Presented at:
XIX Int'l Congress
Transplantation, Miami, FL, Aug 25-30, 2002
Mixed chimerism: from bench to bedside
Megan Sykes,
Harvard University, Boston, MA
Mixed hematopoietic chimerism provides a powerful means of achieving transplantation tolerance. Mixed allogeneic chimerism can be induced in mice receiving allogeneic bone marrow transplantation (BMT) after non-myeloablative host conditioning with depleting anti-T cell monoclonal antibodies (mAbs), low dose (3 Gy) total body irradiation (TBI), and local thymic irradiation (TI) (7 Gy). These mice are specifically tolerant to donor and host antigens. The amount of conditioning used to achieve mixed chimerism can be reduced by using high stem cell doses in place of TBI, and by replacing both recipient T cell depletion and thymic irradiation with a short course of costimulatory blockade. Lasting chimerism and donor-specific tolerance were achieved with a regimen that requires no host pre-conditioning, by giving a high dose of fully MHC-mismatched donor marrow followed by a single injection of each of two costimulatory blockers. Similar results can be achieved by giving a depleting anti-CD8 mAb on day -1 in combination with one injection of anti-CD40L. Engrafted allogeneic stem cells provide a life-long source of progenitor cells that seed the thymus, giving rise to APC which mediate clonal deletion of donor-reactive T cells. Host antigen-presenting cells also populate the thymus, so intrathymic deletion of cells recognizing both the donor and the host occurs. In the regimens involving costimulatory blockade, peripheral mechanisms, including deletion, are involved in the tolerization of the pre-existing donor-reactive T cell repertoire. "Passive cell death" appears to play a more important role than activation-induced cell death in the peripheral deletion of donor-reactive CD4 cells, and signalling through CD40L plays no significant role in the highly reliable tolerization of CD4 cells by BMT with anti-CD40L. Regulatory cells do not appear to play a significant role in this highly reliable approach to tolerizing peripheral CD4 T cells (J. Kurtz et al, unpublished).
In the above murine studies, graft-vs-host disease (GVHD) has been avoided in part because the same reagents used to deplete or inactivate recipient alloreactive T cells are still present in the recipient when donor marrow is given, so the donor T cells in these inocula are also depleted or inactivated. Several of these approaches have been extended to large animal models. Mixed chimerism can also be used to exploit the graft-vs-leukemia (GVL) effect of GVH-reactive donor T cells while avoiding GVHD. When non-tolerant donor lymphocytes (donor lymphocyte infusions, DLI) are given to mixed chimeras weeks or months following BMT, mixed chimeras convert to full donor chimeras, and GVL (M. Mapara et al, unpublished) effects are observed, but GVHD does not occur. Based on these observations, we initiated a clinical trial of mixed chimerism induction followed by DLI for the treatment of advanced hematologic malignancies. Following success with this approach, our group has induced donor-specific allograft tolerance in several patients with multiple myeloma who simultaneously received non-myeloablative BMT and a kidney transplant from the same donor. Thus, non-myeloablative BMT can be used to achieve clinical organ allograft tolerance.