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Regulation of interactions of endotoxin with host cells

Departments of Internal Medicine, Division of Infectious Diseases and The Inflammation Program, University of Iowa, Iowa City, Iowa, Department of Biochemistry, University of Iowa, Iowa City, Iowa

Athmane Teghanemt

Departments of Internal Medicine, Division of Infectious Diseases and The Inflammation Program, University of Iowa, Iowa City, Iowa

Kol A. Zarember

Department of Molecular Biology, Genentech, Inc., South San Francisco, California, USA

Jerrold P. Weiss

Departments of Internal Medicine, Division of Infectious Diseases and The Inflammation Program, University of Iowa, Iowa City, Iowa, jerrold-weiss{at}uiowa.edu, Microbiology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, Iowa, Iowa City Veterans' Administration Medical Center, Iowa City, Iowa, USA

Potent Toll-like receptor 4 (TLR4)-dependent cell activation by endotoxin requires lipopolysaccharide-binding protein (LBP) and CD14-dependent delivery of endotoxin to cells containing MD-2 and TLR4. We have used metabolically labeled [¹⁴C] meningococcal lipooligosaccharide (LOS), purified recombinant endotoxin-binding proteins, and cultured endothelial cells to better define protein: endotoxin intermediates key in cell activation in the absence of functional membrane (m) CD14. Protein:endotoxin complexes or aggregates (agg) were purified by gel sieving and characterized by immunocapture and bio-assays. Cell activation closely correlated with LBP, albumin and soluble (s) CD14-dependent conversion of endotoxin agg (M_r 20 x 10⁶) to monomeric (M ~55 x 10 ³) endotoxin:sCD14 complexes. Ordered interaction of LBP (+ albumin) and sCD14 with ^rLOSagg was required for the efficient formation of a bioactive endotoxin:sCD14 complex and potent cell activation. Increasing the ratio of LBP/sCD14 or addition of bactericidal/permeability-increasing protein (BPI) reduced accumulation of endotoxin:sCD14 complexes and instead yielded aggregates of endotoxin (M_r ~1—20 x 10⁶) containing LBP or BPI that were taken up by cells in a CD14- and TLR4-independent manner without inducing pro-inflammatory responses. These findings strongly suggest that host machinery linked to TLR4-dependent cellular activation or TLR4-independent cellular clearance of endotoxin selectively recognizes different protein:endotoxin complexes. At the outset of infection, the low concentrations of LBP present and absence of extracellular BPI favor formation of pro-inflammatory endotoxin:CD14 complexes. The mobilization of LBP and BPI that is triggered by inflammation directs endotoxin for clearance and hence resolution of endotoxin-triggered inflammation.

Journal of Endotoxin Research, Vol. 9, No. 6, 401-408 (2003)
DOI: 10.1177/09680519030090060301


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