Ch 5

• Primary immunoglobulin gene rearrangements and expression

  The process by which complete immunoglobulin genes are generated through the somatic recombination of separate gene segments

  
• 1. cell receptor gene rearrangement


  The process by which T-cell receptor genes are generated through gene rearrangement

  
• Chapter contents


  • Primary immunoglobulin gene rearrangements and expression
  • T-cell receptor gene rearrangement

  
• Complete immunoglobulin genes are generated by the somatic recombination of separate gene segments

  
• Immunoglobulin variable region


  • 3 hypervariable (HV) regions are encoded within a single V-region exon
  • Ig fold supported by framework regions (FRs) (9 β strands → 2 β sheets) → hold 3 HV regions (loops) that determine antigen specificity
  • A complete variable region in a lymphocyte is encoded within a single exon of the full antigen-receptor gene: 3 HV regions are interspersed between 4 FRs

  
• CDR3 region


  Originates from 2 or more individual gene segments that are joined during lymphocyte development

  
• Light-chain variable region


  Constructed from 2 segments: a variable (V) and a joining (J) gene segment in genomic DNA are joined to form a complete light-chain V-region exon

  
• Heavy-chain variable region


  Constructed from 3 gene segments: Diversity (D) and J gene segments join → V gene segment joins to the combined DJ sequence, forming a complete VH exon

  
• Somatic recombination of Ig genes

  1. Germline configuration of the Ig gene locus

  2. D-J rearrangement

  3. V-DJ rearrangement

  4. Transcription

  5. Pre-mRNA

  6. Splicing

  7. Mature mRNA

  8. Translation

  9. Ig

  
• Recombination signal sequences (RSSs)

  • Conserved heptamer and nonamer sequences that flank V, D, and J gene segments of Ig
  • Heptamers are found at 3′ end of V segments, 5′ end of J segments, and both sides of D segments
  • Joining of gene segments involves a 12-bp RSS and a 23-bp RSS — the 12/23 rule
  • RAG-1 recombinase cuts DNA precisely between last nucleotide of V gene segment and first C of heptamer or between last G of heptamer and first nucleotide of D or J gene segment

  
• Palindromic P-nucleotides


  Generated by the endonuclease activity of Artemis during Ig/TCR gene recombination

  
• Terminal deoxynucleotidyl transferase (TdT)


  A specialized DNA polymerase expressed in immature pre-B and pre-T cells that adds non-templated N-nucleotides to V, D, and J exons of TCR and BCR genes during gene recombination, contributing to junctional diversity

  
• Processes that generate diversity of the immunoglobulin repertoire

  • Combinational diversity: the multiple inherited gene segments are used in different combinations
  • Junctional diversity: variable addition and subtraction of nucleotides at the junctions between gene segments contributes to the diversity of the third hypervariable region

  
• IgM and IgD


  Derived from the same pre-mRNA transcript and are both expressed on the surface of mature B cells

  
• Generation of transmembrane and secreted forms of immunoglobulin

  1. Cleavage and polyadenylation at the second site (pA2) and splicing → transmembrane form of the heavy chain (mIgM)

  2. Polyadenylation at the first poly(A) addition site (pA1) → secreted form of heavy chain (sIgM)

  
• 1. cell receptor gene segments

  • Arranged in a pattern similar to immunoglobulin gene segments and are rearranged by the same enzymes
  • TCRα locus: 70–80 Vα gene segments, each preceded by an exon encoding the leader sequence (L), a cluster of 61 Jα gene segments located a considerable distance from the Vα gene segments, and a single Cα gene
  • TCRβ locus: a cluster of 52 functional V gene segments located distant from two separate clusters that each contain a single D gene segment together with six or seven J gene segments and a single C gene

  
• 1. cell receptor diversity


  Concentrated in the third hypervariable region, where the highly variable CDR3 regions mainly contact the unique peptide component of the ligand

  
• γδ T-cell receptors are also generated by gene rearrangement

  
• Sources of T-cell receptor diversity compared to immunoglobulins

  • Ig heavy chains and TCR β chains both consist of D segments, with Ig heavy chains having more (23) compared to the TCR β chains (2)
  • Ig heavy chain D segments are rarely read in 3 frames where TCR β chain D segments are often read in 3 frames