Plasma Lipoproteins Promote the Release of Bacterial Lipopolysaccharide from the Monocyte Cell Surface

Abstract

Gram-negative bacterial lipopolysaccharide (LPS)1 (endotoxin) is one of the most potent and ubiquitous of the known bacterial signal molecules. Animals have sensitive mechanisms for recognizing the presence of LPS in tissues. Monocytes, macrophages, and neutrophils, which express the LPS binding receptor, CD14 (1, 2), and its signal transducer, Toll-like receptor-4 (3), are particularly sensitive to LPS (4). They respond by producing inflammatory mediators that amplify and diversify the LPS signal, triggering host defenses that wall off and destroy invading bacteria. For reasons that are not entirely clear, however, the response to LPS can also be harmful, resulting in severe sepsis, septic shock, and even death. Mechanisms that regulate responses to LPS are therefore likely to be very important for the host.HDL is the most abundant of the lipoproteins in human plasma and interstitial fluids. It can remove both phospholipids and unesterified cholesterol from cells (reviewed in Ref. 5), and HDL-mediated cellular cholesterol efflux and the subsequent delivery of HDL-cholesterol to the liver (reverse cholesterol transport) are thought to help protect animals from atherosclerosis. This study addresses another facet of the HDL particle: its ability to bind LPS and neutralize its biological activity. There is abundant evidence that HDL and other plasma lipoproteins play an important role in controlling host responses to LPS. Numerous studies have shown that complexes of LPS with HDL and other lipoproteins have little or no stimulatory activity, either in vitro or in vivo (6 -9), and there is now strong evidence that HDL and other lipoproteins can neutralize endotoxin in vivo (10 -17).Two plasma lipid transfer proteins, LPS-binding protein (LBP) (18,19) and phospholipid transfer protein (PLTP) (20), promote the binding of purified LPS to lipoproteins, whereas only LBP can facilitate LPS binding to CD14 on cell membranes (mCD14) or soluble CD14 (sCD14) in plasma (1,21,22). sCD14 can rapidly transfer LPS to mCD14 on cells (23), and it also facilitates the activation of cells that do not express mCD14 (24). Although sCD14 can also transfer LPS to HDL (19), it has been said to contribute very little to the movement of LPS to lipoproteins in whole plasma (25).In this study, we sought to determine whether HDL and plasma LPS transfer proteins could remove LPS from host cells. We found that HDL facilitates the release of cell-bound LPS, that sCD14 enhances this release, and that the removal of LPS from the cells attenuates proinflammatory responses. This new pathway of LPS traffic to lipoproteins may be important for regulating host responses to Gram-negative bacteria. These findings also raise the possibility that efflux of microbial ligands from macrophages to HDL contributes to the antiinflammatory potency of HDL in processes such as atherosclerosis. EXPERIMENTAL PROCEDURESCells and Reagents-Human monocytic THP-1 cells were cultured as described previously (26). CD14 expression was induced either by culturing the cells in 1...

Lipopolysaccharide (LPS)-binding Protein Inhibits Responses to Cell-bound LPS

“…Released and cellassociated [ 3 H]LPS were measured as previously described (35). LPS internalization was measured by protease protection of [ 3 H]LPS and by surface quenching of fluorescent LPS with trypan blue as previously described (31,34).…”

Thematic review series: The Pathogenesis of Atherosclerosis. Effects of infection and inflammation on lipid and lipoprotein metabolism mechanisms and consequences to the host

“…Additionally, infusion of HDL reduces CD14 expression on monocytes (318). Once LPS is bound to monocytes, lipoproteins have been shown to promote the release of LPS from the cell surface, further attenuating the cellular response to LPS (328). Collectively, these studies suggest that lipoproteins can help neutralize the lethal effects of LPS by accelerating its clearance from the plasma, redirecting it away from monocytes and macrophages, decreasing immune cell activation, and reducing the release of cytokines, thus attenuating LPS toxicity.…”

Acute inflammation and infection maintain circulating phospholipid levels and enhance lipopolysaccharide binding to plasma lipoproteins

“…When LPS is added to plasma or serum, the majority of the lipoprotein-bound LPS is found in the HDL fraction, whereas most of the remainder is found in LDL (22)(23)(24). In septic patients, HDL levels decline more than those of any other lipoprotein class (25).…”