Background: Understanding the complex orchestrated inflammation in atopic dermatitis (AD), one of the most common chronic inflammatory diseases worldwide, is essential for therapeutic approaches. However, a comparative analysis on the single‐cell level of the inflammation signatures correlated with the severity is missing so far.Methods: We applied single‐cell RNA and T‐cell receptor (TCR) sequencing on immune cells enriched from skin biopsies and matched blood samples of AD in comparison with psoriasis (PS) patients.Results: Clonally propagated skin‐derived T cells showed disease‐specific TCR motifs shared between patients which was more pronounced in PS compared to AD. The disease‐specific T‐cell clusters were mostly of a Th2/Th22 sub‐population in AD and Th17/Tc17 in PS, and their numbers were associated with severity scores in both diseases. Herein, we provide for the first time a list that associates cell type‐specific gene expression with the severity of the two most common chronic inflammatory skin diseases. Investigating the cell signatures in the patients´ PBMCs and skin stromal cells, a systemic involvement of type‐3 inflammation was clearly detectable in PS circulating cells, while in AD inflammatory signatures were most pronounced in fibroblasts, pericytes, and keratinocytes. Compositional and functional analyses of myeloid cells revealed the activation of antiviral responses in macrophages in association with disease severity in both diseases.Conclusion: Different disease‐driving cell types and subtypes which contribute to the hallmarks of type‐2 and type‐3 inflammatory signatures and are associated with disease activities could be identified by single‐cell RNA‐seq and TCR‐seq in AD and PS.

NoIL-33 is a member of the IL-1 family of cytokines that is constitutively expressed in healthy skin and was found to be increased in the skin of patients with atopic dermatitis (AD). Because it can be released after tissue damage or physical stress including scratching of the skin,1 it has been classified as an alarmin concerned with alerting the immune system.2 It enhances TH2 responses by inducing IL-5 and IL-13 as well as TH1 responses via upregulation of IFN-γ. Keratinocytes are known producer cells of IL-33 and also express the receptor complex consisting of ST2 and IL-1RAcP on their surface. The aim of this study was to investigate the effect of IL-33 on keratinocytes, skin biopsies, and living skin equivalents with regard to the regulation of the skin barrier molecule filaggrin (FLG)

Atopic dermatitis (AD) is one of the most common chronic inflammatory skin diseases leading to pruritic skin lesions. A subset of AD patients exhibits a disseminated severe HSV infection called eczema herpeticum (EH) that can cause life‐threatening complications. This review gives an overview of the clinical picture, and characteristics of the patients as well as the diagnosis and therapy of EH. A special focus lies on the pathophysiological hallmarks identified so far that predispose for EH. This aspect covers genetic aberrations, immunological changes, and environmental influences displaying a complex multifactorial situation, which is not completely understood. Type 2 skewing of virus‐specific T cells in ADEH⁺ patients has been implicated in immune profile abnormalities, along with impaired functions of dendritic cells and natural killer cells. Furthermore, aberrations in interferon pathway‐related genes such as IFNG and IFNGR1 have been identified to increase the risk of EH. IL‐4, IL‐25, and thymic stromal lymphopoietin (TSLP) are overexpressed in EH, whereas antimicrobial peptides like human β‐defensins and LL‐37 are reduced. Concerning the epidermal barrier, single nucleotide polymorphisms (SNPs) in skin barrier proteins such as filaggrin were identified in ADEH⁺ patients. A dysbalance of the skin microbiome also contributes to EH due to an increase of Staphylococcus aureus, which provides a supporting role to the viral infection via secreted toxins such as α‐toxin. The risk of EH is reduced in AD patients treated with dupilumab. Further research is needed to identify and specifically target risk factors for EH in AD patients.

Interleukin (IL)-26 is a T_H17 cytokine with known antimicrobial and pro-inflammatory functions. However, the precise role of IL-26 in the context of pathogenic T_H17 responses is unknown. Here we identify a population of blood T_H17 intermediates that produce high levels of IL-26 and differentiate into IL-17A-producing T_H17 cells upon TGF-β1 exposure. By combining single cell RNA sequencing, TCR sequencing and spatial transcriptomics we show that this process occurs in psoriatic skin. In fact, IL-26+ T_H17 intermediates infiltrating psoriatic skin induce TGF-β1 expression in basal keratinocytes and thereby promote their own differentiation into IL-17A-producing cells. Thus, our study identifies IL-26-producing cells as an early differentiation stage of T_H17 cells that infiltrates psoriatic skin and controls its own maturation into IL17A-producing T_H17 cells, via epithelial crosstalk involving paracrine production of TGF-β1.