Exchange of the VP5 of infectious bursal disease virus in a serotype Ⅰ strain with that of a serotype Ⅱ strain reduced the viral replication and cytotoxicity

Infectious bursal disease virus (IBDV), belonging to Avibirnavirus genus in the Birnaviridae family, consists of two segments of double-strand RNA. There are two distinct serotypes of IBDV, the pathogenic serotype Ⅰ and the non-pathogenic serotype Ⅱ. Comparison of the deduced amino acid sequences of a panel of VP5 genes retrieved from GenBank revealed a high identity among strains within the serotype Ⅰ or serotype Ⅱ group but a low identity between strains across two serotypes. In this study, we rescued two mosaic viruses, rGtGxVP5 and rGt2382VP5 by exchanging the VP5 gene of a cell culture-adapted serotype Ⅰ Gt strain with its counterpart of the very virulent IBDV Gx strain, or a non-pathogenic 23/82 strain of the serotype Ⅱ. In comparison to the parental strain rGt virus, the rGtGxVP5 showed the similar viral replication, cytotoxicity and the ability of inducing apoptosis; however, the other mosaic virus rGt2382VP5 had a lower titer and a reduced cytotoxicity. Although exchange of VP5 within serotype Ⅰ group did not alter the viral replication and cytotoxicity of Gt strain, exchange of VP5 in the serotype Ⅰ with that of a serotype Ⅱ reduced the viral replication and cytotoxicity on chicken embryo fibroblast (CEF) cells. Therefore, the VP5 of serotype Ⅱ may be one of the factors responsible for the distinct pathogenic features of two serotypes.