Dermal Fibroblasts, Not Keratinocytes, Dominate IL-17A/TNF-Driven Inflammation.

Chronic inflammatory skin diseases such as psoriasis and hidradenitis suppurativa are driven by cytokines, including IL-17A and TNF. Although biologics targeting these cytokines have transformed therapy, the transcriptional contributions of individual skin-resident cell types remain unclear, and dermal fibroblasts have been largely overlooked compared with keratinocytes. To address this, we compared the transcriptional responses of primary human dermal fibroblasts and keratinocytes following in vitro stimulation with IL-17A, TNF, or both, using bulk RNA sequencing and Western blotting. Dermal fibroblasts mounted a stronger and broader proinflammatory response than keratinocytes. This was particularly evident in response to TNF and combined TNF/IL-17A stimulation, with enrichment for immune signalling and chemotaxis pathways and robust induction of chemokine genes, including CCL20, CXCL8, and IL6. Keratinocytes primarily upregulated genes associated with epithelial differentiation, barrier function, and protein regulation, including IL36G, S100A7A, and DEFB4A. The heightened fibroblast responsiveness correlated with increased TNF sensitivity and substantially higher TNFR2 (TNFRSF1B) expression and signalling compared with keratinocytes, suggesting a fibroblast-specific mechanism amplifying inflammatory responses. These findings challenge the keratinocyte-centric view of skin inflammation and identify dermal fibroblasts as active contributors and potential therapeutic targets in TNF- and Th17-driven skin diseases.