Expansion of a unique region in the Marek's disease virus genome occurs concomitantly with attenuation but is not sufficient to cause attenuation

Viral proteogenomic and expression profiling during productive replication of a skin-tropic herpesvirus in the natural host

Efficient transmission of herpesviruses is essential for dissemination in host populations; however, little is known about the viral genes that mediate transmission, mostly due to a lack of natural virus-host model systems. Marek's disease is a devastating herpesviral disease of chickens caused by Marek's disease virus (MDV) and an excellent natural model to study skin-tropic herpesviruses and transmission. Like varicella zoster virus that causes chicken pox in humans, the only site where infectious cell-free MD virions are efficiently produced is in epithelial skin cells, a requirement for host-to-host transmission. Here, we enriched for heavily infected feather follicle epithelial skin cells of live chickens to measure both viral transcription and protein expression using combined short- and long-read RNA sequencing and LC/MS-MS bottom-up proteomics. Enrichment produced a previously unseen breadth and depth of viral peptide sequencing. We confirmed protein translation for 84 viral genes at high confidence (1% FDR) and correlated relative protein abundance with RNA expression levels. Using a proteogenomic approach, we confirmed translation of most well-characterized spliced viral transcripts and identified a novel, abundant isoform of the 14 kDa transcript family via IsoSeq transcripts, short-read intron-spanning sequencing reads, and a high-quality junction-spanning peptide identification. We identified peptides representing alternative start codon usage in several genes and putative novel microORFs at the 5' ends of two core herpesviral genes, pUL47 and ICP4, along with strong evidence of independent transcription and translation of the capsid scaffold protein pUL26.5. Using a natural animal host model system to examine viral gene expression provides a robust, efficient, and meaningful way of validating results gathered from cell culture systems.

First report of Serotype-1 Marek's disease virus (MDV-1) with oncogenic form in backyard turkeys in Turkey: a molecular analysis study

Background

Marek’s disease (MD) is a lymphoproliferative disease caused by Gallid alphaherpesvirus 2 (GaHV-2, MDV-1), which primarily affects chickens. However, the virus is also able to induce tumors and polyneuritis in turkeys, albeit less frequently than in chickens.

Results

This is the first study in Turkey reporting the molecular characterization of a MDV-1 strain detected in a flock of backyard turkeys exhibiting visceral lymphoma. Here, MEQ, vIL-8, pp38 and 132-bp tandem repeat regions, which are frequently preferred in the pathotyping of MDV-1, were examined. It was determined that the MEQ gene of MDV-1/TR-21/turkey strain obtained in the present study encoded 339 amino acids (1020 nt) and had four proline-rich repeat regions (PPPP). Based on the nucleotide sequence of the MEQ gene of the MDV-1/TR-21/turkey strain, a phylogenetic tree was created using the MEGA-X software with the Maximum Likelihood Method (in 1000 replicates). Our strain was highly identical (> 99.8) to the Italian/Ck/625/16, Polish (Polen5) and some Turkish (Layer-GaHV-2-02-TR-2017, Tr/MDV-1/19) MDV-1 strains. Also, nt and aa sequences of the MEQ gene of our strain were 99.1 and 99.41% identical to another Turkish strain (MDV/Tur/2019) originated from chickens. Sequence analysis of pp38 and vIL-8 genes also supported the above finding. The identity ratios of nucleotide and amino acid sequences of vIL-8 and pp38 genes of MDV-1/TR-21/turkey strain were 99.64–100% and 99.79–100%, respectively, when compared with those of the Polish strain. According to 132-bp tandem repeat PCR results, the MDV-1/TR-21/turkey strain had five copies.

Conclusions

These results suggested that the MDV-1/TR-21/turkey strain obtained from backyard turkeys can be either very virulent or very virulent plus pathotype, though experimental inoculation is required for precise pathotyping.

Role of Marek's Disease Virus (MDV)-Encoded US3 Serine/Threonine Protein Kinase in Regulating MDV Meq and Cellular CREB Phosphorylation

Marek's disease (MD) is a neoplastic disease of chickens caused by Marek's disease virus (MDV), a member of the subfamily Alphaherpesvirinae Like other alphaherpesviruses, MDV encodes a serine/threonine protein kinase, U_S3. The functions of U_S3 have been extensively studied in other alphaherpesviruses; however, the biological functions of MDV U_S3 and its substrates have not been studied in detail. In this study, we investigated potential cellular pathways that are regulated by MDV U_S3 and identified chicken CREB (chCREB) as a substrate of MDV U_S3. We show that wild-type MDV U_S3, but not kinase-dead U_S3 (U_S3-K220A), increases CREB phosphorylation, leading to recruitment of phospho-CREB (pCREB) to the promoter of the CREB-responsive gene and activation of CREB target gene expression. Using U_S3 deletion and U_S3 kinase-dead recombinant MDV, we identified U_S3-responsive MDV genes during infection and found that the majority of U_S3-responsive genes were located in the MDV repeat regions. Chromatin immunoprecipitation sequencing (ChIP-seq) studies determined that some U_S3-regulated genes colocalized with Meq (an MDV-encoded oncoprotein) recruitment sites. Chromatin immunoprecipitation-PCR (ChIP-PCR) further confirmed Meq binding to the ICP4/LAT region, which is also regulated by U_S3. Furthermore, biochemical studies demonstrated that MDV U_S3 interacts with Meq in transfected cells and MDV-infected chicken embryonic fibroblasts in a phosphorylation-dependent manner. Finally, in vitro kinase studies revealed that Meq is a U_S3 substrate. MDV U_S3 thus acts as an upstream kinase of the CREB signaling pathway to regulate the transcription function of the CREB/Meq heterodimer, which targets cellular and viral gene expression.IMPORTANCE MDV is a potent oncogenic herpesvirus that induces T-cell lymphoma in infected chickens. Marek's disease continues to have a significant economic impact on the poultry industry worldwide. U_S3 encoded by alphaherpesviruses is a multifunctional kinase involved in the regulation of various cellular pathways. Using an MDV genome quantitative reverse transcriptase PCR (qRT-PCR) array and chromatin immunoprecipitation, we elucidated the role of MDV U_S3 in viral and cellular gene regulation. Our results provide insights into how viral kinase regulates host cell signaling pathways to activate both viral and host gene expression. This is an important step toward understanding host-pathogen interaction through activation of signaling cascades.

Natural co-infection with two virulent wild strains of Marek's disease virus in a commercial layer flock

Marek's disease (MD) is a highly contagious lymphoproliferative poultry disease caused by the oncogenic herpesvirus, Marek's disease virus (MDV). MDV strains have shown a continued evolution of virulence leading to immune failure, and MD cases continue to occur. Co-infection of virulent MDV strains is an important factor leading to viral evolution and host immune failure. This study conducted a laboratory diagnosis and analysis of a MDV infected flock. Testing showed that all samples were MDV positive. PCR detection identified a variable 132-base pair repeat (132-bpr) sequence copy number. This indicated that two virulent strains of MDV were co-infecting the flock. Therefore, we performed homology, sequence alignment, and phylogenetic tree analysis of MDV variant genes including meq, pp38, and RLORF4. Two MDV strains had co-infected the flock; one was the 132bpr two-copy characteristic strain (AH2C) and the other was a 132bpr three-copy characteristic strain (AH3C). Specific mutations in AH3C were found, suggesting that it is a new variant strain. Furthermore, the viral load of the two strains in vivo indicated that both strains had high and similar replication ability. There was no significant difference in the proportion of positive samples of the two strains causing disease. In the whole flock, neither strain displayed an obvious advantage. However, there was a dominant strain in individual chickens, with the exception of one sample. This study reported the co-infection regularity of two virulent MDV strains in the same flock, and even in the same chicken in field conditions. In the context of overall epidemiology, this study is a useful reference.

Isolation and purification of Gallid herpesvirus 2 strains currently distributed in Japan

Gallid herpesvirus 2 (GaHV-2) causes malignant lymphomas in chickens (Marek’s disease, MD). Although MD is controlled through vaccination efforts, field isolates of GaHV-2 have increased in virulence worldwide and even cause MD in vaccinated chickens. GaHV-2 strains are classified into four categories (mild, virulent, very virulent and very virulent +) based on the virulence exhibited in experimental infection in unvaccinated or MD-vaccinated susceptible chickens. Although MD cases are sporadically reported in Japan, the recent field strains of GaHV-2 in Japan have not been characterized. During isolation of recent field strains by using primary chicken kidney cell cultures, a method classically used for GaHV-2 isolation, vaccine strains were simultaneously isolated. Therefore, it is necessary to separate vaccine strains to characterize the virulence and pathogenicity of the GaHV-2 strains currently distributed in Japan. In this study, we prepared cell suspensions from the spleens of MD-symptomatic chickens, inoculated day-old-chicks and isolated GaHV-2 strains by primary chicken kidney cell cultures at 2−3 weeks post inoculation. The isolated strains were passaged several times on chicken embryo fibroblast cells, and PCR analysis revealed that the isolated strains were not contaminated with vaccine strains. Moreover, the contaminant vaccine strains were completely removed by the purification of plaques observed in chicken kidney cells. These procedures are necessary to isolate GaHV-2 field strains from vaccine strains in order to carry out future studies to characterize these strains and glean insights into GaHV-2 virulence and pathogenicity.

Characterizing the molecular basis of attenuation of Marek's disease virus via in vitro serial passage identifies de novo mutations in the helicase-primase subunit gene UL5 and other candidates associated with reduced virulence

Marek's disease (MD) is a lymphoproliferative disease of chickens caused by the oncogenic Gallid herpesvirus 2, commonly known as Marek's disease virus (MDV). MD vaccines, the primary control method, are often generated by repeated in vitro serial passage of this highly cell-associated virus to attenuate virulent MDV strains. To understand the genetic basis of attenuation, we used experimental evolution by serially passing three virulent MDV replicates generated from an infectious bacterial artificial chromosome (BAC) clone. All replicates became completely or highly attenuated, indicating that de novo mutation, and not selection among quasispecies existing in a strain, is the primary driving force for the reduction in virulence. Sequence analysis of the attenuated replicates revealed 41 to 95 single-nucleotide variants (SNVs) at 2% or higher frequency in each population and several candidate genes containing high-frequency, nonsynonymous mutations. Five candidate mutations were incorporated into recombinant viruses to determine their in vivo effect. SNVs within UL42 (DNA polymerase auxiliary subunit) and UL46 (tegument) had no measurable influence, while two independent mutations in LORF2 (a gene of unknown function) improved survival time of birds but did not alter disease incidence. A fifth SNV located within UL5 (helicase-primase subunit) greatly reduced in vivo viral replication, increased survival time of birds, and resulted in only 0 to 11% disease incidence. This study shows that multiple genes, often within pathways involving DNA replication and transcriptional regulation, are involved in de novo attenuation of MDV and provides targets for the rational design of future MD vaccines.

IMPORTANCE

Marek's disease virus (MDV) is a very important pathogen in chickens that costs the worldwide poultry industry $1 billion to $2 billion annually. Marek's disease (MD) vaccines, the primary control method, are often produced by passing virulent strains in cell culture until attenuated. To understand this process, we identified all the changes in the viral genome that occurred during repeated cell passage. We find that a single mutation in the UL5 gene, which encodes a viral protein necessary for DNA replication, reduces disease incidence by 90% or more. In addition, other candidate genes were identified. This information should lead to the development of more effective and rationally designed MD vaccines leading to improved animal health and welfare and lower costs to consumers.

Identification of a neurovirulence factor from Marek's disease virus

In addition to tumors, Marek's disease (MD) virus (MDV) can induce a variety of syndromes linked to the central nervous system. In fact, early descriptions of MD suggested that it was a condition affecting mainly the nervous system. Cytokines and other immune-related genes have been suggested to play a crucial role in MDV-mediated neuropathology, but the mechanisms behind the viral-induced neurologic dysfunction are still poorly understood. In the present study we have used reverse genetic strategies to show that pp14 is not involved in the oncogenic phenotype of MDV1 and is not required for viral replication; however, we provide evidence indicating that the absence of pp14 expression is correlated with increased survival of MDV1-infected chickens, and that its expression is associated with enhanced viral neurovirulence. Our data identify for the first time pp14 as a neurovirulence factor from MDV1 and open the possibility to investigate the molecular mechanisms by which pp14 mediates the damage to the avian nervous system.

Properties of a meq-deleted rmd5 Marek's disease vaccine: protection against virulent MDV challenge and induction of lymphoid organ atrophy are simultaneously attenuated by serial passage in vitro

We have previously shown that deletion of the meq gene from the genome of Cosmid-cloned rMd5 strain of Marek's disease virus (MDV-1) resulted in loss of transformation and oncogenic capacity of the virus. The rMd5deltaMeq (Meq null) virus has been shown to be an excellent vaccine in maternal antibody positive (MAb+) chickens challenged with a very virulent plus (vv+) strain of MDV, 648A. The only drawback was that it retained its ability to induce bursa and thymus atrophy (BTA) like that of the parental rMd5 in maternal antibody negative (MAb-) chickens. We recently reported that the attenuated Meq null virus did not induce BTA at the 40th cell culture passage onward. Its protective ability against challenge with vv+ MDV, strain 686 was similar to the original virus at the 19th passage in MAb- chickens. In this study, we compared the same series of attenuated meq null viruses in commercial chickens. In commercial chickens with MAb, the attenuated viruses quickly lost protection with increasing cell culture attenuation. These data suggest that although attenuation of these meq null viruses eliminated BTA, it had no influence on their protective efficacy in MAb- chickens. However, in commercial chickens (MAb+), the best protection was provided by the original 19th passage; the attenuated 40th passage was as good as one of the currently commercial CVI988/Rispens vaccine, and it did not induce BTA. Therefore, protection against virulent MDV challenge and induction of lymphoid organ atrophy are simultaneously attenuated by serial passage in vitro.

Latency and tumorigenesis in Marek's disease

Despite the remarkable progress in our understanding of Marek's disease (MD) and the causative Marek's disease virus (MDV) biology, a number of major features of this complex viral disease remain unknown. Significant information on critical aspects of virus latency in lymphoid cells, and the virus-host interaction in MDV-induced lymphoma, remains to be identified. Moreover, the nature of the unique milieu of the feather follicle epithelial cell that allows cytolytic infection to continue, despite maintaining the latent infection in the lymphoid cells, is not fully understood. Although there has been significant progress in our understanding of the functions of a number of viral genes in the pathogenesis of the disease, the characteristics of the latent infection, how it differs from tumor phase, and whether latency is a prerequisite for the tumor phase are all important questions still to be answered. Reticuloendotheliosis virus-transformed cell lines have been shown to support MDV latency in a manner almost identical to that seen in MDV-transformed cell lines. There are increasing data on the role of epigenetic regulation, including DNA methylation and histone modifications, in maintaining viral latency. Onset of MD tumor is relatively rapid, and recent studies based on chromosomal integration and T-cell repertoire analysis demonstrated the clonal nature of MD lymphomas. Among the viral determinants of oncogenicity, the basic leucine zipper protein Meq is considered to be the most important and the most extensively studied. Deleting the Meq proteins or abolishing some of the important interactions does affect the oncogenicity of the virus. In addition, the noncoding sequences in the viral genome, such as the viral telomerase RNA and the virus-encoded microRNAs, also have significant influence on MDV-encoded oncogenesis.

Competition between two virulent Marek's disease virus strains in vivo

Previous studies have demonstrated the presence of multiple strains of Marek's disease virus simultaneously circulating within poultry flocks, leading to the assumption that individual birds are repeatedly exposed to a variety of virus strains in their lifetime. Virus competition within individual birds may be an important factor that influences the outcome of co-infection under field conditions, including the potential outcome of emergence or evolution of more virulent strains. A series of experiments was designed to evaluate virus competition within chickens following simultaneous challenge with two virulent serotype 1 Marek's disease virus strains, using either pathogenically similar (rMd5 and rMd5/pp38CVI) or dissimilar (JM/102W and rMd5/pp38CVI) virus pairs. Bursa of Fabricius, feather follicle epithelium, spleen, and tumour samples were collected at multiple time points to determine the frequency and distribution of each virus present using pyrosequencing, immunohistochemistry and virus isolation. In the similar pair, rMd5 appeared to have a competitive advantage over rMd5/pp38CVI, which in turn had a competitive advantage over the less virulent JM/102W in the dissimilar virus pair. Dominance of one strain over the other was not absolute for either virus pair, as the subordinate virus was rarely eliminated. Interestingly, competition between two viruses with either pair rarely ended in a draw. Further work is needed to identify factors that influence virus-specific dominance to better understand what characteristics favour emergence of one strain in chicken populations at the expense of other strains.

Dynamic equilibrium of Marek's disease genomes during in vitro serial passage

Attenuation of Gallid herpesvirus-2 (GaHV-2), the causative agent of Marek's disease, can occur through serial passage of a virulent field isolate in avian embryo fibroblasts. In order to gain a better understanding of the genes involved in attenuation and associate observed changes in phenotype with specific genetic variations, the genomic DNA sequence of a single GaHV-2 virulent strain (648A) was determined at defined passage intervals. Biological characterization of these "interval-isolates" in chickens previously indicated that the ability to induce transient paralysis was lost by passages 40 and the ability to induce persistent neurological disease was lost after passage 80, coincident with the loss of neoplastic lesion formation. Deep sequencing of the interval-isolates allowed for a detailed cataloguing of the mutations that exist within a single passage population and the frequency with which a given mutation occurs across passages. Gross genetic alterations were identified in both novel and well-characterized genes and cis-acting regions involved in replication and cleavage/packaging. Deletions in genes encoding the virulence factors vLipase, vIL8, and RLORF4, as well as a deletion in the promoter of ICP4, appeared between passages 61 and 101. Three mutations in the virus-encoded telomerase which predominated in late passages were also identified. Overall, the frequency of mutations fluctuated greatly during serial passage and few genetic changes were absolute. This indicates that serial passage of GaHV-2 results in the generation of a collection of genomes with limited sequence heterogeneity.

Pathotyping of Australian isolates of Marek's disease virus and association of pathogenicity with meq gene polymorphism

We report the pathotyping of six Australian isolates of Marek's disease virus-1 (MDV1) isolated between 1992 and 2004 and association of virulence with meq gene polymorphism. Unvaccinated and herpesvirus of turkeys (HVT)-vaccinated specific pathogen free chickens were challenged at day 5 with 500 plaque forming units of Marek's disease virus. The isolates induced gross Marek's disease lesions in 53 to 94% of unvaccinated chickens, and HVT induced a protective index ranging from 38 to 100% by 56 days post challenge. This experiment provides evidence that current Australian isolates of MDV1 vary significantly in pathogenicity. However, there was no clear evidence that the most virulent recent isolates were more pathogenic than isolates from the 1980s or that any of the isolates belong to the highest pathotype category of very virulent plus. Evidence is presented that virulence can be predicted by measurements taken as early as 13 days post challenge. The meq gene sequences of five of the isolates used in the experiment were determined. When compared with the very virulent US isolate Md5, there was a 177 base-pair insertion and distinct point mutations in each of the five isolates. There were no individual mutations in the meq sequences that correlated with levels of virulence. However, amino acid alignment of the five Australian and 14 international isolates revealed that the number of repeat sequences of four prolines (PPPP repeats) in the meq gene (overall range 2 to 8) was strongly associated with virulence across all isolates, with the most pathogenic isolates having the fewest number of repeats. The results suggest that the presence of the 177 base-pair insertion alone is not an indicator of attenuation. Rather, the number of PPPP repeats, independent of the presence of the insertion, is a better indicator of pathogenicity.

Cell culture attenuation eliminates rMd5ΔMeq-induced bursal and thymic atrophy and renders the mutant virus as an effective and safe vaccine against Marek's disease

Marek's disease virus (MDV) encodes a basic leucine zipper oncoprotein, Meq, which structurally resembles jun/fos family of transcriptional activators. It has been clearly demonstrated that deletion of Meq results in loss of transformation and oncogenic capacity of MDV. The rMd5ΔMeq virus provided superior protection than CVI988/Rispens vaccine in 15×7 chickens when challenged with a very virulent plus (vv+) strain of MDV, 648A. The rMd5ΔMeq construct was also shown to be an effective vaccine in commercial chickens that were challenged under field conditions by exposure to seeder chicken inoculated with MDV strain 686, a vv+ and arguably the most pathogenic strain of MDV. Although deletion of Meq gene renders the virus non-oncogenic, it still induces lymphoid organ atrophy like that of the parental rMd5, in highly susceptible MDV maternal antibody negative (MAb-) chickens. We have generated 50 cell culture passages of attenuated rMd5ΔMeq viruses and found no significant lymphoid organ atrophy beginning at 40(th) passage onward when compared with the normal control chickens. The protective ability of these attenuated Meq null viruses against challenge with vv+ MDV strain 686 is similar to the original virus at 19(th) passage in maternal antibody negative chickens. The data indicate that attenuation of these Meq null viruses has no influence on their protective efficacy, but eliminated lymphoid organ atrophy and rendered them safe to use even in MAb- chickens, a characteristic that should facilitate commercialization and licensing by vaccine manufacturers.

Accumulation of attenuating mutations in varying proportions within a high passage very virulent plus strain of Gallid herpesvirus type 2

Marek's disease (MD) is controlled through mass vaccination. Although these vaccines reduce or delay tumor formation they fail to induce sterilizing immunity and prevent virus shedding. Relatively little is known about the genetic changes that lead to attenuation. It has been established that serial passage of virulent strains in avian cell lines results in the generation of attenuated progeny at some undefined passage level. A detailed cataloging of the mutations needed for attenuation will be important for advancing our understanding of MD biology and should facilitate the development of better vaccines. Using deep sequencing, the complete nucleotide sequence of the very virulent plus (vv+) strain 648A representing the 101 st passage was determined. Pathotyping studies have indicated that 648Ap101 is indeed attenuated. Comparative sequence analysis with the 648A strain at passage 11 has identified numerous gross genetic changes and subtle single nucleotide polymorphisms scattered throughout the genome. Relative to the strain (648Ap11), deletions were identified in MD-specific genes located in the repeat long (R(L)), unique long (U(L)) and repeat short (R(S)) regions. A deletion in the R(L) region, present in 33% of the p101 sequences, mapped to the genes encoding viral interleukin 8 (vIL8), RLORF4 and RLORF5. An R(S) deletion was mapped to the Meq oncoprotein binding site within the ICP4 promoter and was present in 97.8% of the p101 sequences. A short deletion in the U(L) region mapped to the 3' terminus of the gene encoding vLipase and was present in 54% of the p101 sequences. The cis-acting sites involved in DNA replication and packaging also contained deletions in varying proportions (64% and 100%, respectively). Three mutations, present in 100% of p101 sequences, were identified in the overlapping genes encoding the Arg-rich protein (MDV002/079) and the virus-encoding telomerase (vTR). Varying proportions of 12 single nucleotide polymorphisms (SNPs) were identified within 11 open reading frames (ORFs) and 3 noncoding regions. This comparative sequencing study has provided a wealth of information regarding genetic changes which have occurred during the attenuation process and has indicated that serial passage results in the generation of mixed populations.

Variations in the H/ACA box sequence of viral telomerase RNA of isolates of CVI988 Rispens vaccine

The use of the complete DNA sequence for the Marek's disease virus (MDV) serotype 1 vaccine strain CVI988 Rispens in comparative genomic studies with virulent strains of MDV has revealed the presence of a number of insertions, deletions and single-nucleotide polymorphisms. In this study, we investigated a SNP in the H/ACA box of the viral RNA subunit of telomerase (vTR). We sequenced vTR from four different batches of CVI988 vaccine originating from a single commercial company. The A-to-G mutation defining the SNP in the H/ACA box of CVI988 vTR was present in only some of the batches. Thus, although this mutation affects CVI988 vTR function, it is not shared by all CVI988 isolates and may be a stochastic rather than causative event in CVI988 attenuation.

Sequence determination of a mildly virulent strain (CU-2) of Gallid herpesvirus type 2 using 454 pyrosequencing

The complete DNA sequence of the mildly virulent Gallid herpesvirus type 2 strain CU-2 was determined and consists of 176,922 bp with an overall gene organization typical of class E herpesviruses. Phylogenetically, this strain partitions in its own branch between the virulent strains RB-1B, Md11, and Md5, and the vaccine strain CVI988. Overall, the genome of CU-2 is more similar to that of CVI988, with identically sized unique short regions of 11,651 bp. As in CVI988, an insertion of 177 bp was identified in the overlapping genes encoding the Meq, RLORF6, and 23 kDa proteins within the repeat long region of the genome. A total of 15 single nucleotide polymorphisms (SNPs) common to both CU-2 and CVI988, and not occurring in virulent strains, were identified in the genes encoding UL29, UL45, UL50, UL52, LORF10, RLORF14a, RLORF12, Meq(RLORF7), 23kDa, ICP4, US3, and two hypothetical proteins MDV071.4 and MDV076.4. Each gene encoding UL29 and Meq contained two SNPs. Only one major open reading frame (ORF) encoding UL41, the virus host shutoff (VHS) ribonuclease, was disrupted in the CU-2 genome. An additional cytosine after the 25 codon is predicted to produce a truncated protein of 97 aa. Since GaHV-2 mutants lacking UL41 have been reported to retain their virulence, other factors are likely responsible for the low virulence of CU-2. It is largely suspected that SNPs in common with CVI988 along with the insertions in the Meq loci are responsible for its phenotype. Conversely, we identified 43 nonsynonymous mutations (within 23 genes) that may contribute to the virulence of CU-2. These SNPs are shared exclusively with all sequenced virulent strains (Md5, Md11, and RB-1B) and not present within the CVI988 genome. Although most occur in proteins of unknown function, a significant percentage is in proteins involved in virion assembly.

Characterization of LORF11, a unique gene common to the three Marek's disease virus serotypes

The unique open reading frame 11 (LORF11) of Marek's disease virus (MDV) is present in all three serotypes of MDV and is located in the unique long region of the MDV genome. In the serotype 1 Md5 genome, LORF11 comprises 2711 nucleotides and encodes a predicted protein of 903 amino acids. In order to study the biological function of LORF11 we deleted it from the MDV cosmid A6 by using the RecA-assisted restriction endonuclease cleavage method. The recombinant cosmid, A6DeltaLORF11, was transfected into duck embryo fibroblasts (DEF) in conjunction with parental SN5, P89, SN16, and B40 cosmid clones. Recombinant rMd5DeltaLORF11 plaques were evident at 12-13 days after transfection. Polymerase chain reaction amplification of DEF cells infected with rMd5DeltaLORF11 viruses confirmed the deletion of a 2.57-kb fragment resulting in a 296-bp fragment. Three rMd5DeltaLORF11 mutants were generated and their biological functions were studied in vitro and in vivo. In vitro growth characteristics of rMd5DeltaLORF11 viruses were similar to those of parental rMd5, indicating that LORF11 is not essential for replication in vitro. In vivo studies of rMd5DeltaLORF11 mutants showed that they were impaired in viral replication in the lymphoid organs and had 100x lower viremia than chickens infected with the parental rMd5 virus. Furthermore, rMd5-infected chickens horizontally transmitted the virus to contact controls whereas no horizontal transmission occurred in rMd5DeltaLORF11-infected chickens. Three independent deletion mutants were tested and showed the same phenotypes, so it is unlikely that the observed phenotype is because of any random mutation in the genome. Therefore the LORF11 gene of MDV is essential for normal virus replication in chickens and deletion of LORF11 renders an attenuated virus.

Marek's disease virus unique genes pp38 and pp24 are essential for transactivating the bi-directional promoters for the 1.8 kb mRNA transcripts

The pp38 and pp24 genes of Marek's diseases virus (MDV) share the same promoter, which controls the transcription of pp38 or pp24 and a 1.8-kb mRNA bi-directionally. To understand the trans-activating activity of pp38 and pp24 on the bi-directional promoter, both genes were cloned into pcDNA-3 or pBudCE4.1 vectors either singly or in combination. These plasmids were expressed in transfected chicken embryonic fibroblast (CEF) cells. Chloramphenicol acetyltransferase (CAT) activity expressed under the control of the promoter in CEF co-transfected with pP(1.8 kb)-CAT and pBud-pp38-pp24 was significantly higher than that following transfection with only pBud-pp38 or pBud-pp24. This indicates the combination of pp24 and pp38 together are essential for the activation of the promoter. In DNA mobility shift assays, the promoter binds to pp38 and pp24 together, but not to pp38 or pp24 alone. By competitive inhibition tests with a set of DNA fragments from the promoter region, the sequence 5'-CTGCTCATTT-3' was identified as the core sequence for binding by pp38-pp24 in up-regulation of the bi-directional promoter activity.

Sequence determination of variable regions within the genomes of gallid herpesvirus-2 pathotypes

Comparative genomic studies of attenuated and virulent strains of Gallid herpesvirus 2 (GaHV-2) have identified 6 regions of sequence variability. These regions include the open reading frames (ORFs) encoding UL36 and UL49 and regions devoid of large ORFs (132-bp repeats, a-like sequences and the junctions flanking the unique short region). Our data indicate that the carboxyl terminus of UL36 contains regions of heterogeneity that are unique to CVI988-derived attenuated strains. A deletion of the TKSERT domain and a glycine(245) polymorphism in the UL49 proteins were also identified in these derivatives. Phylogenetic analyses of both UL36 and UL49 sequences indicate that CVI988-derived strains partition differently from other attenuated strains (RM-1 and R2/23), indicating that additional mutations contribute to attenuation. In very virulent and very virulent plus strains a single nucleotide polymorphism (SNP) was identified within the 132-bp tandem repeats. Within the junctions flanking the unique short region, these strains also contain deletions in sequences that are predicted to bind the transcription factor NF kappaB. In some attenuated strains, deletions were also identified in the latency-associated transcript (LAT) promoters and adjacent regions encoding microRNAs. These results indicate that virulence is likely multi-factorial with contributions from both multiple genes and cis-acting sites.

Marek's disease virus: lytic replication, oncogenesis and control

Marek's disease (MD) is caused by a ubiquitous, lymphotropic alphaherpesvirus, MD virus (MDV). MD has been a major concern in the poultry industry owing to the emergence of increasingly virulent strains over the last few decades that were isolated in the face of comprehensive vaccination. The disease is characterized by a variety of clinical signs; among them are neurological symptoms, chronic wasting and, most notably, the development of multiple lymphomas that manifest as solid tumors in the viscera and musculature. Much work has been devoted to study MD-induced oncogenesis and the genes involved in this process. Among the many genes encoded by MDV, a number have been shown recently to affect the development of tumors in chickens, one protein directly causing transformation of cells (Meq) and another being involved in maintaining transformed cells (vTR). Other MDV gene products modulate and are involved in early lytic in vivo replication, thereby increasing the chance of transformation occurring. In this review, we will summarize specific genes encoded by MDV that are involved in the initiation and/or maintenance of transformation and will focus mostly on current vaccination and control strategies against MD, particularly how modern molecular biological methods may be used to improve strategies to combat the disease in the future.

Oncogenic Marek’s disease viruses lacking the 132 base pair repeats can still be attenuated by serial in vitro cell culture passages

Marek's disease virus (MDV) can be attenuated by serially passing the virus in cell culture. During cell culture passage, two copies of a 132 bp repeat are expanded to over 30 copies. We deleted the two copies of the 132 bp repeat region in a pathogenic MDV and demonstrated that the virus was still pathogenic. The pattern and frequency of tumors in the parental and mutant virus were the same. Early virus replication, and the appearance of persistent neurological disease were also similar between the parental and deleted virus. Nevertheless, wild-type MDV and the deletion virus could be attenuated by serial in vitro cell culture passages. Based upon analyzing the passage 40 viruses, attenuation of the MDV lacking the 132 bp repeats appears to occur in a manner that is analogous to the process occurring wild-type MDV attenuation. Whatever process is involved in the cell culture attenuation of MDV, the mechanism does not involve the 132 bp repeat region.

The role of pp38 in regulation of Marek's disease virus bi-directional promoter between pp38 and 1.8-kb mRNA

Marek's disease virus (MDV) contains a bi-directional promoters located between pp38 gene and 1.8-kb mRNA in the long inverted repeat region of the viral genome. The involvement of pp38 gene in up-regulating the activity of these promoters was analyzed by transient expression of chloramphenicol acetyltransferase (CAT) reporter gene. Two CAT reporter plasmids, pP(pp38)-CAT and pP(1.8-kb)-CAT, were constructed to express CAT under the control of the bi-directional promoter in both orientations. These plasmids were transfected into chicken embryonic fibroblast (CEF), infected with rMd5 and pp38 deleted rMd5 (rMd5/Deltapp38), respectively. No CAT activity was detected in uninfected CEF as expected. CAT activities in rMd5/Deltapp38 virus infected CEF (rMd5/Deltapp38-CEF) were 3.5-fold lower using pP(pp38)-CAT and 12-fold lower using pP(1.8-kb)-CAT than those of the parental rMd5 infected CEF (rMd5-CEF). The significantly lower promoter activity in the pp38 deletion virus suggests that pp38 can regulate the activity of the bi-directional promoters, especially in the direction of 1.8-kb mRNA family. Co-transfection of pp38-expressing plasmid (pcDNA-pp38) into rMd5/Deltapp38-CEF significantly increased the activity of the bi-directional promoters using either pP(pp38)-CAT or pP(1.8-kb)-CAT. DNA mobility shift assay showed a binding of the 73-bp sequence of the bi-directional promoter with rMd5-CEF but not with rMd5/Deltapp38-CEF or uninfected CEF lysates. However, rMd5/Deltapp38-CEF lysates could bind the same 73-bp promoter sequence when co-transfected with pp38-expressing plasmid (pcDNA-pp38). All these data taken together suggest pp38 plays an important role in regulating the transcriptional activity of the bi-directional promoter.

Stability of Marek's disease virus 132-bp repeats during serial in vitro passages

The Marek's disease virus (MDV) genome contains 2 sets of 132-bp tandem repeat sequences. An increase in 132-bp repeat units has been associated with attenuation of oncogenicity during in vitro passage. By cloning entire genomes, we demonstrated that the copy number of 132-bp repeats can differ within an individual MDV genome. The stability of the 132-bp repeats during cell passage depended on the initial copy number. When both sets of repeats contained 2 copies, the copy number remained stable, while if even 1 set of repeats contained 6 copies, repeat expansion occurred relatively quickly. This expansion did not affect the in vitro growth curve. However, when MDV clones with low and high copy numbers were passed together, genomes with expanded repeats rapidly predominated, mimicking the behavior of naturally-occurring MDV. These results suggest that the preponderance of high-copy repeats after passage reflects intracellular copy number within individual infected cells rather than an influence on the spread of the virus.

Replication kinetics of Marek's disease vaccine virus in feathers and lymphoid tissues using PCR and virus isolation

CVI988 (Rispens), an avirulent strain of Marek's disease virus, is the most widely used vaccine against Marek's disease. The kinetics of replication of CVI988 was examined in tissues of chickens vaccinated at either 1 day or 14 days of age and sampled regularly up to 28 days post-vaccination. Age at vaccination had no significant effect on the kinetics of CVI988 virus replication. During the cytolytic phase of infection (1-7 days), virus levels peaked in the spleen, bursa and thymus with very close correlation among these organs. Virus load in peripheral blood lagged behind and did not reach high levels. Significant numbers of virus genomes were detected in the feather tips only after 7 days, but subsequently rose to levels almost 10(3)-fold greater than in the other tissues. This is the first accurate quantitative data for kinetics of CVI988 replication in a variety of tissues. There was good correlation between data from virus isolation and PCR, with real-time PCR being the preferred method for rapid, accurate and sensitive quantification of virus. Feathers were ideal for non-invasive sampling to detect and measure CVI988 in live chickens and, from 10 days onwards, virus load in feather tips was predictive of virus load in lymphoid tissues where immune responses will occur. The potential for real-time PCR analysis of feather samples for further investigation of the mechanism of vaccinal protection, and to assist optimization of vaccination regimes, is discussed.

Classification of Marek's disease viruses according to pathotype: philosophy and methodology

The concept of pathotype in Marek's disease (MD) probably dates from the recognition of a more virulent form of the disease in the late 1950s (Benton & Cover, 1957). Distinctions between MD virus strains were further expanded with the description of the vv pathotype in the early 1980s and of the vv+ pathotype in the 1990s. Pathotype designations reflect important biological properties that correlate with the break-through of vaccinal immunity in the field. However, pathotyping methods applied by various laboratories have not been uniform, preventing critical comparison of results. Better uniformity of pathotyping procedures is desirable.The Avian Disease and Oncology Laboratory (ADOL) method is based on induction of lymphoproliferative lesions in vaccinated chickens. This method has been used to pathotype more than 45 isolates and is the basis for the current pathotype classification of MD virus strains. Its limitations include requirements for a specific type of chickens (15x7 ab+), large numbers of animals, and a statistical method to compare lesion responses to those of JM/102W and Md5 control strains. Because of these limitations, it has not been and is not likely to be used in other laboratories. Comparability in pathotyping can be improved by the comparison of field isolates with standard prototype strains such as JM/102W, Md5 and 648A (American Type Culture Collection) or their equivalents. Data may be generated by different in vivo procedures that measure tumour induction, neurological disease (both neoplastic and non-neoplastic lesions), or solely non-neoplastic criteria (such as lymphoid organ weights or virus replication). Methods based on neoplastic criteria, especially when generated in MD-immunized chickens, will probably correlate most closely with that of the ADOL method and be most relevant to evolution of MD virus in the field. Based on data from several trials, a modification of the ADOL method that utilizes fewer chickens and can be conducted with commercial specific pathogen free strains is proposed. The modified method is based on "best fit" comparisons with prototype strains, and is expected to provide results generally comparable with the original method. A variety of other alternative criteria (see earlier) are also evaluated both for primary pathotyping and as adjuncts to other pathotyping methods. Advantages and disadvantages of alternative methods are presented.

Attenuation of Marek's disease virus by deletion of open reading frame RLORF4 but not RLORF5a

Marek's disease (MD) in chickens is caused by the alphaherpesvirus MD virus (MDV) and is characterized by the development of lymphoblastoid tumors in multiple organs. The recent identification and cloning of RLORF4 and the finding that four of six attenuated strains of MDV contained deletions within RLORF4 suggested that it is involved in the attenuation process of MDV. To assess the role of RLORF4 in MD pathogenesis, its coding sequence was deleted in the pRB-1B bacterial artificial chromosome clone. Additionally, RLORF5a was deleted separately to examine its importance for oncogenesis. The sizes of plaques produced by MDV reconstituted from pRB-1BdeltaRLORF5a (rRB-1BdeltaRLORF5a) were similar to those produced by the parental pRB-1B virus (rRB-1B). In contrast, virus reconstituted from pRB-1BDeltaRLORF4 (rRB-1BdeltaRLORF4) produced significantly larger plaques. Replication of the latter virus in cultured cells was higher than that of rRB-1B or rRB-1BdeltaRLORF5a using quantitative PCR (qPCR) assays. In vivo, both deletion mutants and rRB-1B replicated at comparable levels at 4, 7, and 10 days postinoculation (p.i.), as determined by virus isolation and qPCR assays. At 14 days p.i., the number of PFU of virus isolated from chickens infected with rRB-1BdeltaRLORF4 was comparable to that from chickens infected with highly attenuated RB-1B and significantly lower than that from rRB-1B-infected birds. The number of tumors and kinetics of tumor production in chickens infected with rRB-1BdeltaRLORF5a were similar to those of P2a chickens infected with rRB-1B. In stark contrast, none of the chickens inoculated with rRB-1BdeltaRLORF4 died up to 13 weeks p.i.; however, two chickens had tumors at the termination of the experiment. The data indicate that RLORF4 is involved in attenuation of MDV, although the function of RLORF4 is still unknown.

Quail cell lines supporting replication of Marek's disease virus serotype 1 and 2 and herpesvirus of turkeys

Marek's disease virus (MDV), a highly cell-associated alphaherpesvirus, can be isolated and propagated in chicken kidney cells (CKC) and chicken or duck embryo fibroblast cells (CEF or DEF, respectively). Two recently developed cell lines, CU447 and CU453, developed from methylcholanthrene-induced tumors in Japanese quail, were examined for their suitability to propagate the three serotypes of MDV. The MDV strain RB-1B (serotype 1) was passaged for more than 30 passages in CU447 without causing cytopathic effects (CPE). Polymerase chain reaction analysis of RB-1B-infected CU447 cells demonstrated the presence of MDV DNA using primers specific for ICP4, pp38, and gB. The 132-bp direct repeats within the BamH1-H and -D fragments were amplified to the same level as RB-1B that was passaged in CKC or CEF. Different passages of RB-1B in CU447 were examined for expression of gB and pp38 transcripts, and pp38, gB, gE, and VP22 protein expression. Irrespective of the passage level, these transcripts and proteins were detected in the RB-1B-infected CU447 cells. Infectious virus was rescued by cocultivation of RB-1B-infected CU447 with CKC. Herpes virus of turkeys was propagated in CU447 and CU453 causing CPE in both cell lines. SB-1 did not cause CPE in either cell line but a few SB-1-infected cells could be detected using a monoclonal antibody specific for serotype 2 MDV.