豬生殖和呼吸道綜合症病毒GP5-M與大腸桿菌熱溶解性腸毒素次單位B(LTB)融合蛋白之基因轉殖菸草口服疫苗於豬隻之免疫效力評估

Abstract

Porcine reproductive and respiratory syndrome virus (PRRSV) is a single-stranded positive RNA virus, which belongs to the family Arteriviridae and is classified in the order of Nidovirales. PRRS has been one of the most economically significant diseases of swine industry since 1980 and is characterized by reproductive failure in sows and respiratory distress in growing pigs. Although PRRSV modified live-attenuated vaccines (MLVs) and inactivated vaccines have been available for more than a decade, the efficacy of both vaccines is limited. The complicate evading immune mechanism by PRRSV makes it difficult to develop vaccine, both effective and safe. The objective of the present study was to investigate the immunogenicity and efficacy of a transgenic tobacco PRRS subunit vaccine containing GP5-M protein and Escherichia coli heat-labile enterotoxin-B subunit (LTB) after oral administration. In addition, Ingelvac MLV vaccine was included in the study. Twenty-four 5-to 6-week old pigs, seronegative for PRRSV, were randomly divided into 4 groups. Pigs of group one were given orally three consecutive doses of equal concentration of recombinant GP5-M protein expressed in the leaves of tobacco (GP5-M-LTB-T) at a 2-week interval. Pigs from the group were inoculated intramuscularly with 2 ml of Ingelvac PRRS MLV vaccine two times. The pigs were challenged with 2 ml 5x105 TCID50 PRRSV MD001 at 8 weeks post-initial immunization (WPI). The efficacy of the two different types of vaccines was evaluated based on clinical parameters, immune responses as well as post-challenge virological profiles. Serologic and saliva responses were measured by ELISA; viremia, saliva and tissue viral loads upon necropsy were measured by quantitative real time PCR. The results of changes in the bodyweights and daily weight gain revealed no statistical significance among groups, and no clinical signs were observed throughout the study. The levels of serum PRRSV ORF5-specific IgG in GP5-M-LTB-T and MLV groups elevated gradually after three or two consecutive immunization, respectively, and were significantly higher than that of the control group. The level of saliva PRRSV ORF5-specific IgA elevated gradually in GP5-M-LTB-T, while the MLV group didn’t demonstrate IgA. The levels of tissue viral load in GP5-M-LTB-T did have statiscally significant differences from control group, while the MLV group generally showed lower mean values than those of control group and GP5-M-LTB-T group. On the contrary, the results of viremia showed no significant differences among the three groups. In summary, the present study has demonstrated that GP5-M-LTB-T transgenic tobacco leaves could induce mucosal and systemic immune responses in pigs, while neither GP5-M-LTB-T nor MLV group could prevent pigs from PRRSV infection. The preliminary results support the foreseeable potential of subunit vaccine development.