Development of a novel S. Typhi and Paratyphi A outer membrane vesicles based bivalent vaccine against enteric fever

Type 5 secretion system antigens as vaccines against Gram-negative bacterial infections

Infections caused by Gram-negative bacteria are leading causes of mortality worldwide. Due to the rise in antibiotic resistant strains, there is a desperate need for alternative strategies to control infections caused by these organisms. One such approach is the prevention of infection through vaccination. While live attenuated and heat-killed bacterial vaccines are effective, they can lead to adverse reactions. Newer vaccine technologies focus on utilizing polysaccharide or protein subunits for safer and more targeted vaccination approaches. One promising avenue in this regard is the use of proteins released by the Type 5 secretion system (T5SS). This system is the most prevalent secretion system in Gram-negative bacteria. These proteins are compelling vaccine candidates due to their demonstrated protective role in current licensed vaccines. Notably, Pertactin, FHA, and NadA are integral components of licensed vaccines designed to prevent infections caused by Bordetella pertussis or Neisseria meningitidis. In this review, we delve into the significance of incorporating T5SS proteins into licensed vaccines, their contributions to virulence, conserved structural motifs, and the protective immune responses elicited by these proteins.

Recent development of oral vaccines (Review)

Oral immunization can elicit an effective immune response and immune tolerance to specific antigens. When compared with the traditional injection route, delivering antigens via the gastrointestinal mucosa offers superior immune effects and compliance, as well as simplicity and convenience, making it a more optimal route for immunization. At present, various oral vaccine delivery systems exist. Certain modified bacteria, such as Salmonella, Escherichia coli and particularly Lactobacillus, are considered promising carriers for oral vaccines. These carriers can significantly enhance immunization efficiency by actively replicating in the intestinal tract following oral administration. The present review provided a discussion of the main mechanisms of oral immunity and the research progress made in the field of oral vaccines. Additionally, it introduced the advantages and disadvantages of the currently more commonly administered injectable COVID-19 vaccines, alongside the latest advancements in this area. Furthermore, recent developments in oral vaccines are summarized, and their potential benefits and side effects are discussed.

Outer membrane vesicles as a platform for the discovery of antibodies to bacterial pathogens

Bacterial outer membrane vesicles (OMVs) are nanosized spheroidal particles shed by gram-negative bacteria that contain biomolecules derived from the periplasmic space, the bacterial outer membrane, and possibly other compartments. OMVs can be purified from bacterial culture supernatants, and by genetically manipulating the bacterial cells that produce them, they can be engineered to harbor cargoes and/or display molecules of interest on their surfaces including antigens that are immunogenic in mammals. Since OMV bilayer-embedded components presumably maintain their native structures, OMVs may represent highly useful tools for generating antibodies to bacterial outer membrane targets. OMVs have historically been utilized as vaccines or vaccine constituents. Antibodies that target bacterial surfaces are increasingly being explored as antimicrobial agents either in unmodified form or as targeting moieties for bactericidal compounds. Here, we review the properties of OMVs, their use as immunogens, and their ability to elicit antibody responses against bacterial antigens. We highlight antigens from bacterial pathogens that have been successfully targeted using antibodies derived from OMV-based immunization and describe opportunities and limitations for OMVs as a platform for antimicrobial antibody development. KEY POINTS: • Outer membrane vesicles (OMVs) of gram-negative bacteria bear cell-surface molecules • OMV immunization allows rapid antibody (Ab) isolation to bacterial membrane targets • Review and analysis of OMV-based immunogens for antimicrobial Ab development.

Vaccine value profile for Salmonella enterica serovar Paratyphi A

In Asia, there are an estimated 12 million annual cases of enteric fever, a potentially fatal systemic bacterial infection caused by Salmonella enterica serovars Typhi (STy) and Paratyphi A (SPA). The recent availability of typhoid conjugate vaccines (TCV), an increasing incidence of disease caused by SPA and growing antimicrobial resistance (AMR) across the genus Salmonella makes a bivalent STy/SPA vaccine a useful public health proposition. The uptake of a stand-alone paratyphoid vaccine is likely low thus, there is a pipeline of bivalent STy/SPA candidate vaccines. Several candidates are close to entering clinical trials, which if successful should facilitate a more comprehensive approach for enteric fever control. Additionally, the World Health Organization (WHO) has made advancing the development of vaccines that protect young children and working aged adults against both agents of enteric fever a priority objective. This "Vaccine Value Profile" (VVP) addresses information related predominantly to invasive disease caused by SPA prevalent in Asia. Information is included on stand-alone SPA candidate vaccines and candidate vaccines targeting SPA combined with STy. Out of scope for the first version of this VVP is a wider discussion on the development of a universal Salmonella combination candidate vaccine, addressing both enteric fever and invasive non-typhoidal Salmonella disease, for use globally. This VVP is a detailed, high-level assessment of existing, publicly available information to inform and contextualize the public health, economic, and societal potential of pipeline vaccines and vaccine-like products for SPA. Future versions of this VVP will be updated to reflect ongoing activities such as vaccine development strategies and "Full Vaccine Value Assessment" that will inform the value proposition of an SPA vaccine. This VVP was developed by an expert working group from academia, non-profit organizations, public-private partnerships, and multi-lateral organizations as well as in collaboration with stakeholders from the WHO South-East Asian Region. All contributors have extensive expertise on various elements of the VVP for SPA and collectively aimed to identify current research and knowledge gaps.

Pentavalent outer membrane vesicles immunized mice sera confers passive protection against five prevalent pathotypes of diarrhoeagenic Escherichia coli in neonatal mice

Diarrhoeagenic Escherichia coli (DEC) pathotypes are one of the major causative agents of diarrhoea induced childhood morbidity and mortality in developing countries. Licensed vaccines providing broad spectrum protection against DEC mediated infections are not available. Outer membrane vesicles (OMVs) are microvesicles released by gram-negative bacteria during the growth phase and contain multiple immunogenic proteins. Based on prevalence of infections, we have formulated a pentavalent outer-membrane vesicles (POMVs) based immunogen targeting five main pathotypes of DEC responsible for diarrhoeal diseases. Following isolation, OMVs from five DEC pathotypes were mixed in equal proportions to formulate POMVs and 10 µg of the immunogen was intraperitoneally administered to adult BALB/c mice. Three doses of POMVs induced significant humoral immune response against whole cell lysates (WCLs), outer membrane proteins (OMPs) and lipopolysaccharides (LPS) isolated from DEC pathotypes along with significant induction of cellular immune response in adult mice. Passive transfer of POMVs immunized adult mice sera protected neonatal mice significantly against DEC infections. Overall, this study finds POMVs to be immunogenic in conferring broad-spectrum passive protection to neonatal mice against five main DEC pathotypes. Altogether, these findings suggest that POMVs can be used as a potent vaccine candidate to ameliorate the DEC-mediated health burden.

Bacterial ghost cell based bivalent candidate vaccine against Salmonella Typhi and Salmonella Paratyphi A: A prophylactic study in BALB/c mice

Typhoid and emerging paratyphoid fever are a severe enteric disease worldwide with high morbidity and mortality. Licensed typhoid vaccines are in the market, but no paratyphoid vaccine is currently available. In the present study we developed a bivalent vaccine against Salmonella Typhi and Paratyphi A using a bacterial ghost platform. Bacterial ghost cells (BGs) are bacteria-derived cell membranes without cytoplasmic contents that retain their cellular morphology, including all cell surface features. Furthermore, BGs have inherent adjuvant properties that promote an enhanced humoral and cellular immune reaction to the target antigen. Sodium hydroxide was used to prepare ghost cells of Salmonella Typhi and Paratyphi A. The bacterial ghost cells were characterised using electron microscopy. Then BALB/c mice were immunized three times (0th, 14th and 28th day) with the bivalent typhoidal bacterial ghost cells. Haematological study of adult mice throughout immunization period reflected that the immunogen was safe to administer and does not affect the animals' health. After complete immunization, we found significant serum antibody titter against whole cell lysate, outer membrane protein and lipopolysaccharide of both bacteria, and cell-mediated immunity was observed in an ex-vivo experiment. CD4+, CD8a+ and CD19+ splenic cell populations were increased in immunized animals. Bivalent Typhoidal ghost cell immunized mice showed better survival, less bacterial colonization in systemic organs, and less inflammation and/or destruction of tissue in histopathological analysis than non-immunized control mice.Serum antibodies of immunized animals can significantly inhibit bacterial motility and mucin penetration ability with better killing properties against Salmonella Typhi and Paratyphi A. Furthermore, significant passive protection was observed through the adoptive transfer of serum antibody and lymphocytes of immunized animals to naïve animals after bacterial infection. In summary, Bivalent Typhoidal Bacterial Ghost cells (BTBGs) enhances immunogenic properties and serves as a safe and effective prevention strategy against Salmonella Typhi and Paratyphi A.

A tetravalent Shigella Outer Membrane Vesicles based candidate vaccine offered cross-protection against all the serogroups of Shigella in adult mice

In today's world and mostly in low and middle income countries, S. flexneri and S. sonnei remains the major causative agent of clinical bacillary dysentery. Based on contemporary epidemiology, a tetravalent Outer Membrane Vesicle (OMVs) based immunogen was formulated using the most commonly circulating Shigella strains, namely, S. flexneri 2a, S. flexneri 3a, S. flexneri 6 and S. sonnei I, in a 1:1:1:1 ratio. Adult BALB/c mice were orally immunized in a prime-boost-boost manner. Tetravalent Shigella OMVs immunogen induced significant and persistent serum and mucosal antibodies against OMVs, Outer Membrane Proteins (OMPs) and lipopolysaccharides (LPS). Tetravalent OMVs also primed cell mediated immune response effectively. Protective efficacy against six heterologous Shigella strains was checked in an intra-peritoneal mouse model. Immunized mice survived lethal infection better than the non-immunized mice cohort with fewer replicating bacteria isolated from their gut. This study establishes the possibilities of tetravalent OMVs immunogen to become a potent vaccine candidate against human shigellosis, overcoming the limitations of sero-specific cross-protection of Shigella species.

Potential of a novel flagellin epitope as a broad-spectrum vaccine candidate against enteric fever

Relentless emergence of antibiotic resistant Salmonella strains, coupled with the drawbacks associated with currently available vaccines against enteric fever, warrants an urgent need to look for new vaccine candidates. Out of the multiple virulence factors harbored by Salmonella, flagella are regarded as one of the most important targets of innate as well as adaptive immune response. Individual Salmonella serotypes alternate between expression of two different antigenic forms encoded by fliC and fljB genes, respectively thereby employing this as a strategy to escape the host immune response. In the present study, using various immunoinformatic approaches, a flagellin epitope, present in both antigenic forms of typhoidal Salmonellae has been targeted. Following B-cell epitope and B-cell derived T-cell epitope prediction and interaction studies with major histocompatibility complexes using molecular docking, a peptide epitope was selected. Further, it was screened for its presence in majority of typhoidal serovars along with other useful attributes, in silico. Thereafter, safety studies were performed with the synthesized peptide. Subsequently, immunization studies were carried out using S. Typhi as well as S. Paratyphi A induced murine peritonitis model. Active immunization with peptide-BSA conjugate resulted in 75% and 80% mice survival following lethal challenge with S. Typhi and S. Paratyphi A respectively, along with a significant IgG antibody titer, thereby highlighting its immunogenic potential. Reduced bacterial burden in vital organs along with improved histoarchitecture and cytokine levels further substantiated the protective efficacy of the proposed candidate. Passive immunization studies with the candidate verified the protective efficacy of the generated antibodies against lethal challenge of bacteria in mice. Given the endemic nature of enteric fever and the antigenic variability observed in Salmonella serotypes, present study highlights the importance of using a vaccine candidate, which, along with generating a strong immune response, also exhibits a broad coverage against both, S. Typhi as well as S. Paratyphi A strains.

Reverse engineering approach: a step towards a new era of vaccinology with special reference to Salmonella

INTRODUCTION

Salmonella is responsible for causing enteric fever, septicemia, and gastroenteritis in humans. Due to high disease burden and emergence of multi- and extensively drug-resistant Salmonella strains, it is becoming difficult to treat the infection with existing battery of antibiotics as we are not able to discover newer antibiotics at the same pace at which the pathogens are acquiring resistance. Though vaccines against Salmonella are available commercially, they have limited efficacy. Advancements in genome sequencing technologies and immunoinformatics approaches have solved the problem significantly by giving rise to a new era of vaccine designing, i.e. 'Reverse engineering.' Reverse engineering/vaccinology has expedited the vaccine identification process. Using this approach, multiple potential proteins/epitopes can be identified and constructed as a single entity to tackle enteric fever.

AREAS COVERED

This review provides details of reverse engineering approach and discusses various protein and epitope-based vaccine candidates identified using this approach against typhoidal Salmonella.

EXPERT OPINION

Reverse engineering approach holds great promise for developing strategies to tackle the pathogen(s) by overcoming the limitations posed by existing vaccines. Progressive advancements in the arena of reverse vaccinology, structural biology, and systems biology combined with an improved understanding of host-pathogen interactions are essential components to design new-generation vaccines.

Paratyphoid Fever A: Infection and Prevention

Enteric fever is caused by Salmonella enterica serovar Typhi, Salmonella enterica serovar Paratyphi A, B, and C. While S. Typhi remains the primary causative agent of enteric fever, S. Paratyphi A is responsible for an increasing portion of enteric fever incidence. However, the current available vaccines for enteric fever are all developed from S. Typhi, and lack adequate cross immune protection against paratyphoid fever A. Therefore, paratyphoid A vaccines are urgently needed. The present paper reviews the latest progresses in pathogenesis, global burden, infection features of paratyphoid fever A, as well as the status of vaccine development, highlighting the necessity for the development of vaccines against paratyphoid fever A.

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.

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.

The need & the issues related to new-generation typhoid conjugate vaccines in India

The low- and middle-income countries bear the highest burden of typhoid fever in the world. India, along with other South Asian countries, has a significant incidence of typhoid fever among young children though there is a paucity of published data on community burden. In spite of the availability of Vi-polysaccharide (Vi-PS) and conjugated Vi-PS vaccines, these are not adequately utilized in India and in the neighbouring countries. To address many shortcomings of the unconjugated Vi-PS vaccines, typhoid conjugate vaccines (TCVs) are developed by conjugating Vi-PS with different carrier proteins. Three such vaccines using tetanus toxoid as a carrier protein are already licensed in India. Several other Vi-PS conjugates are currently in various stages of development. The current review provides an update on the existing and upcoming new TCVs along with a detailed discussion on the various issues involved with their clinical use and limitations.

Bivalent non-typhoidal Salmonella outer membrane vesicles immunized mice sera confer passive protection against gastroenteritis in a suckling mice model

Invasive non-typhoidal Salmonella (iNTS) serovars, especially Salmonella Typhimurium (ST) and Salmonella Enteritidis (SE), cause gastroenteritis worldwide. Due to the emergence of multi-drug resistance in iNTS, a broad-spectrum vaccine is urgently needed for the prevention of iNTS infection. Currently, there is no effective licensed vaccine against iNTS available in the market. We have formulated an outer membrane vesicles (OMVs) based bivalent immunogen as a vaccine candidate to generate broad-spectrum protective immunity against both recently circulating prevalent ST and SE. We have isolated OMVs from ST and SE and formulated the immunogen by mixing both OMVs (1:1 ratio). Three doses of bivalent immunogen significantly induced humoral immune responses against lipopolysaccharides (LPSs) and outer membrane proteins (OMPs) as well as a cell-mediated immune response in adult mice. We also observed that proteins of OMVs act as an adjuvant for generation of high levels of anti-LPS antibodies through T cell activation. We then characterized the one-day old suckling mice model for both ST and SE mediated gastroenteritis and used the model for a passive protection study. In the passive protection study, we found the passive transfer of bivalent OMVs immunized sera significantly reduced ST and SE mediated colonization and gastroenteritis symptoms in the colon of suckling mice compared to non-immunized sera recipients. The overall study demonstrated that OMVs based bivalent vaccine could generate broad-spectrum immunity against prevalent iNTS mediated gastroenteritis. This study also established the suckling mice model as a suitable animal model for vaccine study against iNTS mediated gastroenteritis.

Bacteria-derived membrane vesicles to advance targeted photothermal tumor ablation

Nanoscale outer membrane vesicles (OMVs) secreted by Gram-negative bacteria are often applied in antibacterial treatment as adjuvants or antigens. Recently, OMVs have also been tested in a few anti-tumor treatment studies, in which OMVs are injected multiple times to achieve certain therapeutic effects, showing risks in repeated cytokine storms. Herein, we propose the use a single low dose of OMVs combined with photothermal therapy (PTT) for effective cancer treatment. It was found that single i. v. injection of OMVs could activate the immune system by boosting the secretion levels of anti-tumor related cytokines. In addition, single i. v. injection of OMVs could also lead to extravasation of red blood cells in the tumor mainly owing to the effect of lipopolysaccharide on the OMVs. Such effect was not observed in other normal organs. As the results, the tumors on OMV-treated mice showed obviously darkened color with greatly increased intratumoral optical absorbance in the near-infrared (NIR) region, further enabling effective photothermal ablation of those tumors by the NIR laser. Without causing obvious adverse responses, bacteria-derived OMVs may be a new type of therapeutic agent for cancer treatment with multiple functions.

Mucosal vaccines and technology

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