Systemic immune response of rainbow trout exposed to Flavobacterium psychrophilum infection

Bacterial cold-water disease (BCWD) caused by Flavobacterium psychrophilum is one of the most serious bacterial diseases leading to significant economic loss for rainbow trout (Oncorhynchus mykiss) aquaculture. However, little is known about the systemic immune response of rainbow trout against F. psychrophilum infection. This study investigated the immune response of rainbow trout to F. psychrophilum infection using multiple experiments, including bacterial load detection, phagocyte activity assessment, enzyme activity evaluation, and gene expression profiling. Results showed that the spleen index and intestinal pathogen load reached a peak at 3 days post-infection, with strong pro-inflammatory gene expression observed in rainbow trout. Leukocytes RBA and PKA were significantly elevated in the spleen, blood and intestine at 7 days post-infection. Heat map analysis demonstrated that the spleen had a more substantial pro-inflammatory response compared to the intestine postinfection and exhibited higher expression levels of immune-related genes, including IgM, il1 beta, il6, cd4, cd8a, cd8b, c1q, chathelicidin, inos, and lysozyme. Both Th1 and Th2 polarized responses in the spleen were activated, with Th2 (il4/13a, gata3) (FC > 4) being more intense than Th1 (tnf alpha, t-bet) (FC > 2). Tight junction proteins exhibited down-regulation followed by up-regulation post-infection. Collectively, the results of this study expand our current understanding of the immune response of rainbow trout post F. psychrophilum infection but also provide new avenues for investigation in salmonid aquaculture.