Vaccine prevention of meningococcal disease, coming soon?

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.

Size determination of bacterial capsular oligosaccharides used to prepare conjugate vaccines against Neisseria meningitidis groups Y and W135

The glycoconjugate vaccines against Neisseria meningitidis groups Y and W135 consist of pools of selected oligosaccharides conjugated to the protein carrier (CRM197). Consistent production of these vaccines requires control and thus determination of the average degree of polymerisation of the oligosaccharides used for conjugation. Acid hydrolysis generates group Y and W135 oligosaccharides with N-acetylneuraminic acid at the reducing end. A method, involving NaBH4 reduction and quantification of this terminal N-acetylneuraminic acid by use of high performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) following acid hydrolysis (2M TFA), was developed. The average degree of polymerisation is calculated from the ratio of reduced N-acetylneuraminic acid to total N-acetylneuraminic acid. The assay was qualified by application to group C, Y and W135 oligosaccharide standards characterised by liquid chromatography, mass and NMR spectroscopy.

Meningococcal polysaccharide-protein conjugate vaccines

It is now 5 years since the UK became the first country to introduce the serogroup C meningococcal polysaccharide-protein conjugate vaccines (MenC) into its routine immunisation schedule. This article reviews the global use of MenC with particular reference to the range of immunisation strategies used internationally. To date, concerns that MenC may result in an increase in meningococcal disease due to non-C serogroups have not been realised. The vaccine has proved to be highly safe and effective; however, concerns have arisen regarding the duration of vaccine effectiveness. Although booster doses of MenC may potentially extend the duration of protection offered by the vaccine, there are, as yet, no studies assessing this option. Clinical trials are underway to assess new combination conjugate vaccines (containing A, C, Y, and W polysaccharides), and it is probable that these more broadly protective vaccines will become available in the near future.

Synthesis of structures corresponding to the capsular polysaccharide of Neisseria meningitidis group A

Four differently substituted trimers of the CPS repeating unit have been synthesised in order to investigate the dependence on oligosaccharide size, acetylation and mode of phosphorylation of glycoconjugate vaccines against Neisseria meningitidis group A. A spacer-containing starting monomer, a H-phosphonate elongating monomer and a 6-O-phosphorylated H-phosphonate cap monomer have been synthesised and coupled together to afford, after deprotection, the target trimer structures differing in their acetylation and phosphorylation substitution pattern.

Expression of heterologous antigens in commensal Neisseria spp.: preservation of conformational epitopes with vaccine potential

Commensal neisseriae share with Neisseria meningitidis (meningococcus) a tendency towards overproduction of the bacterial outer envelope, leading to the formation and release during growth of outer membrane vesicles (OMVs). OMVs from both meningococci and commensal neisseriae have shown promise as vaccines to protect against meningococcal disease. We report here the successful expression at high levels of heterologous proteins in commensal neisseriae and the display, in its native conformation, of one meningococcal outer membrane protein vaccine candidate, NspA, in OMVs prepared from such a recombinant Neisseria flavescens strain. These NspA-containing OMVs conferred protection against otherwise lethal intraperitoneal challenge of mice with N. meningitidis serogroup B, and sera raised against them mediated opsonophagocytosis of meningococcal strains expressing this antigen. This development promises to facilitate the design of novel vaccines containing membrane protein antigens that are otherwise difficult to present in native conformation that provide cross-protective efficacy in the prevention of meningococcal disease.

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.

PCR-based assay for detection of Neisseria meningitidis capsular serogroups 29E, X, and Z

PCR-based assays for the identification of Neisseria meningitidis serogroups 29E, X, and Z by detection of specific regions of the ctrA gene are described. The specificities of these assays were confirmed using serogroups A, B, C, 29E, H, W135, X, Y, and Z and nongroupable meningococcal isolates.

Selection and Characterization of Cyclic Peptides that Bind to a Monoclonal Antibody Against Meningococcal L3,7,9 lipopolysaccharides

There is still no general vaccine for prevention of disease caused by group-B meningococcal strains. Meningococcal lipopolysaccharides (LPSs) have received attention as potential vaccine candidates, but concerns regarding their safety have been raised. Peptide mimics of LPS epitopes may represent safe alternatives to immunization with LPS. The monoclonal antibody (MoAb) 9-2-L3,7,9 specific for Neisseria meningitidis LPS immunotype L3,7,9 is bactericidal and does not cross-react with human tissue. To explore the possibility of isolating peptide mimics of the epitope recognized by MoAb 9-2-L3,7,9, we have constructed two phage display libraries of six and nine random amino acids flanked by cysteines. Furthermore, we developed a system for the easy exchange of peptide-encoding sequences from the phage-display system to a hepatitis B core (HBc) expression system. Cyclic peptides that specifically bound MoAb 9-2-L3,7,9 at a site overlapping with the LPS-binding site were selected from both libraries. Three out of four tested peptides which reacted with MoAb 9-2-L3,7,9 were successfully presented as fusions to the immunodominant loop of HBc particles expressed in Escherichia coli. However, both peptide conjugates to keyhole limpet haemocyanin and HBc particle fusions failed to give an anti-LPS response in mice.

Opportunities for Control of Meningococcal Disease in the United States

The United States currently has relatively low rates of meningococcal disease caused by Neisseria meningitidis. Serogroups Y, C, and B are most common. Although most cases are sporadic, a minority are associated with outbreaks. Pediatric populations have disproportionately higher rates of disease, but nearly two thirds of all cases occur in persons aged 15 years and older. The major challenge to control of domestic meningococcal disease is the absence of a vaccine to prevent sporadic cases spanning many age groups. The quadrivalent A/C/Y/W-135 meningococcal polysaccharide vaccine is licensed in the United States, but because of its limited efficacy in children under two years of age, it is recommended for high-risk groups and outbreak response rather than routine childhood immunization. New conjugate meningococcal vaccines have successfully reduced endemic disease in the United Kingdom, and similar vaccines promise to have a dramatic impact on the burden of meningococcal disease in the United States.

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.

Infection with an avirulent phoP mutant of Neisseria meningitidis confers broad cross-reactive immunity

Successful vaccines against serogroup A and C meningococcal strains have been developed, but current serogroup B vaccines provide protection against only a limited range of strains. The ideal meningococcal vaccine would provide cross-reactive immunity against the variety of strains that may be encountered in any community, but it is unclear whether the meningococcus possesses immune targets that have the necessary level of cross-reactivity. We have generated a phoP mutant of the meningococcus by allele exchange. PhoP is a component of a two-component regulatory system which in other bacteria is an important regulator of virulence gene expression. Inactivation of the PhoP-PhoQ system in Salmonella leads to avirulence, and phoP mutants have been shown to confer protection against virulent challenge. These mutants have been examined as potential live attenuated vaccines. We here show that a phoP mutant of the meningococcus is avirulent in a mouse model of infection. Moreover, infection of mice with the phoP mutant stimulated a bactericidal immune response that not only killed the infecting strain but also showed cross-reactive bactericidal activity against a range of strains with different serogroup, serotype, and serosubtyping antigens. Sera from the mutant-infected mice contained immunoglobulin G that bound to the surface of a range of meningococcal strains and mediated opsonophagocytosis of meningococci by human phagocytic cells. The meningococcal phoP mutant is thus a candidate live, attenuated vaccine strain and may also be used to identify cross-reactive protective antigens in the meningococcus.

Water accessibility, aggregation, and motional features of polysaccharide-protein conjugate vaccines

A relaxometric investigation of a nontoxic mutant of diphtheria toxin and of its conjugates with capsular polysaccharides of different groups of Neisseria meningitidis was performed. The insertion of polysaccharides chains alters dramatically the hydrodynamic properties of the protein. The model-free analysis of the (1)H nuclear magnetic relaxation dispersion profiles of their water solutions shows: i), a reduced protein hydration with respect to the carrier protein alone; ii), a much larger flexibility of the conjugates with respect to a compact macromolecule of the same molecular weight; and iii), a strong tendency to aggregate. The above findings are largely independent on the nature of the polysaccharide and thus provide a fairly general picture of the dynamic properties of glycoconjugate proteins.

Vaccines against bacterial meningitis

Meningitis remains an important cause of morbidity and mortality among children >5 years of age and is especially prevalent in developing countries. Effective routine immunization against Hib, pneumococcus and serogroupC meningococcus has had a significant impact on both invasive disease and carriage caused by these encapsulated bacteria. The major challenge in prevention of meningitis remains the delivery of vaccines worldwide, especially to resource-poor regions with the greatest disease burden.

Critical analysis of old and new vaccines against N. meningitidis serogroup C, considering the meningococcal disease epidemiology in Brazil

Worldwide, the impact of meningococcal disease is substantial, and the potential for the introduction and spread of more virulent strains of N. meningitidis or strains with increased resistance to current antibiotics causes concern, making prevention essential.

OBJECTIVES

Review the indications for meningococcal disease vaccines, considering the epidemiological status in Brazil.

METHODS

A critical literature review on this issue using the Medline and Lilacs databases.

RESULTS

In Brazil, MenB and MenC were the most important serogroups identified in the 1990s. Polysaccharide vaccines available against those serogroups can offer only limited protection for infants, the group at highest risk for meningococcal disease. Additionally, polysaccharide vaccines may induce a hypo-responsive state to MenC. New meningococcal C conjugate vaccines could partially solve these problems, but it is unlikely that in the next few years a vaccine against MenB that can promote good protection against multiple strains of MenB responsible for endemic and epidemic diseases will become available.

CONCLUSIONS

In order to make the best decision about recommendations on immunization practices, better quality surveillance data are required. In Brazil, MenC was responsible for about 2,000 cases per year during the last 10 years. New conjugate vaccines against MenC are very effective and immunogenic, and they should be recommended, especially for children less than 5 years old. Polysaccharide vaccines should be indicated only in epidemic situations and for high-risk groups. Until new vaccines against MenC and MenB are available for routine immunization programs, the most important measure for controlling meningococcal disease is early diagnosis of these infections in order to treat patients and to offer chemoprophylaxis to contacts.

The future of meningitis vaccines

Without effective vaccines meningitis remains a substantial worldwide threat with major health-care implications. A number of advances have been made in vaccine design and implementation over the last decade, with new vaccine initiatives providing substantial promise for the future reduction of global disease burden.

Serotype distribution, antibiotic susceptibility, and genetic relatedness of Neisseria meningitidis strains recently isolated in Italy

The availability of new polysaccharide-protein conjugate vaccines against Neisseria meningitidis serogroup C prompted European National Health authorities to carefully monitor isolate characteristics. In Italy, during 1999-2001, the average incidence was 0.4 cases per 100,000 inhabitants. Serogroup B was predominant and accounted for 75% of the isolates, followed by serogroup C with 24%. Serogroup C was isolated almost twice as frequently in cases of septicemia than in cases of meningitis, and the most common phenotypes were C:2a:P1.5 and C:2b:P1.5. Among serogroup B meningococci, the trend of predominant phenotypes has changed from year to year, with a recent increase in the frequency of B:15:P1.4. Only a few meningococci had decreased susceptibility to penicillin, and, in the penA gene, all of these strains had exogenous DNA blocks deriving from the DNA of commensal Neisseria flavescens, Neisseria cinerea, and Neisseria perflava/sicca. Fluorescent amplified fragment-length polymorphism analysis revealed the nonclonal nature of the strains with decreased susceptibility to penicillin.

W135 in Africa: origins, problems and perspectives

Serogroup A meningococci have been the major cause of epidemic meningococcal disease in Africa throughout the last 100 years. The reasons for this unusual pattern of behaviour have remained unclear and there remain significant debates and logistic difficulties around the appropriate use of plain A/C polysaccharide vaccination to control African meningococcal disease. Since the Hajj pilgrimage of 2000 serogroup W135 organisms (of the ST-11 clonal complex) have emerged as a further significant cause of epidemic meningococcal disease in Africa. Whilst advances in molecular biological and genetic techniques have yielded increasing insights into meningococcal epidemiology there remain many unanswered questions about the reason for the emergence of a serogroup W135 clone capable of epidemic behaviour and in particular its relation to past use of group A/C polysaccharide. The high cost and short supply of quadrivalent (A,C,Y, W135) vaccine to protect against W135 disease has added to what was already the significant burden of controlling serogroup A meningococcal disease. The ability of virulent meningococcal clones to acquire new capsule types raises further concerns about the future nature of meningococcal disease in Africa and the strategies of vaccination use and development necessary to contain it.

La inmunización activa frente a Neisseria meningitidis serogrupo B

Serogroup B Neisseria meningitidis causes high morbidity and mortality rates over the world. This article reviews the current vaccination strategies against this microorganism, including vaccines already tested on a large scale, particularly those based on class 1 outer membrane proteins, and vaccines in different stages of development. The latter involve several approaches, such as modification of the polysaccharide capsule composition or conjugation with proteins, and the use of recombinant DNA techniques to obtain vaccines that express the prevalent sero-subtypes in a particular geographical area. The challenges that have emerged with the sequencing of the meningococcus B genome are also addressed.

Safety and efficacy of meningococcal group C conjugate vaccines

The UK was the first country to implement a universal vaccination programme with conjugate polysaccharide vaccines against Neisseria meningitidis group C. This article reviews the pre- and postlicensure data on their efficacy and safety 3 years after the introduction of the programme. Local reactogenicity data compare favourably with other routine vaccinations and no specific increase in adverse reactions has been associated with their use in infant vaccination programmes. Self-limiting systemic reactions such as fever, myalgia, headaches and irritability have commonly been observed in prelicensure studies. Passive postlicensure safety monitoring of suspected adverse reactions has identified a large number of reports, generally of reactions deemed non-serious and reversible. An Expert Working Group has concluded the balance of benefits and risks to be overwhelmingly favourable. Further safety data are expected from large data-linkage studies. Present efficacy estimates, based on active surveillance of case numbers, vaccine failures and coverage rates, are approximately 90% for all age groups. A significant fall in the number of cases attributable to meningococcal group C infection has been observed in the age group of < 20 years. The annual number of fatalities from confirmed meningococcal C disease in the same population has fallen from 67 to 5 cases within a 2-year period.

Use and validation of NMR assays for the identity and O-acetyl content of capsular polysaccharides from Neisseria meningitidis used in vaccine manufacture

We describe a validated NMR (nuclear magnetic resonance) spectroscopic assay for the identity of the capsular polysaccharides (CPSs) from Neisseria meningitidis Groups A, C, W135 and Y used in vaccine manufacture, and to determine the proportion of residues carrying an O-acetyl substituent. Proof of structural identity and quantitation of the O-acetyl content are key control parameters for these vaccines. The meningococcal CPSs have variable levels of O-acetylation, present at multiple sites in the repeat unit, leading to complex NMR spectra. Base-catalysed de-O-acetylation of the Groups A, C, W135 and Y CPSs yields simplified and reproducible spectra suitable for comparison with reference data. The degree of O-acetylation of the original CPS can be determined by integration of the acetate anion resonance and a suitable resonance from the saccharide moiety. The assay was validated using 46 independent samples from five manufacturers, and is shown to be robust and reproducible.

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.