Neuraminic acid is alpha 2-->3 linked in the lipooligosaccharide of Neisseria meningitidis serogroup B strain 6275

Emergence and significance of carbohydrate-specific antibodies

Carbohydrate-specific antibodies are widespread among all classes of immunoglobulins. Despite their broad occurrence, little is known about their formation and biological significance. Carbohydrate-specific antibodies are often classified as natural antibodies under the assumption that they arise without prior exposure to exogenous antigens. On the other hand, various carbohydrate-specific antibodies, including antibodies to ABO blood group antigens, emerge after the contact of immune cells with the intestinal microbiota, which expresses a vast diversity of carbohydrate antigens. Here we explore the development of carbohydrate-specific antibodies in humans, addressing the definition of natural antibodies and the production of carbohydrate-specific antibodies upon antigen stimulation. We focus on the significance of the intestinal microbiota in shaping carbohydrate-specific antibodies not just in the gut, but also in the blood circulation. The structural similarity between bacterial carbohydrate antigens and surface glycoconjugates of protists, fungi and animals leads to the production of carbohydrate-specific antibodies protective against a broad range of pathogens. Mimicry between bacterial and human glycoconjugates, however, can also lead to the generation of carbohydrate-specific antibodies that cross-react with human antigens, thereby contributing to the development of autoimmune disorders.

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.

Lipooligosaccharide structure contributes to multiple steps in the virulence of Neisseria meningitidis

Lipooligosaccharide (LOS) of Neisseria meningitidis has been implicated in meningococcal interaction with host epithelial cells and is a major factor contributing to the human proinflammatory response to meningococci. LOS mutants of the encapsulated N. meningitidis serogroup B strain NMB were used to further determine the importance of the LOS structure in in vitro adherence and invasion of human pharyngeal epithelial cells by meningococci and to study pathogenicity in a mouse (CD46 transgenic) model of meningococcal disease. The wild-type strain [NeuNAc-Galbeta-GlcNAc-Galbeta-Glcbeta-Hep2 (GlcNAc, Glcalpha) 3-deoxy-D-manno-2-octulosonic acid (KDO2)-lipid A; 1,4' bisphosphorylated], although poorly adherent, rapidly invaded an epithelial cell layer in vitro, survived and multiplied early in blood, reached the cerebrospinal fluid, and caused lethal disease in the mouse model. In contrast, the Hep2 (GlcNAc) KDO2-lipid A (pgm) mutant, which was highly adherent to cultured epithelial cells, caused significantly less bacteremia and mortality in the mouse model. The Hep2-KDO2-lipid A (rfaK) mutant was shown to be moderately adherent and to cause levels of bacteremia and mortality similar to those caused by the wild-type strain in the mouse model. The KDO2-lipid A (gmhB) mutant, which lacks the heptose disaccharide in the inner core of LOS, avidly attached to epithelial cells but was otherwise avirulent. Disease development correlated with expression of specific LOS structures and was associated with lower adherence but rapid meningococcal passage to and survival in the bloodstream, induction of proinflammatory cytokines, and the crossing of the blood-brain barrier. Taken together, the results of this study further define the importance of the LOS structure as a virulence component involved in multiple steps in the pathogenesis of N. meningitidis.

Genetic and functional analyses of the lgtH gene, a member of the β-1,4-galactosyltransferase gene family in the genus Neisseria

Lipooligosaccharide (LOS) is a major virulence factor of the pathogenic Neisseria. Three galactosyltransferase genes, lgtB, lgtE and lgtH, responsible for the biosynthesis of LOS oligosaccharide chains, were analysed in five Neisseria species. The function of lgtH in Neisseria meningitidis 6275 was determined by mutagenesis and chemical characterization of the parent and mutant LOS chains. The chemical characterization included SDS-PAGE, immunoblot, hexose and mass spectrometry analyses. Compared with the parent LOS, the mutant LOS lacked galactose, and its oligosaccharide decreased by three or four sugar units in matrix-assisted laser desorption ionization (MALDI)-MS analysis. The results show that lgtH encodes a β-1,4-galactosyltransferase, and that the glucose moiety linked to heptose (Hep) in the α chain is the acceptor site in the biosynthesis of Neisseria LOS. To understand the sequence diversity and relationships of lgtB, lgtE and lgtH, the entire lgt-1 locus was further sequenced in three N. meningitidis strains and three commensal Neisseria strains, and compared with the previously reported lgt genes from Neisseria species. Comparison of the protein sequences of the three enzymes LgtB, LgtE and LgtH showed a conserved N-terminal region, and a highly variable C-terminal region, suggesting functional constraint for substrate and acceptor specificity, respectively. The analyses of allelic variation and evolution of 23 lgtB, 12 lgtE and 14 lgtH sequences revealed a distinct evolutionary history of these genes in Neisseria. For example, the splits graph of lgtE displayed a network evolution, indicating frequent DNA recombination, whereas splits graphs of lgtB and lgtH displayed star-tree-like evolution, indicating the accumulation of point mutations. The data presented here represent examples of the evolution and variation of prokaryotic glycosyltransferase gene families. These imply the existence of multiple enzyme isoforms for biosynthesis of a great diversity of oligosaccharides in nature.

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.

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.

Structural and immunochemical characterization of a Neisseria gonorrhoeae epitope defined by a monoclonal antibody 2C7; the antibody recognizes a conserved epitope on specific lipo-oligosaccharides in spite of the presence of human carbohydrate epitopes

Lipo-oligosaccharides (LOS) produced by Neisseria gonorrhoeae are important antigenic and immunogenic components of the outer membrane complex. Previously, we showed that murine monoclonal antibody (mAb) 2C7 did not cross-react with human glycosphingolipids but identified the LOS epitope that is widely expressed in vivo and in vitro (Gulati, S., McQuillen, D. P., Mandrell, R. E., Jani, D. B., and Rice, P. A. (1996) J. Infect. Dis. 174, 1223-1237). In the present study, we analyzed the structure of gonococcal strain WG LOS containing the 2C7 epitope and investigated the structural requirements for expression of the epitope. We determined that the WG LOS components are Hep[1]-elongated forms of 15253 LOS that have a lactose on both Hep[1] and Hep[2] (Yamasaki, R., Kerwood, D. E., Schneider, H., Quinn, K. P., Griffiss, J. M., and Mandrell, R. E. (1994) J. Biol. Chem. 269, 30345-30351). In addition, we found that expression of the 2C7 epitope within the LOS is blocked when the Hep[2]-lactose is elongated. Based on the structural data of these LOS and the results obtained from immunochemical analyses, we conclude the following: 1) mAb 2C7 requires both the 15253 OS minimum structure and the N-linked fatty acids in the lipoidal moiety for expression of the epitope; 2) mAb 2C7 binds to the LOS that elongates the lactose on Hep[1] of the 15253 OS, but not the one on Hep[2]; and 3) the 2C7 epitope is expressed on gonococcal LOS despite the presence of human carbohydrate epitopes such as a lactosamine or its N-acetylgalactosaminylated (globo) form. Our study shows that the conserved epitope defined by mAb 2C7 could potentially be used as a safe site for the development of a vaccine candidate.

The (alpha2-->8)-linked polysialic acid capsule and lipooligosaccharide structure both contribute to the ability of serogroup B Neisseria meningitidis to resist the bactericidal activity of normal human serum

The molecular basis for the resistance of serogroup B Neisseria meningitidis to the bactericidal activity of normal human sera (NHS) was examined with a NHS-resistant, invasive serogroup B meningococcal isolate and genetically and structurally defined capsule-, lipooligosaccharide (LOS)-, and sialylation-altered mutants of the wild-type strain. Expression of the (alpha2-->8)-linked polysialic acid serogroup B capsule was essential for meningococcal resistance to NHS. The very NHS-sensitive phenotype of acapsular mutants (99.9 to 100% killed in 10, 25, and 50% NHS) was not rescued by complete LOS sialylation or changes in LOS structure. However, expression of the capsule was necessary but not sufficient for a fully NHS-resistant phenotype. In an encapsulated background, loss of LOS sialylation by interrupting the alpha2,3 sialyltransferase gene, lst, increased sensitivity to 50% NHS. In contrast, replacement of the lacto-N-neotetraose alpha-chain (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc) with glucose extensions (GlcN) in a galE mutant resulted in a strain resistant to killing by 50% NHS at all time points. Encapsulated meningococci expressing a Hep2(GlcNAc)-->KDO2-->lipid A LOS without an alpha-chain demonstrated enhanced sensitivity to 50% NHS (98% killed at 30 min) mediated through the antibody-dependent classical complement pathway. Encapsulated LOS mutants expressing truncated Hep2-->KDO2-->lipid A and KDO2-->lipid A structures were also sensitive to 50% NHS (98 to 100% killed at 30 min) but, unlike the wild-type strain and mutants with larger oligosaccharide structures, they were killed by hypogammaglobulinemic sera. These data indicate that encapsulation is essential but that the LOS structure contributes to the ability of serogroup B N. meningitidis to resist the bactericidal activity of NHS.

Characterization of terminal NeuNAcalpha2-3Galbeta1-4GlcNAc sequence in lipooligosaccharides of Neisseria meningitidis

Group B and C Neisseria meningitidis are the major cause of meningococcal disease in the United States and in Europe. N . meningitidis lipooligosaccharide (LOS), a major surface antigen, can be divided into 12 immunotypes of which L1 through L8 were found among Group B and C organisms. Groups B and C but not Group A may sialylate their LOSs with N-acetylneuraminic acid (NeuNAc) at the nonreducing end because they synthesize CMP-NeuNAc. Using sialic acid-galactose binding lectins as probes in an ELISA format, six of the eight LOS immunotypes (L2, L3, L4, L5, L7, and L8) in Groups B and C bound specifically to Maackia amurensis leukoagglutinin (MAL), which recognizes NeuNAcalpha2-3Galbeta1-4GlcNAc/Glc sequence, but not to Sambucus nigra agglutinin, which binds NeuNAcalpha2-6Gal sequence. The combination of SDS-PAGE and MAL-blot analyses revealed that these six LOSs contained only the NeuNAcalpha2-3Galbeta1-4GlcNAc trisaccharide sequence in their 4.1 kDa LOS components, which have a common terminal lacto-N-neotetraose (LNnT, Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc) structure when nonsialylated as shown by previous studies. The LOS-lectin binding was abolished when the LOSs were treated with Newcastle disease viral neuraminidase which cleaves alpha2-->3 linked sialic acid. Methylation analysis of a representative LOS (L2) confirmed that NeuNAc is 2-->3 linked to Gal. Thus, these LOSs structurally mimic certain glycolipids, i.e., paragloboside (LNnT-ceramide) and sialylparagloboside and some glycoproteins in having LNnT and N-acetyllactosamine sequences, respectively, with or without alpha2-->3 linked NeuNAc. The molecular mimicry of the LOSs may play a role in the pathogenesis of N.meningitidis by assisting the organism to evade host immune defenses in man.

Sialylation of Neisseria meningitidis lipooligosaccharide inhibits serum bactericidal activity by masking lacto-N-neotetraose

Exogenous sialylation of gonococcal lipooligosaccharide causes resistance to serum bactericidal activity. The aim of this study was to determine how lipooligosaccharide sialylation affects the serum sensitivities of group C Neisseria meningitidis strains. The relationship between the degree of sialylation or expression of the lipooligosaccharide sialic acid acceptor, lacto-N-neotetraose (LNnT), of nine meningococcal strains and their sensitivities to a pool of normal human sera was assessed. All strains expressed LNnT that was variously endogenously sialylated. Susceptibility to serum bactericidal activity ranged from extremely sensitive to resistant in 50% serum. For endogenously sialylated strains, the amount of killing correlated with the amount of free LNnT above a threshold of expression; strains that expressed less than the threshold survived in 25% serum. All strains added more sialic acid when they were grown in medium that contained cytidine monophospho-N-acetylneuraminic acid. Exogenous sialylation reduced the expression of free LNnT and significantly increased serum resistance. Exogenous sialylation affected killing through both classical and alternative complement pathways. The killing of exogenously sialylated strains also correlated with the amount of free LNnT. The amounts of endogenous, exogenous, and total sialic acid bound to LNnT did not correlate with the resistance of strains to serum bactericidal activity; rather, the loss of free LNnT expression by sialylation was associated with resistance. In conclusion, the expression of free LNnT by group C meningococcal strains is directly associated with the amount of killing of organisms in pooled human sera. Both endogenous and exogenous lipooligosaccharide sialylation are associated with increased serum resistance by masking LNnT.

Structural basis of the Neisseria meningitidis immunotypes including the L4 and L7 immunotypes

The application of high-resolution 1H, 13C and 31P NMR and MS analyses to the oligosaccharide moieties of the L4 and L7 immunotypes of Neisseria meningitidis revealed that they had the following structures: [formula: see text] The fact that the L7 LPS is not sialylated at O-3 of its terminal beta-D-galactopyranosyl residue implies that it is a mutant strain unable to endogenously sialylate its lacto-N-neotetraose antenna. With the structural elucidation of the L4 and L7 LPS immunotypes, a more comprehensive structural profile of the LPS involved in disease isolates can now be assembled. This provides valuable insights into the structural basis of the N. meningitidis immunotyping system which could be of use in formulating an LPS-based vaccine against meningococcal meningitis.

Expression of sialic acid and polysialic acid in serogroup B Neisseria meningitidis: divergent transcription of biosynthesis and transport operons through a common promoter region

We studied capsule-defective (Cap-) serogroup B meningococcal mutants created through Tn916 or omega-fragment mutagenesis. The Cap- phenotypes were the results of insertions in three of four linked genes (synX, synC, and synD) involved in CMP-N-acetylneuraminic acid and polysialic acid capsule biosynthesis, and in ctrA the first of four linked genes involved in capsule membrane transport. Mutations in the CMP-N-acetylneuraminic acid biosynthesis genes synX and synC caused defects in lipooligosaccharide sialylation but not mutations in the putative (alpha2 -> 8)-linked polysialyltransferase (synD) or in ctrA. Reverse transcriptase PCR studies indicated that the four biosynthesis genes (synX to -D) and the capsule transport genes (ctr to -D) were separately transcribed as operons. The operons were separated by a 134-bp intergenic region. Primer extension of synX and ctrA demonstrated that transcription of the operons was divergently initiated from adjacent start sites present in the intergenic region. Both transcriptional start sites were preceded by a perfect -10 Pribnow promoter binding region. The synX to -D, but not the ctrA to -D, transcriptional start site was preceded by a sequence bearing strong homology to the consensus sigma 70 -35 promoter binding sequence. Both promoters showed transcriptional activity when cloned behind a lacZ reporter gene in Escherichia coli. Our results confirm the intrinsic relationship between polysialic acid capsule biosynthesis and lipooligosaccharide sialylation pathways in group B Neisseria meningitidis. Our study also suggests that the intergenic region separating the synX to -D and ctrA to -D operons is an important control point for the regulation of group B capsule expression through coordinated transcriptional regulation of the synX to -D and drA to -D promoters.

Characterization of two transposon mutants from Haemophilus influenzae type b with altered lipooligosaccharide biosynthesis

Two isogenic mutants of Haemophilus influenzae type b (Hib) strain A2 were prepared by random m-Tn3(Cm) insertions into the 7.4-kb lsg (lipooligosaccharide synthesis genes) region of Hib DNA, which consists of seven complete and one partial open reading frame (orfs). Compared to the parent A2 strain which produces a complex mixture of lipooligosaccharides (LOS), the mutant strains 281.25 and 276.4 produced only a few LOS species. The precise locations of transposon insertions into the lsg loci of these mutants were determined (base 3546 in orf 4 for strain 281.25 and base 4402 in orf 5 for strain 276.4), and the effects of these mutations on LOS biosynthesis and epitope expression were evaluated. When the O-deacylated LOS were analyzed by mass spectrometry, both strains contained major LOS species of M(r) 2601, 2439, and 2277, which consisted of a common heptose trisaccharide core structure [Hep3(PEA)Kdo(P)-lipid A, where Hep is L-glycero-D-manno-heptose, Kdo is 3-deoxy-D-manno-octulosonic acid, and PEA is phosphoethanolamine] and four, three, or two hexoses, respectively. These species represent the smallest components of the wild-type LOS mixture. The major LOS oligosaccharide obtained from the strain 281.25 by mild acid hydrolysis was dephosphorylated and shown by composition analysis, methylation analysis, mass spectrometry, and 2D NMR studies to be a triantennary structure consisting of a heptose trisaccharide core with two glucose disaccharide branches: Hep alpha 1 --> (Glc beta 1 --> 4Glc alpha 1 --> 3) 2Hep alpha 1 --> (Glc beta 1 --> 4Glc beta 1 --> 4)3Hep alpha 1 --> anhydroKdo. Unlike the parent A2 strain, mutant strain 281.25 cannot add galactoses to the branches of this octasaccharide. Strain 276.4 is similarly deficient, except that it can still utilize a minor biosynthetic pathway leading to the addition of sialyl-N-acetyllactosamine.

Inner core biosynthesis of lipooligosaccharide (LOS) in Neisseria meningitidis serogroup B: identification and role in LOS assembly of the alpha1,2 N-acetylglucosamine transferase (RfaK)

A lipooligosaccharide (LOS) mutant of Neisseria meningitidis serogroup B strain NMB (immunotype L3,7,9) was identified in a Tn916 (tetM) mutant bank by loss of reactivity with monoclonal antibody 3F11, which recognizes the terminal Galbeta1-->4GlcNAc epitope in the lacto-N-neotetraose moiety of the wild-type LOS structure. The mutant, designated 559, was found to express a truncated LOS of 3.0 kDa. Southern and PCR analyses demonstrated that there was a single intact Tn916 insertion (class I) in the mutant 559 chromosome. Linkage of the LOS phenotype and the Tn916 insertion was confirmed by transformation of the wild-type parent. Nucleotide sequence analysis of the region surrounding the transposition site revealed a 1,065-bp open reading frame (ORF). A homology search of the GenBank/EMBL database revealed that the amino acid sequence of this ORF had 46.8% similarity and 21.2% identity with the alpha1,2 N-acetylglucosamine transferase (RfaK) from Salmonella typhimurium. Glycosyl composition and linkage analysis of the LOS produced by mutant 559 revealed that the lacto-N-neotetraose group which is attached to heptose I (HepI) and the N-acetylglucosamine and glucose residues that are attached to HepII in the inner core of the parental LOS were absent. These analyses also showed that the HepII residue in both the parent and the mutant LOS molecules was phosphorylated, presumably by a phosphoethanolamine substituent. The insertion of nonpolar and polar antibiotic resistance cartridges into the parental rfaK gene resulted in the expression of LOS with the same mobility as that produced by mutant 559. This result indicated that the inability to add the lacto-N-neotetraose group to the 559 LOS is not due to a polar effect on a gene(s) downstream of rfaK. Our data indicate that we have identified the meningococcal alpha1,2 N-acetylglucosamine transferase responsible for the addition of N-acetylglucosamine to HepII. We propose that the lack of alpha-chain extension from HepI in the LOS of mutant 559 may be due to structural constraints imposed by the incomplete biosynthesis of the LOS inner core.

Lipopolysaccharides of Helicobacter pylori strains P466 and MO19: structures of the O antigen and core oligosaccharide regions

Lipopolysaccharides (LPS) from phenol-water extraction of dyspeptic (P466) and asymptomatic (MO19) strains of Helicobacter pylori were each isolated as water-soluble material of high relative molecular mass (high M(r)) and as water-insoluble gels of low M(r). Chemical and spectroscopic analyses of the soluble LPS and oligosaccharides liberated from the water-insoluble gels led to proposed structures for chains comprising the O antigen, intervening, and core regions. As in the LPS from the type strain NCTC 11637 [Aspinall, G. O., et al. (1996) Biochemistry 35, 2489-2497], the O antigen region of the P466 LPS is characterized by the presence of extended chains with fucosylated and nonfucosylated N-acetyllactosamine units, the former carrying alpha-L-fucopyranose units at O-3 of beta-D-GlcNAc residues. This structure differs from that of the type strain in termination of the O chain by a Lewis(y) (Le(y)) antigenic determinant [alpha-L-Fuc(1-->2)beta-D-Gal(1-->4)[alpha-L-Fuc(1-->3)]beta-D-GlcNAc] but also has internal Lewis(x) (Le(x)) units. The inner core region of the P466 LPS is indistinguishable from that in the type strain. In contrast, the O antigen region of the LPS from strain MO19 consists of a single Le(y) epitope linked via a 3-linked beta-D-Gal to an intervening region on the basis of a sequence of 3-linked D-glycero-alpha-D-manno-heptose residues which is in turn linked to an inner core identical to that in the type strain and the P466 strain. Results in this and the preceding paper show that LPS from the three H. pylori strains display molecular mimicry of human cell surface glycoconjugates but may vary in the expression of Le(x) or Le(y) determinants, the degree of O antigen chain extension, or in the presence of an additional region between the inner core and the O antigen.

Lipopolysaccharide of the Helicobacter pylori type strain NCTC 11637 (ATCC 43504): structure of the O antigen chain and core oligosaccharide regions

Smooth- and rough-form lipopolysaccharides from phenol-water extraction of cells from Helicobacter pylori type strain NCTC 11637 were isolated as the water-soluble component of high-M(r) and water-insoluble low-M(r) gel. Structural investigations were performed on the intact water-soluble smooth-form lipopolysaccharide, various oligosaccharides formed as chemical and enzymic degradation products, and three oligosaccharide fractions liberated by acetic acid hydrolysis from the water-insoluble rough-form lipopolysaccharide. A structure is proposed for the complete polysaccharide component of the smooth-form lipopolysaccharide comprising the O antigen chain, an intervening region, and the inner core oligosaccharide on the basis of 1H and 13C NMR experiments, fast atom bombardment/mass spectrometry, and methylation linkage analysis of permethylated oligo- and polysaccharide derivatives. The most striking feature of the O antigen region in the lipopolysaccharide is the presence of extended chains with fucosylated and nonfucosylated N-acetyllactosamine (LacNAc) units that mimic human cell surface glycoconjugates in normal human granulocytes. The chains are terminated by di- or trimeric Lewis(x) (Le(x)) determinants, which are also found in tumor-associated carbohydrate antigens in many adenocarcinomas.

The lipooligosaccharides of pathogenic gram-negative bacteria

Lipooligosaccharides (LOSs) are the major glycolipids expressed on mucosal Gram-negative bacteria, including members of the genera Neisseria, Haemophilus, Bordetella, and Branhamella. They can also be expressed on some enteric bacteria such as Campylobacter jejuni and Campylobacter coli strains. LOS is analogous to the lipopolysaccharide (LPS) found in other Gram-negative families. LOSs share similar lipid A structures with an identical array of functional activities as LPSs. LOSs lack O-antigen units with the LOS oligosaccharide structures limited to 10 saccharide units. The LOS species of pathogenic Neisseria can play a major role in pathogenesis through enhancing the resistance of the organism to killing by normal human serum. Other distinguishing characteristics of LOS are the structural and antigenic similarity of some LOS species to human glycolipids and the potential for certain LOSs to be modified in vivo by host substances or secretions. These modifications of LOS in different environments of the host result in synthesis of new LOS structures that probably benefit the survival of the pathogen. The LOS of N. gonorrhoeae can act as a ligand of human receptors, promoting invasion of host cells. It is becoming clearer that LOSs are crucial factors in the pathogenesis of bacteria that express them.

The lipooligosaccharides of Haemophilus ducreyi are highly sialylated

The major lipooligosaccharides of the sexually transmitted pathogen Haemophilus ducreyi 35000 have been previously found to terminate in N-acetyllactosamine and sialyl-N-acetyllactosamine, Neu5Ac alpha 2-->3Gal beta 1-->4GlcNAc (W. Melaugh, N. J. Phillips, A. A. Campagnari, M. V. Tullius, and B. W. Gibson, Biochemistry 33: 13070-13078, 1994). In this study, mass spectrometry and composition analyses have shown that the lipooligosaccharides from three other H. ducreyi strains also contain N-acetyllactosamine and are highly sialylated (approximately 30 to 50%), although one African strain was found to contain neither of these structural features.

Anti-Gal binds to pili of Neisseria meningitidis: the immunoglobulin A isotype blocks complement-mediated killing

alpha 1,3-Galactosyl antibodies (anti-Gal) are ubiquitous natural human serum and secretory polyclonal antibodies that bind to terminal galactose-alpha 1,3-galactose (alpha-galactosyl) residues. Serum immunoglobulin G (IgG) anti-Gal can block alternative complement pathway-mediated lysis of representative gram-negative enteric bacteria that bind it to lipopolysaccharide alpha-galactosyl structures, thereby promoting survival of such bacteria in the nonimmune host. We wanted to know whether anti-Gal also could bind to the lipooligosaccharides (LOS) of Neisseria meningitidis. To our surprise, we found that serum and secretory anti-Gal bound to pili but not to LOS of certain strains. This suggested the presence of an immunogenic pilus carbohydrate epitope. Mild periodate oxidation of sodium dodecyl sulfate-polyacrylamide gel electrophoresis-separated outer membrane preparations from strains that bound anti-Gal followed by labeling of the neoaldehyde groups resulted in the labeling of bands that corresponded to pilin and LOS, confirming that pilin contains carbohydrate structures. A Bandeiraea simplicifolia lectin that also binds terminal alpha 1,3-galactosyl residues also bound to pilin. Serum IgG, IgA, and IgM anti-Gal as well as colostral secretory IgA anti-Gal bound to pilin, as judged by immunoblotting, and to the pili of intact piliated organisms, as judged by immunoelectron microscopy. Total serum anti-Gal (IgG, IgA, and IgM) and purified serum IgA1 anti-Gal, but not its purified IgG isotype, blocked complement-mediated lysis of a piliated meningococcal strain that bound anti-Gal to its pili. Colostral anti-Gal secretory IgA blocked killing of the same strain. Thus, anti-Gal IgA may promote disease when it binds to the pili of N. meningitidis strains.

Identification of the gonococcal glmU gene encoding the enzyme N-acetylglucosamine 1-phosphate uridyltransferase involved in the synthesis of UDP-GlcNAc

In searching for the gonococcal sialyltransferase gene(s), we cloned a 3.8-kb DNA fragment from gonococcus strain MS11 that hybridized with the oligonucleotide JU07, which was derived from the conserved C terminus of the sialyl motif present in mammalian sialyltransferases. Sequencing of the fragment revealed four putative open reading frames (ORFs), one of which (ORF-1) contained a partial sialyl motif including the amino acid sequence VGSKT, which is highly conserved among sialyltransferases. The gene was flanked by two inverted repeats containing the neisserial DNA uptake sequence and was preceded by a putative sigma 54 promoter. Database searches, however, revealed a high degree of homology between ORF-1 and the N-acetylglucosamine 1-phosphate uridyltransferase (GlmU) of Escherichia coli and Bacillus subtilis and not with any known sialyltransferase. This homology was further established by the successful complementation of an orf-1 mutation by the E. coli glmU gene. Enzyme assays demonstrated that ORF-1 did not possess sialyltransferase activity but mimicked GlmU function catalyzing the conversion of N-acetylglucosamine 1-phosphate into UDP-N-acetylglucosamine, which is a key metabolite in the syntheses of lipopolysaccharide, peptidoglycan, and sialic acids.

Adherence of pilus- Opa+ gonococci to epithelial cells in vitro involves heparan sulfate

Neisseria gonorrhoeae attaches to host epithelial cells via pili and opacity-associated (Opa) outer membrane proteins. Pilus- gonococci (Gc) of strain MS11 adhere to both human and nonhuman cells, but only when particular Opa proteins are expressed; OpaA+ variants adhere best, OpaC+ variants are next best, and the seven other Opa+ variants adhere poorly or not at all. The adherence of OpaA+ Gc to Chinese hamster ovary (CHO) cells is inhibited by heparin or heparan sulfate (HS), but not by chondroitin sulfate. OpaA+ Gc do not adhere to CHO cells devoid of HS proteoglycans; low concentrations of heparin restore OpaA+ Gc adherence to these HS-deficient CHO cells and high concentrations inhibit it. 3H-heparin binding to whole Gc parallels their adherence abilities (OpaA+ > OpaC+ > OpaH+ >> Opas B, D, E, F, G, I = Opa- = 0). Opa proteins separated by SDS-PAGE also bind 3H-heparin. These data suggest that adherence of pilus-, Opa+ Gc involves HS-proteoglycan of eukaryotic cells.

Monoclonal antibodies against Haemophilus lipopolysaccharides: clone DP8 specific for Haemophilus ducreyi and clone DH24 binding to lacto-N-neotetraose

Mouse monoclonal antibodies (MAbs) DP8 [immunoglobulin G1(kappa)] and DH24 [immunoglobulin M(kappa)], which are specific for Haemophilus ducreyi lipopolysaccharide (LPS), were generated by fusing mouse myeloma NS0 cells with spleen cells of BALB/c mice immunized with a total membrane preparation of H. ducreyi. MAb DP8 reacted in whole-cell enzyme immunoassay (EIA) and colony dot immunoblotting with all 50 strains of H. ducreyi but not with any other bacteria tested, which suggests an exposed and species-specific epitope on the H. ducreyi cell surface. This conclusion was supported by the finding that DP8 bound to all six H. ducreyi LPSs tested but not to any of the Haemophilus influenzae or enterobacterial LPSs or synthetic glycoconjugates. The MAb DH24 bound to 43 of 50 strains of H. ducreyi and to few strains of H. influenzae, Neisseria gonorrhoeae, and Neisseria meningitidis, as evaluated by whole-cell EIA and colony dot immunoblotting. The MAb DH24 reacted with five of the six H. ducreyi LPSs tested and with the lacto-N-neotetraose (Gal beta 1-->4GlcNAc beta 1-->3Gal beta 1-->4Glc) series of synthetic glycoconjugates, as determined by EIA. By using polysaccharides obtained after both mild acidic hydrolysis and strong alkali treatment and dephosphorylated samples as inhibitors of the MAbs binding to H. ducreyi LPS antigens, it could be shown that phosphate groups were essential for the binding of DP8 to LPS but that they did not affect antigenic recognition by DH24. None of the MAbs bound to isolated lipid A, but aggregation caused by the fatty acids of lipid A was essential for epitope recognition.

Genetic locus for the biosynthesis of the variable portion of Neisseria gonorrhoeae lipooligosaccharide

A locus involved in the biosynthesis of gonococcal lipooligosaccharide (LOS) has been cloned from gonococcal strain F62. The locus contains five open reading frames. The first and second reading frames are homologous, but not identical, to the fourth and fifth reading frames, respectively. Interposed is an additional reading frame which has distant homology to the Escherichia coli rfaI and rfaI genes, both glucosyl transferases involved in lipopolysaccharide core biosynthesis. The second and fifth reading frames show strong homology to the lex-1 or lic2A gene of Haemophilus influenzae, but do not contain the CAAT repeats found in this gene. Deletions of each of these five genes, of combinations of genes, and of the entire locus were constructed and introduced into parental gonococcal strain F62 by transformation. The LOS phenotypes were then analyzed by SDS-PAGE and reactivity with monoclonal antibodies. Analysis of the gonococcal mutants indicates that four of these genes are the glycosyl transferases that add GalNAc beta 1-->3Gal beta 1-->4GlcNAc beta 1-->3 Gal beta 1--4 to the substrate Glc beta 1-->4Hep--R of the inner core region. The gene with homology to E. coli rfaI/rfaI is involved with the addition of the alpha-linked galactose residue in the biosynthesis of the alternative LOS structure Gal alpha 1-->4Gal beta 1-->4Glc beta 1-->4Hep-->R. Since these genes encode LOS glycosyl transferases they have been named lgtA, lgtB, lgtC, lgtD, and lgtE. The DNA sequence analysis revealed that lgtA, lgtC, and lgtD contained poly-G tracts, which, in strain F62 were, respectively, 17, 10, and 11 bp. Thus, three of the LOS biosynthetic enzymes are potentially susceptible to premature termination by reading frame changes. It is likely that these structural features are responsible for the high-frequency genetic variation of gonococcal LOS.

Current status of meningococcal group B vaccine candidates: capsular or noncapsular?

Meningococcal meningitis is a severe, life-threatening infection for which no adequate vaccine exists. Current vaccines, based on the group-specific capsular polysaccharides, provide short-term protection in adults against serogroups A and C but are ineffective in infants and do not induce protection against group B strains, the predominant cause of infection in western countries, because the purified serogroup B polysaccharide fails to elicit human bactericidal antibodies. Because of the poor immunogenicity of group B capsular polysaccharide, different noncapsular antigens have been considered for inclusion in a vaccine against this serogroup: outer membrane proteins, lipooligosaccharides, iron-regulated proteins, Lip, pili, CtrA, and the immunoglobulin A proteases. Alternatively, attempts to increase the immunogenicity of the capsular polysaccharide have been made by using noncovalent complexes with outer membrane proteins, chemical modifications, and structural analogs. Here, we review the strategies employed for the development of a vaccine for Neisseria meningitidis serogroup B; the difficulties associated with the different approaches are discussed.

Analysis of C3 deposition and degradation on Neisseria meningitidis and Neisseria gonorrhoeae

The deposition and degradation of human complement component C3 on the cell surfaces of Neisseria meningitidis and Neisseria gonorrhoeae were studied. Bacteria were incubated in human serum, and ester-linked C3 fragments were analyzed by hydroxylamine release and immunoblot detection. Similar patterns of C3 degradation were found for both serum-resistant and serum-sensitive meningococcal strains of serogroups A, B, C, Y, and W135, as well as for serum-sensitive gonococcal strains and their sialylated serum-resistant variants. The predominant fragments in all cases were the 40-kDa alpha' 2 chain of iC3b and the 75-kDa beta chain common to both C3b and iC3b. The 67-kDa alpha' 1 chain of iC3b was also detected. The 105-kDa alpha' chain of intact C3b represented a minor proportion of deposited C3. Capsule-specific immunoglobulin G or immunoglobulin A1 did not alter the observed degradation patterns, nor did incubation of meningococci in properdin-deficient serum. The degradation of C3 in C5-, C6-, or C8-deficient serum was the same as that in normal serum, although the deposition of C3 was severely limited, based as indicated by the intensity of the fragments. With the use of an enzyme-linked immunosorbent assay that measured total iC3b and C3, I found that both iC3b deposition and C3 deposition varied among meningococcal and gonococcal strains and that the amounts of iC3b and C3 were independent of the relative quantities of cell surface sialic acid and of serum sensitivity for meningococci but not for gonococci. I conclude that complement activation on neisserial cell surface results in the formation of an identical repertoire of predominantly iC3b fragments of ester-linked C3b molecules regardless of the presence of sialic acid in either the capsule or the lipooligosaccharide or of the sensitivity of the organism to complement-mediated lysis but that the quantities of both ester- and amide-linked iC3b molecules deposited exhibit strain variability.