The present endeavor was conducted to evaluate the role of activated macrophage in the susceptibility of two different rainbow trout (Oncorhynchus mykiss) strains, a susceptible American (T) and a more resistant German (H), to infection with Myxobolus cerebralis. Arginase-2 and inducible nitric oxide synthase (iNOS) genes were used as references to the alternative and classical pathway of macrophage activation. The expression level of both genes was measured using quantitative real-time polymerase chain reaction. The expression level of arginase-2 was significantly upregulated in strain T at 2 h and 8 days post exposure in the strain H. In case of iNOS, the expression level was significantly upregulated from 24 h to 8 days p.e. in strain T and only in 8 days p.e. in strain H. During this study also, the influence of nitric oxide (NO) on the viability of the triactinomyxon spores (TAMs) of M. cerebralis was evaluated using the NO-donor S-nitroso-N-acetyl-penicillamine (SNAP). Rising final concentrations of SNAP from 0.25 to 1 mM at 2, 4, and 24 h resulted in increasing numbers of propidium iodide-positive TAMs detected. The results of this study suggest an inability of strain T to react with an effective immune response against infection with M. cerebralis. Furthermore, the TAMs of M. cerebralis react with significant decrease of viable spores to rising concentration of SNAP and longer incubation, but there is also evidence for some resistance to NO activity.

Expression of immune-regulatory genes that code for cyclooxigenase-2 (COX-2), transforming growth factor beta (TGF-beta), and two isoforms of interleukin-1beta (IL-1beta1 and IL-1beta2) was studied in susceptible and non-susceptible rainbow trout strains for 200 days after exposure to Myxobolus cerebralis. Expression of COX-2, IL-1beta1, and IL-1beta2 increased 5 min post exposure (p.e.) and was always more elevated in the susceptible strain than in the non-susceptible strain. In both strains, expression of COX-2 returned to the control level within a few hours p.e. Expression of IL-1beta1 and IL-1beta2 showed two elevated waves in both strains until 4 days p.e. Expression of TGF-beta in the non-susceptible strain was elevated at nearly all sampling points, but was decreased in the susceptible strain until up-regulation between 4 and 20 days p.e.; TGF-beta was the only gene where the expression in the non-susceptible strain was more elevated than in the susceptible strain. Rainbow trout of the non-susceptible strain appeared to resist infection by M. cerebralis with only minor transcriptional regulation of the genes investigated. Increased transcriptions of genes in the susceptible strain may be the result of an inability to antagonize the infection.