Inhibition of human lymphocyte natural cytotoxicity and antibody-dependent cell-mediated cytotoxicity by K-76 COONa, a reagent that blocks complement activity.

K-76 COONa is a 440 m.w. fungal product that can inhibit complement activity of C5 and Factor I. K-76 COONa abrogated both human natural killer (NK) cell activity and antibody-dependent cell-mediated cytotoxicity (ADCC) (ID50 approximately 1.5 mM). To be effective, K-76 COONa had to be present during the assay, because pretreatment of lymphocytes with highly inhibitory concentrations of K-76 COONa did not inhibit cytolysis. The monocarboxylic K-76 derivative was more inhibitory to NK than the dicarboxylic derivative. This relative efficacy is similar to that observed for inhibition of complement lysis. K-76 COONa inhibited NK when added before NK conjugate formation, but had little effect when added after conjugate formation. The compound also inhibited the formation of conjugates by NK and K cells. Therefore, this reagent selectively affected events that occurred between the initial effector-target cell interactions and the formation of stable conjugates. It had little influence on the post-binding "lethal hit" stage of cytolysis. These data imply a) that if any molecules similar to C5 are activated during the "lethal hit" stage of cytolysis, then they are inaccessible to K-76 COONa , and b) that C3bi-like molecules may be involved in lymphocyte binding.