The availability of umbilical cord blood (UCB) as a source of haematopoietic stem cells (HSC) for transplantation has met an important niche in the field of HSC transplantation (HSCT) as patients unable to find a HLA-matched sibling or unrelated donor have been able to receive less well-matched UCB transplantation (UCBT) with equivalent outcomes. This has led to significant growth in this field resulting in more than 20 000 unrelated donor UCBTs performed to date with about 3000 more performed annually. Growth of UCBT has been further supported by the proliferation of public cord blood banks throughout the world which store UCB at no cost to the donor, making these available for patients all round the world through global search registries like the US National Marrow Donor Program (NMDP), NetCord and the Bone Marrow Donors Worldwide (BMDW). International organizations like the World Marrow Donor Association have also helped to steer these efforts through the formulation and distribution of guidelines and protocols for these cord blood banks and bone marrow registries. The US Food and Drug Administration (FDA) has also stepped in to regulate publicly banked UCB as an Investigational New Drug (IND).
The key limiting factor in UCBT is in the limited number of cells for transplantation (about 10-fold less than donated bone marrow) resulting in delayed engraftment and even non-engraftment, particularly for adult patients for whom UCB cell doses may be insufficient relative to the patient's body size. Efforts to overcome this barrier include the use of concurrent infusion of two differing cord blood units in order to raise the cumulative cell dose. Interestingly, this does not lead to mutual rejection of the CBUs, but appears to result in an additive effect on enhancing engraftment. Other efforts to overcome cell dose constraints of cord blood include direct bone marrow injection, use of homing molecules and ex vivo cord blood expansion.
Cell dose is also an important consideration for cord blood banking as donated UCB that is collected with cell count <800 million nucleated cells has very low chance of utilization by many transplant centres which demand the best cell doses for their patients. As such, not all UCB collected is banked, although many of the low volume cords can still be reassigned to research. Strategies to increase the number of cells collected from each delivery include the use of ex utero devices which apply suction, perfusion or pressure to delivered placenta and umbilical cord in order to maximize HSC collection. Devices which enhance cell recovery during cord blood processing also help to minimize cell loss. Other strategies which might influence obstetric practice are not advised.
As the worldwide experience in UCBT and UCB banking grows, patient outcomes have continued to improve such that UCBT now has a firm place in the HSCT spectrum of care with even greater potential for growth in the years to come. The challenge is for these advances to stay cost-effective so that the majority of patients can still have access to them.