An analogue of lipid A and LPS from Rhodobacter sphaeroides inhibits neutrophil responses to LPS by blocking receptor recognition of LPS and by depleting LPS‐binding protein in plasma

When incubated with lipopolysaccharide (LPS) in the presence of plasma, neutrophils become primed for enhanced release of superoxide in response to triggering by formyl‐Met‐Leu‐Phe (fMLP). The effect of LPS on phagocytes is inhibited by a synthetic lipid A precursor, LA‐14‐PP (lipid IVa) or by LPS from Rhodobacter sphaeroides (Rs). We studied the mechanisms by which LA‐14‐PP or Rs‐LPS inhibited LPS‐induced responses. When neutrophils were exposed to LA‐14‐PP or Rs‐LPS for 3 min and then to Escherichia coli‐LPS, the antagonists inhibited priming for superoxide release, and also blocked up‐regulation of CD11b and adherence. This inhibition was dependent on plasma, was not overcome by higher amounts of E. coli‐LPS or plasma, and was not observed at 0°C, suggesting that E. coli‐LPS was not able to interact with its receptor or other cellular recognition molecule in neutrophils that had been exposed to the antagonists. The alternative possibility that LA‐14‐PP or Rs‐LPS depleted a plasma cofactor, resulting in inhibition of priming, was investigated by using LPS from Porphyromonas gingivalis (Pg) and Bordetella pertussis (Bp). These LPS primed neutrophils in a plasma‐dependent and CD14‐dependent manner, but were not blocked by LA‐14‐PP or Rs‐LPS. When sub‐optimal concentrations of plasma were exposed to LA‐14‐PP or Rs‐LPS, and then mixed with Pg‐IPS or Bp‐LPS, followed by incubation with neutrophils, priming and up‐regulation of GD11b were inhibited, and this inhibition was overcome by increasing the concentration of plasma. Binding of LPS‐binding protein (LBP) in plasma to immobilized E. coli‐LPS was inhibited by pre‐incubation of plasma with LA‐14‐PP or Rs‐LPS. Together with the result that treatment of plasma with anti‐LBP antibody abolished the cofactor activity of plasma, these results indicated that LA‐14‐PP and Rs‐LPS depleted LBP from plasma, resulting in inability of LPS to act on neutrophils. Thus LA‐14‐PP and Rs‐LPS inhibited the action of LPS on neutrophils by at least two mechanisms, blocking of LPS receptor recognition and depletion of the cofactor LBP.