| Infectious bursal disease virus (IBDV) is an avian, segmented double-stranded RNA virus. The genomic segment A encodes all the structural (VP2, VP4, and VP3) and nonstructural (NS) proteins, whereas segment B encodes the viral RNA-dependent RNA polymerase (VP1). To identify the molecular determinants for virulence, pathogenesis, and cell tropism of IBDV, full-length cDNA clones of a virulent strain IM and several chimeric cDNA clones of segments A and B between the attenuated vaccine strain (D78) and the virulent IM strains were constructed. Using the cRNA-based reverse genetics system developed for IBDV, four chimeric viruses were recovered after transfection by electroporation procedures in Vero or chicken embryo fibroblast (CEF) cells, one of which was recovered after propagation in embryonated eggs. To evaluate the characteristics of the recovered viruses in vivo, 3-week-old specific-pathogen-free chickens were inoculated by ocular route with equal doses of D78, IM, or chimeric viruses and their bursa were analyzed for pathological lesions at three days post infection. Viruses in which VP4, VP4+VP3, and VP1 coding sequences of the virulent strain IM were substituted for the corresponding region in the vaccine strain, failed to induce hemorrhagic lesions in the bursa. The virus in which VP2 coding region of the vaccine strain was replaced with the virulent IM strain, caused hemorrhagic lesions in the bursa, equivalent to those seen with the classical virulent strain. These results suggest that the virulence and pathogenesis markers of IBDV reside in VP2, principally due to specific amino acid substitutions in the hypervariable region of this protein. Two of the chimeric viruses, one containing VP1, and the other containing VP2 sequences of the classical virulent strain could only be recovered in CEF cells and embryonated eggs, respectively, suggesting that the cell tropism markers for IBDV reside both in VP1 and VP2. |
