VP5 and the N terminus of VP2 are not responsible for the different pathotype of serotype I and II infectious bursal disease virus

Two serotypes have been identified in infectious bursal disease virus (IBDV ), a member of the family Birnaviridae. A reverse genetics system was used for generation of chimeras in genome segment A of the two serotypes, in whi ch the complete viral VP5 gene and 3' noncoding region (NCR), or parts ther eof, were exchanged. The engineered viruses were characterized in vitro and in vivo in comparison to serotype I and II IBDV. Our results show that IBD V chimeras exhibit a different phenotype in cell culture compared to the wi ld-type viruses. In in vitro-cultivated bursal-derived cells, chimeric viru ses infected B lymphocytes, as does serotype I IBDV, Surprisingly, serotype II virus was also able to infect in vitro-cultivated bursal cells, but the se were neither B lymphocytes nor macrophages. After infection of susceptib le chickens all chimeras replicated in the bursa of Fabricius (BF), and thr ee chimeric viruses caused mild depletion of bursal cells. In contrast, aft er infection of chickens with a chimeric IBDV containing exchanged VP5 as w ell as 3'-NCR, no depletion was detectable. The serotype II strain did not replicate in the BF nor did it cause depletion of bursal cells. Thus, the o rigin of VP5 does not explain the different pathotype of IBDV serotype I an d II.