Clinic and pathophysiology of photosensitivity in lupus erythematosus

Lupus erythematosus (LE) represents an autoimmune disease with great clinical variability in which photosensitivity is a common feature for all forms and subsets. The nature and characteristics of clinical photosensitivity in LE have been elucidated through standardized phototesting procedures. The development of skin lesions after UV-injury is typically delayed starting from a few days up to three weeks after the irradiation, and may persist for months. Therefore, patients may not be aware of the detrimental effects of sunlight for their disease. The most photosensitive subset of LE is LE tumidus, followed by subacute cutaneous LE. Phototesting has also been crucial for studying the pathophysiology of LE-photosensitivity. Abnormalities of generation and clearance of UV-triggered apoptotic cells in LE are an important source of autoantigens. Recent data demonstrate the linkage of innate with adoptive immune pathways in UV-induced autoimmune response. Plasmocytoid dendritic cells (PDC) and their secreted IFN-alpha play a central role in the LE-pathogenesis. The recruitment of relevant leukocyte subsets is dependant on certain chemokines, which have been characterized in recent studies. An amplification cycle has been postulated, in which UV induces apoptosis and necrosis resulting in the production and release of chemokines. Subsequently, effector memory T cells as well as PDCs are recruited and activated perpetuating an amplification process that leads to UV-induced cutaneous LE lesion.