Sphingosine-1-phosphate receptor type 4 is critically involved in the regulation of peritoneal B-1 cell trafficking and distribution in vivo.

B-1 cells are crucially involved in immune defense and regulation of inflammation and autoimmunity. B-1 cells are predominantly located in the peritoneal and pleural cavities, although body cavity B-1 cells recirculate systemically under steady-state conditions. The chemokines CXCL12 and CXCL13 have been identified as the main regulators of peritoneal B-cell trafficking. In mice deficient for sphingosine-1-phosphate receptor 4 (S1PR₄), B-1a and B-1b cell numbers are reduced in the peritoneal cavity by an unknown mechanism. In this study, we show that S1PR₄-mediated S1P signaling modifies the chemotactic response of peritoneal B cells to CXCL13 and CXCL12 in vitro. In vivo, S1PR₄-mediated S1P signaling affects both immigration into and emigration from the peritoneal cavity. Long-term reconstitution experiments of scid mice with wt or s1pr₄ ^-/- peritoneal B cells revealed a distinct distributional pattern in secondary lymphoid organs. As a functional consequence, both plasmatic and mucosal IgM levels, the main product of B-1a cells, are reduced in mice reconstituted with s1pr₄ ^-/- peritoneal cells. In summary, our data identify S1PR₄ as the second S1P receptor (besides S1PR₁), which is critically involved in the regulation of peritoneal B-1 cell function.