
CD
34+
cells
per
cent
age
%
0
10
20
30
40
50
60
70
80
MSC feederlayer
MSC pellet
CD
34+
cells
exp
ansi
on fo
ld
Figure 7 CB CD34+ cells expansion on MSC feeder layer or adherent MSC pellet. (n=3)
A B
Figure 8 MSC pellet morphology under phase contrast microscope (Magnification: 100X). A: MSC pellet adhered to the surface after 24 hours incubation. B: after another 2 days co-culture, MSC pellet spread to the surrounding surface.
37
Summary: The screening experiments showed that MSC can survive in the
serum free StemSpan medium supplemented with growth factors and LDL. MSC’s
metabolic level affected the expansion result. Neither too low or too high metabolic
level was favorable for CB CD34+ cells expansion. The aggregated conformation of
MSCs did not show any significant effect to CB expansion. In conclusion, we set the
optimal co-culture condition as non irradiated MSC feeder layers culturing in 24-well
tissue culture plate.
4.2 Study of the contact and non-contact co-culture
Most of the co-culture research focused on identifying MSC secreted factors
in supporting HSC expansion. Here we focused on another aspect of the problem: the
effect of cell-cell contact in co-culture. We adopted a system for non-contact co-
culture (transwell co culture). In that system MSC feeder layers were separated from
the CB cells by cell culture inserts, but the molecular signals and factors still could
communicate through the porous membranes (pore size: 0.4um) of the cell culture
inserts. So the only difference to normal feeder layer co-culture would be the block of
the cell-cell contact.
In this study, 600 CB CD34+ cells (CD34+CD45+%: 91%; CD34+CD38-
CD45+%: 16.4%) were cultured in the presence of the cytokines SCF, FL, TPO and
IL-3 for 10 days either in direct contact with a confluent layer of MSCs or physically
separated from the MSC layer by transwell culture. The expansion result showed the
incorporation of MSC feeder layer in both conditions was beneficial for CD34+ cells
expansion. When blocked the cell-cell contact between CB cells and MSCs, the non-
contact co-culture resulted 316.5±55.1, 21.1±3.7 fold increase in total cells,
CD34+CD45+ cells respectively, as compared to 545.4±82.0, 47.9±5.2 fold increase
38
in contact co-culture system. The significant decrease of total cells expansion fold,
CD34+ cells expansion fold in non-contact system suggested cell-cell contact must
play a crucial role in supporting CB cells expansion. Due to the system error in
analysis CD34+CD38-CD45+ cells (another combination of markers to identify HSC),
no significant difference was observed between the contact and non-contact system.
0
100
200
300
400
500
600
700
Contact Non contact
Tota
l cel
ls fo
ld in
crea
se
With MSC Without MSC
0
20
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60
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Contact Non contact
CD
34+C
D45
+ ce
lls fo
ld in
crea
se With MSC Without MSC
0
100
200
300
400
Contact Non contact
CD
34+C
D38
-CD
45+
cells
fold
incr
ease With MSC Without MSC
Figure 9 CB CD34+ cells expansion in contact co-culture or non-contact co-culture.
(n>10, p<0.05).
A
0
0.5
1
1.5
2
2.5
3
Contact co culture Non contact coculture
CMFD
A +
cells
%
B
0
10
20
30
40
50
60
70
80
90
Contact co culture Non contact coculture
CD34
+ ce
lls %
CMFDA+ CMFDA-
Figure 10 Cell-cell contact study by CMFDA and CD34 expression. A: Percentage of CMFDA stained cells. B: CD34+ expression over CMFDA+ cells and CMFDA- cells in contact co-culture and non-contact co-culture system. (n=4).
Then we tried to quantify this kind of cell-cell contact, and its direct result to
CD34 surface markers expression. We used long term tracing staining- CMFDA
stained MSC feeder layer first, then co-culturing CB cells for 10 days. If there was
39
cell-cell communication between MSCs and CB cells, CMFDA would be transferred
to the interacted CB cells through plasma exchanges. The non-contact co-culture was
used as a control, to test if there was CMFDA diffusion existed between non-contact
cells. The result (Figure 10)showed that about 1. 65±0.55 % expanded cells were
stained with CMFDA in contact co-culture system, while nearly no expanded cells
stained with CMFDA in non-contact system. 63.9±17.4% of the CMFDA+ cells
expressed CD34, which was about 8 times higher than the CMFDA- cells in contact
co-culture. This highly suggested that cell-cell communication existed between MSCs
and CB cells in contact co-culture, and that kind of cell-cell communication helped to
reserve stem cell primitivity.
4. 3 Function assays of contact and non-contact co-culture
4.3.1 Phenotype screening of the expanded cells I
To investigate the differentiation status of the expanded cells, we analyzed the
surface markers expression on the expanded cells by flow cytometry. The
incorporation of MSC in both systems displayed similar phenotypic surface markers
expression profile in increasing CD34+CD45+ cells %, CD38+CD45+ cells %,
CD19+cells % and decreasing Gly-A+ cells % compared to the TCPS control and
transwell control. These results suggest that the MSC incorporated co-cultures
favored the expansion of primitive cells (CD34) and lymphocytes (CD38), in
particular B-lymphoid cells (CD19) and slowed down erythrocyte (Gly-A)
differentiation. On the other hand, when contact co-culture and non-contact co-culture
systems were compared, direct cell-cell contact resulted in the increase of
CD34+CD45+ %, CD45+ cells %, CD38+CD45+ cells %, CD15+CD45+ cells %
and the decrease of Gly-A+ cells %, CD3+ cells %, CD19+ cells %, but there was no
40
significant difference in CD34+CD38-CD45+ cells % and CD41+ cells %. The results
suggested that direct cell-cell contact favored monocytes / granucytes (CD15)
differentiation, slowed down erythrocyte (Gly-A), B-lymphoid cells (CD19), T-
lymphoid cells (CD3) differentiation, and maintained slightly higher percentages of
more primitive cells, compared with non-contact co-culture.
In control experiments, we investigated whether the doubling the volume of
culture medium in the non-contact system could affect the expansion of CB CD34+
cells. No significant differences in terms of cell expansion and cell phenotype were
observed when compared to cultures without MSC (data not shown). This confirmed
that the medium volume was not confounding factor.
Table 1: Surface markers expression of expanded cells I:
Percentage % CD45+ CD34+CD45+ CD34+CD38-CD45+ CD38+CD45+
Contact co-culture 85.87±5.85* 8.22±1.12 3.15±1.73 29.37±6.24*
TCPS control 71.45±8.45 3.42±1.01 2.00±0.75 10.56±2.07
Non-contact co-culture 73.22±4.02 6.14±0.78 2.77±1.98 14.23±2.38
Transwell control 69.05±18.27 2.74±0.67 1.98±0.83 7.83±1.91
Percentage % CD15+CD45+ Gly-A+ CD3+ CD19+ CD41+
Contact co-culture 43.27±8.87* 20.99±5.84* 0.30±0.07* 1.08±0.33* 5.83±1.74
TCPS control 27.36±5.32 30.56±3.25 0.44±0.15 0.41±0.11 4.19±1.48
Non-contact co-culture 25.43±0.23 28.51±0.41 1.06±0.35 3.39±1.64 6.06±2.52
Transwell control 25.74±2.83 44.8±9.91 0.91±0.10 0.40±0.09 2.44±0.50
CD15: Monocytes and Granulocytes; Gly-A: precursors of Erythrocytes, CD3+: T cells, CD19+: B cells, CD41+: Megakaryocytes, the precursor of plateles. (n>6 for the first 4 categories, n>3 for the left categories, *p<0.05 compared to non-contact co-culture)
41
4.3.2 CFC I
The CFC assay evaluates the multipotency of the cells following expansion.
Figure 11 shows the colony counts derived from the expanded cells, expressed as fold
increase over the number of colonies derived from 600 freshly thawed CB CD34+
cells (total CFC colonies: 132.8 ±8.5; CFC-GM: 72±3.2; CFC-E: 20±3.5; CFC-
GEMM: 40.8±8.7). The contact co-culture produced higher fold increase in CFC-GM
colonies and total CFC colonies, which were consistent with the expansion and
phenotype screening result. In terms of CFC profile (Figure 11B) ex vivo expansion
appear to have induced an increase of CFC-E at the expense of CFC-GEMM in
comparison with unexpanded cells. No significant effect on the CFC-GEMM% was
observed.
A0
50
100
150
200
250
CFC-GM CFC-E CFC-GEMM CFC total
CFU
fold
incr
ease
Contact coTCPS controlNon contact coTranswell control
B0
10
20
30
40
50
60
70
CFC-GM CFC-E CFC-GEMM
Perc
enta
ges
*100
%
UnexpandedContact coTCPS controlNon contact coTranswell control
Figure 11 CFC properties of expanded cells I. 600 CB CD34+ enriched cells (CFC-GM: 72±3.17; CFC-E: 20±3.46; CFC-GEMM: 40.8±8.65) were expanded in contact or non-contact system with or without MSC. A: Colonies fold increase. B: Colonies distribution. (Mean + SD; n>4; *p<0.05 compared with 2D co culture)
4.3.3 Animal engraftment I
The Repopulation assay of human hematopoietic stem cell in NOD/SCID
mouse can quantitatively measure the dual ability of stem cells to self-renew and
42
differentiate. From the literature, the lowest number of CD34+ CB cells that can
engraft NOD/ SCID mice is around 2×104 [Holyoake et al (1999)].
Table 2: The engraftment and lineage expression by human cells in the bone marrow of NOD/SCID mice I
Human Cells (%) Mouse injection Positive
Mice * CD45+ CD41+ CD19+CD45+ CD13+CD45+ GlyA+
20000 uncultured CD34+ CB cells 3/3 0.232;
10.094; 1.132 0.723±0.01 3.056±4.36 0.672±0.74 3.476±0.26
600 uncultured CD34+ CB cells 3/3 2.386;
0.248; 0.739 -0.106±0.3 0.532±0.55 0.606±0.83 1.521±1.60
Injected cells were expanded from 600 CD34+ CB cells in the following systems:
2D co-culture 5/5 1.435; 1.570; 1.683; 1.770; 1.461
0.664±0.9 1.193±0.41 1.268±0.35 3.972±1.06
Non-contact co-culture 4/4 1.041; 0.625;
1.246; 0.589 0.193±0.1 0.86±0.24 0.981±0.32 2.245±0.51
Control (Tissue culture plates) 4/7
0.013; 0.603; 0.487; 1.045; 1.379; 0; 0
0.732±0.4 0.667±0.28 1.082±0.70 3.767±0.58
*Positive mice were identified by at least 0.2% of human CD45+ cells of total lysed bone marrow cells. The lineage expression of human megakaryocytes CD41+; human B cells CD19+CD45+; human monocytes and granulocytes CD13+CD45+; human erythrocytes Gly-A were studied by FACS in all the positive mice.
The ex vivo results suggested that the co-culture produced about 550-320 fold
increase of CD34+CD45+ cells, 50-100 fold increase of CD34+CD38-CD45+ cells and
80-90 fold increase of CFC-GEMM colonies. Based on this reasoning, cells expanded
from about 600 initial CD34+ CB cells would be expected to contain enough
progenitors to reconstitute the hematopoiesis of irradiated NOD/SCID mice. As no
significant difference in expansion results was observed among all the non co-culture
systems, we chose cells expanded from the 2D control (TCPS) as a common control
for all the non co-culture systems. In addition, un-expanded CD34+ CB cells at two
different doses (600 and 20,000 cells) were also transplanted for comparison. Table 2
show the engraftment efficiency of the expanded cells as well as their differentiation
43
potential in the positive mice. The contact co-culture system produced an engraftment
percentage above 1%, which were better than non-contact co-culture system and
TCPS control. The engraftment of the unexpanded cells, particularly at low dose, was
unexpected. Nevertheless, there is high variability among animals in these two groups.
The results highlight the fact that ex vivo expansion would impair stem cells
transplantation ability.
4.4 Discussion
Hematopoietic stem cells, defined by their ability to self-renew and to
differentiate into all mature blood cell types, have by far been the most widely applied
stem cells in the clinic for treatment of such diseases as leukemias and other
malignant cancers as well as several non-malignant blood disorders. However, the
clinical applications of HSC are severely hampered by our limited understanding of
the mechanisms that control HSC self-renewal and differentiation and by the inability
to expand HSC in culture. However, over the past 15 years, rapid advances have been
made in the understanding of HSC biology, particularly in the roles of cytokines and
signaling molecules such as Wnt, Notch etc [Zhang et al (2003); Calvi et al (2003)].
These findings have led to the development of stroma-free ex vivo expansion systems,
which are clinically more well-defined and accepted [Lazzari et al (2001)]. Recently
however, interest has been revived in stroma-supported culture systems and various
cell types including several primary cell types [Breems et al (1998)], immortalized
stroma cell lines [Nishioka et al (2003)], or genetically engineered cytokine-secreting
cells [Balduini et al (1998)]. Feeder cultures are able to provide a more
physiologically relevant repertoire of factors that promotes HSC self-renewal and
maintenance.
44
MSC is a non hematopoietic, well-characterized homogeneous population of
adherent skeletal and connective tissue progenitor cells within the bone marrow
stroma [Haynesworth et al (1992); Majumdar et al (1998); Pittenger et al (1999)]. It is
one of the most promising stem cell types due to their availability and the relatively
simple requirements for ex vivo expansion. Importantly, MSC provides a rich
environment of signals, including cytokines, ECM proteins, adhesion molecules, and
cell-cell interactions, controlling the proliferation, survival, and differentiation of
early lympho- hematopoietic stem cells [Haynesworth et al (1996), Majumdar et al
(2000)]. Accordingly, MSC has been shown to be capable of supporting HSC
expansion ex vivo [Kadereit et al (2002); Wang et al (2004); McNiece et al (2004)].
Similar to Dexter-type stroma cells, MSC can also support LTC-IC colonies during
prolonged ex vivo bone marrow culture [Majumdar et al (2000)].
In addition to the abovementioned advantages, MSC, like other stem cells
which are poor antigen presenting cells, are relatively immunologically compatible,
and show positive augmentative role in HSC transplantation. High dose chemotherapy
or total body irradiation often results in the ablation of the bone marrow stroma,
which is critical for the maintenance of hematopoiesis. Previous animal studies
suggest that the transplantation of healthy stroma components including MSC
enhances the ability of the bone marrow micro environment to support hematopoiesis
following transplantation [Devine et al (2001)]. Recent studies have shown that
autologous MSC expanded ex vivo could accelerate hematopoietic recovery after
HSC transplantation [Koc et al (2000); Westerman and Leboulch (1996)]. Allogeneic
MSC expanded in culture has been successfully co-infused with T-cell depleted
peripheral blood hematopoietic progenitors into a haploidentical receipient and is
currently under further clinical evaluation [Lee et al (2002)]. In summary, all these
45
studies have shown that MSC is a good stroma candidate. We therefore incorporated
it into our CB expansion systems.
Most of the studies which focus on the secretion effect of MSC showed that
MSC secretes, at baseline, IL-6, IL-7, IL-8, IL-11, IL-12, IL-14, and IL-15, MCSF, FL,
and SCF, similar to the cytokines and growth factors expressed by marrow derived
stroma cells [Majumdar et al (1998)]. We wondered whether conditioned medium
could replace the total function of MSC ex vivo, or is the cell-cell contact needed. We
adopted the transwell co-culture system, which blocks the cell-cell contact, but still
permits the molecular signal exchange through porous membrane. Our results
indicated that the contact co-culture increased the expansion of total cells (545.4
vs.316.5 fold), CD34+ cells (47.9 vs. 21.1 fold), CD34+CD38- cells (100.4 vs. 52.8
fold) compared with the non-contact co-culture in transwell system. Similarly, the
pluripotency of cells expanded in contact co-culture as evaluated by CFC assay was
also superior compared to those expanded in non-contact co-culture (CFU-GEMM:
84.1 vs. 76.0 fold; total CFU 150.1 vs. 116.5 fold). The animal engraftment assays
also showed that cells expanded in contact co-culture system appeared to be more
competent in reconstructing short term hematopoiesis. The CMFDA staining showed
that the 63.9% of the anchored cells to the stroma layer (CMFDA+), expressed CD34
surface marker, which is about 8 times higher than suspension cells. The findings
suggested that contact co-culture is effective in maintaining stem cell function,
protecting them from differentiation.
46
CHAPTER 5
MSC AND CB CD34+ CELLS 3D CO-CULTURE
5.1 Screening of 3D scaffolds
In this study we incorporated MSC stroma into the 3D system supporting CB
CD34+ cells expansion, in order to more faithfully replicating the in vivo 3D marrow
configuration.
A B
C D
Figure 12 SEM image of PET meshes and co-culture in meshes. A: PET unwoven mesh; B: CB and MSC co-culture over PET unwoven mesh (Day 10 ). C: PET woven mesh; B: CB and MSC co-culture over PET woven mesh (Day 10 ).
First of all we screened two commonly used 3D systems: unwoven PET mesh
and woven PET mesh. Although serum coated unwoven PET mesh achieved good
expansion result in the literature, the MSC co-culture in non woven PET fibrous
matrices induced quite limited expansion of CD34+ cells. It was probably because of
47
the space limit in structure for co-culture (expanded MSC size: about 20*60 um, CB
cells diameter 3-10 um, compared to the controlled pore size 10-60 um in fibrous
matrices), and difficulties in harvesting the entrapped CB expanded cells from the MSC
stroma in the scaffold. For the application purpose we gave up this scaffold, chose
woven PET mesh, which was relatively easy to isolate cells, while still maintaining part
of the 3D structure. The PET woven mesh comprises of fibers (mesh opening 1um, wire
diameter 34 um, thickness 70 um, and 6.29 fold increase of surface area compared to
the 2D surface, detected by micro CT).
0
100
200
300
400
500
600
700
800
900
1000
Wovenmesh
Unwovenmesh
Tota
l cel
ls e
xpan
sion
fold
With MSC
Without MSC
0
2
4
6
8
10
12
14
Wovenmesh
Unwovenmesh
CD34
+ ce
lls p
erce
ntag
e %
With MSC
Without MSC
0
20
40
60
80
100
120
Wovenmesh
Unwovenmesh
CD
34+
cells
exp
ansi
on fo
ld
With MSC
Without MSC
Figure 13 CB CD34+ cells expansion in PET woven or unwoven mesh. (n=4, p<0.05 compare woven mesh co-culture with the other groups).
5.2 Study of 3D and 2D co-culture
In this study, 600 CB CD34+ cells (CD34+CD45+%: 91%; CD34+CD38-
CD45+%: 16.4%) were cultured in the presence of the cytokines SCF, FL, TPO and
IL-3 for 10 days in direct contact with a confluent layer of MSC growing on 2D or 3D
scaffolds. As shown in Figure 14, the expansion of total cells, CD34+CD45+ cells and
CD34+CD38-CD45+ cells in 3D co-culture were all significantly higher than those of
the other groups. Co-culture in 3D configuration resulted in approximately
48
891.2±163.6, 96.0±20.2, 273.2±106.1 fold increase in total cells, CD34+CD45+, and
CD34+CD38-CD45+ cells, respectively, as compared to 545.4±82.0, 47.9±5.2,
100.4±51.7 fold increase in the 2D co-culture system. These results also suggest that
3D co-culture is superior to 2D co-culture in supporting the expansion of CB CD34+
cells.
0
200
400
600
800
1000
2D 3D
Tota
l cel
ls fo
ld in
crea
se
With MSC
Without MSC
0
20
40
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120
2D 3D
CD
34+C
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+ ce
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Without MSC
0
100
200
300
400
2D 3DC
D34
+CD
38-C
D45
+ ce
lls fo
ld in
crea
se
With MSC
Without MSC
Figure 14 CB CD34+ cells expansion in 2D or 3D systems. (n>10, p<0.05 compare woven mesh co-culture with the other groups).
5.3 Functional assays of 3D and 2D co-culture
5.3.1 Phenotype screening of expanded cells II
To investigate the differentiation status of the expanded cells, we analyzed the
surface markers expression on the expanded cells by flow cytometry. Cells expanded
in the 2D and 3D co-cultures displayed very similar phenotypic surface marker
expression profiles. The notable exceptions are the increase in the CD34+CD38-
CD45+ (5.08% vs. 3.15%; p: 0.019) and CD19+ (2.94% vs. 1.08%; p: 0.041)
populations in the 3D co-culture systems (Table 3). In comparison to the TCPS
controls, the co-cultures in 2D and 3D systems showed increases in CD45+,
CD34+CD45+, CD38+CD45+ and CD19+ cell population but a decrease in Gly-A+
cell population, There is however no significant difference in the CD15+CD45+,
49
CD3+%, and CD41+ populations after expansion. These results suggest that the
contact co-cultures favored the expansion of primitive cells (CD34) and lymphocytes
(CD38), in particular B-lymphoid cells (CD19). It also resulted in a reduction in
erythrocyte (Gly-A) population, and produced no significant effects on the
percentages of monocytes/granulocytes (CD15), T cells (CD3), and megakaryocytes
(CD41).
Table 3 Surface markers expression of expanded cells II
Percentage % CD45+ CD34+CD45+ CD34+CD38-CD45+ CD38+CD45+
2D co-culture 85.87±5.85 8.22±1.12 3.15±1.73 29.37±6.24
TCPS control 71.45±8.45 3.42±1.01 2.00±0.75 10.56±2.07
3D co-culture 85.54±3.77 10.06±2.63 5.08±1.83 26.71±5.39
3D control 61.51±15.44 2.82±0.69 1.80±0.76 8.32±4.32
Percentage % CD15+CD45+ Gly-A+ CD3+ CD19+ CD41+
2D co-culture 43.27±8.87 20.99±5.84 0.30±0.07 1.08±0.33 5.83±1.74
TCPS control 27.36±5.32 30.56±3.25 0.44±0.15 0.41±0.11 4.19±1.48
3D co-culture 37.01±5.68 21.01±2.45 0.99±0.64 2.94±0.87 4.96±1.41
3D control 27.39±6.33 47.77±11.57 0.43±0.10 0.19±0.19 2.34±0.20
CD15: Monocytes and Granulocytes; Gly-A: precursors of Erythrocytes, CD3+: T cells, CD19+: B cells, CD41+: Megakaryocytes, the precursor of platelets. *p<0.05 compared to its internal control: cytokine expansion only. (n>6 for the first 4 categories, n>3 for the left categories)
In control experiments, we investigated whether the presence of 3D PET mesh
alone could affect the expansion of CB CD34+ cells. No significant differences in
terms of cell expansion and cell phenotype were observed when compared to cultures
50
without MSC (data not shown). This confirmed that the PET mesh was not
confounding factor.
5.3.2 CFC II
The CFC assay evaluates the multipotency of the cells following expansion.
Figure 15A shows the colony counts derived from the expanded cells, expressed as
fold increase over the number of colonies derived from 600 freshly thawed CB
CD34+ cells (total CFC colonies: 132.8 ±8.5; CFC-GM: 72±3.2; CFC-E: 20±3.5;
CFC-GEMM: 40.8±8.7). The 3D co-culture produced the highest fold increase across
all CFU types assessed, being statistically significant from the 2D co-culture. The
trend is consistent with the cell expansion result. In terms of CFC profile (Figure 15B),
ex vivo expansion appear to have induced an increase of CFC-E at the expense of
CFC-GEMM in comparison with unexpanded cells. No significant effect on the CFC-
GM% was observed. The 3D co-culture showed a higher CFC-GEMM% than the 2D
co-culture (24.56±1.38% vs. 17.13±2.69%; p: 0.0012).
A0
50
100
150
200
250
300
350
400
CFC-GM CFC-E CFC-GEMM CFC total
CFU
fold
incr
ease
2D co 2D 3D co 3D
B0
10
20
30
40
50
60
70
CFC-GM CFC-E CFC-GEMM
Perc
enta
ges
*100
%
Unexpanded 2D co 2D 3D co 3D
Figure 15 CFC properties of expanded cells II. 600 CB CD34+ enriched cells (CFC-GM: 72±3.17; CFC-E: 20±3.46; CFC-GEMM: 40.8±8.65) were expanded in 2D or 3D system with or without MSC. A: Colonies fold increase. B: Colonies distribution. (Mean + SD; n>4; *p<0.05 compared with 2D co culture)
51
5.3.3 Animal engraftment II
The repopulation assay of human hematopoietic stem cell in NOD/SCID
mouse can quantitatively measure the dual ability of stem cell to self-renew and
differentiate. From the literature, the lowest number of CD34+ CBcell that can engraft
NOD/ SCID mice is around 2×104 [Holyoake et al (1999)]. The ex vivo results
suggested that the co-culture produced about 50-100 fold increase of CD34+CD45+
cells, 100-270 fold increase of CD34+CD38-CD45+ cells and 80-180 fold increase of
CFC-GEMM colonies. Based on this reasoning, cells expanded from about 600 initial
HSC would be expected to contain enough progenitors to reconstitute the
hematopoiesis of irradiated NOD/SCID mice. As no significant difference in
expansion results was observed among all the non co-culture systems, we chose cells
expanded from the 2D control (TCPS) as a common control for all the non co-culture
systems. In addition, un-expanded CD34+ CB cells at two different doses (600 and
20,000 cells) were also transplanted for comparison.
Table 4 showed the engraftment efficiency of the expanded cells as well as
their differentiation potential in the positive mice. The 3D co-culture system
consistently generated engraftment of human CD45+ cells at percentages above 1.5%
in all mice analyzed. Similarly, the 2D co-culture system produced an engraftment
percentage above 1%. The difference between these two systems is statistically
significant (p3D co to 2D co=0.008). Cells expanded in 3D co-culture also yielded higher
human B cells (CD19+CD45+) and monocytes and granunolocytes (CD13+CD45+)
engraftment percentages (B cells: P3D co to 2D co=0.035; monocytes and granulocytes:
P3D co to 2D co=0.008). This result was consistent with the observed higher CD34+ and
CFC-GEMM expansion results. The engraftment of the unexpanded cells, particularly
at low dose, was unexpected. Nevertheless, there is high variability among animals in
52
these two groups. The results highlight the fact that ex vivo expansion would impair
stem cells transplantation ability.
Table 4 The engraftment and lineage expression by human cells in the bone marrow of NOD/SCID mice II
Human Cells (%) Mouse injection Positive
Mice * CD45+ CD41+ CD19+CD45+ CD13+CD45+ GlyA+
20000 uncultured CD34+ CB cells 3/3 0.232;
10.094; 1.132 0.723±0.01 3.056±4.36 0.672±0.73 3.476±0.26
600 uncultured CD34+ CB cells 3/3 2.386;
0.248; 0.739 -0.106±0.32 0.532±0.55 0.606±0.83 1.521±1.61
Injected cells were expanded from 600 CD34+ CB cells in the following systems:
3D co-culture 5/5 2.470; 2.952; 3.187; 2.370; 1.913
1.77±0.99 2.125±0.68 3.008±0.85 6.178±2.25
2D co-culture 5/5 1.435; 1.570; 1.683; 1.770; 1.461
0.664±0.86 1.193±0.41 1.268±0.35 3.972±1.06
Control (Tissue culture plates) 4/7
0.013; 0.603; 0.487; 1.045; 1.379; 0; 0
0.732±0.43 0.667±0.28 1.082±0.70 3.767±0.58
*Positive mice were identified by at least 0.2% of human CD45+ cells of total lysed bone marrow cells. The lineage expression of human megakaryocytes CD41+; human B cells CD19+CD45+; human monocytes and granulocytes CD13+CD45+; human erythrocytes Gly-A were studied by FACS in all the positive mice. (Mean ± SD; *p<0.05 compared with other expanded cells engraftment).
5.4 3D culture of MSC
In an attempt to understand the observed differences in the supportive role of
MSC in the 2D and 3D co-culture systems, we analyzed the attachment, proliferation
and ECM production of the cells cultured on the PET woven fabric and TCPS. The
result (Figure 16) showed that the 3D PET woven structure was more favorable for
cell attachment, or possibly entrapment (77.4±3.3% vs. 60.8±4.5%). Consequently,
the 3D system maintained more cells in confluence compared with the 2D system
(65,000 ± 5,000 cells per well vs. 52,000 ± 2,000 cells per well at day 4). MSCs
grown in the PET mesh also showed higher protein production compared to those
53
grown on TCPS. (7.3±0.6 vs. 6.2±1.7 ug per 10000 cells; p: 0.0157). Scanning
electron microscopy revealed that the MSCs grown on the PET mesh secreted copious
amount of ECM proteins that were organized into an extensive fibrous network
(Figure 17). In contrast, MSCs grown on TCPS displayed relatively sparse ECM
production. The immuno-staining showed that MSCs exhibited a more elongated
morphology in PET mesh than in TCPS surface. Cultured on TCPS, the MSCs
expressed FN but not collagens I, collagen IV, and laminin. Cultured on the PET
mesh, the MSCs expressed FN strongly and also detectable although low amount of
collagen I.
A
0
20
40
60
80
100
3D 2D
MSC
atta
chm
ent *
100%
B
0
20000
40000
60000
80000
3D 2D
No.
of c
ells
per
wel
l
C
0
2
4
6
8
10
3D 2D
Prot
ein
prod
uctio
n (u
g) p
er 1
0000
cel
ls
Figure 16 MSC attachment, proliferation, and protein production in 2D or 3D systems. A: MSC attachment on 3D PET mesh and 2D TCPS after 24 hours incubation. B: MSC proliferation after 4 days culture on 3D PET mesh and 2D TCPS. C: Protein production per 10,000 cells after 4 days culture on 3D PET mesh and 2D TCPS. (Mean ± SD, n=4; Pattachment: 0.0009; Pcell number:0.002; Pprotein production: 0.0157).
5.5 Discussion
Recently, the function of 3D micro environment in hematopoiesis has attracted
more attention and interests. Quite a few 3D systems were designed to enhance cell–
54
cell and cell–matrix interactions, in an effort to mimic the in vivo 3D bone marrow
structure in which more than 95% of normal hematopoiesis occurs; like collagen
coated nylon mesh [Naughton et al (1987) Naughton et al (1990)], porous collagen
micro spheres [Wang et al (1995); Mantalaris et al (1998)], tantalunm coated cellfoam
[Rosenzweig et al (1997)], serum coated unwoven PET fibrous matrices [Li et al
(2001)] in supporting CB or bone marrow cells expansion.
Figure 17 SEM and Confocal images of MSCs culture and co-culture in 2D or 3D systems. A-B: MSCs grown on 3D and 2D surface at confluence; C: MSCs and CB cells after 10 days co-culture; D-E: MSCs grown on 3D and 2D surface; F-G: Fibronectin expression in 3D and 2D culture; H-I: Collagen I expression in 3D and 2D culture. Cells were stained with CMFDA (green); each kind of ECM proteins was stained with TRITC (red) separately.
In this study, we investigated the effects of a combination of MSC feeder layer
with 3D architecture on CB CD34+ cells expansion. MSCs were cultured on woven
55
PET mesh (mesh opening 1um, wire diameter 34um, thickness 70um, and 6.29 fold
increase of surface area compared to the 2D surface, detected by micro CT). Our
results indicated that co-culture of CB CD34+ cells with MSCs in a 3D setting
produced increased expansion of total cells (891.2- vs 545.4-fold), CD34+ cells (96.0-
vs 47.9-fold) or CD34+CD38- cells (273.2- vs 100.4-fold) compared to control
culture. Similarly, the pluripotency of cells expanded on 3D co-cultures as assessed
by CFU assay was also superior compared to those expanded on 2D coculture (CFU-
GEMM:183.6- vs 84.1-fold; total CFU: 229.9- vs 150.1-fold). Additionally, animal
engraftment assays have shown that cells expanded in the 3D co-culture appear to be
more competent in reconstituting short term hematopoiesis. Cytochemical staining
revealed that culture of MSCs on 3D mesh led to increased proliferation which may
contribute to the increase in beneficial effects on CB expansion. More importantly, we
observed high expression of ECM fibrils on MSCs grown in 3D culture, a feature not
seen for cells grown on 2D culture. This result is consistent with the former findings
[Grayson et al (2004)] which showed that MSCs secreted more ECM protein such as
FN when cultured in 3D non-woven PET matrices. Scanning electron microscopy
reveals a tendency of hematopoietic cells to aggregate in crevasses formed by fibrils
and cell structures. This may provide 3D topological cues necessary to maintain the
primitiveness and pluripotency of the CD34+ cells.
In addition to providing mechanical support , the endogenous bone marrow
furnishes a biochemical environment by secreting a complex meshwork of ECM rich
in proteins such as collagen and elastin, specialized proteins such as FN and laminin,
and proteglycans, It has been reported that ECM glycosaminoglycan side chains can
bind soluble growth factors forming concentration gradients that among other roles
aid in cell anchorage, facilitate receptors dimerization and signal transduction,
56
modulate extra cellular proteolysis and protect signal molecules from being degraded
[Ramirez and Rifkinb (2003) ]. Cell adhesion motifs present in the ECM also play
important roles in the regulation of stem cell biology. FN which is secreted by bone
marrow stroma cells, have been found to have important influence on HSC cell cycle.
Its receptors, integrin α4β1 and α5β1, are highly expressed on HSC. This implied
that besides its traditional functions in HSC lodgment, FN may also take part in the
maintenance of the self-renewal ability of HSC within the hematopoietic niche.
Evidence comes from studies that showed that adhesion to FN during ex vivo culture
preserves the regenerative capacity of human PHPCs/HSCs [Dao et al (1998)]. Taken
together, we postulate that the influence of a 3D microenviroment coupled with
elevated deposition of ECM by MSC create a cellular niche more closely resembling
the natural bone marrow thus favoring CD34+ cell proliferation and maintenance of
pluripotency.
57
CHAPTER 6
STUDY OF FN IMMOBILIZATION BIOACTIVITY
Inspired by the 3D co-culture results that FN was specially secreted by MSCs
in the co-culture system and FN’s potential functions in controlling cells adherence
and cell cycles, we hypothesized that FN may play a very important role in CB’s
expansion. Immobilization of FN onto the surface of culture system may represent a
stroma free expansion strategy. The purpose of this sub project was to evaluate two
common protein immobilization methods, and study the change of FN bioactivity
during the immobilization process. The results of this study would be applied to
construct artificial HSC expansion niche in the future.
6.1 Physical and chemical characterization of conjugated and adsorbed FN
6.1.1 FN immobilization efficiency
The amount of the immobilized FN on modified PET surface was determined
by the micro BCA assay, which depends on the number of peptide bonds and the
presence of four amino acids (cysteine, cystine, tryptophan and tyrosine). Figure 18
shows the surface density of FN as a function of the input concentration in solution.
The conjugation method produced a higher FN content onto the surface than the
adsorption method. With adsorption, the content of FN adsorbed reached a saturation
level around 400 ng/cm2 once the FN input concentration exceeded 4 ug/ml. The
saturation level is consistent with conventional adsorption isotherms, representing
approximately the amount of FN necessary to produce a monolayer coating based on
the dimension of the molecules [Williams et al (1982)]. In contrast, the conjugation
method seemed to be able to form multiple layers of FN despite extensive washing.
At an FN input concentration of 30 ug/ml, 918±134 ng/cm2 of FN could be stably
immobilized onto the surface.
58
6.1.2 XPS assay
XPS instruments measure the number and kinetic energy of emitted
photoelectrons. Because the core electrons of each element have characteristic
binding energies, the peaks in the XPS spectra allow identification of all elements,
except H and He. Among the elements detected, the N1s signal percentage as a
function of the FN surface density was shown for both substrates in Figure 19 and
Table 5. The XPS data showed that FN covalent conjugated surface had higher
nitrogen percentages. This result was consistent with the micro-BCA result, that more
protein was immobilized through covalent conjugation method.
0
200
400
600
800
1000
1200
0 3 6 9 12 15 18 21 24 27 30FN input concentration (ug/ml)
FN d
ensi
ty o
n su
rfac
e ( n
g/cm2 )
ConjugationAbsorption
Figure 18 FN immobilization efficiency. FN surface density (by micro-BCA assay) was immobilization methods (conjugation & adsorption) and protein concentration (0.256ug/ml-30ug/ml) dependent. Mean ± SD was obtained from five separate experiments
A0
10000
20000
30000
40000
50000
02004006008001000
Cou
nts
/ s
Binding Energy (eV)
O
C
B0
5000
10000
15000
20000
25000
02004006008001000
Cou
nts
/ s
Binding Energy (eV)
N
C0
C0
5000
10000
15000
20000
25000
02004006008001000
Cou
nts
/ s
Binding Energy (eV)
O
N
C
Figure 19 XPS survey of FN immobilized PET surface. A) Clean PET surface; B) Fn conjugated surface; C) Fn adsorbed surface
59
Table 5 Element proportion on FN immobilized surface
Element Percentages Surface
C O N
Fn conjugated PET 67.4 26.47 6.13
Fn adsorbed PET 70.07 24.14 5.79
6.1.3 Surface contact angle measurements
The contact angle between water and a surface is an inverse measure of the
ability of water to ‘‘wet’’ the surface. A smaller contact angle indicates a hydrophilic
surface, on which water spreads to a greater extent; a larger contact angle indicates a
hydrophobic surface, on which water beads up.
The cleaned and unmodified PET surface showed a static contact angle of
74.1º ± 2.6º. Amination of PET surface with ethylenediamine lower the contact angle
to 62.9º ± 3.1º by introducing the more polar amino groups on the surface. The
conjugation of FN on aminated surface decreased the angle further to 56.7º ± 2.0º).
The ethanolamine blocked surface had similar contact angle with the aminated surface
(61.5º ± 2.9º), and the FN adsorption onto this surface did not change the contact
angle further (61.8º± 2.1º). While the mechanism is not clear, the difference in the
contact angles between the FN- conjugated and the FN-adsorbed surfaces (p<0.05)
suggest dissimilarity in the conformation of FN presented on the surface.
6.1.4 AFM assay
In AFM assay, a sharp tip attached to a cantilever scanned across a surface.
Changes in surface topography that were encountered as the tip moved over the
material’s surface changed inter atomic attractive or repulsive forces between the
60
surface and tip. These forces were sensed and recorded to construct images of surface
3D topography.
50
55
60
65
70
75
80
Clean PET Aminated PET Fn conjugatedPET
Ethanolamineblocked PET
Fn adsorbedPET
Surf
ace
cont
act a
ngle
Figure 20 Surface contact angle on clean, modified or FN immobilized PET surface (n>5). (p<0.05. between Fn conjugated PET and Fn adsorbed PET)
A) B)
Figure 21 AFM image of FN immobilized PET surface. (A) FN conjugated PET surface, scanning area: 4.8um*4.8um. Ra: 119 nm, Rmax: 522 nm. (B) FN adsorbed PET surface, scanning area: 10um*10um Ra: 11.2nm, Rmax: 217nm.
Different morphology was observed when FN was conjugated or adsorbed to
the PET surfaces in the AFM topographic images (Figure 21). On the FN-adsorbed
PET surface, the protein appeared evenly distributed, with a mean roughness of
Ra=11.2 nm and a maximal roughness of 217 nm. In contrast, the protein on the FN-
conjugated PET surface was more closely packed in layers, with a mean roughness of
Ra=119 nm and a maximal roughness of 522 nm. Due to this kind of aggregation, the
FN conjugated surface maintained higher FN density. Neither of the modification
61
processes changed surface area significantly (scanning area: 1um2, conjugation:
1.06um2, adsorption: 1.13 um2).
6.2 Biological characterization of conjugated and adsorbed FN
6.2.1 ELISA test for RGD domain
Although the FN surface density was higher in the FN-conjugated surface
according to the micro-BCA result, the ELISA test for active RGD domains showed
that FN bioactivity in the access to active RGD was lower in FN conjugated surface.
(Figure 22).
0
200
400
600
800
1000
1200
Conjugation Adsorption
FN d
ensi
ty n
g/cm
2
ELISAMicro-BCA
Figure 22 ELISA measurement of active RGD domains in immobilized FN compared with Micro –BCA result. (FN input concentration of 30ng/ml: conjugated FN surface density: 917.6±133.8 ng/cm2; adsorbed FN surface density: 433.1±45.8 ng/cm2 by micro-BCA) Significant difference existed between two methods *p<0.05 (n>4).
There was little difference between the micro-BCA and ELISA measurements
for the surface density of adsorbed FN, 433.1±45.8 ng/cm2 versus 410.8±13.7 ng/cm2,
respectively, at the FN input concentration of 30ug/ml. In contrast, the corresponding
values for the conjugation method were 917.6±133.8 ng/cm2 and 220.3 ± 10.1 ng/cm2,
which indicated a loss of 76% of the active RGD domains.
62
6.2.2 Ex vivo cell experiment
To further assess the bioactivity of FN adsorbed surface and FN conjugated
surface, we compared the measured cell behaviors as a function of surface. The cells
viability over 24 hours on the two FN-immobilized PET surfaces did not show any
significant difference according to the MTT assay. However, the adhesion of BHK21
cells at the 2 hours time point was quite different (Figure 23). The FN-adsorbed PET
surface supported cell adhesion (from 53.6±10.2% to 70.3±9.4%) with the increase of
adsorbed FN surface density (from 0 to 433.1±45.8 ng/cm2). The FN-conjugated
surface on the other hand suppressed cell adhesion (from 53.6±10.2% to 12.2±6.0%)
with the increase of FN density (from 0 to 917.6±133.8 ng/cm2).
0
20
40
60
80
100
0 200 400 600 800 1000FN surface density ng/cm2
Aad
here
nt c
ells
perc
enta
ges %
Conjugation Adsorption
Figure 23 Cell adhesion on FN immobilized PET surface (n>4)
6.3 Discussion
6.3.1 FN conformation and its bioactivity
Many studies have shown that the conformation flexibility influences the
biological properties of FN. Certain activities of FN are latent in the intact molecule
and become fully expressed in its proteolytic fragments [Muir and Manthorpe (1992)].
For example, 29 and 40 kDa heparin-binding proteolytic fragments of FN are potent
63
inhibitors of endothelial growth ex vivo, while native FN is not [Homandberg et al
(1985)]. Generally it is believed that FN molecules is secreted by cells and folded into
globular domains specialized for particular functions, such as binding to integrins,
collagen, heparin sulfate, hyaluronic acid, the change of their conformation would
affect their bioactivity. [Morla and Ruoslahti (1992)]
6.3.2 FN adsorption
Langmuir isotherm was often used for the rudimentary description of protein
adsorption phenomena: the near-linear dependence of adsorbed molecules on the bulk
concentration at low concentration regimens and the saturation effect at higher
adsorbent concentrations [Horbett (1994); Mientus and Knippel (1995)], which was
quite consistent with our result. Saturation level suggested the formation of monolayer
of FN on surface, so the extra FN unable to form stable bonding onto the FN
monolayer, was washed off. Monolayer coverage for proteins theoretically depends
on the shape and size of the protein molecules as well as on its mode of adsorption.
Varying monolayer coverage of protein surface density has been reported to range
between 200ng/cm2 to 500ng/cm2 [Rezania and Healy (1999); Lhoest et al (1998)]. In
this experiment, we reported a saturation surface density around 400 ng/cm2.
Although we could not compare the adsorbed FN with soluble FN by ELISA or cell
adhesion, the AFM photos showed that the adsorbed FN kept its global compact
conformation, uniformly distributed on the surface, having less structure alteration
compared with conjugated FN.
6.3.3 FN conjugation and fibrillogenesis
64
In solution, the compact conformation of FN is maintained by only a small
number of electrostatic associations between distant segments of the molecule: III12–
14/ III2–3, and III12–14/ amino-terminal Heparin I domains [Johnson et al (1999)].
However the electrostatic associations can be destroyed in a regulated stepwise
process, fibrillogenesis, in which soluble FN is converted into insoluble fibrillar
network. In initiation process: integrins at cells surface recognize the cell-binding
domains of FN, usually α5β1 binding to RGD domains or α4β1 binding to CS domains.
It induces FN unfolding that exposes FN binding sites and promotes intermolecular
interactions with other FN. Finally the integrins clustering and binding of additional
FN leads to fibril formation [Bentley et al (1985); Patynowski and Schwarzbauer
(2003)].
According to the protein immobilization result and morphology observed in
AFM, we hypothesize a cell-free fibrillogenesis model in the conjugation process
(Figure 24). At first the amino groups of the protein were randomly immobilized to
the surface, which helped FN to unfold. While the positive charged amino groups
were strongly immobilized to the substrate, the negative charged domains of the
protein were driven away from the surface, forming a negative charge concentrated
“surface”. This negatively charged “surface” could immobilize the amino groups of
the FN by electrostatic association, help the FN unfold, and form new adsorption
“surface” repeatedly. Thus it formed the multiple layers of FN onto the surface, and
initiated the fibrillogenesis process. In summary, this proposed model showed that
the orientation of the FN molecules on amino groups conjugated surface and
subsequent unfolding of the molecule may be the key to the induction of
fibrillogenesis. At high surface charge densities, the created electric field would repel
the negative domains and strongly attract the positive domains, leaving open and
65
accessible the domains implicated in the fibrillogenesis process. By this model we can
explain how conjugation method stably immobilized multiple layers of FN, and how
fibrils like structure formed on surface. Pernodet also reported similar cell-free
fibrillogenesis on surfonated polystyrene surface after 130 hours of 100ug/ml FN
[Pernodet et al (2003)]. Compared his result, our conjugation method showed more
efficiency in initiating fibrillogenesis.
A
B
C
D
Type I Repeat Variable Sequence Type II Repeat Type III Repeat
Figure 24 Hypothetical schemes of FN fibrillogenesis in conjugation process. A: Compact conformation of FN in solution is stabilized by III12-14/III2-3 and III12-14/I domains. B: FN unfolds when the amino groups of the FN are immobilized to the surface. C: The unfolded FN exposes new layer of negatively charged “surface” for extra FN adsorption. D: FN adsorbes and partially unfoldes on the FN layer through electronic association, exposes new “surface” for FN adsorption.
It is increasingly apparent that FN matrix can have profound effects on cell
functions. Data from structural modeling of repeat III10 indicate that the unfolding of
the FN and the following fibrillogenesis process might control the accessibility of the
RGD sequence, and enable cells to retract from FN matrices in order to migrate and
proliferate [Krammer et al (1999)]. Data from endothelial cells adhering assays, also
66
reported that after fibrillogenesis on endothelial cells surface, the FN matrix prevent
the extra cells adhering [Dekker et al (1993)]. Our experiment also showed that the
conjugation induced fibrillogenesis suppressed the accessibility of RGD domains, also
suppressed cells adhesion, which was quite consistent with the literature mentioned
above.
In summary this project studied the regulation of FN involved cell-material
interaction at the level of FN immobilization methods. In the adsorption method, FN
adsorbed as isolated molecules or as a monolayer of protein, and preserved higher
bioactivity in maintaining more RGD domains and favoring cells adhesion. The
conjugation method induced FN fibrillogenesis, which maintained less RGD domains
and suppressed cells adhesion. The combination of these two methods may offer us
the ability to readily control the degree of surface bioactivity, which has particular
significance to the biomaterials applications like microenvironment reconstruction. It
has been noted that it is a relatively easy task to generate a highly adhesive substrate
by direct adsorption of cells adhesion molecules like FN. However a difficulty often
encountered in application is the inability of cells, once attached to an overly adhesive
bridging substrate, to leave the material surface for the comparatively less adhesive
surrounding [Schwab and Bartholdi (1996)]. In this regard, promoting cell migration
away from the device surface probably will require that the surface become less
adhesive in comparison with surrounding tissue, which often is not recognized in the
development of bioactive surfaces.
67
CHAPTER 7
CONCLUSIONS
7.1 Conclusions
The ultimate goal of this thesis project was to develop a HSC ex vivo
expansion system that would be able to produce sufficient and functional HSCs for
the allogeneic transplantation of CB. To fulfill this goal, we aimed to design an
artificial microenvironment that mimicked the hematopoiesis process in vivo, which
can efficiently expanded the cell number and still maintained the “stemness” of the
cells.
We chose MSC, non hematopoietic, well-characterized skeletal and
connective tissue progenitor cells within the bone marrow, as stroma to support HSC
expansion ex vivo. First, MSC is relatively easy to harvest and culture ex vivo. MSC
provides a rich environment of signals, including cytokines, ECM proteins, adhesion
molecules, and cell-cell interactions, controlling the proliferation, survival, and
differentiation of early lympho- hematopoietic stem cells [Haynesworth et al (1996),
Majumdar et al (2000)]. What is more, MSCs, like other stem cells, are poor antigen
presenting and relatively immunologically compatible.
In this study we clearly demonstrated the benefit of MSC inclusion in HSC
expansion ex vivo. We tested the co culture result under different conditions: fresh
stroma & irradiated stroma co-culture; stroma exchanged & non stroma exchanged
co-culture; feeder layer & pellet co-culture; contact & non-contact co-culture; 2D &
3D co-culture. The results showed that MSC can survive in the serum free StemSpan
medium supplemented with growth factors and LDL. MSC’s metabolic level affected
68
the expansion result. Neither too low nor too high metabolic level was favorable for
CB CD34+ cells expansion.
Compared with non-contact co-culture in transwell system, contact co-culture
increased higher expansion of total cells (545.4 vs.316.5 fold; p<0.05), CD34+ cells
(47.9 vs. 21.1 fold; p<0.05), CD34+CD38- cells (100.4 vs. 52.8 fold; p<0.05). Similarly,
the pluripotency of cells expanded in contact co-culture as evaluated by CFC assay was
also superior compared to those expanded in non-contact co-culture (CFU-GEMM:
84.1 vs. 76.0 fold; total CFU 150.1 vs. 116.5 fold; p<0.05). The animal engraftment
assays also showed that cells expanded in contact co-culture system appeared to be
more competent in reconstructing short term hematopoiesis. These results suggested
that cell-cell contact was required for the further expansion of stem cells; only growth
factors in conditioned medium could not fully replace the function of stroma. The long
term tracing staining- CMFDA experiment showed that about 1.6% of the expanded
cells had communicated with stroma in culture, which were stained with CMFDA. And
63.9% of these CMFDA+ cells expressed CD34 surface marker, which was about 8
times higher than CMFDA- suspension cells. The results highly suggested that cell-cell
communication existed between MSCs and CB cells in contact co-culture, and that
kind of cell-cell communication helped to reserve stem cells primitivity and prevent
them from differentiation.
The conformation of the co-culture system also significantly affected stem
cells expansion. The result showed that the direct contact between MSC and HSC in
3D cultures led to statistically-significant higher expansion of CB CD34+ cells
compared to 2D co-cultures: 891- versus 545-fold increase in total cells, 96- versus
48-fold increase of CD34+ cells, and 230- versus 150-fold increase in CFC.
NOD/SCID mouse engraftment assays also indicated a high success rate of
69
hematopoiesis reconstruction with these expanded cells. Cytochemical staining
revealed that culture of MSCs on 3D mesh led to increased proliferation which may
contribute to the increase in beneficial effects on CB expansion. More importantly, we
observed high expression of ECM fibrils on MSCs grown in 3D culture, especially
FN, a feature not seen for cells grown on 2D culture. This 3D scaffold induced fibrous
structure and excessive expression of ECM protein facilitated and regulated CB
CD34+ cells expansion, which probably explained the enhanced result.
Inspired by the finding that FN was the mainly expressed ECM protein by
MSCs, and FN’s influence in HSC cell cycle [Dao et al (1998)], we hypothesize that
the immobilization of ECM protein FN onto the surface of culture systems might
represent another stroma free expansion strategy. A key problem in FN
immobilization is the change of bioactivity during the immobilization process. We
studied FN bioactivity after two common immobilization pathways: conjugation and
adsorption, we found a way to control the function of the immobilized protein on the
level of immobilization methods, which can be helpful for the construction of an
artificial niche in the future. The adsorption method maintained a normal
conformation of FN, leading to less change of its bioactivity in the access of RGD
domains and cell adhesion ability. The covalent conjugation process, on the other
hand, induced FN unfolding and fibrillogenesis ex vivo, which blocked the access of
active RGD domains and suppressed fibroblast cells adhesion. The HSC expansion on
these FN immobilized scaffolds also showed very promising results [Q. Feng et al,
unpublished data].
Taken together, we designed a 3D bio-artificial “niche” coupled with elevated
deposition of ECM by MSC, which more closely resembled the natural bone marrow
microenvironment; therefore, it favored CD34+ cell proliferation and maintenance of
70
pluripotency. We also studied FN bioactivity after immobilization, which helped us to
construct another stroma free system supporting CD34+ cells expansion.
7.2 Future prospect
In summary, a great deal of experiences over the past 15 years in terms of the
ability to isolate and culture primitive hematopoietic stem and progenitor cells has
been attempted. Especially the research in growth factors and other soluble factors of
stem cell proliferation have provided great insight into the biology of stem cells, but
we do not yet understand how to organize these molecules in a physiologically
relevant manner. The perpetual discoveries in molecular biology and developmental
biology help this process cooperatively. For example, recently Lodish group isolated a
new kind of HSC-supportive cells: mouse fetal liver CD3+ cells. Subsequently, they
identified two growth factors: angiopoietin like 2 and angiopoietin like 3 specially
expressed by these HSC-supportive cells by microarray expression analysis. The 10
days expansion in the presence of these two factors, induced up to 30 net expansion of
long-term HSC by reconstitution analysis, which was the highest stem cell expansion
that had ever been reported [Zhang et al 2006]. These researches pave the way to the
establishment of the procedure for clinical application and build up hope for these
patients.
However, there is still a long way to translate these research results into clinics.
To be widely accepted, procedures for ex vivo stem cell expansion will have to meet
the requirements of Federal Good Manufacturing Practices (GMPs). The key concepts
of GMPs are:
o Preventing the transmission of communicable disease.
71
o Processing controls, for example, prevent contamination and preserve integrity
and function.
o Assuring clinical safety and effectiveness, especially with presence of
allogeneic materials, for example the allogeneic MSC in this study
That is to say products (for infusion) must be produced in a manner that ensures that
they are safe and (reproducibly) fit for their intended use.
So far, we seem to only have safety data. Studies Allogeneic MSC expanded
in culture could be successfully co-infused with T-cell depleted peripheral blood
hematopoietic progenitors into a haploidentical receipient and it is currently under
further clinical evaluation [Lee et al (2002)]. And a short term standard lymphocytes
mixture test is recommended before transplantation, to make sure expanded cells with
MSCs will not induce immunological reaction.
The uniformity and effectiveness of expanded products is still under the
developmental stages. As mentioned before, the main limit of CB allogeneic
transplantation is insufficient cells number. In transplantation CB nucleated cells,
about 2,500,000 CD34+ cells per kg body weight is required [Koller et al 1973].
Therefore, for a patient of 70 kg body weight, 175,000,000 CD34+ cells are required.
Based on the 3D co-culture system mentioned above, 96-fold expansion of CD34+
cells can be achieved. about 1,820,000 CD34+ cells are required to start expansion.
More than 3000 scaffolds are needed, which is quite unpractical. A clinical-scale
bioreactor must be developed to accommodate large number of cells and scaffolds
with high cell densities. With this developed culture system of sufficient simplicity,
flexibility and economic efficacy, the true potential of ex vivo expansion of HSCs that
mimic the in vivo conditions will finally be realized.
72
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