Mucosal immunization with Shigella flexneri outer membrane vesicles induced protection in mice

The Role of Bacterial Extracellular Vesicles in the Immune Response to Pathogens, and Therapeutic Opportunities

Pathogenic bacteria have several mechanisms to evade the host's immune response and achieve an efficient infection. Bacterial extracellular vesicles (EVs) are a relevant cellular communication mechanism, since they can interact with other bacterial cells and with host cells. In this review, we focus on the EVs produced by some World Health Organization (WHO) priority Gram-negative and Gram-positive pathogenic bacteria; by spore-producing bacteria; by Mycobacterium tuberculosis (a bacteria with a complex cell wall); and by Treponema pallidum (a bacteria without lipopolysaccharide). We describe the classification and the general properties of bacterial EVs, their role during bacterial infections and their effects on the host immune response. Bacterial EVs contain pathogen-associated molecular patterns that activate innate immune receptors, which leads to cytokine production and inflammation, but they also contain antigens that induce the activation of B and T cell responses. Understanding the many effects of bacterial EVs on the host's immune response can yield new insights on the pathogenesis of clinically important infections, but it can also lead to the development of EV-based diagnostic and therapeutic strategies. In addition, since EVs are efficient activators of both the innate and the adaptive immune responses, they constitute a promising platform for vaccine development.

Elizabethkingia anophelis outer membrane vesicles as a novel vaccine candidate against infection: insights into immune response and potential for passive immunity

IMPORTANCE

Elizabethkingia anophelis, a Gram-negative pathogen, causes infections such as bacteraemia, pneumonia, and neonatal meningitis. The pathogen resists most antimicrobial classes, making novel approaches urgently needed. In natural settings, Gram-negative bacteria secrete outer membrane vesicles (OMVs) that carry important molecules in the bacterial life cycle. These OMVs are enriched with proteins involved in virulence, survival, and carbohydrate metabolism, making them a promising source for vaccine development against the pathogen. This study investigated the efficacy of imipenem-induced OMVs (iOMVs) as a vaccine candidate against E. anophelis infection in a mouse pneumonia model. Mice immunized with iOMVs were completely protected during lethal-dose challenges. Passive immunization with hyperimmune sera and splenocytes conferred protection against lethal pneumonia. Further investigation is needed to understand the mechanisms underlying the protective effects of iOMV-induced passive immunity, such as the action on specific antibody subclasses or T cell subsets.

Transforming parasites into their own foes: parasitic extracellular vesicles as a vaccine platform

Parasitic diseases continue to afflict millions of people globally. However, traditional vaccine development strategies are often difficult to apply to parasites, leaving an immense unmet need for new effective vaccines for the prevention and control of parasitic infections. As parasites commonly use extracellular vesicles (EVs) to interact with, interfere with, or modulate the host immune response from a distance, parasite-derived EVs may provide promising vaccine agents that induce immunity against parasitic infections. We here present achievements to date and the challenges and limitations associated with using parasitic EVs in a clinical context. Despite the many difficulties that need to be overcome, we believe this direction could offer a new and reliable source of therapeutics for various neglected parasitic diseases.

Investigate the immunogenic and protective effect of trivalent chimeric protein containing IpaD-StxB-TolC antigens as a vaccine candidate against S. dysenteri and S. flexneri

BACKGROUND

s: Shigella spp. causes bloody diarrhea and leads to death, especially in children. Chimeric proteins containing virulence factors can prevent Shigella infection. The purpose of this study is to investigate the immunogenic and protective effect of trivalent chimeric protein containing IpaD-StxB-TolC antigens against shiga toxin, S. dysenteri and S. flexneri in vitro and in vivo conditions.

METHODS

Recombinant vector was transferred to E. coli BL21. The expression of the chimeric protein was confirmed by SDS PAGE and purified using the Ni-NTA column. Mice were immunized with recombinant protein and antibody titer was evaluated by ELISA. 10, 25 and 50 LD₅₀ of Shiga toxin neutralization was evaluated in vitro (Vero cell line) and in vivo conditions. Also, the challenge of immunized mice with 10, 25 and 50 LD₅₀ of S. dysentery and S. flexneri was done.

RESULTS

The expression and purification of the recombinant protein with 60.6 kDa was done. ELISA showed increased antibody titer against the chimeric protein. MTT assay indicated that 1/8000 dilution of the sera had a 51% of cell viability against the toxin in Vero cell line. The challenge of mice immunized with toxin showed that the mice had complete protection against 10 and 25 LD₅₀ of toxin and had 40% survival against 50 LD₅₀. Mice receiving 10 and 25 LD₅₀ of S. dysenteri and S. flexneri had 100% protection and in 50 LD₅₀ the survival rate was 60 and 50%, respectively. Organ burden showed that the amount of bacterial colonization in immunized mice was 1 × 10⁴ CFU/mL, which was significantly different from the control group.

CONCLUSION

This study showed that chimeric proteins can create favorable immunogenicity in the host as vaccine candidates.

Exosomes: from biology to immunotherapy in infectious diseases

Exosomes are extracellular vesicles derived from the endosomal compartment, which are released by all kinds of eukaryotic and prokaryotic organisms. These vesicles contain a variety of biomolecules that differ both in quantity and type depending on the origin and cellular state. Exosomes are internalized by recipient cells, delivering their content and thus contributing to cell-cell communication in health and disease. During infections exosomes may exert a dual role, on one hand, they can transmit pathogen-related molecules mediating further infection and damage, and on the other hand, they can protect the host by activating the immune response and reducing pathogen spread. Selective packaging of pathogenic components may mediate these effects. Recently, quantitative analysis of samples by omics technologies has allowed a deep characterization of the proteins, lipids, RNA, and metabolite cargoes of exosomes. Knowledge about the content of these vesicles may facilitate their therapeutic application. Furthermore, as exosomes have been detected in almost all biological fluids, pathogenic or host-derived components can be identified in liquid biopsies, making them suitable for diagnosis and prognosis. This review attempts to organize the recent findings on exosome composition and function during viral, bacterial, fungal, and protozoan infections, and their contribution to host defense or to pathogen spread. Moreover, we summarize the current perspectives and future directions regarding the potential application of exosomes for prophylactic and therapeutic purposes.

Facile synthesis of multi-faceted, biomimetic and cross-protective nanoparticle-based vaccines for drug-resistant Shigella: a flexible platform technology

BACKGROUND

No commercial vaccines are available against drug-resistant Shigella due to serotype-specific/narrow-range of protection. Nanoparticle-based biomimetic vaccines involving stable, conserved, immunogenic proteins fabricated using facile chemistries can help formulate a translatable cross-protective Shigella vaccine. Such systems can also negate cold-chain transportation/storage thus overcoming challenges prevalent in various settings.

METHODS

We explored facile development of biomimetic poly (lactide-co-glycolide)/PLGA 50:50 based nanovaccines (NVs), encapsulating conserved stabilized antigen(s)/immunostimulant of S. dysenteriae 1 origin surface-modified using simple chemistries. All encapsulants (IpaC/IpaB/LPS) and nanoparticles (NPs)-bare and modified (NV), were thoroughly characterized. Effect of IpaC on cellular uptake of NPs was assessed in-vitro. Immunogenicity of the NVs was assessed in-vivo in BALB/c mice by intranasal immunization. Cross-protective efficacy was assessed by intraperitoneally challenging the immunized groups with a high dose of heterologous S. flexneri 2a and observing for visible diarrhea, weight loss and survival. Passive-protective ability of the simplest NV was assessed in the 5-day old progeny of vaccinated mice.

RESULTS

All the antigens and immunostimulant to be encapsulated were successfully purified and found to be stable both before and after encapsulation into NPs. The ~ 300 nm sized NPs with a zeta potential of ~ - 25 mV released ~ 60% antigen by 14th day suggesting an appropriate delivery kinetics. The NPs could be successfully surface-modified with IpaC and/or CpG DNA. In vitro experiments revealed that the presence of IpaC can significantly increase cellular uptake of NPs. All NVs were found to be cytocompatible and highly immunogenic. Antibodies in sera of NV-immunized mice could recognize heterologous Shigella. Immunized sera also showed high antibody and cytokine response. The immunized groups were protected from diarrhea and weight loss with ~ 70-80% survival upon heterologous Shigella challenge. The simplest NV showed ~ 88% survival in neonates.

CONCLUSIONS

Facile formulation of biomimetic NVs can result in significant cross-protection. Further, passive protection in neonates suggest that parental immunization could protect infants, the most vulnerable group in context of Shigella infection. Non-invasive route of vaccination can also lead to greater patient compliance making it amenable for mass-immunization. Overall, our work contributes towards a yet to be reported platform technology for facile development of cross-protective Shigella vaccines.

Emerging role of microbiota derived outer membrane vesicles to preventive, therapeutic and diagnostic proposes

The role of gut microbiota and its products in human health and disease is profoundly investigated. The communication between gut microbiota and the host involves a complicated network of signaling pathways via biologically active molecules generated by intestinal microbiota. Some of these molecules could be assembled within nanoparticles known as outer membrane vesicles (OMVs). Recent studies propose that OMVs play a critical role in shaping immune responses, including homeostasis and acute inflammatory responses. Moreover, these OMVs have an immense capacity to be applied in medical research, such as OMV-based vaccines and drug delivery. This review presents a comprehensive overview of emerging knowledge about biogenesis, the role, and application of these bacterial-derived OMVs, including OMV-based vaccines, OMV adjuvants characteristics, OMV vehicles (in conjugated vaccines), cancer immunotherapy, and drug carriers and delivery systems. Moreover, we also highlight the significance of the potential role of these OMVs in diagnosis and therapy.

Stable Recombinant Invasion Plasmid Antigen C (IpaC)-Based Single Dose Nanovaccine for Shigellosis

Shigellosis, caused by the bacteria Shigella, is the leading cause of bacterial diarrhea and the second leading cause of diarrheal death among children under the age of five. Unfortunately, Shigella strains have acquired resistance to antibiotics, and a commercial vaccine is yet to be available. We have previously demonstrated that Shigella dysenteriae serotype 1 (Sd1)-based recombinant, stabilized, "invasion plasmid antigen C" (IpaC; 42 kDa) protein can induce robust immune responses in BALB/c mice against a challenge of a high dose of heterologous Shigella when immunized via three intranasal doses of IpaC without an adjuvant. In this work, in order to reduce the frequency of dosing and increase possible patient compliance, based on our previous screening, the minimum protective dose of stabilized IpaC (20 μg) was encapsulated in biodegradable polymeric poly(lactide-co-glycolide) nanoparticles (∼370 nm) and intranasally administered in BALB/c mice in a single dose. Interestingly, a single intranasal dose of the developed vaccine particles encapsulating only 20 μg of Sd1 IpaC led to a temporal increase in the antibody production with an improved cytokine response compared to free IpaC administered three times as described in our previous report. Upon intraperitoneal challenge with a high dose of heterologous Shigella flexneri 2a (common in circulation), the immunized animals were protected from diarrhea, lethargy, and weight loss with ∼67% survival, while all the control animals died by 36 h of the challenge. Overall, the developed nanovaccine could be explored as a potential noninvasive, cross-protective, single-dose, single-antigen Shigella vaccine amenable for scale-up and eventual mass immunization.

Materials‐based vaccines for infectious diseases

Infectious diseases that result from pathogen infection are among the leading causes of human death, with pathogens such as human immunodeficiency virus, malaria, influenza, and ongoing SARS‐COV‐2 viruses constantly threatening the global population. While the mechanisms behind various infectious diseases are not entirely clear and thus retard the development of effective therapeutics, vaccines have served as a universal approach to containing infectious diseases. However, conventional vaccines that solely consist of antigens or simply mix antigens and adjuvants have failed to control various highly infective or deadly pathogens. Biomaterials‐based vaccines have provided a promising solution due to their ability to synergize the function of antigens and adjuvants, troubleshoot delivery issues, home and manipulate immune cells in situ. In this review, we will summarize different types of materials‐based vaccines for generating cellular and humoral responses against pathogens and discuss the design criteria for amplifying the efficacy of materials‐based vaccines against infectious diseases.

This article is categorized under:

Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease

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.

Vaccination against Bacterial Infections: Challenges, Progress, and New Approaches with a Focus on Intracellular Bacteria

Many bacterial infections are major health problems worldwide, and treatment of many of these infectious diseases is becoming increasingly difficult due to the development of antibiotic resistance, which is a major threat. Prophylactic vaccines against these bacterial pathogens are urgently needed. This is also true for bacterial infections that are still neglected, even though they affect a large part of the world’s population, especially under poor hygienic conditions. One example is typhus, a life-threatening disease also known as “war plague” caused by Rickettsia prowazekii, which could potentially come back in a war situation such as the one in Ukraine. However, vaccination against bacterial infections is a challenge. In general, bacteria are much more complex organisms than viruses and as such are more difficult targets. Unlike comparatively simple viruses, bacteria possess a variety of antigens whose immunogenic potential is often unknown, and it is unclear which antigen can elicit a protective and long-lasting immune response. Several vaccines against extracellular bacteria have been developed in the past and are still used successfully today, e.g., vaccines against tetanus, pertussis, and diphtheria. However, while induction of antibody production is usually sufficient for protection against extracellular bacteria, vaccination against intracellular bacteria is much more difficult because effective defense against these pathogens requires T cell-mediated responses, particularly the activation of cytotoxic CD8+ T cells. These responses are usually not efficiently elicited by immunization with non-living whole cell antigens or subunit vaccines, so that other antigen delivery strategies are required. This review provides an overview of existing antibacterial vaccines and novel approaches to vaccination with a focus on immunization against intracellular bacteria.

Roles of the Tol/Pal System in Bacterial Pathogenesis and Its Application to Antibacterial Therapy

The Tol/Pal system (also written as “The Tol-Pal system”) is a set of protein complexes produced by most Gram-negative bacteria. It comprises the inner membrane-associated and the outer membrane-anchored subunits composed of the TolA, TolQ, and TolR proteins and the TolB and Pal proteins, respectively. Although the Tol/Pal system was first defined as bacterial proteins involved in colicin uptake of Escherichia coli, its global roles have been characterized in several studies as mentioned in this article. Pathogenesis of many Gram-negative pathogens is sustained by the Tol/Pal system. It is also essential for cell growth and fitness in some pathogens. Therefore, the Tol/Pal system is proposed as a potential target for antimicrobial chemotherapy. Although the tol/pal mutants are low in virulence, they still have the ability to stimulate the immune system. The Pal protein is highly immunogenic and induces both adaptive and innate immune responses. Therefore, the tol/pal mutant strains and Pal proteins also have potential vaccine properties. For these reasons, the Tol/Pal system represents a promising research target in the development of antibacterial therapeutic strategies for refractory infections caused by multi-drug-resistant (MDR), Gram-negative pathogens. In this paper, we summarize studies on the Tol/Pal system associated with bacterial pathogenesis and vaccine development.

Commensal and Pathogenic Bacterial-Derived Extracellular Vesicles in Host-Bacterial and Interbacterial Dialogues: Two Sides of the Same Coin

Extracellular vesicles (EVs) cause effective changes in various domains of life. These bioactive structures are essential to the bidirectional organ communication. Recently, increasing research attention has been paid to EVs derived from commensal and pathogenic bacteria in their potential role to affect human disease risk for cancers and a variety of metabolic, gastrointestinal, psychiatric, and mental disorders. The present review presents an overview of both the protective and harmful roles of commensal and pathogenic bacteria-derived EVs in host-bacterial and interbacterial interactions. Bacterial EVs could impact upon human health by regulating microbiota–host crosstalk intestinal homeostasis, even in distal organs. The importance of vesicles derived from bacteria has been also evaluated regarding epigenetic modifications and applications. Generally, the evaluation of bacterial EVs is important towards finding efficient strategies for the prevention and treatment of various human diseases and maintaining metabolic homeostasis.

Peptide-Based Vaccines for Tuberculosis

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis. As a result of the coronavirus disease 2019 (COVID-19) pandemic, the global TB mortality rate in 2020 is rising, making TB prevention and control more challenging. Vaccination has been considered the best approach to reduce the TB burden. Unfortunately, BCG, the only TB vaccine currently approved for use, offers some protection against childhood TB but is less effective in adults. Therefore, it is urgent to develop new TB vaccines that are more effective than BCG. Accumulating data indicated that peptides or epitopes play essential roles in bridging innate and adaptive immunity and triggering adaptive immunity. Furthermore, innovations in bioinformatics, immunoinformatics, synthetic technologies, new materials, and transgenic animal models have put wings on the research of peptide-based vaccines for TB. Hence, this review seeks to give an overview of current tools that can be used to design a peptide-based vaccine, the research status of peptide-based vaccines for TB, protein-based bacterial vaccine delivery systems, and animal models for the peptide-based vaccines. These explorations will provide approaches and strategies for developing safer and more effective peptide-based vaccines and contribute to achieving the WHO’s End TB Strategy.

The role of TolA, TolB, and TolR in cell morphology, OMVs production, and virulence of Salmonella Choleraesuis

The Tol–Pal system of Gram-negative bacteria is necessary for maintaining outer membrane integrity. It is a multiprotein complex of five envelope proteins, TolQ, TolR, TolA, TolB, and Pal. These proteins were first investigated in E. coli, and subsequently been identified in many other bacterial genera. However, the function of the Tol–Pal system in Salmonella Choleraesuis pathogenesis is still unclear. Here, we reported the role of three of these proteins in the phenotype and biology of S. Choleraesuis. We found that mutations in tolA, tolB, and tolR caused severe damage to the cell wall, which was supported by observing the microstructure of spherical forms, long chains, flagella defects, and membrane blebbing. We confirmed that all the mutants significantly decreased S. Choleraesuis survival when exposed to sodium deoxycholate and exhibited a high sensitivity to vancomycin, which may be explained by the disruption of envelope integrity. In addition, tolA, tolB, and tolR mutants displayed attenuated virulence in a mouse infection model. This could be interpreted as a series of defective phenotypes in the mutants, such as severe defects in envelope integrity, growth, and motility. Further investigation showed that all the genes participate in outer membrane vesicles (OMVs) biogenesis. Interestingly, immunization with OMVs from ΔtolB efficiently enhanced murine viability in contrast to OMVs from the wild-type S. Choleraesuis, suggesting its potential use in vaccination strategies. Collectively, this study provides an insight into the biological role of the S. Choleraesuis Tol–Pal system.

Shigella Outer Membrane Vesicles as Promising Targets for Vaccination

The clinical symptoms of shigellosis, a gastrointestinal infection caused by Shigella spp. range from watery diarrhea to fulminant dysentery. Endemic infections, particularly among children in developing countries, represent the majority of clinical cases. The situation is aggravated due to the high mortality rate of shigellosis, the rapid dissemination of multi-resistant Shigella strains and the induction of only serotype-specific immunity. Thus, infection prevention due to vaccination, encompassing as many of the circulating serotypes as possible, has become a topic of interest. However, vaccines have turned out to be ineffective so far. Outer membrane vesicles (OMVs) are promising novel targets for vaccination. OMVs are constitutively secreted by Gram-negative bacteria including Shigella during growth. They are composed of soluble luminal portions and an insoluble membrane and can contain toxins, bioactive periplasmic and cytoplasmic (lipo-) proteins, (phospho-) lipids, nucleic acids and/or lipopolysaccharides. Thus, OMVs play an important role in bacterial cell–cell communication, growth, survival and pathogenesis. Furthermore, they modulate the secretion and transport of biomolecules, the stress response, antibiotic resistance and immune responses of the host. Thus, OMVs serve as novel secretion machinery. Here, we discuss the current literature and highlight the properties of OMVs as potent vaccine candidates because of their immunomodulatory, antigenic and adjuvant properties.

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.

Recent Progress in Shigella and Burkholderia pseudomallei Vaccines

Significant advancement has been made in the development of vaccines against bacterial pathogens. However, several roadblocks have been found during the evaluation of vaccines against intracellular bacterial pathogens. Therefore, new lessons could be learned from different vaccines developed against unrelated intracellular pathogens. Bacillary dysentery and melioidosis are important causes of morbidity and mortality in developing nations, which are caused by the intracellular bacteria Shigella and Burkholderia pseudomallei, respectively. Although the mechanisms of bacterial infection, dissemination, and route of infection do not provide clues about the commonalities of the pathogenic infectious processes of these bacteria, a wide variety of vaccine platforms recently evaluated suggest that in addition to the stimulation of antibodies, identifying protective antigens and inducing T cell responses are some additional required elements to induce effective protection. In this review, we perform a comparative evaluation of recent candidate vaccines used to combat these two infectious agents, emphasizing the common strategies that can help investigators advance effective and protective vaccines to clinical trials.

Outer Membrane Vesicles Derived from Salmonella enterica Serotype Typhimurium Can Deliver Shigella flexneri 2a O-Polysaccharide Antigen To Prevent Shigella flexneri 2a Infection in Mice

Shigellosis has become a serious threat to health in many developing countries due to the severe diarrhea it causes. Shigella flexneri 2a is the principal species responsible for this endemic disease. Despite multiple attempts to design a vaccine against shigellosis, no effective vaccine has been developed yet. Lipopolysaccharide (LPS) is both an essential virulence factor and an antigen protective against Shigella, due to its outer domain, termed O-polysaccharide antigen. In the present study, S. flexneri 2a O-polysaccharide antigen was innovatively biosynthesized in Salmonella and attached to core-lipid A via the ligase WaaL, with purified outer membrane vesicles (OMVs) utilized as vaccine vectors. Here, we identified the expression of the heterologous O-antigen and have described the isolation, characterization, and immune protection efficiency of the OMV vaccine. Furthermore, the results of animal experiments indicated that immunization of mice with the OMV vaccine induced significant specific anti-Shigella LPS antibodies in the serum, with similar trends in IgA levels from vaginal secretions and fluid from bronchopulmonary lavage, both intranasally and intraperitoneally. The OMV vaccine derived from both routes of administration provided significant protection against virulent S. flexneri 2a infection, as judged by a serum bactericidal assay, opsonization assay, and challenge test. This vaccination strategy represents a novel and improved approach to control shigellosis by the combination of Salmonella glycosyl carrier lipid bioconjugation with OMVs. IMPORTANCEShigella, the cause of shigellosis or bacillary dysentery, is a major public health concern, especially for children in developing countries. An effective vaccine would control the spread of the disease to some extent. However, no licensed vaccine against Shigella infection in humans has so far been developed. The Shigella O-antigen polysaccharide is effective in stimulating the production of protective antibodies and so could represent a vaccine antigen candidate. In addition, bacterial outer membrane vesicles (OMVs) have been used as antigen delivery platforms due to their nanoscale properties and ease of antigen delivery to trigger an immune response. Therefore, the present study provides a new strategy for vaccine design, combining a glycoconjugated vaccine with OMVs. The design concept of this strategy is the expression of Shigella O-antigen via the LPS synthesis pathway in recombinant Salmonella, from which the OMV vaccine is then isolated. Based on these findings, we believe that the novel vaccine design strategy in which polysaccharide antigens are delivered via bacterial OMVs will be effective for the development and clinical application of an effective Shigella vaccine.

Prophylactic vaccine delivery systems against epidemic infectious diseases

Prophylactic vaccines have evolved from traditional whole-cell vaccines to safer subunit vaccines. However, subunit vaccines still face problems, such as poor immunogenicity and low efficiency, while traditional adjuvants are usually unable to meet specific response needs. Advanced delivery vectors are important to overcome these barriers; they have favorable safety and effectiveness, tunable properties, precise location, and immunomodulatory capabilities. Nevertheless, there has been no systematic summary of the delivery systems to cover a wide range of infectious pathogens. We herein summarized and compared the delivery systems for major or epidemic infectious diseases caused by bacteria, viruses, fungi, and parasites. We also included the newly licensed vaccines (e.g., COVID-19 vaccines) and those close to licensure. Furthermore, we highlighted advanced delivery systems with high efficiency, cross-protection, or long-term protection against epidemic pathogens, and we put forward prospects and thoughts on the development of future prophylactic vaccines.

Bacterial Outer Membrane Vesicles: From Discovery to Applications

Secretion of cellular components across the plasma membrane is an essential process that enables organisms to interact with their environments. Production of extracellular vesicles in bacteria is a well-documented but poorly understood process. Outer membrane vesicles (OMVs) are produced in gram-negative bacteria by blebbing of the outer membrane. In addition to their roles in pathogenesis, cell-to-cell communication, and stress responses, OMVs play important roles in immunomodulation and the establishment and balance of the gut microbiota. In this review, we discuss the multiple roles of OMVs and the current knowledge of OMV biogenesis. We also discuss the growing and promising biotechnological applications of OMV. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Demystifying particle-based oral vaccines

Introduction: The oral route of vaccination is pain- and needle-free and can induce systemic and mucosal immunity. However, gastrointestinal barriers and antigen degradation impose significant hurdles in the development of oral vaccines. Live attenuated viruses and bacteria can overcome these barriers but at the risk of introducing safety concerns. As an alternative, particles have been investigated for antigen protection and delivery, yet there are no FDA-approved oral vaccines based on particle-based delivery systems. Our objective was to discover underlying determinants that can explain the current inadequacies and identify paradigms that can be implemented in future for successful development of oral vaccines relying on particle-based delivery systems.Areas covered: We reviewed literature related to the use of particles for oral vaccination and placed special emphasis on formulation characteristics and administration schedules to gain an insight into how these parameters impact production of antigen-specific antibodies in systemic and mucosal compartments.Expert opinion: Despite the long history of vaccines, particle-based oral vaccination is a relative new field with the first study published in 1989. Substantial variability exists between different studies with respect to dosing schedules, number of doses, and the amount of vaccine per dose. Most studies have not used adjuvants in the formulations. Better standardization in vaccination parameters is required to improve comparison between experiments, and adjuvants should be used to enhance the systemic and mucosal immune responses and to reduce the number of doses, which will make oral vaccines more attractive.

Gut Microbe-Derived Outer Membrane Vesicles: A Potential Platform to Control Cecal Load of Campylobacter jejuni

Acute diarrheal illness and gastroenteritis caused by Campylobacter jejuni infection remain significant public health risks in developing countries with substantial mortality and morbidity in humans, particularly in children under the age of five. Genetic diversities among Campylobacter jejuni and limited understanding of immunological correlations of host protection remain primary impediments for developing an effective measure to controlCampylobacter infection. Moreover, the lack of a reliable in vivo model to mimic natural infection against Campylobacter jejuni has substantially delayed the vaccine-development process. Given the role of bacterial outer membrane associated proteins in intestinal adherence and invasion as well as modulating dynamic interplay between host and pathogens, bacterial outer-membrane vesicles have emerged as a potential vaccine target against a number of gut pathogens, including Campylobacter jejuni. Here, we describe a mucosal vaccine strategy using chitosan-coated outer-membrane vesicles to induce specific immune responses against Campylobacter jejuni in mice. To overcome the challenges of mucosal delivery of outer membrane vesicles in terms of exposure to variable pH and risk of enzymatic degradation, we preferentially used chitosan as a nontoxic, mucoadhesive polymer. We show that intragastric delivery of chitosan-coated outer-membrane vesicles imparts significant immune protection against Campylobacter jejuni with high level local and systemic antibody production. Further, immunization with the outer membrane vesicles resulted in potent cellular responses with an increased CD4⁺ and CD8⁺ T cell population. Moreover, significant upregulation of IFN-γ and IL-6 gene expression suggests that mucosal delivery of outer membrane vesicles promotes a Th1/Th2 mixed-type immune response. Together, as an acellular and nonreplicating canonical end product of bacterial secretion, mucosal delivery of outer membrane vesicles may represent a promising platform for developing an effective vaccine againstCampylobacter jejuni.

Virus Eradication and Synthetic Biology: Changes with SARS-CoV-2?

The eradication of infectious diseases has been achieved only once in history, in 1980, with smallpox. Since 1988, significant effort has been made to eliminate poliomyelitis viruses, but eradication is still just out of reach. As the goal of viral disease eradication approaches, the ability to recreate historically eradicated viruses using synthetic biology has the potential to jeopardize the long-term sustainability of eradication. However, the emergence of the severe acute respiratory syndrome-coronavirus (SARS-CoV)-2 pandemic has highlighted our ability to swiftly and resolutely respond to a potential outbreak. This virus has been synthetized faster than any other in the past and is resulting in vaccines before most attenuated candidates reach clinical trials. Here, synthetic biology has the opportunity to demonstrate its truest potential to the public and solidify a footing in the world of vaccines.

Extracellular vesicles of bacteria as potential targets for immune interventions

Bacterial infection is one of the most common and serious diseases. Extracellular vesicles (EVs) expressed by bacterial cells during infection and their biological functions have been a growing field in recent years. The study of the immune interaction mechanism between EVs and bacteria has become more significant. EVs are released into the extracellular microenvironment during bacterial infection. EVs carry various lipids, proteins, nucleic acids, and other substances of host bacteria and participate in various physiological and pathological processes. EV-based vaccines against bacterial infection are also being evaluated. This review focuses on the biological characteristics of EVs, the interaction between EVs and the host immune system, and the potential of EVs as new vaccines. A deeper understanding of the interaction between EVs and the immune system informs on the biological function and heterogeneity of EVs. This knowledge also can facilitate the development and application of EVs and their potential as vaccines.

Outer Membrane Vesicle Induction and Isolation for Vaccine Development

Gram-negative bacteria release vesicular structures from their outer membrane, so called outer membrane vesicles (OMVs). OMVs have a variety of functions such as waste disposal, communication, and antigen or toxin delivery. These vesicles are the promising structures for vaccine development since OMVs carry many surface antigens that are identical to the bacterial surface. However, isolation is often difficult and results in low yields. Several methods to enhance OMV yield exist, but these do affect the resulting OMVs. In this review, our current knowledge about OMVs will be presented. Different methods to induce OMVs will be reviewed and their advantages and disadvantages will be discussed. The effects of the induction and isolation methods used in several immunological studies on OMVs will be compared. Finally, the challenges for OMV-based vaccine development will be examined and one example of a successful OMV-based vaccine will be presented.

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.

Orally-administered outer-membrane vesicles from Helicobacter pylori reduce H. pylori infection via Th2-biased immune responses in mice

As the trend of antibiotic resistance has increased, prevention and treatment of Helicobacter pylori infection have been challenged by the fact that no vaccines preventing H. pylori infection are available. Scientists continue to make sustained efforts to find better vaccine formulations and adjuvants to eradicate this chronic infection. In this study, we systemically analyzed the protein composition and potential vaccine function of outer-membrane vesicles (OMVs) derived from gerbil-adapted H. pylori strain 7.13. In total, we identified 169 proteins in H. pylori OMVs and found that outer-membrane, periplasmic and extracellular proteins (48.9% of the total proteins) were enriched. Furthermore, we evaluated the immune protective response of H. pylori OMVs in a C57BL/6 mouse model, and mice were orally immunized with OMVs or the H. pylori whole cell vaccine (WCV) alone, with or without cholera toxin (CT) as an adjuvant. The data demonstrated that oral immunization with OMVs can elicit a strong humoral and significantly higher mucosal immune response than the group immunized with the WCV plus the CT adjuvant. Moreover, our results also confirmed that OMVs predominantly induced T helper 2 (Th2)-biased immune responses that can significantly reduce bacterial loads after challenging with the H. pylori Sydney Strain 1 (SS1). In summary, OMVs as new antigen candidates in vaccine design would be of great value in controlling H. pylori infection.

Bacterial outer membrane vesicles as a platform for biomedical applications: An update

Outer membrane vesicles (OMVs) are produced by Gram-negative bacteria both in vitro and in vivo. OMVs are nano-sized spherical vehicles formed by lipid bilayer membranes and contain multiple parent bacteria-derived components. Based on the presence of bacterial antigens, pathogen-associated molecular patterns (PAMPs), adhesins, various proteins and the vesicle structure, OMVs have been developed for biomedical applications as bacterial vaccines, adjuvants, cancer immunotherapy agents, drug delivery vehicles, and anti-bacteria adhesion agents. In this review, we analyze the contributions of the structure and composition of OMVs to their applications, summarize the methods used to isolate and characterize OMVs, and highlight recent progress and future perspectives of OMVs in biomedical applications.

Intranasal delivery system of bacterial antigen using thermosensitive hydrogels based on a Pluronic-Gantrez conjugate

Thermosensitive hydrogels have been studied as feasible needle-avoidance alternative to vaccine delivery. In this work, we report the development of a new thermal-sensitive hydrogel for intranasal vaccine delivery. This delivery system was formulated with a combination of the polymer Gantrez® AN119 and the surfactant Pluronic® F127 (PF127), with a high biocompatibility, biodegradability and immunoadjuvant properties. Shigella flexneri outer membrane vesicles were used as the antigen model. A stable and easy-to-produce thermosensitive hydrogel which allowed the incorporation of the OMV-antigenic complex was successfully synthetized. A rapid gel formation was achieved at body temperature, which prolonged the OMV-antigens residence time in the nasal cavity of BALB/c mice when compared to intranasal delivery of free-OMVs. In addition, the bacterial antigens showed a fast release profile from the hydrogel in vitro, with a peak at 30 min of incubation at 37 °C. Hydrogels appeared to be non-cytotoxic in the human epithelial HeLa cell line and nose epithelium as well, as indicated by the absence of histopathological features. Immunohistochemical studies revealed that after intranasal administration the OMVs reached the nasal associated lymphoid tissue. These results support the use of here described thermosensitive hydrogels as a potential platform for intranasal vaccination.

Mucosal vaccines and technology

There is an urgent and unmet need to develop effective vaccines to reduce the global burden of infectious disease in both animals and humans, and in particular for the majority of pathogens that infect via mucosal sites. Here we summarise the impediments to developing mucosal vaccines and review the new and emerging technologies aimed at overcoming the lack of effective vaccine delivery systems that is the major obstacle to developing new mucosal vaccines.

Towards a subunit vaccine from a Shigella flexneri ΔtolR mutant

Disruption of one or more components of the Tol-Pal system, involved in maintaining the integrity of the outer membrane of Gram-negative bacteria, has been proposed as a method to increase the yield obtained from natural production of outer membrane vesicles (OMV). We present a new OMV-based product, obtained from genetically modified Shigella flexneri 2a with a non-polar deletion in tolR and heat-inactivated (HT-ΔtolR). The S. flexneri ΔtolR strain lead to a higher release of vesicles, more than 8-times when compared to the yield obtained from chemically inactivated wild type strain. S. flexneri mutant strain appeared to be more sensitive to different chemical compounds, including antibiotics, bile salts or human complement and it was also less virulent in both in vitro and in vivo assays. The mutation produced some changes in the LPS O-chain and protein expression. S. flexneri ΔtolR was enriched in long and very long LPS O-chain and expressed a different pattern of surface proteins or lipoproteins. In vitro toxicity and activation properties were determined in Raw 267.4 macrophage cell line. HT-ΔtolR antigenic complex was non-cytotoxic and activation markers, such as MHC-II or CD40, were highly expressed during incubation with this product. Finally, preliminary studies on the antibody response elicited by HT-ΔtolR demonstrated a robust and diverse response in mice. Considering these promising results, HT-ΔtolR antigenic extract appears as a new potential vaccine candidate to face shigellosis.

IgG Responses to Porins and Lipopolysaccharide within an Outer Membrane-Based Vaccine against Nontyphoidal Salmonella Develop at Discordant Rates

Antibodies acquired after vaccination or natural infection with Gram-negative bacteria, such as invasive Salmonella enterica serovar Typhimurium, can protect against disease. Immunization with naturally shed outer membrane vesicles from Gram-negative bacteria is being studied for its potential to protect against many infections, since antigens within vesicles maintain their natural conformation and orientation. Shedding can be enhanced through genetic modification, and the resulting particles, generalized modules for membrane antigens (GMMA), not only offer potential as vaccines but also can facilitate the study of B-cell responses to bacterial antigens. Here we show that the response to immunization with GMMA from S. Typhimurium (STmGMMA) provides B-cell-dependent protection and induces antibodies to two immunodominant antigens, lipopolysaccharide (LPS) and porins. Antibodies to LPS O antigen (O-Ag) markedly enhance protection in the spleen, but this effect is less marked in the liver. Strikingly, IgG responses to LPS and porins develop with distinct kinetics. In the first week after immunization, there is a dramatic T-cell-independent B1b-cell-associated induction of all IgG isotypes, except IgG1, to porins but not to LPS. In contrast, production of IgG1 to either antigen was delayed and T cell dependent. Nevertheless, after 1 month, cells in the bone marrow secreting IgG against porins or LPS were present at a similar frequency. Unexpectedly, immunization with O-Ag-deficient STmGMMA did not substantially enhance the anti-porin response. Therefore, IgG switching to all antigens does not develop synchronously within the same complex and so the rate of IgG switching to a single component does not necessarily reflect its frequency within the antigenic complex.

Vaccines save millions of lives, yet for some infections there are none. This includes some types of Salmonella infections, killing hundreds of thousands of people annually. We show how a new type of vaccine, called GMMA, that is made from blebs shed from the Salmonella cell wall, works to protect against infection in mice by inducing host proteins (antibodies) specifically recognizing bacterial components (antigens). The rate of development of IgG antibody to antigens within GMMA occurred with different kinetics. However, the antibody response to GMMA persists and is likely to provide prolonged protection for those who need it. These results help show how antibody responses to bacterial antigens develop and how vaccines like GMMA can work and help prevent infection.

Nanogel-based nasal vaccines for infectious and lifestyle-related diseases

Because the mucosa is the major entry route for most pathogens, the development of mucosal vaccines is a rational approach for protecting against these undesired agents. Mucosal administration of vaccine antigen is useful for non-infectious chronic diseases as well, because of its advantages over injection routes, including comparable efficacy in the induction of systemic immune responses, less pain, and no risk of adverse events at the injection site. However, because it is difficult to effectively induce and regulate antigen-specific mucosal and systemic immune responses when antigen alone is mucosally administered, an appropriate form of mucosal delivery vehicle must be used. Antigen delivery systems involving nanogels, which act as artificial chaperones and mucosal adhesives, are a promising approach to overcoming this problem. Here, we introduce current perspectives regarding the development of nanogel-based nasal vaccines for both infectious and lifestyle-related diseases.

Salmonella Choleraesuis outer membrane vesicles: Proteomics and immunogenicity

Salmonella enterica serotype Choleraesuis (S. Choleraesuis), Gram-negative facultative intracellular pathogen is capable of inducing the cholera in pigs whose symptoms manifest as fever, depression, septicemia, arthritis, and diarrhea. Infections with S. Choleraesuis has resulted in great economic loss for the swine breeding operations. Bacterial outer membrane vesicles (OMVs) play an important role in pathogenicity and host-pathogen interaction. In this study, we purified OMVs released by S. Choleraesuis strain χ3545 and characterized their lipopolysaccharide (LPS) profile. The OMVs contained intact LPS molecules. By using LC-MS/MS, we identified 192 proteins in the OMVs. In addition, the subcellular location and biological functions of the vesicles was predicted. The proteins were mainly derived from outer membranes and cytoplasm. Several proteins were immunoreactive and associated with the secretion pathway. Some putative multi-drug resistance-associated proteins were also identified. Furthermore, immunization experiment via intranasal or intraperitoneal route in mice demonstrated that S. Choleraesuis OMVs could elicit strong humoral and mucosal immune responses. Although OMVs as vaccine did not provide strong protection against clinical strain of wild-type S. Choleraesuis, immunization of OMVs still prolonged the survival time of vaccinated mice after high dose of S. Choleraesuis infection. Overall, this study provides valuable fundamental information toward elucidating the pathogenicity and functions of OMVs secreted from S. Choleraesuis.

Effective protection of mice against Shigella flexneri with a new self-adjuvant multicomponent vaccine

PURPOSE

The aim of this study was to develop an immunogenic protective product against Shigella flexneri by employing a simple and safe heat treatment-based strategy.

METHODOLOGY

The physicochemical characteristics of naturally produced (OMV) and heat-induced (HT) outer-membrane vesicles from S. flexneri were examined, including a comparison of the protein content of the products. Toxicological and biodistribution studies, and a preliminary experiment to examine the protective effectiveness of HT in a murine model of S. flexneri infection, were also included.

RESULTS

This method simultaneously achieves complete bacterial inactivation and the production of the HT vaccine product, leading to a safe working process. The obtained HT complex presented a similar morphology (electron microscopy) and chemical composition to the classical OMV, although it was enriched in some immunogens, such as lipoproteins, OmpA or OmpC, among others. The HT formulation was not toxic and biodistribution studies performed in mice demonstrated that the vaccine product remained in the small intestine after nasal administration. Finally, a single dose of HT administered nasally was able to protect mice against S. flexneri 2a.

CONCLUSION

The convenient and safe manufacturing process, and the preliminary biological evaluation, support the use of the self-adjuvanted HT complex as a new vaccine candidate to face shigellosis. Further development is required, such as additional immune analyses, to evaluate whether this new subunit vaccine can be useful in achieving full protection against Shigella.

Nasal nanovaccines

Nasal administration of vaccines is convenient for the potential stimulation of mucosal and systemic immune protection. Moreover the easy accessibility of the intranasal route renders it optimal for pandemic vaccination. Nanoparticles have been identified as ideal delivery systems and adjuvants for vaccine application. Heterogeneous protocols have been used for animal studies. This complicates the understanding of the formulation influence on the immune response and the comparison of the different nanoparticles approaches developed. Moreover anatomical and immunological differences between rodents and humans provide an additional hurdle in the rational development of nasal nanovaccines. This review will give a comprehensive expertise of the state of the art in nasal nanovaccines in animals and humans focusing on the nanomaterial used.

Drug and Vaccine Development for the Treatment and Prevention of Urinary Tract Infections

Urinary tract infections (UTI) are among the most common bacterial infections in humans, affecting millions of people every year. UTI cause significant morbidity in women throughout their lifespan, in infant boys, in older men, in individuals with underlying urinary tract abnormalities, and in those that require long-term urethral catheterization, such as patients with spinal cord injuries or incapacitated individuals living in nursing homes. Serious sequelae include frequent recurrences, pyelonephritis with sepsis, renal damage in young children, pre-term birth, and complications of frequent antimicrobial use including high-level antibiotic resistance and Clostridium difficile colitis. Uropathogenic E. coli (UPEC) cause the vast majority of UTI, but less common pathogens such as Enterococcus faecalis and other enterococci frequently take advantage of an abnormal or catheterized urinary tract to cause opportunistic infections. While antibiotic therapy has historically been very successful in controlling UTI, the high rate of recurrence remains a major problem, and many individuals suffer from chronically recurring UTI, requiring long-term prophylactic antibiotic regimens to prevent recurrent UTI. Furthermore, the global emergence of multi-drug resistant UPEC in the past ten years spotlights the need for alternative therapeutic and preventative strategies to combat UTI, including anti-infective drug therapies and vaccines. In this chapter, we review recent advances in the field of UTI pathogenesis, with an emphasis on the identification of promising drug and vaccine targets. We then discuss the development of new UTI drugs and vaccines, highlighting the challenges these approaches face and the need for a greater understanding of urinary tract mucosal immunity.

Outer membrane vesicles in service as protein shuttles, biotic defenders, and immunological doppelgängers

Characterization of host microbial interactions typically occurs on the cellular or protein level. Recently, a more thorough and accurate appreciation of cellular interactions has come into better focus with improved understanding of membrane vesicles (OMV). While OMVs are documented primarily in Gram-negative bacteria, certain Gram-positive species generate these structures, despite the obvious physical limitations of the cell envelope. Here, we briefly review the current understanding of OMVs in content and function, their role in pathogenesis, and the consequences of somatic cell gene expression on these events.

Particulate delivery systems for vaccination against bioterrorism agents and emerging infectious pathogens

Bioterrorism agents that can be easily transmitted with high mortality rates and cause debilitating diseases pose major threats to national security and public health. The recent Ebola virus outbreak in West Africa and ongoing Zika virus outbreak in Brazil, now spreading throughout Latin America, are case examples of emerging infectious pathogens that have incited widespread fear and economic and social disruption on a global scale. Prophylactic vaccines would provide effective countermeasures against infectious pathogens and biological warfare agents. However, traditional approaches relying on attenuated or inactivated vaccines have been hampered by their unacceptable levels of reactogenicity and safety issues, whereas subunit antigen-based vaccines suffer from suboptimal immunogenicity and efficacy. In contrast, particulate vaccine delivery systems offer key advantages, including efficient and stable delivery of subunit antigens, co-delivery of adjuvant molecules to bolster immune responses, low reactogenicity due to the use of biocompatible biomaterials, and robust efficiency to elicit humoral and cellular immunity in systemic and mucosal tissues. Thus, vaccine nanoparticles and microparticles are promising platforms for clinical development of biodefense vaccines. In this review, we summarize the current status of research efforts to develop particulate vaccine delivery systems against bioterrorism agents and emerging infectious pathogens. WIREs Nanomed Nanobiotechnol 2017, 9:e1403. doi: 10.1002/wnan.1403 For further resources related to this article, please visit the WIREs website.

Surface display of a borrelial lipoprotein on meningococcal outer membrane vesicles

Outer Membrane Vesicles (OMVs) are gaining attention as vaccine candidates. The successful expression of heterologous antigens in OMVs, with the OMV functioning both as adjuvant and delivery vehicle, has greatly enhanced their vaccine potential. Since there are indications that surface exposed antigens might induce a superior immune response, targeting of heterologous antigens to the OMV surface is of special interest. Several systems for surface display of heterologous antigens on OMVs have been developed. However, these systems have not been used to display lipidated membrane-associated proteins known as lipoproteins, which are emerging as key targets for protective immunity. We were therefore interested to see whether we could express a foreign lipoprotein on the outer surface of OMVs. When outer surface protein A (OspA), a borrelial surface-exposed lipoprotein, was expressed in meningococci, it was found that although OspA was present in OMVs, it was no longer surface-exposed. Therefore, a set of fusions of OspA to different regions of factor H binding protein (fHbp), a meningococcal surface-exposed lipoprotein, were designed and tested for their surface-exposure. An N-terminal part of fHbp was found to be necessary for the successful surface display of OspA on meningococcal OMVs. When mice were immunized with this set of OMVs, an OspA-specific antibody response was only elicited by OMVs with clearly surface-exposed OspA, strengthening the idea that the exact positioning of an antigen in the OMV affects the immune response. This method for the surface display of heterologous lipoproteins on OMVs is a step forward in the development of OMVs as a vaccine platform.

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.

Bacterial membrane vesicles, an overlooked environmental colloid: Biology, environmental perspectives and applications

Phospholipid vesicles play important roles in biological systems. Bacteria are one of the most abundant organisms on Earth, and bacterial membrane vesicles (MVs) were first observed 50 years ago. Many bacteria release MVs to the environment that mainly consist of the cell membrane and typically range from 20 to 400 nm in size. Bacterial MVs are involved in several biological functions, such as delivery of cargo, virulence and gene transfer. MVs can be isolated from laboratory culture and directly from the environment, indicating their high abundance in and impact on ecosystems. Many colloidal particles in the environment ranging in size from 1 nm to 1 μm have been reported but not characterized at the molecular level, and MVs remain to be explored. Hence, MVs can be considered terra incognita in environmental colloid research. Although MV biogenesis and biological roles are yet to be fully understood, the accumulation of knowledge has opened new avenues for their applications. Via genetic engineering, the MV yield can be greatly increased, and the components of MVs can be tailored. Recent studies have demonstrated that MVs have promising potential for applications such as drug delivery systems and nanobiocatalysts. For instance, MV vaccines have been extensively studied and have already been approved in Europe. Recent MV studies have evoked great interest in the fields of biology and biotechnology, but fundamental questions, such as their transport in the environment or physicochemical features of MVs, remain to be addressed. In this review, we present the current understanding of bacterial MVs and environmental perspectives and further introduce their applications.

Vaccination with outer membrane vesicles from Francisella noatunensis reduces development of francisellosis in a zebrafish model

Infection of fish with the facultative intracellular bacterium Francisella noatunensis remains an unresolved problem for aquaculture industry worldwide as it is difficult to vaccinate against without using live attenuated vaccines. Outer membrane vesicles (OMVs) are biological structures shed by Gram-negative bacteria in response to various environmental stimuli. OMVs have successfully been used to vaccinate against both intracellular and extracellular pathogens, due to an ability to stimulate innate, cell-mediated and humoral immune responses. We show by using atomic force and electron microscopy that the fish pathogenic bacterium F. noatunensis subspecies noatunensis (F.n.n.) shed OMVs both in vitro into culture medium and in vivo in a zebrafish infection model. The main protein constituents of the OMV are IglC, PdpD and PdpA, all known Francisella virulence factors, in addition to the outer membrane protein FopA and the chaperonin GroEL, as analyzed by mass spectrometry. The vesicles, when used as a vaccine, reduced proliferation of the bacterium and protected zebrafish when subsequently challenged with a high dose of F.n.n. without causing adverse effects for the host. Also granulomatous responses were reduced in F.n.n.-challenged zebrafish after OMV vaccination. Taken together, the data support the possible use of OMVs as vaccines against francisellosis in fish.

Comparative analysis of the uropathogenic Escherichia coli surface proteome by tandem mass-spectrometry of artificially induced outer membrane vesicles

Uropathogenic Escherichia coli (UPEC) are the major cause of urinary tract infections. For successful colonisation of the urinary tract, UPEC employ multiple surface-exposed or secreted virulence factors, including adhesins and iron uptake systems. Whilst individual UPEC strains and their virulence factors have been the focus of extensive research, there have been no outer membrane (OM) proteomic studies based on large clinical UPEC collections, primarily due to limitations of traditional methods. In this study, a high-throughput method based on tandem mass-spectrometry of EDTA heat-induced outer membrane vesicles (OMVs) was developed for the characterisation of the UPEC surface-associated proteome. The method was applied to compare the OM proteome of fifty-four UPEC isolates, resulting in the identification of 8789 proteins, consisting of 619 unique proteins, which were subsequently interrogated for their subcellular origin, prevalence and homology to characterised virulence factors. Multiple distinct virulence-associated proteins were identified, including two novel putative iron uptake proteins, an uncharacterised type of chaperone-usher fimbriae and various highly prevalent hypothetical proteins. Our results give fundamental insight into the physiology of UPEC and provide a framework for understanding the composition of the UPEC OM proteome.

BIOLOGICAL SIGNIFICANCE

In this study a high-throughput method based on tandem mass-spectrometry of EDTA heat-induced outer membrane vesicles was used to define the outer membrane proteome of a large uropathogenic E. coli (UPEC) collection. Our results provide an inventory of proteins expressed on the surface of UPEC, and provide a framework for understanding the composition of the UPEC OM proteome. The method enables the rapid characterisation of the E. coli surface proteome and could easily be applied to the large-scale outer membrane protein profiling of other Gram-negative bacteria.

An assessment of enterotoxigenic Escherichia coli and Shigella vaccine candidates for infants and children

Despite improvements to water quality, sanitation, and the implementation of current prevention and treatment interventions, diarrhea remains a major cause of illness and death, especially among children less than five years of age in the developing world. Rotavirus vaccines have already begun making a real impact on diarrhea, but several more enteric vaccines will be necessary to achieve broader reductions of illness and death. Among the many causes of diarrheal disease, enterotoxigenic Escherichia coli (ETEC) and Shigella are the two most important bacterial pathogens for which there are no currently licensed vaccines. Vaccines against these two pathogens could greatly reduce the impact of disease caused by these infections. This review describes the approaches to ETEC and Shigella vaccines that are currently under development, including a range of both cellular and subunit approaches for each pathogen. In addition, the review discusses strategies for maximizing the potential benefit of these vaccines, which includes the feasibility of co-administration, consolidation, and combination of vaccine candidates, as well as issues related to effective administration of enteric vaccines to infants. Recent impact studies indicate that ETEC and Shigella vaccines could significantly benefit global public health. Either vaccine, particularly if they could be combined together or with another enteric vaccine, would be an extremely valuable tool for saving lives and promoting the health of infants and children in the developing world, as well as potentially providing protection to travelers and military personnel visiting endemic areas.

Protection from hemolytic uremic syndrome by eyedrop vaccination with modified enterohemorrhagic E. coli outer membrane vesicles

We investigated whether eyedrop vaccination using modified outer membrane vesicles (mOMVs) is effective for protecting against hemolytic uremic syndrome (HUS) caused by enterohemorrhagic E. coli (EHEC) O157:H7 infection. Modified OMVs and waaJ-mOMVs were prepared from cultures of MsbB- and Shiga toxin A subunit (STxA)-deficient EHEC O157:H7 bacteria with or without an additional waaJ mutation. BALB/c mice were immunized by eyedrop mOMVs, waaJ-mOMVs, and mOMVs plus polymyxin B (PMB). Mice were boosted at 2 weeks, and challenged peritoneally with wild-type OMVs (wtOMVs) at 4 weeks. As parameters for evaluation of the OMV-mediated immune protection, serum and mucosal immunoglobulins, body weight change and blood urea nitrogen (BUN)/Creatinin (Cr) were tested, as well as histopathology of renal tissue. In order to confirm the safety of mOMVs for eyedrop use, body weight and ocular histopathological changes were monitored in mice. Modified OMVs having penta-acylated lipid A moiety did not contain STxA subunit proteins but retained non-toxic Shiga toxin B (STxB) subunit. Removal of the polymeric O-antigen of O157 LPS was confirmed in waaJ-mOMVs. The mice group vaccinated with mOMVs elicited greater humoral and mucosal immune responses than did the waaJ-mOMVs and PBS-treated groups. Eyedrop vaccination of mOMVs plus PMB reduced the level of humoral and mucosal immune responses, suggesting that intact O157 LPS antigen can be a critical component for enhancing the immunogenicity of the mOMVs. After challenge, mice vaccinated with mOMVs were protected from a lethal dose of wtOMVs administered intraperitoneally, conversely mice in the PBS control group were not. Collectively, for the first time, EHEC O157-derived mOMV eyedrop vaccine was experimentally evaluated as an efficient and safe means of vaccine development against EHEC O157:H7 infection-associated HUS.