Atopic dermatitis (AD) is a chronic inflammatory skin disease that significantly impacts quality of life. The pathogenesis of AD is a complex combination of skin barrier dysfunction, type II immune response, and pruritus. Progress in the understanding of the immunological mechanisms of AD has led to the recognition of multiple novel therapeutic targets. For systemic therapy, new biologic agents that target IL-13, IL-22, IL-33, the IL-23/IL-17 axis, and OX40-OX40L are being developed. Binding of type II cytokines to their receptors activates Janus kinase (JAK) and its downstream signal, namely signal transduction and activator of transcription (STAT). JAK inhibitors block the activation of the JAK-STAT pathway, thereby blocking the signaling pathways mediated by type II cytokines. In addition to oral JAK inhibitors, histamine H4 receptor antagonists are under investigation as small-molecule compounds. For topical therapy, JAK inhibitors, aryl hydrocarbon receptor modulators, and phosphodiesterase-4 inhibitors are being approved. Microbiome modulation is also being examined for the treatment of AD. This review outlines current and future directions for novel therapies of AD that are currently being investigated in clinical trials, focusing on their mechanisms of action and efficacy. This supports the accumulation of data on advanced treatments for AD in the new era of precision medicine.

Atopic dermatitis (AD) is one of the most common chronic inflammatory skin diseases, and the condition is typified by barrier dysfunction and immune dysregulation. Recent studies have characterized various phenotypes and endotypes of AD and elucidated the mechanism. Numerous topical and systemic narrow targeting therapies for AD have been developed according to these findings. Topical medications, including Janus kinase (JAK) inhibitors, phosphodiesterase 4 inhibitors, and the aryl hydrocarbon receptor agonist tapinarof, are effective and safe for AD compared to topical corticosteroids. Oral JAK inhibitors and monoclonal antibodies targeting interleukin (IL)-4, IL-13, IL-31, IL-33, OX40, thymic stromal lymphopoietin, and sphingosine 1-phosphate signaling have displayed outstanding efficacy against moderate-to-severe AD. We are currently in a new era of AD treatment.

Interleukin (IL)-33 and IL-37 have been identified as novel cytokines involved in various inflammatory diseases. However, their specific roles remain largely unknown. Recent studies have shown that IL-33, which triggers inflammation, and IL-37, which suppresses it, cooperatively regulate the balance between inflammation and anti-inflammation. IL-33 and IL-37 are also deeply involved in the pathogenesis of inflammatory skin diseases such as atopic dermatitis (AD) and psoriasis. Furthermore, a signaling pathway by which aryl hydrocarbon receptor (AHR), a receptor for dioxins, regulates the expression of IL-33 and IL-37 has been revealed. Here, we outline recent findings on the mechanisms regulating IL-33 and IL-37 expression in AD and psoriasis. IL-33 expression is partially dependent on mitogen-activated protein kinase (MAPK) activation, and IL-37 has a role in suppressing MAPK in human keratinocytes. Furthermore, IL-33 downregulates skin barrier function proteins including filaggrin and loricrin, thereby downregulating the expression of IL-37, which colocalizes with these proteins. This leads to an imbalance of the IL-33–IL-37 axis, involving increased IL-33 and decreased IL-37, which may be associated with the pathogenesis of AD and psoriasis. Therefore, AHR-mediated regulation of the IL-33–IL-37 axis may lead to new therapeutic strategies for the treatment of AD and psoriasis.

Atopic dermatitis (AD) is characterized by chronic inflammation, barrier dysfunction, and pruritus, exacerbated by external stimuli, such as scratching. This study investigates the role of extracellular adenosine triphosphate (ATP) in the pathophysiology of AD and assesses the therapeutic potential of clodronate, an ATP release inhibitor. Our research demonstrates that extracellular ATP impairs skin barrier function by reducing the filaggrin expression in the keratinocytes, a critical protein for barrier integrity. Furthermore, ATP release, triggered by IL-4 and mechanical stimuli, amplifies inflammation by promoting cytokine and chemokine production by the immune cells. Clodronate, by inhibiting ATP release, restores the filaggrin levels in the keratinocytes, reduces TARC production in the dendritic cells, and alleviates AD symptoms in a mouse model. These findings suggest that targeting extracellular ATP could offer a novel therapeutic approach to improving skin barrier function and reducing inflammation in AD. Future studies should explore the long-term efficacy and safety of ATP-targeted therapies in clinical settings.

Difamilast, a phosphodiesterase 4 (PDE4) inhibitor, has been shown to be effective in the treatment of atopic dermatitis (AD), although the mechanism involved remains unclear. Since IL-33 plays an important role in the pathogenesis of AD, we investigated the effect of difamilast on IL-33 activity. Since an in vitro model of cultured normal human epidermal keratinocytes (NHEKs) has been utilized to evaluate the pharmacological potential of adjunctive treatment of AD, we treated NHEKs with difamilast and analyzed the expression of the suppression of tumorigenicity 2 protein (ST2), an IL-33 receptor with transmembrane (ST2L) and soluble (sST2) isoforms. Difamilast treatment increased mRNA and protein levels of sST2, a decoy receptor suppressing IL-33 signal transduction, without affecting ST2L expression. Furthermore, supernatants from difamilast-treated NHEKs inhibited IL-33-induced upregulation of TNF-α, IL-5, and IL-13 in KU812 cells, a basophil cell line sensitive to IL-33. We also found that difamilast activated the aryl hydrocarbon receptor (AHR)–nuclear factor erythroid 2-related factor 2 (NRF2) axis. Additionally, the knockdown of AHR or NRF2 abolished the difamilast-induced sST2 production. These results indicate that difamilast treatment produces sST2 via the AHR–NRF2 axis, contributing to improving AD symptoms by inhibiting IL-33 activity.

Crosstalk: skin cells and immune cells in inflammatory skin diseases The skin is the human body's largest organ consisting of two layers: epidermis and dermis, and appendages: hair and sweat glands. The skin not only wraps the body but also protects it from external stimuli and infection, perceives sensations such as pain and itch, and coordinates with various circulating immune cells for immune response/regulation. Recent studies have shown that inflammatory skin diseases, including psoriasis and atopic dermatitis, harbor systemic inflammation/immune abnormalities such as strong Th activation and expansion of specific immune cell subsets. Novel biologics and small molecule inhibitors targeting specific biomarkers and immune signals are much more effective and safer than conventional systemic therapies for these skin diseases. This Research Topic focuses on the interaction between the skin and immune cells and explores how skin cells and immune cells interact with each other and contribute to the pathogenesis of those skin diseases. The review from Dainichi and Iwata can be considered the grand opening of the Research Topic. The epithelial-immune microenvironment (EIME) of epithelial tissues has five common elements (1): microbial flora (2), barrier (3), epithelial cells (4), immune cells, and (5) peripheral nerve endings. EIME provides both constant defense and situationspecific protective responses through three-layered mechanisms comprising barriers, innate immunity, and acquired immunity. The interactions between the five EIME elements of the skin protect against external dangers from the environment. They show five EIME models: atopic dermatitis, psoriasis, SLE, alopecia areata, and acne to simplify the disease pathologies. We should know more about epidermal cells to understand the interactions between the skin and other cells. Epidermal keratinocytes can recognize various cytokines and pathogen-associated molecular patterns and produce a wide variety of inflammatory Frontiers in Immunology frontiersin.org 01