A MECHANISM OF ACQUIRED-RESISTANCE TO COMPLEMENT-MEDIATED LYSIS BY ENTAMOEBA-HISTOLYTICA

Some Entamoeba histolytica strains resist complement-mediated lysis by serum. Susceptible and resistant strains activate the complement syst em equivalently, but resistant amebas evade killing by membrane attack complexes. Our objective was to determine the mechanism by which trop hozoites of E. histolytica resist lysis by human serum. Amebas were ma de resistant to lysis by incubation with increasing concentrations of normal human serum. The possibility that resistant cells ingest membra ne attack complexes was explored by subcellular fractionation of susce ptible and resistant trophozoites treated with sublytic concentrations of human serum containing radiolabeled C9. In both cases, most of the label was in the fractions containing plasma membrane. The susceptibl e strain consistently showed more label associated with these fraction s than the resistant strain. Thus, the possibility that the membrane a ttack complexes were released to the medium was explored. Both resista nt and susceptible trophozoites release to the medium similar amounts of material excluded by Sepharose CL-2B in the presence or absence of normal human serum. Labeled C9 elutes together with the main bulk of p roteins from the medium; this indicates that it is not in vesicles or high molecular weight aggregates. Coincubation of susceptible amebas w ith lysates of resistant trophozoites confers resistance to susceptibl e cells within 30 min. Resistance to lysis by serum can also be acquir ed by susceptible amebas after coincubation with lysates from human er ythrocytes or after feeding them with whole human red blood cells. Res istant but not susceptible trophozoites show intense immunofluorescent staining on their surface with anti-human erythrocytic membrane antib ody. These results suggest that amebas acquire resistance to lysis by serum by incorporating into their membranes complement regulatory prot eins.