Revaccination with the same Theileria annulata infected cell line may not be feasible for boosting immunity against tropical theileriosis.

A model for studying re-immunisation using skin grafting was developed as the allogeneic responses produced by T. annulata cell lines were similar to those produced by skin grafting. Appearance of schizonts and piroplasms post-immunisation was either delayed or prevented by already existing allogeneic responses. Isolation of parasite infected cell lines from lymph node biopsies and peripheral blood after cell line immunisation was also delayed or prevented by already existing allogeneic responses. Ability to isolate cell lines after immunisation correlated with protection i.e. if no parasite infected cell line of donor origin was isolated after immunisation, there was no protection. Allogeneic responses delayed or prevented the appearance of MHC I restricted parasite specific cytotoxic T lymphocytes post-immunisation. If the parasite transfer was prevented after immunisation; animals were fully susceptible to challenge. These experiments showed that allogeneic responses, generated in animals after immunisation with T. annulata schizont cell culture vaccine, can block parasite transfer and further development or enhancement of immunity against the parasite at the time of second immunisation with the same cell line. The observations are of immediate importance in endemic areas where T. annulata infected cell culture vaccines are being used. They are even more relevant in countries where animals are regularly moved between theileriosis free and endemic areas. It may not be advisable to re-immunise animals with the same cell line as that used for first vaccination.