Fig. 2.

Toll-like receptors (TLRs) mediated signal transduction pathway. Toll-like receptors (TLRs) are comprised of at least 11 members. Among 11 members, TLR3, TLR7, TLR8, TLR9 were found to localize on endosome membrane and play an essential role in nucleic acid-sensing. TLR3 can detect dsRNA, and TLR7 and TLR8 recognized ssRNA. And unmethylated CpG motif of ssDNA can activate TLR9. TLRs are comprised of the N-terminal extracellular domain (ECD), transmembrane domain (TM), and C-terminal cytoplasmic Toll/IL-1 receptor domain (TIR). ECD domain is responsible for binding pathogen-associated molecular patterns (PAMPs), and the signaling cascades are conducted through the TIR domain, which recruits TRIF or Myd88 as adapters, to activate NFκB and IRFs