For numerous malignant and non-malignant hematological diseases allogeneic hemato¬poietic stem cell transplantation (HSCT) is the only curative therapy. One of the major complications is the occurrence of acute graft-versus-host-disease (aGVHD). Thirty to eighty percent of patients after HSCT develop aGVHD despite the prophylactic application of different immunosuppressive drugs depending on risk factors such as HLA-match, donor relation, age etc.1-3.