Vaccine potential of the Neisseria meningitidis 2086 lipoprotein

Recombinant outer membrane protein Q and putative lipoprotein from Bordetella pertussis inducing strong humoral response were not protective alone in the murine lung colonization model

Despite high vaccination coverage after introduction of whole cell (wP) and acellular pertussis (aP) vaccines, pertussis resurgence has been reported in many countries. aP vaccines are commonly preferred due to side effects of wP vaccines and formulated with aluminum hydroxide (Alum), which is not an effective adjuvant to eliminate Bordetella pertussis. Low efficiency of current aP vaccines is thought to be the main reason for the resurgence for which newer generation aP vaccines are needed. In the present study, immunogenicity and protective efficacy of outer membrane protein Q (OmpQ) and a putative lipoprotein (Lpp) from B. pertussis were investigated in mice by using two diefrent adjuvants, monophosphoryl lipid A (MPLA) or Alum. OmpQ and putative Lpp were cloned, expressed, and purified from Escherichia coli. The proteins were formulated to immunize mice. Both recombinant OmpQ and putative Lpp induced a significant increase in immunoglobulin G1 (IgG1) and immunoglobulin G2a (IgG2a) responses compared to the control group. Moreover, MPLA-adjuvanted formulations resulted in higher IgG2a levels than Alum-adjuvanted ones. However, there were no significant differences between test and control groups regarding interferon-gamma (IFN-γ) levels, and the mice lung colonization experiments indicated that neither rOmpQ nor rLpp could confer protection alone against B. pertussis challenge.

Genomic Surveillance of 4CMenB Vaccine Antigenic Variants among Disease-Causing Neisseria meningitidis Isolates, United Kingdom, 2010-2016

In September 2015, 4CMenB meningococcal vaccine was introduced into the United Kingdom infant immunization program without phase 3 trial information. Understanding the effect of this program requires enhanced surveillance of invasive meningococcal disease (IMD) Neisseria meningitidis isolates and comparison with prevaccination isolates. Bexsero Antigen Sequence Types (BASTs) were used to analyze whole-genome sequences of 3,073 prevaccine IMD N. meningitidis isolates obtained during 2010−2016. Isolates exhibited 803 BASTs among 31 clonal complexes. Frequencies of antigen peptide variants were factor H binding protein 1, 13.4%; Neisserial heparin-binding antigen 2, 13.8%; Neisseria adhesin A 8, 0.8%; and Porin A-VR2:P1.4,10.9%. In 2015−16, serogroup B isolates showed the highest proportion (35.7%) of exact matches to >1 Bexsero components. Serogroup W isolates showed the highest proportion (93.9%) of putatively cross-reactive variants of Bexsero antigens. Results highlighted the likely role of cross-reactive antigens. BAST surveillance of meningococcal whole-genome sequence data is rapid, scalable, and portable and enables international comparisons of isolates.

The Meningococcal Cysteine Transport System Plays a Crucial Role in Neisseria meningitidis Survival in Human Brain Microvascular Endothelial Cells

Neisseria meningitidis colonizes at a nasopharynx of human as a unique host and has many strains that are auxotrophs for amino acids for their growth. To cause invasive meningococcal diseases (IMD) such as sepsis and meningitis, N. meningitidis passes through epithelial and endothelial barriers and infiltrates into blood and cerebrospinal fluid as well as epithelial and endothelial cells. However, meningococcal nutrients, including cysteine, become less abundant when it more deeply infiltrates the human body even during inflammation, such that N. meningitidis has to acquire nutrients in order to survive/persist, disseminate, and proliferate in humans. This was the first study to examine the relationship between meningococcal cysteine acquisition and the pathogenesis of meningococcal infections. The results of the present study provide insights into the mechanisms by which pathogens with auxotrophs acquire nutrients in hosts and may also contribute to the development of treatments and prevention strategies for IMD.

While Neisseria meningitidis typically exists in an asymptomatic nasopharyngeal carriage state, it may cause potentially lethal diseases in humans, such as septicemia or meningitis, by invading deeper sites in the body. Since the nutrient compositions of human cells are not always conducive to meningococci, N. meningitidis needs to exploit nutrients from host environments. In the present study, the utilization of cysteine by the meningococcal cysteine transport system (CTS) was analyzed for the pathogenesis of meningococcal infections. A N. meningitidis strain deficient in one of the three cts genes annotated as encoding cysteine-binding protein (cbp) exhibited approximately 100-fold less internalization into human brain microvascular endothelial cells (HBMEC) than the wild-type strain. This deficiency was restored by complementation with the three cts genes together, and the infectious phenotype of HBMEC internalization correlated with cysteine uptake activity. However, efficient accumulation of ezrin was observed beneath the cbp mutant. The intracellular survival of the cbp mutant in HBMEC was markedly reduced, whereas equivalent reductions of glutathione concentrations and of resistance to reactive oxygens species in the cbp mutant were not found. The cbp mutant grew well in complete medium but not in synthetic medium supplemented with less than 300 μM cysteine. Taking cysteine concentrations in human cells and other body fluids, including blood and cerebrospinal fluid, into consideration, the present results collectively suggest that the meningococcal CTS is crucial for the acquisition of cysteine from human cells and participates in meningococcal nutrient virulence.

Applications of genomics to slow the spread of multidrug-resistant Neisseria gonorrhoeae

Infections with Neisseria gonorrhoeae, a sexually transmitted pathogen that causes urethritis, cervicitis, and more severe complications, are increasing. Gonorrhea is typically treated with antibiotics; however, N. gonorrhoeae has rapidly acquired resistance to many antibiotic classes, and lineages with reduced susceptibility to the currently recommended therapies are emerging worldwide. In this review, we discuss the contributions of whole genome sequencing (WGS) to our understanding of resistant N. gonorrhoeae. Genomics has illuminated the evolutionary origins and population structure of N. gonorrhoeae and the magnitude of horizontal gene transfer within and between Neisseria species. WGS can be used to predict the susceptibility of N. gonorrhoeae based on known resistance determinants, track the spread of these determinants throughout the N. gonorrhoeae population, and identify novel loci contributing to resistance. WGS has also allowed more detailed epidemiological analysis of transmission of N. gonorrhoeae between individuals and populations than previously used typing methods. Ongoing N. gonorrhoeae genomics will complement other laboratory techniques to understand the biology and evolution of the pathogen, improve diagnostics and treatment in the clinic, and inform public health policies to limit the impact of antibiotic resistance.

Identification and characterization of a novel single-chain variable fragment (scFv) antibody against Neisseria meningitidis factor H-binding protein (fHbp)

Purpose. Neisseria meningitidis is the leading global cause of meningitis and sepsis. Detection, followed by identification, of bacterial pathogens is important in medicine and public health. In the present study, we used the ribosome display technique to select single-chain variable fragments (scFv) that are specific to the surface-exposed fHbp antigen of N. meningitidis. Methodology. The recombinant fHbp protein was used as the antigen for the immunization of BALB/c mice. Anti-fHbp VH/k chain ribosome display libraries were assembled by joining VH and k into the VH/k chain with a specially constructed linker by PCR overlap extension. The scFv library was panned against the recombinant fHbp protein by using a single round of the ribosome display method via a rabbit reticulocyte lysate system.Results/Key findings. The selected anti-fHbp antibody exhibited high affinity and specificity in the enzyme-linked immunosorbent assay (ELISA) and the whole bacterial cell enzyme-linked immunosorbent assay (Bact-ELISA).Conclusion. The affinity of the selected scFv was ~8.65×10⁹ M^-1. The isolated scFv can provide the basis for developing a diagnostic kit.

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.

Molecular characterization of two sub-family specific monoclonal antibodies to meningococcal Factor H binding protein

Factor H binding protein (FHbp) is a component of two licensed vaccines for prevention of sepsis and meningitis caused by serogroup B meningococci. FHbp binds human Factor H (FH), which contributes to evasion of host immunity and FHbp sequence variants can be classified into two sub-families. Antibodies against FHbp elicit complement-mediated killing and can inhibit recruitment of FH to the bacterial surface. We report epitope mapping studies of two murine IgG mAbs, designated JAR 31 and JAR 36, isolated from a mouse immunized with FHbp in sub-family A, which is present in ∼30-40% of invasive isolates. In the present study, we tested the reactivity of mAbs JAR 31 and JAR 36 with seven natural FHbp sequence variants from different phylogenic groups. We screened bacteriophage-displayed peptide libraries to identify amino acid residues contributing to the JAR 36 epitope. Based on the reactivities of mAbs JAR 31 and JAR 36 with the seven FHbp variants, and the frequent occurrences of aspartate (D) and lysine (K) residues in the JAR 36-bound phage peptides, we selected six residues in the carboxyl-terminal region of FHbp for replacement with alanine (A). The D201A and K203A substitutions respectively eliminated and decreased binding of mAbs JAR 31 and JAR 36 to FHbp. These substitutions did not affect binding of the control mAb JAR 33 or of human FH. JAR 31 or JAR 36 mediated cooperative complement-mediated bactericidal activity with other anti-FHbp mAbs. The identification of two amino acid residues involved in the epitopes recognized by these anti-FHbp mAbs may contribute to a more complete understanding of the spatial requirements for cooperative anti-FHbp mAb bactericidal activity.

Distinct evolutionary patterns of Neisseria meningitidis serogroup B disease outbreaks at two universities in the USA

Neisseria meningitidis serogroup B (MnB) was responsible for two independent meningococcal disease outbreaks at universities in the USA during 2013. The first at University A in New Jersey included nine confirmed cases reported between March 2013 and March 2014. The second outbreak occurred at University B in California, with four confirmed cases during November 2013. The public health response to these outbreaks included the approval and deployment of a serogroup B meningococcal vaccine that was not yet licensed in the USA. This study investigated the use of whole-genome sequencing(WGS) to examine the genetic profile of the disease-causing outbreak isolates at each university. Comparative WGS revealed differences in evolutionary patterns between the two disease outbreaks. The University A outbreak isolates were very closely related, with differences primarily attributed to single nucleotide polymorphisms/insertion-deletion (SNP/indel) events. In contrast, the University B outbreak isolates segregated into two phylogenetic clades, differing in large part due to recombination events covering extensive regions (>30 kb) of the genome including virulence factors. This high-resolution comparison of two meningococcal disease outbreaks further demonstrates the genetic complexity of meningococcal bacteria as related to evolution and disease virulence.

Toll-like Receptor Agonist Conjugation: A Chemical Perspective

Toll-like receptors (TLRs) are vital elements of the mammalian immune system that function by recognizing pathogen-associated molecular patterns (PAMPs), bridging innate and adaptive immunity. They have become a prominent therapeutic target for the treatment of infectious diseases, cancer, and allergies, with many TLR agonists currently in clinical trials or approved as immunostimulants. Numerous studies have shown that conjugation of TLR agonists to other molecules can beneficially influence their potency, toxicity, pharmacokinetics, or function. The functional properties of TLR agonist conjugates, however, are highly dependent on the ligation strategy employed. Here, we review the chemical structural requirements for effective functional TLR agonist conjugation. In addition, we provide similar analysis for those that have yet to be conjugated. Moreover, we discuss applications of covalent TLR agonist conjugation and their implications for clinical use.

Predicting the Susceptibility of Meningococcal Serogroup B Isolates to Bactericidal Antibodies Elicited by Bivalent rLP2086, a Novel Prophylactic Vaccine

Bivalent rLP2086 (Trumenba), a vaccine for prevention of Neisseria meningitidis serogroup B (NmB) disease, was licensed for use in adolescents and young adults after it was demonstrated that it elicits antibodies that initiate complement-mediated killing of invasive NmB isolates in a serum bactericidal assay with human complement (hSBA). The vaccine consists of two factor H binding proteins (fHBPs) representing divergent subfamilies to ensure broad coverage. Although it is the surrogate of efficacy, an hSBA is not suitable for testing large numbers of strains in local laboratories. Previously, an association between the in vitro fHBP surface expression level and the susceptibility of NmB isolates to killing was observed. Therefore, a flow cytometric meningococcal antigen surface expression (MEASURE) assay was developed and validated by using an antibody that binds to all fHBP variants from both fHBP subfamilies and accurately quantitates the level of fHBP expressed on the cell surface of NmB isolates with mean fluorescence intensity as the readout. Two collections of invasive NmB isolates (n = 1,814, n = 109) were evaluated in the assay, with the smaller set also tested in hSBAs using individual and pooled human serum samples from young adults vaccinated with bivalent rLP2086. From these data, an analysis based on fHBP variant prevalence in the larger 1,814-isolate set showed that >91% of all meningococcal serogroup B isolates expressed sufficient levels of fHBP to be susceptible to bactericidal killing by vaccine-induced antibodies.IMPORTANCE Bivalent rLP2086 (Trumenba) vaccine, composed of two factor H binding proteins (fHBPs), was recently licensed for the prevention of N. meningitidis serogroup B (NmB) disease in individuals 10 to 25 years old in the United States. This study evaluated a large collection of NmB isolates from the United States and Europe by using a flow cytometric MEASURE assay to quantitate the surface expression of the vaccine antigen fHBP. We find that expression levels and the proportion of strains above the level associated with susceptibility in an hSBA are generally consistent across these geographic regions. Thus, the assay can be used to predict which NmB isolates are susceptible to killing in the hSBA and therefore is able to demonstrate an fHBP vaccine-induced bactericidal response. This work significantly advances our understanding of the potential for bivalent rLP2086 to provide broad coverage against diverse invasive-disease-causing NmB isolates.

Structure-based design of chimeric antigens for multivalent protein vaccines

There is an urgent need to develop vaccines against pathogenic bacteria. However, this is often hindered by antigenic diversity and difficulties encountered manufacturing membrane proteins. Here we show how to use structure-based design to develop chimeric antigens (ChAs) for subunit vaccines. ChAs are generated against serogroup B Neisseria meningitidis (MenB), the predominant cause of meningococcal disease in wealthy countries. MenB ChAs exploit factor H binding protein (fHbp) as a molecular scaffold to display the immunogenic VR2 epitope from the integral membrane protein PorA. Structural analyses demonstrate fHbp is correctly folded and the PorA VR2 epitope adopts an immunogenic conformation. In mice, immunisation with ChAs generates fHbp and PorA antibodies that recognise the antigens expressed by clinical MenB isolates; these antibody responses correlate with protection against meningococcal disease. Application of ChAs is therefore a potentially powerful approach to develop multivalent subunit vaccines, which can be tailored to circumvent pathogen diversity.

Factor H binding protein (fHbp) and PorA are components of experimental serogroup B N. meningitidis vaccines. Here the authors graft the VR2 loop of PorA onto an fHBp-based scaffold to demonstrate proof-of-principle of a chimeric antigen strategy and vaccination against meningococcal disease.

Human protective response induced by meningococcus B vaccine is mediated by the synergy of multiple bactericidal epitopes

4CMenB is the first broad coverage vaccine for the prevention of invasive meningococcal disease caused by serogroup B strains. To gain a comprehensive picture of the antibody response induced upon 4CMenB vaccination and to obtain relevant translational information directly from human studies, we have isolated a panel of human monoclonal antibodies from adult vaccinees. Based on the Ig-gene sequence of the variable region, 37 antigen-specific monoclonal antibodies were identified and produced as recombinant Fab fragments, and a subset also produced as full length recombinant IgG1 and functionally characterized. We found that the monoclonal antibodies were cross-reactive against different antigen variants and recognized multiple epitopes on each of the antigens. Interestingly, synergy between antibodies targeting different epitopes enhanced the potency of the bactericidal response. This work represents the first extensive characterization of monoclonal antibodies generated in humans upon 4CMenB immunization and contributes to further unraveling the immunological and functional properties of the vaccine antigens. Moreover, understanding the mechanistic nature of protection induced by vaccination paves the way to more rational vaccine design and implementation.

Crystal structure reveals vaccine elicited bactericidal human antibody targeting a conserved epitope on meningococcal fHbp

Data obtained recently in the United Kingdom following a nationwide infant immunization program against serogroup B Neisseria meningitidis (MenB) reported >80% 4CMenB vaccine-mediated protection. Factor H-binding protein (fHbp) is a meningococcal virulence factor and a component of two new MenB vaccines. Here, we investigated the structural bases underlying the fHbp-dependent protective antibody response in humans, which might inform future antigen design efforts. We present the co-crystal structure of a human antibody Fab targeting fHbp. The vaccine-elicited Fab 1A12 is cross-reactive and targets an epitope highly conserved across the repertoire of three naturally occurring fHbp variants. The free Fab structure highlights conformational rearrangements occurring upon antigen binding. Importantly, 1A12 is bactericidal against MenB strains expressing fHbp from all three variants. Our results reveal important immunological features potentially contributing to the broad protection conferred by fHbp vaccination. Our studies fuel the rationale of presenting conserved protein epitopes when developing broadly protective vaccines.

Factor H binding protein (fHbp) is a meningococcal virulence factor and a component of vaccines against serogroup B Neisseria meningitidis. Here, the authors characterize the vaccine-elicited human antibody Fab 1A12 and present both the free and the fHbp-bound Fab 1A12 crystal structures.

Development of an FHbp-CTB holotoxin-like chimera and the elicitation of bactericidal antibodies against serogroup B Neisseria meningitidis

The Neisseria meningitidis factor H binding protein (FHbp) is an important virulence factor and vaccine antigen contained in both USA licensed serogroup B meningococcal vaccines. Recent studies in human factor H (hFH) transgenic mice suggest that hFH-FHbp interactions lower FHbp-elicited immunogenicity. To provide tools with which to characterize and potentially improve FHbp immunogenicity, we developed an FHbp-cholera holotoxin-like chimera vaccine expression system in Escherichia coli that utilizes cholera toxin B (CTB) as both a scaffold and adjuvant for FHbp. We developed FHbp-CTB chimeras using a wild-type (WT) FHbp and a low hFH-binding FHbp mutant R41S. Both chimeras bound to G_M1 ganglioside and were recognized by the FHbp-specific monoclonal antibody JAR4. The R41S mutant had greatly reduced hFH binding compared to the WT FHbp-CTB chimera. WT and R41S FHbp-CTB chimeric antigens were compared to equimolar amounts of FHbp admixed with CTB or FHbp alone in mouse immunogenicity studies. The chimeras were significantly more immunogenic than FHbp alone or mixed with CTB, and elicited bactericidal antibodies against a panel of MenB isolates. This study demonstrates a unique and simple method for studying FHbp immunogenicity. The chimeric approach may facilitate studies of other protein-based antigens targeting pathogenic Neisseria and lay groundwork for the development of new protein based vaccines against meningococcal and gonococcal disease.

A Bivalent Meningococcal B Vaccine in Adolescents and Young Adults

BACKGROUND

MenB-FHbp is a licensed meningococcal B vaccine targeting factor H-binding protein. Two phase 3 studies assessed the safety of the vaccine and its immunogenicity against diverse strains of group B meningococcus.

METHODS

We randomly assigned 3596 adolescents (10 to 18 years of age) to receive MenB-FHbp or hepatitis A virus vaccine and saline and assigned 3304 young adults (18 to 25 years of age) to receive MenB-FHbp or saline at baseline, 2 months, and 6 months. Immunogenicity was assessed in serum bactericidal assays that included human complement (hSBAs). We used 14 meningococcal B test strains that expressed vaccine-heterologous factor H-binding proteins representative of meningococcal B epidemiologic diversity; an hSBA titer of at least 1:4 is the accepted correlate of protection. The five primary end points were the proportion of participants who had an increase in their hSBA titer for each of 4 primary strains by a factor of 4 or more and the proportion of those who had an hSBA titer at least as high as the lower limit of quantitation (1:8 or 1:16) for all 4 strains combined after dose 3. We also assessed the hSBA responses to the primary strains after dose 2; hSBA responses to the 10 additional strains after doses 2 and 3 were assessed in a subgroup of participants only. Safety was assessed in participants who received at least one dose.

RESULTS

In the modified intention-to-treat population, the percentage of adolescents who had an increase in the hSBA titer by a factor of 4 or more against each primary strain ranged from 56.0 to 85.3% after dose 2 and from 78.8 to 90.2% after dose 3; the percentages of young adults ranged from 54.6 to 85.6% and 78.9 to 89.7%, after doses 2 and 3, respectively. Composite responses after doses 2 and 3 in adolescents were 53.7% and 82.7%, respectively, and those in young adults were 63.3% and 84.5%, respectively. Responses to the 4 primary strains were predictive of responses to the 10 additional strains. Most of those who received MenB-FHbp reported mild or moderate pain at the vaccination site.

CONCLUSIONS

MenB-FHbp elicited bactericidal responses against diverse meningococcal B strains after doses 2 and 3 and was associated with more reactions at the injection site than the hepatitis A virus vaccine and saline. (Funded by Pfizer; ClinicalTrials.gov numbers, NCT01830855 and NCT01352845 ).

Neisseria cinerea Expresses a Functional Factor H Binding Protein Which Is Recognized by Immune Responses Elicited by Meningococcal Vaccines

Neisseria meningitidis is a major cause of bacterial meningitis and sepsis worldwide. Capsular polysaccharide vaccines are available against meningococcal serogroups A, C, W, and Y. More recently two protein-based vaccines, Bexsero and Trumenba, against meningococcal serogroup B strains have been licensed; both vaccines contain meningococcal factor H binding protein (fHbp). fHbp is a surface-exposed lipoprotein that binds the negative complement regulator complement factor H (CFH), thereby inhibiting the alternative pathway of complement activation. Recent analysis of available genomes has indicated that some commensal Neisseria species also contain genes that potentially encode fHbp, although the functions of these genes and how immunization with fHbp-containing vaccines could affect the commensal flora have yet to be established. Here, we show that the commensal species Neisseria cinerea expresses functional fHbp on its surface and that it is responsible for recruitment of CFH by the bacterium. N. cinerea fHbp binds CFH with affinity similar to that of meningococcal fHbp and promotes survival of N. cinerea in human serum. We examined the potential impact of fHbp-containing vaccines on N. cinerea We found that immunization with Bexsero elicits serum bactericidal activity against N. cinerea, which is primarily directed against fHbp. The shared function of fHbp in N. cinerea and N. meningitidis and cross-reactive responses elicited by Bexsero suggest that the introduction of fHbp-containing vaccines has the potential to affect carriage of N. cinerea and other commensal species.

Contribution of factor H-Binding protein sequence to the cross-reactivity of meningococcal native outer membrane vesicle vaccines with over-expressed fHbp variant group 1

Factor H-binding protein (fHbp) is an important meningococcal vaccine antigen. Native outer membrane vesicles with over-expressed fHbp (NOMV OE fHbp) have been shown to induce antibodies with broader functional activity than recombinant fHbp (rfHbp). Improved understanding of this broad coverage would facilitate rational vaccine design. We performed a pair-wise analysis of 48 surface-exposed amino acids involved in interacting with factor H, among 383 fHbp variant group 1 sequences. We generated isogenic NOMV-producing meningococcal strains from an African serogroup W isolate, each over-expressing one of four fHbp variant group 1 sequences (ID 1, 5, 9, or 74), including those most common among invasive African meningococcal isolates. Mice were immunised with each NOMV, and sera tested for IgG levels against each of the rfHbp ID and for ability to kill a panel of heterologous meningococcal isolates. At the fH-binding site, ID pairs differed by a maximum of 13 (27%) amino acids. ID 9 shared an amino acid sequence common to 83 ID types. The selected ID types differed by up to 6 amino acids, in the fH-binding site. All NOMV and rfHbp induced high IgG levels against each rfHbp. Serum killing from mice immunised with rfHbp was generally less efficient and more restricted compared to NOMV, which induced antibodies that killed most meningococci tested, with decreased stringency for ID type differences. Breadth of killing was mostly due to anti-fHbp antibodies, with some restriction according to ID type sequence differences. Nevertheless, under our experimental conditions, no relationship between antibody cross-reactivity and variation fH-binding site sequence was identified. NOMV over-expressing different fHbp IDs belonging to variant group 1 induce antibodies with fine specificities against fHbp, and ability to kill broadly meningococci expressing heterologous fHbp IDs. The work reinforces that meningococcal NOMV with OE fHbp is a promising vaccine strategy, and provides a basis for rational selection of antigen sequence types for over-expression on NOMV.

Effective protection of mice against Shigella flexneri with a new self-adjuvant multicomponent vaccine

PURPOSE

The aim of this study was to develop an immunogenic protective product against Shigella flexneri by employing a simple and safe heat treatment-based strategy.

METHODOLOGY

The physicochemical characteristics of naturally produced (OMV) and heat-induced (HT) outer-membrane vesicles from S. flexneri were examined, including a comparison of the protein content of the products. Toxicological and biodistribution studies, and a preliminary experiment to examine the protective effectiveness of HT in a murine model of S. flexneri infection, were also included.

RESULTS

This method simultaneously achieves complete bacterial inactivation and the production of the HT vaccine product, leading to a safe working process. The obtained HT complex presented a similar morphology (electron microscopy) and chemical composition to the classical OMV, although it was enriched in some immunogens, such as lipoproteins, OmpA or OmpC, among others. The HT formulation was not toxic and biodistribution studies performed in mice demonstrated that the vaccine product remained in the small intestine after nasal administration. Finally, a single dose of HT administered nasally was able to protect mice against S. flexneri 2a.

CONCLUSION

The convenient and safe manufacturing process, and the preliminary biological evaluation, support the use of the self-adjuvanted HT complex as a new vaccine candidate to face shigellosis. Further development is required, such as additional immune analyses, to evaluate whether this new subunit vaccine can be useful in achieving full protection against Shigella.

The role of anti-NHba antibody in bactericidal activity elicited by the meningococcal serogroup B vaccine, MenB-4C

BACKGROUND

MenB-4C (Bexsero®) is a multicomponent serogroup B meningococcal vaccine. For vaccine licensure, efficacy was inferred from serum bactericidal antibody (SBA) against three antigen-specific indicator strains. The bactericidal role of antibody to the fourth vaccine antigen, Neisserial Heparin binding antigen (NHba), is incompletely understood.

METHODS

We identified nine adults immunized with two or three doses of MenB-4C who had sufficient volumes of sera and >3-fold increases in SBA titer against a strain with high NHba expression, which was mismatched with the other three MenB-4C antigens that elicit SBA. Using 1month-post-immunization sera we measured the effect of depletion of anti-NHba and/or anti-Factor H binding protein (FHbp) antibodies on SBA.

RESULTS

Against three strains matched with the vaccine only for NHba, depletion of anti-NHba decreased SBA titers by an average of 43-79% compared to mock-adsorbed sera (P<0.05). Despite expression of sub-family A FHbp (mismatched with the sub-family B vaccine antigen), depletion of anti-FHbp antibodies also decreased SBA by 45-64% (P<0.05). Depletion of both antibodies decreased SBA by 84-100%. Against a strain with sub-family B FHbp and expression of NHba with 100% identity to the vaccine antigen, depletion of anti-NHba decreased SBA by an average of 26%, compared to mock-adsorbed sera (P<0.0001), and depletion of anti-FHbp antibody decreased SBA by 92% (P<0.0001).

CONCLUSIONS

Anti-NHba antibody can contribute to SBA elicited by MenB-4C, particularly in concert with anti-FHbp antibody. However, some high NHba-expressing strains are resistant, even with an exact match between the amino acid sequence of the vaccine and strain antigens.

Biological Functions of the Secretome of Neisseria meningitidis

Neisseria meningitidis is a Gram-negative bacterial pathogen that normally resides as a commensal in the human nasopharynx but occasionally causes disease with high mortality and morbidity. To interact with its environment, it transports many proteins across the outer membrane to the bacterial cell surface and into the extracellular medium for which it deploys the common and well-characterized autotransporter, two-partner and type I secretion mechanisms, as well as a recently discovered pathway for the surface exposure of lipoproteins. The surface-exposed and secreted proteins serve roles in host-pathogen interactions, including adhesion to host cells and extracellular matrix proteins, evasion of nutritional immunity imposed by iron-binding proteins of the host, prevention of complement activation, neutralization of antimicrobial peptides, degradation of immunoglobulins, and permeabilization of epithelial layers. Furthermore, they have roles in interbacterial interactions, including the formation and dispersal of biofilms and the suppression of the growth of bacteria competing for the same niche. Here, we will review the protein secretion systems of N. meningitidis and focus on the functions of the secreted proteins.

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.

Liposomes or traditional adjuvants: induction of bactericidal activity by the macrophage infectivity potentiator protein (Mip) of Neisseria meningitidis

Currently, one of the main approaches to achieve a vaccine for serogroup B Neisseria meningitidis is based on outer membrane proteins with low antigenic variability among strains. Since these proteins tend to be minor components of the outer membrane, recombinant production is required to obtain them in sufficient amounts for evaluation and development of vaccines. In this study, we analysed the ability of recombinant macrophage infectivity potentiator (rMip) protein to induce protective bactericidal activity in mice. The rMip protein was cloned from N. meningitidis strain H44/76 and was used to immunise mice, and the sera obtained were tested against the homologous and several heterologous N. meningitidis strains. The sera were obtained using the rMip alone, with adjuvant Al(OH)₃ , or after inclusion into liposomes. Bactericidal activity was variable depending on the strain, although high titres were seen against strains H44/76 and NmP27. Liposomes enhanced fourfold the reactivity against the homologous strain. The results presented suggest that the rMip protein should be considered a promising candidate for the improvement of future protein-based vaccines.

Some Gram-negative Lipoproteins Keep Their Surface Topology When Transplanted from One Species to Another and Deliver Foreign Polypeptides to the Bacterial Surface

In Gram-negative bacteria, outer membrane-associated lipoproteins can either face the periplasm or protrude out of the bacterial surface. The mechanisms involved in lipoprotein transport through the outer membrane are not fully elucidated. Some lipoproteins reach the surface by using species-specific transport machinery. By contrast, a still poorly characterized group of lipoproteins appears to always cross the outer membrane, even when transplanted from one organism to another. To investigate such lipoproteins, we tested the expression and compartmentalization in E. coli of three surface-exposed lipoproteins, two from Neisseria meningitidis (Nm-fHbp and NHBA) and one from Aggregatibacter actinomycetemcomitans (Aa-fHbp). We found that all three lipoproteins were lipidated and compartmentalized in the E. coli outer membrane and in outer membrane vesicles. Furthermore, fluorescent antibody cell sorting analysis, proteolytic surface shaving, and confocal microscopy revealed that all three proteins were also exposed on the surface of the outer membrane. Removal or substitution of the first four amino acids following the lipidated cysteine residue and extensive deletions of the C-terminal regions in Nm-fHbp did not prevent the protein from reaching the surface of the outer membrane. Heterologous polypeptides, fused to the C termini of Nm-fHbp and NHBA, were efficiently transported to the E. coli cell surface and compartmentalized in outer membrane vesicles, demonstrating that these lipoproteins can be exploited in biotechnological applications requiring Gram-negative bacterial surface display of foreign polypeptides.

Recent Progress in the Prevention of Serogroup B Meningococcal Disease

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

Neisserial surface lipoproteins: structure, function and biogenesis

The surface of many Gram-negative bacteria contains lipidated protein molecules referred to as surface lipoproteins or SLPs. SLPs play critical roles in host immune evasion, nutrient acquisition and regulation of the bacterial stress response. The focus of this review is on the SLPs present in Neisseria, a genus of bacteria that colonise the mucosal surfaces of animals. Neisseria contains two pathogens of medical interest, namely Neisseria meningitidis and N. gonorrhoeae. Several SLPs have been identified in Neisseria and their study has elucidated key strategies used by these pathogens to survive inside the human body. Herein, we focus on the identification, structure and function of SLPs that have been identified in Neisseria. We also survey the translocation pathways used by these SLPs to reach the cell surface. Specifically, we elaborate on the strategies used by neisserial SLPs to translocate across the outer membrane with an emphasis on Slam, a novel outer membrane protein that has been implicated in SLP biogenesis. Taken together, the study of SLPs in Neisseria illustrates the widespread roles played by this family of proteins in Gram-negative bacteria.

Emerging experience with meningococcal serogroup B protein vaccines

INTRODUCTION

The successful development of two broadly protective vaccines targeting Neisseria meningitidis serogroup B (MenB); 4CMenB and rLP2086, is the most significant recent advance in meningococcal disease prevention. Areas covered: Here we review the principles underlying the development of each vaccine and the novel methods used to estimate vaccine coverage. We update clinical and post-licensure experience with 4CMenB and rLP2086. Expert commentary: The immunogenicity and acceptable safety profile of 4CMenB and rLP2086 has been demonstrated in clinical trials. Continuing uncertainties exist around the appropriate age groups to be immunized, the degree and duration of efficacy, and the impact on nasopharyngeal carriage which has implications for strategies to interrupt transmission and maximize herd protection effects. Universal vaccination programs such as those undertaken in Quebec and the United Kingdom are providing important information on these issues. The potential for MenB vaccines to prevent infection by other serogroups appears promising, and the impact of MenB vaccines on other pathogenic neisserial species with similar surface proteins warrants further investigation.

Prevalence of factor H Binding Protein sub-variants among Neisseria meningitidis in China

OBJECTIVE

To study the prevalence of the fHbp genes in Neisseria meningitidis (N. meningitidis) isolates for further evaluation and development of serogroup B meningococcal vaccines in China.

METHODS

A panel of 1012 N. meningitidis strains was selected from the national culture collection from 1956 to 2016, according to the years of isolation, locations, and strain sources. These were tested by FHbp variant typing. Multi-locus sequence typing (MLST) was performed on 822 of these samples, including 242 strains from clinical strains and 580 carrier-derived strains. Analysis based on sequence types, serogroups, and FHbp variations were used to summarize the prevalence and characteristics of N. meningitidis.

RESULTS

There were 8 serogroups of N. meningitidis as well as a collection of nongroupable strains in this study. 1008 of 1012 N. meningitidis strains tested were positive for the fHbp gene. Serogroup A N. meningitidis (MenA) strains belonging to ST-1 and ST-5 clonal complexes harbored genes only encoding variant 1 (v1) FHbp. All MenW strains encoded v2 FHbp. 61.9% of clinical MenB strains were positive for v2 FHbp vs. 32.1% that were positive for v1. Among fHbp-positive carrier-derived MenB strains, v2 FHbp accounted for 90.8%. 79.7% of clinical MenC strains were positive for v1 FHbp and 20.3% were positive for v2 FHbp. Among carrier-derived MenC strains, v2 FHbp predominated. The number of major serogroups of N. meningitidis analyzed by MLST was 822, and the encoded FHbp showed CC- or ST-specific characteristics.

CONCLUSION

fHbp genes were detected in almost all N. meningitidis strains in this study. Therefore, it is possible that a vaccine against MenB or meningococci irrespective of serogroups, which includes FHbp, could be developed. Meningococcal vaccine development for China is a complex issue and these findings warrant further attention with respect to vaccine development.

Meningococcal carriage in Dutch adolescents and young adults; a cross-sectional and longitudinal cohort study

OBJECTIVES

Current information on rates and dynamics of meningococcal carriage is essential for public health policy. This study aimed to determine meningococcal carriage prevalence, its risk factors and duration in the Netherlands, where meningococcal C vaccine coverage is >90%. Several methods to identify serogroups of meningococcal carriage isolates among adolescent and young adults were compared.

METHODS

Oropharyngeal swabs were collected from 1715 participants 13-23 years of age in 2013-2014; 300 were prospectively followed over 8 months. Cultured isolates were characterized by Ouchterlony, real-time (rt-) PCR or whole-genome sequencing (WGS). Direct swabs were assessed by rt-PCR. Questionnaires on environmental factors and behaviour were also obtained.

RESULTS

A meningococcal isolate was identified in 270/1715 (16%) participants by culture. Of MenB isolates identified by whole genome sequencing, 37/72 (51%) were correctly serogrouped by Ouchterlony, 46/51 (90%) by rt-PCR of cultured isolates, and 39/51 (76%) by rt-PCR directly on swabs. A sharp increase in carriage was observed before the age of 15 years. The age-related association disappeared after correction for smoking, level of education, frequent attendance to crowded social venues, kissing in the previous week and alcohol consumption. Three participants carried the same strain identified at three consecutive visits in an 8-month period. In these isolates, progressively acquired mutations were observed.

CONCLUSIONS

Whole genome sequencing of culture isolates was the most sensitive method for serogroup identification. Based upon results of this study and risk of meningococcal disease, an adolescent meningococcal vaccination might include children before the age of 15 years to confer individual protection and potentially to establish herd protection.

Neisseria meningitidis Serogroup B Vaccine, Bivalent rLP2086, Induces Broad Serum Bactericidal Activity Against Diverse Invasive Disease Strains Including Outbreak Strains

BACKGROUND

Bivalent rLP2086 (Trumenba), 1 of 2 meningococcal serogroup B (MnB) vaccines recently approved in the United States for the prevention of MnB disease in individuals 10-25 years of age, is composed of 2 lipidated factor H binding proteins from subfamilies A and B. This study evaluated the breadth of MnB strain coverage elicited by bivalent rLP2086 measured with serum bactericidal assays using human complement (hSBAs).

METHODS

hSBA responses to diverse MnB clinical strains circulating in the United States and Europe (n = 23), as well as recent US university outbreak strains (n = 4), were evaluated. Individual prevaccination and postvaccination sera from adolescents and young adults previously enrolled in phase 2 clinical studies of bivalent rLP2086 were assessed. Responders were defined by an hSBA titer ≥1:8, which is more stringent than the accepted correlate of protection (hSBA titer ≥1:4).

RESULTS

Baseline hSBA response rates were generally low; robust increases were observed after 2 and 3 doses of bivalent rLP2086, with hSBA responses to all test strains ranging from 31.8% to 100% and 55.6% to 100%, respectively. hSBA responses to strains expressing prevalent subfamily A and B factor H binding protein variants in the United States and Europe, A22 and B24, ranged from 88.0% to 95.0% and 81.0% to 100.0%, respectively, after dose 3. Substantial responses were also observed for recent US outbreak strains.

CONCLUSIONS

Bivalent rLP2086 elicits robust hSBA responses to MnB strains expressing 14 factor H binding protein variants representing approximately 80% of MnB invasive isolates and different from vaccine antigens, suggesting that bivalent rLP2086 confers broad protection against diverse MnB disease-causing strains.

Serum Bactericidal Antibody Responses of Adults Immunized with the MenB-4C Vaccine against Genetically Diverse Serogroup B Meningococci

MenB-4C is a meningococcal vaccine for the prevention of serogroup B disease. The vaccine contains factor H binding protein (FHbp) and three other antigens that can elicit serum bactericidal antibodies (SBA). For vaccine licensure, efficacy was inferred from the SBA responses against three antigen-specific indicator strains. The relation between those results and broad protection against circulating genetically diverse strains is not known. Twenty adults were immunized with two doses of MenB-4C given 1 to 2 months apart. SBA activity against 3 reference strains and 15 serogroup B test strains (6 from college outbreaks) was measured. Compared to the preimmunization titers, 70%, 95%, and 95% of subjects had ≥4-fold increases in the titers of anti-PorA P1.4, anti-NadA, and anti-FHbp antibodies against the reference strains, respectively. In contrast, only 25 to 45% of the subjects had ≥4-fold increases in responses to 10 of the 15 test strains, including 8 that expressed one to three of the antigens in the vaccine. At 1 month, the majority of subjects with <4-fold titer increases had serum titers of ≥1:4, which are considered sufficient for protection. However, the titers against four strains declined to <1:4 by 4 to 6 months in one-third to greater than 50% of the subjects tested. Clinically relevant isolates are often more resistant to SBA than the indicator strains used to measure antigen-specific SBA. A working model is that the percentage of subjects with titers of ≥1:4 at 1 month postimmunization correlates with short-term protection against that strain, whereas the percentage of subjects with ≥4-fold titer increases represents a more robust response. (The protocol used at the Oxford Vaccine Group has been registered at ClinicalTrials.gov under registration no. NCT02398396.).

The role of apolipoprotein N-acyl transferase, Lnt, in the lipidation of factor H binding protein of Neisseria meningitidis strain MC58 and its potential as a drug target

BACKGROUND AND PURPOSE

The level of cell surface expression of the meningococcal vaccine antigen, Factor H binding protein (FHbp) varies between and within strains and this limits the breadth of strains that can be targeted by FHbp-based vaccines. The molecular pathway controlling expression of FHbp at the cell surface, including its lipidation, sorting to the outer membrane and export, and the potential regulation of this pathway have not been investigated until now. This knowledge will aid our evaluation of FHbp vaccines.

EXPERIMENTAL APPROACH

A meningococcal transposon library was screened by whole cell immuno-dot blotting using an anti-FHbp antibody to identify a mutant with reduced binding and the disrupted gene was determined.

KEY RESULTS

In a mutant with markedly reduced binding, the transposon was located in the lnt gene which encodes apolipoprotein N-acyl transferase, Lnt, responsible for the addition of the third fatty acid to apolipoproteins prior to their sorting to the outer membrane. We provide data indicating that in the Lnt mutant, FHbp is diacylated and its expression within the cell is reduced 10 fold, partly due to inhibition of transcription. Furthermore the Lnt mutant showed 64 fold and 16 fold increase in susceptibility to rifampicin and ciprofloxacin respectively.

CONCLUSION AND IMPLICATIONS

We speculate that the inefficient sorting of diacylated FHbp in the meningococcus results in its accumulation in the periplasm inducing an envelope stress response to down-regulate its expression. We propose Lnt as a potential novel drug target for combination therapy with antibiotics.

LINKED ARTICLES

This article is part of a themed section on Drug Metabolism and Antibiotic Resistance in Micro-organisms. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.14/issuetoc.

Meningococcal serogroup B vaccines: Estimating breadth of coverage

Neisseria meningitidis serogroup B (MenB) is an important cause of invasive meningococcal disease. The development of safe and effective vaccines with activity across the diversity of MenB strains has been challenging. While capsular polysaccharide conjugate vaccines have been highly successful in the prevention of disease due to meningococcal serogroups A, C, W, and Y, this approach has not been possible for MenB owing to the poor immunogenicity of the MenB capsular polysaccharide. Vaccines based on outer membrane vesicles have been successful in the prevention of invasive MenB disease caused by the single epidemic strain from which they were derived, but they do not confer broad protection against diverse MenB strains. Thus, alternative approaches to vaccine development have been pursued to identify vaccine antigens that can provide broad protection against the epidemiologic and antigenic diversity of invasive MenB strains. Human factor H binding protein (fHBP) was found to be such an antigen, as it is expressed on nearly all invasive disease strains of MenB and can induce bactericidal responses against diverse MenB strains. A bivalent vaccine (Trumenba®, MenB-FHbp, bivalent rLP2086) composed of equal amounts of 2 fHBP variants from each of the 2 immunologically diverse subfamilies of fHBP (subfamilies A and B) was the first MenB vaccine licensed in the United States under an accelerated approval pathway for prevention of invasive MenB disease. Due to the relatively low incidence of meningococcal disease, demonstration of vaccine efficacy for the purposes of licensure of bivalent rLP2086 was based on vaccine-elicited bactericidal activity as a surrogate marker of efficacy, as measured in vitro by the serum bactericidal assay using human complement. Because bacterial surface proteins such as fHBP are antigenically variable, an important component for evaluation and licensure of bivalent rLP2086 included stringent criteria for assessment of breadth of coverage across antigenically diverse and epidemiologically important MenB strains. This review describes the rigorous approach used to assess broad coverage of bivalent rLP2086. Alternative nonfunctional assays proposed for assessing vaccine coverage are also discussed.

Enhanced protective antibody to a mutant meningococcal factor H-binding protein with low-factor H binding

Meningococcal factor H-binding protein (FHbp) is an antigen in 2 serogroup B meningococcal vaccines. FHbp specifically binds human and some nonhuman primate complement FH. To investigate the effect of binding of FH to FHbp on protective antibody responses, we immunized infant rhesus macaques with either a control recombinant FHbp antigen that bound macaque FH or a mutant antigen with 2 amino acid substitutions and >250-fold lower affinity for FH. The mutant antigen elicited 3-fold higher serum IgG anti-FHbp titers and up to 15-fold higher serum bactericidal titers than the control FHbp vaccine. When comparing sera with similar IgG anti-FHbp titers, the antibodies elicited by the mutant antigen gave greater deposition of complement component C4b on live meningococci (classical complement pathway) and inhibited binding of FH, while the anti-FHbp antibodies elicited by the control vaccine enhanced FH binding. Thus, the mutant FHbp vaccine elicited an anti-FHbp antibody repertoire directed at FHbp epitopes within the FH binding site, which resulted in greater protective activity than the antibodies elicited by the control vaccine, which targeted FHbp epitopes outside of the FH combining site. Binding of a host protein to a vaccine antigen impairs protective antibody responses, which can be overcome with low-binding mutant antigens.

The Dual Role of Lipids of the Lipoproteins in Trumenba, a Self-Adjuvanting Vaccine Against Meningococcal Meningitis B Disease

Trumenba (bivalent rLP2086) is a vaccine licensed for the prevention of meningococcal meningitis disease caused by Neisseria meningitidis serogroup B (NmB) in individuals 10-25 years of age in the USA. The vaccine is composed of two factor H binding protein (fHbp) variants that were recombinantly expressed in Escherichia coli as native lipoproteins: rLP2086-A05 and rLP2086-B01. The vaccine was shown to induce potent bactericidal antibodies against a broad range of NmB isolates expressing fHbp that were different in sequence from the fHbp vaccine antigens. Here, we describe the characterization of the vaccine antigens including the elucidation of their structure which is characterized by two distinct motifs, the polypeptide domain and the N-terminal lipid moiety. In the vaccine formulation, the lipoproteins self-associate to form micelles driven by the hydrophobicity of the lipids and limited by the size of the folded polypeptides. The micelles help to increase the structural stability of the lipoproteins in the absence of bacterial cell walls. Analysis of the lipoproteins in Toll-like receptor (TLR) activation assays revealed their TLR2 agonist activity. This activity was lost with removal of the O-linked fatty acids, similar to removal of all lipids, demonstrating that this moiety plays an adjuvant role in immune activation. The thorough understanding of the structure and function of each moiety of the lipoproteins, as well as their relationship, lays the foundation for identifying critical parameters to guide vaccine development and manufacture.

The Journey of Lipoproteins Through the Cell: One Birthplace, Multiple Destinations

Bacterial lipoproteins are a very diverse group of proteins characterized by the presence of an N-terminal lipid moiety that serves as a membrane anchor. Lipoproteins have a wide variety of crucial functions, ranging from envelope biogenesis to stress response. In Gram-negative bacteria, lipoproteins can be targeted to various destinations in the cell, including the periplasmic side of the cytoplasmic or outer membrane, the cell surface or the external milieu. The sorting mechanisms have been studied in detail in Escherichia coli, but exceptions to the rules established in this model bacterium exist in other bacteria. In this chapter, we will present the current knowledge on lipoprotein sorting in the cell. Our particular focus will be on the surface-exposed lipoproteins that appear to be much more common than previously assumed. We will discuss the different targeting strategies, provide numerous examples of surface-exposed lipoproteins and discuss the techniques used to assess their surface exposure.

Thermoregulation of Meningococcal fHbp, an Important Virulence Factor and Vaccine Antigen, Is Mediated by Anti-ribosomal Binding Site Sequences in the Open Reading Frame

During colonisation of the upper respiratory tract, bacteria are exposed to gradients of temperatures. Neisseria meningitidis is often present in the nasopharynx of healthy individuals, yet can occasionally cause severe disseminated disease. The meningococcus can evade the human complement system using a range of strategies that include recruitment of the negative complement regulator, factor H (CFH) via factor H binding protein (fHbp). We have shown previously that fHbp levels are influenced by the ambient temperature, with more fHbp produced at higher temperatures (i.e. at 37°C compared with 30°C). Here we further characterise the mechanisms underlying thermoregulation of fHbp, which occurs gradually over a physiologically relevant range of temperatures. We show that fHbp thermoregulation is not dependent on the promoters governing transcription of the bi- or mono-cistronic fHbp mRNA, or on meningococcal specific transcription factors. Instead, fHbp thermoregulation requires sequences located in the translated region of the mono-cistronic fHbp mRNA. Site-directed mutagenesis demonstrated that two anti-ribosomal binding sequences within the coding region of the fHbp transcript are involved in fHbp thermoregulation. Our results shed further light on mechanisms underlying the control of the production of this important virulence factor and vaccine antigen.

The bacterium Neisseria meningitidis is exquisitely adapted to survive in the human host, and possesses several mechanisms to interact with host cells in the upper airway and to circumvent immune responses. However, the mechanisms that govern the expression of factors that contribute to colonisation and disease are incompletely understood. In this work, we further characterise how temperature influences the production of factor H binding protein (fHbp) by the meningococcus; fHbp recruits human complement proteins to the surface of the bacterium, and is an important vaccine antigen. We show that thermoregulation of fHbp occurs gradually over a physiological range of temperatures found in the upper airway, the site of colonisation. This regulation does not require specific meningococcal transcription factors, and sequence analysis indicates that fHbp mRNA forms a secondary structure which could act as an RNA thermosensor. Additional studies demonstrate that there are two specific sequences within the coding region of fHbp mRNA are important for thermosensing and could base-pair to the ribosome binding site, thus blocking translation of this protein. As fHbp is thermoregulated, vaccines that target this antigen might not impose a high level of selective pressure on the bacterium at the mucosal surface, thereby limiting herd immunity induce by fHbp containing vaccines.

Bivalent rLP2086 Vaccine (Trumenba(®)): A Review in Active Immunization Against Invasive Meningococcal Group B Disease in Individuals Aged 10-25 Years

Bivalent rLP2086 vaccine (Trumenba(®)) [hereafter referred to as rLP2086] is a Neisseria meningitidis serogroup B (MenB) vaccine recently licensed in the USA for active immunization to prevent invasive disease caused by MenB in individuals 10-25 years of age. rLP2086, which contains two variants of the meningococcal surface protein factor H-binding protein (fHBP), was approved by the FDA under the accelerated approval pathway after the immunogenicity of the vaccine was demonstrated in several phase II trials. This article reviews the immunogenicity and reactogenicity of rLP2086 as demonstrated in the trials with a focus on the US setting and on use of the vaccine as per FDA-approved labeling. rLP2086 is approved in the USA as a three-dose series administered in a 0-, 2-, and 6-month schedule. In the phase II trials, rLP2086 elicited a robust immune response against a panel of MenB test strains. A strong immune response was evident in a marked proportion of subjects after two vaccine doses, with a further increase after a third dose. The four primary test strains used were selected to be representative of MenB strains prevalent in the USA, with each expressing an fHBP variant heterologous to the vaccine antigens. rLP2086 was generally well tolerated in the trials, with most adverse reactions being mild to moderate in severity. Although some questions remain, including the duration of the protective response, rLP2086 vaccine has the potential to be a valuable tool for the prevention of invasive MenB disease.

Prevention of Meningococcal Infection in the United States: Current Recommendations and Future Considerations

Neisseria meningitidis is a common cause of bacterial meningitis and septicemia that can lead to permanent sequelae or death. N meningitidis is classified into serogroups based on the composition of the capsular polysaccharide, with serogroups A, B, C, W, X, and Y recognized as the major disease-causing organisms. The unpredictability of infection coupled with the poor prognosis for some patients suggests immunization as an effective preventive strategy. Importantly, four of the six disease-causing serogroups (A, C, Y, and W) may be prevented with available quadrivalent capsular polysaccharide-protein conjugate vaccines; these vaccines have been successfully implemented into immunization programs in the United States. Unfortunately, quadrivalent conjugate vaccines are not effective against serogroup B, now the most common cause of invasive meningococcal disease. Two recombinant protein vaccines recently were licensed for prevention of serogroup B disease. Recommendations for use of these serogroup B vaccines in the United States have been made by the Advisory Committee on Immunization Practices. This article will discuss all available meningococcal vaccines, current recommendations for use, lessons learned from previous experiences, and future considerations, with the hope of further understanding how use of these vaccines may help reduce incidence of meningococcal disease in the United States.

Characteristics of a new meningococcal serogroup B vaccine, bivalent rLP2086 (MenB-FHbp; Trumenba®)

Neisseria meningitidis is a common cause of bacterial meningitis, often leading to permanent sequelae or death. N. meningitidis is classified into serogroups based on the composition of the bacterial capsular polysaccharide; the 6 major disease-causing serogroups are designated A, B, C, W, X, and Y. Four of the 6 disease-causing serogroups (A, C, Y, and W) can be effectively prevented with available quadrivalent capsular polysaccharide protein conjugate vaccines; however, capsular polysaccharide conjugate vaccines are not effective against meningococcal serogroup B (MnB). There is no vaccine available for serogroup X. The public health need for an effective serogroup B vaccine is evident, as MnB is the most common cause of meningococcal disease in the United States and is responsible for almost half of all cases in persons aged 17 to 22 years. In fact, serogroup B meningococci were responsible for the recent meningococcal disease outbreaks on college campuses. However, development of a suitable serogroup B vaccine has been challenging, as serogroup B polysaccharide-based vaccines were found to be poorly immunogenic. Vaccine development for MnB focused on identifying potential outer membrane protein targets that elicit broadly protective immune responses across strains from the vast number of proteins that exist on the bacterial surface. Human factor H binding protein (fHBP; also known as LP2086), a conserved surface-exposed bacterial lipoprotein, was identified as a promising vaccine candidate. Two recombinant protein-based serogroup B vaccines that contain fHBP have been successfully developed and licensed in the United States under an accelerated approval process: bivalent rLP2086 (MenB-FHbp; Trumenba®) and 4CMenB (MenB-4 C; Bexsero®). This review will focus on bivalent rLP2086 only, including vaccine components, mechanism of action, and potential coverage across serogroup B strains in the United States.

Temporal Changes in BEXSERO® Antigen Sequence Type Associated with Genetic Lineages of Neisseria meningitidis over a 15-Year Period in Western Australia

Neisseria meningitidis is the causative agent of invasive meningococcal disease (IMD). The BEXSERO^® vaccine which is used to prevent serogroup B disease is composed of four sub-capsular protein antigens supplemented with an outer membrane vesicle. Since the sub-capsular protein antigens are variably expressed and antigenically variable amongst meningococcal isolates, vaccine coverage can be estimated by the meningococcal antigen typing system (MATS) which measures the propensity of the strain to be killed by vaccinated sera. Whole genome sequencing (WGS) which identifies the alleles of the antigens that may be recognised by the antibody response could represent, in future, an alternative estimate of coverage. In this study, WGS of 278 meningococcal isolates responsible for 62% of IMD in Western Australia from 2000–2014 were analysed for association of genetic lineage (sequence type [ST], clonal complex [cc]) with BEXSERO^® antigen sequence type (BAST) and MATS to predict the annual vaccine coverage. A hyper-endemic period of IMD between 2000–05 was caused by cc41/44 with the major sequence type of ST-146 which was not predicted by MATS or BAST to be covered by the vaccine. An increase in serogroup diversity was observed between 2010–14 with the emergence of cc11 serogroup W in the adolescent population and cc23 serogroup Y in the elderly. BASTs were statistically associated with clonal complex although individual antigens underwent antigenic drift from the major type. BAST and MATS predicted an annual range of 44–91% vaccine coverage. Periods of low vaccine coverage in years post-2005 were not a result of the resurgence of cc41/44:ST-146 but were characterised by increased diversity of clonal complexes expressing BASTs which were not predicted by MATS to be covered by the vaccine. The driving force behind the diversity of the clonal complex and BAST during these periods of low vaccine coverage is unknown, but could be due to immune selection and inter-strain competition with carriage of non-disease causing meningococci.

Immunogenicity, Safety, and Tolerability of Bivalent rLP2086 Meningococcal Group B Vaccine Administered Concomitantly With Diphtheria, Tetanus, and Acellular Pertussis and Inactivated Poliomyelitis Vaccines to Healthy Adolescents

KEY POINTS

Concomitant administration of bivalent rLP2086 (Trumenba [Pfizer, Inc] and diphtheria, tetanus, and acellular pertussis and inactivated poliovirus vaccine (DTaP/IPV) was immunologically noninferior to DTaP/IPV and saline and was safe and well tolerated. Bivalent rLP2086 elicited robust and broad bactericidal antibody responses to diverse Neisseria meningitidis serogroup B strains expressing antigens heterologous to vaccine antigens after 2 and 3 vaccinations.

BACKGROUND

Bivalent rLP2086, a Neisseria meningitidis serogroup B (MnB) vaccine (Trumenba [Pfizer, Inc]) recently approved in the United States to prevent invasive MnB disease in individuals aged 10-25 years, contains recombinant subfamily A and B factor H binding proteins (fHBPs). This study evaluated the coadministration of Repevax (diphtheria, tetanus, and acellular pertussis and inactivated poliovirus vaccine [DTaP/IPV]) (Sanofi Pasteur MSD, Ltd) and bivalent rLP2086.

METHODS

Healthy adolescents aged ≥11 to <19 years received bivalent rLP2086 + DTaP/IPV or saline + DTaP/IPV at month 0 and bivalent rLP2086 or saline at months 2 and 6. The primary end point was the proportion of participants in whom prespecified levels of antibodies to DTaP/IPV were achieved 1 month after DTaP/IPV administration. Immune responses to bivalent rLP2086 were measured with serum bactericidal assays using human complement (hSBAs) against 4 MnB test strains expressing fHBP subfamily A or B proteins different from the vaccine antigens.

RESULTS

Participants were randomly assigned to receive bivalent rLP2086 + DTaP/IPV (n = 373) or saline + DTaP/IPV (n = 376). Immune responses to DTaP/IPV in participants who received bivalent rLP2086 + DTaP/IPV were noninferior to those in participants who received saline + DTaP/IPV.The proportions of bivalent rLP2086 + DTaP/IPV recipients with prespecified seroprotective hSBA titers to the 4 MnB test strains were 55.5%-97.3% after vaccination 2 and 81.5%-100% after vaccination 3. The administration of bivalent rLP2086 was well tolerated and resulted in few serious adverse events.

CONCLUSIONS

Immune responses to DTaP/IPV administered with bivalent rLP2086 to adolescents were noninferior to DTaP/IPV administered alone. Bivalent rLP2086 was well tolerated and elicited substantial and broad bactericidal responses to diverse MnB strains in a high proportion of recipients after 2 vaccinations, and these responses were further enhanced after 3 vaccinations.ClinicalTrials.gov identifier NCT01323270.

A Phase 2, Randomized, Active-controlled, Observer-blinded Study to Assess the Immunogenicity, Tolerability and Safety of Bivalent rLP2086, a Meningococcal Serogroup B Vaccine, Coadministered With Tetanus, Diphtheria and Acellular Pertussis Vaccine and Serogroup A, C, Y and W-135 Meningococcal Conjugate Vaccine in Healthy US Adolescents

BACKGROUND

Bivalent rLP2086, targeting meningococcal serogroup B, will extend prevention of meningococcal disease beyond that provided by quadrivalent serogroup ACWY vaccines; coadministration with recommended vaccines may improve adherence to vaccine schedules. This phase 2, randomized, active-controlled, observer-blinded study assessed whether immune responses induced by coadministration of Menactra (meningococcal A, C, Y and W-135 polysaccharide conjugate vaccine [MCV4]) and Adacel (tetanus toxoid, reduced diphtheria toxoid, acellular pertussis vaccine [Tdap]) with bivalent rLP2086 (Trumenba [meningococcal serogroup B vaccine], approved in the United States) were noninferior to MCV4 + Tdap or bivalent rLP2086 alone.

METHODS

Healthy adolescents aged 10 to <13 years received MCV4 + Tdap + bivalent rLP2086, MCV4 + Tdap or bivalent rLP2086. Bivalent rLP2086 response was assessed with serum bactericidal assays using human complement with 2 meningococcal serogroup B test strains expressing vaccine-heterologous factor H-binding protein variants; MCV4 with SBAs using rabbit complement; and Tdap with multiplexed Luminex assays. Safety was evaluated.

RESULTS

Two thousand six hundred forty-eight subjects were randomized. Immune responses to MCV4 + Tdap + bivalent rLP2086 were noninferior to MCV4 + Tdap or bivalent rLP2086 alone. Seroprotective serum bactericidal assays using human complement titers were documented for 62.3%-68.0% and 87.5%-90% of MCV4 + Tdap + bivalent rLP2086 recipients after doses 2 and 3, respectively. A ≥4-fold rise in serum bactericidal assays using human complement titers from baseline was achieved by 56.3%-64.3% and 84.0%-85.7% of subjects after doses 2 and 3, respectively. Bivalent rLP2086 alone induced similar responses. Concomitant administration did not substantially increase reactogenicity compared with bivalent rLP2086 alone.

CONCLUSIONS

Bivalent rLP2086 given concomitantly with MCV4 + Tdap met all noninferiority immunogenicity criteria without a clinically meaningful increase in reactogenicity. MCV4 and bivalent rLP2086 coadministration would provide coverage against the 5 major disease-causing serogroups.

Meningococcal Serogroup B Bivalent rLP2086 Vaccine Elicits Broad and Robust Serum Bactericidal Responses in Healthy Adolescents

BACKGROUND

Neisseria meningitidis serogroup B (MnB) is a leading cause of invasive meningococcal disease in adolescents and young adults. A recombinant factor H binding protein (fHBP) vaccine (Trumenba(®); bivalent rLP2086) was recently approved in the United States in individuals aged 10-25 years. Immunogenicity and safety of 2- or 3-dose schedules of bivalent rLP2086 were assessed in adolescents.

METHODS

Healthy adolescents (11 to <19 years) were randomized to 1 of 5 bivalent rLP2086 dosing regimens (0,1,6-month; 0,2,6-month; 0,2-month; 0,4-month; 0,6-month). Immunogenicity was assessed by serum bactericidal antibody assay using human complement (hSBA). Safety assessments included local and systemic reactions and adverse events.

RESULTS

Bivalent rLP2086 was immunogenic when administered as 2 or 3 doses; the most robust hSBA responses occurred with 3 doses. The proportion of subjects with hSBA titers ≥1:8 after 3 doses ranged from 91.7% to 95.0%, 98.9% to 99.4%, 88.4% to 89.0%, and 86.1% to 88.5% for MnB test strains expressing vaccine--heterologous fHBP variants A22, A56, B24, and B44, respectively. After 2 doses, responses ranged from 90.8% to 93.5%, 98.4% to 100%, 69.1% to 81.1%, and 70.1% to 77.5%. Geometric mean titers (GMTs) were highest among subjects receiving 3 doses and similar between the 2- and 3-dose regimens. After 2 doses, GMTs trended numerically higher among subjects with longer intervals between the first and second dose (6 months vs 2 and 4 months). Bivalent rLP2086 was well tolerated.

CONCLUSIONS

Bivalent rLP2086 was immunogenic and well tolerated when administered in 2 or 3 doses. Three doses yielded the most robust hSBA response rates against MnB strains expressing vaccine-heterologous subfamily B fHBPs.

Meningococcal Factor H Binding Protein Vaccine Antigens with Increased Thermal Stability and Decreased Binding of Human Factor H

Neisseria meningitidis causes cases of bacterial meningitis and sepsis. Factor H binding protein (FHbp) is a component of two licensed meningococcal serogroup B vaccines. FHbp recruits the complement regulator factor H (FH) to the bacterial surface, which inhibits the complement alternative pathway and promotes immune evasion. Binding of human FH impairs the protective antibody responses to FHbp, and mutation of FHbp to decrease binding of FH can increase the protective responses. In a previous study, we identified two amino acid substitutions in FHbp variant group 2 that increased its thermal stability by 21°C and stabilized epitopes recognized by protective monoclonal antibodies (MAbs). Our hypothesis was that combining substitutions to increase stability and decrease FH binding would increase protective antibody responses in the presence of human FH. In the present study, we generated four new FHbp single mutants that decreased FH binding and retained binding of anti-FHbp MAbs and immunogenicity in wild-type mice. From these mutants, we selected two, K219N and G220S, to combine with the stabilized double-mutant FHbp antigen. The two triple mutants decreased FH binding >200-fold, increased the thermal stability of the N-terminal domain by 21°C, and bound better to an anti-FHbp MAb than the wild-type FHbp. In human-FH-transgenic mice, the FHbp triple mutants elicited 8- to 15-fold-higher protective antibody responses than the wild-type FHbp antigen. Collectively, the data suggest that mutations to eliminate binding of human FH and to promote conformational stability act synergistically to optimize FHbp immunogenicity.

Meningococcal vaccine antigen diversity in global databases

The lack of an anti-capsular vaccine against serogroup B meningococcal disease has necessitated the exploration of alternative vaccine candidates, mostly proteins exhibiting varying degrees of antigenic variation. Analysis of variants of antigen-encoding genes is facilitated by publicly accessible online sequence repositories, such as the Neisseria PubMLST database and the associated Meningitis Research Foundation Meningococcus Genome Library (MRF-MGL). We investigated six proposed meningococcal vaccine formulations by deducing the prevalence of their components in the isolates represented in these repositories. Despite high diversity, a limited number of antigenic variants of each of the vaccine antigens were prevalent, with strong associations of particular variant combinations with given serogroups and genotypes. In the MRF-MGL and globally, the highest levels of identical sequences were observed with multicomponent/multivariant vaccines. Our analyses further demonstrated that certain combinations of antigen variants were prevalent over periods of decades in widely differing locations, indicating that vaccine formulations containing a judicious choice of antigen variants have potential for long-term protection across geographic regions. The data further indicated that formulations with multiple variants would be especially relevant at times of low disease incidence, as relative diversity was higher. Continued surveillance is required to monitor the changing prevalence of these vaccine antigens.