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.

Control of endotoxin activity and interleukin-1 production through regulation of lipopolysaccharide-lipoprotein binding by a macrophage factor.

Lipopolysaccharides (LPSs) extracted from gram-negative bacteria are much less active when bound to serum lipoproteins. We present evidence here that the binding of radiolabeled LPS extracted from Escherichia coli O113 and Salmonella typhimurium to lipoproteins in rabbit serum is increased 8 to 24 h after a single intravenous injection of homologous or heterologous LPS. Supernatants of activated macrophages containing interleukin-1 also stimulate increased binding. The isolated product of this binding does not induce the production of interleukin-1 by macrophages in vivo or in vitro and is unable itself to stimulate increased binding of LPS to lipoprotein. Normal rabbit sera spiked with lipoprotein fractions prepared from tolerant but not normal rabbit sera bind increased amounts of LPS. These data suggest that there may exist a self-regulated mechanism for decreasing the toxicity of LPS and the production of LPS-induced interleukin-1; this mechanism is controlled by a macrophage factor and functions through altering the binding of LPS to certain serum lipoproteins.