INDUCTION OF COMPLEMENT RESISTANCE IN CLONED PATHOGENIC ENTAMOEBA-HISTOLYTICA

The lytic effect of complement activated through the alternative pathw ay (AP) was studied on pathogenic and nonpathogenic Entamoeba histolyt ica recently isolated from stool samples. Recent nonpathogenic isolate s were nearly unaffected by exposure to AP whereas recent pathogenic s tool isolates were highly susceptible to AP dependent complement-media ted lysis. Complement susceptible pathogenic stool isolates developed complement resistance in vivo during hamster liver passage and in vitr o during cultivation in the presence of increasing concentrations of n ormal human serum (NHS). Since a clone of pathogenic HM-1:IMSS which i nitially was highly susceptible also acquired complement resistance du ring cultivation in the presence of NHS, it is concluded that compleme nt resistance was caused by induction rather than by selection alone. Because cultivation in the presence of heat-inactivated NHS did not af fect complement susceptibility of the cloned HM-1:IMSS, complement act ivation itself might induce complement resistance in pathogenic E. his tolytica.