Dendritic cells (DC) comprise a small subpopulation of lymphoid cells, with distinct morphologic features, surface phenotypes and a potent accessory function in T cell-dependent immune responses. In the present study, we investigated the in vitro effects of a tumor promoter, phorbol myristate acetate (PMA), and calcium ionophore A23187 on the accessory cell function of mouse spleen DCs in the primary mixed lymphocyte reaction (1 degree MLR) and oxidative mitogenesis (OM). A multi-step purification procedure was used to procure a highly enriched DC population from mouse spleen. The accessory cell activity of the DCs so obtained was much stronger than that of M phi s in both MLR and OM. The effects of PMA, a protein kinase C (PKC) activator, on DC were dose-dependent. If pretreated with 50 ng/ml of PMA for 3 h, DC activity was enhanced by about two-fold; whereas 200 ng/ml decreased DC activity with an inhibition rate of about 50%. However, in the latter situation, a moderate increase in DC activity was seen in the early phase of the response. When pretreated with 0.5-1.0 mumol/L A23187 for 6-8 h, the accessory cell activity of DCs was twice as potent as that of the control, and the enhancing effect was sustained in both MLR and OM. Our results indicate that the function of DCs, a cell type with constitutively high accessory cell activity, can be further promoted by A23187 or a low dose of PMA. This is also circumstantial evidence of an up-regulation of DC activity via PKC activation and/or an increase in cytoplasmic calcium.(ABSTRACT TRUNCATED AT 250 WORDS)