Inhibition of murine renal carcinoma pulmonary metastases by systemic administration of interferon γ: Mechanism of action and potential for combination with interleukin 4

We have previously demonstrated that IFN-γ causes cell growth inhibition and up-regulation of MHC antigens in human renal cell carcinoma cell lines. In this study, we have investigated the therapeutic potential of IFN-γ for the treatment of 5-day established pulmonary metastases induced by i.v. injection of Renca cells, a murine renal adenocarcinoma. We found that systemic injections of IFN-γ significantly reduced the number of lung metastases in a dose-dependent manner and increased mouse survival. Histological evaluation of IFN-γ-treated lungs showed residual small tumor nodules containing extensive necrosis and mononuclear infiltrates. Immunohistochemistry studies on lung sections showed macrophage infiltration into tumor nodules, and in vivo depletion of macrophages partially inhibited IFN-γ antitumor effect, suggesting a role for the macrophages in tumor destruction. Lymphocyte depletion of either natural killer (NK) cells or CD4+ or CD8+ T-cell subsets or both T-cell subsets did not affect the IFN-γ, effect, whereas depletion of both NK and T cells decreased the antitumor activity of IFN-γ. These data indicate that neither T cells nor NK cells are essential for this activity but that either lymphocyte population can contribute to the IFN-γ effect. An optimal dose of IFN-γ inhibited by 60% the growth of Renca cells treated for 3 days in vitro, but this effect was transient and less pronounced in a long-term colony assay, suggesting that IFN-γ direct growth inhibition may play a role but may not be sufficient to mediate its antitumor effect in vivo. In vitro, IFN-γ caused up-regulation of class I MHC antigens and induction of class II antigen expression in Renca cells, an effect that may enhance Renca immunogenicity but may be relevant only when a T-cell response is elicited. A sequential administration of IFN- γ followed by interleukin 4 was therapeutically better than IFN-γ alone for the treatment of advanced pulmonary metastases, probably due to different antitumor mechanisms induced by these two cytokines.