Induction of Interleukin-22 (IL-22) production in CD4+ T Cells by IL-17A Secreted from CpG-Stimulated Keratinocytes

BACKGROUND Interleukin-17A (IL-17A) is mainly secreted from Th17 cells that are activated by various stimuli including CpG oligodeoxynucleotide, a Toll-like receptor 9 (TLR9) ligand. Recently, it has been demonstrated that keratinocytes play an important role in the pathogenesis of psoriasis. OBJECTIVE To investigate the potential role of keratinocytes, we examined whether TLR9 ligand CpG induces IL-17A expression in keratinocytes. METHODS We used HaCaT keratinocytes as a model system, and determined CpG-induced IL-17A using enzyme-linked immunosorbent assay and Western blot. RESULTS When HaCaT keratinocytes were treated with CpG, the expression of several cytokines including IL-17A, tumor necrosis factor-α and CCL20 was markedly increased. Treatment with nuclear factor (NF)-κB inhibitor significantly blocked the CpG-induced IL-17A production, indicating that CpG induced IL-17A expression through the NF-κB signaling pathway. In addition, IL-17A secreted from keratinocytes stimulated the CD4+ T cells, resulting in strong induction of IL-22 production. CONCLUSION Since IL-22 is an important mediator for psoriatic inflammation, our data suggest that keratinocytes can participate in the pathogenesis of psoriasis via the TLR9-dependent IL-17A production.