Proteomic insights into intestinal immune modulation in Atlantic salmon (Salmo salar) fed fucoidan from brown seaweed (Saccharina latissima)

Fucoidan is a sulfated polysaccharide from brown seaweed such as sugar kelp ( Saccharina latissima ) that has bioactive properties including immunomodulatory, antioxidant, and antimicrobial effects. This current study investigated the modulation of the intestinal immune profile of pre-smolts Atlantic salmon ( Salmo salar ) by dietary fucoidan. To do this, a 5-week experiment, consisting of triplicate tanks with 40 fish per tank, was conducted using either a commercial-like diet (control) or a diet containing 0.2% fucoidan. Afterwards, shotgun proteomics, enrichment analysis, and the detection of specific biomarkers (by immunohistochemistry or ELISA) were performed. Key results in distal intestine (DI) revealed that fish fed fucoidan increased the presence of CD8 + cells, as well as proteins linked to degradation of branched-chain amino acids, tight junctions, NOD-like receptors, TGF-beta and PPAR signalling. Furthermore, immunological pathways (e.g., complement, antigen processing and presentation, T cell receptor signalling, leukocyte migration, and neutrophil extracellular trap formation) were enriched in fucoidan group along with higher number of chromosome-associated proteins (e.g., histone H2A, nucleosome assembly protein 1-like 4), chaperones (e.g., HSP60, HSP90A, HSPB1) and lectins (e.g., mannose-binding proteins). Also, DI from the fucoidan group showed the enrichment of CD markers including CD29, CD61, CD63 or CD243, which have been reported during cell trafficking (including CD8 + cells). This analysis proposes the potential involvement of CD8 + cells along with other leukocytes in the remodeling of intestinal immunity of fish under nutritional programming. Overall, the current findings showed that fucoidan can modulate innate and adaptive immune mechanisms, making it an interesting candidate for functional aquafeeds.