ABSTRACT
Macrophage-mediated effector mechanisms against the protozoan parasite Entamoeba histolytica were studied. Unstimulated macrophages were inefficient at killing E. histolytica trophozoites in vitro and were killed by the trophozoites. Conversely, immature cells of the mononuclear phagocyte lineage (promonocytes) were shown to display a strong spontaneous amebicidal activity. The acquisition of macrophage amebicidal activity following cytokine treatment was investigated. Gamma interferon, tumor necrosis factor alpha, and macrophage colony-stimulating factor 1, or combinations thereof, were shown to endow murine bone marrow-derived macrophages with significant amebicidal activity. Low doses of gamma interferon and tumor necrosis factor alpha and of gamma interferon and colony-stimulating factor 1 were shown to act synergistically in this phenomenon. This enhancement of amebicidal activity was shown to operate on bone marrow-derived macrophages, elicited peritoneal macrophages, and, to a much lesser extent, spleen macrophages. Although acquisition of amebicidal activity was associated with a strong respiratory burst, the addition of oxygen-free radical scavengers showed that the killing activity was approximately 45% H2O2 dependent. In addition, amebicidal activity by macrophages was shown to be contact dependent and was inhibited by 61% with the protease inhibitor tosyl lysyl chloromethyl ketone. Our results indicate that immunologic production of gamma interferon, tumor necrosis factor alpha, and colony-stimulating factor 1 could be important in the activation of macrophages for host defense against amebiasis and that promonocytes are strong effector cells against virulent amebae.
