Varicella-zoster virus proteins in skin lesions: implications for a novel role of ORF29p in chickenpox

Creating the "Dew Drop on a Rose Petal": the Molecular Pathogenesis of Varicella-Zoster Virus Skin Lesions

Varicella-zoster virus (VZV) is a human alphaherpesvirus that causes varicella (chicken pox) as the primary infection in a susceptible host. Varicella is very contagious through its transmission by direct contact with vesicular skin lesions that contain high titers of infectious virus and respiratory droplets. While the clinical manifestations of primary VZV infection are well recognized, defining the molecular mechanisms that drive VZV pathogenesis in the naive host before adaptive antiviral immunity is induced has been a challenge due to species specificity. This review focuses on advances made in identifying the differentiated human host cells targeted by VZV to cause varicella, the processes involved in viral takeover of these heterogenous cell types, and the host cell countermeasures that typically culminate in a benign illness. This work has revealed many unexpected and multifaceted mechanisms used by VZV to achieve its high prevalence and persistence in the human population.

Simian Varicella Virus Pathogenesis in Skin during Varicella and Zoster

Primary simian varicella virus (SVV) infection and reactivation in nonhuman primates is a valuable animal model in the study of varicella zoster virus disease [varicella (chickenpox) and herpes zoster (shingles)]. To understand SVV pathogenesis in skin, we inoculated 10 rhesus macaques with SVV, resulting in varicella rash. After the establishment of latency, eight of the monkeys were immunosuppressed using tacrolimus with or without irradiation and prednisone and two monkeys were not immunosuppressed. Zoster rash developed in all immunosuppressed monkeys and in one non-immunosuppressed monkey. Five monkeys had recurrent zoster. During varicella and zoster, SVV DNA in skin scrapings ranged from 50 to 10⁷ copies/100 ng of total DNA and 2–127 copies/100 ng of total DNA, respectively. Detection of SVV DNA in blood during varicella was more frequent and abundant compared to that of zoster. During varicella and zoster, SVV antigens colocalized with neurons expressing β-III tubulin in epidermis, hair follicles, and sweat glands, suggesting axonal transport of the virus. Together, we have demonstrated that both SVV DNA and antigens can be detected in skin lesions during varicella and zoster, providing the basis for further studies on SVV skin pathogenesis, including immune responses and mechanisms of peripheral spread.

The Biology of Varicella-Zoster Virus Replication in the Skin

The replication of varicella-zoster virus (VZV) in skin is critical to its pathogenesis and spread. Primary infection causes chickenpox, which is characterised by centrally distributed skin blistering lesions that are rich in infectious virus. Cell-free virus in the cutaneous blistering lesions not only spreads to cause further cases, but infects sensory nerve endings, leading to the establishment of lifelong latency in sensory and autonomic ganglia. The reactivation of virus to cause herpes zoster is again characterised by localised painful skin blistering rash containing infectious virus. The development of in vitro and in vivo models of VZV skin replication has revealed aspects of VZV replication and pathogenesis in this important target organ and improved our understanding of the vaccine strain vOKa attenuation. In this review, we outline the current knowledge on VZV interaction with host signalling pathways, the viral association with proteins associated with epidermal terminal differentiation, and how these interconnect with the VZV life cycle to facilitate viral replication and shedding.

Molecular Aspects of Varicella-Zoster Virus Latency

Primary varicella-zoster virus (VZV) infection causes varicella (chickenpox) and the establishment of a lifelong latent infection in ganglionic neurons. VZV reactivates in about one-third of infected individuals to cause herpes zoster, often accompanied by neurological complications. The restricted host range of VZV and, until recently, a lack of suitable in vitro models have seriously hampered molecular studies of VZV latency. Nevertheless, recent technological advances facilitated a series of exciting studies that resulted in the discovery of a VZV latency-associated transcript (VLT) and provide novel insights into our understanding of VZV latency and factors that may initiate reactivation. Deducing the function(s) of VLT and the molecular mechanisms involved should now be considered a priority to improve our understanding of factors that govern VZV latency and reactivation. In this review, we summarize the implications of recent discoveries in the VZV latency field from both a virus and host perspective and provide a roadmap for future studies.

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.

[Varicella-zoster virus (VZV)]

Varicella-zoster virus (VZV) causes varicella in primary infection and zoster after reactivation from latency. Both herpes simplex virus (HSV) and VZV are classified into the same alpha-herpesvirus subfamily. Although most VZV genes have their HSV homologs, VZV has many unique biological characteristics. In this review, we summarized recent studies on 1) animal models for VZV infection and outcomes from studies using the models, including 2) viral dissemination processes from respiratory mucosa, T cells, to skin, 3) cellular receptors for VZV entry, 4) functions of viral genes required uniquely for in vivo growth and for establishment of latency, 5) host immune responses and viral immune evasion mechanisms, and 6) varicella vaccine and anti-VZV drugs.

Absence or overexpression of the Varicella-Zoster Virus (VZV) ORF29 latency-associated protein impairs late gene expression and reduces VZV latency in a rodent model

Varicella-zoster virus (VZV) ORF29 encodes the viral single-stranded DNA binding protein and is expressed during latency in human ganglia. We constructed an ORF29 deletion mutant virus and showed that the virus could replicate only in cells expressing ORF29. An ORF29-repaired virus, in which ORF29 was driven by a cytomegalovirus promoter, grew to peak titers similar to those seen with the parental virus. The level of ORF29 protein in cells infected with the repaired virus was greater than that seen with parental virus. Infection of cells with either the ORF29 deletion or repaired virus resulted in similar levels of VZV immediate-early proteins but reduced levels of glycoprotein E compared to those observed with parental virus. Cotton rats infected with the ORF29 deletion mutant had a markedly reduced frequency of latent infection in dorsal root ganglia compared with those infected with parental virus (P < 0.00001). In contrast, infection of animals with the ORF29 deletion mutant resulted in a frequency of ganglionic infection at 3 days similar to that seen with the parental virus. Animals infected with the ORF29-repaired virus, which overexpresses ORF29, also had a reduced frequency of latent infection compared with those infected with parental virus (P = 0.0044). These studies indicate that regulation of ORF29 at appropriate levels is critical for VZV latency in a rodent model.

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.

Memory cytotoxic T cell responses to viral tegument and regulatory proteins encoded by open reading frames 4, 10, 29, and 62 of varicella-zoster virus

Cytotoxic T cell recognition of tegument and regulatory proteins encoded by open reading frames (ORFs) 4, 10, 29, and 62 of varicella-zoster virus (VZV) was evaluated using limiting dilution conditions to estimate the precursor frequencies of memory T cells specific for these proteins in immune subjects. Responder cell frequencies for ORFs 4, 10, and 62 gene products, which are virion tegument components and function as immediate early viral transactivating proteins, were equivalent. CTLp recognition of VZV proteins made in latently infected cells, which include ORF4 and ORF62 proteins, was not maintained preferentially when compared to ORF10 protein, which has not been shown to be expressed during latency. T cell recognition of ORF29 protein, the major DNA binding protein, which is expressed during replication but not incorporated into the virion tegument, was less common than responses to ORFs 4, 10, and 62 gene products. Older individuals had diminished numbers of memory CTLp that lysed autologous targets expressing IE62 protein; these responses were increased after immunization with live attenuated varicella vaccine to the range observed in younger adults. Adaptive immunity to VZV is characterized by a broad repertoire of memory CTL responses to proteins that comprise the virion tegument and regulate viral gene expression in infected cells.

Mutational analysis of the repeated open reading frames, ORFs 63 and 70 and ORFs 64 and 69, of varicella-zoster virus

Varicella-zoster virus (VZV) open reading frame 63 (ORF63), located between nucleotides 110581 and 111417 in the internal repeat region, encodes a nuclear phosphoprotein which is homologous to herpes simplex virus type 1 (HSV-1) ICP22 and is duplicated in the terminal repeat region as ORF70 (nucleotides 118480 to 119316). We evaluated the role of ORFs 63 and 70 in VZV replication, using recombinant VZV cosmids and PCR-based mutagenesis to make single and dual deletions of these ORFs. VZV was recovered within 8 to 10 days when cosmids with single deletions were transfected into melanoma cells along with the three intact VZV cosmids. In contrast, VZV was not detected in transfections carried out with a dual deletion cosmid. Infectious virus was recovered when ORF63 was cloned into a nonnative AvrII site in this cosmid, confirming that failure to generate virus was due to the dual ORF63/70 deletion and that replication required at least one gene copy. This requirement may be related to our observation that ORF63 interacts directly with ORF62, the major immediate-early transactivating protein of VZV. ORF64 is located within the inverted repeat region between nucleotides 111565 and 112107; it has some homology to the HSV-1 Us10 gene and is duplicated as ORF69 (nucleotides 117790 to 118332). ORF64 and ORF69 were deleted individually or simultaneously using the VZV cosmid system. Single deletions of ORF64 or ORF69 yielded viral plaques with the same kinetics and morphology as viruses generated with the parental cosmids. The dual deletion of ORF64 and ORF69 was associated with an abnormal plaque phenotype characterized by very large, multinucleated syncytia. Finally, all of the deletion mutants that yielded recombinants retained infectivity for human T cells in vitro and replicated efficiently in human skin in the SCIDhu mouse model of VZV pathogenesis.

Varicella zoster virus in human and rat tissue specimens

The limited supple of appropriate tissues for study has been an impediment to investigations of varicella zoster virus (VZV) latency. Human dorsal root ganglia (DRG) harboring latent virus are not plentiful and are not amenable to manipulation for studying the events surrounding the establishment, maintenance, and cessation of latency. An alternative to studies in human DRG is the rat model of latency, which appears to provide a reliable method of investigating VZV latency. Other alternatives include studies in other human tissues involved in VZV pathogenesis. In order to improve our understanding of the establishment and cessation of latency, we performed comparative immunohistochemical analysis of chickenpox and zoster skin lesions. This analysis revealed that during primary infection and reactivation productive VZV infection occurs in a variety of cell types and that the major VZV DNA binding protein, ORF29p, is present in peripheral axons early during the course of chickenpox. VZV latency was studied in the rat model by in situ hybridization and compared with similar studies performed in human DRG containing latent virus, confirming that VZV DNA persists in the same sites in DRG of the two species.

Open reading frame S/L of varicella-zoster virus encodes a cytoplasmic protein expressed in infected cells

We report the discovery of a novel gene in the varicella-zoster virus (VZV) genome, designated open reading frame (ORF) S/L. This gene, located at the left end of the prototype VZV genome isomer, expresses a polyadenylated mRNA containing a splice within the 3' untranslated region in virus-infected cells. Sequence analysis reveals significant differences between the ORF S/Ls of wild-type and attenuated strains of VZV. Antisera raised to a bacterially expressed portion of ORF S/L reacted specifically with a 21-kDa protein synthesized in cells infected with a VZV clinical isolate and with the original vaccine strain of VZV (Oka-ATCC). Cells infected with other VZV strains, including a wild-type strain that has been extensively passaged in tissue culture and commercially produced vaccine strains of Oka, synthesize a family of proteins ranging in size from 21 to 30 kDa that react with the anti-ORF S/L antiserum. MeWO cells infected with recombinant VZV harboring mutations in the C-terminal region of the ORF S/L gene lost adherence to the stratum and adjacent cells, resulting in an altered plaque morphology. Immunohistochemical analysis of VZV-infected cells demonstrated that ORF S/L protein localizes to the cytoplasm. ORF S/L protein was present in skin lesions of individuals with primary or reactivated infection and in the neurons of a dorsal root ganglion during virus reactivation.