Antigenic structure analysis of VP3 of infectious bursal disease virus

VP3 is one of the major structural proteins of infectious bursal disease virus (IBDV), but the epitopes of VP3 have not been precisely identified. To further identify its epitopes, VP3 of Gx strain was cloned and expressed as a recombinant protein in Escherichia coli BL21 (DE3). Female BALB/c mice were immunized with the purified VP3 and then four VP3-specific monoclonal antibodies (MAbs) were developed. The MAbs specifically reacted with chicken embryo fibroblasts (CEF) infected with IBDV. A set of 17 partially overlapping or consecutive peptides (P1-P17) spanning VP3 were expressed for epitope screening by pepscan. Through Western blot and enzyme-linked immunosorbent assay (ELISA), two epitopes of VP3, 109-119aa (864-874aa of polyprotein) and 177-190aa (932-945aa of polyprotein), were identified. The two epitopes are totally homologous in many vvIBDV, classical strains, attenuated strains and serotype 2. Both peptides have good immunogenicity and could induce antibodies against IBDV in BALB/c mice. In addition, the two epitope peptides could react with IBDV positive chicken serum and IBDV VP3 positive mice serum. This is the first time that the linear B cell epitopes on VP3 of IBDV have been identified in such a precise location, which may be a benefit to further understanding VP3 of IBDV.