The structural basis for pyocin resistance in Neisseria gonorrhoeae lipooligosaccharides

Neisseria gonorrhoeae scavenges host sialic acid for Siglec-mediated, complement-independent suppression of neutrophil activation

Gonorrhea, caused by the bacterium Neisseria gonorrhoeae (Gc), is characterized by neutrophilic influx to infection sites. Gc has developed mechanisms to resist killing by neutrophils that include modifications to its surface lipooligosaccharide (LOS). One such LOS modification is sialylation: Gc sialylates its terminal LOS sugars with cytidine-5'-monophosphate-N-acetylneuraminic acid, which is scavenged from the host using LOS sialyltransferase (Lst) since Gc cannot make its sialic acid. Sialylation enables sensitive strains of Gc to resist complement-mediated killing in a serum-dependent manner. However, little is known about the contribution of sialylation to complement-independent, direct Gc-neutrophil interactions. In the absence of complement, we found sialylated Gc expressing opacity-associated (Opa) proteins decreased the oxidative burst and granule exocytosis from primary human neutrophils. In addition, sialylated Opa+ Gc survived better than vehicle treated or Δlst Gc when challenged with neutrophils. However, Gc sialylation did not significantly affect Opa-dependent association with or internalization of Gc by neutrophils. Previous studies have implicated sialic acid-binding immunoglobulin-type lectins (Siglecs) in modulating neutrophil interactions with sialylated Gc. Blocking neutrophil Siglecs with antibodies that bind to their extracellular domains eliminated the ability of sialylated Opa+ Gc to suppress the oxidative burst and resist neutrophil killing. These findings highlight a new role for sialylation in Gc evasion of human innate immunity, with implications for the development of vaccines and therapeutics for gonorrhea.

IMPORTANCE

Neisseria gonorrhoeae, the bacterium that causes gonorrhea, is an urgent global health concern due to increasing infection rates, widespread antibiotic resistance, and its ability to thwart protective immune responses. The mechanisms by which Gc subverts protective immune responses remain poorly characterized. One way N. gonorrhoeae evades human immunity is by adding sialic acid that is scavenged from the host onto its lipooligosaccharide, using the sialyltransferase Lst. Here, we found that sialylation enhances N. gonorrhoeae survival from neutrophil assault and inhibits neutrophil activation, independently of the complement system. Our results implicate bacterial binding of sialic acid-binding lectins (Siglecs) on the neutrophil surface, which dampens neutrophil antimicrobial responses. This work identifies a new role for sialylation in protecting N. gonorrhoeae from cellular innate immunity, which can be targeted to enhance the human immune response in gonorrhea.

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.

Neisseria gonorrhoeae scavenges host sialic acid for Siglec-mediated, complement-independent suppression of neutrophil activation

Gonorrhea, caused by the bacterium Neisseria gonorrhoeae (Gc), is characterized by neutrophil influx to infection sites. Gc has developed mechanisms to resist killing by neutrophils that include modifications to its surface lipooligosaccharide (LOS). One such LOS modification is sialylation: Gc sialylates its terminal LOS sugars with cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-NANA) scavenged from the host using LOS sialyltransferase (Lst), since Gc cannot make its own sialic acid. Sialylation enables sensitive strains of Gc to resist complement-mediated killing in a serum-dependent manner. However, little is known about the contribution of sialylation to complement-independent, direct Gc-neutrophil interactions. In the absence of complement, we found sialylated Gc expressing opacity-associated (Opa) proteins decreased the oxidative burst and granule exocytosis from primary human neutrophils. In addition, sialylated Opa+ Gc survived better than vehicle treated or Δlst Gc when challenged with neutrophils. However, Gc sialylation did not significantly affect Opa-dependent association with or internalization of Gc by neutrophils. Previous studies have implicated sialic acid-binding immunoglobulin-type lectins (Siglecs) in modulating neutrophil interactions with sialylated Gc. Blocking neutrophil Siglecs with antibodies that bind to their extracellular domains eliminated the ability of sialylated Opa+ Gc to suppress oxidative burst and resist neutrophil killing. These findings highlight a new role for sialylation in Gc evasion of human innate immunity, with implications for the development of vaccines and therapeutics for gonorrhea.

Comparison of lipooligosaccharides from human challenge strains of Neisseria gonorrhoeae

The alarming rise of antibiotic resistance and the emergence of new vaccine technologies have increased the focus on vaccination to control gonorrhea. Neisseria gonorrhoeae strains FA1090 and MS11 have been used in challenge studies in human males. We used negative-ion MALDI-TOF MS to profile intact lipooligosaccharide (LOS) from strains MS11mkA, MS11mkC, FA1090 A23a, and FA1090 1-81-S2. The MS11mkC and 1-81-S2 variants were isolated from male volunteers infected with MS11mkA and A23a, respectively. LOS profiles were obtained after purification using the classical phenol water extraction method and by microwave-enhanced enzymatic digestion, which is more amenable for small-scale work. Despite detecting some differences in the LOS profiles, the same major species were observed, indicating that microwave-enhanced enzymatic digestion is appropriate for MS studies. The compositions determined for MS11mkA and mkC LOS were consistent with previous reports. FA1090 is strongly recognized by mAb 2C7, an antibody-binding LOS with both α- and β-chains if the latter is a lactosyl group. The spectra of the A23a and 1-81-S2 FA1090 LOS were similar to each other and consistent with the expression of α-chain lacto-N-neotetraose and β-chain lactosyl moieties that can both be acceptor sites for sialic acid substitution. 1-81-S2 LOS was analyzed after culture with and without media supplemented with cytidine-5'-monophosphate N-acetylneuraminic acid (CMP-Neu5Ac), which N. gonorrhoeae needs to sialylate its LOS. LOS sialylation reduces the infectivity of gonococci in men, although it induces serum resistance in serum-sensitive strains and reduces killing by neutrophils and antimicrobial peptides. The infectivity of FA1090 in men is much lower than that of MS11mkC, but the reason for this difference is unclear. Interestingly, some peaks in the spectra of 1-81-S2 LOS after bacterial culture with CMP-Neu5Ac were consistent with disialylation of the LOS, which could be relevant to the reduced infectivity of FA1090 in men and could have implications regarding the phase variation of the LOS and the natural history of infection.

Alternative pathway amplification and infections

The alternative pathway (AP) is the phylogenetically oldest arm of the complement system and may have evolved to mark pathogens for elimination by phagocytes. Studies using purified AP proteins or AP-specific serum showed that C3b amplification on bacteria commenced following a lag phase of about 5 min and was highly dependent on the concentration of complement. Most pathogens have evolved several elegant mechanisms to evade complement, including expressing proteases that degrade AP proteins and secreting proteins that block function of C3 convertases. In an example of convergent evolution, many microbes recruit the AP inhibitor factor H (FH) using molecular mechanisms that mimic FH interactions with host cells. In most instances, the AP serves to amplify C3b deposited on microbes by the classical pathway (CP). The role of properdin on microbes appears to be restricted to stabilization of C3 convertases; scant evidence exists for its role as an initiator of the AP on pathogens in the context of serum. Therapeutic complement inhibition carries with it an increased risk of infection. Antibody (Ab)-dependent AP activation may be critical for complement activation by vaccine-elicited Ab when the CP is blocked, and its molecular mechanism is discussed.

Neisseria gonorrhoeae physiology and pathogenesis

Neisseria gonorrhoeae is an obligate human pathogen that is the cause of the sexually transmitted disease gonorrhoea. Recently, there has been a surge in gonorrhoea cases that has been exacerbated by the rapid rise in gonococcal multidrug resistance to all useful antimicrobials resulting in this organism becoming a significant public health burden. Therefore, there is a clear and present need to understand the organism's biology through its physiology and pathogenesis to help develop new intervention strategies. The gonococcus initially colonises and adheres to host mucosal surfaces utilising a type IV pilus that helps with microcolony formation. Other adhesion strategies include the porin, PorB, and the phase variable outer membrane protein Opa. The gonococcus is able to subvert complement mediated killing and opsonisation by sialylation of its lipooligosaccharide and deploys a series of anti-phagocytic mechanisms. N. gonorrhoeae is a fastidious organism that is able to grow on a limited number of primary carbon sources such as glucose and lactate. The utilization of lactate by the gonococcus has been implicated in a number of pathogenicity mechanisms. The bacterium lives mainly in microaerobic environments and can grow both aerobically and anaerobically with the aid of nitrite. The gonococcus does not produce siderophores for scavenging iron but can utilize some produced by other bacteria, and it is able to successful chelate iron from host haem, transferrin and lactoferrin. The gonococcus is an incredibly versatile human pathogen; in the following chapter, we detail the intricate mechanisms used by the bacterium to invade and survive within the host.

Isolation and Characterization of Antibodies Induced by Immunization with TNF-α Inducible Globotetraosylceramide

Glycosphingolipids containing very-long-chain fatty acids (VLCFAs) regulate several immune responses, such as cytokine production, immune signaling, and antibody induction. We previously reported that stimulation with an inflammatory mediator, TNF-α, promotes the expression of glycosphingolipids in vascular endothelial cells. The major component is globotetraosylceramide containing VLCFAs (Gb4Cer-VLCFAs), but its role in inflammatory responses has not been fully investigated. In this study, the antibody-inducing properties of Gb4Cer-VLCFAs were analyzed using serum and hybridoma cells generated from Gb4Cer-VLCFA-immunized mice. The reactivity of serum antibodies against Gb4Cer indicated that immunization with Gb4Cer-VLCFAs immediately induced the production of anti-Gb4Cer antibodies. Over 81% of hybridomas generated from the splenocytes of an immunized mouse produced anti-Gb4Cer antibodies, a subset of which recognized an epitope shared by Gb4Cer and its precursor globotriaosylceramide (Gb3Cer). Further biochemical analyses of established monoclonal antibodies revealed that these antibodies included IgM and IgG3, which specifically react with Gb4Cer and Gb3Cer. These results indicate that immunization with Gb4Cer-VLCFAs can efficiently induce the production of anti-Gb4Cer and -Gb3Cer antibodies by B cells.

Targeting Lipooligosaccharide (LOS) for a Gonococcal Vaccine

The increasing incidence of gonorrhea worldwide and the global spread of multidrug-resistant strains of Neisseria gonorrhoeae, constitute a public health emergency. With dwindling antibiotic treatment options, there is an urgent need to develop safe and effective vaccines. Gonococcal lipooligosaccharides (LOSs) are potential vaccine candidates because they are densely represented on the bacterial surface and are readily accessible as targets of adaptive immunity. Less well-understood is whether LOSs evoke protective immune responses. Although gonococcal LOS-derived oligosaccharides (OSs) are major immune targets, often they undergo phase variation, a feature that seemingly makes LOS less desirable as a vaccine candidate. However, the identification of a gonococcal LOS-derived OS epitope, called 2C7, that is: (i) a broadly expressed gonococcal antigenic target in human infection; (ii) a virulence determinant, that is maintained by the gonococcus and (iii) a critical requirement for gonococcal colonization in the experimental setting, circumvents its limitation as a potential vaccine candidate imposed by phase variation. Difficulties in purifying structurally intact OSs from LOSs led to "conversion" of the 2C7 epitope into a peptide mimic that elicited cross-reactive IgG anti-OS antibodies that also possess complement-dependent bactericidal activity against gonococci. Mice immunized with the 2C7 peptide mimic clear vaginal colonization more rapidly and reduce gonococcal burdens. 2C7 vaccine satisfies criteria that are desirable in a gonococcal vaccine candidate: broad representation of the antigenic target, service as a virulence determinant that is also critical for organism survival in vivo and elicitation of broadly cross-reactive IgG bactericidal antibodies when used as an immunogen.

Innate immune response to lipooligosaccharide: pivotal regulator of the pathobiology of invasive Neisseria meningitidis infections

Infections due to Neisseria meningitidis afflict more than one million people worldwide annually and cause death or disability in many survivors. The clinical course of invasive infections has been well studied, but our understanding of the cause of differences in patient outcomes has been limited because these are dependent on multiple factors including the response of the host, characteristics of the bacteria and interactions between the host and the bacteria. The meningococcus is a highly inflammatory organism, and the lipooligosaccharide (LOS) on the outer membrane is the most potent inflammatory molecule it expresses due to the interactions of the lipid A moiety of LOS with receptors of the innate immune system. We previously reported that increased phosphorylation of hexaacylated neisserial lipid A is correlated with greater inflammatory potential. Here we postulate that variability in lipid A phosphorylation can tip the balance of innate immune responses towards homeostatic tolerance or proinflammatory signaling that affects adaptive immune responses, causing disease with meningitis only, or septicemia with or without meningitis, respectively. Furthermore, we propose that studies of the relationship between bacterial virulence and gene expression should consider whether genetic variation could affect properties of biosynthetic enzymes resulting in LOS structural differences that alter disease pathobiology.

Functional and structural studies on the Neisseria gonorrhoeae GmhA, the first enzyme in the glycero-manno-heptose biosynthesis pathways, demonstrate a critical role in lipooligosaccharide synthesis and gonococcal viability

Sedoheptulose-7-phosphate isomerase, GmhA, is the first enzyme in the biosynthesis of nucleotide-activated-glycero-manno-heptoses and an attractive, yet underexploited, target for development of broad-spectrum antibiotics. We demonstrated that GmhA homologs in Neisseria gonorrhoeae and N. meningitidis (hereafter called GmhAGC and GmhANM , respectively) were interchangeable proteins essential for lipooligosaccharide (LOS) synthesis, and their depletion had adverse effects on neisserial viability. In contrast, the Escherichia coli ortholog failed to complement GmhAGC depletion. Furthermore, we showed that GmhAGC is a cytoplasmic enzyme with induced expression at mid-logarithmic phase, upon iron deprivation and anaerobiosis, and conserved in contemporary gonococcal clinical isolates including the 2016 WHO reference strains. The untagged GmhAGC crystallized as a tetramer in the closed conformation with four zinc ions in the active site, supporting that this is most likely the catalytically active conformation of the enzyme. Finally, site-directed mutagenesis studies showed that the active site residues E65 and H183 were important for LOS synthesis but not for GmhAGC function in bacterial viability. Our studies bring insights into the importance and mechanism of action of GmhA and may ultimately facilitate targeting the enzyme with small molecule inhibitors.

Internalization of a novel, huge lectin from Ibacus novemdentatus (slipper lobster) induces apoptosis of mammalian cancer cells

An N-acetyl sugar-binding lectin (termed iNoL) displaying cytotoxic activity against human cancer cells was isolated from the slipper lobster Ibacus novemdentatus (family Scyllaridae). iNoL recognized monosaccharides containing N-acetyl group, and glycoproteins (e.g., BSM) containing oligosaccharides with N-acetyl sugar. iNoL was composed of five subunits (330, 260, 200, 140, and 30 kDa), which in turn consisted of 70-, 40-, and 30-kDa polypeptides held together by disulfide bonds. Electron microscopic observations and gel permeation chromatography indicated that iNoL was a huge (500-kDa) molecule and had a polygonal structure under physiological conditions. iNoL displayed cytotoxic (apoptotic) effects against human cancer cell lines MCF7 and T47D (breast), HeLa (ovarian), and Caco2 (colonic), through incorporation (internalization) into cells. The lectin was transported into lysosomes via endosomes. Its cytotoxic effect and incorporation into cells were inhibited by the co-presence of N-acetyl-D-mannosamine (ManNAc). Treatment of HeLa cells with iNoL resulted in DNA fragmentation and chromatin condensation, through activation of caspase-9 and -3. In summary, the novel crustacean lectin iNoL is incorporated into mammalian cancer cells through glycoconjugate interaction, and has cytotoxic (apoptotic) effects.

Phase variable changes in genes lgtA and lgtC within the lgtABCDE operon of Neisseria gonorrhoeae can modulate gonococcal susceptibility to normal human serum

The α-chain of the core oligosaccharide of the lipo-oligosaccharide (LOS) produced by Neisseria gonorrhoeae can undergo reversible and rapid changes in structure due to phase-variable production of certain enzymes employed in the biosynthesis of the lacto- N-neotetraose structure. Five of these enzymes are encoded by the lgtABCDE operon, and polynucleotide tracts within three of these genes ( lgtA, lgtC and lgtD) can be substrates for slipped-strand mispairing events that lead to nucleotide insertions or deletion events which result in variable production of their respective gene products. We now report that phase-variable synthesis of the lgtA and lgtC gene products in strain FA19 results in the production of elongated LOS α-chains and that the presence of these LOS species can result in gonococci being sensitive to the bacteriolytic action of serum-antibody and complement. Hence, phase variation within the lgtABCDE operon can significantly impact the ability of gonococci to subvert this important host defense system.

Review: Lipopolysaccharide inner core oligosaccharide structure and outer membrane stability in human pathogens belonging to the Enterobacteriaceae

In the Enterobacteriaceae, the outer membrane is primarily comprised of lipopolysaccharides. The lipopolysaccharide molecule is important in mediating interactions between the bacterium and its environment and those regions of the molecule extending further away from the cell surface show a higher amount of structural diversity. The hydrophobic lipid A is highly conserved, due to its important role in the structural integrity of the outer membrane. Attached to the lipid A region is the core oligosaccharide. The inner core oligosaccharide (lipid A proximal) backbone is also well conserved. However, non-stoichiometric substitutions of the basic inner core structure lead to structural variation and microheterogeneity. These include the addition of negatively charged groups (phosphate or galacturonic acid), ethanolamine derivatives, and glycose residues (Kdo, rhamnose, galactose, glucosamine, N-acetylglucosamine, heptose, Ko). The genetics and biosynthesis of these substitutions is beginning to be elucidated. Modification of heptose residues with negatively charged molecules (such as phosphate in Escherichia coli and Salmonella and galacturonic acid in Klebsiella pneumoniae ) has been shown to be involved in maintaining membrane stability. However, the biological role(s) of the remaining substitutions is unknown.

Phase-Variable Heptose I Glycan Extensions Modulate Efficacy of 2C7 Vaccine Antibody Directed against Neisseria gonorrhoeae Lipooligosaccharide

Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection, gonorrhea, has developed resistance to most conventional antibiotics. Safe and effective vaccines against gonorrhea are needed urgently. A candidate vaccine that targets a lipooligosaccharide (LOS) epitope recognized mAb 2C7 attenuates gonococcal burden in the mouse vaginal colonization model. Glycan extensions from the LOS core heptoses (HepI and HepII) are controlled by phase-variable LOS glycosyltransferase (lgt) genes; we sought to define how HepI glycan extensions affect mAb 2C7 function. Isogenic gonococcal mutants in which the lgt required for mAb 2C7 reactivity (lgtG) was genetically locked on and the lgt loci required for HepI variation (lgtA, lgtC, and lgtD) were genetically locked on or off in different combinations were created. We observed 100% complement-dependent killing by mAb 2C7 of a mutant that expressed lactose (Gal-Glc) from HepI, whereas a mutant that expressed Gal-Gal-Glc-HepI fully resisted killing (>100% survival). Mutants that elaborated 4- (Gal-GlcNAc-Gal-Glc-HepI) and 5-glycan (GalNAc-Gal-GlcNAc-Gal-Glc-HepI) structures displayed intermediate phenotypes (<50% killing with 2 μg/ml and >95% killing with 4 μg/ml mAb 2C7). The contrasting phenotypes of the lactose-HepI and the Gal-Gal-Glc-HepI LOS structures were recapitulated with phase variants of a recently isolated clinical strain. Despite lack of killing of the Gal-Gal-Glc-HepI mutants, mAb 2C7 deposited sufficient C3 on these bacteria for opsonophagocytic killing by human neutrophils. In conclusion, mAb 2C7 showed functional activity against all gonococcal HepI LOS structures defined by various lgtA/C/D on/off combinations, thereby providing further impetus for use of the 2C7 epitope in a gonococcal vaccine.

Lipid a is more than acyl chains

The acyl chain length, number, and distribution have been considered the major factors contributing to this biological activity of lipid A. The charged head groups on the dihexosamine backbone have also been implicated in contributing to this biology. In Neisseria, it has now been shown that loss of the 4' phosphoethanolamine has an impact on virulence in an animal model and on the organism's susceptibility to cationic antimicrobial peptides. Such studies offer potential insight into targets for novel antimicrobial agents.

The sugar 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) as a characteristic component of bacterial endotoxin — a review of its biosynthesis, function, and placement in the lipopolysaccharide core

The sugar 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) is a characteristic component of bacterial lipopolysaccharide (LPS, endotoxin). It connects the carbohydrate part of LPS with C6 of glucosamine or 2,3-diaminoglucose of lipid A by acid-labile α-ketosidic linkage. The number of Kdo units present in LPS, the way they are connected, and the occurrence of other substituents (P, PEtn, PPEtn, Gal, or β-l-Ara4N) account for structural diversity of the inner core region of endotoxin. In a majority of cases, Kdo is crucial to the viability and growth of bacterial cells. In this paper, the biosynthesis of Kdo and the mechanism of its incorporation into the LPS structure, as well as the location of this unique component in the endotoxin core structures, have been described.

Urethral exudates of men with Neisseria gonorrhoeae infections select a restricted lipooligosaccharide phenotype during transmission

Neisseria gonorrhoeae lipooligosaccharides (LOSs) induce immunoglobulin G that protects men from experimental infection. This raises the possibility that an LOS vaccine might prevent gonorrhea. Gonococci make different LOS molecules, depending on whether 3 genes, lgtA, lgtC, and lgtD, are in frame (IF) or out of frame (OOF). Mispairing of polymeric guanine (polyG) tracts within each gene determines its frame during replication. We amplified lgtA, lgtC, and lgtD from diagnostic slides of urethral exudates and sequenced their polyG tracts. We found that lgtA in exudative bacteria is IF and that lgtC is OOF. The frame of lgtD varied widely: it was OOF in most but not all cases. This genotype would result in synthesis of polylactosamine α chains that could be sialylated. Polylactosamine α chains would enhance virulence, and their sialylation would enable gonococci to survive within polymorphonuclear cells; however, an active LgtD in a few bacteria could provide a survival advantage in other sites of infection.

Human lipooligosaccharide IGG that prevents endemic meningococcal disease recognizes an internal lacto-N-neotetraose structure

Antibodies that initiate complement-mediated killing of Neisseria meningitidis as they enter the bloodstream from the oropharynx protect against disseminated disease. Human IgGs that bind the neisserial L7 lipooligosaccharide (LOS) are bactericidal for L3,7 and L2,4 meningococci in the presence of human complement. These strains share a lacto-N-neotetraose (nLc4) LOS α chain. We used a set of mutants that have successive saccharide deletions from the nLc4 α chain to characterize further the binding and bactericidal activity of nLc4 LOS IgG. We found that the nLc4 α chain conforms at least four different antigens. We separately purified IgG that required the nLc4 (non-reducing) terminal galactose (Gal) for binding and IgG that bound the truncated nLc3 α chain that lacks this Gal residue. IgG that bound the internal nLc3 α chain killed both L3,7 and L2,4 strains, whereas IgG that required the nLc4 terminal Gal residue for binding killed L2,4 stains but not L3,7 strains. These results show that the diversity of LOS antibodies in human serum is as much a function of the conformation of multiple antigens by a single glycoform as of the production of multiple glycoforms. Differences in sensitivity to killing by human nLc4 LOS IgG may account for the fact that fully two-thirds of endemic group B meningococcal disease in infants and children is caused by L3,7 strains, but only 20% is caused by L2,4 stains.

Sequence-based predictions of lipooligosaccharide diversity in the Neisseriaceae and their implication in pathogenicity

Endotoxin [Lipopolysaccharide (LPS)/Lipooligosaccharide (LOS)] is an important virulence determinant in gram negative bacteria. While the genetic basis of endotoxin production and its role in disease in the pathogenic Neisseria has been extensively studied, little research has focused on the genetic basis of LOS biosynthesis in commensal Neisseria. We determined the genomic sequences of a variety of commensal Neisseria strains, and compared these sequences, along with other genomic sequences available from various sequencing centers from commensal and pathogenic strains, to identify genes involved in LOS biosynthesis. This allowed us to make structural predictions as to differences in LOS seen between commensal and pathogenic strains. We determined that all neisserial strains possess a conserved set of genes needed to make a common 3-Deoxy-D-manno-octulosonic acid -heptose core structure. However, significant genomic differences in glycosyl transferase genes support the published literature indicating compositional differences in the terminal oligosaccharides. This was most pronounced in commensal strains that were distally related to the gonococcus and meningococcus. These strains possessed a homolog of heptosyltransferase III, suggesting that they differ from the pathogenic strains by the presence a third heptose. Furthermore, most commensal strains possess homologs of genes needed to synthesize lipopolysaccharide (LPS). N. cinerea, a commensal species that is highly related to the gonococcus has lost the ability to make sialyltransferase. Overall genomic comparisons of various neisserial strains indicate that significant recombination/genetic acquisition/loss has occurred within the genus, and this muddles proper speciation.

The oligosaccharide of gonococcal lipooligosaccharide contains several epitopes that are recognized by human antibodies

Recently, we isolated human IgG from normal human sera (NHS) using lipooligosaccharide (LOS) from gonococcal strain JW31R as an affinity ligand. We provided evidence that the oligosaccharide (OS) moiety of LOS was immunogenic in humans and that NHS contains functional antibodies that bind to the branched OS. The present study aimed to identify bactericidal antibodies that bind to partial core OS structures or their adjacent sites expressed in the 3,4-branched and 2,3:3,4-dibranched neisserial LOSs. Using 15253 LOS from serum-resistant gonococcal strain 15253 as an affinity ligand, we isolated IgG2 and found that this preparation contained at least three different species. (i) One IgG2 species recognized a cross-reactive epitope that is expressed on 3,4-branched and 2,3:3,4-dibranched neisserial LOSs. (ii) Another IgG2 species was specific for JW31R LOS from a pyocin-resistant gonococcal strain; this IgG-defined epitope was not shared with the aforementioned branched LOSs. (iii) The third IgG2 species bound to the "Salmonella minnesota" Rb and Re mutant lipopolysaccharides (LPSs); this IgG2 recognizes a KDOalpha2-4KDO residue at the reducing end of the carbohydrate moiety of each LPS. The IgG2 was also found to be functional and facilitated the killing of strain 15253. The current results show that neisserial LOS contains several epitopes within its OS moiety that are recognized by human antibodies.

Structural characterization of an oligosaccharide made by Neisseria sicca

Neisseria sicca 4320 expresses two carbohydrate-containing components with sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobilities that resemble those of lipooligosaccharide and lipopolysaccharide. Using matrix-assisted laser desorption ionization--time of flight and electrospray ionization mass spectrometry, we characterized a disaccharide carbohydrate repeating unit expressed by this strain. Gas chromatography identified the sugars composing the unit as rhamnose and N-acetyl-D-glucosamine. Glycosidase digestion confirmed the identity of the nonreducing terminal sugar of the disaccharide and established its beta-anomeric configuration. Mass spectrometry analysis and lectin binding were used to verify the linkages within the disaccharide repeat. The results revealed that the disaccharide repeat is [-4) beta-L-rhamnose (1-3) beta-N-acetyl-D-glucosamine (1-] with an N-acetyl-D-glucosamine nonreducing terminus. This work is the first structural characterization of a molecule that possesses rhamnose in the genus Neisseria.

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.

Heptose I glycan substitutions on Neisseria gonorrhoeae lipooligosaccharide influence C4b-binding protein binding and serum resistance

Lipooligosaccharide (LOS) heptose (Hep) glycan substitutions influence gonococcal serum resistance. Several gonococcal strains bind the classical complement pathway inhibitor, C4b-binding protein (C4BP), via their porin (Por) molecule to escape complement-dependent killing by normal human serum (NHS). We show that the proximal glucose (Glc) on HepI is required for C4BP binding to Por1B-bearing gonococcal strains MS11 and 1291 but not to FA19 (Por1A). The presence of only the proximal Glc on HepI (lgtE mutant) permitted maximal C4BP binding to MS11 but not to 1291. Replacing 1291 lgtE Por with MS11 Por increased C4BP binding to levels that paralleled MS11 lgtE, suggesting that replacement of the Por1B molecule dictated the effects of HepI glycans on C4BP binding. The remainder of the strain background did not affect C4BP binding; replacing the Por of strain F62 with MS11 Por (F62 PorMS11) and truncating HepI mirrored the findings in the MS11 background. C4BP binding correlated with resistance to killing by NHS in most instances. F62 PorMS11 and its lgtE mutant were sensitive to NHS despite binding C4BP, secondary to kinetically overwhelming classical pathway activation and possibly increased alternative pathway activation (measured by factor Bb binding) by the F62 background. FA19 lgtF (HepI unsubstituted) resisted killing by only 10% NHS, not 50% NHS, despite binding levels of C4BP similar to those of FA19 and FA19 lgtE (both resistant to 50% serum), suggesting a role for the proximal Glc in serum resistance independently of C4BP binding. This study provides mechanistic insights into how HepI LOS substitutions affect the serum resistance of N. gonorrhoeae.

Affinity-purified human immunoglobulin G that binds a lacto-N-neotetraose-dependent lipooligosaccharide structure is bactericidal for serogroup B Neisseria meningitidis

Despite technological advances, no vaccine to prevent serogroup B meningococcal disease is available. The failure to develop a vaccine has shifted the focus to an alternative outer membrane structure, lipooligosaccharide (LOS), because disseminated disease induces bactericidal immunoglobulin G (IgG) that binds LOS. The purpose of this study was to identify the LOS structure(s) that induces human bactericidal IgG by purification and characterization of these antibodies. Human LOS IgG antibodies were affinity purified by passage of intravenous immunoglobulin through purified, type-specific LOS having a known structure coupled to epoxy-activated Sepharose 6B. Pathogenic group B strains representing the major LOS serotypes were used to examine the binding and bactericidal activities of four LOS-specific IgG preparations. All four LOS-specific IgG preparations bound to strains expressing homologous, as well as heterologous, LOS serotypes as determined by flow cytometry and an enzyme-linked immunosorbent assay. With human complement, IgG that was purified with L7 LOS was bactericidal for strains expressing L3,7 and L2,4 LOS, serotypes expressed by the majority of disease-associated group B and C meningococci. In conclusion, we purified human LOS-specific IgG that binds meningococci across LOS glycose-specific serotypes. An antigen that is dependent on the glycose lacto-N-neotetraose induces IgG in humans that is bactericidal for L2, L3, L4, and L7 strains. A vaccine containing this antigen would have the potential to protect against the vast majority of group B meningococcal strains.

Role of lgtC in resistance of nontypeable Haemophilus influenzae strain R2866 to human serum

We are investigating a nontypeable Haemophilus influenzae (NTHI) strain, R2866, isolated from a child with meningitis. R2866 is unusually resistant to killing by normal human serum. The serum 50% inhibitory concentration (IC50) for this strain is 18%, approaching that of encapsulated H. influenzae. R3392 is a derivative of R2866 that was found to have increased sensitivity to human serum (IC50, 1.5%). Analysis of tetrameric repeat regions within lipooligosaccharide (LOS) biosynthetic genes in both strains indicated that the glycosyltransferase gene lgtC was out of frame ("off") in most colonies of R3392 but in frame with its start codon ("on") in most colonies of the parent. We sought antigenic and biochemical evidence for modification of the LOS structure. In a whole-cell enzyme-linked immunosorbent assay, strain R3392 displayed reduced binding of the Galalpha1,4Gal-specific monoclonal antibody 4C4. Mass spectrometry analysis of LOS from strain R2866 indicated that the primary oligosaccharide glycoform contained four heptose and four hexose residues, while that of R3392 contained four heptose and three hexose residues. We conclude that the R2866 lgtC gene encodes a galactosyltransferase involved in synthesis of the 4C4 epitope, as in other strains, and that expression of lgtC is associated with the high-level serum resistance that has been observed for this strain. This is the first description of the genetic basis of high-level serum resistance in NTHI, as well as the first description of LOS composition in an NTHI strain for which the complete genome sequence has been determined.

Biochemical analysis of Lpt3, a protein responsible for phosphoethanolamine addition to lipooligosaccharide of pathogenic Neisseria

The inner core of neisserial lipooligosaccharide (LOS) contains heptose residues that can be decorated by phosphoethanolamine (PEA). PEA modification of heptose II (HepII) can occur at the 3, 6, or 7 position(s). We used a genomic DNA sequence of lpt3, derived from Neisseria meningitidis MC58, to search the genomic sequence of N. gonorrhoeae FA1090 and identified a homolog of lpt3 in N. gonorrhoeae. A PCR amplicon containing lpt3 was amplified from F62DeltaLgtA, cloned, mutagenized, and inserted into the chromosome of N. gonorrhoeae strain F62DeltaLgtA, producing strain F62DeltaLgtAlpt3::Tn5. LOS isolated from this strain lost the ability to bind monoclonal antibody (MAb) 2-1-L8. Complementation of this mutation by genetic removal of the transposon insertion restored MAb 2-1-L8 binding. Mass spectrometry analysis of LOS isolated from the F62DeltaLgtA indicated that this strain contained two PEA modifications on its LOS. F62DeltaLgtAlpt3::Tn5 lacked a PEA modification on its LOS, a finding consistent with the hypothesis that lpt3 encodes a protein mediating PEA addition onto gonococcal LOS. The DNA encoding lpt3 was cloned into an expression vector and Lpt3 was purified. Purified Lpt3 was able to mediate the addition of PEA to LOS isolated from F62DeltaLgtAlpt3::Tn5.

Separation and identification of neisserial lipooligosaccharide oligosaccharides using high-performance anion-exchange chromatography with pulsed amperometric detection

We determined the optimal conditions for high-performance anion-exchange chromatography with pulsed amperometric detection (HPAE-PAD) of oligosaccharides (OS) released from neisserial lipooligosaccharides (LOS) by mild acid hydrolysis. We efficiently obtained detailed composition, sequence, and linkage information about high Mr LOS. We found that HPAE-PAD can discriminate isobaric (same Mr) molecules of different structure, for example, nLc4 and Gb4, distinguish alpha from beta chain extensions, and determine the number of phosphoethanolamine (PEA) substituents. HPAE-PAD provided quantitative information that could be used to compare the relative abundances of OS. We used HPAE-PAD to identify all of the known LOS alpha chain antennae. When used with antibody-binding profiles and exoglycosidase digestion results, HPAE-PAD can provide nearly complete structures rapidly.

Enhanced factor H binding to sialylated Gonococci is restricted to the sialylated lacto-N-neotetraose lipooligosaccharide species: implications for serum resistance and evidence for a bifunctional lipooligosaccharide sialyltransferase in Gonococci

We isolated serologically identical (by serovar determination and porin variable region [VR] typing) strains of Neisseria gonorrhoeae from an infected male and two of his monogamous female sex partners. One strain (termed 398078) expressed the L1 (Galalpha1 --> 4 [corrected] Galbeta1 --> 4Glcbeta1 --> 4HepI) lipooligosaccharide (LOS) structure exclusively; the other (termed 398079) expressed the lacto-N-neotetraose (LNT; Galbeta1 --> 4GlcNAcbeta1 --> 3Galbeta1 --> 4Glcbeta1 --> 4HepI) LOS structure. The strain from the male index case expressed both glycoforms and exhibited both immunotypes. Nuclear magnetic resonance analysis revealed that sialic acid linked to the terminal Gal of L1 LOS via an alpha2 --> 6 linkage and, as expected, to the terminal Gal of LNT LOS via an alpha2--> 3 linkage. Insertional inactivation of the sialyltransferase gene (known to sialylate LNT LOS) abrogated both L1 LOS sialylation and LNT LOS sialylation, suggesting a bifunctional nature of this enzyme in gonococci. Akin to our previous observations, sialylation of the LNT LOS of strain 398079 enhanced the binding of the complement regulatory molecule, factor H. Rather surprisingly, factor H did not bind to sialylated strain 398078. LOS sialylation conferred the LNT LOS-bearing strain complete (100%) resistance to killing by even 50% nonimmune normal human serum (NHS), whereas sialylation of L1 LOS conferred resistance only to 10% NHS. The ability of gonococcal sialylated LNT to bind factor H confers high-level serum resistance, which is not seen with sialylated L1 LOS. Thus, serum resistance mediated by sialylation of gonococcal L1 and LNT LOS occurs by different mechanisms, and specificity of factor H binding to sialylated gonococci is restricted to the LNT LOS species.

Gonococcal porin IB activates NF-kappaB in human urethral epithelium and increases the expression of host antiapoptotic factors

Infection of human urethral epithelial cells (UECs) with Neisseria gonorrhoeae increases the transcription of several host antiapoptotic genes, including bfl-1, cox-2, and c-IAP-2. In order to identify the bacterial factor(s) responsible for eliciting these changes, the transcriptional status of apoptotic machinery was monitored in UECs challenged with certain gonococcal membrane components. Initially, we observed that infection of UECs with gentamicin-killed gonococci increased the expression of the antiapoptotic Bcl-2 family member, bfl-1. This observation indicated that viable, replicating bacteria are not required for induction of antiapoptotic gene expression. Confirming this observation, treatment of UECs with purified gonococcal membrane increased the expression of bfl-1, cox-2, and c-IAP-2. This finding suggested that a factor or multiple factors present in the outer membrane (OM) are responsible for altering UEC antiapoptotic gene expression. Interestingly, treatment of UECs with gonococcal porin IB (PorB IB), a major constituent of the OM, significantly increased the transcription of bfl-1, cox-2, and c-IAP-2. The upregulation of these genes by PorB IB was determined to be dependent on NF-kappaB activation, as inhibiting NF-kappaB blocked induced expression of these genes. This work demonstrates the altered expression of host apoptotic factors in response to gonococcal PorB IB and supports a model whereby UEC cell death may be modulated as a potential mechanism of bacterial survival and proliferation.

Infection of human urethral epithelium with Neisseria gonorrhoeae elicits an upregulation of host anti-apoptotic factors and protects cells from staurosporine-induced apoptosis

In order to better understand the host response to an infection with Neisseria gonorrhoeae, microarray technology was used to analyse the gene expression profile between uninfected and infected human urethral epithelium. The anti-apoptotic genes bfl-1, cox-2 and c-IAP-2 were identified to be upregulated approximately eight-, four- or twofold, respectively, following infection. Subsequent assays including RT-PCR, real time RT-PCR and RNase protection confirmed the increased expression of these apoptotic regulators, and identified that a fourth anti-apoptotic factor, mcl-1, is also upregulated. RT-PCR and RNase protection also showed that key pro-apoptotic factors including bax, bad and bak do not change in expression. Furthermore, our studies demonstrated that infection with the gonococcus partially protects urethral epithelium from apoptosis induced by the protein kinase inhibitor, staurosporine (STS). This work shows that following infection with Neisseria gonorrhoeae, several host anti-apoptotic factors are upregulated. In addition, a gonococcal infection protects host cells from subsequent STS-induced death. The regulation of host cell death by the gonococcus may represent a mechanism employed by this pathogen to survive and proliferate in host epithelium.

Activation of toll-like receptor 2 (TLR2) and TLR4/MD2 by Neisseria is independent of capsule and lipooligosaccharide (LOS) sialylation but varies widely among LOS from different strains

Lipooligosaccharide (LOS) structure and capsular polysaccharide of Neisseria meningitidis each greatly influence the virulence of the organism and the quality of host innate immune responses. In this study, we found that production of the proinflammatory cytokine tumor necrosis factor (TNF) by a human monocyte-derived cell line (THP-1) exposed to strains of N. meningitidis lacking capsule and/or with truncated LOS was similar to that elicited by the isogenic wild-type strain. These mutants also exhibited no difference in induction of the interleukin-8 (IL-8) promoter in a transfected HeLa cell system of Toll-like receptor 2 (TLR2) and TLR4/MD2 signaling. However, purified LOS from diverse strains of Neisseria (both N. meningitidis and N. gonorrhoeae) caused widely variant levels of IL-8 promoter induction in cells expressing MD2 that correlated with the production of TNF from THP-1 cells. These data suggest that although modification of the oligosaccharide chain of LOS and/or absence of capsule do not affect cell signaling mediated by TLR4/MD2, fine-structural differences in the LOS do influence signaling through TLR4/MD2 and, through this pathway, influence some of the proinflammatory responses elicited by Neisseria.

Biochemical properties of Neisseria gonorrhoeae LgtE

A fragment of chromosomal DNA encoding the lgtE gene of Neisseria gonorrhoeae strain F62 was amplified by PCR and cloned into the expression vector pET15b. Functional LgtE was purified and its biochemical properties were determined. The purified enzyme was maximally active in buffer containing manganese; minimal activity was obtained in buffer containing other divalent cations. LgtE was only able to mediate the addition of UDP-galactose into neisserial lipooligosaccharides (LOSs). We used a variety of genetically defined and chemically verified LOS structures to determine acceptor specificity. LgtE was able to mediate the addition of galactose into a variety of LOS structures, indicating the this enzyme possesses broad acceptor specificity. Furthermore, it was able to add multiple galactose residues onto LOS. We also determined that this enzyme was capable of adding galactose onto both the alpha and beta chains of neisserial LOS.

Galectin-3 binds lactosaminylated lipooligosaccharides from Neisseria gonorrhoeae and is selectively expressed by mucosal epithelial cells that are infected

Galectins are a family of beta-galactoside binding proteins that have been proposed as host receptors for bacteria because beta-galactoside carbohydrates are common in bacterial membrane glycolipid lipooligosaccharides (LOS) and lipopolysaccharides. We investigated the interaction of galectin-3 with gonococcal LOS that make lactosyl (Lc2 or Lac), paraglobosyl (nLc4; LNnT; lacto-N-neotetraose), gangliosyl (IV3GalNAcnLc4), and neolactohexaosyl (nLc6, lactonorhexaosyl) oligosaccharides. All but gangliosyl LOS terminate in beta-galactoside. Galectin-3 had the highest affinity for the nLc6 LOS, which is made by a strain that is highly infectious for the male urethra, but also bound nLc4 LOS and to a Lac LOS. The lacto-N-neotetraose tetrasaccharide was a more potent inhibitor of galectin-3 binding to LOS than either lactose or N-acetyllactosamine. The relative affinity of galectin-3 for gonococci mirrored its affinity for purified LOS. Western blot analysis revealed expression of galectin-3 by human endometrial adenocarcinoma and prostatic epithelial cells that can be invaded by gonococci. Immunohistochemistry of human fallopian tube epithelium showed localized expression of galectin-3 by non-ciliated cells, the specific cell gonococci invade in this tissue. We conclude that because of its location and affinity for gonococcal LOS galectin-3 could play a role in gonococcal infection.

Neisseria gonorrhoeae strain PID2 simultaneously expresses six chemically related lipooligosaccharide structures

Neisseria gonorrhoeae strain PID2 was isolated from a woman suffering from pelvic inflammatory disease. When LOS expressed by this strain is analyzed on SDS-PAGE gels, at least six different lipooligosaccharide (LOS) components are visualized. We characterized the LOSs made by this strain by exoglycosidase digestion, sugar composition analysis, mass spectrometry, and analysis of the genes needed for its synthesis. DNA sequence analysis showed that the lgt gene cluster in this strain has undergone a rearrangement and that it possesses two copies of lgtA, one copy of lgtB and lgtC, and a hybrid gene containing sequences from lgtB and lgtE. We determined that the hybrid lgtB/E gene retained the lgtE gene function. DNA sequence analysis of the gene organization suggested that an intramolecular recombination between lgtA and lgtD and lgtB and lgtE had occurred via homologous recombination between similar sequences. Our studies demonstrated that fluorophore-assisted carbohydrate electrophoresis can be utilized to rapidly determine the composition of LOS. By combining exoglycosidase digestion, in combination with mass spectrometry analysis and compositional analysis, the data indicate that all of the LOS components produced by PID2 extend off of the alpha chain. The longest alpha chain oligosaccharide structure is Gal-GlcNAc-Gal-GlcNAc-Gal-Glc-Heptose I, and the six LOS components are built up by sequentially adding sugars onto the first heptose. PID2 LOS is the first Neisserial LOS to be shown to be devoid of phosphoethanolamine modifications. Because PID2 can surface express its LOS, it indicates that the addition of phosphoethanolamine is not required for LOS surface expression.

Strategies for mimicking Neisserial saccharide epitopes as vaccines

Monoclonal antibody (mAb) 2C7 recognizes a conserved and widely expressed oligosaccharide (OS) epitope on Neisseria gonorrhoeae. This OS epitope evokes a significant bactericidal and opsonic immune response after natural infection and vaccination. The OS epitope structure represents an excellent target for a potential protective gonococcal vaccine. Because carbohydrate antigens are T-cell independent, inducing weak antibody responses, OS molecules are not useful immunogens. We developed and examined two different strategies to mimic the 2C7 OS epitope: (i) an anti-idiotope (mAb CA1); and (ii) a peptide (PEP-1). These surrogate immunogens elicited antibody responses in mice (CA1 and PEP-1) and rabbits (CA1) that were bactericidal in vitro against gonococci. Both CA1 and PEP-1 are true immunologic mimics of OS and may form a basis for the development of vaccine candidates for human immunization against N. gonorrhoeae.

Mass spectrometric strategy for the characterization of lipooligosaccharides from Neisseria gonorrhoeae 302 using FTICR

The lipooligosaccharides (LOS) of Neisseria gonorrhoeae 302 were profiled using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Using techniques developed in this laboratory, the topology and some of the linkages of the LOS were determined. Mass spectrometric analysis in the negative ion mode yielded a glycoform of the composition: Hex3 Hep2 Hxn1 PEA1 KDO2 DPLA. The composition was confirmed through exact mass measurements, which showed only a 2 ppm error between the exact mass and theoretical mass. Although the core structure has been postulated previously, the positioning of the three hexose moieties were in question for this particular strain of N. gonorrhoeae. Topology assignment was performed through collision-induced dissociation analysis of the O-deacylated glycoform in the negative ion mode followed by submission to the saccharide topology analysis tool (STAT) computer program, which confirmed the topology assignment. It was found that the three hexoses were added to the Hep[I] of the conserved core of N. gonorrhoeae in a linear fashion, while Hep[II] remains unbranched. Linkage position analysis was performed through application of a mild acid hydrolysis technique followed by collision-induced dissociation of the sodiated precursor ions, yielding a 1 --> 4 linkage between the terminating and penultimate hexoses.

Regulation of the mannan-binding lectin pathway of complement on Neisseria gonorrhoeae by C1-inhibitor and alpha 2-macroglobulin

We examined complement activation by Neisseria gonorrhoeae via the mannan-binding lectin (MBL) pathway in normal human serum. Maximal binding of MBL complexed with MBL-associated serine proteases (MASPs) to N. gonorrhoeae was achieved at a concentration of 0.3 microg/ml. Preopsonization with MBL-MASP at concentrations as low as 0.03 microg/ml resulted in approximately 60% killing of otherwise fully serum-resistant gonococci. However, MBL-depleted serum (MBLdS) reconstituted with MBL-MASP before incubation with organisms (postopsonization) failed to kill at a 100-fold higher concentration. Preopsonized organisms showed a 1.5-fold increase in C4, a 2.5-fold increase in C3b, and an approximately 25-fold increase in factor Bb binding; enhanced C3b and factor Bb binding was classical pathway dependent. Preopsonization of bacteria with a mixture of pure C1-inhibitor and/or alpha(2)-macroglobulin added together with MBL-MASP, all at physiologic concentrations before adding MBLdS, totally reversed killing in 10% reconstituted serum. Reconstitution of MBLdS with supraphysiologic (24 microg/ml) concentrations of MBL-MASP partially overcame the effects of inhibitors (57% killing in 10% reconstituted serum). We also examined the effect of sialylation of gonococcal lipooligosaccharide (LOS) on MBL function. Partial sialylation of LOS did not decrease MBL or C4 binding but did decrease C3b binding by 50% and resulted in 80% survival in 10% serum (lacking bacteria-specific Abs) even when sialylated organisms were preopsonized with MBL. Full sialylation of LOS abolished MBL, C4, and C3b binding, resulting in 100% survival. Our studies indicate that MBL does not participate in complement activation on N. gonorrhoeae in the presence of "complete" serum that contains C1-inhibitor and alpha(2)-macroglobulin.

Intracellular survival of Neisseria gonorrhoeae in male urethral epithelial cells: importance of a hexaacyl lipid A

Neisseria gonorrhoeae is a strict human pathogen that invades and colonizes the urogenital tracts of males and females. Lipooligosaccharide (LOS) has been shown to play a role in gonococcal pathogenesis. The acyl transferase MsbB is involved in the biosynthesis of the lipid A portion of the LOS. In order to determine the role of an intact lipid A structure on the pathogenesis of N. gonorrhoeae, the msbB gene was cloned and sequenced, a deletion and insertion mutation was introduced into N. gonorrhoeae, and the mutant strain was designated 1291A11K3. Mass spectrometric analyses of 1291A11K3 LOS determined that this mutation resulted in a pentaacyl rather than a hexaacyl lipid A structure. These analyses also demonstrated an increase in the phosphorylation of lipid A and an increase in length of the oligosaccharide of a minor species of the msbB LOS. The interactions of this mutant with male urethral epithelial cells (uec) were examined. Transmission and scanning electron microscopy studies indicated that the msbB mutants formed close associations with and were internalized by the uec at levels similar to those of the parent strain. Gentamicin survival assays performed with 1291A11K3 and 1291 bacteria demonstrated that there was no difference in the abilities of the two strains to adhere to uec; however, significantly fewer 1291A11K3 bacteria than parent strain bacteria were recovered from gentamicin-treated uec. These studies suggest that the lipid A modification in the N. gonorrhoeae msbB mutant may render it more susceptible to innate intracellular killing mechanisms when internalized by uec.

Synthesis of alpha-lactosyl-(1-->3)-L-glycero-alpha-D-manno-heptopyranoside, a partial oligosaccharide structure expressed within the lipooligosaccharide produced by Neisseria gonorrhoeae strain 15253

The glycosyl donor, hepta-O-benzyl-beta-lactosyl trichloroacetimidate (4) was prepared by treating hepta-O-benzyl-lactose with trichloroacetonitrile in the presence of potassium carbonate. The acceptor, methyl 2-O-benzyl-4,6-O-benzylidene-7,8-dideoxy-alpha-D-manno-oct-7-enopyranoside (8) was synthesized by hydrolysis of a 3,4-butane diacetal of methyl L-glycero-alpha-D-manno-oct-enopyranoside and subsequent benzylidenation. Glycosidation of the donor 4 with the acceptor 8 in 1,4-dioxane using Me(3)SiOTf as a promoter for 1 h at room temperature gave methyl (2,3,4,6-tetra-O-benzyl-beta-D-galactopyranosyl)-(1-->4)-(2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1-->3)-2-O-benzyl-4,6-O-benzylidene-7,8-dideoxy-alpha-D-manno-oct-7-enopyranoside (9) as a major product (59%). The oct-enopyranoside moiety of the trisaccharide 9 was converted to a heptopyranoside (80%) by oxidative cleavage with OsO(4)-NaIO(4) and subsequent reduction. Hydrogenolysis of the resulting trisaccharide and subsequent acetylation gave the peracetate of alpha-lactosyl-(1-->3)-Hep. Deacetylation of the peracetate afforded the title trisaccharide.

Receptor-mediated endocytosis of Neisseria gonorrhoeae into primary human urethral epithelial cells: the role of the asialoglycoprotein receptor

Urethral epithelial cells are invaded by Neisseria gonorrhoeae during gonococcal infection in men. To understand further the mechanisms of gonococcal entry into host cells, we used the primary human urethral epithelial cells (PHUECs) tissue culture system recently developed by our laboratory. These studies showed that human asialoglycoprotein receptor (ASGP-R) and the terminal lactosamine of lacto-N-neotetraose-expressing gonococcal lipooligosaccharide (LOS) play an important role in invasion of PHUECs. Microscopy studies showed that ASGP-R traffics to the cell surface after gonococcal challenge. Co-localization of ASGP-R with gonococci was observed. As ASGP-R-mediated endocytosis is clathrin dependent, clathrin localization in PHUECs was examined after infection. Infected PHUECs showed increased clathrin recruitment and co-localization of clathrin and gonococci. Preincubating PHUECs in 0.3 M sucrose or monodansylcadaverine (MDC), which both inhibit clathrin-coated pit formation, resulted in decreased invasion. N. gonorrhoeae strain 1291 produces a single LOS glycoform that terminates with Gal(beta1-4)GlcNac(beta1-3)Gal(beta1-4)Glc (lacto-N-neotetraose). Invasion assays showed that strain 1291 invades significantly more than four isogenic mutants expressing truncated LOS. Sialylation of strain 1291 LOS inhibited invasion significantly. Preincubation of PHUECs in asialofetuin (ASF), an ASGP-R ligand, significantly reduced invasion. A dose-response reduction in invasion was observed in PHUECs preincubated with increasing concentrations of NaOH-deacylated 1291 LOS. These studies indicated that an interaction between lacto-N-neotetraose-terminal LOS and ASGP-R allows gonococcal entry into PHUECs.

Complex O-acetylation in Legionella pneumophila serogroup 1 lipopolysaccharide. Evidence for two genes involved in 8-O-acetylation of legionaminic acid

A putative gene encoding an O-acetyl transferase, lag-1, is involved in biosynthesis of the O-polysaccharide (polylegionaminic acid) in some Legionella pneumophila serogroup 1 strains. To study the effect of the presence and absence of the gene on the O-polysaccharide O-acetylation, lag-1 from strain Philadelphia 1 was expressed in trans in the naturally lag-1-negative OLDA strain RC1, and immunoblot analysis revealed that the lag-1-encoded O-acetyl transferase is active. O-Polysaccharides of different size were prepared from the lipopolysaccharides of wild-type and transformant strains by mild acid degradation followed by gel-permeation chromatography. Using NMR spectroscopy and MALDI-TOF mass spectrometry, it was found that O-acetylation of the first three legionaminic acid residues next to the core occurs in the short-chain O-polysaccharide (<10 sugars) from both strains. Hence, there is another O-acetyl transferase encoded by a gene different from lag-1. In the longer-chain O-polysaccharide, a legionaminic acid residue proximal to the core is N-methylated and could be further 8-O-acetylated in the lag-1-dependent manner. Only strains expressing a functional lag-1 gene were recognized in Western blot analysis by monoclonal antibody 3/1 requiring 8-O-acetylated polylegionaminic acid for binding. The highly O-acetylated outer core region of the lipopolysaccharide is involved in the epitope of another serogroup 1-specific monoclonal antibody termed LPS-1. The O-acetylation pattern of the L. pneumophila serogroup 1 core oligosaccharide was revised using MALDI-TOF mass spectrometry. lag-1-independent O-acetylation of the core and short-chain O-polysaccharide was found to be a common feature of L. pneumophila serogroup 1 strains. The biological importance of conserved lag-1-independent and variable lag-1-dependent O-acetylation is discussed.

Analysis of lipooligosaccharide biosynthesis in the Neisseriaceae

Neisserial lipooligosaccharide (LOS) contains three oligosaccharide chains, termed the alpha, beta, and gamma chains. We used Southern hybridization experiments on DNA isolated from various Neisseria spp. to determine if strains considered to be nonpathogenic possessed DNA sequences homologous with genes involved in the biosynthesis of these oligosaccharide chains. The presence or absence of specific genes was compared to the LOS profiles expressed by each strain, as characterized by their mobilities on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gel and their reactivities with various LOS-specific monoclonal antibodies. A great deal of heterogeneity was seen with respect to the presence of genes encoding glycosyltransferases in Neisseria. All pathogenic species were found to possess DNA sequences homologous with the lgt gene cluster, a group of genes needed for the synthesis of the alpha chain. Some of these genes were also found to be present in strains considered to be nonpathogenic, such as Neisseria lactamica, N. subflava, and N. sicca. Some nonpathogenic Neisseria spp. were able to express high-molecular-mass LOS structures, even though they lacked the DNA sequences homologous with rfaF, a gene whose product must act before gonococcal and meningococcal LOS can be elongated. Using a PCR amplification strategy, in combination with DNA sequencing, we demonstrated that N. subflava 44 possessed lgtA, lgtB, and lgtE genes. The predicted amino acid sequence encoded by each of these genes suggested that they encoded functional proteins; however, structural analysis of LOS isolated from this strain indicated that the bulk of its LOS was not modified by these gene products. This suggests the existence of an additional regulatory mechanism that is responsible for the limited expression of these genes in this strain.

Structural and immunochemical characterization of the lipooligosaccharides expressed by Neisseria subflava 44

Neisserial lipooligosaccharides (LOSs) are a family of complex cell surface glycolipids. We used mass spectrometry techniques (electrospray ionization, collision-induced dissociation, and multiple step), combined with fluorophore-assisted carbohydrate electrophoresis monosaccharide composition analysis, to determine the structure of the two low-molecular-mass LOS molecules (LOSI and LOSII) expressed by Neisseria subflava 44. We determined that LOSI contains one glucose on both the alpha and beta chains. LOSII is structurally related to LOSI and differs from it by the addition of a hexose (either glucose or galactose) on the alpha chain. LOSI and LOSII were able to bind monoclonal antibody (MAb) 25-1-LC1 when analyzed by Western blotting experiments. We used a set of genetically defined Neisseria gonorrhoeae mutants that expressed single defined LOS epitopes and a group of Neisseria meningitidis strains that expresses chemically defined LOS components to determine the structures recognized by MAb 25-1-LC1. We found that extensions onto the beta-chain glucose of LOSI block the recognition by this MAb, as does further elongation from the LOSII alpha chain. The LOSI structure was determined to be the minimum structure that is recognized by MAb 25-1-LC1.

Molecular Mimicry of Host Structures by Lipooligosaccharides of Neisseria Meningitidis: Characterization of Sialylated and Nonsialylated Lacto-N-Neotetraose (Galß1-4GlcNAcß1-3Galβ1-4Glc) Structures in Lipooligosaccharides Using Monoclonal Antibodies and Specific Lectins

Neisseria meningitidis lipooligosaccharides (LOSs) are classified into 12 immunotypes. Most LOSs are heterogeneous in having a few components by SDS-PAGE analysis that differ antigenically and chemically. We have utilized a monoclonal antibody that recognizes lacto-N-neotetraose (LNnT) and the lectin, Maackia amurensis leukoagglutinin (MAL), which is specific for NeuNAcalpha2-3Galbeta1-4GlcNAc trisacchride sequence to characterize the 12 N. meningitidis LOSs. Using the combination of ELISA, SDS-PAGE, Western blotting, and other chemical analyses, we have shown that the LNnT (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc) sequence was present in the 4.0-kDa LOS components of seven immunotype LOSs seen on SDS-PAGE. Six of the seven LNnT-containing LOSs also bound the MAL lectin indicating that N-acetylneuraminic acid (NeuNAc) was alpha2,3-linked to the LNnT sequence in the LOSs. Sialylation of the terminal Gal of LNnT-containing 4.0-kDa component caused only a slight increase in its apparent MW to 4100 on SDS-PAGE. The one LOS with the LNnT-containing component, but not MAL-binding, was from a Group A N. meningitidis, which does not synthesize CMP-NeuNAc, the substrate needed for LOS sialylation. Thus, it is concluded (1) a common LNnT sequence is present in seven immunotype LOSs in addition to their immunotype epitopes, and (2) NeuNAc is alpha2 --> 3 linked to the terminal Gal of LNnT if a organism synthesizes CMP-NeuNAc such as Groups B and C organisms. The above conclusions are consistent with the published structures of N. meningitidis LOSs. The results also demonstrate that specific carbohydrate-binding lectins and monoclonal antibodies can be used as simple yet effective tools to characterize specific carbohydrate sequences in a bacterial LOS or LPS such as N. meningitidis LOS. It is intriguing that N. meningitidis LOSs mimic certain glycosphingolipids, such as paragloboside (LNnT-ceramide) and sialylparagloboside, and some glycoproteins of the host in having LNnT and N-acetyllactosamine sequences respectively with or without alpha2 --> 3 linked NeuNAc. Epidemiological studies of N. meningitidis suggest that the molecular mimicry of host structures by its LOS plays a role in the pathogenesis of N. meningitidis by helping the organism to evade host immune defenses in man. The molecular mimicry of host structures by LOS or LPS is also found in other human pathogens such as N. gonorrhoeae, Haemophilus ducreyi, H. influenaze, Moraxella catarrhalis, Campylobacter jejuni, and Helicobacter pylori.

Lipooligosaccharide P(k) (Galalpha1-4Galbeta1-4Glc) epitope of moraxella catarrhalis is a factor in resistance to bactericidal activity mediated by normal human serum

Moraxella catarrhalis is a respiratory pathogen responsible for acute bacterial otitis media in children and exacerbation of chronic bronchitis in adults. M. catarrhalis strains are frequently resistant to the bactericidal activity of normal human serum. In order to determine if the lipooligosaccharide (LOS) of M. catarrhalis has a role in serum resistance, the UDP-glucose-4-epimerase (galE) gene was identified, cloned, and sequenced and a deletion/insertion mutation was introduced into M. catarrhalis strain 2951. GalE enzymatic activity, measured in whole-cell lysates, was ablated in M. catarrhalis 2951 galE. Mass spectrometric analysis of LOS isolated with hot phenol-water confirmed that strain 2951 produced a type A LOS. These studies showed that the LOS from 2951 galE had lost two hexose residues due to the galE mutation and that the resultant LOS structure lacked the (Galalpha1-4Galbeta1-4Glc) P(k) epitope found on M. catarrhalis 2951. Wild-type M. catarrhalis 2951 is resistant to complement-mediated serum bactericidal activity. In contrast, a greater than 2-log(10)-unit reduction in CFU occurred after incubation of 2951 galE in either 50 or 25% pooled human serum (PNHS), and CFU in 10% PNHS decreased by about 1 log(10) unit. These studies suggest that the P(k) epitope of the LOS may be an important factor in the resistance of M. catarrhalis to the complement-mediated bactericidal effect of normal human serum.

Non-typeable Haemophilus influenzae adhere to and invade human bronchial epithelial cells via an interaction of lipooligosaccharide with the PAF receptor

Adherence and invasion are thought to be key events in the pathogenesis of non-typeable Haemophilus influenzae (NTHi). The role of NTHi lipooligosaccharide (LOS) in adherence was examined using an LOS-coated polystyrene bead adherence assay. Beads coated with NTHi 2019 LOS adhered significantly more to 16HBE14 human bronchial epithelial cells than beads coated with truncated LOS isolated from an NTHi 2019 pgmB:ermr mutant (P = 0.037). Adherence was inhibited by preincubation of cell monolayers with NTHi 2019 LOS (P = 0.0009), but not by preincubation with NTHi 2019 pgmB:ermr LOS. Competitive inhibition studies with a panel of compounds containing structures found within NTHi LOS suggested that a phosphorylcholine (ChoP) moiety was involved in adherence. Further experiments revealed that mutations affecting the oligosaccharide region of LOS or the incorporation of ChoP therein caused significant decreases in the adherence to and invasion of bronchial cells by NTHi 2019 (P < 0.01). Analysis of infected monolayers by confocal microscopy showed that ChoP+ NTHi bacilli co-localized with the PAF receptor. Pretreatment of bronchial cells with a PAF receptor antagonist inhibited invasion by NTHi 2109 and two other NTHi strains expressing ChoP+ LOS glycoforms exhibiting high reactivity with an anti-ChoP antibody on colony immunoblots. These data suggest that a particular subset of ChoP+ LOS glycoforms could mediate NTHi invasion of bronchial cells by means of interaction with the PAF receptor.

Gonococcal lipooligosaccharide is a ligand for the asialoglycoprotein receptor on human sperm

In the present study, we show that Neisseria gonorrhoeae lipooligosaccharide (LOS) can bind to the asialoglycoprotein receptor (ASGP-R) on human sperm. This work demonstrates the presence of ASGP-R on human sperm. Binding of purified ASGP-R ligand decreased in the presence of gonococci. Binding of purified iodinated gonococcal LOS identified a protein of molecular weight corresponding to that of human ASGP-R. The presence of excess unlabelled LOS blocked binding of iodinated gonococcal LOS. Binding of wild-type gonococcal LOS to sperm was higher than that of mutant LOS lacking the galactose ligand for ASGP-R. These data suggest that the ASGP-R on human sperm cells recognizes and binds wild-type gonococcal LOS. This interaction may contribute to the transmission of gonorrhea from infected males to their sexual partners.

Structural relationships and sialylation among meningococcal L1, L8, and L3,7 lipooligosaccharide serotypes

Eighteen of 34 endemic meningococcal case strains were of the L8 lipooligosaccharide (LOS) type; four of these were both L3 and L7 (L3,7), and seven were L1. L1 structures arose by alternative terminal Gal substitutions of lactosyl diheptoside L8 structures, as determined by electrospray ionization and other mass spectrometric techniques, and enzymatic and chemical degradations (Structures L1 and L1a). [see text for structure] The more abundant molecule, designated L1, had a trihexose globosyl alpha chain; the less abundant one, designated L1a, had a beta-lactosyl alpha chain and a parallel alpha-lactosaminyl gamma chain. A P(k) globoside (Galalpha1-->4Galbeta1-->4 Glc-R) monoclonal antibody bound 9/10 L1 strains, but a P(1) globoside (Galalpha1-->4Galbeta1-->4GlcNAc-R) mAb bound none of them. alpha-Galactosidase caused loss of both L1 structures and creation of L8 structures; beta-galactosidase caused loss of the L8 determinant. The L1/P(k) glycose was partially sialylated. Some LOS also had unsubstituted basal beta-GlcNAc additions. These structural relationships explain co-expression of L8, L1, and L3,7 serotypes.