Abstract Introduction: Graft-versus-host disease (GvHD), a severe complication of allogeneic hematopoietic stem cell transplantation (allo-HSCT), results in post-transplant morbidity and mortality. Steroids with strong immunosuppressive and anti-inflammatory effects remain the gold standard for initial management of GvHD. However, high doses of steroids are usually accompanied by an increased risk of infections and secondary malignancies. ECP, due to its good clinical response, is recommended as a second-line treatment for GvHD. Notably, no clinical data showed that ECP therapy results in relapse and infection. However, the mechanism of action behind that is barely known. Therefore, we conducted this study to figure out how ECP preserves the anti-viral and anti-leukemia effects. Materials and Methods: 34 patients with steroid-refractory/resistant aGvHD ≥ °II and moderate to severe cGvHD received ECP therapy at the University Hospitals Heidelberg, Greifswald and Tel Hashomer. ECP therapy was performed according to the current European guidelines. For clinical staging of aGvHD under ECP treatment patients were evaluated according to Glucksberg criteria, for quality of life according to Karnofsky and for clinical staging of cGvHD according to NIH criteria. Phenotypical analysis of different cellular subsets was evaluated by multicolor flow cytometry. The quantity and quality of virus-specific CD8+ T cells were evaluated by Tetramer staining and IFN-γ-ELISPOT. NK activity in terms of killing function and cytokine release function was monitored by chromium-51 release assay and intracellular cytokine staining. Results: ECP seems to be favorable in the treatment of GvHD. Clinically, an overall response of 75% for aGvHD and 78% for cGvHD patients was achieved. A steroid-sparing effect in addition to the resolution of GvHD manifestations was observed in responders. Our patients showed no increased susceptibility to infections. On the cellular level, the frequency of cytotoxic CD8+ T cells, the most important mediators of GvL activity, as well as CD4+CD8+ T cells, γδ T cells, NKT cells and CD56dimCD57+NKG2C+ NK cells as other well-established protective cell subsets remained constant under ECP therapy. Moreover, neither frequency nor IFN-γ release of CMV specific CD8+ T cells was hampered by ECP. 51Cr-release assay revealed stable functional cytotoxicity of NK cells. Additionally, ECP therapy did not affect the capacity of cytokine release by NK cells, including quality, quantity and multifunctional release. Furthermore, no significant influence of ECP therapy on antigen recall function of NK cells, CD4+ and CD8+ T cells could be observed. Conclusion: ECP therapy represents an attractive strategy to treat GvHD. Moreover, our results reflect that ECP therapy preserves immunity against infections as well as the GvL effect via maintaining the quality and quantity of effector cells. Disclosures Hilgendorf: Novartis: Other: Travel support, Research Funding; Medac: Other: Travel support, Research Funding.