We investigated mechanisms by which interferons alpha/beta (IFN), a family of antiviral immune genes, inhibit reactivation of murine gammaherpesvirus 68 (MHV68). Computer virus lacking the M2 ISRE (ISRE) overexpresses M2 mRNA and displays uncontrolled acute replicationin vivo, higher latent viral weight, and aberrantly high reactivation from latency. These phenotypes of the ISRE mutant are B-cell-specific, require IRF2, and correlate with a significant increase in virulence in a model of acute viral pneumonia. We therefore identify a mechanism by which a gammaherpesvirus subverts host IFN signaling in a surprisingly cooperative manner, to directly repress viral replication and reactivation and enforce latency, thereby minimizing acute host disease. Since we find ISREs 5 to the major lymphocyte latency genes of multiple rodent, primate, and human gammaherpesviruses, we propose that cooperative subversion of IFN-induced IRFs to promote latent infection is an ancient strategy that ensures a stable, minimally-pathogenic virus-host relationship. == Author Summary == Herpesviruses establish life-long infection in a non-replicating state termed latency. During immune compromise, herpesviruses can reactivate and cause severe disease, including cancer. We investigated mechanisms by which interferons alpha/beta (IFN), a family of antiviral immune genes, inhibit reactivation of murine gammaherpesvirus 68 (MHV68). MHV68 is related to Epstein-Barr computer virus and Kaposi’s sarcoma-associated herpesvirus, human gammaherpesviruses associated with multiple cancers. We made the surprising discovery that during latency, MHV68 cooperates with IFN to inhibit its own replication. Specifically, a viral gene required for reactivation has evolved to be directly repressed by an IFN-induced transcription factor, IRF2. Once computer virus replication has triggered sufficient IFN production, expression of this viral gene is usually reduced and reactivation efficiency decreases. This strategy safeguards the health of the host, since a mutant computer virus that cannot respond to IRF2 replicates uncontrollably and is more virulent. Viral sensing of IFN is also potentially subversive, since it allows MHV68 to detect periods of localized immune quiescence during which it can reactivate and spread to a new host. Thus, we highlight a novel path of virus-host coevolution, toward cooperative subversion of the antiviral immune response. These observations may illuminate new targets for drugs to inhibit herpesvirus reactivation or eliminate herpesvirus-associated tumors. == Introduction == The gammaherpesviruses (HVs) establish life-long latent contamination in memory B lymphocytes. The human HVs Epstein-Barr computer virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV) are the causes of infectious mononucleosis and Kaposi’s sarcoma (KS), respectively[1],[2]. HV latency is a cofactor in the development of lymphomas, sarcomas, and carcinomas. Viral reactivation and neoplasms increase in immune compromised individuals, highlighting the need for immune surveillance to prevent severe disease[2]. Mechanisms of immune control of latent EBV and KSHV are T-3775440 hydrochloride not completely understood due to their human-specific host range. Murine gammaherpesvirus 68 (MHV68) is usually closely related to the human HVs and provides a genetic model to study HV-immune interactions that regulate pathogenesis[3],[4]. We previously uncovered an unexpected role for type I (alpha/beta) interferons (IFN) during MHV68 latency[5]. IFN are a family of antiviral cytokines whose expression is triggered by cellular sensors of viral nucleic acid that activate interferon regulatory factor (IRF) family transcription factors[6]. IRFs bind to interferon stimulated response elements (ISREs) in IFN gene promoters to induce expression of IFN. IFN signaling via its heterodimeric receptor (IFNAR1/2) induces a large family of interferon-stimulated genes (ISGs) that inhibit viral replication by multiple mechanisms. Once computer virus infection has been cleared, the IFN-induced transcriptional repressor IRF2 exerts a negative feedback role to terminate IFN expression and prevent inflammatory pathology[7]. Many viruses antagonize IFN expression or ISG function to maximize replication[8]. However, T-3775440 hydrochloride the interactions between latent viruses and IFN are largely unexplored. We found that mice lacking the IFN receptor (IFNAR1-/-) exhibit increased Rabbit polyclonal to AMIGO1 MHV68 reactivation from latency in both splenic B cells and peritoneal macrophages[5]. This was unexpected since viral molecules that induce IFN production should be largely absent during T-3775440 hydrochloride latency, when infectious computer virus is usually undetectable using classical virologic assays. In addition, known antiviral functions of IFN are crucial during acute viral contamination, but are thought to be dispensable once replication is usually controlled[6]. One clue to the mechanism of IFN function during MHV68 T-3775440 hydrochloride latency came from our observation that this MHV68 latent gene M2 is usually specifically upregulated in splenocytes from IFNAR1-/- mice[5]. M2 is required for establishment of latency in splenic B cells following mucosal contamination and is essential for reactivation from B cells[9]. While the precise function of M2 T-3775440 hydrochloride is not known, it interacts with B cell signaling molecules including fyn and vav1, resulting in efficient entry of infected B cells into a germinal center.