Lymphocyte proliferative responses in immunized mice after in vitro stimulation with purified PRRSV LV antigen. Each group of mice (n = 6) was immunized with 100 μg of DNA vaccine at 0 and 3 weeks. Five weeks after the last inoculation, the mice were sacrificed and their splenocytes stimulated with PRRSV virus. After 72 h of stimulation, WST-1 was added and OD values were determined after a further 4 h of incubation. The samples were assayed in triplicate. Data are presented as the mean ± standard error. * Indicates a significant difference (P < 0.05) between the groups. Data are shown as the mean ± S.D.