A novel mimetic antigen eliciting protective antibody to Neisseria meningitidis

Inhibition of the alternative pathway of nonhuman infant complement by porin B2 contributes to virulence of Neisseria meningitidis in the infant rat model

Neisseria meningitidis utilizes capsular polysaccharide, lipooligosaccharide (LOS) sialic acid, factor H binding protein (fHbp), and neisserial surface protein A (NspA) to regulate the alternative pathway (AP) of complement. Using meningococcal mutants that lacked all four of the above-mentioned molecules (quadruple mutants), we recently identified a role for PorB2 in attenuating the human AP; inhibition was mediated by human fH, a key downregulatory protein of the AP. Previous studies showed that fH downregulation of the AP via fHbp or NspA is specific for human fH. Here, we report that PorB2-expressing quadruple mutants also regulate the AP of baby rabbit and infant rat complement. Blocking a human fH binding region on PorB2 of the quadruple mutant of strain 4243 with a chimeric protein that comprised human fH domains 6 and 7 fused to murine IgG Fc enhanced AP-mediated baby rabbit C3 deposition, which provided evidence for an fH-dependent mechanism of nonhuman AP regulation by PorB2. Using isogenic mutants of strain H44/76 that differed only in their PorB molecules, we confirmed a role for PorB2 in resistance to killing by infant rat serum. The PorB2-expressing strain also caused higher levels of bacteremia in infant rats than its isogenic PorB3-expressing counterpart, thus providing a molecular basis for increased survival of PorB2 isolates in this model. These studies link PorB2 expression with infection of infant rats, which could inform the choice of meningococcal strains for use in animal models, and reveals, for the first time, that PorB2-expressing strains of N. meningitidis regulate the AP of baby rabbits and rats.

Meningococcal serogroup B infections: a search for a broadly protective vaccine

Meningococcal disease is mainly caused by serogroup B in many West European countries. Recently, a highly efficacious vaccine against infections caused by serogroup C has been introduced in the UK and The Netherlands. However, an effective vaccine against serogroup B has not yet become available. Outer membrane vesicle vaccines against serogroup B were previously tested in large Phase III trials but showed a low efficacy in young children. In addition, the high variability of the vaccines' main component, porin A, potentially diminishes its efficacy. Therefore, several approaches in either optimizing these outer membrane vesicle vaccines or searching for novel, highly conserved antigens are currently under investigation. The sequencing of the meningococcal genome has provided new opportunities to detect additional immunogenic epitopes. In this review, the developments in the search for a broadly protective meningococcal serogroup B vaccine will be discussed.

Transcriptional regulation of the nadA gene in Neisseria meningitidis impacts the prediction of coverage of a multicomponent meningococcal serogroup B vaccine

The NadA adhesin is a major component of 4CMenB, a novel vaccine to prevent meningococcus serogroup B (MenB) infection. Under in vitro growth conditions, nadA is repressed by the regulator NadR and poorly expressed, resulting in inefficient killing of MenB strains by anti-NadA antibodies. Interestingly, sera from children infected with strains that express low levels of NadA in laboratory growth nevertheless recognize the NadA antigen, suggesting that NadA expression during infection may be different from that observed in vitro. In a strain panel covering a range of NadA levels, repression was relieved through deleting nadR. All nadR knockout strains expressed high levels of NadA and were efficiently killed by sera from subjects immunized with 4CMenB. A selected MenB strain, NGP165, mismatched for other vaccine antigens, is not killed by sera from immunized infants when the strain is grown in vitro. However, in an in vivo passive protection model, the same sera effectively protected infant rats from bacteremia with NGP165. Furthermore, we identify a novel hydroxyphenylacetic acid (HPA) derivative, reported by others to be produced during inflammation, which induces expression of NadA in vitro, leading to efficient antibody-mediated killing. Finally, using bioluminescent reporters, nadA expression in the infant rat model was induced in vivo at 3 h postinfection. Our results suggest that during infectious disease, NadR repression is alleviated due to niche-specific signals, resulting in high levels of NadA expression from any nadA-positive (nadA(+)) strain and therefore efficient killing by anti-NadA antibodies elicited by the 4CMenB vaccine.

The lytic transglycosylases of Neisseria gonorrhoeae

Neisseria gonorrhoeae encodes five lytic transglycosylases (LTs) in the core genome, and most gonococcal strains also carry the gonococcal genetic island that encodes one or two additional LTs. These peptidoglycan (PG)-degrading enzymes are required for a number of processes that are either involved in the normal growth of the bacteria or affect the pathogenesis and gene transfer aspects of this species that make N. gonorrhoeae highly inflammatory and highly genetically variable. Systematic mutagenesis determined that two LTs are involved in producing the 1,6-anhydro PG monomers that cause the death of ciliated cells in Fallopian tubes. Here, we review the information available on these enzymes and discuss their roles in bacterial growth, cell separation, autolysis, type IV secretion, and pathogenesis.

Polypeptide folding on a conformational-space network: dependence of network topology on the structural discretization procedure

Mapping the conformational space of a polypeptide onto a network of conformational states involves a number of subjective choices, mostly in relation to the definition of conformation and its discrete nature in a network framework. Here, we evaluate the robustness of the topology of conformational-space networks derived from Molecular Dynamics (MD) simulations with respect to the use of different discretization (clustering) methods, variation of their parameters, simulation length and analysis time-step, and removing high-frequency motions from the coordinate trajectories. In addition, we investigate the extent to which polypeptide dynamics can be reproduced on the resulting networks when assuming Markovian behavior. The analysis is based on eight 500 ns and eight 400 ns MD simulations in explicit water of two 10-residue peptides. Three clustering algorithms were used, two of them based on the pair-wise root-mean-square difference between structures and one on dihedral-angle patterns. A short characteristic path length and a power-law behavior of the probability distribution of the node degree are obtained irrespective of the clustering method or the value of any of the tested parameters. The average cliquishness is consistently one or two orders of magnitude larger than that of a random realization of a network of corresponding size and connectivity. The cliquishness as function of node degree and the kinetic properties of the networks are found to be most dependent on clustering method and/or parameters. Although Markovian simulations on the networks reproduce cluster populations accurately, their kinetic properties most often differ from those observed in the MD simulations.

Influence of serogroup B meningococcal vaccine antigens on growth and survival of the meningococcus in vitro and in ex vivo and in vivo models of infection

A novel vaccine against serogroup B meningococcal disease - containing a combination of protein antigens identified by reverse vaccinology: fHBP fused to GNA2091, GNA2132 fused to GNA1030, and NadA - is currently in Phase III clinical trials. In order to determine the role of these antigens in the growth, survival and fitness of the meningococcus, we generated a mutant lacking the expression of all five protein antigens (5KO), a mutant lacking the three main antigens (fHBP, GNA2132 and NadA; 3KO), as well as strains lacking the single antigens. Our results show that abrogation of expression of these antigens in Neisseria meningitidis results in reduced growth in vitro, increased sensitivity of the bacterium to stresses it may encounter in the host, as well as reduced fitness in ex vivo models of infection and in an in vivo infant rat competitive index assay. These results support a multivalent vaccine approach, which was undertaken to strengthen the protective activity of the vaccine antigens, increase the breadth of MenB strains targeted by the vaccine, and limit the potential for selection of vaccine escape mutants.

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.

The RNA chaperone Hfq is involved in stress response and virulence in Neisseria meningitidis and is a pleiotropic regulator of protein expression

The well-conserved protein Hfq has emerged as the key modulator of riboregulation in bacteria. This protein is thought to function as an RNA chaperone and to facilitate base pairing between small regulatory RNA (sRNA) and mRNA targets, and many sRNAs are dependent on the Hfq protein for their regulatory functions. To address the possible role of Hfq in riboregulated circuits in Neisseria meningitidis, we generated an Hfq mutant of the MC58 strain, and the knockout mutant has pleiotropic phenotypes; it has a general growth phenotype in vitro in culture media, and it is sensitive to a wide range of stresses, including those that it may encounter in the host. Furthermore, the expression profile of a vast number of proteins is clearly altered in the mutant, and we have identified 27 proteins by proteomics. All of the phenotypes tested to date are also restored by complementation of Hfq expression in the mutant strain. Importantly, in ex vivo and in vivo models of infection the Hfq mutant is attenuated. These data indicate that Hfq plays a key role in stress response and virulence, and we propose a major role for Hfq in regulation of gene expression. Moreover, this study suggests that in meningococcus there is a large Hfq-mediated sRNA network which so far is largely unexplored.

Identification by genomic immunization of a pool of DNA vaccine candidates that confer protective immunity in mice against Neisseria meningitidis serogroup B

We have shown previously that expression library immunization is viable alternative approach to induce protective immunity against Neisseria meningitidis serogroup B. In this study we report that few rounds of library screening allow identification of protective pools of defined antigens. A previously reported protective meningococcal library (L8, with 600 clones) was screened and two sub-libraries of 95 clones each were selected based on the induction of bactericidal and protective antibodies in BALB/c mice. After sequence analysis of each clone within these sub-libraries, we identified a pool of 20 individual antigens that induced protective immune responses in mice against N. meningitidis infection, and the observed protection was associated with the induction of bactericidal antibodies. Our studies demonstrate for the first time that ELI combined with sequence analysis is a powerful and efficient tool for identification of candidate antigens for use in a meningococcal vaccine.

Role of FNR and FNR-regulated, sugar fermentation genes in Neisseria meningitidis infection

While it is generally accepted that anaerobic metabolism is required during infection, supporting experimental data have only been described in a limited number of studies. To provide additional evidence on the role of anaerobic metabolism in bacterial pathogens while invading mammalian hosts, we analysed the effect of the inactivation of FNR, the major regulatory protein involved in the adaptation to oxygen restrictive conditions, and of two of the FNR-regulated genes on the survival of Neisseria meningitidis serogroup B (MenB) in vivo. We found that fnr deletion resulted in more than 1 log reduction in the meningococcal capacity to proliferate both in infant rats and in mice. To identify which of the FNR-regulated genes were responsible for this attenuated phenotype, we defined the FNR regulon by combining DNA microarray analysis and FNR-DNA binding studies. Under oxygen-restricted conditions, FNR positively controlled the transcription of nine transcriptional units, the most upregulated of which were the two operons NMB0388-galM and mapA-pgmbeta implicated in sugar metabolism and fermentation. When galM and mapA were knocked out, the mutants were attenuated by 2 and 3 logs respectively. As the operons are controlled by FNR, from these data we conclude that MenB survival in the host anatomical sites where oxygen is limiting is supported by sugar fermentation.

A universal vaccine for serogroup B meningococcus

Meningitis and sepsis caused by serogroup B meningococcus are two severe diseases that still cause significant mortality. To date there is no universal vaccine that prevents these diseases. In this work, five antigens discovered by reverse vaccinology were expressed in a form suitable for large-scale manufacturing and formulated with adjuvants suitable for human use. The vaccine adjuvanted by aluminum hydroxide induced bactericidal antibodies in mice against 78% of a panel of 85 meningococcal strains representative of the global population diversity. The strain coverage could be increased to 90% and above by the addition of CpG oligonucleotides or by using MF59 as adjuvant. The vaccine has the potential to conquer one of the most devastating diseases of childhood.

Reverse vaccinology — In search of a genome-derived meningococcal vaccine

Although significant advances have been made toward the control of bacterial meningitis in children with the development of capsular polysaccharide protein conjugate vaccines, this approach has proven problematic for the serogroup B meningococcus. Non-capsular vaccines based upon outer membrane vesicles of Neisseria meningitidis have been useful in control of clonal serogroup B outbreaks, although due to variability of PorA, these vaccines may be less useful in control of endemic disease. Genome-based vaccine discovery was evaluated in an attempt to produce a candidate capable of conferring a broadly protective vaccine against a diversity of meningococcal B strains.

Proteomic analysis of Neisseria lactamica and N eisseria meningitidis outer membrane vesicle vaccine antigens

Vaccines to prevent meningococcal disease have been developed from the outer membrane vesicles (OMVs) of Neisseria meningitidis and the related commensal organism Neisseria lactamica. In addition to lipopolysaccharide and the major porins, these vaccines contain a large number of proteins that are incompletely characterised. Here we describe comparative proteomic analyses of the N. lactamica OMV vaccine and OMVs from a serogroup B strain of N. meningitidis. Tandem mass-spectrometry data for trypsinised N. lactamica OMV vaccine were matched to an incompletely assembled genome sequence from the same strain to give 65 robust protein identifications and a further 122 single- or two-peptide matches. Fifty-seven N. meningitidis K454 proteins were identified robustly (and a further 68 from single- or two-peptide matches) by inference from the N. meningitidis MC58 genome. The results suggest that OMVs have a hitherto unappreciated complexity and pinpoint novel candidate antigens for further characterisation.

The immunogenicity of a conformationally restricted peptide mimetic of meningococcal lipooligosaccharide

Life-threatening meningitis and septicaemia caused by Neisseria meningitidis are a public health priority, and their prevention by vaccination is a major objective. Meningococcal capsular polysaccharide-based vaccines are effective against the major invasive serogroups, except for serogroup B, the capsule of which mimics human polysaccharides and is poorly immunogenic. An alternative vaccine candidate that has the potential to offer cross-protection against antigenically diverse meningococci is the lipooligosaccharide (LOS). The structurally constrained peptide mimetic, C22, of a bactericidal antibody epitope within LOS was previously shown to elicit cross-reactive antibodies to meningococcal LOS when complexed to NeutrAvidintrade mark as a carrier protein. The immunogenicity of this antigen in H-2(d) (BALB/c) and H-2(k) (C3H/HeN) haplotype mice was further investigated. Anti-LOS immunoglobulin G (IgG) antibody titres increased with the vaccine dose and correlated with the anti-C22 peptide antibody titres in both haplotypes. Antigen-stimulated Th1/Th2 cytokine secretion by splenocytes and antibody isotypes indicated a Th2-type immune response with IgG1 antibodies and a low titre of IgG2b. There was no serum bactericidal activity observed against the meningococcus.

Towards an improved serogroup BNeisseria meningitidisvaccine

Meningococcal disease, presenting primarily as septicaemia and meningitis, continues to be a devastating problem around the world. Over the last century, vaccine development has been undertaken in earnest for the prevention of this disease. Polysaccharide vaccines have been available for almost 40 years, yet they are poorly immunogenic in young children who are at the highest risk. Since their introduction into some routine immunisation schedules in 1999, polysaccharide-protein conjugate vaccines for the prevention of serogroup C meningococcal infection have proven efficacious. A quadrivalent polysaccharide-protein conjugate vaccine against serogroups A, C, W135 and Y, which is being introduced in the US this year, is hoped to control disease caused by these serogroups. To date, however, the development of a universally safe, immunogenic and effective serogroup B Neisseria meningitidis vaccine has remained a challenge. This review details the many conventional vaccine strategies and the more recent genome-derived technological approaches being used in serogroup B vaccine development. The future prevention of serogroup B disease will rely on both outer membrane vesicle vaccines being used for serosubtype-specific outbreaks and new vaccines containing multiple other antigens. Investment by the pharmaceutical industry in preclinical research and development provides hope that an efficacious serogroup B meningococcal vaccine can be developed.

T-cell-stimulating protein A elicits immune responses during meningococcal carriage and human disease

In recognition of the need for immunological memory-inducing components for future Neisseria meningitidis group B vaccines, we previously searched the proteome of N. meningitidis and identified T-cell-stimulating protein A (TspA). This study was designed to confirm the immunogencity of TspA and to examine the subset of T-helper cell responses to the protein in patients and nasopharyngeal carriers. The tspA gene was reconstructed, cloned, and expressed in Escherichia coli, and the recombinant TspA (rTspA) protein was affinity purified. T-cell proliferative responses to rTspA were detected in the peripheral blood mononuclear cells (PBMCs) of convalescent patients and carriers, confirming that TspA-specific T-cell responses were stimulated by invasive disease and nasopharyngeal colonization. Following stimulation of PBMCs with meningococcal lysate, increased frequencies of both Th1 and Th2 cells were observed, indicating that, as during carriage, invasive meningococcal disease induced an unbiased T-helper subset response. A similar unbiased T-helper response was also detected against rTspA in the PBMCs of convalescent patients. The response of PBMCs from the carriers to TspA stimulation, however, was very weak, and the frequencies of cytokine-positive CD4 cells were not significantly greater than the frequencies in unstimulated control cultures. All of the patients and carriers responded with serum antimeningococcal immunoglobulin G (IgG) antibodies, while four of six samples from patients and 5 of 14 samples from carriers contained detectable anti-rTspA IgG antibodies. Taken together, the results of this study confirmed the immunogenicity of TspA in humans during natural meningococcal infection, and therefore, TspA is worthy of further investigation as a possible T-cell stimulating component of future vaccines.

Protective antibody responses elicited by a meningococcal outer membrane vesicle vaccine with overexpressed genome-derived neisserial antigen 1870

Background. Meningococcal outer membrane vesicle (OMV) vaccines are efficacious in humans but have serosubtype-specific serum bactericidal antibody responses directed at the porin protein PorA and the potential for immune selection of PorA-escape mutants.Methods. We prepared an OMV vaccine from a Neisseria meningitidis strain engineered to overexpress genome-derived neisserial antigen (GNA) 1870, a lipoprotein discovered by genome mining that is being investigated for use in a vaccine.Results. Mice immunized with the modified GNA1870-OMV vaccine developed broader serum bactericidal antibody responses than control mice immunized with a recombinant GNA1870 protein vaccine or an OMV vaccine prepared from wild-type N. meningitidis or a combination of vaccines prepared from wild-type N. meningitidis and recombinant protein. Antiserum from mice immunized with the modified GNA1870-OMV vaccine also elicited greater deposition of human C3 complement on the surface of live N. meningitidis bacteria and greater passive protective activity against meningococcal bacteremia in infant rats. A N. meningitidis mutant with decreased expression of PorA was more susceptible to bactericidal activity of anti-GNA1870 antibodies.Conclusions. The modified GNA1870-OMV vaccine elicits broader protection against meningococcal disease than recombinant GNA1870 protein or conventional OMV vaccines and also has less risk of selection of PorA-escape mutants than a conventional OMV vaccine.

The region comprising amino acids 100 to 255 of Neisseria meningitidis lipoprotein GNA 1870 elicits bactericidal antibodies

GNA 1870 is a novel surface-exposed lipoprotein, identified by genome analysis of Neisseria meningitidis strain MC58, which induces bactericidal antibodies. Three sequence variants of the protein were shown to be sufficient to induce bactericidal antibodies against a panel of strains representative of the diversity of serogroup B meningococci. Here, we studied the antigenic and immunogenic properties of GNA 1870, which for convenience was divided into domains A, B, and C. The immune responses of mice immunized with each of the three variants were tested using overlapping peptides scanning the entire protein length and using recombinant fragments. We found that while most of the linear epitopes are located in the A domain, the bactericidal antibodies are directed against conformational epitopes located in the BC domain. This was also confirmed by the isolation of a bactericidal murine monoclonal antibody, which failed to recognize linear peptides on the A, B, and C domains separately but recognized a conformational epitope formed only by the combination of the B and C domains. Arginine in position 204 was identified as important for binding of the monoclonal antibody. The identification of the region containing bactericidal epitopes is an important step in the design of new vaccines against meningococci.

Biotechnology and vaccines: application of functional genomics to Neisseria meningitidis and other bacterial pathogens

Since its introduction, vaccinology has been very effective in preventing infectious diseases. However, in several cases, the conventional approach to identify protective antigens, based on biochemical, immunological and microbiological methods, has failed to deliver successful vaccine candidates against major bacterial pathogens. The recent development of powerful biotechnological tools applied to genome-based approaches has revolutionized vaccine development, biological research and clinical diagnostics. The availability of a genome provides an inclusive virtual catalogue of all the potential antigens from which it is possible to select the molecules that are likely to be more effective. Here, we describe the use of "reverse vaccinology", which has been successful in the identification of potential vaccines candidates against Neisseria meningitidis serogroup B and review the use of functional genomics approaches as DNA microarrays, proteomics and comparative genome analysis for the identification of virulence factors and novel vaccine candidates. In addition, we describe the potential of these powerful technologies in understanding the pathogenesis of various bacteria.

Genome-based vaccines

Vaccination is an effective possibility to prevent many bacterial or viral infections, but for several important pathogens still no vaccines are available. The sequences of complete genomes are now decoded for an increasing number of bacterial pathogens and offer the possibility for comprehensive screenings to identify targets for vaccine development. In this article current genomic approaches to identify antigenic proteins of Neisseria meningitidis, Streptococcus pneumoniae, Staphylococcus aureus, and Chlamydia pneumoniae are summarized.

GNA33 of Neisseria meningitidis is a lipoprotein required for cell separation, membrane architecture, and virulence

GNA33 is a membrane-bound lipoprotein with murein hydrolase activity that is present in all Neisseria species and well conserved in different meningococcal isolates. The protein shows 33% identity to a lytic transglycolase (MltA) from Escherichia coli and has been shown to be involved in the degradation of both insoluble murein sacculi and unsubstituted glycan strands. To study the function of the gene and its role in pathogenesis and virulence, a knockout mutant of a Neisseria meningitidis serogroup B strain was generated. The mutant exhibited retarded growth in vitro. Transmission electron microscopy revealed that the mutant grows in clusters which are connected by a continuous outer membrane, suggesting a failure in the separation of daughter cells. Moreover, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of culture supernatant revealed that the mutant releases several proteins in the medium. The five most abundant proteins, identified by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry analysis, belong to the outer membrane protein family. Finally, the mutant showed an attenuated phenotype, since it was not able to cause bacteremia in the infant rat model. We conclude that GNA33 is a highly conserved lipoprotein which plays an important role in peptidoglycan metabolism, cell separation, membrane architecture, and virulence.

Vaccine potential of the Neisseria meningitidis 2086 lipoprotein

A novel antigen that induces cross-reactive bactericidal antibodies against a number of Neisseria meningitidis strains is described. This antigen, a approximately 28-kDa lipoprotein called LP2086, was first observed within a complex mixture of soluble outer membrane proteins (sOMPs) following a series of fractionation, protein purification, and proteomics steps. Approximately 95 different neisserial isolates tested positive by Western blotting and PCR screening methods for the presence of the protein and the gene encoding LP2086. The strains tested included isolates of N. meningitidis serogroups A, B, C, W135, and Y, Neisseria gonorrhoeae, and Neisseria lactamica. To better understand the microheterogeneity of this protein, the 2086 genes from 63 neisserial isolates were sequenced. Two different subfamilies of LP2086 were identified based on deduced amino acid sequence homology. A high degree of amino acid sequence similarity exists within each 2086 subfamily. The highest degree of genetic diversity was seen between the two subfamilies which share approximately 60 to 75% homology at the nucleic acid level. Flow cytometry (fluorescence-activated cell sorting) analyses and electron microscopy indicated that the LP2086 is localized on the outer surface of N. meningitidis. Antiserum produced against a single protein variant was capable of eliciting bactericidal activity against strains expressing different serosubtype antigens. Combining one recombinant lipidated 2086 (rLP2086) variant from each subfamily with two rPorA variants elicited bactericidal activity against all strains tested. The rLP2086 family of antigens are candidates worthy of further vaccine development.

Conformational epitopes recognized by protective anti-neisserial surface protein A antibodies

NspA is a conserved membrane protein that elicits protective antibody responses in mice against Neisseria meningitidis. A recent crystallographic study showed that NspA adopts an eight-stranded beta-barrel structure when reconstituted in detergent. In order to define the segments of NspA-containing epitopes recognized by protective murine anti-NspA antibodies, we studied the binding of two bactericidal and protective anti-NspA monoclonal antibodies (MAbs), AL12 and 14C7. Neither MAb binds to overlapping synthetic peptides (10-mers, 12-mers, and cyclic 12-mers) corresponding to the entire mature sequence of NspA, or to denatured recombinant NspA (rNspA), although binding to the protein can be restored by refolding in liposomes. Based on the ability of the two MAbs to bind to Escherichia coli microvesicles prepared from a set of rNspA variants created by site-specific mutagenesis, the most important contacts between the MAbs and NspA appear to be located within the LGG segment of loop 3. The conformation of loop 2 also appears to be an important determinant, as particular combinations of residues in this segment resulted in loss of antibody binding. Thus, the two anti-NspA MAbs recognize discontinuous conformational epitopes that result from the close proximity of loops 2 and 3 in the three-dimensional structure of NspA. The data suggest that optimally immunogenic vaccines using rNspA will require formulations that permit proper folding of the protein.

Bioinformatics: how it is being used to identify bacterial vaccine candidates

Genomic sequencing has provided a tremendous amount of information that can be useful in vaccine target identification. The sheer volume of information available necessitates the use of new research disciplines and techniques. Using bioinformatics, researchers sift through available data to identify appropriate candidates for biological analysis. This review provides an overview of available bioinformatic techniques for vaccine candidate identification and a few examples of how these techniques are being applied to specific bacterial pathogens.

Vaccination against Neisseria meningitidis using three variants of the lipoprotein GNA1870

Sepsis and meningitis caused by serogroup B meningococcus are devastating diseases of infants and young adults, which cannot yet be prevented by vaccination. By genome mining, we discovered GNA1870, a new surface-exposed lipoprotein of Neisseria meningitidis that induces high levels of bactericidal antibodies. The antigen is expressed by all strains of N. meningitidis tested. Sequencing of the gene in 71 strains representative of the genetic and geographic diversity of the N. meningitidis population, showed that the protein can be divided into three variants. Conservation within each variant ranges between 91.6 to 100%, while between the variants the conservation can be as low as 62.8%. The level of expression varies between strains, which can be classified as high, intermediate, and low expressors. Antibodies against a recombinant form of the protein elicit complement-mediated killing of the strains that carry the same variant and induce passive protection in the infant rat model. Bactericidal titers are highest against those strains expressing high yields of the protein; however, even the very low expressors are efficiently killed. The novel antigen is a top candidate for the development of a new vaccine against meningococcus.

Identification and characterization of genomic loci unique to the Brazilian purpuric fever clonal group of H. influenzae biogroup aegyptius: functionality explored using meningococcal homology

Brazilian purpuric fever (BPF) is a fulminant septicaemic infection of young children, caused by a clonal group of strains of Haemophilus influenzae biogroup aegyptius (Hae), an organism previously solely associated with conjunctivitis. Their special capacity to invade from the initial site of conjunctival infection is unexplained. A polymerase chain reaction (PCR)-amplified subtractive hybridization technique was used to identify genes specific to the BPF clonal group. A copy of bacteriophage HP1 and 46 further chromosomal loci were identified in the BPF but not in the conjunctivitis strain of Hae. Sixteen were characterized further, and one - encoding an analogue of the Legionella pneumophila epithelial cell entry-enhancing protein EnhC - was investigated in depth. Two genes, bpf001 and bpf002, unique to the BPF clonal group were identified between homologues of HI1276 and HI1277 in a complex locus close to H. influenzae genetic island 1, recently identified in pathogenic H. influenzae type b. Bpf001 encodes a protein homologous to EnhC and to the previously uncharacterized product of the meningococcal gene NMB0419. Functional studies of bpf001 proving intractable, NMB0419 was chosen as a surrogate for investigation and shown to modulate bacterial interaction with monolayers of human respiratory epithelial cells, promoting invasion, the first stage (for Hae) in the pathogenesis of BPF.

GNA33 from Neisseria meningitidis serogroup B encodes a membrane-bound lytic transglycosylase (MltA)

In a previous study, we used the genome of serogroup B Meningococcus to identify novel vaccine candidates. One of these molecules, GNA33, is well conserved among Meningococcus B strains, other Meningococcus serogroups and Gonococcus and induces bactericidal antibodies as a result of being a mimetic antigen of the PorA epitope P1.2. GNA33 encodes a 48-kDa lipoprotein that is 34.5% identical with membrane-bound lytic transglycosylase A (MltA) from Escherichia coli. In this study, we expressed GNA33, i.e. Meningococcus MltA, as a lipoprotein in E. coli. The lipoprotein nature of recombinant MltA was demonstrated by incorporation of [3H]palmitate. MltA lipoprotein was purified to homogeneity from E. coli membranes by cation-exchange chromatography. Muramidase activity was confirmed when MltA was shown to degrade insoluble murein sacculi and unsubstituted glycan strands. HPLC analysis demonstrated the formation of 1,6-anhydrodisaccharide tripeptide and tetrapeptide reaction products, confirming that the protein is a lytic transglycosylase. Optimal muramidase activity was observed at pH 5.5 and 37 degrees C and enhanced by Mg2+, Mn2+ and Ca2+. The addition of Ni2+ and EDTA had no significant effect on activity, whereas Zn2+ inhibited activity. Triton X-100 stimulated activity 5.1-fold. Affinity chromatography indicated that MltA interacts with penicillin-binding protein 2 from Meningococcus B, and, like MltA from E. coli, may form part of a multienzyme complex.

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.

Bacterial pathogen genomics and vaccines

Infectious diseases remain a major cause of deaths and disabilities in the world, the majority of which are caused by bacteria. Although immunisation is the most cost effective and efficient means to control microbial diseases, vaccines are not yet available to prevent many major bacterial infections. Examples include dysentery (shigellosis), gonorrhoea, trachoma, gastric ulcers and cancer (Helicobacter pylori). Improved vaccines are needed to combat some diseases for which current vaccines are inadequate. Tuberculosis, for example, remains rampant throughout most countries in the world and represents a global emergency heightened by the pandemic of HIV. The availability of complete genome sequences has dramatically changed the opportunities for developing novel and improved vaccines and facilitated the efficiency and rapidity of their development. Complete genomic databases provide an inclusive catalogue of all potential candidate vaccines for any bacterial pathogen. In conjunction with adjunct technologies, including bioinformatics, random mutagenesis, microarrays, and proteomics, a systematic and comprehensive approach to identifying vaccine discovery can be undertaken. Genomics must be used in conjunction with population biology to ensure that the vaccine can target all pathogenic strains of a species. A proof in principle of the utility of genomics is provided by the recent exploitation of the complete genome sequence of Neisseria meningitidis group B.