Double cord blood transplants: filling a niche?

Over the last decade, umbilical cord blood (CB) has become a viable option for hematopoietic stem cell transplants. A limiting factor has been the 10-fold lower cell dose in a CB unit compared with harvested bone marrow or peripheral blood stem cells. The low cell dose leads to delayed engraftment and an increase in graft failure. Double cord blood transplants represent a novel strategy to overcome this limitation. This review will cover the rationale behind and clinical results of double cord blood transplant studies. The chimerism data from these studies reveal that typically only one CB "wins the battle" for engraftment. These results may have important scientific implications in terms of understanding the nature of the hematopoietic stem cell niche and how modulation of this niche may impact transplant outcomes.