A recombinant Fc-fused nanobody provides complete protection against feline parvovirus infection

Feline panleukopenia (FPL), caused by feline parvovirus (FPV), is a fatal infectious disease in cats characterized by leukopenia, fever, and severe enteritis, for which no specific antiviral treatment is available. Here, we report the development of a potent neutralizing nanobody-based therapeutic against FPV. A high-diversity camelid VHH library (2.95 × 10⁹ CFU) was constructed and screened to identify Nb35, a nanobody with high affinity and specificity for FPV. To enhance its stability and in vivo persistence, Nb35 was fused to the human IgG1 Fc domain, yielding the recombinant fusion antibody 35Fc. The Fc fusion preserved the favorable biophysical properties of Nb35 while markedly improving pharmacokinetic behavior, enabling sustained viral neutralization. Biochemical and biophysical analyses revealed that 35Fc exhibits strong binding affinity and superior neutralizing potency compared with conventional monoclonal antibodies. Molecular docking and dynamics simulations demonstrated that 35Fc interacts stably with the VP2 capsid protein via an extensive network of hydrogen bonds, salt bridges, and π-cation interactions, with a binding free energy of -63.11 kcal/mol. Key interface residues, including LYS76 and TYR561, etc., play critical roles in maintaining this high-affinity complex. In an FPV-infected cat model, 35Fc treatment achieved 100 % survival, alleviated clinical symptoms, and almost completely suppressed viral replication and shedding across fecal, ocular-nasal, and intestinal tissues, while preserving organ integrity. Collectively, these results demonstrate that 35Fc effectively blocks FPV infection by targeting the VP2 capsid and provide structural and functional evidence for the development of Fc-fused nanobody therapeutics in veterinary medicine.