Differential quantification of cloned CVI988 vaccine strain and virulent RB-1B strain of Marek’s disease viruses in chicken tissues, using real-time PCR

Dynamic Changes in Viral Loads during Co-Infection with a Recombinant Turkey Herpesvirus Vector Vaccine and Very Virulent Marek's Disease Virus In Vivo

Marek's disease (MD), caused by the Marek's disease virus (MDV), is a common infectious tumor disease in chickens and was the first neoplastic disease preventable by vaccination. However, the vaccine cannot completely prevent virulent MDV infections, allowing both the vaccine and virulent MDV to coexist in the same chicken for extended periods. This study aims to investigate the changes in viral load of the very virulent strain Md5 and the rHVT-IBD vaccine in different chicken tissues using a real-time PCR assay. The results showed that the rHVT-IBD vaccine significantly reduced the viral load of MDV-Md5 in different organs, while the load of rHVT-IBD was significantly increased when co-infected with Md5. Additionally, co-infection with Md5 and rHVT-IBD in chickens not only changed the original viral load of both viruses but also affected the positive rate of Md5 at 14 days post-vaccination. The positive rate decreased from 100% to 14.29% (feather tips), 0% (skin), 33.33% (liver), 16.67% (spleen), 28.57% (thymus), 33.33% (bursa), and 66.67% (PBL), respectively. This study enhances our understanding of the interactions between HVT vector vaccines and very virulent MDV in chickens and provides valuable insights for the future development of MD vaccines.

Novel variant infectious bursal disease virus diminishes FAdV-4 vaccination and enhances pathogenicity of FAdV-4

Infectious bursal disease virus (IBDV) caused an acute and highly contagious infectious disease characterized by severe immunosuppression, causing considerable economic losses to the poultry industry globally. Although this disease was well-controlled under the widely use of commercial vaccines in the past decades, the novel variant IBDV strains emerged recently because of the highly immunized-selection pressure in the field, posting new threats to poultry industry. Here, we reported novel variant IBDV is responsible for a disease outbreak, and assessed the epidemic and pathogenicity of IBDV in this study. Moreover, we constructed a challenge model using Fowl adenovirus serotype 4 (FAdV-4) to study on the immunosuppressive effect. Our findings underscore the importance of IBDV surveillance, and provide evidence for understanding the pathogenicity of IBDV.

Regulation of Wnt/β-catenin signaling by Marek's disease virus in vitro and in vivo

Marek's disease virus (MDV) infection causes immunosuppression in the host, ultimately inducing tumor formation and causing significant economic losses to the poultry industry. While the abnormal activation of the Wnt/β-catenin signaling pathway is closely associated with the occurrence and development of tumors. However, the relationship between MDV and the Wnt/β-catenin pathway remains unclear. In this study, we found that the MDV RB1B strain, but not the MDV vaccine strain CVI988, activated the Wnt/β-catenin signaling pathway by increasing the phosphorylation level of GSK-3β in chicken embryo fibroblast (CEF). In vivo infection experiments in SPF chickens also confirmed that the RB1B strain activated the Wnt/β-catenin signaling pathway, while the CVI988 strain did not lead to its activation. Moreover, unlike the Meq protein encoded by the CVI988 strain, the Meq protein encoded by the RB1B strain specifically activated the Wnt/β-catenin signaling pathway in CEF cells. The findings from these studies extend our understanding of the regulation of Wnt/β-catenin signaling by MDV, which make a new contribution to understanding the virus-host interactions of MDV.

Pathogenicity of Field Marek’s Disease Virus Serotype-1 and Vaccine Efficacy Test in Chicken in Eastern Shewa Ethiopia

Background

Marek’s disease is a chicken lymphoproliferative viral illness. As new viruses emerge, vaccination immunity is being broken and hence pathogenecity assessment and vaccine evaluation related to the pathogen is critical for developing vaccine immunity in the field.

Methods

An experimental investigation was conducted to determine the pathogenicity of field isolates against Marek’s disease in antibody-free chicks and to assess the protective efficacy of the Marek’s disease vaccination. The viral isolates in question were discovered during an outbreak investigation for a previous study. The pathogenicity and effectiveness trial used a complete random design.

Results

In the pathogenicity trial, chickens inoculated with Bishoftu and Mojo field isolate had lower body weight 77.7±3.757 and 78.15±1.95 g at 10 dpi, respectively, when compared to un-inoculated controls, 89.85±3.838 g at 10 dpi. Incidence of early mortality syndrome (35% and 25%), lymphoma (53.8% and 40%), and overall mortality (50% and 45%) between Bishoftu and Mojo isolates, respectively, was discovered. Vaccinations with Herpes virus of turkey challenged chickens were provided complete protection against Marek’s disease.

Conclusion

Based on the findings in pathogenecity assessment experimental trials, Bishoftu and Mojo isolates were designated as virulent Marek’s disease viruses. Regular vaccinations with Herpes virus of turkey vaccine and supported by biosecurity measures in poultry farms are important to prevent the disease.

Morphological and Immunohistochemical Examination of Lymphoproliferative Lesions Caused by Marek's Disease Virus in Breeder Chickens

Simple Summary

The poultry industry is the most intensive and fastest growing among all livestock production systems, and, in the last decades, it has expanded exponentially due to an increasing demand for meat and eggs. Marek’s disease is a highly contagious and rapidly progressive lymphoproliferative disease. It is one of the most dangerous diseases of those affecting the sector because it causes important economic losses. Although widely controlled by vaccination programs, sometimes chickens are not totally protected, and the presence of virulent field strains can allow outbreaks. This case describes the occurrence of Marek’s disease observed in a breeder chicken flock that reported an increase in mortality rate (+0.4–0.6%) after the 32nd week. Histological analysis has highlighted severe lesions on visceral organs of chickens caused by Marek’s disease, especially in the intestinal tract of a hen that had a tumor mass in the distal part of the cloaca. Immunohistochemical staining confirmed the disease-associated tumor. The aim of this study was to underline the importance of vaccine administration related to the maintenance of proper biosecurity practice, especially in the first week of the raising cycle. In addition, monitoring for disease even after vaccination is crucial to minimize economic loss.

Abstract

Marek’s disease is widely controlled by vaccination programs; however, chickens are not totally protected, especially immediately after the vaccination when a strong challenge could interfere with the effectiveness of vaccination in the absence of proper biosecurity practice. This case report describes the occurrence of Marek’s disease (MD) observed in a breeder chicken flock reared southeast of Sicily. MD outbreak occurred from 32 to 47 weeks with an increase in weekly mortality rate (+0.4–0.6%). Overall, mortality rate related to Marek’s disease was about 6% at the end of the cycle. Carcasses of chickens found during the occurrence of disease underwent necropsy, and tissues were collected to confirm the infection. Gizzard, cecal tonsil, intestine, spleen and tumor mass were collected and analyzed from a carcass of one hen, 32 weeks old and apparently asymptomatic. Multiplex real-time PCR performed on spleen tissues detected the presence of MD virus pathogenic strain. Macroscopic and microscopic evaluation of the rest of the samples confirmed the neoplastic disease. Moreover, the immunophenotype of the tumor cells was identified as CD3 positive by immunohistochemical (IHC) staining. The vaccinated flock had become rapidly infected with the MD virus, which proves that the challenge of the MD virus was too strong in the rearing house at the beginning of the cycle, causing the outbreak.

Molecular characterisation and phylogenetic analysis of Marek's disease virus in Turkish layer chickens

1. There is no current data about the genotypes of Marek's disease virus (MDV) in Turkish poultry flocks; hence, this study was performed to analyse CVI988/Rispens, turkey herpesvirus (HVT) vaccine viruses and MDV field viruses as well as to perform phylogenetic analysis of MDV in Turkish layer chickens. 2. In 2017 and 2018, a total of 602 spleen samples from 49 layer flocks were collected from the Marmara, West Black Sea and Aegean regions. DNA was extracted from the spleen samples and the samples were analysed by real-time PCR probe assay to detect CVI988/Rispens and HVT vaccine viruses and MDV field strains. Samples found positive for MDV by real-time PCR were subjected to PCR using the Meq gene primers for phylogenetic analysis. 3. Amongst 49 flocks, virulent MDV was detected in nine flocks. CVI988/Rispens and HVT vaccine strains were detected in 47 flocks and HVT in all 49 flocks. Splenomegaly, hepatomegaly and tumours in the oviduct were observed in chickens of affected flocks. Virulent MDV was detected in 120 out of 602 spleen samples. Sequencing and phylogenetic analyses showed that MDVs detected in this study were closely related to MDV strains from Italy, Poland, Saudi Arabia, Iraq, India and China but showed diversity with MDV strains from Egypt and Hungary. Multiple sequence analysis of the Meq protein revealed several point mutations in deduced amino acid sequences. Interestingly, CVI988/Rispens vaccine virus from China (AF493555) showed mutations at position 66 (G66R) and 71 (S66A) along with two other vaccine strains from China (GU354326.1) and Russia (EU032468.1), in comparison with the other vaccine strain CVI988/Rispens (DQ534538). The molecular analyses of the Meq gene suggested that Turkish field strains of MDV are in the class of virulent or very virulent pathotypes. 4. The results have shown that MDV still affects poultry health, and the phylogenetic and amino acid variation data obtained will help in vaccination and control strategies.

Epidemiology, molecular characterization, and recombination analysis of chicken anemia virus in Guangdong province, China

Chicken anemia virus (CAV) causes severe anemia and immunosuppression in young chickens and a compromised immune response in older birds, resulting in great economic losses to the poultry industry worldwide. Here, we report the molecular epidemiology and characterization of CAV circulating in poultry in Guangdong province, China. Ninety-one of 277 chickens collected from 2016 to 2017 were CAV positive. Full-genome sequencing revealed the presence of eight separate strains. Phylogenetic analysis based on the genome sequences obtained in this study and related sequences available in the GenBank database showed that all of the CAV isolates exhibit a close relationship to each other and belong to the same genotypic group. Putative recombination events were also detected in the genomes of the newly isolated CAVs. Collectively, our findings underscore the importance of CAV surveillance and provide information that will lead to a better understanding of the evolution of CAV.

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.

Pervasive Differential Splicing in Marek's Disease Virus can Discriminate CVI-988 Vaccine Strain from RB-1B Very Virulent Strain in Chicken Embryonic Fibroblasts

Marek's disease is a major scourge challenging poultry health worldwide. It is caused by the highly contagious Marek's disease virus (MDV), an alphaherpesvirus. Here, we showed that, similar to other members of its Herpesviridae family, MDV also presents a complex landscape of splicing events, most of which are uncharacterised and/or not annotated. Quite strikingly, and although the biological relevance of this fact is unknown, we found that a number of viral splicing isoforms are strain-specific, despite the close sequence similarity of the strains considered: very virulent RB-1B and vaccine CVI-988. We validated our findings by devising an assay that discriminated infections caused by the two strains in chicken embryonic fibroblasts on the basis of the presence of some RNA species. To our knowledge, this study is the first to accomplish such a result, emphasizing how relevant a comprehensive picture of the viral transcriptome is to fully understand viral pathogenesis.

Glutaminolysis and Glycolysis Are Essential for Optimal Replication of Marek's Disease Virus

Viruses can manipulate host cellular metabolism to provide energy and essential biosynthetic requirements for efficient replication. Marek’s disease virus (MDV), an avian alphaherpesvirus, causes a deadly lymphoma in chickens and hijacks host cell metabolism. This study provides evidence for the importance of glycolysis and glutaminolysis, but not fatty acid β-oxidation, as an essential energy source for the replication and spread of MDV. Moreover, it suggests that in MDV infection, as in many tumor cells, glutamine is used for generation of energetic and biosynthetic requirements of the MDV infection, while glucose is used biosynthetically.

Viruses may hijack glycolysis, glutaminolysis, or fatty acid β-oxidation of host cells to provide the energy and macromolecules required for efficient viral replication. Marek’s disease virus (MDV) causes a deadly lymphoproliferative disease in chickens and modulates metabolism of host cells. Metabolic analysis of MDV-infected chicken embryonic fibroblasts (CEFs) identified elevated levels of metabolites involved in glutamine catabolism, such as glutamic acid, alanine, glycine, pyrimidine, and creatine. In addition, our results demonstrate that glutamine uptake is elevated by MDV-infected cells in vitro. Although glutamine, but not glucose, deprivation significantly reduced cell viability in MDV-infected cells, both glutamine and glucose were required for virus replication and spread. In the presence of minimum glutamine requirements based on optimal cell viability, virus replication was partially rescued by the addition of the tricarboxylic acid (TCA) cycle intermediate, α-ketoglutarate, suggesting that exogenous glutamine is an essential carbon source for the TCA cycle to generate energy and macromolecules required for virus replication. Surprisingly, the inhibition of carnitine palmitoyltransferase 1a (CPT1a), which is elevated in MDV-infected cells, by chemical (etomoxir) or physiological (malonyl-CoA) inhibitors, did not reduce MDV replication, indicating that MDV replication does not require fatty acid β-oxidation. Taken together, our results demonstrate that MDV infection activates anaplerotic substrate from glucose to glutamine to provide energy and macromolecules required for MDV replication, and optimal MDV replication occurs when the cells do not depend on mitochondrial β-oxidation.

IMPORTANCE Viruses can manipulate host cellular metabolism to provide energy and essential biosynthetic requirements for efficient replication. Marek’s disease virus (MDV), an avian alphaherpesvirus, causes a deadly lymphoma in chickens and hijacks host cell metabolism. This study provides evidence for the importance of glycolysis and glutaminolysis, but not fatty acid β-oxidation, as an essential energy source for the replication and spread of MDV. Moreover, it suggests that in MDV infection, as in many tumor cells, glutamine is used for generation of energetic and biosynthetic requirements of the MDV infection, while glucose is used biosynthetically.

Biotic concerns in generating molecular diagnosis matrixes for 4 avian viruses with emphasis on Marek's disease virus

The great advance in the field of diagnosis of avian viruses is reflecting the highly sophisticated molecular assays of the human and general virology in providing highly sensitive and fast methods of diagnosis. The present review will discuss the biotic factors and the complexities that became evident with the evolution of the novel molecular diagnostic assays with emphasis on 4 avian viruses, chicken anemia, infectious laryngotracheitis, turkey meningoencephalitis, but mainly on Marek's disease virus. To create a biologically meaningful diagnosis, attention should be dedicated to various biotic factors and not only of the diagnostic assay. Included among the important factors are, (a) the sample examined and the sampling strategy, (b) the outcomes of the pathogen amplification ex vivo, (c) the sampling time and its reflection on the disease diagnosis, (d) the impact of simultaneous multiple virus-infections regarding the ability to demonstrate all pathogens and inter- and intra-interactions between the pathogens. A concerted consideration of the relevant factors and the use of advanced molecular diagnostic assay would yield biologically significant diagnosis in real-time that would beneficiate the poultry industry.

Molecular characterization of Marek's disease virus in a poultry layer farm from Colombia

Marek's disease (MD) is a lymphoproliferative disease caused by an Alphaherpesvirus, genus Mardivirus, serotype 1 (Gallid Herpesvirus 2, GaHV-2) that includes all known pathogenic strains. In addition to Marek's disease virus (MDV) serotype 1, the genus includes 2 distinct nonpathogenic serotypes: serotype 2 (GaHV-3) and serotype 3 (Meleagridis Herpesvirus 1, MeHV-1) which are used in commercially available vaccines against MD. As a result of vaccination, clinical signs are not commonly observed, and new cases are usually associated with emerging variant strains against which the vaccines are less effective. In this study, a commercial layer farm showing clinical signs compatible with MDV infection was evaluated. Histological lesions and positive immunohistochemistry in the sciatic nerve and thymus were compatible with cytolytic phase of MD. GaHV-2, GaHV-3 and MeHV-1 were identified by PCR and qPCR in blood samples from 17 birds with suspected MD. Analysis of the Meq gene of the Colombian GaHV-2 isolate revealed a 99% sequence identity with Asian strains, and in the phylogenetic analysis clustered with vv+ MDV. The analysis of amino acid alignments demonstrated an interruption of the proline rich region in P176A, P217A and P233L positions, which are generally associated with vv+ strains. Some of these changes, such as P233L and L258S positions have not been reported previously. In addition, primary cell cultures inoculated with lymphocytes isolated from the spleen showed typical cytopathic effect of GaHV-2 at 5 d post infection. Based on the molecular analysis, the results from this study indicate the presence of vv+ MDV infection in commercial birds for the first time in Colombia. It is recommended to perform further assays in order to demonstrate the pathotype characteristics in vivo.

Differential amplification of Marek's disease CVI988 vaccine and of wild-type isolates from organs of commercial chickens using single or duplexed probes in real-time PCR

The differentiation of Marek's disease virus (MDV)-infected and vaccinated animal (DIVA) test, based on the MDV pp38 gene was described by Baigent et al. [(2016). Real-time PCR for differential quantification of CVI988 vaccine and virulent MDV strains. Journal of Virological Methods, 233, 23-36], using similar primers and alternate probes for virulent MDV-1 and the vaccine CVI988 virus. We explored the assay's applicability for commercial vaccines and commercial chickens, as the above-mentioned study employed tissue-cultured MDV strains and tissues from experimental trials. DNA of visceral organs and feathers of vaccinated or naturally infected chickens was used. Further, the applicability of the DIVA assay was evaluated using single or duplexed probes for the two viruses in the same amplification tube. Due to the high viral content in the commercial vaccines and in the clinical cases of MDV-1 infected commercial chickens, their examination by the MDV-1 DIVA real-time PCR was performed in one step. However, for the feather DNAs of commercially vaccinated birds, a step of pre-amplification was required. The MDV-1 DIVA real-time PCR performed as single probe in separate tubes using the Vir3 probe was very sensitive for virulent MDV-1 strains, but not very specific, as it also gave a clear signal with CVI988 vaccine virus. In contrast, the CVI vaccine probe was specific for CVI988, and did not recognize the MDV-1 strains. When both probes were present in one tube, the CVI probe showed a greater sensitivity for CV1988, while the Vir3 probe showed a much better specificity for virulent MDV-1.

Marek's disease herpesvirus vaccines integrate into chicken host chromosomes yet lack a virus-host phenotype associated with oncogenic transformation

Marek's disease (MD) is a lymphotropic and oncogenic disease of chickens that can lead to death in susceptible and unvaccinated host birds. The causative pathogen, MD virus (MDV), a highly oncogenic alphaherpesvirus, integrates into host genome near the telomeres. MD occurrence is controlled across the globe by biosecurity, selective breeding for enhanced MD genetic resistance, and widespread vaccination of flocks using attenuated serotype 1 MDV or other serotypes. Despite over 40 years of usage, the specific mechanism(s) of MD vaccine-related immunity and anti-tumor effects are not known. Here we investigated the cytogenetic interactions of commonly used MD vaccine strains of all three serotypes (HVT, SB-1, and Rispens) with the host to determine if all were equally capable of host genome integration. We also studied the dynamic profiles of chromosomal association and integration of the three vaccine strains, a first for MD vaccine research. Our cytogenetic data provide evidence that all three MD vaccine strains tested integrate in the chicken host genome as early as 1 day after vaccination similar to oncogenic strains. However, a specific, transformation-associated virus-host phenotype observed for oncogenic viruses is not established. Our results collectively provide an updated model of MD vaccine-host genome interaction and an improved understanding of the possible mechanisms of vaccinal immunity. Physical integration of the oncogenic MDV genome into host chromosomes along with cessation of viral replication appears to have joint signification in MDV's ability to induce oncogenic transformation. Whereas for MD vaccine serotypes, a sustained viral replication stage and lack of the chromosome-integrated only stage were shared traits during early infection.

Real-time PCR for differential quantification of CVI988 vaccine virus and virulent strains of Marek's disease virus

CVI988/Rispens vaccine, the 'gold standard' vaccine against Marek's disease in poultry, is not easily distinguishable from virulent strains of Marek's disease herpesvirus (MDV). Accurate differential measurement of CVI988 and virulent MDV is commercially important to confirm successful vaccination, to diagnose Marek's disease, and to investigate causes of vaccine failure. A real-time quantitative PCR assay to distinguish CVI988 and virulent MDV based on a consistent single nucleotide polymorphism in the pp38 gene, was developed, optimised and validated using common primers to amplify both viruses, but differential detection of PCR products using two short probes specific for either CVI988 or virulent MDV. Both probes showed perfect specificity for three commercial preparations of CVI988 and 12 virulent MDV strains. Validation against BAC-sequence-specific and US2-sequence-specific q-PCR, on spleen samples from experimental chickens co-infected with BAC-cloned pCVI988 and wild-type virulent MDV, demonstrated that CVI988 and virulent MDV could be quantified very accurately. The assay was then used to follow kinetics of replication of commercial CVI988 and virulent MDV in feather tips and blood of vaccinated and challenged experimental chickens. The assay is a great improvement in enabling accurate differential quantification of CVI988 and virulent MDV over a biologically relevant range of virus levels.

Absolute quantification of a very virulent Marek's disease virus dynamic quantity and distributions in different tissues

Chickens infected with Marek's disease virus (MDV) carry the virus consistently for a long time, which increases the incidence and rate of virus-induced multi-organ tumors and increases its potential for horizontal transmission. There is a positive correlation between very virulent (vv) MDV quantity and the pathology. The purpose of this study was to determine the vvMDV loads dynamics in different phases, and the correlation between the viral quantity and tumor development. We used a SYBR Green duplex real-time quantitative PCR (q-PCR) assay to detect and quantify MDV loads and distributions in different tissues, targeting the Eco-Q protein gene (meq) of the virus and the house-keeping ovotransferrin (ovo) gene of chickens. The q-PCR was performed using different tissue DNA preparations derived from chickens which were infected with 1,000 pfu of the SDWJ1302 strain and tissue samples were collected from control and MDV-infected birds on 7, 10, 15, 21, 28, 40, 60, and 90 d post-infection (DPI). The data indicated that the MDV genome was almost quantifiable in immune organs of infected chickens as early as 7 DPI, and the number of MDV genome copies in the blood and different organs peaked by 28 DPI, but then gradually decreased by 40 DPI. The levels of viral quantity in the lymphocytes, liver, and spleen were all higher than those in other organs, and that in the feather follicles was the highest among different phases of MDV infection. The vvMDV could still be detected in peripheral blood and tissues by 90 DPI, and the vast existence of virus will stimulate tissue destruction. The data provided further evidence of viral infection involving multi-organ distribution and mainly involving immune organ proliferation, resulting in immunosuppression.

Skin involvement in lymphomas caused by Marek's disease virus infection in Silkie chickens

The Silkie is a typical Chinese breed of chicken. In 2012, batches of Silkies were found to have diffuse tumor-like nodules on their skin after feather removal, when they were slaughtered at about 60 days old. Gross examination showed no visible neoplastic lesions on the visceral organs and peripheral nerves, except slight splenomegaly in individual chickens. The disease was prevalent, with high condemnation rates for skin lesions, which caused great economic losses to the company. Tissues, including skin, visceral organs, and peripheral nerves, were collected for histologic examination. Heparinized blood samples were collected for virus isolation and identification. Marek's disease virus (MDV), Reticuloendotheliosis virus (REV), and Avian leukosis virus (ALV) were analyzed, using polymerase chain reaction (PCR) tests. Histologic examination showed that all of the tumor-like nodules on the skin were lymphomas. Lymphoproliferative lesions occurred mostly on the skin and only a few on the viscera, including the liver and proventriculus. Infected chick embryo fibroblasts showed clear cytopathic effects; indirect fluorescent antibody test for envelope glycoprotein B was positive. In addition, PCR indicated the presence of MDV serotype 1 infection without REV and ALV. A phylogenetic tree of the Meq gene showed that the isolate (SD121201) and Chinese reference strains, which are very virulent MDVs, are in the same clade. It was concluded that the Silkies tested were infected with MDV serotype 1. The Marek's disease epidemic has been controlled using CVI988/Rispens vaccines.

Fluorescent tagging of VP22 in N-terminus reveals that VP22 favors Marek's disease virus (MDV) virulence in chickens and allows morphogenesis study in MD tumor cells

Marek’s disease virus (MDV) is an alpha-herpesvirus causing Marek’s disease in chickens, mostly associated with T-cell lymphoma. VP22 is a tegument protein abundantly expressed in cells during the lytic cycle, which is essential for MDV spread in culture. Our aim was to generate a pathogenic MDV expressing a green fluorescent protein (EGFP) fused to the N-terminus of VP22 to better decipher the role of VP22 in vivo and monitor MDV morphogenesis in tumors cells. In culture, rRB-1B EGFP22 led to 1.6-fold smaller plaques than the parental virus. In chickens, the rRB-1B EGFP22 virus was impaired in its ability to induce lymphoma and to spread in contact birds. The MDV genome copy number in blood and feathers during the time course of infection indicated that rRB-1B EGFP22 reached its two major target cells, but had a growth defect in these two tissues. Therefore, the integrity of VP22 is critical for an efficient replication in vivo, for tumor formation and horizontal transmission. An examination of EGFP fluorescence in rRB-1B EGFP22-induced tumors showed that about 0.1% of the cells were in lytic phase. EGFP-positive tumor cells were selected by cytometry and analyzed for MDV morphogenesis by transmission electron microscopy. Only few particles were present per cell, and all types of virions (except mature enveloped virions) were detected unequivocally inside tumor lymphoid cells. These results indicate that MDV morphogenesis in tumor cells is more similar to the morphorgenesis in fibroblastic cells in culture, albeit poorly efficient, than in feather follicle epithelial cells.

Relationship between levels of very virulent MDV in poultry dust and in feather tips from vaccinated chickens

To assess the effect of various vaccine strains on replication and shedding of virulent Marek's disease virus from experimentally infected chickens, quantitative PCR (q-PCR) methods were developed to accurately quantify viral DNA in infected chickens and in the environment in which they were housed. Four groups of 10 chickens, kept in poultry isolators, were vaccinated at 1 day old with one of four vaccines covering each of the three vaccine serotypes, then challenged with very virulent MDV strain Md5 at 8 days of age. At regular time-points, feather tips were collected from each chicken and poultry dust was collected from the air-extract prefilter of each isolator. DNA was extracted from feather and dust samples and subjected to real-time q-PCR, targeting the U(S)2 gene of MDV-1, in order to measure Md5 level per 10(4) feather tip cells or per microgram of dust. Accuracy of DNA extraction from dust and real-time q-PCR were validated by comparing either q-PCR cycle threshold values or the calculated MDV genome level; for use in q-PCR, DNA was extracted from serial dilutions of MDV-infected dust diluted with noninfected dust, or DNA from MDV-infected dust was diluted with DNA from noninfected dust. The results confirmed the accuracy and sensitivity of dust DNA extraction and subsequent q-PCR and showed that differences in virus levels between dust samples truly reflect differences in shedding. Vaccination delayed both replication of Md5 in feather tips and shedding of Md5. First detection of Md5 in feather tips always preceded or coincided with first detection in dust in each group. pCVI988 and HVT+SB-1 were the most efficient vaccines in reducing both replication and shedding of Md5. There was close correlation between mean virus level in feathers of each group and mean virus level in the dust shed by that group. This relationship was similar in each of the vaccinated groups, demonstrating that measurement of the virus in dust can be used to monitor accurately both the infection status of the chickens and environmental contamination by MDV.

Influence of vaccination with CVI988/Rispens on load and replication of a very virulent Marek's disease virus strain in feathers of chickens

Several highly efficacious vaccines are currently available for control of Marek's disease, a lymphoproliferative disease in chickens. However, these vaccines are unable to prevent infection with Marek's disease virus (MDV) in vaccinated birds. This leads to shedding of virulent MDV from feather follicle epithelium and skin epithelial cells of vaccinated and infected chickens. The objective of the present study was to study the interactions between a vaccine strain (CVI988/Rispens) and a very virulent strain of MDV (RB1B) in feathers. We examined genome load and replication of CVI988 and MDV-RB1B strains at various time points post infection. Moreover, we evaluated cytokine expression in feathers as indicators of immunity generated in response to vaccines against MDV. Analysis of feathers collected between 4 and 21 days post infection (d.p.i.) revealed a steady level of CVI988 genome load in the presence or absence of RB1B. Infection with MDV resulted in a significant increase in RB1B genome load peaking at 14 d.p.i. Importantly, vaccination with CVI988 resulted in a significant reduction in accumulation of MDV-RB1B in feathers. RB1B genome accumulation in feather tips was associated with increased expression of interferon-α at 14 d.p.i. and interferon-Sγ at earlier time points, 4 and 7 d.p.i. compared with 10 and 14 d.p.i. Interleukin-10 and interleukin-6 were up-regulated at 14 d.p.i. in the infected groups. This study expands our understanding of the dynamics of replication of vaccine and virulent MDV strains in the feathers and illuminates mechanisms associated with immunity to Marek's disease.

The oncogenic microRNA OncomiR-21 overexpressed during Marek's disease lymphomagenesis is transactivated by the viral oncoprotein Meq

Gallid herpesvirus 2 (GaHV-2) is an oncogenic herpesvirus that causes T lymphoma in chicken. GaHV-2 encodes a basic leucine zipper (bZIP) protein of the AP-1 family, Meq. Upon formation of homo- or heterodimers with c-Jun, Meq may modulate the expression of viral and cellular genes involved in lymphomagenesis. GaHV-2 also encodes viral microRNAs (miRNAs) involved in latency and apoptosis escape. However, little is known about cellular miRNA deregulation during the development of GaHV-2-associated lymphoma. We determined the cellular miRNA expression profiles of chickens infected with a very virulent strain (RB-1B) or a vaccine strain (CVI988) or noninfected. Among the most deregulated cellular miRNAs, we focused our efforts on gga-miR-21, which is upregulated during GaHV-2 infection. We mapped the gga-miR-21 promoter to the 10th intron of the TMEM49 gene and found it to be driven by AP-1- and Ets-responsive elements. We show here that the viral oncoprotein Meq binds to this promoter, thereby transactivating gga-miR-21 expression. We confirmed that this miRNA targets chicken programmed death cell 4 (PDCD4) and promotes tumor cell growth and apoptosis escape. Finally, gga-miR-21 was overexpressed only during infection with a very virulent strain (RB-1B) and not during infection with a nononcogenic strain (CVI988), providing further evidence for its role in GaHV-2 lymphomagenesis. Our data therefore suggest an additional role for Meq in GaHV-2-mediated lymphomagenesis through the induction of miR-21 expression.