DIRECT ACTIVATION OF MURINE PERITONEAL-MACROPHAGES FOR NITRIC-OXIDE PRODUCTION AND TUMOR-CELL KILLING BY INTERFERON-GAMMA

Interferon-gamma (IFN-gamma) is known to prime macrophages for tumor c ell lysis and nitric oxide (NO) production as measured by enhanced sen sitivity to lipopolysaccharide (LPS). In the present study, the abilit y of IFN-gamma to directly activate peritoneal macrophages from C57BL/ 6 and Balb/c mice for tumor cytotoxicity and NO production was evaluat ed. Macrophage-mediated tumor cell killing was measured by an 18 h Cr- 51 release assay using P815 mastocytoma cells as targets. Concurrent N O production was measured as nitrite in the supernatants of macrophage cultures. Incubation of macrophages with IFN-gamma resulted in activa tion of macrophages for tumor cell lysis. IFN-gamma alone also activat ed macrophages for NO production under identical conditions. Addition of LPS along with IFN-gamma resulted in synergism in the activation of macrophages for both cytolysis and NO production. LPS contamination o f the IFN-gamma preparation was absent as evidenced by the following c riteria: (1) the IFN-gamma preparation as well as the reagents used we re shown to be free of LPS contamination based on LAL endotoxin tests (sensitivity 25 pg/ml), (2) the ability of IFN-gamma to activate macro phages was not abrogated by prior treatment of the cytokine with polym yxin B, whereas the effect of LPS was inhibited (70-100%) under simila r conditions, (3) pretreatment of the IFN-gamma preparation with a spe cific endotoxin neutralizing protein did not abrogate the ability of I FN-gamma to induce macrophage activation, and (4) heat treatment of so lutions containing IFN-gamma alone or IFN-gamma + LPS abolished only t he effect of IFN-gamma, not that of LPS. A comparison of the IFN-gamma responsiveness in the peritoneal macrophages from C57BL/6 and Balb/c macrophages revealed that C57BL/6 macrophages are more responsive to I FN-gamma. These results demonstrate that IFN-gamma alone is capable of stimulating macrophages for tumor cell killing and NO production and that the magnitude of IFN-gamma responsiveness varies with the strain of mice studied.