Recombinant African swine fever virus p30-flagellin fusion protein promotes p30-specific humoral and cellular immune responses in mice.

African swine fever (ASF) is a transmissible and deadly viral disease caused by the African swine fever virus (ASFV) that has considerably affected the global pig industry. Vaccination is considered a potentially effective method to control ASF. However, live attenuated vaccines can't protect against all circulating virus isolates. Subunit vaccines can induce both cellular and humoral immune responses, but often require the addition of adjuvants. Flagellin, a stimulator of Toll-like receptor 5 (TLR5), functions as a potent adjuvant by enhancing cellular and humoral immune responses. However, its high antigenicity may cause severe systemic inflammation. In this study, an Escherichia coli expression system was used to express ASFV p30 protein (p30) fused with Salmonella Typhimurium FliCΔD2D3 (without the D2 and D3 domains of FliC). The immunological effect of p30-FlicΔD2D3 protein in mice was evaluated. Results revealed that the ASFV p30 protein and the p30-FlicΔD2D3 fusion protein were effectively expressed by the E. coli expression system. In vitro activity analysis showed that the p30-FlicΔD2D3 fusion protein could be recognized by ASFV-positive serum, had good immunoreactivity, and remarkably promoted IL-8 secretion related to TLR5 activity in HEK293-mTLR5 cells. However, p30-FlicΔD2D3 induced significantly lower levels of inflammatory factor IL-8 than that induced by wild-type flagellin. Immunization with the p30-FlicΔD2D3 fusion protein considerably promoted cellular and humoral immune responses in mice. Therefore, the p30-FlicΔD2D3 protein retained good immune reactivity and TLR5 agonist efficacy. It also enhanced humoral and cellular immune responses in mice. This work offered valuable information that will be helpful to develop ASF subunit vaccines.