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 C D14 (sCD14) and movement of LPS from micelles to reconstituted high de nsity lipoprotein (R-HDL) particles. Here we show that LBP facilitates a third lipid transfer reaction: movement of LPS from LPS-sCD14 compl exes to R-HDL particles. This action of LBP is catalytic, with one mol ecule of LBP enabling the movement of multiple LPS molecules into R-HD L. LBP-catalyzed movement of LPS from LPS-sCD14 complexes to R-HDL neu tralizes 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 accelerat ed more than 30-fold by addition of sCD14. Several observations sugges t that sCD14 accelerates this reaction by serving as a shuttle for LPS : addition of LBP and sCD14 to LPS micelles resulted in LPS-sCD14 comp lexes that could diffuse through a 100-kD cutoff filter; LPS-sCD14 com plexes appeared transiently during movement of LPS to R-HDL facilitate d 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 LP S and sCD14 were formed transiently when LPS was incubated in plasma, suggesting that these complexes may play a role as intermediates in th e neutralization of LPS under physiological conditions. These findings detail a new activity for sCD14 and suggest a novel mechanism for lip id transfer by LBP.