Potent neutralizing equine antibodies raised against recombinant SARS-CoV-2 spike protein for COVID-19 passive immunization therapy

We used the trimeric spike (S) glycoprotein (residues 1-1208) in the prefusion conformation to immunize horses for production of hyperimmune globulins against SARS-CoV-2. Serum antibody titers measured by anti-spike ELISA were above 1:1,000,000, and neutralizing antibody titer was 1:14,604 (average PRNT ₉₀ ), which is 140-fold higher than the average neutralizing titer of plasma from three convalescent COVID-19 patients analyzed for comparison. Using the same technology routinely used for industrial production of other horse hyperimmune products, plasma from immunized animals was pepsin digested to remove the Fc portion and purified, yielding a F(ab’) ₂ preparation with PRNT ₉₀ titers 150-fold higher than the neutralizing titers in human convalescent plasma. Repeating the hyperimmunization in a second group of horses confirmed the very high neutralizing titers in serum and in a GMP clinical F(ab’) ₂ lot. Virus-neutralizing activity in samples from mice that received the F(ab’) ₂ preparation was detected even three days after injection, indicating an appropriate half-life for therapeutic intervention. These results supported the design of a clinical trial (identifier <ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="clintrialgov" xlink:href="NCT04573855">NCT04573855</ext-link> ) to evaluate safety and efficacy of this horse F(ab’) ₂ preparation.