Immature DC Isolated After Co-Culture with PUVA-Treated Peripheral Blood Mononuclear Cells Downregulate Graft-Versus-Host Reactions in the Human Skin Explant Model

Graft-versus-host disease (GvHD) remains the major barrier to successful allogeneic hematopoietic stem cell transplantation (HSCT). Extracorporeal photopheresis (ECP) is a potent immunomodulatory treatment option for GvHD. In contrast to conventional immunosuppressants, ECP is considered not to increase relapse and infection rates resulting from generalised immunosuppression. ECP involves the mechanical separation of 5-10% of patient peripheral blood mononuclear cells, which are then exposed to psoralen and UVA light (PUVA) before they are returned to the patient. ECP has been shown to induce apoptosis in various cell types, in particular lymphocytes. Several studies describe downregulation of pro-inflammatory cytokines as well as promotion of peripheral tolerance through enhanced production of Tregulatory cells in the course of ECP-treatment. Modulation of antigen-presenting cells such as dendritic cells (DC) by PUVA-treated lymphocytes might be implicated in these regulatory processes. We evaluated the impact of PUVA-treated lymphocytes on immature DC and further demonstrated the functional capacity of such modified DC to modulate GVH reactions using a well-established human skin-explant model of GvHD. Addition of immature DC isolated after co-culture with PUVA-treated but not untreated MLR cells significantly downregulated skin-GvH reactions (p=0.023, Mann-Whitney-Test). IFN-gamma levels were non-significantly decreased in MLR and skin supernatants. We observed a non-significant increase in PD-L1 expression in iDC after co-culture with PUVA-treated MLR cells whereas expression levels of IDO and ILT-3 were not affected. We conclude that iDC modulated by PUVA-induced apoptotic cells potently downregulate allogeneic immune responses possibly through PD-L1- dependent signaling. Keywords: Allogeneic hematopoietic stem cell transplantation, extracorporeal photopheresis, ECP, graft-versus-host disease, dendritic cells, apoptosis, tolerance, immunoregulation.