title: Human lipopolysaccharide-binding protein (LBP) and CD14 independently deliver triacylated lipoproteins to Toll-like receptor 1 (TLR1) and TLR2 and enhance formation of the ternary signaling complex.
abstract: Bacterial lipoproteins are the most potent microbial agonists for the Toll-like receptor 2 (TLR2) subfamily, and this pattern recognition event induces cellular activation, leading to host immune responses. Triacylated bacterial lipoproteins coordinately bind TLR1 and TLR2, resulting in a stable ternary complex that drives intracellular signaling. The sensitivity of TLR-expressing cells to lipoproteins is greatly enhanced by two lipid-binding serum proteins known as lipopolysaccharide-binding protein (LBP) and soluble CD14 (sCD14); however, the physical mechanism that underlies this increased sensitivity is not known. To address this, we measured the ability of LBP and sCD14 to drive ternary complex formation between soluble extracellular domains of TLR1 and TLR2 and a synthetic triacylated lipopeptide agonist. Importantly, addition of substoichiometric amounts of either LBP or sCD14 significantly enhanced formation of a TLR1·TLR2 lipopeptide ternary complex as measured by size exclusion chromatography. However, neither LBP nor sCD14 was physically associated with the final ternary complex. Similar results were obtained using outer surface protein A (OspA), a naturally occurring triacylated lipoprotein agonist from Borrelia burgdorferi. Activation studies revealed that either LBP or sCD14 sensitized TLR-expressing cells to nanogram levels of either the synthetic lipopeptide or OspA lipoprotein agonist. Together, our results show that either LBP or sCD14 can drive ternary complex formation and TLR activation by acting as mobile carriers of triacylated lipopeptides or lipoproteins.
authors: Department of Microbiology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801.
authors: Diana Rose E
authors: DRE
authors: Ranoa
authors: Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801.
authors: Stacy L
authors: SL
authors: Kelley
authors: Department of Microbiology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801; Department of College of Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801. Electronic address: tapping@illinois.edu.
authors: Richard I
authors: RI
authors: Tapping
mesh_terms: D000209:Acute-Phase Proteins; D016470:Bacteremia / Q000382:microbiology; D001426:Bacterial Proteins / Q000378:metabolism; D001667:Binding, Competitive; D002352:Carrier Proteins; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods; D057809:HEK293 Cells; D006801:Humans; D007113:Immunity, Innate; D007249:Inflammation; D055666:Lipopeptides / Q000737:chemistry; D018950:Lipopolysaccharide Receptors / Q000096:biosynthesis*; D008562:Membrane Glycoproteins; D011485:Protein Binding; D011994:Recombinant Proteins / Q000737:chemistry; D018805:Sepsis / Q000382:microbiology; D015398:Signal Transduction; D051194:Toll-Like Receptor 1 / Q000378:metabolism*; D051195:Toll-Like Receptor 2 / Q000378:metabolism*; D000099231:Lipopolysaccharide-Binding Protein
chemical_list: D000209:Acute-Phase Proteins; D001426:Bacterial Proteins; D002352:Carrier Proteins; D055666:Lipopeptides; D018950:Lipopolysaccharide Receptors; D008562:Membrane Glycoproteins; D011994:Recombinant Proteins; D051194:Toll-Like Receptor 1; D051195:Toll-Like Receptor 2; D000099231:Lipopolysaccharide-Binding Protein