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. 2020 Nov 20;21(22):8790. doi: 10.3390/ijms21228790

Figure 1.

Figure 1

Toll-like receptors (TLR)-mediated signaling pathway in keratinocytes. TLR1, 2, 4, 5, and 6 are present on the cell membrane, while TLR3 and 9 reside in keratinocytes’ intracellular compartments. TLR2 recognizes a broad range of pathogen-associated molecular patterns, including triacyl lipopeptide (in combination with TLR1), diacyl lipopeptide (in combination with TLR6), peptidoglycan (PGN), lipoteichoic acid (LTA), and Zymosan. TLR5 recognizes flagellin, while TLR4 recognizes lipopolysaccharides (LPS), which is aided by two accessory proteins cluster of differentiation (CD14) and myeloid differentiation factor 2 (MD2). TLR3 and TLR9 reside on the endosomal membrane and recognize double-stranded RNA (dsRNA) and unmethylated CpG DNA, respectively. Upon stimulation, these TLRs activate the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) signaling pathway to induce the expression of inflammatory cytokines. TLR3 and TLR4 can also activate interferon regulatory factor 3 (IRF3), while TLR9 activates interferon regulatory factor 7 (IRF7) to induce the expression of type 1 interferons (IFNs).