Potency and efficacy of VP20-based vaccine against tilapia lake virus using different prime-boost vaccination regimens in tilapia

Tilapia lake virus (TiLV) is a newly emerged pathogen responsible for high mortality and economic losses in the global tilapia industry. Vaccination is the most effective way to prevent and control viral diseases. There is currently no vaccine on the market to control TiLV infections. Recently, we identified that TiLV segment 8 encoded a protein (VP20) with immunogenicity sufficient for use as a vaccine antigen. In this study, the immune responses and protective efficacy elicited by VP20 with different prime-boost vaccination regimens (DNA only, protein only, and DNA plus protein) in tilapia was evaluated. The results indicated that both pV-optiVP20 plasmid and rVP20 protein induced humoral and cellular immune responses. Tilapia in the DNA prime-protein boost group developed significantly higher levels of antibody response compared with those immunized with either the DNA or the protein alone (P < 0.05). Furthermore, the highest mRNA levels of the genes TNF-α, IL-1β, IgM, CD4, MHC-Ia and MHC-II were induced following priming with the pV-optiVP20 and boosting with the rVP20. Additionally, tilapia inoculated with the pV-optiVP20 prime-rVP20 boost regimen had a 72.5% survival rate against challenge with the lethal TiLV 2017A compared with 50% or 52.5% survival using the pV-optiVP20 and the rVP20 vaccinations alone. These findings demonstrated that VP20-based vaccine could elicit both humoral and cellular immune responses and significantly protected the fish against infectious challenge by TiLV. The DNA prime-protein boost immunization strategy showed the potential advantage over a solo vaccination.