Inhibitory Activity of Constitutive Nitric Oxide on the Expression of Alpha/Beta Interferon Genes in Murine Peritoneal Macrophages

We investigated the role of the constitutive nitric oxide (NO) in the expression of interferon (IFN) genes in mouse peritoneal macrophages (PM). The treatment of PM with l-arginine-N(G)-amine (AA), a potent inhibitor of NO-producing enzymes, resulted in a marked accumulation of IFN-α4 mRNA and, to a minor extent, of IFN-β mRNA. In contrast, the expression of IFN-γ mRNA, as well as tumor necrosis factor alpha and interleukin-6 mRNA, was not affected. Furthermore, a remarkable increase in the expression of the IFN regulating factor 1 (IRF-1), but not of IRF-2, mRNA was detected in AA-treated PM. To investigate whether the AA-induced activation of the IFN system correlates with the production and antiviral activity of IFN, the extent of encephalomyocarditis virus (EMCV) replication was monitored in AA-treated PM with respect to control cultures. AA treatment strongly inhibited, in a dose-dependent manner, EMCV yields in PM. Likewise, similar results were obtained by the addition of the NO-scavenger carboxyphenyl-tetramethylimidazoline-oxyl-oxide. In addition, inhibition of NO synthesis by N(G)-mono-methyl-l-arginine in PM strongly decreased virus replication in coculture of PM and EMCV-infected L929 cells, whereas no antiviral effect was observed in L929 cells alone. Moreover, the AA-mediated antiviral activity was abrogated in the presence of antibody to IFN-α/β, whereas antibody to IFN-γ was completely ineffective. Taken together, these results indicate that low levels of NO, constitutively released by resting PM, negatively regulate the expression and activity of IFN-α/β in PM. We suggest that NO acts as a homeostatic agent in the regulation of IFN pathway expression in macrophages.