Vaccines against botulism

Tetanus toxin and botulinum neurotoxin-derived fusion molecules are effective bivalent vaccines

Tetanus toxin (TeNT) and botulinum neurotoxins (BoNTs) are neuroprotein toxins, with the latter being the most toxic known protein. They are structurally similar and contain three functional domains: an N-terminal catalytic domain (light chain), an internal heavy-chain translocation domain (HN domain), and a C-terminal heavy chain receptor binding domain (Hc domain or RBD). In this study, fusion functional domain molecules consisting of the TeNT RBD (THc) and the BoNT/A RBD (AHc) (i.e., THc-Linker-AHc and AHc-Linker-THc) were designed, prepared, and identified. The interaction of each Hc domain and the ganglioside receptor (GT1b) or the receptor synaptic vesicle glycoprotein 2 (SV2) was explored in vitro. Their immune response characteristics and protective efficacy were investigated in animal models. The recombinant THc-linker-AHc and AHc-linker-THc proteins with the binding activity had the correct size and structure, thus representing novel subunit vaccines. THc-linker-AHc and AHc-linker-THc induced high levels of specific neutralizing antibodies, and showed strong immune protective efficacy against both toxins. The high antibody titers against the two novel fusion domain molecules and against individual THc and AHc suggested that the THc and AHc domains, as antigens in the fusion functional domain molecules, do not interact with each other and retain their full key epitopes responsible for inducing neutralizing antibodies. Thus, the recombinant THc-linker-AHc and AHc-linker-THc molecules are strong and effective bivalent biotoxin vaccines, protecting against two biotoxins simultaneously. Our experimental design will be valuable to develop recombinant double-RBD fusion molecules as potent bivalent subunit vaccines against bio-toxins. KEY POINTS: • Double-RBD fusion molecules from two toxins had the correct structure and activity. • THc-linker-AHc and AHc-linker-THc efficiently protected against both biotoxins. • Such bivalent biotoxin vaccines based on the RBD are a valuable experimental design.

Recent Developments in Vaccine Design: From Live Vaccines to Recombinant Toxin Vaccines

Vaccines are one of the most effective strategies to prevent pathogen-induced illness in humans. The earliest vaccines were based on live inoculations with low doses of live or related pathogens, which carried a relatively high risk of developing the disease they were meant to prevent. The introduction of attenuated and killed pathogens as vaccines dramatically reduced these risks; however, attenuated live vaccines still carry a risk of reversion to a pathogenic strain capable of causing disease. This risk is completely eliminated with recombinant protein or subunit vaccines, which are atoxic and non-infectious. However, these vaccines require adjuvants and often significant optimization to induce robust T-cell responses and long-lasting immune memory. Some pathogens produce protein toxins that cause or contribute to disease. To protect against the effects of such toxins, chemically inactivated toxoid vaccines have been found to be effective. Toxoid vaccines are successfully used today at a global scale to protect against tetanus and diphtheria. Recent developments for toxoid vaccines are investigating the possibilities of utilizing recombinant protein toxins mutated to eliminate biologic activity instead of chemically inactivated toxins. Finally, one of the most contemporary approaches toward vaccine design utilizes messenger RNA (mRNA) as a vaccine candidate. This approach was used globally to protect against coronavirus disease during the COVID-19 pandemic that began in 2019, due to its advantages of quick production and scale-up, and effectiveness in eliciting a neutralizing antibody response. Nonetheless, mRNA vaccines require specialized storage and transport conditions, posing challenges for low- and middle-income countries. Among multiple available technologies for vaccine design and formulation, which technology is most appropriate? This review focuses on the considerable developments that have been made in utilizing diverse vaccine technologies with a focus on vaccines targeting bacterial toxins. We describe how advancements in vaccine technology, combined with a deeper understanding of pathogen-host interactions, offer exciting and promising avenues for the development of new and improved vaccines.

Botulinum Toxin as a Biological Warfare Agent: Poisoning, Diagnosis and Countermeasures

Botulinum toxin is a neurotoxin produced by Clostridium botulinum and some other relative species. It causes a lethal disease called botulism. It can enter the body via infections by Clostridium (e.g. wound and children botulism) or by direct contact with the toxin or eating contaminated food (food-borne botulism). Botulinum toxin is also considered as a relevant biological warfare agent with an expected high number of causalities when misused for bioterrorist or military purposes. The current paper surveys the actual knowledge about botulinum toxin pathogenesis, the manifestation of poisoning, and current trends in diagnostics and therapeutics. Relevant and recent literature is summarized in this paper.

Immunological characterisation and immunoprotective efficacy of functional domain antigens of botulinum neurotoxin serotype A

Botulinum neurotoxins (BoNTs) are highly toxic proteins that mediate their effects by binding to neuronal receptors and block the neutralizing ability of therapeutic antibodies. Vaccination is currently the most effective strategy to prevent botulism. In this study, a series of recombinant functional domain antigens of BoNT/A were prepared and identified, and their immunoprotective efficacies were explored and compared. Our results showed that all antigens produced strong humoral immune responses, although their protective effects against the toxin were different. Only the Hc and HN-L antigens produced strong protective effects and afforded complete immunoprotection. In addition, the combined vaccine groups showed that there was no synergistic effect on immune responses after antigen combination, suggesting that the integrity of the toxin antigen or domain is crucial to the immune effects. Studies of the dose-dependent immunoprotective effects further confirmed that the Hc domain antigen afforded more effective protective potency than the HN-L antigen, equivalent to the immune effect of the full-length toxin (Hc + HN-L combination group). Overall, our results demonstrated that the Hc domain elicited a strong protective immune response and also provided basic data and theoretical support for the development of Hc-based BoNT/A subunit vaccine. Therefore, the receptor binding domain Hc is implicated as a promising target antigen of the BoNT/A recombinant subunit vaccine as an alternative to the toxoid vaccine.

Vaccine Production to Protect Animals Against Pathogenic Clostridia

Clostridium is a broad genus of anaerobic, spore-forming, rod-shaped, Gram-positive bacteria that can be found in different environments all around the world. The genus includes human and animal pathogens that produce potent exotoxins that cause rapid and potentially fatal diseases responsible for countless human casualties and billion-dollar annual loss to the agricultural sector. Diseases include botulism, tetanus, enterotoxemia, gas gangrene, necrotic enteritis, pseudomembranous colitis, blackleg, and black disease, which are caused by pathogenic Clostridium. Due to their ability to sporulate, they cannot be eradicated from the environment. As such, immunization with toxoid or bacterin-toxoid vaccines is the only protective method against infection. Toxins recovered from Clostridium cultures are inactivated to form toxoids, which are then formulated into multivalent vaccines. This review discusses the toxins, diseases, and toxoid production processes of the most common pathogenic Clostridium species, including Clostridiumbotulinum, Clostridiumtetani, Clostridiumperfringens, Clostridiumchauvoei, Clostridiumsepticum, Clostridiumnovyi and Clostridiumhemolyticum.

Development and evaluation of candidate subunit vaccine against botulinum neurotoxin serotype B

Botulinum neurotoxins (BoNTs) are potential biological weapons because of their high toxicity and mortality. Vaccination is an effective strategy to prevent botulism. The carboxyl-terminus of the heavy chain (Hc domain) is nontoxic and sufficient to generate protective immune responses against natural BoNTs in animals. To produce a vaccine suitable for human use, a recombinant non His-tagged isoform of the Hc domain of botulinum neurotoxin serotype B (BHc) was expressed in Escherichia coli and purified by sequential chromatography. The immunogenicity of recombinant E.coli-expressed BHc and the yeast-expressed mBHc antigens was explored and compared in Balb/c mice. BHc provided comparable protective potency but elicited significantly higher antibody titer and neutralization potency against BoNT/B after twice immunization, indicating that the recombinant BHc protein expressed in E.coli have better immunogenicity than the yeast-expressed mBHc. Moreover, a frequency and dose-dependent effect was observed in mice immunized with BHc subunit vaccine and the anti-BHc ELISA antibody titers correlated well with neutralizing antibody titers and protection potency. In summary, the Alhydrogel-formulated BHc subunit vaccine afforded effective protection against BoNT/B challenge. Therefore, the non-His-tagged and homogeneous BHc expressed in E.coli represents a good potential candidate subunit vaccine for human use.

Crystal structure of the BoNT/A2 receptor-binding domain in complex with the luminal domain of its neuronal receptor SV2C

A detailed molecular understanding of botulinum neurotoxin (BoNT)/host-cell-receptor interactions is fundamental both for developing strategies against botulism and for generating improved BoNT variants for medical applications. The X-ray crystal structure of the receptor-binding domain (H_C) of BoNT/A1 in complex with the luminal domain (LD) of its neuronal receptor SV2C revealed only few specific side-chain - side-chain interactions that are important for binding. Notably, two BoNT/A1 residues, Arg 1156 and Arg 1294, that are crucial for the interaction with SV2, are not conserved among subtypes. Because it has been suggested that differential receptor binding of subtypes might explain their differences in biological activity, we determined the crystal structure of BoNT/A2-H_C in complex with SV2C-LD. Although only few side-chain interactions are conserved between the two BoNT/A subtypes, the overall binding mode of subtypes A1 and A2 is virtually identical. In the BoNT/A2-H_C - SV2C complex structure, a missing cation-π stacking is compensated for by an additional salt bridge and an anion-π stacking interaction, which explains why the binding of BoNT/A subtypes to SV2C tolerates variable side chains. These findings suggest that motif extensions and a shallow binding cleft in BoNT/A-H_C contribute to binding specificity.

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

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

Induction of protective neutralizing antibody responses against botulinum neurotoxin serotype C using plasmid carried by PLGA nanoparticles

Botulinum neurotoxin (BoNT) is a lethal neurotoxin, for which there is currently not an approved vaccine. Recent efforts in developing vaccine candidates against botulism have been directed at the heavy chain fragment of BoNT, because antibodies against this region have been shown to prevent BoNT from binding to its receptor and thus to nerve cell surface, offering protection against BoNT intoxication. In the present study, it was shown that immunization with plasmid DNA that encodes the 50 KDa C-terminal fragment of the heavy chain of BoNT serotype C (i.e., BoNT/C-Hc50) and is carried by cationic poly (lactic-co-glycolic) acid (PLGA) nanoparticles induces stronger BoNT/C-specific antibody responses, as compared to immunization with the plasmid alone. Importantly, the antibodies have BoNT/C-neutralizing activity, protecting the immunized mice from a lethal dose of BoNT/C challenge. A plasmid DNA vaccine encoding the Hc50 fragments of BoNT serotypes that cause human botulism may represent a viable vaccine candidate for protecting against botulinum neurotoxin intoxication.

Recombinant Botulinum Toxoids: A Practical Guide for Production

Clostridium botulinum is a Gram-positive, spore-forming, anaerobic bacillus that produces a potent neurotoxin. Botulinum neurotoxins (BoNTs) are classified from serotypes A to H, and even though they have similar mechanisms of action, they show preferential hosts. In veterinary medicine, BoNT serotypes C and D are the most important, once several animal species are susceptible to them. Since BoNTs are the most potent toxins known in nature, the best way to control botulism in animals is through vaccination. However, current commercial vaccines are based on inactivated toxins (toxoids) and cells (bacterins) and present many drawbacks, such as a time-consuming production with variable antigen yield and biosafety risks. Recombinant vaccines, especially those produced by Escherichia coli expression system, have proved to be an interesting alternative to overcome these problems. E. coli is a very well-known microorganism that allows the production of large amounts of nontoxic recombinant antigens in a short period using simple culture medium reducing the production complexity and decreasing most of the biosafety risks involved in the process. We describe herein a method for the production of recombinant vaccines for veterinary medicine application, involving initial steps of gene design up to vaccine formulation and evaluation itself.

Structure of the BoNT/A1--receptor complex

Botulinum neurotoxin A causes botulism but is also used for medical and cosmetic applications. A detailed molecular understanding of BoNT/A--host receptor interactions is therefore fundamental for improving current clinical applications and for developing new medical strategies targeting human disorders. Towards this end, we recently solved an X-ray crystal structure of BoNT/A1 in complex with its neuronal protein receptor SV2C. Based on our findings, we discuss the potential implications for BoNT/A function.

Preparation of egg yolk antibodies against BoNT/B and their passive protection in mouse models

Botulism in human is a devastating intoxication caused mainly by type A, B, and E botulinum neurotoxins (BoNTs). The most effective treatment of botulism is injection of BoNT antiserum in the first 24 h. In this study, a recombinant C-terminal heavy chain of BoNT/B (BHc) was successfully expressed in E. coli. The soluble BHc was used as an antigen to immunize laying hens for yolk immunoglobulin (IgY) production. The purified IgY against BHc subunit, preincubated with the BoNT/B, was predominantly involved in the neutralization of BoNT/B toxicity. Furthermore, both intraperitoneal and intragastric administration of the IgY could protect mice from death caused by injection of toxin at a lethal dose. Our results therefore suggest that anti-BHc IgY directed to the Hc domain is effectively involved in the neutralization of BoNT/B toxin and may be considered as preventive and therapeutic intervention in the case of botulism.

Intranasal vaccination with an engineered influenza virus expressing the receptor binding subdomain of botulinum neurotoxin provides protective immunity against botulism and influenza

Influenza virus is a negative segmented RNA virus without DNA intermediate. This makes it safer as a vaccine delivery vector than most DNA viruses that have potential to integrate their genetic elements into host genomes. In this study, we developed a universal influenza viral vector, expressing the receptor binding subdomain of botulinum neurotoxin A (BoNT/A). We tested the growth characters of the engineered influenza virus in chicken eggs and Madin–Darby canine kidney epithelial cells (MDCK), and showed that it can be produced to a titer of 5 × 10⁶ plaque forming unites/ml in chicken eggs and MDCK cells. Subsequently, mice intranasally vaccinated with the engineered influenza virus conferred protection against challenge with lethal doses of active BoNT/A toxin and influenza virus. Our results demonstrated the feasibility to develop a dual purpose nasal vaccine against both botulism and influenza.

Characterization and immunological activity of different forms of recombinant secreted Hc of botulinum neurotoxin serotype B products expressed in yeast

The recombinant Hc proteins of botulinum neurotoxins and tetanus toxin are exclusively produced by intracellular heterologous expression in Pichia pastoris for use in subunit vaccines; the same Hc proteins produced by secreted heterologous expression are hyper-glycosylated and immunologically inert. Here, several different recombinant secreted Hc proteins of botulinum neurotoxin serotype B (BHc) were expressed in yeast and we characterized and assessed their immunological activity in detail. Recombinant low-glycosylated secreted BHc products (BSK) were also immunologically inert, similar to hyper-glycosylated BHc products (BSG), although deglycosylation restored their immunological activities. Unexpectedly, deglycosylated proBHc contained an unexpected pro-peptide of an α-factor signal and fortuitous N-linked glycosylation sites in the non-cleaved pro-peptide sequences, but not in the BHc sequences. Notably, a non-glycosylated secreted homogeneous BHc isoform (mBHc), which we successfully prepared after deleting the pro-peptide and removing its single potential glycosylation site, was immunologically active and could confer effective protective immunity, similarly to non-glycosylated rBHc. In summary, we conclude that a non-glycosylated secreted BHc isoform can be prepared in yeast by deleting the pro-peptide of the α-factor signal and mutating its single potential glycosylation site. This approach provides a rational and feasible strategy for the secretory expression of botulism or other toxin antigens.

Production and evaluation of a recombinant subunit vaccine against botulinum neurotoxin serotype B using a 293E expression system

Although Escherichia coli and yeast were commonly used to express recombinant Hc of botulinum neurotoxins, as an alternative, in current study, a 293E expression system was used to express the Hc of botulinum neurotoxin serotype B (BHc) as soluble recombinant protein for experimental vaccine evaluation. Our results demonstrated that the 293E expression system could produce high level of recombinant secreted BHc protein, which was immunorecognized specifically by anti-botulinum neurotoxin serotype B (BoNT/B) sera and showed ganglioside binding activities. The serological response and efficacy of recombinant BHc formulated with aluminum hydroxide adjuvant were evaluated in mice. Immunization with Alhydrogel-formulated BHc subunit vaccine afforded the effective protection against BoNT/B challenge. A frequency- and dose-dependent effect to immunization with BHc subunit vaccine was observed and the ELISA antibody titers correlated well with neutralizing antibody titers and protection. And a solid-phase assay showed that the neutralizing antibodies from the BHc-immunized mice inhibited the binding of BHc to the ganglioside GT1b. Our results also show that the plasmid pABE293SBHc derived of the 293E expression system as DNA vaccine is capable of inducing stronger humoral response and protective efficacy against BoNT/B than the pVAX1SBHc. In summary, immunization with the 293E-expressed BHc protein generates effective immune protection against BoNT/B as E. coli or yeast-expressed BHc, so the efficient expression of botulinum Hc protein for experimental vaccine can be prepared using the 293E expression system.

Botulinum neurotoxins: new questions arising from structural biology

Botulinum neurotoxins (BoNTs) are the most toxic substances known and cause botulism in vertebrates. They have also emerged as effective and powerful reagents for cosmetic and medical applications. One important prerequisite for understanding BoNT function in disease, and the further development of the toxins for cosmetic and medical applications, is a detailed knowledge of BoNT interactions with non-toxic neurotoxin-associated proteins and cell surface receptors. Based on the substantial recent progress in obtaining high-resolution crystal structures of key BoNT complexes, we summarize the major advances in understanding BoNT interactions and discuss the resulting potential implications, in particular those relating to BoNT serotype A.

Recombinant rabies virus particles presenting botulinum neurotoxin antigens elicit a protective humoral response in vivo

Botulinum neurotoxins are one of the most potent toxins found in nature, with broad medical applications from cosmetics to the treatment of various neuropathies. Additionally, these toxins are classified as Category A-Tier 1 agents, with human lethal doses calculated at as little as 90 ng depending upon the route of administration. Of the eight distinct botulinum neurotoxin serotypes, the most common causes of human illness are from serotypes /A, /B, and /E. Protection can be achieved by eliciting antibody responses against the receptor-binding domain of the neurotoxin. Our previous research has shown that recombinant rabies virus–based particles can effectively present heterologous antigens. Here, we describe a novel strategy using recombinant rabies virus particles that elicits a durable humoral immune response against the botulinum neurotoxin receptor binding domains from serotypes /A, /B, and /E. Following intramuscular administration of β-propiolactone-inactivated rabies virus particles, mice elicited specific immune responses against the cognate antigen. Administration of a combination of these vectors also demonstrated antibody responses against all three serotypes based on enzyme-linked immunosorbent assay (ELISA) measurements, with minimal decay within the study timeline. Complete protection was achieved against toxin challenge from the serotypes /A and /B and partial protection for /E, indicating that a multivalent approach is feasible.

New targets in the search for preventive and therapeutic agents for botulism

Botulism is a severe neuroparalytic disease resulting from exposure to one of the most poisonous toxins to humans. Because of this high potency and the use of toxins as biological weapons, botulism is a public health concern and each case represents an emergency. Current therapy involves respiratory supportive care and anti-toxins administration. As a preventive measure, vaccination against toxins represents an effective strategy but is undesirable due the rarity of botulism and the effectiveness of toxins in treating several neuromuscular disorders. This paper summarizes the current issues in botulism treatment and prevention, highlighting the challenge for future researches.

Botulinum neurotoxins: genetic, structural and mechanistic insights

Botulinum neurotoxins (BoNTs) are produced by anaerobic bacteria of the genus Clostridium and cause a persistent paralysis of peripheral nerve terminals, which is known as botulism. Neurotoxigenic clostridia belong to six phylogenetically distinct groups and produce more than 40 different BoNT types, which inactivate neurotransmitter release owing to their metalloprotease activity. In this Review, we discuss recent studies that have improved our understanding of the genetics and structure of BoNT complexes. We also describe recent insights into the mechanisms of BoNT entry into the general circulation, neuronal binding, membrane translocation and neuroparalysis.

DNA electroporation in rabbits as a method for generation of high-titer neutralizing antisera: examples of the botulinum toxins types A, B, and E

Raising high titer antibodies in animals is usually performed by protein immunization, which requires the long and sometimes difficult step of production of the recombinant protein. DNA immunization is an alternative to recombinant proteins, only requiring the building of an eukaryotic expression plasmid. Thanks to efficient DNA delivery techniques such as in vivo electroporation, DNA vaccination has proven useful the last few years. In this work, we have shown that it is possible to raise very high antibody titers in rabbit by DNA electroporation of an antigen encoding plasmid in the skeletal muscle with the right set of electrodes and rabbit strain. In a model of botulinum toxins types A and E, the neutralizing titers obtained after three treatments were high enough to fit the European Pharmacopeia, while it did not for type B toxin. Furthermore, the raised antibodies have high avidity and are suitable for in vitro and in vivo immunodetection of proteins.

Vaccine design: emerging concepts and renewed optimism

Arguably, vaccination represents the single most effective medical intervention ever developed. Yet, vaccines have failed to provide any or adequate protection against some of the most significant global diseases. The pathogens responsible for these vaccine-recalcitrant diseases have properties that allow them to evade immune surveillance and misdirect or eliminate the immune response. However, genomic and systems biology tools, novel adjuvants and delivery systems, and refined molecular insight into protective immunity have started to redefine the landscape, and results from recent efficacy trials of HIV and malaria vaccines have instilled hope that another golden age of vaccines may be on the horizon.