CT-223 Extracorporeal Photopheresis Induces NETosis in Neutrophils Derived From Patients With Graft-Versus-Host Disease

Introduction: Extracorporeal photopheresis (ECP) serves as a second-line treatment for patients with acute or chronic graft versus host disease (GVHD) and demonstrates efficacy in ameliorating GVHD in this setting. The mechanism by which ECP acts against GVHD is not fully understood. We aimed to assess the influence of ECP on the formation of neutrophil extracellular traps (NETs) among patients with GVHD. Methods: We assessed several patients with GVHD who were treated with ECP at Rabin Medical Center. Blood samples were obtained at three different time points: before the onset of an ECP cycle, at the end of the first day of treatment, and 24 hours following the initiation of the ECP treatment cycle. Neutrophils were harvested from all blood samples. NET formation was assessed by measurement of neutrophil elastase activity and by immunofluorescence. Results: Six patients (two women and four men) with chronic GVHD (cGVHD) were included in our study. The underlying hematologic disease was acute myeloid leukemia in four patients, B-cell acute lymphocytic leukemia in one patient, and myelodysplastic syndrome in another patient. We observed a sharp increase in NET formation among all the six patients following ECP. Neutrophil elastase activity level was elevated from a mean value of 2.21mU/mL (+/-0.6mU/mL) at baseline to a mean value of 13.82mU/mL (+/-5.54mU/mL) immediately after the treatment and to a peak value of 17.35mU/mL (+/-10,74mU/mL) 24 hours following the initiation of the ECP cycle. Conclusions: Our preliminary data indicate that ECP induces NET formation among patients with GVHD. Given the central role of neutrophils in the pathogenesis of GVHD, the contribution of NETs to GVHD, and to ECP mode of action, should be further investigated.