Varicella-zoster virus glycoprotein E is a critical determinant of virulence in the SCID mouse-human model of neuropathogenesis

Varicella zoster virus glycoprotein E facilitates PINK1/Parkin-mediated mitophagy to evade STING and MAVS-mediated antiviral innate immunity

Viruses have evolved to control mitochondrial quality and content to facilitate viral replication. Mitophagy is a selective autophagy, in which the damaged or unnecessary mitochondria are removed, and thus considered an essential mechanism for mitochondrial quality control. Although mitophagy manipulation by several RNA viruses has recently been reported, the effect of mitophagy regulation by varicella zoster virus (VZV) remains to be fully determined. In this study, we showed that dynamin-related protein-1 (DRP1)-mediated mitochondrial fission and subsequent PINK1/Parkin-dependent mitophagy were triggered during VZV infection, facilitating VZV replication. In addition, VZV glycoprotein E (gE) promoted PINK1/Parkin-mediated mitophagy by interacting with LC3 and upregulating mitochondrial reactive oxygen species. Importantly, VZV gE inhibited MAVS oligomerization and STING translocation to disrupt MAVS- and STING-mediated interferon (IFN) responses, and PINK1/Parkin-mediated mitophagy was required for VZV gE-mediated inhibition of IFN production. Similarly, carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-mediated mitophagy induction led to increased VZV replication but attenuated IFN production in a three-dimensional human skin organ culture model. Our results provide new insights into the immune evasion mechanism of VZV gE via PINK1/Parkin-dependent mitophagy.

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.

Human immunoglobulins are transported to HCMV viral envelope by viral Fc gamma receptors-dependent and independent mechanisms

Human cytomegaloviruses (HCMVs) employ many different mechanisms to escape and subvert the host immune system, including expression of the viral IgG Fcγ receptors (vFcγRs) RL11 (gp34), RL12 (gp95), RL13 (gpRL13), and UL119 (gp68) gene products. The role of vFcγRs in HCMV pathogenesis has been reported to operate in infected cells by interfering with IgG-mediated effector functions. We found that gp34 and gp68 are envelope proteins that bind and internalize human IgGs on the surface of infected cells. Internalized IgGs are then transported on the envelope of viral particles in a vFcR-dependent mechanism. This mechanism is also responsible for the incorporation on the virions of the anti-gH neutralizing antibody MSL-109. Intriguingly, we show that gp68 is responsible for MSL-109 incorporation, but it is dispensable for other anti-HCMV antibodies that do not need this function to be transported on mature virions. HCMV-infected cells grown in presence of anti-HCMV monoclonal antibodies generate a viral progeny still infective and possible to be neutralized. This is the first example of a virus carrying neutralizing IgGs on its surface and their possible role is discussed.

Varicella Zoster Virus Neuronal Latency and Reactivation Modeled in Vitro

Latency and reactivation in neurons are critical aspects of VZV pathogenesis that have historically been difficult to investigate. Viral genomes are retained in many human ganglia after the primary infection, varicella; and about one-third of the naturally infected VZV seropositive population reactivates latent virus, which most often clinically manifests as herpes zoster (HZ or Shingles). HZ is frequently complicated by acute and chronic debilitating pain for which there remains a need for more effective treatment options. Understanding of the latent state is likely to be essential in the design of strategies to reduce reactivation. Experimentally addressing VZV latency has been difficult because of the strict human species specificity of VZV and the fact that until recently, experimental reactivation had not been achieved. We do not yet know the neuron subtypes that harbor latent genomes, whether all can potentially reactivate, what the drivers of VZV reactivation are, and how immunity interplays with the latent state to control reactivation. However, recent advances have enabled a picture of VZV latency to start to emerge. The first is the ability to detect the latent viral genome and its expression in human ganglionic tissues with extraordinary sensitivity. The second, the subject of this chapter, is the development of in vitro human neuron systems permitting the modeling of latent states that can be experimentally reactivated. This review will summarize recent advances of in vitro models of neuronal VZV latency and reactivation, the limitations of the current systems, and discuss outstanding questions and future directions regarding these processes using these and yet to be developed models. Results obtained from the in vitro models to date will also be discussed in light of the recent data gleaned from studies of VZV latency and gene expression learned from human cadaver ganglia, especially the discovery of VZV latency transcripts that seem to parallel the long-studied latency-associated transcripts of other neurotropic alphaherpesviruses.

The Structures and Functions of VZV Glycoproteins

The virions of all enveloped viruses, including those of the Herpesviridae, must bind to the cell surface then undergo a process of membrane fusion between the cell plasma membrane and the virus particle envelope. As for all herpesviruses, glycoproteins in the virion envelope are the modus operandi of these events.

Varicella-zoster virus: molecular controls of cell fusion-dependent pathogenesis

Varicella-zoster virus (VZV) is the causative agent of chicken pox (varicella) and shingles (zoster). Although considered benign diseases, both varicella and zoster can cause complications. Zoster is painful and can lead to post herpetic neuralgia. VZV has also been linked to stroke, related to giant cell arteritis in some cases. Vaccines are available but the attenuated vaccine is not recommended in immunocompromised individuals and the efficacy of the glycoprotein E (gE) based subunit vaccine has not been evaluated for the prevention of varicella. A hallmark of VZV pathology is the formation of multinucleated cells termed polykaryocytes in skin lesions. This cell-cell fusion (abbreviated as cell fusion) is mediated by the VZV glycoproteins gB, gH and gL, which constitute the fusion complex of VZV, also needed for virion entry. Expression of gB, gH and gL during VZV infection and trafficking to the cell surface enables cell fusion. Recent evidence supports the concept that cellular processes are required for regulating cell fusion induced by gB/gH-gL. Mutations within the carboxyl domains of either gB or gH have profound effects on fusion regulation and dramatically restrict the ability of VZV to replicate in human skin. This loss of regulation modifies the transcriptome of VZV infected cells. Furthermore, cellular proteins have significant effects on the regulation of gB/gH-gL-mediated cell fusion and the replication of VZV, exemplified by the cellular phosphatase, calcineurin. This review provides the current state-of-the-art knowledge about the molecular controls of cell fusion-dependent pathogenesis caused by VZV.

Comparative Antibody Responses to the Live-Attenuated and Recombinant Herpes Zoster Vaccines

Two herpes zoster (HZ) vaccines licensed in the United States are recommended by the Advisory Committee on Immunization Practices (ACIP): (i) live-attenuated vaccine (ZVL) using vOka strain varicella-zoster virus (VZV) and (ii) recombinant adjuvanted vaccine (RZV) containing recombinant varicella-zoster virus (VZV) glycoprotein E (gE). Two phase 3 clinical trials of RZV led the Advisory Committee on Immunization Practices (ACIP) to recommend it with preferred status. VZV T cell-mediated immunity (CMI), but not humoral immunity, is considered essential for protection against HZ. Published studies of humoral immunity focused on VZV-specific IgG concentration. To complement reports comparing the CMI responses to these vaccines, we compared humoral responses in ZVL and RZV recipients, emphasizing functional qualities (avidity and neutralization). Baseline avidities to a VZV glycoprotein mixture (gp) were near the upper limit of detection, but avidity to gE was much lower. Small increases in gp avidity were observed for both RZV and ZVL vaccination (19 and 12 avidity index units [AIU], respectively). RZV boosted both gE avidity and VZV neutralizing antibody significantly more than ZVL (mean gE avidity boost, 47 AIU versus 22 AIU; mean neutralizing antibody boost, 22-fold versus 8-fold). Increases in neutralizing antibodies strongly correlated with gE avidity increases (r = 0.5) and moderately with gp avidity increases (r = 0.23). After 1 year, 81% of RZV recipients and only 18% of ZVL recipients retained >50% of their peak avidity boosts. These results are consistent with the CMI responses to these vaccines: RZV responses are skewed to long-term memory, whereas ZVL preferentially induces transient effector responses.IMPORTANCE These observations further distinguish the immunogenicity and duration of the immune response of the two vaccines. In addition, measurements of functional humoral immunity (IgG avidity and neutralizing antibody) in response to zoster immunization, alone or combined with other immune markers, might contribute to practical in vitro correlates of protection. Combined with previous observations of the cell-mediated response to these vaccines, this study suggests that vaccine development will benefit from more expansive and granular assessments of acquired immunity during early phase 1 immunogenicity trials.

Insights into the pathogenesis of varicella viruses

Purpose of review

Varicella zoster virus (VZV) is a highly contagious, neurotropic alpha herpes virus that causes varicella (chickenpox). VZV establishes lifelong latency in the sensory ganglia from which it can reactivate to induce herpes zoster (HZ), a painful disease that primarily affects older individuals and those who are immune-suppressed. Given that VZV infection is highly specific to humans, developing a reliable in vivo model that recapitulates the hallmarks of VZV infection has been challenging. Simian Varicella Virus (SVV) infection in nonhuman primates reproduces the cardinal features of VZV infections in humans and allows the study of varicella virus pathogenesis in the natural host. In this review, we summarize our current knowledge about genomic and virion structure of varicelloviruses as well as viral pathogenesis and antiviral immune responses during acute infection, latency and reactivation. We also examine the immune evasion mechanisms developed by varicelloviruses to escape the host immune responses and the current vaccines available for protecting individuals against chickenpox and herpes zoster.

Recent findings

Data from recent studies suggest that infected T cells are important for viral dissemination to the cutaneous sites of infection as well as site of latency and that a viral latency-associated transcript might play a role in the transition from lytic infection to latency and then reactivation.

Summary

Recent studies have provided exciting insights into mechanisms of varicelloviruses pathogenesis such as the critical role of T cells in VZV/SVV dissemination from the respiratory mucosa to the skin and the sensory ganglia; the ability of VZV/SVV to interfere with host defense; and the identification of VLT transcripts in latently infected ganglia. However, our understanding of these phenomena remains poorly understood. Therefore, it is critical that we continue to investigate host-pathogen interactions during varicelloviruses infection. These studies will lead to a deeper understanding of VZV biology as well as novel aspects of cell biology.

Adjuvanted Recombinant Glycoprotein E Herpes Zoster Vaccine

The adjuvanted recombinant glycoprotein E herpes zoster (HZ) vaccine is superior to the live attenuated HZ vaccine, with an efficacy >90% against HZ in healthy immunocompetent adults aged ≥50 years after vaccination. In pivotal studies, the efficacy of the new vaccine varied very little with the age of the vaccinee and decreased only by 5-10% in the 3.5 years after immunization. This nonlive vaccine was successfully administered to small cohorts of immunocompromised individuals; initial trials showed efficacy of >60-80% in several such settings. Potential drawbacks include the requirement for 2 vaccine doses separated by 2-6 months, local and systemic reactogenicity that is significantly greater than observed with commonly used vaccines, and the inclusion of a strong adjuvant that has been minimally studied in clinical settings where it might be problematic, such as in people with autoimmune diseases. Postmarketing studies are underway to address some of the drawbacks.

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.

Varicella Virus-Host Interactions During Latency and Reactivation: Lessons From Simian Varicella Virus

Varicella zoster virus (VZV) is a neurotropic alphaherpesvirus and the causative agent of varicella (chickenpox) in humans. Following primary infection, VZV establishes latency in the sensory ganglia and can reactivate to cause herpes zoster, more commonly known as shingles, which causes significant morbidity, and on rare occasions mortality, in the elderly. Because VZV infection is highly restricted to humans, the development of a reliable animal model has been challenging, and our understanding of VZV pathogenesis remains incomplete. As an alternative, infection of rhesus macaques with the homologous simian varicella virus (SVV) recapitulates the hallmarks of VZV infection and thus constitutes a robust animal model to provide critical insights into VZV pathogenesis and the host antiviral response. In this model, SVV infection results in the development of varicella during primary infection, generation of an adaptive immune response, establishment of latency in the sensory ganglia, and viral reactivation upon immune suppression. In this review, we discuss our current knowledge about host and viral factors involved in the establishment of SVV latency and reactivation as well as the important role played by T cells in SVV pathogenesis and antiviral immunity.

The C-terminus of varicella-zoster virus glycoprotein M contains trafficking motifs that mediate skin virulence in the SCID-human model of VZV pathogenesis

Knowledge about the function of varicella-zoster virus glycoprotein M is limited; the requirement of gM for skin and neural tropism are unknown. VZV gM contains two predicted YXXΦ trafficking motifs and a dileucine motif in the carboxyl-terminus. We constructed a recombinant VZV with gM truncated from the first YXXΦ and five additional viruses with YXXΦ tyrosine substitutions, alone and in combination with dileucine substitution. All recombinant viruses grew to high titer but mutation of the membrane-proximal YXXΦ motif reduced plaque size in cultured cells and altered gM localization. C-terminus truncation had a pronounced effect on virion morphogenesis and plaque size, but not on overall replication kinetics in vitro. Mutation of gM trafficking motifs and truncation attenuated replication in human skin xenografts in vivo; gM truncation did not alter neurotropism. Our results demonstrate that the gM C-terminus is dispensable for virus replication in cultured cells but is important for skin pathogenesis.

ORF7 of Varicella-Zoster Virus Is Required for Viral Cytoplasmic Envelopment in Differentiated Neuronal Cells

Although a varicella-zoster virus (VZV) vaccine has been used for many years, the neuropathy caused by VZV infection is still a major health concern. Open reading frame 7 (ORF7) of VZV has been recognized as a neurotropic gene in vivo, but its neurovirulent role remains unclear. In the present study, we investigated the effect of ORF7 deletion on VZV replication cycle at virus entry, genome replication, gene expression, capsid assembly and cytoplasmic envelopment, and transcellular transmission in differentiated neural progenitor cells (dNPCs) and neuroblastoma SH-SY5Y (dSY5Y) cells. Our results demonstrate that the ORF7 protein is a component of the tegument layer of VZV virions. Deleting ORF7 did not affect viral entry, viral genome replication, or the expression of typical viral genes but clearly impacted cytoplasmic envelopment of VZV capsids, resulting in a dramatic increase of envelope-defective particles and a decrease in intact virions. The defect was more severe in differentiated neuronal cells of dNPCs and dSY5Y. ORF7 deletion also impaired transmission of ORF7-deficient virus among the neuronal cells. These results indicate that ORF7 is required for cytoplasmic envelopment of VZV capsids, virus transmission among neuronal cells, and probably the neuropathy induced by VZV infection.IMPORTANCE The neurological damage caused by varicella-zoster virus (VZV) reactivation is commonly manifested as clinical problems. Thus, identifying viral neurovirulent genes and characterizing their functions are important for relieving VZV related neurological complications. ORF7 has been previously identified as a potential neurotropic gene, but its involvement in VZV replication is unclear. In this study, we found that ORF7 is required for VZV cytoplasmic envelopment in differentiated neuronal cells, and the envelopment deficiency caused by ORF7 deletion results in poor dissemination of VZV among neuronal cells. These findings imply that ORF7 plays a role in neuropathy, highlighting a potential strategy to develop a neurovirulence-attenuated vaccine against chickenpox and herpes zoster and providing a new target for intervention of neuropathy induced by VZV.

Varicella-Zoster Virus Glycoproteins: Entry, Replication, and Pathogenesis

Varicella-zoster virus (VZV), an alphaherpesvirus that causes chicken pox (varicella) and shingles (herpes zoster), is a medically important pathogen that causes considerable morbidity and, on occasion, mortality in immunocompromised patients. Herpes zoster can afflict the elderly with a debilitating condition, postherpetic neuralgia, triggering severe, untreatable pain for months or years. The lipid envelope of VZV, similar to all herpesviruses, contains numerous glycoproteins required for replication and pathogenesis.

PURPOSE OF REVIEW

To summarize the current knowledge about VZV glycoproteins and their roles in cell entry, replication and pathogenesis.

RECENT FINDINGS

The functions for some VZV glycoproteins are known, such as gB, gH and gL in membrane fusion, cell-cell fusion regulation, and receptor binding properties. However, the molecular mechanisms that trigger or mediate VZV glycoproteins remains poorly understood.

SUMMARY

VZV glycoproteins are central to successful replication but their modus operandi during replication and pathogenesis remain elusive requiring further mechanistic based studies.

Role for the αV Integrin Subunit in Varicella-Zoster Virus-Mediated Fusion and Infection

Varicella-zoster virus (VZV) is an alphaherpesvirus that causes varicella and herpes zoster. Membrane fusion is essential for VZV entry and the distinctive syncytium formation in VZV-infected skin and neuronal tissue. Herpesvirus fusion is mediated by a complex of glycoproteins gB and gH-gL, which are necessary and sufficient for VZV to induce membrane fusion. However, the cellular requirements of fusion are poorly understood. Integrins have been implicated to facilitate entry of several human herpesviruses, but their role in VZV entry has not yet been explored. To determine the involvement of integrins in VZV fusion, a quantitative cell-cell fusion assay was developed using a VZV-permissive melanoma cell line. The cells constitutively expressed a reporter protein and short hairpin RNAs (shRNAs) to knock down the expression of integrin subunits shown to be expressed in these cells by RNA sequencing. The αV integrin subunit was identified as mediating VZV gB/gH-gL fusion, as its knockdown by shRNAs reduced fusion levels to 60% of that of control cells. A comparable reduction in fusion levels was observed when an anti-αV antibody specific to its extracellular domain was tested in the fusion assay, confirming that the domain was important for VZV fusion. In addition, reduced spread was observed in αV knockdown cells infected with the VZV pOka strain relative to that of the control cells. This was demonstrated by reductions in plaque size, replication kinetics, and virion entry in the αV subunit knockdown cells. Thus, the αV integrin subunit is important for VZV gB/gH-gL fusion and infection.

IMPORTANCE

Varicella-zoster virus (VZV) is a highly infectious pathogen that causes chickenpox and shingles. A common complication of shingles is the excruciating condition called postherpetic neuralgia, which has proven difficult to treat. While a vaccine is now available, it is not recommended for immunocompromised individuals and its efficacy decreases with the recipient's age. These limitations highlight the need for new therapies. This study examines the role of integrins in membrane fusion mediated by VZV glycoproteins gB and gH-gL, a required process for VZV infection. This knowledge will further the understanding of VZV entry and provide insight into the development of better therapies.

Immunogenicity of varicella zoster virus glycoprotein E DNA vaccine

In the present study a eukaryotic expression vector of varicella zoster virus (VZV) glycoprotein E (gE) was constructed and enabled to express in COS7 cells. Furthermore, a specific immune response against the VZV gE eukaryotic expression plasmid was induced in BALB/c mice. The VZV gE gene was amplified using polymerase chain reaction (PCR) and cloned into a eukaryotic expression vector, pcDNA3.1. The recombinant vector was subsequently transfected into COS7 cells using a liposome transfection reagent. The recombinant protein was instantaneously expressed by the transfected cells, as detected by immunohistochemistry, and the recombinant pcDNA-VZV gE plasmid was subsequently used to immunize mice. Tissue expression levels were analyzed by reverse transcription-PCR. In addition, the levels of serum antibodies and spleen lymphocyte proliferation activity were investigated. The amplified target gene included the full-length gE gene (~2.7 kb), and the recombinant expression vector induced gE expression in COS7 cells. In addition, the expression plasmid induced sustained expression in vivo following immunization of mice. Furthermore, the plasmid was capable of inducing specific antibody production and effectively stimulating T cell proliferation. Effective humoral and cellular immunity was triggered in the mice immunized with the VZV gE eukaryotic expression vector. The results of the present study laid the foundation for future research into a VZV DNA vaccine.

A highly conserved epitope-vaccine candidate against varicella-zoster virus induces neutralizing antibodies in mice

Varicella-zoster virus (VZV) is a highly infectious agent of varicella and herpes zoster (HZ). Vaccination is by far the most effective way to prevent these diseases. More safe, stable and efficient vaccines, such as epitope-based vaccines, now have been increasingly investigated by many researchers. However, only a few VZV neutralizing epitopes have been identified to date. We have previously identified a linear epitope between amino acid residues 121 and 135 of gE. In this study, we validated that this epitope is highly conserved amongst different VZV strains that covered five existing phylogenetic clades with an identity of 100%. We evaluated the immunogenicity of the recombinant hepatitis B virus core (HBc) virus-like particles (VLPs) which included amino acids (121-135). VZV-gE-specific antibodies were detected in immunized mouse serum using ELISA. The anti-peptide antiserum positively detected VZV via Western blot and immunofluorescent staining assays. More importantly, these peptides could neutralize VZV, indicating that these peptides represented neutralizing epitopes. These findings have important implications for the development of epitope-based protective VZV vaccines.

Extensive CD4 and CD8 T Cell Cross-Reactivity between Alphaherpesviruses

The Alphaherpesvirinae subfamily includes HSV types 1 and 2 and the sequence-divergent pathogen varicella zoster virus (VZV). T cells, controlled by TCR and HLA molecules that tolerate limited epitope amino acid variation, might cross-react between these microbes. We show that memory PBMC expansion with either HSV or VZV enriches for CD4 T cell lines that recognize the other agent at the whole-virus, protein, and peptide levels, consistent with bidirectional cross-reactivity. HSV-specific CD4 T cells recovered from HSV-seronegative persons can be explained, in part, by such VZV cross-reactivity. HSV-1-reactive CD8 T cells also cross-react with VZV-infected cells, full-length VZV proteins, and VZV peptides, as well as kill VZV-infected dermal fibroblasts. Mono- and cross-reactive CD8 T cells use distinct TCRB CDR3 sequences. Cross-reactivity to VZV is reconstituted by cloning and expressing TCRA/TCRB receptors from T cells that are initially isolated using HSV reagents. Overall, we define 13 novel CD4 and CD8 HSV-VZV cross-reactive epitopes and strongly imply additional cross-reactive peptide sets. Viral proteins can harbor both CD4 and CD8 HSV/VZV cross-reactive epitopes. Quantitative estimates of HSV/VZV cross-reactivity for both CD4 and CD8 T cells vary from 10 to 50%. Based on these findings, we hypothesize that host herpesvirus immune history may influence the pathogenesis and clinical outcome of subsequent infections or vaccinations for related pathogens and that cross-reactive epitopes and TCRs may be useful for multi-alphaherpesvirus vaccine design and adoptive cellular therapy.

Neuronal Subtype and Satellite Cell Tropism Are Determinants of Varicella-Zoster Virus Virulence in Human Dorsal Root Ganglia Xenografts In Vivo

Varicella zoster virus (VZV), a human alphaherpesvirus, causes varicella during primary infection. VZV reactivation from neuronal latency may cause herpes zoster, post herpetic neuralgia (PHN) and other neurologic syndromes. To investigate VZV neuropathogenesis, we developed a model using human dorsal root ganglia (DRG) xenografts in immunodeficient (SCID) mice. The SCID DRG model provides an opportunity to examine characteristics of VZV infection that occur in the context of the specialized architecture of DRG, in which nerve cell bodies are ensheathed by satellite glial cells (SGC) which support neuronal homeostasis. We hypothesized that VZV exhibits neuron-subtype specific tropism and that VZV tropism for SGC contributes to VZV-related ganglionopathy. Based on quantitative analyses of viral and cell protein expression in DRG tissue sections, we demonstrated that, whereas DRG neurons had an immature neuronal phenotype prior to implantation, subtype heterogeneity was observed within 20 weeks and SGC retained the capacity to maintain neuronal homeostasis longterm. Profiling VZV protein expression in DRG neurons showed that VZV enters peripherin+ nociceptive and RT97+ mechanoreceptive neurons by both axonal transport and contiguous spread from SGC, but replication in RT97+ neurons is blocked. Restriction occurs even when the SGC surrounding the neuronal cell body were infected and after entry and ORF61 expression, but before IE62 or IE63 protein expression. Notably, although contiguous VZV spread with loss of SGC support would be predicted to affect survival of both nociceptive and mechanoreceptive neurons, RT97+ neurons showed selective loss relative to peripherin+ neurons at later times in DRG infection. Profiling cell factors that were upregulated in VZV-infected DRG indicated that VZV infection induced marked pro-inflammatory responses, as well as proteins of the interferon pathway and neuroprotective responses. These neuropathologic changes observed in sensory ganglia infected with VZV may help to explain the neurologic sequelae often associated with zoster and PHN.

Varicella zoster virus (VZV) causes varicella; herpes zoster results from VZV reactivation and is associated with post herpetic neuralgia (PHN). We hypothesized that VZV exhibits neuron-subtype specific tropism and that VZV tropism for satellite glial cells (SGC) results in loss of SGC functions that support neurons and contributes to VZV-related ganglionopathy. Using human DRG xenografts in SCID mice, we demonstrated that initial VZV access to neuronal cell bodies occurs by the axonal route, followed by axonal and contiguous spread between neuron-satellite cell complexes. VZV replication is restricted in mechanoreceptive neurons compared to nociceptive neurons. Despite restricted infection, mechanoreceptive neurons were selectively depleted in association with SGC loss following acute DRG infection. VZV infection of DRG triggers release of pro-inflammatory cytokines that cause neuronal damage. These observations may help to explain the neurologic sequelae often associated with herpes zoster and PHN.

Molecular mechanisms of varicella zoster virus pathogenesis

Varicella zoster virus (VZV) is the causative agent of varicella (chickenpox) and zoster (shingles). Investigating VZV pathogenesis is challenging as VZV is a human-specific virus and infection does not occur, or is highly restricted, in other species. However, the use of human tissue xenografts in mice with severe combined immunodeficiency (SCID) enables the analysis of VZV infection in differentiated human cells in their typical tissue microenvironment. Xenografts of human skin, dorsal root ganglia or foetal thymus that contains T cells can be infected with mutant viruses or in the presence of inhibitors of viral or cellular functions to assess the molecular mechanisms of VZV-host interactions. In this Review, we discuss how these models have improved our understanding of VZV pathogenesis.

Varicella-zoster virus and herpes simplex virus 1 can infect and replicate in the same neurons whether co- or superinfected

The two human neurotropic alphaherpesviruses varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV1) both establish latency in sensory ganglia. Human trigeminal ganglia are known to frequently harbor both viruses, and there is evidence to suggest the presence of both VZV and HSV1 DNA in the same neuron. We ask here whether VZV and HSV1 can exclude themselves and each other and whether they can productively infect the same cells in human neurons and human foreskin fibroblasts (HFF). Simultaneous infection (coinfection) or consecutive infection (superinfection) was assessed using cell-free HSV1 and VZV expressing fluorescent reporter proteins. Automated analysis was carried out to detect singly and dually infected cells. We demonstrate that VZV and HSV1 both display efficient superinfection exclusion (SE) in HFF, with each virus excluding either itself or the other virus. While SE also occurred in neurons, it was with much lower efficiency. Both alphaherpesviruses productively infected the same neurons, whether applied simultaneously or even consecutively, albeit at lower frequencies.

IMPORTANCE

Superinfection exclusion by VZV for itself or the related neurotropic alphaherpesvirus HSV1 has been studied here for the first time. We find that while these viruses display classic SE in fibroblasts, SE is less efficient for both HSV1 and VZV in human neurons. The ability of multiple VZV strains to productively infect the same neurons has important implications in terms of recombination of both wild-type and vaccine strains in patients.

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.

ORF7 of varicella-zoster virus is a neurotropic factor

Varicella-zoster virus (VZV) is the causative agent of chickenpox and herpes zoster (shingles). After the primary infection, the virus remains latent in sensory ganglia and reactivates upon weakening of the cellular immune system due to various conditions, erupting from sensory neurons and infecting the corresponding skin tissue. The current varicella vaccine is highly attenuated in the skin and yet retains its neurovirulence and may reactivate and damage sensory neurons. The factors involved in neuronal invasion and establishment of latency are still elusive. Previously, we constructed a library of whole-gene deletion mutants carrying a bacterial artificial chromosome sequence and a luciferase marker in order to perform a comprehensive VZV genome functional analysis. Here, screening of dispensable gene deletion mutants in differentiated neuronal cells led to the identification of ORF7 as the first known, likely a main, VZV neurotropic factor. ORF7 is a virion component localized to the Golgi compartment in infected cells, whose deletion causes loss of polykaryon formation in epithelial cell culture. Interestingly, ORF7 deletion completely abolishes viral spread in human nervous tissue ex vivo and in an in vivo mouse model. This finding adds to our previous report that ORF7 is also a skin-tropic factor. The results of our investigation will not only lead to a better understanding of VZV neurotropism but could also contribute to the development of a neuroattenuated vaccine candidate against shingles or a vector for delivery of other antigens.

Herpes virus fusion and entry: a story with many characters

Herpesviridae comprise a large family of enveloped DNA viruses all of whom employ orthologs of the same three glycoproteins, gB, gH and gL. Additionally, herpesviruses often employ accessory proteins to bind receptors and/or bind the heterodimer gH/gL or even to determine cell tropism. Sorting out how these proteins function has been resolved to a large extent by structural biology coupled with supporting biochemical and biologic evidence. Together with the G protein of vesicular stomatitis virus, gB is a charter member of the Class III fusion proteins. Unlike VSV G, gB only functions when partnered with gH/gL. However, gH/gL does not resemble any known viral fusion protein and there is evidence that its function is to upregulate the fusogenic activity of gB. In the case of herpes simplex virus, gH/gL itself is upregulated into an active state by the conformational change that occurs when gD, the receptor binding protein, binds one of its receptors. In this review we focus primarily on prototypes of the three subfamilies of herpesviruses. We will present our model for how herpes simplex virus (HSV) regulates fusion in series of highly regulated steps. Our model highlights what is known and also provides a framework to address mechanistic questions about fusion by HSV and herpesviruses in general.

Investigation of varicella-zoster virus neurotropism and neurovirulence using SCID mouse-human DRG xenografts

Varicella-zoster virus (VZV) is a medically important human alphaherpesvirus. Investigating pathogenic mechanisms that contribute to VZV neurovirulence are made difficult by a marked host restriction. Our approach to investigating VZV neurotropism and neurovirulence has been to develop a mouse-human xenograft model in which human dorsal root ganglia (DRG) are maintained in severe compromised immunodeficient (SCID) mice. In this review, we will describe our key findings using this model in which we have demonstrated that VZV infection of SCID DRG xenograft results in rapid and efficient spread, enabled by satellite cell infection and polykaryon formation, which facilitates robust viral replication and release of infectious virus. In neurons that persist following this acute replicative phase, VZV genomes are present at low frequency with limited gene transcription and no protein synthesis, a state that resembles VZV latency in the natural human host. VZV glycoprotein I and interaction between glycoprotein I and glycoprotein E are critical for neurovirulence. Our work demonstrates that the DRG model can reveal characteristics about VZV replication and long-term persistence of latent VZV genomes in human neuronal tissues, in vivo, in an experimental system that may contribute to our knowledge of VZV neuropathogenesis.

Simian varicella virus gene expression during acute and latent infection of rhesus macaques

Varicella zoster virus (VZV) is a neurotropic α-herpesvirus that causes chickenpox during primary infection and establishes latency in sensory ganglia. Reactivation of VZV results in herpes zoster and other neurological complications. Our understanding of the VZV transcriptome during acute and latent infection in immune competent individuals remains incomplete. Infection of rhesus macaques with the homologous simian varicella virus (SVV) recapitulates the hallmarks of VZV infection. We therefore characterized the SVV transcriptome by quantitative real-time reverse transcriptase PCR during acute infection in bronchial alveolar lavage (BAL) cells and peripheral blood mononuclear cells, and during latency in sensory ganglia obtained from the same rhesus macaques. During acute infection, all known SVV open reading frames (ORFs) were detected, and the most abundantly expressed ORFs are involved in virus replication and assembly such as the transcriptional activator ORF 63 and the structural proteins ORF 41 and ORF 49. In contrast, latent SVV gene expression is highly restricted. ORF 61, a viral transactivator and latency-associated transcript, is the most prevalent transcript detected in sensory ganglia. We also detected ORFs A, B, 4, 10, 63, 64, 65, 66, and 68 though significantly less frequently than ORF 61. This comprehensive analysis has revealed genes that potentially play a role in the establishment and/or maintenance of SVV latency.

Differentiated neuroblastoma cells provide a highly efficient model for studies of productive varicella-zoster virus infection of neuronal cells

Varicella-zoster virus (VZV) is a highly species-specific herpesvirus that targets sensory ganglionic neurons. This species specificity has limited the study of many aspects of VZV pathogenesis, including neuronal infection. We report development of a highly efficient neuroblastoma cell model to study productive VZV infection of neuronal cells. We show that differentiation of SH-SY5Y neuroblastoma cells yields a homogenous population of neuron-like cells that are permissive to the full VZV replicative cycle. These cells supported productive infection by both laboratory and clinical VZV isolates, including the live varicella vaccine. This model may enable rapid identification of genetic determinants facilitating VZV neurotropism.

Mutagenesis of varicella-zoster virus glycoprotein I (gI) identifies a cysteine residue critical for gE/gI heterodimer formation, gI structure, and virulence in skin cells

Varicella-zoster virus (VZV) is the alphaherpesvirus that causes chicken pox (varicella) and shingles (zoster). The two VZV glycoproteins gE and gI form a heterodimer that mediates efficient cell-to-cell spread. Deletion of gI yields a small-plaque-phenotype virus, ΔgI virus, which is avirulent in human skin using the xenograft model of VZV pathogenesis. In the present study, 10 mutant viruses were generated to determine which residues were required for the typical function of gI. Three phosphorylation sites in the cytoplasmic domain of gI were not required for VZV virulence in vivo. Two deletion mutants mapped a gE binding region in gI to residues 105 to 125. A glycosylation site, N116, in this region did not affect virulence. Substitution of four cysteine residues highly conserved in the Alphaherpesvirinae established that C95 is required for gE/gI heterodimer formation. The C95A and Δ105-125 (with residues 105 to 125 deleted) viruses had small-plaque phenotypes with reduced replication kinetics in vitro similar to those of the ΔgI virus. The Δ105-125 virus was avirulent for human skin in vivo. In contrast, the C95A mutant replicated in vivo but with significantly reduced kinetics compared to those of the wild-type virus. In addition to abolished gE/gI heterodimer formation, gI from the C95A or the Δ105-125 mutant was not recognized by monoclonal antibodies that detect the canonical conformation of gI, demonstrating structural disruption of gI in these viruses. This alteration prevented gI incorporation into virus particles. Thus, residues C95 and 105 to 125 are critical for gI structure required for gE/gI heterodimer formation, virion incorporation, and ultimately, effective viral spread in human skin.