Antagonistic lipopolysaccharides block E. coli lipopolysaccharide function at human TLR4 via interaction with the human MD‐2 lipopolysaccharide binding site

Summary Lipopolysaccharides containing underacylated lipid A structures exhibit reduced abilities to activate the human (h) Toll‐like receptor 4 (TLR4) signalling pathway and function as potent antagonists against lipopolysaccharides bearing canonical lipid A structures. Expression of underacylated lipopolysaccharides has emerged as a novel mechanism utilized by microbial pathogens to modulate host innate immune responses. Notably, antagonistic lipopolysaccharides are prime therapeutic candidates for combating Gram negative bacterial sepsis. Penta‐acylated msb B and tetra‐acylated Porphyromonas gingivalis lipopolysaccharides functionally antagonize hexa‐acylated Escherichia coli lipopolysaccharide‐dependent activation of hTLR4 through the coreceptor, hMD‐2. Here, the molecular mechanism by which these antagonistic lipopolysaccharides act at hMD‐2 is examined. We present evidence that both msb B and P. gingivalis lipopolysaccharides are capable of direct binding to hMD‐2. These antagonistic lipopolysaccharides can utilize at least two distinct mechanisms to block E. coli lipopolysaccharide‐dependent activation of hTLR4. The main mechanism consists of direct competition between the antagonistic lipopolysaccharides and E. coli lipopolysaccharide for the same binding site on hMD‐2, while the secondary mechanism involves the ability of antagonistic lipopolysaccharide–hMD‐2 complexes to inhibit E. coli lipopolysaccharide–hMD‐2 complexes function at hTLR4. It is also shown that both hTLR4 and hMD‐2 contribute to the species‐specific recognition of msb B and P. gingivalis lipopolysaccharides as antagonists at the hTLR4 complex.