Lipopolysaccharide Binding Protein-mediated Complexation of Lipopolysaccharide with Soluble CD14

Abstract

Endotoxin (lipopolysaccharide; LPS) activates a wide variety of host defense mechanisms. In mammals LPS binding protein (LBP) and CD14 interact with LPS to mediate cellular activation. Using sucrose density gradients and a fluorescent endotoxin derivative we have investigated the mechanism of LPS binding to LBP and the soluble form of CD14 (sCD14). LPS binds to LBP to form two types of complex; at low ratios of LPS to LBP complexes with one molecule of LBP and 1-2 molecules of LPS predominate, while at high ratios of LPS to LBP a large aggregate of LBP and LPS predominates. Complexes of LPS with sCD14 do not form large aggregates, consisting of only 1-2 LPS bound to a single sCD14 even at high multiples of LPS to sCD14. LBP catalyzes LPS binding to sCD14. Catalysis by LBP apparently occurs because LBP provides a pathway for LPS to bind to sCD14 which avoids the necessity for LPS monomers in aqueous solution. The dissociation constants for LPS.LBP and LPS.sCD14 complexes were determined to be 3.5 x 10(-9) and 29 x 10(-9) M, respectively. These numbers suggest that when LBP and sCD14 are present at roughly equal concentrations as they are in normal human plasma and compete for limited LPS, the LPS will predominantly associate with LBP.

Synthetic Endotoxin-Binding Peptides Block Endotoxin- Triggered TNF-α Production by Macrophages In Vitro and In Vivo and Prevent Endotoxin-Mediated Toxic Shock

“…To further increase the efficiency of endotoxin-binding peptides, we created a series of dimeric peptides, i.e., peptides containing two endotoxin-binding domains. The idea for the development of dimeric peptides was based on the rationale to increase the avidity of the peptide for endotoxin and that natural endotoxin-binding proteins like LBP may bind more than one endotoxin molecule per molecule of LBP (44). The dimeric peptide LL-10-H-14 consisting of the linear LALF-derived peptide LL-10 and the cyclic LBPderived peptide H-14 displayed the highest activity of the series of peptides tested here to block the release of TNF-␣ by human and murine cells in repeated experiments (Figs.…”

Characterization of recombinant human HBP/CAP37/azurocidin, a pleiotropic mediator of inflammation‐enhancing LPS‐induced cytokine release from monocytes

“…, an acute-phase protein present in small amounts in serum, is known to be necessary for LPS responses of CD14 receptor-bearing cells, such as neutrophils and monocytes [17][18][19]. LBP lowers the threshold stimulatory concentration of LPS and markedly enhances the LPS-induced cytokine release [20,21].…”

STAT‐1‐mediated repression of monocyte interleukin‐10 gene expression in vivo

“…The gram-negative bacterial cell wall component lipopolysaccharide (LPS) is an endotoxin that triggers a myriad of immunoregulatory cytokines that participate in eliminating the bacterial insult [1]. LPS associates with a 60-kD serum protein called lipidbinding protein and this complex subsequently associates with CD14, a 50-kD glycoprotein surface receptor constitutively expressed primarily by macrophages and monocytes, two cellular components of the innate immune system [2,3]. Once bound to CD14, the LPS/ CD14 complex associates with a separate membranebound receptor Toll-like receptor (TLR)4, a member of a family of specialized pattern recognition receptors called the TLR, so named for their homology to the Drosophila Toll proteins [4,5].…”