Detection of lipopolysaccharide (LPS)-binding membrane proteins by immuno-coprecipitation with LPS and anti-LPS antibodies

Decay-accelerating factor (DAF/CD55) is a functional active element of the LPS receptor complex

Previously, we identified an 80 kDa membrane protein (LMP80) that is capable of binding to LPS and lipid A in the presence of LBP and sCD14. LMP80 could also be detected after immuno-coprecipitation of cell membranes with LPS and lipid A, indicating a physical contact of LMP80 and LPS/lipid A. Further analysis and peptide sequencing revealed that LMP80 is identical to CD55 (decay accelerating factor, DAF), a regulatory molecule of the complement cascade. Transfection of LPS-hyporesponsive Chinese hamster ovary (CHO) cells with human CD55 resulted in the translocation of NF-κB upon stimulation with LPS or lipid A. Our results demonstrate a new functional role of CD55 as a molecule able to mediate LPS-induced activation of cells that may be part of a multimeric LPS receptor complex.

A biophysical approach towards an understanding of endotoxin-induced signal transduction

The purpose of our studies is to define the physicochemical parameters involved in the activation of host cells by endotoxin and to characterize the processes operative during endotoxin/membrane interaction with the aim of understanding transmembrane signal transduction mechanisms. To this end, we determined the molecular conformation of the lipid A component of various endotoxins (endotoxic conformation) using X-ray small angle diffraction, their intercalation into reconstituted macrophage membranes with fluorescence resonance energy transfer spectroscopy, and their IL-6 inducing capacity in whole blood. We also investigated their influence on ion channels as a possible primary event in cell activation applying patch-clamp techniques to macrophages. We found a strong influence of the molecular charge on the molecular conformation, and we could show that the presence of charged groups and a cone- or wedge-like molecular conformation of lipid A are prerequisites for the expression of bioactivity. We also obtained strong evidence supporting the idea that the interaction of endotoxin with ion channels is one of the very early events in the interaction with the cell and, most likely, in signal transduction.

The Combining Sites of Anti-lipid A Antibodies Reveal a Widely Utilized Motif Specific for Negatively Charged Groups

Lipopolysaccharide dispersed in the blood by Gram-negative bacteria can be a potent inducer of septic shock. One research focus has been based on antibody sequestration of lipid A (the endotoxic principle of LPS); however, none have been successfully developed into a clinical treatment. Comparison of a panel of anti-lipid A antibodies reveals highly specific antibodies produced through distinct germ line precursors. The structures of antigen-binding fragments for two homologous mAbs specific for lipid A, S55-3 and S55-5, have been determined both in complex with lipid A disaccharide backbone and unliganded. These high resolution structures reveal a conserved positively charged pocket formed within the complementarity determining region H2 loops that binds the terminal phosphates of lipid A. Significantly, this motif occurs in unrelated antibodies where it mediates binding to negatively charged moieties through a range of epitopes, including phosphorylated peptides used in diagnostics and therapeutics. S55-3 and S55-5 have combining sites distinct from anti-lipid A antibodies previously described (as a result of their separate germ line origin), which are nevertheless complementary both in shape and charge to the antigen. S55-3 and S55-5 display similar avidity toward lipid A despite possessing a number of different amino acid residues in their combining sites. Binding of lipid A occurs independent of the acyl chains, although the GlcN-O6 attachment point for the core oligosaccharide is buried in the combining site, which explains their inability to recognize LPS. Despite their lack of therapeutic potential, the observed motif may have significant immunological implications as a tool for engineering recombinant antibodies.

Renal endothelial injury and microvascular dysfunction in acute kidney injury

The kidney is comprised of heterogeneous cell populations that function together to perform a number of tightly controlled, complex and interdependent processes. Renal endothelial cells contribute to vascular tone, regulation of blood flow to local tissue beds, modulation of coagulation and inflammation, and vascular permeability. Both ischemia and sepsis have profound effects on the renal endothelium, resulting in microvascular dysregulation resulting in continued ischemia and further injury. In recent years, the concept of the vascular endothelium as an organ that is both the source of and target for inflammatory injury has become widely appreciated. Here we revisit the renal endothelium in the light of ever evolving molecular advances.

Structural Basis for Antibody Recognition of Lipid A: INSIGHTS TO POLYSPECIFICITY TOWARD SINGLE-STRANDED DNA

Septic shock is a leading cause of death, and it results from an inflammatory cascade triggered by the presence of microbial products in the blood. Certain LPS from Gram-negative bacteria are very potent inducers and are responsible for a high percentage of septic shock cases. Despite decades of research, mAbs specific for lipid A (the endotoxic principle of LPS) have not been successfully developed into a clinical treatment for sepsis. To understand the molecular basis for the observed inability to translate in vitro specificity for lipid A into clinical potential, the structures of antigen-binding fragments of mAbs S1-15 and A6 have been determined both in complex with lipid A carbohydrate backbone and in the unliganded form. The two antibodies have separate germ line origins that generate two markedly different combining-site pockets that are complementary both in shape and charge to the antigen. mAb A6 binds lipid A through both variable light and heavy chain residues, whereas S1-15 utilizes exclusively the variable heavy chain. Both antibodies bind lipid A such that the GlcN-O6 attachment point for the core oligosaccharide is buried in the combining site, which explains the lack of LPS recognition. Longstanding reports of polyspecificity of anti-lipid A antibodies toward single-stranded DNA combined with observed homology of S1-15 and A6 and the reports of several single-stranded DNA-specific mAbs prompted the determination of the structure of S1-15 in complex with single-stranded DNA fragments, which may provide clues about the genesis of autoimmune diseases such as systemic lupus erythematosus, thyroiditis, and rheumatic autoimmune diseases.

Human keratinocytes express functional CD14 and toll-like receptor 4

CD14 and the toll-like receptor 4 have been known to play an important role in lipopolysaccharide-induced cellular responses in bacterial infections. Although CD14 and toll-like receptor 4 expression has been demonstrated in a number of myeloid cells, much less is known about the expression and function of these lipopolysaccharide receptors on nonleukocytes. In this study, we demonstrate that human keratinocytes are capable of expressing functional CD14 and toll-like receptor 4. Keratinocytes were found to constitutively express CD14 and toll-like receptor 4 mRNA that was augmented by exposure to lipopolysaccharide. Cell surface expression of keratinocyte CD14 and toll-like receptor 4 was detected by flow cytometry. Lipopolysaccharide binding to keratinocyte CD14 and toll-like receptor 4 resulted in a rapid intracellular Ca2+ response, nuclear factor-kappaB nuclear translocation, and the secretion of proinflammatory cytokines and chemokines. These results have important implications for our understanding of cutaneous innate immunity to bacterial infections of the skin.

Lipopolysaccharide recognition: CD14, TLRs and the LPS-activation cluster

Recognition of bacterial lipopolysaccharide (LPS) by the innate immune system elicits strong pro-inflammatory responses that can eventually cause a fatal sepsis syndrome in humans. LPS-mediated activation of mammalian cells is believed to involve the interaction of LPS with lipopolysaccharide-binding protein (LBP) in the serum and, subsequently with CD14. Although there is no doubt that CD14 binds LPS, CD14 is not capable of initiating a transmembrane activation signal because it is a glycosylphosphatidylinositol (GPI)-anchored protein. Accumulating evidence has suggested that LPS must interact with a transmembrane receptor(s) that is responsible for signal transduction. Integrins CD11c and/or CD18, Toll-like receptors (TLRs), as well as CD55, have been suggested to serve this function. Recently, we have revealed that a signalling complex of receptors is formed following LPS stimulation, which comprises heat-shock proteins (Hsps) 70 and 90, chemokine receptor 4 (CXCR4) and growth differentiation factor 5 (GDF5). Taking into account the discovery of the TLRs and the LPS-activation cluster, we propose a new model of LPS recognition.

Interactions of bacterial lipopolysaccharide and peptidoglycan with a 70 kDa and an 80 kDa protein on the cell surface of CD14+ and CD14- cells

Bacterial cell wall components, lipopolysaccharide (LPS), lipoteichoic acid (LTA), and peptidoglycan (PGN) are known to stimulate cells of the immune, inflammatory and vascular systems contributing to septic shock. CD14 has been identified as the main LPS receptor, a process that is accelerated by the serum protein LPS-binding protein (LBP). CD14 has also been found to bind LTA and PGN from the cell wall of gram positive bacteria. Recently, toll-like receptor proteins TLR-2 and TLR-4 have been shown to be required for LPS and LTA-induced intracellular signalling. Although CD14 functions as either a glycosylphosphatidylinositol (GPI)-anchored molecule that does not transverse the cell membrane or as a soluble serum protein, the mechanisms by which the CD14-LPS/LTA complex interacts with the TLRs remains to be elucidated. We have looked directly for cell surface protein(s) that bind LPS or LTA in a CD14-dependent manner. Using biochemical approaches we have identified two proteins of molecular weight 70 kDa (LAP-1) and 80 kDa (LAP-2) that can be precipitated from both CD14(+) and CD14(-) cells with LPS- or LTA-specific antibodies. Binding of LPS and LTA to LAP-1 and -2 required serum. While soluble CD14 (sCD14) was sufficient to allow precipitation of these two proteins from CD14(-) cells, serum could not be replaced by purified sCD14 and/or LBP when mCD14-expressing cells were used.

The interaction of human peripheral blood eosinophils with bacterial lipopolysaccharide is CD14 dependent

Bacterial lipopolysaccharide (LPS, endotoxin) is a ubiquitous component of dust and air pollution and is suspected to contribute after inhalation to an activation of eosinophils in bronchial tissues of asthmatic patients, provoking inflammatory and allergic processes. We were therefore interested in the interaction of eosinophil granulocytes with LPS and have examined the activation of and uptake to human peripheral blood eosinophils by LPS. Eosinophils were stimulated by LPS and the endotoxic component lipid A and the release of tumor necrosis factor alpha (TNF-alpha) and of the eosinophil-specific granule protein eosinophil cationic protein (ECP) was estimated. The results show induction of TNF-alpha and ECP-release by LPS and lipid A in a dose-dependent manner. Anti-CD14 monoclonal antibody (moAb) (clone MEM-18) and the synthetic lipid A partial structure 406 blocked the release of TNF-alpha and ECP by LPS-stimulated eosinophils. Studies with radioactively labeled LPS showed dose-dependent uptake of (3)H-LPS to eosinophils. The (3)H-LPS uptake was found to be specific because preincubation with unlabeled LPS, compound 406 and also anti-CD14 antibodies inhibited uptake of (3)H-LPS to eosinophil granulocytes. By flow cytometry using anti-CD14 moAb and by reverse transcriptase-polymerase chain reaction (RT-PCR) technique, CD14 expression was detectable. Furthermore, messenger RNA (mRNA) expression of Toll-like receptors (TLR) 2 and TLR 4 was detected, indicating the presence of these CD14 coreceptors. The results indicate that eosinophils can take up LPS and can be stimulated by LPS in a CD14-dependent manner. Hence, in addition to allergens, eosinophils interact with endotoxin, a process that possibly exacerbates ongoing inflammatory and allergic processes.

The dual role of lipopolysaccharide as effector and target molecule

Lipopolysaccharides (LPS) are major integral components of the outer membrane of Gram-negative bacteria being exclusively located in its outer leaflet facing the bacterial environment. Chemically they consist in different bacterial strains of a highly variable O-specific chain, a less variable core oligosaccharide, and a lipid component, termed lipid A, with low structural variability. LPS participate in the physiological membrane functions and are, therefore, essential for bacterial growth and viability. They contribute to the low membrane permeability and increase the resistance towards hydrophobic agents. They are also the primary target for the attack of antibacterial drugs and proteins such as components of the host's immune response. When set free LPS elicit, in higher organisms, a broad spectrum of biological activities. They play an important role in the manifestation of Gram-negative infection and are therefore termed endotoxins. Physico-chemical parameters such as the molecular conformation and the charges of the lipid A portion, which is responsible for endotoxin-typical biological activities and is therefore termed the 'endotoxic principle' of LPS, are correlated with the biological activity of chemically different LPS.

Identification of the 80-kDa LPS-binding protein (LMP80) as decay-accelerating factor (DAF, CD55)

The activation of immunocompetent cells by lipopolysaccharide (LPS) during severe Gram-negative infections is responsible for the pathophysiological reactions, possibly resulting in the clinical picture of sepsis. Monocytes recognize LPS mainly through the LPS receptor CD14, however, other cellular binding structures have been assumed to exist. In previous studies, we have described an 80-kDa LPS-binding membrane protein (LMP80), which is present on human monocytes as well as endothelial cells. Here we demonstrate that LMP80 is widely distributed and that it forms complexes together with LPS and sCD14. Furthermore, we report on the biochemical purification of LMP80 and its identification as decay-accelerating factor, CD55, by amino acid sequencing and cloning techniques. Our results imply a new feature of CD55 as a molecule which interacts with LPS/sCD14 complexes. However, the involvement of CD55 in LPS-induced signaling remains to be elucidated.