[Unrelated allogeneic umbilical cord blood transplantation: present status, problems and countermeasures]

As unrelated allogeneic umbilical cord blood transplantation (UCBT) has been developed for 20 years already since 1988, more than ten thousands cases have cumulatively undergone UCBT over the world. A huge number of clinical data confirmed that UCBT had unique characters with low rate of severe GVHD. The efficacy and data on TRM, relapse and EFS of allogeneic UCBT with HLA 0-1 mismatched are similar to those in HLA matched BMT. UCBT has become the optimal choice for source of hematopoietic stem cells for allogeneic stem cell transplant especially when HLA-matched or haploidentical donors are not available in time. In most developed countries, unrelated allogeneic UCBT developed successively, and in recent years HLA mismatched UCBT with double units performed in adults increased even more rapidly than in children. Another recent trend of UCBT has been extending to treat some non-malignant but refractory diseases in pediatrics, such as severe combined immunodeficiency, thalassemia major, bone marrow failure syndrome and metabolic disorders. The clinical successful practice of double units for cord blood transplantation inspires to ponder over questions remaining mystery. What is the conflict like between two mismatched donor cells in vivo, which does not spoil the whole transplantation but enable the patient to be engrafted successfully without any increment of the dosage by the sum of two doses together? How can they both be taken at the same time firstly by the recipient, but why does only one predominate later? What are the factors enable the donor cells of the winner to sustain? With the references of the international experiences, how to solve the clinical encountered problems, perspective of unrelated allogeneic UCBT and proper strategies to be enacted are reviewed.