CRITICAL AMINO-ACID CHANGES IN VP2 VARIABLE DOMAIN ARE ASSOCIATED WITH TYPICAL AND ATYPICAL ANTIGENICITY IN VERY VIRULENT INFECTIOUS BURSALDISEASE VIRUSES

Classical serotype 1 infectious bursal disease viruses (IBDV), but not very virulent (vv) isolates, react with neutralizing monoclonal antib ody (NMab) 3 in virus neutralization tests or antigen-capture ELISA. T wo other NMabs, 6 and 8, bind to both classical and most vv strains, b ut not to the atypical 94 432 and 91 168 vv strains, respectively. The basis for such reactivities was investigated by sequencing the genome region encoding the VP2 major immunogenic domain. In classical, varia nt, vaccine or vv IBDV strains, negative reactions with NMab3 were ass ociated with changes in the Proline-Glycine pair at amino-acid (aa) po sitions 222-223 (hydrophilic peak A), and negative reactions with NMab s 6 and 8 with aa changes from positions 318 to 324 (hydrophilic peak B). The 91 168 and 94 432 viruses are the first vvIBDVs to present aa changes in peak B.