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

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

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

Synergistic activity of antibodies in the multicomponent 4CMenB vaccine

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

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

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

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

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

Reciprocal Packaging of the Main Structural Proteins of Type 1 Fimbriae and Flagella in the Outer Membrane Vesicles of "Wild Type" Escherichia coli Strains

Fundamental aspects of outer membrane vesicle (OMV) biogenesis and the engineering of producer strains have been major research foci for many in recent years. The focus of this study was OMV production in a variety of Escherichia coli strains including wild type (WT) (K12 and BW25113), mutants (from the Keio collection) and proprietary [BL21 and BL21 (DE3)] strains. The present study investigated the proteome and prospective mechanism that underpinned the key finding that the dominant protein present in E. coli K-12 WT OMVs was fimbrial protein monomer (FimA) (a polymerizable protein which is the key structural monomer from which Type 1 fimbriae are made). However, mutations in genes involved in fimbriae biosynthesis (ΔfimA, B, C, and F) resulted in the packaging of flagella protein monomer (FliC) (the major structural protein of flagella) into OMVs instead of FimA. Other mutations (ΔfimE, G, H, I, and ΔlrhA-a transcriptional regulator of fimbriation and flagella biosynthesis) lead to the packaging of both FimA and Flagellin into the OMVs. In the majority of instances shown within this research, the production of OMVs is considered in K-12 WT strains where structural appendages including fimbriae or flagella are temporally co-expressed throughout the growth curve as shown previously in the literature. The hypothesis, proposed and supported within the present paper, is that the vesicular packaging of the major FimA is reciprocally regulated with the major FliC in E. coli K-12 OMVs but this is abrogated in a range of mutated, non-WT E. coli strains. We also demonstrate, that a protein of interest (GFP) can be targeted to OMVs in an E. coli K-12 strain by protein fusion with FimA and that this causes normal packaging to be disrupted. The findings and underlying implications for host interactions and use in biotechnology are discussed.

Immunogenicity profiling of protein antigens from capsular group B Neisseria meningitidis

Outer membrane vesicle (OMV)- based vaccines have been used to provide strain-specific protection against capsular group B Neisseria meningitidis infections, but the full breadth of the immune response against the components of the OMV has not been established. Sera from adults vaccinated with an OMV vaccine were used to screen 91 outer membrane proteins (OMPs) incorporated in an antigen microarray panel. Antigen-specific IgG levels were quantified pre-vaccination, and after 12 and 18 weeks. These results were compared with IgG levels from mice vaccinated with the same OMV vaccine. The repertoires of highly responding antigens in humans and mice overlapped, but were not identical. The highest responding antigens to human IgG comprised four integral OMPs (PorA, PorB, OpcA and PilQ), a protein which promotes the stability of PorA and PorB (RmpM) and two lipoproteins (BamC and GNA1162). These observations will assist in evaluating the role of minor antigen components within OMVs in providing protection against meningococcal infection. In addition, the relative dominance of responses to integral OMPs in humans emphasizes the importance of this subclass and points to the value of maintaining conformational epitopes from integral membrane proteins in vaccine formulations.

The Therapeutic Benefit of Bacterial Membrane Vesicles

The therapeutic potential of extracellular vesicles from eukaryotes has gained strong interest in recent years. However, research into the therapeutic application of their bacterial counterparts, known as bacterial membrane vesicles, is only just beginning to be appreciated. Membrane vesicles (MVs) from both Gram-positive and Gram-negative bacteria offer significant advantages in therapeutic development, including large-scale, cost effective production and ease of molecular manipulation to display foreign antigens. The nanoparticle size of MVs enables their dissemination through numerous tissue types, and their natural immunogenicity and self-adjuvanting capability can be harnessed to induce both cell-mediated and humoral immunity in vaccine design. Moreover, the ability to target MVs to specific tissues through the display of surface receptors raises their potential use as targeted MV-based anti-cancer therapy. This review discusses recent advances in MV research with particular emphasis on exciting new possibilities for the application of MVs in therapeutic design.

Cell membrane-derived nanomaterials for biomedical applications

The continued evolution of biomedical nanotechnology has enabled clinicians to better detect, prevent, manage, and treat human disease. In order to further push the limits of nanoparticle performance and functionality, there has recently been a paradigm shift towards biomimetic design strategies. By taking inspiration from nature, the goal is to create next-generation nanoparticle platforms that can more effectively navigate and interact with the incredibly complex biological systems that exist within the body. Of great interest are cellular membranes, which play essential roles in biointerfacing, self-identification, signal transduction, and compartmentalization. In this review, we explore the major ways in which researchers have directly leveraged cell membrane-derived biomaterials for the fabrication of novel nanotherapeutics and nanodiagnostics. Such emerging technologies have the potential to significantly advance the field of nanomedicine, helping to improve upon traditional modalities while also enabling novel applications.

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.

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

Objectives

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

Methods

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

Results

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

Conclusion

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

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

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

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

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

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