Antigenic domains analysis of classical swine fever virus E2 glycoprotein by mutagenesis and conformation-dependent monoclonal antibodies

Glycoprotein E2 of classical swine fever virus (CSFV) is the major antigenic protein exposed on the outer surface of the virion that induces main neutralizing antibodies during infection in pigs. This study displays the differences in antigenicity of E2 between vaccine and field strains of CSFV by their variable reaction patterns between expressed proteins and monoclonal antibodies (mAbs). The D/A domains of various CSFVs were relatively conserved and recognized by all mAbs against the A domain. However, mAbs against B/C domains were able to differentiate field viruses TD/96/TWN (subgroup 2.1) and 94.4/IL/94/TWN (subgroup 3.4) from the vaccine virus LPC/AHRI (subgroup 1.1). By analysis of expressed truncated proteins, the epitope(s) on B/C domains were mapped to the N-terminal 90 residues of E2 between amino acids 690 and 779. Site-directed mutagenesis further showed that residues 693C, 737C, 771L, 772L, 773F and 774D were critical for the reactivity of E2 protein with mAbs. Thus, the B/C domains are responsible for antigen specificity among various CSFVs, and the disulfide bond and motif 771LLFD774 are essential for the structural integrity of its conformational recognition. These data significantly increase our understanding of the antigenic structure of E2 for antibody binding.