Toll-like receptor signaling in teleosts

Toll-like receptors (TLRs) sit at the top of the immune system pyramid. They form a paramount family of immune sentinels capable of sensing diverse microbe-associated molecular patterns (MAPMs), danger/damage-associated molecular patterns (DAMPs), and other signals. These perceptions trigger immediate innate immunity and instruct subsequent adaptive immunity. TLRs are highly glycosylated type I transmembrane glycoproteins that share a conserved tripartite domain architecture (LRR, TM and TIR domains), classified into six subfamilies (TLR1, TLR3, TLR4, TLR5, TLR7, TLR11) in vertebrates. Upon ligand engagement, TLRs form homodimers or heterodimers to activate immune responses via SMOCs, orchestrated by intrinsic and pathogen-directed negative regulators, glycosylation modification, etc. TLR signaling culminates in the production of inflammatory cytokines, interferons, inflammasomes, immune cell activation, apoptosis, etc. Teleosts, as the largest and most diverse group among the extant vertebrates, manifest important economic value and are crucial for understanding the evolution of vertebrate immunity. To date, teleosts contain 20 TLRs (TLR1-5, TLR7-9, TLR13, TLR14, TLR18-23, TLR25-28) with expansions and losses in different species, and most of them possess more or less variants. Almost all teleostean TLRs localize in organelles, such as endosomes and lysosomes, sensing not only pathogens and DAMPs but also trophic factors and environmental stresses (hypoxia, temperature, microplastics, etc.). Most ligands for TLRs remain undetermined in teleosts. The adaptors consist of MyD88, TIRAP, TRIF, SARM1, BCAP and SCIMP, but without TRAM; however, half of the corresponding relationships between TLRs and adaptors remain unknown in teleosts. Neofunctionalization often emerges during evolution in teleostean TLRs. Here, a systematic review of TLR signaling in teleosts, from the perspective of comparative immunology, presents the current understanding of the functions and mechanisms of teleosts. Additionally, it provides strong evidence of a divergent TLR signaling repertoire with the species-specific variation among teleosts. These are expected to benefit novel adjuvants, aquaculture, fish immunology, and comparative immunology.