Direct Visualization of Ebola Virus Fusion Triggering in the Endocytic Pathway

Spence, Jennifer S.; Krause, Tyler B.; Mittler, Eva; Jangra, Rohit K.; Chandran, Kartik

Previous work has revealed that a conformational epitope at the GP1-GP2 interface in the GP base is targeted by multiple neutralizing antibodies, including two of three components of the ZMapp therapeutic cocktail (7, 44, 45) , leading to the hypothesis that this epitope is an "Achilles heel" for filoviruses. It was proposed on structural grounds that these base-binding antibodies potently neutralize by blocking fusogenic rearrangement and membrane insertion by GP2. Here, we provide the first direct evidence that neutralizing antibodies targeting the EBOV GP base inhibit viral fusion triggering (Fig. 3C) . Interestingly, higher concentrations of neutralizing antibody were required to inhibit lipid mixing to the same extent as infection, but this may typify fusion by enveloped viruses. A recent study by Otterstrom et al. (57) examined the levels of antibody binding needed to inhibit influenza virus hemifusion. Using a pair of potent, broadly effective neutralizing antibodies, whose inhibitory profiles against influenza virus strains are very similar to those of KZ52 and ZMapp against filoviruses (58, 59) , the authors showed that only high concentrations of these stem-binding antibodies were able to block the majority of hemifusion events. Although their assay (viral particles bound to a supported bilayer) differed from ours, their data strongly corroborate our results, with approximately 50 to 150 g/ml of either anti-influenza virus antibody needed to reduce in vitro hemifusion by~90%. The requirement for antibody concentrations beyond relevance to infection indicates a far lower threshold for lipid mixing than for full fusion and content release.