Plasma lipoproteins promote the release of bacterial lipopolysaccharide from the monocyte cell surface

When bacterial lipopolysaccharide (LPS) enters the bloodstream, it is thoug ht to have two general fates. If LPS binds to circulating leukocytes, it tr iggers innate host defense mechanisms and often elicits toxic reactions. If instead LPS binds to plasma lipoproteins, its bioactivity is largely neutr alized. This study shows that lipoproteins can also take up LPS that has fi rst bound to leukocytes, When monocytes were loaded with [H-3]LPS and then incubated in plasma, they released over 70% of the cell-associated [H-3]LPS into lipoproteins (predominantly high density lipoprotein), whereas in ser um-free medium the [H-3]LPS remained tightly associated with the cells. The transfer reaction could be reproduced in the presence of pure native lipop roteins or reconstituted high density lipoprotein. Plasma immunodepletion e xperiments and experiments using recombinant LPS transfer proteins revealed that soluble CD14 significantly enhances LPS release from the cells, high concentrations of EPS-binding protein have a modest effect, and phospholipi d transfer protein is unable to facilitate LPS release, Essentially all of the LPS on the monocyte cell surface can be released. Lipoprotein-mediated LPS release was accompanied by a reduction in several cellular responses to the LPS, suggesting that the movement of LPS from leukocytes into lipoprot eins may attenuate host responses to LPS in vivo.