Soluble CD14 acts as a shuttle in the neutralization of lipopolysaccharide (LPS) by LPS-binding protein and reconstituted high density lipoprotein.
We have recently shown that lipopolysaccharide (LPS)-binding protein (LBP) is a lipid transfer protein that catalyzes two distinct reactions: movement of bacterial LPS (endotoxin) from LPS micelles to soluble CD14 (sCD14) and movement of LPS from micelles to reconstituted high density lipoprotein (R-HDL) particles. Here we show that LBP facilitates a third lipid transfer reaction: movement of LPS from LPS-sCD14 complexes to R-HDL particles. This action of LBP is catalytic, with one molecule of LBP enabling the movement of multiple LPS molecules into R-HDL. LBP-catalyzed movement of LPS from LPS-sCD14 complexes to R-HDL neutralizes the capacity of LPS to stimulate polymorphonuclear leukocytes. Our findings show that LPS may be transferred to R-HDL either by the direct action of LBP or by a two-step reaction in which LPS is first transferred to sCD14 and subsequently to R-HDL. We have observed that the two-step pathway of LPS transfer to R-HDL is strongly favored over direct transfer. Neutralization of LPS by LBP and R-HDL was accelerated more than 30-fold by addition of sCD14. Several observations suggest that sCD14 accelerates this reaction by serving as a shuttle for LPS: addition of LBP and sCD14 to LPS micelles resulted in LPS-sCD14 complexes that could diffuse through a 100-kD cutoff filter; LPS-sCD14 complexes appeared transiently during movement of LPS to R-HDL facilitated by purified LBP; and sCD14 could facilitate transfer of LPS to R-HDL without becoming part of the final LPS-R-HDL complex. Complexes of LPS and sCD14 were formed transiently when LPS was incubated in plasma, suggesting that these complexes may play a role as intermediates in the neutralization of LPS under physiological conditions. These findings detail a new activity for sCD14 and suggest a novel mechanism for lipid transfer by LBP.
Wurfel
M M
MM
Laboratory of Cellular Physiology and Immunology, Rockefeller University, New York 10021, USA.
Hailman
E
E
Wright
S D
SD
D000209:Acute-Phase Proteins; D015703:Antigens, CD / Q000378:metabolism*; D015214:Antigens, Differentiation, Myelomonocytic / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism / Q000494:pharmacology*; D006801:Humans; D018950:Lipopolysaccharide Receptors; D008070:Lipopolysaccharides / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008075:Lipoproteins, HDL / Q000378:metabolism / Q000494:pharmacology*; D008562:Membrane Glycoproteins; D008970:Molecular Weight; D000099231:Lipopolysaccharide-Binding Protein
D000209:Acute-Phase Proteins; D015703:Antigens, CD; D015214:Antigens, Differentiation, Myelomonocytic; D002352:Carrier Proteins; D018950:Lipopolysaccharide Receptors; D008070:Lipopolysaccharides; D008075:Lipoproteins, HDL; D008562:Membrane Glycoproteins; D000099231:Lipopolysaccharide-Binding Protein