Booster Vaccination With GVGH Shigella sonnei 1790GAHB GMMA Vaccine Compared to Single Vaccination in Unvaccinated Healthy European Adults: Results From a Phase 1 Clinical Trial

Testing S. sonnei GMMA with and without Aluminium Salt-Based Adjuvants in Animal Models

Shigellosis is one of the leading causes of diarrheal disease in low- and middle-income countries, particularly in young children, and is more often associated with antimicrobial resistance. Therefore, a preventive vaccine against shigellosis is an urgent medical need. We have proposed Generalised Modules for Membrane Antigens (GMMA) as an innovative delivery system for Shigella sonnei O-antigen, and an Alhydrogel formulation (1790GAHB) has been extensively tested in preclinical and clinical studies. Alhydrogel has been used as an adsorbent agent with the main purpose of reducing potential GMMA systemic reactogenicity. However, the immunogenicity and systemic reactogenicity of this GMMA-based vaccine formulated with or without Alhydrogel have never been compared. In this work, we investigated the potential adjuvant effect of aluminium salt-based adjuvants (Alhydrogel and AS37) on S. sonnei GMMA immunogenicity in mice and rabbits, and we found that S. sonnei GMMA alone resulted to be strongly immunogenic. The addition of neither Alhydrogel nor AS37 improved the magnitude or the functionality of vaccine-elicited antibodies. Interestingly, rabbits injected with either S. sonnei GMMA adsorbed on Alhydrogel or S. sonnei GMMA alone showed a limited and transient body temperature increase, returning to baseline values within 24 h after each vaccination. Overall, immunisation with unadsorbed GMMA did not raise any concern for animal health. We believe that these data support the clinical testing of GMMA formulated without Alhydrogel, which would allow for further simplification of GMMA-based vaccine manufacturing.

Putative correlates of protection against shigellosis assessing immunomarkers across responses to S. sonnei investigational vaccine

Shigella spp. are a leading bacterial cause of diarrhea. No widely licensed vaccines are available and there is no generally accepted correlate of protection. We tested a S. sonnei Generalized Modules for Membrane Antigen (GMMA)-based vaccine (1790GAHB) in a phase 2b, placebo-controlled, randomized, controlled human infection model study (NCT03527173) enrolling healthy United States adults aged 18-50 years. We report analyses evaluating immune responses to vaccination, with the aim to identify correlates of risk for shigellosis among assessed immunomarkers. We found that 1790GAHB elicited S. sonnei lipopolysaccharide specific α4β7+ immunoglobulin (Ig) G and IgA secreting B cells which are likely homing to the gut, indicating the ability to induce a mucosal in addition to a systemic response, despite parenteral delivery. We were unable to establish or confirm threshold levels that predict vaccine efficacy facilitating the evaluation of vaccine candidates. However, serum anti-lipopolysaccharide IgG and bactericidal activity were identified as potential correlates of risk for shigellosis.

Quality by Design Framework Applied to GMMA Purification

In recent years, Generalized Modules for Membrane Antigens (GMMA) have received increased attention as an innovative vaccine platform against bacterial pathogens, particularly attractive for low- and middle-income countries because of manufacturing simplicity. The assessment of critical quality attributes (CQAs), product-process interactions, identification of appropriate in process analytical methods, and process modeling is part of a robust quality by design (QbD) framework to support further development and control of manufacturing processes. QbD implementation in the context of the GMMA platform will ensure robust manufacturing of batches with desired characteristics, facilitating technical transfer to local manufacturers, regulatory approval, and commercialization of vaccines based on this technology. Here, we summarize the methodology suggested, applied to a first step of GMMA manufacturing process.

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.

Outer Membrane Vesicle Vaccine Platforms

Outer membrane vesicles (OMVs) are spontaneously released by many gram-negative bacteria during their growth and constitute an important virulence factor for bacteria, helping them to survive through harsh environmental conditions. Native OMVs, naturally-released from bacteria, are produced at a level too low for vaccine manufacturing, requiring chemical treatment (detergent-extracted) or genetic manipulation, resulting in generalized modules for membrane antigens (GMMAs). Over the years, the nature and properties of OMVs have made them a viable platform for vaccine development. There are a few licensed OMV vaccines mainly for the prevention of meningitis caused by Neisseria meningitidis serogroup B (MenB) and Haemophilus influenzae type b (Hib). There are several candidates in clinical development against other gram-negative organisms from which the OMVs are derived, but also against heterologous targets in which the OMVs are used as carriers (e.g. coronavirus disease 2019 [COVID-19]). The use of OMVs for targets other than those from which they are derived is a major advancement in OMV technology, improving its versatility by being able to deliver protein or polysaccharide antigens. Other advances include the range of genetic modifications that can be made to improve their safety, reduce reactogenicity, and increase immunogenicity and protective efficacy. However, significant challenges remain, such as identification of general tools for high-content surface expression of heterologous proteins on the OMV surface. Here, we outline the progress of OMV vaccines to date, particularly discussing licensed OMV-based vaccines and candidates in clinical development. Recent trends in preclinical research are described, mainly focused on genetic manipulation and chemical conjugation for the use of OMVs as carriers for heterologous protein and polysaccharide antigens. Remaining challenges with the use of OMVs and directions for future research are also discussed.

Comparison of Shigella GMMA and glycoconjugate four-component formulations in animals

Shigellosis is leading bacterial cause of diarrhea with high prevalence in children younger than 5 years in low- and middle-income countries, and increasing number of reports of Shigella cases associated to anti-microbial resistance. No vaccines against Shigella are still licensed, but different candidates based on the O-antigen portion of lipopolysaccharides are in clinic. Generalized Modules for Membrane Antigens (GMMA) have been proposed as an alternative delivery system for the O-antigen, and a 4-component vaccine candidate (altSonflex1-2-3), containing GMMA from S. sonnei and S. flexneri 1b, 2a and 3a is being tested in a phase 1/2 clinical trial, with the aim to elicit broad protection against the most prevalent Shigella serotypes. Here, the 4-component GMMA vaccine candidate has been compared to a more traditional glycoconjugate formulation for the ability to induce functional antibodies in mice and rabbits. In mice, in the absence of Alhydrogel, GMMA induce higher IgG antibodies than glycoconjugates and stronger bactericidal titers against all Shigella serotypes. In the presence of Alhydrogel, GMMA induce O-antigen specific IgG levels similar to traditional glycoconjugates, but with a broader range of IgG subclasses, resulting in stronger bactericidal activity. In rabbits, GMMA elicit higher functional antibodies than glycoconjugates against S. sonnei, and similar responses to S. flexneri 1b, 2a and 3a, independently from the presence of Alhydrogel. Different O-antigen based vaccines against Shigella are now in clinical stage and it will be of particular interest to understand how the preclinical findings in the different animal models translate in humans.

A next-generation GMMA-based vaccine candidate to fight shigellosis

Shigellosis is a leading cause of diarrheal disease in low-middle-income countries (LMICs). Effective vaccines will help to reduce the disease burden, exacerbated by increasing antibiotic resistance, in the most susceptible population represented by young children. A challenge for a broadly protective vaccine against shigellosis is to cover the most epidemiologically relevant serotypes among >50 Shigella serotypes circulating worldwide. The GMMA platform has been proposed as an innovative delivery system for Shigella O-antigens, and we have developed a 4-component vaccine against S. sonnei, S. flexneri 1b, 2a and 3a identified among the most prevalent Shigella serotypes in LMICs. Driven by the immunogenicity results obtained in clinic with a first-generation mono-component vaccine, a new S. sonnei GMMA construct was generated and combined with three S. flexneri GMMA in a 4-component Alhydrogel formulation (altSonflex1-2-3). This formulation was highly immunogenic, with no evidence of negative antigenic interference in mice and rabbits. The vaccine induced bactericidal antibodies also against heterologous Shigella strains carrying O-antigens different from those included in the vaccine. The Monocyte Activation Test used to evaluate the potential reactogenicity of the vaccine formulation revealed no differences compared to the S. sonnei mono-component vaccine, shown to be safe in several clinical trials in adults. A GLP toxicology study in rabbits confirmed that the vaccine was well tolerated. The preclinical study results support the clinical evaluation of altSonflex1-2-3 in healthy populations, and a phase 1-2 clinical trial is currently ongoing.

Characterization of Enzyme-Linked Immunosorbent Assay (ELISA) for Quantification of Antibodies against Salmonella Typhimurium and Salmonella Enteritidis O-Antigens in Human Sera

Nontyphoidal Salmonella (NTS) is a leading cause of morbidity and mortality caused by enteric pathogens worldwide in both children and adults, and vaccines are not yet available. The measurement of antigen-specific antibodies in the sera of vaccinated or convalescent individuals is crucial to understand the incidence of disease and the immunogenicity of vaccine candidates. A solid and standardized assay used to determine the level of specific anti-antigens IgG is therefore of paramount importance. In this work, we presented the characterization of a customized enzyme-linked immunosorbent assay (ELISA) with continuous readouts and a standardized definition of EU/mL. We assessed various performance parameters: standard curve accuracy, dilutional linearity, intermediate precision, specificity, limits of blanks, and quantification. The simplicity of the assay, its high sensitivity and specificity coupled with its low cost and the use of basic consumables and instruments without the need of high automation makes it suitable for transfer and application to different laboratories, including resource-limiting settings where the disease is endemic. This ELISA is, therefore, fit for purpose to be used for quantification of antibodies against Salmonella Typhimurium and Salmonella Enteritidis O-antigens in human samples, both for vaccine clinical trials and large sero-epidemiological studies.

The Two Faces of Bacterial Membrane Vesicles: Pathophysiological Roles and Therapeutic Opportunities

Bacterial membrane vesicles (MVs) are nanosized lipid particles secreted by lysis or blebbing mechanisms from Gram-negative and -positive bacteria. It is becoming increasingly evident that MVs can promote antimicrobial resistance but also provide versatile opportunities for therapeutic exploitation. As non-living facsimiles of parent bacteria, MVs can carry multiple bioactive molecules such as proteins, lipids, nucleic acids, and metabolites, which enable them to participate in intra- and interspecific communication. Although energetically costly, the release of MVs seems beneficial for bacterial fitness, especially for pathogens. In this review, we briefly discuss the current understanding of diverse MV biogenesis routes affecting MV cargo. We comprehensively highlight the physiological functions of MVs derived from human pathogens covering in vivo adaptation, colonization fitness, and effector delivery. Emphasis is given to recent findings suggesting a vicious cycle of MV biogenesis, pathophysiological function, and antibiotic therapy. We also summarize potential therapeutical applications, such as immunotherapy, vaccination, targeted delivery, and antimicrobial potency, including their experimental validation. This comparative overview identifies common and unique strategies for MV modification used along diverse applications. Thus, the review summarizes timely aspects of MV biology in a so far unprecedented combination ranging from beneficial function for bacterial pathogen survival to future medical applications.

Toward a Shigella Vaccine: Opportunities and Challenges to Fight an Antimicrobial-Resistant Pathogen

Shigellosis causes more than 200,000 deaths worldwide and most of this burden falls on Low- and Middle-Income Countries (LMICs), with a particular incidence in children under 5 years of age. In the last decades, Shigella has become even more worrisome because of the onset of antimicrobial-resistant strains (AMR). Indeed, the WHO has listed Shigella as one of the priority pathogens for the development of new interventions. To date, there are no broadly available vaccines against shigellosis, but several candidates are being evaluated in preclinical and clinical studies, bringing to light very important data and information. With the aim to facilitate the understanding of the state-of-the-art of Shigella vaccine development, here we report what is known about Shigella epidemiology and pathogenesis with a focus on virulence factors and potential antigens for vaccine development. We discuss immunity after natural infection and immunization. In addition, we highlight the main characteristics of the different technologies that have been applied for the development of a vaccine with broad protection against Shigella.

Exploring the Role of GMMA Components in the Immunogenicity of a 4-Valent Vaccine against Shigella

Shigellosis is the leading cause of diarrheal disease, especially in children of low- and middle-income countries, and is often associated with anti-microbial resistance. Currently, there are no licensed vaccines widely available against Shigella, but several candidates based on the O-antigen (OAg) portion of lipopolysaccharides are in development. We have proposed Generalized Modules for Membrane Antigens (GMMA) as an innovative delivery system for OAg, and a quadrivalent vaccine candidate containing GMMA from S. sonnei and three prevalent S. flexneri serotypes (1b, 2a and 3a) is moving to a phase II clinical trial, with the aim to elicit broad protection against Shigella. GMMA are able to induce anti-OAg-specific functional IgG responses in animal models and healthy adults. We have previously demonstrated that antibodies against protein antigens are also generated upon immunization with S. sonnei GMMA. In this work, we show that a quadrivalent Shigella GMMA-based vaccine is able to promote a humoral response against OAg and proteins of all GMMA types contained in the investigational vaccine. Proteins contained in GMMA provide T cell help as GMMA elicit a stronger anti-OAg IgG response in wild type than in T cell-deficient mice. Additionally, we observed that only the trigger of Toll-like Receptor (TLR) 4 and not of TLR2 contributed to GMMA immunogenicity. In conclusion, when tested in mice, GMMA of a quadrivalent Shigella vaccine candidate combine both adjuvant and carrier activities which allow an increase in the low immunogenic properties of carbohydrate antigens.

Generalized Modules for Membrane Antigens (GMMA), an outer membrane vesicle-based vaccine platform, for efficient viral antigen delivery

Vaccine platforms enable fast development, testing, and manufacture of more affordable vaccines. Here, we evaluated Generalized Modules for Membrane Antigens (GMMA), outer membrane vesicles (OMVs) generated by genetically modified Gram-negative bacteria, as a vaccine platform for viral pathogens. Influenza A virus hemagglutinin (HA), either physically mixed with GMMA (HA+STmGMMA mix), or covalently linked to GMMA surface (HA-STmGMMA conjugate), significantly increased antigen-specific humoral and cellular responses, with HA-STmGMMA conjugate inducing further enhancement than HA+STmGMMA mix. HA-STmGMMA conjugate protected mice from lethal challenge. The versatility for this platform was confirmed by conjugation of rabies glycoprotein (RABVG) onto GMMA through the same method. RABVG+STmGMMA mix and RABVG-STmGMMA conjugate exhibited similar humoral and cellular response patterns and protection efficacy as the HA formulations, indicating relatively consistent responses for different vaccines based on the GMMA platform. Comparing to soluble protein, GMMA was more efficiently taken up in vivo and exhibited a B-cell preferential uptake in the draining lymph nodes (LNs). Together, GMMA enhances immunity against viral antigens, and the platform works well with different antigens while retaining similar immunomodulatory patterns. The findings of our study imply the great potential of GMMA-based vaccine platform also against viral infectious diseases.

Antigen presentation by Follicular Dendritic cells to cognate B cells is pivotal for Generalised Modules for Membrane Antigens (GMMA) immunogenicity

GMMA has been proposed as a potent technology platform for the design of safe, effective and affordable vaccines. As GMMA are vesicles blebbing out of the outer membrane of Gram-negative bacteria, they contain lipopolysaccharides, lipoproteins and peptidoglycans that stimulate immune cells via Toll-like Receptors 4 (TLR4) or TLR2. Being basically nanoparticles, GMMA can be efficiently captured by Follicular Dendritic Cells (FDC) for antigen presentation to cognate B cells. GMMA have shown to be highly immunogenic in preclinical and clinical studies and the engagement of TLR4 and TLR2 or antigen presentation by FDC may have a prominent role in GMMA immunogenicity, which is well worth investigating. By using GMMA derived from Shigella sonnei and Salmonella Typhimurium, we show for the first time that the antigen presentation by FDC to cognate B cells plays a major role in the induction of an effective humoral immune response upon immunization with GMMA by using both models. The engagement of TLR4 is critical to elicit an optimal antibody production, but its effect on antibody functionality is dependent on GMMA type and is dispensable when immunizing with Alum adjuvant, whereas TLR2 does not have any role for GMMA immunogenicity. Our findings represent a substantial advancement of the knowledge on GMMA mode of action and shed a light on novel perspectives for the design of safer and more effective GMMA-based vaccines. ONE SENTENCE SUMMARY: The study demonstrated that the antigen presentation by FDC to cognate B cells plays a major role for GMMA immunogenicity.

Complement-mediated serum bactericidal activity of antibodies elicited by the Shigella sonnei GMMA vaccine in adults from a shigellosis-endemic country: Exploratory analysis of a Phase 2a randomized study

Shigella is associated with a significant burden of disease worldwide among individuals of all ages and is the major cause of moderate and severe diarrhea in children under five years of age in low- and middle-income countries. Several candidate vaccines against Shigella species are currently under clinical development. The investigational 1790GAHB vaccine against Shigella sonnei is based on GMMA (Generalized Modules for Membrane Antigens) technology. The vaccine was well tolerated and induced high antibody levels in early-phase clinical trials in both Shigella-endemic and non-endemic settings. The present analysis assessed the bactericidal activity of antibodies induced by 1790GAHB in healthy Kenyan adults during a phase 2a, controlled, randomized study (NCT02676895). Participants received two doses of 1790GAHB 4 weeks apart containing either 1.5/25 µg or 6/100 µg O antigen/protein, or active comparator vaccines (Control). Serum bactericidal activity (SBA) against S. sonnei was assessed at pre-vaccination (D1), 28 days post-first dose (D29) and 28 days post-second dose (D57), using a luminescence-based assay. Most participants had SBA titers above the lower limit of quantification of the assay at D1. SBA geometric mean titers increased 3.4-fold in the 1.5/25 µg group and 6.3-fold in the 6/100 µg group by D29 and were maintained at D57. There was no increase in SBA geometric mean titers in the Control group. A strong correlation was observed between SBA titers and anti-S. sonnei lipopolysaccharide serum immunoglobulin G antibody concentrations (Pearson correlation coefficient = 0.918), indicating that SBA can effectively complement enzyme-linked immunosorbent assay data by indicating the functionality of 1790GAHB-induced antibodies.

Engineered bacterial membrane vesicles are promising carriers for vaccine design and tumor immunotherapy

Bacterial membrane vesicles (BMVs) have emerged as novel and promising platforms for the development of vaccines and immunotherapeutic strategies against infectious and noninfectious diseases. The rich microbe-associated molecular patterns (MAMPs) and nanoscale membrane vesicle structure of BMVs make them highly immunogenic. In addition, BMVs can be endowed with more functions via genetic and chemical modifications. This article reviews the immunological characteristics and effects of BMVs, techniques for BMV production and modification, and the applications of BMVs as vaccines or vaccine carriers. In summary, given their versatile characteristics and immunomodulatory properties, BMVs can be used for clinical vaccine or immunotherapy applications.

Towards a Four-Component GMMA-Based Vaccine against Shigella

Shigellosis remains a major public health problem around the world; it is one of the leading causes of diarrhoeal disease in low- and middle-income countries, particularly in young children. The increasing reports of Shigella cases associated with anti-microbial resistance are an additional element of concern. Currently, there are no licensed vaccines widely available against Shigella, but several vaccine candidates are in development. It has been demonstrated that the incidence of disease decreases following a prior Shigella infection and that serum and mucosal antibody responses are predominantly directed against the serotype-specific Shigella O-antigen portion of lipopolysaccharide membrane molecules. Many Shigella vaccine candidates are indeed O-antigen-based. Here we present the journey towards the development of a potential low-cost four-component Shigella vaccine, eliciting broad protection against the most prevalent Shigella serotypes, that makes use of the GMMA (Generalized Modules for Membrane Antigens) technology, a novel platform based on bacterial outer membranes for delivery of the O-antigen to the immune system.

Progress in Alternative Strategies to Combat Antimicrobial Resistance: Focus on Antibiotics

Antibiotic resistance, and, in a broader perspective, antimicrobial resistance (AMR), continues to evolve and spread beyond all boundaries. As a result, infectious diseases have become more challenging or even impossible to treat, leading to an increase in morbidity and mortality. Despite the failure of conventional, traditional antimicrobial therapy, in the past two decades, no novel class of antibiotics has been introduced. Consequently, several novel alternative strategies to combat these (multi-) drug-resistant infectious microorganisms have been identified. The purpose of this review is to gather and consider the strategies that are being applied or proposed as potential alternatives to traditional antibiotics. These strategies include combination therapy, techniques that target the enzymes or proteins responsible for antimicrobial resistance, resistant bacteria, drug delivery systems, physicochemical methods, and unconventional techniques, including the CRISPR-Cas system. These alternative strategies may have the potential to change the treatment of multi-drug-resistant pathogens in human clinical settings.

Dissecting in Vitro the Activation of Human Immune Response Induced by Shigella sonnei GMMA

Generalized Modules for Membrane Antigens (GMMA) are outer membrane exosomes purified from Gram-negative bacteria genetically mutated to increase blebbing and reduce risk of reactogenicity. This is commonly achieved through modification of the lipid A portion of lipopolysaccharide. GMMA faithfully resemble the bacterial outer membrane surface, and therefore represent a powerful and flexible platform for vaccine development. Although GMMA-based vaccines have been demonstrated to induce a strong and functional antibody response in animals and humans maintaining an acceptable reactogenicity profile, the overall impact on immune cells and their mode of action are still poorly understood. To characterize the GMMA-induced immune response, we stimulated human peripheral blood mononuclear cells (hPBMCs) with GMMA from Shigella sonnei. We studied GMMA both with wild-type hexa-acylated lipid A and with the corresponding less reactogenic penta-acylated form. Using multicolor flow cytometry, we assessed the activation of immune cell subsets and we profiled intracellular cytokine production after GMMA stimulation. Moreover, we measured the secretion of thirty cytokines/chemokines in the cell culture supernatants. Our data indicated activation of monocytes, dendritic, NK, B, and γδ T cells. Comparison of the cytokine responses showed that, although the two GMMA have qualitatively similar profiles, GMMA with modified penta-acylated lipid A induced a lower production of pro-inflammatory cytokines/chemokines compared to GMMA with wild-type lipid A. Intracellular cytokine staining indicated monocytes and dendritic cells as the main source of the cytokines produced. Overall, these data provide new insights into the activation of key immune cells potentially targeted by GMMA-based vaccines.

GMMA Technology for the Development of Safe Vaccines: Meta-Analysis of Individual Patient Data to Assess the Safety Profile of Shigella sonnei 1790GAHB Vaccine in Healthy Adults, with Special Focus on Neutropenia

Introduction

Shigellosis is a major health concern among children < 5 years of age from developing countries, and there are no widely available vaccines to prevent it. The GMMA-based 1790GAHB investigational vaccine against Shigella sonnei was well tolerated and immunogenic in phase 1 and 2 studies conducted in healthy adults from Shigella endemic and non-endemic populations. Based on pooled data of five individual trials, we assessed the association between vaccine administration and the risk of neutropenia as well as the overall safety profile of 1790GAHB.

Methods

The risk ratio (RR) of neutropenia was evaluated between participants receiving 1790GAHB (vaccinees) and active comparator/placebo (controls) using different ethnicity-specific absolute neutrophil count (ANC) thresholds established to define neutropenia. Safety was assessed in terms of solicited, unsolicited, and serious adverse events (AEs).

Results

Of the 279 participants, 11 (5.5%) vaccinees and 4 (5.0%) controls had ANC below the appropriate threshold within 7 days post-vaccination. RR was 0.96 [95% confidence interval (CI) 0.54–1.70]. When neutrophil counts of participants of African descent were measured against an ethnicity non-specific threshold, they resulted in neutropenia episodes in 30 (37.0%) vaccinees and 16 (30.2%) controls, while only 2 (2.5%) vaccinees and 1 (1.9%) control had neutropenia when the ethnicity-specific threshold was applied. RRs were 0.98 (95% CI 0.75–1.28) and 1.30 (95% CI 0.1–17.6), respectively. Solicited and unsolicited AEs were slightly more frequent among vaccinees than controls. No serious AEs, other than neutropenia cases, were recorded in the vaccine group.

Conclusion

By applying the appropriate threshold, no increased risk of neutropenia was identified in vaccinees compared with the controls. The frequency of neutropenia events varied drastically when ethnicity-appropriate thresholds were applied. This observation highlights the importance of selecting appropriate cut-off values according to the correct population reference. Overall, the 1790GAHB vaccine demonstrated an acceptable safety profile.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40121-022-00596-x.

The Ongoing Journey of a Shigella Bioconjugate Vaccine

Shigellosis is a serious disease with a major impact, especially in low-income countries where mortality and morbidity are high. In addition, shigellosis among travelers and military personnel is a cause of significant morbidity and contributes to the increase in antimicrobial resistance. The World Health Organization (WHO) considers the development of a Shigella vaccine a priority for public health. Over the past 60 years, several efforts to develop a Shigella vaccine have been pursued, without success. The principle of preventing shigellosis with a conjugate vaccine was demonstrated in the 1990′s, but this vaccine was not further developed. Bioconjugation is an innovative technology that allows the production of conjugate vaccines in a biological environment to preserve native immunogenic structures. In this review, we describe the journey of the bioconjugate Shigella vaccine, one of the most advanced clinical programs for a Shigella vaccine.

GMMA as a 'plug and play' technology to tackle infectious disease to improve global health: context and perspectives for the future

INTRODUCTION

Generalized-Modules-for-Membrane-Antigens (GMMA) is a technology platform developed to design outer membrane vesicle (OMV)-based vaccines. GMMA are basically OMVs derived from a bacterial strain specifically engineered to obtain a fit-for-purpose and affordable vaccine by potentiating, or deleting, expression of specific genes. OMVs can be used as a carrier for antigens by inducing their expression on them, with the aim to improve antigen immunogenicity and design multivalent combination vaccines.

AREAS COVERED

We expanded this finding to show that the chemical conjugation of different proteic and/or polysaccharidic antigens, to GMMA, is a methodology complementary to the genetic manipulation to obtain highly effective combination vaccines. Here we discuss our findings with a specific focus on the impact that GMMA technology can have on global health, as this technology platform is particularly suited to support the development of affordable vaccines for low-income countries.

EXPERT OPINION

We believe that it is critical to elucidate the mode of action of GMMA immunogenicity and have provided a summarized description of the immunological questions to be addressed in the near future. The improved knowledge of GMMA might lead to designing more effective and safer GMMA-based vaccines to tackle the most serious vaccine-preventable diseases.

Combined effects of glycan chain length and linkage type on the immunogenicity of glycoconjugate vaccines

The development and use of antibacterial glycoconjugate vaccines have significantly reduced the occurrence of potentially fatal childhood and adult diseases such as bacteremia, bacterial meningitis, and pneumonia. In these vaccines, the covalent linkage of bacterial glycans to carrier proteins augments the immunogenicity of saccharide antigens by triggering T cell-dependent B cell responses, leading to high-affinity antibodies and durable protection. Licensed glycoconjugate vaccines either contain long-chain bacterial polysaccharides, medium-sized oligosaccharides, or short synthetic glycans. Here, we discuss factors that affect the glycan chain length in vaccines and review the available literature discussing the impact of glycan chain length on vaccine efficacy. Furthermore, we evaluate the available clinical data on licensed glycoconjugate vaccine preparations with varying chain lengths against two bacterial pathogens, Haemophilus influenzae type b and Neisseria meningitidis group C, regarding a possible correlation of glycan chain length with their efficacy. We find that long-chain glycans cross-linked to carrier proteins and medium-sized oligosaccharides end-linked to carriers both achieve high immunogenicity and efficacy. However, end-linked glycoconjugates that contain long untethered stretches of native glycan chains may induce hyporesponsiveness by T cell-independent activation of B cells, while cross-linked medium-sized oligosaccharides may suffer from suboptimal saccharide epitope accessibility.

Development of a Self-Adjuvanting, Cross-Protective, Stable Intranasal Recombinant Vaccine for Shigellosis

With the acquirement of antibiotic resistance, Shigella has resulted in multiple epidemics of shigellosis, an infectious diarrheal disease, causing thousands of deaths per year. Unfortunately, there are no licensed vaccines, primarily due to low or serotype-specific immunogenicity. Thus, conserved subunit vaccines utilizing recombinant invasion plasmid antigens (Ipa) have been explored as cross-protective vaccine candidates. However, achieving cross-protection against Shigella dysenteriae 1, which caused multiple pandemics/epidemics in the recent past, has been difficult. Therefore, a rational approach to improve cross-protection in the preparation for a possible pandemic should involve conserved proteins from S. dysenteriae 1 (Sd1). IpaC is one such conserved immunogenic protein that is less explored as an independent vaccine due to its instability/aggregation. Therefore, to improve cross-protection and potential immunogenicity and to be prepared for a future epidemic/pandemic, herein, we stabilized recombinant Sd1 IpaC, expressed without its chaperone, using a previously reported stabilizing detergent (LDAO) in a modified protocol and assessed its vaccine potential without an adjuvant. The protein assembled into heterogeneous complex spherical structures in the presence of LDAO and showed improved stability at storage temperatures of -80, -20, 4, 25, and 37 °C while providing enhanced yield and concentration. The protein could also be stably lyophilized and reconstituted, increasing the convenience of transportation and storage. Upon intranasal administration in BALB/c mice, the stabilized-IpaC-immunized groups generated significant antibody response and were not only protected against a high intraperitoneal dose of homologous S. dysenteriae 1 but also showed 100% survival against heterologous Shigella flexneri 2a without an adjuvant, while the control animals showed visible diarrhea (bloody-Sd1 challenge), lethargy, and weight loss with 0% survival. Overall, this work demonstrates that stabilized IpaC can be explored as a minimalist, self-adjuvanting, cross-protective, intranasal, single-antigen Shigella vaccine.

Impact of O-Acetylation on S. flexneri 1b and 2a O-Antigen Immunogenicity in Mice

Shigellosis is a diarrheal disease caused prevalently by Shigella flexneri and S. sonnei and representing a major global health risk, particularly in developing countries. Bacterial O-antigen (OAg) is the primary target of the host immune response and modifications of its oligosaccharide units, including O-acetylation, are responsible for the variability among the circulating S. flexneri serotypes. No vaccines are widely available against shigellosis and the understanding of the immunogenicity induced by the OAg is fundamental for the design of a vaccine that could cover the most prevalent Shigella serotypes. To understand whether a different O-acetylation pattern could influence the immune response elicited by S. flexneri OAg, we employed as a vaccine technology GMMA purified from S. flexneri 2a and 1b strains that were easily engineered to obtain differently O-acetylated OAg. Resulting GMMA were tested in mice, demonstrating not only no major impact of O-acetyl decorations on the immune response elicited by the two OAg against the homologous strains, but also that the O-acetylation of the Rhamnose III residue (O-factor 9), shared among serotypes 1b, 2a and 6, does not induce cross-reactive antibodies against these serotypes. This work contributes to the optimization of vaccine design against Shigella, providing indication about the ability of shared epitopes to elicit broad protection against S. flexneri serotypes and supporting the identification of critical quality attributes of OAg-based vaccines.

Rationalizing the design of a broad coverage Shigella vaccine based on evaluation of immunological cross-reactivity among S. flexneri serotypes

No vaccine to protect against an estimated 238,000 shigellosis deaths per year is widely available. S. sonnei is the most prevalent Shigella, and multiple serotypes of S. flexneri, which change regionally and globally, also cause significant disease. The leading Shigella vaccine strategies are based on the delivery of serotype specific O-antigens. A strategy to minimize the complexity of a broadly-protective Shigella vaccine is to combine components from S. sonnei with S. flexneri serotypes that induce antibodies with maximum cross-reactivity between different serotypes. We used the GMMA-technology to immunize animal models and generate antisera against 14 S. flexneri subtypes from 8 different serotypes that were tested for binding to and bactericidal activity against a panel of 11 S. flexneri bacteria lines. Some immunogens induced broadly cross-reactive antibodies that interacted with most of the S. flexneri in the panel, while others induced antibodies with narrower specificity. Most cross-reactivity could not be assigned to modifications of the O-antigen, by glucose, acetate or phosphoethanolamine, common to several of the S. flexneri serotypes. This allowed us to revisit the current dogma of cross-reactivity among S. flexneri serotypes suggesting that a broadly protective vaccine is feasible with limited number of appropriately selected components. Thus, we rationally designed a 4-component vaccine selecting GMMA from S. sonnei and S. flexneri 1b, 2a and 3a. The resulting formulation was broadly cross-reactive in mice and rabbits, inducing antibodies that killed all S. flexneri serotypes tested. This study provides the framework for a broadly-protective Shigella vaccine which needs to be verified in human trials.

A strategy to optimize the composition for a broadly-protective Shigella vaccine is to combine components directed against S. sonnei with S. flexneri serotypes to induce antibody responses with the maximum cross-reactivity between different serotypes. Based on mouse and rabbit immunogenicity, we selected 4 GMMA-immunogens, derived from S. sonnei and S. flexneri 1b, 2a and 3a, able to induce antibodies that were broadly bactericidal against most epidemiologically significant S. flexneri strains in mice and rabbits. This was not predicted on the basis of O-antigen modifications conferring serotype or group specificities and allowed revisiting the dogma of cross-protection among S. flexneri serotypes. Overall, this study provides a framework for the rational design of a broadly-protective vaccine that will be evaluated in upcoming human vaccine trials. It also tackles a key issue regarding Shigella vaccine development that is balancing a sufficient number of antigenic components in the vaccine to provide adequate coverage of serotype diversity while minimizing complexity.

Efficacy, safety, and immunogenicity of the Shigella sonnei 1790GAHB GMMA candidate vaccine: Results from a phase 2b randomized, placebo-controlled challenge study in adults

BACKGROUND

Shigellosis accounts for substantial morbidity and mortality worldwide and is the second most common cause of moderate and severe diarrhoea in children.

METHODS

This phase 2b study (NCT03527173), conducted between August 2018 and November 2019, evaluated vaccine efficacy (VE), safety, and immunogenicity of a Shigella sonnei GMMA candidate vaccine (1790GAHB) in adults, using a S. sonnei 53 G controlled human infection model. Participants (randomized 1:1) received two doses of 1790GAHB or placebo (GAHB-Placebo), at day (D) 1 and D29, and an oral challenge of S. sonnei 53 G at D57. VE was evaluated using several endpoints, reflecting different case definitions of shigellosis. For the primary endpoint, the success criterion was a lower limit of the 90% confidence interval >0.

FINDINGS

Thirty-six and 35 participants received 1790GAHB or placebo, respectively; 33 and 29 were challenged, 15 and 12 developed shigellosis. VE was not demonstrated for any endpoint. Adverse events were more frequent in 1790GAHB versus placebo recipients post-vaccination. Anti-S. sonnei lipopolysaccharide (LPS) IgG responses increased at D29 and remained stable through D57 in group 1790GAHB; no increase was shown in placebo recipients.

INTERPRETATION

1790GAHB had an acceptable safety profile and induced anti-LPS IgG responses but did not demonstrate clinical efficacy against shigellosis. Baseline/pre-challenge antibody levels were higher in participants who did not develop shigellosis post-challenge, suggesting a role of anti-LPS IgG antibodies in clinical protection, although not fully elucidated in this study. For further vaccine development an increased S. sonnei O-antigen content is likely needed to enhance anti-LPS immune responses.

FUNDING

GlaxoSmithKline Biologicals SA, Bill and Melinda Gates Foundation.

GMMA-Based Vaccines: The Known and The Unknown

Generalized Modules for Membrane Antigens (GMMA) are outer membrane vesicles derived from Gram-negative bacteria engineered to provide an over-vesiculating phenotype, which represent an attractive platform for the design of affordable vaccines. GMMA can be further genetically manipulated to modulate the risk of systemic reactogenicity and to act as delivery system for heterologous polysaccharide or protein antigens. GMMA are able to induce strong immunogenicity and protection in animal challenge models, and to be well-tolerated and immunogenic in clinical studies. The high immunogenicity could be ascribed to their particulate size, to their ability to present to the immune system multiple antigens in a natural conformation which mimics the bacterial environment, as well as to their intrinsic self-adjuvanticity. However, GMMA mechanism of action and the role in adjuvanticity are still unclear and need further investigation. In this review, we discuss progresses in the development of the GMMA vaccine platform, highlighting successful applications and identifying knowledge gaps and potential challenges.

Development of a Monocyte Activation Test as an Alternative to the Rabbit Pyrogen Test for Mono- and Multi-Component Shigella GMMA-Based Vaccines

Generalised modules for membrane antigens (GMMA)-based vaccines comprise the outer membrane from genetically modified Gram-negative bacteria containing membrane proteins, phospholipids and lipopolysaccharides. Some lipoproteins and lipopolysaccharides are pyrogens; thus, GMMA-based vaccines are intrinsically pyrogenic. It is important to control the pyrogenic content of biological medicines, including vaccines, to prevent adverse reactions such as febrile responses. The rabbit pyrogen test (RPT) and bacterial endotoxin test (BET) are the most commonly employed safety assays used to detect pyrogens. However, both tests are tailored for detecting pyrogenic contaminants and have considerable limitations when measuring the pyrogen content of inherently pyrogenic products. We report the adaptation of the monocyte activation test (MAT) as an alternative to the RPT for monitoring the pyrogenicity of Shigella GMMA-based vaccines. The European Pharmacopoeia endorses three MAT methods (A–C). Of these, method C, the reference lot comparison test, was identified as the most suitable. This method was evaluated with different reference materials to ensure parallelism and consistency for a mono- and multi-component Shigella GMMA vaccine. We demonstrate the drug substance as a promising reference material for safety testing of the matched drug product. Our results support the implementation of MAT as an alternative to the RPT and use of the defined parameters can be extended to GMMA-based vaccines currently in development, aiding vaccine batch release.

Neisseria meningitidis Factor H Binding Protein Surface Exposure on Salmonella Typhimurium GMMA Is Critical to Induce an Effective Immune Response against Both Diseases

GMMA, outer membrane vesicles resulting from hyperblebbing mutated bacterial strains, are a versatile vaccine platform for displaying both homologous and heterologous antigens. Periplasmic expression is a popular technique for protein expression in the lumen of the blebs. However, the ability of internalized antigens to induce antibody responses has not been extensively investigated. Herein, the Neisseria meningitidis factor H binding protein (fHbp) was heterologously expressed in the lumen of O-antigen positive (OAg+) and O-antigen negative (OAg−) Salmonella Typhimurium GMMA. Only the OAg− GMMA induced an anti-fHbp IgG response in mice if formulated on Alum, although it was weak and much lower compared to the recombinant fHbp. The OAg− GMMA on Alum showed partial instability, with possible exposure of fHbp to the immune system. When we chemically conjugated fHbp to the surface of both OAg+ and OAg− GMMA, these constructs induced a stronger functional response compared to the fHbp immunization alone. Moreover, the OAg+ GMMA construct elicited a strong response against both the target antigens (fHbp and OAg), with no immune interference observed. This result suggests that antigen localization on GMMA surface can play a critical role in the induction of an effective immune response and can encourage the development of GMMA based vaccines delivering key protective antigens on their surface.

Long-Term Anti-Bacterial Immunity against Systemic Infection by Salmonella enterica Serovar Typhimurium Elicited by a GMMA-Based Vaccine

Salmonella Typhimurium (STm) represents the most prevalent cause of invasive non-typhoidal Salmonella (iNTS) disease, and currently no licensed vaccine is available. In this work we characterized the long-term anti-bacterial immunity elicited by a STm vaccine based on Generalized Modules of Membrane Antigens (GMMA) delivering O:4,5 antigen, using a murine model of systemic infection. Subcutaneous immunization of mice with STmGMMA/Alhydrogel elicited rapid, high, and persistent antigen-specific serum IgG and IgM responses. The serum was bactericidal in vitro. O:4,5-specific IgG were also detected in fecal samples after immunization and positively correlated with IgG observed in intestinal washes. Long-lived plasma cells and O:4,5-specific memory B cells were detected in spleen and bone marrow. After systemic STm challenge, a significant reduction of bacterial load in blood, spleen, and liver, as well as a reduction of circulating neutrophils and G-CSF glycoprotein was observed in STmGMMA/Alhydrogel immunized mice compared to untreated animals. Taken together, these data support the development of a GMMA-based vaccine for prevention of iNTS disease.

Antibodies Elicited by the Shigella sonnei GMMA Vaccine in Adults Trigger Complement-Mediated Serum Bactericidal Activity: Results From a Phase 1 Dose Escalation Trial Followed by a Booster Extension

Shigella is the second most deadly diarrheal disease among children under five years of age, after rotavirus, with high morbidity and mortality in developing countries. Currently, no vaccine is widely available, and the increasing levels of multidrug resistance make Shigella a high priority for vaccine development. The single-component candidate vaccine against Shigella sonnei (1790GAHB), developed using the GMMA technology, contains the O antigen (OAg) portion of lipopolysaccharide (LPS) as active moiety. The vaccine was well tolerated and immunogenic in early-phase clinical trials. In a phase 1 placebo-controlled dose escalation trial in France (NCT02017899), three doses of five different vaccine formulations (0.06/1, 0.3/5, 1.5/25, 3/50, 6/100 µg of OAg/protein) were administered to healthy adults. In the phase 1 extension trial (NCT03089879), conducted 2–3 years following the parent study, primed individuals who had undetectable antibody levels before the primary series received a 1790GAHB booster dose (1.5/25 µg OAg/protein). Controls were unprimed participants immunized with one 1790GAHB dose. The current analysis assessed the functionality of sera collected from both studies using a high-throughput luminescence-based serum bactericidal activity (SBA) assay optimized for testing human sera. Antibodies with complement-mediated bactericidal activity were detected in vaccinees but not in placebo recipients. SBA titers increased with OAg dose, with a persistent response up to six months after the primary vaccination with at least 1.5/25 µg of OAg/protein. The booster dose induced a strong increase of SBA titers in most primed participants. Correlation between SBA titers and anti-S. sonnei LPS serum immunoglobulin G levels was observed. Results suggest that GMMA is a promising OAg delivery system for the generation of functional antibody responses and persistent immunological memory.

Salmonella Paratyphi A Outer Membrane Vesicles Displaying Vi Polysaccharide as a Multivalent Vaccine against Enteric Fever

Typhoid and paratyphoid fevers have a high incidence worldwide and coexist in many geographical areas, especially in low-middle-income countries (LMIC) in South and Southeast Asia. There is extensive consensus on the urgent need for better and affordable vaccines against systemic Salmonella infections. Generalized modules for membrane antigens (GMMA), outer membrane exosomes shed by Salmonella bacteria genetically manipulated to increase blebbing, resemble the bacterial surface where protective antigens are displayed in their native environment. Here, we engineered S Paratyphi A using the pDC5-viaB plasmid to generate GMMA displaying the heterologous S Typhi Vi antigen together with the homologous O:2 O antigen. The presence of both Vi and O:2 was confirmed by flow cytometry on bacterial cells, and their amount was quantified on the resulting vesicles through a panel of analytical methods. When tested in mice, such GMMA induced a strong antibody response against both Vi and O:2, and these antibodies were functional in a serum bactericidal assay. Our approach yielded a bivalent vaccine candidate able to induce immune responses against different Salmonella serovars, which could benefit LMIC residents and travelers.

Stability of Outer Membrane Vesicles-Based Vaccines, Identifying the Most Appropriate Methods to Detect Changes in Vaccine Potency

Ensuring the stability of vaccines is crucial to successfully performing global immunization programs. Outer Membrane Vesicles (OMV) are receiving great attention as vaccine platforms. OMV are complex molecules and few data have been collected so far on their stability. OMV produced by bacteria, genetically modified to increase their spontaneous release, simplifying their production, are also known as Generalized Modules for Membrane Antigens (GMMA). We have performed accelerated stability studies on GMMA from different pathogens and verified the ability of physico-chemical and immunological methods to detect possible changes. High-temperature conditions (100 °C for 40 min) did not affect GMMA stability and immunogenicity in mice, in contrast to the effect of milder temperatures for a longer period of time (37 °C or 50 °C for 4 weeks). We identified critical quality attributes to monitor during stability assessment that could impact vaccine efficacy. In particular, specific recognition of antigens by monoclonal antibodies through competitive ELISA assays may replace in vivo tests for the potency assessment of GMMA-based vaccines.

Advances in the development of Salmonella-based vaccine strategies for protection against Salmonellosis in humans

Salmonella spp. are important human pathogens globally causing millions of cases of typhoid fever and non‐typhoidal salmonellosis annually. There are only a few vaccines licensed for use in humans which all target Salmonella enterica serovar Typhi. Vaccine development is hampered by antigenic diversity between the thousands of serovars capable of causing infection in humans. However, a number of attenuated candidate vaccine strains are currently being developed. As facultative intracellular pathogens with multiple systems for transporting effector proteins to host cells, attenuated Salmonella strains can also serve as ideal tools for the delivery of foreign antigens to create multivalent live carrier vaccines for simultaneous immunization against several unrelated pathogens. Further, the ease with which Salmonella can be genetically modified and the extensive knowledge of the virulence mechanisms of this pathogen means that this bacterium has often served as a model organism to test new approaches. In this review we focus on (1) recent advances in live attenuated Salmonella vaccine development, (2) improvements in expression of foreign antigens in carrier vaccines and (3) adaptation of attenuated strains as sources of purified antigens and vesicles that can be used for subunit and conjugate vaccines or together with attenuated vaccine strains in heterologous prime‐boosting immunization strategies. These advances have led to the development of new vaccines against Salmonella which have or will soon be tested in clinical trials.

Effect of O-Antigen Chain Length Regulation on the Immunogenicity of Shigella and Salmonella Generalized Modules for Membrane Antigens (GMMA)

Recently, generalized modules for membrane antigens (GMMA) technology has been proposed as an alternative approach to traditional glycoconjugate vaccines for O-antigen delivery. Saccharide length is a well-known parameter that can impact the immune response induced by glycoconjugates both in terms of magnitude and quality. However, the criticality of O-antigen length on the immune response induced by GMMA-based vaccines has not been fully elucidated. Here, Shigella and Salmonella GMMA-producing strains were further mutated in order to display homogeneous polysaccharide populations of different sizes on a GMMA surface. Resulting GMMA were compared in mice immunization studies. Athymic nude mice were also used to investigate the involvement of T-cells in the immune response elicited. In contrast with what has been reported for traditional glycoconjugate vaccines and independent of the pathogen and the sugar structural characteristics, O-antigen length did not result in being a critical parameter for GMMA immunogenicity. This work supports the identification of critical quality attributes to optimize GMMA vaccine design and improve vaccine efficacy and gives insights on the nature of the immune response induced by GMMA.

The identification of novel immunogenic antigens as potential Shigella vaccine components

BACKGROUND

Shigella is a major diarrheal pathogen for which there is presently no vaccine. Whole genome sequencing provides the ability to predict and derive novel antigens for use as vaccines. Here, we aimed to identify novel immunogenic Shigella antigens that could serve as Shigella vaccine candidates, either alone, or when conjugated to Shigella O-antigen.

METHODS

Using a reverse vaccinology approach, where genomic analysis informed the Shigella immunome via an antigen microarray, we aimed to identify novel immunogenic Shigella antigens. A core genome analysis of Shigella species, pathogenic and non-pathogenic Escherichia coli, led to the selection of 234 predicted immunogenic Shigella antigens. These antigens were expressed and probed with acute and convalescent serum from microbiologically confirmed Shigella infections.

RESULTS

Several Shigella antigens displayed IgG and IgA seroconversion, with no difference in sero-reactivity across by sex or age. IgG sero-reactivity to key Shigella antigens was observed at birth, indicating transplacental antibody transfer. Six antigens (FepA, EmrK, FhuA, MdtA, NlpB, and CjrA) were identified in in vivo testing as capable of producing binding IgG and complement-mediated bactericidal antibody.

CONCLUSIONS

These findings provide six novel immunogenic Shigella proteins that could serve as candidate vaccine antigens, species-specific carrier proteins, or targeted adjuvants.

Dissecting the contribution of O-Antigen and proteins to the immunogenicity of Shigella sonnei generalized modules for membrane antigens (GMMA)

GMMA are exosomes released from engineered Gram-negative bacteria resembling the composition of outer membranes. We applied the GMMA technology for the development of an O-Antigen (OAg) based vaccine against Shigella sonnei, the most epidemiologically relevant cause of shigellosis. S. sonnei OAg has been identified as a key antigen for protective immunity, and GMMA are able to induce anti-OAg-specific IgG response in animal models and healthy adults. The contribution of protein-specific antibodies induced upon vaccination with GMMA has never been fully elucidated. Anti-protein antibodies are induced in mice upon immunization with either OAg-negative and OAg-positive GMMA. Here we demonstrated that OAg chains shield the bacteria from anti-protein antibody binding and therefore anti-OAg antibodies were the main drivers of bactericidal activity against OAg-positive bacteria. Interestingly, antibodies that are not targeting the OAg are functional against OAg-negative bacteria. The immunodominant protein antigens were identified by proteomic analysis. Our study confirms a critical role of the OAg on the immune response induced by S. sonnei GMMA. However, little is known about OAg length and density regulation during infection and, therefore, protein exposure. Hence, the presence of protein antigens on S. sonnei GMMA represents an added value for GMMA vaccines compared to other OAg-based formulations.

The role of vaccines in combatting antimicrobial resistance

The use of antibiotics has enabled the successful treatment of bacterial infections, saving the lives and improving the health of many patients worldwide. However, the emergence and spread of antimicrobial resistance (AMR) has been highlighted as a global threat by different health organizations, and pathogens resistant to antimicrobials cause substantial morbidity and death. As resistance to multiple drugs increases, novel and effective therapies as well as prevention strategies are needed. In this Review, we discuss evidence that vaccines can have a major role in fighting AMR. Vaccines are used prophylactically, decreasing the number of infectious disease cases, and thus antibiotic use and the emergence and spread of AMR. We also describe the current state of development of vaccines against resistant bacterial pathogens that cause a substantial disease burden both in high-income countries and in low- and medium-income countries, discuss possible obstacles that hinder progress in vaccine development and speculate on the impact of next-generation vaccines against bacterial infectious diseases on AMR.

Outer membrane vesicle vaccines

Outer Membrane Vesicles (OMV) have received increased attention in recent years as a vaccine platform against bacterial pathogens. OMV from Neisseria meningitidis serogroup B have been extensively explored. Following the success of the MeNZB OMV vaccine in controlling an outbreak of N. meningitidis B in New Zealand, additional research and development resulted in the licensure of the OMV-containing four-component 4CMenB vaccine, Bexsero. This provided broader protection against multiple meningococcal B strains. Advances in the field of genetic engineering have permitted further improvements in the platform resulting in increased yields, reduced endotoxicity and decoration with homologous and heterologous antigens to enhance immuno genicity and provide broader protection. The OMV vaccine platform has been extended to many other pathogens. In this review, we discuss progress in the development of the OMV vaccine delivery platform, highlighting successful applications, together with potential challenges and gaps.

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.

Characterization of Competitive ELISA and Formulated Alhydrogel Competitive ELISA (FAcE) for Direct Quantification of Active Ingredients in GMMA-Based Vaccines

Generalized modules for membrane antigens (GMMA) represent a technology particularly attractive for designing affordable vaccines against Gram-negative bacteria. We explored such technology for the development of O-antigen-based vaccines against Shigella and nontyphoidal Salmonella. Adsorption of GMMA on Alhydrogel was required for abrogation of pyrogenicity in rabbits, and Shigella sonnei GMMA on Alhydrogel was well tolerated and immunogenic in humans. Quantification of key antigens in formulated vaccines was fundamental for release and to check stability overtime. Traditionally, the direct quantification of antigens adsorbed on aluminum salts has been challenging, and the quantification of each active ingredient in multicomponent formulated vaccines has been even more complicated. To directly quantify each active ingredient and unbound drug substances in formulated vaccines, we developed the Formulated Alhydrogel competitive ELISA (FAcE) and the competitive ELISA method, respectively. The methods were both fully characterized, assessing specificity, repeatability, intermediate precision, and accuracy, for S. sonnei OAg quantification, both in a single component or multicomponent GMMA formulation also containing S. flexneri GMMA. The developed immunological methods allowed us to fully characterize Shigella GMMA drug products, supporting their preclinical and clinical development. The same methods, already extended to GMMA from nontyphoidal Salmonella and Neisseria meningitidis, could be potentially extended to any antigen formulated on Alhydrogel.

Pathogenic signature of invasive non-typhoidal Salmonella in Africa: implications for vaccine development

Invasive non-typhoidal Salmonella (iNTS) infections are a leading cause of bacteremia in Sub-Saharan Africa (sSA), thereby representing a major public health threat. Salmonella Typhimurium clade ST313 and Salmonella Enteriditis lineages associated with Western and Central/Eastern Africa are among the iNTS serovars which are of the greatest concern due to their case-fatality rate, especially in children and in the immunocompromised population. Identification of pathogen-associated features and host susceptibility factors that increase the risk for invasive non-typhoidal salmonellosis would be instrumental for the design of targeted prevention strategies, which are urgently needed given the increasing spread of multidrug-resistant iNTS in Africa. This review summarizes current knowledge of bacterial traits and host immune responses associated with iNTS infections in sSA, then discusses how this knowledge can guide vaccine development while providing a summary of vaccine candidates in preclinical and early clinical development.

Outer membrane vesicles: moving within the intricate labyrinth of assays that can predict risks of reactogenicity in humans

Outer membrane vesicles (OMV) are exosomes naturally released from the surface of Gram-negative bacteria. Since the ’80s, OMVs have been proposed as powerful vaccine platforms due to their intrinsic self-adjuvanticity and ability to present multiple antigens in natural conformation. However, the presence of several pathogen-associated molecular patterns (PAMPs), especially lipid A, has raised concerns about potential systemic reactogenicity in humans. Recently, chemical and genetic approaches allowed to efficiently modulate the balance between reactogenicity and immunogenicity for the use of OMV in humans. Several assays (monocyte activation test, rabbit pyrogenicity test, limulus amebocyte lysate, human transfectant cells, and toxicology studies) were developed to test, with highly predictive potential, the risk of reactogenicity in humans before moving to clinical use. In this review, we provide a historical perspective on how different assays were and can be used to successfully evaluate systemic reactogenicity during clinical development and after licensure.

OMV Vaccines and the Role of TLR Agonists in Immune Response

Outer Membrane Vesicles (OMVs) are bacterial nanoparticles that are spontaneously released during growth both in vitro and in vivo by Gram-negative bacteria. They are spherical, bilayered membrane nanostructures that contain many components found within the external surface of the parent bacterium. Naturally, OMVs serve the bacteria as a mechanism to deliver DNA, RNA, proteins, and toxins, as well as to promote biofilm formation and remodel the outer membrane during growth. On the other hand, as OMVs possess the optimal size to be uptaken by immune cells, and present a range of surface-exposed antigens in native conformation and Toll-like receptor (TLR) activating components, they represent an attractive and powerful vaccine platform able to induce both humoral and cell-mediated immune responses. This work reviews the TLR-agonists expressed on OMVs and their capability to trigger individual TLRs expressed on different cell types of the immune system, and then focuses on their impact on the immune responses elicited by OMVs compared to traditional vaccines.

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

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

GMMA and Glycoconjugate Approaches Compared in Mice for the Development of a Vaccine against Shigella flexneri Serotype 6

Shigella infections are one of the top causes of diarrhea throughout the world, with Shigella flexneri being predominant in developing countries. Currently, no vaccines are widely available and increasing levels of multidrug-resistance make Shigella a high priority for vaccine development. The serotype-specific O-antigen moiety of Shigella lipopolysaccharide has been recognized as a key target for protective immunity, and many O-antigen based candidate vaccines are in development. Recently, the Generalized Modules for Membrane Antigens (GMMA) technology has been proposed as an alternative approach to traditional glycoconjugate vaccines for O-antigen delivery. Here, these two technologies are compared for a vaccine against S. flexneri serotype 6. Genetic strategies for GMMA production, conjugation approaches for linkage of the O-antigen to CRM₁₉₇ carrier protein, and a large panel of analytical methods for full vaccine characterization have been put in place. In a head-to-head immunogenicity study in mice, GMMA induced higher anti-O-antigen IgG than glycoconjugate administered without Alhydrogel. When formulated on Alhydrogel, GMMA and glycoconjugate elicited similar levels of persistent anti-O-antigen IgG with bactericidal activity. Glycoconjugates are a well-established bacterial vaccine approach, but can be costly, particularly when multicomponent preparations are required. With similar immunogenicity and a simpler manufacturing process, GMMA are a promising strategy for the development of a vaccine against Shigella.

Glycoconjugate vaccines: current approaches towards faster vaccine design

Introduction: Over the last decades, glycoconjugate vaccines have been proven to be a successful strategy to prevent infectious diseases. Many diseases remain to be controlled, especially in developing countries, and emerging antibiotic-resistant bacteria present an alarming public-health threat. The increasing complexity of future vaccines, and the need to accelerate development processes have triggered the development of faster approaches to glycoconjugate vaccines design. Areas covered: This review provides an overview of recent progress in glycoconjugation technologies toward faster vaccine design. Expert opinion: Among the different emerging approaches, glycoengineering has the potential to combine glycan assembly and conjugation to carrier systems (such as proteins or outer membrane vesicles) in one step, resulting in a simplified manufacturing process and fewer analytical controls. Chemical and enzymatic strategies, and their automation can facilitate glycoepitope identification for vaccine design. Other approaches, such as the liposomal encapsulation of polysaccharides, potentially enable fast and easy combination of numerous antigens in the same formulation. Additional progress is envisaged in the near future, and some of these systems still need to be further validated in humans. In parallel, new strategies are needed to accelerate the vaccine development process, including the associated clinical trials, up to vaccine release onto the market.