Selection of an infectious bursal disease virus mutant with increased immunogenicity following passage under humoral immune pressure.

It is generally known that the pathogenicity of infectious bursal disease virus (IBDV) strains decreases following passage in cell culture. However, there is no information about the effect of passage under immune pressure on the phenotypic and molecular properties of IBDV. In the present study, a small plaque mutant virus with poor neutralization capability, but showing similar growth characteristics as the parental virus strain, QC2, was isolated after serial passage in Vero cells in presence of IBDV serotype 1 chicken polyclonal antiserum. This mutant virus showed reduced pathogenicity but enhanced immunogenicity compared to the parental virus. Sequence analysis of the non-coding regions of the genome revealed 4 and 3 nucleotide changes in the 3' non-coding regions of segments A and B, respectively, and none in the 5' non-coding regions. Restriction enzyme analysis of selected coding regions of the IBDV genome in both viruses revealed a loss of the PstI site in the VP2 region of the mutant virus. Selection of such mutant viruses by passaging under immune pressure may offer an improved method for developing safer and more effective attenuated vaccine strains against infectious bursal disease of chickens.