Synergistic anti-tumor effects with co-expression of GM-CSF and IFN-γ in murine tumors

We explored the potential therapeutic benefit of introducing GM-CSF, IFN-gamma or a combination of both factors into CT26 tumor cells. CT26 cells secreting either GM-CSF or IFN-gamma exhibited delayed tumorigenicity; however, cells expressing both GM-CSF and IFN-gamma did not form tumors. Even when wild type CT26 cells were introduced into a distant site of mice that had been inoculated with CT26/GM-CSF/IFN-gamma cells, no tumors were generated. Furthermore, when we injected GM-CSF + IFN-gamma cells into animals bearing established tumors, the tumors were either rejected or their development was delayed, suggesting that synergistic effects were induced against these tumors via a systemic immune response. Histopathological examination of the tumors injected with cells expressing GM-CSF and IFN-gamma combined showed necrosis and few signs of malignancy. The growth of tumors from mice treated with CT26/GM-CSF/IFN-gamma cells exhibited a delay in tumor formation and no effects were seen in athymic nude mice, which are deficient in T lymphocytes, or in splenectomized nude mice, which are deficient in natural killer (NK) cells, respectively. Our data indicate a dual role for T and NK cells in mediating the anti-tumor activity of this therapy. Our results suggest that transduction of tumor cells with both GM-CSF + IFN-gamma results in a powerful synergistic effect of the 2 cytokines that is of greater therapeutic benefit than transduction with either cytokine alone.