Identification of a conserved linear B-cell epitope at the N-terminus of the E2 glycoprotein of Classical swine fever virus by phage-displayed random peptide library

The E2 protein of Classical swine fever virus (CSFV) is an important envelope glycoprotein, which is responsible for inducing protective immune response in swine. Four antigenic domains, A–D, have been mapped on the E2 protein. The present study describes the identification of a linear B-cell epitope at the N-terminus of the E2 protein by screening a phage-displayed random 12-peptide library with the neutralizing monoclonal antibody (mAb) HQ06 directed against the E2 protein. HQ06 was found to bind to the phages displaying a consensus motif LFDGSNP, which is highly homologous to 772LFDGTNP778 of the CSFV polyprotein, locating on the border between antigenic domains B/C and A of the E2 protein. Considering that HQ06 showed reactivity with the motif 772LFDGTNP778 expressed as GST fusion in Western blot and indirect ELISA, we propose that the motif 772LFDGTNP778 represents a linear B-cell epitope of the E2 protein. The motif 773FDGTNP778 is the minimal requirement for the reactivity as demonstrated by analysis of the reactivity of HQ06 with several truncated peptides derived from the motif. Alignment of amino acid sequences from a number of CSFV isolates indicated that the epitope is well conserved among different subgroups of CSFV. Substitutions of the individual residues within the epitope 773FDGTNP778 demonstrated that residues 773F, 775G, and 778P constitute the core of the epitope. The identified epitope will be useful for development of diagnostic assays and epitope-based marker vaccines against CSFV.