Elaboration of a 3.6-kilodalton lipooligosaccharide, antibody against which is absent from human sera, is associated with serum resistance of Neisseria gonorrhoeae

Functional characterization of the gonococcal polyphosphate pseudo-capsule

Neisseria gonorrhoeae is an exclusively human pathogen able to evade the host immune system through multiple mechanisms. Gonococci accumulate a large portion of phosphate moieties as polyphosphate (polyP) on the exterior of the cell. Although its polyanionic nature has suggested that it may form a protective shield on the cell surface, its role remains controversial. Taking advantage of a recombinant His-tagged polyP-binding protein, the presence of a polyP pseudo-capsule in gonococcus was demonstrated. Interestingly, the polyP pseudo-capsule was found to be present in specific strains only. To investigate its putative role in host immune evasion mechanisms, such as resistance to serum bactericidal activity, antimicrobial peptides and phagocytosis, the enzymes involved in polyP metabolism were genetically deleted, generating mutants with altered polyP external content. The mutants with lower polyP content on their surface compared to the wild-type strains, became sensitive to complement-mediated killing in presence of normal human serum. Conversely, naturally serum sensitive strains that did not display a significant polyP pseudo-capsule became resistant to complement in the presence of exogenous polyP. The presence of polyP pseudo-capsule was also critical in the protection from antibacterial activity of cationic antimicrobial peptide, such as cathelicidin LL-37. Results showed that the minimum bactericidal concentration was lower in strains lacking polyP than in those harboring the pseudo-capsule. Data referring to phagocytic killing resistance, assessed by using neutrophil-like cells, showed a significant decrease in viability of mutants lacking polyP on their cell surface in comparison to the wild-type strain. The addition of exogenous polyP overturned the killing phenotype of sensitive strains suggesting that gonococcus could exploit environmental polyP to survive to complement-mediated, cathelicidin and intracellular killing. Taken together, data presented here indicate an essential role of the polyP pseudo-capsule in the gonococcal pathogenesis, opening new perspective on gonococcal biology and more effective treatments.

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.

Role of Gonococcal Neisserial Surface Protein A (NspA) in Serum Resistance and Comparison of Its Factor H Binding Properties with Those of Its Meningococcal Counterpart

Neisseria gonorrhoeae, the causative agent of gonorrhea, has evolved several mechanisms to subvert complement, including binding of the complement inhibitor factor H (FH). We previously reported FH binding to N. gonorrhoeae independently of lipooligosaccharide (LOS) sialylation. Here we report that factor H-like protein 1 (FHL-1), which contains FH domains 1 through 7 and possesses complement-inhibitory activity, also binds to N. gonorrhoeae The ligand for both FH and FHL-1 was identified as neisserial surface protein A (NspA), which has previously been identified as a ligand for these molecules on Neisseria meningitidis As with N. meningitidis NspA (Nm-NspA), N. gonorrhoeae NspA (Ng-NspA) bound FH/FHL-1 through FH domains 6 and 7. Binding of FH/FHL-1 to NspA was human specific; the histidine (H) at position 337 of domain 6 contributed to human-specific FH binding to both Ng- and Nm-NspA. FH/FHL-1 bound Nm-NspA better than Ng-NspA; introducing Q at position 73 (loop 2, present in Ng-NspA) or replacing V and D at positions 112 and 113 in Nm-NspA loop 3 with A and H (Ng-NspA), respectively, reduced FH/FHL-1 binding. The converse Ng-NspA to Nm-NspA mutations increased FH/FHL-1 binding. Binding of FH/FHL-1 through domains 6 and 7 to N. gonorrhoeae increased with truncation of the heptose I (HepI) chain of LOS and decreased with LOS sialylation. Loss of NspA significantly decreased serum resistance of N. gonorrhoeae with either wild-type or truncated LOS. This report highlights the role for NspA in enabling N. gonorrhoeae to subvert complement despite LOS phase variation. Knowledge of FH-NspA interactions will inform the design of vaccines and immunotherapies against the global threat of multidrug-resistant gonorrhea.

A Novel Sialylation Site on Neisseria gonorrhoeae Lipooligosaccharide Links Heptose II Lactose Expression with Pathogenicity

Sialylation of lacto-N-neotetraose (LNnT) extending from heptose I (HepI) of gonococcal lipooligosaccharide (LOS) contributes to pathogenesis. Previously, gonococcal LOS sialyltransterase (Lst) was shown to sialylate LOS in Triton X-100 extracts of strain 15253, which expresses lactose from both HepI and HepII, the minimal structure required for monoclonal antibody (MAb) 2C7 binding. Ongoing work has shown that growth of 15253 in cytidine monophospho-N-acetylneuraminic acid (CMP-Neu5Ac)-containing medium enables binding to CD33/Siglec-3, a cell surface receptor that binds sialic acid, suggesting that lactose termini on LOSs of intact gonococci can be sialylated. Neu5Ac was detected on LOSs of strains 15253 and an MS11 mutant with lactose only from HepI and HepII by mass spectrometry; deleting HepII lactose rendered Neu5Ac undetectable. Resistance of HepII lactose Neu5Ac to desialylation by α2-3-specific neuraminidase suggested an α2-6 linkage. Although not associated with increased factor H binding, HepII lactose sialylation inhibited complement C3 deposition on gonococci. Strain 15253 mutants that lacked Lst or HepII lactose were significantly attenuated in mice, confirming the importance of HepII Neu5Ac in virulence. All 75 minimally passaged clinical isolates from Nanjing, China, expressed HepII lactose, evidenced by reactivity with MAb 2C7; MAb 2C7 was bactericidal against the first 62 (of 75) isolates that had been collected sequentially and were sialylated before testing. MAb 2C7 effectively attenuated 15253 vaginal colonization in mice. In conclusion, this novel sialylation site could explain the ubiquity of gonococcal HepII lactose in vivo Our findings reinforce the candidacy of the 2C7 epitope as a vaccine antigen and MAb 2C7 as an immunotherapeutic antibody.

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.

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.

Properdin is critical for antibody-dependent bactericidal activity against Neisseria gonorrhoeae that recruit C4b-binding protein

Gonorrhea, a sexually transmitted disease caused by Neisseria gonorrhoeae, is an important cause of morbidity worldwide. A safe and effective vaccine against gonorrhea is needed because of emerging resistance of gonococci to almost every class of antibiotic. A gonococcal lipooligosaccharide epitope defined by the mAb 2C7 is being evaluated as a candidate for development of an Ab-based vaccine. Immune Abs against N. gonorrhoeae need to overcome several subversive mechanisms whereby gonococcus evades complement, including binding to C4b-binding protein (C4BP; classical pathway inhibitor) and factor H (alternative pathway [AP] inhibitor). The role of AP recruitment and, in particular, properdin in assisting killing of gonococci by specific Abs is the subject of this study. We show that only those gonococcal strains that bind C4BP require properdin for killing by 2C7, whereas strains that do not bind C4BP are efficiently killed by 2C7 even when AP function is blocked. C3 deposition on bacteria mirrored killing. Recruitment of the AP by mAb 2C7, as measured by factor B binding, occurred in a properdin-dependent manner. These findings were confirmed using isogenic mutant strains that differed in their ability to bind to C4BP. Immune human serum that contained bactericidal Abs directed against the 2C7 lipooligosaccharide epitope as well as murine antigonococcal antiserum required functional properdin to kill C4BP-binding strains, but not C4BP-nonbinding strains. Collectively, these data point to an important role for properdin in facilitating immune Ab-mediated complement-dependent killing of gonococcal strains that inhibit the classical pathway by recruiting C4BP.

Lipooligosaccharide Structure is an Important Determinant in the Resistance of Neisseria Gonorrhoeae to Antimicrobial Agents of Innate Host Defense

The strict human pathogen Neisseria gonorrhoeae has caused the sexually transmitted infection termed gonorrhea for thousands of years. Over the millennia, the gonococcus has likely evolved mechanisms to evade host defense systems that operate on the genital mucosal surfaces in both males and females. Past research has shown that the presence or modification of certain cell envelope structures can significantly impact levels of gonococcal susceptibility to host-derived antimicrobial compounds that bathe genital mucosal surfaces and participate in innate host defense against invading pathogens. In order to facilitate the identification of gonococcal genes that are important in determining levels of bacterial susceptibility to mediators of innate host defense, we used the Himar I mariner in vitro mutagenesis system to construct a transposon insertion library in strain F62. As proof of principle that this strategy would be suitable for this purpose, we screened the library for mutants expressing decreased susceptibility to the bacteriolytic action of normal human serum (NHS). We found that a transposon insertion in the lgtD gene, which encodes an N-acetylgalactosamine transferase involved in the extension of the α-chain of lipooligosaccharide (LOS), could confer decreased susceptibility of strain F62 to complement-mediated killing by NHS. By complementation and chemical analyses, we demonstrated both linkage of the transposon insertion to the NHS-resistance phenotype and chemical changes in LOS structure that resulted from loss of LgtD production. Further truncation of the LOS α-chain or loss of phosphoethanolamine (PEA) from the lipid A region of LOS also impacted levels of NHS-resistance. PEA decoration of lipid A also increased gonococcal resistance to the model cationic antimicrobial polymyxin B. Taken together, we conclude that the Himar I mariner in vitro mutagenesis procedure can facilitate studies on structures involved in gonococcal pathogenesis.

Phosphoethanolamine substitution of lipid A and resistance of Neisseria gonorrhoeae to cationic antimicrobial peptides and complement-mediated killing by normal human serum

The capacity of Neisseria gonorrhoeae to cause disseminated gonococcal infection requires that such strains resist the bactericidal action of normal human serum. The bactericidal action of normal human serum against N. gonorrhoeae is mediated by the classical complement pathway through an antibody-dependent mechanism. The mechanism(s) by which certain strains of gonococci resist normal human serum is not fully understood, but alterations in lipooligosaccharide structure can affect such resistance. During an investigation of the biological significance of phosphoethanolamine extensions from lipooligosaccharide, we found that phosphoethanolamine substitutions from the heptose II group of the lipooligosaccharide beta-chain did not impact levels of gonococcal (strain FA19) resistance to normal human serum or polymyxin B. However, loss of phosphoethanolamine substitution from the lipid A component of lipooligosaccharide, due to insertional inactivation of lptA, resulted in increased gonococcal susceptibility to polymyxin B, as reported previously for Neisseria meningitidis. In contrast to previous reports with N. meningitidis, loss of phosphoethanolamine attached to lipid A rendered strain FA19 susceptible to complement killing. Serum killing of the lptA mutant occurred through the classical complement pathway. Both serum and polymyxin B resistance as well as phosphoethanolamine decoration of lipid A were restored in the lptA-null mutant by complementation with wild-type lptA. Our results support a role for lipid A phosphoethanolamine substitutions in resistance of this strict human pathogen to innate host defenses.

Modification of lipooligosaccharide with phosphoethanolamine by LptA in Neisseria meningitidis enhances meningococcal adhesion to human endothelial and epithelial cells

The lipooligosaccharide (LOS) of Neisseria meningitidis can be decorated with phosphoethanolamine (PEA) at the 4' position of lipid A and at the O-3 and O-6 positions of the inner core of the heptose II residue. The biological role of PEA modification in N. meningitidis remains unclear. During the course of our studies to elucidate the pathogenicity of the ST-2032 (invasive) meningococcal clonal group, disruption of lptA, the gene that encodes the PEA transferase for 4' lipid A, led to a approximately 10-fold decrease in N. meningitidis adhesion to four kinds of human endothelial and epithelial cell lines at an multiplicity of infection of 5,000. Complementation of the lptA gene in a Delta lptA mutant restored wild-type adherence. By matrix-assisted laser desorption ionization-time-of-flight mass spectrometry analysis, PEA was lost from the lipid A of the Delta lptA mutant compared to that of the wild-type strain. The effect of LptA on meningococcal adhesion was independent of other adhesins such as pili, Opc, Opa, and PilC but was inhibited by the presence of capsule. These results indicate that modification of LOS with PEA by LptA enhances meningococcal adhesion to human endothelial and epithelial cells in unencapsulated N. meningitidis.

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.

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.

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.

C4bp binding to porin mediates stable serum resistance of Neisseria gonorrhoeae

Screening of 29 strains of Neisseria gonorrhoeae revealed that 16/21 serum resistant strains and 0/8 serum sensitive strains bound C4bp, suggesting that C4bp binding to gonococci could contribute to serum resistance. C4bp bound to gonococci retained cofactor (C4b-degrading) function. Using allelic exchange to construct strains with hybrid Por1A/B molecules, we demonstrate that the N-terminal loop (loop 1) of Por1A is required for C4bp binding. Serum resistant Por1B gonococcal strains also bind C4bp via their Por molecule. Using allelic exchange and site-directed mutagenesis, we have shown that loops 5 and 7 together form a negatively charged C4bp binding domain. C4bp-Por1B interactions are ionic in nature (inhibited by high salt as well as by heparin), while the C4bp-Por1A bond is hydrophobic. mAbs directed against SCR1 of the alpha-chain of C4bp inhibit C4bp binding to both Por1A and Por1B. Furthermore, only recombinant C4bp mutant molecules that contain alpha-chain SCR1 bind both Por1A and Por1B gonococci, confirming that SCR1 contains Por binding sites. C4bp alpha-chain monomers do not bind strains with either Por molecule, suggesting that the polymeric form of C4bp is required for binding to gonococci. Inhibition of C4bp binding to serum resistant Por1A and Por1B strains in a serum bactericidal assay using fAb fragments against C4bp SCR1 results in complete killing at 30 min of otherwise fully serum resistant strains in only 10% normal serum, underscoring the role of C4bp in mediating gonococcal serum resistance.

Binding of C4b-binding protein to porin: a molecular mechanism of serum resistance of Neisseria gonorrhoeae

We screened 29 strains of Neisseria gonorrhoeae and found 16/21 strains that resisted killing by normal human serum and 0/8 serum sensitive strains that bound the complement regulator, C4b-binding protein (C4bp). Microbial surface-bound C4bp demonstrated cofactor activity. We constructed gonococcal strains with hybrid porin (Por) molecules derived from each of the major serogroups (Por1A and Por1B) of N. gonorrhoeae, and showed that the loop 1 of Por1A is required for C4bp binding. Por1B loops 5 and 7 of serum-resistant gonococci together formed a negatively charged C4bp-binding domain. C4bp-Por1B interactions were ionic in nature (inhibited by high salt or by heparin), whereas the C4bp-Por1A bond was hydrophobic. Only recombinant C4bp mutant molecules containing the NH2-terminal alpha-chain short consensus repeat (SCR1) bound to both Por1A and Por1B gonococci, suggesting that SCR1 contained Por binding sites. C4bp alpha-chain monomers did not bind gonococci, indicating that the polymeric form of C4bp was required for binding. Using fAb fragments against C4bp SCR1, C4bp binding to Por1A and Por1B strains was inhibited in a complement-dependent serum bactericidal assay. This resulted in complete killing of these otherwise fully serum resistant strains in only 10% normal serum, underscoring the importance of C4bp in mediating gonococcal serum resistance.

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.

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.

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.

Selection of Opa+ Neisseria gonorrhoeae by limited availability of normal human serum

Experimental infections of human male volunteers with Neisseria gonorrhoeae have provided valuable insights into the early stages of gonorrheal disease. Bacterial variants expressing outer membrane opacity (Opa) proteins appear to be selected from the inoculum during a period in which total recoverable numbers of bacteria decrease rapidly. This apparent survival advantage occurs simultaneously with the onset of an inflammatory response, characterized by local production of interleukin 6 (IL-6) and IL-8 and the appearance of leukocytes in urine. Since the inflammatory response may also result in the presence of serum factors on the mucosal surface, we investigated the possibility that killing in normal human serum (NHS) leads to the selection of Opa+ variants. We therefore studied killing of separate populations and mixtures of Opa- and Opa+ N. gonorrhoeae MS11mk in NHS. Expression of an Opa protein conferred a survival advantage upon the organism; i.e., the Opa+ variants were more serum resistant than their isogenic Opa- counterparts, resulting in a selection for Opa+ phenotypes when a mixture of Opa+ and Opa- gonococci (GC) was exposed to submaximal doses of NHS. This selection was observed in three different lipooligosaccharide (LOS) backgrounds, indicating that it was not due to a difference in LOS expression between Opa- and Opa+ phenotypes. Incubation in NHS of sialylated GC resulted in a similar selection for Opa+ variants. The presence of normal human urine during the serum killing assay had no effect on the selection phenomenon but drastically depleted NHS of bactericidal activity, which was found to be at least partly due to complement inhibition. The results suggest that serum killing may contribute to the transition from Opa- to Opa+ phenotypes during the early stages of infection of the male urethra.

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.

Molecular mechanisms and implications for infection of lipopolysaccharide variation in Neisseria

The lipopolysaccharides of the pathogenic Neisseria species are subject to structural variation owing to a combination of intrinsic changes in lipopolysaccharide (LPS) biosynthesis and external modification of the LPS molecule with sialic acid. This variation appears to control bacterial behaviour by altering their ability to interact with human cells and to evade host immune defences. This interconversion of LPS phenotypes, which is also observed during the natural infection, is probably due to environmental regulation of LPS biosynthesis superimposed on spontaneous changes in the DNA of distinct LPS loci. LPS variation may be a common strategy of mucosal pathogens to colonize and persist within the human host.

A mutation in the Neisseria gonorrhoeae rfaD homolog results in altered lipooligosaccharide expression

The gonococcal lsi-6 locus was cloned and shown by DNA sequence analysis to have homology with the E. coli rfaD gene, which encodes ADP-L-glycero-D-mannoheptose epimerase. This enzyme is involved in the biosynthesis of the lipopolysaccharide precursor ADP-L-glycero-D-mannoheptose. A site-directed frameshift mutation in lsi-6 was constructed by PCR amplification and introduced into the chromosome of Neisseria gonorrhoeae MS11 P+ by transformation. The lipooligosaccharides (LOS) of mutant and parental strains were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The lsi-6 mutant produced LOS components with apparent molecular masses of 2.6 and 3.6 kDa as compared with a 3.6-kDa band of the MS11 P+ strain. The parental LOS phenotype was expressed when a revertant was constructed by transformation of the cloned wild-type gene into the lsi-6 mutant. The immunoreactivity of LOS from parental and constructed strains was examined by SDS-PAGE and Western blotting. Only the parental and reconstructed wild-type strains produced a 3.6-kDa LOS component that reacted with monoclonal antibody 2-1-L8. These results suggest that the lsi-6 locus is involved in gonococcal LOS biosynthesis and that the nonreactive mutant 3.6-kDa LOS component contains a conformational change or altered saccharide composition that interferes with immunoreactivity.

Heparin protects Opa+ Neisseria gonorrhoeae from the bactericidal action of normal human serum

The pathobiological significance of lipooligosaccharide (LOS) and outer membrane opacity protein (Opa) changes in gonorrheal disease are poorly understood. We assessed variants of strain MS11mk with different LOS and Opa phenotypes for their liability to killing by normal human sera. LOS differences correlated with strikingly disparate susceptibilities to serum killing; LOSa variants were serum resistant, LOSb variants were serum sensitive, and sialylation of LOSb variants enhanced their survival (as reported previously). Opa phenotype had little influence on the killing of serum-sensitive LOSb cells that were incubated directly in normal human sera, but preincubation of Opa+ LOSb variants in heparin increased their serum resistance whereas Opa- LOSb variants showed no change. Some Opa proteins conferred slightly higher resistance than others, but heparin preincubation increased serum resistance for variants expressing each of seven Opa proteins. These in vitro phenomena may relate to conditions within the male urethra where sulfate-containing proteoglycans are abundant and where antibody and complement may transude from blood plasma. The results suggest that the selective advantage for Opa+ Neisseria gonorrhoeae bacteria observed in vivo may reflect their ability to utilize host cell components to resist killing by host defenses.

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.

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.

Production and characterization of monoclonal antibodies to type 8 lipooligosaccharide of Neisseria meningitidis

Eight monoclonal antibodies (MAbs) to lipooligosaccharides (LOSs) of Neisseria meningitidis were produced by immunizing mice with purified LOS from group A meningococcal strain A1. The specificities of the MAbs were examined by enzyme-linked immunosorbent assay (ELISA), immunodot assay, and ELISA inhibition by using the homologous A1 LOS, 12 immunotype LOSs of N. meningitidis (L1 through L12), and LOSs or lipopolysaccharides from other gram-negative bacteria. Two of the MAbs, 4385G7 (immunoglobulin G2b [IgG2b]) and 4387A5 (IgG2a), had the strongest reactivities with the homologous A1 LOS, moderate reactivities with the M978 (L8) LOS, but no reactivity with other LOSs. The other six MAbs (4 IgM and 2 IgG3) reacted with the A1 LOS and with several or many of the 12 LOSs. ELISA inhibition at 50% showed that the inhibitory activities of the LOSs from strains A1 and BB431 (a group B strain) to the specific MAb 4387A5 were about 10 to 20 times greater than that of the M978 (L8) LOS. When compared with MAb 2-1-L8 (L8) by Western blot (immunoblot) analysis and ELISA inhibition, the two specific MAbs recognized a different epitope in the 3.6-kDa LOSs of strains A1 and BB431. We propose that the new epitope is L8a, since the MAbs also reacted with the M978 (L8) LOS. The expression of the L8a epitope in the A1 LOS requires a few monosaccharide residues in its oligosaccharide moiety, and the fatty acid residues in its lipid A moiety also play a role. In a whole-cell ELISA, the two specific MAbs bound specifically to the homologous strain A1 and the L8 prototype strain M978 but not to any other LOS prototype strains. These results suggest that the two specific MAbs can be used for LOS typing of N. meningitidis.

The interaction of naturally elaborated blebs from serum-susceptible and serum-resistant strains of Neisseria gonorrhoeae with normal human serum

We studied the interaction of normal human serum immunoglobulins with outer-membrane bleb antigens of Neisseria gonorrhoeae. Gonococcal 68,000 Dalton and Lip (H.8 antigen) outer-membrane proteins were recognized by normal human serum immunoglobulins in blebs from serum-resistant strains, but not in blebs from serum-susceptible strains. The addition of blebs from a serum-resistant strain to bactericidal assays resulted in significantly greater inhibition of serum killing than the addition of blebs from a serum-susceptible strain. Our results indicate that blebs from two serum-resistant gonococcal strains have an enhanced ability to bind and remove cell-targeted bactericidal factors, and that outer-membrane blebbing may contribute to serum resistance.

Characterization of naturally elaborated blebs from serum-susceptible and serum-resistant strains of Neisseria gonorrhoeae

Outer-membrane blebs from two serum-susceptible and two serum-resistant strains of Neisseria gonorrhoeae were characterized. In general, bleb surfaces resembled cell surfaces, but there were qualitative and quantitative protein differences in blebs released by serum-susceptible and serum-resistant strains. Relative to blebs from serum-resistant strains, blebs from serum-susceptible strains expressed reduced amounts of major outer-membrane proteins I and III, and little if any 68,000 Dalton outer-membrane protein.

Expression of paragloboside-like lipooligosaccharides may be a necessary component of gonococcal pathogenesis in men

To learn how lipooligosaccharide (LOS) phase variations affect pathogenesis, we studied two male volunteers who were challenged intraurethrally with Neisseria gonorrhoeae that make a single LOS of 3,600 daltons and sequentially followed LOS expression by gonococci as urethritis developed. LOS variation occurred in vivo. Signs and symptoms of gonorrhea began with the appearance of variants making 4,700-dalton LOS that are immunochemically similar to glycosphingolipids of human hematopoietic cells (Mandrell, R.E., J.M. Griffiss, and B.A. Macher. 1989. J. Exp. Med. 168:107) and that have acceptors for sialic acid. A variant that appeared at the onset of leukorrhoea was shed by 34/36 men with naturally acquired gonorrhea at the time they sought medical attention; the other two shed the variant associated with dysuria. None shed the challenge variant. These data show that in vivo phase shifts to higher molecular mass LOS that mimic human cell membrane glycolipids are associated with the development of gonococcal leukorrhea.

Epitope expression and partial structural characterization of F62 lipooligosaccharide (LOS) of Neisseria gonorrhoeae: IgM monoclonal antibodies (3F11 and 1-1-M) recognize non-reducing termini of the LOS components

F62 LOS of Neisseria gonorrhoeae consists of two components. The higher molecular weight (MW) component is recognized by monoclonal antibody (MAb) 1-1-M and the smaller MW component by MAb 3F11. Epitope expression of the two LOS components and their partial structures were investigated by treating the F62 LOS with several glycosidases and then monitoring their antigenicity with the two mouse IgM MAbs. The 1-1-M-defined LOS component was cleaved with both beta-N-acetylhexosaminidase and endo-beta-galactosidase, and each cleavage resulted in the loss of expression of the 1-1-M-defined epitope. The N-acetylhexosamine (HexNAc) released by the hexosaminidase was found to be GalNAc, and the smaller oligosaccharide released by the endo enzyme was identified to be a dimer GalNAc beta----Gal. In contrast, the MAb 3F11-defined LOS component was not digested by the endo galactosidase, but it was cleaved with alpha and beta-galactosidase, and expression of the MAb 3F11-defined LOS epitope expression of the MAb 3F11-defined LOS was abolished by the treatment with each of two exo enzymes. MAb 3F11 bound to the 1-1-M-defined LOS component resulting from the removal of the beta-GalNAc residue, and the resulting LOS was further cleaved with beta-galactosidase, but not with alpha-galactosidase. From these results, we conclude the following: (1) MAbs 1-1-M and 3F11 both recognize the non-reducing termini of the LOS components; (2) the 1-1-M-defined LOS component has the GalNAc beta----Gal beta 1----4-Glc (or GlcNAc) structure, and the GalNAc beta----Gal residue is involved in the MAb 1-1-M-defined epitope; (3) the MAb 3F11-defined LOS component may not have a Gal beta 1----4GlcNAc beta 1----4Gal beta 1----4Glc structure within the molecule. However, it has beta-Gal residue at its non-reducing terminus, and this residue is involved in the MAb 3F11-defined epitope; (4) the two LOS components share a similar antigenic structure, and the 3F11-defined epitope structure is present in the MAb 1-1-M-defined LOS component. Expression of this epitope within the 1-1-M-defined LOS molecule is blocked by the beta-GalNAc residue; however, the beta-GalNAc residue at the non-reducing end may be not the only structural difference between the two components.

Alterations of the LPS determine virulence of Neisseria gonorrhoeae in guinea-pig subcutaneous chambers

The virulence of lipopolysaccharide (LPS) variants of Neisseria gonorrhoeae strain Gc40 was studied in vivo using the guinea-pig subcutaneous chamber model. Survival of variants D1, D2, D4 and D5 was assessed by viable counts made on chamber fluid at various times after inoculation. Chemotactic effect was measured by counts of white cells in the chambers. Differential cell counts and assessments of the location of the gonococci were made on Giemsa-stained smears of chamber fluid. Sensitivity of the variants to normal guinea-pig serum was determined by in vitro bactericidal assays. D1 and D5 had relatively high Mr LPS which was shed in the medium, were serum resistant, produced intense infections and were mainly extracellular. Large number of damaged white cells were present. D2 and D4, had low Mr LPS which was poorly shed in the medium, were serum sensitive and produced low grade infections. D2 was the least infective and was seen mainly inside neutrophils. Collectively the data indicates that the type of LPS on the gonococcal surface and possibly the amount of shed LPS strongly influence the fate of gonococci in vivo, in an environment in which antibodies, complement and phagocytic cells are freely available. This may be decisive at some stages of the human infection.

Neisseria gonorrhoeae LPS variation, serum resistance and its induction by cytidine 5'-monophospho-N-acetylneuraminic acid

Inherent serum resistance and the effect of the serum resistance inducing factor cytidine 5'-monophospho-N-acetylneuraminic acid (CMP-NANA) were studied on Neisseria gonorrhoeae with different lipopolysaccharides (LPS). Strain M01 and LPS variants of strain Gc40 (variants D1, D2, D4 and D5) were examined after incubation in the presence or absence of CMP-NANA by bactericidal assays using normal human or immune sera and by SDS-PAGE followed by silver staining or Western blotting. The blots were probed with monoclonal antibody CC1, specific to epitope C of the LPS. Variants D1 and D5 were inherently serum resistant, variants D2 and D4 and strain M01 were susceptible. CMP-NANA induced marked changes in the LPS of all gonococci. However, only some gonococci were converted to serum resistance. Gonococci which were converted to serum resistance had LPS with components of relatively large molecular mass, expressing epitope C. Variants which did not convert to serum resistance had LPS with low molecular mass components only, without epitope C. Conversion to serum resistance increased the size of the large LPS components without affecting the expression of epitope C. The results indicate that conversion to serum resistance by CMP-NANA is not a general occurrence but depends on the quality of the LPS.

Meningococcal molecular mimicry and the search for an ideal vaccine

The carbohydrates expressed on the surface of meningococcal strains of groups B and C mimic those commonly found on human cells and thus are not functionally antigenic in infancy. In order to develop an effective vaccine, it will be necessary to find ways of circumventing this molecular mimicry. Three possible ways of achieving this are discussed. (i) The surface polysaccharides can theoretically present conformationally different epitopes, some of which might be recognized as antigenic by the host. Experimental evidence is presented that such differences do indeed exist; what is needed is to determine which of these conformations are unique to the organism and hence potentially antigenic. (ii) Precursors of the surface lipooligosaccharides may be unable to mimic human antigens, and so may be potential candidates for vaccine development. (iii) Natural immunity to some strains of meningococci develops in young children who are colonized with strains of Neisseria lactamica, and it is possible that its development could be enhanced by widespread intentional colonization by N. lactamica strains that are particularly efficient inducers of broad immunity.

Characterization of fourteen strains of Neisseria gonorrhoeae: structural analyses and serum reactivities

Resistance to normal human serum (NHS) killing in Neisseria gonorrhoeae has been associated with particular types of Protein I (PI) and lipopolysaccharide (LPS), but many exceptions exist, and the role of these structures in determining serum reactivities remains controversial. In reality, the response of the gonococcus to NHS is probably governed by several parameters involving a number of outer-membrane (OM) components. We surveyed the serum reactivities of 14 strains of N. gonorrhoeae and characterized each of their major OM components. The strains presented a spectrum of sensitivity to pooled NHS. As assessed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, immunoblotting, and peptide mapping, the strains were also quite heterogeneous in terms of PI, H.8 antigen, and LPS type, and the presence of the 2-1-L8 epitope. Five of the strains had identical PIAs in varying LPS and H.8 backgrounds, and four had identical PIBs in varying LPS and H.8 backgrounds. As assessed by electrophoretic migration and monoclonal antibody binding, Protein III and the 44,000 Dalton protein were identical in these strains. We found no association between PI subclass and serum sensitivity, while H.8 and LPS variation appeared to be related to bactericidal responses. The diversity and close interaction of gonococcal components in the OM are undoubtedly involved in differential abilities to survive NHS killing.

Measurement of the human immune response to meningococcal lipooligosaccharide antigens by using serum to inhibit monoclonal antibody binding to purified lipooligosaccharide

We developed a human inhibition monoclonal enzyme-linked immunosorbent assay (HIMELISA) to investigate the human immune response to the lipooligosaccharides (LOS) of Neisseria meningitidis. Monoclonal antibodies (MAb) were used to define seven epitopes on four LOS molecules of a meningococcal strain (126E) previously shown to express immunogenic LOS epitopes. The assay could distinguish epitope-specific antibody within whole sera. Neither the specificity nor the amount of the antibody measured by HIMELISA in sera of vaccinates changed during the immune response to meningococcal capsular polysaccharides, a chemically unrelated antigen. By using the HIMELISA, it was determined that sera from adults convalescing from meningococcal disease strongly inhibited MAb binding to two of the seven defined epitopes. The 3.6-kilodalton LOS of strain 126E expressed both of these epitopes. In addition, one of the inhibited epitopes was also expressed on the 4.0-kilodalton LOS of strain 126E. The convalescent-phase sera inhibited MAb binding to these two epitopes when they were expressed on LOS of diverse meningococcal strains. An acute-phase serum blocked MAb to the two epitopes to a lesser degree than did a convalescent-phase serum from the same patient. Immunoblotting the sodium dodecyl sulfate-polyacrylamide gel electrophoresis-separated LOS with convalescent-phase sera confirmed the specificity of the human anti-LOS antibody identified by HIMELISA.

Construction of isogenic gonococci with variable porin structure: effects on susceptibility to human serum and antibiotics

Protein I (PI) is the most abundant protein on the gonococcal cell surface and besides its porin function it may have important properties contributing to pathogenicity. By allelic exchange using cloned PI genes from FA19 (PIA) and MS11 (PIB) and a selectable marker introduced closely downstream of these genes, we constructed sets of isogenic gonococcal strains that differ only in their PI gene. Analysis revealed that PI has a major effect on stable resistance to normal human serum, and a slight effect on low-level resistance to antibiotics. All PIA/B hybrids were hypersusceptible to serum, suggesting a possible explanation for why such hybrids do not occur in nature.

Distribution of gonococcal lipopolysaccharide biosynthesis genes among strains of Neisseria gonorrhoeae and other neisserial species

A plasmid, pTME6, containing Neisseria gonorrhoeae lipopolysaccharide biosynthesis genes was used as a probe to analyze DNA from strains of N. gonorrhoeae, N. meningitidis and various commensal Neisseria by Southern blotting. Chromosomal DNA from 26 gonococcal strains probed with 32P-labeled pTME6 produced five different hybridization patterns. No correlation between hybridization pattern and auxotype, serotype, serum sensitivity or SDS-urea-PAGE migration of LPS was observed. DNA from strains of N. meningitidis, N. lactamica and N. cinerea, but not other commensal Neisseria species, hybridized strongly to pTME6.

Molecular analysis of lipooligosaccharide biosynthesis in Neisseria gonorrhoeae

A HindIII gene bank of Neisseria gonorrhoeae MUG116 was constructed in the cosmid vector pHC79. A cosmid (pSY81) was isolated that was able to convert N. gonorrhoeae FA5100 to reactivity with monoclonal antibody (MAb) 2-1-L8. Several MAb-reactive transformants were isolated and characterized with respect to lipooligosaccharide (LOS) production as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, their ability to react with two other LOS-specific MAbs (3F11 and O6B4), and Southern blot analysis. Escherichia coli containing the clone had altered lipopolysaccharide expression as determined by electrophoretic analysis; however, no reactivity was seen with gonococcus-specific MAbs. The introduction of pSY81 into FA5100 had a pleiomorphic effect, giving rise to transformants having the full parental phenotype or transformants lacking reactivity to a combination of LOS-specific MAbs. Southern blot analysis indicated that the LOS biosynthetic mutation in FA5100 was not due to chromosomal rearrangement or large deletions.

Human immune response to meningococcal outer membrane protein epitopes after natural infection or vaccination

Antibody levels in 41 sets of human acute- and convalescent-phase meningococcal sera were compared with those in 23 sets of human prevaccination and 2-week postvaccination sera. We used a modification of a solid-phase radioimmunoassay (SPRIA) technique to test each of the human serum samples as inhibitors of monoclonal antibodies (MAbs) that bind (HIMSPRIA) to the outer membrane complex from a 2a:P1.2:P5.1 strain. We used three murine MAbs specific for the 2a, P1.2, and P5.1 epitopes on meningococcal class 1, 2, and 5 proteins, respectively, to detect antibodies with similar specificities in human sera. Each of 40 available matching strains from patients were also screened with the three MAbs in a nitrocellulose spot blot assay. A total of 37 (92%) were positive for the 2a epitope, 36 (90%) were positive for the P1.2 epitope, and 16 (40%) were positive for the P5.1 epitope. Of 38 available convalescent-phase sera, 27 (71%) matched with these strains and had detectable inhibiting antibody for each of the MAb-defined protein epitopes of the infecting strain. Three convalescent-phase sera had no HIMSPRIA activity for MAb-defined epitopes that were present on the infecting strain; others had activity for one or two of the epitopes. The results were similar for pre- and postvaccination sera. The average level of HIMSPRIA activity for the P1.2 epitope was greater than fivefold higher in postvaccination sera compared with that in convalescent-phase sera. Sera with distinct patterns of HIMSPRIA activity also were tested by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis and showed a correlation between the HIMSPRIA activity for particular epitopes and the level of antibody binding to the immunoblotted proteins possessing those epitopes. A comparison of the HIMSPRIA and the bactericidal activity of selected postvaccination sera indicated a possible correlation between HIMSPRIA and bactericidal activity, but it also suggested the presence of bactericidal antibodies with specificities other than those defined by the MAbs.

Comparison of isolates of Neisseria gonorrhoeae causing meningitis and report of gonococcal meningitis in a patient with C8 deficiency

We studied a previously healthy 20-year-old woman who presented with gonococcal meningitis. The gonococcal isolate, HT-1, was prototrophic by auxotyping, was protein I serovar IB-1, and agglutinated with wheat germ lectin. This isolate differed from the proline-requiring, serovar IA-1 and IB-4, wheat germ-agglutination-negative gonococcal isolates recovered from three patients during a recent outbreak of gonococcal meningitis in Philadelphia. HT-1 was killed by normal pooled human sera (greater than or equal to 98% at 30 min) but not effectively killed by the convalescent-phase sera of the patient (greater than 30% survival at 30 min). Similar results were obtained when mucosal and cerebrospinal fluid isolates from a Philadelphia patient were exposed to these sera, but mucosal and blood isolates from another Philadelphia case showed increased resistance to killing by normal pooled human sera. Further characterization revealed multiple differences in outer membrane and cellular proteins and lipopolysaccharide between case isolates. Absence of the L8 lipopolysaccharide epitope was noted for all isolates. Sera of our patient were found to have low total hemolytic complement (CH100 = 21 U/ml; normal = 55 to 100 U/ml) due to deficiency of C8 (C8 less than 1,000 CH50 U/ml; normal = greater than or equal to 16,000 CH50 U/ml). This is the first reported case of gonococcal meningitis occurring in a patient with a terminal-complement deficiency. Gonococcal meningitis is a rare complication of gonococcal bacteremia. Both defects in host defenses (e.g., terminal-complement deficiency) and organisms with unusual virulence appear to contribute to the pathogenesis of this complication of gonococcal bacteremia.

Neisseria lactamica and Neisseria meningitidis share lipooligosaccharide epitopes but lack common capsular and class 1, 2, and 3 protein epitopes

Neisseria lactamica, a common human pharyngeal commensal, contributes to acquired immunity to Neisseria meningitidis. To define the surface antigens shared between these two species, we used monoclonal antibodies (MAbs) to study 35 N. lactamica strains isolated in various parts of the world for cross-reactivity with meningococcal capsules, outer membrane proteins, and lipooligosaccharides (LOS). No N. lactamica strain reacted significantly with MAbs specific for capsular group A, B, C, Y, or W, and we were unable to extract capsular polysaccharide from them. Only 2 of 33 strains reacted weakly with MAbs against class 2 serotype proteins P2b and P2c. None reacted with MAbs specific for meningococcal class 1 protein P1.2 or P1.16 or class 2/3 serotype protein P2a or P15. Most N. lactamica strains (30 of 35) bound one or more of seven LOS-specific MAbs. Two LOS epitopes, defined by MAbs O6B4 and 3F11, that are commonly found on pathogenic Neisseria species were found on 25 of 35 N. lactamica. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting showed that the LOS of N. lactamica are composed of multiple components that are physically and antigenically similar to the LOS of pathogenic Neisseria species. Among four other commensal neisserial species, only Neisseria cinerea shared LOS epitopes defined by MAbs O6B4 and 3F11. Previous studies have shown that pharyngeal colonization with N. lactamica induces bactericidal antibodies against the meningococcus. We postulate that shared N. lactamica and meningococcal LOS epitopes may play an important role in the development of natural immunity to the meningococcus.

Electromorphic characterization and description of conserved epitopes of the lipooligosaccharides of group A Neisseria meningitidis

We studied the lipooligosaccharides (LOS) of 28 group A Neisseria meningitidis of epidemiologically diverse origins to investigate whether each of the LOS serotypes found in serogroup A could be identified physically as well as antigenically. Using a dot blot assay with LOS-specific monoclonal antibodies (MAbs), we identified four epitopes that were serotype specific. The LOS from strains of each serotype were electromorphically and antigenically distinct when analyzed by silver-stained sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting. The LOS of L8 strains contained a 3,600-Mr component that bound the L8 MAb. The LOS of L9 strains contained two major components of 4,500 and 4,200 Mr. They bound the L9 MAb to the larger component. The LOS of L10 strains had a single major component of 4,000 Mr that bound the L10 MAb. The LOS of L11 strains contained a major 3,600-Mr component that could not be distinguished from the 3,600-Mr LOS of L8 strains by SDS-PAGE but that bound the L11 MAb. LOS of group A strains contained a highly conserved epitope in addition to a serotype-specific epitope. This was identified by a MAb that bound to all the strains on dot-blots and to multiple LOS components of various Mrs on immunoblots. We conclude that the LOS which bear the L9, L10, and L11 determinants are physically distinct and can be identified by SDS-PAGE or MAb binding or both. L8 and L11 are both borne on a 3.6-kilodalton LOS and can only be distinguished serologically.

Epitope expression of gonococcal lipooligosaccharide (LOS). Importance of the lipoidal moiety for expression of an epitope that exists in the oligosaccharide moiety of LOS

Antigenic expression of lipooligosaccharide (LOS) of strain F62 of Neisseria gonorrhoeae, was investigated with mouse monoclonal IgM antibody 3F11. F62 LOS was modified in various ways in order to understand structural requirements for expression of the 3F11-defined epitope. When the LOS was partially deacylated by treating it with 50 mM NaOH at 80 degrees C for 20 min or with anhydrous hydrazine at 80 degrees C for 20 min, the binding of 3F11 to those deacylated LOS samples decreased significantly. Removal of phosphate groups by treatment of the LOS with HF (4 days at 4 degrees C) did not affect the antigenicity at all. Neither did reduction of carboxyl groups in the LOS molecule (by activation of carboxyl groups with a carbodiimide followed by treatment with NaBH4) alter epitope expression. On oxidation with NaIO4, the LOS lost its antigenicity completely. The presence of Mg2+ did not change the circular dichroism (CD) behavior of F62 LOS. However, the partially deacylated LOS samples showed significantly different CD patterns in the 190-200 nm region compared with F62 LOS, which suggests conformational changes of F62 LOS due to the loss of fatty acids in the lipoidal moiety. Oligosaccharide (OS) and lipoidal components obtained after hydrolysis of F62 LOS with 1% acetic acid, were not recognized by the antibody. The antigenicity of OS was not retained by non-stereospecific acylation of OS with decanoyl chloride. We conclude the following: (1) 3F11-defined epitope exists in the OS moiety of F62 LOS; however, for it to be expressed, the carbohydrate moiety must be in a certain conformation that is defined by an overall structure of the LOS molecule. This structure is significantly influenced by some of the fatty acids in the lipoidal moiety of the LOS molecule; (2) the presence of phosphate or 3-deoxy-manno-2-octulosonic acid (dOclA) is not essential for expression of the 3F11-defined epitope; (3) the presence of divalent cations does not affect epitope expression.

Purification and characterization of DNA methyltransferases from Neisseria gonorrhoeae

Three DNA methyltransferases, M.NgoAI, and M.NgoBI and M.NgoBII, free of any nuclease activities were isolated from Neisseria gonorrhoeae strains WR220 and MUG116 respectively. M.NgoAI recognizes the sequence 5' GGCC 3' and methylates the first 5' cytosine on both strands. M.NgoBI and M.NgoBII recognize 5' TCACC 3' and 5' GTAN5CTC 3' respectively. M.NgoBII methylates cytosine on only one strand to produce 5' GTAN5mCTC 3'.

Instability of expression of lipooligosaccharides and their epitopes in Neisseria gonorrhoeae

We assessed variation in the expression of lipooligosaccharide (LOS) components and their epitopes within populations of a strain of Neisseria gonorrhoeae by using the monoclonal antibodies (MAbs) O6B4 and 3F11 and immunoenzymatic, immuno-colloidal gold electron microscopic, and sodium dodecyl sulfate-polyacrylamide gel electrophoretic procedures. Wild-type organisms varied in binding of both MAbs. We used the intensity of immunoenzymatic colony blot color to distinguish four binding variants for each MAb: red (R), pink (P), and colorless (nonreactive [N]) and an N back to R (N-R) revertant. R to P to R and R to N to R variation occurred at frequencies of 0.2% and 0.02%, respectively. The electrophoretic LOS profiles and MAb immunoblot patterns of the R, P, and N-R variants were the same as those of the wild type. LOSs of the N variants, in contrast, were of lower Mr, bound neither 3F11 nor O6B4 MAb, and contained as their major component the 3.6-kilodalton LOS that bears the L8LOS epitope of N. meningitidis. Results of immunoelectron microscopic studies were consistent with LOS binding patterns. Large number of colloidal gold particles were deposited about both R and P variants, distally from R organisms, but proximally from P organisms. N variant organisms, like their LOS, bound neither of the MAbs. N-R variant organisms were like the wild type in that they showed much variation in the amounts of MAb they bound.