VP1 protein of infectious bursal disease virus modulates the virulence in vivo

Infectious bursal disease viruses (IBDVs), belonging to the Birnaviridae family, cause severe immunodeficiency in young chickens by destroying the precursors of antibody-producing B cells in the bursa of Fabricius (BF). Different pathotypes of IBDVs, including cell culture-adapted viruses, differ markedly in virulence, which is characterized by mortality and bursal damage. To study the molecular determinants of virulence in IBDV, the genomic segments A and B of GLS bursa-derived (GLSBD) and tissue culture-adapted (GLSTC) viruses were cloned and sequenced. Comparison of the deduced amino acid sequences of segments A and B revealed only two amino acid substitutions at positions 87 (Q --> R) and 261 (P --> L) in segment B, and at positions 253 (Q --> H) and 284 (A --> T) in segment A; the latter of which has been shown to be involved in tissue culture adaptation and attenuation of the virus. To study the function of VP1 protein encoded by segment B, reassortant viruses between tissue culture-adapted strains, GLSTC and D78, and GLSBD were recovered using the reverse genetics system. The recombinant virus rGLSBDB containing segment B of GLSBD was able to replicate in Vero and chicken embryo fibroblast (CEF) cells but exhibited delayed replication kinetics. To evaluate the characteristics of these viruses in vivo, 3-week-old chickens were given equal doses of parental viruses or reassortant viruses by ocular inoculation. The pathological lesions and viral antigen distribution in BF were analyzed at 1, 2, or 3 days postinfection. Parental GLSBD and the recovered rGLSBDB viruses propagate most efficiently in the BF and cause severe bursal lesions, whereas the tissue culture-adapted GLSTC virus replicates less efficiently and induces mild bursal lesions at 3 days postinfection. Taken together, our results demonstrate that the VP1 protein of IBDV is involved in the efficiency of viral replication and modulates the virulence in vivo.