Effects of a subunit vaccine (FlaA) and immunostimulant (CpG-ODN 1668) against Vibrio anguillarum in tilapia (Oreochromis niloticus)

• CpG-ODN 1668 (CpG) showed potential for use as an adjuvant for vaccines.
• Recombinant flagellin-A (rFlaA) is a promising subunit vaccine for prophylaxis.
• The rFlaA and CpG could be used as a suitable vaccine-adjuvant combination for vaccine-based protection against infectious Vibrio anguillarum in fish.

Vibriosis, caused by Vibrio anguillarum, is a common bacterial disease in cultured fish. Treatment of vibriosis can lead to the development of antibiotic resistance or tolerance. Therefore, alternative methods for combatting vibriosis are needed. In this study, we investigated the effects of a subunit vaccine (recombinant flagellin-A [rFlaA]) and adjuvant (CpG-ODN 1668 [CpG]) candidate against V. anguillarum in tilapia (Oreochromis niloticus). In an in vivo experiment, fish in the rFlaA + CpG and rFlaA-only groups were first treated with CpG or PBS, followed by vaccination with rFlaA the next day. In the control group, both vaccine and adjuvant were replaced by PBS. The highest survival rate (60%) was observed in fish treated with rFlaA + CpG, compared with 0% in PBS-treated fish or 35% in fish immunized with rFlaA-only. Moreover, sera purified from tilapia treated with rFlaA + CpG exhibited significantly stronger agglutination activity (approximately 3-fold higher) than sera obtained from control fish, whereas sera from fish treated with rFlaA-only showed values 2.5-fold higher than those in control fish. The bacterial survival rates were also lower in sera from tilapia treated with rFlaA + CpG (41%) than in sera from fish treated with rFlaA alone (61%) or control fish (100%). Thus, fish treated with both rFlaA and CpG exhibited the highest immune activation in terms of agglutination and bactericidal activity. In summary, FlaA could be a promising subunit vaccine, and CpG-ODN 1668 could be applied as an adjuvant for immunization against V. anguillarum.