Interferon-γ, But Not Interferon-αβ, Synergizes with Tumor Necrosis Factor-α and Lipid A in the Induction of Nitric Oxide Production by Murine L929 Cells

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Abstract
Recently, we have demonstrated that tumor necrosis factor (TNF)-sensitive tumor cells produce nitric oxide (NO) in response to TNF whereas TNF-resistant cells do not. Because the addition of interferon-γ (IFN-γ) augmented NO production, we were interested in investigating this phenomenon further and comparing the effects of IFN-γ with those of IFN-αβ. We found that cell lines that are sensitive to TNF-mediated cytotoxicity (TMC) produced NO in response to TNF and IFN-γ, but not in response to IFN-αβ. The effect of IFN-γ on NO production was dose dependent, but IFN-γ by itself did not induce NO production. A TNF-resistant cell line (MCA) did not produce NO under any of the conditions tested. Different results were obtained when the effect of IFNs on TMC was assayed. TNF-sensitive L929 cells were rendered less sensitive to TNF after treatment with both types of IFN. In contrast, another TNF-sensitive cell, WEHI 164, was rendered more sensitive to TMC after treatment with both types of IFN. The effect of IFNs on WEHI cells was dose dependent. Neither IFN had any effect on TNF sensitivity of TNF-resistant MCA cells. The addition of lipid A (LA) had no effect on TMC under any condition. However, L929 cells treated with LA, TNF, and IFN-γ produced twice as much NO as cells treated with TNF and IFN-γ only. Northern analysis for cytokine-inducible NO synthase (NOS) mRNA steady-state levels indicated that TNF synergized with IFN-γ to induce increased accumulation of NOS mRNA. However, equal levels of NOS message were induced by TNF and IFN-γ in the presence and absence of LA. The data provide further evidence that the production of NO by adherent murine tumor cells does not participate in the mechanism of TMC, and that IFN-γ, but not IFN-αβ, augments NO production by these cells.