Dissecting key contributions of TH2 and TH17 cytokines to atopic dermatitis pathophysiology

In atopic dermatitis (AD), epidermal disease hallmarks are driven by a complex cutaneous inflammatory milieu that varies between patients. How these variable inflammatory signals affect cellular and molecular epidermal AD phenotypes is difficult to study in vivo. We aimed to unravel which AD-associated cytokines drive specific epidermal disease hallmarks. We utilized primary and immortalized keratinocyte-derived human epidermal equivalents stimulated with T helper (Th)2, -17 and -22 cytokines. Morphological, functional and transcriptomic analyses revealed that Th2 cytokines interleukin (IL)-4 and IL-13 were main inducers of a pro-inflammatory and hyperproliferative epidermis. The presence of IL-17A or IL-22 in the Th2 milieu, and especially Th2 + IL-22, most closely resembled AD hallmarks including spongiosis, more severe keratinocyte differentiation defects and epidermal barrier dysfunction. Single-cell spatial transcriptomics showed expansion of keratinocytes expressing high levels of proliferation genes, and downregulation of differentiation genes in the upper epidermal layers. The transcriptomic comparison to in vivo AD lesional skin indicated that the Th2 + IL-22 AD model demonstrated greatest resemblance and identified AD disease marker genes altered by Th2 + IL-22 such as downregulated ACER1 and AKR1C3. Gene expression levels were restored by combinatory exposure to aryl hydrocarbon receptor (AHR) ligand tapinarof and Janus Kinase (JAK) inhibitor tofacitinib. This combined therapeutic approach also completely restored epidermal barrier function and improved morphological disease hallmarks. Our results reveal the important role of IL-22 in the Th2 driven acute AD pathophysiology and highlight the potential of combinatory medicine in targeted treatment of AD.