Endogenous accumulation of IFN-gamma in IFN-gamma-R(-/-) mice increases resistance to B16F10-Nex2 murine melanoma: a model for direct IFN-gamma anti-tumor cytotoxicity in vitro and in vivo.

Syngeneic IFN-gamma(-/-) and IRF-1(-/-) mice are very sensitive to B16F10-Nex2 murine melanoma cells implanted subcutaneously. In contrast, IFN-gamma-R(-/-) (GRKO) mice are remarkably resistant to tumor development. Only 0-30% of these animals, challenged with a high dose of melanoma cells (5 x 10(5)), developed tumors at a late stage. The hypothesis of interferon gamma (IFN-gamma) accumulation and consequent cytotoxicity to implanted tumor cells was confirmed in vitro and ex vivo. IFN-gamma reduced tumor-cell growth in vitro in 60-81%, added alone or with LPS. Splenocytes and peritoneal macrophages from naïve GRKO mice activated with anti-CD3 and interleukin-12 (IL-12), respectively, accumulated IFN-gamma at levels 10-fold those of the wild-type. Supernatants of IL-12-activated macrophages from GRKO mice were toxic to B16F10-Nex2 cells, an effect reversible by anti-IFN-gamma antibody treatment. IL-12-activated macrophages from iNOS(-/-) mice were still highly cytotoxic to B16F10-Nex2 cells, but IL-12-activated macrophages from IFN-gamma-deficient mice were not inhibitory. In vivo, a single injection of anti-IFN-gamma antibody 18 h after tumor-cell challenge in GRKO mice rendered all animals susceptible to B16F10-Nex2 melanoma development. No tumors developed in the untreated GRKO mice during up to 45 days of observation. This model can be useful in understanding immune responses that involve IFN-gamma as a direct cytotoxic factor.