Regulation of human natural killing by levamisole

The effect of levamisole on human natural killing (NK) has been studied. In short-term chromium release assays, levamisole at a concentration of 10(-3) M was inhibitory to NK when present in the assays. Pretreatment of NK effector cells and K562 target cells with levamisole separately indicated that the effect was on effector cell activity and was not due to any change in target cell susceptibility. Inactivation of the effector cells required greater than 4 h pretreatment with levamisole if NK activity was subsequently tested in the absence of the drug. Pretreatment with levamisole for up to 19 h had no effect on the lymphocyte proliferative response to phytohemagglutinin (PHA). NK activity of drug-inactivated effector cells recovered after further incubation in levamisole-free medium. Levamisole at 10(-4) M or less had no effect on NK either by pretreatment or by its presence in the NK assays.