Antigen B of the vaccine strains of Marek's disease virus and herpesvirus of turkeys presents heat-labile group and serotype specific epitopes

Out of Sight, but Not Out of Mind: Aspects of the Avian Oncogenic Herpesvirus, Marek's Disease Virus

Marek's disease virus is an economically important avian herpesvirus that causes tumors and immunosuppression in chickens and turkeys. The virus, disease, and vaccines have been known for more than 50 years, but as knowledge gaps still exists, intensive research is still ongoing. The understanding of MDV complexity can provide scientific insight in topics that cannot be experimented in humans, providing a unique model that is dually useful for the benefit of the poultry industry and for studying general herpesvirology. The present review presents the following topics: the MDV biology, the vaccine's and virulent virus' peculiar presence in feathers, protection by vaccination. In addition, two relatively behind the scenes topics are reviewed; first, the meq MDV oncogene and its recent implication in molecular epidemiology and in the MDV virulence determination, and second, the functionality of conformational epitopes of the MDV immunodominant protein, glycoprotein B. Our studies were particular, as they were the only ones describing three-dimensional MDV gB oligomers. MDV gB (glycoprotein B) continuous and discontinuous epitopes were shown to possess distinctive neutralization activities. In contrast, the significance of oligomerization of the viral membrane proteins for the creation of discontinuous epitopes in other herpesviruses was explored extensively.

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.

Sequence analysis of Meq oncogene among Indian isolates of Marek's disease herpesvirus

Marek's disease (MD), caused by Marek's disease virus (MDV), is a highly contagious neoplastic disease of chicken that can be prevented by vaccination. However, in recent years many cases of vaccine failure have been reported worldwide as chickens develop symptoms of MD in spite of proper vaccination. Distinct polymorphism and point mutations in Meq gene of MDV have been reported to be associated with virulence and oncogenicity. The present study was carried out with the objective to isolate and characterize field isolates of MDV on the basis of Meq gene. Twenty five samples of suspected cases of MD were collected and processed for virus isolation in duck embryo fibroblast (DEF) primary culture where 28% (7 of 25) samples showed characteristic cytopathic effects of MDV in the form of plaques and syncytia. Additional evidence of presence of MDV in these samples was confirmed by PCR. To analyze diversity in all seven isolates of MDV, a polymorphism study was carried out by cloning and sequencing of full length of Meq gene (1020 bp). Sequence homology of 7 isolates with 23 reference strains showed 98.10-99.40% similarity in nucleotide and 95.90-98.50% similarity in amino acid sequences. Six isolates revealed 5 repeat sequences of 4 prolines (PPPP) whereas, one isolate revealed only 4 repeats. In phylogenetic analysis, these isolates formed a separate cluster showing close relatedness to the Chinese isolates. The study indicates a high mutation rate in field isolates of MDV that may be probable cause of vaccination failure.

Marek's disease in turkeys. II. Characterization of the viral glycoprotein B gene and antigen of a turkey strain of Marek's disease virus

Marek's disease virus (MDV) causes immunosuppression and tumors in chickens. As sporadic cases of Marek's disease (MD) were recorded in turkeys, the antigenic and genomic characteristics of the MDV glycoprotein B (gB) gene and antigen of turkeys were compared to the chicken MDV gB. The whole chicken and turkey gB genes were sequenced and found identical. By immunoblotting of infected-cell culture lysates using chicken convalescent and gB monoclonal antibodies, the antigenic epitopes of the chicken and turkey viruses were found to differ. The turkey MDV had a unique epitope, compared to the chicken MDV and compared with our previous findings. While the chicken MDV had two epitope types, heat-labile but dithiothreitol (DTT)-stable and heat-stable but DTT-labile, the turkey MDV gB epitope is both heat and DTT-labile.

Molecular identification of the Marek's disease virus vaccine strain CVI988 in vaccinated chickens

The study describes three polymerase chain reaction (PCR) systems for the CVI988 vaccine virus: the meq gene, the MDV BamHI-D/H 132 bp tandem repeat fragment and the MDV-gB gene. Whereas the PCR product of virulent MDV strains and of the CVI988 virus strain with the meq and the 132 bp primer sets differed for the two templates, the MDV-gB PCR products were similar. The sensitivity of the three PCRs was determined for the two templates: the CVI988 DNA was detected up to 2.48 plaque forming units, and a MDV-1 DNA, was amplified with the 132 bp primers up to the 10(-3) DNA dilution, and up to the 10(-2) with the MDV-gB and meq gene primers. As conventional detection for the CVI988 vaccine virus is by tissue culture, the aim was to analyse the feasibility of the molecular detection of the vaccine virus in the vaccinated chick. In two experimental trials employing specific pathogen free and commercial Lohmann chicks, respectively, the vaccine virus replicated to a limited extent; it was detected only in the spleen of up to 60% chicks at 2-4 weeks and in one chick at 3 weeks, respectively. The survey of three commercial Lohmann flocks, kept in biosecurity conditions, revealed the vaccine virus only in the spleen of 40% of 30-day-old chicks. The present study shows that CV1988 DNA is present in vaccinated chicks in a low quantity and it is difficult to detect directly from the chick, probably because vaccine viruses are latent in vivo. For an efficient detection it is pertinent to cultivate the vaccine virus on chicken embryo fibroblasts (CEF), as then the virus escapes the latent state, enters into the productive mode of replication, and a high viral copy number is produced.

Virus-neutralization domains on the oligomeric (230 kDa) forms of antigen B of herpesvirus of turkeys and Marek's disease virus differ in cross-serotypic activity

Herpesvirus of turkeys (HVT) is frequently used to protect chickens against Marek's disease (MD). The HVT and MDV native antigen B complex shares common epitopes. To determine whether these oligomers present virus-neutralizing domains, monospecific antibodies to the HVT and MDV native 230 kDa oligomers were produced. The monospecific antibody immunopurified from an anti-HVT avian serum neutralized the in vitro infectivity of the oncogenic isolate MDV-B and the vaccine strains CVI988, SB1 and HVT and immunoblotted the 230 kDa oligomers of HVT and CVI988. As a result of the immunofluorescence analysis on infected cells, the monospecific antibody revealed foci of diffuse cytoplasmic immunofluorescence. A second monospecific antibody to the heat-stable 130 kDa monomer of HVT had limited neutralizing activity against HVT and CVI988 only, immunoblotted only the native HVT oligomer, and was not active in immunofluorescence. The monospecific antibody to the MDV-B 230 kDa oligomer neutralized and immunoblotted only the two MDV-1 strains but stained cells infected with MDVs of the three serotypes in immunofluorescence. It is concluded that the cross-protective neutralizing epitopes of HVT are located on heat-labile oligomeric forms of antigen B.

Common antigenic epitopes are present on heat-labile oligomers of MDV glycoprotein B and on HSV glycoprotein B

The antigenic cross-reactivity between the Marek's disease virus glycoprotein B (MDV gB) and glycoprotein B (gB) of herpes simplex virus type 1 and 2 (HSV1 and HSV2) was analysed by the immunoblotting method. We studied cell lysates in both denatured and in undenatured form (i.e., unheated) and reacted them with convalescent sera from chickens infected with the RBIB MDV strain and with human anti-HSV1 gB. Both sera detected the heat-labile MDV gB and the HSV gB oligomers. In addition, monospecific antibodies to the MDV gB 230 kDa oligomer (strain CVI988) were immunoaffinity purified from both the chicken and the human sera. The chicken and human monospecific antibodies detected the homologous and the heterologous gB oligomers in native MDV- and HSV1-infected cell lysates. 15 human sera were tested by immunoblotting and by immunofluorescence on HSV1-, CVI988-and herpes virus of turkeys (HVT)-infected cells. By both assays about half of the human sera reacted with MDV-infected cells. This study demonstrates that the MDV gB heat-labile oligomers possess conformational epitopes shared with the human alpha-herpes virus HSV1 and HSV2 gB heat-labile oligomers.

Open reading frames in a 4556 nucleotide sequence within MDV-1 BamHI-D DNA fragment: evidence for splicing of mRNA from a new viral glycoprotein gene

A DNA segment of the MDV-1 BamHI-D fragment was sequenced, and the open reading frames (ORFs) present in the 4556 nucleotide fragment were analyzed by computer programs. Computer analysis identified 19 putative ORFs in the sequence ranging from a coding capacity of 37 amino acids (aa) (ORF-1a) to 684aa (ORF-1). The special properties of four ORFs (1a, 1, 2, and 3) were investigated. Two adjacent ORFs, ORF-1a and ORF-1, were found by computer analysis to have the properties of two introns encoding a glycoprotein: ORF-1a encodes an aa sequence with the properties of a signal peptide, and ORF-1 encodes a polypeptide with a membrane anchor domain and putative N-glycosylation sites in the aa sequence. ORF-1a and ORF-1 were found to be transcribed in MDV-1-infected cells. Two RNA transcripts were detected: a precursor RNA and its spliced form. Both are transcribed from a promoter located 5' to ORF-1a, and splice donor and acceptor sites are used to splice the mRNA after cleavage of a 71-nucleotide sequence. This finding suggest that ORF-1a and ORF-1 are two introns of a new MDV-1 glycoprotein gene. The DNA sequence containing ORF-1 was transiently expressed in COS-1 cells, and the viral protein produced in these cells was found to react with anti-MDV serotype-1 Antigen B-specific monoclonal antibodies. These studies indicate that the protein encoded by ORF-1 has antigenic properties resembling Antigen B of MDV-1. A gene homologous to ORF-1 was detected in the genome of both MDV-2(SB1) and MDV-3(HVT), which serve as commercial vaccine strains. Two additional ORFs were noted in the 4556 nucleotide sequence: ORF-2, which encodes a 333 aa polypeptide initiating in the UL and terminating in the TRL prior to the putative origin of replication, and ORF-3, which encodes a 155 aa polypeptide that is partly homologous to the phosphoprotein pp38 encoded by the BamHI-H sequence. The 65 N-terminal aa of the two gene products are identical, both being derived from the nucleotide sequences in the TRL and IRL, respectively. Additional homologous aa sequences are the hydrophobic aa domain in the middle of both proteins. The functions of ORF-2, ORF-3, and additional ORFs are under study.