Monophosphoryl Lipid A Induces Endotoxin Tolerance in Endothelial Cells

Abstract

Monophosphoryl lipid A (MPLA) is a lipid A TLR4 agonist that exhibits a mild inflammatory effect compared to lipopolysaccharide (LPS). As endothelial cells regulate vascular inflammation, we hypothesized that MPLA treatment would induce tolerance in endothelial cells subsequently challenged with LPS. We cultured human umbilical vein endothelial cells (HUVECs) and exposed them to MPLA or LPS for 16 hours and then allowed them to sit in fresh media for 24 hours. Afterwards, cells were exposed to LPS for an additional 6 hours. Inflammation was assessed by quantifying IL‐6 and G‐CSF levels in the culture media. Pre‐treatment with MPLA reduced LPS‐induced IL‐6 secretion by 26% ± 5% compared to LPS alone. LPS pretreatment induced a similar reduction in IL‐6 after repeat LPS exposure (25% ± 7%, n=4 per group). MPLA and LPS also induced endotoxin tolerance of GCSF secretion compared to LPS alone with a reduction of GCSF levels by 46% and 38%, respectively. MPLA alone induced only minimal secretion of IL‐6 and GCSF (227.04 ± 9.68 pg/ml and 18.81 ± 1.19 pg/ml, respectively) compared to LPS alone (1634.92 ± 76.92 pg/ml for IL‐6 and 291.29 ± 33.18 pg/ml for GCSF). These studies demonstrate the ability of MPLA to induce endotoxin tolerance in HUVECs with potency equal to that of LPS, but without the associated pro‐inflammatory response.