Induction of DNA Damages upon Marek's Disease Virus Infection: Implication in Viral Replication and Pathogenesis

Temporal Dynamics of Purinergic Receptor Expression in the Lungs of Marek's Disease (MD) Virus-Infected Chickens Resistant or Susceptible to MD

Marek's disease virus (MDV) is an economic concern for the poultry industry due to its poorly understood pathophysiology. Purinergic receptors (PRs) are potential therapeutic targets for viral infections, including herpesviruses, prompting our investigation into their role in MDV pathogenesis. The current study is part of an experimental series analyzing the expression of PRs during MDV infection. To address the early or short-acting P2 PR responses during natural MDV infection, we performed an "exposure" experiment where age-matched chickens were exposed to experimentally infected shedders to initiate natural infection. In addition, select non-PR regulatory gene responses were measured. Two groups of naïve contact chickens (n = 5/breed/time point) from MD-resistant (White Leghorns: WL) and -susceptible (Pure Columbian) chicken lines were housed separately with experimentally infected PC (×PC) and WL (×WL) chickens for 6 or 24 h. Whole lung lavage cells (WLLC) were collected, RNA was extracted, and RT-qPCR assays were used to measure specific PR responses. In addition, other potentially important markers in pathophysiology were measured. Our study revealed that WL chickens exhibited higher P1 PR expression during natural infection. WL chickens also showed higher expression of P1A3 and P2X3 at 6 and 24 h when exposed to PC-infected chickens. P2X5 and P2Y1 showed higher expression at 6 h, while P2Y5 showed higher expression at 6 and 24 h; regardless of the chicken line, PC chickens exhibited higher expression of P2X2, P2Y8, P2Y10, P2Y13, and P2Y14 when exposed to either group of infected chickens. In addition, MDV infection altered the expression of DDX5 in both WL and PC groups exposed to PC-infected birds only. However, irrespective of the source of exposure, BCL2 and ANGPTL4 showed higher expression in both WL and PC. The expression of STAT1A and STAT5A was influenced by time and breed, with major changes observed in STAT5A. CAT and SOD1 expression significantly increased in both WL and PC birds, regardless of the source of infection. GPX1 and GPX2 expression also increased in both WL and PC, although overall lower expression was observed in PC chickens at 24 h compared to 6 h. Our data suggest systemic changes in the host during early infection, indicated by the altered expression of PRs, DDX5, BCL2, ANGPTL4, and other regulatory genes during early MDV infection. The relative expression of these responses in PC and WL chickens suggests they may play a key role in their response to natural MDV infection in the lungs and long-term pathogenesis and survival.

Immune escape of avian oncogenic Marek's disease herpesvirus and antagonistic host immune responses

Marek's disease virus (MDV) is a highly pathogenic and oncogenic alpha herpesvirus that causes Marek's disease (MD), which is one of the most important immunosuppressive and rapid-onset neoplastic diseases in poultry. The onset of MD lymphomas and other clinical diseases can be efficiently prevented by vaccination; these vaccines are heralded as the first demonstration of a successful vaccination strategy against a cancer. However, the persistent evolution of epidemic MDV strains towards greater virulence has recently resulted in frequent outbreaks of MD in vaccinated chicken flocks worldwide. Herein, we provide an overall review focusing on the discovery and identification of the strategies by which MDV evades host immunity and attacks the immune system. We have also highlighted the decrease in the immune efficacy of current MD vaccines. The prospects, strategies and new techniques for the development of efficient MD vaccines, together with the possibilities of antiviral therapy in MD, are also discussed.

Porcine IGFBP3 promotes porcine circovirus type 2 replication via PERK/eIF2α mediated DNA damage

The infection of porcine circovirus type 2 (PCV2) triggers activation of the protein kinase RNA-like endoplasmic reticulum kinase (PERK) pathway and leads to DNA damage. Insulin-like growth factor-binding protein 3 (IGFBP3) may interact with the endoplasmic reticulum (ER). It remains unclear whether IGFBP3 regulates DNA damage via ER stress to mediate PCV2 replication. In this study, we observed an upregulation of porcine IGFBP3 expression during PCV2 infection, and overexpression of IGFBP3 enhanced the expression of PCV2 Cap protein, PCV2 DNA copy number, and viral titers in PK-15 B6 cells and 3D4/21 cells. Additionally, overexpression of IGFBP3 induced an increase in the DNA damage marker γH2AX by activating the PERK/eIF2α pathway without concomitant activation of ATF4, IRE1α, and ATF6α/GRP78 pathways in PK-15 B6 cells and 3D4/21 cells. Knockdown of IGFBP3 had a reverse effect on PCV2 replication in PK-15 B6 cells and 3D4/21 cells. Furthermore, treatment with etoposide enhanced PCV2 replication while KU57788 decreased it. GSK2606414 and salubrinal limited both DNA damage and viral replication. Therefore, our findings suggest that porcine IGFBP3 promotes PCV2 replication through the PERK/eIF2α pathway-mediated induction of DNA damage in PK-15 B6 cells and 3D4/21 cells. Our study provides a basis for exploring novel antiviral strategies via the extensive understanding of the relationships between host cellular proteins and viral replication.

Mechanism of herpesvirus UL24 protein regulating viral immune escape and virulence

Herpesviruses have evolved a series of abilities involved in the process of host infection that are conducive to virus survival and adaptation to the host, such as immune escape, latent infection, and induction of programmed cell death for sustainable infection. The herpesvirus gene UL24 encodes a highly conserved core protein that plays an important role in effective viral infection. The UL24 protein can inhibit the innate immune response of the host by acting on multiple immune signaling pathways during virus infection, and it also plays a key role in the proliferation and pathogenicity of the virus in the later stage of infection. This article reviews the mechanism by which the UL24 protein mediates herpesvirus immune escape and its effects on viral proliferation and virulence by influencing syncytial formation, DNA damage and the cell cycle. Reviewing these studies will enhance our understanding of the pathogenesis of herpesvirus infection and provide evidence for new strategies to combat against viral infection.

In vitro infection of primary chicken lymphocytes with Marek's disease virus

Marek's disease virus (MDV) is a highly oncogenic alphaherpesvirus that infects immune cells and causes a deadly lymphoproliferative disease in chickens. Cytokines and monoclonal antibodies promote the survival of chicken lymphocytes in vitro. Here, we describe protocols for the isolation, maintenance, and efficient MDV infection of primary chicken lymphocytes and lymphocyte cell lines. This facilitates the investigation of key aspects of the MDV life cycle in the primary target cells of viral replication, latency, genome integration, and reactivation. For complete details on the use and execution of this protocol, please refer to Schermuly et al.,¹ Bertzbach et al. (2019),² and You et al.³ For a comprehensive background on MDV, please see Osterrieder et al.⁴ and Bertzbach et al. (2020).⁵.

DNA Damage Response Signaling Is Crucial for Effective Chikungunya Virus Replication

Viruses utilize a plethora of strategies to manipulate the host pathways and hijack host machineries for efficient replication. Several DNA and few RNA viruses are reported to interact with proteins involved in DNA damage responses (DDRs). As the DDR pathways have never been explored in alphaviruses, this investigation intended to understand the importance of the DDR pathways in chikungunya virus (CHIKV) infection in vitro, in vivo, and ex vivo models. The study revealed that CHIKV infection activated the Chk2 and Chk1 proteins associated with the DDR signaling pathways in Vero, RAW264.7, and C2C12 cells. The comet assay revealed an increase in DNA damage by 95%. Inhibition of both ATM-ATR kinases by the ATM/ATR kinase inhibitor (AAKi) showed a drastic reduction in the viral particle formation in vitro. Next, the treatment of CHIKV-infected C57BL/6 mice with this drug reduced the disease score substantially with a 93% decrease in the viral load. The same was observed in human peripheral blood mononuclear cell (hPBMC)-derived monocyte-macrophage populations. Additionally, silencing of Chk2 and Chk1 reduced viral progeny formation by 91.2% and 85.5%, respectively. Moreover, CHIKV-nsP2 was found to interact with Chk2 and Chk1 during CHIKV infection. Furthermore, CHIKV infection induced cell cycle arrest in G₁ and G₂ phases. In conclusion, this work demonstrated for the first time the mechanistic insights regarding the induction of the DDR pathways by CHIKV that might contribute to the designing of effective therapeutics for the control of this virus infection in the future. IMPORTANCE Being intracellular parasites, viruses require several host cell machineries for effectively replicating their genome, along with virus-encoded enzymes. One of the strategies involves hijacking of the DDR pathways. Several DNA and few RNA viruses interact with the cellular proteins involved in the DDR pathways; however, reports regarding the involvement of Chk2 and Chk1 in alphavirus infection are limited. This is the first study to report that modulation of DDR pathways is crucial for effective CHIKV infection. It also reveals an interaction of CHIKV-nsP2 with two crucial host factors, namely, Chk2 and Chk1, for efficient viral infection. Interestingly, CHIKV infection was found to cause DNA damage and arrest the cell cycle in G₁ and G₂ phases for efficient viral infection. This information might facilitate the development of effective therapeutics for controlling CHIKV infection in the future.

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.

Hypoxia and HIF-1 Trigger Marek’s Disease Virus Reactivation in Lymphoma-Derived Latently Infected T Lymphocytes

Latency is a hallmark of herpesviruses, allowing them to persist into their host without virions production. Acute exposure to hypoxia (below 3% O 2 ) was identified as a trigger of latent-to-lytic switch (reactivation) for human oncogenic gamma-herpesviruses (KSHV and EBV). Therefore, we hypothesized that hypoxia could also induce reactivation of Marek’s disease virus (MDV), sharing biological properties with EBV and KSHV (notably oncogenic properties), into lymphocytes. Acute exposure to hypoxia (1% O 2 ) of two MDV-latently infected cell lines derived from MD tumors (3867K and MSB-1) induced MDV reactivation. A bioinformatic analysis of the RB-1B MDV genome revealed 214 putative hypoxia-response element consensus sequences on 119 open reading frames. RT-qPCR analysis showed five MDV genes strongly upregulated early after hypoxia. In 3867K cells under normoxia, pharmacological agents mimicking hypoxia (MLN4924 and CoCl 2 ) increased MDV reactivation, but to a lower level than real hypoxia. Overexpression of wild-type or stabilized human hypoxia inducible factor-1α (HIF-1α) in MSB-1 cells in normoxia also promoted MDV reactivation. In such conditions, lytic cycle was detected in cells with a sustainable HIF-1α expression, but also in HIF-1α negative cells, indicating that MDV reactivation is mediated by HIF-1, in a direct and/or indirect manner. Lastly, we demonstrated by a reporter assay that HIF-1α overexpression induced the transactivation of two viral promoters, shown upregulated in hypoxia. These results suggest that hypoxia may play a crucial role in the late lytic replication phase observed in vivo in MDV-infected chickens exhibiting tumors, since a hypoxic microenvironment is a hallmark of most solid tumors.

IMPORTANCE

Latent-to-lytic switch of herpesviruses (aka reactivation) is responsible for pathology recurrences and/or viral shedding. Studying physiological triggers of reactivation is therefore important for health to limit lesions and viral transmission. Marek's disease virus (MDV) is a potent oncogenic alpha-herpesvirus establishing latency in T-lymphocytes and causing lethal T-lymphomas in chickens. In vivo , a second lytic phase is observed during tumoral stage. Hypoxia being a hallmark of tumors, we wondered whether hypoxia induces MDV reactivation in latently-infected T-lymphocytes, like previously shown for EBV and KSHV in B-lymphocytes. In this study, we demonstrated that acute hypoxia (1% O2) triggers MDV reactivation in two MDV transformed T-cell lines. We provide some molecular basis of this reactivation by showing that hypoxia inducible factor (HIF-1) overexpression induces MDV reactivation to a similar extend than hypoxia after 24 hours. Hypoxia is therefore a reactivation stimulus shared by mammalian and avian oncogenic herpesviruses of different genus.

A Cell Culture System to Investigate Marek's Disease Virus Integration into Host Chromosomes

Marek’s disease virus (MDV) is a highly oncogenic alphaherpesvirus that causes a devastating neoplastic disease in chickens. MDV has been shown to integrate its genome into the telomeres of latently infected and tumor cells, which is crucial for efficient tumor formation. Telomeric repeat arrays present at the ends of the MDV genome facilitate this integration into host telomeres; however, the integration mechanism remains poorly understood. Until now, MDV integration could only be investigated qualitatively upon infection of chickens. To shed further light on the integration mechanism, we established a quantitative integration assay using chicken T cell lines, the target cells for MDV latency and transformation. We optimized the infection conditions and assessed the establishment of latency in these T cells. The MDV genome was efficiently maintained over time, and integration was confirmed in these cells by fluorescence in situ hybridization (FISH). To assess the role of the two distinct viral telomeric repeat arrays in the integration process, we tested various knockout mutants in our in vitro integration assay. Efficient genome maintenance and integration was thereby dependent on the presence of the telomeric repeat arrays in the virus genome. Taken together, we developed and validated a novel in vitro integration assay that will shed light on the integration mechanism of this highly oncogenic virus into host telomeres.

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.

Mardivirus Infection and Persistence in Feathers of a Chicken Model Harboring a Local Autoimmune Response

Herpesvirus of turkey (HVT) is commonly used as a vaccine to protect chickens against Marek’s disease. Following vaccination, HVT infects feathers where it can be detected in all chicken lines examined. Unlike the parental Brown line (BL), Smyth line (SL) chickens develop vitiligo, due to autoimmune destruction of melanocytes in feathers. Previous reports showed a strong inflammatory response in Smyth chickens’ feathers at vitiligo onset, that subsided once melanocytes were destroyed, and depigmentation was complete. Here, we questioned whether the local autoimmune response in the Smyth model influences HVT infection and persistence in feathers. For this, one-day-old SL and BL chickens were vaccinated with Newcastle disease (rHVT-ND). Vitiligo was scored and HVT loads in pigmented and non-pigmented growing feathers were quantified regularly over 20 weeks. Chickens of both lines showed moderate HVT loads in feathers. At the onset of active vitiligo, the HVT load was significantly higher in SL compared to BL feathers. However, no difference in HVT loads was noticed between pigmented and non-pigmented feathers from SL chickens. Therefore, surprisingly, the inflammatory response in feathers of SL chickens did not inhibit HVT infection and persistence, but on the contrary, temporarily promoted HVT infection in feathers.

Alphaherpesvirus Major Tegument Protein VP22: Its Precise Function in the Viral Life Cycle

Alphaherpesviruses are zoonotic pathogens that can cause a variety of diseases in humans and animals and severely damage health. Alphaherpesvirus infection is a slow and orderly process that can lie dormant for the lifetime of the host but may be reactivated when the immune system is compromised. All alphaherpesviruses feature a protein layer called the tegument that lies between the capsid and the envelope. Virus protein (VP) 22 is one of the most highly expressed tegument proteins; there are more than 2,000 copies of this protein in each viral particle. VP22 can interact with viral proteins, cellular proteins, and chromatin, and these interactions play important roles. This review summarizes the latest literature and discusses the roles of VP22 in viral gene transcription, protein synthesis, virion assembly, and viral cell-to-cell spread with the purpose of enhancing understanding of the life cycle of herpesviruses and other pathogens in host cells. The molecular interaction information herein provides important reference data.

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.

TOP2β-Dependent Nuclear DNA Damage Shapes Extracellular Growth Factor Responses via Dynamic AKT Phosphorylation to Control Virus Latency

The mTOR pathway integrates both extracellular and intracellular signals and serves as a central regulator of cell metabolism, growth, survival, and stress responses. Neurotropic viruses, such as herpes simplex virus-1 (HSV-1), also rely on cellular AKT-mTORC1 signaling to achieve viral latency. Here, we define a novel genotoxic response whereby spatially separated signals initiated by extracellular neurotrophic factors and nuclear DNA damage are integrated by the AKT-mTORC1 pathway. We demonstrate that endogenous DNA double-strand breaks (DSBs) mediated by Topoisomerase 2β-DNA cleavage complex (TOP2βcc) intermediates are required to achieve AKT-mTORC1 signaling and maintain HSV-1 latency in neurons. Suppression of host DNA-repair pathways that remove TOP2βcc trigger HSV-1 reactivation. Moreover, perturbation of AKT phosphorylation dynamics by downregulating the PHLPP1 phosphatase led to AKT mis-localization and disruption of DSB-induced HSV-1 reactivation. Thus, the cellular genome integrity and environmental inputs are consolidated and co-opted by a latent virus to balance lifelong infection with transmission.

Oxidative Stress in Poultry: Lessons from the Viral Infections

Reactive species (RS), generally known as reactive oxygen species (ROS) and reactive nitrogen species (RNS), are produced during regular metabolism in the host and are required for many cellular processes such as cytokine transcription, immunomodulation, ion transport, and apoptosis. Intriguingly, both RNS and ROS are commonly triggered by the pathogenic viruses and are famous for their dual roles in the clearance of viruses and pathological implications. Uncontrolled production of reactive species results in oxidative stress and causes damage in proteins, lipids, DNA, and cellular structures. In this review, we describe the production of RS, their detoxification by a cellular antioxidant system, and how these RS damage the proteins, lipids, and DNA. Given the widespread importance of RS in avian viral diseases, oxidative stress pathways are of utmost importance for targeted therapeutics. Therefore, a special focus is provided on avian virus-mediated oxidative stresses. Finally, future research perspectives are discussed on the exploitation of these pathways to treat viral diseases of poultry.