Effects of TLR agonists and viral infection on cytokine and TLR expression in Atlantic salmon (Salmo salar)

• Head kidney and spleen are the main TLR expressing organs in Atlantic salmon.
• Several TLR ligands induced expression of inflammatory cytokines in salmon head kidney leucocytes.
• Poly I:C and imiquimod induced expression of interferons and Mx in salmon head kidney leucocytes.
• TLR3, TLR7 and TLR8a1 were induced in vivo after viral infection.

The development of efficient and cheap vaccines against several aquatic viruses is necessary for a sustainable fish farming industry. Toll-like receptor (TLR) ligands have already been used as good adjuvants in human vaccines. With more understanding of TLR expression, function, and ligand specificity in fish, more efficient adjuvants for fish viral vaccines can be developed. In this paper, we examine all known TLRs in Atlantic salmon (Salmo salar) and demonstrate that head kidney and spleen are the main organs expressing TLRs in salmon. We also show that adherent head kidney leucocytes from salmon are able to respond to many of the known agonists for human TLRs, and that viral infection can induce up-regulation of several TLRs. These findings substantiate these receptors’ role in immune responses to pathogens in salmonids making their ligands attractive as vaccine adjuvant candidates.