Herpes simplex virus DNA and mRNA sequences in acutely and chronically infected trigeminal ganglia of mice

Critical Role of Regulatory T Cells in the Latency and Stress-Induced Reactivation of HSV-1

Herpes simplex virus 1 (HSV-1) spreads in populations through a latency entry and reactivation cycle. The role of host immune-suppressive factor regulatory T cells (Treg cells) in controlling latency establishment and reactivation is not completely understood. Here, using an HSV-1 ocular infection murine model, we observe a positive correlation between the level of Treg cells and viral infectivity and demonstrate the requirement for Treg cells in latency establishment. Furthermore, we show that host stress leads to HSV-1 reactivation via increased Treg cell control of CD8⁺ T cells, permitting viral replication under diminished immune surveillance. Together, we propose that Treg cell regulation may serve as a key target for controlling HSV infection.

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.

Serological and biomolecular survey on canine herpesvirus-1 infection in a dog breeding kennel

Canine herpesvirus-1 (CaHV-1) is a globally distributed pathogen causing reproductive, respiratory, ocular and neurological disorders in adult dogs and neonatal death in puppies. This pathogen is considered poorly immunogenic, and neutralizing antibodies are found for only a short time following exposure. Further, seroprevalence can be affected by several epidemiological factors. A virological survey was conducted in a high-density population breeding kennel in Central Italy. There were several factors predisposing animals to CaHV-1 infection, such as age, number of pregnancies, experience with mating and dog shows, cases of abortion, management and environmental hygiene. Serum neutralization (SN) and nested PCR assays were used to estimate prevalence of CaHV-1. None of the submitted samples tested positive for nested PCR, and none of the sera tested CaHV-1 positive. No association was observed between antibody titers and risk factors, and no sign of viral reactivation was detected in either males or females. These results suggest that CaHV-1 is not circulating within this kennel and that further studies are needed in order to better understand the distribution of the virus within Italy.

CD8(+) T-cell attenuation of cutaneous herpes simplex virus infection reduces the average viral copy number of the ensuing latent infection

During an initial encounter with herpes simplex virus type 1 (HSV-1) it takes several days for an adaptive immune response to develop and for herpes-specific CD8(+) T cells to infiltrate sites of infection. By this time the virus has firmly established itself within the innervating sensory nervous system where it then persists indefinitely. Preventing the establishment of viral latency would require blocking the skin to nervous system transmission of the virus. We wished to examine if CD8(+) T cells present early during acute HSV-1 infection could block this transmission. We show that effector CD8(+) T cells failed to prevent the establishment of HSV latency even when present prior to infection. This lack of blocking likely reflects the delayed infiltration of the CD8(+) T cells into the infected skin. Examination of the kinetics of HSV-1 infection highlighted the rapidity at which the virus infects the sensory ganglia and singled out early viral replication within the skin as an important factor in determining the magnitude of the ensuing latent infection. Though unable to prevent the establishment of latency, CD8(+) T cells could reduce the average viral copy number of the residual latent infection by dampening the skin infection and thus limiting the skin-to-nerve transmission of virus.

Spontaneous molecular reactivation of herpes simplex virus type 1 latency in mice

Infection of the mouse trigeminal ganglia (TG) is the most commonly used model for the study of herpes simplex virus type 1 (HSV-1) latency. Its popularity is caused, at least in part, by the perception that latent infection can be studied in this system in the absence of spontaneous viral reactivation. However, this perception has never been rigorously tested. To carefully study this issue, the eyes of Swiss-Webster mice were inoculated with HSV-1 (KOS), and 37-47 days later the TG were dissected, serial-sectioned, and probed for HSV-1 ICP4, thymidine kinase, glycoprotein C, and latency-associated transcript RNA by in situ hybridization. Serial sections of additional latently infected TG were probed with HSV-1-specific polyclonal antisera. Analysis of thousands of probed sections revealed abundant expression of viral transcripts, viral protein, and viral DNA replication in about 1 neuron per 10 TG tested. These same neurons were surrounded by a focal white cell infiltrate, indicating the presence of an antigenic stimulus. We conclude that productive cycle viral genes are abundantly expressed in rare neurons of latently infected murine TG and that these events are promptly recognized by an active local immune response. In the absence of detectable infectious virus in these ganglia, we propose the term "spontaneous molecular reactivation" to describe this ongoing process.

Detection of canine herpesvirus DNA in the ganglionic neurons and the lymph node lymphocytes of latently infected dogs

To determine the site of latent infection of canine herpesvirus (CHV), tissues from dogs convalescent from acute infection with CHV were examined for the presence of viral genome DNA by the nested polymerase chain reaction. CHV DNA was detected in the trigeminal ganglia and the retropharyngeal lymph nodes. In situ hybridization study of the tissues revealed that CHV genome persisted in the nuclei of ganglionic neurons and lymphocytes.

Quantification of transcripts from the ICP4 and thymidine kinase genes in mouse ganglia latently infected with herpes simplex virus

Herpes simplex virus establishes latency in nervous tissue in which it is maintained for the life of the mammalian host, with occasional reactivation leading to subsequent spread. Latency-associated transcripts are abundant during latency, but viral proteins and productive cycle RNAs have not been detected. Using sensitive, quantitative PCR assays, we have quantified certain viral RNAs specific to productive-cycle genes in mouse ganglia latently infected with herpes simplex virus type 1. Sense-strand RNA specific to the essential immediate-early gene, ICP4, was present in most ganglia in variable amounts relative to the amount of viral DNA, with one to seven molecules of RNA per viral genome in about 20% of ganglia. In contrast, the amount of latency-associated transcripts was much less variable, at an average of 4 x 10(4) molecules per viral genome. The amounts of ICP4-specific RNA were similar at 30 and 60 days postinfection, and at least some of these transcripts initiated within a region consistent with utilization of the ICP4 promoter. RNA specific to the thymidine kinase gene, whose transcription in productive infection is dependent on ICP4, was present in latently infected ganglia at a maximum level of 3.2 x 10(6) molecules per ganglion (500 molecules per viral genome). ICP4-specific and tk-specific RNAs measured from the same samples showed a positive correlation extending over 2 orders of magnitude. We conclude that ICP4-specific RNA is expressed in the absence of detectable reactivation and discuss possible implications of our findings for latent gene expression.

Two open reading frames (ORF1 and ORF2) within the 2.0-kilobase latency-associated transcript of herpes simplex virus type 1 are not essential for reactivation from latency

The herpes simplex virus type 1 (HSV-1) latency-associated transcripts (LATs) are dispensable for establishment and maintenance of latent infection. However, the LATs have been implicated in reactivation of the virus from its latent state. Since the reported LAT deletion and/or insertion variants that are reactivation impaired contain deletions in the putative LAT promoter, it is not known which LAT sequences are involved in reactivation. To examine the role of the 2.0-kb LAT in the process of reactivation and the functional importance of the putative open reading frames (ORF1 and ORF2) contained within the 2.0-kb LAT, we have constructed an HSV-1 variant that contains a precise deletion and insertion within the LAT-specific DNA sequences using site-directed mutagenesis. The HSV-1 variant FS1001K contains an 1,186-bp deletion starting precisely from the 5' end of the 2.0-kb LAT and, for identification, a XbaI restriction endonuclease site insertion. The FS1001K genome contains no other deletions and/or insertions as analyzed by a variety of restriction endonucleases. The deletion in FS1001K removes the entire 556-bp intron within the 2.0-kb LAT, the first 229 nucleotides of ORF1, and the first 159 nucleotides of ORF2 without having an affect on the RL2 (ICP0) gene. Explant cocultivation reactivation assays indicated that this deletion had a minimal effect on reactivation of the variant FS1001K compared with the parental wild-type virus using a mouse eye model. As expected, Northern (RNA) blot analyses have shown that the variant virus (FS1001K) does not produce the 2.0-kb LAT or the 1.45- to 1.5-kb LAT either in vitro or in vivo; however, FS1001K produces an intact RL2 transcript in tissue culture. These data suggest that the 2.0-kb LAT putative ORF1 and ORF2 (or the first 1,186 bp of the 2.0-kb LAT) are dispensable for explant reactivation of latent HSV-1.

Molecular biology of herpes simplex virus type 1 latency in the nervous system

Herpes simplex virus (HSV) is one of the best studied examples of viral ability to remain latent in the human nervous system and to cause recurrent disease by reactivation. Intensive effort was directed in recent years to unveil the molecular viral mechanisms and the virus-host interactions associated with latent HSV infection. The discovery of the state of the latent viral DNA in nervous tissues and of the presence of latency-associated gene expression during latent infection, both differing from the situation during viral replication, provided important clues relevant to the pathogenesis of latent HSV infection. This review summarizes the current state of knowledge on the site of latent infection, the molecular phenomena of latency, and the mechanisms of the various stages of latency: acute infection, establishment and maintenance of latency, and reactivation. This information paved the way to recent trials aiming to use herpes viruses as vectors to deliver genes into the nervous system, an issue that is also addressed in this review.

Detection of herpes simplex virus in gingival tissue

The presence of herpes simplex virus (HSV) antigens was shown by immunofluorescence staining in 26 of 66 (39.3%) specimens of clinically healthy gingiva, but only one sample contained infectious virus. HSV DNA sequences were clearly identified in intact gingival cells by dot blot hybridization in one specimen, and a weak pattern in a second one. Both specimens harbored viral antigens. These findings of viral genome and protein expression suggest that the virus is present in the latent form in the gingiva.

The role of cyclic nucleotide mediators in latency and reactivation of HSV-1 infected neuroblastoma cells

The mechanisms that control herpes simplex virus type 1 latency and reactivation are still poorly understood. We developed an in vitro murine neuroblastoma cell HSV-infected, acyclovir suppressed model to study the influence of different cyclic nucleotide mediators on the latency and reactivation of HSV-1. A positive cDNA 'in situ' hybridisation for HSV genome was used to prove the establishment of a viral-host cell nuclear relationship. An ABC-immunoperoxidase reaction to cell surface HSV mature glycoproteins was also performed to determine the time of viral reactivation with formation of mature virions. Supernates of cultured cells were placed on Vero cells for confirmation of reactivation by classic cytopathic effect. Theophylline (50 micrograms/ml) and dibutyryl-cAMP (0.1, 0.5, 1 mg/ml) produced the most pronounced response, accelerating HSV reactivation time by 150%. Epinephrine (10, 20 micrograms/ml) had an intermediate effect on accelerating viral reactivation; and verapamil (20, 50 micrograms/ml), theophylline and epinephrine at lower doses had a smaller effect. Carbamylcholine (10 micrograms/ml) prolonged the time to viral reactivation by 100%, 36 hours compared to control time of 18 hours. Insulin (0.1, 0.5, 1 mg/ml) also prolonged HSV 'latency' by six hours. Exogenous dibutyryl-cGMP and carbamylcholine at lower concentrations did not have an effect on viral reactivation. These findings suggest that there is a relationship between changes of intracellular concentration of cyclic nucleotides and HSV latency and reactivation.

A review of the molecular mechanism of HSV-1 latency

The neurotropic herpes viruses, as typified by herpes simplex virus type 1, are noted for their ability to form latent infections. The latent infection differs from the acute infection both in gene expression and the physical state of the viral genome. Latency can be divided into several stages--establishment, maintenance of reactivation--each of which are active areas of research. This review describes the molecular biology of HSV-1 latency and presents the current level of understanding of the molecular mechanism of HSV-1 latency.

Reactivation of latent herpes simplex virus by adenovirus recombinants encoding mutant IE-0 gene products

We have previously shown that adenovirus recombinants expressing functional ICP0 reactivate latent herpes simplex virus type 2 (HSV-2) in an in vitro latency system. This study demonstrated that ICP0, independent of other HSV gene products, is sufficient to reactivate latent HSV-2 in this in vitro system. To assess the effects of defined mutations in the sequence encoding ICP0 (IE-0) on reactivation, seven in-frame insertion and three in-frame deletion mutants were moved into an adenovirus expression vector. Each recombinant directed the synthesis of stable ICP0 of the correct size. The transactivation activity of the mutated sequences in these recombinants was similar to that when they were tested in plasmids. When these recombinants were examined for their ability to reactivate in the in vitro latency system, mutants with dramatic defects in transactivation (Ad-0/125, Ad-0/89, Ad-0/2/7, and Ad-0/88/93) were unable to reactivate latent HSV-2 independent of the multiplicity of infection. An exception to this correlation was the finding that Ad-0/89, which transactivated poorly, was able to reactivate latent virus after prolonged incubation whereas other transactivation-deficient mutants could not. Moreover, the presence of ICP4 did not compensate for the inability of any of the recombinants tested to reactivate HSV-2. These results show that (i) the transactivation domains of ICP0 are also used in reactivation, (ii) the presence of another essential HSV regulatory protein ICP4 does not alter the pattern of reactivation by ICP0, and (iii) mutations in some regions of IE-0 previously shown to affect viral growth and plaque formation did not alter its ability to reactivate in this in vitro system.

Regulation and cell-type-specific activity of a promoter located upstream of the latency-associated transcript of herpes simplex virus type 1

To identify promoter regions which control expression of the latency-associated transcript (LAT) of herpes simplex virus type 1 (HSV-1), we constructed a series of recombinant vectors in which various sequences upstream of LAT were linked to the chloramphenicol acetyltransferase gene and tested for expression efficiency by transfection into tissue culture cells. In HeLa cells no activity was observed from the region (-250 to +201) immediately surrounding the nominal 5' end of LAT, but high levels of activity were observed by using different fragments within the region -1267 to -594. This promoter activity was largely contained within the 140-base-pair region from -797 to -658 and was 20- to 50-fold stronger than typical HSV delayed-early promoters and at least as strong as the activity from the simian virus 40 (SV40) enhancer-promoter region or the HSV immediate-early 110,000-Mr (IE110K) promoter region. In human neuroblastoma cells (IMR-32), there was a dramatic switch in relative activities in favor of the LAT promoter, so that it was 45- and 200-fold stronger than the IE110K and SV40 constructs, respectively. Furthermore, optimal activity in the neuroblastoma cells required sequences within the region -1267 to -797. This region had little effect on activity in HeLa cells. We also show that the LAT promoter activity was very efficiently repressed by the IE175K protein. From internal deletion analysis, the site of repression was located within a 55-base-pair region just downstream of a potential TATA box. This region exhibited a high degree of homology with the IE175K cap site and may be a binding site for the IE175K protein.

Spread of herpes simplex virus type 1 in the central nervous system during experimentally reactivated encephalitis

Because many of the features of reactivated herpes simplex virus type 1 (HSV-1) central nervous systems (CNS) infections in vivo are incompletely understood, we used an animal model to study the development of the morphological, ultrastructural, radiological and immunological changes which occurred during acute and experimentally reactivated diseases. Rabbits were intranasally inoculated with HSV-1, and their latent trigeminal ganglionic and CNS infections were reactivated by intravenous injection of cyclophosphamide and dexamethasone. Technetium brain scans were performed to localize areas of blood-brain barrier breakdown, and cerebrospinal fluid (CSF) was analysed for IgG content by radial immunodiffusion assays. Nervous system tissues were studied by in situ hybridization and by immunofluorescent, light and electron microscopic techniques. Diffuse uptake of technetium was observed as HSV-1 spread transsynaptically into the brain during the acute phase of infection, and viral antigens and nucleic acids were detected in both the CNS olfactory and trigeminal systems. During latency, viral RNA was detected in the nuclei of neurons within the CNS olfactory cerebral and entorhinal cortices, indicating that HSV-1 became latent within the same CNS structures that were involved during the acute phase of infection. Following drug-induced reactivation, the brain scans revealed a more focal breakdown of the blood-brain barrier, and both neurons and neuronal processes in the entorhinal and olfactory cortices contained viral nucleic acids which correlated with the ultrastructural presence of HSV-1 virions. During the reactivated phase of infection a marked increase in the CSF IgG index occurred without an increase in the CSF: serum albumen ratio indicating a prompt intrathecal response in infected rabbits as compared to controls. To some extent, the CSF IgG index reflected the degree of histopathological damage.

Establishment of latent ganglionic infection with herpes simplex virus via maxillary gingiva and viral re-activation in vivo after trauma

Herpes simplex virus can remain latent for months or years in sensory and automatic ganglia of animals and man, and can be re-activated in vivo by several procedures such as neurectomy, irritation of epithelial surfaces, and administration of immunosuppressive agents. The objective of this study was to determine whether dental stimuli can cause re-activation of the latent herpes simplex virus. Homogenization and explanation of ganglia from mice showed that herpes simplex virus (type 1) traveled from maxillary gingiva to trigeminal ganglia, and remained latent. It was also shown that mice passively immunized with rabbit antibody to herpes simplex virus, following the inoculation of herpes simplex virus by the maxillary gingiva route, developed a latent infection in the trigeminal ganglia. Neutralizing antibody was cleared from the circulation and could not be detected in most of these animals after five weeks. A neutralizing test showed that antibody-negative mice with latent infection were able to produce antibody to re-infection with herpes simplex virus, suggesting that re-activation can be identified by measurement of serum antibody. By use of this mouse model system, it was shown that when maxillary gingiva was traumatized with dry ice, viral re-activation occurred in 58% of these animals, as demonstrated by the appearance of neutralizing antibody. Irradiation by a Stomalaser beam had no effect on the re-activation of latent herpes simplex virus. Our mouse model system may serve as a useful model for obtaining new information on re-activating or inhibitory factors in dentistry.

Trichloroethylene cranial neuropathy: is it really a toxic neuropathy or does it activate latent herpes virus?

The mechanism of the cranial neuropathy associated with heavy exposure to trichloroethylene (or dichloroethylene) is unknown. In severe cases there is destructive spread of the neuropathic process from the Vth cranial nerve nuclei up and down the brain stem in a manner that is difficult to explain on accepted neurotoxicological principles. However, there is a close association reported of this form of trigeminal neuropathy with reactivation of orofacial herpes simplex that suggests the possibility that the chemical, which readily gains entrance into the nervous system, may be responsible for reactivating the latent virus. This novel hypothesis is discussed in the light of current understanding of latency in herpes simplex infection in nervous tissue.

Expression of herpes simplex virus type 1 (HSV-1) latency-associated transcripts and transcripts affected by the deletion in avirulent mutant HFEM: evidence for a new class of HSV-1 genes

During latent herpes simplex virus type 1 (HSV-1) infection in the trigeminal ganglia of mice, three virus-specific transcripts, 2.0, 1.5, and 1.45 kilobases (kb), are detectable by Northern (RNA) blot analysis, but only the 2.0-kb transcript can be detected in HSV-1-infected tissue culture cells (J.G. Spivack and N. W. Fraser, J. Virol. 61:3842-3847, 1987). Since these latency-associated genes map to a diploid region of the genome, transcription from the deletion mutant HFEM, which contains only one complete copy of these genes, was investigated to determine the effect of gene dosage. The 4.1-kb HFEM deletion is located between the alpha genes ICP0 and ICP27. ICP0 mRNA and the 2.0-kb latency-associated transcript were present at normal levels during HFEM infection, but ICP27 mRNA and 0.9- and 1.1-kb transcripts that map near the deletion were not readily detectable. The levels of expression of one or more of these genes might be an important determinant of HSV-1 virulence in animal hosts. ICP27 mRNA accumulated when protein synthesis was inhibited before HFEM infection, implying that the deletion may affect ICP27 regulatory rather than coding elements. Expression of the 2.0-kb latency-associated transcript was characterized in infected CV-1 cells with metabolic inhibitors and strand-specific probes. On the basis of metabolic inhibitor studies, the gene encoding the 2.0-kb latency-associated transcript is not an alpha gene. During HSV-1 replication in infected tissue culture cells, the beta and gamma genes require the prior expression of alpha gene products. However, the latency-associated RNAs are expressed in the absence of detectable levels of alpha transcripts in latently infected mice. Thus, this latency-associated gene family appear to be regulated quite differently than alpha, beta, or gamma genes. For these reasons, and because the latency-associated genes may perform latent rather than replicative functions, we propose that they should be considered members of a new HSV-1 gene class, the lambda genes.

Expression of herpes simplex virus type 1 latency-associated transcripts in the trigeminal ganglia of mice during acute infection and reactivation of latent infection

Herpes simplex virus type 1 (HSV-1) establishes a latent infection in the trigeminal ganglia of mice infected via the eye. In these ganglia three viral transcripts, of 2.0, 1.5, and 1.45 kilobases (kb), which are at least partially colinear, have been identified by Northern (RNA) blot analysis. These RNAs partially overlap ICPO, but are transcribed in the opposite direction (J. G. Spivack and N. W. Fraser, J. Virol. 61:3841-3847, 1987). The accumulation of these latency-associated transcripts, as well as other viral RNAs, was studied during an acute infection and the reactivation of a latent HSV-1 infection in mice. The 2.0-kb latency-associated transcript was detected in trigeminal ganglia of mice as early as 4 days postinfection, and the 1.45- and 1.5-kb RNA doublet was detected at 14 days postinfection. The levels of these latency-associated transcripts increased steadily over a 60-day period. In contrast, other HSV-1 transcripts were detected at 2 to 3 days postinfection, reached a peak on day 4, and rapidly declined below detectable levels by day 7. The data indicate that the temporal expression of the latency-associated genes during acute infection in the trigeminal ganglia of mice is different from the temporal expression of genes involved in HSV-1 replication. During the reactivation of latent HSV-1 from explanted trigeminal ganglia, the latency-associated RNAs decreased about twofold, but were present at significant levels even after HSV-1 DNA increased and infectious virus was recovered. The decrease of the latency-associated transcripts occurred when reactivation was blocked by phosphonoacetic acid or novobiocin, which suggests that this decrease may be an early event in the entry of latent HSV-1 into the viral replication cycle.

Latent herpes simplex virus type 1 transcripts in peripheral and central nervous system tissues of mice map to similar regions of the viral genome

Herpes simplex virus type 1 (HSV-1) DNA and RNA have been detected in peripheral nervous system (PNS) and central nervous system (CNS) tissues of latently infected mice. However, explant methods are successful in reactivating HSV-1 only from latently infected PNS tissues. In this report, latent herpesvirus infections in mouse PNS and CNS tissues were compared by in situ hybridization to determine whether the difference in reactivation was at the level of the virus or the host tissue. It was demonstrated that the HSV-1 transcripts present during latency in the mouse PNS and CNS originated from the same region of the genome, the repeats which bracket the long unique sequence. Therefore, the difference in reactivation with PNS and CNS tissues cannot be accounted for by differences in the extent of the HSV-1 genome transcribed during herpesvirus latency. Latent HSV-1 RNA was detected in the trigeminal ganglia (PNS) and the trigeminal system in the CNS from the mesencephalon to the spinal cord as well as other regions of the CNS not noted previously. Latent HSV-1 RNA was found predominantly in neurons but also in a small number of cells which could not be identified as neuronal cells. It is suggested that host differences in CNS and PNS tissues, rather than differences in latent virus transcription, may be important determinants in the HSV-1 reactivation process in explanted tissues.

Latent herpes simplex virus in human trigeminal ganglia. Detection of an immediate early gene "anti-sense" transcript by in situ hybridization

We used in situ hybridization to study the expression of herpes simplex virus type 1 genes during latent infections of human sensory ganglia. Trigeminal ganglia were recovered at autopsy from 24 subjects with no evidence of an active herpetic infection. These ganglia were hybridized to 35S-labeled single-stranded RNA probes spanning 72 percent of the herpes simplex genome. In the ganglia of 16 subjects, 0.2 to 4.3 percent of the neuronal cells contained abundant nuclear signals for viral RNA. Ganglia from three patients with low or undetectable levels of antibodies to herpes simplex type 1 lacked viral RNA signals, whereas the ganglia from all of six patients with elevated antibody titers showed viral RNA signals. Transcription was detected only from the region of the viral genome containing a gene that encodes an immediate early protein known as the infected-cell protein number zero (ICP0). However, normal ("sense") transcripts of this gene, which are prominent in an acute infection, were not detected. In contrast, a novel transcript was found overlapping with, but opposite in direction to, the ICP0 transcript (and was therefore "anti-sense"). Although this transcript has been only partially characterized, we believe that it may have a role in maintaining the latency of herpes simplex virus.

Detection of latency-related viral RNAs in trigeminal ganglia of rabbits latently infected with herpes simplex virus type 1

Using a combination of in situ hybridization and Northern (RNA) blot analysis, we investigated herpes simplex virus type 1 (HSV-1) transcriptional activity in an ocular rabbit model of HSV-1 latency. Radioactively labeled cloned fragments, representing virtually the entire HSV-1 genome, were individually hybridized to RNA in sections of trigeminal ganglia taken from rabbits during the latent phase of infection with HSV-1 (McKrae). Our results suggest that two discrete latency-related RNAs (LR-RNAs) may be present. The LR-RNAs were localized mainly in the nuclei of neurons. The more abundant LR-RNA was detected in approximately 3% of all neurons examined and was designated major LR-RNA. The other LR-RNA, designated minor LR-RNA, was detected in approximately 0.3% of neurons from latently infected rabbits. The genes for the LR-RNAs mapped in the vicinity of the immediate-early gene ICP0 (also designated IE110). The gene for the major LR-RNA partially overlapped the left (3') end of the ICP0 gene. In situ hybridization with single-stranded RNA probes showed that this LR-RNA was of complementary sense to that of ICP0 mRNA. Northern blot analysis gave an approximate size for this LR-RNA of 1.8 to 2.2 kilobases. The minor LR-RNA mapped to or near the right (5') end of the ICP0 gene. The detection of LR-RNAs suggests the possibility that these RNAs or their products may play significant roles in the initiation and/or maintenance of HSV-1 latency.

Continued expression of a poly(A)+ transcript of herpes simplex virus type 1 in trigeminal ganglia of latently infected mice

Radioactively labeled cDNAs were prepared by using as the template poly(A)+ mRNA from trigeminal ganglia of mice latently infected with herpes simplex virus type 1. These cDNAs were used as hybridization probes for Southern blots of cloned herpes simplex virus type 1 DNA fragments. Specific hybridization to fragments from the terminal repetition of the L segment was detected with probes derived from mRNAs obtained as early as 3 weeks and as late as 17 months postinoculation. Fine mapping of the region of hybridization showed that the viral transcripts originated from DNA sequences coding for the immediate-early gene IE-1 (alpha-0). These results indicate that IE-1, or an as yet unidentified gene colinear with it, is continuously expressed during latency.

Search for viral nucleic acid sequences in the post mortem brains of patients with schizophrenia and individuals who have committed suicide

Molecular hybridisation has been used to screen several areas of post mortem brain from 20 patients with schizophrenia, 23 individuals who were suspected of having committed suicide and 21 control cases, for viral nucleic acids. Cloned probes were able to detect picogram levels of viral DNA and to quantify herpes simplex type 1 DNA from encephalitic brain, but no sequences hybridising to cytomegalovirus, varicella zoster virus, herpes simplex type I or JC or BK papovaviruses were detected in any of the experimental samples. These findings are discussed with reference to the viral hypothesis of the aetiology of psychiatric disease.

Peripheral adrenergic stimulation and indomethacin in experimental ocular shedding of HSV

The application of 6-hydroxydopamine to the cornea by iontophoresis, followed by topical epinephrine, effectively induces herpes simplex virus (HSV) shedding from the external eye of latently infected rabbits. In this study the beta adrenergic blocker, Timolol, reduced virus shedding when applied immediately before the epinephrine, but continued administration resulted in increased viral shedding. While indomethacin, a prostaglandin synthesis inhibitor decreased HSV replication in cell culture, it failed to decrease virus shedding when applied topically to the eye in adrenergically stimulated animals. Timolol may act then by its effect on the peripheral cells of the eye rather than by stimulation of virus production in ganglionic neurons. These same animals were subsequently tested for latent infection of the trigeminal and superior cervical ganglia and corneas 14 months after primary infection. Only 2 of 14 animals had virus in the trigeminal ganglia, a finding which suggests that latent virus may be depleted by repeated reactivations. Virus was recovered from corneas of five rabbits by co-cultivation so it is possible that corneal latency occurs in this rabbit model as it does in humans.

Herpes simplex virus, type 1 invasion of the rabbit and mouse nervous systems revealed by in situ hybridization

Using a 3H-labelled virion DNA probe applied to tissue sections, we have previously identified the precise microscopic anatomical location of herpes simplex virus (HSV) during the acute and latent stages of infection of the mouse trigeminal ganglia and central nervous system (CNS). In the present investigation, we compared the mouse and the rabbit with respect to their ability to support acute and latent infections of trigeminal ganglionic and central nervous system neurons. We found that HSV-1, strain F, produced acute and latent infection of trigeminal ganglion cells in both mice and rabbits; however, lower levels of HSV-1 RNA were expressed in rabbit neurons as compared to mouse neurons, and many fewer neurons of the rabbit supported an acute infection than in the mouse. Studies of the trigeminal system within the CNS revealed that HSV-1 established latency more readily in the mouse than in the rabbit. The histopathology observed in acutely infected rabbit brain was less intense and less widespread than in mouse brain.

Study of HSV-1 DNA species from trigeminal ganglia of rabbits during acute and latent infections

Recurrent herpetic keratitis remains a major cause of corneal blindness in developed countries. A fundamental unanswered question regarding herpes simplex virus infection concerns the relationship between the virus and host cell DNA during latency. In the present study DNA was extracted from trigeminal ganglia during both acute and latent infection following ocular inoculation. Extracted, purified DNA was utilized for transfection and for hybridization studies using a 32P-labeled HSV-1 DNA probe. DNA extracted during acute infection was complete, linear and non-integrated. Autoradiographic patterns of DNA isolated during latent infection were suggestive of two separate DNA species.

Herpesvirus (pseudorabies virus) latency in swine: occurrence and physical state of viral DNA in neural tissues

The occurrence of the pseudorabies virus (PRV, herpes suis 1) genome in various neural tissues of latently infected pigs was investigated. During the latent phase of infection, between 7 and 52 weeks p.i., the average amount of PRV DNA ranged between 0.3 and 0.05 genome copies per cell. The results obtained by in situ cytohybridization and reassociation kinetic experiments indicated that each latently infected cell harbored at least 30 viral genome copies. PRV DNA could be demonstrated in similar frequencies (about 30% of cases) in the trigeminal ganglia, the olfactory bulb, and the medulla oblongata, and less frequently in the brain stem and the spinal cord. Southern blot analysis showed that in general the physical state of the latent genome was linear and nonintegrated. Only in 2 of 15 animals could the presence of circular or concatemeric viral DNA be observed. Thus, we could show that over a period of 13 months after infection the PRV genome persisted both qualitatively and quantitatively in a stable state in different areas of both the peripheral and the central nervous system.

Speculations on the role of transmissible agents in the pathogenesis of Alzheimer's disease

Unconventional agents and conventional viruses provide model systems to investigate the pathogenesis of Alzheimer's disease (AD). The essay which follows examines the hypothetical role of herpes simplex in AD and presents some generally applicable experimental approaches to detecting genes in brain tissues. The concluding section, on parallels between AD and diseases of the brain caused by unconventional viruses, defines strategies for isolating genes related to pathology.

Persistent viral infection. The carrier state

A persistent viral infection is one in which the virus in a replicating or non-replicating form persists in the host beyond the normal recovery and elimination period for that particular viral infection. The clinical significance and mechanisms of persistence, when known, are discussed for the important viral infections of dogs and cats. Particular emphasis is given to feline viral rhinotracheitis, feline calicivirus, canine distemper, and feline leukemia.

Herpes simplex virus immediate-early protein ICP4 in murine models of latency

Herpes simplex virus-1 (HSV) infection of trigeminal and dorsal root ganglia was established in mice via corneal scarification and footpad injection, respectively. Trigeminal and dorsal root ganglia were removed during the acute and latent stages of infection and processed for the immediate-early HSV polypeptide ICP4 (VP175) using both a monoclonal reagent and polyclonal antiserum and the avidin biotin complex immunoperoxidase method. ICP4 (VP175) antigen was readily detected during the acute infection of both dorsal and trigeminal ganglia, but not during latency. This antigen could again be detected by reactivation of the latent virus by explanation and organ culture. The detection of ICP4 (VP175) during latency in a rabbit model but not in a murine model may correlate with biologic differences in each system (e.g., rabbits spontaneously reactivate). Alternatively, the discrepancy could reflect viral strain characteristics or may imply that ICP4 (VP175) need not be constitutively expressed (at detectable levels) in all models of latent infection.

Detection of HSV1 DNA by in situ hybridisation in human brain after immunosuppression

Human brain cells were examined for the presence of herpes simplex virus type 1 (HSV1) DNA sequences by in situ hybridisation. Viral genome was detected in immunosuppressed patients with virological evidence of past HSV infection but not in immunosuppressed patients with no such evidence. In patients who had not been immunosuppressed, no HSV DNA sequences were detectable.

Detection of two viral genomes in single cells by double-label hybridization in situ and color microradioautography

Double labeling and color microradioautography were used in a new method of hybridization in situ to identify different genes in individual cells. The method is based on the unequal penetration of 3H and 35S into two layers of nuclear track emulsion separated by a thin barrier film. Hybridization of a 35S-labeled probe specific for one kind of gene results in silver grains over cells in both layers of emulsion; a 3H-labeled probe for a second gene provides grains only in the first layer of emulsion. Silver grains are converted to magenta-colored grains in the first layer and to cyan-colored grains in the second to facilitate enumeration of grains in each layer. This technique should be widely applicable in analyses of differential gene expression in single cells or in discrete populations of cells.

Herpes simplex virus persistence in mouse neuroblastoma (C 1300) cell cultures: role of interferon

The Mp strain of herpes simplex virus type 1 (HSV1) induced a persistent infection in the mouse C 1300 neuronal cell line (clone N 115). C 1300 cultures infected at an MOI of 0.01 or 0.001 survived the initial infection and continued to produce infectious virus and viral antigens for 185 days and 31 days, respectively. Viral antigens were not detected in cultures no longer producing infectious virus; these "cured" cultures had comparable susceptibility to reinfection with HSV as previously uninfected C 1300 cells. While significant amounts of interferon were produced by C 1300 cells when challenged with Newcastle Disease Virus (NDV) or when treated with poly I:C, HSV-induced interferon could not be detected in either the acutely or persistently infected cell lines. The persistent state was not significantly altered by the addition of 1,000 units/ml of murine interferon alpha plus beta (MuIFN alpha + beta), nor was it affected by the addition of antibody to MuIFN. It appears that IFN does not play an important role in the establishment and/or maintenance of viral persistence in this neuronal system.

Herpes simplex virus infections

Infections with herpes simplex virus (HSV) are extremely common. HSV infection may be asymptomatic or may cause any one of a wide variety of disease syndromes. In this review, the physical properties and mode of replication of HSV are briefly described, and an outline of the different clinical manifestations associated with HSV infection is presented. Principles of diagnosis, treatment, and prevention of these infections are also discussed.

Neuritic uptake and transport of antiviral drugs modifying herpes simplex virus infection of rat sensory neurons

The neuritic uptake and transport of three antiviral drugs were studied in a cell culture system with dissociated cells of rat dorsal root ganglia. Cultured sensory neurons extended neuritic projections which penetrated a vacuum grease sealed diffusion barrier in the culture. The peripheral infection with herpes simplex virus (HSV) type 1 (McIntyre) resulted in uptake and transport of HSV by neuritic extensions causing a neuronal infection inside the diffusion barrier. By varying the route of administration and concentration of drug and by manipulating the nerve cell culture system, neuritic uptake and transport also of the antiviral drugs (acyclovir, adenine-arabinoside and foscarnet) were demonstrable. The findings are discussed in relation to axonal transport and antiviral treatment of HSV infections of the nervous system.

A variant of human cytomegalovirus derived from a persistently infected culture

Infection of cells derived from an osteogenic sarcoma (HOS) with human cytomegalovirus (HCMV) resulted in persistent infection. It appears that persistent infection is due to a balance between release of virus and the growth of uninfected cells. Viruses derived from the persistently infected cultures were not temperature sensitive nor were they defective interfering particles. However, hybridization experiments using the Q-labeled probe from the XbaI Q fragment indicated that one copy of the repeat sequences contained in fragments Q and O of CMV, Towne DNA have been completely deleted from the virus DNA derived from the persistent culture. Thus the mechanism of persistent infection is probably due in part to a variant of CMV present in the cultures.

Efficacy of the acyclic nucleoside 9-(1,3-dihydroxy-2-propoxymethyl)guanine against primary and recrudescent genital herpes simplex virus type 2 infections in guinea pigs

The acyclic nucleoside 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG) was evaluated for its efficacy in protecting guinea pigs from primary and recrudescent infections of herpes simplex virus type 2. Vaginally infected guinea pigs were treated twice a day with DHPG at 25 mg/kg per dose for 3 weeks. Subcutaneous doses were started 3 h, 24 h, or 5 weeks after virus inoculation. Treatment starting at 3 or 24 h reduced the severity of the primary infection by greater than 70% when lesions were graded for 3 weeks; lesion duration was lessened by greater than 55%. For 6 weeks after treatment, the number of recrudescent lesions was reduced by greater than 60%, and the duration of the recrudescences declined by greater than 40%. When dosing was started at 3 h postinfection, 33% of the animals did not develop any sign of primary or recrudescent infection throughout the 9-week test. By comparison, all the animals treated with DHPG starting at 24 h or with saline became infected. A 3-week DHPG regimen starting 5 weeks postinfection reduced the number of animals that developed recrudescent lesions by 70%. When treatment ended, however, recrudescent episodes in the animals increased to the level of saline-treated controls. These results suggest that (i) DHPG is highly effective in reducing the severity of both primary and recrudescent lesions of herpes simplex virus type 2, (ii) early treatment of a primary infection or treatment of recrudescences reduces the incidence of recrudescences, and (iii) the drug appears to have no effect on the latent form of the virus, as the incidence of recrudescences increases when DHPG treatment is ended.

Specific reduction in Na currents after infection with herpes simplex virus in cultured mammalian nerve cells

Tissue-cultured nerve cells originating from dorsal root ganglia of adult guinea-pigs were infected with type 2 herpes simplex virus (HSV) in vitro, and changes in membrane properties were examined (48 h later) by conventional electrophysiological technique using a glass microelectrode. The nerve cells infected with HSV (HSV-NC) failed to generate full-sized Na spikes, which was associated with a marked reduction in the maximum rate of rise of the Na spike, i.e. Na currents. Some SV-NC failed to generate any Na spike. By contrast, the HSV-NC generated unchanged, full-sized Ca spikes when bathed in a Na-free solution containing tetraethylammonium+. Changes in resting membrane properties, such as resting potential, input resistance and capacitance, remained small after the infection. We thus consider that HSV reduces Na currents in a specific fashion in this early period of the infection.

Morphological and physiological studies on cultured nerve cells from guinea pigs infected with herpes simplex virus in vivo

Adult guinea pigs were inoculated with type 2 herpes simplex virus (HSV) on the whole back skin, and nerve cells from the dorsal root ganglia (DRG) 6 days after infection were grown in tissue culture. Morphological and physiological properties of the cultured nerve cells from HSV-infected animals (HSV-NC) were compared to those of nerve cells of DRG from the control, non-virus infected animals (CON-NC). During the early period of the culture (0-4 days) growth of nerve cells and non-neuronal cells from the HSV infected animals was essentially the same as that from the control animals. HSV-antigen was present in only a small percentage of the HSV-NC by immunofluorescence (IF). Electrophysiological examination revealed that most of the HSV-NC exhibited a reduced capability of generating spikes, which was quantitatively described as a reduction in the Vmax of the Na spikes. This was interpreted as a reduction in the number of Na channel molecules in the plasma membrane of the HSV-NC, while the resting membrane properties of the same cells, such as the resting membrane potential, input resistance and capacitance, were essentially the same as those of the CON-NC. On 4-5 days in culture, some HSV-NC regained a full capability to generate Na spikes. We considered these nerve cells to have overcome the HSV infection and were now entering a latent period of HSV infection. About 1/3 of the HSV-NC still remained incapable of generating Na spikes. Viral antigen was detected in only 10% of the nerve cells. In the late stage of the culture, HSV infection in vitro was first observed as lysis of non-neuronal cells growing close to some HSV-NC. Nerve cells then started to lose their neurites and became spherical. Finally on 8-9 days most of the cells, including the nerve cells, were lost from the dishes as a result of a generalized infection of supporting cells, i.e. fibroblasts and Schwann cells. This study confirms our previous finding that the electrophysiological technique is much more sensitive than the IF method for the detection of HSV-infection in nerve cells. The results indicate that some nerve cells infected with HSV overcome the infection in vitro. This is interpreted as the entering of these nerve cells into a latent period of HSV infection in vitro.

Homology between murine and human cellular DNA sequences and the terminal repetition of the S component of herpes simplex virus type 1 DNA

DNA from trigeminal ganglia of mice latently infected with herpes simplex virus type 1 was cloned in bacteriophage lambda Charon 27. Three recombinant clones that cross-hybridize to each other and hybridize to the S-segment terminal repeat region of the herpesvirus genome were obtained. The region of homology maps within a 1.1 kb Sma l fragment approximately 400 bp from the S-segment terminal repeat terminus, outside the domain of the a sequences. The insert DNAs in these clones, as well as the Sma l fragment of viral DNA that contains the region of homology, hybridize to uninfected mouse brain DNA with a pattern indicative of the presence of repeated sequences, and also hybridize to discrete Eco Rl fragments of human DNA. These recombinant clones contain intermediate repetitive mouse genomic DNA sequences related to a small region of the viral S-segment terminal repeat, and those sequences are evolutionarily conserved in mammalian DNA.