Reconstitution of Marek's disease virus serotype 1 (MDV-1) from DNA cloned as a bacterial artificial chromosome and characterization of a glycoprotein B-negative MDV-1 mutant

Impact of viral telomeric repeat sequences on herpesvirus vector vaccine integration and persistence

Marek's disease virus (MDV) vaccines were the first vaccines that protected against cancer. The avirulent turkey herpesvirus (HVT) was widely employed and protected billions of chickens from a deadly MDV infection. It is also among the most common vaccine vectors providing protection against a plethora of pathogens. HVT establishes latency in T-cells, allowing the vaccine virus to persist in the host for life. Intriguingly, the HVT genome contains telomeric repeat arrays (TMRs) at both ends; however, their role in the HVT life cycle remains elusive. We have previously shown that similar TMRs in the MDV genome facilitate its integration into host telomeres, which ensures efficient maintenance of the virus genome during latency and tumorigenesis. In this study, we investigated the role of the TMRs in HVT genome integration, latency, and reactivation in vitro and in vivo. Additionally, we examined HVT infection of feather follicles. We generated an HVT mutant lacking both TMRs (vΔTMR) that efficiently replicated in cell culture. We could demonstrate that wild type HVT integrates at the ends of chromosomes containing the telomeres in T-cells, while integration was severely impaired in the absence of the TMRs. To assess the role of TMRs in vivo, we infected one-day-old chickens with HVT or vΔTMR. vΔTMR loads were significantly reduced in the blood and hardly any virus was transported to the feather follicle epithelium where the virus is commonly shed. Strikingly, latency in the spleen and reactivation of the virus were severely impaired in the absence of the TMRs, indicating that the TMRs are crucial for the establishment of latency and reactivation of HVT. Our findings revealed that the TMRs facilitate integration of the HVT genome into host chromosomes, which ensures efficient persistence in the host, reactivation, and transport of the virus to the skin.

LORF9 of Marek's disease virus is involved in the early cytolytic replication of B lymphocytes and can act as a target for gene deletion vaccine development

IMPORTANCE

Marek's disease virus (MDV) is a highly infectious and oncogenic virus that can induce severe T cell lymphomas in chickens. MDV encodes more than 100 genes, most of which have unknown functions. This work indicated that the LORF9 gene is necessary for MDV early cytolytic replication in B lymphocytes. In addition, we have found that the LORF9 deletion mutant has a comparative immunological protective effect with CVI988/Rispens vaccine strain against very virulent MDV challenge. This is a significant discovery that LORF9 can be exploited as a possible target for the development of an MDV gene deletion vaccine.

Fully Attenuated meq and pp38 Double Gene Deletion Mutant Virus Confers Superior Immunological Protection against Highly Virulent Marek's Disease Virus Infection

Marek's disease virus (MDV) induces immunosuppression and neoplastic disease in chickens. The virus is controllable via an attenuated meq deletion mutant virus, which has the disadvantage of retaining the ability to induce lymphoid organ atrophy. To overcome this deficiency and produce more vaccine candidates, a recombinant MDV was generated from the highly virulent Md5BAC strain, in which both meq and a cytolytic replication-related gene, pp38, were deleted. Replication of the double deletion virus, Md5BAC ΔmeqΔpp38, was comparable with that of the parental virus in vitro. The double deletion virus was shown to be fully attenuated and to reduce lymphoid organ atrophy in vivo. Crucially, Md5BAC ΔmeqΔpp38 confers superior protection against highly virulent virus compared with a commercial vaccine strain, CVI988/Rispens. Transcriptomic profiling indicated that Md5BAC ΔmeqΔpp38 induced a different host immune response from CVI988/Rispens. In summary, a novel, effective, and safe vaccine candidate for prevention and control of MD caused by highly virulent MDV is reported. IMPORTANCE MDV is a highly contagious immunosuppressive and neoplastic pathogen. The virus can be controlled through vaccination via an attenuated meq deletion mutant virus that retains the ability to induce lymphoid organ atrophy. In this study, we overcame the deficiency by generating meq and pp38 double deletion mutant virus. Indeed, the successfully generated meq and pp38 double deletion mutant virus had significantly reduced replication capacity in vivo but not in vitro. It was fully attenuated and conferred superior protection efficacy against very virulent MDV challenge. In addition, the possible immunological protective mechanism of the double deletion mutant virus was shown to be different from that of the gold standard MDV vaccine, CVI988/Rispens. Overall, we successfully generated an attenuated meq deletion mutant virus and widened the range of potential vaccine candidates. Importantly, this study provides for the first time the theoretical basis of vaccination induced by fully attenuated gene-deletion mutant virus.

Cytomegalovirus-vaccine-induced unconventional T cell priming and control of SIV replication is conserved between primate species

Strain 68-1 rhesus cytomegalovirus expressing simian immunodeficiency virus (SIV) antigens (RhCMV/SIV) primes MHC-E-restricted CD8+ T cells that control SIV replication in 50%-60% of the vaccinated rhesus macaques. Whether this unconventional SIV-specific immunity and protection is unique to rhesus macaques or RhCMV or is intrinsic to CMV remains unknown. Here, using cynomolgus CMV vectors expressing SIV antigens (CyCMV/SIV) and Mauritian cynomolgus macaques, we demonstrate that the induction of MHC-E-restricted CD8+ T cells requires matching CMV to its host species. RhCMV does not elicit MHC-E-restricted CD8+ T cells in cynomolgus macaques. However, cynomolgus macaques vaccinated with species-matched 68-1-like CyCMV/SIV mounted MHC-E-restricted CD8+ T cells, and half of the vaccinees stringently controlled SIV post-challenge. Protected animals manifested a vaccine-induced IL-15 transcriptomic signature that is associated with efficacy in rhesus macaques. These findings demonstrate that the ability of species-matched CMV vectors to elicit MHC-E-restricted CD8+ T cells that are required for anti-SIV efficacy is conserved in nonhuman primates, and these data support the development of HCMV/HIV for a prophylactic HIV vaccine.

In vivo imaging reveals novel replication sites of a highly oncogenic avian herpesvirus in chickens

In vivo bioluminescence imaging facilitates the non-invasive visualization of biological processes in living animals. This system has been used to track virus infections mostly in mice and ferrets; however, until now this approach has not been applied to pathogens in avian species. To visualize the infection of an important avian pathogen, we generated Marek’s disease virus (MDV) recombinants expressing firefly luciferase during lytic replication. Upon characterization of the recombinant viruses in vitro, chickens were infected and the infection visualized in live animals over the course of 14 days. The luminescence signal was consistent with the known spatiotemporal kinetics of infection and the life cycle of MDV, and correlated well with the viral load measured by qPCR. Intriguingly, this in vivo bioimaging approach revealed two novel sites of MDV replication, the beak and the skin of the feet covered in scales. Feet skin infection was confirmed using a complementary fluorescence bioimaging approach with MDV recombinants expressing mRFP or GFP. Infection was detected in the intermediate epidermal layers of the feet skin that was also shown to produce infectious virus, regardless of the animals’ age at and the route of infection. Taken together, this study highlights the value of in vivo whole body bioimaging in avian species by identifying previously overlooked sites of replication and shedding of MDV in the chicken host.

In vivo bioluminescence imaging is a powerful tool to track virus infection in the whole body of living animals. This system has been successfully used in mice, ferrets, rats and even fishes, but until now never in birds. In this study, we performed the first in vivo imaging assessing the spread of an important avian pathogen, the highly oncogenic Marek’s disease virus (MDV). Using a recombinant virus expressing firefly luciferase, we visualized the course of MDV infection in chicks for 14 days. The bioluminescent signal was consistent with the known kinetics and sites of dissemination of MDV, notably in feathers. With this new approach, we also discovered two novels sites of early infection and replication that may contribute to persistent virus shedding. Both novel sites represent hard skin appendages like the feathers: the beak and the skin of the feet that are covered in scales. These results were confirmed with two recombinant viruses expressing fluorescent proteins. Fifty-five years after the discovery of MDV and thanks to in vivo imaging, we provide new insights in MDV life cycle in vivo, highlighting the importance of bioluminescence imaging of the entire body in living animals.

The Diverse Major Histocompatibility Complex Haplotypes of a Common Commercial Chicken Line and Their Effect on Marek's Disease Virus Pathogenesis and Tumorigenesis

The major histocompatibility complex (MHC) is crucial for appropriate immune responses against invading pathogens. Chickens possess a single predominantly-expressed class I molecule with strong associations between disease resistance and MHC haplotype. For Marek's disease virus (MDV) infections of chickens, the MHC haplotype is one of the major determinants of genetic resistance and susceptibility. VALO specific pathogen free (SPF) chickens are widely used in biomedical research and vaccine production. While valuable findings originate from MDV infections of VALO SPF chickens, their MHC haplotypes and associated disease resistance remained elusive. In this study, we used several typing systems to show that VALO SPF chickens possess MHC haplotypes that include B9, B9:02, B15, B19 and B21 at various frequencies. Moreover, we associate the MHC haplotypes to MDV-induced disease and lymphoma formation and found that B15 homozygotes had the lowest tumor incidence while B21 homozygotes had the lowest number of organs with tumors. Finally, we found transmission at variable levels to all contact birds except B15/B21 heterozygotes. These data have immediate implications for the use of VALO SPF chickens and eggs in the life sciences and add another piece to the puzzle of the chicken MHC complex and its role in infections with this oncogenic herpesvirus.

Direct cloning of a herpesvirus genome for rapid generation of infectious BAC clones

INTRODUCTION

The herpesviridae are DNA viruses with large and complicated genomes. The herpesvirus bacterial artificial chromosomes (BACs) have been useful for generating recombinant viruses to study the biology and pathogenesis. However, the conventional method using homologous recombination is not only time consuming but also prone to accumulate attenuating mutations during serial passage of the virus in cells. Elimination of the BAC vector from the recombinant viral genome requires additional step for phenotypically consistence with the original strain.

OBJECTIVES

To generate a streamlined approach for generating infectious BAC clones of herpesvirus.

METHODS

The 142-kb pseudorabies virus genome was directly cloned into a bacterial artificial chromosome (BAC) in Escherichia coli by Exonuclease Combined with RecET recombination (ExoCET). Placement of the BAC vector at the terminus of the linear virus genome enabled excision of the BAC backbone from the viral genome by restriction endonuclease for delivery into mammalian cells, with the subsequent rapid rescue of virus that was genetically identical to the original strain.

RESULTS

This new approach for molecular cloning of the genome from a large DNA virus and isolation of pure virus lacking the BAC vector from transfected mammalian cells bypass the tedious and time-consuming method of multiple rounds of plaque purification. The viral BAC was stable in E. coli, allowing further mutagenesis mediated by the Red system or various site-specific recombination methods.

CONCLUSION

An efficient method for construction of infectious clones of herpesvirus was established. It is expected to be potentially useful for other viruses with large double-stranded DNA genomes.

Effect of Insertion and Deletion in the Meq Protein Encoded by Highly Oncogenic Marek’s Disease Virus on Transactivation Activity and Virulence

Marek’s disease virus (MDV) causes malignant lymphoma in chickens (Marek’s disease, MD). Although MD is currently controlled by vaccination, MDV strains have continuously increased in virulence over the recent decades. Polymorphisms in Meq, an MDV-encoded oncoprotein that serves as a transcription factor, have been associated with the enhanced virulence of the virus. In addition, insertions and deletions in Meq have been observed in MDV strains of higher virulence, but their contribution to said virulence remains elusive. In this study, we investigated the contribution of an insertion (L-Meq) and a deletion in the Meq gene (S-Meq) to its functions and MDV pathogenicity. Reporter assays revealed that both insertion and deletion enhanced the transactivation potential of Meq. Additionally, we generated RB-1B-based recombinant MDVs (rMDVs) encoding each Meq isoform and analyzed their pathogenic potential. rMDV encoding L-Meq indueced the highest mortality and tumor incidence in infected animals, whereas the rMDV encoding S-Meq exhibited the lowest pathogenicity. Thus, insertion enhanced the transactivation activity of Meq and MDV pathogenicity, whereas deletion reduced pathogenicity despite having increased transactivation activity. These data suggest that other functions of Meq affect MDV virulence. These data improve our understanding of the mechanisms underlying the evolution of MDV virulence.

Visualization of Marek’s Disease Virus Genomes in Living Cells during Lytic Replication and Latency

Visualization of the herpesvirus genomes during lytic replication and latency is mainly achieved by fluorescence in situ hybridization (FISH). Unfortunately, this technique cannot be used for the real-time detection of viral genome in living cells. To facilitate the visualization of the Marek’s disease virus (MDV) genome during all stages of the virus lifecycle, we took advantage of the well-established tetracycline operator/repressor (TetO/TetR) system. This system consists of a fluorescently labeled TetR (TetR-GFP) that specifically binds to an array of tetO sequences. This tetO repeat array was first inserted into the MDV genome (vTetO). Subsequently, we fused TetR-GFP via a P2a self-cleaving peptide to the C-terminus of the viral interleukin 8 (vIL8), which is expressed during lytic replication and latency. Upon reconstitution of this vTetO-TetR virus, fluorescently labeled replication compartments were detected in the nucleus during lytic replication. After validating the specificity of the observed signal, we used the system to visualize the genesis and mobility of the viral replication compartments. In addition, we assessed the infection of nuclei in syncytia as well as lytic replication and latency in T cells. Taken together, we established a system allowing us to track the MDV genome in living cells that can be applied to many other DNA viruses.

The LORF5 Gene Is Non-essential for Replication but Important for Duck Plague Virus Cell-to-Cell Spread Efficiently in Host Cells

Duck plague virus (DPV) can cause high morbidity and mortality in many waterfowl species within the order Anseriformes. The DPV genome contains 78 open reading frames (ORFs), among which the LORF2, LORF3, LORF4, LORF5, and SORF3 genes are unique genes of avian herpesvirus. In this study, to investigate the role of this unique LORF5 gene in DPV proliferation, we generated a recombinant virus that lacks the LORF5 gene by a two-step red recombination system, which cloned the DPV Chinese virulent strain (DPV CHv) genome into a bacterial artificial chromosome (DPV CHv-BAC); the proliferation law of LORF5-deleted mutant virus on DEF cells and the effect of LORF5 gene on the life cycle stages of DPV compared with the parent strain were tested. Our data revealed that the LORF5 gene contributes to the cell-to-cell transmission of DPV but is not relevant to virus invasion, replication, assembly, and release formation. Taken together, this study sheds light on the role of the avian herpesvirus-specific gene LORF5 in the DPV proliferation life cycle. These findings lay the foundation for in-depth functional studies of the LORF5 gene in DPV or other avian herpesviruses.

A Cell Culture System to Investigate Marek's Disease Virus Integration into Host Chromosomes

Marek’s disease virus (MDV) is a highly oncogenic alphaherpesvirus that causes a devastating neoplastic disease in chickens. MDV has been shown to integrate its genome into the telomeres of latently infected and tumor cells, which is crucial for efficient tumor formation. Telomeric repeat arrays present at the ends of the MDV genome facilitate this integration into host telomeres; however, the integration mechanism remains poorly understood. Until now, MDV integration could only be investigated qualitatively upon infection of chickens. To shed further light on the integration mechanism, we established a quantitative integration assay using chicken T cell lines, the target cells for MDV latency and transformation. We optimized the infection conditions and assessed the establishment of latency in these T cells. The MDV genome was efficiently maintained over time, and integration was confirmed in these cells by fluorescence in situ hybridization (FISH). To assess the role of the two distinct viral telomeric repeat arrays in the integration process, we tested various knockout mutants in our in vitro integration assay. Efficient genome maintenance and integration was thereby dependent on the presence of the telomeric repeat arrays in the virus genome. Taken together, we developed and validated a novel in vitro integration assay that will shed light on the integration mechanism of this highly oncogenic virus into host telomeres.

Marek's disease virus encoded miR-M6 and miR-M10 are dispensable for virus replication and pathogenesis in chickens

MicroRNAs (miRNAs) are a class of approximately 22 nucleotides long non-coding RNAs, and virus-encoded miRNAs play an important role in pathogenesis. Marek's disease virus (MDV) is an oncogenic avian alphaherpesvirus that causes immunosuppression and tumors in its natural host, chicken. In the MDV genome, 14 miRNA precursors and 26 mature miRNAs were identified, thus MDV has been used as a model to study the function of viral miRNAs in vivo. Recently, a cluster of miRNAs encoded by MDV, Cluster 3 miRNAs (miR-M8-M10), has been shown to restrict early cytolytic replication and pathogenesis of MDV. In this study, we further analyzed the role of miR-M6 and miR-M10, members of cluster miR-M8-M10, in MDV replication and pathogenicity. We found that, compared to parental MDV, deletion of miR-M6-5p significantly enhanced the replication of MDV in cell culture, but not in chickens. The replication of miR-M6-5p deletion MDV was restored once the deleted sequences were re-inserted. Our results also showed that deletion of miR-M10-5p did not affect the replication of MDV in vitro and in vivo. In addition, our animal study results showed that deletion of miR-M6-5p or miR-M10-5p did not alter the pathogenesis of MDV. In conclusion, our study shows that both miR-M6 and miR-M10 are dispensable for MDV replication and pathogenesis in chickens, while also suggests a repressive role of miR-M6 in MDV replication in cell culture.

Characterization of a Novel Viral Interleukin 8 (vIL-8) Splice Variant Encoded by Marek's Disease Virus

Marek’s disease virus (MDV) is a highly cell-associated oncogenic alphaherpesvirus that causes lymphomas in various organs in chickens. Like other herpesviruses, MDV has a large and complex double-stranded DNA genome. A number of viral transcripts are generated by alternative splicing, a process that drastically extends the coding capacity of the MDV genome. One of the spliced genes encoded by MDV is the viral interleukin 8 (vIL-8), a CXC chemokine that facilitates the recruitment of MDV target cells and thereby plays an important role in MDV pathogenesis and tumorigenesis. We recently identified a novel vIL-8 exon (vIL-8-E3′) by RNA-seq; however, it remained elusive whether the protein containing the vIL-8-E3′ is expressed and what role it may play in MDV replication and/or pathogenesis. To address these questions, we first generated recombinant MDV harboring a tag that allows identification of the spliced vIL-8-E3′ protein, revealing that it is indeed expressed. We subsequently generated knockout viruses and could demonstrate that the vIL-8-E3′ protein is dispensable for MDV replication as well as secretion of the functional vIL-8 chemokine. Finally, infection of chickens with this vIL-8-E3′ knockout virus revealed that the protein is not important for MDV replication and pathogenesis in vivo. Taken together, our study provides novel insights into the splice forms of the CXC chemokine of this highly oncogenic alphaherpesvirus.

Methods for the Manipulation of Herpesvirus Genome and the Application to Marek's Disease Virus Research

Herpesviruses are a group of double-strand DNA viruses that infect a wide range of hosts, including humans and animals. In the past decades, numerous methods have been developed to manipulate herpesviruses genomes, from the introduction of random mutations to specific genome editing. The development of genome manipulation methods has largely advanced the study of viral genes function, contributing not only to the understanding of herpesvirus biology and pathogenesis, but also the generation of novel vaccines and therapies to control and treat diseases. In this review, we summarize the major methods of herpesvirus genome manipulation with emphasis in their application to Marek’s disease virus research.

Comprehensive profiling analysis of the N6-methyladenosine-modified circular RNA transcriptome in cultured cells infected with Marek's disease virus

Marek's disease virus (MDV) induces severe immunosuppression and lymphomagenesis in the chicken, its natural host, and results in a condition that investigated the pathogenesis of MDV and have begun to focus on the expression profiling of circular RNAs (circRNAs). However, little is known about how the expression of circRNAs is referred to as Marek's disease. Previous reports have is regulated during MDV replication. Here, we carried out a comprehensive profiling analysis of N6-methyladenosine (m⁶A) modification on the circRNA transcriptome in infected and uninfected chicken embryonic fibroblast (CEF) cells. Methylated RNA immunoprecipitation sequencing (MeRIP-Seq) revealed that m⁶A modification was highly conserved in circRNAs. Comparing to the uninfected group, the number of peaks and conserved motifs were not significantly different in cells that were infected with MDV, although reduced abundance of circRNA m⁶A modifications. However, gene ontology and Kyoto encyclopedia of genes and genomes (KEGG) pathway analyses revealed that the insulin signaling pathway was associated with the regulation of m⁶A modified circRNAs in MDV infection. This is the first report to describe alterations in the transcriptome-wide profiling of m⁶A modified circRNAs in MDV-infected CEF cells.

Latest Insights into Unique Open Reading Frames Encoded by Unique Long (UL) and Short (US) Regions of Marek's Disease Virus

Marek’s disease virus (MDV) is an oncogenic avian alphaherpesvirus whose genome consists of unique long (UL) and short (US) regions that are flanked by inverted repeat regions. More than 100 open reading frames (ORFs) have been annotated in the MDV genome, and are involved in various aspects of MDV biology and pathogenesis. Within UL and US regions of MDV, there are several unique ORFs, some of which have recently been shown to be important for MDV replication and pathogenesis. In this review, we will summarize the current knowledge on these ORFs and compare their location in different MDV strains.

A Novel Effective and Safe Vaccine for Prevention of Marek's Disease Caused by Infection with a Very Virulent Plus (vv+) Marek's Disease Virus

Marek’s disease virus (MDV) is a highly contagious alphaherpesvirus that causes rapid onset lymphoma in chickens. Marek’s disease (MD) is effectively controlled using vaccination; however, MDV continues to break through vaccinal immunity, due to the emergence of highly virulent field strains. Earlier studies revealed that deletion of the meq gene from MDV resulted in an attenuated virus that protects against MD in chickens challenged with highly virulent field strains. However, the meq deleted virus retains the ability to induce significant lymphoid organ atrophy. In a different study, we found that the deletion of the vIL8 gene resulted in the loss of lymphoid organ atrophy in inoculated chickens. Here, we describe the generation of a recombinant MDV from which both meq and vIL8 genes were deleted. In vitro studies revealed that the meq and vIL8 double deletion virus replicated at levels similar to the parental very virulent plus (vv+) virus. In addition, in vivo studies showed that the double deletion mutant virus (686BAC-ΔMeqΔvIL8) conferred protection comparable to CVI988, a commercial vaccine strain, when challenged with a vv+ MDV virus, and significantly reduced lymphoid organ atrophy, when compared to meq null virus, in chickens. In conclusion, our study describes the development of a safe and effective vaccine candidate for prevention of MD in chickens.

Marek's Disease Virus Requires Both Copies of the Inverted Repeat Regions for Efficient In Vivo Replication and Pathogenesis

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus of chickens. The MDV genome consists of two unique regions that are both flanked by inverted repeat regions. These repeats harbor several genes involved in virus replication and pathogenesis, but it remains unclear why MDV and other herpesviruses harbor these large sequence duplications. In this study, we set to determine if both copies of these repeat regions are required for MDV replication and pathogenesis. Our results demonstrate that MDV mutants lacking the entire internal repeat region (ΔIR_LS) efficiently replicate and spread from cell-to-cell in vitro However, ΔIR_LS replication was severely impaired in infected chickens and the virus caused significantly less frequent disease and tumors compared to the controls. In addition, we also generated recombinant viruses that harbor a deletion of most of the internal repeat region, leaving only short terminal sequences behind (ΔIR_LS-HR). These remaining homologous sequences facilitated rapid restoration of the deleted repeat region, resulting in a virus that caused disease and tumors comparable to the wild type. Therefore, ΔIR_LS-HR represents an excellent platform for rapid genetic manipulation of the virus genome in the repeat regions. Taken together, our study demonstrates that MDV requires both copies of the repeats for efficient replication and pathogenesis in its natural host.IMPORTANCE Marek's disease virus (MDV) is a highly oncogenic alphaherpesvirus that infects chickens and causes losses in the poultry industry of up to $2 billion per year. The virus is also widely used as a model to study alphaherpesvirus pathogenesis and virus-induced tumor development in a natural host. MDV and most other herpesviruses harbor direct or inverted repeats regions in their genome. However, the role of these sequence duplications in MDV remains elusive and has never been investigated in a natural virus-host model for any herpesvirus. Here, we demonstrate that both copies of the repeats are needed for efficient MDV replication and pathogenesis in vivo, while replication was not affected in cell culture. With this, we further dissect herpesvirus genome biology and the role of repeat regions in Marek's disease virus replication and pathogenesis.

Combinatorial Drug Treatments Reveal Promising Anticytomegaloviral Profiles for Clinically Relevant Pharmaceutical Kinase Inhibitors (PKIs)

Human cytomegalovirus (HCMV) is a human pathogenic herpesvirus associated with a variety of clinical symptoms. Current antiviral therapy is not always effective, so that improved drug classes and drug-targeting strategies are needed. Particularly host-directed antivirals, including pharmaceutical kinase inhibitors (PKIs), may help to overcome problems of drug resistance. Here, we focused on utilizing a selection of clinically relevant PKIs and determined their anticytomegaloviral efficacies. Particularly, PKIs directed to host or viral cyclin-dependent kinases, i.e., abemaciclib, LDC4297 and maribavir, exerted promising profiles against human and murine cytomegaloviruses. The anti-HCMV in vitro activity of the approved anti-cancer drug abemaciclib was confirmed in vivo using our luciferase-based murine cytomegalovirus (MCMV) animal model in immunocompetent mice. To assess drug combinations, we applied the Bliss independence checkerboard and Loewe additivity fixed-dose assays in parallel. Results revealed that (i) both affirmative approaches provided valuable information on anti-CMV drug efficacies and interactions, (ii) the analyzed combinations comprised additive, synergistic or antagonistic drug interactions consistent with the drugs’ antiviral mode-of-action, (iii) the selected PKIs, especially LDC4297, showed promising inhibitory profiles, not only against HCMV but also other α-, β- and γ-herpesviruses, and specifically, (iv) the combination treatment with LDC4297 and maribavir revealed a strong synergism against HCMV, which might open doors towards novel clinical options in the near future. Taken together, this study highlights the potential of therapeutic drug combinations of current developmental/preclinical PKIs.

Distinct polymorphisms in a single herpesvirus gene are capable of enhancing virulence and mediating vaccinal resistance

Modified-live herpesvirus vaccines are widely used in humans and animals, but field strains can emerge that have a higher virulence and break vaccinal protection. Since the introduction of the first vaccine in the 1970s, Marek’s disease virus overcame the vaccine barrier by the acquisition of numerous genomic mutations. However, the evolutionary adaptations in the herpesvirus genome responsible for the vaccine breaks have remained elusive. Here, we demonstrate that point mutations in the multifunctional meq gene acquired during evolution can significantly alter virulence. Defined mutations found in highly virulent strains also allowed the virus to overcome innate cellular responses and vaccinal protection. Concomitantly, the adaptations in meq enhanced virus shedding into the environment, likely providing a selective advantage for the virus. Our study provides the first experimental evidence that few point mutations in a single herpesviral gene result in drastically increased virulence, enhanced shedding, and escape from vaccinal protection.

Viruses can acquire mutations during evolution that alter their virulence. An example of a virus that has shown repeated shifts to higher virulence in response to more efficacious vaccines is the oncogenic Marek’s disease virus (MDV) that infects chickens. Until now, it remained unknown which mutations in the large virus genome are responsible for this increase in virulence. We could demonstrate that very few amino acid changes in the meq oncogene of MDV can significantly alter the virulence of the virus. In addition, these changes also allow the virus to overcome vaccinal protection and enhance the shedding into the environment. Taken together, our data provide fundamental insights into evolutionary changes that allow this deadly veterinary pathogen to evolve towards greater virulence.

Abrogation of Marek's disease virus replication using CRISPR/Cas9

Marek's disease virus (MDV) is a highly cell-associated alphaherpesvirus that causes deadly lymphomas in chickens. While vaccination protects against clinical symptoms, MDV field strains can still circulate in vaccinated flocks and continuously evolve towards greater virulence. MDV vaccines do not provide sterilizing immunity, allowing the virus to overcome vaccine protection, and has increased the need for more potent vaccines or alternative interventions. In this study, we addressed if the CRISPR/Cas9 system can protect cells from MDV replication. We first screened a number of guide RNAs (gRNAs) targeting essential MDV genes for their ability to prevent virus replication. Single gRNAs significantly inhibited virus replication, but could result in the emergence of escape mutants. Strikingly, combining two or more gRNAs completely abrogated virus replication and no escape mutants were observed upon serial passaging. Our study provides the first proof-of-concept, demonstrating that the CRISPR/Cas9 system can be efficiently used to block MDV replication. The presented findings lay the foundation for future research to completely protect chickens from this deadly pathogen.

Transcriptional Analyses of Innate and Acquired Immune Patterning Elicited by Marek's Disease Virus Vaccine Strains: Turkey Herpesvirus (HVT), Marek's Disease Virus 2 (strain SB1), and Bivalent Vaccines (HVT/SB1 and HVT-LT/SB1)

Marek's disease (MD) is a complex pathology of chickens caused by MD virus (MDV) 1 and is observed as paralysis, immune suppression, neurologic signs, and the rapid formation of T-cell lymphomas. The incidence of MD in commercial broilers is largely controlled via vaccination, either in ovo or at hatch with live attenuated vaccines, i.e., turkey herpesvirus (HVT) or a bivalent combination of HVT with the MDV 2 strain (SB1). To further extend the protection conferred by bivalent HVT/SB-1, recombinant HVTs encoding transgenes of other avian viruses have similarly been used for in ovo administration. Despite decades of use, the specific mechanisms associated with vaccine-induced protection remain obscure. Additionally, the mechanistic basis for vaccine synergism conferred by bivalent HVT/SB-1, compared with HVT or SB-1 administered alone, is largely unknown. In the present study, we report on temporal changes in innate and acquired immune-patterning gene expression by using ex vivo splenocyte infection and in ovo vaccination models. We report that in the ex vivo splenocyte infection model, by 72 hr postinfection, vaccines induced IFN and IFN-stimulated gene expression, with lesser proinflammatory cytokine induction. For several genes (TLR3, IFN-γ, OASL, Mx1, NOS2A, and IL-1β), the effects on gene expression were additive for HVT, SB1, and HVT/SB1 infection. We observed similar patterns of induction in in ovo-vaccinated commercial broiler embryos and chicks with HVT/SB-1 or recombinant HVT-based bivalent combination (HVT-LT/SB-1). Furthermore, HVT/SB-1 or HVT-LT/SB-1 in ovo vaccination appeared to hasten immune maturation, with expression patterns suggesting accelerated migration of T and natural killer cells into the spleen. Finally, HVT/SB-1 vaccination resulted in a coordinated induction of IL-12p40 and downregulation of suppressors of cytokine signaling 1 and 3, indicative of classical macrophage 1 and T-helper 1 patterning.

Efficient Mutagenesis of Marek's Disease Virus-Encoded microRNAs Using a CRISPR/Cas9-Based Gene Editing System

The virus-encoded microRNAs (miRNAs) have been demonstrated to have important regulatory roles in herpesvirus biology, including virus replication, latency, pathogenesis and/or tumorigenesis. As an emerging efficient tool for gene editing, the clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 system has been successfully applied in manipulating the genomes of large DNA viruses. Herein, utilizing the CRISPR/Cas9 system with a double-guide RNAs transfection/virus infection strategy, we have established a new platform for mutagenesis of viral miRNAs encoded by the Marek's disease virus serotype 1 (MDV-1), an oncogenic alphaherpesvirus that can induce rapid-onset T-cell lymphomas in chickens. A series of miRNA-knocked out (miR-KO) mutants with deletions of the Meq- or the mid-clustered miRNAs, namely RB-1B∆Meq-miRs, RB-1B∆M9-M2, RB-1B∆M4, RB-1B∆M9 and RB-1B∆M11, were generated from vvMDV strain RB-1B virus. Interestingly, mutagenesis of the targeted miRNAs showed changes in the in vitro virus growth kinetics, which is consistent with that of the in vivo proliferation curves of our previously reported GX0101 mutants produced by the bacterial artificial chromosome (BAC) clone and Rec E/T homologous recombination techniques. Our data demonstrate that the CRISPR/Cas9-based gene editing is a simple, efficient and relatively nondisruptive approach for manipulating the small non-coding genes from the genome of herpesvirus and will undoubtedly contribute significantly to the future progress in herpesvirus biology.

Vaccinal efficacy of molecularly cloned Gallid alphaherpesvirus 3 strain 301B/1 against very virulent Marek's disease virus challenge

Marek's disease virus (MDV), a causative agent of Marek's disease, has evolved its virulence partly because the current control strategies fail to provide sterilizing immunity. Gallid alphaherpesvirus 3 (GaHV-3) and turkey herpesvirus have been developed as bivalent vaccines to improve upon the level of protection elicited by single formulations. Since the in vitro passage of vaccines can result in attenuation, a GaHV-3 strain 301B/1 was cloned as a bacterial artificial chromosome (BAC) by inserting the mini-F replicon into the virus genome. A fully infectious virus, v301B-BAC, was reconstituted from the 301B/1 BAC clone and had similar growth kinetics comparable to that of the parental 301B/1 virus with strong reactivity against anti-301B/1 chicken sera. Protective efficacies of v301B-BAC, parental 301B/1, and SB-1 vaccine were evaluated against a very virulent MDV Md5 challenge. Clinical signs were significantly lower in the v301B-BAC vaccinated groups (24-25 %), parental 301B/1 (29 %) compare to that of non-vaccinated control (100%) and the removal of BAC sequences from v301B-BAC genome further reduced this to 17 %. The protective indices of v301B-BACs (75-76 %) were comparable with those of both the 301B/1 and the SB-1 vaccine (71%). Removal of the mini-F replicon resulted in a reconstituted virus with a protective index of 83 %. The shedding of challenge virus was notably lower in the v301B-BAC, and v301B-delBAC vaccinated groups. Overall, the protective efficacy of the 301B-BAC-derived vaccine virus against a very virulent MDV challenge was comparable to that of the parental 301B/1 virus as well as the SB-1 vaccine virus.

Marek's Disease Virus Infection of Natural Killer Cells

Natural killer (NK) cells are key players in the innate immune response. They kill virus-infected cells and are crucial for the induction of adaptive immune responses. Marek’s disease virus (MDV) is a highly contagious alphaherpesvirus that causes deadly T cell lymphomas in chickens. Host resistance to MDV is associated with differences in NK cell responses; however, the exact role of NK cells in the control of MDV remains unknown. In this study, we assessed if MDV can infect NK cells and alter their activation. Surprisingly, we could demonstrate that primary chicken NK cells are very efficiently infected with very virulent RB-1B MDV and the live-attenuated CVI988 vaccine. Flow cytometry analysis revealed that both RB-1B and CVI988 enhance NK cell degranulation and increase interferon gamma (IFNγ) production in vitro. In addition, we could show that the MDV Eco Q-encoded oncogene (meq) contributes to the induction of NK cell activation using meq knockout viruses. Taken together, our data revealed for the first time that NK cells are efficiently infectable with MDV and that this oncogenic alphaherpesvirus enhances NK cell degranulation and increased IFNγ production in vitro.

A proofreading-impaired herpesvirus generates populations with quasispecies-like structure

RNA virus populations are composed of highly diverse individuals that form a cloud of related sequences commonly referred to as a 'quasispecies'^1-3. This diversity arises as a consequence of low-fidelity genome replication^4,5. By contrast, DNA virus populations contain more uniform individuals with similar fitness⁶. Genome diversity is often correlated with increased fitness in RNA viruses, while DNA viruses are thought to require more faithful genome replication. During DNA replication, erroneously incorporated bases are removed by a 3'-5' exonuclease, a highly conserved enzymatic function of replicative DNA but not RNA polymerases. This proofreading process enhances replication fidelity and ensures the genome integrity of DNA organisms, including large DNA viruses⁷. Here, we show that a herpesvirus can tolerate impaired exonucleolytic proofreading, resulting in DNA virus populations, which, as in RNA viruses⁸, are composed of highly diverse genotypes of variable individual fitness. This indicates that herpesvirus mutant diversity may compensate for individual fitness loss. Notably, in vivo infection with diverse virus populations results in a marked increase in virulence compared to genetically homogenous parental virus. While we cannot exclude that the increase in virulence is caused by selection of and/or interactions between individual genotypes, our findings are consistent with quasispecies dynamics. Our results contrast with traditional views of DNA virus replication and evolution, and indicate that a substantial increase in population diversity can lead to higher virulence.

Lorf9 deletion significantly eliminated lymphoid organ atrophy induced by meq-deleted very virulent Marek's disease virus

Marek's disease virus (MDV) is a highly contagious alphaherpesvirus that causes rapid onset of T cell lymphomas in chickens. MDV continues to break through vaccinal immunity due to the emergence of highly virulent field strains. Earlier studies revealed that deletion of the meq gene from MDV results in attenuated vaccines that protect against disease when chickens are infected with highly virulent strains. However, meq-deleted viruses still retain the ability to induce lymphoid organ atrophy, which raises safety concerns. In an earlier study, we found that deletion of lorf9 counteracts this lymphoid organ atrophy. Here, we describe the generation of a double deletion mutant virus lacking virus-encoded meq and lorf9. In vitro studies revealed that during replication, the mutant virus had kinetic characteristics similar to the parental virus; however, in vivo the replication capability was significantly reduced. Results of animal studies revealed no obvious MDV-specific symptoms and lesions. Importantly, the double deletion mutant virus lost the capacity to induce lymphoid organ atrophy, which has been the main obstacle during development of a good vaccine candidate.

Chicken endothelial cells are highly responsive to viral innate immune stimuli and are susceptible to infections with various avian pathogens

It is well established that the endothelium plays a prominent role in the pathogenesis of various infectious diseases in mammals. However, little is known about the role of endothelial cells (EC) as targets for avian pathogens and their contribution to the pathogenesis of infectious diseases in galliform birds. First, we explored the innate immune response of primary chicken aortic endothelial cells (pchAEC), obtained from 18-day-old embryos, to stimulation with pathogen-associated molecular patterns or recombinant chicken interferons (type I, II and III IFNs). In spite of the abundant expression of a number of innate immune receptors, marked cytokine responses to stimulation with pathogen-associated molecular patterns were only seen in pchAEC treated with the TLR3 agonist polyI:C (pI:C) and the MDA5 agonist liposome-complexed polyI:C (L-pI:C), as was assessed by quantitative PCR and luciferase-based IFN-I/NFκB reporter assays. Treatments of pchAEC with IFN-α, IFN-γ and IFN-λ resulted in STAT1-phosphorylation/activation, as was revealed by immunoblotting. Next, we demonstrated that pchAEC are susceptible to infection with a variety of poultry pathogens, including Marek's disease virus (MDV), infectious bursal disease virus (IBDV), avian pathogenic Escherichia coli (APEC) and Eimeria tenella. Our data highlight that chicken EC are potential targets for viral, bacterial and protozoan pathogens in gallinaceous poultry and may partake in the inflammatory and antimicrobial response. The pchAEC infection model used herein will allow further studies interrogating avian pathogen interactions with vascular EC. RESEARCH HIGHLIGHTS Use of a well-defined primary chicken aortic endothelial cell (pchAEC) culture model for studying avian host-pathogen interactions. pchAEC are responsive to innate immune stimulation with viral pathogen-associated molecular patterns and chicken type I, II and III interferons. pchAEC are susceptible to infections with economically important poultry pathogens, including MDV, IBDV, APEC and Eimeria tenella.

A gD&gC-substituted pseudorabies virus vaccine strain provides complete clinical protection and is helpful to prevent virus shedding against challenge by a Chinese pseudorabies variant

Background

Since 2011, pseudorabies caused by a variant PRV has re-emerged in many Chinese Bartha-K61-vaccinated pig farms. An efficacious vaccine is necessary to control this disease. We described the construction of a gD&gC-substituted pseudorabies virus (PRV B-gD&gC^S) from the Bartha-K61 (as backbone) and AH02LA strain (as template for gD and gC genes) through bacterial artificial chromosome (BAC) technology using homologous recombination. The growth kinetics of PRV B-gD&gC^S was compared with Bartha-K61. Its safety was evaluated in 28-day-old piglets. Protection efficacy was tested in piglets by lethal challenge with AH02LA at 7 days post vaccination, including body temperature, clinical symptoms, virus shedding, mortality rate, and lung lesions.

Results

The results showed that a BAC clone of Bartha-K61 and a B-gD&gC^S clone were successfully generated. The growth kinetics of PRV B-gD&gC^S strain on ST (Swine testicular) cells was similar to that of the Bartha-K61 strain. No piglets inoculated intramuscularly with PRV B-gD&gC^S strain exhibited any clinical symptoms or virus shedding. After AH02LA challenge, all piglets in PRV B-gD&gC^S and Bartha-K61 groups (n = 5 each) survived without exhibiting any clinical symptoms and high body temperature. More importantly, PRV B-gD&gC^S strain completely prevented virus shedding in 2 piglets and reduced virus shedding post challenge in the other 3 piglets as compared with Bartha-K61 group.

Conclusions

Our results suggest that PRV B-gD&gC^S strain is a promising vaccine candidate for the effective control of current severe epidemic pseudorabies in China.

Polymorphisms affecting the gE and gI proteins partly contribute to the virulence of a newly-emergent highly virulent Chinese pseudorabies virus

An outbreak of a highly virulent pseudorabies virus strain, ZJ01, occurred in PRV-vaccinated pigs in China in 2011. In this study, ZJ01 caused fatal diseases, while the Chinese prototypic PRV strain LA caused mild respiratory disorders. Full-genome sequencing results indicate the two viruses can be classified into two sub-clusters that distinct from traditional European and US strains. To examine the potential role of the gE and gI proteins in ZJ01 virulence, we generated several recombinant viruses. In two chimeric viruses (rZJ01-LA/gEI and rLA-ZJ01/gEI), the gE and gI genes were swapped using corresponding genes from ZJ01 and LA. rZJ01-LA/gEI and the parental virus rZJ01 retained high virulence in piglets, although the survival time for rZJ01-LA/gEI infected piglets was obviously prolonged. In contrast, rLA-ZJ01/gEI exhibited higher virulence than its parental virus rLA. We conclude that changes in gE and gI proteins partly contribute to the enhanced virulence of ZJ01 strain.

A phylogenomic analysis of Marek's disease virus reveals independent paths to virulence in Eurasia and North America

Virulence determines the impact a pathogen has on the fitness of its host, yet current understanding of the evolutionary origins and causes of virulence of many pathogens is surprisingly incomplete. Here, we explore the evolution of Marek's disease virus (MDV), a herpesvirus commonly afflicting chickens and rarely other avian species. The history of MDV in the 20th century represents an important case study in the evolution of virulence. The severity of MDV infection in chickens has been rising steadily since the adoption of intensive farming techniques and vaccination programs in the 1950s and 1970s, respectively. It has remained uncertain, however, which of these factors is causally more responsible for the observed increase in virulence of circulating viruses. We conducted a phylogenomic study to understand the evolution of MDV in the context of dramatic changes to poultry farming and disease control. Our analysis reveals evidence of geographical structuring of MDV strains, with reconstructions supporting the emergence of virulent viruses independently in North America and Eurasia. Of note, the emergence of virulent viruses appears to coincide approximately with the introduction of comprehensive vaccination on both continents. The time‐dated phylogeny also indicated that MDV has a mean evolutionary rate of ~1.6 × 10⁻⁵ substitutions per site per year. An examination of gene‐linked mutations did not identify a strong association between mutational variation and virulence phenotypes, indicating that MDV may evolve readily and rapidly under strong selective pressures and that multiple genotypic pathways may underlie virulence adaptation in MDV.

A Tiny RNA that Packs a Big Punch: The Critical Role of a Viral miR-155 Ortholog in Lymphomagenesis in Marek's Disease

MicroRNAs (miRNAs) are small non-coding RNAs that have been identified in animals, plants, and viruses. These small RNAs play important roles in post-transcriptional regulation of various cellular processes, including development, differentiation, and all aspects of cancer biology. Rapid-onset T-cell lymphoma of chickens, namely Marek’s disease (MD), induced by Gallid alphaherpesvirus 2 (GaHV2), could provide an ideal natural animal model for herpesvirus-related cancer research. GaHV2 encodes 26 mature miRNAs derived from 14 precursors assembled in three distinct gene clusters in the viral genome. One of the most highly expressed GaHV2 miRNAs, miR-M4-5p, shows high sequence similarity to the cellular miR-155 and the miR-K12-11 encoded by Kaposi’s sarcoma-associated herpesvirus, particularly in the miRNA “seed region.” As with miR-K12-11, miR-M4-5p shares a common set of host and viral target genes with miR-155, suggesting that they may target the same regulatory cellular networks; however, differences in regulatory function between miR-155 and miR-M4-5p may distinguish non-viral and viral mediated tumorigenesis. In this review, we focus on the functions of miR-M4-5p as the viral ortholog of miR-155 to explore how the virus mimics a host pathway to benefit the viral life cycle and trigger virus-induced tumorigenesis.

ESCDL-1, a new cell line derived from chicken embryonic stem cells, supports efficient replication of Mardiviruses

Marek’s disease virus is the etiological agent of a major lymphoproliferative disorder in poultry and the prototype of the Mardivirus genus. Primary avian somatic cells are currently used for virus replication and vaccine production, but they are largely refractory to any genetic modification compatible with the preservation of intact viral susceptibility. We explored the concept of induction of viral replication permissiveness in an established pluripotent chicken embryonic stem cell-line (cES) in order to derive a new fully susceptible cell-line. Chicken ES cells were not permissive for Mardivirus infection, but as soon as differentiation was triggered, replication of Marek’s disease virus was detected. From a panel of cyto-differentiating agents, hexamethylene bis (acetamide) (HMBA) was found to be the most efficient regarding the induction of permissiveness. These initial findings prompted us to analyse the effect of HMBA on gene expression, to derive a new mesenchymal cell line, the so-called ESCDL-1, and monitor its susceptibility for Mardivirus replication. All Mardiviruses tested so far replicated equally well on primary embryonic skin cells and on ESCDL-1, and the latter showed no variation related to its passage number in its permissiveness for virus infection. Viral morphogenesis studies confirmed efficient multiplication with, as in other in vitro models, no extra-cellular virus production. We could show that ESCDL-1 can be transfected to express a transgene and subsequently cloned without any loss in permissiveness. Consequently, ESCDL-1 was genetically modified to complement viral gene deletions thus yielding stable trans-complementing cell lines. We herein claim that derivation of stable differentiated cell-lines from cES cell lines might be an alternative solution to the cultivation of primary cells for virology studies.

Preliminary study of the UL55 gene based on infectious Chinese virulent duck enteritis virus bacterial artificial chromosome clone

Background

Lethal Duck Enteritis Virus (DEV) infection can cause high morbidity and mortality of many species of waterfowl within the order Anseriformes. However, little is known about the function of viral genes including the conserved UL55 gene among alpha herpes virus due to the obstacles in maintenance and manipulation of DEV genome in host cells.

Methods

In this paper, we constructed an infectious bacteria artificial chromosome (BAC) clone of the lethal clinical isolate duck enteritis virus Chinese virulent strain (DEV CHv) by inserting a transfer vector containing BAC mini-F sequence and selection marker EGFP into UL23 gene using homologous recombination. UL55 deletion and its revertant mutant were generated by two-step RED recombination in E. coli on basis of rescued recombinant virus. The function of UL55 gene in DEV replication and its effect on distribution of UL26.5 protein were carried out by growth characteristics and co-localization analysis.

Results

The complete genome of DEV CHv can be stably maintained in E. coli as a BAC clone and reconstituted again in DEF cells. The generated UL55 deletion mutant based on DEV CHv-BAC-G displayed similar growth curves, plaque morphology and virus titer of its parental virus in infected Duck Embryo Fibroblast (DEF) cells. Immunofluorescence assay indicated that the loss of UL55 gene do not affect the distribution of UL26.5 protein in intracellular. These data also suggest infectious BAC clone of DEV CHv will facilitate the gene function studies of DEV genome.

Conclusions

We have successfully developed an infectious BAC clone of lethal clinical isolate DEV CHv for the first time. The generated UL55 gene mutant based on that demonstrated this platform would be a very useful tool for functional study of DEV genes. We found the least known DEV UL55 is dispensable for virus replication and UL26.5 distribution, and it could be a very promise candidate locus for developing bivalent vaccine. Experiment are now in progress for testifying the possibility of UL55 gene locus as an exogenous gene insertion site for developing DEV vectored vaccine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0748-y) contains supplementary material, which is available to authorized users.

Impairment of infectious laryngotracheitis virus replication by deletion of the UL[-1] gene

Infectious laryngotracheitis virus (ILTV) encodes several unique genes, including a pair of unique nuclear proteins UL0 and UL[-1] that are expressed during replication in cell culture. Although the UL0 gene has been shown to be dispensable for replication, the role of UL[-1] has not been elucidated. In this study a deletion mutant of ILTV lacking the UL[-1] gene was constructed using homologous recombination. The coding sequences of the gene were replaced with the gene for enhanced green fluorescent protein and the cytomegalovirus major immediate early promoter element. The progeny virus carrying the reporter gene was readily identified using fluorescent microscopy, but was unable to propagate in the permissive cells in the absence of wild type ILTV. Even after plaque purification and fluorescent associated cell sorting the recombinant virus deficient in UL[-1] gene could not be successfully isolated. Our findings suggest that the UL[-1] gene has an important role in ILTV replication.

Comparative pathogenicity and immunogenicity of triple and double gene-deletion pseudorabies virus vaccine candidates

Pseudorabies (PR) outbreaks have re-emerged in many pig farms with vaccination coverage in China, which suggests that current vaccine strains provide poor protection against novel, epidemic strains. In this study, based on the highly virulent PRV ZJ01 variant, a PR virus (PRV) thymidine kinase (TK)/gE/gI deleted strain-rZJ01ΔTK/gE/gI-was generated, which showed similar growth features in vitro compared to the parent strain PRV ZJ01 and its gE/gI deleted strain rZJ01ΔgE/gI. The results of a piglet experiment (with 10 piglets each group) showed that the rZJ01ΔTK/gE/gI vaccine generated similar levels of neutralizing antibodies against ZJ01 compared to the rZJ01ΔgE/gI vaccine (p>0.05). However, rZJ01ΔgE/gI inoculation resulted in slight inflammatory cell infiltrations, hemorrhages, and congestion in the brain and lungs. After a ZJ01 challenge, all animals in the rZJ01ΔTK/gE/gI- and rZJ01ΔgE/gI-vaccinated groups survived without exhibiting any clinical symptoms, whereas all non-vaccinated control animals died within 7days post-challenge. Furthermore, microscopic lesions and virus loads in the brains and lungs in the two vaccinated groups were significantly lower than those in the control group. Meanwhile, the virus levels in the brains of piglets in the rZJ01ΔTK/gE/gI group were significantly lower than those in the rZJ01ΔgE/gI group. These results indicate that the triple gene-deleted PRV rZJ01ΔTK/gE/gI strain has lower pathogenicity and higher protective efficacy against variant PRV challenge compared with the double gene-deleted PRV rZJ01ΔgE/gI strain. Together, all these data indicate that the PRV rZJ01ΔTK/gE/gI strain should be an ideal vaccine candidate for the prevention of PR in China.

An inactivated gE-deleted pseudorabies vaccine provides complete clinical protection and reduces virus shedding against challenge by a Chinese pseudorabies variant

Background

Since the end of 2011 an outbreak of pseudorabies affected Chinese pig herds that had been vaccinated with the commercial vaccine made of Bartha K61 strain. It is now clear that the outbreak was caused by an emergent PRV variant. Even though vaccines made of PRV Bartha K61 strain can confer certain cross protection against PRV variants based on experimental data, less than optimal clinical protection and virus shedding reduction were observed, making the control or eradication of this disease difficult.

Results

An infectious clone of PRV AH02LA strain was constructed to generate a gE deletion mutant PRV(LA-A^B) strain. PRV(LA-A^B) strain can reach a titer of 10^8.43 TCID₅₀ /mL (50% tissue culture infectious dose) on BHK-21 cells. To evaluate the efficiency of the inactivated vaccine made of PRV(LA-A^B) strain, thirty 3-week-old PRV-negative piglets were divided randomly into six groups for vaccination and challenge test. All five piglets in the challenge control showed typical clinical symptoms of pseudorabies post challenge. Sneezing and nasal discharge were observed in four and three piglets in groups C(vaccinated with inactivated PRV Bartha K61 strain vaccine) and D(vaccinated with live PRV Bartha K61 strain vaccine) respectively. In contrast, piglets in both groups A(vaccinated with inactivated PRV LA-AB strain vaccine) and B(vaccinated with inactivated PRV LA-A^B strain vaccine with adjuvant) presented mild or no clinical symptoms. Moreover, viral titers detected via nasal swabs were approximately 100 times lower in group B than in the challenge control, and the duration of virus shedding (3–4 days) was shorter than in either the challenge control (5–10 days) or groups C and D (5–6 days).

Conclusions

The infectious clone constructed in this study harbors the whole genome of the PRV variant AH02LA strain. The gE deletion mutant PRV(LA-A^B)strain generated from PRV AH02LA strain can reach a high titer on BHK-21 cells. An inactivated vaccine of PRV LA-A^B provides clinical protection and significantly reduces virus shedding post challenge, especially if accompanied by the adjuvant CVC1302. While Inactivated or live vaccines made of PRV Barth K61 strain can provide only partial protection in this test.

Recombinant Marek's Disease Virus as a Vector-Based Vaccine against Avian Leukosis Virus Subgroup J in Chicken

Avian leukosis virus subgroup J (ALV-J) is an immunosuppressive virus that causes considerable economic losses to the chicken industry in China. However, there is currently no effective vaccine to prevent ALV-J infection. In order to reduce the losses caused by ALV-J, we constructed two effective ALV-J vaccines by inserting the ALV-J (strain JL093-1) env or gag+env genes into the US2 gene of the Marek’s disease herpesviruses (MDV) by transfection of overlapping fosmid DNAs, creating two recombinant MDVs, rMDV/ALV-gag+env and rMDV/ALV-env. Analysis of cultured chicken embryo fibroblasts infected with the rMDVs revealed that Env and Gag were successfully expressed and that there was no difference in growth kinetics in cells infected with rMDVs compared with that of cells infected with the parent MDV. Chickens vaccinated with either rMDV revealed that positive serum antibodies were induced. Both rMDVs also effectively reduced the rate of positive viremia in chicken flocks challenged with ALV-J. The protective effect provided by rMDV/ALV-env inoculation was slightly stronger than that provided by rMDV/ALV-gag+env. This represents the first study where a potential rMDV vaccine, expressing ALV-J antigenic genes, has been shown to be effective in the prevention of ALV-J. Our study also opens new avenues for the control of MDV and ALV-J co-infection.

Deletion of the BAC sequences from recombinant meq-null Marek's disease (MD) virus increases immunosuppression while maintaining protective efficacy against MD

Marek's disease virus (MDV) GX0101 is a field strain of MDV with a naturally occurring insertion of the reticuloendotheliosis virus (REV) long terminal repeat (LTR) fragment. Both copies of the meq gene were knocked out in the GX0101 bacterial artificial chromosome (BAC) clone to construct the recombinant virus SC9-1, resulting in a complete lack of pathogenicity and providing better protection against MD than CVI988/Rispens. In the present study, the BAC sequences in SC9-1 were removed using a cre-loxP system, and the virus termed SC9-2. SC9-2 showed a significant increase in replication in vitro and in vivo. There was a significant decrease in chicken weight, immune organ index, and antibody levels compared with those of SC9-1-inoculated chickens. The immune protection index of SC9-2 was similar to that of SC9-1, and the difference was not significant. The results of our studies demonstrate that the SC9-2 virus provides protection in specific pathogen free (SPF) chickens when challenged with a very virulent MDV rMd5, but it induces immunosuppressive effects in SPF chickens.

Virus and host genomic, molecular, and cellular interactions during Marek's disease pathogenesis and oncogenesis

Marek's Disease Virus (MDV) is a chicken alphaherpesvirus that causes paralysis, chronic wasting, blindness, and fatal lymphoma development in infected, susceptible host birds. This disease and its protective vaccines are highly relevant research targets, given their enormous impact within the poultry industry. Further, Marek's disease (MD) serves as a valuable model for the investigation of oncogenic viruses and herpesvirus patterns of viral latency and persistence--as pertinent to human health as to poultry health. The objectives of this article are to review MDV interactions with its host from a variety of genomic, molecular, and cellular perspectives. In particular, we focus on cytogenetic studies, which precisely assess the physical status of the MDV genome in the context of the chicken host genome. Combined, the cytogenetic and genomic research indicates that MDV-host genome interactions, specifically integration of the virus into the host telomeres, is a key feature of the virus life cycle, contributing to the viral achievement of latency, transformation, and reactivation of lytic replication. We present a model that outlines the variety of virus-host interactions, at the multiple levels, and with regard to the disease states.

Sodium tanshinone IIA sulfonate affects Marek's disease virus replication by inhibiting gB expression

CONTEXT

Previous studies demonstrated that sodium tanshinone IIA sulfonate (STS) could inhibit MDV replication in vitro. The mechanism about how STS inhibits MDV replication is still not well understood.

OBJECTIVE

In this study, we evaluated the effect of STS on gB gene/protein of Marek's disease virus (MDV).

MATERIALS AND METHODS

The concentration of 0.25 mg/ml of STS was used in this study. Meanwhile, 0.25 mg/ml of acyclovir (ACV) was used as a positive control. About 9-11-d-old embryonated specific-pathogen-free (SPF) chicken eggs were used to prepare CEF cells. CEF cells were infected with MDV 2 h, followed by treatment with STS. Real-time PCR and western blot assay were used to measure the gB (UL27) gene/protein expression in STS treatment group at 24, 48, 72, and 96 h post-infection.

RESULTS

Compared with MDV control, the gB gene copies were significantly decreased in STS and ACV treatment groups at 72 h and 96 h (p < 0.05), both in the DNA and in the mRNA level. Furthermore, the expression of gB protein was also inhibited by STS at 24, 72, and 96 h.

DISCUSSION AND CONCLUSION

Our study demonstrated that STS could effectively inhibit the MDV replication by suppressing gB gene/protein expression in cell culture.

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.

Construction of a recombinant duck enteritis virus (DEV) expressing hemagglutinin of H5N1 avian influenza virus based on an infectious clone of DEV vaccine strain and evaluation of its efficacy in ducks and chickens

Background

Highly pathogenic avian influenza virus (AIV) subtype H5N1 remains a threat to poultry. Duck enteritis virus (DEV)-vectored vaccines expressing AIV H5N1 hemagglutinin (HA) may be viable AIV and DEV vaccine candidates.

Methods

To facilitate the generation and further improvement of DEV-vectored HA(H5) vaccines, we first constructed an infectious clone of DEV Chinese vaccine strain C-KCE (DEV^C-KCE). Then, we generated a DEV-vectored HA(H5) vaccine (DEV-H5(UL55)) based on the bacterial artificial chromosome (BAC) by inserting a synthesized HA(H5) expression cassette with a pMCMV IE promoter and a consensus HA sequence into the noncoding area between UL55 and LORF11. The immunogenicity and protective efficacy of the resulting recombinant vaccine against DEV and AIV H5N1 were evaluated in both ducks and chickens.

Results

The successful construction of DEV BAC and DEV-H5(UL55) was verified by restriction fragment length polymorphism analysis. Recovered virus from the BAC or mutants showed similar growth kinetics to their parental viruses. The robust expression of HA in chicken embryo fibroblasts infected with the DEV-vectored vaccine was confirmed by indirect immunofluorescence and western blotting analyses. A single dose of 10⁶ TCID₅₀ DEV-vectored vaccine provided 100 % protection against duck viral enteritis in ducks, and the hemagglutination inhibition (HI) antibody titer of AIV H5N1 with a peak of 8.2 log₂ was detected in 3-week-old layer chickens. In contrast, only very weak HI titers were observed in ducks immunized with 10⁷ TCID₅₀ DEV-vectored vaccine. A mortality rate of 60 % (6/10) was observed in 1-week-old specific pathogen free chickens inoculated with 10⁶ TCID₅₀ DEV-vectored vaccine.

Conclusions

We demonstrate the following in this study. (i) The constructed BAC is a whole genome clone of DEV^C-KCE. (ii) The insertion of an HA expression cassette sequence into the noncoding area between UL55 and LORF11 of DEV^C-KCE affects neither the growth kinetics of the virus nor its protection against DEV. (iii) DEV-H5(UL55) can generate a strong humoral immune response in 3-week-old chickens, despite the virulence of this virus observed in 1-week-old chickens. (iv) DEV-H5(UL55) induces a weak HI titer in ducks. An increase in the HI titers induced by DEV-vectored HA(H5) will be required prior to its wide application.

In vitro model for lytic replication, latency, and transformation of an oncogenic alphaherpesvirus

Marek's disease virus (MDV) is an alphaherpesvirus that causes deadly T-cell lymphomas in chickens and serves as a natural small animal model for virus-induced tumor formation. In vivo, MDV lytically replicates in B cells that transfer the virus to T cells in which the virus establishes latency. MDV also malignantly transforms CD4+ T cells with a T(reg) signature, ultimately resulting in deadly lymphomas. No in vitro infection system for primary target cells of MDV has been available due to the short-lived nature of these cells in culture. Recently, we characterized cytokines and monoclonal antibodies that promote survival of cultured chicken B and T cells. We used these survival stimuli to establish a culture system that allows efficient infection of B and T cells with MDV. We were able to productively infect with MDV B cells isolated from spleen, bursa or blood cultured in the presence of soluble CD40L. Virus was readily transferred from infected B to T cells stimulated with an anti-TCRαVβ1 antibody, thus recapitulating the in vivo situation in the culture dish. Infected T cells could then be maintained in culture for at least 90 d in the absence of TCR stimulation, which allowed the establishment of MDV-transformed lymphoblastoid cell lines (LCL). The immortalized cells had a signature comparable to MDV-transformed CD4+ α/β T cells present in tumors. In summary, we have developed a novel in vitro system that precisely reflects the life cycle of an oncogenic herpesivrus in vivo and will allow us to investigate the interaction between virus and target cells in an easily accessible system.

Marek's disease: Genetic regulation of gallid herpesvirus 2 infection and latency

Gallid herpesvirus-2 (GaHV-2) is an oncogenic α-herpesvirus that causes Marek's disease (MD), a T cell lymphosarcoma (lymphoma) of domestic fowl (chickens). The GaHV-2 genome integrates by homologous recombination into the host genome and, by modulating expression of viral and cellular genes, induces transformation of latently infected cells. MD is a unique model of viral oncogenesis. Mechanisms implicated in the regulation of viral and cellular genes during GaHV-2 infection operate at transcriptional, post-transcriptional and post-translational levels, with involvement of viral and cellular transcription factors, along with epigenetic modifications, alternative splicing, microRNAs and post-translational modifications of viral proteins. Meq, the major oncogenic protein of GaHV-2, is a viral transcription factor that modulates expression of viral genes, for example by binding to the viral bidirectional promoter of the pp38-pp24/1.8 kb mRNA, and also modulates expression of cellular genes, such as Bcl-2 and matrix metalloproteinase 3. GaHV-2 expresses viral telomerase RNA subunit (vTR), which forms a complex with the cellular telomerase reverse transcriptase (TERT), thus contributing to tumorigenesis, while vTR independent of telomerase activity is implicated in metastasis. Expression of a viral interleukin 8 homologue may contribute to lymphomagenesis. Inhibition of expression of the pro-apoptotic factors JARID2 and SMAD2 by viral microRNAs may promote the survival and proliferation of GaHV-2 latently infected cells, thus enhancing tumorigenesis, while inhibition of interleukin 18 by viral microRNAs may be involved in evasion of immune surveillance. Viral envelope glycoproteins derived from glycoprotein B (gp60 and gp49), as well as glycoprotein C, may also play a role in immune evasion.

Efficient Marek's disease virus (MDV) and herpesvirus of turkey infection of the QM7 cell line that does not contain latent MDV genome

Marek's disease virus (MDV; also known as Gallid herpesvirus 2, MDV-1) causes oncogenic disease in chickens producing clinical signs that include lymphomas, visceral tumours, nerve lesions, and immunosuppression. MDV vaccines are widely used and mostly produced using primary cells: chicken embryo fibroblast cells, duck embryo fibroblast cells, chicken embryo kidney cells or chicken kidney cells. An immortalized cell line that can be used to manufacture the virus has long been desired. In this report, we demonstrate that QM7 cells were susceptible to infection with either MDV or herpesvirus of turkey (HVT; also known as Meleagrid herpesvirus 1, MDV-3). Polymerase chain reaction analysis with primers amplifying selected MDV genes revealed that QM7 cells did not contain these sequences. However, MDV genes were detected in QT35 cells, which have been reported to harbour latent MDV virus. Transfection of naked MDV DNA initiated efficient infection of QM7 cells. In addition, QM7 cell lysate, clarified supernatant, and QM7 cell pellet infected with MDV were negative for reverse transcriptase activity, indicating absence of endogenous retrovirus. QM7 cells were also found to be free of other avian pathogens in a chick embryo inoculation test. In vivo studies of MDV growing in QM7 cells showed the virus retained its pathogenicity and virulence. In ovo experiments demonstrated that both HVT and MDV propagated in QM7 cells did not interfere with hatchability of injected eggs, and viruses could be re-isolated from hatched chicks. The results suggest that QM7 could be a good host cell line for growing both MDV and HVT.

Expression of fluorescent proteins within the repeat long region of the Marek's disease virus genome allows direct identification of infected cells while retaining full pathogenicity

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus and causes Marek's disease (MD) in chickens. RLORF4 is an MDV-specific gene located in the repeat long (RL) regions of the genome and is directly involved in attenuation. In this report, we generated recombinant (r)MDVs in which eGFP or mRFP was inserted in-frame of the 3' end of the RLORF4 gene. In vitro growth was unaffected and infected cells could be identified by using fluorescent microscopy. Interestingly, though inserted in-frame with RLORF4, eGFP and mRFP were expressed alone, confirming mRNA expression and splicing within the RL of MDV is complex. In vivo, rMDVs expressing mRFP or eGFP caused tumors similar to wild-type MDV. Fluorescent protein expression could be seen in spleen, tumor, and feather follicle epithelial cells. These results show that expression of fluorescent proteins within the RL region results in fluorescent rMDVs that still maintains full pathogenicity in the chicken.

Herpesvirus mutagenesis facilitated by infectious bacterial artificial chromosomes (iBACs)

A critical factor in the study of herpesviruses, their genes and gene functions is the capacity to derive mutants that harbor deletions, truncations, or insertions within the genetic elements of interest. Once constructed the impact of the introduced mutation on the phenotypic properties of the rescued virus can be determined in either in vitro or in vivo systems. However, the construction of such mutants by traditional virological mutagenesis techniques can be a difficult and laborious undertaking. The maintenance of a viral genome as an infectious bacterial artificial chromosome (iBAC), however, endows the capacity to manipulate the viral genome for mutagenesis studies with relative ease. Here, the construction and characterization of two gene deletion mutants of an alphaherpesvirus maintained as iBAC in combination with an inducible homologous recombination system in Escherichia coli is detailed. The methodology is generally applicable to any iBAC and is demonstrated to be a highly efficient and informative approach for mutant virus construction.