Antibody specificities and effect of meningococcal carriage in icelandic teenagers receiving the Norwegian serogroup B outer membrane vesicle vaccine

Outer membrane vesicles as a platform for the discovery of antibodies to bacterial pathogens

Bacterial outer membrane vesicles (OMVs) are nanosized spheroidal particles shed by gram-negative bacteria that contain biomolecules derived from the periplasmic space, the bacterial outer membrane, and possibly other compartments. OMVs can be purified from bacterial culture supernatants, and by genetically manipulating the bacterial cells that produce them, they can be engineered to harbor cargoes and/or display molecules of interest on their surfaces including antigens that are immunogenic in mammals. Since OMV bilayer-embedded components presumably maintain their native structures, OMVs may represent highly useful tools for generating antibodies to bacterial outer membrane targets. OMVs have historically been utilized as vaccines or vaccine constituents. Antibodies that target bacterial surfaces are increasingly being explored as antimicrobial agents either in unmodified form or as targeting moieties for bactericidal compounds. Here, we review the properties of OMVs, their use as immunogens, and their ability to elicit antibody responses against bacterial antigens. We highlight antigens from bacterial pathogens that have been successfully targeted using antibodies derived from OMV-based immunization and describe opportunities and limitations for OMVs as a platform for antimicrobial antibody development. KEY POINTS: • Outer membrane vesicles (OMVs) of gram-negative bacteria bear cell-surface molecules • OMV immunization allows rapid antibody (Ab) isolation to bacterial membrane targets • Review and analysis of OMV-based immunogens for antimicrobial Ab development.

4CMenB vaccine and its role in preventing transmission and inducing herd immunity

INTRODUCTION

: Vaccination is the most effective method of protecting people from invasive meningococcal disease (IMD). Of all the capsular groups, B is the most common cause of invasive meningococcal disease in many parts of the world. Despite this, adolescent meningococcal B vaccine programs have not been implemented globally, partly due to the lack of evidence for herd immunity afforded by meningococcal B vaccines.

AREAS COVERED

This review aims to synthesise the available evidence on recombinant 4CMenB vaccines' ability to reduce pharyngeal carriage and therefore provide indirect (herd) immunity against IMD.

EXPERT OPINION

There is some evidence that the 4CMenB vaccine may induce cross-protection against non-B carriage of meningococci. However, the overall body of evidence does not support a clinically significant reduction in carriage of disease-associated or group B meningococci following 4CMenB vaccination. No additional cost-benefit from herd immunity effects should be included when modelling the cost-effectiveness of 4CMenB vaccine programs against group B IMD. 4CMenB immunisation programs should focus on direct (individual) protection for groups at greatest risk of meningococcal disease.

Future meningococcal B and combination vaccines being developed should consider the impact of the vaccine on carriage as part of their clinical evaluation.

Effect of Vaccine-Elicited Antibodies on Colonization of Neisseria meningitidis Serogroup B and C Strains in a Human Bronchial Epithelial Cell Culture Model

Capsular polysaccharide-protein conjugate vaccines protect individuals from invasive disease and decrease carriage, which reduces spread of the organism in the population. In contrast, antibodies elicited by plain polysaccharide or protein antigen-based meningococcal (Men) vaccines have little or no effect on decreasing carriage. In this study, we investigated the mechanism by which vaccine-induced human immunoglobulin G (IgG) antibodies affect colonization by meningococcal serogroup B (MenB) or C (MenC) strains using a human bronchial epithelial cell culture model (16HBE14o-). Fluorescence microscopy showed that bacteria colonizing the apical side of 16HBE14o- monolayers had decreased capsular polysaccharide on the bacterial surface that resulted from shedding the capsule and not decreased production of polysaccharide. Capsular polysaccharide shedding depended on the presence of 16HBE14o- cells and bacteria but not direct adherence of the bacteria to the cells. Treatment of bacteria and cells with postimmunization MenC-conjugate IgG or murine anti-MenB polysaccharide monoclonal antibodies (MAbs) inhibited capsule shedding, microcolony dispersal, and invasion of the 16HBE14o- cell monolayer. In contrast, the IgG responses elicited by immunization with MenC polysaccharide (PS), MenB outer membrane vesicle (OMV)-based, or factor H binding protein (FHbp)-based vaccines were not different than preimmune IgG or no-treatment response. The results provide new insights on the mechanism by which high-avidity anticapsular antibodies elicited by polysaccharide-conjugate vaccines affect meningococcal colonization. The data also suggest that any effect on colonization by IgG elicited by OMV- or FHbp-based vaccines may involve a different mechanism.

Recent Progress in the Prevention of Serogroup B Meningococcal Disease

The widespread use of meningococcal polysaccharide conjugate vaccines has highlighted the challenge of providing protection against serogroup B disease. Over a period of 4 decades, vaccine development has focused on subcapsular protein antigens, first with outer membrane vesicle (OMV) vaccines against epidemic outbreaks, and more recently on new multicomponent vaccines designed to offer better cross-protection against the antigenically diverse strains responsible for endemic disease. Because of the low incidence of meningococcal disease, the protective efficacy of these vaccines has not been determined in clinical studies, and their licensure has been based on serological data; however, the serological assays used to predict protective coverage have limitations. As a result, evidence of the effectiveness of these vaccines against different strains and the contribution of specific antigens to protection can only be provided by epidemiological analyses following their implementation in sufficiently large populations. The recent inclusion of the four-component meningococcal serogroup B (4CMenB) vaccine, Bexsero, in the infant immunization program in the UK has provided preliminary evidence that the vaccine is effective. Ongoing surveillance will provide valuable data on its longer-term impact and antigenic coverage. Further development of protein-based vaccines against meningococcal disease is anticipated to improve antigenic coverage and adjust to changes in circulating strains. At the same time, alternative immunization strategies may be explored to improve overall vaccine effectiveness by, for example, protecting the youngest infants or providing herd protection.

A novel meningococcal outer membrane vesicle vaccine with constitutive expression of FetA: A phase I clinical trial

Objectives

Outer membrane vesicle (OMV) vaccines are used against outbreaks of capsular group B Neisseria meningitidis (MenB) caused by strains expressing particular PorA outer membrane proteins (OMPs). Ferric enterobactin receptor (FetA) is another variable OMP that induces type-specific bactericidal antibodies, and the combination of judiciously chosen PorA and FetA variants in vaccine formulations is a potential approach to broaden protection of such vaccines.

Methods

The OMV vaccine MenPF-1 was generated by genetically modifying N. meningitidis strain 44/76 to constitutively express FetA. Three doses of 25 μg or 50 μg of MenPF-1 were delivered intra-muscularly to 52 healthy adults.

Results

MenPF-1 was safe and well tolerated. Immunogenicity was measured by serum bactericidal assay (SBA) against wild-type and isogenic mutant strains. After 3 doses, the proportion of volunteers with SBA titres ≥1:4 (the putative protective titre) was 98% for the wild-type strain, and 77% for the strain 44/76 FetA_onPorA_off compared to 51% in the strain 44/76 FetA_offPorA_off, demonstrating that vaccination with MenPF-1 simultaneously induced FetA and PorA bactericidal antibodies.

Conclusion

This study provides a proof-of-concept for generating bactericidal antibodies against FetA after OMV vaccination in humans. Prevalence-based choice of PorA and FetA types can be used to formulate a vaccine for broad protection against MenB disease.

•

MenB OMV vaccines' efficacy is strain-restricted by the variable antigen PorA.

•

FetA is another variable antigen, but has iron-dependent expression.

•

The combination of only a few PorA and FetA can induce broad-protection.

•

A mutated OMV was created containing one PorA and one FetA.

•

FetA induces bactericidal antibody response in addition to the PorA response in a Phase I trial.

Novel blocking human IgG directed against the pentapeptide repeat motifs of Neisseria meningitidis Lip/H.8 and Laz lipoproteins

Ab-initiated, complement-dependent killing contributes to host defenses against invasive meningococcal disease. Sera from nonimmunized individuals vary widely in their bactericidal activity against group B meningococci. We show that IgG isolated from select individuals can block killing of group B meningococci by human sera that are otherwise bactericidal. This IgG also reduced the bactericidal efficacy of Abs directed against the group B meningococcal protein vaccine candidates factor H-binding protein currently undergoing clinical trials and Neisserial surface protein A. Immunoblots revealed that the blocking IgG was directed against a meningococcal Ag called H.8. Killing of meningococci in reactions containing bactericidal mAbs and human blocking Abs was restored when binding of blocking Ab to meningococci was inhibited using either synthetic peptides corresponding to H.8 or a nonblocking mAb against H.8. Furthermore, genetic deletion of H.8 from target organisms abrogated blocking. The Fc region of the blocking IgG was required for blocking because F(ab')(2) fragments were ineffective. Blocking required IgG glycosylation because deglycosylation with peptide:N-glycanase eliminated blocking. C4b deposition mediated by an anti-factor H-binding protein mAb was reduced by intact blocking IgG, but not by peptide:N-glycanase-treated blocking IgG, suggesting that blocking resulted from inhibition of classical pathway of complement. In conclusion, we have identified H.8 as a meningococcal target for novel blocking Abs in human serum. Such blocking Abs may reduce the efficacy of select antigroup B meningococcal protein vaccines. We also propose that outer membrane vesicle-containing meningococcal vaccines may be more efficacious if purged of subversive immunogens such as H.8.

Functional and specific antibody responses in adult volunteers in new zealand who were given one of two different meningococcal serogroup B outer membrane vesicle vaccines

This study presents detailed analyses of total and specific serum antibody levels among 26 and 24 adult volunteers before vaccination and after the third dose of the meningococcal serogroup B outer membrane vesicle (OMV) vaccines MeNZB and MenBvac, respectively, in a clinical trial in New Zealand (V. Thornton, D. Lennon, K. Rasanathan, J. O'Hallahan, P. Oster, J. Stewart, S. Tilman, I. Aaberge, B. Feiring, H. Nokleby, E. Rosenqvist, K. White, S. Reid, K. Mulholland, M. J. Wakefield, and D. Martin, Vaccine 24:1395-1400, 2006). With the homologous vaccine strains as targets, both vaccines induced significant increases in serum bactericidal and opsonophagocytic activities and in the levels of immunoglobulin G (IgG) to OMV antigens in an enzyme-linked immunosorbent assay (ELISA) and to live meningococci by flow cytometry. They also induced high levels of activity against the heterologous strains, particularly in terms of opsonophagocytic activity and IgG binding to live bacteria. The antibody levels with the homologous and heterologous strains in the four assays showed high and significant positive correlations. Specific IgG binding to 10 major OMV antigens in each vaccine was measured by scanning of immunoblots; ELISAs for two antigens, lipopolysaccharide and Neisseria surface protein A (NspA), were also performed. Both vaccines elicited significant increases in IgG binding to all homologous and heterologous OMV antigens except NspA. The total IgG band intensity on the blots correlated significantly with the IgG levels determined by the OMV ELISA and flow cytometry. In conclusion, the results of the various immunological assays showed that both OMV vaccines gave rise to high levels of specific and cross-reacting antibodies.

Comparison and correlation of neisseria meningitidis serogroup B immunologic assay results and human antibody responses following three doses of the Norwegian meningococcal outer membrane vesicle vaccine MenBvac

The prediction of efficacy of Neisseria meningitidis serogroup B (MenB) vaccines is currently hindered due to the lack of an appropriate correlate of protection. For outer membrane vesicle (OMV) vaccines, immunogenicity has primarily been determined by the serum bactericidal antibody (SBA) assay and OMV enzyme-linked immunosorbent assay (ELISA). However, the opsonophagocytic assay (OPA), surface labeling assay, whole blood assay (WBA), and salivary antibody ELISA have been developed although correlation with protection is presently undetermined. Therefore, the aim of the study was to investigate further the usefulness of, and relationships between, MenB immunologic assays. A phase II trial of the OMV vaccine, MenBvac, with proven efficacy was initiated to compare immunologic assays incorporating the vaccine and six heterologous strains. Correlations were achieved between the SBA assay, OMV ELISA, and OPA using human polymorphonuclear leukocytes and human complement but not between an OPA using HL60 phagocytic cells and baby rabbit complement. Correlations between the surface labeling assay, the SBA assay, and the OMV ELISA were promising, although target strain dependent. Correlations between the salivary antibody ELISA and other assays were poor. Correlations to the WBA were prevented since many samples had results greater than the range of the assay. The study confirmed the immunogenicity and benefit of a third dose of MenBvac against the homologous vaccine strain using a variety of immunologic assays. These results emphasize the need for standardized methodologies that would allow a more robust comparison of assays between laboratories and promote their further evaluation as correlates of protection against MenB disease.

Proteomic analysis of a meningococcal outer membrane vesicle vaccine prepared from the group B strain NZ98/254

In the absence of a suitable carbohydrate-based vaccine, outer membrane vesicle (OMV) vaccines have been used to disrupt outbreaks of serogroup B meningococcal disease for more than 20 years. Proteomic technology provides physical methods with the potential to assess the composition and consistency of these complex vaccines. 2-DE, combined with MS, were used to generate a proteome map of an OMV vaccine, developed to disrupt a long-running outbreak of group B disease in New Zealand. Seventy four spots from the protein map were identified including the outer membrane protein (OMP) antigens: PorA, PorB, RmpM and OpcA. Protein identification indicates that, in addition to OMPs, OMV vaccines contain periplasmic, membrane-associated and cytoplasmic proteins. 2-D-DIGE technology highlighted differences between preclinical development batches of vaccines from two different manufacturers.

Structural variation and immune recognition of the P1.2 subtype meningococcal antigen

Neisseria meningitidis is a globally important cause of bacterial meningitis and septicemia. No comprehensive antimeningococcal vaccine is available, largely as a consequence of the high sequence diversity of those surface proteins that could function as components of a vaccine. One such component is the protein PorA, a major surface porin of this Gram-negative organism that has been used in a number of experimental and licensed vaccines. Here we describe a series of experiments designed to investigate the consequences for antibody recognition of sequence diversity within a PorA antigen. The binding of a 14-residue peptide, corresponding to the P1.2 subtype antigen, to the MN16C13F4 monoclonal antibody was sensitive to mutation of five out of the six residues within the epitope sequence. The crystal structure of the antibody Fab fragment, determined in complex with the peptide antigen, shows a remarkably hydrophobic binding site and interactions between the antigen and antibody are dominated by apolar residues. Nine intrachain hydrogen bonds are formed within the antigen which maintain the beta-hairpin conformation of the peptide. These hydrogen bonds involve residues that are highly conserved amongst different P1.2 sequence variants, suggesting that some positions may be conserved for structural reasons in these highly polymorphic regions. The sensitivity of antibody recognition of the antigen towards mutation provides a structural explanation for the widespread sequence variation seen in different PorA sequences in this region. Single point mutations are sufficient to remove binding capability, providing a rationale for the manner in which different meningococcal PorA escape variants arise.

Passive protection in the infant rat protection assay by sera taken before and after vaccination of teenagers with serogroup B meningococcal outer membrane vesicle vaccines

From a previous published clinical trial among teenagers in Iceland [Perkins BA, Jonsdottir K, Briem H, Griffiths E, Plikaytis BD, Høiby EA, et al. J Infect Dis 1998;177:683--91], we evaluated a 25% stratified subset of sera, collected before vaccination and 6 weeks after the second vaccination with either the Norwegian (n=37) or the Cuban (n=35) serogroup B meningococcal outer membrane vesicle (OMV) vaccine or the control serogroup A/C capsular polysaccharide vaccine (n=20), for protective activity in an infant rat protection assay (IRPA). Protection was assessed with both the Norwegian (44/76-SL, B:15:P1.7,16:L3,7) and the Cuban (Cu 385, B:4:P1.19,15:L3,7) vaccine strain, and the results compared with serum bactericidal assay (SBA) titres and anti-OMV IgG antibody concentrations. An IRPA response was defined as a >or=10-fold rise in protective activity compared to pre-vaccination level. Forty-six percent (42/92) of the pre-vaccination sera showed protection with strain 44/76-SL compared to only 12% (11/92) with strain Cu 385. After the second dose, 22% (8/37) of those given the Norwegian vaccine showed IRPA responses with the homologous strain compared to 65% (24/37) in SBA. The corresponding numbers with the homologous strain for the Cuban vaccinees were 14% (5/35) and 29% (10/35), respectively. Among the controls, 15% (3/20) showed IRPA responses to 44/76-SL but none to Cu 385. Correlation between IRPA activity and SBA titres or anti-OMV IgG was low, especially for pre-vaccination sera against strain 44/76-SL. We conclude that the sensitivity of IRPA described herein may not be sufficient to evaluate serogroup B OMV vaccine responses from clinical samples.

Mucosal immunity and optimizing protection with meningococcal serogroup B vaccines

Candidate Neisseria meningitidis serogroup B vaccines that are based on outer-membrane vesicles induce protective immunity in adults but provide neither crossprotection for infants nor long-lasting immunity. We suggest that this lack of vaccine efficacy is not solely because the best antigens are yet to be identified but also results from inappropriate programming of the immune response. Natural carriage of N. meningitidis and related bacteria leads to the development of protective immunity both at the mucosal surface and in the circulation. We propose that vaccine strategies that mimic this natural immunization process would better-optimize vaccine-induced protective immunity. Thus, mucosal immunization before a systemic booster vaccination could provide the solution and reduce the necessity for multiple injections to achieve immunity.

Protection by natural human immunoglobulin M antibody to meningococcal serogroup B capsular polysaccharide in the infant rat protection assay is independent of complement-mediated bacterial lysis

Neisseria meningitidis, an important cause of bacterial meningitis and septicemia worldwide, is associated with high mortality and serious sequelae. Natural immunity against meningococcal disease develops with age, but the specificity and functional activity of natural antibodies associated with protection are poorly understood. We addressed this question by using a selected subset of prevaccination sera (n = 26) with convergent or discrepant serum bactericidal activity (SBA) and infant rat protective activity (IRPA) against the serogroup B meningococcal strain 44/76-SL (B:15:P1.7,16) from Icelandic teenagers. The sera were analyzed by opsonophagocytic activity (OPA) assay, immunoblotting, immunoglobulin G (IgG) quantitation against live meningococcal cells by flow cytometry, and enzyme immunosorbent assay (EIA). High levels of SBA and OPA were reflected in distinct IgG binding to major outer membrane proteins and/or lipopolysaccharide in immunoblots. However, we could not detect any specific antibody patterns on blots that could explain IRPA. Only IgM antibody to group B capsular polysaccharide (B-PS), measured by EIA, correlated positively (r = 0.76, P < 0.001) with IRPA. Normal human sera (NHS; n = 20) from healthy Finnish children of different ages (7, 14, and 24 months and 10 years) supported this finding and showed an age-related increase in IRPA that coincided with the acquisition of B-PS specific IgM antibody. The protection was independent of complement-mediated bacterial lysis, as detected by the inability of NHS to augment SBA in the presence of human or infant rat complement and the equal protective activity of NHS in rat strains with fully functional or C6-deficient complement.

Stability of PorA during a meningococcal disease epidemic

Meningococci causing New Zealand's epidemic, which began in 1991, are defined as group B, serosubtype P1.4 (subtype P1.7-2,4), belonging to the ST-41/ST-44 complex, lineage III. Of the 2,358 group B isolates obtained from disease cases from 1991 through 2003, 85.7% (2,021 of 2,358) were determined to be serosubtype P1.4. Of the remaining isolates, 156 (6.6%) were not serosubtypeable (NST). Molecular analysis of the porA gene from these B:NST meningococcal isolates was used to determine the reason. Most NST isolates (156, 88.5%) expressed a PorA that was distinct from P1.7-2,4 PorA. Fifteen isolates expressed variants of P1.7-2,4 PorA, and a further three expressed P1.7-2,4 PorA without any sequence variation. These three isolates expressed P1.7-2,4 PorA at very low levels, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, and showed variation in the porA promoter region. Among the 15 meningococcal isolates expressing variants of P1.7-2,4 PorA, 11 different sequence variations were found. Compared with the P1.7-2,4 PorA sequence, the sequences of these variants contained deletions, insertions, or single-nucleotide substitutions in the VR2 region of the protein. Multilocus restriction typing was used to assess the clonal derivations of B:NST case isolates. Meningococcal isolates expressing distinct PorA proteins belonged mostly to clonal types that were unrelated to the epidemic strain, whereas all meningococcal isolates expressing variants of P1.7-2,4 PorA belonged to the ST-41/ST-44 complex, lineage III. These results, together with those obtained serologically, demonstrate that the P1.7-2,4 PorA protein of meningococci responsible for New Zealand's epidemic has remained relatively stable over 13 years and support the use of a strain-specific outer membrane vesicle vaccine to control the epidemic.

Molecular genetic methods in diagnosis and direct characterization of acute bacterial central nervous system infections

Acute bacterial infection of the central nervous system requires rapid and adequate management. Etiological diagnosis is hence crucial. Moreover, the epidemic threat of certain bacteria necessitates a reliable characterization of the involved bacterial strains to follow the spread of epidemic strains. Conventional identification and characterization of etiological agents are basically based on culture and identification of bacterial markers most frequently by serological assays. Molecular identification and characterization of bacteria have been employed. They provide more reliable analysis of bacterial isolates. Molecular methods for non-culture diagnosis of bacterial infections have recently been developed. In many cases, the molecular assays have decreased the identification time of positive cultures and rescued detection of pathogens in culture-negative clinical samples.