Nerve growth factor deprivation results in the reactivation of latent herpes simplex virus in vitro

A5-positive primary sensory neurons are nonpermissive for productive infection with herpes simplex virus 1 in vitro

Herpes simplex viruses 1 and 2 (HSV-1 and HSV-2) establish latency and express the latency-associated transcript (LAT) preferentially in different murine sensory neuron populations, with most HSV-1 LAT expression in A5(+) neurons and most HSV-2 LAT expression in KH10(+) neurons. To study the mechanisms regulating the establishment of HSV latency in specific subtypes of neurons, cultured dissociated adult murine trigeminal ganglion (TG) neurons were assessed for relative permissiveness for productive infection. In contrast to that for neonatal TG, the relative distribution of A5(+) and KH10(+) neurons in cultured adult TG was similar to that seen in vivo. Productive infection with HSV was restricted, and only 45% of cultured neurons could be productively infected with either HSV-1 or HSV-2. A5(+) neurons supported productive infection with HSV-2 but were selectively nonpermissive for productive infection with HSV-1, a phenomenon that was not due to restricted viral entry or DNA uncoating, since HSV-1 expressing β-galactosidase under the control of the neurofilament promoter was detected in ∼90% of cultured neurons, with no preference for any neuronal subtype. Infection with HSV-1 reporter viruses expressing enhanced green fluorescent protein (EGFP) from immediate early (IE), early, and late gene promoters indicated that the block to productive infection occurred before IE gene expression. Trichostatin A treatment of quiescently infected neurons induced productive infection preferentially from non-A5(+) neurons, demonstrating that the nonpermissive neuronal subtype is also nonpermissive for reactivation. Thus, HSV-1 is capable of entering the majority of sensory neurons in vitro; productive infection occurs within a subset of these neurons; and this differential distribution of productive infection is determined at or before the expression of the viral IE genes.

Nature and duration of growth factor signaling through receptor tyrosine kinases regulates HSV-1 latency in neurons

Herpes simplex virus-1 (HSV-1) establishes life-long latency in peripheral neurons where productive replication is suppressed. While periodic reactivation results in virus production, the molecular basis of neuronal latency remains incompletely understood. Using a primary neuronal culture system of HSV-1 latency and reactivation, we show that continuous signaling through the phosphatidylinositol 3-kinase (PI3-K) pathway triggered by nerve growth factor (NGF)-binding to the TrkA receptor tyrosine kinase (RTK) is instrumental in maintaining latent HSV-1. The PI3-K p110α catalytic subunit, but not the β or δ isoforms, is specifically required to activate 3-phosphoinositide-dependent protein kinase-1 (PDK1) and sustain latency. Disrupting this pathway leads to virus reactivation. EGF and GDNF, two other growth factors capable of activating PI3-K and PDK1 but that differ from NGF in their ability to persistently activate Akt, do not fully support HSV-1 latency. Thus, the nature of RTK signaling is a critical host parameter that regulates the HSV-1 latent-lytic switch.

Tegument protein control of latent herpesvirus establishment and animation

Herpesviruses are successful pathogens that infect most vertebrates as well as at least one invertebrate species. Six of the eight human herpesviruses are widely distributed in the population. Herpesviral infections persist for the life of the infected host due in large part to the ability of these viruses to enter a non-productive, latent state in which viral gene expression is limited and immune detection and clearance is avoided. Periodically, the virus will reactivate and enter the lytic cycle, producing progeny virus that can spread within or to new hosts. Latency has been classically divided into establishment, maintenance, and reactivation phases. Here we focus on demonstrated and postulated molecular mechanisms leading to the establishment of latency for representative members of each human herpesvirus family. Maintenance and reactivation are also briefly discussed. In particular, the roles that tegument proteins may play during latency are highlighted. Finally, we introduce the term animation to describe the initiation of lytic phase gene expression from a latent herpesvirus genome, and discuss why this step should be separated, both molecularly and theoretically, from reactivation.

Zhangfei, a novel regulator of the human nerve growth factor receptor, trkA

The replication of herpes simplex virus (HSV) in epithelial cells, and during reactivation from latency in sensory neurons, depends on a ubiquitous cellular protein called host cell factor (HCF). The HSV transactivator, VP16, which initiates the viral replicative cycle, binds HCF as do some other cellular proteins. Of these, the neuronal transcription factor Zhangfei suppresses the ability of VP16 to initiate the replicative cycle. It also suppresses Luman, another cellular transcription factor that binds HCF. Interactions of nerve growth factor (NGF) and its receptor tropomyosin-related kinase (trkA) appear to be critical for maintaining HSV latency. Because the neuronal transcription factor Brn3a, which regulates trkA expression, has a motif for binding HCF, we investigated if Zhangfei had an effect on its activity. We found that Brn3a required HCF for activating the trkA promoter and Zhangfei suppressed its activity in non-neuronal cells. However, in neuron-like NGF-differentiated PC12 cells, both Brn3a and Zhangfei activated the trkA promoter and induced the expression of endogenous trkA. In addition, capsaicin, a stressor, which activates HSV in in vitro models of latency, decreased levels of Zhangfei and trkA transcripts in NGF-differentiated PC12 cells.

Efficient quiescent infection of normal human diploid fibroblasts with wild-type herpes simplex virus type 1

Quiescent infection of cultured cells with herpes simplex virus type 1 (HSV-1) provides an important, amenable means of studying the molecular mechanics of a nonproductive state that mimics key aspects of in vivo latency. To date, establishing high-multiplicity nonproductive infection of human cells with wild-type HSV-1 has proven challenging. Here, we describe simple culture conditions that established a cell state in normal human diploid fibroblasts that supported efficient quiescent infection using wild-type virus and exhibited many important properties of the in vivo latent state. Despite the efficient production of immediate early (IE) proteins ICP4 and ICP22, the latter remained unprocessed, and viral late gene products were only transiently and inefficiently produced. This low level of virus activity in cultures was rapidly suppressed as the nonproductive state was established. Entry into quiescence was associated with inefficient production of the viral trans-activating protein ICP0, and the accumulation of enlarged nuclear PML structures normally dispersed during productive infection. Lytic replication was rapidly and efficiently restored by exogenous expression of HSV-1 ICP0. These findings are in agreement with previous models in which quiescence was established with HSV mutants disrupted in their expression of IE gene products that included ICP0 and, importantly, provide a means to study cellular mechanisms that repress wild-type viral functions to prevent productive replication. We discuss this model in relation to existing systems and its potential as a simple tool to study the molecular mechanisms of quiescent infection in human cells using wild-type HSV-1.

Topical treatment with nerve growth factor in an animal model of herpetic keratitis

BACKGROUND

In vitro and in vivo studies demonstrated the antiviral efficacy of nerve growth factor (NGF) and its cyto-protective effect in herpes simplex virus (HSV)-infected cells. The aims of this study were to evaluate the role of endogenous NGF in HSV corneal infection, and the effects of topical NGF treatment on herpetic keratitis.

METHODS

Herpetic keratitis was induced in 40 rabbits with the HSV-1 McKrae strain. Animals were divided into four groups, and treated with topical neutralizing anti-NGF antibodies, NGF, acyclovir or balanced salt solution (BSS) respectively. The clinical course of HSV keratitis was evaluated and scored by slit-lamp examination. In addition, biochemical (immunohistochemistry for glycoprotein D) and molecular (nested PCR for glycoprotein D) analyses were carried out to estimate viral replication.

RESULTS

Treatment with anti-NGF antibodies induced a more severe keratitis associated with increased biochemical and molecular markers of active viral replication. Two animals in this group developed lethal HSV encephalitis. Conversely, topical treatment with NGF induced a significant amelioration of clinical and laboratory parameters when compared to the BSS treated group (control). No significant differences were observed between NGF- and acyclovir-treated groups.

CONCLUSIONS

This study demonstrated the crucial role of endogenous NGF in herpetic keratitis. The comparable effects of NGF and acyclovir confirm the antiviral activity of NGF, and indicate a potential use of topical NGF in herpetic keratitis.

Herpes simplex virus type-1 latency inhibits dendritic growth in sympathetic neurons

Herpes simplex virus type-1 (HSV-1) initially infects mucoepithelial tissues of the orofacial region, the eye and to a lesser extent the genitalia. Subsequently, the virus is retrogradely transported through the axons of the sensory and sympathetic neurons to their nuclei, where the virus establishes a life-long latent infection. During this latency period, the viral genome is transcriptionally silent except for a single region encoding the latency-associated transcript (LAT). LAT has been shown to affect apoptosis, but little else is known regarding its effects on neurons. To understand how HSV-1 latency might affect dendrites in sympathetic neurons, we transfected primary cultures of sympathetic neurons obtained from rat embryos, with LAT expressing plasmids. LAT inhibited initial dendritic growth and induced dendritic retraction in sympathetic neurons. Latent HSV-1 infection of cultured sympathetic neurons inhibited dendritic growth indicating that this is likely also a consequence of natural infection.

Latent herpes simplex virus infection of sensory neurons alters neuronal gene expression

The persistence of herpes simplex virus (HSV) and the diseases that it causes in the human population can be attributed to the maintenance of a latent infection within neurons in sensory ganglia. Little is known about the effects of latent infection on the host neuron. We have addressed the question of whether latent HSV infection affects neuronal gene expression by using microarray transcript profiling of host gene expression in ganglia from latently infected versus mock-infected mouse trigeminal ganglia. (33)P-labeled cDNA probes from pooled ganglia harvested at 30 days postinfection or post-mock infection were hybridized to nylon arrays printed with 2,556 mouse genes. Signal intensities were acquired by phosphorimager. Mean intensities (n = 4 replicates in each of three independent experiments) of signals from mock-infected versus latently infected ganglia were compared by using a variant of Student's t test. We identified significant changes in the expression of mouse neuronal genes, including several with roles in gene expression, such as the Clk2 gene, and neurotransmission, such as genes encoding potassium voltage-gated channels and a muscarinic acetylcholine receptor. We confirmed the neuronal localization of some of these transcripts by using in situ hybridization. To validate the microarray results, we performed real-time reverse transcriptase PCR analyses for a selection of the genes. These studies demonstrate that latent HSV infection can alter neuronal gene expression and might provide a new mechanism for how persistent viral infection can cause chronic disease.

Neurovirological methods and their applications

Over the last 30 years neurovirology has emerged as a major discipline which has much relevance to both human disease and many aspects of neuroscience. This overview of the field aims to define briefly most of the major neurovirological techniques, both "classical" and more recent, and to indicate how these have been used to gain knowledge about the pathogenesis, clinical investigation, and treatment of viral infections of the central nervous system.

Caspase-3-dependent reactivation of latent herpes simplex virus type 1 in sensory neuronal cultures

Life-long latent herpes simplex virus type 1 (HSV-1) is harbored in sensory neurons where sporadic reactivation occurs. Reactivation stimuli may involve activation of apoptotic signaling in the neuron. Previous experiments have demonstrated that reactivation of latent HSV-1 in dorsal root ganglion (DRG) neuronal cultures occurred following nerve growth factor (NGF) deprivation. NGF deprivation stimulates apoptotic signaling by activating the proapoptotic proteolytic enzyme, caspase-3. When DRG neuronal cultures harboring latent HSV-1 were treated with a caspase-3-specific inhibitor, NGF deprivation-induced reactivation was significantly reduced. Interestingly, the caspase-3 inhibitor had no effect on productive HSV-1 infection. Furthermore, activation of caspase-3 with either C2-ceramide or a recombinant adenovirus expressing caspase-3 caused significant HSV-1 reactivation.

Capsaicin-induced reactivation of latent herpes simplex virus type 1 in sensory neurons in culture

Herpes simplex virus type 1 (HSV-1) produces a life-long latent infection in neurons of the peripheral nervous system, primarily in the trigeminal and dorsal root ganglia. Neurons of these ganglia express high levels of the capsaicin receptor, also known as the vanilloid receptor-1 (VR-1). VR-1 is a non-selective ion channel, found on sensory neurons, that primarily fluxes Ca(2+) ions in response to various stimuli, including physiologically acidic conditions, heat greater than 45 degrees C and noxious compounds such as capsaicin. Using an in vitro neuronal model to study HSV-1 latency and reactivation, we found that agonists of the VR-1 channel - capsaicin and heat - resulted in reactivation of latent HSV-1. Capsaicin-induced reactivation of HSV-1 latently infected neurons was dose-dependent. Additionally, activation of VR-1 at its optimal temperature of 46 degrees C caused a significant increase in virus titres, which could be attenuated with the VR-1 antagonist, capsazepine. VR-1 activation that resulted in HSV-1 reactivation was calcium-dependent, since the calcium chelator BAPTA significantly reduced reactivation following treatment with caspsaicin and forskolin. Taken together, these results suggest that activation of the VR-1 channel, often associated with increases in intracellular calcium, results in HSV-1 reactivation in sensory neurons.

Herpesvirus quiescence (QIF) in neuronal cells VI: Correlative analysis demonstrates usefulness of QIF-PC12 cells to examine HSV-1 latency, reactivation and genes implicated in its regulation

PURPOSE

To compare the usefulness of the in vitro quiescently infected (QIF)-PC12 cell model(30) with the in vivo rabbit eye model of latency for the study of herpes simplex virus (HSV) genes implicated in reactivation from latency.

METHODS

HSV-1 strains 17+/pR20.5/5 and 17+/pR20.5/5/LAT, that were previously constructed by insertion of genes encoding beta-galactosidase, green fluorescent protein (GFP) or the latency associated transcript (LAT) open reading frame in the U(S)5 region,(34) were used to examine viral growth and inducible reactivation in the two models.

RESULTS

17+/pR20.5/5 exhibited diminished reactivation phenotype when compared with wild type 17+ in neuronal cells (i.e., QIF-PC12 cell model) and the rabbit eye model of latency. 17+/pR20.5/5/LAT, which contains the deregulated LAT gene, reactivated at wild type levels. Analysis of growth in neurally differentiated (ND)-PC12 cells demonstrated a low proportion of QIF cells expressed virus-encoded signals during the quiescent infection and a direct relationship between lytic viral growth in neuronal cells and reactivation phenotype. Even though 17+/pR20.5/5/LAT produced a more severe acute infection in the rabbit cornea, the different reactivation efficiency of 17+/pR20.5/5 and 17+/pR20.5/5/LAT in vivo and in vitro was not attributed to different viral genome copy number in the cells harboring cryptic genomes.

CONCLUSIONS

We conclude that 1) viral growth in neuronal cells correlates with reactivation phenotype in vivo and in vitro, 2) 17+/pR20.5/5 is attenuated in viral growth and reactivation in both models, and 3) 17+/pR20.5/5/LAT demonstrates wild-type phenotype for reactivation in both models. Attenuation of 17+/pR20.5/5 could be the result of the disruption of U(S)5 or a second site mutation. If the attenuation is the result of U(S)5 disruption, a gene that provides anti-apoptotic functions,( 41,42) this attenuation is more than compensated for by the expression of the LAT ORF. Overall, the findings indicate that the QIF-PC12 cell model is useful for segregating phases of reactivation, and particularly studying the inductive events involved in reactivation of a cryptic viral genome in neurally differentiated cells.

Entry of herpes simplex virus type 1 into primary sensory neurons in vitro is mediated by Nectin-1/HveC

Primary cultures of rat and mouse sensory neurons were used to study the entry of herpes simplex virus type 1 (HSV-1). Soluble, truncated nectin-1 but not HveA prevented viral entry. Antibodies against nectin-1 also blocked infection of rat neurons. These results indicate that nectin-1 is the primary receptor for HSV-1 infection of sensory neurons.

The roles of inflammation, STAT transcription factors, and nerve growth factor in viral reactivation and herpes keratitis

Nerve growth factor (NGF) has an inhibitory effect while inflammatory cytokines may stimulate herpes simplex virus (HSV) reactivation. NGF binds to its receptor trkA on terminal axons and signals the neuron cell body in the ganglion. Many cytokines may also signal neurons through their specific receptors, affecting STAT transcription factors within the cell bodies. We studied the effects of trigeminal ganglion (TG) explantation, a powerful HSV reactivation stimulus, on the NGF/trkA and the JAK/STAT signaling pathways. Immunohistochemistry and gel shifting experiments were performed using anti-STAT, anti-trkA, or negative control antibodies on mouse TGs. The expression of neuronal trkA was greatly reduced or eliminated by 3 days postexplantation. In contrast, the expression of STAT1, STAT3, and STAT5b, as well as phosphotyrosine-STAT3 were relatively preserved in these explanted TGs. Gel-shifting experiments indicated that TG nuclear extracts bind specifically to the HSV-1 LAT promoter, an important viral gene that regulates reactivation. STAT1, but not STAT3 or STAT5b, was detected as a component of this LAT binding complex. These studies suggest that the inhibitory effects of NGF/trkA signaling are lost after TG explantation while STAT expression is maintained, allowing HSV-1 reactivation to proceed.

Lack of effect of treatment with penciclovir or acyclovir on the establishment of latent HSV-1 in primary sensory neurons in culture

Recent studies suggest reductions in establishment of herpes simplex virus, type 1 (HSV-1) latency using the nucleoside analog penciclovir compared with acyclovir in the murine model. These observations raise the possibility that the new analogs may have novel activities that directly interfere with the establishment of the latent infection, suggesting a mechanism other than simply blocking the productive infection. To determine if penciclovir has a direct action on the establishment of latency, we compared the effects of penciclovir versus acyclovir in an in vitro model of HSV-1 latency in rat dorsal root ganglia neurons in culture. In neurons in culture, both penciclovir and acyclovir were highly effective in blocking the productive infection. However, neither penciclovir nor acyclovir blocked establishment of latency as demonstrated by similar percentages of neurons expressing the latency-associated transcript (LAT). Following removal of the respective nucleoside analog, latency was maintained until reactivation was induced by nerve growth factor deprivation. Similar virus titers were recovered after induction of reactivation of latent infections, which were established in the presence of either penciclovir or acyclovir. These results indicate that neither penciclovir nor acyclovir treatment directly prevents the establishment of latent HSV-1 infections in primary sensory neurons in culture.

Signal transducers and activators of transcription (Stat) are detectable in mouse trigeminal ganglion neurons

We studied signal transducers and activators of transcription (Stat) expression in mouse trigeminal ganglia (TG) to gain an understanding of herpes simplex virus (HSV) infection and reactivation. Mouse TG were harvested and were either frozen for Western blot analysis or preserved in 4% paraformaldehyde for subsequent immunohistochemistry study. The thawed specimens were homogenized, and nuclear/cytoplasmic extractions were performed for Western blots and immunoprecipitation. Immunohistochemistry showed that Stat1, Stat3, Stat4, Stat5b, and phosphotyrosine Stat3 localized to TG neurons, not surrounding satellite cells. Western blot of TG nuclear and cytoplasmic extracts confirmed the presence of these Stat at the appropriate molecular weights. Stat2 was undetectable in TG by these methods. Immunoprecipitation of TG nuclear extracts did not confirm the presence of Stat-Stat dimers in these specimens. These studies show that several Stat, including phosphotyrosine Stat3, are present in TG neurons, the site of HSV latency, where they could act upon latent viral DNA to effect reactivation.

HSV-1 Vector-Delivered FGF2 to the Retina Is Neuroprotective but Does Not Preserve Functional Responses

Fibroblast growth factor 2 (bFGF, FGF2) exhibits mitogenic, angiogenic, wound healing, and neuroprotective properties. Infusion of FGF2 in vivo to treat neurodegenerative disorders in animal models results in increased survival of damaged neurons, but these effects are transient. To test the feasibility of HSV vector-delivered FGF2 for neuroprotection, we inserted the FGF2 gene under the control of the HCMV immediate-early promoter into an attenuated avirulent HSV-1 vector. Transduction with FGF2/HSV-1 virus promoted survival of PC12 cells, induced differentiation of these cells to the neuronal phenotype in vitro, and protected PC12 neuronal cells from death induced by nerve growth factor withdrawal. The attenuated FGF2/HSV-1 virus was able to deliver and direct expression of the FGF2 gene in the eye. Delivery prior to light exposure in a rat model of retinal degeneration resulted in significant protection against photoreceptor loss. However, functional ERG responses were not detected. Treatment of normal eyes with the vector alone suppressed ERGs, which were only partially restored in eyes receiving the FGF2 vector. Thus, although the FGF2-HSV-1 virus induced preservation of cell and tissue structure, this was not sufficient to protect photoreceptor function.

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.

Herpes simplex encephalitis occurring after chemotherapy, surgery, and stereotactic radiotherapy for medulloblastoma

Reactivation of latent herpes simplex virus (HSV) in the trigeminal ganglion most commonly gives rise to recurrent herpes labialis and rarely to herpes simplex encephalitis. The mechanisms underlying reactivation of latent trigeminal HSV are complex. Here we report the case history of a 25-year-old woman who developed a fatal, bilateral necrotizing destructive temporal lobe lesion following surgical removal of a cerebellar medulloblastoma and combined radiotherapy and chemotherapy for recurrent tumor. Neuropathologic examination of the brain revealed minimal inflammatory changes, but immunohistochemistry was positive for HSV protein, and HSV deoxyribonucleic acid (DNA) was recovered from formalin-fixed paraffin-embedded brain tissue. The temporal proximity of the surgery, chemotherapy, and radiotherapy to the onset of disease suggests that these factors may have acted as triggers that precipitated conversion of latent HSV to overt HSV.

Fibril formation and neurotoxicity by a herpes simplex virus glycoprotein B fragment with homology to the Alzheimer's A beta peptide

Despite significant progress in the elucidation of the genetic basis of early-onset familial Alzheimer's disease (AD), the etiology of sporadic cases remains elusive. Although certain genetic loci play a role in conferring susceptibility in some sporadic AD cases, it is likely that the etiology is multifactorial; hence, the majority of cases cannot be attributed to genetic factors alone, indicating that environmental factors may modulate the onset and/or progression of the disease. Head injury and infectious agents are environmental factors that have been periodically implicated, but no plausible mechanisms have been clearly identified. With regard to infectious agents, speculation has often centered on the neurotropic herpes viruses, with herpes simplex virus 1 (HSV1) considered a likely candidate. We report that an internal sequence of HSV1 glycoprotein B (gB) is homologous to the carboxyl-terminal region of the A beta peptide that accumulates in diffuse and neuritic plaques in AD. Synthetic peptides were generated and the biophysical and biological properties of the viral peptide compared to those of A beta. Here we show that this gB fragment forms beta-pleated sheets, self-assembles into fibrils that are thioflavin-positive and ultrastructurally indistinguishable from A beta, accelerates the formation of A beta fibrils in vitro, and is toxic to primary cortical neurons at doses comparable to those of A beta. These findings suggest a possible role for this infectious agent in the pathophysiology of sporadic cases of AD.

CD8(+) T cells can block herpes simplex virus type 1 (HSV-1) reactivation from latency in sensory neurons

Recurrent herpes simplex virus type 1 (HSV-1) disease usually results from reactivation of latent virus in sensory neurons and transmission to peripheral sites. Therefore, defining the mechanisms that maintain HSV-1 in a latent state in sensory neurons may provide new approaches to reducing susceptibility to recurrent herpetic disease. After primary HSV-1 corneal infection, CD8(+) T cells infiltrate the trigeminal ganglia (TGs) of mice, and are retained in latently infected ganglia. Here we demonstrate that CD8(+) T cells that are present in the TGs at the time of excision can maintain HSV-1 in a latent state in sensory neurons in ex vivo TG cultures. Latently infected neurons expressed viral genome and some expressed HSV-1 immediate early and early proteins, but did not produce HSV-1 late proteins or infectious virions. Addition of anti-CD8alpha monoclonal antibody 5 d after culture initiation induced HSV-1 reactivation, as demonstrated by production of viral late proteins and infectious virions. Thus, CD8(+) T cells can prevent HSV-1 reactivation without destroying the infected neurons. We propose that when the intrinsic capacity of neurons to inhibit HSV-1 reactivation from latency is compromised, production of HSV-1 immediate early and early proteins might activate CD8(+) T cells aborting virion production.

Neural precursor cells for delivery of replication-conditional HSV-1 vectors to intracerebral gliomas

Cellular delivery of a replication-conditional herpes simplex virus type 1 (HSV-1) vector provides a means for gene therapy of invasive tumor cells. LacZ-bearing neural precursor cells, which can migrate and differentiate in the brain, were infected with a ribonucleotide reductase-deficient HSV-1 mutant virus (rRp450) that replicates only in dividing cells. Replication of rRp450 in neural precursor cells was blocked prior to implantation into the tumor by growth arrest in late G1 phase through treatment with mimosine. Viral titers in the medium of mimosine-treated, rRp450-infected neural precursor cells were below detection levels 3 days after infection. In culture, after removal of mimosine and passaging, cells resumed growth and replication of rRp450 so that, 7 days later, virus was present in the medium and cell death was evident. Mimosine-treated neural precursor cells injected into established intracerebral CNS-1 gliomas in nude mice migrated extensively throughout the tumor and into the surrounding parenchyma beyond the tumor over 3 days. Mimosine-treated neural precursor cells, infected with rRp450 and injected into intracerebral CNS-1 tumors, also migrated within the tumor with the appearance of foci of HSV-thymidine kinase-positive (TK+) cells, presumably including tumor cells, distributed throughout the tumor and in the surrounding parenchyma over a similar period. This migratory cell delivery method has the potential to expand the range of delivery of HSV-1 vectors to tumor cells in the brain.

Valacyclovir for the prevention of recurrent herpes simplex virus eye disease after excimer laser photokeratectomy

PURPOSE

A variety of factors have been reported as inducing the reactivation of latent herpes simplex virus (HSV), among them stress, trauma, and UV radiation. Excimer laser photorefractive keratectomy (PRK) is a surgical procedure utilizing a 193 nm ultraviolet light to alter the curvature of the cornea and hence correct vision. Reactivation of ocular herpes simplex keratitis following such excimer laser PRK has been reported. All published cases of HSV reactivation following excimer laser treatment in humans are reviewed. The present study evaluates whether stress, trauma of the corneal de-epithelialization prior to the laser, or the excimer laser treatment itself to the stromal bed induces this ocular reactivation of the latent HSV, and whether a systemic antiviral agent, valacyclovir, would prevent such laser PRK-induced reactivation of the HSV.

METHODS

Forty-three normal 1.5- to 2.5-kg New Zealand white rabbits were infected on the surface of the cornea with HSV-1, strain RE. The animals were monitored until resolution, and then all animals were divided into 5 treatment groups: (1) de-epithelialization only, intraperitoneal (i.p.) saline for 14 days; (2) de-epithelialization plus laser, i.p. saline for 14 days; (3) de-epithelialization plus laser, valacyclovir 50 mg/kg per day i.p. for 14 days; (4) de-epithelialization plus laser, valacyclovir 100 mg/kg per day i.p. for 14 days; (5) de-epithelialization plus laser, valacyclovir 150 mg/kg per day i.p. for 14 days. Animals were evaluated in a masked fashion by clinical examination biweekly and viral cultures biweekly through day 28.

RESULTS

The reactivation rates were as follows: group 1, 0%; group 2, 67%; group 3, 50%; group 4, 17%; and group 5, 0%. Viral titers were negative in animals that had no reactivation but persistently positive in those that had reactivation (day 6 through day 28).

CONCLUSIONS

Excimer laser (193 nm) treatment can trigger reactivation of ocular herpes disease (67%) and viral shedding in the latently infected rabbit. De-epithelialization alone is not sufficient to cause reactivation or viral shedding. Prophylaxis with intraperitoneal valacyclovir decreases the recurrence rate in a dose-response fashion. At 150 mg/kg per day, there are no recurrences. The presence of persistent viral shedding in reactivated animals may correlate with cases of late HSV recurrence reported in humans undergoing excimer treatment. The data suggest that humans undergoing excimer laser procedures for correction of refractive errors or treatment of corneal scars with a history of herpetic keratitis are at increased risk for reactivation. Such patients, however, may appropriately be considered for prophylactic systemic antiviral medication at the time of the laser procedure in order to decrease the possibility of recurrence.

Long-term transgene expression in mice infected with a herpes simplex virus type 1 mutant severely impaired for immediate-early gene expression

The role of viral immediate-early (IE) gene expression in herpes simplex virus type 1 (HSV-1) latency was investigated. The HSV-1 multiple mutant in1312, defective for the expression of the virion transactivator VP16 and the IE proteins ICP0 and ICP4, was used as the parent for these studies. The coding sequences of the Escherichia coli lacZ gene, preceded by the encephalomyocarditis virus internal ribosome entry site, were inserted into the region of in1312 that encodes the latency-associated transcripts (LATs) such that transcription of the transgene was controlled by the LAT promoter. This insert has previously been shown to direct long-term latent-phase expression of beta-galactosidase in a wild-type HSV-1 genome (R. H. Lachmann and S. Efstathiou, J. Virol. 71, 3197-3207, 1997). The resulting recombinant, in1388, was apathogenic after inoculation into mice via the footpad and did not detectably replicate in dorsal root ganglia (DRG) or footpads. Mutant in1388 established latency in DRG, and beta-galactosidase was expressed in increasing numbers of neurons over the first 25 days of infection. During latency, more than 1% of neurons in ganglia that innervate the footpad expressed beta-galactosidase, with the number of positive cells remaining constant for at least 5 months. Rescue of the VP16, ICP0, or ICP4 mutations of in1388 did not affect the number of beta-galactosidase-expressing neurons detected during latency. The results demonstrate that HSV-1 mutants severely impaired for IE gene expression are capable of establishing latency and efficiently expressing a foreign gene product under control of the LAT promoter.

Nerve growth factor is an autocrine factor essential for the survival of macrophages infected with HIV

Nerve growth factor (NGF) is a neurotrophin with the ability to exert specific effects on cells of the immune system. Human monocytes/macrophages (M/M) infected in vitro with HIV type 1 (HIV-1) are able to produce substantial levels of NGF that are associated with enhanced expression of the high-affinity NGF receptor (p140 trkA) on the M/M surface. Treatment of HIV-infected human M/M with anti-NGF Ab blocking the biological activity of NGF leads to a marked decrease of the expression of p140 trkA high-affinity receptor, a concomitant increased expression of p75(NTR) low-affinity receptor for NGF, and the occurrence of apoptotic death of M/M. Taken together, these findings suggest a role for NGF as an autocrine survival factor that rescues human M/M from the cytopathic effect caused by HIV infection.

Transactivation of herpes simplex virus type 1 immediate-early gene expression by virion-associated factors is blocked by an inhibitor of cyclin-dependent protein kinases

Initiation of productive infection by human herpes simplex virus type 1 (HSV-1) requires cell cycle-dependent protein kinase (cdk) activity. Treatment of cells with inhibitors of cdks blocks HSV-1 replication and prevents accumulation of viral transcripts, including immediate-early (IE) transcripts (26). Inhibition of IE transcript accumulation suggests that virion proteins, such as VP16, require functional cdks to activate viral transcription. In this report, we show that a cdk inhibitor, Roscovitine, blocks VP16-dependent IE gene expression. In the presence of Roscovitine, the level of virion-induced activation of a transfected reporter gene (the gene encoding chloramphenicol acetyltransferase) linked to the promoter-regulatory region of the ICP0 gene was reduced 40-fold relative to that of untreated samples. Roscovitine had little effect on the interaction of VP16 with VP16-responsive DNA sequences as measured by electrophoretic mobility shift assays. These data indicate that VP16-dependent activation of IE gene expression requires functional cdks and that this requirement is independent of the ability of VP16 to bind to DNA.

Reactivation of herpes simplex virus: the role of cytokines and intracellular factors

Considerable progress has been achieved in relating environmental stimuli and viral genetics to herpes simplex virus reactivation. The cytokines IL-1, IL-6, nerve growth factor NGF, and interferons have been implicated in herpes simplex virus reactivation. These molecules may act as signals which convey important information about the environment to the latent viral genome. Herpes simplex virus latency associated transcript expression is important for viral reactivation. The function of these unique viral RNA's is not completely understood, but they appear to be necessary for the efficient establishment of a latent infection in the ganglion. A better understanding of the mechanisms underlying the herpes simplex virus reactivation can be expected to lead to novel effective treatments for herpes simplex diseases.

Nuclear localization of the C1 factor (host cell factor) in sensory neurons correlates with reactivation of herpes simplex virus from latency

After a primary infection, herpes simplex virus is maintained in a latent state in neurons of sensory ganglia until complex stimuli reactivate viral lytic replication. Although the mechanisms governing reactivation from the latent state remain unknown, the regulated expression of the viral immediate early genes represents a critical point in this process. These genes are controlled by transcription enhancer complexes whose assembly requires and is coordinated by the cellular C1 factor (host cell factor). In contrast to other tissues, the C1 factor is not detected in the nuclei of sensory neurons. Experimental conditions that induce the reactivation of herpes simplex virus in mouse model systems result in rapid nuclear localization of the protein, indicating that the C1 factor is sequestered in these cells until reactivation signals induce a redistribution of the protein. The regulated localization suggests that C1 is a critical switch determinant of the viral lytic-latent cycle.

Herpes simplex virus type 1 vector-mediated expression of nerve growth factor protects dorsal root ganglion neurons from peroxide toxicity

Nerve growth factor beta subunit (beta-NGF) transgene delivery and expression by herpes simplex virus type 1 (HSV-1) vectors was examined in a cell culture model of neuroprotection from hydrogen peroxide toxicity. Replication-competent (tk- K mutant background) and replication-defective (ICP4(-);tk- S mutant background) vectors were engineered to contain the murine beta-NGF cDNA under transcriptional control of either the human cytomegalovirus immediate-early gene promoter (HCMV IEp) (e.g., KHN and SHN) or the latency-active promoter 2 (LAP2) (e.g., KLN and SLN) within the viral thymidine kinase (tk) locus. Infection of rat B103 and mouse N2A neuronal cell lines, 9L rat glioma cells, and Vero cells with the KHN or SHN vectors resulted in the production of beta-NGF-specific transcripts and beta-NGF protein reaching a maximum at 3 days postinfection (p.i.). NGF protein was released into the culture media in amounts ranging from 10.83 to 352.86 ng/ml, with the highest levels being achieved in B103 cells, and was capable of inducing neurite sprouting of PC-12 cells. The same vectors produced high levels of NGF in primary dorsal root ganglion (DRG) cultures at 3 days. In contrast to HCMV IEp-mediated expression, the LAP2-NGF vectors showed robust expression in primary DRG neurons at 14 days. The neuroprotective effect of vector produced NGF was assessed by its ability to inhibit hydrogen peroxide-induced neuron toxicity in primary DRG cultures. Consistent with the kinetics of vector-mediated NGF expression, HCMV-NGF vectors were effective in abrogating the toxic effects of peroxide at 3 but not 14 days p.i. whereas LAP2-NGF vector transduction inhibited apoptosis in DRG neurons at 14 days p.i. but was ineffective at 3 days p.i. Similar kinetics of NGF expression were observed with the KHN and KLN vectors in latently infected mouse trigeminal ganglia, where high levels of beta-NGF protein expression were detected at 4 wks p.i. only from the LAP2; HCMV-NGF-driven expression peaked at 3 days but could not be detected during HSV latency at 4 weeks. Together, these results indicate that (i) NGF vector-infected cells produce and secrete mature, biologically active beta-NGF; (ii) vector-synthesized NGF was capable of blocking peroxide-induced apoptosis in primary DRG cultures; and (iii) the HCMV-IEp functioned to produce high levels of NGF for several days; but (iv) only the native LAP2 was capable of long-term expression of a therapeutic gene product in latently infected neurons in vivo.

Characterization of a nerve growth factor-inducible cellular activity that enhances herpes simplex virus type 1 gene expression and replication of an ICP0 null mutant in cells of neural lineage

Herpes simplex virus type 1 (HSV-1) ICP0 is required for efficient viral gene expression during lytic infection, especially at low multiplicities. A series of cellular activities that can substitute for ICP0 has been identified, suggesting that when the activity of ICP0 is limiting, these activities can substitute for ICP0 to activate viral gene expression. The cellular activities may be especially important during reactivation of HSV from neuronal latency when viral gene expression is initiated in the absence of prior viral protein synthesis. Consistent with this hypothesis, we have identified an inducible activity in cells of neural lineage (PC12) that can complement the low-multiplicity growth phenotype of an ICP0 null mutant, n212. Pretreatment of PC12 cells with nerve growth factor (NGF) or fibroblast growth factor (FGF) prior to infection produced a 10- to 20-fold increase in the 24-h yield of n212 but only a 2- to 4-fold increase in the yield of wild-type virus relative to mock treatment. Slot blot analysis of nuclear DNA isolated from infected cells treated or mock treated with NGF indicated that NGF treatment does not significantly affect viral entry. The NGF-induced activity in PC12 cells was expressed transiently, with peak complementing activity observed when cells were treated with NGF 12 h prior to infection. Addition of NGF 3 h after infection had little effect on virus yield. The NGF-induced cellular activity was inhibited by pretreatment of PC12 cells with kinase inhibitors that have high specificity for kinases involved in NGF/FGF-dependent signal transduction. RNase protection assays demonstrated that the NGF-inducible PC12 cell activity, like that of ICP0, functions to increase the level of viral mRNA during low-multiplicity infection. These results suggest that activation of viral transcription by ICP0 and transcriptional activation of cellular genes by NGF and FGF utilize common signal transduction pathways in PC12 cells.

The herpes simplex virus type 1 immediate-early protein ICP0 is necessary for the efficient establishment of latent infection

The immediate-early protein ICP0 of herpes simplex virus type 1 (HSV-1) is not essential for viral replication. However, ICP0 is important for efficient viral replication during the productive infection and for reactivation of latent HSV-1 in vivo. The in vitro model of HSV-1 latency in dorsal root ganglia neurons was used to examine the role of ICP0 in the individual steps that could lead to the appearance of a decreased reactivation phenotype of ICP0 mutant viruses. After establishment of latent infections in the neuronal cultures, induction of reactivation by nerve growth factor (NGF) deprivation resulted in the production of infectious virus with delayed kinetics and a burst size that was significantly decreased for the ICP0 mutants compared with wild-type HSV-1. The efficiency of establishment of latency with the ICP0 mutants was similarly decreased at least 10-fold, as measured by three criteria: (i) the percentage of neurons expressing the major latency-associated transcript during the latent infection, (ii) the amount of viral DNA detected in the neuronal cultures, and (iii) the percentage of neurons expressing ICP4 immunoreactivity after the induction of reactivation. The most striking finding was that ICP0 supplied by an adenovirus vector significantly restored the ability of an ICP0 mutant to establish latency and reactivation. These results strongly indicate a critical role for ICP0 in the establishment of the latent HSV-1 infection in the in vitro neuronal model.

The herpes simplex virus type 1 latency-associated transcript promoter is activated through Ras and Raf by nerve growth factor and sodium butyrate in PC12 cells

Herpes simplex virus establishes latent infections in the nuclei of sensory neurons. These infections are characterized by the abundant expression of a series of 5' coterminal transcripts termed the latency-associated transcripts (LATs). Available evidence indicates that LAT expression is specifically regulated in latently infected neurons. Although previous studies have examined the regulation of LAT expression in neuronal and nonneuronal cells, the mechanism of regulation of LAT expression in neuronal cells in response to external factors has not been investigated. To address this question, we characterized the activity of LAT promoter fusion constructs in PC12 cells following treatment with nerve growth factor (NGF) and/or sodium butyrate (NaB), agents that affect expression of cell cycle-associated genes. Expression from the LAT promoter was induced 8- to 12-fold by either NGF or NaB alone and 40- to 60-fold when the two agents were added simultaneously. Fibroblast growth factor also induced expression from the LAT promoter but to a lesser extent than NGF. Treatment with factors such as epidermal growth factor, phorbol myristate acetate, cyclic AMP, or KCI had no significant effect on LAT promoter activity. Notably, promoter-reporter constructs containing immediate-early (ICP0 and ICP4), early (ICP8 and UL9), and late (UL10 and UL30) viral promoters were induced only two- to fourfold by NGF, suggesting that the LAT promoter may be unusual among herpes simplex virus genes in the magnitude of its response to this factor. To identify pathways leading to LAT activation in vitro, we characterized the response of the LAT promoter to NGF and/or NaB in PC12-derived cell lines containing mutations in specific signal transduction pathways. We found that activation of the LAT promoter requires Ras activation and that activation of the serine/threonine kinase, Raf, is sufficient to induce LAT expression. Together, these results indicate that the LAT promoter is regulated via the Ras/Raf signal transduction pathway in response to external factors such as NGF and NaB and that LAT expression may be regulated by NGF in latently infected neurons.

Effects of nerve growth factor and phorbol derivative on reactivation of herpes simplex virus type 1 in cultured cells of latently infected adult mouse trigeminal ganglia

Reactivation of herpes simplex virus type 1 (HSV-1) occurred rapidly in cells of latently infected adult mouse trigeminal ganglia which were cultured in serum-free medium in the presence of sufficient nerve growth factor (NGF). However, HSV-1 reactivation was delayed significantly in ganglionic cultures in the absence of exogenous NGF or in cultures treated with 2-aminopurine in the presence of NGF. The delayed viral reactivation in ganglionic cultures without NGF was accelerated by treatment with phorbol myristate acetate or dibutyryl cyclic AMP. Culture conditions which affected HSV-1 reactivation did not affect replication of HSV-1 in normal ganglionic cultures.

Herpes simplex virus gene expression in neurons: viral DNA synthesis is a critical regulatory event in the branch point between the lytic and latent pathways

Herpes simplex virus establishes a latent infection in peripheral neurons. We examined viral gene expression in rat peripheral neurons in vitro and determined that viral gene expression is attenuated and delayed in these neurons compared with that in Vero cells. In addition, using pharmacologic and genetic blocks to viral DNA synthesis, we found that viral alpha and beta gene expression was upregulated by viral DNA synthesis. Although maximal gene expression in neurons requires viral DNA synthetic activity, activation of viral gene expression was seen even in the presence of herpes simplex virus DNA polymerase inhibitors, but not in the absence of the origin-binding protein. Initiation of viral DNA synthesis is apparently a key regulatory event in the balance between the lytic and latent pathways in peripheral neurons.

Transcription factors interacting with herpes simplex virus alpha gene promoters in sensory neurons

Interference with VP16-mediated activation of herpes virus immediate-early (or alpha) genes is thought to be the major cause of establishing viral latency in sensory neurons. This could be brought about by lack of a key activating transcription factor(s) or active repression. In this study we find that sensory neurons express all important components for VP16-mediated alpha gene induction, such as the POU transcription factor Oct-1, host cell factor (HCF) and GABP alpha/beta. However, Oct-1 and GABP alpha/beta are only present at low levels and the VP16-induced complex (VIC) appears different. We do not find protein expression of the transcription factor Oct-2, implicated by others as an alpha gene repressor. The POU factor N-Oct3 (Brn 2 or POU3F2) is also present in sensory neurons and binds viral TAATGARAT motifs with higher affinity than Oct-1, indicating that it may be a candidate repressor for competitive binding to TAATGARAT motifs. When transfected into HeLa cells, where Oct-1 and GABP alpha/beta are highly abundant, N-Oct3 represses model promoters with multimerized TAATGARAT motifs, but fails to repress complete alpha gene promoters. Taken together our findings suggest that modulation of alpha gene promoters could contribute to viral latency when low concentrations of the activating transcription factors Oct-1 and GABP alpha/beta prevail. Our data, however, refute the notion that competing Oct factors are able to block alpha gene transcription to achieve viral latency.

Long-term expression in sensory neurons in tissue culture from herpes simplex virus type 1 (HSV-1) promoters in an HSV-1-derived vector

Amplicons, defective herpes simplex virus type 1 (HSV-1) vectors, were constructed to use four HSV-1 promoters, from the immediate-early (IE) 1 IE 3, IE 4/5, and late glycoprotein C (gC) genes, to regulate expression of the Escherichia coli lacZ gene, encoding beta-galactosidase, and packaged into infectious particles. Infection of sensory neurons in vitro with amplicons containing the IE 1, IE 3, or IE 4/5 promoter resulted in stable long-term expression of beta-galactosidase from 2 to 10 weeks after gene transfer. The number of neurons expressing beta-galactosidase was not changed by treatments previously shown to produce reactivation of latent HSV-1. In addition, the latency-associated transcript was detected in many of the same neurons that expressed beta-galactosidase, indicating that the viral IE promoters in the amplicons can function in the same neurons that harbor latent virus. Delivery of beta-galactosidase protein directly into neurons by microinjection indicated that the half-life for histochemical detection of beta-galactosidase was between 24 and 48 h, suggesting that the persistence of beta-galactosidase histochemical staining cannot be explained by the stability of the reporter protein alone. In contrast to the IE promoters, the gC promoter of the late gene class did not support long-term expression of beta-galactosidase; instead, beta-galactosidase was detected in only a few neurons per culture at 2 weeks after infection, and superinfection with wild-type HSV-1 did not increase the level of expression from the gC promoter. These results suggest that the HSV-1 IE promoters in the amplicons are not subject to the promoter inactivation that occurs with many types of virus vectors and that the IE promoters in the context of the amplicon avoid the promoter inactivation observed from the same promoters in the HSV-1 genome during latency.

Propranolol suppresses reactivation of herpesvirus

Herpes simplex virus type 1 (HSV-1) reactivates from the nervous system and causes recurrent disease in end organs such as the eye and the lips. We found that the beta-adrenergic receptor blocker, propranolol, reduces HSV-1 reactivation in an animal model. Mice latent for McKrae strain HSV-1 were injected with propranolol or saline once a day for 3 successive days, and subjected to a brief period of hyperthermia on the second day to induce reactivation. Twenty-four hours after the third injection, swabs of the ocular surface and homogenates of the corneas and trigeminal ganglia were analyzed for the presence of infectious virus and viral DNA. Treatment with propranolol significantly decreased the appearance of infectious virus in the tear film, cornea, and trigeminal ganglia (P < 0.05, chi 2-test). The results suggest a possible new pharmacologic approach to suppressing herpesvirus reactivation in the nervous system and thereby preventing recurrent disease.

Heat shock-induced reactivation of herpes simplex virus type 1 in latently infected mouse trigeminal ganglion cells in dissociated culture

An in vitro herpes simplex virus type 1 (HSV-1) latency model was established, using trigeminal ganglia from latently infected mice. When heat-treated at 43 degrees C for 3 h, reactivation followed in 76.6% of the cultures, while reactivation was not observed in non-heat-treated controls.

Effect of exogenous nerve growth factor on neurotoxicity of and neuronal gene delivery by a herpes simplex amplicon vector in the rat brain

We have previously shown that local destruction of neural tissue by wild-type herpes simplex virus type 1 (HSV-1) is attenuated by intracerebral infusion of nerve growth factor (NGF). To investigate the effect of NGF on the extent of neurolysis and efficacy of neuronal gene transfer mediated by an HSV-1 amplicon vector system in vivo, rats were stereotaxically injected in the striatum with an amplicon preparation, pHSVlac. This amplicon contains the Escherichia coli lacZ gene under the transcriptional control of the HSV-1 immediate early 4/5 promoter and is packaged by an HSV-1 helper virus carrying a deletion in the immediate early 3 gene. Vector injection was followed by continuous intracerebral infusion of NGF-beta (total dose 5 micrograms) or vehicle solution over 7 days. Animals were sacrificed at the end of the 7-day infusion period for histological analysis of the brains. A distinct zone of inflammation and necrosis surrounded the injection site in all vector-inoculated animals. The volume of striatal tissue destruction was significantly smaller in NGF-treated animals (1.27 +/- 0.19 mm3; mean +/- SEM) than in the vehicle-treated controls (2.16 +/- 0.37 mm3; P < 0.05 by t-test). Immunohistochemical staining for HSV and beta-galactosidase (beta-Gal) in vehicle-treated animals revealed that many striatal cells harbored HSV antigens (3,678 +/- 636), but only a small number expressed the reporter gene at 7 days post-injection (294 +/- 60). NGF infusion did not significantly affect the number of HSV-immunoreactive cells (4,224 +/- 618), or the number of cells expressing beta-Gal (330 +/- 72) at this time.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of nerve growth factor in modulating herpes simplex virus reactivation in vivo

The role of nerve growth factor (NGF) in the modulation of herpes simplex virus (HSV) latency and reactivation was investigated in a mouse model. To determine whether NGF depletion would reactivate latent virus, concentrated anti-NGF serum antibodies were administered intraperitoneally to latently infected mice for 9 consecutive days. To determine whether NGF given prophylactically could suppress UV-B-induced viral reactivation, mice were irradiated with UV-B while being treated with NGF using diverse regimes over a 4-day period. Following intraperitoneal administration of anti-NGF antibodies, viral shedding was detected in a small number (10%) of mice, but it was not possible to pharmacologically suppress UV-B-induced viral reactivation with NGF. It would appear, therefore, that HSV latency in neurons innervating the cornea can be sustained and disrupted by physiological factors independent of NGF levels.

Induction of reactivation of herpes simplex virus in murine sensory ganglia in vivo by cadmium

Herpes simplex viruses maintained in a latent state in sensory neurons in mice do not reactivate spontaneously, and therefore the factors or procedures which cause the virus to reactivate serve as a clue to the mechanisms by which the virus is maintained in a latent state. We report that cadmium sulfate induces latent virus to reactivate in 75 to 100% of mice tested. The following specific findings are reported. (i) The highest frequency of induction was observed after two to four daily administrations of 100 micrograms of cadmium sulfate. (ii) Zinc, copper, manganese, or nickel sulfate administered in equimolar amounts under the same regimen did not induce viral reactivation; however, zinc sulfate in molar ratios 25-fold greater than those of cadmium induced viral replication in 2 of 16 ganglia tested. (iii) Administration of zinc, nickel, or manganese prior to the cadmium sulfate reduced the incidence of ganglia containing infectious virus. (iv) Administration of cadmium daily during the first week after infection and at 2-day intervals to 13 days after infection resulted in the recovery from ganglia of infectious virus in titers 10- to 100-fold higher than those obtained from animals given saline. Moreover, infectious virus was recovered as late as 11 days after infection compared with 6 days in mice administered saline. (v) Administration of cadmium immediately after infection or repeatedly after establishment of latency did not exhaust the latent virus harbored by sensory neurons, inasmuch as the fraction of ganglia of mice administered cadmium and yielding infectious virus was similar to that observed in mice treated with saline. We conclude that induction of cadmium tolerance precludes reactivation of latent virus. If the induction of metallothionein genes was the sole factor required to cause reactivation of latent virus, it would have been expected that all metals which induce metallothioneins would also induce reactivation, which was not observed. The results therefore raise the possibility that in addition to inducing the metallothionein genes, cadmium inactivates the factors which maintain the virus in latent state.

Gene delivery to neurons: is herpes simplex virus the right tool for the job?

Herpes simplex virus (HSV)-derived vectors are currently being developed for the introduction of foreign DNA into neurons. HSV vectors can facilitate a range of molecular studies on postmitotic neurons and may ultimately be used for somatic cell gene therapy for certain neurologic diseases. In this article, the salient features of the pathologenesis and molecular biology of HSV relevant to its use as a vector are described, along with an overview of the methods used to derive these vectors. The accomplishments which have been made to date using the HSV vector system are discussed, with emphasis on the issues of this technology which remain to be addressed. HSV has the potential to be a most useful tool for neuronal cell transgenesis and it is likely that important neurobiological questions will be answered using this vector system.

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.

Herpes simplex virus type 1 mutant strain in1814 establishes a unique, slowly progressing infection in SCID mice

Ocular infection of immunocompetent (BALB/c) mice with wild-type herpes simplex virus type 1 (HSV-1) 17+ may lead to acute fatal encephalitis; however, in surviving animals, a latent (nonproductive) infection of the nervous system is established. In contrast, 17+ infection invariably kills mice with severe combined immunodeficiency (SCID mice) within 2 weeks. Ocular infection of immunocompetent mice with a mutant HSV-1 strain, in1814, which does not produce a functional alpha-transinducing protein, results in no detectable viral replication in the nervous system during the time corresponding to the acute phase of infection, no mortality, and the establishment of latency. In SCID mice, however, the in1814 virus establishes a unique, slowly progressing infection. In studying the courses of in1814 infection in SCID and BALB/c mice, we found that although intact B- and/or T-lymphocytic functions were required for the control of viral replication in the nervous system, some of the infected neurons of SCID mice seemed to be able to restrict in1814 replication and harbor the virus in a latent state.