The role of Toll-like Receptors in innate immunity and infectious diseases of teleost

The immune defence mechanism depends mainly on germ-line encoded pattern-recognition receptors (PRRs). These PRRs respond to many exogenous pathogens and/or endogenous serious signals, by recognizing some highly conserved structures such as pathogen-associated molecular patterns (PAMPs) and danger/damage associated molecular patterns (DAMPs). Till date, the most studied PRRs are Toll-like receptors (TLRs). Upon activation of TLRs, there is production of inflammatory cytokines and type I interferons (IFNs) via myeloid differentiation primary response gene 88 (MyD88)-dependent or -independent signalling, respectively, modulating innate and adaptive immunity, as well as inflammatory responses. In fish species studied to date, there are more than 17 TLRs that are identified with some showing homology to mammals, and some are unique for teleost. In the present review, more light are to be shed on the classification, structure and specific ligands of TLRs, with focuses on their signal pathways and different biological activities. Studies of TLRs and their role in the innate immune will potentially have implications for the prevention and treatment of fish diseases.