The recombinant Hc subunit of Clostridium botulinum neurotoxin serotype A is an effective botulism vaccine candidate

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.

Development and evaluation of a tetravalent botulinum vaccine

Botulinum neurotoxins (BoNTs) are the most toxic known proteins. Naturally occurring botulism in humans is caused by botulinum serotypes A, B, E, and F. Vaccination is an effective strategy to prevent botulism. In this study, a tetravalent botulinum vaccine (TBV) that can prevent serotypes A, B, E, and F was developed using the C-terminal receptor-binding domain of BoNT (Hc) as an antigen. To develop a suitable vaccine formulation, in vitro binding experiments of antigens and aluminum adjuvant in different buffers, and in vivo experiments of TBV at different antigen concentrations, were conducted. Our results showed that the optimal vaccine formulation buffer was a pH 6.0 phosphate buffer, and the suitable antigen concentration was 40 or 80 µg/ml of each antigen. A pilot-scale TBV was then prepared and evaluated for immunogenicity and stability. The results showed that TBV could elicit strong protective efficacy against each BoNT in mice, and remain effective after two years of storage at 4ºC, indicating that the preparation was stable and highly effective. Adsorption experiments also showed that the antigens could be well adsorbed by the aluminum adjuvant after 2 years of storage. Our results provide valuable experimental data supporting the development of a tetravalent botulinum vaccine, which is a promising candidate for the prevention of botulinum serotypes A, B, E, and F.

Functional EL-HN Fragment as a Potent Candidate Vaccine for the Prevention of Botulinum Neurotoxin Serotype E

Clostridium botulinum produces botulinum neurotoxin (BoNT), which is the most toxic known protein and the causative agent of human botulism. BoNTs have similar structures and functions, comprising three functional domains: catalytic domain (L), translocation domain (HN), and receptor-binding domain (Hc). In the present study, BoNT/E was selected as a model toxin to further explore the immunological significance of each domain. The EL-HN fragment (L and HN domains of BoNT/E) retained the enzymatic activity without in vivo neurotoxicity. Extensive investigations showed EL-HN functional fragment had the highest protective efficacy and contained some functional neutralizing epitopes. Further experiments demonstrated the EL-HN provided a superior protective effect compared with the EHc or EHc and EL-HN combination. Thus, the EL-HN played an important role in immune protection against BoNT/E and could provide an excellent platform for the design of botulinum vaccines and neutralizing antibodies. The EL-HN has the potential to replace EHc or toxoid as the optimal immunogen for the botulinum vaccine.

Intratracheal inoculation of AHc vaccine induces protection against aerosolized botulinum neurotoxin A challenge in mice

Botulinum neurotoxin (BoNT), produced by Clostridium botulinum, is generally known to be the most poisonous of all biological toxins. In this study, we evaluate the protection conferred by intratracheal (i.t.) inoculation immunization with recombinant Hc subunit (AHc) vaccines against aerosolized BoNT/A intoxication. Three AHc vaccine formulations, i.e., conventional liquid, dry powder produced by spray freeze drying, and AHc dry powder reconstituted in water are prepared, and mice are immunized via i.t. inoculation or subcutaneous (s.c.) injection. Compared with s.c.-AHc-immunized mice, i.t.-AHc-immunized mice exhibit a slightly stronger protection against a challenge with 30,000× LD₅₀ aerosolized BoNT/A. Of note, only i.t.-AHc induces a significantly higher level of toxin-neutralizing mucosal secretory IgA (SIgA) production in the bronchoalveolar lavage of mice. In conclusion, our study demonstrates that the immune protection conferred by the three formulations of AHc is comparable, while i.t. immunization of AHc is superior to s.c. immunization against aerosolized BoNT/A intoxication.

Engineering Botulinum Neurotoxins for Enhanced Therapeutic Applications and Vaccine Development

Botulinum neurotoxins (BoNTs) show increasing therapeutic applications ranging from treatment of locally paralyzed muscles to cosmetic benefits. At first, in the 1970s, BoNT was used for the treatment of strabismus, however, nowadays, BoNT has multiple medical applications including the treatment of muscle hyperactivity such as strabismus, dystonia, movement disorders, hemifacial spasm, essential tremor, tics, cervical dystonia, cerebral palsy, as well as secretory disorders (hyperhidrosis, sialorrhea) and pain syndromes such as chronic migraine. This review summarizes current knowledge related to engineering of botulinum toxins, with particular emphasis on their potential therapeutic applications for pain management and for retargeting to non-neuronal tissues. Advances in molecular biology have resulted in generating modified BoNTs with the potential to act in a variety of disorders, however, in addition to the modifications of well characterized toxinotypes, the diversity of the wild type BoNT toxinotypes or subtypes, provides the basis for innovative BoNT-based therapeutics and research tools. This expanding BoNT superfamily forms the foundation for new toxins candidates in a wider range of therapeutic options.

Evaluation of a recombinant tetanus toxin subunit vaccine

Tetanus is an acute, fatal disease caused by exotoxin produced by Clostridium tetani. The current vaccine against tetanus is based on inactivated tetanus toxin (TeNT). To develop a recombinant TeNT vaccine suitable for replacement of full-length tetanus toxoid (TT) vaccine for use in humans, a recombinant non-tagged isoform of the Hc domain of the tetanus toxin (THc) was expressed in Escherichia coli and purified by sequential chromatography steps. The immunogenicity and protective effect of the THc antigen were explored and compared with those of TT in Balb/c mice. The THc-based subunit vaccine provided complete protection against TeNT challenge following a high dosage as a toxoid vaccine. While the anti-THc and neutralising antibody titres were higher for the THc-based vaccine than the TT vaccine because protective epitopes are located on the THc domain. Frequency- and dose-dependent immunoprotection were also observed in THc-immunised mice. Mice immunised with one injection of 1 μg or 4 μg THc antigen were completely protected against 10² or 10³ 50% mouse lethal dose (LD₅₀) of TeNT, respectively. Furthermore, the THc protein was found to recognise and bind to ganglioside GT1b in a dose-dependent manner, and anti-THc sera antibodies also inhibited binding between THc and GT1b. Antigen on the form of recombinant non-tagged THc domain expressed in E. coli achieved strong immunoprotective potency, suggesting that it could be developed into a candidate subunit vaccine against tetanus as an alternative to the current TT vaccine.

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.

Development and evaluation of candidate subunit vaccine and novel antitoxin against botulinum neurotoxin serotype E

Botulinum neurotoxins (BoNTs) are among the most toxic proteins. Vaccination is an effective strategy to prevent botulism. To generate a vaccine suitable for human use, a recombinant non-His-tagged isoform of the Hc domain of botulinum neurotoxin serotype E (rEHc) was expressed in Escherichia coli and purified by sequential chromatography. The immunogenicity of rEHc was evaluated in mice and dose- and time-dependent immune responses were observed in both antibody titers and protective potency. Then, the pilot-scale expression and purification of rEHc were performed, and its immunological activity was characterized. Our results showed rEHc has good immunogenicity and can elicit strong protective potency against botulinum neurotoxin serotype E (BoNT/E) in mice, indicating that rEHc is an effective botulism vaccine candidate. Further, we developed a novel antitoxin against BoNT/E by purifying F(ab')₂ from pepsin-digested serum IgG of rEHc-inoculated horses. The protective effect of the F(ab')₂ antitoxin was determined in vitro and in vivo. The results showed that our F(ab')₂ antitoxin can prevent botulism in BoNT/E-challenged mice and effectively alleviate the progression of paralysis caused by BoNT/E to achieve therapeutic effects. Therefore, our results provide valuable experimental data for the production of a novel antitoxin, which is a promising candidate for the treatment of BoNT/E-induced botulism.

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.

Enhanced effects of DNA vaccine against botulinum neurotoxin serotype A by targeting antigen to dendritic cells

As dendritic cells (DCs) play a critical role in priming antigen-specific immune responses, the efficacy of DNA vaccines may be enhanced by targeting the encoded antigen proteins to DCs. In this study, we constructed a DC-targeted DNA vaccine encoding the Hc domain of botulinum neurotoxin serotype A (AHc) fused with scDEC, a single-chain Fv antibody (scFv) specific for the DC-restricted antigen-uptake receptor DEC205. Intramuscular injections of mice with the DC-targeted DNA vaccine (pVAX1-scDEC-AHc) stimulated more DCs to mature than the non-targeted DNA vaccine (pVAX1-SAHc) in the splenocytes. The DC-targeted DNA vaccine could induce more DCs maturation at the site of inoculation. The DC-targeted DNA vaccine induced stronger AHc-specific humoral immune responses, lymphocyte proliferative responses and protective potency against BoNT/A in mice than did pVAX1-SAHc. Moreover, the DC-targeting DNA vaccine provided effective protection after only two inoculations. In summary, these results showed that the DC-targeted fusion DNA vaccine could generate strong immunity, indicating that maturation of DCs induced by pVAX1-scDEC-AHc may be helpful for priming and boosting immune responses. Thus, we propose that the strategy of targeting antigen to DCs in vivo via DEC205 can enhance effectively the potency of DNA vaccines against BoNTs or other pathogens in an animal model.

Substrate-based inhibitors exhibiting excellent protective and therapeutic effects against Botulinum Neurotoxin A intoxication

Potent inhibitors to reverse Botulinum neurotoxins (BoNTs) activity in neuronal cells are currently not available. A better understanding of the substrate recognition mechanism of BoNTs enabled us to design a novel class of peptide inhibitors which were derivatives of the BoNT/A substrate, SNAP25. Through a combination of in vitro, cellular based, and in vivo mouse assays, several potent inhibitors of approximately one nanomolar inhibitory strength both in vitro and in vivo have been identified. These compounds represent the first set of inhibitors that exhibited full protection against BoNT/A intoxication in mice model with undetectable toxicity. Our findings validated the hypothesis that a peptide inhibitor targeting the two BoNT structural regions which were responsible for substrate recognition and cleavage respectively could exhibit excellent inhibitory effect, thereby providing insight on future development of more potent inhibitors against BoNTs.

In Vivo Toxicity and Immunological Characterization of Detoxified Recombinant Botulinum Neurotoxin Type A

PURPOSE

A double-mutant E224A/E262A full-length botulinum neurotoxin (BoNT) Type A with structural similarity to native BoNT/A but lacking the endopeptidase activity provides an ideal surrogate for testing pharmacokinetics and immunochemical characteristics of BoNT.

METHODS

We determined lethality (LD50) of deactivated recombinant botulinum neurotoxin (drBoNT/A) to be 24.0 μg by intraperitoneal route (i.p). The polypeptide drBoNT/A labeled with near infra-red dye 800 (NIR 800) was used to examine its distribution to different organs using whole body imaging when administered to mice via intravenous (i.v) or i.p route. Also, drBoNT/A was used to evaluate its immunogenicity in Balb/C mice model.

RESULTS

drBoNT/A was found to be highly immunogenic when tested under various in vivo conditions in Balb/C mice model. For the first time we have demonstrated that a full length 150 kDa drBoNT/A, by administering via inhalation route in mice model, has evoked both circulating immunoglobulin levels of IgG and secretory IgA at the mucosal surface. The immunoglobulin levels were sufficient enough to protect against the challenge dose of native BoNT toxin in mice model. Tissue distribution of drBoNT/A seems to be similar to that of native toxin.

CONCLUSIONS

Based on the characteristics described in this report this nontoxic holotoxin protein will assist us to explore the window of opportunity available for therapeutic treatment in case of unnatural poisoning, and also it can be an effective vaccine candidate.

Antigenic sites on the HN domain of botulinum neurotoxin A stimulate protective antibody responses against active toxin

Botulinum neurotoxins (BoNTs) are the most toxic substances known. BoNT intoxicates cells in a highly programmed fashion initiated by binding to the cell surface, internalization and enzymatic cleavage of substrate, thus, inhibiting synaptic exocytosis. Over the past two decades, immunological significance of BoNT/A C-terminal heavy chain (H_C) and light chain (L_C) domains were investigated extensively leading to important findings. In the current work, we explored the significance of BoNT/A heavy chain N-terminal (H_N) region as a vaccine candidate. Mice were immunized with recombinant H_N519–845 generating antibodies (Abs) that were found to be protective against lethal dose of BoNT/A. Immuno-dominant regions of H_N519–845 were identified and individually investigated for antibody response along with synthetic peptides within those regions, using in vivo protection assays against BoNT/A. Results were confirmed by patch-clamp analysis where anti-H_N antibodies were studied for the ability to block toxin-induced channel formation. This data strongly indicated that H_N519–593 is an important region in generating protective antibodies and should be valuable in a vaccine design. These results are the first to describe and dissect the protective activity of the BoNT/A H_N domain.

Pentavalent replicon vaccines against botulinum neurotoxins and tetanus toxin using DNA-based Semliki Forest virus replicon vectors

The clostridial neurotoxin (CNT) family includes botulinum neurotoxin (BoNT), serotypes A, B, E, and F of which can cause human botulism, and tetanus neurotoxin (TeNT), which is the causative agent of tetanus. This suggests that the greatest need is for a multivalent or multiagent vaccine that provides protection against all 5 agents. In this study, we investigated the feasibility of generating several pentavalent replicon vaccines that protected mice against BoNTs and TeNT. First, we evaluated the potency of individual replicon DNA or particle vaccine against TeNT, which induced strong antibody and protective responses in BALB/c mice following 2 or 3 immunizations. Then, the individual replicon TeNT vaccines were combined with tetravalent BoNTs vaccines to prepare 4 types of pentavalent replicon vaccines. These replicon DNA or particle pentavalent vaccines could simultaneously and effectively induce antibody responses and protect effects against the 5 agents. Finally, a solid-phase assay showed that the sera of pentavalent replicon formulations-immunized mice inhibited the binding of THc to the ganglioside GT1b as the sera of individual replicon DNA or particle-immunized mice. These results indicated these pentavalent replicon vaccines could protect against the 4 BoNT serotypes and effectively neutralize and protect the TeNT. Therefore, our studies demonstrate the utility of combining replicon DNA or particle vaccines into multi-agent formulations as potent pentavalent vaccines for eliciting protective responses against BoNTs and TeNT.

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.

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.

Production and evaluation of a recombinant chimeric vaccine against clostridium botulinum neurotoxin types C and D

Bovine botulism is a fatal disease that is caused by botulinum neurotoxins (BoNTs) produced by Clostridium botulinum serotypes C and D and that causes great economic losses, with nearly 100% lethality during outbreaks. It has also been considered a potential source of human food-borne illness in many countries. Vaccination has been reported to be the most effective way to control bovine botulism. However, the commercially available toxoid-based vaccines are difficult and hazardous to produce. Neutralizing antibodies targeted against the C-terminal fragment of the BoNT heavy chain (HC) are known to confer efficient protection against lethal doses of BoNTs. In this study, a novel recombinant chimera, consisting of Escherichia coli heat-labile enterotoxin B subunit (LTB), a strong adjuvant of the humoral immune response, fused to the HC of BoNT serotypes C and D, was produced in E. coli. Mice vaccinated with the chimera containing LTB and an equivalent molar ratio of the chimera without LTB plus aluminum hydroxide (Al(OH)3) developed 2 IU/mL of antitoxins for both serotypes. Guinea pigs immunized with the recombinant chimera with LTB plus Al(OH)3 developed a protective immune response against both BoNT/C (5 IU/mL) and BoNT/D (10 IU/mL), as determined by a mouse neutralization bioassay with pooled sera. The results achieved with guinea pig sera fulfilled the requirements of commercial vaccines for prevention of botulism, as determined by the Brazilian Ministry of Agriculture, Livestock and Food, Supply. The presence of LTB was essential for the development of a strong humoral immune response, as it acted in synergism with Al(OH)3. Thus, the vaccine described in this study is a strong candidate for the control of botulism in cattle.

The receptor binding domain of botulinum neurotoxin serotype A (BoNT/A) inhibits BoNT/A and BoNT/E intoxications in vivo

The receptor binding domain of botulinum neurotoxin (BoNT), also designated the C terminus of the heavy chain (H(C)), is a promising vaccine candidate against botulism. In this study, a highly efficient expression system for the protein was developed in Escherichia coli, which provided yields that were 1 order of magnitude higher than those reported to date (350 mg H(C) per liter). The product was highly immunogenic, protecting mice from a challenge with 10(5) 50% lethal dose (LD(50)) after a single vaccination and generating a neutralizing titer of 49.98 IU/ml after three immunizations. In addition, a single boost with HC increased neutralizing titers by up to 1 order of magnitude in rabbits hyperimmunized against toxoid. Moreover, we demonstrate here for the first time in vivo inhibition of BoNT/A intoxication by H(C)/A, presumably due to a blockade of the neurotoxin protein receptor SV2. Administration of HC/A delayed the time to death from 10.4 to 27.3 h in mice exposed to a lethal dose of BoNT/A (P = 0.0005). Since BoNT/A and BoNT/E partially share SV2 isoforms as their protein receptors, the ability of H(C)/A to cross-inhibit BoNT/E intoxication was evaluated. The administration of H(C)/A together with BoNT/E led to 50% survival and significantly delayed the time to death for the nonsurviving mice (P = 0.003). Furthermore, a combination of H(C)/A and a subprotective dose of antitoxin E fully protected mice against 850 mouse LD(50) of BoNT/E, suggesting complementary mechanisms of protection consisting of toxin neutralization by antibodies and receptor blocking by H(C)/A.

Potent tetravalent replicon vaccines against botulinum neurotoxins using DNA-based Semliki Forest virus replicon vectors

Human botulism is commonly associated with botulinum neurotoxin (BoNT) serotypes A, B, E and F. This suggests that the greatest need is for a tetravalent vaccine that provides protection against all four of these serotypes. In current study, we investigated the feasibility of generating several tetravalent vaccines that protected mice against the four serotypes. Firstly, monovalent replicon vaccine against BoNT induced better antibody response and protection than that of corresponding conventional DNA vaccine. Secondly, dual-expression DNA replicon pSCARSE/FHc or replicon particle VRP-E/FHc vaccine was well resistant to the challenge of BoNT/E and BoNT/F mixture as a combination vaccine composed of two monovalent replicon vaccines. Finally, the dual-expression DNA replicon or replicon particle tetravalent vaccine could simultaneously and effectively neutralize and protect the four BoNT serotypes. Protection correlated directly with serum ELISA titers and neutralization antibody levels to BoNTs. Therefore, replicon-based DNA or particle might be effective vector to develop BoNT vaccines, which might be more desirable for use in clinical application than the conventional DNA vaccines. Our studies demonstrate the utility of combining dual-expression DNA replicon or replicon particle vaccines into multi-agent formulations as potent tetravalent vaccines for eliciting protective responses to four serotypes of BoNTs.

Oral vaccination with an adenovirus-vectored vaccine protects against botulism

We have previously shown that an adenovirus vectored vaccine delivered intramuscularly or intranasally was effective in protection against botulism in a mouse model. The adenoviral vector encodes a human codon-optimized heavy chain C-fragment (H(C)50) of botulinum neurotoxin type C (BoNT/C). Here, we evaluate the same vaccine candidate as an oral vaccine against BoNT/C in a mouse model. To elicit protective immunity, the mice were orally vaccinated with a single dose of 1×10(4) to 1×10(7)plaque forming units (pfu) of the adenoviral vector. The immune sera, collected six weeks after oral vaccination with 2×10(7)pfu adenovirus, have shown an ability to neutralize the biological activity of BoNT/C in vitro. Additionally, animals receiving a single dose of 2×10(6)pfu adenovirus or greater were completely protected against challenge with 100×MLD(50) of BoNT/C. The data demonstrated the feasibility to develop an adenovirus-based oral vaccine against botulism.

Enhanced potency of replicon vaccine using one vector to simultaneously co-express antigen and interleukin-4 molecular adjuvant

We evaluated the utility of interleukin-4 (IL-4) as molecular adjuvant of replicon vaccines for botulinum neurotoxin serotype A (BoNT/A) in mouse model. In both Balb/c and C57/BL6 mice that received the plasmid DNA replicon vaccines derived from Semliki Forest virus (SFV) encoding the Hc gene of BoNT/A (AHc), the immunogenicity was significantly modulated and enhanced by co-delivery or co-express of the IL-4 molecular adjuvant. The enhanced potencies were also produced by co-delivery or co-expression of the IL-4 molecular adjuvant in mice immunized with the recombinant SFV replicon particles (VRP) vaccines. In particular, when AHc and IL-4 were co-expressed within the same replicon vaccine vector using dual-expression or bicistronic IRES, the anti-AHc antibody titers, serum neutralization titers and survival rates of immunized mice after challenged with BoNT/A were significantly increased. These results indicate IL-4 is an effective Th2-type adjuvant for the replicon vaccines in both strain mice, and the co-expression replicon vaccines described here may be an excellent candidate for further vaccine development in other animals or humans. Thus, we described a strategy to design and develop efficient vaccines against BoNT/A or other pathogens using one replicon vector to simultaneously co-express antigen and molecular adjuvant.

High sensitivity ELISA for detection of botulinum neurotoxin serotype F

Botulinum neurotoxins (BoNTs) are classified as category A biological threat agents by the Centers for Disease Control and Prevention (CDC) in the United States for its hazardous and potential bioterrorist threat to the public. About 1% naturally occurring botulisms are caused by Botulinum neurotoxin serotype F (BoNT/F). Most of the immunoassays for detecting BoNTs focus on the serotypes A and B, but few methods have been established for the detection of BoNT/F. Recently, the recombinant Hc subunit of botulinum neurotoxin type F (rFHc) was expressed as an effective vaccine against BoNT/F, indicating that this rFHc could be an effective immunogen to raise monoclonal antibodies (MAbs) for the detection and neutralization of BoNT/F. Here we present a novel sandwich enzyme-linked immunosorbent assay (ELISA) based on two MAbs against rFHc, which were FMMU-BTF-8 and FMMU-BTF-29 as capture antibody and detection antibody, respectively. The limit of detection (LOD) of this ELISA reached 12.09 pg/mL, much less than that of the other reported immunoassays. A simple, sensitive ELISA for detecting and quantifying BoNT/F was established, which can be used as a valuable method to detect and quantify BoNT/F.

Three types of human CpG motifs differentially modulate and augment immunogenicity of nonviral and viral replicon DNA vaccines as built-in adjuvants

NakedDNA vaccines given by intramuscular injection are efficient in mouse models, but they require improvement for human use. As the immunogenicity of DNA vaccines depends, to a large extent, on the presence of CpG motifs as built-in adjuvants, we addressed this issue by inserting three types of human CpG motifs (A-type, B-type, and C-type) into the backbone of nonviral DNA and viral DNA replicon vectors with distinct immunostimulatory activities on human PBMCs. The adjuvant effects of CpG modifications in DNA vaccines expressing three types of antigens (β-Gal, AHc, or PA4) were then characterized in mice and found to significantly enhance antigen-specific humoral and cell-mediated immune responses. The three types of CpG motifs also differentially affected and modulated immune responses and protective potency against botulinum neurotoxin serotype A and Bacillus anthracis A16R challenge. Taken together, these results demonstrate that insertion of human CpG motifs can differentially modulate the immunogenicity of nonviral DNA vaccines as well as viral DNA replicon vaccines. Our study provides not only a better understanding of the in vivo activities of CpG motif adjuvants but implications for the rational design of such motifs as built-in adjuvants for DNA vectors targeting specific antigens.

Epitope mapping of botulinum neurotoxins light chains

Botulinum neurotoxins (BoNTs) are listed among the most potent biothreat agents. Simultaneously, two out of seven known serotypes of these toxins are used in medicine and cosmetics. This situation calls for development of detailed epitope maps of these toxins. Such maps will help to develop new ways for decreasing damage caused by these toxins if they were to be used as weapons while retaining the therapeutic effect of these toxins used as medicine. Here, we used a library of random fragments of DNA encoding the catalytic domain of botulinum neurotoxin serotype A to identify short epitope-forming sequences. We demonstrated that knowledge of such sequences in a BoNT of one serotype can be used for identification of epitope-forming sequences in other serotypes of BoNTs. We also demonstrated a serodiagnostic value of identified sequences and their ability to retain epitope-specific structures and trigger production of corresponding antibodies, even when they are transferred into a background of a completely alien carrier protein.

Development of highly sensitive chemiluminescence enzyme immunoassay based on the anti-recombinant H(C) subunit of botulinum neurotoxin type A monoclonal antibodies

Botulinum neurotoxins (BoNTs) are the most poisonous substances ever known. The early detection of these toxins could bear more time for appropriate medical intervention. The standard method for detecting BoNTs is the mouse bioassay, which is time consuming (up to 4 days) and requires a large number of laboratory animals. The immunologic detection methods could detect the toxins within a day, but most of these methods are less sensitive compared with the mouse bioassay due to the lack of high-affinity antibodies. Recently, the recombinant H(C) subunit of botulinum neurotoxin type A (rAH(C)) was expressed as an effective vaccine against botulism, indicating that the rAH(C) could be an effective immunogen that raises the monoclonal antibody (mAb) for detecting BoNT/A. After immunized BALB/c mice with rAH(C), 56 mAbs were generated. Two of these mAbs were selected to establish a highly sensitive sandwich chemiluminescence enzyme immunoassay (CLEIA), in which FMMU-BTA-49 and FMMU-BTA-22 were used as capture antibody and detection antibody, respectively. The calculated limit of detection (LOD) based on molecular weight of rAH(C) and BoNT/A reached 0.45 pg mL(-1). This CLEIA can be used in the detection of BoNT/A in matrices such as milk and beef extract. This method has 20-40 fold lower LOD than that of the mouse bioassay and takes only 3 h to complete the detection, indicating that it can be used as a valuable method to detect and quantify BoNT/A.

Co-expression of tetanus toxin fragment C in Escherichia coli with thioredoxin and its evaluation as an effective subunit vaccine candidate

The receptor-binding domain of tetanus toxin (THc), which mediates the binding of the toxin to the nerve cells, is a candidate subunit vaccine against tetanus. In this study one synthetic gene encoding the THc was constructed and highly expressed in Escherichia coli by co-expression with thioredoxin (Trx). The purified THc-vaccinated mice were completely protected against an active toxin challenge in mouse models of disease and the potency of two doses of THc was comparable to that of three doses of toxoid vaccine. And a solid-phase assay showed that the anti-THc sera inhibited the binding of THc or toxoid to the ganglioside GT1b as the anti-tetanus toxoid sera. Furthermore, mice were vaccinated once or twice at four different dosages of THc and a dose-response was observed in both the antibody titer and protective efficacy with increasing dosage of THc and number of vaccinations. The data presented in the report showed that the recombinant THc expressed in E. coli is efficacious in protecting mice against challenge with tetanus toxin suggesting that the THc protein may be developed into a human subunit vaccine candidate designed for the prevention of tetanus.

Enhancement of the immunogenicity of DNA replicon vaccine of Clostridium botulinum neurotoxin serotype A by GM-CSF gene adjuvant

Granulocyte-macrophage clony-stimulating factor (GM-CSF) is an attractive adjuvant for a DNA vaccine on account of its ability to recruit antigen-presenting cells to the site of antigen synthesis as well as stimulate the maturation of dendritic cells.This study evaluated the utility of GM-CSF as a plasmid DNA replicon vaccine adjuvants for botulinum neurotoxin serotype A (BoNT/A) in mouse model. In balb/c mice that received the plasmid DNA replicon vaccines derived from Semliki Forest virus (SFV) carrying the Hc gene of BoNT/A (AHc), both antibody and lymphoproliferative response specific to AHc were induced, the immunogenicity was enhanced by co-delivery or coexpress of the GM-CSF gene. In particular, when AHc and GM-CSF were coexpressed within the SFV based DNA vaccine, the anti-AHc antibody titers and survival rates of immunized mice after challenged with BoNT/A were significantly increased, and further enhanced by coimmunization with aluminum phosphate adjuvant.

Immunization of mice with the non-toxic HC50 domain of botulinum neurotoxin presented by rabies virus particles induces a strong immune response affording protection against high-dose botulinum neurotoxin challenge

We previously showed that rabies virus (RABV) virions are excellent vehicles for antigen presentation. Here, a reverse genetic approach was applied to generate recombinant RABV that express a chimeric protein composed of the heavy chain carboxyterminal half (HC50) of botulinum neurotoxin type A (BoNT/A) and RABV glycoprotein (G). To promote surface expression and incorporation of HC50/A into RABV virions, the RABV glycoprotein (G) ER translocation sequence, various fragments of RABV ectodomain (ED) and cytoplasmic domain were fused to HC50/A. The HC50/A chimeric proteins were expressed on the surface of cells infected with all of the recombinant RABVs, however, the highest level of surface expression was detected by utilizing 30 amino acids of the RABV G ED (HV50/A-E30). Our results also indicated that this chimeric protein was effectively incorporated into RABV virions. Immunization of mice with inactivated RABV-HC50/A-E30 virions induced a robust anti-HC50/A IgG antibody response that efficiently neutralized circulating BoNT/A in vivo, and protected mice against 1000 fold the lethal dose of BoNT/A.

Enhanced potency of individual and bivalent DNA replicon vaccines or conventional DNA vaccines by formulation with aluminum phosphate

DNA vaccines against botulinum neurotoxin (BoNTs) induce protective humoral immune responses in mouse model, but when compared with conventional vaccines such as toxoid and protein vaccines, DNA vaccines often induce lower antibody level and protective efficacy and are still necessary to increase their potency. In this study we evaluated the potency of aluminum phosphate as an adjuvant of DNA vaccines to enhance antibody responses and protective efficacy against botulinum neurotoxin serotypes A and B in Balb/c mice. The administration of these individual and bivalent plasmid DNA replicon vaccines against botulinum neurotoxin serotypes A and B in the presence of aluminum phosphate improved both antibody responses and protective efficacy. Furthermore, formulation of conventional plasmid DNA vaccines encoding the same Hc domains of botulinum neurotoxin serotypes A and B with aluminum phosphate adjuvant increased both antibody responses and protective efficacy. These results indicate aluminum phosphate is an effective adjuvant for these two types of DNA vaccines (i.e., plasmid DNA replicon vaccines and conventional plasmid DNA vaccines), and the vaccine formulation described here may be an excellent candidate for further vaccine development against botulinum neurotoxins.

Principles of antidote pharmacology: an update on prophylaxis, post-exposure treatment recommendations and research initiatives for biological agents

The use of biological agents has generally been confined to military-led conflicts. However, there has been an increase in non-state-based terrorism, including the use of asymmetric warfare, such as biological agents in the past few decades. Thus, it is becoming increasingly important to consider strategies for preventing and preparing for attacks by insurgents, such as the development of pre- and post-exposure medical countermeasures. There are a wide range of prophylactics and treatments being investigated to combat the effects of biological agents. These include antibiotics (for both conventional and unconventional use), antibodies, anti-virals, immunomodulators, nucleic acids (analogues, antisense, ribozymes and DNAzymes), bacteriophage therapy and micro-encapsulation. While vaccines are commercially available for the prevention of anthrax, cholera, plague, Q fever and smallpox, there are no licensed vaccines available for use in the case of botulinum toxins, viral encephalitis, melioidosis or ricin. Antibiotics are still recommended as the mainstay treatment following exposure to anthrax, plague, Q fever and melioidosis. Anti-toxin therapy and anti-virals may be used in the case of botulinum toxins or smallpox respectively. However, supportive care is the only, or mainstay, post-exposure treatment for cholera, viral encephalitis and ricin - a recommendation that has not changed in decades. Indeed, with the difficulty that antibiotic resistance poses, the development and further evaluation of techniques and atypical pharmaceuticals are fundamental to the development of prophylaxis and post-exposure treatment options. The aim of this review is to present an update on prophylaxis and post-exposure treatment recommendations and research initiatives for biological agents in the open literature from 2007 to 2009.

Development and evaluation of candidate vaccine and antitoxin against botulinum neurotoxin serotype F

To produce a vaccine suitable for human use, a recombinant non His-tagged isoform of the Hc domain of botulinum neurotoxin serotype F (rFHc) was expressed in Escherichia coli and purified by sequential chromatography. The rFHc was evaluated as a subunit vaccine candidate in mouse model of botulism. A dose-response was observed in both antibody titer and protective efficacy with increasing dosage of rFHc and number of vaccinations. These findings suggest that the rFHc is an effective botulism vaccine candidate. Further, we developed a new antitoxin against botulinum neurotoxin serotype F (BoNT/F) by purifying F(ab')(2) fragments from pepsin digested serum IgGs of horses inoculated with rFHc. The protective effect of the F(ab')(2) antitoxin against BoNT/F was determined both in vitro and in vivo. The results showed that the F(ab')(2) antitoxin could prevent botulism in mice challenged with BoNT/F and effectively delayed progression of paralysis from botulism in the therapeutic setting. Thus, our results provide valuable experimental data for this new antitoxin as a potential candidate for treatment of botulism caused by BoNT/F.

Development and preclinical evaluation of a new F(ab')₂ antitoxin against botulinum neurotoxin serotype A

Concern about the malicious applications of botulinum neurotoxin has highlighted the need for a new generation of safe and highly potent antitoxins. In this study, we developed and evaluated the preclinical pharmacology and safety of a new F(ab')₂ antitoxin against botulinum neurotoxin serotype A (BoNT/A). As an alternative to formalin-inactivated toxoid, the recombinant Hc domain of botulinum neurotoxin serotype A (rAHc) was used to immunize horses, and the IgGs from the hyperimmune sera were digested to obtain F(ab')₂ antitoxin. The protective effect of the new F(ab')₂ antitoxin against BoNT/A was determined both in vitro and in vivo. The results showed that the F(ab')₂ antitoxin could prevent botulism in mice challenged with BoNT/A and effectively delayed progression of paralysis from botulism in the therapeutic setting. The preclinical safety of the new F(ab')₂ antitoxin was also evaluated, and it showed neither harmful effects on vital functions nor adverse effects such as acute toxicity, or immunological reactions in mice and dogs. Thus, our results provide valuable experimental data for this new antitoxin as a potential candidate for treatment of botulism caused by BoNT/A, and our findings support the safety of the new F(ab')₂ antitoxin for clinical use. Our study further demonstrates the proof of concept for development of a similar strategy for obtaining potent antitoxin against other BoNT serotypes.

Efficacy of a potential trivalent vaccine based on Hc fragments of botulinum toxins A, B, and E produced in a cell-free expression system

Botulinum toxins produced by the anaerobic bacterium Clostridium botulinum are the most potent biological toxins in nature. Traditionally, people at risk are immunized with a formaldehyde-inactivated toxin complex. Second generation vaccines are based on the recombinant carboxy-terminal heavy-chain (Hc) fragment of the neurotoxin. However, the materialization of this approach is challenging, mainly due to the high AT content of clostridial genes. Herein, we present an alternative strategy in which the native genes encoding Hc proteins of botulinum toxins A, B, and E were used to express the recombinant Hc fragments in a cell-free expression system. We used the unique property of this open system to introduce different combinations of chaperone systems, protein disulfide isomerase (PDI), and reducing/oxidizing environments directly to the expression reaction. Optimized expression conditions led to increased production of soluble Hc protein, which was successfully scaled up using a continuous exchange (CE) cell-free system. Hc proteins were produced at a concentration of more than 1 mg/ml and purified by one-step Ni(+) affinity chromatography. Mice immunized with three injections containing 5 microg of any of the in vitro-expressed, alum-absorbed, Hc vaccines generated a serum enzyme-linked immunosorbent assay (ELISA) titer of 10(5) against the native toxin complex, which enabled protection against a high-dose toxin challenge (10(3) to 10(6) mouse 50% lethal dose [MsLD(50)]). Finally, immunization with a trivalent HcA, HcB, and HcE vaccine protected mice against the corresponding trivalent 10(5) MsLD(50) toxin challenge. Our results together with the latest developments in scalability of the in vitro protein expression systems offer alternative routes for the preparation of botulinum vaccine.