Dissecting the role of virus-specific T cells in providing protection against Marek's Disease Virus in chickens

Infection with Marek's disease virus (MDV) causes lymphoma in chickens, and is responsible for annual losses of approximately £1.5 billion to the global poultry industry. Approximately 22 billion doses of MDV vaccine are annually used to control tumour formation, but the vaccine fails to control virus replication and the virus continues to evolve and form increasingly virulent strains. It is unknown how the vaccine can control tumour formation whilst failing to control viral infection. Understanding the mechanism(s) involved in vaccine failure is crucial for the development of more effective MDV vaccines.

In animal model studies, it has been shown that T lymphocytes are involved in the control of tumour formation and viral infection via production of inflammatory proteins, cytokines, and their cytotoxic abilities. Here, we aim to (1) determine the role of cytokine producing T lymphocytes in the control of tumour formation in vaccinated chickens (2) examine whether a lipid mediator, PGE2, which is produced by MDV infected cells, suppresses cytotoxic function of T lymphocytes in the infected chickens, and this may be involved in the failure of vaccine-induced T cell responses to control infection. To achieve these objectives, we will analyse whether adoptive transfer of cytokine producing T lymphocytes from vaccinated chickens to naïve chickens can control tumour formation. Second, we will determine whether administration of chemical inhibitors of PGE2 and vaccination against viral proteins or adoptive transfer of T lymphocytes can rescue the cytotoxic function of T lymphocytes and thus protects the chickens from both tumour formation and viral infection.

Marek's disease virus (MDV), an alphaherpesvirus, infects chickens and causes a deadly lymphoma formation. Current MDV vaccines protect the birds from tumour development but not from infection. Thus, the oncogenic virus replicates in the vaccinated birds and this virus is shed from the infected birds, leading to the generation of hyper-virulent strains. Because of the cell-associated nature of the virus, it is postulated that vaccine-induced T cells plays a crucial role in the control of the disease. It has been shown that gB, PP38, MEQ, and ICP4 of MDV are T cell target antigens, and immunisation with viral vector expressing gB provides protection against lymphoma formation but not viral infection and shedding. Our preliminary data shows that MDV infection impairs virus-specific T cell degranulation without influencing on their ability to produce cytokines production. Moreover, we have demonstrated that MDV infection activates the COX-2/PGE2 pathway, which can impair T cell degranulation without inhibiting their cytokine production in vitro. In this proposal, we will initially determine whether vaccine-induced immunity to PP38, MEQ and ICP4 can protect chickens from tumour formation, and generate MDV-specific T cells from vaccinated chickens for adoptive T cell transfer experiments. Adoptive cell transfer (ACT) of MDV-specific T cells to naïve birds will provide information on the role of T cells in conferring protection against lymphoma formation. Finally, we will administer chemical inhibitors of the COX-2/PGE2 pathway in combination with MDV vaccine or ACT of MDV-specific T cells and determine the effects of these treatments on both lymphoma formation and viral infection. Together, these studies will determine the role of virus-specific T cells in vaccine-induced protection and will provide insight on interference of viral-infection with cytotoxicity as a viral strategy to escape the immune control.

The aim of this proposal is to provide information on the role of T lymphocytes in vaccine-induced protection against Marek's Disease (MD), and identify mechanism involved in the failure of vaccines to control infection. Although the impact of this research will initially benefit the academic community, the results from this study will have long-term impact on vaccine development, and may lead to the development of more effective vaccines against infection with Marek's disease virus (MDV). In the long-term, the critical impact of these findings will be generating vaccines that can inhibit infection and virus shedding, which will have a major impact on improving MD control, with accompanying economic and societal benefits. Poultry and poultry products are the cheapest and most accessible source of animal protein and demands for these products have increased exponentially. Compared to the other livestock sectors, the modern poultry production methods have the most efficient feed-to-meat conversion ratios with lowest global warming potential. It is estimated that poultry will account for 46% of meat consumed by 2022 around the world (http://oecd.org/site/oecdfaoagriculturaloutlook/O-FAO). Poultry meat is important to the UK, not just because of the food it provides but because it contributes £3.3 billion to UK GDP, with every £1 billion generating another £1.3 billion in the rest of UK economy. The industry supports 35,400 direct and a further 37,900 indirect jobs in the wider supply chain (Oxford Economics, 2013). Infectious diseases are a continuous threat to poultry industry through losses or reduction in production and animal welfare. MD is one of the major diseases of poultry which causes serious economic losses and the global estimate of losses from MD is approximately US $2 billion annually. The outcome of this research will provide crucial information which could be used to generate more effective vaccines providing protections against infection. Although the focus in this proposal is on MD, our results will have a wider impact by providing underpinning knowledge for similar studies with other economically important avian pathogens, as MDV strains are used as a vector for generation of immunity against other infectious diseases. The results of the project will be communicated to The Pirbright Institute stakeholders, such as BBSRC and DEFRA. The reagents and scientific knowledge generated will be published in peer-reviewed journals, and in presentations at meetings for scientists, and the impact of the project will also be publicized by communication with the farming community and the public through our website, workshops, training courses and exhibitions. The Pirbright Institute is committed to the development of highly skilled researchers and the postdoctoral scientists who will be recruited for this post will receive excellent training in T cell epitope mapping, flow cytometry, cellular and molecular techniques, thereby contributing to the wider training, innovation, skills and capability of the UK science with consequent boosting to science based industries and the UK economy.