A Hot-Spring Water Improves Inflammatory Conditions in an Injury-Induced Atopic Dermatitis Mouse Model by Regulating Skin Barrier Function

Background: Atopic dermatitis (AD) is a common inflammatory skin condition in which skin barrier function plays a crucial role. Hot spring water is known for its beneficial effects on skin health. This study investigates the impact of a hot spring water on AD pathology, focusing on skin barrier function. Methods: Using the tape-stripping method, we induced an AD mouse model, treated the mice with either hot-spring water or tap water, and assessed time-dependent changes in skin barrier function, histology, and AD-related proteins. Results: Transepithelial water loss (TEWL) was significantly increased after tape-stripping, which began to decrease from day 2 in both treatment groups. On day 3, water loss was significantly decreased in hot-spring-treated mice than tap water-treated mice. Histological analysis revealed thickening and vacuolization of the stratum spinosum from day 2, becoming more pronounced on day 3 in tap-water-treated mice. However, in hot-spring-treated mice, the stratum spinosum was significantly less thickened, and the stratum granulosum was better formed. Immunostaining showed that transient receptor potential vanilloid 4 (TRPV4) levels decreased at day 2 but returned to baseline by day 3, with no significant differences between groups. Filaggrin, a key skin barrier protein, was markedly low in tape-stripped areas at day 0, but increased progressively, with a higher level in the upper epidermis of hot-spring-treated mice compared to tap-water-treated counterparts. Additionally, hot spring water treatment significantly reduced CD8+ T cell numbers and IL-4 cytokine levels, mitigating inflammation. Conclusions: Threfore, hot spring water enhances skin barrier recovery and reduces inflammation in AD.