Mixed chimerism as an approach for the induction of transplantation tolerance

Chimerism and tolerance: past, present and future strategies to prolong renal allograft survival

PURPOSE OF REVIEW

Immunological factors are a major cause of kidney allograft loss. Calcineurin inhibitors (CNIs) have improved short-term kidney allograft survival; however, they in turn contribute to long-term kidney allograft loss from chronic CNI nephrotoxicity. Tolerance induction in transplantation can avoid the long-term adverse effects of immunosuppressive medications. This review aims to critically discuss recent efforts in inducing transplantation tolerance.

RECENT FINDINGS

Tolerance induction mediated by chimerism has shown some promise in minimizing or even complete withdrawal of immunosuppressive treatments in kidney allograft recipients. There has been a number of approaches as varied as the number of centres conducting these trials. However, they can be grouped into those mediated by transient microchimerism and those facilitated by more stable macro or full donor chimerism. The success rates in terms of long-term drug-free graft survival has been limited in microchimerism-mediated tolerance induction approaches. Mixed macrochimerism of less than 50% donor may be unstable with mostly the recipient's native immune system overpowering the donor chimeric status.Tolerance induction leading to chimerism has been limited to living donor kidney transplantation and additional long-term outcomes are required. Furthermore, immune monitoring after tolerance induction has faced a limitation in studying due to a lack of sufficient study participants and appropriate study controls.

SUMMARY

Tolerance induction is one of several strategies used to prolong kidney allograft survival, but it has not been routinely utilized in clinical practice. However, future applications from the trials to clinical practice remain limited to living donor kidney transplantation. Once further data regarding tolerance inductions exist and practicality becomes widely accepted, tolerance induction may shift the paradigm in the field of kidney transplantation to achieve the best possible outcome of 'One Organ for Life'.

Immunological Consequences of In Utero Exposure to Foreign Antigens

Immunologic tolerance refers to a state of immune nonreactivity specific to particular antigens as an important issue in the field of transplantation and the management of autoimmune diseases. Tolerance conceptually originated from Owen’s observation of blood cell sharing in twin calves. Owen’s conceptual framework subsequently constituted the backbone of Medawar’s “actively acquired tolerance” as the major tenet of modern immunology. Based upon this knowledge, the delivery of genetically distinct hematopoietic stem cells into pre-immune fetuses represented a novel and unique approach to their engraftment without the requirement of myeloablation or immunosuppression. It might also make fetal recipients commit donor alloantigens to memory of their patterns as “self” so as to create a state of donor-specific tolerance. Over the years, the effort made experimentally or clinically toward in utero marrow transplantation could not reliably yield sufficient hematopoietic chimerism for curing candidate diseases as anticipated, nor did allogeneic graft tolerance universally develop as envisaged by Medawar following in utero exposure to various forms of alloantigens from exosomes, lymphocytes or marrow cells. Enduring graft tolerance was only conditional on a state of significant hematopoietic chimerism conferred by marrow inocula. Notably, fetal exposure to ovalbumin, oncoprotein and microbial antigens did not elicit immune tolerance, but instead triggered an event of sensitization to the antigens inoculated. These fetal immunogenic events might be clinically relevant to prenatal imprinting of atopy, immune surveillance against developmental tumorigenesis, and prenatal immunization against infectious diseases. Briefly, the immunological consequences of fetal exposure to foreign antigens could be tolerogenic or immunogenic, relying upon the type or nature of antigens introduced. Thus, the classical school of “actively acquired tolerance” might oversimplify the interactions between developing fetal immune system and antigens. Such interactions might rely upon fetal macrophages, which showed up earlier than lymphocytes and were competent to phagocytose foreign antigens so as to bridge toward antigen-specific adaptive immunity later on in life. Thus, innate fetal macrophages may be the potential basis for exploring how the immunological outcome of fetal exposure to foreign antigens is determined to improve the likelihood and reliability of manipulating fetal immune system toward tolerization or immunization to antigens.

Flt3-L Augments the Engraftment of Donor-Derived Bone Marrow Cells When Combined with Sublethal Irradiation and Costimulatory (CD28/B7 and CD40/CD40L) Blockade

T-cell costimulatory blockade as a constituent for recipient conditioning prior to bone marrow transplantation has led to the development of less toxic protocols for the establishment of donor cell chimerism. We therefore hypothesized that the addition of the hematopoietic growth factor, Flt3-ligand (Flt3-L), to the perioperative inhibition of the CD28/B7 and CD40/CD40 ligand costimulatory pathways would enhance the engraftment of allogeneic bone marrow. Recipient BALB/c ByJ (H-2d, Mlsc, Vβ6+/Vβ8+ TCR) received a single sublethal dose of total body irradiation (300 rad) 6 h prior to transplantation IV with unfractionated donor CBA/J (H-2k, Mlsd, Vβ6-/Vβ8+ TCR) bone marrow cells. CTLA4-Ig and/or MR1 were administered at 500 μg IP on days 0, 2, 4, and 6 posttransplantation. Flt3-L was administered at 10 μg IP on days 0–6. Donor cell chimerism was determined on days 30–90 by flow cytometric analysis. Donor-specific tolerance was assessed by skin grafting. In vitro TCR cross-linking assays and flow cytometry were utilized to explore the deletion of donor-reactive T cells. Recipients receiving CTLA4-Ig and MR1 engrafted allogeneic bone marrow cells in the peripheral blood (3/6; 50%) with chimerism being detected at 2–31%. Addition of Flt3-L to this preconditioning regimen enhanced the incidence of engraftment of donor bone marrow cells (10/13; 3–70%). Long-term survival of donor but not third-party-specific skin grafts demonstrated that donor-specific tolerance had been achieved in the chimeric recipients. Deletion of the donor-reactive T cells within the chimeric recipients was also observed. The addition of hematopoietic growth factors and cytokines to the nonmyeloablative regimen of sublethal irradiation and T-cell costimulatory blockade provides a novel strategy for the establishment of donor cell chimerism and for the induction of stable and robust donor-specific tolerance. The deletion of donor-reactive T cells using this protocol suggests the reliability and feasibility of this protocol for clinical transplantation.

Depletion of foxp3(+) T cells abrogates tolerance of skin and heart allografts in murine mixed chimeras without the loss of mixed chimerism

The relative contribution of central and peripheral mechanisms to the generation and maintenance of allograft tolerance is of considerable interest. Here, we present new evidence that regulatory T cells (Foxp3(+) ) maintain skin and heart allograft tolerance in mixed hematopoietic chimeric mice. Transient depletion of both donor- and recipient-derived Foxp3(+) cells was necessary and sufficient to induce decisive rejection of long-accepted skin and heart allografts. In contrast, stable hematopoietic chimerism remained, and there was no detectable induction of donor-specific reactivity to hematopoietic cells. Foxp3(+) cell depletion did not result in the rejection of skin grafts of only MHC-disparate donors (B6.C-H2(d) /bByJ), indicating that MHC antigens were not the target in the graft. We conclude that two different mechanisms of tolerance are present in mixed chimeras. Hematopoietic chimerism, resistant to Foxp3(+) depletion, is probably due to deletional tolerance to MHC antigens, as supported by previous studies. In contrast, regulatory tolerance mechanisms involving Foxp3(+) cells are required to control reactivity against non-MHC antigens not present on hematopoietic lineages.

Recipients with in utero induction of tolerance upregulated MHC class I in the engrafted donor skin

The alterations in MHC class I expression play a crucial step in immune evasion of cancer or virus-infected cells. This study aimed to examine whether tolerized grafts modified MHC class I expression. FVB/N mice were rendered tolerant of C57BL/6 alloantigens by in utero transplantation of C57BL/6 marrows. Postnatally, engrafted donor skins and leukocytes were examined for their MHC expression by quantitative real-time PCR and flow cytometry. Engrafted donor skins upregulated their MHC class I related gene transcripts after short-term (1~2 weeks) or long-term (>1 month) engraftment. This biological phenomenon was simultaneously associated with upregulation of TAP1 gene transcripts, suggesting an important role of TAP1 in the regulation of MHC class I pathway. The surface MHC class I molecules of H-2K^b in engrafted donor leukocytes consistently showed overexpression. Conclusively, the induction of allograft tolerance involved biological modifications of donor transplants. The overexpression of MHC class I within engrafted transplants of tolerant mice might be used as the tolerance biomarkers for identifying a state of graft tolerance.

Role of regulatory T cells in transferable immunological tolerance to bone marrow donor in murine mixed chimerism model

Constructing a bone marrow chimera prior to graft transplantation can induce donor-specific immune tolerance. Mixed chimerism containing hematopoietic cells of both recipient- and donor-origin has advantages attributed from low dose of total body irradiation. In this study, we explored the mechanism of mixed chimerism supplemented with depletion of Natural Killer cells. Mixed chimerism with C57BL/6 bone marrow cells was induced in recipient BALB/c mice which were given 450 cGy of γ-ray irradiation (n = 16). As revealed by reduced proliferation and cytokine production in mixed leukocyte reaction and ELISpot assay (24.6 vs 265.5), the allo-immune response to bone marrow donor was reduced. Furthermore, the induction of transferable immunological tolerance was confirmed by adoptive transfer and subsequent acceptance of C57BL/6 skin graft (n = 4). CD4(+)FoxP3(+) regulatory T cells were increased in the recipient compartment of the mixed chimera (19.2% → 33.8%). This suggests that regulatory T cells may be therapeutically used for the induction of graft-specific tolerance by mixed chimerism.

Induction of tolerance of vascularized composite allografts

Vascularized composite allotransplantation has become established as a clinical specialty since the first successful hand transplant was performed in 1998. Data now available indicate that hand and face transplants offer patients good functional outcomes and significant improvements in quality of life. Despite the debilitating nature of the injuries treated by such transplants, the defects are generally not life threatening, making it difficult for physicians to recommend life-long immunosuppression that can itself have grave consequences. One potential solution to this dilemma is the induction of immunologic tolerance of the tissue transplants because tolerance would eliminate the need for such immunosuppression. Transplant tolerance may also prevent chronic rejection, a significant source of late graft loss after organ transplantation.Induction of mixed hematopoietic chimerism is a robust approach to establishing such transplant tolerance, which recently led to the first clinical application of a tolerance induction protocol for kidney transplantation. In this manuscript, we review the current status of VCA and of research directed toward bringing a tolerance approach to the VCA field. We also discuss the potential clinical significance of these studies and outline the remaining obstacles to introduction of a tolerance induction protocol to clinical practice in hand or face transplantation.

Immune tolerance and transplantation

Successful allogeneic hematopoietic stem cell transplantation (HSCT) and solid organ transplantation require development of a degree of immune tolerance against allogeneic antigens. T lymphocytes play a critical role in allograft rejection, graft failure, and graft-versus-host disease (GVHD). T-cell tolerance occurs by two different mechanisms: (1) depletion of self-reactive T cells during their maturation in the thymus (central tolerance), and (2) suppression/elimination of self-reactive mature T cells in the periphery (peripheral tolerance). Induction of transplant tolerance improves transplantation outcomes. Adoptive immunotherapy with immune suppressor cells including regulatory T cells, natural killer (NK)-T cells, veto cells, and facilitating cells are promising therapies for modulation of immune tolerance. Achieving mixed chimerism with the combination of thymic irradiation and T-cell-depleting antibodies, costimulatory molecule blockade with/without inhibitory signal activation, and elimination of alloreactive T cells with varying methods including pre- or post-transplant cyclophosphamide administration appear to be effective in inducing transplant tolerance.

Challenges in allogeneic mesenchymal stem cell-mediated cardiac repair

Autologous mesenchymal stem cells (MSCs) have been proven safe in phase I and II clinical trials in patients who have suffered a myocardial infarction. However, their potential for proliferation and differentiation decreases with age, which limits their efficacy in elderly patients. Allogeneic MSCs offer several key advantages over autologous MSCs, including a high regenerative potential and availability for clinical use without the delay required for expansion. It was believed that allogeneic MSCs were immune privileged and thus able to escape the recipient's immune system. In several preclinical studies, allogeneic MSCs were successful in regenerating the myocardium, and the transplanted MSCs improved heart function early after implantation. However, the long-term ability of allogeneic MSCs to preserve heart function is limited because of a transition from an immune privileged to an immunogenic phenotype after the cells differentiate. The initial phase I/II clinical study using allogeneic MSCs in patients with acute myocardial infarction was safe, and no side effects were observed. However, the long-term safety and efficacy of allogeneic MSCs remain to be established. In this review, we discuss the challenges of using allogeneic MSCs for cardiac repair and present strategies to prevent the immune rejection of allogeneic MSCs to increase their potential for use in cardiac patients.

Allogeneic hematopoietic stem cell transplantation in children with sickle cell disease

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) represents the only curative treatment for sickle cell disease (SCD), being successful in around 85-90% of patients. Mortality and long-term morbidity (including infertility, gonadal failure, and chronic graft-vs.-host disease) associated with conventional approaches curtail the number of patients who undergo allo-HSCT. Recently, it has been demonstrated that cord blood is as effective as and possibly safer than bone marrow in pediatric patients with SCD. Likewise, transplant strategies based on the use of reduced-intensity regimens and the induction of mixed chimerism have been explored to decrease allo-HSCT short- and long-term complications.

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

Challenges in the pursuit of immune tolerance

Strategies for inducing immune tolerance are fundamentally similar across a spectrum of immune-mediated disorders, including allergic disease, autoimmunity, and rejection of allografts. In each case, the objective of establishing an immunoregulatory balance is challenged by variable upswings in effector cell populations and proinflammatory mediators of immunity, requiring careful, and innovative therapeutic intervention to restore stability. The Immune Tolerance Network, an international consortium sponsored by the National Institutes of Health, seeks to advance both the scientific understanding and the clinical success of immune therapies for these disorders, through an innovative and collaborative effort involving clinical trials and mechanistic studies. Over the last decade, scientists have evaluated cell-based ablation and deviation strategies in trials using lymphocyte-specific targeting, induction of host-donor hematopoietic chimerism, induction of antigen-specific immune regulation, and a variety of antigen desensitization approaches. In this article, we review some of the highlights of this experience and discuss the potential for progress, utilizing new insights into regulatory mechanisms and biomarker signatures of tolerance.

Host alloreactive memory T cells influence tolerance to kidney allografts in nonhuman primates

Transplant tolerance, defined as indefinite allograft survival without immunosuppression, has been regularly achieved in laboratory mice but not in nonhuman primates or humans. In contrast to laboratory mice, primates regularly have high frequencies of alloreactive memory T cells (TMEMs) before transplantation. These TMEMs are poorly sensitive to conventional immunosuppression and costimulation blockade, and the presence of donor-reactive TMEMs in primates may account for their resistance to transplant tolerance protocols that have proven consistently effective in mice. We measured the frequencies of anti-donor TMEMs before and after transplantation in a series of rejecting and tolerant monkeys that underwent nonmyeloablative conditioning, short-term immunosuppression, and combined allogeneic kidney/cell transplantation. Transplants were acutely rejected in all the monkeys with high numbers of donor-specific TMEMs before transplantation. In contrast, long-term survival was observed in the recipients harboring lower frequencies of anti-donor TMEMs before transplantation. Similar amounts of TMEM homeostatic expansion were recorded in all transplanted monkeys upon hematopoietic reconstitution; however, only the tolerant monkeys had no expansion or activation of donor-reactive TMEMs after transplantation. These results indicate that the presence of high frequencies of host donor-reactive TMEMs before transplantation impairs tolerance induction to kidney allografts in this nonhuman primate model. Indeed, recipients harboring a low anamnestic reactivity to their donor before transplantation were successfully rendered tolerant via infusion of donor cells and short-term immunosuppression. This suggests that selection of allogeneic donors with low memory responses in recipients may be essential to successful transplant tolerance induction in patients.

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.

Hematopoietic stem cell transplantation in thalassemia

Almost 30 years have passed since the first successful hematopoietic stem cell transplantation in thalassemia and that first patient is now a healthy young adult with a completely normal life. Since that time, more than 3000 such transplants have been performed worldwide. This review provides a brief history of hematopoietic stem cell transplantation in thalassemia and reassesses current clinical results with the objective to provide outcome predictions based on modern transplant technologies. The role of hematopoietic stem cell transplantation in the oral chelation era and implications for possible closure in the approach to future gene therapy will also be discussed.

Transplantation of a whole ear allograft by immunological induction of donor-specific tolerance by bone marrow transplantation: An experimental study in rabbits

Previous studies in our laboratory have shown that giving bone marrow cells through the portal vein or intraosseous route is likely to be beneficial to tolerance of induction of allografts in rabbits. Using this model, we tested whether a less severe regimen for conditioning of the host can prevent rejection of allografts. Rabbits were given a single intraosseous injection of donor bone marrow cells and total body irradiation (7 Gy) and transplantation of skin and ear allografts. Mean skin allograft survival for this treatment was 88 days, which was similar to the results of our earlier study. A donor ear was accepted for more than a year with no signs of rejection. These results suggest that a single intraosseous injection of donor bone marrow cells is sufficient for induction of donor-specific tolerance in rabbits and that immunosuppressive agents may not be needed.

Fas mediates cardiac allograft acceptance in mice with impaired T-cell-intrinsic NF-kappaB signaling

The transcription factor NF-kappaB is critical for T-cell activation and survival. We have shown that mice expressing a T-cell-restricted NF-kappaB superrepressor (IkappaBalphaDeltaN-Tg) permanently accept heart but not skin allografts. Overexpression of the prosurvival factor Bcl-x(L) in T cells restored heart rejection, suggesting that graft acceptance in IkappaBalphaDeltaN-Tg mice was attributable to deletion of alloreactive T cells.In vitro, the increased death of IkappaBalphaDeltaN-Tg T cells upon TCR stimulation when compared with wildtype T cells was mostly because of Fas/FasL interaction. Similarly, Fas played a key role in cardiac allograft acceptance by IkappaBalphaDeltaN-Tg mice as both genetic and antibody-mediated inhibition of Fas-signaling restored cardiac allograft rejection. Rejection correlated with graft infiltration by T cells and splenic production of IFN-gamma upon allostimulation. These results indicate that T-cell inhibition of NF-kappaB results in cardiac allograft acceptance because of increased susceptibility to Fas-mediated cell death.

Host and donor immune responses contribute to antiviral effects of amotosalen-treated donor lymphocytes following early posttransplant cytomegalovirus infection

We have previously shown that amotosalen-treated splenocytes rescued allorecipients from a lethal dose of mouse CMV (MCMV) administered on day 0 in experimental parent C57BL/6-->CB6F1 allogeneic bone marrow transplant. In this study, we investigated the mechanism of antiviral activity of amotosalen-treated donor splenocytes when sublethal MCMV infections were administered 7 days posttransplant. Recipients of 3 x 10(6) untreated splenocytes were used as control. Following MCMV infection, recipients of untreated splenocytes had 40% early mortality due to acute graft-vs-host disease compared with no deaths among recipients of 10 x 10(6) treated splenocytes. However, recipients of both types of donor splenocytes effectively cleared MCMV from their liver. Like the untreated CD8(+) T cells, amotosalen-treated CD8(+) T cells equally retained their in vivo CTL activity against MCMV early peptide-pulsed targets and expressed similar levels of granzyme B within 11 days of infection. In contrast to full donor chimerism in recipients of untreated splenocytes, recipients of amotosalen-treated splenocytes showed mixed chimerism with both donor spleen- and host-derived anti-MCMV CD8(+) T cells in their blood and lymphoid organs, with significantly higher numbers of host-derived CD4(-)CD8(-) (double negative) T cells in the spleens of recipients of treated splenocytes compared with the recipients of untreated splenocytes. Additionally, recipients of amotosalen-treated splenocytes had lower levels of serum IFN-gamma and TNF-alpha in response to MCMV infection compared with untreated recipients. Thus, adoptive immunotherapy with treated T cells is a novel therapeutic approach that facilitates hematopoietic engraftment and permits antiviral immunity of both donor and host T cells without graft-vs-host disease.

Chimerism and tolerance in a recipient of a deceased-donor liver transplant

Complete hematopoietic chimerism and tolerance of a liver allograft from a deceased male donor developed in a 9-year-old girl, with no evidence of graft-versus-host disease 17 months after transplantation. The tolerance was preceded by a period of severe hemolysis, reflecting partial chimerism that was refractory to standard therapies. The hemolysis resolved after the gradual withdrawal of all immunosuppressive therapy.

Immune tolerance: mechanisms and application in clinical transplantation

The achievement of immune tolerance, a state of specific unresponsiveness to the donor graft, has the potential to overcome the current major limitations to progress in organ transplantation, namely late graft loss, organ shortage and the toxicities of chronic nonspecific immumnosuppressive therapy. Advances in our understanding of immunological processes, mechanisms of rejection and tolerance have led to encouraging developments in animal models, which are just beginning to be translated into clinical pilot studies. These advances are reviewed here and the appropriate timing for clinical trials is discussed.

Immunologic approaches to composite tissue allograft

This article discusses the immunologic principles and the most promising immunologic approaches for composite tissue allograft tolerance. We have previously reviewed some of the pharmacologic approaches for composite tissue allo-transplantation. In this review, we will summarize the range of options that may address the challenge of transplantation in reconstructive surgery.

Induction of tolerance across fully mismatched barriers by a nonmyeloablative treatment excluding antibodies or irradiation use

A mixed-chimerism approach is a major goal to circumvent sustained immunosuppression, but most of the proposed protocols need antibody treatment or host irradiation. Another promising experience involves busulfan combined with cyclophosphamide treatment. Additionally, recent publications demonstrated that, differing from busulfan, treosulfan administration does not present severe organ or hemato toxicities. Currently, Duchenne muscular dystrophy (DMD) patients are treated with chronic immunosuppression for muscle precursor cell transplantation (MT). We have developed a safe tolerance approach within this cellular allotransplantation therapy background. Thus, we have conditioned, prior to a donor BALB/c MT, the dystrophic mouse model C57Bl10J mdx/mdx, with our treatment based on a donor-specific transfusion, then a treosulfan treatment combined with single cyclophosphamide dose, and finally a donor bone marrow transplantation (TTCB). A first MT was performed in all mixed chimeric mice resulting from the TTCB treatment in the left tibialis anterior (TA) muscles. A second MT from the same donor strain was performed 100 days later in the right TA without any additional therapy. Results show that all treated mice developed permanent mixed chimerism. Long-lasting donor-positive fibers were present in both TAs of the mice, which received MT after the TTCB treatment. Only a basal level of infiltration was observed around donor fibers and mixed chimeric mice rejected third-party haplotype skin grafts. Thus, mixed chimerism development with this TTCB conditioning regimen promotes donor-specific stable tolerance, avoiding costimulatory blockade antibodies or irradiation use and side effects of sustained immunosuppressive treatments. This protocol could be eventually applied for MT to DMD patients or others tissue transplantations.

The immunity of splenic and peritoneal F4/80(+) resident macrophages in mouse mixed allogeneic chimeras

Mixed allogeneic chimeras are emerging as a prospective approach to induce immune tolerance in clinics. However, the immunological function of macrophages in mixed chimeras has not been evaluated. Using a B6-->BALB/c mixed chimera model, we investigated the phenotype and function of F4/80(+) resident peritoneal exudate macrophage (PEMs) and splenic macrophages (SPMs) in vitro and in vivo. Recipient F4/80(+)PEMs and SPMs in mixed chimeras expressed significantly lower levels of MHC-II, CD54, and CD23 than those in non-chimeric mice before lipopolysaccharide stimulation. Recipient F4/80(+)PEMs and SPMs in mixed chimeras induced normal cell proliferation and delayed-type hypersensitivity of allo-T cells, but they induced more IFN-gamma and IL-2 products and less IL-10 and TGF-beta products of allo-T cells compared with those of non-chimeras. Furthermore, recipient F4/80(+)PEMs and SPMs had significantly higher phagocytotic capacity against chicken red blood cells or allo-T cells than those of controls while they had normal phagocytosis to Escherichia coli. Although some slight but significant alterations of recipient macrophages have been detected, these results provide direct evidences for the efficient immunity of recipient macrophages in mixed allogeneic chimeras. The present study also, for the first time, offered basic information for macrophages maturing in heterogeneous environments.

Progress and potential for regenerative medicine

Regenerative medicine focuses on new therapies to replace or restore lost, damaged, or aging cells in the human body to restore function. This goal is being realized by collaborative efforts in nonmammalian and human development, stem cell biology, genetics, materials science, bioengineering, and tissue engineering. At present, understanding existing reparative processes in humans and exploring the latent ability to regenerate tissue remains the focus in this field. This review covers recent work in limb regeneration, fetal wound healing, stem cell biology, somatic nuclear transfer, and tissue engineering as a foundation for developing new clinical therapies to augment and stimulate human regeneration.

Induction of chimerism in rhesus macaques through stem cell transplant and costimulation blockade-based immunosuppression

A strategy for producing high-level hematopoietic chimerism after non-myeloablative conditioning has been established in the rhesus macaque. This strategy relies on hematopoietic stem cell transplantation after induction with a non-myeloablative dose of busulfan and blockade of the IL2-receptor in the setting of mTOR inhibition with sirolimus and combined CD28/CD154 costimulation blockade. Hematopoietic stem cells derived from bone marrow and leukopheresis products both were found to be successful in inducing high-level chimerism. Mean peripheral blood peak donor chimerism was 81% with a median chimerism duration of 145 days. Additional immune modulation strategies, such as pre-transplant CD8 depletion, donor-specific transfusion, recipient thymectomy or peritransplant deoxyspergualin treatment did not improve the level or durability of chimerism. Recipient immunologic assessment suggested that chimerism occurred amidst donor-specific down-regulation of alloreactive T cells, and the reappearance of vigorous T-mediated alloreactivity accompanied rejection of the transplants. Furthermore, viral reactivation constituted a significant transplant-related toxicity and may have negatively impacted the ability to achieve indefinite survival of transplanted stem cells. Nevertheless, this chimerism-induction regimen induced amongst the longest-lived stem cell chimerism reported to date for non-human primates and thus represents a platform upon which to evaluate emerging tolerance-induction strategies.

Acquired immunologic tolerance: with particular reference to transplantation

The first unequivocally successful bone marrow cell transplantation in humans was recorded in 1968 by the University of Minnesota team of Robert A. Good (Gatti et al. Lancet 2: 1366-1369, 1968). This achievement was a direct extension of mouse models of acquired immunologic tolerance that were established 15 years earlier. In contrast, organ (i.e. kidney) transplantation was accomplished precociously in humans (in 1959) before demonstrating its feasibility in any experimental model and in the absence of a defensible immunologic rationale. Due to the striking differences between the outcomes with the two kinds of procedure, the mechanisms of organ engraftment were long thought to differ from the leukocyte chimerism-associated ones of bone marrow transplantation. This and other concepts of alloengraftment and acquired tolerance have changed over time. Current concepts and their clinical implications can be understood and discussed best from the perspective provided by the life and times of Bob Good.

Tolerance, xenotransplantation: future therapies

Solid organ transplantation is limited by an insufficient number of organs to meet the needs of a growing population of patients with end-organ failure. A second problem is that, after successful transplantation, many organs fail owing in large part to chronic immunologic injury or so called "chronic rejection". In other circumstances, the organ "outlives" the recipient, often because the recipient succumbs to medical conditions related to chronic immunosuppression. This review focuses on two future therapies that could solve these problems, specifically, tolerance induction to permit long-term patient and graft survival and xenotransplantation to provide an unlimited supply of donor organs. The primary focuses are the most promising ongoing transplantation research that could be encountered by clinical surgeons in the near future.

Stem cell based therapies to treat muscular dystrophy

Muscular dystrophies comprise a heterogeneous group of neuromuscular disorders, characterized by progressive muscle wasting, for which no satisfactory treatment exists. Multiple stem cell populations, both of adult or embryonic origin, display myogenic potential and have been assayed for their ability to correct the dystrophic phenotype. To date, many of these described methods have failed, underlying the need to identify the mechanisms controlling myogenic potential, homing of donor populations to the musculature, and avoidance of the immune response. Recent results focus on the fresh isolation of satellite cells and the use of multiple growth factors to promote mesangioblast migration, both of which promote muscle regeneration. Throughout this chapter, various stem cell based therapies will be introduced and evaluated based on their potential to treat muscular dystrophy in an effective and efficient manner.

Induction of transplantation tolerance in non-human primate preclinical models

Short-term outcomes following organ transplantation have improved considerably since the availability of cyclosporine ushered in the modern era of immunosuppression. In spite of this, many of the current limitations to progress in the field are directly related to the existing practice of relatively non-specific immunosuppression. These include increased risks of opportunistic infection and cancer, and toxicity associated with long-term immunosuppressive drug exposure. In addition, long-term graft loss continues to result in part from a failure to adequately control the anti-donor immune response. The development of a safe and reliable means of inducing tolerance would ameliorate these issues and improve the lives of transplant recipients, yet given the improving clinical standard of care, the translation of new therapies has become appropriately more cautious and dependent on increasingly predictive preclinical models. While convenient and easy to use, rodent tolerance models have not to date been reliably capable of predicting a therapy's potential efficacy in humans. Non-human primates possess an immune system that more closely approximates that found in humans, and have served as a more rigorous preclinical testing ground for novel therapies. Prior to clinical adaptation therefore, tolerance regimens should be vetted in non-human primates to ensure that there is sufficient potential for efficacy to justify the risk of its application.

The unfinished legacy of liver transplantation: emphasis on immunology

Liver transplantation radically changed the philosophy of hepatology practice, enriched multiple areas of basic science, and had pervasive ripple effects in law, public policy, ethics, and theology. Why organ engraftment was feasible remained enigmatic, however, until the discovery in 1992 of donor leukocyte microchimerism in long-surviving liver, and other kinds of organ recipients. Following this discovery, the leukocyte chimerism-associated mechanisms were elucidated that directly linked organ and bone marrow transplantation and eventually clarified the relationship of transplantation immunology to the immunology of infections, neoplasms, and autoimmune disorders. We describe here how the initially controversial paradigm shift mandated revisions of cherished dogmas. With the fresh insight, the reasons for numerous inexplicable phenomena of transplantation either became obvious or have become susceptible to discriminate experimental testing. The therapeutic implications of the "new immunology" in hepatology and in other medical disciplines, have only begun to be explored. Apart from immunology, physiologic investigations of liver transplantation have resulted in the discovery of growth factors (beginning with insulin) that are involved in the regulation of liver size, ultrastructure, function, and the capacity for regeneration. Such studies have partially explained functional and hormonal relationships of different abdominal organs, and ultimately they led to the cure or palliation by liver transplantation of more than 2 dozen hepatic-based inborn errors of metabolism. Liver transplantation should not be viewed as a purely technologic achievement, but rather as a searchlight whose beams have penetrated the murky mist of the past, and continue to potentially illuminate the future.

NK cells mediate costimulation blockade-resistant rejection of allogeneic stem cells during nonmyeloablative transplantation

Although T-cell CD28/CD40 costimulation blockade represents a powerful mechanism to promote immune tolerance during murine allotransplantation, it has not yet been successfully translated to clinical transplantation. We determined the impact of natural killer (NK) cells on costimulation blockade-resistant rejection of donor bone marrow. We found that NK cells represent a potent barrier to engraftment: host NK depletion led to increased donor stem cell survival, increased mixed hematopoietic chimerism and to engraftment of low doses of donor marrow (1 x 10(8)/kg) that were otherwise rejected. To understand the mechanisms of NK alloreactivity, we employed an in vivo NK-specific cytotoxicity assay. We found that an increased proportion of target cells were killed between days 2 and 8 after cell transfer, and that NK killing of parental targets was inducible: NK cells preprimed with allotargets were more efficient at their elimination upon reexposure. Finally, both transplant and in vivo NK-killing models were used to determine the contribution of LFA-1 to NK alloreactivity. Blockade of LFA-1 led to decreased NK-mediated killing, and increased alloengraftment. These results identify NK alloreactivity as an integral component to costimulation blockade-resistant rejection, and suggest that its inhibition may represent an important target in the clinical translation of tolerance-induction transplantation.

Reduced-Intensity Regimens in Allogeneic Hematopoietic Stem Cell Transplantation for Hemoglobinopathies

The only well-established curative therapy for patients with hemoglobinopathies is allogeneic hematopoietic stem cell transplantation (HSCT), which, in the last 20 years, has been mainly performed from an HLA-matched, related donor, using bone marrow as source of hematopoietic progenitors. More recent studies indicate that HSCT from unrelated donors may offer results comparable to those obtained with HLA-identical family donors, provided that stringent criteria of compatibility are employed for selecting the donor. Cord blood transplantation was also suggested to be an equally effective, but safer, procedure than bone marrow transplantation, due to the lower incidence and severity of both acute and chronic graft-versus-host disease. In view of the early, as well as late, morbidity and mortality associated with conventional myeloablative transplantation in patients with hemoglobinopathies, it is not surprising that great interest and relevant expectations for patients with hemoglobinopathies have been raised by the introduction in the clinical practice of reduced-intensity preparative regimens. However, few reports have demonstrated the feasibility of using reduced-intensity preparative regimens for successfully treating these patients and many treatment failures, mainly due to the lack of sustained donor engraftment, have been reported. Despite these limitations, some of the concepts obtained from the use of reduced intensity regimens, such as the substitution of fludarabine for cyclophosphamide, may be important to further improve the outcome of patients with hemoglobinopathies, especially of those with poor prognostic characteristics, given HSCT.

Combined Treatment with CD40 Costimulation Blockade, T-Cell Depletion, Low-Dose Irradiation, and Donor Bone Marrow Transfusion in Limb Allograft Survival

To determine the efficacy of a regimen based on CD40 costimulation blockade and donor bone marrow in the limb allograft model, C57Bl/6 mice received limb allografts from Balb/c mice and either no treatment or a combination of MR1 (anti-CD40 ligand monoclonal antibody), CD4+ and CD8+ T-cell-depleting antibodies, low-dose irradiation, and bone marrow transfusion from Balb/c donors for 1 or 2 weeks. Recipients treated for 1 week showed rejection at 38.2 +/- 5.4 (mean +/- SEM) days, while those treated for 2 weeks had allograft survival of 56.5 +/- 9.9, with a range up to 91 days. Histology demonstrated rejection which was less cell-mediated and suggestive of transplant vasculopathy. Differential rejection of skin occurred first. Thus, a combined regimen based on CD40 costimulatory blockade and donor marrow significantly prolonged allograft survival. However, tolerance was not achieved, and histology suggests chronic rejection as a possible cause of allograft loss.

Advances in strategies for inducing central tolerance in organ allograft recipients

Encouraging results in large animal models and from the clinic have been reported recently suggesting that the deliberate induction of transplantation tolerance using central deletional protocols may be closer to becoming a reality. The induction of central tolerance would be especially applicable to pediatric organ transplant recipients. In this review, we discuss three promising protocols of central tolerance induction and why they are relevant to pediatric organ transplantation.

Prolonged limb allograft survival with CD40 costimulation blockade, T-cell depletion, and megadose donor bone-marrow transfusion

The purpose of this study was to determine the efficacy of a treatment regimen consisting of CD 40 costimulation blockade, T-cell depletion, and megadose donor bone marrow transfusion in the limb allograft model. C57Bl/6 mice underwent limb transplantation from Balb/c mice and received MR1 (anti-CD 40 ligand monoclonal antibody), and CD4(+) and CD8(+) T cell-depleting antibodies with and without 120 x 10(6) donor bone-marrow transfusion. Recipients treated only with antibodies showed rejection at 51.4+/-17 (mean+/-SEM) days, while those who also received donor bone marrow had allograft survival of 67+/-16.4 days, with a range up to 91 days. Treated specimens with rejection had less lymphocytic infiltration than untreated controls. Recipients of donor bone marrow also demonstrated early mixed chimerism, which disappeared after 1 month. While allograft survival was prolonged, tolerance was not achieved, and the mechanism of rejection was more consistent with a chronic process.

Degeneration of stroma reduces retention of homed cells in bone marrow of lethally irradiated mice

Cytotoxic drugs or irradiation are generally administered before bone marrow (BM) transplantation because of the idea that host bone marrow 'niches' become available to the donor cells for engraftment. How BM stromal cells respond to the radiation, which ultimately modulates grafting of donor cells, is poorly understood. In this study, we examined homing and marrow retention of PKH26+ donor cells in BM of age-matched C57BL/6J mice conditioned at different doses of irradiation. When we injected donor cells into mice that received 900 cGy, the percent homing was highest (15.8 +/- 1.5%) as compared to the lower doses of radiation. Despite the highest levels of homing of donor cells in these mice, about 70% (p < 0.005) homed cells were detached from the marrow within 72 h of transplantation. In contrast, a 2- to 2.5-fold (p < 0.03) multiplication of homed PKH-26+ Sca-1+ cells was observed in sublethally irradiated mice. While determining that CD45- CD106+ cells in BM of the mice received 900 cGy, we found that more than 80% of cells were depleted. It was also revealed from this investigation that grafted cells conferred partial protection to the endogenous myeloid colony-forming cells from radiation injury. Collectively, the present study implicates radiation-induced degeneration of stroma as a cause of poor retention of donor cells in BM of lethally irradiated mice. These results may have important clinical implications in designing conditioning regimens for BM transplantation.

The CD28 peptidemimic can induce mixed chimerism and prolong the survival of cardiac allografts

Costimulatory blockade with CD28 peptidemimic (CD28PM, CD28 PM was synthesized by solid phase synthetic methods) prolongs cardiac allograft survival in mice, but has not reliably induced tolerance when used alone. In the current studies, we evaluated the effect of adding B7 blockade to a chimerism inducing nonmyeloablative regimen in mice and observed a significant improvement of donor bone marrow cells (BMC) engraftment, which had been associated with mixed chimerism and long-term survival of cardiac allografts. The mixed lymphocyte reaction (MLR) and the ear pinna cardiac transplantation model were performed to evaluate the effects of CD28PM in induction of specific immune hypo-response and extension of allograft survival. The expressed rates of B7.1 and B7.2 on the C57BL/6 splenocytes were 56.25% and 20.52%, respectively. The specific hypo-response status was established after immunization with CD28PM pre-treated donor splenocytes and the average inhibition rate was only 43% compared with normal control. Subsequently, a total number of 2 x 10(7) bone marrow cells per mouse were implanted to the recipients. The allogenic chimerism was obviously observed with the rate as high as 8.84% (mean) at the time point of day 14. During the first 50 days post bone marrow transfusion (BMT) the chimerism rate declined stepwise. But from 50 to 100 days, the chimerism rate sustained in a range of 3.35% to 4.6%. The results of transplantation experiments showed the survival of allgenic cardiac grafts were maintained over 100 days in recipients. Thus, donor BMC engraftment with mixed chimerism appears essential for induction of allograft tolerance using this conditioning regimen. Mixed chimerism approach, by the addition of CD28-B7 costimulatory blockade with CD28PM, has been shown to establish mixed chimerism and induce cardiac allograft tolerance in mice.

Tolerogenic protocols for intestinal transplantation

The intestine has long been considered as a "forbidden" organ to transplant [Ann. Surg. 216 (1992) 223-33]. This is due to the particularly challenging nature of the immunological conflict that an intestinal graft may cause: a particularly vigorous rejection response, in addition to the capacity to mount a graft-versus-host disease (GVHD) [Transplantation 37 (1984) 429]. Currently, the short-term success of intestinal transplantation (Itx) depends upon the chronic delivery of profound immunosuppression but this causes infection, malignancies--in particular posttransplant lymphoproliferative disorder (PTLD)--and direct drug toxicity. For these reasons, the results of Itx remain inferior to those of other solid organ transplants in the middle and in the long term (Intestinal Transplant Registry: www.small-bowel-transplant.org). Improved results and wider application of Itx requires the development of protocols that would facilitate acceptance of the new intestine thereby allowing to reduce the need for immunosuppression with its attending complications. Relevant experimental data and the recent evolution in the clinical strategies used to promote acceptance of intestinal grafts are reviewed.

Allogeneic bone marrow–derived flk-1+Sca-1− mesenchymal stem cells leads to stable mixed chimerism and donor-specific tolerance

OBJECTIVE

To investigate the possibility of flk-1+Sca-1- bone marrow-derived mesenchymal stem cells (bMSCs) to induce stable mixed chimerism and donor-specific graft tolerance.

METHODS

Allogeneic flk-1+Sca-1- bMSCs and syngeneic bone marrow (BM) cells were cotransplanted into lethally irradiated (8.5 Gy) recipient mice. FACS was used to analyze the chimerism 150 days later. Donor-type skin transplantation was performed to observe donor-specific immunotolerance in recipient mice. Mixed lymphocyte reaction (MLR) and mitogen proliferative assays were performed to evaluate proliferative response of splenocytes from recipient mice.

RESULTS

More than 5% donor-derived CD3+ cells were detected in splenocytes of recipient mice. Long-term survival of donor-type skin grafts was observed. MLR and mitogen proliferative assays showed that recipient mice had low immunoresponse to donor cells but retained normal ConA-induced proliferative response compared with normal mice.

CONCLUSION

Our results show for the first time that induction of stable mixed hematopoietic chimerism can be achieved with allogeneic flk-1+Sca-1- bMSC transplantation, which leads to permanent donor-specific immunotolerance in allogeneic host and results in long-term allogeneic skin graft acceptance.

CD154 blockade for induction of mixed chimerism and prolonged renal allograft survival in nonhuman primates

Costimulatory blockade with anti-CD154 monoclonal antibody (aCD154) prolongs allograft survival in nonhuman primates, but has not reliably induced tolerance when used alone. In the current studies, we evaluated the effect of adding CD154 blockade to a chimerism inducing nonmyeloablative regimen in primates. We observed a significant improvement of donor bone marrow (DBM) engraftment, which has been associated with a lower incidence of acute rejection and long-term survival of renal allografts without the need for previously required splenectomy. Among the long-term survivors, four never showed evidence of rejection, with the longest survival exceeding 1700 days following discontinuation of immunosuppression. Nevertheless, late chronic rejection was observed in three of eight recipients, indicating the necessity of further modifications of the regimen. Control recipients receiving no DBM or donor splenocytes in place of DBM rejected their allografts. Thus, DBM engraftment with, at least, transient mixed chimerism appears essential for induction of allograft tolerance using this conditioning regimen. Modification of the original mixed chimerism approach, by the addition of costimulatory blockade, has been shown to enhance mixed chimerism and induce renal allograft tolerance with less morbidity in nonhuman primates.

Stem cell–mediated tolerance inducing strategies in organ transplantation

The scope of possible tools to modulate the recipients immune response towards tolerance induction basically includes deletional and non deletional mechanisms, which are currently targeted by various strategies including monoclonal antibodies, cytokine deviation, chimerism induction and the support of regulating T-cells. Here we summarize the main findings in the field derived from experimental animal studies and currently performed clinical studies. This review focuses to give a clinically relevant overview over relevant tolerance inducing concepts, taking into consideration risk profiles and clinical efficacy associated with specific immunosuppressive regiments currently applied in the clinical setting of transplantation.

Mixed chimerism and transplantation tolerance

Establishment of mixed hematopoietic chimerism carries with it the induction of transplantation tolerance to any other tissue or organ from the same donor. This strategy has been studied extensively for induction of tolerance in mice. During the past decade, we have extended the same strategy, with modifications, to cynomolgus monkeys and most recently to renal transplant patients. In this report we review the evolution of these studies from preclinical applications to our current clinical experience with two therapeutic protocols sponsored by the Immune Tolerance Network. The first of these studies is for patients with myeloma and end-stage renal disease with an HLA-matched sibling donor; the second for patients with end-stage renal disease and HLA-mismatched donors. Although it is still early in the course of these studies, the results to date are very encouraging.