Pathogenesis of Marek's disease virus infection

Effect of In Ovo Supplementation of Slab51 Probiotic Mixture, Associated with Marek's Disease Vaccine, on Growth Performance, Intestinal Morphology and Eimeria spp. Infection in Broiler Chickens

The interest for in ovo feeding has grown in the last decades mainly concerning probiotics, live microorganisms that can actively interact with the embryo. The aim of this study was to evaluate the effects of a multi-strain probiotic diluted in Marek's disease vaccine (MDV) on zootechnical performances, intestinal morphology and Eimeria spp. infection. One hundred and twenty eggs of Ross 308 broiler chickens were incubated until 18 d, when 105 fertilised and vital eggs were randomly allocated into three groups. A control group (C) was inoculated with MDV; two treated groups (P1 and P2) were inoculated with MDV and different concentrations of probiotics: 1 × 105 CFU/100 μL in P1 and 1 × 106 CFU/100 μL in P2. After hatching, chickens were separated into three replicates (10/replicate). Zootechnical parameters were determined. At the end of the cycle (35 d), chickens were slaughtered, and the intestine was collected for morphological analysis from nine chickens per group (three/replicate). Eimeria spp. oocyst shedding was determined weekly, and parasitic lesions were analysed on the histological sample. In ovo treatment with probiotic did not influence hatching rate but significantly improved body weight and positively influenced intestinal morphometric data compared to C. Oocyst shedding in faeces resulted in an increase in C, with significant differences at sampling performed at 14, 21 and 28 d of age. These results suggest that the tested probiotic compound is safe for in ovo supplementation and effectively improves zootechnical performances and coccidian resistance.

DCLK1 mediated cooperative acceleration of EMT by avian leukosis virus subgroup J and Marek's disease virus via the Wnt/β-catenin pathway promotes tumor metastasis

Co-infection with oncogenic retrovirus and herpesvirus significantly facilitates tumor metastasis in human and animals. Co-infection with avian leukosis virus subgroup J (ALV-J) and Marek's disease virus (MDV), which are typical oncogenic retrovirus and herpesvirus, respectively, leads to enhanced oncogenicity and accelerated tumor formation, resulting in increased mortality of affected chickens. Previously, we found that ALV-J and MDV cooperatively promoted tumor metastasis. However, the molecular mechanism remains elusive. Here, we found that doublecortin-like kinase 1 (DCLK1) mediated cooperative acceleration of epithelial-mesenchymal transition (EMT) by ALV-J and MDV promoted tumor metastasis. Mechanistically, DCLK1 induced EMT via activating Wnt/β-catenin pathway by interacting with β-catenin, thereby cooperatively promoting tumor metastasis. Initially, we screened and found that DCLK1 was a potential mediator for the cooperative activation of EMT by ALV-J and MDV, and enhanced cell proliferation, migration, and invasion. Subsequently, we revealed that DCLK1 physically interacted with β-catenin to promote the formation of the β-catenin-TCF4 complex, inducing transcription of the Wnt target gene, c-Myc, promoting EMT by increasing the expression of N-cadherin, Vimentin, and Snail, and decreasing the expression of E-cadherin. Taken together, we discovered that jointly activated DCLK1 by ALV-J and MDV accelerated cell proliferation, migration and invasion, and ultimately activated EMT, paving the way for tumor metastasis. This study elucidated the molecular mechanism underlying cooperative metastasis induced by co-infection with retrovirus and herpesvirus.

IMPORTANCE

Tumor metastasis, a complex phenomenon in which tumor cells spread to new organs, is one of the greatest challenges in cancer research and is the leading cause of cancer-induced death. Numerous studies have shown that oncoviruses and their encoded proteins significantly affect metastasis, especially the EMT process. ALV-J and MDV are classic tumorigenic retrovirus and herpesvirus, respectively. We found that ALV-J and MDV synergistically promoted EMT. Further, we identified the tumor stem cell marker DCLK1 in ALV-J and MDV co-infected cells. DCLK1 directly interacted with β-catenin, promoting the formation of the β-catenin-TCF4 complex. This interaction activated the Wnt/β-catenin pathway, thereby inducing EMT and paving the way for synergistic tumor metastasis. Exploring the molecular mechanisms by which ALV-J and MDV cooperate during EMT will contribute to our understanding of tumor progression and metastasis. This study provides new insights into the cooperative induced tumor metastasis by retroviruses and herpesviruses.

Unraveling the role of γδ T cells in the pathogenesis of an oncogenic avian herpesvirus

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus that causes deadly lymphomas in chickens. In chickens, up to 50% of all peripheral T cells are gamma delta (γδ) T cells. Until now, their role in MDV pathogenesis and tumor formation remains poorly understood. To investigate the role of γδ T cells in MDV pathogenesis, we infected recently generated γδ T cell knockout chickens with very virulent MDV. Strikingly, disease and tumor incidence were highly increased in the absence of γδ T cells, indicating that γδ T cells play an important role in the immune response against MDV. In the absence of γδ T cells, virus replication was drastically increased in the thymus and spleen, which are potential sites of T cell transformation. Taken together, our data provide the first evidence that γδ T cells play an important role in the pathogenesis and tumor formation of this highly oncogenic herpesvirus.IMPORTANCEGamma delta (γδ) T cells are the most abundant T cells in chickens, but their role in fighting pathogens remains poorly understood. Marek's disease virus (MDV) is an important veterinary pathogen, that causes one of the most frequent cancers in animals and is used as a model for virus-induced tumor formation. Our study revealed that γδ T cells play a crucial role in combating MDV, as disease and tumor incidence drastically increased in the absence of these cells. γδ T cells restricted virus replication in the key lymphoid organs, thereby decreasing the likelihood of causing tumors and disease. This study provides novel insights into the role of γδ T cells in the pathogenesis of this highly oncogenic virus.

Novel lncRNA 803 related to Marek's disease inhibits apoptosis of DF-1 cells

Marek's disease (MD) is a neoplastic disease that significantly affects the poultry industry. Long non-coding RNAs (lncRNAs) are crucial regulatory factors in various biological processes, including tumourigenesis. However, the involvement of novel lncRNAs in the course of MD virus (MDV) infection is still underexplored. Here, we present the first comprehensive characterization of differentially expressed lncRNAs in chicken spleen at different stages of MDV infection. A series of differentially expressed lncRNAs was identified at each stage of MDV infection through screening. Notably, our investigation revealed a novel lncRNA, lncRNA 803, which exhibited significant differential expression at different stages of MDV infection and was likely to be associated with the p53 pathway. Further analyses demonstrated that the overexpression of lncRNA 803 positively regulated the expression of p53 and TP53BP1 in DF-1 cells, leading to the inhibition of apoptosis. This is the first study to focus on the lncRNA expression profiles in chicken spleens during MDV pathogenesis. Our findings highlight the potential role of the p53-related novel lncRNA 803 in MD pathogenesis and provide valuable insights for decoding the molecular mechanism of MD pathogenesis involving non-coding RNA.RESEARCH HIGHLIGHTS Differentially expressed lncRNAs in spleens of chickens infected with Marek's disease virus at different stages were identified for the first time.The effects of novel lncRNA 803 on p53 pathway and apoptosis of DF-1 cells were reported for the first time.

Interactions between avian viruses and skin in farm birds

This article reviews the avian viruses that infect the skin of domestic farm birds of primary economic importance: chicken, duck, turkey, and goose. Many avian viruses (e.g., poxviruses, herpesviruses, Influenza viruses, retroviruses) leading to pathologies infect the skin and the appendages of these birds. Some of these viruses (e.g., Marek's disease virus, avian influenza viruses) have had and/or still have a devasting impact on the poultry economy. The skin tropism of these viruses is key to the pathology and virus life cycle, in particular for virus entry, shedding, and/or transmission. In addition, for some emergent arboviruses, such as flaviviruses, the skin is often the entry gate of the virus after mosquito bites, whether or not the host develops symptoms (e.g., West Nile virus). Various avian skin models, from primary cells to three-dimensional models, are currently available to better understand virus-skin interactions (such as replication, pathogenesis, cell response, and co-infection). These models may be key to finding solutions to prevent or halt viral infection in poultry.

The genome evolution of Marek's disease viruses in chickens and turkeys in China

The virus that causes Marek's disease (MD) is globally ubiquitous in chickens, continuously evolving, and poses a significant threat to the poultry industry. Although vaccines are extensively used, MD still occurs frequently and the virus has evolved increased virulence in China. Here, we report an outbreak of MD in vaccinated chickens and unvaccinated turkeys in a backyard farm in Guangdong province, China, in 2018. Phylogenetic analysis revealed two lineages of MDVs at this farm, with one lineage, containing isolates from two turkeys and five chickens, clustering with virulent Chinese strains and displays a relatively high genetic divergence from the vaccine strains. These new isolates appear to have broken through vaccine immunity, yielding this outbreak of MD in chickens and turkeys. The second lineage included four chicken isolates that clustered with the CVI988 and 814 vaccine strains. The large diversity of MDVs in this single outbreak reveals a complex circulation of MDVs in China. Poor breeding conditions and the weak application of disease prevention and control measures make backyard farms a hotbed for the evolution of viruses that cause infectious diseases. This is especially important in MDV as the MD vaccines do not provide sterilizing immunity, which allows the replication and shedding of virulent field viruses by vaccinated individuals and supporting the continuous evolution of MDVs. Hence, constant monitoring of the evolution of MDVs is necessary to understand the evolution of these field viruses and potential expansions of their host range.

Survey of the Clinical, Cytological, and Histopathological Features Associated with Neoplasms in Captive Avian Species in Melbourne, Australia

Over a 3.5-year period, tissue samples from 141 companion and aviary birds with clinically suspected, naturally occurring solid neoplasms were collected via surgical biopsy (n = 53) or at necropsy examination (n = 88) from a population of birds presenting to an avian veterinary clinic in Melbourne, Australia. Neoplastic processes were identified in 73.7% (n = 104) of samples, with 83.7% (n = 87) being interpreted as malignant neoplasia and 16.3% (n = 17) being interpreted as benign neoplasia. The overall estimated prevalence of neoplasia in the study population (n = 5125) was 2.45% (95% confidence interval, 2-2.97%). The gastrointestinal and reproductive tracts were the most commonly affected systems. The most common presenting signs were nonspecific and included lethargy, coelomic distension, and inappetence. In 59 cases, fine-needle aspirates (FNAs) and impression smears were also obtained and evaluated cytologically. The accuracy of cytology for detecting neoplasia in birds and for determining whether a neoplastic process was benign or malignant was assessed by using histopathology as the "gold standard." There was complete agreement between the cytological and histopathological diagnoses in 72.8% (43/59) of cases. Cytology correctly identified 87.5% of these cases (35/40) as malignant neoplasms and 55.6% (5/9) as benign processes. There was no significant difference between the use of cytology and histopathology for the detection of malignant neoplasia in birds (P = 0.185). The accuracies of FNAs and impression smears for examining avian tumors were also compared. Overall, the best cell preservation was obtained by performing impression smears from tissues, with 62.2% (n = 28) returning high cellularity for cytological examination, compared to 53.8% (n = 14) when samples were obtained by FNA. This study provides an overview of the types and prevalence of neoplasms in a captive bird population from Australia, correlates physical examination findings with tumor types, and provides evidence that cytology is a reliable preliminary diagnostic tool for detecting neoplasia in birds.

Characterization of integrated Marek's disease virus genomes supports a model of integration by homology-directed recombination and telomere-loop-driven excision

IMPORTANCE

Marek's disease virus (MDV) is a ubiquitous chicken pathogen that inflicts a large economic burden on the poultry industry, despite worldwide vaccination programs. MDV is only partially controlled by available vaccines, and the virus retains the ability to replicate and spread between vaccinated birds. Following an initial infection, MDV enters a latent state and integrates into host telomeres and this may be a prerequisite for malignant transformation, which is usually fatal. To understand the mechanism that underlies the dynamic relationship between integrated-latent and reactivated MDV, we have characterized integrated MDV (iMDV) genomes and their associated telomeres. This revealed a single orientation among iMDV genomes and the loss of some terminal sequences that is consistent with integration by homology-directed recombination and excision via a telomere-loop-mediated process.

Effect of Pre-Treatment with a Recombinant Chicken Interleukin-17A on Vaccine Induced Immunity against a Very Virulent Marek's Disease Virus

The host response to pathogenic microbes can lead to expression of interleukin (IL)-17, which has antimicrobial and anti-viral activity. However, relatively little is known about the basic biological role of chicken IL-17A against avian viruses, particularly against Marek's disease virus (MDV). We demonstrate that, following MDV infection, upregulation of IL-17A mRNA and an increase in the frequency of IL-17A+ T cells in the spleen occur compared to control chickens. To elaborate on the role of chIL-17A in MD, the full-length chIL-17A coding sequence was cloned into a pCDNA3.1-V5/HIS TOPO plasmid. The effect of treatment with pcDNA:chIL-17A plasmid in combination with a vaccine (HVT) and very virulent(vv)MDV challenge or vvMDV infection was assessed. In combination with HVT vaccination, chickens that were inoculated with the pcDNA:chIL-17A plasmid had reduced tumor incidence compared to chickens that received the empty vector control or that were vaccinated only (66.6% in the HVT + empty vector group and 73.33% in HVT group versus 53.3% in the HVT + pcDNA:chIL-17A). Further analysis demonstrated that the chickens that received the HVT vaccine and/or plasmid expressing IL-17A had lower MDV-Meq transcripts in the spleen. In conclusion, chIL-17A can influence the immunity conferred by HVT vaccination against MDV infection in chickens.

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.

Effect of light intensity on the tear production, central corneal thickness, and intraocular pressure in broiler chickens

OBJECTIVE

The objective of the present study was to examine the effect of different light intensities on tear production, corneal thickness, and intraocular pressure in broilers.

ANIMALS STUDIED

Both eyes of 72 male broilers were evaluated in this study.

MATERIALS AND METHODS

Broilers were divided into the following three groups: low light intensity (n = 24, 5 lux), moderate light intensity (n = 24, 20 lux), and high light intensity (n = 24, 80 lux). The eyes of all birds (n = 72) underwent a complete ophthalmic examination, which included the Schirmer tear test (STT-I), intraocular pressure (IOP), and the central cornea thickness measurement (CCT). The effect of light intensity on the Schirmer test, intraocular pressure, and central corneal thickness values was examined at eye and animal level (right and left eyes separately and cumulatively/no distinguishing left or right) by using one-way ANOVA.

RESULTS

At the animal level, without discrimination of left and right eye measurements, statistically significant differences were found between 5-20 and 20-80 lux groups on IOP measurements (p < .05). The difference in CCT measurements between the 5 and 20 lux groups was statistically significant (p < .05), and the corneal thickness of the 5 lux group animals was found to be statistically significant and higher than the 20 lux group (p < .05).

CONCLUSIONS

In conclusion, light intensity has an influence on eye health in broilers. Present results may attribute to the future studies as a reference value for broilers raised under different light intensities.

Identification of a Novel Insertion Site HVT-005/006 for the Generation of Recombinant Turkey Herpesvirus Vector

Turkey herpesvirus (HVT) has been widely used as a successful live virus vaccine against Marek's disease (MD) in chickens for more than five decades. Increasingly, HVT is also used as a highly effective recombinant vaccine vector against multiple avian pathogens. Conventional recombination, or recombineering, techniques that involve the cloning of viral genomes and, more recently, gene editing methods have been used for the generation of recombinant HVT-based vaccines. In this study, we used NHEJ-dependent CRISPR/Cas9-based approaches to insert the mCherry cassette for the screening of the HVT genome and identifying new potential sites for the insertion of foreign genes. A novel intergenic site HVT-005/006 in the unique long (UL) region of the HVT genome was identified, and mCherry was found to be stably expressed when inserted at this site. To confirm whether this site was suitable for the insertion of other exogenous genes, haemagglutinin (HA) of the H9N2 virus was inserted into this site, and a recombinant HVT-005/006-HA was rescued. The recombinant HVT-HA can grow well and express HA protein stably, which demonstrated that HVT-005/006 is a promising site for the insertion of foreign genes.

Development of monoclonal antibodies specific to Marek disease virus-EcoRI-Q (Meq) for the immunohistochemical diagnosis of Marek disease using formalin-fixed, paraffin-embedded samples

Marek disease (MD) is a viral disease characterized by the development of lymphoma in poultry. Although morphologic confirmation of lymphoma is used to diagnose MD, immunohistochemical detection of MD virus-EcoRI-Q (Meq), which is a viral protein that is expressed exclusively in MD tumor cells, would further improve the accuracy of diagnosis. We developed monoclonal antibodies (mAbs) that specifically detect Meq by immunohistochemistry (IHC) using formalin-fixed, paraffin-embedded (FFPE) sections. We evaluated the sensitivity and specificity of 14 mAbs that we produced, using FFPE samples of MDCC-MSB1 cells, MD tumor tissues, and tissues of uninfected chickens. Four different antigen retrieval conditions were investigated. Thirteen mAbs reacted with Meq in FFPE sections, but immunohistochemical reactivity and specificity varied depending on the mAb and antigen retrieval condition; heat-induced antigen retrieval (HIAR) was more effective at detecting Meq than the other tested conditions. HIAR pH 9 tended to increase immunoreactivity and decrease specificity. Of the 5 mAbs that immunoreacted strongly with Meq without nonspecific reactions under the optimal antigen retrieval conditions, 3 mAbs (1C1-121, 3A3-112, 5F7-82) did not produce background staining of tumor or non-tumor tissues; 2 mAbs (2C5-11, 4A5-54) produced background staining. The mAb 6B5-128 reacted moderately with Meq without nonspecific reactions and background staining. The remaining mAbs showed weak immunoreactivity or problematic nonspecific reactions. Our results suggest that some of our developed mAbs can be used in IHC to detect Meq in FFPE sections with high specificity, and that the use of IHC may greatly improve the diagnosis of MD.

The sulphated polysaccharides extract ulvans from Ulva armoricana limits Marek's disease virus dissemination in vitro and promotes viral reactivation in lymphoid cells

Background

Marek’s disease (MD) is a highly contagious lymphoproliferative disease of chickens caused by an alphaherpesvirus, Marek’s disease virus (MDV). MD is presently controlled by systematic vaccination of animals, which protects efficiently against the development of clinical disease. However, MDV vaccines do not prevent the multiplication and spread of MDV field strains and may favor the emergence of strains with increased virulence. Therefore, MDV persists to be a major problem for the poultry industry and the development of new alternative strategies to control MDV is needed. Seaweed extracts have previously been shown to exert immunomodulatory and antiviral activities, especially against herpesviruses. The objective of the present study was to explore the effect of Ulva armoricana extracts on MDV infection in vitro.

Results

We could demonstrate that the ulvan extract as well as its vitamin-enriched formulation reduce the viral load by about 80% at 24 h post-infection in infected chicken fibroblasts at concentrations that are innocuous for the cells. We also observed a substantial decrease in MDV plaque size suggesting that ulvans impede MDV cell-to-cell spread in vitro. Moreover, we showed that ulvan extract could promote MDV reactivation in lymphoid cells.

Conclusions

Our data provide the first evidence that the use of the ulvan extract could be a good alternative to limit MDV infection in poultry.

B cells do not play a role in vaccine-mediated immunity against Marek's disease

BACKGROUND

Marek's disease virus (MDV), a highly oncogenic α-herpesvirus, is the etiological agent of Marek's disease (MD) in chickens. The antiviral activity of vaccine-induced immunity against MD reduces the level of early cytolytic infection, production of cell-free virions in the feather follicle epithelial cells (FFE), and lymphoma formation. Despite the success of several vaccines that have greatly reduced the economic losses from MD, the mechanism of vaccine-induced immunity is poorly understood.

METHODS

To provide insight into possible role of B cells in vaccine-mediated protection, we bursectomized birds on day of hatch and vaccinated them eight days later. The birds were challenged 10 days post vaccination with or without receiving adoptive lymphocytes from age-matched control birds prior to inoculation. The study also included vaccinated/challenged and non-vaccinated challenged intact birds. Flowcytometric analysis of PBMN cells were conducted twice post bursectomy to confirm B cell depletion and assess the effect of surgery on T cell population. Immunohistochemical analysis and viral genome copy number assessment in the skin samples at termination was performed to measure the replication rate of MDV in the FFE of the skin tissues of the challenged birds.

RESULTS

The non-vaccinated/challenged birds developed typical clinical signs of MD while the vaccinated/challenged and bursectomized, vaccinated/challenged groups with or without adoptive lymphocyte transfer, were fully protected with no sign of transient paralysis, weight loss, or T cell lymphomas. Immunohistochemical analysis and viral genome copy number evaluation in the skin samples revealed that unlike the vaccinated/challenged birds a significant number of virus particles were produced in the FFE of the non-vaccinated/challenged birds at termination. In the bursectomized, vaccinated/challenged groups, only a few replicating virions were detected in the skin of birds that received adoptive lymphocytes prior to challenge.

CONCLUSIONS

The study shows that B cells do not play a critical role in MD vaccine-mediated immunity.

The Conserved Herpesviridae Protein Kinase (CHPK) of Gallid alphaherpesvirus 3 (GaHV3) Is Required for Horizontal Spread and Natural Infection in Chickens

We have formerly identified the conserved herpesvirus protein kinase (CHPK) as essential for horizontal transmission of Marek’s disease virus (MDV). Thus far, it has been confirmed that the mutation of the invariant lysine (K) of CHPKs abrogates kinase activity and that CHPK activity is required for MDV horizontal transmission. Since CHPK is conserved among all members of the Herpesviridae, we hypothesized that CHPK, and specifically its kinase activity, is important for the horizontal transmission of other herpesviruses. To test this hypothesis, we utilized our experimental and natural infection model in chickens with MD vaccine strain 301B/1 of Gallid alphaherpesvirus 3 (GaHV3). First, we mutated the invariant lysine (K) 157 of 301B/1 CHPK to alanine (A) and determined whether it was required for horizontal transmission. To confirm the requirement of 301B/1 CHPK activity for transmission, a rescued virus was generated in which the A157 was changed back to a K (A157K). Despite both the CHPK mutant (K157A) and rescuant (A157K) viruses having replication defects in vivo, only the CHPK mutant (K157A) was unable to spread to contact chickens, while both wild-type and rescuant (A157K) viruses transmitted efficiently, confirming the importance of CHPK activity for horizontal spread. The data confirm that CHPK is required for GaHV3 transmission and suggest that the requirement of avian CHPKs for natural infection is conserved.

Occurrence of Marek's disease in vaccinated Algerian broiler breeder flocks: A histopathological survey

BACKGROUND AND AIM

Marek's disease (MD) is a lymphoproliferative disease that occurs in chickens. In the absence of control measures, MD causes devastating losses to commercial poultry flocks. Vaccination has enabled dramatic success in the prevention and control of MD. However, the MD vaccination program has failed frequently, and occasional clinical outbreaks have been reported in the vaccinated flocks as well. The present study aimed to describe the clinical and histopathological characteristics of the field cases of MD in broiler breeder flocks.

MATERIALS AND METHODS

A survey on the update of MD occurrence in Algerian broiler breeder flocks was conducted from June 2020 to September 2020. Ten vaccinated broiler breeder flocks located in Central Algeria and having progressive tumors in different visceral organs were evaluated for MD virus infection by conducting a histopathological examination of the birds.

RESULTS

The age of the birds affected with MD ranged from 13 to 22 weeks. The mortality rate varied sensitively from 4% to 10%. The clinical symptoms reported in the affected flocks included locomotor, nervous, digestive, and respiratory symptoms. Necropsy of the dead or euthanized birds revealed visceral lymphomatosis in several organs and macroscopic changes in the peripheral nerves (including loss of longitudinal striation, color change [grayish], and volume increase). The histopathological findings included the infiltration and proliferation of lymphocytes and blast cells (lymphoblasts) in various organs of the birds, which are the typical characteristics of MD and, therefore, confirmed the field infection of MD in these birds.

CONCLUSION

The present study provided evidence for the high prevalence of MD in the broiler breeder flocks vaccinated with a bivalent vaccine (turkey herpesvirus+Rispens) at the hatchery. The findings of the present study may indicate high-level failure of vaccination in these birds.

Marek's disease virus prolongs survival of primary chicken B-cells by inducing a senescence-like phenotype

Marek’s disease virus (MDV) is an alphaherpesvirus that causes immunosuppression and deadly lymphoma in chickens. Lymphoid organs play a central role in MDV infection in animals. B-cells in the bursa of Fabricius facilitate high levels of MDV replication and contribute to dissemination at early stages of infection. Several studies investigated host responses in bursal tissue of MDV-infected chickens; however, the cellular responses specifically in bursal B-cells has never been investigated. We took advantage of our recently established in vitro infection system to decipher the cellular responses of bursal B-cells to infection with a very virulent MDV strain. Here, we demonstrate that MDV infection extends the survival of bursal B-cells in culture. Microarray analyses revealed that most cytokine/cytokine-receptor-, cell cycle- and apoptosis-associated genes are significantly down-regulated in these cells. Further functional assays validated these strong effects of MDV infections on cell cycle progression and thus, B-cell proliferation. In addition, we confirmed that MDV infections protect B-cells from apoptosis and trigger an accumulation of the autophagy marker Lc3-II. Taken together, our data indicate that MDV-infected bursal B-cells show hallmarks of a senescence-like phenotype, leading to a prolonged B-cell survival. This study provides an in-depth analysis of bursal B-cell responses to MDV infection and important insights into how the virus extends the survival of these cells.

Upon MDV entry via the respiratory tract, B-cells are among the first cells to be infected in the lung and allow an efficient amplification of the virus. B-cells ensure the transmission of the virus to activated T-cells in which it replicates and ultimately transforms CD4-positive T-cells. Although playing a pivotal role in the MDV life cycle, the response of B-cells to MDV is currently not fully understood. Here, by using an in vitro infection model of primary bursal B-cells, we show that MDV infection leads to a prolonged B-cell survival resulting from decreased cell proliferation, protection from apoptosis and activation of autophagy. Our study provides new insights into the B-cell response to MDV infection, demonstrating that MDV triggers a senescence-like phenotype in B-cells that could potentiate their role in MDV pathogenesis.

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.

Lymphoma in Psittacine Birds: A Histological and Immunohistochemical Assessment

In psittacine birds, round cell neoplasms that originate from lymphocytes, plasma cells, histiocytes, or mast cells are sporadic and poorly described. The lack of morphological and immunohistochemical diagnostic criteria or grading schemes make specific diagnoses and prognoses challenging. We assessed cases of psittacine birds diagnosed with round cell neoplasia from 3 North American veterinary diagnostic laboratories to describe the diagnostic features of these tumors. For all cases, demographic data, anatomic distribution, histological features, and immunoreactivity for T (CD3) and B (Pax5 and MUM-1) cell markers were assessed using tissue microarrays and whole slide mounts. Thirty-eight psittacine birds representing 14 species were included. Tumors were mainly infiltrative and multicentric, were composed of homogenous sheets of round to polygonal cells, and commonly presented with a high mitotic count (average 21 mitoses per high-power field). Based on Pax5 immunoreactivity, B-cell lymphoma was most common (19/38 [50%]), and was significantly associated with involvement of the gastrointestinal and urogenital systems. Of the 38 cases, 6 (16%) were consistent with T-cell lymphoma, 3 (8%) with plasma cell tumor, and 3 (8%) were double-reactive for both B- and T-lymphocyte markers. This is the first study to describe morphologic and immunohistochemical features of round cell neoplasia in a large number of psittacine birds, and provides benchmark data for future studies aimed at elucidating the diagnosis and prognosis of these neoplasms. These data also provide useful information about reactivity of commercially available antibodies as lymphocyte markers in tissues of multiple psittacine species.

Comprehensive profiling analysis of the N6-methyladenosine-modified circular RNA transcriptome in cultured cells infected with Marek's disease virus

Marek's disease virus (MDV) induces severe immunosuppression and lymphomagenesis in the chicken, its natural host, and results in a condition that investigated the pathogenesis of MDV and have begun to focus on the expression profiling of circular RNAs (circRNAs). However, little is known about how the expression of circRNAs is referred to as Marek's disease. Previous reports have is regulated during MDV replication. Here, we carried out a comprehensive profiling analysis of N6-methyladenosine (m⁶A) modification on the circRNA transcriptome in infected and uninfected chicken embryonic fibroblast (CEF) cells. Methylated RNA immunoprecipitation sequencing (MeRIP-Seq) revealed that m⁶A modification was highly conserved in circRNAs. Comparing to the uninfected group, the number of peaks and conserved motifs were not significantly different in cells that were infected with MDV, although reduced abundance of circRNA m⁶A modifications. However, gene ontology and Kyoto encyclopedia of genes and genomes (KEGG) pathway analyses revealed that the insulin signaling pathway was associated with the regulation of m⁶A modified circRNAs in MDV infection. This is the first report to describe alterations in the transcriptome-wide profiling of m⁶A modified circRNAs in MDV-infected CEF cells.

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.

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.

The dominantly expressed class II molecule from a resistant MHC haplotype presents only a few Marek's disease virus peptides by using an unprecedented binding motif

Viral diseases pose major threats to humans and other animals, including the billions of chickens that are an important food source as well as a public health concern due to zoonotic pathogens. Unlike humans and other typical mammals, the major histocompatibility complex (MHC) of chickens can confer decisive resistance or susceptibility to many viral diseases. An iconic example is Marek's disease, caused by an oncogenic herpesvirus with over 100 genes. Classical MHC class I and class II molecules present antigenic peptides to T lymphocytes, and it has been hard to understand how such MHC molecules could be involved in susceptibility to Marek's disease, given the potential number of peptides from over 100 genes. We used a new in vitro infection system and immunopeptidomics to determine peptide motifs for the 2 class II molecules expressed by the MHC haplotype B2, which is known to confer resistance to Marek's disease. Surprisingly, we found that the vast majority of viral peptide epitopes presented by chicken class II molecules arise from only 4 viral genes, nearly all having the peptide motif for BL2*02, the dominantly expressed class II molecule in chickens. We expressed BL2*02 linked to several Marek's disease virus (MDV) peptides and determined one X-ray crystal structure, showing how a single small amino acid in the binding site causes a crinkle in the peptide, leading to a core binding peptide of 10 amino acids, compared to the 9 amino acids in all other reported class II molecules. The limited number of potential T cell epitopes from such a complex virus can explain the differential MHC-determined resistance to MDV, but raises questions of mechanism and opportunities for vaccine targets in this important food species, as well as providing a basis for understanding class II molecules in other species including humans.

Marek's disease virus US3 protein kinase phosphorylates chicken HDAC 1 and 2 and regulates viral replication and pathogenesis

Marek’s disease virus (MDV) is a potent oncogenic alphaherpesvirus that elicits a rapid onset of malignant T-cell lymphomas in chickens. Three MDV types, including GaHV-2 (MDV-1), GaHV-3 (MDV-2) and MeHV-1 (HVT), have been identified and all encode a U_S3 protein kinase. MDV-1 U_S3 is important for efficient virus growth in vitro. To study the role of U_S3 in MDV replication and pathogenicity, we generated an MDV-1 U_S3-null virus and chimeric viruses by replacing MDV-1 U_S3 with MDV-2 or HVT U_S3. Using MD as a natural virus-host model, we showed that both MDV-2 and HVT U_S3 partially rescued the growth deficiency of MDV-1 U_S3-null virus. In addition, deletion of MDV-1 U_S3 attenuated the virus resulting in higher survival rate and lower MDV specific tumor incidence, which could be partially compensated by MDV-2 and HVT U_S3. We also identified chicken histone deacetylase 1 (chHDAC1) as a common U_S3 substrate for all three MDV types while only U_S3 of MDV-1 and MDV-2 phosphorylate chHDAC2. We further determined that U_S3 of MDV-1 and HVT phosphorylate chHDAC1 at serine 406 (S406), while MDV-2 U_S3 phosphorylates S406, S410, and S415. In addition, MDV-1 U_S3 phosphorylates chHDAC2 at S407, while MDV-2 U_S3 targets S407 and S411. Furthermore, biochemical studies show that MDV U_S3 mediated phosphorylation of chHDAC1 and 2 affect their stability, transcriptional regulation activity, and interaction network. Using a class I HDAC specific inhibitor, we showed that MDV U_S3 mediated phosphorylation of chHDAC1 and 2 is involved in regulation of virus replication. Overall, we identified novel substrates for MDV U_S3 and characterized the role of MDV U_S3 in MDV pathogenesis.

Marek’s disease virus (MDV) is a highly contagious and oncogenic avian alphaherpesvirus that causes T-cell lymphomas in chickens. Alphaherpesviruses encoded U_S3 is a multifunctional protein kinase involved in viral replication, apoptosis resistance, and cell-to-cell spread. In this study, we evaluated the importance of MDV U_S3 in regulating MDV replication and pathogenesis in chickens. Our results provide first evidence that MDV U_S3 protein kinase is involved in the replication and pathogenicity of MDV in its natural host. We also identified chicken histone deacetylase 1 and 2 (chHDAC1 and 2) as novel substrates of U_S3 for MDV and characterized the potential impacts of MDV U_S3 induced phosphorylation in their protein stability, transcriptional regulation and protein interactions; to our knowledge, this is the first comparative study of the functions of U_S3 from all three MDV types. This is an important finding towards a better understanding of the functions of alphaherpesviruses encoded U_S3 protein kinase.

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 Meq oncoprotein interacts with chicken HDAC 1 and 2 and mediates their degradation via proteasome dependent pathway

Marek's disease virus (MDV) encodes a basic-leucine zipper (BZIP) protein, Meq, which is considered the major MDV oncoprotein. It has been reported that the oncogenicity of Meq is associated with its interaction with C-terminal binding protein 1 (CtBP), which is also an interaction partner of Epstein-Barr virus encoded EBNA3A and EBNA3C oncoproteins. Since both EBNA3C and CtBP interact with histone deacetylase 1 (HDAC1) and HDAC2, we examined whether Meq shares this interaction with chicken HDAC1 (chHDAC1) and chHDAC2. Using confocal microscopy analysis, we show that Meq co-localizes with chHDAC1 and chHDAC2 in the nuclei of MDV lymphoblastoid tumor cells. In addition, immunoprecipitation assays demonstrate that Meq interacts with chHDAC1 and chHDAC2 in transfected cells and MDV lymphoblastoid tumor cells. Using deletion mutants, interaction domains were mapped to the N-terminal dimerization domain of chHDAC1 and chHDAC2, and the BZIP domain of Meq. Our results further demonstrate that this interaction mediates the degradation of chHDAC1 and chHDAC2 via the proteasome dependent pathway. In addition, our results show that Meq also induces the reduction of global ubiquitinated proteins through a proteasome dependent pathway. In conclusion, our results provide evidence that Meq interacts with chHDAC1 and chHDAC2, and induces their proteasome dependent degradation.

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.

The Emergence of a vv + MDV Can Break through the Protections Provided by the Current Vaccines

Marek’s disease (MD) is an infectious malignant T-cell lymphoma proliferative disease caused by Marek’s disease virus (MDV). In recent years, the emergence of very virulent (vv) and/or very virulent plus (vv +) strains of MDV in the field has been suggested as one of the causes of vaccination failure. The pathogenicity of the MDV strain GX18NNM4, isolated from a clinical outbreak in a broiler breeder flock that was vaccinated with CVI988/Rispens, was investigated. In the vaccination-challenge test, GX18NNM4 was able to break through the protections provided by the vaccines CVI988 and 814. It also significantly reduced body weight gain and caused marked gross lesions and a large area of infiltration of neoplastic lymphocyte cells in the heart, liver, pancreas, etc. of the infected birds. In addition, the expressions of programmed death 1 (PD-1) and its ligand, programmed death ligand 1 (PD-L1), in the spleens and cecal tonsils (CTs) of the unvaccinated challenged birds were significantly increased compared to those in the vaccinated challenged birds, indicating that the PD-1/PD-L1 pathway is related to immune evasion mechanisms. The results showed that the GX18NNM4 strain could cause severe immunosuppression and significantly decrease the protections provided by the current commercial vaccines, thus showing GX18NNM4 to be a vv + MDV strain.

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.

Genome-wide characterization of copy number variations in the host genome in genetic resistance to Marek's disease using next generation sequencing

Background

Marek’s disease (MD) is a highly neoplastic disease primarily affecting chickens, and remains as a chronic infectious disease that threatens the poultry industry. Copy number variation (CNV) has been examined in many species and is recognized as a major source of genetic variation that directly contributes to phenotypic variation such as resistance to infectious diseases. Two highly inbred chicken lines, 6₃ (MD-resistant) and 7₂ (MD-susceptible), as well as their F₁ generation and six recombinant congenic strains (RCSs) with varied susceptibility to MD, are considered as ideal models to identify the complex mechanisms of genetic and molecular resistance to MD.

Results

In the present study, to unravel the potential genetic mechanisms underlying resistance to MD, we performed a genome-wide CNV detection using next generation sequencing on the inbred chicken lines with the assistance of CNVnator. As a result, a total of 1649 CNV regions (CNVRs) were successfully identified after merging all the nine datasets, of which 90 CNVRs were overlapped across all the chicken lines. Within these shared regions, 1360 harbored genes were identified. In addition, 55 and 44 CNVRs with 62 and 57 harbored genes were specifically identified in line 6₃ and 7₂, respectively. Bioinformatics analysis showed that the nearby genes were significantly enriched in 36 GO terms and 6 KEGG pathways including JAK/STAT signaling pathway. Ten CNVRs (nine deletions and one duplication) involved in 10 disease-related genes were selected for validation by using quantitative real-time PCR (qPCR), all of which were successfully confirmed. Finally, qPCR was also used to validate two deletion events in line 7₂ that were definitely normal in line 6₃. One high-confidence gene, IRF2 was identified as the most promising candidate gene underlying resistance and susceptibility to MD in view of its function and overlaps with data from previous study.

Conclusions

Our findings provide valuable insights for understanding the genetic mechanism of resistance to MD and the identified gene and pathway could be considered as the subject of further functional characterization.

Genome-wide analysis of circular RNAs involved in Marek's disease tumourigenesis in chickens

Marek's disease (MD), induced by Marek's disease virus (MDV), is a lymphotropic neoplastic disease and causes huge economic losses to the poultry industry. Non-coding RNAs (ncRNAs) play important regulatory roles in disease pathogenesis. To investigate host circular RNA (circRNA) and microRNA (miRNA) expression profile, RNA sequencing was performed in tumourous spleens (TS), spleens from the survivors (SS) without any lesion after MDV infection, and non-infected chicken spleens (NS). A total of 2,169 circRNAs were identified and more than 80% of circRNAs were derived from exon. The flanking introns of 1,744 exonic circRNAs possessed 579 reverse complementary matches (RCMs), which mainly overlapped with chicken repeat 1 family (CR1F). It suggested that CR1F mediated the cyclization of exons by intron pairing. Out of 2,169 circRNAs, 113 were differentially expressed circRNAs (DECs). The Q-PCR and Rnase R digestion experiments showed circRNA possessed high stability compared with their linear RNAs. Integrated with previous transcriptome data, we profiled regulatory networks of circRNA/long non-coding RNA (lncRNA)-miRNA-mRNA. Extensive competing endogenous RNA (ceRNA) networks were predicted to be involved in MD tumourigenesis. Interestingly, circZMYM3, an intronic circRNA, interacted with seven miRNAs which targeted some immune genes, such as SWAP70 and CCL4. Gga-miR-155 not only interacted with circGTDC1 and circMYO1B, but also targeted immune-related genes, such as GATA4, which indicated the roles of non-coding RNAs played to mediate immune responsive genes. Collectively, this is the first study that integrated RNA expression profiles in MD model. Our results provided comprehensive interactions of ncRNAs and mRNA in MD tumourigenesis.

Transcriptional Profiles Associated with Marek's Disease Virus in Bursa and Spleen Lymphocytes Reveal Contrasting Immune Responses during Early Cytolytic Infection

Marek's disease virus (MDV), an alpha herpes virus, causes a lymphoproliferative state in chickens known as Marek's disease (MD), resulting in severe monetary losses to the poultry industry. Because lymphocytes of bursa of Fabricius and spleen are prime targets of MDV replication during the early cytolytic phase of infection, the immune response in bursa and spleen should be the foundation of late immunity induced by MDV. However, the mechanism of the MDV-mediated host immune response in lymphocytes in the early stage is poorly understood. The present study is primarily aimed at identifying the crucial genes and significant pathways involved in the immune response of chickens infected with MDV CVI988 and the very virulent RB1B (vvRB1B) strains. Using the RNA sequencing approach, we analyzed the generated transcriptomes from lymphocytes isolated from chicken bursa and spleen. Our findings validated the expression of previously characterized genes; however, they also revealed the expression of novel genes during the MDV-mediated immune response. The results showed that after challenge with CVI988 or vvRB1B strains, 634 and 313 differentially expressed genes (DEGs) were identified in splenic lymphocytes, respectively. However, 58 and 47 DEGs were observed in bursal lymphocytes infected with CVI988 and vvRB1B strains, respectively. Following MDV CVI988 or vvRB1B challenge, the bursal lymphocytes displayed changes in IL-6 and IL-4 gene expression. Surprisingly, splenic lymphocytes exhibited an overwhelming alteration in the expression of cytokines and cytokine receptors involved in immune response signaling. On the other hand, there was no distinct trend between infection with CVI988 and vvRB1B and the expression of cytokines and chemokines, such as IL-10, IFN-γ, STAT1, IRF1, CCL19, and CCL26. However, the expression profiles of IL-1β, IL-6, IL8L1, CCL4 (GGCL1), and CCL5 were significantly upregulated in splenic lymphocytes from chickens infected with CVI988 compared with those of chickens infected with vvRB1B. Because these cytokines and chemokines are considered to be associated with B cell activation and antigenic signal transduction to T cells, they may indicate differences of immune responses initiated by vaccinal and virulent strains during the early phase of infection. Collectively, our study provides valuable data on the transcriptional landscape using high-throughput sequencing to understand the different mechanism between vaccine-mediated protection and pathogenesis of virulent MDV in vivo.

Latest Insights into Marek's Disease Virus Pathogenesis and Tumorigenesis

Marek’s disease virus (MDV) infects chickens and causes one of the most frequent cancers in animals. Over 100 years of research on this oncogenic alphaherpesvirus has led to a profound understanding of virus-induced tumor development. Live-attenuated vaccines against MDV were the first that prevented cancer and minimized the losses in the poultry industry. Even though the current gold standard vaccine efficiently protects against clinical disease, the virus continuously evolves towards higher virulence. Emerging field strains were able to overcome the protection provided by the previous two vaccine generations. Research over the last few years revealed important insights into the virus life cycle, cellular tropism, and tumor development that are summarized in this review. In addition, we discuss recent data on the MDV transcriptome, the constant evolution of this highly oncogenic virus towards higher virulence, and future perspectives in MDV research.

Marek's disease vaccines-induced differential expression of known and novel microRNAs in primary lymphoid organ bursae of White Leghorn

Marek’s disease (MD) is a contagious disease of domestic chickens caused by MD viruses. MD has been controlled primarily by vaccinations, yet sporadic outbreaks of MD take place worldwide. Commonly used MD vaccines include HVT, SB-1 and CVI988/Rispens and their efficacies are reportedly dependent of multiple factors including host genetics. Our previous studies showed protective efficacy of a MD vaccine can differ drastically from one chicken line to the next. Advanced understanding on the underlying genetic and epigenetic factors that modulate vaccine efficacy would greatly improve the strategy in design and development of more potent vaccines. Two highly inbred lines of White Leghorn were inoculated with HVT and CVI988/Rispens. Bursa samples were taken 26 days post-vaccination and subjected to small RNA sequencing analysis to profile microRNAs (miRNA). A total of 589 and 519 miRNAs was identified in one line, known as line 6₃, 490 and 630 miRNAs were identified in the other, known as line 7₂, in response to HVT or CVI988/Rispens inoculation, respectively. HVT and CVI988/Rispens induced mutually exclusive 4 and 13 differentially expressed (DE) miRNAs in line 6₃ birds in contrast to a non-vaccinated group of the same line. HVT failed to induce any DE miRNA and CVI988/Rispens induced a single DE miRNA in line 7₂ birds. Thousands of target genes for the DE miRNAs were predicted, which were enriched in a variety of gene ontology terms and pathways. This finding suggests the epigenetic factor, microRNA, is highly likely involved in modulating vaccine protective efficacy in chicken.

Expression of the Conserved Herpesvirus Protein Kinase (CHPK) of Marek's Disease Alphaherpesvirus in the Skin Reveals a Mechanistic Importance for CHPK during Interindividual Spread in Chickens

The Herpesviridae encode many conserved genes, including the conserved herpesvirus protein kinase (CHPK) that has multifunctional properties. In most cases, herpesviruses lacking CHPK can propagate in cell culture to various degrees, depending on the virus and cell culture system. However, in the natural animal model system of Marek's disease alphaherpesvirus (MDV) in chickens, CHPK is absolutely required for interindividual spread from chicken to chicken. The lack of biological reagents for chicken and MDV has limited our understanding of this important gene during interindividual spread. Here, we engineered epitope-tagged proteins in the context of virus infection in order to detect CHPK in the host. Using immunofluorescence assays and Western blotting during infection in cell culture and in chickens, we determined that the invariant lysine 170 (K170) of MDV CHPK is required for interindividual spread and autophosphorylation of CHPK and that mutation to methionine (M170) results in instability of the CHPK protein. Using these newly generated viruses allowed us to examine the expression of CHPK in infected chickens, and these results showed that mutant CHPK localization and late viral protein expression were severely affected in feather follicles wherein MDV is shed, providing important information on the requirement of CHPK for interindividual spread.IMPORTANCE Marek's disease in chickens is caused by Gallid alphaherpesvirus 2, better known as Marek's disease alphaherpesvirus (MDV). Current vaccines only reduce tumor formation but do not block interindividual spread from chicken to chicken. Understanding MDV interindividual spread provides important information for the development of potential therapies to protect against Marek's disease while also providing a reliable natural host in order to study herpesvirus replication and pathogenesis in animals. Here, we studied the conserved Herpesviridae protein kinase (CHPK) in cell culture and during infection in chickens. We determined that MDV CHPK is not required for cell-to-cell spread, for disease induction, and for oncogenicity. However, it is required for interindividual spread, and mutation of the invariant lysine (K170) results in stability issues and aberrant expression in chickens. This study is important because it addresses the critical role CHPK orthologs play in the natural host.

Virus-induced immunosuppression in turkeys (Meleagris gallopavo): A review

Immunosuppression is characterized by a dysfunction of humoral and/or cellular immune response leading to increase of susceptibility to secondary infections, increase of mortality and morbidity, poor productivity, and welfare and vaccination failures. Humoral immune response depression is due to perturbation of soluble factors, as complement and chemokines in innate immunity and antibodies or cytokines in adaptive immunity. At the cellular immune response, immunosuppression is the consequence of the dysfunction of T-cells, B-cells, heterophils, monocytes, macrophages, and natural Killer cells. Immunosuppression in turkeys can be caused by numerous, non-infectious, and infectious agents, having variable pathological and molecular mechanisms. Interactions between them are very complex. This paper reviews the common viruses inducing clinical and sub-clinical immunosuppression in turkeys, and enteric and neoplastic viruses in particular, as well as the interactions among them. The evaluation of immunosuppression is currently based on classical approach; however, new technique such as the microarray technology is being developed to investigate immunological mediator’s genes detection. Controlling of immunosuppression include, in general, biosecurity practices, maintaining appropriate breeding conditions and vaccination of breeders and their progeny. Nevertheless, few vaccines are available against immunosuppressive viruses in turkey’s industry. The development of new control strategies is reviewed.

RNA Sequencing revealed differentially expressed genes functionally associated with immunity and tumor suppression during latent phase infection of a vv + MDV in chickens

Very virulent plus Marek's disease (MD) virus (vv + MDV) induces tumors in relatively resistant lines of chickens and early mortality in highly susceptible lines of chickens. The vv + MDV also triggers a series of cellular responses in both types of chickens. We challenged birds sampled from a highly inbred chicken line (line 6₃) that is relatively resistant to MD and from another inbred line (line 7₂) that is highly susceptible to MD with a vv + MDV. RNA-sequencing analysis was performed with samples extracted from spleen tissues taken at 10-day and 21-day post infection (dpi). A total of 64 and 106 differentially expressed genes was identified in response to the vv + MDV challenge at latent phase in the resistant and susceptible lines of chickens, respectively. Direct comparisons between samples of the two lines identified 90 and 126 differentially expressed genes for control and MDV challenged groups, respectively. The differentially expressed gene profiles illustrated that intensive defense responses were significantly induced by vv + MDV at 10 dpi and 21 dpi but with slight changes in the resistant line. In contrast, vv + MDV induced a measurable suppression of gene expression associated with host defense at 10 dpi but followed by an apparent activation of the defense response at 21 dpi in the susceptible line of chickens. The observed difference in gene expression between the two genetic lines of chickens in response to MDV challenge during the latent phase provided a piece of indirect evidence that time points for MDV reactivation differ between the genetic lines of chickens with different levels of genetic resistance to MD. Early MDV reactivation might be necessary and potent to host defense system readiness for damage control of tumorigenesis and disease progression, which consequently results in measurable differences in phenotypic characteristics including early mortality (8 to 20 dpi) and tumor incidence between the resistant and susceptible lines of chickens. Combining differential gene expression patterns with reported GO function terms and quantitative trait loci, a total of 27 top genes was selected as highly promising candidate genes for genetic resistance to MD. These genes are functionally involved with virus process (F13A1 and HSP90AB1), immunity (ABCB1LB, RGS5, C10ORF58, OSF-2, MMP7, CXCL12, GAL1, GAL2, GAL7, HVCN1, PDE4D, IL4I1, PARP9, EOMES, MPEG1, PDK4, CCLI10, K60 and FST), and tumor suppression (ADAMTS2, LXN, ARRDC3, WNT7A, CLDN1 and HPGD). It is anticipated that these findings will facilitate advancement in the fundamental understanding on mechanisms of genetic resistance to MD. In addition, such advancement may also provide insights on tumor virus-induced tumorigenesis in general and help the research community recognize MD study may serve as a good model for oncology study involving tumor viruses.

Virus-Induced Immunosuppression in Chickens

A healthy immune system is a cornerstone for poultry production. Any factor diminishing the immune responses will affect production parameters and increase cost. There are numerous factors, infectious and noninfectious, causing immunosuppression (IS) in chickens. This paper reviews the three viral diseases that most commonly induce IS or subclinical IS in chickens: Marek's disease virus (MDV), chicken infectious anemia virus (CIAV), and infectious bursal disease virus (IBDV), as well as the interactions among them. MDV-induced IS (MDV-IS) affects both humoral and cellular immune responses. It is very complex, poorly understood, and in many cases underdiagnosed. Vaccination protects against some but not all aspects of MDV-IS. CIAV induces apoptosis of the hemocytoblasts resulting in anemia, hemorrhages, and increased susceptibility to bacterial infections. It also causes apoptosis of thymocytes and dividing T lymphocytes, affecting T helper functions, which are essential for antibody production and cytotoxic T lymphocyte (CTL) functions. Control of CIAV is based on vaccination of breeders and maternal antibodies (MAbs). However, subclinical IS can occur after MAbs wane. IBDV infection affects the innate immune responses during virus replication and humoral immune responses as a consequence of the destruction of B-cell populations. Vaccines with various levels of attenuation are used to control IBDV. Interactions with MAbs and residual virulence of the vaccines need to be considered when designing vaccination plans. The interaction between IBDV, CIAV, and MDV is critical although underestimated in many cases. A proper control of IBDV is a must to have proper humoral immune responses needed to control CIAV. Equally, long-term control of MDV is not possible if chickens are coinfected with CIAV, as CIAV jeopardizes CTL functions critical for MDV control.

Differential expression of type I interferon mRNA and protein levels induced by virulent Marek's disease virus infection in chickens

Marek's disease virus (MDV), an α-herpesvirus targeting avian species, causes fatal Marek's disease (MD) in chickens. The host interferon (IFN) responses play a key role in resisting viral infection. However, host IFN responses following MDV infection in the chicken central immune organs (thymus and bursa of Fabricius), which contain numerous MDV target cells, is poorly understood. In this study, we performed animal experiments in specific pathogen-free chickens infected with two virulent MDV strains (BS/15 and Md5) or without infection as negative controls. Specifically, the type I IFN (IFN-α and IFN-β) transcriptional and proteomic expression levels at 7, 10, 14, 17, and 21 days post infection (dpi) were detected and analyzed. Our results indicated that the mRNA and protein expression levels of IFN-α and IFN-β in the thymus and bursa of Fabricius were mainly downregulated in cytolytic infection (such as 10 dpi) and reactivation (such as 17 dpi) stages, but not the latent (such as 14 dpi) stage of MDV infection, which was determined by comprehensively analyzing the MDV viral load and immune organ damage caused by MDV infection. These data suggest that MDV could inhibit the expression of host type I IFNs, which may be involved in the MDV-induced host immunosuppression and contribute to the immune escape of MDV from host immunity. Furthermore, we found that the downregulated expression of the host type I IFNs induced by BS/15 and Md5 infection was significantly different, which we speculated may be related to the diverse virulence and pathogenicity of MDV strains. In conclusion, our study demonstrated that MDV mostly inhibited the expression of type I IFNs in infected hosts, which may be associated to its pathogenesis.

Marek's Disease Virus Disables the ATR-Chk1 Pathway by Activating STAT3

Oncogenic virus replication often leads to genomic instability, causing DNA damage and inducing the DNA damage response (DDR) pathway. The DDR pathway is a cellular pathway that senses DNA damage and regulates the cell cycle to maintain genomic stability. Therefore, the DDR pathway is critical for the viral lifecycle and tumorigenesis. Marek's disease virus (MDV), an alphaherpesvirus that causes lymphoma in chickens, has been shown to induce DNA damage in infected cells. However, the interaction between MDV and the host DDR is unclear. In this study, we observed that MDV infection causes DNA strand breakage in chicken fibroblast (CEF) cells along with an increase in the DNA damage markers p53 and p21. Interestingly, we showed that phosphorylation of STAT3 was increased during MDV infection, concomitantly with a decrease of Chk1 phosphorylation. In addition, we found that MDV infection was enhanced by VE-821, an ATR-specific inhibitor, but attenuated by hydroxyurea, an ATR activator. Moreover, inhibition of STAT3 phosphorylation by Stattic eliminates the ability of MDV to inhibit Chk1 phosphorylation. Finally, we showed that MDV replication was decreased by Stattic treatment. Taken together, these results suggest that MDV disables the ATR-Chk1 pathway through STAT3 activation to benefit its replication.IMPORTANCE MDV is used as a biomedical model to study virus-induced lymphoma due to the similar genomic structures and physiological characteristics of MDV and human herpesviruses. Upon infection, MDV induces DNA damage, which may activate the DDR pathway. The DDR pathway has a dual impact on viruses because it manipulates repair and recombination factors to facilitate viral replication and also initiates antiviral action by regulating other signaling pathways. Many DNA viruses evolve to manipulate the DDR pathway to promote virus replication. In this study, we identified a mechanism used by MDV to inhibit ATR-Chk1 pathways. ATR is a cellular kinase that responds to broken single-stranded DNA, which has been less studied in MDV infection. Our results suggest that MDV infection activates STAT3 to disable the ATR-Chk1 pathway, which is conducive to viral replication. This finding provides new insight into the role of STAT3 in interrupting the ATR-Chk1 pathway during MDV replication.

The Herpesviridae Conserved Multifunctional Infected-Cell Protein 27 (ICP27) Is Important but Not Required for Replication and Oncogenicity of Marek's Disease Alphaherpesvirus

The Herpesviridae conserved infected-cell protein 27 (ICP27) is essential for cell culture-based replication of most herpesviruses studied. For members of the Alphaherpesvirinae, ICP27 regulates the expression of many viral genes, including expression of pUL44 (gC), pUL47 (VP13/14), and pUL48 (VP16). These three viral proteins are dysregulated during Marek's disease alphaherpesvirus (MDV) replication in cell culture. MDV replicates in a highly cell-associated manner in cell culture, producing little to no infectious virus. In contrast, infectious cell-free MDV is produced in specialized feather follicle epithelial (FFE) cells of infected chickens, in which these three genes are abundantly expressed. This led us to hypothesize that MDV ICP27, encoded by gene UL54, is a defining factor for the dysregulation of gC, pUL47, and pUL48 and, ultimately, ineffective virus production in cell culture. To address ICP27's role in MDV replication, we generated recombinant MDV with ICP27 deleted (vΔ54). Interestingly, vΔ54 replicated, but plaque sizes were significantly reduced compared to those of parental viruses. The reduced cell-to-cell spread was due to ICP27 since plaque sizes were restored in rescued viruses, as well as when vΔ54 was propagated in cells expressing ICP27 in trans In chickens, vΔ54 replicated, induced disease, and was oncogenic but was unable to transmit from chicken to chicken. To our knowledge, this is the first report showing that the Herpesviridae conserved ICP27 protein is dispensable for replication and disease induction in its natural host.IMPORTANCE Marek's disease (MD) is a devastating oncogenic disease that affects the poultry industry and is caused by MD alphaherpesvirus (MDV). Current vaccines block induction of disease but do not block chicken-to-chicken transmission. There is a knowledge gap in our understanding of how MDV spreads from chicken to chicken. We studied the Herpesviridae conserved ICP27 regulatory protein in cell culture and during MDV infection in chickens. We determined that MDV ICP27 is important but not required for replication in both cell culture and chickens. In addition, MDV ICP27 was not required for disease induction or oncogenicity but was required for chicken-to-chicken transmission. This study is important because it addresses the role of ICP27 during infection in the natural host and provides important information for the development of therapies to protect chickens against MD.

Macrophages from Susceptible and Resistant Chicken Lines have Different Transcriptomes following Marek's Disease Virus Infection

Despite successful control by vaccination, Marek’s disease (MD) has continued evolving to greater virulence over recent years. To control MD, selection and breeding of MD-resistant chickens might be a suitable option. MHC-congenic inbred chicken lines, 6₁ and 7₂, are highly resistant and susceptible to MD, respectively, but the cellular and genetic basis for these phenotypes is unknown. Marek’s disease virus (MDV) infects macrophages, B-cells, and activated T-cells in vivo. This study investigates the cellular basis of resistance to MD in vitro with the hypothesis that resistance is determined by cells active during the innate immune response. Chicken bone marrow-derived macrophages from lines 6₁ and 7₂ were infected with MDV in vitro. Flow cytometry showed that a higher percentage of macrophages were infected in line 7₂ than in line 6₁. A transcriptomic study followed by in silico functional analysis of differentially expressed genes was then carried out between the two lines pre- and post-infection. Analysis supports the hypothesis that macrophages from susceptible and resistant chicken lines display a marked difference in their transcriptome following MDV infection. Resistance to infection, differential activation of biological pathways, and suppression of oncogenic potential are among host defense strategies identified in macrophages from resistant chickens.

Transcripts of antibacterial peptides in chicken erythrocytes infected with Marek's disease virus

BACKGROUND

Chicken erythrocytes are involved in immunity through binding of toll-like receptors (TLRs) with their ligands to activate downstream signaling and lead to cytokine production in erythrocytes. Some avian β-defensins (AvBDs) are constitutively expressed in tissues and some others can be induced by various bacteria and viruses. However, the expression of AvBDs in erythrocytes has not yet been studied extensively.

RESULTS

The transcripts of eight AvBDs (AvBD1 to AvBD7, and AvBD9) and liver-expressed antimicrobial peptide-2 (LEAP-2) were found in normal chicken erythrocytes. The expression levels of AvBD2, 4 and 7 were significantly increased (P < 0.01), whereas the levels of AvBD1, 6 and 9 were significantly decreased (P < 0.01) after Marek's disease virus (MDV) infection. The mRNA expression level of LEAP-2 was not significantly changed after MDV infection. Highest viral nucleic acid (VNA) of MDV in the feather tips among the tested time points was found at 14 days post-infection (d.p.i.). In addition, 35 MD5-related gene segments were detected in the erythrocytes at 14 d.p.i. by transcriptome sequencing.

CONCLUSIONS

These results suggest that the AvBDs in chicken erythrocytes may participate in MDV-induced host immune responses.

Marek's disease vaccine activates chicken macrophages

To provide insights into the role of innate immune responses in vaccine-mediated protection, we investigated the effect of Marek's disease (MD) vaccine, CVI988/Rispens, on the expression patterns of selected genes associated with activation of macrophages in MD-resistant and MD-susceptible chicken lines. Upregulation of interferon γ, interleukin (IL)-1β, IL-8, and IL-12 at different days post-inoculation (dpi) revealed activation of macrophages in both chicken lines. A strong immune response was induced in cecal tonsils of the susceptible line at 5 dpi. The highest transcriptional activities were observed in spleen tissues of the resistant line at 3 dpi. No increase in the population of CD3⁺ T cells was observed in duodenum of vaccinated birds at 5 dpi indicating a lack of involvement of the adaptive immune system in the transcriptional profiling of the tested genes. There was, however, an increase in the number of macrophages in the duodenum of vaccinated birds. The CVI988/Rispens antigen was detected in the duodenum and cecal tonsils of the susceptible line at 5 dpi but not in the resistant line. This study sheds light on the role of macrophages in vaccine-mediated protection against MD and on the possible development of new recombinant vaccines with enhanced innate immune system activation properties.

Gga-miR-130b-3p inhibits MSB1 cell proliferation, migration, invasion, and its downregulation in MD tumor is attributed to hypermethylation

Marek's disease is an oncogenic and lymphoproliferative disease of chickens caused by Marek's disease virus. Hypermethylation or hypomethylation of CpG islands in gene promoter region are involved in the initiation and progression of carcinogenesis. In this study, we analyzed differential methylation levels of upstream region of gga-miR-130b-3p gene between Marek's disease virus-infected tumorous and non-infected spleens. Around the upstream 1 kb of gga-miR-130b-3p gene, two amplicons were designed that covered 616 bp. There were forty-eight CpG sites in this region. CpG sites in this region presented higher methylation level in tumorous spleens compared with that in non-infected ones. There were eight CpG sites significantly hypermethylated in tumorous spleens. The expression level of three DNA methyltransferases including DNMT1, DNMT3a and DNMT3b increased and the expression level of Tet ten-eleven translocation protein 2 remarkably decreased in tumorous spleens. Hypermethylation in the upstream region of gga-miR-130b-3p gene might be a direct reason for its downregulation in MD tumorous tissues. Moreover, cell proliferation of Marek's disease lymphoblastoid cell line MDCC-MSB1 was remarkably inhibited at 24, 36, 48, 60 and 72 h post-gga-miR-130b-3p-agomir transfection. The transwell migration assay indicated cell number of migration was significantly lower in miRNA agomir transfection group. Matrix metalloproteinases MMP2 and MMP9 are involved in tumor invasion, and their protein levels were significantly downregulated at 72 h post-miRNA-agomir transfection. Collectively, these results indicated that hypermethylation in upstream region of gga-miR-130b-3p gene contributed to its downregulation in tumorous tissues. Gga-miR-130b-3p plays an inhibitory role in lymphomatous cell transformation.