Efficacy of a composite aluminum adjuvant containing inulin in protecting turbot (Scophthalmus maximus) against Aeromonas salmonicida infection.

Inulin is a type of plant polysaccharide that is widely distributed in nature. Research has demonstrated that inulin possesses adjuvant activity, stimulating balanced cellular and humoral immune responses. However, the role of inulin as an adjuvant has been relatively underexplored within aquatic vaccine research. In this study, a composite aluminum adjuvant containing inulin was developed, and an inactivated vaccine against Aeromonas salmonicida was formulated based on this adjuvant. Subsequently, the immunoprotective effects of the vaccine in turbot were evaluated to assess the potential application of inulin as an adjuvant in aquaculture vaccines. As a result, the composite of aluminum and inulin significantly promoted immune cell recruitment and antigen uptake at the injection site. Vaccination with the composite adjuvant significantly reduced mortality following A. salmonicida challenge, achieving a relative percent survival (RPS) of 61.54% compared to the control group. Moreover, the vaccine exhibited increased levels of specific IgM along with elevated serum protein levels and lysozyme activity. Furthermore, gene expression analysis revealed significant upregulation of MHC-I, MHC-IIβ, CD4, CD8 as well as some cytokines including IL-4, IL-10, IL-1β, IFN-γ, IL-12 and TNF-α, indicating a robust humoral and cellular immune response. In brief, these findings suggest that inulin acts as a natural immune stimulant and, when combined with aluminum, confers enhanced protective efficacy compared with either adjuvant alone, while balancing both humoral and cellular immune responses. Further studies are needed to clarify the nature of this complementary effect. This novel composite of adjuvants presents a leading candidate strategy for improving fish vaccination practices within aquaculture aimed at bolstering disease prevention efforts. Further studies are warranted to optimize this formulation and investigate the mechanisms underlying immune enhancement provided by the composite adjuvant system for teleost fish.