Experimental simian varicella virus infection of St. Kitts vervet monkeys

Insertion of foreign genes into the simian varicella virus genome by Tn7-mediated site-specific transposition

A Tn7-transposition approach was utilized for site-specific insertion of foreign genes into the genome of simian varicella virus (SVV), the causative agent of simian varicella in nonhuman primates. The severe acute respiratory syndrome coronavirus (SARS-CoV-2) nucleocapsid (N) gene and receptor binding domain (RBD) of the spike gene were inserted into the ORF 14 region of the SVV genome cloned into a bacterial artificial chromosome and then transfected into Vero cells to generate the infectious recombinant SVV (rSVV). The rSVV replicated efficiently in infected Vero cells and expressed the N and RBD antigens as indicated by immunoblot and immunofluorescence assays. Tn7-mediated transposition provides a rapid and efficient method for constructing rSVVs which may be evaluated as live-attenuated vaccines.

Simian Varicella Virus: Molecular Virology and Mechanisms of Pathogenesis

Simian varicella virus (SVV) was first isolated in 1966 from African green monkeys (Cercopithecus aethiops) imported from Nairobi, Kenya, to the Liverpool School of Tropical Medicine in the United Kingdom (UK) (Clarkson et al., Arch Gesamte Virusforsch 22:219-234, 1967). SVV infection caused severe disease that resulted in a 56% case fatality rate (CFR) in the imported animals within 48 h of the appearance of a varicella-like rash (Clarkson et al., Arch Gesamte Virusforsch 22:219-234, 1967; Hemme et al., Am J Trop Med Hyg 94:1095-1099, 2016). The deceased animals presented with fever, widespread vesicular rash, and multiple hemorrhagic foci throughout the lungs, liver, and spleen (Clarkson et al., Arch Gesamte Virusforsch 22:219-234, 1967). This outbreak was quickly followed by a second outbreak in 47 patas monkeys (Erythrocebus patas) imported from Chad and Nigeria by Glaxo Laboratories (London, England, UK), which quickly spread within the facility (McCarthy et al., Lancet 2:856-857, 1968).

Recombinant Simian Varicella Virus-Simian Immunodeficiency Virus Vaccine Induces T and B Cell Functions and Provides Partial Protection against Repeated Mucosal SIV Challenges in Rhesus Macaques

HIV vaccine mediated efficacy, using an expanded live attenuated recombinant varicella virus-vectored SIV rSVV-SIVgag/env vaccine prime with adjuvanted SIV-Env and SIV-Gag protein boosts, was evaluated in a female rhesus macaques (RM) model against repeated intravaginal SIV challenges. Vaccination induced anti-SIV IgG responses and neutralizing antibodies were found in all vaccinated RMs. Three of the eight vaccinated RM remained uninfected (vaccinated and protected, VP) after 13 repeated challenges with the pathogenic SIVmac251-CX-1. The remaining five vaccinated and infected (VI) macaques had significantly reduced plasma viral loads compared with the infected controls (IC). A significant increase in systemic central memory CD4+ T cells and mucosal CD8+ effector memory T-cell responses was detected in vaccinated RMs compared to controls. Variability in lymph node SIV-Gag and Env specific CD4+ and CD8+ T cell cytokine responses were detected in the VI RMs while all three VP RMs had more durable cytokine responses following vaccination and prior to challenge. VI RMs demonstrated predominately SIV-specific monofunctional cytokine responses while the VP RMs generated polyfunctional cytokine responses. This study demonstrates that varicella virus-vectored SIV vaccination with protein boosts induces a 37.5% efficacy rate against pathogenic SIV challenge by generating mucosal memory, virus specific neutralizing antibodies, binding antibodies, and polyfunctional T-cell responses.

An enzyme-linked immunosorbent assay (ELISA) to determine Simian Varicella Virus antibody titers in infected rhesus monkeys (Macaca mulatta)

BACKGROUND

Simian varicella virus (SVV) is a primate herpesvirus that causes a natural varicella-like disease in Old World monkeys and may cause epizootics in facilities housing nonhuman primates. SVV infection of nonhuman primates is used as an experimental model to investigate varicella pathogenesis and to develop antiviral strategies.

METHODS

An indirect enzyme-linked immunosorbent assay (ELISA) was developed to detect SVV antibodies in infected rhesus macaque monkeys.

RESULTS

An ELISA determined SVV antibody titers following experimental infection. SVV IgG was detected by day 14 post-infection and remained elevated for at least 84 days.

CONCLUSIONS

The SVV ELISA is a safe and rapid approach to confirm SVV seropositivity and to determine SVV antibody titers in naturally and experimentally SVV-infected monkeys. In addition to being a useful diagnostic assay to rapidly confirm acute disease or past SVV infection, the SVV ELISA is a valuable epidemiological tool to determine the incidence of SVV in non-human primate facilities.

Alveolar barrier disruption in varicella pneumonia is associated with neutrophil extracellular trap formation

Primary varicella-zoster virus (VZV) infection in adults is often complicated by severe pneumonia, which is difficult to treat and is associated with high morbidity and mortality. Here, the simian varicella virus (SVV) nonhuman primate (NHP) model was used to investigate the pathogenesis of varicella pneumonia. SVV infection resulted in transient fever, viremia, and robust virus replication in alveolar pneumocytes and bronchus-associated lymphoid tissue. Clearance of infectious virus from lungs coincided with robust innate immune responses, leading to recruitment of inflammatory cells, mainly neutrophils and lymphocytes, and finally severe acute lung injury. SVV infection caused neutrophil activation and formation of neutrophil extracellular traps (NETs) in vitro and in vivo. Notably, NETs were also detected in lung and blood specimens of varicella pneumonia patients. Lung pathology in the SVV NHP model was associated with dysregulated expression of alveolar epithelial cell tight junction proteins (claudin-2, claudin-10, and claudin-18) and alveolar endothelial adherens junction protein VE-cadherin. Importantly, factors released by activated neutrophils, including NETs, were sufficient to reduce claudin-18 and VE-cadherin expression in NHP lung slice cultures. Collectively, the data indicate that alveolar barrier disruption in varicella pneumonia is associated with NET formation.

Current In Vivo Models of Varicella-Zoster Virus Neurotropism

Varicella-zoster virus (VZV), an exclusively human herpesvirus, causes chickenpox and establishes a latent infection in ganglia, reactivating decades later to produce zoster and associated neurological complications. An understanding of VZV neurotropism in humans has long been hampered by the lack of an adequate animal model. For example, experimental inoculation of VZV in small animals including guinea pigs and cotton rats results in the infection of ganglia but not a rash. The severe combined immune deficient human (SCID-hu) model allows the study of VZV neurotropism for human neural sub-populations. Simian varicella virus (SVV) infection of rhesus macaques (RM) closely resembles both human primary VZV infection and reactivation, with analyses at early times after infection providing valuable information about the extent of viral replication and the host immune responses. Indeed, a critical role for CD4 T-cell immunity during acute SVV infection as well as reactivation has emerged based on studies using RM. Herein we discuss the results of efforts from different groups to establish an animal model of VZV neurotropism.

Pathogenesis of varicelloviruses in primates

Varicelloviruses in primates comprise the prototypic human varicella-zoster virus (VZV) and its non-human primate homologue, simian varicella virus (SVV). Both viruses cause varicella as a primary infection, establish latency in ganglionic neurons and reactivate later in life to cause herpes zoster in their respective hosts. VZV is endemic worldwide and, although varicella is usually a benign disease in childhood, VZV reactivation is a significant cause of neurological disease in the elderly and in immunocompromised individuals. The pathogenesis of VZV infection remains ill-defined, mostly due to the species restriction of VZV that impedes studies in experimental animal models. SVV infection of non-human primates parallels virological, clinical, pathological and immunological features of human VZV infection, thereby providing an excellent model to study the pathogenesis of varicella and herpes zoster in its natural host. In this review, we discuss recent studies that provided novel insight in both the virus and host factors involved in the three elementary stages of Varicellovirus infection in primates: primary infection, latency and reactivation.

Recombinant varicella-zoster virus vaccines as platforms for expression of foreign antigens

Varicella-zoster virus (VZV) vaccines induce immunity against childhood chickenpox and against shingles in older adults. The safety, efficacy, and widespread use of VZV vaccines suggest that they may also be effective as recombinant vaccines against other infectious diseases that affect the young and the elderly. The generation of recombinant VZV vaccines and their evaluation in animal models are reviewed. The potential advantages and limitations of recombinant VZV vaccines are addressed.

T-Cell tropism of simian varicella virus during primary infection

Varicella-zoster virus (VZV) causes varicella, establishes a life-long latent infection of ganglia and reactivates to cause herpes zoster. The cell types that transport VZV from the respiratory tract to skin and ganglia during primary infection are unknown. Clinical, pathological, virological and immunological features of simian varicella virus (SVV) infection of non-human primates parallel those of primary VZV infection in humans. To identify the host cell types involved in virus dissemination and pathology, we infected African green monkeys intratracheally with recombinant SVV expressing enhanced green fluorescent protein (SVV-EGFP) and with wild-type SVV (SVV-wt) as a control. The SVV-infected cell types and virus kinetics were determined by flow cytometry and immunohistochemistry, and virus culture and SVV-specific real-time PCR, respectively. All monkeys developed fever and skin rash. Except for pneumonitis, pathology produced by SVV-EGFP was less compared to SVV-wt. In lungs, SVV infected alveolar myeloid cells and T-cells. During viremia the virus preferentially infected memory T-cells, initially central memory T-cells and subsequently effector memory T-cells. In early non-vesicular stages of varicella, SVV was seen mainly in perivascular skin infiltrates composed of macrophages, dendritic cells, dendrocytes and memory T-cells, implicating hematogenous spread. In ganglia, SVV was found primarily in neurons and occasionally in memory T-cells adjacent to neurons. In conclusion, the data suggest the role of memory T-cells in disseminating SVV to its target organs during primary infection of its natural and immunocompetent host.

Varicella-zoster virus (VZV) causes varicella, establishes life-long latent infection in ganglia and reactivates later in life to cause zoster. VZV is acquired via the respiratory route, with skin rash occurring up to 3 weeks after exposure. The cell types that transport VZV to skin and ganglia during primary infection are unknown. Simian varicella virus (SVV) infection of non-human primates mimics clinical, pathological and immunological features of human VZV infection. African green monkeys were infected with recombinant SVV expressing enhanced green fluorescent protein (SVV-EGFP) or wild-type SVV (SVV-wt) as a control. By visualizing SVV-EGFP−infected cells in the living animal and in tissue samples, we identified the virus-infected cell types in blood, lungs, skin and ganglia during primary infection. Our data demonstrate that during viremia, SVV predominantly infects peripheral blood memory T-cells. Detection of SVV-infected memory T-cells in lungs, in early varicella skin lesions and also, albeit to a lesser extent, in ganglia suggests a role for memory T-cells in transporting virus to these organs. Our study provides novel insights into the cell types involved in virus dissemination and the overall pathology of varicella in a non-human primate model.

Simian varicella virus infection of Chinese rhesus macaques produces ganglionic infection in the absence of rash

Varicella-zoster virus (VZV) causes varicella (chickenpox), becomes latent in ganglia along the entire neuraxis, and may reactivate to cause herpes zoster (shingles). VZV may infect ganglia via retrograde axonal transport from infected skin or through hematogenous spread. Simian varicella virus (SVV) infection of rhesus macaques provides a useful model system to study the pathogenesis of human VZV infection. To dissect the virus and host immune factors during acute SVV infection, we analyzed four SVV-seronegative Chinese rhesus macaques infected intratracheally with cell-associated 5 × 10³ plaque-forming units (pfu) of SVV-expressing green fluorescent protein (n = 2) or 5 × 10⁴ pfu of wild-type SVV (n = 2). All monkeys developed viremia and SVV-specific adaptive B- and T-cell immune responses, but none developed skin rash. At necropsy 21 days postinfection, SVV DNA was found in ganglia along the entire neuraxis and in viscera, and SVV RNA was found in ganglia, but not in viscera. The amount of SVV inoculum was associated with the extent of viremia and the immune response to virus. Our findings demonstrate that acute SVV infection of Chinese rhesus macaques leads to ganglionic infection by the hematogenous route and the induction of a virus-specific adaptive memory response in the absence of skin rash.

Review: The neurobiology of varicella zoster virus infection

Varicella zoster virus (VZV) is a neurotropic herpesvirus that infects nearly all humans. Primary infection usually causes chickenpox (varicella), after which virus becomes latent in cranial nerve ganglia, dorsal root ganglia and autonomic ganglia along the entire neuraxis. Although VZV cannot be isolated from human ganglia, nucleic acid hybridization and, later, polymerase chain reaction proved that VZV is latent in ganglia. Declining VZV-specific host immunity decades after primary infection allows virus to reactivate spontaneously, resulting in shingles (zoster) characterized by pain and rash restricted to one to three dermatomes. Multiple other serious neurological and ocular disorders also result from VZV reactivation. This review summarizes the current state of knowledge of the clinical and pathological complications of neurological and ocular disease produced by VZV reactivation, molecular aspects of VZV latency, VZV virology and VZV-specific immunity, the role of apoptosis in VZV-induced cell death and the development of an animal model provided by simian varicella virus infection of monkeys.

Recombinant varicella vaccines induce neutralizing antibodies and cellular immune responses to SIV and reduce viral loads in immunized rhesus macaques

The development of an effective AIDS vaccine remains one of the highest priorities in HIV research. The live, attenuated varicella-zoster virus (VZV) Oka vaccine, safe and effective for prevention of chickenpox and zoster, also has potential as a recombinant vaccine against other pathogens, including human immunodeficiency virus (HIV). The simian varicella model, utilizing simian varicella virus (SVV), offers an approach to evaluate recombinant varicella vaccine candidates. Recombinant SVV (rSVV) vaccine viruses expressing simian immunodeficiency virus (SIV) env and gag antigens were constructed. The hypothesis tested was that a live, attenuated rSVV-SIV vaccine will induce immune responses against SIV in the rhesus macaques and provide protection against SIV challenge. The results demonstrated that rSVV-SIV vaccination induced low levels of neutralizing antibodies and cellular immune responses to SIV in immunized rhesus macaques and significantly reduced viral loads following intravenous challenge with pathogenic SIVmac251-CX-1.

Simian herpesviruses and their risk to humans

A high level of genetic and physiological homology with humans has rendered non-human primates (NHP) an essential animal model for biomedical research. As such NHP offer a unique opportunity to study host-pathogen interactions in a species that closely mimics human biology but can yet be maintained under tight laboratory conditions. Indeed, studies using NHP have been critical to our understanding of pathogenesis as well as the development of vaccines and therapeutics. This further facilitated by the fact that NHPs are susceptible to a variety of pathogens that bear significant homology to human pathogens. Unfortunately, these same viruses pose a potential health issue to humans. In this review we discuss the simian herpesviruses and their potential to cause disease in researchers that come into close contact with them.

Simian varicella virus pathogenesis

Because varicella zoster virus (VZV) is an exclusively human pathogen, the development of an animal model is necessary to study pathogenesis, latency, and reactivation. The pathological, virological, and immunological features of simian varicella virus (SVV) infection in nonhuman primates are similar to those of VZV infection in humans. Both natural infection of cynomolgus and African green monkeys as well as intrabronchial inoculation of rhesus macaques with SVV provide the most useful models to study viral and immunological aspects of latency and the host immune response. Experimental immunosuppression of monkeys latently infected with SVV results in zoster, thus providing a new model system to study how the loss of adaptive immunity modulates virus reactivation.

Simian varicella virus infection of rhesus macaques recapitulates essential features of varicella zoster virus infection in humans

Simian varicella virus (SVV), the etiologic agent of naturally occurring varicella in primates, is genetically and antigenically closely related to human varicella zoster virus (VZV). Early attempts to develop a model of VZV pathogenesis and latency in nonhuman primates (NHP) resulted in persistent infection. More recent models successfully produced latency; however, only a minority of monkeys became viremic and seroconverted. Thus, previous NHP models were not ideally suited to analyze the immune response to SVV during acute infection and the transition to latency. Here, we show for the first time that intrabronchial inoculation of rhesus macaques with SVV closely mimics naturally occurring varicella (chickenpox) in humans. Infected monkeys developed varicella and viremia that resolved 21 days after infection. Months later, viral DNA was detected only in ganglia and not in non-ganglionic tissues. Like VZV latency in human ganglia, transcripts corresponding to SVV ORFs 21, 62, 63 and 66, but not ORF 40, were detected by RT-PCR. In addition, as described for VZV, SVV ORF 63 protein was detected in the cytoplasm of neurons in latently infected monkey ganglia by immunohistochemistry. We also present the first in depth analysis of the immune response to SVV. Infected animals produced a strong humoral and cell-mediated immune response to SVV, as assessed by immunohistology, serology and flow cytometry. Intrabronchial inoculation of rhesus macaques with SVV provides a novel model to analyze viral and immunological mechanisms of VZV latency and reactivation.

Recombinant simian varicella viruses expressing respiratory syncytial virus antigens are immunogenic

Recombinant simian varicella viruses (rSVVs) were engineered to express respiratory syncytial virus (RSV) antigens. The RSV surface glycoprotein G and second matrix protein M2 (22k) genes were cloned into the SVV genome, and recombinant viruses were characterized in vitro and in vivo. rSVVs were also engineered to express the membrane-anchored or secreted forms of the RSV-G protein as well as an RSV G lacking its chemokine mimicry motif (CX3C), which may have different effects on priming the host immune response. The RSV genes were efficiently expressed in rSVV/RSV-infected Vero cells as RSV-G and -M2 transcripts were detected by RT-PCR, and RSV antigens were detected by immunofluorescence and immunoblot assays. The rSVVs replicated efficiently in Vero cell culture. Rhesus macaques immunized with rSVV/RSV-G and rSVV/RSV-M2 vaccines produced antibody responses to SVV and RSV antigens. The results demonstrate that recombinant varicella viruses are suitable vectors for the expression of RSV antigens and may represent a novel vaccine strategy for immunization against both pathogens.

Simian varicella virus expresses a latency-associated transcript that is antisense to open reading frame 61 (ICP0) mRNA in neural ganglia of latently infected monkeys

Simian varicella virus (SVV) and varicella-zoster virus (VZV) are closely related alphaherpesviruses that cause varicella (chickenpox) in nonhuman primates and humans, respectively. After resolution of the primary disease, SVV and VZV establish latent infection of neural ganglia and may later reactivate to cause a secondary disease (herpes zoster). This study investigated SVV gene expression in neural ganglia derived from latently infected vervet monkeys. SVV transcripts were detected in neural ganglia, but not in liver or lung tissues, of latently infected animals. A transcript mapping to open reading frame (ORF) 61 (herpes simplex virus type 1 [HSV-1] ICP0 homolog) was consistently detected in latently infected trigeminal, cervical, and lumbar ganglia by reverse transcriptase PCR. Further analysis confirmed that this SVV latency-associated transcript (LAT) was oriented antisense to the gene 61 mRNA. SVV ORF 21 transcripts were also detected in 42% of neural ganglia during latency. In contrast, SVV ORF 28, 29, 31, 62, and 63 transcripts were not detected in ganglia, liver, or lung tissues of latently infected animals. The results demonstrate that viral gene expression is limited during SVV latency and that a LAT antisense to an ICP0 homolog is expressed. In this regard, SVV gene expression during latency is similar to that of HSV-1 and other neurotropic animal alphaherpesviruses but differs from that reported for VZV.

Recombinant simian varicella viruses induce immune responses to simian immunodeficiency virus (SIV) antigens in immunized vervet monkeys

The varicella-zoster virus (VZV) Oka vaccine offers potential as a recombinant vaccine against other pathogens. In this study, recombinant simian varicella viruses (rSVV) expressing simian immunodeficiency virus (SIV) envelope (env, gp130) and gag antigens were constructed. Expression of the SIV env and gag transcripts and antigens in rSVV-infected Vero cells was confirmed. The rSVV-SIVenv and rSVV-SIVgag viruses replicated as efficiently as wild-type SVV in cell culture. The immunogenicity of rSVV-SIVenv and rSVV-SIVgag was investigated in immunized vervet monkeys. Humoral immune responses to the SIV gp130 and gag antigens were detected as early as 4 weeks after the initial immunization with higher antibody titers following a booster immunization. Cellular immune responses against the SIV gp130 antigen were detected by ELISPOT assay. The rSVV established latent infection in neural ganglia. A subsequent study will evaluate the ability of rSVV vaccines expressing SIV antigens to protect nonhuman primates against simian AIDS.

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.

Simian varicella virus gene 61 encodes a viral transactivator but is non-essential for in vitro replication

Simian varicella virus (SVV) is closely related to varicella-zoster virus (VZV), the causative agent of chickenpox and shingles. The SVV and VZV gene 61 polypeptides are homologs of the HSV-1 ICP0, a viral transactivator which appears to play a role in viral latency and reactivation. In this study, the molecular properties of the SVV 61 were characterized. The SVV open reading frame (ORF) 61 encodes a 54.1-kDa polypeptide with 37% amino acid identity to the VZV 61. Homology to the HSV-1 ICP-0 is limited to a conserved RING finger motif at the amino terminus of the protein. A nuclear localization sequence (nls) at the carboxy-terminus directs the SVV 61 to the cell nucleus, while a SVV 61nls(-) mutant is confined to the cell cytoplasm. The SVV 61 transactivates its own promoter as well as SVV immediate early (IE, ORF 62), early (ORFs 28 and 29), and late (ORF 68) gene promoters in transfected Vero cells. The RING finger and nls motifs are required for efficient SVV 61 transactivation. The SVV 61 has no effect on the ability of the major SVV transactivator (IE62) to induce SVV promoters. Generation and propagation of a SVV gene 61 deletion mutant demonstrated that the SVV 61 is non-essential for in vitro replication. SVV gene 61 is expressed in liver, lung, and neural ganglia of infected monkeys during acute simian varicella.

Simian varicella: a model for human varicella-zoster virus infections

Simian varicella virus (SVV) causes a natural varicella-like disease in nonhuman primates. Epizootics of simian varicella occur sporadically in facilities housing Old World monkeys. SVV is antigenically and genetically related to varicella-zoster virus (VZV), the etiologic agent of varicella (chickenpox) and herpes zoster (shingles) in humans. The SVV and VZV genomes are similar in size and structure, share 70%-75% DNA homology and are co-linear with respect to gene organisation. Simian varicella is a highly contagious disease characterised by fever and vesicular skin rash and may progress to pneumonia and hepatitis. Infected monkeys may resolve the disease within 2 weeks although epizootics are sometimes associated with high morbidity and mortality. SVV, like VZV, establishes life-long latent infection, as indicated by detection of viral DNA within neural ganglia. Subsequently, SVV may reactivate to cause secondary disease and spread of the virus to susceptible monkeys. The relatedness of VZV and SVV and the similarities in the clinical symptoms and pathogenesis of human and simian varicella make SVV infection of nonhuman primates an excellent animal model to investigate VZV pathogenesis and latency, and to evaluate potential antiviral strategies.

Pathogenesis of simian varicella virus

Simian varicella virus (SVV) is closely related to varicella-zoster virus (VZV) and induces a natural varicella-like disease in nonhuman primates. Therefore, simian varicella is a useful model to investigate varicella pathogenesis and to evaluate antiviral therapies. In this report, we review recent studies on SVV pathogenesis and latency. Experimental infection of African green monkeys is followed by a 7-10 day incubation period during which a viremia disseminates the virus throughout the body. Clinical disease is characterized by fever and vesicular skin rash. Pneumonia and hepatitis may occur during more severe infections. Examination of acutely infected tissues reveals histopathology including necrosis and hemorrhage in the skin, lung, liver, and spleen. In contrast, the neural ganglia exhibit minimal histopathology. SVV DNA, immediate early, early, and late gene transcripts, and viral antigens are detected in the tissues of acutely infected monkeys. Host immune responses are induced which resolve the acute infection within 21 days. During or after acute infection, SVV establishes latent infection in the ganglia of surviving monkeys. The virus may reactivate later in life to cause secondary disease and viral transmission to susceptible monkeys.

Viral gene expression during acute simian varicella virus infection

Simian varicella virus (SVV) causes a natural varicella-like disease in nonhuman primates. Outbreaks of simian varicella occur sporadically in primate facilities. Simian varicella is used as a model for investigation of varicella-zoster virus (VZV) pathogenesis and latency. In this study, SVV gene expression and histopathology were analysed in tissues of acutely infected vervet monkeys. RT-PCR analysis demonstrated expression of specific SVV immediate early, early and late genes in the skin, lung, liver and ganglia tissues of acutely infected monkeys. Viral antigen expression and histopathology, including necrosis and inflammation, were detected in the skin, lungs, liver and spleen of infected monkeys by immunohistochemical analysis. Viral antigen expression, but little or no histopathology, was evident in the neural ganglia, the eventual site of viral latency. The study provides a foundation for further investigation on the role of viral genes in varicella pathogenesis and latency.

Expression of the simian varicella virus glycoprotein E

Simian varicella virus (SVV) is closely related to human varicella-zoster virus (VZV) and induces a varicella-like disease in nonhuman primates. The SVV genome encodes a glycoprotein E (gE) which is homologous to the gE of VZV and other alphaherpesviruses. The SVV gE was expressed in Escherichia coli and rabbits were immunized with the recombinant gE fusion proteins to generate polyclonal gE antiserum. Immunofluorescence and immunoprecipitation analyses demonstrated that the SVV gE is expressed on the surface and within SVV-infected cells. The gE is also expressed on SVV virions as indicated by serum neutralization assay. The mature SVV gE is glycosylated and is similar in size ( approximately 100 kd) to the mature VZV gE. Immunohistochemical analysis detected gE within skin vesicles and lung tissue of SVV-infected monkeys. Analysis of the humoral immune response to gE in an SVV-infected monkey determined that anti-gE antibody is induced as early as day 9 postinfection and persists at high titer for longer than 4 months. The simian varicella model offers an opportunity to investigate the role of gE in viral pathogenesis and immunity and to evaluate its potential as a varicella vaccine.