Toll-like receptors (TLRs) signalling and expression pattern

Toll is a Drosophila gene essential for ontogenesis and antimicrobial resis tance. Several homologues of Toll have been identified and cloned in verteb rates, namely Toll-like receptors (TLRs). TLRs are structurally characteriz ed by a cytoplasmic Toll/interleukin-1R (TIR) domain and by extracellular l eucine rich repeats. TLRs characterized so far activate the MyD88/IRAK casc ade which activates NF-kappaB transcription factor. Genetic, gene transfer, and dominant negative approaches have involved TLR family members (TLR2 an d TLR4) in distinct bacterial components for recognition and signalling. Ho wever, very little information is available regarding other TLRs. Here we p ropose to classify TLRs based on their expression pattern, in ubiquitous (T LR 1), restricted (TLR2, 4, 5) and specific (TLR3) TLRs. Differential expre ssion and regulation as well as distinct, though overlapping, ligand recogn ition patterns may underlie the existence of a such a large, seemingly redu ndant, TLR family.