To investigate the specific antibody response, serum samples was obtained from vaccinated mice 7 days after each DNA vaccination and 3 days after boosting with rWR-PRRSV-M. An iELISA was performed according to methods described previously [16] . The purified M protein was used as the coating antigen, the tested sera applied at a 1:10 dilution, and an HRP-conjugated goat anti-mouse IgG antibody (Bio-Rad) used as the secondary antibody. The microplates were developed using orthophenylene diamine (OPDA, AMERSCO) and H 2 O 2 for 10 min, after which the reaction was stopped by the addition of 1 M H 2 SO 4 . Finally, the optical density (OD) was read at 492 nm. An anti-PRRSV-M monoclonal antibody was used as a positive control. Serum containing antibodies against Akabane virus and the sera of control group mice served as negative controls.