Entamoeba histolytica: cytopathogenicity, including serum effects on contact-dependent and toxin-induced lysis of hamster kidney cell monolayers

Some properties of toxin, isolated from extracts of axenically cultivated Entamoeba histolytica or excreted by intact amoebae, were investigated using a toxicity assay in microplates with monolayers of baby hamster kidney cells. Preparative isoelectric focusing showed that the highest cytotoxic activity was present in a fraction of antigen containing protein bands with an isoelectric point between 4.5 and 5. Activity of the toxin was stable between pH 4 and 10. Nonimmune rabbit serum and concanavalin A, coupled to Sepharose beads, were able to bind a large amount of toxin. Cytotoxicity of antigen was inhibited by specific immune IgG and by unknown factors in nonimmune serum with a molecular weight between 50,000 and 100,000. The toxin was inactivated by trypsin, but not by trypsin inhibitor. Its activity was thiol dependent. Serum also had a marked inhibitory effect on contact lysis of BHK cells induced by intact trophozoites. A considerable reduction of both contactdependent and toxin-induced Cytopathogenicity was observed when Diamond's TP-S-1 medium was used in the assay, in which the TP broth had been autoclaved. It is suggested that Entamoeba histolytica exocytozes toxin, which acts on adjacent cells during close contact.