KU80 IS REQUIRED FOR IMMUNOGLOBULIN ISOTYPE SWITCHING

Isotype switching is the DNA recombination mechanism by which antibody genes diversity immunoglobulin effector functions. HPT contrast to V( D)J recombination, which is mediated by RAG1, RAG2 and DNA double-stra nded break (DSB) repair proteins, little is known about the mechanism of switching. We have investigated tbe role of DNA DSB repair in switc h recombination in mice that are unable to repair DSBs due to a defici ency in Ku80 (Ku80(-/-)). B-cell development is arrested at the pro-B cell stage in Ku80(-/-) mice because of abnormalities in V(D)J recombi nation, and there are no mature B cells. To reconstitute the B-cell co mpartment in Ku80(-/-) mice, pre-rearranged V(B1-8)DJ(H)2 (mu(1)) and V(3-83)J(K)2 (kappa(1)) genes were introduced into the Ku80(-/-) backg round (Ku80(-/-)mu(1/+)kappa(1/+)) Ku80(-/-)mu(1/+)kappa(1/+) mice dev elop mature mIgM(+) B cells that respond normally to lipopolysaccharid e (LPS) or LPS pins interleukin-4 (IL-4) by producing specific germlin e Ig constant region transcripts and by forming snitch region-specific DSBs. However, Ku80(-/-)mu(1/+)kappa(1/+) B cells are unable to produ ce immunoglobulins of secondary isotypes, and fail to complete switch recombination. Thus, Ku80 is essential for switch recombination in viv o, suggesting a significant overlap between the molecular machinery th at mediates DNA DSB repair, V(D)J recombination and isotype switching.