IL-17C is a functionally distinct member of the IL-17 family that binds IL-17RE/A to promote innate defense in epithelial cells and regulate Th17 cell differentiation. We demonstrate that IL-17C (not IL-17A) is the most abundant IL-17 isoform in lesional psoriasis skin (1058pg/ml vs. 8pg/ml; p<0.006) and localizes to keratinocytes (KCs), endothelial cells (ECs) and leukocytes. ECs stimulated with IL-17C produce increased TNFα and KCs stimulated with IL-17C/TNFα produce similar inflammatory gene response patterns as those elicited by IL-17A/TNFα, including increases in IL-17C, TNFα, IL-8, IL-1α/β, IL-1F5, IL-1F9, IL-6, IL-19, CCL20, S100A7/A8/A9, DEFB4, LCN2 and PI3 (p<0.05); indicating a positive pro-inflammatory feedback loop between the epidermis and ECs. Psoriasis patients treated with etanercept rapidly decrease cutaneous IL-17C levels, suggesting IL-17C/TNFα-mediated inflammatory signaling is critical for psoriasis pathogenesis. Mice genetically engineered to overexpress IL-17C in KCs develop well-demarcated areas of erythematous, flakey “involved” skin adjacent to areas of normal appearing “uninvolved” skin despite increased IL-17C expression in both areas (p<0.05). Uninvolved skin displays increased angiogenesis and elevated S100A8/A9expression (p<0.05) but no epidermal hyperplasia; whereas involved skin exhibits robust epidermal hyperplasia, increased angiogenesis and leukocyte infiltration and upregulated TNFα, IL-1α/β, IL-17A/F, IL-23p19, VEGF, IL-6 and CCL20 (p<0.05) suggesting that IL-17C, when coupled with other pro-inflammatory signals, initiates the development of psoriasiform dermatitis. This skin phenotype was significantly improved following 8 weeks of TNFα inhibition. These findings identify a role for IL-17C in skin inflammation and suggest a pathogenic function for the elevated IL-17C observed in lesional psoriasis skin.

The IL-1 family members IL-36α (IL-1F6), IL-36β (IL-1F8) and IL-36γ (IL-1F9) and the receptor antagonist IL-36Ra (IL-1F5) constitute a novel signaling system that is poorly understood. We now show that these cytokines have profound effects on the skin immune system. Treatment of human keratinocytes with IL-36 cytokines significantly increased the expression of CXCL1, CXCL8, CCL3, CCL5, and CCL20, potent chemotactic agents for activated leukocytes, and IL-36α injected intradermally resulted in chemokine expression, leukocyte infiltration and acanthosis of mouse skin. Blood monocytes, myeloid dendritic cells (DC) and monocyte-derived DC (MO-DC) expressed IL-36R and responded to IL-36. In contrast, no direct effects of IL-36 on resting or activated human CD4+ or CD8+ T cells, or blood neutrophils, could be demonstrated. Monocytes expressed IL-1A, IL-1B and IL-6 mRNA and IL-1β and IL-6 protein and mDC upregulated surface expression of CD83, CD86 and HLADR and secretion of IL-1β and IL-6 after treatment with IL-36. Furthermore, IL-36α-treated MO-DC enhanced allogeneic CD4+ T cell proliferation, demonstrating that IL-36 can stimulate the maturation and function of DC and drive T cell proliferation. These data indicate that IL-36 cytokines actively propagate skin inflammation via the activation of keratinocytes, antigen presenting cells and, indirectly, T cells.

The clinical features of psoriasis, characterized by sharply demarcated scaly erythematous plaques, are typically so distinctive that a diagnosis can easily be made on these grounds alone. However, there is great variability in treatment response between individual patients, and this may reflect heterogeneity of inflammatory networks driving the disease. In this study, whole-genome transcriptional profiling was used to characterize inflammatory and cytokine networks in 62 lesional skin samples obtained from patients with stable chronic plaque psoriasis. We were able to stratify lesions according to their inflammatory gene expression signatures, identifying those associated with strong (37% of patients), moderate (39%) and weak inflammatory infiltrates (24%). Additionally, we identified differences in cytokine signatures with heightened cytokine-response patterns in one sub-group of lesions (IL-13-strong; 50%) and attenuation of these patterns in a second sub-group (IL-13-weak; 50%). These sub-groups correlated with the composition of the inflammatory infiltrate, but were only weakly associated with increased risk allele frequency at some psoriasis susceptibility loci (e.g., REL, TRAF3IP2 and NOS2). Our findings highlight variable points in the inflammatory and cytokine networks known to drive chronic plaque psoriasis. Such heterogeneous aspects may shape clinical course and treatment responses, and can provide avenues for development of personalized treatments.

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