Equine-Origin Immunoglobulin Fragments Protect Nonhuman Primates from Ebola Virus Disease

Ebola virus (EBOV) infections result in aggressive hemorrhagic fever in humans, with fatality rates reaching 90% and with no licensed specific therapeutics to treat ill patients. Advances over the past 5 years have firmly established monoclonal antibody (MAb)-based products as the most promising therapeutics for treating EBOV infections, but production is costly and quantities are limited; therefore, MAbs are not the best candidates for mass use in the case of an epidemic. To address this need, we generated EBOV-specific polyclonal F(ab')₂ fragments from horses hyperimmunized with an EBOV vaccine. The F(ab')₂ was found to potently neutralize West African and Central African EBOV in vitro Treatment of nonhuman primates (NHPs) with seven doses of 100 mg/kg F(ab')₂ beginning 3 or 5 days postinfection (dpi) resulted in a 100% survival rate. Notably, NHPs for which treatment was initiated at 5 dpi were already highly viremic, with observable signs of EBOV disease, which demonstrated that F(ab')₂ was still effective as a therapeutic agent even in symptomatic subjects. These results show that F(ab')₂ should be advanced for clinical testing in preparation for future EBOV outbreaks and epidemics.IMPORTANCE EBOV is one of the deadliest viruses to humans. It has been over 40 years since EBOV was first reported, but no cure is available. Research breakthroughs over the past 5 years have shown that MAbs constitute an effective therapy for EBOV infections. However, MAbs are expensive and difficult to produce in large amounts and therefore may only play a limited role during an epidemic. A cheaper alternative is required, especially since EBOV is endemic in several third world countries with limited medical resources. Here, we used a standard protocol to produce large amounts of antiserum F(ab')₂ fragments from horses vaccinated with an EBOV vaccine, and we tested the protectiveness in monkeys. We showed that F(ab')₂ was effective in 100% of monkeys even after the animals were visibly ill with EBOV disease. Thus, F(ab')₂ could be a very good option for large-scale treatments of patients and should be advanced to clinical testing.