Outer membrane vesicles obtained from Bordetella pertussis Tohama expressing the lipid A deacylase PagL as a novel acellular vaccine candidate

State of the Art and Emerging Technologies in Vaccine Design for Respiratory Pathogens

In this review, we present the efforts made so far in developing effective solutions to prevent infections caused by seven major respiratory pathogens: influenza virus, respiratory syncytial virus (RSV), the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), Bordetella pertussis, Streptococcus pneumoniae (pneumococcus), Mycobacterium tuberculosis, and Pseudomonas aeruginosa. Advancements driven by the recent coronavirus disease 2019 (COVID-19) crisis have largely focused on viruses, but effective prophylactic solutions for bacterial pathogens are also needed, especially in light of the antimicrobial resistance (AMR) phenomenon. Here, we discuss various innovative key technologies that can help address this critical need, such as (a) the development of Lung-on-Chip ex vivo models to gain a better understanding of the pathogenesis process and the host-microbe interactions; (b) a more thorough investigation of the mechanisms behind mucosal immunity as the first line of defense against pathogens; (c) the identification of correlates of protection (CoPs) which, in conjunction with the Reverse Vaccinology 2.0 approach, can push a more rational and targeted design of vaccines. By focusing on these critical areas, we expect substantial progress in the development of new vaccines against respiratory bacterial pathogens, thereby enhancing global health protection in the framework of the increasingly concerning AMR emergence.

Brucella suis ΔmapB outer membrane vesicles as an acellular vaccine against systemic and mucosal B. suis infection

Introduction

Swine brucellosis, caused by Brucella suis, is a worldwide infectious zoonotic disease. Currently, there are no available human or porcine vaccines to protect against B. suis infection, which is primarily acquired through the mucosa. We recently described B. suis MapB, the homologous protein of TamB, the inner membrane component of the TAM system. Our findings indicate that MapB is involved in bacterial cell envelope homeostasis. In this study, we characterize the outer membrane vesicles (OMVs) of B. suis 1330 (wt) and those of B. suis ΔmapB (ΔmapB) mutant strain and evaluate their vaccine potential in mice.

Methods

OMVs were isolated using the ultracentrifugation method and characterized through electron microscopy, Dynamic Light Scattering, SDS-PAGE and proteomics. Immunogenicity was assessed by intramuscular immunization of mice with wt OMVs or ΔmapB OMVs, followed by the measurement of antigen-specific antibody levels and functional assays to evaluate the protective capacity of the antibodies. Cellular immunity was assessed by characterizing cytokine secretion through ELISA after in vitro stimulation of spleen cells with heat-killed B. suis. To determine the level of protection conferred by immunization, mice were challenged with virulent B. suis via intraperitoneal or intratracheal routes, and the bacterial load was quantified post-challenge.

Results

Dynamic Light Scattering of the OMVs from both strains revealed the presence of spherical structures of 90-130 nm. Proteomic analysis identified 94 and 95 proteins in the wt and ΔmapB OMVs, respectively, including several known Brucella immunogens. Both OMVs showed immunoreactivity with sera from Brucella-infected pigs. Intramuscular immunization of mice with both OMVs induced antigen-specific IgG in serum, with the ΔmapB OMVs group showing higher titers compared to the wt OMVs group. Serum antibodies from both OMVs groups reduced B. suis adherence and invasion of lung epithelial cells and enhanced its phagocytosis by macrophages. Upon in vitro antigen stimulation, spleen cells from mice immunized with ΔmapB OMVs secreted higher levels of interleukin-17 and especially gamma interferon compared to cells from mice immunized with wt OMVs, suggesting the induction of a stronger T helper 1 response in the ΔmapB OMVs group. While immunization with both wt and ΔmapB OMVs achieved the same level of protection following intratracheal infection with B. suis (p<0.01), immunization with ΔmapB OMVs provided higher levels of protection against intraperitoneal infection.

Discussion

Overall, these results demonstrate that the B. suis ΔmapB OMVs are immunogenic and capable of inducing both cellular and humoral immune responses that protect against mucosal and systemic B. suis challenges.

Mucosal vaccination with outer membrane vesicles derived from Bordetella pertussis reduces nasal bacterial colonization after experimental infection

Introduction

We previously identified Bordetella pertussis-derived outer membrane vesicles (OMVs) as a promising immunogen for improving pertussis vaccines. In this study, we evaluated the efficacy of our vaccine prototype in immunization strategies aimed at reducing disease transmission by targeting colonization in the upper airways while maintaining protection against severe disease by reducing colonization in the lower respiratory tract.

Methods

We assessed different mucosal administration strategies in a murine model, including homologous mucosal 2-dose prime-boost schedules and heterologous prime-boost strategies combining intramuscular (IM) systemic immunization with mucosal routes (intranasal, IN; or sublingual, SL). We utilized alum and c-di-AMP as adjuvants for the systemic and mucosal formulations of the OMV vaccine prototype, respectively. A homologous prime/boost IM immunization schedule and commercial vaccines were used for comparisons.

Results

All tested heterologous schemes induced higher levels of specific IgG with significant avidity, as well as higher levels of IgG1 and IgG2c, compared to the corresponding homologous prime-boost 2-dose schemes via mucosal routes (OMVIN-IN or OMVSL-SL). High IgA levels were observed post-B. pertussis challenge following OMVIN-IN treatments and heterologous treatments where the second dose was administered via a mucosal route (prime-pull scheme). Furthermore, schemes involving the intranasal route, whether in a homologous or heterologous scheme, induced the highest levels of IL-17 and IFN-γ. Accordingly, these schemes showed superior efficacy against nasal colonization than the commercial vaccines. Homologous intranasal immunization exhibited the highest protective capacity against nasal colonization while maintaining an excellent level of protection in the lower respiratory tract. To further enhance protection against nasal colonization, we performed a comparative analysis of formulations containing either single or combined adjuvants, administered via homologous intranasal route. These assays revealed that the use of alum combined with c-di-AMP, did not enhance the immune protective capacity in comparison with that observed for the formulation containing c-di-AMP alone.

Conclusions

All the experiments presented here demonstrate that the use of OMVs, regardless of the scheme applied (except for OMVSL-SL), significantly outperformed acellular pertussis (aP) vaccines, achieving a higher reduction in bacterial colonization in the upper respiratory tract (p<0.01).

A novel outer membrane vesicle adjuvant improves vaccine protection against Bordetella pertussis

Pertussis is a vaccine-preventable respiratory disease caused by the Gram negative coccobacillus Bordetella pertussis. The licensed acellular pertussis (aP) vaccines protect against disease but do not prevent bacterial colonization and transmission. Here, we developed and tested an intranasal vaccine composed of aP antigens combined with T-vant, a novel adjuvant derived from bacterial outer membrane vesicles, that elicits both mucosal and systemic immune responses. We hypothesized that immunization of mice with aP-T-vant would enhance mucosal immunity and eliminate B. pertussis in the respiratory tract. In contrast to mice immunized intramuscularly with the licensed aP vaccine, intranasal immunization with aP-T-vant eliminated bacteria in both the lung and nasopharynx. Protection was associated with IFN-gamma and IL-17-producing, non-circulating CD4 + T cells in the lung and nasopharynx, and sterilizing immunity in the nasopharynx was dependent on IL-17. Novel mucosal adjuvants, such as T-vant, warrant further investigation to enhance the efficacy of next generation pertussis vaccines.

AI-2 quorum sensing controlled delivery of cytolysin-A by tryptophan auxotrophic low-endotoxic Salmonella and its anticancer effects in CT26 mice with colon cancer

INTRODUCTION

The limitations of conventional cancer therapies necessitate target-oriented, highly invasive, and safe treatment approaches. Hence, the intrinsic anti-tumor activity of Salmonella can offer better options to combat cancers.

OBJECTIVES

This study aims to utilize attenuated Salmonella and deliver cytolytic protein cytolysin A (ClyA) under quorum sensing (QS) signaling for precise localized expression in tumors but not in healthy organs.

METHODS

The therapeutic delivery strain was imposed with tryptophan auxotroph for selective colonization in tumors by trpA and trpE deletion, and lipid-A and O-antigen were altered by pagL and rfaL deletions using lambda red recombination method. The strain was transformed with the designed QS-controlled ClyA expression vector which was validated by western blot. The in vivo passaged therapeutic strain was used for treatment four times at a weekly interval, with a dose of 5 × 106 CFU/mouse for cancer therapy.

RESULTS

The attenuated strain induced minimal endotoxicity-related cytokines TNF-α, IL-1β, and IFN-γ and exhibited adequate colonization despite earlier exposure in mice. The QS-controlled ClyA expression was confirmed by western blot from bacterial cultures grown at different cell densities. The results demonstrated that the in vivo passaged strain preferentially colonized the tumor after vacating the spleen, liver, and lung, leaving no outward histological scars. The anti-cancer effect of the designed construct was evaluated in the murine CT26 colon cancer model. The expression of ClyA increased tumoricidal activity by 67 % compared to vector control.

CONCLUSION

Hence, the anti-tumor effect of the engineered Salmonella strain was improved by ClyA expression via QS activation after achieving the threshold bacterial cell density. Further, immunohistochemical staining of the tumor and other organs corroborated the QS-controlled tumor-specific expression of ClyA. Overall, the results imply that the developed anti-cancer Salmonella has low endotoxicity and QS-controlled expression of ClyA as beneficial safety elements and supports recurrent Salmonella inoculation by O-antigen deficiency.

Outer membrane vesicles derived from Bordetella pertussis are potent adjuvant that drive Th1-biased response

For several years, we have been committed to exploring the potential of Bordetella pertussis-derived outer membrane vesicles (OMVBp) as a promising third-generation vaccine against the reemerging pertussis disease. The results of our preclinical trials not only confirm its protective capacity against B. pertussis infection but also set the stage for forthcoming human clinical trials. This study delves into the examination of OMVBp as an adjuvant. To accomplish this objective, we implemented a two-dose murine schedule to evaluate the specific immune response induced by formulations containing OMVBp combined with 3 heterologous immunogens: Tetanus toxoid (T), Diphtheria toxoid (D), and the SARS-CoV-2 Spike protein (S). The specific levels of IgG, IgG1, and IgG2a triggered by the different tested formulations were evaluated using ELISA in dose-response assays for OMVBp and the immunogens at varying levels. These assays demonstrated that OMVBp exhibits adjuvant properties even at the low concentration employed (1.5 μg of protein per dose). As this effect was notably enhanced at medium (3 μg) and high concentrations (6 μg), we chose the medium concentration to determine the minimum immunogen dose at which the OMV adjuvant properties are significantly evident. These assays demonstrated that OMVBp exhibits adjuvant properties even at the lowest concentration tested for each immunogen. In the presence of OMVBp, specific IgG levels detected for the lowest amount of antigen tested increased by 2.5 to 10 fold compared to those found in animals immunized with formulations containing adjuvant-free antigens (p<0.0001). When assessing the adjuvant properties of OMVBp compared to the widely recognized adjuvant alum, we detected similar levels of specific IgG against D, T and S for both adjuvants. Experiments with OMVs derived from E. coli (OMVE.coli) reaffirmed that the adjuvant properties of OMVs extend across different bacterial species. Nonetheless, it's crucial to highlight that OMVBp notably skewed the immune response towards a Th1 profile (p<0.05). These collective findings emphasize the dual role of OMVBp as both an adjuvant and modulator of the immune response, positioning it favorably for incorporation into combined vaccine formulations.

Recent advances in various bio-applications of bacteria-derived outer membrane vesicles

Outer membrane vesicles (OMVs) are spherical nanoparticles released from gram-negative bacteria. OMVs were originally classified into native 'nOMVs' (produced naturally from budding of bacteria) and non-native (produced by mechanical means). nOMVs and detergent (dOMVs) are isolated from cell supernatant without any detergent cell disruption techniques and through detergent extraction, respectively. Growth stages and conditions e.g. different stress factors, including temperature, nutrition deficiency, and exposure to hazardous chemical agents can affect the yield of OMVs production and OMVs content. Because of the presence of bacterial antigens, pathogen-associated molecular patterns (PAMPs), various proteins and the vesicle structure, OMVs have been developed in many biomedical applications. OMVs due to their size can be phagocytized by APCs, enter lymph vessels, transport antigens efficiently, and induce both T and B cells immune responses. Non-engineered OMVs have been frequently used as vaccines against different bacterial and viral infections, and various cancers. OMVs can also be used in combination with different antigens as an attractive vaccine adjuvant. Indeed, foreign antigens from target microorganisms can be trapped in the lumen of nonpathogenic vesicles or can be displayed on the surface through bacterial membrane protein to increase the immunogenicity of the antigens. In this review, different factors affecting OMV production including time of cultivation, growth media, stress conditions and genetic manipulations to enhance vesiculation will be described. Furthermore, recent advances in various biological applications of OMVs such as vaccine, drug delivery, cancer therapy, and enzyme carrier are discussed. Generally, the application of OMVs as vaccine carrier in three categories (i.e., non-engineered OMVs, OMVs as an adjuvant, recombinant OMVs (rOMVs)), as delivery system for small interfering RNA and therapeutic agents, and as enzymes carrier will be discussed.

Microbe-host interactions: structure and functions of Gram-negative bacterial membrane vesicles

Bacteria-host interaction is a common, relevant, and intriguing biological phenomena. The host reacts actively or passively to the bacteria themselves, their products, debris, and so on, through various defense systems containing the immune system, the bacteria communicate with the local or distal tissues of the host via their own surface antigens, secreted products, nucleic acids, etc., resulting in relationships of attack and defense, adaptation, symbiosis, and even collaboration. The significance of bacterial membrane vesicles (MVs) as a powerful vehicle for the crosstalk mechanism between the two is growing. In the recent decade, the emergence of MVs in microbial interactions and a variety of bacterial infections, with multiple adhesions to host tissues, cell invasion and evasion of host defense mechanisms, have brought MVs to the forefront of bacterial pathogenesis research. Whereas MVs are a complex combination of molecules not yet fully understood, research into its effects, targeting and pathogenic components will advance its understanding and utilization. This review will summarize structural, extraction and penetration information on several classes of MVs and emphasize the role of MVs in transport and immune response activation. Finally, the potential of MVs as a therapeutic method will be highlighted, as will future research prospects.

Association between the Respiratory Microbiome and Plasma Microbial Extracellular Vesicles in Intubated Patients

Extracellular vesicles (EVs) regulate various cellular and immunological functions in human diseases. There is growing interest in the clinical role of microbial EVs in pneumonia. However, there is a lack of research on the correlation between lung microbiome with microbial EVs and the microbiome of other body sites in pneumonia. We investigated the co-occurrence of lung microbiome and plasma microbe-derived EVs (mEVs) in 111 samples obtained from 60 mechanically ventilated patients (41 pneumonia and 19 non-pneumonia cases). The microbial correlation between the two samples was compared between the pneumonia and non-pneumonia cases. Bacterial composition of the plasma mEVs was distinct from that of the lung microbiome. There was a significantly higher correlation between lung microbiome and plasma mEVs in non-pneumonia individuals compared to pneumonia patients. In particular, Acinetobacter and Lactobacillus genera had high correlation coefficients in non-pneumonia patients. This indicates a beneficial effect of mEVs in modulating host lung immune response through EV component transfer.

Impact of maternal whole-cell or acellular pertussis primary immunization on neonatal immune response

With the introduction of pertussis immunization for pregnant women in many countries, there has been renewed interest in the impact of whole-cell pertussis vaccine (wP) versus acellular vaccine (aP) on disease control, particularly regarding the best approach for priming. To gather evidence on this topic, we analyzed the impact of aP or wP priming on aP vaccination during pregnancy (aPpreg) in mice. Two-mother vaccination schemes were employed (wP-wP-aPpreg and aP-aP-aPpreg), and the immune response in the mothers and their offspring, as well as the protection of the offspring against Bordetella pertussis challenge, were assessed. Pertussis toxin (PTx)-specific IgG responses were detected in mothers after both the second and third doses, with higher titers after the third dose, regardless of the vaccination schedule. However, a significant reduction in PTx-IgG levels was observed after 22 weeks post aPpreg immunization in mothers with the aP-aP-aPpreg scheme but not in the wP-wP-aPpreg immunized mothers. The aP-aP-aPpreg schedule triggered a murine antibody response mainly to a Th2-profile, while wP-wP-aPpreg induced a Th1/Th2 mixed profile. Both immunization schemes administered to the mothers protected the offspring against pertussis, but the wP-wP-aPpreg vaccination conferred offspring protection in all pregnancies at least up to 20 weeks after receiving the aPpreg-dose. In contrast, the immunity induced by aP-aP-aPpreg began to decline in births that occurred 18 weeks after receiving the aPpreg dose. For the aP-aP-aPpreg scheme, pups born from gestations furthest from aPpreg (+22 weeks) had lower PTx-specific IgG levels than those born closer to the application of the dose during pregnancy. In contrast, for pups born to wP-wP-aPpreg vaccinated mothers, the PTx-specific IgG levels were maintained over time, even for those born at the longest time studied (+22 weeks). It is noteworthy that only the pups born from mothers with aP-aP-aPpreg and receiving a neonatal dose of either aP or wP were more susceptible to B. pertussis infection than mice with only maternal immunity, suggesting interference with the induced immunity (p<0.05). However, it should be noted that mice with maternal immunity, whether vaccinated or not with neonatal doses, are better protected against colonization with B. pertussis than mice without maternal immunity but vaccinated with aP or wP.

Bordetella pertussis and outer membrane vesicles

Bordetella pertussis is the causative agent of a respiratory infection called pertussis (whooping cough) that can be fatal in newborns and infants. The pathogen produces a variety of antigenic compounds which alone or simultaneously can damage various host cells. Despite the availability of pertussis vaccines and high vaccination coverage around the world, a resurgence of the disease has been observed in many countries. Reasons for the increase in pertussis cases may include increased awareness, improved diagnostic techniques, low vaccine efficacy, especially acellular vaccines, and waning immunity. Many efforts have been made to develop more effective strategies to fight against B. pertussis and one of the strategies is the use of outer membrane vesicles (OMVs) in vaccine formulations. OMVs are attracting great interest as vaccine platforms since they can carry immunogenic structures such as toxins and LPS. Many studies have been carried out with OMVs from different B. pertussis strains and they revealed promising results in the animal challenge and human preclinical model. However, the composition of OMVs differs in terms of isolation and purification methods, strains, culture, and stress conditions. Although the vesicles from B. pertussis represent an attractive pertussis vaccine candidate, further studies are needed to advance clinical research for next-generation pertussis vaccines. This review summarizes general information about pertussis, the history of vaccines against the disease, and the immune response to these vaccines, with a focus on OMVs. We discuss progress in developing an OMV-based pertussis vaccine platform and highlight successful applications as well as potential challenges and gaps.

Coping Strategies for Pertussis Resurgence

Pertussis (whooping cough) is a respiratory disease caused primarily by Bordetella pertussis, a Gram-negative bacteria. Pertussis is a relatively contagious infectious disease in people of all ages, mainly affecting newborns and infants under 2 months of age. Pertussis is undergoing a resurgence despite decades of high rates of vaccination. To better cope with the challenge of pertussis resurgence, we evaluated its possible causes and potential countermeasures in the narrative review. Expanded vaccination coverage, optimized vaccination strategies, and the development of a new pertussis vaccine may contribute to the control of pertussis.

Evaluation of Whole-Cell and Acellular Pertussis Vaccines in the Context of Long-Term Herd Immunity

After the pertussis vaccine had been introduced in the 1940s and was shown to be very successful in reducing the morbidity and mortality associated with the disease, the possibility of improving both vaccine composition and vaccination schedules has become the subject of continuous interest. As a result, we are witnessing a considerable heterogeneity in pertussis vaccination policies, which remains beyond universal consensus. Many pertussis-related deaths still occur in low- and middle-income countries; however, these deaths are attributable to gaps in vaccination coverage and limited access to healthcare in these countries, rather than to the poor efficacy of the first generation of pertussis vaccine consisting in inactivated and detoxified whole cell pathogen (wP). In many, particularly high-income countries, a switch was made in the 1990s to the use of acellular pertussis (aP) vaccine, to reduce the rate of post-vaccination adverse events and thereby achieve a higher percentage of children vaccinated. However the epidemiological data collected over the past few decades, even in those high-income countries, show an increase in pertussis prevalence and morbidity rates, triggering a wide-ranging debate on the causes of pertussis resurgence and the effectiveness of current pertussis prevention strategies, as well as on the efficacy of available pertussis vaccines and immunization schedules. The current article presents a systematic review of scientific reports on the evaluation of the use of whole-cell and acellular pertussis vaccines, in the context of long-term immunity and vaccines efficacy.

Novel approaches to reactivate pertussis immunity

INTRODUCTION

Whole cell and acellular pertussis vaccines have been very effective in decreasing the deaths of neonates and infants from Bordetella pertussis. Despite high vaccine coverage worldwide, pertussis remains one of the most common vaccine-preventable diseases, thus suggesting that new pertussis vaccination strategies are needed. Several candidates are currently under development, such as acellular pertussis vaccines that use genetically detoxified pertussis toxin, acellular pertussis vaccines delivered with new adjuvants or new delivery systems, or an intranasally delivered, live attenuated vaccine.

AREAS COVERED

This review discusses the different possibilities for improving current pertussis vaccines and the present state of knowledge on the pertussis vaccine candidates under development.

EXPERT OPINION

Until there is a safe, effective, and affordable alternative to the two types of existing vaccines, we should maintain sufficient childhood coverage and increase the vaccination of pregnant women, adolescents, and young adults.

Involvement of Bacterial Extracellular Membrane Nanovesicles in Infectious Diseases and Their Application in Medicine

Bacterial extracellular membrane nanovesicles (EMNs) are attracting the attention of scientists more and more every year. These formations are involved in the pathogenesis of numerous diseases, among which, of course, the leading role is occupied by infectious diseases, the causative agents of which are a range of Gram-positive and Gram-negative bacteria. A separate field for the study of the role of EMN is cancer. Extracellular membrane nanovesicles nowadays have a practical application as vaccine carriers for immunization against many infectious diseases. At present, the most essential point is their role in stimulating immune response to bacterial infections and tumor cells. The possibility of nanovesicles' practical use in several disease treatments is being evaluated. In our review, we listed diseases, focusing on their multitude and diversity, for which EMNs are essential, and also considered in detail the possibilities of using EMNs in the therapy and prevention of various pathologies.

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.

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.

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.

Biomimetic Nanoparticles Coated with Bacterial Outer Membrane Vesicles as a New-Generation Platform for Biomedical Applications

The biomedical field is currently reaping the benefits of research on biomimetic nanoparticles (NPs), which are synthetic nanoparticles fabricated with natural cellular materials for nature-inspired biomedical applications. These camouflage NPs are capable of retaining not only the physiochemical properties of synthetic nanoparticles but also the original biological functions of the cellular materials. Accordingly, NPs coated with cell-derived membrane components have achieved remarkable growth as prospective biomedical materials. Particularly, bacterial outer membrane vesicle (OMV), which is a cell membrane coating material for NPs, is regarded as an important molecule that can be employed in several biomedical applications, including immune response activation, cancer therapeutics, and treatment for bacterial infections with photothermal activity. The currently available cell membrane-coated NPs are summarized in this review. Furthermore, the general features of bacterial OMVs and several multifunctional NPs that could serve as inner core materials in the coating strategy are presented, and several methods that can be used to prepare OMV-coated NPs (OMV-NPs) and their characterization are highlighted. Finally, some perspectives of OMV-NPs in various biomedical applications for future potential breakthrough are discussed. This in-depth review, which includes potential challenges, will encourage researchers to fabricate innovative and improvised, new-generation biomimetic materials through future biomedical applications.

Evaluation of Outer Membrane Vesicles Obtained from Predominant Local Isolate of Boredetella pertussis as a Vaccine Candidate

Background:

Pertussis is a current contagious bacterial disease caused by Bp. Given the prevalence of pertussis, development of new vaccines is important. This study was attempted to evaluate the expression of main virulence factors (PTX, PRN, and FHA) from Bp predominant strains and also compare the expression of these factors in the OMVs obtained from predominant circulating Bp isolate.

Methods:

The physicochemical features of the prepared OMVs were analyzed by electron microscopy and SDS-PAGE. The presence of the mentioned virulence factors was confirmed by Western blotting. BALB/c mice (n = 21) immunized with characterized OMVs were challenged intranasally with sublethal doses of Bp, to examine their protective capacity.

Results:

Electron microscopic examination of the OMVs indicated vesicles within the range of 40 to 200 nm. SDS-PAGE and Western blotting demonstrated the expression of all three main protective immunogens (PTX, PRN, and FHA), prevalent in the predominant, challenge, and vaccine strains, and OMVs of the predominant IR37 strain and BP134 vaccine strain. Significant differences were observed in lung bacterial counts between the immunized mice with OMV (30 CFU/lung) compared to the negative control group ((6 10⁴ CFU/lung; p < 0.001). In mice immunized with OMVs (3 µg), the number of lungs recovered colonies after five days dropped at least five orders of magnitude compared to the control group.

Conclusion:

OMVs obtained from circulating isolates with the predominant profile may constitute a highly promising vaccine quality. They also can be proposed as a potential basic material for the development of new pertussis vaccine candidate.

Pertussis Vaccine Candidate Based on Outer Membrane Vesicles Derived From Biofilm Culture

Outer membrane vesicles (OMV) derived from Bordetella pertussis-the etiologic agent of the resurgent disease called pertussis-are safe and effective in preventing bacterial colonization in the lungs of immunized mice. Vaccine formulations containing those OMV are capable of inducing a mixed Th1/Th2/Th17 profile, but even more interestingly, they may induce a tissue-resident memory immune response. This immune response is recommended for the new generation of pertussis-vaccines that must be developed to overcome the weaknesses of current commercial acellular vaccines (second-generation of pertussis vaccine). The third-generation of pertussis vaccine should also deal with infections caused by bacteria that currently circulate in the population and are phenotypically and genotypically different [in particular those deficient in the expression of pertactin antigen, PRN(-)] from those that circulated in the past. Here we evaluated the protective capacity of OMV derived from bacteria grown in biofilm, since it was observed that, by difference with older culture collection vaccine strains, circulating clinical B. pertussis isolates possess higher capacity for this lifestyle. Therefore, we performed studies with a clinical isolate with good biofilm-forming capacity. Biofilm lifestyle was confirmed by both scanning electron microscopy and proteomics. While scanning electron microscopy revealed typical biofilm structures in these cultures, BipA, fimbria, and other adhesins described as typical of the biofilm lifestyle were overexpressed in the biofilm culture in comparison with planktonic culture. OMV derived from biofilm (OMVbiof) or planktonic lifestyle (OMVplank) were used to formulate vaccines to compare their immunogenicity and protective capacities against infection with PRN(+) or PRN(-) B. pertussis clinical isolates. Using the mouse protection model, we detected that OMVbiof-vaccine was more immunogenic than OMVplank-vaccine in terms of both specific antibody titers and quality, since OMVbiof-vaccine induced antibodies with higher avidity. Moreover, when OMV were administered at suboptimal quantity for protection, OMVbiof-vaccine exhibited a significantly adequate and higher protective capacity against PRN(+) or PRN(-) than OMVplank-vaccine. Our findings indicate that the vaccine based on B. pertussis biofilm-derived OMV induces high protection also against pertactin-deficient strains, with a robust immune response.

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.

Vaccine-Induced Cellular Immunity against Bordetella pertussis: Harnessing Lessons from Animal and Human Studies to Improve Design and Testing of Novel Pertussis Vaccines

Pertussis ('whooping cough') is a severe respiratory tract infection that primarily affects young children and unimmunised infants. Despite widespread vaccine coverage, it remains one of the least well-controlled vaccine-preventable diseases, with a recent resurgence even in highly vaccinated populations. Although the exact underlying reasons are still not clear, emerging evidence suggests that a key factor is the replacement of the whole-cell (wP) by the acellular pertussis (aP) vaccine, which is less reactogenic but may induce suboptimal and waning immunity. Differences between vaccines are hypothesised to be cell-mediated, with polarisation of Th1/Th2/Th17 responses determined by the composition of the pertussis vaccine given in infancy. Moreover, aP vaccines elicit strong antibody responses but fail to protect against nasal colonisation and/or transmission, in animal models, thereby potentially leading to inadequate herd immunity. Our review summarises current knowledge on vaccine-induced cellular immune responses, based on mucosal and systemic data collected within experimental animal and human vaccine studies. In addition, we describe key factors that may influence cell-mediated immunity and how antigen-specific responses are measured quantitatively and qualitatively, at both cellular and molecular levels. Finally, we discuss how we can harness this emerging knowledge and novel tools to inform the design and testing of the next generation of improved infant pertussis 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.

Exosome-Based Vaccines: History, Current State, and Clinical Trials

Extracellular vesicles (EVs) are released by most cell types as part of an intracellular communication system in crucial processes such as inflammation, cell proliferation, and immune response. However, EVs have also been implicated in the pathogenesis of several diseases, such as cancer and numerous infectious diseases. An important feature of EVs is their ability to deliver a wide range of molecules to nearby targets or over long distances, which allows the mediation of different biological functions. This delivery mechanism can be utilized for the development of therapeutic strategies, such as vaccination. Here, we have highlighted several studies from a historical perspective, with respect to current investigations on EV-based vaccines. For example, vaccines based on exosomes derived from dendritic cells proved to be simpler in terms of management and cost-effectiveness than dendritic cell vaccines. Recent evidence suggests that EVs derived from cancer cells can be leveraged for therapeutics to induce strong anti-tumor immune responses. Moreover, EV-based vaccines have shown exciting and promising results against different types of infectious diseases. We have also summarized the results obtained from completed clinical trials conducted on the usage of exosome-based vaccines in the treatment of cancer, and more recently, coronavirus disease.

Mucosal Immunization Against Pertussis: Lessons From the Past and Perspectives

Background

Current vaccination strategies against pertussis are sub-optimal. Optimal protection against Bordetella pertussis, the causative agent of pertussis, likely requires mucosal immunity. Current pertussis vaccines consist of inactivated whole B. pertussis cells or purified antigens thereof, combined with diphtheria and tetanus toxoids. Although they are highly protective against severe pertussis disease, they fail to elicit mucosal immunity. Compared to natural infection, immune responses following immunization are short-lived and fail to prevent bacterial colonization of the upper respiratory tract. To overcome these shortcomings, efforts have been made for decades, and continue to be made, toward the development of mucosal vaccines against pertussis.

Objectives

In this review we systematically analyzed published literature on protection conferred by mucosal immunization against pertussis. Immune responses mounted by these vaccines are summarized.

Method

The PubMed Library database was searched for published studies on mucosal pertussis vaccines. Eligibility criteria included mucosal administration and the evaluation of at least one outcome related to efficacy, immunogenicity and safety.

Results

While over 349 publications were identified by the search, only 63 studies met the eligibility criteria. All eligible studies are included here. Initial attempts of mucosal whole-cell vaccine administration in humans provided promising results, but were not followed up. More recently, diverse vaccination strategies have been tested, including non-replicating and replicating vaccine candidates given by three different mucosal routes: orally, nasally or rectally. Several adjuvants and particulate formulations were tested to enhance the efficacy of non-replicating vaccines administered mucosally. Most novel vaccine candidates were only tested in animal models, mainly mice. Only one novel mucosal vaccine candidate was tested in baboons and in human trials.

Conclusion

Three vaccination strategies drew our attention, as they provided protective and durable immunity in the respiratory tract, including the upper respiratory tract: acellular vaccines adjuvanted with lipopeptide LP1569 and c-di-GMP, outer membrane vesicles and the live attenuated BPZE1 vaccine. Among all experimental vaccines, BPZE1 is the only one that has advanced into clinical development.

Bacterial Extracellular Vesicles and the Gut-Microbiota Brain Axis: Emerging Roles in Communication and Potential as Therapeutics

Bacterial extracellular vesicles (BEVs) have emerged as candidate signaling vectors for long-distance interkingdom communication within the gut-microbiota brain axis. Most bacteria release these nanosized vesicles, capable of signaling to the brain via their abundant protein and small RNA cargo, possibly directly via crossing the blood-brain barrier. BEVs have been shown to regulate brain gene expression and induce pathology at most stages of neuroinflammation and neurodegeneration, and thus they may play a causal role in diseases such as Alzheimer's, Parkinson's, and depression/anxiety. On the other hand, BEVs have intrinsic therapeutic properties that may be relevant to probiotic therapy and can also be engineered to function as drug delivery vehicles and vaccines. Thus, BEVs may be both a cause of and solution to neuropathological conditions. In this review, current knowledge of the physiological roles of BEVs as well as state of the art pertaining to the development of therapeutic BEVs in the context of the microbiome-gut-brain axis are summarized.

Bacterial Membrane Vesicles in Pneumonia: From Mediators of Virulence to Innovative Vaccine Candidates

Pneumonia due to respiratory infection with most prominently bacteria, but also viruses, fungi, or parasites is the leading cause of death worldwide among all infectious disease in both adults and infants. The introduction of modern antibiotic treatment regimens and vaccine strategies has helped to lower the burden of bacterial pneumonia, yet due to the unavailability or refusal of vaccines and antimicrobials in parts of the global population, the rise of multidrug resistant pathogens, and high fatality rates even in patients treated with appropriate antibiotics pneumonia remains a global threat. As such, a better understanding of pathogen virulence on the one, and the development of innovative vaccine strategies on the other hand are once again in dire need in the perennial fight of men against microbes. Recent data show that the secretome of bacteria consists not only of soluble mediators of virulence but also to a significant proportion of extracellular vesicles-lipid bilayer-delimited particles that form integral mediators of intercellular communication. Extracellular vesicles are released from cells of all kinds of organisms, including both Gram-negative and Gram-positive bacteria in which case they are commonly termed outer membrane vesicles (OMVs) and membrane vesicles (MVs), respectively. (O)MVs can trigger inflammatory responses to specific pathogens including S. pneumonia, P. aeruginosa, and L. pneumophila and as such, mediate bacterial virulence in pneumonia by challenging the host respiratory epithelium and cellular and humoral immunity. In parallel, however, (O)MVs have recently emerged as auspicious vaccine candidates due to their natural antigenicity and favorable biochemical properties. First studies highlight the efficacy of such vaccines in animal models exposed to (O)MVs from B. pertussis, S. pneumoniae, A. baumannii, and K. pneumoniae. An advanced and balanced recognition of both the detrimental effects of (O)MVs and their immunogenic potential could pave the way to novel treatment strategies in pneumonia and effective preventive approaches.

The Path to New Pediatric Vaccines against Pertussis

Whooping cough, or pertussis, mostly caused by Bordetella pertussis, is a respiratory disease that affects all age groups, but severe and fatal pertussis occurs almost exclusively in young children. The widespread use of whole-cell and, more recently, of acellular vaccines has substantially reduced the disease incidence. However, it has not been eliminated in any part of the world and has made a worrisome rebound in several areas. Cocoon and maternal immunization have been implemented in several countries but have their intrinsic limitations. To effectively control pertussis, novel vaccines are needed that protect against disease and prevent B. pertussis infection and transmission, which is not the case for current vaccines. Several approaches are contemplated, including alternative administration routes, such as nasal immunization, improvement of acellular vaccines by adding more antigens and T-cell-promoting adjuvants, and the development of novel vaccines, such as outer membrane vesicles and live attenuated vaccines. Among them, only a live attenuated vaccine has so far been assessed for safety and immunogenicity in preclinical models other than mice and is in clinical development. Before any of these vaccines can be used in neonates, extensive safety and immunogenicity assessment in pre-clinical neonatal models and in carefully designed clinical trials is necessary. The aim of this review is to discuss the current pertussis problem, implemented strategies to resolve it, the value of animal models and novel vaccine approaches.

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.

Membrane Microvesicles as Potential Vaccine Candidates

The prevention and control of infectious diseases is crucial to the maintenance and protection of social and public healthcare. The global impact of SARS-CoV-2 has demonstrated how outbreaks of emerging and re-emerging infections can lead to pandemics of significant public health and socio-economic burden. Vaccination is one of the most effective approaches to protect against infectious diseases, and to date, multiple vaccines have been successfully used to protect against and eradicate both viral and bacterial pathogens. The main criterion of vaccine efficacy is the induction of specific humoral and cellular immune responses, and it is well established that immunogenicity depends on the type of vaccine as well as the route of delivery. In addition, antigen delivery to immune organs and the site of injection can potentiate efficacy of the vaccine. In light of this, microvesicles have been suggested as potential vehicles for antigen delivery as they can carry various immunogenic molecules including proteins, nucleic acids and polysaccharides directly to target cells. In this review, we focus on the mechanisms of microvesicle biogenesis and the role of microvesicles in infectious diseases. Further, we discuss the application of microvesicles as a novel and effective vaccine delivery system.

Recent Advances in the Pursuit of an Effective Acinetobacter baumannii Vaccine

Acinetobacter baumannii has been a major cause of nosocomial infections for decades. The absence of an available vaccine coupled with emerging multidrug resistance has prevented the medical community from effectively controlling this human pathogen. Furthermore, the ongoing pandemic caused by SARS-CoV-2 has increased the risk of hospitalized patients developing ventilator-associated pneumonia caused by bacterial opportunists including A. baumannii. The shortage of antibiotics in the development pipeline prompted the World Health Organization to designate A. baumannii a top priority for the development of new medical countermeasures, such as a vaccine. There are a number of important considerations associated with the development of an A. baumannii vaccine, including strain characteristics, diverse disease manifestations, and target population. In the past decade, research efforts have revealed a number of promising new immunization strategies that could culminate in a safe and protective vaccine against A. baumannii. In this review, we highlight the recent progress in the development of A. baumannii vaccines, discuss potential challenges, and propose future directions to achieve an effective intervention against this human pathogen.

Next-Generation Pertussis Vaccines Based on the Induction of Protective T Cells in the Respiratory Tract

Immunization with current acellular pertussis (aP) vaccines protects against severe pertussis, but immunity wanes rapidly after vaccination and these vaccines do not prevent nasal colonization with Bordetella pertussis. Studies in mouse and baboon models have demonstrated that Th1 and Th17 responses are integral to protective immunity induced by previous infection with B. pertussis and immunization with whole cell pertussis (wP) vaccines. Mucosal Th17 cells, IL-17 and secretory IgA (sIgA) are particularly important in generating sustained sterilizing immunity in the nasal cavity. Current aP vaccines induce potent IgG and Th2-skewed T cell responses but are less effective at generating Th1 and Th17 responses and fail to prime respiratory tissue-resident memory T (TRM) cells, that maintain long-term immunity at mucosal sites. In contrast, a live attenuated pertussis vaccine, pertussis outer membrane vesicle (OMV) vaccines or aP vaccines formulated with novel adjuvants do induce cellular immune responses in the respiratory tract, especially when delivered by the intranasal route. An increased understanding of the mechanisms of sustained protective immunity, especially the role of respiratory TRM cells, will facilitate the development of next generation pertussis vaccines that not only protect against pertussis disease, but prevent nasal colonization and transmission of B. pertussis.

Canonical and Non-canonical Inflammasome Activation by Outer Membrane Vesicles Derived From Bordetella pertussis

Outer Membrane Vesicles (OMVs) derived from different Gram-negative bacteria have been proposed as an attractive vaccine platform because of their own immunogenic adjuvant properties. Pertussis or whooping cough is a highly contagious vaccine-preventable respiratory disease that resurged during the last decades in many countries. In response to the epidemiological situation, new boosters have been incorporated into vaccination schedules worldwide and new vaccine candidates have started to be designed. Particularly, our group designed a new pertussis vaccine candidate based on OMVs derived from Bordetella pertussis (BpOMVs). To continue with the characterization of the immune response induced by our OMV based vaccine candidate, this work aimed to investigate the ability of OMVs to activate the inflammasome pathway in macrophages. We observed that NLRP3, caspase-1/11, and gasdermin-D (GSDMD) are involved in inflammasome activation by BpOMVs. Moreover, we demonstrated that BpOMVs as well as transfected B. pertussis lipooligosaccharide (BpLOS) induce caspase-11 (Casp11) and guanylate-binding proteins (GBPs) dependent non-canonical inflammasome activation. Our results elucidate the mechanism by which BpOMVs trigger one central pathway of the innate response activation that is expected to skew the adaptive immune response elicited by BpOMVs vaccination.

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

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

Intranasal immunization with outer membrane vesicle pertussis vaccine confers broad protection through mucosal IgA and Th17 responses

A vaccine based on outer membrane vesicles of pertussis (omvPV) is protective in a mouse-challenge model and induces a broad antibody and mixed Th1/Th2/Th17 response against multiple antigens following subcutaneous immunization. However, this route did not result in mucosal immunity and did not prevent nasopharyngeal colonization. In this study, we explored the potential of intranasal immunization with omvPV. Only intranasal immunization induced strong mucosal immune responses that encompasses enhanced pulmonary and nasal IgA antibody levels, mainly directed against Vag8 and LPS. Furthermore, high numbers of IgA- and IgG-producing plasma cells were detected as well as lung-resident IgA memory B-cells. Finally, only intranasal immunization induced pulmonary Th1/Th17-related cytokine responses. The magnitude and type of systemic immunity was comparable between both routes and included high systemic IgG antibody levels, strong IgG-producing plasma cell responses, memory B-cells residing in the spleen and systemic Th1/Th2/Th17-related cytokine responses. Importantly, only intranasal immunization prevented colonization in both the lungs and the nasal cavity. In conclusion, intranasal omvPV immunization induces mucosal IgA and Th17-mediated responses without influencing the systemic immunity profile. These responses resulted in prevention of Bordetella pertussis colonization in the respiratory tract, including the nasal cavity, thereby potentially preventing transmission.

Use of a Neonatal-Mouse Model to Characterize Vaccines and Strategies for Overcoming the High Susceptibility and Severity of Pertussis in Early Life

Newborns and unvaccinated infants, compared to other age groups, are more susceptible to pertussis infection, manifesting severe symptoms leading to a higher mortality. The recent increase in pertussis cases demands more effective strategies to overcome this major health problem. In parallel with maternal-immunization, neonatal-immunization (NI) is a strategy needing revision. Here, using the intranasal-challenge-mouse-model we evaluated the protective capacity of NI in both naïve-mice and those with maternally acquired immunity. We tested our acellular-vaccine-candidate based on outer-membrane-vesicles derived from Bordetella pertussis (OMVP) that induces Th2-profile but also the recommended Th-profile for protection: Th1/Th17-profile and CD4 T-memory-cells that reside in the lungs. Commercial acellular-vaccine (aP) and whole cell-vaccine (wP) inducing mainly Th2-profile and Th1-profile, respectively, were also tested. Analyzing the induced immunity and protection capability of NI included in 1- or 2-dose schedules with the same or different types of vaccine, we detected that the aP-vaccine administered in either single- or 2-dose schedules protected against sublethal B. pertussis infection. Schedules consisting of doses of aP neonatally and of OMVP or wP vaccine during infancy greatly reduced bacterial lung colonization while inducing the highest levels of high-avidity anti-pertussis toxin (PTx) IgG. That OMVP or wP neonatal dose did not interfere with the protection of transferred maternal immunity was especially encouraging. Moreover, OMVP- or wP used as a neonatal dose enhanced the quality of the humoral immune response in immunized pups. Antibodies generated by OMVP-or wP-vaccinated mice born to aP-immunized mothers were of higher avidity than those from mice that harbored only maternal immunity; but when mothers and neonates were immunized with the same aP-vaccine, the humoral response in the neonates was partially suppressed through the blunting of the level of anti-PTx IgG induced by the neonatal aP dose. These results demonstrated that neonatal immunization is a possible strategy to be considered to improve the current pertussis epidemiology. For neonates without maternal-immunity, mixed-vaccination schedules that include the aP- and OMVP-vaccines appear to be the most appropriate to induce protection in the pups. For offspring from immune mothers, to avoid blunting-effect, NI should be carried out with vaccines other than those applied during pregnancy.

A Pertussis Outer Membrane Vesicle-Based Vaccine Induces Lung-Resident Memory CD4 T Cells and Protection Against Bordetella pertussis, Including Pertactin Deficient Strains

Pertussis is a respiratory infectious disease that has been resurged during the last decades. The change from the traditional multi-antigen whole-cell pertussis (wP) vaccines to acellular pertussis (aP) vaccines that consist of a few antigens formulated with alum, appears to be a key factor in the resurgence of pertussis in many countries. Though current aP vaccines have helped to reduce the morbidity and mortality associated with pertussis, they do not provide durable immunity or adequate protection against the disease caused by the current circulating strains of Bordetella pertussis, which have evolved in the face of the selection pressure induced by the vaccines. Based on the hypothesis that a new vaccine containing multiple antigens could overcome deficiencies in the current aP vaccines, we have designed and characterized a vaccine candidate based on outer membrane vesicle (OMVs). Here we show that the OMVs vaccine, but not an aP vaccine, protected mice against lung infection with a circulating pertactin (PRN)-deficient isolate. Using isogenic bacteria that in principle only differ in PRN expression, we found that deficiency in PRN appears to be largely responsible for the failure of the aP vaccine to protect against this circulating clinical isolates. Regarding the durability of induced immunity, we have already reported that the OMV vaccine is able to induce long-lasting immune responses that effectively prevent infection with B. pertussis. Consistent with this, here we found that CD4 T cells with a tissue-resident memory (T_RM) cell phenotype (CD44⁺CD62L^lowCD69⁺ and/or CD103⁺) accumulated in the lungs of mice 14 days after immunization with 2 doses of the OMVs vaccine. CD4 T_RM cells, which have previously been shown to play a critical role sustained protective immunity against B. pertussis, were also detected in mice immunized with wP vaccine, but not in the animals immunized with a commercial aP vaccine. The CD4 T_RM cells secreted IFN-γ and IL-17 and were significantly expanded through local proliferation following respiratory challenge of mice with B. pertussis. Our findings that the OMVs vaccine induce respiratory CD4 T_RM cells may explain the ability of this vaccine to induce long-term protection and is therefore an ideal candidate for a third generation vaccine against B. pertussis.

New Pertussis Vaccines: A Need and a Challenge

Effective diphtheria, tetanus toxoids, whole-cell pertussis (wP) vaccines were used for massive immunization in the 1950s. The broad use of these vaccines significantly reduced the morbidity and mortality associated with pertussis. Because of reports on the induction of adverse reactions, less-reactogenic acellular vaccines (aP) were later developed and in many countries, especially the industrialized ones, the use of wP was changed to aP. For many years, the situation of pertussis seemed to be controlled with the use of these vaccines, however in the last decades the number of pertussis cases increased in several countries. The loss of the immunity conferred by the vaccines, which is faster in the individuals vaccinated with the acellular vaccines, and the evolution of the pathogen towards geno/phenotypes that escape more easily the immunity conferred by the vaccines were proposed as the main causes of the disease resurgence. According to their composition of few immunogens, the aP vaccines seem to be exerting a greater selection pressure on the circulating bacterial population causing the prevalence of bacterial isolates defective in the expression of vaccine antigens. Under this context, it is clear that new vaccines against pertussis should be developed. Several vaccine candidates are in preclinical development and few others have recently completed phaseI/phaseII trials. Vaccine candidate based on OMVs is a promising candidate since appeared overcoming the major weaknesses of current aP-vaccines. The most advanced development is the live attenuated-vaccine BPZE1 which has successfully completed a first-in-man clinical trial.

Outer-Membrane-Vesicle-Associated O Antigen, a Crucial Component for Protecting Against Bordetella parapertussis Infection

Bordetella parapertussis is a respiratory-disease pathogen producing symptomatology similar to that of pertussis but of underestimated incidence and with no specific vaccine existing. We recently designed a vaccine candidate from B. parapertussis outer-membrane vesicles (OMVs) that proved to be safe and protective in a murine-infection model. Based on protection recently reported for the B. parapertussis O antigen in aqueous solution, we assessed here whether the B. parapertussis O-antigen-containing lipopolysaccharide (BppLPS-O⁺) embedded in the membranes, as present in B. parapertussis-derived OMVs (OMVs(Bpp-LPS-O⁺)), was the component responsible for that previously observed protection by OMVs. By performing a comparative study with OMVs from a human strain with undetectable O antigen (OMVs(Bpp-LPS-O^-)), we demonstrated that the OMVs(Bpp-LPS-O⁺), but not the OMVs(Bpp-LPS-O^-), protected mice against sublethal B. parapertussis infections. Indeed, the B. parapertussis loads were significantly reduced in the lungs of OMVs(Bpp-LPS-O⁺) -vaccinated animals, with the CFUs recovered being decreased by 4 log units below those detected in the non-immunized animals or in the animals treated with the OMVs(Bpp-LPS-O^-), (p < 0.001). We detected that the OMVs(Bpp-LPS-O⁺) induced IgG antibodies against B. parapertussis whole-cell lysates, which immunocomponents recognized, among others, the O antigen and accordingly conferred protection against B. parapertussis infection, as observed in in-vivo-passive-transfer experiments. Of interest was that the OMVs(Bpp-LPS-O⁺) -generated sera had opsonophagocytic and bactericidal capabilities that were not detected with the OMVs(Bpp-LPS-O^-)-induced sera, suggesting that those activities were involved in the clearance of B. parapertussis. Though stimulation of cultured spleen cells from immunized mice with formulations containing the O antigen resulted in gamma interferon (IFN-γ) and interleukin-17 production, spleen cells from OMVs(Bpp-LPS-O⁺) -immunized mice did not significantly contribute to the observed protection against B. parapertussis infection. The protective capability of the B. parapertussis O antigen was also detected in formulations containing both the OMVs derived from B. pertussis and purified BppLPS-O⁺. This combined formulation protected mice against B. pertussis along with B. parapertussis.

Molecular and cellular signatures underlying superior immunity against Bordetella pertussis upon pulmonary vaccination

Mucosal immunity is often required for protection against respiratory pathogens but the underlying cellular and molecular mechanisms of induction remain poorly understood. Here, systems vaccinology was used to identify immune signatures after pulmonary or subcutaneous immunization of mice with pertussis outer membrane vesicles. Pulmonary immunization led to improved protection, exclusively induced mucosal immunoglobulin A (IgA) and T helper type 17 (Th17) responses, and in addition evoked elevated systemic immunoglobulin G (IgG) antibody levels, IgG-producing plasma cells, memory B cells, and Th17 cells. These adaptive responses were preceded by unique local expression of genes of the innate immune response related to Th17 (e.g., Rorc) and IgA responses (e.g., Pigr) in addition to local and systemic secretion of Th1/Th17-promoting cytokines. This comprehensive systems approach identifies the effect of the administration route on the development of mucosal immunity, its importance in protection against Bordetella pertussis, and reveals potential molecular correlates of vaccine immunity to this reemerging pathogen.

Versatile effects of bacterium-released membrane vesicles on mammalian cells and infectious/inflammatory diseases

Gram-negative bacterium-released outer-membrane vesicles (OMVs) and Gram-positive bacterium-released membrane vesicles (MVs) share significant similarities with mammalian cell-derived MVs (eg, microvesicles and exosomes) in terms of structure and their biological activities. Recent studies have revealed that bacterial OMVs/MVs could (1) interact with immune cells to regulate inflammatory responses, (2) transport virulence factors (eg, enzymes, DNA and small RNAs) to host cells and result in cell injury, (3) enhance barrier function by stimulating the expression of tight junction proteins in intestinal epithelial cells, (4) upregulate the expression of endothelial cell adhesion molecules, and (5) serve as natural nanocarriers for immunogenic antigens, enzyme support and drug delivery. In addition, OMVs/MVs can enter the systemic circulation and induce a variety of immunological and metabolic responses. This review highlights the recent advances in the understanding of OMV/MV biogenesis and their compositional remodeling. In addition, interactions between OMVs/MVs and various types of mammalian cells (ie, immune cells, epithelial cells, and endothelial cells) and their pathological/preventive effects on infectious/inflammatory diseases are summarized. Finally, methods for engineering OMVs/MVs and their therapeutic potential are discussed.

Membrane Vesicles Derived from Bordetella bronchiseptica: Active Constituent of a New Vaccine against Infections Caused by This Pathogen

Bordetella bronchiseptica, a Gram-negative bacterium, causes chronic respiratory tract infections in a wide variety of mammalian hosts, including humans (albeit rarely). We recently designed Bordetella pertussis and Bordetella parapertussis experimental vaccines based on outer membrane vesicles (OMVs) derived from each pathogen, and we obtained protection against the respective infections in mice. Here, we demonstrated that OMVs derived from virulent-phase B. bronchiseptica (OMVBbvir⁺) protected mice against sublethal infections with different B. bronchiseptica strains, two isolated from farm animals and one isolated from a human patient. In all infections, we observed that the B. bronchiseptica loads were significantly reduced in the lungs of vaccinated animals; the lung-recovered CFU were decreased by ≥4 log units, compared with those detected in the lungs of nonimmunized animals (P < 0.001). In the OMVBbvir⁺-immunized mice, we detected IgG antibody titers against B. bronchiseptica whole-cell lysates, along with an immune serum having bacterial killing activity that both recognized B. bronchiseptica lipopolysaccharides and polypeptides such as GroEL and outer membrane protein C (OMPc) and demonstrated an essential protective capacity against B. bronchiseptica infection, as detected by passive in vivo transfer experiments. Stimulation of cultured splenocytes from immunized mice with OMVBbvir⁺ resulted in interleukin 5 (IL-5), gamma interferon (IFN-γ), and IL-17 production, indicating that the vesicles induced mixed Th2, Th1, and Th17 T-cell immune responses. We detected, by adoptive transfer assays, that spleen cells from OMVBbvir⁺-immunized mice also contributed to the observed protection against B. bronchiseptica infection. OMVs from avirulent-phase B. bronchiseptica and the resulting induced immune sera were also able to protect mice against B. bronchiseptica infection.IMPORTANCEBordetella bronchiseptica, a Gram-negative bacterium, causes chronic respiratory tract infections in a wide variety of mammalian hosts, including humans (albeit rarely). Several vaccines aimed at preventing B. bronchiseptica infection have been developed and used, but a safe effective vaccine is still needed. The significance and relevance of our research lie in the characterization of the OMVs derived from B. bronchiseptica as the source of a new experimental vaccine. We demonstrated here that our formulation based on OMVs derived from virulent-phase B. bronchiseptica (OMVBbvir⁺) was effective against infections caused by B. bronchiseptica isolates obtained from different hosts (farm animals and a human patient). In vitro and in vivo characterization of humoral and cellular immune responses induced by the OMVBbvir⁺ vaccine enabled a better understanding of the mechanism of protection necessary to control B. bronchiseptica infection. Here we also demonstrated that OMVs derived from B. bronchiseptica in the avirulent phase and the corresponding induced humoral immune response were able to protect mice from B. bronchiseptica infection. This realization provides the basis for the development of novel vaccines not only against the acute stages of the disease but also against stages of the disease or the infectious cycle in which avirulence factors could play a role.

What Is Wrong with Pertussis Vaccine Immunity? The Problem of Waning Effectiveness of Pertussis Vaccines

Pertussis is resurgent in some countries, particularly those in which children receive acellular pertussis (aP) vaccines in early infancy and boosters later in life. Immunologic studies show that, whereas whole-cell pertussis (wP) vaccines orient the immune system toward Th1/Th17 responses, acellular pertussis vaccines orient toward Th1/Th2 responses. Although aP vaccines do provide protection during the first years of life, the change in T-cell priming results in waning effectiveness of aP as early as 2-3 years post-boosters. Although other factors, such as increased virulence of pertussis strains, better diagnosis, and better surveillance may play a role, the increase in pertussis appears to be the result of waning immunity. In addition, studies in baboon models, requiring confirmation in humans, show that aP is less able to prevent nasopharyngeal colonization of Bordetella pertussis than wP or natural infection.

Bacterial outer membrane vesicles: New insights and applications

Outer membrane vesicles (OMVs) (∼50-250 nm in diameter) are produced by both pathogenic and nonpathogenic bacteria as a canonical end product of secretion. In this review, we focus on the OMVs produced by gram-negative bacteria. We provide an overview of the OMV structure, various factors regulating their production, and their role in modulating host immune response using a few representative examples. In light of the importance of the diverse cargoes carried by OMVs, we discuss the different modes of their entry into the host cell and advances in the high-throughput detection of these OMVs. A conspicuous application of OMVs lies in the field of vaccination; we discuss its success in immunization against human diseases such as pertussis, meningitis, shigellosis and aqua-farming endangering diseases like edwardsiellosis.

Physiopathological roles of spontaneously released outer membrane vesicles of Bordetella pertussis

AIM

Bordetella pertussis has been shown to release outer membrane vesicles (OMV) both in vitro and in vivo but little is known about their biological role during the initial phases of B. pertussis infection of the airways.

RESULTS

We have demonstrated that OMV are released by B. pertussis in a human ciliated-airway cell model and purified vesicles can interact with host cells. Binding and uptake are strictly Bvg-regulated and OMV-associated pertussis toxin contributes to host-cell intoxication. Furthermore, we have shown that OMV act as iron-delivery systems complementing the B. pertussis growth defect in iron-limiting conditions.

CONCLUSION

We have proved that OMV play different roles in B. pertussis physiopathology and we opened new perspectives to be further investigated.

Pertussis Maternal Immunization: Narrowing the Knowledge Gaps on the Duration of Transferred Protective Immunity and on Vaccination Frequency

Maternal safety through pertussis vaccination and subsequent maternal–fetal-antibody transfer are well documented, but information on infant protection from pertussis by such antibodies and by subsequent vaccinations is scarce. Since mice are used extensively for maternal-vaccination studies, we adopted that model to narrow those gaps in our understanding of maternal pertussis immunization. Accordingly, we vaccinated female mice with commercial acellular pertussis (aP) vaccine and measured offspring protection against Bordetella pertussis challenge and specific-antibody levels with or without revaccination. Maternal immunization protected the offspring against pertussis, with that immune protection transferred to the offspring lasting for several weeks, as evidenced by a reduction (4–5 logs, p < 0.001) in the colony-forming-units recovered from the lungs of 16-week-old offspring. Moreover, maternal-vaccination-acquired immunity from the first pregnancy still conferred protection to offspring up to the fourth pregnancy. Under the conditions of our experimental protocol, protection to offspring from the aP-induced immunity is transferred both transplacentally and through breastfeeding. Adoptive-transfer experiments demonstrated that transferred antibodies were more responsible for the protection detected in offspring than transferred whole spleen cells. In contrast to reported findings, the protection transferred was not lost after the vaccination of infant mice with the same or other vaccine preparations, and conversely, the immunity transferred from mothers did not interfere with the protection conferred by infant vaccination with the same or different vaccines. These results indicated that aP-vaccine immunization of pregnant female mice conferred protective immunity that is transferred both transplacentally and via offspring breastfeeding without compromising the protection boostered by subsequent infant vaccination. These results—though admittedly not necessarily immediately extrapolatable to humans—nevertheless enabled us to test hypotheses under controlled conditions through detailed sampling and data collection. These findings will hopefully refine hypotheses that can then be validated in subsequent human studies.

Proteome Analysis Is a Valuable Tool to Monitor Antigen Expression during Upstream Processing of Whole-Cell Pertussis Vaccines

Physicochemical and immunochemical assays were applied to substantiate the relation between upstream processing and the quality of whole-cell pertussis vaccines. Bordetella pertussis bacteria were cultured on a chemically defined medium using a continuous cultivation process in stirred tank reactors to obtain uniform protein expression. Continuous culture favors the consistent production of proteins known as virulence factors. Magnesium sulfate was added during the steady state of the culture in order to diminish the expression of virulence proteins. Changes in gene expression and antigen composition were measured by microarrays, mass spectrometry and ELISA. Transcriptome and proteome data revealed high similarity between the biological triplicates demonstrating consistent cultivation of B. pertussis. The addition of magnesium sulfate resulted in an instant downregulation of the virulence genes in B. pertussis, but a gradual decrease of virulence proteins. The quantity of virulence proteins concurred highly with the potency of the corresponding whole-cell pertussis vaccines, which were determined by the Kendrick test. In conclusion, proteome analysis provided detailed information on the composition and proportion of virulence proteins present in the whole-cell preparations of B. pertussis. Moreover, proteome analysis is a valuable method to monitor the production process of whole-cell biomass and predict the product quality of whole-cell pertussis vaccines.