Background: Mechanisms of action of extracorporeal photochemotherapy (ECP) in graft-versus-host disease are incompletely understood. It has been proposed that phagocytosis of apoptotic bodies by monocytes and macrophages induces their activation, a process that increases production of immunosuppressive cytokines. We analyzed apoptosis and cytokine secretion in an autologous coculture system combining peripheral blood lymphocytes (PBL) obtained after ex vivo ECP treatment and nonirradiated peripheral blood mononuclear cells (PBMC).
Methods: We studied 11 leukapheresis products treated by ECP from six patients with resistant limited or extensive chronic graft-versus-host disease. Purified PBL obtained by monocyte depletion of the buffy coat from leukapheresis products were mixed with nonirradiated PBMC. Nonirradiated PBL were used as control. Cytokine production was tested at the mRNA level by real-time reverse transcriptase-polymerase chain reaction for interleukin (IL)-10, IL-1 receptor antagonist (IL-1Ra), IL-1beta, tumor necrosis factor-alpha, and IL-12p40.
Results: Morphologic analysis and flow cytometry displayed important lymphocyte apoptotic features culminating at 24 to 48 hr. Coculture of patient's PBMC with ultraviolet-irradiated PBL as compared with nonirradiated PBL resulted in significant increase of IL-10 mRNA (3418+/-1015 versus 10596+/-3402 mRNA copy numbers; P=0.001) and IL-1Ra mRNA (23890+/-6166 versus 41767+/-10181 mRNA copy numbers; P=0.001). Incubation with a neutralizing anti-IL-10 monoclonal antibody resulted in a marked decrease of IL-1Ra mRNA levels.
Conclusion: Our findings are consistent with the fact that ECP modifies patient's autologous lymphocytes by inducing a process of apoptosis that activates monocytes and macrophages, leading to increased synthesis of IL-10 and IL-1Ra mRNAs. The induction of this latter mediator is dependent on IL-10.