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

Linyuan Feng,Zemin Deng,Tianhong Zhou,Fenyong Liu,Yujun Liu,Chingman Foo,Xu Sun,Marco Paliza-Carre,Gia-Phong Vu,Jingxue Sheng,Yanhong Ran,Sangwei Lu,Songbin Wu,Phong Trang,Hongjian Li,Xiaoping Yang,Robert F Kalejta

doi:10.1371/journal.ppat.1006867

Abstract

Interferon-γ (IFN-γ) 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-γ induced STAT1-dependent transcription of many genes responsible for IFN-γ immune responses. However, no proteins encoded by herpesviruses have been reported to interact with Nmi and inhibit Nmi-mediated activation of IFN-γ immune responses to achieve immune evasion from IFN-γ 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-γ stimulated genes, and (b) decreased levels of the induction of the transcription of IFN-γ stimulated genes. UL23-deficient HCMV mutants induced higher transcription of IFN-γ stimulated genes and exhibited lower titers than parental and control revertant viruses expressing functional UL23 in IFN-γ 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-γ induced responses and an increase of viral resistance to IFN-γ. Our results further highlight the roles of UL23-Nmi interactions in facilitating viral immune escape from IFN-γ responses and enhancing viral resistance to IFN antiviral effects.

Highlights

Human cytomegalovirus (CMV), a member of the human herpesvirus family, is a common opportunistic virus causing severe ailments and deaths in people with immature or compromised immune systems [1,2,3,4]
We show strong evidence that the UL23 protein of human cytomegalovirus (HCMV), a human herpesvirus, interacts with N-myc interactor (Nmi) protein
These results suggest that interfering with Nmi function may represent an effective mechanism for a herpesvirus to block Nmi-mediated IFN-γ responses and increase viral resistance to IFNγ

Summary

Introduction

Human cytomegalovirus (CMV), a member of the human herpesvirus family, is a common opportunistic virus causing severe ailments and deaths in people with immature or compromised immune systems [1,2,3,4]. HCMV expresses viral proteins to modulate the host immune responses at every step of its life cycle, which play a crucial role in viral pathogenesis [7,8,9]. Interferons (IFNs) such as IFN-γ are part of the innate immune response to viral infections and confer potent antiviral effects. These cytokines have important roles in immune-surveillance for malignant cells [10,11]. The transcription of type II IFN (IFN-γ)-dependent genes is regulated by gamma-activated sequence (GAS) elements, and the signal transducer and activator of transcription (STAT) protein, STAT1 is the most important transcription factor for the regulation of these transcriptional responses. The homodimers are transported to the nucleus and bind to GAS elements to induce transcription of various interferon stimulated genes [19,20,21,22]

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Conclusion