CIKS/Act1-mediated signaling by IL-17 cytokines in context: implications for how a CIKS gene variant may predispose to psoriasis.

Psoriasis is a relapsing skin disease characterized by abnormal keratinocyte proliferation and differentiation and by an influx of inflammatory immune cells. Recently, IL-17 cytokines have been strongly implicated as critical for the pathogenesis of this disease. IL-17A (also known as IL-17) and IL-17F are the signature cytokines of Th17 cells, but are also produced by innate cells, including γδ T cells present in skin, whereas epithelial cells, including keratinocytes, may produce IL-17C. IL-17 cytokines signal via the adaptor protein connection to IκB kinase and stress-activated protein kinases (CIKS)/Act1. Psoriasis is a disease with a strong genetic predisposition, and the gene encoding CIKS has recently been identified as a susceptibility locus. Unexpectedly, one predisposing gene variant features a mutation that impairs rather than enhances CIKS-mediated IL-17 cytokine signaling, counter to the predicted role for IL-17 cytokines in psoriatic inflammation. In this study, we demonstrate, however, that this mutant adaptor does not impair the IL-17-specific contributions to the genetic response when combined with TNF-α, a cytokine also prominent in psoriatic inflammation. Interestingly, TNF-α signals compensate IL-17 signaling defects imposed by this mutant adaptor even for genes that are not induced by TNF-α alone, including the transcription factors CCAAT/enhancer binding protein δ and IκBζ, which help regulate secondary gene expression in response to IL-17. Based on these findings we discuss a scenario in which the mutant adaptor may interfere with homeostatic maintenance of epithelial barriers, thereby potentially enabling the initiation of inflammatory responses to insults, whereas this same mutant adaptor would still be able to mediate IL-17-specific contributions to inflammation once TNF-α is present.