Specific binding of lipopolysaccharides to mouse macrophages--I. Characteristics of the interaction and inefficiency of the polysaccharide region

Do endotoxin aggregates intercalate into phospholipid membranes in a nonspecific, hydrophobic manner?

The interaction of endotoxin aggregates with phospholipid liposomes of different composition was investigated applying fluorescence polarization spectroscopy with the fluorophore diphenylhexatriene and the resonance energy transfer technique using N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-PE and N-(Rhodamine B sulfonyl)-PE. Fluorescence polarization data at constant temperature could be interpreted in favor of an intercalation of lipopolysaccharide into phospholipid liposomes even in the absence of Ca2+. Intercalation, however, could be clearly excluded from determinations performed as a function of temperature. Experiments employing the resonance energy transfer technique clearly showed that a nonspecific, hydrophobic intercalation of endotoxin aggregates into phospholipid liposomes only takes place in the presence of excess molar concentrations of divalent cations and/or after long-term incubation at elevated temperature (37°C). These findings indicate that under (near) physiological conditions nonspecific intercalation of aggregated lipopolysaccharide into phospholipid membranes represents an unlikely event. The significance of these results for an understanding of the fundamental mechanisms of cell activation by endotoxin is discussed.

Cellular antiendotoxin activities of lung surfactant protein C in lipid vesicles

The respiratory system is continuously exposed to airborne particles containing lipopolysaccharide. Our laboratory established previously that the hydrophobic surfactant protein C (SP-C) binds to lipopolysaccharide and to one of its cellular receptors, CD14. Here we examined the influence of SP-C, and of a synthetic analog, on some cellular in vitro effects of lipopolysaccharide. When associated with vesicles of dipalmitoylphosphatidylcholine, SP-C inhibits the binding of a tritium-labeled lipopolysaccharide to the macrophage cell line RAW 264.7. Under similar conditions of presentation, SP-C inhibits the mitogenic effect of lipopolysaccharide on mouse splenocytes, and inhibits the lipopolysaccharide-induced production of tumor necrosis factor-alpha by peritoneal and alveolar macrophages, and of nitric oxide by RAW 264.7 cells. In contrast, tumor necrosis factor-alpha production induced by a lipopeptide, and nitric oxide production induced by picolinic acid, were not affected by SP-C. The lipopolysaccharide-binding capacity of SP-C is resistant to peroxynitrite, a known mediator of acute lung injury formed by reaction of nitric oxide with superoxide anions. These results indicate that SP-C may play a role in lung defense; SP-C resists degradation under inflammatory conditions and traps lipopolysaccharide, preventing it from inducing production of noxious mediators in alveolar cells.

An essential role for albumin in the interaction of endotoxin with lipopolysaccharide-binding protein and sCD14 and resultant cell activation

Experiments utilizing endotoxin aggregates, lipooligosaccharides (LOS) isolated from metabolically labeled Neisseria meningitidis serotype group B, demonstrate that albumin is an essential component of lipopolysaccharide binding protein- (LBP) and sCD14-dependent 1) disaggregation of LOS and 2) LOS activation of human umbilical vein endothelial cells (HUVEC). Aggregates of LOS (LOS(agg)) with an apparent M(r) >or= 2 x 10(7) were isolated by gel sieving on Sephacryl HR S500 in buffered balanced salts solution plus albumin. Incubation of LOS(agg) with LBP and sCD14 promoted LOS(agg) disaggregation in an albumin-dependent fashion to complexes that contain LOS and sCD14, but no LBP, with an apparent M(r) approximately 60,000 (LOS:sCD14) as determined by Sephacryl S200 chromatography. Isolation by gel filtration of LOS(agg):protein aggregates formed by the interaction of LOS(agg) with either LBP or sCD14 alone revealed that the sequence of LOS-protein interactions as well as the step(s) at which albumin is necessary for the production of bioactive LOS:sCD14 were specific. Efficient generation of LOS:sCD14 required 1) interaction of LOS(agg) with LBP before interaction with CD14 and 2) the presence of albumin during the interaction of LBP with LOS(agg). Activation of HUVEC by LOS(agg), as measured by IL-8 production, required both LBP and sCD14 and was thirty times more potent in the presence of albumin. In contrast, LOS:sCD14 did not require additional LBP, sCD14, or albumin to activate HUVEC but depended on the presence of albumin for optimal solubility/stability once formed. The albumin effect is apparently specific, because neither ovalbumin nor gelatin substituted for albumin in facilitating LBP:sCD14-dependent disaggregation of LOS(agg) or activation of endothelial cells. These results indicate that albumin is an essential facilitator of LBP/sCD14-induced LOS disaggregation that is required for activation of endothelial cells by LOS(agg).

Mechanisms of acute vasodilator response to bacterial lipopolysaccharide in the rat coronary microcirculation

1. In this study the mechanisms of the acute vasodilator action of bacterial lipopolysaccharide (LPS) were investigated in the rat Langendorff perfused heart. 2. Infusion of LPS (5 microg ml(-1)) caused a rapid and sustained fall in coronary perfusion pressure (PP) of 59 +/- 4 mmHg (n = 12) and a biphasic increase in NO levels determined in the coronary effluent by chemiluminescent detection. Both the fall in PP and the increase in NO release were completely abolished (n = 3) by pretreatment of hearts with the NO synthase inhibitor L-NAME (50 microM). 3. LPS-induced vasodilatation was markedly attenuated to 5 +/- 4 mmHg (n 3) by pretreatment of hearts with the B2 kinin receptor antagonist Hoe-140 (100 nM). 4. Vasodilator responses to LPS were also blocked by brief pretreatment with mepacrine (0.5 microM, n = 3) or nordihydroguaiaretic acid (0.1 microM, n = 4) and markedly attenuated by WEB 2086 (3 microM, n = 4). 5. Thirty minutes pretreatment of hearts with dexamethasone (1 nM), but not progesterone (1 microM), significantly modified responses to LPS. The action of dexamethasone was time-dependent, having no effect when applied either simultaneously with or pre-perfused for 5 min before the administration of LPS but inhibiting the response to LPS by 91 +/- 1% (n = 4) when pre-perfused for 15 min. The inhibition caused by dexamethasone was blocked by 15 min pretreatment with the glucocorticoid receptor antagonist RU-486 (100 nM) or by 2 min pre-perfusion of a 1:200 dilution of LCPS1, a selective antilipocortin 1 (LC1) neutralizing antibody. 6. Treatment with the protein synthesis inhibitor, cycloheximide (10 microM, for 15 min) selectively blunted LPS-induced vasodilatation, reducing the latter to 3 +/- 5 mmHg (n = 3), while having no effect on vasodilator responses to either bradykinin or sodium nitroprusside. 7. These results indicate that LPS-induced vasodilatation in the rat heart is dependent on activation of kinin B2 receptors and synthesis of NO. In addition, phospholipase A2 (PLA2) is activated by LPS resulting in the release of platelet-activating factor (PAF) and lipoxygenase but not cyclo-oxygenase products. These effects are dependent on de novo synthesis of an intermediate protein which remains to be identified.

Functional lipopolysaccharide receptors of low affinity are constitutively expressed on mouse bone marrow cells

Although lipopolysaccharide (LPS)-induced overproduction of cytokines, involved in the pathogenesis of septic shock, occupies the spotlight of endotoxin research, another LPS effect, the differentiation of various cell types including haematopoietic bone marrow cells (BMC), which is probably related to its radioprotective activity, deserves equal attention. We have previously established that nanomolar concentrations of LPS trigger in human BMC the expression of CD14 by an induction mechanism independent of CD14 or any other molecule anchored to the cell membrane by a glycosyl phosphatidylinositol glycolipid. We now show that this LPS-induced stimulation is triggered by the binding of a small number of LPS molecules (13,000 molecules/cell) to constitutive LPS receptors of low affinity (Kd = 480 nM). This interaction, which was inhibited by a synthetic LPS antagonist, appeared specific, reversible, saturable, time- and temperature-dependent, but was independent of divalent cations, and was inhibited by serum. Exposure of BMC to LPS did not induce a down-modulation of these receptors, but enhanced their sensitivity to trypsin degradation. Inhibition of LPS binding following different treatments correlated with inhibition of BMC stimulation, thus suggesting that the sparse constitutive receptors of low affinity are efficient signalling receptors for LPS.

Differential recovery of macrophages from endotoxin-tolerant states elicited by lipopolysaccharide and enzymatic treatments

Exposure of macrophages to endotoxin (lipopolysaccharide, LPS) leads to a suppression of their capacity to bind LPS and to produce cytokines after reexposure to LPS. This phenomenon is termed endotoxin tolerance, or LPS-induced desensitization. LPS also stimulates the secretion of serine proteases in macrophages, and activates membrane phospholipases. We have investigated the role of trypsin (a serine protease) and of a phosphatidylinositol-specific phospholipase C (PI-PLC, which cleaves GPI-anchored molecules such as CD14), on LPS-induced desensitization. The results obtained by treatment with PI-PLC or in the presence of protease inhibitors, suggested that activation of phospholipases and proteases are not involved in LPS-induced desensitization. However, trypsin treatment of macrophages abolished both LPS binding and cytokine responses. The recovery of macrophages from this trypsin-induced tolerance (restoration of TNF-alpha synthesis without reexpression of LPS-binding sites) was very different from that following LPS-induced tolerance (reexpression of LPS-binding sites without restoration of TNF-alpha synthesis). The results are consistent with the hypothesis that signaling LPS-receptors might be synthesized de novo after trypsin degradation, whereas non-signaling LPS-receptors might be internalized and recycled after preexposure to LPS.

CD14 and CD11b mediate serum-independent binding to human monocytes of an acylpolygalactoside isolated from Klebsiella pneumoniae

A water-soluble acylpolygalactosyl (APG) of 34 kDa was obtained from the Klebsiella pneumoniae membrane by alkaline hydrolysis and delipidation. APG comprises a poly(1,3)galactose chain, a core, and a lipid moiety made of a glucosamine disaccharide with two N-linked beta OH-myristates. The monocyte binding sites for APG were investigated by flow cytometry. Biotin-labelled APG (Biot-APG) bound to monocytes at 4 degrees C in the absence of serum, calcium, and magnesium. The binding was dose dependent, saturable, and displaced by unlabelled APG. Neither the polysaccharide chain present in APG-related molecules nor the PPi group or additional ester-linked myristates and palmitates were required for APG binding. The role of CD11b and CD14 was demonstrated by competitive inhibition with monoclonal antibodies and by the uptake of APG by these solubilized proteins. APG was rapidly internalized into monocytes at 37 degrees C while CD14 and CD11b/CD18 molecules were partially down-modulated. Lipopolysaccharides (LPS) from the same K. pneumoniae strain and from Escherichia coli and Salmonella minnesota partially competed for Biot-APG binding in the absence but not in the presence of serum. When altered by alkaline hydrolysis, those LPS became strong competitors for APG binding. It was concluded that alkaline hydrolysis of the K. pneumoniae membrane yielded molecules structurally related to LPS which bind to LPS membrane receptors in the absence of serum.

Anti-endotoxin monoclonal antibodies inhibit secretion of tumor necrosis factor-alpha by two distinct mechanisms

OBJECTIVE

To determine whether monoclonal antibodies (mAbs) directed against lipopolysaccharide (LPS, endotoxin) act by promoting LPS neutralization, LPS uptake by macrophages, or both processes, the authors assessed the effects of these agents on LPS-induced cytokine secretion and cellular uptake of LPS.

SUMMARY BACKGROUND DATA

MAbs directed against LPS have been shown to attenuate LPS-induced macrophage tumor necrosis factor-alpha (TNF-alpha) secretion, a process that may contribute to protective capacity. The mechanisms by which this process occurs have not been established.

METHODS

MAbs directed against LPS were evaluated in vitro for their capacity to (1) inhibit TNF-alpha secretion, and (2) alter fluorescein isothiocyanate-labeled LPS uptake (employing flow cytometry analysis and fluorescence microscopy) by the macrophage-like cell line RAW 264.7.

RESULTS

MAb 8G9, an IgG3 directed against the O-antigen polysaccharide region of Escherichia coli 0111:B4 LPS, significantly reduced LPS-induced TNF-alpha secretion and promoted a more than 40-fold increase in LPS uptake by macrophages. The authors established that this was mediated by a Fc receptor-mediated process because 8G9 F(ab')2 fragments that lack the Fc portion of the IgG molecule were capable of inhibiting TNF-alpha secretion, but did not promote increased LPS uptake to the same degree. Cross-reactive, anti-deep core/lipid A mAb 1B6, an IgG2a, also promoted uptake of E. coli 0111:B4 LPS and O-antigen polysaccharide-deficient E. coli J5 LPS, but only inhibited TNF-alpha secretion induced by E. coli J5 LPS to which it binds most efficiently. MAb 3D10, an IgM also directed against the O-antigen polysaccharide region of E. coli 0111:B4 LPS, inhibited TNF-alpha secretion but did not increase cellular uptake of LPS, presumably acting solely due to LPS neutralization. Polymyxin B, an antibiotic that binds stoichiometrically to the lipid A portion of LPS, inhibited TNF-alpha secretion and prevented cellular LPS uptake.

CONCLUSIONS

These results suggest that IgG and IgM anti-LPS mAbs exert protective capacity by extracellular neutralization of LPS, while IgG Fc receptor-mediated cellular uptake also may serve to bypass macrophage activation and TNF-alpha secretion by promoting internalization and intracellular neutralization.

A synthetic analog of the 3-deoxy-D-manno-2-octulosonic acid disaccharide moiety of rough-type endotoxins does not bind to mouse peritoneal macrophages and human monocytes

Strong evidence supports the concept that lipid A is the main biologically active region of endotoxins and is recognized by specific binding sites of different cell types. However, receptors for carbohydrates are also present on mononuclear phagocytes, and it has been suggested that one of these lectin-like proteins may be specific for the 3-deoxy-D-manno-2-octolosonic acid (Kdo) residues of endotoxins. To reexamine this hypothesis, we prepared a 125I-labeled conjugate consisting of a synthetic Kdo-2,4-Kdo disaccharide covalently linked to bovine serum albumin (125I-Kdo2-BSA). The Kdo disaccharide residues of this radiolabeled conjugate were fully accessible to a monoclonal antibody which reacts specifically with this epitope. However, 125I-Kdo2-BSA did not exhibit any detectable specific binding on thioglycolate-elicited mouse peritoneal macrophages or on human monocytes. Furthermore, the specific binding of biotin-labeled lipopolysaccharide derivatives to mouse macrophages and human monocytes was not inhibited by a soluble synthetic Kdo-2,4-Kdo-polyacrylamide copolymer or by a synthetic glycolipid consisting of an alpha-Kdo residue glycosidically linked to O-6 of allyl-4-O-phosphoryl-N-3-hydroxytetradecanoyl-beta-D-glucosaminide. These results indicate that binding sites specific for Kdo are not present (or not accessible) on the surface of mouse macrophages and human monocytes.

Bacterial endotoxin rapidly stimulates prolonged endothelium-dependent vasodilatation in the rat isolated perfused heart

1. The effects of bacterial lipopolysaccharide (Escherichia coli 0111-B4; LPS) on coronary vascular tone were examined in the isolated perfused heart of the rat. The role of nitric oxide and/or prostaglandin products of the cyclo-oxygenase pathway in mediating the actions of LPS were also investigated. 2. Coronary vascular tone was raised and maintained by a continuous perfusion of the thromboxane-mimetic U46619 (5 nM). LPS perfusion (0.1-100 micrograms ml-1) caused a concentration-dependent fall in coronary tone without any significant change in the force of cardiac contractility. 3. At 5 micrograms ml-1, LPS reduced perfusion pressure by 38 +/- 9 mmHg. This effect was rapid in onset, maximal within the first 5 min and sustained for 90 +/- 10 min (n = 6). 4. The vasodilatation induced by LPS was dependent on the presence of an intact endothelium and abolished following endothelial damage caused by air embolism. 5. NG-nitro-L-arginine methylester (L-NAME; 50 microM) or NG-nitro-L-arginine (L-NOARG; 50 microM) blocked the vasodilatation induced by LPS (5 micrograms ml-1). The inhibition caused by these arginine analogues was partially reversed by 1 mM L- but not D-arginine. 6. The vasodilator action of LPS was also completely blocked by the glucocorticoid, dexamethasone (10 microM) but unaffected by indomethacin (10 microM). 7. These results suggest that LPS evokes rapid release of nitric oxide (NO) in the microvasculature of the rat isolated heart presumably via activation of the constitutive L-arginine-NO pathway in the endothelium. Furthermore, the lack of effect of indomethacin suggests that prostaglandins released via the cyclo-oxygenase pathway are not involved in mediating this action of LPS.

Biochemical and electron microscopy analysis of the endotoxin binding to microtubules in vitro

The mechanisms involved in cellular activation and damage by bacterial endotoxins are not completely defined. In particular, there is little information about possible intracellular targets of endotoxins. Recently, the participation of a microtubule associated protein in endotoxin actions on macrophages has been suggested. In the present work, we have studied the effect of E. coli lipopolysaccharide on the polymerization of microtubular protein in vitro. Electrophoretic analysis of the polymerization mixtures showed that the endotoxin inhibited the polymerization when present at high concentrations. At lower concentrations, LPS selectively displaced the microtubule associated protein MAP-2 from the polymerized microtubules. Electron microscopy showed that LPS binds to microtubules of tubulin + MAPs and to microtubules of purified tubulin (without MAPs) polymerized with taxol. Gel filtration experiments confirmed the binding of LPS to tubulin, and by ligand blot assays an interaction LPS-MAP-2 was detected. The ability of LPS to interact with microtubular proteins suggests a possible participation of microtubules on the cellular effects of endotoxins.

Binding of a membrane proteoglycan from Klebsiella pneumoniae and its derivatives to human leukocytes

The binding of a membrane proteoglycan from a non-encapsulated strain of Klebsiella pneumoniae (Kp-MPG) and four derivatives thereof, to human leukocytes, was investigated by indirect immunofluorescence using biotinylated F(ab')2 fragments of anti-Kp-MPG antibodies and the streptavidin-phycoerythrin amplification system in flow cytometry. Four Kp-MPG derivatives were studied: 1/ an acylpoly(1,3)galactoside (APG), 2/ an APG preparation submitted to acid hydrolysis which removed all fatty acids, but left intact the galactose chain of APG (GC-APG), 3/ a preparation obtained by mild alkaline hydrolysis, containing additional ester-linked C14 and C16 fatty acids bound to the APG molecule (EFA-APG) and 4/ a polymer of the latter compound (APG pol). Kp-MPG, APG and EFA-APG were shown to bind exclusively to monocytes at the lowest concentrations (from 0.15 to 3 microM APG). At higher concentrations, these compounds interacted with polymorphonuclear leukocytes, and with lymphocyte subsets in the following decreasing order: B cells, NK cells, CD8+ and CD4+ lymphocytes. Neither APG pol or GC-APG nor K. pneumoniae smooth LPS showed significant binding to leukocytes. However Kp-LPS treated by drastic alkaline hydrolysis displayed binding properties similar to those of APG. Removal of the ester-linked C14 and C16 fatty acids from EFA-APG did not affect the binding of the molecule. The capacity of cells from the myelomonocytic lineage to bind Kp-MPG and APG was very low in phenotypically immature cell lines (HL60 and U937) as compared with monocytes or polymorphonuclear cells. Treatment of U937 cells with interferon-gamma up-regulated their APG binding capacity along with the expression of the integrin CD 11 b and the CD 14 molecule, whereas monocytes exposed to interferon-gamma showed an increased binding of APG associated with an elevated expression of the galactose specific lectin Mac-2. The data demonstrate a preferential binding of Kp-MPG and APG to cells of the monocyte/macrophage lineage. APG binding does not involve the poly (1,3) galactose chain and the ester-linked C14 and C16 fatty acids but requires the presence of the hydrophobic part of the molecule.

Binding of a bacterial acylpoly(1,3)galactoside to human blood leucocytes

The binding of a 34-kDa (mol. wt.) acylpoly(1,3)galactoside (APG) extracted from a membrane proteoglycan of Klebsiella pneumoniae to human blood leucocytes was investigated. APG is made of a long poly(1,3)galactose chain, a core-like region and a lipid moiety which comprises two glucosamine residues bound to a phosphate group and two beta OH myristic acids. Fluoresceinated APG was shown to bind preferentially to monocytes and to a lesser extent to polymorphonuclear neutrophils, as determined by flow cytometry. Binding of fluoresceinated APG was inhibited by unlabelled APG; it was concentration dependent, but not saturable, with rapid kinetics. It occurred at +4 degrees C but was markedly increased at 37 degrees C. It involved trypsin-sensitive molecules on the membrane of monocytes. Neither the parent proteoglycan nor lipopolysaccharide from K. pneumoniae or Salmonella minnesota competed for APG binding. A minor non-specific binding to lymphocytes, occurring predominantly on B cells, was observed. Unlike that of lipopolysaccharide, the APG binding was not blocked by polymyxin B sulphate. Interaction between the galactose chain of APG and the galactose receptor does not account for the binding of APG to monocytes because the galactose receptor (Mac-2) is expressed at high density on activated macrophages but not on monocytes. Despite its strong binding to human blood monocytes, APG displayed a much weaker activity than K. pneumoniae membrane proteoglycan with respect to induction of monocyte cytokine synthesis. When administered as a Technetium 99 conjugate, APG was shown to label inflammatory foci in experimental animals, and its property as a marker of macrophages is currently being evaluated in clinical trials.

Binding of endotoxin to macrophages; interactions of spin-labelled saccharide residues

The molecular mechanisms of endotoxin action are poorly understood. A prerequisite to cellular activation by this agent must be interaction (binding) with the plasma membrane. In this study we have investigated the role of the polysaccharide region of endotoxin (LPS) in binding to macrophages and macrophage-like cell lines. The LPS molecules, from Escherichia coli O111.B4, J5 and the lipid-A, were spin labelled with 2,2,6,6-tetramethylpiperidine-N-oxyl] (Tempo) free radical in their sugar residues, and examined by electron spin resonance spectroscopy. This is the first report of the synthesis of spin-labelled endotoxins. Measurement of the rotational correlation times (Tc) indicated that the saccharide resides do not bind to membrane surface structures and suggests that the binding of LPS to macrophages is mediated by the lipid acyl chains. Anti-sera to LPS from E. coli O111.B4 was effective in binding to the polysaccharide of the same LPS bound to the cell surface.

Biosynthetic radiolabeling of bacterial lipopolysaccharide to high specific activity

We describe a method for producing radiolabeled lipopolysaccharide (LPS) by incorporating [3H]acetate into an aceEF, gltA strain of Escherichia coli K12. The LPS has substantially greater specific radioactivity (2 microCi per microgram LPS, or approximately 8 Ci/mmol) than has been reported previously for biosynthetically radiolabeled LPS. The 3H is incorporated into the fatty acyl chains of the lipid A moiety. LPS prepared by this method has several attractive features for biological studies, including native structure and bioactivity, long radioactive half-life, and high specific activity.

Uptake, distribution and fate of bacterial lipopolysaccharides in monocytes and macrophages: an ultrastructural and functional correlation

Bacterial lipopolysaccharides (LPS), which are important components of the cell wall of gram-negative bacteria, induce a number of host responses both beneficial and harmful. The present review elucidates the uptake, distribution and functions of LPS in mononuclear phagocytes in an attempt to gain an insight into the mechanisms which control the pathogenesis of LPS mediated septic shock. The unique feature of LPS bilayer structure, the tagged LPS and antibodies to LPS provide means for studying binding, uptake, fate and subcellular distribution of LPS in tissues and cells. LPS bind to monocytes and macrophages by specific interaction via receptors such as scavenger receptors, CD14 and CD18 and by non-specific interactions, and enter the cells via receptor-mediated endocytosis, absorptive pinocytosis, phagocytosis, and diffusion. The ingested LPS are localized in pinocytic vesicles, phagocytic vacuoles, cytoplasm, mitochondria, rough endoplasmic reticulum, Golgi apparatus, and nucleus. The interactions of LPS with monocytes and macrophages trigger a broad spectrum of cellular responses, including production of important bioactive factors or mediators, such as IL-1, TNF, interferons, prostaglandins, and macrophage-derived growth factor, which are implicated in the pathogenesis of septic shock and wound healing. However, there is no conclusive evidence indicating that production of the mediators can only be induced through specific interactions.

Binding of endotoxin to macrophages: distinct effects of serum constituents

The respective roles of serum lipoproteins, and of the complement component C3, in the binding of endotoxin (LPS) to macrophages were analyzed by an in vitro assay using [3H]LPS. The addition of an anti-C3 serum in the medium induced an apparent abolishment of the specific binding of LPS to mouse macrophages, but this effect appeared to be due to an actual increase of nonspecific binding. Isolated complexes of LPS with lipoproteins of high density (HDL3) and of very high density (VHDL) did not bind to macrophages. Furthermore, addition of HDL3 and VHDL in the incubation medium inhibited the specific binding of LPS to macrophages. These results suggest that C3 reduces nonspecific interactions between LPS and macrophages whereas associations between LPS and HDL3 or VHDL inhibit specific LPS-macrophage interactions.

Specific binding of lipopolysaccharides to mouse macrophages--II. Involvement of distinct lipid a substructures

The interaction of lipopolysaccharide-binding sites of mouse macrophages with the Lipid A region of endotoxins (LPS) was demonstrated by direct binding of labeled Lipid A conjugates, by inhibition of the binding of labeled LPS with anti-Lipid A monoclonal antibodies, and by the considerable reduction of this binding after chemical and enzymatic removal of the fatty acid esters of the LPS. The substructures of Lipid A required for the specific binding of LPS to macrophages were analyzed by the use of synthetic lipids consisting of mono- or disaccharide derivatives of glucosamine. The two phosphate groups of Lipid A (at positions 1 and 4') as well as certain hydroxyl groups, appeared to play a critical role in the binding. However, the reactivities of the synthetic lipids with the macrophage surface, as compared with those with anti-Lipid A antibodies, could hardly be explained by the existence of a single LPS receptor, and suggested the presence, on the macrophage surface, of different LPS-binding sites that recognize different substructures or spatial configurations of the lipid moiety of endotoxins.