Lecture 22 - Diversity of IG and TCR Ag Specificity PDF

Lecture 22 – Diversity of IG and TCR Ag specificity BIOL233 – Immune Systems Wood 3rd Ed. (Pg 107-116) Kuby Immunology 8th Ed. (Pg 449- 509) Cassie [email protected] SU6: Lymphocyte development, Gene organisation and Expression Lecture 21: Lymphocyt...

Lecture 22 – Diversity of IG and TCR Ag specificity BIOL233 – Immune Systems Wood 3rd Ed. (Pg 107-116) Kuby Immunology 8th Ed. (Pg 449- 509) 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 Learning Outcomes Describe The purpose of V(D)J recombination during: Ig gene rearrangement TCR gene rearrangement Steps of V(D)J recombination  Mechanisms that increase Diversity of Ig & TCR Ag specificities During Lymphocyte development: There is a “mixing & matching” of V, D, and J genes – called V(D)J recombination = large increase in possible combinations from a small no. of genes ~5 x 106 Possible Ig and TCR molecules  Antigen Receptor Genes in B and T lymphocytes Genes that encode diverse antigen receptors of B and T lymphocytes Generated by rearrangement in individual lymphocyte genes comprising of Different Variable (V) region gene segments with Diversity (D) and Joining (J) gene segments  Mechanisms that increase Diversity : IG and TCR Rearrangement V(D)J recombination Directed by recombination signal sequences (RSS) – Found at: 3’ end of V segment Both sides of D segment 5’ end of J segment  Mechanisms that increase Diversity : IG and TCR Rearrangement V(D)J recombination Directed by recombination signal sequences (RSS) – Found at: Made up of: Heptamer (7bp) and Nonamer (9bp) – separated by 12 or 23 bp spacers Whole Heptamer + 12 or 23 bp spacer + Nonamer = RSS  Mechanisms that increase Diversity : V(D)J recombination  Ig and TCR Rearrangement V(D)J recombination process – consists of FIVE steps  V(D)J Recombination 1. Alignment of RSS nonamers Nonamers with 12 bp spacer brought together with nonamers with 23 bp spacer 12/23 rule – Recombination occurs between one region with a 12-bp spacer and a second region with a 23 bp spacer: A 12 bp spacer can bind only to an RSS with a 23 bp spacer Ensures that when Ig Heavy chain and TCR β genes rearrange: V rearranges to D and not to J  V(D)J Recombination 2. Cleavage of DNA Once nonamers are aligned: DNA strand within heptamers – cleaved by endonucleases Intervening DNA is excised or “looped out” Ends of V and D genes now contain part of heptamer sequences  V(D)J Recombination 3. Formation of cleavage hairpins & generation of P-nucleotides Two strands of DNA at ends of V and D genes – join to form a hairpin Hairpins are cleaved – each DNA strand contains nucleotides from the heptamer These nucleotides called P- nucleotides Formation of hairpins and cleavage Increases variety of DNA sequences  V(D)J Recombination 4. Addition of N-nucleotides Nucleotides are added randomly to the end of DNA strands These nucleotides – called N- nucleotides Done by the enzyme terminal deoxynucleotide transferase Further increases variability of DNA sequences in different cells  V(D)J Recombination 5. Filling in of DNA Gaps created during cleavage ect. Gaps need to be filled Requires several enzymes: Exonucleases – removes bases from coding ends DNA polymerase – Inserts additional two nucleotides Makes the two ends compatible for joining DNA ligase – ligates coding ends together DNA repair and completion of the joint formation Lecture 23 – IG and TCR GENE Rearrangemen t – Regulation and control BIOL233 – Immune Systems Cassie [email protected]