Phorbol ester-induced enhancement in lytic activity of CD8+ splenic T cells from low-dose melphalan-treated MOPC-315-tumor bearers.

We have previously shown that while spleen cells from untreated mice bearing a large MOPC-315 tumor are not cytotoxic in vitro for MOPC-315 tumor cells, spleen cells obtained from such mice on day 7 after low-dose melphalan (L-phenylalanine mustard); L-PAM therapy exert a substantial anti-MOPC-315 cytotoxicity [Mokyr et al. (1989) Cancer Res 49: 4597]. Here we show that this anti-MOPC-315 lytic activity is evident by day 5, and peaks on day 7 after the low-dose chemotherapy, at a time when the mice are actively engaged in tumor eradication. Short-term exposure of spleen cells from mice bearing a MOPC-315 tumor and treated with low-dose L-PAM (L-PAM TuB mice) to phorbol 12-myristate 13-acetate (PMA) was found to enhance greatly the ability of these spleen cells to lyse MOPC-315 tumor cells. The highest level of anti-MOPC-315 cytotoxicity was obtained when spleen cells from tumor-bearing mice that had received chemotherapy 7 days earlier were exposed to PMA at a concentration of 1-10 ng/ml. The exertion of the enhanced anti-MOPC-315 lytic activity by L-PAM TuB spleen cells exposed to PMA was found to require CD8+, but not CD4+, T cells. The apparent specificity of the lytic activity exerted by the PMA-stimulated L-PAM TuB spleen cells was illustrated not only by the inability of the spleen cells to lyse an allogeneic, antigenically unrelated thymoma (EL4), but also by their relatively weak lytic activity for two antigenically related syngeneic plasmacytomas. In addition, when EL4 target cells were admixed with MOPC-315 tumor cells, the lytic activity triggered in the L-PAM TuB spleen cells by the MOPC-315 tumor cells plus PMA was not effective in lysing the antigenically unrelated target cells. Moreover, even in the presence of the calcium-specific ionophore, ionomycin, L-PAM TuB spleen cells exposed to PMA were unable to lyse the EL4 target cells. Thus, fresh CD8+ splenic T cells from L-PAM TuB mice that are in the process of eradicating a large MOPC-315 tumor as a consequence of low-dose L-PAM therapy can be triggered with PMA to exert enhanced lytic activity against MOPC-315 tumor cells.(ABSTRACT TRUNCATED AT 400 WORDS)