• Vaccine Name: BoNT/B(Hc)
• Target Pathogen: Clostridium botulinum
• Target Disease: Botulism
• Vaccine Ontology ID: VO_0004077
• Type: Subunit vaccine
• Adjuvant:
  • Adjuvant name:
  • VO adjuvant ID: VO_0001241
  • Description: Botulinum neurotoxins (BoNT) are a group of seven (A–G) antigenically distinct proteins produced by Clostridium botulinum, a gram-positive, spore-forming, obligate anaerobic bacillus found in soil and marine sediments from all parts of the world. BoNTs inhibit the release of acetylcholine (ACH) at the synapse of motor neurons, which initially causes muscle weakness, progressing to flaccid paralysis, subsequent ventilatory failure, and death. This can happen at very low concentrations of BoNT (10−9 g/kg), making the toxin one of the most potent poisons known.The most common exposures to BoNT are food-borne, infant, and wound botulism. The likely mode of delivery of BoNT in bio-warfare or terrorist scenarios is through aerosol delivery. Toxicity from BoNT can be lethal without early detection and intensive supportive therapy. Trivalent equine-based antitoxins (A, B, and E) are available and are used clinically to neutralize and clear BoNT from the circulatory system. A number of different products have been proposed as candidate vaccines against BoNT. For BoNT to express its deleterious effects, sequential mechanistic events are carried out by three discrete structural domains of the protein. The BoNT exert toxicity only after they bind specifically to a neuron receptor at the neuromuscular junction. This receptor-toxin complex is then internalized within a vesicle followed by translocation of a portion of the toxin into the neuronal cytosol. This part of the molecule is responsible for the inherent toxicity that arises from its endoprotease activity on neurovesicle transport proteins. Once enzymatically inactivated by the toxin, these "SNARE" proteins cannot facilitate ACH-containing neurovesicles docking to the plasma membrane at the neuromuscular junction. This inhibits the vesicle from expelling its contents (ACH) into the synaptic space. The different toxin events are brought about by distinct protein domains of the holotoxin that are nearly equal in mass, which have been structurally and functionally defined. The holotoxin, approximately 150 kDa, consists of a heavy chain (approximately 100 kDa) that extends from the carboxy terminus to a disulfide bridge where the light chain (approximately 50 kDa) is attached. The receptor-binding domain is found in the carboxy portion of the heavy chain known as the C fragment or HC region. The remaining fragment of the heavy chain is known as the HN and, with the carboxy terminus of the light chain, spans the disulfide bridge to comprise the translocation domain, which facilitates light-chain entry into the neuronal cytoplasm. The light chain contains the zinc-dependent endoprotease domain responsible for inactivating docking proteins, inhibiting exocytosis, and the inherent toxicity of BoNT. Because BoNT toxicity is neuro-specific and enzymatic, while substrate is limited, relatively few molecules of BoNT are necessary to inhibit neurovesicle docking and expulsion. A number of regions of the BoNT have been investigated and cloned for potential use as immunogens for recombinant vaccine candidates A and B serotypes. The C fragment of BoNT was a logical choice for a vaccine candidate based on its native function, immunogenicity, and lack of observed signs of toxicity (Boles et al., 2006).
• Preparation: Purified rBoNTB(Hc) vaccine (Lot No. 0490) was produced under current Good Manufacturing Practices at the Department of Biologics Research, Pilot Bio-Production Facility, Walter Reed Army Institute of Research, Forest Glen, MD. The pilot production lot of rBoNTB(HC) was expressed by the yeast vector Pichia pastoris, producing an intracellular protein product that was subsequently highly purified to yield a product of 2.34 g of rBoNTB(HC) per 6 kg wet yeast mass. Final dilutions to a 0.5 ml injection volume was made with the addition of 0.8% normal sterile saline (Boles et al., 2006).
• Virulence: (Boles et al., 2006)
• Description: Botulinum neurotoxins (BoNT) are a group of seven (A–G) antigenically distinct proteins produced by Clostridium botulinum, a gram-positive, spore-forming, obligate anaerobic bacillus found in soil and marine sediments from all parts of the world. BoNTs inhibit the release of acetylcholine (ACH) at the synapse of motor neurons, which initially causes muscle weakness, progressing to flaccid paralysis, subsequent ventilatory failure, and death. This can happen at very low concentrations of BoNT (10−9 g/kg), making the toxin one of the most potent poisons known.The most common exposures to BoNT are food-borne, infant, and wound botulism. The likely mode of delivery of BoNT in bio-warfare or terrorist scenarios is through aerosol delivery. Toxicity from BoNT can be lethal without early detection and intensive supportive therapy. Trivalent equine-based antitoxins (A, B, and E) are available and are used clinically to neutralize and clear BoNT from the circulatory system. A number of different products have been proposed as candidate vaccines against BoNT. For BoNT to express its deleterious effects, sequential mechanistic events are carried out by three discrete structural domains of the protein. The BoNT exert toxicity only after they bind specifically to a neuron receptor at the neuromuscular junction. This receptor-toxin complex is then internalized within a vesicle followed by translocation of a portion of the toxin into the neuronal cytosol. This part of the molecule is responsible for the inherent toxicity that arises from its endoprotease activity on neurovesicle transport proteins. Once enzymatically inactivated by the toxin, these "SNARE" proteins cannot facilitate ACH-containing neurovesicles docking to the plasma membrane at the neuromuscular junction. This inhibits the vesicle from expelling its contents (ACH) into the synaptic space. The different toxin events are brought about by distinct protein domains of the holotoxin that are nearly equal in mass, which have been structurally and functionally defined. The holotoxin, approximately 150 kDa, consists of a heavy chain (approximately 100 kDa) that extends from the carboxy terminus to a disulfide bridge where the light chain (approximately 50 kDa) is attached. The receptor-binding domain is found in the carboxy portion of the heavy chain known as the C fragment or HC region. The remaining fragment of the heavy chain is known as the HN and, with the carboxy terminus of the light chain, spans the disulfide bridge to comprise the translocation domain, which facilitates light-chain entry into the neuronal cytoplasm. The light chain contains the zinc-dependent endoprotease domain responsible for inactivating docking proteins, inhibiting exocytosis, and the inherent toxicity of BoNT. Because BoNT toxicity is neuro-specific and enzymatic, while substrate is limited, relatively few molecules of BoNT are necessary to inhibit neurovesicle docking and expulsion. A number of regions of the BoNT have been investigated and cloned for potential use as immunogens for recombinant vaccine candidates A and B serotypes. The C fragment of BoNT was a logical choice for a vaccine candidate based on its native function, immunogenicity, and lack of observed signs of toxicity (Boles et al., 2006).

Monkey Response

• Host Strain: Rhesus monkeys (Macaca mulatta)
• Vaccination Protocol: Twenty-eight monkeys were randomly chosen to receive either vehicle (Alhydrogel only, n=2) or 1 μg (n=6) or 5 μg (n=6) of the recombinant vaccine. All animals received the same lot no. of rBoNTB(Hc) vaccine or vehicle, administered 3 times, 4 weeks apart, in the thigh, in a final volume of 0.5 ml saline, all of which contained a final concentration of 0.2% alhydrogel adjuvant. The challenge phase consisted of 14 monkeys that were aerosol challenged with BoNTB as described below at 6 weeks after the last vaccine dose. The remaining 14 animals were treated identically with the exception that blood was drawn at more distant time points for the duration of immunity phase of the study and were not challenged. Phlebotomies and aerosol exposures were performed under light anesthesia via tiletamine-zolazepam (Boles et al., 2006).
• Persistence: ELISA and SNA titers in non-human primates demonstrated a response for up to 2 years. In addition, the rBoNTB(HC) vaccine candidate produced SNA titers that were protective for at least the monkeys at 14 week from the first phase, whether these same or higher titers are protective for the second phase monkeys up to 2 years was not assessed (Boles et al., 2006).
• Side Effects: Daily observations by the veterinary staff revealed no adverse reactions to the recombinant vaccine itself, consistent with the findings of other studies evaluating rBoNT(HC) vaccines for other serotypes (Boles et al., 2006).
• Efficacy: All rhesus monkeys inoculated with either 1 or 5 μg of rBoNTB(Hc) survived the aerosol challenge at 14 weeks after the first injection (Boles et al., 2006).
• Description: A number of different products have been proposed as candidate vaccines against BoNT and some, including BoNTB(Hc), use a development strategy based on known structure activity relationships. For BoNT to express its deleterious effects, sequential mechanistic events are carried out by three discrete structural domains of the protein. The BoNT exert toxicity only after they bind specifically to a neuron receptor at the neuromuscular junction. This receptor-toxin complex is then internalized within a vesicle followed by translocation of a portion of the toxin into the neuronal cytosol. This part of the molecule is responsible for the inherent toxicity that arises from its endoprotease activity on neurovesicle transport proteins. Once enzymatically inactivated by the toxin, the SNARE proteins cannot facilitate ACH-containing neurovesicles docking to the plasma membrane at the neuromuscular junction. This, in effect, inhibits the vesicle from expelling its contents (ACH) into the synaptic space.
  
  The different toxin events are brought about by distinct protein domains of the holotoxin that are nearly equal in mass, which have been structurally and functionally defined. The holotoxin, approximately 150 kDa, consists of a heavy chain (approximately 100 kDa) that extends from the carboxy terminus to a disulfide bridge where the light chain (approximately 50 kDa) is attached. The receptor-binding domain is found in the carboxy portion of the heavy chain known as the C fragment or HC region. The remaining fragment of the heavy chain is known as the HN and, with the carboxy terminus of the light chain, spans the disulfide bridge to comprise the translocation domain, which facilitates light-chain entry into the neuronal cytoplasm. The light chain contains the zinc-dependent endoprotease domain responsible for inactivating docking proteins, inhibiting exocytosis, and the inherent toxicity of BoNT. Because BoNT toxicity is neuro-specific and enzymatic, while substrate is limited, relatively few molecules of BoNT are necessary to inhibit neurovesicle docking and expulsion (Boles et al., 2006).