In vitro effects of leukocyte interferon on the cytotoxic potential of human monocytes

The in vitro cytotoxicity of human monocytes was enhanced by leukocyte interferon (IFN-alpha). In contrast to lymphokine activation which is obtained gradually under long-term contact with monocytes, maximal activation of effector cells was obtained within 4 hours of culture with IFN-alpha prior to medium exchange and target cell addition. The degree of activation was clearly dose-dependent, reaching a plateau level of cytolysis at 400 U/ml of IFN-alpha. In cytostasis no such plateau was demonstrable. IFN-alpha enhanced the level of cytolysis mediated by human monocytes without inducing any change in the kinetics of lysis, regardless of whether IFN-alpha was used for activation of monocytes prior to or simultaneously with the cytolysis assay. The direct effect of IFN-alpha on K-562 target cells was sparse. The incorporation of thymidine was modestly depressed in the presence of IFN-alpha. Exposure of K-562 cells to IFN-alpha did not have any effect on the spontaneous isotope leakage from prelabelled target cells. The exposure of K-562 cells to IFN-alpha prior to the start of co-culture with monocytes did not change the level of cytolysis reached, either with non-activated or with IFN-alpha activated monocytes, and irrespective of the in vitro age of the monocytes. The effect of IFN-alpha thus seems to be mediated via the effector cell population and not via target cell modulation.