2021-22 Lymphocyte Development PDF

Block 1.2 lectures 2024-2025 lecture Highlighter key Writer Reviewer Doctor explanati...

Block 1.2 lectures 2024-2025 lecture Highlighter key Writer Reviewer Doctor explanation Abbreviation Key information Book >> Anwar hani >> Eithar zaki Anwar hani 221-222-223 notes References Student explaintion Deleted Eithar zaki 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. 2 progenitors  Steps of DEVELOPMENT OF IMMUNE REPERTOIRES ① production of pro B,T lymphocytes in BM Early lymphocyte development Early lymphocyte development ② proliferation of lymphocytes ③ lymphocytes will express different unique antigen Production of diverse antigen receptors receptors Final results Maturation and selection B-cells: Formed and mature in the bone B Lymphocyte In Bone marrow ( BM ) marrow. T-cells: Formed in the bone marrow, then T Lymphocyte In Thymus gland mature in the thymus. 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 By somatic genetic recombination LYMPHOCYTE MATURATION B lymphocytes mature in bone marrow T lymphocytes mature in thymus immature stages > central lymphoid organs(BM, thymus) STEPS It could be T or B cells Functional ( able to recognize and response to antigen ) 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 ‫السلايده لفهم اللي فوق من تفريغ ‪222‬‬  LYMPHOCYTE MATURATION ( selection of lymphocytes ) 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 Results express antigen receptors die by apoptosis  LYMPHOCYTE MATURATION Types of selection Immature T cells selected to recognize self MHC molecules in the thymus -positive selection Aim 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 Which means : Eliminated of B,T cells thats strongly bind to self antigen to prevent autoimmune diseases  There are 2 checkpoints : 1- Elimination of cells that don’t express any antigen receptors 2- Elimination of cells that express antigen receptors to self antigen Second Checkpoint The lymphocytes are undergoing First proliferation ‘just dividing” Checkpoint  Explaination of the previous slide 3.Pre-B/T Cell: Expresses one First Checkpoint: Cells must chain of the antigen receptor. If 1.Common Lymphoid Progenitor: 2.Pro-B/T Cells: The early express a functional pre- the receptor is not properly The starting point where stem cells stage of B and T cell antigen receptor. Failure to formed, the cell undergoes differentiate into lymphoid cells lineage commitment. do so results in cell death. programmed cell death. 4.Immature B/T Cell: 5.Second Checkpoint: Involves Positive Selection: Occurs if the antigen receptor Completes the expression of a positive and negative selection shows weak antigen recognition, leading to the functional antigen receptor. processes survival of the cell. Negative Selection: Occurs if the antigen receptor strongly recognizes self-antigens, leading to cell death to prevent autoimmunity.  Germline genes Functional genes the gene that are present in The gene that after the cell recombination develops into these germlines 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. ① variable region contains multiple genes one or few C region genes present in these cells. ② constant region contains few genes 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). Which means that the recombination of this gens ( V, D, J ) will lead to generate functional gens ( for both receptors “ Ig , TCR” ) PRODUCTION OF DIVERSE ANTIGEN RECEPTORS All antigen receptor gene loci = V, J,C genes Only Ig heavy-chain and TCRβ-chain loci = V, J,C + D gene segments VDJ recombination  VDJ recombination D-J recombination V-DJ recombination VDJ recombinase enzyme The DNA is then transcribed, and RNA splicing processes the RNA to produce mRNA encoding the Ig heavy chain.  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  Junctional diversity the doctor’s example: The VDJ when they are recombined, subsequently there are going to be some nucleotide changes, addition or deletion. So, this will bring about further diversity. For example, let's say, we have VH2, DH2 and JH2 recombine gene in one of the lymphocytes. Another lymphocyte has the same recombination because spontaneous random process VH2, DH2, JH2 That means both these clones of lymphocyte are expressing the same antigen receptor. Yes or no? no , It can have different antigen specifications. How? Because in the first lymphocyte of the same recombine gene, there could be addition of some nucleotides between the junction of these three recombine genes or deletion of some nucleotide sequences that will make the antigen receptor a bit different from the second one. 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 1. The recombination between D + J will happen = DJ recombined gene 2. V will join to DJ recombination = VDJ recombined gene, at the junction of this three gens the changes will happened so it will be Junctional diversity, if not it will be Combinatorial diversity 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 intopre-B cellsin the cytoplasm. µchain and surrogate light chains associate with Igαand Igβsignaling molecules to form pre-B cell receptor (pre-BCR) complex There is a genetic recombination from light chain and heavy chain and Which means that pre B cell receptor complex consist of = µ chain + Igα and Igβ ( signaling molecules ) that will leads to diversity MATURATION AND SELECTION OF B LYMPHOCYTES pre-BCR delivers signals promoting survival and proliferation of B lineage cells Progenitors Pre-B cells that make nonproductive rearrangements fail to make µprotein, 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 Proteins Once a functional light chain is made, Rag 1 and Rag 2 expression is turned off Immature B cell interaction with self-antigen: if one chromosome starts the recombination , other chromosomes, will be there is no recombination. Receptor editing -B cells can continue to rearrange Ig L genes So, it is a shut-off. It is “closed-off. ”There is no recombination in the second alylic.  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 Anergyis a term inimmunobiologythat describes a lack of reaction by the body's defense mechanisms to foreign substances, and consists of a direct induction of peripheral lymphocytetolerance. An individual in a state of anergy often indicates that theimmune systemis unable to mount a normal immune response against a specificantigen, 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 beingclonal deletionandimmunoregulation) called as clonal energy. And clonal energy is one of the mechanism of immunological tolerance. That is, what is the immunological tolerance? Self-tolerance. There is one called as self-tolerance. Then the immunological tolerance in the broader sense is different. Self-tolerance means what? Don't move the self-energy. No immune response against self-energy,  Clonal deletion Clonal deletionis the elimination ofB andT cells that express receptors for self-antigens and before they develop into fully immunocompetent lymphocytes Responsible forimmune 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 Double negative Immature progenitors pro-T cells formed, not express CD4 or CD8 CD4,CD8 pro T cell Cells expand in influence of IL-7 produced in the thymus VDJ recombination is successful , TCRβ-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.  Weak recognition will The T lymphocyte will interact with lead to + selection antigen presenting cell “APCs” in thymus which are presenting self antigen any lymphocyte that binds with the peptide antigen MSI complex weakly will it be selected or not? It will be selected. why? because it is binding to the MSC, the portion non-polymorphic if there is no recognition of area of the MSC. It is not recognising self-peptide. MSC plus peptide that means It is not auto-reactive but this weak interaction is there is no recognition of due to the MSC recognition of self-MSC. even self-MSC. That means we do not want such mature t-lymphocyte. That means it has to undergo a process and elimination. Why are we call it the pre-T lymphocyte? Because it is expressing one chain of T- cell receptor Summary The genes that encode antigen receptors are present in the germline and brought together during maturation of lymphocytes In B cells, Iggene 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