Selective breeding influences innate immune responses of Rainbow trout (Onchorhynchus mykiss) challenged with Lactococcus garvieae in controlled conditions

This study investigates the differential immune response in selectively bred lines i.e. resistant (R) and susceptible (S) of rainbow trout (O. mykiss) challenged with L. garvieae under controlled experimental conditions. To assess non-specific immune responses, lysozyme activity, bactericidal activity, peroxidase activity, total protein levels, and immunoglobulin M (IgM) concentrations in serum samples were measured. Except total protein and IgM concentrations, no significant differences (p ≥0.05) were observed in any fish groups. In parallel, the expression of immune-related genes i.e. cytokine IL-8, MHC-II, antimicrobial peptides (Cathelecidin-1 & 2, hepcidin), complement factor-3 and 5 and caspase-3 and 8 including reference genes (β-act, elongation factor-1 and 60S) in spleen, head kidney, fat and liver of L. garvieae infected R and S rainbow trout were assessed to provide molecular insights into the host’s defence mechanisms. Results indicated that almost all the genes considered showed a significant difference in expression between 1-dpi and 3-dpi given the acute development of the infection, thus showing their involvement in the mechanisms of response to the infection and fish immunity. Our results indicate distinct immunological profiles between the selected lines, with significant differences observed in both humoral and cellular responses. These findings suggest that selective breeding may influence the innate immune capacity of rainbow trout, offering a potential pathway for enhancing disease resistance in aquaculture.