HSV-1 (17) replicated to high titer in human corneal fibroblasts (> 108 PFU/105 cells) following infection at one PFU per 100 cells. Pretreatment of the cells for 24 h with 50 U/ml recombinant human tumor necrosis factor alpha (TNF-α) or 5 IU/ml of human interferon gamma (IFN-γ) resulted in only modest reduction (2- to 19-fold) in virus yield. However, when the two cytokines were combined the antiviral effect was dramatically increased. There was > 1000-fold reduction in virus titer in 8 of 8 trials. In contrast, the combinations of 50 U/ml TNF-α with 5 IU/ml IFN-α or IFN-β did not produce a synergistic effect. The pronounced synergistic antiviral activity of TNF-α + IFN-γ could be demonstrated in fibroblast cultures from different donors, and HSV-2 as well as HSV-1 strains were inhibited. There was no evidence that dual cytokine treatment was toxic for uninfected or HSV-infected cells. Insight into the mechanism responsible for the synergistic effect was provided by the observation that TNF-α + IFN-γ induced IFN-β. In addition, anti-IFN-β but not anti-IFN-α antibodies could reverse the antiviral effect, and reconstitution with IFN-β could duplicate the phenomenon. We conclude that the combination of TNF-α and IFN-γ at low concentrations can exert a powerful anti-herpes effect in human corneal fibroblasts which can be chiefly attributed to the induction of IFN-β.
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