*2.1. Cells and Virus*

PCV-free Porcine Kidney 15 (PK15) (BH0370, ATCC, Manassas, VA, USA) cells were supplied by China Institute of Veterinary Drugs Control and cultured in Opti-MEM (11095-072, Thermo Fisher, Waltham, MA, USA) with 10% fetal bovine serum (10099-141-FBS, Thermo Fisher). PCV2 strain HZ0201 (GenBank accession no. AY188355.1, 106.5TCID50/mL) was propagated in PK15 cells [33]. PCV2 virions were enriched using ultra-centrifugation.
