Mechanisms of neutralization of a human anti-α-toxin antibody.

MEDI4893 is a neutralizing human monoclonal antibody that targets α-toxin (AT) and is currently undergoing evaluation in the field of Staphylococcus aureus-mediated diseases. We have solved the crystal structure of MEDI4893 Fab bound to monomeric AT at a resolution of 2.56 Å and further characterized its epitope using various engineered AT variants. We have found that MEDI4893 recognizes a novel epitope in the so-called "rim" domain of AT and exerts its neutralizing effect through a dual mechanism. In particular, MEDI4893 not only sterically blocks binding of AT to its cell receptor but also prevents it from adopting a lytic heptameric trans-membrane conformation.