Active and inactive influenza virus induction of tumor necrosis factor-α and nitric oxide in J774.1 murine macrophages: modulation by interferon-γ and failure to induce apoptosis

Infection of J774.1 murine macrophages by influenza A virus (IAV) induces two major responses, production of host defense molecules and death by apoptosis. We investigated whether induction of two cytotoxic compounds, tumor necrosis factor-alpha (TNF-α) and nitric oxide (NO), directly caused IAV-induced apoptosis, and whether induction could be modulated by interferon-γ (IFN-γ) or the replication competence of the virus. Live IAV potently induced production of both TNF-α and NO, but UV inactivated virus was a poor inducer of both molecules. When cells were pre-treated with IFN-γ, inactive IAV became as effective an inducer of NO, but not TNF-α, as live IAV. Amantadine, which antagonizes viral entry and replication, partly inhibited TNF-α and NO production in unprimed cells, but did not inhibit NO in IFN-γ primed cells. IAV-induced cytotoxicity was not due to the induction of TNF-α or NO. Cells were insensitive to either TNF-α-containing supernatants or to recombinant TNF-α. Anti-TNF-α antibody did not protect cells from IAV-induced cell death, and anti-oxidants that inhibited TNF-α production also failed to increase cell survival. Inhibitors of NO production did not protect from IAV-induced cell death, either alone or in combination with superoxide dismutase (SOD). We conclude that, even though IAV was a potent inducer of TNF-α and NO in macrophages, IAV-induced apoptosis was not mediated directly by them. Importantly, viral replication was not required for the induction of TNF-α or NO, and the action of inactive IAV could be potentiated by IFN-γ.