Lipopolysaccharide Binding Protein Binds to Triacylated and Diacylated Lipopeptides and Mediates Innate Immune Responses

Abstract

LPS binding protein (LBP) is an acute-phase protein synthesized predominantly in the liver of the mammalian host. It was first described to bind LPS of Gram-negative bacteria and transfer it via a CD14-enhanced mechanism to a receptor complex including TLR-4 and MD-2, initiating a signal transduction cascade leading to the release of proinflammatory cytokines. In recent studies, we found that LBP also mediates cytokine induction caused by compounds derived from Gram-positive bacteria, including lipoteichoic acid and peptidoglycan fragments. Lipoproteins and lipopeptides have repeatedly been shown to act as potent cytokine inducers, interacting with TLR-2, in synergy with TLR-1 or -6. In this study, we show that these compounds also interact with LBP and CD14. We used triacylated lipopeptides, corresponding to lipoproteins of Borrelia burgdorferi, mycobacteria, and Escherichia coli, as well as diacylated lipopeptides, corresponding to, e.g., 2-kDa macrophage activating lipopeptide of Mycoplasma spp. Activation of Chinese hamster ovary cells transfected with TLR-2 by both lipopeptides was enhanced by cotransfection of CD14. Responsiveness of human mononuclear cells to these compounds was greatly enhanced in the presence of human LBP. Binding of lipopeptides to LBP as well as competitive inhibition of this interaction by LPS was demonstrated in a microplate assay. Furthermore, we were able to show that LBP transfers lipopeptides to CD14 on human monocytes using FACS analysis. These results support that LBP is a pattern recognition receptor transferring a variety of bacterial ligands including the two major types of lipopeptides to CD14 present in different receptor complexes.

Oxidized Phospholipid Inhibition of Toll-like Receptor (TLR) Signaling Is Restricted to TLR2 and TLR4

“…Notably, it has been established that OxPAPC binds to serum accessory proteins such as LBP and CD14 and competitively inhibits the binding of LPS to these proteins (17). Likewise, it has been shown that both di-acyl and tri-acyl bacterial lipopeptides bind to CD14 and LBP (35,47) and that CD14 and LBP are required for enhanced cellular sensitivity to bacterial lipopeptides (32,34). Thus, it is likely that the established binding of OxPAPC to CD14 and LBP (17) also contributes to the inhibition of cellular responsiveness to lipopeptides via TLR2.…”

TLR2 Stimulation Regulates the Balance between Regulatory T Cell and Th17 Function: A Novel Mechanism of Reduced Regulatory T Cell Function in Multiple Sclerosis

“…In particular, most in vivo relevant lipopeptides are triacylated and, therefore, are adequately modeled by Pam3Cys used in our study (28,64). Respiratory viruses that can activate TLR2 (29,63) also have been associated with MS exacerbations (31,32,61).…”

Pathogen Recognition by Toll-like Receptor 2 Activates Weibel-Palade Body Exocytosis in Human Aortic Endothelial Cells

“…3A). It has been shown that PGN directly binds TLR2 per se (40), whereas bacterial lipopeptides are thought to directly interact with TLR2-associated molecules such as CD14 and LBP but not with TLR2 per se (7,41,42), suggesting the existence of different ligand-recognition mechanisms by TLR2. Furthermore, a novel family of PGN-binding proteins such as peptidoglycan recognition proteins has been found (43) and might enable discrimination of PGN from other TLR2 agonists.…”