Cells, viruses, and plasmids.

The MA104-derived monkey kidney cell line MARC-145 was cultured in Dulbecco’s modified Eagle’s medium (DMEM) (Gibco) with 10% fetal bovine serum (FBS) (Gibco) and penicillin (50 U/mL) and streptomycin (50 μg/mL) at 37°C under a humid 5% CO₂ atmosphere. PAMs, the target cells of PRRSV, prepared from 1-month-old SPF pigs were maintained in RPMI 1640 (Gibco) medium, containing 10% FBS and penicillin (50 U/mL) and streptomycin (50 μg/mL) as previously described (41). The full-length infectious cDNA clone plasmids (pWSK-JXwn) of HP-PRRSV JXwn06 (GenBank accession number {"type":"entrez-nucleotide","attrs":{"text":"EF641008","term_id":"149929787","term_text":"EF641008"}}EF641008) and the rescued viruses (RvJXwn) were used in this study (33).