Regulatory T cells induced by ultraviolet radiation.

Regulatory T cells belong to a subset of T lymphocytes which suppress immune reactions in an antigen-specific fashion. They play an important role in the prevention of autoimmune diseases. Ultraviolet (UV) radiation was also found to suppress the immune system in an antigen-specific fashion mediated by UV-induced regulatory T cells. Induction of these cells by UV radiation is an active process which requires antigen presentation by UV-damaged but still viable Langerhans cells in the lymph nodes. UV-induced regulatory T cells have been recently characterized to express CD4 and CD25 and to release the immunosuppressive cytokine interleukin-10 upon activation. Once activated in an antigen-specific fashion, they suppress immune responses in a general fashion via the release of interleukin-10, a phenomenon called bystander suppression. Upon intravenous injection, UV-induced regulatory T cells primarily migrate into the lymph nodes, explaining why they preferentially suppress sensitization. Recently, the development of regulatory T cells was demonstrated in an experimental model of photopheresis, a therapeutic regimen which is used for the therapy of autoimmune diseases, transplant rejection and graft-versus-host disease. Further characterization of these cells will determine whether they can be applied therapeutically in the future with the ultimate aim to induce specific immunosuppression.