In vitro proliferative and cytotoxic responses of PBL from Theileria annulata-immune cattle.

Peripheral blood lymphocytes were isolated from healthy calves and were subsequently infected with sporozoites of Theileria annulata in vitro. The infected cells were passaged for 50 times and thereafter inoculated into animals from which they were previously isolated. Within 4-5 days, schizont-containing cells were demonstrable in the lymph nodes of all animals. Few days later, merozoites were detected in erythrocytes. A slight decrease in the counts of lymphocytes and leucocytes was also found. After 2 months these animals and a group of uninfected calves were heavily infected by tick-infestation and showed severe symptoms of theileriosis with 60% schizont-containing cells in the lymph nodes and a parasitaemia of about 35%. Because of the severity of the infection, all control calves were treated with Halofuginone. In contrast, the initially immunized cattle (by inoculation of culture cells), survived the infection without chemotherapy. Less than 10% of their lymph node cells contained schizonts, whereas less than 1% of their erythrocytes were found to be infected with merozoites. In all immunized animals, specific cytotoxic PBL, with the capacity to lyse autologous but not allogeneic infected cells, were demonstrated. In addition, a population of PBL were found to be able to inhibit the growth of T.annulata-infected culture cells in vitro. However, in comparison to PBL of immune animals, PBL of acute infected calves were superior in their capacity to inhibit the proliferation of schizont-containing cells. In mixed lymphocyte reactions, T. annulata-infected cells could induce a more pronounced proliferative response in PBL from immune than in PBL of uninfected animals.