Amino acid mutations in the capsid protein produce novel porcine circovirus type 2 neutralizing epitopes.

Porcine circovirus type 2 (PCV2) is associated with postweaning multisystemic wasting syndrome in pigs. A monoclonal antibody (mAb) 8E4 against the PCV2 capsid protein has the capacity to neutralize the virus. However, this mAb can only react with some PCV2a strains (LG, CL, and JF2; mAb 8E4-positive strains), but does not cross-react with some PCV2b strains (YJ and JF; mAb 8E4-negative strains). In the present study, site-directed mutagenesis was performed targeting the external amino acids of the capsid proteins, which are different between mAb 8E4-positive and -negative strains. A mutation of arginine to alanine at position 59 in the capsid protein of strain JF allowed the mutant to be recognized and neutralized by mAb 8E4. Likewise, mutations of arginine to alanine at position 59 together with alanine to threonine at position 60 in the capsid protein of the YJ strain resulted in a gain of neutralization and recognition by mAb 8E4. Here, we demonstrated that the amino acids at positions 59 and 60 in the capsid protein of PCV2 participate in the formation of conformational neutralizing epitopes and mutations at positions 59 or 59/60 result in novel neutralizing epitopes of mAb 8E4-negative strains. This study provides valuable information for further in-depth mapping of the conformational neutralizing epitopes, clarification of antigenic differences among PCV2 strains, and development of a useful vaccine candidate for control of PCV2-associated diseases.