Neisseria meningitidis serogroup C polysaccharide and serogroup B outer membrane vesicle conjugate as a bivalent meningococcus vaccine candidate

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.

Preclinical evaluation of MenB vaccines: prerequisites for clinical development

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

Bioanalysis of meningococcal vaccines

Meningococcal meningitis is feared because of the rapid onset of severe disease from mild symptoms and, therefore, is an important target for vaccine research. Five serogroups, defined by the structures of their capsular polysaccharides, are responsible for the vast majority of disease. Protection against four of these five serogroups can be obtained with polysaccharide or glycoconjugate vaccines, in which fragments of the capsular polysaccharides attached to a carrier protein generate anticarbohydrate immune responses, whilst protection against group B disease requires protein immunogens, often presented in vesicles containing outer membrane proteins. Glycoconjugate vaccines are now an established technology, but outer-membrane protein vaccines are still under development and present significant challenges. This review discusses physicochemical approaches to the characterization and quality control of these vaccines, as well as highlighting the problems and differences in vaccine design required for protection against different serogroups of the same species of pathogen.

Preparation and evaluation of vibrio cholerae O1 EL Tor Ogawa lipopolysaccharide-tetanus toxoid conjugates

The lipopolysaccharide (LPS) of Vibrio cholerae is considered one of the most important antigens from the point of view of immunogenicity in these bacteria. We have undertaken detoxification of this LPS by basic hydrolysis and the resultant amine groups were used for their conjugation to tetanus toxoid as carrier protein using carbodiimide-mediated coupling. The resulting conjugates were inoculated in Balb/c mice for immunogenicity studies. The anti-LPS IgG and vibriocidal antibodies were measured in serum. The antigenicity of this conjugated was evaluated by ELISA, with serums of humans vaccinated with a strain genetically modified. The conjugated elicited: high titers of IgG anti-LPS, high titers of vibriocidal antibodies and there was recognition of LPS by antibodies from cholerae immunised human serum. These results show that the conjugated LPS obtained by us, could be evaluated like a potential vaccine for human use.

Adjuvant can improve protection induced by OMV vaccine against Neisseria meningitidis serogroups B/C in neonatal mice

Meningococcal outer membrane vesicle (OMV) vaccines are weak antigens in infants. This study aimed at investigating alternative adjuvants for induction of functional antibodies in newborn mice. Serogroup B/C anti-meningococcal vaccines, consisting of capsular polysaccharide from serogroup C (PSC) conjugated to OMV from one serogroup B serosubtype prevalent in Brazil, combined with OMV from another prevalent serosubtype, were tested in newborn and adult mice with the following adjuvants: aluminum hydroxide, MPL (monophosphoryl lipid A), Titermax and MF59. Total IgG, IgG avidity index determination and bactericidal assay were performed with sera from immunized mice. Antibodies induced against PSC in newborn mice showed avidity and bactericidal titers, similar to those obtained in adult mice, independently of the adjuvant. Evidence is presented that the inclusion of MF59 enhanced the immune response against OMV in newborn mice.

Activation of human dendritic cells by the PorA protein of Neisseria meningitidis

The major porin proteins present in the outer membrane of Neisseria meningitidis, the causative agent of life-threatening meningitis and septicaemia, are believed to have potent immunostimulatory effects. In this study, the interactions between human monocyte-derived dendritic cells (mo-DC) and the PorA porin were investigated, in order to reveal the role of this protein in promoting innate and adaptive immune responses. Recombinant (r)PorA induced mo-DC maturation, as reflected by reduced receptor-mediated endocytosis, increased production of the chemokines IL-8, RANTES, MIP-1 alpha and MIP-1 beta and augmented expression of the surface markers CD40, CD54, CD80, CD86 and major histocompatibility complex class II molecules. However, rPorA induced either low level or no significant secretion of pro-inflammatory cytokines from mo-DC. The protein potently augmented the capacity of mo-DC to activate both allogeneic CD4(+) memory T-cells and CD4(+)RA(+) naïve T-cells. In addition, rPorA appeared to inhibit the production of IL-12p70 that follows from the interaction between CD40 on the mo-DC and CD40-ligand on T-cells, thereby directing T-cell differentiation towards a Th2 type response. These data demonstrate that PorA is involved in DC activation and in influencing the nature of the T-helper immune response, which are important properties for generating antibody responses required for protective immunity against meningococci and for determining the immuno-adjuvant effects of this protein.

Isolation, structural characterization, and immunological evaluation of a high-molecular-weight exopolysaccharide from Staphylococcus aureus

Colonization of implanted medical devices by coagulase-negative staphylococci such as Staphylococcus epidermidis is mediated by the bacterial polysaccharide intercellular adhesin (PIA), a polymer of beta-(1-->6)-linked glucosamine substituted with N-acetyl and O-succinyl constituents. The icaADBC locus containing the biosynthetic genes for production of PIA has been identified in both S. epidermidis and S. aureus. Whereas it is clear that PIA is a constituent that contributes to the virulence of S. epidermidis, it is less clear what role PIA plays in infection with S. aureus. Recently, identification of a novel polysaccharide antigen from S. aureus termed poly N-succinyl beta-(1-->6)-glucosamine (PNSG) has been reported. This polymer was composed of the same glycan backbone as PIA but was reported to contain a high proportion of N-succinylation rather than acetylation. We have isolated a glucosamine-containing exopolysaccharide from the constitutive over-producing MN8m strain of S. aureus in order to prepare polysaccharide-protein conjugate vaccines. In this report we demonstrate that MN8m produced a high-molecular-weight (>300,000 Da) polymer of beta-(1-->6)-linked glucosamine containing 45-60% N-acetyl, and a small amount of O-succinyl (approx 10% mole ratio to monosaccharide units). By detailed NMR analyses of polysaccharide preparations, we show that the previous identification of N-succinyl was an analytical artifact. The exopolysaccharide we have isolated is active in in vitro hemagglutination assays and is immunogenic in mice when coupled to a protein carrier. We therefore conclude that S. aureus strain MN8m produces a polymer that is chemically and biologically closely related to the PIA produced by S. epidermidis.

Immune response to native NadA from Neisseria meningitidis and its expression in clinical isolates in Brazil

A mAb against the NadA protein from Neisseria meningitidis strain 3006 (serosubtype B : 2b : P1.2 : P5.2,8) demonstrated strong bactericidal activity against Brazilian epidemic serogroup B strain N44/89 (B : 4,7 : P1.19,15 : P5.5,7) and a serogroup C strain, IMC 2135 (C : 2a : P1.5,2), but not against another serogroup C strain, N1002/90 (C : 2b : P1.3 : P5.8). The immunogenicity of native NadA in an outer-membrane vesicle (OMV) preparation was also tested. Serum from mice immunized with OMV from serogroup B strain N44/89, which contains the NadA protein, showed bactericidal activity against serogroup B and C strains possessing NadA. In dot-blot analysis of 100 serogroup B and 100 serogroup C isolates from Brazilian patients, the mAb to NadA recognized about 60 % of the samples from both serogroups. The molecular mass of the NadA protein from strain N44/89 determined by mass spectrometry was 37 971 Da and the peptide sequences were identical to those of NadA from N. meningitidis strain MC58.

Bordetella pertussis fimbriae are effective carrier proteins in Neisseria meningitidis serogroup C conjugate vaccines

Serogroup C meningococcal conjugate vaccines generally use diphtheria or tetanus toxoids as the protein carriers. The use of alternative carrier proteins may allow multivalent conjugate vaccines to be formulated into a single injection and circumvent potential problems of immune suppression in primed individuals. Bordetella pertussis fimbriae were assessed as carrier proteins for Neisseria meningitidis serogroup C polysaccharide. Fimbriae were conjugated to the polysaccharide using modifications of published methods and characterised by size exclusion chromatography; co-elution of protein and polysaccharide moieties confirmed conjugation. The conjugates elicited boostable IgG responses to fimbriae and serogroup C polysaccharide in mice, and IgG:IgM ratios indicated that the responses were thymus-dependent. High bactericidal antibody titres against a serogroup C strain of N. meningitidis were also observed. In a mouse infection model, the conjugate vaccine protected against lethal infection with N. meningitidis. Therefore, B. pertussis fimbriae are effective carrier proteins for meningococcal serogroup C polysaccharide and could produce a vaccine to protect against meningococcal disease and to augment protection against pertussis.

New therapies and vaccines for meningococcal disease

Meningococcal disease (MCD) is an important cause of morbidity and mortality. The pathophysiology consists of a complex interaction of bacterial and host factors, triggered by the release of endotoxin which initiates the inflammatory cascade, resulting in multi-organ failure, coagulopathy, capillary leak, metabolic derangement and eventually death. Prompt recognition and aggressive management are essential in reducing mortality. Over the past decade, there has been intense research into novel therapies and vaccines, with largely disappointing results. Therapies have been broadly divided into anti-endotoxin and anti-TNF-alpha therapies, treatment aimed at correcting coagulopathy and at blood purification and anti-inflammatory cytokine therapy. The reasons for the disappointing results in the search for new therapeutic strategies are difficult to identify. The disordered physiology in MCD results from a complex interaction of several mediators; therefore attempts to correct this by altering just one step represents a gross oversimplification of the process. In addition, the experimental model of endotoxaemia, which is often used, is a poor representation of an acutely ill patient with rapidly progressive shock. There have been several small or poorly designed trials, which have failed to reach definite conclusions. In order to yield conclusive results any future trials must be multicentre, randomised, controlled trials, but these are expensive and, in practice, difficult to conduct. The BPI trial (vide infra) was a significant step forward in this regard and demonstrated the ability to organise a large multicentred trial which can act as a template for future trials. Although the results were not significant there was an overall trend towards improved outcome in the treatment arm. Whilst the development of effective therapies and vaccines are awaited, the priorities at present must be the prompt recognition and aggressive management of disease.

Vaccines against meningococcal disease: current and future technologies

Development of the meningococcal serogroup C conjugate vaccine and its national implementation in the UK has been a major breakthrough in the prevention of meningococcal disease. New technologies are increasing the likelihood that research towards a vaccine against group B meningococcus will be successful. This review covers the recent development of vaccines against meningococcal disease and examines future vaccine candidates. The development of meningococcal polysaccharide vaccines was based on the virulence of the bacterial capsule components. The immunogenicity of these vaccines has been improved by covalent linkage to proteins in the new meningococcal C conjugate vaccines. However, the most promising developments for serogroup B disease have stemmed from other virulence determinants such as outer membrane proteins (OMPs) and lipopolysaccharides (LPS). New genome sequencing technology promises a way forward to developing a broadly cross-protective vaccine for this important pathogen.