Vaccination of mice with DNA encoding a large fragment of botulinum neurotoxin serotype A

Intradermal immunization with botulinum neurotoxin serotype E DNA vaccine induces humoral and cellular immunity and protects against lethal toxin challenge

Botulinum neurotoxins (BoNTs) produced by the spore-forming, gram-positive, anaerobic bacterium Clostridium botulinum are the most toxic substances known and cause botulism, flaccid paralysis, or death. Owing to their high lethality, BoNTs are classified as category A agents by the Centers for Disease Control (CDC). Currently, there are no vaccines available to protect against BoNTs, so the rapid development of a safe and effective vaccine is important. DNA-based vaccines have recently drawn great attention because they can be developed quickly and can be applied in mass vaccination strategies to prevent disease outbreaks. Here, we report on the immunogenic and protective efficacy of a DNA vaccine, encoding a 50-kDa carboxy-terminal fragment of the BoNT serotype E heavy chain, which is delivered via an intradermal route. This plasmid DNA vaccine induced robust humoral and cellular BoNT/E-specific immune responses and completely protected animals against lethal challenge with BoNT/E. These results not only indicate that DNA vaccines could be further developed as safe and effective candidates for vaccines against BoNTs but also suggest a possible approach for developing vaccines that protect against bio-threat toxins.

Vaccines against Botulism

Botulinum neurotoxins (BoNT) cause the flaccid paralysis of botulism by inhibiting the release of acetylcholine from motor neurons. There are seven serotypes of BoNT (A-G), with limited therapies, and no FDA approved vaccine for botulism. An investigational formalin-inactivated penta-serotype-BoNT/A-E toxoid vaccine was used to vaccinate people who are at high risk of contracting botulism. However, this formalin-inactivated penta-serotype-BoNT/A-E toxoid vaccine was losing potency and was discontinued. This article reviews the different vaccines being developed to replace the discontinued toxoid vaccine. These vaccines include DNA-based, viral vector-based, and recombinant protein-based vaccines. DNA-based vaccines include plasmids or viral vectors containing the gene encoding one of the BoNT heavy chain receptor binding domains (HC). Viral vectors reviewed are adenovirus, influenza virus, rabies virus, Semliki Forest virus, and Venezuelan Equine Encephalitis virus. Among the potential recombinant protein vaccines reviewed are HC, light chain-heavy chain translocation domain, and chemically or genetically inactivated holotoxin.

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.

Induction of Acute Lung Injury after Intranasal Administration of Toxin Botulinum A Complex

The inhalation of aerozolized botulinum toxin may represent a potential significant hazard to both military and civilian personnel. Since the lung is the primary target organ for inhaled toxin, the investigation reported herein was conducted to examine lung function in mice exposed to botulinum toxin A complex by intranasal route. Data includes lethality, symptomatology, measurement of respiratory function (minute ventilation, respiratory frequency, and tidal volume), and histopathology of the lungs. The clinical signs of intoxication are similar to those observed in foodborne botulism. Plethysmography revealed severe impairment of all respiratory parameters tested from 7 hours postexposure. Severe lung lesions, possibly secondary to the intoxication, were observed in mice who survived 14 days after the toxin challenge. These included intra-alveolar hemorrhage and interstitial edema. Mice immunized by the pentavalent (ABCDE) toxoid were protected against the neurotoxin (4 LD50) as revealed by the decrease of lethality and severity of nervous signs of intoxication, but not against histopathological changes in the lungs. These effects are nonspecific and require further experiments in order to specify the relationships between the pathology and the inflammatory process in the lung due to mediators such as cytokines, and possibly permanent physiological sequelae.

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.

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.

Progress toward development of an inhalation vaccine against botulinum toxin

The looming threat of bioterrorism has enhanced interest in the development of vaccines against agents such as botulinum toxin. This in turn has stimulated efforts to create vaccines that are effective by the oral and inhalation routes. Recently, considerable progress has been made in creating an inhalation vaccine against botulism. This work stems from the discovery that a polypeptide that represents a third of the toxin molecule retains the ability to be adsorbed from the airway and to evoke an immune response but retains none of the adverse effects of the native toxin. Interestingly, this polypeptide can also serve as a carrier molecule in the creation of inhalation vaccines against other pathogens.

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.

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.

Sublingual immunization with adenovirus F protein-based vaccines stimulates protective immunity against botulinum neurotoxin A intoxication

Sublingual (s.l.) vaccination is an efficient way to induce elevated levels of systemic and mucosal immune responses. To mediate mucosal uptake, ovalbumin (OVA) was genetically fused to adenovirus 2 fiber protein (OVA-Ad2F) to assess whether s.l. immunization was as effective as an alternative route of vaccination. Ad2F-delivered vaccines were efficiently taken up by dendritic cells and migrated mostly to submaxillary gland lymph nodes, which could readily stimulate OVA-specific CD4(+) T cells. OVA-Ad2F + cholera toxin (CT)-immunized mice elicited significantly higher OVA-specific serum IgG, IgA and mucosal IgA antibodies among the tested immunization groups. These were supported by elevated OVA-specific IgG and IgA antibody-forming cells. A mixed T(h)-cell response was induced as evident by the enhanced IL-4, IL-10, IFN-γ and TNF-α-specific cytokine-forming cells. To assess whether this approach can stimulate neutralizing antibodies, immunizations were performed with the protein encumbering the β-trefoil domain of C-terminus heavy chain (Hcβtre) from botulinum neurotoxin A (BoNT/A) as well as when fused to Ad2F. Hcβtre-Ad2F + CT-dosed mice showed the greatest serum IgG, IgA and mucosal IgA titers among the immunization groups. Hcβtre-Ad2F alone also induced elevated antibody production in contrast to Hcβtre alone. Plasma from Hcβtre + CT- and Hcβtre-Ad2F + CT-immunized groups neutralized BoNT/A and protected mice from BoNT/A intoxication. Most importantly, Hcβtre-Ad2F + CT-immunized mice were protected from BoNT/A intoxication relative to Hcβtre + CT-immunized mice, which only showed ∼60% protection. This study shows that s.l. immunization with Ad2F-based vaccines is effective in conferring protective immunity.

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.

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.

Adenovirus F protein as a delivery vehicle for botulinum B

Background

Immunization with recombinant carboxyl-terminal domain of the heavy chain (Hc domain) of botulinum neurotoxin (BoNT) stimulates protective immunity against native BoNT challenge. Most studies developing a botulism vaccine have focused on the whole Hc; however, since the principal protective epitopes are located within β-trefoil domain (Hcβtre), we hypothesize that immunization with the Hcβtre domain is sufficient to confer protective immunity. In addition, enhancing its uptake subsequent to nasal delivery prompted development of an alternative vaccine strategy, and we hypothesize that the addition of targeting moiety adenovirus 2 fiber protein (Ad2F) may enhance such uptake during vaccination.

Results

The Hcβtre serotype B immunogen was genetically fused to Ad2F (Hcβtre/B-Ad2F), and its immunogenicity was tested in mice. In combination with the mucosal adjuvant, cholera toxin (CT), enhanced mucosal IgA and serum IgG Ab titers were induced by nasal Hcβtre-Ad2F relative to Hcβtre alone; however, similar Ab titers were obtained upon intramuscular immunization. These BoNT/B-specific Abs induced by nasal immunization were generally supported in large part by Th2 cells, as opposed to Hcβtre-immunized mice that showed more mixed Th1 and Th2 cells. Using a mouse neutralization assay, sera from animals immunized with Hcβtre and Hcβtre-Ad2F protected mice against 2.0 LD₅₀.

Conclusion

These results demonstrate that Hcβtre-based immunogens are highly immunogenic, especially when genetically fused to Ad2F, and Ad2F can be exploited as a vaccine delivery platform to the mucosa.

Generation of high-titer neutralizing antibodies against botulinum toxins A, B, and E by DNA electrotransfer

Botulinum neurotoxins are known to be among the most toxic known substances. They produce severe paralysis by preventing the release of acetylcholine at the neuromuscular junction. Thus, new strategies for efficient production of safe and effective anti-botulinum neurotoxin antisera have been a high priority. Here we describe the use of DNA electrotransfer into the skeletal muscle to enhance antiserum titers against botulinum toxin serotypes A, B, and E in mice. We treated animals with codon-optimized plasmid DNA encoding the nontoxic but highly immunogenic C-terminal heavy chain fragment of the toxin. By employing both codon optimization and the electrotransfer procedure, the immune response and corresponding neutralizing antiserum titers were markedly increased. The cellular localization of the antigen and the immunization regimens were also shown to increase neutralizing titers to >100 IU/ml. This study demonstrates that DNA electrotransfer is an effective procedure for raising neutralizing antiserum titers to remarkably high levels.

The strange case of the botulinum neurotoxin: using chemistry and biology to modulate the most deadly poison

In the classic novella "The Strange Case of Dr. Jekyll and Mr. Hyde", Robert Louis Stevenson paints a stark picture of the duality of good and evil within a single man. Botulinum neurotoxin (BoNT), the most potent known toxin, possesses an analogous dichotomous nature: It shows a pronounced morbidity and mortality, but it is used with great effect in much lower doses in a wide range of clinical scenarios. Recently, tremendous strides have been made in the basic understanding of the structure and function of BoNT, which have translated into widespread efforts towards the discovery of biomacromolecules and small molecules that specifically modulate BoNT activity. Particular emphasis has been placed on the identification of inhibitors that can counteract BoNT exposure in the event of a bioterrorist attack. This Review summarizes the current advances in the development of therapeutics, including vaccines, peptides, and small-molecule inhibitors, for the prevention and treatment of botulism.

Production and characterization of monoclonal antibody to botulinum neurotoxin type B light chain by phage display

A monoclonal antibody to the light chain of botulinum neurotoxin type B (BoNT/B) was generated and its protective activity was evaluated in vivo. A chimeric rabbit/human Fab library was generated using bone marrow and spleen cDNAs of rabbits immunized with the BoNT/B light chain, and three monoclonal antibodies specific to the catalytic domain of BoNT/B were isolated. One of these clones, BCXRH1, was specific to a conformation-dependent epitope, and partially neutralized the BoNT/B complex in vivo.

Botulism: cause, effects, diagnosis, clinical and laboratory identification, and treatment modalities

Botulism is a neuroparalytic disease caused by neurotoxins produced by the bacteria Clostridium botulinum. Botulinum neurotoxins (BoNTs) are among the most potent naturally occurring toxins and are a category A biological threat agent. The 7 toxin serotypes of BoNTs (serotypes A-G) have different toxicities, act through 3 different intracellular protein targets, and exhibit different durations of effect. Botulism may follow ingestion of food contaminated with BoNT, from toxin production of C botulinum present in the intestine or wounds, or from inhalation of aerosolized toxin. Intoxication classically presents as an acute, symmetrical, descending flaccid paralysis. Early diagnosis is important because antitoxin therapy is most effective when administered early. Confirmatory testing of botulism with BoNT assays or C botulinum cultures is time-consuming, and may be insensitive in the diagnosis of inhalational botulism and in as many as 32% of food-borne botulism cases. Therefore, the decision to initiate botulinum antitoxin therapy is primarily based on symptoms and physical examination findings that are consistent with botulism, with support of epidemiological history and electrophysiological testing. Modern clinical practice and antitoxin treatment has reduced botulism mortality rates from approximately 60% to < or =10%. The pentavalent botulinum toxoid is an investigational product and has been used for more than 45 years in at-risk laboratory workers to protect against toxin serotypes A to E. Due to declining immunogenicity and potency of the pentavalent botulinum toxoid, novel vaccine candidates are being developed.

Enhanced immune responses using plasmid DNA replicon vaccine encoding the Hc domain of Clostridium botulinum neurotoxin serotype A

In current study, the immunogenicity of a plasmid DNA replicon vaccine (pSCARSHc) encoding the Hc domain of Clostridium botulinum neurotoxin serotype A (AHc) was investigated and compared with a conventional plasmid DNA vaccine (pcDNASHc) encoding the same antigen. In vitro, pSCARSHc incorporating Semliki Forest virus (SFV) replicon could express AHc protein and induce apoptosis of transfected cells. Comparison with the conventional plasmid DNA vaccine (pcDNASHc) yielded several interesting results. First, our self-designed pSCARSHc could induce relatively higher AHc-specific antibodies and lymphocyte proliferative responses in immunized Balb/c mice, especially at low doses. Second, while both pSCARSHc and pcDNASHc induced Th2-type immune responses, the ratio of IgG1 to IgG2a was lower in pSCARSHc groups and the Th2- and Th1-type humoral immune responses induced by pSCARSHc were also stronger than that of the pcDNASHc vaccine. Third, it was shown that the sera from pSCARSHc-vaccinated mice conferred more efficient protection than those from pcDNASHc-vaccinated mice by BoNT/A neutralization assay. Finally, mice immunized with pSCARSHc could also elicit more efficient protection against BoNT/A than pcDNASHc. These results indicate that our plasmid DNA replicon vaccine can provide strong immunogenicity and should be a potential alternative strategy to conventional DNA vaccines in developing an efficacious vaccine against C. botulinum neurotoxin serotype A.

Protective immunity against botulism provided by a single dose vaccination with an adenovirus-vectored vaccine

Botulinum neurotoxins cause botulism, a neuroparalytic disease in humans and animals. We constructed a replication-incompetent adenovirus encoding a synthesized codon-optimized gene for expression of the heavy chain C-fragment (H(C)50) of botulinum neurotoxin type C (BoNT/C). This recombinant human serotype 5 adenoviral vector (Ad5) was evaluated as a genetic vaccine candidate against botulism caused by BoNT/C in a mouse model. A one-time intramuscular injection with 10(5) to 2 x 10(7)pfu of adenoviral vectors elicited robust serum antibody responses against H(C)50 of BoNT/C as assessed by ELISA. Immune sera showed high potency in neutralizing the active BoNT/C in vitro. After a single dose of 2 x 10(7)pfu adenoviral vectors, the animals were completely protected against intraperitoneal challenge with 100 x MLD(50) of active BoNT/C. The protective immunity appeared to be vaccine dose-dependent. The anti-toxin protective immunity could last for at least 7 months without a booster injection. In addition, we observed that pre-existing immunity to the wild-type Ad5 in the host had no significant influence on the protective efficacy of vaccination. The data suggest that an adenovirus-vectored genetic vaccine is a highly efficient prophylaxis candidate against botulism.

DNA vaccines for biodefence

The advantages associated with DNA vaccines include the speed with which they may be constructed and produced at large-scale, the ability to produce a broad spectrum of immune responses, and the ability for delivery using non-invasive means. In addition, DNA vaccines may be manipulated to express multiple antigens and may be tailored for the induction of appropriate immune responses. These advantages make DNA vaccination a promising approach for the development of vaccines for biodefence. In this review, the potential of DNA vaccines for biodefence is discussed.

Production of vaccines against leading biowarfare toxins can utilize DNA scientific technology

There are a significant number of different natural toxins that are potential biological warfare agents against which a vaccine is needed. DNA science has been a key to the development of potential vaccines against the top threat toxin and should contribute such effects for other toxin's vaccines. Several different DNA technological scientific techniques have been used to accomplish the general goals of (1) cloning of the toxin or large toxin fragments, (2) altering the specific gene sequence to obtain high level expression of vaccine candidate production in alternate species (3) placement of the vaccine gene in very different presentation types of species.

Clostridium botulinum: a bug with beauty and weapon

Clostridium botulinum, a Gram-positive, anaerobic spore-forming bacteria, is distinguished by its significant clinical applications as well as its potential to be used as bioterror agent. Growing cells secrete botulinum neurotoxin (BoNT), the most poisonous of all known poisons. While BoNT is the causative agent of deadly neuroparalytic botulism, it also serves as a remarkably effective treatment for involuntary muscle disorders such as blepharospasm, strabismus, hemifacial spasm, certain types of spasticity in children, and other ailments. BoNT is also used in cosmetology for the treatment of glabellar lines, and is well-known as the active component of the anti-aging medications Botox and Dysport. In addition, recent reports show that botulinum neurotoxin can be used as a tool for pharmaceutical drug delivery. However, BoNT remains the deadliest of all toxins, and is viewed by biodefense researchers as a possible agent of bioterrorism (BT). Among seven serotypes, C. botulinum type A is responsible for the highest mortality rate in botulism, and thus has the greatest potential to act as biological weapon. Genome sequencing of C. botulinum type A Hall strain (ATCC 3502) is now complete, and has shown the genome size to be 3.89 Mb with a G+C content of approximately 28.2%. The bacterium harbors a 16.3 kb plasmid with a 26.8% G+C content--slightly lower than that of the chromosome. Most of the virulence factors in C. botulinum are chromosomally encoded; bioinformatic analysis of the genome sequence has shown that the plasmid does not harbor toxin genes or genes for related virulence factors. Interestingly, the plasmid does harbor genes essential to replication, including dnaE, which encodes the alpha subunit of DNA polymerase III which has close similarity with its counterpart in C. perfringens strain 13. The plasmid also contains similar genes to those that encode the ABC-type multidrug transport ATPase, and permease. The presence of ABC-type multidrug transport ATPase, and permease suggests putative involvement of efflux pumps in bacteriocin production, modification, and export in C. botulinum. The C. botulinum plasmid additionally harbors genes for LambdaBa04 prophage and site-specific recombinase that are similar to those found in the Ames strain of Bacillus anthracis; these genes and their products may play a role in genomic rearrangement. Completion of genome sequencing for C. botulinum will provide an opportunity to design genomic and proteomic-based systems for detecting different serotypes of C. botulinum strains in the environment. The completed sequence may also facilitate identification of potential virulence factors and drug targets, as well as help characterize neurotoxin-complexing proteins, their polycistronic expression, and phylogenetic relationships between different serotypes.

Defensive applications of gene transfer technology in the face of bioterrorism: DNA-based vaccines and immune targeting

Gene transfer involves the introduction of an engineered gene into a person's cells with the expectation that the protein expressed from the gene will produce a therapeutic benefit. Strategies based on this principle have led to the approval of > 600 clinical trials and enrollment of approximately 3500 subjects worldwide in attempts to treat diseases ranging from cancer to AIDS to cystic fibrosis. While gene therapy has met with limited success and still has many hurdles to overcome before it sees wide application, it may be useful as a defensive strategy against bioterrorism agents including infectious microbes and toxins. Although many defensive strategies are possible, immunological strategies are currently the most developed and are being actively applied to the development of strategies against several of the most virulent potential bio-weapons. While most of these strategies are not yet ready for human application, DNA-based vaccines appear to be among the most promising in the fight against bioterrorism.

DNA vaccination protects against botulinum neurotoxin type F

A DNA vaccine was constructed which expressed the binding domain of Clostridium botulinum neurotoxin serotype F fused to a signal peptide. Three intra-muscular doses fully protected Balb/c mice against 10(4) MLD of serotype F toxin. Priming of the immune response by DNA vaccination followed by a single booster with type F binding domain protein resulted in high levels of antibody against the binding domain. This study demonstrates the utility of DNA vaccination for protection against botulinum neurotoxin type F and indicates that a prime-boost regimen could be an efficient method of generating antibody for passive immune therapy in cases of botulism involving serotype F toxin.

Evaluation of a botulinum fragment C-based ELISA for measuring the humoral immune response in primates

An enzyme-linked immunosorbent assay (ELISA) using botulinum neurotoxin serotype B recombinant fragment C (rBoNTB(HC)) was developed to measure specific humoral immune responses of monkeys vaccinated with a vaccine consisting of rBoNTB(HC). Several fundamental parameters for a bioassay were evaluated. The evaluation results demonstrated that using BoNTB(HC) as the capture antigen led to a specific and sensitive ELISA for botulinum type B antibody with excellent precision, accuracy, and linearity. There was a good correlation (r=0.91) between ELISA titers and neutralization bioassay titers. Experimental results suggested that the ELISA could be useful for detecting botulinum type B antibody levels and may supplement mouse neutralization bioassays during planned clinical manufacturing and clinical trials of rBoNTB(HC) vaccine.

Vaccines for Category A bioterrorism diseases

Vaccination programmes are very successful as a preventive strategy against many infectious diseases which have had a major impact on human morbidity and mortality. One of these diseases, smallpox, has been eliminated as a natural infection. The recent concern about biological attacks has turned attention to the use of an immunisation programme to prevent infection with what are considered the most significant potentially harmful biowarfare pathogens. This review puts into perspective the available information on current immunisation and newer vaccine options for anthrax, smallpox, tularaemia, plague and botulism.

Candidate vaccine against botulinum neurotoxin serotype A derived from a Venezuelan equine encephalitis virus vector system

A candidate vaccine against botulinum neurotoxin serotype A (BoNT/A) was developed by using a Venezuelan equine encephalitis (VEE) virus replicon vector. This vaccine vector is composed of a self-replicating RNA containing all of the VEE nonstructural genes and cis-acting elements and also a heterologous immunogen gene placed downstream of the subgenomic 26S promoter in place of the viral structural genes. In this study, the nontoxic 50-kDa carboxy-terminal fragment (H(C)) of the BoNT/A heavy chain was cloned into the replicon vector (H(C)-replicon). Cotransfection of BHK cells in vitro with the H(C)-replicon and two helper RNA molecules, the latter encoding all of the VEE structural proteins, resulted in the assembly and release of propagation-deficient, H(C) VEE replicon particles (H(C)-VRP). Cells infected with H(C)-VRP efficiently expressed this protein when analyzed by either immunofluorescence or by Western blot. To evaluate the immunogenicity of H(C)-VRP, mice were vaccinated with various doses of H(C)-VRP at different intervals. Mice inoculated subcutaneously with H(C)-VRP were protected from an intraperitoneal challenge of up to 100,000 50% lethal dose units of BoNT/A. Protection correlated directly with serum enzyme-linked immunosorbent assay titers to BoNT/A. The duration of the immunity achieved was tested at 6 months and at 1 year postvaccination, and mice challenged at these times remained refractory to challenge with BoNT/A.