Lipid A-like molecules that antagonize the effects of endotoxins on human monocytes.

Abstract

Lipopolysaccharide (LPS) endotoxin is implicated as the bacterial product responsible for the clinical syndrome of Gram-negative septicemia. Although the lipid A domain of LPS appears to be responsible for the toxicity of endotoxin, lipid A from the photosynthetic bacterium Rhodobacter sphaeroides (RSLA) and a disaccharide precursor of lipid A from enteric bacteria, termed lipid IVA, have little activity on human cells. Using the human promonomyelocytic cell line THP-1 and human monocytic cells, we now show that both lipid IVA and RSLA are antagonists of LPS. Complete, apparently competitive, inhibition of LPS activity is possible at a 10-100-fold excess of antagonist, as judged by measuring the release of cytokines and prostaglandin E2. Both antagonists prevent monocyte stimulation by endotoxin extracted from a variety of Gram-negative bacteria. Cells pretreated with either inhibitor and subsequently washed still show attenuated responses to LPS. Stimulation of monocytes by whole Gram-negative bacteria is also antagonized in a dose-dependent manner. Lipid X has no inhibitory effect in the same dose range as lipid IVA and RSLA. These findings rule out LPS sequestration as the explanation for the observed antagonism. Neither inhibitor alters monocyte stimulation by phorbol 12-myristate 13-acetate, Staphylococcus aureus, or purified protein derivative, demonstrating specificity for LPS. Although RSLA appears to inhibit LPS when tested with macrophages from both humans and mice, lipid IVA had the unique ability to act as an LPS antagonist with human-derived cells but to exhibit LPS-like effects with murine-derived cells. Like LPS, lipid IVA stimulated the release of both tumor necrosis factor alpha and arachidonic acid from murine-derived RAW 264.7 macrophage tumor cells. The range of concentrations necessary for lipid IVA to induce LPS-like effects in murine cells was similar to that necessary to antagonize the actions of LPS in human monocytes. The agonist activities of lipid IVA were completely inhibitable by RSLA. This unique species-dependent pharmacology observed with lipid IVA may reflect differences between human and murine LPS receptors. RSLA and lipid IVA may be useful in defining the role of LPS in Gram-negative bacterial infections and may prove to be prototypical therapeutic agents for the treatment of Gram-negative septicemia.