Phorbol esters inhibit the functional activation of cytotoxic precursors in mixed lymphocyte cultures.

We have investigated the effect of phorbol esters on T cell activation and generation of suppressor and cytotoxic activity in mixed lymphocyte cultures (MLC). The presence of 30 nM P(Bu)2 during the sensitization phase inhibited the generation of allospecific cytotoxicity and also decreased the killing potential against NK-sensitive targets. The inhibition was not mediated by direct blocking of the lytic capacity nor by suppression of clonal expansion of cytotoxic cells through modulation of lymphokine production. The presence of P(Bu)2 enhanced cell proliferation, but inhibited the functional activation of lymphocytes and consequent generation of Dr antigen-positive T cells. Because the presence of the compound did not affect the MLC-induced generation of suppressor activity, it is likely that P(Bu)2 selectively blocks the maturation of cytotoxic precursors. Surface-marker analysis with OKT monoclonal antibodies revealed that the effects on lymphocyte activation were associated with a decrease in OKT3 and OKT4 reactivity and an increase in the percentage of OKT8-positive cells. The decrease in OKT4 reactivity was not due to selective loss of this lymphocyte subpopulation, because P(Bu)2 was equally mitogenic for the purified OKT4- and OKT8-positive subsets. The results suggest that the effect of P(Bu)2 on cell differentiation and its ability to modulate the expression of functional markers in lymphocyte subsets may interfere with T-T cell cooperation that controls the functional maturation of cytotoxic precursors.