Synergistic Activation of Macrophages by Lymphokine and Lipopolysaccharide: Evidence for Lymphokine as the Primer and Interferon as the Trigger

Abstract

Murine macrophages can be activated to develop cytotoxicity against P815 mastocytoma tumor cells following incubation in vitro with either bacterial lipopolysaccharide (LPS; 10 μg/ml) or cell-free culture supernatant from mitogen-stimulated BALB/c lymphocytes (lymphokine; 10% v/v). Under optimal conditions in an in vitro growth inhibition assay, tumor cytostasis by these treated macrophages did not exceed 35%. The combination of extremely low concentrations of lymphokine (1.25%) with subthreshold amounts of LPS (0.1 μg/ml) synergistically activated macrophages with >50% inhibition of target cell growth. Previous studies have indicated that direct macrophage activation by LPS and various other polyanions results via interferon (IFN) induction. Pretreatment of macrophages with subthreshold levels of lymphokine potentiated the tumor cytostasis of macrophages subsequently exposed to interferon. The synergism for macrophage activation between preparations of lymphokine and either LPS or IFN required a defined sequence of events. To demonstrate synergism, macrophages had to be either incubated with lymphokine and the other activating agent simultaneously or treated first with lymphokine. Thus, macrophages exposed to low concentrations of lymphokine supernatants enter into a receptive, primed state in which they are not yet cytotoxic but can be triggered by another signal (LPS or IFN) to develop full cytotoxicity.