CD59a deficient mice display reduced B cell activity and antibody production in response to T-dependent antigens

CD59a is the primary regulator of membrane attack complex in mice. Recently, we have shown that CD59a-deficient (Cd59a−/−) mice exhibit enhanced CD4+ T cell responses. Here, we explored the effects of CD59a on B cell function and antibody production. Contrary to our expectations, Cd59a−/− mice showed a decreased humoral immune response to a T cell dependent antigen, sheep red blood cells. We found that the decreased humoral immune response was associated with a reduction in plasma cell number in vivo and reduced ability to respond to stimuli during in vitro culture experiments. Using MLR studies in which purified wild type or Cd59a−/− CD4+ T cells were mixed with purified B cells from each source, we found that the reduced B cell activation was largely due to the absence of CD59a on CD4+ T cells. Furthermore, a CD59a fusion protein bound specifically to mouse B cells, and enhanced B cell proliferation in a MLR, demonstrating that B cells express an as yet unidentified ligand for CD59a that aids in B cell activation.