15 and 16-Receptor Diversity

• MHC diversity


  Determined by co dominant expression of alleles
• TCR/Ig diversity

  Produced by recombination within each T or B cells
• Immunoglobulin Gene Organization

  1. Germline DNA

  2. Somatic recombination

  3. D-J joined, rearranged DNA

  4. V-J or V-DJ joined, rearranged DNA

  5. Transcription

  6. Splicing

  7. Translation
• Mechanism of VDJ recombination

  • Mediated by specific DNA sequences
  • Recombination Signal Sequences (RSS; Dictate where things are cut)
  • Ensure joins occur at right place with the right components
  • Controlled recombination activating genes : RAG1/2
  • Occurs seperately in each cell
  • Each B cell will only produce a single immunoglobulin binding site
• RSS


  Structure: Heptamer--- Spacer with either 12 or 23 base pairs---nonamer
• Rules of Recombination


  • Only occurs between segments on the same chromosome
  • A 12 bp RSS must combine with a 23 RSS
• Order of Recombination

  1. D to J

  2. V to DJ
• Steps of VDJ recombination

  1. RAG 1/2 randomly choose a RSS and bind. It aligns the V and J (on top of each other)

  2. RAG 1/2 nick ONE strand at 5' border of heptamer on RSS

  3. The free OH group at the nick naturally attacks the P on the non-coding strand

  4. DNA ligase IV joins the loose ends together in the signal joint: forms circle

  5. Hair pin loop is opened by an enzyme with a knick at a random spot

  6. V and J regions are ligated together in light chain

  7. TDT adds non-templated nucleotides to the coding joint before ligation
• How we get so much diversity

  • Different combinations of gene segments
  • Addition of P/N nucleotides or deletion during ligation
  • Heavy Chain V, D and J
  • Light Chain: just V and J
• B Cell Recombination

  1. Has to recombine all the segments needed for productional of functional heavy and light chains

  2. Has to ensure there is only one light chain and 1 heavy chain protein produced per cell

  3. Has to ensure resulting immunoglobulin does not bind to self antigens and cause autoimmunity

  4. Allelic Exclusion: only one chromosome recombines at a time

  5. Start with heavy chains: Recombine D-J on both alleles, Only ONE allele recombines V-DJ

  6. If it makes a productive protein, cell moves on. If not successful can try the other allele

  7. Test pre-BCR using a surrogate light chain as the actual one hasn't been recombined yet

  8. Once you have a good heavy chain don't want any more rearrangements and instead focus on light chain

  9. If there are non-productive joins can undergo receptor editing (go back and try again)
• Selecting Antibody Isotypes


  1. Early B cells express IgM first

  2. Cytokines from T cells tell the activated B cell which isotype it needs

  3. DNA cut at AID and like before removed DNA forms circle and is deleted (can't go back)

  4. Alternative Splicing to produce membrane-bound and secreted forms
• T Cell Recombination

  1. Has to recombine all the segments needed for production of functional beta and alpha chains

  2. Has to ensure only one aa and one b chain protein in each cell

  3. Has to ensure TCR does not cause autoimmunity and react to self antigens

  4. Uses same mechanism as B cells (RAGs, RSS, TdT, allelic exclusion)

  5. No alternative splicing because doesn't need isotype switching
• TCR diversity

  • Has similar numbers of V, D and J as B cells
  • More diversity as there is TdT activity in both the alpha and beta chains
• a/B vs y/ST cells

  • CD4 and CD8 all have a/B TCR
  • Small amount of T cells have y/S TCR instead