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Infectious bursal disease virus (IBDV) is a pathogen of worldwide significance to the poultry industry. Although the P DE  and P FG  domains of the capsid protein VP2 contribute significantly to virulence and fitness, the detailed molecular basis for the pathogenicity of IBDV is still not fully understood. Because residues 253 and 284 of VP2 are not the sole determinants of virulence, we hypothesized that other residues involved in virulence and fitness might exist in the P DE  and P FG  domains of VP2. To test this, five amino acid changes selected by sequence comparison of the P DE  and P FG  domains of VP2 were introduced individually using a reverse genetics system into the virulent strain (rGx-F9VP2). Then reverse mutations of the selected residues 249 and 256 were introduced individually into the attenuated strain (rGt). Seven modified viruses were generated and evaluated  in vitro  (CEF cells) and  in vivo  (SPF chicken). For residue 249, Q249R could elevate  in vitro  and reduce  in vivo  the replication of rGx-F9VP2 while R249Q could reduce  in vitro  and elevate  in vivo  the replication of rGt; meanwhile Q249R reduced the virulence of rGx-F9VP2 while R249Q increased the virulence of rGt, which indicated that residue 249 significantly contributed to the replication and virulence of IBDV. For residue 256, I256V could elevate  in vitro  and reduce  in vivo  the replication of rGx-F9VP2 while V256I could reduce  in vitro  but didn’t change  in vivo  the replication of rGt; although V256I didn’t increase the virulence of rGt, I256V obviously reduced the virulence of virulent IBDV. The present results demonstrate for the first time, to different extent, residues 249 and 256 of VP2 are involved in the replication efficiency and virulence of IBDV; this is not only beneficial to further understanding of pathogenic mechanism but also to the design of newly tailored vaccines against IBDV.

Mutations of Residues 249 and 256 in VP2 Are Involved in the Replication and Virulence of Infectious Bursal Disease Virus