Protection by herpes simplex virus glycoprotein D against Fas-mediated apoptosis: role of nuclear factor kappaB

Comparison of the Severity of Zoster-Associated Pain and Incidence of Postherpetic Neuralgia in Patients with and without Pre-Existing Spinal Disorders at the Same Spinal Nerve Level: A Retrospective Multicenter Study

The incidences of herpes zoster (HZ) and postherpetic neuralgia (PHN) are significantly influenced by age. As individuals age, the occurrence of spinal disorders increases, thereby raising the likelihood of HZ and PHN coexistence. Considering this, our study aimed to explore the potential impact of pre-existing spinal disorders at the nerve level where HZ developed, on the severity of zoster-associated pain (ZAP) and the incidence of PHN. For our investigation, we retrospectively analyzed a total of 237 patients who presented with HZ and ZAP at various sensory levels (cervical, thoracic, lumbar, and sacral) with or without pre-existing spinal disorders. The presence or absence of spinal disorders at the sensory level affected by HZ was determined using computed tomography or magnetic resonance imaging. Our study results revealed that the group with spinal disorders at the sensory level where HZ developed did not exhibit an increased incidence of PHN. However, 3-6 months after HZ onset, this same group showed significantly higher ZAP scores compared to the group without spinal disorders. It implies a need for heightened pain management, as the coexistence of these conditions can increase pain severity. This study furnishes an initial standpoint to delve into intricate interactions between two diseases.

Immunopathogenesis of viral infections in neurological autoimmune disease

Autoimmune diseases develop due to self-tolerance failure in recognizing self and non-self-antigens. Several factors play a role in inducing autoimmunity, including genetic and environmental elements. Several studies demonstrated the causative role of viruses; however, some studies showed the preventive effect of viruses in the development of autoimmunity. Neurological autoimmune diseases are classified based on the targets of autoantibodies, which target intracellular or extracellular antigens rather than neurons. Several theories have been hypothesized to explain the role of viruses in the pathogenesis of neuroinflammation and autoimmune diseases. This study reviewed the current data on the immunopathogenesis of viruses in autoimmunity of the nervous system.

Manipulation of Host Cell Death Pathways by Herpes Simplex Virus

Herpes simplex virus (HSV)-1 and HSV-2 are ubiquitous human pathogens that infect keratinized epithelial surfaces and establish lifelong latent infection in sensory neurons of the peripheral nervous system. HSV-1 causes oral cold sores, and HSV-2 causes genital lesions characterized by recurrence at the site of the initial infection. In multicellular organisms, cell death plays a pivotal role in host defense by eliminating pathogen-infected cells. Apoptosis and necrosis are readily distinguished types of cell death. Apoptosis, the main form of programmed cell death, depends on the activity of certain caspases, a family of cysteine proteases. Necroptosis, a regulated form of necrosis that is unleashed when caspase activity is compromised, requires the activation of receptor-interacting protein (RIP) kinase 3 (RIPK3) through its interaction with other RIP homotypic interaction motif (RHIM)-containing proteins such as RIPK1. To ensure lifelong infection in the host, HSV carries out sophisticated molecular strategies to evade host cell death responses during viral infection. HSV-1 is a well-characterized pathogen that encodes potent viral inhibitors that modulate both caspase activation in the apoptosis pathway and RIPK3 activation in the necroptosis pathway in a dramatic, species-specific fashion. The viral UL39-encoded viral protein ICP6, the large subunit of the virus-encoded ribonucleotide reductase, functions as a suppressor of both caspase-8 and RHIM-dependent RIPK3 activities in the natural human host. In contrast, ICP6 RHIM-mediated recruitment of RIPK3 in the nonnatural mouse host drives the direct activation of necroptosis. This chapter provides an overview of the current state of the knowledge on molecular interactions between HSV-1 viral proteins and host cell death pathways and highlights how HSV-1 manipulates cell death signals for the benefit of viral propagation.

Apoptosis and Phagocytosis as Antiviral Mechanisms

Viruses are infectious entities that make use of the replication machinery of their hosts to produce more progenies, causing disease and sometimes death. To counter viral infection, metazoan hosts are equipped with various defense mechanisms, from the rapid-evoking innate immune responses to the most advanced adaptive immune responses. Previous research demonstrated that cells in fruit flies and mice infected with Drosophila C virus and influenza, respectively, undergo apoptosis, which triggers the engulfment of apoptotic virus-infected cells by phagocytes. This process involves the recognition of eat-me signals on the surface of virus-infected cells by receptors of specialized phagocytes, such as macrophages and neutrophils in mice and hemocytes in fruit flies, to facilitate the phagocytic elimination of virus-infected cells. Inhibition of phagocytosis led to severe pathologies and death in both species, indicating that apoptosis-dependent phagocytosis of virus-infected cells is a conserved antiviral mechanism in multicellular organisms. Indeed, our understanding of the mechanisms underlying apoptosis-dependent phagocytosis of virus-infected cells has shed a new perspective on how hosts defend themselves against viral infection. This chapter explores the mechanisms of this process and its potential for developing new treatments for viral diseases.

Caspase-8 is required for HSV-1-induced apoptosis and promotes effective viral particle release via autophagy inhibition

Regulated cell death (RCD) plays an important role in the progression of viral replication and particle release in cells infected by herpes simplex virus-1 (HSV-1). However, the kind of RCD (apoptosis, necroptosis, others) and the resulting cytopathic effect of HSV-1 depends on the cell type and the species. In this study, we further investigated the molecular mechanisms of apoptosis induced by HSV-1. Although a role of caspase-8 has previously been suggested, we now clearly show that caspase-8 is required for HSV-1-induced apoptosis in a FADD-/death receptor-independent manner in both mouse embryo fibroblasts (MEF) and human monocytes (U937). While wild-type (wt) MEFs and U937 cells exhibited increased caspase-8 and caspase-3 activation and apoptosis after HSV-1 infection, respective caspase-8-deficient (caspase-8−/−) cells were largely impeded in any of these effects. Unexpectedly, caspase-8−/− MEF and U937 cells also showed less virus particle release associated with increased autophagy as evidenced by higher Beclin-1 and lower p62/SQSTM1 levels and increased LC3-I to LC3-II conversion. Confocal and electron microscopy revealed that HSV-1 stimulated a strong perinuclear multivesicular body response, resembling increased autophagy in caspase-8−/− cells, entrapping virions in cellular endosomes. Pharmacological inhibition of autophagy by wortmannin restored the ability of caspase-8−/− cells to release viral particles in similar amounts as in wt cells. Altogether our results support a non-canonical role of caspase-8 in both HSV-1-induced apoptosis and viral particle release through autophagic regulation.

Viral-mediated activation and inhibition of programmed cell death

Viruses are ubiquitous intracellular genetic parasites that heavily rely on the infected cell to complete their replication life cycle. This dependency on the host machinery forces viruses to modulate a variety of cellular processes including cell survival and cell death. Viruses are known to activate and block almost all types of programmed cell death (PCD) known so far. Modulating PCD in infected hosts has a variety of direct and indirect effects on viral pathogenesis and antiviral immunity. The mechanisms leading to apoptosis following virus infection is widely studied, but several modalities of PCD, including necroptosis, pyroptosis, ferroptosis, and paraptosis, are relatively understudied. In this review, we cover the mechanisms by which viruses activate and inhibit PCDs and suggest perspectives on how these affect viral pathogenesis and immunity.

Quantitative Evaluation of Very Low Levels of HIV-1 Reverse Transcriptase by a Novel Highly Sensitive RT-qPCR Assay

Based on previous experience in our laboratory, we developed a real-time reverse transcriptase (RT) quantitative PCR (RT-qPCR) assay for the assessment of very low levels of HIV-1 RT activity. The RNA, acting as a template for reverse transcription into cDNA by HIV-1 RT, consisted of a synthetic RNA ad hoc generated by in vitro transcription and included a coding sequence for HSV-1 gD (gD-RNA-synt). Different conditions of variables involved in the RT-qPCR reaction, notably different amounts of gD-RNA-synt, different mixes of the reaction buffer, and different dNTP concentrations, were tested to optimize the assay. The results indicated that the gD-RNA-synt-based RT assay, in its optimized formulation, could detect a specific cDNA reverse transcription even in the presence of 1 × 10^-9 U of HIV RT. This achievement greatly improved the sensitivity of the assay over previous versions. In summary, this constructed RT-qPCR assay may be considered a promising tool for providing accurate information on very low HIV-1 RT activity.

Human Herpesvirus 6A Tegument Protein U14 Induces NF-κB Signaling by Interacting with p65

Viral infection induces host cells to mount a variety of immune responses, which may either limit viral propagation or create conditions conducive to virus replication in some instances. In this regard, activation of the NF-κB transcription factor is known to modulate virus replication. Human herpesvirus 6A (HHV-6A), which belongs to the Betaherpesvirinae subfamily, is frequently found in patients with neuroinflammatory diseases, although its role in disease pathogenesis has not been elucidated. In this study, we found that the HHV-6A-encoded U14 protein activates NF-κB signaling following interaction with the NF-κB complex protein, p65. Through induction of nuclear translocation of p65, U14 increases the expression of interleukin-6 (IL-6), IL-8, and monocyte chemoattractant protein 1 transcripts. We also demonstrated that activation of NF-κB signaling is important for HHV-6A replication, since inhibition of this pathway reduced virus protein accumulation and viral genome copy number. Taken together, our results suggest that HHV-6A infection activates the NF-κB pathway and promotes viral gene expression via late gene products, including U14. IMPORTANCE Human herpesvirus 6A (HHV-6A) is frequently found in patients with neuro-inflammation, although its role in the pathogenesis of this disease has not been elucidated. Most viral infections activate the NF-κB pathway, which causes the transactivation of various genes, including those encoding proinflammatory cytokines. Our results indicate that HHV-6A U14 activates the NF-κB pathway, leading to upregulation of proinflammatory cytokines. We also found that activation of the NF-κB transcription factor is important for efficient viral replication. This study provides new insight into HHV-6A U14 function in host cell signaling and identifies potential cellular targets involved in HHV-6A pathogenesis and replication.

Herpes Simplex Virus Type 2 Glycoprotein D Inhibits NF-κB Activation by Interacting with p65

NF-κB plays a crucial role in regulating cell proliferation, inflammation, apoptosis, and immune responses. HSV type 2 (HSV-2) is one of the most predominant sexually transmitted pathogens worldwide, and its infection increases the risk of HIV type 1 (HIV-1) acquisition and transmission. HSV-2 glycoprotein D (gD), highly homologous to HSV-1 gD, is essential for viral adhesion, fusion, entry, and spread. It is known that HSV-1 gD can bind herpesvirus entry mediator (HVEM) to trigger NF-κB activation and thereby facilitate viral replication at the early stage of infection. In this study, we found that purified HSV-2 gD triggered NF-κB activation at the early stage of infection, whereas ectopic expression of HSV-2 gD significantly downregulated TNF-α-induced NF-κB activity as well as TNF-α-induced IL-6 and IL-8 expression. Mechanistically, HSV-2 gD inhibited NF-κB, but not IFN-regulatory factor 3 (IRF3), activation and suppressed NF-κB activation mediated by overexpression of TNFR-associated factor 2 (TRAF2), IκB kinase α (IKKα), IKKβ, or p65. Coimmunoprecipitation and binding kinetic analyses demonstrated that HSV-2 gD directly bound to the NF-κB subunit p65 and abolished the nuclear translocation of p65 upon TNF-α stimulation. Mutational analyses further revealed that HSV-2 gD interacted with the region spanning aa 19-187 of p65. Findings in this study together demonstrate that HSV-2 gD interacts with p65 to regulate p65 subcellular localization and thereby prevents NF-κB-dependent gene expression, which may contribute to HSV-2 immune evasion and pathogenesis.

Herpes simplex virus 1 infection induces ubiquitination of UBE1a

Herpes simplex virus 1 (HSV-1) is a human DNA virus that causes cold sores, keratitis, meningitis, and encephalitis. Ubiquitination is a post-translational protein modification essential for regulation of cellular events, such as proteasomal degradation, signal transduction, and protein trafficking. The process is also involved in events for establishing viral infection and replication. The first step in ubiquitination involves ubiquitin (Ub) binding with Ub-activating enzyme (E1, also termed UBE1) via a thioester linkage. Our results show that HSV-1 infection alters protein ubiquitination pattern in host cells, as evidenced by MS spectra and co-immunoprecipitation assays. HSV-1 induced ubiquitination of UBE1a isoform via an isopeptide bond with Lys604. Moreover, we show that ubiquitination of K604 in UBE1a enhances UBE1a activity; that is, the activity of ubiquitin-transfer to E2 enzyme. Subsequently, we investigated the functional role of UBE1a and ubiquitination of K604 in UBE1a. We found that UBE1-knockdown increased HSV-1 DNA replication and viral production. Furthermore, overexpression of UBE1a, but not a UBE1a K604A mutant, suppressed viral replication. Furthermore, we found that UBE1a and ubiquitination at K604 in UBE1a retarded expression of HSV-1 major capsid protein, ICP5. Our findings show that UBE1a functions as an antiviral factor that becomes activated upon ubiquitination at Lys604.

Appraisal of a Simple and Effective RT-qPCR Assay for Evaluating the Reverse Transcriptase Activity in Blood Samples from HIV-1 Patients

Testing HIV-1 RNA in plasma by PCR is universally accepted as the ultimate standard to confirm diagnosis of HIV-1 infection and to monitor viral load in patients under treatment. However, in some cases, this assay could either underestimate or overestimate the replication capacity of a circulating or latent virus. In the present study, we performed the assessment of evaluating the HIV-1 reverse transcriptase (RT) activity by means of a new assay for the functional screening of the status of HIV-1 patients. To this purpose, we utilized, for the first time on blood samples, an adapted version of a real-time RT quantitative PCR assay, utilized to evaluate the HIV-1-RT inhibitory activity of compounds. The study analyzed blood samples from 28 HIV-1-infected patients, exhibiting a wide range of viremia and immunological values. Results demonstrated that plasma HIV-1 RT levels, expressed as cycle threshold values obtained with the assay under appraisal, were inversely and highly significantly correlated with the plasma HIV-1-RNA levels of the patients. Thus, an HIV-1 RT quantitative PCR assay was created which we describe in this study, and it may be considered as a promising basis for an additional tool capable of furnishing information on the functional virological status of HIV-1-infected patients.

UBE1a Suppresses Herpes Simplex Virus-1 Replication

Herpes simplex virus-1 (HSV-1) is the causative agent of cold sores, keratitis, meningitis, and encephalitis. HSV-1-encoded ICP5, the major capsid protein, is essential for capsid assembly during viral replication. Ubiquitination is a post-translational modification that plays a critical role in the regulation of cellular events such as proteasomal degradation, protein trafficking, and the antiviral response and viral events such as the establishment of infection and viral replication. Ub-activating enzyme (E1, also named UBE1) is involved in the first step in the ubiquitination. However, it is still unknown whether UBE1 contributes to viral infection or the cellular antiviral response. Here, we found that UBE1a suppressed HSV-1 replication and contributed to the antiviral response. The UBE1a inhibitor PYR-41 increased HSV-1 production. Immunofluorescence analysis revealed that UBE1a highly expressing cells presented low ICP5 expression, and vice versa. UBE1a inhibition by PYR-41 and shRNA increased ICP5 expression in HSV-1-infected cells. UBE1a reduced and retarded ICP5 protein expression, without affecting transcription of ICP5 mRNA or degradation of ICP5 protein. Additionally, UBE1a interacted with ICP27, and both partially co-localized at the Hsc70 foci/virus-induced chaperone-enriched (VICE) domains. PYR-41 reduced the co-localization of UBE1a and ICP27. Thus, our findings provide insights into the mechanism of UBE1a in the cellular response to viral infection.

The Race between Host Antiviral Innate Immunity and the Immune Evasion Strategies of Herpes Simplex Virus 1

Herpes simplex virus 1 (HSV-1) is very successful in establishing acute and latent infections in humans by counteracting host antiviral innate immune responses. HSV-1 has evolved various strategies to evade host antiviral innate immunity and some cellular survival-associated pathways. Since there is still no vaccine available for HSV-1, a continuous update of information regarding the interaction between HSV-1 infection and the host antiviral innate immunity will provide novel insights to develop new therapeutic strategies for HSV-1 infection and its associated diseases. Here, we update recent studies about how HSV-1 evades the host antiviral innate immunity, specifically how HSV-1 proteins directly or indirectly target the adaptors in the antiviral innate immunity signaling pathways to downregulate the signal transduction. Additionally, some classical intracellular stress responses, which also play important roles in defense of viral invasion, will be discussed here. With a comprehensive review of evasion mechanisms of antiviral innate immunity by HSV-1, we will be able to develop potential new targets for therapies and a possible vaccine against HSV-1 infections.

Herpes Simplex Virus 1 UL2 Inhibits the TNF-α-Mediated NF-κB Activity by Interacting With p65/p50

Herpes simplex virus 1 (HSV-1) is a large double-stranded DNA virus that encodes at least 80 viral proteins, many of which are involved in the virus-host interaction and are beneficial to the viral survival and reproduction. However, the biological functions of some HSV-1-encoded proteins are not fully understood. Nuclear factor κB (NF-κB) activation is the major antiviral innate response, which can be triggered by various signals induced by cellular receptors from different pathways. Here, we demonstrated that HSV-1 UL2 protein could antagonize the tumor necrosis factor α (TNF-α)-mediated NF-κB activation. Co-immunoprecipitation assays showed that UL2 could interact with the NF-κB subunits p65 and p50, which also revealed the region of amino acids 9 to 17 of UL2 could suppress the NF-κB activation and interact with p65 and p50, and UL2 bound to the immunoglobulin-like plexin transcription factor functional domain of p65. However, UL2 did not affect the formation of p65/p50 dimerization and their nuclear localizations. Yet, UL2 was demonstrated to inhibit the NF-κB activity by attenuating TNF-α-induced p65 phosphorylation at Ser536 and therefore decreasing the expression of downstream inflammatory chemokine interleukin 8. Taken together, the attenuation of NF-κB activation by UL2 may contribute to the escape of host's antiviral innate immunity for HSV-1 during its infection.

MG132 exerts anti-viral activity against HSV-1 by overcoming virus-mediated suppression of the ERK signaling pathway

Herpes simplex virus 1 (HSV-1) causes a number of clinical manifestations including cold sores, keratitis, meningitis and encephalitis. Although current drugs are available to treat HSV-1 infection, they can cause side effects such as nephrotoxicity. Moreover, owing to the emergence of drug-resistant HSV-1 strains, new anti-HSV-1 compounds are needed. Because many viruses exploit cellular host proteases and encode their own viral proteases for survival, we investigated the inhibitory effects of a panel of protease inhibitors (TLCK, TPCK, E64, bortezomib, or MG132) on HSV-1 replication and several host cell signaling pathways. We found that HSV-1 infection suppressed c-Raf-MEK1/2-ERK1/2-p90RSK signaling in host cells, which facilitated viral replication. The mechanism by which HSV-1 inhibited ERK signaling was mediated through the polyubiquitination and proteasomal degradation of Ras-guanine nucleotide-releasing factor 2 (Ras-GRF2). Importantly, the proteasome inhibitor MG132 inhibited HSV-1 replication by reversing ERK suppression in infected cells, inhibiting lytic genes (ICP5, ICP27 and UL42) expression, and overcoming the downregulation of Ras-GRF2. These results indicate that the suppression of ERK signaling via proteasomal degradation of Ras-GRF2 is necessary for HSV-1 infection and replication. Given that ERK activation by MG132 exhibits anti-HSV-1 activity, these results suggest that the proteasome inhibitor could serve as a novel therapeutic agent against HSV-1 infection.

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.

The Latency-Associated Transcript Inhibits Apoptosis via Downregulation of Components of the Type I Interferon Pathway during Latent Herpes Simplex Virus 1 Ocular Infection

The herpes simplex virus (HSV-1) latency-associated transcript (LAT) has been shown to inhibit apoptosis via inhibiting activation of proapoptotic caspases. However, the mechanism of LAT control of apoptosis is unclear, because LAT is not known to encode a functional protein, and the LAT transcript is found largely in the nucleus. We hypothesized that LAT inhibits apoptosis by regulating expression of genes that control apoptosis. Consequently, we sought to establish the molecular mechanism of antiapoptosis functions of LAT at a transcriptional level during latent HSV-1 ocular infection in mice. Our results suggest the following. (i) LAT likely inhibits apoptosis via upregulation of several components of the type I interferon (IFN) pathway. (ii) LAT does not inhibit apoptosis via the caspase cascade at a transcriptional level or via downregulating Toll-like receptors (TLRs). (iii) The mechanism of LAT antiapoptotic effect is distinct from that of the baculovirus inhibitor of apoptosis (cpIAP) because replacement of LAT with the cpIAP gene resulted in a different gene expression pattern than in either LAT+ or LAT- viruses. (iv) Replacement of LAT with the cpIAP gene does not cause upregulation of CD8 or markers of T cell exhaustion despite their having similar levels of latency, further supporting that LAT and cpIAP function via distinct mechanisms.IMPORTANCE The HSV-1 latency reactivation cycle is the cause of significant human pathology. The HSV-1 latency-associated transcript (LAT) functions by regulating latency and reactivation, in part by inhibiting apoptosis. However, the mechanism of this process is unknown. Here we show that LAT likely controls apoptosis via downregulation of several components in the JAK-STAT pathway. Furthermore, we provide evidence that immune exhaustion is not caused by the antiapoptotic activity of the LAT.

NF-κB-Dependent Production of ROS and Restriction of HSV-1 Infection in U937 Monocytic Cells

Herpes simplex virus 1 (HSV-1) can infect a wide range of cell types, including cells of the adaptive and innate immunity but, normally, it completes a fully-permissive replication cycle only in epithelial or neural cells. Complex mechanisms controlling this delicate balance in immune cells and consequent restriction of HSV-1 infection in these cells have not been completely elucidated. We have recently demonstrated that the transcription factor nuclear factor kappa B (NF-κB) can act as a main permissiveness regulator of HSV-1 infection in monocytic cells, however, mediators involved in this regulation have not been identified. To better define mechanisms involved in this phenomenon and, particularly, the possible involvement of ROS, wild type U937 cells or U937 cells stably transfected with a dominant-negative (DN) IκB-mutant and selenium-containing compounds, as anti-oxidants, were utilized. The main results can be summarized as follows. HSV-1 infection induces an immediate ROS production in U937 monocytic cells that can efficiently activate NF-κB but not in DN-IκB-mutant cells. Treatment with selenium-containing antioxidants efficiently inhibited HSV-1-induced ROS generation while producing increased levels of HSV-1 replication and a reduction of HSV-1-induced NF-κB activation in U937 monocytic cells. Our results suggest a scenario in which an efficient NF-κB-dependent ROS production in response to infection could contribute in limiting HSV-1 replication in monocytes/macrophages, thus avoiding possible irreparable damage to the innate immune system of the host during HSV-1 infection.

Pyrimidine 2,4-Diones in the Design of New HIV RT Inhibitors

The pyrimidine nucleus is a versatile core in the development of antiretroviral agents. On this basis, a series of pyrimidine-2,4-diones linked to an isoxazolidine nucleus have been synthesized and tested as nucleoside analogs, endowed with potential anti-HIV (human immunodeficiency virus) activity. Compounds 6a-c, characterized by the presence of an ethereal group at C-3, show HIV reverse transcriptase (RT) inhibitor activity in the nanomolar range as well as HIV-infection inhibitor activity in the low micromolar with no toxicity. In the same context, compound 7b shows only a negligible inhibition of RT HIV.

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.

Herpes Simplex Virus Type 1 Infection of the Central Nervous System: Insights Into Proposed Interrelationships With Neurodegenerative Disorders

Herpes simplex virus type 1 (HSV-1) is highly prevalent in humans and can reach the brain without evident clinical symptoms. Once in the central nervous system (CNS), the virus can either reside in a quiescent latent state in this tissue, or eventually actively lead to severe acute necrotizing encephalitis, which is characterized by exacerbated neuroinflammation and prolonged neuroimmune activation producing a life-threatening disease. Although HSV-1 encephalitis can be treated with antivirals that limit virus replication, neurological sequelae are common and the virus will nevertheless remain for life in the neural tissue. Importantly, there is accumulating evidence that suggests that HSV-1 infection of the brain both, in symptomatic and asymptomatic individuals could lead to neuronal damage and eventually, neurodegenerative disorders. Here, we review and discuss acute and chronic infection of particular brain regions by HSV-1 and how this may affect neuron and cognitive functions in the host. We review potential cellular and molecular mechanisms leading to neurodegeneration, such as protein aggregation, dysregulation of autophagy, oxidative cell damage and apoptosis, among others. Furthermore, we discuss the impact of HSV-1 infection on brain inflammation and its potential relationship with neurodegenerative diseases.

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.

First Impressions-the Potential of Altering Initial Host-Virus Interactions for Rational Design of Herpesvirus Vaccine Vectors

Purpose

The earliest host-virus interactions occur during virus attachment and entry into cells. These initial steps in the virus lifecycle influence the outcome of infection beyond delivery of the viral genome into the cell. Herpesviruses alter host signaling pathways and processes during attachment and entry to facilitate virus infection and modulate innate immune responses. We suggest in this review that understanding these early signaling events may inform the rational design of therapeutic and prevention strategies for herpesvirus infection, as well as the engineering of viral vectors for immunotherapy purposes.

Recent Findings

Recent reports demonstrate that modulation of Herpes Simplex Virus Type-1 (HSV-1) entry results in unexpected enhancement of antiviral immune responses.

Summary

A variety of evidence suggests that herpesviruses promote specific cellular signaling responses that facilitate viral replication after binding to cell surfaces, as well as during virus entry. Of particular interest is the ability of the virus to alter innate immune responses through these cellular signaling events. Uncovering the underlying immune evasion strategies may lead to the design of live-attenuated vaccines that can generate robust and protective anti-viral immune responses against herpesviruses. These adjuvant properties may be extended to a variety of heterologous antigens expressed by herpesviral vectors.

Virus Infection and Death Receptor-Mediated Apoptosis

Virus infection can trigger extrinsic apoptosis. Cell-surface death receptors of the tumor necrosis factor family mediate this process. They either assist persistent viral infection or elicit the elimination of infected cells by the host. Death receptor-mediated apoptosis plays an important role in viral pathogenesis and the host antiviral response. Many viruses have acquired the capability to subvert death receptor-mediated apoptosis and evade the host immune response, mainly by virally encoded gene products that suppress death receptor-mediated apoptosis. In this review, we summarize the current information on virus infection and death receptor-mediated apoptosis, particularly focusing on the viral proteins that modulate death receptor-mediated apoptosis.

The Us2 Gene Product of Herpes Simplex Virus 2 modulates NF-κB activation by targeting TAK1

HSV-2 is one of the most common sexually transmitted pathogens worldwide and HSV-2 infection triggers cytokine and chemokine production. However, little is known about which HSV-2 genes engage in the regulation of NF-κB signaling and what mechanisms are involved. In a screen of the unique short (Us) regions of HSV-2, we observed that HSV-2 Us2 activates NF-κB signaling. We additionally indicated that deficiencies of Us2 decrease HSV-2 WT mediated NF-κB activation and cytokine and chemokine production, and overexpression of Us2 showed opposite effects. Co-immunoprecipitations indicated that Us2 interacted with TGF-β activated kinase 1 (TAK1), a serine/threonine kinase essential for NF-κB activation, and Us2 has the ability to regulate the TAK1-mediated pathway and induces TAK1 downstream signaling. Further studies verified that Us2 induced the phosphorylation of TAK1, resulting in the activation of TAK1 mediated downstream signaling. The role of Us2 in HSV-2 induced NF-κB pathways was also confirmed in the Us2-deficient mutant and HSV-2 WT infected mice. Our results indicate that HSV-2 Us2 gene product binds to TAK1 to positively regulate NF-κB signaling and, for the first time, provide insights into the molecular mechanism.

Almond Skin Extracts Abrogate HSV-1 Replication by Blocking Virus Binding to the Cell

The aim of the present research was to determine the effect of almond skin extracts on herpes simplex virus 1 (HSV-1) replication. Drug-resistant strains of HSV frequently develop following therapeutic treatment. Therefore, the discovery of novel anti-HSV drugs deserves great effort. Here, we tested both natural (NS) and blanched (BS) polyphenols-rich almond skin extracts against HSV-1. HPLC analysis showed that the prevalent compounds in NS and BS extracts contributing to their antioxidant activity were quercetin, epicatechin and catechin. Results of cell viability indicated that NS and BS extracts were not toxic to cultured Vero cells. Furthermore, NS extracts were more potent inhibitors of HSV-1 than BS extracts, and this trend was in agreement with different concentrations of flavonoids. The plaque forming assay, Western blot and real-time PCR were used to demonstrate that NS extracts were able to block the production of infectious HSV-1 particles. In addition, the viral binding assay demonstrated that NS extracts inhibited HSV-1 adsorption to Vero cells. Our conclusion is that natural products from almond skin extracts are an extraordinary source of antiviral agents and provide a novel treatment against HSV-1 infections.

The suppression of apoptosis by α-herpesvirus

Apoptosis, an important innate immune mechanism that eliminates pathogen-infected cells, is primarily triggered by two signalling pathways: the death receptor pathway and the mitochondria-mediated pathway. However, many viruses have evolved various strategies to suppress apoptosis by encoding anti-apoptotic factors or regulating apoptotic signalling pathways, which promote viral propagation and evasion of the host defence. During its life cycle, α-herpesvirus utilizes an elegant multifarious anti-apoptotic strategy to suppress programmed cell death. This progress article primarily focuses on the current understanding of the apoptosis-inhibition mechanisms of α-herpesvirus anti-apoptotic genes and their expression products and discusses future directions, including how the anti-apoptotic function of herpesvirus could be targeted therapeutically.

Participation of 3-O-sulfated heparan sulfates in the protection of macrophages by herpes simplex virus-1 glycoprotein D and cyclophilin B against apoptosis

Heparan sulfates (HS) are involved in numerous biological processes, which rely on their ability to interact with a large panel of proteins. Although the reaction of 3‐O‐sulfation can be catalysed by the largest family of HS sulfotransferases, very few mechanisms have been associated with this modification and to date, only glycoprotein D (gD) of herpes simplex virus‐1 (HSV‐1 gD) and cyclophilin B (CyPB) have been well‐described as ligands for 3‐O‐sulfated HS. Here, we hypothesized that both ligands could induce the same responses via a mechanism dependent on 3‐O‐sulfated HS. First, we checked that HSV‐1 gD was as efficient as CyPB to induce the activation of the same signalling events in primary macrophages. We then demonstrated that both ligands efficiently reduced staurosporin‐induced apoptosis and modulated the expression of apoptotic genes. In addition to 3‐O‐sulfated HS, HSV‐1 gD was reported to interact with other receptors, including herpes virus entry mediator (HVEM), nectin‐1 and ‐2. Thus, we decided to identify the contribution of each binding site in the responses triggered by HSV‐1 gD and CyPB. We found that knock‐down of 3‐O‐sulfotransferase 2, which is the main 3‐O‐sulfated HS‐generating enzyme in macrophages, strongly reduced the responses induced by both ligands. Moreover, silencing the expression of HVEM rendered macrophages unresponsive to either HSV‐1 gD and CyPB, thus indicating that both proteins induced the same responses by interacting with a complex formed by 3‐O‐sulfated HS and HVEM. Collectively, our results suggest that HSV‐1 might hijack the binding sites for CyPB in order to protect macrophages against apoptosis for efficient infection.

HSV-1-induced activation of NF-κB protects U937 monocytic cells against both virus replication and apoptosis

The transcription factor nuclear factor-kappa B (NF-κB) is a crucial player of the antiviral innate response. Intriguingly, however, NF-κB activation is assumed to favour herpes simplex virus (HSV) infection rather than restrict it. Apoptosis, a form of innate response to viruses, is completely inhibited by HSV in fully permissive cells, but not in cells incapable to fully sustain HSV replication, such as immunocompetent cells. To resolve the intricate interplay among NF-κB signalling, apoptosis and permissiveness to HSV-1 in monocytic cells, we utilized U937 monocytic cells in which NF-κB activation was inhibited by expressing a dominant-negative IκBα. Surprisingly, viral production was increased in monocytic cells in which NF-κB was inhibited. Moreover, inhibition of NF-κB led to increased apoptosis following HSV-1 infection, associated with lysosomal membrane permeabilization. High expression of late viral proteins and induction of apoptosis occurred in distinct cells. Transcriptional analysis of known innate response genes by real-time quantitative reverse transcription-PCR excluded a contribution of the assayed genes to the observed phenomena. Thus, in monocytic cells NF-κB activation simultaneously serves as an innate process to restrict viral replication as well as a mechanism to limit the damage of an excessive apoptotic response to HSV-1 infection. This finding may clarify mechanisms controlling HSV-1 infection in monocytic cells.

The interplay between human herpes simplex virus infection and the apoptosis and necroptosis cell death pathways

Human herpes simplex virus (HSV) is a ubiquitous human pathogen that establishes a lifelong latent infection and is associated with mucocutaneous lesions. In multicellular organisms, cell death is a crucial host defense mechanism that eliminates pathogen-infected cells. Apoptosis is a well-defined form of programmed cell death executed by a group of cysteine proteases, called caspases. Studies have shown that HSV has evolved strategies to counteract caspase activation and apoptosis by encoding anti-apoptotic viral proteins such as gD, gJ, Us3, LAT, and the ribonucleotide reductase large subunit (R1). Recently, necroptosis has been identified as a regulated form of necrosis that can be invoked in the absence of caspase activity. Receptor-interacting kinase 3 (RIP3 or RIPK3) has emerged as a central signaling molecule in necroptosis; it is activated via interaction with other RIP homotypic interaction motif (RHIM)-containing proteins such as RIP1 (or RIPK1). There is increasing evidence that HSV R1 manipulates necroptosis via the RHIM-dependent inactivation or activation ofRIP3 in a species-specific manner. This review summarizes the current understanding of the interplay between HSV infection and cell death pathways, with an emphasis on apoptosis and necroptosis.

Phylogenetically Distant Viruses Use the Same BH3-Only Protein Puma to Trigger Bax/Bak-Dependent Apoptosis of Infected Mouse and Human Cells

Viruses can trigger apoptosis of infected host cells if not counteracted by cellular or viral anti-apoptotic proteins. These protective proteins either inhibit the activation of caspases or they act as Bcl-2 homologs to prevent Bax/Bak-mediated outer mitochondrial membrane permeabilization (MOMP). The exact mechanism by which viruses trigger MOMP has however remained enigmatic. Here we use two distinct types of viruses, a double stranded DNA virus, herpes simplex virus-1 (HSV-1) and a positive sense, single stranded RNA virus, Semliki Forest virus (SFV) to show that the BH3-only protein Puma is the major mediator of virus-induced Bax/Bak activation and MOMP induction. Indeed, when Puma was genetically deleted or downregulated by shRNA, mouse embryonic fibroblasts and IL-3-dependent monocytes as well as human colon carcinoma cells were as resistant to virus-induced apoptosis as their Bax/Bak double deficient counterparts (Bax/Bak-/-). Puma protein expression started to augment after 2 h postinfection with both viruses. Puma mRNA levels increased as well, but this occurred after apoptosis initiation (MOMP) because it was blocked in cells lacking Bax/Bak or overexpressing Bcl-xL. Moreover, none of the classical Puma transcription factors such as p53, p73 or p65 NFκB were involved in HSV-1-induced apoptosis. Our data suggest that viruses use a Puma protein-dependent mechanism to trigger MOMP and apoptosis in host cells.

How do viruses control mitochondria-mediated apoptosis?

There is no doubt that viruses require cells to successfully reproduce and effectively infect the next host. The question is what is the fate of the infected cells? All eukaryotic cells can "sense" viral infections and exhibit defence strategies to oppose viral replication and spread. This often leads to the elimination of the infected cells by programmed cell death or apoptosis. This "sacrifice" of infected cells represents the most primordial response of multicellular organisms to viruses. Subverting host cell apoptosis, at least for some time, is therefore a crucial strategy of viruses to ensure their replication, the production of essential viral proteins, virus assembly and the spreading to new hosts. For that reason many viruses harbor apoptosis inhibitory genes, which once inside infected cells are expressed to circumvent apoptosis induction during the virus reproduction phase. On the other hand, viruses can take advantage of stimulating apoptosis to (i) facilitate shedding and hence dissemination, (ii) to prevent infected cells from presenting viral antigens to the immune system or (iii) to kill non-infected bystander and immune cells which would limit viral propagation. Hence the decision whether an infected host cell undergoes apoptosis or not depends on virus type and pathogenicity, its capacity to oppose antiviral responses of the infected cells and/or to evade any attack from immune cells. Viral genomes have therefore been adapted throughout evolution to satisfy the need of a particular virus to induce or inhibit apoptosis during its life cycle. Here we review the different strategies used by viruses to interfere with the two major apoptosis as well as with the innate immune signaling pathways in mammalian cells. We will focus on the intrinsic mitochondrial pathway and discuss new ideas about how particular viruses could activately engage mitochondria to induce apoptosis of their host.

The epithelial αvβ3-integrin boosts the MYD88-dependent TLR2 signaling in response to viral and bacterial components

TLR2 is a cell surface receptor which elicits an immediate response to a wide repertoire of bacteria and viruses. Its response is usually thought to be proinflammatory rather than an antiviral. In monocytic cells TLR2 cooperates with coreceptors, e.g. CD14, CD36 and αMβ2-integrin. In an earlier work we showed that αvβ3-integrin acts in concert with TLR2 to elicit an innate response to HSV, and to lipopolysaccharide. This response is characterized by production of IFN-α and -β, a specific set of cytokines, and NF-κB activation. We investigated the basis of the cooperation between αvβ3-integrin and TLR2. We report that β3-integrin participates by signaling through Y residues located in the C-tail, known to be involved in signaling activity. αvβ3-integrin boosts the MYD88-dependent TLR2 signaling and IRAK4 phosphorylation in 293T and in epithelial, keratinocytic and neuronal cell lines. The replication of ICP0minus HSV is greatly enhanced by DN versions of MYD88, of Akt – a hub of this pathway, or by β3integrin-silencing. αvβ3-integrin enables the recruitment of TLR2, MAL, MYD88 at lipid rafts, the platforms from where the signaling starts. The PAMP of the HSV-induced innate response is the gH/gL virion glycoprotein, which interacts with αvβ3-integrin and TLR2 independently one of the other, and cross-links the two receptors. Given the preferential distribution of αvβ3-integrin to epithelial cells, we propose that αvβ3-integrin serves as coreceptor of TLR2 in these cells. The results open the possibility that TLR2 makes use of coreceptors in a variety of cells to broaden its spectrum of activity and tissue specificity.

In an earlier work we showed that a relevant contribution to the overall IFN-based antiviral response of the cell to herpes simplex virus is exerted by αvβ3-integrin which acts in concert with TLR2 in eliciting this response. Major characteristics of this branch of the innate response are the secretion of IFN-α and -β, of a specific set of cytokines, and the activation of NF-κB. The response is elicited also by LPS, indicating that the αvβ3-integrin TLR2 sentinels sense both bacteria and viruses. The IFN response is usually thought to be elicited by the endosomal and cytoplasmic sensors. Here we have investigated the basis of the αvβ3-integrin–TLR2 response, and found that αvβ3-integrin acts through its signaling C-tail, and boosts the MYD88- IRAK4-dependent TLR2 response. This is seen also in epithelial and neuronal cells which exemplify targets of HSV infection. Altogether, the results argue that αvβ3-integrin may serve as a coreceptor of TLR2 in epithelial cells. A point of novelty is that the TLR2 coreceptors known to date - CD14, CD36 and αMβ2-integrins - are typical of monocytic-derived cells (macrophages, DCs). To our knowledge a TLR2 coreceptor for epithelial cells was not known to date.

Oligonucleotides designed to inhibit TLR9 block Herpes simplex virus type 1 infection at multiple steps

Herpes simplex virus type 1 (HSV-1) is an important human pathogen which requires activation of nuclear factor-kappa B (NFκB) during its replication cycle. The persistent nature of HSV-1 infection, and the emergence of drug-resistant strains, highlights the importance of research to develop new antiviral agents. Toll-like receptors (TLRs) play a prominent role during the early antiviral response by recognizing viral nucleic acid and gene products, activating NFκB, and stimulating the production of inflammatory cytokines. We demonstrate a significant effect on HSV-1 replication in ARPE-19 and Vero cells when oligonucleotides designed to inhibit TLR9 are added 2h prior to infection. A greater than 90% reduction in the yield of infectious virus was achieved at oligonucleotide concentrations of 10-20 μM. TLR9 inhibitory oligonucleotides prevented expression of essential immediate early herpes gene products as determined by immunofluorescence microscopy and Western blotting. TLR9 oligonucleotides also interfered with viral attachment and entry. A TLR9 inhibitory oligonucleotide containing five adjacent guanosine residues (G-ODN) exhibited virucidal activity and inhibited HSV-1 replication when added post-infection. The antiviral effect of the TLR9 inhibitory oligonucleotides did not depend on the presence of TLR9 protein, suggesting a mechanism of inhibition that is not TLR9 specific. TLR9 inhibitory oligonucleotides also reduced NFκB activity in nuclear extracts. Studies using these TLR inhibitors in the context of viral infection should be interpreted with caution.

Inhibition of Bim enhances replication of varicella-zoster virus and delays plaque formation in virus-infected cells

Programmed cell death (apoptosis) is an important host defense mechanism against intracellular pathogens, such as viruses. Accordingly, viruses have evolved multiple mechanisms to modulate apoptosis to enhance replication. Varicella-zoster virus (VZV) induces apoptosis in human fibroblasts and melanoma cells. We found that VZV triggered the phosphorylation of the proapoptotic proteins Bim and BAD but had little or no effect on other Bcl-2 family members. Since phosphorylation of Bim and BAD reduces their proapoptotic activity, this may prevent or delay apoptosis in VZV-infected cells. Phosphorylation of Bim but not BAD in VZV-infected cells was dependent on activation of the MEK/extracellular signal-regulated kinase (ERK) pathway. Cells knocked down for Bim showed delayed VZV plaque formation, resulting in longer survival of VZV-infected cells and increased replication of virus, compared with wild-type cells infected with virus. Conversely, overexpression of Bim resulted in earlier plaque formation, smaller plaques, reduced virus replication, and increased caspase 3 activity. Inhibition of caspase activity in VZV-infected cells overexpressing Bim restored levels of virus production similar to those seen with virus-infected wild-type cells. Previously we showed that VZV ORF12 activates ERK and inhibits apoptosis in virus-infected cells. Here we found that VZV ORF12 contributes to Bim and BAD phosphorylation. In summary, VZV triggers Bim phosphorylation; reduction of Bim levels results in longer survival of VZV-infected cells and increased VZV replication.

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

NF-κB plays central roles in regulation of diverse biological processes, including innate and adaptive immunity and inflammation. HSV-1 is the archetypal member of the alphaherpesviruses, with a large genome encoding over 80 viral proteins, many of which are involved in virus-host interactions and show immune modulatory capabilities. In this study, we demonstrated that the HSV-1 ICP0 protein, a viral E3 ubiquitin ligase, was shown to significantly suppress tumor necrosis factor alpha (TNF-α)-mediated NF-κB activation. ICP0 was demonstrated to bind to the NF-κB subunits p65 and p50 by coimmunoprecipitation analysis. ICP0 bound to the Rel homology domain (RHD) of p65. Fluorescence microscopy demonstrated that ICP0 abolished nuclear translocation of p65 upon TNF-α stimulation. Also, ICP0 degraded p50 via its E3 ubiquitin ligase activity. The RING finger (RF) domain mutant ICP0 (ICP0-RF) lost its ability to inhibit TNF-α-mediated NF-κB activation and p65 nuclear translocation and degrade p50. Notably, the RF domain of ICP0 was sufficient to interact with p50 and abolish NF-κB reporter gene activity. Here, it is for the first time shown that HSV-1 ICP0 interacts with p65 and p50, degrades p50 through the ubiquitin-proteasome pathway, and prevents NF-κB-dependent gene expression, which may contribute to immune evasion and pathogenesis of HSV-1.

Herpes simplex virus 1 DNA polymerase processivity factor UL42 inhibits TNF-α-induced NF-κB activation by interacting with p65/RelA and p50/NF-κB1

Herpes simplex virus 1 (HSV-1) is the archetypal member of the alphaherpesvirus with a large genome encoding over 80 viral proteins, many of which are involved in virus-host interactions and show immune modulatory capabilities. In this study, we demonstrated that the HSV-1 UL42 protein, a DNA polymerase processivity factor, was a novel antagonism of the canonical NF-κB signaling pathway. UL42 was shown to significantly suppress TNF-α mediated NF-κB activation. Co-immunoprecipitation experiment revealed that UL42 bound to the NF-κB subunits p65 and p50. Fluorescence microscopy demonstrated that UL42 abolished nuclear translocation of p65 and p50 upon TNF-α-stimulation. But the inhibiting capacity of UL42 2R/2A (R279A, R280A) and UL42 3R/3A (R113A, R279A and R280A) mutants were less than wild type UL42. Also UL42 bound to the Rel homology domain of the NF-κB subunit p65 and p50. Notably, the N-terminal of UL42 was sufficient to interact with p65 and p50 and abolished NF-κB reporter gene activity. Thus, it was first time we demonstrated that HSV-1 UL42 appeared to prevent NF-κB-dependent gene expression by retaining p65 and p50 in the cytoplasm, and UL42-dependent transcriptional activation were inherently coupled to promote HSV-1 lytic replication, which also may contribute to immune evasion and pathogenesis of HSV-1.

Herpes simplex virus US3 tegument protein inhibits Toll-like receptor 2 signaling at or before TRAF6 ubiquitination

Herpes simplex virus (HSV) has evolved multiple strategies to modulate host immune responses. In a screen of HSV open reading frames to identify additional HSV-encoded proteins that affect NF-κB signaling, we identified the viral US3 tegument protein as an inhibitor of NF-κB signaling. We found that the US3 protein is required for inhibition of TLR2 signaling induced by viral infection and that this inhibition occurs at very early times post-infection. Expression of US3 in transfected cells inhibits TLR2 signaling induced by Zymosan, and this inhibition occurs at or downstream of MyD88 and upstream of p65. Polyubiquitination of TRAF6 is critical for its function in TLR2 signaling. Using US3-null and US3 kinase-defective mutant viruses, we demonstrate that HSV US3 reduces TRAF6 polyubiquitination and that the kinase activity of US3 is necessary for this effect. Therefore, US3 is necessary and sufficient for inhibiting TLR2 signaling at or before the stage of TRAF6 ubiquitination.

Herpes simplex virus glycoproteins gH/gL and gB bind Toll-like receptor 2, and soluble gH/gL is sufficient to activate NF-κB

A number of sentinels sense incoming herpes simplex virus (HSV) virions and initiate an immediate innate response. The first line of defense at the cell surface is TLR2 (Toll-like receptor 2), whose signature signaling activity leads to activation of the key transcription factor NF-κB. We report that the HSV pathogen-associated molecular patterns for TLR2 are the virion glycoproteins gH/gL and gB, which constitute the conserved fusion core apparatus across the members of the Herpesviridae family. Specifically, virions devoid singly of one of essential fusion glycoproteins (gD, gB, or gH null), able to attach to cells but defective in fusion/entry, were sufficient to elicit the first wave of NF-κB response to HSV. The most effective were the gD-null virions, positive for gH/gL and gB. A soluble form of gB, truncated upstream of the transmembrane sequence (gB(730t-st)), was produced in human cells and purified by means of a Strep tag. gH/gL and gB were each able to physically interact with TLR2 in coimmunoprecipitation assays, one independently of the other, yet gH(t-st)/gL, but not gB(730t-st), elicited an NF-κB response. Thus, whereas both HSV gH/gL and gB are ligands to TLR2, only gH/gL is sufficient to initiate a signaling cascade which leads to NF-κB activation.

The tug-of-war between dendritic cells and human chronic viruses

The human immune system is under constant challenge from many viruses, some of which the body is successfully able to clear. Other viruses have evolved to escape the host immune responses and thus persist, leading to the development of chronic diseases. Dendritic cells are professional antigen-presenting cells that play a major role in both innate and adaptive immunity against different pathogens. This review focuses on the interaction of different chronic viruses with dendritic cells and the viruses' ability to exploit this critical cell type to their advantage so as to establish persistence within the host.

The virion host shutoff protein of herpes simplex virus 1 blocks the replication-independent activation of NF-κB in dendritic cells in the absence of type I interferon signaling

Immune evasion is a defining feature of the virus-host relationship. During infection, herpes simplex virus type 1 (HSV-1) utilizes multiple proteins to manipulate the host immune response. In the present study, we investigated the mechanism by which the virion host shutoff (vhs) protein blocks the activation of dendritic cells (DCs). Previously, we found that coinfection of wild-type HSV-1 with a panel of RNA viruses resulted in a block to DC activation that was attributable to vhs. These observations led us to hypothesize that the vhs-mediated inhibition was dependent on signaling through the RIG-I-like receptor (RLR) signaling pathway. By examining DCs generated from MAVS (IPS-1) knockout (KO) mice, we determined that RLR/MAVS signaling is not essential for the DC response to HSV-1. We also evaluated the requirement for the type I interferon (IFN) signaling pathway in DC activation following infection with HSV-1 and found that stimulation of DCs with wild-type HSV-1 required intact type I IFN signaling for the production of cytokines, whereas the vhs deletion (vhs(-)) mutant virus activated DCs without the need for exogenous IFN signaling. Comparisons of transcription factor activation in DCs infected with wild-type HSV and the vhs(-) mutant virus revealed that NF-κB activation was inhibited by vhs in the early phase of the infection. In contrast, IRF3 activation was not influenced by vhs. In these studies, measurement of proinflammatory cytokines and type I IFN release from the infected DCs reflected the activation status of these transcription factors. Taken together, the work presented here (i) describes a novel role for the vhs protein as an inhibitor of the early activation of NF-κB during HSV-1 infection of DCs and (ii) offers a mechanistic explanation of how this protein interferes with DC activation.

Herpes virus infection can cause intervertebral disc degeneration

It has been proposed that intervertebral disc degeneration might be caused by low-grade infection. The purpose of the present study was to assess the incidence of herpes viruses in intervertebral disc specimens from patients with lumbar disc herniation. A polymerase chain reaction based assay was applied to screen for the DNA of eight different herpes viruses in 16 patients and two controls. DNA of at least one herpes virus was detected in 13 specimens (81.25%). Herpes Simplex Virus type-1 (HSV-1) was the most frequently detected virus (56.25%), followed by Cytomegalovirus (CMV) (37.5%). In two patients, co-infection by both HSV-1 and CMV was detected. All samples, including the control specimens, were negative for Herpes Simplex Virus type-2, Varicella Zoster Virus, Epstein Barr Virus, Human Herpes Viruses 6, 7 and 8. The absence of an acute infection was confirmed both at the serological and mRNA level.

To our knowledge this is the first unequivocal evidence of the presence of herpes virus DNA in intervertebral disc specimens of patients with lumbar disc herniation suggesting the potential role of herpes viruses as a contributing factor to the pathogenesis of degenerative disc disease.

Susceptibility of primary HTLV-1 isolates from patients with HTLV-1-associated myelopathy to reverse transcriptase inhibitors

Since human T-lymphotropic virus type 1 (HTLV-1)-associated diseases are associated with a high HTLV-1 load, reducing this load may treat or prevent disease. However, despite in vitro evidence that certain nucleoside/nucleotide analogue reverse transcriptase inhibitors (NRTIs) are active against HTLV-1, in vivo results have been disappointing. We therefore assayed the sensitivity of HTLV-1 primary isolates to a panel of RT inhibitors. HTLV-1 primary isolates were obtained, pre- and post- NRTI treatment, from patients with HTLV-1-associated myelopathy. Sensitivity to azidothymidine (AZT), lamivudine (3TC), tenofovir (TDF) and three phosphonated carbocyclic 2’-oxa-3’aza nucleosides (PCOANs) was assessed in a RT inhibitor assay. With the exception of 3TC, HTLV RT from primary isolates was less sensitive to all tested inhibitors than HTLV-1 RT from MT-2 cells. HTLV-1 RT from primary isolates and from chronically infected, transformed MT-2 cells was insensitive to 3TC. Sensitivity of primary isolates to RT inhibitors was not reduced following up to 12 months of patient treatment with AZT plus 3TC. The sensitivity of HTLV-1 primary isolates to NRTIs differs from that of cell lines and may vary among patients. Failure of NRTIs to reduce HTLV-1 viral load in vivo was not due to the development of phenotypic NRTI resistance. AZT and the three PCOANs assayed all consistently inhibited primary isolate HTLV-1 RT.

Role of Fas/FasL in regulation of inflammation in vaginal tissue during HSV-2 infection

To assess the role of Fas in lesion development during genital HSV-2 infection, we used a well-established HSV-2 murine model applied to MRL-Fas(lpr)/J (Fas-/-) and C3-Fasl(gld)/J (FasL-/-) C57BL6 mice. In vitro infection of murine keratinocytes and epithelial cells was used to clarify molecular details of HSV-2 infection. Despite upregulation of Fas and FasL, HSV-2-infected keratinocytes and epithelial cells showed a moderate level of apoptosis due to upregulated expression of the anti-apoptotic factors Bcl-2, Akt kinase and NF-κB. Inflammatory lesions within the HSV-2-infected epithelium of C57BL6 mice consisted of infected cells upregulating Fas, FasL and Bcl-2, uninfected cells upregulating Fas and neutrophils expressing both Fas and FasL. Apoptosis was detected in HSV-2-infected cells and to even higher extent in non-infected cells surrounding HSV-2 infection sites. HSV-2 infection of Fas- and FasL-deficient mice led to increased apoptosis and stronger recruitment of neutrophils within the infection sites. We conclude that the Fas pathway participates in regulation of inflammatory response in the vaginal epithelium at the initial stage of HSV-2 infection.

Inhibition of NF-κB activation sensitizes U937 cells to 3'-azido-3'-deoxythymidine induced apoptosis

In this study, we investigated molecular mechanisms underlying low susceptibility to apoptosis induced by the nucleoside analog azidothymidine (AZT) and the role of nuclear factor-κB (NF-κB) activation in these phenomena. A preliminary screening in different cell lines indicated U937 monocytic cell line as suitable to this purpose. Treatment of U937 cells even with suprapharmacological concentrations of AZT induced only moderate levels of apoptosis. Surprisingly, SuperArray analysis showed that AZT induced the transcriptional activity of both pro- and anti-apoptotic genes. Interestingly, moreover, several genes upregulated by AZT were NF-κB related. In fact, AZT, after an initial inhibition of NF-κB activation with respect to control, induced a transient, but consistent, increase in NF-κB-binding activity. Inhibition of NF-κB activation in U937 cells, stably transfected with a dominant-negative IκBα or by pharmacological treatment, sensitized them to apoptosis induced by AZT and impaired the upregulation of anti-apoptotic genes in response to AZT treatment, with respect to control cells. These results indicate that NF-κB activation by AZT has a role in protecting target cells from apoptotic cell death, improving our understanding of the toxicology and the therapeutic usage of this drug.

Herpes simplex virus glycoprotein D interferes with binding of herpesvirus entry mediator to its ligands through downregulation and direct competition

To initiate membrane fusion and virus entry, herpes simplex virus (HSV) gD binds to a cellular receptor such as herpesvirus entry mediator (HVEM). HVEM is a tumor necrosis factor (TNF) receptor family member with four natural ligands that either stimulate (LIGHT and LTα) or inhibit (BTLA and CD160) T cell function. We hypothesized that the interaction of gD with HVEM affects the binding of natural ligands, thereby modulating the immune response during infection. Here, we investigated the effect that gD has on the interaction of HVEM with its natural ligands. First, HSV gD on virions or cells downregulates HVEM from the cell surface. Similarly, trans-interaction with BTLA or LIGHT also downregulates HVEM from the cell surface, suggesting that HSV may subvert a natural mechanism for regulating HVEM activity. Second, we showed that wild-type gD had the lowest affinity for HVEM compared with the four natural ligands. Moreover, gD directly competed for binding to HVEM with BTLA but not LTα or LIGHT, indicating the possibility that gD selectively controls HVEM signals. On the other hand, natural ligands influence the use of HVEM by HSV. For instance, soluble BTLA, LTα, and LIGHT inhibited the binding of wild-type gD to HVEM, and soluble BTLA and LTα blocked HSV infection of HVEM-expressing cells. Thus, gD is at the center of the interplay between HVEM and its ligands. It can interfere with HVEM function in two ways, by competing with the natural ligands and by downregulating HVEM from the cell surface.

Herpes simplex virus immediate-early ICP0 protein inhibits Toll-like receptor 2-dependent inflammatory responses and NF-kappaB signaling

The discovery of the Toll-like receptors (TLRs) and their importance in the regulation of host responses to infection raised attention to the complex interplay between viral gene products and the host innate immune responses in determining the outcome of virus infection. Robust inflammatory cytokine responses are observed in herpes simplex virus (HSV)-infected animals and cells. Our studies have demonstrated that Toll-like receptor 2 (TLR2) activation by HSV results in NF-κB activation with concomitant inflammatory cytokine production and that TLR2 activation plays a critical role in HSV-induced pathology and mortality. Here we demonstrate that the HSV-1 immediate-early ICP0 protein reduces the TLR2-mediated inflammatory response to HSV 1 (HSV-1) infection. Expression of ICP0 alone is sufficient to block TLR2-driven responses to both viral and nonviral ligands at or downstream of the MyD88 adaptor and upstream of p65. ICP0 alone can also reduce the levels of MyD88 and Mal (TIRAP). In HSV-infected cells, the E3 ligase function of ICP0 and cellular proteasomal activity are required for the inhibitory activity. Our results argue for a model in which ICP0 promotes the degradation of TLR adaptor molecules and inhibition of the inflammatory response, much as it inhibits the interferon response by sequestration and degradation of interferon regulatory factor 3 (IRF-3).