RAD51 EXPRESSION AND LOCALIZATION IN B-CELLS CARRYING OUT CLASS SWITCH RECOMBINATION

Rad51 is a highly conserved eukaryotic homolog of the prokaryotic reco mbination protein RecA, which has been shown to function in both recom binational repair of DNA damage and meiotic recombination in yeast. In primary murine B cells cultured with lipopolysaccharide (LPS) to stim ulate heavy chain class switch recombination, Rad51 protein levels are dramatically induced. Immunofluorescent microscopy shows that anti-Ra d51 antibodies stain foci that are localized within the nuclei of swit ching B cells. Immunohistochemical analysis of splenic sections shows that clusters of cells that stain brightly with anti-Rad51 antibodies are evident within several days after primary immunization and that Ra d51 staining in vivo is confined to B cells that are switching from ex pression of IgM to IgG antibodies. Following switch recombination, B c ells populate splenic germinal centers, where somatic hypermutation an d clonal proliferation occur. Germinal center B cells are not stained by anti-Rad51 antibodies. Rad51 expression is therefore not coincident with somatic hypermutation, nor does Rad51 expression correlate simpl y,vith cell proliferation. These data suggest that Rad51, or a highly related member of the conserved RecA family, may function in class swi tch recombination.