Biomedical subjects

S D Wright

Publications and source records attributed to S D Wright.

At least 55 records · Page 3Linked to original sources

PubMed1997

Targeted deletion of the lipopolysaccharide (LPS)-binding protein gene leads to profound suppression of LPS responses ex vivo, whereas in vivo responses remain intact.

Gram-negative bacterial lipopolysaccharide (LPS) stimulates phagocytic leukocytes by interacting with the cell surface protein CD14. Cellular responses to LPS are markedly potentiated by the LPS-binding protein (LBP), a lipid-transfer protein that binds LPS aggregates and transfers LPS monomers to CD14. LBP also transfers LPS to lipoproteins, thereby promoting the neutralization of LPS. LBP present in normal plasma has been shown to enhance the LPS responsiveness of cells in vitro. The role of LBP in promoting LPS responsiveness in vivo was tested in LBP-deficient mice produced by gene targeting in embryonic stem cells. Whole blood from LBP-deficient animals was 1,000-fold less responsive to LPS as assessed by the release of tumor necrosis factor (TNF)-alpha. Blood from gene-targeted mice was devoid of immunoreactive LBP, essentially incapable of transferring LPS to CD14 in vitro, and failed to support cellular responses to LPS. These activities were restored by the addition of exogenous recombinant murine LBP to the plasma. Despite these striking in vitro findings, no significant differences in TNF-alpha levels were observed in plasma from wild-type and LBP-deficient mice injected with LPS. These data suggest the presence of an LBP-independent mechanism for responding to LPS. These LBP knockout mice may provide a tool for discovering the nature of the presumed second mechanism for transferring LPS to responsive cells.

Acute-Phase Proteins↗

PubMed1997

Mice genetically hyporesponsive to lipopolysaccharide (LPS) exhibit a defect in endocytic uptake of LPS and ceramide.

We have recently shown that monomeric bacterial LPS is rapidly delivered from the plasma membrane to an intracellular site and that agents that block vesicular transport block responses of neutrophils to lipopolysaccharide (LPS) (Detmers, P.A., N. Thiéblemont, T. Vasselon, R. Pironkova, D.S. Miller, and S.D. Wright. 1996. J. Immunol. 157:5589-5596). To examine further the connection between intracellular transport of LPS and signaling, we observed internalization of fluorescently labeled LPS in cells from LPS-hyporesponsive (Lpsd) mice. Binding of fluorescent LPS from LPS-soluble CD14 (sCD14) complexes by peritoneal macrophages from Lpsd and control (Lpsn) mice was quantitatively similar, and confocal images obtained from these cells exhibited an identical appearance immediately after labeling. Incubation of labeled Lpsn macrophages at 37 degrees C caused movement of the fluorescence from the cell perimeter in one or two spots in the perinuclear region. However, in Lpsd cells the fluorescence remained dispersed, suggesting a defect in vesicular transport. LPS resembles ceramide, and Lpsd mice fail to respond to ceramide. As with LPS, we found that binding of fluorescent ceramide by Lpsd and Lpsn macrophages was quantitatively similar, and the label moved rapidly to one to two spots in the perinuclear region in Lpsn mice. However, in Lpsd macrophages the fluorescence remained dispersed. These results show that cells deficient in responses to LPS exhibit defective vesicular transport of LPS and ceramide and point to a role for vesicular transport in responses to these mediators.

Animals↗

PubMed1997

Tissue factor pathway inhibitor blocks cellular effects of endotoxin by binding to endotoxin and interfering with transfer to CD14.

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type plasma protease inhibitor that inhibits factor Xa and the factor VIIa/tissue factor catalytic complex. It plays an important role in feedback inhibition of the coagulation cascade (Broze, Annu Rev Med 46:103, 1995). TFPI has also been used successfully to prevent lethality and attenuate coagulopathic responses in a baboon model of septic shock (Creasey et al, J Clin Invest 91:2850, 1993; and Carr et al, Circ Shock 44:126, 1995). However, the mechanism of reduced mortality in these animals could not be explained merely by the anticoagulant effect of TFPI, because TFPI-treated animals also had a significantly depressed interleukin-6 response. Moreover, inhibition of coagulopathic responses by other anticoagulants has failed to block the organ damage or lethal effect of endotoxic shock (Coalson et al, Circ Shock 5:423, 1978; Warr et al, Blood 75:1481, 1990; and Taylor et al, Blood 78:364, 1991). We show here that recombinant TFPI can bind to endotoxin in vitro. This binding prevents interaction of endotoxin with both lipopolysaccharide binding protein and CD14, thereby blocking cellular responses.

Anti-Inflammatory Agents, Non-Steroidal↗

PubMed1997

Lipopolysaccharide-binding protein and soluble CD14 transfer lipopolysaccharide to phospholipid bilayers: preferential interaction with particular classes of lipid.

LPS-binding protein (LBP) catalyzes the movement of LPS (endotoxin) from micelles directly to high density lipoprotein (HDL) particles, and this activity results in neutralization of the biologic activities of LPS. LBP also catalyzes the transfer of LPS to HDL by a two-step mechanism in which LPS is transferred to soluble CD14 (sCD14), and then from LPS-sCD14 complexes to HDL. In this work, we show that the phospholipid component of HDL, phosphatidylcholine (PC), is both necessary and sufficient for LBP-catalyzed neutralization of LPS through either mechanism. Our observation that LBP and sCD14 can transport LPS into phospholipid bilayers suggests that LBP and membrane CD14 may transport LPS into the phospholipid bilayer of cells such as monocytes and neutrophils. Studies with a variety of purified phospholipids showed that: 1) PC, phosphatidylserine, phosphatidylinositol, and sphingomyelin can neutralize LPS, while phosphatidylethanolamine, ceramide, and lactosylceramide cannot. 2) PC containing saturated long chain acyl groups (distearoyl-PC) does not neutralize LPS, but PC containing unsaturated long chain acyl groups (dioleoyl-PC) rapidly neutralizes LPS. 3) Inclusion of sCD14 is absolutely necessary to observe LBP-dependent neutralization of LPS by sphingomyelin, globoside, and phosphatidylserine. 4) Inclusion of sCD14 enhances movement to longer chain PC vesicles, but slows movement to certain short chain vesicles. These findings indicate that LBP and sCD14 will rapidly transfer LPS to certain membranes based on the kinetics of the movement of LPS into these membranes. This discrimination may target LPS to certain classes of lipoprotein, certain cell types, or even certain lipid domains at the cell surface.

Acute-Phase Proteins↗

PubMed1997

Inhibitory effect of growth hormone on TNF-alpha secretion and nuclear factor-kappaB translocation in lipopolysaccharide-stimulated human monocytes.

Several studies have pointed to a link between immune and endocrine systems, including a regulatory function of GH on monocyte activation. The present study demonstrates that human THP-1 promonocytic cells, engineered by gene transfer to constitutively produce human growth hormone (hGH), secreted depressed amounts of TNF-alpha in response to challenge by LPS. The effect of GH appears to occur in an autocrine fashion, since the inhibitory effect on TNF-alpha secretion by constitutive GH production could be abolished in the presence of anti-hGH mAb. The GH-induced inhibitory effect was also observed using normal human monocytes and monocyte-derived macrophages. Inhibition of TNF-alpha production by THP-1-hGH-transfected cells cultured in the presence of LPS is dependent on a selective pathway, since no inhibition of TNF-alpha production was observed when cells were cultured in the presence of PMA. Inhibition of TNF-alpha secretion by LPS-stimulated THP-1-hGH cells was associated with a decrease in nuclear translocation of nuclear factor-kappaB. The capacity of GH to inhibit LPS-induced TNF-alpha production by monocytes without altering other pathways leading to TNF-alpha production may be of potential relevance in septic shock, since GH is available for clinical use.

CD4 Antigens↗

PubMed1997

Lipopolysaccharide binding protein and soluble CD14 catalyze exchange of phospholipids.

Lipopolysaccharide binding protein (LBP) is a plasma protein known to facilitate the diffusion of bacterial LPS (endotoxin). LBP catalyzes movement of LPS monomers from LPS aggregates to HDL particles, to phospholipid bilayers, and to a binding site on a second plasma protein, soluble CD14 (sCD14). sCD14 can hasten transfer by receiving an LPS monomer from an LPS aggregate, and then surrendering it to an HDL particle, thus acting as a soluble "shuttle" for an insoluble lipid. Here we show that LBP and sCD14 shuttle not only LPS, but also phospholipids. Phosphatidylinositol (PI), phosphatidylcholine, and a fluorescently labeled derivative of phosphatidylethanolamine (R-PE) are each transferred by LBP from membranes to HDL particles. The transfer could be observed using recombinant LBP and sCD14 or whole human plasma, and the plasma-mediated transfer of PI could be blocked by anti-LBP and partially inhibited by anti-CD14. sCD14 appears to act as a soluble shuttle for phospholipids since direct binding of PI and R-PE to sCD14 was observed and because addition of sCD14 accelerated transfer of these lipids. These studies define a new function for LBP and sCD14 and describe a novel mechanism for the transfer of phospholipids in blood. In further studies, we show evidence suggesting that LBP transfers LPS and phospholipids by reciprocal exchange: LBP-catalyzed binding of R-PE to LPS x sCD14 complexes was accompanied by the exit of LPS from sCD14, and LBP-catalyzed binding of R-PE to sCD14 was accelerated by prior binding of LPS to sCD14. Binding of one lipid is thus functionally coupled with the release of a second. These results suggest that LBP acts as a lipid exchange protein.

Acute-Phase Proteins↗

PubMed1996

Potential role of membrane internalization and vesicle fusion in adhesion of neutrophils in response to lipopolysaccharide and TNF.

Human polymorphonuclear leukocytes (PMN) respond to LPS with strongly increased integrin-mediated adhesion. While the first step of this process has been identified as the interaction of LPS with CD14 on the cell surface, subsequent steps remain to be elucidated. The experiments presented here suggest that monomeric LPS is internalized in vesicles, and uptake may be required for signaling. Fluorescently labeled LPS presented as monomeric complexes with soluble CD14 appeared in the plasma membrane of PMN by 5 min and was concentrated in cytoplasmic vesicles by 20 min. Adhesion in response to LPS/soluble CD14 occurred only after a 15- to 20-min lag period, consistent with endocytosis occurring before signal generation. In contrast, there was no time lag for adhesion in response to the formyl peptide formyl-norleucyl-leucyl-phenylalanine (fNLLP). Adhesion in response to LPS, but not fNLLP, was completely blocked by lowering the temperature to 19 degrees C, a procedure that prevents vesicle fusion. These studies indicated that an event with the time and temperature dependence of endocytosis precedes signaling by LPS. Cytochalasin D, an inhibitor of phagocytosis, and wortmannin, an inhibitor of phosphatidylinositol 3-kinase that blocks vesicle fusion and phagocytosis, both completely blocked adhesion in response to LPS but not in response to fNLLP. These results support the idea that LPS internalization and early endosomal fusion may be required for signal transduction. Parallel studies showed that the adhesion response to TNF had time, temperature, and inhibitor sensitivities nearly identical with those of LPS, suggesting that responses to TNF may also include an obligate vesicle fusion step.

Cell Adhesion↗

PubMed1996

Chemical characteristics of Staphylococcus aureus molecules that have CD14-dependent cell-stimulating activity.

We have previously shown that Staphylococcus aureus contains molecules that bind CD14 and induce cellular responses. We have partially purified this activity and named the product active fraction (AF). Here, we further explore the chemical characteristics of AF. We show that 1) the activity is sensitive to base and hydrofluoric acid hydrolysis; 2) AF binds to Mono Q, indicating that it is negatively charged; 3) most activity in AF runs as a large species in aqueous buffer, but a small proportion runs with a size of <15 kDa by gel chromatography; and 4) native PAGE of AF yields a band of high apparent m.w., and SDS-PAGE yields a ladder of bands of lower apparent m.w. These data suggest that AF is a mixture of heterogeneous amphiphilic molecules that form tight micelles. These characteristics are shared by LPS, suggesting that AF has both functional and structural similarity to LPS. S. aureus cell wall has been shown to stimulate cells in a CD14-dependent fashion, and peptidoglycan (PGN) has thus been described as a ligand for membrane CD14. However, we found that the ability of S. aureus cell wall to stimulate CD14-bearing monocytes was enhanced by preincubation with soluble CD14 (sCD14). Upon treatment with sCD14, the cell-stimulating activity appeared predominantly in the supernatant, not in the insoluble PGN pellet. These findings are not consistent with a direct interaction of membrane CD14 with insoluble PGN. They suggest, rather, that the activity of cell wall may be caused by a sCD14-binding molecule that associates noncovalently with PGN.

Cell Line↗

PubMed1996

Effects of site-directed mutagenesis of basic residues (Arg 94, Lys 95, Lys 99) of lipopolysaccharide (LPS)-binding protein on binding and transfer of LPS and subsequent immune cell activation.

LPS-binding protein (LBP) is a 60-kDa acute phase glycoprotein capable of binding the LPS of Gram-negative bacteria and facilitating its diffusion. This process is thought to be of potential importance in inflammatory reactions and pathogenic states such as septic shock syndrome. Here, we report on the identification of a LPS binding domain within the LBP molecule and on the identification of single amino acids important for binding of LPS by LBP. Several synthetic LBP peptides inhibited LPS-LBP interaction, and amino acids Arg 94 and Lys 95 were centrally located in these inhibitory peptides. LBP mutants with amino acid exchanges within this region were expressed and tested in five different functional assays: binding to immobilized LPS; facilitation of binding of LPS aggregates to monocytes; transfer of LPS monomers from aggregates to soluble CD14; transfer of soluble CD14-bound LPS monomers to high density lipoprotein (HDL); and enhancement of LPS-induced cell activation. The double mutant Glu 94/Glu 95 was completely lacking LPS binding, transfer, and cell stimulatory activity, indicating that the integrity of amino acids 94 and 95 is required for LBP function. While mutations of amino acids Arg 94 or Lys 95 into alanine reduced the LPS binding activity of LBP dramatically, the ability to facilitate binding of LPS aggregates to membrane CD14 at the cell surface was retained. These findings emphasize the distinction between binding of LPS aggregates to cells, which is not associated with cell stimulation, and binding of LPS monomers to CD14, which leads to cell stimulation.

Acute-Phase Proteins↗

PubMed1996

Human leukocyte elastase is an endogenous ligand for the integrin CR3 (CD11b/CD18, Mac-1, alpha M beta 2) and modulates polymorphonuclear leukocyte adhesion.

Integrin CR3 (CD11b/CD18, Mac-1, alpha M beta 2) mediates the transient adhesion of polymorphonuclear leukocytes (PMN) to surfaces coated with fibrinogen, C3bi, ICAM-1, and other ligands. Recent studies (Cai, T.-Q., and S.D. Wright 1995. J. Biol. Chem. 270:14358) suggest that adhesion may be favored by stimulus-dependent changes in the kinetics of ligand binding by CR3. Cell detachment, on the other hand, must occur by a different mechanism because binding kinetics cannot affect cell adhesion after binding of ligand has occurred. We have sought a mechanism that would reverse binding of ligand to CR3 and report here that lysates of PMN contain an endogenous ligand that binds CR3 and competes the binding of C3bi. Purification and sequence analysis identified the structurally homologous azurophilic granule proteins, elastase, protease 3, and azurocidin as candidates. Studies with purified elastase and azurocidin showed that each bound specifically to purified, immobilized CR3. Elastase may play a role in modulating integrin-mediated cell adhesion because it is expressed at the cell surface, and the expression level is inversely proportional to cell adhesivity. Furthermore, a monoclonal antibody against elastase prevented detachment of PMN from fibrinogen-coated surfaces and blocked chemotaxis, confirming a role for this protein in regulating integrin-mediated adhesion. These studies suggest a model for release of integrin-mediated cell adhesion in which endogenous ligands such as elastase may release adhesion by "'eluting" substrate-bound ligand. A role for the proteolytic activity of elastase appears likely but is not demonstrated in this study.

Amino Acid Sequence↗

PubMed1996

Plasma lipopolysaccharide-binding protein is found associated with a particle containing apolipoprotein A-I, phospholipid, and factor H-related proteins.

Neutrophils exhibit a dramatic enhancement of integrin-mediated cell adhesion in response to lipopolysaccharide (LPS). This response requires CD14 on the neutrophil and plasma proteins in solution. We have purified the factor from plasma that facilitates the adhesive response of neutrophil to LPS by using a combination of affinity and ion-exchange chromatography. Previous work has shown that the activity is associated with apolipoprotein A-I (apoA-I), and here we show that this activity is associated with an apoA-I-bearing complex of protein and phospholipid. Native polyacrylamide gel electrophoresis (PAGE) analysis showed a ladder of bands in the Mr 200,000 region, and electron microscopy revealed round, indented particles of 11.4 +/- 0.12 nm in diameter. Characterization of these particles revealed a density of 1.219-1.264 g/ml and approximately 10 molecules of lipid phosphate per Mr 200,000 complex. SDS-PAGE showed that each of the bands seen in native PAGE was composed of several polypeptides. These were identified as apoA-I, LPS binding protein (LBP), and factor H-related proteins (FHRPs). Physical association of apoA-I, LBP, and FHRP in these particles was further confirmed using double immunodiffusion, and association of LBP and FHRP in plasma was confirmed by coimmunoprecipitation. FHRPs are the numerically dominant protein components in these particles, and all plasma FHRP-1 appears to be associated with these particles. We suggest that FHRPs may be the defining constituent of this novel "lipoprotein" particle.

Acute-Phase Proteins↗

PubMed1996

Stimulation of macrophages and neutrophils by complexes of lipopolysaccharide and soluble CD14.

Sensitive responses of monocytes, macrophages, and neutrophils to bacterial LPS require membrane-bound CD14 (mCD14) and a plasma protein called LPS-binding protein (LBP). Cells lacking mCD14 respond to complexes of LPS and soluble CD14 (sCD14); these responses do not require LBP. To determine whether LBP is necessary for responses of mCD14-bearing cells to LPS, we measured responses of macrophages and neutrophils to complexes of LPS and sCD14 formed in the absence of LBP. We found that the amount of LPS needed to induce adhesive responses of neutrophils or cytokine production by macrophages was the same whether LPS was added with LBP or as LPS-sCD14 complexes, and was >100-fold less than when LPS was added alone. This result supports the view that LBP transfers LPS to CD14, but is not directly involved in responses of CD14-bearing cells to LPS. Responses of neutrophils to LPS-sCD14 complexes could be inhibited partially by blocking mCD14, suggesting that LPS may move rapidly from sCD14 to mCD14. Additionally, we found that responses of neutrophils to LBP and smooth LPS were made 30 to 100 times more sensitive when sCD14 was added. Our findings show that LBP is not necessary for the activation of CD14-bearing cells with LPS, and suggest that LPS-sCD14 complexes are an important intermediate in the inflammatory responses of leukocytes to LPS.

Acute-Phase Proteins↗

PubMed1996

Neutralization and transfer of lipopolysaccharide by phospholipid transfer protein.

Phospholipid transfer protein (PLTP) and lipopolysaccharide-binding protein (LPB) are lipid transfer proteins found in human plasma. PLTP shares 24% sequence similarity with LBP. PLTP mediates the transfer and exchange of phospholipids between lipoprotein particles, whereas LBP transfers bacterial lipopolysaccharide (LPS) either to lipoprotein particles or to CD14, a soluble and cell-surface receptor for LPS. We asked whether PLTP could interact with LPS and mediate the transfer of LPS to lipoproteins or to CD14. PLTP was able to bind and neutralize LPS: incubation of LPS with purified recombinant PLTP (rPLTP) resulted in the inhibition of the ability of LPS to stimulate adhesive responses of neutrophils, and addition of rPLTP to blood inhibited cytokine production in response to LPS. Transfer of LPS by rPLTP was examined using fluorescence dequenching experiments and native gel electrophoresis. The results suggested that rPLTP was able to mediate the exchange of LPS between micelles and the transfer of LPS to reconstituted HDL particles, but it did not transfer LPS to CD14. Consonant with these findings, rPLTP did not mediate CD14-dependent adhesive responses of neutrophils to LPS. These results suggest that while PLTP and LBP both bind and transfer LPS, PLTP is unable to transfer LPS to CD14 and thus does not mediate responses of cells to LPS.

Boron Compounds↗

PubMed1996

Catalytic properties of lipopolysaccharide (LPS) binding protein. Transfer of LPS to soluble CD14.

Lipopolysaccharide (LPS) binding protein (LBP) is a lipid transfer protein that catalyzes transfer of LPS monomers from micelles to a binding site on soluble CD14 (sCD14) and transfer of LPS from LPS.sCD14 complexes to HDL particles. To characterize the first of these two reactions, LPS covalently derivatized with the fluorophore, boron dipyrromethene difluoride (BODIPY), was used to monitor LBP-catalyzed movement of LPS in real time. The fluorescence efficiency of micelles of BODIPY-LPS was low but was strongly increased upon dissolution in detergent or upon binding to sCD14. Spontaneous binding of BODIPY-LPS to sCD14 was very slow but was accelerated by substoichiometric concentration of LBP, and the rate of binding was measured under a variety of conditions. LBP-catalyzed transfer was first order with respect to both sCD14 and LPS concentration, and the apparent Km values were 1 approximately 2 microg/ml for sCD14 and 100 ng/ml for LPS. The maximum turnover number for LBP was approximately 150 molecules of LPS min-1 LBP-1. LBP alone caused a small but measurable increase in the fluorescence of BODIPY-LPS, suggesting that it bound LPS aggregates but did not readily remove LPS monomers. The subsequent addition of sCD14 caused a large fluorescence increase, suggesting transfer of BODIPY-LPS to sCD14. These and other observations suggest that LPS is transferred by an ordered ternary complex reaction mechanism in which LBP transfers LPS monomer from LPS aggregates to sCD14 without dissociating from the LPS aggregate.

Acute-Phase Proteins↗

PubMed1996

IB4, a monoclonal antibody against the CD18 leukocyte adhesion protein, reduces intracranial pressure following thromboembolic stroke in the rabbit.

Neutrophil activation and accumulation as a consequence of cerebral ischemia-reperfusion has been suggested to exacerbate tissue injury. The current study is designed to examine the effect of IB4, a monoclonal antibody directed against the neutrophil adhesion protein, CD18, in a rabbit model of thromboembolic stroke. New Zealand rabbits (3-3.5 kg n=8 each group), were given an autologous clot embolus, delivered to the anterior circulation of the brain via the internal carotid artery. Immediately following thromboembolism, the mean arterial pressure in all animals was reduced to 30 mmHg by controlled exsanguination for a period of 45 min. All animals were mechanically ventilated and following parameters were monitored hourly: arterial blood gases, intracranial pressure, regional cerebral blood flow, hematocrit, and core temperature. Rabbits were given either IB4 (1 mg kg(-1)), or vehicle (1 percent albumin, IV) 30 min following the thromboembolic event. The mean arterial pressure of all animals was restored to the baseline value of 50-60 mmHg for the remainder of the 4-h experiment. Following the thromboembolic event, the intracranial pressure rose in both groups, although this was significantly less in the IB4-treated group, with the final values being 195.9 +/- 38.3 vs. 135.5 +/- 26.0 percent of baseline (mean +/- SEM, p < 0.05). However, regional cerebral blood flow and infarct size (TTC staining) were virtually identical in both groups. It is concluded that blockade of the neutrophil adhesion protein, CD18, may contribute to a reduction in the intracranial pressure following cerebral ischemia and reperfusion, providing further evidence that activated neutrophils may contribute to cerebral edema.

Animals↗

PubMed1996

Exploring the Caregiver Burden Inventory (CBI): further evidence for a multidimensional view of burden.

The purpose of this study was to examine the multidimensional nature of caregiver burden by specifically analyzing the patterns of association between five dimensions of burden as measured by the Caregiver Burden Inventory [1] and selected demographic, health, functioning, and well-being indicators. Subscales measuring each dimension were internally consistent and relatively independent in a sample of 160 caregivers. Time dependence burden was most influenced by patient impairment and caregiving involvement, whereas emotional burden was largely a function of caregiving satisfaction. Most of the variance in developmental burden was explained by depression and caregiving satisfaction. Contrary to expectations, physical health measures explained little variance in physical burden, of which most was explained by depression. Less than 10 percent of the variance in social burden was explained by depression and caregiver days sick. The findings lend support to a multidimensional view of burden and with minor modifications, the CBI appears to be a promising instrument with which to measure the construct.

Absenteeism↗

PubMed1995

Molecules from Staphylococcus aureus that bind CD14 and stimulate innate immune responses.

Mammals mount a rapid inflammatory response to gram-negative bacteria by recognizing lipopolysaccharide (LPS, endotoxin). LPS binds to CD14, and the resulting LPS-CD14 complex induces synthesis of cytokines and up-regulation of adhesion molecules in a variety of cell types. Gram-positive bacteria provoke a very similar inflammatory response, but the molecules that provoke innate responses to these bacteria have not been defined. Here we show that protein-free, phenol extracts of Staphylococcus aureus contain a minor component that stimulates adhesion of neutrophils and cytokine production in monocytes and in the astrocytoma cell line, U373. Responses to this component do not absolutely require CD14, but addition of soluble CD14 enhances sensitivity of U373 cells by up to 100-fold, and blocking CD14 on monocytes decreases sensitivity nearly 1,000-fold. Deletion of residues 57-64 of CD14, which are required for responses to LPS, also eliminates CD14-dependent responses to S. aureus molecules. The stimulatory component of S. aureus binds CD14 and blocks binding of radioactive LPS. Unlike LPS, the activity of S. aureus molecules was neither enhanced by LPS binding protein nor inhibited by bactericidal/permeability increasing protein. The active factor in extracts of S. aureus is also structurally and functionally distinct from the abundant species known as lipoteichoic acid (LTA). Cell-stimulating activity fractionates differently from LTA on a reverse-phase column, pure LTA fails to stimulate cells, and LTA antagonizes the action of LPS in assays of IL-6 production. These studies suggest that mammals may use CD14 in innate responses to both gram-negative and gram-positive bacteria, and that gram-positive bacteria may contain an apparently unique, CD14-binding species that initiates cellular responses.

Antigens, Bacterial↗

PubMed1995

Identification of a domain in soluble CD14 essential for lipopolysaccharide (LPS) signaling but not LPS binding.

CD14 is a 55-kDa glycoprotein that binds lipopolysaccharide (LPS) and enables LPS-dependent responses in a variety of cells. Monoclonal antibodies of CD14 such as 3C10 and MEM-18 are known to neutralize biological activity of CD14. Recently, it has been demonstrated that MEM-18 recognizes the LPS-binding site of CD14, between amino acids 57 and 64. It has also been shown that 3C10 recognizes a distinct epitope from that of MEM-18, indicating that 3C10 may yet define another functional domain of CD14. In order to identify the epitope for 3C10, we constructed a series of alanine substitution mutants of soluble CD14 (sCD14). BIAcore analyses showed that regions between amino acids 7 and 10 and between amino acids 11 and 14 are required for 3C10 binding. To assess the effect of altering the 3C10 epitope in CD14, we generated a stable cell line expressing a mutant sCD14 containing alanine substitutions in the region between amino acids 7 and 10, sCD14(7-10)A, and purified this protein to homogeneity. sCD14(7-10)A has impaired ability to mediate LPS-dependent IL-6 up-regulation in U373 cells, integrin activation in neutrophils, and NF-kappa B activation in U373 cells. Purified sCD14(7-10)A was, however, capable of forming a stable complex with LPS in an LPS binding protein-facilitated and LPS binding protein-independent fashion. The ability of sCD14(7-10)A to bind LPS was also demonstrated in assays in which excess sCD14(7-10)A inhibited LPS-mediated tumor necrosis factor-alpha production in whole blood and adhesion of polymorphonuclear leukocytes to fibrinogen. These data strongly suggest that a region recognized by neutralizing monoclonal antibody 3C10 contains a domain required for cellular signaling but not for LPS binding.

Acute-Phase Proteins↗