Single-cell analysis implicates Th17 to Th2 cell plasticity in the pathogenesis of palmoplantar pustulosis

Background\ud\udPalmoplantar pustulosis (PPP) is a severe inflammatory skin disorder, characterised by eruptions of painful, neutrophil-filled pustules on the palms and soles. While PPP has a profound effect on quality of life, it remains poorly understood and notoriously difficult to treat.\udObjective\ud\udWe sought to investigate the immune pathways that underlie the pathogenesis of PPP.\udMethods\ud\udWe applied bulk- and single-cell RNA-sequencing methods to the analysis of skin biopsies and peripheral blood mononuclear cells. We validated our results by flow cytometry and immune fluorescence microscopy\udResults\ud\udBulk RNA-sequencing of patient skin detected an unexpected signature of T-cell activation, with a significant overexpression of several Th2 genes typically upregulated in atopic dermatitis. To further explore these findings, we carried out single-cell RNA-sequencing in peripheral blood mononuclear cells of healthy and affected individuals. We found that the memory CD4+T-cells of PPP patients were skewed towards a Th17 phenotype, a phenomenon that was particularly significant among CLA+ skin-homing cells. We also identified a subset of memory CD4+ T-cells which expressed both Th17 (KLRB1/CD161) and Th2 (GATA3) markers, with pseudo-time analysis suggesting that the population was the result of Th17 to Th2 plasticity. Interestingly, the GATA3+/CD161+ cells were over-represented among the PBMCs of affected individuals, both in the scRNA-seq dataset and in independent flow-cytometry experiments. Dual positive cells were also detected in patient skin by means of immune fluorescence microscopy.\udConclusions\ud\udThese observations demonstrate that PPP is associated with complex T-cell activation patterns and may explain why biologics that target individual T-helper populations have shown limited therapeutic efficacy.\udClinical implications\ud\udThe simultaneous activation of Th17 and Th2 responses in PPP supports the therapeutic use of agents that inhibit multiple T-cell pathways.