title: Antigenic analysis of classical swine fever virus E2 glycoprotein using pig antibodies identifies residues contributing to antigenic variation of the vaccine C-strain and group 2 strains circulating in China.
abstract: BACKGROUND
  Glycoprotein E2, the immunodominant protein of classical swine fever virus (CSFV), can induce neutralizing antibodies and confer protective immunity in pigs. Our previous phylogenetic analysis showed that subgroup 2.1 viruses branched away from subgroup 1.1, the vaccine C-strain lineage, and became dominant in China. The E2 glycoproteins of CSFV C-strain and recent subgroup 2.1 field isolates are genetically different. However, it has not been clearly demonstrated how this diversity affects antigenicity of the protein.
  
  
  RESULTS
  Antigenic variation of glycoprotein E2 was observed not only between CSFV vaccine C-strain and subgroup 2.1 strains, but also among strains of the same subgroup 2.1 as determined by ELISA-based binding assay using pig antisera to the C-strain and a representative subgroup 2.1 strain QZ-07 currently circulating in China. Antigenic incompatibility of E2 proteins markedly reduced neutralization efficiency against heterologous strains. Single amino acid substitutions of D705N, L709P, G713E, N723S, and S779A on C-strain recombinant E2 (rE2) proteins significantly increased heterologous binding to anti-QZ-07 serum, suggesting that these residues may be responsible for the antigenic variation between the C-strain and subgroup 2.1 strains. Notably, a G713E substitution caused the most dramatic enhancement of binding of the variant C-strain rE2 protein to anti-QZ-07 serum. Multiple sequence alignment revealed that the glutamic acid residue at this position is conserved within group 2 strains, while the glycine residue is invariant among the vaccine strains, highlighting the role of the residue at this position as a major determinant of antigenic variation of E2. A variant Simpson's index analysis showed that both codons and amino acids of the residues contributing to antigenic variation have undergone similar diversification.
  
  
  CONCLUSIONS
  These results demonstrate that CSFV vaccine C-strain and group 2 strains circulating in China differ in the antigenicity of their E2 glycoproteins. Systematic site-directed mutagenesis of the antigenic units has revealed residues that limit cross-reactivity. Our findings may be useful for the development of serological differential assays and improvement of immunogenicity of novel classical swine fever vaccines.
authors.affiliation: Institute of Preventive Veterinary Medicine, Zhejiang Provincial Key Laboratory of Preventive Veterinary Medicine, Zhejiang University, Hangzhou 310029, PR China.
authors.forename: Ning
authors.initials: N
authors.lastname: Chen
authors.forename: Chao
authors.initials: C
authors.lastname: Tong
authors.forename: Dejiang
authors.initials: D
authors.lastname: Li
authors.forename: Jing
authors.initials: J
authors.lastname: Wan
authors.forename: Xuemei
authors.initials: X
authors.lastname: Yuan
authors.forename: Xiaoliang
authors.initials: X
authors.lastname: Li
authors.forename: Jinrong
authors.initials: J
authors.lastname: Peng
authors.forename: Weihuan
authors.initials: W
authors.lastname: Fang
mesh_terms: D000595:Amino Acid Sequence / Q000235:genetics; D000818:Animals; D057134:Antibodies, Neutralizing / Q000235:genetics / Q000276:immunology; D000940:Antigenic Variation / Q000235:genetics / Q000276:immunology; D000956:Antigens, Viral / Q000235:genetics / Q000276:immunology*; D002681:China; D006691:Classical Swine Fever / Q000276:immunology; D006692:Classical Swine Fever Virus / Q000235:genetics* / Q000276:immunology*; D003062:Codon / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics; D014644:Genetic Variation; D007106:Immune Sera / Q000276:immunology; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D011994:Recombinant Proteins / Q000235:genetics / Q000276:immunology; D013552:Swine; D014759:Viral Envelope Proteins / Q000235:genetics* / Q000276:immunology*
chemical_list: D057134:Antibodies, Neutralizing; D000956:Antigens, Viral; D003062:Codon; D007106:Immune Sera; D011994:Recombinant Proteins; D014759:Viral Envelope Proteins; C087086:glycoprotein E2, classical swine fever virus