In vitro induction of intracellular killing of parasitic protozoa by macrophages.

Macrophages play an important role in the defence of the host against infection by certain parasitic protozoa. Aspects of the interaction in vitro between macrophages and Trypanosoma cruzi, Toxoplasma gondii, and Leishmania spp. are reviewed. These three types of parasites are internalized by phagocytosis; the relationship established between the parasites and intracellular organelles vary, however. Whereas T. cruzi appears to escape from the phagocytic vacuole, T. gondii inhibits phagosome-lysosome fusion, and Leishmania proliferates within the phagolysosomal environment. Macrophages can be activated in vitro, by incubation with lymphokine-rich media, to kill these parasites. Destruction of these microorganisms requires that an adequate supply of oxygen metabolites be produced. Variations in the levels of endogenous scavengers of such metabolites in both macrophages and parasites may explain differences in the susceptibility of parasites to killing by activated macrophages.