*2.1. Cells and Virus*

PRRSV strains, including the classical VR2332 strain (GenBank accession no. U87392.3; lineage 5.1), highly pathogenic XH-GD (GenBank accession no. EU624117; lineage 8.7) [21], and the NADC30-like strain HNLY (isolated in a sow with reproductive problem and saved in our lab; lineage 1), were saved

in our lab. Highly pathogenic JXA1 (GenBank accession no. EF112445.1; lineage 8.7) was generously offered by Professor Tian [9].

All of the PRRSV strains were propagated and titrated in Marc-145 cells. Virus titers of each strain were calculated by using a Reed-Muench method.
