IL-17 Ligand and Receptor Family Members Are Differentially Expressed by Keratinocyte Subpopulations and Modulate Their Differentiation and Inflammatory Phenotype.

Psoriasis is a chronic inflammatory skin disease characterized by dysregulation of the interleukin 17 (IL-17) signaling axis. Given that psoriasis development depends on keratinocyte stem cells and early progenitors' sensitivity to differentiation, we analyzed IL-17 ligands and the expression and function of in a novel subset of keratinocyte subpopulations: keratinocyte stem cells (KSC) and early and late Transit Amplifying (ETA or LTA, respectively) cells. We found that all subpopulations expressed all IL-17 variants, predominantly in ETA and LTA. Conversely, IL-17 receptor expression resulted in more heterogeneity, with IL-17RA, -C, and -E being the most differentially regulated. Stimulus with IL-17A, IL-17-F, IL-17-A/F, and IL-17C promotes the upregulation of CXCL1, CXCL8, and DEFB4 mRNAs expression in both KSC and ETA. Moreover, IL-17A and IL-17A/F mainly decrease KSC proliferation and promote cell cycle block. Globally, IL-17A and IL-17A/F modulated the expression of proliferation, differentiation, and psoriasis-associated markers. Furthermore, KSC- and ETA-derived 3D reconstructions displayed increased epidermal thickness and upregulated KRT16 expression after treatment with IL-17A or IL-17A/F. Therefore, our data demonstrated that IL-17 family members perform distinctive functions in a specific keratinocyte subpopulation and define IL-17 signaling as a critical modulator of KSC behavior, proving its role in epidermal homeostasis dysregulation of psoriasis.