Botulism is caused by exposure to protein toxins called botulinum neurotoxins (BoNTs) that are produced by Clostridium botulinum. BoNTs are CDC Tier 1 select agent for which no antidote currently exists. Seven different BoNT serotypes have been discovered to date (BoNT/A-G), many having numerous additional BoNT subtypes. However the only currently available treatments are serum based antitoxin products derived from large animals that are only effective if administered soon after BoNT intoxication. The challenge of developing BoNT therapeutics is exacerbated by the fact that the seven known BoNT serotypes are each distinct toxins with distinct receptor specificities and proteases that cleave at distinct sites on SNARE proteins to disrupt nerve transmission. Due to the severity of the risk, the paucity of treatment options, and the complexity of the challenge, novel approaches to the prevention and treatment of BoNT intoxication are clearly needed. We now have extensive evidence in multiple toxin models demonstrating that bispecific VHH-based neutralizing agents (VNAs), consisting of two covalently linked, toxin-neutralizing VHHs, are antitoxins with potencies that often exceed that of current monoclonal and polyclonal antitoxin agents. Furthermore, VNAs offer substantial advantages over serum and mAb antitoxin products as they are economical to produce and highly versatile; offering innovative new prevention and treatment strategies for toxin exposures and infections with toxin-producing pathogens such as gene therapies and direct delivery to enteric and pulmonary sites of challenge. In this proposal, we test the hypothesis that integrating structural and mechanistic information into VNA design will lead to even greater antitoxin efficacy and versatility.
The Specific Aims are to (1) determine the crystal structures of selected BoNT-binding VHHs in complex with their target BoNTs; (2) define the mechanisms by which VHHs selected in Aim 1 block BoNT toxicity, and; (3) design and test bispecific VNAs with enhanced antitoxin properties by exploiting structure/function data from Aims 1 and 2. This will be the first comprehensive structural mapping of BoNT neutralizing epitopes, which will be complemented with mechanistic studies of BoNT function and BoNT-host interactions. Furthermore, this study will improve general understanding of how structural and mechanistic information can inform the design of even more effective VNA antitoxin agents and should permit rapid development of commercial antitoxin therapeutics to treat exposures to all BoNT serotypes and other toxin biothreat agents.

Public Health Relevance

Botulinum neurotoxins (BoNTs) are bacterial toxins that cause the paralytic botulism syndrome. They are extremely dangerous, CDC Tier 1 biodefense threat that are widely available, easily produced, exceedingly toxic and for which no antidote is available. We propose to perform the first comprehensive structural mapping of BoNT neutralizing epitopes, which will lead to improved understanding of BoNT function and BoNT-host interactions. The structural and mechanistic information will help the development of novel, potent VHH-based neutralizing agents to prevent and treat botulism.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Research Project (R01)
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Bacterial Pathogenesis Study Section (BACP)
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Ranallo, Ryan
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University of California Irvine
Schools of Medicine
United States
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Lam, Kwok-Ho; Qi, Ruifeng; Liu, Shun et al. (2018) The hypothetical protein P47 of Clostridium botulinum E1 strain Beluga has a structural topology similar to bactericidal/permeability-increasing protein. Toxicon 147:19-26
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