Loop PDE of viral capsid protein is involved in immune escape of the emerging novel variant infectious bursal disease virus

Infectious bursa disease (IBD) is an acute, highly contactable, lethal, immunosuppressive infectious disease caused by the Infectious bursa disease virus (IBDV). Currently, the emerged novel variant IBDV (nVarIBDV) and the sustainedly prevalent very virulent IBDV (vvIBDV) are the two most prevalent strains of IBDV in China. The antigenic properties of the two prevalent strains differed significantly, which led to the escape of nVarIBDV from the immune protection provided by the existing vvIBDV vaccine. However, the molecular basis of the nVarIBDV immune escape remains unclear. In this study, we demonstrated, for the first time, that residues 252, 254, and 256 in the PDE of VP2 are involved in the immune escape of the emerging nVarIBDV. Firstly, the IFA-mediated antigen-antibody affinity assay showed that PBC and PDE of VP2 could affect the affinity of vvIBDV antiserum to VP2, of which PDE was more significant. The key amino acids of PDE influencing the antigen-antibody affinity were also identified, with G254N being the most significant, followed by V252I and I256V. Then the mutated virus with point or combined mutations was rescued by reverse genetics. it was further demonstrated that mutations of V252I, G254N, and I256V in PDE could individually or collaboratively reduce antigen–antibody affinity and interfere with antiserum neutralization, with G254N being the most significant. This study revealed the reasons for the widespread prevalence of nVarIBDV in immunized chicken flocks and provided innovative ideas for designing novel vaccines that match the antigen of the epidemic strain.