IL-36α from Skin-Resident Cells Plays an Important Role in the Pathogenesis of Imiquimod-Induced Psoriasiform Dermatitis by Forming a Local Autoamplification Loop

IL-36 alpha (gene symbol Il1f6), a member of the IL-36 family, is closely associated with inflammatory diseases, including colitis and psoriasis. In this study, we found that Il1f6(-/-) mice developed milder psoriasiform dermatitis upon treatment with imiquimod, a ligand for TLR ligand 7 (TLR7) and TLR8, whereas Il1f6(-/-) mice showed similar susceptibility to dextran sodium sulfate-induced colitis to wild-type mice. These effects were observed in both cohoused and separately housed conditions, and antibiotic treatment did not cancel the resistance of Il1f6(-/-) mice to imiquimod-induced dermatitis. Bone marrow (BM) cell transfer revealed that IL-36 alpha expression in skin-resident cells is important for the pathogenesis of dermatitis in these mice. Following stimulation with IL-36 alpha, the expression of Il1f6 and Il1f9 (IL-36 gamma), but not Il1f8 (IL-36 beta), was enhanced in murine BM-derived Langerhans cells (BMLCs) and murine primary keratinocytes but not in fibroblasts from mice. Upon stimulation with agonistic ligands of TLRs and C-type lectin receptors (CLRs), Il1f6 expression was induced in BMLCs and BM-derived dendritic cells. Furthermore, IL-36 alpha stimulation resulted in significantly increased gene expression of psoriasis-associated Th17-related cytokines and chemokines such as IL-1 alpha, IL-1 beta, IL-23, CXCL1, and CXCL2 in BMLCs and fibroblasts, and IL-1 alpha, IL-1 beta, IL-17C, and CXCL2 in keratinocytes. Collectively, these results suggest that TLR/CLR signaling-induced IL-36 alpha plays an important role for the development of psoriasiform dermatitis by enhancing Th17-related cytokine/chemokine production in skin-resident cells via a local autoamplification loop.