Abstract
Although neuronal Toll-like receptors (TLRs) (e.g., TLR2, TLR3, and TLR7) have been implicated in itch sensation, the roles of keratinocyte TLRs in chronic itch are elusive. Herein, we evaluated the roles of keratinocyte TLR2 and TLR7 in chronic itch under dry skin and psoriasis conditions, which was induced by either acetone-ether-water treatment or 5% imiquimod cream in mice, respectively. We found that TLR2 and TLR7 signaling were significantly upregulated in dry skin and psoriatic skin in mice. Chronic itch and epidermal hyperplasia induced by dry skin or psoriasis were comparably reduced in TLR2 and TLR7 knockout mice. In the dry skin model, the enhanced messenger RNA (mRNA) expression levels of pruritic CXCL1/2, IL-31, IL-33, ST2, IL-6, IL-17A, TNF-α, and IFN-γ were inhibited in TLR2-/- mice, while CXCL2, IL-31, and IL-6 were inhibited in TLR7-/- mice. In psoriasis model, the enhanced mRNA expression levels of pruritic CXCL1/2, IL-31, IL-33, ST2, IL-6, and TNF-α were inhibited in TLR2-/- mice, while CXCL1/2, IL-31, IL-33, ST2, IL-6, IL-17A, and TNF-α were inhibited in TLR7-/- mice. Incubation with Staphylococcus aureus (S. aureus) peptidoglycan (PGN-SA) (a TLR2 agonist), imiquimod (a TLR7 agonist), and miR142-3p (a putative TLR7 agonist) were sufficient to upregulate the expression of pruritic cytokines or chemokines in cultured keratinocyte HaCaT cells. Finally, pharmacological blockade of C-X-C Motif Chemokine Receptor 1/2 and high mobility group box protein 1 dose-dependently attenuated acute and chronic itch in mice. Together, these results indicate that keratinocyte TLR2 and TLR7 signaling pathways are distinctly involved in the pathogenesis of chronic itch.