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 v irus (IBDV) strains decreases following passage in cell culture. However, t here is no information about the effect of passage under immune pressure on the phenotypic and molecular properties of IBDV. Ln the present study, a s mall plaque mutant virus with poor neutralization capability, but showing s imilar growth characteristics as the parental virus strain, QC2, was isolat ed after serial passage in Vero cells in presence of IBDV serotype 1 chicke n polyclonal antiserum. This mutant virus showed reduced pathogenicity but enhanced immunogenicity compared to the parental virus. Sequence analysis o f 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 developin g safer and more effective attenuated vaccine strains against infectious bu rsal disease of chickens.