Several studies have suggested that genetic differences in PCV2 are associated with the geographical region from which the isolates originated, and a classification system that has been proposed divides PCV2 into three genotypes (a, b and c); a and b are the two major genotypes of PCV2 [8, 22–26], but c is only isolated in Demark . Therefore, PCV2c was not used in the present study. Until now, only one serotype has been identified among strains of PCV2. However, mAbs directed against PCV2 (except PCV2c) have shown some differences in reactivity with different PCV2 strains [7, 14].
MAb 8E4 generated in the present study reacted with PCV2a (LG, CL and JF2), by the IPMA and capture ELISA, and had the capacity to neutralize PCV2a (LG, CL and JF2). Therefore, using mAb 8E4, three strains of PCV2a could be differentiated from three PCV2b strains. However, mAb 8E4 did not give a positive reaction by western blot analysis. Thus, the above results suggest that mAb 8E4 recognizes a conformational epitope in the capsid protein of PCV2.
There were several regions of diversity identified by alignment of the amino acid sequences of the capsid protein between PCV2a and PCV2b strains in the present study. The first 46 residues at the N terminus of the capsid protein are probably not involved in the formation of conformational epitopes. This region contains residues rich in basic amino acids and thus may be involved in the formation of the interior surface of the virion, and may interact with the negative charges of genomic DNA during virus assembly, as reported for many icosahedral viruses [27–29]. Amino acids from residue 47 to the C terminus within the capsid protein may be important for formation of PCV2 capsid protein. Several epitopes in the PCV2 capsid protein that are involved in reactions with antibodies are also within this range [6, 7, 30]. Therefore, five regions (aa 47-72, 80-94, 110-154, 190-211 and 230-235) were chosen for construction of PCV2-ORF2-CL/YJ chimeras that included amino acids that differed between PCV2a and PCV2b.
A chimera in which 230PLKP233 of the C terminus of PCV2a/CL-Cap was replaced by 230PLNLNE235 of the C terminus of the PCV2b/YJ-Cap was constructed to analyze whether elongation of the C terminus affected the conformational neutralizing epitope against mAb 8E4. Elongation of the C terminus by two amino acids did not change the reactivity of mAb 8E4 against PCV2a/CL in the IPMA (Figure 1a). Furthermore, rJF2-ORF2, derived from PCV2a/JF2, in which the C terminus was elongated by three amino acids, had the same reactivity with mAb 8E4 as rCL-ORF2 and rCL-YJ-5 in the IPMA (Figure 1c).
In previous studies, analysis of the reactivity of PCV1/PCV2 chimeras has suggested that the amino acid sequences from aa 47-62 and 165-200, as well as the last four C-terminal amino acids of the capsid protein, are likely to be in close proximity and may form a cluster of conformational epitopes on the surface of the PCV2 virion . In the present study, the replacement of an amino acid residue (A59R) in the capsid protein altered the reaction of PCV2a (LG, CL, and JF2) with mAb 8E4. Therefore, it could be concluded that the alanine at position 59 was a critical amino acid in the conformational neutralizing epitope recognized by mAb 8E4. Alanine is a nonpolar hydrophobic amino acid with a molecular weight (MW) of 89 Da, whereas arginine is a polar basic hydrophilic amino acid with a MW of 174. Due to the differences in size, charge and hydrophobicity between alanine and arginine, this may have major consequences on the secondary and tertiary structure of the PCV2 capsid protein. Therefore, it could be concluded that the replacement of an amino acid residue (A59R) in the capsid protein of PCV2a (CL, LG and JF2) disrupted the binding of mAb 8E4 completely. Furthermore, the amino acid at position 59 is located on loop BC of the capsid protein . This loop together with loop DE and HI are on the exterior surface of the PCV2 to form the highest protrusion . Therefore, this position may be more easily recognized by B cell receptor and with a high possibility to become a conformational B cell epitope.
It was confirmed that another mutant (rYJ-CL-1-59), which contained a single amino acid mutation of R to A at position 59, did not have the ability to react with mAb 8E4. We suggest that the amino acid at position 59 of capsid protein is a necessary but not sufficient residue for epitope recognition by mAb 8E4. The 3D structure of capsid protein and mAb 8E4 complex should be studied to gain full knowledge of the conformational epitope against mAb 8E4.