An effective combined vaccine against vibriosis and typical and atypical furunculosis in Atlantic salmon (Salmo salar).

This study evaluated the safety and efficacy of a novel trivalent inactivated vaccine targeting domestic strains of Vibrio anguillarum (VA), Aeromonas salmonicida subsp. salmonicida (ASS), and A. salmonicida subsp. masoucida (ASM) in Atlantic salmon (Salmo salar). While furunculosis and vibriosis pose significant threats to the Korean aquaculture industry, the inclusion of local ASM strains in polyvalent vaccines remains rare. Three vaccine formulations-Mix1, Mix2, and Mix3-with decreasing antigen concentrations were evaluated. Comprehensive safety assessments showed that no adverse effects were detected under the conditions tested. Notably, the Mix1 group achieved the same weight gain (111.11%) as the PBS control group, and serological parameters (AST, ALT, ALP, and glucose) as well as tissue histopathology remained within normal physiological ranges. In challenge trials, the Mix1 formulation demonstrated superior protective efficacy, achieving Relative Percent Survival (RPS) of 100%, 77.5%, and 97.5% against VA, ASS, and ASM, respectively. Immunological analysis revealed that vaccine-induced protection may be associated with the activation of both humoral and cellular immune pathways, as suggested in gene expression profiles. Results showed significantly elevated antibody titers and the upregulation of key markers, including BCL6 (associated with T follicular helper cell differentiation) and sIgM. Notably, T-bet gene expression was significantly upregulated in the Mix1 group, suggesting a potential association with protective immune responses. In contrast, the low-dose Mix3 formulation showed no protection against ASS (0% RPS). Despite significant antibody production, mortality in this group was associated with a marked pathological surge in IL-1β expression (19.67-fold) concurrently with the downregulation of T-bet (0.49-fold) following challenge. In conclusion, the data suggest that antigen concentration may influence the transition from a protective cellular immune response to a dysregulated inflammatory response, providing useful insights for optimizing polyvalent vaccine formulations in the salmon aquaculture industry. Indeed, this study suggests that T-bet, BCL6, and IL-1β may serve as candidate correlates of immune responses following vaccination.