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Placental/umbilical cord blood (PCB) stem cells for transplantation: early clinical outcomes and the status of ex vivo expansion strategies.

Placental/umbilical cord blood (PCB) stem cells for transplantation provide a potentially useful alternative for patients who do not have an HLA-matched family or unrelated bone marrow donor. Concerns regarding this source of stem cells include the limited number of stem cells in a PCB unit and the delayed time to platelet engraftment. Because of the limited number of stem cells, there is a very clear cell dose effect for both success of engraftment and time to engraftment. As a result, many transplant centers will only consider PCB stem cells as a second choice for transplanting adults, despite the very favorable profile of post-transplant graft-versus-host disease (GvHD). This has resulted in considerable interest in the development of ex vivo stem cell expansion strategies. This review outlines the current status of PCB transplant outcomes as well as the status of our understanding of stem cell expansion with the currently available technologies. A stem cell dose-limiting effect on outcome will result in a narrower window of clinical indications for the use of this stem cell source, despite the acknowledged reduction in GvHD. The trade-offs between poor engraftment and reduction in fatal or severe chronic GvHD remain to be quantitated.