Mapping of the antibody-binding regions on the HN-domain (residues 449-859) of botulinum neurotoxin A with antitoxin antibodies from four host species. Full profile of the continuous antigenic regions of the H-chain of botulinum neurotoxin A.

Previously, we mapped the antibody (Ab) and T-cell recognition regions on the HC domain (residues 855-1296) of the 848-residue heavy (H) chain of botulinum neurotoxin A (BoNT/A). We have mapped here the HN-domain (residues 449-859) regions that bind protective anti-BoNT/A Abs raised in four different species. We synthesized, purified, and characterized 29 19-residue peptides that spanned the entire HN and overlapped consecutively by 5 residues, and also region L218-231 around the L-chain's substrate-binding site. Human, horse, mouse, and chicken anti-BoNT/A Abs did not bind to the L-peptide but recognized similar HN regions within peptides 519-537/533-551/547-565/561-579 (with slight left- or right-shifts), 743-761, 785-803, and 813-831/827-845 overlap. Recognition of other peptides that bound lower Ab levels showed similarities and also some differences. Peptide 463-481, strongly immunodominant with horse antisera, did not bind human, mouse, and chicken Abs. However, peptide 449-467 bound Abs in these three antisera, and the region may have shifted to the right (peptide 463-481) with horse Abs. The overlap 659-677/673-691 reacted strongly with human Abs whereas with mouse and chicken antisera, only peptide 673-691 showed low reactivity. Horse antisera had no detectable Ab binding to region(s) 659-691. The Ab-recognition regions on the H chain occupy surface locations in BoNT/A three-dimensional structure, but the great part of the surface is not immunogenic. Regions recognized by the protective antisera of the four different species are prime candidates for inclusion in synthetic vaccine designs.