The Functions of Autophagy Genes and Interferon Regulatory Factor 1 in Interferon Gamma Mediated Control of Murine Norovirus Infection

Human noroviruses (HuNVs) account for the majority of the non-bacterial gastroenteritis worldwide. Symptoms of HuNV infection are generally self-limiting and typically resolve within 72 hrs implicating innate immune mechanisms in their control. Interferons (IFNs) are central to early host defense and are likely involved in restricting HuNV infection. However, in the absence of a culturable HuNV, studies of the immune mechanisms that restrict HuNV replication are limited. To study the role of IFNs, in particular interferon gamma, (IFNγ) and the antiviral machinery they employ to control norovirus (NV) infection, we utilized murine norovirus (MNV) as a model of NV pathogenesis and IFN-mediated control. We found that the autophagy proteins, Atg 5, 7 and 16 are important for control of MNV by IFNγ in macrophages. These proteins are best known as essential participants in macroautophapy (herein autophagy) which is the process by which cells engulf and digest self components. Here, we demonstrate that the action of these molecules in control of MNV represent a non-canonical function of these proteins. Furthermore, we have shown that IFNγ acts downstream of initial viral protein translation but upstream of replication of the viral genomes in an Atg5 dependent manner. Moreover we confirmed the in vivo relevance of Atg5-mediated IFNγ effects. Furthermore, we found that the transcription factor, interferon regulatory factor (IRF-1), was important for control of MNV replication in vivo and required for IFNγ mediated inhibition of MNV in cultured cells. Despite reported roles of IRF-1 in coordinating type I IFN induction and action, we establish that IFNγ mediated inhibition of MNV was IRF-1 dependent but IFNα/β independent. Furthermore, we have used microarray analyses to identify genes that are transcriptionally regulated by STAT-1 and IRF-1 and which we speculate contribute to IFNγ direct antiviral effects. These studies identify a novel IFNγ-mediated antiviral mechanism involving autophagy proteins. Furthermore, they demonstrate a role of IRF-1 in control of MNV infection and provide new targets for investigation that may further elucidate the molecular mechanisms of IFNγ direct antiviral effects.