Ultraviolet light-induced regulatory (suppressor) T cells: an approach for promoting induction of operational allograft tolerance?

Ultraviolet (UV) light is known to induce skin cancers by causing DNA gene mutations and inducing immunosuppression. Taking advantage of these immunosuppressive capacities, UV light has been used, with different modalities, as an immunosuppressive therapy in a variety of diseases including allograft rejection and graft-versus-host disease. Phototherapy includes UVB irradiation, UVA irradiation, oral psoralen (+)UVA irradiation (PUVA), photodynamic therapy, and extracorporeal photopheresis, which consists of infusion of UVA-irradiated autologous leukocytes collected by apheresis and incubated with 8-methoxypsoralen. According to numerous experimental models and human data, there is increasing evidence that UVB irradiation and extracorporeal photopheresis can induce regulatory T cells and anticlonotypic activity. These therapies induce apoptosis of activated T cells or of extracorporally treated mononuclear cells, and up-regulate the expression of costimulary molecules and adhesion molecules on antigen presenting cells. UVB- or UVA-induced apoptotic cells could secrete immune suppressive cytokines (interleukin (IL)-4, IL-10). The processing and presentation of apoptotic T cell antigens from clones of pathogenic T cells by activated antigen presenting cells might explain the induction of systemic anticlonotypic activity by photopheresis. This induction of cell-mediated suppressive activity opens up future prospects with the aim of expanding regulatory T cells and/or anticlonotypic activity, especially by photopheresis in organ and cell transplantation.