Mammalian TLR comprise a large family consisting of at least 11 members. TLR1–9 were found to be conserved between humans and mice. TLR10 is presumably functional in humans but non-functional in mice. Similarly, mouse TLR11 is functional, but there is a stop codon in the human TLR11 gene, which results in a lack of production of human TLR11. The cytoplasmic INCB024360 nmr portion of TLR shows high similarity to that of the
interleukin (IL)-1 receptor family, and is termed a Toll/IL-1 receptor (TIR) domain. Despite this similarity, the extracellular portions of both types of receptors are structurally unrelated. The IL-1 receptors possess an immunoglobulin-like domain, whereas TLR bear leucine-rich repeats in the extracellular domain. Functionally, a critical role of TLR4 in the
recognition of the microbial component was initially characterized. Subsequently, it has been established that individual TLR play important roles in recognizing specific microbial components derived from pathogens including bacteria, fungi, protozoa and viruses. Toll-like receptor 2 is essential selleck kinase inhibitor in the recognition of microbial lipopeptides and peptidoglycan derived from Gram-positive bacteria. TLR1 and TLR6 cooperate with TLR2 to discriminate subtle differences between triacyl and diacyl lipopeptides, respectively. TLR2 forms heterophilic dimers with TLR1 and TLR6, both of which are structurally related to TLR2. TLR4 is the receptor for LPS derived from the outer membrane of Gram-negative bacteria. TLR5 recognizes flagellin. TLR3 is implicated in the recognition of viral dsRNA associated with viral replication, whereas TLR7 and TLR8 are implicated in viral-derived ssRNA recognition. Thus, polyinosinic–polycytidylic acid (polyI:C), which is a synthetic mimetic for dsRNA, can induce TLR3 signaling. TLR9 is essential in unmethylated
(CpG) DNA recognition. There are two types of ligands, exogenous and endogenous, for TLR4. As described above, TLR4 is an essential receptor for bacterial endotoxin or LPS recognition. In addition Ergoloid to LPS, other exogenous ligands are F protein from respiratory syncytial virus, chlamydial heat shock protein (Hsp)60 and taxol, a plant-derived anticancer reagent that mimics the action of LPS in mice but not in humans. Endogenous ligands of TLR4 comprise fibrinogen, fibronectin, heparan sulfate, hyaluronic acid, and Hsp60 and Hsp70. However, all of these endogenous ligands require very high concentration to activate TLR4. It has been shown that contamination of LPS in Hsp70 preparation confers ability to activate TLR4. LPS is a very potent immuno-activator and accordingly, TLR4 can be activated by a very small amount of LPS, contaminating these endogenous ligand preparations.[4, 19-22] Therefore, we need careful attention in biological research using these endogenous ligands. The different TLR and their corresponding ligands are described in Table 1.