Amino acid changes in the variable region of VP2 in three infectious bursal disease viruses with different virulence, originating from a common ancestor

An infectious bursal disease virus (IBDV), Hyd(C), from an outbreak was iso lated and plaque-purified in BGM-70 cells. From the moderately virulent pla que-purified virus, two IBDVs with relatively high and low virulence were o btained by passaging the virus in specific pathogen free chickens and BGM-7 0 cells 10 and seven times, respectively. Comparison of amino acid sequence s of the VP2 variable region of these viruses revealed that three amino aci ds at positions 279, 284 and 300 (Asp, Thr and Glu, respectively, in the pl aque-purified virus) were changed. In in vitro-passaged virus, amino acid r esidues 279, 284 and 300 were Asn, Thr and Glu, whereas these were Asp, Ala and Gln in the in vivo-passaged virus. Change of residue 284 (Thr --> Ala) had a critical role in cell culture infectivity, whereas the change in res idue 279 (Asp --> Asn) was associated with attenuation of the virus. No cor relation could be observed between amino acid changes at position 300 and v irulence or cell culture infectivity. Moreover, residue 330 (Arg) in heptap eptide motif SWSAR(330) GS was not found to be associated with the cell cul ture infectivity or virulence.