An ultrastructural study of the interaction in vitro between Trypanosoma theileri and bovine leucocytes

Bovine neutrophils, eosinophils and macrophages obtained from the mammary gland were cytotoxic to Trypanosoma theileri epimastigotes in vitro in the presence of specific antibody. A detailed ultrastructural study revealed that all types of effector cell phagocytosed T. theileri despite the relatively large size of this parasite (10-130 microns). Phagocytosis proceeded in the classical manner by envelopment of the trypanosome by long pseudopodia. With eosinophils and macrophages structural damage of the parasite was only observed following ingestion. There was evidence of inter-granule fusion in eosinophils prior to the discharge of granule contents into the parasitophorous vacuole; following degradation of the parasite in eosinophils some phagolysosomes opened to the exterior and released what appeared to be trypanosome debris. While eosinophils required less than 30 min to initiate serious damage to the parasite, macrophages required in excess of 2 h to produce similar damage. Conversely, neutrophils caused severe damage within 5 min. An important feature of neutrophil-mediated cytotoxicity was the extensive structural damage to the parasite that was observed before closure of the phagosome. Initially this damage was observed at the end of the trypanosome that was in contact with the neutrophil. This finding suggests that neutrophil-mediated killing of T. theileri is a combination of both extracellular and intracellular mechanisms.