The structure of the L9 immunotype lipooligosaccharide from Neisseria meningitidis NMA Z2491

Glycoengineering with neuraminic acid analogs to label lipooligosaccharides and detect native sialyltransferase activity in gram-negative bacteria

Lipooligosaccharides are the most abundant cell surface glycoconjugates on the outer membrane of Gram-negative bacteria. They play important roles in host-microbe interactions. Certain Gram-negative pathogenic bacteria cap their lipooligosaccharides with the sialic acid, N-acetylneuraminic acid (Neu5Ac), to mimic host glycans that among others protects these bacteria from recognition by the hosts immune system. This process of molecular mimicry is not fully understood and remains under investigated. To explore the functional role of sialic acid-capped lipooligosaccharides at the molecular level, it is important to have tools readily available for the detection and manipulation of both Neu5Ac on glycoconjugates and the involved sialyltransferases, preferably in live bacteria. We and others have shown that the native sialyltransferases of some Gram-negative bacteria can incorporate extracellular unnatural sialic acid nucleotides onto their lipooligosaccharides. We here report on the expanded use of native bacterial sialyltransferases to incorporate neuraminic acids analogs with a reporter group into the lipooligosaccharides of a variety of Gram-negative bacteria. We show that this approach offers a quick strategy to screen bacteria for the expression of functional sialyltransferases and the ability to use exogenous CMP-Neu5Ac to decorate their glycoconjugates. For selected bacteria we also show this strategy complements two other glycoengineering techniques, Metabolic Oligosaccharide Engineering and Selective Exo-Enzymatic Labeling, and that together they provide tools to modify, label, detect and visualize sialylation of bacterial lipooligosaccharides.

Neisseria gonorrhoeae lipooligosaccharide glycan epitopes recognized by bactericidal IgG antibodies elicited by the meningococcal group B-directed vaccine, MenB-4C

Introduction

Outer membrane vesicles (OMVs) of Neisseria meningitidis in the group B-directed vaccine MenB-4C (BexseroR) protect against infections with Neisseria gonorrhoeae. The immunological basis for protection remains unclear. N. meningitidis OMV vaccines generate human antibodies to N. meningitidis and N. gonorrhoeae lipooligosaccharide (LOS/endotoxin), but the structural specificity of these LOS antibodies is not defined.

Methods

Ten paired human sera obtained pre- and post-MenB-4C immunization were used in Western blots to probe N. meningitidis and N. gonorrhoeae LOS. Post-MenB-4C sera (7v5, 19v5, and 17v5), representing individual human variability in LOS recognition, were then used to interrogate structurally defined LOSs of N. meningitidis and N. gonorrhoeae strains and mutants and studied in bactericidal assays.

Results and discussion

Post-MenB-4C sera recognized both N. meningitidis and N. gonorrhoeae LOS species, ~10% of total IgG to gonococcal OMV antigens. N. meningitidis and N. gonorrhoeae LOSs were broadly recognized by post-IgG antibodies, but with individual variability for LOS structures. Deep truncation of LOS, specifically a rfaK mutant without α-, β-, or γ-chain glycosylation, eliminated LOS recognition by all post-vaccine sera. Serum 7v5 IgG antibodies recognized the unsialyated L1 α-chain, and a 3-PEA-HepII or 6-PEA-HepII was part of the conformational epitope. Replacing the 3-PEA on HepII with a 3-Glc blocked 7v5 IgG antibody recognition of N. meningitidis and N. gonorrhoeae LOSs. Serum 19v5 recognized lactoneotetrose (LNT) or L1 LOS-expressing N. meningitidis or N. gonorrhoeae with a minimal α-chain structure of Gal-Glc-HepI (L8), a 3-PEA-HepII or 6-PEA-HepII was again part of the conformational epitope and a 3-Glc-HepII blocked 19v5 antibody binding. Serum 17v5 LOS antibodies recognized LNT or L1 α-chains with a minimal HepI structure of three sugars and no requirement for HepII modifications. These LOS antibodies contributed to the serum bactericidal activity against N. gonorrhoeae. The MenB-4C vaccination elicits bactericidal IgG antibodies to N. gonorrhoeae conformational epitopes involving HepI and HepII glycosylated LOS structures shared between N. meningitidis and N. gonorrhoeae. LOS structures should be considered in next-generation gonococcal vaccine design.

The sweet side of the pathogenic Neisseria: the role of glycan interactions in colonisation and disease

Glycomics is a rapidly growing field that focuses on the structure and function of carbohydrates (glycans) in biological systems. Glycan interactions play a major role in infectious disease, at all stages of colonisation and disease progression. Neisseria meningitidis, the cause of meningococcal sepsis and meningitis, and Neisseria gonorrhoeae, which causes the sexually transmitted infection gonorrhoea, are responsible for significant morbidity and mortality worldwide. Neisseria meningitidis displays a range of surface glycosylations including capsule polysaccharide, lipooligosaccharide and O-linked glycoproteins. While N. gonorrhoeae does not have a capsule, it does express both lipooligosaccharide and O-linked glycoproteins. Neisseria gonorrhoeae also has the ability to scavenge host sialic acids, while several N. meningitidis serogroups can synthesise sialic acid. Surface expressed sialic acid is key in serum resistance and survival in the host. On the host side, the pathogenic Neisseria protein adhesins such as Opc and NHBA bind to host glycans for adherence and colonisation of host cells. Essentially, from both the bacterial and host perspective, glycan interactions are fundamental in colonisation and disease of pathogenic Neisseria. The key aspects of glycobiology of the pathogenic Neisseria are reviewed herein.

Lipooligosaccharide Structures of Invasive and Carrier Isolates of Neisseria meningitidis Are Correlated with Pathogenicity and Carriage

The degree of phosphorylation and phosphoethanolaminylation of lipid A on neisserial lipooligosaccharide (LOS), a major cell-surface antigen, can be correlated with inflammatory potential and the ability to induce immune tolerance in vitro. On the oligosaccharide of the LOS, the presence of phosphoethanolamine and sialic acid substituents can be correlated with in vitro serum resistance. In this study, we analyzed the structure of the LOS from 40 invasive isolates and 25 isolates from carriers of Neisseria meningitidis without disease. Invasive strains were classified as groups 1-3 that caused meningitis, septicemia without meningitis, and septicemia with meningitis, respectively. Intact LOS was analyzed by high resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Prominent peaks for lipid A fragment ions with three phosphates and one phosphoethanolamine were detected in all LOS analyzed. LOS from groups 2 and 3 had less abundant ions for highly phosphorylated lipid A forms and induced less TNF-α in THP-1 monocytic cells compared with LOS from group 1. Lipid A from all invasive strains was hexaacylated, whereas lipid A of 6/25 carrier strains was pentaacylated. There were fewer O-acetyl groups and more phosphoethanolamine and sialic acid substitutions on the oligosaccharide from invasive compared with carrier isolates. Bioinformatic and genomic analysis of LOS biosynthetic genes indicated significant skewing to specific alleles, dependent on the disease outcome. Our results suggest that variable LOS structures have multifaceted effects on homeostatic innate immune responses that have critical impact on the pathophysiology of meningococcal infections.

The biology of Neisseria adhesins

Members of the genus Neisseria include pathogens causing important human diseases such as meningitis, septicaemia, gonorrhoea and pelvic inflammatory disease syndrome. Neisseriae are found on the exposed epithelia of the upper respiratory tract and the urogenital tract. Colonisation of these exposed epithelia is dependent on a repertoire of diverse bacterial molecules, extending not only from the surface of the bacteria but also found within the outer membrane. During invasive disease, pathogenic Neisseriae also interact with immune effector cells, vascular endothelia and the meninges. Neisseria adhesion involves the interplay of these multiple surface factors and in this review we discuss the structure and function of these important molecules and the nature of the host cell receptors and mechanisms involved in their recognition. We also describe the current status for recently identified Neisseria adhesins. Understanding the biology of Neisseria adhesins has an impact not only on the development of new vaccines but also in revealing fundamental knowledge about human biology.

Galectin-3 binds Neisseria meningitidis and increases interaction with phagocytic cells

Galectin-3 is expressed and secreted by immune cells and has been implicated in multiple aspects of the inflammatory response. It is a glycan binding protein which can exert its functions within cells or exogenously by binding cell surface ligands, acting as a molecular bridge or activating signalling pathways. In addition, this lectin has been shown to bind to microorganisms. In this study we investigated the interaction between galectin-3 and Neisseria meningitidis, an important extracellular human pathogen, which is a leading cause of septicaemia and meningitis. Immunohistochemical analysis indicated that galectin-3 is expressed during meningococcal disease and colocalizes with bacterial colonies in infected tissues from patients. We show that galectin-3 binds to N. meningitidis and we demonstrate that this interaction requiresfull-length, intact lipopolysaccharide molecules. We found that neither exogenous nor endogenous galectin-3 contributes to phagocytosis of N. meningitidis; instead exogenous galectin-3 increases adhesion to monocytes and macrophages but not epithelial cells. Finally we used galectin-3 deficient (Gal-3(-/-) ) mice to evaluate the contribution of galectin-3 to meningococcal bacteraemia. We found that Gal-3(-/-) mice had significantly lower levels of bacteraemia compared with wild-type mice after challenge with live bacteria, indicating that galectin-3 confers an advantage to N. meningitidis during systemic infection.

Formulation of meningococcal capsular polysaccharide vaccine-loaded microparticles with robust innate immune recognition

Neisseria meningitidis is a leading cause of bacterial meningitis and sepsis associated with a high mortality rate. Capsular polysaccharides (CPSs) are a major virulence factor and form the basis for serogroup designation and protective vaccines. The current polysaccharide meningococcal vaccines are available but are very expensive and require chemical conjugation. Here, we report a novel meningococcal vaccine formulation consisting of meningococcal CPS polymers encapsulated in albumin-based biodegradable microparticles that slowly release antigen and induce robust innate immune responses. Vaccines that elicit innate immunity are reported to have enhanced and protective adaptive immune responses. In this study, the meningococcal CPS-loaded microparticles, but not the empty microparticles, induced the release of IL-8, TNF-α and IL-1β, enhanced phagocytic capacity and induced robust autophagy in macrophages. The novel meningococcal vaccine microparticles are robustly taken up by macrophages and elicit strong innate immune responses that enhance antigen presentation which is a prerequisite for inducing adaptive immunity.

Neisseria meningitidis serogroup B lipooligosaccharide genotyping reveals high prevalence of L2 strains in Spain and unexpected relationship with factor H-binding protein expression

Neisseria meningitidis may be classified according to the lipooligosaccharide immunotype. We show that this classification can be achieved by PCR genotyping of the genes involved in the lipooligosaccharide inner-core biosynthesis, lpt3, lpt6, lgtG and lot3. Genotyping data correlated well (90-100%) with mass spectrometry data and was, therefore, applied to screen a random subset of recent N. meningitidis serogroup B isolates from Europe. Analysis of the proportion of the different lipooligosaccharide types highlighted the predominance of L3 strains. Surprisingly, high rates of L2 type strains were found in Spain (17%, versus 2.5% in Germany and 1.9% in the United Kingdom). Therefore, we also investigated further these Spanish L2 strains in an attempt to explain such prevalence despite the known sensitivity of L2 immunotype to complement. We explored the hypothesis that these strains express high amounts of factor H-binding protein (fHbp), but we found, on the contrary, that L2 strains express low or undetectable amounts of fHbp. Our findings suggest that, in addition to a genetic analysis, a multivalent approach may be necessary to estimate the effectiveness of a N. meningitidis serogroup B vaccine.

Improvement of immunogenicity of meningococcal lipooligosaccharide by coformulation with lipidated transferrin-binding protein B in liposomes: implications for vaccine development

Among various meningococcal antigens, lipooligosaccharide (LOS) and recombinant lipidated transferrin-binding protein B (rlip-TbpB) are considered to be putative vaccine candidates against group B Neisseria meningitidis. In the present work, we report the development of a new liposome-based vaccine formulation containing both rlip-TbpB and L8 LOS. The endotoxic activity of the liposomal LOS was evaluated in vitro using the Limulus Amebocyte Lysate assay and compared to the endotoxic activity of free LOS. Above a 250:1 lipid/LOS molar ratio, liposomes were shown to effectively detoxify the LOS as the endotoxic activity of the LOS was reduced by more than 99%. Immunogenicity studies in rabbits showed that the presence of rlip-TbpB dramatically increased the immunogenicity of the LOS. While the formulation raised a strong anti-TbpB response, it elicited a higher anti-LOS IgG level than the liposomal LOS alone. Sera from rabbits immunized with rlip-TbpB/liposomal LOS displayed increased ability to recognize LOS on live bacteria expressing the L8 immunotype and increased anti-LOS-specific bactericidal activity compared to sera from rabbits immunized with liposomal LOS alone. Measurement of interleukin-8 (IL-8) produced by HEK293 cells transfected with Toll-like receptor (TLR) after stimulation with rlip-TbpB showed that the protein is a TLR2 agonist, which is in accordance with the structure of its lipid. Furthermore, an in vivo study demonstrated that the lipid moiety is not only required for its adjuvant effect but also has to be linked to the protein. Overall, the rlip-TbpB/LOS liposomal formulation was demonstrated to induce an effective anti-LOS response due to the adjuvant effect of rlip-TbpB on LOS.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Importance of antibodies to lipopolysaccharide in natural and vaccine-induced serum bactericidal activity against Neisseria meningitidis group B

Analysis of the specificity of bactericidal antibodies in normal, convalescent, and postvaccination human sera is important in understanding human immunity to meningococcal infections and can aid in the design of an effective group B vaccine. A collection of human sera, including group C and group B convalescent-phase sera, normal sera with naturally occurring cross-reactive bactericidal activity, and some postvaccination sera, was analyzed to determine the specificity of cross-reactive bactericidal antibodies. Analysis of human sera using a bactericidal antibody depletion assay demonstrated that a significant portion of the bactericidal activity could be removed by purified lipopolysaccharide (LPS). LPS homologous to that expressed on the bactericidal test strain was most effective, but partial depletion by heterologous LPS suggested the presence of antibodies with various degrees of cross-reactivity. Binding of anti-L3,7 LPS bactericidal antibodies was affected by modification of the core structure, suggesting that these functional antibodies recognized epitopes consisting of both core structures and lacto-N-neotetraose (LNnT). When the target strain was grown with 5'-cytidinemonophospho-N-acetylneuraminic acid (CMP-NANA) to increase LPS sialylation, convalescent-phase serum bactericidal titers were decreased by only 2- to 4-fold, and most remaining bactericidal activity was still depleted by LPS. Highly sialylated LPS was ineffective in depleting bactericidal antibodies. We conclude that natural infections caused by strains expressing L3,7 LPS induce persistent, protective bactericidal antibodies and appear to be directed against nonsialylated bacterial epitopes. Additionally, subsets of these bactericidal antibodies are cross-reactive, binding to several different LPS immunotypes, which is a useful characteristic for an effective group B meningococcal vaccine antigen.

Characterization of the carbohydrate backbone of Vibrio parahaemolyticus O6 lipopolysaccharides

Structural characterization studies have been carried out on the carbohydrate backbone of Vibrio parahaemolyticus serotype O6 lipopolysaccharides (LPS). The carbohydrate backbone isolated from O6 LPS by sequential derivatization, that is, dephosphorylation, O-deacylation, pyridylamination, N-deacylation and N-acetylation, is a nonasaccharide consisting of 3 mol of D-glucosamine (GlcN) (of which one is pyridylaminated), 2 mol of L-glycero-D-manno-heptose (Hep), and 1 mol each of D-galactose (Gal), D-glucose (Glc), D-glucuronic acid (GlcA) and 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo). Structural analyses by nuclear magnetic resonance spectroscopy and fast-atom bombardment mass spectrometry demonstrated that the carbohydrate backbone is β-Galp-(1→2)-α-Hepp-(1→3)-α-Hepp-(1→5)-α-Kdop-(2→6)-β-GlcpNAc-(1→6)-GlcNAc-PA, in which the 3-substituted α-Hepp is further substituted by β-GlcpNAc-(1→4)-β-Glcp at position 4 and by β-GlcpA at position 2. In native O6 LPS, an additional 1 mol of D-galacturonic acid, which is liberated by dephosphorylation in hydrofluoric acid, is present at an unknown position. A previous study by the present authors reported that, of 13 O-serotype LPS of V. parahaemolyticus, the only LPS from which Kdo was detected was from O6 LPS after mild acid hydrolysis. In the present study, we have demonstrated that only 1 mol of Kdo is present at the lipid A proximal position, a component which is common to the LPS in all serotypes of the bacterium, and that there is no additional Kdo in the carbohydrate backbone of O6 LPS. ELISA and ELISA inhibition analysis using antisera against O6 and Salmonella enterica Minnesota R595 and LPS of both strains further revealed that Kdo is not involved as an antigenic determinant of O6 LPS.

Genotypic and phenotypic modifications of Neisseria meningitidis after an accidental human passage

A scientist in our laboratory was accidentally infected while working with Z5463, a Neisseria meningitidis serogroup A strain. She developed severe symptoms (fever, meningism, purpuric lesions) that fortunately evolved with antibiotic treatment to complete recovery. Pulse-field gel electrophoresis confirmed that the isolate obtained from the blood culture (Z5463BC) was identical to Z5463, more precisely to a fourth subculture of this strain used the week before the contamination (Z5463PI). In order to get some insights into genomic modifications that can occur in vivo, we sequenced these three isolates. All the strains contained a mutated mutS allele and therefore displayed an hypermutator phenotype, consistent with the high number of mutations (SNP, Single Nucleotide Polymorphism) detected in the three strains. By comparing the number of SNP in all three isolates and knowing the number of passages between Z5463 and Z5463PI, we concluded that around 25 bacterial divisions occurred in the human body. As expected, the in vivo passage is responsible for several modifications of phase variable genes. This genomic study has been completed by transcriptomic and phenotypic studies, showing that the blood strain used a different haemoglobin-linked iron receptor (HpuA/B) than the parental strains (HmbR). Different pilin variants were found after the in vivo passage, which expressed different properties of adhesion. Furthermore the deletion of one gene involved in LOS biosynthesis (lgtB) results in Z5463BC expressing a different LOS than the L9 immunotype of Z2491. The in vivo passage, despite the small numbers of divisions, permits the selection of numerous genomic modifications that may account for the high capacity of the strain to disseminate.

The human host defense peptide LL-37 interacts with Neisseria meningitidis capsular polysaccharides and inhibits inflammatory mediators release

Capsular polysaccharides (CPS) are a major virulence factor in meningococcal infections and form the basis for serogroup designation and protective vaccines. Our work has identified meningococcal CPS as a pro-inflammatory ligand that functions through TLR2 and TLR4-MD2-dependent activation. We hypothesized that human cationic host defense peptides interact with CPS and influence its biologic activity. Accordingly, the interaction of meningococcal CPS with the human-derived cationic peptide LL-37, which is expressed by phagocytic and epithelial cells that interface with meningococci during infection, was investigated. LL-37 neutralized the pro-inflammatory activity of endotoxin-free CPS as assessed by TLR2 and TLR4-MD-2-dependent release of TNFα, IL-6 and IL-8 from human and murine macrophages. The cationic and hydrophobic properties of LL-37 were crucial for this inhibition, which was due to binding of LL-37 to CPS. LL-37 also inhibited the ability of meningococcal CPS to induce nitric oxide release, as well as TNFα and CXCL10 (IP-10) release from TLR4-sufficient and TLR4-deficient murine macrophages. Truncated LL-37 analogs, especially those that retained the antibacterial domain, inhibited vaccine grade CPS and meningococcal CPS prepared from the major serogroups (A, B C, Y and W135). Thus, LL-37 interaction with CPS was independent of specific glucan structure. We conclude that the capacity of meningococcal CPS to activate macrophages via TLR2 and TLR4-MD-2 can be inhibited by the human cationic host defense peptide LL-37 and propose that this impacts CPS-based vaccine responses.

Genetic and structural characterization of L11 lipooligosaccharide from Neisseria meningitidis serogroup A strains

The lipooligosaccharide (LOS) of immunotype L11 is unique within serogroup A meningococci. In order to resolve its molecular structure, we conducted LOS genotyping by PCR analysis of genes responsible for alpha-chain sugar addition (lgtA, -B, -C, -E, -H, and -F) and inner core substituents (lgtG, lpt-3, and lpt-6). For this study, we selected seven strains belonging to subgroup III, a major clonal complex responsible for meningococcal meningitis epidemics in Africa. In addition, we sequenced the homopolymeric tract regions of three phase-variable genes (lgtA, lgtG, and lot-3) to predict gene functionality. The fine structure of the L11 LOS of each strain was determined using composition and glycosyl linkage analyses, NMR, and mass spectrometry. The masses of the dephosphorylated oligosaccharides were consistent with an oligosaccharide composed of two hexoses, one N-acetyl-hexosamine, two heptoses, and one KDO, as proposed previously. The molar composition of LOS showed two glucose residues to be present, in agreement with lgtH sequence prediction. Despite phosphoethanolaminetransferase genes lpt-3 and lpt-6 being present in all seven Neisseria meningitidis strains, phosphoethanolamine (PEtn) was found at both O-3 and O-6 of HepII among the three ST-5 strains, whereas among the four ST-7 strains, only one PEtn was found and located at O-3 of the HepII. The L11 LOS was found to be O-acetylated, as was indicated by the presence of the lot-3 gene being in-frame in all of the seven N. meningitidis strains. To our knowledge, these studies represent the first full genetic and structural characterization of the L11 LOS of N. meningitidis. These investigations also suggest the presence of further regulatory mechanisms affecting LOS structure microheterogeneity in N. meningitidis related to PEtn decoration of the inner core.