Experimental extracorporeal photopheresis inhibits the sensitization and effector phases of contact hypersensitivity via two mechanisms: generation of IL-10 and induction of regulatory T cells.

Extracorporeal photopheresis (ECP) is used to treat immune-mediated diseases including transplant rejection and graft-vs-host-disease. An experimental murine model of ECP utilizing contact hypersensitivity (CHS) revealed that ECP inhibits the sensitization of CHS and induces regulatory T cells (Treg). In this study, we find that ECP inhibits not only the sensitization but also the effector phase of CHS, although Treg only inhibited sensitization. IL-10 was determined to be a critical component of the effector phase inhibition and also a driving force in developing Treg. Thus, we propose that the inhibition of the effector phase of CHS by ECP is a process that does not require Treg but may be mediated via enhanced IL-10 as suggested by the use of IL-10-deficient mice. This suggests that ECP has at least two mechanisms of action, one inhibiting the effector phase of CHS and one generating Treg, which in turn can inhibit CHS sensitization and is responsible for the transferable protection. Together, this may help explain the clinical benefits of ECP in prophylactic, acute, and therapeutic settings.