Replication ability of three highly protective Marek's disease vaccines: implications in lymphoid organ atrophy and protection

Temporal Changes in Splenic Immune Cell Populations following Infection with a Very Virulent plus MDV in Commercial Meat-Type Chickens

Marek's disease virus (MDV) can cause severe immunosuppression in chickens. Our previous study showed that infection with very virulent plus (vv+) MDV strains of one-day-old commercial meat-type chickens possessing maternal antibodies against MDV resulted in severe depletion of splenocytes at 28-30 days of age. In the present study, we have investigated the effect of vv+MDV strain 686 on splenic immunophenotypes at 6, 20, and 30 days post-infection (dpi). Both live and dead cells were analyzed, and the data were statistically compared to the uninfected control. The results revealed a decrease in the total live cell population starting on day 20, primarily affecting B cells, CD8β+, and gamma delta (γδ) T cells, while the frequencies of both live and dead CD3+ and CD4+ T cells were increased. The MHC-I expression of CD3+ and CD4+ T cells was higher at 20 and 30 dpi, while the expression of MHC-II on these cells was downregulated at 6 dpi but was upregulated at 30 dpi. Collectively, these results suggest that maternal antibodies seem to delay the negative effects of vv+MDV on the splenic lymphoid populations, albeit being non-protective. Our results emphasize the importance of MD vaccination in vv+MDV endemic areas.

Pseudorabies virus hijacks DDX3X, initiating an addictive "mad itch" and immune suppression, to facilitate viral spread

Infections with defined Herpesviruses, such as Pseudorabies virus (PRV) and Varicella zoster virus (VZV) can cause neuropathic itch, referred to as "mad itch" in multiple species. The underlying mechanisms involved in neuropathic "mad itch" are poorly understood. Here, we show that PRV infections hijack the RNA helicase DDX3X in sensory neurons to facilitate anterograde transport of the virus along axons. PRV induces re-localization of DDX3X from the cell body to the axons which ultimately leads to death of the infected sensory neurons. Inducible genetic ablation of Ddx3x in sensory neurons results in neuronal death and "mad itch" in mice. This neuropathic "mad itch" is propagated through activation of the opioid system making the animals "addicted to itch". Moreover, we show that PRV co-opts and diverts T cell development in the thymus via a sensory neuron-IL-6-hypothalamus-corticosterone stress pathway. Our data reveal how PRV, through regulation of DDX3X in sensory neurons, travels along axons and triggers neuropathic itch and immune deviations to initiate pathophysiological programs which facilitate its spread to enhance infectivity.

Marek's Disease Virus Telomeric Integration Profiles of Neoplastic Host Tissues Reveal Unbiased Chromosomal Selection and Loss of Cellular Diversity during Tumorigenesis

The avian α-herpesvirus known as Marek's disease virus (MDV) linearly integrates its genomic DNA into host telomeres during infection. The resulting disease, Marek's disease (MD), is characterized by virally-induced lymphomas with high mortality. The temporal dynamics of MDV-positive (MDV+) transformed cells and expansion of MD lymphomas remain targets for further understanding. It also remains to be determined whether specific host chromosomal sites of MDV telomere integration confer an advantage to MDV-transformed cells during tumorigenesis. We applied MDV-specific fluorescence in situ hybridization (MDV FISH) to investigate virus-host cytogenomic interactions within and among a total of 37 gonad lymphomas and neoplastic splenic samples in birds infected with virulent MDV. We also determined single-cell, chromosome-specific MDV integration profiles within and among transformed tissue samples, including multiple samples from the same bird. Most mitotically-dividing cells within neoplastic samples had the cytogenomic phenotype of 'MDV telomere-integrated only', and tissue-specific, temporal changes in phenotype frequencies were detected. Transformed cell populations composing gonad lymphomas exhibited significantly lower diversity, in terms of heterogeneity of MDV integration profiles, at the latest stages of tumorigenesis (>50 days post-infection (dpi)). We further report high interindividual and lower intraindividual variation in MDV integration profiles of lymphoma cells. There was no evidence of integration hotspots into a specific host chromosome(s). Collectively, our data suggests that very few transformed MDV+ T cell populations present earlier in MDV-induced lymphomas (32-50 dpi), survive, and expand to become the dominant clonal population in more advanced MD lymphomas (51-62 dpi) and establish metastatic lymphomas.

Characterization of Md5-BAC-REV-LTR virus as Marek's disease vaccine in commercial meat-type chickens: protection and immunosuppression

Md5-BAC-REV-LTR is a recombinant Marek's disease virus (MDV), with an insertion of the long terminal repeat (LTR) of reticuloendotheliosis virus (REV) into the genome of the highly virulent MDV strain rMd5. It has been shown that Md5-BAC-REV-LTR does not induce tumours and confers high protection against challenge with MDV in 15 × 7 chickens. The objective of the present study was to evaluate the protection and safety (in terms of oncogenicity and immunosuppression) of Md5-BAC-REV-LTR in commercial meat-type chickens bearing maternal antibodies against MDV. Our results show that sub-cutaneous administration of Md5-BAC-REV-LTR at 1 day of age conferred high protection (protection index PI = 84.2) against an early challenge (1 day) by contact exposure to shedder birds infected with the vv+ MDV 648A strain. In such stringent challenge conditions, Md5-BAC-REV-LTR was more protective than a commercial CVI988 (PI = 12.4) and similar to the experimental vaccine Md5-BACΔmeq (PI = 92.4). Furthermore, Md5-BAC-REV-LTR did not induce either tumours or immunosuppression in this study. Immunosuppression was evaluated by the relative lymphoid organ weights and also by the ability of the vaccine to induce late-MDV-induced immunosuppression associated with reactivation of the virus. This study shows that Md5-BAC-REV-LTR has the potential to be used as a MD vaccine and is highly protective against early challenge with vv+ MDV. RESEARCH HIGHLIGHTSMd5-BAC-REV-LTR is highly protective against early challenge with vv+ MDV in commercial meat-type chickens.Md5-BAC-REV-LTR does not cause early immunosuppression.Md5-BAC-REV-LTR does not cause late immunosuppression.Unlike other serotype 1 vaccines, Md5-BAC-REV-LTR is not detected in feather pulp at 7 days post vaccination.

Marek’s Disease Is a Threat for Large Scale Poultry Production

Marek’s disease (MD) is one of the widespread infectious diseases that causes huge losses in large-scale poultry production. This is due to weight loss, poorer feed conversion and an increased number of deaths among infected birds. The etiological agent is a Marek’s disease virus (MDV) belonging to the Herpesviridae family. It is mainly described in poultry, however, it is also found in geese. There are three MDV serotypes, and four patotypes within serotype 1. Currently, Marek’s disease is very rare in its classical form. There are non-specific clinical symptoms, and anatomopathological changes are mainly observed in the liver, spleen and the reproductive system. This may be due to the evolution in the pathogenicity of MDV field strains over the past several decades. The presence of MDV and number of molecular diagnostic tests based on the detection of viral nucleic acids and viral proteins is already found in birds that have several weeks old. Laboratory diagnostics are based mainly on molecular biology (mainly PCR) methods. The only relatively effective method instead of biosecurity measures, of preventing MD is prophylactic vaccination of 1-day-old chickens or in ovo vaccination. Nevertheless, Marek’s disease is still recorded in poultry flocks around the world, with estimated losses reaching several million dollars.

Infection-Associated Thymic Atrophy

The thymus is a vital organ of the immune system that plays an essential role in thymocyte development and maturation. Thymic atrophy occurs with age (physiological thymic atrophy) or as a result of viral, bacterial, parasitic or fungal infection (pathological thymic atrophy). Thymic atrophy directly results in loss of thymocytes and/or destruction of the thymic architecture, and indirectly leads to a decrease in naïve T cells and limited T cell receptor diversity. Thus, it is important to recognize the causes and mechanisms that induce thymic atrophy. In this review, we highlight current progress in infection-associated pathogenic thymic atrophy and discuss its possible mechanisms. In addition, we discuss whether extracellular vesicles/exosomes could be potential carriers of pathogenic substances to the thymus, and potential drugs for the treatment of thymic atrophy. Having acknowledged that most current research is limited to serological aspects, we look forward to the possibility of extending future work regarding the impact of neural modulation on thymic atrophy.

Vaccination and Host Marek's Disease-Resistance Genotype Significantly Reduce Oncogenic Gallid alphaherpesvirus 2 Telomere Integration in Host Birds

Marek's disease (MD) is an infectious disease characterized by lymphomas and high mortality in susceptible chickens. The causative and ubiquitous alpha-herpesvirus known as MD virus (MDV) integrates into host telomeres during early infection through latency, known to be an important phase for oncogenic transformation. Herein, we sought to determine the influence of vaccination and host genetics on the temporal dynamics of MDV-host genome interactions. We studied integration profiles using 2 MD vaccines that vary in protective efficacy in 2 genetic lines that differ in MD resistance/susceptibility. Virus integration of both oncogenic MDV and vaccine strains was observed in both MD susceptible and resistant birds, however, the lines differed in their dynamic telomere-integration profiles. Notably, the resistant host genotype exhibited a smaller percentage of replicating cells with the virus telomere-integrated only phenotype as compared to the susceptible genotype. Vaccination with Rispens, the most protective MD vaccine, also reduced the establishment of the virus telomere-integrated only phenotype, suggesting a significant role of the phenotype in MD lymphoma development. The effect of Rispens vaccination was most dramatic in the susceptible genotype. These results suggest important connections between vaccinal immunity, MDV telomere integration, virus-induced oncogenesis, and virus-host genome interactions in the context of host genetics and disease susceptibility.

Addition of a UL5 helicase-primase subunit point mutation eliminates bursal-thymic atrophy of Marek's disease virus ∆Meq recombinant virus but reduces vaccinal protection

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus and the causative agent of Marek's disease (MD), characterized by immunosuppression, paralysis, nerve enlargement and induction of T-cell lymphomas in chickens. Despite widespread usage of vaccines since the 1970s to control MD, more virulent field strains of MDV have emerged that overcome vaccinal protection, necessitating the development of new and more protective MD vaccines. The ∆Meq virus, a recombinant Md5 strain MDV lacking the viral oncogene Meq, is one candidate MD vaccine with great potential but unfortunately it also causes bursal-thymic atrophy (BTA) in maternal antibody negative chickens, raising concerns that impede commercial use as a vaccine. Previously, we identified a point mutation within UL5 that reduced in vivo replication in attenuated viruses. We proposed that introduction of the UL5 point mutation into the ∆Meq virus would reduce in vivo replication and eliminate BTA yet potentially retain high protective abilities. In birds, the ∆Meq+UL5 recombinant MDV had reduced replication compared to the original ∆Meq virus, while weights of lymphoid organs indicated that ∆Meq+UL5 did not induce BTA, supporting the hypothesis that reduction of in vivo replication would also abolish BTA. Vaccine trials of the ∆Meq+UL5 virus compared to other ∆Meq-based viruses and commercial vaccines show that, while the ∆Meq+UL5 does provide vaccinal protection, this protection was also reduced compared to the original ∆Meq virus. Therefore, it appears that a very delicate balance is required between levels of replication able to induce high vaccinal protection, yet not so high as to induce BTA.

Marek's disease herpesvirus vaccines integrate into chicken host chromosomes yet lack a virus-host phenotype associated with oncogenic transformation

Marek's disease (MD) is a lymphotropic and oncogenic disease of chickens that can lead to death in susceptible and unvaccinated host birds. The causative pathogen, MD virus (MDV), a highly oncogenic alphaherpesvirus, integrates into host genome near the telomeres. MD occurrence is controlled across the globe by biosecurity, selective breeding for enhanced MD genetic resistance, and widespread vaccination of flocks using attenuated serotype 1 MDV or other serotypes. Despite over 40 years of usage, the specific mechanism(s) of MD vaccine-related immunity and anti-tumor effects are not known. Here we investigated the cytogenetic interactions of commonly used MD vaccine strains of all three serotypes (HVT, SB-1, and Rispens) with the host to determine if all were equally capable of host genome integration. We also studied the dynamic profiles of chromosomal association and integration of the three vaccine strains, a first for MD vaccine research. Our cytogenetic data provide evidence that all three MD vaccine strains tested integrate in the chicken host genome as early as 1 day after vaccination similar to oncogenic strains. However, a specific, transformation-associated virus-host phenotype observed for oncogenic viruses is not established. Our results collectively provide an updated model of MD vaccine-host genome interaction and an improved understanding of the possible mechanisms of vaccinal immunity. Physical integration of the oncogenic MDV genome into host chromosomes along with cessation of viral replication appears to have joint signification in MDV's ability to induce oncogenic transformation. Whereas for MD vaccine serotypes, a sustained viral replication stage and lack of the chromosome-integrated only stage were shared traits during early infection.

Protective efficacy of a recombinant bacterial artificial chromosome clone of a very virulent Marek's disease virus containing a reticuloendotheliosis virus long terminal repeat

Marek's disease virus (MDV), an alphaherpesvirus, causes Marek's disease (MD), a lymphoproliferative disease in poultry characterized by T-cell lymphomas, nerve lesions, and mortality. Vaccination is used worldwide to control MD, but increasingly virulent field strains can overcome this protection, driving a need to create new vaccines. Previous studies revealed that insertion of reticuloendotheliosis virus (REV) long terminal repeat (LTR) into a bacterial artificial chromosome (BAC) clone of a very virulent strain of MDV, Md5, rendered the resultant recombinant virus, rMd5 REV-LTR BAC, fully attenuated in maternal antibody positive (Mab+) chickens at passage 40. In the current study, the protective efficacy of rMd5 REV-LTR BAC was evaluated. First, passage 70 was identified as being fully attenuated in maternal antibody negative chickens and chosen as the optimal passage level for use in protective efficacy studies. Second, three protective efficacy trials were conducted comparing the rMd5 REV-LTR p70 BAC to the CVI988/Rispens vaccine. Groups of Mab+ and Mab- 15I₅ × 7₁ chickens were vaccinated in ovo at 18 days of embryonation or intra-abdominally at day of hatch, and challenged at 5 days post-hatch with the vv+MDV strain 686. Vaccination at day of hatch and in ovo with rMd5 REV-LTR p70 BAC protected chickens against MDV-induced bursa and thymic atrophy, but did not provide the same level of protection against MD tumours as that afforded by the commercial vaccine, CVI988/Rispens.

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.