Nitric oxide (NO), produced in interferon (IFN)-gamma-activated murine macrophages by the enzyme inducible nitric oxide synthase (iNOS), has been found to have antiviral properties. We have previously shown that herpes simplex virus type 2 (HSV-2) infection of macrophages synergistically enhances IFN-gamma-induced NO production, and we now extend these findings by providing evidence that virus-induced tumour necrosis factor (TNF)-alpha mediates activation of the transcription factor nuclear factor (NF)-kappaB, which in turn is responsible for the synergistic effect. HSV-2 infection and IFN-gamma stimulation of macrophages synergistically induced TNF-alpha secretion and nuclear translocation of NF-kappaB, which bound to a sequence corresponding to a kappaB site in the iNOS promoter. The effect of HSV-2 on NF-kappaB and NO production was eliminated when cells were treated with antibodies to TNF-alpha, and direct inhibition of NF-kappaB activation with pyrrolidinedithiocarbamate (PDTC) also blocked the effect of HSV-2 infection on NO production. The effect of the NF-kappaB activation inhibitor was not mediated through inhibition of the production of interferon regulatory factor (IRF)-1 or of TNF-alpha itself, and a possible alternative mechanism of activation of NF-kappaB through virus-induced activation of the kinase PKR was also ruled out. Thus, our data indicate that NF-kappaB activation, through virus-induced autocrine TNF-alpha secretion, is responsible for the synergistic effect of HSV-2 infection on IFN-gamma-induced NO production, and that such activation might constitute a mechanism by which high-output NO production is targeted to infectious foci.


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