A 308‐nm Excimer Lamp Ameliorates <scp>MC903</scp> ‐Induced Atopic Dermatitis With Reductions of Intraepidermal Nerve Fiber Density and Expression of Thymic Stromal Lymphopoietin

Atopic dermatitis (AD) is a chronic inflammatory skin disorder characterized by pruritus, barrier dysfunction, and complex immune dysregulation. Phototherapy with narrow-band ultraviolet B (NB-UVB) or 308 nm excimer devices is clinically effective for AD, but the underlying mechanisms remain incompletely understood. In this study, we investigated the therapeutic effects of a 308-nm excimer lamp using a murine model of AD induced by topical application of the vitamin D3 analog MC903. C57BL/6 mice were treated with MC903 for 13 consecutive days, with or without irradiation by a 308-nm excimer lamp at 100 mJ/cm² every other day. Clinical severity was assessed daily, and histological, immunohistochemical, and molecular analyses were performed on lesional skin. Excimer irradiation significantly alleviated MC903-induced dermatitis from days 4 to 12, with reduced clinical severity scores, epidermal thickness, and acanthosis compared with non-irradiated controls. Immunohistochemical analysis revealed a significant decrease in intraepidermal PGP9.5-positive nerve fibers in irradiated mice, suggesting that excimer therapy attenuates pruritus through neuroimmune modulation. Furthermore, excimer irradiation increased dermal infiltration of Foxp3-positive regulatory T cells and decreased plasma IgE levels, although no significant upregulation of IL-10 mRNA was detected. Importantly, quantitative PCR demonstrated that excimer irradiation reduced thymic stromal lymphopoietin (TSLP) expression in lesional skin, while levels of other Th2-related cytokines such as IL-4 and IL-13 remained unchanged. These findings indicate that 308-nm excimer lamp therapy ameliorates AD-like dermatitis through dual mechanisms: suppression of intraepidermal nerve fibers and downregulation of epithelial-derived TSLP, together with enhanced regulatory T-cell infiltration. This study provides new mechanistic insights into the immunological and neurocutaneous effects of excimer phototherapy, supporting its clinical use in AD and potentially other inflammatory skin diseases.