RATIONALE:: Systemic inflammation has emerged as a key pathophysiological process which induces multi-organ injury and causes serious human diseases. Endothelium is critical in maintaining cellular and inflammatory homeostasis, controlling systemic inflammation, and progression of inflammatory diseases. We postulated that endothelium produces and releases endogenous soluble factors to modulate inflammatory responses and protect against systemic inflammation. OBJECTIVE:: To identify endothelial cell-released soluble factors that protect against endothelial barrier dysfunction and systemic inflammation. METHODS AND RESULTS:: We found that conditioned medium (CM) of endothelial cells (ECs) inhibited cyclooxgenase-2 and interleukin-6 expression in macrophages stimulated with lipopolysaccharide (LPS). Analysis of CM extracts by liquid chromatography–mass spectrometry showed the presence of 5-methoxytryptophan (5-MTP) but not other related tryptophan metabolites. Furthermore, endothelial cells-derived 5-MTP suppressed LPS-induced inflammatory responses and signaling in macrophages and endotoxemic lung tissues. LPS suppressed 5-MTP level in EC-CM and reduced serum 5-MTP level in the murine sepsis model. Intraperitoneal injection of 5-MTP restored serum 5-MTP accompanied by inhibition of LPS-induced endothelial leakage and suppression of LPS- or cecal ligation and puncture (CLP)-mediated pro-inflammatory mediators overexpression. 5-MTP administration rescued lungs from LPS-induced damages and prevented sepsis-related mortality. Importantly, compared with healthy subjects, serum 5-MTP level in septic patients was decreased by 65%, indicating an important clinical relevance. CONCLUSIONS:: We conclude that 5-MTP belongs to a novel class of endothelium-derived protective molecules which defend against endothelial barrier dysfunction and excessive systemic inflammatory responses.
