Neutral polymers elicit, and antibodies to spectrin, band 4.1 protein and cytoplasmic domain of band 3 protein inhibit the concanavalin A-mediated agglutination of human erythrocytes.

Concanavalin A (Con A) is known to agglutinate human erythrocytes if the cells are pre-treated with a proteinase or neuraminidase. We report that untreated cells can also be made to agglutinate with the lectin if the lectin-bound cells are treated with anti-Con A antibodies, or if a neutral polymer such as serum albumin, polyvinylpyrrolidone or Ficoll is added. Thus, Con A falls in the category of 'incomplete' lectins. The polymer induces Con A-agglutinability without altering the receptor number, or deformability of the cells. If the polymer is sequestered within erythrocyte ghosts, Con A is unable to agglutinate them; but the presence of the polymer only on the outer surface (as in intact cells) or on both the surfaces permits agglutinability. Thus, the site of the polymer effect resides on the outer surface of the membrane. The polymer, however, is unable to induce agglutinability in erythrocyte vesicles, whose membrane lacks skeletal proteins. The result suggests a positive role for the membrane skeleton in the process of agglutination brought about by the polymer, as is true also for the agglutination of proteinase-treated cells. In order to obtain detailed information on the proteins participating in agglutination, monospecific antibodies to spectrins, band 4.1 protein, ankyrin and the cytoplasmic domain of band 3 protein were internalized in erythrocytes. It is found that anti-spectrin and anti-band 3 cytoplasmic domain, but not their Fab's, inhibit the Con A-mediated agglutinability partially, and anti-4.1 antibodies, as well as the Fab's, inhibit the agglutinability substantially. Anti-ankyrin, however, was without any effect. The results confirm a positive role for the membrane skeleton in the Con A-mediated agglutination of normal erythrocytes in the presence of a neutral polymer, or in proteinase treated cells. We also provide evidence for requirement of Mg-ATP in the agglutination process.