Induction of apoptosis accelerates reactivation of latent HSV-1 in ganglionic organ cultures and replication in cell cultures

Enhanced therapeutic efficacy for glioblastoma immunotherapy with an oncolytic herpes simplex virus armed with anti-PD-1 antibody and IL-12

Glioblastoma is the most common and aggressive malignant brain tumor and has limited treatment options. Hence, innovative approaches are urgently needed. Oncolytic virus therapy is emerging as a promising modality for cancer treatment due to its tumor-specific targeting and immune-stimulatory properties. In this study, we developed a new generation of oncolytic herpes simplex virus C5252 by deletion of a 15-kb internal repeat region and both copies of γ34.5 genes. Additionally, C5252 was armed with anti-programmed cell death protein 1 antibody and interleukin-12 to enhance its therapeutic efficacy for glioblastoma immune-virotherapy. In vitro and in vivo experiments demonstrate that C5252 has a remarkable safety profile and potent anti-tumor activity against glioblastoma. Mechanistic studies demonstrated that C5252 specifically induces cell apoptosis by caspase-3/7 activation via downregulating ciliary neurotrophic factor receptor α. Furthermore, the enhanced anti-tumor therapeutic efficacy of C5252 in a subcutaneous glioblastoma model and an orthotopic glioblastoma model was confirmed. Moreover, syngeneic mouse models showed that the murine surrogate of C5252 has superior anti-tumor activity compared to the unarmed backbone virus, with enhanced immune activation. Taken together, our findings support C5252 as a promising therapeutic option for glioblastoma treatment, positioning it as a highly promising candidate for clinical translation.

c-Jun signaling during initial HSV-1 infection modulates latency to enhance later reactivation in addition to directly promoting the progression to full reactivation

Herpes simplex virus-1 (HSV-1) establishes a latent infection in peripheral neurons and periodically reactivates to permit transmission, which can result in clinical manifestations. Viral transactivators required for lytic infection are largely absent during latent infection, and therefore, HSV-1 relies on the co-option of neuronal host signaling pathways to initiate its gene expression. The activation of the neuronal c-Jun N-terminal kinase (JNK) cell stress pathway is central to initiating biphasic reactivation in response to multiple stimuli. However, how host factors work with JNK to stimulate the initial wave of gene expression (known as Phase I) or the progression to full Phase II reactivation remains unclear. Here, we found that c-Jun, the primary target downstream of neuronal JNK cell stress signaling, functions during reactivation but not during the JNK-mediated initiation of Phase I gene expression. Instead, c-Jun was required to transition from Phase I to full HSV-1 reactivation and was detected in viral replication compartments of reactivating neurons. Interestingly, we also identified a role for both c-Jun and enhanced neuronal stress during initial neuronal infection in promoting a more reactivation-competent form of HSV-1 latency. Therefore, c-Jun functions at multiple stages during the HSV latent infection of neurons to promote reactivation but not during the initial JNK-dependent Phase I. Importantly, by demonstrating that initial infection conditions can contribute to later reactivation abilities, this study highlights the potential for latently infected neurons to maintain a molecular scar of previous exposure to neuronal stressors.IMPORTANCEThe molecular mechanisms that regulate the reactivation of herpes simplex virus-1 (HSV-1) from latent infection are unknown. The host transcription and pioneer factor c-Jun is the main target of the JNK cell stress pathway that is known to be important in exit of HSV from latency. Surprisingly, we found that c-Jun does not act with JNK during exit from latency but instead promotes the transition to full reactivation. Moreover, c-Jun and enhanced neuronal stress during initial neuronal infection promoted a more reactivation-competent form of HSV-1 latency. c-Jun, therefore, functions at multiple stages during HSV-1 latent infection of neurons to promote reactivation. Importantly, this study contributes to a growing body of evidence that de novo HSV-1 infection conditions can modulate latent infection and impact future reactivation events, raising important questions on the clinical impact of stress during initial HSV-1 acquisition on future reactivation events and consequences.

Intimate Relationship Between Stress and Human Alpha‑Herpes Virus 1 (HSV‑1) Reactivation from Latency

Purpose of Review

Numerous studies concluded stress (acute, episodic acute, or chronic) increases the incidence of human alpha-herpes virus 1 (HSV-1) reactivation from latency in neurons. This review will summarize how stress stimulates viral gene expression, replication, and reactivation from latency.

Recent Findings

Stress (capital S) stress-mediated activation of the glucocorticoid receptor (GR) accelerates reactivation from latency, whereas a corticosteroid-specific antagonist impairs viral replication and reactivation from latency. GR and specific stress-induced cellular transcription factors also stimulate viral promoters that drive expression of key viral transcriptional regulators: infected cell protein 0 (ICP0), ICP4, ICP27 and viral tegument protein (VP16). Hence, GR is predicted to initially stimulate viral gene expression. GR-mediated immune-inhibitory functions are also predicted to enhance viral replication and viral spread.

Summary

Identifying cellular factors and viral regulatory proteins that trigger reactivation from latency in neurons may provide new therapeutic strategies designed to reduce the incidence of reactivation from latency.

Impaired glucocorticoid receptor function attenuates herpes simplex virus 1 production during explant-induced reactivation from latency in female mice

IMPORTANCE

A correlation exists between stress and increased episodes of human alpha-herpes virus 1 reactivation from latency. Stress increases corticosteroid levels; consequently, the glucocorticoid receptor (GR) is activated. Recent studies concluded that a GR agonist, but not an antagonist, accelerates productive infection and reactivation from latency. Furthermore, GR and certain stress-induced transcription factors cooperatively transactivate promoters that drive the expression of infected cell protein 0 (ICP0), ICP4, and VP16. This study revealed female mice expressing a GR containing a serine to alanine mutation at position 229 (GRS229A) shed significantly lower levels of infectious virus during explant-induced reactivation compared to male GRS229A or wild-type parental C57BL/6 mice. Furthermore, female GRS229A mice contained fewer VP16 + TG neurons compared to male GRS229A mice or wild-type mice during the early stages of explant-induced reactivation from latency. Collectively, these studies revealed that GR transcriptional activity has female-specific effects, whereas male mice can compensate for the loss of GR transcriptional activation.

Slug, a Stress-Induced Transcription Factor, Stimulates Herpes Simplex Virus 1 Replication and Transactivates a cis-Regulatory Module within the VP16 Promoter

Stress-mediated activation of the glucocorticoid receptor (GR) and specific stress-induced transcription factors stimulate herpes simplex virus 1 (HSV-1) productive infection, explant-induced reactivation, and immediate early (IE) promoters that drive expression of infected cell protein 0 (ICP0), ICP4, and ICP27. Several published studies concluded the virion tegument protein VP16, ICP0, and/or ICP4 drives early steps of reactivation from latency. Notably, VP16 protein expression was induced in trigeminal ganglionic neurons of Swiss Webster or C57BL/6J mice during early stages of stress-induced reactivation. If VP16 mediates reactivation, we hypothesized stress-induced cellular transcription factors would stimulate its expression. To address this hypothesis, we tested whether stress-induced transcription factors transactivate a VP16 cis-regulatory module (CRM) located upstream of the VP16 TATA box (-249 to -30). Initial studies revealed the VP16 CRM cis-activated a minimal promoter more efficiently in mouse neuroblastoma cells (Neuro-2A) than mouse fibroblasts (NIH-3T3). GR and Slug, a stress-induced transcription factor that binds enhancer boxes (E-boxes), were the only stress-induced transcription factors examined that transactivated the VP16 CRM construct. GR- and Slug-mediated transactivation was reduced to basal levels when the E-box, two 1/2 GR response elements (GREs), or NF-κB binding site was mutated. Previous studies revealed GR and Slug cooperatively transactivated the ICP4 CRM, but not ICP0 or ICP27. Silencing of Slug expression in Neuro-2A cells significantly reduced viral replication, indicating Slug-mediated transactivation of ICP4 and VP16 CRM activity correlates with enhanced viral replication and reactivation from latency. IMPORTANCE Herpes simplex virus 1 (HSV-1) establishes lifelong latency in several types of neurons. Periodically cellular stressors trigger reactivation from latency. Viral regulatory proteins are not abundantly expressed during latency, indicating cellular transcription factors mediate early stages of reactivation. Notably, the glucocorticoid receptor (GR) and certain stress-induced transcription factors transactivate cis-regulatory modules (CRMs) essential for expression of infected cell protein 0 (ICP0) and ICP4, key viral transcriptional regulatory proteins linked to triggering reactivation from latency. Virion protein 16 (VP16) specifically transactivates IE promoter and was also reported to mediate early stages of reactivation from latency. GR and Slug, a stress-induced enhancer box (E-box) binding protein, transactivate a minimal promoter downstream of VP16 CRM, and these transcription factors occupy VP16 CRM sequences in transfected cells. Notably, Slug stimulates viral replication in mouse neuroblastoma cells suggesting Slug, by virtue of transactivating VP16 and ICP4 CRM sequences, can trigger reactivation in certain neurons.

The Bovine Herpesvirus 1 Latency-Reactivation Cycle, a Chronic Problem in the Cattle Industry

Bovine alphaherpesvirus 1 (BoHV-1) is a persistent and recurring disease that affects cattle worldwide. It is a major contributor to bovine respiratory disease and reproductive failure in the US. A major complication of BoHV-1 arises from the lifelong latent infection established in the sensory ganglia of the peripheral nervous system following acute infection. Lifelong latency is marked by periodic reactivation from latency that leads to virus transmission and transient immunosuppression. Physiological and environmental stress, along with hormone fluctuations, can drive virus reactivation from latency, allowing the virus to spread rapidly. This review discusses the mechanisms of the latency/reactivation cycle, with particular emphasis on how different hormones directly regulate BoHV-1 gene expression and productive infection. Glucocorticoids, including the synthetic corticosteroid dexamethasone, are major effectors of the stress response. Stress directly regulates BoHV-1 gene expression through multiple pathways, including β-catenin dependent Wnt signaling, and the glucocorticoid receptor. Related type 1 nuclear hormone receptors, the androgen and progesterone receptors, also drive BoHV-1 gene expression and productive infection. These receptors form feed-forward transcription loops with the stress-induced Krüppel-like transcription factors KLF4 and KLF15. Understanding these molecular pathways is critical for developing novel therapeutics designed to block reactivation and reduce virus spread and disease.

Topical phenylbutyrate antagonizes NF-κB signaling and resolves corneal inflammation

Chronic inflammation of the immune privileged cornea originating from viral or nonviral conditions results in significant vision loss. Topical corticosteroids are the common treatments for corneal inflammation, but the drugs cause serious and potentially blinding side effects in the long term. Therefore, new standalone and/or synergistic anti-inflammatory therapies with lower side effects are desperately needed. Here, we show that the aromatic fatty acid phenylbutyrate (PBA) acts as a potent inhibitor of inflammation in preclinical ocular-inflammation models. PBA prevents the transcription as well as translation of pro-inflammatory cytokines by LPS and poly(I:C) via persistent inhibition of NF-κB signaling. PBA quickens the resolution of ocular inflammation in mice by decreasing corneal thickness and immune cell infiltration. More importantly, PBA can synergize with the dexamethasone to antagonize NF-κB signaling at lower drug concentrations. Our results demonstrate that PBA therapy exerts previously unreported anti-inflammatory effects in the eye and facilitates corneal healing during persistent inflammation.

Stress Triggers Expression of Bovine Herpesvirus 1 Infected Cell Protein 4 (bICP4) RNA during Early Stages of Reactivation from Latency in Pharyngeal Tonsil

Bovine herpesvirus 1 (BoHV-1), an important pathogen of cattle, establishes lifelong latency in sensory neurons within trigeminal ganglia (TG) after acute infection. The BoHV-1 latency-reactivation cycle, like other alphaherpesvirinae subfamily members, is essential for viral persistence and transmission. Notably, cells within pharyngeal tonsil (PT) also support a quiescent or latent BoHV-1 infection. The synthetic corticosteroid dexamethasone, which mimics the effects of stress, consistently induces BoHV-1 reactivation from latency allowing early stages of viral reactivation to be examined in the natural host. Based on previous studies, we hypothesized that stress-induced cellular factors trigger expression of key viral transcriptional regulatory genes. To explore this hypothesis, RNA-sequencing studies compared viral gene expression in PT during early stages of dexamethasone-induced reactivation from latency. Strikingly, RNA encoding infected cell protein 4 (bICP4), which is translated into an essential viral transcriptional regulatory protein, was detected 30 min after dexamethasone treatment. Ninety minutes after dexamethasone treatment bICP4 and, to a lesser extent, bICP0 RNA were detected in PT. All lytic cycle viral transcripts were detected within 3 h after dexamethasone treatment. Surprisingly, the latency related (LR) gene, the only viral gene abundantly expressed in latently infected TG neurons, was not detected in PT during latency. In TG neurons, bICP0 and the viral tegument protein VP16 are expressed before bICP4 during reactivation, suggesting distinct viral regulatory genes mediate reactivation from latency in PT versus TG neurons. Finally, these studies confirm PT is a biologically relevant site for BoHV-1 latency, reactivation from latency, and virus transmission. IMPORTANCE BoHV-1, a neurotropic herpesvirus, establishes, maintains, and reactivates from latency in neurons. BoHV-1 DNA is also detected in pharyngeal tonsil (PT) from latently infected calves. RNA-sequencing studies revealed the viral infected cell protein 4 (bICP4) RNA was expressed in PT of latently infected calves within 30 min after dexamethasone was used to initiate reactivation. As expected, bICP4 RNA was not detected during latency. All lytic cycle viral genes were expressed within 3 h after dexamethasone treatment. Conversely, bICP0 and the viral tegument protein VP16 are expressed prior to bICP4 in trigeminal ganglionic neurons during reactivation. The viral latency related gene, which is abundantly expressed in latently infected neurons, was not abundantly expressed in PT during latency. These studies provide new evidence PT is a biologically relevant site for BoHV-1 latency and reactivation. Finally, we predict other alphaherpesvirinae subfamily members utilize PT as a site for latency and reactivation.

Non-Targeted Metabolomic Analysis of Chicken Kidneys in Response to Coronavirus IBV Infection Under Stress Induced by Dexamethasone

Stress in poultry can lead to changes in body metabolism and immunity, which can increase susceptibility to infectious diseases. However, knowledge regarding chicken responses to viral infection under stress is limited. Dexamethasone (Dex) is a synthetic glucocorticoid similar to that secreted by animals under stress conditions, and has been widely used to induce stress in chickens. Herein, we established a stress model in 7-day-old chickens injected with Dex to elucidate the effects of stress on IBV replication in the kidneys. The metabolic changes, immune status and growth of the chickens under stress conditions were comprehensively evaluated. Furthermore, the metabolic profile, weight gain, viral load, serum cholesterol levels, cytokines and peripheral blood lymphocyte ratio were compared in chickens treated with Dex and infected with IBV. An LC-MS/MS-based metabolomics method was used to examine differentially enriched metabolites in the kidneys. A total of 113 metabolites whose abundance was altered after Dex treatment were identified, most of which were lipids and lipid-like molecules. The principal metabolic alterations in chicken kidneys caused by IBV infection included fatty acid, valine, leucine and isoleucine metabolism. Dex treatment before and after IBV infection mainly affected the host’s tryptophan, phenylalanine, amino sugar and nucleotide sugar metabolism. In addition, Dex led to up-regulation of serum cholesterol levels and renal viral load in chickens, and to the inhibition of weight gain, peripheral blood lymphocytes and IL-6 production. We also confirmed that the exogenous cholesterol in DF-1 cells promoted the replication of IBV. However, whether the increase in viral load in kidney tissue is associated with the up-regulation of cholesterol levels induced by Dex must be demonstrated in future experiments. In conclusion, chick growth and immune function were significantly inhibited by Dex. Host cholesterol metabolism and the response to IBV infection are regulated by Dex. This study provides valuable insights into the molecular regulatory mechanisms in poultry stress, and should support further research on the intrinsic link between cholesterol metabolism and IBV replication under stress conditions.

Independent Cis-Regulatory Modules within the Herpes Simplex Virus 1 Infected Cell Protein 0 (ICP0) Promoter Are Transactivated by Krüppel-like Factor 15 and Glucocorticoid Receptor

A corticosteroid antagonist impairs Herpes Simplex Virus 1 (HSV-1) productive infection and explant-induced reactivation from latency, suggesting corticosteroids and the glucocorticoid receptor (GR) mediate certain aspects of these complex virus-host interactions. GR-hormone complexes regulate transcription positively and negatively, in part, by binding GR response elements (GREs). Recent studies revealed infected cell protein 0 (ICP0), ICP4, and ICP27 promoter/cis-regulatory modules (CRMs) are cooperatively transactivated by GR and Krüppel-like factor 15 (KLF15), which forms a feed-forward transcription loop. We hypothesized the ICP0 promoter contains independent CRMs that are transactivated by GR, KLF15, and the synthetic corticosteroid dexamethasone (DEX). This hypothesis is based on the finding that the ICP0 promoter contains multiple transcription factor binding sites, and GR and KLF15 cooperatively transactivate the full-length ICP0 promoter. ICP0 promoter sequences spanning -800 to -635 (fragment A) were efficiently transactivated by GR, KLF15, and DEX in monkey kidney cells (Vero), whereas GR and DEX significantly enhanced promoter activity in mouse neuroblastoma cells (Neuro-2A). Furthermore, ICP0 fragment B (-458 to -635) was efficiently transactivated by GR, KLF15, and DEX in Vero cells, but not Neuro-2A cells. Finally, fragment D (-232 to -24) was transactivated significantly in Vero cells by GR, KLF15, and DEX, whereas KLF15 and DEX were sufficient for transactivation in Neuro-2A cells. Collectively, these studies revealed efficient transactivation of three independent CRMs within the ICP0 promoter by GR, KLF15, and/or DEX. Finally, GC-rich sequences containing specificity protein 1 (Sp1) binding sites were essential for transactivation.

The Hippocampal Vulnerability to Herpes Simplex Virus Type I Infection: Relevance to Alzheimer's Disease and Memory Impairment

Herpes simplex virus type 1 (HSV-1) as a possible infectious etiology in Alzheimer’s disease (AD) has been proposed since the 1980s. The accumulating research thus far continues to support the association and a possible causal role of HSV-1 in the development of AD. HSV-1 has been shown to induce neuropathological and behavioral changes of AD, such as amyloid-beta accumulation, tau hyperphosphorylation, as well as memory and learning impairments in experimental settings. However, a neuroanatomical standpoint of HSV-1 tropism in the brain has not been emphasized in detail. In this review, we propose that the hippocampal vulnerability to HSV-1 infection plays a part in the development of AD and amnestic mild cognitive impairment (aMCI). Henceforth, this review draws on human studies to bridge HSV-1 to hippocampal-related brain disorders, namely AD and aMCI/MCI. Next, experimental models and clinical observations supporting the neurotropism or predilection of HSV-1 to infect the hippocampus are examined. Following this, factors and mechanisms predisposing the hippocampus to HSV-1 infection are discussed. In brief, the hippocampus has high levels of viral cellular receptors, neural stem or progenitor cells (NSCs/NPCs), glucocorticoid receptors (GRs) and amyloid precursor protein (APP) that support HSV-1 infectivity, as well as inadequate antiviral immunity against HSV-1. Currently, the established diseases HSV-1 causes are mucocutaneous lesions and encephalitis; however, this review revises that HSV-1 may also induce and/or contribute to hippocampal-related brain disorders, especially AD and aMCI/MCI.

Regulation of Krüppel-Like Factor 15 Expression by Herpes Simplex Virus Type 1 or Bovine Herpesvirus 1 Productive Infection

Expression of Krüppel-like factor 15 (KLF15), a stress-induced transcription factor, is induced during bovine herpesvirus 1 (BoHV-1) reactivation from latency, and KLF15 stimulates BoHV-1 replication. Transient transfection studies revealed that KLF15 and glucocorticoid receptor (GR) cooperatively transactivate the BoHV-1-immediate-early transcription unit 1 (IEtu1), herpes simplex virus type 1 (HSV-1) infected cell protein 0 (ICP0), and ICP4 promoters. The IEtu1 promoter drives expression of bICP0 and bICP4, two key BoHV-1 transcriptional regulatory proteins. Based on these studies, we hypothesized infection is a stressful stimulus that increases KLF15 expression and enhances productive infection. New studies demonstrated that silencing KLF15 impaired HSV-1 productive infection, and KLF15 steady-state protein levels were increased at late stages of productive infection. KLF15 was primarily localized to the nucleus following infection of cultured cells with HSV-1, but not BoHV-1. When cells were transfected with a KLF15 promoter construct and then infected with HSV-1, promoter activity was significantly increased. The ICP0 gene, and to a lesser extent, bICP0 transactivated the KLF15 promoter in the absence of other viral proteins. In contrast, BoHV-1 or HSV-1 encoded VP16 had no effect on KLF15 promoter activity. Collectively, these studies revealed that HSV-1 and BoHV-1 productive infection increased KLF15 steady-state protein levels, which correlated with increased virus production.

Transactivation of Herpes Simplex Virus 1 (HSV-1) Infected Cell Protein 4 Enhancer by Glucocorticoid Receptor and Stress-Induced Transcription Factors Requires Overlapping Krüppel-Like Transcription Factor 4/Sp1 Binding Sites

Following acute infection, herpes simplex virus 1 (HSV-1) lytic cycle viral gene expression is silenced; consequently, lifelong latency in neurons is established. Certain external stimuli that trigger reactivation from latency also activate the glucocorticoid receptor (GR). The synthetic corticosteroid dexamethasone, but not a GR-specific antagonist, increases the frequency of explant-induced reactivation from latency and stimulates productive infection. Furthermore, dexamethasone increases expression of cellular transcription factors in trigeminal ganglionic neurons: for example, SLUG and three Krüppel-like transcription factor (KLF) family members, KLF4, KLF15, and promyelocytic leukemia zinc finger protein (PLZF). Consequently, we hypothesized that stress-induced transcription factors stimulate expression of ICP4, a viral transcriptional regulator required for productive infection. New studies demonstrated that GR and KLF4, PLZF, or SLUG cooperatively transactivate the ICP4 enhancer upstream of a minimal promoter in monkey kidney cells (Vero) and mouse neuroblastoma cells (Neuro-2A). Strikingly, mutagenesis of two KLF4/Sp1 binding sites reduced GR- plus KLF4-, PLZF-, or SLUG-mediated transactivation to basal levels. A consensus enhancer (E)-Box adjacent to a KLF4/Sp1 binding site was also required for GR- and SLUG-, but not KLF family member-, mediated transactivation of the ICP4 promoter. Chromatin immunoprecipitation studies (ChIP) revealed GR and stress-induced transcription factors occupy ICP4 enhancer sequences. Conversely, specific binding was generally reduced in the KLF4/Sp1 mutant. Furthermore, GR and SLUG occupancy of ICP4 enhancer sequences was reduced in the E-Box mutant. Based on these studies, we suggest stressful stimuli can trigger productive infection because GR and specific stress-induced transcription factors activate ICP4 expression.IMPORTANCE Certain stressful stimuli activate the glucocorticoid receptor (GR) and increase the incidence of herpes simplex virus 1 (HSV-1) reactivation from latency. For example, a corticosteroid antagonist impairs productive infection and virus shedding following explant of trigeminal ganglia from latently infected mice. Infected cell protein 4 (ICP4) is the only immediate early viral transcriptional regulator required for productive infection, suggesting stressful stimuli stimulate ICP4 expression. New studies revealed GR and stress-induced transcription factors identified during reactivation from latency, SLUG and three Krüppel-like transcription factor family members (KLF4, KLF15, and promyelocytic leukemia zinc finger protein), cooperatively transactivate the ICP4 enhancer. Two KLF4 consensus binding sites were crucial for cooperative transactivation of the ICP4 enhancer. A consensus enhancer-box also mediated cooperative transactivation of the ICP4 enhancer by GR and SLUG. The ability of GR and stress-induced transcription factors to transactivate ICP4 enhancer activity is predicted to trigger productive infection following stressful stimuli.

Antisense Transcripts and Antisense Protein: A New Perspective on Human Immunodeficiency Virus Type 1

It was first predicted in 1988 that there may be an Open Reading Frame (ORF) on the negative strand of the Human Immunodeficiency Virus type 1 (HIV-1) genome that could encode a protein named AntiSense Protein (ASP). In spite of some controversy, reports began to emerge some years later describing the detection of HIV-1 antisense transcripts, the presence of ASP in transfected and infected cells, and the existence of an immune response targeting ASP. Recently, it was established that the asp gene is exclusively conserved within the pandemic group M of HIV-1. In this review, we summarize the latest findings on HIV-1 antisense transcripts and ASP, and we discuss their potential functions in HIV-1 infection together with the role played by antisense transcripts and ASPs in some other viruses. Finally, we suggest pathways raised by the study of antisense transcripts and ASPs that may warrant exploration in the future.

Termination of Transcription of LAT Increases the Amounts of ICP0 mRNA but Does Not Alter the Course of HSV-1 Infection in Latently Infected Murine Ganglia

On entering sensory ganglia, herpes simplex viruses 1 (HSV-1) establishes a latent infection with the synthesis of a latency associated transcript (LAT) or initiates productive infection with expression of a set of immediate early viral proteins. The precise mechanisms how expression of α genes is suppressed during the latency are unknown. One mechanism that has been proposed is illustrated in the case of ICP0, a key immediate early viral regulatory protein. Specifically, the 2 kb LAT intron is complementary to the 3' terminal portion of ICP0 mRNA. To test the hypothesis that accumulation of LAT negatively affects the accumulation of ICP0 mRNA, we inserted a DNA fragment encoding two poly(A) sequences into LAT to early terminate LAT transcript without interrupting the complementary sequence of ICP0 transcript (named as SR1603). Comparisons of the parent (SR1601) and mutant (SR1603) HSV-1 viruses showed the following: Neurons harboring latent SR1603 virus accumulated equivalent amounts of viral DNA but higher amounts of ICP0 mRNA and lower amounts of LAT, when compared to neurons harboring the SR1601 virus. One notable difference between the two viruses is that viral RNA accumulation in explanted ganglia harboring SR1603 virus initiated significantly sooner than that in neurons harboring SR1601 virus, suggesting that ICP0 may act as an activator of viral gene expression in permissive cells. Collectively, these data suggest that increased ICP0 mRNA by suppressed LAT did not affect the establishment of latency in latently infected murine ganglia.

Identification of candidate microRNAs from Ostreid herpesvirus-1 (OsHV-1) and their potential role in the infection of Pacific oysters (Crassostrea gigas)

Oyster production is an economic activity of great interest worldwide. Recently, oysters have been suffering significant mortalities from OsHV-1infection, which has resulted in substantial economic loses in several countries around the world. Understanding viral pathogenicity mechanisms is of central importance for the establishment of disease control measures. Thus, the present work aimed to identify and characterize miRNAs from OsHV-1 as well as to predict their target transcripts in the virus and the host. OsHV-1 genome was used for the in silico discovery of pre-miRNAs. Subsequently, viral and host target transcripts of the OsHV-1 miRNAs were predicted according to the base pairing interaction between mature miRNAs and mRNA 3' untranslated regions (UTRs). Six unique pre-miRNAs were found in different regions of the viral genome, ranging in length from 85 to 172 nucleotides. A complex network of self-regulation of viral gene expression mediated by the miRNAs was identified. These sequences also seem to have a broad ability to regulate the expression of host immune-related genes, especially those associated with pathogen recognition. Our results suggest that OsHV-1 encodes miRNAs with important functions in the infection process, inducing self-regulation of viral transcripts, as well as affecting the regulation of Pacific oyster transcripts related to immunity. Understanding the molecular basis of host-pathogen interactions can help mitigate the recurrent events of oyster mass mortalities by OsHV-1 observed worldwide.

Disturbed Yin-Yang balance: stress increases the susceptibility to primary and recurrent infections of herpes simplex virus type 1

Herpes simplex virus type 1 (HSV-1), a neurotropic herpes virus, is able to establish a lifelong latent infection in the human host. Following primary replication in mucosal epithelial cells, the virus can enter sensory neurons innervating peripheral tissues via nerve termini. The viral genome is then transported to the nucleus where it can be maintained without producing infectious progeny, and thus latency is established in the cell. Yin–Yang balance is an essential concept in traditional Chinese medicine (TCM) theory. Yin represents stable and inhibitory factors, and Yang represents the active and aggressive factors. When the organism is exposed to stress, especially psychological stress caused by emotional stimulation, the Yin–Yang balance is disturbed and the virus can re-engage in productive replication, resulting in recurrent diseases. Therefore, a better understanding of the stress-induced susceptibility to HSV-1 primary infection and reactivation is needed and will provide helpful insights into the effective control and treatment of HSV-1. Here we reviewed the recent advances in the studies of HSV-1 susceptibility, latency and reactivation. We included mechanisms involved in primary infection and the regulation of latency and described how stress-induced changes increase the susceptibility to primary and recurrent infections.

Herpesviral Latency-Common Themes

Latency establishment is the hallmark feature of herpesviruses, a group of viruses, of which nine are known to infect humans. They have co-evolved alongside their hosts, and mastered manipulation of cellular pathways and tweaking various processes to their advantage. As a result, they are very well adapted to persistence. The members of the three subfamilies belonging to the family Herpesviridae differ with regard to cell tropism, target cells for the latent reservoir, and characteristics of the infection. The mechanisms governing the latent state also seem quite different. Our knowledge about latency is most complete for the gammaherpesviruses due to previously missing adequate latency models for the alpha and beta-herpesviruses. Nevertheless, with advances in cell biology and the availability of appropriate cell-culture and animal models, the common features of the latency in the different subfamilies began to emerge. Three criteria have been set forth to define latency and differentiate it from persistent or abortive infection: 1) persistence of the viral genome, 2) limited viral gene expression with no viral particle production, and 3) the ability to reactivate to a lytic cycle. This review discusses these criteria for each of the subfamilies and highlights the common strategies adopted by herpesviruses to establish latency.

Antagonizing the Glucocorticoid Receptor Impairs Explant-Induced Reactivation in Mice Latently Infected with Herpes Simplex Virus 1

Herpes simplex virus 1 (HSV-1) establishes lifelong latent infections in neurons. Reactivation from latency can lead to serious recurrent disease, including stromal keratitis, corneal scarring, blindness, and encephalitis. Although numerous studies link stress to an increase in the incidence of reactivation from latency and recurrent disease, the mechanism of action is not well understood. We hypothesized that stress, via corticosteroid-mediated activation of the glucocorticoid receptor (GR), stimulates viral gene expression and productive infection during reactivation from latency. Consequently, we tested whether GR activation by the synthetic corticosteroid dexamethasone influenced virus shedding during reactivation from latency using trigeminal ganglion (TG) explants from Swiss Webster mice latently infected with HSV-1, strain McKrae. TG explants from the latently infected mice shed significantly higher levels of virus when treated with dexamethasone. Conversely, virus shedding from TG explants was significantly impaired when they were incubated with medium containing a GR-specific antagonist (CORT-108297) or stripped fetal bovine serum, which lacks nuclear hormones and other growth factors. TG explants from latently infected, but not uninfected, TG contained significantly more GR-positive neurons following explant when treated with dexamethasone. Strikingly, VP16 protein expression was detected in TG neurons at 8 hours after explant whereas infected-cell protein 0 (ICP0) and ICP4 protein expression was not readily detected until 16 hours after explant. Expression of all three viral regulatory proteins was stimulated by dexamethasone. These studies indicated corticosteroid-mediated GR activation increased the number of TG neurons expressing viral regulatory proteins, which enhanced virus shedding during explant-induced reactivation from latency.IMPORTANCE Herpes simplex virus 1 (HSV-1) establishes lifelong latent infections in neurons within trigeminal ganglia (TG); periodically, reactivation from latency occurs, leading to virus transmission and recurrent disease. Chronic or acute stress increases the frequency of reactivation from latency; how this occurs is not well understood. Here, we demonstrate that the synthetic corticosteroid dexamethasone stimulated explant-induced reactivation from latency. Conversely, a glucocorticoid receptor (GR) antagonist significantly impaired reactivation from latency, indicating that GR activation stimulated explant-induced reactivation. The viral regulatory protein VP16 was readily detected in TG neurons prior to infected-cell protein 0 (ICP0) and ICP4 during explant-induced reactivation. Dexamethasone induced expression of all three viral regulatory proteins following TG explant. Whereas the immunosuppressive properties of corticosteroids would facilitate viral spread during reactivation from latency, these studies indicate GR activation increases the number of TG neurons that express viral regulatory proteins during early stages of explant-induced reactivation.

Comparison of HEp-2 and Vero Cell Responses Reveal Unique Proapoptotic Activities of the Herpes Simplex Virus Type 1 α0 Gene Transcript and Product

Previous studies have provided evidence suggesting a role for apoptosis in the control of Herpes Simplex Virus 1 (HSV-1) latency. HSV-1 induces and then later blocks apoptosis in infected cells. The immediate early viral gene α0, which synthesizes the ICP0 protein, is necessary and sufficient for HSV-1-induced apoptosis in human epithelial (HEp-2) cells. While previous research showed that ICP0 protein synthesis is not necessary for HSV-1-induced apoptosis in infected HEp-2 cells, circumstantial evidence suggested that it might be needed in infected African green monkey kidney (Vero) cells. In this study, we determined the specific aspects of α0 needed to trigger apoptosis in these two cell types. HEp-2 cells transfected with α0 expressing plasmids that generated either full-length, truncated, or no detectable (multiple stop codons) ICP0 protein died through apoptosis. This indicates that ICP0 protein is not necessary for α0-induced apoptosis and that α0 mRNA alone has apoptotic induction properties in HEp-2 cells. We next investigated the primary structure of α0's mRNA to better define its proapoptotic ability. Since α0 is one of the few HSV-1 genes that are spliced, we transfected cells with a plasmid expressing ICP0 from cDNA copy, pcDNAICP0. The cells transfected with pcDNAICP0 underwent apoptosis at a level equivalent to those transfected with the genomic copy of α0, which indicates that neither splicing events nor introns are required for the apoptotic function of α0 in HEp-2 cells. Next, we studied the ability of α0 to cause apoptosis in Vero cells. Since HSV-1-induced apoptosis in Vero cells requires protein synthesis early in infection, proteins synthesized with immediate early kinetics may facilitate apoptosis. Vero cells were transfected with plasmids producing either full-length ICP0 or ICP0 truncated at codon 212. Full-length ICP0, but not truncated ICP0, induced apoptosis in Vero cells. Together, these results suggest that α0 gene expression triggers apoptosis, but ICP0 protein is needed to facilitate apoptosis in Vero cells. In addition, ICP0's facilitation activity may lie in its carboxyl-terminated domain. Thus, our results demonstrate that α0's mRNA and protein possess proapoptotic properties. The requirement for ICP0 protein during HSV-dependent apoptosis appears to be cell type specific.

Disseminated zoster in an adult patient with extensive burns: a case report

Background

Shingles (localized zoster) and disseminated zoster are caused by the reactivation of latent varicella zoster virus (VZV). Reactivation of VZV is related to impaired cell-mediated immunity. Extensive burns affecting a patient result in burn-related immunosuppression and cytokine storm. Despite immunosuppression in burn patients, the reactivation of VZV is extremely rare, whereas eczema herpeticum, caused by reactivation of latent herpes simplex virus (HSV), is common. We have found only 1 published case of VZV reactivation during burn treatment in the literature.

Case presentation

A 51-year-old man was burned in a fire, which affected 60% of his total body surface area (TBSA), and also received inhalation injury (day 0). Despite fluid resuscitation, he showed persistent renal failure. Continuous hemodialysis and filtration (CHDF) combined with polymyxin B-immobilized fiber column direct hemoperfusion (PMX-DHP) therapy was used for cytokine modulation. Autologous and allogeneic skin grafting was performed. On day 15, multiple-drug-resistant Pseudomonas aeruginosa (MDRP) was detected from a blood specimen, and the patient developed multiple organ failure (MOF). On day 31, compact aggregations of small vesicles appeared on the intact skin of his left knee and left buttock. The vesicles were located within the 4th lumbar (L4) spinal dermatome. From day 32 to day 34, similar new vesicles arose on his intact skin and epithelializing skin-graft donor sites. We diagnosed disseminated zoster, based on the patient’s age, the characteristic occurrence of the initial vesicles within a limited area of intact skin in the left L4 dermatome, and a positive Tzank smear. Serologic testing on day 36 showed a high level of anti-VZV immunoglobulin (Ig)G with low levels of anti-VZV IgM, anti-HSV IgG, and anti-HSV IgM. The patient was isolated in a negative-pressure room to avoid air-borne spread of VZV. On day 52, the patient died.

Conclusions

To the best of our knowledge, our patient is the second case of reactivation of VZV during burn treatment. It is unclear why reactivation of VZV is rare in patients with burn-related immunosuppression, whereas HSV reactivation is common. Cytokine modulation throughout the treatment period using CHDF combined with PMX-DHP might have been related to the rare reactivation of VZV in our patient. Our case provides an additional information on the relationship between the immune status of a patient with extensive burns and reactivation of latent VZV or HSV.

The Glucocorticoid Receptor (GR) Stimulates Herpes Simplex Virus 1 Productive Infection, in Part Because the Infected Cell Protein 0 (ICP0) Promoter Is Cooperatively Transactivated by the GR and Krüppel-Like Transcription Factor 15

Following acute infection, herpes simplex virus 1 (HSV-1) establishes lifelong latency in neurons. Physical, emotional, and chemical stresses are linked to increasing the incidence of reactivation from latency, but the mechanism of action is not well understood. In general, stress increases corticosteroid levels, leading to activation of the glucocorticoid receptor (GR), a pioneer transcription factor. Consequently, we hypothesized that stress-mediated activation of the GR can stimulate productive infection and viral gene expression. New studies demonstrated that the GR-specific antagonist (CORT-108297) significantly reduced HSV-1 productive infection in mouse neuroblastoma cells (Neuro-2A). Additional studies demonstrated that the activated GR and Krüppel-like transcription factor 15 (KLF15) cooperatively transactivated the infected cell protein 0 (ICP0) promoter, a crucial viral regulatory protein. Interestingly, the synthetic corticosteroid dexamethasone and GR or KLF15 alone had little effect on ICP0 promoter activity in transfected Neuro-2A or Vero cells. Chromatin immunoprecipitation (ChIP) studies revealed that the GR and KLF15 occupied ICP0 promoter sequences important for transactivation at 2 and 4 h after infection; however, binding was not readily detected at 6 h after infection. Similar results were obtained for cells transfected with the full-length ICP0 promoter. ICP0 promoter sequences lack a consensus "whole" GR response element (GRE) but contain putative half-GREs that were important for dexamethasone induced promoter activity. The activated GR stimulates expression of, and interacts with, KLF15; consequently, these data suggest KLF15 and the GR form a feed-forward loop that activates viral gene expression and productive infection following stressful stimuli.IMPORTANCE The ability of herpes simplex virus 1 (HSV-1) to periodically reactivate from latency results in virus transmission and recurrent disease. The incidence of reactivation from latency is increased by chronic or acute stress. Stress increases the levels of corticosteroids, which bind and activate the glucocorticoid receptor (GR). Since GR activation is an immediate early response to stress, we tested whether the GR influences productive infection and the promoter that drives infected cell protein 0 (ICP0) expression. Pretreatment of cells with a GR-specific antagonist (CORT-108297) significantly reduced virus replication. Although the GR had little effect on ICP0 promoter activity alone, the Krüppel-like transcription factor 15 (KLF15) cooperated with the GR to stimulate promoter activity in transfected cells. In transfected or infected cells, the GR and KLF15 occupied ICP0 sequences important for transactivation. Collectively, these studies provide insight into how stress can directly stimulate productive infection and viral gene expression.

B Cell Receptor Activation and Chemical Induction Trigger Caspase-Mediated Cleavage of PIAS1 to Facilitate Epstein-Barr Virus Reactivation

Epstein-Barr virus (EBV) in tumor cells is predominately in the latent phase, but the virus can undergo lytic reactivation in response to various stimuli. However, the cellular factors that control latency and lytic replication are poorly defined. In this study, we demonstrated that a cellular factor, PIAS1, restricts EBV lytic replication. PIAS1 depletion significantly facilitated EBV reactivation, while PIAS1 reconstitution had the opposite effect. Remarkably, we found that various lytic triggers promote caspase-dependent cleavage of PIAS1 to antagonize PIAS1-mediated restriction and that caspase inhibition suppresses EBV replication through blocking PIAS1 cleavage. We further demonstrated that a cleavage-resistant PIAS1 mutant suppresses EBV replication upon B cell receptor activation. Mechanistically, we demonstrated that PIAS1 acts as an inhibitor for transcription factors involved in lytic gene expression. Collectively, these results establish PIAS1 as a key regulator of EBV lytic replication and uncover a mechanism by which EBV exploits apoptotic caspases to antagonize PIAS1-mediated restriction.

Strength in diversity: Understanding the pathways to herpes simplex virus reactivation

Herpes simplex virus (HSV) establishes a latent infection in peripheral neurons and can periodically reactivate to cause disease. Reactivation can be triggered by a variety of stimuli that activate different cellular processes to result in increased HSV lytic gene expression and production of infectious virus. The use of model systems has contributed significantly to our understanding of how reactivation of the virus is triggered by different physiological stimuli that are correlated with recrudescence of human disease. Furthermore, these models have led to the identification of both common and distinct mechanisms of different HSV reactivation pathways. Here, we summarize how the use of these diverse model systems has led to a better understanding of the complexities of HSV reactivation, and we present potential models linking cellular signaling pathways to changes in viral gene expression.

The Wnt Signaling Pathway Is Differentially Expressed during the Bovine Herpesvirus 1 Latency-Reactivation Cycle: Evidence That Two Protein Kinases Associated with Neuronal Survival, Akt3 and BMPR2, Are Expressed at Higher Levels during Latency

Sensory neurons in trigeminal ganglia (TG) of calves latently infected with bovine herpesvirus 1 (BoHV-1) abundantly express latency-related (LR) gene products, including a protein (ORF2) and two micro-RNAs. Recent studies in mouse neuroblastoma cells (Neuro-2A) demonstrated ORF2 interacts with β-catenin and a β-catenin coactivator, high-mobility group AT-hook 1 (HMGA1) protein, which correlates with increased β-catenin-dependent transcription and cell survival. β-Catenin and HMGA1 are readily detected in a subset of latently infected TG neurons but not TG neurons from uninfected calves or reactivation from latency. Consequently, we hypothesized that the Wnt/β-catenin signaling pathway is differentially expressed during the latency and reactivation cycle and an active Wnt pathway promotes latency. RNA-sequencing studies revealed that 102 genes associated with the Wnt/β-catenin signaling pathway were differentially expressed in TG during the latency-reactivation cycle in calves. Wnt agonists were generally expressed at higher levels during latency, but these levels decreased during dexamethasone-induced reactivation. The Wnt agonist bone morphogenetic protein receptor 2 (BMPR2) was intriguing because it encodes a serine/threonine receptor kinase that promotes neuronal differentiation and inhibits cell death. Another differentially expressed gene encodes a protein kinase (Akt3), which is significant because Akt activity enhances cell survival and is linked to herpes simplex virus 1 latency and neuronal survival. Additional studies demonstrated ORF2 increased Akt3 steady-state protein levels and interacted with Akt3 in transfected Neuro-2A cells, which correlated with Akt3 activation. Conversely, expression of Wnt antagonists increased during reactivation from latency. Collectively, these studies suggest Wnt signaling cooperates with LR gene products, in particular ORF2, to promote latency.IMPORTANCE Lifelong BoHV-1 latency primarily occurs in sensory neurons. The synthetic corticosteroid dexamethasone consistently induces reactivation from latency in calves. RNA sequencing studies revealed 102 genes associated with the Wnt/β-catenin signaling pathway are differentially regulated during the latency-reactivation cycle. Two protein kinases associated with the Wnt pathway, Akt3 and BMPR2, were expressed at higher levels during latency but were repressed during reactivation. Furthermore, five genes encoding soluble Wnt antagonists and β-catenin-dependent transcription inhibitors were induced during reactivation from latency. These findings are important because Wnt, BMPR2, and Akt3 promote neurogenesis and cell survival, processes crucial for lifelong viral latency. In transfected neuroblastoma cells, a viral protein expressed during latency (ORF2) interacts with and enhances Akt3 protein kinase activity. These findings provide insight into how cellular factors associated with the Wnt signaling pathway cooperate with LR gene products to regulate the BoHV-1 latency-reactivation cycle.

1, 25(OH)2 D3 Induces Reactivation and Death of Kaposi's Sarcoma-Associated Herpesvirus of Primary Effusion Lymphoma cells

Kaposi's sarcoma associated herpesvirus (KSHV) a gammaherpesvirus establishes perennial latency in the host with periodic reactivation. Occasionally change in the physiological condition like hypoxia, host cell differentiation can trigger the lytic switch and reactivation of the virus. The biologically active form of 1, 25(OH)2 D3 plays a critical role in the regulation of various physiological processes (e.g. regulation of mineral homeostasis and control of bone metabolism). Apart from its role in host physiology, 1, 25(OH)2 D3 has been implicated as a potential agent for the prevention and/or treatment of many a tumors. Here we show that 1, 25(OH)2 D3 induces both death of Kaposi sarcoma associated herpesvirus infected PEL cells and KSHV replication. 1, 25(OH)2 D3 mediated inhibition of proliferation was associated with apoptosis of the PEL cells, and virus reactivation. In addition, p38 signalling is required for KSHV reactivation. Furthermore, treatment of PEL cells with p38 inhibitor abrogated the expression of ORF57, thus blocking lytic switch. Furthermore, silencing of VDR resulted in reduced ORF57 expression compared to the control cells, signifying the potential role of 1, 25(OH)2 D3 in KSHV reactivation. Thus, our studies have revealed a novel role of 1, 25(OH)2 D3 in the regulation of KSHV reactivation and PEL cell death.

Haemocytes collected from experimentally infected Pacific oysters, Crassostrea gigas: Detection of ostreid herpesvirus 1 DNA, RNA, and proteins in relation with inhibition of apoptosis

Recent transcriptomic approaches focused on anti-viral immunity in molluscs lead to the assumption that the innate immune system, such as apoptosis, plays a crucial role against ostreid herpesvirus type 1 (OsHV-1), infecting Pacific cupped oyster, Crassostrea gigas. Apoptosis constitutes a major mechanism of anti-viral response by limiting viral spread and eliminating infected cells. In this way, an OsHV-1 challenge was performed and oysters were monitored at three times post injection to investigate viral infection and host response: 2h (early after viral injection in the adductor muscle), 24h (intermediate time), and 48h (just before first oyster mortality record). Virus infection, associated with high cumulative mortality rates (74% and 100%), was demonstrated in haemocytes by combining several detection techniques such as real-time PCR, real-time RT PCR, immunofluorescence assay, and transmission electron microscopy examination. High viral DNA amounts ranged from 5.46×104 to 3.68×105 DNA copies ng-1 of total DNA, were detected in dead oysters and an increase of viral transcripts was observed from 2, 24, and 48hpi for the five targeted OsHV-1 genes encoding three putative membrane proteins (ORFs 25, 41, and 72), a putative dUTPase (ORF 75), and a putative apoptosis inhibitor (ORF 87). Apoptosis was studied at molecular and cellular levels with an early marker (phosphatidyl-serine externalisation measured by flow cytometry and epifluorescence microscopy) and a later parameter (DNA fragmentation by terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay (TUNEL)). The down-regulation of genes encoding proteins involved in the activation of the apoptotic pathway (TNF and caspase 3) and the up-regulation of genes encoding anti-apoptotic proteins (IAP-2, and Bcl-2) suggested an important anti-apoptosis phenomenon in haemocytes from OsHV-1 infected oysters at 24 and 48hpi. Additionally, more phosphatidyl-serines were externalized and more cells with DNA fragmentation were observed in haemocytes collected from artificial seawater injected oysters than in haemocytes collected from OsHV-1 infected oysters at 24 and 48hpi, suggesting an inhibition of the apoptotic process in presence of the virus. In conclusion, this study is the first to focus on C. gigas haemocytes, cells involved in the host immune defense, during an OsHV-1 challenge in controlled conditions by combining various and original approaches to investigate apoptosis at molecular and cellular levels.

Differentially expressed genes during spontaneous lytic switch of Marek's disease virus in lymphoblastoid cell lines determined by global gene expression profiling

Marek's disease virus (MDV), an alphaherpesvirus of poultry, causes Marek's disease and is characterized by visceral CD4+TCRαβ+ T-cell lymphomas in susceptible hosts. Immortal cell lines harbouring the viral genome have been generated from ex vivo cultures of MD tumours. As readily available sources of large numbers of cells, MDV-transformed lymphoblastoid cell lines (LCLs) are extremely valuable for studies of virus-host interaction. While the viral genome in most cells is held in a latent state, minor populations of cells display spontaneous reactivation identifiable by the expression of lytic viral genes. Spontaneous reactivation in these cells presents an opportunity to investigate the biological processes involved in the virus reactivation. For detailed characterization of the molecular events associated with reactivation, we used two lymphoblastoid cell lines derived from lymphomas induced by pRB1B-UL47eGFP, a recombinant MDV engineered to express enhanced green fluorescent protein (EGFP) fused with the UL47. We used fluorescence-activated cell sorting to purify the low-frequency EGFP-positive cells with a spontaneously activating viral genome from the majority EGFP-negative cells and analysed their gene expression profiles by RNA-seq using Illumina HiSeq2500. Ingenuity pathway analysis on more than 2000 differentially expressed genes between the lytically infected (EGFP-positive) and latently infected (EGFP-negative) cell populations identified the biological pathways involved in the reactivation. Virus-reactivating cells exhibited differential expression of a significant number of viral genes, with hierarchical differences in expression levels. Downregulation of a number of host genes including those directly involved in T-cell activation, such as CD3, CD28, ICOS and phospholipase C, was also noticed in the LCL undergoing lytic switch.

HSV1 latent transcription and non-coding RNA: A critical retrospective

Virologists have invested great effort into understanding how the herpes simplex viruses and their relatives are maintained dormant over the lifespan of their host while maintaining the poise to remobilize on sporadic occasions. Piece by piece, our field has defined the tissues in play (the sensory ganglia), the transcriptional units (the latency-associated transcripts), and the responsive genomic region (the long repeats of the viral genomes). With time, the observed complexity of these features has compounded, and the totality of viral factors regulating latency are less obvious. In this review, we compose a comprehensive picture of the viral genetic elements suspected to be relevant to herpes simplex virus 1 (HSV1) latent transcription by conducting a critical analysis of about three decades of research. We describe these studies, which largely involved mutational analysis of the notable latency-associated transcripts (LATs), and more recently a series of viral miRNAs. We also intend to draw attention to the many other less characterized non-coding RNAs, and perhaps coding RNAs, that may be important for consideration when trying to disentangle the multitude of phenotypes of the many genetic modifications introduced into recombinant HSV1 strains.

Effects of the synthetic corticosteroid dexamethasone on bovine herpesvirus 1 productive infection

Sensory neurons are a primary site for life-long latency of bovine herpesvirus 1 (BoHV-1). The synthetic corticosteroid dexamethasone induces reactivation from latency and productive infection, in part because the BoHV-1 genome contains more than 100 glucocorticoid receptor (GR) responsive elements (GREs). Two GREs in the immediate early transcription unit 1 promoter are required for dexamethasone induction. Recent studies also demonstrated that the serum and glucocorticoid receptor protein kinase (SGK) family stimulated BoHV-1 replication. Consequently, we hypothesized that dexamethasone influences several aspects of productive infection. In this study, we demonstrated that dexamethasone increased expression of the immediate early protein bICP4, certain late transcripts, and UL23 (thymidine kinase) by four hours after infection. SGK1 expression and Akt phosphorylation were also stimulated during early stages of infection and dexamethasone treatment further increased this effect. These studies suggest that stress, as mimicked by dexamethasone treatment, has the potential to stimulate productive infection by multiple pathways.

Dexamethasone treatment decreases replication of viral hemorrhagic septicemia virus in Epithelioma papulosum cyprini cells

The expression of Mx1 in EPC cells after treatment with poly(I:C) or infection with viral hemorrhagic septicemia virus (VHSV) was significantly suppressed by treatment with dexamethasone. However, the titer of VHSV did not increase but instead decreased after dexamethasone treatment. This suggests that dexamethasone not only downregulates type I IFN but also affects certain factors that are necessary for VHSV replication. An important effect of HSP90 on replication of RNA viruses and downregulation of HSP90 by glucocorticoids have been reported. In this study, dexamethasone downregulated HSP90α expression in EPC cells that were stimulated with poly(I:C) or infected with VHSV. Furthermore, cells treated with an HSP90 inhibitor, geldanamycin, showed significantly decreased titers of VHSV, suggesting that HSP90 may be an important host component involved in VHSV replication, and HSP90 inhibition might be one of the causes for the observed reduction in viral titer caused by dexamethasone treatment.

Neuronal Stress Pathway Mediating a Histone Methyl/Phospho Switch Is Required for Herpes Simplex Virus Reactivation

Herpes simplex virus (HSV) reactivation from latent neuronal infection requires stimulation of lytic gene expression from promoters associated with repressive heterochromatin. Various neuronal stresses trigger reactivation, but how these stimuli activate silenced promoters remains unknown. We show that a neuronal pathway involving activation of c-Jun N-terminal kinase (JNK), common to many stress responses, is essential for initial HSV gene expression during reactivation. This JNK activation in neurons is mediated by dual leucine zipper kinase (DLK) and JNK-interacting protein 3 (JIP3), which direct JNK toward stress responses instead of other cellular functions. Surprisingly, JNK-mediated viral gene induction occurs independently of histone demethylases that remove repressive lysine modifications. Rather, JNK signaling results in a histone methyl/phospho switch on HSV lytic promoters, a mechanism permitting gene expression in the presence of repressive lysine methylation. JNK is present on viral promoters during reactivation, thereby linking a neuronal-specific stress pathway and HSV reactivation from latency.

The serum and glucocorticoid-regulated protein kinases (SGK) stimulate bovine herpesvirus 1 and herpes simplex virus 1 productive infection

Serum and glucocorticoid-regulated protein kinases (SGK) are serine/threonine protein kinases that contain a catalytic domain resembling other protein kinases: AKT/protein kinase B, protein kinase A, and protein kinase C-Zeta for example. Unlike these constitutively expressed protein kinases, SGK1 RNA and protein levels are increased by growth factors and corticosteroids. Stress can directly stimulate SGK1 levels as well as stimulate bovine herpesvirus 1 (BoHV-1) and herpes simplex virus 1 (HSV-1) productive infection and reactivation from latency suggesting SGK1 can stimulate productive infection. For the first time, we provide evidence that a specific SGK inhibitor (GSK650394) significantly reduced BoHV-1 and HSV-1 replication in cultured cells. Proteins encoded by the three BoHV-1 immediate early genes (bICP0, bICP4, and bICP22) and two late proteins (VP16 and gE) were consistently reduced by GSK650394 during early stages of productive infection. In summary, these studies suggest SGK may stimulate viral replication following stressful stimuli.

Epstein-Barr Virus MicroRNA miR-BART20-5p Suppresses Lytic Induction by Inhibiting BAD-Mediated caspase-3-Dependent Apoptosis

Epstein-Barr virus (EBV) is a human gammaherpesvirus associated with a variety of tumor types. EBV can establish latency or undergo lytic replication in host cells. In general, EBV remains latent in tumors and expresses a limited repertoire of latent proteins to avoid host immune surveillance. When the lytic cycle is triggered by some as-yet-unknown form of stimulation, lytic gene expression and progeny virus production commence. Thus far, the exact mechanism of EBV latency maintenance and the in vivo triggering signal for lytic induction have yet to be elucidated. Previously, we have shown that the EBV microRNA miR-BART20-5p directly targets the immediate early genes BRLF1 and BZLF1 as well as Bcl-2-associated death promoter (BAD) in EBV-associated gastric carcinoma. In this study, we found that both mRNA and protein levels of BRLF1 and BZLF1 were suppressed in cells following BAD knockdown and increased after BAD overexpression. Progeny virus production was also downregulated by specific knockdown of BAD. Our results demonstrated that caspase-3-dependent apoptosis is a prerequisite for BAD-mediated EBV lytic cycle induction. Therefore, our data suggest that miR-BART20-5p plays an important role in latency maintenance and tumor persistence of EBV-associated gastric carcinoma by inhibiting BAD-mediated caspase-3-dependent apoptosis, which would trigger immediate early gene expression.

IMPORTANCE

EBV has an ability to remain latent in host cells, including EBV-associated tumor cells hiding from immune surveillance. However, the exact molecular mechanisms of EBV latency maintenance remain poorly understood. Here, we demonstrated that miR-BART20-5p inhibited the expression of EBV immediate early genes indirectly, by suppressing BAD-induced caspase-3-dependent apoptosis, in addition to directly, as we previously reported. Our study suggests that EBV-associated tumor cells might endure apoptotic stress to some extent and remain latent with the aid of miR-BART20-5p. Blocking the expression or function of BART20-5p may expedite EBV-associated tumor cell death via immune attack and apoptosis.

Role of activating transcription factor 3 in the synthesis of latency-associated transcript and maintenance of herpes simplex virus 1 in latent state in ganglia

A key property of herpes simplex viruses (HSVs) is their ability to establish latent infection in sensory or autonomic ganglia and to reactivate on physical, hormonal, or emotional stress. In latently infected ganglia, HSVs express a long noncoding RNA, a latency-associated transcript (LAT), which plays a key role in maintaining latently infected neurons, but not viral proteins. To investigate the events leading to reactivation, we examined the use of ganglionic organ cultures that enable rapid reactivation in medium containing antibody to nerve growth factor (NGF) or delayed reactivation in medium containing NGF and epidermal growth factor (EGF). Here we report the discovery that activating transcription factor 3 (ATF3), a stress response protein, profoundly affects the interaction of HSV with its host. Specifically, (i) ATF3 is induced by stress, such as inhibition of protein synthesis or infection; (ii) in infected cells, ATF3 enhances the accumulation of LAT by acting on the response elements in the promoter of the LAT precursor RNA; (iii) ATF3 is induced nearly 100-fold in ganglionic organ cultures; and (iv) ATF3 plays a key role in the maintenance of the latent state, inasmuch as expression of ATF3 bereft of the C-terminal activation domain acts as a dominant negative factor, inducing HSV gene expression in ganglionic organ cultures harboring latent virus and incubated in medium containing NGF and EGF. Thus, ATF3 is a component of a cluster of cellular proteins that together with LAT maintain the integrity of the neurons harboring latent virus.

Bovine herpesvirus 1 productive infection and immediate early transcription unit 1 promoter are stimulated by the synthetic corticosteroid dexamethasone

The primary site for life-long latency of bovine herpesvirus 1 (BHV-1) is sensory neurons. The synthetic corticosteroid dexamethasone consistently induces reactivation from latency; however the mechanism by which corticosteroids mediate reactivation is unclear. In this study, we demonstrate for the first time that dexamethasone stimulates productive infection, in part, because the BHV-1 genome contains more than 100 potential glucocorticoid receptor (GR) response elements (GREs). Immediate early transcription unit 1 (IEtu1) promoter activity, but not IEtu2 or VP16 promoter activity, was stimulated by dexamethasone. Two near perfect consensus GREs located within the IEtu1 promoter were necessary for dexamethasone-mediated stimulation. Electrophoretic mobility shift assays and chromatin immunoprecipitation studies demonstrated that the GR interacts with IEtu1 promoter sequences containing the GREs. Although we hypothesize that DEX-mediated stimulation of IEtu1 promoter activity is important during productive infection and perhaps reactivation from latency, stress likely has pleiotropic effects on virus-infected cells.

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) protects cells against cold-shock-induced apoptosis by maintaining phosphorylation of protein kinase B (AKT)

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) blocks apoptosis and inhibits caspase-3 activation. We previously showed that serum starvation (removal of serum from tissue culture media), which takes several days to induce apoptosis, results in decreased levels of both AKT (protein kinase B) and phosphorylated AKT (pAKT) in cells not expressing LAT. In contrast in mouse neuroblastoma cells expressing LAT, AKT, and pAKT levels remained high. AKT is a serine/threonine protein kinase that promotes cell survival. To examine the effect of LAT on AKT-pAKT using a different and more rapid method of inducing apoptosis, a stable cell line expressing LAT was compared to non-LAT expressing cells as soon as 15 min following recovery from cold-shock-induced apoptosis. Expression of LAT appeared to inhibit dephosphorylation of pAKT. This protection correlated with blocking numerous pro-apoptotic events that are inhibited by pAKT. These results support the hypothesis that inhibiting dephosphorylation of pAKT may be one of the pathways by which LAT protects cells against apoptosis.

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.

Innate lymphotoxin receptor mediated signaling promotes HSV-1 associated neuroinflammation and viral replication

Host anti-viral innate immunity plays important roles in the defense against HSV-1 infection. In this study, we find an unexpected role for innate LT/LIGHT signaling in promoting HSV-1 replication and virus induced inflammation in immunocompromised mice. Using a model of footpad HSV-1 infection in Rag1^–/– mice, we observed that blocking LT/LIGHT signaling with LTβR-Ig could significantly delay disease progression and extend the survival of infected mice. LTβR-Ig treatment reduced late proinflammatory cytokine release in the serum and nervous tissue, and inhibited chemokine expression and inflammatory cells infiltration in the dorsal root ganglia (DRG). Intriguingly, LTβR-Ig treatment restricted HSV-1 replication in the DRG but not the footpad. These findings demonstrate a critical role for LT/LIGHT signaling in modulating innate inflammation and promoting HSV-1 replication in the nervous system, and suggest a new target for treatment of virus-induced adverse immune response and control of severe HSV-1 infection.

Apoptosis-induced activation of HIV-1 in latently infected cell lines

Background

Despite much work, safe and effective approaches to attack and deplete the long-lived reservoir of cells latently infected with HIV-1 remain an elusive goal. Patients infected with HIV-1 treated with cytotoxic agents or bone marrow transplantation can experience decreases in the reservoir of HIV-1 latently infected cells. Other viruses capable of long-term latency, such as herpesviruses, can sense host cell apoptosis and respond by initiating replication. These observations suggest that other viruses capable of long-term latency, like HIV-1, might also sense when its host cell is about to undergo apoptosis and respond by initiating replication.

Results

Pro-monocytic (U1) and lymphoid (ACH-2) HIV-1 persistently infected cell lines were treated with cytotoxic drugs – doxorubicin, etoposide, fludarabine phosphate, or vincristine – and activation of latent HIV-1 was evaluated using assays for HIV-1 RNA and p24 production. Both cell lines showed dose-dependent increases in apoptosis and associated HIV-1 activation following exposure to the cytotoxic agents. Pretreatment of the cells with the pan-caspase inhibitor Z-VAD-FMK prior to exposure to the cytotoxic agents inhibited apoptosis and viral activation. Direct exposure of the latently infected cell lines to activated caspases also induced viral replication. HIV-1 virions produced in association with host cell apoptosis were infectious.

Conclusions

The results indicate that latent HIV-1 can sense when its host cell is undergoing apoptosis and responds by completing its replication cycle. The results may help explain why patients treated with cytotoxic regimens for bone marrow transplantation showed reductions in the reservoir of latently infected cells. The results also suggest that the mechanisms that HIV-1 uses to sense and respond to host cell apoptosis signals may represent helpful new targets for approaches to attack and deplete the long-lived reservoir of cells latently infected with HIV-1.

Bovine herpesvirus 1 regulatory proteins are detected in trigeminal ganglionic neurons during the early stages of stress-induced escape from latency

Bovine herpesvirus 1 (BHV-1) establishes latency in sensory neurons. The synthetic corticosteroid dexamethasone consistently induces reactivation from latency. Within 90 min after latently infected calves are treated with dexamethasone, two BHV-1 regulatory proteins, BHV-1-infected cell protein 0 (bICP0) and viral protein 16 (VP16), are expressed in the same neuron. In this study, we demonstrate that VP16 and bICP0 can be detected at 22 and 33 min after dexamethasone (DEX) treatment of latently infected calves. However, we were unable to discern whether VP16 or bICP0 was expressed at early times after reactivation. VP16+ neurons consistently express the glucocorticoid receptor suggesting corticosteroid-mediated activation of its receptor rapidly stimulates reactivation from latency.

Manipulation of apoptosis and necroptosis signaling by herpesviruses

Like apoptosis, necroptosis is an innate immune mechanism that eliminates pathogen-infected cells. Receptor-interacting protein kinase (RIP)3 (also called RIPK3) mediates necrotic death by phosphorylating an executioner protein, MLKL, leading to plasma membrane leakage. The pathway is triggered against viruses that block caspase 8. In murine CMV, the viral inhibitor of caspase 8 activation prevents extrinsic apoptosis but also has the potential to unleash necroptosis. This virus encodes the viral inhibitor of RIP activation to prevent RIP homotypic interaction motif (RHIM)-dependent signal transduction and necroptosis. Recent investigations reveal a similar mechanism at play in the human alpha-herpesviruses, herpes simplex virus (HSV)1 and HSV2, where RHIM competitor function and caspase 8 suppression are carried out by the virus-encoded large subunit of ribonucleotide reductase (R1). In human cells, R1 inhibition of caspase 8 prevents TNF-induced apoptosis, but sensitizes to TNF-induced necroptosis. The RHIM and caspase 8 interaction domains of R1 collaborate to prevent RIP3-dependent steps and enable both herpesviruses to deflect host cell death machinery that would cut short infection. In mouse cells, HSV1 infection by itself triggers necroptosis by driving RIP3 protein kinase activity. HSV1 R1 contributes to the activation of RIP3 adaptor function in mice, a popular host animal for experimental infection. Based on these studies, infection of RIP3-kinase inactive mice should be explored in models of pathogenesis and latency. The necrotic death pathway that is suppressed during infection in the natural host becomes a cross-species barrier to infection in a non-natural host.

The number of alphaherpesvirus particles infecting axons and the axonal protein repertoire determines the outcome of neuronal infection

Infection by alphaherpesviruses invariably results in invasion of the peripheral nervous system (PNS) and establishment of either a latent or productive infection. Infection begins with long-distance retrograde transport of viral capsids and tegument proteins in axons toward the neuronal nuclei. Initial steps of axonal entry, retrograde transport, and replication in neuronal nuclei are poorly understood. To better understand how the mode of infection in the PNS is determined, we utilized a compartmented neuron culturing system where distal axons of PNS neurons are physically separated from cell bodies. We infected isolated axons with fluorescent-protein-tagged pseudorabies virus (PRV) particles and monitored viral entry and transport in axons and replication in cell bodies during low and high multiplicities of infection (MOIs of 0.01 to 100). We found a threshold for efficient retrograde transport in axons between MOIs of 1 and 10 and a threshold for productive infection in the neuronal cell bodies between MOIs of 1 and 0.1. Below an MOI of 0.1, the viral genomes that moved to neuronal nuclei were silenced. These genomes can be reactivated after superinfection by a nonreplicating virus, but not by a replicating virus. We further showed that viral particles at high-MOI infections compete for axonal proteins and that this competition determines the number of viral particles reaching the nuclei. Using mass spectrometry, we identified axonal proteins that are differentially regulated by PRV infection. Our results demonstrate the impact of the multiplicity of infection and the axonal milieu on the establishment of neuronal infection initiated from axons.

Alphaherpesvirus genomes may remain silent in peripheral nervous system (PNS) neurons for the lives of their hosts. These genomes occasionally reactivate to produce infectious virus that can reinfect peripheral tissues and spread to other hosts. Here, we use a neuronal culture system to investigate the outcome of axonal infection using different numbers of viral particles and coinfection assays. We found that the dynamics of viral entry, transport, and replication change dramatically depending on the number of virus particles that infect axons. We demonstrate that viral genomes are silenced when the infecting particle number is low and that these genomes can be reactivated by superinfection with UV-inactivated virus, but not with replicating virus. We further show that viral invasion rapidly changes the profiles of axonal proteins and that some of these axonal proteins are rate limiting for efficient infection. Our study provides new insights into the establishment of silent versus productive alphaherpesvirus infections in the PNS.

Necroptosis: The Trojan horse in cell autonomous antiviral host defense

Herpesviruses suppress cell death to assure sustained infection in their natural hosts. Murine cytomegalovirus (MCMV) encodes suppressors of apoptosis as well as M45-encoded viral inhibitor of RIP activation (vIRA) to block RIP homotypic interaction motif (RHIM)-signaling and recruitment of RIP3 (also called RIPK3), to prevent necroptosis. MCMV and human cytomegalovirus encode a viral inhibitor of caspase (Casp)8 activation to block apoptosis, an activity that unleashes necroptosis. Herpes simplex virus (HSV)1 and HSV2 incorporate both RHIM and Casp8 suppression strategies within UL39-encoded ICP6 and ICP10, respectively, which are herpesvirus-conserved homologs of MCMV M45. Both HSV proteins sensitize human cells to necroptosis by blocking Casp8 activity while preventing RHIM-dependent RIP3 activation and death. In mouse cells, HSV1 ICP6 interacts with RIP3 and, surprisingly, drives necroptosis. Thus, herpesviruses have illuminated the contribution of necoptosis to host defense in the natural host as well as its potential to restrict cross-species infections in nonnatural hosts.

A comparison of herpes simplex virus type 1 and varicella-zoster virus latency and reactivation

Herpes simplex virus type 1 (HSV-1; human herpesvirus 1) and varicella-zoster virus (VZV; human herpesvirus 3) are human neurotropic alphaherpesviruses that cause lifelong infections in ganglia. Following primary infection and establishment of latency, HSV-1 reactivation typically results in herpes labialis (cold sores), but can occur frequently elsewhere on the body at the site of primary infection (e.g. whitlow), particularly at the genitals. Rarely, HSV-1 reactivation can cause encephalitis; however, a third of the cases of HSV-1 encephalitis are associated with HSV-1 primary infection. Primary VZV infection causes varicella (chickenpox) following which latent virus may reactivate decades later to produce herpes zoster (shingles), as well as an increasingly recognized number of subacute, acute and chronic neurological conditions. Following primary infection, both viruses establish a latent infection in neuronal cells in human peripheral ganglia. However, the detailed mechanisms of viral latency and reactivation have yet to be unravelled. In both cases latent viral DNA exists in an 'end-less' state where the ends of the virus genome are joined to form structures consistent with unit length episomes and concatemers, from which viral gene transcription is restricted. In latently infected ganglia, the most abundantly detected HSV-1 RNAs are the spliced products originating from the primary latency associated transcript (LAT). This primary LAT is an 8.3 kb unstable transcript from which two stable (1.5 and 2.0 kb) introns are spliced. Transcripts mapping to 12 VZV genes have been detected in human ganglia removed at autopsy; however, it is difficult to ascribe these as transcripts present during latent infection as early-stage virus reactivation may have transpired in the post-mortem time period in the ganglia. Nonetheless, low-level transcription of VZV ORF63 has been repeatedly detected in multiple ganglia removed as close to death as possible. There is increasing evidence that HSV-1 and VZV latency is epigenetically regulated. In vitro models that permit pathway analysis and identification of both epigenetic modulations and global transcriptional mechanisms of HSV-1 and VZV latency hold much promise for our future understanding in this complex area. This review summarizes the molecular biology of HSV-1 and VZV latency and reactivation, and also presents future directions for study.

Characterization of herpes simplex virus 2 primary microRNA Transcript regulation

In order to understand factors that may influence latency-associated transcription and latency-associated transcript (LAT) phenotypes, we studied the expression of the herpes simplex virus 2 (HSV-2) LAT-associated microRNAs (miRNAs). We mapped the transcription initiation sites of all three primary miRNA transcripts and identified the ICP4-binding sequences at the transcription initiation sites of both HSV-2 LAT (pri-miRNA for miR-I and miR-II, which target ICP34.5, and miR-III, which targets ICP0) and L/ST (a pri-miRNA for miR-I and miR-II) but not at that of the primary miR-H6 (for which the target is unknown). We confirmed activity of the putative HSV-2 L/ST promoter and found that ICP4 trans-activates the L/ST promoter when the ICP4-binding site at its transcription initiation site is mutated, suggesting that ICP4 may play a dual role in regulating transcription of L/ST and, consequently, of miR-I and miR-II. LAT exon 1 (containing LAT enhancer sequences), together with the LAT promoter region, comprises a bidirectional promoter required for the expression of both LAT-encoded miRNAs and miR-H6 in latently infected mouse ganglia. The ability of ICP4 to suppress ICP34.5-targeting miRNAs and to activate lytic viral genes suggests that ICP4 could play a key role in the switch between latency and reactivation.

IMPORTANCE

The HSV-2 LAT and viral miRNAs expressed in the LAT region are the most abundant viral transcripts during HSV latency. The balance between the expression of LAT and LAT-associated miRNAs and the expression of lytic viral transcripts from the opposite strand appears to influence whether individual HSV-infected neurons will be latently or productively infected. The outcome of neuronal infection may thus depend on regulation of gene expression of the corresponding primary miRNAs. In the present study, we characterize promoter sequences responsible for miRNA expression, including identification of the primary miRNA 5' ends and evaluation of ICP4 response. These findings provide further insight into the virus' strategy to tightly control expression of lytic cycle genes (especially the neurovirulence factor, ICP34.5) and suggest a mechanism (via ICP4) for the transition from latency to reactivated productive infection.

A herpes simplex virus type 1 mutant disrupted for microRNA H2 with increased neurovirulence and rate of reactivation

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) encodes several microRNAs. One of these, miR-H2, overlaps and is antisense to the ICP0 gene and appears to decrease expression of the ICP0 protein. To determine if miR-H2 plays a role in the HSV-1 latency-reactivation cycle, we constructed a mutant, McK-ΔH2, in which this microRNA has been disrupted without altering the predicted amino acid sequence of ICP0. McK-ΔH2 produced increased amounts of ICP0. Although replication of McK-ΔH2 was similar to that of its wild-type (wt) McKrae parental virus in RS cells and mouse eyes, McK-ΔH2 was more neurovirulent in Swiss-Webster mice than McKrae based on the percent of mice that died from herpes encephalitis following ocular infection. In addition, using a mouse trigeminal ganglia (TG) explant model of induced reactivation, we show here for the first time that miR-H2 appears to play a role in modulating HSV-1 reactivation. Although the percent of TG from which virus reactivated by day 10 after explant was similar for McK-ΔH2, wt McKrae, and the marker-rescued virus McK-ΔH2Res, at earlier times, significantly more reactivation was seen with McK-ΔH2. Our results suggest that in the context of the virus, miR-H2 downregulates ICP0 and this moderates both HSV-1 neurovirulence and reactivation.

A virulent bioluminescent and fluorescent dual-reporter Marek's disease virus unveils an alternative spreading pathway in addition to cell-to-cell contact

Marek's disease virus (MDV) is a growing threat for the poultry industry. Unfortunately, despite successful vaccination against the disease, MDV remains in circulation within vaccinated flocks, leading to the selection of increasingly virulent pathotypes. Detailed knowledge of the virus biology and the host-virus interaction is required to improve the vaccine efficiency. In the present study, I engineered an original, dual-reporter MDV to track and quantify virus replication in vitro and in vivo.

An inquiry into the molecular basis of HSV latency and reactivation

Herpes simplex virus (HSV) evolved an elegant strategy that enables the virus to impact a large fraction of the human population. The virus replicates at the portal of entry (mouth, genitals) and concurrently it is transported retrograde to sensory neurons. In sensory neurons it establishes a silent (latent) infection. A variety of stimuli can reactivate the virus. The reactivated virus is transmitted anterograde to a site at the portal of entry for transmission by physical contact between infected and uninfected tissues to other individuals. The central issue is how a virus that vigorously replicates and successfully blocks the innate immune defenses of the host at the portal of entry into the body remains silent in sensory neurons. The presentation focuses on three key issues: (a) current assessment of the impact of HSV on human health, (b) the mechanisms by which the virus overcomes a key host defense mechanism at the portal of entry into the body and yet is silenced in latently infected neurons, and (c) the mechanisms by which the virus reactivates.