Evaluation of the vaccine potiential of recombinant live attenuated listeria monocytogenes expressing CD8+ T cell epitopes of theileria parva in cattle

Abstract

East Coast fever (ECF) is a highly fatal lympho-proliferative disease of cattle caused by tick-borne protozoan Theileria parva. The intra-lymphocytec schizont stage induces a cancer-like transformation of the infected cell which is responsible for the pathology and ultimately death of infected animals. Immunity from the cattle that recover from ECF or treated using live vaccines is mediated by parasite-specific CD8+ cytotoxic T lymphocytes (CTL), which target and clear schizont-infected lymphocytes. Although the identified CTL target antigens have been evaluated using DNA and attenuated pox virus vectors and have been shown to induce CD8+ CTL responses in a proportion of cattle that have correlated with reduced disease severity upon parasite challenge, the inability to consistently trigger the protective CTL responses is a major impediment that needs to be overcome. This study aimed at evaluating the potential of recombinant live attenuated Listeria monocytogenes (Lm AactADinlB) expressing three defined T. pcirva CTL epitopes to stimulate protective CTL responses against T. parva in cattle upon lethal parasite challenge. In vitro studies using recombinant Lm AactADinlB indicated that one of the T. parva CTL epitopes could be weakly expressed and recognized by antigenspecific CD8+ T cell lines in one of the experiments. Cattle were screened against L. monocytogenes T cell target antigenic peptide pool, listeriolysin-O (LLO), and the vaccine select animals based on those with no or low background responses to LLO. Induction of epitope-specific CD8+ T cell IFN-y immune responses were detected in three out of nine vaccinated animals while T cell responses in PBMC to LLO were observed in eight animals immunized with either wild type or recombinant Lm AactADinlB. Cytotoxic T cell responses were detected after boosting in only one of nine immunized animals. However these responses did not translate into a significant protective effect after challenge. This may be in part due to poor expression of recombinant antigens leading to sub-optimal induction of protective antigen-specific CD8+ T cells. Although this immunization regime failed to induce protective CD8+ T cells against T. parva in all the animals. By modifying the antigen formulation, dose, mode and route of inoculation a more efficient protocol can be developed. This kind of formulation will find application in vaccine preparation in other diseases including cancer where the induction of CD8+ Tcell immunity is critical.