Heterogeneity of the antigenic site responsible for the induction of neutralizing antibodies in infectious bursal disease virus.

Using neutralizing monoclonal antibodies, three categories of escape mutants were selected from a stock of wild-type infectious bursal disease virus (IBDV). Additional mutants were found, where alterations coexisted in two or three of these epitopes. Although each group of mutants had a distinct reaction pattern with neutralizing monoclonal antibodies, all types of mutants were neutralized by convalescent chicken sera to the same extent. In spite of the lack of homogeneity in these antigenic sites located on IBDV structural polypeptide VP2, all neutralizing monoclonal antibodies reacted with epitopes in extracts prepared from the bursa of Fabricius from animals that had died during recent outbreaks of infectious bursal disease in the F.R.G. and Africa. Since binding to VP2 of the escape mutants, demonstrable by immunoprecipitation, correlated with the neutralizing capacity of these antibodies, a combined immunoprecipitation-immunoblotting technique was established as equivalent for a neutralization assay. The results of our experiments indicate that IBDV did not undergo a major antigenic variation in these two areas of Europe and Africa. The significance of protein conformation for the interaction of VP2 with neutralizing antibodies was underlined by the finding that renatured VP2 was capable of binding neutralizing antibodies; the antibodies induced in animals by immunization with this protein, however, were not neutralizing.