Experimental investigation of herpes simplex virus latency

Asymptomatic neonatal herpes simplex virus infection in mice leads to persistent CNS infection and long-term cognitive impairment

Neonatal herpes simplex virus (nHSV) is a devastating infection impacting approximately 14,000 newborns globally each year. nHSV infection is associated with high neurologic morbidity and mortality, making early intervention critical. Clinical outcomes of symptomatic nHSV infections are well-studied, but little is known about the frequency of, or outcomes following, subclinical or asymptomatic nHSV. Given the ubiquitous nature of HSV infection and frequency of asymptomatic shedding in adults, subclinical infections are underreported and could contribute to long-term neurological damage. To assess potential neurological morbidity associated with subclinical nHSV infection, we developed a low-dose (100 PFU) intranasal HSV infection model in neonatal wild-type C57BL/6 mice. At this dose, HSV DNA was detected in the brain by quantitative PCR (qPCR) but was not associated with acute clinical signs of infection. However, months after neonatal inoculation with this low dose of HSV, we observed impaired mouse performance on a range of cognitive and memory tests. Memory impairment was induced by infection with either HSV-1 or HSV-2 wild-type viruses, indicating that the cognitive impairment associated with neonatal infection was not strain-specific. Maternal immunization reduced neonate central nervous system (CNS) viral burden and prevented offspring from developing neurological sequelae following nHSV infection. Altogether, these results support the idea that subclinical neonatal infections may lead to cognitive decline in adulthood and that maternal vaccination is an effective strategy for reducing neurological sequelae in infected offspring. These findings may have profound implications for understanding and modeling the etiology of human neurodegenerative disorders such as Alzheimer's Disease.

Asymptomatic neonatal herpes simplex virus infection in mice leads to long-term cognitive impairment

Neonatal herpes simplex virus (nHSV) is a devastating infection impacting approximately 14,000 newborns globally each year. Infection is associated with high neurologic morbidity and mortality, making early intervention and treatment critical. Clinical outcomes of symptomatic nHSV infections are well-studied, but little is known about the frequency of, or outcomes following, sub-clinical or asymptomatic nHSV. Given the ubiquitous nature of HSV infection and frequency of asymptomatic shedding in adults, subclinical infections are underreported, yet could contribute to long-term neurological damage. To assess potential neurological morbidity associated with subclinical nHSV infection, we developed a low-dose (100 PFU) HSV infection protocol in neonatal C57BL/6 mice. At this dose, HSV DNA was detected in the brain by PCR but was not associated with acute clinical symptoms. However, months after initial inoculation with 100 PFU of HSV, we observed impaired mouse performance on a range of cognitive and memory performance tasks. Memory impairment was induced by infection with either HSV-1 or HSV-2 wild-type viruses, but not by a viral mutant lacking the autophagy-modulating Beclin-binding domain of the neurovirulence gene γ34.5. Retroviral expression of wild type γ34.5 gene led to behavioral pathology in mice, suggesting that γ34.5 expression may be sufficient to cause cognitive impairment. Maternal immunization and HSV-specific antibody treatment prevented offspring from developing neurological sequelae following nHSV-1 infection. Altogether, these results support the idea that subclinical neonatal infections may lead to cognitive decline in adulthood, with possible profound implications for research on human neurodegenerative disorders such as Alzheimer's Disease.

Effector functions are required for broad and potent protection of neonatal mice with antibodies targeting HSV glycoprotein D

Multiple failed herpes simplex virus (HSV) vaccine candidates induce robust neutralizing antibody (Ab) responses in clinical trials, raising the hypothesis that Fc-domain-dependent effector functions may be critical for protection. While neonatal HSV (nHSV) infection results in mortality and lifelong neurological morbidity in humans, it is uncommon among neonates with a seropositive birthing parent, supporting the hypothesis that Ab-based therapeutics could protect neonates from HSV. We therefore investigated the mechanisms of monoclonal Ab (mAb)-mediated protection in a mouse model of nHSV infection. For a panel of glycoprotein D (gD)-specific mAbs, neutralization and effector functions contributed to nHSV-1 protection. In contrast, effector functions alone were sufficient to protect against nHSV-2, exposing a functional dichotomy between virus types consistent with vaccine trial results. Effector functions are therefore crucial for protection by these gD-specific mAbs, informing effective Ab and vaccine design and demonstrating the potential of polyfunctional Abs as therapeutics for nHSV infections.

A viral lncRNA tethers HSV-1 genomes at the nuclear periphery to establish viral latency

IMPORTANCE

Herpes simplex virus 1 (HSV-1) establishes lifelong latency in neuronal cells. Following a stressor, the virus reactivates from latency, virus is shed at the periphery and recurrent disease can occur. During latency, the viral lncRNA termed the latency-associated transcript (LAT) is known to accumulate to high abundance. The LAT is known to impact many aspects of latency though the molecular events involved are not well understood. Here, we utilized a human neuronal cell line model of HSV latency and reactivation (LUHMES) to identify the molecular-binding partners of the LAT during latency. We found that the LAT binds to both the cellular protein, TMEM43, and HSV-1 genomes in LUHMES cells. Additionally, we find that knockdown of TMEM43 prior to infection results in a decreased ability of HSV-1 to establish latency. This work highlights a potential mechanism for how the LAT facilitates the establishment of HSV-1 latency in human neurons.

Targeting carcinoembryonic antigen-expressing tumors using a novel transcriptional and translational dual-regulated oncolytic herpes simplex virus type 1

VG2025 is a recombinant oncolytic herpes simplex virus type 1 (HSV-1) that uses transcriptional and translational dual regulation (TTDR) of critical viral genes to enhance virus safety and promote tumor-specific virus replication without reducing virulence. The TTDR platform is based on transcriptional control of the essential HSV-1 immediate-early protein ICP27 using a tumor-specific carcinoembryonic antigen (CEA) promoter, coupled with translational control of the neurovirulence factor ICP34.5 using multiple microRNA (miR)-binding sites. VG2025 further incorporates IL-12 and the IL-15/IL-15 receptor alpha subunit complex to enhance the antitumor and immune stimulatory properties of oncolytic HSVs. The TTDR strategy was verified in vitro and shown to be highly selective. Strong in vivo antitumor efficacy was observed following both intratumoral and intravenous administration. Clear abscopal and immune memory effects were also evident, indicating a robust antitumor immune response. Gene expression profiling of treated tumors revealed increased immune cell infiltration and activation of multiple immune-signaling pathways when compared with the backbone virus. Absence of neurotoxicity was verified in mice and in rhesus monkeys. Taken together, the enhanced tumor clearance, excellent safety profile, and positive correlation between CEA levels and viral replication efficiency may provide an opportunity for using biomarker-based precision medicine in oncolytic virotherapy.

Herpes simplex virus protein UL56 inhibits cGAS-Mediated DNA sensing to evade antiviral immunity

After herpes simplex virus type 1 (HSV-1) infection, the cytosolic sensor cyclic GMP-AMP synthase (cGAS) recognizes DNA and catalyzes synthesis of the second messenger 2'3'-cGAMP. cGAMP binds to the ER-localized adaptor protein MITA (also known as STING) to activate downstream antiviral responses. Conversely, HSV-1-encoded proteins evade antiviral immune responses via a wide variety of delicate mechanisms, promoting viral replication and pathogenesis. Here, we identified HSV-1 envelop protein UL56 as a negative regulator of cGAS-mediated innate immune responses. Overexpression of UL56 inhibited double-stranded DNA-triggered antiviral responses, whereas UL56-deficiency increased HSV-1-triggered induction of downstream antiviral genes. UL56-deficiency inhibited HSV-1 replication in wild-type but not MITA-deficient cells. UL56-deficient HSV-1 showed reduced replication in the brain of infected mice and was less lethal to infected mice. Mechanistically, UL56 interacted with cGAS and inhibited its DNA binding and enzymatic activity. Furthermore, we found that UL56 homologous proteins from different herpesviruses had similar roles in antagonizing cGAS-mediated innate immune responses. Our findings suggest that UL56 is a component of HSV-1 evasion of host innate immune responses by antagonizing the DNA sensor cGAS, which contributes to our understanding of the comprehensive mechanisms of immune evasion by herpesviruses.

Single-cell transcriptomics identifies Gadd45b as a regulator of herpesvirus-reactivating neurons

Single-cell RNA sequencing (scRNA-seq) is a powerful technique for dissecting the complexity of normal and diseased tissues, enabling characterization of cell diversity and heterogeneous phenotypic states in unprecedented detail. However, this technology has been underutilized for exploring the interactions between the host cell and viral pathogens in latently infected cells. Herein, we use scRNA-seq and single-molecule sensitivity fluorescent in situ hybridization (smFISH) technologies to investigate host single-cell transcriptome changes upon the reactivation of a human neurotropic virus, herpes simplex virus-1 (HSV-1). We identify the stress sensor growth arrest and DNA damage-inducible 45 beta (Gadd45b) as a critical antiviral host factor that regulates HSV-1 reactivation events in a subpopulation of latently infected primary neurons. We show that distinct subcellular localization of Gadd45b correlates with the viral late gene expression program, as well as the expression of the viral transcription factor, ICP4. We propose that a hallmark of a "successful" or "aborted" HSV-1 reactivation state in primary neurons is determined by a unique subcellular localization signature of the stress sensor Gadd45b.

Non-cytopathic herpes simplex virus type-1 isolated from acyclovir-treated patients with recurrent infections

Herpes simplex virus (HSV) usually produces cytopathic effect (CPE) within 24-72 h post-infection (P.I.). Clinical isolates from recurrent HSV infections in patients on Acyclovir therapy were collected between 2016 and 2019 and tested in cell cultures for cytopathic effects and further in-depth characterization. Fourteen such isolates did not show any CPE in A549 or Vero cell lines even at 120 h P.I. However, these cultures remained positive for HSV-DNA after several passages. Sequence analysis revealed that the non-CPE isolates were all HSV-1. Analysis of the thymidine kinase gene from the isolates revealed several previously reported and two novel ACV-resistant mutations. Immunofluorescence and Western blot data revealed a low-level expression of the immediate early protein, ICP4. Late proteins like ICP5 or capsid protein, VP16 were almost undetectable in these isolates. AFM imaging revealed that the non-CPE viruses had structural deformities compared to wild-type HSV-1. Our findings suggest that these strains are manifesting an unusual phenomenon of being non-CPE herpesviruses with low level of virus protein expressions over several passages. Probably these HSV-1 isolates are evolving towards a more "cryptic" form to establish chronic infection in the host thereby unraveling yet another strategy of herpesviruses to evade the host immune system.

Modulating glutamine metabolism to control viral immuno-inflammatory lesions

Infection of the cornea with HSV results in an immune-inflammatory reaction orchestrated by proinflammatory T cells that is a major cause of human vision impairment. The severity of lesions can be reduced if the representation of inflammatory T cells is changed to increase the presence of T cells with regulatory function. This report shows that inhibiting glutamine metabolism using 6-Diazo-5-oxo-l-norleucine (DON) administered via intraperitoneal (IP) starting 6 days after ocular infection and continued until day 15 significantly reduced the severity of herpetic stromal keratitis lesions. The therapy resulted in reduced neutrophils, macrophages as well proinflammatory CD4 Th1 and Th17 T cells in the cornea, but had no effect on levels of regulatory T cells. A similar change in the representation of inflammatory and regulatory T cells occurred in the trigeminal ganglion (TG) the site where HSV infection establishes latency. Glutamine metabolism was shown to be required for the in-vitro optimal induction of both Th1 and Th17 T cells but not for the induction of Treg that were increased when glutamine metabolism was inhibited. Inhibiting glutamine metabolism also changed the ability of latently infected TG cells from animals previously infected with HSV to reactivate and produce infectious virus.

MicroRNAs: Harbingers and shapers of periodontal inflammation

Periodontal disease is an inflammatory reaction of the periodontal tissues to oral pathogens. In the present review we discuss the intricate effects of a regulatory network of gene expression modulators, microRNAs (miRNAs), as they affect periodontal morphology, function and gene expression during periodontal disease. These miRNAs are small RNAs involved in RNA silencing and post-transcriptional regulation and affect all stages of periodontal disease, from the earliest signs of gingivitis to the regulation of periodontal homeostasis and immunity and to the involvement in periodontal tissue destruction. MiRNAs coordinate periodontal disease progression not only directly but also through long non-coding RNAs (lncRNAs), which have been demonstrated to act as endogenous sponges or decoys that regulate the expression and function of miRNAs, and which in turn suppress the targeting of mRNAs involved in the inflammatory response, cell proliferation, migration and differentiation. While the integrity of miRNA function is essential for periodontal health and immunity, miRNA sequence variations (genetic polymorphisms) contribute toward an enhanced risk for periodontal disease progression and severity. Several polymorphisms in miRNA genes have been linked to an increased risk of periodontitis, and among those, miR-146a, miR-196, and miR-499 polymorphisms have been identified as risk factors for periodontal disease. The role of miRNAs in periodontal disease progression is not limited to the host tissues but also extends to the viruses that reside in periodontal lesions, such as herpesviruses (human herpesvirus, HHV). In advanced periodontal lesions, HHV infections result in the release of cytokines from periodontal tissues and impair antibacterial immune mechanisms that promote bacterial overgrowth. In turn, controlling the exacerbation of periodontal disease by minimizing the effect of periodontal HHV in periodontal lesions may provide novel avenues for therapeutic intervention. In summary, this review highlights multiple levels of miRNA-mediated control of periodontal disease progression, (i) through their role in periodontal inflammation and the dysregulation of homeostasis, (ii) as a regulatory target of lncRNAs, (iii) by contributing toward periodontal disease susceptibility through miRNA polymorphism, and (iv) as periodontal microflora modulators via viral miRNAs.

Biological and environmental factors associated with the detection of elephant endotheliotropic herpesvirus in Asian elephants (Elephas maximus) in Thailand

Elephant endotheliotropic herpesvirus (EEHV) infection is one of the most common diseases in young elephants, causing severe fatal hemorrhagic disease. Subclinical infection was previously described; however, information about the factors associated with virus shedding and reactivation were scarce. To identify the biological and environmental factors related with EEHV detection, blood and oral swab samples were collected from nine captive Asian elephants in Thailand for one year and tested for EEHV presence using real-time PCR. Data including hematological values, management, environmental temperature, and serum cortisol levels were also recorded and analyzed. Results showed that the viral detection frequency ranged from 0-25%. The highest detection frequency was found in the two youngest elephants, aged less than 15 years. Three types of viruses, EEHV1, EEHV4, and EEHV5, were found in this study, which also detected mixed infection in five elephants. Additionally, the study found that sample type, changes in hematological values, management and health issues, and serum cortisol levels were not associated with herpesvirus detection in the elephants. However, EEHV detection percentage was significantly increased in the summer (mid-Feb to mid-May), possibly due to body fitness reduction from food source limitation and low nutrient content. To obtain a broad aspect of EEHV management, long-term EEHV monitoring is highly recommended in every captive elephant herd.

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

[Evaluation and management of infectious esophagitis in immunocompromised patients and immunocompetent individuals]

Among the many causes of the inflammatory process in the esophagus, infectious diseases are becoming increasingly important due to their steady growth. Previously esophageal infections have traditionally been associated with immunodeficiency syndromes, but now in clinical practice, these disorders are becoming increasingly recognized in immunocompetent individuals. Early diagnosis of infectious esophagitis is necessary to develop effective treatment tactics, and, as a result, reduce the risk of complications and adverse outcomes of the disease. This study reviewed the most clinical relevant pathogens of infectious esophagitis, both among patients with immunodeficiency and among healthy individuals. Specific diagnostic, risk factors, clinical presentation and therapeutic features were considered depending on the immune status of patients.

Herpes Simplex Virus 1 Strains 17syn + and KOS(M) Differ Greatly in Their Ability To Reactivate from Human Neurons In Vitro

Herpes simplex virus 1 (HSV-1) establishes a lifelong latent infection in peripheral nerve ganglia. Periodically, the virus reactivates from this latent reservoir and is transported to the original site of infection. Strains of HSV-1 have been noted to vary greatly in their virulence and reactivation efficiencies in animal models. While HSV-1 strain 17syn ⁺ can be readily reactivated, strain KOS(M) shows little to no reactivation in the mouse and rabbit models of induced reactivation. Additionally, 17syn ⁺ is markedly more virulent in vivo than KOS. This has raised questions regarding potential strain-specific differences in neuroinvasion and neurovirulence and their contribution to differences in the establishment of latency (or ability to spread back to the periphery) and to the reactivation phenotype. To determine if any difference in the ability to reactivate between strains 17syn ⁺ and KOS(M) is manifest at the level of neurons, we utilized a recently characterized human neuronal cell line model of HSV latency and reactivation (LUHMES). We found that KOS(M) established latency with a higher number of viral genomes than strain 17syn ⁺ Strikingly, we show that the KOS(M) viral genomes have a higher burden of heterochromatin marks than strain 17syn ⁺ The increased heterochromatin profile for KOS(M) correlates with the reduced expression of viral lytic transcripts during latency and impaired induced reactivation compared to that of 17syn ⁺ These results suggest that genomes entering neurons from HSV-1 infections with strain KOS(M) are more prone to rapid heterochromatinization than those of 17syn ⁺ and that this results in a reduced ability to reactivate from latency.IMPORTANCE Herpes simplex virus 1 (HSV-1) establishes a lifelong infection in neuronal cells. The virus periodically reactivates and causes recurrent disease. Strains of HSV-1 vary greatly in their virulence and potential to reactivate in animal models. Although these differences are phenotypically well defined, factors contributing to the strains' abilities to reactivate are largely unknown. We utilized a human neuronal cell line model of HSV latency and reactivation (LUHMES) to characterize the latent infection of two HSV-1 wild-type strains. We find that strain-specific differences in reactivation are recapitulated in LUHMES. Additionally, these differences correlate with the degree of heterochromatinization of the latent genomes. Our data suggest that the epigenetic state of the viral genome is an important determinant of reactivation that varies in a strain-specific manner. This work also shows the first evidence of strain-specific differences in reactivation outside the context of the whole animal at a human neuronal cell level.

Deletion of Herpes Simplex Virus 1 MicroRNAs miR-H1 and miR-H6 Impairs Reactivation

During all stages of infection, herpes simplex virus 1 (HSV-1) expresses viral microRNAs (miRNAs). There are at least 20 confirmed HSV-1 miRNAs, yet the roles of individual miRNAs in the context of viral infection remain largely uncharacterized. We constructed a recombinant virus lacking the sequences for miR-H1-5p and miR-H6-3p (17dmiR-H1/H6). The seed sequences for these miRNAs are antisense to each other and are transcribed from divergent noncoding RNAs in the latency-associated transcript (LAT) promoter region. Comparing phenotypes exhibited by the recombinant virus lacking these miRNAs to the wild type (17syn+), we found that during acute infection in cell culture, 17dmiR-H1/H6 exhibited a modest increase in viral yields. Analysis of pathogenesis in the mouse following footpad infection revealed a slight increase in virulence for 17dmiR-H1/H6 but no significant difference in the establishment or maintenance of latency. Strikingly, explant of latently infected dorsal root ganglia revealed a decreased and delayed reactivation phenotype. Further, 17dmiR-H1/H6 was severely impaired in epinephrine-induced reactivation in the rabbit ocular model. Finally, we demonstrated that deletion of miR-H1/H6 increased the accumulation of the LAT as well as several of the LAT region miRNAs. These results suggest that miR-H1/H6 plays an important role in facilitating efficient reactivation from latency.IMPORTANCE While HSV antivirals reduce the severity and duration of clinical disease in some individuals, there is no vaccine or cure. Therefore, understanding the mechanisms regulating latency and reactivation as a potential to elucidate targets for better therapeutics is important. There are at least 20 confirmed HSV-1 miRNAs, yet the roles of individual miRNAs in the context of viral infection remain largely uncharacterized. The present study focuses on two of the miRNAs (miR-H1/H6) that are encoded within the latency-associated transcript (LAT) region, a portion of the genome that has been associated with efficient reactivation. Here, we demonstrate that the deletion of the seed sequences of these miRNAs results in a severe reduction in reactivation of HSV-1 in the mouse and rabbit models. These results suggest a linkage between these miRNAs and reactivation.

Latent pseudorabies virus infection in medulla oblongata from quarantined pigs

Pseudorabies virus (PRV) is a major pathogen in pig husbandry and is also a risk to human well-being. Pigs with latent PRV infection carry the virus lifelong, and it can be activated under conducive conditions. This poses a very important challenge to the control of the virus and may even prevent its elimination. To investigate latent infection with wild-type (wt) PRV, and also infection due to the use of live attenuated vaccines on farms, 80 pigs from two large-scale swine operations were traced. At 6 months old, the quarantined pigs were slaughtered and brain samples were collected. A PCR assay targeting the gB and gE genes was developed to detect PRV DNA fragments in medulla oblongata. Five of the samples (6.3%) were gB and gE gene fragment double-positive, 60 of the samples (75%) were gB single-positive, and 15 samples (18.7%) showed double-negative. A portion of latency-associated transcripts (LATs), EP0 mRNA, were found to be present in the gB gene fragment positive samples. Furthermore, the five double-positive samples were transmitted blindly, and apparent cytopathic effects were found in three of the five samples in the fourth generation. By means of Western blotting, PCR and sequencing, two of the isolated viruses were found to be related to vaccine strain Bartha-K61. Another was closely related to domestic epidemic strains HN1201 and LA and relatively unrelated to other Asian isolates. These results suggest that the live vaccines are latently present in brains, in a manner similar to wt PRV, and this poses potential safety issues in the pig husbandry industry. Wt PRV and live vaccine viruses were found to co-exist in pigs, demonstrating that the live vaccines were unable to confer complete sterilizing immunity, which may explain outbreaks of pseudorabies on vaccinated farms.

MicroRNA Involvement in Signaling Pathways During Viral Infection

The study of miRNAs started in 1993, when Lee et al. observed their involvement in the downregulation of a crucial protein known as LIN-14 in the nematode Caenorhabditis elegans. Since then, great progress has been made regarding research on microRNAs, which are now known to be involved in the regulation of various physiological and pathological processes in both animals and humans. One such example is represented by their interaction with various signaling pathways during viral infections. It has been observed that these pathogens can induce the up-/downregulation of various host miRNAs in order to elude the host's immune system. In contrast, some miRNAs studied could have an antiviral effect, enabling the defense mechanisms to fight the infection or, at the very least, they could induce the pathogen to enter a latent state. At the same time, some viruses encode their own miRNAs, which could further modulate the host's signaling pathways, thus favoring the survival and replication of the virus. The goal of this extensive literature review was to present how miRNAs are involved in the regulation of various signaling pathways in some of the most important and well-studied human viral infections. Further on, knowing which miRNAs are involved in various viral infections and what role they play could aid in the development of antiviral therapeutic agents for certain diseases that do not have a definitive cure in the present. The clinical applications of miRNAs are extremely important, as miRNAs targeted inhibition may have substantial therapeutic impact. Inhibition of miRNAs can be achieved through many different methods, but chemically modified antisense oligonucleotides have shown the most prominent effects. Though scientists are far from completely understanding all the molecular mechanisms behind the complex cross-talks between miRNA pathways and viral infections, the general knowledge is increasing on the different roles played by miRNAs during viral 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.

Gold Nanoparticles Crossing Blood-Brain Barrier Prevent HSV-1 Infection and Reduce Herpes Associated Amyloid-βsecretion

Infections caused by HSV-1 and their typical outbreaks invading the nervous system have been related to neurodegenerative diseases. HSV-1 infection may deregulate the balance between the amyloidogenic and non-amyloidogenic pathways, raising the accumulation of amyloid-β peptides, one of the hallmarks in the neurodegenerative diseases. An effective treatment against both, HSV-1 infections and neurodegeneration, is a major therapeutic target. Therefore, gold nanoparticles (NPAus) have been previously studied in immunotherapy, cancer and cellular disruptions with very promising results. Our study demonstrates that a new NPAus family inhibits the HSV-1 infection in a neural-derived SK-N-MC cell line model and that this new NPAus reduces the HSV-1-induced β-secretase activity, as well as amyloid-β accumulation in SK-APP-D1 modifies cell line. We demonstrated that NPAuG3-S8 crosses the blood-brain barrier (BBB) and does not generate cerebral damage to in vivo CD1 mice model. The NPAuG3-S8 could be a promising treatment against neuronal HSV-1 infections and neuronal disorders related to the Aβ peptides.

HSV Mutant Generation and Dual Detection Methods for Gaining Insight into Latent/Lytic Cycles In Vivo

Two important components of a useful strategy to examine viral gene function, regulation, and pathogenesis in vivo are (1) a highly efficient protocol to generate viral mutants that limits undesired mutation and retains full replication competency in vivo, and (2) an efficient system to detect and quantify viral promoter activity and gene expression in rare cells in vivo and to gain insight into the surrounding tissue environment. Our strategy and protocols for generating, characterizing, and employing HSV viral promoter/reporter mutants in vivo are provided in this two-part chapter.

Using Primary SCG Neuron Cultures to Study Molecular Determinants of HSV-1 Latency and Reactivation

We describe a primary neuronal culture system suitable for molecular characterization of herpes simplex virus type 1 (HSV-1) infection, latency, and reactivation. While several alternative models are available, including infections of live animal or explanted ganglia, these are complicated by the presence of multiple cell types, including immune cells, and difficulties in manipulating the neuronal environment. The highly pure neuron culture system described here can be readily manipulated and is ideal for molecular studies that focus exclusively on the relationship between the virus and host neuron, the fundamental unit of latency. As such this model allows for detailed investigations of both viral and neuronal factors involved in the establishment and maintenance of HSV-1 latency and in viral reactivation induced by defined stimuli.

Herpes Simplex Virus 1 Induces Brain Inflammation and Multifocal Demyelination in the Cotton Rat Sigmodon hispidus

Our work demonstrates for the first time a direct association between infection with herpes simplex virus 1, a ubiquitous human pathogen generally associated with facial cold sores, and multifocal brain demyelination in an otherwise normal host, the cotton rat Sigmodon hispidus. For a long time, demyelinating diseases were considered to be autoimmune in nature and were studied by indirect methods, such as immunizing animals with myelin components or feeding them toxic substances that induce demyelination. Treatment against demyelinating diseases has been elusive, partially because of their unknown etiology. This work provides the first experimental evidence for the role of HSV-1 as the etiologic agent of multifocal brain demyelination in a normal host and suggests that vaccination against HSV-1 can help to combat demyelinating disorders.

Demyelinating central nervous system (CNS) disorders like multiple sclerosis (MS) and acute disseminated encephalomyelitis (ADEM) have been difficult to study and treat due to the lack of understanding of their etiology. Numerous cases point to the link between herpes simplex virus (HSV) infection and multifocal CNS demyelination in humans; however, convincing evidence from animal models has been missing. In this work, we found that HSV-1 infection of the cotton rat Sigmodon hispidus via a common route (lip abrasion) can cause multifocal CNS demyelination and inflammation. Remyelination occurred shortly after demyelination in HSV-1-infected cotton rats but could be incomplete, resulting in “scars,” further supporting an association between HSV-1 infection and multifocal demyelinating disorders. Virus was detected sequentially in the lip, trigeminal ganglia, and brain of infected animals. Brain pathology developed primarily on the ipsilateral side of the brain stem, in the cerebellum, and contralateral side of the forebrain/midbrain, suggesting that the changes may ascend along the trigeminal lemniscus pathway. Neurologic defects occasionally detected in infected animals (e.g., defective whisker touch and blink responses and compromised balance) could be representative of the brain stem/cerebellum dysfunction. Immunization of cotton rats with a split HSV-1 vaccine protected animals against viral replication and brain pathology, suggesting that vaccination against HSV-1 may protect against demyelinating disorders.

IMPORTANCE Our work demonstrates for the first time a direct association between infection with herpes simplex virus 1, a ubiquitous human pathogen generally associated with facial cold sores, and multifocal brain demyelination in an otherwise normal host, the cotton rat Sigmodon hispidus. For a long time, demyelinating diseases were considered to be autoimmune in nature and were studied by indirect methods, such as immunizing animals with myelin components or feeding them toxic substances that induce demyelination. Treatment against demyelinating diseases has been elusive, partially because of their unknown etiology. This work provides the first experimental evidence for the role of HSV-1 as the etiologic agent of multifocal brain demyelination in a normal host and suggests that vaccination against HSV-1 can help to combat demyelinating disorders.

Persistent Infection with Herpes Simplex Virus 1 and Alzheimer's Disease-A Call to Study How Variability in Both Virus and Host may Impact Disease

Increasing attention has focused on the contributions of persistent microbial infections with the manifestation of disease later in life, including neurodegenerative conditions such as Alzheimer’s disease (AD). Current data has shown the presence of herpes simplex virus 1 (HSV-1) in regions of the brain that are impacted by AD in elderly individuals. Additionally, neuronal infection with HSV-1 triggers the accumulation of amyloid beta deposits and hyperphosphorylated tau, and results in oxidative stress and synaptic dysfunction. All of these factors are implicated in the development of AD. These data highlight the fact that persistent viral infection is likely a contributing factor, rather than a sole cause of disease. Details of the correlations between HSV-1 infection and AD development are still just beginning to emerge. Future research should investigate the relative impacts of virus strain- and host-specific factors on the induction of neurodegenerative processes over time, using models such as infected neurons in vitro, and animal models in vivo, to begin to understand their relationship with cognitive dysfunction.

[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.

Lund Human Mesencephalic (LUHMES) Neuronal Cell Line Supports Herpes Simplex Virus 1 Latency In Vitro

Lund human mesencephalic (LUHMES) cells are human embryonic neuronal precursor cells that can be maintained as proliferating cells due to the expression of a tetracycline-regulatable (Tet-off) v-myc transgene. They can be differentiated to postmitotic neurons by the addition of tetracycline, glial cell-derived neurotrophic factor (GDNF), and dibutyryl cAMP. We demonstrate that these cells can be infected with herpes simplex virus 1 (HSV-1) at a multiplicity of infection (MOI) of 3 with the majority of cells surviving. By 6 days postinfection, there is a loss of lytic gene transcription and an increase in the numbers of neurons that express the latency-associated transcripts (LATs). Importantly, the virus can then be reactivated by the addition of a phosphoinositide 3-kinase inhibitor, which has previously been shown to reactivate HSV-1 in rat neuron cultures. While rodent primary culture neuron systems have been described, these are limited by their lack of scalability, as it is difficult to obtain more than 500,000 neurons to employ for a given experiment. Several recent papers have described a human dorsal root ganglion (DRG) neuron culture model and human induced pleuripotent stem cell (iPSC) neuron culture models that are scalable, but they require that the presence of an antiviral suppression be maintained following HSV-1 infection. The human LUHMES cell model of HSV-1 infection described here may be especially useful for studying HSV-1 latency and reactivation on account of its scalability, its amenability to maintenance of latency without the continual use of antiviral inhibitors, and its latent gene expression profile which mirrors many properties observed in vivo, importantly, the heterogeneity of cells expressing the LATs.IMPORTANCE Herpes simplex virus (HSV) is responsible for significant morbidity in humans due to its ability to cause oral and genital lesions, ocular disease, and encephalitis. While antivirals can attenuate the severity and frequency of disease, there is no vaccine or cure. Understanding the molecular details of HSV latency and reactivation is key to the development of new therapies. One of the difficulties in studying HSV latency has been the need to rely on establishment of latent infections in animal models. While rodent primary neuron culture models have shown promise, they yield relatively small numbers of latently infected neurons for biochemical and molecular analyses. Here we present the use of a human central nervous system (CNS)-derived conditionally proliferating cell line that can be differentiated into mature neurons and latently infected with HSV-1. This model shows promise as a scalable tool to study molecular and biochemical aspects of HSV-1 latency and reactivation in human neurons.

Latent versus productive infection: the alpha herpesvirus switch

Alpha herpesviruses are common pathogens of mammals. They establish a productive infection in many cell types, but a life-long latent infection occurs in PNS neurons. A vast majority of the human population has latent HSV-1 infections. Currently, there is no cure to clear latent infections. Even though HSV-1 is among the best studied viral pathogens, regulation of latency and reactivation is not well understood due to several challenges including a lack of animal models that precisely recapitulate latency/reactivation episodes; a difficulty in modeling in vitro latency; and a limited understanding of neuronal biology. In this review, we discuss insights gained from in vitro latency models with a focus on the neuronal and viral factors that determine the mode of infection.

The potential of currently unavailable herpes virus vaccines

INTRODUCTION

Despite overwhelming experimental work, there are no licensed vaccines against the most frequent Alphaherpesviruses, namely herpes simplex virus 1 and 2 (HSV1 and 2) nor against the Epstein-Barr virus (EBV), a member of the subfamily Gammaherpesvirus.

AREAS COVERED

Since the DNAs of both HSVs reside in the regional sensory ganglia in a latent state (i.e. as circularized episomal molecules), a corresponding vaccine might be useful for immunotherapy rather than for prevention of primary infection. Here we describe the design of a purified subunit vaccine as well as the preparation and efficacy of a recombinant fusion protein consisting of the gD ectodomain from our domestic attenuated HSV1 strain HSZP. The EBV vaccines considered so far, were destined for prevention of infectious mononucleosis (IM) or to prevent formation of EBV related tumors. To design the EBV peptide vaccine, at least 15 carefully selected immunogenic epitopes coming from 12 virus coded proteins were bound to synthetic micro-particle carriers along with a non-specific pathogen recognizing receptor (PRR) stimulating both the T as well as B lymphocytes.

EXPERT COMMENTARY

The efficacy of a novel EBV peptide in the rabbit model was based on criteria such as antibody formation (EA-D detected by ELISA, early and capsid proteins tested by immunoblot), presence of LMP1 antigen and of viral DNA in peripheral white blood cells. Out of 19 peptide combinations used for vaccination, at least 6 showed a satisfactory protective effect.

Shared ancestry of herpes simplex virus 1 strain Patton with recent clinical isolates from Asia and with strain KOS63

Herpes simplex virus 1 (HSV-1) is a widespread pathogen that persists for life, replicating in surface tissues and establishing latency in peripheral ganglia. Increasingly, molecular studies of latency use cultured neuron models developed using recombinant viruses such as HSV-1 GFP-US11, a derivative of strain Patton expressing green fluorescent protein (GFP) fused to the viral US11 protein. Visible fluorescence follows viral DNA replication, providing a real time indicator of productive infection and reactivation. Patton was isolated in Houston, Texas, prior to 1973, and distributed to many laboratories. Although used extensively, the genomic structure and phylogenetic relationship to other strains is poorly known. We report that wild type Patton and the GFP-US11 recombinant contain the full complement of HSV-1 genes and differ within the unique regions at only eight nucleotides, changing only two amino acids. Although isolated in North America, Patton is most closely related to Asian viruses, including KOS63.

First demonstration of equid gammaherpesviruses within the gastric mucosal epithelium of horses

Horses commonly develop gastric mucosal ulcers, similar to humans, a condition known as equine gastric ulcer syndrome (EGUS) that can lead to poor performance and lost training time and care expenses. Unlike humans, however, an infectious bacterial cause of ulcers has not been conclusively identified. Herpesviruses, while well-established causative agents of diseases such as cold sores, genital lesions, and certain types of cancer, have also been implicated in the development of a subset of gastric ulcers in humans. The presence of equid herpesviruses in the gastrointestinal tract and their potential contribution to EGUS has not been evaluated. Here, we provide the first evidence of equid gammaherpesviruses 2 and 5 (EHV-2 and -5) within the epithelium of the gastric mucosa of horses. These viruses were initially detected by a nested PCR screen of gastric tissue samples obtained from client- and university-owned horses with and without ulcers; however, no association with EGUS was found in this limited sample set. We then validated a highly sensitive in situ hybridization (ISH) assay and used this assay to characterize the distribution of these viruses in necropsy gastric tissue samples from five racehorses. Analyses revealed frequent EHV-2 and EHV-5 co-infections within the gastric mucosal epithelium, regardless of the ulcer status. These results are the first to demonstrate the presence of equid gammaherpesviruses in the gastric mucosa of horses and warrants further investigation to determine the contribution of these viruses to the development of EGUS and/or other gastrointestinal diseases.

Herpes Simplex Virus Establishment, Maintenance, and Reactivation: In Vitro Modeling of Latency

All herpes viruses establish lifelong infections (latency) in their host, and herpes simplex viruses (HSVs) are highly prevalent worldwide. Recurrence of HSV infections contributes to significant disease burden in people and on rare occasion can be fatal. Cell culture models that recapitulate latent infection provide valuable insight on the host processes regulating viral establishment and maintenance of latency. More robust and rapid than infections in live animal studies, advancements in neuronal culture techniques have made the systematic analysis of viral reactivation mechanisms feasible. Only recently have human neuronal cell lines been available, but models in the natural host cell are a critical addition to the currently available models.

Modeling HSV-1 Latency in Human Embryonic Stem Cell-Derived Neurons

Herpes simplex virus 1 (HSV-1) uses latency in peripheral ganglia to persist in its human host, however, recurrent reactivation from this reservoir can cause debilitating and potentially life-threatening disease. Most studies of latency use live-animal infection models, but these are complex, multilayered systems and can be difficult to manipulate. Infection of cultured primary neurons provides a powerful alternative, yielding important insights into host signaling pathways controlling latency. However, small animal models do not recapitulate all aspects of HSV-1 infection in humans and are limited in terms of the available molecular tools. To address this, we have developed a latency model based on human neurons differentiated in culture from an NIH-approved embryonic stem cell line. The resulting neurons are highly permissive for replication of wild-type HSV-1, but establish a non-productive infection state resembling latency when infected at low viral doses in the presence of the antivirals acyclovir and interferon-α. In this state, viral replication and expression of a late viral gene marker are not detected but there is an accumulation of the viral latency-associated transcript (LAT) RNA. After a six-day establishment period, antivirals can be removed and the infected cultures maintained for several weeks. Subsequent treatment with sodium butyrate induces reactivation and production of new infectious virus. Human neurons derived from stem cells provide the appropriate species context to study this exclusively human virus with the potential for more extensive manipulation of the progenitors and access to a wide range of preexisting molecular tools.

An Immortalized Human Dorsal Root Ganglion Cell Line Provides a Novel Context To Study Herpes Simplex Virus 1 Latency and Reactivation

A defining characteristic of alphaherpesviruses is the establishment of lifelong latency in host sensory ganglia with occasional reactivation causing recurrent lytic infections. As an alternative to rodent models, we explored the use of an immortalized cell line derived from human dorsal root ganglia. HD10.6 cells proliferate by virtue of a transduced tetracycline-regulated v-myc oncogene. In the presence of doxycycline, HD10.6 cells mature to exhibit neuronal morphology and express sensory neuron-associated markers such as neurotrophin receptors TrkA, TrkB, TrkC, and RET and the sensory neurofilament peripherin. Infection of mature HD10.6 neurons by herpes simplex virus 1 (HSV-1) results in a delayed but productive infection. However, infection at a low multiplicity of infection (MOI) in the presence of acyclovir results in a quiescent infection resembling latency in which viral genomes are retained in a low number of neurons, viral gene expression is minimal, and infectious virus is not released. At least some of the quiescent viral genomes retain the capacity to reactivate, resulting in viral DNA replication and release of infectious virus. Reactivation can be induced by depletion of nerve growth factor; other commonly used reactivation stimuli have no significant effect.IMPORTANCE Infections by herpes simplex viruses (HSV) cause painful cold sores or genital lesions in many people; less often, they affect the eye or even the brain. After the initial infection, the virus remains inactive or latent in nerve cells that sense the region where that infection occurred. To learn how virus maintains and reactivates from latency, studies are done in neurons taken from rodents or in whole animals to preserve the full context of infection. However, some cellular mechanisms involved in HSV infection in rodents are different from those in humans. We describe the use of a human cell line that has the properties of a sensory neuron. HSV infection in these cultured cells shows the properties expected for a latent infection, including reactivation to produce newly infectious virus. Thus, we now have a cell culture model for latency that is derived from the normal host for this virus.

Linear Multiepitope (Glyco)peptides for Type-Specific Serology of Herpes Simplex Virus (HSV) Infections

Detection of type-specific antibodies is an important and essential part of accurate diagnosis, even in silent carriers of herpes simplex virus (HSV)-1 (oral) and HSV-2 (genital) infections. Serologic assays that identify HSV-1 and HSV-2 type-specific antibodies have been commercially available for more than a decade but often face problems related to cross-reactivity and similar issues. Attempts to identify type-specific peptide epitopes for use in serology for both HSV-1 and HSV-2 have been limited. We recently demonstrated epitope mapping of envelope glycoprotein G2 and identified a type-specific glycopeptide epitope that broadly recognized HSV-2 infected individuals. In the present work we have performed a comprehensive glycopeptide synthesis and microarray epitope mapping of 14 envelope proteins from HSV-1 and HSV-2, namely, gB, gC, gD, gE, gG, gH, and gI, using sera from HSV-1- and HSV-2-infected individuals and control sera. Several unique type-specific peptide epitopes with high sensitivity were identified and synthesized as one large linear multiepitope sequence using microwave-assisted solid-phase (glyco)peptide synthesis. Microarray validation with clinically defined HSV and Varicella Zoster (VZV) sera confirmed excellent cumulative specificities and sensitivities.

Immune response of T cells during herpes simplex virus type 1 (HSV-1) infection

Herpes simplex virus type 1 (HSV-1), a neurotropic member of the alphaherpes virus family, is among the most prevalent and successful human pathogens. HSV-1 can cause serious diseases at every stage of life including fatal disseminated disease in newborns, cold sores, eye disease, and fatal encephalitis in adults. HSV-1 infection can trigger rapid immune responses, and efficient inhibition and clearance of HSV-1 infection rely on both the innate and adaptive immune responses of the host. Multiple strategies have been used to restrict host innate immune responses by HSV-1 to facilitate its infection in host cells. The adaptive immunity of the host plays an important role in inhibiting HSV-1 infections. The activation and regulation of T cells are the important aspects of the adaptive immunity. They play a crucial role in host-mediated immunity and are important for clearing HSV-1. In this review, we examine the findings on T cell immune responses during HSV-1 infection, which hold promise in the design of new vaccine candidates for HSV-1.

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.

Neutralizing human recombinant antibodies against herpes simplex virus type 1 glycoproteins B from a phage-displayed scFv antibody library

The HSV-1 envelope glycoprotein B (gB) plays a critical role in virus entry into host cells. Neutralizing antibodies can therefore potentially prevent virus entry into target cells and cell-to-cell spread of infection. Our present study focused on the selection of neutralizing single-chain Fv (scFv) antibodies of a phage-displayed nonimmune human scFv antibody library against gB of HSV-1. To enrich specific scFvs, two phage antibodies were isolated against amino acid residues 31–43 derived from the N-terminal part of gB using panning technique. Two scFvs, scFv-gB₁ and scFv-gB₂, with frequencies of 45% and 20% were obtained from scFv clones after performing PCR and MvaI fingerprinting. In phage ELISA analysis, both gB₁ and gB₂ scFvs demonstrated high reactivity with the gB peptide. In the neutralization assay, scFv-gB₁ and scFv-gB₂ represented neutralizing effects of 55% and 59%, respectively. Upon further enhancement of the neutralizing effects of these antibodies, they can be considered as new potential alternatives in the treatment and prophylaxis of HSV-1 infections.

Convergent evolution of twintron-like configurations: One is never enough

Introns inserted within introns are commonly referred to as twintrons, however the original definition for twintron implied that splicing of the external member of the twintron could only proceed upon splicing of the internal member. This review examines the various types of twintron-like arrangements that have been reported and assigns them to either nested or twintron categories that are subdivided further into subtypes based on differences of their mode of splicing. Twintron-like arrangements evolved independently by fortuitous events among different types of introns but once formed they offer opportunities for the evolution of new regulatory strategies and/or novel genetic elements.

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 1 Reactivates from Autonomic Ciliary Ganglia Independently from Sensory Trigeminal Ganglia To Cause Recurrent Ocular Disease

Herpes simplex virus 1 (HSV-1) and HSV-2 establish latency in sensory and autonomic neurons after ocular or genital infection, but their recurrence patterns differ. HSV-1 reactivates from latency to cause recurrent orofacial disease, and while HSV-1 also causes genital lesions, HSV-2 recurs more efficiently in the genital region and rarely causes ocular disease. The mechanisms regulating these anatomical preferences are unclear. To determine whether differences in latent infection and reactivation in autonomic ganglia contribute to differences in HSV-1 and HSV-2 anatomical preferences for recurrent disease, we compared HSV-1 and HSV-2 clinical disease, acute and latent viral loads, and viral gene expression in sensory trigeminal and autonomic superior cervical and ciliary ganglia in a guinea pig ocular infection model. HSV-2 produced more severe acute disease, correlating with higher viral DNA loads in sensory and autonomic ganglia, as well as higher levels of thymidine kinase expression, a marker of productive infection, in autonomic ganglia. HSV-1 reactivated in ciliary ganglia, independently from trigeminal ganglia, to cause more frequent recurrent symptoms, while HSV-2 replicated simultaneously in autonomic and sensory ganglia to cause more persistent disease. While both HSV-1 and HSV-2 expressed the latency-associated transcript (LAT) in the trigeminal and superior cervical ganglia, only HSV-1 expressed LAT in ciliary ganglia, suggesting that HSV-2 is not reactivation competent or does not fully establish latency in ciliary ganglia. Thus, differences in replication and viral gene expression in autonomic ganglia may contribute to differences in HSV-1 and HSV-2 acute and recurrent clinical disease.

Analyses of herpes simplex virus type 1 latency and reactivation at the single cell level using fluorescent reporter mice

Herpes simplex virus type 1 (HSV-1) establishes a latent infection in sensory neurons from which the virus can periodically reactivate. Whilst latency establishment is thought to result from a failure to express immediate-early genes, we have previously shown that subpopulations of the latent neuronal reservoir have undergone lytic promoter activation prior to latency establishment. In the present study, we have investigated the biological properties of such latently infected neuronal subpopulations using Ai6 fluorescent reporter mice. Using this system we have determined that prior ICP0 or TK promoter activation does not correlate with increased latent virus DNA loads within individual cells and that neurons with evidence of historical lytic cycle promoter activity exhibit a comparable frequency of reactivation to that of the general latent cell population. Comparison of viral DNA content within cells harbouring latent HSV-1 genomes and those undergoing the earliest stages of reactivation has revealed that reactivation can initiate from cells harbouring a wide range of HSV-1 genome copies, but that exiting latency is biased towards cells bearing higher latent virus DNA loads.

[Herpes simplex virus type 1 latency and reactivation: an update]

Following primary infections HSV-1 replicates productively in epithelial cells and enters sensory neurons via nerve termini. After retrograde transport the virus genome is delivered into the cell nucleus, where it establishes lifelong latent infections. During latency, the virus genome remains as a chromatinized episome expressing only a set of latency-associated transcripts (LAT) and a group of microRNAs that inhibit expression of key lytic viral functions. Periodically the virus can reactivate to reinitiate lytic, secondary infections at peripheral tissues. The ability to establish both lytic and latent infections relies on the coexistence in the virus genome of two alternative gene expression programs, under the control of epigenetic mechanisms. Latency is an adaptive phenotype that allows the virus to escape immune host responses and to reactivate and disseminate to other hosts upon recognizing danger signals such as stress, neurologic trauma or growth factor deprivation.

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.

Animal models of herpes simplex virus immunity and pathogenesis

Herpes simplex viruses are ubiquitous human pathogens represented by two distinct serotypes: herpes simplex virus (HSV) type 1 (HSV-1); and HSV type 2 (HSV-2). In the general population, adult seropositivity rates approach 90% for HSV-1 and 20-25% for HSV-2. These viruses cause significant morbidity, primarily as mucosal membrane lesions in the form of facial cold sores and genital ulcers, with much less common but more severe manifestations causing death from encephalitis. HSV infections in humans are difficult to study in many cases because many primary infections are asymptomatic. Moreover, the neurotropic properties of HSV make it much more difficult to study the immune mechanisms controlling reactivation of latent infection within the corresponding sensory ganglia and crossover into the central nervous system of infected humans. This is because samples from the nervous system can only be routinely obtained at the time of autopsy. Thus, animal models have been developed whose use has led to a better understanding of multiple aspects of HSV biology, molecular biology, pathogenesis, disease, and immunity. The course of HSV infection in a spectrum of animal models depends on important experimental parameters including animal species, age, and genotype; route of infection; and viral serotype, strain, and dose. This review summarizes the animal models most commonly used to study HSV pathogenesis and its establishment, maintenance, and reactivation from latency. It focuses particularly on the immune response to HSV during acute primary infection and the initial invasion of the ganglion with comparisons to the events governing maintenance of viral latency.

A role for H/ACA and C/D small nucleolar RNAs in viral replication

We have employed gene-trap insertional mutagenesis to identify candidate genes whose disruption confer phenotypic resistance to lytic infection, in independent studies using 12 distinct viruses and several different cell lines. Analysis of >2,000 virus-resistant clones revealed >1,000 candidate host genes, approximately 20 % of which were disrupted in clones surviving separate infections with 2–6 viruses. Interestingly, there were 83 instances in which the insertional mutagenesis vector disrupted transcripts encoding H/ACA-class and C/D-class small nucleolar RNAs (SNORAs and SNORDs, respectively). Of these, 79 SNORAs and SNORDs reside within introns of 29 genes (predominantly protein-coding), while 4 appear to be independent transcription units. siRNA studies targeting candidate SNORA/Ds provided independent confirmation of their roles in infection when tested against cowpox virus, Dengue Fever virus, influenza A virus, human rhinovirus 16, herpes simplex virus 2, or respiratory syncytial virus. Significantly, eight of the nine SNORA/Ds targeted with siRNAs enhanced cellular resistance to multiple viruses suggesting widespread involvement of SNORA/Ds in virus–host interactions and/or virus-induced cell death.

Lytic gene expression is frequent in HSV-1 latent infection and correlates with the engagement of a cell-intrinsic transcriptional response

Herpes simplex viruses (HSV) are significant human pathogens that provide one of the best-described examples of viral latency and reactivation. HSV latency occurs in sensory neurons, being characterized by the absence of virus replication and only fragmentary evidence of protein production. In mouse models, HSV latency is especially stable but the detection of some lytic gene transcription and the ongoing presence of activated immune cells in latent ganglia have been used to suggest that this state is not entirely quiescent. Alternatively, these findings can be interpreted as signs of a low, but constant level of abortive reactivation punctuating otherwise silent latency. Using single cell analysis of transcription in mouse dorsal root ganglia, we reveal that HSV-1 latency is highly dynamic in the majority of neurons. Specifically, transcription from areas of the HSV genome associated with at least one viral lytic gene occurs in nearly two thirds of latently-infected neurons and more than half of these have RNA from more than one lytic gene locus. Further, bioinformatics analyses of host transcription showed that progressive appearance of these lytic transcripts correlated with alterations in expression of cellular genes. These data show for the first time that transcription consistent with lytic gene expression is a frequent event, taking place in the majority of HSV latently-infected neurons. Furthermore, this transcription is of biological significance in that it influences host gene expression. We suggest that the maintenance of HSV latency involves an active host response to frequent viral activity.

Primary herpes simplex virus (HSV) infections are characterized by acute disease that resolves rapidly, but the virus persists in a latent form in sensory neurons that can be a source of renewed disease. Analyzing gene expression in single mouse neurons harboring latent HSV, we show directly that HSV latency is dynamic and heterogeneous. HSV lytic gene transcripts were frequently detected in latently infected neurons and often in combinations. Expression of selected cellular anti-viral and survival genes showed that transcriptional profiles differed between latently infected and uninfected neurons from the same ganglia. The pattern of host gene expression also differed between latently infected neurons that were and were not experiencing HSV lytic gene expression. Our study suggests that HSV latency is characterized by very frequent switching on of lytic genes and a rapid response by the host, presumably to halt progression to reactivation.

Virus infections in the nervous system

Virus infections usually begin in peripheral tissues and can invade the mammalian nervous system (NS), spreading into the peripheral (PNS) and more rarely the central (CNS) nervous systems. The CNS is protected from most virus infections by effective immune responses and multilayer barriers. However, some viruses enter the NS with high efficiency via the bloodstream or by directly infecting nerves that innervate peripheral tissues, resulting in debilitating direct and immune-mediated pathology. Most viruses in the NS are opportunistic or accidental pathogens, but a few, most notably the alpha herpesviruses and rabies virus, have evolved to enter the NS efficiently and exploit neuronal cell biology. Remarkably, the alpha herpesviruses can establish quiescent infections in the PNS, with rare but often fatal CNS pathology. Here we review how viruses gain access to and spread in the well-protected CNS, with particular emphasis on alpha herpesviruses, which establish and maintain persistent NS infections.

Complexity of Interferon-γ Interactions with HSV-1

The intricacies involving the role of interferon-gamma (IFN-γ) in herpesvirus infection and persistence are complex. Herpes simplex virus type 1 (HSV-1) uses a variety of receptors to enter cells and is transported to and from the host cell nucleus over the microtubule railroad via retrograde and anterograde transport. IFN-γ exerts dual but conflicting effects on microtubule organization. IFN-γ stimulates production of suppressors of cytokine signaling 1 and 3 (SOCS1 and SOCS3), which are involved in microtubule stability and are negative regulators of IFN-γ signaling when overexpressed. IFN-γ also interferes with the correct assembly of microtubules causing them to undergo severe bundling, contributing to its anti-viral effect. Factors leading to the decision for a replicative virus lytic cycle or latency in the trigeminal ganglion (TG) occur on histone 3 (H3), involve IFN-γ produced by natural killer cells and non-cytolytic CD8⁺T cells, SOCS1, SOCS3, and M2 anti-inflammatory microglia/macrophages maintained by inhibitory interleukin 10 (IL-10). Both M2 microglia and CD4⁺CD25⁺Foxp3⁺ Treg cells produce IL-10. Histone deacetylases (HDACs) are epigenetic regulators maintaining chromatin in an inactive state necessary for transcription of IFN-γ-activated genes and their anti-viral effect. Following inhibition of HDACs by stressors such as ultraviolet light, SOCS1 and SOCS3 are acetylated, and chromatin is relaxed and available for virus replication. SOCS1 prevents expression of MHC class 1 molecules on neuronal cells and SOCS3 attenuates cytokine-induced inflammation in the area. A model is presented to unify the effects of IFN-γ, SOCS1, SOCS3, and HSV-1 on H3 and chromatin structure in virus latency or reactivation. HSV-1 latency in the TG is viewed as an active ongoing process involving maintenance of microglia in an M2 anti-inflammatory state by IL-10. IL-10 is produced in an autocrine manner by the M2 microglia/macrophages and by virus-specific CD4⁺Foxp3⁺ Treg cells interacting with virus-specific non-cytolytic CD8⁺ T cells.

Detection of the genome and transcripts of a persistent DNA virus in neuronal tissues by fluorescent in situ hybridization combined with immunostaining

Single cell codetection of a gene, its RNA product and cellular regulatory proteins is critical to study gene expression regulation. This is a challenge in the field of virology; in particular for nuclear-replicating persistent DNA viruses that involve animal models for their study. Herpes simplex virus type 1 (HSV-1) establishes a life-long latent infection in peripheral neurons. Latent virus serves as reservoir, from which it reactivates and induces a new herpetic episode. The cell biology of HSV-1 latency remains poorly understood, in part due to the lack of methods to detect HSV-1 genomes in situ in animal models. We describe a DNA-fluorescent in situ hybridization (FISH) approach efficiently detecting low-copy viral genomes within sections of neuronal tissues from infected animal models. The method relies on heat-based antigen unmasking, and directly labeled home-made DNA probes, or commercially available probes. We developed a triple staining approach, combining DNA-FISH with RNA-FISH and immunofluorescence, using peroxidase based signal amplification to accommodate each staining requirement. A major improvement is the ability to obtain, within 10 µm tissue sections, low-background signals that can be imaged at high resolution by confocal microscopy and wide-field conventional epifluorescence. Additionally, the triple staining worked with a wide range of antibodies directed against cellular and viral proteins. The complete protocol takes 2.5 days to accommodate antibody and probe penetration within the tissue.