Escherichia coli Braun lipoprotein induces a lipopolysaccharide-like endotoxic response from primary human endothelial cells

All bacteria contain proteins in which their amino-terminal cysteine residu e is modified with N-acyl S-diacylglycerol functions, and peptides and prot eins bearing this modification are immunomodulatory. The major outer membra ne lipoprotein of Escherichia coli, the Braun lipoprotein (BLP), is the pro totypical triacylated cysteinyl-modified protein. We find it is as active a s LPS in stimulating human endothelial cells to an inflammatory phenotype, and a BLP-negative mutant of E. coli was less inflammatory than its parenta l strain. While the lipid modification was essential, the lipidated protein was more potent than a lipid-modified peptide. BLP associates with CD14, b ut this interaction, unlike that with LPS, was not required to elicit endot helial cell activation. BLP stimulated endothelial cell E-selectin surface expression, IL-6 secretion, and up-regulation of the same battery of cytoki ne mRNAs induced by LPS. Quantitative microarray analysis of 4400 genes sho wed the same 30 genes were induced by BLP and LPS, and that there was near complete concordance in the level of gene induction. We conclude that the l ipid modification of at least one abundant Gram-negative protein is essenti al for endotoxic activity, but that the protein component also influences a ctivity. The equivalent potency of BLP and LPS, and their complete concorda nce in the nature and extent of endothelial cell activation show that E. co li endotoxic activity is not due to just LPS. The major outer membrane prot ein of E. coli is a fully active endotoxic agonist for endothelial cells.