Development and phase 1 clinical testing of a conjugate vaccine against meningococcus A and C

Exploring the variables influencing the immune response of traditional and innovative glycoconjugate vaccines

Vaccines are cost-effective tools for reducing morbidity and mortality caused by infectious diseases. The rapid evolution of pneumococcal conjugate vaccines, the introduction of tetravalent meningococcal conjugate vaccines, mass vaccination campaigns in Africa with a meningococcal A conjugate vaccine, and the recent licensure and introduction of glycoconjugates against S. Typhi underlie the continued importance of research on glycoconjugate vaccines. More innovative ways to produce carbohydrate-based vaccines have been developed over the years, including bioconjugation, Outer Membrane Vesicles (OMV) and the Multiple antigen-presenting system (MAPS). Several variables in the design of these vaccines can affect the induced immune responses. We review immunogenicity studies comparing conjugate vaccines that differ in design variables, such as saccharide chain length and conjugation chemistry, as well as carrier protein and saccharide to protein ratio. We evaluate how a better understanding of the effects of these different parameters is key to designing improved glycoconjugate vaccines.

Historical advances in structural and molecular biology and how they impacted vaccine development

Vaccines are among the greatest tools for prevention and control of disease. They have eliminated smallpox from the planet, decreased morbidity and mortality for major infectious diseases like polio, measles, mumps, and rubella, significantly blunted the impact of the COVID-19 pandemic, and prevented viral induced cancers such as cervical cancer caused by human papillomavirus. Recent technological advances, in genomics, structural biology, and human immunology have transformed vaccine development, enabling new technologies such as mRNA vaccines to greatly accelerate development of new and improved vaccines. In this review, we briefly highlight the history of vaccine development, and provide examples of where advances in genomics and structural biology, paved the way for development of vaccines for bacterial and viral diseases.

Impact and Control of Sugar Size in Glycoconjugate Vaccines

Glycoconjugate vaccines have contributed enormously to reducing and controlling encapsulated bacterial infections for over thirty years. Glycoconjugate vaccines are based on a carbohydrate antigen that is covalently linked to a carrier protein; this is necessary to cause T cell responses for optimal immunogenicity, and to protect young children. Many interdependent parameters affect the immunogenicity of glycoconjugate vaccines, including the size of the saccharide antigen. Here, we examine and discuss the impact of glycan chain length on the efficacy of glycoconjugate vaccines and report the methods employed to size polysaccharide antigens, while highlighting the underlying reaction mechanisms. A better understanding of the impact of key parameters on the immunogenicity of glycoconjugates is critical to developing a new generation of highly effective vaccines.

Cross reacting material (CRM197) as a carrier protein for carbohydrate conjugate vaccines targeted at bacterial and fungal pathogens

This paper gives an overview of conjugate glycovaccines which contain recombinant diphtheria toxoid CRM197 as a carrier protein. A special focus is given to synthetic methods used for preparation of neoglycoconjugates of CRM197 with oligosaccharide epitopes of cell surface carbohydrates of pathogenic bacteria and fungi. Syntheses of commercial vaccines and laboratory specimen on the basis of CRM197 are outlined briefly.

Immunobiology of Carbohydrates: Implications for Novel Vaccine and Adjuvant Design Against Infectious Diseases

Carbohydrates are ubiquitous molecules expressed on the surface of nearly all living cells, and their interaction with carbohydrate-binding proteins is critical to many immunobiological processes. Carbohydrates are utilized as antigens in many licensed vaccines against bacterial pathogens. More recently, they have also been considered as adjuvants. Interestingly, unlike other types of vaccines, adjuvants have improved immune response to carbohydrate-based vaccine in humans only in a few cases. Furthermore, despite the discovery of many new adjuvants in the last years, aluminum salts, when needed, remain the only authorized adjuvant for carbohydrate-based vaccines. In this review, we highlight historical and recent advances on the use of glycans either as vaccine antigens or adjuvants, and we review the use of currently available adjuvants to improve the efficacy of carbohydrate-based vaccines. A better understanding of the mechanism of carbohydrate interaction with innate and adaptive immune cells will benefit the design of a new generation of glycan-based vaccines and of immunomodulators to fight both longstanding and emerging diseases.

Generalized Modules for Membrane Antigens as Carrier for Polysaccharides: Impact of Sugar Length, Density, and Attachment Site on the Immune Response Elicited in Animal Models

Nanoparticle systems are being explored for the display of carbohydrate antigens, characterized by multimeric presentation of glycan epitopes and special chemico-physical properties of nano-sized particles. Among them, outer membrane vesicles (OMVs) are receiving great attention, combining antigen presentation with the immunopotentiator effect of the Toll-like receptor agonists naturally present on these systems. In this context, we are testing Generalized Modules for Membrane Antigens (GMMA), OMVs naturally released from Gram-negative bacteria mutated to increase blebbing, as carrier for polysaccharides. Here, we investigated the impact of saccharide length, density, and attachment site on the immune response elicited by GMMA in animal models, using a variety of structurally diverse polysaccharides from different pathogens (i.e., Neisseria meningitidis serogroup A and C, Haemophilus influenzae type b, and streptococcus Group A Carbohydrate and Salmonella Typhi Vi). Anti-polysaccharide immune response was not affected by the number of saccharides per GMMA particle. However, lower saccharide loading can better preserve the immunogenicity of GMMA as antigen. In contrast, saccharide length needs to be optimized for each specific antigen. Interestingly, GMMA conjugates induced strong functional immune response even when the polysaccharides were linked to sugars on GMMA. We also verified that GMMA conjugates elicit a T-dependent humoral immune response to polysaccharides that is strictly dependent on the nature of the polysaccharide. The results obtained are important to design novel glycoconjugate vaccines using GMMA as carrier and support the development of multicomponent glycoconjugate vaccines where GMMA can play the dual role of carrier and antigen. In addition, this work provides significant insights into the mechanism of action of glycoconjugates.

Development of an automated platform for the optimal production of glycoconjugate vaccines expressed in Escherichia coli

Protein Glycan Coupling Technology (PGCT) uses purposely modified bacterial cells to produce recombinant glycoconjugate vaccines. This vaccine platform holds great potential in this context, namely due to its modular nature, the simplified production process in comparison to traditional chemical conjugation methods, and its amenability to scaled-up operations. As a result, a considerable reduction in production time and cost is expected, making PGCT-made vaccines a suitable vaccine technology for low-middle income countries, where vaccine coverage remains predominantly low and inconsistent. This work aims to develop an integrated whole-process automated platform for the screening of PGCT-made glycoconjugate vaccine candidates. The successful translation of a bench scale process for glycoconjugate production to a microscale automated setting was achieved. This was integrated with a numerical computational software that allowed hands-free operation and a platform adaptable to biological variation over the course of a production process. Platform robustness was proven with both technical and biological replicates and subsequently the platform was used to screen for the most favourable conditions for production of a pneumococcal serotype 4 vaccine candidate. This work establishes an effective automated platform that enabled the identification of the most suitable E. coli strain and genetic constructs to be used in ongoing early phase research and be further brought into preclinical trials.

Structure of a protective epitope reveals the importance of acetylation of Neisseria meningitidis serogroup A capsular polysaccharide

Meningococcal meningitis remains a substantial cause of mortality and morbidity worldwide. Until recently, countries in the African meningitis belt were susceptible to devastating outbreaks, largely attributed to serogroup A Neisseria meningitidis (MenA). Vaccination with glycoconjugates of MenA capsular polysaccharide led to an almost complete elimination of MenA clinical cases. To understand the molecular basis of vaccine-induced protection, we generated a panel of oligosaccharide fragments of different lengths and tested them with polyclonal and monoclonal antibodies by inhibition enzyme-linked immunosorbent assay, surface plasmon resonance, and competitive human serum bactericidal assay, which is a surrogate for protection. The epitope was shown to optimize between three and six repeating units and to be O-acetylated. The molecular interactions between a protective monoclonal antibody and a MenA capsular polysaccharide fragment were further elucidated at the atomic level by saturation transfer difference NMR spectroscopy and X-ray crystallography. The epitope consists of a trisaccharide anchored to the antibody via the O- and N-acetyl moieties through either H-bonding or CH-π interactions. In silico docking showed that 3-O-acetylation of the upstream residue is essential for antibody binding, while O-acetate could be equally accommodated at three and four positions of the other two residues. These results shed light on the mechanism of action of current MenA vaccines and provide a foundation for the rational design of improved therapies.

GMMA Is a Versatile Platform to Design Effective Multivalent Combination Vaccines

Technology platforms are an important strategy to facilitate the design, development and implementation of vaccines to combat high-burden diseases that are still a threat for human populations, especially in low- and middle-income countries, and to address the increasing number and global distribution of pathogens resistant to antimicrobial drugs. Generalized Modules for Membrane Antigens (GMMA), outer membrane vesicles derived from engineered Gram-negative bacteria, represent an attractive technology to design affordable vaccines. Here, we show that GMMA, decorated with heterologous polysaccharide or protein antigens, leads to a strong and effective antigen-specific humoral immune response in mice. Importantly, GMMA promote enhanced immunogenicity compared to traditional formulations (e.g., recombinant proteins and glycoconjugate vaccines), without negative impact to the anti-GMMA immune response. Our findings support the use of GMMA as a "plug and play" technology for the development of effective combination vaccines targeting different bugs at the same time.

A Bioorthogonal Click Chemistry Toolbox for Targeted Synthesis of Branched and Well-Defined Protein-Protein Conjugates

Bioorthogonal chemistry holds great potential to generate difficult-to-access protein-protein conjugate architectures. Current applications are hampered by challenging protein expression systems, slow conjugation chemistry, use of undesirable catalysts, or often do not result in quantitative product formation. Here we present a highly efficient technology for protein functionalization with commonly used bioorthogonal motifs for Diels-Alder cycloaddition with inverse electron demand (DAinv ). With the aim of precisely generating branched protein chimeras, we systematically assessed the reactivity, stability and side product formation of various bioorthogonal chemistries directly at the protein level. We demonstrate the efficiency and versatility of our conjugation platform using different functional proteins and the therapeutic antibody trastuzumab. This technology enables fast and routine access to tailored and hitherto inaccessible protein chimeras useful for a variety of scientific disciplines. We expect our work to substantially enhance antibody applications such as immunodetection and protein toxin-based targeted cancer therapies.

Improving efficacy of glycoconjugate vaccines: from chemical conjugates to next generation constructs

Glycoconjugate vaccines are among the safest and most successful vaccines developed during the past 30 years. Since the first semisynthetic chemical conjugate vaccine licensed in the 1980's to protect human against Haemophilus influenzae type b infection, conjugate vaccines against Neisseria meningitidis and Streptococcus pneumoniae have been developed and registered using the same approach (i.e. bacterial growth to produce capsular polysaccharide antigen and chemical coupling to carrier protein). Other types of conjugate vaccines have been recently developed and tested in clinical trials, prepared by coupling chemically synthesized oligosaccharides to proteins, by engineering Escherichia coli to directly produce bioconjugates or by delivering the native carbohydrate antigen in engineered membrane vesicles (i.e. Generalized Modules for Membrane Antigens, GMMA). Through this review, the reader will have an insight regarding the history and the characteristics of different types of conjugate vaccines, and the attributes that might affect their immunogenicity and their potential for future applications.

Higher mass meningococcal group C-tetanus toxoid vaccines conjugated with carbodiimide correlate with greater immunogenicity

To examine the link between meningococcal C (MenC) vaccine size and immunogenic response, a panel of MenC glycoconjugate vaccines were prepared differing in chain length, molar mass and hydrodynamic volume. The preparations consisted of different lengths of MenC polysaccharide (PS) covalently linked to monomeric purified tetanus toxoid (TT) carrier protein using the coupling reagent ethylcarbodiimide hydrochloride (EDC). Size exclusion chromatography with multi-angle light scattering (SEC-MALS) and viscometry analysis confirmed that the panel of MenC-TT conjugates spanned masses of 191,500 to 2,348,000 g/mol, and hydrodynamic radii ranging from 12.1 to 47.9 nm. The two largest conjugates were elliptical in shape, whereas the two smallest conjugates were more spherical. The larger conjugates appeared to fit a model described by multiple TTs with cross-linked PS, typical of lattice-like networks described previously for TT conjugates, while the smaller conjugates were found to fit a monomeric or dimeric TT configuration. The effect of vaccine conjugate size on immune responses was determined using a two-dose murine immunization. The two larger panel vaccine conjugates produced higher anti-MenC IgG1 and IgG2b titres after the second dose. Larger vaccine conjugate size also stimulated greater T-cell proliferative responses in an in vitro recall assay, although cytokines indicative of a T-helper response were not measurable. In conclusion, larger MenC-TT conjugates up to 2,348,000 g/mol produced by EDC chemistry correlate with greater humoral and cellular murine immune responses. These observations suggest that conjugate size can be an important modulator of immune response.

A preliminary study on the application of PspA as a carrier for group A meningococcal polysaccharide

This study aimed to explore the feasibility of pneumococcal surface protein A (PspA) as a carrier protein. Three recombinant pneumococcal surface proteins from three different clades were expressed by the prokaryotic expression system and conjugated to group A meningococcal polysaccharide (GAMP) to generate three polysaccharide-protein conjugates. The conjugates, unconjugated proteins, GAMP, and GAMP-TT vaccine bulk (used as positive control) were immunized into mice, and their immune effects were assessed by the methods of enzyme-linked immunosorbent assay (ELISA), flow cytometry (FCM), and serum bactericidal assay (SBA). The results showed that the polysaccharide-protein conjugates could produce higher levels of anti-GAMP IgG titers (P < 0.05), higher ratios of Th1/Th2 (P < 0.05), and higher levels of serum bactericidal activity (P < 0.05), compared with the unconjugated GAMP. The conjugation of PspAs to GAMP also enhanced the anti-PspA responses compared with unconjugated PspAs except for PspA3. In conclusion, the results indicated that the three PspAs were appropriate carrier proteins, as demonstrated by the characteristics of T-cell dependent responses to the GAMP, and might protect against group A of epidemic cerebrospinal meningitis.

Vaccines and global health: In search of a sustainable model for vaccine development and delivery

Most vaccines for diseases in low- and middle-income countries fail to be developed because of weak or absent market incentives. Conquering diseases such as tuberculosis, HIV, malaria, and Ebola, as well as illnesses caused by multidrug-resistant pathogens, requires considerable investment and a new sustainable model of vaccine development involving close collaborations between public and private sectors.

Mitigating base-catalysed degradation of periodate-oxidized capsular polysaccharides: Conjugation by reductive amination in acidic media

Reductive amination coupling an aldehyde-containing polysaccharide, generated by periodate oxidation, with the amino groups in protein has been widely used in the synthesis of glycoconjugate vaccines. The conjugation is often achieved under slightly basic conditions via a Schiff's base intermediate followed by its reduction with sodium cyanoborohydride. We observed that oxidized capsular polysaccharides such as Streptococcus pneumoniae type 6B (Pn-6B) and Haemophilus influenzae type a (HiA) underwent significant degradation during the conjugation in slightly basic media leading to sub-optimal glycoconjugates. Further study on oxidized Pn-3, Pn-6A, Pn-6C, Pn-2 polysaccharides and dextran provided evidence that the degradation is a result of base-catalysed β-elimination. In contrast to HiA, Pn-2, Pn-3, Pn-6B polysaccharides and dextran, oxidized Pn-6A and Pn-6C polysaccharides were stable under basic conditions due to lack of the leaving group at the β-position of the aldehyde. By performing conjugation of oxidized polysaccharides to bovine serum albumin (BSA) in phosphate buffer at pH 6.0, 6.8, 7.2 and 8.0, we concluded that the reductive amination proceeds best in slightly acidic media, particularly with those β-elimination susceptible polysaccharides.

Combined Chemical Synthesis and Tailored Enzymatic Elongation Provide Fully Synthetic and Conjugation-Ready Neisseria meningitidis Serogroup X Vaccine Antigens

Studies on the polymerization mode of Neisseria meningitidis serogroup X capsular polymerase CsxA recently identified a truncated construct that can be immobilized and used for length controlled on-column production of oligosaccharides. Here, we combined the use of a synthetic acceptor bearing an appendix for carrier protein conjugation and the on-column process to a novel chemo-enzymatic strategy. After protein coupling of the size optimized oligosaccharide produced by the one-pot elongation procedure, we obtained a more homogeneous glycoconjugate compared to the one previously described starting from the natural polysaccharide. Mice immunized with the conjugated fully synthetic oligomer elicited functional antibodies comparable to controls immunized with the current benchmark MenX glycoconjugates prepared from the natural capsule polymer or from fragments of it enzymatically elongated. This pathogen-free technology allows the fast total in vitro construction of predefined bacterial polysaccharide fragments. Compared to conventional synthetic protocols, the procedure is more expeditious and drastically reduces the number of purification steps to achieve the oligomers. Furthermore, the presence of a linker for conjugation in the synthetic acceptor minimizes manipulations on the enzymatically produced glycan prior to protein conjugation. This approach enriches the methods for fast construction of complex bacterial carbohydrates.

Fast and accurate dynamic estimation of field effectiveness of meningococcal vaccines

BACKGROUND

Estimating the effectiveness of meningococcal vaccines with high accuracy and precision can be challenging due to the low incidence of the invasive disease, which ranges between 0.5 and 1 cases per 100,000 in Europe and North America. Vaccine effectiveness (VE) is usually estimated with a screening method that combines in one formula the proportion of meningococcal disease cases that have been vaccinated and the proportion of vaccinated in the overall population. Due to the small number of cases, initial point estimates are affected by large uncertainties and several years may be required to estimate VE with a small confidence interval.

METHODS

We used a Monte Carlo maximum likelihood (MCML) approach to estimate the effectiveness of meningococcal vaccines, based on stochastic simulations of a dynamic model for meningococcal transmission and vaccination. We calibrated the model to describe two immunization campaigns: the campaign against MenC in England and the Bexsero campaign that started in the UK in September 2015. First, the MCML method provided estimates for both the direct and indirect effects of the MenC vaccine that were validated against results published in the literature. Then, we assessed the performance of the MCML method in terms of time gain with respect to the screening method under different assumptions of VE for Bexsero.

RESULTS

MCML estimates of VE for the MenC immunization campaign are in good agreement with results based on the screening method and carriage studies, yet characterized by smaller confidence intervals and obtained using only incidence data collected within 2 years of scheduled vaccination. Also, we show that the MCML method could provide a fast and accurate estimate of the effectiveness of Bexsero, with a time gain, with respect to the screening method, that could range from 2 to 15 years, depending on the value of VE measured from field data.

CONCLUSIONS

Results indicate that inference methods based on dynamic computational models can be successfully used to quantify in near real time the effectiveness of immunization campaigns against Neisseria meningitidis. Such an approach could represent an important tool to complement and support traditional observational studies, in the initial phase of a campaign.

Factors contributing to the immunogenicity of meningococcal conjugate vaccines

Various glycoprotein conjugate vaccines have been developed for the prevention of invasive meningococcal disease, having significant advantages over pure polysaccharide vaccines. One of the most important features of the conjugate vaccines is the induction of a T-cell dependent immune response, which enables both the induction of immune memory and a booster response after repeated immunization. The nature of the carrier protein to which the polysaccharides are chemically linked, is often regarded as the main component of the vaccine in determining its immunogenicity. However, other factors can have a significant impact on the vaccine's profile. In this review, we explore the physico-chemical properties of meningococcal conjugate vaccines, which can significantly contribute to the vaccine's immunogenicity. We demonstrate that the carrier is not the sole determining factor of the vaccine's profile, but, moreover, that the conjugate vaccine's immunogenicity is the result of multiple physico-chemical structures and characteristics.

Evaluation of the non-toxic mutant of the diphtheria toxin K51E/E148K as carrier protein for meningococcal vaccines

Diphtheria toxin mutant CRM197 is a common carrier protein for glycoconjugate vaccines, which has been proven an effective protein vector for, among others, meningococcal carbohydrates. The wide-range use of this protein in massive vaccine production requires constant increase of production yields and adaptability to an ever-growing market. Here we compare CRM197 with the alternative diphtheria non-toxic variant DT-K51E/E148K, an inactive mutant that can be produced in the periplasm of Escherichia coli. Biophysical characterization of DT-K51E/E148K suggested high similarity with CRM197, with main differences in their alpha-helical content, and a suitable purity for conjugation and vaccine preparation. Meningococcal serogroup A (MenA) glycoconjugates were synthesized using CRM197 and DT-K51E/E148K as carrier proteins, obtaining the same conjugation yields and comparable biophysical profiles. Mice were then immunized with these CRM197 and DT-K51E/E148K conjugates, and essentially identical immunogenic and protective effects were observed. Overall, our data indicate that DT-K51E/E148K is a readily produced protein that now allows the added flexibility of E. coli production in vaccine development and that can be effectively used as protein carrier for a meningococcal conjugate vaccine.

Investigating the immunodominance of carbohydrate antigens in a bivalent unimolecular glycoconjugate vaccine against serogroup A and C meningococcal disease

Multicomponent constructs, obtained by coupling different glycans to the carrier protein, have been proposed as a way to co-deliver multiple surface carbohydrates targeting different strains of one pathogen and reduce the number of biomolecules in the formulation of multivalent vaccines. To assess the feasibility of this approach for anti-microbial vaccines and investigate the potential immunodominance of one carbohydrate antigen over the others in these constructs, we designed a bivalent unimolecular vaccine against serogroup A (MenA) and C (MenC) meningococci, with the two different oligomers conjugated to same molecule of carrier protein (CRM197). The immune response elicited in mice by the bivalent MenAC construct was compared with the ones induced by the monovalent MenA and MenC vaccines and their combinations. After the second dose, the bivalent construct induced good levels of anti-MenA and anti-MenC antibodies with respect to the controls. However, the murine sera from the MenAC construct exhibited good anti-MenC bactericidal activity, and very low anti-MenA functionality when compared to the monovalent controls. This result was explained with the diverse relative avidities against MenA and MenC polysaccharides, which were measured in the generated sera. The immunodominant effect of the MenC antigen was fully overcome following the third immunization, when sera endowed with higher avidity and excellent bactericidal activity against both MenA and MenC expressing strains were elicited. Construction of multicomponent glycoconjugate vaccines against microbial pathogens is a feasible approach, but particular attention should be devoted to study and overcome possible occurrence of immune interference among the carbohydrates.

From empiricism to rational design: a personal perspective of the evolution of vaccine development

Vaccination, which is the most effective medical intervention that has ever been introduced, originated from the observation that individuals who survived a plague or smallpox would not get the disease twice. To mimic the protective effects of natural infection, Jenner - and later Pasteur - inoculated individuals with attenuated or killed disease-causing agents. This empirical approach inspired a century of vaccine development and the effective prophylaxis of many infectious diseases. From the 1980s, several waves of new technologies have enabled the development of novel vaccines that would not have been possible using the empirical approach. The technological revolution in the field of vaccination is now continuing, and it is delivering novel and safer vaccines. In this Timeline article, we provide our views on the transition from empiricism to rational vaccine design.

Immunoactivity of protein conjugates of carba analogues from Neisseria meningitidis a capsular polysaccharide

Neisseria meningitidis type A (MenA) is a Gram-negative encapsulated bacterium that is a major cause of epidemic meningitis, especially in the sub-Saharan region of Africa. The development and manufacture of a liquid glycoconjugate vaccine against MenA are hampered by the poor hydrolytic stability of its capsular polysaccharide (CPS), consisting of (1→6)-linked 2-acetamido-2-deoxy-α-d-mannopyranosyl phosphate repeating units. The replacement of the ring oxygen with a methylene group to generate a carbocyclic analogue leads to enhancement of its chemical stability. Herein, we report conjugation of carbocyclic analogue monomer, dimer, and trimer to the protein carrier CRM197. After immunization in mice, only the conjugated trimer was able to induce specific anti-MenA polysaccharide IgG antibodies with in vitro bactericidal activity, although to a lesser extent than pentadecamer and hexamer oligomers obtained from mild acid hydrolysis of the native polysaccharide conjugated to the same protein carrier. This study represents the first proof-of-concept that hydrolytically stable structural analogues of saccharide antigens can be used for the development of efficacious antimicrobial preventative therapies. Conjugates with longer carbocyclic oligomers and/or precise acetylation patterns could further increase the induced immune response to a level comparable with those of commercially available anti-meningococcal glycoconjugate vaccines.

Development of a glycoconjugate vaccine to prevent meningitis in Africa caused by meningococcal serogroup X

Neisseria meningitidis is a major cause of bacterial meningitis worldwide, especially in the African meningitis belt, and has a high associated mortality. The meningococcal serogroups A, W, and X have been responsible for epidemics and almost all cases of meningococcal meningitis in the meningitis belt over the past 12 y. Currently no vaccine is available against meningococcal X (MenX). Because the development of a new vaccine through to licensure takes many years, this leaves Africa vulnerable to new epidemics of MenX meningitis at a time when the epidemiology of meningococcal meningitis on the continent is changing rapidly, following the recent introduction of a glycoconjugate vaccine against serogroup A. Here, we report the development of candidate glycoconjugate vaccines against MenX and preclinical data from their use in animal studies. Following optimization of growth conditions of our seed MenX strain for polysaccharide (PS) production, a scalable purification process was developed yielding high amounts of pure MenX PS. Different glycoconjugates were synthesized by coupling MenX oligosaccharides of varying chain length to CRM197 as carrier protein. Analytical methods were developed for in-process control and determination of purity and consistency of the vaccines. All conjugates induced high anti-MenX PS IgG titers in mice. Antibodies were strongly bactericidal against African MenX isolates. These findings support the further development of glycoconjugate vaccines against MenX and their assessment in clinical trials to produce a vaccine against the one cause of epidemic meningococcal meningitis that currently cannot be prevented by available vaccines.

Relative stability of meningococcal serogroup A and X polysaccharides

Prior to the introduction of the MenAfriVac™ serogroup A glycoconjugate vaccine in September 2010, serogroup A was the major epidemic disease-causing meningococcal serogroup in the African meningitis belt. However, recently serogroup X meningococcal (MenX) disease has received increased attention because of outbreaks recorded in this region, with increased endemic levels of MenX disease over the past 2 years. Whereas polysaccharide-protein conjugate vaccines against meningococcal serogroups A, C, W and Y (MenA, MenC, MenW, MenY) are on the market, a vaccine able to protect against MenX has never been achieved. The structure of serogroup A, C, W and Y meningococcal polysaccharides has been already fully elucidated by NMR. MenX capsular polysaccharide (MenX CPS) structure is also documented but fewer characterization data have been published. We have applied here (1)H NMR, (31)P NMR and HPLC to evaluate the stability of MenX CPS in aqueous solution as compared to MenA capsular polysaccharide (MenA CPS). The stability study demonstrated that MenA CPS is more susceptible to hydrolytic degradation than MenX CPS. The different stereochemistry of the N-acetyl group at position C(2) of mannosamine (MenA CPS) and glucosamine (MenX CPS) respectively might play a fundamental role in this susceptibility to polysaccharide chain degradation. The satisfactory stability of MenX CPS predicts the possibility that a stable fully-liquid MenX polysaccharide or glycoconjugate vaccine could be developed.

Toward a meningitis-free world

Meningococcal meningitis is a devastating disease that is often fatal. Vaccines against the five major meningococcal serogroups causing disease are about to become available, a conjugate vaccine against meningococcus A is in use for mass vaccination in Africa, and a protein-based vaccine against meningococcal B is ready for licensure. With the availability of these new vaccines, the world can finally be rid of meningococcal meningitis, thus rewriting a new chapter in medical history.

The design of semi-synthetic and synthetic glycoconjugate vaccines

INTRODUCTION

Glycoconjugate vaccines are among the safest and most efficacious vaccines developed during the last 30 years. They are a potent tool for prevention of life-threatening bacterial infectious diseases like meningitis and pneumonia. The concept of hapten-carrier conjugation is now being extended to other disease areas.

AREAS COVERED

This is an overview of the history and current status of glycoconjugate vaccines. The authors discuss the approaches for their preparation and quality control as well as those variables which might affect their product profile. The authors also look at the potential to develop fully synthetic conjugate vaccines based on the progress of organic chemistry. Additionally, new applications of conjugate vaccines technology in the field of non-infectious diseases are discussed. Through this review, the reader will have an insight regarding the issues and complexities involved in the preparation and characterization of conjugate vaccines, the variables that might affect their immunogenicity and the potential for future applications.

EXPERT OPINION

The immunogenicity of weak T-independent antigens can be increased in quantity and quality by conjugation to protein carriers, which provide T-cell help. Glycoconjugate vaccines are among the safest and most efficacious vaccines developed so far. Various conjugation procedures and carrier proteins can be used. Many variables impact on the immunogenicity of conjugate vaccines and a tight control through physicochemical tests is important to ensure manufacturing and clinical consistency. New and challenging targets for conjugate vaccines are represented by cancer and other non-infectious diseases.

The challenge of developing universal vaccines

Antigenic variability of immunodominant antigens is a common mechanism used by pathogens to escape the immune response. Frequently, the proposed solution is a universal vaccine based on conserved antigens present on all strains of the pathogen. Indeed, a lot of progress has been made in the development of vaccines that induce broad immune responses. However, truly universal vaccines are not easy to produce and still face many challenges, mostly because in those pathogens that use antigenic variability to escape the immune response, conserved antigens have been selected by evolution to be poorly immunogenic. This review describes the progress made towards the development of vaccines inducing broad protection against Neisseria meningitidis, influenza, HIV, and Candida and the challenges of developing truly universal vaccines.

WITHDRAWN: Conjugate vaccines for preventing meningococcal C meningitis and septicaemia

BACKGROUND

Meningococcal polysaccharide (MPLS) vaccines protect against Serogroup C disease, but do not produce an immune response in infants less than two years of age. This limitation can be overcome by linking C polysaccharide to carrier proteins ('conjugating'), to create meningococcal serogroup C conjugate (MCC) vaccines. In the absence of trial data, the immune response to vaccination has been considered to be a reasonable surrogate for vaccine protection.

OBJECTIVES

To assess the immunogenicity, safety and efficacy of MCC vaccines for preventing meningitis and septicaemia.

SEARCH STRATEGY

We searched the Cochrane Central Register Controlled Trials (CENTRAL) (The Cochrane Library 2005, issue 3); MEDLINE (1966 to September, Week 1 2005); and EMBASE (1990 to June 2005) and references of studies.

SELECTION CRITERIA

Randomised controlled trials (RCTs) and controlled clinical trials (CCTs) in humans comparing MCC vaccines against a control vaccine or none. In the absence of any trials on vaccine efficacy, population-based observational studies about effectiveness were included.

DATA COLLECTION AND ANALYSIS

Two authors independently screened the results of the literature searches, selected eligible studies, extracted the data and evaluated the quality of them.

MAIN RESULTS

The studies showed that MCC vaccine was highly immunogenic in infants after two and three doses, in toddlers after one and two doses and in older age groups after one dose. In general higher titres were generated after MCC than after MPLS vaccines. Immunological hypo-responsiveness seen after repeated doses of MPLS vaccine may be overcome with MCC. Observational studies have documented a significant decline in meningococcal C disease in countries where MCC vaccines have been widely used. The timing of the vaccinations schedules, the specific conjugate used, and the vaccines given concomitantly or combined, may be important.

AUTHORS' CONCLUSIONS

The MCC vaccine appears to be safe, immunogenic and able to induce immunological memory in all age groups. Observational studies strongly suggest that MCC is clinically effective.

Toward self-adjuvanting subunit vaccines: model peptide and protein antigens incorporating covalently bound toll-like receptor-7 agonistic imidazoquinolines

Toll-like receptor (TLR)-7 agonists show prominent Th1-biased immunostimulatory activities. A TLR7-active N(1)-(4-aminomethyl)benzyl substituted imidazoquinoline 1 served as a convenient precursor for the syntheses of isothiocyanate and maleimide derivatives for covalent attachment to free amine and thiol groups of peptides and proteins. 1 was also amenable to direct reductive amination with maltoheptaose without significant loss of activity. Covalent conjugation of the isothiocyanate derivative 2 to α-lactalbumin could be achieved under mild, non-denaturing conditions, in a controlled manner and with full preservation of antigenicity. The self-adjuvanting α-lactalbumin construct induced robust, high-affinity immunoglobulin titers in murine models. The premise of covalently decorating protein antigens with adjuvants offers the possibility of drastically reducing systemic exposure of the adjuvant, and yet eliciting strong, Th1-biased immune responses.

Qualitative and quantitative assessment of meningococcal antigens to evaluate the potential strain coverage of protein-based vaccines

A unique multicomponent vaccine against serogroup B meningococci incorporates the novel genome-derived proteins fHbp, NHBA, and NadA that may vary in sequence and level of expression. Measuring the effectiveness of such vaccines, using the accepted correlate of protection against invasive meningococcal disease, could require performing the serum bactericidal assay (SBA) against many diverse strains for each geographic region. This approach is impractical, especially for infants, where serum volumes are very limited. To address this, we developed the meningococcal antigen typing system (MATS) by combining a unique vaccine antigen-specific ELISA, which detects qualitative and quantitative differences in antigens, with PorA genotyping information. The ELISA correlates with killing of strains by SBA and measures both immunologic cross-reactivity and quantity of the antigens NHBA, NadA, and fHbp. We found that strains exceeding a threshold value in the ELISA for any of the three vaccine antigens had ≥80% probability of being killed by immune serum in the SBA. Strains positive for two or more antigens had a 96% probability of being killed. Inclusion of multiple different antigens in the vaccine improves breadth of coverage and prevents loss of coverage if one antigen mutates or is lost. The finding that a simple and high-throughput assay correlates with bactericidal activity is a milestone in meningococcal vaccine development. This assay allows typing of large panels of strains and prediction of coverage of protein-based meningococcal vaccines. Similar assays may be used for protein-based vaccines against other bacteria.

Measuring antigen-specific bactericidal responses to a multicomponent vaccine against serogroup B meningococcus

Serum bactericidal activity using human complement is the basis for established correlates of protection against invasive meningococcal disease. During the development of multicomponent protein-based vaccines against meningococcus B, it is necessary to measure antigen-specific bactericidal responses. This is not straightforward because each strain may be killed by antibodies to multiple antigens. We characterized a large panel of strains and, using a competitive inhibition SBA, we identified four strains that are each specifically killed by bactericidal antibodies to one of the major vaccine components. These strains provide a straightforward approach to demonstrate protective responses to each component of the vaccine and demonstrate that each of the antigens in the vaccine is sufficient to provide a potentially protective level of bactericidal activity.

Serum bactericidal antibody response 1 year after meningococcal polysaccharide vaccination of patients with common variable immunodeficiency

Some patients with common variable immunodeficiency (CVID) can generate an antibody response following vaccination with Neisseria meningitidis polysaccharide, but the duration of this protection is unknown. In this study, serum bactericidal antibody (SBA) responses to serogroup C N. meningitidis of 23 patients with CVID and 23 sex- and age-matched controls were measured 1 year after vaccination with the plain A/C meningococcal polysaccharide vaccine. The fold rise in serum bactericidal antibody geometric mean titers of the control group from prevaccination to 1 year postvaccination was significantly higher than that of the patient group (5.41- versus 2.96-fold, P = 0.009). Of 23 CVID patients, 8 had a poor response to vaccine (<4-fold rise) 3 weeks after vaccination, and low titers remained when measured 1 year later. Of the 15 CVID patients who had a normal response to vaccine (>/=4-fold rise) 3 weeks after vaccination, 6 cases failed to maintain protective SBA titers, whereas the remaining 9 had protective titers 1 year after vaccination. Only one of the 23 controls, who developed protective SBA titers after 3 weeks, lost the protective titers after 1 year. Among the patients, the presence of bronchiectasis and/or splenomegaly at enrollment was associated with poor SBA response to vaccine at 3 weeks and/or failure to maintain protective levels at 1 year. The results of this study demonstrate that a number of CVID patients can produce protective antibody titers that can persist for 1 year after vaccination, which lends strong support to the inclusion of polysaccharide vaccine in the immunization program for CVID patients.

Structural organization of NadADelta(351-405), a recombinant MenB vaccine component, by its physico-chemical characterization at drug substance level

The physico-chemical characterization of NadADelta(351-405), a recombinant protein discovered by reverse vaccinology, component of a candidate vaccine against Neisseria meningitidis serotype B is presented. Analytical methods like mass spectrometry, electrophoresis, optical spectroscopy and SEC-MALLS have been applied to unveil the structure of NadADelta(351-405), and to evaluate Product-Related Substances. Moreover, analysis of the protein after intentional denaturation has been applied in order to challenge the chosen methods and to determine their appropriateness and specificity. All the obtained results were inserted in a model allowing in-depth understanding of the antigen NadADelta(351-405): it is present in solution as a homo-trimer, retaining a high percentage of alpha-helix secondary structure, and able to reassemble from monomeric subunits after thermal denaturation; this structural organization is consistent with that foreseen for MenB NadA (Neisseria Adhesin A). The analytical data sets produced during process development for clinical phases I-III material confirm product quality and manufacturing consistency.

Neonatal immune response and serum bactericidal activity induced by a meningococcal conjugate vaccine is enhanced by LT-K63 and CpG2006

Neonates have a poorly developed immune system. Therefore it is important to develop vaccination strategies that induce protective immunity and immunological memory against pathogens early in life. The immunogenicity of a meningococcal serogroup C polysaccharide conjugate (MenC-CRM(197)) was assessed in neonatal mice, and effects of LT-K63 and CpG2006 and immunisation routes were compared. Neonatal mice were primed subcutaneously (s.c.) or intranasally (i.n.) with MenC-CRM(197) with or without LT-K63 or CpG2006 and re-immunised 16 and 30 days later by the same route and formulation. Antibody levels were measured and generation of immunological memory assessed by affinity maturation and kinetics of the Ab response. Serum bactericidal activity (SBA) was measured to evaluate protective efficacy. The second and third dose of MenC-CRM(197) mixed with either LT-K63 or CpG2006 induced a rapid increase in MenC-specific IgG antibodies, to levels higher than elicited by MenC-CRM(197) alone (P<0.01) and in unimmunised mice (P<0.001), indicating efficient generation of memory by priming through both s.c. and i.n. routes. SBA was detected after three s.c. immunisations with MenC-CRM(197) s.c. alone. However, only two doses of MenC-CRM(197)+LT-K63 or MenC-CRM(197)+CpG2006 were needed to induce SBA levels>16. LT-K63 and CpG2006 enhanced neonatal antibody responses, affinity maturation, immunological memory to the conjugate MenC-CRM(197) and protective immunity. These results encourage the development of neonatal vaccination strategies to induce protective immunity and immunological memory against meningococcal disease.