Studies on the calcium dependence of human NK cell killing

T lymphocytes and NK cells depend on extracellular Ca2+ to mediate cellular cytotoxicity. In the present work, we have used pharmacological tools to analyze the nature of this calcium dependence. Ca2+ channel blockers like nifedipine greater than or equal to diltiazem greater than verapamil greater than cobalt chloride inhibited NK killing but at concentrations higher than those sufficient to block voltage-operated Ca2+ channels. Quercetin and TMB-8 also suppressed killing. Depolarization or NK cells with high K+ concentration resulted in partial inhibition of lysis in contrast to hyperpolarization with K+ ionophore valinomycin which had no effect. Depolarization of hyperpolarization in the presence of a protein kinase C activator (phorbol ester, TPA) did not initiate killing of NK resistant target cells. Of the two K+ channel inhibitors tested, 4-AP and TEA, only 4-AP was inhibitory for NK killing. No release of membrane-bound Ca2+ as judged by chlorotetracycline fluorescence could be detected in the NK cell population during binding to target cells although an influx of 45Ca2+ into the NK cell population was found. Treatment of NK cells with calcium ionophore A23187 did not trigger killing, but lysis could be induced by simultaneous stimulation with A23187 and TPA. The results indicate that NK killing depends on Ca2+ channels that are different from voltage operated channels and that intracellular Ca2+ may act in concert with protein kinase C activation.