Binding and neutralization of endotoxin by Limulus antilipopolysaccharide factor.

In order to examine the ability of Limulus antilipopolysaccharide factor (LALF) to bind lipopolysaccharide (LPS), we purified LALF to homogeneity from Limulus amoebocyte lysate and coupled it covalently to agarose beads. LALF-coupled beads captured more tritiated LPS from rough and smooth strains of gram-negative bacteria than did control human serum albumin-coupled beads. Unlabeled homologous and heterologous LPS competed for the binding of 3H-LPS to LALF-coupled beads. LALF bound LPS in a dose-dependent manner as assessed by the precipitation of LPS-LALF complexes with 50% saturated ammonium sulfate. We also studied the ability of LALF to neutralize LPS. LPS preincubated with LALF was less mitogenic for murine splenocytes, was less pyrogenic in the rabbit fever assay, was less lethal in mice which had been sensitized to LPS with actinomycin D, and induced less fever, neutropenia, and pulmonary hypertension when infused into sheep. Our findings extend prior studies which suggested that LALF binds to and neutralizes LPS from multiple strains of gram-negative bacteria.

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.

Role of interleukin-1 in augmenting serum neutralization of bacterial lipopolysaccharide.

We have previously described an assay to quantify the serum neutralization of bacterial lipopolysaccharide which is based on a spectrophotometric Limulus amoebocyte lysate test (T.J. Novitsky, P.F. Roslansky, G.R. Siber, and H.S. Warren, J. Clin. Microbiol. 21:211-216, 1985). Studies since have shown that serum samples drawn from patients with leukemia and fever, gram-negative or gram-positive bacterial infections, or shock caused by gram-negative bacteria neutralize approximately 10-fold more lipopolysaccharide than do samples from normal controls. These findings suggested that the increased neutralization might reflect an acute-phase response and raised the question of whether it might be under the control of interleukin-1. To answer this question, we studied the neutralization of lipopolysaccharide in serum samples drawn from rabbits before and after the administration of crude interleukin-1, prepared from activated macrophage supernatants, and recombinant human interleukin-1. Crude interleukin-1 induced a 5.7-fold increase in serum neutralization 24 h after intravenous injection, and cloned interleukin-1 induced a 3.0-fold increase (P less than or equal to 0.01 and 0.05, respectively). In individual rabbits given identical doses of crude interleukin-1 on a weight basis, the serum-neutralizing ability correlated significantly with three activities of interleukin-1: rise in temperature (r2 = 0.558; P less than or equal to 0.01), decrease in serum iron (r2 = 0.534; P less than or equal to 0.01), and increase in serum copper (r2 = 0.323; P less than or equal to 0.05). We conclude that the increase in neutralization of bacterial lipopolysaccharide by serum samples drawn from patients with inflammatory states is mediated, at least in part, by interleukin-1, presumably through the induction of acute-phase serum proteins.