Expression, purification and characterization of the receptor-binding domain of botulinum neurotoxin serotype B as a vaccine candidate

• An order of magnitude improved yield for the expression of soluble HC/B in E. coli.
• HC/B exhibits good protective immunogenicity and inhibits botulism in vivo.
• A simple method to remove ArnA, a common His-tagged contaminating protein, was developed.

The receptor-binding domain of botulinum neurotoxins (the HC fragment) is a promising vaccine candidate. Among the HC fragments of the seven BoNT serotypes, the expression of HC/B in Escherichia coli is considered especially challenging due to its accumulation as a non-soluble protein aggregate. In this study, the effects of different parameters on the expression of soluble HC/B were evaluated using a screening assay that included growing the bacterium at a small scale, a chemical cell lysis step, and a specific ELISA. The highest soluble HC/B expression levels were obtained when the bacterium E. coli BL21(DE3) + pET-9a-HC/B was grown in terrific broth media at 18 °C without induction. Under these conditions, the yield was an order of magnitude higher than previously reported. Standard purification of the protein using a nickel column resulted in a low purity of HC/B. However, the addition of an acidic wash step prior to protein elution released a major protein contaminant and significantly increased the purity level. Mass spectrometry analysis identified the contaminant as ArnA, an E. coli protein that often contaminates recombinant His-tagged protein preparations. The purified HC/B was highly immunogenic, protecting mice from a 106 LD50 challenge after a single vaccination and generating a neutralizing titer of 50 IU/ml after three immunizations. Moreover, the functionality of the protein was preserved, as it inhibited BoNT/B intoxication in vivo, presumably due to blockade of the neurotoxin protein receptor synaptotagmin.