Necrosis versus apoptosis as the mechanism of target cell death induced by Entamoeba histolytica.

The human pathogen Entamoeba histolytica is known to kill a variety of host cells, including leukocytes. Using human myeloid cells as targets, we studied whether cytotoxicity of amoebic trophozoites in vitro is equivalent to the induction of apoptosis or whether these target cells die via necrosis. Based upon morphological criteria, incubation of target cells with amoebae resulted in necrosis, with cell swelling, rupture of plasma membrane, and release of cell contents including nucleic acids being detected by light and transmission electron microscopy. On the other hand, the characteristic features of apoptosis such as cell shrinking, surface blebbing, and chromatin condensation were not observed. Moreover, internucleosomal fragmentation of genomic DNA within target cells as a characteristic feature of apoptotic cell death did not occur as judged by the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling technique in combination with flow cytometry. Consistently, cleavage of DNA was detectable upon agarose gel electrophoresis only after a substantial part of the target cell population had already been lysed. We also analyzed the mechanism of cell death induced by amoebapores, pore-forming peptides and primary candidate molecules for mediating the cytolytic activity of E. histolytica. At a time point at which the majority of target cells showed membrane injury upon incubation with purified amoebapores, no DNA degradation was detectable in the victim cells. The data suggest that the target cells used in our study undergo necrosis rather than apoptosis when they are killed by viable trophozoites as well as by isolated amoebapores.