and cancer risk. ATM is a serine/threonine kinase, like DNA-PKcs, and functions during V(D)J recombination by activating pathways that prevent the chromosomal translocations and large DNA deletions that can sometimes occur during resolution of DNA double-strand breaks. Some V(D)J recombination can occur in the absence of ATM, since the immune deficiencies seen in ataxia telangiectasia, which include low numbers of B and T cells and/or a deficiency in antibody class switching, are variable in their severity and are less severe than in SCID. Evidence that ATM and DNA-PKcs are partially redundant in their functions comes from the observation that B cells lacking both kinases show much more severely abnormal signal joining sequences compared with B cells lacking either enzyme alone.
5-6 The diversity of the immunoglobulin repertoire is generated by four main processes.
The gene rearrangements that combine gene segments to form a complete V-region exon generate diversity in two ways. First, there are multiple different copies of each type of gene segment, and different combinations of gene segments can be used in different rearrangement events. This combinatorial diversity is responsible for a substantial part of the diversity of V regions. Second, junctional diversity is introduced at the joints between the different gene segments as a result of the addition and subtraction of nucleotides by the recombination process. A third source of diversity is also combinatorial, arising from the many possible different combinations of heavyand light-chain V regions that pair to form the antigen-binding site in the immunoglobulin molecule. The two means of generating combinatorial diversity alone could give rise, in theory, to approximately 1.9 × 106 different antibody molecules, as we will see below. Coupled with junctional diversity, it is estimated that at least 1011 different receptors could make up the repertoire of receptors expressed by naive B cells, and diversity could even be several orders of magnitude greater, depending on how one calculates junctional diversity. Finally, somatic hypermutation, which we describe in Chapter 10, occurs only in B cells after the initiation of an immune response and introduces point mutations into the rearranged V-region genes. This process generates further diversity in the antibody repertoire that can be selected for enhanced binding to antigen.
5-7 The multiple inherited gene segments are used in different combinations.
There are multiple copies of the V, D, and J gene segments, each of which can contribute to an immunoglobulin V region. Many different V regions can therefore be made by selecting different combinations of these segments. For human κ light chains, there are approximately 40 functional Vκ gene segments and 5 Jκ gene segments, and thus potentially 200 different combinations of complete Vκ regions. For λ light chains there are approximately 30 functional Vλ gene segments and 4 to 5 Jλ gene segments, yielding at least 120 possible Vλ regions (see Fig. 5.4). So, in all, 320 different light chains can be made as a result of combining different light-chain gene segments. For the heavy chains of humans, there are 40 functional VH gene segments, approximately 25 DH gene segments, and 6 JH gene segments, and thus around 6000 different possible VH regions (40 ×25 ×6 = 6000). During B-cell development, rearrangement at the heavy-chain gene locus to produce a heavy chain is followed by several rounds of cell division before light-chain gene rearrangement takes place, resulting in the same heavy chain being paired with different light chains in different cells. Because both the heavyand the light-chain V regions contribute to antibody specificity, each of the 320 different light chains could be combined with each of the approximately 6000 heavy chains to give around 1.9 × 106 different antibody specificities.