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The inhibition of LPS-induced cell activation by specific antagonists is a long-known phenomenon; however, the underlying mechanisms are still poorly understood. It is commonly accepted that the membrane-bound receptors mCD14 and TLR4 are involved in the activation of mononuclear cells by LPS and that activation may be enhanced by soluble LPS-binding protein (LBP). Hexaacylated Escherichia coli lipid A has the highest cytokine-inducing capacity, whereas lipid A with four fatty acids (precursor IVa, synthetic compound 406) is endotoxically inactive, but expresses antagonistic activity against active LPS. Seeking to unravel basic molecular principles underlying antagonism, we investigated phospholipids with structural similarity to compound 406 with respect to their antagonistic activity. The tetraacylated diphosphatidylglycerol (cardiolipin, CL) exhibits high structural similarity to 406, and our experiments showed that CL strongly inhibited LPS-induced TNF-alpha release when added to the cells before stimulation or as a CL/LPS mixture. Also negatively charged and to a lesser degree zwitterionic diacyl phospholipids inhibited LPS-induced cytokine production. Using Abs against LBP, we could show that the activation of cells by LPS was dependent on the presence of cell-associated LBP, thus making LBP a possible target for the antagonistic action of phospholipids. In experiments investigating the LBP-mediated intercalation of LPS and phospholipids into phospholipid liposomes mimicking the macrophage membrane, we could show that preincubation of soluble LBP with phospholipids leads to a significant reduction of LPS intercalation. In summary, we show that LBP is a target for the inhibitory function of phospholipids.

Phospholipids Inhibit Lipopolysaccharide (LPS)-Induced Cell Activation: A Role for LPS-Binding Protein