Herpes simplex virus 1 tegument protein US11 downmodulates the RLR signaling pathway via direct interaction with RIG-I and MDA-5

Cytosolic Innate Immune Sensing and Signaling upon Infection

Cytosolic sensing of pathogens is essential to a productive immune response. Recent reports have emphasized the importance of signaling platforms emanating from organelles and cytosolic sensors, particularly during the response to intracellular pathogens. Here, we highlight recent discoveries identifying the key mediators of nucleic acid and cyclic nucleotide sensing and discuss their importance in host defense. This review will also cover strategies evolved by pathogens to manipulate these pathways.

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.

Human Herpesvirus 6A U14 Is Important for Virus Maturation

Human herpesvirus 6A (HHV-6A) U14 is a virion protein with little known function in virus propagation. Here, we elucidated its function by constructing and analyzing U14-mutated viruses. We found that U14 is essential for HHV-6A propagation. We then constructed a mutant virus harboring dysfunctional U14. This virus showed severely reduced growth and retarded maturation. Taken together, these data indicate that U14 plays an important role during HHV-6A maturation.

Designing Herpes Viruses as Oncolytics

Oncolytic herpes simplex virus (oHSV) was one of the first genetically-engineered oncolytic viruses. Because herpes simplex virus (HSV) is a natural human pathogen that can cause serious disease, it is incumbent that it be genetically-engineered or significantly attenuated for safety. Here we present a detailed explanation of the functions of HSV-1 genes frequently mutated to endow oncolytic activity. These genes are non-essential for growth in tissue culture cells but are important for growth in post-mitotic cells, interfering with intrinsic antiviral and innate immune responses or causing pathology, functions dispensable for replication in cancer cells. Understanding the function of these genes leads to informed creation of new oHSVs with better therapeutic efficacy. Virus infection and replication can also be directed to cancer cells through tumor-selective receptor binding and transcriptional- or post-transcriptional miRNA-targeting, respectively. In addition to the direct effects of oHSV on infected cancer cells and tumors, oHSV can be 'armed' with transgenes that are: reporters, to track virus replication and spread; cytotoxic, to kill uninfected tumor cells; immune modulatory, to stimulate anti-tumor immunity; or tumor microenvironment altering, to enhance virus spread or to inhibit tumor growth. In addition to HSV-1, other alphaherpesviruses are also discussed for their oncolytic activity.

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.

Innate responses to small interfering RNA pools inhibiting herpes simplex virus infection in astrocytoid and epithelial cells

Herpes simplex virus (HSV) is a human pathogen that can cause severe diseases such as encephalitis, keratitis and neonatal herpes. Control of HSV infection may be achieved by using small interfering (si)RNAs. We have designed and enzymatically produced pools of siRNAs targeting HSV. In addition to the target-specific effects, such siRNAs may induce innate immunity responses that may contribute to antiviral effects. HSV has versatile ways of modulating innate immunity, and it remains unclear whether HSV-specific antiviral treatment would benefit from the potential immunostimulatory effects of siRNAs. To address this, cell lines derived from epithelium and nervous system were studied for innate immunity reactions to HSV infection, to siRNA treatment, and to a combination of treatment and infection. In addition, the outcome of HSV infection was quantitated. We show that innate immunity reactions vary drastically between the cell lines. Moreover, our findings indicate only a minimal relation between the antiviral effect and the treatment-induced innate immunity responses. Thus, the antiviral effect is mainly sequence specific and the inhibition of HSV infection is not ascribed to the slight innate immunity induction.

The Z proteins of pathogenic but not nonpathogenic arenaviruses inhibit RIG-I-like receptor-dependent interferon production

Arenavirus pathogens cause a wide spectrum of diseases in humans ranging from central nervous system disease to lethal hemorrhagic fevers with few treatment options. The reason why some arenaviruses can cause severe human diseases while others cannot is unknown. We find that the Z proteins of all known pathogenic arenaviruses, lymphocytic choriomeningitis virus (LCMV) and Lassa, Junin, Machupo, Sabia, Guanarito, Chapare, Dandenong, and Lujo viruses, can inhibit retinoic acid-inducible gene 1 (RIG-i) and Melanoma Differentiation-Associated protein 5 (MDA5), in sharp contrast to those of 14 other nonpathogenic arenaviruses. Inhibition of the RIG-i-like receptors (RLRs) by pathogenic Z proteins is mediated by the protein-protein interactions of Z and RLRs, which lead to the disruption of the interactions between RLRs and mitochondrial antiviral signaling (MAVS). The Z-RLR interactive interfaces are located within the N-terminal domain (NTD) of the Z protein and the N-terminal CARD domains of RLRs. Swapping of the LCMV Z NTD into the nonpathogenic Pichinde virus (PICV) genome does not affect virus growth in Vero cells but significantly inhibits the type I interferon (IFN) responses and increases viral replication in human primary macrophages. In summary, our results show for the first time an innate immune-system-suppressive mechanism shared by the diverse pathogenic arenaviruses and thus shed important light on the pathogenic mechanism of human arenavirus pathogens.

IMPORTANCE

We show that all known human-pathogenic arenaviruses share an innate immune suppression mechanism that is based on viral Z protein-mediated RLR inhibition. Our report offers important insights into the potential mechanism of arenavirus pathogenesis, provides a convenient way to evaluate the pathogenic potential of known and/or emerging arenaviruses, and reveals a novel target for the development of broad-spectrum therapies to treat this group of diverse pathogens. More broadly, our report provides a better understanding of the mechanisms of viral immune suppression and host-pathogen interactions.

DNA virus uncoating

Virus genomes are condensed and packaged inside stable proteinaceous capsids that serve to protect them during transit from one cell or host organism, to the next. During virus entry, capsid shells are primed and disassembled in a complex, tightly-regulated, multi-step process termed uncoating. Here we compare the uncoating-programs of DNA viruses of the pox-, herpes-, adeno-, polyoma-, and papillomavirus families. Highlighting the chemical and mechanical cues virus capsids respond to, we review the conformational changes that occur during stepwise disassembly of virus capsids and how these culminate in the release of viral genomes at the right time and cellular location to assure successful replication.

HSV-2 immediate-early protein US1 inhibits IFN-β production by suppressing association of IRF-3 with IFN-β promoter

HSV-2 is the major cause of genital herpes, and its infection increases the risk of HIV-1 acquisition and transmission. After initial infection, HSV-2 can establish latency within the nervous system and thus maintains lifelong infection in humans. It has been suggested that HSV-2 can inhibit type I IFN signaling, but the underlying mechanism has yet to be determined. In this study, we demonstrate that productive HSV-2 infection suppresses Sendai virus (SeV) or polyinosinic-polycytidylic acid-induced IFN-β production. We further reveal that US1, an immediate-early protein of HSV-2, contributes to such suppression, showing that US1 inhibits IFN-β promoter activity and IFN-β production at both mRNA and protein levels, whereas US1 knockout significantly impairs such capability in the context of HSV-2 infection. US1 directly interacts with DNA binding domain of IRF-3, and such interaction suppresses the association of nuclear IRF-3 with the IRF-3 responsive domain of IFN-β promoter, resulting in the suppression of IFN-β promoter activation. Additional studies demonstrate that the 217-414 aa domain of US1 is critical for the suppression of IFN-β production. Our results indicate that HSV-2 US1 downmodulates IFN-β production by suppressing the association of IRF-3 with the IRF-3 responsive domain of IFN-β promoter. Our findings highlight the significance of HSV-2 US1 in inhibiting IFN-β production and provide insights into the molecular mechanism by which HSV-2 evades the host innate immunity, representing an unconventional strategy exploited by a dsDNA virus to interrupt type I IFN signaling pathway.

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.

IFI16 restricts HSV-1 replication by accumulating on the hsv-1 genome, repressing HSV-1 gene expression, and directly or indirectly modulating histone modifications

Interferon-γ inducible factor 16 (IFI16) is a multifunctional nuclear protein involved in transcriptional regulation, induction of interferon-β (IFN-β), and activation of the inflammasome response. It interacts with the sugar-phosphate backbone of dsDNA and modulates viral and cellular transcription through largely undetermined mechanisms. IFI16 is a restriction factor for human cytomegalovirus (HCMV) and herpes simplex virus (HSV-1), though the mechanisms of HSV-1 restriction are not yet understood. Here, we show that IFI16 has a profound effect on HSV-1 replication in human foreskin fibroblasts, osteosarcoma cells, and breast epithelial cancer cells. IFI16 knockdown increased HSV-1 yield 6-fold and IFI16 overexpression reduced viral yield by over 5-fold. Importantly, HSV-1 gene expression, including the immediate early proteins, ICP0 and ICP4, the early proteins, ICP8 and TK, and the late proteins gB and Us11, was reduced in the presence of IFI16. Depletion of the inflammasome adaptor protein, ASC, or the IFN-inducing transcription factor, IRF-3, did not affect viral yield. ChIP studies demonstrated the presence of IFI16 bound to HSV-1 promoters in osteosarcoma (U2OS) cells and fibroblasts. Using CRISPR gene editing technology, we generated U2OS cells with permanent deletion of IFI16 protein expression. ChIP analysis of these cells and wild-type (wt) U2OS demonstrated increased association of RNA polymerase II, TATA binding protein (TBP) and Oct1 transcription factors with viral promoters in the absence of IFI16 at different times post infection. Although IFI16 did not alter the total histone occupancy at viral or cellular promoters, its absence promoted markers of active chromatin and decreased those of repressive chromatin with viral and cellular gene promoters. Collectively, these studies for the first time demonstrate that IFI16 prevents association of important transcriptional activators with wt HSV-1 promoters and suggest potential mechanisms of IFI16 restriction of wt HSV-1 replication and a direct or indirect role for IFI16 in histone modification.

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.

The herpes simplex virus 1 virion host shutoff protein enhances translation of viral late mRNAs by preventing mRNA overload

We recently demonstrated that the virion host shutoff (vhs) protein, an mRNA-specific endonuclease, is required for efficient herpes simplex virus 1 (HSV-1) replication and translation of viral true-late mRNAs, but not other viral and cellular mRNAs, in many cell types (B. Dauber, J. Pelletier, and J. R. Smiley, J. Virol. 85:5363-5373, 2011, http://dx.doi.org/10.1128/JVI.00115-11). Here, we evaluated whether the structure of true-late mRNAs or the timing of their transcription is responsible for the poor translation efficiency in the absence of vhs. To test whether the highly structured 5' untranslated region (5'UTR) of the true-late gC mRNA is the primary obstacle for translation initiation, we replaced it with the less structured 5'UTR of the γ-actin mRNA. However, this mutation did not restore translation in the context of a vhs-deficient virus. We then examined whether the timing of transcription affects translation efficiency at late times. To this end, we engineered a vhs-deficient virus mutant that transcribes the true-late gene US11 with immediate-early kinetics (IEUS11-ΔSma). Interestingly, IEUS11-ΔSma showed increased translational activity on the US11 transcript at late times postinfection, and US11 protein levels were restored to wild-type levels. These results suggest that mRNAs can maintain translational activity throughout the late stage of infection if they are present before translation factors and/or ribosomes become limiting. Taken together, these results provide evidence that in the absence of the mRNA-destabilizing function of vhs, accumulation of viral mRNAs overwhelms the capacity of the host translational machinery, leading to functional exclusion of the last mRNAs that are made during infection.

IMPORTANCE

The process of mRNA translation accounts for a significant portion of a cell's energy consumption. To ensure efficient use of cellular resources, transcription, translation, and mRNA decay are tightly linked and highly regulated. However, during virus infection, the overall amount of mRNA may increase drastically, possibly overloading the capacity of the translation apparatus. Our results suggest that the HSV-1 vhs protein, an mRNA-specific endoribonuclease, prevents mRNA overload during infection, thereby allowing translation of late viral mRNAs. The requirement for vhs varies between cell types. Further studies of the basis for this difference likely will offer insights into how cells regulate overall mRNA levels and access to the translational apparatus.

Commentary on the regulation of viral proteins in autophagy process

The ability to subvert intracellular antiviral defenses is necessary for virus to survive as its replication occurs only in the host cells. Viruses have to modulate cellular processes and antiviral mechanisms to their own advantage during the entire virus life cycle. Autophagy plays important roles in cell regulation. Its function is not only to catabolize aggregate proteins and damaged organelles for recycling but also to serve as innate immunity to remove intracellular pathogenic elements such as viruses. Nevertheless, some viruses have evolved to negatively regulate autophagy by inhibiting its formation. Even more, some viruses have employed autophagy to benefit their replication. To date, there are more and more growing evidences uncovering the functions of many viral proteins to regulate autophagy through different cellular pathways. In this review, we will discuss the relationship between viruses and autophagy and summarize the current knowledge on the functions of viral proteins contributing to affect autophagy process.

Recognition of herpes simplex viruses: toll-like receptors and beyond

Herpes simplex viruses (HSVs) are human pathogens that establish lytic and latent infections. Reactivation from latency occurs intermittently, which represents a lifelong source of recurrent infection. In this complex process, HSV triggers and neutralizes innate immunity. Therefore, a dynamic equilibrium between HSV and the innate immune system determines the outcome of viral infection. Detection of HSV involves pathogen recognition receptors that include Toll-like receptors, retinoic acid-inducible gene I-like receptors, and cytosolic DNA sensors. Moreover, innate components or pathways exist to sense membrane fusion upon viral entry into host cells. Consequently, this surveillance network activates downstream transcription factors, leading to the induction of type I interferon and inflammatory cytokines. Not surprisingly, with the capacity to establish chronic infection HSV has evolved strategies that modulate or evade innate immunity. In this review, we describe recent advances pertinent to the interplay of HSV and the induction of innate immunity mediated by pathogen recognition receptors or pathways.

Master sensors of pathogenic RNA - RIG-I like receptors

Initiating the immune response to invading pathogens, the innate immune system is constituted of immune receptors (pattern recognition receptors, PRR) that sense microbe-associated molecular patterns (MAMPs). Detection of pathogens triggers intracellular defense mechanisms, such as the secretion of cytokines or chemokines to alarm neighboring cells and attract or activate immune cells. The innate immune response to viruses is mostly based on PRRs that detect the unusual structure, modification or location of viral nucleic acids. Most of the highly pathogenic and emerging viruses are RNA genome-based viruses, which can give rise to zoonotic and epidemic diseases or cause viral hemorrhagic fever. As viral RNA is located in the same compartment as host RNA, PRRs in the cytosol have to discriminate between viral and endogenous RNA by virtue of their structure or modification. This challenging task is taken on by the homologous cytosolic DExD/H-box family helicases RIG-I and MDA5, which control the innate immune response to most RNA viruses. This review focuses on the molecular basis for RIG-I like receptor (RLR) activation by synthetic and natural ligands and will discuss controversial ligand definitions.

Suppression of PACT-induced type I interferon production by herpes simplex virus 1 Us11 protein

Herpes simplex virus 1 (HSV-1) Us11 protein is a double-stranded RNA-binding protein that suppresses type I interferon production through the inhibition of the cytoplasmic RNA sensor RIG-I. Whether additional cellular mediators are involved in this suppression remains to be determined. In this study, we report on the requirement of cellular double-stranded RNA-binding protein PACT for Us11-mediated perturbation of type I interferon production. Us11 associates with PACT tightly to prevent it from binding with and activating RIG-I. The Us11-deficient HSV-1 was indistinguishable from the Us11-proficient virus in the suppression of interferon production when PACT was compromised. More importantly, HSV-1-induced activation of interferon production was abrogated in PACT knockout murine embryonic fibroblasts. Our findings suggest a new mechanism for viral evasion of innate immunity through which a viral double-stranded RNA-binding protein interacts with PACT to circumvent type I interferon production. This mechanism might also be used by other PACT-binding viral interferon-antagonizing proteins such as Ebola virus VP35 and influenza A virus NS1.

Herpes simplex virus 1 serine/threonine kinase US3 hyperphosphorylates IRF3 and inhibits beta interferon production

Viral infection initiates a series of signaling cascades that lead to the transcription of interferons (IFNs), finally inducing interferon-stimulated genes (ISGs) to eliminate viruses. Viruses have evolved a variety of strategies to modulate host IFN-mediated immune responses. Herpes simplex virus 1 (HSV-1) US3, a Ser/Thr kinase conserved in alphaherpesviruses, was previously reported to counteract host innate immunity; however, the molecular mechanism is elusive. In this study, we report that US3 blocks IFN-β production by hyperphosphorylating IFN regulatory factor 3 (IRF3). Ectopic expression of US3 protein significantly inhibited Sendai virus (SeV)-mediated activation of IFN-β and IFN-stimulated response element (ISRE) promoters and the transcription of IFN-β, ISG54, and ISG56. US3 was also shown to block SeV-induced dimerization and nuclear translocation of IRF3. The kinase activity was indispensable for its inhibitory function, as kinase-dead (KD) US3 mutants K220M and D305A could not inhibit IFN-β production. Furthermore, US3 interacted with and hyperphosphorylated IRF3 at Ser175 to prevent IRF3 activation. Finally, the US3 KD mutant viruses were constructed and denoted K220M or D305A HSV-1, respectively. Cells and mice infected with both mutant viruses produced remarkably larger amounts of IFN-β than those infected with wild-type HSV-1. For the first time, these findings provide convincing evidence that US3 hyperphosphorylates IRF3, blocks the production of IFN-β, and subverts host innate immunity.

Middle East respiratory syndrome coronavirus accessory protein 4a is a type I interferon antagonist

Middle East respiratory syndrome coronavirus (MERS-CoV) causes severe acute respiratory infection with as yet unclear epidemiology. We previously showed that MERS-CoV counteracts parts of the innate immune response in human bronchiolar cells. Here we analyzed accessory proteins 3, 4a, 4b, and 5 for their abilities to inhibit the type I interferon response. Accessory protein 4a was found to block interferon induction at the level of melanoma differentiation-associated protein 5 (MDA5) activation presumably by direct interaction with double-stranded RNA.

Herpes simplex virus 1 ubiquitin-specific protease UL36 inhibits beta interferon production by deubiquitinating TRAF3

Interferon (IFN)-mediated innate immune defense is a potent antiviral mechanism. Viruses evade innate immunity and limit secretion of beta interferon (IFN-β) to replicate and survive in the host. The largest tegument protein of herpes simplex virus 1 (HSV-1), UL36, contains a novel deubiquitinase (DUB) motif embedded in its N terminus, denoted UL36 ubiquitin-specific protease (UL36USP). In the present study, we demonstrate that HSV-1 UL36USP inhibits Sendai virus (SeV)-induced interferon regulatory factor 3 (IRF3) dimerization, promoter activation, and transcription of IFN-β. The DUB activity of UL36USP is essential to block IFN-β production. UL36USP also inhibited IFN-β promoter activity induced by overexpression of the N terminus of RIG-I (RIG-IN) and MAVS, but not TBK-1, IκB kinase ε (IKKε), and IRF3/5D. UL36USP was subsequently shown to deubiquitinate TRAF3 and prevent the recruitment of the downstream adaptor TBK1. The recombinant HSV-1 lacking UL36USP DUB activity was generated. Cells infected with the mutant virus produced more IFN-β than wild-type (WT) HSV-1-infected cells. These findings demonstrate HSV-1 UL36USP removes polyubiquitin chains on TRAF3 and counteracts the IFN-β pathway.

Herpes simplex virus 1-encoded tegument protein VP16 abrogates the production of beta interferon (IFN) by inhibiting NF-κB activation and blocking IFN regulatory factor 3 to recruit its coactivator CBP

Host cells activate innate immune signaling pathways to defend against invading pathogens. To survive within an infected host, viruses have evolved intricate strategies to counteract host immune responses. Herpesviruses, including herpes simplex virus type 1 (HSV-1), have large genomes and therefore have the capacity to encode numerous proteins that modulate host innate immune responses. Here we define the contribution of HSV-1 tegument protein VP16 in the inhibition of beta interferon (IFN-β) production. VP16 was demonstrated to significantly inhibit Sendai virus (SeV)-induced IFN-β production, and its transcriptional activation domain was not responsible for this inhibition activity. Additionally, VP16 blocked the activation of the NF-κB promoter induced by SeV or tumor necrosis factor alpha treatment and expression of NF-κB-dependent genes through interaction with p65. Coexpression analysis revealed that VP16 selectively blocked IFN regulatory factor 3 (IRF-3)-mediated but not IRF-7-mediated transactivation. VP16 was able to bind to IRF-3 but not IRF-7 in vivo, based on coimmunoprecipitation analysis, but it did not affect IRF-3 dimerization, nuclear translocation, or DNA binding activity. Rather, VP16 interacted with the CREB binding protein (CBP) coactivator and efficiently inhibited the formation of the transcriptional complexes IRF-3-CBP in the context of HSV-1 infection. These results illustrate that VP16 is able to block the production of IFN-β by inhibiting NF-κB activation and interfering with IRF-3 to recruit its coactivator CBP, which may be important to the early events leading to HSV-1 infection.

Synergistic control of herpes simplex virus pathogenesis by IRF-3, and IRF-7 revealed through non-invasive bioluminescence imaging

Interferon regulatory factors IRF-3 and IRF-7 are central to the establishment of the innate antiviral response. This study examines HSV-1 pathogenesis in IRF-3(-/-), IRF-7(-/-) and double-deleted IRF3/7(-/-) (DKO) mice. Bioluminescence imaging of infection revealed that DKO mice developed visceral infection following corneal inoculation, along with increased viral burdens in all tissues relative to single knockout mice. While all DKO mice synchronously reached endpoint criteria 5 days post infection, the IRF-7(-/-) mice survived longer, indicating that although IRF-7 is dominant, IRF-3 also plays a role in controlling disease. Higher levels of systemic pro-inflammatory cytokines were found in IRF7(-/-) and DKO mice relative to wild-type and IRF-3(-/-) mice, and IL-6 and G-CSF, indicative of sepsis, were increased in the DKO mice relative to wild-type or single-knockout mice. In addition to controlling viral replication, IRF-3 and -7 therefore play coordinating roles in modulation of inflammation during HSV infection.

Analysis of the early steps of herpes simplex virus 1 capsid tegumentation

Herpes simplex virus type 1 particles are multilayered structures with a DNA genome surrounded by a capsid, tegument, and envelope. While the protein content of mature virions is known, the sequence of addition of the tegument and the intracellular compartments where this occurs are intensely debated. To probe this process during the initial stages of egress, we used two approaches: an in vitro nuclear egress assay, which reconstitutes the exit of nuclear capsids to the cytoplasm, and a classical nuclear capsid sedimentation assay. As anticipated, in vitro cytoplasmic capsids did not harbor UL34, UL31, or viral glycoproteins but contained US3. In agreement with previous findings, both nuclear and in vitro capsids were positive for ICP0 and ICP4. Unexpectedly, nuclear C capsids and cytoplasmic capsids produced in vitro without any cytosolic viral proteins also scored positive for UL36 and UL37. Immunoelectron microscopy confirmed that these tegument proteins were closely associated with nuclear capsids. When cytosolic viral proteins were present in the in vitro assay, no additional tegument proteins were detected on the capsids. As previously reported, the tegument was sensitive to high-salt extraction but, surprisingly, was stabilized by exogenous proteins. Finally, some tegument proteins seemed partially lost during egress, while others possibly were added at multiple steps or modified along the way. Overall, an emerging picture hints at the early coating of capsids with up to 5 tegument proteins at the nuclear stage, the shedding of some viral proteins during nuclear egress, and the acquisition of others tegument proteins during reenvelopment.

The herpes simplex virus 1 Us11 protein inhibits autophagy through its interaction with the protein kinase PKR

Autophagy is now known to be an essential component of host innate and adaptive immunity. Several herpesviruses have developed various strategies to evade this antiviral host defense. Herpes simplex virus 1 (HSV-1) blocks autophagy in fibroblasts and in neurons, and the ICP34.5 protein is important for the resistance of HSV-1 to autophagy because of its interaction with the autophagy machinery protein Beclin 1. ICP34.5 also counteracts the shutoff of protein synthesis mediated by the double-stranded RNA (dsRNA)-dependent protein kinase PKR by inhibiting phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α) in the PKR/eIF2α signaling pathway. Us11 is a late gene product of HSV-1, which is also able to preclude the host shutoff by direct inhibition of PKR. In the present study, we unveil a previously uncharacterized function of Us11 by demonstrating its antiautophagic activity. We show that the expression of Us11 is able to block autophagy and autophagosome formation in both HeLa cells and fibroblasts. Furthermore, immediate-early expression of Us11 by an ICP34.5 deletion mutant virus is sufficient to render the cells resistant to PKR-induced and virus-induced autophagy. PKR expression and the PKR binding domain of Us11 are required for the antiautophagic activity of Us11. However, unlike ICP34.5, Us11 did not interact with Beclin 1. We suggest that the inhibition of autophagy observed in cells infected with HSV-1 results from the activity of not only ICP34.5 on Beclin 1 but also Us11 by direct interaction with PKR.

Nuclear IFI16 induction of IRF-3 signaling during herpesviral infection and degradation of IFI16 by the viral ICP0 protein

Innate sensing of microbial components is well documented to occur at many cellular sites, including at the cell surface, in the cytosol, and in intracellular vesicles, but there is limited evidence of nuclear innate signaling. In this study we have defined the mechanisms of interferon regulatory factor-3 (IRF-3) signaling in primary human foreskin fibroblasts (HFF) infected with herpes simplex virus 1 (HSV-1) in the absence of viral gene expression. We found that the interferon inducible protein 16 (IFI16) DNA sensor, which is required for induction of IRF-3 signaling in these cells, is nuclear, and its localization does not change detectably upon HSV-1 d109 infection and induction of IRF-3 signaling. Consistent with the IFI16 sensor being nuclear, conditions that block viral DNA release from incoming capsids inhibit IRF-3 signaling. An unknown factor must be exported from the nucleus to activate IRF-3 through cytoplasmic STING, which is required for IRF-3 activation and signaling. However, when the viral ICP0 protein is expressed in the nucleus, it causes the nuclear relocalization and degradation of IFI16, inhibiting IRF-3 signaling. Therefore, HSV-1 infection is sensed in HFF by nuclear IFI16 upon release of encapsidated viral DNA into the nucleus, and the viral nuclear ICP0 protein can inhibit the process by targeting IFI16 for degradation. Together these results define a pathway for nuclear innate sensing of HSV DNA by IFI16 in infected HFF and document a mechanism by which a virus can block this nuclear innate response.

The Akt1 isoform is required for optimal IFN-β transcription through direct phosphorylation of β-catenin

IFN-β is a critical antiviral cytokine that is capable of modulating the systemic immune response. The transcriptional induction of IFN-β is a highly regulated process, involving the activation of pattern recognition receptors and their downstream signaling pathways. The Akt family of serine/threonine kinases includes three isoforms. The specific role for the individual Akt isoforms in pattern recognition and signaling remains unclear. In this article, we report that the TLR3-mediated expression of IFN-β is blunted in cells that lack Akt1. The expression of IFN-β-inducible genes such as CCL5 and CXCL10 was also reduced in Akt1-deficient cells; the induction of TNF-α and CXCL2, whose expression does not rely on IFN-β, was not reduced in the absence of Akt1. Macrophages from Akt1(-/-) mice displayed deficient clearance of HSV-1 along with reduced IFN-β expression. Our results demonstrate that Akt1 signals through β-catenin by phosphorylation on Ser(552), a site that differs from the glycogen synthase kinase 3 β phosphorylation site. Stimulation of a chemically activated version of Akt1, in the absence of other TLR3-dependent signaling, was sufficient for accumulation and phosphorylation of β-catenin at Ser(552). Taken together, these results demonstrate that the Akt1 isoform is required for β-catenin-mediated promotion of IFN-β transcription downstream of TLR3 activation.