Herpes simplex virus type 1 DNA replication and gene expression during explant-induced reactivation of latently infected murine sensory ganglia

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.

HSV-1 LAT Promoter Deletion Viruses Exhibit Strain-Specific and LAT-Dependent Epigenetic Regulation of Latent Viral Genomes in Human Neurons

Herpes simplex virus 1 (HSV-1) establishes latency in neurons and expresses long noncoding RNAs termed the latency-associated transcripts (LATs). Two previous studies using HSV-1 recombinants containing deletions in the LAT promoter revealed opposing effects of the promoter deletion regarding the heterochromatinization of latent viral genomes in mice ganglia. Confounding variables in these studies include viral strains utilized (17syn+ versus KOS), anatomical infection site (footpad versus eye) and infectious virus dose (500 versus 1 × 105 PFU), and to date the basis for the differences between the two studies remains unresolved. We recently reported that 17syn+ and KOS display distinct differences in heterochromatin levels during latency in human neurons. This raised the possibility that the discrepancy seen between the two previous studies could be explained by strain-specific differences within the LAT region. Here, we examine two recombinants containing orthologous 202 bp LAT promoter deletions, 17ΔPst and KOSΔPst, in a human neuronal model of latency and reactivation (LUHMES). We found that LUHMES neurons recapitulate previous observations in mice where deletion of the LAT promoter results in an increase in H3K27me3 deposition on the viral genome compared to the parental strain 17syn+ but a decrease compared to the parental strain KOS. We also found distinct strain-specific differences in the production of viral transcripts and proteins during latency. These results indicate that the function and/or regulation of the LATs differs between HSV-1 strains and may shed light on some discrepancies found in the literature when examining the function of the LATs. IMPORTANCE Herpes simplex virus 1 (HSV-1) establishes a lifelong infection in neuronal cells. Periodically, the virus reactivates from this latent state and causes recurrent disease. Mechanisms that control entry into and maintenance of latency are not well understood, though epigenetic posttranslational modification of histones associated with the viral genome are known to play an important role. During latency, the latency-associated transcript (LAT) is known to impact epigenetic marks, but the ultimate effect has been a point of controversy. Here, we utilize a human neuronal cell line model of HSV latency and reactivation (LUHMES) to characterize latency for two HSV-1 wild-type strains and their respective LAT promoter deletion viruses. We find that the LAT acts in a strain-specific manner to influence levels of chromatin marks, viral transcription, and viral protein production. This work highlights the need to account for strain-specific differences when characterizing the LAT's function and the dynamics of reactivation.

Herpes Simplex Virus 1 MicroRNA miR-H8 Is Dispensable for Latency and Reactivation In Vivo

The regulatory functions of 10 individual viral microRNAs (miRNAs) that are abundantly expressed from the herpes simplex virus 1 (HSV-1) latency-associated transcript (LAT) region remain largely unknown. Here, we focus on HSV-1 miRNA miR-H8, which is within the LAT 3p exon, antisense to the first intron of ICP0, and has previously been shown to target a host glycosylphosphatidylinositol (GPI)-anchoring pathway. However, the functions of this miRNA have not been assessed in the context of the viral genome during infection. Therefore, we constructed a recombinant virus lacking miR-H8 (17dmiR-H8) and compared it to the parental wild-type and rescue viruses to characterize phenotypic differences. In rabbit skin cells, 17dmiR-H8 exhibited only subtle reductions in viral yields. In contrast, we found significant decreases in both viral yields (8-fold) and DNA replication (9.9-fold) in murine neuroblastoma cells, while 17dmiR-H8 exhibited a 3.6-fold increase in DNA replication in differentiated human neuronal cells (Lund human mesencephalic [LUHMES] cells). These cell culture phenotypes suggested potential host- and/or neuron-specific roles for miR-H8 in acute viral replication. To assess whether miR-H8 plays a role in HSV latency or reactivation, we used a human in vitro reactivation model as well as mouse and rabbit reactivation models. In the LUHMES cell-induced reactivation model, there was no difference in viral yields at 48 h postreactivation. In the murine dorsal root ganglion explant and rabbit ocular adrenergic reactivation models, the deletion of miR-H8 had no detectable effect on genome loads during latency or reactivation. These results indicate that miR-H8 is dispensable for the establishment of HSV-1 latency and reactivation.IMPORTANCE Herpesviruses have a remarkable ability to sustain lifelong infections by evading host immune responses, establishing a latent reservoir, and maintaining the ability to reactivate the lytic cascade to transmit the virus to the next host. The HSV-1 latency-associated transcript region is known to regulate many aspects of HSV-1 latency and reactivation, although the mechanisms for these functions remain unknown. To this end, we characterize an HSV-1 recombinant containing a deletion of a LAT-encoded miRNA, miR-H8, and demonstrate that it plays no detectable role in the establishment of latency or reactivation in differentiated human neurons (LUHMES cells) and mouse and rabbit models. Therefore, this study allows us to exclude miR-H8 from phenotypes previously attributed to the LAT region. Elucidating the genetic elements of HSV-1 responsible for establishment, maintenance, and reactivation from latency may lead to novel strategies for combating persistent herpesvirus infections.

The latency-associated transcript locus of herpes simplex virus 1 is a virulence determinant in human skin

Herpes simplex virus 1 (HSV-1) infects skin and mucosal epithelial cells and then travels along axons to establish latency in the neurones of sensory ganglia. Although viral gene expression is restricted during latency, the latency-associated transcript (LAT) locus encodes many RNAs, including a 2 kb intron known as the hallmark of HSV-1 latency. Here, we studied HSV-1 infection and the role of the LAT locus in human skin xenografts in vivo and in cultured explants. We sequenced the genomes of our stock of HSV-1 strain 17syn⁺ and seven derived viruses and found nonsynonymous mutations in many viral proteins that had no impact on skin infection. In contrast, deletions in the LAT locus severely impaired HSV-1 replication and lesion formation in skin. However, skin replication was not affected by impaired intron splicing. Moreover, although the LAT locus has been implicated in regulating gene expression in neurones, we observed only small changes in transcript levels that were unrelated to the growth defect in skin, suggesting that its functions in skin may be different from those in neurones. Thus, although the LAT locus was previously thought to be dispensable for lytic infection, we show that it is a determinant of HSV-1 virulence during lytic infection of human skin.

Herpes simplex virus type 1 (HSV-1) infects and destroys the outer layer of skin cells, producing lesions known as cold sores. Although these lesions heal, the virus persists in the host for the lifetime and can reactivate to cause new lesions. This is possible because the virus enters the axons of neurones in the skin and moves to their cell bodies located in spinal or cranial nerve bundles called ganglia, where the virus becomes dormant (latent). The most abundant viral RNAs expressed during this state are the latency associated transcripts (LATs), which have been considered a hallmark of HSV-1 latency. Here, we studied HSV-1 infection and spread in human skin. Unexpectedly, we found that the LAT locus is necessary for lesion formation in skin. HSV-1 viruses that were genetically mutated to delete the start of the locus could not spread in skin, whereas viruses with many other genetic mutations had this capacity. Our results suggest that an antiviral drug that inhibits transcripts from this region of the viral genome could block viral spread in skin, or a vaccine could possibly be produced by genetically modifying the virus at the LAT locus and by doing so, limit the virus’ ability become latent in neurones.

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.

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

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

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.

R430: A potent inhibitor of DNA and RNA viruses

Acyclovir (ACV) is an effective antiviral agent for treating lytic Herpes Simplex virus, type 1 (HSV-1) infections, and it has dramatically reduced the mortality rate of herpes simplex encephalitis. However, HSV-1 resistance to ACV and its derivatives is being increasingly documented, particularly among immunocompromised individuals. The burgeoning drug resistance compels the search for a new generation of more efficacious anti-herpetic drugs. We have previously shown that trans-dihydrolycoricidine (R430), a lycorane-type alkaloid derivative, effectively inhibits HSV-1 infections in cultured cells. We now report that R430 also inhibits ACV-resistant HSV-1 strains, accompanied by global inhibition of viral gene transcription and enrichment of H3K27me3 methylation on viral gene promoters. Furthermore, we demonstrate that R430 prevents HSV-1 reactivation from latency in an ex vivo rodent model. Finally, among a panel of DNA viruses and RNA viruses, R430 inhibited Zika virus with high therapeutic index. Its therapeutic index is comparable to standard antiviral drugs, though it has greater toxicity in non-neuronal cells than in neuronal cells. Synthesis of additional derivatives could enable more efficacious antivirals and the identification of active pharmacophores.

Herpes Simplex Virus 2 Latency-Associated Transcript (LAT) Region Mutations Do Not Identify a Role for LAT-Associated MicroRNAs in Viral Reactivation in Guinea Pig Genital Models

Despite the long-standing observation that herpes simplex virus (HSV) latency-associated transcript (LAT) promoter deletion viruses show impaired recurrence phenotypes in relevant animal models, the mechanism by which these sequences exert this phenotypic effect is unknown. We constructed and evaluated four mutant HSV-2 isolates with targeted mutations in the LAT promoter and LAT-associated microRNAs (miRNAs) affecting (i) the LAT TATA box; (ii) the LAT ICP4-binding site; (iii) miRNA I (miR-I) and miR-II (miR-I/II), which both target ICP34.5; and (iv) miR-III, which targets ICP0. While the LAT TATA box mutant caused milder acute infections than wild-type (WT) virus, there was no difference in the recurrence phenotype between these viruses. LAT and miRNA expression during latency was not impaired by this mutation, suggesting that other promoter elements may be more important for latent HSV-2 LAT expression. Mutation of the LAT ICP4-binding site also did not cause an in vivo phenotypic difference between mutant and WT viruses. Acute infection and reactivation from latency of the miR-I/II mutant were similar to those of its rescuant, although the acute infection was slightly reduced in severity relative to that caused by the wild-type virus. The miR-III mutant also exhibited WT phenotypes in acute and recurrent phases of infection. While they do not rule out an effect of these elements in human latency or reactivation, these findings do not identify a specific role for LAT or LAT-associated miRNAs in the HSV-2 LAT promoter deletion phenotype in guinea pigs. Thus, other sequences in this region may play a more important role in the long-studied LAT-associated phenotype in animals.IMPORTANCE While it has been known for several decades that specific HSV-1 and HSV-2 sequences near the LAT promoter are required for efficient viral reactivation in animal models, the mechanism is still not known. We constructed four mutant viruses with the goal of identifying critical sequence elements and of specifically testing the hypothesis that microRNAs that are expressed during latency play a role. Determination that specific LAT promoter sequences and miRNA sequences do not influence viral reactivation of HSV-2 helps to narrow down the search for the mechanism by which the virus controls its latency and recurrence phenotype.

CTCF Binding Sites in the Herpes Simplex Virus 1 Genome Display Site-Specific CTCF Occupation, Protein Recruitment, and Insulator Function

There are seven conserved CTCF binding domains in the herpes simplex virus 1 (HSV-1) genome. These binding sites individually flank the latency-associated transcript (LAT) and the immediate early (IE) gene regions, suggesting that CTCF insulators differentially control transcriptional domains in HSV-1 latency. In this work, we show that two CTCF binding motifs in HSV-1 display enhancer blocking in a cell-type-specific manner. We found that CTCF binding to the latent HSV-1 genome was LAT dependent and that the quantity of bound CTCF was site specific. Following reactivation, CTCF eviction was dynamic, suggesting that each CTCF site was independently regulated. We explored whether CTCF sites recruit the polycomb-repressive complex 2 (PRC2) to establish repressive domains through a CTCF-Suz12 interaction and found that Suz12 colocalized to the CTCF insulators flanking the ICP0 and ICP4 regions and, conversely, was removed at early times postreactivation. Collectively, these data support the idea that CTCF sites in HSV-1 are independently regulated and may contribute to lytic-latent HSV-1 control in a site-specific manner.IMPORTANCE The role of chromatin insulators in DNA viruses is an area of interest. It has been shown in several beta- and gammaherpesviruses that insulators likely control the lytic transcriptional profile through protein recruitment and through the formation of three-dimensional (3D) chromatin loops. The ability of insulators to regulate alphaherpesviruses has been understudied to date. The alphaherpesvirus HSV-1 has seven conserved insulator binding motifs that flank regions of the genome known to contribute to the establishment of latency. Our work presented here contributes to the understanding of how insulators control transcription of HSV-1.

CCCTC-Binding Factor Acts as a Heterochromatin Barrier on Herpes Simplex Viral Latent Chromatin and Contributes to Poised Latent Infection

Herpes simplex virus 1 (HSV-1) establishes latent infection in neurons via a variety of epigenetic mechanisms that silence its genome. The cellular CCCTC-binding factor (CTCF) functions as a mediator of transcriptional control and chromatin organization and has binding sites in the HSV-1 genome. We constructed an HSV-1 deletion mutant that lacked a pair of CTCF-binding sites (CTRL2) within the latency-associated transcript (LAT) coding sequences and found that loss of these CTCF-binding sites did not alter lytic replication or levels of establishment of latent infection, but their deletion reduced the ability of the virus to reactivate from latent infection. We also observed increased heterochromatin modifications on viral chromatin over the LAT promoter and intron. We therefore propose that CTCF binding at the CTRL2 sites acts as a chromatin insulator to keep viral chromatin in a form that is poised for reactivation, a state which we call poised latency.

Herpes simplex virus 1 (HSV-1) is a human pathogen that persists for the lifetime of the host as a result of its ability to establish latent infection within sensory neurons. The mechanism by which HSV-1 transitions from the lytic to latent infection program is largely unknown; however, HSV-1 is able to coopt cellular silencing mechanisms to facilitate the suppression of lytic gene expression. Here, we demonstrate that the cellular CCCTC-binding factor (CTCF)-binding site within the latency associated transcript (LAT) region is critical for the maintenance of a specific local chromatin structure. Additionally, loss of CTCF binding has detrimental effects on the ability to reactivate from latent infection. These results argue that CTCF plays a critical role in epigenetic regulation of viral gene expression to establish and/or maintain a form of latent infection that can reactivate efficiently.

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.

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.

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.

History and genomic sequence analysis of the herpes simplex virus 1 KOS and KOS1.1 sub-strains

A collection of genomic DNA sequences of herpes simplex virus (HSV) strains has been defined and analyzed, and some information is available about genomic stability upon limited passage of viruses in culture. The nature of genomic change upon extensive laboratory passage remains to be determined. In this report we review the history of the HSV-1 KOS laboratory strain and the related KOS1.1 laboratory sub-strain, also called KOS (M), and determine the complete genomic sequence of an early passage stock of the KOS laboratory sub-strain and a laboratory stock of the KOS1.1 sub-strain. The genomes of the two sub-strains are highly similar with only five coding changes, 20 non-coding changes, and about twenty non-ORF sequence changes. The coding changes could potentially explain the KOS1.1 phenotypic properties of increased replication at high temperature and reduced neuroinvasiveness. The study also provides sequence markers to define the provenance of specific laboratory KOS virus stocks.

Increased neurovirulence and reactivation of the herpes simplex virus type 1 latency-associated transcript (LAT)-negative mutant dLAT2903 with a disrupted LAT miR-H2

At least six microRNAs (miRNAs) appear to be encoded by the latency-associated transcript (LAT) of herpes simplex virus type 1 (HSV-1). The gene for ICP0, an important immediate early (IE) viral protein, is anti-sense to, and overlaps with, the region of LAT from which miRNA H2 (miR-H2) is derived. We recently reported that a mutant (McK-ΔH2) disrupted for miR-H2 on the wild-type HSV-1 strain McKrae genomic background has increased ICP0 expression, increased neurovirulence, and slightly more rapid reactivation. We report here that HSV-1 mutants deleted for the LAT promoter nonetheless make significant amounts of miR-H2 during lytic tissue culture infection, presumably via readthrough transcription from an upstream promoter. To determine if miR-H2 might also play a role in the HSV-1 latency/reactivation cycle of a LAT-negative mutant, we constructed dLAT-ΔH2, in which miR-H2 is disrupted in dLAT2903 without altering the predicted amino acid sequence of the overlapping ICP0 open reading frame. Similar to McK-ΔH2, dLAT-ΔH2 expressed more ICP0, was more neurovirulent, and had increased reactivation in the mouse TG explant-induced reactivation model of HSV-1 compared with its parental virus. Interestingly, although the increased reactivation of McK-ΔH2 compared with its parental wild-type (wt) virus was subtle and only detected at very early times after explant TG induced reactivation, the increased reactivation of dLAT-ΔH2 compared with its dLAT2903 parental virus appeared more robust and was significantly increased even at late times after induction. These results confirm that miR-H2 plays a role in modulating the HSV-1 reactivation phenotype.

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.

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.

CTCF occupation of the herpes simplex virus 1 genome is disrupted at early times postreactivation in a transcription-dependent manner

In herpes simplex virus 1 (HSV-1), binding clusters enriched in CTCF during latency have been previously identified. We hypothesized that CTCF binding to CTCF clusters in HSV-1 would be disrupted in a reactivation event. To investigate, CTCF occupation of three CTCF binding clusters in HSV-1 was analyzed following sodium butyrate (NaB)- and explant-induced reactivation in the mouse. Our data show that the CTCF domains positioned within the HSV-1 genome, specifically around the latency-associated transcript (LAT) and ICP0 and ICP4 regions of the genome, lose CTCF occupancy following the application of reactivation stimuli in wild-type virus. We also found that CTCF binding clusters upstream of the ICP0 and ICP4 promoters both function as classical insulators capable of acting as enhancer blockers of the LAT enhancer. Finally, our results suggest that CTCF occupation of domains in HSV-1 may be differentially regulated both during latency and at early times following reactivation by the presence of lytic transcripts and further implicate epigenetic regulation of HSV-1 as a critical component of the latency-reactivation transition.

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.

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.

Numerous conserved and divergent microRNAs expressed by herpes simplex viruses 1 and 2

Certain viruses use microRNAs (miRNAs) to regulate the expression of their own genes, host genes, or both. Previous studies have identified a limited number of miRNAs expressed by herpes simplex viruses 1 and 2 (HSV-1 and -2), some of which are conserved between these two viruses. To more comprehensively analyze the miRNAs expressed by HSV-1 or HSV-2 during productive and latent infection, we applied a massively parallel sequencing approach. We were able to identify 16 and 17 miRNAs expressed by HSV-1 and HSV-2, respectively, including all previously known species, and a number of previously unidentified virus-encoded miRNAs. The genomic positions of most miRNAs encoded by these two viruses are within or proximal to the latency-associated transcript region. Nine miRNAs are conserved in position and/or sequence, particularly in the seed region, between these two viruses. Interestingly, we did not detect an HSV-2 miRNA homolog of HSV-1 miR-H1, which is highly expressed during productive infection, but we did detect abundant expression of miR-H6, whose seed region is conserved with HSV-1 miR-H1 and might represent a functional analog. We also identified a highly conserved miRNA family arising from the viral origins of replication. In addition, we detected several pairs of complementary miRNAs and we found miRNA-offset RNAs (moRs) arising from the precursors of HSV-1 and HSV-2 miR-H6 and HSV-2 miR-H4. Our results reveal elements of miRNA conservation and divergence that should aid in identifying miRNA functions.

Towards an understanding of the herpes simplex virus type 1 latency-reactivation cycle

Infection by herpes simplex virus type 1 (HSV-1) can cause clinical symptoms in the peripheral and central nervous system. Recurrent ocular shedding can lead to corneal scarring and vision loss making HSV-1 a leading cause of corneal blindness due to an infectious agent. The primary site of HSV-1 latency is sensory neurons within trigeminal ganglia. Periodically, reactivation from latency occurs resulting in virus transmission and recurrent disease. During latency, the latency-associated transcript (LAT) is abundantly expressed. LAT expression is important for the latency-reactivation cycle in animal models, in part, because it inhibits apoptosis, viral gene expression, and productive infection. A novel transcript within LAT coding sequences (AL3) and small nonprotein coding RNAs are also expressed in trigeminal ganglia of latently infected mice. In this review, an update of viral factors that are expressed during latency and their potential roles in regulating the latency-reactivation cycle is discussed.

Epigenetic regulation of latent HSV-1 gene expression

Like other alpha-herpesviruses, Herpes Simplex Virus Type 1 (HSV-1) possesses the ability to establish latency in sensory ganglia as a non-integrated, nucleosome-associated episome in the host cell nucleus. Transcription of the genome is limited to the Latency-Associated Transcript (LAT), while the lytic genes are maintained in a transcriptionally repressed state. This partitioning of the genome into areas of active and inactive transcription suggests epigenetic control of HSV-1 latent gene expression. During latency viral transcription is not regulated by DNA methylation but likely by post-translational histone modifications. The LAT region is the only region of the genome enriched in marks indicative of transcriptional permissiveness, specifically dimethyl H3 K4 and acetyl H3 K9, K14, while the lytic genes appear under-enriched in those same marks. In addition, facultative heterochromatin marks, specifically trimethyl H3 K27 and the histone variant macroH2A, are enriched on lytic genes during latency. The distinct epigenetic domains of the LAT and the lytic genes appear to be separated by chromatin insulators. Binding of CTCF, a protein that binds to all known vertebrate insulators, to sites within the HSV-1 genome likely prevents heterochromatic spreading and blocks enhancer activity. When the latent viral genome undergoes stress-induced reactivation, it is possible that CTCF binding and insulator function are abrogated, enabling lytic gene transcription to ensue. In this review we summarize our current understanding of latent HSV-1 epigenetic regulation as it pertains to infections in both the rabbit and mouse models. CTCF insulator function and regulation of histone tail modifications will be discussed. We will also present a current model of how the latent genome is carefully controlled at the epigenetic level and how stress-induced changes to it may trigger reactivation.

Latency-associated transcript (LAT) exon 1 controls herpes simplex virus species-specific phenotypes: reactivation in the guinea pig genital model and neuron subtype-specific latent expression of LAT

Herpes simplex virus 1 (HSV-1) and HSV-2 cause similar acute infections but differ in their abilities to reactivate from trigeminal and lumbosacral dorsal root ganglia. During latency, HSV-1 and HSV-2 also preferentially express their latency-associated transcripts (LATs) in different sensory neuronal subtypes that are positive for A5 and KH10 markers, respectively. Chimeric virus studies showed that LAT region sequences influence both of these viral species-specific phenotypes. To further map the LAT region sequences responsible for these phenotypes, we constructed the chimeric virus HSV2-LAT-E1, in which exon 1 (from the LAT TATA to the intron splice site) was replaced by the corresponding sequence from HSV-1 LAT. In intravaginally infected guinea pigs, HSV2-LAT-E1 reactivated inefficiently relative to the efficiency of its rescuant and wild-type HSV-2, but it yielded similar levels of viral DNA, LAT, and ICP0 during acute and latent infection. HSV2-LAT-E1 preferentially expressed the LAT in A5+ neurons (as does HSV-1), while the chimeric viruses HSV2-LAT-P1 (LAT promoter swap) and HSV2-LAT-S1 (LAT sequence swap downstream of the promoter) exhibited neuron subtype-specific latent LAT expression phenotypes more similar to that of HSV-2 than that of HSV-1. Rescuant viruses displayed the wild-type HSV-2 phenotypes of efficient reactivation in the guinea pig genital model and a tendency to express LAT in KH10+ neurons. The region that is critical for HSV species-specific differences in latency and reactivation thus lies between the LAT TATA and the intron splice site, and minor differences in the 5' ends of chimeric sequences in HSV2-LAT-E1 and HSV2-LAT-S1 point to sequences immediately downstream of the LAT TATA.

The polycomb group protein Bmi1 binds to the herpes simplex virus 1 latent genome and maintains repressive histone marks during latency

The mechanism by which herpes simplex virus 1 (HSV-1) establishes latency in sensory neurons is largely unknown. Recent studies indicate that epigenetic modifications of the chromatin associated with the latent genome may play a key role in the transcriptional control of lytic genes during latency. In this study, we found both constitutive and facultative types of heterochromatin to be present on the latent HSV-1 genome. Deposition of the facultative marks trimethyl H3K27 and histone variant macroH2A varied at different sites on the genome, whereas the constitutive marker trimethyl H3K9 did not. In addition, we show that in the absence of the latency-associated transcript (LAT), the latent genome shows a dramatic increase in trimethyl H3K27, suggesting that expression of the LAT during latency may act to promote an appropriate heterochromatic state that represses lytic genes but is still poised for reactivation. Due to the presence of the mark trimethyl H3K27, we examined whether Polycomb group proteins, which methylate H3K27, were present on the HSV-1 genome during latency. Our data indicate that Bmi1, a member of the Polycomb repressive complex 1 (PRC1) maintenance complex, associates with specific sites in the genome, with the highest level of enrichment at the LAT enhancer. To our knowledge, these are the first data demonstrating that a virus can repress its gene transcription to enter latency by exploiting the mechanism of Polycomb-mediated repression.

Investigation of the mechanism by which herpes simplex virus type 1 LAT sequences modulate preferential establishment of latent infection in mouse trigeminal ganglia

We previously demonstrated that herpes simplex virus type 1 (HSV-1) preferentially establishes latent infection in monoclonal antibody (MAb) A5-positive ganglionic neurons and that a 2.8-kb portion of the HSV-1 genome, corresponding to the 5' end of the LAT (latency-associated transcript) coding region, is responsible for this phenotype (38, 65). In the current study we carried out further genetic mapping of this latency phenotype and investigated some of the mechanisms that might be responsible. Studies with the chimeric virus HSV-1 17syn+/LAT2, an HSV-1 virus engineered to express HSV-2 LAT, demonstrated that this virus exhibited an HSV-2 latency phenotype, preferentially establishing latency in MAb KH10-positive neurons. This result is complementary to that previously described for the chimeric virus HSV-2 333/LAT1 and indicate that the HSV-1 latency phenotype can be changed to that of HSV-2 by substitution of a 2.8-kb piece of complementary viral DNA. Sequential studies in which we evaluated the pattern of HSV-1 latent infection of the mouse trigeminal ganglion following ocular inoculation with viruses with deletions of functional thymidine kinase, glycoprotein E, ICP0, and US9 protein demonstrate that preferential establishment of HSV-1 latent infection in A5-positive neurons is not a consequence of (i) differential access of HSV-1 to A5-positive neurons,(ii) differential cell-to-cell spread of HSV-1 to A5-positive neurons, (iii) differential "round-trip" spread of HSV-1 to A5-positive neurons, or (iv) expression of ICP0. Additional mapping studies with the HSV-1 LAT deletion viruses dLAT371, 17DeltaSty, and 17Delta348 indicate that most of the LAT 5' exon is not required for HSV-1 to preferentially establish latent infection in A5-positive neurons.

Impairment in reactivation of a latency associated transcript (LAT)-deficient HSV-2 is not solely dependent on the latent viral load or the number of CD8(+) T cells infiltrating the ganglia

The HSV latency-associated transcript (LAT) is abundantly expressed during virus latency. Previous studies have shown that the latent viral load and CD8(+) T cells in ganglia influence the rate of reactivation of HSV. While LAT is important for efficient reactivation and establishment of latency, it is uncertain how LAT affects either the HSV latent viral load or CD8(+) T cell infiltration of ganglia. We infected mice with LAT-deficient or LAT-restored HSV-2 at a wide range of inocula. We found that the reduced rate of spontaneous ex-vivo reactivation of the LAT-deficient virus was not associated with a higher number of CD8(+) T cells in the ganglia. Reactivation rates were lower for LAT-deficient than LAT restored HSV-2 even when the latent viral loads were similar, indicating that differences in reactivation were not solely dependent on the latent viral load. Therefore, LAT likely has additional functions important for reactivation.

Heat-induced reactivation of HSV-1 in latent mice: upregulation in the TG of CD83 and other immune response genes and their LAT-ICP0 locus

PURPOSE

To determine changes in host gene expression in HSV-1 latent trigeminal ganglia (TG) after hyperthermic stress.

METHODS

Scarified corneas of 6-week-old female BALB/c mice were inoculated with either HSV-1 17Syn(+) (high phenotypic reactivator) or 17DeltaPst(LAT(-)) (low phenotypic reactivator) at 10(4) plaque-forming units/eye. At 28 days after infection, viral reactivation was induced in some of the infected mice with hyperthermic stress, and the mice were killed after 1 hour. Heat-treated uninfected mice served as the control. Labeled cRNA derived from TG-isolated total RNA was hybridized to 430 2.0 chips containing 14,000 mouse genes. Gene expression was confirmed by quantitative real-time PCR.

RESULTS

There was no difference in gene expression in the non-heat-treated mice. Gene expression in the TG of each of the heat-treated mouse groups (17Syn(+), 17DeltaPst(LAT(-)) and uninfected) yielded upregulation of more than twofold of a group of the same genes, designated as heat stress-induced gene expression. Twenty-nine genes (0.2%) were significantly upregulated (2- to 17-fold) when the heat stress-induced gene expression was subtracted from the gene expression of 17Syn(+) latent TG relative to 17DeltaPst(LAT(-)) latent TG 1 hour after mouse hyperthermic stress. Nine host adaptive immunity genes comprising Ig molecules, CD83, CD8A, ADA, and CCL8 were the largest subset upregulated, and all were confirmed by real-time PCR. Others identified included genes involved in hypothalamic-pituitary gland functions.

CONCLUSIONS

Hyperthermic stress-induced reactivation of the HSV-1 high phenotypic reactivator can upregulate gene expression involved in B-cell function and in T-cell function. CD83 is implicated in HSV-1 latency, suggesting it could also be involved in immune-mediated mechanisms of viral reactivation.

Ocular HSV-1 latency, reactivation and recurrent disease

Ocular infection with HSV-1 continues to be a serious clinical problem despite the availability of effective antivirals. Primary infection with HSV-1 can involve ocular and adenaxial sites and can manifest as blepharitis, conjunctivitis, or corneal epithelial keratitis. After initial ocular infection, HSV-1 can establish latent infection in the trigeminal ganglia for the lifetime of the host. During latency, the viral genome is retained in the neuron without producing viral proteins. However, abundant transcription occurs at the region encoding the latency-associated transcript, which may play significant roles in the maintenance of latency as well as neuronal reactivation. Many host and viral factors are involved in HSV-1 reactivation from latency. HSV-1 DNA is shed into tears and saliva of most adults, but in most cases this does not result in lesions. Recurrent disease occurs as HSV-1 is carried by anterograde transport to the original site of infection, or any other site innervated by the latently infected ganglia, and can reinfect the ocular tissues. Recurrent corneal disease can lead to corneal scarring, thinning, stromal opacity and neovascularization and, eventually, blindness. In spite of intensive antiviral and anti-inflammatory therapy, a significant percentage of patients do not respond to chemotherapy for herpetic necrotizing stromal keratitis. Therefore, the development of therapies that would reduce asymptomatic viral shedding and lower the risks of recurrent disease and transmission of the virus is key to decreasing the morbidity of ocular herpetic disease. This review will highlight basic HSV-1 virology, and will compare the animal models of latency, reactivation, and recurrent ocular disease to the current clinical data.

During herpes simplex virus type 1 infection of rabbits, the ability to express the latency-associated transcript increases latent-phase transcription of lytic genes

Trigeminal ganglia (TG) from rabbits latently infected with either wild-type herpes simplex virus type 1 (HSV-1) or the latency-associated transcript (LAT) promoter deletion mutant 17DeltaPst were assessed for their viral chromatin profile and transcript abundance. The wild-type 17syn+ genomes were more enriched in the transcriptionally permissive mark dimethyl H3 K4 than were the 17DeltaPst genomes at the 5' exon and ICP0 and ICP27 promoters. Reverse transcription-PCR analysis revealed significantly more ICP4, tk, and glycoprotein C lytic transcripts in 17syn+ than in 17DeltaPst. These results suggest that, for efficient reactivation from latency in rabbits, the LAT is important for increased transcription of lytic genes during latency.

Chromatin control of herpes simplex virus lytic and latent infection

Herpes simplex viruses (HSV) can undergo a lytic infection in epithelial cells and a latent infection in sensory neurons. During latency the virus persists until reactivation, which leads to recurrent productive infection and transmission to a new host. How does HSV undergo such different types of infection in different cell types? Recent research indicates that regulation of the assembly of chromatin on HSV DNA underlies the lytic versus latent decision of HSV. We propose a model for the decision to undergo a lytic or a latent infection in which HSV encodes gene products that modulate chromatin structure towards either euchromatin or heterochromatin, and we discuss the implications of this model for the development of therapeutics for HSV infections.

Relaxed repression of herpes simplex virus type 1 genomes in Murine trigeminal neurons

The expression of herpes simplex virus (HSV) genomes in the absence of viral regulatory proteins in sensory neurons is poorly understood. Previously, our group reported an HSV immediate early (IE) mutant (d109) unable to express any of the five IE genes and encoding a model human cytomegalovirus immediate early promoter-green fluorescent protein (GFP) transgene. In cultured cells, GFP expressed from this mutant was observed in only a subset of infected cells. The subset exhibited cell type dependence, as the fractions of GFP-expressing cells varied widely among the cell types examined. Herein, we characterize this mutant in murine embryonic trigeminal ganglion (TG) cultures. We found that d109 was nontoxic to neural cultures and persisted in the cultures throughout their life spans. Unlike with some of the cultured cell lines and strains, expression of the GFP transgene was observed in a surprisingly large subset of neurons. However, very few nonneuronal cells expressed GFP. The abilities of ICP0 and an inhibitor of histone deacetylase, trichostatin A (TSA), to activate GFP expression from nonexpressing cells were also compared. The provision of ICP0 by infection with d105 reactivated quiescent genomes in nearly every cell, whereas reactivation by TSA was much more limited and restricted to the previously nonexpressing neurons. Moreover, we found that d109, which does not express ICP0, consistently reactivated HSV type 1 (KOS) in latently infected adult TG cultures. These results suggest that the state of persisting HSV genomes in some TG neurons may be more dynamic and more easily activated than has been observed with nonneuronal cells.

Stable cell lines expressing high levels of the herpes simplex virus type 1 LAT are refractory to caspase 3 activation and DNA laddering following cold shock induced apoptosis

The herpes simplex virus type 1 (HSV-1) latency associated transcript (LAT) gene's anti-apoptosis activity plays a central, but not fully elucidated, role in enhancing the virus's reactivation phenotype. In transient transfection experiments, LAT increases cell survival following an apoptotic insult in the absence of other HSV-1 genes. However, the high background of untransfected cells has made it difficult to demonstrate that LAT inhibits specific apoptotic factors such as caspases. Here we report that, in mouse neuroblastoma cell lines (C1300) stably expressing high levels of LAT, cold shock induced apoptosis was blocked as judged by increased survival, protection against DNA fragmentation (by DNA ladder assay), and inhibition of caspase 3 cleavage and activation (Western blots). To our knowledge, this is the first report providing direct evidence that LAT blocks two biochemical hallmarks of apoptosis, caspase 3 cleavage and DNA laddering, in the absence of other HSV-1 gene products.

Introducing point mutations into the ATGs of the putative open reading frames of the HSV-1 gene encoding the latency associated transcript (LAT) reduces its anti-apoptosis activity

The herpes simplex virus type 1 (HSV-1) latency associated transcript (LAT) gene has anti-apoptosis activity that directly or indirectly enhances the virus's reactivation phenotype in small animal models. The first 1.5 kb of the primary 8.3 kb LAT is sufficient and some or all of it is necessary for LAT's anti-apoptosis in transient transfection assays and for LAT's ability to enhance the reactivation phenotype. Based on LAT's genomic sequence, the first 1.5 kb contains eight potential open reading frames (ORFs) defined as an ATG followed by an in frame termination codon. In this study, point mutations were introduced into the ATGs of ORFs present in the 1.5 kb fragment of LAT. Mutagenesis of all eight ATGs in LAT ORFs consistently reduced the anti-apoptotic activity of LAT in transiently transfected mouse neuroblastoma cells regardless of whether apoptosis was induced by caspase 8 or caspase 9. Mutation of the six ATGs located in the stable intron sequences within the 1.5 kb LAT had a dramatic effect on caspase 9, but not caspase 8, induced apoptosis. For both caspase 8 and caspase 9 induced apoptosis, mutating the two ATGs in the exon of the LAT 1.5 kb fragment reduced, but did not eliminate the anti-apoptotic activity of LAT. These studies suggest that altering the fine structure of regulatory RNA or expression of a putative LAT ORF regulates the anti-apoptosis activity of LAT. These studies also indicate that more than one function is present in the 1.5 kb LAT fragment.

Reactivation phenotype in rabbits of a herpes simplex virus type 1 mutant containing an unrelated antiapoptosis gene in place of latency-associated transcript

Latency-associated transcript (LAT) significantly enhances the spontaneous reactivation phenotype of herpes simplex virus type 1 (HSV-1). The mechanism by which LAT accomplishes this has been elusive. To determine if LAT's antiapoptosis activity is involved, the authors used a rabbit eye model to analyze the spontaneous reactivation phenotype of an HSV-1 mutant in which LAT was replaced by an unrelated antiapoptosis gene. This virus, dLAT-cpIAP, contains the open reading frame of the baculovirus inhibitor of apoptosis protein gene (cpIAP) in place of LAT, under control of the LAT promoter. The authors report here that in a rabbit ocular model of infection, dLAT-cpIAP had a spontaneous reactivation phenotype similar to wild-type virus and significantly higher than LAT(-) viruses. This was consistent with their previous findings using the mouse trigeminal ganglia explant-induced reactivation model. Whether LAT (and in the case of dLAT-cpIAP, cpIAP) enhances the spontaneous reactivation phenotype by functioning during establishment of latency, maintenance of latency, or reactivation from latency, or during two or more of these periods, remains to be determined. Regardless, the results presented in this study strongly support the hypothesis that LAT's antiapoptosis activity is the dominant function that enhances HSV-1's spontaneous reactivation phenotype.

Reactivation from quiescence does not coincide with a global induction of herpes simplex virus type 1 transactivators

Herpes simplex virus type 1 (HSV-1) reactivates from a small fraction of latently infected neurons in vivo and neuronally differentiated (ND), quiescently infected (QIF)-PC12 cells in vitro. This may be the result of reactivation initiating in only a few cells, or reactivation followed by premature termination of the productive virus life cycle in many or even a majority of cells. To examine the viral stress response, HSV-1 promoters of representative alpha, beta, and gamma class genes were examined in ND- and QIF-PC12 cells after treatments with agents known to induce reactivation. HSV-1 promoters displayed variable levels of basal gene expression in ND-PC12 cells ranging from 2 to 1,200 times the level of the control vector pGL3-Basic. Expression of the latency associated transcript (LAT) was greatest, with representatives of the alpha class exhibiting greater expression than the beta and gamma classes. The HSV-1 promoters examined did not respond dramatically to stress treatments. The viral gene response was also measured during the initiation of reactivation of a cryptic HSV-1 genome after forskolin treatment, under conditions that restricted DNA replication. During the first 24 h after stress induction the response was limited. By 48 h post-forskolin treatment, only modest increases occurred for ICP0, ICP4, and LAT transcripts, reaching levels of no greater than 2.2 times mock treated levels. In contrast, ICP27, ribonucleotide reductase (RR), and VP16 promoters did not respond. These findings indicate that reactivation from QIF-PC12 cells does not result in a global response of the specific HSV-1 genes tested, when assessed at the population level. These data support the hypothesis that stress-induced reactivation initiates in a minority of cells.

Tissue-specific splicing of the herpes simplex virus type 1 latency-associated transcript (LAT) intron in LAT transgenic mice

To study the regulation of herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) expression and processing in the absence of other cis and trans viral functions, a transgenic mouse containing the region encompassing the LAT promoter (LAP1) and the LAT 5' exon through the 2.0-kb intron was created. LAT expression was detectable by reverse transcriptase PCR (RT-PCR) in a number of tissues, including the dorsal root ganglia (DRG), trigeminal ganglia (TG), brain, skin, liver, and kidney. However, when the accumulation of the 2.0-kb LAT intron was analyzed at the cellular level by in situ hybridization, little or no detectable accumulation was observed in the brain, spinal cord, kidney, or foot, although the 2.0-kb LAT intron was detected at high levels (over 90% of neurons) in the DRG and TG. Northern blot analysis detected the stable 2.0-kb LAT intron only in the sensory ganglia. When relative amounts of the spliced and unspliced LAT within the brain, liver, kidney, spinal cord, TG, and DRG were analyzed by real-time RT-PCR, splicing of the 2.0-kb LAT intron was significantly more efficient in the sensory ganglia than in other tissues. Finally, infection of both transgenic mice and nontransgenic littermates with HSV-1 revealed no differences in lytic replication, establishment of latency, or reactivation, suggesting that expression of the LAT transgene in trans has no significant effect on those functions. Taken together, these data indicate that the regulation of expression and processing of LAT RNA within the mouse is highly cell-type specific and occurs in the absence of other viral cis- and trans-acting factors.

Laser-capture microdissection: refining estimates of the quantity and distribution of latent herpes simplex virus 1 and varicella-zoster virus DNA in human trigeminal Ganglia at the single-cell level

There remains uncertainty and some controversy about the percentages and types of cells in human sensory nerve ganglia that harbor latent herpes simplex virus 1 (HSV-1) and varicella-zoster virus (VZV) DNA. We developed and validated laser-capture microdissection and real-time PCR (LCM/PCR) assays for the presence and copy numbers of HSV-1 gG and VZV gene 62 sequences in single cells recovered from sections of human trigeminal ganglia (TG) obtained at autopsy. Among 970 individual sensory neurons from five subjects, 2.0 to 10.5% were positive for HSV-1 DNA, with a median of 11.3 copies/positive cell, compared with 0.2 to 1.5% of neurons found to be positive by in situ hybridization (ISH) for HSV-1 latency-associated transcripts (LAT), the classical surrogate marker for HSV latency. This indicates a more pervasive latent HSV-1 infection of human TG neurons than originally thought. Combined ISH/LCM/PCR assays revealed that the majority of the latently infected neurons do not accumulate LAT to detectable levels. We detected VZV DNA in 1.0 to 6.9% of individual neurons from 10 subjects. Of the total 1,722 neurons tested, 4.1% were VZV DNA positive, with a median of 6.9 viral genomes/positive cell. After removal by LCM of all visible neurons on a slide, all surrounding nonneuronal cells were harvested and assayed: 21 copies of HSV-1 DNA were detected in approximately 5,200 nonneuronal cells, while nine VZV genomes were detected in approximately 14,200 nonneuronal cells. These data indicate that both HSV-1 and VZV DNAs persist in human TG primarily, if not exclusively, in a moderate percentage of neuronal cells.

Herpes simplex virus 1 immediate-early and early gene expression during reactivation from latency under conditions that prevent infectious virus production

The program of gene expression exhibited by herpes simplex virus during productive infection of cultured cells is well established; however, less is known about the regulatory controls governing reactivation from latency in neurons. One difficulty in examining gene regulation during reactivation lies in distinguishing between events occurring in initial reactivating cells versus events occurring in secondarily infected cells. Thus, two inhibitors were employed to block production of infectious virus: acyclovir, which inhibits viral DNA synthesis, and WAY-150138, which permits viral DNA synthesis but inhibits viral DNA encapsidation. Latently infected murine ganglia were explanted in the presence of either inhibitor, and then amounts of RNA, DNA, or infectious virus were quantified. In ganglia explanted for 48 h, the levels of five immediate-early and early RNAs did not exhibit meaningful differences between acyclovir and WAY-150138 treatments when analyzed by in situ hybridization or quantitative reverse transcription-PCR. However, comparative increases in viral DNA and RNA content in untreated ganglia suggested that virus was produced before 48 h postexplant. This was confirmed by the detection of infectious virus as early as 14 h postexplant. Together, these results suggest that high levels of viral gene expression at 48 h postexplant are due largely to the production of infectious virus and subsequent spread through the tissue. These results lead to a reinterpretation of previous results indicating a role for DNA replication in immediate-early and early viral gene expression; however, it remains possible that viral gene expression is regulated differently in neurons than in cultured cells.

A herpes simplex virus type 1 mutant expressing a baculovirus inhibitor of apoptosis gene in place of latency-associated transcript has a wild-type reactivation phenotype in the mouse

The latency-associated transcript (LAT) is essential for the wild-type herpes simplex virus type 1 (HSV-1) high-reactivation phenotype since LAT- mutants have a low-reactivation phenotype. We previously reported that LAT can decrease apoptosis and proposed that this activity is involved in LAT's ability to enhance the HSV-1 reactivation phenotype. The first 20% of the primary 8.3-kb LAT transcript is sufficient for enhancing the reactivation phenotype and for decreasing apoptosis, supporting this proposal. For this study, we constructed an HSV-1 LAT- mutant that expresses the baculovirus antiapoptosis gene product cpIAP under control of the LAT promoter and in place of the LAT region mentioned above. Mice were ocularly infected with this mutant, designated dLAT-cpIAP, and the reactivation phenotype was determined using the trigeminal ganglion explant model. dLAT-cpIAP had a reactivation phenotype similar to that of wild-type virus and significantly higher than that of (i) the LAT- mutant dLAT2903; (ii) dLAT1.5, a control virus containing the same LAT deletion as dLAT-cpIAP, but with no insertion of foreign DNA, thereby controlling for potential readthrough transcription past the cpIAP insert; and (iii) dLAT-EGFP, a control virus identical to dLAT-cpIAP except that it contained the enhanced green fluorescent protein open reading frame (ORF) in place of the cpIAP ORF, thereby controlling for expression of a random foreign gene instead of the cpIAP gene. These results show that an antiapoptosis gene with no sequence similarity to LAT can efficiently substitute for the LAT function involved in enhancing the in vitro-induced HSV-1 reactivation phenotype in the mouse.

Human cytomegalovirus 5-kilobase immediate-early RNA is a stable intron

Immediate-early viral gene products of human cytomegalovirus (HCMV) are derived from several genomic loci and largely serve to establish a cellular environment conducive to viral replication. We have further examined an unusual immediate-early transcript known as the 5-kb RNA, concluding that it is a stable intron encoded by HCMV. The 5-kb RNA is highly AT rich in sequence and lacks open reading frames likely to be translated into protein. We confirmed the absence of polyadenylation of the transcript and showed that it is primarily nuclear localized during viral infection. We mapped the 5' end of the 5-kb RNA to a consensus splice donor site and localized the 3' end in the vicinity of a splice acceptor site. In transfection studies, we showed that the 5-kb RNA can be spliced from a heterologous primary transcript. Using bacterial artificial chromosome technology, we constructed a viral recombinant containing a mutation in the 5' splice donor site that defines the 5' end of the RNA and found that this mutation eliminates expression of the 5-kb RNA during viral infection. This mutant grows in human fibroblasts without complementation. Taken together, these data support the conclusion that the 5-kb RNA is a stable intron expressed by HCMV.

Construction of a herpes simplex virus type 1 mutant with only a three-nucleotide change in the branchpoint region of the latency-associated transcript (LAT) and the stability of its two-kilobase LAT intron

Previous studies using a eukaryotic expression system indicated that the unusual stability of the latency-associated transcript (LAT) intron was due to its nonconsensus branchpoint sequence (T.-T Wu, Y.-H. Su, T. M. Block, and J. M. Taylor, Virology, 243:140-149, 1998). The present study investigated the role of the branchpoint sequence in the stability of the intron expressed from the herpes simplex virus type 1 (HSV-1) genome and the role of LAT intron stability in the HSV-1 life cycle. A branchpoint mutant called Sy2 and the corresponding rescued viruses, SyRA and SyRB, were constructed. To preserve the coding sequence of the immediate early gene icp0, which overlaps with the branchpoint region of the 2-kb LAT, a 3-nucleotide mutation into the branchpoint region of the 2-kb LAT was introduced, resulting in a branchpoint that is 85% identical to the consensus intron branchpoint sequence of eukaryotic cells. As anticipated, there was a 90- to 96-fold reduction in 2-kb LAT accumulation following productive infection in tissue culture and latent infection in mice with Sy2, as determined by Northern blot analysis. These results clearly suggest that the accumulation of the 2-kb intron in tissue culture and in vivo is, at least in part, due to the nonconsensus branchpoint sequence of the LAT intron. Interestingly, a failure to accumulate LAT was associated with greater progeny production of Sy2 at a low multiplicity of infection (0.01) in tissue culture, but not in mice. However, the ability of mutant Sy2 to reactivate from trigeminal ganglia (TG) derived from latently infected mice was indistinguishable from that of wild-type virus, as assayed in the mouse TG explant reactivation system.

Comparison of herpes simplex virus reactivation in ganglia in vivo and in explants demonstrates quantitative and qualitative differences

The in vivo ganglionic environment directs the latent herpes simplex virus transcriptional program. Since stress-driven perturbations in sensory neurons are thought to play a critical role in the transition from latency to reactivation, a primary concern in the selection of a valid model of the molecular interactions leading to reactivation is the faithful recapitulation of these environments. In this study reactivation of latently infected ganglia excised and cultured in vitro (explanted) is compared to reactivation occurring in latently infected ganglia in vivo following hyperthermic stress. Three notable points emerged. (i). Neurons in explanted ganglia exhibited marked morphological changes within 2 to 3 h postexplant. DNA fragmentation in neuronal nuclei was detected at 3 h, and atypical expression of cell cycle- and stress-regulated proteins such as geminin, cdk2, cdk4, and cytochrome c became apparent at 2 to 48 h. These changes were associated with axotomy and explant and not with the initiation or progression of reactivation and were not observed in ganglia following in vivo hyperthermic stress. (ii). Despite these differences, during the first 22 h primary reactivation events were restricted to a very small number of neurons in vivo and in explanted ganglia. This suggests that at any given time only a few latently infected neurons are competent to reactivate or that the probability of reactivation occurring in any particular neuron is very low. Importantly, the marked changes detected in explanted ganglia were not correlated with increased reactivation, demonstrating that these changes were not associated with the reactivation process per se. (iii). Secondary spread of virus was evident in explanted ganglia within 36 h, an event not observed in vivo. We conclude that explant reactivation may provide an ancillary system for selected studies of the early events in reactivation. However, clear signs of neuronal degeneration within 2 to 3 h postexplant indicate that these ganglia are undergoing major physiological changes not associated with the reactivation process. This ongoing neurodegeneration could alter even the early virus-host interactions in reactivation, and thus caution in the extrapolation of results obtained in explants to the in vivo interactions initiating reactivation is warranted.

A non-consensus branch point plays an important role in determining the stability of the 2-kb LAT intron during acute and latent infections of herpes simplex virus type-1

Herpes simplex virus type 1 (HSV-1) establishes lifelong latent infection in sensory neurons of the peripheral nervous system. During HSV latency, the latency-associated transcripts (LATs) are the only viral transcripts abundantly expressed. The most abundant form of LATs is a 2-kb stable intron spliced from a primary transcript (mLAT). It has been previously reported that a non-consensus branch point influences the stability of the intron (in vitro) in cells transfected with plasmid constructs (J. Virol. 71 (1997) 5849; J. Virol. 71 (1997) 4199). However, it is unknown whether this branch point is important in determining LAT stability in vivo (in the context of virus). To study the role of this stable intron in HSV-1 infection, we have constructed a mutant virus KOS-CONS in which the branch point has been mutated to consensus branch point nucleotides. The accumulation of the 2-kb intron in KOS-CONS-infected cells was greatly reduced. The LAT intron was not detectable in KOS-CONS-infected mouse trigeminal ganglia (TG) during acute and latent phase infection by Northern blot analysis. Replication of the KOS-CONS and the wild-type KOS viruses on Vero cells was determined to be similar, as was the level of HSV-1 DNA in mouse trigeminal ganglia during acute and latent phase infection. Using the mouse TG explant model, the reactivation pattern of both viruses was shown to be similar. Our data suggest that the unique branch point plays a significant role in determining the stability of LAT intron in vivo, but that the stability of the intron does not appear to affect HSV-1 replication, the establishment of latency, or viral reactivation.

Methods for detecting the HSV-1 LAT anti-apoptosis activity in virus infected tissue culture cells

Plasmids expressing the herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) reduce apoptosis in transient transfection assays in tissue culture. LAT also reduces apoptosis in the context of the virus in trigeminal ganglia of rabbits and mice at approximately 6-7 days post-infection during the switch from acute to latent HSV-1 infection, a time at which LAT is the only abundantly transcribed viral gene. Analysis of LAT's anti-apoptosis function is complicated in tissue culture by the expression of at least five additional viral gene products that can block apoptosis, and by the fact that apoptosis usually occurs in only a fraction of the cells. Here, we present two approaches for detecting LAT's anti-apoptosis activity in the context of the whole virus in tissue culture. Using a combination of serum starvation to both partially synchronize the cells and induce apoptosis, and Hoechst staining to detect chromatin condensation, we found that there was a small window of time post-infection during which Schwann cells infected with the LAT(-) mutant dLAT2903 reproducibly had more apoptotic nuclei than identically treated cells infected with the LAT(+) parental virus HSV-1 strain McKrae. Using serum starvation and/or UV treatment and a method to isolate fragmented DNA away from large chromosomal DNA, we found a similar window of time post-infection during which Neuro2A cells infected with dLAT2903 had increased DNA fragmentation (as judged by a DNA laddering assay) compared to identically treated cells infected with wild type McKrae or the LAT(+) marker rescued dLAT2903R virus. These assays should permit the use of culture assays, rather than labor intensive animal models, to examine LAT's anti-apoptosis activity in the context of the virus in a large number of existing LAT mutant viruses.

Wide variations in herpes simplex virus type 1 inoculum dose and latency-associated transcript expression phenotype do not alter the establishment of latency in the rabbit eye model

The latency-associated transcript (LAT) is required for efficient reactivation of herpes simplex virus type 1 from latent infection in the rabbit eye model, but LAT's mechanism of action is unknown. In addition to reactivation, the LAT region seems to correspond to multiple functions, with some LAT deletion mutants exhibiting increased virulence, increased neuronal death, and restricted establishment of latency. While a LAT promoter deletion mutant (17DeltaPst) seems to be primarily restricted in reactivation in the rabbit, subtle effects on virulence or the establishment of latency cannot be precluded at the normal high levels of virus inoculum used in the rabbit model. Since such additional LAT phenotypes may be more evident with lower doses of virus, we evaluated the influence of initial viral inoculum and LAT expression on the progression of acute infection and the establishment of latency. We have assayed both virus recovery rates and viral genome loads in rabbit corneas and trigeminal ganglia. Our results show that (i) in the corneas and trigeminal ganglia, the maximum amount of virus present during acute infection is independent of the LAT genotype and inoculum dose, although greater viral yields are obtained earlier with higher inoculum doses, and (ii) the range in numbers of latent genomes detected in the ganglia is independent of the inoculum dose and the LAT genotype and therefore no difference in establishment of latency is observed.

Transgenic mouse with the herpes simplex virus type 1 latency-associated gene: expression and function of the transgene

During herpes simplex virus type 1 (HSV-1) latent infection in human peripheral sensory ganglia, the major viral gene transcribed is the latency-associated transcript (LAT) gene. In order to facilitate the study of this gene, we generated a transgenic mouse that contains the DNA fragment that transcribes the LAT RNAs (2.0 kb and its 1.5-kb spliced transcript) under control of the cytomegalovirus promoter. The tissue distribution of these transcripts and their effects upon HSV-1 replication, latency, and reactivation in the transgenic-mouse model were examined. Different steady-state amounts of both transcripts were found in various tissues. While the highest levels of the 2.0-kb RNA were detected in heart and skeletal muscle, the 1.5-kb transcript was found at elevated levels in the brain and at much higher levels in the trigeminal ganglia (TG). Replication of both the wild-type and a LAT-negative mutant virus was suppressed in primary embryonic fibroblasts obtained from LAT-expressing transgenic mice compared to that in cells obtained from normal mice. HSV-1 DNA amounts in latently infected TG of transgenic mice were similar to those in normal mice. Reactivation of latent HSV-1 LAT-negative mutants by explant cocultivation of TG from transgenic mice was more efficient than reactivation from normal-mouse TG. Considering our present and previous results, we propose that the significantly higher steady-state level of the 1.5-kb RNA in the TG may link this transcript to latency functions and that by inhibition of virus replication, the LAT gene may protect ganglion cells and thereby increase the probability of reactivation.