The Glucocorticoid-Induced Leucine Zipper and Immunomodulation in Extracorporeal Photochemotherapy

Extracorporeal photochemotherapy (ECP) induces antigen-specific immune tolerance in graft-versus-host disease and solid-organ transplant rejection, and involves the ex vivo exposure of peripheral blood mononuclear cells to 8-methoxypsoralen plus ultraviolet A light (PUVA). In this study, we developed an in vitro model of ECP to decipher the immunomodulatory mechanisms of PUVA. The glucocorticoid-induced leucine zipper (GILZ) is both necessary and sufficient for the generation of dexamethasone-induced tolerogenic dendritic cells (DCs). We hypothesized that PUVA-induced activation of GILZ may contribute to the immune tolerance observed after ECP therapy in graft-versus-host disease and solid-organ transplantation. We report that PUVA acts via two pathways culminating in GILZ up-regulation in human monocyte-derived dendritic cells (MoDCs). Firstly, PUVA directly induces GILZ expression in MoDCs in a dose-dependent fashion (p < 0.01). Secondly, PUVA acts indirectly through the generation of apoptotic lymphocytes to induce GILZ expression in an apoptotic cell dose-dependent fashion (p < 0.01). MoDCs treated with PUVA, and/or exposed to lymphocytes rendered apoptotic by PUVA, up-regulated GILZ, down-regulated CD80, CD86 and CD83, became resistant to LPS-induced maturation, increased IL-10 production, and decreased production of pro-inflammatory cytokines (IL-12, IFN-gamma, IL-6, TNF-alpha), and chemokines (IL-8, MCP-1, MIP-1beta;, RANTES) (all p < 0.05). Knockdown of GILZ via transient transfection with GILZ siRNA, compared to scramble siRNA, reduced IL-10 production and increased IL-12 production, demonstrating that GILZ is necessary for generating this tolerogenic cytokine profile. This study uncovers a potential molecular explanation for the immunomodulatory effects of PUVA, specifically through the induction of GILZ and polarization of immature MoDCs into tolerogenic DCs, and has implications for better understanding how ECP induces antigen-specific immunosuppression in vivo.