Bactericidal Activity Of Antibodies Research Articles

A new candidate epitope-based vaccine against PspA PhtD of Streptococcus pneumoniae: a computational experimental approach.

Pneumococcus is an important respiratory pathogen that is associated with high rates of death in newborn children and the elderly. Given the disadvantages of current polysaccharide-based vaccines, the most promising alternative for developing improved vaccines may be to use protein antigens with different roles in pneumococcus virulence. PspA and PhtD, highly immunogenic surface proteins expressed by almost all pneumococcal strains, are capable of eliciting protective immunity against lethal infections. In this study using immunoinformatics approaches, we constructed one fusion construct (called PAD) by fusing the immunodominant regions of PspA from families 1 & 2 (PA) to the immunodominant regions of PhtD (PD). The objective of this project was to test the immunogenicity of the fusion protein PAD and to compare its protective activity against S. pneumoniae infection with PA or PD alone and a combination of PA and PD. The prediction of physicochemical properties, antigenicity, allergenicity, toxicity, and 3D-structure of the constructs, as well as molecular docking with HLA receptor and immune simulation were performed using computational tools. Finally, mice were immunized and the serum levels of antibodies/cytokines and functionality of antibodies in vitro were evaluated after immunization. The mice survival rates and decrease of bacterial loads in the blood/spleen were examined following the challenge. The computational analyses indicated the proposed constructs could be antigenic, non-allergenic, non-toxic, soluble and able to elicit robust immune responses. The results of actual animal experiments revealed the candidate vaccines could induce the mice to produce high levels of antibodies and cytokines. The complement-mediated bactericidal activity of antibodies was confirmed and the antibodies provided favorable survival in immunized mice after bacterial challenge. In general, the experimental results verified the immunoinformatics studies. For the first time this report presents novel peptide-based vaccine candidates consisting of immunodominant regions of PspA and PhtD antigens. The obtained findings confirmed that the fusion formulation could be relatively more efficient than the individual and combination formulations. The results propose that the fusion protein alone could be used as a serotype-independent pneumococcal vaccine or as an effective partner protein for a conjugate polysaccharide vaccine.

A Serum Bactericidal Assay for the Complement-Mediated Bactericidal Activity of Antibodies

A Serum Bactericidal Assay for the Complement-Mediated Bactericidal Activity of Antibodies

A Serum Bactericidal Assay for the Complement-Mediated Bactericidal Activity of Antibodies

A Serum Bactericidal Assay for the Complement-Mediated Bactericidal Activity of Antibodies

A recombinant commensal bacteria elicits heterologous antigen-specific immune responses during pharyngeal carriage.

The human nasopharynx contains a stable microbial ecosystem of commensal and potentially pathogenic bacteria, which can elicit protective primary and secondary immune responses. Experimental intranasal infection of human adults with the commensal Neisseria lactamica produced safe, sustained pharyngeal colonization. This has potential utility as a vehicle for sustained release of antigen to the human mucosa, but commensals in general are thought to be immunologically tolerated. Here, we show that engineered N. lactamica, chromosomally transformed to express a heterologous vaccine antigen, safely induces systemic, antigen-specific immune responses during carriage in humans. When the N. lactamica expressing the meningococcal antigen Neisseria Adhesin A (NadA) was inoculated intranasally into human volunteers, all colonized participants carried the bacteria asymptomatically for at least 28 days, with most (86%) still carrying the bacteria at 90 days. Compared to an otherwise isogenic but phenotypically wild-type strain, colonization with NadA-expressing N. lactamica generated NadA-specific immunoglobulin G (IgG)- and IgA-secreting plasma cells within 14 days of colonization and NadA-specific IgG memory B cells within 28 days of colonization. NadA-specific IgG memory B cells were detected in peripheral blood of colonized participants for at least 90 days. Over the same period, there was seroconversion against NadA and generation of serum bactericidal antibody activity against a NadA-expressing meningococcus. The controlled infection was safe, and there was no transmission to adult bedroom sharers during the 90-day period. Genetically modified N. lactamica could therefore be used to generate beneficial immune responses to heterologous antigens during sustained pharyngeal carriage.

Bactericidal antibodies against hypervirulent Neisseria meningitidis C field strains following MenC-CRM or MenACWY-CRM priming and MenACWY-CRM booster in children

ABSTRACT An increase in invasive meningococcal disease (IMD) incidence was observed in Tuscany in 2015/2016, mainly due to hypervirulent clonal complex (cc) 11 strains. In a post-hoc analysis, we assessed bactericidal activity of antibodies in sera from children primed with MenACWY-CRM or MenC-CRM conjugate vaccines and receiving a MenACWY-CRM booster dose against 5 meningococcal C (MenC) strains isolated from IMD cases. Sera collected from 90 infants/toddlers who participated in a phase III, open-label study (NCT00667602) and its extension (NCT01345721) were tested by serum bactericidal activity assay with human complement (hSBA). Children were primed with either MenACWY-CRM at 6–8 and 12 months of age (group 2_MenACWY; N = 30), MenACWY-CRM (group 1_MenACWY; N = 30), or MenC-CRM at 12 months of age (group 1_MenC; N = 30); all received MenACWY-CRM booster dose at 22–45 months of age. Four tested strains (FI001–FI004) were C:P1.5–1,10-8:F3-6:ST-11 (cc11) and 1 (FI005) was C:P1.7–4,14-6:F3-9:ST-1031 (cc334). Overall, immune responses tended to be higher against Fl002–FI004 than Fl001 and Fl005. Geometric mean titers were high in group 2_MenACWY (range: 94.8 [FI005]–588.1 [FI004]) and very high post-boosting with MenACWY-CRM in all groups (176.9 [FI005]–3911.0 [FI004]). Seroresponse rates tended to be higher in group 1_MenC (33.3% [FI005]–93.3% [FI004]) than in group 1_MenACWY (16.7% [FI005]–73.3% [FI004]). Irrespective of strains tested or the identity/number of priming doses, ≥96.7% of children had hSBA titers ≥1:8 post-MenACWY-CRM booster dose. MenACWY-CRM and MenC-CRM elicited bactericidal antibodies and immunological memory against hypervirulent cc11 and cc334 MenC strains responsible for IMD outbreaks.

Intra-Laboratory Evaluation of Luminescence Based High-Throughput Serum Bactericidal Assay (L-SBA) to Determine Bactericidal Activity of Human Sera against Shigella.

Despite the huge decrease in deaths caused by Shigella worldwide in recent decades, shigellosis still causes over 200,000 deaths every year. No vaccine is currently available, and the morbidity of the disease coupled with the rise of antimicrobial resistance renders the introduction of an effective vaccine extremely urgent. Although a clear immune correlate of protection against shigellosis has not yet been established, the demonstration of the bactericidal activity of antibodies induced upon vaccination may provide one means of the functionality of antibodies induced in protecting against Shigella. The method of choice to evaluate the complement-mediated functional activity of vaccine-induced antibodies is the Serum Bactericidal Assay (SBA). Here we present the development and intra-laboratory characterization of a high-throughput luminescence-based SBA (L-SBA) method, based on the detection of ATP as a proxy of surviving bacteria, to evaluate the complement-mediated killing of human sera. We demonstrated the high specificity of the assay against a homologous strain without any heterologous aspecificity detected against species-related and non-species-related strains. We assessed the linearity, repeatability and reproducibility of L-SBA on human sera. This work will guide the bactericidal activity assessment of clinical sera raised against S. sonnei. The method has the potential of being applicable with similar performances to determine the bactericidal activity of any non-clinical and clinical sera that rely on complement-mediated killing.

Complement-Dependent Serum Bactericidal Assays for Neisseria gonorrhoeae.

The global spread of multidrug-resistant gonorrhea has spurred efforts to develop a safe and effective vaccine against gonorrhea. Complement plays an important role in host defenses against Neisseria infections. Complement-dependent bactericidal activity of antibodies (either natural antibodies or those elicited by immunization) is a well-established correlate of protection against meningococcal infections. Although correlates of protection against gonococcal infection have not been defined, there is evidence to suggest that complement-mediated killing may also predict vaccine efficacy against this disease. This chapter describes methods to prepare human complement sources and perform bactericidal assays against Neisseria gonorrhoeae.

Immunoprotective potential of BamA, the outer membrane protein assembly factor, against MDR Acinetobacter baumannii

AbstarctAcinetobacter baumannii infections are responsible for major health problems in immunocompromised patients particularly in intensive care units. Due to rapid acquisition of and also inherent drug resistance, a vaccine is an effective treatment option against this pathogen. BamA, an outer membrane β-barrel assembly protein, was identified in A. baumannii as potential vaccine candidate by in silico analysis. The immunoprotective efficacy of this highly conserved protein was investigated against a virulent multidrug resistant clinical isolate using murine pneumonia model. Recombinant BamA elicited a high IgG antibody titer (160000) in mice. Opsonophagocytic killing assay showed non-neutrilizing, opsonizing antibodies with combinatorial bactericidal activity of antibodies and complement components. Active and passive immunization protected 80 and 60% mice respectively against intranasal challenge with lethal dose (109 CFU) of virulent A. baumannii along with efficient clearance of bacteria in mice lungs and reduction in levels of pro-inflammatory cytokines viz. TNF-α, IL-6 and IL-1β in sera and lung tissue homogenate. Increase in levels of IL-10, an anti-inflammatory cytokine and reduction of neutrophils in lungs facilitated the control of infection. This study demonstrates the potential of BamA as effective vaccine candidate and a promising target for antibody-based therapy to protect against MDR A. baumannii infections.

Construction of an inactivated typhoid vaccine candidate expressing Escherichia coli heat-labile enterotoxin B subunit and evaluation of its immunogenicity in a murine model.

Purpose. Typhoid fever caused by Salmonella enterica serovar Typhi has contributed to the global public health burden, particularly in developing countries. In this study, an S. Typhi ghost was developed and its capacity as a vaccine candidate against typhoid fever was assessed.Methodology. An asd + plasmid pJHL187 harbouring a ghost cassette comprising the PhiX 174 Elysis gene tightly controlled under the convergent promotor system was transformed into an asd gene-deleted mutant S.Typhi strain (STG). The eltB gene encoding the E. coli heat-labile enterotoxin (LTB) protein was subcloned into a foreign antigen delivery cassette of pJHL187 to increase mucosal immunity.Results. The stringent repression and expression of the lethal E lysis gene in the system allowed stable production of the ghost strain and secretion of LTB, which was confirmed by immune blot analysis. The level of IgG and sIgA was significantly increased in the mice subcutaneously immunized with STG-LTB compared to the non-immunized mice (P<0.05). The CD3+CD4+ T cell subpopulation was augmented in the immunized group (P<0.05) and showed the increment of immunomodulatory cytokines IL-2, IL-6, IL-12, IL-17 and IFN-γ in in vitro restimulated splenocytes isolated from the inoculated mice. The serum bactericidal activity of antibodies generated in the rabbits injected with STG-LTB was proved by the elimination of approximately 87.5 % of wild-type S. Typhi in the presence of exogenous complement.Conclusion. The results demonstrated that the STG-LTB ghost effectively enhanced the immunological responses, meaning that STG-LTB is potentially available as a vaccine candidate against typhoid fever.

A novel approach for construction of an inactivated typhoid vaccine candidate that effectively augments both humoral and cellular immune responses

Salmonella enterica serovar Typhi ghost was constructed as a vaccine candidate against typhoid fever. An asd+ plasmid pJHL187 harboring a ghost cassette comprised of PhiX 174 E lysis gene stringently controlled under the convergent promotor components and was transformed into the asd gene-deleted mutant S. Typhi Ty21a strain (STG). Twenty female BALB/c mice randomly assigned into two groups were subcutaneously vaccinated at 5weeks of age to assess immunogenic characteristics of the constructed STG. The level of serum IgG in the immunized mice was significantly increased during the observational period (P<0.001) as the mice showed the significant elevation of secretory IgA at week 6 compared to those in the non-immunized mice (P<0.05). The CD3+CD4+ T cell subpopulation in the primed splenocytes showed approximately twofold increase in the immunized group. Further, the gene expression of various immunomodulatory cytokines associated with Th-1, Th-2 and Th-17 immunity was observed in in vitro restimulated splenocytes isolated from the immunized mice. Serum Bactericidal activity of antibodies produced in the rabbits immunized with STG was proved by the elimination of almost all of wild-type S. Typhi in the presence of exogenous complement over an hour at week 6 after the first immunization. The immuno-stimulatory traits of STG demonstrated that the construct effectively enhanced the immunological responses, providing a potential of STG as the vaccine candidate against typhoid fever.

Effectiveness of Meningococcal B Vaccine against Endemic HypervirulentNeisseriameningitidisW Strain, England

Serum samples from children immunized with a meningococcal serogroup B vaccine demonstrated potent serum bactericidal antibody activity against the hypervirulent Neisseria meningitidis serogroup W strain circulating in England. The recent introduction of this vaccine into the United Kingdom national immunization program should also help protect infants against this endemic strain.

Development of a serological assay to predict antibody bactericidal activity against non-typeable Haemophilus influenzae

BackgroundNon-typeable Haemophilus influenzae (NTHi) is a Gram negative microorganism residing in the human nasopharyngeal mucosa and occasionally causing infections of both middle ear and lower respiratory airways. A broadly protective vaccine against NTHi has been a long-unmet medical need, as the high genetic variability of this bacterium has posed great challenges.ResultsIn this study, we developed a robust serum bactericidal assay (SBA) to optimize the selection of protective antigens against NTHi. SBA takes advantage of the complement-mediated lysis of bacterial cells and is a key in vitro method for measuring the functional activity of antibodies. As a proof of concept, we assessed the bactericidal activity of antibodies directed against antigens known to elicit a protective response, including protein D used as carrier protein in the Synflorix pneumococcal polysaccharide conjugate vaccine. Prior to SBA screening, the accessibility of antigens to antibodies and the capacity of the latter to induce C3 complement deposition was verified by flow cytometry. Using baby rabbit serum as a source of complement, the proposed assay not only confirmed the bactericidal activity of the antibodies against the selected vaccine candidates, but also showed a significant reproducibility.ConclusionsConsidering the rapidity and cost-effectiveness of this novel SBA protocol, we conclude that it is likely to become an important tool to prove the capability of antibodies directed against recombinant antigens to induce NTHi in vitro killing and to both select new protective vaccine candidates, and predict vaccine efficacy.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0420-x) contains supplementary material, which is available to authorized users.

Re-evaluation of the immunogenicity of meningococcal serogroups A and C polysaccharide vaccine among a healthy population aged 5 to 59

Objective To re-evaluate the immunogenicity of meningococcal serogroups A and C polysaccharide vaccine among a healthy population of age 5 to 59.Methods Pre and post-vaccination ser-um samples were collected from the subjects involved in a randomized , controlled clinical trial conducted in 2005 at Hechi, Guangxi, China.Enzyme-linked immunosorbent assay （ELISA） was performed for the quan-titative detection of specific anti-capsule IgG antibody against meningococcal serogroups A and C in serum samples.Serum bactericidal assay （ SBA） was used to measure the bactericidal antibody activity against se-rougroups A and C bacterial strains .Results Geometric mean concentrations （GMCs） of specific anti-cap-sule IgG antibody were 23.66 μg/ml and 78.83 μg/ml for serogroup A （P〈0.05）, and 1.23 μg/ml and 51.25 μg/ml for serogroup C （P〈0.05） in serum samples of pre and post-vaccination, respectively.Be-fore immunization , 99% of serum samples （ 97/98 ） showed serogroup A-specific IgG concentration ≥2μg/ml, which reached up to 100%（98/98） after vaccination （P〉0.05）.The percentage of serum sam-ples with serogroup C-specific IgG concentration ≥2 μg/ml rose from 20%to 99% after vaccination （ P〈0.05）.The ratio of positive serum samples with at least four times of increases in concentration of specific IgG against serogroup A and serogroup C after vaccine immunization were 34% and 100%, respectively . The rSBA geometric mean titers （ GMTs） for serogroups A and C in serum sample of post-vaccination were＆amp;nbsp;respectively elevated from 1164 to 5530 （P〈0.05）, and from 4 to 6225 （P〈0.05）.The percentage of se-rum samples with rSBA GMTs≥128 increased from 91% to 99% for serogroup A （P〉0.05）, and from 14%to 96%for serogroup C （P〈0.05） after vaccination.The rSBA GMTs with at least four times of in-creases after vaccination were detected respectively in 53% and 100% of serogroups A and C vaccinated subjects.Pearson correlation coefficient between IgG concentration and rSBA GMTs was r=0.15 （pre） and r=0.23 （post） for serogroup A, and r=-0.14 （pre） and r=0.58 （post） for serogroup C （P〈0.05）. Conclusion This study has demonstrated an efficient and sustained immunogenicity with meningococcal sero -groups A and C polysaccharide vaccine as evidenced by the data from standardized assays of ELISA and SBA . Key words: Meningococcal serogroups A and C polysaccharide vaccine; Immuogenicity

Neisseria meningitidis Escape from the Bactericidal Activity of a Monoclonal Antibody Is Mediated by Phase Variation of lgtG and Enhanced by a Mutator Phenotype

Bacteria adapt to environmental changes through high-frequency switches in expression of specific phenotypes. Localized hypermutation mediated by simple sequence repeats is an important mechanism of such phase variation (PV) in Neisseria meningitidis. Loss or gain of nucleotides in a poly(C) tract located in the reading frame results in switches in expression of lgtG and determines whether a glucose or a phosphoethanolamine (PEtn) is added at a specific position in the inner core lipopolysaccharide (LPS). Monoclonal antibody (MAb) B5 is bactericidal for N. meningitidis strain 8047 when PEtn is present in the inner core LPS and lgtG is switched "off." Escape from the bactericidal activity of this antibody was examined by subjecting strain 8047 to multiple cycles of growth in the presence of MAb B5 and human serum. Escape variants with alterations in the lgtG repeat tract rapidly accumulated in bacterial populations during selection with this antibody. Strain 8047 was outcompeted in this assay by the 8047 Delta mutS strain due to the elevated PV rate of this mismatch repair mutant and hence the greater proportion of preexisting phase variants of lgtG in the inoculum. This mutS mutant was also more virulent than strain 8047 during escape from passive protection by MAb B5 in an in vivo infant rat model of bacteremia. These results provide an example of how PV rates can modulate the occurrence and severity of infection and have important implications for understanding the evolution of bacterial fitness in species subject to environmental variations that occur during persistence within and transmission between hosts.

Complement‐Dependent Synergistic Bactericidal Activity of Antibodies against Factor H–Binding Protein, a Sparsely Distributed Meningococcal Vaccine Antigen

Antibodies to factor H (fH)-binding protein (fHBP), a meningococcal vaccine antigen, activate classical complement pathway serum bactericidal activity (SBA) and block binding of the complement inhibitor fH. To understand these 2 functions in protection, we investigated the interactions of human complement and 2 anti-fHBP monoclonal antibodies (MAbs) with encapsulated Neisseria meningitidis. JAR 3 (IgG3) blocks fH binding and elicits SBA against 2 strains with naturally high fHBP expression and a low-expressing strain genetically engineered to express high fHBP levels. JAR 4 (IgG2a) does not block fH binding or elicit SBA. Neither MAb alone elicits SBA against 2 other strains with low fHBP expression, but together the MAbs increase C4b binding and elicit SBA; JAR 3 alone also is bactericidal in whole blood. In nonimmune blood, fHBP knockout mutants from high-expressing stains do not survive, but mutants of low-expressing strains do. Expression of fHBP is a prerequisite for bacterial survival in blood only by strains with naturally high fHBP expression. In low-expressing strains, combinations of 2 nonbactericidal anti-fHBP MAbs can bind to nonoverlapping epitopes, engage C1q, activate C4, and mediate classical complement pathway SBA. In the absence of sufficient C4b binding for SBA, an individual MAb can have opsonophagocytic bactericidal activity.

Age-specific seroprevalence of serogroup C meningococcal serum bactericidal antibody activity and serogroup A, C, W135 and Y-specific IgG concentrations in the Turkish population during 2005

Like many other developing countries; there is no accurate information about the antibody levels against Neisseria meningitidis in Turkey. We collected serum samples from four health centers located in different geographic regions and stratified according to age in order to obtain a baseline seroprevalence of protective antibodies to meningococcal serogroup C and provide data on seroprevalence of IgG antibodies to serogroups A, C, W135 and Y. Sera were tested for serum bactericidal antibodies (SBA) to serogroup C meningococci using rabbit serum as the complement source and by a bead based assay for serogroup A, C, W135 and Y-specific IgG. It was observed that 30% and 12% of individuals within the study population had SBA titers of > or =8 and > or =128, respectively. Overall; at least 70% of the population are susceptible (SBA titer <8) to meningococcal serogroup C disease. The rate of susceptibility was highest in infants aged 7-12 months and young children (1-4 years). Regardless of age, for serogroup A, C, W135 and Y, 60.5%, 27.2%, 12.3% and 19.2% of subjects, respectively, had serogroup-specific IgG concentrations > or =2 microg/mL. These data highlight that a large proportion of the Turkish population are susceptible to serogroups C, W135 and Y and should be considered, along with serogroup-specific disease incidence data, in future decisions on possible meningococcal vaccination programmes.

Additive and Synergistic Bactericidal Activity of Antibodies Directed against Minor Outer Membrane Proteins of Neisseria meningitidis

Neisseria meningitidis serogroup B is a major cause of bacterial meningitis in younger populations. The available vaccines are based on outer membrane vesicles obtained from wild-type strains. In children less than 2 years old they confer protection only against strains expressing homologous PorA, a major, variable outer membrane protein (OMP). We genetically modified a strain in order to eliminate PorA and to overproduce one or several minor and conserved OMPs. Using a mouse model mimicking children's PorA-specific bactericidal activity, it was demonstrated that overproduction of more than one minor OMP is required to elicit antibodies able to induce complement-mediated killing of strains expressing heterologous PorA. It is concluded that a critical density of bactericidal antibodies needs to be reached at the surface of meningococci to induce complement-mediated killing. With minor OMPs, this threshold is reached when more than one antigen is targeted, and this allows cross-protection.

Carriage ofNeisseria meningitidisSerogroup W135 ST-2881

Serogroup W135 ST-2881 meningococci caused a cluster of meningitis cases in Niger in 2003. Of 80 healthy persons in the patients' villages, 28 (35%) carried meningococci; 20 of 21 W135 carrier strains were ST-2881. Ten months later, 5 former carriers were still carriers of W135 ST-2881 strains. The serum bactericidal antibody activity changed according to carrier status.

Improved immunogenicity of a H44/76 group B outer membrane vesicle vaccine with over-expressed genome-derived Neisserial antigen 1870

A broadly protective vaccine against meningococcal group B disease is not available. We previously reported that an outer membrane vesicle (OMV) vaccine containing over-expressed genome-derived antigen (GNA) 1870 elicited broader protective antibody responses than recombinant GNA1870 or conventional OMV vaccines prepared from a strain that naturally expresses low amounts of GNA1870. Certain wildtype strains such as H44/76 naturally express larger amounts of GNA1870 and, potentially, could be used to prepare an improved OMV vaccine without genetic over-expression of the antigen. We transformed H44/76 with a shuttle vector to over-express variant 1 (v.1) GNA1870 and compared the immunogenicity in mice of OMV vaccines prepared from wildtype H44/76 (v.1), the mutant, and a recombinant v.1 GNA1870 vaccine. Mice immunized with OMV with over-expressed GNA1870 developed broader serum bactericidal and/or greater C3 deposition activity on the surface of encapsulated strains of N. meningitidis than control mice immunized with the OMV vaccine prepared from the wildtype strain, or the rGNA1870 vaccine. When a panel of group B strains from patients in California was tested, sera from mice immunized with the OMV vaccine containing over-expressed GNA1870 were bactericidal against 100% of the v.1 strains. In contrast, only 20% of isolates that expressed subvariants of the v.1 GNA1870 protein were susceptible to bactericidal activity of antibodies elicited by the rGNA1870 or conventional OMV vaccines. Thus, even a modest increase in GNA1870 expression in a strain that naturally is a high producer of GNA1870 results in an OMV vaccine that elicits broader protection against meningococcal disease.

Bactericidal activity of antibodies elicited against the Neisseria meningitidis 37-kDa ferric binding protein (FbpA) with different adjuvants.

The 37-kDa ferric binding protein, FbpA, from three Neisseria meningitidis strains was purified to homogeneity with iron-affinity chromatography and used for immunisation of mice employing four different adjuvants: aluminium hydroxide, Freund's, the saponin Quil-A, and a Ribi adjuvant system (RAS). Controls immunised without adjuvant were also included. All sera obtained were monospecific for the meningococcal FbpA, with antibody titres higher when RAS and Quil-A were used (256), PBS resulting in titres similar to those of Freund's (64), and, surprisingly, with no antibodies elicited when aluminium hydroxide, the only approved adjuvant for use in humans, was used. All anti-FbpA sera bound to intact meningococcal cells, showing a complete cross-reactivity, but the bactericidal activity of anti-FbpA antibodies, demonstrated for the first time in this work, was low (32% of killing with the homologous strain), and the analysis of immunoglobulin isotypes showed that the non-bactericidal IgG1 was predominant. The results confirm that the FbpA is surface-exposed, antigenic, and able to elicit bactericidal antibodies, although, in the conditions and with the adjuvants tested, killing efficacy was low and cross-killing was very variable, not supporting the inclusion of this protein in vaccine formulations. Nevertheless, given the high conservation of the FbpA in the genus Neisseria, its surface exposure and its antigenicity, studies on immunisation with peptides corresponding to the exposed epitopes and/or new adjuvant systems could improve the bactericidal response to this protein, making it suitable for vaccine development.