Selection and characterization of human antibodies neutralizing Bacillus anthracis toxin

A less than adequate therapeutic plan for the treatment of anthrax in the 2001 bioterrorism attacks has highlighted the importance of developing alternative or complementary therapeutic approaches for biothreat agents. In these regards passive immunization possesses several important advantages over active vaccination and the use of antibiotics, as it can provide immediate protection against Bacillus anthracis. Herein, we report the selection and characterization of several human monoclonal neutralizing antibodies against the toxin of B. anthracis from a phage displayed human scFv library. In total 15 clones were selected with distinct sequences and high specificity to protective antigen and thus were the subject of a series of both biophysical and cell-based cytotoxicity assays. From this panel of antibodies a set of neutralizing antibodies were identified, of which clone A8 recognizes the lethal (and/or edema) factor binding domain, and clones F1, G11, and G12 recognize the cellular receptor binding domain found within the protective antigen. It was noted that all clones distinguish a conformational epitope existing on the protective antigen; this steric relationship was uncovered using a sequential epitope mapping approach. For each neutralizing antibody, the kinetic constants were determined by surface plasmon resonance, while the potency of protection was established using a two-tier macrophage cytotoxicity assay. Among the neutralizing antibodies identified, clone F1 possessed the highest affinity to protective antigen, and provided superior protection from lethal toxin in the cell cytotoxicity assay. The data presented provide the ever-growing arsenal of immunological and functional analysis of monoclonal antibodies to the exotoxins of anthrax. In addition it grants new candidates for the prophylaxis and therapeutic treatment against this toxin.