Spontaneous reactivation of herpes simplex virus type 1 in latently infected murine sensory ganglia

Mechanisms and consequences of mRNA destabilization during viral infections

During viral infection there is dynamic interplay between the virus and the host to regulate gene expression. In many cases, the host induces the expression of antiviral genes to combat infection, while the virus uses "host shut-off" systems to better compete for cellular resources and to limit the induction of the host antiviral response. Viral mechanisms for host shut-off involve targeting translation, altering host RNA processing, and/or inducing the degradation of host mRNAs. In this review, we discuss the diverse mechanisms viruses use to degrade host mRNAs. In addition, the widespread degradation of host mRNAs can have common consequences including the accumulation of RNA binding proteins in the nucleus, which leads to altered RNA processing, mRNA export, and changes to transcription.

Binding of herpesvirus entry mediator (HVEM) and HSV-1 gD affect reactivation but not latency levels

Previously we reported that the HSV-1 latency associated transcript (LAT) specifically upregulates the cellular herpesvirus entry mediator (HVEM) but no other known HSV-1 receptors. HSV-1 glycoprotein D (gD) binds to HVEM but the effect of this interaction on latency-reactivation is not known. We found that the levels of latent viral genomes were not affected by the absence of gD binding to HVEM. However, reactivation of latent virus in trigeminal ganglia explant cultures was blocked in the absence of gD binding to HVEM. Neither differential HSV-1 replication and spread in the eye nor levels of latency influenced reactivation. Despite similar levels of latency, reactivation in the absence of gD binding to HVEM correlated with reduced T cell exhaustion. Our results indicate that HVEM-gD signaling plays a significant role in HSV-1 reactivation but not in ocular virus replication or levels of latency. The results presented here identify gD binding to HVEM as an important target that influences reactivation and survival of ganglion resident T cells but not levels of latency. This concept may also apply to other herpesviruses that engages HVEM.

Persistent Herpes Simplex Virus Type 1 Infection of Enteric Neurons Triggers CD8+ T Cell Response and Gastrointestinal Neuromuscular Dysfunction

Behind the central nervous system, neurotropic viruses can reach and persist even in the enteric nervous system (ENS), the neuronal network embedded in the gut wall. We recently reported that immediately following orogastric (OG) administration, Herpes simplex virus (HSV)-1 infects murine enteric neurons and recruits mononuclear cells in the myenteric plexus. In the current work, we took those findings a step forward by investigating the persistence of HSV-1 in the ENS and the local adaptive immune responses against HSV-1 that might contribute to neuronal damage in an animal model. Our study demonstrated specific viral RNA transcripts and proteins in the longitudinal muscle layer containing the myenteric plexus (LMMP) up to 10 weeks post HSV-1 infection. CD3⁺CD8⁺INFγ⁺ lymphocytes skewed towards HSV-1 antigens infiltrated the myenteric ganglia starting from the 6^th week of infection and persist up to 10 weeks post-OG HSV-1 inoculation. CD3⁺CD8⁺ cells isolated from the LMMP of the infected mice recognized HSV-1 antigens expressed by infected enteric neurons. In vivo, infiltrating activated lymphocytes were involved in controlling viral replication and intestinal neuromuscular dysfunction. Indeed, by depleting the CD8⁺ cells by administering specific monoclonal antibody we observed a partial amelioration of intestinal dysmotility in HSV-1 infected mice but increased expression of viral genes. Our findings demonstrate that HSV-1 persistently infects enteric neurons that in turn express viral antigens, leading them to recruit activated CD3⁺CD8⁺ lymphocytes. The T-cell responses toward HSV-1 antigens persistently expressed in enteric neurons can alter the integrity of the ENS predisposing to neuromuscular dysfunction.

Crosstalk Between Epithelial Cells, Neurons and Immune Mediators in HSV-1 Skin Infection

Herpes simplex virus type 1 (HSV-1) infection is highly prevalent in humans, with approximately two-thirds of the world population living with this virus. However, only a fraction of those carrying HSV-1, which elicits lifelong infections, are symptomatic. HSV-1 mainly causes lesions in the skin and mucosae but reaches the termini of sensory neurons innervating these tissues and travels in a retrograde manner to the neuron cell body where it establishes persistent infection and remains in a latent state until reactivated by different stimuli. When productive reactivations occur, the virus travels back along axons to the primary infection site, where new rounds of replication are initiated in the skin, in recurrent or secondary infections. During this process, new neuron infections occur. Noteworthy, the mechanisms underlying viral reactivations and the exit of latency are somewhat poorly understood and may be regulated by a crosstalk between the infected neurons and components of the immune system. Here, we review and discuss the immune responses that occur at the skin during primary and recurrent infections by HSV-1, as well as at the interphase of latently-infected neurons. Moreover, we discuss the implications of neuronal signals over the priming and migration of immune cells in the context of HSV-1 infection.

Longitudinal transcriptomic characterization of viral genes in HSV-1 infected tree shrew trigeminal ganglia

BACKGROUND

Following acute infection, Herpes Simplex virus-1 (HSV-1) establishes lifelong latency and recurrent reactivation in the sensory neurons of trigeminal ganglia (TG). Infected tree shrew differs from mouse and show characteristics similar to human infection. A detailed transcriptomic analysis of the tree shrew model could provide mechanistic insights into HSV-1 infection in humans.

METHODS

We sequenced the transcriptome of infected TGs from tree shrews and mice, and 4 human donors, then examined viral genes expression up to 58 days in infected TGs from mouse and tree shrew, and compare the latency data with that in human TGs.

RESULTS

Here, we found that all HSV-1 genes could be detected in mouse TGs during acute infection, but 22 viral genes necessary for viral transcription, replication and viral maturation were not expressed in tree shrew TGs during this stage. Importantly, during latency, we found that LAT could be detected both in mouse and tree shrew, but the latter also has an ICP0 transcript signal absent in mouse but present in human samples. Importantly, we observed that infected human and tree shrew TGs have a more similar LAT region transcription peak. More importantly, we observed that HSV-1 spontaneously reactivates from latently infected tree shrews with relatively high efficiency.

CONCLUSIONS

These results represent the first longitudinal transcriptomic characterization of HSV-1 infection in during acute, latency and recurrent phases, and revealed that tree shrew infection has important similar features with human infection.

Development and application of a combined molecular and tissue culture-based approach to detect latent infectious laryngotracheitis virus (ILTV) in chickens

Infectious laryngotracheitis virus (ILTV) causes severe respiratory disease in chickens. ILTV can establish latency and reactivate later in life, but there have been few investigations of ILTV latency. This study aimed to contribute to the methodologies available to detect latent ILTV. A nested PCR was developed which was more sensitive than three other molecular methods investigated in this study. This nested PCR was then used in conjunction with in vitro reactivation culture methods that were optimized and applied to trigeminal ganglia (TG) and tracheal samples from ILTV-vaccinated commercial layer birds (n = 30). ILTV DNA was detected by nested PCR in the upper respiratory tract (URT) or eye of 22 birds. Of the remaining 8 birds, ILTV could be detected by co-culture in TG of 5 birds, with reactivated virus mostly detected 6 days post-explant (dpe). ILTV was also detected in tracheal cultures by 6 dpe. In the ILTV-positive URT samples, the virus could be characterised as vaccine strains SA2 (n = 9) or A20 (n = 5). This study provides evidence for reactivation and shedding of vaccine ILTV in commercial layer birds. Moreover, this study produced a molecular and in-vitro culture method to detect latent viral infection.

Roles of Non-coding RNAs During Herpesvirus Infection

Non-coding RNAs (ncRNAs) play essential roles in multiple aspects of the life cycles of herpesviruses and contribute to lifelong persistence of herpesviruses within their respective hosts. In this chapter, we discuss the types of ncRNAs produced by the different herpesvirus families during infection, some of the cellular ncRNAs manipulated by these viruses, and the overall contributions of ncRNAs to the viral life cycle, influence on the host environment, and pathogenesis.

[Mechanisms of herpes simplex virus latency and reactivation]

Herpes simplex virus (HSV), including HSV-1 and HSV-2, is an important pathogen that can cause many diseases. Usually these diseases are recurrent and incurable. After lytic infection on the surface of peripheral mucosa, HSV can enter sensory neurons and establish latent infection during which viral replication ceases. Moreover, latent virus can re-enter the replication cycle by reactivation and return to peripheral tissues to start recurrent infection. This ability to escape host immune surveillance during latent infection and to spread during reactivation is a viral survival strategy and the fundamental reason why no drug can completely eradicate the virus at present. Although there are many studies on latency and reactivation of HSV, and much progress has been made, many specific mechanisms of the process remain obscure or even controversial due to the complexity of this process and the limitations of research models. This paper reviews the major results of research on HSV latency and reactivation, and discusses future research directions in this field.

CCR2+ migratory macrophages with M1 status are the early-responders in the cornea of HSV-1 infected mice

Complex interactions between HSV-1 and infiltrating immune cells play important roles in establishing localized, acute virus replication as well as chronic latent infection. The extent and duration of initial virus replication are the key determinants of subsequent pathologic inflammatory responses and therefore, the accumulation of immune cell populations at this time point is a key target for prevention. Therefore, we evaluated the role of various immune cell infiltrates between 1 h and 28 days post-infection (PI) using mice infected with virulent HSV-1 strain McKrae without corneal scarification. The effect of corneal scarification on immune cell infiltrates was also determined. We first determined the activation status and origin of macrophage infiltrates as early as 1 h PI. We found a sharp increase in the total macrophage population after 12 h PI, that was primarily due to infiltration of CCR2⁺ migratory macrophages, mostly in M1 status (MHC II⁺). The number of CCR2^- resident macrophages, mostly unpolarized (M0), increased gradually over time and peaked at 48 h PI. Interestingly, some of the resident macrophages gained an M2-like phenotype (CD206^Low), which peaked at 12 h PI, concurrent with M1 macrophage infiltration. From 1–7 days PI, infiltration of various immune cells correlated strongly with HSV-1 replication, with neutrophils showing the biggest increase, and NKT cells the biggest decrease, after infection. The presence of geographical ulcer did not correlate with increased infiltration, while mice with corneal scarring had significantly more immune cell infiltration than those without corneal scarring. Overall, we showed time-dependent infiltration of various immune cells in the eye of HSV-1 infected mice. Initial infiltration of macrophages followed by infiltration of T cells at later times PI demonstrates the importance of targeting macrophages rather than other immune cells type, for therapeutic treatment of HSV-1.

Infectious Herpes Simplex Virus in the Brain Stem Is Correlated with Reactivation in the Trigeminal Ganglia

Latent herpes simplex virus (HSV) DNA has been detected in the central nervous systems (CNS) of humans postmortem, and infection with HSV has been correlated with the development of neurodegenerative diseases. However, whether HSV can directly reactivate in the CNS and/or infectious virus can be transported to the CNS following reactivation in peripheral ganglia has been unclear. In this study, infectious virus was recovered from both the trigeminal ganglia and the brain stem of latently infected mice following a reactivation stimulus, but a higher frequency of reactivation and increased titers of infectious virus were recovered from the trigeminal ganglia. Viral proteins were detected in neurons of the trigeminal ganglia, but a cellular source of infectious virus could not be identified in the brain stem. These results suggest that infectious virus is transported from the ganglia to the CNS following reactivation but do not exclude the potential for direct reactivation in the CNS.

Herpes simplex virus (HSV) establishes latency in neurons of the peripheral and central nervous systems (CNS). Evidence is mounting that HSV latency and reactivation in the nervous system has the potential to promote neurodegenerative processes. Understanding how this occurs is an important human health goal. In the mouse model, in vivo viral reactivation in the peripheral nervous system, triggered by hyperthermic stress, has been well characterized with respect to frequency and cell type. However, characterization of in vivo reactivation in the CNS is extremely limited. Further, it remains unclear whether virus reactivated in the peripheral nervous system is transported to the CNS in an infectious form, how often this occurs, and what parameters underlie the efficiency and outcomes of this process. In this study, reactivation was quantified in the trigeminal ganglia (TG) and the brain stem from the same latently infected animal using direct assays of equivalent sensitivity. Reactivation was detected more frequently in the TG than in the brain stem and, in all but one case, the amount of virus recovered was greater in the TG than that detected in the brain stem. Viral protein positive neurons were observed in the TG, but a cellular source for reactivation in the brain stem was not identified, despite serially sectioning and examining the entire tissue (0/6 brain stems). These findings suggest that infectious virus detected in the brain stem is primarily the result of transport of reactivated virus from the TG into the brain stem.

IMPORTANCE Latent herpes simplex virus (HSV) DNA has been detected in the central nervous systems (CNS) of humans postmortem, and infection with HSV has been correlated with the development of neurodegenerative diseases. However, whether HSV can directly reactivate in the CNS and/or infectious virus can be transported to the CNS following reactivation in peripheral ganglia has been unclear. In this study, infectious virus was recovered from both the trigeminal ganglia and the brain stem of latently infected mice following a reactivation stimulus, but a higher frequency of reactivation and increased titers of infectious virus were recovered from the trigeminal ganglia. Viral proteins were detected in neurons of the trigeminal ganglia, but a cellular source of infectious virus could not be identified in the brain stem. These results suggest that infectious virus is transported from the ganglia to the CNS following reactivation but do not exclude the potential for direct reactivation in the CNS.

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.

Herpes Simplex Virus 1 Latency and the Kinetics of Reactivation Are Regulated by a Complex Network of Interactions between the Herpesvirus Entry Mediator, Its Ligands (gD, BTLA, LIGHT, and CD160), and the Latency-Associated Transcript

Recently, we reported that the herpesvirus entry mediator (HVEM; also called TNFRSF14 or CD270) is upregulated by the latency-associated transcript (LAT) of herpes simplex virus 1 (HSV-1) and that the absence of HVEM affects latency reactivation but not primary infection in ocularly infected mice. gD has been shown to bind to HVEM. LIGHT (TNFSF14), CD160, and BTLA (B- and T-lymphocyte attenuator) also interact with HVEM and can interfere with HSV gD binding. It was not known if LIGHT, CD160, or BTLA affected the level of latency reactivation in the trigeminal ganglia (TG) of latently infected mice. To address this issue, we ocularly infected LIGHT^-/-, CD160^-/-, and BTLA^-/- mice with LAT(+) and LAT(-) viruses, using similarly infected wild-type (WT) and HVEM^-/- mice as controls. The amount of latency, as determined by the levels of gB DNA in the TG of the LIGHT^-/-, CD160^-/-, and BTLA^-/- mice infected with either LAT(+) or LAT(-) viruses, was lower than that in WT mice infected with LAT(+) virus and was similar in WT mice infected with LAT(-) virus. The levels of LAT RNA in HVEM^-/-, LIGHT^-/-, CD160^-/-, and BTLA^-/- mice infected with LAT(+) virus were similar and were lower than the levels of LAT RNA in WT mice. However, LIGHT^-/-, CD160^-/-, and BTLA^-/- mice, independent of the presence of LAT, had levels of reactivation similar to those of WT mice infected with LAT(+) virus. Faster reactivation correlated with the upregulation of HVEM transcript. The LIGHT^-/-, CD160^-/-, and BTLA^-/- mice had higher levels of HVEM expression, and this, along with the absence of BTLA, LIGHT, or CD160, may contribute to faster reactivation, while the absence of each molecule, independent of LAT, may have contributed to lower latency. This study suggests that, in the absence of competition with gD for binding to HVEM, LAT RNA is important for WT levels of latency but not for WT levels of reactivation.IMPORTANCE The effects of BTLA, LIGHT, and CD160 on latency reactivation are not known. We show here that in BTLA, LIGHT, or CD160 null mice, latency is reduced; however, HVEM expression is upregulated compared to that of WT mice, and this upregulation is associated with higher reactivation that is independent of LAT but dependent on gD expression. Thus, one of the mechanisms by which BTLA, LIGHT, and CD160 null mice enhance reactivation appears to be the increased expression of HVEM in the presence of gD. Thus, our results suggest that blockade of HVEM-LIGHT-BTLA-CD160 contributes to reduced HSV-1 latency and reactivation.

An M2 Rather than a TH2 Response Contributes to Better Protection against Latency Reactivation following Ocular Infection of Naive Mice with a Recombinant Herpes Simplex Virus 1 Expressing Murine Interleukin-4

We found previously that altering macrophage polarization toward M2 responses by injection of colony-stimulating factor 1 (CSF-1) was more effective in reducing both primary and latent infections in mice ocularly infected with herpes simplex virus 1 (HSV-1) than M1 polarization by gamma interferon (IFN-γ) injection. Cytokines can coordinately regulate macrophage and T helper (T_H) responses, with interleukin-4 (IL-4) inducing type 2 T_H (T_H2) as well as M2 responses and IFN-γ inducing T_H1 as well as M1 responses. We have now differentiated the contributions of these immune compartments to protection against latency reactivation and corneal scarring by comparing the effects of infection with recombinant HSV-1 in which the latency-associated transcript (LAT) gene was replaced with either the IL-4 (HSV-IL-4) or IFN-γ (HSV-IFN-γ) gene using infection with the parental (LAT-negative) virus as a control. Analysis of peritoneal macrophages in vitro established that the replacement of LAT with the IL-4 or IFN-γ gene did not affect virus infectivity and promoted polarization appropriately. Protection against corneal scarring was significantly higher in mice ocularly infected with HSV-IL-4 than in those infected with HSV-IFN-γ or parental virus. Levels of primary virus replication in the eyes and trigeminal ganglia (TG) were similar in the three groups of mice, but the numbers of gC⁺ cells were lower on day 5 postinfection in the eyes of HSV-IL-4-infected mice than in those infected with HSV-IFN-γ or parental virus. Latency and explant reactivation were lower in both HSV-IL-4- and HSV-IFN-γ-infected mice than in those infected with parental virus, with the lowest level of latency being associated with HSV-IL-4 infection. Higher latency correlated with higher levels of CD8, PD-1, and IFN-γ mRNA, while reduced latency and T-cell exhaustion correlated with lower gC⁺ expression in the TG. Depletion of macrophages increased the levels of latency in all ocularly infected mice compared with their undepleted counterparts, with macrophage depletion increasing latency in the HSV-IL-4 group greater than 3,000-fold. Our results suggest that shifting the innate macrophage immune responses toward M2, rather than M1, responses in HSV-1 infection would improve protection against establishment of latency, reactivation, and eye disease.IMPORTANCE Ocular HSV-1 infections are among the most frequent serious viral eye infections in the United States and a major cause of virus-induced blindness. As establishment of a latent infection in the trigeminal ganglia results in recurrent infection and is associated with corneal scarring, prevention of latency reactivation is a major therapeutic goal. It is well established that absence of latency-associated transcripts (LATs) reduces latency reactivation. Here we demonstrate that recombinant HSV-1 expressing IL-4 (an inducer of T_H2/M2 responses) or IFN-γ (an inducer of T_H1/M1 responses) in place of LAT further reduced latency, with HSV-IL-4 showing the highest overall protective efficacy. In naive mice, this higher protective efficacy was mediated by innate rather than adaptive immune responses. Although both M1 and M2 macrophage responses were protective, shifting macrophages toward an M2 response through expression of IL-4 was more effective in curtailing ocular HSV-1 latency reactivation.

Molecular Mechanisms for Herpes Simplex Virus Type 1 Pathogenesis in Alzheimer's Disease

This review focuses on research in the areas of epidemiology, neuropathology, molecular biology and genetics that implicates herpes simplex virus type 1 (HSV-1) as a causative agent in the pathogenesis of sporadic Alzheimer’s disease (AD). Molecular mechanisms whereby HSV-1 induces AD-related pathophysiology and pathology, including neuronal production and accumulation of amyloid beta (Aβ), hyperphosphorylation of tau proteins, dysregulation of calcium homeostasis, and impaired autophagy, are discussed. HSV-1 causes additional AD pathologies through mechanisms that promote neuroinflammation, oxidative stress, mitochondrial damage, synaptic dysfunction, and neuronal apoptosis. The AD susceptibility genes apolipoprotein E (APOE), phosphatidylinositol binding clathrin assembly protein (PICALM), complement receptor 1 (CR1) and clusterin (CLU) are involved in the HSV lifecycle. Polymorphisms in these genes may affect brain susceptibility to HSV-1 infection. APOE, for example, influences susceptibility to certain viral infections, HSV-1 viral load in the brain, and the innate immune response. The AD susceptibility gene cholesterol 25-hydroxylase (CH25H) is upregulated in the AD brain and is involved in the antiviral immune response. HSV-1 interacts with additional genes to affect cognition-related pathways and key enzymes involved in Aβ production, Aβ clearance, and hyperphosphorylation of tau proteins. Aβ itself functions as an antimicrobial peptide (AMP) against various pathogens including HSV-1. Evidence is presented supporting the hypothesis that Aβ is produced as an AMP in response to HSV-1 and other brain infections, leading to Aβ deposition and plaque formation in AD. Epidemiologic studies associating HSV-1 infection with AD and cognitive impairment are discussed. Studies are reviewed supporting subclinical chronic reactivation of latent HSV-1 in the brain as significant in the pathogenesis of AD. Finally, the rationale for and importance of clinical trials treating HSV-1-infected MCI and AD patients with antiviral medication is discussed.

New Paradigms for the Study of Ocular Alphaherpesvirus Infections: Insights into the Use of Non-Traditional Host Model Systems

Ocular herpesviruses, most notably human alphaherpesvirus 1 (HSV-1), canid alphaherpesvirus 1 (CHV-1) and felid alphaherpesvirus 1 (FHV-1), infect and cause severe disease that may lead to blindness. CHV-1 and FHV-1 have a pathogenesis and induce clinical disease in their hosts that is similar to HSV-1 ocular infections in humans, suggesting that infection of dogs and cats with CHV-1 and FHV-1, respectively, can be used as a comparative natural host model of herpesvirus-induced ocular disease. In this review, we discuss both strengths and limitations of the various available model systems to study ocular herpesvirus infection, with a focus on the use of these non-traditional virus-natural host models. Recent work has demonstrated the robustness and reproducibility of experimental ocular herpesvirus infections in dogs and cats, and, therefore, these non-traditional models can provide additional insights into the pathogenesis of ocular herpesvirus infections.

The loss of binary: Pushing the herpesvirus latency paradigm

Purpose of Review

Herpesvirus latency has been viewed as a binary state where replication is either on or off. During latency, gene expression is thought to be restricted to non-coding RNAs or very few proteins so that the virus avoids detection by the immune system. However, a number of recent studies across herpesvirus families call into question the existence of a binary switch for latency, and suggest that latency is far more dynamic than originally presumed. These studies are the focus of this review.

Recent Findings

Highly sensitive and global approaches to investigate viral gene expression in the context of latency have revealed low level viral transcripts, and in some cases protein, from each of the three kinetic gene classes during the latent alpha and beta herpesvirus infection either in vitro or in vivo. Further, low level, asymptomatic virus shedding persists following acute infection. Together, these findings have raised questions about how silent the latent infection truly is.

Summary

Emerging evidence suggests that viral gene expression associated with latent states may be broader and more dynamic than originally presumed during herpesvirus latency. This is an important possibility to consider in understanding the molecular programs associated with the establishment, maintenance and reactivation of herpesvirus latency. Here, we review these findings and detail how they contribute to the emergence of a biphasic model of reactivation.

Herpesvirus Entry Mediator and Ocular Herpesvirus Infection: More than Meets the Eye

As its name suggests, the host receptor herpesvirus entry mediator (HVEM) facilitates herpes simplex virus (HSV) entry through interactions with a viral envelope glycoprotein. HVEM also bridges several signaling networks, binding ligands from both tumor necrosis factor (TNF) and immunoglobulin (Ig) superfamilies with diverse, and often opposing, outcomes. While HVEM was first identified as a viral entry receptor for HSV, it is only recently that HVEM has emerged as an important host factor in immunopathogenesis of ocular HSV type 1 (HSV-1) infection. Surprisingly, HVEM exacerbates disease development in the eye independently of entry. HVEM signaling has been shown to play a variety of roles in modulating immune responses to HSV and other pathogens, and there is increasing evidence that these effects are responsible for HVEM-mediated pathogenesis in the eye. Here, we review the dual branches of HVEM function during HSV infection: entry and immunomodulation. HVEM is broadly expressed; intersects two important immunologic signaling networks; and impacts autoimmunity, infection, and inflammation. We hope that by understanding the complex range of effects mediated by this receptor, we can offer insights applicable to a wide variety of disease states.

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.

Herpes Simplex Virus Type 1 and Other Pathogens are Key Causative Factors in Sporadic Alzheimer's Disease

This review focuses on research in epidemiology, neuropathology, molecular biology, and genetics regarding the hypothesis that pathogens interact with susceptibility genes and are causative in sporadic Alzheimer's disease (AD). Sporadic AD is a complex multifactorial neurodegenerative disease with evidence indicating coexisting multi-pathogen and inflammatory etiologies. There are significant associations between AD and various pathogens, including Herpes simplex virus type 1 (HSV-1), Cytomegalovirus, and other Herpesviridae, Chlamydophila pneumoniae, spirochetes, Helicobacter pylori, and various periodontal pathogens. These pathogens are able to evade destruction by the host immune system, leading to persistent infection. Bacterial and viral DNA and RNA and bacterial ligands increase the expression of pro-inflammatory molecules and activate the innate and adaptive immune systems. Evidence demonstrates that pathogens directly and indirectly induce AD pathology, including amyloid-β (Aβ) accumulation, phosphorylation of tau protein, neuronal injury, and apoptosis. Chronic brain infection with HSV-1, Chlamydophila pneumoniae, and spirochetes results in complex processes that interact to cause a vicious cycle of uncontrolled neuroinflammation and neurodegeneration. Infections such as Cytomegalovirus, Helicobacter pylori, and periodontal pathogens induce production of systemic pro-inflammatory cytokines that may cross the blood-brain barrier to promote neurodegeneration. Pathogen-induced inflammation and central nervous system accumulation of Aβ damages the blood-brain barrier, which contributes to the pathophysiology of AD. Apolipoprotein E4 (ApoE4) enhances brain infiltration by pathogens including HSV-1 and Chlamydophila pneumoniae. ApoE4 is also associated with an increased pro-inflammatory response by the immune system. Potential antimicrobial treatments for AD are discussed, including the rationale for antiviral and antibiotic clinical trials.

Lytic Promoters Express Protein during Herpes Simplex Virus Latency

Herpes simplex virus (HSV) has provided the prototype for viral latency with previously well-defined acute or lytic and latent phases. More recently, the deep quiescence of HSV latency has been questioned with evidence that lytic genes can be transcribed in this state. However, to date the only evidence that these transcripts might be translated has come from immunological studies that show activated T cells persist in the nervous system during latency. Here we use a highly sensitive Cre-marking model to show that lytic and latent phases are less clearly defined in two significant ways. First, around half of the HSV spread leading to latently infected sites occurred beyond the initial acute infection and second, we show direct evidence that lytic promoters can drive protein expression during latency.

Herpes simplex virus, which causes cold sores and genital herpes, has active and inactive (or latent) phases of infection that have been considered to be distinct. In this study we found that the active phase of infection, including spread in the nervous system, continues longer than has been previously appreciated. We also show evidence that virus genes previously only associated with active infection are turned on during latency. These genes are of particular interest because other work has found that they are targets of the immune response to HSV. The extent and nature of residual viral activity during latency is important to understand because it may suggest therapeutic targets to reduce recurrent HSV disease.

Herpes simplex virus type-1: replication, latency, reactivation and its antiviral targets

Infection by herpes simplex virus type-1 (HSV-1) causes several diseases, ranging from cutaneous, oral and genital infections to fatal encephalitis. Despite the availability of antiviral therapies on the market, their efficacies are incomplete, and new cases of resistant strains arise, mainly in the immunocompromised, but also recently documented in immunocompetent patients. Over the last decades a lot has been discovered about the molecular basis of infection which has been of great benefit to the investigation of new anti-HSV-1 molecules. In this review we summarize replication, latency and reactivation highlighting potential antiviral targets and new molecules described in the past several years in the literature.

Detection of Murine Herpesvirus 68 (MHV-68) in Dermacentor reticulatus Ticks

Murid herpesvirus 4 (MuHV 4) strain 68 (MHV-68) is a natural pathogen of murid rodents, which serves as hosts to Dermacentor reticulatus ticks. These ticks are known to transmit multiple pathogens, which can cause diseases in humans and animals. Recently, the detection of MHV-68 antibodies in the blood of animals living in the same biotope as virus-infected mice has suggested the role of ticks in pathogen circulation in nature. Herein, to identify MHV-68 in D. reticulatus ticks, DNA samples from 432 adults were collected at two sites in southwestern Slovakia from 2011 to 2014. Samples were examined by polymerase chain reaction (PCR), targeting ORF50 of MHV-68. Ignoring season and locality, we have found 25.9 % of the male and 44.9 % of the female ticks to be positive. Within ticks collected in Vojka, 40 % (125/312) became positive, at a rate of approximately 6.8 times higher in spring than in autumn (66 vs 9.7 %). In addition, in the spring, 1.4 times more females were positive than males. Within ticks collected in Gabčíkovo, 23.3 % (28/120) became positive, with positive females being twice as frequent. The infecting virus was identified by analyzing amplified products via sequencing and restriction fragment length polymorphism (RFLP) analyses. Using an explantation/co-cultivation procedure, we examined the salivary glands, intestines, and ovaries of five females for live MHV-68. In all organs of two ticks, we identified a virus capable of replication in mammalian cells. This is the first report of MHV-68 detection in D. reticulatus ticks and of a live virus in their organs. Findings encourage further study to determine whether this potential arbovirus, found in salivary glands, is transmissible. It further supports the hypothesis regarding the mediating role of ticks in MHV-68 circulation in nature.

Therapeutic immunization with a mixture of herpes simplex virus 1 glycoprotein D-derived “asymptomatic” human CD8+ T-cell epitopes decreases spontaneous ocular shedding in latently infected HLA transgenic rabbits: association with low frequency of local PD-1+ TIM-3+ CD8+ exhausted T cells

Most blinding ocular herpetic disease is due to reactivation of herpes simplex virus 1 (HSV-1) from latency rather than to primary acute infection. No herpes simplex vaccine is currently available for use in humans. In this study, we used the HLA-A*02:01 transgenic (HLA Tg) rabbit model of ocular herpes to assess the efficacy of a therapeutic vaccine based on HSV-1 gD epitopes that are recognized mainly by CD8(+) T cells from "naturally" protected HLA-A*02:01-positive, HSV-1-seropositive healthy asymptomatic (ASYMP) individuals (who have never had clinical herpes disease). Three ASYMP CD8(+) T-cell epitopes (gD(53-61), gD(70-78), and gD(278-286)) were linked with a promiscuous CD4(+) T-cell epitope (gD(287-317)) to create 3 separate pairs of CD4-CD8 peptides, which were then each covalently coupled to an Nε-palmitoyl-lysine moiety, a Toll-like receptor 2 (TLR-2) ligand. This resulted in the construction of 3 CD4-CD8 lipopeptide vaccines. Latently infected HLA Tg rabbits were immunized with a mixture of these 3 ASYMP lipopeptide vaccines, delivered as eye drops in sterile phosphate-buffered saline (PBS). The ASYMP therapeutic vaccination (i) induced HSV-specific CD8(+) T cells that prevent HSV-1 reactivation ex vivo from latently infected explanted trigeminal ganglia (TG), (ii) significantly reduced HSV-1 shedding detected in tears, (iii) boosted the number and function of HSV-1 gD epitope-specific CD8(+) T cells in draining lymph nodes (DLN), conjunctiva, and TG, and (iv) was associated with fewer exhausted HSV-1 gD-specific PD-1(+) TIM-3+ CD8(+) T cells. The results underscore the potential of an ASYMP CD8(+) T-cell epitope-based therapeutic vaccine strategy against recurrent ocular herpes.

IMPORTANCE

Seventy percent to 90% of adults harbor herpes simplex virus 1 (HSV-1), which establishes lifelong latency in sensory neurons of the trigeminal ganglia. This latent state sporadically switches to spontaneous reactivation, resulting in viral shedding in tears. Most blinding herpetic disease in humans is due to reactivation of HSV-1 from latency rather than to primary acute infection. To date, there is no licensed therapeutic vaccine that can effectively stop or reduce HSV-1 reactivation from latently infected sensory ganglia and the subsequent shedding in tears. In the present study, we demonstrated that topical ocular therapeutic vaccination of latently infected HLA transgenic rabbits with a lipopeptide vaccine that contains exclusively human “asymptomatic” CD8(+) T-cell epitopes successfully decreased spontaneous HSV-1 reactivation, as judged by a significant reduction in spontaneous shedding in tears. The findings should guide the clinical development of a safe and effective T-cell-based therapeutic herpes vaccine.

Decreased reactivation of a herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) mutant using the in vivo mouse UV-B model of induced reactivation

Blinding ocular herpetic disease in humans is due to herpes simplex virus type 1 (HSV-1) reactivations from latency, rather than to primary acute infection. The cellular and molecular immune mechanisms that control the HSV-1 latency-reactivation cycle remain to be fully elucidated. The aim of this study was to determine if reactivation of the HSV-1 latency-associated transcript (LAT) deletion mutant (dLAT2903) was impaired in this model, as it is in the rabbit model of induced and spontaneous reactivation and in the trigeminal ganglia (TG) explant-induced reactivation model in mice. The eyes of mice latently infected with wild-type HSV-1 strain McKrae (LAT((+)) virus) or dLAT2903 (LAT((-)) virus) were irradiated with UV-B, and reactivation was determined. We found that compared to LAT((-)) virus, LAT((+)) virus reactivated at a higher rate as determined by shedding of virus in tears on days 3 to 7 after UV-B treatment. Thus, the UV-B-induced reactivation mouse model of HSV-1 appears to be a useful small animal model for studying the mechanisms involved in how LAT enhances the HSV-1 reactivation phenotype. The utility of the model for investigating the immune evasion mechanisms regulating the HSV-1 latency/reactivation cycle and for testing the protective efficacy of candidate therapeutic vaccines and drugs is discussed.

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.

Establishment of HSV1 latency in immunodeficient mice facilitates efficient in vivo reactivation

The establishment of latent infections in sensory neurons is a remarkably effective immune evasion strategy that accounts for the widespread dissemination of life long Herpes Simplex Virus type 1 (HSV1) infections in humans. Periodic reactivation of latent virus results in asymptomatic shedding and transmission of HSV1 or recurrent disease that is usually mild but can be severe. An in-depth understanding of the mechanisms regulating the maintenance of latency and reactivation are essential for developing new approaches to block reactivation. However, the lack of a reliable mouse model that supports efficient in vivo reactivation (IVR) resulting in production of infectious HSV1 and/or disease has hampered progress. Since HSV1 reactivation is enhanced in immunosuppressed hosts, we exploited the antiviral and immunomodulatory activities of IVIG (intravenous immunoglobulins) to promote survival of latently infected immunodeficient Rag mice. Latently infected Rag mice derived by high dose (HD), but not low dose (LD), HSV1 inoculation exhibited spontaneous reactivation. Following hyperthermia stress (HS), the majority of HD inoculated mice developed HSV1 encephalitis (HSE) rapidly and synchronously, whereas for LD inoculated mice reactivated HSV1 persisted only transiently in trigeminal ganglia (Tg). T cells, but not B cells, were required to suppress spontaneous reactivation in HD inoculated latently infected mice. Transfer of HSV1 memory but not OVA specific or naïve T cells prior to HS blocked IVR, revealing the utility of this powerful Rag latency model for studying immune mechanisms involved in control of reactivation. Crossing Rag mice to various knockout strains and infecting them with wild type or mutant HSV1 strains is expected to provide novel insights into the role of specific cellular and viral genes in reactivation, thereby facilitating identification of new targets with the potential to block reactivation.

Although mouse models have been very useful in studies of HSV1 latency, the inability to efficiently reactivate latent HSV1 in vivo has impeded studies of reactivation. Reasoning that reactivation would be much more efficient in the absence of T cells, we exploited IVIG to promote survival of latently infected Rag mice lacking B and T cells. We established a threshold inoculum dose that was higher for B6- compared to 129-Rag mice, which determined whether HSV1 could be efficiently reactivated in vivo resulting in encephalitis. We showed directly that memory T cells are required to control spontaneous and induced reactivation in mice inoculated at high dose but are dispensable for maintaining latency in low dose inoculated mice. Incorporating different knockout strains into the Rag latency model by adoptive transfer of cells or crossbreeding will facilitate studying the role of various cellular genes involved in regulating neuronal gene expression and innate and adaptive immunity in the control of HSV1 reactivation. The potential of this powerful latency model to unravel the molecular and immune mechanisms regulating latency will be realized only after it is adopted and refined by researchers in the field.

The 3 facets of regulation of herpes simplex virus gene expression: A critical inquiry

On entry into the body herpes simplex viruses (HSV) replicate in a series of steps that involves derepression of viral DNA activated by VP16, a virion protein, and sequential transcription of viral genes in a cascade fashion. HSV also enters into neurons in which viral DNA maintained as heterochromatin and with few exceptions viral gene expression is silenced. A third face of the interaction of HSV with its host cells takes place at the moment when the silenced viral genome in neurons is abruptly derepressed. The available data do no reveal evidence that HSV encodes different regulatory programs for each facet of its interaction with its host cells. Rather the data point to significant gaps in our knowledge of the mechanisms by which each facet is initiated and the roles of the infected cells at each facet of the interaction of viral gene products with the host cell.

Persistent infection by HSV-1 is associated with changes in functional architecture of iPSC-derived neurons and brain activation patterns underlying working memory performance

BACKGROUND

Herpes simplex virus, type 1 (HSV-1) commonly produces lytic mucosal lesions. It invariably initiates latent infection in sensory ganglia enabling persistent, lifelong infection. Acute HSV-1 encephalitis is rare and definitive evidence of latent infection in the brain is lacking. However, exposure untraceable to encephalitis has been repeatedly associated with impaired working memory and executive functions, particularly among schizophrenia patients.

METHODS

Patterns of HSV-1 infection and gene expression changes were examined in human induced pluripotent stem cell (iPSC)-derived neurons. Separately, differences in blood oxygenation level-dependent (BOLD) responses to working memory challenges using letter n-back tests were investigated using functional magnetic resonance imaging (fMRI) among schizophrenia cases/controls.

RESULTS

HSV-1 induced lytic changes in iPSC-derived glutamatergic neurons and neuroprogenitor cells. In neurons, HSV-1 also entered a quiescent state following coincubation with antiviral drugs, with distinctive changes in gene expression related to functions such as glutamatergic signaling. In the fMRI studies, main effects of schizophrenia (P = .001) and HSV-1 exposure (1-back, P = 1.76 × 10(-4); 2-back, P = 1.39 × 10(-5)) on BOLD responses were observed. We also noted increased BOLD responses in the frontoparietal, thalamus, and midbrain regions among HSV-1 exposed schizophrenia cases and controls, compared with unexposed persons.

CONCLUSIONS

The lytic/quiescent cycles in iPSC-derived neurons indicate that persistent neuronal infection can occur, altering cellular function. The fMRI studies affirm the associations between nonencephalitic HSV-1 infection and functional brain changes linked with working memory impairment. The fMRI and iPSC studies together provide putative mechanisms for the cognitive impairments linked to HSV-1 exposure.

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.

Role of polycomb proteins in regulating HSV-1 latency

Herpes simplex virus (HSV) establishes a latent infection within sensory neurons of humans. Latency is characterized by the transcriptional repression of lytic genes by the condensation of lytic gene regions into heterochromatin. Recent data suggest that facultative heterochromatin predominates, and that cellular Polycomb proteins are involved in the establishment and maintenance of transcriptional repression during latency. This review summarizes these data and discusses the implication of viral and cellular factors in regulating heterochromatin composition.

Antiherpes virus-specific treatment and cognition in schizophrenia: a test-of-concept randomized double-blind placebo-controlled trial

OBJECTIVE

To test our hypothesis that valacyclovir, an antiherpes virus-specific medication, added to antipsychotics (APs) would improve cognitive performance and psychopathology among schizophrenia subjects exposed to neurotropic herpes simplex virus, type 1 (HSV1).

METHODS

Using a double-blind placebo-controlled design, we randomized 24 HSV1-seropositive schizophrenia subjects to receive either valacyclovir (n = 12) or placebo (n = 12) for 18 weeks in addition to stable doses of APs. Valacyclovir dose was stabilized at 1.5 g twice daily orally. At each visit, subjects were evaluated for severity of psychopathology and side effects using standardized scales and a study-specific semistructured checklist. A computerized neurocognitive battery validated on both schizophrenia and healthy subjects was administered at baseline and follow-up. Intent-to-treat analysis, using linear regression models that included all randomized subjects, were used to examine differential changes in cognition and psychopathology scores over 18 weeks between valacyclovir and placebo, accounting for placebo response.

RESULTS

Valacyclovir group improved in verbal memory, working memory, and visual object learning compared with placebo group. The effect sizes (Cohen's d) were 0.79 for working memory, 1.14 for immediate verbal memory, and 0.97 for the visual object learning. Psychotic symptom severity did not improve.

CONCLUSIONS

Supplemental valacyclovir may alleviate impairments in cognitive domains that are often observed in schizophrenia but not psychotic symptoms in those exposed to HSV1. If replicated, this approach could provide a novel strategy to treat cognitive impairments in a subgroup of schizophrenia subjects who can be reliably identified using a blood test.

Herpes simplex virus 1 tropism for human sensory ganglion neurons in the severe combined immunodeficiency mouse model of neuropathogenesis

The tropism of herpes simplex virus (HSV-1) for human sensory neurons infected in vivo was examined using dorsal root ganglion (DRG) xenografts maintained in mice with severe combined immunodeficiency (SCID). In contrast to the HSV-1 lytic infectious cycle in vitro, replication of the HSV-1 F strain was restricted in human DRG neurons despite the absence of adaptive immune responses in SCID mice, allowing the establishment of neuronal latency. At 12 days after DRG inoculation, 26.2% of human neurons expressed HSV-1 protein and 13.1% expressed latency-associated transcripts (LAT). Some infected neurons showed cytopathic changes, but HSV-1, unlike varicella-zoster virus (VZV), only rarely infected satellite cells and did not induce fusion of neuronal and satellite cell plasma membranes. Cell-free enveloped HSV-1 virions were observed, indicating productive infection. A recombinant HSV-1-expressing luciferase exhibited less virulence than HSV-1 F in the SCID mouse host, enabling analysis of infection in human DRG xenografts for a 61-day interval. At 12 days after inoculation, 4.2% of neurons expressed HSV-1 proteins; frequencies increased to 32.1% at 33 days but declined to 20.8% by 61 days. Frequencies of LAT-positive neurons were 1.2% at 12 days and increased to 40.2% at 33 days. LAT expression remained at 37% at 61 days, in contrast to the decline in neurons expressing viral proteins. These observations show that the progression of HSV-1 infection is highly restricted in human DRG, and HSV-1 genome silencing occurs in human neurons infected in vivo as a consequence of virus-host cell interactions and does not require adaptive immune control.

Rabbit and mouse models of HSV-1 latency, reactivation, and recurrent eye diseases

The exact mechanisms of HSV-1 establishment, maintenance, latency, reactivation, and also the courses of recurrent ocular infections remain a mystery. Comprehensive understanding of the HSV-1 disease process could lead to prevention of HSV-1 acute infection, reactivation, and more effective treatments of recurrent ocular disease. Animal models have been used for over sixty years to investigate our concepts and hypotheses of HSV-1 diseases. In this paper we present descriptions and examples of rabbit and mouse eye models of HSV-1 latency, reactivation, and recurrent diseases. We summarize studies in animal models of spontaneous and induced HSV-1 reactivation and recurrent disease. Numerous stimuli that induce reactivation in mice and rabbits are described, as well as factors that inhibit viral reactivation from latency. The key features, advantages, and disadvantages of the mouse and rabbit models in relation to the study of ocular HSV-1 are discussed. This paper is pertinent but not intended to be all inclusive. We will give examples of key papers that have reported novel discoveries related to the review topics.

A cultured affair: HSV latency and reactivation in neurons

After replicating in surface epithelia, herpes simplex virus type-1 (HSV-1) enters the axonal terminals of peripheral neurons. The viral genome translocates to the nucleus, where it establishes a specialized infection known as latency, re-emerging periodically to seed new infections. Studies using cultured neuron models that faithfully recapitulate the molecular hallmarks of latency and reactivation defined in live animal models have provided fresh insight into the control of latency and connections to neuronal physiology. With this comes a growing appreciation for how the life cycles of HSV-1 and other herpesviruses are governed by key host pathways controlling metabolic homeostasis and cell identity.

Influence of herpes simplex virus 1 latency-associated transcripts on the establishment and maintenance of latency in the ROSA26R reporter mouse model

Herpes simplex virus 1 (HSV-1) can establish life-long latent infection in sensory neurons, from which periodic reactivation can occur. During latency, viral gene expression is largely restricted to the latency-associated transcripts (LATs). While not essential for any phase of latency, to date the LATs have been shown to increase the efficiency of both establishment and reactivation of latency in small-animal models. We sought to investigate the role of LAT expression in the frequency of latency establishment within the ROSA26R reporter mouse model utilizing Cre recombinase-encoding recombinant viruses harboring deletions of the core LAT promoter (LAP) region. HSV-1 LAT expression was observed to influence the number of latently infected neurons in trigeminal but not dorsal root ganglia. Furthermore, the relative frequencies of latency establishment of LAT-positive and LAT-negative viruses are influenced by the inoculum dose following infection of the mouse whisker pads. Finally, analysis of the infected cell population at two latent time points revealed a relative loss of latently infected cells in the absence of LAT expression. We conclude that the HSV-1 LATs facilitate the long-term stability of the latent cell population within the infected host and that interpretation of LAT establishment phenotypes is influenced by infection methodology.

Entry of herpes simplex virus type 1 (HSV-1) into the distal axons of trigeminal neurons favors the onset of nonproductive, silent infection

Following productive, lytic infection in epithelia, herpes simplex virus type 1 (HSV-1) establishes a lifelong latent infection in sensory neurons that is interrupted by episodes of reactivation. In order to better understand what triggers this lytic/latent decision in neurons, we set up an organotypic model based on chicken embryonic trigeminal ganglia explants (TGEs) in a double chamber system. Adding HSV-1 to the ganglion compartment (GC) resulted in a productive infection in the explants. By contrast, selective application of the virus to distal axons led to a largely nonproductive infection that was characterized by the poor expression of lytic genes and the presence of high levels of the 2.0-kb major latency-associated transcript (LAT) RNA. Treatment of the explants with the immediate-early (IE) gene transcriptional inducer hexamethylene bisacetamide, and simultaneous co-infection of the GC with HSV-1, herpes simplex virus type 2 (HSV-2) or pseudorabies virus (PrV) helper virus significantly enhanced the ability of HSV-1 to productively infect sensory neurons upon axonal entry. Helper-virus-induced transactivation of HSV-1 IE gene expression in axonally-infected TGEs in the absence of de novo protein synthesis was dependent on the presence of functional tegument protein VP16 in HSV-1 helper virus particles. After the establishment of a LAT-positive silent infection in TGEs, HSV-1 was refractory to transactivation by superinfection of the GC with HSV-1 but not with HSV-2 and PrV helper virus. In conclusion, the site of entry appears to be a critical determinant in the lytic/latent decision in sensory neurons. HSV-1 entry into distal axons results in an insufficient transactivation of IE gene expression and favors the establishment of a nonproductive, silent infection in trigeminal neurons.

Upon primary infection of the oronasal mucosa, herpes simplex virus type 1 (HSV-1) rapidly reaches the ganglia of the peripheral nervous system via axonal transport and establishes lifelong latency in surviving neurons. Central to the establishment of latency is the ability of HSV-1 to reliably switch from productive, lytic spread in epithelia to nonproductive, latent infection in sensory neurons. It is not fully understood what specifically disposes incoming particles of a highly cytopathogenic, fast-replicating alphaherpesvirus to nonproductive, latent infection in sensory neurons. The present study shows that selective entry of HSV-1 into the distal axons of trigeminal neurons strongly favors the establishment of a nonproductive, latent infection, whereas nonselective infection of neurons still enables HSV-1 to induce lytic gene expression. Our data support a model of latency establishment in which the site of entry is an important determinant of the lytic/latent decision in the infected neuron. Productive infection of the neuron ensues if particles enter the soma of the neuron directly. In contrast, previous retrograde axonal transport of incoming viral particles creates a distinct scenario that abrogates VP16-dependent transactivation of immediate-early gene expression and precludes the expression of lytic genes to an extent sufficient to prevent the initiation of massive productive infection of trigeminal neurons.

Ocular herpes simplex virus: how are latency, reactivation, recurrent disease and therapy interrelated?

Most humans are infected with herpes simplex virus (HSV) type 1 in early childhood and remain latently infected throughout life. While most individuals have mild or no symptoms, some will develop destructive HSV keratitis. Ocular infection with HSV-1 and its associated sequelae account for the majority of corneal blindness in industrialized nations. Neuronal latency in the peripheral ganglia is established when transcription of the viral genome is repressed (silenced) except for the latency-associated transcripts and microRNAs. The functions of latency-associated transcripts have been investigated since 1987. Roles have been suggested relating to reactivation, establishment of latency, neuronal protection, antiapoptosis, apoptosis, virulence and asymptomatic shedding. Here, we review HSV-1 latent infections, reactivation, recurrent disease and antiviral therapies for the ocular HSV diseases.

Control of HSV-1 latency in human trigeminal ganglia--current overview

Although recurrent Herpes simplex virus type 1 (HSV-1) infections are quite common in humans, little is known about the exact molecular mechanisms involved in latency and reactivation of the virus from its stronghold, the trigeminal ganglion. After primary infection, HSV-1 establishes latency in sensory neurons, a state that lasts for the life of the host. Reactivation of the virus leads to recurrent disease, ranging from relatively harmless cold sores to ocular herpes. If herpes encephalitis-often a devastating disease-is also caused by reactivation or a new infection, is still a matter of debate. It is widely accepted that CD8(+) T cells as well as host cellular factors play a crucial role in maintaining latency. At least in the animal model, IFNγ and Granzyme B secretion of T cells were shown to be important for control of viral latency. Furthermore, the virus itself expresses factors that regulate its own latency-reactivation cycle. In this regard, the latency associated transcript, immediate-early proteins, and viral miRNAs seem to be the key players that control latency and reactivation on the viral side. This review focuses on HSV-1 latency in humans in the light of mechanisms learned from animal models.

Expression of herpes simplex virus 1-encoded microRNAs in human trigeminal ganglia and their relation to local T-cell infiltrates

Herpes simplex type 1 (HSV-1) is a neurotropic virus which establishes lifelong latency in human trigeminal ganglia (TG). Currently, two nonexclusive control mechanisms of HSV-1 latency are discussed: antiviral CD8(+) T cells and viral microRNAs (miRNAs) encoded by the latency associated transcript (LAT). We investigate here to what extent these mechanisms may contribute to the maintenance of HSV-1 latency. We show that only a small proportion of LAT(+) neurons is surrounded by T cells in human TG. This indicates that viral latency in human TG might be controlled by other mechanisms such as viral miRNAs. Therefore, we assessed TG sections for the presence of HSV-1 miRNA, DNA, and mRNA by combining LAT in situ hybridization, T-cell immunohistochemistry, and single cell analysis of laser-microdissected sensory neurons. Quantitative reverse transcription-PCR (RT-PCR) revealed that LAT(+) neurons with or without surrounding T cells were always positive for HSV-1 miRNAs and DNA. Furthermore, ICP0 mRNA could rarely be detected only in LAT(+) neurons, as analyzed by single-cell RT-PCR. In contrast, in LAT(-) neurons that were surrounded by T cells, neither miRNAs nor the DNA of HSV-1, HSV-2, or varicella-zoster virus could be detected. These data indicate that the majority of LAT(+) neurons is not directly controlled by T cells. However, miRNA expression in every latently infected neuron would provide an additional checkpoint before viral replication is initiated.

Herpes simplex virus 1 microRNAs expressed abundantly during latent infection are not essential for latency in mouse trigeminal ganglia

Several herpes simplex virus 1 microRNAs are encoded within or near the latency associated transcript (LAT) locus, and are expressed abundantly during latency. Some of these microRNAs can repress the expression of important viral proteins and are hypothesized to play important roles in establishing and/or maintaining latent infections. We found that in lytically infected cells and in acutely infected mouse ganglia, expression of LAT-encoded microRNAs was weak and unaffected by a deletion that includes the LAT promoter. In mouse ganglia latently infected with wild type virus, the microRNAs accumulated to high levels, but deletions of the LAT promoter markedly reduced expression of LAT-encoded microRNAs and also miR-H6, which is encoded upstream of LAT and can repress expression of ICP4. Because these LAT deletion mutants establish and maintain latent infections, these microRNAs are not essential for latency, at least in mouse trigeminal ganglia, but may help promote it.

The herpes simplex virus 1 latency-associated transcript promotes functional exhaustion of virus-specific CD8+ T cells in latently infected trigeminal ganglia: a novel immune evasion mechanism

Following ocular herpes simplex virus 1 (HSV-1) infection of C57BL/6 mice, HSV-specific (HSV-gB(498-505) tetramer(+)) CD8(+) T cells are induced, selectively retained in latently infected trigeminal ganglia (TG), and appear to decrease HSV-1 reactivation. The HSV-1 latency-associated transcript (LAT) gene, the only viral gene that is abundantly transcribed during latency, increases reactivation. Previously we found that during latency with HSV-1 strain McKrae-derived viruses, more of the total TG resident CD8 T cells expressed markers of exhaustion with LAT(+) virus compared to LAT(-) virus. Here we extend these findings to HSV-1 strain 17syn+-derived LAT(+) and LAT(-) viruses and to a virus expressing just the first 20% of LAT. Thus, the previous findings were not an artifact of HSV-1 strain McKrae, and the LAT function involved mapped to the first 1.5 kb of LAT. Importantly, to our knowledge, we show here for the first time that during LAT(+) virus latency, most of the HSV-1-specific TG resident CD8 T cells were functionally exhausted, as judged by low cytotoxic function and decreased gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α) production. This resulted in LAT(-) TG having more functional HSV-gB(498-505) tetramer(+) CD8(+) T cells compared to LAT(+) TG. In addition, LAT expression, in the absence of other HSV-1 gene products, appeared to be able to directly or indirectly upregulate both PD-L1 and major histocompatibility complex class I (MHC-I) on mouse neuroblastoma cells (Neuro2A). These findings may constitute a novel immune evasion mechanism whereby the HSV-1 LAT directly or indirectly promotes functional exhaustion (i.e., dysfunction) of HSV-specific CD8(+) T cells in latently infected TG, resulting in increased virus reactivation.

Of mice and not humans: how reliable are animal models for evaluation of herpes CD8(+)-T cell-epitopes-based immunotherapeutic vaccine candidates?

Herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2)-specific CD8(+) T cells that reside in sensory ganglia, appear to control recurrent herpetic disease by aborting or reducing spontaneous and sporadic reactivations of latent virus. A reliable animal model is the ultimate key factor to test the efficacy of therapeutic vaccines that boost the level and the quality of sensory ganglia-resident CD8(+) T cells against spontaneous herpes reactivation from sensory neurons, yet its relevance has been often overlooked. Herpes vaccinologists are hesitant about using mouse as a model in pre-clinical development of therapeutic vaccines because they do not adequately mimic spontaneous viral shedding or recurrent symptomatic diseases, as occurs in human. Alternatives to mouse models are rabbits and guinea pigs in which reactivation arise spontaneously with clinical herpetic features relevant to human disease. However, while rabbits and guinea pigs develop spontaneous HSV reactivation and recurrent ocular and genital disease none of them can mount CD8(+) T cell responses specific to Human Leukocyte Antigen- (HLA-)restricted epitopes. In this review, we discuss the advantages and limitations of these animal models and describe a novel "humanized" HLA transgenic rabbit, which shows spontaneous HSV-1 reactivation, recurrent ocular disease and mounts CD8(+) T cell responses to HLA-restricted epitopes. Adequate investments are needed to develop reliable preclinical animal models, such as HLA class I and class II double transgenic rabbits and guinea pigs to balance the ethical and financial concerns associated with the rising number of unsuccessful clinical trials for therapeutic vaccine formulations tested in unreliable mouse models.

Virus reactivation: a panoramic view in human infections

Viruses are obligate intracellular parasites, relying to a major extent on the host cell for replication. An active replication of the viral genome results in a lytic infection characterized by the release of new progeny virus particles, often upon the lysis of the host cell. Another mode of virus infection is the latent phase, where the virus is 'quiescent' (a state in which the virus is not replicating). A combination of these stages, where virus replication involves stages of both silent and productive infection without rapidly killing or even producing excessive damage to the host cells, falls under the umbrella of a persistent infection. Reactivation is the process by which a latent virus switches to a lytic phase of replication. Reactivation may be provoked by a combination of external and/or internal cellular stimuli. Understanding this mechanism is essential in developing future therapeutic agents against viral infection and subsequent disease. This article examines the published literature and current knowledge regarding the viral and cellular proteins that may play a role in viral reactivation. The focus of the article is on those viruses known to cause latent infections, which include herpes simplex virus, varicella zoster virus, Epstein-Barr virus, human cytomegalovirus, human herpesvirus 6, human herpesvirus 7, Kaposi's sarcoma-associated herpesvirus, JC virus, BK virus, parvovirus and adenovirus.

The role of LAT in increased CD8+ T cell exhaustion in trigeminal ganglia of mice latently infected with herpes simplex virus 1

Herpes simplex virus (HSV) infection is a classic example of latent viral infection in humans and experimental animal models. The HSV-1 latency-associated transcript (LAT) plays a major role in the HSV-1 latency reactivation cycle and thus in recurrent disease. Whether the presence of LAT leads to generation of dysfunctional T cell responses in the trigeminal ganglia (TG) of latently infected mice is not known. To address this issue, we used LAT-positive [LAT(+)] and LAT-deficient [LAT(-)] viruses to evaluate the effect of LAT on CD8 T cell exhaustion in TG of latently infected mice. The amount of latency as determined by quantitative reverse transcription-PCR (qRT-PCR) of viral DNA in total TG extracts was 3-fold higher with LAT(+) than with LAT(-) virus. LAT expression and increased latency correlated with increased mRNA levels of CD8, PD-1, and Tim-3. PD-1 is both a marker for exhaustion and a primary factor leading to exhaustion, and Tim-3 can also contribute to exhaustion. These results suggested that LAT(+) TG contain both more CD8(+) T cells and more CD8(+) T cells expressing the exhaustion markers PD-1 and Tim-3. This was confirmed by flow cytometry analyses of expression of CD3/CD8/PD-1/Tim-3, HSV-1, CD8(+) T cell pentamer (specific for a peptide derived from residues 498 to 505 of glycoprotein B [gB(498-505)]), interleukin-2 (IL-2), and tumor necrosis factor alpha (TNF-α). The functional significance of PD-1 and its ligands in HSV-1 latency was demonstrated by the significantly reduced amount of HSV-1 latency in PD-1- and PD-L1-deficient mice. Together, these results may suggest that both PD-1 and Tim-3 are mediators of CD8(+) T cell exhaustion and latency in HSV-1 infection.

Immunization with different viral antigens alters the pattern of T cell exhaustion and latency in herpes simplex virus type 1-infected mice

We have shown previously that immunization with herpes simplex virus type 1 (HSV-1) glycoprotein K (gK) exacerbated corneal scarring (CS) in ocularly infected mice. In this study, we investigated whether higher levels of CS were correlated with higher levels of latency and T cell exhaustion in gK-immunized mice. BALB/c mice were vaccinated with baculovirus-expressed gK or gD or mock immunized. Twenty-one days after the third immunization, mice were ocularly infected with 2 × 10(4) PFU/eye of virulent HSV-1 strain McKrae. On day 5 postinfection, virus replication in the eye was measured, and on day 30 postinfection, infiltration of the trigeminal ganglia (TG) by CD4, CD8, programmed death 1 (PD-1), and T cell immunoglobulin and mucin domain-containing protein 3 (Tim-3) was monitored by immunohistochemistry and quantitative real-time PCR (qRT-PCR). This study demonstrated that higher levels of CS were correlated with higher levels of latency, and this was associated with the presence of significantly higher numbers of CD4(+)PD-1(+) and CD8(+)PD-1(+) cells in the TG of the gK-immunized group than in both the gD- and mock-immunized groups. Levels of exhaustion associated with Tim-3 were the same among gK- and mock-vaccinated groups but higher than levels in the gD-vaccinated group. In this study, we have shown for the first time that both PD-1 and Tim-3 contribute to T cell exhaustion and an increase of latency in the TG of latently infected mice.

A novel HLA (HLA-A*0201) transgenic rabbit model for preclinical evaluation of human CD8+ T cell epitope-based vaccines against ocular herpes

We introduced a novel humanized HLA-A*0201 transgenic (HLA Tg) rabbit model to assess the protective efficacy of a human CD8(+) T cell epitope-based vaccine against primary ocular herpes infection and disease. Each of the three immunodominant human CD8(+) T cell peptide epitopes from HSV-1 glycoprotein D (gD(53-61), gD(70-78), and gD(278-286)) were joined with a promiscuous human CD4(+) T cell peptide epitope (gD(49-82)) to construct three separate pairs of CD4-CD8 peptides. Each CD4-CD8 peptide pair was then covalently linked to an N(epsilon)-palmitoyl-lysine residue via a functional base lysine amino group to construct CD4-CD8 lipopeptides. HLA Tg rabbits were immunized s.c. with a mixture of the three CD4-CD8 HSV-1 gD lipopeptides. The HSV-gD-specific T cell responses induced by the mixture of CD4-CD8 lipopeptide vaccine and the protective efficacy against acute virus replication and ocular disease were determined. Immunization induced HSV-gD(49-82)-specific CD4(+) T cells in draining lymph node (DLN); induced HLA-restricted HSV-gD(53-61), gD(70-78), and gD(278-286)-specific CD8(+) T cells in DLN, conjunctiva, and trigeminal ganglia and reduced HSV-1 replication in tears and corneal eye disease after ocular HSV-1 challenge. In addition, the HSV-1 epitope-specific CD8(+) T cells induced in DLNs, conjunctiva, and the trigeminal ganglia were inversely proportional with corneal disease. The humanized HLA Tg rabbits appeared to be a useful preclinical animal model for investigating the immunogenicity and protective efficacy of human CD8(+) T cell epitope-based prophylactic vaccines against ocular herpes. The relevance of HLA Tg rabbits for future investigation of human CD4-CD8 epitope-based therapeutic vaccines against recurrent HSV-1 is discussed.