VDJ Recombination

Question 1

What is the purpose of VDJ recombination?

Answer 1

VDJ recombination is the mechanism B and T cells uses to acheive diversity within their lymphocyte receptors.

V = Variable

D = Diversity

J = Joining/Junction

Question 2

What are the key enzymes involved in VDJ recombination?

Answer 2

• RAG1 and RAG2 = lymphocyte-specific DNA-modifying enzymes (they make the hairpin loop and do the cutting)
• Terminal deoxynucleotidyl transferase (TdT) = enzyme that adds **non-germline **nucleotides after the cutting out of the hairpin loop

Question 3

Beyond VDJ recombination, how is additional diversity introduced?

Answer 3

• Further diversity is introduced at gene segment junctions through imprecise joining mechanisms at Complementarity Determining Region (CDR3) that lies at the center of antigen-binding site (it is encoded at the V-D-J or V-J junctions of the antigen receptor genes)
• Independent association of the two chains of Ig or TCR
• Mature B cells activated by antigen undergo somatic point mutation of the Variable region DNA (somatic hypermutation)

Question 4

C/C BCRs and TCRs in terms of structure, function, and level of diversity

Answer 4

• Igs exist in both membrane-bound (BCRs) and secretied forms (antibodies)
  
  • This is due to differential splicing of mRNA to include exons for either the hydrohobic mb anchor or the secreted tailpiece (respectively)
• Havey chain Constant regions contain 3 or 4 Ig domains, whereas TCR chains have only one
• Antibodies have a variety of effector functions mediated by their Constant regions
  
  • There are also several different alternative heavy chain Constant regions for Igs, each which specific effector functions
• In BCRs, the same Variable region can be expressed along with different Constant regions through isotype switching, which maximizes the possible effector fnuctions of any given antibody

Question 5

Four processes that generate the diversity of Igs

Answer 5

• Gene segment combinatorial diversity
  
  • Due to presence in the germline of multiple different copies of each type of gene segment (i.e., multiple Vs, Ds, and Js)
• Junctional diversity
  
  • Due to the addition and subtraction of nucleotides at the joints b/w the different V, D, and J segments in the recombination process
  • Concentrated in 3rd hypervariable region (CDR3 region)
• Chain-pairing
  
  • Due to the many possible different combinations of heavy- and light-chain variable regions (alpha and beta chains in TCRs) that pair to form the antigen-binding site
• Somatic hypermutation
  
  • Only in BCRs (not in TCRs)