Reduced isotype switching in splenic B cells from mice deficient in mismatch repair enzymes

Mice deficient in various mismatch repair (MMR) enzymes were examined to de termine whether this repair pathway is involved in antibody class switch re combination. Splenic B cells from mice deficient in Msh2, Mlh1, Pms2, or Ml h1 and Pms2 were stimulated in culture with lipopolysaccharide (LPS) to ind uce immunoglobulin (Ig)G2b and IgG3, LPS and interleukin (IL)-4 to induce I gG1, or LPS, anti-delta-dextran, IL-4, IL-5, and transforming growth factor (TGF)-beta 1 to induce IgA. After 4 d in culture, cells were surface stain ed for IgM and non-IgM isotypes and analyzed by FACS(R). B cells from MMR-d eficient mice show a 35-75% reduction in isotype switching, depending on th e isotype and on the particular MMR enzyme missing. IgG2b is the most affec ted, reduced by 75% in Mlh1-deficient animals. The switching defect is not due to a lack of maturation of the B cells, as purified IgM(+)IgD(+) B cell s show the same reduction. MMR deficiency had no effect on cell proliferati on, viability, or apoptosis, as detected by [H-3]thymidine incorporation an d by propidium iodide staining. The reduction in isotype switching was demo nstrated to be at the level of DNA recombination by digestion-circularizati on polymerase chain reaction (DC-PCR). A model of the potential role for MM R enzymes in class switch recombination is presented.