Varicella-zoster virus infection of human neural cells in vivo

A Guide to Preclinical Models of Zoster-Associated Pain and Postherpetic Neuralgia

Reactivation of latent varicella-zoster virus (VZV) causes herpes zoster (HZ), which is commonly accompanied by acute pain and pruritus over the time course of a zosteriform rash. Although the rash and associated pain are self-limiting, a considerable fraction of HZ cases will subsequently develop debilitating chronic pain states termed postherpetic neuralgia (PHN). How VZV causes acute pain and the mechanisms underlying the transition to PHN are far from clear. The human-specific nature of VZV has made in vivo modeling of pain following reactivation difficult to study because no single animal can reproduce reactivated VZV disease as observed in the clinic. Investigations of VZV pathogenesis following primary infection have benefited greatly from human tissues harbored in immune-deficient mice, but modeling of acute and chronic pain requires an intact nervous system with the capability of transmitting ascending and descending sensory signals. Several groups have found that subcutaneous VZV inoculation of the rat induces prolonged and measurable changes in nociceptive behavior, indicating sensitivity that partially mimics the development of mechanical allodynia and thermal hyperalgesia seen in HZ and PHN patients. Although it is not a model of reactivation, the rat is beginning to inform how VZV infection can evoke a pain response and induce long-lasting alterations to nociception. In this review, we will summarize the rat pain models from a practical perspective and discuss avenues that have opened for testing of novel treatments for both zoster-associated pain and chronic PHN conditions, which remain in critical need of effective therapies.

The Ban on US Government Funding Research Using Human Fetal Tissues: How Does This Fit with the NIH Mission to Advance Medical Science for the Benefit of the Citizenry?

Some have argued that human fetal tissue research is unnecessary and/or immoral. Recently, the Trump administration has taken the drastic––and we believe misguided––step to effectively ban government-funded research on fetal tissue altogether. In this article, we show that entire lines of research and their clinical outcomes would not have progressed had fetal tissue been unavailable. We argue that this research has been carried out in a manner that is ethical and legal, and that it has provided knowledge that has saved lives, particularly those of pregnant women, their unborn fetuses, and newborns. We believe that those who support a ban on the use of fetal tissue are halting medical progress and therefore endangering the health and lives of many, and for this they should accept responsibility. At the very least, we challenge them to be true to their beliefs: if they wish to short-circuit a scientific process that has led to medical advances, they should pledge to not accept for themselves the health benefits that such advances provide.

Zoster Vaccination in Older Adults: Efficacy and Public Health Implications

Shingles and its most common disabling complication, post-herpetic neuralgia, represent a serious public health challenge in the older population. The decline in the T-cell-mediated immune response to varicella zoster virus after age 50 is clearly associated with increased risk of viral reactivation, causing an acutely painful zoster rash, which may have a severe prodrome of dermatomal pain and persist as seriously debilitating post-herpetic neuralgia well beyond the resolution of the rash. However, new vaccines and adjuvants are being developed and trialed and are now more effective in preventing shingles and the sequelae of post-herpetic neuralgia. Those vaccines that possess the ability to enhance antigen presentation and reverse memory T-cell exhaustion, as well as diminish the immune suppressive effects of regulatory T cells, are most likely to be effective in older adults.

Manipulation of the Innate Immune Response by Varicella Zoster Virus

Varicella zoster virus (VZV) is the causative agent of chickenpox (varicella) and shingles (herpes zoster). VZV and other members of the herpesvirus family are distinguished by their ability to establish a latent infection, with the potential to reactivate and spread virus to other susceptible individuals. This lifelong relationship continually subjects VZV to the host immune system and as such VZV has evolved a plethora of strategies to evade and manipulate the immune response. This review will focus on our current understanding of the innate anti-viral control mechanisms faced by VZV. We will also discuss the diverse array of strategies employed by VZV to regulate these innate immune responses and highlight new knowledge on the interactions between VZV and human innate immune cells.

Molecular Genetic Insights Into Varicella Zoster Virus (VZV), the vOka Vaccine Strain, and the Pathogenesis of Latency and Reactivation

Genetic tools for molecular typing of varicella zoster virus (VZV) have been used to understand the spread of virus, to differentiate wild-type and vaccine strains, and to understand the natural history of VZV infection in its cognate host. Molecular genetics has identified 7 clades of VZV (1-6 and 9), with 2 more mooted. Differences between the vOka vaccine strain and wild-type VZVs have been used to distinguish the cause of postimmunization events and to provide insight into the natural history of VZV infections. Importantly molecular genetics has shown that reinfection with establishment of latency by the reinfecting strain is common, that dual infections with different viruses can occur, and that reactivation of the superinfecting genotype can both occur. Whole-genome sequencing of the vOka vaccine has been used to show that vesicles form from a single virion, that latency is established within a few days of inoculation, and that all vaccine strains are capable of establishing latency and reactivating. Novel molecular tools have characterized the transcripts expressed during latent infection in vitro.

Immunosenescence Modulation by Vaccination

A decline in immune function is a hallmark of aging that leads to complicated illness from a variety of infectious diseases, cancer and other immune-mediated disorders, and may limit the ability to appropriately respond to vaccination. How vaccines might alter the senescent immune response and what are the immune correlates of protection will be addressed from the perspective of (1) stimulating a previously primed response as in the case of vaccines for seasonal influenza and herpes zoster, (2) priming the response to novel antigens such as pandemic influenza or West Nile virus, (3) vaccination against bacterial pathogens such as pneumococcus and pertussis, (4) vaccines against bacterial toxins such as tetanus and Clostridium difficile, and (5) vaccine approaches to mitigate effects of cytomegalovirus on immune senescence. New or improved vaccines developed over recent years demonstrate the considerable opportunity to improve current vaccines and develop new vaccines as a preventive approach to a variety of diseases in older adults. Strategies for selecting appropriate immunologic targets for new vaccine development and evaluating how vaccines may alter the senescent immune response in terms of potential benefits and risks in the preclinical and clinical trial phases of vaccine development will be discussed.

[Results of laser confocal microscopy of the cornea in viral uveitis (a preliminary report)]

Herpesviruses involve neurotropic activity (they affect nerve cells) and have the ability to induce an immune response (a special tropism for immune tissue), which provides a valid reason for studying the possibilities of visualizing nerve fibers of the cornea and Langerhans cells (LC) in viral uveitis (with prospective applications in early diagnosis).

PURPOSE

To evaluate the results of laser corneal confocal microscopy (CCM) in viral uveitis of varying localization.

MATERIAL AND METHODS

The main study group included 23 patients (23 eyes) diagnosed with unilateral herpesviral uveitis (chorioretinitis), the patients' age varied from 18 to 79 years. The control group comprised 19 healthy volunteers (38 eyes) aged 20 to 75 years. In addition, the paired eyes of the main group patients were examined. In all patients, standard ophthalmologic examination was complemented with CCM performed on the HRT III device with a corneal module, followed by analysis of the course and structure of corneal nerve fibers (CNF) using copyrighted software Liner 1.2.

CONCLUSION

The preliminary results achieved in this study outline the prospects for further research on the state of cornea (in particular, changes in the course and structure of CNF, and the presence of dendritiform cells of Langerhans) with laser CCM in patients with uveitis of various etiologies. These morphological changes also has potential use as diagnostic markers of inflammation of the uveal tract. The main criteria for assessing the state of cornea in viral uveitis include the following: increased tortuosity of CNF, increase in the number and size of Langerhans cells. Further research - in particular, studying the integrated use of diagnostic methods necessary for the verification of viral uveitis, as well as detailed analysis of the history and clinical picture of the disease - is required to substantiate the inclusion of laser confocal microscopy method in the algorithm for the diagnosis of viral uveitis.

Varicella zoster virus infection of human fetal lung cells alters mitochondrial morphology

Varicella zoster virus (VZV) is a ubiquitous alphaherpesvirus that establishes latency in ganglionic neurons throughout the neuraxis after primary infection. Here, we show that VZV infection induces a time-dependent significant change in mitochondrial morphology, an important indicator of cellular health, since mitochondria are involved in essential cellular functions. VZV immediate-early protein 63 (IE63) was detected in mitochondria-rich cellular fractions extracted from infected human fetal lung fibroblasts (HFL) by Western blotting. IE63 interacted with cytochrome c oxidase in bacterial 2-hybrid analyses. Confocal microscopy of VZV-infected HFL cells at multiple times after infection revealed the presence of IE63 in the nucleus, mitochondria, and cytoplasm. Our data provide the first evidence that VZV infection induces alterations in mitochondrial morphology, including fragmentation, which may be involved in cellular damage and/or death during virus infection.

Virus infection, antiviral immunity, and autoimmunity

As a group of disorders, autoimmunity ranks as the third most prevalent cause of morbidity and mortality in the Western World. However, the etiology of most autoimmune diseases remains unknown. Although genetic linkage studies support a critical underlying role for genetics, the geographic distribution of these disorders as well as the low concordance rates in monozygotic twins suggest that a combination of other factors including environmental ones are involved. Virus infection is a primary factor that has been implicated in the initiation of autoimmune disease. Infection triggers a robust and usually well-coordinated immune response that is critical for viral clearance. However, in some instances, immune regulatory mechanisms may falter, culminating in the breakdown of self-tolerance, resulting in immune-mediated attack directed against both viral and self-antigens. Traditionally, cross-reactive T-cell recognition, known as molecular mimicry, as well as bystander T-cell activation, culminating in epitope spreading, have been the predominant mechanisms elucidated through which infection may culminate in an T-cell-mediated autoimmune response. However, other hypotheses including virus-induced decoy of the immune system also warrant discussion in regard to their potential for triggering autoimmunity. In this review, we discuss the mechanisms by which virus infection and antiviral immunity contribute to the development of autoimmunity.

The participation of varicella zoster virus in relapses of multiple sclerosis

OBJECTIVE

Recent studies have documented the apparent participation of varicella zoster virus (VZV) in the etiopathogenesis of multiple sclerosis (MS). The present study aimed to corroborate the possible presence of VZV during exacerbations of MS.

DESIGN

Fifty-three patients with definite MS were included; of them, 31 were studied during the first week of a clinical relapse, whereas 16 were studied during remission; 6 patients with progressive MS were also studied. Genes from 5 herpes viruses: varicella zoster, herpes simplex 1 and 2, Epstein-Barr and herpes 6 were studied by polymerase chain reaction in cerebrospinal fluid and in peripheral blood mononuclear cells (PBMC). As controls 21 patients with inflammatory or functional neurological disorders were included.

RESULTS

DNA from varicella zoster virus was found in the CSF from all MS patients studied during relapse (100%) and in the PBMC from 28 of them (90%). However, VZV DNA was found in the CSF only in 5 MS patients studied during remission (31%) and in the PBMC from 3 of them (19%). VZV DNA was also found, but in lower amounts, in the CSF (83%) and PBMC (33%) from patients with progressive MS. In contrast, VZV was not found either in CSF or in PBMC from controls. Results from the other herpes viruses tested were similar in MS patients and in controls.

CONCLUSIONS

Our results corroborate the conspicuous, but ephemeral presence of VZV during relapses of MS and support the idea of VZV involvement in the etiopathogenesis of MS. Recent epidemiological and molecular studies as well as reports of severe VZV infections triggered by specifically induced immunosuppression during therapy of MS give additional support to this potential association.

Interaction of allergy history and antibodies to specific varicella-zoster virus proteins on glioma risk

Glioma is the most common cancer of the central nervous system but with few confirmed risk factors. It has been inversely associated with chicken pox, shingles and seroreactivity to varicella virus (VZV), as well as to allergies and allergy-associated IgE. The role of antibody reactivity against individual VZV antigens has not been assessed. Ten VZV-related proteins, selected for high immunogenicity or known function, were synthesized and used as targets for antibody measurements in the sera of 143 glioma cases and 131 healthy controls selected from the San Francisco Bay Area Adult Glioma Study. Glioma cases exhibited significantly reduced seroreactivity compared to controls for six antigens, including proteins IE63 [odds ratio (OR) = 0.26, 95% confidence interval (CI): 0.12-0.58, comparing lowest quartile to highest) and the VZV-unique protein ORF2p (OR = 0.44, 95% CI: 0.21-0.96, lowest quartile to highest). When stratifying the study population into those with low and high self-reported allergy history, VZV protein seroreactivity was only associated inversely with glioma among individuals self-reporting more than two allergies. The data provide insight into both allergy and VZV effects on glioma: strong anti-VZV reactions in highly allergic individuals are associated with reduced occurrence of glioma. This result suggests a role for specificity in the anti-VZV immunity in brain tumor suppression for both individual VZV antigens and in the fine-tuning of the immune response by allergy. Anti-VZV reactions may also be a biomarker of effective CNS immunosurveillance owing to the tropism of the virus.

Pathogenesis and current approaches to control of varicella-zoster virus infections

Varicella-zoster virus (VZV) was once thought to be a fairly innocuous pathogen. That view is no longer tenable. The morbidity and mortality due to the primary and secondary diseases that VZV causes, varicella and herpes zoster (HZ), are significant. Fortunately, modern advances, including an available vaccine to prevent varicella, a therapeutic vaccine to diminish the incidence and ameliorate sequelae of HZ, effective antiviral drugs, a better understanding of VZV pathogenesis, and advances in diagnostic virology have made it possible to control VZV in the United States. Occult forms of VZV-induced disease have been recognized, including zoster sine herpete and enteric zoster, which have expanded the field. Future progress should include development of more effective vaccines to prevent HZ and a more complete understanding of the consequences of VZV latency in the enteric nervous system.

Varicella zoster virus (VZV)-human neuron interaction

Varicella zoster virus (VZV) is a highly neurotropic, exclusively human herpesvirus. Primary infection causes varicella (chickenpox), wherein VZV replicates in multiple organs, particularly the skin. Widespread infection in vivo is confirmed by the ability of VZV to kill tissue culture cells in vitro derived from any organ. After varicella, VZV becomes latent in ganglionic neurons along the entire neuraxis. During latency, virus DNA replication stops, transcription is restricted, and no progeny virions are produced, indicating a unique virus-cell (neuron) relationship. VZV reactivation produces zoster (shingles), often complicated by serious neurological and ocular disorders. The molecular trigger(s) for reactivation, and thus the identity of a potential target to prevent it, remains unknown due to an incomplete understanding of the VZV-neuron interaction. While no in vitro system has yet recapitulated the findings in latently infected ganglia, recent studies show that VZV infection of human neurons in SCID mice and of human stem cells, including induced human pluripotent stem cells and normal human neural progenitor tissue-like assemblies, can be established in the absence of a cytopathic effect. Usefulness of these systems in discovering the mechanisms underlying reactivation awaits analyses of VZV-infected, highly pure (>90%), terminally differentiated human neurons capable of prolonged survival in vitro.

Three-dimensional normal human neural progenitor tissue-like assemblies: a model of persistent varicella-zoster virus infection

Varicella-zoster virus (VZV) is a neurotropic human alphaherpesvirus that causes varicella upon primary infection, establishes latency in multiple ganglionic neurons, and can reactivate to cause zoster. Live attenuated VZV vaccines are available; however, they can also establish latent infections and reactivate. Studies of VZV latency have been limited to the analyses of human ganglia removed at autopsy, as the virus is strictly a human pathogen. Recently, terminally differentiated human neurons have received much attention as a means to study the interaction between VZV and human neurons; however, the short life-span of these cells in culture has limited their application. Herein, we describe the construction of a model of normal human neural progenitor cells (NHNP) in tissue-like assemblies (TLAs), which can be successfully maintained for at least 180 days in three-dimensional (3D) culture, and exhibit an expression profile similar to that of human trigeminal ganglia. Infection of NHNP TLAs with cell-free VZV resulted in a persistent infection that was maintained for three months, during which the virus genome remained stable. Immediate-early, early and late VZV genes were transcribed, and low-levels of infectious VZV were recurrently detected in the culture supernatant. Our data suggest that NHNP TLAs are an effective system to investigate long-term interactions of VZV with complex assemblies of human neuronal cells.

Varicella-zoster virus (VZV), the alphaherpesvirus that typically causes childhood chickenpox and shingles in adults, becomes latent in neurons, thus remaining in the body for a lifetime. Unfortunately, few models are available to study the establishment of VZV latency since the virus infects only humans and establishes persistent infections and latency only in neurons, a slowly proliferating, short-lived cell in culture. We have successfully maintained normal human neural progenitor cells (NHNP) in tissue-like assemblies (TLAs) in 3-dimensional (3D) cultures for up to 6 months. The 3D NHNP TLAs show some characteristics as those found in the human trigeminal ganglia, the site of VZV latency. NHNP TLAs infected with VZV remain viable for 3 months during which time VZV DNA replicates and remains genetically stable, virus genes are transcribed, and infectious VZV is sporadically released. The ability to maintain VZV infected NHNP cells in culture for extended times provides the unique opportunity to study the molecular interactions between this important human pathogen and neuronal tissue to an extent previously unattainable.

Human sensory neurons derived from induced pluripotent stem cells support varicella-zoster virus infection

After primary infection, varicella-zoster virus (VZV) establishes latency in neurons of the dorsal root and trigeminal ganglia. Many questions concerning the mechanism of VZV pathogenesis remain unanswered, due in part to the strict host tropism and inconsistent availability of human tissue obtained from autopsies and abortions. The recent development of induced pluripotent stem (iPS) cells provides great potential for the study of many diseases. We previously generated human iPS cells from skin fibroblasts by introducing four reprogramming genes with non-integrating adenovirus. In this study, we developed a novel protocol to generate sensory neurons from iPS cells. Human iPS cells were exposed to small molecule inhibitors for 10 days, which efficiently converted pluripotent cells into neural progenitor cells (NPCs). The NPCs were then exposed for two weeks to growth factors required for their conversion to sensory neurons. The iPS cell-derived sensory neurons were characterized by immunocytochemistry, flow cytometry, RT-qPCR, and electrophysiology. After differentiation, approximately 80% of the total cell population expressed the neuron-specific protein, βIII-tubulin. Importantly, 15% of the total cell population co-expressed the markers Brn3a and peripherin, indicating that these cells are sensory neurons. These sensory neurons could be infected by both VZV and herpes simplex virus (HSV), a related alphaherpesvirus. Since limited neuronal populations are capable of supporting the entire VZV and HSV life cycles, our iPS-derived sensory neuron model may prove useful for studying alphaherpesvirus latency and reactivation.

Varicella zoster virus infection of highly pure terminally differentiated human neurons

In vitro analyses of varicella zoster virus (VZV) reactivation from latency in human ganglia have been hampered by the inability to isolate virus by explantation or cocultivation techniques. Furthermore, attempts to study interaction of VZV with neurons in experimentally infected ganglion cells in vitro have been impaired by the presence of nonneuronal cells, which become productively infected and destroy the cultures. We have developed an in vitro model of VZV infection in which highly pure (>95 %) terminally differentiated human neurons derived from pluripotent stem cells were infected with VZV. At 2 weeks post-infection, infected neurons appeared healthy compared to VZV-infected human fetal lung fibroblasts (HFLs), which developed a cytopathic effect (CPE) within 1 week. Tissue culture medium from VZV-infected neurons did not produce a CPE in uninfected HFLs and did not contain PCR-amplifiable VZV DNA, but cocultivation of infected neurons with uninfected HFLs did produce a CPE. The nonproductively infected neurons contained multiple regions of the VZV genome, as well as transcripts and proteins corresponding to VZV immediate-early, early, and late genes. No markers of the apoptotic caspase cascade were detected in healthy-appearing VZV-infected neurons. VZV infection of highly pure terminally differentiated human neurons provides a unique in vitro system to study the VZV-neuronal relationship and the potential to investigate mechanisms of VZV reactivation.

Varicella-zoster virus infection of differentiated human neural stem cells

Primary varicella-zoster virus (VZV) infection in humans produces varicella (chickenpox), after which the virus becomes latent in ganglionic neurons. Analysis of the physical state of viral nucleic acid and virus gene expression during latency requires postmortem acquisition of fresh human ganglia. To provide an additional way to study the VZV-host relationship in neurons, we developed an in vitro model of infected differentiated human neural stem cells (NSCs). NSCs were induced to differentiate in culture dishes coated with poly-l-lysine and mouse laminin in the presence of fibroblast growth factor 2 (FGF-2), nerve growth factor (NGF), brain-derived neurotropic factor (BDNF), dibutyryl cyclic AMP, and retinoic acid. Immunostaining with neuronal (MAP2a and β-tubulin), astrocyte (GFAP), and oligodendrocyte (CNPase) markers revealed that differentiated neurons constituted approximately 90% of the cell population. These neurons were maintained in culture for up to 8 weeks. No cytopathic effect (CPE) developed in neurons infected with cell-free VZV (Zostavax vaccine) compared to human fetal lung fibroblasts infected with VZV. Weeks later, VZV DNA virus-specific transcripts (open reading frames [ORFs] 21, 29, 62, and 63) were detected in infected neurons, and dual immunofluorescence staining revealed the presence of VZV IE63 and gE exclusively in healthy-appearing neurons, but not in astrocytes. Neither the tissue culture medium nor a homogenate prepared from VZV-infected neurons produced a CPE in fibroblasts. VZV induced apoptosis in fibroblasts, as shown by activation of caspase 3 and by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) staining, but not in neurons. This model provides a unique in vitro system to study the VZV-neuronal relationship.

Varicella-zoster virus neurotropism in SCID mouse-human dorsal root ganglia xenografts

Varicella-zoster virus (VZV) is a neurotropic human alphaherpesvirus and the causative agent of varicella and herpes zoster. VZV reactivation from latency in sensory nerve ganglia is a direct consequence of VZV neurotropism. Investigation of VZV neuropathogenesis by infection of human dorsal root ganglion xenografts in immunocompromised (SCID) mice has provided a novel system in which to examine VZV neurotropism. Experimental infection with recombinant VZV mutants with targeted deletions or mutations of specific genes or regulatory elements provides an opportunity to assess gene candidates that may mediate neurotropism and neurovirulence. The SCID mouse-human DRG xenograft model may aid in the development of clinical strategies in the management of herpes zoster as well as in the development of "second generation" neuroattenuated vaccines.

Immunosenescence Modulation by Vaccination

A decline in immune function is a hallmark of aging that leads to complicated illness from a variety of infectious diseases, cancer and other immune-mediated disorders, and may limit the ability to appropriately respond to vaccination. How vaccines might alter the senescent immune response and what are the immune correlates of protection will be addressed from the perspective of 1) stimulating a previously primed response as in the case of vaccines for seasonal influenza and herpes zoster, 2) priming the response to novel antigens such as pandemic influenza or other viruses, 3) vaccination against bacterial pathogens such as pneumococcus, and 4) altering the immune response to an endogenous protein as in the case of a vaccine against Alzheimer’s disease. In spite of the often limited efficacy of vaccines for older adults, influenza vaccination remains the only cost-saving medical intervention in this population. Thus, considerable opportunity exists to improve current vaccines and develop new vaccines as a preventive approach to a variety of diseases in older adults. Strategies for selecting appropriate immunologic targets for new vaccine development and evaluating how vaccines may alter the senescent immune response in terms of potential benefits and risks in the preclinical and clinical trial phases of vaccine development will be discussed.

Varicella-zoster virus ORF 58 gene is dispensable for viral replication in cell culture

Background

Open reading frame 58 (ORF58) of varicella-zoster virus (VZV) lies at the 3'end of the Unique long (U_L) region and its functional is unknown. In order to clarify whether ORF58 is essential for the growth of VZV, we constructed a deletion mutant of ORF58 (pOka-BACΔ58) from the Oka parental genome cloned into a bacterial artificial chromosome (pOka-BAC).

Results

The ORF58-deleted virus (rpOkaΔ58) was reconstituted from the pOka-BACΔ58 genome in MRC-5 cells, indicating that the ORF58 gene is non-essential for virus growth. Comparison of the growth rate of rpOkaΔ58 and recombinant wild-type virus by assessing plaque sizes revealed no significant differences between them both in MRC-5 cells and malignant melanoma cells.

Conclusion

This study shows that the ORF58 gene is dispensable for viral replication and does not affect the virus' ability to form plaques in vitro.

Productive varicella-zoster virus infection of cultured intact human ganglia

Varicella-zoster virus (VZV) is a species-specific herpesvirus which infects sensory ganglia. We have developed a model of infection of human intact explant dorsal root ganglia (DRG). Following exposure of DRG to VZV, viral antigens were detected in neurons and nonneuronal cells. Enveloped virions were visualized by transmission electron microscopy in neurons and nonneuronal cells and within the extracellular space. Moreover, rather than remaining highly cell associated during infection of cultured cells, such as fibroblasts, cell-free VZV was released from infected DRG. This model enables VZV infection of ganglionic cells to be studied in the context of intact DRG.

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.

Varicella-zoster virus ORF63 inhibits apoptosis of primary human neurons

Virus-encoded modulation of apoptosis may serve as a mechanism to enhance cell survival and virus persistence. The impact of productive varicella-zoster virus (VZV) infection on apoptosis appears to be cell type specific, as infected human sensory neurons are resistant to apoptosis, yet human fibroblasts readily become apoptotic. We sought to identify the viral gene product(s) responsible for this antiapoptotic phenotype in primary human sensory neurons. Treatment with phosphonoacetic acid to inhibit viral DNA replication and late-phase gene expression did not alter the antiapoptotic phenotype, implicating immediate-early (IE) or early genes or a virion component. Compared to the parental VZV strain (rOKA), a recombinant virus unable to express one copy of the diploid IE gene ORF63 (rOka deltaORF63) demonstrated a significant induction of apoptosis in infected neurons, as determined by three methods: annexin V staining, deoxynucleotidyltransferase-mediated dUTP-biotin nick end label staining, and transmission electron microscopy. Furthermore, neurons transfected with a plasmid expressing ORF63 resisted apoptosis induced by nerve growth factor withdrawal. These results show that ORF63 can suppress apoptosis of neurons and provide the first identification of a VZV gene encoding an antiapoptotic function. As ORF63 is expressed in neurons during both productive and latent infection, it may play a significant role in viral pathogenesis by promoting neuron survival during primary and reactivated infections.

Chickenpox

PURPOSE OF REVIEW

Varicella-zoster virus (VZV) remains a public health issue around the globe despite the availability of a live attenuated vaccine and several highly active antiviral agents. A program of universal infant vaccination against varicella was introduced in the US almost 10 years ago. Epidemiological data continue to accumulate that will inform decision-making on vaccine use elsewhere. These findings, together with relevant advances in VZV virology, form the substance of this review.

RECENT FINDINGS

Understanding of the pathogenesis of varicella has significantly advanced with the demonstration that the cation-independent mannose 6-phosphate receptor is critical to both entry and egress of enveloped VZV. While our knowledge of intervening events remains sketchy, the future study of VZV will be facilitated by the recent successful cloning of the VZV genome into a bacterial artificial chromosome. Models of latency and reactivation are also being developed, which may help us to understand the epidemiology of herpes zoster in vaccinated populations. Continued evidence of decline in the incidence of varicella, associated hospitalizations and deaths suggests that the vaccine as used in the US is highly effective. However, rates of breakthrough disease are significant and sufficient to sustain outbreaks, even among highly vaccinated populations. This is so despite the generally reduced infectiousness of varicella occurring in vaccinated individuals. There is some evidence of attrition of the immune response over time following immunization in a small proportion of vaccinees.

SUMMARY

Our ability to prevent and treat varicella still outstrips our knowledge of pathogenetic and immune mechanisms. Further clinical advances are likely to arise from growing understanding of VZV biology.

Varicella-zoster virus infection of human dorsal root ganglia in vivo

Varicella-zoster virus (VZV) causes varicella and establishes latency in sensory ganglia. VZV reactivation results in herpes zoster. We developed a model using human dorsal root ganglion (DRG) xenografts in severe combined immunodeficient (SCID) mice to investigate VZV infection of differentiated neurons and satellite cells in vivo. DRG engrafted under the kidney capsule and contained neurons and satellite cells within a typical DRG architecture. VZV clinical isolates infected the neurons within DRG. At 14 days postinfection, VZ virions were detected by electron microscopy in neuronal cell nuclei and cytoplasm but not in satellite cells. The VZV genome copy number was 7.1 x 10(7) to 8.0 x 10(8) copies per 10(5) cells, and infectious virus was recovered. This initial phase of viral replication was followed within 4-8 weeks by a transition to VZV latency, characterized by the absence of infectious virus release, the cessation of virion assembly, and a reduction in VZV genome copies to 3.7 x 10(5) to 4.7 x 10(6) per 10(5) cells. VZV persistence in DRG was achieved without any requirement for VZV-specific adaptive immunity and was associated with continued transcription of the ORF63 regulatory gene. The live attenuated varicella vaccine virus exhibited the same pattern of short-term replication, persistence of viral DNA, and prominent ORF63 transcription as the clinical isolates. VZV-infected T cells transferred virus from the circulation into DRG, suggesting that VZV lymphotropism facilitates its neurotropism. DRG xenografts may be useful for investigating neuropathogenic mechanisms of other human viruses.

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

Members of the Herpesviridae family of viruses are classified into the alpha, beta and gamma subfamilies. The alpha subfamily is estimated to have diverged from the beta and gamma subfamilies 200-220 million years ago. The ancestors of the herpes simplex virus (HSV) and the varicella-zoster virus (VZV), two ubiquitous and clinically important human pathogens, appeared 70-80 million years ago. As these viruses coevolved with their specific primate hosts, genetic rearrangements led to the development of the contemporary alphaherpesviruses and their distinct complement of genes. Here the distinct features of HSV and VZV are discussed in terms of their transmissibility, clinical picture, tissue tropism, establishment of latency/reactivation and immune evasion, which can, at least in part, be explained by differences in their genomes.