Lipid A heterogeneity and its role in the host interactions with pathogenic and commensal bacteria

Lipopolysaccharide (LPS) is for most but not all Gram-negative bacteria an essential component of the outer leaflet of the outer membrane. LPS contributes to the integrity of the outer membrane, which acts as an effective permeability barrier to antimicrobial agents and protects against complement-mediated lysis. In commensal and pathogenic bacteria LPS interacts with pattern recognition receptors (e.g LBP, CD14, TLRs) of the innate immune system and thereby plays an important role in determining the immune response of the host. LPS molecules consist of a membrane-anchoring lipid A moiety and the surface-exposed core oligosaccharide and O-antigen polysaccharide. While the basic lipid A structure is conserved among different bacterial species, there is still a huge variation in its details, such as the number, position and chain length of the fatty acids and the decoration of the glucosamine disaccharide with phosphate, phosphoethanolamine or amino sugars. New evidence has emerged over the last few decades on how this lipid A heterogeneity confers distinct benefits to some bacteria because it allows them to modulate host responses in response to changing host environmental factors. Here we give an overview of what is known about the functional consequences of this lipid A structural heterogeneity. In addition, we also summarize new approaches for lipid A extraction, purification and analysis which have enabled analysis of its heterogeneity.

Activation of Canine, Mouse and Human TLR2 and TLR4 by Inactivated Leptospira Vaccine Strains

Canine Leptospira vaccines contain inactivated strains of pathogenic Leptospira, the causative agents of leptospirosis. For an effective response to vaccination, activation of the innate immune system via pattern recognition receptors such as TLRs is crucial. However, it is not known which TLRs are activated by Leptospira in dogs. To investigate the involvement of canine TLR2, TLR4, and TLR5 in the recognition of Leptospira, we stimulated canine moDC and reporter cells expressing canine TLR2 with either whole-inactivated bacteria or purified LPS of Leptospira strains, representing the serogroups generally used in canine leptospirosis vaccines. Using the endotoxin neutralizing reagent polymyxin B and TLR4 antagonist RS-LPS, we demonstrate that Leptospira LPS and canine TLR4 are involved in IL-1β production as well as in the uptake of inactivated Leptospira in canine moDC. Furthermore, polymyxin B only partially inhibited IL-1β production induced by inactivated Leptospira, suggesting that next to TLR4, also other TLRs may be involved. The observed activation of canine TLR2-expressing reporter cells by inactivated Leptospira strains indicates that TLR2 could be one of these TLRs. Next, we analyzed TLR2 and TLR4 activating capabilities by the same Leptospira strains using human and mouse TLR-expressing reporter cells. Inactivated Leptospira and leptospiral LPS activated not only mouse, but also human TLR4 and this activation was shown to be LPS dependent in both cases. Additionally, inactivated Leptospira activated mouse and human TLR2-expressing reporter cell lines. In our study, we could not identify significant species differences in the recognition of Leptospira by TLR2 and TLR4 between dog, human and mouse. Lastly, we show that these inactivated Leptospira strains are recognized by both mouse and human TLR5 reporter cells only after exposure to additional heat-treatment. Unfortunately, we were not able to confirm this in the canine system. Our data show that TLR2 and TLR4 are involved in the recognition of Leptospira strains used in the production of canine Leptospira vaccines. This study contributes to the understanding of Leptospira-induced innate immune responses in dogs, humans, and mice. Future studies are needed to further explore the role of canine TLR2, TLR4 and TLR5 in the induction of vaccine-mediated immunity against Leptospira.

Nanoflow LC-MS Method Allowing In-Depth Characterization of Natural Heterogeneity of Complex Bacterial Lipopolysaccharides

The variable modification of the outer membrane lipopolysaccharide (LPS) in Gram-negative bacteria contributes to bacterial pathogenesis through various mechanisms, including the development of antibiotic resistance and evasion of the immune response of the host. Characterizing the natural structural repertoire of LPS is challenging due to the high heterogeneity, branched architecture, and strong amphipathic character of these glycolipids. To address this problem, we have developed a method enabling the separation and structural profiling of complex intact LPS mixtures by using nanoflow reversed-phase high-performance liquid chromatography (nLC) coupled to electrospray ionization Fourier transform mass spectrometry (ESI-FT-MSⁿ). Nanogram quantities of rough-type LPS mixtures from Neisseria meningitidis could be separated and analyzed by nLC-ESI-FT-MS. Furthermore, the method enabled the analysis of highly heterogeneous smooth (S)-type LPS from pathogenic enteric bacteria such as Salmonella enterica serotype Typhimurium and Escherichia coli serotype O111:B4. High-resolution, accurate mass spectra of intact LPS containing various lengths of the O-specific polysaccharide in the range of 3 and 15 kDa were obtained. In addition, MS/MS experiments with collision-induced dissociation of intact LPS provided detailed information on the composition of oligo/polysaccharides and lipid A domains of single S-type LPS species. The structural heterogeneity of S-type LPS was characterized by unprecedented details. Our results demonstrate that nLC-ESI-FT-MSⁿ is an attractive strategy for the structural profiling of small quantities of complex bacterial LPS mixtures in their intact form.

Naturally circulating pertactin-deficient Bordetella pertussis strains induce distinct gene expression and inflammatory signatures in human dendritic cells

Respiratory infections caused by Bordetella pertussis are reemerging despite high pertussis vaccination coverage. Since the introduction of the acellular pertussis vaccine in the late twentieth century, circulating B. pertussis strains increasingly lack expression of the vaccine component pertactin (Prn). In some countries, up to 90% of the circulating B. pertussis strains are deficient in Prn. To better understand the resurgence of pertussis, we investigated the response of human monocyte-derived dendritic cells (moDCs) to naturally circulating Prn-expressing (Prn-Pos) and Prn-deficient (Prn-Neg) B. pertussis strains from 2016 in the Netherlands. Transcriptome analysis of moDC showed enriched IFNα response-associated gene expression after exposure to Prn-Pos B. pertussis strains, whereas the Prn-Neg strains induced enriched expression of interleukin- and TNF-signaling genes, as well as other genes involved in immune activation. Multiplex immune assays confirmed enhanced proinflammatory cytokine secretion by Prn-Neg stimulated moDC. Comparison of the proteomes from the Prn-Pos and Prn-Neg strains revealed, next to the difference in Prn, differential expression of a number of other proteins including several proteins involved in metabolic processes. Our findings indicate that Prn-deficient B. pertussis strains induce a distinct and stronger immune activation of moDCs than the Prn-Pos strains. These findings highlight the role of pathogen adaptation in the resurgence of pertussis as well as the effects that vaccine pressure can have on a bacterial population.

Substrate specificity of the pyrophosphohydrolase LpxH determines the asymmetry of Bordetella pertussis lipid A

Lipopolysaccharides are anchored to the outer membrane of Gram-negative bacteria by a hydrophobic moiety known as lipid A, which potently activates the host innate immune response. Lipid A of Bordetella pertussis, the causative agent of whooping cough, displays unusual structural asymmetry with respect to the length of the acyl chains at the 3 and 3' positions, which are 3OH-C10 and 3OH-C14 chains, respectively. Both chains are attached by the acyltransferase LpxA, the first enzyme in the lipid A biosynthesis pathway, which, in B. pertussis, has limited chain length specificity. However, this only partially explains the strict asymmetry of lipid A. In attempts to modulate the endotoxicity of B. pertussis lipid A, here we expressed the gene encoding LpxA from Neisseria meningitidis, which specifically attaches 3OH-C12 chains, in B. pertussis This expression was lethal, suggesting that one of the downstream enzymes in the lipid A biosynthesis pathway in B. pertussis cannot handle precursors with a 3OH-C12 chain. We considered that the UDP-diacylglucosamine pyrophosphohydrolase LpxH could be responsible for this defect as well as for the asymmetry of B. pertussis lipid A. Expression of meningococcal LpxH in B. pertussis indeed resulted in new symmetric lipid A species with 3OH-C10 or 3OH-C14 chains at both the 3 and 3' positions, as revealed by MS analysis. Furthermore, co-expression of meningococcal lpxH and lpxA resulted in viable cells that incorporated 3OH-C12 chains in B. pertussis lipid A. We conclude that the asymmetry of B. pertussis lipid A is determined by the acyl chain length specificity of LpxH.

Modulating endotoxin activity by combinatorial bioengineering of meningococcal lipopolysaccharide

Neisseria meningitidis contains a very potent hexa-acylated LPS that is too toxic for therapeutic applications. We used systematic molecular bioengineering of meningococcal LPS through deletion of biosynthetic enzymes in combination with induction of LPS modifying enzymes to yield a variety of novel LPS mutants with changes in both lipid A acylation and phosphorylation. Mass spectrometry was used for detailed compositional determination of the LPS molecular species, and stimulation of immune cells was done to correlate this with endotoxic activity. Removal of phosphethanolamine in lipid A by deletion of lptA slightly reduces activity of hexa-acylated LPS, but this reduction is even more evident in penta-acylated LPS. Surprisingly, expression of PagL deacylase in a penta-acylated lpxL1 mutant increased LPS activity, contradicting the general rule that tetra-acylated LPS is less active than penta-acylated LPS. Further modification included expression of lpxP, an enzyme known to add a secondary 9-hexadecenoic acid to the 2’ acyl chain. The LpxP enzyme is temperature-sensitive, enabling control over the ratio of expressed modified hexa- and penta-acylated LPS by simply changing the growth temperature. These LPS derivatives display a broad range of TLR4 activity and differential cytokine induction, which can be exploited for use as vaccine adjuvant or other TLR4-based therapeutics.

Vaccine-Mediated Activation of Human TLR4 Is Affected by Modulation of Culture Conditions during Whole-Cell Pertussis Vaccine Preparation

The potency of whole-cell pertussis (wP) vaccines is still determined by an intracerebral mouse protection test. To allow development of suitable in vitro alternatives to this test, insight into relevant parameters to monitor the consistency of vaccine quality is essential. To this end, a panel of experimental wP vaccines of varying quality was prepared by sulfate-mediated suppression of the BvgASR master virulence regulatory system of Bordetella pertussis during cultivation. This system regulates the transcription of a range of virulence proteins, many of which are considered important for the induction of effective host immunity. The protein compositions and in vivo potencies of the vaccines were BvgASR dependent, with the vaccine containing the highest amount of virulence proteins having the highest in vivo potency. Here, the capacities of these vaccines to stimulate human Toll-like receptors (hTLR) 2 and 4 and the role these receptors play in wP vaccine-mediated activation of antigen-presenting cells in vitro were studied. Prolonged BvgASR suppression was associated with a decreased capacity of vaccines to activate hTLR4. In contrast, no significant differences in hTLR2 activation were observed. Similarly, vaccine-induced activation of MonoMac-6 and monocyte-derived dendritic cells was strongest with the highest potency vaccine. Blocking of TLR2 and TLR4 showed that differences in antigen-presenting cell activation could be largely attributed to vaccine-dependent variation in hTLR4 signalling. Interestingly, this BvgASR-dependent decrease in hTLR4 activation coincided with a reduction in GlcN-modified lipopolysaccharides in these vaccines. Accordingly, expression of the lgmA-C genes, required for this glucosamine modification, was significantly reduced in bacteria exposed to sulfate. Together, these findings demonstrate that the BvgASR status of bacteria during wP vaccine preparation is critical for their hTLR4 activation capacity and suggest that including such parameters to assess consistency of newly produced vaccines could bring in vitro testing of vaccine quality a step closer.

Lipopolysaccharide engineering in Neisseria meningitidis: structural analysis of different pentaacyl lipid A mutants and comparison of their modified agonist properties

Engineering the lipopolysaccharide (LPS) biosynthetic pathway offers the potential to obtain modified derivatives with optimized adjuvant properties. Neisseria meningitidis strain H44/76 was modified by expression of the pagL gene encoding lipid A 3-O-deacylase from Bordetella bronchiseptica and by inactivation of the lgtB gene encoding the terminal oligosaccharide galactosyltransferase. Mass spectrometry analysis of purified mutant LPS was used for detailed compositional analysis of all present molecular species. This determined that the modified LPS was mainly pentaacylated, demonstrating high efficiency of conversion from the hexaacyl to the 3-O-deacylated form by heterologous lipid A 3-O-deacylase (PagL) expression. MS analyses also provided evidence for expression of only one major oligosaccharide glycoform, which lacked the terminal galactose residue as expected from inactivation of the lgtB gene. The immunomodulatory properties of PagL-deacylated LPS were compared with another pentaacyl form obtained from an lpxL1(-) mutant, which lacks the 2' secondary acyl chain. Although both LPS mutants displayed impaired capacity to induce production of the pro-inflammatory cytokine IL-6 in the monocytic cell line Mono Mac 6, induction of the Toll-interleukin-1 receptor domain-containing adaptor-inducing interferon-β-dependent chemokine interferon-γ-induced protein 10 was largely retained only for the lgtB(-)/pagL(+) mutant. Removal of remaining hexaacyl species exclusively present in lgtB(-)/pagL(+) LPS demonstrated that these minor species potentiate but do not determine the activity of this LPS. These results are the first to indicate a qualitatively different response of human innate cells to pentaacyl lpxL1(-) and pagL(+) LPS and show the importance of detailed structure-function analysis when working with modified lipid A structures. The pagL(+) LPS has significant potential as immune modulator in humans.

Intact rough- and smooth-form lipopolysaccharides from Escherichia coli separated by preparative gel electrophoresis exhibit differential biologic activity in human macrophages

We established a new preparative separation procedure, based on DOC/PAGE, to isolate intact lipopolysaccharide (LPS) fractions from natural LPS preparations of Escherichia coli. Analysis of the chemical integrity of LPS fractions by MS showed that no significant chemical modifications were introduced by the procedure. Contamination with toll-like receptor 2 (TLR2)-reactive cell-wall components present in the natural LPS mixture was effectively removed by the procedure, as determined by the absence of reactivity of the purified fractions in a HEK293-TLR2 cell line. Biologic analysis of LPS fractions derived from E. coli O111 in human macrophages demonstrated that the rough (R), semirough (SR) and smooth (S) LPS fractions were highly active at inducing tumor necrosis factor-alpha (TNF-α) in the presence of human serum; however, on a weight basis the R-LPS and SR-LPS fractions were more active, by a factor of 10-100, than was the S-LPS fraction. Under serum-free conditions, the natural LPS mixture, as well as the R-LPS and SR-LPS fractions, showed dose-dependent activation of macrophages, although the response was attenuated by about 10- to 100-fold. In contrast, the S-LPS fraction failed to induce TNF-α. Remarkably, the dose-response of the natural LPS mixture resembled that of the R-LPS and SR-LPS fractions, supporting that short-chain (R and SR) forms of LPS dominate the innate immune response of human macrophages to LPS in vitro. Biologic activity to the S-LPS fraction under serum-free conditions could be restored by the addition of recombinant lipopolysaccharide-binding protein (LBP). In contrast, soluble cluster of differentiation antigen 14 was not able to confer activity of the S-LPS fraction, indicating a crucial role of LBP in the recognition of S-LPS by human macrophages.

Isolation of smooth-type lipopolysaccharides to electrophoretic homogeneity

Polyacrylamide slab gel electrophoresis in the presence of sodium dodecyl sulfate or sodium deoxycholate (SDS- or DOC-slab-PAGE) is a powerful technique for the separation of smooth(S)-type bacterial lipopolysaccharides (LPS). In order to recover the individual LPS species from the polyacrylamide gel for subsequent analyses, a sensitive, nondestructive reverse staining of slab-PAGE-separated LPS has been developed. The individual reverse-stained LPS bands can be rapidly and efficiently recovered into an aqueous 5% triethylamine solution when they are extruded to produce fine gel microparticles. Based on these principles, an isolation methodology that combines preparative slab-PAGE, reverse staining, extrusion, and passive elution can be used to isolate, to electrophoretic homogeneity, micrograms to hundreds of micrograms of individual LPS species successfully from smooth-type LPS mixtures.

Isolation of smooth-type lipopolysaccharides to electrophoretic homogeneity

The high structural heterogeneity of smooth-type lipopolysaccharides (LPS) enormously complicates the isolation of their constituent molecular species. Proof of concept is given here on the feasibility of using preparative slab-PAGE to isolate highly homogeneous smooth-type LPS glycoforms. LPS species (from 3.6 to 14.2 kDa) from Escherichia coli K-235 were separated by preparative slab-PAGE and recovered by utilizing the combined on-gel LPS reverse staining, extrusion, and passive elution techniques. As a result, 15 electrophoretically pure LPS fractions were obtained. The LPS content in the recovered fractions ranged from 280 ng (intermediate mobility glycoforms) to 411 mug (highest mobility glycoforms). The quantities of LPS fractions were sufficient to allow quantitation of the Limulus amebocyte lysate (LAL) activities of these distinct-molecular-mass LPS species, in the range from (1.1 +/- 0.1)x10(3) to (8.7 +/- 0.3)x10(5) endotoxin units (EU)/mL, by standard LAL assay. We have thus definitively demonstrated that slab-PAGE may be a suitable platform to more selectively purify individual glycoform fractions from smooth-type LPS.

Preparation of plasmid DNA-containing liposomes using a high-pressure homogenization–extrusion technique

High-pressure homogenization-extrusion (HPHE) is a method that can be used for downsizing large lipid vesicles with commercially available instrumentation (e.g., from Avestin Inc., Canada), which covers a full range of processing capacities from laboratory (0.5-3.5 mL) to large-scale continuous (1-1000 L/h) production. Consequently, the feasibility (at the laboratory scale) of using HPHE for producing DNA-loaded liposomes by the conventional dehydration-rehydration method was explored. HPHE-generated small unilamellar vesicles had a mean size in the range of 27-76 nm depending on the number of processing cycles and lipid (PC:DOPE:DOTAP or PC:DOPE:Ethyl-DOPC, 1:0.5:0.5, mol/mol) formulation. The size could be further regulated by the pore size (50 or 100 nm) of the extrusion membrane. Using plasmids for the V3 loop of HIV-1, and the capsid, E1 and E2 of hepatitis C, entrapment yields of 72-98.2% into dehydrated-rehydrated vesicles (DRV) were obtained over a wide range (0.309-2.5 mg) of DNA quantities. Most of the plasmid DNA was retained by liposomes even in the presence of sodium dodecyl sulfate (from 0.05% to 0.3%) and efficiently protected from nuclease-mediated degradation. Although the encapsulation process slightly decreased (in the range of 42.8-65.7%) the relative abundance of plasmid super coiled isoforms, the transfection efficiency of monkey kidney COS-7 cells with the plasmid DNA extracted from liposomes (9+/-0.4%) was similar to that of the non-treated DNA (8.7+/-0.2%), using the commercial SuperFect(R) Transfection Reagent. Also, it was found that an appreciable loss of lipid mass-either associated with the HPHE or the dehydration-rehydration steps-occurs during the liposome manufacturing process. These results at the bench scale are a useful reference for planning pilot or large-scale manufacture of DNA vaccine-containing liposomes.

Matrix-assisted laser desorption/ionization-time of flight-mass spectrometry of lipopolysaccharide species separated by slab-polyacrylamide gel electrophoresis: High-resolution separation and molecular weight determination of lipooligosaccharides fromVibrio fischeri strain HMK

We recently demonstrated that the combined use of lipopolysaccharide (LPS) reverse staining and high-efficiency passive elution techniques can be successfully used as a suitable interface between LPS slab-gel separation and electrospray ionization-mass spectrometry (ESI-MS) of LPS-derived oligosaccharides. Here, we extend our micropurification strategy for the analysis of O-deacylated LPS forms from Vibrio fischeri HMK after recovery from single reverse-stained LPS bands using matrix-assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS). The quantities (30-40 microg) obtained from the two gel-resolved LPS bands were sufficient to allow MALDI-TOF-MS detection of O-deacylated LPS glycoforms at m/z 3767.1, 3890.1 for the high-molecular-weight or at m/z 2522.5, 2645.4, 2725.7, and 2848.7 for the low-molecular-weight LPS band. These LPS band heterogeneities resulted not only from variations in the oligosaccharide region of the LPS but also from two phosphorylation states of the lipid A (diphosphoryl and diphosphoryl plus a single phosphoethanolamine substitution). On the other hand, MALDI-TOF mass spectra of the separated LPS bands displayed reduced heterogeneity and increased signal-to-noise ratios as compared to spectra of the unpurified LPS. Furthermore, micropurification of LPS bands prior MALDI-TOF-MS led to a higher sensitivity of detection of less abundant low-molecular-weight LPS glycoforms. Taken together, this and our previous study on gel-micropurified LPS using ESI definitively show how one can unambiguously determine the different molecular species contained within each gel-separated LPS band, their relative abundance and oligosaccharide sequences.

Linking Mass Spectrometry and Slab-Polyacrylamide Gel Electrophoresis by Passive Elution of Lipopolysaccharides from Reverse-Stained Gels: Analysis of Gel-Purified Lipopolysaccharides from Haemophilus influenzae Strain Rd

Haemophilus influenzae is an important cause of human disease, and its lipopolysaccharide (LPS) is known to be a major virulence factor. H. influenzae produces short-chain LPS of which the heterogeneity is often visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) using silver staining for detection. Individual bands have not previously been recovered by this method in quantities sufficient for mass spectrometry. In an attempt toward the development of sensitive mass spectrometrical strategies to be used in structural studies of H. influenzae LPS and LPS from other bacteria, we have applied here our previously described slab-PAGE-based micropurification method to obtain unmodified LPS fractions of high purity (>95%) from a crude LPS preparation of H. influenzae strain Rd. Two LPS-fractions were obtained which, after a procedure including mild acid hydrolysis, dephosphorylation, and permethylation of the resulting oligosaccharides, were subjected to tandem electrospray ionization mass spectrometry (ESI-MS/MS). The quantities of micropurified LPS fractions-the recovery of LPS in terms of total mass was 30%-were found sufficient to allow the characterization of LPS glycoforms. The ESI-MS spectra of the individual bands showed reduced heterogeneity. Furthermore, the integrity of the micropurified LPS was confirmed. The spectra-displayed molecular ions showed improved intensity, increased respective signal-to-noise ratios demonstrating the sensitivity of analysis. Consequently, both the direct determination of the molecular masses of the gel-separated LPS glycoforms and sequence analyses using ESI-MS/MS were possible.

High-efficiency passive elution of bacterial lipopolysaccharides from polyacrylamide gels

We recently described a method for recovering polyacrylamide-gel-separated bacterial lipopolysaccharides (LPS) based on the sensitive on-gel LPS detection (1-10 ng/band) with zinc-imidazole followed by passive elution from 32 microm average size gel microparticles into water. With this procedure, the recovery of rough- or semismooth-type LPS after 3 h elution is about 70-80%, while that of smooth LPS is only about 10%. Here we evaluated whether a simple replacement of water with other eluents would enhance the passive diffusion of LPS. We found that solutions of the detergents sodium dodecyl sulfate (SDS), sodium deoxycholate (DOC) and Triton X-100, or mixtures of the organic solvents acetonitrile and triethylamine and water, increased the recovery of a smooth LPS band from Vibrio cholerae O1 in a concentration-dependent manner. Furthermore, a quantitative recovery of rough or smooth LPS from V. cholerae O1, Escherichia coli O111:B4, E. coli K-235, or Serratia marcescens was feasible in 1% SDS or DOC after 3 h or in 5% triethylamine after only 2 min. A simple dilution of SDS or DOC or evaporation of triethylamine rendered the eluted LPS preparations compatible with biochemical activity determination, as tested by Limulus amebocyte lysate assay. Thus, this improved micropurification method may be a suitable interface between analytical gel electrophoresis and further characterization or use of LPS.

A mutant streptokinase lacking the C-terminal 42 amino acids is less reactive with preexisting antibodies in patient sera

Streptokinase (SK) is an efficacious thrombolytic drug for the treatment of myocardial infarction. Because of its immunogenicity, patients receiving SK therapy develop high anti-SK antibody (Ab) titers, which might provoke severe allergic reactions and neutralize SK activity. In this report we studied the reactivity of a synthetic 42-residue peptide resembling SKC-2 C-terminus with patient sera. SKC-2(373-414) peptide was recognized by 39 and 64% of patients, before and after SKC-2 therapy, respectively. An SKC-2 deletion mutant (mut-C42), lacking the same 42 C-terminal residues, was constructed and expressed in Escherichia coli. Recognition of mut-C42 by preexisting Abs from patient sera was 51 and 68% of reactivity to SKC-2, as assessed by direct binding and competition assays, respectively. For most of the patients, mut-C42-neutralizing activity titer (NAT) significantly decreased with respect to SKC-2-NAT. This study opens the possibility of producing a less immunogenic variant of SK, which could constitute a preferred alternative for thrombolytic therapy.

Mice immunization with gel electrophoresis-micropurified bacterial lipopolysaccharides

Some evidence on the possible use of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to elicit antibodies against smooth- or rough-type bacterial lipopolysaccharides (LPS) is shown. Gel-separated LPS were negatively stained with zinc-imidazole to precisely localize the bands of interest under fully reversible conditions. Then the bands of interest were excised and the resulting gel slices washed in a solution of a zinc-complexing agent (e.g., 100 mM EDTA), after which they were extruded through a metal sieve of 32 microm average size contained in a 1 mL syringe, to generate homogeneous gel microparticles. The LPS-containing gel slurries were used directly to immunize female BALB/c mice. Using this procedure, positive mouse polyclonal antibody responses against gel-purified smooth- or rough-LPS forms from Escherichia coli K-235 or Bordetella pertussis were elicited, as tested by a dot-immunoblotting assay. Our results may encourage the use of SDS-PAGE-micropurified LPS to develop optimized immunization procedures for the generation of specific antibodies against LPS bands of defined sizes, and therefore they constitute an intermediate step toward that aim.

Elution of lipopolysaccharides from polyacrylamide gels

A simple procedure for elution in water of bacterial lipopolysaccharides (LPS) from sodium dodecyl sulfate-polyacrylamide gels is described. It consists of the combination of three principal steps: first, highly sensitive on-gel LPS detection (1-10 ng/band) with zinc-imidazole (negative or reverse staining); second, washing of the individual LPS band in a solution of a zinc-complexing agent (e.g., 100 mM EDTA); and finally, elution of the LPS (100-200 microliters water for a 0.5-microgram LPS band) from gel microparticles for 3 h at room temperature. Using this procedure, we have successfully eluted a variety of LPS forms from Bordetella pertussis, Escherichia coli 0111:B4, E. coli K-235, Salmonella enteritidis, and Pseudomonas aeruginosa. Elution recovery of rough or semismooth LPS was about 70-80%, while that of smooth LPS was only about 10%. Eluted LPS was biologically active as tested by limulus amebocyte lysate and TNF-alpha assays.

Sensitive reverse staining of bacterial lipopolysaccharides on polyacrylamide gels by using zinc and imidazole salts

We present a new method for visualizing bacterial lipopolysaccharides (LPS)/lipooligosaccharides (LOS) electrophoresed in sodium dodecyl sulfate-polyacrylamide gels. After electrophoresis, gels are washed in boiling water to appreciably remove remaining electrophoresis reagents, then incubated in 10 mM zinc sulfate for 15 min, and subsequently immersed in 0.2 M imidazole for 3 min. As a result, zinc salts precipitate all over the gel surface except in the zones occupied by LPS/LOS, which appear as transparent, colorless bands. Gels can be stored in distilled water for weeks without loss of the negative image. The sensitivity of this stain is similar to that of silver. We believe that zinc-imidazole may be a suitable nontoxic alternative to silver in the rapid analysis of LPS/LOS by polyacrylamide gel electrophoresis.

Negative staining with zinc-imidazole of gel electrophoresis-separated nucleic acids

Zinc and imidazole salts were applied for the detection of nucleic acids on either polyacrylamide of agarose gels. After electrophoresis, polyacrylamide gels are washed in distilled water to remove most of the residual electrophoresis reagents, then incubated in 10 mM zinc sulfate for 10 min, and subsequently immersed in 0.2 M imidazole for 3 min. As a result, zinc salts precipitate on the gel surface, except in the positions occupied by nucleic acids, which appear as transparent, colorless bands. Staining of nucleic acids on agarose gels can be performed by incubation in 40 mM zinc sulfate for 10 min, followed by immersion in 0.2 M imidazole for 5 min to form a deep white-stained background. On soaking in 2 M imidazole for 45 min, the imidazole-induced zinc precipitate is removed from the positions were nucleic acids are located resulting in a negative image of colorless and transparent nucleic acid bands against a white background. The sensitivity of this stain ranges from 5 to 7 ng/band for small (from 1 to 0.2 kbp) DNA, from 7.8 to 13 ng/band for different 22-base oligonucleotides, from 62 to 125 ng/band for large (from 20 to 2 kbp) DNA, and is 1 microgram/band for human peripheral-blood monocyte RNA. After chelation of zinc with EDTA, the nucleic acids can be quantitatively recovered from the gel. The principal advantage of this technique over ethidium bromide staining is evident for preparative purposes. Using zinc-imidazole in the detection of purified pBACIB.1 (2.8 kbp) plasmid DNA and anti-HBsAg single chain Fv antibody fragment (0.7 kbp) DNA, followed by elution from gel slices, ligation and transformation of competent E. coli XL-1 Blue cells, the number of transformants notably increased from 280 (obtained with conventional ethidium bromide staining plus UV-irradiation at 312 nm) to 10,000.

Correspondence between radioactive and functional methods in the quality control of DNA restriction and modifying enzymes

We evaluated the use of two radiolabeled lambda DNA/Hpa II substrates to detect 5'-->3', 3'-->5' single and double stranded DNA dependent exonuclease and phosphatase activities found as contaminants in restriction and modifying enzyme preparations. Looking for the meaning of the radioactive assays results in a real cloning experience, we performed a cloning simulation assay using the same conditions established for the radioactive assay (enzyme units and pmols of DNA ends). As a result, we found that for degradation percentages of the radioactive DNA substrate per enzyme unit below 0.5, the false positives in the cloning stimulation assay were less than 5%. This conditions could ensure a good performance of the enzyme preparations for cloning experiments. Finally, we described the use of the radiolabeled [gamma 33P] ATP lambda Hpa II DNA substrate to detect 5'-->3' single stranded DNA dependent exonuclease and phosphatase contaminating activities in some critical steps of the purification process of the restriction enzyme Kpn I.

Zinc-imidazole positive: a new method for DNA detection after electrophoresis on agarose gels not interfering with DNA biological integrity

Nucleic acids separated by gel electrophoresis are commonly detected within the gel matrix with ethidium bromide staining, followed by gel irradiation with ultraviolet (UV) light. When the separated nucleic acids are to be recovered for further characterization or use, this methodology is unsuitable (i) because a significant number of chemical lesions to the nucleic acid molecules are caused, heavily compromising their biological activity, and (ii) because of health hazards due to accumulative direct contact with ethidium bromide and exposure to UV-light. As an alternative, for preparative purposes, a new nontoxic detection method employing zinc and imidazole salts is described. After electrophoresis, the gel is first washed in distilled water to substantially remove remaining electrophoresis reagents, then incubated in 40 mM zinc sulfate for 10 min to allow binding of Zn2+ to the DNA, and subsequently washed with distilled water to remove unbound Zn2+ from gel regions devoid of DNA. On soaking in 0.2 M imidazole for a few minutes, zinc-DNA complexes are visualized as deep-white (positive) stained bands against a slightly opaque background. The sensitivity is similar to that of ethidium bromide. Gels can be kept in distilled water for months without loss of staining. After zinc chelation, e.g. with EDTA, it is feasible to quantitatively recover chemically intact and biologically active DNA from the gels, as shown by reelectrophoresis and transformation experiments.