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 target s, we studied whether cytotoxicity of amoebic trophozoites in vitro is equivalent to the induction of apoptosis or whether these target cell s die via necrosis, Based upon morphological criteria, incubation of t arget cells with amoebae resulted in necrosis, with cell swelling, rup ture of plasma membrane, and release of cell contents including nuclei c acids being detected by light and transmission electron microscopy, On the other hand, the characteristic features of apoptosis such as ce ll shrinking, surface blebbing, and chromatin condensation were not ob served, 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-media ted dUTP nick end labeling technique in combination with flow cytometr y, Consistently, cleavage of DNA was detectable upon agarose gel elect rophoresis 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 mo lecules for mediating the cytolytic activity off. histolytica. At a ti me point at which the majority of target cells showed membrane injury upon incubation with purified amoebapores, no DNA degradation was dete ctable in the victim cells, The data suggest that the target cells use d in our study undergo necrosis rather than apoptosis when they are ki lled by viable trophozoites as well as by isolated amoebapores.