2021-22 Lymphocyte Development PDF

Title: Lymphocyte Development CRN No: 15569(Male), 15581 (Female) Block: 1.2 Subject/Discipline: Immunology Expert: Dr.Sayed A.Quadri Block Coordinator: Dr.Sayed A.Quadri Learning objectives Explain how diversity develops in the specificity of immunoglobulins and T cell receptors. How mature B...

Title: Lymphocyte Development CRN No: 15569(Male), 15581 (Female) Block: 1.2 Subject/Discipline: Immunology Expert: Dr.Sayed A.Quadri Block Coordinator: Dr.Sayed A.Quadri Learning objectives Explain how diversity develops in the specificity of immunoglobulins and T cell receptors. How mature B lymphocytes are produced in the bone marrow from common lymphoid progenitor cells Discuss the role of the rearrangement of the membrane immunoglobulin genes in this process. Also explain the term ‘allelic exclusion’ and the relevance of this phenomenon. Learning objectives How is the maturation of autoreactive cells prevented during the development of B lymphocytes in the bone marrow (central tolerance). Describe how T lymphocytes are produced in the thymus and how this involves rearrangement of the T cell receptor genes Explain how the interaction between the TCR and MHC determines the fate of double positive thymocytes. Describe the difference between positive and negative selection and how potential threats by auto reactive cells are prevented.  DEVELOPMENT OF IMMUNE REPERTOIRES Early lymphocyte development Production of diverse antigen receptors Maturation and selection B Lymphocyte T Lymphocyte DEVELOPMENT OF IMMUNE REPERTOIRES A complex process 1000 Million lymphocyte clones 325 Million base pairs of genes Not enough genes in human genome to encode every possible receptor Immune system developed extremely diverse antigen receptors from limited genes Linked to B and T lymphocyte maturation process LYMPHOCYTE MATURATION B lymphocytes mature in bone marrow T lymphocytes mature in thymus STEPS LYMPHOCYTE MATURATION Common lymphoid progenitors in BM B-cell or T-Cell progenitors (activation of transcription factors, ↑ accessibility of Ig, TCR gene). Proliferation of Immature lymphocytes- Stimulated by IL-7 (stromal cells of BM, thymus) ARs are encoded by several genes present in germline which recombine to produce unique combination: AR gene recombination Lymphocytes are selected to mature at different stages of their maturation: Lymphocyte selection LYMPHOCYTE MATURATION Lymphocytes selected at multiple steps during maturation to preserve useful specificities Selection of lymphocytes after proliferation is based on intact antigen receptor expression & recognition Cells with intact, functional antigen receptors selected to survive and proliferate Pre-lymphocytes and immature lymphocytes that fail to express antigen receptors die by apoptosis LYMPHOCYTE MATURATION Immature T cells selected to recognize self MHC molecules in the thymus - positive selection They ensures complete maturation of cell, capable of recognizing antigens displayed by the same MHC molecules on APCs Strongly self-reactive B or T cells are eliminated to prevent the development of autoimmune responses - negative selection  Second Checkpoint First Checkpoint Germline genes Functional genes Recall Lymphocyte Antigen Antigen Receptor Production of diverse antigen receptors Somatic recombination of AR variable region leads to production of unique antigen specificities of lymphocytes. Hematopoietic stem cells and early lymphoid progenitors contain multiple V region gene segments. one or few C region genes present in these cells. Between V and C genes is a group of D and J genes. Random selection and recombination of these gene segments (V,D,J) may lead to generation of functional genes with unique variable region of ARs (Ig and TCR). PRODUCTION OF DIVERSE ANTIGEN RECEPTORS All antigen receptor gene loci = V, J,C genes Only Ig heavy-chain and TCR β-chain loci = V, J,C + D gene segments VDJ recombination  VDJ recombination D-J recombination V-DJ recombination VDJ recombinase enzyme  VDJ recombination Lymphocyte progenitor → B lymphocyte = recombination of first one D gene segment with one J segment, followed by rearrangement of a V segment to the fused D-J element Somatic recombination of V and J or V, D, J, gene segments is mediated by a lymphoid- specific enzyme - VDJ recombinase VDJ recombinase expressed only in immature B and T lymphocytes composed of the recombinase-activating gene 1 and 2 (RAG-1 and RAG-2) Recognizes DNA sequences of antigen receptor V, D, and J gene segments  VDJ recombination Recombinase brings 2 Ig or TCR gene segments close together and cleaves the DNA at specific sites DNA breaks then repaired by ligases, producing a full-length recombined V-J or V-D-J without the intervening DNA segments Intact Ig heavy-chain and light-chain genes are rearranged and expressed only in B cells TCR α and β genes are rearranged and expressed only in T cells VDJ recombination - Diversity Combinatorial diversity - diversity of antigen receptors produced by use of different combinations of V, D, and J gene segments in different clones of lymphocytes Junctional diversity by changes produced in nucleotide sequences introduced at the junctions of the recombining V, D, and J gene segments Combinatorial diversity is limited by the number of available V, D, and J gene segments VDJ recombination Junctional diversity is unlimited Junctional diversity produced by three types of sequence changes MATURATION AND SELECTION OF B LYMPHOCYTES Progenitors proliferate pro-B cells formed Cells that make productive VDJ rearrangements develop into pre-B cells in the cytoplasm. µ chain and surrogate light chains associate with Igα and Igβ signaling molecules to form pre-B cell receptor (pre-BCR) complex MATURATION AND SELECTION OF B LYMPHOCYTES pre-BCR delivers signals promoting survival and proliferation of B lineage cells Pre-B cells that make nonproductive rearrangements fail to make µ protein, cannot express pre-BCR signals & die by apoptosis  Allelic exclusion Express only one of two Ig alleles is called “allelic exclusion” MECHANISM- Once a functional heavy chain is made, the pre-BCR can assemble and send signals, this redirects the Rag1 and Rag2 proteins away from the Ig H locus and toward the Ig L loci Once a functional light chain is made, Rag 1 and Rag 2 expression is turned off Immature B cell interaction with self-antigen: Receptor editing -B cells can continue to rearrange Ig L genes  Allelic exclusion Allelic exclusion is a process by which only one allele of a gene is expressed while the other allele is silenced In B lymphocytes, successful heavy chain gene rearrangement of genetic material occur from one chromosome resulting in shutting down of rearrangement of genetic material from the second chromosome Anergy = Non-responsiveness  Clonal anergy Anergy is a term in immunobiology that describes a lack of reaction by the body's defense mechanisms to foreign substances, and consists of a direct induction of peripheral lymphocyte tolerance. An individual in a state of anergy often indicates that the immune system is unable to mount a normal immune response against a specific antigen, usually a self-antigen. Lymphocytes are said to be anergic when they fail to respond to their specific antigen. Anergy is one of three processes that induce tolerance, modifying the immune system to prevent self-destruction (the others being clonal deletion and immunoregulation)  Clonal deletion Clonal deletion is the elimination of B and T cells that express receptors for self-antigens and before they develop into fully immunocompetent lymphocytes Responsible for immune tolerance Occasionally, T cells and B cells are produced that react to proteins that are expressed by the body's own cells, called autoantigens. Such cells have to be neutralized before entering into circulation as they could potentially attack healthy tissue During development T cells are rendered tolerant to self antigens. T cell receptors can be tolerated by processes that result in physical elimination (clonal deletion) or functional inactivation (clonal anergy) MATURATION AND SELECTION OF T LYMPHOCYTES T cell progenitors migrate from the bone marrow to the thymus Immature progenitors pro-T cells formed, not express CD4 or CD8 Cells expand in influence of IL-7 produced in the thymus VDJ recombination is successful , TCR β-chain protein is synthesized, it is expressed on surface of protein pre-Tα, to form pre-T cells MATURATION AND SELECTION OF T LYMPHOCYTES POSITIVE AND NEGATIVE SELECTION NEGATIVE SELECTION: Results from strong interaction/ recognition of self peptide/MHC complex with the TCR of a Thymocyte POSITIVE SELECTION: Results from weak interaction/recognition of a self peptide/MHC complex with the TCR of a Thymocyte then T cells are more likely to be useful Self-antigen is always present, but foreign antigens are generally not present at sites of development of immune cells. Summary The genes that encode antigen receptors are present in the germline and brought together during maturation of lymphocytes In B cells, Ig gene segments undergo recombination as the cells mature in the bone marrow In T cells, The TCR gene segments undergo recombination during maturation in the Thymus Summary Receptors of different specificities are generated by different combinations of V,D and J gene segments MHC restriction Immature lymphocytes that strongly recognize self antigens are negatively selected and are eliminated. THANK YOU