Heterologous prime-boost: an important candidate immunization strategy against Tembusu virus

Tembusu virus (TMUV), a newly emerging pathogenic flavivirus, spreads rapidly between ducks, causing massive economic losses in the Chinese duck industry. Vaccination is the most effective method to prevent TMUV. Therefore, it is urgent to look for an effective vaccine strategy against TMUV. Heterologous prime-boost regimens priming with vaccines and boosting with recombinant adenovirus vaccines have been proven to be successful strategies for protecting against viruses in experimental animal models.

In this study, heterologous and homologous prime-boost strategies using an attenuated salmonella vaccine and a recombinant adenovirus vaccine expressing prM-E or the E gene of TMUV were evaluated to protect ducks against TMUV infection for the first time, including priming and boosting with the attenuated salmonella vaccine, priming and boosting with the recombinant adenovirus vaccine, and priming with the attenuated salmonella vaccine and boosting with the recombinant adenovirus vaccine. Humoral and cellular immune responses were detected and evaluated. We then challenged the ducks with TMUV at 12 days after boosting to assay for clinical symptoms, mortality, viral loads and histopathological lesions after these different strategies.

Compared with the homologous prime-boost strategies, the heterologous prime-boost regimen produced higher levels of neutralizing antibodies and IgG antibodies against TMUV. Additionally, it could induce higher levels of IFN-γ than homologous prime-boost strategies in the later stage. Interestingly, the heterologous prime-boost strategy induced higher levels of IL-4 in the early stage, but the IL-4 levels gradually decreased and were even lower than those induced by the homologous prime-boost strategy in the later stage. Moreover, the heterologous prime-boost strategy could efficiently protect ducks, with low viral titres, no clinical symptoms and histopathological lesions in this experiment after challenge with TMUV, while slight clinical symptoms and histopathological lesions were observed with the homologous prime-boost strategies.

Our results indicated that the heterologous prime-boost strategy induced higher levels of humoral and cellular immune responses and better protection against TMUV infection in ducks than the homologous prime-boost strategies, suggesting that the heterologous prime-boost strategy is an important candidate for the design of a novel vaccine strategy against TMUV.