An alternative conditioning regimen for induction of specific skin graft tolerance across full major histocompatibility complex barriers

Engrafting fetal liver cells into multiple tissues of healthy adult mice without the use of immunosuppressants

We have shown the fetal liver cell engraftments into multiple tissues of adult healthy mice, achieved without suppressing the animals' immune systems. Fetal cells from the livers of male C57Bl/6J Black lineage mice at day 13 to 15 of gestation were injected intravenously into female adult CC57W/MY White mice. The grafting was evaluated by Y-chromosome-specific PCR, cytometric analysis of fluorescently stained donor cells, and histological analysis. All the methods consistently showed the presence of multiple engraftments randomly distributed through the various organs of the recipients. After 60 days, the grafts still constituted 0.1 to 2.75% of the tissues. The grafted cells did not change their appearance in any of the organs except the brain, where they became enlarged. Inflammatory reactions were not detected in any of the histological preparations. The frequency of engraftments was higher in the liver, indicating that similarity between the donor and recipient cells facilitates engraftment. The high inherent plasticity of fetal liver cells underlies their ability to integrate into healthy recipient organs, which can be governed by environmental conditions and connections with neighboring cells rather than by the initial cellular developmental programs. The fact that fetal liver cells can be grafted into multiple tissues of healthy animals indicates that they can be used to replace the natural loss of cells in adult organisms.

Towards a myeloablative regimen with clinical potential: I. Treosulfan conditioning and bone marrow transplantation allow induction of donor-specific tolerance for skin grafts across full MHC barriers

To investigate whether we could create a radiation-free conditioning regimen to induce permanent mixed and multilineage chimerism and donor-specific tolerance, we treated recipient mice with anti-T-cell antibodies, varying and fractionated doses of Treosulfan and fully MHC disparate bone marrow cells. Treosulfan is mainly used in the treatment of ovarian cancer. It is a structural analog of busulfan, but it does not induce severe hepatotoxicity or veno-occlusive disease at or above the maximum tolerated dose, lacks significant nonhematological toxicity and has limited organ toxicity. We report here the successful induction of permanent mixed multilineage chimerism and donor-specific tolerance as was proven by skin transplantation and IFN-gamma ELISPOT. In conclusion, because of its lower nonhematological toxicity, compared with other myeloablative regimens (eg irradiation or busulfan admin- istration), Treosulfan could be a better candidate for conditioning to induce donor-specific allograft tolerance.

Nonmyeloablative conditioning is sufficient to allow engraftment of EGFP-expressing bone marrow and subsequent acceptance of EGFP-transgenic skin grafts in mice

Immunologic reactions against gene therapy products may prove to be a frequent problem in clinical gene therapy protocols. Enhanced green fluorescence protein (EGFP) is commonly used as a marker in gene transfer protocols, and immune responses against EGFP-expressing cells have been documented. The present study was designed to investigate the effect of a pharmacologic, nonmyeloablative, conditioning regimen on the development of EGFP+ donor/recipient mixed bone marrow chimerism and ensuing tolerance to EGFP-expressing transplants. To this end, C57BL/6J (B6) mice were treated with soluble formulations of either busulfan (Busulfex) or the closely related compound treosulfan, followed by transplantation of bone marrow cells from EGFP-transgenic (B6-EGFP.Tg) donor mice. Such conditioning regimens resulted in long-term persistence of donor EGFP+ cells among various hematopoietic lineages from blood, bone marrow, and thymus. Stable hematopoietic chimeras transplanted at 10 to 17 weeks after bone marrow transplantation (BMT) with B6-EGFP.Tg skin grafts all accepted their transplants, whereas non-EGFP chimeric B6 control animals were able to mount rejection of the EGFP+ B6 skin grafts. Control third-party grafts from major histocompatibility complex (MHC)-mismatched mice were rejected within 20 days, indicating that acceptance of EGFP-expressing skin grafts was the result of specific immune tolerance induction by the transplantation of EGFP-transgenic bone marrow. Long-term tolerance to EGFP in chimeric recipients was confirmed by the absence of anti-EGFP-reactive T cells and antibodies. These results broaden the therapeutic potential for using hematopoietic molecular chimerism in nonmyeloablated recipients as a means of preventing rejection of genetically modified cells.

Durable engraftment of major histocompatibility complex-incompatible cells after nonmyeloablative conditioning with fludarabine, low-dose total body irradiation, and posttransplantation cyclophosphamide

Treatment of leukemia by myeloablative conditioning and transplantation of major histocompatibility complex (MHC)-mismatched stem cells is generally avoided because of the high risk of graft rejection or lethal graft-versus-host disease (GVHD). This study shows that MHC-incompatible cells can engraft stably after nonmyeloablative conditioning with immunosuppressive chemotherapy and low-dose total body irradiation (TBI). Long-term mixed hematopoietic chimerism, clonal deletion of donor-reactive T cells, and bidirectional cytotoxic T-cell tolerance were achieved by transplanting MHC-mismatched marrow cells into recipients conditioned with pretransplantation fludarabine or cyclophosphamide (Cy), 50 to 200 cGy TBI on day -1, and Cy 200 mg/kg intraperitoneally on day 3. In this model, long-term donor chimerism was proportional to the dose of TBI or donor marrow cells. Pretransplantation fludarabine and posttransplantation Cy were both required for alloengraftment, but the drugs had additional effects. For example, fludarabine sensitized host stem cells to the toxicity of TBI, because animals conditioned with both agents had higher chimerism than animals conditioned with TBI alone (P <.05). Also, posttransplantation Cy attenuated lethal and nonlethal GVH reactions, because F(1) recipients of host-reactive, parental spleen cells survived longer (P <.05) and had lower donor cell chimerism (P <.01) if they received posttransplantation Cy than if they did not. Finally, delayed infusions of donor lymphocytes into mixed chimeras prolonged survival after leukemia challenge (P <.0001) without causing lethal GVHD. These results indicate that stable engraftment of MHC-incompatible cells can be induced after fludarabine-based, nonmyeloablative conditioning and that it serves as a platform for adoptive immunotherapy with donor lymphocyte infusions.

Transplantation tolerance induced by mixed chimerism

Although short- and long-term results after organ transplantation have improved considerably in recent years, morbidity and mortality rates in graft recipients remain high. The induction of lifelong donor-specific tolerance would dramatically improve outcome after organ transplantation. Although many tolerance protocols have been successful in rodent studies, most of these approaches have failed when attempted in large animals or humans. Robust tolerance, in contrast, has been demonstrated with mixed chimerism regimens not only in rodents but also in large animal models, including non-human primates. Furthermore, mixed chimerism protocols have been developed that would be feasible in cadaveric, and thus in thoracic, transplantation. The induction of mixed hematopoietic chimerism is therefore an attractive experimental approach for development of clinical tolerance protocols. One of the obstacles to widespread clinical application of this concept is the remaining toxicity of the host conditioning. Recent advances, however, have led to substantially milder protocols that could become clinically acceptable in the foreseeable future. This article provides a short overview of the basic mechanisms by which immunologic tolerance may be induced, describes the concept of mixed chimerism as a promising approach for clinical tolerance induction, and reviews recent progress in developing clinically feasible mixed chimerism protocols.

Mixed chimerism and transplantation tolerance

Achieving transplantation tolerance is an important goal in the effort to reduce long-term morbidity and mortality in organ transplant recipients. Robust, lifelong, donor-specific tolerance can be reliably achieved by induction of mixed chimerism in various animal models. To date, the clinical application of these proto-cols has been impeded partly by the potential toxicity of the required host conditioning regimens and the lack of successful studies in large animals. This article reviews the progress achieved in recent years in developing considerably milder conditioning protocols in rodents, and in extending some of these models to achieve permanent mixed chimerism and tolerance in large animals. Advances in the induction of xenogeneic tolerance through mixed chimerism are also discussed.