| Bluetongue virus(BTV) belongs to the genus Orbivirus, family Reoviridae. This virus is transmitted by adult Culicoides midges, infecting ruminant animals(sheep, cattle, and certain species of deer) and causing fever, weight loss, and reproductive dysfunction. There are 26 serotypes, which are determined based on the outer protein VP2 and that hardly show cross-protection. Seven serotypes, or BTV-1, BTV-2, BTV-3, BTV-4, BTV-12, BTV-15, and BTV-16, have been identified in China. The serotypes BTV-1 and BTV-16 are the two most prevalent serotypes in China.Inactivated and live-attenuated vaccines are now widely being used to control BTV infection in domestic animals. However, both inactivated vaccines and live-attenuated vaccines cannot distinguish between infected and vaccinated animals. Live-attenuated vaccines may also result in genome segment re-assortment between vaccine and field strains and may even induce residual virulence, abortions and teratological effects in experimental animals. Therefore, developing new vaccines that are safe and effective is necessary.In this study, we successfully constructed three DNA vaccines in the p CAGGS vector expressing BTV-1 VP2 alone or VP5 alone or co-expressing VP2 and VP5 and r FPV vaccines expressing VP2 alone or VP5 alone or co-expressing VP2 and VP5. These vaccines were named p CAG-VP2, p CAG-VP5, p CAG-(VP2+VP5), r FPV-VP2, r FPV-VP5, and r FPV-(VP2+VP5), respectively.In the mouse study, DNA vaccines and recombinant fowlpox virus(r FPV) vaccines expressing VP2 alone or VP2 in combination with VP5 or co-expressing VP2 and VP5 proteins of BTV-1 were evaluated in both mice and sheep. Several strategies were tested in mice, including DNA vaccine prime and boost, r FPV vaccine prime and boost, and DNA vaccine prime and r FPV vaccine boost. Our results showed that both homologous(DNA/DNA and r FPV/r FPV) and heterologous(DNA/r FPV) prime-boost vaccinations expressing either BTV-1 VP2 alone or a combination of the two major BTV proteins(VP2 and VP5) or co-expressing VP2 and VP5 led to the development of high Nab titers against BTV-1 compared with titers in the control group. However, DNA prime-r FPV boost vaccination regimens were more effective than DNA/DNA or r FPV/r FPV regimens were, and notably, among all immunized groups, p CAG-(VP2+VP5)/r FPV-(VP2+VP5) was significantly more effective than the other 8 approaches. We then chose p CAG-(VP2+VP5)/r FPV-(VP2+VP5) to evaluate immune responses in sheep.In the sheep study, our results indicated that a strategy combining a DNA vaccine prime(co-expressing VP2 and VP5) followed by an r FPV vaccine boost(co-expressing VP2 and VP5) could induce a high titer of neutralizing antibodies in sheep. The highest Nab titer was 188.36 at five weeks, which was much higher than that induced by the recombinant canarypox virus vaccine and BTV-1 VLPs.We did not challenge the vaccinated sheep with BTV-1 because of the need for restriction to a Biosafety Level 3 facility, but previous studies have proven that Nabs play a key role in protecting vaccinated animals against BTV infection. In our current study, the p CAG-(VP2+VP5)/r FPV-(VP2+VP5) vaccine strategy could induce notably high Nab titers and T cell responses. We have reason to speculate that the p CAG-(VP2+VP5)/r FPV-(VP2+VP5) vaccine strategy can completely protect sheep from BTV infection, and our findings highlight the potential value of such a prime-boost vaccination strategy using p CAG-(VP2+VP5)/r FPV-(VP2+VP5) in prevention of BTV infection.Therefore, the data in our study suggest that a DNA vaccine consisting of a p CAG-(VP2+VP5) prime and an r FPV-(VP2+VP5) boost could be an important candidate for the design of a novel vaccine against BTV-1. |
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