Lymphocytes (T, B and NK cells) The lymphocytes are of 3 types: 1. T- cells (Bone-Marrow Derived & Thymus-Developed Cells) 2. B- cells (Bone-Marrow Derived in Human; or Bursa of Fabricius Derived in Birds). 3. NK- cells (Natural- Killer Cells; Bone- Marrow Derived).
T- cells:The T-cells originate from the bone-marrow from the pluripotential cells (PPC) which is the mother cells of all haematopoietic cells. There are 2 stem cells that originate from the PPC:1. Lymphoid stem cell (LSC) that gives origin to the lymphoid cells (T, B & probably NK).2. Myeloid stem cell.The PPC is driven to become LSC by Stem Cell Factor (SCF). Then the LSC will be driven to migrate to the Thymus by SCF & IL-3. Then from the Thymus to the peripheral blood the precursor cells will be driven by SCF, IL-2, IL-7 and thymic hormones to become mature virgin T- cells “Naive T cells” (See Fig.).
Development of T –cells in the Thymus:There are 3 stages of development of thymocytes in the Thymus:I. Triple Negative Cells:The subcapsular cortical precursor cells arrive in the Thymus and they are CD3(-), CD4(-), and CD8(-), but they are CD22(+), and CD44(+). These cells represent 10% of the thymocytes.2. Double Positive Cells:These cells represent the majority (80%) of the thymocytes. They are CD4(+), and CD8(+), but become CD22(-) & CD44(-).These cells are also positive for CD1, CD2, & CD7.All the cortical thymocytes will be educated to recognize self from non-self antigens by contact with the HLA antigens (Class I & II) of the cortical stromal (non-lymphoid) cells. Those cells which do NOT react against self antigens and can recognize the HLA antigen at low concentration (amount) will be allowed to migrate to the Medulla of the Thymus, while those which can not will be deleted or killed by “Apoptosis”. This process of selection is called “Positive Selection”.
III. Single Positive Cells: These are the medullary thymocytes. They represent about 10% of the thymocytes. These cells are either CD4 positive (T-helper) or CD8 positive (T- cytotoxic); never both. These cells are also positive for CD1, CD2, CD3, and CD7, but negative for both CD22, and CD44. The normal CD4:CD8 ratio is 2:1. Also, these cells in order to be allowed to migrate to the peripheral blood they have to recognize the self HLA antigen in high concentration on the medullary epithelial cells. However, those cells which can not, they are killed by apoptosis and this selection process is called “Negative Selection”.
T-Helper/Inducer Cells (Th, CD4): Generally have the following functions: 1. Cytokine production to promote T-cells differentiation and division, and Th- memory cells production. 2. Acceleration of T- cytotoxic (Tc, CD8) cells maturity. 3. Attraction of macrophages, and enhance their bactericidal activity. 4. Recruit neutrophils, eosinophils, and basophils to the site of infection and inflammation. 5. Stimulation of NK cells activity. 6. Help B-cells activation. Note: CD4 antigen is also the receptor for the Human Immunodeficiency Virus (HIV) causing AIDS.
Subsets of CD4 Cells:Today, SEVEN distinct CD4 T cell subsets exist. These cells come from the Naive conventional CD4 T-cells according to the signal they receive during their interaction with the antigens and have different functions:1.TH12. TH23. Treg (induced “iTreg”, and natural “TH3, nTreg, or TregS; S= Suppressor)4. TH175. NKT (type 1, Type 2, and NKT-like)6. TFH (Follicular B- Helper)7. TH9
Natural Killer T-cells (NKT cells) These cells can perform functions of both CD4 (T helper) and CD8 (T cytotoxic) cells. They recognize lipid/ glycolipid antigen presented by CD1d molecule Deficiency/dysfunction of these cells is important in autoimmune diseases (DM, atherosclerosis) and asthma . Recognition of antigens: CD4 cells recognize antigens in combination with MHC-II (class 2) NK cells recognition of antigens: 1. FC-gamma R (CD16) 2. IL (12, 15, 18, 2, Chemokine CCL5) 3. Activating/Inhibitory Receptors (CD94, Ly49, KIR, LIR)
Th1 cells: 1. Are critical for immunity to intracellular microorganisms, including Mycobacterium infections. 2. Their abnormal activation is an important event in most organ specific autoimmune diseases. 3. Produce mainly IL-2, lymphotoxin-alpha, and INF-gamma. IL-2: Important for memory CD4 cells together with INF-gamma, and an important role in the stimulation of CD8 cells. lymphotoxin-alpha: Increased in multiple sclerosis. INF-gamma: Activates macrophages & increase their bactericidal activity. 4. Th1 cells can be activated by INF-gamma, and IL-12. 5. Help B cells to produce IgG2, and IgM antibodies.
Th2 Cells: 1. Are critical for many extracellular pathogens including helminths. 2. Their activation is responsible for allergic inflammatory diseases and asthma. 3. Help B cells to produce IgG1 and IgE. 4. Produce IL-4, IL-5, IL-9, IL-10, IL-13, IL-25, and amphiregulin. 5. Th2 cells are activated by IL-4, and IL-2.
Cytokines of TH2: IL-4: Positive feedback cytokine for Th2. IL-5: Attraction of eosinophils. IL-9: Effective against mast cells, lymphocytes, and enhances mucin production by epithelial cells in allergy. IL-10: Suppress Th1 proliferation and dendritic cell functions. IL-13: Effective in expulsion of helminthes, and in air way hypersensitivity. IL-25: Enhances the production of the above mentioned cytokines, and also produced by lung epithelial cells in allergy. Amphiregulin: Induces epithelial proliferation, and have a role in Trichuris muris expulsion and air way hypersensitivity.
Th17 Cells: 1. Play a critical function in protection against microbial challenges particularly extracellular bacteria and fungi. 2. Important in some autoimmune responses (previously attributed to Th1 cells) as experimental autoimmune encephalitis (EAE), collagen induced arthritis (CIA), and some form of inflammatory bowel diseases (IBD). 3. Produce IL-17 of a and f types ( these cells derived their name from this cytokine), IL-21, IL-22. 4. Th17 cells are activated by TGF-beta, IL-6, IL-21, and IL-23, while IL-4 and INF-gamma suppress their functions.
Regulator CD4 cells:Regulator CD4 T-cells (previously known as T-suppressor cells; these cells play an important role in maintaining self tolerance) and can be grouped into:I. Naturally-occurring Regulator CD4 T cells (nTreg): Are CD4+, CD25+, and FoxP3+ cells which arise in the thymus.They link T cells with CD11c + and CD123 + dendritic cells.II. Inducer Regulator (iTreg) CD4 T cells: 1. Produce Transforming growth factor-bata (TGF-beta), IL-10, and IL-35. 2. iTreg cells are activated by TGF-beta and IL-2. 3. These cells cab be FOXP3 - (Th3 or Tr1). 4. iTreg cells are activated by TGF-beta and IL-2. They play a major role in mucosal immunity such as that of the gut besides their ability to inhibit TH1 and TH2. Notes: 1. IL-10 production by all CD4 T cells serves as a negative regulatory mechanism for limiting the immune responses to prevent host tissue damage. 2. Discovery of Treg cells is a “revolution” in modern immunology.
Follicular B helper T cells (Follicular helper T cells or TFH)These are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleen and Peyer’s patches. TFH cells are found within B cell follicles and mediate antigen specific naпve or memory B cell activation, which triggers germinal center formation, probably through the expression of CD40L and the secretion of IL-21 and IL-4.TH9: They produce IL9 and defend against helminthic infestation.
T- Cell Receptor for Antigens (TCR): The TCR are composed of 2 parts which are disulfide Linked Heterodimer (DSLH) + CD3. The TCR recognize the antigens in combination with MHC (HLA) molecules (i.e., CD4 with MHC II, while CD8 with MHC I) : 1. DSLH : This is composed of 2 chains; alpha- chain of 40-45 KD and beta- chain of 45-55 KD. These chains are located mainly extracellularly , with small intracellular portion. These chains are linked by disulfide bonds. The alpha chain have 3 portions; variable ( V ) for specificity, Joining ( J ) and Constant ( C ), while beta- chain have V, Diversity ( D ), J, and C. The combination of these segments give vast array of specificities. About 95-99 % of DSLH have alpha / beta chains. However, 1-5% have delta / gamma chains. The latter chains are either CD4 (-) / CD3(-) or CD4(-) / CD8(+). The possible functions of these cells is to defend skin and mucous membranes since they are located nearby these areas. 2. CD3 : Composed of 6 chains; 4 of them (1 delta, 2 epsilon, and 1 gamma) have half extracellularly and half intracellularly. The other 2 chains are maily intracellularly ( theta ). The functions of CD3 is to support the DSLH and to transfer the immunological signals from extracellular to intracellular compartments.
CD8 Lymphocytes: These cells predominate in human B.M and gut associated tissues and constitute 35% of the peripheral T cells. These Tc cells recognize antigens in association with class I MHC (HLA) molecules. They directly kill virus infected cells and tumour cells. After killing, activated TC disengage from the target to kill other cells. Their action differs from that of NK cells in being specific and MHC restricted. The main target cells of Tc are: 1. Through the release of cytotoxic proteins which include perforins and lymphotoxins, these produce pores in the target cell membrane causing lysis. 2. By activation of the factor of apoptotic signal ( FAS ) protein, which causes programmed cell death (apoptosis). 3. Through the release of TNF- beta and INF- gamma which facilitate apoptosis
Natural Killer (NK) Cells:These are large granular lymphocytes which lack most suface markers of B and T cells. In contrast to T cells, they are CD3 –ve, CD11b +ve, and CD 56+ve. They comprise 10-15% of the peripheral lymphocytes. They have spontaneous non-specific cytotoxic activity on tumour or mutated cells, graft cells / tissues, and virus infected cells. They are not MHC restricted and MHC I inhibits their killing functions. Their activity is increased by INF, IL-12 and IL-2. The NK cells kill through the release of perforins which cause pores in the target cell membrane. They produce a protective protein which prevents perforin self-lysis. They also kill by induction of apoptosis in a manner similar to Tc cells..
Cont./……The NK cells release numerous cytokines during their interaction with target cells, including INF-gamma, TNF-alpha and GM-CSF. They have receptors for Fc portion of IgG (CD16) and therefore can interact with and kill antibody coated cells,i.e., antibody Dependent Cell Mediated Cytotoxicity (ADCC). The IL-2 activate NK cells (and CD8 Tc cells) are called lymphokine activated killer (LAK) cells which can efficiently kill. LAK cells have shown promising results in treatment of metastatic cancer in clinical studies. Recently NK1-T cells that has some of the characteristics of both T cells and NK cells has been recognized
Memory T cells are a subset of antigen-specific T cells that persist long-term after an infection has resolved. They quickly expand to large numbers of effector T cells upon re-exposure to their cognate antigen, thus providing the immune system with "memory" against past infections. Memory T cells comprise two subtypes: central memory T cells (TCM cells) and effector memory T cells (TEM cells). Memory cells may be either CD4+ or CD8+. Memory T cells typically express the cell surface protein CD45RO (vs naпve T cells “TH0” which are CD45RA).
B-Lymphocytes (B-Cells)
B- Lymphocytes These cells originate from PPC in the B.M which become lymphoid stem cell (LSC) under the effect of SCF. Then the LSC under the influence of SCF & IL-3 becomes B-cell precursor which transform to mature virgin B cell under the influence of SCF & IL-7. The development from the B.M. to mature B cells is under genetic control and occurs in every normal human being. Any abnormality in this process results in an immunodeficiency state.
Development of B Cells: The details of the development of B cells are well studied by monoclonal antibodies (McAb). These cells pass from LSC to become pro-B, early pre-B, late pre-B, immature B and lastly mature B cells. items LSC Pro-B Er.Pre-B Lt.pre-B immature B matureB CD34 + + - - - - CD19 - + + + + + CD10 - + - - - - IgD - - - - - sIgD IgM - - - cIgM sIgM sIgM
Mature B-cells: The mature-B cells are positive for CD19, CD20, CD21, CD22, CD23, CD24, CD25, CD32, CD35, CD79a, CD119, CD150 and MHC class I and II. CD21= Receptor for EBV and C3d (complement component). CD32= IgG Fc receptor. The normal mature B- cells are CD5(-), but the malignant counter parts (lymphocytic lymphoma/ leukaemia) are CD5(+). The mature B cells are resting, and re-circulating cells. They have sIgM and sIgD which are the B cells receptors for antigens (BCR). The sIgD is more abundant than the sIgM (100,000 IgM /cell) on normal cells ,but opposite on malignant CD5(+) cells. These cells represent about 15% of the peripheral lymphocytes and are present in the primary lymphoid follicles , mantle zone of secondary follicles of L.N., and white pulp (marginal zone & germinal centre) in the spleen. The B cells when catch an antigen in the blood they go the spleen, while those present in the lymph they go to the L.N.
B – Cells Markers:Each of the normal stages have its counter malignant part which may express comporable markers. Example : common acute lymphoblastic leukaemia (cALL) is the malignancy of pro-B cells, so the markers of leukaemic cells are CD10(+), & CD19(+).
B- T cells co-operation:Most of the B cells depend on T cells in their activation. The B-T cells co-operation occurs according to the following:1. Intimate contact between B & T cells after the B cells catch an antigen by BCR. The antigen is internalized, then re-expressed incombination with HLA class II (Ag + HLA-II). This will be recognized (dual recognition of Ag & HLA-II) by CD4 (T-helper cells). The B-T contact is mediated by: A. MHC II + Ag on B-cells with TCR + CD4 on T cells (Co-Receptors). B. CD80 or B7 on B cells combines with CD28 on T cells. C. CD40 on B cells combines with CD40L (L= ligand) on T- cells. B and C = Co-Stimulatory Pairs. D. ICAM-1 on B cells and LFA-1 on T cells. E. LFA – 3 on B cells and CD2 on T cells. D and E = Adhesion Pairs.
Cont/...T-B cells co-operation 2. Release of appropriate cytokines. Th1 controls the production of IgM & IgG2, while Th2 controls IgE & IgG1 (see the cytokines of these cells). Memory B- cells: Some of the B cells become memory cells after antigenic stimulation. These cells may express IgG, IgA or IgE. B- cells Education: The B cells are educated in the B.M. to recognize the self- HLA molecules. The cells which can not recognize self antigens are deleted by apoptosis (positive and negative selection) which is controlled by the macrophages in the B.M.
B cells Activation: B cells is triggered when it encounters a matching antigen. The B cell engulfs the antigen and digest it. Then it displays (presents) antigen fragments bound to its specific MHC molecules. This combination of antigen and MHC attracts the help of a mature, matching T cell. Cytokines secreted by the T cell help the B cell to multiply and mature into antibody producing plasma cells. The antibodies are released from the plasma cells into the blood to lock onto matching antigens. The antigen-antibody complexes are the cleared by the complement cascade or by the liver and spleen.
Morphological forms of B cells during activation:After exposure of B-cells to antigenic stimulation, the mature B cells enter the G.C. of the lymphoid follicle and loose their SIgD first, then SIgM and become centroblast and centrocytes (both are germinal center cells). Then the cells leave the G.C. to the interfollicular area (cortex) and become immunoblasts. Then these cells migrate towards the medulla and become plasmacytoid cells which start to synthesize Igs, then plasma cells which are present in the medullary cords and synthesize thousands of Igs molecules per second. The half life of plasma cells is about 3-10 days. Each of these stages have their malignant counter parts. Examples: CB & CC = Germinal centre cells lymphoma (Centroblastic, centrocytic lymphoma), Immunoblast = Immunoblastic lymphoma, plasmacytoid = Waldentrom’s macroglobulinaemia, plasma cells = multiple myeloma.