Peritoneal B cells respond to phorbol esters in the absence of co-mitogen.

B cells obtained by irrigation of the peritoneal cavity differ from splenic B cells in signaling requirements for the initiation of DNA synthesis. Splenic B cells are stimulated to enter S phase by phorbol esters in conjunction with a second signal provided by calcium ionophore; however, splenic B cells are not stimulated by phorbol ester alone. In contrast, peritoneal B cells from NZB and BALB/c mice were stimulated to incorporate tritiated thymidine by each of the phorbol esters, PMA and phorbol dibutyrate, acting alone. Stimulation of peritoneal B cells was apparent when cells were cultured at lower than usual cell densities, and responses were unaffected by coculture with splenic B cells. Responding cells adhered to plastic petri dishes coated with anti-mouse IgM antibody, but were not completely removed by treatment with anti-Ly-1.2 antibody plus C. These results indicate that phorbol esters constitute a complete signal that stimulates some peritoneal B cells to enter S phase.