Regulation of differentiation of peritoneal B-1a (CD5+) B cells. Activated peritoneal macrophages release prostaglandin E2, which inhibits IgM secretion by peritoneal B-1a cells.

The B-1a (CD5+) subset of B cells comprises the majority of B cells in the peritoneal cavity and is implicated in the pathogenesis of certain autoimmune diseases and lymphoproliferative disorders. When we stimulated purified B-1a cells with LPS, they produced more than four times as much IgM as similarly stimulated whole peritoneal cells (containing the same number of B-1a cells). Reconstitution experiments using FACS-purified peritoneal cell populations revealed that resident peritoneal macrophages (Mac1+, B220-) profoundly inhibited the LPS response of peritoneal B-1a cells. Culture of B-1a cells with peritoneal macrophages at a ratio of 3:1 (reflecting the in vivo ratio) resulted in a fivefold or greater reduction in the IgM response to LPS. LPS activation of macrophages resulted in production of a soluble factor that inhibited LPS-induced B cell differentiation by 86% when used at a concentration of 5%. When [3H]arachidonic acid-pulsed macrophages were stimulated with LPS, the major arachidonic acid metabolite secreted was PGE2 (a potent inhibitor of B cell differentiation). The inhibitory capacity of the macrophage-derived supernatant was reversed by the addition of anti-PGE2. These findings indicate that macrophage-derived PGE2 functions as an important regulator of polyclonal response of B-1a cells to LPS.