Plasmodium lophurae: Lectin-mediated agglutination of infected red cells and cytochemical fine-structure detection of lectin binding sites on parasite and host cell membranes

Abstract

Erythrocytes infected with the avian malaria Plasmodium lophurae were specifically agglutinated with the lectin concanavalin A (Con A) at room temperature, but not at 0 ° C or when cells were fixed prior to lectin exposure; by contrast, normal duckling cells were not agglutinated by Con A. The degree of agglutination of infected red cells was correlated with the size of the parasite. No differences were observed in the agglutination reactions of normal and infected cells with the lectins soybean agglutinin (SBA), wheat germ agglutinin (WGA), and ricin. The degree of agglutination was: WGA > ricin > SBA > Con A. Fixed cells and cells exposed at 0 C to WGA, ricin, and SBA gave similar patterns of agglutination as did cells reacted with these lectins at room temperature. Erythrocyte-free P. lophurae, fixed or living, pretreated with trypsin-deoxyribonuclease or untreated with these enzymes, were not agglutinated with any of the lectins tested. Lectin-mediated agglutination was inhibited and reversed by the presence of lectin-specific saccharides. Trypsinized normal red cells showed an increased agglutinability with Con A; this agglutinability was only slightly reduced by low temperature. The degree of agglutination of trypsinized infected cells by Con A was similar to untreated infected red cells at room temperature. Lectin binding to the cell surface was visualized electron microscopically by the use of ferritin-conjugated lectins. Ferritin was distributed densely and randomly with ricin and WGA; a less dense distribution was observed with SBA, and with Con A the ferritin-lectin was sparse and patchy. The parasitophorous vacuolar membrane (PVM) and the plasma membrane (PM) of free parasites did not bind ricin, SBA, or WGA; on rare occasions localized patches of ferritin-Con A were seen on the surface membranes of the parasite. The results indicate: the agglutination of P. lophurae-infected cells by Con A is a consequence of a decrease in the electrostatic repulsion of infected red cells and a clustering of the surface saccharides rather than an increase in the number of Con A binding sites; the specific saccharides, i.e., glucose, galactose, and N-acetylglucosamine, present on the erythrocyte surface are either cryptic, totally absent, or present at exceedingly low levels in the parasite surface membranes; the PVM formed by red cell endocytosis of the parasite is modified in such a way that its surface characteristics are plasmodium-like.