In vitro influence of endotoxin on human mononuclear phagocyte structure and function. 2. Enhancement of the expression of cytoststic and cytolytic activity of normal and lymphokine-activated monocytes.

The presence of non-toxic concentrations of E. coli endotoxin (LPS) during the in vitro interaction of normal human monocytes and a human tumour cell line (NHIK 3025) enhanced monocyte-mediated target cell cytostasis and cytolysis. Monocyte responsiveness to LPS was greatest at an intermediate stage of in vitro differentiation. The expression of cytostatic and cytolytic activity by human monocytes activated with mediators from Corynebacterium parvum-stimulated human lymphocytes was also enhanced by LPS. Lymphokine activation did not induce additional LPS responsiveness in the monocytes. Monocytes activated with lymphokines and subsequently deactivated by in vitro culture did not show any increase in LPS responsiveness. A soluble cytostatic factor, which is probably not cold thymidine, was released from monocytes exposed first to lymphokines and then to LPS. While LPS is ineffective as an induction signal of monocyte cytotoxicity to tumour cells in this system, it enhances the expression of cytotoxicity induced by prolonged in vitro culture or lymphokine activation.