Induction of tolerance in organ recipients by hematopoietic stem cell transplantation

The use of hematopoietic stem cell transplantation (HSCT) for the establishment of mixed chimerism represents a viable and attractive approach for generating tolerance in transplantation biology, as it generally leads to durable immune tolerance, enabling the subsequent engraftment of organ transplants without the need for a deleterious continuous immunosuppressive therapy. However, in order to apply HSCT to patients in a manner that enables long term survival, transplant-related mortality must be minimized by eliminating the risk for graft-versus-host-disease (GVHD) and by reducing the toxicity of the conditioning protocol. T-cell depleted bone marrow transplants (TDBMT) have been shown to adequately eliminate GVHD. However, even in leukemia patients undergoing supralethal conditioning, mismatched TDBMT are vigorously rejected. This barrier can be overcome through the modulatory activity of CD34 cells, which are endowed with veto activity, by the use of megadose stem cell transplants. In mice, megadoses of Sca+lin-hematopoietic stem cells can induce mixed chimerism following sub-lethal conditioning. Nevertheless, the number of human CD34 cells that can be harvested is not likely to be sufficient to overcome rejection under reduced intensity conditioning (RIC), which might be acceptable in recipients of organ transplantation. To address this challenge, we investigated a novel source of veto cells, namely anti 3rd-party cytotoxic T cells (CTLs) which are depleted of GVH reactivity, combined with megadoses of purified stem cells and a RIC protocol. This approach might provide a safer modality for the induction of durable chimerism.