Allelic diversity of the two transferrin binding protein B gene isotypes among a collection of Neisseria meningitidis strains representative of serogroup B disease: implication for the composition of a recombinant TbpB-based vaccine

Genome-Wide Association Studies Identify an Association of Transferrin Binding Protein B Variation and Invasive Serogroup Y Meningococcal Disease in Older Adults

BACKGROUND

Neisseria meningitidis serogroup Y, especially ST-23 clonal complex (Y:cc23), represents a larger proportion of invasive meningococcal disease (IMD) in older adults compared to younger individuals. This study explored the meningococcal genetic variation underlying this association.

METHODS

Maximum-likelihood phylogenies and the pangenome were analyzed using whole-genome sequence (WGS) data from 200 Y:cc23 isolates in the Neisseria PubMLST database. Genome-wide association studies (GWAS) were performed on WGS data from 250 Y:cc23 isolates from individuals with IMD aged ≥65 years versus < 65 years.

RESULTS

Y:cc23 meningococcal variants did not cluster by age group or disease phenotype in phylogenetic analyses. Pangenome comparisons found no differences in presence or absence of genes in IMD isolates from the different age groups. GWAS identified differences in nucleotide polymorphisms within the transferrin-binding protein B (tbpB) gene in isolates from individuals ≥65 years of age. TbpB structure modelling suggests these may impact binding of human transferrin.

CONCLUSIONS

These data suggest differential iron scavenging capacity amongst Y:cc23 meningococci isolated from older compared to younger patients. Iron acquisition is essential for many bacterial pathogens including the meningococcus. These polymorphisms may facilitate colonization, thereby increasing the risk of disease in vulnerable older people with altered nasopharyngeal microbiomes and nutritional status.

Targeting bacterial transferrin and lactoferrin receptors for vaccines

A substantial disease burden in vertebrates is due to Gram-negative bacteria that exclusively inhabit the upper respiratory or genitourinary tracts of their hosts and rely on directly acquiring iron from the host iron-binding glycoproteins through surface receptor proteins. The receptors enable these bacteria to proliferate independently from their neighbors on the mucosal surface and during invasive infection of the host. The diversity in these receptors evolved over millions of years of evolution, which thus bodes well for long-lasting vaccine coverage. Experiments in food production animals provide proof of concept for the use of engineered antigens derived from the receptor proteins to prevent colonization and invasive infection in the natural host, strongly supporting development of these vaccines for use in humans.

Comprehensive Bioinformatic Assessments of the Variability of Neisseria gonorrhoeae Vaccine Candidates

A protective vaccine is the only viable way to stop the spread of gonorrhea in the face of rising antibiotic resistance. However, the notorious phase and antigenic variation of Neisseria gonorrhoeae surface proteins remains one of the challenges in vaccine development. To facilitate vaccine advancement efforts, we carried out comprehensive bioinformatic analyses of sequence variation by comparing 34 gonorrhea antigen candidates among >5,000 clinical N. gonorrhoeae isolates deposited in the Neisseria PubMLST database. Eight protein antigens showed exceptional conservation by having a single allele variant distributed in >80% of isolates. An additional 18 vaccine candidates were represented by ≤3 alleles in >50% of N. gonorrhoeae isolates globally. Phylogenetic analyses highlighted closely related antigen variants and additionally showed that AniA and FetB were the closest between N. gonorrhoeae and N. meningitidis Up to 44% of N. meningitidis alleles for both antigens have premature stop codons, suggesting differential expression. Mapping polymorphisms to the available three-dimensional structures of 12 antigens revealed low-frequency surface polymorphisms. PorB and TbpB possessed numerous high-prevalence polymorphic sites. While TbpA was also highly variable, conserved loops were nonetheless identified. A high degree of sequence conservation, the distribution of a single antigen variant among N. gonorrhoeae strains globally, or low-frequency sequence polymorphisms in surface loops make ACP, AniA, BamA, BamE, MtrE, NspA, NGO0778, NGO1251, NGO1985, OpcA, PldA, Slam2, and ZnuD promising candidates for a gonorrhea vaccine. Finally, the commonly used N. gonorrhoeae FA1090 strain emerges as a vaccine prototype, as it carries antigen sequence types identical to the most broadly distributed antigen variants.IMPORTANCE Neisseria gonorrhoeae, the Gram-negative bacterium responsible for the sexually transmitted infection gonorrhea, is categorized as a high-priority pathogen for research and development efforts. N. gonorrhoeae's "superbug" status, its high morbidity, and the serious health impact associated with gonorrhea highlight the importance of vaccine development. One of the longstanding barriers to developing an effective vaccine against N. gonorrhoeae is the remarkable variability of surface-exposed antigens. In this report, we addressed this roadblock by applying extensive bioinformatic analyses to 34 gonorrhea antigen candidates among >5,000 clinical N. gonorrhoeae isolates. Our studies are important, as they reveal promising, conserved gonorrhea vaccine candidates and aid structural vaccinology. Moreover, these approaches are broadly applicable to other infectious diseases where surface antigen variability impedes successful vaccine design.

Oral administration of recombinant Neisseria meningitidis PorA genetically fused to H. pylori HpaA antigen increases antibody levels in mouse serum, suggesting that PorA behaves as a putative adjuvant

The Neisseria meningitidis outer membrane protein PorA from a Chilean strain was purified as a recombinant protein. PorA mixed with AbISCO induced bactericidal antibodies against N. meningitidis in mice. When PorA was fused to the Helicobacter pylori HpaA antigen gene, the specific response against H. pylori protein increased. Splenocytes from PorA-immunized mice were stimulated with PorA, and an increase in the secretion of IL-4 was observed compared with that of IFN-γ. Moreover, in an immunoglobulin sub-typing analysis, a substantially higher IgG₁ level was found compared with IgG_2a levels, suggesting a Th2-type immune response. This study revealed a peculiar behavior of the purified recombinant PorA protein per se in the absence of AbISCO as an adjuvant. Therefore, the resistance of PorA to proteolytic enzymes, such as those in the gastrointestinal tract, was analyzed, because this is an important feature for an oral protein adjuvant. Finally, we found that PorA fused to the H. pylori HpaA antigen, when expressed in Lactococcus lactis and administered orally, could enhance the antibody response against the HpaA antigen approximately 3 fold. These observations strongly suggest that PorA behaves as an effective oral adjuvant.

Sequence and structural diversity of transferrin receptors in Gram-negative porcine pathogens

Actinobacillus pleuropneumoniae, Actinobacillus suis, and Haemophilus parasuis are bacterial pathogens from the upper respiratory tract that are responsible for a substantial burden of porcine disease. Although reduction of disease has been accomplished by intensive management practices, immunization remains an important strategy for disease prevention, particularly when intensive management practices are not feasible or suitable. An attractive target for vaccine development is the surface receptor involved in acquiring iron from host transferrin, since it is common to all three pathogenic species and has been shown to be essential for survival and disease causation. It has also recently been demonstrated that an engineered antigen derived from the lipoprotein component of the receptor, transferrin-binding protein B (TbpB), was more effective at preventing infection by H. parasuis than a commercial vaccine product. This study was initiated to explore the genetic and immunogenic diversity of the transferrin receptor system from these species. Nucleic acid sequences were obtained from a geographically and temporally diverse collection of isolates, consisting of 41 A. pleuropneumoniae strains, 30 H. parasuis strains, and 2 A. suis strains. Phylogenetic analyses demonstrated that the receptor protein sequences cluster independently of species, suggesting that there is genetic exchange between these species such that receptor-based vaccines should logically target all three species. To evaluate the cross-reactive response of TbpB-derived antigens, pigs were immunized with the intact TbpB, the TbpB N-lobe and the TbpB C-lobe from A. pleuropneumoniae strain H49 and the resulting sera were tested against a representative panel of TbpBs; demonstrating that the C-lobe induces a broadly cross-reactive response. Overall our results indicate that there is a common reservoir for transferrin receptor antigenic variation amongst these pathogens. While this could present a challenge to future vaccine development, our results suggest a rationally designed TbpB-based vaccine may provide protection against all three pathogens.

Patterns of structural and sequence variation within isotype lineages of the Neisseria meningitidis transferrin receptor system

Neisseria meningitidis inhabits the human upper respiratory tract and is an important cause of sepsis and meningitis. A surface receptor comprised of transferrin-binding proteins A and B (TbpA and TbpB), is responsible for acquiring iron from host transferrin. Sequence and immunological diversity divides TbpBs into two distinct lineages; isotype I and isotype II. Two representative isotype I and II strains, B16B6 and M982, differ in their dependence on TbpB for in vitro growth on exogenous transferrin. The crystal structure of TbpB and a structural model for TbpA from the representative isotype I N. meningitidis strain B16B6 were obtained. The structures were integrated with a comprehensive analysis of the sequence diversity of these proteins to probe for potential functional differences. A distinct isotype I TbpA was identified that co-varied with TbpB and lacked sequence in the region for the loop 3 α-helix that is proposed to be involved in iron removal from transferrin. The tightly associated isotype I TbpBs had a distinct anchor peptide region, a distinct, smaller linker region between the lobes and lacked the large loops in the isotype II C-lobe. Sequences of the intact TbpB, the TbpB N-lobe, the TbpB C-lobe, and TbpA were subjected to phylogenetic analyses. The phylogenetic clustering of TbpA and the TbpB C-lobe were similar with two main branches comprising the isotype 1 and isotype 2 TbpBs, possibly suggesting an association between TbpA and the TbpB C-lobe. The intact TbpB and TbpB N-lobe had 4 main branches, one consisting of the isotype 1 TbpBs. One isotype 2 TbpB cluster appeared to consist of isotype 1 N-lobe sequences and isotype 2 C-lobe sequences, indicating the swapping of N-lobes and C-lobes. Our findings should inform future studies on the interaction between TbpB and TbpA and the process of iron acquisition.

The structure of lactoferrin-binding protein B from Neisseria meningitidis suggests roles in iron acquisition and neutralization of host defences

Pathogens have evolved a range of mechanisms to acquire iron from the host during infection. Several Gram-negative pathogens including members of the genera Neisseria and Moraxella have evolved two-component systems that can extract iron from the host glycoproteins lactoferrin and transferrin. The homologous iron-transport systems consist of a membrane-bound transporter and an accessory lipoprotein. While the mechanism behind iron acquisition from transferrin is well understood, relatively little is known regarding how iron is extracted from lactoferrin. Here, the crystal structure of the N-terminal domain (N-lobe) of the accessory lipoprotein lactoferrin-binding protein B (LbpB) from the pathogen Neisseria meningitidis is reported. The structure is highly homologous to the previously determined structures of the accessory lipoprotein transferrin-binding protein B (TbpB) and LbpB from the bovine pathogen Moraxella bovis. Docking the LbpB structure with lactoferrin reveals extensive binding interactions with the N1 subdomain of lactoferrin. The nature of the interaction precludes apolactoferrin from binding LbpB, ensuring the specificity of iron-loaded lactoferrin. The specificity of LbpB safeguards proper delivery of iron-bound lactoferrin to the transporter lactoferrin-binding protein A (LbpA). The structure also reveals a possible secondary role for LbpB in protecting the bacteria from host defences. Following proteolytic digestion of lactoferrin, a cationic peptide derived from the N-terminus is released. This peptide, called lactoferricin, exhibits potent antimicrobial effects. The docked model of LbpB with lactoferrin reveals that LbpB interacts extensively with the N-terminal lactoferricin region. This may provide a venue for preventing the production of the peptide by proteolysis, or directly sequestering the peptide, protecting the bacteria from the toxic effects of lactoferricin.

Transcriptional profiling of Neisseria meningitidis interacting with human epithelial cells in a long-term in vitro colonization model

Neisseria meningitidis is a commensal of humans that can colonize the nasopharyngeal epithelium for weeks to months and occasionally invades to cause life-threatening septicemia and meningitis. Comparatively little is known about meningococcal gene expression during colonization beyond those first few hours. In this study, the transcriptome of adherent serogroup B N. meningitidis strain MC58 was determined at intervals during prolonged cocultivation with confluent monolayers of the human respiratory epithelial cell line 16HBE14. At different time points up to 21 days, 7 to 14% of the meningococcal genome was found to be differentially regulated. The transcriptome of adherent meningococci obtained after 4 h of coculture was markedly different from that obtained after prolonged cocultivation (24 h, 96 h, and 21 days). Genes persistently upregulated during prolonged cocultivation included three genes (hfq, misR/phoP, and lrp) encoding global regulatory proteins. Many genes encoding known adhesins involved in epithelial adherence were upregulated, including those of a novel locus (spanning NMB0342 to NMB0348 [NMB0342-NMB0348]) encoding epithelial cell-adhesive function. Sixteen genes (including porA, porB, rmpM, and fbpA) encoding proteins previously identified by their immunoreactivity to sera from individuals colonized long term with serogroup B meningococci were also upregulated during prolonged cocultivation, indicating that our system models growth conditions in vivo during the commensal state. Surface-expressed proteins downregulated in the nasopharynx (and thus less subject to selection pressure) but upregulated in the bloodstream (and thus vulnerable to antibody-mediated bactericidal activity) should be interesting candidate vaccine antigens, and in this study, three new proteins fulfilling these criteria have been identified: NMB0497, NMB0866, and NMB1882.

The distribution and 'in vivo' phase variation status of haemoglobin receptors in invasive meningococcal serogroup B disease: genotypic and phenotypic analysis

Two haemoglobin-binding proteins, HmbR and HpuAB, contribute to iron acquisition by Neisseria meningitidis. These receptors are subject to high frequency, reversible switches in gene expression--phase variation (PV)--due to mutations in homopolymeric (poly-G) repeats present in the open reading frame. The distribution and PV state of these receptors was assessed for a representative collection of isolates from invasive meningococcal disease patients of England, Wales and Northern Ireland. Most of the major clonal complexes had only the HmbR receptor whilst the recently expanding ST-275-centred cluster of the ST-269 clonal complex had both receptors. At least one of the receptors was in an 'ON' configuration in 76.3% of the isolates, a finding that was largely consistent with phenotypic analyses. As PV status may change during isolation and culture of meningococci, a PCR-based protocol was utilised to confirm the expression status of the receptors within contemporaneously acquired clinical specimens (blood/cerebrospinal fluid) from the respective patients. The expression state was confirmed for all isolate/specimen pairs with <15 tract repeats indicating that the PV status of these receptors is stable during isolation. This study therefore establishes a protocol for determining in vivo PV status to aid in determining the contributions of phase variable genes to invasive meningococcal disease. Furthermore, the results of the study support a putative but non-essential role of the meningococcal haemoglobin receptors as virulence factors whilst further highlighting their vaccine candidacy.

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

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

Microcalorimetry: a response to challenges in modern biotechnology

Almost any process in life is accompanied by heat changes which can be monitored by isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC). Both techniques are now established tools in fundamental research but over the last decade a clear tendency towards more problem-driven applications is noted. This review aims at summarizing these problem-oriented applications of microcalorimetry and the solutions both techniques can provide to problems in biotechnology. The biotechnological issues to which microcalorimetry has been successfully applied are as diverse as rational drug design, overcoming drug resistance, optimization of long-term stability of proteins, estimation of the bioavailability of drugs, control of complex pharmaceutical products or the optimization of gene delivery efficiency. The main limitation of microcalorimetry, which is the relatively large amounts of sample necessary for analysis, is less important in the biotechnology sector which frequently uses large-scale produced bulk products for analysis. The recently developed high-throughput DSC and ITC microcalorimeters will additionally reduce the labour intensity of these techniques. Due to the precision of microcalorimetric analyses and the versatility of processes which can be studied, it is expected that ITC and DSC will soon be key technologies in biotechnological research.

Evidence for a common gene pool and frequent recombinational exchange of the tbpBA operon in Mannheimia haemolytica, Mannheimia glucosida and Bibersteinia trehalosi

The tbpBA operon was sequenced in 42 representative isolates of Mannheimia haemolytica (32), Mannheimia glucosida (6) and Bibersteinia trehalosi (4). A total of 27 tbpB and 20 tbpA alleles were identified whilst the tbpBA operon was represented by 28 unique alleles that could be assigned to seven classes. There were 1566 (34.8% variation) polymorphic nucleotide sites and 482 (32.1% variation) variable inferred amino acid positions among the 42 tbpBA sequences. The tbpBA operons of serotype A2 M. haemolytica isolates are, with one exception, substantially more diverse than those of the other M. haemolytica serotypes and most likely have a different ancestral origin. The tbpBA phylogeny has been severely disrupted by numerous small- and large-scale intragenic recombination events. In addition, assortative (entire gene) recombination events, involving either the entire tbpBA operon or the individual tbpB and tbpA genes, have played a major role in shaping tbpBA structure and it's distribution in the three species. Our findings indicate that a common gene pool exists for tbpBA in M. haemolytica, M. glucosida and B. trehalosi. In particular, B. trehalosi, M. glucosida and ovine M. haemolytica isolates share a large portion of the tbpA gene, and this probably reflects selection for a conserved TbpA protein that provides effective iron uptake in sheep. Bovine and ovine serotype A2 lineages have very different tbpBA alleles. Bovine-like tbpBA alleles have been partially, or completely, replaced by ovine-like tbpBA alleles in ovine serotype A2 isolates, suggesting that different transferrin receptors are required by serotype A2 isolates for optimum iron uptake in cattle and sheep. Conversely, the tbpBA alleles of bovine-pathogenic serotype A1 and A6 isolates are very similar to those of closely related ovine isolates, suggesting a recent and common evolutionary origin.

Variation of the factor H-binding protein of Neisseria meningitidis

There is currently no comprehensive meningococcal vaccine, due to difficulties in immunizing against organisms expressing serogroup B capsules. To address this problem, subcapsular antigens, particularly the outer-membrane proteins (OMPs), are being investigated as candidate vaccine components. If immunogenic, however, such antigens are often antigenically variable, and knowledge of the extent and structuring of this diversity is an essential part of vaccine formulation. Factor H-binding protein (fHbp) is one such protein and is included in two vaccines under development. A survey of the diversity of the fHbp gene and the encoded protein in a representative sample of meningococcal isolates confirmed that variability in this protein is structured into two or three major groups, each with a substantial number of alleles that have some association with meningococcal clonal complexes and serogroups. A unified nomenclature scheme was devised to catalogue this diversity. Analysis of recombination and selection on the allele sequences demonstrated that parts of the gene are subject to positive selection, consistent with immune selection on the protein generating antigenic variation, particularly in the C-terminal region of the peptide sequence. The highest levels of selection were observed in regions corresponding to epitopes recognized by previously described bactericidal monoclonal antibodies.

Meningococcal protein antigens and vaccines

The development of a comprehensive vaccine against meningococcal disease has been challenging. Recent developments in molecular genetics have provided both explanations for these challenges and possible solutions. Since genome sequence data became available there has been a marked increase in number of protein antigens that have been suggested as prospective vaccine components. This review catalogues the proposed vaccine candidates and examines the evidence for their inclusion in potential protein vaccine formulations.

Secreted proteins of Neisseria meningitidis protect mice against infection

We addressed the hypothesis that meningococcal secreted proteins (MSPs) can elicit protective immunity against meningococcal disease. Endotoxin-depleted MSP preparations were used to immunise a group of 15 six-week-old BALB/c mice (25microg MSPs/dose mixed with Freund's complete adjuvant) on days 0, 14 and 21. Mice were challenged 2 weeks later with 10(7) colony forming units of live Neisseria meningitidis strain MC58 (serogroup B, ET-5). Negative and positive control groups of 15 mice each were injected with adjuvant only, or a live attenuated strain of MC58, respectively. Seven out of 15 mice (47%) from the negative control group died after 72h of challenge, whereas none of test or positive control group died. Protection afforded by the anti-MSP immune response can be at least partly attributed to complement-mediated bacterial lysis, detectable in vitro using the serum of immunised mice. Murine anti-MC58 MSP sera were bactericidal against homologous and five unrelated ET-5 serogroup B strains. However, failed to kill strains from other hypervirulent clonal lineages belonging to the same or different serogroups, despite the presence of cross-reactive antibodies detectable by immunoblotting. Similar sera raised against MSPs from an isolate belonging to the ET-37 electropherotype lineage were bactericidal against all tested isolates of this lineage and, in addition, against some but not all isolates belonging to the ET-5 lineage. FACS analysis of intact bacteria treated with anti-MSPs confirmed surface-binding of antibodies.

Growth deficiencies of Neisseria meningitidis pfs and luxS mutants are not due to inactivation of quorum sensing

The activated methyl cycle (AMC) is a central metabolic pathway used to generate (and recycle) several important metabolites and enable methylation. Pfs and LuxS are considered integral components of this pathway because they convert S-adenosylhomocysteine (SAH) to S-ribosylhomocysteine (SRH) and S-ribosylhomocysteine to homocysteine (HCY), respectively. The latter reaction has a second function since it also generates the precursor of the quorum-sensing molecule autoinducer 2 (AI-2). By demonstrating that there was a complete lack of AI-2 production in pfs mutants of the causative agent of meningitis and septicemia, Neisseria meningitidis, we showed that the Pfs reaction is the sole intracellular source of the AI-2 signal. Analysis of lacZ reporters and real-time PCR experiments indicated that pfs is expressed constitutively from a promoter immediately upstream, and careful study of the pfs mutants revealed a growth defect that could not be attributed to a lack of AI-2. Metabolite profiling of the wild type and of a pfs mutant under various growth conditions revealed changes in the concentrations of several AMC metabolites, particularly SRH and SAH and under some conditions also HCY. Similar studies established that an N. meningitidis luxS mutant also has metabolite pool changes and growth defects in line with the function of LuxS downstream of Pfs in the AMC. Thus, the observed growth defect of N. meningitidis pfs and luxS mutants is not due to quorum sensing but is probably due to metabolic imbalance and, in the case of pfs inactivation, is most likely due to toxic accumulation of SAH.

Distribution of transferrin binding protein B gene (tbpB) variants among Neisseria species

BACKGROUND

Transferrin binding protein B (tbpB), an outer membrane lipoprotein, is required for the acquisition of iron from human transferrin. Two tbpB families have been documented in Neisseria meningitidis: an isotype I tbpB gene of 1.8 kb and an isotype II tbpB gene of 2.1 kb, the former expressed by meningococci in the disease-associated ST-11 clonal complex and the latter found among meningococci belonging to the hyper-invasive clonal complexes including ST-8, ST-18, ST-32, ST-41/44 as well as N. gonorrhoeae isolates. The origin of the isotype I tbpB gene is unknown, however several features in common with non-pathogenic Neisseria and the ST-11 clonal complex N. meningitidis isolate FAM18 have been documented leading to the hypothesis that the isotype I tbpB gene may also be shared between non-pathogenic Neisseria and ST-11 meningococci. As a result, the diversity of the tbpB gene was investigated in a defined collection of Neisseria species.

RESULTS

Two families of isotype I tbpB were identified: family A containing conserved genes belonging to ST-11 meningococci, N. polysaccharea and N. lactamica isolates and family B including more diverse isotype I tbpB genes from N. sicca, N. mucosa, N. flava, N. subflava as well as N. cinerea, N. flavescens and N. polysaccharea isolates. Three isotype II tbpB families were identified with: family C containing diverse tbpB genes belonging to N. polysaccharea, N. lactamica, N. gonorrhoeae and N. meningitidis isolates, family D including another subset of isotype II tbpB genes from N. lactamica isolates and family E solely composed of N. gonorrhoeae tbpB genes.

CONCLUSION

This study reveals another instance of similarity between meningococci of the ST-11 clonal complex and non-pathogenic Neisseria with the origin of the isotype I tbpB gene resulting from a horizontal genetic transfer event occurring between these two populations.

Additive and synergistic bactericidal activity of antibodies directed against minor outer membrane proteins of Neisseria meningitidis

Neisseria meningitidis serogroup B is a major cause of bacterial meningitis in younger populations. The available vaccines are based on outer membrane vesicles obtained from wild-type strains. In children less than 2 years old they confer protection only against strains expressing homologous PorA, a major, variable outer membrane protein (OMP). We genetically modified a strain in order to eliminate PorA and to overproduce one or several minor and conserved OMPs. Using a mouse model mimicking children's PorA-specific bactericidal activity, it was demonstrated that overproduction of more than one minor OMP is required to elicit antibodies able to induce complement-mediated killing of strains expressing heterologous PorA. It is concluded that a critical density of bactericidal antibodies needs to be reached at the surface of meningococci to induce complement-mediated killing. With minor OMPs, this threshold is reached when more than one antigen is targeted, and this allows cross-protection.

Outer membrane vesicles of the VA-MENGOC-BC vaccine against serogroup B of Neisseria meningitidis: Analysis of protein components by two-dimensional gel electrophoresis and mass spectrometry

Neisseria meningitidis is a Gram-negative bacterium responsible for significant mortality worldwide. While effective polysaccharides-based vaccines exist against serogroups A, C, W135, and Y, no similar vaccine is suitable for children under 4 years against disease caused by serogroup B strains. Therefore, major vaccine efforts against this serogroup are based on outer membrane vesicles (OMVs), containing major outer membrane proteins. The OMV-based vaccine produced by the Finlay Institute in Cuba (VA-MENGOC-BC) contributed to the rapid decline of the epidemic in this Caribbean island. While the content of major proteins in this vaccine has been discussed, no detailed work of an outer membrane proteomic map of this, or any other, commercially available OMV-derived product has been published so far. Since OMVs exhibit a large bias toward a few major proteins and usually contain a high content of lipids, establishing the adequate conditions for high resolution, 2-DE of this kind of preparation was definitely a technical challenge. In this work, 2-DE and MS have been used to generate a proteomic map of this product, detailing the presence of 31 different proteins, and it allows the identification of new putative protective protein components it contains.

Identification of pathogen-specific genes through microarray analysis of pathogenic and commensal Neisseria species

The release of the complete genome sequences of Neisseria meningitidis MC58 and Z2491 along with access to the sequences of N. meningitidis FAM18 and Neisseria gonorrhoeae FA1090 allowed the construction of a pan-Neisseria microarray, with every gene in all four genomes represented. The microarray was used to analyse a selection of strains including all N. meningitidis serogroups and commensal Neisseria species. For each strain, genes were defined as present, divergent or absent using gack analysis software. Comparison of the strains identified genes that were conserved within N. meningitidis serogroup B strains but absent from all commensal strains tested, consisting of mainly virulence-associated genes and transmissible elements. The microarray was able to distinguish between pilin genes, pilC orthologues and serogroup-specific capsule biosynthetic genes, and to identify dam and drg genotypes. Previously described N. meningitidis genes involved in iron response, adherence to epithelial cells, and pathogenicity were compared to the microarray analysis. The microarray data correlated with other genetic typing methods and were able to predict genotypes for uncharacterized strains and thus offer the potential for a rapid typing method. The subset of pathogen-specific genes identified represents potential drug or vaccine targets that would not eliminate commensal neisseriae and the associated naturally acquired immunity.

Progress in vaccines for sexually transmitted diseases

The development pipeline for vaccines to control sexually transmitted infections holds greater promise than ever before. Preclinical studies are encouraging in the development of chlamydia and gonococcal vaccines, and for the first time, recent clinical trials have shown the feasibility of creating vaccines to control genital herpes and cervical human papillomavirus infections. Behavioral research suggests that these vaccines will likely find acceptance among health care providers and consumers.

Identification of a conserved Moraxella catarrhalis haemoglobin-utilization protein, MhuA

Moraxella catarrhalis is a leading cause of acute otitis media in children and is a cause of respiratory disease in adults with underlying lung disease. This organism is a strict human pathogen that has an absolute requirement for iron in order to grow and cause disease. Previous studies identified transferrin and lactoferrin receptors used by M. catarrhalis to obtain iron from the human host, yet other iron-acquisition systems remain undefined. In this study, it is demonstrated that this strict mucosal pathogen can utilize haemoglobin (Hb) as a sole source of iron for growth. A novel 107 kDa outer-membrane protein involved in Hb utilization by this pathogen was also identified. An isogenic mutant defective in this Moraxella Hb-utilization protein (MhuA), 7169 : : mhuA, showed a significant lag during growth in the presence of Hb as the sole iron source. This protein appears to be expressed constitutively, regardless of growth conditions, and a mAb directed to MhuA demonstrated that this protein contains highly conserved, surface-exposed epitopes. Data demonstrating that expression of MhuA may be highly specific to isolates of M. catarrhalis are also presented, suggesting a potential role as a diagnostic marker. To our knowledge, this is the first report demonstrating that M. catarrhalis expresses an Hb-binding protein and that this bacterium can utilize Hb as a sole iron source for growth.

Overcoming antigenic diversity and improving vaccines using DNA shuffling and screening technologies

Viral, bacterial and parasitic pathogens have evolved multiple strategies to evade the immune response, facilitate transmission and establish chronic infections. One of the underlying strategies that pathogens have evolved is antigenic variation of immune response targets that reduce the affinity of antigen binding to antibodies and major histocompatability complex class I and II receptors. Vaccine candidates generally target a limited number of these antigen variants or combine antigens from several variants to include in multivalent vaccine formulations. DNA shuffling and screening technologies, also known as MolecularBreeding (Maxygen, Inc.) directed molecular evolution, have been successfully used to identify and develop novel and chimaeric vaccine candidates capable of inducing immune responses that recognise and control multiple antigenic variants. DNA shuffling and screening strategies also select vaccine candidates with improved immunogenicity, increased expression as recombinant polypeptides and improved growth of whole viruses in cell culture. As DNA shuffling and screening strategies can be applied to many pathogens, there remain numerous applications of DNA shuffling to solve challenging problems in vaccine process development and manufacture.

Meningococcal vaccines

Meningococcal disease is one of the most feared and serious infections in the young and its prevention by vaccination is an important goal. The high degree of antigenic variability of the organism makes the meningococcus a challenging target for vaccine prevention. Meningococcal polysaccharide vaccines against serogroup A and C are efficacious and have been widely used, often in combination with serogroup Y and W135 components. Their relative lack of immunogenicity in young children and infants can be overcome by conjugation to a protein carrier. The effectiveness of serogroup C glycoconjugate vaccines in children of all ages has been demonstrated and they have now been introduced into routine vaccination schedules. Conjugate vaccines against other serogroups, including A, Y, and W135 will soon be available and it is hoped they may emulate this success. Prevention of serogroup B disease has proven more elusive. Several serogroup B vaccines based on outer membrane vesicles have been shown to be immunogenic and reasonably effective in adults and older children, but the protection offered by them is chiefly strain-specific. Multivalent recombinant PorA vaccines have been developed to broaden the protective effect, but no efficacy data are available as yet. Intensive efforts have been directed at other outer membrane protein vaccine candidates and lipopolysaccharide, and some of these have been shown to offer protection in experimental animal models. Nonpathogenic Neisseriae spp. such as Neisseria lactamica are also possible vaccine candidates. Previously unknown proteins have been identified from in silico analysis of the meningococcal genome and their vaccine potential explored. However, none of these has yet been presented as the 'universal' protective antigen and work in this field continues to be held back by our limited knowledge concerning the mechanisms of natural protection against serogroup B meningococci.

Protective activity of monoclonal antibodies to genome-derived neisserial antigen 1870, a Neisseria meningitidis candidate vaccine

Genome-derived neisserial Ag (GNA) 1870 is a meningococcal vaccine candidate that can be subdivided into three variants based on amino acid sequence variability. Variant group 1 accounts for approximately 60% of disease-producing group B isolates. The Ag went unrecognized until its discovery by genome mining because it is expressed in low copy number by most strains. To investigate the relationship between Ab binding to GNA1870 and complement-mediated protective functions, we prepared a panel of four murine IgG mAbs against rGNA1870 (variant 1) and evaluated their activity against nine genetically diverse encapsulated Neisseria meningitidis strains expressing subvariants of variant 1 GNA1870. Based on flow cytometry with live encapsulated bacteria, surface accessibility of the epitopes recognized by the mAbs appeared to be low in most strains. Yet mAb concentrations <1 to 5 micro g/ml were sufficient to elicit bactericidal activity with human complement and/or activate C3b deposition on the bacterial surface. Certain combinations of mAbs were highly bactericidal against strains that were resistant to bactericidal activity of the respective individual mAbs. The mAbs conferred passive protection against bacteremia in infant rats challenged by strains resistant to bacteriolysis, and the protective activity paralleled the ability of the mAb to activate C3b deposition. Thus, despite low GNA1870 surface exposure, anti-GNA1870 variant 1 Abs are bactericidal and/or elicit C3b deposition and confer protection against bacteremia caused by encapsulated N. meningitidis strains expressing GNA1870 subvariant 1 proteins. The data support GNA1870 as a promising vaccine candidate for prevention of meningococcal group B disease caused by GNA1870 variant 1 strains.

Role of transferrin receptor from a Neisseria meningitidis tbpB isotype II strain in human transferrin binding and virulence

Neisseria meningitidis acquires iron through the action of the transferrin (Tf) receptor, which is composed of the Tf-binding proteins A and B (TbpA and TbpB). Meningococci can be classified into isotype I and II strains depending on whether they harbor a type I or II form of TbpB. Both types of TbpB have been shown to differ in their genomic, biochemical, and antigenic properties. Here we present a comparative study of isogenic mutants deficient in either or both Tbps from the isotype I strain B16B6 and isotype II strain M982. We show that TbpA is essential in both strains for iron uptake and growth with iron-loaded human Tf as a sole iron source. No growth has also been observed for the TbpB- mutant of strain B16B6, as shown previously, whereas the growth of the analogous mutant in M982 was similar to that in the wild type. This indicates that TbpB in the latter strain plays a facilitating but not essential role in iron uptake, which has been observed previously in similar studies of other bacteria. These data are discussed in relation to the fact that isotype II strains represent more than 80% of serogroup B meningococcal strains. The contribution of both subunits in the bacterial virulence of strain M982 has been assessed in a murine model of bacteremia. Both the TbpB- TbpA- mutant and the TbpA- mutant are shown to be nonvirulent in mice, whereas the virulence of the TbpB- mutant is similar to that of the wild type.

Immunogenicity of gonococcal transferrin binding proteins during natural infections

In this study, we examined the immune response during gonococcal infection to the individual transferrin binding proteins by using a quantitative enzyme-linked immunosorbent assay (ELISA). Recombinant transferrin binding protein A (rTbpA) and rTbpB were purified under nondenaturing conditions for use as ELISA antigens. Sera and secretions from culture-positive individuals were analyzed for antibodies to rTbpA and rTbpB and compared to samples from individuals with no history of gonococcal infection. Although antibodies to both rTbpA and rTbpB were detected in serum, in most cases the antibody levels were not significantly different from those measured in the control population. Also, previous history of gonococcal infection did not increase antibody levels in serum, suggesting the lack of an anamnestic response. Analysis of secretion samples revealed antibody levels that were generally below the limits of detection in our assay. Overall, this study demonstrated a paucity of systemic and local antibody responses to rTbps as a result of natural infection and represents a baseline over which a protective antibody response will have to be generated in order to develop an efficacious gonococcal vaccine.

Transferrin-binding protein B of Neisseria meningitidis: sequence-based identification of the transferrin-Binding site confirmed by site-directed mutagenesis

A sequence-based prediction method was employed to identify three ligand-binding domains in transferrin-binding protein B (TbpB) of Neisseria meningitidis strain B16B6. Site-directed mutagenesis of residues located in these domains has led to the identification of two domains, amino acids 53 to 57 and 240 to 245, which are involved in binding to human transferrin (htf). These two domains are conserved in an alignment of different TbpB sequences from N. meningitidis and Neisseria gonorrhoeae, indicating a general functional role of the domains. Western blot analysis and BIAcore and isothermal titration calorimetry experiments demonstrated that site-directed mutations in both binding domains led to a decrease or abolition of htf binding. Analysis of mutated proteins by circular dichroism did not provide any evidence for structural alterations due to the amino acid replacements. The TbpB mutant R243N was devoid of any htf-binding activity, and antibodies elicited by the mutant showed strong bactericidal activity against the homologous strain, as well as against several heterologous tbpB isotype I strains.

Insight into the structure and function of the transferrin receptor from Neisseria meningitidis using microcalorimetric techniques

The transferrin receptor of Neisseria meningitidis is composed of the transmembrane protein TbpA and the outer membrane protein TbpB. Both receptor proteins have the capacity to independently bind their ligand human transferrin (htf). To elucidate the specific role of these proteins in receptor function, isothermal titration calorimetry was used to study the interaction between purified TbpA, TbpB or the entire receptor (TbpA + TbpB) with holo- and apo-htf. The entire receptor was shown to contain a single high affinity htf-binding site on TbpA and approximately two lower affinity binding sites on TbpB. The binding sites appear to be independent. Purified TbpA was shown to have strong ligand preference for apo-htf, whereas TbpA in the receptor complex with TbpB preferentially binds the holo form of htf. The orientation of the ligand specificity of TbpA toward holo-htf is proposed to be the physiological function of TbpB. Furthermore, the thermodynamic mode of htf binding by TbpB of isotypes I and II was shown to be different. A protocol for the generation of active, histidine-tagged TbpB as well as its individual N- and C-terminal domains is presented. Both domains are shown to strongly interact with each other, and isothermal titration calorimetry and circular dichroism experiments provide clear evidence for this interaction causing conformational changes. The N-terminal domain of TbpB was shown to be the site of htf binding, whereas the C-terminal domain is not involved in binding. Furthermore, the interactions between TbpA and the different domains of TbpB have been demonstrated.

Reverse vaccinology: a genome-based approach for vaccine development

During the last century several approaches have been followed for the development of vaccines. These include live-attenuated viruses and bacteria, killed microorganisms and the subunit vaccines [1]. With the introduction of recombinant DNA technologies, new approaches have been exploited for vaccine manufacturing. However, the major problem remains the rapid identification of highly immunogenic and protective antigens suitable for vaccine development, which still relies on standard biochemical and microbiological techniques. The advent of genomics has greatly contributed to providing a new impulse to the microbial field. The complete genomic sequence of a human pathogen represents a new unexploited field, to be used for the design of novel vaccines and antimicrobial drugs. In the case of meningococcus B, four decades of continuous efforts, using conventional technologies of purifying antigens from the microorganism, had not been sufficient to deliver an effective and universal vaccine. It was therefore decided to obtain the genomic sequence of serogroup B Neisseria meningitidis (MenB) and use this information to identify vaccine candidates. This approach was named "reverse vaccinology"[2].

Challenges for the development of vaccines against Haemophilus influenzae and Neisseria meningitidis

The development of protein-polysaccharide conjugate vaccines has had a major impact on Haemophilus influenzae type b disease. The application of this technology to Neisseria meningitidis is also striking, particularly for serogroup C. However, significant challenges exist for the development of vaccines against non-typeable H. influenzae and against N. meningitidis serogroup B. Issues such as non-vaccine-strain replacement and correlates of protection need to be addressed as well as the longer-term implications of vaccination against what are essentially 'normal' microflora.

The meningococcus tamed?

Serogroup B Neisseria meningitidis is a frequent cause of invasive meningococcal disease, yet there are no effective vaccines suitable for routine immunisation. Limited efficacy has been shown with meningococcal outer membrane vacccines in children 4 years and older. Here we review the status of current research and consider new approaches to development of meningococcal serogroup B vaccines.

Development of vaccines against meningococcal disease

Neisseria meningitidis is a major cause of bacterial meningitis and sepsis. Polysaccharide-protein conjugate vaccines for prevention of group C disease have been licensed in Europe. Such vaccines for prevention of disease caused by groups A (which is associated with the greatest disease burden worldwide), Y, and W135 are being developed. However, conventional approaches to develop a vaccine for group B strains, which are responsible for most cases in Europe and the USA, have been largely unsuccessful. Capsular polysaccharide-based vaccines can elicit autoantibodies to host polysialic acid, whereas the ability of most non-capsular antigens to elicit broad-based immunity is limited by their antigenic diversity. Many new membrane proteins have been discovered during analyses of genomic sequencing data. These antigens are highly conserved and, in mice, elicit serum bactericidal antibodies, which are the serological hallmark of protective immunity in man. Therefore, there are many promising new vaccine candidates, and improved prospects for development of a broadly protective vaccine for group B disease, and for control of all meningococcal disease.

Role of Neisseria meningitidis luxS in cell-to-cell signaling and bacteremic infection

Numerous pathogenic bacteria contain luxS, which is required for autoinducer-2 production. Here, we demonstrate that Neisseria meningitidis contains a functional copy of luxS that is necessary for full meningococcal virulence; strains with a luxS deletion are defective for bacteremia, a prerequisite of meningococcal pathogenesis.

Affinity-purification of Transferrin-binding protein B under nondenaturing conditions

The commonly used purification procedures for Transferrin-binding protein B (TbpB) are based on an affinity chromatography step using resins onto which human transferrin had been immobilized. These protocols involve protein elution using denaturing buffer solutions. Here we present an improved protocol which permits protein elution under nondenaturing conditions using chelating agents such as phosphate or compounds containing a pyrophosphate group. Furthermore, isothermal titration calorimetry experiments of the purified protein with holotransferrin have been shown to be a reliable method to assess the purity and activity of the purified material.

Specific ligand binding attributable to individual epitopes of gonococcal transferrin binding protein A

The gonococcal transferrin receptor complex comprises two iron-regulated proteins, TbpA and TbpB. TbpA is essential for transferrin-iron uptake and is a TonB-dependent integral outer membrane protein. TbpB is thought to increase the efficiency of iron uptake from transferrin and is lipid modified and surface exposed. To evaluate the structure-function relationships in one of the components of the receptor, TbpA, we created constructs that fused individual putative loops of TbpA with amino-terminal affinity tags. The recombinant proteins were then overexpressed in Escherichia coli, and the fusions were recovered predominately from inclusion bodies. Inclusion body proteins were solubilized, and the epitope fusions were renatured by slow dialysis. To assess transferrin binding capabilities, the constructs were tested in a solid-phase dot blot assay followed by confirmatory quantitative chemiluminescent enzyme-linked immunosorbent assays. The constructs with only loop 5 and with loops 4 and 5 demonstrated dose-dependent specific ligand binding in spite of being out of the context of the intact receptor. The immunogenicities of individual TbpA-specific epitopes were investigated by generating rabbit polyclonal antisera against the fusion proteins. Most of the fusion proteins were immunogenic under these conditions, and the resulting sera recognized full-length TbpA in immunoblots. These results suggest that individual epitopes of TbpA are both immunogenic and functional with respect to ligand binding capabilities, and the vaccine implications of these findings are discussed.

Vaccine prevention of meningococcal disease, coming soon?

The past century has seen the use of a number of vaccines for prevention and control of meningococcal disease with varied success. The use of polysaccharide vaccines for the control of outbreaks of serogroup C infections in teenagers and young adults and epidemic serogroup A disease has been established for 30 years and an effective protein-polysaccharide conjugate vaccine against serogroup C was introduced into the UK infant immunisation schedule in 2000. The next generation of these glycoconjugate vaccines will be on the shelf soon, eventually offering the prospect of eradication of serogroups A, C, Y and W135 through routine infant immunisation. Despite these exciting prospects, serogroup B meningococci still account for a majority of infections in industrialised nations but development of safe, immunogenic and effective serogroup B meningococcal vaccines has been an elusive goal. Outer membrane vesicle vaccines for B disease are already used in some countries, and will likely be used more widely in the next few years, but efficacy for endemic disease in children has so far been disappointing. However, the innovations arising from the availability of the meningococcal genome sequence, public and scientific interest in the disease and recent pharmaceutical company investment in development of serogroup B vaccines may have started the countdown to the end of meningococcal infection in children.

Production of Neisseria meningitidis transferrin-binding protein B by recombinant Bordetella pertussis

Neisseria meningitidis serogroup B infections are among the major causes of fulminant septicemia and meningitis, especially severe in young children, and no broad vaccine is available yet. Because of poor immunogenicity of the serogroup B capsule, many efforts are now devoted to the identification of protective protein antigens. Among those are PorA and, more recently, transferrin-binding protein B (TbpB). In this study, TbpB of N. meningitidis was genetically fused to the N-terminal domain of the Bordetella pertussis filamentous hemagglutinin (FHA), and the fha-tbpB hybrid gene was expressed in B. pertussis either as a plasmid-borne gene or as a single copy inserted into the chromosome. The hybrid protein was efficiently secreted by the recombinant strains, despite its large size, and was recognized by both anti-FHA and anti-TbpB antibodies. A single intranasal administration of recombinant virulent or pertussis-toxin-deficient, attenuated B. pertussis to mice resulted in the production of antigen-specific systemic immunoglobulin G (IgG), as well as local IgG and IgA. The anti-TbpB serum antibodies were of the IgG1, IgG2a, and IgG2b isotypes and were found to express complement-mediated bactericidal activity against N. meningitidis. These observations indicate that recombinant B. pertussis may be a promising vector for the development of a mucosal vaccine against serogroup B meningococci.

Genetic isolation of meningococci of the electrophoretic type 37 complex

Neisseria meningitidis (the meningococcus) is a naturally competent bacterial species in which intra- and interspecific horizontal gene transfer is a major source of genetic diversity. In strains of the electrophoretic type 37 (ET-37) complex and of the A4 cluster, we identified genomic DNA coding for a novel restriction-modification system and for the tail of a previously unidentified prophage. Furthermore, a novel 7.2-kb DNA segment restricted to clones of the ET-37 complex and the A4 cluster was isolated and shown to occur both as a plasmid (pJS-B) and as a chromosomal integration. Neither the genomic loci nor pJS-B was present in ET-5 complex, lineage 3, or serogroup A meningococci. The differential distribution of the DNA segments described herein, as well as of opcA, porB, nmeAI, nmeBI, and nmeDI described previously, supports the concept of genetic isolation of hypervirulent lineages responsible for most cases of serogroup C disease worldwide.