BACKGROUND AND OBJECTIVE: Peripheral blood stem cells (PBSC) are being increasingly used as an alternative to conventional allogeneic bone marrow (BM) transplantation. This has prompted the Working Group on CD34-Positive Hematopoietic Cells to evaluate the current utilization of allogeneic PBSC in clinical hematology. EVIDENCE AND INFORMATION SOURCES: The method employed for preparing this review was that of informal consensus development. Members of the Working Group met three times, and the participants at these meetings examined a list of problems previously prepared by the chairman. They discussed the single points in order to reach an agreement on different opinions and eventually approved the final manuscript. Some of the authors of the present review have been working in the field of stem cell transplantation and have contributed original papers in peer-reviewed journals. In addition, the material examined in the present review includes articles and abstracts published in journals covered by the Science Citation Index and Medline. STATE OF THE ART: Review of the current literature shows that unmanipulated allogeneic PBSC give prompt and stable engraftment in HLA-identical sibling recipients. Despite the much higher number of T-cells infused, the incidence and severity of acute GVHD after PBSC transplant seems comparable to that observed with bone marrow (BM) cells. In comparison to the latter, PBSC probably ensure faster immunologic reconstitution in the early post-transplant period. Controversial results on the incidence and severity of acute-GVHD have been reported when CD34+ selection methods are used. Prospective randomized trials are underway to compare the results of PBSC and BM allogeneic transplantation. In mismatched family donor transplants, T-cell depleted PBSC successfully engraft immune-myeloablated recipients through a megacell-dose effect able to overcome the HLA barrier. Experience with PBSC in the context of unrelated donor transplants is currently anecdotal and prospective trials should be completed before that practice becomes routine. Finally, there is also limited evidence that, following induction chemotherapy, the addition of PBSC to donor lymphocyte infusion (DLI) for treatment of leukemia relapse after BMT may improve the safety and effectiveness of DLI itself. Concerning cord blood (CB) transplants, the most interesting aspects are the ease of CB collection and storage, the low risk of viral contamination and the low immune reactivity of CB cells. This last property has its clinical counterpart in an apparently reduced incidence and severity of acute GVHD both in sibling and unrelated CB transplants, probably making the level of donor/recipient HLA disparity acceptable a greater degree with respect to what is required for transplants from other sources.
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