Construction and characterization of Marek's disease viruses having green fluorescent protein expression tied directly or indirectly to phosphoprotein 38 expression

Temperature-induced reactivation of Marek's disease virus-transformed T cells ex vivo

Marek's disease virus (MDV) establishes latency in chicken T lymphocytes that can lead to T cell transformation and cancer. Transformed Marek's disease chicken cell lines (MDCCs) can be expanded ex vivo and provide a valuable model to study latency, transformation, and reactivation. Here, we developed MDCCs from chickens infected with MDV that fluoresce during lytic replication and reactivation. Sodium butyrate treatment increased fluorescent protein expression as evidenced by fluorescent microscopy, flow cytometry, and western blotting; however, it caused significant apoptosis and necrosis. Treatment of MDCCs by decreasing the temperature resulted in robust MDV reactivation without significant induction of apoptosis and necrosis. Furthermore, MDV reactivation was significantly affected by the time in culture that can affect downstream reactivation analyses. In all, our data show that fluorescent protein expression during reactivation is a robust tool to examine viral replication in live cells ex vivo, and temperature treatment is an efficient technique to induce reactivation without punitive effects on cell viability seen with chemical treatment.

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

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

Exploration of Alternative Splicing (AS) Events in MDV-Infected Chicken Spleens

Marek’s disease (MD) was an immunosuppression disease induced by Marek’s disease virus (MDV). MD caused huge economic loss to the global poultry industry, but it also provided an ideal model for studying diseases induced by the oncogenic virus. Alternative splicing (AS) simultaneously produced different isoform transcripts, which are involved in various diseases and individual development. To investigate AS events in MD, RNA-Seq was performed in tumorous spleens (TS), spleens from the survivors (SS) without any lesion after MDV infection, and non-infected chicken spleens (NS). In this study, 32,703 and 25,217 AS events were identified in TS and SS groups with NS group as the control group, and 1198, 1204, and 348 differently expressed (DE) AS events (p-value < 0.05 and FDR < 0.05) were identified in TS vs. NS, TS vs. SS, SS vs. NS, respectively. Additionally, Function enrichment analysis showed that ubiquitin-mediated proteolysis, p53 signaling pathway, and phosphatidylinositol signaling system were significantly enriched (p-value < 0.05). Small structural variations including SNP and indel were analyzed based on RNA-Seq data, and it showed that the TS group possessed more variants on the splice site region than those in SS and NS groups, which might cause more AS events in the TS group. Combined with previous circRNA data, we found that 287 genes could produce both circular and linear RNAs, which suggested these genes were more active in MD lymphoma transformation. This study has expanded the understanding of the MDV infection process and provided new insights for further analysis of resistance/susceptibility mechanisms.

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

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

Recombinant duck enteritis viruses expressing major structural proteins of the infectious bronchitis virus provide protection against infectious bronchitis in chickens

To design an alternative vaccine for control of infectious bronchitis in chickens, three recombinant duck enteritis viruses (rDEVs) expressing the N, S, or S1 protein of infectious bronchitis virus (IBV) were constructed using conventional homologous recombination methods, and were designated as rDEV-N, rDEV-S, and rDEV-S1, respectively. Chickens were divided into five vaccinated groups, which were each immunized with one of the rDEVs, covalent vaccination with rDEV-N & rDEV-S, or covalent vaccination with rDEV-N & rDEV-S1, and a control group. An antibody response against IBV was detectable and the ratio of CD4⁺/CD8⁺ T-lymphocytes decreased at 7 days post-vaccination in each vaccinated group, suggesting that humoral and cellular responses were elicited in each group as early as 7 days post-immunization. After challenge with a homologous virulent IBV strain at 21 days post-immunization, vaccinated groups showed significant differences in the percentage of birds with clinical signs, as compared to the control group (p < 0.01), as the two covalent-vaccination groups and the rDEV-S group provided better protection than the rDEV-N- or rDEV-S1-vaccinated group. There was less viral shedding in the rDEV-N & rDEV-S- (2/10) and rDEV-N & rDEV-S1- (2/10) vaccinated groups than the other three vaccinated groups. Based on the clinical signs, viral shedding, and mortality rates, rDEV-N & rDEV-S1 covalent vaccination conferred better protection than use of any of the single rDEVs.

•

Infectious bronchitis virus (IBV) causes a respiratory disease in domestic chickens worldwide.

•

Duck enteritis virus (DEV) was used as viral vaccine vector in chickens.

•

Recombinant DEV (rDEV) expressing IBV N, S, or S1 conferred protection against IBV.

•

Covalent vaccination with rDEV-N & rDEV-S1 conferred higher level of protection against IBV.

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

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

Selection of a recombinant Marek's disease virus in vivo through expression of the Marek's EcoRI-Q (Meq)-encoded oncoprotein: characterization of an rMd5-based mutant expressing the Meq of strain RB-1B

Marek's disease (MD) is a highly contagious viral disease of chickens (Gallus gallus domesticus) caused by MD virus (MDV), characterized by paralysis, neurologic signs, and the rapid onset of T-cell lymphomas. MDV-induced T-cell transformation requires a basic leucine zipper protein called Marek's EcoRI-Q-encoded protein (Meq). We have identified mutations in the coding sequence of Meq that correlated with virus pathotype (virulent, very virulent, and very virulent plus). The aim of this study was to determine whether recombinant viruses could be isolated based on Meq expression through in vivo selection. Chicken embryo fibroblasts (CEFs) were cotransfected with an rMd5 strain-based Meq deletion virus (rMd5deltaMeq) and meq loci from strains representing different pathotypes of MDV. Transfected CEFs were inoculated into chickens in two independent studies. We were able to isolate a single recombinant virus, rMDV-1137, in a contact-exposed chicken. rMDV-1137 had recombined two copies of the meq gene of RB-1B and was found to have pathogenicity similar to both RB-1B and rMd5 parental strains. We found the RB-1B- and rMd5-induced lymphomas showed differences in composition and that rMDV-1137-induced lymphomas were intermediate in their composition. We were able to establish cell lines from both RB-1B- (MDCC-UD35, -UD37) and rMDV-1137 (MDCC-UD36, -UD38)-induced, but not rMd5-induced, lymphomas. To date, no rMd5- or parent Md5-transformed T-cell lines have been reported. Our results suggest that 1) a recombinant MDV can be selected on the basis of oncogenicity; 2) changes in Meq sequence seem to affect tumor composition and the ability to establish cell lines; and 3) in addition to meq, other genomic loci affect MDV pathogenicity and oncogenicity.

Fluorescently tagged pUL47 of Marek's disease virus reveals differential tissue expression of the tegument protein in vivo

Marek's disease virus (MDV), a lymphotropic alphaherpesvirus, causes Marek's disease (MD) in chickens. MD is characterized by neurological signs, chronic wasting, and T cell lymphomas that predominate in the visceral organs. MDV replicates in a highly cell-associated manner in vitro and in vivo, with infectious virus particles being released only from feather follicle epithelial (FFE) cells in the skin. Virus produced and shed from FFE cells allows transmission of MDV from infected to naïve chickens, but the mechanisms or roles of differential virus gene expression have remained elusive. Here, we generated recombinant MDV in which we fused enhanced green fluorescent protein (EGFP) to the C terminus of the tegument protein pUL47 (vUL47-EGFP) or pUL49 (vUL49-EGFP). While vUL49-EGFP was highly attenuated in vitro and in vivo, vUL47-EGFP showed unaltered pathogenic potential and stable production of pUL47-EGFP, which facilitated direct analysis of pUL47 expression in cells and tissues. Our studies revealed that pUL47-EGFP is expressed at low levels and localizes to the nucleus during lytic replication in vitro and in lymphocytes in the spleen in vivo, while it is undetectable in tumors. In contrast, pUL47-EGFP is highly abundant and localizes predominantly in the cytoplasm in FFE cells in the skin, where MDV is shed into the environment. We concluded that differential expression and localization of MDV pUL47-EGFP tegument protein is potentially important for the unique cell-associated nature of MDV in vitro and in lymphocytes in vivo, as well as production of free virus in FFE cells.

Clustering of mutations within the inverted repeat regions of a serially passaged attenuated gallid herpesvirus type 2 strain

Marek's disease (MD) is the leading cause of losses in chicken production in the world. Over the past 40 years significant progress has been made in the control of MD through the use of vaccines which reduce or delay tumor formation in vaccinated flocks. However, these vaccines fail to induce an immune response that protects against infection and virus shedding. Little is known about the genetic changes that lead to attenuation and are necessary for the generation of vaccine strains. Previous research has demonstrated that serial passage of virulent strains in cell culture results in the generation of attenuated progeny. Obtaining detailed knowledge of the changes which are needed for attenuation will be important for advancing our understanding of MD biology and should facilitate the development of more potent vaccines. We have determined the complete nucleotide sequence of a bacterial artificial chromosome (BAC) construct representing the 80th passage of a very virulent plus (vv+) MD virus strain termed 584A. Pathotyping studies have indicated that this strain (584Ap80) is indeed attenuated. Bioinformatic analysis of the sequencing data has identified numerous gross genetic changes clustering in the inverted repeat regions of the genome, as well as subtle changes (single nucleotide polymorphisms or SNPs) scattered throughout the genome. Relative to the parental strain (584Ap9), insertional mutations were identified in the MD-specific genes encoding RLORF1, RLORF3, RLORF6, 23 kDa, RLORF7 (Meq), vIL8, vLip, RSORF1, and five uncharacterized novel genes. Deletions were found in four locations within the 584Ap80 genome. A large deletion (297nt) was found in the diploid genes 85.6/98.6 and a 321 nt deletion within the intergenic region between the U(L)3 and U(L)3.5 genes is predicted to create a fusion polypeptide. A single nucleotide deletion was identified within the origin of replication. Both insertions and deletions were found in the dipoid genes MDV3.4/78.3 encoding the virulence factor RLORF4. The sequencing of the attenuated strain 584Ap80 and comparison to that of the virulent parent 584A passage 9 (584Ap9) has provided a wealth of information regarding genetic changes which have occurred during the attenuation process.

Expression of Marek's disease virus phosphorylated polypeptide pp38 produces splice variants and enhances metabolic activity

The phosphorylated polypeptide (pp)38 of oncogenic Marek's disease (MD) herpesvirus (MDV) is expressed during lytic infections in vivo and in vitro, but its functions have not been fully elucidated. The quail cell line QT-35, latently infected with MDV, was used to generate QTP32 in which pp38 is expressed under control of a tetracycline controlled promoter to examine possible functions of pp38. Induction of pp38 did not influence late MDV genes expression, but it enhanced mitochondrial dehydrogenase activity significantly. Two new pp38 splice variants were found in induced QTP32 cells, in additional in vitro systems and MDV-infected chickens. Differential expression of full-length pp38 and splice variants suggests that the splice variants are important during latency and perhaps transformation. Polypeptides of 40 and 20kDa were detected by Western blot using monoclonal antibody H19. These polypeptides were also produced in DF-1 cells transfected with a pp38 construct in which the splice acceptor sites had been mutated. Our results add important new information to the role of pp38 in the pathogenesis of MD. The data suggest that pp38 and the two newly described splice variants may influence metabolic activity, which may have important consequences for the understanding of latency and tumor development.