Extracorporeal Photopheresis Attenuates Murine Graft-versus-Host Disease via Bone Marrow–Derived Interleukin-10 and Preserves Responses to Dendritic Cell Vaccination

Extracorporeal photopheresis (ECP) is emerging as a therapy for graft-versus-host-disease (GVHD), but the full mechanism of action and the impact on immunity have not been fully established. After murine minor histocompatibility antigen–mismatched bone marrow (BM) transplantation (allo-BMT), coinfusion of ECP-treated splenocytes with T cell–replete BM attenuated GVHD irrespective of the donor strain of the ECP-treated splenocytes, and was associated with increased numbers of regulatory T cells. Coculture of myeloid dendritic cells (DCs) with ECP-treated splenocytes resulted in increased interleukin (IL)-10 production after submaximal stimulation with lipopolysaccharide. Furthermore, male myeloid DCs exposed to ECP-treated splenocytes were less potent at inducing CD8+ HY responses when used as a vaccine in vivo. The efficacy of ECP-treated splenocytes was enhanced when administered just before delayed donor lymphocyte infusion following T cell–depleted allo-BMT, allowing for the administration of sufficient numbers of T cells to respond to myeloid DC vaccination in the absence of a thymus. Finally, the therapeutic effect of ECP-treated splenocytes was lost in recipients of IL-10–deficient BM. We demonstrate that ECP-treated splenocytes attenuate GVHD irrespective of the source of ECP-treated cells via a mechanism that likely involves modulation of DCs and requires IL-10 produced by BM-derived cells. Importantly, the attenuation of GVHD by ECP-treated splenocytes permits donor lymphocyte infusion–dependent responses to DC vaccines after allo-BMT.