Role of normal serum in the binding of lipopolysaccharide to IgG fractions from rabbit antisera to Escherichia coli J5 and other gram-negative bacteria.

Because lipopolysaccharide (LPS) bound to lipoprotein is less active than unbound LPS in multiple assay systems, the binding of radiolabeled LPS to lipoproteins in sera prepared from normal rabbits and rabbits made hyperimmune to Escherichia coli J5 were compared. LPS-lipoprotein binding in hyperimmune sera to E. coli J5 was not greater than that in normal serum as assessed by ultracentrifugation, but more LPS was precipitated from hyperimmune antisera than normal sera under conditions designed to precipitate LPS-lipoprotein complexes with calcium and dextran. Radiolabeled LPS was precipitated by delipidated antisera and fractions of IgG purified by anion exchange chromatography, but the precipitation was dependent on the presence of normal serum in the reaction mixture. These data suggest that a fluid-phase RIA done in the presence of normal serum may facilitate the detection of IgG in antisera raised to E. coli J5 that binds to heterologous smooth LPS.