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

Purinergic signaling during Marek's disease in chickens

Purinergic receptors (PRs) have been reported as potential therapeutic targets for many viral infections including herpesviruses, which urges the investigation into their role in Marek's disease (MD), a herpesvirus induced cancer in chickens that is an important pathogen for the poultry industry. MD is caused by MD virus (MDV) that has a similar viral life cycle as human varicella zoster virus in that it is shed from infected epithelial skin cells and enters the host through the respiratory route. In this report, PR responses during natural MDV infection and disease progression was examined in MD-resistant white Leghorns (WL) and MD-susceptible Pure Columbian (PC) chickens during natural infection. Whole lung lavage cells (WLLC) and liver tissue samples were collected from chickens infected but showing no clinical signs of MD (Infected) or presenting with clinical disease (Diseased). RNA was extracted followed by RT-qPCR analysis with gene specific primers against members of the P1, P2X, and P2Y PR families. Differential expression (p < 0.05) was observed in breed and disease conditions. Some PRs showed tissue specific expression (P1A1, P2X1, and P2X6 in WLLC) whereas others responded to MDV infection only in MD-susceptible (PC) chickens (P1A2A, P2X1, P2X5, P2X7). P2Y PRs had differential expression in both chicken lines in response to MDV infection and MD progression. This study is the first to our knowledge to examine PR responses during MDV infection and disease progression. These results suggest PR signaling may an important area of research for MDV replication and MD.

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.

In vivo imaging reveals novel replication sites of a highly oncogenic avian herpesvirus in chickens

In vivo bioluminescence imaging facilitates the non-invasive visualization of biological processes in living animals. This system has been used to track virus infections mostly in mice and ferrets; however, until now this approach has not been applied to pathogens in avian species. To visualize the infection of an important avian pathogen, we generated Marek’s disease virus (MDV) recombinants expressing firefly luciferase during lytic replication. Upon characterization of the recombinant viruses in vitro, chickens were infected and the infection visualized in live animals over the course of 14 days. The luminescence signal was consistent with the known spatiotemporal kinetics of infection and the life cycle of MDV, and correlated well with the viral load measured by qPCR. Intriguingly, this in vivo bioimaging approach revealed two novel sites of MDV replication, the beak and the skin of the feet covered in scales. Feet skin infection was confirmed using a complementary fluorescence bioimaging approach with MDV recombinants expressing mRFP or GFP. Infection was detected in the intermediate epidermal layers of the feet skin that was also shown to produce infectious virus, regardless of the animals’ age at and the route of infection. Taken together, this study highlights the value of in vivo whole body bioimaging in avian species by identifying previously overlooked sites of replication and shedding of MDV in the chicken host.

In vivo bioluminescence imaging is a powerful tool to track virus infection in the whole body of living animals. This system has been successfully used in mice, ferrets, rats and even fishes, but until now never in birds. In this study, we performed the first in vivo imaging assessing the spread of an important avian pathogen, the highly oncogenic Marek’s disease virus (MDV). Using a recombinant virus expressing firefly luciferase, we visualized the course of MDV infection in chicks for 14 days. The bioluminescent signal was consistent with the known kinetics and sites of dissemination of MDV, notably in feathers. With this new approach, we also discovered two novels sites of early infection and replication that may contribute to persistent virus shedding. Both novel sites represent hard skin appendages like the feathers: the beak and the skin of the feet that are covered in scales. These results were confirmed with two recombinant viruses expressing fluorescent proteins. Fifty-five years after the discovery of MDV and thanks to in vivo imaging, we provide new insights in MDV life cycle in vivo, highlighting the importance of bioluminescence imaging of the entire body in living animals.

Advances on Innate Immune Evasion by Avian Immunosuppressive Viruses

Innate immunity is not only the first line of host defense against pathogenic infection, but also the cornerstone of adaptive immune response. Upon pathogenic infection, pattern recognition receptors (PRRs) of host engage pathogen-associated molecular patterns (PAMPs) of pathogens, which initiates IFN production by activating interferon regulatory transcription factors (IRFs), nuclear factor-kappa B (NF-κB), and/or activating protein-1 (AP-1) signal transduction pathways in host cells. In order to replicate and survive, pathogens have evolved multiple strategies to evade host innate immune responses, including IFN-I signal transduction, autophagy, apoptosis, necrosis, inflammasome and/or metabolic pathways. Some avian viruses may not be highly pathogenic but they have evolved varied strategies to evade or suppress host immune response for survival, causing huge impacts on the poultry industry worldwide. In this review, we focus on the advances on innate immune evasion by several important avian immunosuppressive viruses (infectious bursal disease virus (IBDV), Marek’s disease virus (MDV), avian leukosis virus (ALV), etc.), especially their evasion of PRRs-mediated signal transduction pathways (IFN-I signal transduction pathway) and IFNAR-JAK-STAT signal pathways. A comprehensive understanding of the mechanism by which avian viruses evade or suppress host immune responses will be of help to the development of novel vaccines and therapeutic reagents for the prevention and control of infectious diseases in chickens.

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

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

The Evasion of Antiviral Innate Immunity by Chicken DNA Viruses

The innate immune system constitutes the first line of host defense. Viruses have evolved multiple mechanisms to escape host immune surveillance, which has been explored extensively for human DNA viruses. There is growing evidence showing the interaction between avian DNA viruses and the host innate immune system. In this review, we will survey the present knowledge of chicken DNA viruses, then describe the functions of DNA sensors in avian innate immunity, and finally discuss recent progresses in chicken DNA virus evasion from host innate immune responses.

Marek's Disease Virus RLORF4 Inhibits Type I Interferon Production by Antagonizing NF-κB Activation

Marek's disease virus (MDV), which causes T cell lymphomas in chickens, is economically important and has contributed to knowledge of herpesvirus-associated oncogenicity. The DNA-sensing pathway induces innate immune responses against DNA virus infection, and nuclear factor κB (NF-κB) signaling is critical for the establishment of innate immunity. Here, we report that RLORF4, an MDV-specific protein directly involved in viral attenuation, is an inhibitor of the DNA-sensing pathway. The results showed that ectopically expressed RLORF4 blocked beta interferon (IFN-β) promoter activation induced by cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING). RLORF4 selectively inhibited the activation of NF-κB but not IFN-regulatory factor 7. RLORF4 was found to bind the endogenous NF-κB subunits p65 and p50, and it also bound to the Rel homology domains of these subunits. Furthermore, RLORF4 suppressed the nuclear translocation of p65 and p50 mediated by tumor necrosis factor alpha and interferon-stimulatory DNA. Finally, deletion of RLORF4 from the MDV genome promoted IFN-β and interleukin-6 (IL-6) production in vitro and in vivo In the absence of RLORF4, the host cellular immunity was significantly increased, and reduced viral titers were observed during infection of chickens. Our results suggest that the RLORF4-mediated suppression of the host antiviral innate immunity might play an important role in MDV pathogenesis.IMPORTANCE Marek's disease virus (MDV) RLORF4 has been shown to be directly involved in the attenuation of MDV upon serial passages in vitro; however, the exact function of this protein during viral infection was not well characterized. This study demonstrated that RLORF4 significantly inhibits cGAS-STING-mediated NF-κB activation by binding to the Rel homology domains of the NF-κB subunits p65 and p50, interrupting their translocation to the nuclei and thereby inhibiting IFN-β production. Furthermore, RLORF4 deficiency promoted the induction of IFN-β and downstream IFN-stimulated genes during MDV infection in chickens. Our results suggest that the contribution of RLORF4 to MDV virulence may stem from its inhibition of viral DNA-triggered IFN-β responses.

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.