Human cytomegalovirus UL23 inhibits transcription of interferon-γ stimulated genes and blocks antiviral interferon-γ responses by interacting with human N-myc interactor protein

Interferon-gamma (IFN-gamma) represents one of the most important innate immunity responses in a host to combat infections of many human viruses including human herpesviruses. Human N-myc interactor (Nmi) protein, which has been shown to interact with signal transducer and activator of transcription (STAT) proteins including STAT1, is important for the activation of IFN-gamma induced STAT1-dependent transcription of many genes responsible for IFN-gamma immune responses. However, no proteins encoded by herpesviruses have been reported to interact with Nmi and inhibit Nmi-mediated activation of IFN-gamma immune responses to achieve immune evasion from IFN-gamma responses. In this study, we show strong evidence that the UL23 protein of human cytomegalovirus (HCMV), a human herpesvirus, specifically interacts with Nmi. This interaction was identified through a yeast two-hybrid screen and co-immunoprecipitation in human cells. We observed that Nmi, when bound to UL23, was not associated with STAT1, suggesting that UL23 binding of Nmi disrupts the interaction of Nmi with STAT1. In cells overexpressing UL23, we observed (a) significantly reduced levels of Nmi and STAT1 in the nuclei, the sites where these proteins act to induce transcription of IFN-gamma stimulate genes, and (b) decreased levels of the induction of the transcription of IFN-gamma stimulated genes. UL23-deficient HCMV mutants induced higher transcription of IFN-gamma stimulated genes and exhibited lower titers than parental and control revertant viruses expressing functional UL23 in IFN-gamma treated cells. Thus, UL23 appears to interact directly with Nmi and inhibit nuclear translocation of Nmi and its associated protein STAT1, leading to a decrease of IFN-gamma induced responses and an increase of viral resistance to IFN-gamma. Our results further highlight the roles of UL23-Nmi interactions in facilitating viral immune escape from IFN-gamma responses and enhancing viral resistance to IFN antiviral effects.