Intense pulsed light (IPL) treatment helps clear rosacea by calming over‑active mast cells and lowering inflammation‑related proteins, as shown in patient skin samples, mouse models, and cell experiments.

Rosacea, a chronic inflammatory disorder of the facial skin, is effectively treated by intense pulsed light (IPL). To explore the potential molecular mechanism underlying the photobiomodulation effect of IPL for rosacea treatment. Skin samples from patients with rosacea were subjected to histological and immunohistological staining. Ten patients were followed up after IPL treatment using the VISIA^® skin analysis system, and the severity was assessed. In vivo, skin changes in mice with rosacea-like inflammation induced by intradermal injection of 320 μM LL-37 with or without IPL treatment were evaluated using L*a*b colorimetry as well as histological and immunological staining. In vitro, LL-37-stimulated mast cells (MCs) with or without IPL treatment were evaluated for protein expression of matrix metalloproteinase (MMP)-9, kallikrein-related peptidase 5 (KLK5), and cathelicidin using western blotting and qRT-PCR. Profound infiltration of inflammatory cells and evident MC degranulation were found in rosacea skin lesions. The expression of rosacea-related biomarkers and inflammatory cytokines was higher in lesional areas than in non-lesional areas, as demonstrated via immunochemical staining. In all patients, rosacea severity reduced after IPL therapy. In vivo, IPL alleviated inflammation in mice with rosacea-like inflammation, as demonstrated by the significantly decreased MMP-9, KLK5, and cathelicidin expression and reduced percentage of degranulating MCs. In vitro, IPL decreased MMP-9, KLK5, and cathelicidin expression in P815 cells, reducing the release of inflammatory cytokines and inhibiting rosacea-like inflammatory reactions. The photobiomodulation effect of IPL for rosacea treatment may inhibit MC degranulation and alleviate inflammatory reactions.