Herpes simplex virus and varicella-zoster virus: why do these human alphaherpesviruses behave so differently from one another?

Distinguishing Features of Anterior Uveitis Caused by Herpes Simplex Virus, Varicella-Zoster Virus, and Cytomegalovirus

PURPOSE

To determine distinguishing features of the clinical characteristics of anterior uveitis (AU) caused by herpes simplex virus (HSV), varicella-zoster virus (VZV), and cytomegalovirus (CMV).

DESIGN

Retrospective, multicenter case series.

METHODS

Consecutive patients with herpetic AU examined at 11 tertiary centers in Japan between January 2012 and December 2017 and who were followed for ≥3 months were evaluated. Diagnosis was made by polymerase chain reaction (PCR) for HSV, VZV, or CMV in the aqueous humor, or classical signs of herpes zoster ophthalmicus.

RESULTS

This study enrolled 259 herpetic AU patients, including PCR-proven HSV-AU (30 patients), VZV-AU (50), and CMV-AU (147), and herpes zoster ophthalmicus (32). All HSV-AU and VZV-AU patients were unilateral, while 3% of CMV-AU patients were bilateral. Most HSV-AU and VZV-AU patients were sudden onset with an acute clinical course, while CMV-AU had a more insidious onset and chronic course. There were no significant differences for all surveyed symptoms, signs, and complications between HSV-AU and VZV-AU. However, significant differences were detected for many items between CMV-AU and the other two herpetic AU types. Ocular hyperemia and pain, blurring of vision, ciliary injection, medium-to-large keratic precipitates (KPs), cells and flare in the anterior chamber, and posterior synechia significantly more often occurred in HSV-AU and VZV-AU vs CMV-AU. In contrast, small KPs, coin-shaped KPs, diffuse iris atrophy, elevated intraocular pressure, and glaucoma surgery were significantly more frequent in CMV-AU vs HSV-AU and VZV-AU.

CONCLUSION

This multicenter, retrospective study identified distinguishing features of HSV-AU, VZV-AU, and CMV-AU.

Vaccines for Herpes Simplex: Recent Progress Driven by Viral and Adjuvant Immunology

Herpes simplex viruses (HSV) types 1 and 2 are ubiquitous. They both cause genital herpes, occasionally severe disease in the immunocompromised, and facilitate much HIV acquisition globally. Despite more than 60 years of research, there is no licensed prophylactic HSV vaccine and some doubt as to whether this can be achieved. Nevertheless, a previous HSV vaccine candidate did have partial success in preventing genital herpes and HSV acquisition and another immunotherapeutic candidate reduced viral shedding and recurrent lesions, inspiring further research. However, the entry pathway of HSV into the anogenital mucosa and the subsequent cascade of immune responses need further elucidation so that these responses could be mimicked or improved by a vaccine, to prevent viral entry and colonization of the neuronal ganglia. For an effective novel vaccine against genital herpes the choice of antigen and adjuvant may be critical. The incorporation of adjuvants of the vaccine candidates in the past, may account for their partial efficacy. It is likely that they can be improved by understanding the mechanisms of immune responses elicited by different adjuvants and comparing these to natural immune responses. Here we review the history of vaccines for HSV, those in development and compare them to successful vaccines for chicken pox or herpes zoster. We also review what is known of the natural immune control of herpes lesions, via interacting innate immunity and CD4 and CD8 T cells and the lessons they provide for development of new, more effective vaccines.

Cytoskeletons in the Closet-Subversion in Alphaherpesvirus Infections

Actin filaments, microtubules and intermediate filaments form the cytoskeleton of vertebrate cells. Involved in maintaining cell integrity and structure, facilitating cargo and vesicle transport, remodelling surface structures and motility, the cytoskeleton is necessary for the successful life of a cell. Because of the broad range of functions these filaments are involved in, they are common targets for viral pathogens, including the alphaherpesviruses. Human-tropic alphaherpesviruses are prevalent pathogens carried by more than half of the world’s population; comprising herpes simplex virus (types 1 and 2) and varicella-zoster virus, these viruses are characterised by their ability to establish latency in sensory neurons. This review will discuss the known mechanisms involved in subversion of and transport via the cytoskeleton during alphaherpesvirus infections, focusing on protein-protein interactions and pathways that have recently been identified. Studies on related alphaherpesviruses whose primary host is not human, along with comparisons to more distantly related beta and gammaherpesviruses, are also presented in this review. The need to decipher as-yet-unknown mechanisms exploited by viruses to hijack cytoskeletal components—to reveal the hidden cytoskeletons in the closet—will also be addressed.

Oncolytic Virotherapy by HSV

Oncolytic virotherapy is a kind of antitumor therapy using viruses with natural or engineered tumor-selective replication to intentionally infect and kill tumor cells. An early clinical trial has been performed in the 1950s using wild-type and non-engineered in vitro-passaged virus strains and vaccine strains (first generation oncolytic viruses). Because of the advances in biotechnology and virology, the field of virotherapy has rapidly evolved over the past two decades and innovative recombinant selectivity-enhanced viruses (second generation oncolytic viruses). Nowadays, therapeutic transgene-delivering "armed" oncolytic viruses (third generation oncolytic viruses) have been engineered using many kinds of viruses. In this chapter, the history, mechanisms, rationality, and advantages of oncolytic virotherapy by herpes simplex virus (HSV) are mentioned. Past and ongoing clinical trials by oncolytic HSVs (G207, HSV1716, NV1020, HF10, Talimogene laherparepvec (T-VEC, OncoVEX^GM-CSF)) are also summarized. Finally, the way of enhancement of oncolytic virotherapy by gene modification or combination therapy with radiation, chemotherapy, or immune checkpoint inhibitors are discussed.

Real-World Effectiveness and Safety of a Live-Attenuated Herpes Zoster Vaccine: A Comprehensive Review

Herpes zoster (HZ) is a common, painful and debilitating disease caused by the reactivation of latent varicella-zoster virus in ganglia. This clinical event occurs more frequently in the elderly and those who are immunocompromised. The most common complication of HZ is post-herpetic neuralgia (PHN) which is responsible for the highest HZ-related burden of illness and is challenging to treat. Due to the important clinical and economic impact of HZ and PHN, and the suboptimal treatments that are currently available, HZ vaccination is an important approach to reduce the burden of illness. Currently, one-dose, live-attenuated vaccine is licensed in the United States and Europe to prevent HZ and it is included in some national immunization programs. The clinical efficacy, safety and tolerability of the vaccine has been demonstrated in two large phase III clinical trials, involving more than 38,000 and 22,000 individuals aged ≥60 and 50-59 years, respectively. This comprehensive review summarizes the extensive "real-world" effectiveness and safety data from both immunocompetent and immunocompromised individuals. These data confirm those from the clinical trials, supporting the use of HZ vaccine in clinical practice and provide evidence that the current recommendations for immunocompromised individuals should be revised.

FUNDING

Funding for the editorial assistance, article processing charges, and open access fee for this publication was provided by Sanofi Pasteur MSD.

Role of gB and pUS3 in Equine Herpesvirus 1 Transfer between Peripheral Blood Mononuclear Cells and Endothelial Cells: a Dynamic In Vitro Model

Infected peripheral blood mononuclear cells (PBMC) effectively transport equine herpesvirus type 1 (EHV-1), but not EHV-4, to endothelial cells (EC) lining the blood vessels of the pregnant uterus or central nervous system, a process that can result in abortion or myeloencephalopathy. We examined, using a dynamic in vitro model, the differences between EHV-1 and EHV-4 infection of PBMC and PBMC-EC interactions. In order to evaluate viral transfer between infected PBMC and EC, cocultivation assays were performed. Only EHV-1 was transferred from PBMC to EC, and viral glycoprotein B (gB) was shown to be mainly responsible for this form of cell-to-cell transfer. For addressing the more dynamic aspects of PBMC-EC interaction, infected PBMC were perfused through a flow channel containing EC in the presence of neutralizing antibodies. By simulating capillary blood flow and analyzing the behavior of infected PBMC through live fluorescence imaging and automated cell tracking, we observed that EHV-1 was able to maintain tethering and rolling of infected PBMC on EC more effectively than EHV-4. Deletion of US3 reduced the ability of infected PBMC to tether and roll compared to that of cells infected with parental virus, which resulted in a significant reduction in virus transfer from PBMC to EC. Taking the results together, we conclude that systemic spread and EC infection by EHV-1, but not EHV-4, is caused by its ability to infect and/or reprogram mononuclear cells with respect to their tethering and rolling behavior on EC and consequent virus transfer.

IMPORTANCE

EHV-1 is widespread throughout the world and causes substantial economic losses through outbreaks of respiratory disease, abortion, and myeloencephalopathy. Despite many years of research, no fully protective vaccines have been developed, and several aspects of viral pathogenesis still need to be uncovered. In the current study, we investigated the molecular mechanisms that facilitate the cell-associated viremia, which is arguably the most important aspect of EHV-1 pathogenesis. The newly discovered functions of gB and pUS3 add new facets to their previously reported roles. Due to the conserved nature of cell-associated viremia among numerous herpesviruses, these results are also very relevant for viruses such as varicella-zoster virus, pseudorabies virus, human cytomegalovirus, and others. In addition, the constructed mutant and recombinant viruses exhibit potent in vitro replication but have significant defects in certain stages of the disease course. These viruses therefore show much promise as candidates for future live vaccines.

Deciphering Human Cell-Autonomous Anti-HSV-1 Immunity in the Central Nervous System

Herpes simplex virus 1 (HSV-1) is a common virus that can rarely invade the human central nervous system (CNS), causing devastating encephalitis. The permissiveness to HSV-1 of the various relevant cell types of the CNS, neurons, astrocytes, oligodendrocytes, and microglia cells, as well as their response to viral infection, has been extensively studied in humans and other animals. Nevertheless, human CNS cell-based models of anti-HSV-1 immunity are of particular importance, as responses to any given neurotropic virus may differ between humans and other animals. Human CNS neuron cell lines as well as primary human CNS neurons, astrocytes, and microglia cells cultured/isolated from embryos or cadavers, have enabled the study of cell-autonomous anti-HSV-1 immunity in vitro. However, the paucity of biological samples and their lack of purity have hindered progress in the field, which furthermore suffers from the absence of testable primary human oligodendrocytes. Recently, the authors have established a human induced pluripotent stem cells (hiPSCs)-based model of anti-HSV-1 immunity in neurons, oligodendrocyte precursor cells, astrocytes, and neural stem cells, which has widened the scope of possible in vitro studies while permitting in-depth explorations. This mini-review summarizes the available data on human primary and iPSC-derived CNS cells for anti-HSV-1 immunity. The hiPSC-mediated study of anti-viral immunity in both healthy individuals and patients with viral encephalitis will be a powerful tool in dissecting the disease pathogenesis of CNS infections with HSV-1 and other neurotropic viruses.

A Cohort Historical Analysis of the Relationship between Thyroid Hormone Malady and Alpha-Human Herpesvirus Activation

Background

A number of physiological factors have been suggested to participate in the alpha- Human Herpesvirus (αHHV) reactivation, such as hormonal aberration. Thyroid hormone (TH) was shown to play a suppressive role in Herpes Simplex Virus Type-1 (HSV-1) gene expression and replication in cell culture and animal models. We hypothesize that reactivation of αHHV in humans may be due to, at least in part, by TH status.

Methods

Prior to implementing a full-scale population-based prospective inquiry into this hypothesis, a pilot study using a medical claims data base and a case-controlled, retrospective cohort investigation was conducted to develop a hypothetical link between TH complication and αHHV reactivation. Using diagnostic codes for treating thyroid disorders and αHHV infections as proxies for biologic/clinic outcomes, we queried a large, comprehensive hospital data base to construct two patient cohorts: Cohort 1 was comprised of patients receiving TH diagnoses over a twelve-year period, and Cohort 2 was composed of patients not receiving TH diagnoses during this period. Diagnoses of αHHV were recorded for each cohort and the difference in the frequency was examined for statistical significance. Demographic analyses such as age, gender, etc were also performed.

Results

Using 2×2 contingency table analyses and Statistical Analysis Software (SAS), an Odds Ratio (OR) of 2.83 was observed for the total population of 21 years old and above with a chi-square of 61.55 and p < 0.001, confirming that a severe significant difference was found between these two cohorts. This result suggested that patients with αHHV diagnosis have higher chances to have TH disorders. Additional investigation revealed that female were at higher/significant probability to have both TH and αHHV diagnosis, indicating a link of αHHV reactivation to a complex hormonal profile difference between genders. Our observation indicated that female patients of 21 years of age and above exhibited a very high incidence (OR of 3.40, p < 0.001) compared to the male groups (OR of 1.91, p < 0.05), indicating the possibility that hormonal alteration in females maybe transient but robust and can lead to αHHV reactivation more often than the males.

Conclusion

These results indicated that TH dysfunction may have implication in αHHV pathogenesis and females exhibited much higher probability to suffer αHHV reactivation due to TH disruption. Although the results from this pilot study have limitations and require additional controlled clinical examination such as more detailed patient records, lab data, therapeutic outcome, etc, it provides a tool to assess the effects of hormone imbalance on virus reactivation by retrospective analyses using existing large scale data base.

Herpes simplex virus US3 protein kinase regulates host responses and determines neurovirulence

The US3 of HSV encodes a serine/threonine protein kinase that is highly conserved among members of the alphaherpesviruses. It is an accessory gene that is not required for viral replication in cultured cells but appears essential for viral survival in humans. Although accumulating in vitro evidence suggested that the viral protein kinase is multifunctional, little information is available about its functions in vivo. Several reports point out that, upon invasion into the peripheral nervous system, HSV blocks virus-induced neuronal apoptosis, while presumably subverting host immune responses, largely through actions of the US3 protein kinase. In addition, the US3 protein kinase confers the viral neurovirulence. In the present article, functions of the HSV US3 protein kinase are briefly reviewed, with special attention given to its role in regulating host responses and neurovirulence.

Molecular studies of the Oka varicella vaccine

Varicella zoster virus (VZV) is one of eight members of the Herpesviridae family for which humans are the primary host; it causes two distinct diseases, varicella (chickenpox) and zoster (shingles). Varicella results from primary infection, during which the virus establishes latency in sensory neurons, a characteristic of all members of the Alphaherpesvirinae subfamily. Zoster is caused by reactivation of latent virus, which typically occurs when cellular immunity is impaired. VZV is the first human herpesvirus for which a vaccine has been licensed. The vaccine preparation, v-Oka, is a live-attenuated virus stock produced by the classic method of tissue culture passage in animal and human cell lines. Over 90 million doses of the vaccine have been administered in countries worldwide, including the USA, where varicella morbidity and mortality has declined dramatically. Over the last decade, several laboratories have been committed to investigating the mechanism by which the Oka vaccine is attenuated. Mutations have accumulated across the genome of the vaccine during the attenuation process; however, studies of the contribution of these changes to vaccine attenuation have been hampered by the lack of a suitable animal model of VZV disease and by the heterogeneity that exists among the viral population within the vaccine preparation. Notwithstanding, a wealth of data has been generated using various laboratory methodologies. Studies of the vaccine virus in human xenografts implanted in severe combined immunodeficiency-hu mice, have enabled analyses of the replication dynamics of the vaccine in dorsal root ganglia, T lymphocytes and skin. In vitro assays have been used to investigate the effect of vaccine mutations on viral gene expression and sequence analysis of vaccine rash viruses has permitted investigations into spread of the vaccine virus in a human host. We present here a review of what has been learned thus far about the molecular and phenotypic characteristics of the Oka vaccine.

Herpesviruses and intermediate filaments: close encounters with the third type

Intermediate filaments (IF) are essential to maintain cellular and nuclear integrity and shape, to manage organelle distribution and motility, to control the trafficking and pH of intracellular vesicles, to prevent stress-induced cell death, and to support the correct distribution of specific proteins. Because of this, IF are likely to be targeted by a variety of pathogens, and may act in favor or against infection progress. As many IF functions remain to be identified, however, little is currently known about these interactions. Herpesviruses can infect a wide variety of cell types, and are thus bound to encounter the different types of IF expressed in each tissue. The analysis of these interrelationships can yield precious insights into how IF proteins work, and into how viruses have evolved to exploit these functions. These interactions, either known or potential, will be the focus of this review.

Mammalian alphaherpesvirus miRNAs

Mammalian alphaherpesviruses are major causes of human and veterinary disease. During productive infection, these viruses exhibit complex and robust patterns of gene expression. These viruses also form latent infections in neurons of sensory ganglia in which productive cycle gene expression is highly repressed. Both modes of infection provide advantageous opportunities for regulation by microRNAs. Thus far, published data regarding microRNAs are available for six mammalian alphaherpesviruses. No microRNAs have yet been detected from varicella zoster virus. The five other viruses-herpes simplex viruses-1 and -2, herpes B virus, bovine herpesvirus-1, and pseudorabies virus-representing both genera of mammalian alphaherpesviruses have been shown to express microRNAs. In this article, we discuss these microRNAs in terms of where they are encoded in the viral genome relative to other viral transcripts; whether they are expressed during productive or latent infection; their potential targets; what little is known about their actual targets and functions during viral infection; and what little is known about the interactions of these viruses with the host microRNA machinery. This article is part of a Special Issue entitled: "MicroRNAs in viral gene regulation".

Detection of varicella-zoster virus DNA in 414 human trigeminal ganglia from cadavers by the polymerase chain reaction: a comparison of the detection rate of varicella-zoster virus and herpes simplex virus type 1

Investigation of varicella-zoster virus (VZV) is important epidemiologically, and determination of its prevalence rate in human trigeminal ganglia is important to provide surveillance data. To date, studies on VZV detection in trigeminal ganglia have used specimens obtained from a relatively limited number of cadavers. This study attempted to detect VZV DNA as well as Herpes simplex virus type 1 (HSV-1) DNA by the polymerase chain reaction (PCR) from 414 samples of trigeminal ganglia obtained from 207 cadavers selected at random. The detection rate was examined to determine whether there were significant differences in the positive rate between the left and right trigeminal ganglia, males and females, and among age groups. A relationship was found between the positive rates for VZV and HSV-1. VZV DNA was detected in 391 of the trigeminal ganglia (94.4%) and 201 of the cadavers (97.1%) in 121/124 males and 80/83 females. HSV-1 DNA was detected in 251 of the samples (60.6%) and 134 of the cadavers (64.7%) in 72/124 males and 62/83 females. There was no significant difference for either virus in the detection rates between the left and right trigeminal ganglia and males and females. Age and positivity for HSV-1, but not VZV, showed a significant relationship. All 134 cadavers positive for HSV-1 were also positive for VZV. VZV and HSV-1 become latent in bilateral trigeminal ganglia, and are not affected by gender. The prevalence of HSV-1 was greater in advanced age, and the HSV-1-positive rate was correlated with the VZV-positive rate.

Dominant-negative proteins in herpesviruses - from assigning gene function to intracellular immunization

Investigating and assigning gene functions of herpesviruses is a process, which profits from consistent technical innovation. Cloning of bacterial artificial chromosomes encoding herpesvirus genomes permits nearly unlimited possibilities in the construction of genetically modified viruses. Targeted or randomized screening approaches allow rapid identification of essential viral proteins. Nevertheless, mapping of essential genes reveals only limited insight into function. The usage of dominant-negative (DN) proteins has been the tool of choice to dissect functions of proteins during the viral life cycle. DN proteins also facilitate the analysis of host-virus interactions. Finally, DNs serve as starting-point for design of new antiviral strategies.

Medical application of herpes simplex virus

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are important human pathogens that cause a variety of diseases from mild skin diseases such as herpes labialis and herpes genitalis to life-threatening diseases such as herpes encephalitis and neonatal herpes. A number of studies have elucidated the roles of this virus in viral replication and pathogenicity, the regulation of gene expression, interaction with the host cell and immune evasion from the host system. This research has allowed the development of potential therapeutic agents and vectors for human diseases. This review focuses on the basic functions and roles of HSV gene products and reviews the current knowledge of medical applications of genetically engineered HSV mutants using different strategies. These major HSV-derived vectors include: (i) amplicons for gene delivery vectors; (ii) replication-defective HSV recombinants for vaccine vectors; (iii) replication-attenuated HSV recombinants for oncolytic virotherapy.

Regulation of the retinoblastoma proteins by the human herpesviruses

Viruses are obligate intracellular parasites that alter the environment of infected cells in order to replicate more efficiently. One way viruses achieve this is by modulating cell cycle progression. The main regulators of progression out of G0, through G1, and into S phase are the members of the retinoblastoma (Rb) family of tumor suppressors. Rb proteins repress the transcription of genes controlled by the E2F transcription factors. Because the expression of E2F-responsive genes is required for cell cycle progression into the S phase, Rb arrests the cell cycle in G0/G1. A number of viral proteins directly target Rb family members for inactivation, presumably to create an environment more hospitable for viral replication. Such viral proteins include the extensively studied oncoproteins E7 (from human papillomavirus), E1A (from adenovirus), and the large T (tumor) antigen (from simian virus 40). Elucidating how these three viral proteins target and inactivate Rb has proven to be an invaluable approach to augment our understanding of both normal cell cycle progression and carcinogenesis. In addition to these proteins, a number of other virally-encoded inactivators of the Rb family have subsequently been identified including a surprising number encoded by human herpesviruses. Here we review how the human herpesviruses modulate Rb function during infection, introduce the individual viral proteins that directly or indirectly target Rb, and speculate about what roles Rb modulation by these proteins may play in viral replication, pathogenesis, and oncogenesis.

Childhood viral exanthems

Many viral infections exhibit cutaneous lesions. Recognition of the exanthems associated with these infections and the broader clinical scenarios in which they occur can lead to more rapid diagnosis and appropriate treatment for affected patients.

The cycle of human herpes simplex virus infection: virus transport and immune control

After infection of skin or mucosa, herpes simplex virus enters the sensory nerve endings and is conveyed by retrograde axonal transport to the dorsal root ganglion, where the virus develops lifelong latency. Intermittent reactivation, which is spontaneous in humans, leads to anterograde transport of virus particles and proteins to the skin or mucosa, where the virus is shed and/or causes disease. Immune control of viral infection and replication occurs at the level of skin or mucosa during initial or recurrent infection and also within the dorsal root ganglion, where immune mechanisms control latency and reactivation. This article examines current views on the mechanisms of retrograde and anterograde transport of the virus in axons and the mechanisms of innate and adaptive immunity that control infection in the skin or mucosa and in the dorsal root ganglion--in particular, the role of interferons, myeloid and plasmacytoid dendritic cells, CD4(+) and CD8(+) T cells, and interferon- gamma and other cytokines, including their significance in the development of vaccines for genital herpes.

Molecular studies of Varicella zoster virus

VZV is a highly cell-associated member of the Herpesviridae family and one of the eight herpesviruses to infect humans. The virus is ubiquitous in most populations worldwide, primary infection with which causes varicella, more commonly known as chickenpox. Characteristic of members of the alphaherpesvirus sub-family, VZV is neurotropic and establishes latency in sensory neurones. Reactivation from latency, usually during periods of impaired cellular immunity, causes herpes zoster (shingles). Despite being one of the most genetically stable human herpesviruses, nucleotide alterations in the virus genome have been used to classify VZV strains from different geographical regions into distinct clades. Such studies have also provided evidence that, despite pre-existing immunity to VZV, subclinical reinfection and reactivation of reinfecting strains to cause zoster is also occurring. During both primary infection and reactivation, VZV infects several PBMC and skin cell lineages. Difficulties in studying the pathogenesis of VZV because of its high cell association and narrow host range have been overcome through the development of the VZV severe combined immunodeficient mouse model carrying human tissue implants. This model has provided a valuable tool for studying the importance of individual viral proteins during both the complex intracellular replication and assembly of new virions and for understanding the underlying mechanism of attenuation of the live varicella vaccine. In addition, a rat model has been developed and successfully used to uncover which viral proteins are important for both the establishment and maintenance of latent VZV infection.

Herpes simplex virus US3 protein kinase regulates virus-induced apoptosis in olfactory and vomeronasal chemosensory neurons in vivo

A role for the US3 protein kinase of herpes simplex virus (HSV) in regulating virus-induced neuronal apoptosis was investigated in an experimental mouse system, in which wild-type HSV invades the central nervous system (CNS) via the olfactory and vomeronasal systems upon intranasal infection. Wild-type HSV-2 strain 186 infected a fraction of olfactory and vomeronasal chemosensory neurons without inducing apoptosis and was transmitted to the CNS, precipitating lethal encephalitis. In sharp contrast, an US3-disrupted mutant, L1BR1, induced neuronal apoptosis in these peripheral conduits upon infection, blocking viral transmission to the CNS and causing no signs of disease. An US3-repaired mutant, L1B(-)11, behaved similarly to the wild-type virus. Only 5 p.f.u. of L1BR1 was sufficient to compromise mice when the mutant virus was introduced directly into the olfactory bulb, a viral entry site of the CNS. These results suggest that the US3 protein kinase of HSV regulates virus-induced neuronal apoptosis in peripheral conduits and determines the neuroinvasive phenotype of HSV. Furthermore, virus-induced neuronal apoptosis of peripheral nervous system cells may be a protective host response that blocks viral transmission to the CNS.