Macrophage activation for tumor cytotoxicity: induction of macrophage tumoricidal activity by lymphokines from EL-4, a continuous T cell line.

Supernatant culture fluids from a phorbol myristate acetate (PMA) stimulated variant of the murine EL-4 thymoma cell line activated inflammatory macrophages for nonspecific tumoricidal activity in vitro; active supernatants fluids were not directly toxic to tumor target cells in the absence of macrophages. Macrophages treated with culture fluids of unstimulated and concanavalin A-stimulated EL-4 or PMA-stimulated cells derived from any of ten other continuous T cell lines were not cytotoxic. The titer of activity in lymphokines from PMA-stimulated EL-4 was similar to that from antigen-stimulated spleen cell cultures. Spleen cell and EL-4 derived activities were both retained within a dialysis membrane and destroyed by heating at 100 degrees C for 15 min. However, unlike the activity in spleen cell-derived LK, activity in EL-4 culture fluids was stable to dialysis against pH 2 buffer. Further physicochemical characterization of the EL-4 activity showed two different factors that activated macrophages for cytotoxicity: one was indistinguishable from gamma interferon (IFN) (antiviral activity, 50,000 m.w., pI 4.2 to 6.0, pH 2 labile and neutralized by anti-gamma IFN serum); the other was clearly distinct (no antiviral activity, 23,000 m.w., pH 2 stable and unaffected by anti-gamma IFN serum). These results suggest that the EL-4 cell line may produce one or more of the macrophage activation factors present in spleen cell-derived lymphokines.