Toll-like receptors (TLR) and its signaling pathway in teleost

Fish is the most diverse group of vertebrates. The immune system protects fish from pathogenic infection in their adaptation and evolution. Toll-like receptors constitute the first line of defense against the invading pathogens, and play a crucial role in connecting the innate immune response and acquiring immune response. Although the protein structure and function of TLR families from lower invertebrates to higher mammals are relatively conservative, the TLR of fish have shown greater diversity, including some fish-specific TLR and more alternative splicing TLR. The immune recognition, signaling transduction, activation and regulation of TLR pathway in teleost have demonstrated more differences compared with higher vertebrates. In this review, we summarized the molecular characterizations, subfamilies, functions, and diversity of TLR in fish. In addition, immune signaling transduction and regulation of TLR pathway were also reviewed for better understanding the immune response of fish.