PorB3 outer membrane protein on Neisseria meningitidis is poorly accessible for antibody binding on live bacteria

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.

Challenges for the development of a universal vaccine against leptospirosis revealed by the evaluation of 22 vaccine candidates

Leptospirosis is a neglected disease of man and animals that affects nearly half a million people annually and causes considerable economic losses. Current human vaccines are inactivated whole-cell preparations (bacterins) of Leptospira spp. that provide strong homologous protection yet fail to induce a cross-protective immune response. Yearly boosters are required, and serious side-effects are frequently reported so the vaccine is licensed for use in humans in only a handful of countries. Novel universal vaccines require identification of conserved surface-exposed epitopes of leptospiral antigens. Outer membrane β-barrel proteins (βb-OMPs) meet these requirements and have been successfully used as vaccines for other diseases. We report the evaluation of 22 constructs containing protein fragments from 33 leptospiral βb-OMPs, previously identified by reverse and structural vaccinology and cell-surface immunoprecipitation. Three-dimensional structures for each leptospiral βb-OMP were predicted by I-TASSER. The surface-exposed epitopes were predicted using NetMHCII 2.2 and BepiPred 2.0. Recombinant constructs containing regions from one or more βb-OMPs were cloned and expressed in Escherichia coli. IMAC-purified recombinant proteins were adsorbed to an aluminium hydroxide adjuvant to produce the vaccine formulations. Hamsters (4-6 weeks old) were vaccinated with 2 doses containing 50 – 125 μg of recombinant protein, with a 14-day interval between doses. Immunoprotection was evaluated in the hamster model of leptospirosis against a homologous challenge (10 – 20× ED₅₀) with L. interrogans serogroup Icterohaemorrhagiae serovar Copenhageni strain Fiocruz L1-130. Of the vaccine formulations, 20/22 were immunogenic and induced significant humoral immune responses (IgG) prior to challenge. Four constructs induced significant protection (100%, P < 0.001) and sterilizing immunity in two independent experiments, however, this was not reproducible in subsequent evaluations (0 – 33.3% protection, P > 0.05). The lack of reproducibility seen in these challenge experiments and in other reports in the literature, together with the lack of immune correlates and commercially available reagents to characterize the immune response, suggest that the hamster may not be the ideal model for evaluation of leptospirosis vaccines and highlight the need for evaluation of alternative models, such as the mouse.

Heat and Chemical Treatments Affect the Viability, Morphology, and Physiology of Staphylococcus aureus and Its Subsequent Antibody Labeling for Flow Cytometric Analysis

The effects of heat and chemical treatments on Staphylococcus aureus viability and physiology and their subsequent effects on antibody binding ability and cell morphology were measured. Treatments included lethal and sublethal heat; exposure to organic acids, salt, and sodium hydroxide; and freeze-thawing. Strain-related differences in viability were noted depending on treatment and were reflected in changes in physiology as monitored by flow cytometry (FCM) using three different staining protocols: SYTO 9/propidium iodide (PI), DiOC₂(3), or calcein acetoxymethyl ester (calcein-AM)/PI. Treatments that resulted in significant losses in viability as measured by plate counting were reflected better by the first two staining combinations, as intracellular calcein-AM uptake may have been impaired by certain treatments. FCM analysis using labeling by commercial anti-S. aureus antibodies indicated that differences in cell physiology as a result of treatments influenced immunofluorescence detection. The ratio of the mean fluorescence intensities of stained cells to those of unstained cells [MFI/MFI_(us)] varied with treatment, five of these treatments, including freeze-thaw, citric acid, oxalic acid, NaCl, and NaOH treatments, resulted in significantly lower fluorescence values compared to controls.IMPORTANCE FCM data indicated that cells conventionally considered to be dead and which would not give rise to CFU in a plate count assay, e.g., cells heated to 80°C, were labeled by antibody staining. This finding suggests that without the inclusion of a live/dead discriminating dye, these cells would be erroneously detected as viable within an FCM assay. Reductions in antibody staining due to physicochemical treatment were strain related, reflecting the complexity of the phenomenon under study and illustrating that substantial validation of any new antibody detection-based method, including physiological staining and cell sorting, should be undertaken. Researchers should be aware of physicochemical treatments causing false-negative results: in this study, freeze-thawing severely reduced antibody binding without affecting the viability of a substantial percentage of cells. Scanning electron microscopy carried out on treated cells revealed a range of morphological changes resulting from physicochemical treatments which may have hindered antibody binding.

The K1 capsular polysaccharide of Acinetobacter baumannii strain 307-0294 is a major virulence factor

Acinetobacter baumannii is a pathogen of increasing medical importance with a propensity to be multidrug resistant, thereby making treatment challenging. Little is known of virulence traits in A. baumannii. To identify virulence factors and potential drug targets, random transposon (Tn) mutants derived from the A. baumannii strain AB307-0294 were screened to identify genes essential for growth in human ascites fluid in vitro, an inflammatory exudative fluid. These studies led to the identification of two genes that were predicted to be required for capsule polymerization and assembly. The first, ptk, encodes a putative protein tyrosine kinase (PTK), and the second, epsA, encodes a putative polysaccharide export outer membrane protein (EpsA). Monoclonal antibodies used in flow cytometric and Western analyses confirmed that these genes are required for a capsule-positive phenotype. A capsule-positive phenotype significantly optimized growth in human ascites fluid, survival in human serum, and survival in a rat soft tissue infection model. Importantly, the clearance of the capsule-minus mutants AB307.30 (ptk mutant, capsule minus) and AB307.45 (epsA mutant, capsule minus) was complete and durable. These data demonstrated that the K1 capsule from AB307-0294 was an important protectin. Further, these data suggested that conserved proteins, which contribute to the capsule-positive phenotype, are potential antivirulence drug targets. Therefore, the results from this study have important biologic and translational implications and, to the best of our knowledge, are the first to address the role of capsule in the pathogenesis of A. baumannii infection.

Characterisation and immune responses to meningococcal recombinant porin complexes incorporated into liposomes

We have analysed the structure of meningococcal outer membrane complexes and found that the main complexes are formed by different combinations of PorA and/or PorB molecules, associated to other proteins such as RmpM. In view of the growing knowledge of the importance of conformational epitopes in the immune responses to many pathogens, our aim in this study was to analyse the interactions of PorA and PorB by reconstitution of both recombinant porins into liposomes and determine the relevance of these interactions for the immune response. Recombinant PorA and PorB incorporated into liposomes associate forming complexes that are homomeric when only one of the porins is present, but heteromeric when both neisserial porins are present, mimicking those found previously in native outer membrane vesicles (OMVs). Association of PorA and PorB to form heterocomplexes modifies the immunogenicity of at least PorB, allowing the production of antibodies that recognise conformational epitopes, and produces new epitopes that react with a 50 kDa outer membrane protein not yet identified.

Meningococcal protein antigens and vaccines

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

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.

Opsonophagocytic activity and other serological indications of Bordetella pertussis infection in military recruits in Norway

Bordetella pertussis is the causative agent of pertussis (whooping cough). Despite high vaccination coverage, pertussis remains a significant disease in many countries. Besides vaccination, transient carriage of Bordetella spp. or other cross-reacting organisms adds to the immunity against pertussis. However, the various immunological mechanisms conferring protection remain largely unknown. In this study, paired serum samples from 464 healthy Norwegian military recruits were collected, the first at enrolment and the second about 8 months later. The prevalence of pertussis during military service was examined by comparing the paired serum samples for immunoglobulin G (IgG) antibodies against pertussis toxin (PT) by enzyme-linked immunosorbent assay (ELISA). Seventy-eight percent of the recruits had low levels of IgG antibodies against PT in both samples. Conversely, 8.4% of the recruits demonstrated high anti-PT IgG levels in the first sample, indicative of recent pertussis prior to enrolment. One recruit experienced seroconversion, indicating pertussis during service. A subset of 248 serum samples with low, medium, and high anti-PT IgG titers were analyzed by a different ELISA kit for IgG and IgA antibodies against PT and filamentous hemagglutinin (FHA) and for opsonophagocytic activity (OPA), for induction of C3b deposition products, and for IgG binding with live B. pertussis as the antigen. We observed high correlations between OPA and IgG against live bacteria (r = 0.83), between OPA and IgG anti-FHA (r = 0.79), between OPA and anti-PT IgG (r = 0.68), and between OPA and C3b binding (r = 0.70) (P < 0.0001 for all). Anti-PT IgA did not correlate closely with the other assays.

Persisting immune responses indicating long-term protection after booster dose with meningococcal group B outer membrane vesicle vaccine

MenBvac is an outer membrane vesicle vaccine against systemic meningococcal disease caused by serogroup B Neisseria meningitidis. In this placebo-controlled double-blind study including 374 healthy adolescents, the safety and immunogenicity of a schedule of three primary doses 6 weeks apart followed by a fourth dose a year later were evaluated. Antibody responses to the vaccine strain and heterologous strains (non-vaccine-type strains) and the persistence of these antibodies were measured by the serum bactericidal assay (SBA) and enzyme-linked immunosorbent assay up to 1 year after the last dose. The proportion of subjects with SBA titers of > or = 4 against the vaccine strain increased from 3% prevaccination to 65% after the third dose. Ten months later, this proportion had declined to 28%. The fourth dose induced a booster response demonstrated by 93% of subjects achieving a titer of > or = 4. One year after the booster dose, 64% still showed SBA titers of > or = 4. Cross-reacting antibodies were induced against all heterologous strains tested, although the magnitude of SBA titers differed widely between the different strains. All four doses of MenBvac were safe. Both MenBvac and the placebo had reactogenicity profiles of mild to moderate local and systemic reactions. Pain, the most common reaction, was reported with similar frequencies in both groups. No serious adverse events occurred in the MenBvac group. This study confirmed the good immunogenicity of the primary course of MenBvac and demonstrated prolonged persistence and increased cross-reactivity of functional antibodies elicited by a booster dose.

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.

Streptococcus pneumoniae enolase is important for plasminogen binding despite low abundance of enolase protein on the bacterial cell surface

Enolase represents one of the anchorless surface proteins of Streptococcus pneumoniae and has previously been identified as a plasminogen-binding protein, endowing this pathogen with host proteolytic activity. In this study the mAb 245,C-6 (IgG1) was produced in a BALB/c mouse after immunizing with a protein fraction from S. pneumoniae. The mAb reacted with recombinant pneumococcal enolase both under non-denaturing and denaturing conditions. The epitope for the mAb was mapped to residues 55DKSRYGGLG63 of pneumococcal enolase using a peptide array. By applying the previously reported structure of enolase, this epitope was localized in a surface-exposed loop in each of the monomers of the octameric enolase. Previous immunoelectron microscopic studies, using polyclonal rabbit antibodies against enolase, depicted enolase on the cell surface but did not quantify the amount of surface-exposed enolase on viable pneumococci. Here, flow cytometry revealed no binding of mAb 245,C-6 to viable pneumococci, including TIGR4 and its non-encapsulated isogenic mutant, and only a minor increase of fluorescence intensity was measured when the polyclonal anti-enolase antibodies were used. In contrast, control antibodies recognizing the choline-binding proteins (CBPs) PspA and PspC showed high reactivities. The non-encapsulated TIGR4 did not show increased levels of antibody binding for mAb 245,C-6 or polyclonal anti-enolase antibodies, but revealed increased binding of polyclonal antibodies reacting with PspA or PspC. These results suggest that, compared to other surface-exposed proteins such as CBPs, the amount of enolase under the selected conditions is low. Flow cytometry, however, with FITC-labelled plasminogen demonstrated that the amount of surface-exposed enolase is important for plasminogen binding and, therefore, is also important for pneumococcal pathogenesis.

Development and characterisation of outer membrane vesicle vaccines against serogroup A Neisseria meningitidis

Neisseria meningitidis bacteria of serogroup A are causing recurring meningitis epidemics on the African continent. An outer membrane vesicle (OMV) vaccine against serogroup A meningococci made from a subgroup III serogroup A meningococcal strain was previously shown to induce antibodies with serum bactericidal activity (SBA) in mice. We have here further investigated the properties of OMV vaccines made from five different subgroup III serogroup A meningococcal strains grown in a synthetic medium with low iron content. In addition to the major outer membrane proteins (PorA, PorB, RmpM, Opa and OpcA), small amounts of the NadA, TdfH, Omp85, FetA, FbpA and NspA outer membrane proteins, as well as lipooligosaccharides, were detected in the vaccines. The OMV vaccines were used to immunise mice. Anti-meningococcal IgG antibodies in the mouse sera were analysed by immunoblotting and by enzyme-linked immunosorbent assay against OMVs, and against live meningococcal cells in SBA and a flow-cytometric assay. The vaccines induced antibodies with high SBA and opsonophagocytic activity. The strongest IgG responses were directed against PorA. Significant SBA responses were also observed against a subgroup III strain, which did not express PorA, whereas no SBA was observed against a clone IV-1 serogroup A strain. An OMV vaccine from serogroup A meningococci may be an alternative to polysaccharide and conjugate polysaccharide vaccines for Africa.

Immunogenicity and bactericidal activity in mice of an outer membrane protein vesicle vaccine against Neisseria meningitidis serogroup A disease

Serogroup A Neisseria meningitidis organisms of the subgroup III have caused epidemics of meningitis in sub-Saharan Africa since their introduction into the continent in 1987. The population structure of these bacteria is basically clonal, and these meningococci are strikingly similar in their major outer membrane antigens PorA and PorB. Protein-based vaccines might be an alternative to prevent epidemics caused by these meningococci; thus, we developed an outer membrane vesicle (OMV) vaccine from a serogroup A meningococcal strain of subgroup III. The serogroup A OMV vaccine was highly immunogenic in mice and elicited significant bactericidal activity towards several other serogroup A meningococci of subgroup III. The IgG antibodies generated were in immunoblot shown to be mainly directed towards the PorA outer membrane protein. The results presented demonstrate the potential of an OMV vaccine as an optional strategy to protect against meningococcal disease caused by serogroup A in Africa.

The four mouse IgG isotypes differ extensively in bactericidal and opsonophagocytic activity when reacting with the P1.16 epitope on the outer membrane PorA protein of Neisseria meningitidis

Mouse monoclonal antibodies (MoAbs) of the four IgG isotypes, all specific for the P1.16 epitope on the meningcoccal PorA protein, were tested for functional activities. The avidities of the antibodies, measured by NH4SCN elution in enzyme-linked immunosorbent assay, showed similar values for all the MoAbs. The serum bactericidal activity (SBA) defined as the lowest concentration of antibodies giving 50% reduction in the number of meningococcal colony-forming units using human serum as complement, showed a hierarchy of IgG3 >> IgG2b > IgG2a >> IgG1. For the opsonophagocytosis (OP), the hierarchy was IgG3 > IgG2b = IgG2a >> IgG1. OP was measured in flow cytometry using log-phase live meningococci as target cells, normal human peripheral blood polymorphonuclear cells (PMNs) as effector cells and human serum as a complement source. The mouse MoAbs were negative in OP when using human PMNs in the absence of complement. The results demonstrate the importance of choosing the right isotype of mouse MoAbs when using them to judge the potential vaccine importance of their corresponding antigen. If such MoAbs should be used for passive vaccination against infectious diseases, the isotype would presumably play an important role for their anticipated clinical effects.

Inhibition of C5a-induced inflammation with preserved C5b-9-mediated bactericidal activity in a human whole blood model of meningococcal sepsis

The complement system plays an important role in the initial defense against Neisseria meningitidis. In contrast, uncontrolled activation in meningococcal sepsis contributes to the development of tissue damage and shock. In a novel human whole blood model of meningococcal sepsis, we studied the effect of complement inhibition on inflammation and bacterial killing. Monoclonal antibodies (mAbs) blocking lectin and alternative pathways inhibited complement activation by N meningitidis and oxidative burst induced in granulocytes and monocytes. Oxidative burst was critically dependent on CD11b/CD18 (CR3) expression but not on Fc gamma-receptors. Specific inhibition of C5a using mAb 137-26 binding the C5a moiety of C5 before cleavage prohibited CR3 up-regulation, phagocytosis, and oxidative burst but had no effect on C5b-9 (TCC) formation, lysis, and bacterial killing. An mAb-blocking cleavage of C5, preventing C5a and TCC formation, showed the same effect on CR3, phagocytosis, and oxidative burst as the anti-C5a mAb but additionally inhibited TCC formation, lysis, and bacterial killing, consistent with a C5b-9-dependent killing mechanism. In conclusion, the anti-C5a mAb 137-26 inhibits the potentially harmful effects of N meningitidis-induced C5a formation while preserving complement-mediated bacterial killing. We suggest that this may be an attractive approach for the treatment of meningococcal sepsis.

Construction and functional activities of chimeric mouse-human immunoglobulin G and immunoglobulin M antibodies against the Neisseria meningitidis PorA P1.7 and P1.16 epitopes

We studied the in vitro protective activities of human immunoglobulin G1 (IgG1), IgG3, and IgM antibodies against group B meningococci by constructing sets of chimeric mouse-human antibodies (chIgG1, chIgG3, and chIgM, respectively) with identical binding regions against the P1.7 and P1.16 epitopes on PorA. This was done by cloning the V genes of three mouse hybridoma antibodies and subsequently transfecting vectors containing the homologous heavy- and light-chain genes into NSO cells. Cell clones secreting intact human chIgG1, chIgG3, or chIgM antibodies originating from three parent mouse antibodies were isolated. The functional affinities appeared to be similar for all human isotypes and surprisingly also for the pentameric chIgM antibody. chIgG1 exhibited greater serum bactericidal activity (SBA) than chIgG3, while chIgG3 was more efficient in inducing a respiratory burst (RB) associated with opsonophagocytosis than chIgG1 was. On the other hand, chIgM exhibited SBA similar to that of chIgG1, but it exhibited much higher RB activity than chIgG3 and chIgG1 exhibited. The antibodies against the P1.16 epitope were more efficient in terms of SBA than the antibodies against the P1.7 epitope were; thus, 10- to 40-fold-lower concentrations of antibodies against P1.16 than of antibodies against P1.7 were needed to induce SBA. On the other hand, antibodies against these epitopes were equally effective in inducing RB. Our results revealed differences in the functional activities of human chIgG1, chIgG3, and chIgM antibodies against meningococci, which might influence their protective effects against meningococcal disease.

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

Antibody specificities of pre- and postvaccination serum samples from 40 (53%) teenagers who received three doses of the Norwegian Neisseria meningitidis serogroup B vaccine (B:15:P1.7,16) during a previous trial in Iceland (Perkins et al., J. Infect. Dis. 177:683-691, 1998) were analyzed with serum bactericidal activity (SBA) and immunoblotting assays with reference and isogenic meningococcal H44/76 vaccine strains. The H44/76 variants demonstrated significant vaccine-induced SBA to P1.7,16 PorA and Opc but not to PorB, Opa5.5, and a heterologous PorA protein. On blots, immunoglobulin G levels to all these proteins increased significantly after vaccination. Measurement of SBA to the two main variable regions (P1.7 and P1.16) on the P1.7,16 PorA with PorA deletion mutants revealed significantly higher activity to the P1.7,- and P1.-,16 mutants compared to the P1.7,16 strain, indicating exposure of new accessible epitopes. Only 12 (30%) serum samples showed distinct decreases with these or the P1.-,- mutant, with most samples containing SBA to the P1.7 and P1.16 combination. In contrast, P1.16-specific antibodies were mainly found on blots. Thirteen of the vaccinees (32.5%) were carriers of meningococci at the time of the third dose, of whom four (30.8%) harbored strains of the ET-5 complex. Carriage of P1.15 strains was generally reflected in > or =4-fold increases in SBA and distinct immunoglobulin G binding to the P1.19,15 PorA on blots. Although vaccination did not elicit bactericidal activity to the serotype 15 PorB, most carriers of serotype 15 strains showed > or =4-fold increases in SBA to this antigen.

Cloning, sequencing and expression of immunoglobulin variable regions of murine monoclonal antibodies specific for the P1.7 and P1.16 PorA protein loops of Neisseria meningitidis

The P1.7 and P1.16 epitopes on the PorA protein on the outer membrane of Neisseria meningitidis can induce protective antibodies upon vaccination. Structural analysis of antibodies to these targets can give information on the immune response induced by these epitopes and can reveal any structural similarities among the antibodies. To do so, we have isolated the immunoglobulin (Ig) variable genes from four mouse hybridomas expressing antibodies against the P1.7 and P1.16 epitopes. These V genes were successfully expressed as functional chimeric (ch) mouse/human IgG1 antibodies by subcloning them into expression vectors containing the constant genes of human heavy and light chains. Sequencing the two sets of V genes against P1.16 revealed a high degree of homology, similar to that previously published for P1.7 V genes. The close homology allowed us to interchange heavy and light chains between antibodies in some instances to construct new antibodies that bind the original antigen. This study demonstrates that the immune response in mice against the meningococcal PorA protein epitopes P1.7 as well as P1.16 is limited to few and very similar germline genes, and therefore the P1.7- and P1.16-specific antibodies share high degree of similarities amongst each other. These V genes were used to construct chimeric antibodies with conserved antigen-binding activity.

Neisseria meningitidis serogroup B: laboratory correlates of protection

Meningococcal disease in the Western countries is frequently caused by Neisseria meningitidis serogroup B. Major efforts have been made to develop a safe and efficacious vaccine against this serogroup which is suitable for use in infants and young children. To assess the quality of the immune response after vaccination with candidate vaccines, laboratory correlates of protection are needed. For serogroups A and C, serum bactericidal activity (SBA) is a well established predictor for protection, but for serogroup B other mechanisms besides SBA may also be involved in conferring protection from disease. Several laboratory methods for identification and evaluation of the immunogenicity of possible vaccine antigens are described in this review.

Phylogenetic Evidence for Frequent Positive Selection and Recombination in the Meningococcal Surface Antigen PorB

Previous estimates of rates of synonymous (d(S)) and nonsynonymous (d(N)) substitution among Neisseria meningitidis gene sequences suggested that the surface loops of the variable outer membrane protein PorB were under only weak selection pressure from the host immune response. These findings were consistent with studies indicating that PorB variants were not always protective in immunological and microbiological assays and questioned the suitability of this protein as a vaccine component. PorB, which is expressed at high levels on the surface of the meningococcus, has been implicated in mechanisms of pathogenesis and has also been used as a typing target in epidemiological investigations. In this work, using more precise estimates of selection pressures and recombination rates, we have shown that some residues in the surface loops of PorB are under very strong positive selection, as great as that observed in human immunodeficiency virus-1 surface glycoproteins, whereas amino acids within the loops and the membrane-spanning regions of the protein are under purifying selection, presumably because of structural constraints. Congruence tests showed that recombination occurred at a rate that was not sufficient to erase all phylogenetic similarity and did not greatly bias selection analysis. Homology models of PorB structure indicated that many strongly selected sites encoded residues that were predicted to be exposed to host immune responses, implying that this protein is under strong immune selection and requires further examination as a potential vaccine candidate. These data show that phylogenetic inference can be used to complement immunological and biochemical data in the choice of vaccine candidates.

Immunization with the recombinant PorB outer membrane protein induces a bactericidal immune response against Neisseria meningitidis

Infections with Neisseria meningitidis are characterized by life-threatening meningitis and septicemia. The meningococcal porin proteins from serogroup B meningococci have been identified as candidates for inclusion in vaccines to prevent such infections. In this study, we investigated the vaccine potential of the PorB porin protein free of other meningococcal components. The porB gene from a strain of Neisseria meningitidis expressing the class 3 outer membrane porin protein (PorB3) was cloned into the pRSETB vector, and the protein was expressed at high levels in a heterologous host Escherichia coli. The recombinant protein was purified to homogeneity by affinity chromatography and used for immunization after incorporation into liposomes and into micelles composed either of zwitterionic detergent or nondetergent sulfobetaine. The immunogenicity of these preparations was compared to recombinant PorB protein adsorbed to Al(OH)(3) adjuvant as a control. Although sera raised against the protein adsorbed to Al(OH)(3) reacted with the purified recombinant protein, sera raised against liposomes and micelles showed greater activity with native protein, as measured by enzyme immunoassay with outer membranes and by whole-cell immunofluorescence. Reactivity with native protein was considerably enhanced by incorporation of the adjuvant monophosphoryl lipid A into the liposome or micelle preparations. Recognition of the native protein was in a serotype-specific manner and was associated with the ability of the antisera to promote high levels of serotype-specific complement-mediated killing of meningococci. These results demonstrate that the PorB protein should be considered as a component of a vaccine designed to prevent serogroup B meningococcal infection.