A Rhesus Rhadinovirus Viral Interferon (IFN) Regulatory Factor Is Virion Associated and Inhibits the Early IFN Antiviral Response.

UNLABELLED: The interferon (IFN) response is the earliest host immune response dedicated to combating viral infection. As such, viruses have evolved strategies to subvert this potent antiviral response. Two closely related gammaherpesviruses, Kaposi's sarcoma-associated herpesvirus (KSHV) and rhesus macaque rhadinovirus (RRV), are unique in that they express viral homologues to cellular interferon regulatory factors (IRFs), termed viral IRFs (vIRFs). Cellular IRFs are a family of transcription factors that are particularly important for the transcription of type I IFNs. Here, we demonstrate a strategy employed by RRV to ensure rapid inhibition of virus-induced type I IFN induction. We found that RRV vIRF R6, when expressed ectopically, interacts with a transcriptional coactivator, CREB-binding protein (CBP), in the nucleus. As a result, phosphorylated IRF3, an important transcriptional regulator in beta interferon (IFN-ß) transcription, fails to effectively bind to the IFN-ß promoter, thus inhibiting the activation of IFN-ß genes. In addition, we found R6 within RRV virion particles via immunoelectron microscopy and, furthermore, that virion-associated R6 is capable of inhibiting the type I IFN response by preventing efficient binding of IRF3/CBP complexes to the IFN-ß promoter in the context of infection. The work shown here is the first example of a vIRF being associated with either the KSHV or RRV virion. The presence of this immunomodulatory protein in the RRV virion provides the virus with an immediate mechanism to evade the host IFN response, thus enabling the virus to effectively establish an infection within the host. IMPORTANCE: Kaposi's sarcoma-associated herpesvirus (KSHV) and the closely related rhesus macaque rhadinovirus (RRV) are the only viruses known to encode viral homologues to cellular interferon regulatory factors (IRFs), known as vIRFs. In KSHV, these proteins have been shown to play major roles in a variety of cellular processes and are particularly important in the evasion of the host type I interferon (IFN) response. In this study, we delineate the immunomodulatory mechanism of an RRV vIRF and its ability to assist the virus in rapid immune evasion by being prepackaged within the virion, thus providing evidence, for the first time, of a virion-associated vIRF. This work further contributes to our understanding of the mechanisms behind immunomodulation by the RRV vIRFs during infection.

Genomic characterization of Japanese macaque rhadinovirus, a novel herpesvirus isolated from a nonhuman primate with a spontaneous inflammatory demyelinating disease.

Japanese macaque rhadinovirus (JMRV) is a novel gamma-2 herpesvirus that was isolated from a Japanese macaque (JM) with an inflammatory demyelinating encephalomyelitis referred to as Japanese macaque encephalomyelitis, a disease that possesses clinical and histopathological features resembling multiple sclerosis in humans. Genomic DNA sequence analysis reveals that JMRV is a gammaherpesvirus closely related to rhesus macaque rhadinovirus (RRV) and human herpesvirus 8. We describe here the complete nucleotide sequence and structure of the JMRV genome, as well as the sequence of two plaque isolates of this virus. Analysis of the JMRV genome not only demonstrates that this virus shares a number of genes with RRV that may be involved in pathogenesis but also indicates the presence of unique JMRV genes that could potentially contribute to disease development. The knowledge of the genomic sequence of JMRV, and the ability to easily propagate the virus in vitro, make JMRV infection of JM an attractive model for examining the potential role of an infectious viral agent in the development of demyelinating encephalomyelitis disease in vivo.

Poxviruses deploy genomic accordions to adapt rapidly against host antiviral defenses.

In contrast to RNA viruses, double-stranded DNA viruses have low mutation rates yet must still adapt rapidly in response to changing host defenses. To determine mechanisms of adaptation, we subjected the model poxvirus vaccinia to serial propagation in human cells, where its antihost factor K3L is maladapted against the antiviral protein kinase R (PKR). Viruses rapidly acquired higher fitness via recurrent K3L gene amplifications, incurring up to 7%-10% increases in genome size. These transient gene expansions were necessary and sufficient to counteract human PKR and facilitated the gain of an adaptive amino acid substitution in K3L that also defeats PKR. Subsequent reductions in gene amplifications offset the costs associated with larger genome size while retaining adaptive substitutions. Our discovery of viral "gene-accordions" explains how poxviruses can rapidly adapt to defeat different host defenses despite low mutation rates and reveals how classical Red Queen conflicts can progress through unrecognized intermediates.

Viral interferon regulatory factors decrease the induction of type I and type II interferon during rhesus macaque rhadinovirus infection.

Kaposi's sarcoma-associated herpesvirus and rhesus macaque rhadinovirus (RRV), two closely related gammaherpesviruses, are unique in their expression of viral homologs of cellular interferon regulatory factors (IRFs), termed viral IRFs (vIRFs). To assess the role of vIRFs during de novo infection, we have utilized the bacterial artificial chromosome clone of wild-type RRV(17577) (WT(BAC) RRV) to generate a recombinant virus with all 8 of the vIRFs deleted (vIRF-ko RRV). The infection of primary rhesus fibroblasts and peripheral blood mononuclear cells (PBMCs) with vIRF-ko RRV resulted in earlier and increased induction of type I interferon (IFN) (IFN-α/ß) and type II IFN (IFN-γ). Additionally, plasmacytoid dendritic cells maintained higher levels of IFN-α production in PBMC cultures infected with vIRF-ko RRV than in cultures infected with WT(BAC) RRV. Moreover, the nuclear accumulation of phosphorylated IRF-3, which is necessary for the induction of type I IFN, was also inhibited following WT(BAC) RRV infection. These findings demonstrate that during de novo RRV infection, vIRFs are inhibiting the induction of IFN at the transcriptional level, and one potential mechanism for this is the disruption of the activation and localization of IRF-3.