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

OpcA and PorB are novel bactericidal antigens of the 4CMenB vaccine in mice and humans

The ability of Neisseria meningitidis Outer Membrane Vesicles (OMV) to induce protective responses in humans is well established and mainly attributed to Porin A (PorA). However, the contribution of additional protein antigens to protection remains to be elucidated. In this study we dissected the immunogenicity of antigens originating from the OMV component of the 4CMenB vaccine in mice and humans. We collected functional data on a panel of strains for which bactericidal responses to 4CMenB in infants was attributable to the OMV component and evaluated the role of 30 OMV-specific protein antigens in cross-coverage. By using tailor-made protein microarrays, the immunosignature of OMV antigens was determined. Three of these proteins, OpcA, NspA, and PorB, triggered mouse antibodies that were bactericidal against several N. meningitidis strains. Finally, by genetic deletion and/or serum depletion studies, we demonstrated the ability of OpcA and PorB to induce functional immune responses in infant sera after vaccination. In conclusion, while confirming the role of PorA in eliciting protective immunity, we identified two OMV antigens playing a key role in protection of infants vaccinated with the 4CMenB vaccine against different N. meningitidis serogroup B strains.

Intranasal and Intramuscular Immunization with Outer Membrane Vesicles from Serogroup C Meningococci Induced Functional Antibodies and Immunologic Memory

BACKGROUND

Immunization is the key to prevent invasive meningococcal disease (IMD), caused by Neisseria meningitidis. Outer membrane vesicles (OMVs) can be used as meningococcal antigens.

METHODS

Isogenic mice A/Sn (H2^a) were immunized with low antigenic doses of OMVs of an N. meningitidis C:2a:P1.5 strain, via intranasal/intramuscular route, adjuvanted by cholera toxin subunit B (CTB) or via intramuscular route only, adjuvanted by aluminium hydroxide (AH). Mice were followed until old age and humoral and cellular responses were assessed by ELISA, Immunoblotting, Dot-blot, Serum-bactericidal assay, Immunohistochemistry and ELISpot.

RESULTS

OMV+CTB and OMV+AH groups presented statistically higher antibodies titers, which persisted until middle and old ages. IgG isotypes point to a Th2 type of response. Avidity indexes were considered high, regardless of adjuvant use, but only groups immunized with OMVs and adjuvants (OMV+CTB and OMV+AH) presented bactericidal activity. The antibodies recognized antigens of molecular weights attributed to porin and cross-reactivity proteins. Although the spleen of old mice did not present differences in immunohistochemistry marking of CD68+, CD4+, CD79+ and CD25+ cells, splenocytes of immune groups secreted IL-4 and IL-17 when stimulated with OMVs and meningococcal C polysaccharide.

CONCLUSION

We concluded that both adjuvants, CTB and AH, improved the immunogenicity of low doses of OMVs and contributed to a persistent immune response. Even though AH is well established in the vaccinology area, CTB seems to be a promising adjuvant candidate for meningococcal vaccines: it is suitable for mucosal delivery and supports a Th2 type of response. Therefore, OMVs are still a relevant vaccine platform.

Immunological Effects of Dimethyldioctadecylammonium Bromide and Saponin as Adjuvants for Outer Membrane Vesicles from Neisseria meningitidis

The meningococcal disease is a global health threat, but is preventable through vaccination. Adjuvants improve meningococcal vaccines and are able to trigger different aspects of the immune response. The present work evaluated the immune response of mice against Neisseria meningitidis outer membrane vesicles (OMV) complexed with the adjuvants aluminium hydroxide (AH), via subcutaneous route; and dimethyldioctadecylammonium bromide (DDA) or Saponin (Sap), via intranasal/subcutaneous routes. ELISA demonstrated that all adjuvants increased IgG titers after the booster dose, remaining elevated for 18 months. Additionally, adjuvants increased the avidity of the antibodies and the bactericidal titer: OMVs alone were bactericidal until 1:4 dilution but, when adjuvanted by Alum, DDA or Sap, it increased to 1/32. DDA and Sap increased all IgG isotypes, while AH improved IgG1 and IgG2a levels. Thus, Sap led to the recognition of more proteins in Immunoblot, followed by DDA and AH. Sap and AH induced higher IL-4 and IL-17 release, respectively. The use of adjuvants improved both cellular and humoral immune response, however, each adjuvant contributed to particular parameters. This demonstrates the importance of studying different adjuvant options and their suitability to stimulate different immune mechanisms, modulating the immune response.

Synergistic activity of antibodies in the multicomponent 4CMenB vaccine

INTRODUCTION

: Vaccines based on multiple antigens often induce an immune response which is higher than that triggered by each single component, with antibodies acting cooperatively and synergistically in tackling the infection.

AREAS COVERED

An interesting example is the antibody response induced by the 4CMenB vaccine, currently licensed for the prevention of Neisseria meningitidis serogroup B (MenB). It contains four antigenic components: Factor H binding protein (fHbp), Neisseria adhesin A (NadA), Neisserial Heparin Binding Antigen (NHBA) and Outer Membrane Vesicles (OMV). Monoclonal and polyclonal antibodies raised by vaccination with 4CMenB show synergistic activity in complement-dependent bacterial killing. This review summarizes published and unpublished data and provides evidence of the added value of multicomponent vaccines.

EXPERT OPINION

: The ability of 4CMenB vaccine to elicit antibodies targeting multiple surface-exposed antigens is corroborated by the recent data on real world evidences. Bactericidal activity is generally mediated by antibodies that bind to antigens highly expressed on the bacterial surface and immunologically related. However, simultaneous binding of antibodies to various surface-exposed antigens can overcome the threshold density of antigen-antibody complexes needed for complement activation. The data discussed in this review highlight the interplay between antibodies targeting major and minor antigens and their effect on functionality.

Clinical trial registration: www.clinicaltrials.gov identifiers of studies with original data mentioned in the article: NCT00937521, NCT00433914, NCT02140762 and NCT02285777.

Human B Cell Responses to Dominant and Subdominant Antigens Induced by a Meningococcal Outer Membrane Vesicle Vaccine in a Phase I Trial

Neisseria meningitidis outer membrane vesicle (OMV) vaccines are safe and provide strain-specific protection against invasive meningococcal disease (IMD) primarily by inducing serum bactericidal antibodies against the outer membrane proteins (OMP). To design broader coverage vaccines, knowledge of the immunogenicity of all the antigens contained in OMVs is needed. In a Phase I clinical trial, an investigational meningococcal OMV vaccine, MenPF1, made from a meningococcus genetically modified to constitutively express the iron-regulated FetA induced bactericidal responses to both the PorA and the FetA antigen present in the OMP. Using peripheral blood mononuclear cells collected from this trial, we analyzed the kinetics of and relationships between IgG, IgA, and IgM B cell responses against recombinant PorA and FetA, including (i) antibody-secreting cells, (ii) memory B cells, and (iii) functional antibody responses (opsonophagocytic and bactericidal activities). Following MenPF1vaccination, PorA-specific IgG secreting cell responses were detected in up to 77% of participants and FetA-specific responses in up to 36%. Memory B cell responses to the vaccine were low or absent and mainly detected in participants who had evidence of preexisting immunity (P = 0.0069). Similarly, FetA-specific antibody titers and bactericidal activity increased in participants with preexisting immunity and is consistent with the idea that immune responses are elicited to minor antigens during asymptomatic Neisseria carriage, which can be boosted by OMV vaccines.

IMPORTANCENeisseria meningitidis outer membrane vesicles (OMV) are a component of the capsular group B meningococcal vaccine 4CMenB (Bexsero) and have been shown to induce 30% efficacy against gonococcal infection. They are composed of multiple antigens and are considered an interesting delivery platform for vaccines against several bacterial diseases. However, the protective antibody response after two or three doses of OMV-based meningococcal vaccines appears short-lived. We explored the B cell response induced to a dominant and a subdominant antigen in a meningococcal OMV vaccine in a clinical trial and showed that immune responses are elicited to minor antigens. However, memory B cell responses to the OMV were low or absent and mainly detected in participants who had evidence of preexisting immunity against the antigens. Failure to induce a strong B cell response may be linked with the low persistence of protective responses.

Broad vaccine protection against Neisseria meningitidis using factor H binding protein

Neisseria meningitidis, the causative agent of invasive meningococcal disease (IMD), is classified into different serogroups defined by their polysaccharide capsules. Meningococcal serogroups A, B, C, W, and Y are responsible for most IMD cases, with serogroup B (MenB) causing a substantial percentage of IMD cases in many regions. Vaccines using capsular polysaccharides conjugated to carrier proteins have been successfully developed for serogroups A, C, W, and Y. However, because the MenB capsular polysaccharide is poorly immunogenic, MenB vaccine development has focused on alternative antigens. The 2 currently available MenB vaccines (MenB-4C and MenB-FHbp) both include factor H binding protein (FHbp), a surface-exposed protein harboured by nearly all meningococcal isolates that is important for survival of the bacteria in human blood. MenB-4C contains a nonlipidated FHbp from subfamily B in addition to other antigens, including Neisserial Heparin Binding Antigen, Neisserial adhesin A, and outer membrane vesicles, whereas MenB-FHbp contains a lipidated FHbp from each subfamily (A and B). FHbp is highly immunogenic and a main target of bactericidal activity of antibodies elicited by both licensed MenB vaccines. FHbp is also an important vaccine component, in contrast to some other meningococcal antigens that may have limited cross-protection across strains, as FHbp-specific antibodies can provide broad cross-protection within each subfamily. Limited cross-protection between subfamilies necessitates the inclusion of FHbp variants from both subfamilies to achieve broad FHbp-based vaccine coverage. Additionally, immune responses to the lipidated form of FHbp have a superior cross-reactive profile to those elicited by the nonlipidated form. Taken together, the inclusion of lipidated FHbp variants from both FHbp subfamilies is expected to provide broad protection against the diverse disease-causing meningococcal strains expressing a wide range of FHbp sequence variants. This review describes the development of vaccines for MenB disease prevention, with a focus on the FHbp antigen.

Immunogenicity of trimeric autotransporter adhesins and their potential as vaccine targets

The current problem of increasing antibiotic resistance and the resurgence of numerous infections indicate the need for novel vaccination strategies more than ever. In vaccine development, the search for and the selection of adequate vaccine antigens is the first important step. In recent years, bacterial outer membrane proteins have become of major interest, as they are the main proteins interacting with the extracellular environment. Trimeric autotransporter adhesins (TAAs) are important virulence factors in many Gram-negative bacteria, are localised on the bacterial surface, and mediate the first adherence to host cells in the course of infection. One example is the Neisseria adhesin A (NadA), which is currently used as a subunit in a licensed vaccine against Neisseria meningitidis. Other TAAs that seem promising vaccine candidates are the Acinetobacter trimeric autotransporter (Ata), the Haemophilus influenzae adhesin (Hia), and TAAs of the genus Bartonella. Here, we review the suitability of various TAAs as vaccine candidates.

Evaluation of the immune responses following co-administration of PilQ and type b-flagellin from Pseudomonas aeruginosa in the burn mouse model

Considering the increased antibiotic resistance of Pseudomonas aeruginosa, the evaluation of immune response against the antigens of this bacterium seems necessary. In this study, the protective efficacy and immunological properties of P. aeruginosa recombinant PilQ (r-PilQ) and type b-flagellin (FLB) proteins was evaluated in the burn mouse model of infection. The inbred BALB/c mice were immunized with r-PilQ and FLB antigens. To investigate the type of induced immune response, sera were analyzed by ELISA for total IgG, IgG1, and IgG2a isotypes. After the final immunization, the IL-4, IFN-γ, and IL-17 cytokines level were examined in the spleen of non-challenged mice. Fifty days after lethal challenge, the survival rate and bacterial burden in the skin and other internal organs of experimental mice were assessed. The in vivo administration of r-PilQ, FLB and combined antigen resulted in a significant increase in the survival of mice (66%, 75%, and 83%, respectively) infected by the PAO1 strain of P. aeruginosa in the burn model of infection. Immunization of mice with r-PilQ and FLB mixture induced high titers of IL-4 and IL-17 cytokines compared to control groups (P < 0.05). The high titer of antisera raised against combined antigen was able to inhibit the systemic spread of the PAO1 strain from the site of infection to the internal organs. We concluded that the parallel role of IL-4 and IL-17 is necessary for elimination of the bacteria and promotion of survival in the immunized burn mice.

Potential large scale production of meningococcal vaccines by stable overexpression of fHbp in the rice seeds

Factor H binding protein (fHbp) is the most promising vaccine candidate against serogroup B of Neisseria meningitidis which is a major cause of morbidity and mortality in children. In order to facilitate large scale production of a commercial vaccine, we previously used transgenic Arabidopsis thaliana, but plant-derived fHbp is still far away from a commercial vaccine due to less biomass production. Herein, we presented an alternative route for the production of recombinant fHbp from the seeds of transgenic rice. The OsrfHbp gene encoding recombinant fHbp fused protein was introduced into the genome of rice via Agrobacterium-mediated transformation. The both stable integration and transcription of the foreign OsrfHbp were confirmed by Southern blotting and RT-PCR analysis respectively. Further, the expression of fHbp protein was measured by immunoblotting analysis and quantified by ELISA. The results indicated that fHbp was successfully expressed and the highest yield of fHbp was 0.52 ± 0.03% of TSP in the transgenic rice seeds. The purified fHbp protein showed good antigenicity and immunogenicity in the animal model. The results of this experiment offer a novel approach for large-scale production of plant-derived commercial vaccine fHbp.

Novel approaches to Neisseria meningitidis vaccine design

A range of vaccines is available for preventing life-threatening diseases caused by infection with Neisseria meningitidis (meningococcus, Men). Capsule polysaccharide (CPS)-conjugate vaccines are successful prophylactics for serogroup MenA, MenC, MenW and MenY infections, and outer membrane vesicle (OMV) vaccines have been used successfully for controlling clonal serogroup MenB infections. MenB vaccines based on recombinant proteins identified by reverse vaccinology (Bexsero™) and proteomics (Trumenba™) approaches have recently been licensed and Bexsero™ has been introduced into the UK infant immunisation programme. In this review, we chart the development of these licensed vaccines. In addition, we discuss the plethora of novel vaccinology approaches that have been applied to the meningococcus with varying success in pre-clinical studies, but which provide technological platforms for application to other pathogens. These strategies include modifying CPS, lipooligosaccharide and OMV; the use of recombinant proteins; structural vaccinology approaches of designing synthetic peptide/mimetope vaccines, DNA vaccines and engineered proteins; epitope presentation on biological and synthetic particles; through vaccination with live-attenuated pathogen(s), or with heterologous bacteria expressing vaccine antigens, or to competitive occupation of the nasopharyngeal niche by commensal bacterial spp. After close to a century of vaccine research, it is possible that meningococcal disease may be added, shortly, to the list of diseases to have been eradicated worldwide by rigorous vaccination campaigns.

Bioinformatic analysis of meningococcal Msf and Opc to inform vaccine antigen design

Neisseria meningitidis is an antigenically and genetically variable Gram-negative bacterium and a causative agent of meningococcal meningitis and septicaemia. Meningococci encode many outer membrane proteins, including Opa, Opc, Msf, fHbp and NadA, identified as being involved in colonisation of the host and evasion of the immune response. Although vaccines are available for the prevention of some types of meningococcal disease, none currently offer universal protection. We have used sequences within the Neisseria PubMLST database to determine the variability of msf and opc in 6,500 isolates. In-silico analysis revealed that although opc is highly conserved, it is not present in all isolates, with most isolates in clonal complex ST-11 lacking a functional opc. In comparison, msf is found in all meningococcal isolates, and displays diversity in the N-terminal domain. We identified 20 distinct Msf sequence variants (Msf SV), associated with differences in number of residues within the putative Vn binding motifs. Moreover, we showed distinct correlations with certain Msf SVs and isolates associated with either hyperinvasive lineages or those clonal complexes associated with a carriage state. We have demonstrated differences in Vn binding between three Msf SVs and generated a cross reactive Msf polyclonal antibody. Our study has highlighted the importance of using large datasets to inform vaccine development and provide further information on the antigenic diversity exhibited by N. meningitidis.

Meningococcal Antigen Typing System (MATS)-Based Neisseria meningitidis Serogroup B Coverage Prediction for the MenB-4C Vaccine in the United States

Neisseria meningitidis is the most common cause of bacterial meningitis in children and young adults worldwide. A 4-component vaccine against N. meningitidis serogroup B (MenB) disease (MenB-4C [Bexsero]; GSK) combining factor H binding protein (fHBP), neisserial heparin binding protein (NHBA), neisserial adhesin A (NadA), and PorA-containing outer membrane vesicles was recently approved for use in the United States and other countries worldwide. Because the public health impact of MenB-4C in the United States is unclear, we used the meningococcal antigen typing system (MATS) to assess the strain coverage in a panel of strains representative of serogroup B (NmB) disease in the United States. MATS data correlate with killing in the human complement serum bactericidal assay (hSBA) and predict the susceptibility of NmB strains to killing in the hSBA, the accepted correlate of protection for MenB-4C vaccine. A panel of 442 NmB United States clinical isolates (collected in 2000 to 2008) whose data were down weighted with respect to the Oregon outbreak was selected from the Active Bacterial Core Surveillance (ABCs; CDC, Atlanta, GA) laboratory. MATS results examined to determine strain coverage were linked to multilocus sequence typing and antigen sequence data. MATS predicted that 91% (95% confidence interval [CI₉₅], 72% to 96%) of the NmB strains causing disease in the United States would be covered by the MenB-4C vaccine, with the estimated coverage ranging from 88% to 97% by year with no detectable temporal trend. More than half of the covered strains could be targeted by two or more antigens. NHBA conferred coverage to 83% (CI₉₅, 45% to 93%) of the strains, followed by factor H-binding protein (fHbp), which conferred coverage to 53% (CI₉₅, 46% to 57%); PorA, which conferred coverage to 5.9%; and NadA, which conferred coverage to 2.5% (CI₉₅, 1.1% to 5.2%). Two major clonal complexes (CC32 and CC41/44) had 99% strain coverage. The most frequent MATS phenotypes (39%) were fHbp and NHBA double positives. MATS predicts over 90% MenB-4C strain coverage in the United States, and the prediction is stable in time and consistent among bacterial genotypes. IMPORTANCE The meningococcal antigen typing system (MATS) is an enzyme-linked immunosorbent assay (ELISA)-based system that assesses the levels of expression and immune reactivity of the three recombinant MenB-4C antigens and, in conjunction with PorA variable 2 (VR2) sequencing, provides an estimate of the susceptibility of NmB isolates to killing by MenB-4C-induced antibodies. MATS assays or similar antigen phenotype analyses assume importance under conditions in which analyses of vaccine coverage predictions are not feasible with existing strategies, including large efficacy trials or functional antibody screening of an exhaustive strain panel. MATS screening of a panel of NmB U.S. isolates (n = 442) predicts high MenB-4C vaccine coverage in the United States.

A deeper mining on the protein composition of VA-MENGOC-BC®: An OMV-based vaccine against N. meningitidis serogroup B and C

The protein composition of an Outer Membrane Vesicle (OMV) preparation that constitutes the active pharmaceutical ingredient of VA-MENGOC-BC®, an effective vaccine against Neisseria meningitidis serogroups B, and C is presented. This preparation has a high lipid content and five abundant membrane proteins (FetA, PorA, PorB, RmpM, and Opc), constituting approximately 70% of the total protein mass. The protein composition was determined by combining the use of the Hexapeptide Ligand Library and an orthogonal tandem fractionation of tryptic peptides by reverse-phase chromatography at alkaline and acid pH. This approach equalizes the concentration of tryptic peptides derived from low- and high-abundance proteins as well as considerably simplifying the number of peptides analyzed by LC-MS/MS, enhancing the possibility of identifying low-abundance species. Fifty-one percent of the proteins originally annotated as membrane proteins in the genome of the MC58 strain were identified. One hundred and sixty-eight low-abundance cytosolic proteins presumably occluded within OMV were also identified. Four (NadA, NUbp, GNA2091, and fHbp), out of the five antigens constituting the Bexsero® vaccine, were detected in this OMV preparation. In particular, fHbp is also the active principle of the Trumenba® vaccine developed by Pfizer. The HpuA and HpuB gene products (not annotated in the MC58 genome) were identified in the CU385 strain, a clinical isolate that is used to produce this OMV. Considering the proteins identified here and previous work done by our group, the protein catalogue of this OMV preparation was extended to 266 different protein species.

The next chapter for group B meningococcal vaccines

The majority of invasive meningococcal disease (IMD) in the developed world is caused by capsular group B Neisseria meningitidis, however success with vaccination against organisms bearing this capsule has previously been restricted to control of geographically limited clonal outbreaks. As we enter a new era, with the first routine program underway to control endemic group B meningococcal disease for infants in the UK, it is timely to review the key landmarks in group B vaccine development, and discuss the issues determining whether control of endemic group B disease will be achieved. Evidence of a reduction in carriage acquisition of invasive group B meningococcal strains, after vaccination among adolescents, is imperative if routine immunization is to drive population control of disease beyond those who are vaccinated (i.e. through herd immunity). The need for multiple doses to generate a sufficiently protective response and reactogenicity remain significant problems with the new generation of vaccines. Despite these limitations, early data from the UK indicate that new group B meningococcal vaccines have the potential to have a major impact on meningococcal disease, and to provide new insight into how we might do better in the future.

Phase variation of Opa proteins of Neisseria meningitidis and the effects of bacterial transformation

Opa proteins are major proteins involved in meningococcal colonization of the nasopharynx and immune interactions. Opa proteins undergo phase variation (PV) due to the presence of the 5'-CTCTT-3' coding repeat (CR) sequence. The dynamics of PV of meningococcal Opa proteins is unknown. Opa PV, including the effect of transformation on PV, was assessed using a panel of Opa-deficient strains of Neisseria meningitidis. Analysis of Opa expression from UK disease-causing isolates was undertaken. Different opa genes demonstrated variable rates of PV, between 6.4 × 10(-4) and 6.9 × 10(-3) per cell per generation. opa genes with a longer CR tract had a higher rate of PV (r(2) = 0.77, p = 0.1212). Bacterial transformation resulted in a 180-fold increase in PV rate. The majority of opa genes in UK disease isolates (315/463, 68.0%) were in the 'on' phase, suggesting the importance of Opa proteins during invasive disease. These data provide valuable information for the first time regarding meningococcal Opa PV. The presence of Opa PV in meningococcal populations and high expression of Opa among invasive strains likely indicates the importance of this protein in bacterial colonization in the human nasopharynx. These findings have potential implications for development of vaccines derived from meningococcal outer membranes.

Apa2H1, the first head domain of Apa2 trimeric autotransporter adhesin, activates mouse bone marrow-derived dendritic cells and immunization with Apa2H1 protects against Actinobacillus pleuropneumoniae infection

Actinobacillus pleuropneumoniae is the causative pathogen of porcine pleuropneumonia, which results in large economic losses in the pig industry worldwide. There are, however, no effective subunit vaccines are available in the market owing to the various serotypes and the absence of cross-protection against this pathogen. Therefore, the selection of protective components is of great significance for vaccine development. We previously showed that trimeric autotransporter adhesins are important virulence factors of A. pleuropneumoniae. To determine the potential role in vaccine development of the functional head domain (Apa2H1) of Apa2, a trimeric autotransporter adhesin found in A. pleuropneumoniae, we obtained nature-like trimeric Apa2H1 using a prokaryotic expression system and co-culture of Apa2H1 with bone marrow derived dendritic cells (BMDCs) in vitro resulted in maturation of BMDCs, characterised by the up-regulation of CD83, MHC-II, CCR7, ICAM-I and the increased expression of factors related to B lymphoid cells stimulation, such as proliferation-inducing ligand (APRIL), B lymphocyte stimulator (BLyS) and B cell activating factor (BAFF). The in vivo results showed that vaccination with Apa2H1 resulted in the robust production of antigen-specific antibodies, modestly induced mixed Th1 and Th2 immunity, impaired bacterial colonization and dissemination, and improved mouse survival rates. This study is the first to show that Apa2H1 is antigenic and can be used as a component of a subunit vaccine against A. pleuropneumoniae infection, providing valuable reference material for the development of an effective vaccine against A. pleuropneumoniae.

Improving the immunogenicity of a trivalent Neisseria meningitidis native outer membrane vesicle vaccine by genetic modification

Trivalent native outer membrane vesicles (nOMVs) derived from three genetically modified Neisseria meningitidis serogroup B strains have been previously evaluated immunologically in mice and rabbits. This nOMV vaccine elicited serum bactericidal activity (SBA) against multiple N. meningitidis serogroup B strains as well as strains from serogroups C, Y, W, and X. In this study, we used trivalent nOMVs isolated from the same vaccine strains and evaluated their immunogenicity in an infant Rhesus macaque (IRM) model whose immune responses to the vaccine are likely to be more predictive of the responses in human infants. IRMs were immunized with trivalent nOMV vaccines and sera were evaluated for exogenous human serum complement-dependent SBA (hSBA). Antibody responses to selected hSBA generating antigens contained within the trivalent nOMVs were also measured and we found that antibody titers against factor H binding protein variant 2 (fHbpv2) were very low in the sera from animals immunized with these original nOMV vaccines. To increase the fHbp content in the nOMVs, the vaccine strains were further genetically altered by addition of another fHbp gene copy into the porB locus. Trivalent nOMVs from the three new vaccine strains had higher fHbp antigen levels and generated higher anti-fHbp antibody responses in immunized mice and IRMs. As expected, fHbp insertion into the porB locus resulted in no PorB expression. Interestingly, higher expression of PorA, an hSBA generating antigen, was observed for all three modified vaccine strains. Compared to the trivalent nOMVs from the original strains, higher PorA levels in the improved nOMVs resulted in higher anti-PorA antibody responses in mice and IRMs. In addition, hSBA titers against other strains with PorA as the only hSBA antigen in common with the vaccine strains also increased.

Over-expression of fHbp in Arabdopsis for development of meningococcal serogroup B subunit vaccine

Due to lack of commercial vaccine against the serogroup B (MenB) of Neisseria meningitides, the incidence of meningococcal disease remains high. To solve the issue, transgenic plants are used as bioreactors to produce a plant-derived fHbp subunit vaccine. In this study, the fHbp gene was optimized according to the codon usage bias of Arabidopsis thaliana, synthesized artificially, cloned into an expression vector, driven by a seed-specific promoter, and introduced into A. thaliana by Agrobacterium-mediated floral-dip transformation. Transgenic plants were identified by glufosinate selection, quickstix strips for PAT/bar tests and PCR analysis. The five plants showing higher expression of recombinant fHbp were screened through indirect ELISA. Southern blot analysis showed that the transgenic line rHF-22 had a single-copy integration and the highest expression of fHbp. Recombinant fHbp was purified from seeds of rHF-22 by nitrilotriacetic acid-mediated affinity chromatography, and the purity was 82.5%. BALB/c mice were tested for fHbp vaccine protection from lethal MenB infection, and the relative percent survival was found to be 80%. This study indicates that the recombinant fHbp produced from seeds of rHF-22 is a potential candidate for commercial MenB vaccine. It also provides a reference for safe, cheap and large-scale production of other plant-made vaccines.

FetA Antibodies Induced by an Outer Membrane Vesicle Vaccine Derived from a Serogroup B Meningococcal Isolate with Constitutive FetA Expression

Invasive meningococcal disease causes over 3500 cases each year in Europe, with particularly high incidence among young children. Among serogroup B meningococci, which cause most of the cases, high diversity in the outer membrane proteins (OMPs) is observed in endemic situations; however, comprehensive molecular epidemiological data are available for the diversity and distribution of the OMPs PorA and FetA and these can be used to rationally design a vaccine with high coverage of the case isolates. The aim of this study was to determine whether outer membrane vesicles (OMVs) derived from an isolate with constitutive FetA expression (MenPF-1 vaccine) could be used to induce antibodies against both the PorA and FetA antigens. The immunogenicity of various dose levels and number of doses was evaluated in mice and rabbits, and IgG antibody responses tested against OMVs and recombinant PorA and FetA proteins. A panel of four isogenic mutants was generated and used to evaluate the relative ability of the vaccine to induce serum bactericidal activity (SBA) against FetA and PorA. Sera from mice were tested in SBA against the four target strains. Results demonstrated that the MenPF-1 OMVs were immunogenic against PorA and FetA in both animal models. Furthermore, the murine antibodies induced were bactericidal against isogenic mutant strains, suggesting that antibodies to both PorA and FetA were functional. The data presented indicate that the MenPF-1 vaccine is a suitable formulation for presenting PorA and FetA OMPs in order to induce bactericidal antibodies, and that proceeding to a Phase I clinical trial with this vaccine candidate is justified.

Meningococcal Antigen Typing System Development and Application to the Evaluation of Effectiveness of Meningococcal B Vaccine and Possible Use for Other Purposes

Development of the 4-component meningococcal serogroup B vaccine (4CMenB) has required new assays for the reliable evaluation of the expression and cross-reactivity of those specific antigen variants that are predicted to be targeted by bactericidal antibodies elicited by the vaccine in different isolates. Existing laboratory techniques, such as multilocus sequence typing, are poorly suited to this purpose, since they do not provide information on the contribution of single vaccine components and therefore cannot be applied to estimate the potential coverage of the multicomponent vaccine. The hSBA, the only correlate of protection against invasive meningococcal disease accepted thus far, cannot conveniently be used to test large number of strains. To overcome these issues, the meningococcal antigen typing system (MATS) has been specifically developed in order to predict 4CMenB coverage of individual meningococcus serogroup B strains. To date, MATS has proved advantageous for several reasons, including its ability to assess both qualitative and quantitative aspects of surface antigens of single strains in a highly reproducible, rapid, and resource-saving manner, while its shortcomings include a possible underestimation of 4CMenB coverage and the use of pooled sera to calculate the positive bactericidal threshold. This paper provides an overview of MATS development and its field application.

Multivalent meningococcal serogroup B vaccines: challenges in predicting protection and measuring effectiveness

Vaccines targeting Neisseria meningitidis serogroup B (MenB) have been attempted for 40 years. Monovalent outer membrane vesicle vaccines targeted at epidemic outbreaks have been successfully developed. Newer vaccines aim to induce antibodies to cross-reactive antigens, such as factor H binding protein (rLP2086) or a mix of outer membrane vesicle, factor H binding protein and other minor antigens (4CMenB). The true protective coverage among circulating MenB isolates afforded by these vaccines is unknown. Carefully conducted Phase IV post-implementation evaluations designed to measure specific effectiveness against major circulating MenB clonal lineages are needed to address the critical question of which antigens are linked to protection. Progress with whole-genome sequencing and bio-informatics may allow the composition of antigen mozaics based on two major outer membrane proteins: PorA and FetA.

Meningococcal serogroup B vaccine (4CMenB): Booster dose in previously vaccinated infants and primary vaccination in toddlers and two-year-old children

OBJECTIVE

The multicomponent, recombinant serogroup B vaccine, 4CMenB, is approved in Europe, Canada and Australia from two months of age. We investigated persistence to booster doses at 12 months of age following infant vaccination, and immune response to catch-up vaccination of toddlers and children up to two years of age.

METHODS

We assessed persistence of immune responses after one year in participants vaccinated as infants, and responses to two doses at 12-15 or 24-26 months of age in vaccine-naïve children, as serum bactericidal activity with human complement (hSBA) against indicator strains for four vaccine antigens. Adverse events were recorded after each vaccination.

RESULTS

High antibody titers were induced against all four 4CMenB components following booster vaccination in infant-primed toddlers and after two doses in previously unvaccinated toddlers or two-year-olds. Antibodies waned over 12 months, particularly those against NZ OMV. Systemic reactogenicity in toddlers was lower than in infants, and lower again in vaccine-naïve two-year-olds. Local reactogenicity was common in all groups.

CONCLUSIONS

Four infant or two toddler 4CMenB vaccinations elicit immune responses believed to be protective for the first two years of life, which can be boosted. Reactogenicity is lower in toddlers than in infants.

New findings on the function and potential applications of the trimeric autotransporter adhesin

Trimeric autotransporter adhesins (TAAs) are located on the surface of many pathogenic Gram-negative bacteria. TAAs belong to the autotransporter protein family and consist of three identical monomers. These obligate homotrimeric proteins are secreted through the bacterial type Vc secretion system and share a common molecular organization that each monomer consists of a N-terminal "passenger" domain and a C-terminal translocation domain. TAAs are important virulence factors that are involved in bacterial life cycle and participate in mediating infection, invasion, dissemination and evasion of host immune responses. TAAs have also proved to be useful for many applications, such as vaccines and disease biomarkers. We here mainly focused on new findings on bio-function and application of TAAs in addition to their common structure and secretion mechanisms.

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.

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

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

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

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

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

Identification and therapeutic potential of a vitronectin binding region of meningococcal msf

The human pathogen Neisseria meningitides (Nm) attains serum resistance via a number of mechanisms, one of which involves binding to the host complement regulator protein vitronectin. We have shown previously that the Meningococcal surface fibril (Msf), a trimeric autotransporter, binds to the activated form of vitronectin (aVn) to increase Nm survival in human serum. In this study, we aimed to identify the aVn-binding region of Msf to assess its potential as an antigen which can elicit antibodies that block aVn binding and/or possess bactericidal properties. Using several recombinant Msf fragments spanning its surface-exposed region, the smallest aVn-binding recombinants were found to span residues 1-86 and 39-124. The use of further deletion constructs and overlapping recombinant Msf fragments suggested that a region of Msf comprising residues 39-82 may be primarily important for aVn binding and that other regions may also be involved but to a lesser extent. Molecular modelling implicated K66 and K68, conserved in all available Msf sequences, to be involved in the interaction. Recombinant fragments which bound to aVn were able to reduce the survival advantage conveyed by aVn-interaction in serum bactericidal assays. Antibodies raised against one such fragment inhibited aVn binding to Msf. In addition, the antibodies enhanced specific killing of Msf-expressing Nm in a dose-dependent manner. Overall, this study identifies an aVn-binding region of Msf, an adhesin known to impart serum resistance properties to the pathogen; and shows that this region of Msf can elicit antibodies with dual properties which reduce pathogen survival within the host and thus has potential as a vaccine antigen.

Phase variation mediates reductions in expression of surface proteins during persistent meningococcal carriage

Asymptomatic and persistent colonization of the upper respiratory tract by Neisseria meningitidis occurs despite elicitation of adaptive immune responses against surface antigens. A putative mechanism for facilitating host persistence of this bacterial commensal and pathogen is alterations in expression of surface antigens by simple sequence repeat (SSR)-mediated phase variation. We investigated how often phase variation occurs during persistent carriage by analyzing the SSRs of eight loci in multiple isolates from 21 carriers representative of 1 to 6 months carriage. Alterations in repeat number were detected by a GeneScan analysis and occurred at 0.06 mutations/gene/month of carriage. The expression states were determined by Western blotting and two genes, fetA and nadA, exhibited trends toward low expression states. A critical finding from our unique examination of combinatorial expression states, "phasotypes," was for significant reductions in expression of multiple phase-variable surface proteins during persistent carriage of some strains. The immune responses in these carriers were examined by measuring variant-specific PorA IgG antibodies, capsular group Y IgG antibodies and serum bactericidal activity in concomitant serum samples. Persistent carriage was associated with high levels of specific IgG antibodies and serum bactericidal activity while recent strain acquisition correlated with a significant induction of antibodies. We conclude that phase-variable genes are driven into lower expression states during long-term persistent meningococcal carriage, in part due to continuous exposure to antibody-mediated selection, suggesting localized hypermutation has evolved to facilitate host persistence.

The Long Road to an Effective Vaccine for Meningococcus Group B (MenB)

Neisseria meningitidis infection can cause life-threatening meningitis and meningococcal septicaemia. Over the past 40 years, vaccines against most of the main meningococcal serogroups have offered increasingly good protection from disease, with one major exception in the developed world: serogroup B meningococcus (MenB). In the United States, MenB accounts for about a quarter of cases of meningococcal meningitis, with the bulk of the rest caused by meningococcus serogroups C (MenC) and Y (MenY). In the UK, where a vaccine against MenC is widely used, MenB is now responsible for nearly 90% of cases of invasive meningococcal disease. Recent attempts to create a universal MenB vaccine have been thwarted by the variability of the surface proteins of MenB and by the similarity of the MenB capsule to human glycoproteins. This review discusses current meningococcal vaccine strategies and their limitations with regard to MenB, and examines a promising new strategy for the rational design of a MenB vaccine. Thanks to a fusion of a rational reverse genetics approach and a membrane vesicle approach, a MenB vaccine, 4CMenB (Bexsero(®)), has finally gained regulatory approval in Europe and could be in clinical use by the end of 2013.

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.

Evaluation of truncated NhhA protein as a candidate meningococcal vaccine antigen

NhhA (Neisseria hia homologue) is an outer membrane protein from Neisseria meningitidis, the causative agent of meningococcal disease. The protein is surface exposed and its expression in a wide range of meningococcal strains suggests it is a promising vaccine candidate. In addition, immunization of mice with outer membrane vesicles of strains that overexpress NhhA in conjunction with one of TbpA, Omp85 or NspA results in synergistic bactericidal responses. We previously showed that the NhhA sequence is highly conserved between strains, with the majority of the differences localized to four distinct variable regions located in the amino-terminal region of the mature protein. In this study, N. meningitidis strains were constructed that over-express wild-type NhhA. Strains expressing truncated versions of NhhA, with deletions from the amino-terminal region that removed the most variable regions, were also made. These expression strains were also modified so that immunodominant, phase- and antigenically-variable outer membrane proteins were not expressed, truncated lipooligosaccharide (LOS) expression was genetically fixed (no phase variability), and capsular polysaccharide expression abolished. Outer membrane vesicles derived from these strains were used to immunize mice. As previously observed, a synergistic effect involving another antigen, TbpA, was required to demonstrate bactericidal activity. The highest bactericidal response against a heterologous strain was obtained with a truncated variant of NhhA. These results indicate that removal of (a) variable region(s) does not reduce bactericidal responses against NhhA, and that bactericidal targets exist in regions other than the variable N-teminus. This provides the basis for future examination of responses against truncated NhhA in protecting against heterologous NhhA strains, and further evaluation of truncated NhhA as a candidate for inclusion in a vaccine against all serogroups of N. meningitidis.

Meningococcal omp85 in detergent-extracted outer membrane vesicle vaccines induces high levels of non-functional antibodies in mice

The vaccine potential of meningococcal Omp85 was studied by comparing the immune responses of genetically modified deoxycholate-extracted outer membrane vesicles, expressing five-fold higher levels of Omp85, with wild-type vesicles. Groups (n = 6-12) of inbred and outbred mouse strains (Balb/c, C57BL/6, OFI and NMRI) were immunized with the two vaccines, and the induced antibody levels and bactericidal and opsonic activities measured. Except for Balb/c mice, which were low responders, the genetically modified vaccine raised high Omp85 antibody levels in all mouse strains. In comparison, the wild-type vaccine gave lower antibody levels, but NMRI mice responded to this vaccine with the same high levels as the modified vaccine in the other strains. Although the vaccines induced strain-dependent Omp85 antibody responses, the mouse strains showed high and similar serum bactericidal titres. Titres were negligible with heterologous or PorA-negative meningococcal target strains, demonstrating the presence of the dominant bactericidal PorA antibodies. The two vaccines induced the same opsonic titres. Thus, the genetically modified vaccine with high Omp85 antibody levels and the wild-type vaccine induced the same levels of functional activities related to protection against meningococcal disease, suggesting that meningococcal Omp85 is a less attractive vaccine antigen.

Preclinical evaluation of MenB vaccines: prerequisites for clinical development

Despite the widespread use of polysaccharide and conjugate vaccines against disease caused by several serogroups of Neisseria meningitidis, vaccines targeting meningococci expressing the serogroup B capsule (MenB) have focused on subcapsular antigens, due to crossreactivity of the polysaccharide with human glycoproteins. Protein vaccines composed of outer membrane vesicles have been used successfully to control epidemics of MenB disease in several countries; however, these are specific for epidemic strains. Currently, a single serogroup B vaccine, aiming to provide comprehensive coverage, has been approved for use, and several others are undergoing clinical trials. Data on potential new vaccine candidates, from discovery to initial preclinical evaluation, are regularly published. In this review, the data required to progress from preclinical to clinical development of MenB vaccines are outlined, with reference to relevant regulatory guidelines. The issues caused by a lack of reliable animal models, particularly with respect to determination of protective efficacy, are also discussed.

Dual pili post-translational modifications synergize to mediate meningococcal adherence to platelet activating factor receptor on human airway cells

Pili of pathogenic Neisseria are major virulence factors associated with adhesion, twitching motility, auto-aggregation, and DNA transformation. Pili of N. meningitidis are subject to several different post-translational modifications. Among these pilin modifications, the presence of phosphorylcholine (ChoP) and a glycan on the pilin protein are phase-variable (subject to high frequency, reversible on/off switching of expression). In this study we report the location of two ChoP modifications on the C-terminus of N. meningitidis pilin. We show that the surface accessibility of ChoP on pili is affected by phase variable changes to the structure of the pilin-linked glycan. We identify for the first time that the platelet activating factor receptor (PAFr) is a key, early event receptor for meningococcal adherence to human bronchial epithelial cells and tissue, and that synergy between the pilin-linked glycan and ChoP post-translational modifications is required for pili to optimally engage PAFr to mediate adherence to human airway cells.

Neisseria meningitidis is an important human pathogen that can cause rapidly progressing, life threatening meningitis and sepsis in humans. There is no fully protective vaccine against this pathogen in current use and the key processes that dictate the transition from harmless carriage of the bacterium in the airway (the case for the vast majority of colonised hosts) to invasive disease are largely undefined. A key missing link in this organism's interaction with the human host is the identity of the receptor that is the first point of contact for the organism within the airway. In this study, we report that the receptor for this important human pathogen on airway epithelial cells is the platelet activating factor receptor (PAFr), an immunomodulatory molecule shown by others to play a role in promoting bacterial sepsis. We also show that two post-translational modifications, glycosylation and phosphorylcholine, are subject to phase-variation (high frequency, reversible switching of gene expression). They are closely associated on adjacent pilin subunits, and synergy between both are required for the efficient engagement with the PAFr. These data define a new role for these post-translational modifications in meningococcal adherence and also provide an insight into the selective pressures that underlie their phase variable expression.

ZnuD, a potential candidate for a simple and universal Neisseria meningitidis vaccine

Neisseria meningitidis serogroup B (MenB) is a major cause of bacterial sepsis and meningitis, with the highest disease burden in young children. Available vaccines are based on outer membrane vesicles (OMVs) obtained from wild-type strains. However, particularly in toddlers and infants, they confer protection mostly against strains expressing the homologous protein PorA, a major and variable outer membrane protein. In the quest for alternative vaccine antigens able to provide broad MenB strain coverage in younger populations, but potentially also across all age groups, ZnuD, a protein expressed under zinc-limiting conditions, may be considered a promising candidate. Here, we have investigated the potential value of ZnuD and show that it is a conserved antigen expressed by all MenB strains tested except for some strains of clonal complex ST-8. In mice and guinea pigs immunized with ZnuD-expressing OMVs, antibodies were elicited that were able to trigger complement-mediated killing of all the MenB strains and serogroup A, C, and Y strains tested when grown under conditions of zinc limitation. ZnuD is also expressed during infection, since anti-ZnuD antibodies were detected in sera from patients. In conclusion, we confirm the potential of ZnuD-bearing OMVs as a component of an effective MenB vaccine.

Predicted strain coverage of a meningococcal multicomponent vaccine (4CMenB) in Europe: a qualitative and quantitative assessment

BACKGROUND

A novel multicomponent vaccine against meningococcal capsular group B (MenB) disease contains four major components: factor-H-binding protein, neisserial heparin binding antigen, neisserial adhesin A, and outer-membrane vesicles derived from the strain NZ98/254. Because the public health effect of the vaccine, 4CMenB (Novartis Vaccines and Diagnostics, Siena, Italy), is unclear, we assessed the predicted strain coverage in Europe.

METHODS

We assessed invasive MenB strains isolated mainly in the most recent full epidemiological year in England and Wales, France, Germany, Italy, and Norway. Meningococcal antigen typing system (MATS) results were linked to multilocus sequence typing and antigen sequence data. To investigate whether generalisation of coverage applied to the rest of Europe, we also assessed isolates from the Czech Republic and Spain.

FINDINGS

1052 strains collected from July, 2007, to June, 2008, were assessed from England and Wales, France, Germany, Italy, and Norway. All MenB strains contained at least one gene encoding a major antigen in the vaccine. MATS predicted that 78% of all MenB strains would be killed by postvaccination sera (95% CI 63-90, range of point estimates 73-87% in individual country panels). Half of all strains and 64% of covered strains could be targeted by bactericidal antibodies against more than one vaccine antigen. Results for the 108 isolates from the Czech Republic and 300 from Spain were consistent with those for the other countries.

INTERPRETATION

MATS analysis showed that a multicomponent vaccine could protect against a substantial proportion of invasive MenB strains isolated in Europe. Monitoring of antigen expression, however, will be needed in the future.

FUNDING

Novartis Vaccines and Diagnostics.

Phase variation of PorA, a major outer membrane protein, mediates escape of bactericidal antibodies by Neisseria meningitidis

Several outer membrane proteins of Neisseria meningitidis are subject to phase variation due to alterations in simple sequence repeat tracts. The PorA protein is a major outer membrane protein and a target for protective host immune responses. Phase variation of PorA is mediated by a poly-G repeat tract present within the promoter, leading to alterations in protein expression levels. N. meningitidis strain 8047 was subjected to serial passage in the presence of P1.2, a PorA-specific bactericidal monoclonal antibody. Rapid development of resistance to bactericidal activity was associated with a switch in the PorA repeat tract from 11G to 10G. Phase variants with a 10G repeat tract exhibited a 2-fold reduction in surface expression of PorA protein. A mutS mutant of strain 8047, with an elevated phase variation rate, exhibited a higher rate of escape and an association of escape with 10G and 9G variants, the latter having a 13-fold reduction in surface expression of PorA. We conclude that graduated reductions in the surface expression of outer membrane proteins mediated by phase variation enable meningococci to escape killing in vitro by bactericidal antibodies. These findings indicate how phase variation could have a major impact on immune escape and host persistence of meningococci.

History of meningococcal vaccines and their serological correlates of protection

For over a hundred years Neisseria meningitidis has been known to be one of the major causes of bacterial meningitis. However, effective vaccines were not developed until the latter part of the 20th century. The first of these were based on purified high molecular weight capsular polysaccharides and more recently the development of glycoconjugate vaccines has made paediatric immunisation programmes possible. The prevention of group B meningococcal disease has remained a challenge throughout this period. This review charts the history of the development of meningococcal vaccines and the importance of serological correlates of protection in their evaluation.

The meningococcal adhesin NhhA provokes proinflammatory responses in macrophages via toll-like receptor 4-dependent and -independent pathways

Activation of macrophages by Toll-like receptors (TLRs) and functionally related proteins is essential for host defense and innate immunity. TLRs recognize a wide variety of pathogen-associated molecules. Here, we demonstrate that the meningococcal outer membrane protein NhhA has immunostimulatory functions and triggers release of proinflammatory cytokines from macrophages. NhhA-induced cytokine release was found to proceed via two distinct pathways in RAW 264.7 macrophages. Interleukin-6 (IL-6) secretion was dependent on activation of TLR4 and required the TLR signaling adaptor protein MyD88. In contrast, release of tumor necrosis factor (TNF) was TLR4 and MyD88 independent. Both pathways involved NF-κB-dependent gene regulation. Using a PCR-based screen, we could identify additional targets of NhhA-dependent gene activation such as the cytokines and growth factors IL-1α, IL-1β, granulocyte colony-stimulating factor (G-CSF), and granulocyte-macrophage colony-stimulating factor (GM-CSF). In human monocyte-derived macrophages, G-CSF, GM-CSF, and IL-6 were found to be major targets of NhhA-dependent gene regulation. NhhA induced transcription of IL-6 and G-CSF mRNA via TLR4-dependent pathways, whereas GM-CSF transcription was induced via TLR4-independent pathways. These data provide new insights into the role of NhhA in host-pathogen interaction.

Loss of outer membrane protein C in Escherichia coli contributes to both antibiotic resistance and escaping antibody-dependent bactericidal activity

Outer membrane proteins (OMPs) serve as the permeability channels for nutrients, toxins, and antibiotics. In Escherichia coli, OmpA has been shown to be involved in bacterial virulence, and OmpC is related to multidrug resistance. However, it is unclear whether OmpC also has a role in the virulence of E. coli. The aims of this study were to characterize the role of OmpC in antimicrobial resistance and bacterial virulence in E. coli. The ompC deletion mutant showed significantly decreased susceptibility to carbapenems and cefepime. To investigate the survival of E. coli exposed to the innate immune system, a human blood bactericidal assay showed that the ompC mutant increased survival in blood and serum but not in complement-inactivated serum. These effects were also demonstrated in the natural selection of OmpC mutants. Also, C1q interacted with E. coli through a complex of antibodies bound to OmpC as a major target. Bacterial survival was increased in the wild-type strain in a dose-dependent manner by adding free recombinant OmpC protein or anti-C1q antibody to human serum. These results demonstrated that the interaction of OmpC-specific antibody and C1q was the key step in initiating the antibody-dependent classical pathway for the clearance of OmpC-expressing E. coli. Anti-OmpC antibody was detected in human sera, indicating that OmpC is an immunogen. These data indicate that the loss of OmpC in E. coli is resistant to not only antibiotics, but also the serum bactericidal effect, which is mediated from the C1q and anti-OmpC antibody-dependent classical pathway.

Structural basis for iron piracy by pathogenic Neisseria

Neisseria are obligate human pathogens causing bacterial meningitis, septicaemia and gonorrhoea. Neisseria require iron for survival and can extract it directly from human transferrin for transport across the outer membrane. The transport system consists of TbpA, an integral outer membrane protein, and TbpB, a co-receptor attached to the cell surface; both proteins are potentially important vaccine and therapeutic targets. Two key questions driving Neisseria research are how human transferrin is specifically targeted, and how the bacteria liberate iron from transferrin at neutral pH. To address these questions, we solved crystal structures of the TbpA-transferrin complex and of the corresponding co-receptor TbpB. We characterized the TbpB-transferrin complex by small-angle X-ray scattering and the TbpA-TbpB-transferrin complex by electron microscopy. Our studies provide a rational basis for the specificity of TbpA for human transferrin, show how TbpA promotes iron release from transferrin, and elucidate how TbpB facilitates this process.

Vaccine development against Neisseria meningitidis

Meningococcal disease is communicable by close contact or droplet aerosols. Striking features are high case fatality rates and peak incidences of invasive disease in infants, toddlers and adolescents. Vaccine development is hampered by bacterial immune evasion strategies including molecular mimicry.As for Haemophilus influenzae and Streptococcus pneumoniae, no vaccine has therefore been developed that targets all serogroups of Neisseria meningitidis. Polysaccharide vaccines available both in protein conjugated and non-conjugated form, have been introduced against capsular serogroups A, C,W-135 and Y, but are ineffective against serogroup B meningococci, which cause a significant burden of disease in many parts of the world. Detoxified outer membrane vesicles are used since decades to elicit protection against epidemic serogroup B disease. Genome mining and biochemical approaches have provided astounding progress recently in the identification of immunogenic, yet reasonably conserved outer membrane proteins. As subcapsular proteins nevertheless are unlikely to immunize against all serogroup B variants, thorough investigation by surrogate assays and molecular epidemiology approaches are needed prior to introduction and post-licensure of protein vaccines. Research currently addresses the analysis of life vaccines, meningococcus B polysaccharide modifications and mimotopes, as well as the use of N. lactamica outer membrane vesicles.

Recombinant outer membrane secretin PilQ(406-770) as a vaccine candidate for serogroup B Neisseria meningitidis

Secretin PilQ is an antigenically conserved outer membrane protein which is present on most meningococci. This protein naturally expressed at high levels and is essential for meningococcal pilus expression at the cell surface. A 1095 bp fragment of C-terminal of secretin pilQ from serogroup B Neisseria meningitidis was cloned into prokaryotic expression vector pET-28a. Recombinant protein was overexpressed with IPTG and affinity-purified by Ni-NTA agarose. BALB/c mice were immunized subcutaneously with purified rPilQ(406-770) mixed with Freund's adjuvant. Serum antibody responses to serogroups A and B N. meningitidis whole cells or purified rPilQ(406-770) and functional activity of antibodies were determined by ELISA and SBA, respectively. The output of rPilQ(406-770) was approximately 50% of the total bacterial proteins. Serum IgG responses were significantly increased in immunized group with PilQ(406-770) mixed with Freund's adjuvant in comparison with control groups. Antisera produced against rPilQ(406-770) demonstrated strong surface reactivity to serogroups A and B N. meningitidis tested by whole-cell ELISA. Surface reactivity to serogroup B N. meningitidis was higher than serogroup A. The sera from PilQ(406-770) immunized animals were strongly bactericidal against serogroups A and B. These results suggest that rPilQ(406-770) is a potential vaccine candidate for serogroup B N. meningitidis.