Extracorporeal photochemotherapy

Although extracorporeal photochemotherapy (ECP) is reported to be effective for a wide variety of diseases, such as cutaneous T cell lymphoma, autoimmune diseases, organ graft rejection, and graft versus host disease, its mechanism of action remains unclear. The basis of extracorporeal photopheresis is the reinfusion of leukocytes previously exposed to 8-methoxypsoralen (8-MOP) and ultraviolet A radiation. Although only 5-10% of circulating mononuclear cells is treated during one ECP procedure, the treatment has long-lasting immunomodulatory effects. The advantage of photopheresis treatment is the low frequency of side effects related to treatment. The disadvantages, however, are the practical efforts required and the high treatment cost. Recent studies demonstrated that ECP downregulates the immune response and induces tolerance by regulatory T cells. Other studies suggest that the mechanism of ECP also involves the recruitment and involvement of additional immune cells. Although immune tolerance induced by ECP is the most likely mechanism of the clinical efficacy of ECP it is not clear how ECP, both activates tumor immunity against cutaneous T-cell lymphoma and induces tolerance in autoreactive disorders. Further studies are necessary to determine the details of the underlying mechanisms of action.