ZMapp, a cocktail of three monoclonal antibodies (MAbs; c2G4, c4G7, and c13C6) against the ebolavirus (EBOV) glycoprotein (GP), shows promise for combatting outbreaks of EBOV, as occurred in West Africa in 2014. Prior studies showed that Fabs from these MAbs bind a soluble EBOV GP ectodomain and that MAbs c2G4 and c4G7, but not c13C6, neutralize infections in cell cultures. Using cryo-electron tomography, we extended these findings by characterizing the structures of c2G4, c4G7, and c13C6 IgGs bound to native, full-length GP from the West African 2014 isolate embedded in filamentous viruslike particles (VLPs). As with the isolated ectodomain, c13C6 bound to the glycan cap, whereas c2G4 and c4G7 bound to the base region of membrane-bound GP. The tomographic data suggest that all three MAbs bind with high occupancy and that the base-binding antibodies can potentially bridge neighboring GP spikes. Functional studies indicated that c2G4 and c4G7, but not c13C6, competitively inhibit entry of VLPs bearing EBOV GP into the host cell cytoplasm, without blocking trafficking of VLPs to NPC1+ endolysosomes, where EBOV fuses. Moreover, c2G4 and c4G7 bind to and can block entry mediated by the primed (19-kDa) form of GP without impeding binding of the C-loop of NPC1, the endolysosomal receptor for EBOV. The most likely mode of action of c2G4 and c4G7 is therefore by inhibiting conformational changes in primed, NPC1-bound GP that initiate fusion between the viral and target membranes, similar to the action of certain broadly neutralizing antibodies against influenza hemagglutinin and HIV Env.
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