Study Unit 6 - Lectures 21 (1).pptx

Study Unit 6: Lymphocytes – Development Gene Organization & Expression BIOL233 – Immune Systems Cassie [email protected] SU6: Lymphocyte development, Gene organisation and Expression Lecture 21: Lymphocyte production & Lymphocyte Receptor Genes Lecture 22: Diversity of IG...

Study Unit 6: Lymphocytes – Development Gene Organization & Expression BIOL233 – Immune Systems Cassie [email protected] SU6: Lymphocyte development, Gene organisation and Expression Lecture 21: Lymphocyte production & Lymphocyte Receptor Genes Lecture 22: Diversity of IG and TCR Ag-Specificity Lecture 23: IG and TCR Gene Rearrangement – Regulation and control  Lecture 21: Lymphocyte Production & Receptor Genes BIOL233 – Immune Systems Kuby Immunology 8th Ed (Pg 434- 509, 599-603, 653-655) Wood 3rd Ed (Pg 92-100, 100-107) Cassie  Learning Outcomes Lymphocyte Production Stages of lymphocyte development in: B lymphocytes T lymphocytes (CD4+ CD8+) Organisation of Ig and TCR genes Ig chains – Kappa and lambda light chains & Ig heavy chains TCR chains – alpha chain & beta chain Gene segments that make up Ig and TCR genes Ig chains TCR chains Describe segments encoding CDR regions of Ig and TCR V(D)J recombination  Lymphocyte Production Lymphocytes = white blood cells B lymphocytes T lymphocytes Originate from pluripotent stem cells in bone marrow Lymphoid Progenator (Lymphoid Stem cells) Produce lymphocytes through cell division & differentiation During Differentiation – Cell function changes based on changes in Gene expression Express different proteins – indicate if it becomes B or T cell Note: Pluripotent stem cells = Immature cells – capable of giving rise to several different cell types Lymphocyte Production – Stages of development Bone marrow Lymphocyte Production What makes it become a B lymphocyte or T lymphocyte? Determined by TWO factors: 1. Anatomical site of development B lymphocytes develop in the bone marrow T lymphocytes develop in the thymus 2. Gene expression Alterations in gene expression during differentiation Causes different proteins to be expressed in different cells B Lymphocyte Development T Lymphocyte Development There are THREE types of T cells 1. CD4 T cells Express the α/β TCR 2. CD8 T cells Express the α/β TCR 3. γ/δ T cells Express the γ/δ TCR Note: CD4 and CD8 express the same types of receptors but γ/δ is different TCR = T cell Receptor T Lymphocyte Development B vs T Lymphocyte Development Lymphocyte Receptor Genes B cells – express immunoglobulins (Igs) T cells – express T cell receptors (TCRs) During differentiation: B and T cells generate receptors for different antigens Receptors must have different Ag specificities Ag-binding side of Igs and TCRs need to recognise Ags from nearly all pathogens  Lymphocyte Receptor Genes Lymphocytes use a limited no. of genes – produce receptors with variable specificity How? Genes coding for Igs and TCRs – made up of segments V(D)J recombination Genetic recombination mechanism – occurs in developing lymphocytes Rearranges different gene segments Generates large repertoire of Igs and TCRs with different specificities  Lymphocyte Receptor Genes - Recap Igs – 2 Heavy chains & 2 light chains TCRs – α-chain and β-chain Encoded by ~300 different gene segments Must produce 109 – 1014 surface receptors (Igs and TCRs)  B-Lymphocyte Receptor Genes Igs – 2 Heavy chains & 2 light chains Comprise a constant (C) and Variable (V) region Encoded by THREE loci: 1. Ig heavy locus 2. Ig Kappa (k) locus 3. Ig lambda (λ) locus Each H and L chain genes Have multiple copies of three different types of gene segments for variable regions  B-Lymphocyte Receptor Genes Encoded by THREE loci: 1. Ig heavy locus On chromosome 14 with gene segments for Ig heavy chain 2. Ig Kappa (k) locus On chromosome 2 with gene segments for part of Ig Light chain 3. Ig lambda (λ) locus On chromosome 22 with gene segments for part of Ig Light chain  B-Lymphocyte Receptor Genes Ig K-light chain Consists of constant and variable region Constant region encoded by a single exon Ck gene segment Variable region encoded for by one of the many available exons – in total two exons “chosen” Vk gene segment (~76 exons) Jk (Joining) gene segment (~5 exons) K-light chain gene – contains Ck, Vk, and Jk gene segments B-Lymphocyte Receptor Genes Ig K-light chain – gene rearrangement 1. DNA encoding k-chain folds One V region joins to one J region (V3 and J3) 2. Looped DNA excised – V3 Joins J3 3. Gene rearrangement 4. Primary transcript - mRNA 5. mRNA transcript – translated into k-chain protein Note: Different B cells – different Ig-K light chains B-Lymphocyte Receptor Genes  B-Lymphocyte Receptor Genes Ig K-light chain Variable regions of Ig – have 3 Complementarity Determining Regions (CDRs) Vary extensively in aa sequences Different Ag-binding specificities CD1 and CD2 Encoded for exclusively by V gene segments CDR3 Encoded for by part of the V & part of the J segment Genes from both segments - influence antigen specificity  B-Lymphocyte Receptor Genes Ig λ-light chain Slightly different from k-light chain 30 functional Vλ genes – four repeats of functional J and C genes λ-light chain proteins – produced from each of the 4 Cλ genes – structurally the same  B-Lymphocyte Receptor Genes Ig heavy chain Basic organisation of Ig H genes – similar to k-light chain gene Variable regions encoded by V and J segments as well as the diversity (D) segment Heavy chain generated from one gene from each segment  B-Lymphocyte Receptor Genes Two gene rearrangements occur during B-cell development Joining of one gene from D segment to one gene from J segment Joining of one gene from V segment to one gene from D segment  B-Lymphocyte Receptor Genes Ig heavy chain Variable regions – Contain CDRs CDR3 encoded for by part of V gene and all of D and J gene Constant region genes – downstream of J genes (3’) Encodes constant regions of heavy chain One Ig heavy chain constant region for each class and subclass of Ab Total of 9 constant genes in humans  T-Lymphocyte Receptor Genes TCR gene Encode for α chain and β chain of TCR Basic arrangement – similar to Ig genes  T-Lymphocyte Receptor Genes TCR α gene One C region – coded for by 4 exons 50-70 Vα genes and 50 Jα genes CDR1 and CDR 2 – coded for by the V genes CDR3 – coded for by part of V gene and part of J gene  T-Lymphocyte Receptor Genes TCR β gene Variable region – encoded for by genes in V, D, and J segments 50 Vβ genes Two repeats of a cluster of one D-region (Two Dβ genes) Two repeats of a cluster of seven J-region genes (13 Jβ genes – one is pseudogene and not expressed) Constant region Two constant region genes (two Cβ genes)  Lecture 22 – Diversity of IG and TCR Ag specificity BIOL233 – Immune Systems Cassie [email protected]