Cathelicidin Antimicrobial Peptide LL37 Induces Toll-Like Receptor 8 and Amplifies IL-36γ and IL-17C in Human Keratinocytes

LL37 is produced by skin injury and bacterial infection and plays an important role in the early stages of psoriasis. In particular, the intracellular receptors toll-like receptors (TLR)3, TLR7, TLR8, and TLR9 are thought to be involved in the pathogenesis of psoriasis in conjunction with LL37, but the interaction between TLR7/8 and LL37 in keratinocytes (KCs) remains unclear. This study aimed to clarify the relationship between LL37 and TLR7/8 in KCs and their involvement in the pathogenetic pathways seen in psoriasis using cultured KCs and skin samples of patients with psoriasis. TLR7/8 was induced by LL37 in KCs. TLR8 but not TLR7 functionally induced many psoriasis-related molecules, whereas IL-17C was not altered by the blockade of TLR7/8. Although costimulation of LL37 with self-RNA/DNA did not show any interaction, LL37 itself would promote psoriasis-related genes. IL-36 receptor antagonistic antibody suppressed IL-17C induced by LL37. In psoriatic epidermis, LL37, TLRs, IL-17C, and IL-36γ expressions were increased and coexpressed with each other. Thus, we concluded that LL37 activates TLR8 in KCs and induces IL-17C through the induction of IL-36γ. Regulation of TLR8 or LL37 in KCs could be a potential therapeutic strategy for psoriatic inflammation.