Passive endocytosis of sporozoites of Theileria parva in macrophages at 1-2 degrees C.

Phagocytosis by macrophages is generally considered to involve two steps which can be experimentally dissociated: attachment, which is not energy-dependent, and interiorization, which does not occur below 15 degrees C. We have previously reported entry of the sporozoites of Theileria parva into bovine lymphocytes at 2 degrees C by a process of passive endocytosis which appears to depend entirely upon progressive circumferential binding of ligands on the parasite to receptors on the host cell membrane. We report here similar entry into macrophages in vitro at 1-2 degrees C. It is suggested that in vivo small (1 micron) spherical parasites bearing appropriate ligands can invade phagocytic or non-phagocytic host cells by a process of endocytosis that requires little or no expenditure of energy.