Immunogenic fusion proteins induce neutralizing SARS-CoV-2 antibodies in the serum and milk of sheep

•Novel antigen fusions of SARS-CoV-2 RBD to both ovine IgG2a Fc domains and GM-CSF were designed and tested in a large ruminant model.•Immunization produced neutralizing antibody responses that were transferred to colostrum/milk.•IgG2a Fc fusions had higher antigenicity than native antigens.•Our approach suggests an improved strategy for generating hyperimmune milk products with enhanced bioactivity. Antigen-specific polyclonal immunoglobulins derived from the serum, colostrum, or milk of immunized ruminant animals have potential as scalable therapeutics for the control of viral diseases including COVID-19. Here we show that the immunization of sheep with fusions of the SARS-CoV-2 receptor binding domain (RBD) to ovine IgG2a Fc domains promotes significantly higher levels of antigen-specific antibodies compared to native RBD or full-length spike antigens. This antibody population contained elevated levels of neutralizing antibodies that suppressed binding between the RBD and hACE2 receptors in vitro. A second immune-stimulating fusion candidate, Granulocyte-macrophage colony-stimulating factor (GM-CSF), induced high neutralizing responses in select animals but narrowly missed achieving significance. We further demonstrated that the antibodies induced by these fusion antigens were transferred into colostrum/milk and possessed cross-neutralizing activity against diverse SARS-CoV-2 variants. Our findings highlight a new pathway for recombinant antigen design in ruminant animals with applications in immune milk production and animal health.