Identification of sequence changes responsible for the attenuation of highly virulent infectious bursal disease virus

The genetic changes responsible for the attenuation of infectious bursal disease virus (IBDV) have not been defined at the molecular level, although passage of the virus in cell culture results in the loss of virulence. To understand the molecular basis of IBDV virulence and attenuation, the IBDV genome segment encoding the precursor polyprotein (NH2-VP2-VP4-VP3-COOH) of a cell culture-adapted OKYMT strain derived from highly virulent OKYM was cloned as cDNA, and the nucleotide sequence was determined. Comparison of the identified nucleotide and deduced amino acid sequences of the attenuated strain with the parental virulent OKYM strain revealed only five amino acid differences: four in the VP2 variable domain and one in the VP3. Two amino acid substitutions at positions 279 (Asp-->Asn) and 284 (Ala-->Thr) in the VP2 variable domain were commonly predicted in another cell culture-adapted strain. These two amino acid changes resulted in reduced hydrophila of this region and deletion of the alpha-helix which might alter the conformation of the virion surface structures. These results may imply that the amino acid residues at position 279 and 284 in VP2 variable domain contribute to virulence of IBDV.