B cell receptor stimulation inhibits class switch recombination through elevated DNA repair.

Antibody diversity in B cells arises from the activity of activation-induced deaminase (AID), which introduces uracils into DNA of the immunoglobulin loci. The presence of uracil initiates a cascade of mutagenic events, resulting in somatic hypermutation and class switch recombination (CSR). To produce CSR, uracils are removed from DNA by uracil DNA glycosylase (UNG), resulting in formation of an abasic site, which is subsequently transformed into a double-stand break and promotes recombination to another constant gene. While the classical model indicates that B cells are activated when the IgM receptor binds antigen, stimulation of cells ex vivo with anti-IgM is insufficient for CSR. In fact, anti-IgM stimulation has a strong inhibitory effect on CSR when added together with lipopolysaccharide (LPS). To determine the mechanism of anti-IgM inhibition of CSR, we examined the known components required for CSR: AID and UNG. After stimulation with anti-IgM and LPS, AID was expressed and recruited to the immunoglobulin loci, and UNG levels increased and prevented uracil accumulation. We also linked the increased expression of UNG to the cell-cycle, as the presence of anti-IgM allowed cells to enter S-phase sooner. Therefore, for efficient CSR, AID-induced uracils must accumulate to sufficient levels before initiating S-phase, to provide more substrate for UNG and endonuclease to generate mutagenic strand breaks. If cell division occurs too early, as in the case of stimulation with anti-IgM and LPS, UNG is quickly upregulated and removes the uracils before they can accumulate, resulting in faithful DNA repair and less CSR.