Imaging of the varicella zoster virion in the viral highways: comparison with herpes simplex viruses 1 and 2, cytomegalovirus, pseudorabies virus, and human herpes viruses 6 and 7

Varicella-Zoster Virus-Genetics, Molecular Evolution and Recombination

This chapter first details the structure, organization and coding content of the VZV genome to provide a foundation on which the molecular evolution of the virus can be projected. We subsequently describe the evolution of molecular profiling approaches from restriction fragment length polymorphisms to single nucleotide polymorphism profiling to modern day high-throughput sequencing approaches. We describe how the application of these methodologies led to our current model of VZV phylogeograpy including the number and structure of geographic clades and the role of recombination in reshaping these.

Exocytosis of Progeny Infectious Varicella-Zoster Virus Particles via a Mannose-6-Phosphate Receptor Pathway without Xenophagy following Secondary Envelopment

The literature on the egress of different herpesviruses after secondary envelopment is contradictory. In this report, we investigated varicella-zoster virus (VZV) egress in a cell line from a child with Pompe disease, a glycogen storage disease caused by a defect in the enzyme required for glycogen digestion. In Pompe cells, both the late autophagy pathway and the mannose-6-phosphate receptor (M6PR) pathway are interrupted. We have postulated that intact autophagic flux is required for higher recoveries of VZV infectivity. To test that hypothesis, we infected Pompe cells and then assessed the VZV infectious cycle. We discovered that the infectious cycle in Pompe cells was remarkably different from that of either fibroblasts or melanoma cells. No large late endosomes filled with VZV particles were observed in Pompe cells; only individual viral particles in small vacuoles were seen. The distribution of the M6PR pathway (trans-Golgi network to late endosomes) was constrained in infected Pompe cells. When cells were analyzed with two different anti-M6PR antibodies, extensive colocalization of the major VZV glycoprotein gE (known to contain M6P residues) and the M6P receptor (M6PR) was documented in the viral highways at the surfaces of non-Pompe cells after maximum-intensity projection of confocal z-stacks, but neither gE nor the M6PR was seen in abundance at the surfaces of infected Pompe cells. Taken together, our results suggested that (i) Pompe cells lack a VZV trafficking pathway within M6PR-positive large endosomes and (ii) most infectious VZV particles in conventional cell substrates are transported via large M6PR-positive vacuoles without degradative xenophagy to the plasma membrane.IMPORTANCE The long-term goal of this research has been to determine why VZV, when grown in cultured cells, invariably is more cell associated and has a lower titer than other alphaherpesviruses, such as herpes simplex virus 1 (HSV1) or pseudorabies virus (PRV). Data from both HSV1 and PRV laboratories have identified a Rab6 secretory pathway for the transport of single enveloped viral particles from the trans-Golgi network within small vacuoles to the plasma membrane. In contrast, after secondary envelopment in fibroblasts or melanoma cells, multiple infectious VZV particles accumulated within large M6PR-positive late endosomes that were not degraded en route to the plasma membrane. We propose that this M6PR pathway is most utilized in VZV infection and least utilized in HSV1 infection, with PRV's usage being closer to HSV1's usage. Supportive data from other VZV, PRV, and HSV1 laboratories about evidence for two egress pathways are included.

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.

Varicella-Zoster Virus ORF9p Binding to Cellular Adaptor Protein Complex 1 Is Important for Viral Infectivity

ORF9p (homologous to herpes simplex virus 1 [HSV-1] VP22) is a varicella-zoster virus (VZV) tegument protein essential for viral replication. Even though its precise functions are far from being fully described, a role in the secondary envelopment of the virus has long been suggested. We performed a yeast two-hybrid screen to identify cellular proteins interacting with ORF9p that might be important for this function. We found 31 ORF9p interaction partners, among which was AP1M1, the μ subunit of the adaptor protein complex 1 (AP-1). AP-1 is a heterotetramer involved in intracellular vesicle-mediated transport and regulates the shuttling of cargo proteins between endosomes and the trans-Golgi network via clathrin-coated vesicles. We confirmed that AP-1 interacts with ORF9p in infected cells and mapped potential interaction motifs within ORF9p. We generated VZV mutants in which each of these motifs was individually impaired and identified leucine 231 in ORF9p to be critical for the interaction with AP-1. Disrupting ORF9p binding to AP-1 by mutating leucine 231 to alanine in ORF9p strongly impaired viral growth, most likely by preventing efficient secondary envelopment of the virus. Leucine 231 is part of a dileucine motif conserved among alphaherpesviruses, and we showed that VP22 of Marek's disease virus and HSV-2 also interacts with AP-1. This indicates that the function of this interaction in secondary envelopment might be conserved as well.IMPORTANCE Herpesviruses are responsible for infections that, especially in immunocompromised patients, can lead to severe complications, including neurological symptoms and strokes. The constant emergence of viral strains resistant to classical antivirals (mainly acyclovir and its derivatives) pleads for the identification of new targets for future antiviral treatments. Cellular adaptor protein (AP) complexes have been implicated in the correct addressing of herpesvirus glycoproteins in infected cells, and the discovery that a major constituent of the varicella-zoster virus tegument interacts with AP-1 reveals a previously unsuspected role of this tegument protein. Unraveling the complex mechanisms leading to virion production will certainly be an important step in the discovery of future therapeutic targets.

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.

Prevalence and distribution of VZV in temporal arteries of patients with giant cell arteritis

OBJECTIVE

Varicella-zoster virus (VZV) infection may trigger the inflammatory cascade that characterizes giant cell arteritis (GCA).

METHODS

Formalin-fixed, paraffin-embedded GCA-positive temporal artery (TA) biopsies (50 sections/TA) including adjacent skeletal muscle and normal TAs obtained postmortem from subjects >50 years of age were examined by immunohistochemistry for presence and distribution of VZV antigen and by ultrastructural examination for virions. Adjacent regions were examined by hematoxylin & eosin staining. VZV antigen-positive slides were analyzed by PCR for VZV DNA.

RESULTS

VZV antigen was found in 61/82 (74%) GCA-positive TAs compared with 1/13 (8%) normal TAs (p < 0.0001, relative risk 9.67, 95% confidence interval 1.46, 63.69). Most GCA-positive TAs contained viral antigen in skip areas. VZV antigen was present mostly in adventitia, followed by media and intima. VZV antigen was found in 12/32 (38%) skeletal muscles adjacent to VZV antigen-positive TAs. Despite formalin fixation, VZV DNA was detected in 18/45 (40%) GCA-positive VZV antigen-positive TAs, in 6/10 (60%) VZV antigen-positive skeletal muscles, and in one VZV antigen-positive normal TA. Varicella-zoster virions were found in a GCA-positive TA. In sections adjacent to those containing VZV, GCA pathology was seen in 89% of GCA-positive TAs but in none of 18 adjacent sections from normal TAs.

CONCLUSIONS

Most GCA-positive TAs contained VZV in skip areas that correlated with adjacent GCA pathology, supporting the hypothesis that VZV triggers GCA immunopathology. Antiviral treatment may confer additional benefit to patients with GCA treated with corticosteroids, although the optimal antiviral regimen remains to be determined.

Aberrant virion assembly and limited glycoprotein C production in varicella-zoster virus-infected neurons

Highly pure (>95%) terminally differentiated neurons derived from pluripotent stem cells appear healthy at 2 weeks after infection with varicella-zoster virus (VZV), and the cell culture medium contains no infectious virus. Analysis of the healthy-appearing neurons revealed VZV DNA, transcripts, and proteins corresponding to the VZV immediate early, early, and late kinetic phases of replication. Herein, we further characterized virus in these neuronal cells, focusing on (i) transcription and expression of late VZV glycoprotein C (gC) open reading frame 14 (ORF14) and (ii) ultrastructural features of virus particles in neurons. The analysis showed that gC was not expressed in most infected neurons and gC expression was markedly reduced in a minority of VZV-infected neurons. In contrast, expression of the early-late VZV gE glycoprotein (ORF68) was abundant. Transcript analysis also showed decreased gC transcription compared with gE. Examination of viral structure by high-resolution transmission electron microscopy revealed fewer viral particles than typically observed in cells productively infected with VZV. Furthermore, viral particles were more aberrant, in that most capsids in the nuclei lacked a dense core and most enveloped particles in the cytoplasm were light particles (envelopes without capsids). Together, these results suggest a considerable deficiency in late-phase replication and viral assembly during VZV infection of neurons in culture.

Directional transneuronal spread of α-herpesvirus infection

Most α-herpesviruses are pantropic, neuroinvasive pathogens that establish a reactivateable, latent infection in the PNS of their natural hosts. Various manifestations of herpes disease rely on extent and direction of the spread of infection between the surface epithelia and the nervous system components that innervate that surface. One aspect of such controlled spread of infection is the capacity for synaptically defined, transneuronal spread, a property that makes α-herpesviruses useful tools for determining the connectivity of neural circuits. The current understanding of intra-axonal transport and transneuronal spread of α-herpesviruses is reviewed, focusing on work with herpes simplex virus and pseudorabies virus, the available in vitro technology used to study viral transport and spread is evaluated and how certain viral mutants can be used to examine neural circuit architecture is described in this article.

Egress of light particles among filopodia on the surface of Varicella-Zoster virus-infected cells

Varicella-zoster virus (VZV) is renowned for its very low titer when grown in cultured cells. There remains no single explanation for the low infectivity. In this study, viral particles on the surfaces of infected cells were examined by several imaging technologies. Few surface particles were detected at 48 h postinfection (hpi), but numerous particles were observed at 72 and 96 hpi. At 72 hpi, 75% of the particles resembled light (L) particles, i.e., envelopes without capsids. By 96 hpi, 85% of all particles resembled L particles. Subsequently, the envelopes of complete virions and L particles were investigated to determine their glycoprotein constituents. Glycoproteins gE, gI, and gB were detected in the envelopes of both types of particles in similar numbers; i.e., there appeared to be no difference in the glycoprotein content of the L particles. The viral particles emerged onto the cell surface amid actin-based filopodia, which were present in abundance within viral highways. Viral particles were easily detected at the base of and along the exterior surfaces of the filopodia. VZV particles were not detected within filopodia. In short, these results demonstrate that VZV infection of cultured cells produces a larger proportion of aberrant coreless particles than has been seen with any other previously examined alphaherpesvirus. Further, these results suggested a major disassociation between capsid formation and envelopment as an explanation for the invariably low VZV titer in cultured cells.

Molecular studies of Varicella zoster virus

VZV is a highly cell-associated member of the Herpesviridae family and one of the eight herpesviruses to infect humans. The virus is ubiquitous in most populations worldwide, primary infection with which causes varicella, more commonly known as chickenpox. Characteristic of members of the alphaherpesvirus sub-family, VZV is neurotropic and establishes latency in sensory neurones. Reactivation from latency, usually during periods of impaired cellular immunity, causes herpes zoster (shingles). Despite being one of the most genetically stable human herpesviruses, nucleotide alterations in the virus genome have been used to classify VZV strains from different geographical regions into distinct clades. Such studies have also provided evidence that, despite pre-existing immunity to VZV, subclinical reinfection and reactivation of reinfecting strains to cause zoster is also occurring. During both primary infection and reactivation, VZV infects several PBMC and skin cell lineages. Difficulties in studying the pathogenesis of VZV because of its high cell association and narrow host range have been overcome through the development of the VZV severe combined immunodeficient mouse model carrying human tissue implants. This model has provided a valuable tool for studying the importance of individual viral proteins during both the complex intracellular replication and assembly of new virions and for understanding the underlying mechanism of attenuation of the live varicella vaccine. In addition, a rat model has been developed and successfully used to uncover which viral proteins are important for both the establishment and maintenance of latent VZV infection.

Varicella vaccination of children in the United States: assessment after the first decade 1995-2005

Live attenuated varicella vaccine (strain Oka) was approved for administration to healthy children in the United States in 1995. Over the past 10 years, varicella vaccine has been given to millions of children, usually at ages between 12 and 18 months. In states such as California, Michigan, and Texas, there has been a marked decline in the number of reported cases of varicella. Furthermore, there has been a 75% decrease in varicella-related hospitalizations across the United States, as well as a similar decrease in the number of deaths caused by complications of chickenpox. The main unanticipated result has been a growing number of outbreaks of varicella among immunized children ("breakthrough varicella"). The most cited risk factors for breakthrough varicella include the following: (1) 3-5-year interval since immunization and (2) immunization at the youngest ages, especially 12 months. Explanations for breakthrough varicella include a lessened immune response among the youngest recipients of the vaccine. Another possibility is genetic variation among circulating VZV strains. VZV strains can be separated into two geographic clades called European/North American and Asian, based on single nucleotide polymorphisms. Two mutant North American strains have been isolated from patients in the last 10 years. Several genomic differences between Oka vaccine strain and other strains have also been identified, including one site at the DNA origin of replication. Since breakthrough disease among vaccine recipients appears to be more common in the United States than in Japan, further comparisons between the varicella vaccination programs in Japan and the United States are warranted. In addition, data from varicella vaccination programs in Europe should provide further insight into the effectiveness of varicella vaccination in different geographic and ethnic populations.

Infection with an endemic human herpesvirus disrupts critical glial precursor cell properties

Human herpesvirus 6 (HHV-6), a common resident virus of the human CNS, has been implicated in both acute and chronic inflammatory--demyelinating diseases. Although HHV-6 persists within the human CNS and has been described to infect mature oligodendrocytes, nothing is known about the susceptibility of glial precursors, the ancestors of myelin-producing oligodendrocytes, to viral infection. We show that HHV-6 infects human glial precursor cells in vitro. Active infection was demonstrated by both electron microscopy and expression of viral gene transcripts and proteins, with subsequent formation of cell syncytia. Infection leads to alterations in cell morphology and impairment of cell replication but not increased cell death. Infected cells showed decreased proliferation as measured by bromodeoxyuridine uptake, which was confirmed by blunting of the cell growth rate of infected cells compared with uninfected controls over time. The detailed analysis using novel, fluorescent-labeled HHV-6A or HHV-6B reagents demonstrated strong G1/S phase inhibition in infected precursor cells. Cell cycle arrest in HHV-6-infected cells was associated with a profound decrease in the expression of the glial progenitor cell marker A2B5 and a corresponding increase in the oligodendrocyte differentiation marker GalC. These data demonstrate for the first time that infection of primary human glial precursor cells with a neurologically relevant human herpesvirus causes profound alterations of critical precursor cell properties. In light of recent observations that repair of CNS demyelination is dependent on the generation of mature oligodendrocytes from the glial precursor cell pool, these findings may have broad implications for both the ineffective repair seen in demyelinating diseases and the disruption of normal glial maturation.

Effectiveness of live varicella vaccine

The disease burden of chickenpox to children has been described, and a lower force of neutralising antibody to varicella-zoster virus (VZV) than against measles, either after natural infection or after vaccination, has been reported. In the case of VZV, strong cell-mediated immunity may work efficiently to prevent the spread of the virus. The lower force of humoral antibody to VZV might be related to the occurrence of "breakthrough" varicella cases in a small portion of the vaccine recipients. Safety and high effectiveness of the varicella vaccine--approximately 85% effective for all diseases and 95-100% effective for moderate-to-severe diseases--have been reported. Vaccine-induced immunity persists for 10-20 years. However, concerns have been raised that universal immunisation in children may shift the susceptibility from children to adults, whose symptoms are usually moderate-to-severe. In addition, other concerns have been expressed that, due to lack of exposure to varicella in children, the elderly may develop zoster infections more frequently than before. A clear answer is difficult to give at present, although, for several reasons, such situations may be unlikely to occur.

Wild-type Kaposi's sarcoma-associated herpesvirus isolated from the oropharynx of immune-competent individuals has tropism for cultured oral epithelial cells

Based on the observation that wild-type Kaposi's sarcoma-associated herpesvirus (KSHV) DNA can be detected in the oral cavity of healthy, immunocompetent individuals, we hypothesized that epithelial cells could be infected in vitro by wild-type (WT) KSHV isolated from immunocompetent individuals. Primary oral epithelial (P-EPI) cells and telomerase-immortalized oral epithelial cells were generated from human gingival tissue and were then infected in vitro with WT KSHV isolated from throat wash samples. Markers of lytic and latent KSHV infection were detected in cultures by 24 h postinfection by immunofluorescence confocal microscopic assays. The infectivity of the WT and BCBL virus was blocked by neutralizing antibodies against KSHV gB. The presence of KSHV DNA in these cells was confirmed by real-time PCR amplification of different regions of the viral genome. The significant in vitro viral replication that had occurred was inhibited by ganciclovir and by neutralizing antibodies against gB. When infected cultures were examined by scanning electron microscopy, thousands of KSHV particles were clearly visible across the surfaces of P-EPI cells. The detection of enveloped particles indicated that the infectious cycle had proceeded through assembly and egress. We thus demonstrated that oral WT KSHV isolated from immunocompetent individuals was able to infect and replicate in vitro in a relevant primary cell type. Furthermore, our results provide compelling evidence for KSHV transmission within infected oral epithelial cells derived from healthy, immunocompetent populations.