The antibody loci contain various gene segments that rearrange in developing B cells to generate a highly variable repertoire of antibodies. The variable region of the light-chain (LC) is assembled from two gene segments – a long variable (V) segment and a short joining (J) segment (Figure 2). The heavy chain (HC) variable region is assembled from a V segment, J segment, and a diversity (D) segment.
Figure 2. Organization of gene segments in human heavy and light chain loci.
Recombination and joining of V, D, and J segments create a functional variable immunoglobulin region (Figure 3). For human κ LCs, 40 V segments can combine with any of the 5 J segments resulting in 200 possible combinations encoded by this pool. The genes for human λ LCs contain 30 V segments and 4 J segments resulting in 120 possible variable λ regions. HCs contain 65 V segments that can join with any of the 6 J and 27 D segments to form roughly 11,000 possible variable HC regions.
A process called site-specific recombination mediates V(D)J recombination. Chromosomal DNA double stranded breaks are introduced by RAG recombinase. This enzyme complex binds and cleaves the DNA at conserved sequences that flank each V, D, and J segment. Selected V, D, and J segments are rearranged and joined to form the V(D)J exon. DNA ends are repaired by DNA repair enzymes, resulting in deletions or inversions of gene segments.
Nucleotides are often lost or inserted at the joining sites during recombination resulting in a frameshift mutation. This process known as junctional diversification introduces an additional level of antibody diversity in the variable region, specifically in the third complementarity-determining region (CDR3). The formed V(D)J exon is transcribed and translated into a functional HC or LC (Figure 3).
Figure 3. V(D)J recombination forms the variable region of heavy and light chains. (Adapted from Janeway et al. 2001)
In a single B cell, any possible HC can be recombined and produced together with any possible LC. Since both H+L chains provide antigen specificity at the antigen-binding site, this results in over 3 million unique antibodies. Combinatorial diversity of the antigen-binding site is achieved through different combinations of segment joining and H+LC pairing.