The effect of extracorporeal photopheresis on T cell response in chronic graft-versus-host disease.

Extracorporeal photopheresis (ECP) is a safe and effective immunoregulatory therapy for steroid-refractory chronic graft-versus-host disease (cGVHD) but its mechanism of action is poorly understood. In this study, we evaluated the effect of ECP in a sample of cGVHD patients. Our data showed that ECP-treated patients had lower CD4 T and B cells, and substantially higher NK cells than untreated patients. T regulatory (Treg) cells were similar between the two groups of patients. Interestingly, Treg cells were higher in ECP-treated patients and ECP-responders who had no history of aGVHD or sclerosis, than in those who had one of them or both. These findings suggest that at least one of the mechanisms of immunomodulation by ECP targets the Treg cell population and that an increase in Treg cells may be associated with response in patients with cGVHD. Together, the results of ECP are different depending on the patients' clinical condition.