Acute infectious bursal disease in poultry: Immunological and molecular basis of antigenicity of a highly virulent strain

Abstract

This study has confirmed, by the use of immunological and molecular tools, that the recent failures of vaccination against infectious bursal disease (IBD) encountered in Europe were not related to antigenic variation, but to increased virulence of the circulating IBD virus strains. Neutralizing monoclonal antibodies (Mabs) showed that the vaccines of intermediate virulence and the pathogenic strain 849VB had a similar pattern of reactivity in ss neutralization tests. Four distinct epitopes could be defined in seroneutralization and addition ELISA tests. All neutralizing Mabs bound to the structural VP2 protein only in its native form. Moreover, Mabs which did not neutralize some strains precipitated well the VP2 protein from extracts of cells infected with the same virus. This suggests that slight changes in the conformation of the epitope were sufficient to allow the virus to escape to neutralization. VP2 sequencing results confirmed that the neutralizing epitopes are clustered in the variable domain which is highly hydrophobic and flanked by two major hy-drophilic peaks. Three potential 'minor' antigenic sites were identified within the hydrophobic region. Comparison of the VP2 sequence of 849VB strain with other highly virulent isolates showed that they are close together and clearly distinct from 'classical' strains. Moreover, sequencing of IBD vaccines revealed that some of them had not been cloned.