Challenges in Autologous Graft Manipulation

Ex vivo processing of autologous cells for hematopoietic rescue poses numerous challenges to physicians, scientists, and technologists. These challenges include choosing the source of the graft, monitoring tumor involvement, determining whether to purge tumor cells from the graft and what purging method to use, quantitating purging efficacy, evaluating hematopoietic potential in vitro, utilizing cryopreservation techniques and posttransplant immunotherapy, and complying with an evolving strategy on how to regulate these technologies. Although the addition of growth factors to chemotherapy regimens has improved the reproducibility of blood progenitor cell mobilization kinetics, it is clear that it is generally more difficult to harvest cells from patients than from normal donors. There are data to suggest that comobilization of tumor cells may occur, which becomes of increasing concern in poor mobilizers, in whom large numbers of aphereses must be used to achieve a target CD34+ cell dose. The impact of purging on clinical outcome is still disputed, although emerging data show a trend toward benefit. Techniques have generally moved from negative selection to a combination of enrichment of CD34+ cells and tumor-directed negative selection. This can add $5000 to $15,000 to the cost of processing, at a time when managed health care is “unwilling to underwrite research” and has capitated transplantation costs. Molecular purging approaches are now appearing, but are unlikely to be less costly. Central to evaluating the use of purging is accurate quantitation of tumor involvement. Substantial progress has been made in this area with the use of highsensitivity immunocytochemical and molecular biology techniques. These should allow us to study, in a more quantitative fashion, the relationship between graft contamination, purging efficacy, and clinical outcome. Evaluation of engraftment potential in vitro and cryopreservation techniques have been under-studied, although they could radically improve or simplify autograft processing. Resurgence of interest in immunotherapy posttransplant in allograft recipients is based largely on the minimal residual disease model. If tumor cells found within grafts are inherently more metastatic, their contribution to relapse posttransplant may be out of proportion to their numbers. This would reinforce the need to purge