Aims of conditioning.

Nuclear warfare research and treatment of radiation accident victims uncovered the potential of hemopoietic stem cell transplants. Prior to transplantation of hemopoietic stem cells patients receive "conditioning" agents: high-dose total-body irradiation and/or high-dose chemotherapy. High-dose conditioning causes at least 20% procedure-related mortality. Recent efforts to reduce procedure-related mortality by the use of low-dose conditioning included low-dose total-body irradiation, immunosuppressive agents, and the replacement of high-dose chemotherapy by donor lymphocytes for graft-vs-tumor effects. Procedure-related mortality remains high (10-30%). Tumor recurrence at 1 year is over 50%. In this review, the aims of conditioning (creation of space, prevention of hemopoietic stem cell rejection, eradication of immune memory, and eradication of tumor cells) are reexamined in those patient and animal studies that explore quantitative and mechanistic conditioning issues. Translational experimental animal models provide the best opportunities for the development of less toxic conditioning agents for human patients and require an analysis of the consequences of the effects of new conditioning agents on host-vs-graft as well as graft-vs-host reactions. Total-body irradiation or other forms of radiation create space, prevent rejection of histocompatible stem cells, and can eliminate immune memory to autoimmune antigens at modest, nontoxic doses. The transplantation of histoincompatible stem cells and the eradication of large loads of tumor cells remain problematic. The therapeutic index of allogeneic stem cell transplants will increase if new conditioning agents are targeted only to those host tissues that need conditioning: hemopoietic system, immune system, and tumor masses. Radiolabeled immunoglobulins are among the most promising new, low-toxicity conditioning agents.