Polymyxin B Suppresses the Endotoxin Inhibition of Concanavalin A-mediated Erythrocyte Agglutination

The lectin agglutinability of human erythrocytes has been utilized to examine interactions of gram-negative endotoxin with mammalian cell plasma membranes. Erythrocytes treated in buffer with Escherichia coli 0127:B8 lipopolysaccharide (LPS) or Salmonella minnesota Re595 glycolipid for 1 h became resistant to agglutination by the lectin concanavalin A (ConA) in buffer free of LPS or glycolipid. Polymyxin B, a cationic cyclic lipopeptide which specifically binds to the lipid A toxophore, was tested for possible effects on the LPS and glycolipid inhibition of Con A erythrocyte agglutination. The presence of polymyxin B during the initial 1-h treatment with LPS or glycolipid blocked the ability of the endotoxins to render erythrocytes refractory to agglutination by ConA. Inhibition by polymyxin B was stoichiometric, and in repeated experiments, LPS was completely suppressed in the hemagglutination assay at a polymyxin B:LPS weight ratio of 1:4.1 (increasing polymyxin concentration, constant LPS concentration) and 1:5.1 (constant polymyxin concentration, increasing LPS concentration). These stoichiometry values are similar to values obtained for inhibition by polymyxin B of LPS lymphoid cell activation. It was concluded, therefore, that endotoxin inhibition of ConA erythrocyte agglutination reflects interactions of erythrocyte membranes with the lipid A region of endotoxin. In addition, the stoichiometry of polymyxin B inhibition suggests a similar extent of lipid A-dependent LPS interaction with erythrocytes and lymphoid cells.