Pathogenic and nonpathogenic strains of Entamoeba histolytica can be differentiated by monoclonal antibodies to the galactose-specific adherence lectin.

Entamoeba histolytica infection results in either asymptomatic colonization or invasive colitis and liver abscess. E. histolytica isolates from patients with invasive disease have characteristic isoenzyme profiles (pathogenic zymodemes), suggesting a role for parasite factors in determining the severity of infection. A galactose-specific cell surface lectin from a pathogenic zymodeme was shown to mediate in vitro adherence to human colonic mucins and contact-dependent killing of target cells. Six nonoverlapping antigenic determinants were identified on the 170-kilodalton heavy subunit of the pathogenic lectin. Anti-lectin monoclonal antibodies (MAb) directed against epitopes 1 and 2 enhanced adherence whereas MAb to epitopes 3 through 6 either inhibited or had no effect on adherence. We tested 50 pathogenic and nonpathogenic strains for reactivity to these anti-lectin MAb by radioimmunoassay. MAb to epitopes 1 through 6 reacted in the radioimmunoassay with all 16 pathogenic zymodeme strains tested. In contrast, only MAb to epitopes 1 and 2 bound to the lectin from nonpathogenic strains. Western immunoblots with anti-lectin antibodies showed that the 170-kilodalton heavy subunit was present in the nonpathogenic amebae. Adherence of the nonpathogenic SAW 760 strain to human erythrocytes was enhanced by MAb to epitope 1 and blocked by galactose, confirming the presence of a functionally active lectin. A lectin radioimmunoassay based on MAb to epitopes 1 and 3 proved to be a simple and rapid method to distinguish pathogenic from nonpathogenic amebae in culture. Further exploration of the functional consequences of the antigenic differences demonstrated for the lectin may lead to a better understanding of its role in pathogenesis.