Different mechanisms control peripheral and central tolerance in hematopoietic chimeric mice

Selective expansion of regulatory T cells using an orthogonal IL-2/IL-2 receptor system facilitates transplantation tolerance

Adoptive transfer of Tregs has been shown to improve alloengraftment in animal models. However, it is technically challenging to expand Tregs ex vivo for the purpose of infusing large numbers of cells in the clinic. We demonstrate an innovative approach to engineering an orthogonal IL-2/IL-2 receptor (IL-2R) pair, the parts of which selectively interact with each other, transmitting native IL-2 signals, but do not interact with the natural IL-2 or IL-2R counterparts, thereby enabling selective stimulation of target cells in vivo. Here, we introduced this orthogonal IL-2R into Tregs. Upon adoptive transfer in a murine mixed hematopoietic chimerism model, orthogonal IL-2 injection significantly promoted orthogonal IL-2R+Foxp3GFP+CD4+ cell proliferation without increasing other T cell subsets and facilitated donor hematopoietic cell engraftment followed by acceptance of heart allografts. Our data indicate that selective target cell stimulation enabled by the engineered orthogonal cytokine receptor improves Treg potential for the induction of organ transplantation tolerance.

Mechanisms of Tolerance Induction by Hematopoietic Chimerism: The Immune Perspective

Hematopoietic chimerism is one of the effective approaches to induce tolerance to donor‐derived tissue and organ grafts without administration of life‐long immunosuppressive therapy. Although experimental efforts to develop such regimens have been ongoing for decades, substantial cumulative toxicity of combined hematopoietic and tissue transplants precludes wide clinical implementation. Tolerance is an active immunological process that includes both peripheral and central mechanisms of mutual education of coresident donor and host immune systems. The major stages include sequential suppression of early alloreactivity, establishment of hematopoietic chimerism and suppressor cells that sustain the state of tolerance, with significant mechanistic and temporal overlap along the tolerization process. Efforts to devise less toxic transplant strategies by reduction of preparatory conditioning focus on modulation rather than deletion of residual host immunity and early reinstitution of regulatory subsets at the central and peripheral levels. Stem Cells Translational Medicine2017;6:700–712

Long-term Tolerance Toward Haploidentical Vascularized Composite Allograft Transplantation in a Canine Model Using Bone Marrow or Mobilized Stem Cells

BACKGROUND

The development of safe and reliable protocols for the transplantation of the face and hands may be accomplished with animal modeling of transplantation of vascularized composite allografts (VCA). Previously, we demonstrated that tolerance to a VCA could be achieved after canine recipients were simultaneously given marrow from a dog leukocyte antigen-identical donor. In the present study, we extend those findings across a dog leukocyte antigen mismatched barrier.

METHODS

Eight recipient dogs received total body irradiation (4.5 cGy), hematopoietic cell transplantation (HCT), either marrow (n = 4) or granulocyte-colony stimulating factor mobilized peripheral blood stem cells (n = 4), and a VCA transplant from the HCT donor. Post grafting immunosuppression consisted of mycophenolate mofetil (28 days) and cyclosporine (35 days).

RESULTS

In 4 dogs receiving bone marrow, 1 accepted both its marrow transplant and demonstrated long-term tolerance to the donor VCA (>52 weeks). Three dogs rejected both their marrow transplants and VCA at 5 to 7 weeks posttransplant. Dogs receiving mobilized stem cells all accepted their stem cell transplant and became tolerant to the VCA. However, 3 dogs developed graft-versus-host disease, whereas 1 dog rejected its stem cell graft by week 15 but exhibited long-term tolerance toward its VCA (>90 weeks).

CONCLUSIONS

The data suggest that simultaneous transplantation of mobilized stem cells and a VCA is feasible and leads to tolerance toward the VCA in a haploidentical setting. However, there is a higher rate of donor stem cell engraftment compared with marrow HCT and an increase in the incidence of graft-versus-host disease.

A novel approach inducing transplant tolerance by activated invariant natural killer T cells with costimulatory blockade

Invariant natural killer T (iNKT) cells are one of the innate lymphocytes that regulate immunity, although it is still elusive how iNKT cells should be manipulated for transplant tolerance. Here, we describe the potential of a novel approach using a ligand for iNKT cells and suboptimal dosage of antibody for CD40-CD40 ligand (L) blockade as a powerful method for mixed chimerism establishment after allogenic bone marrow transplantation in sublethally irradiated fully allo recipients. Mixed-chimera mice accepted subsequent cardiac allografts in a donor-specific manner. High amounts of type 2 helper T cytokines were detected right after iNKT cell activation, while subsequent interferon-gamma production by NK cells was effectively inhibited by CD40/CD40L blockade. Tolerogenic components, such as CD11c(low) mPDCA1(+) plasmacytoid dendritic cells and activated regulatory T cells (Tregs) expressing CD103, KLRG-1 and PD-1, were subsequently augmented. Those activating Tregs seem to be required for the establishment of chimerism because depletion of the Tregs 1 day before allogenic cell transfer resulted in a chimerism brake. These results collectively suggest that our new protocol makes it possible to induce donor-specific tolerance by enhancement of the innate ability for immune tolerance in place of the conventional immunosuppression.

Signal one and two blockade are both critical for non-myeloablative murine HSCT across a major histocompatibility complex barrier

Non-myeloablative allogeneic haematopoietic stem cell transplantation (HSCT) is rarely achievable clinically, except where donor cells have selective advantages. Murine non-myeloablative conditioning regimens have limited clinical success, partly through use of clinically unachievable cell doses or strain combinations permitting allograft acceptance using immunosuppression alone. We found that reducing busulfan conditioning in murine syngeneic HSCT, increases bone marrow (BM):blood SDF-1 ratio and total donor cells homing to BM, but reduces the proportion of donor cells engrafting. Despite this, syngeneic engraftment is achievable with non-myeloablative busulfan (25 mg/kg) and higher cell doses induce increased chimerism. Therefore we investigated regimens promoting initial donor cell engraftment in the major histocompatibility complex barrier mismatched CBA to C57BL/6 allo-transplant model. This requires full myeloablation and immunosuppression with non-depleting anti-CD4/CD8 blocking antibodies to achieve engraftment of low cell doses, and rejects with reduced intensity conditioning (≤75 mg/kg busulfan). We compared increased antibody treatment, G-CSF, niche disruption and high cell dose, using reduced intensity busulfan and CD4/8 blockade in this model. Most treatments increased initial donor engraftment, but only addition of co-stimulatory blockade permitted long-term engraftment with reduced intensity or non-myeloablative conditioning, suggesting that signal 1 and 2 T-cell blockade is more important than early BM niche engraftment for transplant success.

Endogenous memory CD8 T cells are activated within cardiac allografts without mediating rejection

Endogenous memory CD8 T cells infiltrate MHC-mismatched cardiac allografts within 12-24 h posttransplant in mice and are activated to proliferate and produce IFN-γ. To more accurately assess the graft injury directly imposed by these endogenous memory CD8 T cells, we took advantage of the ability of anti-LFA-1 mAb given to allograft recipients on days 3 and 4 posttransplant to inhibit the generation of primary effector T cells. When compared to grafts from IgG-treated recipients on day 7 posttransplant, allografts from anti-LFA-1 mAb-treated recipients had increased numbers of CD8 T cells but these grafts had marked decreases in expression levels of mRNA encoding effector mediators associated with graft injury and decreases in donor-reactive CD8 T cells producing IFN-γ. Despite this decreased activity within the allograft, CD8 T cells in allografts from recipients treated with anti-LFA-1 mAb continued to proliferate up to day 7 posttransplant and did not upregulate expression of the exhaustion marker LAG-3 but did have decreased expression of ICOS. These results indicate that endogenous memory CD8 T cells infiltrate and proliferate in cardiac allografts in mice but do not express sufficient levels of functions to mediate overt graft injury and acute rejection.

Optimizing chimerism level through bone marrow transplantation and irradiation to induce long-term tolerance to composite tissue allotransplantation

BACKGROUND

Mixed chimerism with long-term composite tissue allotransplant (CTA) acceptance can be achieved through allogeneic bone marrow transplantation (BMT). The present study investigated the optimal chimerism level by giving different irradiation dosages to recipients to induce tolerance to CTA.

METHODS

Chimera were prepared using Brown-Norway and Lewis rats with strong major histocompatibility complex incompatibility. The Lewis rats received 5 mg antilymphocyte globulin (day -1 and 10) and 16 mg/kg cyclosporine (day 0-10) and were separated into groups 1, 2, 3, 4, and 5 according to the day -1 irradiation dosage: 0, 200, 400, 600, and 950 cGy, respectively. The Lewis rats were then reconstituted with 100 × 10(6) T-cell-depleted Brown-Norway bone marrow cells (day 0) and received vascularized Brown-Norway-CTA on day 28. Chimerism was assessed monthly by flow cytometry starting on day 28 after BMT. Graft-versus-host disease (GVHD) was assessed clinically and histologically.

RESULTS

Chimerism, 4 weeks after BMT, averaged 0.2%, 9.2%, 30.7%, 58%, and 99.3% in groups 1 to 5, respectively. GVHD occurred as follows: groups 1 and 2, none; group 3, 1 case of GVHD; group 4, 7 cases of GVHD (of which 3 died); and group 5, 10 cases of GVHD (of which 6 died). The percentage of long-term CTA acceptance was 0%, 0%, 90%, 70%, and 40% in groups 1 to 5, respectively. The percentage of regulatory T cells was significantly lower in high-chimerism (≥ 20%, n = 15) than in low-chimerism (<20%, n = 5) rats that accepted CTA long-term .

CONCLUSIONS

The chimerism level correlated positively with GVHD occurrence and long-term CTA acceptance but correlated negatively with regulatory T-cell levels. Optimal chimerism for CTA acceptance through pre-CTA BMT and irradiation occurs at 20-50% at day 28 after BMT in the rat model.

Survival time of cardiac allografts prolonged by isogeneic BMT in mice

To find an approach to prolong the survival time of cardiac allografts in a BALB/c-to-C57/BL6 heterotopic heart transplant model and to try to figure out related chemokines and cytokines, isogeneic and allogeneic BM cells were obtained from pregnant C57/BL6 (♀C57/BL6 × ♂BALB/c) and regular BALB/c mice and injected to the half lethally irradiated C57/BL6 mice 1 day before heart transplantation. Recipients were treated with CsA or phosphate-buffered saline for 7 days. Isogeneic BMT (iBMT) from pregnant C57/BL6 mice was observed to significantly prolong the survival of BALB/c allografts and reduce the lymphocyte infiltration. Allogeneic BMT (aBMT) and iBMT both exhibited signicantly less T-cell proliferation reactivity and the similar degree of chimerism. There was no significant difference in these groups of IFN-γ and IL-4 production. The level of chemokine MIG (CXCL9) dramatically decreased in aBMT and iBMT groups compared with the control group. But there were no significant differences between aBMT and iBMT group. IL-17 and RORγ(t) (receptor-related orphan receptor) production were downregulated in iBMT recipients. These results indicate that iBMT can prolong the survival of cardiac allografts. IL-17 production downregulated in iBMT recipients. This means that iBMT may have important therapeutic implications.

Effects of chimerism on the mice heart transplanted survival with the bone marrow infusion

AIMS

To evaluate the effects of chimerism on the mice heart transplanted survival with the bone marrow infusion.

METHODS

Bone marrow cells (BMCs) were obtained from BALB/c mice. These BMCs were injected into the irradiated (2Gy-Co60) C57BL/6 mice through femoral vein. Then Group A mice were treated with Cyclosporine (1mg/kg) for 21days and Group B were not treated with Cyclosporine. Group C were treated as the control group without BMCs infusion. Group D were treated with Cyclosporine (1mg/kg) for 21days pre-hearttransplantation without BMCs infusion. After 21days, the C57BL/6 mice received heart allografts from BALB/c. To determine the degree of chimerism in BMCs infusion recipients, peripheral blood were isolated on day 7, 14, 21. Allografts were harvested 10days after heart transplantation for the histological analysis.

RESULTS

(1) Chimerism detected in the peripheral blood of Group A mice on day 7 after BMCs infusion was 6.1±2.5%, on day 14 was 15.4±2.9% and on day 21 was 10.7±2.6%. For the Group B mice on day 7 after BMCs infusion, the chimerism was 2.8±1.1%, on day 14 was 11.2±4.8% and on day 21 was 7.4±3.7%. For the Groups C and D mice, no chimerism was observed. Group A mice had the tendency toward improved level of chimerism than Group B mice. (2) The survival time of Group A (n=6) was 13.0±1.4days which was significantly longer than Group B (n=6) with the survival time was 8.5±1.3days (p<0.001), also longer than the mice in Groups C and D, the survival time of which were 10.0±1.3days (p=0.008) and 9.4±1.1days (p=0.004). There is no significant difference among Groups B, C, and D. (3) The HE staining showed the much more seriously heart rejection in Groups B, C and D than Group A.

CONCLUSIONS

The chimerism was found in the BMCs infusion groups. Without the CsA treatment combined with chimerism could not protect the transplanted heart. There was no obvious evidence showed that the chimerism alone could improve the survival time of cardiac allografts in mice.

Characterization of tolerance induction through prenatal marrow transplantation: the requirement for a threshold level of chimerism to establish rather than maintain postnatal skin tolerance

Hematopoietic chimerism resulting from prenatal marrow transplantation does not consistently result in allotolerance for unidentified causes. In a C57BL/6-into-FVB/N murine model, we transplanted T-cell-depleted adult marrow on gestational day 14 to elucidate the immunological significance of chimerism towards postnatal tolerance. Postnatally, chimerism was examined by flow cytometry, and tolerance by skin transplantation and mixed lymphocyte reaction. Regulatory T cells were quantified by FoxP3 expression. Peripheral chimerism linearly related to thymic chimerism, and predicted the degree of graft acceptance with levels >3% at skin placement, yielding consistent skin tolerance. Low- and high-level chimeras had lower intrathymic CD3(high) expression than microchimeras or untransplanted mice. Regardless of the skin tolerance status in mixed chimeras, donor-specific alloreactivity by lymphocytes was suppressed but could be partially restored by exogenous interleukin-2. Recipients that lost peripheral chimerism did not accept donor skin unless prior donor skin had engrafted at sufficient chimerism levels, suggesting that complete tolerance can develop as a consequence of chimerism-related immunosuppression of host lymphocytes and the tolerogenic effects of donor skin. Thus, hematopoietic chimerism exerted immunomodulatory effects on the induction phase of allograft tolerance. Once established, skin tolerance did not fade away along with spontaneous regression of peripheral and tissue chimerism, as well as removal of engrafted donor skin. Neither did it break following in vivo depletion of increased regulatory T cells, and subcutaneous interleukin-2 injection beneath the engrafted donor skin. Those observations indicate that the maintenance of skin tolerance is multifaceted, neither solely dependent upon hematopoietic chimerism and engrafted donor skin nor on the effects of regulatory T cells or clonal anergy. We conclude that hematopoietic chimerism generated by in utero hematopoietic stem cell transplantation is critical to establish rather than maintain postnatal skin tolerance. Therefore, the diminution of hematopoietic chimerism below a threshold level does not nullify an existing tolerance state, but lessens the chance of enabling complete tolerance.

Potential role of host effector memory CD8+ T cells in marrow rejection after mixed chimerism induction in cynomolgus monkeys

Mixed hematopoietic chimerism provides a powerful means of achieving transplantation tolerance. We investigated the efficacy of combined blockade of the CD40/CD154 and CD28/B7 costimulation pathways to induce sustained mixed chimerism in cynomolgus monkeys following major histocompatibility complex-mismatched bone marrow (BM) transplants. A nonmyeloablative conditioning regimen of busulfan, intravenous and intraosseous ifosfamide, and anti-thymocyte globulin was used. BM transplantation was followed by a one-week course of CTLA4-Ig/anti-CD154 monoclonal antibodies. Three recipients achieved a wide range of transient chimerism (10.8-79.8%). A rapid proliferation of host effector memory (CD28(low)CD95(high)) CD8(+) T cells was observed in conditioned animals whether or not they received allogeneic BM, and this expansion occurred concurrently with the loss of chimerism in BM recipients. CD8(+) T cells from the recipients had increased reactivity to donor stimulators vs. third-party stimulators. Additional immunosuppression with tacrolimus or deoxyspergualin after transplantation delayed post-transplant proliferation of effector memory CD8(+) T cells but did not promote chimerism. A one-month course of costimulatory blockade also did not prevent marrow rejection. These studies demonstrate that combined CD40/CD154 and CD28/B7 costimulatory blockade supports transient mixed chimerism induction following nonmyeloablative conditioning in primates, but is insufficient to overcome host immune resistance likely mediated by effector memory CD8(+) T cells.

Induction of allogeneic mixed chimerism by immature dendritic cells and bone marrow transplantation leads to prolonged tolerance to major histocompatibility complex disparate allografts

Mixed chimerism has been shown to lead to prolonged major histocompatibility complex (MHC) disparate allograft survival and immune-specific tolerance; however, traditional conditioning regimes often involve myeloablation, which may pose a significant safety risk. In this study we examined the use of donor C57BL/6 (H-2(b)) immature dendritic cells (imDCs) to tolerize the BALB/c (H-2(d)) recipient to bone marrow transplantation (BMT), allowing the induction of mixed chimerism without immunosuppression or myeloablation. We showed that successful mismatched bone marrow engraftment can be achieved using imDCs given up to 3 days prior to BMT and that mixed chimerism can be established and detected in excess of 100 days post-BMT without evidence of graft-versus-host disease. Furthermore, we showed that imDCs can suppress lymphocyte proliferation in response to mismatched MHC stimulation, leading to increased expression of interleukin (IL)-4 and IL-10 and decreased expression of IL-2 and interferon-gamma (IFN-gamma). The induction of stable chimeras through pre-conditioning of mice with donor imDCs followed by BMT led to tolerance, allowing the long-term survival (> 110 days) of mismatched cardiac allografts and the prolonged survival of mismatched skin allografts without the need for immunosuppression or myeloablation. Transplantation with third-party C3H allografts were rapidly rejected in this model, suggesting that immune-specific tolerance was achieved. The induction of immune-specific tolerance without the need for immunosuppression or myeloablation represents a significant advance in transplant immunology and may provide clinicians with a plausible alternative in combating organ rejection following transplantation.

TLR agonists prevent the establishment of allogeneic hematopoietic chimerism in mice treated with costimulation blockade

Activation of TLR4 by administration of LPS shortens the survival of skin allografts in mice treated with costimulation blockade through a CD8 T cell-dependent, MyD88-dependent, and type I IFN receptor-dependent pathway. The effect of TLR activation on the establishment of allogeneic hematopoietic chimerism in mice treated with costimulation blockade is not known. Using a costimulation blockade protocol based on a donor-specific transfusion (DST) and a short course of anti-CD154 mAb, we show that LPS administration at the time of DST matures host alloantigen-presenting dendritic cells, prevents the establishment of mixed allogeneic hematopoietic chimerism, and shortens survival of donor-specific skin allografts. LPS mediates its effects via a mechanism that involves both CD4(+) and CD8(+) T cells and results from signaling through either the MyD88 or the type I IFN receptor pathways. We also document that timing of LPS administration is critical, as injection of LPS 24 h before treatment with DST and anti-CD154 mAb does not prevent hematopoietic engraftment but administration the day after bone marrow transplantation does. We conclude that TLR4 activation prevents the induction of mixed allogeneic hematopoietic chimerism through type I IFN receptor and MyD88-dependent signaling, which leads to the up-regulation of costimulatory molecules on host APCs and the generation of alloreactive T cells. These data suggest that distinct but overlapping cellular and molecular mechanisms control the ability of TLR agonists to block tolerance induction to hematopoietic and skin allografts in mice treated with costimulation blockade.

TLR agonists abrogate co-stimulation blockade-induced mixed chimerism and transplantation tolerance

We investigated the mechanisms by which Toll-like receptor (TLR) agonists affect the induction of mixed chimerism and skin allograft survival in mice treated with co-stimulation blockade (CB). We report that TLR agonists prevent the generation of mixed chimerism by breaking tolerance in the alloreactive CD4(+) and CD8(+) T cell compartments, and that type I interferon (IFN) is important in this process. Understanding how environmental perturbations affect CB-induced transplantation tolerance may lead to more effective regimens that can be used as an approach for the treatment of type I diabetes, for which the transplantation of pancreatic islets is a promising therapy.

Viral infection: a potent barrier to transplantation tolerance

Transplantation of allogeneic organs has proven to be an effective therapeutic for a large variety of disease states, but the chronic immunosuppression that is required for organ allograft survival increases the risk for infection and neoplasia and has direct organ toxicity. The establishment of transplantation tolerance, which obviates the need for chronic immunosuppression, is the ultimate goal in the field of transplantation. Many experimental approaches have been developed in animal models that permit long-term allograft survival in the absence of chronic immunosuppression. These approaches function by inducing peripheral or central tolerance to the allograft. Emerging as some of the most promising approaches for the induction of tolerance are protocols based on costimulation blockade. However, as these protocols move into the clinic, there is recognition that little is known as to their safety and efficacy when confronted with environmental perturbants such as virus infection. In animal models, it has been reported that virus infection can prevent the induction of tolerance by costimulation blockade and, in at least one experimental protocol, can lead to significant morbidity and mortality. In this review, we discuss how viruses modulate the induction and maintenance of transplantation tolerance.

CD8+ T-Cell depletion and rapamycin synergize with combined coreceptor/stimulation blockade to induce robust limb allograft tolerance in mice

The growing development of composite tissue allografts (CTA) highlights the need for tolerance induction protocols. Herein, we developed a mouse model of heterotopic limb allograft in a stringent strain combination in which potentially tolerogenic strategies were tested taking advantage of donor stem cells in the grafted limb. BALB/c allografts were transplanted into C57BL/6 mice treated with anti-CD154 mAb, nondepleting anti-CD4 combined to either depleting or nondepleting anti-CD8 mAbs. Some groups received additional rapamycin. Both depleting and nondepleting mAb combinations without rapamycin only delayed limb allograft rejection, whereas the addition of rapamycin induced long-term allograft survival in both combinations. Nevertheless, robust donor-specific tolerance, defined by the acceptance of a fresh donor-type skin allograft and simultaneous rejection of third-party grafts, required initial CD8(+) T-cell depletion. Mixed donor-recipient chimerism was observed in lymphoid organs and recipient bone marrow of tolerant but not rejecting animals. Tolerance specificity was confirmed by the inability to produce IL-2, IFN-gamma and TNF-alpha in MLC with donor antigen while significant alloreactivity persisted against third- party alloantigens. Collectively, these results show that robust CTA tolerance and mixed donor-recipient chimerism can be achieved in response to the synergizing combination of rapamycin, transient CD8(+) T-cell depletion and costimulation/coreceptor blockade.

Contribution of regulatory T cells and effector T cell deletion in tolerance induction by costimulation blockade

Blocking of costimulatory signals for T cell activation leads to tolerance in several transplantation models, but the underlying mechanisms are incompletely understood. We analyzed the involvement of regulatory T cells (Treg) and deletion of alloreactive cells in the induction and maintenance of tolerance after costimulation blockade in a mouse model of graft-vs-host reaction. Injection of splenocytes from the C57BL/6 parent strain into a sublethally irradiated F(1) offspring (C57BL/6 x C3H) induced a GVHR characterized by severe pancytopenia. Treatment with anti-CD40L mAb and CTLA4-Ig every 3 days during 3 wk after splenocyte injection prevented disease development and induced a long-lasting state of stable mixed chimerism (>120 days). In parallel, host-specific tolerance was achieved as demonstrated by lack of host-directed alloreactivity of donor-type T cells in vitro and in vivo. Chimerism and tolerance were also obtained after CD25(+) cell-depleted splenocyte transfer, showing that CD25(+) natural Treg are not essential for tolerance induction. We further show that costimulation blockade results in enhanced Treg cell activity at early time points (days 6-30) after splenocyte transfer. This was demonstrated by the presence of a high percentage of Foxp3(+) cells among donor CD4(+) cells in the spleen of treated animals, and our finding that isolated donor-type T cells at an early time point (day 30) after splenocyte transfer displayed suppressive capacity in vitro. At later time points (>30 days after splenocyte transfer), clonal deletion of host-reactive T cells was found to be a major mechanism responsible for tolerance.

Manipulating the immune system for anti-tumor responses and transplant tolerance via mixed hematopoietic chimerism

SUMMARY

Stem cells (SCs) with varying potentiality have the capacity to repair injured tissues. While promising animal data have been obtained, allogeneic SCs and their progeny are subject to immune-mediated rejection. Here, we review the potential of hematopoietic stem cells (HSCs) to promote immune tolerance to allogeneic and xenogeneic organs and tissues, to reverse autoimmunity, and to be used optimally to cure hematologic malignancies. We also review the mechanisms by which hematopoietic cell transplantation (HCT) can promote anti-tumor responses and establish donor-specific transplantation tolerance. We discuss the barriers to clinical translation of animal studies and describe some recent studies indicating how they can be overcome. The recent achievements of durable mixed chimerism across human leukocyte antigen barriers without graft-versus-host disease and of organ allograft tolerance through combined kidney and bone marrow transplantation suggest that the potential of this approach for use in the treatment of many human diseases may ultimately be realized.