Bacterial lipopolysaccharides (LPS) stimulate mouse peritoneal macrophages to kill tumor cells in vitro. Lysis is confined to tumor cells where it is nonspecific; both allogeneic and syngeneic cells being susceptible. Stimulation by LPS appears to be due to direct interaction between LPS and macrophages and does not involve participation by lymphocytes. After exposure to LPS, a latent period must elapse before macrophages can lyse tumor cells. The cytolytic mechanism requires contact between target cells and viable effector cells which maintain their lytic capacity for a sustained period and can kill on repeated occasions. The generation of a macrophage cytolytic effect by LPS is critically dependent upon the absolute number of macrophages which must be sufficient to produce confluent monolayers. These findings indicate that LPS stimulation of macrophages in vitro represents a valuable model system for the study of the mechanisms of macrophage stimulation and of the mediation of tumor cell death.

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