Abstract
Role of methionine supplementation in immune modulation and resistance of Atlantic salmon (Salmo salar) to Salmon Louse (Lepeophtheirus salmonis)
Salmo salar aquaculture is severely affected by Lepeophtheirus salmonis infections, resulting in substantial economic losses. In recent years, numerous studies have investigated the host-pathogen interaction in order to develop effective treatments or prophylactic measures against salmon louse. Dietary supplementation with methionine in fish diets has shown to enhance resistance to bacterial infections. However, no studies have been developed to assess the immunomodulatory potential of this amino acid on parasite incidence in aquaculture. The present study aimed to evaluate on the effect of dietary methionine supplementation on the immune response of Atlantic salmon to sea lice infection.
Juvenile fish (±120g) were fed for four weeks with either a control diet (CTRL) or a methionine-supplemented diet (MET, 2x requirement level of L-methionine), and subsequently sampled (n=12/diet). The remaining fish were infected by cohabitation with sea lice (100 copepods/fish) and sampled two weeks post-challenge. Blood, plasma, liver and head-kidney tissues were collected, for haematological analysis, evaluation of the systemic humoral response, oxidative stress parameters, and gene expression analysis, respectively. Additionally, fish skin was collected for proteomic analysis and single-cell RNA sequencing (scRNA-seq) before, and after infection (parasite attachment sites).
Parasite counts per fish revealed that 70% were in the chalimus-II while the remainder were pre-adults stage, with no differences found between diet. At chamilus-II stage, the parasite feeds solely on mucus and skin, which may explain the limited haematological response observed, though infection seems to increase white blood cells counts. These results, together with lack of oxidative stress biomarkers and immune-related genes modulation in the head kidney, suggest that although the presence of the ectoparasite was confirmed, it did not induce a significant systemic response in the host. This is further supported by the absence of lipid peroxidation in tissues. Altogether, this reinforces the notion that at early developmental stages the parasite induces immune responses mainly locally, at the skin. These responses mainly caused by wounds may not impose a systemic response. Nonetheless, plasma protease activity in the MET-diet group increased significantly following infestation when compared to the basal-diet and the non-infected group.
The results of proteomics and scRNA-seq of the skin where lice were attached are currently being analysed in order to gain a better understanding of the immunomodulatory capacity of methionine and the local response involved in resistance to salmon louse.
Work funded by the European Union’s Horizon Europe research and innovation programme (GA No.101079467 project GRINNAQUA).