Fig 2: IL26+ cells differentiate into IL-17A-producing cells at the clone level.a UMAP projection of the single-cell transcriptomes of nonlesional and lesional psoriatic skin CD4 T cells from two merged datasets colored according to the inferred pseudotime. b Dynamics of IL17A and IL26 gene expression in skin CD4 T cells over pseudotime. Solid lines show the expression average. c Venn diagram of the number of shared TCR clonotypes between CD4 T cells at early (<10), intermediate, and late (>15) pseudotime. d Sankey diagram of the shared TCR clonotypes between late IL17A+ CD4 T cells and early IL26+ or IL26- CD4 T cells. e UMAP projection of the single-cell transcriptomes of two different TCR clones colored according to the inferred pseudotime. f Percentages of IL26+ and IL17A+ cells among CD4+ T cells from healthy (HS), nonlesional (NL), and lesional (L) skin of atopic dermatitis (AD) and psoriasis (PSO) patients. Data were statistically analyzed using two-way ANOVA followed by Tukey’s multiple comparisons test. *p = 0.0181, **p = 0.0094, ****p < 0.0001. Data represent the mean ± SEM of 3 independent patient samples. g UMAP projection of the single-cell transcriptomes of dermal CD4 T cells from AD lesional skin colored according to the expression level of IL26 (green), and IL17A (red). h UMAP projection of the single-cell transcriptomes of dermal CD4 T cells as in (g) colored according to the inferred pseudotime. Solid lines are trajectories of gene expression changes learned by Monocle 3. Source data are provided as a Source Data file.

Fig 3: Rapid TGF-ß1 independent differentiation of IL26+ but not IL17A+ TH17 cells.a Flow-FISH analysis of IL-26 and IL-17A mRNA expression in naive CD4 T cells stimulated for 7 days in the presence of either single cytokines or a mix of the TH17 polarizing cytokines IL-1ß, IL-6, IL-23, with or without TGF-ß1. Data from one representative donor are shown. b Percentages of IL26+ and IL17A+ T cells in cultures described in (a). Data are mean + SEM of 5 independent donors. Data were statistically analyzed using one-way ANOVA followed by Tukey’s multiple comparisons test. *p = 0.0255; ***p = 0.0003. c ELISA measurement of secreted IL-26 and IL-17A by T cells obtained by a 7-day stimulation of naive CD4 T cells as in (a). Data are mean ± SEM of 4 independent donors. d Flow-FISH analysis of the expression kinetics of IL-26 and IL-17A mRNA in naive CD4 T cells stimulated for 7 days either alone, or in the presence of TH17 polarizing cytokines IL-1ß, IL-6, IL-23, with or without TGF-ß1. Data from one representative donor are shown. e Percentages of IL26+ and IL17A+ T cells cultured as in (d). Data are mean ± SD of 3 independent donors. Source data are provided as a Source Data file.

Fig 4: Circulating TH17 cells contain a large cell population expressing IL26 but not IL17A.a Flow-FISH analysis of IL-26, IL-17A, IFN-?, and IL-22 mRNA expression in memory TH17 cells isolated from peripheral blood. Data shown are from one representative donor. b Frequencies of IL26 and IL17A single producing T cells and IL26/IL17A double producers among blood memory TH17 cells. Data represent the mean ± SD (n = 4 independent donors). c Flow-FISH analysis of IL-26, IL-17A, IFN?, and IL-22 mRNA expression in four independent TH17 cell clones (B101, C2, C4, and C8). d Frequencies of IL26 and IL17A single-positive cells, and IL26/IL17A double positive cells among TH17 cell clones Data are mean ± SD (n = 4). Source data are provided as a Source Data file.

Fig 5: Increased TGF-β1 expression by psoriatic keratinocytes induced by IL-26.a Expression fold change of TGFB1 in healthy skin (n = 40) and nonlesional (n = 199) and lesional (n = 120) skin from psoriasis patients using the SSF Bioinformatics hub. Data are represented as a box plot which bounds extends from the 25th to 75th percentiles, a middle line is plotted at the median, and whiskers go from the minimal to maximal value Data were statistically analyzed using one-way ANOVA followed by Tukey’s multiple comparisons test. ****p < 0.0001. b UMAP projection of the single-cell transcriptomes of total cells from nonlesional (top) and lesional (bottom) skin of 3 psoriasis patients colored according to the expression level of TGFB1. LC Langherans cells, KC keratinocytes, Mac/DC macrophages and dendritic cells, Fb fibroblasts, VE vascular endothelial cells. c Confocal microscopy image of lesional psoriatic skin representative of 5 patients stained for TGF-β mRNA. Dashed line delineates the dermo-epidermal junction. d Violin plots of TGFB1 expression by basal (KRT5+, KRT14+), proliferating (CDK1+, PCNA+), and differentiated suprabrasal (KRT1+, KRT10+) keratinocytes using single-cell RNAseq data from nonlesional and lesional skin of atopic dermatitis and psoriasis patients. e, Expression of TGFB1 in NHEK cells treated with different cytokines for 16 h. Data represent mean ± SEM of 3 biological replicates from one representative donor. Data were statistically analyzed using one-way ANOVA followed by Tukey’s multiple comparisons test. *p = 0.036; **p = 0.0063, ****p < 0.0001. f Expression of TGFB1 in healthy skin treated with IL-26 for 4 h. Data represent 3 independent donors. Data were statistically analyzed using two-tailed paired Student’s t-test. *p = 0.0122. g Expression of TGFB1 in NHEK cells treated overnight with IL-26 in the presence of blocking antibodies against IL-26, IL10R2, and IL20R1. Data represent mean ± SEM of 4 biological replicates from one donor. Data were statistically analyzed using one-way ANOVA followed by Tukey’s multiple comparisons test. ****p < 0.0001. h Expression of TGFB1 in WT or IL20R1−/− HaCaT cells treated overnight with IL-26. Data represent mean ± SEM of 7 biological replicates. Data were statistically analyzed using one-way ANOVA followed by Tukey’s multiple comparisons test. ****p < 0.0001. i, Amounts of TGF-β1 produced by HaCaT cells treated overnight with TH17 cell supernatants (from 5 independent donors) in the presence of blocking antibodies against IL10R2 and IL20R1, or control isotype. Data represent mean ± SEM. Data were statistically analyzed using one-way ANOVA followed by Tukey’s multiple comparisons test. *p = 0.0124 (aIL10R2), *p = 0.018 (aIL20R1). j Amounts of IL-26 (left) and IL-17A (right) produced by blood TH17 cells (from 4 independent donors) treated for 7 days with IL-26-treated HaCaT cell supernatants in the presence of blocking antibodies against TGF-β. Data represent mean ± SEM. Data were statistically analyzed using two-tailed unpaired Student’s t-test. *p = 0.014. e–h Expression values measured by RT-qPCR were normalized to the reference gene GAPDH. Source data are provided as a Source Data file.