The effect of the host lymphocyte lysosomal system on the developing intracellular Theileria annulata sporozoites.

The fate of developing intracellular Theileria annulata sporozoites within bovine peripheral blood lymphocytes (PBL) was investigated in vitro using acid phosphatase ultracytochemistry. The intracellular sporozoites, while they developed into trophozoites, fed, and transformed into schizonts, provoked intense host lysosomal activity as early as 30 min after they were mixed with lymphocytes. Although viable developing trophozoites failed to fuse with lysosomal vesicles, thereby avoiding enzymatic digestion, the dead parasites readily formed 'phagolysosomes', resulting in lysosomal degranulation and trophozoite digestion. Possible factors responsible for the observed lack of lysosomal fusion with viable trophozoites are discussed.