A novel role for C5a in regulating B1 cell homeostasis and function

Abstract B1 cells are innate-like B cells producing natural IgM (nIgM) antibodies without previous antigen exposure. They ensure first line humoral defense and long-lasting immunity against T-cell independent antigens found in encapsulated bacteria. B-1 cells are predominantly located inside body cavities and in lower numbers in spleen and bone marrow. Previous findings suggest that their egress is contolled through several integrins and the chemokines CXCL12 and CXCL13. B1 cell need to migrate from the peritoneal cavity to the spleen to become nIgM producing plasma cells. The role of complement in B1 cell homeostasis and migration has not been explored. Here, we determined the impact of C5a and the C5a receptors (C5aR) 1 and 2 on B1 cell biology. We found significantly reduced B1 cell numbers in the peritoneal cavity of C5aR1−/− or C5aR2−/− as compared with wildtype mice that were associated with increased B1 cell numbers in the spleen. Consequently, C5aR1−/− mice had elevated serum levels of nIgM against phosphorylcholine and different polysaccharides of S. pneumoniae under steady state conditions. Using a GFP-C5aR1 reporter mouse, we further demonstrate that C5aR1 expression on B1 cells is upregulated whereas the CXCL13 receptor CXCR5, CXCR4, IgM and aM (CD11b) integrin expression is downregulated before B1 cells leave the peritoneal cavity upon TLR2 stimulation in vivo. In line with this observation, we found that in C5aR1−/− mice, TLR2-driven B1 cell egress was diminished, as B1 cell numbers in C5aR1−/− mice were unchanged whereas they markedly dropped in wildtype mice. Thus, our data suggest a novel role for C5a in the regulation of B1 homeostasis in the peritoneal cavity, nIgM production and TLR-driven egress from the peritoneum into the spleen.