Deletions of structural glycoprotein E2 of classical swine fever virus strain Alfort/187 resolve a linear epitope of monoclonal antibody WH303 and the minimal N-terminal domain essential for binding immunoglobulin G antibodies of a pig hyperimmune serum

The major structural glycoprotein E2 of classical swine fever virus (CSFV) is responsible for eliciting neutralizing antibodies and conferring protect ive immunity. The current structural model of this protein predicts its sur face-exposed region at the N terminus with a short stretch of the C-termina l residues spanning the membrane envelope. In this study, the N-terminal re gion of 221 amino acids (aa) covering as 690 to 910 of the CSFV strain Alfo rt/187 E2, expressed as a fusion product in Escherichia coli, was shown to contain the epitope recognized by a monoclonal antibody (WH303) with affini ty for various CSFV strains but not for the other members of the Pestivirus genus, bovine viral diarrhea virus (BVDV) and border disease virus (BDV). This region also contains the sites recognized by polyclonal immunoglobulin G (IgG) antibodies of a pig hyperimmune serum. Serial deletions of this re gion precisely defined the epitope recognized by WH303 to be TAVSPTTLR (aa 829 to 837) of E2. Comparison of the sequences around the WH303-binding sit e among the E2 proteins of pestiviruses indicated that the sequence TAVSPTT LR is strongly conserved in CSFV strains but highly divergent among BVDV an d BDV strains. These results provided a structural basis for the reactivity patterns of WH303 and also useful information for the design of a peptide containing this epitope for potential use in the detection and identificati on of CSFV. By deletion analysis, an antigenic domain capable of reacting w ith pig polyclonal IgG was found 17 as from the WH303 epitope within the N- terminal 123 residues (aa 690 to 812). Small N- or C-terminal deletions int roduced into the domain disrupt its reactivity with pig polyclonal IgG, sug gesting that this is the minimal antigenic domain required for binding to p ig antibodies. This domain could have eliminated or reduced the cross-react ivity with other pestiviruses and may thus have an application for the sero logical detection of CSFV infection; evaluation of this is now possible, si nce the domain has been expressed in E. coli in large amounts and purified to homogeneity by chromatographic methods.