T-Cell Receptors and Development PDF

Lymphocytes and their receptors Dr Felix N. Toka DVM, PhD, DSc., DACVM Professor, Immunology & Virology Objectives Describe the development of main T and B lymphocyte receptors Describe the lymphocyte selection process Lymphocytes Lymphocytes are important for the adaptive i...

Lymphocytes and their receptors Dr Felix N. Toka DVM, PhD, DSc., DACVM Professor, Immunology & Virology Objectives Describe the development of main T and B lymphocyte receptors Describe the lymphocyte selection process Lymphocytes Lymphocytes are important for the adaptive immune response 3 major types of lymphocytes: T lymphocytes (T cells) that regulate adaptive immunity and are responsible for cell-mediated immune responses B lymphocytes (B cells) that are responsible for antibody production Natural killer (NK) cells that play a role in innate immunity (LGL, large granular lymphocytes) Morphology of lymphocytes Lymphocytes are small, round cells, 7 to 15 µm in diameter Have a large, round nucleus The nucleus is surrounded by a thin rim of cytoplasm Giemsa stain Development of lymphocytes T-cell development T-cell development begins well before birth Pre-T cells leave the bone marrow and migrate to the thymus and are called thymocytes The T-cell receptor begins to form Cells that express the β chain become αβ T- cells Cells that express γ or δ chains become γδ T- cells Specificity of a response to an antigen Complementarity between the antigen and the TCR dictates the specificity of a response One TCR for one antigen (epitope) In spite of this animals can respond to many antigens How is this possible? How is diversity possible? Random rearrangement of germline gene segments encoding TCR components A feature only unique to T cells (and B cells) Takes place in the thymus (for T cells)  Recombination of gene segments V1 V2 V3 D1 D2 J1 J2 CM CD DNA of B/T-cell progenitor cells ~ 40 ~25 6 ~10 Selection of gene segments through rearrangement V3 D2 J1 CM CD DNA of mature B/T cells V = Variable; J = Joining; D = Diversity; C = Constant  Development of the TCR Genes that encode α and β chains of the TCR are not contiguous They are found in segments in different regions of the DNA Segments include V, J, D and C regions Arrangement of TCR genes on mammalian chromosomes V = Variable; J = Joining; D = Diversity; C = Constant Development of the TCR cont’d TCR gene rearrangement Begins with rearrangement of β-chain genes in the pre-T cells Random joining of a Dβ and a Jβ gene segment Random linking of the new DJβ segment with a Vβ segment Formation of the variable region of a β chain Gene rearrangement cont’d The α-chain gene segment rearrangement begins after the β chain Vα and Jα gene segments join to make a complete α-chain variable region Then VJα segment is joined with a Cα region segment Gene rearrangement cont’d T-cell α/β antigen-specific receptor A fully rearranged β chain cannot be expressed before the α chain has been rearranged Gene rearrangement cont’d Rearrangement of segments is made possible by products of 2 genes called recombination activating gene 1 and 2 (RAG1 and RAG2) DNA rearrangement during TCR α-chain formation Activity of RAG1 and RAG2 DNA rearrangement during TCR α-chain formation Enzymes, such as DNA-dependent protein kinase, excise the intervening DNA to increase diversity of the TCR http://www.dogbreedplus.com/dog_breeds/frisian_water_dog.php Selection of T cells To be of use, the T cell must express a functional TCR, i.e., a TCR that can recognize and react with the MHC–antigen complex on the surface of an APC This sorting or selection process occurs in two steps - positive selection and negative selection. Selection of T cells takes place in the thymus Positive selection A test of whether a newly created TCR can bind to the MHC I or MHC II on thymic cortical epithelial cells (APCs). The T-cell should bind the MHC in order to pass the positive selection If a T-cell does not bind the MHC, it fails the positive selection and is eliminated by apoptosis Why positive selection? Elimination of thymocytes that: have failed to assemble a TCR have assembled a useless TCR Commitment of T cells to either a CD4+ or a CD8+ lineage As much as 96% of thymocytes fail positive selection and undergo apoptosis. Negative selection If a T cell’s TCR binds with high affinity to an MHC-antigen complex present on an APC, the T cell dies through apoptosis Negative selection eliminates most thymocytes that react with self as represented in the thymus  Early stage thymocytes lack CD4 and CD8, they are double-negative cells CD4 and CD8 are expressed once the αβ TCR is expressed – they become double positive T cells. Mature T cells are single positive, i.e., express CD4 or CD8 only CD4 and CD8 are co-receptors on T cells Commitment to be a CD4+ or a CD8 + T cell If the TCR has an affinity for an MHC I, the thymocyte turns off the CD4 gene and begins to express solely CD8 Commitment to CD4+ or CD8+ T cells cont’d If the newly formed TCR has affinity for MHC class II molecules, that lymphocyte switches off the CD8 gene and expresses only CD4 Selection of T cells cont’d T cells that survive positive and negative selection leave the thymus via the blood CD8+ T cells become cytotoxic T cells CD4+ T cells become helper T cells (eventually differentiate into Th1, Th2, Th17 etc. after antigen encounter) Some CD4+ T cells differentiate into regulatory T cells Selection of T cells cont’d Selection of non-self reacting T cells is made easier because the thymus has representation of non-thymus self antigens The autoimmune regulator (AIRE) gene allows thymic epithelial cells to express many of the proteins usually found in other tissues of the body Major groups of receptors on T cells Antigen receptor (TCR) Regulatory receptors Cytokine receptors Transport receptors Antibody receptors Chemokine receptors Complement receptors Adhesion molecules (e.g., integrins, selectins) The T cell receptor (TCR) – antigen receptor  Other receptors on T cells Major surface receptors of T cells, their ligands, and their functions T Cell surface molecules * *WC = Workshop Cluster  Development of B lymphocytes The process that leads to development of B cells that eventually produce antibodies is divided into three stages Maturation - generation of mature immunocompetent B lymphocytes Activation - contact of B lymphocytes with specific antigens Differentiation - activated B cells become plasma cells that produce antibodies or become memory B cells For most vertebrates (particularly humans and mice) B lymphocytes develop in the bone marrow  B lymphocyte maturation Development begins in fetal life and later throughout the lifetime The yolk sac, fetal liver and bone marrow are locations of development, but after birth development takes place only in the bone marrow Cells differentiate from lymphoid precursor cells into B lymphocyte progenitor cells (Pro-B) Later they differentiate into B lymphocyte precursor cells in the presence of IL-7 and bone marrow stromal cells The B lymphocyte receptor (BCR) C H H V = Variable; D = Diversity; J = Joining; C = Constant  Susumu Tonegawa 1987 Physiology or Medicine Antibody diversity  Rearrangement of Ig genes The first to appear on pro-B cells are heavy chains (H) (DHJH) followed a VHDHJH rearrangement and further the α and β chains of poly-Ig Light chains are rearranged at the pre-B cell stage  Because of alleleic exclusion only one type of light chain can be expressed (VLJL), κ or λ After the pre-B cell stage immature B cells emerge with antigen specificity resulting from the rearrangement of V, D and J regions of H chains and V and J regions of the L chains  Immature B cells posses membrane bound IgM (mIgM) and Igα/Igβ chains, forming the B lymphocyte cell receptor (BCR) complex The chain rearrangement process is regulated by two recombinases RAG1 and RAG2 Immature B lymphocytes are not fully functional. Stimulation through the BCR at this stage leads to cell death or anergy (lack of response)  Full maturity is attained after membrane bound IgD is expressed. However, this requires a change in RNA coding to allow coding of both heavy chains i.e., μ and δ (IgM and IgD) Checkpoints of B cell development  Of the 5 x 1010 B cells produced in bone marrow daily, only 10% reach the circulation. The rest are eliminated through negative selection or clonal deletion Negative selection or clonal deletion applies to B cells that have high affinity for self antigens Mature B cells leave the bone marrow and arrive at the periphery where they undergo activation and differentiation into plasma cells  The B cell receptor Has 4 polypeptide chains 2 identical H (heavy) chains and 2 identical L (light) chains The L chains are bound to the H chains by disulfide bonds The H chains are bound to each other similarly  The B cell receptor The C-terminal of the H chains span the cell membrane and are identical for each Ig isotype The N-terminal (membrane-distal) portions of the H chains are highly variable Similarly, the N-terminal portions of L chains are highly variable Igα/Igβ are signalling molecules  Other receptors on B cells Major surface receptors of B cells, their ligands, and their functions. Location of lymphocytes in the body Lymphocyte populations in different animal species