Activation and proliferation signals in mouse B cells. VIII. Induction of DNA synthesis in B cells by a combination of calcium ionophores and phorbol myristate acetate.

Mouse B cells cultured with either phorbol myristate acetate (PMA), or with Ca2+ ionophores enter a transitional activated state, between quiescence (G0) and G1, but do not synthesize DNA. It is shown here that the combination of PMA plus the ionophore ionomycin induces resting B cells to synthesize DNA, but not to secrete antibody. B cells from CBA/N mice carrying the xid defect, and those from the lipopolysaccharide-unresponsive C3H/HeJ strain also respond to this combination. Suboptimal doses of the two stimuli synergize with B cell-stimulating factor 1 in promoting proliferation of resting B cells, but the co-mitogen does not substitute for type II B cell growth factor in the BCL1 lymphoma. Furthermore, (as predicted) the combination of these two agents does not induce the breakdown of inositol phospholipids in B cells. These data are consistent with the hypothesis that elevation of intracellular Ca2+ (by the ionophore), plus activation of protein kinase C (by PMA) leads to DNA synthesis in B cells. The combination of Ca2+ ionophore and PMA thus appears to essentially mimic the biochemical effects of ligation of surface immunoglobulin receptors on B cells, by providing the two second messengers normally emanating from the receptor-mediated breakdown of polyphosphoinositides.