Transcriptional analysis of host responses to Marek's disease virus infection in chicken thymus

Metabolomic profiling of Marek's disease virus infection in host cell based on untargeted LC-MS

Marek's disease (MD) caused by Marek's disease virus (MDV), poses a serious threat to the poultry industry by inducing neurological disease and malignant lymphoma in infected chickens. However, the underlying mechanisms how MDV disrupts host cells and causes damage still remain elusive. Recently, the application of metabolomics has shown great potential for uncovering the complex mechanisms during virus-host interactions. In this study, chicken embryo fibroblasts (CEFs) infected with MDV were subjected to ultrahigh-performance liquid chromatography-quadrupole time-of-flight tandem mass spectrometry (UHPLC-QTOF-MS) and multivariate statistical analysis. The results showed that 261 metabolites were significantly altered upon MDV infection, with most changes occurring in amino acid metabolism, energy metabolism, nucleotide metabolism, and lipid metabolism. Notably, MDV infection induces an up-regulation of amino acids in host cells during the early stages of infection to provide the energy and intermediary metabolites necessary for efficient multiplication of its own replication. Taken together, these data not only hold promise in identifying the biochemical molecules utilized by MDV replication in host cells, but also provides a new insight into understanding MDV-host interactions.

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.

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.

Influence of Marek's disease virus vaccines on chicken melanoma differentiation-associated gene 5-dependent-type I interferon signal transduction pathway with a highlight on their secondary impact on the immune responses post Newcastle disease virus vaccination

Marek's disease virus (MDV) leads to a lytic infection of B-lymphocytes in chickens, and also latently infects T-lymphocytes. Although Marek's disease vaccines have been widely in use, little is known about the innate immune response of this important livestock vaccine. In this study, we tested the effect of different commercially applied Marek's disease vaccines on the expression pattern of selected genes related to chicken interferon-alpha (chIFN-α) (melanoma differentiation associated gene 5 "MDA5″ dependent) signal transduction pathway. Both MDV serotype I (Rispens) and serotype III (Herpesvirus of turkey "HVT") vaccines could stimulate MDA5 dependent-type I interferon response as early as three days post vaccination in a dose-dependent manner. The stimulation continued up to 10 days in the instance of HVT vaccine and declined in the case of Rispens. Surprisingly, increasing the doses of the two vaccines led to dose-dependent down-regulation in the expression pattern of the investigated pathway, five and ten days post vaccination. Additionally, to shed the light on the consequent effect on the immune responses of the other viral vaccine, another experimental model based on Newcastle disease virus (NDV) vaccines was designed using HVT, HVT-VP2 and Rispens MDV vaccines. The three MDV vaccines were found to reduce chicken humoral immune response post NDV vaccination. However, only Rispens and HVT-VP2 had suppressive effects on the expression of MDA5-dependent-chIFN-α related cytokines. Consistent with this finding, the protection rate and NDV- humoral immune response post challenge with virulent NDV strain was lower in case of Rispens and HVT-VP2 vaccines.

Dynamic Changes in the Splenic Transcriptome of Chickens during the Early Infection and Progress of Marek's Disease

Gallid alphaherpesvirus 2 (GaHV2) is an oncogenic avian herpesvirus inducing Marek’s disease (MD) and rapid-onset T-cell lymphomas. To reveal molecular events in MD pathogenesis and tumorigenesis, the dynamic splenic transcriptome of GaHV2-infected chickens during early infection and pathogenic phases has been determined utilizing RNA-seq. Based on the significant differentially expressed genes (DEGs), analysis of gene ontology, KEGG pathway and protein-protein interaction network has demonstrated that the molecular events happening during GaHV2 infection are highly relevant to the disease course. In the ‘Cornell Model’ description of MD, innate immune responses and inflammatory responses were established at early cytolytic phase but persisted until lymphoma formation. Humoral immunity in contrast began to play a role firstly in the intestinal system and started at late cytolytic phase. Neurological damage caused by GaHV2 is first seen in early cytolytic phase and is then sustained throughout the following phases over a long time period. During the proliferative phase many pathways associated with transcription and/or translation were significantly enriched, reflecting the cell transformation and lymphoma formation. Our work provides an overall view of host responses to GaHV2 infection and offers a meaningful basis for further studies of MD biology.

Comparative transcriptomics of genetically divergent lines of chickens in response to Marek's disease virus challenge at cytolytic phase

Marek's disease (MD), caused by Marek's disease virus (MDV), remains an economically significant threat to the poultry industry worldwide. Genetic resistance to MD is a promising alternative strategy to augment current control measures (vaccination and management). However, only a few functional genes reportedly conferring MD resistance have been identified. Here, we performed a comparative transcriptomics analysis of two highly inbred yet genetically divergent lines of chickens (line 63 and 72) that are resistant and susceptible to MD, respectively, in response to a very virulent plus strain of MDV (vv+MDV) challenge at cytolytic phase. A total of 203 DEGs in response to MDV challenge were identified in the two lines. Of these, 96 DEGs were in common for both lines, in addition to 36 and 71 DEGs that were specific for line 63 and 72, respectively. Functional enrichment analysis results showed the DEGs were significantly enriched in GO terms and pathways associated with immune response. Especially, the four DEGs, FGA, ALB, FN1, and F13A1 that reportedly facilitate virus invasion or immunosuppression, were found to be significantly up-regulated in the susceptible line 72 but down-regulated in the resistant line 63 birds. These results provide new resources for future studies to further elucidate the genetic mechanism conferring MD resistance.

Investigation of Marek's disease virus from chickens in central Ethiopia

Marek's disease (MD) is a lymphoproliferative and neuropathic disease of domestic chickens and less commonly, turkeys and quails, caused by a highly contagious, cell-associated, oncogenic herpesvirus. In Ethiopia, MD is believed to be introduced with importation of exotic and crossbred to improve the poultry production and has been reported to be a potential threat to the poultry sector both in backyard and commercial farming systems. This study was aimed at isolation and molecular analysis of MD virus isolates circulating in chicken population in the central part of Ethiopia where commercial farms are populated. From September 2013 to January 2014, clinical and post-mortem examination were conducted on diseased chickens suspected of MD virus infection. Representative spleen and feather follicle samples were collected following sterile procedure, and infectious virus isolation was performed using primary chicken fibroblast cell culture. Cell culture inoculated with suspension of pathological samples developed characteristic MD virus cytopathic effect of rounding of the cells and small plaques. Further analysis of the virus was conducted by conventional PCR amplifying the ICP4 gene fragment from eleven tissue samples using MD virus specific primers. PCR products were further sequenced and analyzed. Nucleotide sequence similarity search of the local isolates resulted a high degree of sequence similarity with Gallid Herpes virus type 2 strain (Marek's disease virus type 1, JN034558). To our knowledge, the present study is the first report conducted on virus isolation and molecular characterization of MD virus isolates circulated in Ethiopia. Eleven ICP4-like gene fragment (318 bp) sequences generated in the present study were uploaded in the public database (KU842366-76). Further research on virus isolation, genetic characterization, and infection dynamics is recommended targeting chickens of all age groups reared in different agro-ecological zones under different production system.

Virus and host genomic, molecular, and cellular interactions during Marek's disease pathogenesis and oncogenesis

Marek's Disease Virus (MDV) is a chicken alphaherpesvirus that causes paralysis, chronic wasting, blindness, and fatal lymphoma development in infected, susceptible host birds. This disease and its protective vaccines are highly relevant research targets, given their enormous impact within the poultry industry. Further, Marek's disease (MD) serves as a valuable model for the investigation of oncogenic viruses and herpesvirus patterns of viral latency and persistence--as pertinent to human health as to poultry health. The objectives of this article are to review MDV interactions with its host from a variety of genomic, molecular, and cellular perspectives. In particular, we focus on cytogenetic studies, which precisely assess the physical status of the MDV genome in the context of the chicken host genome. Combined, the cytogenetic and genomic research indicates that MDV-host genome interactions, specifically integration of the virus into the host telomeres, is a key feature of the virus life cycle, contributing to the viral achievement of latency, transformation, and reactivation of lytic replication. We present a model that outlines the variety of virus-host interactions, at the multiple levels, and with regard to the disease states.