ALLOGRAFT RESPONSES CAN INTERFERE WITH THE DEVELOPMENT OF IMMUNITY AGAINST THEILERIA-ANNULATA FOLLOWING VACCINATION WITH PARASITE INFECTED CELL-LINES

Theileria annulata macroschizont-infected cell lines are successfully used as vaccines in several countries. The inoculated animals produce a strong allogeneic response against the MHC antigens of the immunizin g cell line followed by an anti-parasite response. Immunity against th e parasite wanes in the absence of challenge and re-immunization is so metimes recommended However, it is not known if allogeneic responses g enerated by the first immunization with a T. annulata infected cell li ne will interfere with the boosting of immunity against the parasite a t the time of re-immunization with the same cell line. Animals were pr imed against MHC antigens by skin grafting followed by immunization wi th a T. annulata infected cell line prepared from the skin donor. A st rong anti-MHC response was produced. This interfered with parasite tra nsfer and the development of an anti-parasite immune response; the eff ect was more marked when a low vaccine cell dose was used. There was a negative correlation between the ease of isolating infected cells fro m the animals after cell line immunization, and the subsequent respons e to challenge. Where no cell lines could be isolated, the animals wer e fully susceptible to sporozoite challenge. These observations are of immediate importance in endemic areas where cell lines of T. annulata schizonts are being used as vaccines to control the disease.