CLINICAL UPDATE: ALLOGENEIC PERIPHERAL BLOOD STEM CELL TRANSPLANTATION

Allogeneic PBSC donors are selected in the same manner as allogeneic bone marrow transplantation (BMT) donors, that is, by considering their HLA identity or partial identity with the proposed transplant recipient, as well as the CMV serological status and other features of the proposed donor and the recipient. As for BMT, allogeneic donors may be family members (usually siblings) or unrelated persons and may be fully or partially HLA-matched with the potential recipient. Like BMT, PBSC transplantation is given as a transfusion of allogeneic cells; therefore, the allogeneic PBSC product is subject to the same scrutiny and regulations that are applied to all other blood products for transfusion, including the necessity of screening donors for transfusion transmitted diseases within 30 days of harvesting the PBSC.

The actual collection process for PBSC from allogeneic donors is essentially the same as for autologous donors and uses the identical techniques and types of apheresis equipment, although the majority of allogeneic donors have sufficient peripheral venous access to permit collection without the insertion of a central catheter. The "mobilization" process differs for autologous and allogeneic donors. In patients with malignancy who are to receive autologous stem cells as rescue from chemotherapy-induced marrow hypoplasia, PBSC harvesting is maximally efficient when chemotherapy is first used to suppress the bone marrow (and further diminish the cancer cell burden), followed by a stimulatory cytokine/growth factor, most often granulocyte-colony stimulating factor (G-CSF), for additional marrow stimulation during the recovery phase. By contrast, the use of chemotherapy in healthy allogeneic donors is unacceptable because of the side effects of these highly toxic agents. Therefore, allogeneic PBSC donors are mobilized with growth factors alone, specifically G-CSF here at YNHH.

White blood cell counts (WBC) in normal individuals may rise to very high levels during G-CSF administration, and concerns have been raised over the theoretical complications of leukostasis, including myocardial infarction and stroke. In fact, a recent editorial has suggested that the WBC during G-CSF therapy should not be allowed to exceed 70,000/ml. However, serious complications have not been reported, and growth factors are thought to be safe for administration to normal persons in both the short- and long-term. Growth factors have been approved by the FDA for allogeneic PBSC harvesting. Outpatient G-CSF is administered daily as a subcutaneous injection to the allogeneic donor. The peripheral WBC begins to increase within 24-48 hours and rises steadily; the stem cell count (CD34-positive cells) peaks on or about day 5 of G-CSF therapy. PBSC collection begins at that point, and similar to autologous donors, the goal of collection is to harvest a total of ( 2.5 x 106 CD34 cells per kilogram of the recipient's body weight for each anticipated transplant. To achieve this goal, most allogeneic donors require two collections, and only rarely more than two. CD34 is a cell surface antigen which is characteristic of hematopoietic stem cells; the clinical laboratory identifies and quantitates stem cells using flow cytometry and fluorescent monoclonal antibodies specific for the CD34 antigen.

The current practice with PBSC is to freeze the stem cells in an identical fashion as would be done for autologous PBSC collections. If ABO-incompatibility exists between the recipient and the donor, it is possible to avoid an intravascular hemolytic transfusion reaction at the time of transplantation by physically removing the allogeneic red cells; this is done prior to freezing and can be accomplished using a number of techniques, including density-gradient centrifugation, buffy coat preparation through differential centrifugation, automated cell washing, and column-based cell separation.

Unlike autologous PBSC collections which may be contaminated by circulating malignant cells and hence are sometimes subjected to a selection process in order to deplete neoplastic cells while enriching for stem cells, allogeneic PBSC transplantations are free of malignant cells. The greater risk in allogeneic transplant is the development of graft-versus-host-disease (GVHD) which is mediated by donor lymphocytes. Therefore, it may be preferable to reduce or even "titrate" the risk of GVHD in recipients by depleting PBSC collections of T lymphocytes or T lymphocyte subsets; transplants across significant HLA barriers are likely to require such T cell-depletion. Conversely, separate apheresis collections of donor lymphocytes may be required for transfusion into patients who have relapsed early after transplantation in hopes of boosting a graft-versus-leukemia effect. Future Lab News updates will focus on the specific strategies for optimal manipulation of both allogeneic and autologous PBSC.

References

1. Teshima A, Harada M. Mobilization of peripehral blood progenitor cells for allogeneic transplantation. Cytokines Cell Mol Ther 3:101-14, 1997
2. Dreger P, Glass B, Uharek L, Zeis M, Schmitz N. Allogeneic transplantation of mobilized peripheral blood progenitor cells: towards tailored cell therapy. Int J Hematol 66:1-11, 1997

Steven Mechanic, M.D.