Ultraviolet Radiation: Both Friend and Foe in Systemic Autoimmune Diseases?

Ultraviolet radiation (UVR) has been claimed to be one of the environmental factors involved in triggering both the onset and the exacerbations of systemic autoimmune diseases, such as systemic lupus erythematosus (SLE), dermatomyositis (DM), and systemic sclerosis (SSc). Therefore, patients affected by these diseases are instructed to avoid sun exposure and to use regularly sunscreens. On the other hand, there is increasing knowledge about the effects of different wavebands of UVR on the cells of the immune system (effector T cells, regulatory T cells, Langerhans cells, monocyte/macrophages). These effects can be exerted both locally in the skin and systemically and have been exploited to treat patients with UVA1 phototherapy and extracorporeal photopheresis. The principles of photobiology will be reviewed and the implications of UVR as "disease trigger" or "therapeutic agent" will be discussed for SLE, DM, and SSc.