Macrophage-mediated innate host defense against protozoan parasites

Macrophages are immune cells that play a pivotal role in the detection and elimination of pathogenic microorganisms. Macrophages possess a variety of surface receptors devoted to the recognition of non-self by discriminating between host and pathogen-derived structures. Recognition of foreign microorganisms by the macrophage ultimately results in phagocytosis and the eventual destruction of microorganisms by lysosomal enzymes, toxic reactive oxygen and nitrogen intermediates, and/or nutrient deprivational mechanisms. However, protozoan parasites such as Toxoplasma gondii, Trypanosoma cruzi, and Leishmania spp., parasitize macrophages, utilizing them as a host cell for their growth, replication, and/or maintenance of their life cycles. The protozoan parasites of the genus Leishmania are unique in that their intracellular replication in the host is predominantly restricted to a single cell type, the macrophage. This review focuses on the cellular processes involved in macrophage-mediated host defense against protozoan parasites, from the initial host-parasite interactions that mediate recognition to the mechanisms employed by macrophages to destroy and eliminate the pathogen. As an example model system of experimental study, we describe in more more detail the cellular interactions between macrophages and the obligate intracellular parasite of mammalian macrophages, Leishmania spp.