Ku80 is required for immunoglobulin isotype switching

Isotype switching is the DNA recombination mechanism by which antibody genes diversify immunoglobulin effector functions. In contrast to V(D)J recombination, which is mediated by RAG1, RAG2 and DNA double-stranded break (DSB) repair proteins, little is known about the mechanism of switching. We have investigated the role of DNA DSB repair in switch recombination in mice that are unable to repair DSBs due to a deficiency in Ku80 (Ku80(-/-)). B-cell development is arrested at the pro-B cell stage in Ku80(-/-) mice because of abnormalities in V(D)J recombination, and there are no mature B cells. To reconstitute the B-cell compartment in Ku80(-/-) mice, pre-rearranged VB1-8 DJH2 (mu i) and V3-83JK2 (kappa i) genes were introduced into the Ku80(-/-) background (Ku80(-/-)mu i/+kappa i/+). Ku80(-/-)mu i/+ kappai/+ mice develop mature mIgM+ B cells that respond normally to lipopolysaccharide (LPS) or LPS plus interleukin-4 (IL-4) by producing specific germline Ig constant region transcripts and by forming switch region-specific DSBs. However, Ku80(-/-)mu i/+kappa i/+ B cells are unable to produce immunoglobulins of secondary isotypes, and fail to complete switch recombination. Thus, Ku80 is essential for switch recombination in vivo, suggesting a significant overlap between the molecular machinery that mediates DNA DSB repair, V(D)J recombination and isotype switching.