Expression of foreign LpxA acyltransferases in Neisseria meningitidis results in modified lipid A with reduced toxicity and retained adjuvant activity

Reducing the endotoxic activity or enhancing the vaccine immunogenicity by altering the length of lipid A acyl chain in Salmonella

The balance of the attenuation and reactogenicity is an issue in the development of recombinant attenuated Salmonella vaccines (RASV). Some reactogenic strains produced side effects are partially induced by lipid A. As reported, the number of lipid A acyl chains influence the strength and outcome of immune responses. However, there is rarely any study to investigate the modifications of acyl chain length on the effect of the toxicity and immunogenicity in Salmonella. In this study, foreign acyltransferase genes lpxA and lpxD were introduced into S. Typhimurium, which produced the S006 (ΔaraBAD::P_lppCtlpxA_C10) or S007 (ΔproBA::P_lppSslpxD_C16) strains with C10 or C16 acyl chains respectively. The results showed that the increased polymyxin B susceptibility, reduced swimming and invasion capabilities were observed in the S006. In addition, it also exhibited a lower endotoxicity and colonization ability compared to the parent strain. The result indicated the introduction of C10 acyl chains could be as a candidate choice for lipid A detoxifying strategy in engineering bacteria. However, the longer acyl chain modification didn't obviously change these abilities. Parallelly, these modifications were introduced into a Salmonella vaccine strain to determine their influences on the immune responses against Pneumonia. After inoculation by the strain V003 (ΔaraBAD ΔproBA::P_lppSslpxD_C16 χ9241), the mice produced robust levels of anti-PspA IgG, and a balanced Th1/Th2 immunity, which resulted in a significant survival improvement of mice with challenging against Streptococcus pneumonia. Therefore, the combination of lipid A modification with C16 acyl chain may be a better strategy for the development of ideal RASVs.

Heat shock enhances outer-membrane vesicle release in Bordetella spp

Pertussis, also known as whooping cough, is caused by the Gram-negative bacterium Bordetella pertussis, an obligate human pathogen. Despite high vaccination rates in high-income countries, resurgence of pertussis cases is an occurring problem that urges the necessity of developing an improved vaccine. Likewise, the efficacy of vaccines for Bordetella bronchiseptica, which causes similar disease in pigs and companion animals, is debatable. A promising approach for novel vaccines is the use of outer membrane vesicles (OMVs). However, spontaneous OMV (sOMV) release by Bordetella spp. is too low for cost-effective vaccine production. Therefore, we investigated the influence of growth in various media commonly used for culturing Bordetella in the Bvg+, i.e. virulent, phase and of a heat shock applied to inactivate the cells on OMV production. Inactivation of the bacterial cells at 56 °C before OMV isolation greatly enhanced OMV release in both Bordetella spp. without causing significant cell lysis. The growth medium used barely affected the efficiency of OMV release but did affect the protein pattern of the OMVs. Differences were found to be related, at least in part, to different availability of the nutrient metals iron and zinc in the media and include expression of potentially relevant vaccine antigens, such as the receptors FauA and ZnuD. The protein content of OMVs released by heat shock was comparable to that of sOMVs as determined by SDS-PAGE and Western blot analysis, and their heat-modifiable electrophoretic mobility suggests that also protein conformation is unaffected. However, significant differences were noticed between the protein content of OMVs and that of a purified outer membrane fraction, with two major outer membrane proteins, porin OmpP and the peptidoglycan-associated RmpM, being underrepresented in the OMVs. Altogether, these results indicate that the application of a heat shock is potentially an important step in the development of cost-effective, OMV-based vaccines for both Bordetella spp.

Shortening the Lipid A Acyl Chains of Bordetella pertussis Enables Depletion of Lipopolysaccharide Endotoxic Activity

Whooping cough, or pertussis, is an acute respiratory infectious disease caused by the Gram-negative bacterium Bordetella pertussis. Whole-cell vaccines, which were introduced in the fifties of the previous century and proved to be effective, showed considerable reactogenicity and were replaced by subunit vaccines around the turn of the century. However, there is a considerable increase in the number of cases in industrialized countries. A possible strategy to improve vaccine-induced protection is the development of new, non-toxic, whole-cell pertussis vaccines. The reactogenicity of whole-cell pertussis vaccines is, to a large extent, derived from the lipid A moiety of the lipopolysaccharides (LPS) of the bacteria. Here, we engineered B. pertussis strains with altered lipid A structures by expressing genes for the acyltransferases LpxA, LpxD, and LpxL from other bacteria resulting in altered acyl-chain length at various positions. Whole cells and extracted LPS from the strains with shorter acyl chains showed reduced or no activation of the human Toll-like receptor 4 in HEK-Blue reporter cells, whilst a longer acyl chain increased activation. Pyrogenicity studies in rabbits confirmed the in vitro assays. These findings pave the way for the development of a new generation of whole-cell pertussis vaccines with acceptable side effects.

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.

Sulfate depletion triggers overproduction of phospholipids and the release of outer membrane vesicles by Neisseria meningitidis

Outer membrane vesicles (OMVs) produced by bacteria are interesting vaccine candidates. OMVs are nanoparticles that contain many immunogenic components, are self-adjuvating, and non-replicative. Despite recent insights in the biogenesis of OMVs, there is no consensus on a conserved mechanism of OMV release and the OMV yield from bacterial cultures remains low. For Neisseria meningitidis, a Gram-negative human pathogen causing meningitis and sepsis, a feasible OMV production method based on triggering OMV release by cysteine depletion has been described. In this study, we investigated the mechanism behind this external trigger for OMV release to improve the production process. Since enhanced OMV release upon cysteine depletion was associated with oxidative stress and redox responses, we investigate the influence of more oxidized sulfur sources on OMV release. We show that N. meningitidis grows similarly on sulfate, the most oxidized sulfur source, and OMV release is triggered by sulfur depletion in general. Sulfate depletion induced increased release of OMVs over cysteine depletion. Proteomics showed that sulfur depletion resulted in oxidative stress responses and upregulated phospholipid and LPS biosynthesis. Furthermore, OMVs produced by sulfur depletion were enriched in phospholipids. Mechanistically, we hypothesize that sulfur depletion results in overproduction of phospholipids causing increased bulging of the outer membrane and subsequent OMV release.

Outer membrane vesicles as platform vaccine technology

Outer membrane vesicles (OMVs) are released spontaneously during growth by many Gram-negative bacteria. They present a range of surface antigens in a native conformation and have natural properties like immunogenicity, self-adjuvation and uptake by immune cells which make them attractive for application as vaccines against pathogenic bacteria. In particular with Neisseria meningitidis, they have been investigated extensively and an OMV-containing meningococcal vaccine has recently been approved by regulatory agencies. Genetic engineering of the OMV-producing bacteria can be used to improve and expand their usefulness as vaccines. Recent work on meningitis B vaccines shows that OMVs can be modified, such as for lipopolysaccharide reactogenicity, to yield an OMV product that is safe and effective. The overexpression of crucial antigens or simultaneous expression of multiple antigenic variants as well as the expression of heterologous antigens enable expansion of their range of applications. In addition, modifications may increase the yield of OMV production and can be combined with specific production processes to obtain high amounts of well-defined, stable and uniform OMV particle vaccine products. Further improvement can facilitate the development of OMVs as platform vaccine product for multiple applications.

Biosynthetically engineered lipopolysaccharide as vaccine adjuvant

Lipopolysaccharide (LPS), a dominant component of the Gram-negative bacterial outer membrane, is a strong activator of the innate immune system, and thereby an important determinant in the adaptive immune response following bacterial infection. This adjuvant activity can be harnessed following immunization with bacteria-derived vaccines that naturally contain LPS, and when LPS or molecules derived from it are added to purified vaccine antigens. However, the downside of the strong biological activity of LPS is its ability to contribute to vaccine reactogenicity. Modification of the LPS structure allows triggering of a proper immune response needed in a vaccine against a particular pathogen while at the same time lowering its toxicity. Extensive modifications to the basic structure are possible by using our current knowledge of bacterial genes involved in LPS biosynthesis and modification. This review focuses on biosynthetic engineering of the structure of LPS and implications of these modifications for generation of safe adjuvants.

Involvement of Neisseria meningitidis lipoprotein GNA2091 in the assembly of a subset of outer membrane proteins

GNA2091 of Neisseria meningitidis is a lipoprotein of unknown function that is included in the novel 4CMenB vaccine. Here, we investigated the biological function and the subcellular localization of the protein. We demonstrate that GNA2091 functions in the assembly of outer membrane proteins (OMPs) because its absence resulted in the accumulation of misassembled OMPs. Cell fractionation and protease accessibility experiments showed that the protein is localized at the periplasmic side of the outer membrane. Pulldown experiments revealed that it is not stably associated with the β-barrel assembly machinery, the previously identified complex for OMP assembly. Thus, GNA2091 constitutes a novel outer membrane-based lipoprotein required for OMP assembly. Furthermore, its location at the inner side of the outer membrane indicates that protective immunity elicited by this antigen cannot be due to bactericidal or opsonic activity of antibodies.

Characterization of lipid A profiles from Shigella flexneri variant X lipopolysaccharide

RATIONALE

In developing countries, Shigella flexneri (Sf) is the major causative agent of the endemic shigellosis (bacillary dysentery) responsible annually for one million fatalities mostly among infants. Lipopolysaccharides (LPSs) are characteristic components of the outer membrane of the overwhelming majority of Gram-negative bacteria. Since lipid A is essential for the viability of the Gram-negative bacteria, it is subject to extensive chemical studies with new analytical techniques.

METHODS

Lipid A was released by mild acid hydrolysis from the lipopolysaccharide which was obtained via the phenol/water extraction, purified and analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and matrix-assisted laser desorption/ionization laser-induced dissociation tandem mass spectrometry (MALDI-LID-MS/MS).

RESULTS

A detailed structural study of the whole lipid A obtained from S. flexneri variant X was carried out for the first time. Thus, we have shown that lipid A is a heterogeneous mixture having different numbers of acylated and phosphoethanolamine groups attached to the diglucosamine backbone. Furthermore, we found in the phenol phase an unusual hepta-acylated lipid A species, although the abundance was very low.

CONCLUSIONS

MALDI-TOF-MS allowed us to unravel the lipid A heterogeneity, which was not previously reported in Sf LPS. It is well known that slight variations of the chemical structure of lipid A may change its biological activity. Thus, the knowledge of the detailed chemical structure represents an essential step for further development of new preventive or therapeutically active compounds.

The LptD chaperone LptE is not directly involved in lipopolysaccharide transport in Neisseria meningitidis

The biosynthesis of lipopolysaccharide (LPS) in gram-negative bacteria is well understood, in contrast to the transport to its destination, the outer leaflet of the outer membrane. In Escherichia coli, synthesis and transport of LPS are essential processes. Neisseria meningitidis, conversely, can survive without LPS and tolerates inactivation of genes involved in LPS synthesis and transport. Here, we analyzed whether the LptA, LptB, LptC, LptE, LptF, and LptG proteins, recently implicated in LPS transport in E. coli, function similarly in N. meningitidis. None of the analyzed proteins was essential in N. meningitidis, consistent with their expected roles in LPS transport and additionally demonstrating that they are not required for an essential process such as phospholipid transport. As expected, the absence of most of the Lpt proteins resulted in a severe defect in LPS transport. However, the absence of LptE did not disturb transport of LPS to the cell surface. LptE was found to be associated with LptD, and its absence affected total levels of LptD, suggesting a chaperone-like role for LptE in LptD biogenesis. The absence of a direct role of LptE in LPS transport was substantiated by bioinformatic analyses showing a low conservation of LptE in LPS-producing bacteria. Apparently, the role of LptE in N. meningitidis deviates from that in E. coli, suggesting that the Lpt system does not function in a completely conserved manner in all gram-negative bacteria.

Role of the periplasmic chaperones Skp, SurA, and DegQ in outer membrane protein biogenesis in Neisseria meningitidis

The periplasmic chaperones Skp, SurA, and DegP are implicated in the biogenesis of outer membrane proteins (OMPs) in Escherichia coli. Here, we investigated whether these chaperones exert similar functions in Neisseria meningitidis. Although N. meningitidis does not contain a homolog of the protease/chaperone DegP, it does possess a homolog of another E. coli protein, DegQ, which can functionally replace DegP when overproduced. Hence, we examined whether in N. meningitidis, DegQ acts as a functional homolog of DegP. Single skp, surA, and degQ mutants were easily obtained, showing that none of these chaperones is essential in N. meningitidis. Furthermore, all combinations of double mutants were generated and no synthetic lethality was observed. The absence of SurA or DegQ did not affect OMP biogenesis. In contrast, the absence of Skp resulted in severely lower levels of the porins PorA and PorB but not of other OMPs. These decreased levels were not due to proteolytic activity of DegQ, since porin levels remained low in a skp degQ double mutant, indicating that neisserial DegQ is not a functional homolog of E. coli DegP. The absence of Skp resulted in lower expression of the porB gene, as shown by using a P(porB)-lacZ fusion. We found no cross-species complementation when Skp of E. coli or N. meningitidis was heterologously expressed in skp mutants, indicating that Skp functions in a species-specific manner. Our results demonstrate an important role for Skp but not for SurA or DegQ in OMP biogenesis in N. meningitidis.

Coincorporation of LpxL1 and PagL mutant lipopolysaccharides into liposomes with Neisseria meningitidis opacity protein: influence on endotoxic and adjuvant activity

Wild-type lipopolysaccharide (LPS) of Neisseria meningitidis normally contains six acyl chains. Penta-acylated LPS forms were generated through inactivation of the lpxL1 gene or through the expression of the Bordetella bronchiseptica pagL gene in N. meningitidis. The resulting LPS species, designated LpxL1 LPS and PagL LPS, respectively, display reduced endotoxic activity compared to wild-type LPS. Here, we determined the adjuvant potential of PagL LPS by comparison with the broadly used LpxL1 LPS. We also investigated the potential benefit for adjuvanticity of coincorporating these LPS species, together with the meningococcal opacity-associated protein OpaJ as a model antigen, in a liposomal delivery system. PagL LPS showed a higher endotoxic activity than LpxL1 LPS, and their incorporation into liposomes significantly reduced their endotoxic activity as determined by measuring the induction of interleukin-6 (IL-6) production in a murine macrophage cell line. To determine the adjuvant effect, BALB/c mice were immunized with OpaJ-containing liposomes and either free LPS or LPS coincorporated into the proteoliposomes. OpaJ-containing liposomes adjuvanted with AlPO(4) or not adjuvanted at all were included as control groups. In the appropriate dose, PagL LPS showed a superior adjuvant effect compared with LpxL1 LPS, and for both LPS types, free LPS showed a higher adjuvant effect than when coincorporated into the liposomes, as evidenced by higher titers of IgG2a and IgG2b antibodies against OpaJ(+) meningococci and higher bactericidal titers. In conclusion, PagL LPS is a better adjuvant than LpxL1 LPS, but coincorporation of either LPS into proteoliposomes did not improve their adjuvant activity.

Profiles of structural heterogeneity in native lipooligosaccharides of Neisseria and cytokine induction

Fine differences in the phosphorylation and acylation of lipooligosaccharide (LOS) from Neisseria species are thought to profoundly influence the virulence of the organisms and the innate immune responses of the host, such as signaling through toll-like receptor 4 (TLR4) and triggering receptor expressed on myeloid cells (TREM). MALDI time-of-flight (TOF) mass spectrometry was used to characterize heterogeneity in the native LOS from Neisseria gonorrheae and N. meningitidis. A sample preparation methodology previously reported for Escherichia coli lipopolysaccharide (LPS) employing deposition of untreated LOS on a thin layer of a film composed of 2,4,6-trihydroxyacetophenone and nitrocellulose was used. Prominent peaks were observed corresponding to molecular ions and to fragment ions primarily formed by cleavage between the 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) and the lipid A (LA). Analyses of these data and comparison with spectra of the corresponding O-deacylated or hydrogen fluoride-treated LOS enabled the detection of novel species that apparently differed by the expression of up to three phosphates with one or more phosphoethanolamine (PEA) groups on the LA. We found that the heterogeneity profile of acylation and phosphorylation correlates with the induction of proinflammatory cytokines in THP-1 monocytic cells. This methodology enabled us to rapidly profile components of structural variants of native LOS that are of importance biologically.

Differential activation of human and mouse Toll-like receptor 4 by the adjuvant candidate LpxL1 of Neisseria meningitidis

Neisseria meningitidis LpxL1 lipopolysaccharide (LPS) bearing penta-acylated lipid A is considered a promising adjuvant candidate for inclusion in future N. meningitidis vaccines, as it elicits a markedly reduced endotoxic response in human macrophages relative to that in wild-type (hexa-acylated) LPS, while it is an equally effective adjuvant in mice. As dendritic cells (DC) and Toll-like receptors (TLR) are regarded as central mediators in the initiation of an immune response, here we evaluated the ability of LpxL1 LPS to mature and to activate human DC and examined its TLR4-/MD-2-activating properties. Unexpectedly, purified LpxL1 LPS displayed minimal human DC-stimulating properties compared to wild-type LPS. Although whole bacteria induced DC maturation and activation irrespective of their type of LPS, the LpxL1 mutant failed to activate the human recombinant TLR4/MD-2 complex expressed in HeLa cells. Similarly, purified LpxL1 LPS was unable to activate human TLR4/MD-2 and it even acted as an antagonist of wild-type LPS. Both wild-type and LpxL1 LPSs activated the murine TLR4/MD-2 complex, consistent with their abilities to induce maturation and activation of murine DC. Assays with cells transfected with different combinations of human and murine TLR4 and MD-2 indicated that TLR4 was a more-major determinant of the LPS response than MD-2. The species-specific activation of the TLR4/MD-2 complex by LpxL1 LPS may have an impact on the use of LpxL1 LPS as an adjuvant and the use of murine immunization models in human meningococcal vaccine development.

Supplementation of whole-cell pertussis vaccines with lipopolysaccharide analogs: modification of vaccine-induced immune responses

Lipopolysaccharide (LPS) is one of the main constituents of the Gram-negative bacterial outer membrane. Besides being an endotoxin, LPS also possesses a powerful adjuvant activity. Previously, it has been shown that changes in the chemical composition of the lipid A domain of LPS modulate its biological activity. For example, monophosphoryl lipid A (MPL) has been shown to be a non-toxic immunostimulatory compound. Moreover, several LPS analogs have been shown to antagonise LPS-induced signalling in eukaryotic cells. In the present study, we show that supplementation of a whole-cell pertussis (wP) vaccine with LPS analogs modulates the vaccine-induced immune responses. We show in a mouse-model system that addition of MPL to a wP vaccine increases vaccine efficacy without altering vaccine-induced serum pro-inflammatory cytokine levels. Furthermore, we show that Neisseria meningitidis LpxL2 LPS, an LPS species derived from a N. meningitidis lpxL2 mutant, antagonises wP and LPS-stimulated interleukin-6 (IL-6) production by macrophages in vitro, and that addition of this LPS-derivative to the wP vaccine decreases vaccine-induced serum IL-6 levels and increases vaccine efficacy.

Biogenesis of the gram-negative bacterial outer membrane

The cell envelope of gram-negative bacteria consists of two membranes, the inner and the outer membrane, that are separated by the periplasm. The outer membrane consists of phospholipids, lipopolysaccharides, integral membrane proteins, and lipoproteins. These components are synthesized in the cytoplasm or at the inner leaflet of the inner membrane and have to be transported across the inner membrane and through the periplasm to assemble eventually in the correct membrane. Recent studies in Neisseria meningitidis and Escherichia coli have led to the identification of several machineries implicated in these transport and assembly processes.

Consequences of the expression of lipopolysaccharide-modifying enzymes for the efficacy and reactogenicity of whole-cell pertussis vaccines

Lipopolysaccharide is one of the major constituents of the Gram-negative bacterial outer membrane and is, due to its endotoxic activity, responsible for the relatively high reactogenicity of whole-cell vaccines. In addition, lipopolysaccharide has strong immune stimulating properties, which makes it, potentially, an interesting vaccine component. In a previous study, we have shown that expression of two lipopolysaccharide-modifying enzymes, i.e., PagP and PagL, modulates the endotoxic activity of the Gram-negative bacterium Bordetella pertussis, the causative agent of whooping cough. To assess the consequences of PagP and PagL expression on the efficacy and reactogenicity of whole-cell pertussis vaccines, we have immunised mice and challenged them intranasally with wild-type B. pertussis. Vaccine efficacy, B. pertussis-specific antibody responses, and cytokine profiles were evaluated. The results show that expression of PagL, but not of PagP, significantly increases vaccine efficacy without altering vaccine reactogenicity. Therefore, PagL-expressing B. pertussis strains may form a basis for the development of a new and safer whole-cell pertussis vaccine, as higher vaccine efficacies may allow a reduced vaccine dosage. These data show, for the first time, that lipopolysaccharide composition is an important determinant for the efficacy of whole-cell pertussis vaccines.

TLR4-dependent adjuvant activity of Neisseria meningitidis lipid A

The adjuvant activity of Neisseria meningitidis serogroup B lipopoly(oligo)saccharide (LOS) from wild-type and genetically defined LOS mutants and unglycosylated meningococcal lipid A was assessed in C3H/HeN and C3H/HeJ mice. Meningococcal lipid A, a weak agonist for TLR4/MD-2 in human macrophages, was found to have adjuvant activity similar to that of wild-type and KDO(2)-lipid A LOS in C3H/HeN mice. All meningococcal LOS structures as adjuvants induced high titers of IgG1, IgG2a and IgG2b but very little IgG3 to OMP compared to no adjuvant PBS controls. In addition, induced OMP antibodies were shown to have high bactericidal activity against serogroup B meningococci. Purified LOS and lipid A structures failed to induce any adjuvant activity in C3H/HeJ mice indicating that meningococcal LOS as an adjuvant was TLR4-dependent. Unglycosylated meningococcal lipid A because of its weak agonist activity for human macrophages and retention of adjuvant activity may be a candidate for use in serogroup B meningococcal OMP and OMV vaccines and for use as an adjuvant in other vaccines.

Expression of the lipopolysaccharide-modifying enzymes PagP and PagL modulates the endotoxic activity of Bordetella pertussis

Lipopolysaccharide (LPS) is one of the major constituents of the gram-negative bacterial cell envelope. Its endotoxic activity causes the relatively high reactogenicity of whole-cell vaccines. Several bacteria harbor LPS-modifying enzymes that modulate the endotoxic activity of the LPS. Here we evaluated whether two such enzymes, i.e., PagP and PagL, could be useful tools for the development of an improved and less reactogenic whole-cell pertussis vaccine. We showed that expression of PagP and PagL in Bordetella pertussis leads to increased and decreased endotoxic activity of the LPS, respectively. As expected, PagP activity also resulted in increased endotoxic activity of whole bacterial cells. However, more unexpectedly, this was also the case for PagL. This paradoxical result may be explained, in part, by an increased release of LPS, which we observed in the PagL-expressing cells.

An outer membrane enzyme encoded by Salmonella typhimurium lpxR that removes the 3'-acyloxyacyl moiety of lipid A

The Salmonella and related bacteria modify the structure of the lipid A portion of their lipopolysaccharide in response to environmental stimuli. Some lipid A modifications are required for virulence and resistance to cationic antimicrobial peptides. We now demonstrate that membranes of Salmonella typhimurium contain a novel hydrolase that removes the 3'-acyloxyacyl residue of lipid A in the presence of 5 mM Ca2+. We have identified the gene encoding the S. typhimurium lipid A 3'-O-deacylase, designated lpxR, by screening an ordered S. typhimurium genomic DNA library, harbored in Escherichia coli K-12, for expression of Ca2+-dependent 3'-O-deacylase activity in membranes. LpxR is synthesized with an N-terminal type I signal peptide and is localized to the outer membrane. Mass spectrometry was used to confirm the position of lipid A deacylation in vitro and the release of the intact 3'-acyloxyacyl group. Heterologous expression of lpxR in the E. coli K-12 W3110, which lacks lpxR, resulted in production of significant amounts of 3'-O-deacylated lipid A in growing cultures. Orthologues of LpxR are present in the genomes of E. coli O157:H7, Yersinia enterocolitica, Helicobacter pylori, and Vibrio cholerae. The function of LpxR is unknown, but it could play a role in pathogenesis because it might modulate the cytokine response of an infected animal.

Crystal structure and catalytic mechanism of the LPS 3-O-deacylase PagL from Pseudomonas aeruginosa

Pathogenic gram-negative bacteria can modify the lipid A portion of their lipopolysaccharide in response to environmental stimuli. 3-O-deacylation of lipid A by the outer membrane enzyme PagL modulates signaling through Toll-like receptor 4, leading to a reduced host immune response. We found that PagL is widely disseminated among gram-negative bacteria. Only four residues are conserved: a Ser, His, Phe, and Asn residue. Here, we describe the crystal structure of PagL from Pseudomonas aeruginosa to 2.0-A resolution. It consists of an eight-stranded beta-barrel with the axis tilted by approximately 30 degrees with respect to the lipid bilayer. The structure reveals that PagL contains an active site with a Ser-His-Glu catalytic triad and an oxyanion hole that comprises the conserved Asn. The importance of active site residues was confirmed in mutagenesis studies. Although PagL is most likely active as a monomer, its active site architecture shows high resemblance to that of the dimeric 12-stranded outer membrane phospholipase A. Modeling of the substrate lipid X onto the active site reveals that the 3-O-acyl chain is accommodated in a hydrophobic groove perpendicular to the membrane plane. In addition, an aspartate makes a hydrogen bond with the hydroxyl group of the 3-O-acyl chain, probably providing specificity of PagL toward lipid A.

Differentiation of Campylobacter coli, Campylobacter jejuni, Campylobacter lari, and Campylobacter upsaliensis by a multiplex PCR developed from the nucleotide sequence of the lipid A gene lpxA

We describe a multiplex PCR assay to identify and discriminate between isolates of Campylobacter coli, Campylobacter jejuni, Campylobacter lari, and Campylobacter upsaliensis. The C. jejuni isolate F38011 lpxA gene, encoding a UDP-N-acetylglucosamine acyltransferase, was identified by sequence analysis of an expression plasmid that restored wild-type lipopolysaccharide levels in Escherichia coli strain SM105 [lpxA(Ts)]. With oligonucleotide primers developed to the C. jejuni lpxA gene, nearly full-length lpxA amplicons were amplified from an additional 11 isolates of C. jejuni, 20 isolates of C. coli, 16 isolates of C. lari, and five isolates of C. upsaliensis. The nucleotide sequence of each amplicon was determined, and sequence alignment revealed a high level of species discrimination. Oligonucleotide primers were constructed to exploit species differences, and a multiplex PCR assay was developed to positively identify isolates of C. coli, C. jejuni, C. lari, and C. upsaliensis. We characterized an additional set of 41 thermotolerant isolates by partial nucleotide sequence analysis to further demonstrate the uniqueness of each species-specific region. The multiplex PCR assay was validated with 105 genetically defined isolates of C. coli, C. jejuni, C. lari, and C. upsaliensis, 34 strains representing 12 additional Campylobacter species, and 24 strains representing 19 non-Campylobacter species. Application of the multiplex PCR method to whole-cell lysates obtained from 108 clinical and environmental thermotolerant Campylobacter isolates resulted in 100% correlation with biochemical typing methods.

Dissemination of lipid A deacylases (pagL) among gram-negative bacteria: identification of active-site histidine and serine residues

Lipopolysaccharide (LPS) is one of the main constituents of the Gram-negative bacterial outer membrane. It usually consists of a highly variable O-antigen, a less variable core oligosaccharide, and a highly conserved lipid moiety, designated lipid A. Several bacteria are capable of modifying their lipid A architecture in response to external stimuli. The outer membrane-localized lipid A 3-O-deacylase, encoded by the pagL gene of Salmonella enterica serovar Typhimurium, removes the fatty acyl chain from the 3 position of lipid A. Although a similar activity was reported in some other Gram-negative bacteria, the corresponding genes could not be identified. Here, we describe the presence of pagL homologs in a variety of Gram-negative bacteria. Although the overall sequence similarity is rather low, a conserved domain could be distinguished in the C-terminal region. The activity of the Pseudomonas aeruginosa and Bordetella bronchiseptica pagL homologs was confirmed upon expression in Escherichia coli, which resulted in the removal of an R-3-hydroxymyristoyl group from lipid A. Upon deacylation by PagL, E. coli lipid A underwent another modification, which was the result of the activity of the endogenous palmitoyl transferase PagP. Furthermore, we identified a conserved histidine-serine couple as active site residues, suggesting a catalytic mechanism similar to serine hydrolases. The biological function of PagL remains unclear. However, because PagL homologs were found in both pathogenic and nonpathogenic species, PagL-mediated deacylation of lipid A probably does not have a dedicated role in pathogenicity.

Stability of mono- and trivalent meningococcal outer membrane vesicle vaccines

The stability during storage of outer membrane vesicles (OMVs) of Neisseria meningitidis group B was studied. Three types of OMVs were compared for their stability, containing either one (monovalent) or three different PorA subtypes (trivalent), the latter with and without class 4 outer membrane protein (OMO, RmpM). Aqueous formulations were stored freeze-dried (4 degrees C), frozen (-70 degrees C) and in liquid form at 4, 37 and 56 degrees C. Physico-chemical properties and immunogenicity of the OMVs as well as PorA conformation and antigenicity (P1.7-2,4, the subtype present in all formulations) were monitored during 1 year. At -70 or 4 degrees C, the structure and immunogenicity of OMVs was preserved. Storage of OMVs at high temperatures (37 or 56 degrees C) induced destruction of the OMV structure and denaturation of PorA, followed by chemical degradation. Immunogenicity decreased or was lost completely. Changes observed in the fluorescence spectra of degraded OMVs were also seen in tryptophan (Trp) and tyrosine (Tyr) derivatives incubated at 56 degrees C, indicating the occurrence of chemical degradation of tryptophan and tyrosine residues in PorA. Trivalent OMVs were slightly more stable at 37 degrees C than monovalent OMVs as assessed by in vitro methods, but these differences did not result in differences in the immunogenicity. The stability of trivalent OMVs was not affected by the presence of RmpM. Both trivalent and monovalent OMVs could be freeze-dried with preservation of their immunogenicity. In conclusion, OMVs are sensitive to elevated temperatures, but are stable in the frozen or freeze-dried state or when stored at 4 degrees C in the liquid state.