Mobilization of intrasporozoite Ca2+ is essential for Theileria parva sporozoite invasion of bovine lymphocytes.

The entry of Theileria parva sporozoites into bovine lymphocytes occurs rapidly and involves a defined series of events. In the present study the role of calcium in sporozoite entry was examined. Depletion of Ca2+ from the external medium had little effect on sporozoite entry suggesting that the initial sporozoite-host cell interaction is a Ca(2+)-independent process. Sporozoite entry could, however, be inhibited by a range of Ca2+ channel blockers (verapamil, nicardipine, diltiazem) and calmodulin antagonists (TPF, chloropromazine, W7 and calmidazolium). Evidence is also presented that demonstrates that sporozoite entry is dependent on changes in sporozoite cytosolic Ca2+ caused by the release of Ca2+ from intrasporozoite stores. First, reagents that produced an influx of Ca2+ into the parasite (A23187) blocked entry. Second, depletion of intrasporozoite Ca2+ levels (10 microM A23187 + 1.0 mM EGTA) or an increase in the cytoplasmic buffering capacity of the sporozoite cytoplasm (by preloading sporozoites with MAPT/AM) inhibited invasion. Third, sporozoite entry was inhibited by TMB-8 which blocks the release of Ca2+ from intracellular stores. Lastly, treatment of sporozoites with the Ca(2+)-mobilizing agents thapsigargin, cyclopiazonic acid but not InsP3 prevented sporozoite entry. In these cases the premature release of intrasporozoite Ca2+ inhibited sporozoite binding to the host cell surface; sporozoites that bound became internalized at rates comparable to the controls. In contrast, treatment of lymphocytes with these reagents had no significant effect on sporozoite entry. Collectively these results demonstrate that the mobilization of Ca2+ from intrasporozoite stores following sporozoite binding to the host cell surface is essential for successful parasite invasion.