Engineering hematopoietic grafts using elutriation and positive cell selection to reduce GVHD

A systematic approach to hematopoietic graft manipulation has minimized several of the variables inherent to allogeneic BMT. Through this approach, we have been able to significantly impact on morbidity and quality of life following allogeneic transplantation. Acute and chronic GVHD, blood product and antibiotic usage, in patient hospitalization, acuity, costs and survival (especially in patients older than 40) have been improved. The HLA barrier still presents a formidable obstacle to achieving a more widespread use of this therapy. The complications encountered in HLA matched/TCD grafts occur with even greater magnitude in the HLA-mismatched or unrelated donor setting. Several centers are now engaged in studies using TCD grafts that are augmented with high doses of CD34+ cells to ensure engraftment while reducing the incidence of GVHD (50-53). Mobilized allogeneic PBSC appear to be an excellent source of stem cells for BMT (5,6). The earlier reports showed decreased rates of GVHD, despite having T cell burdens 10 times higher than those found in unmanipulated bone marrow. However, several of these centers now report an unacceptably high incidence of chronic GVHD (along with its attendant morbidity) following allogeneic PBSC transplantation (54-55). Initial results of TCD in these PBSC grafts using CD34+ selection are disappointing in that recipients developed unexpectedly high incidences of both acute and chronic GVHD (56). No doubt, significant differences exist between marrow and PBSC ancillary cell populations. For example, two laboratories now report the presence of natural suppressor cells in these allogeneic PBSC products in both mice (57) and humans (58). Thus, the same, step-wise approach would be expected to improve graft performance when using PBSC, cord blood, fetal tissue, xenografts or genetically engineered products as a stem cell source. Indeed, there are new reports of improved clinical outcome (especially in the incidence of GVHD) in the PMRD setting using both CD34+ selected (59) and sequential CD34+/CD2+ selected (60) PBSC grafts. It is hoped that future graft engineering approaches will be as successful as previous studies and will extend this form of therapy to an even larger patient population.