Herpes simplex virus 1 E3 ubiquitin ligase ICP0 protein inhibits tumor necrosis factor alpha-induced NF-κB activation by interacting with p65/RelA and p50/NF-κB1

Herpes Simplex Virus Type 1-Encoded miR-H2-3p Manipulates Cytosolic DNA-Stimulated Antiviral Innate Immune Response by Targeting DDX41

Herpes simplex virus type 1 (HSV-1), one of the human pathogens widely epidemic and transmitted among various groups of people in the world, often causes symptoms known as oral herpes or lifelong asymptomatic infection. HSV-1 employs many sophisticated strategies to escape host antiviral immune response based on its multiple coding proteins. However, the functions involved in the immune evasion of miRNAs encoded by HSV-1 during lytic (productive) infection remain poorly studied. Dual-luciferase reporter gene assay and bioinformatics revealed that Asp-Glu-Ala-Asp (DEAD)-box helicase 41 (DDX41), a cytosolic DNA sensor of the DNA-sensing pathway, was a putative direct target gene of HSV-1-encoded miR-H2-3p. The transfection of miR-H2-3p mimics inhibited the expression of DDX41 at the level of mRNA and protein, as well as the expression of interferon beta (IFN-β) and myxoma resistance protein I (MxI) induced by HSV-1 infection in THP-1 cells, and promoted the viral replication and its gene transcription. However, the transfection of miR-H2-3p inhibitor showed opposite effects. This finding indicated that HSV-1-encoded miR-H2-3p attenuated cytosolic DNA-stimulated antiviral immune response by manipulating host DNA sensor molecular DDX41 to enhance virus replication in cultured cells.

Characteristics of herpes simplex virus infection and pathogenesis suggest a strategy for vaccine development

Herpes simplex virus (HSV) can cause oral or genital ulcerative lesions and even encephalitis in various age groups with high infection rates. More seriously, HSV may lead to a wide range of recurrent diseases throughout a lifetime. No vaccines against HSV are currently available. The accumulated clinical research data for HSV vaccines reveal that the effects of HSV interacting with the host, especially the host immune system, may be important for the development of HSV vaccines. HSV vaccine development remains a major challenge. Thus, we focus on the research data regarding the interactions of HSV and host immune cells, including dendritic cells (DCs), innate lymphoid cells (ILCs), macrophages, and natural killer (NK) cells, and the related signal transduction pathways involved in immune evasion and cytokine production. The aim is to explore possible strategies to develop new effective HSV vaccines.

Discovery of Small-Molecule Inhibitors Targeting the E3 Ubiquitin Ligase Activity of the Herpes Simplex Virus 1 ICP0 Protein Using an In Vitro High-Throughput Screening Assay

Herpes simplex virus 1 (HSV-1) has infected more than 80% of the population. Reactivation of the virus causes diseases ranging in severity from benign cold sores to fatal encephalitis. Current treatments involve viral DNA replication inhibitors, but the emergence of drug-resistant mutants is observed frequently, highlighting the need for novel antiviral therapies. Infected cell protein 0 (ICP0) of HSV-1 is encoded by an immediate early gene and plays a fundamental role during infection, because it enables viral gene expression and blocks antiviral responses. One mechanism by which ICP0 functions is through an E3 ubiquitin ligase activity that induces the degradation of targeted proteins. A ΔICP0 virus or mutants with deficiencies in E3 ligase activity cannot counteract beta interferon (IFN-β)-induced restriction of viral infection, are highly immunogenic, are avirulent, and fail to spread. Thus, small molecules interfering with essential and conserved ICP0 functions are expected to compromise HSV-1 infection. We have developed a high-throughput screening assay, based on the autoubiquitination properties of ICP0, to identify small-molecule inhibitors of ICP0 E3 ubiquitin ligase activity. Through a pilot screening procedure, we identified nine compounds that displayed dose-dependent inhibitory effects on ICP0 but not on Mdm2, a control E3 ubiquitin ligase. Following validation, one compound displayed ICP0-dependent inhibition of HSV-1 infection. This compound appeared to bind ICP0 in a cellular thermal shift assay, it blocked ICP0 self-elimination, and it blocked wild-type but not ICP0-null virus gene expression. This scaffold displays specificity and could be used to develop optimized ICP0 E3 ligase inhibitors.IMPORTANCE Since acyclovir and its derivatives were launched for herpesviruses control almost four decades ago, the search for novel antivirals has waned. However, as human life expectancy has increased, so has the number of immunocompromised individuals who receive prolonged treatment for HSV recurrences. This has led to an increase in unresponsive patients due to acquired viral drug resistance. Thus, novel treatments need to be explored. Here we explored the HSV-1 ICP0 E3 ligase as a potential antiviral target because (i) ICP0 is expressed before virus replication, (ii) it is essential for infection in vivo, (iii) it is required for efficient reactivation of the virus from latency, (iv) inhibition of its E3 ligase activity would sustain host immune responses, and (v) it is shared by other herpesviruses. We report a compound that inhibits HSV-1 infection in an ICP0-dependent manner by inhibiting ICP0 E3 ligase activity.

Herpes Simplex Virus 1 Tegument Protein UL46 Inhibits TANK-Binding Kinase 1-Mediated Signaling

TANK-binding kinase 1 (TBK1) is a key component of the antiviral immunity signaling pathway. It activates downstream interferon regulatory factor 3 (IRF3) and subsequent type I interferon (IFN-I) production. Herpes simplex virus type 1 (HSV-1) can antagonize host antiviral immune responses and lead to latent infection. Here, HSV-1 tegument protein UL46 was demonstrated to downregulate TBK1-dependent antiviral innate immunity. UL46 interacted with TBK1 and reduced TBK1 activation and its downstream signaling. Our results showed that UL46 impaired the interaction of TBK1 and IRF3 and downregulated the activation of IRF3 by inhibiting the dimerization of TBK1 to reduce the IFN-I production induced by TBK1 and immunostimulatory DNA. The IFN-I and its downstream antiviral genes induced by UL46-deficient HSV-1 (ΔUL46 HSV-1) were higher than those of wild-type HSV-1 (WT HSV-1). In addition, the stable knockdown of TBK1 facilitated the replication of ΔUL46 HSV-1, but not WT HSV-1. Together, these findings reveal a novel mechanism of immune evasion by HSV-1.IMPORTANCE HSV-1 has evolved multiple strategies to evade host antiviral responses and establish a lifelong latent infection, but the molecular mechanisms by which HSV-1 interrupts antiviral innate immunity are not completely understood. As TBK1 is very critical for antiviral innate immunity, it is of great interest to reveal the immune evasion mechanism of HSV-1 by targeting TBK1. In the present study, HSV-1 UL46 was found to inhibit the activation of IFN-I by targeting TBK1, suggesting that the evasion of TBK1 mediated antiviral innate immunity by HSV-1 UL46. Findings in this study will provide new insights into the host-virus interaction and help develop new approaches against HSV-1 infection.

HSV-1\EGFP stimulates miR-146a expression in a NF-κB-dependent manner in monocytic THP-1 cells

The nuclear factor κB (NF-κB) pathway plays a key role in innate and adaptive immunity, cell proliferation and survival, inflammation and tumors development. MiR-146a is an immune system regulator that has anti-inflammatory function in multiple cell types and conditions. Here we demonstrate activation of canonical NF-κB pathway in monocytic cells upon HSV-1 replication. By constructing and using a recombinant HSV-1\EGFP virus, we monitored the capability of the virus to recruit NF-κB and we report that the phosphorylation of p65 protein correlates with an active virus replication at single-cell level. In addition, we found that upregulation of miR-146a during viral replication is strictly dependent on NF-κB activation and correlates with tight control of the interleukin-1 receptor-associate kinase 1 (IRAK1). Accordingly, THP-1 DN IκBα cells, expressing a dominant negative mIκBα, did not show upregulation of miR-146a upon HSV-1 infection. Our data suggest that the expression of miRNA-146a modulates NF-κB activation through targeting IRAK1 during HSV-1 replication in THP-1 cells.

Suppression of NF-κB Activity: A Viral Immune Evasion Mechanism

Nuclear factor-κB (NF-κB) is an important transcription factor that induces the expression of antiviral genes and viral genes. NF-κB activation needs the activation of NF-κB upstream molecules, which include receptors, adaptor proteins, NF-κB (IκB) kinases (IKKs), IκBα, and NF-κB dimer p50/p65. To survive, viruses have evolved the capacity to utilize various strategies that inhibit NF-κB activity, including targeting receptors, adaptor proteins, IKKs, IκBα, and p50/p65. To inhibit NF-κB activation, viruses encode several specific NF-κB inhibitors, including NS3/4, 3C and 3C-like proteases, viral deubiquitinating enzymes (DUBs), phosphodegron-like (PDL) motifs, viral protein phosphatase (PPase)-binding proteins, and small hydrophobic (SH) proteins. Finally, we briefly describe the immune evasion mechanism of human immunodeficiency virus 1 (HIV-1) by inhibiting NF-κB activity in productive and latent infections. This paper reviews a viral mechanism of immune evasion that involves the suppression of NF-κB activation to provide new insights into and references for the control and prevention of viral diseases.

Herpes Simplex Virus 1 Tegument Protein VP22 Abrogates cGAS/STING-Mediated Antiviral Innate Immunity

Cytosolic DNA arising from intracellular pathogens is sensed by cyclic GMP-AMP synthase (cGAS) and triggers a powerful innate immune response. However, herpes simplex virus 1 (HSV-1), a double-stranded DNA virus, has developed multiple mechanisms to attenuate host antiviral machinery and facilitate viral infection and replication. In the present study, we found that HSV-1 tegument protein VP22 acts as an inhibitor of cGAS/stimulator of interferon genes (cGAS/STING)-mediated production of interferon (IFN) and its downstream antiviral genes. Our results showed that ectopic expression of VP22 decreased cGAS/STING-mediated IFN-β promoter activation and IFN-β production. Infection with wild-type (WT) HSV-1, but not VP22-deficient virus (ΔVP22), inhibited immunostimulatory DNA (ISD)-induced activation of the IFN signaling pathway. Further study showed that VP22 interacted with cGAS and inhibited the enzymatic activity of cGAS. In addition, stable knockdown of cGAS facilitated the replication of ΔVP22 virus but not the WT. In summary, our findings indicate that HSV-1 VP22 acts as an antagonist of IFN signaling to persistently evade host innate antiviral responses.IMPORTANCE cGAS is very important for host defense against viral infection, and many viruses have evolved ways to target cGAS and successfully evade the attack by the immune system of their susceptible host. This study demonstrated that HSV-1 tegument protein VP22 counteracts the cGAS/STING-mediated DNA-sensing antiviral innate immunity signaling pathway by inhibiting the enzymatic activity of cGAS. The findings in this study will expand our understanding of the interaction between HSV-1 replication and the host DNA-sensing signaling pathway.

Defining the Role of Stress Granules in Innate Immune Suppression by the Herpes Simplex Virus 1 Endoribonuclease VHS

In response to virus-induced shutoff host protein synthesis, dynamic aggregates containing mRNA, RNA-binding proteins and translation factors termed stress granules (SGs) often accumulate within the cytoplasm. SGs typically form following phosphorylation and inactivation of the eukaryotic translation initiation factor 2α (eIF2α), a substrate of the double-stranded RNA (dsRNA)-activated kinase protein kinase R (PKR). The detection of innate immune sensors and effectors like PKR at SGs suggests a role in pathogen nucleic acid sensing. However, the functional importance of SGs in host innate responses is unclear and has primarily been examined in response to infection with select RNA viruses. During infection with the DNA virus herpes simplex virus 1 (HSV-1), the virus-encoded virion host shutoff (VHS) endoribonuclease is required to restrict interferon production, PKR activation, and SG formation, although the relationship between these activities remains incompletely understood. Here, we show that in cells infected with a VHS-deficient HSV-1 (ΔVHS) dsRNA accumulated and localized to SGs. Surprisingly, formation of dsRNA and its concentration at SGs was not required for beta interferon mRNA induction, indicating that suppression of type I interferon induction by VHS does not stem from its control of dsRNA accumulation. Instead, STING signaling downstream of cGMP-AMP synthase (cGAS)-dependent DNA sensing is required for beta interferon induction. In contrast, significantly less PKR activation is observed when SG assembly is disrupted by ISRIB, an inhibitor of phosphorylated eIF2α-mediated translation repression, or depleting SG scaffolding proteins G3BP1 or TIA1. This demonstrates that PKR activation is intimately linked to SG formation and that SGs form important hubs to potentiate PKR activation during infection.IMPORTANCE Formation of cytoplasmic stress granules that are enriched for innate immune sensors and effectors is suppressed during many viral infections. It is unclear, however, to what extent this is a side effect of viral efforts to maintain protein synthesis or intentional disruption of a hub for innate immune sensing. In this study, we utilize a herpes simplex virus 1 mutant lacking the RNA nuclease VHS which upon infection induces SGs, PKR activation, and beta interferon to address this question. We show that dsRNA is localized to SGs and that SGs can function to promote PKR activation in the context of a DNA virus infection, but we find no evidence to support their importance for interferon induction during HSV-1 infection.

Innate Immune Mechanisms and Herpes Simplex Virus Infection and Disease

Innate immune responses play a major role in the control of herpes simplex virus (HSV) infections, and a multiplicity of mechanisms have emerged as a result of human evolution to sense and respond to HSV infections. HSV in turn has evolved a number of ways to evade immune detection and to blunt human innate immune responses. In this review, we summarize the major host innate immune mechanisms and the HSV evasion mechanisms that have evolved. We further discuss how disease can result if this equilibrium between virus and host response is disrupted.

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.

RING-Domain E3 Ligase-Mediated Host-Virus Interactions: Orchestrating Immune Responses by the Host and Antagonizing Immune Defense by Viruses

The RING-domain E3 ligases (RING E3s), a group of E3 ligases containing one or two RING finger domains, are involved in various cellular processes such as cell proliferation, immune regulation, apoptosis, among others. In the host, a substantial number of the RING E3s have been implicated to inhibit viral replication through regulating immune responses, including activation and inhibition of retinoic acid-inducible gene I-like receptors, toll-like receptors, and DNA receptor signaling pathways, modulation of cell-surface expression of major histocompatibility complex, and co-stimulatory molecules. During the course of evolution and adaptation, viruses encode RING E3s to antagonize host immune defense, such as the infected cell protein 0 of herpes simplex virus type 1, the non-structural protein 1 of rotavirus, and the K3 and K5 of Kaposi’s sarcoma-associated herpesvirus. In addition, recent studies suggest that viruses can hijack the host RING E3s to facilitate viral replication. Based on emerging and interesting discoveries, the RING E3s present novel links among the host and viruses. Herein, we focus on the latest research progresses in the RING E3s-mediated host–virus interactions and discuss the outlooks of the RING E3s for future research.

Transcriptional Dysregulation and Post-translational Modifications in Polyglutamine Diseases: From Pathogenesis to Potential Therapeutic Strategies

Polyglutamine (polyQ) diseases are hereditary neurodegenerative disorders caused by an abnormal expansion of a trinucleotide CAG repeat in the coding region of their respective associated genes. PolyQ diseases mainly display progressive degeneration of the brain and spinal cord. Nine polyQ diseases are known, including Huntington's disease (HD), spinal and bulbar muscular atrophy (SBMA), dentatorubral-pallidoluysian atrophy (DRPLA), and six forms of spinocerebellar ataxia (SCA). HD is the best characterized polyQ disease. Many studies have reported that transcriptional dysregulation and post-translational disruptions, which may interact with each other, are central features of polyQ diseases. Post-translational modifications, such as the acetylation of histones, are closely associated with the regulation of the transcriptional activity. A number of groups have studied the interactions between the polyQ proteins and transcription factors. Pharmacological drugs or genetic manipulations aimed at correcting the dysregulation have been confirmed to be effective in the treatment of polyQ diseases in many animal and cellular models. For example, histone deaceylase inhibitors have been demonstrated to have beneficial effects in cases of HD, SBMA, DRPLA, and SCA3. In this review, we describe the transcriptional and post-translational dysregulation in polyQ diseases with special focus on HD, and we summarize and comment on potential treatment approaches targeting disruption of transcription and post-translation processes in these diseases.

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.

Modulation of the NFκb Signalling Pathway by Human Cytomegalovirus

Many viruses trigger innate and adaptive immune responses and must circumvent the negative consequences to successfully establish infection in their hosts. Human Cytomegalovirus (HCMV) is no exception, and devotes a significant portion of its coding capacity to genes involved in immune evasion. Activation of the NFκB signalling pathway by viral binding and entry results in induction of antiviral and pro-inflammatory genes that have significant negative effects on HCMV infection. However, NFκB signalling stimulates transcription from the Major Immediate Early Promoter (MIEP) and pro-inflammatory signalling is crucial for cellular differentiation and viral reactivation from latency. Accordingly, HCMV encodes proteins that act to both stimulate and inhibit the NFκB signalling pathway. In this Review we will highlight the complex interactions between HCMV and NFκB, discussing the known agonists and antagonists encoded by the virus and suggest why manipulation of the pathway may be critical for both lytic and latent infections.

Herpes simplex virus type 1 abrogates the antiviral activity of Ch25h via its virion host shutoff protein

Cholesterol 25-hydroxylase (Ch25h) is an interferon-inducible protein, and recent studies have demonstrated that it inhibited the replication of many enveloped viruses. However, in this study, we found that cells infected with wild-type (WT) HSV-1 reduced the expression of Ch25h, and ectopic expression of Ch25h could not inhibit the replication of WT-HSV-1. By screening assay, HSV-1 UL41 protein was found to down-regulate the expression of Ch25h. In addition, UL41 abrogated the antiviral activity of Ch25h via degrading its mRNA. Furthermore, ectopic expression of Ch25h inhibited the replication of UL41-null mutant HSV-1 (R2621), but not WT-HSV-1, and knockdown of Ch25h did not affect the replication of WT-HSV-1, but promoted the replication of the R2621. For the first time, HSV-1 UL41 was demonstrated to evade the antiviral function of Ch25h via its endonuclease activity.

Inhibition of herpes simplex-1 virus replication by 25-hydroxycholesterol and 27-hydroxycholesterol

Oxysterols are known pleiotropic molecules whose antiviral action has been recently discovered. Here reported is the activity of a panel of oxysterols against HSV-1 with the identification of a new mechanism of action. A marked antiviral activity not only of 25HC but also of 27HC against HSV-1 was observed either if the oxysterols were added before or after infection, suggesting an activity unrelated to the viral entry inhibition as proposed by previous literature. Therefore, the relation between the pro-inflammatory activity of oxysterols and the activation of NF-kB and IL-6 induced by HSV-1 in the host cell was investigated. Indeed, cell pre-incubation with oxysterols further potentiated IL-6 production as induced by HSV-1 infection with a consequent boost of the interleukin's total cell secretion. Further, a direct antiviral effect of IL-6 administration to HSV-1 infected cells was demonstrated, disclosing an additional mechanism of antiviral action by both 25HC and 27HC.

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25HC and 27HC markedly inhibit HSV-1 replication in a standard cell culture system.

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Cell pre-incubation with oxysterols potentiates IL-6 production as induced by HSV-1.

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The concentration of IL-6 induced by oxysterols actually inhibits HSV-1 replication.

Herpes Simplex Virus 1 UL24 Abrogates the DNA Sensing Signal Pathway by Inhibiting NF-κB Activation

Cyclic GMP-AMP synthase (cGAS) is a newly identified DNA sensor that recognizes foreign DNA, including the genome of herpes simplex virus 1 (HSV-1). Upon binding of viral DNA, cGAS produces cyclic GMP-AMP, which interacts with and activates stimulator of interferon genes (STING) to trigger the transcription of antiviral genes such as type I interferons (IFNs), and the production of inflammatory cytokines. HSV-1 UL24 is widely conserved among members of the herpesviruses family and is essential for efficient viral replication. In this study, we found that ectopically expressed UL24 could inhibit cGAS-STING-mediated promoter activation of IFN-β and interleukin-6 (IL-6), and UL24 also inhibited interferon-stimulatory DNA-mediated IFN-β and IL-6 production during HSV-1 infection. Furthermore, UL24 selectively blocked nuclear factor κB (NF-κB) but not IFN-regulatory factor 3 promoter activation. Coimmunoprecipitation analysis demonstrated that UL24 bound to the endogenous NF-κB subunits p65 and p50 in HSV-1-infected cells, and UL24 was also found to bind the Rel homology domains (RHDs) of these subunits. Furthermore, UL24 reduced the tumor necrosis factor alpha (TNF-α)-mediated nuclear translocation of p65 and p50. Finally, mutational analysis revealed that the region spanning amino acids (aa) 74 to 134 of UL24 [UL24(74-134)] is responsible for inhibiting cGAS-STING-mediated NF-κB promoter activity. For the first time, UL24 was shown to play an important role in immune evasion during HSV-1 infection.IMPORTANCE NF-κB is a critical component of the innate immune response and is strongly induced downstream of most pattern recognition receptors (PRRs), leading to the production of IFN-β as well as a number of inflammatory chemokines and interleukins. To establish persistent infection, viruses have evolved various mechanisms to counteract the host NF-κB pathway. In the present study, for the first time, HSV-1 UL24 was demonstrated to inhibit the activation of NF-κB in the DNA sensing signal pathway via binding to the RHDs of the NF-κB subunits p65 and p50 and abolishing their nuclear translocation.

Herpes Simplex Virus 1 Abrogates the cGAS/STING-Mediated Cytosolic DNA-Sensing Pathway via Its Virion Host Shutoff Protein, UL41

Cyclic GMP-AMP synthase (cGAS) is a key DNA sensor capable of detecting microbial DNA and activating the adaptor protein stimulator of interferon genes (STING), leading to interferon (IFN) production and host antiviral responses. Cells exhibited reduced type I IFN production in response to cytosolic DNA in the absence of cGAS. Although the cGAS/STING-mediated DNA-sensing signal is crucial for host defense against many viruses, especially for DNA viruses, few viral components have been identified to specifically target this signaling pathway. Herpes simplex virus 1 (HSV-1) is a DNA virus that has evolved multiple strategies to evade host immune responses. In the present study, we found that HSV-1 tegument protein UL41 was involved in counteracting the cGAS/STING-mediated DNA-sensing pathway. Our results showed that wild-type (WT) HSV-1 infection could inhibit immunostimulatory DNA-induced activation of the IFN signaling pathway compared with the UL41-null mutant virus (R2621), and ectopic expression of UL41 decreased cGAS/STING-mediated IFN-β promoter activation and IFN-β production. Further study indicated that UL41 reduced the accumulation of cGAS to abrogate host recognition of viral DNA. In addition, stable knockdown of cGAS facilitated the replication of R2621 but not WT HSV-1. For the first time, HSV-1 UL41 was demonstrated to evade the cGAS/STING-mediated DNA-sensing pathway by degrading cGAS via its RNase activity.IMPORTANCE HSV-1 is well known for its ability to evade host antiviral responses and establish a lifelong latent infection while triggering reactivation and lytic infection under stress. Currently, whether HSV-1 evades the cytosolic DNA sensing and signaling is still poorly understood. In the present study, we found that tegument protein UL41 targeted the cGAS/STING-mediated cellular DNA-sensing pathway by selectively degrading cGAS mRNA. Knockdown of endogenous cGAS could facilitate the replication of R2621 but not WT HSV-1. Furthermore, UL41 was shown for the first time to act directly on cGAS. Findings in this study could provide new insights into the host-virus interaction and help develop new approaches against HSV-1.

Herpes simplex virus 1 infection dampens the immediate early antiviral innate immunity signaling from peroxisomes by tegument protein VP16

Background

Herpes simplex virus 1 (HSV-1) is an archetypal member of the alphaherpesvirus subfamily with a large genome encoding over 80 proteins, many of which play a critical role in virus-host interactions and immune modulation. Upon viral infections, the host cells activate innate immune responses to restrict their replications. Peroxisomes, which have long been defined to regulate metabolic activities, are reported to be important signaling platforms for antiviral innate immunity. It has been verified that signaling from peroxisomal MAVS (MAVS-Pex) triggers a rapid interferon (IFN) independent IFN-stimulated genes (ISGs) production against invading pathogens. However, little is known about the interaction between DNA viruses such as HSV-1 and the MAVS-Pex mediated signaling.

Results

HSV-1 could activate the MAVS-Pex signaling pathway at a low multiplicity of infection (MOI), while infection at a high MOI dampens MAVS-Pex induced immediately early ISGs production. A high-throughput screen assay reveals that HSV-1 tegument protein VP16 inhibits the immediate early ISGs expression downstream of MAVS-Pex signaling. Moreover, the expression of ISGs was recovered when VP16 was knockdown with its specific short hairpin RNA.

Conclusion

HSV-1 blocks MAVS-Pex mediated early ISGs production through VP16 to dampen the immediate early antiviral innate immunity signaling from peroxisomes.

Herpes Simplex Virus 1 Ubiquitin-Specific Protease UL36 Abrogates NF-κB Activation in DNA Sensing Signal Pathway

The DNA sensing pathway triggers innate immune responses against DNA virus infection, and NF-κB signaling plays a critical role in establishing innate immunity. We report here that the herpes simplex virus 1 (HSV-1) ubiquitin-specific protease (UL36USP), which is a deubiquitinase (DUB), antagonizes NF-κB activation, depending on its DUB activity. In this study, ectopically expressed UL36USP blocked promoter activation of beta interferon (IFN-β) and NF-κB induced by cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING). UL36USP restricted NF-κB activation mediated by overexpression of STING, TANK-binding kinase 1, IκB kinase α (IKKα), and IKKβ, but not p65. UL36USP was also shown to inhibit IFN-stimulatory DNA-induced IFN-β and NF-κB activation under conditions of HSV-1 infection. Furthermore, UL36USP was demonstrated to deubiquitinate IκBα and restrict its degradation and, finally, abrogate NF-κB activation. More importantly, the recombinant HSV-1 lacking UL36USP DUB activity, denoted as C40A mutant HSV-1, failed to cleave polyubiquitin chains on IκBα. For the first time, UL36USP was shown to dampen NF-κB activation in the DNA sensing signal pathway to evade host antiviral innate immunity.IMPORTANCE It has been reported that double-stranded-DNA-mediated NF-κB activation is critical for host antiviral responses. Viruses have established various strategies to evade the innate immune system. The N terminus of the HSV-1 UL36 gene-encoded protein contains the DUB domain and is conserved across all herpesviruses. This study demonstrates that UL36USP abrogates NF-κB activation by cleaving polyubiquitin chains from IκBα and therefore restricts proteasome-dependent degradation of IκBα and that DUB activity is indispensable for this process. This study expands our understanding of the mechanisms utilized by HSV-1 to evade the host antiviral innate immune defense induced by NF-κB signaling.

Herpes Simplex Virus 1 UL41 Protein Suppresses the IRE1/XBP1 Signal Pathway of the Unfolded Protein Response via Its RNase Activity

During viral infection, accumulation of viral proteins can cause stress in the endoplasmic reticulum (ER) and trigger the unfolded protein response (UPR) to restore ER homeostasis. The inositol-requiring enzyme 1 (IRE1)-dependent pathway is the most conserved of the three UPR signal pathways. Upon activation, IRE1 splices out an intron from the unspliced inactive form of X box binding protein 1 [XBP1(u)] mRNA and produces a transcriptionally potent spliced form [XBP1(s)]. Previous studies have reported that the IRE1/XBP1 pathway is inhibited upon herpes simplex virus 1 (HSV-1) infection; however, the underlying molecular mechanism is still elusive. Here, we uncovered a role of the HSV-1 UL41 protein in inhibiting the IRE1/XBP1 signal pathway. Ectopic expression of UL41 decreased the expression of XBP1 and blocked XBP1 splicing activation induced by the ER stress inducer thapsigargin. Wild-type (WT) HSV-1, but not the UL41-null mutant HSV-1 (R2621), decreased XBP1 mRNA induced by thapsigargin. Nevertheless, infection with both WT HSV-1 and R2621 without drug pretreatment could reduce the mRNA and protein levels of XBP1(s), and additional mechanisms might contribute to this inhibition of XBP1(s) during R2621 infection. Taking these findings together, our results reveal XBP1 as a novel target of UL41 and provide insights into the mechanism by which HSV-1 modulates the IRE1/XBP1 pathway.

IMPORTANCE

During viral infection, viruses hijack the host translation apparatus to produce large amounts of viral proteins, which leads to ER stress. To restore ER homeostasis, cells initiate the UPR to alleviate the effects of ER stress. The IRE1/XBP1 pathway is the most conserved UPR branch, and it activates ER-associated protein degradation (ERAD) to reduce the ER load. The IRE1/XBP1 branch is repressed during HSV-1 infection, but little is known about the underlying molecular mechanism. Our results show for the first time that UL41 suppresses the IRE1/XBP1 signal pathway by reducing the accumulation of XBP1 mRNA, and characterization of the underlying molecular mechanism provides new insight into the modulation of UPR by HSV-1.

Herpes Simplex Virus 1 Tegument Protein UL41 Counteracts IFIT3 Antiviral Innate Immunity

The interferon-induced protein with tetratricopeptide repeat 3 (IFIT3 or ISG60) is a host-intrinsic antiviral factor that restricts many instances of DNA and RNA virus replication. Herpes simplex virus 1 (HSV-1), a DNA virus bearing a large genome, can encode many viral proteins to counteract the host immune responses. However, whether IFIT3 plays a role upon HSV-1 infection is little known. In this study, we show for the first time that HSV-1 tegument protein UL41, a viral endoribonuclease, plays an important role in inhibiting the antiviral activity of IFIT3. Here, we demonstrated that ectopically expressed IFIT3 could restrict the replication of vesicular stomatitis virus (VSV) but had little effect on the replication of wild-type (WT) HSV-1. Further study showed that WT HSV-1 infection downregulated the expression of IFIT3, and ectopic expression of UL41, but not the immediate-early protein ICP0, notably reduced the expression of IFIT3. The underlying molecular mechanism was that UL41 diminished the accumulation of IFIT3 mRNA to abrogate its antiviral activity. In addition, our results illustrated that ectopic expression of IFIT3 inhibited the replication of UL41-null mutant virus (R2621), and stable knockdown of IFIT3 facilitated its replication. Taking these findings together, HSV-1 was shown for the first time to evade the antiviral function of IFIT3 via UL41.

IMPORTANCE

The tegument protein UL41 of HSV-1 is an endoribonuclease with the substrate specificity of RNase A, which plays an important role in viral infection. Upon HSV-1 infection, interferons are critical cytokines that regulate immune responses against viral infection. Host antiviral responses are significantly boosted or crippled in the presence or absence of IFIT3; however, whether IFIT3 plays a role during HSV-1 infection is still unknown. Our data show for the first time that IFIT3 has little effect on HSV-1 replication, as UL41 decreases the accumulation of IFIT3 mRNA and subverts its antiviral activity. This study identifies IFIT3 as a novel target of the tegument protein UL41 and provides new insight into HSV-1-mediated immune evasion.

Diverse mechanisms evolved by DNA viruses to inhibit early host defenses

In mammalian cells, early defenses against infection by pathogens are mounted through a complex network of signaling pathways shepherded by immune-modulatory pattern-recognition receptors. As obligate parasites, the survival of viruses is dependent on the evolutionary acquisition of mechanisms that tactfully dismantle and subvert the cellular intrinsic and innate immune responses. Here, we review the diverse mechanisms by which viruses that accommodate DNA genomes are able to circumvent activation of cellular immunity. We start by discussing viral manipulation of host defense protein levels by either transcriptional regulation or protein degradation. We next review viral strategies used to repurpose or inhibit these cellular immune factors by molecular hijacking or by regulating their post-translational modification status. Additionally, we explore the infection-induced temporal modulation of apoptosis to facilitate viral replication and spread. Lastly, the co-evolution of viruses with their hosts is highlighted by the acquisition of elegant mechanisms for suppressing host defenses via viral mimicry of host factors. In closing, we present a perspective on how characterizing these viral evasion tactics both broadens the understanding of virus-host interactions and reveals essential functions of the immune system at the molecular level. This knowledge is critical in understanding the sources of viral pathogenesis, as well as for the design of antiviral therapeutics and autoimmunity treatments.

Herpes Simplex Virus 1 Serine Protease VP24 Blocks the DNA-Sensing Signal Pathway by Abrogating Activation of Interferon Regulatory Factor 3

The interferon (IFN)-mediated antiviral response is a central aspect of host defense; however, viruses have evolved multiple strategies to counteract IFN-mediated responses in order to successfully infect the host. Herpes simplex virus 1 (HSV-1), a typical human-restricted DNA virus, is capable of counteracting host immune responses via several distinct viral proteins, thus establishing a lifelong latent infection. In this study, we demonstrate that the VP24 protein, a serine protease of HSV-1 essential for the formation and maturation of capsids, is a novel antagonist of the beta interferon (IFN-β) pathway. Here, VP24 was shown for the first time to dampen interferon stimulatory DNA (ISD)-triggered IFN-β production and inhibit IFN-β promoter activation induced by cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING) and by STING, respectively. Further study demonstrated that ectopic expression of VP24 selectively blocked IFN regulatory factor 3 (IRF3) but not NF-κB promoter activation. In addition, VP24 was demonstrated to downregulate ISD-induced phosphorylation and dimerization of IRF3 during HSV-1 infection with a VP24 stable knockdown human foreskin fibroblast cell line. The underlying molecular mechanism is that VP24 abrogates the interaction between TANK-binding kinase 1 (TBK1) and IRF3, hence impairing IRF3 activation. These results illustrate that VP24 is able to block the production of IFN-β by inhibiting IRF3 activation, which may represent a critical adaptation to enable viral effective replication within the host.

IMPORTANCE

This study demonstrated that HSV-1 protein VP24 could inhibit IFN-β production and promoter activation triggered by ISD, cGAS and STING and by STING, respectively. VP24 selectively blocked IRF3 promoter activation and ISD-induced phosphorylation and dimerization of IRF3 without affecting the NF-κB promoter activation during viral infection. VP24 also inhibited IRF3 activation by impeding the interaction between TBK1 and IRF3 during viral infection. This study provides new insights into the immune evasion mediated by HSV-1 and identifies VP24 as a crucial effector for HSV-1 to evade the host DNA-sensing signal pathway.

The Ubiquitination of NF-κB Subunits in the Control of Transcription

Nuclear factor (NF)-κB has evolved as a latent, inducible family of transcription factors fundamental in the control of the inflammatory response. The transcription of hundreds of genes involved in inflammation and immune homeostasis require NF-κB, necessitating the need for its strict control. The inducible ubiquitination and proteasomal degradation of the cytoplasmic inhibitor of κB (IκB) proteins promotes the nuclear translocation and transcriptional activity of NF-κB. More recently, an additional role for ubiquitination in the regulation of NF-κB activity has been identified. In this case, the ubiquitination and degradation of the NF-κB subunits themselves plays a critical role in the termination of NF-κB activity and the associated transcriptional response. While there is still much to discover, a number of NF-κB ubiquitin ligases and deubiquitinases have now been identified which coordinate to regulate the NF-κB transcriptional response. This review will focus the regulation of NF-κB subunits by ubiquitination, the key regulatory components and their impact on NF-κB directed transcription.

Evasion of host antiviral innate immunity by HSV-1, an update

Herpes simplex virus type 1 (HSV-1) infection triggers a rapid induction of host innate immune responses. The type I interferon (IFN) signal pathway is a central aspect of host defense which induces a wide range of antiviral proteins to control infection of incoming pathogens. In some cases, viral invasion also induces DNA damage response, autophagy, endoplasmic reticulum stress, cytoplasmic stress granules and other innate immune responses, which in turn affect viral infection. However, HSV-1 has evolved multiple strategies to evade host innate responses and facilitate its infection. In this review, we summarize the most recent findings on the molecular mechanisms utilized by HSV-1 to counteract host antiviral innate immune responses with specific focus on the type I IFN signal pathway.

The N-Terminal of Aquareovirus NS80 Is Required for Interacting with Viral Proteins and Viral Replication

Reovirus replication and assembly occurs within viral inclusion bodies that formed in specific intracellular compartments of cytoplasm in infected cells. Previous study indicated that aquareovirus NS80 is able to form inclusion bodies, and also can retain viral proteins within its inclusions. To better understand how NS80 performed in viral replication and assembly, the functional regions of NS80 associated with other viral proteins in aquareovirus replication were investigated in this study. Deletion mutational analysis and rotavirus NSP5-based protein association platform were used to detect association regions. Immunofluorescence images indicated that different N-terminal regions of NS80 could associate with viral proteins VP1, VP4, VP6 and NS38. Further co-immunoprecipitation analysis confirmed the interaction between VP1, VP4, VP6 or NS38 with different regions covering the N-terminal amino acid (aa, 1–471) of NS80, respectively. Moreover, removal of NS80 N-terminal sequences required for interaction with proteins VP1, VP4, VP6 or NS38 not only prevented the capacity of NS80 to support viral replication in NS80 shRNA-based replication complementation assays, but also inhibited the expression of aquareovirus proteins, suggesting that N-terminal regions of NS80 are necessary for viral replication. These results provided a foundational basis for further understanding the role of NS80 in viral replication and assembly during aquareovirus infection.

Recent advances on viral manipulation of NF-κB signaling pathway

NF-κB transcription factors regulate the expression of hundreds of genes primarily involved in immune responses. Signaling events leading to NF-κB activation constitute a major antiviral immune pathway. To replicate and persist within their hosts, viruses have evolved diverse strategies to evade and exploit cellular NF-κB immune signaling cascades for their benefit. We summarize recent studies concerning viral manipulation of the NF-κB signaling pathway downstream of pattern recognition receptors. Signal transduction mediated by pattern recognition receptors is a research frontier for both infectious disease and innate immunology.

A Fas associated factor negatively regulates anti-bacterial immunity by promoting Relish degradation in Bombyx mori

Negative regulation is required to keep NF-κB-dependent immune response under tight control. In previous study, we have identified a Fas associated factor (FAF) family member in Bombyx mori, BmFAF, and proposed it may act as a negative regulator in immune response. In this study, we found knock-down of BmFAF by RNAi led to a remarkable increase in transcriptional level of several antimicrobial peptide genes, including BmCecropinA1 and BmMoricin, and higher survival rate to Gram-negative bacterial infection. We also confirmed the regulatory role of BmFAF in suppressing NF-κB-dependent transcription by employing an inducible promoter in BmE cells. Consistent with these physiological phenotypes, BmFAF suppressed the activity of the essential transcription factor, Relish, in IMD signaling pathway by promoting its proteasomal degradation through direct interaction. In addition, by constructing various truncation mutants, we further demonstrated that UBA domain in BmFAF is required for the inhibitory role, and potential ubiquitination also occurs in this domain. Taken together, our results suggest that BmFAF is a negative regulator of IMD pathway by mediating degradation of Relish.

Neurons versus herpes simplex virus: the innate immune interactions that contribute to a host-pathogen standoff

Herpes simplex virus (HSV) is a prevalent neurotropic virus, which establishes lifelong latent infections in the neurons of sensory ganglia. Despite our long-standing knowledge that HSV predominately infects sensory neurons during its life cycle, little is known about the neuronal antiviral response to HSV infection. Recent studies show that while sensory neurons have impaired intrinsic immunity to HSV infection, paracrine IFN signaling can potentiate a potent antiviral response. Additionally, antiviral autophagy plays an important role in neuronal control of HSV infection. Here we review the literature of antiviral signaling and autophagy in neurons, the mechanisms by which HSV can counteract these responses, and postulate how these two pathways may synergize to mediate neuronal control of HSV infection and yet result in lifelong persistence of the virus.

Evasion of early antiviral responses by herpes simplex viruses

Besides overcoming physical constraints, such as extreme temperatures, reduced humidity, elevated pressure, and natural predators, human pathogens further need to overcome an arsenal of antimicrobial components evolved by the host to limit infection, replication and optimally, reinfection. Herpes simplex virus-1 (HSV-1) and herpes simplex virus-2 (HSV-2) infect humans at a high frequency and persist within the host for life by establishing latency in neurons. To gain access to these cells, herpes simplex viruses (HSVs) must replicate and block immediate host antiviral responses elicited by epithelial cells and innate immune components early after infection. During these processes, infected and noninfected neighboring cells, as well as tissue-resident and patrolling immune cells, will sense viral components and cell-associated danger signals and secrete soluble mediators. While type-I interferons aim at limiting virus spread, cytokines and chemokines will modulate resident and incoming immune cells. In this paper, we discuss recent findings relative to the early steps taking place during HSV infection and replication. Further, we discuss how HSVs evade detection by host cells and the molecular mechanisms evolved by these viruses to circumvent early antiviral mechanisms, ultimately leading to neuron infection and the establishment of latency.

Innate recognition of alphaherpesvirus DNA

Alphaherpesviruses include human and animal pathogens, such as herpes simplex virus type 1, which establish life-long latent infections with episodes of recurrence. The immunocompetence of the infected host is an important determinant for the outcome of infections with alphaherpesviruses. Recognition of pathogen-associated molecular patterns by pattern recognition receptors is an essential, early step in the innate immune response to pathogens. In recent years, it has been discovered that herpesvirus DNA is a strong inducer of the innate immune system. The viral genome can be recognized in endosomes by TLR9, as well as intracellularly by a variety of DNA sensors, the best documented being cGAS, RNA Pol III, IFI16, and AIM2. These DNA sensors use converging signaling pathways to activate transcription factors, such as IRF3 and NF-κB, which induce the expression of type I interferons and other inflammatory cytokines and activate the inflammasome. This review summarizes the recent literature on the innate sensing of alphaherpesvirus DNA, the mechanisms of activation of the different sensors, their mechanisms of signal transduction, their physiological role in defense against herpesvirus infection, and how alphaherpesviruses seek to evade these responses to allow establishment and maintenance of infection.

Identification of TRIM27 as a novel degradation target of herpes simplex virus 1 ICP0

The herpes simplex virus 1 (HSV-1) immediate early protein ICP0 performs many functions during infection, including transactivation of viral gene expression, suppression of innate immune responses, and modification and eviction of histones from viral chromatin. Although these functions of ICP0 have been characterized, the detailed mechanisms underlying ICP0's complex role during infection warrant further investigation. We thus undertook an unbiased proteomic approach to identifying viral and cellular proteins that interact with ICP0 in the infected cell. Cellular candidates resulting from our analysis included the ubiquitin-specific protease USP7, the transcriptional repressor TRIM27, DNA repair proteins NBN and MRE11A, regulators of apoptosis, including BIRC6, and the proteasome. We also identified two HSV-1 early proteins involved in nucleotide metabolism, UL39 and UL50, as novel candidate interactors of ICP0. Because TRIM27 was the most statistically significant cellular candidate, we investigated the relationship between TRIM27 and ICP0. We observed rapid, ICP0-dependent loss of TRIM27 during HSV-1 infection. TRIM27 protein levels were restored by disrupting the RING domain of ICP0 or by inhibiting the proteasome, arguing that TRIM27 is a novel degradation target of ICP0. A mutant ICP0 lacking E3 ligase activity interacted with endogenous TRIM27 during infection as demonstrated by reciprocal coimmunoprecipitation and supported by immunofluorescence data. Surprisingly, ICP0-null mutant virus yields decreased upon TRIM27 depletion, arguing that TRIM27 has a positive effect on infection despite being targeted for degradation. These results illustrate a complex interaction between TRIM27 and viral infection with potential positive or negative effects of TRIM27 on HSV under different infection conditions.

IMPORTANCE

During productive infection, a virus must simultaneously redirect multiple cellular pathways to replicate itself while evading detection by the host's defenses. To orchestrate such complex regulation, viruses, including herpes simplex virus 1 (HSV-1), rely on multifunctional proteins such as the E3 ubiquitin ligase ICP0. This protein regulates various cellular pathways concurrently by targeting a diverse set of cellular factors for degradation. While some of these targets have been previously identified and characterized, we undertook a proteomic screen to identify additional targets of this activity to further characterize ICP0's role during infection. We describe a set of candidate interacting proteins of ICP0 identified through this approach and our characterization of the most statistically significant result, the cellular transcriptional repressor TRIM27. We present TRIM27 as a novel degradation target of ICP0 and describe the relationship of these two proteins during infection.

Herpes simplex virus 1 counteracts viperin via its virion host shutoff protein UL41

The interferon (IFN)-inducible viperin protein restricts a broad range of viruses. However, whether viperin plays a role during herpes simplex virus 1 (HSV-1) infection is poorly understood. In the present study, it was shown for the first time that wild-type (WT) HSV-1 infection couldn't induce viperin production, and ectopically expressed viperin inhibited the replication of UL41-null HSV-1 but not WT viruses. The underlying molecular mechanism is that UL41 counteracts viperin's antiviral activity by reducing its mRNA accumulation.

HSV-1 ICP0: An E3 Ubiquitin Ligase That Counteracts Host Intrinsic and Innate Immunity

The herpes simplex virus type 1 (HSV-1) encoded E3 ubiquitin ligase, infected cell protein 0 (ICP0), is required for efficient lytic viral replication and regulates the switch between the lytic and latent states of HSV-1. As an E3 ubiquitin ligase, ICP0 directs the proteasomal degradation of several cellular targets, allowing the virus to counteract different cellular intrinsic and innate immune responses. In this review, we will focus on how ICP0’s E3 ubiquitin ligase activity inactivates the host intrinsic defenses, such as nuclear domain 10 (ND10), SUMO, and the DNA damage response to HSV-1 infection. In addition, we will examine ICP0’s capacity to impair the activation of interferon (innate) regulatory mediators that include IFI16 (IFN γ-inducible protein 16), MyD88 (myeloid differentiation factor 88), and Mal (MyD88 adaptor-like protein). We will also consider how ICP0 allows HSV-1 to evade activation of the NF-κB (nuclear factor kappa B) inflammatory signaling pathway. Finally, ICP0’s paradoxical relationship with USP7 (ubiquitin specific protease 7) and its roles in intrinsic and innate immune responses to HSV-1 infection will be discussed.

Herpes simplex virus 1 protein kinase US3 hyperphosphorylates p65/RelA and dampens NF-κB activation

Nuclear factor κB (NF-κB) plays important roles in innate immune responses by regulating the expression of a large number of target genes involved in the immune and inflammatory response, apoptosis, cell proliferation, differentiation, and survival. To survive in the host cells, viruses have evolved multiple strategies to evade and subvert the host immune response. Herpes simplex virus 1 (HSV-1) bears a large DNA genome, with the capacity to encode many different viral proteins to counteract the host immune responses. In the present study, we demonstrated that HSV-1 protein kinase US3 significantly inhibited NF-κB activation and decreased the expression of inflammatory chemokine interleukin-8 (IL-8). US3 was also shown to hyperphosphorylate p65 at serine 75 and block its nuclear translocation. Two US3 mutants, K220M and D305A, still interacted with p65; however, they could not hyperphosphorylate p65, indicating that the kinase activity of US3 was indispensable for the function. The attenuation of NF-κB activation by HSV-1 US3 protein kinase may represent a critical adaptation to enable virus persistence within the host. Importance: This study demonstrated that HSV-1 protein kinase US3 significantly inhibited NF-κB activation and decreased the expression of inflammatory chemokine interleukin-8 (IL-8). US3 hyperphosphorylated p65 at serine 75 to inhibit NF-κB activation. The kinase activity of US3 was indispensable for its hyperphosphorylation of p65 and abrogation of the nuclear translocation of p65. The present study elaborated a novel mechanism of HSV-1 US3 to evade the host innate immunity.

The ubiquitin-conjugating system: multiple roles in viral replication and infection

Through the combined action of ubiquitinating and deubiquitinating enzymes, conjugation of ubiquitin to a target protein acts as a reversible post-translational modification functionally similar to phosphorylation. Indeed, ubiquitination is more and more recognized as a central process for the fine regulation of many cellular pathways. Due to their nature as obligate intracellular parasites, viruses rely on the most conserved host cell machineries for their own replication. Thus, it is not surprising that members from almost every viral family are challenged by ubiquitin mediated mechanisms in different steps of their life cycle and have evolved in order to by-pass or exploit the cellular ubiquitin conjugating system to maximize their chance to establish a successful infection. In this review we will present several examples of the complex interplay that links viruses and the ubiquitin conjugation machinery, with a special focus on the mechanisms evolved by the human immunodeficiency virus to escape from cellular restriction factors and to exit from infected cells.