Multifunctional Pan-ebolavirus Antibody Recognizes a Site of Broad Vulnerability on the Ebolavirus Glycoprotein

摘要

class="head no_bottom_margin" id="sec1title">IntroductionEbola virus, a member of the Filoviridae family, causes severe disease in humans with 25% to 90% mortality rates and significant epidemic potential. There are no licensed ebolavirus vaccines or treatments. The largest 2013–2016 Ebola epidemic in West Africa, with a total of 28,646 cases of Ebola virus disease (EVD) and 11,323 deaths reported (), highlighted the need to accelerate EVD therapeutics development.There are five known species—Zaire ebolavirus, Bundibugyo ebolavirus, Sudan ebolavirus, Tai Forest ebolavirus, and Reston ebolavirus—which are represented, respectively, by Zaire (EBOV), Bundibugyo (BDBV), Sudan (SUDV), Tai Forest (TAFV), and Reston (RESTV) viruses. EBOV, BDBV, and SUDV are clinically relevant viruses that are known to cause lethal disease in humans (). Infections with RESTV are usually asymptomatic in humans, and only one case of non-lethal human infection has been reported for TAFV ().The Ebola virus envelope contains a single surface protein, glycoprotein (GP), which forms a trimer. The GP protomer consists of two subunits, GP1 and GP2. The GP1 subunit has a heavily glycosylated mucin-like domain and a glycan cap, which shields the host receptor binding site (RBS) that binds to domain C of its endosomal receptor, the protein Niemann-Pick C1 (NPC1-C). The GP2 subunit contains the internal fusion loop (IFL) and stalk and is anchored into the viral membrane by a transmembrane domain (). The GP is solely responsible for viral attachment to the host cell, endosomal entry, and membrane fusion (), and thus it is also the major target for neutralizing monoclonal antibodies (mAbs) and vaccine design.An experimental therapeutic mAb mixture, ZMapp, comprising three murine-human chimeric EBOV GP-specific mAbs (2G4 and 4G7 recognizing the base region and c13C6 recognizing the glycan cap), fully protected non-human primates (NHPs) from lethal EBOV challenge (). This cocktail also exhibited activity when used as treatment of EVD in humans in incomplete clinical trial testing during the recent epidemic (). ZMapp mAbs bind only to EBOV, however, and do not recognize BDBV or SUDV. We and others have isolated hundreds of new ebolavirus GP-specific mAbs from EBOV or BDBV survivors since the last EVD outbreak. New mAbs have been described that recognize diverse antigenic sites on GP, including epitopes on the glycan cap, the IFL, the GP1 head, the GP1/GP2 interface, the RBS, and the stalk (, , , , , , , , href="#bib49" rid="bib49" class=" bibr popnode">Wec et al., 2017, href="#bib54" rid="bib54" class=" bibr popnode">Zhao et al., 2017). Most of the mAbs isolated to date neutralize only one or two ebolavirus species. There is a medical need for mAb therapeutics that exhibit a pan-ebolavirus pattern of breadth, because the nature of future EVD outbreaks cannot be predicted. Recently, investigators identified the IFL as a site of vulnerability on GP, and they reported the isolation of three rare broadly neutralizing mAbs (bNAbs) that also possessed protective capacity against EBOV, BDBV, and SUDV. One antibody was derived from the B cells of immunized NHPs (href="#bib54" rid="bib54" class=" bibr popnode">Zhao et al., 2017), and two others were isolated from the B cells of a human survivor (href="#bib49" rid="bib49" class=" bibr popnode">Wec et al., 2017). These studies demonstrate that rare bNAbs against ebolaviruses are generated in response to natural infection or vaccination. Identifying potent bNAbs that can resist the emergence of viral escape mutants, and systematic analysis to define unique molecular and immunological features that mediate broad protection by these antibodies, are important next steps for rational selection of therapeutic mAb candidates.Here, we isolated two human bNAbs, designated EBOV-515 and -520, that bound to sites of vulnerability on ebolavirus GP and mediated protection against EBOV, BDBV, and SUDV. In-depth analysis of the mechanism of action revealed key features that contributed to the broad reactivity, neutralization, and protection mediated by these mAbs. Together, the findings suggested high promise for these newly identified human mAbs as candidate pan-ebolavirus therapeutics.