Short-term immunosuppression facilitates induction of mixed chimerism and tolerance after bone marrow transplantation without cytoreductive conditioning

Combining Treg Therapy With Donor Bone Marrow Transplantation: Experimental Progress and Clinical Perspective

Donor-specific tolerance remains a goal in transplantation because it could improve graft survival and reduce morbidity. Cotransplantation of donor hematopoietic cells to achieve chimerism is a promising approach for tolerance induction, which was successfully tested in clinical trials. However, current protocols are associated with side effects related to the myelosuppressive recipient conditioning, which makes it difficult to introduce them as standard therapy. More recently, adoptive cell therapy with polyclonal or donor-specific regulatory T cells (Treg) proved safe and feasible in several transplant trials, but it is unclear whether it can induce tolerance on its own. The combination of both approaches-Treg therapy and hematopoietic cell transplantation-leads to chimerism and tolerance without myelosuppressive treatment in murine models. Treg therapy promotes engraftment of allogeneic hematopoietic cells, reducing conditioning requirements and enhancing regulatory mechanisms maintaining tolerance. This review discusses possible modes of action of transferred Treg in experimental chimerism models and describes translational efforts investigating the potent synergy of Treg and chimerism.

Chimerism, Transplant Tolerance, and Beyond

The present review discusses current developments in tolerance induction for solid organ transplantation with a particular emphasis on chimerism-based approaches. It explains the basic mechanisms of chimerism-based tolerance and provides an update on ongoing clinical tolerance trials. The concept of "delayed tolerance" is presented, and ongoing preclinical studies in the nonhuman primate setting-including current limitations and hurdles regarding this approach-are illustrated. In addition, a brief overview and update on cell-based tolerogenic clinical trials is provided. In a critical approach, advantages, limitations, and potential implications for the future of these different regimens are discussed.

Stability of Chimerism in Non-Obese Diabetic Mice Achieved By Rapid T Cell Depletion Is Associated With High Levels of Donor Cells Very Early After Transplant

Stable mixed hematopoietic chimerism is a robust method for inducing donor-specific tolerance with the potential to prevent rejection of donor islets in recipients with autoimmune type-1 diabetes. However, with reduced intensity conditioning, fully allogeneic chimerism in a tolerance resistant autoimmune-prone non-obese diabetic (NOD) recipient has rarely been successful. In this setting, successful multilineage chimerism has required either partial major histocompatability complex matching, mega doses of bone marrow, or conditioning approaches that are not currently clinically feasible. Irradiation free protocols with moderate bone marrow doses have not generated full tolerance; donor skin grafts were rejected. We tested whether more efficient recipient T cell depletion would generate a more robust tolerance. We show that a combination of donor-specific transfusion-cyclophosphamide and multiple T cell depleting antibodies could induce stable high levels of fully allogeneic chimerism in NOD recipients. Less effective T cell depletion was associated with instability of chimerism. Stable chimeras appeared fully donor-specific tolerant, with clonal deletion of allospecific T cells and acceptance of donor skin grafts, while recovering substantial immunocompetence. The loss of chimerism months after transplant was significantly associated with a lower level of chimerism and donor T cells within the first 2 weeks after transplant. Thus, rapid and robust recipient T cell depletion allows for stable high levels of fully allogeneic chimerism and robust donor-specific tolerance in the stringent NOD model while using a clinically feasible protocol. In addition, these findings open the possibility of identifying recipients whose chimerism will later fail, stratifying patients for early intervention.

Transplantation Tolerance through Hematopoietic Chimerism: Progress and Challenges for Clinical Translation

The perception that transplantation of hematopoietic stem cells can confer tolerance to any tissue or organ from the same donor is widely accepted but it has not yet become a treatment option in clinical routine. The reasons for this are multifaceted but can generally be classified into safety and efficacy concerns that also became evident from the results of the first clinical pilot trials. In comparison to standard immunosuppressive therapies, the infection risk associated with the cytotoxic pre-conditioning necessary to allow allogeneic bone marrow engraftment and the risk of developing graft-vs.-host disease (GVHD) constitute the most prohibitive hurdles. However, several approaches have recently been developed at the experimental level to reduce or even overcome the necessity for cytoreductive conditioning, such as costimulation blockade, pro-apoptotic drugs, or Treg therapy. But even in the absence of any hazardous pretreatment, the recipients are exposed to the risk of developing GVHD as long as non-tolerant donor T cells are present. Total lymphoid irradiation and enriching the stem cell graft with facilitating cells emerged as potential strategies to reduce this peril. On the other hand, the long-lasting survival of kidney allografts, seen with transient chimerism in some clinical series, questions the need for durable chimerism for robust tolerance. From a safety point of view, loss of chimerism would indeed be favorable as it eliminates the risk of GVHD, but also complicates the assessment of tolerance. Therefore, other biomarkers are warranted to monitor tolerance and to identify those patients who can safely be weaned off immunosuppression. In addition to these safety concerns, the limited efficacy of the current pilot trials with approximately 40-60% patients becoming tolerant remains an important issue that needs to be resolved. Overall, the road ahead to clinical routine may still be rocky but the first successful long-term patients and progress in pre-clinical research provide encouraging evidence that deliberately inducing tolerance through hematopoietic chimerism might eventually make it from dream to reality.

Combining Treg therapy with mixed chimerism: Getting the best of both worlds

Deliberate establishment of donor-specific immunologic tolerance is considered to be the "Holy Grail" in transplantation medicine, but clinical tolerance protocols for routine organ transplantation are still an unmet need. Mixed hematopoietic chimerism is an attractive tolerance strategy with considerable potential. Recent pilot trials provide proof-of-principle that mixed chimerism can induce tolerance in renal transplant recipients. Routine clinical translation, however, is impeded by the side effects of the cytotoxic recipient conditioning necessary for the transient engraftment of HLA-mismatched BM. In murine studies recently published in The American Journal of Transplantation, we demonstrated that the therapeutic application of polyclonal recipient regulatory T cells (Tregs) leads to engraftment of practicable doses of fully allogeneic BM and to donor-specific tolerance without any cytotoxic conditioning, thereby eliminating a major impediment for the clinical translation of the mixed chimerism strategy in the experimental setting. The background and the implications of these findings are discussed.

Murine models of transplantation tolerance through mixed chimerism: advances and roadblocks

Organ transplantation is the treatment of choice for patients with end-stage organ failure, but chronic immunosuppression is taking its toll in terms of morbidity and poor efficacy in preventing late graft loss. Therefore, a drug-free state would be desirable where the recipient permanently accepts a donor organ while remaining otherwise fully immunologically competent. Mouse studies unveiled mixed chimerism as an effective approach to induce such donor-specific tolerance deliberately and laid the foundation for a series of clinical pilot trials. Nevertheless, its widespread clinical implementation is currently prevented by cytotoxic conditioning and limited efficacy. Therefore, the use of mouse studies remains an indispensable tool for the development of novel concepts with potential for translation and for the delineation of underlying tolerance mechanisms. Recent innovations developed in mice include the use of pro-apoptotic drugs or regulatory T cell (Treg ) transfer for promoting bone marrow engraftment in the absence of myelosuppression and new insight gained in the role of innate immunity and the interplay between deletion and regulation in maintaining tolerance in chimeras. Here, we review these and other recent advances in murine studies inducing transplantation tolerance through mixed chimerism and discuss both the advances and roadblocks of this approach.

Advances in targeting co-inhibitory and co-stimulatory pathways in transplantation settings: the Yin to the Yang of cancer immunotherapy

In the past decade, the power of harnessing T-cell co-signaling pathways has become increasingly understood to have significant clinical importance. In cancer immunotherapy, the field has concentrated on two related modalities: First, targeting cancer antigens through highly activated chimeric antigen T cells (CAR-Ts) and second, re-animating endogenous quiescent T cells through checkpoint blockade. In each of these strategies, the therapeutic goal is to re-ignite T-cell immunity, in order to eradicate tumors. In transplantation, there is also great interest in targeting T-cell co-signaling, but with the opposite goal: in this field, we seek the Yin to cancer immunotherapy's Yang, and focus on manipulating T-cell co-signaling to induce tolerance rather than activation. In this review, we discuss the major T-cell signaling pathways that are being investigated for tolerance induction, detailing preclinical studies and the path to the clinic for many of these molecules. These include blockade of co-stimulation pathways and agonism of coinhibitory pathways, in order to achieve the delicate state of balance that is transplant tolerance: a state which guarantees lifelong transplant acceptance without ongoing immunosuppression, and with preservation of protective immune responses. In the context of the clinical translation of immune tolerance strategies, we discuss the significant challenge that is embodied by the fact that targeted pathway modulators may have opposing effects on tolerance based on their impact on effector vs regulatory T-cell biology. Achieving this delicate balance holds the key to the major challenge of transplantation: lifelong control of alloreactivity while maintaining an otherwise intact immune system.

Transplant Tolerance Induction in Newborn Infants: Mechanisms, Advantages, and Potential Strategies

Although several tolerance induction protocols have been successfully implemented in adult renal transplantation, no tolerance induction approach has, as yet, been defined for solid organ transplantations in young infants. Pediatric transplant recipients have a pressing demand for the elaboration of tolerance induction regimens. Indeed, since they display a longer survival time, they are exposed to a higher level of risks linked to long-term immunosuppression (IS) and to chronic rejection. Interestingly, central tolerance induction may be of great interest in newborns, because of their immunological immaturity and the important role of the thymus at this early stage in life. The present review aims to clarify mechanisms and strategies of tolerance induction in these immunologically premature recipients. We first introduce the discovery and mechanisms of neonatal tolerance in murine experimental models and subsequently analyze tolerance induction in human newborn infants. Hematopoietic mixed chimerism in neonates is also discussed based on in utero hematopoietic stem cell (HSC) transplant studies. Then, we review the recent advances in tolerance induction approaches in adults, including the infusion of HSCs associated with less toxic conditioning regimens, regulatory T cells/facilitating cells/mesenchymal stem cells transplantation, costimulatory blockade, and thymus manipulation. Finally, IS withdrawal in pediatric solid organ transplant is discussed. In conclusion, the establishment of transplant tolerance induction in infants is promising and deserves further investigations. Future studies could focus on the selection of patients, on less toxic conditioning regimens, and on biomarkers for IS minimization or withdrawal.

Alloreactive Regulatory T Cells Allow the Generation of Mixed Chimerism and Transplant Tolerance

The induction of donor-specific transplant tolerance is one of the main goals of modern immunology. Establishment of a mixed chimerism state in the transplant recipient has proven to be a suitable strategy for the induction of long-term allograft tolerance; however, current experimental recipient preconditioning protocols have many side effects, and are not feasible for use in future therapies. In order to improve the current mixed chimerism induction protocols, we developed a non-myeloablative bone-marrow transplant (NM-BMT) protocol using retinoic acid (RA)-induced alloantigen-specific Tregs, clinically available immunosuppressive drugs, and lower doses of irradiation. We demonstrate that RA-induced alloantigen-specific Tregs in addition to a NM-BMT protocol generates stable mixed chimerism and induces tolerance to allogeneic secondary skin allografts in mice. Therefore, the establishment of mixed chimerism through the use of donor-specific Tregs rather than non-specific immunosuppression could have a potential use in organ transplantation.

Rapamycin and CTLA4Ig synergize to induce stable mixed chimerism without the need for CD40 blockade

The mixed chimerism approach achieves donor-specific tolerance in organ transplantation, but clinical use is inhibited by the toxicities of current bone marrow (BM) transplantation (BMT) protocols. Blocking the CD40:CD154 pathway with anti-CD154 monoclonal antibodies (mAbs) is exceptionally potent in inducing mixed chimerism, but these mAbs are clinically not available. Defining the roles of donor and recipient CD40 in a murine allogeneic BMT model, we show that CD4 or CD8 activation through an intact direct or CD4 T cell activation through the indirect pathway is sufficient to trigger BM rejection despite CTLA4Ig treatment. In the absence of CD4 T cells, CD8 T cell activation via the direct pathway, in contrast, leads to a state of split tolerance. Interruption of the CD40 signals in both the direct and indirect pathway of allorecognition or lack of recipient CD154 is required for the induction of chimerism and tolerance. We developed a novel BMT protocol that induces mixed chimerism and donor-specific tolerance to fully mismatched cardiac allografts relying on CD28 costimulation blockade and mTOR inhibition without targeting the CD40 pathway. Notably, MHC-mismatched/minor antigen-matched skin grafts survive indefinitely whereas fully mismatched grafts are rejected, suggesting that non-MHC antigens cause graft rejection and split tolerance.

Influence of immune responses in gene/stem cell therapies for muscular dystrophies

Muscular dystrophies (MDs) are a heterogeneous group of diseases, caused by mutations in different components of sarcolemma, extracellular matrix, or enzymes. Inflammation and innate or adaptive immune response activation are prominent features of MDs. Various therapies under development are directed toward rescuing the dystrophic muscle damage using gene transfer or cell therapy. Here we discussed current knowledge about involvement of immune system responses to experimental therapies in MDs.

Mechanistic and therapeutic role of regulatory T cells in tolerance through mixed chimerism

PURPOSE OF REVIEW

Although substantial advances in transplantation medicine have improved short-term graft survival, long-term outcome after organ transplantation is unsatisfactory. The induction of donor-specific tolerance as a potential solution remains an unmet need. Mixed chimerism established through transplantation of donor bone marrow is an appealing tolerance strategy, but widespread clinical application is prevented by the toxicity of recipient conditioning, which is required for achieving bone marrow engraftment. Clonal deletion - both central and peripheral - has long been recognized as a cardinal mechanism in experimental mixed chimerism models.

RECENT FINDINGS

Several recent studies have delineated the importance of nondeletional, regulatory mechanisms for the induction of tolerance through mixed chimerism. Moreover, the therapeutic application of recipient regulatory T cells (Tregs) has been combined with the transplantation of donor bone marrow. Such a 'Treg-chimerism' protocol leads to engraftment of conventional doses of fully allogeneic bone marrow and to donor-specific tolerance without the need for any cytotoxic conditioning.

SUMMARY

Regulatory mechanisms play a major role in mixed chimerism protocols. Treg therapy is exceptionally effective in achieving bone marrow engraftment without cytotoxic recipient treatment, thereby eliminating a major toxic factor preventing widespread application of the mixed chimerism strategy.

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.

Mixed chimerism through donor bone marrow transplantation: a tolerogenic cell therapy for application in organ transplantation

PURPOSE OF REVIEW

Organ transplantation is the state-of-the-art treatment for end-stage organ failure; however, long-term graft survival is still unsatisfactory. Despite improved immunosuppressive drug therapy, patients are faced with substantial side effects and the risk of chronic rejection with subsequent graft loss. The transplantation of donor bone marrow for the induction of mixed chimerism has been recognized to induce donor-specific tolerance a long time ago, but safety concerns regarding toxicities of current bone marrow transplantation (BMT) protocols impede widespread application.

RECENT FINDINGS

Recent studies in nonhuman primates and kidney transplant patients have demonstrated successful induction of allograft tolerance even though--in contrast to murine models--only transient chimerism was achieved. Progress toward the development of nontoxic murine BMT protocols revealed that Treg therapy is a potent therapeutic adjunct eliminating the need for cytotoxic recipient conditioning. Furthermore, new insight into the mechanisms underlying tolerization of CD4 and CD8 T cells in mixed chimeras has been gained and has identified possible difficulties impeding clinical translation.

SUMMARY

This review will address the recent advances in murine models as well as findings from the first clinical trials for the induction of tolerance through mixed chimerism. Both the potential for more widespread clinical application and the remaining hurdles and challenges of this tolerance approach will be discussed.

Anti-LFA-1 or rapamycin overcome costimulation blockade-resistant rejection in sensitized bone marrow recipients

While costimulation blockade-based mixed chimerism protocols work well for inducing tolerance in rodents, translation to preclinical large animal/nonhuman primate models has been less successful. One recognized cause for these difficulties is the high frequency of alloreactive memory T cells (Tmem) found in the (pre)clinical setting as opposed to laboratory mice. In the present study, we therefore developed a murine bone marrow transplantation (BMT) model employing recipients harboring polyclonal donor-reactive Tmem without concomitant humoral sensitization. This model was then used to identify strategies to overcome this additional immune barrier. We found that B6 recipients that were enriched with 3 × 10(7) T cells isolated from B6 mice that had been previously grafted with Balb/c skin, rejected Balb/c BM despite costimulation blockade with anti-CD40L and CTLA4Ig (while recipients not enriched developed chimerism). Adjunctive short-term treatment of sensitized BMT recipients with rapamycin or anti-LFA-1 mAb was demonstrated to be effective in controlling Tmem in this model, leading to long-term mixed chimerism and donor-specific tolerance. Thus, rapamycin and anti-LFA-1 mAb are effective in overcoming the potent barrier that donor-reactive Tmem pose to the induction of mixed chimerism and tolerance despite costimulation blockade.

IDO-competent-DCs induced by IFN-γ attenuate acute rejection in rat liver transplantation

PURPOSE

We established a stable rat model of liver transplantation using Sprague-Dawley rats and Wistar rats in order to investigate the role of the IDO gene in acute rejection after rat liver transplantation.

METHODS

IDO gene expression and IDO enzyme activity were quantified in liver syngeneic grafts and allografts using microdialysis-HPLC. Liver allografts were evaluated for IDO expression by histopathology. We measured liver function-related biomarkers in liver allografts which were re-infused with untreated or IFN-γ-treated dendritic cells (DCs).

RESULTS

We found a significant increase in IDO gene expression and IDO enzyme activity in liver allografts compared the sham and syngeneic graft groups. There was a significant correlation between the number of IDO-positive cells and severity of acute rejection. IDO gene expression and enzyme activity was upregulated in the IFN-γ-treated DC group within 7 days after transplantation compared to the untreated DC group and survival rates were significantly improved.

CONCLUSIONS

Our results suggested that IDO gene expression correlates with the severity of acute rejection and that IFN-γ-induced IDO-positive DCs may attenuate acute rejection and catalyze local tryptophan metabolism via IDO enzyme expression, leading to immune tolerance after liver transplantation.

Therapeutic efficacy of polyclonal tregs does not require rapamycin in a low-dose irradiation bone marrow transplantation model

BACKGROUND

Mixed chimerism is an effective strategy for the induction of transplantation tolerance but the toxicity of recipient conditioning makes current bone marrow (BM) transplantation (BMT) protocols unsuitable for widespread clinical application. Therapies promoting BM engraftment under minimal conditioning would facilitate translation of this concept to the clinic. Recently, we have shown that regulatory T cell (Treg) therapy has potent engraftment-enhancing effects in an irradiation-free noncytotoxic BMT protocol, but only if it is combined with rapamycin treatment.

METHODS

Here, we investigated whether polyclonal Treg therapy is effective in promoting chimerism and tolerance in an otherwise unsuccessful BMT protocol using low-dose total body irradiation (1 Gy) and costimulation blockade and determined whether Tregs do so on their own without rapamycin.

RESULTS

The application of polyclonal FoxP3-transduced recipient Tregs led to durable multilineage chimerism and donor-specific skin graft tolerance whereas recipients receiving costimulation blockade alone or green flourescent protein (GFP)-transduced cells failed to develop chimerism. Infused Tregs had a limited life span as indicated by polymerase chain reaction analysis but rather contribute to de novo induction of subsequent Treg generations. Deletion of donor-reactive T cells was observed but progressed more slowly over time compared with recipients of a nonmyeloablative BMT protocol using 3 Gy total body irradiation.

CONCLUSIONS

In conclusion, Treg therapy promotes BM engraftment on its own in a low-dose irradiation BMT protocol, leading to chimerism and tolerance maintained through deletional and nondeletional mechanisms.

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.

Cell-based therapy in allergy

IgE-mediated allergy is an immunological disorder occurring in response to otherwise harmless environmental antigens (i.e., allergens). Development of effective therapeutic or preventive approaches inducing robust tolerance toward allergens remains an unmet goal. Several experimental tolerance approaches have been described. The therapeutic use of regulatory T cells (Tregs) and the establishment of molecular chimerism are two cell-based strategies that are of particular interest. Treg therapy is close to clinical application, but its efficacy remains to be fully defined. Recent proof-of-concept studies demonstrated that transplantation of syngeneic hematopoietic stem cells modified in vitro to express a major allergen leads to molecular chimerism and robust allergen-specific tolerance. Here we review cell-based tolerance strategies in allergy, discussing their potentials and limitations.

Induction of mixed chimerism in mice by employing different conditioning protocols and bone marrow cell transplantation

Mixed chimerism has been suggested to produce allograft tolerance. Since this phenomenon is not fully understood, the aim of our study was to evaluate various protocols for chimerism induction in a mouse model. B6.SJL-Ptprc(a)Pep3(b) mice were injected with 20 to 30 x 10(6) bone marrow cells from Balb C mice. Conditioning consisted of total body gamma irradiation with 9.5, 5, and 3 Gy on "-1 day" of the experiment, with 200 mg/kg cyclophosphamide (CP) ("+2 day"). Additionally, one group of mice received blocking antibody against CD40L on days 0, 1, 4, and 7. The presence of mixed chimerism in peripheral blood was assessed at 1, 2, 3, 4, 6, and 8 weeks using flow cytometry to detect CD45.1 or CD45.2 antigen expression. Moreover, the chimerism was examined in CD4, CD8, CD45/B220, Mac-1alpha subpopulations in peripheral blood and bone marrow cells at 8 weeks. We also compared chimerism in peripheral blood, bone marrow, and spleen leukocyte populations. We observed that the most effective conditioning method with relatively low toxicity was based on concomitant use of 5 Gy total body irradiation and CP. The percentage of donor cells differed among peripheral blood subpopulations and bone marrow cells, but was similar in leukocyte populations derived from various sources. Our experiments sought to optimize the induction of stable mixed chimerism.

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.

Murine mobilized peripheral blood stem cells have a lower capacity than bone marrow to induce mixed chimerism and tolerance

Allogeneic bone marrow transplantation (BMT) under costimulation blockade allows induction of mixed chimerism and tolerance without global T-cell depletion (TCD). The mildest such protocols without recipient cytoreduction, however, require clinically impracticable bone marrow (BM) doses. The successful use of mobilized peripheral blood stem cells (PBSC) instead of BM in such regimens would provide a substantial advance, allowing transplantation of higher doses of hematopoietic donor cells. We thus transplanted fully allogeneic murine granulocyte colony-stimulating factor (G-CSF) mobilized PBSC under costimulation blockade (anti-CD40L and CTLA4Ig). Unexpectedly, PBSC did not engraft, even when very high cell doses together with nonmyeloablative total body irradiation (TBI) were used. We show that, paradoxically, T cells contained in the donor PBSC triggered rejection of the transplanted donor cells. Rejection of donor BM was also triggered by the cotransplantation of unmanipulated donor T cells isolated from naïve (nonmobilized) donors. Donor-specific transfusion and transient immunosuppression prevented PBSC-triggered rejection and mixed chimerism and tolerance were achieved, but graft-versus-host disease (GVHD) occurred. The combination of in vivo TCD with costimulation blockade prevented rejection and GVHD. Thus, if allogeneic PBSC are transplanted instead of BM, costimulation blockade alone does not induce chimerism and tolerance without unacceptable GVHD-toxicity, and the addition of TCD is required for success.

Rapamycin, mycophenolate mofetil, methylprednisolone, and cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin-based conditioning regimen to induce partial tolerance to hind limb allografts without cytoreductive conditioning

BACKGROUND

Composite tissue allograft transplantation may represent the next frontier in the field of reconstructive surgery. However, the main obstacles precluding the routine use of composite tissue allotransplants are rejection and toxicity associated with life-long immunosuppressive therapy. In this study, we investigated a nontoxic immunosuppressant and cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin (CTLA4-Ig)-based protocol to induce donor-specific tolerance to hind limb allografts in rats.

METHODS

Fully mismatched, 4- to 10-week-old Brown Norway (BN, RT1n) and Lewis (RT1) rats were used as cell/organ donors and recipients, respectively. Recipients were treated with CTLA4-Ig (2 mg/kg/d) on days -30, -28, -26, -24, and -22, rapamycin, mycophenolate mofetil, and methylprednisolone (RAPA/MMF/MP) combined therapy (from days -30 to day 100), a single dose of anti-lymphocyte serum (10 mg, on day -30), and donor bone marrow (10 x 10(7) T-cell-depleted cells) transplantation (BMT, on day -30). Thirty days after BMT, chimeric animals received hind limb allotransplantations (on day 0). The RAPA/MMF/MP combined therapy was changed to Cyclosporine (CsA, 8 mg/kg/d) on day 100 and maintained thereafter at this level.

RESULTS

Hematopoietic chimerism of 17.6 +/- 9.5% at day 0, was stable (15.2 +/- 5.6%) at 230 days post-BMT; there was no sign of graft-versus-host disease. Chimeric recipients (Lewis) permanently accepted (>200 days) donor (BN)-specific (RT11, n = 6) hind limbs, yet rapidly rejected (20 +/- 2 days) third-party hind limbs (Wistar Furth [WF]). Lymphocytes of graft-tolerant animals demonstrated hyporesponsiveness in mixed lymphocyte cultures in a donor-specific manner. Tolerant graft histology showed no signs of acute and chronic rejection.

CONCLUSIONS

The immunosuppressant and CTLA4-Ig-based conditioning regimen with donor BMT produced mixed chimerism and induced partial donor-specific tolerance to hind limb allografts.

Expansion of effector memory TCR Vbeta4+ CD8+ T cells is associated with latent infection-mediated resistance to transplantation tolerance

Therapies that control largely T cell-dependent allograft rejection in humans also possess the undesirable effect of impairing T cell function, leaving transplant recipients susceptible to opportunistic viruses. Prime among these opportunists are the ubiquitous herpesviruses. To date, studies are lacking that address the effect of viruses that establish a true latent state on allograft tolerance or the effect of tolerance protocols on the immune control of latent viruses. By using a mixed chimerism-based tolerance-induction protocol, we found that mice undergoing latent infection with gammaHV68, a murine gamma-herpesvirus closely related to human gamma-herpesviruses such as EBV and Kaposi's sarcoma-associated herpesvirus, significantly resist tolerance to allografts. Limiting the degree of virus reactivation or innate immune response did not reconstitute chimerism in latently infected mice. However, gammaHV68-infected mice showed increased frequency of CD8+ T cell alloreactivity and, interestingly, expansion of virus-induced, alloreactive, "effector/effector memory" TCR Vbeta4+CD8+ T cells driven by the gammaHV68-M1 gene was associated with resistance to tolerance induction in studies using gammaHV68-M1 mutant virus. These results define the viral gene and immune cell types involved in latent infection-mediated resistance to allograft tolerance and underscore the influence of latent herpesviruses on allograft survival.

Different mechanisms control peripheral and central tolerance in hematopoietic chimeric mice

Regulatory T cells (Treg) are important in peripheral tolerance, but their role in establishing and maintaining hematopoietic mixed chimerism and generating central tolerance is unclear. We now show that costimulation blockade using a donor-specific transfusion and anti-CD154 antibody applied to mice given bone marrow and simultaneously transplanted with skin allografts leads to hematopoietic chimerism and permanent skin allograft survival. Chimeric mice bearing intact skin allografts fail to generate effector/memory T cells against allogeneic targets as shown by the absence of IFNgamma-producing CD44(high)CD8+ T cells and in vivo cytotoxicity. Depletion of Tregs by injection of anti-CD4 or anti-CD25 antibody prior to costimulation blockade prevents chimerism, shortens skin allograft survival and leads to generation of effector/memory cytotoxic T cells. Depletion of Tregs by injection of anti-CD4 or anti-CD25 antibody two months after transplantation leads to loss of skin allografts even though mice remain chimeric and exhibit little in vivo cytotoxicity. In contrast, chimerism is lost, but skin allografts survive following naïve T-cell injection. We conclude that hematopoietic chimerism and peripheral tolerance may be maintained by different mechanisms in mixed hematopoietic chimeras.

Recent Progress in Tolerance Induction through Mixed Chimerism

Organ transplant recipients require life-long treatment with immunosuppressive drugs. Currently available immunosuppression is associated with substantial morbidity and mortality, and is ineffective in inhibiting chronic rejection and graft loss. Therefore, a permanent state of donor-specific tolerance remains a primary goal for transplantation research. The induction of mixed hematopoietic chimerism is an attractive concept in this regard. Hematopoietic chimerism modulates the immunologic repertoire by extending the mechanisms of self-tolerance to donor-specific allotolerance. Despite recent progress in developing nontoxic bone marrow transplantation protocols for rodents, translation to large animals has remained difficult. Here, we outline the concept of tolerance via mixed chimerism, and review recent progress and remaining challenges in bringing this approach to the clinical setting.

Allogeneic bone marrow transplantation in models of experimental autoimmune encephalomyelitis: evidence for a graft-versus-autoimmunity effect

Autologous hematopoietic stem cell transplantation (HSCT) is being explored in the treatment of severe multiple sclerosis (MS), and is based on the concept of "resetting" the immune system. The use of allogeneic HSCT may offer additional advantages, such as the replacement of the autoreactive immune compartment by healthy allogeneic cells and development of a graft-versus-autoimmunity (GVA) effect. However, in clinical practice, the genetic susceptibility to MS of allogeneic stem cell donors is generally unknown, and GVA may therefore be an important mechanism of action. Experimental autoimmune encephalomyelitis (EAE)-susceptible and -resistant mouse strains were used to determine the roles of genetic susceptibility, level of donor-chimerism, and alloreactivity in the therapeutic potential of syngeneic versus allogeneic bone marrow transplant (BMT) for EAE. After transplantation and EAE induction, animals were evaluated for clinical EAE and ex vivo myelin oligodendrocyte glycoprotein-specific proliferation. Early after BMT, both syngeneic and allogeneic chimeras were protected from EAE development. On the longer term, allogeneic but not syngeneic BMT conferred protection, but this required high-level donor-chimerism from EAE-resistant donors. Importantly, when EAE-susceptible donors were used, robust protection from EAE was obtained when active alloreactivity, induced by donor lymphocyte infusions, was provided. Our findings indicate the requirement of a sufficient level of donor-chimerism from a nonsusceptible donor in the therapeutic effect of allogeneic BMT. Importantly, the data indicate that, independently of genetic susceptibility, active alloreactivity is associated with a GVA effect, thereby providing new evidence to support the potential role of allogeneic BMT in the treatment of MS.

Role of thymus in operational tolerance induction in limb allograft transplant model

BACKGROUND

In this study, we evaluated the role of host thymus in tolerance induction in composite tissue allografts (CTA) across major histocompatibility complex (MHC) barrier during a 7-day alphabeta- T-cell receptor (TCR)/ cyclosporine A (CsA) protocol.

MATERIALS AND METHODS

A total of 62 limb allograft transplants were studied. Euthymic (group A) and thymectomized (group B) Lewis recipients (LEW, RT1(1)) received vascularized hind-limb allografts from hybrid Lewis x Brown-Norway (F1), (LBN, RT1(1+n)) donors. Mixed lymphocyte reaction (MLR) and skin grafting assessed donor-specific tolerance in vitro and in vivo, respectively. Flow cytometry determined the efficacy of immunosuppressive protocols and the presence of donor-specific chimerism. Immunocytochemistry revealed the presence of donor-specific cells in the lymphoid organs of recipients.

RESULTS

Isograft transplants survived indefinitely. For thymectomized rats, the median survival time (MST) of limb allograft in non-treated recipients was 7 days; monotherapy with alphabeta-TCR extended MST to 16 days, and CsA therapy extended it to 30 days. Using the alphabeta-TCR/CsA protocol, the MST of allografts was 51 days. For euthymic rats, the MST of limb allograft in non-treated recipients was 7 days; monotherapy with alphabeta-TCR or CsA extended MST to 13 or 22 days, respectively. Treatment with alphabeta-TCR/CsA resulted in indefinite allografts survival (MST=370 days). MLR and skin grafting confirmed donor-specific tolerance in euthymic recipients. Flow cytometry showed stable chimerism in the euthymic rats and transient chimerism in thymectomized limb recipients. Immunoperoxidase staining revealed the persistence of donor-derived cells in the lymphoid tissues of euthymic recipients.

CONCLUSION

We found that the presence of thymus was imperative for the induction of donor-specific tolerance in rat hind-limb composite tissue allografts using a alphabeta-TCR/CsA protocol.

Immunomodulation of transgene responses following naked DNA transfer of human factor VIII into hemophilia A mice

A robust humoral immune response against human factor VIII (hFVIII) following naked DNA transfer into immunocompetent hemophilia A mice completely inhibits circulating FVIII activity despite initial high-level hFVIII gene expression. To prevent this undesirable response, we compared transient immunomodulation strategies. Eight groups of mice (n = 4-9 per group) were treated with naked DNA transfer of pBS-HCRHPI-hFVIIIA simultaneously with immunosuppressive reagents that included cyclosporine A (CSA), rapamycin (RAP), mycophenylate mofetil (MMF), a combination of CSA and MMF, a combination of RAP and MMF, a monoclonal antibody against murine CD40 ligand (MR1), recombinant murine Ctla4Ig, and a combination of MR1 and Ctla4Ig. All animals except those receiving only CSA exhibited delayed or absent immune responses against hFVIII. The most effective immunosuppressive regimen, the combination of Ctla4Ig and MR1, prevented inhibitor formation in 8 of 9 animals; the ninth had transient low-titer antibodies. All 9 mice of this group produced persistent, therapeutic levels of hFVIII for more than 6 months. When challenged with the T-dependent antigen bacteriophage Phix174, tolerized mice exhibited normal primary and secondary antibody responses, suggesting that transient immunomodulation to disrupt B/T-cell interaction at the time of plasmid injection effectively promoted long-term immune tolerance specific for hFVIII.

Stem cells and diabetes treatment

Diabetes mellitus types 1 and 2 are characterized by absolute versus relative lack of insulin-producing beta cells, respectively. Reconstitution of a functional beta-cell mass by cell therapy--using organ donor islets of Langerhans--has been demonstrated to restore euglycaemia in the absence of insulin treatment. This remarkable achievement has stimulated the search for appropriate stem cell sources from which adequate expansion and maturation of therapeutic beta cells can be achieved. This recent activity is reviewed and presented with particular focus on directed differentiation from pluripotent embryonic stem cells (versus other stem/progenitor cell sources) based on knowledge from pancreatic beta-cell development and the parallel approach to controlling endogenous beta-cell neogenesis.

Induction of Mixed Chimerism through Transplantation of CD45-Congenic Mobilized Peripheral Blood Stem Cells after Nonmyeloablative Irradiation

Clinical translation of the mixed-chimerism approach for inducing transplantation tolerance would be facilitated if mobilized peripheral blood stem cells (mPBSCs) could be used instead of bone marrow cells (BMCs). Because the use of mPBSCs for this purpose has not been investigated in nonmyeloablative murine protocols, we explored the engraftment potential of mPBSCs in a CD45-congenic model as a first step. After 2, 1.5, or 1 Gy of total body irradiation, CD45.1 B6 hosts received unseparated granulocyte colony-stimulating factor-mobilized CD45.2 B6 PBSCs or unseparated CD45.2 B6 BMCs. The same total cell numbers, or aliquots of mPBSCs and BMCs containing similar numbers of c-kit+ cells, were transplanted both with and without a short course of rapamycin-based immunosuppression (IS). Transplantation of mPBSCs induced long-term multilineage macrochimerism, but chimerism levels were significantly lower than among recipients of the same number of BMCs. Transplanting aliquots containing similar numbers of c-kit+ cells reduced the difference between mPBSCs and BMCs, but lower levels of chimerism were nonetheless observed in mPBSC recipients. Chimerism levels correlated more closely with the number of transplanted progenitor cells as determined by colony-forming unit assays. IS did not affect chimerism levels, indicating that the donor CD45 isoform or other minor disparities do not pose a major barrier to engraftment. Our findings indicate that under nonmyeloablative conditions, progenitor cells contained in mPBSCs have an engraftment capacity similar to progenitor cells from BMCs, allowing induction of lasting mixed chimerism with moderate cell numbers. On a cell-per-cell basis, unseparated BMCs have some advantages that may be minimized if the number of progenitor cells is equalized. These results are expected to facilitate the development of mPBSC-based allogeneic tolerance protocols.