Differential accumulation of herpes simplex virus type 1 latency-associated transcripts in sensory and autonomic ganglia

An Insight into Current Treatment Strategies, Their Limitations, and Ongoing Developments in Vaccine Technologies against Herpes Simplex Infections

Herpes simplex virus (HSV) infection, the most prevalent viral infection that typically lasts for a lifetime, is associated with frequent outbreaks of oral and genital lesions. Oral herpes infection is mainly associated with HSV-1 through oral contact, while genital herpes originates due to HSV-2 and is categorized under sexually transmitted diseases. Immunocompromised patients and children are more prone to HSV infection. Over the years, various attempts have been made to find potential targets for the prevention of HSV infection. Despite the global distress caused by HSV infections, there are no licensed prophylactic and therapeutic vaccines available on the market against HSV. Nevertheless, there are numerous promising candidates in the pre-clinical and clinical stages of study. The present review gives an overview of two herpes viruses, their history, and life cycle, and different treatments adopted presently against HSV infections and their associated limitations. Majorly, the review covers the recent investigations being carried out globally regarding various vaccine strategies against oral and genital herpes virus infections, together with the recent and advanced nanotechnological approaches for vaccine development. Consequently, it gives an insight to researchers as well as people from the health sector about the challenges and upcoming solutions associated with treatment and vaccine development against HSV infections.

Herpes Simplex Virus Latency Is Noisier the Closer We Look

During herpes simplex virus (HSV) latency, the viral genome is harbored in peripheral neurons in the absence of infectious virus but with the potential to restart infection. Advances in epigenetics have helped explain how viral gene expression is largely inhibited during latency. Paradoxically, at the same time, the view that latency is entirely silent has been eroding. This low-level noise has implications for our understanding of HSV latency and should not be ignored.

Herpes Simplex Virus 2 in Autonomic Ganglia: Evidence for Spontaneous Reactivation

Herpes simplex virus 2 (HSV-2) can be transmitted in the presence or absence of lesions, allowing efficient spread among the general population. Recurrent HSV genital lesions are thought to arise from reactivated latent virus in sensory cell bodies of the dorsal root ganglia (DRG). However, HSV-2 has also been found latent in autonomic ganglia. Spontaneous reactivation or a low level of chronic infection could theoretically also occur in these peripheral nervous tissues, contributing to the presence of infectious virus in the periphery and to viral transmission. Use of a recently described, optimized virus with a monomeric mNeonGreen protein fused to viral capsid protein 26 (VP26) permitted detection of reactivating virus in explanted ganglia and cryosections of DRG and the sacral sympathetic ganglia (SSG) from latently infected guinea pigs. Immediate early, early, and late gene expression were quantified by droplet digital reverse transcription-PCR (ddRT-PCR), providing further evidence of viral reactivation not only in the expected DRG but also in the sympathetic SSG. These findings indicate that viral reactivation from autonomic ganglia is a feature of latent viral infection and that these reactivations likely contribute to viral pathogenesis.IMPORTANCE HSV-2 is a ubiquitous important human pathogen that causes recurrent infections for the life of its host. We hypothesized that the autonomic ganglia have important roles in viral reactivation, and this study sought to determine whether this is correct in the clinically relevant guinea pig vaginal infection model. Our findings indicate that sympathetic ganglia are sources of reactivating virus, helping explain how the virus causes lifelong recurrent disease.

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.

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.

Herpes simplex virus type 1 latently infected neurons differentially express latency-associated and ICP0 transcripts

During the latent phase of herpes simplex virus type 1 (HSV-1) infection, the latency-associated transcripts (LATs) are the most abundant viral transcripts present in neurons, but some immediate-early viral transcripts, such as those encoding ICP0, have also been reported to be transcribed in latently infected mouse trigeminal ganglia (TG). A murine oro-ocular model of herpetic infection was used to study ICP0 gene expression in the major anatomical sites of HSV-1 latency, including the TG, superior cervical ganglion, spinal cord, and hypothalamus. An HSV-1 recombinant strain, SC16 110LacZ, revealed ICP0 promoter activity in several neurons in latently infected ganglia, and following infection with wild-type HSV-1 strain SC16, in situ hybridization analyses identified ICP0 transcripts in the nuclei of neurons at times consistent with the establishment of latency. Reverse transcription (RT)-PCR assays performed on RNA extracted from latently infected tissues indicated that ICP0 transcripts were detected in all anatomical sites of viral latency. Furthermore, quantitative real-time RT-PCR showed that neurons differentially expressed the LATs and ICP0 transcripts, with splicing of ICP0 transcripts being dependent on the anatomical location of latency. Finally, TG neurons were characterized by high-level expression of LATs and detection of abundant unspliced ICP0 transcripts, a pattern markedly different from those of other anatomical sites of HSV-1 latency. These results suggest that LATs might be involved in the maintenance of HSV-1 latency through the posttranscriptional regulation of ICP0 in order to inhibit expression of this potent activator of gene expression during latency.

Stable Levels of Long-Term Transgene Expression Driven by the Latency-Associated Transcript Promoter in a Herpes Simplex Virus Type 1 Vector

Previous gene transfer studies of the herpes simplex virus type 1 (HSV-1) using the latency-associated transcript (LAT) promoter have reported a decrease in transgene expression in the brain over time, but the extent of this decrease has not been measured and it is unknown if expression eventually stabilizes. We examined LAT promoter-mediated transgene expression in the mouse brain for 1 year following intracranial injection with a HSV-1 vector expressing human beta-glucuronidase (GUSB). The vector genome copy number remained stable from 2 to 52 weeks. Quantitative reverse transcriptase PCR detected a peak of LAT intron expression at 2 weeks (corresponding to the end of the acute phase of viral infection), followed by stable expression during latency (13-52 weeks). The number of GUSB-positive cells also had a peak in the acute phase and then was stable during latency (13-52 weeks). GUSB enzymatic activity was maintained at 11% of normal at 6 and 12 months, indicating that the LAT promoter is capable of driving stable transgene expression in the brain.

Development of a reverse transcriptase-polymerase chain reaction assay to detect feline herpesvirus-1 latency-associated transcripts in the trigeminal ganglia and corneas of cats that did not have clinical signs of ocular disease

OBJECTIVE

To develop a reverse transcriptase-polymerase chain reaction (RT-PCR) assay to detect feline herpesvirus-1 (FHV-1) latency-associated transcripts (LATs) in the corneas and trigeminal ganglia of cats that did not have clinical signs of ocular disease.

SAMPLE POPULATION

Corneas and trigeminal ganglia obtained from 21 cats necropsied at the Indiana Animal Disease Diagnostic Laboratory and 25 cats euthanatized at a humane shelter; none of the cats had a recent history of respiratory tract or ocular disease, and all had normal results for ophthalmic examinations.

PROCEDURE

Both corneas and both trigeminal ganglia were harvested from each cat. An initial PCR assay detected FHV-1 DNA in the corneas and trigeminal ganglia. The RNA was then isolated from samples positive for FHV-1 DNA, and an RT-PCR assay was used to detect LATs.

RESULTS

FHV-1 DNA was detected in 45 of 92 (48.9%) corneas and 38 of 92 (41.3%) trigeminal ganglia. In many samples, the RNA had degraded and RT-PCR assay was not possible. Of the samples subjected to RT-PCR assay, none of the 39 corneas but 4 of 16 trigeminal ganglia had positive results when tested for LATs.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of the results indicated that a high percentage of cats that did not have clinical signs of ocular disease had detectable FHV-1 DNA in their corneas and trigeminal ganglia. This study documents that the RT-PCR assay can successfully identify LATs and may serve as a tool to better understand the biologic characteristics of FHV-1 and its relationship to clinical disease.

Are latent, immediate-early genes of herpes simplex virus-1 essential in causing trigeminal neuralgia?

The etiology and pathogenesis of major trigeminal neuralgia remain largely unknown, but are believed to result from an irritative lesion near the semilunar ganglion. We suggest that its primary cause is a single, active DNA sequence in the persistent but non-integrated genome of latent herpes simplex virus type 1 commonly observed in a few infected A-delta nerve fibers of the cheek. Facial pain occurs as a result of herpes virus reactivation and when supplies of neurotrophins controlling normal transport functions of axolemmal ion channels become depleted.

The relationship of herpes simplex virus latency associated transcript expression to genome copy number: a quantitative study using laser capture microdissection

To investigate the quantitative relationship of latent herpes simplex virus (HSV) genomes to the expression of latency associated transcripts (LATs) we used a combination of laser capture microdissection (LCM), polymerase chain reaction (PCR), and quantitative real-time PCR to determine the number of HSV genomes in individual neurons of the mouse trigeminal ganglion (TG) during viral latency. Both LAT-positive and LAT-negative neurons detected by in situ hybridization (ISH) and lifted by LCM contained HSV genomes detected by PCR for HSV ICP47. The number of genomes/cell determined by real-time PCR with probes for HSV UL44 following LCM demonstrated a Poisson distribution with a predicted mean count of 178 genomes/LAT-positive neuron, and 68 genomes/LAT-negative neuron. The range was similar between the LAT-positive and LAT-negative neurons, and there was a substantial overlap in the distributions. These results suggest that the expression of LATs in an amount that is detectable by ISH does not depend only on the number of HSV genomes in the cell, and by implication suggests that neuron-specific factors play a role in the regulation of LAT expression during latency.

Herpes simplex virus type 1 promoter activity during latency establishment, maintenance, and reactivation in primary dorsal root neurons in vitro

A neonatal rat dorsal root ganglion-derived neuronal culture system has been utilized to study herpes simplex virus (HSV) latency establishment, maintenance, and reactivation. We present our initial characterization of viral gene expression in neurons following infection with replication-defective HSV recombinants carrying beta-galactosidase and/or green fluorescent protein reporter genes under the control of lytic cycle- or latency-associated promoters. In this system lytic virus reporter promoter activity was detected in up to 58% of neurons 24 h after infection. Lytic cycle reporter promoters were shut down over time, and long-term survival of neurons harboring latent virus genomes was demonstrated. Latency-associated promoter-driven reporter gene expression was detected in neurons from early times postinfection and was stably maintained in up to 83% of neurons for at least 3 weeks. In latently infected cultures, silent lytic cycle promoters could be activated in up to 53% of neurons by nerve growth factor withdrawal or through inhibition of histone deacetylases by trichostatin A. We conclude that the use of recombinant viruses containing reporter genes, under the regulation of lytic and latency promoter control in neuronal cultures in which latency can be established and reactivation can be induced, is a potentially powerful system in which to study the molecular events that occur during HSV infection of neurons.

A herpes simplex virus type 1 vector as marker for retrograde neuronal tracing: characterization of lacZ transcription and localization of labelled neuronal cells in sensory and autonomic ganglia after inoculation of the anterior segment of the eye

Herpes simplex virus type 1 (HSV-1) is a human, neurotropic pathogen which also can infect experimental animals. Much interest has been focused on genetic modification of HSV-1 so that it can be used as a vector for gene delivery and for tracing neuronal connections. For expression of a foreign gene inserted into the HSV-1 genome, both the site of insertion and the promoter activity are important. We have used a previously described HSV-1 vector, KOS/58, to demonstrate that the beta-galactosidase gene inserted together with a neurofilament L promoter into the coding region of the glycoprotein C (gC) gene is under the control of the foreign promoter rather than under that of the gC gene. This was performed by isolation of RNA from infected, neuron-like PC12 cells and Northern blotting using probes from various regions of the modified part of the genome. The vector was then inoculated in the cornea, subconjunctivally, or into the anterior chamber of the mouse eye. Whole mounts of the trigeminal, superior cervical and pterygopalatine ganglions were stained for beta-galactosidase. The localization of labelled neurons was consistent with retrograde axonal transport as the principal way of neuronal infection indicating that KOS/58 could be used as a retrograde tracer. The position of the labelled cells suggests a somatotopic organization of the mouse trigeminal and superior cervical ganglion similar to that of rats and rabbits.

Genetic studies exposing the splicing events involved in herpes simplex virus type 1 latency-associated transcript production during lytic and latent infection

Herpes simplex virus type 1 (HSV-1) establishes latency in sensory neurons, a state in which the viral lytic genes are silenced and only the latency locus is transcriptionally active, producing the 2. 0- and 1.5-kb latency-associated transcripts (LATs). Previous experimental evidence indicates that the LATs are stable introns, and it has been reported that LAT formation is abolished by debilitating substitution mutations in the predicted splice sites during lytic infection but not latency (J. L. Arthur et al., J. Gen. Virol. 79:107-116, 1998). We have independently studied a set of deletion mutations to explore the roles of the proposed splice sites during lytic and latent infection. HSV-1 mutant viruses missing the invariant intron-terminal 5'-G(T/C) or 3'-AG dinucleotides were analyzed for LAT formation during lytic infection in vitro, when only the 2-kb LAT is produced, and during latency in mouse trigeminal ganglia, where both LATs are expressed. Northern blot analysis of total RNAs from different productively infected cell lines showed that the lytic (2-kb) LAT was not expressed by the various splice site deletion mutants. In vivo studies using a mouse eye model of latency similarly showed that the latent (2- and 1. 5-kb) LATs were not expressed by the mutants. PCR analysis with primers flanking the LAT sequence revealed the expected splice junction for LAT excision in RNA from sensory neurons latently infected with wild-type but not mutant virus. Using a virus mutant deleted in the splicing signals flanking the 556-bp region of LAT whose absence distinguishes the 1.5- and 2-kb LATs, we observed selective elimination of 1.5-kb LAT expression in latency, supporting previous suggestions that the internal region is removed by splicing. Taken together, these results demonstrate that the 2-kb LAT is formed during both lytic and latent infection by splicing at the predicted splice sites and that an additional splicing event is involved in the latency-restricted production of the 1.5-kb LAT. We have also mapped the 3' end of the lytic 2-kb LAT and discuss our results in the context of previous models addressing the unusual stability of the LATs.

Infection of rat brain cell aggregates with neurovirulent and nonneurovirulent strains of herpes simplex virus type 1

Rat brain cell aggregates represent a three-dimensional tissue culture system of brain tissue in the form of small, multicellular spheroids. In the present work, we have infected these "minibrains" with neurovirulent, nonneurovirulent, and nonreplicating strains of HSV-1. The neurovirulent strains 17(+) and KOS(M) spread rapidly through the aggregates, while the nonreplicating ICP4 deletion mutant KD6 infected cells only at the periphery of the aggregates. Spread and replication of the nonneurovirulent strains RE6 and tk-7, and to some extent also of R13/1, were restricted. The interaction between different strains of HSV-1 and the rat brain cell aggregates is thus comparable to that seen in the brain, suggesting that the aggregates represent a useful tool for studying HSV-1 infection of brain tissue in vitro.

Comparative analyses of the latency-associated transcript promoters from herpes simplex virus type 1 strains H129, +GC and KOS-63

We have analyzed the activity of a specific portion of the latency-associated transcript (LAT) promoter of three strains of herpes simplex virus type 1 (HSV-1) in neuronal and non-neuronal cell types. Restriction fragments containing the LAT promoter sequences and the 5'-end of the LATs were isolated from HSV-1 strains H129, +GC and KOS-63, sequenced and cloned into a chloramphenicol transferase (CAT) plasmid vector. These vectors were separately assayed for CAT production in human (SknSH) and mouse (C-1300) neuroblastoma cell lines and a human continuous cell line (HeLa). Strain KOS-63 contained a C to T base substitution within the LAT promoter binding factor element upstream of the cAMP response element binding sequence. In replicate experiments, in which the construct DNA was used for transfection, the CAT constructs from strains H129 and +GC functioned equally well in all three cell lines. In contrast, the strain KOS-63 CAT construct functioned significantly better in HeLa cells than in neuroblastoma cell lines and better than the identical CAT constructs from strains H129 and +GC. In addition, the construct from strain KOS-63 functioned less well in the human neuroblastoma cell line than in HeLa or C-1300 neuroblastoma cells. When LAT expression was examined directly in vivo by in situ hybridization, strain KOS-63 produced slightly less LAT RNA than strain H129 within trigeminal ganglionic neurons of latently infected rabbits. However, utilizing competitive gel-shift assays, DNA fragments containing the LAT promoter binding element from all three strains bound equivalent amounts of HeLa cell nuclear proteins. Together, these results suggest that the activity expressed by the strain KOS-63 LAT promoter in vivo and in vitro may relate to positive or negative effects of DNA binding proteins on LAT transcription, and that these effects are cell-type dependent.

Analysis of the 2-kilobase latency-associated transcript expressed in PC12 cells productively infected with herpes simplex virus type 1: evidence for a stable, nonlinear structure

The major latency-associated transcript (LAT) expressed in PC12 cells productively infected with herpes simplex virus type 1 is a 2-kb, nonpolyadenylated RNA molecule that accumulates in the nuclei of infected cells. In actinomycin D-treated cells, the 2-kb LAT gene transcript has a half-life considerably greater than 12 h. After polyacrylamide gel electrophoresis, two species of the transcript were observed, a major species that was retarded in the gel and a minor species that migrated as a 1.96-kb RNA molecule. RNase H digestion after hybridization of the RNA with an oligonucleotide complementary to positions -80 to -101 relative to the 3' end of the 2-kb LAT gene transcript changed the mobility of the retarded species into that of the rapidly migrating species. Our data indicate that the 2-kb LAT gene transcript expressed in productively infected PC12 cells is present in a stable, nonlinear form.

cis-acting elements involved in transcriptional regulation of the herpes simplex virus type 1 latency-associated promoter 1 (LAP1) in vitro and in vivo

Latency-associated promoter 1 (LAP1) of herpes simplex virus type 1 is required to generate a series of latency-associated transcripts (LATs) in sensory neurons of latently infected animals. Sequence analysis and DNA binding studies have suggested the existence of several cis-acting elements within LAP1 that are potentially important for promoter function, although their role in LAT gene expression during latency is largely unexplored. In this report, we present evidence that the LAP1 TATA box is essential for transcription initiation in vitro. A reduction in LAT synthesis measured by in situ hybridization and reverse transcription-PCR (RT-PCR) of rat brain tissue latently infected with a LAP1 TATA substitution virus demonstrated that this sequence was required for full LAP1 activity in vivo. Analysis of additional site-directed and 5'-deletion mutants of LAP1 by in vitro transcription-primer extension assays showed that upstream elements including the USF and cyclic AMP response element (CRE) site specifically contributed to LAP1 function and that sequences beginning at position -620 relative to the transcription start site were essential for full promoter activity. The combination of deleting USF, CRE, and TATA completely abolished LAT expression in the brain, identifying these as essential elements for the neuron-specific functioning of LAP1 during latency. Mutation of the transcription start site did not abolish transcription, suggesting the absence of an initiator element. However, one of the most exciting findings from this study is that the region downstream of the TATA box appears to contain a true enhancer that is not only essential for transcription, but also functional when positioned 1.6 kb downstream of the start site of transcription. It was concluded that (i) the TATA box was essential for full transcriptional activity from LAP1 both in vitro and in vivo, (ii) the USF element and CRE contribute to LAP1 function during latency in combination with the TATA element, (iii) multiple trans-acting factors besides the USF- and CRE-binding proteins were required for full promoter activity in vitro, and (iv) sequences downstream of the TATA box enhanced promoter activity in vitro.

Quantitation of herpes simplex virus type 1 DNA and latency-associated transcripts in rabbit trigeminal ganglia demonstrates a stable reservoir of viral nucleic acids during latency

In this investigation we determined the dynamics of herpes simplex virus type 1 (HSV-1) DNA and latency-associated transcripts (LAT) in the latently infected rabbit trigeminal ganglion. Rabbit eyes were infected with either the McKrae strain or the l7Syn+ strain of HSV-1. Rabbits were sacrificed between 5 and 360 days after infection and their trigeminal ganglia were analyzed for the number of HSV DNA genomes and the number of neuronal cells expressing LAT. There was no statistically significant change in the number of HSV genomes or the number of neuronal cells expressing LAT in these ganglia between 20 and 360 days after infection. For both strains, the amount of HSV DNA averaged 16.8 genomes per 100 cells, and 9.2% of the neurons expressed LAT. There were 17 to 34 HSV genomes per LAT-expressing neuronal cell. The number of LAT-expressing neurons did not change over the 360 days. Spontaneous reactivation (HSV-1 recovery in tear film) and recurrence (HSV-1-specific epithelial lesions) occurred during the period of this study; however, these events did not alter the quantity of HSV-1 DNA or the number of LAT-expressing cells. These results suggest that after the latent infection is established, the viral DNA in the ganglia does not replicate to any measurable extent over long periods of latency, since no significant change in the number of HSV genomes occurs. The results also suggest that only a very small number of latently infected neuronal cells are needed to produce infectious HSV-1 during reactivation.

A 348-base-pair region in the latency-associated transcript facilitates herpes simplex virus type 1 reactivation

Latency-associated transcript (LAT) promoter deletion mutants of herpes simplex virus type 1 have a reduced capacity to reactivate following adrenergic induction in the rabbit eye model. We have mapped a reactivation phenotype within LAT and describe the construction of recombinants in which poly(A) addition sites have been placed at intervals within the LAT region to form truncated LAT transcripts. These mutants localize the induced reactivation phenotype to the 5' end of LAT. To further define this region, we constructed a recombinant containing a 348-bp deletion located 217 bp downstream of the transcription start site of the 8.5-kb LAT. This virus, 17delta348, expresses LAT but exhibits a significantly reduced ability to reactivate following epinephrine iontophoresis into the cornea. Quantitative DNA PCR analysis reveals that 17delta 348 establishes a latent infection within rabbit trigeminal ganglia with the same efficiency as does either the rescuant or wild-type virus. The region deleted in 17delta348 encodes three potential translational initiators (ATGs) which we have mutated and demonstrated to be dispensable for epinephrine-induced reactivation. In addition, three smaller deletions within this region have been constructed and were shown to reactivate at wild-type (parent) frequencies. These studies indicate that an undefined portion of the 348-bp region is required to facilitate induced reactivation. Sequence analysis of this 348-bp region revealed a CpG island which extends into the LAT promoter and which possesses homology to conserved elements within the mouse and human XIST transcript encoded on the X chromosome. Possible implications of these elements in the regulation of LAT expression are discussed.

The ecology and pathology of Epstein-Barr virus

Epstein-Barr virus achieves its ubiquitous and uniform epidemiological distribution by a dual strategy of latency to guarantee lifelong persistence and intermittent replication to guarantee transmission. These two functions appear to dictate residence in different cell types: latency in B lymphocytes and replication in epithelial cells. Both of these cell compartments are potential sites for EBV-associated malignancies.

Correlation of virus replication, cytokine (TNF-alpha and IL-1) producing cells, neuronal necrosis and inflammation after intranasal infection of mice with herpes simplex virus strains of different virulence

The number of TNF-alpha and IL-1 beta producing cells was investigated during the acute replication phase of herpes simplex virus (HSV) in trigeminal ganglia after intranasal infection with strains of different virulence. The highly virulent strain WAL replicated strongly and induced many cytokine producing cells early in the ganglia. The low virulent strain HFEM replicated less, only few cytokine producing cells were detected late. The thymidine-kinase negative (TK-) virus 1301 did not replicate but produced some lymphocytic inflammation. The higher the virulence of strains of HSV-1 or -2 was, the stronger was the extent of histopathological lesions; moreover, a dissociation in time between replication and cellular reaction (granulocytic and lymphocytic) could be observed after infection with strains HFEM and TK- virus 1301. CD4 and CD8 positive cells could be detected mainly at the rim of necrotic areas, TNF-alpha and IL-1 beta producing cells, however, were scattered throughout the ganglia.

Long-term expression of a reporter gene from latent herpes simplex virus in the rat hippocampus

A problem in utilizing herpes simplex virus (HSV) as a vector for expression of foreign genes in CNS neurons has been the inability to facilitate long-term expression of the engineered genes. Previously, we showed that the murine moloney leukemia virus LTR would drive beta-galactosidase (beta-gal) transcription for extended periods from the latent viral genome in sensory, but not motor neurons. In this communication we further evaluate the utility of the LTR promoter for use in long-term expression vectors. Following stereotactic injection of 8117/43 (an ICP4 minus, non-replicating virus with the LTR driving the beta-gal gene, or KD6 (an ICP4 minus non-replicating virus not expressing beta-gal) into the hippocampus of rats, polymerase chain reaction (PCR) analysis of viral DNA after 2 months indicated that latent infections were established. Assaying by both x-gal staining and reverse transcriptase PCR we demonstrate that (1) beta-gal can be detected for at least 6 months in hippocampal neurons, and (2) although the number of beta-gal transcripts in these cells drops considerably by 2 weeks, they can be detected during the period studied. These studies indicate that the LTR promoter is active and affords long-term expression in the CNS, albeit at comparatively low levels compared to those observed at acute times.

Experimental herpes simplex virus type 1 (HSV-1) infection of the spinal cord and dorsal root ganglia

Host factors determining the outcome of herpes simplex virus type 1 (HSV-1) infection within neurons are poorly understood. This paper aims to identify regional differences in the behaviour of HSV-1 within the nervous system as an approach to investigating the role of the host environment in determining the outcome of infection. We describe a mouse model of HSV infection focused on motor neurons of the spinal cord, resulting from intramuscular injection (i.m.) and compare this with the behaviour of virus within sensory neurons following scarification of virus on to skin. Viral antigen was detectable immunohistochemically by 2 days in both models and disappeared by 9-11 days. The time course of acute infection was reflected in the i.m. group by quantitative plaque assay for virus. Inflammation and cell destruction occurred in both models, but clinical features and histological destruction were greater in the group infected via the intramuscular route. In the sensory ganglia, a latent state from which virus could be reactivated by explanation, was established with LATS expression detectable in many neurons at 35 days post-infection (p.i.), but not in non-neuronal cells. Expression of latency associated transcript (LATS) was detected in motor neutrons in spinal cords at 35 days p.i. providing evidence for establishment of a LATS-positive latent state at this site, and continued to be detectable up to 6 months post-infection. In addition, LATS was detected in white matter at late times, suggesting a non-neuronal site of latency. In contrast to the behaviour in sensory ganglia, induced reactivation from spinal cords, by explanation and nerve section, was a very rare event. We have shown that a LATS-positive latent state can be established within motor neurons of the CNS, but that there are regional differences in the biology and outcome of infection between the CNS and peripheral nervous system. We propose that this may be a useful model to study reproducibly, the behaviour of HSV-1 in a CNS environment and, by comparison with sensory ganglion infection, to explore host factors which may underlie these regional differences. The relevance of this model for using HSV-1 as a therapeutic vector for motor neurons is also discussed.

Enhancement by TNF-alpha of reactivation and replication of latent herpes simplex virus from trigeminal ganglia of mice

The influence of tumor-necrosis-factor-alpha (TNF-alpha), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukine-1 (IL-1) and IL-3 on the in vitro reactivation frequency and replication rate of trigeminal ganglia of mice latently infected with herpes simplex virus (HSV) strain KOS was studied. It could be demonstrated that TNF-alpha and possibility GM-CSF, but not IL-1 and IL-3, enhanced the reactivation frequency and replication of HSV. Interferon alpha/beta (IFN alpha/beta) prevented reactivation and replication.

In vivo deletion analysis of the herpes simplex virus type 1 latency-associated transcript promoter

During herpes simplex virus latency, transcripts accumulate from a single transcription unit of the viral genome. The promoter for these latency-associated transcripts (LAT) has been located, and a number of studies have documented the specific regions of this promoter which are important in transient assays of neuronal cells in culture. To examine the regulation of this promoter from the viral genome, both in vitro and in vivo, a series of seven promoter deletion viruses which drive the expression of the reporter gene beta-galactosidase was constructed. Rabbit skin cells were infected in cell culture with viruses bearing each promoter mutation, and the LAT promoter activity was compared with that obtained by infecting two neuronal cell lines, ND7 cells and C1300 neuroblastoma cells. Mouse dorsal root ganglia were also infected with these recombinant viruses by footpad inoculations, and beta-galactosidase activity was measured. Infected neuronal cells lines and dorsal root ganglia exhibit much more LAT promoter activity than infected rabbit skin cells, suggesting that the region upstream of -250 may contain one or several neuronal specific DNA-binding sites. However, a comparison of LAT promoter activities within the deletion series revealed many differences between neurons of the dorsal root ganglia infected in vivo and the two neuronal cell lines infected in vitro. These results suggest that neurons may vary extensively in the quantity or kind of transcription factors they contain.

PCR-based analysis of herpes simplex virus type 1 latency in the rat trigeminal ganglion established with a ribonucleotide reductase-deficient mutant

Competitive quantitative PCR and reverse transcriptase-PCR were used to quantitate DNA and RNA from an attenuated ribonucleotide reductase-deleted herpes simplex virus type 1 (HSV-1) mutant in the rat trigeminal ganglion after peripheral inoculation following corneal scarification. Amplification of ganglionic DNA with oligonucleotide primers specific for the HSV-1 glycoprotein B (gB) gene and for the latency-associated transcript (LAT) gene indicated that there were approximately 2 x 10(5) genome equivalents per ganglion at 2 days, 7 days, and 8 weeks after inoculation. Amplification of ganglionic RNA with primers specific for HSV-1 LAT indicated that the amount of LAT RNA was also stable over 8 weeks, with 10(7) LAT molecules per ganglion at 2 days and at 7 days postinoculation and 1.4 x 10(7) LAT molecules per ganglion at 8 weeks. In situ hybridization with a digoxigenin-labeled riboprobe specific for LAT detected an average of one to two LAT-positive cells in each positive 6-microns section of trigeminal ganglion. In situ PCR detection of HSV-1 genomes in similar sections, using digoxigenin-labeled nucleotides with primers specific for HSV-1 gB, identified as many as 120 genome-positive cells per section. These results indicate that there are approximately 50 LAT molecules per latent HSV-1 genome in the trigeminal ganglion, compared with 15 LAT molecules per latent HSV-1 genome in the central nervous system (R. Ramakrishnan, D. J. Fink, G. Jiang, P. Desai, J. C. Glorioso, and M. Levine, J. Virol. 68:1864-1873, 1994), but that cells with detectable LATs by in situ hybridization represent only a small proportion of those ganglionic neurons containing HSV-1 genomes. The presence of latent HSV-1 genomes in a large number of neurons suggests that HSV-1 may be more efficient in establishing the latent state than would be anticipated from previous reports.

Competitive quantitative PCR analysis of herpes simplex virus type 1 DNA and latency-associated transcript RNA in latently infected cells of the rat brain

Competitive quantitative PCRs were used to examine the consequences of stereotactically injecting a highly attenuated herpes simplex virus type 1 mutant into rat brains. This mutant virus, designated RR1CAT/RR2lacZ, was engineered so that coding sequences of the genes UL39 and UL40 specifying the subunits of the viral ribonucleotide reductase were replaced by the chloramphenicol acetyltransferase (CAT) and the lacZ gene coding sequences, respectively. Stereotactic injection of this virus into the hippocampal region of the rat brain resulted in a localized infection. Viral gene products were visualized by immunochemical, cytochemical, or in situ hybridization techniques in the injected hippocampal region at 2 days postinjection. Viral genomes, represented by glycoprotein B (gB), latency-associated transcript (LAT), and lacZ sequences could be amplified by PCR from templates obtained by scraping hippocampal tissue off single 10-microns frozen sections. Both gB message and LAT could be detected by reverse transcriptase (RT)-PCR. At day 7 postinjection, neither CAT message, gB message, nor beta-galactosidase activity could be visualized by the same techniques, although viral DNA was detected by PCR and LAT could be detected by RT-PCR. A similar pattern was seen at 8 weeks, suggesting that latency was established by the mutant virus in cells of the injected hippocampus. By competitive quantitative PCR, hippocampal sections were determined to contain 2.6 x 10(5) genome equivalents (represented by the gB gene) on day 2, 6.2 x 10(4) on day 7, and 8.3 x 10(4) at 8 weeks. By competitive quantitative RT-PCR, the numbers of LAT molecules at the same time points were 3.2 x 10(6), 1.3 x 10(6), and 1.2 x 10(6), respectively. The numbers of LAT molecules per genome equivalent were 12.5, 20.3, and 14.5, respectively, being approximately the same for each of the three time points. The data permit the conclusion that the RR mutant virus establishes latency in the rat brain with the persistence of the viral genome and the production of LAT molecules. Once latency is established, the numbers of viral genomes and LAT RNA molecules remain constant. Thus the competitive quantitative PCR and RT-PCR techniques provide very sensitive and reliable methods to quantitate viral DNA and RNA present in infected tissue.