Current status of reduced-intensity allogeneic stem cell transplantation using alternative donors

The optimal donor for a patient undergoing reduced-intensity stem cell transplantation remains a human leukocyte antigen (HLA)-matched relative. Alternative donors such as matched unrelated donors (MUDs), mismatched related donors (haploidentical), or unrelated umbilical cord blood (UCB) units have emerged as options as well. The most experience thus far has been with MUD donors, mostly attributed to the development of allele-specific DNA-based HLA-typing methods. The biggest drawback remains the significant delay needed to locate a donor. Haploidentical donors exist for almost all patients, and offer the convenience of a living related donor. However, significant rates of graft-versus-host disease (GVHD) and other toxicities continue to complicate such HLA mismatching. UCB is the most recent option for source of stem cells. Frozen cord blood units can be acquired almost immediately and are able to safely traverse 1 or 2-HLA antigen mismatch barriers. The experience with UCB has been limited by the low cell dose, although recent innovations are attempting to overcome this. In this review, we summarize the current experience and knowledge with alternative donors as stem cell sources for reduced-intensity transplantation.