Varicella-zoster virus (VZV) open reading frame 10 protein, the homolog of the essential herpes simplex virus protein VP16, is dispensable for VZV replication in vitro

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.

Duck Plague Virus pUL48 Protein Activates the Immediate-Early Gene to Initiate the Transcription of the Virus Gene

Duck plague caused by the duck plague virus (DPV) is an infectious disease that seriously harms the waterfowl breeding industry. The VP16 protein of α herpesvirus can bind to specific cis-acting elements upstream of the promoter of the immediate-early (IE, α) gene to promote the transcription of the IE gene, so it is also called the trans-inducer of IE gene (α-TIF). However, no studies on DPV α-TIF have been reported. This study investigated the DPV pUL48, a homolog of HSV-1 VP16, transcriptional activation region, target sequence, and viral protein affecting its transcriptional activation using a dual-luciferase reporter gene detection system, and pUL48 was identified as the α-TIF of DPV. (1) The regulation of pUL48 on DPV different gene promoters showed that pUL48 could activate all the promoters of IE genes (ICP4, ICP22, and ICP27) but not the promoters of early and late genes. (2) The activity of pUL48 to ICP4 and ICP22 promoters with different upstream lengths showed that pUL48 activated ICP4 and ICP22 promoters by acting on TAATGA (T) TAT element upstream of ICP4 promoter and TAATTATAT element upstream of ICP22 promoter, respectively. (3) Transcriptional activation of IE gene by truncated proteins of different lengths at the N-terminal of pUL48 was detected. The results showed that the transcriptional activation domain of pUL48 was amino acids 1–60 at the N-terminal, and amino acids 1–20 was its core region. In addition, it was found that pUL14, pUL46, and pUL47 significantly promoted the transcriptional activation of pUL48. The effects of loss of pUL47 and its nuclear localization signal on the nuclear entry and transcriptional activation function of pUL48 were further examined. The results showed that pUL47 could promote the nuclear entry of pUL48 through its nuclear localization signal at positions 40–50 and 768–777 amino acids, thus, enhancing the transcriptional activation function of pUL48 and synergistic promotion of viral gene transcription.

The Role of VP16 in the Life Cycle of Alphaherpesviruses

The protein encoded by the UL48 gene of alphaherpesviruses is named VP16 or alpha-gene-transactivating factor (α-TIF). In the early stage of viral replication, VP16 is an important transactivator that can activate the transcription of viral immediate-early genes, and in the late stage of viral replication, VP16, as a tegument, is involved in viral assembly. This review will explain the mechanism of VP16 acting as α-TIF to activate the transcription of viral immediate-early genes, its role in the transition from viral latency to reactivation, and its effects on viral assembly and maturation. In addition, this review also provides new insights for further research on the life cycle of alphaherpesviruses and the role of VP16 in the viral life cycle.

Identification of non-essential loci within the Meleagrid herpesvirus 1 genome

Background

Meleagrid herpesvirus 1 (MeHV-1) infectious bacterial artificial chromosomes (iBACs) are ideal vectors for the development of recombinant vaccines for the poultry industry. However, the full potential of iBACS as vectors can only be realised after thorough genetic characterisation, including identification of those genetic locations that are non-essential for virus replication. Generally, transposition has proven to be a highly effective strategy for rapid and efficient mutagenesis of iBAC clones. The current study describes the characterisation of 34 MeHV-1 mutants containing transposon insertions within the pMeHV1-C18 iBAC genome.

Methods

Tn5 and MuA transposition methods were used to generate a library of 76 MeHV-1 insertion mutants. The capacity of each mutant to facilitate the recovery of infectious MeHV-1 was determined by the transfection of clone DNA into chicken embryo fibroblasts.

Results

Attempts to recover infectious virus from the modified clones identified 14 genetic locations that were essential for MeHV-1 replication in cell culture. Infectious MeHV-1 was recovered from the remaining 14 intragenic insertion mutants and six intergenic insertion mutants, suggesting that the respective insertion locations are non-essential for MeHV-1 replication in cell culture.

Conclusions

The essential and non-essential designations for those MeHV-1 genes characterised in this study were generally in agreement with previous reports for other herpesviruses homologues. However, the requirement for the mardivirus-specific genes LORF4A and LORF5 are reported for the first time. These findings will help direct future work on the development of recombinant poultry vaccines using MeHV-1 as a vector by identifying potential transgene insertion sites within the viral genome.

Electronic supplementary material

The online version of this article (doi10.1186/s12985-015-0362-9) contains supplementary material, which is available to authorized users.

A comparison of herpes simplex virus type 1 and varicella-zoster virus latency and reactivation

Herpes simplex virus type 1 (HSV-1; human herpesvirus 1) and varicella-zoster virus (VZV; human herpesvirus 3) are human neurotropic alphaherpesviruses that cause lifelong infections in ganglia. Following primary infection and establishment of latency, HSV-1 reactivation typically results in herpes labialis (cold sores), but can occur frequently elsewhere on the body at the site of primary infection (e.g. whitlow), particularly at the genitals. Rarely, HSV-1 reactivation can cause encephalitis; however, a third of the cases of HSV-1 encephalitis are associated with HSV-1 primary infection. Primary VZV infection causes varicella (chickenpox) following which latent virus may reactivate decades later to produce herpes zoster (shingles), as well as an increasingly recognized number of subacute, acute and chronic neurological conditions. Following primary infection, both viruses establish a latent infection in neuronal cells in human peripheral ganglia. However, the detailed mechanisms of viral latency and reactivation have yet to be unravelled. In both cases latent viral DNA exists in an 'end-less' state where the ends of the virus genome are joined to form structures consistent with unit length episomes and concatemers, from which viral gene transcription is restricted. In latently infected ganglia, the most abundantly detected HSV-1 RNAs are the spliced products originating from the primary latency associated transcript (LAT). This primary LAT is an 8.3 kb unstable transcript from which two stable (1.5 and 2.0 kb) introns are spliced. Transcripts mapping to 12 VZV genes have been detected in human ganglia removed at autopsy; however, it is difficult to ascribe these as transcripts present during latent infection as early-stage virus reactivation may have transpired in the post-mortem time period in the ganglia. Nonetheless, low-level transcription of VZV ORF63 has been repeatedly detected in multiple ganglia removed as close to death as possible. There is increasing evidence that HSV-1 and VZV latency is epigenetically regulated. In vitro models that permit pathway analysis and identification of both epigenetic modulations and global transcriptional mechanisms of HSV-1 and VZV latency hold much promise for our future understanding in this complex area. This review summarizes the molecular biology of HSV-1 and VZV latency and reactivation, and also presents future directions for study.

ORF9p phosphorylation by ORF47p is crucial for the formation and egress of varicella-zoster virus viral particles

The role of the tegument during the herpesvirus lytic cycle is still not clearly established, particularly at the late phase of infection, when the newly produced viral particles need to be fully assembled before being released from the infected cell. The varicella-zoster virus (VZV) protein coded by open reading frame (ORF) 9 (ORF9p) is an essential tegument protein, and, even though its mRNA is the most expressed during the productive infection, little is known about its functions. Using a GalK positive/negative selection technique, we modified a bacterial artificial chromosome (BAC) containing the complete VZV genome to create viruses expressing mutant versions of ORF9p. We showed that ORF9p is hyperphosphorylated during the infection, especially through its interaction with the viral Ser/Thr kinase ORF47p; we identified a consensus site within ORF9p recognized by ORF47p and demonstrated its importance for ORF9p phosphorylation. Strikingly, an ultrastructural analysis revealed that the mutation of this consensus site (glutamate 85 to arginine) strongly affects viral assembly and release, reproducing the ORF47 kinase-dead VZV phenotype. It also slightly diminishes the infectivity toward immature dendritic cells. Taken together, our results identify ORF9p as a new viral substrate of ORF47p and suggest a determinant role of this phosphorylation for viral infectivity, especially during the process of viral particle formation and egress.

Marek's disease virus expresses multiple UL44 (gC) variants through mRNA splicing that are all required for efficient horizontal transmission

Marek's disease (MD) is a devastating oncogenic viral disease of chickens caused by Gallid herpesvirus 2, or MD virus (MDV). MDV glycoprotein C (gC) is encoded by the alphaherpesvirus UL44 homolog and is essential for the horizontal transmission of MDV (K. W. Jarosinski and N. Osterrieder, J. Virol. 84:7911-7916, 2010). Alphaherpesvirus gC proteins are type 1 membrane proteins and are generally anchored in cellular membranes and the virion envelope by a short transmembrane domain. However, the majority of MDV gC is secreted in vitro, although secondary-structure analyses predict a carboxy-terminal transmembrane domain. In this report, two alternative mRNA splice variants were identified by reverse transcription (RT)-PCR analyses, and the encoded proteins were predicted to specify premature stop codons that would lead to gC proteins that lack the transmembrane domain. Based on the size of the intron removed for each UL44 (gC) transcript, they were termed gC104 and gC145. Recombinant MDV viruses were generated in which only full-length viral gC (vgCfull), gC104 (vgC104), or gC145 (vgC145) was expressed. Predictably, gCfull was expressed predominantly as a membrane-associated protein, while both gC104 and gC145 were secreted, suggesting that the dominant gC variants expressed in vitro are the spliced variants. In experimentally infected chickens, the expression of each of the gC variants individually did not alter replication or disease induction. However, horizontal transmission was reduced compared to that of wild-type or revertant viruses when the expression of only a single gC was allowed, indicating that all three forms of gC are required for the efficient transmission of MDV in chickens.

Herpes simplex virus and varicella zoster virus, the house guests who never leave

Human alphaherpesviruses including herpes simplex viruses (HSV-1, HSV-2) and varicella zoster virus (VZV) establish persistent latent infection in sensory neurons for the life of the host. All three viruses have the potential to reactivate causing recurrent disease. Regardless of the homology between the different virus strains, the three viruses are characterized by varying pathologies. This review will highlight the differences in infection pattern, immune response, and pathogenesis associated with HSV-1 and VZV.

Probing of the nuclear import and export signals and subcellular transport mechanism of varicella-zoster virus tegument protein open reading frame 10

Varicella-zoster virus open reading frame 10 (ORF10), a tegument protein present in the virion, is a member of the alphaherpesvirus UL48 gene family that shares considerable amino acid sequence homology with the UL48 prototype, herpes simplex virus type 1 VP16. VP16 serves multiple functions, including transcriptional activation of viral immediate-early genes. Furthermore, VP16 has been shown to be involved in some aspects of virus assembly and/or maturation. However, little is known concerning the function of ORF10. Here, we found that transient expression of ORF10 fused to enhanced yellow fluorescent protein (EYFP) in COS-7 cells showed the predominantly nuclear localization in the absence of other viral proteins. By constructing a series of ORF10 variants fused to EYFP, a bona fide bipartite nuclear localization signal of ORF10 was, for the first time, determined and mapped to amino acids (aa) 302-347. Additionally, the nuclear export signal (NES) was identified and found to be in a leucine-rich region at aa 226-236. Finally, ORF10 was demonstrated to be targeted to the cytoplasm through the functional NES by chromosomal region maintenance 1-dependent pathway, and to the nucleus via Ran and importin β1-dependent pathway that does not require importin α5.

Analysis of the interaction between the essential herpes simplex virus 1 tegument proteins VP16 and VP1/2

The incorporation of tegument proteins into the herpes simplex virus 1 (HSV-1) virion during virion assembly is thought to be a complex, multistage process occurring via numerous interactions between the tegument and the capsid, within the tegument, and between the tegument and the envelope. Here, we set out to examine if the direct interaction between two essential tegument proteins VP1/2 and VP16 is required for connecting the inner tegument with the outer tegument. By using glutathione S-transferase (GST) pulldowns, we identified an essential role of lysine 343 in VP16, mutation of which to a neutral amino acid abrogated the interaction between VP1/2 and VP16. When the K343A substitution was inserted into the gene encoding VP16 (UL48) of the viral genome, HSV-1 replicated successfully although its growth was delayed, and final titers were reduced compared to titers of wild-type virus. Surprisingly, the mutated VP16 was incorporated into virions at levels similar to those of wild-type VP16. However, the analysis of VP16 on cytoplasmic capsids by fluorescence microscopy showed that VP16 associated with cytoplasmic capsids less efficiently when the VP16-VP1/2 interaction was inhibited. This implies that the direct interaction between VP1/2 and VP16 is important for the efficiency/timing of viral assembly but is not essential for HSV-1 replication in cell culture. These data also support the notion that the incorporation of tegument proteins into the herpesviruses is a very complex process with significant redundancy.

Cloning the simian varicella virus genome in E. coli as an infectious bacterial artificial chromosome

Simian varicella virus (SVV) is closely related to human varicella-zoster virus and causes varicella and zoster-like disease in nonhuman primates. In this study, a mini-F replicon was inserted into a SVV cosmid, and infectious SVV was generated by co-transfection of Vero cells with overlapping SVV cosmids. The entire SVV genome, cloned as a bacterial artificial chromosome (BAC), was stably propagated upon serial passage in E. coli. Transfection of pSVV-BAC DNA into Vero cells yielded infectious SVV (rSVV-BAC). The mini-F vector sequences flanked by loxP sites were removed by co-infection of Vero cells with rSVV-BAC and adenovirus expressing Cre-recombinase. Recombinant SVV generated using the SVV-BAC genetic system has similar molecular and in vitro replication properties as wild-type SVV. To demonstrate the utility of this approach, a SVV ORF 10 deletion mutant was created using two-step Red-mediated recombination. The results indicate that SVV ORF 10, which encodes a homolog of the HSV-1 virion VP-16 transactivator protein, is not essential for in vitro replication but is required for optimal replication in cell culture.

Varicella zoster virus latency

Primary infection by varicella zoster virus (VZV) typically results in childhood chickenpox, at which time latency is established in the neurons of the cranial nerve, dorsal root and autonomic ganglia along the entire neuraxis. During latency, the histone-associated virus genome assumes a circular episomal configuration from which transcription is epigenetically regulated. The lack of an animal model in which VZV latency and reactivation can be studied, along with the difficulty in obtaining high-titer cell-free virus, has limited much of our understanding of VZV latency to descriptive studies of ganglia removed at autopsy and analogy to HSV-1, the prototype alphaherpesvirus. However, the lack of miRNA, detectable latency-associated transcript and T-cell surveillance during VZV latency highlight basic differences between the two neurotropic herpesviruses. This article focuses on VZV latency: establishment, maintenance and reactivation. Comparisons are made with HSV-1, with specific attention to differences that make these viruses unique human pathogens.

Genome-wide mutagenesis reveals that ORF7 is a novel VZV skin-tropic factor

The Varicella Zoster Virus (VZV) is a ubiquitous human alpha-herpesvirus that is the causative agent of chicken pox and shingles. Although an attenuated VZV vaccine (v-Oka) has been widely used in children in the United States, chicken pox outbreaks are still seen, and the shingles vaccine only reduces the risk of shingles by 50%. Therefore, VZV still remains an important public health concern. Knowledge of VZV replication and pathogenesis remains limited due to its highly cell-associated nature in cultured cells, the difficulty of generating recombinant viruses, and VZV's almost exclusive tropism for human cells and tissues. In order to circumvent these hurdles, we cloned the entire VZV (p-Oka) genome into a bacterial artificial chromosome that included a dual-reporter system (GFP and luciferase reporter genes). We used PCR-based mutagenesis and the homologous recombination system in the E. coli to individually delete each of the genome's 70 unique ORFs. The collection of viral mutants obtained was systematically examined both in MeWo cells and in cultured human fetal skin organ samples. We use our genome-wide deletion library to provide novel functional annotations to 51% of the VZV proteome. We found 44 out of 70 VZV ORFs to be essential for viral replication. Among the 26 non-essential ORF deletion mutants, eight have discernable growth defects in MeWo. Interestingly, four ORFs were found to be required for viral replication in skin organ cultures, but not in MeWo cells, suggesting their potential roles as skin tropism factors. One of the genes (ORF7) has never been described as a skin tropic factor. The global profiling of the VZV genome gives further insights into the replication and pathogenesis of this virus, which can lead to improved prevention and therapy of chicken pox and shingles.

The Varicella Zoster Virus (VZV) is the causative agent of chicken pox and shingles. The long-term efficacy of the current chickenpox vaccine is yet to be determined, and the current shingles vaccine fails to provide protective immunity for a substantial number of individuals. Shingles can also lead to post-herpetic neuralgia (PHN), a debilitating condition associated with an intractable pain that can linger for life. Therefore, VZV remains an important public health concern. We use growth-rate analysis of our genome-wide deletion library to determine the essentiality of all known VZV genes, including novel annotations for 51% of the VZV proteome. We also discovered a novel skin-tropic factor encoded by ORF7. Overall, our identification of genes essential for VZV replication and pathogenesis will serve as the basis for multiple in-depth genetic studies of VZV, which can lead to improved prevention and therapy of chicken pox and shingles. For example, essential genes may be appealing drug targets and genes whose deletion causes a substantial growth defect may be prospective candidates for novel live attenuated vaccines.

Improving the yeast two-hybrid system with permutated fusions proteins: the Varicella Zoster Virus interactome

Background

Yeast two-hybrid (Y2H) screens have been among the most powerful methods to detect and analyze protein-protein interactions. However, they suffer from a significant degree of false negatives, i.e. true interactions that are not detected, and to a certain degree from false positives, i.e. interactions that appear to take place only in the context of the Y2H assay. While the fraction of false positives remains difficult to estimate, the fraction of false negatives in typical Y2H screens is on the order of 70-90%. Here we present novel Y2H vectors that significantly decrease the number of false negatives and help to mitigate the false positive problem.

Results

We have constructed two new vectors (pGBKCg and pGADCg) that allow us to make both C-terminal fusion proteins of DNA-binding and activation domains. Both vectors can be combined with existing vectors for N-terminal fusions and thus allow four different bait-prey combinations: NN, CC, NC, and CN. We have tested all ~4,900 pairwise combinations of the 70 Varicella-Zoster-Virus (VZV) proteins for interactions, using all possible combinations. About ~20,000 individual Y2H tests resulted in 182 NN, 89 NC, 149 CN, and 144 CC interactions. Overlap between screens ranged from 17% (NC-CN) to 43% (CN-CC). Performing four screens (i.e. permutations) instead of one resulted in about twice as many interactions and thus much fewer false negatives. In addition, interactions that are found in multiple combinations confirm each other and thus provide a quality score. This study is the first systematic analysis of such N- and C-terminal Y2H vectors.

Conclusions

Permutations of C- and N-terminal Y2H vectors dramatically increase the coverage of interactome studies and thus significantly reduce the number of false negatives. We suggest that future interaction screens should use such vector combinations on a routine basis, not the least because they provide a built-in quality score for Y2H interactions that can provide a measure of reproducibility without additional assays.

The varicella-zoster virus genome

The varicella-zoster virus (VZV) genome contains at least 70 genes, and all but six have homologs in herpes simplex virus (HSV). Cosmids and BACs corresponding to the VZV parental Oka and vaccine Oka viruses have been used to "knockout" 34 VZV genes. Seven VZV genes (ORF4, 5, 9, 21, 29, 62, and 68) have been shown to be required for growth in vitro. Recombinant viruses expressing several markers (e.g., beta-galactosidase, green fluorescence protein, luciferase) and several foreign viral genes (from herpes simplex, Epstein-Barr virus, hepatitis B, mumps, HIV, and simian immunodeficiency virus) have been constructed. Further studies of the VZV genome, using recombinant viruses, may facilitate the development of safer and more effective VZV vaccines. Furthermore, VZV might be useful as a vaccine vector to immunize against both VZV and other viruses.

Effects of varicella-zoster virus on cell cycle regulatory pathways

Varicella-zoster virus (VZV) grows efficiently in quiescent cells in vivo and in culture, and virus infection activates cell cycle and signaling pathways without cell division. VZV ORFs have been identified that determine the tissue tropism for nondividing skin, T cells, and neurons in SCID-Hu mouse models. The normal cell cycle status of human foreskin fibroblasts was characterized and was dysregulated upon infection by VZV. The expression of cyclins A, B1, and D3 was highly elevated but did not correspond with extensive cellular DNA synthesis. Cell cycle arrest may be due to activation of the DNA damage response during VZV DNA replication. Other host regulatory proteins were induced in infected cells, including p27, p53, and ATM kinase. A possible explanation for the increase in cell cycle regulatory proteins is activation of transcription factors during VZV infection. There is evidence that VZV infection activates transcription factors through the mitogen-activated protein kinase pathways extracellular-regulated kinase (ERK) and c-Jun N-terminal (transpose these parts of the compound noun) kinase (JNK), which could selectively increase cyclin levels. Some of these perturbed cell functions are essential for VZV replication, such as cyclin-dependent kinase (CDK) activity, and reveal targets for interventions.

Rodent models of varicella-zoster virus neurotropism

Inoculation of rodents with varicella-zoster virus (VZV) results in a latent infection in dorsal root ganglia with expression of at least five of the six VZV transcripts and one of the viral proteins that are reported to be expressed during latency in human ganglia. Rats develop allodynia and hyperalgesia in the limb distal to the site of injection and the resulting exaggerated withdrawal response to stimuli is reduced by treatment with gabapentin and amitryptyline, but not by antiviral therapy. Inoculation of rats with VZV mutants show that most viral genes are dispensable for latency, but that some genes (e.g., ORF4, 29, and ORF63) that are expressed during latency are important for the establishment of latency in rodents, but not for infection of rodent ganglia. The rodent model for VZV latency allows one to study ganglia removed immediately after death, avoiding the possibility of reactivation, and helps to identify VZV genes required for latency.

The Marek's disease virus (MDV) protein encoded by the UL17 ortholog is essential for virus growth

Marek’s disease virus type 1 (MDV-1) shows a strict dependency on the direct cell-to-cell spread for its propagation in cell culture. As MDV-1 shows an impaired nuclear egress in cell culture, we wished to address the characterization of capsid/tegument genes which may intervene in the maturation of intranuclear capsids. Orthologs of UL17 are present in all herpesviruses and, in all reported case, were shown to be essential for viral growth, playing a role in capsid maturation and DNA packaging. As only HSV-1 and PrV UL17 proteins have been characterized so far, we wished to examine the role of MDV-1 pUL17 in virus replication. To analyze MDV-1 UL17 gene function, we created deletion mutants or point mutated the open reading frame (ORF) to interrupt its coding phase. We established that a functional ORF UL17 is indispensable for MDV-1 growth. We chose to characterize the virally encoded protein by tagging the 729 amino-acid long protein with a repeat of the HA peptide that was fused to its C-terminus. Protein pUL17 was identified in infected cell extracts as an 82 kDa protein which localized to the nucleus, colocalizing with VP5, the major capsid protein, and VP13/14, a major tegument protein. By using green fluorescent protein fusion and HA tagged proteins expressed under the cytomegalovirus IE gene enhancer/promoter (P_{CMV IE}), we showed that MDV-1 pUL17 nuclear distribution in infected cells is not an intrinsic property. Although our results strongly suggest that another viral protein retains (or relocate) pUL17 to the nucleus, we report that none of the tegument protein tested so far were able to mediate pUL17 relocation to the nucleus.

Complete DNA sequences of two oka strain varicella-zoster virus genomes

Varicella-zoster virus (VZV) is a herpesvirus and is the causative agent of chicken pox (varicella) and shingles (herpes zoster). Active immunization against varicella became possible with the development of live attenuated varicella vaccine. The Oka vaccine strain was isolated in Japan from a child who had typical varicella, and it was then attenuated by serial passages in cell culture. Several manufacturers have obtained this attenuated Oka strain and, following additional passages, have developed their own vaccine strains. Notably, the vaccines Varilrix and Varivax are produced by GlaxoSmithKline Biologicals and Merck & Co., Inc., respectively. Both vaccines have been well studied in terms of safety and immunogenicity. In this study, we report the complete nucleotide sequence of the Varilrix (Oka-V(GSK)) and Varivax (Oka-V(Merck)) vaccine strain genomes. Their genomes are composed of 124,821 and 124,815 bp, respectively. Full genome annotations covering the features of Oka-derived vaccine genomes have been established for the first time. Sequence analysis indicates 36 nucleotide differences between the two vaccine strains throughout the entire genome, among which only 14 are involved in unique amino acid substitutions. These results demonstrate that, although Oka-V(GSK) and Oka-V(Merck) vaccine strains are not identical, they are very similar, which supports the clinical data showing that both vaccines are well tolerated and elicit strong immune responses against varicella.

Functions of the ORF9-to-ORF12 gene cluster in varicella-zoster virus replication and in the pathogenesis of skin infection

The gene cluster composed of varicella-zoster virus (VZV) open reading frame 9 (ORF9) to ORF12 encodes four putative tegument proteins and is highly conserved in most alphaherpesviruses. In these experiments, the genes within this cluster were deleted from the VZV parent Oka (POKA) individually or in combination, and the consequences for VZV replication were evaluated with cultured cells in vitro and with human skin xenografts in SCID mice in vivo. As has been reported for ORF10, ORF11 and ORF12 were dispensable for VZV replication in melanoma and human embryonic fibroblast cells. In contrast, deletion of ORF9 was incompatible with the recovery of infectious virus. ORF9 localized to the virion tegument and formed complexes with glycoprotein E, which is an essential protein, in VZV-infected cells. Recombinants lacking ORF10 and ORF11 (POKADelta10/11), ORF11 and ORF12 (POKADelta11/12), or ORF10, ORF11 and ORF12 (POKADelta10/11/12) were viable in cultured cells. Their growth kinetics did not differ from those of POKA, and nucleocapsid formation and virion assembly were not disrupted. In addition, these deletion mutants showed no differences compared to POKA in infectivity levels for primary human tonsil T cells. Deletion of ORF12 had no effect on skin infection, whereas replication of POKADelta11, POKADelta10/11, and POKADelta11/12 was severely reduced, and no virus was recovered from skin xenografts inoculated with POKADelta10/11/12. These results indicate that with the exception of ORF9, the individual genes within the ORF9-to-ORF12 gene cluster are dispensable and can be deleted simultaneously without any apparent effect on VZV replication in vitro but that the ORF10-to-ORF12 cluster is essential for VZV virulence in skin in vivo.

Horizontal transmission of Marek's disease virus requires US2, the UL13 protein kinase, and gC

Marek's disease virus (MDV) causes a general malaise in chickens that is mostly characterized by the development of lymphoblastoid tumors in multiple organs. The use of bacterial artificial chromosomes (BACs) for cloning and manipulation of the MDV genome has facilitated characterization of specific genes and genomic regions. The development of most MDV BACs, including pRB-1B-5, derived from a very virulent MDV strain, involved replacement of the US2 gene with mini-F vector sequences. However, when reconstituted viruses based on pRB-1B were used in pathogenicity studies, it was discovered that contact chickens housed together with experimentally infected chickens did not contract Marek's disease (MD), indicating a lack of horizontal transmission. Staining of feather follicle epithelial cells in the skins of infected chickens showed that virus was present but was unable to be released and/or infect susceptible chickens. Restoration of US2 and removal of mini-F sequences within viral RB-1B did not alter this characteristic, although in vivo viremia levels were increased significantly. Sequence analyses of pRB-1B revealed that the UL13, UL44, and US6 genes encoding the UL13 serine/threonine protein kinase, glycoprotein C (gC), and gD, respectively, harbored frameshift mutations. These mutations were repaired individually, or in combination, using two-step Red mutagenesis. Reconstituted viruses were tested for replication, MD incidence, and their abilities to horizontally spread to contact chickens. The experiments clearly showed that US2, UL13, and gC in combination are essential for horizontal transmission of MDV and that none of the genes alone is able to restore this phenotype.

Marek's disease virus research in the post-sequencing era: new tools for the study of gene functions and virus-host interactions

Despite the fact that the causative agent of Marek's disease was described more than 30 years ago, and that subsequently many classical biological studies have been carried out on the Marek's disease virus (MDV), detailed analysis of its gene functions has been hampered by lack of suitable research tools. Information on the primary structure of MDV-1 and its serologically related viruses, MDV-2 and herpesvirus of turkeys, is now available. This review focuses on the introduction of the modern and highly efficient technology of bacterial artificial chromosome (BAC) cloning and mutagenesis for rapid manipulation of the MDV genome, with the aim of studying the functions of its genes and non-coding regions. Constructed MDV BACs carry the complete genome of MDV that can be multiplied in Escherichia coli and manipulated using the tools provided by bacterial genetics. The novel approach of MDV DNA mutagenesis using BAC technology will be explained by examples, and we will discuss gene functions in comparison with their counterparts in other herpesviruses. In addition, we have shown that MDV BAC DNA can be used as a polynucleotide vaccine to protect against Marek's disease, thus opening a new chapter in strategies for control of this disease.

Relevance of the interaction between alphaherpesvirus UL3.5 and UL48 proteins for virion maturation and neuroinvasion

The UL3.5 and UL48 genes, which are conserved in most alphaherpesvirus genomes, are important for maturation of pseudorabies virus (PrV) particles in the cytoplasm of infected cells (W. Fuchs, B. G. Klupp, H. J. Rziha, and T. C. Mettenleiter, J. Virol. 70:3517-3527, 1996; W. Fuchs, H. Granzow, B. G. Klupp, M. Kopp and T. C. Mettenleiter, J. Virol. 76:6729-6742, 2002). In bovine herpesvirus 1 (BoHV-1), the homologous gene products pUL3.5 and pUL48 have been demonstrated to interact physically (N. Lam and G. Letchworth, J. Virol. 74:2876-2884, 2000). Moreover, BoHV-1 pUL3.5 partially complemented a pUL3.5 defect in PrV (W. Fuchs, H. Granzow, and T. C. Mettenleiter, J. Virol. 71:8886-8892, 1997). By using coimmunoprecipitation and yeast two-hybrid studies, we observed a similar interaction between pUL3.5 and pUL48 of PrV, as well as a heterologous interaction between the PrV and BoHV-1 gene products. The relevant domain could be confined to the first 43 amino acids of PrV pUL3.5. Unlike its BoHV-1 homologue, PrV pUL3.5 is processed by proteolytic cleavage, and only an abundant 14-kDa fragment consisting of amino acids 1 to >or=116 could be detected by peptide mass fingerprint analysis of purified wild-type PrV particles, which also contain the pUL48 tegument component. To determine the biological relevance of the protein-protein interaction, pUL3.5-, pUL48-, and double-negative PrV mutants were analyzed in parallel. All deletion mutants were replication competent but exhibited significantly reduced plaque sizes and virus titers in cultured rabbit kidney cells compared to wild-type and rescued viruses, which correlated with a delayed neuroinvasion in intranasally infected mice. Remarkably, the defects of the double-negative mutant were similar to those of pUL48-negative virus. Electron microscopy of cells infected with either deletion mutant revealed the retention of naked nucleocapsids in the cytoplasm and the absence of mature virus particles. In summary, our studies for the first time demonstrate the relevance of the pUL3.5-pUL48 interaction for secondary envelopment of an alphaherpesvirus, give a molecular basis for the observed trans-complementation between the PrV and BHV-1 pUL3.5 homologs, yield conclusive evidence for the incorporation of a proteolytically processed pUL3.5 into PrV virions, and demonstrate the importance of both proteins for neuroinvasion and neurovirulence of PrV.

The ubiquitous cellular transcriptional factor USF targets the varicella-zoster virus open reading frame 10 promoter and determines virulence in human skin xenografts in SCIDhu mice in vivo

Varicella-zoster virus (VZV) open reading frame 10 (ORF10) is a determinant of virulence in SCIDhu skin xenografts but not in human T cells in vivo. In this analysis of the regulation of ORF10 transcription, we have identified four ORF10-related transcripts, including a major 1.3-kb RNA spanning ORF10 only and three other read-through transcripts. Rapid-amplification-of-cDNA-ends experiments indicated that the 1.3-kb transcript of ORF10 has single initiation and termination sites. In transient expression assays, the ORF10 promoter was strongly stimulated by the major VZV transactivator, IE62. Deletion analyses revealed approximate boundaries for the full ORF10 promoter activity between -75 and -45 and between +5 and -8, relative to the ORF10 transcription start site. The recombinant virus POKA10-Deltapro, with the ORF10 promoter deletion, blocked transcription of ORF10 and also of ORF9A and ORF9 mRNAs, whereas expression of read-through ORF9A/9/10 and ORF9/10 transcripts was increased, compensating for the loss of the monocistronic mRNAs. The cellular factor USF bound specifically to its consensus site within the ORF10 promoter and was required for IE62 transactivation, whereas disrupting the predicted TATA boxes or Oct-1 binding elements had no effect. The USF binding site was disrupted in the recombinant virus, POKA10-proDeltaUSF, and no ORF10 protein was produced. Both ORF10 promoter mutants reduced VZV replication in SCIDhu skin xenografts. These observations provided further evidence of the contribution of the ORF10 protein to VZV pathogenesis in skin and demonstrated that VZV depends upon the cellular transcriptional factor USF to support its virulence in human skin in vivo.

Identification of transcripts and protein products of the UL31, UL37, UL46, UL47, UL48, UL49 and US4 gene homologues of avian infectious laryngotracheitis virus

In the present study, the transcription and protein expression of seven genes of infectious laryngotracheitis virus (ILTV) were investigated: UL31 and UL37 possess homologues in all known avian and mammalian herpesviruses, whereas UL46–UL49 and US4 are only conserved in most alphaherpesviruses. A peculiarity of the ILTV genome is the translocation of UL47 from the unique long region to a position upstream of US4 within the unique short region. Northern blot analyses revealed that all of the analysed genes were transcribed most abundantly during the late (γ) phase of replication, but the only true late (γ2) gene was UL47. Using monospecific rabbit antisera, the protein products of all of the genes could be detected and localized in ILTV-infected cells. Considerable amounts of the UL31, UL47 and UL48 gene products were found in the cell nuclei, whereas the other proteins were restricted largely to the cytoplasm. Like the respective tegument proteins of other herpesviruses, the UL37 and UL46–UL49 gene products of ILTV were incorporated into virus particles, whereas the UL31 protein and the glycoprotein encoded by US4 (gG) were not detectable in purified virions. It was also demonstrated that the UL48 protein of ILTV is able to activate an alphaherpesvirus immediate-early gene promoter, which is also a typical feature of other UL48 homologues. Taken together, these results indicate that the functions of all of the investigated ILTV proteins are related to those of their homologues in other alphaherpesviruses.

The varicella-zoster virus (VZV) ORF9 protein interacts with the IE62 major VZV transactivator

The varicella-zoster virus (VZV) ORF9 protein is a member of the herpesvirus UL49 gene family but shares limited identity and similarity with the UL49 prototype, herpes simplex virus type 1 VP22. ORF9 mRNA is the most abundantly expressed message during VZV infection; however, little is known concerning the functions of the ORF9 protein. We have found that the VZV major transactivator IE62 and the ORF9 protein can be coprecipitated from infected cells. Yeast two-hybrid analysis localized the region of the ORF9 protein required for interaction with IE62 to the middle third of the protein encompassing amino acids 117 to 186. Protein pull-down assays with GST-IE62 fusion proteins containing N-terminal IE62 sequences showed that amino acids 1 to 43 of the acidic transcriptional activation domain of IE62 can bind recombinant ORF9 protein. Confocal microscopy of transiently transfected cells showed that in the absence of other viral proteins, the ORF9 protein was localized in the cytoplasm while IE62 was localized in the nucleus. In VZV-infected cells, the ORF9 protein was localized to the cytoplasm whereas IE62 exhibited both nuclear and cytoplasmic localization. Cotransfection of plasmids expressing ORF9, IE62, and the viral ORF66 kinase resulted in significant colocalization of ORF9 and IE62 in the cytoplasm. Coimmunoprecipitation experiments with antitubulin antibodies indicate the presence of ORF9-IE62-tubulin complexes in infected cells. Colocalization of ORF9 and tubulin in transfected cells was visualized by confocal microscopy. These data suggest a model for ORF9 protein function involving complex formation with IE62 and possibly other tegument proteins in the cytoplasm at late times in infection.

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-zoster virus open reading frame 10 is a virulence determinant in skin cells but not in T cells in vivo

The open reading frame 10 (ORF10) of varicella-zoster virus (VZV) encodes a tegument protein that enhances transactivation of VZV genes and has homology to herpes simplex virus type 1 (HSV-1) VP16. While VP16 is essential for HSV replication, ORF10 is dispensable for vaccine OKA (VOKA) growth in vitro. We used parent OKA (POKA) cosmids to delete ORF10, producing POKA delta10; point mutations that disrupted the acidic activation domain and the putative motif for binding human cellular factor 1 (HCF-1) in ORF10 protein yielded POKA10-Phe28Ala, POKA10-Phe28Ser, and POKA10-mHCF viruses. Deleting ORF10 or mutating these two functional domains had no effect on VZV replication, immediate-early gene transcription, or virion assembly in vitro. However, deleting ORF10 reduced viral titers and the extent of cutaneous lesions significantly in SCIDhu skin xenografts in vivo compared to POKA. Epidermal cells infected with POKA delta10 had significantly fewer DNA-containing nucleocapsids and complete virions compared to POKA; 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 replication in vivo. Thus, the decreased pathogenic potential of POKA delta10 in skin could not be attributed to absence of these ORF10 protein functions. In contrast to skin cells, deleting ORF10 did not impair VZV T-cell tropism in vivo, as assessed by infectious virus yields. We conclude that ORF10 protein is required for efficient VZV virion assembly and is a specific determinant of VZV virulence in epidermal and dermal cells in vivo.

The alpha-TIF (VP16) homologue (ETIF) of equine herpesvirus 1 is essential for secondary envelopment and virus egress

The equine herpesvirus 1 (EHV-1) alpha-trans-inducing factor homologue (ETIF; VP16-E) is a 60-kDa virion component encoded by gene 12 (ORF12) that transactivates the immediate-early gene promoter. Here we report on the function of EHV-1 ETIF in the context of viral infection. An ETIF-null mutant from EHV-1 strain RacL11 (vL11deltaETIF) was constructed and analyzed. After transfection of vL11deltaETIF DNA into RK13 cells, no infectious virus could be reconstituted, and only single infected cells or small foci containing up to eight infected cells were detected. In contrast, after transfection of vL11deltaETIF DNA into a complementing cell line, infectious virus could be recovered, indicating the requirement of ETIF for productive virus infection. The growth defect of vL11deltaETIF could largely be restored by propagation on the complementing cell line, and growth on the complementing cell line resulted in incorporation of ETIF in mature and secreted virions. Low- and high-multiplicity infections of RK13 cells with phenotypically complemented vL11deltaETIF virus resulted in titers of virus progeny similar to those used for infection, suggesting that input ETIF from infection was recycled. Ultrastructural studies of vL11deltaETIF-infected cells demonstrated a marked defect in secondary envelopment at cytoplasmic membranes, resulting in very few enveloped virions in transport vesicles or extracellular space. Taken together, our results demonstrate that ETIF has an essential function in the replication cycle of EHV-1, and its main role appears to be in secondary envelopment.

Phosphorylation of the varicella-zoster virus (VZV) major transcriptional regulatory protein IE62 by the VZV open reading frame 66 protein kinase

IE62, the major transcriptional regulatory protein encoded by varicella-zoster virus (VZV), is nuclear at early times of VZV infection but then becomes predominantly cytoplasmic as a result of expression of the protein kinase encoded by open reading frame 66 (ORF66). Cytoplasmic forms of IE62 are required for its inclusion as an abundant VZV virion tegument protein. Here we show that ORF66 directly phosphorylates IE62 at two residues, with phosphorylation at S686 being sufficient to regulate IE62 nuclear import. Phosphotryptic peptide analyses established an ORF66 kinase-mediated phosphorylation of the complete IE62 protein in transfected and VZV-infected cells. Using truncated and point-mutated IE62 peptides, ORF66-directed phosphorylation was mapped to residues S686 and S722, immediately downstream of the IE62 nuclear localization signal. An IE62 protein with an S686A mutation retained efficient nuclear import activity, even in the presence of functional ORF66 protein kinase, but an IE62 protein containing an S686D alteration was imported into the nucleus inefficiently. In contrast, the nuclear import of IE62 carrying an S722A mutation was still modulated by ORF66 expression, and IE62 with an S722D mutation was imported efficiently into the nucleus. An in vitro phosphorylation assay was developed using bacterially expressed IE62-maltose binding protein fusions as substrates for immunopurified ORF66 protein kinase from recombinant baculovirus-infected insect cells. ORF66 kinase phosphorylated the IE62 peptides, with similar specificities for residues S686 and S722. These results indicate that IE62 nuclear import is modulated as a result of direct phosphorylation of IE62 by ORF66 kinase. This represents an interaction that is, so far, unique among the alphaherpesviruses.

Varicella-zoster virus open reading frame 47 (ORF47) protein is critical for virus replication in dendritic cells and for spread to other cells

Varicella-zoster virus infects human dendritic cells (DCs). We found that VZV infection of DCs resulted in down-regulation of Fas expression on the surface of cells. VZV ORF47 was critical for replication of virus in human immature, but not mature DCs. Immature DCs infected with a mutant virus unable to express ORF47 expressed similar levels of a VZV immediate-early protein as cells infected with parental virus; however, cells infected with the ORF47 mutant expressed lower levels of glycoprotein E. Thus, in the absence of ORF47 protein, there is a block in viral replication between immediate-early and late gene expression. VZV unable to express ORF47 was severely impaired for spread of virus from DCs to melanoma cells. Infection of DCs with parental VZV resulted in a different pattern of phosphoproteins compared with the ORF47 mutant virus. Thus, VZV ORF47 is important for replication in immature DCs and for spread to other cells.

High-level expression of Marek's disease virus glycoprotein C is detrimental to virus growth in vitro

Expression levels of Marek's disease virus (MDV) glycoprotein C (gC) are significantly reduced after serial virus passage in cell culture. Reduced gC expression coincides with enhanced MDV growth in vitro and attenuation. To analyze this phenomenon in detail, a full-length infectious MDV clone was modified by Red-based and shuttle mutagenesis in Escherichia coli. Besides a gC-negative deletion mutant harboring a kanamycin resistance gene, a markerless mutant with the U(L)44 gene deleted was constructed. On the basis of this deletion mutant, the original or a modified U(L)44 gene with a mutated start codon (AUG-->ACG) was reinserted into the authentic locus. Similarly, mutants expressing authentic gC or the start codon mutation under the control of a strong constitutive promoter were generated. In vitro studies demonstrated that gC deletion mutants induced twofold-larger plaques than the parental virus did, whereas constitutive overexpression of the glycoprotein resulted in a more than twofold reduction in plaque size. In addition, plaque sizes of the gC deletion mutant were reduced when virus was grown using supernatants from cells infected with parental virus, but supernatants obtained from cells infected with the gC deletion mutant had no measurable effect on plaque size. The results indicated that (i) expression of MDV gC, albeit at low levels in a highly passaged virus, had a significant negative impact on the cell-to-cell spread capabilities of the virus, which was alleviated in its absence and exacerbated by its overexpression, and that (ii) this activity was mediated by the secreted form of MDV gC.

Role of the varicella-zoster virus gene product encoded by open reading frame 35 in viral replication in vitro and in differentiated human skin and T cells in vivo

Although genes related to varicella-zoster virus (VZV) open reading frame 35 (ORF35) are conserved in the herpesviruses, information about their contributions to viral replication and pathogenesis is limited. Using a VZV cosmid system, we deleted ORF35 to produce two null mutants, designated rOkaDelta35(#1) and rOkaDelta35(#2), and replaced ORF35 at a nonnative site, generating two rOkaDelta35/35@Avr mutants. ORF35 Flag-tagged recombinants were made by inserting ORF35-Flag at the nonnative Avr site as the only copy of ORF35, yielding rOkaDelta35/35Flag@Avr, or as a second copy, yielding rOka35Flag@Avr. Replication of rOkaDelta35 viruses was diminished in melanoma and Vero cells in a 6-day analysis of growth kinetics. Plaque sizes of rOkaDelta35 mutants were significantly smaller than those of rOka in melanoma cells. Infection of melanoma cells with rOkaDelta35 mutants was associated with disrupted cell fusion and polykaryocyte formation. The small plaque phenotype was not corrected by growth of rOkaDelta35 mutants in melanoma cells expressing the major VZV glycoprotein E, gE. The rOkaDelta35/35@Avr viruses displayed growth kinetics and plaque morphologies that were indistinguishable from those of rOka. Analysis with ORF35-Flag recombinants showed that the ORF35 gene product localized predominantly to the nuclei of infected cells. Evaluations in the SCIDhu mouse model demonstrated that ORF35 was required for efficient VZV infection of skin and T-cell xenografts, although the decrease in infectivity was most significant in skin. These mutagenesis experiments indicated that ORF35 was dispensable for VZV replication, but deleting ORF35 diminished growth in cultured cells and was associated with attenuated VZV infection of differentiated human skin and T cells in vivo.

Genetic profile of an Oka varicella vaccine virus variant isolated from an infant with zoster

Varicella virus vaccine strain Oka (V-Oka) has in rare cases caused zoster in vaccinated people. Despite broad usage of V-Oka, little is known about varicella-zoster virus genomic sequence variation of strains in vaccine and isolates from patients with vaccine adverse events. Direct sequencing of 20 regions of V-Oka-GSK was compared to the sequences of the original V-Oka-Biken, GlaxoSmithKline Oka vaccine (V-Oka-GSK), and Oka-parental (P-Oka) strains. We analyzed single nucleotide polymorphisms (SNP) differentiating the Oka parental and Oka vaccine strains identified in open reading frames (ORFs) 6, 9A, 10, 21, 31, 39, 50, 51, 52, 54, 55, and 59 and eight base substitutions within ORF 62. Sixteen of these SNP impose an amino acid change in the corresponding gene product. The genotypic analysis revealed that (i) both V-Oka-GSK and V-Oka-Biken comprise mixtures of strains represented in variable proportion from lot to lot; (ii) V-Oka-GSK/zoster isolated from the zoster patient had six wild-type SNP in ORF 9A, 10, 21, 52, 55, and 62 (mutation 108838); (iii) none of the six revertant SNP would reliably discriminate Oka vaccine from the wild type; and (iv) the genomic variation found in V-Oka/zoster might be associated with changes in the biological behavior of the virus. Further studies will be needed to identify potential virulence factors in variant vaccine strains.

A pseudorabies virus recombinant simultaneously lacking the major tegument proteins encoded by the UL46, UL47, UL48, and UL49 genes is viable in cultured cells

The UL46, UL47, UL48, and UL49 genes, which encode major tegument proteins, are conserved in most alphaherpesvirus genomes. However, the relative importance of each of these proteins for replication of individual alphaherpesviruses appears to be different. Recently, we demonstrated that single deletions of UL47 or UL48 impair maturation and egress of pseudorabies virus (PrV) particles to different extents, whereas deletions of UL46 or UL49 have no significant effects on virus replication in cell culture (W. Fuchs, H. Granzow, B. G. Klupp, M. Kopp, and T. C. Mettenleiter, J. Virol. 76:6729-6742, 2002; M. Kopp, B. G. Klupp, H. Granzow, W. Fuchs, and T. C. Mettenleiter, J. Virol. 76:8820-8833, 2002). To test for possible functional redundancy between the four tegument proteins, a quadruple gene deletion mutant (PrV-DeltaUL46-49) was generated and characterized in vitro. Although plaque formation by this mutant was almost abolished and maximum titers were reduced more than 100-fold compared to those of parental wild-type virus, PrV-DeltaUL46-49 could be propagated and serially passaged in noncomplementing porcine and rabbit kidney cells. Electron-microscopic studies revealed that nucleocapsid formation and egress of PrV-DeltaUL46-49 from the host cell nucleus were not affected, but secondary envelopment of nucleocapsids in the cytoplasm was only rarely observed. The replication defect of PrV-DeltaUL46-49 could be fully corrected by reinsertion of the UL46-to-UL49 gene cluster. Plaque sizes and virus titers were only slightly increased after restoration of only UL47 expression, whereas repair of only UL48 resulted in a significant increase in replication capacity to the level of a UL47 deletion mutant. In conclusion, we show that none of the UL46 to UL49 tegument proteins is absolutely required for productive replication of PrV. Moreover, our data indicate that the UL47 and UL48 proteins function independently during cell-to-cell spread and virus egress.

Varicella-zoster virus ORF47 protein kinase, which is required for replication in human T cells, and ORF66 protein kinase, which is expressed during latency, are dispensable for establishment of latency

Varicella-zoster virus (VZV) results in a lifelong latent infection in human sensory and cranial nerve ganglia after primary infection. VZV open reading frame 47 (ORF47) and ORF66 encode protein kinases that phosphorylate several viral proteins, including VZV glycoprotein gE and ORF32, ORF62, and ORF63 proteins. Here we show that the ORF47 protein kinase also phosphorylates gI. While ORF47 is essential for virus replication in human T cells and skin, we found the gene to be dispensable for establishment of latent infection in dorsal root ganglia of rodents. ORF66 protein is expressed during latency. Rodents infected with VZV unable to express ORF66 developed latent infection at a rate similar to that for the parental virus. ORF63 transcripts, a hallmark of VZV latency, were also detected in similar numbers of animals infected with the ORF47 and ORF66 mutants and with the parental virus. VZV mutants unable to express four of the six genes that do not have herpes simplex virus (HSV) homologs (ORFs 1, 13, 32, 57) were also unimpaired for establishment of latency. While a truncated HSV VP16 mutant was previously reported to be unable to establish latency in a mouse model, we found that VZV with a deletion of ORF10, the homolog of HSV VP16, was dispensable for establishment of latency. Thus, seven genes, including one expressed during latency, are dispensable for establishing latent VZV infection.

The UL48 tegument protein of pseudorabies virus is critical for intracytoplasmic assembly of infectious virions

The pseudorabies virus (PrV) homolog of the tegument protein encoded by the UL48 gene of herpes simplex virus type 1 (HSV-1) was identified by using a monospecific rabbit antiserum against a bacterial fusion protein. UL48-related polypeptides of 53, 55, and 57 kDa were detected in Western blots of infected cells and purified virions. Immunofluorescence studies demonstrated that the PrV UL48 protein is predominantly localized in the cytoplasm but is also found in the nuclei of infected cells. Moreover, it is a constituent of extracellular virus particles but is absent from primary enveloped perinuclear virions. In noncomplementing cells, a UL48-negative PrV mutant (PrV-DeltaUL48) exhibited delayed growth and significantly reduced plaque sizes and virus titers, deficiencies which were corrected in UL48-expressing cells. RNA analyses indicated that, like its HSV-1 homolog, the PrV UL48 protein is involved in regulation of immediate-early gene expression. However, the most salient effect of the UL48 gene deletion was a severe defect in virion morphogenesis. Late after infection, electron microscopy of cells infected with PrV-DeltaUL48 revealed retention of newly formed nucleocapsids in the cytoplasm, whereas enveloped intracytoplasmic or extracellular complete virions were only rarely observed. In contrast, capsidless particles were produced and released in great amounts. Remarkably, the intracytoplasmic capsids were labeled with antibodies against the UL36 and UL37 tegument proteins, whereas the capsidless particles were labeled with antisera directed against the UL46, UL47, and UL49 tegument proteins. These findings suggested that the UL48 protein is involved in linking capsid and future envelope-associated tegument proteins during virion formation. Thus, like its HSV-1 homolog, the UL48 protein of PrV functions in at least two different steps of the viral life cycle. The drastic inhibition of virion formation in the absence of the PrV UL48 protein indicates that it plays an important role in virion morphogenesis prior to secondary envelopment of intracytoplasmic nucleocapsids. However, the UL48 gene of PrV is not absolutely essential, and concomitant deletion of the adjacent tegument protein gene UL49 also did not abolish virus replication in cell culture.

Characterization of Marek's disease virus serotype 1 (MDV-1) deletion mutants that lack UL46 to UL49 genes: MDV-1 UL49, encoding VP22, is indispensable for virus growth

Experiments were conducted to investigate the roles of Marek's disease virus serotype 1 (MDV-1) major tegument proteins VP11/12, VP13/14, VP16, and VP22 in viral growth in cultured cells. Based on a bacterial artificial chromosome clone of MDV-1 (BAC20), mutant viruses were constructed in which the MDV-1 homologs of UL46, UL47, UL48, or UL49 were deleted alone and in various combinations. It could be demonstrated that the UL46, UL47, and UL48 genes are dispensable for MDV-1 growth in chicken embryonic skin and quail muscle QM7 cells, although the generated virus mutants exhibited reduced plaque sizes in all cell types investigated. In contrast, a UL49-negative MDV-1 (20 Delta 49) and a UL48-UL49 (20 Delta 48-49) doubly negative mutant were not able to produce MDV-1-specific plaques on either cell type. It was confirmed that this growth restriction is dependent on the absence of VP22 expression, because growth of these mutant viruses could be partially restored on cells that were cotransfected with a UL49 expression plasmid. In addition, we were able to demonstrate that cell-to-cell spread of MDV-1 conferred by VP22 is dependent on the expression of amino acids 37 to 187 of MDV-1 VP22, because expression plasmids containing MDV-1 UL49 mutant genes with deletions of amino acids 1 to 37 or 188 to 250 were still able to restore partial growth of the 20 Delta 49 and 20 Delta 48-49 viruses. These results demonstrate for the first time that an alphaherpesvirus UL49-homologous gene is essential for virus growth in cell culture.

The pseudorabies virus VP22 homologue (UL49) is dispensable for virus growth in vitro and has no effect on virulence and neuronal spread in rodents

The tegument of herpesvirus virions is a distinctive structure whose assembly and function are not well understood. The herpes simplex virus type 1 VP22 tegument protein encoded by the UL49 gene is conserved among the alphaherpesviruses. Using cell biology and viral genetics, we provide an initial characterization of the pseudorabies virus (PRV) VP22 homologue. We identified three isoforms of VP22 present in PRV-infected cells that can be resolved by polyacrylamide gel electrophoresis. The predominant form is not phosphorylated and is present in virions, while the other two species are phosphorylated and excluded from virions. VP22 localized to the nucleus by 6 h postinfection, as determined by immunofluorescence and cell fractionation. VP22 immunofluorescence in the nucleus was both diffuse and in punctate structures. The punctate nuclear localization was the most pronounced form of staining and did not localize exclusively to sites of viral DNA replication. Unexpectedly, a VP22 null mutant had no obvious phenotypes during tissue culture infections and was similar to the wild type in all respects. Moreover, the VP22 null mutant was as virulent and neuroinvasive as the wild-type virus after infection of the rodent eye and spread to the brain using both anterograde and retrograde neuronal circuits.

Molecular characterizations of the equine herpesvirus 1 ETIF promoter region and translation initiation site

The equine herpesvirus 1 (EHV-1) homolog of the herpes simplex virus type 1 (HSV-1) tegument phosphoprotein, alphaTIF (Vmw65; VP16), was identified previously as the product of open reading frame 12 (ORF12), was shown to trans-activate immediate-early (IE) gene promoters, and was described as a 60-kDa virion component designated ETIF. However, the ETIF promoter region and transcription initiation site were not identified. The poly(A) signal of the gene 11 (UL49 homolog) lies just upstream of the first ETIF translation initiation codon, indicating that the first ATG may not be used for initiating ETIF translation. Another in-frame translation initiation codon (ATG2) is located 88 bp downstream of the first ETIF initiation codon (ATG1). Western blot analysis showed that the expressed ETIF protein migrated in SDS-PAGE with an apparent molecular mass of approximately 56 kDa, the same molecular weight identified in SDS-PAGE analysis of the KyD EHV-1 virion preparations. The ETIF expression vector pCETIF, which contains ATG2, trans-activated the IE promoter more efficiently than the pC12 containing both ATG1 and ATG2. S1 nuclease analyses mapped the 5' initiation site of the 1.4-kb transcript approximately 17 to 21 nt downstream of the ATG1. The nucleotide sequence upstream of the ATG1 did not have any promoter activity, while the nucleotide sequence upstream of the ATG2 had promoter activity. In transient transfection assays, the pETIFM2 vector, which was mutated in the ATG2, did not trans-activate the IE promoter; however, the pETIFM1 vector, which was mutated in the ATG1, trans-activated the IE promoter. These results demonstrated that the ATG2 of the ETIF ORF is the ETIF translation initiation codon. ETIF trans-activated only the IE promoter, not early (EICP0, EICP22, EICP27, and thymidine kinase) or late (IR5) promoters, confirming that EICP0, EICP22, and EICP27 are early genes.

Mutagenesis of the varicella-zoster virus genome: lessons learned

The varicella-zoster virus (VZV) genome encodes at least 70 genes. We have developed a cosmid based system to inactivate individual viral genes or to insert foreign genes into the genome. We have shown that many VZV genes are not required for replication of the virus in cell culture. Several of these genes, including VZV ORF61, ORF47, and ORF10, have unexpected phenotypes in cell culture and differ from their homologs in the better studied herpes simplex virus (HSV). We have also used the Oka strain of VZV as a live virus vaccine vector. Guinea pigs vaccinated with recombinant VZV expressing HSV-2 glycoprotein D and challenged with HSV-2 have reduced severity of primary genital herpes and reduced mortality compared to animals receiving parental VZV. Recently we have inserted the human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) glycoprotein 160 genes into the Oka strain of VZV and have shown that these proteins are expressed in recombinant virus-infected cells. Thus, directed mutagenesis of the VZV genome is providing new insights into viral pathogenesis and may provide new candidate vaccines.

Essential role played by the C-terminal domain of glycoprotein I in envelopment of varicella-zoster virus in the trans-Golgi network: interactions of glycoproteins with tegument

Varicella-zoster virus (VZV) is enveloped in the trans-Golgi network (TGN). Here we report that glycoprotein I (gI) is required within the TGN for VZV envelopment. Enveloping membranous TGN cisternae were microscopically identified in cells infected with intact VZV. These sacs curved around, and ultimately enclosed, nucleocapsids. Tegument coated the concave face of these sacs, which formed the viral envelope, but the convex surface was tegument-free. TGN cisternae of cells infected with VZV mutants lacking gI (gI(Delta)) or its C (gI(DeltaC))- or N-terminal (gI(DeltaN))-terminal domains were uniformly tegument coated and adhered to one another, forming bizarre membranous stacks. Viral envelopment was compromised, and no virions were delivered to post-Golgi structures. The TGN was not gI-immunoreactive in cells infected with the gI(Delta) or gI(DeltaN) mutants, but it was in cells infected with gI(DeltaC) (because the ectodomains of gI and gE interact). The presence in the TGN of gI lacking a C-terminal domain, therefore, was not sufficient to maintain enveloping cisternae. In cells infected with intact VZV or with gI(Delta), gI(DeltaN), or gI(DeltaC) mutants, ORF10p immunoreactivity was concentrated on the cytosolic face of TGN membranes, suggesting that it interacts with the cytosolic domains of glycoproteins. Because of the gE-gI interaction, cotransfected cells that expressed gE or gI were able to target truncated forms of the other to the TGN. Our data suggest that the C-terminal domain of gI is required to segregate viral and cellular proteins in enveloping TGN cisternae.

Evidence that herpes simplex virus VP16 is required for viral egress downstream of the initial envelopment event

During infection with herpes simplex virus type 1 (HSV-1), VP16 serves multiple functions, including transcriptional activation of viral immediate early genes and downregulation of the virion host shutoff protein vhs. Furthermore, VP16 has been shown to be involved in some aspect of virus assembly and/or maturation. Experiments with a VP16 null virus, 8MA, suggested that VP16 plays a direct role in virion assembly, since removal of VP16 from the HSV-1 genome results in reduced levels of encapsidated DNA and a failure to produce extracellular enveloped particles. However, VP16 null mutants display a severe translational arrest due to unrestrained vhs activity, thus complicating interpretation of these data. We examine here the role of VP16 in virion assembly and egress in the context of a vhs null background, using the virus 8MA/DeltaSma (VP16(-) vhs(-)). Comparison of 8MA and 8MA/DeltaSma with respect to viral DNA accumulation and encapsidation and accumulation of the major capsid protein, VP5, revealed that the 8MA lethal phenotype is only partially due to uncontrolled vhs activity, indicating that VP16 is required in HSV-1 virion formation. Electron microscopy confirmed these results and further showed that VP16 is required for HSV-1 egress beyond the perinuclear space. In addition, we describe the isolation and characterization of an 8MA derivative capable of propagation on Vero cells, due to second site mutations in the vhs and UL53 (gK) genes. Taken together, these results show that VP16 is required for viral egress downstream of the initial envelopment step and further underscore the importance of VP16 in controlling vhs activity within an infected cell.

Varicella-zoster virus immune evasion

CD4+ and CD8+ T cells play dual roles in varicella-zoster virus (VZV) pathogenesis. The first role is to deliver the virus to cutaneous sites during primary VZV infection, permitting replication at these sites and the successful transmission of the virus to other susceptible individuals. The second contribution of T cells is to provide the critical antigen-specific adaptive immunity needed to stop viral replication and maintain VZV latency in sensory ganglia. The equilibrium between VZV and the host can be predicted to be served by immune evasion mechanisms in at least two important ways, including the facilitation of cell-associated viremia during primary VZV infection and silent persistence in dorsal root ganglia. Interference with antigen presentation by MHC class I downregulation may be expected to play a role in both circumstances. Transient interference with MHC class II expression in varicella skin lesions should facilitate local replication and transmission. In addition, when VZV reactivates, the capacity of viral gene products to block the upregulation of MHC class II expression triggered by interferon-gamma should permit a sufficient period of viral replication to cause the lesions of herpes zoster, despite the presence of VZV-specific T cells, and to allow transmission of the virus to susceptible individuals. Although the effort is at an early stage compared to studies of other viral pathogens, identifying the VZV gene products that exert these effects and their mechanisms of interference has the potential to reveal novel aspects of MHC class I and class II antigen processing and presentation.

Structural and antigenic identification of the ORF12 protein (alpha TIF) of equine herpesvirus 1

The equine herpesvirus 1 (EHV-1) homolog of the herpes simplex virus type 1 (HSV-1) tegument phosphoprotein, alpha TIF (Vmw65; VP16), was identified previously as the product of open reading frame 12 (ORF12) and shown to transactivate immediate early (IE) gene promoters. However, a specific virion protein corresponding to the ORF12 product has not been identified definitively. In the present study the ORF12 protein, designated ETIF, was identified as a 60-kDa virion component on the basis of protein fingerprint analyses in which the limited proteolysis profiles of the major 60-kDa in vitro transcription/ translation product of an ORF12 expression vector (pT7-12) were compared to those of purified virion proteins of similar size. ETIF was localized to the viral tegument in Western blot assays of EHV-1 virions and subvirion fractions using polyclonal antiserum and monoclonal antibodies generated against a glutathione-S-transferase-ETIF fusion protein. Northern and Western blot analyses of EHV-1-infected cell lysates prepared under various metabolic blocks indicated that ORF12 is expressed as a late gene, and cross reaction of polyclonal anti-GST-ETIF with a 63.5-kDa HSV-1 protein species suggested that ETIF and HSV-1 alpha TIF are antigenically related. Last, DNA band shift assays used to assess ETIF-specific complex formation indicated that ETIF participates in an infected cell protein complex with the EHV-1 IE promoter TAATGARAT motif.

Lessons to be learned from varicella-zoster virus

Varicella-zoster virus (VZV) is an alphaherpesvirus responsible for two human diseases: chicken pox and shingles. The virus has a respiratory port of entry. After two successive viremias, it reaches the skin where it causes typical lesions. There, it penetrates the peripheral nervous system and it remains latent in dorsal root ganglia. It is still debatable whether VZV persists in neurons or in satellite cells. During latency, VZV expresses a limited set of transcripts of its immediate early (IE) and early (E) genes but no protein has been detected. Mechanisms of reactivation from ganglia have not been identified. However, dysfunction of the cellular immune system appears to be involved in this process. The cell-associated nature of VZV has made it difficult to identify a temporal order of gene expression, but there appears to be a cascade mechanism as for HSV-1. The lack of high titre cell-free virions or recombination mutants has hindered so far the understanding of VZV gene functions. Five genes, ORFs 4, 10, 61, 62, and 63 that encode regulatory proteins could be involved in VZV latency. ORF4p activates gene promoters with basal activities. ORF10p seems to activate the ORF 62 promoter. ORF61p has trans-activating and trans-repressing activities. The major IE protein ORF62p, a virion component, has DNA-binding and regulatory functions, transactivates many VZV promoters and even regulates its own expression. ORF63p is a nuclear IE protein of yet unclear regulatory functions, abundantly expressed very early in infection. We have established an animal model of VZV latency in the rat nervous system, enabling us to study the expression of viral mRNA and protein expression during latency, and yielding results similar to those found in humans. This model is beginning to shed light on the molecular events in VZV persistent infection and on the regulatory mechanisms that maintain the virus in a latent stage in nerve cells.

Live attenuated varicella vaccine

Varicella-zoster virus (VZV) is a ubiquitous human pathogen that causes varicella, commonly called chicken pox; establishes latency; and reactivates as herpes zoster, referred to as shingles. A live attenuated varicella vaccine, derived from the Oka strain of VZV has clinical efficacy for the prevention of varicella. The vaccine induces persistent immunity to VZV in healthy children and adults. Immunization against VZV also has the potential to lower the risk of reactivation of latent virus. The varicella vaccine may eventually reduce or eliminate herpes zoster, which is a serious problem for elderly and immunocompromised individuals.