Synergistic inhibition of human cytomegalovirus replication by interferon-alpha/beta and interferon-gamma

Background: Recent studies have shown that gamma interferon (IFN-gamma) synergizes with the innate IFNs (IFN-alpha and IFN-beta) to inhibit herpes simplex virus type 1 (HSV-1) replication in vitro. To determine whether this phenomenon is shared by other herpesviruses, we investigated the effects of IFNs on human cytomegalovirus (HCMV) replication. Results: We have found that as with HSV-1, IFN-gamma synergizes with the innate IFNs (IFN-alpha/beta) to potently inhibit HCMV replication in vitro. While pre-treatment of human foreskin fibroblasts (HFFs) with IFN-alpha, IFN-beta or IFN-gamma alone inhibited HCMV plaque formation by similar to 30 to 40-fold, treatment with IFN-alpha and IFN-gamma or IFN-beta and IFN-gamma inhibited HCMV plaque formation by 163- and 662-fold, respectively. The generation of isobole plots verified that the observed inhibition of HCMV plaque formation and replication in HFFs by IFN-alpha/beta and IFN-gamma was a synergistic interaction. Additionally, real-time PCR analyses of the HCMV immediate early (IE) genes (IE1 and IE2) revealed that IE mRNA expression was profoundly decreased in cells stimulated with IFN-alpha/beta and IFN-gamma (similar to 5-11-fold) as compared to vehicle-treated cells. Furthermore, decreased IE mRNA expression was accompanied by a decrease in IE protein expression, as demonstrated by western blotting and immunofluorescence. Conclusion: These findings suggest that IFN-alpha/beta and IFN-gamma synergistically inhibit HCMV replication through a mechanism that may involve the regulation of IE gene expression. We hypothesize that IFN-gamma produced by activated cells of the adaptive immune response may potentially synergize with endogenous type I IFNs to inhibit HCMV dissemination in vivo.