Double umbilical cord blood transplant: more than a cell dose?

Cord blood transplant (CBT) is characterized by the limited amount of hematopoietic stem cells (HSCs), and exhibits higher incidence of engraftment failure. Transplant using two (double) CB units (DCBT) is the most promising strategy to date to augment the total transplanted HSC dose. DCBT increases the availability of transplant to adults, for whom there is only a small chance of finding a matched CB suitable for a single unit transplant. Better selection of CB units, optimization of conditioning regimen, and GVHD prophylaxis are important to further improve DCBT outcomes. In this review we attempt to summarize recent progress in the field of DCBT, to present the strategies applied, and to point out directions for the future.