Cytotoxic T lymphocytes from cattle immunized against Theileria parva exhibit pronounced cross-reactivity among different strain-specific epitopes of the Tp1 antigen.

The protozoan parasite Theileria parva causes a usually fatal disease in cattle, known as East Coast fever. Cattle can be vaccinated by injecting live parasites simultaneously with long acting oxytetracycline (the infection and treatment method, ITM). The immunity induced by ITM is believed to be mediated by cytotoxic T lymphocytes (CTL). Although effective, the ITM vaccine has disadvantages such as the need for a liquid nitrogen cold chain and a complex production process, which may be overcome by the development of a subunit vaccine. However, the high level of antigenic polymorphism among different strains of T. parva may hinder the development of a subunit vaccine aimed at induction of a protective CTL response. In this study, the CTL cross-reactivity among T. parva strains was examined. The Tp1(214-224) epitope has previously been shown to be recognized by cattle of the A18 BoLA type. Three different variants of this epitope have been identified from different T. parva strains. Here, bulk CTL and CTL clones were generated from two animals using both the live sporozoite vaccine composed of three different strains and a Muguga strain for immunization. The cross-reactivity of these CTL with the three variant Tp1 epitopes was examined in interferon gamma ELISPOT assays and CTL killing assays. CD8(+) cells from both animals cross-reacted with the three variant CTL epitopes in interferon gamma ELISPOT assays, although the CD8(+) cells from the Muguga-immunized animal showed a more epitope restricted response. Clones from the vaccine immunized animal showed diverse response patterns with clones responding to each variant peptide. Although some variability in the cytotoxic response was observed, overall strong cross-reactivity among the variant Tp1 epitopes was seen in both animals. Such epitope polymorphism does not, in this case, serve as a potential challenge in a putative subunit vaccine as it would be sufficient to only include one of the variant epitopes.