Studies On The Virulence Conversion And Its Immunological Interferences To Other Vaccine Antigens For High Pathogenic Vaccine Strain TJM-F92 Of Porcine Reproductive And Respiratory Syndrome

Porcine reproductive and respiratory syndrome virus(PRRSV)is causative agent for porcine reproductive and respiratory syndrome(PRRS), which is characterized by reproductive failure in sows and respiratory distress in piglets. The disease has brought great economic losses to the swine industry. Vaccination is the most effective and feasible way to control PRRS. In general, the killed-virus vaccines have good biosecurity, but they do not provide sufficient protection. Modified-live virus vaccines can induce both cellular and humoral immune responses, but the live vaccines have the potential risk of virulence reversion in animals. Furthermore, the modified-live PRRSV vaccine developed by the traditional method is generally difficult to change the immunosuppressive effects of PRRSV, and inoculation of live vaccine usually can cause immune suppression to animals, and interfere the immune response of animals to other live vaccines inoculated at the same time.Highly pathogenic porcine reproductive and respiratory syndrome virus TJM-F92(HP-PRRSV TJM-F92) is a live-attenuated vaccine strain which contains a discontinuous deletion of 30 amino acids(aa) and a continuous deletion of 120 aa in Nsp2 gene. The aim of this study was to investigate whether the virus have the risk of virulence reversion through five pig-to-pig passages in susceptible healthy piglets inoculated with PRRSV TJM-F92 vaccine. Also, studies on the impact of PRRSV TJM-F92 strain to classical swine fever lapinized attenuated virus(CSFV C strain) and pseudorabies virus(PRV Bartha-K61 strain) were carried out to determine whether PRRSV TJM-F92 vaccine has immune interference effect on CSFV C strain vaccine and PRV Bartha-K61 strain vaccine.Healthy susceptible piglets, negative for both PRRSV antigen and antibody, were inoculated with PRRSV TJM-F92 strain. Serum samples were then collected during viremia peak time(days 5-9 after inoculation). Then, new groups of piglets were inoculated with the collected serum samples and the pig-to-pig passages in vivo were through five times in total. The Nsp2 genes of viruses isolated from the piglet serum in the five passages were sequenced and analyzed to test the biosecurity of PRRSV TJM-F92 strain and the stability of 120 aa in the infected animal. The susceptible pigs in each passage did not show any classical clinical signs of PRRS after inoculation with virus and rectal temperatures of animals were normal. At necropsy, lung, spleen, kidney, liver, heart, tonsil and lymph nodes were examined and there had no obvious changes both in the vaccinated and control groups. Histopathology observation indicated that the structure of alveolar was normal and clear. There was a small amount of lymphocytes around the bronchus and no lesion caused by interstitial pneumonia was observed. There were no significant differences between vaccinated and control groups. The deletion of 120 aa in Nsp2 gene of virus still existed after five passages, indicating that the deletion of the 120 aa stabilized genetically in piglets. Those results showed that PRRSV vaccine strain TJM-F92 has no risk of virulence reversion.This study also include the experiments to evaluation the immune interferences of high dose of PRRSV TJM-F92 strain on low dose of CSFV C strain and PRV Bartha-K61 strain and the effect of HP-PRRSV and PRV combo live vaccine on immune function of pigs. Studies compared the change of each immunology index after vaccination and challenge between each groups from the aspects of humoral immune, cellular immune, protection rate and pathological changes. Results demonstrated that vaccination of high doses of PRRSV TJM-F92 strain had no immune interference effects on the antibody level of animals to the low dose of CSFV C strain and PRV Bartha-K61 strain, either vaccinated simultaneously or with interval of 7 days or 14 days after PRRSV TIM-92 vaccination. After challenge of classical swine fever or pseudorabies virulent virus, there was no significant difference of clinical symptoms, body temperature and protection rate between co-immunization, respective-immunization and the single-immunization groups. Animals in all vaccinated groups were surviving and effectively protected against the challenges. 4-6-week-old health susceptible piglets were immunized with PRRSV TJM-F92 and PRV Bartha-K61 combo vaccine. Immunocytes in peripheral blood from the experimental animals were analyzed by flow cytometry, while the antibody level, challenge protection rate and pathological changes were also measured and analyzed. Comparison between the co-immunization and single-immunization groups indicated that the immunocytes of animals in co-immunization with PRRSV and PRV and PRV single-immunization groups did not decreased, with no damage on immune system. Those results showed that PRRSV TJM-F92 strain had no immunosuppressive effects on CSFV C strain or PRV Bartha-K61 strain live vaccine.In conclusion, study results demonstrated that PRRSV TJM-F92 strain has no risk of virulence reversion and it was a very safe candidate vaccine strain. Immune interference test revealed that PRRSV TJM-F92 strain has no immunosuppressive on CSFV C strain and PRV Bartha-K61 strain, providing a solid foundation for developing PRRSV and CSFV combo vaccine, PRRSV and PRV combo vaccine and polyvalent live vaccines based on PRRSV TJM-F92 strain.