Characterisation of the T cell response to the porcine reproductive and respiratory syndrome virus and its application towards the development of improved vaccines

Porcine reproductive and respiratory syndrome (PRRS) is one of the most important pig diseases worldwide. The causative PRRS virus (PRRSV) is rapidly evolving and there is an urgent need for the development of safer and more efficacious vaccines to improve PRRS control. Immunity to PRRSV is not well understood but there are data suggesting that virus-specific T cell IFN-γ responses play an important role. \nTherefore, this project focussed on characterising the T cell response PRRSV and applied this knowledge to develop a novel vaccine strategy. The first part of the \nproject aimed to identify the antigenic targets of the T cell response to PRRSV by utilising a proteome-wide synthetic peptide library and a cohort of PRRSV immune animals. The T cell IFN-γ response was directed at a range of viral proteins but the M and NSP5 proteins stood out as major antigens. Further experiments confirmed M and NSP5 as well conserved targets of in many instances dominant T cell responses. Characterisation of the responding T cell populations showed NSP5-specific responders to be CD8 T cells with a predominant CD44highCD62LlowCD27lowCD25- phenotype. The majority of cells were polyfunctional as assessed by co-expression of TNF-α and mobilisation of the cytotoxic degranulation marker CD107a. Both CD8 and CD4 T cells responded to M with a comparable phenotype to that observed for NSP-specific T cells. In addition, conserved antigenic regions of each protein were identified and specificity shown to associate with major histocompatibility complex haplotype, rather than PRRSV strain. Finally, a vaccine study was conducted using M and NSP5 proteins as T cell antigens formulated as a particulate vaccine with a molecular adjuvant. Vaccination primed antigen-specific CD4 but not CD8 T cell responses and did not confer significant protection of animals from viraemia upon challenge infection. Analysis of the lungs during the resolution of infection showed high levels of virus and M/NSP5 specific CD8 T cell IFN-γ responses, suggesting that vaccine priming of a CD8 T cell response is required for protection from PRRSV infection. It is hoped that this work will inform future PRRSV vaccine design, as well as contributing to the wider field of T cell vaccinology.