Enhancing effect of concanavalin A on the hemolytic activity of anti-Forssman IgG: the role of C1.

Native tetravalent Con A and the divalent acetylated derivative increased the hemolytic titer (i.e., the reciprocal of the antibody dilution required to give an average of one lytic site per sheep erythrocyte) of IgG antibodies against Forssman antigen by up to 225% with guinea pig and human complement. Although the average number of lytic sites generated at each antibody concentration increased, the slope of the titration curve did not change. Other lectins with the same or different sugar specificity either augmented or inhibited hemolysis but were less potent than Con A. Augmentation by Con A was consistent with the ability of lectin on the cell surface to bind but not activate guinea pig C1. Thus it appears that cell-bound Con A and IgG yield a complex that behaves like a doublet of IgG antibody molecules in its ability to fix and activate C1, when activation is dependent on the IgG component. In contrast, the highest dose of Con A inhibited by at least 50% the hemolytic activity of IgG antibodies against either a sugar-free protein (HSA) or a protein reactive with Con A (human myeloma IgE) using cells to which these antigens were coupled with chromic chloride. This suggests that the identity, density, and/or mode of presentation of the antigen on the cell surface may be important determinants of lectin-induced augmentation. Although both the enhancement or inhibition by Con A in the presence of whole C correlated with the number of C1 molecules bound and activated, there was no correlation with the ability of the lectin to agglutinate the cells.