Porcine reproductive and respiratory syndrome (PRRS) has become one of the most economically important diseases to the global pork industry.Cunrently available commercial vaccines provide limited protection due to delayed and weak neutralizing antibody production and cell-mediated immunity.We investigated the effect of swine interleukin 4 (IL-4) on the development of virus-specific immune responses elicited by modified live PRRSV (MLV) vaccine.The level of antibodies against PRRSV membrane proteins in pigs elicited by MLV vaccine plus recombinant plasmid encoding IL-4 (group 3) were significantly higher than those by MLV vaccine alone (group 1) and MLV vaccine plus empty plasmid (group 2),with the exception of primary 14 days post-inoculation.Similarly,the neutralizing efficacy of sera from group 3 was markedly enhanced compared with group 1 and group 2.In cellular immunity,the ratio of CD3+CD4+/CD3+CD8+ T lymphocytes subpopulations from group 3 monitored by flow cytometry (FCM) was significantly higher than those from groupl and group 2 from 42 days post-inoculation to 21 days post-challenge.After challenge,pigs in group 3 showed significantly lower virus loads in peripheral blood measured by a quantitative real-time PCR (RT-qPCR),as compared to those in group 1 and group 2.Pigs in group 1 and group 2 had a low fever and displayed slight inappetence,lethargy,rough hair coats and no lung lesions,meanwhile those in group 3 showed almost no clinical signs,no ltmg lesions.The scores of clinical signs of pigs in group 3 were significantly lower than those in both group 1 and group 2.The scores of lung lesions in group 3 were only numerically lower than those in the latter two groups,but it had no significant difference among three groups.This study indicated swine IL-4 markedly enhanced the protective immune response of hosts and improved the efficacy of the MLV vaccine in preventing PRRS disease.