Asymmetric and Non-Stoichiometric Recognition Results in Broad Protection Against Ebolaviruses by a Two-Antibody Cocktail

There are multiple known ebolaviruses that each cause outbreaks of severe disease. Medical countermeasure development has thus far focused on just one, Ebola virus, resulting in two vaccines and several monoclonal antibodies that provide clinical benefit. However, none of these countermeasures can be used to prevent disease or treat patients infected with other ebolaviruses such as Sudan virus. Further, all treatments evaluated clinically thus far contain antibodies that cross-react to sGP, the abundantly shed version of the glycoprotein, which threatens to absorb otherwise neutralizing antibody responses. Using sera from survivors of Ebola virus infection, we performed targeted sorting of memory B cells that recognize multiple ebolaviruses. We isolated two mAbs, 1C3 and 1C11, that do not cross-react to sGP, potently neutralize and protect rodents from disease. The broad reactivity of both antibodies is based on their recognition of quaternary epitopes of the ebolavirus glycoprotein complex: 1C11 bridges the fusion loop of one monomer to the adjacent monomer in the glycoprotein trimer, whereas 1C3 binds to the center apex of the trimer, in a position that enables a single antigen-binding fragment to anchor simultaneously to the three ebolavirus receptor-binding sites on the timers. In this unique tripartite epitope, identical residues on the three GP monomers each form different interactions with a separate portion of the antibody paratope. The combination of both antibodies completely protected non-human primates from Ebola and Sudan virus infection, indicating their potential future use in the clinic.