L. Gutierrezkobeh et al., A MECHANISM OF ACQUIRED-RESISTANCE TO COMPLEMENT-MEDIATED LYSIS BY ENTAMOEBA-HISTOLYTICA, The Journal of parasitology, 83(2), 1997, pp. 234-241
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.