[Entamoeba histolytica: I. Mechanism of cytotoxic activity]

Cytotoxic action against K562-tissue culture cells was investigated under various conditions with a Chromium-release-assay. When amoebae and target cells were centrifuged together, pathogenic strains of amoebae induced a very fast increase of target cell lysis (up to 50% of maximum lysis after 10 minutes). Only a minor degree of target cell lysis resulted, however, when amoebae and K562 cells were kept in suspension. When amoebae were eliminated selectively by addition of complement 10 minutes after starting the experiment, this fast increase of lysis could not be prevented. These observations suggest that the cytotoxic action might take place in two distinct phases. The first step ("lethal hit") seems to be temperature-independent, whereas a temperature of 37 degrees C is necessary for the second step to occur during which cytoplasmic material is released (chromium release). The presence of amoebae is not necessary for the second step. When amoebae together with and target cells are kept in suspension, amoebae lost their capability of setting the "lethal hit" with increasing time of coincubation. It seems, as if the "lethal hit" cannot be accomplished effectively under the conditions of suspension: cytotoxic substances released by the amoebae cannot be transferred to the target cells and are lost in the fluid phase. Thereby, the amoebae are depleted of such substances. Thus, a stable contact between amoebae and target cells for at least a few minutes seems to be necessary for the expression of cytotoxicity.