Effects of low-dose narrowband UV radiation on skin tissue and mast cells in a canine model

Narrowband ultraviolet radiation (NB-UVR) phototherapy is widely used to treat various skin diseases such as atopic dermatitis and psoriasis. Although the mechanisms of NB-UVR at doses exceeding the minimal erythema dose (MED) have been extensively studied, the mechanisms of sub-MED irradiation remain unclear. This study investigated the effects of NB-UVR on naturally occurring canine atopic dermatitis and analyzed the underlying mechanisms of sub-MED irradiation using in vivo and in vitro canine models. Clinically, NB-UVR irradiation at 310 and 320 nm improved lesion scores, with a significant effect observed with low-dose 320 nm irradiation (n=6, P<0.05). To compare the effects of sub-MED of NB-UVR wavelengths (310, 320, and 330 nm), intradermal skin tests in experimental beagles showed significant alleviation of anaphylactoid reactions across all wavelengths (n=3, P<0.01). The number of cutaneous mast cells was significantly reduced following 310 nm irradiation (n=3, P<0.05). In vitro, WST assays and DAPI staining demonstrated reduced cell viability at 310 and 320 nm (P<0.01), with apoptosis specifically induced at 310 nm (P<0.01). RNA sequencing identified differentially expressed genes associated with muscle cell components and function after 320 nm exposure (n=3, q<0.1). These findings suggest that sub-MED NB-UVR, especially at 320 nm, may offer non-erythemogenic and safer phototherapy, with potential benefits for skin repair.