STRAIN VARIABILITY AND LOCALIZATION OF IMPORTANT EPITOPES ON THE MAJOR STRUCTURAL PROTEIN (VP2) OF INFECTIOUS PANCREATIC NECROSIS VIRUS

Infectious pancreatic necrosis virus (IPNV), a birnavirus, is an impor tant pathogen in fish farms. Analyses of viral proteins showed that VP 2 is the major structural and immunogenic polypeptide of the virus. Al l neutralizing monoclonal antibodies (mAbs) against IPNV are specific to VP2 and bind to continuous or discontinuous epitopes. In order to d etermine which parts of the protein are involved in antigenic variatio ns, five IPNV strains were sequenced over the VP2 coding region. Compa rison of the sequences obtained with three previously published strain s revealed a central variable domain (positions 183 to 335) which enco mpasses two hydrophilic hypervariable segments. Viral mutants which es caped neutralization were then selected with anti-VP2 mAbs directed ag ainst discontinuous epitopes. Sequencing of three mutants revealed a s ingle amino acid mismatch in each of them. All of these substitutions occurred in the hypervariable segments, suggesting that these regions are involved in the formation of a discontinuous epitope. Finally, exp ression of different truncated VP2s in Escherichia coli allowed locali zation of the binding site for neutralizing mAbs which recognize conti nuous epitopes. One of these mAbs bound to the region adjacent to the C-terminus of the variable domain of VP2, while two others reacted wit h the central and C-terminal parts of the variable domain. No antibody reacted with the N-terminus of VP2. These results suggest that the va riable domain of VP2 and the 20 adjacent amino acids of the conserved C-terminal part are the most important in inducing an immune response for the protection of animals. (C) 1995 Academic Press, Inc.