A BCWD-Resistant line of rainbow trout is less sensitive to cortisol implant-induced changes in IgM response as compared to a susceptible (control) line.

In salmonids, stress responses increase cortisol levels and disease susceptibility, including to Flavobacterium psychrophilum (Fp), the causative agent of BCWD. A BCWD-resistant line (R-line) of rainbow trout was used here to investigate potential differences in immunoglobulin response after a combined treatment of cortisol and Fp, as compared to a susceptible (S-line) control line. Expression of membrane and secreted immunoglobulin heavy chains mu and tau were determined by RT-qPCR in spleen and anterior kidney. Cortisol treatment did not affect B cell development in the anterior kidney, but delayed IgM responses at the early stage of infection in the spleen of both lines. An earlier IgM response was a determining factor in differential disease progression between resistant- and susceptible fish after Fp-challenge. S-line fish had a delayed and exacerbated IgM response after cortisol implant indicative of a detrimental cycle of sustained IgM responses and high pathogen loads. In contrast, R-line fish had delayed but milder, and protective IgM responses and cleared pathogen successfully. Fp challenge after cortisol implant increased serum cortisol levels on days 3 and 5 compared to mock treatments in S-line fish, but only on day 3 in R-line. Hence, combined cortisol treatment and Fp challenge differentially modulated B cell activation and Fp loads in BCWD-resistant and susceptible lines of rainbow trout. We propose that under conditions of increased stress, BCWD-resistant fish may remain immunologically better equipped to respond to infections compared to BCWD susceptible fish.