Maimun ZA

LYMPHOCYTES

1. Site of Lymphocyte MaturationLymphocyte arise from bone marrow-derive

precursor = hematopoetic cellsFrom pluripotent stem cellsThe sites: * B cells maturation foetus in liver baby in bone marrow. * T cells maturation in bone marrow migrate to thymus

Maturation of lymphocyte

Lymphocyte develope from precursors in the generative lymphoid organs (the bone marror and thymus). Mature lymphocyte enter the peripheral organs, where they respon to foreign antigens and from where they recirculate in the blood and lymph.

2. General features of Lymphocyte maturationMaturation stadium genetic and cellular

changesEarly maturation: Forming somatic recombination and receptor

gene. Receptor expression (for antigen) “marker” +. B & T precursors are not well

defined by phenotype marker can be demonstrated by genetic abnormalities

Cell proliferation induced by cytokinesThe sequence of maturation:• Stem cells Pro lymphocyte Pre Lymphocyte Immature Lymphocyte Mature Lymphocyte Lymphocyte effector (differenciated/spesific).

Gene Recombination Receptor and expressionThe key event in lymphocyte maturation For the generation of a diverse repertoir and processes

selective survival with useful specificities B or T cell clons are produce antigen receptor Gene recombination receptor composed by somatic recombination coded by gene in a certain locus gene segment forming from some DNA sequences Molecular mechanism of Ig or TCR gene recombinations are similar, functionaly, Ig only in B cells - TCR genes in T cells. Express on the surface cells, antigen receptors are

expressed before encounter with antigen.

Receptor expression on the surface cell • As the result of gene recombination, • Many immature limphocyte clons with

many receptor genes are formed• Useful clones positive selection• Unuseful clones negative selection

programme cell death (apoptosis)• Clones that fail to express receptor

negative selection.

ReceptorMarker: Signal makerFormed via gene recombinationExpressed on cell surface as antigen receptor. B cells B Cell receptor (BCR)

Immunoglobulin (Ig) T cells T Cell Receptor (TCR).Example: Incomplete receptor (Pre-lymphocyte) used: signal marker for the next maturation

process avoid lymphocyte from apoptosis. Complete receptor (lymphocyte immature) used: to give signal to survive, proliferation and

maturation. Unformed programmed to cell death (apoptosis)

3. Maturation Process a. Early maturation

- Stem cells have been determined (lymphocyte lineage)

- Several genes are involved in the maturation process.

- Signed by high mitotic activity- Some molecules are involved in this process,

such as: – Cytokine Interleukin-7 (Il-7), – Membrane protein,– Thymus stromal cells.

b. B lymphocyte maturation

The sequence begin from

arrangement of gene recombination

Ig gene expression immature

cells proliferation selection of the

cells.

B cells maturation stadium Need sequence stages: Pro-B cell • Not yet produced Ig, • Differenciated from the other immature B

cells CD19 and CD10 molecules expression.

Pre-B cell• Incomplete Ig receptor Not expressed Ig yet (antigen reseptor for B

cell) • Receptor importancy stimulate proliferation

and maturation avoid from apoptosis.• Next step forming complete Ig complex

expressed on the surface cell as IgM (IgMs)

Immature B lymphocyte • IgMs• Begin to form another complexes IgM

and IgD.

Mature B lymphocyte • IgMs and IgDs• responsive to outer antigen • Leaving bone marrow peripheral

lymphoid tissue.

Sel B naive B cells• Present in peripheral lymphoid tissue &

blood• Not yet activated by antigen• IgD+IgM+

NB:• During B cells maturation selection

process occurs (by self antigen-MHC), to eliminate or non activate another B cell precursor from unclear/unuseful clones.

Steps of B cells maturation

Steps in the maturation and selection of B lymphocytes . The maturation of B lymphocytes proceeds through sequensial steps, each of which is characterized by particular changes in Ig gene expression and in the patterns of Ig protein expression. At the pro B cell and pre-B cell stages, failure to express functional antigen receptors (Ig heavy chain and Ig light chain, respectively) resut in death of the cells by default pathway of apoptosis.

c. T lymphocyte maturation- Principally, occurs in bone marrow, similar with B

cell maturation.

- Thymus is the main site for T cells maturation.

T cell is called Thymocyte,

- Thymocytes, not yet mature outher cortex area, not expressed TCR yet, move to cortex, express TCRs to medulla, mature T cells (CD4+ or CD8+) peripheral lymphoid tissue or blood.

T cells maturation stadiumImmature T cells in cortex thymus, Pro-T cells or Double-negative thymocytes • Not expressed TCR yet CD4- or CD8-

Pre-T cells or Double-positive thymocytes (T CD4+CD8+ cells) • express CD4 and CD8• Expression of CD4 or CD8 are important ability to

respond to antigen. • Pre-T cells then move to medulla

Mature T cells • Express CD4+CD8־ or CD8+ CD4 ־• Single-positive thymocyte. • Detected from the expression of CD4 or CD8

mol. (spesific). • Specificity base on the function of - CD4+ T cells produce cytokines that “help” B

cells and macrophage againts antigen stimulation.

- T CD8+ cells has ability to produce molecules that can dissolve other cells or regulate antibody forming.

• Then the mature thymocyte single-positif move to peripheral lymphoid tissue.

The maturation of T lymphocyte in the thymus through sequential steps that are often defined by the expression of the CD4 and CD8 coreceptors. The TCR β chain is first expressed at the double-negative pre-T cell stage, and the complete TCR is expressed in double-positive cells. Maturation culminates in the development of CD4+ and CD8+ single positive T cells. As in B cells, failure to express antigen receptors at any stage leads to apoptosis (programme cell death).

Steps of T cells maturation

4. Selection Process that Shape The B and T Lymphocyte Repertoires

• Positive selection Lymphocytes are positively selected to

recognize peptide antigens displayed by self MHC molecules considered “self” by immune system escape from apoptosis (programmed cell death).

Negative selection Growing lymphocytes elimination in

conection with self-MHC lymphocytes are assumed not “self” programmed apoptosis.

• Growing lymphocyte (B and T) sensitive to negative selection soon after they express their receptors on surface cells.

Lymphocytes programmed to apoptosis are: (1) Functionaly fail to express useful antigen

receptor (2) Receptors that are expressed do not

recognize ligands which induced positive selection

(3) Immature lymphocytes are recognized by self antigen with high avidity, or Immature lymphocytes that strongly recognize self antigen are negatively selected.

• Final result of maturation process above ≤ 10% lymphocytes move to blood and lymph.

Steps in maturation and selection of MHC-restricted T lymphocyte. During their maturation, B and T lymphocytes go through cycles of proliferation and expression of receptor chains by gene recombination. Cells that fail to express useful receptor die by apoptosis, because they do not receive necessary survival signals. At the end od the process, the cells undergo positive and negative selection.

Selection process to T and B cells

T Lymphocytes

Characteristic Phenotype is shown by the marker molecules (CD4/CD8) that are expressed on the surface membrane cell. Functions: - effector of cell mediated immunity - regulator of immune respons

Subpopulation T lymphocyte

2 main subpopulations: CD4+ and CD8+ (2:1) CD4+ (Activated by Antigen-MHC II complex) - T helper Subpopulation Th1 & Th2 - T regulator TH1 dan TH2 CD8+ (Activated by Antigen-MHC I complex) - Cytotoxic T Lymphocyte (CTL)

Steps in activation of T lymphocyteAntigen recognation

Lymphocyte activation

Clonal expansion& Differentiation

Effector functions

Steps in activation of T lymphocyte

Naive T cells recognize MHC-associated peptida antigens displayed on APCs + other signal (not shown). The T cells respond by producing cytokines, such as IL-2, and expresing receptors for these cytokines, leading to an autocrine pathway of cell proliferation. The result is clonal expansion of the T cells. Some of the progeny differenciate into effector cells, which serve various functions of cell-mediated immunity, and memory cells, which survive for long periods.

The role of costimulator in

T cells activation.

Naive T cells that have recognized antigen without costimulation may become unresponsive to subsequent exposure to antigen, even if costimulations are present anergy. Microbes, and cytokines produced during innate immune responses to microbes, induce the expression of costimulation, such as B7 moleculs, on the APCs. The B7 costimulators are recognized by the CD28 receptor on naive T cells, providing “signal 2” and in conjunction with antigen recognition (“signal 1”). The recognition initiates T cell responses.

Activation

The molecules that are involvedin T cell activation

Antigen Presenting Cell (APC): peptide-MHC II complex. B7 mol, Cytokines (costimulator)CD4+ T cell: TCR CD4 mol (coreceptor) CD3, , CD28 molecules (costimulator)

Effector function of CD4+ T cell CD4+ T cells that have

differentiated into effector cells express CD40L and secrete cytokines. CD40L binds to CD40 on macro-phages or B lymphocytes, and cytokines bind to their the receptor on the same cells. The combination of signals delivered by CD40 and cytokine receptors activates macrophages in cell-mediated immunity (A) and activates B cells to produce antibodies in humeral immune responses (B).

Activation of CD8 Tcells

A). In some interactions, APCs may ingest infected cells and present microbial antigens to CD8+ T cells and to CD4+ helper cells. The helper cells then produce cytokines that stimulate the expansion and differentiation of the CD8+ T cells. It is also thought that helper cells may activate APCs to make them competent at stimulating CD8+ T cells (not shown).

B). CD8+ T cells recognise class I MHC-associated peptides and receives costimulatory signal if a profesional APC harbors a cytoplasmic microbe.

Development of Memory T Lymphocyes A fraction of antigen-activated T lymphocytes

defferenciates into long-lived memory T cells. • Memory cells survive even after the infection is

eradicated • They can be found in lymphoid tissue, in

mucosal barriers, and in the circulation. • Memory T cells do not continue to produce

cytokines or kill infected cells, but they do so rapidly on encountering the antigen that they recognize. Thus, memory cells are pool of lymphocyte waiting for the infection to return.

B cell Lymphocytes

Function: Secreting antibody plasmacyte APC professionalMarker on surface cell: Imunoglobulin (M), MHC-I, MHC-II, IL-2

R, transferin, receptors, CD19, CD20, CD21, CD22, CD23, CD37, CD39, CD40 and CD45. (Kishimoto, T and Hirano, T, 1989).

B cells activation2 kinds: Direct & Indirect activation• Direct activation of B cell is stimulated

by: Type 1 thymus independent antigens

polyclonal antigen activators IgsR on spesific B cells.

Type 2 thymus independent antigens (polymeric molecules) crossreaction with many IgsR long live a sustain signal to B cells forming antibody.

B-cell recognition of (a) type 1 and (b) type 2 thymus-independent antigens.

• The complex gives a sustained signal to the B-cell because of the long half-life of this type of molecule. a/ activation signal; surface Ig receptor; b/ cross-linking of receptors.

Indirect B cells activation• Thymus dependent antigens

cooperate with T-helper activate resting/naive B cells. (see also CD4 Tcell activation)

The Role of B cells • Signal from Ag. T independent Ag: B cells (via IgsR)

cross-linking the growth & differentiation (helped by other signals) Plasmacyte produce antibody.

T dependent Ag: professional APC present antigen-MHC II complex to CD4 Tcell CD40L produce IL-4 & IL-6 B cells Plasmacyte Antibody Forming Cell (AFC).

Development of Memory B Lymphocytes

Similar with The Development of Memory T Lymphocytes

Classes of lymphocyte Different classes of limphocytes

recoqnize distinc types of antigens and differentiate into effector cels whose function is to eliminate the antigens.

B lymphocytes recognize soluble or cell surface antigens and differentiate into antibody-secreting cells.

Helper T limphocyte recoqnize antigens on the surface of antigen-presenting cells and secrete cytokines. Which stimulate different mechanisms of immunity and inflamation.

CT L recoqnize antigens on infected cells and kill these cells.

Note: T limphocytes recoqnize peptides that

are displayed by MHC molecules