Immune divergence in rainbow trout lines selectively bred for F. psychrophilum resistance or susceptibility revealed by challenge with Weissella tructae

Authors: Wiens, Gregory; CHADWICK, CHRISTOPHER - Life Diagnostics, Inc; Osbourn, Keira; FU, QIN - Cornell University; REID, ROSS - Orise Fellow; Moreland, Travis; Welch, Timothy; Leeds, Timothy; MARANCIK, DAVID - St George'S University

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2026
Publication Date: 5/1/2026
Citation: Wiens, G.D., Chadwick, C., Osbourn, K.E., Fu, Q., Reid, R., Moreland, T.D., Welch, T.J., Leeds, T.D., Marancik, D. 2026. Immune divergence in rainbow trout lines selectively bred for F. psychrophilum resistance or susceptibility revealed by challenge with Weissella tructae. Fish and Shellfish Immunology. 174. Article 111387. https://doi.org/10.1016/j.fsi.2026.111387.
DOI: https://doi.org/10.1016/j.fsi.2026.111387

Interpretive Summary: Weissellosis is a newly emerging disease of rainbow trout that can cause high losses. In this study, we investigated the immune response of two rainbow trout genetic lines, one resistant and one susceptible to a different bacterial pathogen, after infecting them with Weissella tructae. While survival and bacterial load didn't significantly differ between the lines, the susceptible line showed a greater reduction in circulating white blood cells, whereas the resistant line exhibited higher numbers of granulocytes, suggesting different cellular immune responses. Certain blood proteins (cathelicidin, GDF-15, leptin, haptoglobin) increased in both lines post-infection, but specific biomarker proteins were differentially expressed, with tumor necrosis factor alpha (TNFa) 6-fold higher in the susceptible line, while serum amyloid A (SAA) trended higher in the resistant line. Complement factor H-like 1 protein (CFHL-1) was consistently higher in the resistant line regardless of infection, and a significant decrease in plasma fibrinogen, was associated with blood clots in the heart of infected fish. These findings highlight both shared and distinct immune responses to Weissellosis in genetically diverse rainbow trout lines. This study identified biomarkers that may be useful for early diagnosis of Weissellosis in rainbow trout.

Technical Abstract: Host genetic resistance can be improved through selective breeding, yet underlying immune mechanisms often remain unknown due to varying pathogen burden. In previous research, rainbow trout lines, ARS-Fp-R (resistant, R-line) and ARS-Fp-S (susceptible, S-line), were bred for differential resistance to Flavobacterium psychrophilum, but mechanisms remain unclear and study has been confounded by the higher F. psychrophilum load in the susceptible line. Here, we separate line-specific traits from pathogen-load driven effects, by challenging with Weissella tructae, a pathogen for which both lines exhibit equivalent susceptibility. After 3 to 6 days post-challenge with W. tructae, we report the divergent and shared cellular and cytokine profiles. The infected R-line fish exhibited higher granulocyte levels post-challenge, a trait also observed in its response to F. psychrophilum, while S-line fish had a more severe depletion of circulating leukocytes and IgM+ B cells. Proteomic analysis identified 50 infection-induced blood proteins. Using novel salmonid biomarker assays, the S-line exhibited a 6-fold higher tumor necrosis factor-alpha (TNFa) and elevated precerebellin (PCB). In contrast, the R-line showed 3-fold higher baseline complement factor H-like-1 protein (CFHL-1), and a trend toward elevated serum amyloid A (SAA). Other biomarkers, including Cath2, GDF-15, haptoglobin, and C1q-LP3 increased post-infection but did not differ between lines. Both lines exhibited similar granulomatous epicarditis and decreased plasma fibrinogen. Despite equivalent susceptibility to W. tructae, these lines demonstrate divergent cellular and cytokine responses, suggesting a shift in immune architecture due to selective breeding. The novel biomarker assays provide valuable tools for monitoring immune health and precision breeding in salmonid aquaculture.