*2.2. Reagent and Antibodies*

Epigallocatechin gallate (EGCG, 99.91% purity, HY-13653, Medchemexpress, Monmouth, NJ, USA), epicatechin (EC, 99.00% purity, HY-N0001, Medchemexpress) and heparin (99.00% purity, H3393, Sigma-Aldrich, St. Louis, MO, USA) were dissolved into ddH2O at a concentration of 50 mM for storage. The monoclonal antibody (mAb) 5E11 against PCV2 capsid was generated in our laboratory [34]. The fluorescein isothiocyanate (FITC) conjugated goat anti-mouse IgG (ab6785, Abcam, Cambridge, MA, USA), horse radish peroxidase (HRP) conjugated goat anti-mouse IgG (074-1802, KPL, Milford, MA, USA) and Mouse IgG1 Isotype Control (564416, BD Horizon™, San Jose, CA, USA) were purchased from Abcam, Kirkegaard & Perry Laboratories and BD Biosciences, respectively.

#### *2.3. Cytotoxicity Assay*

The cytotoxicity assay of compounds was performed using enhanced CCK8 kit (C0014, Beyotime, Shanghai, China) according to the standard instructions. PK-15 cells seeded into 96-well plate was treated with different concentrations of EGCG or EC for 12 h. Subsequently, 10 μL CCK8 reagent per well was added into the culture medium and after 1 h of incubation, absorbance value (A450) was measured by a spectrophotometer. The cell viability was calculated as (A450compound/A450mock) × 100%.

### *2.4. Determination of Virus Titer*

PK-15 cells seeded in the 96-well plates were added with 10-fold serially diluted virus samples. At 96 hpi (hours post-infection) the virus titers were determined by indirect immunofluorescent assay (IFA) with mAb 5E11 against PCV2 capsid and FITC-conjugated goat anti-mouse IgG. The 50% tissue culture infective dose (TCID50) was calculated according to the Reed-Muench method. The multiplicity of infection (MOI) was defined as the number of plaques forming units (PFUs) computed as PFU = 0.7 × TCID50.
