The anti-interferon activity of conserved viral dUTPase ORF54 is essential for an effective MHV-68 infection

Multifaceted roles for STAT3 in gammaherpesvirus latency revealed through in vivo B cell knockout models

Cancers associated with the oncogenic gammaherpesviruses, Epstein-Barr virus and Kaposi sarcoma herpesvirus, are notable for their constitutive activation of the transcription factor signal transducer and activator of transcription 3 (STAT3). To better understand the role of STAT3 during gammaherpesvirus latency and the B cell response to infection, we used the model pathogen murine gammaherpesvirus 68 (MHV68). Genetic deletion of STAT3 in B cells of CD19cre/+Stat3f/f mice reduced peak MHV68 latency approximately sevenfold. However, infected CD19cre/+Stat3f/f mice exhibited disordered germinal centers and heightened virus-specific CD8 T cell responses compared to wild-type (WT) littermates. To circumvent the systemic immune alterations observed in the B cell-STAT3 knockout mice and more directly evaluate intrinsic roles for STAT3, we generated mixed bone marrow chimeric mice consisting of WT and STAT3 knockout B cells. We discovered a dramatic reduction in latency in STAT3 knockout B cells compared to their WT B cell counterparts in the same lymphoid organ. RNA sequencing of sorted germinal center B cells revealed that MHV68 infection shifts the gene signature toward proliferation and away from type I and type II IFN responses. Loss of STAT3 largely reversed the virus-driven transcriptional shift without impacting the viral gene expression program. STAT3 promoted B cell processes of the germinal center, including IL-21-stimulated downregulation of surface CD23 on B cells infected with MHV68 or EBV. Together, our data provide mechanistic insights into the role of STAT3 as a latency determinant in B cells for oncogenic gammaherpesviruses.IMPORTANCEThere are no directed therapies to the latency program of the human gammaherpesviruses, Epstein-Barr virus and Kaposi sarcoma herpesvirus. Activated host factor signal transducer and activator of transcription 3 (STAT3) is a hallmark of cancers caused by these viruses. We applied the murine gammaherpesvirus pathogen system to explore STAT3 function upon primary B cell infection in the host. Since STAT3 deletion in all CD19+ B cells of infected mice led to altered B and T cell responses, we generated chimeric mice with both normal and STAT3-deleted B cells. B cells lacking STAT3 failed to support virus latency compared to normal B cells from the same infected animal. Loss of STAT3 impaired B cell proliferation and differentiation and led to a striking upregulation of interferon-stimulated genes. These findings expand our understanding of STAT3-dependent processes that are key to its function as a pro-viral latency determinant for oncogenic gammaherpesviruses in B cells and may provide novel therapeutic targets.

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.

B cell-intrinsic STAT3-mediated support of latency and interferon suppression during murine gammaherpesvirus 68 infection revealed through an in vivo competition model

Cancers associated with the oncogenic gammaherpesviruses, Epstein-Barr virus and Kaposi sarcoma herpesvirus, are notable for their constitutive activation of the transcription factor STAT3. To better understand the role of STAT3 during gammaherpesvirus latency and immune control, we utilized murine gammaherpesvirus 68 (MHV68) infection. Genetic deletion of STAT3 in B cells of CD19cre/+Stat3f/f mice reduced peak latency approximately 7-fold. However, infected CD19cre/+Stat3f/f mice exhibited disordered germinal centers and heightened virus-specific CD8 T cell responses compared to WT littermates. To circumvent the systemic immune alterations observed in the B cell-STAT3 knockout mice and more directly evaluate intrinsic roles for STAT3, we generated mixed bone marrow chimeras consisting of WT and STAT3-knockout B cells. Using a competitive model of infection, we discovered a dramatic reduction in latency in STAT3-knockout B cells compared to their WT B cell counterparts in the same lymphoid organ. RNA sequencing of sorted germinal center B cells revealed that STAT3 promotes proliferation and B cell processes of the germinal center but does not directly regulate viral gene expression. Last, this analysis uncovered a STAT3-dependent role for dampening type I IFN responses in newly infected B cells. Together, our data provide mechanistic insight into the role of STAT3 as a latency determinant in B cells for oncogenic gammaherpesviruses.

Targeting Epstein-Barr virus dUTPase, an immunomodulatory protein using anti-viral, anti-inflammatory and neuroprotective phytochemicals

Although primary infection of Epstein-Barr virus is generally non-lethal, viral reactivation is often associated with fatal outcomes. Regardless, there is no FDA-approved treatment available for this viral infection. The current investigation targets viral maintenance and reactivation by inhibiting the functioning of viral deoxyuridine-triphosphatase (dUTPase) using phytochemicals. The EBV-dUTPase is essential for the maintenance of nucleotide balance and thus, plays a vital role in the viral replication cycle. Additionally, the protein has shown neuroinflammatory effects on the host. To selectively target the protein and possibly alter its activity, we utilized a virtual screening approach and screened 45 phytochemicals reported to have antiviral, anti-inflammatory, and neuroprotective properties. The analysis revealed several phytochemicals bound to the target protein with high affinity. In-silico ADMET and Lipinski's rule analysis predicted favorable druggability of Dehydroevodiamine (DHE) among all the phytochemicals. Further, we corroborated our findings by molecular dynamic simulation and binding affinity estimation. Our outcomes ascertained a stable binding of DHE to EBV-dUTPase primarily through electrostatic interactions. We identified that the protein-ligand binding involves the region around His71, previously reported as a potent drug target site. Conclusively, the phytochemical DHE showed a promising future as a drug development candidate against EBV-dUTPase.

Viral dUTPases: Modulators of Innate Immunity

Most free-living organisms encode for a deoxyuridine triphosphate nucleotidohydrolase (dUTPase; EC 3.6.1.23). dUTPases represent a family of metalloenzymes that catalyze the hydrolysis of dUTP to dUMP and pyrophosphate, preventing dUTP from being incorporated into DNA by DNA polymerases, maintaining a low dUTP/dTTP pool ratio and providing a necessary precursor for dTTP biosynthesis. Thus, dUTPases are involved in maintaining genomic integrity by preventing the uracilation of DNA. Many DNA-containing viruses, which infect mammals also encode for a dUTPase. This review will summarize studies demonstrating that, in addition to their classical enzymatic activity, some dUTPases possess novel functions that modulate the host innate immune response.

Evasion of I Interferon-Mediated Innate Immunity by Pseudorabies Virus

Type I interferon (IFN-I) mediated innate immunity serves as the first line of host defense against viral infection, ranging from IFN-I production upon viral detection, IFN-I triggered signaling pathway that induces antiviral gene transcription the antiviral effects of IFN-I induced gene products. During coevolution, herpesviruses have developed multiple countermeasures to inhibit the various steps involved to evade the IFN response. This mini-review focuses on the strategies used by the alphaherpesvirus Pseudorabies virus (PRV) to antagonize IFN-I mediated innate immunity, with a particular emphasis on the mechanisms inhibiting IFN-I induced gene transcription through the JAK-STAT pathway. The knowledge obtained from PRV enriches the current understanding of the alphaherpesviral immune evasion mechanisms and provides insight into the vaccine development for PRV control.

The dUTPase of caprine arthritis-encephalitis virus negatively regulates interferon signaling pathway

Background

Deoxyuracil triphosphate nucleotide (dUTP) pyrophosphatase (dUTPase, DU) is an enzyme of caprine arthritis-encephalitis virus (CAEV) that minimizes incorporation of dUTP into the DNA. Caprine arthritis-encephalitis virus relies partly on its ability to escape from innate immunity to cause persistent infections. Interferon β (IFN-β) is an important marker for evaluating the innate immune system, and it has a broad spectrum of antiviral activity.

Aims

This study was conducted to investigate the details of the IFN-β response to CAEV infection.

Methods

The expression of IFN-β and the proliferation of Sendai virus (SeV) and vesicular stomatitis virus (VSV) were determined by real-time quantitative polymerase chain reaction (qPCR). The effect of DU on the IFN signaling pathway was evaluated using luciferase reporter assays.

Results

In our study, the expression of IFN-β was significantly inhibited and the proliferation of SeV and VSV was promoted in cells overexpressing CAEV-DU. DU affected interferon stimulated response element (ISRE) and IFN-β promoter activities induced by RIG-I/MDA5/MAVS/TBK1 pathway, while did not affect them induced by interferon regulatory factor 3 (IRF3-5D).

Conclusion

DU protein downregulated the production of IFN-β by inhibiting the activity of the signal transduction molecules upstream of IRF3, thereby, helping CAEV escape innate immunity. Findings of this work provide an evidence to understand the persistent infection and multiple system inflammation of CAEV.

Conquering the Host: Determinants of Pathogenesis Learned from Murine Gammaherpesvirus 68

Gammaherpesviruses are an important class of oncogenic pathogens that are exquisitely evolved to their respective hosts. As such, the human gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi sarcoma herpesvirus (KSHV) do not naturally infect nonhuman primates or rodents. There is a clear need to fully explore mechanisms of gammaherpesvirus pathogenesis, host control, and immune evasion in the host. A gammaherpesvirus pathogen isolated from murid rodents was first reported in 1980; 40 years later, murine gammaherpesvirus 68 (MHV68, MuHV-4, γHV68) infection of laboratory mice is a well-established pathogenesis system recognized for its utility in applying state-of-the-art approaches to investigate virus-host interactions ranging from the whole host to the individual cell. Here, we highlight recent advancements in our understanding of the processes by which MHV68 colonizes the host and drives disease. Lessons that inform KSHV and EBV pathogenesis and provide future avenues for novel interventions against infection and virus-associated cancers are emphasized.

Activation and Evasion of Innate Immunity by Gammaherpesviruses

Gammaherpesviruses are ubiquitous pathogens that establish lifelong infections in the vast majority of adults worldwide. Importantly, these viruses are associated with numerous malignancies and are responsible for significant human cancer burden. These virus-associated cancers are due, in part, to the ability of gammaherpesviruses to successfully evade the innate immune response throughout the course of infection. In this review, we will summarize the current understanding of how gammaherpesviruses are detected by innate immune sensors, how these viruses evade recognition by host cells, and how this knowledge can inform novel therapeutic approaches for these viruses and their associated diseases.

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.

Regulation of cGAS-Mediated Immune Responses and Immunotherapy

Early detection of infectious nucleic acids released from invading pathogens by the innate immune system is critical for immune defense. Detection of these nucleic acids by host immune sensors and regulation of DNA sensing pathways have been significant interests in the past years. Here, current understandings of evolutionarily conserved DNA sensing cyclic GMP-AMP (cGAMP) synthase (cGAS) are highlighted. Precise activation and tight regulation of cGAS are vital in appropriate innate immune responses, senescence, tumorigenesis and immunotherapy, and autoimmunity. Hence, substantial insights into cytosolic DNA sensing and immunotherapy of indispensable cytosolic sensors have been detailed to extend limited knowledge available thus far. This Review offers a critical, in-depth understanding of cGAS regulation, cytosolic DNA sensing, and currently established therapeutic approaches of essential cytosolic immune agents for improved human health.

The First Structure of an Active Mammalian dCTPase and its Complexes With Substrate Analogs and Products

Precise regulation of dNTPs within the cellular nucleotide pool is essential for high accuracy of DNA replication and is critical for retaining the genomic integrity. Recently, human dCTPase (deoxycytidine triphosphatase), also known as DCTPP1 (human all-alpha dCTP pyrophosphatase 1), has been revealed to be a key player in the balance of pyrimidine nucleotide concentrations within cells, with DCTPP1 deficiency causing DNA damage and genetic instability in both chromosomal and mitochondrial DNA. DCTPP1 also exhibits an additional "house cleaning" function as it has been shown to be highly active against modified cytidine triphosphates, such as 5-methyl-dCTP, which, if incorrectly incorporated into DNA can introduce undesirable epigenetic marking. To date, structural studies of mammalian dCTPase have been limited to inactive constructs, which do not provide information regarding the catalytic mechanism of this important enzyme. We present here the first structures of an active mammalian dCTPase from M. musculus in complex with the nonhydrolyzable substrate analog dCMPNPP and the products 5-Me-dCMP and dCMP. These structures provide clear insights into substrate binding and catalysis and clearly elucidate why previous structures of mammalian dCTPase were catalytically inactive. The overall structure of M. musculus dCTPase is highly similar to enzymes from the all-alpha NTP phosphohydrolase superfamily. Comparison of M. musculus dCTPase with homologs from a diverse range of mammals, including humans, shows that the residues, which contribute to substrate recognition, are entirely conserved, further supporting the importance of this enzyme in the protection of genomic integrity in mammalian cells.

IFN-λ Decreases Murid Herpesvirus-4 Infection of the Olfactory Epithelium but Fails to Prevent Virus Reactivation in the Vaginal Mucosa

Murid herpesvirus-4 (MuHV-4), a natural gammaherpesvirus of rodents, can infect the mouse through the nasal mucosa, where it targets sustentacular cells and olfactory neurons in the olfactory epithelium before it propagates to myeloid cells and then to B cells in lymphoid tissues. After establishment of latency in B cells, viral reactivation occurs in the genital tract in 80% of female mice, which can lead to spontaneous sexual transmission to co-housed males. Interferon-lambda (IFN-λ) is a key player of the innate immune response at mucosal surfaces and is believed to limit the transmission of numerous viruses by acting on epithelial cells. We used in vivo plasmid-mediated IFN-λ expression to assess whether IFN-λ could prophylactically limit MuHV-4 infection in the olfactory and vaginal mucosae. In vitro, IFN-λ decreased MuHV-4 infection in cells that overexpressed IFN-λ receptor 1 (IFNLR1). In vivo, prophylactic IFN-λ expression decreased infection of the olfactory epithelium but did not prevent virus propagation to downstream organs, such as the spleen where the virus establishes latency. In the olfactory epithelium, sustentacular cells readily responded to IFN-λ. In contrast, olfactory neurons did not respond to IFN-λ, thus, likely allowing viral entry. In the female genital tract, columnar epithelial cells strongly responded to IFN-λ, as did most vaginal epithelial cells, although with some variation from mouse to mouse. IFN-λ expression, however, failed to prevent virus reactivation in the vaginal mucosa. In conclusion, IFN-λ decreased MuHV-4 replication in the upper respiratory epithelium, likely by protecting the sustentacular epithelial cells, but it did not protect olfactory neurons and failed to block virus reactivation in the genital mucosa.

Combinatorial Loss of the Enzymatic Activities of Viral Uracil-DNA Glycosylase and Viral dUTPase Impairs Murine Gammaherpesvirus Pathogenesis and Leads to Increased Recombination-Based Deletion in the Viral Genome

Unrepaired uracils in DNA can lead to mutations and compromise genomic stability. Herpesviruses have hijacked host processes of DNA repair and nucleotide metabolism by encoding a viral UNG that excises uracils and a viral dUTPase that initiates conversion of dUTP to dTTP. To better understand the impact of these processes on gammaherpesvirus pathogenesis, we examined the separate and collaborative roles of vUNG and vDUT upon MHV68 infection of mice. Simultaneous disruption of the enzymatic activities of both vUNG and vDUT led to a severe defect in acute replication and establishment of latency, while also revealing a novel, combinatorial function in promoting viral genomic stability. We propose that herpesviruses require these enzymatic processes to protect the viral genome from damage, possibly triggered by misincorporated uracil. This reveals a novel point of therapeutic intervention to potentially block viral replication and reduce the fitness of multiple herpesviruses.

Misincorporation of uracil or spontaneous cytidine deamination is a common mutagenic insult to DNA. Herpesviruses encode a viral uracil-DNA glycosylase (vUNG) and a viral dUTPase (vDUT), each with enzymatic and nonenzymatic functions. However, the coordinated roles of these enzymatic activities in gammaherpesvirus pathogenesis and viral genomic stability have not been defined. In addition, potential compensation by the host UNG has not been examined in vivo. The genetic tractability of the murine gammaherpesvirus 68 (MHV68) system enabled us to delineate the contribution of host and viral factors that prevent uracilated DNA. Recombinant MHV68 lacking vUNG (ORF46.stop) was not further impaired for acute replication in the lungs of UNG^−/− mice compared to wild-type (WT) mice, indicating host UNG does not compensate for the absence of vUNG. Next, we investigated the separate and combinatorial consequences of mutating the catalytic residues of the vUNG (ORF46.CM) and vDUT (ORF54.CM). ORF46.CM was not impaired for replication, while ORF54.CM had a slight transient defect in replication in the lungs. However, disabling both vUNG and vDUT led to a significant defect in acute expansion in the lungs, followed by impaired establishment of latency in the splenic reservoir. Upon serial passage of the ORF46.CM/ORF54.CM mutant in either fibroblasts or the lungs of mice, we noted rapid loss of the nonessential yellow fluorescent protein (YFP) reporter gene from the viral genome, due to recombination at repetitive elements. Taken together, our data indicate that the vUNG and vDUT coordinate to promote viral genomic stability and enable viral expansion prior to colonization of latent reservoirs.

IFITM1 expression is crucial to gammaherpesvirus infection, in vivo

The oncogenic gammaherpesviruses, Epstein–Barr virus (EBV) and Kaposi’s sarcoma herpesvirus (KSHV), are etiologically associated with a variety of human cancers, including Burkitt’s lymphoma (BL), Hodgkin lymphoma (HL), Kaposi’s sarcoma (KS), and primary effusion lymphoma (PEL). Recently, we demonstrated KSHV infection of B- and endothelial cells to significantly upregulate the expression of interferon induced transmembrane protein 1 (IFITM1) which in turn enhances virus entry. This is an extension of the above study. In here, we determined EBV infection of cells to trigger IFITM1 expression, in vitro. Silencing IFITM1 expression using siRNA specifically lowered gammaherpesvirus infection of cells at a post binding stage of entry. A natural model system to explore the effect of IFITM1 on gammaherpesvirus infection in vivo is infection of BALB/c mice with murine gammaherpesvirus 68 (MHV-68). Priming mice with siRNA specific to IFITM1 significantly lowered MHV-68 titers in the lung specimens compared to priming with (NS)siRNA or PBS. MHV-68 titers were monitored by plaque assay and qPCR. Taken together, for the first time, this study provides insight into the critical role of IFITM1 to promoting in vivo gammaherpesvirus infections.

Viral dedication to vigorous destruction of interferon receptors

Interferons (IFNs) exhibit forceful inhibitory activities against numerous viruses by inducing synthesis of anti-viral proteins or promoting immune cell functions, which help eradicate the vicious microbes. Consequently, the degree to which viruses evade or counterattack IFN responses influences viral pathogenicity. Viruses have developed many strategies to interfere with the synthesis of IFNs or IFN receptor signaling pathway. Furthermore, multiple viruses decrease levels of IFN receptors via diverse tactics, which include decreasing type I IFN receptor mRNA expression, blocking post-translational modification of the receptor, and degrading IFN receptors. Recently, influenza virus was found to induce CK1α-induced phosphorylation and subsequent degradation of the receptor for type I and II IFNs. In this review, viral mechanisms that remove IFN receptors are summarized with an emphasis on the mechanisms for virus-induced degradation of IFN receptors.

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.

While viruses have developed various mechanisms to evade immune responses, hosts also have mechanisms to negatively regulate the antiviral signaling pathways to minimize potential damage. In this study, we show that MHV-68, a gamma-herpesvirus, is able to stimulate macrophages to produce the cellular protein SOCS1, which reduces the antiviral effect initiated by IFN-γ, in a cell type specific manner. These findings provide one more example to support the concept that viruses utilize SOCS1 as an immune evasion mechanism. We also show that TLR3-NF-κB signaling is responsible for the induced production of SOCS1. Our finding that TLR3/NF-κB/SOCS1 impedes the action of IFN-γ/STAT1 on RTA might offer a reasonable explanation of how virus-host interaction achieves a balance to facilitate intra-host spreading and transmission.

Murine cytomegalovirus M72 promotes acute virus replication in vivo and is a substrate of the TRiC/CCT complex

Betaherpesvirus dUTPase homologs are core herpesvirus proteins, but little is known about their role during infection. Human cytomegalovirus (HCMV) UL72 and murine cytomegalovirus (MCMV) M72 have been designated dUTPase homologs, and previous studies indicate UL72 is dispensable for replication and enzymatically inactive. Here, we report the initial characterization of MCMV M72. M72 does not possess dUTPase activity, and is expressed as a leaky-late gene product with multiple protein isoforms. Importantly, M72 augments MCMV replication in vitro and during the early stage of acute infection in vivo. We identify and confirm interaction of M72 with the eukaryotic chaperonin tailless complex protein -1 (TCP-1) ring complex (TRiC) or chaperonin containing tailless complex polypeptide 1 (CCT). Accumulating biochemical evidence indicates M72 forms homo-oligomers and is a substrate of TRiC/CCT. Taken together, we provide the first evidence of M72's contribution to viral pathogenesis, and identify a novel interaction with the TRiC/CCT complex.

Feline Herpesvirus 1 US3 Blocks the Type I Interferon Signal Pathway by Targeting Interferon Regulatory Factor 3 Dimerization in a Kinase-Independent Manner

As a prevalent agent in cats, feline herpesvirus 1 (FHV-1) infection contributes to feline respiratory disease and acute and chronic conjunctivitis. FHV-1 can successfully evade the host innate immune response and persist for the lifetime of the cat. Several mechanisms of immune evasion by human herpesviruses have been elucidated, but the mechanism of immune evasion by FHV-1 remains unknown. In this study, we screened for FHV-1 open reading frames (ORFs) responsible for inhibiting the type I interferon (IFN) pathway with an IFN-β promoter reporter and analysis of IFN-β mRNA levels in HEK 293T cells and the Crandell-Reese feline kidney (CRFK) cell line, and we identified the Ser/Thr kinase US3 as the most powerful inhibitor. Furthermore, we found that the anti-IFN activity of US3 depended on its N terminus (amino acids 1 to 75) and was independent of its kinase activity. Mechanistically, the ectopic expression of US3 selectively inhibited IFN regulatory factor 3 (IRF3) promoter activation. Furthermore, US3 bound to the IRF association domain (IAD) of IRF3 and prevented IRF3 dimerization. Finally, US3-deleted recombinant FHV-1 and US3-repaired recombinant FHV-1 (rFHV-dUS3 and rFHV-rUS3, respectively) were constructed. Compared with wild-type FHV-1 and rFHV-rUS3, infection with rFHV-dUS3 induced large amounts of IFN-β in vitro and in vivo More importantly, US3 deletion significantly attenuated virulence, reduced virus shedding, and blocked the invasion of trigeminal ganglia. These results indicate that FHV-1 US3 efficiently inhibits IFN induction by using a novel immune evasion mechanism and that FHV-1 US3 is a potential regulator of neurovirulence.IMPORTANCE Despite widespread vaccination, the prevalence of FHV-1 remains high, suggesting that it can successfully evade the host innate immune response and infect cats. In this study, we screened viral proteins for inhibiting the IFN pathway and identified the Ser/Thr kinase US3 as the most powerful inhibitor. In contrast to other members of the alphaherpesviruses, FHV-1 US3 blocked the host type I IFN pathway in a kinase-independent manner and via binding to the IRF3 IAD and preventing IRF3 dimerization. More importantly, the depletion of US3 attenuated the anti-IFN activity of FHV-1 and prevented efficient viral replication in vitro and in vivo Also, US3 deletion significantly attenuated virulence and blocked the invasion of trigeminal ganglia. We believe that these findings not only will help us to better understand the mechanism of how FHV-1 manipulates the host IFN response but also highlight the potential role of US3 in the establishment of latent infection in vivo.

Type I Interferon Signaling to Dendritic Cells Limits Murid Herpesvirus 4 Spread from the Olfactory Epithelium

Murid herpesvirus 4 (MuHV-4) is a B cell-tropic gammaherpesvirus that can be studied in vivo Despite viral evasion, type I interferons (IFN-I) limit its spread. After MuHV-4 inoculation into footpads, IFN-I protect lymph node subcapsular sinus macrophages (SSM) against productive infection; after peritoneal inoculation, they protect splenic marginal zone macrophages, and they limit MuHV-4 replication in the lungs. While invasive infections can be used to test specific aspects of host colonization, it is also important to understand natural infection. MuHV-4 taken up spontaneously by alert mice enters them via olfactory neurons. We determined how IFN-I act in this context. Blocking IFN-I signaling did not increase neuronal infection but allowed the virus to spread to the adjacent respiratory epithelium. In lymph nodes, a complete IFN-I signaling block increased MuHV-4 lytic infection in SSM and increased the number of dendritic cells (DC) expressing viral green fluorescent protein (GFP) independently of lytic infection. A CD11c⁺ cell-directed signaling block increased infection of DC only. However, this was sufficient to increase downstream infection, consistent with DC providing the main viral route to B cells. The capacity of IFN-I to limit DC infection indicated that viral IFN-I evasion was only partly effective. Therefore, DC are a possible target for IFN-I-based interventions to reduce host colonization.IMPORTANCE Human gammaherpesviruses infect B cells and cause B cell cancers. Interventions to block virus binding to B cells have not stopped their infection. Therefore, we must identify other control points that are relevant to natural infection. Human infections are difficult to analyze. However, gammaherpesviruses colonize all mammals. A related gammaherpesvirus of mice reaches B cells not directly but via infected dendritic cells. We show that type I interferons, an important general antiviral defense, limit gammaherpesvirus B cell infection by acting on dendritic cells. Therefore, dendritic cell infection is a potential point of interferon-based therapeutic intervention.

The regulatory role of protein phosphorylation in human gammaherpesvirus associated cancers

Activation of specific sets of protein kinases by intracellular signal molecules has become more and more apparent in the past decade. Phosphorylation, one of key posttranslational modification events, is activated by kinase or regulatory protein and is vital for controlling many physiological functions of eukaryotic cells such as cell proliferation, differentiation, malignant transformation, and signal transduction mediated by external stimuli. Moreovers, the reversible modification of phosphorylation and dephosphorylation can result in different features of the target substrate molecules including DNA binding, protein-protein interaction, subcellular location and enzymatic activity, and is often hijacked by viral infection. Epstein-Barr virus (EBV) and Kaposi’s sarcomaassociated herpesvirus (KSHV), two human oncogenic gamma-herpesviruses, are shown to tightly associate with many malignancies. In this review, we summarize the recent progresses on understanding of molecular properties and regulatory modes of cellular and viral proteins phosphorylation influenced by these two tumor viruses, and highlight the potential therapeutic targets and strategies against their related cancers.

Pseudorabies Virus dUTPase UL50 Induces Lysosomal Degradation of Type I Interferon Receptor 1 and Antagonizes the Alpha Interferon Response

Alphaherpesviruses that establish persistent infections rely partly on their ability to evade host antiviral responses, notably the type I interferon (IFN) response. However, the mechanisms employed by alphaherpesviruses to avoid this response are not well understood. Pseudorabies virus (PRV) is an economically important pathogen and a useful model system for studying alphaherpesvirus biology. To identify PRV proteins that antagonize type I IFN signaling, we performed a screen by using an IFN-stimulated response element reporter in the swine cell line CRL. Unexpectedly, we identified the dUTPase UL50 as a strong inhibitor. We confirmed that UL50 has the ability to inhibit type I IFN signaling by performing ectopic expression of UL50 in cells and deletion of UL50 in PRV. Mechanistically, UL50 impeded type I IFN-induced STAT1 phosphorylation, likely by accelerating lysosomal degradation of IFN receptor 1 (IFNAR1). In addition, this UL50 activity was independent of its dUTPase activity and required amino acids 225 to 253 in the C-terminal region. The UL50 encoded by herpes simplex virus 1 (HSV-1) also possessed similar activity. Moreover, UL50-deleted PRV was more susceptible to IFN than UL50-proficient PRV. Our results suggest that in addition to its dUTPase activity, the UL50 protein of alphaherpesviruses possesses the ability to suppress type I IFN signaling by promoting lysosomal degradation of IFNAR1, thereby contributing to immune evasion. This finding reveals UL50 as a potential antiviral target.IMPORTANCE Alphaherpesviruses can establish lifelong infections and cause many diseases in humans and animals. Pseudorabies virus (PRV) is a swine alphaherpesvirus that threatens pig production. Using PRV as a model, we found that this alphaherpesvirus could utilize its encoded dUTPase UL50 to induce IFNAR1 degradation and inhibit type I IFN signaling in an enzymatic activity-independent manner. Our finding reveals a mechanism employed by an alphaherpesvirus to evade the immune response and indicates that UL50 is an important viral protein in pathogenesis and is a potential target for antiviral drug development.

A Herpesvirus Protein Selectively Inhibits Cellular mRNA Nuclear Export

Nuclear mRNA export is highly regulated to ensure accurate cellular gene expression. Viral inhibition of cellular mRNA export can enhance viral access to the cellular translation machinery and prevent anti-viral protein production but is generally thought to be nonselective. We report that ORF10 of Kaposi's sarcoma-associated herpesvirus (KSHV), a nuclear DNA virus, inhibits mRNA export in a transcript-selective manner to control cellular gene expression. Nuclear export inhibition by ORF10 requires an interaction with an RNA export factor, Rae1. Genome-wide analysis reveals a subset of cellular mRNAs whose nuclear export is blocked by ORF10 with the 3' UTRs of ORF10-targeted transcripts conferring sensitivity to export inhibition. The ORF10-Rae1 interaction is important for the virus to express viral genes and produce infectious virions. These results suggest that a nuclear DNA virus can selectively interfere with RNA export to restrict host gene expression for optimal replication.

Type I Interferons and NK Cells Restrict Gammaherpesvirus Lymph Node Infection

Gammaherpesviruses establish persistent, systemic infections and cause cancers. Murid herpesvirus 4 (MuHV-4) provides a unique window into the early events of host colonization. It spreads via lymph nodes. While dendritic cells (DC) pass MuHV-4 to lymph node B cells, subcapsular sinus macrophages (SSM), which capture virions from the afferent lymph, restrict its spread. Understanding how this restriction works offers potential clues to a more comprehensive defense. Type I interferon (IFN-I) blocked SSM lytic infection and reduced lytic cycle-independent viral reporter gene expression. Plasmacytoid DC were not required, but neither were SSM the only source of IFN-I, as IFN-I blockade increased infection in both intact and SSM-depleted mice. NK cells restricted lytic SSM infection independently of IFN-I, and SSM-derived virions spread to the spleen only when both IFN-I responses and NK cells were lacking. Thus, multiple innate defenses allowed SSM to adsorb virions from the afferent lymph with relative impunity. Enhancing IFN-I and NK cell recruitment could potentially also restrict DC infection and thus improve infection control.

IMPORTANCE

Human gammaherpesviruses cause cancers by infecting B cells. However, vaccines designed to block virus binding to B cells have not stopped infection. Using a related gammaherpesvirus of mice, we have shown that B cells are infected not via cell-free virus but via infected myeloid cells. This suggests a different strategy to stop B cell infection: stop virus production by myeloid cells. Not all myeloid infection is productive. We show that subcapsular sinus macrophages, which do not pass infection to B cells, restrict gammaherpesvirus production by recruiting type I interferons and natural killer cells. Therefore, a vaccine that speeds the recruitment of these defenses might stop B cell infection.

Type I Interferons Direct Gammaherpesvirus Host Colonization

Gamma-herpesviruses colonise lymphocytes. Murid Herpesvirus-4 (MuHV-4) infects B cells via epithelial to myeloid to lymphoid transfer. This indirect route entails exposure to host defences, and type I interferons (IFN-I) limit infection while viral evasion promotes it. To understand how IFN-I and its evasion both control infection outcomes, we used Mx1-cre mice to tag floxed viral genomes in IFN-I responding cells. Epithelial-derived MuHV-4 showed low IFN-I exposure, and neither disrupting viral evasion nor blocking IFN-I signalling markedly affected acute viral replication in the lungs. Maximising IFN-I induction with poly(I:C) increased virus tagging in lung macrophages, but the tagged virus spread poorly. Lymphoid-derived MuHV-4 showed contrastingly high IFN-I exposure. This occurred mainly in B cells. IFN-I induction increased tagging without reducing viral loads; disrupting viral evasion caused marked attenuation; and blocking IFN-I signalling opened up new lytic spread between macrophages. Thus, the impact of IFN-I on viral replication was strongly cell type-dependent: epithelial infection induced little response; IFN-I largely suppressed macrophage infection; and viral evasion allowed passage through B cells despite IFN-I responses. As a result, IFN-I and its evasion promoted a switch in infection from acutely lytic in myeloid cells to chronically latent in B cells. Murine cytomegalovirus also showed a capacity to pass through IFN-I-responding cells, arguing that this is a core feature of herpesvirus host colonization.

Murine gammaherpesvirus targets type I IFN receptor but not type III IFN receptor early in infection

The innate immune response represents a primary line of defense against invading viral pathogens. Since epithelial cells are the primary site of gammaherpesvirus replication during infection in vivo and there are no information on activity of IFN-III signaling against gammaherpesviruses in this cell type, in present study, we evaluated the expression profile and virus-host interactions in mouse mammary epithelial cell (NMuMG) infected with three strains of murine gammaherpesvirus, MHV-68, MHV-72 and MHV-4556. Studying three strains of murine gammaherpesvirus, which differ in nucleotide sequence of some structural and non-structural genes, allowed us to compare the strain-dependent interactions with host organism. Our results clearly demonstrate that: (i) MHV-68, MHV-72 and MHV-4556 differentially interact with intracellular signaling and dysregulate IFN signal transduction; (ii) MHV-68, MHV-72 and MHV-4556 degrade type I IFN receptor in very early stages of infection (2-4hpi), but not type III IFN receptor; (iii) type III IFN signaling might play a key role in antiviral defense of epithelial cells in early stages of murine gammaherpesvirus replication; (iv) NMuMG cells are an appropriate model for study of not only type I IFN signaling, but also type III IFN signaling pathway. These findings are important for better understanding of individual virus-host interactions in lytic as well as in persistent gammaherpesvirus replication and help us to elucidate IFN-III function in early events of virus infection.

Immune control of oncogenic γ-herpesviruses

Human γ-herpesviruses contain Epstein Barr virus (EBV), the first human tumor virus that was identified in man, and Kaposi Sarcoma associated herpesvirus (KSHV), one of the most recently identified human oncogenic pathogens. Both of these have co-evolved with humans to cause tumors only in a minority of infected individuals, despite their exquisite ability to establish persistent infections. In this review we will summarize the fine-tuned balance between immune responses, immune escape and cellular transformation by these viruses, which results in life-long persistent, but asymptomatic infection with immune control in most virus carriers. A detailed understanding of this balance is required to immunotherapeutically reinstall it in patients that suffer from EBV and KSHV associated malignancies.

dUTPase: the frequently overlooked enzyme encoded by many retroviruses

Retroviruses are among the best studied viruses in last decades due to their pivotal involvement in cellular processes and, most importantly, in causing human diseases, most notably-acquired immunodeficiency syndrome (AIDS) that is triggered by human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2, respectively). Numerous studied were conducted to understand the involvement of the three cardinal retroviral enzymes, reverse transcriptase, integrase and protease, in the life cycle of the viruses. These studies have led to the development of many inhibitors of these enzymes as anti-retroviral specific drugs that are used for routine treatments of HIV/AIDS patients. Interestingly, a fourth virus-encoded enzyme, the deoxyuridine 5'-triphosphate nucleotidohydrolase (dUTPase) is also found in several major retroviral groups. The presence and the importance of this enzyme to the life cycle of retroviruses were usually overlooked by most retrovirologists, although the occurrence of dUTPases, particularly in beta-retroviruses and in non-primate retroviruses, is known for more than 20 years. Only more recently, retroviral dUTPases were brought into the limelight and were shown in several cases to be essential for viral replication. Therefore, it is likely that future studies on this enzyme will advance our knowledge to a level that will allow designing novel, specific and potent anti-dUTPase drugs that are effective in combating retroviral diseases. The aim of this review is to give concise background information on dUTPases in general and to summarize the most relevant data on retroviral dUTPases and their involvement in the replication processes and pathogenicity of the viruses, as well as in possibly-associated human diseases.

Mesenchymal stem cells detect and defend against gammaherpesvirus infection via the cGAS-STING pathway

Mesenchymal stem cells (MSCs) are widely used in clinical settings to treat tissue injuries and autoimmune disorders due to their multipotentiality and immunomodulation. Long-term observations reveal several complications after MSCs infusion, especially herpesviral infection. However, the mechanism of host defense against herpesviruses in MSCs remains largely unknown. Here we showed that murine gammaherpesvirus-68 (MHV-68), which is genetically and biologically related to human gammaherpesviruses, efficiently infected MSCs both in vitro and in vivo. Cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS) was identified as the sensor of MHV-68 in MSCs for the first time. Moreover, the cytosolic DNA sensing pathway mediated a potent anti-herpesviral effect through the adaptor STING and downstream kinase TBK1. Furthermore, blockade of IFN signaling suggested that cytosolic DNA sensing triggered both IFN-dependent and -independent anti-herpesviral responses. Our findings demonstrate that cGAS-STING mediates innate immunity to gammaherpesvirus infection in MSCs, which may provide a clue to develop therapeutic strategy.

Human herpesviruses-encoded dUTPases: a family of proteins that modulate dendritic cell function and innate immunity

We have previously shown that Epstein-Barr virus (EBV)-encoded dUTPase can modulate innate immune responses through the activation of TLR2 and NF-κB signaling. However, whether this novel immune function of the dUTPase is specific for EBV or a common property of the Herpesviridae family is not known. In this study, we demonstrate that the purified viral dUTPases encoded by herpes simplex virus type 2 (HSV-2), human herpesvirus-6A (HHV-6A), human herpesvirus-8 (HHV-8) and varicella-zoster virus (VZV) differentially activate NF-κB through ligation of TLR2/TLR1 heterodimers. Furthermore, activation of NF-κB by the viral dUTPases was inhibited by anti-TLR2 blocking antibodies (Abs) and the over-expression of dominant-negative constructs of TLR2, lacking the TIR domain, and MyD88 in human embryonic kidney 293 cells expressing TLR2/TLR1. In addition, treatment of human dendritic cells and PBMCs with the herpesviruses-encoded dUTPases from HSV-2, HHV-6A, HHV-8, and VZV resulted in the secretion of the inflammatory cytokines IL-1β, IL-6, IL-8, IL-12, TNF-α, IL-10, and IFN-γ. Interestingly, blocking experiments revealed that the anti-TLR2 Ab significantly reduced the secretion of cytokines by the various herpesviruses-encoded dUTPases (p < 0.05). To our knowledge, this is the first report demonstrating that a non-structural protein encoded by herpesviruses HHV-6A, HHV-8, VZV and to a lesser extent HSV-2 is a pathogen-associated molecular pattern. Our results reveal a novel function of the virus-encoded dUTPases, which may be important to the pathophysiology of diseases caused by these viruses. More importantly, this study demonstrates that the immunomodulatory functions of dUTPases are a common property of the Herpesviridae family and thus, the dUTPase could be a potential target for the development of novel therapeutic agents against infections caused by these herpesviruses.

Interplay between Kaposi's sarcoma-associated herpesvirus and the innate immune system

Understanding of the innate immune response to viral infections is rapidly progressing, especially with regards to the detection of DNA viruses. Kaposi's sarcoma-associated herpesvirus (KSHV) is a large dsDNA virus that is responsible for three human diseases: Kaposi's sarcoma, primary effusion lymphoma and multicentric Castleman's disease. The major target cells of KSHV (B cells and endothelial cells) express a wide range of pattern recognition receptors (PRRs) and play a central role in mobilizing inflammatory responses. On the other hand, KSHV encodes an array of immune evasion genes, including several pirated host genes, which interfere with multiple aspects of the immune response. This review summarizes current understanding of innate immune recognition of KSHV and the role of immune evasion genes that shape the antiviral and inflammatory responses.

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.

Identification of alternative transcripts encoding the essential murine gammaherpesvirus lytic transactivator RTA

The essential immediate early transcriptional activator RTA, encoded by gene 50, is conserved among all characterized gammaherpesviruses. Analyses of a recombinant murine gammaherpesvirus 68 (MHV68) lacking both of the known gene 50 promoters (G50DblKo) revealed that this mutant retained the ability to replicate in the simian kidney epithelial cell line Vero but not in permissive murine fibroblasts following low-multiplicity infection. However, G50DblKo replication in permissive fibroblasts was partially rescued by high-multiplicity infection. In addition, replication of the G50DblKo virus was rescued by growth on mouse embryonic fibroblasts (MEFs) isolated from IFN-α/βR-/- mice, while growth on Vero cells was suppressed by the addition of alpha interferon (IFN-α). 5' rapid amplification of cDNA ends (RACE) analyses of RNAs prepared from G50DblKo and wild-type MHV68-infected murine macrophages identified three novel gene 50 transcripts initiating from 2 transcription initiation sites located upstream of the currently defined proximal and distal gene 50 promoters. In transient promoter assays, neither of the newly identified gene 50 promoters exhibited sensitivity to IFN-α treatment. Furthermore, in a single-step growth analysis RTA levels were higher at early times postinfection with the G50DblKo mutant than with wild-type virus but ultimately fell below the levels of RTA expressed by wild-type virus at later times in infection. Infection of mice with the MHV68 G50DblKo virus demonstrated that this mutant virus was able to establish latency in the spleen and peritoneal exudate cells (PECs) of C57BL/6 mice with about 1/10 the efficiency of wild-type virus or marker rescue virus. However, despite the ability to establish latency, the G50DblKo virus mutant was severely impaired in its ability to reactivate from either latently infected splenocytes or PECs. Consistent with the ability to rescue replication of the G50DblKo mutant by growth on type I interferon receptor null MEFs, infection of IFN-α/βR-/- mice with the G50DblKo mutant virus demonstrated partial rescue of (i) acute virus replication in the lungs, (ii) establishment of latency, and (iii) reactivation from latency. The identification of additional gene 50/RTA transcripts highlights the complex mechanisms involved in controlling expression of RTA, likely reflecting time-dependent and/or cell-specific roles of different gene 50 promoters in controlling virus replication. Furthermore, the newly identified gene 50 transcripts may also act as negative regulators that modulate RTA expression.

IMPORTANCE

The viral transcription factor RTA, encoded by open reading frame 50 (Orf50), is well conserved among all known gammaherpesviruses and is essential for both virus replication and reactivation from latently infected cells. Previous studies have shown that regulation of gene 50 transcription is complex. The studies reported here describe the presence of additional alternatively initiated, spliced transcripts that encode RTA. Understanding how expression of this essential viral gene product is regulated may identify new strategies for interfering with infection in the setting of gammaherpesvirus-induced diseases.

Phosphorylation of a herpes simplex virus 1 dUTPase by a viral protein kinase, Us3, dictates viral pathogenicity in the central nervous system but not at the periphery

Herpes simplex virus 1 (HSV-1) encodes Us3 protein kinase, which is critical for viral pathogenicity in both mouse peripheral sites (e.g., eyes and vaginas) and in the central nervous systems (CNS) of mice after intracranial and peripheral inoculations, respectively. Whereas some Us3 substrates involved in Us3 pathogenicity in peripheral sites have been reported, those involved in Us3 pathogenicity in the CNS remain to be identified. We recently reported that Us3 phosphorylated HSV-1 dUTPase (vdUTPase) at serine 187 (Ser-187) in infected cells, and this phosphorylation promoted viral replication by regulating optimal enzymatic activity of vdUTPase. In the present study, we show that the replacement of vdUTPase Ser-187 by alanine (S187A) significantly reduced viral replication and virulence in the CNS of mice following intracranial inoculation and that the phosphomimetic substitution at vdUTPase Ser-187 in part restored the wild-type viral replication and virulence. Interestingly, the S187A mutation in vdUTPase had no effect on viral replication and pathogenic effects in the eyes and vaginas of mice after ocular and vaginal inoculation, respectively. Similarly, the enzyme-dead mutation in vdUTPase significantly reduced viral replication and virulence in the CNS of mice after intracranial inoculation, whereas the mutation had no effect on viral replication and pathogenic effects in the eyes and vaginas of mice after ocular and vaginal inoculation, respectively. These observations suggested that vdUTPase was one of the Us3 substrates responsible for Us3 pathogenicity in the CNS and that the CNS-specific virulence of HSV-1 involved strict regulation of vdUTPase activity by Us3 phosphorylation.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) encodes a viral protein kinase Us3 which is critical for pathogenicity both in peripheral sites and in the central nervous systems (CNS) of mice following peripheral and intracranial inoculations, respectively. Whereas some Us3 substrates involved in Us3 pathogenicity in peripheral sites have been reported, those involved in Us3 pathogenicity in the CNS remain to be identified. Here, we report that Us3 phosphorylation of viral dUTPase (vdUTPase) at serine 187 (Ser-187), which has been shown to promote the vdUTPase activity, appears to be critical for viral virulence in the CNS but not for pathogenic effects in peripheral sites. Since HSV proteins critical for viral virulence in the CNS are, in almost all cases, also involved in viral pathogenicity at peripheral sites, this phosphorylation event is a unique report of a specific mechanism involved in HSV-1 virulence in the CNS.

On the catalytic mechanism of dimeric dUTPases

We present the structure of the T. brucei dimeric dUTPase in open and closed conformations and probe the reaction mechanism through the binding of transition state mimics. We confirm that the nucleophilic attack occurs on the β-phosphate.

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.

Recent advances in understanding viral evasion of type I interferon

The type I interferon (IFN) system mediates a wide variety of antiviral effects and represents an important first barrier to virus infection. Consequently, viruses have developed an impressive diversity of tactics to circumvent IFN responses. Evasion strategies can involve preventing initial virus detection, via the disruption of the Toll‐like receptors or the retinoic acid inducible gene I (RIG‐I) ‐like receptors, or by avoiding the initial production of the ligands recognized by these receptors. An alternative approach is to preclude IFN production by disarming or degrading the transcription factors involved in the expression of IFN, such as interferon regulatory factor 3 (IRF3)/IRF7, nuclear factor‐κB (NF‐κB), or ATF‐2/c‐jun, or by inducing a general block on host cell transcription. Viruses also oppose IFN signalling, both by disturbing the type I IFN receptor and by impeding JAK/STAT signal transduction upon IFN receptor engagement. In addition, the global expression of IFN‐stimulated genes (ISGs) can be obstructed via interference with epigenetic signalling, and specific ISGs can also be selectively targeted for inhibition. Finally, some viruses disrupt IFN responses by co‐opting negative regulatory systems, whereas others use antiviral mechanisms to their own advantage. Here, we review recent developments in this field.

dUTPases, the unexplored family of signalling molecules

Deciphering the molecular mechanisms that control relevant cellular processes is of utmost importance to understand how viruses, prokaryotic and eukaryotic cells work. The diversity of living organisms suggests that there are novel regulators still to be discovered, which may uncover new regulatory paradigms. dUTPases (Duts) are assumed to be ubiquitous enzymes regulating cellular dUTP levels to prevent misincorporation of uracil into DNA. Recently however, Duts have been involved in the control of several relevant cellular processes, including transfer of mobile genetic elements, regulation of the immune system, autoimmunity or apoptosis, suggesting that they perform regulatory functions. This review aims at investigating the unexplored impact of Duts as novel signalling molecules.

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.

Kaposi's sarcoma-associated herpesvirus ORF54/dUTPase downregulates a ligand for the NK activating receptor NKp44

Kaposi's sarcoma-associated herpesvirus (KSHV) establishes long-term latent infection in humans and can cause cancers in endothelial and B cells. A functioning immune system is vital for restricting viral proliferation and preventing KSHV-dependent neoplasms. While natural killer (NK) lymphocytes are known to target virus-infected cells for destruction, their importance in the anti-KSHV immune response is not currently understood. Activating receptors on NK cells recognize ligands on target cells, including the uncharacterized ligand(s) for NKp44, termed NKp44L. Here we demonstrate that several NK ligands are affected when KSHV-infected cells are induced to enter the lytic program. We performed a screen of most of the known KSHV genes and found that the product of the ORF54 gene could downregulate NKp44L. The ORF54-encoded protein is a dUTPase; however, dUTPase activity is neither necessary nor sufficient for the downregulation of NKp44L. In addition, we find that ORF54 can also target proteins of the cytokine receptor family and the mechanism of downregulation involves perturbation of membrane protein trafficking. The ORF54-related proteins of other human herpesviruses do not possess this activity, suggesting that the KSHV homolog has evolved a novel immunoregulatory function and that the NKp44-NKp44L signaling pathway contributes to antiviral immunity.

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.