Mixed Chimerism and Transplant Tolerance

Tolerance induced by mixed chimerism in place of chronic immunosuppressive therapy would be a drastic departure from current clinical practice in allograft recipients. Thus, efforts to translate from the above rodent models into clinical trials must be interceded by preclinical, large animal studies to document the safety and efficacy of the approach. Chimerism has generally been more difficult to induce in large animals than in rodents, in large part due to differences in the types and amounts of irradiation and reagents used in the different species. For example, T cell–depleting antibodies that accomplish the level of T cell depletion that has been achieved in mouse models for mixed chimerism induction have not been evaluated in primate models. Stable mixed chimerism has recently been achieved in MHC-identical (but not MHC-mismatched) dogs using a non-myeloablative irradiation protocol involving a limited course of pharmacological immunosuppression after BMT (Storb et al., 1997xStable mixed hematopoietic chimerism in DLA-identical littermate dogs given sublethal total body irradiation before and pharmacological immunosuppression after marrow transplantation. Storb, R., Yi, C., Wagner, J.L., Deeg, J., Nash, R.A., Kiem, H.-P., Leisenring, W., and Shulman, H. Blood. 1997; 89: 3048–3054PubMedSee all References(Storb et al., 1997). Recent results in a porcine model (Fuchimoto et al. 2000xMixed chimerism and tolerance without whole body irradiation in a large animal model. Fuchimoto, Y., Huang, C.A., Yamada, K., Shimizu, A., Kitamura, H., Colvin, R.B., Ferrara, V., Murphy, M.C., Sykes, M., White-Scharf, M. et al. J. Clin. Invest. 2000; 105: 1779–1789Crossref | PubMedSee all References, Huang et al. 2000xStable mixed chimerism and tolerance using a non-myeloablative preparative regimen in a large animal model. Huang, C.A., Fuchimoto, Y., Sheir-Dolberg, R., Murphy, M.C., Neville, D.M. Jr., and Sachs, D.H. J. Clin. Invest. 2000; 105: 173–181Crossref | PubMedSee all References) have applied approaches developed in mice to achieve mixed chimerism across MHC barriers. Cynomolgus monkeys conditioned with anti-thymocyte globulin (ATG), fractionated TBI (3 Gy), local TI (7 Gy), and splenectomy before transplantation of MHC-mismatched bone marrow and kidney grafts, followed by 4 weeks of CyA treatment, achieve transient multilineage chimerism in association with long-term kidney graft acceptance (Kawai et al., 1995xMixed allogeneic chimerism and renal allograft tolerance in cynomologous monkeys. Kawai, T., Cosimi, A.B., Colvin, R.B., Powelson, J., Eason, J., Kozlowski, T., Sykes, M., Monroy, R., Tanaka, M., and Sachs, D.H. Transplantation. 1995; 59: 256–262Crossref | PubMedSee all References(Kawai et al., 1995). While these studies provide important proof of principle, in both large animal studies the level of T cell depletion achieved with available reagents was far inferior to that achieved in the murine model upon which they are based.Donor lymphocyte infusions (DLI) convert mixed chimeras to full chimeras without causing GVHD (Sykes et al., 1988xGraft-versus-host-related immunosuppression is induced in mixed chimeras by alloresponses against either host or donor lymphohematopoietic cells. Sykes, M., Sheard, M.A., and Sachs, D.H. J. Exp. Med. 1988; 168: 2391–2396Crossref | PubMedSee all References(Sykes et al., 1988). This observation provided an opportunity to evaluate a protocol for mixed chimerism induction with non-myeloablative conditioning in patients with hematologic malignancies (Spitzer et al., 1999bxThe intentional induction of mixed chimerism and achievement of anti-tumor responses following non-myeloablative conditioning therapy and HLA-matched and mismatched donor bone marrow transplantation for refractory hematologic malignancies. Spitzer, T.R., MCafee, S., Sackstein, R., Colby, C., Toh, H.C., Multani, P., Saidman, S., Weymouth, D., Preffer, F., Poliquin, C. et al. Biol. Blood Marrow Transplant. 1999; 6: 309–320Abstract | Full Text PDFSee all References(Spitzer et al., 1999b), in an effort to exploit lymphohematopoietic GVH reactions induced by DLI in mixed chimeras to achieve graft-versus-tumor effects without GVHD. The potential of this approach to induce transplantation tolerance was evaluated in a patient who had a hematologic malignancy, multiple myeloma, and consequent renal failure. She received a simultaneous bone marrow and renal allograft from her HLA-identical sister, and has now accepted her kidney graft without any immunosuppression for over 2 years. Similar to the primate model described above, in which BMT has been shown to be essential for tolerance induction (Kawai et al., 1995xMixed allogeneic chimerism and renal allograft tolerance in cynomologous monkeys. Kawai, T., Cosimi, A.B., Colvin, R.B., Powelson, J., Eason, J., Kozlowski, T., Sykes, M., Monroy, R., Tanaka, M., and Sachs, D.H. Transplantation. 1995; 59: 256–262Crossref | PubMedSee all References(Kawai et al., 1995), chimerism in this patient was only transient (Spitzer et al., 1999axCombined HLA-matched donor bone marrow and renal transplantation for multiple myeloma with end stage renal disease (the induction of allograft tolerance through mixed lymphohematopoietic chimerism) . Spitzer, T.R., Delmonico, F., Tolkoff-Rubin, N., MCafee, S., Sackstein, R., Saidman, S., Colby, C., Sykes, M., Sachs, D.H., and Cosimi, A.B. Transplantation. 1999; 68: 480–484Crossref | PubMed | Scopus (307)See all References(Spitzer et al., 1999a), suggesting that the kidney graft itself may participate in tolerance induction and/or maintenance after chimerism has played its initial role. Because T cell depletion is only partial in these models, it is clear that the long-term central, deletional tolerance described above in murine models has not yet been achieved with non-myeloablative conditioning in monkeys or humans.Other clinical trials have involved the administration of donor bone marrow from HLA-mismatched cadaveric or living renal or liver allograft donors, without host myelosuppressive or lymphoablative treatment, with the use of conventional chronic immunosuppressive therapy posttransplant. These studies were based on the observation that many long-term organ allograft recipients spontaneously exhibit lasting microchimerism (Starzl et al., 1992xCell migration, chimerism and graft acceptance. Starzl, T.E., Demetris, A.J., Murase, N., Ildstad, S., Ricordi, C., and Trucco, M. Lancet. 1992; 339: 1579–1582Abstract | PubMed | Scopus (799)See all References(Starzl et al., 1992), which is only measurable by highly sensitive techniques. “Microchimerism” should be distinguished from the mixed chimerism discussed above, in which multilineage chimerism is readily measurable by flow cytometry. Numerous attempts have been made to demonstrate a relationship between microchimerism and tolerance. While under certain circumstances microchimerism may contribute to a state of operational tolerance by poorly defined mechanisms, tolerance is by no means assured in the presence of microchimerism (Schlitt et al., 1994xSystemic microchimerism of donor-type associated with irreversible acute liver graft rejection eight years after transplantation. Schlitt, H.J., Hundreiser, J., Ringe, B., and Pichlmayr, R. N. Engl. J. Med. 1994; 330: 646–647Crossref | PubMed | Scopus (114)See all References(Schlitt et al., 1994). The administration of donor marrow with solid organ allografts with standard chronic immunosuppressive therapy has augmented microchimerism, but no significant impact on acute rejection episodes or immunosuppressive medication doses has yet been reported (Rugeles et al., 1997xEvidence for the presence of multilineage chimerism and progenitors of donor dendritic cells in the peripheral blood of bone marrow-augmented organ transplant recipients. Rugeles, M.T., Aitouche, A., Zeevi, A., Fung, J.J., Watkins, S.C., Starzl, T.E., and Rao, A.S. Transplantation. 1997; 64: 735–741Crossref | PubMed | Scopus (23)See all References(Rugeles et al., 1997).The recent development of depleting, humanized monoclonal antibodies against human T cells and against costimulatory molecules raises hope that the advances in rodent models described above in the development of minimal, nontoxic host conditioning regimens for mixed chimerism induction will soon be applied in large animal models and humans. Such efforts are likely to require more than one of these new reagents and thus will depend on the willingness of biotechnology firms to share technologies in order to optimize progress toward this goal.