Safety and immunogenicity of a 4-component GMMA-based Shigella vaccine in healthy European adults: Stage 1 of a randomized, controlled phase I/II clinical trial.

BACKGROUND: We report data from Stage 1 of an ongoing two-staged, phase I/II randomized clinical trial (NCT05073003) with a 4-component Generalized Modules for Membrane Antigens-based vaccine against Shigella sonnei and S. flexneri 1b, 2a and 3a (altSonflex1-2-3, GSK). METHODS: 18-50-year-old Europeans (N=102) were randomized (2:1) to receive two injections of altSonflex1-2-3 or placebo at 3- or 6-month interval. Safety and immunogenicity were assessed at pre-specified timepoints. RESULTS: The most common solicited administration-site event (until 7 days post-each injection) and unsolicited adverse event (until 28 days post-each injection) were pain (altSonflex1-2-3: 97.1%; Placebo: 58.8%) and headache (32.4%; 23.5%), respectively. All serotype-specific functional IgG antibodies peaked 14-28 days post-injection 1 and remained substantially higher than pre-vaccination at 3 or 6 months post-vaccination; the second injection did not boost but restored the initial immune response. The highest seroresponse rates (≥4-fold increase in titers over baseline) were obtained against S. flexneri 2a (ELISA: post-injection 1: 91.0%; post-injection 2 [Day {D}113; D197]: 100%; 97.0%; serum bactericidal activity (SBA): post-injection 1: 94.4%; post-injection 2: 85.7%; 88.9%) followed by S. sonnei (ELISA: post-injection 1: 77.6%; post-injection 2: 84.6%; 78.8%; SBA: post-injection 1: 83.3%; post-injection 2: 71.4%; 88.9%). Immune responses against S. flexneri 1b and S. flexneri 3a, as measured by both ELISA and SBA, were numerically lower compared to those against S. sonnei and S. flexneri 2a. CONCLUSIONS: No safety signals or concerns were identified. altSonflex1-2-3 induced functional serotype-specific immune responses, allowing further clinical development in the target population.

What is the context? Shigella bacteria cause severe and often bloody diarrhea, called shigellosis, that affects mostly young children and can be life-threatening. Shigellosis is particularly common in low- and middle-income countries due to inadequate sanitation and limited access to healthcare. Since the immune response to Shigella is serotype-specific, an ideal vaccine should include multiple Shigella serotypes to ensure broad protection. What is new? We developed a novel vaccine against Shigella that includes Shigella sonnei and three prevalent Shigella flexneri serotypes. In Stage 1 (phase I) of the study, healthy European adults received two vaccine injections given 3 or 6 months apart. We found that: The vaccine was well tolerated, and no safety signals or concerns were identified.Regardless of the interval between injections, specific antibodies were elicited against all four Shigella serotypes, with highest levels against Shigella flexneri 2a and Shigella sonnei.Functional antibody levels peaked after the first injection, remaining higher than the baseline up to 6 months. A second injection did not boost responses but restored functional antibody levels to those after the first injection. What is the impact? The vaccine can now be tested in Stage 2 (phase II) of the study in Africa, a region highly affected by shigellosis.

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.

Molecular Signature of Monocytes Shaped by the Shigella sonnei 1790-Generalized Modules for Membrane Antigens Vaccine.

Shigellosis, an acute gastroenteritis infection caused by Shigella species, remains a public health burden in developing countries. Recently, many outbreaks due to Shigella sonnei multidrug-resistant strains have been reported in high-income countries, and the lack of an effective vaccine represents a major hurdle to counteract this bacterial pathogen. Vaccine candidates against Shigella sonnei are under clinical development, including a Generalized Modules for Membrane Antigens (GMMA)-based vaccine. The mechanisms by which GMMA-based vaccines interact and activate human immune cells remain elusive. Our previous study provided the first evidence that both adaptive and innate immune cells are targeted and functionally shaped by the GMMA-based vaccine. Here, flow cytometry and confocal microscopy analysis allowed us to identify monocytes as the main target population interacting with the S. sonnei 1790-GMMA vaccine on human peripheral blood. In addition, transcriptomic analysis of this cell population revealed a molecular signature induced by 1790-GMMA mostly correlated with the inflammatory response and cytokine-induced processes. This also impacts the expression of genes associated with macrophages' differentiation and T cell regulation, suggesting a dual function for this vaccine platform both as an antigen carrier and as a regulator of immune cell activation and differentiation.

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.

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.

Comparative immunogenicity and efficacy of equivalent outer membrane vesicle and glycoconjugate vaccines against nontyphoidal Salmonella.

Nontyphoidal Salmonellae cause a devastating burden of invasive disease in sub-Saharan Africa with high levels of antimicrobial resistance. Vaccination has potential for a major global health impact, but no licensed vaccine is available. The lack of commercial incentive makes simple, affordable technologies the preferred route for vaccine development. Here we compare equivalent Generalized Modules for Membrane Antigens (GMMA) outer membrane vesicles and O-antigen-CRM197 glycoconjugates to deliver lipopolysaccharide O-antigen in bivalent Salmonella Typhimurium and Enteritidis vaccines. Salmonella strains were chosen and tolR deleted to induce GMMA production. O-antigens were extracted from wild-type bacteria and conjugated to CRM197 Purified GMMA and glycoconjugates were characterized and tested in mice for immunogenicity and ability to reduce Salmonella infection. GMMA and glycoconjugate O-antigen had similar structural characteristics, O-acetylation, and glucosylation levels. Immunization with GMMA induced higher anti-O-antigen IgG than glycoconjugate administered without Alhydrogel adjuvant. With Alhydrogel, antibody levels were similar. GMMA induced a diverse antibody isotype profile with greater serum bactericidal activity than glycoconjugate, which induced almost exclusively IgG1. Immunization reduced bacterial colonization of mice subsequently infected with SalmonellaS Typhimurium numbers were lower in tissues of mice vaccinated with GMMA compared with glycoconjugate. S. Enteritidis burden in the tissues was similar in mice immunized with either vaccine. With favorable immunogenicity, low cost, and ability to induce functional antibodies and reduce bacterial burden, GMMA offer a promising strategy for the development of a nontyphoidal Salmonella vaccine compared with established glycoconjugates. GMMA technology is potentially attractive for development of vaccines against other bacteria of global health significance.

Novel Simple Conjugation Chemistries for Decoration of GMMA with Heterologous Antigens.

Outer Membrane Vesicles (OMV) constitute a promising platform for the development of efficient vaccines. OMV can be decorated with heterologous antigens (proteins or polysaccharides), becoming attractive novel carriers for the development of multicomponent vaccines. Chemical conjugation represents a tool for linking antigens, also from phylogenetically distant pathogens, to OMV. Here we develop two simple and widely applicable conjugation chemistries targeting proteins or lipopolysaccharides on the surface of Generalized Modules for Membrane Antigens (GMMA), OMV spontaneously released from Gram-negative bacteria mutated to increase vesicle yield and reduce potential reactogenicity. A Design of Experiment approach was used to identify optimal conditions for GMMA activation before conjugation, resulting in consistent processes and ensuring conjugation efficiency. Conjugates produced by both chemistries induced strong humoral response against the heterologous antigen and GMMA. Additionally, the use of the two orthogonal chemistries allowed to control the linkage of two different antigens on the same GMMA particle. This work supports the further advancement of this novel platform with great potential for the design of effective vaccines.

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.

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.

Vaccines for low-income countries.

Low-income countries typically lag behind industrialised nations, where the introduction of new vaccines is commonly tailored to the pressures of the commercial market. Happily in recent years this paradigm has started to change with the introduction of a univalent meningococcal A conjugate vaccine that is specifically targeted for the prevention of epidemic meningitis in Africa. The declaration of the 2010s as a New Decade of Vaccines, together with Millennium Development Goals 4 and 5, provide a strong mandate for a new approach to the development of vaccines for low-income countries, so that there has never been a more exciting time to work in this field. This review considers the opportunities and challenges of developing these new vaccines in the context of innovations in vaccinology, the need to induce protective immunity in the populations at risk and the requirement for strong partnership between the countries that will use these vaccines and different elements of the vaccine industry.

Quantitative proteomic analysis of Shigella flexneri and Shigella sonnei Generalized Modules for Membrane Antigens (GMMA) reveals highly pure preparations.

Outer membrane blebs are naturally shed by Gram-negative bacteria and are candidates of interest for vaccines development. Genetic modification of bacteria to induce hyperblebbing greatly increases the yield of blebs, called Generalized Modules for Membrane Antigens (GMMA). The composition of the GMMA from hyperblebbing mutants of Shigella flexneri 2a and Shigella sonnei were quantitatively analyzed using high-sensitivity mass spectrometry with the label-free iBAQ procedure and compared to the composition of the solubilized cells of the GMMA-producing strains. There were 2306 proteins identified, 659 in GMMA and 2239 in bacteria, of which 290 (GMMA) and 1696 (bacteria) were common to both S. flexneri 2a and S. sonnei. Predicted outer membrane and periplasmic proteins constituted 95.7% and 98.7% of the protein mass of S. flexneri 2a and S. sonnei GMMA, respectively. Among the remaining proteins, small quantities of ribosomal proteins collectively accounted for more than half of the predicted cytoplasmic protein impurities in the GMMA. In GMMA, the outer membrane and periplasmic proteins were enriched 13.3-fold (S. flexneri 2a) and 8.3-fold (S. sonnei) compared to their abundance in the parent bacteria. Both periplasmic and outer membrane proteins were enriched similarly, suggesting that GMMA have a similar surface to volume ratio as the surface to periplasmic volume ratio in these mutant bacteria. Results in S. flexneri 2a and S. sonnei showed high reproducibility indicating a robust GMMA-producing process and the low contamination by cytoplasmic proteins support the use of GMMA for vaccines. Data are available via ProteomeXchange with identifier PXD002517.

Modulation of endotoxicity of Shigella generalized modules for membrane antigens (GMMA) by genetic lipid A modifications: relative activation of TLR4 and TLR2 pathways in different mutants.

Outer membrane particles from Gram-negative bacteria are attractive vaccine candidates as they present surface antigens in their natural context. We previously developed a high yield production process for genetically derived particles, called generalized modules for membrane antigens (GMMA), from Shigella. As GMMA are derived from the outer membrane, they contain immunostimulatory components, especially lipopolysaccharide (LPS). We examined ways of reducing their reactogenicity by modifying lipid A, the endotoxic part of LPS, through deletion of late acyltransferase genes, msbB or htrB, in GMMA-producing Shigella sonnei and Shigella flexneri strains. GMMA with resulting penta-acylated lipid A from the msbB mutants showed a 600-fold reduced ability, and GMMA from the S. sonnei ΔhtrB mutant showed a 60,000-fold reduced ability compared with GMMA with wild-type lipid A to stimulate human Toll-like receptor 4 (TLR4) in a reporter cell line. In human peripheral blood mononuclear cells, GMMA with penta-acylated lipid A showed a marked reduction in induction of inflammatory cytokines (S. sonnei ΔhtrB, 800-fold; ΔmsbB mutants, 300-fold). We found that the residual activity of these GMMA is largely due to non-lipid A-related TLR2 activation. In contrast, in the S. flexneri ΔhtrB mutant, a compensatory lipid A palmitoleoylation resulted in GMMA with hexa-acylated lipid A with ∼10-fold higher activity to stimulate peripheral blood mononuclear cells than GMMA with penta-acylated lipid A, mostly due to retained TLR4 activity. Thus, for use as vaccines, GMMA will likely require lipid A penta-acylation. The results identify the relative contributions of TLR4 and TLR2 activation by GMMA, which need to be taken into consideration for GMMA vaccine development.

Combining biophysical methods for the analysis of protein complex stoichiometry and affinity in SEDPHAT.

Reversible macromolecular interactions are ubiquitous in signal transduction pathways, often forming dynamic multi-protein complexes with three or more components. Multivalent binding and cooperativity in these complexes are often key motifs of their biological mechanisms. Traditional solution biophysical techniques for characterizing the binding and cooperativity are very limited in the number of states that can be resolved. A global multi-method analysis (GMMA) approach has recently been introduced that can leverage the strengths and the different observables of different techniques to improve the accuracy of the resulting binding parameters and to facilitate the study of multi-component systems and multi-site interactions. Here, GMMA is described in the software SEDPHAT for the analysis of data from isothermal titration calorimetry, surface plasmon resonance or other biosensing, analytical ultracentrifugation, fluorescence anisotropy and various other spectroscopic and thermodynamic techniques. The basic principles of these techniques are reviewed and recent advances in view of their particular strengths in the context of GMMA are described. Furthermore, a new feature in SEDPHAT is introduced for the simulation of multi-method data. In combination with specific statistical tools for GMMA in SEDPHAT, simulations can be a valuable step in the experimental design.

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.

GMMA Can Stabilize Proteins Across Different Functional Constraints.

Stabilizing proteins without otherwise hampering their function is a central task in protein engineering and design. PYR1 is a plant hormone receptor that has been engineered to bind diverse small molecule ligands. We sought a set of generalized mutations that would provide stability without affecting functionality for PYR1 variants with diverse ligand-binding capabilities. To do this we used a global multi-mutant analysis (GMMA) approach, which can identify substitutions that have stabilizing effects and do not lower function. GMMA has the added benefit of finding substitutions that are stabilizing in different sequence contexts and we hypothesized that applying GMMA to PYR1 with different functionalities would identify this set of generalized mutations. Indeed, conducting FACS and deep sequencing of libraries for PYR1 variants with two different functionalities and applying a GMMA analysis identified 5 substitutions that, when inserted into four PYR1 variants that each bind a unique ligand, provided an increase of 2-6 °C in thermal inactivation temperature and no decrease in functionality.

O-Antigen decorations in Salmonella enterica play a key role in eliciting functional immune responses against heterologous serovars in animal models.

Introduction: Different serovars of Salmonella enterica cause systemic diseases in humans including enteric fever, caused by S. Typhi and S. Paratyphi A, and invasive nontyphoidal salmonellosis (iNTS), caused mainly by S. Typhimurium and S. Enteritidis. No vaccines are yet available against paratyphoid fever and iNTS but different strategies, based on the immunodominant O-Antigen component of the lipopolysaccharide, are currently being tested. The O-Antigens of S. enterica serovars share structural features including the backbone comprising mannose, rhamnose and galactose as well as further modifications such as O-acetylation and glucosylation. The importance of these O-Antigen decorations for the induced immunogenicity and cross-reactivity has been poorly characterized. Methods: These immunological aspects were investigated in this study using Generalized Modules for Membrane Antigens (GMMA) as delivery systems for the different O-Antigen variants. This platform allowed the rapid generation and in vivo testing of defined and controlled polysaccharide structures through genetic manipulation of the O-Antigen biosynthetic genes. Results: Results from mice and rabbit immunization experiments highlighted the important role played by secondary O-Antigen decorations in the induced immunogenicity. Moreover, molecular modeling of O-Antigen conformations corroborated the likelihood of cross-protection between S. enterica serovars. Discussion: Such results, if confirmed in humans, could have a great impact on the design of a simplified vaccine composition able to maximize functional immune responses against clinically relevant Salmonella enterica serovars.

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.

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.

Enhanced Systemic Humoral Immune Response Induced in Mice by Generalized Modules for Membrane Antigens (GMMA) Is Associated with Affinity Maturation and Isotype Switching.

Generalized Modules for Membrane Antigens (GMMA) are outer membrane vesicles derived from Gram-negative bacteria that can be used to design affordable subunit vaccines. GMMA have been observed to induce a potent humoral immune response in preclinical and clinical studies. In addition, in preclinical studies, it has been found that GMMA can be exploited as optimal antigen carriers for both protein and saccharide antigens, as they are able to promote the enhancement of the antigen-specific humoral immune response when the antigen is overexpressed or chemically conjugated to GMMA. Here we investigated the mechanism of this GMMA carrier effect by immunizing mice and using factor H binding protein and GMMA of Neisseria meningitidis B as an antigen-GMMA model. We confirmed that the antigen displayed on the GMMA surface increased the antigen-specific IgG production and, above all, the antibody functionality measured by the serum bactericidal activity. We found that the enhancement of the bactericidal capacity induced by GMMA carrying the antigen on the surface was associated with the increase in antibody affinity to the antigen, and with the switching toward IgG subclasses with more bactericidal potential. Thus, we conclude that the potent carrier effect of GMMA is due to their ability to promote a better quality of humoral immunity.

From an in vivo to an in vitro relative potency (IVRP) assay to fully characterize a multicomponent O-antigen based vaccine against Shigella.

Outer membrane vesicles (OMV) represent an innovative platform for the design of polysaccharide based vaccines. Generalized Modules for Membrane Antigens (GMMA), OMV released from engineered Gram-negative bacteria, have been proposed for the delivery of the O-Antigen, key target for protective immunity against several pathogens including Shigella. altSonflex1-2-3 is a GMMA based vaccine, including S. sonnei and S. flexneri 1b, 2a and 3a O-Antigens, with the aim to elicit broad protection against the most prevalent Shigella serotypes, especially affecting children in low-middle income countries. Here we developed an In Vitro Relative Potency assay, based on recognition of O-Antigen by functional monoclonal antibodies selected to bind the key epitopes of the different O-Antigen active ingredients, directly applied to our Alhydrogel-formulated vaccine. Heat-stressed altSonflex1-2-3 formulations were generated and extensively characterized. The impact of detected biochemical changes in in vivo and in vitro potency assays was assessed. The overall results showed how the in vitro assay can replace the use of animals, overcoming the inherently high variability of in vivo potency studies. The entire panel of physico-chemical methods developed will contribute to detect suboptimal batches and will be valuable to perform stability studies. The work on Shigella vaccine candidate can be easily extended to other O-Antigen based vaccines.