Lipid X, a precursor of
lipid A (the toxic moiety of
endotoxin), has been shown to protect animals from the lethal effects of
endotoxin challenge. We investigated the mechanism of action of
lipid X and
3-aza-lipid X, a diamino-analogue, in vitro, using the ability of
lipopolysaccharide (LPS) to prime neutrophils for an enhanced release of toxic
oxygen radicals.
Lipid X and
3-aza-lipid X inhibited LPS-induced neutrophil priming in a concentration-dependent manner. At high concentrations,
3-aza-lipid X was a partial agonist of priming.
Lipid X was found to inhibit LPS-induced priming by directly interacting with the neutrophil in contrast to
polymyxin B, which neutralized LPS by binding to it. Increasing concentrations of
lipid X shifted the LPS dose response curve of neutrophils rightward but did not prevent maximum priming at higher LPS concentrations, a finding consistent with competitive inhibition. These results suggest that
lipid X, a compound structurally related to
lipid A, may block neutrophil priming by competing with LPS for cellular binding sites.
Lipid X appears to have a novel mechanism of inhibiting LPS effect and may have efficacy in the treatment of gram-negative
sepsis.