Enhancement Of Vaccine Protective Efficacy Through Heterologous Prime-boost Vaccination Strategy Against Schistosoma Japonicum Infection In Mice Model

In China, schistosomiasis, caused by Schistosoma japonicum infection, is still one of the most serious parasitic diseases. Developing transmission blocking verterinary vaccines was urgently needed for efficient control of the diseases. In the present study, we constructed a replication-defective adenoviral vector-based vaccine with optimized triosephosphate isomerase (SjTPI.opt) gene, evaluated the specific immune responses and protective efficacy through different immunization routes in mice model firstly. To screen a heterologous prime-boost vaccination strategy with high protective efficacy, we then prepared DNA vaccines (pcDNA3.1-SjTPI.opt), recombinant protein vaccines (rSjTPI) and recombinant adenoviral vaccines (rAdV-SjTPI.opt) and evaluated the specific immune responses and protective efficacy induced by different heterologous prime-boost combinations including DNA immunized intramuscularly priming rAdV-SjTPI.opt immunized intramuscularly boosting, rAdV-SjTPI.opt immunized intramuscularly priming subcutaneously boosting and rAdV-SjTPI.opt immunized intramuscularly priming rSjTPI immunized subcutaneously boosting. Finally, to optimized the priming and boosting times of the higher heterologous prime-boost strategy and evaluate the protective efficacy lasting time, we carried out the last study in mice model. Data obtained from the present study could provide a foundation for transmission-blocking veterinary vaccine development.We constructed a novel vaccine based on replication-defective adenoviral vector, rAdV-SjTPI.opt, and this vaccine could express the insert gene successfully in vitro. Data from animal challenge experiments showed that rAdV-SjTPI.opt immunized intramuscularly induced Thl biased immune responses in the host, while rAdV-SjTPI.opt immunized subcutaneously induced Th2predominant immune responses. rAdV-SjTPI.opt immunized orally induced low levels of immune responses and no significant protective. Intramuscular rAdV-SjTPI.opt provided a higher protective effect than rAdV-SjTPI.opt immunized subcutaneously (54.92%versus37.50%for worm reduction rate) in mice. These findings may be due to the associated higher levels of specific Thl, antibody responses and lower level of IL-17A.In the next part, the heterologous prime-boost immunization strategies based on three vaccines were evaluated and the specific immune responses were also observed. Data showed that when primed with DNA vaccine intramuscularly (i.m.) and boosted with rAdV-SjTPI.opt i.m., a higher Thl immune response was detected than DNA vaccination group, however the protective effect induced by this strategy was lower than rAdV-SjTPI.opt immunized i.m.. The worm reduction rate, female reduction rate and egg reduction rate in liver induced by this strategy were45.13%,50.11%and54.57%, respectively. An antagonistic effect was observed when primed with rAdV-SjTPI.opt (i.m.) and boosted with rAdV-SjTPI.opt (s.c.). The worm reduction rate, female reduction rate and egg reduction rate in liver induced by this strategy were44.41%,44.51%and57.10%, respectively. The levels of specific cytokines and antibodies were lower than rAdV-SjTPI.opt (i.m.), but higher than rAdV-SjTPI.opt (s.c.) when immunized through homologous prime-boost strategy. The antagonistic effect may be due to the anti-vector role in the present strategy. A synergistic effect was observed when primed with rAdV-SjTPI.opt (i.m.) and boosted with rSjTPI subcutaneously (s.c.). The worm reduction rate, female reduction rate and egg reduction rate in liver induced by this strategy were72.09%,72.73%and72.13%, respectively.The broad spectrum specific immune responses were induced by this strategy. The protective effect was higher than rAdV-SjTPI.opt (i.m.) and rSjTPI (s.c.) immunized through homologous prime-boost strategy. In this part we obtained a novel heterologous prime-boost vaccination strategy, primed with rAdV-SjTPI.opt (i.m.) and boosted with rSjTPI (s.c.) and the worm reduction rate could reach to70%.To optimize the strategy, different times of priming and boosting immunization were carried out and the protective effects were evaluated in mice. Data showed that rAdV-SjTPI.opt primed3times and boost once with rSjTPI (V+V+V+P) could induce high levels of Thl type immune response and the titer of IgG,relatively low levels of Th2and Th17immune response and produce highest protective effect. The worm reduction rate, female reduction rate and egg reduction rate in liver were66.01%,69.52%and67.57%, respectively. Along with the decrease of the rAdV-SjTPi.opt priming times or increase of the rSjTPI boosting times, the specific Thl cytokine levels and protective effect were decreased gradually, however the specific Th2and Th17cytokine levels were increased gradually. To evaluate the protective lasting time of the strategy, challenge experiments were carried out and the protective effects were observed. Data showed that V+V+V+P strategy produced a50%worm reduction rate at10weeks after the last immunization. Therefore, V+V+V+P was a stable and high effective vacciniation strategy against Schistosoma japonicum in mice model.