Photosensitivity in rheumatic diseases.

There have been a number of recent advances in the genetic understanding of photosensitive rheumatic diseases, especially subacute cutaneous lupus erythematosus and dermatomyositis. These advances support the concept that increased numbers of ultraviolet light-induced apoptotic cells in skin lead to a supra-threshold concentration of antigenic peptides. The current genetic data suggest that increased keratinocyte apopotosis can result from increased amounts of TNF-alpha that induce apoptosis due to a ultraviolet light-sensitive TNF promoter polymorphism or to decreased clearance of apototic cells due to polymorphisms associated with decreased serum levels of collectins such as C1q and mannose-binding lectin. These diseases are frequently oligogenic, and other yet to be elucidated genes will, in individual patients, lead to increased numbers of apoptotic cells associated with these cutaneous autoimmune diseases. In the presence of specific MHC class I and II genes, antigen-presenting cells initiate a primary immune response that leads to cutaneous, and likely systemic, autoimmune disease.