Week 7 S - Immunogenetics

Overview of Immunogenetics

• Focus on mechanisms generating antibody (Ab) and T-cell receptor (TCR) diversity.
• Key processes include:
  • Structure of immunoglobulin loci
  • DNA rearrangement and loss
  • V(D)J recombination mechanism
  • Variability sources
  • Isotype switching
  • Antibody maturation

Structure of Immunoglobulin Loci

• Three loci:
  • kappa (k) light chain
  • lambda (λ) light chain
  • heavy chain (H)
• Each locus contains gene segments:
  • V (variable), D (diversity for heavy chain), J (joining), C (constant).
• Locus structure differs in germline vs. B cells:
  • In B cells, DNA is rearranged and some segments are lost.

DNA Loss During Rearrangement

• V(D)J joining involves the loss of DNA during recombination:
  • Recombination leads to the loss of loops of DNA.
  • Randomness in joining contributes to diversity.

Mechanism of V(D)J Rejoining:

• Recombination Signal Sequences (RSS):
  • Comprised of nonamer and heptamer sequences separated by 12 or 23 bp.
  • Important for aligning gene segments for recombination.
• RAG1 and RAG2 Enzymes:
  • Recognize RSS and initiate DNA breaks, forming hairpin structures.
• Joining process can be either deletional or inversional.

Sources of Variation

• Rejoining of segments is not precise, leading to:
  • Non-productive gene arrangements.
  • Additional variation through P-nucleotide addition (during hairpin cleavage) and N-nucleotide addition (by TdT).
• Resulting diversity is concentrated in complementarity-determining regions (CDR) of both heavy and light chains.

Isotype Switching

• Isotype switching allows B cells to change the type of antibody produced without altering the specificity for antigen:
  • Initially, heavy chain transcripts contain both IgM and IgD.
  • Selection of polyadenylation site determines which constant region is expressed (IgM vs. IgD).
  • The process is facilitated by switch regions between C regions of heavy chain locus.
• Enzyme AID (Activation-induced cytidine deaminase) plays a crucial role in deaminating cytosines, facilitating mutations and switch regions.

Antibody Maturation

• Antibody maturation includes:
  • Isotype switching: Changing of heavy chain regions.
  • Affinity maturation: Increased binding affinity for antigens through somatic hypermutation, focused primarily on CDR1 and CDR2 sequences.
  • Transcription dependence: Both processes rely on transcriptional changes and are initiated by AID.

Summary of Key Terms

• V(D)J Recombination: Process generating diverse antibodies and T-cell receptors.
• RSS: Recombination signal sequences crucial for alignment of gene segments.
• AID: Enzyme responsible for editing cytosines to uracils, critical to diversification processes.
• Isotype switching: Mechanism enabling a B cell to produce different classes of heavy chain antibodies (IgM, IgD, etc.).
• Affinity maturation: Process of increasing the binding strength of antibodies to antigens over repeated exposures.