Characterization of lethal factor binding and cell receptor binding domains of protective antigen of Bacillus anthracis using monoclonal antibodies

Abstract

Lethal toxin from bacillus anthracis is composed of protective antigen (PA) and lethal factor (LF). Anti-PA mAbs that neutralized lethal toxin activity, either in vivo or in vitro identified three non-overlapping antigenic regions on PA. Two distinct antigenic regions were recognized by the four mAbs that neutralized lethal toxin activity by inhibiting the binding of ¹²⁵I-LF to cell-bound PA. Mapping showed that one mab, 1G3_PA63, recognized an epitope on a 17 kda fragment located between amino acid residues Ser-168 and Phe-314. The three other mabs, 2D3_PA, 2D5_PA and 10D2_PA, recognized an epitope between amino acids Ile-581 and Asn-601. A single antigenic region was recognized by the three mAbs, 3B6_PA, 14B7_PA and 10E10_PA63, that inhibited binding of ¹²⁵I-PA to cells. This region was located between amino acids Asp-671 and Ile-721. These results confirm previously defined functional domains of PA and suggest that LF may interact with two different sites on PA to form lethal toxin.