Synergistic action of calcium ionophore A23187 and protein kinase C activator bryostatin 1 on human B cell activation and proliferation

In this study we have examined the immunostimulatory effects of the macrocyclic lactone bryostatin 1 on various aspects of B cell activation and proliferation using human tonsillar B cells. Bryostatin 1 is an activator of protein kinase C (PKC) and its properties were compared to those of the classical PKC activator phorbol 12‐myristate 13‐acetate (PMA), a phorbol ester. Time‐course kinetics and dose‐response curves of RNA and DNA synthesis induced by bryostatin 1 or PMA were comparable, albeit the phorbol ester was significantly more potent. The responses triggered by both bryostatin 1 and PMA could be blocked by the PKC inhibitor H7. Bryostatin 1 and PMA mediated similar effects with regard to the activation parameters, increase in cell size, expression of activation‐associated antigens and hyperexpression of major histocompatibility complex class II antigens. Addition of the calcium ionophore A23187 to bryostatin 1‐treated cultures resulted in synergistically enhanced activation and proliferation responses, and this potentiation by A23187 could be inhibited by cyclosporin A. Bryostatin 1 antagonized the effects of PMA‐triggered stimulation in a time‐ and dose‐dependent manner. The basis for this modulation of PMA‐induced effects and the reason for the difference in the abilities of the two agents to stimulate B cells is unclear; possibly, bryostatin 1 and PMA activate different isoforms of PKC and elicit different signals on intracellular biochemical pathways. Bryostatin 1 lacks the tumor‐promoting activity of PMA and is a potent anti‐neoplastic substance. These features together with its immunomodulatory properties qualify bryostatin 1 as a candidate for in vivo use as a biological response modifier.