Production of equine sera as a potential immunotherapy against COVID-19

Emerging viruses such as the COVID-19-inducing virus, SARSCoV- 2, represent a threat to human health, unless effective vaccines, drugs or alternative treatments, such as passive immunization, become accessible. Animal-derived immunoglobulins, such as equine immunoglobulins might be useful as immunoprophylaxis or immunotherapy against this viral disease. Therapeutic antibodies (Abs) for SARS-CoV-2 were obtained from hyperimmune equine plasma using the Spike protein receptor binding domain (RBD) as an immunogen. The presence of anti-RBD antibodies was evaluated by ELISA and the titres of neutralizing antibodies were determined in viral cell culture. Immunized horses generated high-titre of anti-RBD antibodies with antiviral neutralizing activity on Vero-E6 cells of 1/1,000. To minimize potential adverse effects, the immunoglobulins were digested with pepsin, and purified to obtain the F(ab’)2 fragments with the protocol standardized by Biotecfar C.A for the production of snake antivenom. Pre-immune serum displayed an unexpected anti-RBD reactivity by ELISA (titre up to 1/900) and Western Blot, but no angioneutralizing activity. Modelling of the RBD of equine coronavirus showed that some of the known epitopes of SARS-CoV-2 RBD were structurally conserved in the equine coronavirus protein. This might suggest that some of the reactivity observed in the pre-immune serum to the SARS-CoV-2 RBD might be due to a previous exposure to equine coronavirus.