Atlantic salmon scale explants in bacteria-host interaction studies: in vitro challenge model

Cell lines and primary cultures are widely used to study host-pathogen interactions, reducing the need for challenge trials with live fish, which are costly and raise ethical concerns. We present Atlantic salmon scale explants (SE) as a promising model for bacterial challenges. Comparison with the body map (transcriptomes of seventeen cells and tissues) revealed strong immune activity in SE. SE ranked first in antiviral gene expression and second and third in antibacterial and inflammatory responses, respectively. Additionally, SE shows high expression of skin-specific genes encoding structural proteins of the cytoskeleton and extracellular matrix. Contrast to the epidermis, dermis, and whole skin highlights immune genes, including markers of acute inflammation and stress. Exposure to pathogens (Moritella viscosa and Tenacibaculum finnmarkense) and commensal bacteria consistently upregulated a suite of immune genes in a dose response manner. This panel includes chemokines, cytokines, genes involved in eicosanoid metabolism, and both humoral and cellular effectors. The prevalence of genes related to signaling and communication suggests that salmon scales function as sentinels, mobilizing immune responses upon encountering bacteria. While common immune responses were evident across all trials, scales also revealed differences between bacterial species. Combining commensal bacteria with M. viscosa enhanced responses to pathogens, and differences were observed between M. viscosa and T. finnmarkense. The effects of commensal bacteria varied in magnitude. Exposed cells showed changes in morphology and increased iNOS expression. Future development of this model will focus on time-course experiments and expanding the analytical repertoire to include pathology detection methods.