Characteristics of Ebolavirus Glycoprotein Internal Fusion Loop Recognition Determine Broad Cross-Reactivity

Single particle negative stain electron microscopy of 6D6 Fab fragment bound to EBOV GP (side view, left and top view, right). The GP and Fab are shown in grey and blue, respectively with the crystal structures shown as ribbon diagrams for reference.

The 2014-2016 Ebola virus disease (EVD) outbreak in West Africa caused by Ebola virus (EBOV) resulted in over 28,000 cases of EVD, but prior to 2013 more than 40% of all EVD cases were caused by other ebolavirus species. Thus, to combat future EVD outbreaks, antibody treatments that have pan-ebolavirus activity are needed.

The murine monoclonal antibody termed 6D6 neutralizes vesicular stomatitis virus (VSV) pseudotyped with glycoproteins (GP) from all five ebolavirus (Ebola virus (EBOV), Bundibugyo virus (BDBV) and Sudan virus (SUDV), Reston virus (RESTV) and Taï Forest (TAFV)) (see Furuyama et al. Scientific Reports).  Milligan et al. describe the structural analysis of the 6D6 Fab domain alone and in complex with EBOV and BDBV GPs in a Journal of Infectious Diseases article that appeared on September 10, 2018.

The crystal structure of the 6D6 Fab domain indicated that the interface with GP is largely hydrophobic, whereas the complementarity determining region (CDR) H2 has two arginine residues that could participate in electrostatic interactions with GP. Single particle electron microscopy (EM) reconstructions of 6D6 Fab in complex with EBOV and BDBV GPs allowed the first side-by-side comparison of a pan-ebolavirus antibody bound to different GPs.

Both BDBV and EBOV have similar patterns of interaction with 6D6 that involve hydrophobic and electrostatic components, as predicted from the crystal structure. The 6D6 footprint completely covers the internal fusion loop (IFL) and closely resembles that of the pan-ebolavirus antibody ADI-15878 characterized by West et al. The angle of approach for both 6D6 and ADI-15878 are highly similar. However, the 6D6 footprint differs from previously described antibodies: the macaque-derived, broadly cross-reactive CA45 and the human-derived, EBOV- and BDBV-specific ADI-15946, which both bind the base and stem of the IFL. Meanwhile, the epitope for the EBOV-specific antibody mAb100 lies in the general area of the IFL, but its binding site is situated lower on the GP and does not completely bury the IFL. Together, these results suggest that the higher position and recognition of the IFL itself are key factors that confer broad neutralization activity.