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. 4-mm lesional and non-lesional punch biopsies were collected from the acral skin of three patients with palmoplantar pustulosis (PPP) and five healthy controls. Single-cell suspensions were prepared and processed using the 10X Genomics Chromium system, followed by sequencing on an Illumina NovaSeq 6000 platform. Raw reads were aligned to the human reference genome (hg38) using Cell Ranger. Downstream analysis included quality control filtering, removal of ambient RNA contamination (SoupX), doublet detection (scDblFinder), data normalization, and batch correction (Harmony) before clustering with Seurat (v5.0.2).