Improved production process for native outer membrane vesicle vaccine against Neisseria meningitidis

Outer membrane vesicles generated by an exogenous bacteriophage lysin and protection against Acinetobacter baumannii infection

BACKGROUND

The outer membrane vesicles (OMVs) produced by Gram-negative bacteria can modulate the immune system and have great potentials for bacterial vaccine development.

RESULTS

A highly active Acinetobacter baumannii phage lysin, LysP53, can stimulate the production of OMVs after interacting with A. baumannii, Escherichia coli, and Salmonella. The OMVs prepared by the lysin (LOMVs) from A. baumannii showed better homogeneity, higher protein yield, lower endotoxin content, and lower cytotoxicity compared to the naturally produced OMVs (nOMVs). The LOMVs contain a significantly higher number of cytoplasmic and cytoplasmic membrane proteins but a smaller number of periplasmic and extracellular proteins compared to nOMVs. Intramuscular immunization with either LOMVs or nOMVs three times provided robust protection against A. baumannii infections in both pneumonia and bacteremia mouse models. Intranasal immunization offered good protection in the pneumonia model but weaker protection (20-40%) in the bacteremia model. However, with a single immunization, LOMVs demonstrated better protection than the nOMVs in the pneumonia mouse model.

CONCLUSIONS

The novel lysin approach provides a superior choice compared to current methods for OMV production, especially for vaccine development.

Bacterial extracellular vesicles: biotechnological perspective for enhanced productivity

Bacterial extracellular vesicles (BEVs) are non-replicative nanostructures released by Gram-negative and Gram-positive bacteria as a survival mechanism and inter- and intraspecific communication mechanism. Due to BEVs physical, biochemical, and biofunctional characteristics, there is interest in producing and using them in developing new therapeutics, vaccines, or delivery systems. However, BEV release is typically low, limiting their application. Here, we provide a biotechnological perspective to enhance BEV production, highlighting current strategies. The strategies include the production of hypervesiculating strains through gene modification, bacteria culture under stress conditions, and artificial vesicles production. We discussed the effect of these production strategies on BEVs types, morphology, composition, and activity. Furthermore, we summarized general aspects of BEV biogenesis, functional capabilities, and applications, framing their current importance and the need to produce them in abundance. This review will expand the knowledge about the range of strategies associated with BEV bioprocesses to increase their productivity and extend their application possibilities.

Recombinant expression of Yersinia ruckeri outer membrane proteins in Escherichia coli extracellular vesicles

The secretion of extracellular vesicles (EVs) is a common process in Gram-negative bacteria and can be exploited for biotechnological applications. EVs pose a self-adjuvanting, non-replicative vaccine platform, where membrane and antigens are presented to the host immune system in a non-infectious fashion. The secreted quantity of EVs varies between Gram-negative bacterial species and is comparatively high in the model bacterium E. coli. The outer membrane proteins OmpA and OmpF of the fish pathogen Y. ruckeri have been proposed as vaccine candidates to prevent enteric redmouth disease in aquaculture. In this work, Y.ruckeri OmpA or OmpF were expressed in E. coli and recombinant EVs were isolated. To avoid competition between endogenous E. coli OmpA or OmpF, Y. ruckeri OmpA and OmpF were expressed in E. coli strains lacking ompA, ompF, and in a quadruple knockout strain where the four major outer membrane protein genes ompA, ompC, ompF and lamB were removed. Y.ruckeri OmpA and OmpF were successfully expressed in EVs derived from the E. coli mutants as verified by SDS-PAGE, heat modifiability and proteomic analysis using mass-spectrometry. Transmission electron microscopy revealed the presence of EVs in all E. coli strains, and increased EV concentrations were detected when expressing Y. ruckeri OmpA or OmpF in recombinant EVs compared to empty vector controls as verified by nanoparticle tracking analysis. These results show that E. coli can be utilized as a vector for production of EVs expressing outer membrane antigens from Y. ruckeri.

Optimizing Conditions for the Production of Bacterial Extracellular Vesicles of Vibrio vulnificus and Analysis of the Inner Small RNA Compositions

Chemical and physical elements affecting the production of bacterial extracellular vesicles (BEVs) of the human pathogen Vibrio vulnificus were quantitatively assessed to optimize the conditions for the BEV production by using the western blot quantification for an outer membrane porin OmpU and by fluorescent dye FM4-64. When cells were cultured at 37°C in an enriched medium (2 × Luria Bertani; 2 × LB) in the presence of EDTA, they produced about 70% more BEVs. BEVs were purified from the cells cultured in the established optimal conditions by the density gradient ultracentrifugation. The dynamic light scattering measurement of the purified BEVs showed that the diameter of them ranged from approximately 25 nm to 161 nm. We hypothesized that there may be some features in nucleotide sequences specific to RNAs packaged in BEVs compared to those in cellular RNA molecules. We compared the nucleotide sequences and abundance of sRNAs between in the cellular fraction and in BEVs through next-generation sequencing (NGS). While no distinct feature was observed in the nucleotide sequences of sRNAs between two groups, the length of sRNA fragments from BEVs were significantly shorter than those in cytoplasm.

Outer membrane vesicle-based intranasal vaccines

Delivery of vaccines via the mucosal route is regarded as the most effective mode of immunization to counteract infectious diseases that enter via mucosal tissues, including oral, nasal, pulmonary, intestinal, and urogenital surfaces. Mucosal vaccines not only induce local immune effector elements, such as secretory Immunoglobulin A (IgA) reaching the luminal site of the mucosa, but also systemic immunity. Moreover, mucosal vaccines may trigger immunity in distant mucosal tissues because of the homing of primed antigen-specific immune cells toward local and distant mucosal tissue via the common mucosal immune system. While most licensed intramuscular vaccines induce only systemic immunity, next-generation mucosal vaccines may outperform parenteral vaccination strategies by also eliciting protective mucosal immune responses that block infection and/or transmission. Especially the nasal route of vaccination, targeting the nasal-associated lymphoid tissue, is attractive for local and distant mucosal immunization. In numerous studies, bacterial outer membrane vesicles (OMVs) have proved attractive as vaccine platform for homologous bacterial strains, but also as antigen delivery platform for heterologous antigens of nonbacterial diseases, including viruses, parasites, and cancer. Their application has also been extended to mucosal delivery. Here, we will summarize the characteristics and clinical potential of (engineered) OMVs as vaccine platform for mucosal, especially intranasal delivery.

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.

Bacteria-based immunotherapy for cancer: a systematic review of preclinical studies

Immunotherapy has been emerging as a powerful strategy for cancer management. Recently, accumulating evidence has demonstrated that bacteria-based immunotherapy including naive bacteria, bacterial components, and bacterial derivatives, can modulate immune response via various cellular and molecular pathways. The key mechanisms of bacterial antitumor immunity include inducing immune cells to kill tumor cells directly or reverse the immunosuppressive microenvironment. Currently, bacterial antigens synthesized as vaccine candidates by bioengineering technology are novel antitumor immunotherapy. Especially the combination therapy of bacterial vaccine with conventional therapies may further achieve enhanced therapeutic benefits against cancers. However, the clinical translation of bacteria-based immunotherapy is limited for biosafety concerns and non-uniform production standards. In this review, we aim to summarize immunotherapy strategies based on advanced bacterial therapeutics and discuss their potential for cancer management, we will also propose approaches for optimizing bacteria-based immunotherapy for facilitating clinical translation.

The secretome of the fish pathogen Tenacibaculum maritimum includes soluble virulence-related proteins and outer membrane vesicles

Tenacibaculum maritimum, the etiological agent of tenacibaculosis in marine fish, constitutively secretes extracellular products (ECPs) in which protein content has not been yet comprehensively studied. In this work, the prevalence of extracellular proteolytic and lipolytic activities related to virulence was analyzed in 64 T. maritimum strains belonging to the O1-O4 serotypes. The results showed the existence of a great intra-specific heterogeneity in the enzymatic capacity, particularly within serotype O4. Thus, the secretome of a strain belonging to this serotype was characterized by analyzing the protein content of ECPs and the possible production of outer membrane vesicles (OMVs). Notably, the ECPs of T. maritimum SP9.1 contain a large amount of OMVs that were characterized by electron microscopy and purified. Thus, ECPs were divided into soluble (S-ECPs) and insoluble fractions (OMVs), and their protein content was analyzed by a high-throughput proteomic approach. A total of 641 proteins were identified in ECPs including some virulence-related factors, which were mainly found in one of the fractions, either OMVs or S-ECPs. Outer membrane proteins such as TonB-dependent siderophore transporters and the type IX secretion system (T9SS)-related proteins PorP, PorT, and SprA appeared to be mainly associated with OMVs. By contrast, putative virulence factors such as sialidase SiaA, chondroitinase CslA, sphingomyelinase Sph, ceramidase Cer, and collagenase Col were found only in the S-ECPs. These findings clearly demonstrate that T. maritimum releases, through surface blebbing, OMVs specifically enriched in TonB-dependent transporters and T9SS proteins. Interestingly, in vitro and in vivo assays also showed that OMVs could play a key role in virulence by promoting surface adhesion and biofilm formation and maximizing the cytotoxic effects of the ECPs. The characterization of T. maritimum secretome provides insights into ECP function and can constitute the basis for future studies aimed to elucidate the full role of OMVs in the pathogenesis of fish tenacibaculosis.

Correlation between bacterial extracellular vesicles and antibiotics: A potentially antibacterial strategy

Bacterial extracellular vesicles (BEVs) are proteoliposome nanoparticles that are secreted by both Gram-negative (G^-) and Gram-positive (G⁺) bacteria. BEVs have significant roles in various physiological processes of bacteria, including driving inflammatory responses, regulating bacterial pathogenesis, and promoting bacterial survival in diverse environments. Recently, there has been increasing interest in the use of BEVs as a potential solution to antibiotic resistance. BEVs have shown great promise as a new approach to antibiotics, as well as a drug-delivery tool in antimicrobial strategies. In this review, we provide a summary of recent scientific advances in BEVs and antibiotics, including BEV biogenesis, ability to kill bacteria, potential for delivering antibiotics, and their role in the development of vaccines or as immune adjuvants. We propose that BEVs provide a novel antimicrobial strategy that would be beneficial against the increasing threat of antibiotic resistance.

Microencapsulated IL-12 Drives Genital Tract Immune Responses to Intranasal Gonococcal Outer Membrane Vesicle Vaccine and Induces Resistance to Vaginal Infection with Diverse Strains of Neisseria gonorrhoeae

An experimental gonococcal vaccine consisting of outer membrane vesicles (OMVs) and microsphere (ms)-encapsulated interleukin-12 (IL-12 ms) induces Th1-driven immunity, with circulating and genital antibodies to Neisseria gonorrhoeae, after intravaginal (i.vag.) administration in female mice, and generates resistance to vaginal challenge infection. Because i.vag. administration is inapplicable to males and may not be acceptable to women, we determined whether intranasal (i.n.) administration would generate protective immunity against N. gonorrhoeae. Female and male mice were immunized i.n. with gonococcal OMVs plus IL-12 ms or blank microspheres (blank ms). Responses to i.n. immunization were similar to those with i.vag. immunization, with serum IgG, salivary IgA, and vaginal IgG and IgA antigonococcal antibodies induced when OMVs were administered with IL-12 ms. Male mice responded with serum IgG and salivary IgA antibodies similarly to female mice. Gamma interferon (IFN-γ) production by CD4⁺ T cells from iliac lymph nodes was elevated after i.n. or i.vag. immunization with OMVs plus IL-12 ms. Female mice immunized with OMVs plus IL-12 ms by either route resisted challenge with N. gonorrhoeae to an equal extent, and resistance generated by i.n. immunization extended to heterologous strains of N. gonorrhoeae. Detergent-extracted OMVs, which have diminished lipooligosaccharide, generated protective immunity to challenge similar to native OMVs. OMVs from mutant N. gonorrhoeae, in which genes for Rmp and LpxL1 were deleted to eliminate the induction of blocking antibodies against Rmp and diminish lipooligosaccharide endotoxicity, also generated resistance to challenge infection similar to wild-type OMVs when administered i.n. with IL-12 ms. IMPORTANCE We previously demonstrated that female mice can be immunized intravaginally with gonococcal outer membrane vesicles (OMVs) plus microsphere (ms)-encapsulated interleukin-12 (IL-12 ms) to induce antigonococcal antibodies and resistance to genital tract challenge with live Neisseria gonorrhoeae. However, this route of vaccination may be impractical for human vaccine development and is inapplicable to males. Because intranasal immunization has previously been shown to induce antibody responses in both male and female genital tracts, we have evaluated this route of immunization with gonococcal OMVs plus IL-12 ms. In addition, we have refined the composition of gonococcal OMVs to reduce the endotoxicity of lipooligosaccharide and to eliminate the membrane protein Rmp, which induces countereffective blocking antibodies. The resulting vaccine may be more suitable for ultimate translation to human application against the sexually transmitted infection gonorrhea, which is becoming increasingly resistant to treatment with antibiotics.

Bacterial extracellular vesicle applications in cancer immunotherapy

Cancer therapy is undergoing a paradigm shift toward immunotherapy focusing on various approaches to activate the host immune system. As research to identify appropriate immune cells and activate anti-tumor immunity continues to expand, scientists are looking at microbial sources given their inherent ability to elicit an immune response. Bacterial extracellular vesicles (BEVs) are actively studied to control systemic humoral and cellular immune responses instead of using whole microorganisms or other types of extracellular vesicles (EVs). BEVs also provide the opportunity as versatile drug delivery carriers. Unlike mammalian EVs, BEVs have already made it to the clinic with the meningococcal vaccine (Bexsero®). However, there are still many unanswered questions in the use of BEVs, especially for chronic systemically administered immunotherapies. In this review, we address the opportunities and challenges in the use of BEVs for cancer immunotherapy and provide an outlook towards development of BEV products that can ultimately translate to the clinic.

The Discovery of the Role of Outer Membrane Vesicles against Bacteria

Gram-negative bacteria are intrinsically resistant to many commercialized antibiotics. The outer membrane (OM) of Gram-negative bacteria prevents the entry of such antibiotics. Outer membrane vesicles (OMV) are naturally released from the OM of Gram-negative bacteria for a range of purposes, including competition with other bacteria. OMV may carry, as part of the membrane or lumen, molecules with antibacterial activity. Such OMV can be exposed to and can fuse with the cell surface of different bacterial species. In this review we consider how OMV can be used as tools to deliver antimicrobial agents. This includes the characteristics of OMV production and how this process can be used to create the desired antibacterial activity of OMV.

Bacterial outer membrane vesicle-based cancer nanovaccines

Tumor vaccines, a type of personalized tumor immunotherapy, have developed rapidly in recent decades. These vaccines evoke tumor antigen-specific T cells to achieve immune recognition and killing of tumor cells. Because the immunogenicity of tumor antigens alone is insufficient, immune adjuvants and nanocarriers are often required to enhance anti-tumor immune responses. At present, vaccine carrier development often integrates nanocarriers and immune adjuvants. Among them, outer membrane vesicles (OMVs) are receiving increasing attention as a delivery platform for tumor vaccines. OMVs are natural nanovesicles derived from Gram-negative bacteria, which have adjuvant function because they contain pathogen associated molecular patterns. Importantly, OMVs can be functionally modified by genetic engineering of bacteria, thus laying a foundation for applications as a delivery platform for tumor nanovaccines. This review summarizes 5 aspects of recent progress in, and future development of, OMV-based tumor nanovaccines: strain selection, heterogeneity, tumor antigen loading, immunogenicity and safety, and mass production of OMVs.

Bacterial membrane vesicles in inflammatory bowel disease

Inflammatory bowel disease (IBD) is characterized by chronic intestinal inflammation with no cure. The intestine is fundamental in controlling human health. Disruption of the microbial ecosystem in the intestine is considered an important cause of IBD. The interaction between the host and microbiota significantly impacts the intestinal epithelial barrier and immune function. Bacterial membrane vesicles (MVs) are vital participants in bacteria-bacteria and host-microbiota communication. Currently, MVs have been found to exhibit many important regulating effects for intestinal microecology and have excellent application potential in clinical disease therapies. In the present review, we review the current knowledge on MVs, and specifically focus on gut bacterial MVs and their roles in the IBD. In addition, we summarized the potential utility of MVs as a novel therapeutic approach in IBD patients.

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.

Outer membrane vesicles as biomimetic vaccine carriers against infections and cancers

In the last decade, nanoparticle-based therapeutic modalities have emerged as promising treatment options for cancer and infectious diseases. To improve prognosis, chemotherapeutic and antimicrobial drugs must be delivered selectively to the target sites. Researchers have increasingly focused their efforts on improving drug delivery, with a particular emphasis on cancer and infectious diseases. When drugs are administered systemically, they become diluted and can diffuse to all tissues but only until the immune system intervenes and quickly removes them from circulation. To enhance and prolong the systemic circulation of drugs, nanocarriers have been explored and used; however, nanocarriers have a major drawback in that they can trigger immune responses. Numerous nanocarriers for optimal drug delivery have been developed using innovative and effective biointerface technologies. Autologous cell-derived drug carriers, such as outer membrane vesicles (OMVs), have demonstrated improved bioavailability and reduced toxicity. Thus, this study investigates the use of biomimetic OMVs as biomimetic vaccine carriers against infections and cancers to improve our understanding in the field of nanotechnology. In addition, discussion on the advantages, disadvantages, and future prospects of OMVs will also be explored. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Membrane Organization Strategies in Vesicular Antibiotic Delivery

Small molecule antibiotics are often derived from microorganisms that thrive in competitive environments. Their importance as therapeutics for infectious disease in humans has been established over many years. It has now become clear that antibiotic-producing organisms use packaging and delivery in the form of vesicles in many cases. A similar strategy has evolved in recent decades in the pharmaceutical industry for formulation of antibiotic therapies. The top-down approach that has evolved over millions of years in various micro-organisms has generated complex, efficient delivery systems that we are just now beginning to understand. The bottom-up formulation approach involves simple, safe compositions, which are being continually enhanced by trying to add features of which the natural systems inform us. A comparison is made here of these paradigms. Despite the differences, there are a number of common features in the basic physical and biological requirements that must be satisfied. In this review, illustration and comparison of some of these requirements is given, demonstrating the ongoing challenges in this area of research.

An Intranasal OMV-Based Vaccine Induces High Mucosal and Systemic Protecting Immunity Against a SARS-CoV-2 Infection

The development of more effective, accessible, and easy to administer COVID-19 vaccines next to the currently marketed mRNA, viral vector, and whole inactivated virus vaccines is essential to curtailing the SARS-CoV-2 pandemic. A major concern is reduced vaccine-induced immune protection to emerging variants, and therefore booster vaccinations to broaden and strengthen the immune response might be required. Currently, all registered COVID-19 vaccines and the majority of COVID-19 vaccines in development are intramuscularly administered, targeting the induction of systemic immunity. Intranasal vaccines have the capacity to induce local mucosal immunity as well, thereby targeting the primary route of viral entry of SARS-CoV-2 with the potential of blocking transmission. Furthermore, intranasal vaccines offer greater practicality in terms of cost and ease of administration. Currently, only eight out of 112 vaccines in clinical development are administered intranasally. We developed an intranasal COVID-19 subunit vaccine, based on a recombinant, six-proline-stabilized, D614G spike protein (mC-Spike) of SARS-CoV-2 linked via the LPS-binding peptide sequence mCramp (mC) to outer membrane vesicles (OMVs) from Neisseria meningitidis. The spike protein was produced in CHO cells, and after linking to the OMVs, the OMV-mC-Spike vaccine was administered to mice and Syrian hamsters via intranasal or intramuscular prime-boost vaccinations. In all animals that received OMV-mC-Spike, serum-neutralizing antibodies were induced upon vaccination. Importantly, high levels of spike-binding immunoglobulin G (IgG) and A (IgA) antibodies in the nose and lungs were only detected in intranasally vaccinated animals, whereas intramuscular vaccination only induced an IgG response in the serum. Two weeks after their second vaccination, hamsters challenged with SARS-CoV-2 were protected from weight loss and viral replication in the lungs compared to the control groups vaccinated with OMV or spike alone. Histopathology showed no lesions in lungs 7 days after challenge in OMV-mC-Spike-vaccinated hamsters, whereas the control groups did show pathological lesions in the lung. The OMV-mC-Spike candidate vaccine data are very promising and support further development of this novel non-replicating, needle-free, subunit vaccine concept for clinical testing.

PMAP-36 reduces the innate immune response induced by Bordetella bronchiseptica-derived outer membrane vesicles

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Sub-lethal PMAP-36 treatment of bacteria increases outer membrane vesicle release.

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Lipidomic analysis revealed the OMV lipidome upon PMAP-36 or heat treatment.

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Supplementation with PMAP-36 attenuated undesirable OMV-induced immune responses.

Host defense peptides (HDPs), such as cathelicidins, are small, cationic, amphipathic peptides and represent an important part of the innate immune system. Most cathelicidins, including the porcine PMAP-36, are membrane active and disrupt the bacterial membrane. For example, a chicken cathelicidin, CATH-2, has been previously shown to disrupt both Escherichia coli membranes and to release, at sub-lethal concentrations, outer membrane vesicles (OMVs). Since OMVs are considered promising vaccine candidates, we sought to investigate the effect of sub-bactericidal concentrations of PMAP-36 on both OMV release by a porcine strain of Bordetella bronchiseptica and on the modulation of immune responses to OMVs. PMAP-36 treatment of bacteria resulted in a slight increase in OMV release. The characteristics of PMAP-36-induced OMVs were compared with those of spontaneously released OMVs and OMVs induced by heat treatment. The stability of both PMAP-36- and heat-induced OMVs was decreased compared to spontaneous OMVs, as shown by dynamic light scattering. Furthermore, treatment of bacteria with PMAP-36 or heat resulted in an increase in negatively charged phospholipids in the resulting OMVs. A large increase in lysophospholipid content was observed in heat-induced OMVs, which was at least partially due to the activity of the outer-membrane phospholipase A (OMPLA). Although PMAP-36 was detected in OMVs isolated from PMAP-36-treated bacteria, the immune response of porcine bone-marrow-derived macrophages to these OMVs was similar as those against spontaneous or heat-induced OMVs. Therefore, the effect of PMAP-36 addition after OMV isolation was investigated. This did decrease cytokine expression of OMV-stimulated macrophages. These results indicate that PMAP-36 is a promising molecule to attenuate undesirable immune responses, for instance in vaccines.

Correlates of protection for meningococcal surface protein vaccines: lessons from the past

INTRODUCTION

Recombinant surface protein meningococcal serogroup B (MenB) vaccines are available but with different antigen compositions, leading to differences between vaccines in their immunogenicity and likely breadth of coverage. The serology and breadth of coverage assessment for MenB vaccines are multifaceted areas, and a comprehensive understanding of these complexities is required to appropriately compare licensed vaccines and those under development.

AREAS COVERED

In the first of two companion papers that comprehensively review the serology and breadth of coverage assessment for MenB vaccines, the history of early meningococcal vaccines is considered in this narrative review to identify transferable lessons applicable to the currently licensed MenB vaccines and those under development, as well as their serology.

EXPERT OPINION

Understanding correlates of protection and the breadth of coverage assessment for meningococcal surface protein vaccines is significantly more complex than that for capsular polysaccharide vaccines. Determination and understanding of the breadth of coverage of surface protein vaccines are clinically important and unique to each vaccine formulation. It is essential to estimate the proportion of MenB cases that are preventable by a specific vaccine to assess its overall potential impact and to compare the benefits and limitations of different vaccines in preventing invasive meningococcal disease.

Outer membrane vesicles as versatile tools for therapeutic approaches

Budding of the bacterial surface results in the formation and secretion of outer membrane vesicles, which is a conserved phenomenon observed in Gram-negative bacteria. Recent studies highlight that these sphere-shaped facsimiles of the donor bacterium's surface with enclosed periplasmic content may serve multiple purposes for their host bacterium. These include inter- and intraspecies cell-cell communication, effector delivery to target cells and bacterial adaptation strategies. This review provides a concise overview of potential medical applications to exploit outer membrane vesicles for therapeutic approaches. Due to the fact that outer membrane vesicles resemble the surface of their donor cells, they represent interesting nonliving candidates for vaccine development. Furthermore, bacterial donor species can be genetically engineered to display various proteins and glycans of interest on the outer membrane vesicle surface or in their lumen. Outer membrane vesicles also possess valuable bioreactor features as they have the natural capacity to protect, stabilize and enhance the activity of luminal enzymes. Along these features, outer membrane vesicles not only might be suitable for biotechnological applications but may also enable cell-specific delivery of designed therapeutics as they are efficiently internalized by nonprofessional phagocytes. Finally, outer membrane vesicles are potent modulators of our immune system with pro- and anti-inflammatory properties. A deeper understanding of immunoregulatory effects provoked by different outer membrane vesicles is the basis for their possible future applications ranging from inflammation and immune response modulation to anticancer therapy.

Microbiota-host communications: Bacterial extracellular vesicles as a common language

Both gram-negative and gram-positive bacteria release extracellular vesicles (EVs) that contain components from their mother cells. Bacterial EVs are similar in size to mammalian-derived EVs and are thought to mediate bacteria–host communications by transporting diverse bioactive molecules including proteins, nucleic acids, lipids, and metabolites. Bacterial EVs have been implicated in bacteria–bacteria and bacteria–host interactions, promoting health or causing various pathologies. Although the science of bacterial EVs is less developed than that of eukaryotic EVs, the number of studies on bacterial EVs is continuously increasing. This review highlights the current state of knowledge in the rapidly evolving field of bacterial EV science, focusing on their discovery, isolation, biogenesis, and more specifically on their role in microbiota–host communications. Knowledge of these mechanisms may be translated into new therapeutics and diagnostics based on bacterial EVs.

Machine learning applied for metabolic flux-based control of micro-aerated fermentations in bioreactors

Various bio-based processes depend on controlled micro-aerobic conditions to achieve a satisfactory product yield. However, the limiting oxygen concentration varies according to the micro-organism employed, while for industrial applications, there is no cost-effective way of measuring it at low levels. This study proposes a machine learning procedure within a metabolic flux-based control strategy (SUPERSYS_MCU) to address this issue. The control strategy used simulations of a genome-scale metabolic model to generate a surrogate model in the form of an artificial neural network, to be used in a micro-aerobic fermentation strategy (MF-ANN). The meta-model provided setpoints to the controller, allowing adjustment of the inlet air flow to control the oxygen uptake rate. The strategy was evaluated in micro-aerobic batch cultures employing industrial Saccharomyces cerevisiae yeast, with defined medium and glucose as the carbon source, as a case study. The performance of the proposed control scheme was compared with a conventional fermentation and with three previously reported micro-aeration strategies, including respiratory quotient-based control and constant air flow rate. Due to maintenance of the oxidative balance at the anaerobiosis threshold, the MF-ANN provided volumetric ethanol productivity of 4.16 g·L^-1 ·h^-1 and a yield of 0.48 g_ethanol .g_substrate ^-1 , which were higher than the values achieved for the other conditions studied (maximum of 3.4 g·L^-1 ·h^-1 and 0.35-0.40 g_ethanol ·g_substrate ^-1 , respectively). Due to its modular character, the MF-ANN strategy could be adapted to other micro-aerated bioprocesses.

Immunomodulatory roles and novel applications of bacterial membrane vesicles

Bacteria release extracellular vesicles (EVs) known as bacterial membrane vesicles (BMVs) during their normal growth. Gram-negative bacteria produce BMVs termed outer membrane vesicles (OMVs) that are composed of a range of biological cargo and facilitate numerous bacterial functions, including promoting pathogenesis and mediating disease in the host. By contrast, less is understood about BMVs produced by Gram-positive bacteria, which are referred to as membrane vesicles (MVs), however their contribution to mediating bacterial pathogenesis has recently become evident. In this review, we summarise the mechanisms whereby BMVs released by Gram-negative and Gram-positive bacteria are produced, in addition to discussing their key functions in promoting bacterial survival, mediating pathogenesis and modulating host immune responses. Furthermore, we discuss the mechanisms whereby BMVs produced by both commensal and pathogenic organisms can enter host cells and interact with innate immune receptors, in addition to how they modulate host innate and adaptive immunity to promote immunotolerance or drive the onset and progression of disease. Finally, we highlight current and emerging applications of BMVs in vaccine design, biotechnology and cancer therapeutics.

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

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

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.

Pathogenesis Mediated by Bacterial Membrane Vesicles

The release of extracellular vesicles (EVs) is a process conserved across the three domains of life. Amongst prokaryotes, EVs produced by Gram-negative bacteria, termed outer membrane vesicles (OMVs), were identified more than 50 years ago and a wealth of literature exists regarding their biogenesis, composition and functions. OMVs have been implicated in benefiting numerous metabolic functions of their parent bacterium. Additionally, OMVs produced by pathogenic bacteria have been reported to contribute to pathology within the disease setting. By contrast, the release of EVs from Gram-positive bacteria, known as membrane vesicles (MVs), has only been widely accepted within the last decade. As such, there is a significant disproportion in knowledge regarding MVs compared to OMVs. Here we provide an overview of the literature regarding bacterial membrane vesicles (BMVs) produced by pathogenic and commensal bacteria. We highlight the mechanisms of BMV biogenesis and their roles in assisting bacterial survival, in addition to discussing their functions in promoting disease pathologies and their potential use as novel therapeutic strategies.

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.

Diversification in immunogenicity genes caused by selective pressures in invasive meningococci

We studied population genomics of 486 Neisseria meningitidis isolates causing meningitis in the Netherlands during the period 1979-2003 and 2006-2013 using whole-genome sequencing to evaluate the impact of a hyperendemic period of serogroup B invasive disease. The majority of serogroup B isolates belonged to ST-41/44 (41 %) and ST-32 complex (16 %). Comparing the time periods, before and after the decline of serogroup B invasive disease, there was a decrease of ST-41/44 complex sequences (P=0.002). We observed the expansion of a sub-lineage within ST-41/44 complex sequences being associated with isolation from the 1979-2003 time period (P=0.014). Isolates belonging to this sub-lineage expansion within ST-41/44 complex were marked by four antigen allele variants. Presence of these allele variants was associated with isolation from the 1979-2003 time period after correction for multiple testing (Wald test, P=0.0043 for FetA 1-5; P=0.0035 for FHbp 14; P=0.012 for PorA 7-2.4 and P=0.0031 for NHBA two peptide allele). These sequences were associated with 4CMenB vaccine coverage (Fisher's exact test, P<0.001). Outside of the sub-lineage expansion, isolates with markedly lower levels of predicted vaccine coverage clustered in phylogenetic groups showing a trend towards isolation in the 2006-2013 time period (P=0.08). In conclusion, we show the emergence and decline of a sub-lineage expansion within ST-41/44 complex isolates concurrent with a hyperendemic period in meningococcal meningitis. The expansion was marked by specific antigen peptide allele combinations. We observed preliminary evidence for decreasing 4CMenB vaccine coverage in the post-hyperendemic period.

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.

Continuous production of Neisseria meningitidis outer membrane vesicles

Outer membrane vesicles (OMVs) are nanoparticles secreted by Gram-negative bacteria that can be used for diverse biotechnological applications. Interesting applications have been developed, where OMVs are the basis of drug delivery, enzyme carriers, adjuvants, and vaccines. Historically, OMV research has mainly focused on vaccines. Therefore, current OMV production processes have been based on batch processes. The production of OMVs in batch mode is characterized by relatively low yields and high costs. Transition of OMV production processes from batch to continuous processes could increase the volumetric productivity, reduce the production and capital costs, and result in a higher quality product. Here, we study the continuous production of Neisseria meningitidis OMVs to improve volumetric productivity. Continuous cultivation of N. meningitidis resulted in a steady state with similar high OMV concentrations as are reached in current batch processes. The steady state was reproducible and could be maintained for at least 600 h. The volumetric productivity of a continuous culture reached 4.0 × 1014 OMVs per liter culture per day, based on a dilution rate of 1/day. The tested characteristics of the OMVs did not change during the experiments showing feasibility of a continuous production process for the production of OMVs for any application.

Metabolic fluxes-oriented control of bioreactors: a novel approach to tune micro-aeration and substrate feeding in fermentations

Background

Fine-tuning the aeration for cultivations when oxygen-limited conditions are demanded (such as the production of vaccines, isobutanol, 2–3 butanediol, acetone, and bioethanol) is still a challenge in the area of bioreactor automation and advanced control. In this work, an innovative control strategy based on metabolic fluxes was implemented and evaluated in a case study: micro-aerated ethanol fermentation.

Results

The experiments were carried out in fed-batch mode, using commercial Saccharomyces cerevisiae, defined medium, and glucose as carbon source. Simulations of a genome-scale metabolic model for Saccharomyces cerevisiae were used to identify the range of oxygen and substrate fluxes that would maximize ethanol fluxes. Oxygen supply and feed flow rate were manipulated to control oxygen and substrate fluxes, as well as the respiratory quotient (RQ). The performance of the controlled cultivation was compared to two other fermentation strategies: a conventional “Brazilian fuel-ethanol plant” fermentation and a strictly anaerobic fermentation (with ultra-pure nitrogen used as the inlet gas). The cultivation carried out under the proposed control strategy showed the best average volumetric ethanol productivity (7.0 g L⁻¹ h⁻¹), with a final ethanol concentration of 87 g L⁻¹ and yield of 0.46 g_ethanol g_substrate⁻¹. The other fermentation strategies showed lower yields (close to 0.40 g_ethanol g_substrate⁻¹) and ethanol productivity around 4.0 g L⁻¹ h⁻¹.

Conclusion

The control system based on fluxes was successfully implemented. The proposed approach could also be adapted to control several bioprocesses that require restrict aeration.

Spontaneously released Neisseria meningitidis outer membrane vesicles as vaccine platform: production and purification

Outer membrane vesicles (OMVs) are nanoparticles produced by Gram-negative bacteria that can be used as vaccines. The application of OMVs as vaccine component can be expanded by expressing heterologous antigens on OMVs, creating an OMV-based antigen presenting platform. This study aims to develop a production process for such OMV-based vaccines and studies a production method based on meningococcal OMVs that express heterologous antigens on their surface. As a proof of concept, the Borrelia burgdorferi antigens OspA and OspC were expressed on Neisseria meningitidis OMVs to create a concept anti-Lyme disease vaccine. Production of OMVs released in the culture supernatant was induced by high dissolved oxygen concentrations and purification was based on scalable unit operations. A crude recovery of 90 mg OMV protein could be obtained per liter culture. Expressing heterologous antigens on the OMVs did result in minor reduction of bacterial growth, while OMV production remained constant. The antigen expression did not alter the OMV characteristics. This study shows that production of well characterized OMVs containing heterologous antigens is possible with high yields by combining high oxygen concentrations with an optimized purification process. It is concluded that heterologous OMVs show potential as a vaccine platform.

Emerging Technologies for Low-Cost, Rapid Vaccine Manufacture

To stop the spread of future epidemics and meet infant vaccination demands in low- and middle-income countries, flexible, rapid and low-cost vaccine development and manufacturing technologies are required. Vaccine development platform technologies that can produce a wide range of vaccines are emerging, including: a) humanized, high-yield yeast recombinant protein vaccines; b) insect cell-baculovirus ADDomer vaccines; c) Generalized Modules for Membrane Antigens (GMMA) vaccines; d) RNA vaccines. Herein, existing and future platforms are assessed in terms of addressing challenges of scale, cost, and responsiveness. To assess the risk and feasibility of the four emerging platforms, the following six metrics are applied: 1) technology readiness; 2) technological complexity; 3) ease of scale-up; 4) flexibility for the manufacturing of a wide range of vaccines; 5) thermostability of the vaccine product at tropical ambient temperatures; and 6) speed of response from threat identification to vaccine deployment. The assessment indicated that technologies in the order of increasing feasibility and decreasing risk are the yeast platform, ADDomer platform, followed by RNA and GMMA platforms. The comparative strengths and weaknesses of each technology are discussed in detail, illustrating the associated development and manufacturing needs and priorities.

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.

High throughput nanoparticle tracking analysis for monitoring outer membrane vesicle production

Outer membrane vesicles (OMVs) are spherical membrane nanoparticles released by Gram-negative bacteria. OMVs can be quantified in complex matrices by nanoparticle tracking analysis (NTA). NTA can be performed in static mode or with continuous sample flow that results in analysis of more particles in a smaller time-frame. Flow measurements must be performed manually despite the availability of a sample changer on the NanoSight system. Here we present a method for automated measurements in flow mode. OMV quantification in flow mode results in lower variance in particle quantification (coefficient of variation (CV) of 6%, CV static measurements of 14%). Sizing of OMVs was expected to be less favorable in flow mode due to the increased movement of the particles. However, we observed a CV of 3% in flow mode and a CV of 8% in static measurements. Flow rates of up to 5 µL/min displayed correct size and particle measurements, however, particle concentration was slightly lower than in static measurements. The automated method was used to assess OMV release of batch cultures of Neisseria meningitidis. The bacteria released more OMVs in stationary growth phase, while the size of the vesicles remained constant throughout the culture. Taken together, this study shows that automated measurements in flow mode can be established with advanced scripting to reduce the workload for the user.

The next chapter for group B meningococcal vaccines

The majority of invasive meningococcal disease (IMD) in the developed world is caused by capsular group B Neisseria meningitidis, however success with vaccination against organisms bearing this capsule has previously been restricted to control of geographically limited clonal outbreaks. As we enter a new era, with the first routine program underway to control endemic group B meningococcal disease for infants in the UK, it is timely to review the key landmarks in group B vaccine development, and discuss the issues determining whether control of endemic group B disease will be achieved. Evidence of a reduction in carriage acquisition of invasive group B meningococcal strains, after vaccination among adolescents, is imperative if routine immunization is to drive population control of disease beyond those who are vaccinated (i.e. through herd immunity). The need for multiple doses to generate a sufficiently protective response and reactogenicity remain significant problems with the new generation of vaccines. Despite these limitations, early data from the UK indicate that new group B meningococcal vaccines have the potential to have a major impact on meningococcal disease, and to provide new insight into how we might do better in the future.

A Meningococcal Outer Membrane Vesicle Vaccine Incorporating Genetically Attenuated Endotoxin Dissociates Inflammation from Immunogenicity

Background

Group B Neisseria meningitidis, an endotoxin-producing Gram-negative bacterium, causes the highest incidence of group B meningococcus (MenB) disease in the first year of life. The Bexsero vaccine is indicated in Europe from 8 weeks of age. Endotoxin components of outer membrane vesicles (OMVs) or soluble lipopolysaccharide (LPS) represent a potential source of inflammation and residual reactogenicity. The purpose of this study was to compare novel candidate MenB vaccine formulations with licensed vaccines, including Bexsero, using age-specific human in vitro culture systems.

Methods

OMVs from wild type- and inactivated lpxL1 gene mutant-N. meningitidis strains were characterized in human neonatal and adult in vitro whole blood assays and dendritic cell (DC) arrays. OMVs were benchmarked against licensed vaccines, including Bexsero and whole cell pertussis formulations, with respect to Th-polarizing cytokine and prostaglandin E2 production, as well as cell surface activation markers (HLA-DR, CD86, and CCR7). OMV immunogenicity was assessed in mice.

Results

ΔlpxLI native OMVs (nOMVs) demonstrated significantly less cytokine induction in human blood and DCs than Bexsero and most of the other pediatric vaccines (e.g., PedvaxHib, EasyFive, and bacillus Calmette–Guérin) tested. Despite a much lower inflammatory profile in vitro than Bexsero, ΔlpxLI nOMVs still had moderate DC maturing ability and induced robust anti-N. meningitidis antibody responses after murine immunization.

Conclusion

A meningococcal vaccine comprised of attenuated LPS-based OMVs with a limited inflammatory profile in vitro induces robust antigen-specific immunogenicity in vivo.

Outer membrane vesicles of Gallibacterium anatis induce protective immunity in egg-laying hens

Gallibacterium anatis causes infections in the reproductive tract of egg-laying hens and induce increased mortality and decreased egg production. New prophylactic measures are needed in order to improve animal welfare and production efficiency. Bacterial outer membrane vesicles (OMVs) have previously shown promising results in protection against infections and we hypothesized that OMVs could serve as an immunogen to protect egg-laying hens against G. anatis. To investigate the immunogenic potential of G. anatis OMVs, two in vivo studies in egg-laying hens were made. The trials assessedthe degree of protection provided by immunization with G. anatis OMV against challenge and the IgY responses in serum after immunization and challenge, respectively. A total of 64 egg-laying hens were included in the trials. OMVs for immunization were produced and purified from a high-producing G. anatis ΔtolR mutant. Challenge was done with G. anatis 12656-12 and evaluated by scoring lesions and bacterial re-isolation rates from peritoneum. Finally, levels of OMV-specific IgY in sera were assayed by ELISA. Immunization with OMVs decreased the lesions scores significantly, while the bacterial re-isolation remained unchanged. Furthermore, a high OMV-specific IgY response was induced by immunization and subsequent challenge of the hens. The results strongly indicate that immunization with G. anatis OMVs provides significant protection against G. anatis challenge and induces specific antibody responses with high titers of OMV-specific IgY in serum. The results therefore show great promise for OMV based vaccines aiming at providing protecting against G. anatis in egg-laying hens.

Development and application of HPLC-RI and HPLC-MS/MS based methods for quantification of residual deoxycholate levels in pneumococcal polysaccharides

The analysis of residual sodium deoxycholate (DOC); a detergent of biological origin used in manufacturing of polysaccharide vaccines is challenging due to complex sample matrices and the lack of suitable methods. Here we report, rapid and sensitive high-performance liquid chromatography-refractive index (HPLC-RI) and tandem mass spectrometry (HPLC-MS/MS) methods for estimation of residual DOC in pneumococcal polysaccharides. For HPLC-RI method, separation was achieved using Luna C18 column and mobile phase compositions of acetonitrile: methanol: 20 mM sodium acetate (60:05:35% v/v). For HPLC-MS/MS method, separation was achieved using a Hypersil BDS C18 column with gradient elution of methanol and water (0.1% formic acid). MS/MS method showed linearity (r² = 0.997) over the range of 10-320 ng/mL with limits of detection (LOD) and lower limit of quantitation (LOQ) of 3 and 10 ng/mL respectively. Precision (% RSD) and accuracy (% recovery) for both methods were in the range of 0.74-8.29% and 82.33-117.86% respectively. Sample matrices interferences were addressed following novel sample clean-up method based on liquid-liquid extraction. Both methods enabled traceable quantitation of DOC in intermediate and purified pneumococcal polysaccharides of serotypes: 1, 5, 6A, 6B, 7F, 9V, 14, 19A, 19F and 23F.

Meningococcal Outer Membrane Vesicle Composition-Dependent Activation of the Innate Immune Response

Meningococcal outer membrane vesicles (OMVs) have been extensively investigated and successfully implemented as vaccines. They contain pathogen-associated molecular patterns, including lipopolysaccharide (LPS), capable of triggering innate immunity. However, Neisseria meningitidis contains an extremely potent hexa-acylated LPS, leading to adverse effects when its OMVs are applied as vaccines. To create safe OMV vaccines, detergent treatment is generally used to reduce the LPS content. While effective, this method also leads to loss of protective antigens such as lipoproteins. Alternatively, genetic modification of LPS can reduce its toxicity. In the present study, we have compared the effects of standard OMV isolation methods using detergent or EDTA with those of genetic modifications of LPS to yield a penta-acylated lipid A (lpxL1 and pagL) on the in vitro induction of innate immune responses. The use of detergent decreased both Toll-like receptor 4 (TLR4) and TLR2 activation by OMVs, while the LPS modifications reduced only TLR4 activation. Mutational removal of PorB or lipoprotein factor H binding protein (fHbp), two proteins known to trigger TLR2 signaling, had no effect, indicating that multiple TLR2 ligands are removed by detergent treatment. Detergent-treated OMVs and lpxL1 OMVs showed similar reductions of cytokine profiles in the human monocytic cell line MM6 and human dendritic cells (DCs). OMVs with the alternative penta-acylated LPS structure obtained after PagL-mediated deacylation showed reduced induction of proinflammatory cytokines interleukin-6 (IL-6) and IL-1β but not of IP-10, a typical TRIF-dependent chemokine. Taken together, these data show that lipid A modification can be used to obtain OMVs with reduced activation of innate immunity, similar to what is found after detergent treatment.

Label-free quantitative mass spectrometry for analysis of protein antigens in a meningococcal group B outer membrane vesicle vaccine

The development of a multivalent outer membrane vesicle (OMV) vaccine where each strain contributes multiple key protein antigens presents numerous analytical challenges. One major difficulty is the ability to accurately and specifically quantitate each antigen, especially during early development and process optimization when immunoreagents are limited or unavailable. To overcome this problem, quantitative mass spectrometry methods can be used. In place of traditional mass assays such as enzyme-linked immunosorbent assays (ELISAs), quantitative LC-MS/MS using multiple reaction monitoring (MRM) can be used during early-phase process development to measure key protein components in complex vaccines in the absence of specific immunoreagents. Multiplexed, label-free quantitative mass spectrometry methods using protein extraction by either detergent or 2-phase solvent were developed to quantitate levels of several meningococcal serogroup B protein antigens in an OMV vaccine candidate. Precision was demonstrated to be less than 15% RSD for the 2-phase extraction and less than 10% RSD for the detergent extraction method. Accuracy was 70 to 130% for the method using a 2-phase extraction and 90-110% for detergent extraction. The viability of MS-based protein quantification as a vaccine characterization method was demonstrated and advantages over traditional quantitative methods were evaluated. Implementation of these MS-based quantification methods can help to decrease the development time for complex vaccines and can provide orthogonal confirmation of results from existing antigen quantification techniques.

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.

Outer membrane vesicles as platform vaccine technology

Outer membrane vesicles (OMVs) are released spontaneously during growth by many Gram-negative bacteria. They present a range of surface antigens in a native conformation and have natural properties like immunogenicity, self-adjuvation and uptake by immune cells which make them attractive for application as vaccines against pathogenic bacteria. In particular with Neisseria meningitidis, they have been investigated extensively and an OMV-containing meningococcal vaccine has recently been approved by regulatory agencies. Genetic engineering of the OMV-producing bacteria can be used to improve and expand their usefulness as vaccines. Recent work on meningitis B vaccines shows that OMVs can be modified, such as for lipopolysaccharide reactogenicity, to yield an OMV product that is safe and effective. The overexpression of crucial antigens or simultaneous expression of multiple antigenic variants as well as the expression of heterologous antigens enable expansion of their range of applications. In addition, modifications may increase the yield of OMV production and can be combined with specific production processes to obtain high amounts of well-defined, stable and uniform OMV particle vaccine products. Further improvement can facilitate the development of OMVs as platform vaccine product for multiple applications.

Multivalent meningococcal serogroup B vaccines: challenges in predicting protection and measuring effectiveness

Vaccines targeting Neisseria meningitidis serogroup B (MenB) have been attempted for 40 years. Monovalent outer membrane vesicle vaccines targeted at epidemic outbreaks have been successfully developed. Newer vaccines aim to induce antibodies to cross-reactive antigens, such as factor H binding protein (rLP2086) or a mix of outer membrane vesicle, factor H binding protein and other minor antigens (4CMenB). The true protective coverage among circulating MenB isolates afforded by these vaccines is unknown. Carefully conducted Phase IV post-implementation evaluations designed to measure specific effectiveness against major circulating MenB clonal lineages are needed to address the critical question of which antigens are linked to protection. Progress with whole-genome sequencing and bio-informatics may allow the composition of antigen mozaics based on two major outer membrane proteins: PorA and FetA.

Biosynthetically engineered lipopolysaccharide as vaccine adjuvant

Lipopolysaccharide (LPS), a dominant component of the Gram-negative bacterial outer membrane, is a strong activator of the innate immune system, and thereby an important determinant in the adaptive immune response following bacterial infection. This adjuvant activity can be harnessed following immunization with bacteria-derived vaccines that naturally contain LPS, and when LPS or molecules derived from it are added to purified vaccine antigens. However, the downside of the strong biological activity of LPS is its ability to contribute to vaccine reactogenicity. Modification of the LPS structure allows triggering of a proper immune response needed in a vaccine against a particular pathogen while at the same time lowering its toxicity. Extensive modifications to the basic structure are possible by using our current knowledge of bacterial genes involved in LPS biosynthesis and modification. This review focuses on biosynthetic engineering of the structure of LPS and implications of these modifications for generation of safe adjuvants.

A novel approach for purification and selective capture of membrane vesicles of the periodontopathic bacterium, Porphyromonas gingivalis: membrane vesicles bind to magnetic beads coated with epoxy groups in a noncovalent, species-specific manner

Membrane vesicles (MVs) of Porphyromonas gingivalis are regarded as an offensive weapon of the bacterium, leading to tissue deterioration in periodontal disease. Therefore, isolation of highly purified MVs is indispensable to better understand the pathophysiological role of MVs in the progression of periodontitis. MVs are generally isolated by a conventional method based on ultracentrifugation of the bacterial culture supernatant. However, the resulting MVs are often contaminated with co-precipitating bacterial appendages sheared from the live bacteria. Here, we report an intriguing property of P. gingivalis MVs–their ability to bind superparamagnetic beads coated with epoxy groups (SB-Epoxy). Analysis of fractions collected during the purification revealed that all MVs of five tested P. gingivalis stains bound to SB-Epoxy. In contrast, free fimbriae in the crude MV preparation did not bind to the SB-Epoxy. The SB-Epoxy-bound MVs were easily dissociated from the SB-Epoxy using a mild denaturation buffer. These results suggest that the surface chemistry conferred by epoxy on the beads is responsible for the binding, which is mediated by noncovalent bonds. Both the structural integrity and purity of the isolated MVs were confirmed by electron microscopy. The isolated MVs also caused cell detachment from culture dishes at a physiologically relevant concentration. Assays of competitive binding between the SB-Epoxy and mixtures of MVs from five bacterial species demonstrated that only P. gingivalis MVs could be selectively eliminated from the mixtures. We suggest that this novel approach enables efficient purification and selective elimination of P. gingivalis MVs.

Meningococcal disease and the complement system

Despite considerable advances in the understanding of the pathogenesis of meningococcal disease, this infection remains a major cause of morbidity and mortality globally. The role of the complement system in innate immune defenses against invasive meningococcal disease is well established. Individuals deficient in components of the alternative and terminal complement pathways are highly predisposed to invasive, often recurrent meningococcal infections. Genome-wide analysis studies also point to a central role for complement in disease pathogenesis. Here we review the pathophysiologic events pertinent to the complement system that accompany meningococcal sepsis in humans. Meningococci use several often redundant mechanisms to evade killing by human complement. Capsular polysaccharide and lipooligosaccharide glycan composition play critical roles in complement evasion. Some of the newly described protein vaccine antigens interact with complement components and have sparked considerable research interest.