Mixed Chimerism after Allogeneic Stem Cell Transplantation – Focus on Double Cord Blood Transplantation

Allogeneic hematopoietic Stem Cell Transplantation (ASCT) is well established as a curative treatment for many hematological malignancies and non-malignant disorders. The aim of ASCT in these diseases is to achieve sustained donor engraftment to fight leukemic cells in malignant disease, improve hematopoietic function, provide immune competence or normalize enzyme deficiency. Peripheral blood or bone marrow is commonly used to monitor engraftment after ASCT. The presence of mixed donor/recipient chimerism after transplantation, donor/donor chimerism after double cord blood transplantation can be used and interpreted differently based on the initial disease status. In patients with malignant diseases, chimerism is primarily used to detect early relapse but can also indicate threatening rejection. In individuals with non malignant disease, chimerism is merely used to monitor successful engraftment. After double cord blood transplantation, the unique situation with two existing donor immune systems can occur. Most often one of the immune systems rapidly succumbs with one immune system prevailing, but in certain situations mixed donor/donor chimerism can exist for prolonged periods. This review describes the importance of mixed chimerism and the possible interpretation after ASCT in patients with both malignant and non-malignant diseases. It also focuses specifically on the situation and mechanisms donor/ donor chimerism after double cord blood transplantation.