functional anatomy of the adaptive immune response describe the compartments within lymphoid organs...
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Functional Anatomy of the Adaptive Immune Response
• Describe the compartments within lymphoid organs
• Understand the multistep model of leukocyte transmigration
• Understand the process of lymphocyte recirculation
• Understand the concept of chemoattraction
• Explain the process of dendritic cell (DC) maturation and antigen transport
• Explain how B cells encounter antigen
• Explain how lymphocytes exit from lymphoid organs
• Describe the changes that promote T cell ‘homing’ to sites of inflammation
Jason Cyster
Spleen - A filter of the blood• Two main functions of the spleen carried out in separate
regions:1) White-pulp is where immune responses against blood-borne
antigens occur2) Red-pulp is responsible for monitoring and removing old or damaged
RBCs• Red-pulp consists of thin walled venous sinuses and dense
collections of blood cells (including numerous macrophages) that form red-pulp cords (or cords of Billroth)
• Blood supply: branches of central arteries open directly into red-pulp cords, adjacent to the splenic sinuses (open circulation)– Released RBC must cross the sinus walls; interendothelial slits are a
major mechanical barrier and only the most supple, mechanically resilient RBC survive; old and damaged cells are removed by macrophages
Trabecular arteryTrabecular vein
Capsule
Pulpvein
Anatomy of the Spleen
PALS (periarteriolar lymphoid sheath) or T zone
Follicle(B zone)
Red Pulp cord Splenic (venous)
sinus
White-Pulp cord
Lymphatics
One-way valves
Lymph is filtered by Lymph Nodes before
returning to circulation (liters per day)
Lymph contains T and B cells and dendritic cells
(thin walled)
]
afferent lymphatic vessels
efferent lymphatic vessel
capsule
Cortex (T+B zone)
medulla
Primary follicles(B zones)
germinal center(secondary follicle)
Paracortex (T zone)mainly T-cells
Anatomy of a Lymph Node
medullary sinus
subcapsular sinus
radial sinus
Filter antigens from the lymph- for recognition by T and B cells- for destruction by macrophages to
prevent systemic spread
lymph fluid
blood
intestinal contents
ca hev hev
SECONDARY LYMPHOID ORGANS
B cellfollicle
T cellzone
red-pulp
Splenic White-Pulp Cord Lymph Node Mucosal lymphoid tissue (e.g. Tonsil, Appendix)
- filter antigens from body fluids- bring together antigen presenting cells and lymphocytes- help bring together antigen reactive B and T cells
Lymphocytes traverse HEVs to enter lymph nodes and then compartmentalize in B cell follicles and
T cell zones
LN section stained with:B cell markerL-selectin ligand
T cell zone (paracortex)- T cells- DCs (dendritic cells)
HEV(High Endothelial Venule)
Follicle or B zone- B cells - FDCs (follicular
dendritic cells)
The multistep model of leukocyte extravasation
1) Tethering and Rolling
2) Triggering/Activation of Integrins
3) Firm Adhesion 4) Diapedesis
EndothelialCells
Selectins
IntegrinsChemokines
Blood Flow
• Selectins are a specialized type of lectin (sugar binding protein) that bind appropriately glycosylated membrane proteins
• Three types:– L-selectin: restricted to lymphocytes
• ligands on lymphoid tissue HEVs
– P-selectin: made by platelets and activated (inflammed) endothelium
– E-selectin: made by activated (inflammed) endothelium • E- and P-selectin ligands expressed on neutrophils,
monocytes, activated T lymphocytes
Selectins
>40, small secreted proteins
Four families: C, CC, CXC, CX3C
chemokine
chemokine receptor
Chemoattractant-Cytokines or “Chemokines”
Cells with the appropriate receptor migrate (chemotax) up chemokine gradient
Chemokines also promote cell adhesion to endothelium
Lymphoid chemokines – help direct the homeostatic trafficking of cells through
lymphoid tissues (e.g. CCR7 / CCL21; CXCR5 / CXCL13)
Inflammatory chemokines – induced at sites of inflammation; can be expressed by
many cell types; help recruit cells to these sites (e.g. CXCR2 / IL-8; CCR2 / MCP1)
outside
cytoplasm
Integrins• heterodimers of and polypeptide chains• can be in inactive (low affinity for ligand) and active states• intracellular signals can cause ‘inside-out’ signaling in the integrin,
converting it from an inactive to an active state• chemokine and antigen receptor signaling can activate integrins• bind extracellular matrix proteins (e.g. binds fibronectin) or
transmembrane proteins
(e.g. integrin LFA1 binds ICAM1;
integrin binds VCAM1)
• Leukocyte Adhesion Deficiency (LAD) – mutations affecting integrin mediated adhesion• Inhibitor of integrin (Tysabri) in clinical use for treatment of MS & Crohn’s
Multistep cascade of lymphocyte migration to site of infection/inflammation: same logic, different ‘area code’
Prostaglandins & leukotrienes can
function as chemoattractants
Schematic view of a lymph node
AfferentLymphatic
dendriticcell
T zone
B zoneHEV
TT
T
T**
**
B
B
EfferentLymphatic
T
ArteryVein
T
CXCL13
CCL21
B zones produce a B cell attracting chemokineT zone produces T cell and DC attracting chemokines
CXCL13 -> CXCR5 CCL21 -> CCR7
Follicle
Paracortex
Medulla
Schematic view of a lymph node
AfferentLymphatic
dendriticcell
T zone
B zoneHEV
TT
T
T**
**
B
B
EfferentLymphatic
T
ArteryVein
T
CXCL13
CCL21
B zones produce a B cell attracting chemokineT zone produces T cell and DC attracting chemokines
CXCL13 -> CXCR5 CCL21 -> CCR7
Follicle
Paracortex
Medulla
DCs migrate from periphery to lymphoid organ T zone bearing Ag
• immature ‘sentinel’ DCs are present in most tissues, continually sampling for antigen– by pinocytosis, phagocytosis, engulfment of dying cells
• detection of pathogen-derived molecules (e.g. LPS, dsRNA, bacterial DNA) or stressed cells (necrotic cells, TNF, IL-1) causes the DCs to mature – decrease adhesion to local tissue cells (e.g. keratinocytes)– increase expression of receptors (CCR7) for chemokines made by
lymphatic endothelial cells and lymphoid organ T zones– upregulate MHC and costimulatory molecules
• migrate into lymphoid T zone • present antigen to T cells
lymph fluid
hev
B cell antigen encounterantigen
FDC
Sinus Macrophage
• B cells bind intact antigen through their surface Ig / B cell receptor (BCR)• Antigen that enters via blood or lymph reaches the follicle and can be captured
directly by B cells• Follicular dendritic cells (FDCs – tissue resident cells related to fibroblasts) can
display antigen on their surface in an intact form for long periods
Lymph node egress occurs in response to a circulatory lipid (sphingosine-1-phosphate, S1P)
MEDULLARY SINUSES
Dia
gram
cou
rtes
y of
Ted
Yed
nock
- Lymphocytes express a receptor (S1PR1) for S1P
- Egress involves migrating to S1P that is high in blood/lymph and low in the tissue
S1P
blood
thymus, spleen
S1PR1
S1P
S1P lyase
RBC
efferent lymph
lymph node, Peyer’s patch
S1PR1
S1P
S1P lyase
Lymphocytes express S1PR1 and exit lymphoid organs in response to S1P
• S1PR1 is required for T cell egress from thymus and for T and B cell egress from spleen, lymph nodes, tonsil
• Activated lymphocytes transiently down-regulate S1PR1 and are retained in the responding lymphoid tissue until they become effector cells
• FTY720 (Fingolimod) inhibits egress and was FDA approved in 2010 for treatment of relapsing-remitting Multiple Sclerosis
FTY720
Immunosuppressant
Effector T cell Trafficking• Activated T cells exit lymphoid tissue -> circulation
– upregulate S1PR1 to exit lymph nodes• S1PR1 inhibitor (Fingolimod) in use for treatment of Multiple Sclerosis
– ability to re-enter lymphoid tissue is reduced (decrease in CCR7, L-selectin)
• Increased ability to enter inflammed tissue due to increased expression of– ligands for E- and P- selectins – receptors for inflammatory chemokines (e.g. CXCR3) and
lipids (prostaglandin and leukotriene receptors)• Inhibitors of prostaglandin and leukotriene synthesis have mutliple anti-
inflammatory effects, including antagonizing cell recruitment– adhesion molecules (e.g. integrin 1)
• blocking antibody (Natalizumab/Tysabri®) in use for treatment of Multiple Sclerosis and Crohn’s disease