Varicella-Zoster virus proteins encoded by open reading frames 14 and 67 are both dispensable for the establishment of latency in a rat model

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.

Varicella-zoster virus early infection but not complete replication is required for the induction of chronic hypersensitivity in rat models of postherpetic neuralgia

Herpes zoster, the result of varicella-zoster virus (VZV) reactivation, is frequently complicated by difficult-to-treat chronic pain states termed postherpetic neuralgia (PHN). While there are no animal models of VZV-induced pain following viral reactivation, subcutaneous VZV inoculation of the rat causes long-term nocifensive behaviors indicative of mechanical and thermal hypersensitivity. Previous studies using UV-inactivated VZV in the rat model suggest viral gene expression is required for the development of pain behaviors. However, it remains unclear if complete infection processes are needed for VZV to induce hypersensitivity in this host. To further assess how gene expression and replication contribute, we developed and characterized three replication-conditional VZV using a protein degron system to achieve drug-dependent stability of essential viral proteins. Each virus was then assessed for induction of hypersensitivity in rats under replication permissive and nonpermissive conditions. VZV with a degron fused to ORF9p, a late structural protein that is required for virion assembly, induced nocifensive behaviors under both replication permissive and nonpermissive conditions, indicating that complete VZV replication is dispensable for the induction of hypersensitivity. This conclusion was confirmed by showing that a genetic deletion recombinant VZV lacking DNA packaging protein ORF54p still induced prolonged hypersensitivities in the rat. In contrast, VZV with a degron fused to the essential IE4 or IE63 proteins, which are involved in early gene regulation of expression, induced nocifensive behaviors only under replication permissive conditions, indicating importance of early gene expression events for induction of hypersensitivity. These data establish that while early viral gene expression is required for the development of nocifensive behaviors in the rat, complete replication is dispensable. We postulate this model reflects events leading to clinical PHN, in which a population of ganglionic neurons become abortively infected with VZV during reactivation and survive, but host signaling becomes altered in order to transmit ongoing pain.

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.

Varicella-zoster virus glycoprotein I is essential for spread in dorsal root ganglia and facilitates axonal localization of structural virion components in neuronal cultures

Neurons of the sensory ganglia are the major site of varicella-zoster virus (VZV) latency and may undergo productive infection during reactivation. Although the VZV glycoprotein E/glycoprotein I (gE/gI) complex is known to be critical for neurovirulence, few studies have assessed the roles of these proteins during infection of dorsal root ganglia (DRG) due to the high human specificity of the virus. Here, we show that the VZV glycoprotein I gene is an important neurotropic gene responsible for mediating the spread of virus in neuronal cultures and explanted DRG. Inoculation of differentiated SH-SY5Y neuronal cell cultures with a VZV gI gene deletion strain (VZV rOkaΔgI) showed a large reduction in the percentage of cells infected and significantly smaller plaque sizes in a comparison with cultures infected with the parental strain (VZV rOka). In contrast, VZV rOkaΔgI was not significantly attenuated in fibroblast cultures, demonstrating a cell type-specific role for VZV gI. Analysis of rOkaΔgI protein localization by immunofluorescent staining revealed aberrant localization of viral glycoprotein and capsid proteins, with little or no staining present in the axons of differentiated SH-SY5Y cells infected with rOkaΔgI, yet axonal vesicle trafficking was not impaired. Further studies utilizing explanted human DRG indicated that VZV gI is required for the spread of virus within DRG. These data demonstrate a role for VZV gI in the cell-to-cell spread of virus during productive replication in neuronal cells and a role in facilitating the access of virion components to axons.

Apparent expression of varicella-zoster virus proteins in latency resulting from reactivity of murine and rabbit antibodies with human blood group a determinants in sensory neurons

Analyses of varicella-zoster virus (VZV) protein expression during latency have been discordant, with rare to many positive neurons detected. We show that ascites-derived murine and rabbit antibodies specific for VZV proteins in vitro contain endogenous antibodies that react with human blood type A antigens in neurons. Apparent VZV neuronal staining and blood type A were strongly associated (by a χ² test, α = 0.0003). Adsorption of ascites-derived monoclonal antibodies or antiserum with type A erythrocytes or the use of in vitro-derived VZV monoclonal antibodies eliminated apparent VZV staining. Animal-derived antibodies must be screened for anti-blood type A reactivity to avoid misidentification of viral proteins in the neurons of the 30 to 40% of individuals who are blood type A.

Varicella zoster virus latency

Primary infection by varicella zoster virus (VZV) typically results in childhood chickenpox, at which time latency is established in the neurons of the cranial nerve, dorsal root and autonomic ganglia along the entire neuraxis. During latency, the histone-associated virus genome assumes a circular episomal configuration from which transcription is epigenetically regulated. The lack of an animal model in which VZV latency and reactivation can be studied, along with the difficulty in obtaining high-titer cell-free virus, has limited much of our understanding of VZV latency to descriptive studies of ganglia removed at autopsy and analogy to HSV-1, the prototype alphaherpesvirus. However, the lack of miRNA, detectable latency-associated transcript and T-cell surveillance during VZV latency highlight basic differences between the two neurotropic herpesviruses. This article focuses on VZV latency: establishment, maintenance and reactivation. Comparisons are made with HSV-1, with specific attention to differences that make these viruses unique human pathogens.

Overview of varicella-zoster virus glycoproteins gC, gH and gL

The VZV genome is smaller than the HSV genome and only encodes nine glycoproteins. This chapter provides an overview of three VZV glycoproteins: gH (ORF37), gL (ORF60), and gC (ORF14). All three glycoproteins are highly conserved among the alpha herpesviruses. However, VZV gC exhibits unexpected differences from its HSV counterpart gC. In particular, both VZV gC transcription and protein expression are markedly delayed in cultured cells. These delays occur regardless of the virus strain or the cell type, and may account in part for the aberrant assembly of VZV particles. In contrast to VZV gC, the general properties of gH and gL more closely resemble their HSV homologs. VZV gL behaves as a chaperone protein to facilitate the maturation of the gH protein. The mature gH protein in turn is a potent fusogen. Its fusogenic activity can be abrogated when infected cultures are treated with monoclonal anti-gH antibodies.

Experimental models to study varicella-zoster virus infection of neurons

Varicella zoster virus (VZV) infection results in the establishment of latency in human sensory neurons. Reactivation of VZV leads to herpes zoster which can be followed by persistent neuropathic pain, termed post-herpetic neuralgia (PHN). Humans are the only natural host for VZV, and the strict species specificity of the virus has restricted the development of an animal model of infection which mimics all phases of disease. In order to elucidate the mechanisms which control the establishment of latency and reactivation as well as the effect of VZV replication on neuronal function, in vitro models of neuronal infection have been developed. Currently these models involve culturing and infecting dissociated human fetal neurons, with or without their supporting cells, an intact explant fetal dorsal root ganglia (DRG) model, neuroblastoma cell lines and rodent neuronal cell models. Each of these models has distinct advantages as well as disadvantages, and all have contributed towards our understanding of VZV neuronal infection. However, as yet none have been able to recapitulate the full virus lifecycle from primary infection to latency through to reactivation. The development of such a model will be a crucial step towards advancing our understanding of the mechanisms involved in VZV replication in neuronal cells, and the design of new therapies to combat VZV-related disease.

A varicella-zoster virus mutant impaired for latency in rodents, but not impaired for replication in cell culture

While trying to generate a site-directed deletion in the ORF63 latency-associated gene of varicella-zoster virus (VZV) Oka, we constructed a virus with an unexpected rearrangement. The virus has a small deletion in both copies of ORF63 and two copies of a cassette inserted between ORFs 64/65 and 68/69 containing (a) truncated ORF62, (b) ORF63 with a small deletion, and (c) full-length ORF64. The virus was not impaired for growth in human cells, induced higher levels of neutralizing antibodies in guinea pigs, and was impaired for latency in cotton rats compared with parental virus (p=0.0022). Additional mutants containing the same truncation in ORF62, with or without the ORF63 deletion, were less impaired for latency. A VZV Oka mutant, replicating to similar titers and inducing a comparable immune response as parental virus, but impaired for latency, might serve as a safer vaccine and be less likely to reactivate to cause zoster.

VZV infection of keratinocytes: production of cell-free infectious virions in vivo

Varicella-zoster virus (VZV) is the cause of varicella (chickenpox) and zoster (shingles). Varicella is a primary infection that spreads rapidly in epidemics while zoster is a secondary infection that occurs sporadically as a result of the reactivation of previously acquired VZV. Reactivation is made possible by the establishment of latency during the initial episode of varicella. The signature lesions of both varicella and zoster are cutaneous vesicles, which are filled with a clear fluid that is rich in infectious viral particles. It has been postulated that the skin is the critical organ in which both host-to-host transmission of VZV and the infection of neurons to establish latency occur. This hypothesis is built on evidence that the large cation-independent mannose 6-phosphate receptor (MPR(ci)) interacts with VZV in virtually all infected cells, except those of the suprabasal epidermis, in a way that prevents the release of infectious viral particles. Specifically, the virus is diverted in an MPR(ci)-dependent manner from the secretory pathway to late endosomes where VZV is degraded. Because nonepidermal cells are thus prevented from releasing infectious VZV, a slow process, possibly involving fusion of infected cells with their neighbors, becomes the means by which VZV is disseminated. In the epidermis, however, the maturation of keratinocytes to give rise to corneocytes in the suprabasal epidermis is associated uniquely with a downregulation of the MPR(ci). As a result, the diversion of VZV to late endosomes does not occur in the suprabasal epidermis where vesicular lesions occur. The formation of the waterproof, chemically resistant barrier of the epidermis, however, requires that constitutive secretion outlast the downregulation of the endosomal pathway. Infectious VZV is therefore secreted by default, accounting for the presence of infectious virions in vesicular fluid. Sloughing of corneocytes, aided by scratching, then aerosolizes the virus, which can float with dust to be inhaled by susceptible hosts. Infectious virions also bathe the terminals of those sensory neurons that innervate the epidermis. These terminals become infected with VZV and provide a route, retrograde transport, which can conduct VZV to cranial nerve (CNG), dorsal root ganglia (DRG), and enteric ganglia (EG) to establish latency. Reactivation returns VZV to the skin, now via anterograde transport in axons, to cause the lesions of zoster. Evidence in support of these hypotheses includes observations of the VZV-infected human epidermis and studies of guinea pig neurons in an in vitro model system.

Rodent models of varicella-zoster virus neurotropism

Inoculation of rodents with varicella-zoster virus (VZV) results in a latent infection in dorsal root ganglia with expression of at least five of the six VZV transcripts and one of the viral proteins that are reported to be expressed during latency in human ganglia. Rats develop allodynia and hyperalgesia in the limb distal to the site of injection and the resulting exaggerated withdrawal response to stimuli is reduced by treatment with gabapentin and amitryptyline, but not by antiviral therapy. Inoculation of rats with VZV mutants show that most viral genes are dispensable for latency, but that some genes (e.g., ORF4, 29, and ORF63) that are expressed during latency are important for the establishment of latency in rodents, but not for infection of rodent ganglia. The rodent model for VZV latency allows one to study ganglia removed immediately after death, avoiding the possibility of reactivation, and helps to identify VZV genes required for latency.

Complete DNA sequences of two oka strain varicella-zoster virus genomes

Varicella-zoster virus (VZV) is a herpesvirus and is the causative agent of chicken pox (varicella) and shingles (herpes zoster). Active immunization against varicella became possible with the development of live attenuated varicella vaccine. The Oka vaccine strain was isolated in Japan from a child who had typical varicella, and it was then attenuated by serial passages in cell culture. Several manufacturers have obtained this attenuated Oka strain and, following additional passages, have developed their own vaccine strains. Notably, the vaccines Varilrix and Varivax are produced by GlaxoSmithKline Biologicals and Merck & Co., Inc., respectively. Both vaccines have been well studied in terms of safety and immunogenicity. In this study, we report the complete nucleotide sequence of the Varilrix (Oka-V(GSK)) and Varivax (Oka-V(Merck)) vaccine strain genomes. Their genomes are composed of 124,821 and 124,815 bp, respectively. Full genome annotations covering the features of Oka-derived vaccine genomes have been established for the first time. Sequence analysis indicates 36 nucleotide differences between the two vaccine strains throughout the entire genome, among which only 14 are involved in unique amino acid substitutions. These results demonstrate that, although Oka-V(GSK) and Oka-V(Merck) vaccine strains are not identical, they are very similar, which supports the clinical data showing that both vaccines are well tolerated and elicit strong immune responses against varicella.

Aberrant infection and persistence of varicella-zoster virus in human dorsal root ganglia in vivo in the absence of glycoprotein I

Varicella-zoster virus (VZV) causes varicella, establishes latency in sensory ganglia, and reactivates as herpes zoster. Human dorsal root ganglia (DRGs) xenografts in immunodeficient mice provide a model for evaluating VZV neuropathogenesis. Our investigation of the role of glycoprotein I (gI), which is dispensable in vitro, examines the functions of a VZV gene product during infection of human neural cells in vivo. Whereas intact recombinant Oka (rOka) initiated a short replicative phase followed by persistence in DRGs, the gI deletion mutant, rOkaDeltagI, showed prolonged replication with no transition to persistence up to 70 days after infection. Only a few varicella-zoster nucleocapsids and cytoplasmic virions were observed in neurons, and the major VZV glycoprotein, gE, was retained in the rough endoplasmic reticulum in the absence of gI. VZV neurotropism was not disrupted when DRG xenografts were infected with rOka mutants lacking gI promoter elements that bind cellular transactivators, specificity factor 1 (Sp1) and upstream stimulatory factor (USF). Because gI is essential and Sp1 and USF contribute to VZV pathogenesis in skin and T cells in vivo, these DRG experiments indicate that the genetic requirements for VZV infection are less stringent in neural cells in vivo. The observations demonstrate that gI is important for VZV neurotropism and suggest that a strategy to reduce neurovirulence by deleting gI could prolong active infection in human DRGs.

The pattern of viral persistence in monkeys intra-tracheally infected with Simian varicella virus

In situ PCR (ISPCR) and in situ hybridisation (ISH) was performed on 32 tissues from 10 monkeys, intra-tracheally (IT) infected with simian varicella virus (SVV) and 5 tissues from 3 uninfected control animals. The results showed persistence of SVV DNA up to 2 years post-infection (pi) and the localisation of SVV to be confined to neurons except at time points 9 and 10 months pi where SVV positive satellite cells were also detected. There was no evidence for transcription of SVV ORFs 63 and 21 in the ganglia of the one IT infected and 2 naturally infected monkeys investigated using RNA ISH.

Varicella-Zoster virus gene expression at variable periods following death in a rat model of ganglionic infection

We used a rat model of Varicella-Zoster virus (VZV) ganglionic infection, which mirrors some of the features of VZV latency in humans, to determine the temporal pattern of expression of a VZV immediate-early gene (63) and a VZV late gene (40) at 0, 24 and 48 h after death of the animal. The immediate-early VZV gene 63 is known to be abundantly expressed during human ganglionic latency, while the late VZV gene 40 is not expressed during human latency. Using both RNA in situ hybridisation (ISH) and nested RT-PCR, it was found that at all time points in both thoracic and lumbar ganglia, the number of ganglia positive for VZV gene 63 was higher than for gene 40. The expression of gene 40 did not increase with time postmortem (pm) These results provide indirect support for the hypothesis that patterns of expression of VZV genes detected in human tissue at even 48 h pm reflect the pattern of expression during human ganglionic latency.

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.

Epigenetic regulation of varicella-zoster virus open reading frames 62 and 63 in latently infected human trigeminal ganglia

Open reading frames (ORFs) 21, 29, 62, 63, and 66 of varicella-zoster virus (VZV) are transcribed during latency in human ganglia. ORF 63 is the most frequently expressed gene, and ORF 62 encodes a transcriptional activator. The mechanisms regulating the expression of these genes are not well understood, although analyses of other alphaherpesviruses indicate a role for chromatin in virus gene regulation during latent infection. Using chromatin immunoprecipitation (ChIP) assays to analyze the euchromatic state of ORFs 62 and 63 compared to the centromere from human chromosome 4 (heterochromatic) and the human glyceraldehyde-3-phosphate dehydrogenase promoter (euchromatic), we show that the promoters of ORFs 62 and 63 are associated with the histone protein H3K9(Ac) and thus maintained in a euchromatic state during latency. Conversely, the promoters of ORF 36 (thymidine kinase) and ORF 14 (glycoprotein C), genes expressed during lytic but not latent infection, were not enriched in the fraction of latently infected ganglia that bound to anti-H3K9(Ac) antibody. A ChIP assay using productively infected MeWo cells revealed that VZV ORFs 62, 63, 36, and 14 are all euchromatic. Together, these data indicate that the expression of the two latency-related VZV genes, ORFs 62 and 63, is regulated epigenetically through chromatin structure.