Viral culturing and attenuation
Lineage 1 PRRSV strain SD was continuously passaged in Marc-145 cells using Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 2% foetal bovine serum and incubated at 37°C with 5% CO2. The virus was harvested once the virus-infected Marc-145 cells showed ~80% cytopathic effect (CPE). The titres of the wild-type strains SD and HLJWK108-1711 and NADC30-like PRRSV SD at different passages were measured by seeding Marc-145 cells into 96-well cell culture plates 2 d before infection. The 50% tissue culture infective dose (TCID50) was calculated according to the Reed-Muench method. The 125th passage of PRRSV strain SD was harvested and designated lineage 1 PRRSV strain SD-R, which was characterized and evaluated in the present study.