The effect of bryostatin 1 on human lymphocyte-mediated cytotoxicity

We compared the effects of bryostatin 1 (BRYO), a nontumor-promoting protein kinase C activator, and phorbol myristate acetate (PMA) on various types of lymphocyte cytotoxicity. An 18-h preincubation of lymphocytes with 10⁻⁸M BRYO inhibited natural killer (NK) activity but this inhibition was not statistically significant (p = 0.28). In contrast, NK activity was significantly enhanced by preincubation of lymphocytes with 10⁻⁸M PMA (p = 0.0012). Thus, in 13 experiments, the mean LU/10⁶was 21 ± 12 for control cultured cells, 16 ± 14 for BRYO cultured cells, and 40 ± 22 for PMA cultured cells as determined in⁵¹Cr-release assays with K562 target cells. Both BRYO and PMA inhibited lymphocyte antibody-dependent cell-mediated cytotoxicity (ADCC) such that, at a 20:1 effector-to-target ratio, control lymphocytes had a mean 49 ± 12% specific 51Cr release of antibody-coated target cells while BRYO and PMA cultured lymphocytes had 12 ± 6 and 8 ± 12%, respectively, in four experiments. The reduction in ADCC was paralleled by a decrease in Fc receptor (CD16) expression as determined by immunofluorescence analysis. We assessed the ability of BRYO and PMA to induce lymphokine-activated killer (LAK) activity by culturing lymphocytes with optimally mitogenic doses of these compounds and testing for cytotoxicity of Daudi target cells. While both BRYO and PMA induced [³H]thymidine uptake, neither induced LAK activity. Furthermore, both compounds inhibited induction of LAK activity by recombinant interleukin-2 (rIL-2) in cocultures. We also analyzed the effect of BRYO and PMA on preactivated LAK effector cells. Lymphocytes were cultured for 3 days with 100 U/ml of rIL-2, aliquoted, and recultured for 18 h with rIL-2 alone and with the addition of 10⁻⁸M BRYO and 10⁻⁸M PMA, and then tested for LAK activity. Both compounds reduced the cytotoxic activity of LAK effector cells.