Immune Response of Atlantic Salmon ( Salmo salar ) Experimentally Infected by Immersion Challenge With Tenacibaculum dicentrarchi

Tenacibaculosis, caused by Tenacibaculum dicentrarchi, is an emerging bacterial disease affecting salmonid aquaculture worldwide; however, host-pathogen interactions and the immune response of Atlantic salmon (Salmo salar) remain poorly understood. This study evaluated the expression of 20 immune-related genes in Atlantic salmon experimentally infected by immersion with T. dicentrarchi and in uninfected controls under identical conditions. The 10-day challenge trial included sampling every two days (skin/muscle, gills, spleen and head kidney) for gene expression analyses. Koch's postulates were fulfilled in the challenge with the T. dicentrarchi TdCh06, which induced 40% cumulative mortality; deaths began at 2 days post-infection and continued until Day 7. Of the 20 genes analysed by RT-qPCR, seven (MHC-I, MHC-II, Arg-1, IL-1β, IL-2, IL-10 and IFN-α1) were consistently detected across tissues and used for comparative analysis. The results show that T. dicentrarchi induces a rapid but heterogeneous immune response in Atlantic salmon: the spleen exhibits an early, transient systemic response; the head kidney shows delayed but strong transcriptional activation; skin/muscle displays early local activation with sustained IL-1β and IFN-α1 expression; and gills show early mucosal activation with persistent IL-1β induction. Overall, infection triggers a tissue-specific response involving inflammatory, antigen-presentation, interferon-associated and macrophage-related pathways, suggesting sustained immune stimulation rather than effective bacterial clearance. These findings shed light on the host-pathogen interaction during tenacibaculosis and identify key immunological pathways involved in disease progression. Notably, the genes IL-1β, MHC-I, MHC-II, IFN-α1, IL-2, IL-10 and Arg-1 are proposed as potential transcriptional markers of infection, offering valuable tools to characterise host responses. Importantly, these markers may also be useful for evaluating vaccine efficacy against T. dicentrarchi in future studies.