Analysis of Defined Combinations of Monoclonal Antibodies in Anthrax Toxin Neutralization Assays and Their Synergistic Action

Antibodies against the protective antigen (PA) component of anthrax toxin play an important role in protection against disease caused by Bacillus anthracis. In this study, we examined defined combinations of PA-specific monoclonal antibodies for their ability to neutralize anthrax toxin in cell culture assays. We observed additive, synergistic, and antagonistic effects of the anti-bodies depending on the specific antibody combination examined and the specific assay used. Synergistic toxin-neutralizing an-tibody interactions were examined in more detail. We found that one mechanism that can lead to antibody synergy is the bridg-ing of PAmonomers by one antibody, with resultant bivalent binding of the second antibody. These results may aid in optimal design of new vaccines and antibody therapies against anthrax. Inhalation anthrax, caused by the Gram-positive bacterium Ba-cillus anthracis, is a disease that is associated with high rates of morbidity and mortality if not treated early. The potential use of B. anthracis spores as a biological warfare and bioterror agent has spurred significant efforts toward the development of counter-measures for anthrax (16), including new-generation anthrax vac-cines and therapeutics. Most anthrax vaccines and therapeutic antibodies that are currently under development are designed to protect against disease by targeting anthrax toxin, a major viru