Recombinant Antibody Potently Neutralizes Types A, B, and E Botulinum Neurotoxins by a Clearance-Driven Mechanism

We developed a recombinant mAb-based antitoxin to treat botulism resulting from any subtype of botulinum neurotoxins type A, B, or E. Peripheral blood lymphocytes from botulinum toxoid immunized humans were used to create single chain Fv (scFv) yeast displayed antibody libraries. BoNT/A, B, and E binding human scFv antibodies were generated by selecting the libraries on BoNTs by using flow cytometry. Lead antibodies were screened for binding to different BoNT subtypes. Affinities of antibodies cross-reactive with the different subtypes were increased by creating libraries of mutant scFv genes, displaying the gene repertoires on yeast, and selecting for higher affinity binding. For evaluation of in vivo BoNT neutralization, full-length IgG antibodies were expressed and purified. Combinations of 3–4 mAbs binding non-overlapping BoNT/A, B, and E epitopes with high affinity were engineered, which potently neutralize each of the toxins when administered as an equimolar combination. While a number of mechanisms likely contribute to the synergy observed upon combining mAbs, the most important of these is an Fc-mediated acceleration of BoNT clearance from the blood stream. The results demonstrate the feasibility of generating a highly potent human compatible recombinant antitoxin for the treatment or prevention of type A, B, or E botulism.