Photochemotherapy Induces a Faster Apoptosis of Alloreactive Activated T Cells Than of Nonalloreactive Resting T Cells in Graft Versus Host Disease

Graft versus host disease (GvHD) is the main complication after hematopoietic stem-cell transplantation.Extracorporeal photochemotherapy (ECP) is a cell therapy currently used for the treatment of T-cell–mediated diseases and seems as a valuable second-line therapy for patients suffering from steroid-refractory acute or chronic GvHD. ECP induces the apoptosis of treated cells and is believed to elicit a specific immune regulation of alloreactive T cells through repeated apoptotic T-cell infusions. However, its mechanisms of action have not yet been elucidated. In GvHD,alloreactive but not non alloreactive T cells are continuously activated by their environment. We hypothesized that ECP has a differential apoptotic effect on activated compared with resting T cells.