Amino acid mutations of porcine circovirus type 2 (PCV2) capsid protein increase virus binding to host and evade immune responses: An evaluation of viral evolution.

Porcine circovirus type 2 (PCV2), the causative agent of porcine circovirus-associated diseases (PCVAD), has caused huge economic losses in the swine industry. Despite a worldwide disease distribution and numerous reports of the field epidemiology of PCV2, the molecular epidemiology and genetic evolution of PCV2 are not well characterized. In this study, 72 complete genomes of PCV2 strains sequenced from 2016 to 2022 in China were produced; phylogenetic analyses demonstrated three genotypes (PCV2a, PCV2b, and PCV2d). Strains PCV2b and PCV2d were equally represented (30/72, 41.6%) whereas the remainder were PCV2a strains (12/72, 16.6%). Therefore, PCV2b and PCV2d have become dominant genotypes in China. Based on the 1682 capsid protein (Cap) sequences, 27 high-frequency amino acid mutations occurred in Cap, with 18 of 27 mutated amino acids exposed on the capsid surface. That amino acid mutations with increasing positively charged polar amino acids occurred on the capsid surface implied PCV2 may have evolved to better adapt to hosts by enhancing the binding capacity of PCV2 to negatively charged cell receptors. Moreover, these mutations probably enable PCV2 to evade host immune responses by changing epitope antigenicity and decreasing epitope accessibility to the immune system. This study characterized the genotypic evolution of PCV2 and assessed the adaptive value of viral mutations in evading host defences, providing powerful insights to elucidate potential mechanisms of immune evasion and evidence to inform effective prevention and control strategies.