Marek's disease virus-encoded vIL-8 gene is involved in early cytolytic infection but dispensable for establishment of latency

Molecular characterization and phylogenetic analysis of Marek's disease virus in chickens from Ogun State, Nigeria

Marek's disease (MD) is caused by oncogenic MD virus serotype 1 (MDV1) and is characterized by lymphoproliferative lesions resulting in high morbidity and mortality in chickens. Despite being ubiquitous on poultry farms, there is a dearth of information on its molecular characteristics in Nigeria. This study aimed at characterizing three virulence genes (Meq, pp38, and vIL-8) of MDV1 from chickens in Ogun state, Nigeria. Blood, feather quill, and tumour samples of chickens from different commercial poultry farms in Ogun State were pooled, spotted on 107 FTA cards, and screened for MDV1 by polymerase chain reaction (PCR). Phylogenetic analysis was carried out to compare Nigerian MDV1 Meq, pp38, and vIL-8 genes sequences with the published references. Thirteen samples were MDV1-positive and the Meq, as well as pp38, and vIL-8 genes from the different samples were 100% identical. The Meq genes contained 339 amino acids (aa) with three PPPP motifs in the transactivation domain and two interruptions of the PPPP motifs due to proline-to-arginine substitutions at positions 176 and 217 resulting in a 20.88% proline composition. Phylogenetic analysis revealed that the Meq gene clustered with strains from Egypt and very virulent ATE2539 strain from Hungary. Mutations were observed in the pp38 protein (at positions 107 and 109) and vIL-8 protein (at positions 4 and 31). Based on the molecular analysis of the three genes, the results indicate the presence of MDV1 with virulence signatures; therefore, further studies on in vivo pathotyping of Nigerian MDV1 from all states should be performed.RESEARCH HIGHLIGHTS Meq, pp38 and vIL-8 genes were 100% identical between Nigerian MDV strains.Proline content in Nigerian meq gene was 20.88% with two PPPP motifs interruptions.Meq, pp38 and vIL-8 genes of Nigerian MDV were similar to Egyptian and Indian strains.

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

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

V5 and GFP Tagging of Viral Gene pp38 of Marek's Disease Vaccine Strain CVI988 Using CRISPR/Cas9 Editing

Marek's disease virus (MDV) is a member of alphaherpesviruses associated with Marek's disease, a highly contagious neoplastic disease in chickens. The availability of the complete sequence of the viral genome allowed for the identification of major genes associated with pathogenicity using different techniques, such as bacterial artificial chromosome (BAC) mutagenesis and the recent powerful clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-based editing system. Thus far, most studies on MDV genome editing using the CRISPR/Cas9 system have focused on gene deletion. However, analysis of the expression and interactions of the viral proteins during virus replication in infected cells and tumor cells is also important for studying its role in MDV pathogenesis. The unavailability of antibodies against most of the MDV proteins has hindered the progress in such studies. This prompted us to develop pipelines to tag MDV genes as an alternative method for this purpose. Here we describe the application of CRISPR/Cas9 gene-editing approaches to tag the phosphoprotein 38 (pp38) gene of the MDV vaccine strain CVI988 with both V5 and green fluorescent protein (GFP). This rapid and efficient viral-gene-tagging technique can overcome the shortage of specific antibodies and speed up the MDV gene function studies significantly, leading to a better understanding of the molecular mechanisms of MDV pathogenesis.

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

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

Molecular detection and phylogenetic analysis of Marek's disease virus virulence-associated genes from vaccinated flocks in southern India reveals circulation of virulent MDV genotype

Marek's disease (MD) is a re-emerging viral disease of chickens and a serious economic threat to the poultry industry worldwide. Continuous surveillance with molecular investigation is essential to monitor the emergence of virulent Marek's disease virus (MDV) strains and to devise any appropriate vaccination strategy and implement bio-security programmes. In the present study, we investigated the cases of MD outbreaks in vaccinated poultry flocks. The MD outbreak was confirmed through necropsy (mainly visceral tumours), histopathology and viral gene specific PCR. The pathotypes of the field MDV strains were assessed by molecular analysis of three virulence-associated genes, meq, pp38 and vIL-8. The Meq sequence of the field strains analyzed in this study lacked the 59 aa unique to mild strains, indicating that they are potentially virulent strains. Mutation at position 71 and the presence of five proline rich repeats in the transactivation domain, both associated with virulence were observed in these strains; however, the signature sequences specific to very virulent plus strains were absent. Phylogenetic analysis of meq oncogene sequences revealed clustering of the field strains with North Indian strains and with a very virulent plus ATE 2539 strain from Hungary. Analyses of pp38 protein at positions 107 and 109 and vIL-8 protein at positions 4 and 31 showed signatures of virulence. Sequence and phylogenetic analysis of oncogene and virulence-associated genes of field MDVs from vaccinated flock indicated that these strains possessed molecular features of virulent strains.

The role of Meq-vIL8 in regulating Marek's disease virus pathogenesis

Marek's disease virus (MDV) is a highly cell-associated oncogenic alphaherpesvirus that causes T cell lymphoma in chickens. MDV-encoded Meq and vIL8 proteins play important roles in transformation and early cytolytic infection, respectively. Previous studies identified a spliced transcript, meq-vIL8, formed by alternative splicing of meq and vIL8 genes in MDV lymphoblastoid tumour cells. To determine the role of Meq-vIL8 in MDV pathogenesis, we generated a recombinant MDV (MDV-meqΔSD) by mutating the splice donor site in the meq gene to abrogate the expression of Meq-vIL8. As expected, our results show that MDV-meqΔSD virus grows similarly to the parental and revertant viruses in cell culture, suggesting that Meq-vIL8 is dispensable for MDV growth in vitro. We further characterized the pathogenic properties of MDV-meqΔSD virus in chickens. Our results show that lack of Meq-vIL8 did not affect virus replication during the early cytolytic phase, as determined by immunohistochemistry analysis and/or viral genome copy number, but significantly enhanced viral DNA load in the late phase of infection in the spleen and brain of infected chickens. In addition, we observed that abrogation of Meq-vIL8 expression reduced the mean death time and increased the prevalence of persistent neurological disease, common features of highly virulent strains of MDV, in inoculated chickens. In conclusion, our study shows that Meq-vIL8 is an important virulence factor of MDV.

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

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

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

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

Transcriptome-wide N6-methyladenosine modification profiling of long non-coding RNAs during replication of Marek's disease virus in vitro

Background

The newly discovered reversible N6-methyladenosine (m⁶A) modification plays an important regulatory role in gene expression. Long non-coding RNAs (lncRNAs) participate in Marek’s disease virus (MDV) replication but how m⁶A modifications in lncRNAs are affected during MDV infection is currently unknown. Herein, we profiled the transcriptome-wide m⁶A modification in lncRNAs in MDV-infected chicken embryo fibroblast (CEF) cells.

Results

Methylated RNA immunoprecipitation sequencing results revealed that the lncRNA m⁶A modification is highly conserved with MDV infection increasing the expression of lncRNA m⁶A modified sites compared to uninfected cell controls. Gene Ontology and the Kyoto Encyclopedia of Genes and Genomes pathway analysis revealed that lncRNA m⁶A modifications were highly associated with signaling pathways associated with MDV infection.

Conclusions

In this study, the alterations seen in transcriptome-wide m⁶A occurring in lncRNAs following MDV-infection suggest this process plays important regulatory roles during MDV replication. We report for the first time profiling of the alterations in transcriptome-wide m⁶A modification in lncRNAs of MDV-infected CEF cells.

Virus-encoded microRNA-M7 restricts early cytolytic replication and pathogenesis of Marek's disease virus

MicroRNAs (miRNAs) are a class of ∼22 nucleotides non-coding RNAs that are encoded by a wide range of hosts. Viruses, especially herpesviruses, encode a variety of miRNAs that involved in disease progression. Recently, a cluster of virus-encoded miRNAs, miR-M8-M10, have been shown to restrict early cytolytic replication and pathogenesis of Marek's disease virus (MDV), an oncogenic avian alphaherpesvirus that causes lymphoproliferative disease in chickens. In this study, we specifically dissected the role of miR-M7, a member of cluster miR-M8-M10, in regulating MDV replication and pathogenesis. We found that deletion of miR-M7-5p did not affect the virus plaque size and growth in cell culture. However, compared to parental virus, infection of miR-M7-5p deletion virus significantly increased MDV genome copy number at 5 days post infection, suggesting that miR-M7 plays a role to restrict MDV replication during early cytolytic phase. In addition, our results showed that infection of miR-M7-5p deletion virus significantly enhanced the mortality of chickens, even it induced lymphoid organ atrophy similar to parental and revertant viruses. Taken together, our study revealed that the miR-M7 acts as a repressive factor of MDV replication and pathogenesis.

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

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

Marek's Disease Virus Cluster 3 miRNAs Restrict Virus' Early Cytolytic Replication and Pathogenesis

Herpesvirus-encoded microRNAs (miRNAs) have been discovered in infected cells; however, lack of a suitable animal model has hampered functional analyses of viral miRNAs in vivo. Marek’s disease virus (MDV) (Gallid alphaherpesvirus 2, GaHV-2) genome contains 14 miRNA precursors, which encode 26 mature miRNAs, grouped into three clusters. In this study, the role of MDV-encoded cluster 3 miRNAs, also known as mdv1-miR-M8-M10, in pathogenesis was evaluated in chickens, the natural host of MDV. Our results show that deletion of cluster 3 miRNAs did not affect virus replication and plaque size in cell culture, but increased early cytolytic replication of MDV in chickens. We also observed that deletion of cluster 3 miRNAs resulted in significantly higher virus reactivation from peripheral blood lymphocytes. In addition, pathogenesis studies showed that deletion of cluster 3 miRNAs resulted in more severe atrophy of lymphoid organs and reduced mean death time, but did not affect the incidence of MDV-associated visceral tumors. We confirmed these results by generating a cluster 3 miRNA revertant virus in which the parental MDV phenotype was restored. To the best of our knowledge, our study provides the first evidence that MDV cluster 3 miRNAs play an important role in modulating MDV pathogenesis.

Deletion of LORF9 but not LORF10 attenuates Marek's disease virus pathogenesis

Marek's disease virus (MDV) genome contains a number of uncharacterized long open reading frames (LORF) and their role in viral pathogenesis has not been fully investigated. Among them, LORF9 (MDV069) and LORF10 (MDV071) are locate at the right terminus of the MDV genome unique long region (U_L). To investigate their role in MDV pathogenesis, we generated LORF9 or LORF10 deletion and revertant viruses. In vitro growth kinetics results show that both LORF9 and LORF10 are not essential for virus growth in cell culture. However, LORF9, but not LORF10, is involved in MDV early cytolytic replication in vivo, as evidenced by limited viral antigen expression in lymphoid organs of LORF9 deletion virus inoculated chickens. MDV genome copy number data further confirmed that LORF9 is important for MDV replication in spleen during early cytolytic phase. Deletion of LORF9 also partially impairs the replication of MDV in feather follicle epithelium (FFE); however, it can still establish latency and transformation. In addition, pathogenesis studies show that deletion of LORF9, but not LORF10, result in significant reduction of MDV induced mortality and slightly reduce MDV associated tumors of inoculated chickens. Importantly, we confirmed these results with the generation of LORF9 and LORF10 revertant viruses that fully restore the phenotypes of parental MDV. In conclusion, our results show that deletion of LORF9, but not LORF10, significantly impair viral replication in lymphoid organs during early cytolytic phase and attenuate Marek's disease virus pathogenesis.

Attenuation of a recombinant Marek's disease virus lacking the meq oncogene and evaluation on its immune efficacy against Marek's disease virus

SC9-2 is a recombinant Marek's disease virus (MDV) strain lacking the meq oncogene. Previous study demonstrated that SC9-2 virus provides good protection against challenge with a very virulent MDV rMd5, but it induces immunosuppressive effects in specific pathogen-free (SPF) chickens. In the present study, SC9-2 was serially passaged on chicken embryo fibroblast (CEF) cell cultures. The pathogenicity and immune efficacy of SC9-2/10^th and SC9-2/40^th against rMd5 were evaluated. Animal experimental results showed that SC9-2/10^th and SC9-2/40^th showed no lethality or tumorigenicity in SPF chickens. Body weight of chickens inoculated with SC9-2/40^th were significantly higher than that of the chickens inoculated with SC9-2/10^th but lower than that of the uninoculated controls. The severity of bursa and thymus atrophy (BTA) and spleen enlargement in SC9-2/40^th-inoculated chickens were also weaker than the SC9-2/10^th-inoculated ones but stronger than the uninoculated controls. Chickens inoculated with SC9-2/40^th and SC9-2/10^th showed similar antibody levels induced by H9N2 subtype avian influenza virus/Newcastle disease virus inactivated vaccines, both of which were lower than the uninoculated controls. Replication of SC9-2/40^th was significantly lower than SC9-2/10^th in feather follicle epithelium (FFE) of infected chickens. The immune protection index of SC9-2/40^th was also lower than that of SC9-2/10^th, but the difference was not significantly, and both of which were significant higher than that of the commercial MDV vaccine CVI988/Rispens. The results of our studies demonstrated that SC9-2/40^th showed weaker severity of BTA, spleen enlargement, and body weight loss and lower replication level in FFE than SC9-2/10^th in SPF chickens. However, SC9-2/40^th was able to confer better immune protection as compared with CVI988/Rispens vaccination in SPF chickens. In conclusion, serially attenuation of SC9-2 in CEFs reduced the lymphoid organ atrophy and replication in SPF chickens, and the immune protective efficacy of attenuated viruses was still superior than CVI988/Rispens.

Targeted Editing of the pp38 Gene in Marek's Disease Virus-Transformed Cell Lines Using CRISPR/Cas9 System

Marek's disease virus (MDV), a lymphotropic α-herpesvirus associated with T-cell lymphomas in chickens, is an excellent model for herpesvirus biology and virus-induced oncogenesis. Marek's disease (MD) is also one of the cancers against which a vaccine was first used. In the lymphomas and lymphoblastoid cell lines (LCLs) derived from them, MDV establishes latent infection with limited gene expression. Although LCLs are valuable for interrogating viral and host gene functions, molecular determinants associated with the maintenance of MDV latency and lytic switch remain largely unknown, mainly due to the lack of tools for in situ manipulation of the genomes in these cell lines. Here we describe the first application of CRISPR/Cas9 editing approach for precise editing of the viral gene phosphoprotein 38 (pp38), a biomarker for latent/lytic switch in MDV-transformed LCLs MDCC-MSB-1 (Marek's disease cell line MSB-1) and MDCC-HP8. Contradictory to the previous reports suggesting that pp38 is involved in the maintenance of transformation of LCL MSB-1 cells, we show that pp38-deleted cells proliferated at a significant higher rate, suggesting that pp38 is dispensable for the transformed state of these cell lines. Application of CRISPR/Cas9-based gene editing of MDV-transformed cell lines in situ opens up further opportunities towards a better understanding of MDV pathogenesis and virus-host interactions.

Imaging Mass Spectrometry and Proteome Analysis of Marek's Disease Virus-Induced Tumors

The highly oncogenic alphaherpesvirus Marek's disease virus (MDV) causes immense economic losses in the poultry industry. MDV induces a variety of symptoms in infected chickens, including neurological disorders and immunosuppression. Most notably, MDV induces transformation of lymphocytes, leading to T cell lymphomas in visceral organs with a mortality of up to 100%. While several factors involved in MDV tumorigenesis have been identified, the transformation process and tumor composition remain poorly understood. Here we developed an imaging mass spectrometry (IMS) approach that allows sensitive visualization of MDV-induced lymphoma with a specific mass profile and precise differentiation from the surrounding tissue. To identify potential tumor markers in tumors derived from a very virulent wild-type virus and a telomerase RNA-deficient mutant, we performed laser capture microdissection (LCM) and thereby obtained tumor samples with no or minimal contamination from surrounding nontumor tissue. The proteomes of the LCM samples were subsequently analyzed by quantitative mass spectrometry based on stable isotope labeling. Several proteins, like interferon gamma-inducible protein 30 and a 70-kDa heat shock protein, were identified that are differentially expressed in tumor tissue compared to surrounding tissue and naive T cells. Taken together, our results demonstrate for the first time that MDV-induced tumors can be visualized using IMS, and we identified potential MDV tumor markers by analyzing the proteomes of virus-induced tumors.IMPORTANCE Marek's disease virus (MDV) is an oncogenic alphaherpesvirus that infects chickens and causes the most frequent clinically diagnosed cancer in the animal kingdom. Not only is MDV an important pathogen that threatens the poultry industry but it is also used as a natural virus-host model for herpesvirus-induced tumor formation. In order to visualize MDV-induced lymphoma and to identify potential biomarkers in an unbiased approach, we performed imaging mass spectrometry (IMS) and noncontact laser capture microdissection. This study provides a first description of the visualization of MDV-induced tumors by IMS that could be applied also for diagnostic purposes. In addition, we identified and validated potential biomarkers for MDV-induced tumors that could provide the basis for future research on pathogenesis and tumorigenesis of this malignancy.

Application of CRISPR/Cas9 Gene Editing System on MDV-1 Genome for the Study of Gene Function

Marek’s disease virus (MDV) is a member of alphaherpesviruses associated with Marek’s disease, a highly contagious neoplastic disease in chickens. Complete sequencing of the viral genome and recombineering techniques using infectious bacterial artificial chromosome (BAC) clones of Marek’s disease virus genome have identified major genes that are associated with pathogenicity. Recent advances in CRISPR/Cas9-based gene editing have given opportunities for precise editing of the viral genome for identifying pathogenic determinants. Here we describe the application of CRISPR/Cas9 gene editing approaches to delete the Meq and pp38 genes from the CVI988 vaccine strain of MDV. This powerful technology will speed up the MDV gene function studies significantly, leading to a better understanding of the molecular mechanisms of MDV pathogenesis.

Molecular characterization and phylogenetic analysis of a virulent Marek's disease virus field strain in broiler chickens in Japan

Marek's disease is a lymphoproliferative disease causing a serious threat in poultry production. Field strains of Marek's disease virus (MDVs) are continuously re-emerging, causing great economical losses to the poultry industry worldwide in spite of the intensive vaccination and restrictive management policy used. Histopathological and molecular characterizations of MDVs are essential for monitoring the changes of viruses and evaluating the effectiveness of existing vaccines. During 2016, 190 visceral tumour tissues representing 30 vaccinated chicken flocks from the Gifu prefecture, Japan, were analysed. A pathological examination revealed the presence of lymphoproliferative lesions in the visceral organs. Polymerase chain reaction screening of tissue specimens using specific primers for avian leucosis virus, reticuloendotheliosis virus, and MDV was positive only for MDV. The polymerase chain reaction products of meq, pp38, virus-induced IL-8 homology, and glycoprotein MDV genes were sequenced and used for homology, phylogenetic, and similarity level analysis with the published reference of MDVs in the database. The results revealed high similarity between the field isolates, vv and vv+ strains of MDV from the USA and China. Several point mutations in the nucleotide sequence of the field isolates and their deduced amino acid sequences were detected in those genes. The present molecular analyses indicated that nucleotide and amino acid changes could be valuable criteria for differentiation and determination of the pathogenicity and oncogenicity of MDVs according to the Avian Disease and Oncology Laboratory pathotyping in vivo studies. Furthermore, the results suggest that development of a new vaccine must be considered to overcome this devastating avian oncogenic viral disease.

Co-Infection with Marek's Disease Virus and Reticuloendotheliosis Virus Increases Illness Severity and Reduces Marek's Disease Vaccine Efficacy

Marek's disease virus (MDV) and reticuloendotheliosis virus (REV) cause Marek's disease (MD) and reticuloendotheliosis (RE), respectively. Co-infection with MDV and REV is common in chickens, causing serious losses to the poultry industry. However, experimental studies of such co-infection are lacking. In this study, Chinese field strains of MDV (ZW/15) and REV (JLR1501) were used as challenge viruses to evaluate the pathogenicity of co-infection and the influence of MD vaccination in chickens. Compared to the MDV-challenged group, the mortality and tumor rates increased significantly by 20.0% (76.7 to 96.7%) and 26.7% (53.3 to 80.0%), in the co-challenged group, respectively. The protective index of the MD vaccines CVI988 and 814 decreased by 33.3 (80.0 to 47.7) and 13.3 (90.0 to 76.7), respectively. These results indicated that MDV and REV co-infection significantly increased disease severity and reduced the vaccine efficacy. The MDV genome load showed no difference in the feather pulps and spleen, and pathogenicity-related MDV gene expression (meq, pp38, vIL-8, and ICP4) in the spleen significantly increased at some time points in the co-challenged group. Clearly, synergistic pathogenicity occurred between MDV and REV, and the protective efficacy of existing MD vaccines was attenuated by co-infection with Chinese field MDV and REV strains.

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

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

Marek's disease vaccines: Current status, and strategies for improvement and development of vector vaccines

Marek's disease (MD) is a lymphoproliferative viral disease of chickens, which has been controlled through vaccination since 1969. MD vaccines protect against tumors but do not provide sterilizing immunity, and thus it is generally believed that their use has contributed to increase virulence of field strains with the ability to cause MD in vaccinated chickens. Traditional methods of developing vaccines, like cell culture attenuation, have proved unsuccessful for the development of improved vaccines to protect against highly virulent MD virus (MDV) field strains. With the advent of recombinant DNA technology, it is now possible to study MDV gene function and develop rational vaccines that protect against highly pathogenic strains. In addition, the long term protection conferred by MD vaccines, their excellent safety profile, their efficacy when administered early (at hatch or in ovo), and their ability to overcome maternal antibodies, has made MDV an excellent candidate vector to protect not only against MD but also against other important viral poultry diseases. In this review we will discuss the current status of MD vaccines and their use as vector vaccines to control important viral poultry diseases.

Molecular and pathogenicity characterization of Gallid herpesvirus 2 newly isolated in China from 2009 to 2013

During the course of our continuous surveillance of Gallid herpesvirus 2 (GaHV-2), 44 isolates were obtained from GaHV-2-positive chickens of different flocks in China from 2009 to 2013. The meq gene, considered as a major GaHV-2 oncogene, was sequenced and was found to contain an open reading frame of 1020 nucleotides encoding a 339 amino acid (aa) polypeptide in all isolates. Compared with the GaHV-2 GA strain, the meq genes in 15.9 % (7/44) of the isolates analyzed in this study contained an aa substitution mutation at position 88 (A to T) of which is the first report. The main characteristics of Chinese GaHV-2 isolates meq genes included the substitutions K77E, D80Y, V115A, T139A, P176R, and P217A, and the aa substitution frequency at positions 139 and 176 showed an increase. To test the pathogenicity of the isolates, a pathogenicity study and a vaccination-challenge test were performed on three selected isolates (ZY/1203, WC/1203, and WC/1110) and reference strain GA. The results showed that the three isolates induced gross Marek's disease (MD) lesions in 95.0-100 % cases, which was a higher rate than that obtained for strain GA (82.4 %). Three isolates induced mortality in 10-21.1 % of specific-pathogen-free chickens, which was similar to results with strain GA (23.5 %). The commercially available CVI988 vaccine induced lower protective indices (PIs) against ZY/1203 (82.4) and WC/1110 (83.3) as compared to those against WC/1203 (100) and GA (100). These results showed an evolving trend in the meq genes of the isolates; three isolates exhibited higher morbidity as compared to the reference strain and the vaccine induced lower PIs against two isolates as compared to that against the reference strain.

The ORF012 gene of Marek's disease virus type 1 produces a spliced transcript and encodes a novel nuclear phosphoprotein essential for virus growth

Marek's disease virus (MDV), an alphaherpesvirus, is the causative agent of a lethal disease in chickens characterized by generalized nerve inflammation and rapid lymphoma development. The extensive colinearity of the MDV genome with those of related herpesviruses has eased functional characterization of many MDV genes. However, MDV carries a number of unique open reading frames (ORFs) that have not yet been investigated regarding their coding potentials and the functions of their products. Among these unique ORFs are two putative ORFs, ORF011 and ORF012, which are found at the extreme left end of the MDV unique long region. Using reverse transcriptase PCR, we showed that ORF011 and ORF012 are not individual genes but form a single gene through mRNA splicing of a small intron, resulting in the novel ORF012. We generated an ORF012-null virus using an infectious clone of MDV strain RB-1B. The deletion virus had a marked growth defect in vitro and could not be passaged in cultured cells, suggesting an essential role for the ORF012 product in virus replication. Further studies revealed that protein 012 (p012) localized to the nucleus in transfected and infected cells, and we identified by site-directed mutagenesis and green fluorescent protein (GFP) reporter fusion assays a nuclear localization signal (NLS) that was mapped to a 23-amino-acid sequence at the protein's C terminus. Nuclear export was blocked using leptomycin B, suggesting a potential role for p012 as a nuclear/cytoplasmic shuttling protein. Finally, p012 is phosphorylated at multiple residues, a modification that could possibly regulate its subcellular distribution.

IMPORTANCE

Marek's disease virus (MDV) causes a devastating oncogenic disease in chickens with high morbidity and mortality. The costs for disease prevention reach several billion dollars annually. The functional investigation of MDV genes is necessary to understand its complex replication cycle, which eventually could help us to interfere with MDV and herpesviral pathogenesis. We have identified a previously unidentified phosphoprotein encoded by MDV ORF012. We were able to show experimentally that predicted splicing of the gene based on bioinformatics data does indeed occur during replication. The newly identified p012 is essential for MDV replication and localizes to the nucleus due to the presence of a transferable nuclear localization signal at its C terminus. Our results also imply that p012 could constitute a nucleocytoplasmic shuttle protein, a feature that could prove interesting and important.

The influence of host genetics on Marek's disease virus evolution

Since the first report of a polyneuritis in chickens by Joseph Marek in 1907, the clinical nature of the disease has changed. Over the last five decades, the pathogenicity of the Marek's disease virus (MDV) has continued to evolve from the relatively mild strains observed in the 1960s to the more severe strains labeled very virulent plus currently observed in today's outbreaks. To understand the influence of host genetics, specifically the major histocompatibility complex (MHC), on virus evolution, a bacterial artificial chromosome-derived MDV (Md5B40BAC) was passed in vivo through resistant (MHC-B21) and susceptible (MHC-B13) Line 0 chickens. Criteria for selecting virus isolates for in vivo passage were based on virus replication in white blood cells 21 days after challenge and evaluation of MD pathology at necropsy. In the MHC-B13-susceptible line the Md5B40BAC virulence consistently increased from 18% Marek's disease (MD) after in vivo passage 1 (B13-IVP1 Md5B40BAC) to 94% MD after B13-IVP5 Md5B40BAC challenge. In the MHC-B21-resistant line MD virulence fluctuated from 28% at B21-IVP1 Md5B40BAC to a high of 65% in B21-IVP2 Md5B40BAC back to a low of 23% in B21-IVP5 Md5B40BAC-challenged chicks. Although the B21-IVP5 Md5B40BAC isolates were relatively mild in the MHC-B21 chicken line (56% MDV), they were highly virulent in the MHC-B13 line (100% MDV). From this series of experiments it would appear that MDV evolution toward greater virulence occurs in both susceptible and resistant MHC haplotypes, but the resulting increase in pathogenicity is constrained by the resistant MHC haplotype.

Deletion of Marek's disease virus large subunit of ribonucleotide reductase impairs virus growth in vitro and in vivo

Marek's disease virus (MDV), a highly cell-associated lymphotropic alphaherpesvirus, is the causative agent of a neoplastic disease in domestic chickens called Marek's disease (MD). In the unique long (UL) region of the MDV genome, open reading frames UL39 and UL40 encode the large and small subunits of the ribonucleotide reductase (RR) enzyme, named RR1 and RR2, respectively. MDV RR is distinguishable from that present in chicken and duck cells by monoclonal antibody T81. Using recombinant DNA technology we have generated a mutant MDV (Md5deltaRR1) in which RR1 was deleted. PCR amplification of the RR gene in Md5deltaRR1-infected duck embryo fibroblasts (DEF) confirmed the deletion of the 2.4 kb RR1 gene with a resultant amplicon of a 640-bp fragment. Restriction enzyme digests with SalI confirmed a UL39 deletion and the absence of gross rearrangement. The biologic characteristics of Md5deltaRR1 virus were studied in vitro and in vivo. The Md5deltaRR1 replicated in DEF, but significantly slower than parental Md5-BAC, suggesting that RR is important but not essential for replication in fibroblasts. In vivo studies, however, showed that the RR1 deletion virus was impaired for its ability to replicate in chickens. Inoculation of specific-pathogen-free (SPF) chickens with Md5deltaRR1 showed the mutant virus is nonpathogenic and does not induce MD in birds. A revertant virus, Md5deltaRR1/R, was generated with the restored phenotype of the parental Md5-BAC in vivo, indicating that RR is essential for replication of the virus in chickens. Protection studies in SPF chickens indicated that the Md5deltaRR1 virus is not a candidate vaccine against MD.

Cloning of a very virulent plus, 686 strain of Marek's disease virus as a bacterial artificial chromosome

Bacterial artificial chromosome (BAC) vectors were first developed to facilitate propagation and manipulation of large DNA fragments. This technology was later used to clone full-length genomes of large DNA viruses to study viral gene function. Marek's disease virus (MDV) is a highly oncogenic herpesvirus that causes rapid induction of T-cell lymphomas in chickens. Based on the virus's ability to cause disease in vaccinated chickens, MDV strains are classified into pathotypes, with the most virulent strains belonging to the very virulent plus (vv+) pathotype. Here we report the construction of BAC clones of 686 (686-BAC), a vv+ strain of MDV. Transfection of DNA isolated from two independent clones into duck embryo fibroblasts resulted in recovery of infectious virus. Pathogenesis studies showed that the BAC-derived 686 viruses were more virulent than Md5, a vv strain of MDV. With the use of a two-step red-mediated mutagenesis process, both copies of viral interleukin 8 (vIL-8) were deleted from the MDV genome, showing that 686-BACs were amenable to mutagenesis techniques. The generation of BAC clones from a vv+ strain of MDV is a significant step toward understanding molecular basis of MDV pathogenesis.

Marek's disease viral interleukin-8 promotes lymphoma formation through targeted recruitment of B cells and CD4+ CD25+ T cells

Marek's disease virus (MDV) is a cell-associated and highly oncogenic alphaherpesvirus that infects chickens. During lytic and latent MDV infection, a CXC chemokine termed viral interleukin-8 (vIL-8) is expressed. Deletion of the entire vIL-8 open reading frame (ORF) was shown to severely impair disease progression and tumor development; however, it was unclear whether this phenotype was due to loss of secreted vIL-8 or of splice variants that fuse exons II and III of vIL-8 to certain upstream open reading frames, including the viral oncoprotein Meq. To specifically examine the role of secreted vIL-8 in MDV pathogenesis, we constructed a recombinant virus, vΔMetvIL-8, in which we deleted the native start codon from the signal peptide encoding exon I. This mutant lacked secreted vIL-8 but did not affect Meq-vIL-8 splice variants. Loss of secreted vIL-8 resulted in highly reduced disease and tumor incidence in animals infected with vΔMetvIL-8 by the intra-abdominal route. Although vΔMetvIL-8 was still able to spread to naïve animals by the natural route, infection and lymphomagenesis in contact animals were severely impaired. In vitro assays showed that purified recombinant vIL-8 efficiently binds to and induces chemotaxis of B cells, which are the main target for lytic MDV replication, and also interacts with CD4(+) CD25(+) T cells, known targets of MDV transformation. Our data provide evidence that vIL-8 attracts B and CD4(+) CD25(+) T cells to recruit targets for both lytic and latent infection.

Kinetic expression analysis of the cluster mdv1-mir-M9-M4, genes meq and vIL-8 differs between the lytic and latent phases of Marek's disease virus infection

Marek's disease virus (GaHV-2) is an alphaherpesvirus that induces T-cell lymphoma in chickens. The infection includes both lytic and latent stages. GaHV-2 encodes three clusters of microRNAs (miRNAs) located in the internal (I)/terminal (T) repeat (R) regions. We characterized transcripts encompassing the mdv1-mir-M9-M4 and mir-M11-M1 clusters located in the IR(L)/TR(L) region, upstream and downstream from the meq oncogene, respectively. By 5'- and 3'-RACE-PCR and targeted RT-PCR, we showed that mdv1-mir-M9-M4 could be transcribed from an unspliced transcript or from at least 15 alternatively spliced transcripts covering the IR(L)/TR(L) region, encompassing the meq and vIL-8 genes and localizing the mdv1-mir-M9-M4 cluster to the first intron at the 5'-end. However, all these transcripts, whether spliced or unspliced, seemed to start at the same transcriptional start site, their transcription being driven by a single promoter, prmiRM9M4. We demonstrated alternative promoter usage for the meq and vIL-8 genes, depending on the phase of GaHV-2 infection. During the latent phase, the prmiRM9M4 promoter drove transcription of the meq and vIL-8 genes and the mdv1-mir-M9-M4 cluster in the first intron of the corresponding transcripts. By contrast, during the lytic phase, this promoter drove the transcription only of the mdv1-mir-M9-M4 cluster to generate unspliced mRNA, the meq and vIL-8 genes being transcribed principally from their own promoters. Despite the expression of meq and the mdv1-mir-M9-M4 cluster under two different transcriptional processes during the latent and lytic phases, our data provide an explanation for meq expression and mdv1-mir-M4-5P overexpression in miRNA libraries from GaHV-2-infected cells, regardless of the phase of infection.

Epigenetic regulation of the latency-associated region of Marek's disease virus in tumor-derived T-cell lines and primary lymphoma

Meq is the major Marek's disease virus (MDV)-encoded oncoprotein and is essential for T-cell lymphomagenesis. Meq and several noncoding RNAs, including three microRNA (MiR) clusters, are expressed from the repeats of the MDV genome during latent infection of T cells. To investigate the state of the chromatin in this and flanking regions, we carried out chromatin immunoprecipitation (ChIP) analysis of covalent histone modifications and associated bound proteins. T-cell lines and a lymphoma were compared. The chromatin around the promoters for Meq and the noncoding RNAs in both cell lines and the lymphoma were associated with H3K9 acetylation and H3K4 trimethylation, which are marks of transcriptionally active chromatin. These correlated with bound Meq-c-Jun heterodimers. The only binding site for Meq homodimers is located at the lytic origin of replication (OriLyt), next to the lytic gene pp38. This region lacked active marks and was associated with repressive histone modifications (H3K27 and H3K9 trimethylation). DNA CpG methylation was investigated using methylated DNA precipitation (MeDP). In cell lines, DNA methylation was abundant across the repeats but noticeably reduced or absent around the active promoters. In primary tumors, CpG methylation occurred less than 2 months after infection, focused within the ICP4 gene. These data suggest that nonrandom de novo DNA methylation occurs early in lymphomagenesis. In addition, the histone data indicate a role for Meq in the epigenetic regulation of the MDV genome repeats in transformed T cells and suggest that the OriLyt region and the Meq/MiR region might be separated by chromatin boundary elements, and preliminary data on CTCF binding are consistent with this.

Immune responses against Marek's disease virus

It is more than a century since Marek's disease (MD) was first reported in chickens and since then there have been concerted efforts to better understand this disease, its causative agent and various approaches for control of this disease. Recently, there have been several outbreaks of the disease in various regions, due to the evolving nature of MD virus (MDV), which necessitates the implementation of improved prophylactic approaches. It is therefore essential to better understand the interactions between chickens and the virus. The chicken immune system is directly involved in controlling the entry and the spread of the virus. It employs two distinct but interrelated mechanisms to tackle viral invasion. Innate defense mechanisms comprise secretion of soluble factors as well as cells such as macrophages and natural killer cells as the first line of defense. These innate responses provide the adaptive arm of the immune system including antibody- and cell-mediated immune responses to be tailored more specifically against MDV. In addition to the immune system, genetic and epigenetic mechanisms contribute to the outcome of MDV infection in chickens. This review discusses our current understanding of immune responses elicited against MDV and genetic factors that contribute to the nature of the response.

Characterization of a very virulent Marek's disease virus mutant expressing the pp38 protein from the serotype 1 vaccine strain CVI988/Rispens

Marek's disease virus (MDV), a highly cell-associated oncogenic chicken herpesvirus, causes Marek's disease in domestic chickens. A unique phosphoprotein of MDV, pp38, has previously been associated with the maintenance of transformation in MDV-induced tumor cell lines. However, recently, the biological properties of a deletion mutant virus (rMd5Deltapp38) revealed that pp38 is involved in early cytolytic infection in lymphocytes but not in the induction of tumors. Thus, pp38 is important for early cytolytic infection and not for transformation. The pp38 protein of the MDV serotype 1 vaccine strain CVI988/Rispens differs by one amino acid when compared to the pathogenic strains of MDV. Monoclonal antibody, H19, recognizes all serotype 1 MDV strains except CVI988/Rispens. Previous studies have also shown that the unique pp38 epitope in CVI988/Rispens induced high antibody response. In order to study the role of this epitope in the protective properties of CVI988/Rispens, we generated a mutant rMd5 virus in which the wild type pp38 gene has been substituted with that of CVI988/Rispens (rMd5/pp38CVI). The replication properties of rMd5/pp38CVI, both in vitro and in vivo, and tumor induction were examined. We found that the biological properties of rMd5/pp38CVI were similar to the wild type rMd5 virus with regards to in vivo replication, antibody response and tumor induction. This shows that the pp38 derived from CVI988/Rispens is not involved in protective properties as was previously suggested.

Host responses in the bursa of Fabricius of chickens infected with virulent Marek's disease virus

The bursa of Fabricius serves as an important tissue in the process of Marek's disease virus (MDV) pathogenesis, since B cells of the bursa harbor the cytolytic phase of MDV replication cycle. In the present study, host responses associated with MDV infection in the bursa of Fabricius of chickens were investigated. The expression of MDV phosphoprotein (pp)38 antigen, MDV glycoprotein (gB) and MDV viral interleukin (vIL)-8 transcripts was at the highest at 4 days post-infection (d.p.i.) and then showed a declining trend. On the contrary, the expression of meq (MDV EcoRI Q) gene as well as the viral genome load increased gradually until day 14 post-infection. The changes in viral parameters were associated with significantly higher infiltration of macrophages and T cell subsets, particularly CD4+ T cells into the bursa of Fabricius. Of the genes examined, the expression of interferon (IFN)-alpha, IFN-gamma genes and inducible nitric oxide synthase (iNOS) was significantly up-regulated in response to MDV infection in the bursa of Fabricius. The results suggest a role for these cells and cytokines in MDV-induced responses in the bursa of Fabricius.

Recombinant Marek's disease virus (MDV) lacking the Meq oncogene confers protection against challenge with a very virulent plus strain of MDV

Marek's disease virus (MDV) encodes a basic leucine-zipper protein, Meq, that shares homology with the Jun/Fos family of transcriptional factors. Conclusive evidence that Meq is an oncogene of MDV came from recent studies of a Meq-null virus, rMd5 Delta Meq. This virus replicated well in vitro, but was non-oncogenic in vivo. Further characterization of this virus in vivo indicated that the meq gene is dispensable for cytolytic infection since it replicated well in the lymphoid organs and feather follicular epithelium. Since rMd5 Delta Meq virus was apathogenic for chickens, we set out to investigate whether this virus could be a good candidate vaccine. Vaccine efficacy experiments conducted in Avian Disease and Oncology Laboratory (ADOL) 15I(5)x 7(1) chickens vaccinated with rMd5 Delta Meq virus or an ADOL preparation of CVI988/Rispens indicated that the Meq-null virus provided protection superior to CVI988/Rispens, the most efficacious vaccine presently available, following challenge with a very virulent (rMd5) and a very virulent plus (648A) MDV strains.

Virus-encoded chemokines, chemokine receptors and chemokine-binding proteins: new paradigms for future therapy

Over millions of years of coevolution with their hosts, viruses have learned the finest artifices of the immune system defense mechanisms and developed a variety of strategies for evading them. The chemokine system has been a primary target of these viral efforts because of the critical role it plays in the development of effective immune responses. Not only do chemokines control cellular recruitment at the site of infection, they also regulate the magnitude and character of the immune responses. Several viruses, and large DNA viruses in particular, have exploited the chemokine system by hijacking and reprogramming chemokine or chemokine-receptor genes, and/or secreting chemokine-binding proteins. In the past few years there has been intense investigation in this area, driven not only by the prospect of gaining a better understanding of viral-immune evasion mechanisms, but also by the possibility of targeting these molecules as part of future antiviral therapeutic approaches, as well as exploiting viral strategies of chemokine interference as novel therapies for inflammatory or neoplastic diseases.

Comparative full-length sequence analysis of oncogenic and vaccine (Rispens) strains of Marek's disease virus

The complete DNA sequence of the Marek's disease virus serotype 1 vaccine strain CVI988 was determined and consists of 178 311 bp with an overall gene organization identical to that of the oncogenic strains. In examining open reading frames (ORFs), nine differ between vaccine and oncogenic strains. A 177 bp insertion was identified in the overlapping genes encoding the Meq, RLORF6 and 23 kDa proteins of CVI988. Three ORFs are predicted to encode truncated proteins. One, designated 49.1, overlaps the gene encoding the large tegument protein UL36 and encodes a severely truncated protein of 34 aa. The others, ORF5.5/ORF75.91 and ORF3.0/78.0, located in the repeat regions (diploid), encode a previously unidentified ORF of 52 aa and a truncated version of the virus-encoded chemokine (vIL-8), respectively. Subtle genetic changes were identified in the two ORFs encoding tegument proteins UL36 and UL49. Only one diploid ORF (ORF6.2/ORF75.6) present in the genomes of the three virulent strains is absent in the CVI988-BAC genome. Seventy non-synonymous amino acid substitutions were identified that could differentiate CVI988-BAC from all three oncogenic strains collectively. Estimates of the non-synonymous to synonymous substitution ratio (ω) indicate that CVI988 ORFs are generally under purifying selection (ω<1), whereas UL39, UL49, UL50, RLORF6 and RLORF7 (Meq) appear to evolve under relaxed selective constraints. No CVI988 ORF was found to be under positive evolutionary selection (ω≫1).

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.

Modulation of host gene expression by the K15 protein of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) contains several open reading frames (ORFs) encoding proteins capable of initiating signal transduction pathways. Among them is the K15 ORF, which consists of eight exons encoding a protein with 12 predicted transmembrane domains and a cytoplasmic C terminus. When transiently expressed, the 8-exon K15 transcript gives rise to a protein with an apparent molecular mass of 45 kDa. K15 interacts with cellular proteins, TRAF (tumor necrosis factor receptor-associated factor) and Src kinases, and activates AP-1, NF-kappaB, and the mitogen-activated protein kinases (MAPKs) c-jun-N-terminal kinase and extracellular signal-regulated kinase. This signaling activity of K15 is related to phosphorylation of Y(481) of the K15 SH2-B motif Y(481)EEV. In this study we demonstrate the expression of an endogenous 45-kDa K15 protein in KSHV BAC36-infected epithelial cells. This endogenous K15 protein shows the same intracellular localization as transiently expressed K15, and expression kinetic studies suggest it to be a lytic gene. We have further determined the downstream target genes of K15 signaling using DNA oligonucleotide microarrays. We demonstrate that K15 is capable of inducing expression of multiple cytokines and chemokines, including interleukin-8 (IL-8), IL-6, CCL20, CCL2, CXCL3, and IL-1alpha/beta, as well as expression of Dscr1 and Cox-2. In epithelial cells, K15-induced upregulation of most genes was dependent on phosphorylation of Y(481), whereas in endothelial cells mutation of Y(481) did not result in a complete loss of Dscr1 and Cox-2 expression and NFAT-activity. Our study establishes K15 as one of the KSHV lytic genes that are inducing expression of multiple cytokines, which have been shown to play an important role in KSHV-associated pathogenesis.

Polymorphisms in the repeat long regions of oncogenic and attenuated pathotypes of Marek's disease virus 1

The nucleotide sequences of the terminal repeat long (TR(L)) and internal repeat long regions (IR(L)) in the genomes of 13 strains of Marek's disease virus type 1 (MDV-1) were determined and represent the largest collection of sequencing data from a contiguous region (12.8 kb) in the serotype 1 genomes. The collection of strains used in this study has been well characterized with respect to their virulence and contains members of each pathotype (4 attenuated, 1 mildly virulent, 3 virulent, 2 very virulent and 3 very virulent plus). It has previously been reported that two loci (meq and RLORF4) in the RL regions are likely to encode virulence factors based on comparative genomic studies involving vaccine and virulent strains. Additional studies using knockout mutants have provided stronger evidence that indeed RLORF4 and meq or the overlapping genes 23 kD and RLORF6 are involved in virulence. In this report, we provide evidence that additional open reading frames (ORFs) in the RL regions differ significantly between the extremes of the pathotypes (attenuated vs. nonattenuated). A deletion of 10 base pairs has been identified in RLORF12 from two attenuated strains CVI988 BP-5, p48 and RM-1, p40; and the lower virulence strain JM/102W. A deletion of 40 bp was also identified in RLORF4 of the attenuated strain R2/23, passage 106. A 177 bp insertion within the meq loci has been identified in most of the attenuated strains examined. Interestingly, R2/23 did not contain this insertion but instead truncated proteins are predicted for the three overlapping ORFs (meq, 23 kD and RLORF6) due to a frameshift mutation. Single nucleotide polymorphisms (SNPs), which loosely partition between attenuated and nonattenuated strains, have been identified in the ORFs encoding RLORF12, RLORF8, meq, 23 kD, RLORF6, RLORF4, RLORF3 and ICP0 and three previously unidentified short ORFs: MHLS, MLHG and MPSG. Although no single nucleotide polymorphism in the RL regions could predict virulence, their overall contribution to virulence can now be examined in defined mutants containing additional insertions or deletions in ORFs, suspected of encoding virulence factors, identified by this research.