Filoviruses are Category A pathogens which cause a severe and rapidly fatal disease with human case fatality rates of 30-90%. The 2013-2016 epidemic in Western Africa, with over 28,000 cases, was caused by one of these agents, Ebola virus (Zaire species, EBOV), and highlights the urgent need for new countermeasures targeting all virulent filoviruses. Monoclonal antibodies (mAbs) have emerged as the most promising therapeutic platform for treatment and prevention of Ebola virus disease. However, most mAbs and mAb cocktails are narrowly specific for a single filovirus, and are thus of limited use in outbreaks caused by other filoviruses with epidemic potential, including Bundibugyo virus (BDBV), Sudan virus (SUDV), and Marburg virus (MARV). A major gap in the filovirus mAb therapeutic pipeline is the lack of broad- spectrum therapies that are active against multiple disease-causing filoviruses. This proposal leverages an innovative technology platform, bispecific antibody engineering, along with proprietary mAbs from academic (Einstein), government (USAMRIID/Geneva), and commercial (Integrated Biotherapeutics; Mapp Biophamaceutical) partners to develop bispecific antibodies (bis-mAbs) as cross-protective filovirus antibody therapies. We build on a new approach recently developed by our team—a `Trojan Horse' bis-mAb that can coöpt virus particles themselves to `hitch a ride' into cellular endosomes, where it targets crucial and highly conserved viral epitopes that are normally hidden from antibodies. This bis-mAb shows potent, broad neutralization of all ebolaviruses in vitro and provides cross-protection against EBOV and SUDV in mice, thus providing proof-of-concept that bis-mAbs engineered to localize to endosome with viral particles can effectively target sequestered epitopes (Wec et al., Science 2016). Furthermore, this bis-mAb is broadly protective as a single component, thus avoiding the manufacturing and development hurdles of cocktails of mAbs that have been previously shown to be required for broad filovirus protection. To further develop this therapeutic bis-mAb modality, we will expand the range of highly conserved external and endosomal epitopes that are targeted. The goal of this proposal is to optimize and advance bis-mAbs as single component, pan-ebolavirus or pan-filovirus immunotherapies.