Inhibitory effect of cholera toxin on human natural cell‐mediated cytotoxicity and its augmentation by interferon

Cholera toxin inhibits human natural cell‐mediated cytotoxicity in a dose‐ and time‐dependent manner. Pretreatment of lymphocytes with 10 ng/ml of cholera toxin for 2 h almost completely inhibited cytolysis. Interferon augmented human natural cell‐mediated cytolysis, but when lymphocytes were pretreated with cholera toxin before interferon treatment, no enhancement of cytolysis occurred. Cholera toxin could inhibit the enhancement of cytolysis by interferon even when lymphocytes were treated with cholera toxin after 2 h interferon treatment. Cholera toxin subunit B which binds cell surface ganglioside galactosyl‐N‐acetylgalactosaminyl ‐ [N‐ acetylneuraminyl]‐galactosylglucosyl ‐ ceramide (GMI) without activating adenyl cyclase had no effect either on natural cytolysis or on the enhancement of natural cytolysis by interferon, suggesting that mere binding of cholera toxin to the cellular receptor was not enough to inhibit natural cell‐mediated cytolysis. Cyclic adenosine 3′,5′‐monophosphate (cAMP) levels increased in cholera toxin‐treated lymphocytes and the time course of cAMP accumulation was similar to that of cytotoxicity inhibition. Exogenous dibutyryl‐cAMP (db‐cAMP) and theophylline inhibited cytolysis, while exogenous dibutyryl cyclic guanosine 3′,5′‐phosphate (db‐cGMP) enhanced cytolysis slightly, suggesting that the process of inhibition of human natural cell‐mediated cytolysis was at least partly modulated by intracellular cyclic nucleotides.