Peripheral blood and bone marrow hematopoietic stem cells: are they the same?

Hematopoietic stem cells collected from the peripheral blood (PBSCs) have been successfully used for transplantation. Originally, these cells, collected from patients with chronic myelogenous leukemia, were used to restore chronic phase of the disease. More recently, PBSCs have been used principally for autologous transplantation for nonhematologic malignancies, although there are a few recorded cases of allogeneic transplantation with PBSCs. The process of obtaining peripheral blood stem cells is tedious in comparison to the process of harvesting bone marrow for transplantation, requiring multiple lengthy apheresis procedures to collect sufficient cells for transplant. Moreover, the risks associated with PBSC collection (anesthesia risk for catheter placement, risk of introducing infection during collection) are just as great as those associated with marrow harvesting. However, for the patient whose bone marrow is unharvestable due to hypocellularity or fibrosis, PBSC transplantation may reopen the option of treatment by high-dose cytotoxic therapy and autologous hematopoietic stem cell rescue. Peripheral blood stem cell transplantation may also afford the transplant option to the patient whose marrow has been infiltrated with disease. While it has not been demonstrated, it is hypothetically likely that the probability of tumor cell contamination of circulating blood is quite low, due to the lack of adherence necessary for colonization. Recent reports have indicated that autologous transplantation with PBSCs may result in more rapid engraftment relative to autologous transplantation with bone marrow. This has only been reported in programs wherein PBSCs are collected under mobilizing conditions. If it is the case that PBSCs produce more rapid engraftment, however, then the additional effort and cost associated with PBSC collection might be outweighed by the reduced morbidity, mortality, and cost associated with early return of granulocytes in the transplant patient. The biology of the early engraftment phenomenon should be studied, as it is possible that it is related to the mobilizing conditions under which the cells are collected and not due to an intrinsic quality of PBSCs.