Synergy of tumor necrosis factor with protein kinase C activators on T cell activation

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Abstract
The ability of tumor necrosis factor (TNF)‐α to activate T lymphocytes in combination with other stimuli has been studied. TNF was strongly co‐mitogenic with low doses of anti‐CD3 antibodies or phorbol esters (those which are strong activators of protein kinase C, PKC) but poorly with phytohemagglutinin or concanavalin A. No synergism was seen with the calcium ionophore A23187. TNF was co‐mitogenic with several phorbol esters known to activate PKC but was uneffective with inactive phorbol esters such as methyl‐phorbol 12‐myristate 13‐acetate. Furthermore, H‐7 a known inhibitor of PKC, inhibited the proliferative response of T cells induced by esters plus TNF. This effect took place at low doses of TNF and was also observed with purified T lymphocytes indicating that the effect of TNF was not dependent on accessory cells. This proliferative effect of TNF was inhibited by an anti‐interleukin 2 receptor (IL2R) antibody, MAR 108, which blocks IL 2 binding to its receptor. Although PKC activation induced CD25 (IL 2R) expression but very little IL 2 synthesis, TNF did not synergize by augmenting the synthesis of this lymphokine in peripheral blood lymphocytes stimulated with phorbol esters. By contrast, TNF strongly increased the membrane level of CD25 and to a lesser extent that of the activation antigen, 4F2, over the levels already induced by phorbol esters on T cells. More interestingly, TNF significantly increased the number of high‐affinity IL 2R on purified T cells in the presence of phorbol 12,13‐dibutyrate. Our results indicate that TNF is co‐mitogenic with those stimuli which strongly activate PKC and suggest that TNF may play a role on T cell activation increasing the number of effective IL 2/IL 2R interactions when these are limiting.