A virulent bioluminescent and fluorescent dual-reporter Marek's disease virus unveils an alternative spreading pathway in addition to cell-to-cell contact

Senecavirus A Entry Into Host Cells Is Dependent on the Cholesterol-Mediated Endocytic Pathway

Senecavirus A (SVA), an important member of the Picornaviridae family, causes vesicular disease in pigs. Here, we generated an EGFP-expressing recombinant SVA re-SVA-EGFP, which exhibited similar growth kinetics to its parental virus. The reporter SVA was used to study the role of pig ANTXR1 (pANTXR1) in SVA infection in a porcine alveolar macrophage cell line (PAM-Tang cells). Knockdown of the pANTXR1 significantly reduced SVA infection and replication in PAM-Tang cells, while re-expression of the pANTXR1 promoted the cell susceptibility to SVA infection. The results indicated that pANTXR1 is a crucial receptor mediating SVA infection. Subsequently, the viral endocytosis pathways for SVA entry into pig cells were investigated and the results showed that cholesterol played an essential role in receptor-mediated SVA entry. Together, these results demonstrated that SVA entered into host cells through the pANTXR1-mediated cholesterol pathway. Our findings provide potential targets to develop antiviral drugs for the prevention of SVA infection in the pig population.

Cell-to-Cell Transmission of Turkey Herpesvirus in Chicken Embryo Cells via Tunneling Nanotubes

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus that causes immunosuppression, T cell lymphomas, and neuropathic disease in infected chickens. To protect chickens from MDV infection, an avirulent live vaccine of turkey herpesvirus (HVT) has been successfully used in chickens worldwide. Many vaccine manufacturers have used chicken embryo fibroblast (CEF) cells to produce the HVT vaccine. Generally, it has been suggested that HVT is a highly cell-associated herpesvirus that spread via cell-to-cell contact, but it is unclear how HVT is transmitted from infected cells to uninfected target cells. Here, we show via immunofluorescence analysis that nanotubes containing the actin cytoskeleton and HVT antigens from infected CEF cells were observed to contact neighboring cells. When the infected cells were treated with inhibitors for actin polymerization or depolymerization, the formation and extension of the nanotubes from infected cells were greatly inhibited and the intercellular contact was abolished, leading to a drastic reduction in plaque formation and viral titers of the cell-associated virus. Our data indicate that cell-to-cell contacts via nanotubes composed of actin filaments are essential for efficient viral spreading and replication. This finding might contribute to the further improvement of efficient HVT vaccine production.

The Roles of Envelope Glycoprotein M in the Life Cycle of Some Alphaherpesviruses

The envelope glycoprotein M (gM), a surface virion component conserved among alphaherpesviruses, is a multiple-transmembrane domain-containing glycoprotein with a complex N-linked oligosaccharide. The gM mediates a diverse range of functions during the viral life cycle. In this review, we summarize the biological features of gM, including its characterization and function in some specicial alphaherpesviruses. gM modulates the virus-induced membrane fusion during virus invasion, transports other proteins to the appropriate intracellular membranes for primary and secondary envelopment during virion assembly, and promotes egress of the virus. The gM can interact with various viral and cellular components, and the focus of recent research has also been on interactions related to gM. And we will discuss how gM participates in the life cycle of alphaherpesviruses.

Attenuation of Simian Varicella Virus Infection by Enhanced Green Fluorescent Protein in Rhesus Macaques

Simian varicella virus (SVV), the primate counterpart of varicella-zoster virus, causes varicella (chickenpox), establishes latency in ganglia, and reactivates to produce zoster. We previously demonstrated that a recombinant SVV expressing enhanced green fluorescent protein (rSVV.eGFP) is slightly attenuated both in culture and in infected monkeys. Here, we generated two additional recombinant SVVs to visualize infected cells in vitro and in vivo One harbors eGFP fused to the N terminus of open reading frame 9 (ORF9) (rSVV.eGFP-2a-ORF9), and another harbors eGFP fused to the C terminus of ORF66 (rSVV.eGFP-ORF66). Both recombinant viruses efficiently expressed eGFP in cultured cells. Both recombinant SVV infections in culture were comparable to that of wild-type SVV (SVV.wt). Unlike SVV.wt, eGFP-tagged SVV did not replicate in rhesus cells in culture. Intratracheal (i.t.) or i.t. plus intravenous (i.v.) inoculation of rhesus macaques with these new eGFP-tagged viruses resulted in low viremia without varicella rash, although SVV DNA was abundant in bronchoalveolar lavage (BAL) fluid at 10 days postinoculation (dpi). SVV DNA was also found in trigeminal ganglia of one monkey inoculated with rSVV.eGFP-ORF66. Intriguingly, a humoral response to both SVV and eGFP was observed. In addition, monkeys inoculated with the eGFP-expressing viruses were protected from superinfection with SVV.wt, suggesting that the monkeys had mounted an efficient immune response. Together, our results show that eGFP expression could be responsible for their reduced pathogenesis.IMPORTANCE SVV infection in nonhuman primates has served as an extremely useful animal model to study varicella-zoster virus (VZV) pathogenesis. eGFP-tagged viruses are a great tool to investigate their pathogenesis. We constructed and tested two new recombinant SVVs with eGFP inserted into two different locations in the SVV genome. Both recombinant SVVs showed robust replication in culture but reduced viremia compared to that with SVV.wt during primary infection in rhesus macaques. Our results indicate that conclusions on eGFP-tagged viruses based on in vitro results should be handled with care, since eGFP expression could result in attenuation of the virus.

Marek's disease in chickens: a review with focus on immunology

Marek's disease (MD), caused by Marek's disease virus (MDV), is a commercially important neoplastic disease of poultry which is only controlled by mass vaccination. Importantly, vaccines that can provide sterile immunity and inhibit virus transmission are lacking; such that vaccines are only capable of preventing neuropathy, oncogenic disease and immunosuppression, but are unable to prevent MDV transmission or infection, leading to emergence of increasingly virulent pathotypes. Hence, to address these issues, developing more efficacious vaccines that induce sterile immunity have become one of the important research goals for avian immunologists today. MDV shares very close genomic functional and structural characteristics to most mammalian herpes viruses such as herpes simplex virus (HSV). MD also provides an excellent T cell lymphoma model for gaining insights into other herpesvirus-induced oncogenesis in mammals and birds. For these reasons, we need to develop an in-depth knowledge and understanding of the host-viral interaction and host immunity against MD. Similarly, the underlying genetic variation within different chicken lines has a major impact on the outcome of infection. In this review article, we aim to investigate the pathogenesis of MDV infection, host immunity to MD and discuss areas of research that need to be further explored.

Applications of Replicating-Competent Reporter-Expressing Viruses in Diagnostic and Molecular Virology

Commonly used tests based on wild-type viruses, such as immunostaining, cannot meet the demands for rapid detection of viral replication, high-throughput screening for antivirals, as well as for tracking viral proteins or virus transport in real time. Notably, the development of replicating-competent reporter-expressing viruses (RCREVs) has provided an excellent option to detect directly viral replication without the use of secondary labeling, which represents a significant advance in virology. This article reviews the applications of RCREVs in diagnostic and molecular virology, including rapid neutralization tests, high-throughput screening systems, identification of viral receptors and virus-host interactions, dynamics of viral infections in vitro and in vivo, vaccination approaches and others. However, there remain various challenges associated with RCREVs, including pathogenicity alterations due to the insertion of a reporter gene, instability or loss of the reporter gene expression, or attenuation of reporter signals in vivo. Despite all these limitations, RCREVs have become powerful tools for both basic and applied virology with the development of new technologies for generating RCREVs, the inventions of novel reporters and the better understanding of regulation of viral replication.