ID: 76: The MCMV protein M35 is a novel negative regulator of the type I IFN response

The early detection of herpesvirus infection by pattern recognition receptors (PRRs) triggers the production of type I interferons (IFN), which exert potent antiviral effects. Whilst MCMV is recognized to induce strong innate immune responses, UV inactivation of MCMV results in an enhanced type I IFN response, indicating that the virus actively downmodulates innate immune signaling. Using an unbiased luciferase-based assay to screen an MCMV cDNA library, we have identified the M35 protein as a negative regulator of PRR-dependent type I IFN signaling. Infection of primary bone marrow derived macrophages with an MCMV mutant lacking M35 (M35stop) results in increased secretion of type I IFN as compared to wild-type (WT) MCMV. To confirm the physiological role of M35, we infected BALB/c mice with MCMV M35stop to assess the subsequent type I IFN response. MCMV M35stop induces significantly higher type I IFN serum levels than WT MCMV 8 h post infection, which correlates well with expression of tegument M35 protein. MCMV M35stop replicates to significantly reduced titres compared to WT MCMV in the spleen at day 3 post infection. MCMV is then disseminated via secondary viremia to the salivary glands, a site of persistence and dissemination for MCMV. MCMV M35stop is undetectable from the salivary glands from as early as 7 days post infection. This exclusion of M35stop from the salivary glands indicates that M35 is a critical determinant of viral replication in vivo. Collectively, our data demonstrate the first characterisation of M35 as a novel antagonist of type I IFN signaling downstream of PRRs.