Mechanistic and Fc requirements for inhibition of Sudan virus entry and in vivo protection by a synthetic antibody.

The Sudan virus (SUDV), an ebolavirus, causes severe hemorrhagic fever with human case fatality rates of ∼50%. Previous work from our lab demonstrated the synthetic antibody F4 potently inhibits viral entry and protects against lethal virus challenge in mice [Chen et al., ACS Chem. Biol., 2014, 9, 2263-2273]. Here, we explore mechanistic requirements as well as contribution of the Fc region and function on neutralization and in vivo protection. Live cell imaging demonstrates that the antibody colocalizes with vesicular stomatitis virus particles containing the Sudan virus glycoprotein (VSV-GP) and that the antibody is rapidly degraded within cellular endosomes. A viral escape mutant contained substitutions on the N-heptad repeat (NHR) segment of GP2, the fusion subunit. Truncation studies indicated that the size of the Fc impacts virus neutralization potential. Finally, we examined the protective efficacy of Fc-null mutants in mice, and found that Fc function was not required for high levels of protection. Altogether, these results indicate that neutralization of SUDV GP-mediated cell entry likely involves blockade of viral membrane fusion within endosomes, and that inhibition of viral entry is the likely mechanism of in vivo protection.