The European aquaculture of rainbow trout (Oncorhynchus mykiss) is severely affected by pathological events such as outbreaks of viral hemorrhagic septicemia virus (VHSV). Aiming to minimize this issue, numerous efforts have been made to find preventive and therapeutic strategies. Dietary supplementation of methionine (MET) has been recently demonstrated to have beneficial effects on fish resistance to bacterial infections. However, there are no studies regarding the ability of this essential amino acid to modulate host antiviral responses. The present study focused on the ability of dietary MET to modulate the immune response of trout against VHSV. Juvenile trout (5.9 ± 0.9 g) were fed a control diet (CTRL) or a methionine-supplemented (MET, 2x the requirement for the species) diet for a period of 4 weeks and then sampled. Thereafter, the remaining fish were bath challenged with VHSV (105 ml-1 TCID50) and sampled in a time-course manner. Skin and gills were collected for viral load and RNAseq analyses, before (0 h, n = 16) and post-challenge (24, 72 and 120 h, n = 9). By assessing tissue viral load, the peak of infection was recorded at 72 h for both dietary treatments. Curiously, at that time, MET-fed fish showed up-regulation of significant immunological pathways, indicating a positive interaction of this diet at the peak of infection. Particularly in the gills, the MET diet at 72h displayed a strong up-regulation of immune pathways of the general immune response, bacterial recognition agents, DNA and RNA polymerase, pattern recognition receptor pathways and regulatory transcription. These pathways are important against VHSV infections. On the other hand, MET treatment showed to down-regulate pathways involved in cell cycle regulation, biological process and stimulation and apoptosis. In the skin, despite having less positive regulation of differentially expressed genes (DEGs), enrichment of type I interferon related pathway was observed, which is part of the host’s innate immune response against viruses. Some of the most significant DEGs in both tissues have viral immune response functions. In short, MET dietary supplementation allowed an improved response of rainbow trout juveniles against VHSV infection, with the up-regulation of GO terms associated with an improved immune response, despite the similar viral load between the two diets at the peak of infection.
Work funded by the European Union’s Horizon Europe research and innovation programme (GA No.101079467 project GRINNAQUA) and IMMUNAA project (PTDC/CVT-CVT/7741/2020).