Formation of subviral particles of the capsid protein VP2 of infectious bursal disease virus and its application in serological diagnosis

Infectious bursal disease virus (IBDV) is an immunosuppressive disease of young chicken characterized by severe depletion of B-lymphocytes in the bursa of Fabricius. To provide antigen for diagnostic tests, its major structural protein VP2 was expressed in the yeast Saccharomyces cerevisiae. Electron microscopy of purified VP2 protein demonstrated that when expressed from yeast cells VP2 protein forms subviral particles (SVPs) of approximately 20 nm in diameter. A recombinant VP2 antigen-based single serum dilution enzyme linked immunosorbent assay (ELISA) using the SVPs detected IBDV specific antibodies in chickens. A linear relationship was found between the predicted antibody titres at a single working dilution of 1:1000 and the corresponding observed serum titres when determined by the standard serial dilution method. Regression analysis was used to construct a standard curve from which an equation was derived which confirmed their correlation. The equation was then used to convert the corrected absorbance readings of the single working dilution directly into the predicted ELISA antibody titres. The assay proved to be sensitive, specific and accurate as compared to the serum neutralization test and agar gel immunodiffusion test. The recombinant VP2 antigen is a suitable alternative to whole viral antigen.