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J Endotoxin Res. 2005;11(2):117-23.

Monomeric endotoxin:protein complexes are essential for TLR4-dependent cell activation.

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Department of Internal Medicine, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, Iowa 52241, USA.


Potent TLR4-dependent cell activation by Gram-negative bacterial endotoxin depends on sequential endotoxin?protein and protein?protein interactions with LBP, CD14, MD-2 and TLR4. LBP and CD14 combine, in an albumin-dependent fashion, to extract single endotoxin molecules from purified endotoxin aggregates (E(agg)) or the bacterial outer membrane and form monomeric endotoxin:CD14 complexes that are the preferred presentation of endotoxin for transfer to MD-2. Endotoxin in endotoxin:CD14is readily transferred to MD-2, again in an albumin-dependent manner, to form monomeric endotoxin:MD-2 complex. This monomeric endotoxin:protein complex (endotoxin:MD-2) activates TLR4 at picomolar concentrations, independently of albumin, and is, therefore, the apparent ligand in endotoxin-dependent TLR4 activation. Tetra-, penta-, and hexa-acylated forms of meningococcal endotoxin (LOS) react similarly with LBP, CD14, and MD-2 to form endotoxin:MD-2 complexes. However, tetra- and penta-acylated LOS:MD-2 complexes are less potent TLR4 agonists than hexa-acylated LOS:MD-2. This is mirrored in the reduced activity of tetra-, penta- versus hexa-acylated LOS aggregates (LOS(agg)) + LBP toward cells containing mCD14, MD-2, and TLR4. Therefore, changes in agonist potency of under-acylated meninigococcal LOS are determined by differences in properties of monomeric endotoxin:MD-2.

[Indexed for MEDLINE]

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