The Receptor Binding Domain of Botulinum Neurotoxin Serotype A (BoNT/A) Inhibits BoNT/A and BoNT/E Intoxications In Vivo

ABSTRACT 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 H C 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 H C /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.