Single-cell analysis implicates TH17-to-TH2 cell plasticity in the pathogenesis of palmoplantar pustulosis
Daniel McCluskey, Natashia Benzian-Olsson, Satveer K. Mahil, Karoliina Hassi, Christian T. Wohnhaas, A. David Burden, C.E.M. Griffiths, John R Ingram, N. J. Levell, Richard Parslew, Andrew Pink, Nick J. Reynolds, Richard B. Warren, Sudha Visvanathan, Patrick Baum, Jonathan N. Barker, Catherine Smith, Francesca Capon
Abstract
<h3>Background</h3> Palmoplantar pustulosis (PPP) is a severe inflammatory skin disorder characterized by eruptions of painful, neutrophil-filled pustules on the palms and soles. Although PPP has a profound effect on quality of life, it remains poorly understood and notoriously difficult to treat. <h3>Objective</h3> We sought to investigate the immune pathways that underlie the pathogenesis of PPP. <h3>Methods</h3> We applied bulk and single-cell RNA sequencing (RNA-Seq) methods to the analysis of skin biopsy samples and peripheral blood mononuclear cells. We validated our results by flow cytometry and immune fluorescence microscopy <h3>Results</h3> Bulk RNA-Seq of patient skin detected an unexpected signature of T-cell activation, with a significant overexpression of several T<sub>H</sub>2 genes typically upregulated in atopic dermatitis. To further explore these findings, we carried out single-cell RNA-Seq in peripheral blood mononuclear cells of healthy and affected individuals. Memory CD4<sup>+</sup> T cells of PPP patients were skewed toward a T<sub>H</sub>17 phenotype, a phenomenon that was particularly significant among cutaneous lymphocyte-associated antigen–positive skin-homing cells. We also identified a subset of memory CD4<sup>+</sup> T cells that expressed both T<sub>H</sub>17 (<i>KLRB1</i>/CD161) and T<sub>H</sub>2 (<i>GATA3</i>) markers, with pseudotime analysis suggesting that the population was the result of T<sub>H</sub>17 to T<sub>H</sub>2 plasticity. Interestingly, the GATA3<sup>+</sup>/CD161<sup>+</sup> cells were overrepresented among the peripheral blood mononuclear cells of affected individuals, both in the single-cell RNA-Seq data set and in independent flow cytometry experiments. Dual-positive cells were also detected in patient skin by immune fluorescence microscopy. <h3>Conclusions</h3> PPP is associated with complex T-cell activation patterns and may explain why biologic drugs that target individual T helper cell populations have shown limited therapeutic efficacy.