Rta of murine gammaherpesvirus 68 reactivates the complete lytic cycle from latency

A replication-deficient gammaherpesvirus vaccine protects mice from lytic disease and reduces latency establishment

Gammaherpesviruses are oncogenic viruses that establish lifelong infections and are significant causes of morbidity and mortality. Vaccine strategies to limit gammaherpesvirus infection and disease are in development, but there are no FDA-approved vaccines for Epstein-Barr or Kaposi sarcoma herpesvirus. As a new approach to gammaherpesvirus vaccination, we developed and tested a replication-deficient virus (RDV) platform, using murine gammaherpesvirus 68 (MHV68), a well-established mouse model for gammaherpesvirus pathogenesis studies and preclinical therapeutic evaluations. We employed codon-shuffling-based complementation to generate revertant-free RDV lacking expression of the essential replication and transactivator protein encoded by ORF50 to arrest viral gene expression early after de novo infection. Inoculation with RDV-50.stop exposes the host to intact virion particles and leads to limited lytic gene expression in infected cells yet does not produce additional infectious particles. Prime-boost vaccination of mice with RDV-50.stop elicited virus-specific neutralizing antibody and effector T cell responses in the lung and spleen. In contrast to vaccination with heat-inactivated WT MHV68, vaccination with RDV-50.stop resulted in a near complete abolishment of virus replication in the lung 7 days post-challenge and reduction of latency establishment in the spleen 16 days post-challenge with WT MHV68. Ifnar1-/- mice, which lack the type I interferon receptor, exhibit severe disease and high mortality upon infection with WT MHV68. RDV-50.stop vaccination of Ifnar1-/- mice prevented wasting and mortality upon challenge with WT MHV68. These results demonstrate that prime-boost vaccination with a gammaherpesvirus that is unable to undergo lytic replication offers protection against acute replication, impairs the establishment of latency, and prevents severe disease upon the WT virus challenge. Our study also reveals that the ability of a gammaherpesvirus to persist in vivo despite potent pre-existing immunity is an obstacle to obtaining sterilizing immunity.

Replication-dead gammaherpesvirus vaccine protects against acute replication, reactivation from latency, and lethal challenge in mice

Gammaherpesviruses (GHVs) are oncogenic viruses that establish lifelong infections and are significant causes of human morbidity and mortality. While several vaccine strategies to limit GHV infection and disease are in development, there are no FDA-approved vaccines for human GHVs. As a new approach to gammaherpesvirus vaccination, we developed and tested a replication-dead virus (RDV) platform, using murine gammaherpesvirus 68 (MHV68), a well-established mouse model for gammaherpesvirus pathogenesis studies and preclinical therapeutic evaluations. We employed codon-shuffling-based complementation to generate revertant-free RDV lacking expression of the essential replication and transactivator protein (RTA) encoded by ORF50 to arrest viral gene expression early after de novo infection. Inoculation with RDV-50.stop exposes the host to intact virion particles and leads to limited lytic gene expression in infected cells. Prime-boost vaccination of mice with RDV-50.stop elicited virus-specific neutralizing antibody and effector T cell responses in the lung and spleen. Vaccination with RDV-50.stop resulted in a near complete abolishment of virus replication in the lung 7 days post-challenge and virus reactivation from spleen 16 days post-challenge with WT MHV68. Ifnar1-/- mice, which lack the type I interferon receptor, exhibit severe disease upon infection with WT MHV68. RDV-50.stop vaccination of Ifnar1-/- mice prevented wasting and mortality upon challenge with WT MHV68. These results demonstrate that prime-boost vaccination with a GHV that is unable to undergo lytic replication offers protection against acute replication, reactivation, and severe disease upon WT virus challenge.

Rewiring of the Host Cell Metabolome and Lipidome during Lytic Gammaherpesvirus Infection Is Essential for Infectious-Virus Production

Oncogenic virus infections are estimated to cause ~15% of all cancers. Two prevalent human oncogenic viruses are members of the gammaherpesvirus family: Epstein-Barr virus (EBV) and Kaposi's sarcoma herpesvirus (KSHV). We use murine herpesvirus 68 (MHV-68), which shares significant homology with KSHV and EBV, as a model system to study gammaherpesvirus lytic replication. Viruses implement distinct metabolic programs to support their life cycle, such as increasing the supply of lipids, amino acids, and nucleotide materials necessary to replicate. Our data define the global changes in the host cell metabolome and lipidome during gammaherpesvirus lytic replication. Our metabolomics analysis found that MHV-68 lytic infection induces glycolysis, glutaminolysis, lipid metabolism, and nucleotide metabolism. We additionally observed an increase in glutamine consumption and glutamine dehydrogenase protein expression. While both glucose and glutamine starvation of host cells decreased viral titers, glutamine starvation led to a greater loss in virion production. Our lipidomics analysis revealed a peak in triacylglycerides early during infection and an increase in free fatty acids and diacylglyceride later in the viral life cycle. Furthermore, we observed an increase in the protein expression of multiple lipogenic enzymes during infection. Interestingly, pharmacological inhibitors of glycolysis or lipogenesis resulted in decreased infectious virus production. Taken together, these results illustrate the global alterations in host cell metabolism during lytic gammaherpesvirus infection, establish essential pathways for viral production, and recommend targeted mechanisms to block viral spread and treat viral induced tumors. IMPORTANCE Viruses are intracellular parasites which lack their own metabolism, so they must hijack host cell metabolic machinery in order to increase the production of energy, proteins, fats, and genetic material necessary to replicate. Using murine herpesvirus 68 (MHV-68) as a model system to understand how similar human gammaherpesviruses cause cancer, we profiled the metabolic changes that occur during lytic MHV-68 infection and replication. We found that MHV-68 infection of host cells increases glucose, glutamine, lipid, and nucleotide metabolic pathways. We also showed inhibition or starvation of glucose, glutamine, or lipid metabolic pathways results in an inhibition of virus production. Ultimately, targeting changes in host cell metabolism due to viral infection can be used to treat gammaherpesvirus-induced cancers and infections in humans.

RNF213 modulates γ-herpesvirus infection and reactivation via targeting the viral Replication and Transcription Activator

Interferons (IFNs) and the products of interferon-stimulated genes (ISGs) play crucial roles in host defense against virus infections. Although many ISGs have been characterized with respect to their antiviral activity, their target specificities and mechanisms of action remain largely unknown. Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus that is linked to several human malignancies. Here, we used the genetically and biologically related virus, murine gammaherpesvirus 68 (MHV-68) and screened for ISGs with anti-gammaherpesvirus activities. We found that overexpression of RNF213 dramatically inhibited MHV-68 infection, whereas knockdown of endogenous RNF213 significantly promoted MHV-68 proliferation. Importantly, RNF213 also inhibited KSHV de novo infection, and depletion of RNF213 in the latently KSHV-infected iSLK-219 cell line significantly enhanced lytic reactivation. Mechanistically, we demonstrated that RNF213 targeted the Replication and Transcription Activator (RTA) of both KSHV and MHV-68, and promoted the degradation of RTA protein through the proteasome-dependent pathway. RNF213 directly interacted with RTA and functioned as an E3 ligase to ubiquitinate RTA via K48 linkage. Taken together, we conclude that RNF213 serves as an E3 ligase and inhibits the de novo infection and lytic reactivation of gammaherpesviruses by degrading RTA through the ubiquitin-proteasome pathway.

Protein Degradation by Gammaherpesvirus RTAs: More Than Just Viral Transactivators

Kaposi's sarcoma-associated herpesvirus (KSHV) is a member of the Gammaherpesvirus subfamily that encodes several viral proteins with intrinsic E3 ubiquitin ligase activity or the ability to hijack host E3 ubiquitin ligases to modulate the host's immune response and to support the viral life cycle. This review focuses specifically on how the immediate-early KSHV protein RTA (replication and transcription activator) hijacks the host's ubiquitin-proteasome pathway (UPP) to target cellular and viral factors for protein degradation to allow for robust lytic reactivation. Notably, RTA's targets are either potent transcription repressors or they are activators of the innate and adaptive immune response, which block the lytic cycle of the virus. This review mainly focuses on what is currently known about the role of the E3 ubiquitin ligase activity of KSHV RTA in the regulation of the KSHV life cycle, but we will also discuss the potential role of other gammaherpesviral RTA homologs in UPP-mediated protein degradation.

Virus–Host Interplay Between Poly (ADP-Ribose) Polymerase 1 and Oncogenic Gammaherpesviruses

The gammaherpesviruses, include the Epstein–Barr virus, Kaposi’s sarcoma-associated herpesvirus, and murine gammaherpesvirus 68. They establish latent infection in the B lymphocytes and are associated with various lymphoproliferative diseases and tumors. The poly (ADP-ribose) polymerase-1 (PARP1), also called ADP-ribosyltransferase diphtheria-toxin-like 1 (ARTD1) is a nuclear enzyme that catalyzes the transfer of the ADP-ribose moiety to its target proteins and participates in important cellular activities, such as the DNA-damage response, cell death, transcription, chromatin remodeling, and inflammation. In gammaherpesvirus infection, PARP1 acts as a key regulator of the virus life cycle: lytic replication and latency. These viruses also develop various strategies to regulate PARP1, facilitating their replication. This review summarizes the roles of PARP1 in the viral life cycle as well as the viral modulation of host PARP1 activity and discusses the implications. Understanding the interactions between the PARP1 and oncogenic gammaherpesviruses may lead to the identification of effective therapeutic targets for the associated diseases.

The Antiviral Activities of Poly-ADP-Ribose Polymerases

The poly-adenosine diphosphate (ADP)-ribose polymerases (PARPs) are responsible for ADP-ribosylation, a reversible post-translational modification involved in many cellular processes including DNA damage repair, chromatin remodeling, regulation of translation and cell death. In addition to these physiological functions, recent studies have highlighted the role of PARPs in host defenses against viruses, either by direct antiviral activity, targeting certain steps of virus replication cycle, or indirect antiviral activity, via modulation of the innate immune response. This review focuses on the antiviral activity of PARPs, as well as strategies developed by viruses to escape their action.

Th2 Cytokine Modulates Herpesvirus Reactivation in a Cell Type Specific Manner

Gammaherpesviruses, such as Epstein-Barr virus (EBV), Kaposi's sarcoma associated virus (KSHV), and murine γ-herpesvirus 68 (MHV68), establish latent infection in B cells, macrophages, and non-lymphoid cells, and can induce both lymphoid and non-lymphoid cancers. Research on these viruses has relied heavily on immortalized B cell and endothelial cell lines. Therefore, we know very little about the cell type specific regulation of virus infection. We have previously shown that treatment of MHV68-infected macrophages with the cytokine interleukin-4 (IL-4) or challenge of MHV68-infected mice with an IL-4-inducing parasite leads to virus reactivation. However, we do not know if all latent reservoirs of the virus, including B cells, reactivate the virus in response to IL-4. Here we used an in vivo approach to address the question of whether all latently infected cell types reactivate MHV68 in response to a particular stimulus. We found that IL-4 receptor expression on macrophages was required for IL-4 to induce virus reactivation, but that it was dispensable on B cells. We further demonstrated that the transcription factor, STAT6, which is downstream of the IL-4 receptor and binds virus gene 50 N4/N5 promoter in macrophages, did not bind to the virus gene 50 N4/N5 promoter in B cells. These data suggest that stimuli that promote herpesvirus reactivation may only affect latent virus in particular cell types, but not in others.Importance Herpesviruses establish life-long quiescent infections in specific cells in the body, and only reactivate to produce infectious virus when precise signals induce them to do so. The signals that induce herpesvirus reactivation are often studied only in one particular cell type infected with the virus. However, herpesviruses establish latency in multiple cell types in their hosts. Using murine gammaherpesvirus-68 (MHV68) and conditional knockout mice, we examined the cell type specificity of a particular reactivation signal, interleukin-4 (IL-4). We found that IL-4 only induced herpesvirus reactivation from macrophages, but not from B cells. This work indicates that regulation of virus latency and reactivation is cell type specific. This has important implications for therapies aimed at either promoting or inhibiting reactivation for the control or elimination of chronic viral infections.

Deletion of immune evasion genes provides an effective vaccine design for tumor-associated herpesviruses

Vaccines based on live attenuated viruses often induce broad, multifaceted immune responses. However, they also usually sacrifice immunogenicity for attenuation. It is particularly difficult to elicit an effective vaccine for herpesviruses due to an armament of immune evasion genes and a latent phase. Here, to overcome the limitation of attenuation, we developed a rational herpesvirus vaccine in which viral immune evasion genes were deleted to enhance immunogenicity while also attaining safety. To test this vaccine strategy, we utilized murine gammaherpesvirus-68 (MHV-68) as a proof-of-concept model for the cancer-associated human γ-herpesviruses, Epstein-Barr virus and Kaposi sarcoma-associated herpesvirus. We engineered a recombinant MHV-68 virus by targeted inactivation of viral antagonists of type I interferon (IFN-I) pathway and deletion of the latency locus responsible for persistent infection. This recombinant virus is highly attenuated with no measurable capacity for replication, latency, or persistence in immunocompetent hosts. It stimulates robust innate immunity, differentiates virus-specific memory T cells, and elicits neutralizing antibodies. A single vaccination affords durable protection that blocks the establishment of latency following challenge with the wild type MHV-68 for at least six months post-vaccination. These results provide a framework for effective vaccination against cancer-associated herpesviruses through the elimination of latency and key immune evasion mechanisms from the pathogen.

Concurrent Control of the Kaposi's Sarcoma-Associated Herpesvirus Life Cycle through Chromatin Modulation and Host Hedgehog Signaling: a New Prospect for the Therapeutic Potential of Lipoxin A4

Lipoxin A4 (LXA4) is an endogenous lipid mediator with compelling anti-inflammatory and proresolution properties. Studies done to assess the role of arachidonic acid pathways of the host in Kaposi's sarcoma-associated herpesvirus (KSHV) biology helped discover that KSHV infection hijacks the proinflammatory cyclooxygenase-2 (COX-2) and 5-lipoxygenase (5-LO) pathways and concurrently reduces anti-inflammatory LXA4 secretion to maintain KSHV latency in infected cells. Treatment of KSHV-infected cells with LXA4 minimizes the activation of inflammatory and proliferative signaling pathways, including the NF-κB, AKT, and extracellular signal-regulated kinase 1/2 (ERK1/2) pathways, but the exact mechanism of action of LXA4 remains unexplored. Here, using mass spectrometry analysis, we identified components from the minichromosome maintenance (MCM) protein and chromatin-remodeling complex SMARCB1 and SMARCC2 to be LXA4-interacting host proteins in KSHV-infected cells. We identified a higher level of nuclear aryl hydrocarbon receptor (AhR) in LXA4-treated KSHV-infected cells than in untreated KSHV-infected cells, which probably facilitates the affinity interaction of the nucleosome complex protein with LXA4. We demonstrate that SMARCB1 regulates both replication and transcription activator (RTA) activity and host hedgehog (hh) signaling in LXA4-treated KSHV-infected cells. Host hedgehog signaling was modulated in an AMP-activated protein kinase (AMPK)-mammalian target of rapamycin (mTOR)-S6 kinase-dependent manner in LXA4-treated KSHV-infected cells. Since anti-inflammatory drugs are beneficial as adjuvants to conventional and immune-based therapies, we evaluated the potential of LXA4 treatment in regulating programmed death-ligand 1 (PD-L1) on KSHV-carrying tumor cells. Overall, our study identified LXA4-interacting host factors in KSHV-infected cells, which could help provide an understanding of the mode of action of LXA4 and its therapeutic potential against KSHV.IMPORTANCE The latent-to-lytic switch in KSHV infection is one of the critical events regulated by the major replication and transcription activator KSHV protein called RTA. Chromatin modification of the viral genome determines the phase of the viral life cycle in the host. Here, we report that LXA4 interacts with a host chromatin modulator, especially SMARCB1, which upregulates the KSHV ORF50 promoter. SMARCB1 has also been recognized to be a tumor suppressor protein which controls many tumorigenic events associated with the hedgehog (hh) signaling pathway. We also observed that LXA4 treatment reduces PD-L1 expression and that PD-L1 expression is an important immune evasion strategy used by KSHV for its survival and maintenance in the host. Our study underscores the role of LXA4 in KSHV biology and emphasizes that KSHV is strategic in downregulating LXA4 secretion in the host to establish latency. This study also uncovers the therapeutic potential of LXA4 and its targetable receptor, AhR, in KSHV's pathogenesis.

Genome-wide Transcript Structure Resolution Reveals Abundant Alternate Isoform Usage from Murine Gammaherpesvirus 68

The gammaherpesviruses, including Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), and murine gammaherpesvirus 68 (MHV68, MuHV-4, γHV68), are etiologic agents of a wide range of lymphomas and non-hematological malignancies. These viruses possess large and highly dense dsDNA genomes that feature >80 bidirectionally positioned open reading frames (ORFs). The abundance of overlapping transcripts and extensive splicing throughout these genomes have until now prohibited high throughput-based resolution of transcript structures. Here, we integrate the capabilities of long-read sequencing with the accuracy of short-read platforms to globally resolve MHV68 transcript structures using the transcript resolution through integration of multi-platform data (TRIMD) pipeline. This approach reveals highly complex features, including: (1) pervasive overlapping transcript structures; (2) transcripts containing intra-gene or trans-gene splices that yield chimeric ORFs; (3) antisense and intergenic transcripts containing ORFs; and (4) noncoding transcripts. This work sheds light on the underappreciated complexity of gammaherpesvirus transcription and provides an extensively revised annotation of the MHV68 transcriptome.

Ov2 is a modulator of OvHV-2 RTA mediated gene expression

Ovine herpesvirus-2 (OvHV-2) is the causative agent of the sheep-associated form of malignant catarrhal fever, a usually fatal lymphoproliferative disease of bison, deer and cattle. Malignant catarrhal fever is a major cause of cattle loss in Africa with approximately 7% affected annually; and in North America has significant impact on bison farming. Research into the mechanisms by which OvHV-2 induces disease in susceptible species has been hampered by a lack of a cell culture system for the virus. Ov2 is a bZIP protein encoded by OvHV-2. Proteins with bZIP domains in other herpesviruses, such as the Kaposi’s sarcoma-associated herpesvirus K8 protein and the BZLF1 protein of Epstein-Barr virus are known to play important roles in lytic virus replication. Using a reporter based system, we demonstrate that Ov2 can modulate the activity of the major virus transactivator (Replication and Transcriptional Activator protein, RTA) to 1) drive expression of viral genes predicted to be required for efficient reactivation of the virus, including ORF49; and 2) differentially regulate the expression of the two virus encoded Bcl-2 homologues Ov4.5 and Ov9.

Inhibition of murine herpesvirus-68 replication by IFN-gamma in macrophages is counteracted by the induction of SOCS1 expression

Gamma interferon (IFN-γ) is known to negatively regulate murine gammaherpesvirus-68 (MHV-68 or γHV-68) replication. This process involves the suppression of the viral gene replication and transcription activator (RTA) promoter, as well as activation of signal transducers and activators of transcription (STAT1). Notably, this effect is gradually attenuated during MHV-68 infection of bone marrow-derived macrophages (BMMs), which raised the possibility that the virus may utilize a mechanism that counteracts the antiviral effect of IFN-γ. By identifying the cellular factors that negatively regulate JAK-STAT1 signaling, we revealed that the infection of BMMs by MHV-68 induces the expression of suppressor of cytokine signaling 1 (SOCS1) and that depletion of SOCS1 restores the inhibitory effect of IFN-γ on virus replication. Moreover, we demonstrated that the expression of SOCS1 was induced as a result of the Toll-like receptor 3 (TLR3) mediated activation of the NF-κB signaling cascade. In conclusion, we report that TLR3-TRAF-NF-κB signaling pathway play a role in the induction of SOCS1 that counteracts the antiviral effect of IFN-γ during MHV-68 infection. This process is cell type-specific: it is functional in macrophages, but not in epithelial cells or fibroblasts. Our study reveals a mechanism that balances the immune responses and the escape of a gamma-herpesvirus in some antigen-presenting cells.

Murine Gammaherpesvirus 68 Expressing Kaposi Sarcoma-Associated Herpesvirus Latency-Associated Nuclear Antigen (LANA) Reveals both Functional Conservation and Divergence in LANA Homologs

Latency-associated nuclear antigen (LANA) is a multifunctional protein encoded by members of the Rhadinovirus genus of gammaherpesviruses. Studies using murine gammaherpesvirus 68 (MHV68) demonstrated that LANA is important for acute replication, latency establishment, and reactivation in vivo Despite structural similarities in their DNA-binding domains (DBDs), LANA homologs from Kaposi sarcoma-associated herpesvirus (KSHV) and MHV68 exhibit considerable sequence divergence. We sought to determine if KSHV and MHV68 LANA homologs are functionally interchangeable. We generated an MHV68 virus that encodes KSHV LANA (kLANA) in place of MHV68 LANA (mLANA) and evaluated the virus's capacity to replicate, establish and maintain latency, and reactivate. kLANA knock-in (KLKI) MHV68 was replication competent in vitro and in vivo but exhibited slower growth kinetics and lower titers than wild-type (WT) MHV68. Following inoculation of mice, KLKI MHV68 established and maintained latency in splenocytes and peritoneal cells but did not reactivate efficiently ex vivo kLANA repressed the MHV68 promoter for ORF50, the gene that encodes the major lytic transactivator protein RTA, while mLANA did not, suggesting a likely mechanism for the KLKI MHV68 phenotypes. Bypassing this repression by providing MHV68 RTA in trans rescued KLKI MHV68 replication in tissue culture and enabled detection of KLKI MHV68 reactivation ex vivo These data demonstrate that kLANA and mLANA are functionally interchangeable for establishment and maintenance of latency and suggest that repression of lytic replication by kLANA, as previously shown with KSHV, is a kLANA-specific function that is transferable to MHV68.IMPORTANCE Kaposi sarcoma-associated herpesvirus (KSHV) and murine gammaherpesvirus 68 (MHV68) are members of the Rhadinovirus genus of gammaherpesviruses. These viruses establish lifelong infections that place their respective human and murine hosts at risk for cancer. Latency-associated nuclear antigen (LANA) is a conserved Rhadinovirus protein that is necessary for long-term chronic infection by these viruses. To better understand the conserved functions performed by LANA homologs, we generated a recombinant MHV68 virus that encodes the KSHV LANA protein in place of the MHV68 LANA homolog. We determined that the KSHV LANA protein is capable of supporting MHV68 latency in a mouse model of chronic infection but also functions to repress viral replication. This work describes an in vivo model system for defining evolutionarily conserved and divergent functions of LANA homologs in Rhadinovirus infection and disease.

The replication and transcription activator of murine gammaherpesvirus 68 cooperatively enhances cytokine-activated, STAT3-mediated gene expression

Gammaherpesviruses (γHVs) have a dynamic strategy for lifelong persistence, involving productive infection, latency, and intermittent reactivation. In latency reservoirs, such as B lymphocytes, γHVs exist as viral episomes and express few viral genes. Although the ability of γHV to reactivate from latency and re-enter the lytic phase is challenging to investigate and control, it is known that the γHV replication and transcription activator (RTA) can promote lytic reactivation. In this study, we provide first evidence that RTA of murine γΗV68 (MHV68) selectively binds and enhances the activity of tyrosine-phosphorylated host STAT3. STAT3 is a transcription factor classically activated by specific tyrosine 705 phosphorylation (pTyr⁷⁰⁵-STAT3) in response to cytokine stimulation. pTyr⁷⁰⁵-STAT3 forms a dimer that avidly binds a consensus target site in the promoters of regulated genes, and our results indicate that RTA cooperatively enhances the ability of pTyr⁷⁰⁵-STAT3 to induce expression of a STAT3-responsive reporter gene. As indicated by coimmunoprecipitation, in latently infected B cells that are stimulated to reactivate MHV68, RTA bound specifically to endogenous pTyr⁷⁰⁵-STAT3. An in vitro binding assay confirmed that RTA selectively recognizes pTyr⁷⁰⁵-STAT3 and indicated that the C-terminal transactivation domain of RTA was required for enhancing STAT3-directed gene expression. The cooperation of these transcription factors may influence both viral and host genes. During MHV68 de novo infection, pTyr⁷⁰⁵-STAT3 promoted the temporal expression of ORF59, a viral replication protein. Our results demonstrate that MHV68 RTA specifically recognizes and recruits activated pTyr⁷⁰⁵-STAT3 during the lytic phase of infection.

Reactivation and Lytic Replication of Kaposi's Sarcoma-Associated Herpesvirus: An Update

The life cycle of Kaposi’s sarcoma-associated herpesvirus (KSHV) consists of two phases, latent and lytic. The virus establishes latency as a strategy for avoiding host immune surveillance and fusing symbiotically with the host for lifetime persistent infection. However, latency can be disrupted and KSHV is reactivated for entry into the lytic replication. Viral lytic replication is crucial for efficient dissemination from its long-term reservoir to the sites of disease and for the spread of the virus to new hosts. The balance of these two phases in the KSHV life cycle is important for both the virus and the host and control of the switch between these two phases is extremely complex. Various environmental factors such as oxidative stress, hypoxia, and certain chemicals have been shown to switch KSHV from latency to lytic reactivation. Immunosuppression, unbalanced inflammatory cytokines, and other viral co-infections also lead to the reactivation of KSHV. This review article summarizes the current understanding of the initiation and regulation of KSHV reactivation and the mechanisms underlying the process of viral lytic replication. In particular, the central role of an immediate-early gene product RTA in KSHV reactivation has been extensively investigated. These studies revealed multiple layers of regulation in activation of RTA as well as the multifunctional roles of RTA in the lytic replication cascade. Epigenetic regulation is known as a critical layer of control for the switch of KSHV between latency and lytic replication. The viral non-coding RNA, PAN, was demonstrated to play a central role in the epigenetic regulation by serving as a guide RNA that brought chromatin remodeling enzymes to the promoters of RTA and other lytic genes. In addition, a novel dimension of regulation by microPeptides emerged and has been shown to regulate RTA expression at the protein level. Overall, extensive investigation of KSHV reactivation and lytic replication has revealed a sophisticated regulation network that controls the important events in KSHV life cycle.

RTA Occupancy of the Origin of Lytic Replication during Murine Gammaherpesvirus 68 Reactivation from B Cell Latency

RTA, the viral Replication and Transcription Activator, is essential for rhadinovirus lytic gene expression upon de novo infection and reactivation from latency. Lipopolysaccharide (LPS)/toll-like receptor (TLR)4 engagement enhances rhadinovirus reactivation. We developed two new systems to examine the interaction of RTA with host NF-kappaB (NF-κB) signaling during murine gammaherpesvirus 68 (MHV68) infection: a latent B cell line (HE-RIT) inducible for RTA-Flag expression and virus reactivation; and a recombinant virus (MHV68-RTA-Bio) that enabled in vivo biotinylation of RTA in BirA transgenic mice. LPS acted as a second stimulus to drive virus reactivation from latency in the context of induced expression of RTA-Flag. ORF6, the gene encoding the single-stranded DNA binding protein, was one of many viral genes that were directly responsive to RTA induction; expression was further increased upon treatment with LPS. However, NF-κB sites in the promoter of ORF6 did not influence RTA transactivation in response to LPS in HE-RIT cells. We found no evidence for RTA occupancy of the minimal RTA-responsive region of the ORF6 promoter, yet RTA was found to complex with a portion of the right origin of lytic replication (oriLyt-R) that contains predicted RTA recognition elements. RTA occupancy of select regions of the MHV-68 genome was also evaluated in our novel in vivo RTA biotinylation system. Streptavidin isolation of RTA-Bio confirmed complex formation with oriLyt-R in LPS-treated primary splenocytes from BirA mice infected with MHV68 RTA-Bio. We demonstrate the utility of reactivation-inducible B cells coupled with in vivo RTA biotinylation for mechanistic investigations of the interplay of host signaling with RTA.

Identification of Novel Kaposi's Sarcoma-Associated Herpesvirus Orf50 Transcripts: Discovery of New RTA Isoforms with Variable Transactivation Potential

Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus that has been associated with primary effusion lymphoma and multicentric Castleman's disease, as well as its namesake Kaposi's sarcoma. As a gammaherpesvirus, KSHV is able to acutely replicate, enter latency, and reactivate from this latent state. A key protein involved in both acute replication and reactivation from latency is the replication and transcriptional activator (RTA) encoded by the gene Orf50 RTA is a known transactivator of multiple viral genes, allowing it to control the switch between latency and virus replication. We report here the identification of six alternatively spliced Orf50 transcripts that are generated from four distinct promoters. These newly identified promoters are shown to be transcriptionally active in 293T (embryonic kidney), Vero (African-green monkey kidney epithelial), 3T12 (mouse fibroblast), and RAW 264.7 (mouse macrophage) cell lines. Notably, the newly identified Orf50 transcripts are predicted to encode four different isoforms of the RTA which differ by 6 to 10 residues at the amino terminus of the protein. We show the global viral transactivation potential of all four RTA isoforms and demonstrate that all isoforms can transcriptionally activate an array of KSHV promoters to various levels. The pattern of transcriptional activation appears to support a transcriptional interference model within the Orf50 region, where silencing of previously expressed isoforms by transcription initiation from upstream Orf50 promoters has the potential to modulate the pattern of viral gene activation.

IMPORTANCE

Gammaherpesviruses are associated with the development of lymphomas and lymphoproliferative diseases, as well as several other types of cancer. The human gammaherpesvirus, Kaposi's sarcoma-associated herpesvirus (KSHV), is tightly associated with the development of Kaposi's sarcoma and multicentric Castleman's disease, as well as a rare form of B cell lymphoma (primary effusion lymphoma) primarily observed in HIV-infected individuals. RTA is an essential viral gene product involved in the initiation of gammaherpesvirus replication and is conserved among all known gammaherpesviruses. We show here for KSHV that transcription of the gene encoding RTA is complex and leads to the expression of several isoforms of RTA with distinct functions. This observed complexity in KSHV RTA expression and function likely plays a critical role in the regulation of downstream viral and cellular gene expression, leading to the efficient production of mature virions.

Regulation of the viral life cycle by murine gammaherpesvirus 68 microRNAs

γ-Herpesviruses (γHV) such as Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus are important human pathogens involved in lymphoproliferation and tumorigenesis. Murine gammaherpesvirus 68 (MHV-68, γHV-68) is an effective model for the study of γHV pathogenesis and host-virus interaction because it is closely related to human γHV. Similarly to human γHV, MHV-68 encodes 15 microRNAs (miRNAs). Although their functions remain unknown, they are thought to regulate the viral life cycle or host-virus interactions, similarly to other human γHV. Herein, we established stable cell lines expressing MHV-68 miRNAs and investigated the role of MHV-68 miRNAs in the regulation of viral life cycle. We found that mghv-miR-M1-1, -3, -5, -7, -8, -9, -10, -11, -13, and -15 repressed MHV-68 lytic replication by down-regulating expression of the replication and transcription activator (RTA) gene, whereas mghv-miR-M1-2, -4, -6, and -12 induced lytic replication by up-regulating RTA. We confirmed that the decrease in viral replication caused by mghv-miR-M1-1 was abolished by inhibition of miRNA expression via miRNA inhibitor treatment. In addition, we observed that mghv-miR-M1-1 down-regulated c-Jun indirectly and decreased cytokine production, suggesting that mghv-miR-M1-1 may inhibit MHV-68 lytic replication by inhibiting the activator protein 1 (AP-1) signaling pathway.

Latency-Associated Nuclear Antigen E3 Ubiquitin Ligase Activity Impacts Gammaherpesvirus-Driven Germinal Center B Cell Proliferation

Viruses have evolved mechanisms to hijack components of cellular E3 ubiquitin ligases, thus modulating the ubiquitination pathway. However, the biological relevance of such mechanisms for viral pathogenesis in vivo remains largely unknown. Here, we utilized murid herpesvirus 4 (MuHV-4) infection of mice as a model system to address the role of MuHV-4 latency-associated nuclear antigen (mLANA) E3 ligase activity in gammaherpesvirus latent infection. We show that specific mutations in the mLANA SOCS box (V199A, V199A/L202A, or P203A/P206A) disrupted mLANA's ability to recruit Elongin C and Cullin 5, thereby impairing the formation of the Elongin BC/Cullin 5/SOCS (EC5S(mLANA)) complex and mLANA's E3 ligase activity on host NF-κB and Myc. Although these mutations resulted in considerably reduced mLANA binding to viral terminal repeat DNA as assessed by electrophoretic mobility shift assay (EMSA), the mutations did not disrupt mLANA's ability to mediate episome persistence. In vivo, MuHV-4 recombinant viruses bearing these mLANA SOCS box mutations exhibited a deficit in latency amplification in germinal center (GC) B cells. These findings demonstrate that the E3 ligase activity of mLANA contributes to gammaherpesvirus-driven GC B cell proliferation. Hence, pharmacological inhibition of viral E3 ligase activity through targeting SOCS box motifs is a putative strategy to control gammaherpesvirus-driven lymphoproliferation and associated disease.

IMPORTANCE

The gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) cause lifelong persistent infection and play causative roles in several human malignancies. Colonization of B cells is crucial for virus persistence, and access to the B cell compartment is gained by virus-driven proliferation in germinal center (GC) B cells. Infection of B cells is predominantly latent, with the viral genome persisting as a multicopy episome and expressing only a small subset of viral genes. Here, we focused on latency-associated nuclear antigen (mLANA) encoded by murid herpesvirus-4 (MuHV-4), which exhibits homology in sequence, structure, and function to KSHV LANA (kLANA), thereby allowing the study of LANA-mediated pathogenesis in mice. Our experiments show that mLANA's E3 ubiquitin ligase activity is necessary for efficient expansion of latency in GC B cells, suggesting that the development of pharmacological inhibitors of LANA E3 ubiquitin ligase activity may allow strategies to interfere with gammaherpesvirus-driven lymphoproliferation and associated disease.

Interplay of Murine Gammaherpesvirus 68 with NF-kappaB Signaling of the Host

Herpesviruses establish a chronic infection in the host characterized by intervals of lytic replication, quiescent latency, and reactivation from latency. Murine gammaherpesvirus 68 (MHV68) naturally infects small rodents and has genetic and biologic parallels with the human gammaherpesviruses (gHVs), Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. The murine gammaherpesvirus model pathogen system provides a platform to apply cutting-edge approaches to dissect the interplay of gammaherpesvirus and host determinants that enable colonization of the host, and that shape the latent or lytic fate of an infected cell. This knowledge is critical for the development of novel therapeutic interventions against the oncogenic gHVs. The nuclear factor kappa B (NF-κB) signaling pathway is well-known for its role in the promotion of inflammation and many aspects of B cell biology. Here, we review key aspects of the virus lifecycle in the host, with an emphasis on the route that the virus takes to gain access to the B cell latency reservoir. We highlight how the murine gammaherpesvirus requires components of the NF-κB signaling pathway to promote replication, latency establishment, and maintenance of latency. These studies emphasize the complexity of gammaherpesvirus interactions with NF-κB signaling components that direct innate and adaptive immune responses of the host. Importantly, multiple facets of NF-κB signaling have been identified that might be targeted to reduce the burden of gammaherpesvirus-associated diseases.

Infection-Induced Retrotransposon-Derived Noncoding RNAs Enhance Herpesviral Gene Expression via the NF-κB Pathway

Short interspersed nuclear elements (SINEs) are highly abundant, RNA polymerase III-transcribed noncoding retrotransposons that are silenced in somatic cells but activated during certain stresses including viral infection. How these induced SINE RNAs impact the host-pathogen interaction is unknown. Here we reveal that during murine gammaherpesvirus 68 (MHV68) infection, rapidly induced SINE RNAs activate the antiviral NF-κB signaling pathway through both mitochondrial antiviral-signaling protein (MAVS)-dependent and independent mechanisms. However, SINE RNA-based signaling is hijacked by the virus to enhance viral gene expression and replication. B2 RNA expression stimulates IKKβ-dependent phosphorylation of the major viral lytic cycle transactivator protein RTA, thereby enhancing its activity and increasing progeny virion production. Collectively, these findings suggest that SINE RNAs participate in the innate pathogen response mechanism, but that herpesviruses have evolved to co-opt retrotransposon activation for viral benefit.

Short interspersed nuclear elements (SINEs) are noncoding mobile genetic elements that are present at ~10⁶ copies per mammalian genome, roughly comprising 10% of mammalian genomic real estate. SINEs are typically transcriptionally silenced, though in some cases viral infection can promote their expression, yet to an unknown functional outcome. Thus, SINE elements represent the largest class of infection-inducible noncoding RNAs that are functionally uncharacterized. Here, we reveal that SINE RNAs play a critical role in the host-pathogen interaction in that they are required for efficient murine gammaherpesvirus 68 (MHV68) replication and gene expression. We demonstrate that SINE RNAs, both exogenously expressed and infection-induced, are robust activators of the IKKβ kinase, a key signaling molecule in the innate immune response. Activation of the IKKβ kinase by SINE RNA is mediated through both MAVS-dependent and independent mechanisms. Moreover, we demonstrate the activation of the IKKβ via SINE RNA is required to drive the phosphorylation of MHV68 RTA, the main viral transcriptional activator, which enhances its transcriptional activating property. Collectively, we reveal the first example of a role for SINE RNAs in the host-pathogen interaction and identify them as a key immune signaling molecule early during infection. Though SINE RNAs activate the innate immune response, MHV68 has co-opted SINE-mediate innate immune activation to enhance the viral lifecycle.

Murine Gammaherpesvirus 68 ORF48 Is an RTA-Responsive Gene Product and Functions in both Viral Lytic Replication and Latency during In Vivo Infection

Replication and transcription activator (RTA) of gammaherpesvirus is an immediate early gene product and regulates the expression of many downstream viral lytic genes. ORF48 is also conserved among gammaherpesviruses; however, its expression regulation and function remained largely unknown. In this study, we characterized the transcription unit of ORF48 from murine gammaherpesvirus 68 (MHV-68) and analyzed its transcriptional regulation. We showed that RTA activates the ORF48 promoter via an RTA-responsive element (48pRRE). RTA binds to 48pRRE directly in vitro and also associates with ORF48 promoter in vivo. Mutagenesis of 48pRRE in the context of the viral genome demonstrated that the expression of ORF48 is activated by RTA through 48pRRE during de novo infection. Through site-specific mutagenesis, we generated an ORF48-null virus and examined the function of ORF48 in vitro and in vivo. The ORF48-null mutation remarkably reduced the viral replication efficiency in cell culture. Moreover, through intranasal or intraperitoneal infection of laboratory mice, we showed that ORF48 is important for viral lytic replication in the lung and establishment of latency in the spleen, as well as viral reactivation from latency. Collectively, our study identified ORF48 as an RTA-responsive gene and showed that ORF48 is important for MHV-68 replication both in vitro and in vivo.

IMPORTANCE

The replication and transcription activator (RTA), conserved among gammaherpesviruses, serves as a molecular switch for the virus life cycle. It works as a transcriptional regulator to activate the expression of many viral lytic genes. However, only a limited number of such downstream genes have been uncovered for MHV-68. In this study, we identified ORF48 as an RTA-responsive gene of MHV-68 and mapped the cis element involved. By constructing a mutant virus that is deficient in ORF48 expression and through infection of laboratory mice, we showed that ORF48 plays important roles in different stages of viral infection in vivo. Our study provides insights into the transcriptional regulation and protein function of MHV-68, a desired model for studying gammaherpesviruses.

Activation of NF-κB via endosomal Toll-like receptor 7 (TLR7) or TLR9 suppresses murine herpesvirus 68 reactivation

In order to understand and possibly treat B-cell malignancies associated with latent gammaherpesvirus infection, it is vital to understand the factors that control the balance between the two transcriptional states of gammaherpesviruses: latency and lytic replication. We used murine gammaherpesvirus 68 (MHV 68) as a model system to investigate how engagement of endosomal Toll-like receptors (TLRs) impacts reactivation from latency in vitro and establishment of latent infection in vivo. We found that treatment with TLR7 ligand R848 or TLR9 ligand CpG oligodeoxynucleotide (ODN) suppresses reactivation of MHV 68 in vitro. These suppressive effects correlated with the ability to activate cellular transcription factor NF-κB. Downregulation of TLR9 by RNA interference in vitro led to a reduction of nuclear levels of NF-κB p65 and consequently to an increase of spontaneous reactivation in cells latently infected with MHV 68, indicating that the TLR9 pathway suppresses spontaneous reactivation events. In vivo, sustained stimulation of TLR7 by repeated R848 treatment led to an increased frequency of infected splenocytes compared to mock-treated control results. Frequencies of infected splenic B cells in tlr7-/- or tlr9-/- mice after establishment of latency did not differ from those seen with their wild-type counterparts. Nevertheless, MHV 68-infected B cells from tlr9-/- mice showed a higher frequency of reactivation than B cells from wild-type or tlr7-/- mice in ex vivo reactivation assays. Thus, we show a suppressive effect of TLR7 or TLR9 triggering on MHV 68 reactivation that correlates with NF-κB activation and that the mere presence of a functional TLR9 signaling pathway contributes to dampen lytic gammaherpesvirus reactivation in infected cells.

IMPORTANCE

A hallmark of gammaherpesviruses is their establishment of latency in B cells that is reversible through lytic reactivation. Latency can result in B-cell malignancies. Activation of the innate immune system is thought to contribute to controlling the switch between the transcriptional states of latency and reactivation. Nevertheless, the mechanisms involved are not clear. Here, we show that engagement of Toll-like receptor 7 (TLR7) and TLR9 suppresses reactivation of murine gammaherpesvirus MHV 68 in vitro and that stimulation of TLR7 in vivo increases the frequency of infected cells. TLR7 and TLR9 are innate immunity sensors of nucleic acids localized in endosomes. Additionally, we demonstrate that impairment of TLR9 signaling in latently infected B cells leads to increased reactivation. Thus, activated endosomal TLR7 and TLR9 pathways play an important role in promoting establishment of latent gammaherpesvirus infection. Counteracting signaling of these pathways allows reactivation and could represent treatment targets in gammaherpesvirus-associated malignancies.

Interferon regulatory factor 1 restricts gammaherpesvirus replication in primary immune cells

Gammaherpesviruses are ubiquitous pathogens that establish a lifelong infection and are associated with cancer. In spite of the high seroprevalence of infection, the risk factors that predispose the host toward gammaherpesvirus-induced malignancies are still poorly understood. Interferon (IFN) regulatory factor 1 (IRF-1) is a tumor suppressor that is also involved in the regulation of innate and adaptive immune responses. On the basis of its biology, IRF-1 represents a plausible host factor to attenuate gammaherpesvirus infection and tumorigenesis. In this study, we show that IRF-1 restricts gammaherpesvirus replication in primary macrophages, a physiologically relevant immune cell type. In spite of the known role of IRF-1 in stimulating type I IFN expression, induction of a global type I IFN response was similar in IRF-1-deficient and -proficient macrophages during gammaherpesvirus infection. However, IRF-1 was required for optimal expression of cholesterol-25-hydroxylase, a host enzyme that restricted gammaherpesvirus replication in primary macrophages and contributed to the antiviral effects of IRF-1. In summary, the current study provides an insight into the mechanism by which IRF-1 attenuates gammaherpesvirus replication in primary immune cells, a mechanism that is likely to contribute to the antiviral effects of IRF-1 in other virus systems.

IMPORTANCE

Interferon regulatory factor 1 (IRF-1) is a transcription factor that regulates innate and adaptive immune responses and functions as a tumor suppressor. IRF-1 restricts the replication of diverse viruses; however, the mechanisms responsible for the antiviral effects of IRF-1 are still poorly understood. Gammaherpesviruses are ubiquitous pathogens that are associated with the induction of several malignancies. Here we show that IRF-1 expression attenuates gammaherpesvirus replication in primary macrophages, in part by increasing expression of cholesterol-25-hydroxylase (CH25H). CH25H and its product, 25-hydroxycholesterol, restrict replication of diverse virus families. Thus, our findings offer an insight into the mechanism by which IRF-1 attenuates the replication of gammaherpesviruses, a mechanism that is likely to be applicable to other virus systems.

Murine gammaherpesvirus 68 encoding open reading frame 11 targets TANK binding kinase 1 to negatively regulate the host type I interferon response

Upon viral infection, type I interferons, such as alpha and beta interferon (IFN-α and IFN-β, respectively), are rapidly induced and activate multiple antiviral genes, thereby serving as the first line of host defense. Many DNA and RNA viruses counteract the host interferon system by modulating the production of IFNs. In this study, we report that murine gammaherpesvirus 68 (MHV-68), a double-stranded DNA virus, encodes open reading frame 11 (ORF11), a novel immune modulator, to block IFN-β production. ORF11-deficient recombinant viruses induced more IFN-β production in fibroblast and macrophage cells than the MHV-68 wild type or a marker rescue virus. MHV-68 ORF11 decreased IFN-β promoter activation by various factors, the signaling of which converges on TBK1-IRF3 activation. MHV-68 ORF11 directly interacted with both overexpressed and endogenous TBK1 but not with IRF3. Physical interactions between ORF11 and endogenous TBK1 were further confirmed during virus replication in fibroblasts using a recombinant virus expressing FLAG-ORF11. ORF11 efficiently reduced interaction between TBK1 and IRF3 and subsequently inhibited activation of IRF3, thereby negatively regulating IFN-β production. Our domain-mapping study showed that the central domain of ORF11 was responsible for both TBK1 binding and inhibition of IFN-β induction, while the kinase domain of TBK1 was sufficient for ORF11 binding. Taken together, these results suggest a mechanism underlying inhibition of IFN-β production by a gammaherpesvirus and highlight the importance of TBK1 in DNA virus replication.

IMPORTANCE

Gammaherpesviruses are important human pathogens, as they are associated with various kinds of tumors. Upon virus infection, the type I interferon pathway is activated by a series of signaling molecules and stimulates antiviral gene expression. To subvert such interferon antiviral responses, viruses are equipped with multiple factors that can inhibit its critical steps. In this study, we took an unbiased genomic approach using a mutant library of murine gammaherpesvirus 68 to screen a novel viral immune modulator that negatively regulates the type I interferon pathway and identified ORF11 as a strong candidate. ORF11-deficient virus infection produced more interferon than the wild type in both fibroblasts and macrophages. During virus replication, ORF11 directly bound to TBK1, a key regulatory protein in the interferon pathway, and inhibited TBK1-mediated interferon production. Our results highlight a crucial role of TBK1 in controlling DNA virus infection and a viral strategy to curtail host surveillance.

Is murine gammaherpesvirus-68 (MHV-68) a suitable immunotoxicological model for examining immunomodulatory drug-associated viral recrudescence?

Immunosuppressive agents are used for treatment of a variety of autoimmune diseases including rheumatoid arthritis (RA), systemic lupus erythematosis (SLE), and psoriasis, as well as for prevention of tissue rejection after organ transplantation. Recrudescence of herpesvirus infections, and increased risk of carcinogenesis from herpesvirus-associated tumors are related with immunosuppressive therapy in humans. Post-transplant lymphoproliferative disorder (PTLD), a condition characterized by development of Epstein Barr Virus (EBV)-associated B-lymphocyte lymphoma, and Kaposi's Sarcoma (KS), a dermal tumor associated with Kaposi Sarcoma-associated virus (KSHV), may develop in solid organ transplant patients. KS also occurs in immunosuppressed Acquired Immunodeficiency (AIDS) patients. Kaposi Sarcoma-associated virus (KSHV) is a herpes virus genetically related to EBV. Murine gammaherpes-virus-68 (MHV-68) is proposed as a mouse model of gammaherpesvirus infection and recrudescence and may potentially have relevance for herpesvirus-associated neoplasia. The pathogenesis of MHV-68 infection in mice mimics EBV/KSHV infection in humans with acute lytic viral replication followed by dissemination and establishment of persistent latency. MHV-68-infected mice may develop lymphoproliferative disease that is accelerated by disruption of the immune system. This manuscript first presents an overview of gammaherpesvirus pathogenesis and immunology as well as factors involved in viral recrudescence. A description of different types of immunodeficiency then follows, with particular focus on viral association with lymphomagenesis after immunosuppression. Finally, this review discusses different gammaherpesvirus animal models and describes a proposed MHV-68 model to further examine the interplay of immunomodulatory agents and gammaherpesvirus-associated neoplasia.

Murine gammaherpesvirus-68 ORF38 encodes a tegument protein and is packaged into virions during secondary envelopment

Tegument is the unique structure of a herpesvirion which occupies the space between nucleocapsid and envelope. Accumulating data have indicated that interactions among tegument proteins play a key role in virion morphogenesis. Morphogenesis of gammaherpesviruses including Kaposi’s sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) is poorly understood due to the lack of efficient de novo lytic replication in cell culture. Murine gammaherpesvirus-68 (MHV-68) is genetically related to these two human herpesviruses and serves as an effective model to study the lytic replication of gammaherpesviruses. We previously showed that ORF33 of MHV-68 encodes a tegument protein and plays an essential role in virion maturation in the cytoplasm. However, the molecular mechanism of how ORF33 participates in virion morphogenesis has not been elucidated. In this study we demonstrated that ORF38 of MHV-68 is also a tegument protein and is localized to cytoplasmic compartments during both transient transfection and viral infection. Immuno-gold labeling assay showed that ORF38 is only present on virions that have entered the cytoplasmic vesicles, indicating that ORF38 is packaged into virions during secondary envelopment. We further showed that ORF38 co-localizes with ORF33 during viral infection; therefore, the interaction between ORF38 and ORF33 is conserved among herpesviruses. Notably, we found that although ORF33 by itself is distributed in both the nucleus and the cytoplasm, in the presence of ORF38, ORF33 is co-localized to trans-Golgi network (TGN), a site where secondary envelopment takes place.

Primary macrophages rely on histone deacetylase 1 and 2 expression to induce type I interferon in response to gammaherpesvirus infection

Type I interferon is induced shortly following viral infection and represents a first line of host defense against a majority of viral pathogens. Not surprisingly, both replication and latency of gammaherpesviruses, ubiquitous cancer-associated pathogens, are attenuated by type I interferon, although the mechanism of attenuation remains poorly characterized. Gammaherpesviruses also target histone deacetylases (HDACs), a family of pleiotropic enzymes that modify gene expression and several cell signaling pathways. Specifically, we have previously shown that a conserved gammaherpesvirus protein kinase interacts with HDAC1 and -2 to promote gammaherpesvirus replication in primary macrophages. In the current study, we have used genetic approaches to show that expression of HDAC1 and -2 is critical for induction of a type I interferon response following gammaherpesvirus infection of primary macrophages. Specifically, expression of HDAC1 and -2 was required for phosphorylation of interferon regulatory factor 3 (IRF3) and accumulation of IRF3 at the beta interferon promoter in gammaherpesvirus-infected primary macrophages. To our knowledge, this is the first demonstration of a specific role for HDAC1 and -2 in the induction of type I interferon responses in primary immune cells following virus infection. Furthermore, because HDAC1 and -2 are overexpressed in several types of cancer, our findings illuminate potential side effects of HDAC1- and -2-specific inhibitors that are currently under development as cancer therapy agents. IMPORTANCE Gammaherpesviruses establish chronic infection in a majority of the adult population and are associated with several malignancies. Infected cells counteract gammaherpesvirus infection via innate immune signaling mediated primarily through type I interferon. The induction of type I interferon expression proceeds through several stages using molecular mechanisms that are still incompletely characterized. In this study, we show that expression of HDAC1 and -2 by macrophages is required to mount a type I interferon response to incoming gammaherpesvirus. The involvement of HDAC1 and -2 in the type I interferon response highlights the pleiotropic roles of these enzymes in cellular signaling. Interestingly, HDAC1 and -2 are deregulated in cancer and are attractive targets of new cancer therapies. Due to the ubiquitous and chronic nature of gammaherpesvirus infection, the role of HDAC1 and -2 in the induction of type I interferon responses should be considered during the clinical development of HDAC1- and -2-specific inhibitors.

Mouse gammaherpesvirus-68 infection acts as a rheostat to set the level of type I interferon signaling in primary macrophages

Type I interferon (IFN) is a critical antiviral response of the host. We found that Interferon Regulatory Factor 3 (IRF-3) was responsible for induction of type I IFN following mouse gammaherpesvirus-68 (MHV68) infection of primary macrophages. Intriguingly, type I IFN signaling was maintained throughout the entire MHV68 replication cycle, in spite of several known viral IFN antagonists. However, MHV68-infected primary macrophages displayed attenuated responses to exogenous type I IFN, suggesting that MHV68 controls the level of type I IFN signaling that is allowed to occur during replication. Type I IFN receptor and IRF-3 were necessary to attenuate transcription of MHV68 RTA, an immediate early gene critical for replication. Furthermore, higher constitutive activity of RTA promoters was observed in the absence of type I IFN signaling. Our study suggests that MHV68 has preserved the ability to sense type I IFN status of the host in order to limit lytic replication.

A conserved gammaherpesvirus protein kinase targets histone deacetylases 1 and 2 to facilitate viral replication in primary macrophages

Gammaherpesviruses are ubiquitious pathogens that establish lifelong infection and are associated with several malignancies. All gammaherpesviruses encode a conserved protein kinase that facilitates viral replication and chronic infection and thus represents an attractive therapeutic target. In this study, we identify a novel function of gammaherpesvirus protein kinase as a regulator of class I histone deacetylases (HDAC). Mouse gammaherpesvirus 68 (MHV68)-encoded protein kinase orf36 interacted with HDAC1 and 2 and prevented association of these HDACs with the viral promoter driving expression of RTA, a critical immediate early transcriptional activator. Furthermore, the ability to interact with HDAC1 and 2 was not limited to the MHV68 orf36, as BGLF4, a related viral protein kinase encoded by Epstein-Barr virus, interacted with HDAC1 in vitro. Importantly, targeting of HDAC1 and 2 by orf36 was independent of the kinase's enzymatic activity. Additionally, orf36 expression, but not its enzymatic activity, induced changes in the global deacetylase activity observed in infected primary macrophages. Combined deficiency of HDAC1 and 2 rescued attenuated replication and viral DNA synthesis of the orf36 null MHV68 mutant, indicating that the regulation of HDAC1 and 2 by orf36 was relevant for viral replication. Understanding the mechanism by which orf36 facilitates viral replication, including through HDAC targeting, will facilitate the development of improved therapeutics against gammaherpesvirus kinases.

ZAP inhibits murine gammaherpesvirus 68 ORF64 expression and is antagonized by RTA

Zinc finger antiviral protein (ZAP) is an interferon-inducible host antiviral factor that specifically inhibits the replication of certain viruses, including HIV-1 and Ebola virus. ZAP functions as a dimer formed through intermolecular interactions of its N-terminal tails. ZAP binds directly to specific viral mRNAs and inhibits their expression by repressing translation and/or promoting degradation of the target mRNA. ZAP is not a universal antiviral factor, since some viruses grow normally in ZAP-expressing cells. It is not fully understood what determines whether a virus is susceptible to ZAP. We explored the interaction between ZAP and murine gammaherpesvirus 68 (MHV-68), whose life cycle has latent and lytic phases. We previously reported that ZAP inhibits the expression of M2, which is expressed mainly in the latent phase, and regulates MHV-68 latency in cultured cells. Here, we report that ZAP inhibits the expression of ORF64, a tegument protein that is expressed in the lytic phase and is essential for lytic replication. MHV-68 infection induced ZAP expression. However, ZAP did not inhibit lytic replication of MHV-68. We provide evidence showing that the antiviral activity of ZAP is antagonized by MHV-68 RTA, a critical viral transactivator expressed in the lytic phase. We further show that RTA inhibits the antiviral activity of ZAP by disrupting the N-terminal intermolecular interaction of ZAP. Our results provide an example of how a virus can escape ZAP-mediated immunity.

Vaccine prospect of Kaposi sarcoma-associated herpesvirus

Infection of Kaposi sarcoma-associated herpesvirus (KSHV) or human herpesvirus-8 (HHV-8) is estimated to account for 34,000 new cancer cases globally. Unlike other herpesviruses, KSHV is not ubiquitous but is highly prevalent in some areas, such as sub-Saharan Africa where Kaposi sarcoma is the leading cancer among adults. While latent infection of KSHV plays a major and direct role in tumorigenesis, viral lytic replication also makes significant contributions to this process. Efforts to develop a KSHV vaccine are limited, but studies with EBV have provided important lessons. Informative vaccine research has been conducted in the mouse infection model of a closely related rodent virus, murine gammaherpesvirus-68 (MHV-68 or γHV-68). This mouse model has generated fundamental principles for an effective vaccination strategy. KSHV vaccines designed to prevent a naïve host from infection and to boost the immune control of KSHV in persistently infected people will have major impact on individuals who are at a high risk of developing KSHV-associated diseases.

Systematic identification of type I and type II interferon-induced antiviral factors

Type I and type II interferons (IFNs) are cytokines that establish the cellular antiviral state through the induction of IFN-stimulated genes (ISGs). We sought to understand the basis of the antiviral activity induced by type I and II IFNs in relation to the functions of their ISGs. Based on gene expression studies, we systematically identified antiviral ISGs by performing blinded, functional screens on 288 type I and type II ISGs. We assessed and validated the antiviral activity of these ISGs against an RNA virus, vesicular stomatitis virus (VSV), and a DNA virus, murine gammaherpes virus (MHV-68). Overall, we identified 34 ISGs that elicited an antiviral effect on the replication of either one or both viruses. Fourteen ISGs have uncharacterized antiviral functions. We further defined ISGs that affect critical life-cycle processes in expression of VSV protein and MHV-68 immediate-early genes. Two previously undescribed antiviral ISGs, TAP1 and BMP2, were further validated. TAP1-deficient fibroblasts were more susceptible to VSV infection but less so to MHV-68 infection. On the other hand, exogenous BMP2 inhibits MHV-68 lytic growth but did not affect VSV growth. These results delineate common and distinct sets of type I and type II IFN-induced genes as well as identify unique ISGs that have either broad or specific antiviral effects on these viruses.

Tiled microarray identification of novel viral transcript structures and distinct transcriptional profiles during two modes of productive murine gammaherpesvirus 68 infection

We applied a custom tiled microarray to examine murine gammaherpesvirus 68 (MHV68) polyadenylated transcript expression in a time course of de novo infection of fibroblast cells and following phorbol ester-mediated reactivation from a latently infected B cell line. During de novo infection, all open reading frames (ORFs) were transcribed and clustered into four major temporal groups that were overlapping yet distinct from clusters based on the phorbol ester-stimulated B cell reactivation time course. High-density transcript analysis at 2-h intervals during de novo infection mapped gene boundaries with a 20-nucleotide resolution, including a previously undefined ORF73 transcript and the MHV68 ORF63 homolog of Kaposi's sarcoma-associated herpesvirus vNLRP1. ORF6 transcript initiation was mapped by tiled array and confirmed by 5' rapid amplification of cDNA ends. The ∼1.3-kb region upstream of ORF6 was responsive to lytic infection and MHV68 RTA, identifying a novel RTA-responsive promoter. Transcription in intergenic regions consistent with the previously defined expressed genomic regions was detected during both types of productive infection. We conclude that the MHV68 transcriptome is dynamic and distinct during de novo fibroblast infection and upon phorbol ester-stimulated B cell reactivation, highlighting the need to evaluate further transcript structure and the context-dependent molecular events that govern viral gene expression during chronic infection.

Distinct domains in ORF52 tegument protein mediate essential functions in murine gammaherpesvirus 68 virion tegumentation and secondary envelopment

Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus are etiologically associated with several types of human malignancies. However, as these two human gammaherpesviruses do not replicate efficiently in cultured cells, the morphogenesis of gammaherpesvirus virions is poorly understood. Murine gammaherpesvirus 68 (MHV-68) provides a tractable model to define common, conserved features of gammaherpesvirus biology. ORF52 of MHV-68 is conserved among gammaherpesviruses. We have previously shown that this tegument protein is essential for the envelopment and egress of viral particles and solved the crystal structure of ORF52 dimers. To more closely examine its role in virion maturation, we performed immunoelectron microscopy of MHV-68-infected cells and found that ORF52 localized to both mature, extracellular virions and immature viral particles in the cytoplasm. ORF52 consists of three α-helices followed by one β-strand. To understand the structural requirements for ORF52 function, we constructed mutants of ORF52 and examined their ability to complement an ORF52-null MHV-68 virus. Mutations in conserved residues in the N-terminal α1-helix and C terminus, or deletion of the α2-helix, resulted in a loss-of-function phenotype. Furthermore, the α1-helix was crucial for the predominantly punctate cytoplasmic localization of ORF52, while the α2-helix was a key domain for ORF52 dimerization. Immunoprecipitation experiments demonstrated that ORF52 interacts with another MHV-68 tegument protein, ORF42; however, a single point mutation in R95 in the C terminus of ORF52 led to the loss of this interaction. Moreover, the homologues of MHV-68 ORF52 in Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus complement the defect in ORF52-null MHV-68 and interact with MHV-68 ORF52. Taken together, these data uncover the relationship between the α-helical structure and the molecular basis for ORF52 function. This is the first structure-based functional domain mapping study for an essential gammaherpesvirus tegument protein.

The virion-associated open reading frame 49 of murine gammaherpesvirus 68 promotes viral replication both in vitro and in vivo as a derepressor of RTA

Replication and transcription activator (RTA), an immediate-early gene, is a key molecular switch to evoke lytic replication of gammaherpesviruses. Open reading frame 49 (ORF49) is conserved among gammaherpesviruses and shown to cooperate with RTA in regulating virus lytic replication. Here we show a molecular mechanism and in vivo functions of murine gammaherpesvirus 68 (MHV-68 or γHV-68) ORF49. MHV-68 ORF49 was transcribed and translated as a late gene. The ORF49 protein was associated with a virion, interacting with the ORF64 large tegument protein and the ORF25 capsid protein. Moreover, ORF49 directly bound to RTA and its negative cellular regulator, poly(ADP-ribose) polymerase-1 (PARP-1), and disrupted the interactions of RTA and PARP-1. Productive replication of an ORF49-deficient mutant virus (49S) was attenuated in vivo as well as in vitro. Likewise, latent infection was also impaired in the spleen of 49S-infected mice. Taken together, our results suggest that the virion-associated ORF49 protein may promote virus replication both in vitro and in vivo by providing an optimal environment in the early phase of virus infection as a derepressor of RTA.

Gammaherpesvirus gene expression and DNA synthesis are facilitated by viral protein kinase and histone variant H2AX

Gammaherpesvirus protein kinases are an attractive therapeutic target as they support lytic replication and latency. Via an unknown mechanism these kinases enhance expression of select viral genes and DNA synthesis. Importantly, the kinase phenotypes have not been examined in primary cell types. Mouse gammaherpesvirus-68 (MHV68) protein kinase orf36 activates the DNA damage response (DDR) and facilitates lytic replication in primary macrophages. Significantly, H2AX, a DDR component and putative orf36 substrate, enhances MHV68 replication. Here we report that orf36 facilitated expression of RTA, an immediate early MHV68 gene, and DNA synthesis during de novo infection of primary macrophages. H2AX expression supported efficient RTA transcription and phosphorylated H2AX associated with RTA promoter. Furthermore, viral DNA synthesis was attenuated in H2AX-deficient macrophages, suggesting that the DDR system was exploited throughout the replication cycle. The interactions between a cancer-associated gammaherpesvirus and host tumor suppressor system have important implications for the pathogenesis of gammaherpesvirus infection.

Prospects of a novel vaccination strategy for human gamma-herpesviruses

Due to the oncogenic potential associated with persistent infection of human gamma-herpesviruses, including Epstein-Barr virus (EBV or HHV-4) and Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8), vaccine development has focused on subunit vaccines. However, the results using an animal model of mouse infection with a related rodent virus, murine gamma-herpesvirus 68 (MHV-68, γHV-68, or MuHV-4), have shown that the only effective vaccination strategy is based on live attenuated viruses, including viruses engineered to be incapable of establishing persistence. Vaccination with a virus lacking persistence would eliminate many potential complications. Progress in understanding persistent infections of EBV and KSHV raises the possibility of engineering a live attenuated virus without persistence. Therefore, we should keep the option open for developing a live EBV or KSHV vaccine.

In vivo activation of toll-like receptor-9 induces an age-dependent abortive lytic cycle reactivation of murine gammaherpesvirus-68

Infection of mice with murine gammaherpesvirus-68 (γHV-68) serves as a model to understand the pathogenesis of persistent viral infections, including the potential for co-infections to modulate viral latency. We have previously found that infection of neonates (8-day-old mice) with γHV-68 resulted in a high level of persistence of the virus in the lungs as well as the spleen, in contrast to infection of adult mice, for which long-term latency was only readily detected in the spleen. In this study we investigated whether stimulation of toll-like receptor (TLR)9 would modulate viral latency in mice infected with γHV-68 in an age-dependent manner. Pups and adult mice were injected with the synthetic TLR9 ligand CpG ODN at 30 dpi, at which time long-term latency has been established. Three days after CpG injection, the lungs and spleens were removed, and a limiting dilution assay was done to determine the frequency of latently infected cells. RNA was extracted to measure viral transcripts using a ribonuclease protection assay. We observed that CpG injection resulted in an increase in the frequency of latently-infected cells in both the lungs and spleens of infected pups, but only in the spleens of infected adult mice. No preformed virus was detected, suggesting that TLR9 stimulation did not trigger complete viral reactivation. When we examined viral gene expression in these same tissues, we observed expression only of the immediate early lytic genes, rta and K3, but not the early DNA polymerase gene or late gB transcript indicative of an abortive reactivation in the spleen. Additionally, mice infected as pups had greater numbers of germinal center B cells in the spleen following CpG injection, whereas CpG stimulated the expansion of follicular zone B cells in adult mice. These data suggest that stimulation of TLR9 differentially modulates gammaherpesvirus latency via an age-dependent mechanism.

Construction and characterization of an infectious murine gammaherpesivrus-68 bacterial artificial chromosome

Here we describe the cloning of a sequenced WUMS isolate of murine gammaherpesvirus-68 (MHV-68, γHV-68, also known as MuHV-4) as a bacterial artificial chromosome (BAC). We engineered the insertion of the BAC sequence flanked by loxP sites into the left end of the viral genome before the M1 open reading frame. The infectious viruses were reconstituted following transfection of the MHV-68 BAC DNA into cells. The MHV-68 BAC-derived virus replicated indistinguishably from the wild-type virus in cultured cells. Excision of the BAC insert was efficiently achieved by coexpressing the Cre recombinase. Although the BAC insertion did not significantly affect acute productive infection in the lung, it severely compromised the ability of MHV-68 to establish splenic latency. Removal of the BAC sequence restored the wild-type level of latency. Site-specific mutagenesis was carried out by RecA-mediated recombination to demonstrate that this infectious BAC clone can be used for genetic studies of MHV-68.

Histone deacetylases and the nuclear receptor corepressor regulate lytic-latent switch gene 50 in murine gammaherpesvirus 68-infected macrophages

Gammaherpesviruses are important oncogenic pathogens that transit between lytic and latent life cycles. Silencing the lytic gene expression program enables the establishment of latency and a lifelong chronic infection of the host. In murine gammaherpesvirus 68 (MHV68, γHV68), essential lytic switch gene 50 controls the interchange between lytic and latent gene expression programs. However, negative regulators of gene 50 expression remain largely undefined. We report that the MHV68 lytic cycle is silenced in infected macrophages but not fibroblasts and that histone deacetylases (HDACs) mediate silencing. The HDAC inhibitor trichostatin A (TSA) acts on the gene 50 promoter to induce lytic replication of MHV68. HDAC3, HDAC4, and the nuclear receptor corepressor (NCoR) are required for efficient silencing of gene 50 expression. NCoR is critical for transcriptional repression of cellular genes by unliganded nuclear receptors. Retinoic acid, a known ligand for the NCoR complex, derepresses gene 50 expression and enhances MHV68 lytic replication. Moreover, HDAC3, HDAC4, and NCoR act on the gene 50 promoter and are recruited to this promoter in a retinoic acid-responsive manner. We provide the first example of NCoR-mediated, HDAC-dependent regulation of viral gene expression.

A gammaherpesvirus complement regulatory protein promotes initiation of infection by activation of protein kinase Akt/PKB

Background

Viruses have evolved to evade the host's complement system. The open reading frames 4 (ORF4) of gammaherpesviruses encode homologs of regulators of complement activation (RCA) proteins, which inhibit complement activation at the level of C3 and C4 deposition. Besides complement regulation, these proteins are involved in heparan sulfate and glycosaminoglycan binding, and in case of MHV-68, also in viral DNA synthesis in macrophages.

Methodology/Principal Findings

Here, we made use of MHV-68 to study the role of ORF4 during infection of fibroblasts. While attachment and penetration of virions lacking the RCA protein were not affected, we observed a delayed delivery of the viral genome to the nucleus of infected cells. Analysis of the phosphorylation status of a variety of kinases revealed a significant reduction in phosphorylation of the protein kinase Akt in cells infected with ORF4 mutant virus, when compared to cells infected with wt virus. Consistent with a role of Akt activation in initial stages of infection, inhibition of Akt signaling in wt virus infected cells resulted in a phenotype resembling the phenotype of the ORF4 mutant virus, and activation of Akt by addition of insulin partially reversed the phenotype of the ORF4 mutant virus. Importantly, the homologous ORF4 of KSHV was able to rescue the phenotype of the MHV-68 ORF4 mutant, indicating that ORF4 is functionally conserved and that ORF4 of KSHV might have a similar function in infection initiation.

Conclusions/Significance

In summary, our studies demonstrate that ORF4 contributes to efficient infection by activation of the protein kinase Akt and thus reveal a novel function of a gammaherpesvirus RCA protein.

An in vitro system for studying murid herpesvirus-4 latency and reactivation

The narrow species tropisms of Epstein-Barr Virus (EBV) and the Kaposi's Sarcoma -associated Herpesvirus (KSHV) have made Murid Herpesvirus-4 (MuHV-4) an important tool for understanding how gammaherpesviruses colonize their hosts. However, while MuHV-4 pathogenesis studies can assign a quantitative importance to individual genes, the complexity of in vivo infection can make the underlying mechanisms hard to discern. Furthermore, the lack of good in vitro MuHV-4 latency/reactivation systems with which to dissect mechanisms at the cellular level has made some parallels with EBV and KSHV hard to draw. Here we achieved control of the MuHV-4 lytic/latent switch in vitro by modifying the 5′ untranslated region of its major lytic transactivator gene, ORF50. We terminated normal ORF50 transcripts by inserting a polyadenylation signal and transcribed ORF50 instead from a down-stream, doxycycline-inducible promoter. In this way we could establish fibroblast clones that maintained latent MuHV-4 episomes without detectable lytic replication. Productive virus reactivation was then induced with doxycycline. We used this system to show that the MuHV-4 K3 gene plays a significant role in protecting reactivating cells against CD8⁺ T cell recognition.

The de novo methyltransferases DNMT3a and DNMT3b target the murine gammaherpesvirus immediate-early gene 50 promoter during establishment of latency

The role of epigenetic modifications in the regulation of gammaherpesvirus latency has been a subject of active study for more than 20 years. DNA methylation, associated with transcriptional silencing in mammalian genomes, has been shown to be an important mechanism in the transcriptional control of several key gammaherpesvirus genes. In particular, DNA methylation of the functionally conserved immediate-early replication and transcription activator (RTA) has been shown to regulate Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus Rta expression. Here we demonstrate that the murine gammaherpesvirus (MHV68) homolog, encoded by gene 50, is also subject to direct repression by DNA methylation, both in vitro and in vivo. We observed that the treatment of latently MHV68-infected B-cell lines with a methyltransferase inhibitor induced virus reactivation. In addition, we show that the methylation of the recently characterized distal gene 50 promoter represses activity in a murine macrophage cell line. To evaluate the role of de novo methyltransferases (DNMTs) in the establishment of these methylation marks, we infected mice in which conditional DNMT3a and DNMT3b alleles were selectively deleted in B lymphocytes. DNMT3a/DNMT3b-deficient B cells were phenotypically normal, displaying no obvious compromise in cell surface marker expression or antibody production either in naïve mice or in the context of nonviral and viral immunogens. However, mice lacking functional DNMT3a and DNMT3b in B cells exhibited hallmarks of deregulated MHV68 lytic replication, including increased splenomegaly and the presence of infectious virus in the spleen at day 18 following infection. In addition, total gene 50 transcript levels were elevated in the spleens of these mice at day 18, which correlated with the hypomethylation of the distal gene 50 promoter. However, by day 42 postinfection, aberrant virus replication was resolved, and we observed wild-type frequencies of viral genome-positive splenocytes in mice lacking functional DNMT3a and DNMT3b in B lymphocytes. The latter correlated with increased CpG methylation in the distal gene 50 promoter, which was restored to levels similar to those of littermate controls harboring functional DNMT3a and DNMT3b alleles in B lymphocytes, suggesting the existence of an alternative mechanism for the de novo methylation of the MHV68 genome. Importantly, this DNMT3a/DNMT3b-independent methylation appeared to be targeted specifically to the gene 50 promoter, as we observed that the promoters for MHV68 gene 72 (v-cyclin) and M11 (v-bcl2) remained hypomethylated at day 42 postinfection. Taken together, these data provide the first evidence of the importance of DNA methylation in regulating gammaherpesvirus RTA/gene 50 transcription during virus infection in vivo and provide insight into the hierarchy of host machinery required to establish this modification.

Murine gammaherpesvirus 68 has evolved gamma interferon and stat1-repressible promoters for the lytic switch gene 50

Cytokines regulate viral gene expression with important consequences for viral replication and pathogenesis. Gamma interferon (IFN-gamma) is a key regulator of chronic murine gammaherpesvirus 68 (gammaHV68) infection and a potent inhibitor of gammaHV68 reactivation from latency. Macrophages are the cell type that is responsive to the IFN-gamma-mediated control of gammaHV68 reactivation; however, the molecular mechanism of this IFN-gamma action is undefined. Here we report that IFN-gamma inhibits lytic replication of gammaHV68 in primary bone marrow-derived macrophages and decreases transcript levels for the essential lytic switch gene 50. Interestingly, IFN-gamma suppresses the activity of the two known gene 50 promoters, demonstrating that an inflammatory cytokine can directly regulate the promoters for the gammaHV68 lytic switch gene. Stat1, but not IFN-alpha/beta signaling, is required for IFN-gamma action. Moreover, Stat1 deficiency increases basal gammaHV68 replication, gene 50 expression, and promoter activity. Together, these data identify IFN-gamma and Stat1 as being negative regulators of the gammaHV68 lytic cycle and raise the possibility that gammaHV68 maintains IFN-gamma/Stat1-responsive gene 50 promoters to facilitate cell-extrinsic control over the interchange between the lytic and latent cycles.

Induction of protective immunity against murine gammaherpesvirus 68 infection in the absence of viral latency

Human gammaherpesviruses, Epstein-Barr virus, and human herpesvirus 8/Kaposi's sarcoma-associated herpesvirus are important pathogens associated with diseases, including lymphomas and other malignancies. Murine gammaherpesvirus 68 (MHV-68) is used as an experimental model system to study the host immune control of infection and explore novel vaccine strategies based on latency-deficient live viruses. We studied the properties and the potential of a recombinant MHV-68 (AC-RTA) in which the genes required for persistent infection were replaced by a constitutively expressed viral transcription activator, RTA, which dictates the virus to lytic replication. After intranasal infection of mice, replication of AC-RTA in the lung was attenuated, and no AC-RTA virus or viral DNA was detected in the isolated splenocytes, indicating a lack of latency in the spleen. Infection of the AC-RTA virus elicited both cellular immune responses and virus-specific IgG at a level comparable to that elicited by infection of the wild-type virus. Importantly, vaccination of AC-RTA was able to protect mice against subsequent challenge by the wild-type MHV-68. AC-RTA provides a vaccine strategy for preventing infection of human gammaherpesviruses. Furthermore, our results suggest that immunity to the major latent antigens is not required for protection.