Inhibition of lymphocyte invasion by sporozoites and the transformation of trophozoite infected lymphocytes in vitro by serum from Theileria annulata immune cattle.

Serum from calves immune to Theileria annulata inhibited sporozoite-induced infection and transformation of peripheral blood leucocytes (PBL) into macroschizont infected lymphoblastoid cells in vitro. Invasion by sporozoites was suppressed by some sera, especially that obtained after challenge, but in many instances suppression of infection could be attributed only to retardation of development of PBL infected with intracellular trophozoites into proliferating macroschizont infected cells. Suppression occurred both in cultures of PBL from susceptible calves and in cultures of PBL from T. annulata immune calves. More uninfected than infected blast cells were found both in cultures of PBL from susceptible calves and in cultures of PBL from immune animals. Results suggested that proliferation of uninfected PBL from susceptible calves was initiated by the transformation of sporozoite infected cells, whilst proliferation of uninfected PBL from immune calves also incorporated a component which was a mitotic response of sensitised lymphocytes to sporozoites (i.e. parasite antigen). Thymidine incorporation by cultures was a measure of proliferation of both uninfected and infected blast cells. It could be used, therefore, to monitor parasite development only where proliferation of total blast cells was directly related to proliferation of infected blasts, as in cultures of PBL from susceptible calves. This study suggests that resistance of cattle to tick derived challenge with sporozoites is mediated initially by serum factors acting both on sporozoites and on the development of intracellular trophozoites to schizonts; that the proliferation of uninfected lymphocytes in naive cattle infected with tropical theileriosis is a direct response to the proliferation of Theileria infected cells and that the proliferation of uninfected lymphocytes in Theileria immune cattle is a combined response to the proliferation of Theileria infected cells and parasite antigen.