Inhibition of B16 melanoma experimental metastasis by interferon-γ through direct inhibition of cell proliferation and activation of antitumour host mechanisms

Interferon-γ (IFN-γ) has pleiotropic activities other than its antivirus action, including cell growth inhibition, natural killer (NK) cell and cytotoxic T lymphocyte (CTL) activation, and angiogenesis inhibitory activity, and these activities are supposed to be involved in its antitumour activity. However, it has not been completely elucidated which activity is mainly involved in the tumour suppression in vivo. In this study, we analysed inhibitory mechanisms of endogenous IFN-γ against B16 melanoma experimental metastasis. After intravenous injection of tumour cells, tumour deposits in the lungs and liver were increased and life span was shorter in IFN-γ(−/−) mice, indicating important roles for IFN-γ in antitumour mechanisms. Interestingly, tumour deposits were not increased in IFN-γ receptor (R)(−/−) mice. Furthermore, only low levels of cell-mediated immunity against the tumour and activation of NK cells were observed, indicating that antimetastatic effects of IFN-γ is not mediated by host cells. The survival period of B16 melanoma-bearing IFN-γR(−/−) mice was, however, shorter than wild-type mice. These observations suggest that IFN-γ prevents B16 melanoma experimental metastasis by directly inhibiting the cell growth, although antitumour host functions may also be involved in a later phase