A cosmid-based system for inserting mutations and foreign genes into the simian varicella virus genome

Insertion of foreign genes into the simian varicella virus genome by Tn7-mediated site-specific transposition

A Tn7-transposition approach was utilized for site-specific insertion of foreign genes into the genome of simian varicella virus (SVV), the causative agent of simian varicella in nonhuman primates. The severe acute respiratory syndrome coronavirus (SARS-CoV-2) nucleocapsid (N) gene and receptor binding domain (RBD) of the spike gene were inserted into the ORF 14 region of the SVV genome cloned into a bacterial artificial chromosome and then transfected into Vero cells to generate the infectious recombinant SVV (rSVV). The rSVV replicated efficiently in infected Vero cells and expressed the N and RBD antigens as indicated by immunoblot and immunofluorescence assays. Tn7-mediated transposition provides a rapid and efficient method for constructing rSVVs which may be evaluated as live-attenuated vaccines.

A Fosmid-Based System for the Generation of Recombinant Cercopithecine Alphaherpesvirus 2 Encoding Reporter Genes

The transmission of Macacine alphaherpesvirus 1 (McHV-1) from macaques, the natural host, to humans causes encephalitis. In contrast, human infection with Cercopithecine alphaherpesvirus 2 (CeHV-2), a closely related alphaherpesvirus from African vervet monkeys and baboons, has not been reported and it is believed that CeHV-2 is apathogenic in humans. The reasons for the differential neurovirulence of McHV-1 and CeHV-2 have not been explored on a molecular level, in part due to the absence of systems for the production of recombinant viruses. Here, we report the generation of a fosmid-based system for rescue of recombinant CeHV-2. Moreover, we show that, in this system, recombineering can be used to equip CeHV-2 with reporter genes. The recombinant CeHV-2 viruses replicated with the same efficiency as uncloned, wt virus and allowed the identification of cell lines that are highly susceptible to CeHV-2 infection. Collectively, we report a system that allows rescue and genetic modification of CeHV-2 and likely other alphaherpesviruses. This system should aid future analysis of CeHV-2 biology.

Current In Vivo Models of Varicella-Zoster Virus Neurotropism

Varicella-zoster virus (VZV), an exclusively human herpesvirus, causes chickenpox and establishes a latent infection in ganglia, reactivating decades later to produce zoster and associated neurological complications. An understanding of VZV neurotropism in humans has long been hampered by the lack of an adequate animal model. For example, experimental inoculation of VZV in small animals including guinea pigs and cotton rats results in the infection of ganglia but not a rash. The severe combined immune deficient human (SCID-hu) model allows the study of VZV neurotropism for human neural sub-populations. Simian varicella virus (SVV) infection of rhesus macaques (RM) closely resembles both human primary VZV infection and reactivation, with analyses at early times after infection providing valuable information about the extent of viral replication and the host immune responses. Indeed, a critical role for CD4 T-cell immunity during acute SVV infection as well as reactivation has emerged based on studies using RM. Herein we discuss the results of efforts from different groups to establish an animal model of VZV neurotropism.

Varicella Viruses Inhibit Interferon-Stimulated JAK-STAT Signaling through Multiple Mechanisms

Varicella zoster virus (VZV) causes chickenpox in humans and, subsequently, establishes latency in the sensory ganglia from where it reactivates to cause herpes zoster. Infection of rhesus macaques with simian varicella virus (SVV) recapitulates VZV pathogenesis in humans thus representing a suitable animal model for VZV infection. While the type I interferon (IFN) response has been shown to affect VZV replication, the virus employs counter mechanisms to prevent the induction of anti-viral IFN stimulated genes (ISG). Here, we demonstrate that SVV inhibits type I IFN-activated signal transduction via the JAK-STAT pathway. SVV-infected rhesus fibroblasts were refractory to IFN stimulation displaying reduced protein levels of IRF9 and lacking STAT2 phosphorylation. Since previous work implicated involvement of the VZV immediate early gene product ORF63 in preventing ISG-induction we studied the role of SVV ORF63 in generating resistance to IFN treatment. Interestingly, SVV ORF63 did not affect STAT2 phosphorylation but caused IRF9 degradation in a proteasome-dependent manner, suggesting that SVV employs multiple mechanisms to counteract the effect of IFN. Control of SVV ORF63 protein levels via fusion to a dihydrofolate reductase (DHFR)-degradation domain additionally confirmed its requirement for viral replication. Our results also show a prominent reduction of IRF9 and inhibition of STAT2 phosphorylation in VZV-infected cells. In addition, cells expressing VZV ORF63 blocked IFN-stimulation and displayed reduced levels of the IRF9 protein. Taken together, our data suggest that varicella ORF63 prevents ISG-induction both directly via IRF9 degradation and indirectly via transcriptional control of viral proteins that interfere with STAT2 phosphorylation. SVV and VZV thus encode multiple viral gene products that tightly control IFN-induced anti-viral responses.

In this manuscript we demonstrate that the immediate early protein ORF63 encoded by varicella zoster virus (VZV) and simian varicella virus (SVV) interferes with interferon type I-mediated activation of JAK-STAT signaling and thereby inhibits the expression of interferon stimulated genes. ORF63 blocks this pathway by degrading IRF9, which plays a central role in JAK-STAT signaling. In addition, both viruses code for immune evasion mechanisms affecting the JAK-STAT pathway upstream of IRF9, which results in the inhibition of STAT2 phosphorylation. By fusing a degradation domain derived from dihydrofolate reductase (DHFR) to ORF63 we further demonstrate that this protein is essential for SVV growth and gene expression, indicating that ORF63 also affects IFN-signaling indirectly by regulating the expression of other immune evasion genes.

The simian varicella virus ORF A is expressed in infected cells but is non-essential for replication in cell culture

The simian varicella virus (SVV) genome encodes ORF A, a truncated homolog of SVV ORF 4. The SVV ORF A was expressed as a 1.0-kb transcript in SVV-infected Vero cells. The ORF A promoter was active in infected Vero cells and was stimulated by the SVV immediate-early gene ORF 62 product (IE62), a viral transactivator of SVV genes. The SVV ORF A did not transactivate SVV IE, early, or late gene promoters in transfected Vero cells and was unable to augment IE62-mediated transactivation of SVV promoters. A SVV mutant lacking ORF A replicated as efficiently as wild-type SVV in infected Vero cells, indicating that ORF A expression is not essential for in vitro replication.

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.

Simian varicella virus open reading frame 63/70 expression is required for efficient virus replication in culture

Simian varicella virus (SVV) open reading frame (ORF) 63, duplicated in the virus genome as ORF 70, is homologous to varicella zoster virus ORF 63/70. Transfection of bacterial artificial chromosome clones containing the wild-type SVV genome and mutants with stop codons in ORF 70, in both ORFs 63 and 70 and the repaired virus DNA sequences into Vero cells produced a cytopathic effect (CPE). The onset of CPE was much slower with the double-mutant transfectants (10 days vs. 3 days) and plaques were smaller. While SVV ORF 63 is not required for replication in culture, its expression leads to robust virus replication.

Recent advances in cloning herpesviral genomes as infectious bacterial artificial chromosomes

Herpesviruses are common but important pathogens in humans and animals. These viruses have large complex genomes encoding genes with diverse functions in different phases of their life cycle and associated diseases. In the last decade, genomes of herpesviruses cloned as infectious bacterial artificial chromosomes (BACs) have become powerful tools for delineating the functions of viral genes and understanding the pathogenesis of their associated diseases. Here we review the history of herpesviral genetics and recent advances in methods for cloning herpesviral genomes as infectious BACs.

Simian varicella virus: molecular virology

Simian varicella virus (SVV) is a primate herpesvirus that is closely related to varicella-zoster virus (VZV), the causative agent of varicella (chickenpox) and herpes zoster (shingles). Epizootics of simian varicella occur sporadically in facilities housing Old World monkeys. This review summarizes the molecular properties of SVV. The SVV and VZV genomes are similar in size, structure, and gene arrangement. The 124.5 kilobase pair (kbp) SVV genome includes a 104.7 kbp long component covalently linked to a short component, which includes a 4.9 kbp unique short segment flanked by 7.5 kbp inverted repeat sequences. SVV DNA encodes 69 distinct open reading frames, three of which are duplicated within the viral inverted repeats. The viral genome is coordinately expressed, and immediate early (IE), early, and late genes have been characterized. Genetic approaches have been developed to create SVV mutants, which will be used to study the role of SVV genes in viral pathogenesis, latency, and reactivation. In addition, SVV expressing foreign genes are being investigated as potential recombinant varicella vaccines.

The simian varicella virus uracil DNA glycosylase and dUTPase genes are expressed in vivo, but are non-essential for replication in cell culture

Neurotropic herpesviruses express viral deoxyuridine triphosphate nucleotidohydrolase (dUTPase) and uracil DNA glycosylase (UDG) enzymes which may reduce uracil misincorporation into viral DNA, particularly in neurons of infected ganglia. The simian varicella virus (SVV) dUTPase (ORF 8) and UDG (ORF 59) share 37.7% and 53.9% amino acid identity, respectively, with varicella-zoster virus (VZV) homologs. Infectious SVV mutants defective in either dUTPase (SVV-dUTPase(-)) or UDG (SVV-UDG(-)) activity or both (SVV-dUTPase(-)/UDG(-)) were constructed using recA assisted restriction endonuclease cleavage (RARE) and a cosmid recombination system. Loss of viral dUTPase and UDG enzymatic activity was confirmed in CV-1 cells infected with the SVV mutants. The SVV-dUTPase(-), SVV-UDG(-), and SVV-dUTPase(-)/UDG(-) mutants replicated as efficiently as wild-type SVV in cell culture. SVV dUTPase and UDG expression was detected in tissues derived from acutely infected animals, but not in tissues derived from latently infected animals. Further studies will evaluate the pathogenesis of SVV dUTPase and UDG mutants and their potential as varicella vaccines.

Recombinant simian varicella viruses expressing respiratory syncytial virus antigens are immunogenic

Recombinant simian varicella viruses (rSVVs) were engineered to express respiratory syncytial virus (RSV) antigens. The RSV surface glycoprotein G and second matrix protein M2 (22k) genes were cloned into the SVV genome, and recombinant viruses were characterized in vitro and in vivo. rSVVs were also engineered to express the membrane-anchored or secreted forms of the RSV-G protein as well as an RSV G lacking its chemokine mimicry motif (CX3C), which may have different effects on priming the host immune response. The RSV genes were efficiently expressed in rSVV/RSV-infected Vero cells as RSV-G and -M2 transcripts were detected by RT-PCR, and RSV antigens were detected by immunofluorescence and immunoblot assays. The rSVVs replicated efficiently in Vero cell culture. Rhesus macaques immunized with rSVV/RSV-G and rSVV/RSV-M2 vaccines produced antibody responses to SVV and RSV antigens. The results demonstrate that recombinant varicella viruses are suitable vectors for the expression of RSV antigens and may represent a novel vaccine strategy for immunization against both pathogens.

The simian varicella virus genome contains an invertible 665 base pair terminal element that is absent in the varicella zoster virus genome

Simian varicella virus (SVV) causes chickenpox in monkeys, establishes latency and reactivates to produce zoster thus providing a model to study human varicella zoster virus (VZV) infection. Sequence analysis of a recombinant cosmid clone containing the left end of the SVV genome revealed a 665 base pair (bp) segment that is absent in VZV DNA. This segment inverts and contains 507 bp of unique sequences flanked on either side by 79 bp inverted repeats, making the SVV genome to be 124,785 bp in size. Part of the inverted repeat sequence (64 bp) is also present at the junction of the long and short segments of the SVV genome. The terminal DNA sequences are conserved among different SVV isolates and present in tissues from infected monkeys. The terminal region is transcriptionally active and is also present in the genomes of other animal varicelloviruses but absent in the VZV genome.

Recombinant simian varicella viruses induce immune responses to simian immunodeficiency virus (SIV) antigens in immunized vervet monkeys

The varicella-zoster virus (VZV) Oka vaccine offers potential as a recombinant vaccine against other pathogens. In this study, recombinant simian varicella viruses (rSVV) expressing simian immunodeficiency virus (SIV) envelope (env, gp130) and gag antigens were constructed. Expression of the SIV env and gag transcripts and antigens in rSVV-infected Vero cells was confirmed. The rSVV-SIVenv and rSVV-SIVgag viruses replicated as efficiently as wild-type SVV in cell culture. The immunogenicity of rSVV-SIVenv and rSVV-SIVgag was investigated in immunized vervet monkeys. Humoral immune responses to the SIV gp130 and gag antigens were detected as early as 4 weeks after the initial immunization with higher antibody titers following a booster immunization. Cellular immune responses against the SIV gp130 antigen were detected by ELISPOT assay. The rSVV established latent infection in neural ganglia. A subsequent study will evaluate the ability of rSVV vaccines expressing SIV antigens to protect nonhuman primates against simian AIDS.

Simian varicella virus gene 61 encodes a viral transactivator but is non-essential for in vitro replication

Simian varicella virus (SVV) is closely related to varicella-zoster virus (VZV), the causative agent of chickenpox and shingles. The SVV and VZV gene 61 polypeptides are homologs of the HSV-1 ICP0, a viral transactivator which appears to play a role in viral latency and reactivation. In this study, the molecular properties of the SVV 61 were characterized. The SVV open reading frame (ORF) 61 encodes a 54.1-kDa polypeptide with 37% amino acid identity to the VZV 61. Homology to the HSV-1 ICP-0 is limited to a conserved RING finger motif at the amino terminus of the protein. A nuclear localization sequence (nls) at the carboxy-terminus directs the SVV 61 to the cell nucleus, while a SVV 61nls(-) mutant is confined to the cell cytoplasm. The SVV 61 transactivates its own promoter as well as SVV immediate early (IE, ORF 62), early (ORFs 28 and 29), and late (ORF 68) gene promoters in transfected Vero cells. The RING finger and nls motifs are required for efficient SVV 61 transactivation. The SVV 61 has no effect on the ability of the major SVV transactivator (IE62) to induce SVV promoters. Generation and propagation of a SVV gene 61 deletion mutant demonstrated that the SVV 61 is non-essential for in vitro replication. SVV gene 61 is expressed in liver, lung, and neural ganglia of infected monkeys during acute simian varicella.

Simian varicella virus gene 28 and 29 promoters share a common upstream stimulatory factor-binding site and are induced by IE62 transactivation

Simian varicella virus (SVV) is a neurotropic alphaherpesvirus that causes a natural, varicella-like disease in non-human primates. After resolution of the primary disease, SVV, like its human counterpart, varicella-zoster virus (VZV), establishes latent infection in the neural ganglia of the host. In this study, gene expression of SVV open reading frames (ORFs) 28 and 29, which encode the viral DNA polymerase and DNA-binding protein, respectively, was characterized during lytic infection of Vero cells. The results indicate that the intergenic region controlling gene 28 and 29 expression includes overlapping, divergent promoters. The ORF 28 and 29 promoters are active in SVV-infected Vero cells, but not in uninfected cells. The SVV immediate-early gene 62 (IE62) product transactivates ORF 28 and 29 expression, and a cellular upstream stimulatory factor-binding site is important for efficient IE62 induction of genes 28 and 29. DNA sequence analysis of the 185 bp intergenic region identified putative cellular transcription factor-binding sites. Transcriptional analysis mapped ORF 28 and 29 RNA start sites. A recombinant SVV was employed to demonstrate that the ORF 29 promoter can express a heterologous gene (green fluorescent protein) when inserted into a novel site (the ORF 12/13 intergenic region) within the SVV genome. The findings demonstrate similarities between SVV and VZV ORF 28/29 expression and indicate that the simian varicella model may be useful to investigate the differential regulation of viral genes during lytic and latent infection.