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Arvin A, Campadelli-Fiume G, Mocarski E, et al., editors. Human Herpesviruses: Biology, Therapy, and Immunoprophylaxis. Cambridge: Cambridge University Press; 2007.

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Human Herpesviruses: Biology, Therapy, and Immunoprophylaxis.

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Chapter 37VZV: pathogenesis and the disease consequences of primary infection

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VZV is a human alphaherpesvirus that causes varicella (chickenpox) as the primary infection and establishes latency in sensory ganglia. VZV reactivation results in herpes zoster (shingles). During the course of varicella and zoster, VZV infects differentiated human cells that exist within unique tissue microenvironments in humans. The tropism of VZV for skin is the most obvious clinical manifestation of VZV infection, producing the vesicular cutaneous lesions that are associated with varicella and zoster. The site of initial VZV infection in naïve hosts is thought to be mucosal epithelial cells of the upper respiratory tract. Entry is presumed to follow inoculation of the respiratory epithelium with infectious virus transmitted by aerosolized respiratory droplets or by contact with virus in varicella or zoster skin lesions (Arvin, 2001a; Grose, 1981). VZV in respiratory or conjunctival mucosal cells has the opportunity to interact with and infect local immune system cells and those in adjacent lymphoid tissues. Trafficking of infected peripheral blood mononuclear cells (PBMC), which appear to be predominantly T-cells, to the skin is thought to give rise to crops of cutaneous vesicles. Skin lesions contain VZV material associated with necrotic debris and, unlike virus grown in vitro, cell-free, infectious particles are detected in vesicular fluid (Williams et al., 1962). The life cycle of VZV is completed upon its transmission to a susceptible host from an individual with varicella, or it can be postponed for decades by establishing latency in neurons and transmitting to future generations during episodes of zoster.

VZV shares its tropism for epithelial tissues with its relatives, HSV-1 and HSV-2, as well as with the non-human alphaherpesviruses. VZV also shares the neurotropism of these viruses, as discussed elsewhere in this volume. However, VZV seems to be more akin to the betaherpesviruses, HHV6 and HHV7, in its apparent tropism for T-cells (Ku et al., 2002, 2004; Takahashi et al., 1989). VZV infection of T-cells appears to represent a critical phase of its life cycle, providing a mechanism for viral transport from sites of initial infection to the skin. Sensory nerve axons terminate in the dermis and may be infected with VZV, allowing for retrograde transport to sensory ganglia, as it spreads through the skin layers (Annunziato et al., 2000). VZV may also reach neurons by hematogenous spread. The outcome of VZV infection in these various cell types, which are differentiated and non-dividing, depends on interactions between virus proteins and host factors at the cellular level and is modulated by the innate and adaptive immune responses of the infected host (Chapter 39). VZV infection of dendritic cells is described in Chapter 39, and investigations of latency and reactivation in the human host and in rodent models are reviewed in Chapter 38. The goals of this chapter are to discuss VZV infection of T-cells and skin, which are essential target cells of the virus during primary infection, and the disease consequences of varicella in healthy and immunocompromised individuals.

Systems for evaluating determinants of VZV pathogenesis in human skin and T-cells

Immunodeficient (scid/scid) mice with thymus/liver (T-cell) or skin xenografts have provided a useful experimental model for examining VZV pathogenesis in vivo, known as the SCID-hu model of VZV infection (Moffat et al., 1995). In this model, VZV-infected fibroblasts are injected into human tissue xenografts, which are then removed at intervals up to 21–28 days after infection. Initial experiments with T-cell xenografts showed that VZV proteins were expressed in CD4+, CD8+ and CD4/CD8+ T-cells and VZV was cultured from each T-cell subpopulation. T-cells released infectious VZV, which was an important observation because VZV replication in vitro is highly cell-associated. VZV-infected skin implants exhibited epidermal lesions that were indistinguishable from the characteristic lesions of varicella. Experiments with VZV mutants in the SCIDhu model indicate that VZV is challenged to employ more gene functions to replicate successfully in skin and T-cells in vivo, because the cellular environment in intact tissues is fundamentally different from that in cultured cells. Host cell factors such as innate immunity, cellular kinase activation, cellular transactivating proteins and protein trafficking pathways, are likely to affect virulence such that the full range of VZV gene products is needed to modify the tissue environment for effective viral pathogenesis.

Information about the molecular pathogenesis of VZV has been obtained by generating VZV recombinants with selected mutations in the viral genome using cosmid systems. Sets of four or five cosmids have been derived using genomic DNA from the parent Oka strain, pOka, a clinical isolate from Japan from which the live, attenuated varicella vaccine virus, vOka, was made (Niizuma et al., 2003) and from the vOka strains used by Merck & Co., Inc. (Kemble et al., 2000) and the strain deposited at the American Type Culture Collection (Cohen and Seidel, 1993). To obtain mutant viruses, alterations are made in selected VZV open reading frames (ORFs) and introduced into the cosmid that carries the gene of interest. The altered cosmid and the three or four other cosmids are transfected or transduced together into human melanoma cells. Homologous recombination between overlapping sequences generates an intact VZV genome, and if the mutation is not lethal, the virus propagates by cell-cell spread and plaques appear in the transfected cells. To discern phenotypes beyond lethality, mutant viruses are compared to isogenic parent viruses and a repaired strain, also made from cosmids. These “matched sets” of mutant, parent, and repaired viruses are evaluated in vitro using primary cell cultures and in the SCIDhu model in vivo.

Effects of VZV replication on cellular cyclin-dependent kinases and cyclins

The primary reason why VZV pathogenesis must be studied in vivo using the SCIDhu model is that the intracellular environment in human skin and thymus tissues differs greatly from the conditions in cultured cells. VZV is commonly cultivated in human melanoma cells (MeWo) or in subconfluent human fibroblasts (HFF, HELF, MRC-5, WI-38) that contain abundant metabolic precursors and enzymes involved in cell growth and division. In contrast, dermal fibroblasts, differentiated keratinocytes, and single-positive CD4+ or CD8+ T cells in xenografts in SCIDhu mice are not dividing. In these quiescent cells, regulatory proteins such as Rb and p27 suppress biosynthetic pathways for DNA replication and cell division by inhibiting transcription of cyclins and the activity of cyclin-dependent kinases (CDKs) (Olashaw and Pledger, 2002). VZV infection subverts these suppression mechanisms in an unknown manner and causes unscheduled cyclin expression and dysregulation of cyclin-dependent kinases (Fig. 37.1). In VZV-infected, confluent HFFs, high levels of CDK activity are associated with simultaneous expression of cyclins A, B1, and D3 (Leisenfelder and Moffat, 2006). This unusual protein profile is likely induced by VZV since it does not correspond with CDK and cyclin expression patterns found in cellular G0/1, S, G2, or M phases.

Fig. 37.1. Activation of host cyclin-dependent kinases in VZV-infected fibroblasts.

Fig. 37.1

Activation of host cyclin-dependent kinases in VZV-infected fibroblasts. VZV infects quiescent cells such as dermal fibroblasts (HFFs) to cause skin lesions. The photo of VZV plaques in HFFs (arrows) shows that the infected cells are rounded and have (more...)

VZV alters the intracellular environment of resting cells by inducing kinase activity, an effect that appears to be an essential step in virus replication because compounds that inhibit CDK activity prevent VZV spread. Roscovitine and purvalanol A are specific inhibitors of CDKs 1, 2, 5, 7, and 9 that have potent antiviral effects on VZV and other viruses (Moffat et al., 2004b; Taylor et al., 2004). Interestingly, as little as 5μM roscovitine or 2μM purvalanol is needed to prevent VZV replication and yet 10-fold more is needed to cause cell cycle arrest in MeWo cells. Thus VZV is acutely sensitive to levels of CDK activity, which the virus may utilize for initiation of transcription from viral promoters (CDK7 and CDK9), phosphorylation of the C-terminus of glycoprotein I (CDK1) (Ye et al., 1999), and for numerous potential viral and cellular protein targets. An important role for kinase activity associated with cyclin B, presumably CDK1, is phosphorylation of IE62 since these proteins interact in the cytoplasm. Recognition sites for CDK1 are plentiful in IE62, and point mutagenesis confirmed that several are targeted by the kinase in vitro (Leisenfelder and Moffat, unpublished observations).

Investigation of events in the pathogenesis of primary VZV infection in the SCIDhu model

The clinical experience documents that primary VZV infection is initiated by inoculation of the respiratory mucosal epithelium and that the varicella rash appears after a 10–21 day incubation period (Arvin, 2001a; Cohen and Straus, 2001). Given the extreme host-range restriction of VZV, concepts about the pathogenesis of primary VZV infection have been derived from the sequence of events during primary mousepox infection (Grose, 1981). Based on this model, VZV has been thought to reach mononuclear cells in regional lymph nodes, causing a primary viremia that transports the virus to reticuloendothelial organs, such as the liver, for a phase of viral amplification. The theory has been that this amplification stage is followed by a secondary viremia in the late incubation period that carries VZV to skin sites. Recently, work using the SCIDhu model has provided experimental evidence to refine hypotheses about primary VZV pathogenesis (Ku et al., 2004). According to our new model of VZV pathogenesis, VZV tropism for T-cells may facilitate viral transfer to skin (Fig. 37.2). Previous investigations in cultured cells demonstrated that T cells could be infected with VZV (Koropchak et al., 1989; Soong et al., 2000) and that tonsil T-cells, especially activated, memory subpopulations, are highly permissive for VZV infection (Ku et al., 2002). In addition, VZV preferentially infected the activated, memory CD4+ T-cells that constitute >20% of tonsil T-cells. T-cell sub-populations that expressed the skin homing markers, cutaneous leukocyte antigen (CLA), and chemokine receptor CCR4, were also more likely to be infected and VZV did not disrupt the important chemotaxis functions of these cells. In order to investigate the hypothesis that VZV could be transferred to skin by tonsil T-cells, VZV-infected human tonsil T-cells were adoptively transferred to SCIDhu mice via intravenous injection (Ku et al., 2004). The microcirculation within these skin xenografts is formed by human CD31+ endothelial cells, which permits interaction with human T-cells. CD3+ T cells were detected within the epidermis, dermis and around the hair follicles in skin tissues within 24 h after injection into the mouse circulation. T-cells expressing the memory marker, CD45RO, were the predominant population. When skin xenografts were harvested at intervals after inoculation of infected T-cells, characteristic cutaneous VZV lesions were formed and progressed in size and production of infectious virus over 10–21 days. Intravenous injection of VZV-infected fibroblasts did not result in VZV transfer into skin xenografts. T-cell transfer of the virus resulted in lesions expressing the VZV proteins needed for lytic infection, such as the major immediate early ORF62 (IE62) transactivator, the ORF47 kinase and glycoprotein E (gE). VZV infection of skin resulted in extensive formation of multi-nucleated polykaryocytes, a gradual thickening of the epidermis, epidermal cell proliferation, destruction of basement membranes, and cellular degeneration. Foci of VZV-infected cells eventually extended up to surface keratinocytes but only after 14–21 days. Infectious VZV was produced throughout this period of progressive cutaneous lesion formation. Thus, the time required for VZV lesions to penetrate through the keratinocyte layer at the skin surface implies that VZV must reach cutaneous sites of replication at an early, rather than a late stage of the incubation period.

Fig. 37.2. A model of the pathogenesis of primary VZV infection.

Fig. 37.2

A model of the pathogenesis of primary VZV infection. This figure illustrates new concepts about VZV pathogenesis and immunobiology that have emerged from experiments in the SCIDhu mouse model. The upper left panel shows the appearance of a mature VZV (more...)

VZV alters the intracellular environment and produces lytic infection within 2 days in cultured cells in vitro. In contrast, VZV infection in skin xenografts evolved much more slowly. These observations suggested that innate immune mechanisms within the intact cutaneous tissue microenvironment in vivo might modulate VZV replication. Analyses of skin xenografts showed that interferon-α (IFN-α) and interleukin 1-α (IL-1α) were expressed constitutively in the cytoplasm of epidermal cells in uninfected skin (Ku et al., 2002). After VZV infection, IL-1α was translocated to the nuclei of cells expressing VZV proteins, but remained in the cytoplasm of adjacent, uninfected cells. TNF-α expression was not present in uninfected skin and was not induced by VZV infection. Importantly, interferon-α (IFN-α) was not expressed in VZV-infected cells but was up-regulated in neighboring uninfected epidermal cells within the skin xenograft. The phosphorylation state of Stat1 protein is a marker for activation or suppression of the IFN-α pathway because IFN-α binding to its receptors induces Stat1 phosphorylation by JAK kinases. Without phosphorylation, Stat1 is not translocated to cell nuclei and production of IFN-α does not occur. In VZV-infected skin, phosphorylated Stat1 was localized to nuclei in neighboring uninfected epidermal cells, but it was not detected in cells expressing VZV proteins. In uninfected skin, Stat1 was not phosphorylated and Stat1 and IFN-α remained cytoplasmic. Another mediator of the innate immune response, the transcription factor NF-κB, also remained in the cytoplasm of VZV-infected skin cells in SCIDhu implants (Jones and Arvin, 2006). These experiments indicated that VZV replication was associated with expression of a gene product(s) that inhibited antiviral IFN-α production in foci of infected skin cells in vivo by interference with Stat1 activation. To further document the role of the IFN-α response in regulating VZV infection and cell-cell spread between epidermal cells within intact skin tissue in vivo, skin xenografts were inoculated with VZV and SCIDhu mice were given neutralizing antibody against the human IFN-α/β receptor, in order to block type Ⅰ IFN activity. In these experiments, infectious VZV titers were ten-fold higher in skin specimens when IFN signaling was inhibited by receptor blocking and the cutaneous lesions were substantially larger.

Because SCID mice lack the capacity to develop an adaptive, antigen-specific immune response, the progression of VZV infection in skin xenografts is controlled only by innate immunity. In the intact human host, biopsies of varicella skin lesions show a local inflammatory response surrounding the infected cells. The migration of immune cells into damaged tissues is signaled by the up-regulation of adhesion molecules on the vascular endothelial cells. Comparative analyses of infected skin xenografts and human VZV lesion biopsies permitted an examination of whether these changes could be induced by viral replication per se. Cutaneous lesions in patients biopsied at the onset of the varicella rash showed extensive expression of E-selectin, ICAM-1, and VCAM-1, whereas these proteins were not detected in capillaries of infected skin xenografts. Many infiltrating mononuclear cells were detected, most of which expressed CD4 or CD8, and included predominantly CD45RO+ memory T-cells and skin homing CLA+ and CCR4+ T-cells. In contrast, adoptive transfer of PBMC to SCIDhu mice showed no enrichment for these effector T-cell populations in VZV infected skin. These differences in adhesion molecule expression and in mononuclear cell profiles in VZV lesion biopsies and in VZV-infected skin xenografts suggested that recruitment and/or retention of inflammatory T-cells required signals provided by host cellular immunity.

Considered together, data showing VZV infection of T cells in vitro (Ku et al., 2002; Soong et al., 2000) and these experiments in the SCIDhu model suggest that T-cell tropism plays an essential role in VZV pathogenesis. The prolonged varicella incubation period appears to represent the time required for VZV to overcome previously unrecognized, but potent innate antiviral responses, especially IFN-α production, mediated directly by epidermal cells in vivo. The initial phase of VZV pathogenesis is also likely to be facilitated by the failure of VZV to trigger up-regulation of inflammatory adhesion molecules on capillary endothelial cells in skin and virus-mediated modulation of MHC I and MHC Ⅱ expression (see elsewhere in this volume).

The role of VZV glycoproteins in T-cell and skin tropism

As is true of all herpesviruses, VZV glycoproteins have multiple functions that affect tissue tropism both within cells during virus assembly, and outside cells when they are expressed on plasma membranes and virion envelopes. VZV encodes glycoproteins designated gB, gC, gE, gH, gI, gK, gL, gM and gN. However, characterization of the functions of most of these proteins is limited. VZV is unique among the alphaherpesviruses in having no gD homologue. In transfected and VZV-infected cells, glycoproteins form gE/gI and gH/gL heterodimers and gE homodimers; noncovalent interactions between gB/gE and gH/gE have also been identified (Cole and Grose, 2003). Recycling of glycoproteins from the cell surface to the trans-Golgi network (TGN) is regulated to balance envelopment in the TGN, where infectious virions are formed, with the cell fusion that spreads VZV genomes to neighboring cells even when formation of intact virion particles is limited. The VZV glycoproteins, gB, gC, gE, gH, gI, gK, and gL have been shown to be or are likely to be structural components of the virus (Cohen and Straus, 2001; Mo et al., 1999, 2002). In addition to studies of the glycoproteins using expression systems, deletions or targeted mutations of these genes that are not lethal have yielded recombinant VZV mutants for investigation of glycoprotein function in cultured cells and in SCIDhu T cell and skin xenografts in vivo. VZV gB, gE and gK have been demonstrated to be essential for VZV replication in cultured cells, based on failure to generate infectious virus from cosmids with deletions of these genes, and the rescue of infectivity when the deletion is complemented by insertion of the gene into a non-native site and in cell lines. In the SCIDhu model, interactions between VZV glycoproteins at internal sites of virion assembly and surface membranes determine the ability of VZV to replicate in differentiated human T-cells and skin. This summary focuses particularly on evaluations of the contributions of VZV glycoproteins to viral pathogenesis in T-cell and skin xenografts in vivo.

Glycoprotein C

VZV gC is the product of ORF14 (Davison and Scott, 1986). The role of gC in infectivity for human skin was assessed using gC negative mutants of vOka and VZV-Ellen (Moffat et al., 1998). Whereas all of these VZV strains replicated well in tissue culture, only low passage clinical isolates were fully virulent in skin, as shown by infectious virus yields and analysis of xenografts for VZV DNA and viral protein synthesis. All strains except the gC-null Ellen strain retained some capacity to replicate in human skin, but cell-free virus was recovered only from xenografts infected with pOka or VZV-S. An HSV-1 mutant lacking gC expression was also deficient in skin infectivity. These SCID-hu mouse experiments show that gC, which is dispensable for replication in tissue culture, plays a critical role in the virulence of the human alphaherpesviruses, VZV and HSV-1, for human skin.

Glycoprotein E

Whereas the homologous protein in the other alphaherpesviruses is dispensable in cultured cells, VZV gE, encoded by ORF68, is essential for replication (Ku et al., 2002). The functions of gE were analyzed further by creating point mutations or deleting the short 62 amino acid C-terminal domain (Moffat et al., 2004a). These mutants were designed based on observations about functional motifs made using gE expression systems (Cole and Grose, 2003). Mutations were introduced in YAGL (aa582–585), which mediates gE endocytosis, AYRV (aa568–571), the motif that targets gE to the trans-Golgi network (TGN), and SSTT, which is an “acid cluster” comprising a phosphorylation motif (aa588–601). A substitution Y582G in YAGL prevented gE endocytosis, and the Y569A mutation interfered with gE shuttling from the Golgi to the TGN in reports using transient expression methods. These changes were introduced into the viral genome using VZV cosmids and residues S593, S595, T596, and T598 were changed to alanines to alter phosphorylation. These experiments demonstrated a hierarchy in the contributions of gE C-terminal motifs to VZV replication in vitro and to virulence in the SCIDhu model. Deleting the gE C-terminus or mutating the YAGL motif were lethal for VZV replication in vitro. Mutations of AYRV and SSTT were compatible with recovery of VZV, but the AYRV mutation resulted in decreased plaque size and virus production in vitro. When the rOka-gE-AYRV and rOka-gE-SSTT mutants were evaluated in skin and T cell xenografts in SCIDhu mice, interference with TGN targeting was associated with substantial attenuation, especially in skin, whereas the SSTT mutation did not alter VZV infectivity in vivo. Thus, the gE C-terminus contains domains that are essential for VZV replication or are determinants of VZV virulence in differentiated dermal and epidermal cells and T-cells within intact tissue microenvironments in vivo. In addition to C-terminal functions, VZV gE has a unique N-terminal region from amino acids 1–188 (Berarducci et al., 2006). Mutagenesis of this gE ectodomain region identified subdomains essential for replication, cell-cell spread and secondary envelopment and for VZV skin tropism.

Whereas VZV has been considered to be highly antigenically stable, VZV-MSP is a recently discovered wild type virus that has lost an immunodominant B-cell epitope in the gE ectodomain (Mo et al., 2002). This gE “escape mutant” virus exhibited an unusual pattern of egress. When VZV-MSP was evaluated in SCIDhu skin xenografts, the spread of the VZV-MSP variant was accelerated significantly. The cytopathologic changes produced after 21 days by isolates that had the prototypical gE sequence were demonstrated at 14 days in skin xenografts infected with VZV-MSP. Thus, VZV-MSP is a naturally occurring variant with a gE mutation that is associated with a phenotype of enhanced cell–cell spread in vitro and in vivo.

Glycoprotein I

VZV mutants with deletions of gI, encoded by ORF67, can replicate in melanoma cells and fibroblasts, although not in Vero cells (Cohen and Nguyen, 1997; Mallory et al., 1997). Since gI was dispensable in cell culture, gI deletion mutants were evaluated for their capacity to infect human cells in SCIDhu xenografts. Deleting gI was lethal for VZV replication in differentiated skin and T cells in vivo (Moffat et al., 2002). Restoring gI into the mutated VZV genome was associated with the recovery of VZV virulence. Thus, gI is essential for VZV pathogenesis.

Analyses of the gI promoter in expression systems has demonstrated that it contains an activating upstream sequence (AUS) that binds cellular transcription factors Sp1 and USF (Specificity factor 1, Upstream Stimulatory Factor), and the viral transactivator ORF29 DNA binding protein which mediates enhancement of immediate early 62 (IE62)-induced transcription (He et al., 2001). This information was used to design mutants from VZV cosmids in order to evaluate the contributions of these motifs to VZV replication in vitro and in vivo (Ito et al., 2003). Recombinants rOkagI-Sp1 and rOkagI-USF, with two substitutions in Sp1 or USF sites, replicated like rOka in vitro, but infectivity of rOkagI-Sp1 was significantly impaired in skin and T cells in vivo. A double mutant, rOKAgI-Sp1/USF, did not replicate in skin, but yielded low titers of infectious virus in T-cells. The repair, rOkagI:rep-Sp1/USF, was as infectious as rOka. Thus, disrupting gI promoter sites for cellular transactivators altered VZV virulence in vivo, with variable consequences related to the cellular factor and the host cell type. Mutations in the ORF29 responsive element of the gI promoter were made by substituting each of four 10 base pair blocks in this region with a 10 base pair sequence, GATAACTACA, that was predicted to interfere with enhancer effects of the ORF29 protein. One of these mutants, designated rOKAgI-29RE-3, had diminished replication in skin and T-cells, indicating that ORF29 protein-mediated enhancement of gI expression contributes to VZV virulence. These experiments demonstrated that VZV pathogenesis is influenced by interactions of cellular transactivators with the gI promoter. Significantly, comparisons of the effects of the gI promoter mutants on growth in skin and T-cells indicated that cellular transactivators can have consequences for virulence that are cell-type specific. Mutations within promoters of viral genes that are non-essential in vitro should allow construction of recombinant herpesviruses that have altered virulence in specific host cells in vivo, and may be useful for designing herpesvirus gene therapy vectors and attenuated vaccines.

The role of regulatory proteins and viral kinases in T-cell and skin tropism

Although the difficulty of generating sufficient quantities of infectious cell-free virus has prevented experimental analysis, VZV replication is presumed to occur through a sequential expression of immediate early, early and late genes (Chapter 10). VZV regulatory proteins include viral transactivating proteins and viral kinases, which may also be structural components of the virion tegument. While the task is far from complete, some of these genes have been analyzed for their contributions to VZV replication in cultured cells and in the SCIDhu model of VZV pathogenesis in vivo.

IE62 protein

The IE62 protein is the major VZV transactivating protein, required for expression of all viral genes tested to date (Kinchington et al., 1992, 1994). It is encoded by the duplicated genes, ORF62 and ORF71. Experiments in which pOka cosmids were mutated to delete ORF62, ORF71, or the ORF62/71 gene pair demonstrated that at least one copy of ORF62 was required for VZV replication, as expected (Sato et al., 2003a). Restoring a single copy of ORF62 into a non-native site in the US region of the VZV genome resulted in some, albeit reduced, VZV replication in vitro. VZV replication persisted despite introducing targeted mutations in IE62 binding sites that mediate interaction with the IE4 protein. Related experiments demonstrated that the ORF4 gene is essential in VZV (Sato et al., 2003b). Interestingly, when a single copy of ORF62 or ORF71 was deleted, recombination events during cosmid transfection repaired the defective repeat region in some progeny viruses. Mixtures of single copy rOkaΔ62 or rOkaΔ71 and repaired rOka generated by recombination of the single copy deletion mutants was detected in some skin xenografts infected with these recombinants. The diminished replication of the pOka mutants with a single copy of ORF62 at the non-native site was associated with a complete block in VZV infection of skin xenografts in vivo. Although insertion of ORF62 into the non-native site permitted replication in cell culture, ORF62 expression from its native site was necessary for cell-cell spread in differentiated human skin tissues in vivo.

IE63 protein

The IE63 protein is encoded by ORF63 and is duplicated in the VZV genome as ORF70. IE63 protein is a nuclear phosphoprotein with some homology to HSV-1 ICP22. Sequence analysis indicates that IE63 is related to HSV-1 US1.5 protein, which is expressed colinearly with ICP22 (US1) (Baiker et al., 2004). Removing one copy of the duplicated gene, either ORF63 or ORF70, was compatible with VZV replication in vitro (Sommer et al., 2001). VZV was not recovered from transfections done with a dual deletion cosmid, but infectious virus was generated when ORF63 was cloned into the non-native site in the Us region. IE63 protein interacts directly with ORF62, the major immediate early transactivating protein of VZV. The importance of IE62 protein for VZV replication is suggested by the observation that ORF63/ORF70 could be removed and yield infectious virus in vitro if deleted cosmids were transfected along with a plasmid expressing IE62 (Cohen et al., 2004).

The potential functional domains of IE63 protein were analyzed by creating 22 ORF63 mutations in expression plasmids and in the VZV genome. The effects of IE63 phosphorylation and nuclear localization, and IE63 binding to IE62, were evaluated by transient transfection and by replication of the mutant viruses. Briefly, IE63 aa55–67 constituted the IE62 binding site, with R59/L60 being critical residues; S165, S173 and S185 in the IE63 center region were phosphorylated by cellular kinases; and mutations in two putative nuclear localization signal (NLS) sequences changed intracellular IE63 distribution from a nuclear to a cytoplasmic/nuclear pattern. Infectious VZV was recovered with three of the 22 mutations in ORF63. Each of these three IE63 mutants had a single alanine substitution (T171A, S181A or S185A). The IE63 mutants, rOka/ORF63rev[T171], rOka/ORF63rev[S181] and rOka/ORF63rev[S185], replicated less efficiently, had a small plaque phenotype in vitro and had less production of gE and ORF47, indicating that IE63 was involved in expression of these early and late gene products. Virulence of the three IE63 mutants was reduced markedly in skin xenografts, but infection of T-cell xenografts was not affected. The fact that these IE63 mutants were attenuated in skin but not T-cells, suggests that the contribution of the IE63 tegument/regulatory protein to VZV pathogenesis differs depending on the human cell types and tissues that are targeted for infection.

ORF64 protein

ORF64 is duplicated as ORF69 and it has some sequence homology to the HSV-1 Us10 gene, which exists as a single copy. When ORF64 and ORF69 were deleted, either separately or together, one copy at either location in the genome was sufficient to yield infectious virus with growth kinetics and plaque morphology indistinguishable from the parent virus (Sommer et al., 2001). Removing both ORF64 and ORF69 caused an abnormal plaque phenotype made up of very large multinucleated syncytia. Single and dual ORF64/ORF69 mutants were as infectious as the parent and repaired viruses when evaluated in human T-cells in vitro and in human skin xenografts in the SCIDhu mouse model of VZV pathogenesis.

ORF10 protein

ORF10 encodes a tegument protein that enhances transactivation of VZV genes. Analysis of pOkaΔ10 and ORF10 point mutants with disruption of the acidic activation domain and the putative motif for binding human cellular factor-1 (HCF-1) showed no effects on replication, IE gene transcription or virion assembly in vitro (Che et al., 2006). However, epidermal cells in SCIDhu skin xenografts infected with pOkaΔ10 had significantly fewer DNA-containing nucleocapsids and complete virions; extensive aggregates of intracytoplasmic viral particles were also observed. Altering the activation or the putative HCF-1 domains of ORF10 protein had no consequences for VZV skin infection. Deleting ORF10 did not impair VZV T-cell tropism in vivo. Thus, ORF10 protein is necessary for efficient VZ virion assembly and is a VZV virulence determinant in epidermal and dermal cells in vivo.

ORF47 protein

ORF47 encodes a serine/threonine protein kinase that is in a class of conserved herpesvirus proteins that are homologous to HSV-1 UL13. ORF47 protein also has similarities to the casein kinase Ⅱ family of cellular proteins (Cole and Grose, 2003). ORF47 appears to be a component of the virion tegument. VZV mutants that did not express ORF47 protein were made by inserting stop codons into the gene, producing ROka47S, which was shown to replicate as well as intact ROka in an infectious focus assay (Heineman et al., 1996). However, these findings were not predictive of the consequences of blocking ORF47 protein synthesis in vivo, since ORF47 protein was essential for VZV infection of human T cells and skin (Moffat et al., 1998). Restoring ORF47 into the genome of the ROKA47S mutant reconstituted the T cell and skin tropism of the virus. Thus, ORF47 protein functions are necessary in differentiated cells that are involved in VZV pathogenesis in vivo.

In order to further investigate the role of the ORF47 protein, VZV mutants were made that expressed a truncated ORF47 protein, by deleting the C-terminus, and that had mutations that disrupted conserved putative kinase motifs in ORF47 protein (Besser et al., 2003). The mutants were tested for replication, phosphorylation and protein-protein interactions in vitro and allowed an assessment of the effects of specifically eliminating the kinase activity of ORF47 protein on VZV replication in vivo. The ORF47 C-terminal truncation mutants (rOka47ΔC) and those that disrupted the DYS kinase motif (rOka47D-N) had no ORF47 kinase activity. However, binding to IE62 protein was mapped to the N-terminal domain and was preserved. Cells infected with these ORF47 kinase defective mutants exhibited marked nuclear retention of ORF47 and IE62 proteins in vitro. Even though virus titers were not altered based on an infectious focus assay, the electron microscopy analysis of cultured cells infected with the kinase defective mutants showed severely impaired virion assembly and transport of virions to cell surfaces. Normal VZV virion assembly appears to require ORF47 kinase function. Nevertheless, rOka47ΔC or rOka47D-N-infected cells showed VZV-induced cell fusion and syncytia formation.

With regard to pathogenesis, ORF47 protein mutations that eliminated the ORF47 kinase function caused substantial reductions in the capacity to replicate and produce cutaneous lesions in skin xenografts in the SCIDhu model. However, in contrast to the complete ORF47 null mutant, rOKA47S, some replication occurred in skin in vivo if the capacity of ORF47 protein to bind IE62 protein was intact, as shown in experiments with rOka47ΔC and rOka47D-N. ORF47 kinase activity was important for VZV infection and cell-cell spread in human skin in vivo, but preserving the capacity of ORF47 protein to form complexes with IE62 protein, both of which are VZV tegument components, appeared to be the sine qua non for VZV infection of skin in vivo. In contrast to the skin experiments, when the kinase defective rOka47ΔC and rOka47D-N mutants were evaluated in T-cell xenografts, no infectious virus was made in vivo (Besser et al., 2004). These observations were similar to the data obtained in T-cell xenografts infected with ROka47S, when no ORF47 protein was made. The comparison of the growth of kinase-defective ORF47 mutants in skin vs. T-cells suggested the hypothesis that fundamental requirements for VZV pathogenesis in skin and T-cells differ in vivo. Even though virion assembly was much diminished and intracellular trafficking of ORF47 and IE62 proteins, both components of the tegument, and of gE, was aberrant in skin in the absence of ORF47 kinase activity, VZV polykaryocytes were generated by rOka47ΔC and rOka47D-N. Thus, some cell fusion was induced by ORF47 mutants in skin and cell–cell spread occurred even though virion formation was deficient. In contrast, impaired virion assembly by ORF47 mutants was associated with a complete elimination of the capacity to infect T-cells in vivo. Since VZV-infected T-cells do not undergo cell fusion even when most cells in the T cell xenograft have been infected, transfer of incomplete virions by cell–cell fusion does not occur. Instead, virus appears to be released from T-cells for entry into uninfected T-cells in other regions of the xenograft. Considered together, these observations make it plausible to suggest that formation of complete virions and their release is essential for VZV T-cell tropism, creating a differential requirement for virion assembly during the pathogenesis of VZV infection of T-cells and skin.

ORF66 protein

ORF66 encodes a second serine/threonine protein kinase homologous to HSV US 3. Like the ORF47 protein kinase, ORF66 protein was shown to be dispensable for VZV replication in cultured cells by creating ROka66S stop codon mutants. Again, ROka66 mutants replicated as well as intact ROka in cultured cells. Eliminating ORF66 expression did not impair replication in SCIDhu skin xenografts, as compared to the vaccine-derived ROka parent (Moffat et al., 1998). In contrast, ORF66 defective VZV mutants had a significant decrease in their capacity to replicate in T-cell xenografts in vivo. Thus, ORF66 protein appears to be a viral kinase that is necessary to VZV T-cell tropism. When ORF66 expression was blocked in pOka, growth and VZ virion formation was reduced in T-cells in vivo, infected T-cells were more susceptible to apoptosis and pOka66S mutants had less capacity to interfere with induction of the interferon (IFN) signaling pathway (Schaap et al., 2005). Thus, ORF66 kinase appears to have a unique role during T-cell infection and supports VZV T cell tropism by contributing to immune evasion and enhancing survival of infected T-cells.

Disease consequences of primary VZV infection in healthy and immunocompromised hosts

The clinical pattern of primary VZV infection is highly predictable, beginning with an incubation period of 10–21 days following a close exposure of a susceptible individual to another person with varicella or in some cases, herpes zoster (Arvin, 2001b). In contrast to other herpesviruses, primary VZV infection almost always causes symptoms although the diagnosis is missed when the child has only a few lesions and no identified exposure. Varicella often begins with a prodrome of fever, malaise, headache and abdominal pain. These initial symptoms last about 24–48 hours before skin vesicles are noted and are more common in older children and adults. The occurrence of a cell-associated VZV viremia has been well documented during the last few days of the incubation period and for a few days after the cutaneous rash appears, when specimens are tested by tissue culture or for VZV DNA (Asano et al., 1990; Gershon et al., 1978; Koropchak et al., 1989, 1991; Ozaki et al., 1986). Viral cultures of PBMC demonstrate that infectious virus can be recovered from PBMC; VZV was isolated from 11%–24% of PBMC samples taken from healthy individuals with varicella less than 24 h after the rash had appeared. DNA methods are more sensitive, with VZV being detected in 67%–74% of samples tested by in situ hybridization or PCR. Although viral cultures do not yield infectious virus, PCR methods indicate that VZV is present in oropharyngeal specimens just before and after the appearance of skin lesions. The estimated frequency of VZV infection of PBMC from healthy individuals with varicella was approximately 0.01%–0.001%, as detected by in situ hybridization (Koropchak et al., 1989). According to our proposed model of the pathogenesis of primary VZV, this viremic phase may represent the infection of T cells migrating through infected skin sites and re-entering the circulation (Fig. 37.1). This early phase of the illness is usually associated with systemic symptoms, including fever and fatigue, presumably related to cytokine responses; varicella-related fever is usually mild (less than 101.5 °F). The cell-associated VZV viremia is transient, usually resolving within 24–72 hours after the onset of the rash in healthy children and adults. Primary VZV infection is often accompanied by a reduction in the numbers of circulating lymphocytes but this finding is probably secondary, rather than being due to cell destruction by the virus. Mild upper respiratory symptoms and diarrhea may occur but severe respiratory or gastrointestinal illness is rare.

The lesions caused by VZV replication in the skin appear first as small erythematous papules, each of which then evolves within about 12–24 hours to surface vesicles that are filled with clear fluid and surrounded by erythema – the so-called “dew drop on a rose petal.” The first skin lesions in patients with varicella often appear on the face and scalp, or on the chest or back and are pruritic. Formation of multinucleated epithelial cells with intranuclear eosinophilic inclusions and vasculitis involving small blood vessels occurs during the early maculopapular stage. VZV virions are detected in keratinocytes and also in capillary endothelial cells by electron microscopy. VZV is delivered to mucous membrane sites as well as to skin, where it produces ulcers in the oropharynx, conjunctivae and vagina. Vesicles result from a progressive ballooning degeneration of epithelial cells and coalescence of fluid-filled vacuoles between cells. The numbers of VZV-infected cells at the base of the lesion increases during this phase and cell-free virus is released into vesicular fluid. Each lesion begins to become cloudy and crusted within about 48 hours and infectious virus is usually no longer detected after about 72–96 hours. Healing reflects the replacement of epithelial cells at the base of the lesion by cellular proliferation. New skin and mucous membrane lesions continue to develop for a period of 3–5 days in most children, with a range of 1–7 days. Over the 1–7 day course of primary VZV infection, as few as 10 to more than 1500 lesions may appear; on average, healthy children have about 100–300 lesions. Older children and adults, those who are secondary household cases and patients with skin trauma, such as sunburn or eczema are more likely to have more cutaneous and mucous membrane lesions. The crops of lesions that appear later in the clinical course of varicella are usually on the arms and legs. Vesicle formation may be abortive, with little or no infectious virus being detected, presumably due to the induction of antigen-specific T cells by this point in the infection (Chapter 39). VZV lesions are usually superficial and do not leave scars except at the sites of the earliest skin replication; residual scars can often be seen along the hairline or eyebrows.

Secondary bacterial infection of skin lesions is the most common complication of primary VZV infection in healthy children. These infections are most often due to Staphylococcus aureus or Streptococcus pyogenes (group A beta-hemolytic streptococcus) (Dunkle et al., 1991; Jackson et al., 1992). Skin and mucosal damage may provide a portal of entry for these organisms such that bacteremia occurs and the organisms reach deep tissue sites. Thus, varicella may be associated with staphylococcal or streptococcal pneumonia, arthritis or osteomyelitis. Varicella lesions often involve the eyelids and ocular conjunctivae but serious eye complications are rare; unilateral anterior uveitis or corneal lesions may develop but long-term damage is unusual (Liesegang, 1991).

VZV has the capacity to infect the epithelial cells that line the pulmonary alveoli, and to induce edema and an extensive infiltration of mononuclear cells into the alveolar septae. The result of this process can be a severe viral pneumonia. Active VZV replication in the lungs is very unusual in healthy children with varicella. However, the increased morbidity and mortality caused by primary VZV infection in adults is accounted for by their much greater susceptibility to varicella pneumonia (Krugman et al., 1957). Interstitial inflammation and the desquamation of alveolar lining cells into the alveoli has the potential to block the effective transfer of oxygen from the alveolar spaces into the pulmonary capillaries. The consequence is severe hypoxemia and respiratory failure. Most patients with varicella pneumonia develop cough and dyspnea several days after the onset of the cutaneous rash, which suggests that the virus reaches pulmonary epithelial sites during the later viremic phase. Physical abnormalities associated with varicella pneumonia may be difficult to detect because early signs are often limited to fever and tachypnea. The chest radiograph usually shows interstitial pneumonitis with diffuse bilateral infiltrates and perihilar nodular densities but may appear relatively benign even when patients have severe hypoxia. Severe varicella pneumonia may be fatal even with antiviral therapy (Chapter 65) and assisted ventilation.

Healthy children with varicella often have mild, sub clinical hepatitis, detected by minor abnormalities of liver function tests. These abnormalities may reflect an inflammatory response or some limited viral replication in the liver during primary VZV infection. Liver involvement is usually asymptomatic but children with the highest elevation of liver function tests may have severe vomiting. Extensive VZV infection of hepatocytes, with widespread hepatocellular destruction due to virus-induced cell lysis is a rare occurrence but is associated with fulminant hepatic failure.

In addition to its neurotropism for cells in the sensory ganglia, VZV can cause encephalitis and cerebellar ataxia. Meningoencephalitis and cerebellar ataxia are the major clinical signs of VZV-related damage to the central nervous system; some patients have signs of both cerebral and cerebellar disease (Johnson and Milbourn, 1970; Peters et al., 1978). VZV was the cause of encephalitis in 13% of cases of defined etiology in CDC surveillance studies from 1972 and 1977. Although these syndromes are the most common neurologic complications of varicella, information about the pathogenesis of these disorders is limited because most children recover. How primary VZV infection might produce cerebellar ataxia is of interest because VZV is the most common cause of this syndrome in healthy children. VZV has been recovered from the brain tissue of immunocompromised children with fatal varicella encephalitis, suggesting that this syndrome might be caused by direct infection. However, it is speculated that these neurologic manifestations of primary VZV infection may be immune-mediated, for the most part. The symptoms are typically transient but neurologic complications are the second most frequent indication for hospitalization of otherwise healthy children with varicella. The onset of neurologic complications follows the appearance of the rash by several days but a few case reports describe encephalitis and ataxia beginning before skin lesions have appeared. The symptoms of encephalitis are sudden changes in the level of consciousness and generalized seizures; the signs may be meningeal, e.g., nucal rigidity, rather than encephalitic in some cases. The cerebellar syndrome is characterized by a gradual onset of irritability, ataxia, nystagmus and speech disturbances. The cerebrospinal fluid usually shows a mild mononuclear cell inflammatory pattern, with a predominance of lymphocytes, a somewhat elevated protein (<200 mg) and normal glucose, or in some cases, the cerebrospinal fluid may be normal (Gershon et al., 1980). Children under 5 and adults appear to be the most susceptible to central nervous system complications. Not surprisingly, the highest risk of fatal complications appears to be associated with encephalitis rather than cerebellar ataxia. Varicella encephalitis usually resolves within 24–72 hours, even without antiviral therapy. Information about the risk of long-term sequelae after varicella encephalitis is limited; whereas most recover fully, some patients have recurrent seizures and permanent neurologic deficits (Johnson and Milbourn, 1970). The signs of cerebellar ataxia can persist for days or weeks. Among the rare neurologic complications of varicella are transverse myelitis, optic neuritis and very rarely, Guillain-Barre syndrome.

Primary VZV infection can be associated with thrombocytopenia and coagulopathy, although these manifestations are unusual in healthy individuals. The signs of these complications include hemorrhage into the skin vesicles, petechiae, purpura, epistaxis, hematuria and gastrointestinal bleeding. The mechanisms by which thrombocytopenia may be induced include reduced production of platelets and decreased platelet survival; vasculitis, transient hypersplenism or intravascular coagulopathy may be involved. As described for varicella pneumonia, adults are at higher risk for acute hemorrhagic complications of varicella than children. Purpura fulminans, due to arterial thrombosis, is a very rare but life-threatening complication of varicella. Immune-mediated thrombocytopenia may also occur, with symptoms developing from 1 to 2 weeks or longer after varicella. Whether acute or later in their onset, bleeding complications may last for several weeks, but the thrombocytopenia usually resolves completely. Inflammatory damage to the kidneys, presenting as nephritis, is an unusual, late complication in children and adults with varicella; it is possible that this syndrome is due to secondary group A streptococcal infection. A few cases of nephritic syndrome and hemolytic uremic syndrome have been described in children with primary VZV infection. Viral arthritis, diagnosed by the isolation of VZV from joint fluid, is unusual and resolves spontaneously within 3–5 days and has not been associated with residual damage. Myocarditis, pericarditis, pancreatitis and orchitis are other very rare complications of primary VZV infection. The risks of varicella in healthy children and adults have been reduced substantially by the introduction of live attenuated varicella vaccines (Chapter 70).

Varicella in the immunocompromised host

Primary VZV infection was often a life-threatening illness in immunocompromised children before the introduction of acyclovir (Chapter 65) and can be attributed to the delayed or failed induction of VZV-specific cellular immunity (Chapter 39). Most information about the clinical course of varicella in high-risk patients is based on observations in children with leukemia and other childhood malignancies. These children often have prolonged fever and a much more extensive rash and continued formation of new lesions. Serious complications result from unchecked viral dissemination by cell-associated viremia to the lungs, liver and in some cases to the central nervous system. Immunocompromised children develop varicella pneumonia, hepatitis, coagulopathy and meningoencephalitis (Feldman and Lott, 1987; Myers, 1979). Whereas new varicella lesions are unusual after 3–5 days in most healthy children, new lesions may appear for >7 days and resolution of the lesions may take 14 days. Susceptibility to secondary bacterial infections is typically enhanced in children receiving chemotherapy or radiation because of the granulocytopenia induced by treatment of the malignancy. As is the case in healthy adults, most varicella-related deaths result from pneumonia that develops shortly after the appearance of the rash. Before antiviral drugs were available, varicella pneumonia progressed rapidly with most deaths occurring within a few days due to untreatable hypoxemia. Varicella pneumonia is often associated with hepatitis, which can progress to liver failure. Again, as is true in healthy adults, hemorrhage into varicella lesions is a clinical sign of life-threatening coagulopathy, due to thrombocytopenia and altered production of clotting factors. VZV dissemination can also cause meningoencephalitis. Severe abdominal or back pain is a clinical sign of serious primary VZV infection in high-risk patients but the etiology of the symptoms is unknown; it is possible that it is related to early infection of sensory ganglia by hematogenous spread of the virus. Other complications of disseminated varicella in children with malignancy include myocarditis, nephritis, pancreatitis, necrotizing splenitis, esophagitis, and enterocolitis.

Children who receive kidney, liver or other solid organ transplants may also develop progressive varicella as a result of the immunosuppressive drugs given to prevent rejection of the transplanted organ (Feldhoff et al., 1981). Varicella pneumonia appears to be a less frequent complication than hepatitis and coagulopathy in kidney transplant patients. Steroid therapy for chronic diseases, including rheumatoid arthritis, nephrotic syndrome and ulcerative colitis, may lead to severe varicella. Asthma patients given high doses of prednisone, especially during the incubation period, are also at risk, but chronic low-dose steroid therapy does not usually result in varicella complications. The immunologic deficits caused by human immunodeficiency virus (HIV) infection are associated with prolonged, recurrent varicella and with chronic, hyperkaratotic skin lesions but varicella pneumonia, hepatitis and other manifestations of dissemination are unusual compared to children with malignancies or organ transplants (Jura et al., 1989; Kelley et al., 1994). Any of the rare genetic disorders that interfere with the acquisition of antigen-specific T cell immunity results in very high risk of fatal varicella. These diseases include severe combined immunodeficiency disorder, adenosine deaminase deficiency, nucleoside phosphorylase deficiency and cartilage hair hypoplasia/short-limbed dwarfism; serious varicella also occurs in some children with Wiskott–Aldrich syndrome and ataxia telangiectasia.

Varicella in pregnancy and the newborn

Most adults are immune, but susceptible pregnant women appear to be predisposed to severe varicella at rates higher than the enhanced risk associated with primary VZV infection in all healthy adults. Varicella pneumonia is the predominant complication and appears to be more common with varicella acquired in late gestation (Pastuszak et al., 1994). From the limited information available, the risk of fatal varicella, due to pneumonia, appears to be ∼1%–2% (Enders et al., 1994). When primary VZV infection occurs in early pregnancy, the virus can be transferred across the placenta to the developing fetus. The frequency of viral transfer is higher than the risk of fetal damage, as shown by postnatal testing of infants for VZV-specific immunity and the occurrence of zoster in early childhood among infants with no symptoms of intrauterine VZV infection (Dworsky et al., 1980). The estimated incidence of varicella embryopathy is <1%, with most damage due to maternal infection acquired in the first 20 weeks of gestation. The congenital varicella syndrome is most often recognized by unusual cutaneous defects and atrophy of an extremity. Infants often have microcephaly, cortical atrophy and intracranial calcifications secondary to intrauterine VZV encephalitis, with seizures and mental retardation. Damage to the autonomic nervous system is common and produces severe gastroesophogeal reflux and neurogenic bladder, hydroureter and hydronephrosis. Eye damage, manifesting as chorioretinitis, microophthalmia, and cataracts, is typical. Although intrauterine damage is not observed, infants whose mothers develop primary VZV infection just before delivery often develop varicella during the newborn period (Preblud et al., 1985). The risk of transfer of virus to the infant is highest when maternal infection begins 4 days before to 2 days after delivery, suggesting that the virus crosses the placenta during the viremia associated with lesion formation. Because of the early stage of the maternal infection, viral transfer is not associated with transplacental transport of maternal VZV IgG antibodies. Neonatal varicella can be progressive, presumably due to deficiencies in the capacity of the infant to develop VZV-specific T cell responses. Dissemination causes pneumonia and hepatitis, with a risk of meningoencephalitis. These infants require antiviral therapy to prevent such complications. Infants exposed to late gestation maternal varicella can be protected to some extent by administration of passive antibodies, given as varicella immune globulin. Some infants whose mothers have varicella more than 4–5 days before delivery are born with varicella lesions or develop lesions within a few days after birth; these infants appear to be at low risk for complications. Herpes zoster in pregnant women has not been associated with varicella embryopathy.


The principal host cell targets during the life cycle of VZV include the respiratory mucosal epithelium as a portal of entry, immune system cells, especially T-cells, for delivery of the virus to skin sites of replication, and sensory ganglia, where latency is established. VZV transmission to susceptible hosts is ensured by the release of cell-free virus into mucocutaneous lesions during varicella or herpes zoster. Like HSV-1 and HSV-2, VZV is an alphaherpesvirus that has achieved an equilibrium with the human host that has ensured its persistence in the species for millions of years.


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