Screening of immune adjuvants for an attenuated live vaccine derived from the Aeromonas veronii TH0426 strain with a hisJ gene deletion and evaluation of its immune protection in crucian carp.

Aeromonas veronii is a highly pathogenic bacterium in aquaculture, causing fulminant septicemia in farmed fish and resulting in significant economic losses. Vaccination is a primary strategy for the prevention and control of this disease. To screen for the optimal adjuvant to enhance the immune efficacy of an attenuated live vaccine derived from the A. veronii TH0426 strain with a hisJ gene deletion (A. veronii TH0426△hisJ), propolis extract, aluminum hydroxide gel, β-glucan, and Auricularia auricula polysaccharide were combined with the vaccine and administered to crucian carp via intraperitoneal injection. Immune efficacy was evaluated by assessing serum antibody levels, non-specific immune parameters, immune-related gene expression, and protection rates post-challenge. None of the four adjuvants inhibited vaccine strain growth, and no adverse reactions were observed. All adjuvants significantly enhanced vaccine efficacy. The propolis group exhibited the highest serum IgM level (2.58 μg/mL) and phagocytic rate (51.92%), and significantly elevated non-specific immune parameters (ACP, AKP, SOD, LZM, CAT). qPCR analysis revealed significantly upregulated expression of TNF-α, IL-1β, IL-10, and IFN-γ in multiple tissues of the adjuvant groups. In the challenge test, the propolis group showed the highest immune protection rate (77.14%), followed by aluminum hydroxide (74.29%), while the polysaccharide and β-glucan groups both achieved 71.43%, all significantly higher than the vaccine-only group (65.71%). In conclusion, propolis extract is the optimal adjuvant, synergistically enhancing specific and non-specific immunity to provide an effective vaccine strategy against A. veronii.