In the mismatch repair pathway in B cells, but not in other cell types, the DNA lesion is repaired by error-prone DNA polymerases rather than by more accurate polymerases that faithfully copy the undamaged template strand. Individuals with a defect in the translesion polymerase Polη have relatively fewer mutations than usual at A:T, but not at C:G, in their hypermutated immunoglobulin V regions. This fact suggests that Polη is the repair polymerase involved in this pathway of somatic hypermutation. These individuals also have a form of xeroderma pigmentosum, a condition resulting from the inability of their cells to repair DNA damage caused by UV radiation.
10-11 AID initiates class switching to allow the same assembled
VH exon to be associated with different CH genes in the course of an immune response.
All the progeny of a particular B cell activated in an immune response will express the same VH gene that was generated during its development in the bone marrow, although the gene may be modified by somatic hypermutation. In contrast, that B cell’s progeny may express several different C-region isotypes as the cells mature and proliferate during the immune response. The first antigen receptors expressed by B cells are IgM and IgD, and the first antibody produced in an immune response is always IgM. Later in the immune response, the same assembled V region may be expressed in IgG, IgA, or IgE antibodies. This change is known as class switching (or isotype switching), and, unlike the expression of IgD, it involves irreversible DNA recombination. It is stimulated in the course of an immune response by external signals such as cytokines released by TFH cells.
Switching from IgM to the other immunoglobulin classes occurs only after
MOVIE 5.2
B cells have been stimulated by antigen. It is achieved through class switch recombination, which is a type of nonhomologous DNA recombination that is guided by stretches of repetitive DNA known as switch regions. Switch regions lie in the intron between the JH gene segments and the Cμ gene, and at equivalent sites upstream of the genes for each of the other heavy-chain isotypes, with the exception of the δ gene, which does not require DNA rearrangement for its expression (Fig. 10.21, first panel). When a B cell switches from the coexpression of IgM and IgD to the expression of another subtype, DNA recombination occurs between Sμ and the S region immediately upstream of
Fig. 10.21 Class switching involves recombination between specific switch signals. The top panel shows the organization of a rearranged immunoglobulin heavy-chain locus before class switching. Second panel: this figure illustrates switching between the μ and ε isotypes in the mouse heavy-chain locus. Switch regions (S) are repetitive DNA sequences that guide class switching and are found upstream of each of the immunoglobulin C-region genes, with the exception of the δ gene. Switching is guided by the initiation of transcription by RNA polymerase (shaded circle) through these regions from promoters (shown as arrows) located upstream of each S. Due to the repetitive sequences, RNA polymerase can stall within the S regions, allowing these regions to serve as substrates for AID, and subsequently for UNG and APE1. Third panel: these enzymes introduce a high density of single-strand nicks into the non-template DNA strand and the template strand. Staggered nicks are converted to double-strand breaks by a mechanism that is not yet understood. Fourth panel: these breaks are then recognized by the cell’s double-strand break repair machinery, which involves DNA-PKcs, Ku proteins, and other repair proteins. Bottom two panels: the two switch regions, in this case Sμ and Sε, are brought together by the repair proteins, and class switching is completed by excision of the intervening region of DNA (including Cμ and Cδ) and ligation of the Sμ and Sε regions.