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‐a). 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‐a 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‐a. In cytostasis no such plateau was demonstrable. IFN‐a enhanced the level of cytolysis mediated by human monocytes without inducing any change in the kinetics of lysis, regardless of whether IFN‐a was used for activation of monocytes prior to or simultaneously with the cytolysis assay. The direct effect of IFN‐a on K‐562 target cells was sparse. The incorporation of thymidine was modestly depressed in the presence of IFN‐a. Exposure of K‐562 cells to IFN‐a did not have any effect on the spontaneous isotope leakage from prelabelled target cells. The exposure of K‐562 cells to IFN‐a prior to the start of co‐culture with monocytes did not change the level of cytolysis reached, either with non‐activated or with IFN‐a activated monocytes, and irrespective of the in vitro age of the monocytes. The effect of IFN‐a thus seems to be mediated via the effector cell population and not via target cell modulation.