The Role of Toll-like Receptors (TLRs) in Fish Immunity

The innate system’s recognition of non-self and danger signals is mediated by a limited number of germline-encoded pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs). Toll-like receptors (TLRs) are single, non-catalytic, membrane-spanning PRRs present in invertebrates and vertebrates. They act by specifically recognizing PAMPs of a variety of microbes and activate signalling cascades to induce innate immunity. A large number of TLRs has been identified in various teleosts and basal vertebrates. The varied evolutionary lineages of aquatic and warm-blooded higher animals result in some unique characteristics in their TLRs. In innate immunity and pathogen recognition, the majority of them share conserved signalling pathways. Comparing the immunology of teleosts, chondrichthyans, agnathans, and warm-blooded vertebrates requires a functional analysis of novel TLRs in teleosts and basal vertebrates. Furthermore, TLRs play important roles in adaptive immunity in addition to innate immunity. This review describes the discovery of TLRs and their role in fish immunity along their signalling pathway and ligand specificity.