Polymyxin B-horseradish peroxidase conjugates as tools in endotoxin research

Heterogeneity of Lipopolysaccharide as Source of Variability in Bioassays and LPS-Binding Proteins as Remedy

Lipopolysaccharide (LPS), also referred to as endotoxin, is the major component of Gram-negative bacteria's outer cell wall. It is one of the main types of pathogen-associated molecular patterns (PAMPs) that are known to elicit severe immune reactions in the event of a pathogen trespassing the epithelial barrier and reaching the bloodstream. Associated symptoms include fever and septic shock, which in severe cases, might even lead to death. Thus, the detection of LPS in medical devices and injectable pharmaceuticals is of utmost importance. However, the term LPS does not describe one single molecule but a diverse class of molecules sharing one common feature: their characteristic chemical structure. Each bacterial species has its own pool of LPS molecules varying in their chemical composition and enabling the aggregation into different supramolecular structures upon release from the bacterial cell wall. As this heterogeneity has consequences for bioassays, we aim to examine the great variability of LPS molecules and their potential to form various supramolecular structures. Furthermore, we describe current LPS quantification methods and the LPS-dependent inflammatory pathway and show how LPS heterogeneity can affect them. With the intent of overcoming these challenges and moving towards a universal approach for targeting LPS, we review current studies concerning LPS-specific binders. Finally, we give perspectives for LPS research and the use of LPS-binding molecules.

Endotoxin-like properties of an extract from a symbiotic, eukaryotic Chlorella-like green alga

Hot phenol-water extraction of axenic cultures of the eukaryotic symbiotic green alga, Chlorella, yielded a substance having many of the characteristics of bacterial endotoxin or lipopolysaccharide (LPS). This material caused gelation of extracts of Limulus amoebocyte lysate at concentrations similar to those manifest by LPS from Gram-negative enteric bacteria. Activity was reduced substantially by incubation with polymyxin B sulfate and Limulus endotoxin-neutralizing protein (ENP), both of which have been shown to neutralize the biological activity of LPS from Gram-negative enteric bacteria. Partially purified biologically active material was found to contain 3-deoxy-D-manno-octulosonic acid (KDO), a sugar characteristically found in LPS. Acid hydrolysis of the Chlorella extract yielded a precipitate with characteristics of lipid A. Further hydrolysis followed by methylation yielded products with retention times on gas chromatography indistinguishable from 3-hydroxylauric and 3-hydroxymyristic acids. Results of transmission electron microscopy and sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by silver staining are consistent with those obtained with purified enteric bacterial LPS. Exhaustive precautions against potential experimental artifacts due to cross contamination by endotoxin from Gram-negative bacteria, either in algal cultures or associated with glassware allowed the conclusion that the green alga, Chlorella (strain NC64A) synthesizes a LPS-like molecule.

Detection of lipopolysaccharides blotted on nylon membranes

A sensitive method for the detection of Gram-negative bacterial lipopolysaccharide (LPS) blotted on nylon membranes is described. LPSs are separated by SDS-PAGE and then electrophoretically transferred to nylon membranes. Immobilized LPS is oxidized with periodate and then reacted with a hydrazide conjugated to the steroid, digoxigenin. LPS is visualized by alkaline phosphatase labelled antibodies against the steroid and the enzyme substrate 5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium. LPS banding patterns of both rough (R-) and smooth (S-) type LPSs from over 15 different bacterial species are similar to those of silver stained companion gels, but without nonspecific staining of proteins. The detection of S-LPS from Pseudomonas aeruginosa F-D type 1 and R-LPS from Escherichia coli K12 is sensitive to 10-20 ng per lane. The use of this detection system in combination with antibody or lectin studies on identical blots can provide an effective tool in locating the precise position of certain epitopes or sequences in both R- and S-type LPSs on the blots.

A Novel Chemical Biology Approach for Mapping of Polymyxin Lipopeptide Antibody Binding Epitopes

Polymyxins B and E (i.e., colistin) are a family of naturally occurring lipopeptide antibiotics that are our last line of defense against multidrug resistant (MDR) Gram-negative pathogens. Unfortunately, nephrotoxicity is a dose-limiting factor for polymyxins that limits their clinical utility. Our recent studies demonstrate that polymyxin-induced nephrotoxicity is a result of their extensive accumulation in renal tubular cells. The design and development of safer, novel polymyxin lipopeptides is hampered by our limited understanding of their complex structure-nephrotoxicity relationships. This is the first study to employ a novel targeted chemical biology approach to map the polymyxin recognition epitope of a commercially available polymyxin mAb and demonstrate its utility for mapping the kidney distribution of a novel, less nephrotoxic polymyxin lipopeptide. Eighteen novel polymyxin lipopeptide analogues were synthesized with modifications in the polymyxin core domains, namely, the N-terminal fatty acyl region, tripeptide linear segment, and cyclic heptapeptide. Surface plasmon resonance epitope mapping revealed that the monoclonal antibody (mAb) recognition epitope consisted of the hydrophobic domain (N-terminal fatty acyl and position 6/7) and diaminobutyric acid (Dab) residues at positions 3, 5, 8, and 9 of the polymyxin molecule. Structural diversity within the hydrophobic domains and Dab 3 position are tolerated. Enlightened with an understating of the structure-binding relationships between the polymyxin mAb and the core polymyxin scaffold, we can now rationally employ the mAb to probe the kidney distribution of novel polymyxin lipopeptides. This information will be vital in the design of novel, safer polymyxins through chemical tailoring of the core scaffold and exploration of the elusive/complex polymyxin structure-nephrotoxicity relationships.

Polymyxins as novel and safe mucosal adjuvants to induce humoral immune responses in mice

There is currently an urgent need to develop safe and effective adjuvants for enhancing vaccine-induced antigen-specific immune responses. We demonstrate here that intranasal immunization with clinically used polypeptide antibiotics, polymyxin B (PMB) and colistin (CL), along with ovalbumin (OVA), increases OVA-specific humoral immune responses in a dose-dependently manner at both mucosal and systemic compartments. Enhanced immunity by boosting was found to persist during 8 months of observation. Moreover, mice intranasally immunized with OVA plus various doses of PMB or CL showed neither inflammatory responses in the nasal cavity and olfactory bulbs nor renal damages, compared to those given OVA alone. These data suggest that polymyxins may serve as novel and safe mucosal adjuvants to induce humoral immune responses. The polymyxin adjuvanticity was found to be independent of endotoxins liberated by its bactericidal activity, as indicated by similar enhancing effects of PMB in lipopolysaccharide (LPS)-hyporesponsive and LPS-susceptible mice. However, despite the presence of preexisting anti-PMB antibodies, we observed no reduction in the adjuvant function of polymyxins when they were given intranasally. Furthermore, the titers of OVA-specific Abs in mice intranasally immunized with OVA plus PMB or CL were significantly higher than those in mice administered with polymyxin analogues, such as polymyxin B nonapeptide and colistin methanesulfonate. The levels of released β-hexosaminidase and histamine in mast cell culture supernatants stimulated by PMB or CL were also significantly higher than those stimulated by their analogues. These results suggest that both the hydrophobic carbon chain and hydrophilic cationic cyclic peptide contribute to the mucosal adjuvanticity of PMB and CL.

Monodisperse and LPS-free Aggregatibacter actinomycetemcomitans leukotoxin: interactions with human β2 integrins and erythrocytes

Aggregatibacter actinomycetemcomitans is a gram-negative, facultatively anaerobic cocco-bacillus and a frequent member of the human oral flora. It produces a leukotoxin, LtxA, belonging to the repeats-in-toxin (RTX) family of bacterial cytotoxins. LtxA efficiently kills neutrophils and mononuclear phagocytes. The known receptor for LtxA on leukocytes is integrin α(L)β(2) (LFA-1 or CD11a/CD18). However, the molecular mechanisms involved in LtxA-mediated cytotoxicity are poorly understood, partly because LtxA has proven difficult to prepare for experiments as free of contaminants and with its native structure. Here, we describe a protocol for the purification of LtxA from bacterial culture supernatant, which does not involve denaturing procedures. The purified LtxA was monodisperse, well folded as judged by the combined use of synchrotron radiation circular dichroism spectroscopy (SRCD) and in silico prediction of the secondary structure content, and free of bacterial lipopolysaccharide. The analysis by SRCD and similarity to a lipase from Pseudomonas with a known three dimensional structure supports the presence of a so-called beta-ladder domain in the C-terminal part of LtxA. LtxA rapidly killed K562 target cells transfected to express β(2) integrin. Cells expressing α(M)β(2) (CD11b/CD18) or α(X)β(2) (CD11c/CD18) were killed as efficiently as cells expressing α(L)β(2). Erythrocytes, which do not express β(2) integrins, were lysed more slowly. In ligand blotting experiments, LtxA bound only to the β(2) chain (CD18). These data support a previous suggestion that CD18 harbors the major binding site for LtxA as well as identifies integrins α(M)β(2) and α(X)β(2) as novel receptors for LtxA.

Oxidized hemoglobin is an endogenous proinflammatory agonist that targets vascular endothelial cells

Several pathologic conditions are associated with hemolysis, i.e. release of ferrous (Fe(II)) hemoglobin from red blood cells. Oxidation of cell-free hemoglobin produces (Fe(III)) methemoglobin. More extensive oxidation produces (Fe(III)/Fe(IV) O) ferryl hemoglobin. Both cell-free methemoglobin and ferryl hemoglobin are thought to contribute to the pathogenesis of hemolytic disorders. We show hereby that ferryl hemoglobin, but not hemoglobin or methemoglobin, acts as a potent proinflammatory agonist that induces vascular endothelial cells in vitro to rearrange the actin cytoskeleton, forming intercellular gaps and disrupting the integrity of the endothelial cell monolayer. Furthermore, ferryl hemoglobin induces the expression of proinflammatory genes in endothelial cells in vitro, e.g. E-selectin, Icam-1, and Vcam-1, through the activation of the nuclear factor kappaB family of transcription factors. This proinflammatory effect, which requires actin polymerization, involves the activation of the c-Jun N-terminal kinase and the p38 mitogen-activated protein kinase signal transduction pathways. When administered to naïve mice, ferryl hemoglobin induces the recruitment of polymorphonuclear cells, demonstrating that it acts as a proinflammatory agonist in vivo. In conclusion, oxidized hemoglobin, i.e. ferryl hemoglobin, acts as a proinflammatory agonist that targets vascular endothelial cells.

Covalent polymyxin B conjugate with human immunoglobulin G as an antiendotoxin reagent

Polymyxin B (PMB) is a cyclic decapeptide antibiotic which also binds and neutralizes endotoxin. Unfortunately, PMB can be considerably nephrotoxic at clinically utilized doses, thereby limiting its utility as a therapeutic antiendotoxin reagent. We sought to change the pharmacokinetics and toxicity profile of PMB by covalently linking it to a human immunoglobulin G (IgG) carrier. Conjugates of PMB with IgG were prepared by EDAC [1-ethyl-3-(3-dimethylaminopropyl) carbodiimide]-mediated amide formation. Analysis by dot enzyme-linked immunosorbent assay with an anti-PMB monoclonal antibody showed that the purified conjugate contained bound PMB. The IgG-PMB conjugate reacted with lipid A and J5 lipopolysaccharide in Western blot assays in a manner comparable to that of whole antiserum with anti-lipid A reactivity; unconjugated IgG had no reactivity. The PMB bound in the conjugate retained its endotoxin-neutralizing activity compared to that of unbound PMB as evidenced by its dose-dependent inhibition of tumor necrosis factor release by endotoxin-stimulated human monocytes in vitro; unconjugated IgG had no activity. By this assay, the PMB-IgG conjugate was determined to have approximately 3.0 microg of bound functional PMB per 100 microg of total protein of conjugate (five molecules of PMB per IgG molecule). The PMB-IgG conjugate was also bactericidal against clinical strains of Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae relative to unconjugated IgG with MBCs of <4 microg of conjugate per ml for each of the tested strains. The conjugate appeared to be nontoxic at the highest doses deliverable and provided statistically significant protection from death to galactosamine-sensitized, lipopolysaccharide-challenged mice in a dose-dependent fashion when administered prophylactically 2 h before challenge. However, neither free PMB nor the PMB-IgG conjugate could protect mice challenged with endotoxin 2 h after administration. This suggests that these reagents can play a role in prophylaxis but not in therapy of sepsis. These experiments demonstrated that the PMB-IgG conjugate retains bound yet functional PMB as evidenced by its endotoxin-neutralizing activity both in vitro and in vivo. Further work is required to define the role that this or related conjugate compounds may play in the prophylaxis of endotoxin-mediated disease.

Recognition of antigenic epitopes in lipopolysaccharide and protein from Actinobacillus actinomycetemcomitans by serum antibodies in untreated rapidly progressive periodontitis patients

Actinobacillus actinomycetemcomitans has been associated with early-onset periodontitis, including the localized juvenile and rapidly progressive forms. The immunodominant antigens of A. actinomycetemcomitans recognized by rapidly progressive periodontitis patients remain unidentified. Sera from 22 patients with rapidly progressive periodontitis and 20 periodontally normal subjects were tested by enzyme-linked immunosorbent assay (ELISA) for immunoglobulin G antibodies to whole-cell sonicate, protein, purified lipopolysaccharide and lipopolysaccharide fractions of A. actinomycetemcomitans. The median titers of rapidly progressive periodontitis patients and control subjects to whole-cell sonicate were 25.0 and 14.5 ELISA units, respectively (not significantly different). Binding of antibody from patient sera occurred to both the lipopolysaccharide and the protein fractions, with greater binding to lipopolysaccharide than to protein. We show for the first time that patient sera contain antibodies that bind specifically to antigenic epitopes in lipid A and in the core carbohydrate of lipopolysaccharide that were previously considered to be inaccessible and unavailable, as well as to epitopes in the O side chains. Sera manifesting antibody titers 2-fold or greater than the median titer for control sera were judged to be seropositive. More patients were seropositive for lipid A than for any of the other antigen preparations studied, and the median titer for patient sera to lipid A but to none of the other purified lipopolysaccharide fractions was significantly elevated relative to control values. Of 22 patients, 10 were seropositive to whole-cell sonicate, 7 to protein, 8 to lipopolysaccharide, 7 to the high-molecular-weight lipopolysaccharide-polysaccharide fraction rich in O side chains, and 16 to lipid A. The core carbohydrate did not adhere to the test plate surface, and this precluded ELISA measurements. However, when the core carbohydrate was used in the ELISA inhibition assay, it reduced antibody binding to lipopolysaccharide-coated plates by up to 45%, thereby demonstrating antibody binding to core carbohydrate. The core carbohydrate fraction from the Re mutant of Salmonella minnesota known to contain no O-side chains also inhibited binding of specific antibody to plates coated with A actinomycetemcomitans lipopolysaccharide. Overall, there was extreme variation in responses among patients to the various antigen preparations, with no single pattern dominating. Lipopolysaccharide and its components appear to be the immunodominant epitopes, since most rapidly progressive periodontitis patients are seropositive for lipopolysaccharide and/or its components and they have titers relative to those for proteins.

Detection of endotoxin using an evanescent wave fiber-optic biosensor

The lipopolysaccharide endotoxin is the most powerful immune stimulant known and a causative agent in the clinical syndrome known as sepsis. Sepsis is responsible for more than 100,000 deaths annually, in large part due to the lack of a rapid, reliable, and sensitive diagnostic technique. This study describes the detection of LPS from E. coli at concentrations as low as 10 ng/mL, in 30 s using an evanescent wave fiber-optic biosensor. Polymyxin B, covalently immobilized onto the surface of the fiber-optic probe, selectively bound fluorescently labeled LPS. Unlabeled LPS was detected in a competitive assay format using labeled LPS for signal generation. The competitive assay format worked in both buffer and plasma with similar sensitivities. This method can be used with other LPS capture molecules such as antibodies, lectins, or antibiotics, to simultaneously detect LPS and to determine the LPS serotype. The LPS assay using the fiber-optic biosensor is applicable to both clinical and environmental testing.