Mixed hematopoietic chimerism and transplantation tolerance

New insights in immunomodulation for intestinal transplantation

Tolerance is the Holy Grail of solid organ transplantation (SOT) and remains its primary challenge since its inception. In this topic, the seminal contributions of Thomas Starzl at Pittsburgh University outlined foundational principles of graft acceptance and tolerance, with chimerism emerging as a pivotal factor. Immunologically, intestinal transplantation (ITx) poses a unique hurdle due to the inherent characteristics and functions of the small bowel, resulting in increased immunogenicity. This necessitates heavy immunosuppression (IS) while IS drugs side effects cause significant morbidity. In addition, current IS therapies fall short of inducing clinical tolerance and their discontinuation has been proven unattainable in most cases. This underscores the unfulfilled need for immunological modulation to safely reduce IS-related burdens. To address this challenge, the Leuven Immunomodulatory Protocol (LIP), introduced in 2000, incorporates various pro-tolerogenic interventions in both the donor to the recipient, with the aim of facilitating graft acceptance and improving outcome. This review seeks to provide an overview of the current understanding of tolerance in ITx and outline recent advances in this domain.

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.

Immunoresponse to Gene-Modified Hematopoietic Stem Cells

Gene transfer to and correction of hematopoietic stem cells (HSCs) are ideal strategies to cure a number of congenital and acquired disorders. However, transgene products may trigger immunological rejection of modified cells, limiting their therapeutic benefits. Preclinical and clinical data indicate that myeloablative total body irradiation (TBI) allows for efficient engraftment and tolerance to gene-modified HSCs. In contrast, myeloablative chemotherapy using busulfan or similar agents is only sufficient to induce tolerance to gene-modified HSCs producing no or non-immunogenic protein. If cells are modified to produce a protein that is xenogenic or congenitally absent in the patient, additional immunosuppression may be required to prevent an immunological reaction to the transduced cells. New gene editing and in vivo gene therapy techniques could pose additional immune concerns compared to ex vivo gene therapy methods. This review is intended to guide the design of conditioning and immunosuppression therapy in HSC-targeted gene therapy, as well as gene editing.

Mechanisms of Tolerance Induction by Hematopoietic Chimerism: The Immune Perspective

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

Mixed chimerism in SCT: conflict or peaceful coexistence?

Stem cell transplants that follow both myeloablative and non-myeloablative conditioning regimens can result in states of mixed chimerism, which can be stable over time. With widespread availability of Y chromosome FISH in sex-mismatched transplantation and DNA-based methodologies for analysis of chimerism in other donor-recipient pairs, further insights have been gained regarding the implications of the mixed chimeric state. In transplants performed for inherited and acquired marrow failure disorders, disease status can be improved with only 10-20% donor cells, and it appears that stable mixed chimerism at that level is an acceptable outcome often leading to a state of tolerance, but an increasing level of recipient cells often precedes graft rejection. In transplants performed for malignant conditions, increasing levels of mixed chimerism may indicate disease relapse, but some cases with stable levels of mixed chimerism have been compatible with prolonged remission states. Understanding when mixed chimerism is an indication of secondary graft failure or impending graft rejection vs a state of tolerance and ongoing propensity for the establishment of a graft-vs-tumor effect is often difficult with currently available technologies and immunologic assays. The ability to understand the implication of mixed chimerism of multiple cell lineages and of varied lymphocyte subsets will remain important areas for future research to best harness the immunologic and other therapeutic benefits of allogeneic transplantation.

Low-intensity transplant regimens facilitate recruitment of donor-specific regulatory T cells that promote hematopoietic engraftment

Low- or reduced-intensity conditioning regimens for allogeneic hemopoietic stem cell transplantation are effective at establishing donor hematopoietic engraftment and host-vs.-graft (HvG) tolerance. We investigated the mechanisms of HvG tolerance induction and maintenance in an animal model in which transplantation of sublethally irradiated female recipients with bone marrow (BM) from syngeneic male donors produces mixed chimerism. Splenocytes from chimeric mice inhibited HY-specific CD8(+) T cell responses both in vitro and in vivo, and their adoptive transfer facilitated donor hematopoietic engraftment. These properties were contained within the CD4(+)CD25(+) population. The conditioning protocol alone led to a proportional expansion of regulatory T cells (T(regs)), but the inhibitory activity was induced only if male BM was infused. The administration of anti-CD25-depleting antibodies to conditioned recipients at time of BM transplantation prevented donor-recipient chimerism but did not affect engraftment if performed after the establishment of chimerism, thus indicating that recipient T(regs) are required for the generation but not the maintenance of HvG tolerance. We conclude that donor-specific T(regs) of recipient origin are recruited when the donor antigens are present during reduced-intensity conditioning-induced T(reg) expansion.

Tregs and transplantation tolerance

The induction and maintenance of immune tolerance to transplanted tissues constitute an active process involving multiple mechanisms that work cooperatively to prevent graft rejection. These mechanisms are similar to inherent tolerance toward self antigens and have a requirement for active immunoregulation, largely T cell mediated, that promotes specific unresponsiveness to donor alloantigens. This review outlines our current understanding of the Treg subsets that contribute to allotolerance and the mechanisms by which these cells exert their effects as well as their potential for therapy.

Routes to Transplant Tolerance versus Rejection

The alloimmune response can be divided into specific junctures where critical decisions between tolerance and immunity are made which define the outcome of the transplant. At these "decision nodes" various cytokines direct alloresponsive T cells to develop either a proinflammatory response aimed at graft destruction or an immunoregulatory response facilitating graft acceptance. This review will focus on the role of these cytokines in influencing the progression of an alloimmune response leading ultimately to either allograft survival or rejection.

Smooth muscle cells in human coronary atherosclerosis can originate from cells administered at marrow transplantation

Atherosclerosis is the major cause of adult mortality in the developed world, and a significant contributor to atherosclerotic plaque progression involves smooth muscle cell recruitment to the intima of the vessel wall. Controversy currently exists on the exact origin of these recruited cells. Here we use sex-mismatched bone marrow transplant subjects to show that smooth muscle cells throughout the atherosclerotic vessel wall can derive from donor bone marrow. We demonstrate extensive recruitment of these cells in diseased compared with undiseased segments and exclude cell-cell fusion events as a cause for this enrichment. These data have broad implications for our understanding of the cellular components of human atherosclerotic plaque and provide a potentially novel target for future diagnostic and therapeutic strategies.

Islet transplantation, stem cells, and transfusion medicine

Despite the widespread use of exogenous insulin, morbidity and mortality caused by type 1 diabetes mellitus (DM) continue to place a significant burden on society, both in terms of human suffering and cost. The transplantation of vascularized pancreas, usually performed concurrently with renal transplantation, can cure type 1 DM, as shown by results in more than 15000 such transplants over about 30 years. Transplantation of isolated pancreatic islets, instead of the whole organ, however, offers an attractive alternative that minimizes surgery and its complications. Although islet transplantation initially met with only modest success (only about 9% insulin independence at 1 year posttransplant), recent changes in patient selection criteria, number and treatment of islets transplanted, and better immunosuppressive regimens dramatically improved the results; spawning widespread enthusiasm for islet transplantation. Despite this promise, organ/islet availability remains an important limitation to this technology. A solution to the problem of limited materials for transplantation may be in the use of stem/progenitor cells. This article reviews the background of the current enthusiasm for pancreatic islet cell transplantation, highlights future research trends in the field, and suggests that the new islet-related cellular therapies belong within the domain of transfusion medicine.

Growth factor treatment prior to low-dose total body irradiation increases donor cell engraftment after bone marrow transplantation in mice

Low-toxicity conditioning regimens prior to bone marrow transplantation (BMT) are widely explored. We developed a new protocol using hematopoietic growth factors prior to low-dose total body irradiation (TBI) in recipients of autologous transplants to establish high levels of long-term donor cell engraftment. We hypothesized that treatment of recipient mice with growth factors would selectively deplete stem cells, resulting in successful long-term donor cell engraftment after transplantation. Recipient mice were treated for 1 or 7 days with growth factors (stem cell factor [SCF] plus interleukin 11 [IL-11], SCF plus Flt-3 ligand [FL], or granulocyte colony-stimulating factor [G-CSF]) prior to low-dose TBI (4 Gy). Donor cell chimerism was measured after transplantation of congenic bone marrow cells. High levels of donor cell engraftment were observed in recipients pretreated for 7 days with SCF plus IL-11 or SCF plus FL. Although 1-day pretreatments with these cytokines initially resulted in reduced donor cell engraftment, a continuous increase in time was observed, finally resulting in highly significantly increased levels of donor cell contribution. In contrast, G-CSF treatment showed no beneficial effects on long-term engraftment. In vitro stem cell assays demonstrated the effect of cytokine treatment on stem cell numbers. Donor cell engraftment and number of remaining recipient stem cells after TBI were strongly inversely correlated, except for groups treated for 1 day with SCF plus IL-11 or SCF plus FL. We conclude that long-term donor cell engraftment can be strongly augmented by treatment of recipient mice prior to low-dose TBI with hematopoietic growth factors that act on primitive cells.

Heart allograft tolerance without development of posttransplant cardiac allograft vasculopathy in chimerism-based, drug-induced tolerance

BACKGROUND

Recently, we have described a drug (cyclophosphamide [CP] plus busulfan [BU])-induced skin allograft tolerance in mice that can regularly overcome fully H-2-mismatched barriers. Using this method, we have investigated whether or not this regimen can prolong the survival of heart allografts and inhibit the development of posttransplant cardiac allograft vasculopathy (CAV).

METHODS

The components of the method are intravenous administration of 1 x 108 allogeneic spleen cells on day 0, intraperitoneal injection of 200 mg/kg of CP and 30 mg/kg of BU on day 2, and intravenous injection of T cell-depleted 1 x 107 allogeneic bone marrow cells from the same strain of mice on day 3. Heart grafting was performed on day 28. Chimerism in peripheral blood was followed by flow cytometric analysis, and histological analysis was performed at various times after grafting.

RESULTS

In a fully major histocompatability complex (MHC)-mismatched combination of B10.D2 (H-2d, IE+)-->B10 (H-2b, IE-), stable, multilineage-mixed chimerism was observed permanently. B10.D2 heart grafts were accepted permanently in a donor-specific manner, and posttransplant CAV did not develop.

CONCLUSIONS

These results demonstrated that the drug-induced tolerance recently established by us can regularly induce a long-lasting heart allograft tolerance without development of CAV.

Long-term survival of cardiac xenografts in fully xenogeneic (mouse --> rat) bone marrow chimeras

BACKGROUND

The shortage of human hearts remains a major barrier to the efficacy of heart transplantation for the treatment of end-stage heart disease. One potential solution to the supply problem would be the use of hearts from nonhuman donors (xenografts). We have established a model of mouse to rat xenogeneic bone marrow chimerism, and in this study we have hypothesized that such chimeric rats will accept both donor and recipient specific heart grafts while rejecting third-party mouse and rat grafts. We also investigated humoral responses in naive and chimeric rats with and without donor murine cardiac grafts.

METHODS

Recipient Lewis rats (n = 22) were given 1100 cGy lethal total body irradiation and the same day received 300 x 10(6) donor B10.BR mouse bone marrow cells intravenously. Peripheral blood of surviving rats (n = 18) was typed at 4 weeks and then monthly thereafter. Donor and recipient specific and third-party heterotopic heart transplantations were performed at 6 to 8 weeks after reconstitution with bone marrow.

RESULTS

Multilineage bone marrow chimerism was produced in all experimental animals with complete replacement of recipient marrow by donor cells. Murine donor and rat recipient strain hearts transplanted in chimeric rats survived indefinitely. Third-party rat and mouse hearts were rejected, though at a slower rate than bone marrow matched naive controls. High levels of antimouse antibodies were detected in rats with rejected hearts. These antibodies were absent in chimeric animals with long-term surviving heart grafts.

CONCLUSIONS

Long-term multilineage bone marrow chimerism can be produced in a mouse --> rat bone marrow transplant model. Long-term survival of donor specific and recipient specific vascularized cardiac grafts can be produced in these chimeric animals. These animals are clinically normal but show signs of subclinical immunosuppression regimen as they reject third-party hearts later than naive animals. Our results suggest that antibodies also play a significant role in concordant xenograft rejection, and induction of bone marrow chimerism can overcome this barrier.

Induction of cytotoxic T lymphocyte and antibody responses to enhanced green fluorescent protein following transplantation of transduced CD34(+) hematopoietic cells

Genetic modification of hematopoietic stem cells often results in the expression of foreign proteins in pluripotent progenitor cells and their progeny. However, the potential for products of foreign genes introduced into hematopoietic stem cells to induce host immune responses is not well understood. Gene marking and induction of immune responses to enhanced green fluorescent protein (eGFP) were examined in rhesus macaques that underwent nonmyeloablative irradiation followed by infusions of CD34(+) bone marrow cells transduced with a retroviral vector expressing eGFP. CD34(+) cells were obtained from untreated animals or from animals treated with recombinant human granulocyte colony-stimulating factor (G-CSF) alone or G-CSF and recombinant human stem cell factor. Levels of eGFP-expressing cells detected by flow cytometry peaked at 0.1% to 0.5% of all leukocytes 1 to 4 weeks after transplantation. Proviral DNA was detected in 0% to 17% of bone marrow--derived colony-forming units at periods of 5 to 18 weeks after transplantation. However, 5 of 6 animals studied demonstrated a vigorous eGFP-specific cytotoxic T lymphocyte (CTL) response that was associated with a loss of genetically modified cells in peripheral blood, as demonstrated by both flow cytometry and polymerase chain reaction. The eGFP-specific CTL responses were MHC-restricted, mediated by CD8(+) lymphocytes, and directed against multiple epitopes. eGFP-specific CTLs were able to efficiently lyse autologous CD34(+) cells expressing eGFP. Antibody responses to eGFP were detected in 3 of 6 animals. These data document the potential for foreign proteins expressed in CD34(+) hematopoietic cells and their progeny to induce antibody and CTL responses in the setting of a clinically applicable transplantation protocol. (Blood. 2001;97:1951-1959)

Mild preconditioning and low-level engraftment confer methotrexate resistance in mice transplanted with marrow expressing drug-resistant dihydrofolate reductase activity

Effective engraftment of hematopoietic cells targeted for gene transfer is facilitated by cytoreductive preconditioning such as high-dose total body irradiation (TBI). To minimize the adverse side effects associated with TBI, experiments were conducted to determine whether sublethal doses of TBI would allow sufficient engraftment of MTX-resistant hematopoietic cells to confer survival on recipient mice administered MTX. FVB/N animals were administered 1, 2, or 4 Gy TBI (lethal dose, 8.5 Gy), transplanted with 107 FVB/N transgenic marrow cells expressing an MTX-resistant dihydrofolate reductase (DHFR) transgene, and then administered MTX daily for 60 days. Control mice administered 1 Gy with or without subsequent transplantation of normal marrow cells succumbed to MTX toxicity by day 45. In contrast, nearly all animals transplanted with transgenic marrow survived MTX administration, regardless of the TBI dose used for preconditioning. The donor DHFR transgenic marrow engraftment level was proportional to the preconditioning dose of TBI but was surprisingly reduced in animals given 2 or 4 Gy TBI and subsequently administered MTX when compared with control animals administered phosphate-buffered saline. Animals preconditioned with 1 Gy were also protected from MTX toxicity when transplanted with reduced amounts (5 × 106 and 1 × 106 cells) of DHFR transgenic donor marrow, resulting in low-level (approximately 1%) engraftment. In conclusion, very mild preconditioning allows sufficient low-level engraftment of genetically modified stem cells for in vivo manifestation of the modified phenotype, suggesting the usefulness of mild preconditioning regimens in human gene therapy trials targeting hematopoietic stem cells.

Mild preconditioning and low-level engraftment confer methotrexate resistance in mice transplanted with marrow expressing drug-resistant dihydrofolate reductase activity

Effective engraftment of hematopoietic cells targeted for gene transfer is facilitated by cytoreductive preconditioning such as high-dose total body irradiation (TBI). To minimize the adverse side effects associated with TBI, experiments were conducted to determine whether sublethal doses of TBI would allow sufficient engraftment of MTX-resistant hematopoietic cells to confer survival on recipient mice administered MTX. FVB/N animals were administered 1, 2, or 4 Gy TBI (lethal dose, 8.5 Gy), transplanted with 107 FVB/N transgenic marrow cells expressing an MTX-resistant dihydrofolate reductase (DHFR) transgene, and then administered MTX daily for 60 days. Control mice administered 1 Gy with or without subsequent transplantation of normal marrow cells succumbed to MTX toxicity by day 45. In contrast, nearly all animals transplanted with transgenic marrow survived MTX administration, regardless of the TBI dose used for preconditioning. The donor DHFR transgenic marrow engraftment level was proportional to the preconditioning dose of TBI but was surprisingly reduced in animals given 2 or 4 Gy TBI and subsequently administered MTX when compared with control animals administered phosphate-buffered saline. Animals preconditioned with 1 Gy were also protected from MTX toxicity when transplanted with reduced amounts (5 × 106 and 1 × 106 cells) of DHFR transgenic donor marrow, resulting in low-level (approximately 1%) engraftment. In conclusion, very mild preconditioning allows sufficient low-level engraftment of genetically modified stem cells for in vivo manifestation of the modified phenotype, suggesting the usefulness of mild preconditioning regimens in human gene therapy trials targeting hematopoietic stem cells.

Allogeneic hematopoietic chimerism in mice treated with sublethal myeloablation and anti-CD154 antibody: absence of graft-versus-host disease, induction of skin allograft tolerance, and prevention of recurrent autoimmunity in islet-allografted NOD/Lt mice

We describe a tolerance-based stem cell transplantation protocol that combines sublethal radiation with transient blockade of the CD40-CD154 costimulatory pathway using an anti-CD154 antibody. With this protocol, we established hematopoietic chimerism in BALB/c mice transplanted with fully allogeneic C57BL/6 bone marrow. The percentage of donor-origin mononuclear cells in recipients was more than 99%. In addition, all chimeric mice treated with anti-CD154 antibody remained free of graft-versus-host disease (GVHD) and accepted donor-origin but not third-party skin allografts. It was similarly possible to create allogeneic hematopoietic chimerism in NOD/Lt mice with spontaneous autoimmune diabetes. Pancreatic islet allografts transplanted into chimeric NOD/Lt mice were resistant not only to allorejection but also to recurrence of autoimmunity. We conclude that it is possible to establish robust allogeneic hematopoietic chimerism in sublethally irradiated mice without subsequent GVHD by blocking the CD40-CD154 costimulatory pathway using as few as 2 injections of anti-CD154 antibody. We also conclude that chimerism created in this way generates donor-specific allograft tolerance and reverses the predisposition to recurrent autoimmune diabetes in NOD/Lt mice, enabling them to accept curative islet allografts.

Anti-CD154 or CTLA4Ig obviates the need for thymic irradiation in a non-myeloablative conditioning regimen for the induction of mixed hematopoietic chimerism and tolerance

BACKGROUND

Thymic irradiation (TI) or repeated administration of T cell-depleting monoclonal antibodies (TCD mAbs) is required in a previously described non-myeloablative regimen allowing allogeneic marrow engraftment with stable mixed chimerism and tolerance. As both treatments might be associated with toxicity in the clinical setting, we evaluated whether T-cell costimulatory blockade could be used to replace them.

METHODS

C57BL/6 mice received depleting anti-CD4 and anti-CD8 mAbs on day -5, 3 Gy whole body irradiation (day 0), and 15x10(6) fully MHC-mismatched, B10.A bone marrow cells. In addition, hosts were injected with an anti-CD154 mAb (day 0) and/or CTLA4Ig (day +2). Chimerism in peripheral blood was followed by flow cytometric (FACS) analysis, and tolerance was assessed by skin grafting, and also by mixed lymphocyte reaction (MLR) and cell-mediated lympholysis (CML) assays. The frequency of certain Vbeta families was determined by FACS to assess deletion of donor-reactive T cells.

RESULTS

Chimerism was transient and tolerance was not present in animals receiving TCD mAbs on day -5 without costimulatory blockade. The addition of anti-CD154 and CTLA4Ig, alone or in combination, reliably permitted induction of high levels of stable (>6 months) multi-lineage chimerism, with specific tolerance to skin grafts and donor antigens by MLR and CML assays. Long-term chimeras showed deletion of donor-reactive CD4+ peripheral blood lymphocytes, splenocytes, and mature thymocytes. Administration of TCD mAbs only 1 day before bone marrow transplantation plus anti-CD154 also allowed induction of permanent chimerism and tolerance.

CONCLUSIONS

One injection of anti-CD154 or CTLA4Ig overcomes the need for TI or prolonged host TCD in a preclinical model for the induction of mixed chimerism and deletional tolerance and thus further decreases the toxicity of this protocol. Achievement of tolerance with conditioning given over 24 hr suggests applicability to cadaveric organ transplantation.

Induction of immunologic tolerance for transplantation

In the second half of the 20th century, the transplantation of replacement organs and tissues to cure disease has become a clinical reality. Success has been achieved as a direct result of progress in understanding the cellular and molecular biology of the immune system. This understanding has led to the development of immunosuppressive pharmaceuticals that are part of nearly every transplantation procedure. All such drugs are toxic to some degree, however, and their chronic use, mandatory in transplantation, predisposes the patient to the development of infection and cancer. In addition, many of them may have deleterious long-term effects on the function of grafts. New immunosuppressive agents are constantly under development, but organ transplantation remains a therapy that requires patients to choose between the risks of their primary illness and its treatment on the one hand, and the risks of life-long systemic immunosuppression on the other. Alternatives to immunosuppression include modulation of donor grafts to reduce immunogenicity, removal of passenger leukocytes, transplantation into immunologically privileged sites like the testis or thymus, encapsulation of tissue, and the induction of a state of immunologic tolerance. It is the last of these alternatives that has, perhaps, the most promise and most generic applicability as a future therapy. Recent reports documenting long-term graft survival in the absence of immunosuppression suggest that tolerance-based therapies may soon become a clinical reality. Of particular interest to our laboratory are transplantation strategies that focus on the induction of donor-specific T-cell unresponsiveness. The basic biology, protocols, experimental outcomes, and clinical implications of tolerance-based transplantation are the focus of this review.