Damage-associated molecular patterns and Toll-like receptors. Toll-like receptor (TLR) signaling is illustrated as discussed in the text. TLR2 (in association with TLR1 or TLR6) and TLR4 are located at the cell surface. TLR3, TLR7/8 and TLR9 are found in the endosome. All TLRs except TLR3 recruit myeloid-differentiation primary response protein 88 (MyD88) as an adaptor protein, TLR3 and TLR4 recruit Toll/IL-1 receptor domain-containing adaptor-inducing IFN-β (TRIF) protein. TLR signaling pathways lead to NF-κB activation and/or type I interferon (IFN) expression. Endogenous TLR activators are listed based on the receptors they can activate. HMGB1, high-mobility group box-1; IRF, interferon regulatory factor; LDL, low-density lipoprotein; ssRNA, single-strand RNA.