Domain specificity of the human antibody response to Bacillus anthracis protective antigen

Protective antigen (PA) is the cell surface recognition moiety of the Bacillus anthracis A-B toxin system, and the active immunogenic component in the currently licensed human anthrax vaccine (BioThrax™, or AVA). The serum antibody response to the PA protein is polyclonal and complex both in terms of the antibody combining sites utilized to bind PA and the PA-associated epitopes recognized. We have cloned, sequenced, and expressed a large panel of PA-specific human monoclonal antibodies from seven AVA-immunized donors. Dot blots, Western blots, and radiolabeled antigen capture assays employing both proteolytic fragments of PA and engineered PA sub-domain fusion proteins were used to determine the region (domain) of the PA monomer to which each of the cloned human antibodies bound. The domain specificity of the isolated monoclonals was highly biased towards the amino-terminal 20 kDa fragment of PA (PA20), with the majority (62%) of independently arising antibody clones reacting with determinants located on this PA fragment. A similar bias in domain specificity was also demonstrated in the serum response of AVA-vaccinated donors. Since PA20 is cleaved from the remainder of the monomer rapidly following cell surface binding and has no known role in the intoxication process, the immunodominance of PA20-associated epitopes may directly affect the efficacy of PA-based anthrax vaccines.