Chimeras in noncoding regions between serotypes I and II of segment A of infectious bursal disease virus are viable and show pathogenic phenotype in chickens

Institute of Molecular Biology, Friedrich-Loeffler-Institutes, Federal Research Centre for Virus Diseases of Animals, D-17498 Insel Riems, Germany 1 Intervet International BV, NL-5830AA Boxmeer, The Netherlands 2 Author for correspondence: Egbert Mundt. Fax +49 38351 7151. e-mail Egbert.Mundt{at}rie.bfav.de Two serotypes, I and II, have been identified for infectious bursal disease virus (IBDV), a member of the family Birnaviridae . Here, the generation by reverse genetics of IBDV chimeras in segment A of the bisegmented genome is reported. The 5- and 3'-noncoding regions (NCRs) of a serotype II strain were exchanged with the NCRs of a full-length cDNA clone of segment A of a serotype I strain. Isolated chimeric viruses were characterized in cell culture and susceptible chickens. The results show that IBDV chimeras in segment A were able to replicate in cell culture and that VP1 encoded by a serotype I segment B is functionally active with serotype I NCRs as well as with serotype II NCRs. Chimeric viruses infected susceptible chickens and caused mild depletion of bursal cells. Thus, the noncoding regions of segment A are not responsible for the different pathotypes of IBDV serotypes I and II.