Several arenaviruses, chiefly Lassa virus (LASV) in West Africa, cause hemorrhagic fever (HF) disease in humans that is associated with high morbidity and mortality. HF arenaviruses represent an important public health problem in their endemic regions. In addition, the worldwide-distributed prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a neglected clinically important human pathogen. Moreover, several arenaviruses, including LASV and LCMV, represent credible biodefense threats. Public health concerns posed by human arenavirus infections are aggravated by the lack of FDA-licensed vaccines and current anti-arenaviral therapy being limited to the off-label use of ribavirin that is only partially effective. Broadly neutralizing antibodies (BNAb) have been isolated from individuals who recovered from infection with viruses that exhibit a high degree of antigenic variability like HIV and influenza. These BNAb can protect against infection by antigenically divergent viral strains, and therefore they provide a powerful tool for passive immunotherapy. Moreover, information regarding the interaction of these BNAb with their highly conserved target epitopes and their mechanisms of neutralization can facilitate the design and development of novel universal vaccines and antiviral drugs that could confer broad-spectrum protection against arenaviruses. We propose first to evaluate a panel of 103 LASV glycoprotein (GP)-specific human monoclonal antibodies (hMAbs) derived from 14 different Lassa fever (LF) survivors (provided by Dr. Robinson at Tulane University) for their cross-reactivity (Aim 1) and range of neutralizing activities (Aim 2) against a collection of different Old World Arenavirus (OWA) including genetically distantly related strains of LASV and LCMV. We will then identify the GP subunits and specific amino acid residues of LASV GP recognized by selected OWA BNhMAbs by conducting antibody binding competition assays and characterizing selected viral variants resistant to neutralization by the selected BNhMAbs (Aim 3). Finally, we will examine the in vivo neutralizing activity and therapeutic value of selected OWA BNhMAbs with optimal in vitro neutralization profiles (Aim 4). The identification and characterization of OWA BNhMAbs constitutes a first and necessary step for the potential development of immunotherapeutics to treat LF and other OWA-induced disease in humans for which vaccines are not available and existing antivirals are of limited efficacy. Additionally, the identification of the epitopes targeed by these BNhMAbs will provide valuable information for the generation of immunogens able to induce similar arenavirus BNhMAb responses for the development of universal vaccines against HF-causing human OWA, as well as the identification of broad-spectrum anti-arenavirus drugs targeting the structures and activities defined by the highly conserved epitopes recognized by these BNhMAbs.