Th17 Cells with Regulatory Phenotype are the Main IL-17F and IL-26 Producers in Palmoplantar Pustulosis

Palmoplantar pustulosis (PPP) is a chronic inflammatory skin disorder marked by erythematous pustules and desquamation on the palms and soles. While IL-17 pathways are implicated in PPP, IL-17 blockers have shown modest efficacy, underscoring the need for a deeper understanding of IL-17 involvement. To dissect the cellular and spatial architecture of PPP, we performed single-cell RNA sequencing (scRNA-seq) on lesional, nonlesional, and healthy acral skin to examine cellular composition, transcriptomic profiles, and cell-cell interactions. Unbiased clustering revealed nine major cell types, including an inflammatory keratinocyte subset enriched in IL-17A/TNF signatures and marked by high IL-36G expression. Within the lymphocyte compartment, we identified a hybrid “regTh17” population co-expressing regulatory markers (FOXP3, CTLA4, TIGIT), IL17F, and IL26. This regTh17 subset was distinguished by elevated IL1R1 and CD39, suggesting a IL-1β–driven differentiation. Spatial analyses demonstrated significant neighborhood enrichment of regTh17 cells with IL-36G⁺ supraspinous keratinocytes. RegTh17 cells were the predominant source of IL-17F and IL-26 signals, whereas keratinocytes were predicted as their main receivers. We further observed regTh17 co-expressing TNFRSF4 (OX40) and TNFRSF18 (GITR) specifically at sites of IL36G+ keratinocyte interactions, implicating these pathways in amplification of the IL-17/IL-36 inflammatory loop. Together, our integrated single-cell and spatial profiling uncovers Th17 plasticity in PPP, identifies a regTh17-keratinocyte interaction and highlights IL-17F, IL-26, OX40/OX40L, and GITR/GITRL as candidate targets for precision therapies in this challenging disease.