Characterization of a Novel Chimeric Theileria parva p67 Antigen Which Incorporates into Virus-like Particles and Is Highly Immunogenic in Mice

The current method to protect cattle against East Coast Fever (ECF) involves the use of live Theileria parva sporozoites. Although this provides immunity, using live parasites has many disadvantages, such as contributing to the spread of ECF. Subunit vaccines based on the sporozoite surface protein p67 have been investigated as a replacement for the current method. In this study, two DNA vaccines expressing recombinant forms of p67 designed to display on retrovirus-like particles were constructed with the aim of improving immunogenicity. The native leader sequence was replaced with the human tissue plasminogen activator leader in both vaccines. The full-length p67 gene was included in the first DNA vaccine (p67); in the second, the transmembrane domain and cytoplasmic tail were replaced with those of an influenza A virus hemagglutinin 5 (p67HA). Immunofluorescent staining of fixed and live transfected mammalian cells showed that both p67 and p67HA were successfully expressed, and p67HA localised on the cell surface. Furthermore, p67HA was displayed on the surface of both bovine leukaemia virus (BLV) Gag and HIV-1 Gag virus-like particles (VLPs) made in the same cells. Mice vaccinated with DNA vaccines expressing p67 and p67HA alone, or p67HA with BLV or HIV-1 Gag, developed high titres of p67 and BLV Gag-binding antibodies. Here we show that it is possible to integrate a form of p67 containing all known antigenic domains into VLPs. This p67HA–VLP combination has the potential to be incorporated into a vaccine against ECF, as a DNA vaccine or as other vaccine platforms.