Residues of 862, 921 of VP3 are associated with virulence in infectious bursal disease virus strain Harbin-1

Reverse genetics was used to study the effect of particular amino acids of infectious bursal disease virus (IBDV) on virulence. Using site-directed mutagenesis, altering of two amino acids in VP2 (Q253H, A284T) and VP3 (H783Q, V862M, I921V) in the segment A of a Chinese very virulent IBDV field strain Harbin-1, 4 virus mutants including H253/284, H783/862, H862/921, H921/783 were rescued. To evaluate the characteristics of the recovered viruses in vivo, we inoculated 4-week-old chickens with virus mutants and rescued Harbin1 (rHarbin-1), analyzed their bursae for pathological lesions 4 days postinfection. rHarbin-1 and H783/862, H253/284 caused severe bursal lesion, milder lesion for H862/921, mildest for H921/783. However, H253/284 caused the lowest mortality. The results showed that residue at position Q253, A284 of VP2 and V862, I921 of VP3 gene are involved with virulence, but there is difference between VP2 and VP3’s role in virulence. The ability of 862 and 921 to control virulence in VP3 is stronger than 253 and 284.