Immunization Of Animals With A Plasmid DNA Coexpressing Somatostatin Gene And GP5 Gene Of PRRS Virus

Porcine reproductive and respiratory syndrome (PRRS) caused by PRRS virus (PRRSV) is a severe infectious disease in swine herds characterized by reproductive failure in sows and respiratory illness in young pigs. Due to the distinct charateristics of PRRSV, strageties somertimes do not work well and traditional vaccines suffer from some disadvantages. Therefore, development of a safer and more effective vaccine remains a big challenge. The GP5 glycoprotien of PRRSV is involved in inducing neutralizing antibodies, and therefore plays an important role in antiviral immune response. So GP5 has been postulated to be a primary candidate for developing genetic engineering vaccines against PRRS. Somatostatin (SS) is a hormone that inhibits the secretion of growth hormone. Immunization against SS can promote the growth of immunized animals. The previously reported SS-based vaccines promoted the growth of pigs. A genetic engineering vaccine which works both as a growth stimulant and an antiviral vaccine would be promising. This study was designed to explore the effects of DNA immunization on the growth and antibody response in animals immunized with a plasmid DNA encoding SS gene fused with the GP5 gene of PRRSV. A fragment of 180 bp encoding SS gene was amplified by PCR from the genomic DNA of peripheral blood mononuclear cells of the pig, and cloned as a fusion gene with PRRSV GP5 gene into plasmid pISGRTK3 The plasmids pISG-SS/GP5 and pISGRTK3 were purified and used to innoculate BALB/c mice and piglets. Fifteen 4-week-old female BALB/c mice were randomly assigned to three groups. Each mouse in group 1 was injected with 100 μg of pISG-SS/GP5 diluted in 100 μl of normal saline through hind leg muscles. The mice were boosted twice with the same amout of plasmid DNA at 2-week intervals. The other two groups of control mice received the same amount of empty plasmid pISGRTK3 or the same volume of normal saline, respectively, via the same route and frequency. Ten 4-week-old PRRS-free pigs with uniform body weights were randomly assigned to two groups. Five piglets in group 1 were each inoculated 600 μg of pISG-SS/GP5 diluted in 2 ml of normal saline. Five-sixth of the volume was injected into tibialis cranialis muscles of the hind leg using a 26-gauge needle, and the remainder was administered into superficial inguinal lymph nodes using a 27-gauge needle. The piglets were boosted twice at an interval of 3 weeks with the same route and procedure. Five control piglets received the same amount of empty pISGRTK3 vector through the same route and procedure. Serum samples were collected from the tail of immunized mice with negative pressure and from the V. jugularis of immunized pigs, and tested for the presence of anti-GP5 specific antibodies by indirect ELISA and Western blot. The resuls showed that immunization of with the resulting plasmid pISG-SS/GP5 induced anti-GP5 antibodies in BALB/c mice and pigs, as demonstrated by GP5-specific ELISA and immunoblotting. Compared with pigs immunized with empty vector pISGRTK3, the mean body weight gain of pigs immunized with pISG-SS/GP5 increased by 12.2% on the 19th week after the first vaccination. The results indicated the plasmid DNA coexpressing SS and PRRSV GP5 elicited anti-GP5 antibodies and improved the growth performance of immunized pigs, and may represent a promising aniti-PRRS vaccine and growth promoter.