Mixed chimerism and transplantation tolerance

Unlocking Transplant Tolerance with Biomaterials

For patients suffering from organ failure due to injury or autoimmune disease, allogeneic organ transplantation with chronic immunosuppression is considered the god standard in terms of clinical treatment. However, the true "holy grail" of transplant immunology is operational tolerance, in which the recipient exhibits a sustained lack of alloreactivity toward unencountered antigen presented by the donor graft. This outcome is resultant from critical changes to the phenotype and genotype of the immune repertoire predicated by the activation of specific signaling pathways responsive to soluble and mechanosensitive cues. Biomaterials have emerged as a medium for interfacing with and reprogramming these endogenous pathways toward tolerance in precise, minimally invasive, and spatiotemporally defined manners. By viewing seminal and contemporary breakthroughs in transplant tolerance induction through the lens of biomaterials-mediated immunomodulation strategies-which include intrinsic material immunogenicity, the depot effect, graft coatings, induction and delivery of tolerogenic immune cells, biomimicry of tolerogenic immune cells, and in situ reprogramming-this review emphasizes the stunning diversity of approaches in the field and spotlights exciting future directions for research to come.

Donor-Specific Blood Transfusion in Lung Transplantation

Lung transplantation is still hindered by a high rate of chronic rejection necessitating profound immunosuppression with its associated complications. Donor-specific blood transfusion is a pre-transplant strategy aimed at improving graft acceptance. In contrast with standard stored blood or donor-specific regulatory T cells transfusions, this approach utilizes fresh whole blood from the donor prior to allograft transplantation, encompassing all cell types and plasma. The precise mechanisms underlying donor-specific blood transfusion-induced tolerance remain incompletely understood. Associations with regulatory/helper T cells, modulation of mononuclear phagocytic cells or microchimerism have been suggested. While numerous (pre-)clinical studies have explored its application in solid organ transplants like liver, kidney, and intestine, limited attention has been given to the setting of lung transplantation. This comprehensive review summarizes existing knowledge on the mechanisms and outcomes of donor-specific blood transfusion in solid organ transplants both in preclinical and clinical settings. We also address the potential benefits and risks associated with donor-specific blood transfusion in the field of lung transplantation, offering insights into future research directions.

Comparison of blood pressure values and expression of genes associated with hypertension in children before and after hematopoietic cell transplantation

Hypertension is a well-known late effect of hematopoietic cell transplantation (HCT), but no markers predicting its development are known. Our aim was to assess short-term blood pressure (BP) values and expressions of hypertension-associated genes as possible markers of hypertension in children treated with HCT. We measured systolic blood pressure (SBP) and diastolic blood pressure (DBP), using both office procedure and ambulatory BP monitoring (ABPM) in children before HCT and after a median of 6 months after HCT. We compared the results with two control groups, one of healthy children and another of children with simple obesity. We also performed microarray analysis of hypertension-associated genes in patients treated with HCT and children with obesity. We found no significant differences in SBP and DBP in patients before and after HCT. We found significant differences in expressions of certain genes in patients treated with HCT compared with children with obesity. We concluded that BP values in short-term follow-up after HCT do not seem to be useful predictors of hypertension as a late effect of HCT. However, over expressions of certain hypertension-associated genes might be used as markers of hypertension as a late effect of HCT if this is confirmed in larger long-term studies.

Predicting Outcomes of Rat Vascularized Composite Allotransplants through Quantitative Measurement of Chimerism with PCR-Amplified Short Tandem Repeat

Chimerism has been associated with the induction and maintenance of tolerance to vascularized composite allotransplants (VCA). Although most VCA studies have examined chimerism using flow cytometry, we proposed that precision in the measurement of chimerism may be better approximated when complimentary polymerase chain reaction (PCR) is applied to a specific short tandem repeat (STR). We identified a STR, D10Rat25, which exhibited a ~20 bp difference in length between two rat strains (BN and LEW) often utilized as the donor and recipient in many allotransplantation studies. D10Rat25 was PCR-amplified and quantified with capillary electrophoresis. With pure LEW and BN DNA, a standard curve was constructed to measure chimerism with good linearity. When applied to rat VCA, the relationship between systematic (in peripheral blood) or local (at specific organ/tissues) chimerism to allograft outcomes was noted. We found that peripheral chimerism was elevated by up to ~9% postoperative month 1 (POM 1) but then reduced regardless of the final VCA outcome. However, differences in VCA skin chimerism between early rejection and POM 1 (shown as ΔChimerism_POM1-ER) were notable with respect to VCA outcomes. ROC analysis identified the optimum cutoff value as 17.7%. In summary, we have developed a reliable method to quantify the percentage of BN cells/DNA in BN-LEW chimeras. The detection limit was characterized, and the acquired data were comparable with flow cytometry. This method can be applied to solid organ and composite tissue allotransplantation studies.

Immune Tolerance, Xenografts, and Large-Animal Studies in Transplantation

This article summarizes studies in which the author has been involved over several decades, directed toward providing solutions for the three major limitations to the field of transplantation: (1) drug treatment-related complications; (2) chronic rejection; and (3) the availability of transplantable organs. The first two of these limitations may be overcome by induction of transplantation tolerance, while the third will also require a new source of organs, for which great strides are now being made in xenotransplantation through genetic engineering.

Current challenges and future directions for liver transplantation

Liver transplantation is an effective and widely used therapy for several patients with acute and chronic liver diseases. The discrepancy between the number of patients on the waiting list and available donors remains the key issue and is responsible for the high rate of waiting list mortality. The recent news is that the majority of patients with hepatitis C virus related liver disease will be cured by new antivirals therefore we should expect soon a reduction in the need of liver transplantation for these recipients. This review aims to highlight, in two different sections, the main open issues of liver transplantation concerning the current and future strategies to the best use of limited number of organs. The first section cover the strategies to increase the donor pool, discussing the use of older donors, split grafts, living donation and donation after cardiac death and mechanical perfusion systems to improve the preservation of organs before liver transplantation. Challenges in immunosuppressive therapy and operational tolerance induction will be evaluated as potential tools to increase the survival in liver transplant recipients and to reducing the need of re-transplantation. The second section is devoted to the evaluation of possible new indications to liver transplantation, where the availability of organs by implementing the strategies mentioned in the first section and the reduction in the number of waiting transplants for HCV disease is realized. Among these new potential indications for transplantation, the expansion of the Milan criteria for hepatocellular cancer is certainly the most open to question.

Hematopoietic stem cells and solid organ transplantation

Solid organ transplantation provides lifesaving therapy for patients with end stage organ disease. In order for the transplanted organ to survive, the recipient must take a lifelong cocktail of immunosuppressive medications that increase the risk for infections, malignancies and drug toxicities. Data from many animal studies have shown that recipients can be made tolerant of their transplanted organ by infusing stem cells, particularly hematopoietic stem cells, prior to the transplant. The animal data have been translated into humans and now several clinical trials have demonstrated that infusion of hematopoietic stem cells, along with specialized conditioning regimens, can permit solid organ allograft survival without immunosuppressive medications. This important therapeutic advance has been made possible by understanding the immunologic mechanisms by which stem cells modify the host immune system, although it must be cautioned that the conditioning regimens are often severe and associated with significant morbidity. This review discusses the role of hematopoietic stem cells in solid organ transplantation, provides an understanding of how these stem cells modify the host immune system and describes how newer information about adaptive and innate immunity might lead to improvements in the use of hematopoietic stem cells to induce tolerance to transplanted organs.

Recent advances in renal interstitial fibrosis and tubular atrophy after kidney transplantation

Although kidney transplantation has been an important means for the treatment of patients with end stage of renal disease, the long-term survival rate of the renal allograft remains a challenge. The cause of late renal allograft loss, once known as chronic allograft nephropathy, has been renamed “interstitial fibrosis and tubular atrophy” (IF/TA) to reflect the histologic pattern seen on biopsy. The mechanisms leading to IF/TA in the transplanted kidney include inflammation, activation of renal fibroblasts, and deposition of extracellular matrix proteins. Identifying the mediators and factors that trigger IF/TA may be useful in early diagnosis and development of novel therapeutic strategies for improving long-term renal allograft survival and patient outcomes. In this review, we highlight the recent advances in our understanding of IF/TA from three aspects: pathogenesis, diagnosis, and treatment.

Solid organ transplantation following end-organ failure in recipients of hematopoietic stem cell transplantation in children

Hematopoietic stem cell transplantation (HSCT) is an accepted treatment modality for various malignant and non-malignant disorders of the lympho-hematopoietic system. Patient survival rate has increased significantly with the use of this procedure. However, with the increase in disease-free patient survival rates, complications including various organ toxicities are also common. Kidney, liver, lung, heart, and skin are among those solid organs that are commonly affected and frequently lead to organ dysfunction and eventually end-organ disease. Conservative measures may or may not be successful in managing the organ failure in these patients. Solid organ transplantation has been shown to be promising in those patients who fail conservative management. This review will summarize the causes of solid organ (kidney, liver, and lung) dysfunction and the available data on transplantation of these solid organs in post-HSCT recipients.

Induction of tolerance through mixed chimerism

"Mixed chimerism" refers to a state in which the lymphohematopoietic system of the recipient of allogeneic hematopoietic stem cells comprises a mixture of host and donor cells. This state is usually attained through either bone marrow or mobilized peripheral blood stem cell transplantation. Although numerous treatment regimens have led to transplantation tolerance in mice, the induction of mixed chimerism is currently the only treatment modality that has been successfully extended to large animals and to the clinic. Here we describe and compare the use of mixed chimerism to establish transplantation tolerance in mice, pigs, monkeys, and in the clinic. We also attempt to correlate the mechanisms involved in achieving tolerance with the nature of the tolerance that has resulted in each case.

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.

Five-year clinical effects of donor bone marrow cells infusions in kidney allograft recipients: improved graft function and higher graft survival

Augmentation of microchimerism in solid organ transplant recipients by donor bone marrow cells (DBMC) infusion may promote immune hyporesponsiveness and consequently improve long-term allograft survival. Between March 2005 and July 2007, outcomes for 20 living unrelated donor (LURD) primary kidney recipients with concurrent DBMC infusion (an average of 2.19 ± 1.13 x 10⁹ donor cells consisting of 2.66 ± 1.70 x 10⁷ CD34⁺ cells) were prospectively compared with 20 non-infused control allograft recipients given similar conventional immunosuppressive regimens. With five years of clinical follow up, a total of 11 cases experienced rejection episodes (3 DBMI patients vs. 8 controls, p = 0.15). One DBMC-infused patient experienced chronic rejection vs. two episodes (1 biopsy-confirmed) in the control patients. Actuarial and death-censored 5-y graft survival was significantly higher in infused patients compared with controls (p = 0.01 and p = 0.03, respectively). Long-term graft survival was significantly associated with pre-transplant anti-HLA antibodies (p = 0.01), slightly with peripheral microchimerism (p = 0.09) and CD4⁺CD25⁺FoxP3⁺ T cells (p = 0.09). Immunosuppressant dosing was lower in infused patients than controls, particularly for mycophenolate mofetil (p = 0.001). The current findings as well as our previous reports on these patients indicates clinical improvement in long-term graft survival of renal transplant patients resulting from low-dose DBMC infusion given without induction therapy.

Host stem cells repopulate liver allografts: reverse chimerism

Liver transplant has become life-saving therapy for thousands of patients with end stage liver disease in the United States, but chronic rejection and the toxicities of immunosuppression remain significant obstacles to the further expansion of this modality and "transplant tolerance" remains a central goal in the field. So we and others are looking for alternative post-transplant strategies. We set out to 'engineer' repopulation after transplantation in a strain combination [dark agouti (DA) to Lewis green fluorescent protein+ (LEW GFP+)] which rejects liver grafts strongly, a model that more closely resembles the situation in humans. Our central finding is purposeful manipulation of the immune response with low dose immunosuppression and liberation of stem cells for a very short period after transplantation results in long-term transplant acceptance by two mechanisms: transforming the liver (donor) to self (host) phenotype, and auto-suppression of the specific allograft response.

Generation of human-to-pig chimerism to induce tolerance through transcutaneous in utero injection of cord blood-derived mononuclear cells or human bone marrow mesenchymals cells in a preclinical program of liver xenotransplantation: preliminary results

OBJECTIVE

Using a percutaneous ecoguided injection system to obtain chimeric piglets through a less invasive and traumatic technique than previously reported.

MATERIALS AND METHODS

The two types of human cells included umbilical cord blood mononuclear elements and mesenchymal stem cells cultured from bone marrow. Four sows at gestational day 50 were anesthetized. A needle was inserted through the skin and uterine wall to reach the peritoneal cavity of the fetuses under continuous ultrasound guidance. Fourteen piglets were injected with various cell concentrations.

RESULTS

All sows carried pregnancies to term yielding 69 piglets, among which 67 were alive and two mummified. Two piglets died during the first 48 hours of life. Chimerism was detected using flow cytometry and by quantitative polymerase chain reaction (q-PCR) to detect Alu gene in blood or tissues samples. The analysis detected blood chimerism in 13 piglets (21%) by flow cytometry and the presence of the human Alu gene in 33 (51%) by q-PCR. The results suggest cell trafficking between littermates after in utero injection.

CONCLUSIONS

Transcutaneous echo-guided injection succeeded to produce chimeric piglets without disadvantages to the sow or the fetuses and avoiding abortions or fetal death.

Depletion of alloreactive T cells for tolerance induction in a recipient of kidney and hematopoietic stem cell transplantations

The present case report represents a successful attempt to induce transplantation tolerance to organ allograft by combined administration of donor hematopoietic cells and kidney based on in vivo deletion of alloreactive host-vs-graft and graft-vs-host alloreactive T cells following non-myeloablative conditioning. We were able to induce mixed and eventually full donor chimerism and tolerance of kidney allograft in a 15-yr-old male with ESRD after cisplatin treatment and autologous HSCT for mediastinal germ cell tumor. Our approach to induce tolerance was based on preferential depletion of alloreactive T cells induced by exposure to donor's alloantigens and administration of cyclophosphamide at day 2 and day 3 after stem cell infusion. Additional non-specific immunosuppression as part of the conditioning included exposure to two fractions of TLI, treatment with alemtuzumab (monoclonal anti-CD52) and short-term conventional IS treatment to avoid early graft loss, because of request of IRB. Using this approach, with rapid tapering of all conventional IS treatment, the patient maintains good renal functions without evidence of both acute and chronic rejection for 32 months off all medications.

Double negative Treg cells promote nonmyeloablative bone marrow chimerism by inducing T-cell clonal deletion and suppressing NK cell function

The establishment of immune tolerance and prevention of chronic rejection remain major goals in clinical transplantation. In bone marrow (BM) transplantation, T cells and NK cells play important roles for graft rejection. In addition, graft-versus-host-disease (GVHD) remains a major obstacle for BM transplantation. In this study, we aimed to establish mixed chimerism in an irradiation-free condition. Our data indicate that adoptive transfer of donor-derived T-cell receptor (TCR) αβ(+) CD3(+) CD4(-) CD8(-) NK1.1(-) (double negative, DN) Treg cells prior to C57BL/6 to BALB/c BM transplantation, in combination with cyclophosphamide, induced a stable-mixed chimerism and acceptance of C57BL/6 skin allografts but rejection of third-party C3H (H-2k) skin grafts. Adoptive transfer of CD4(+) and CD8(+) T cells, but not DN Treg cells, induced GVHD in this regimen. The recipient T-cell alloreactive responsiveness was reduced in the DN Treg cell-treated group and clonal deletions of TCRVβ2, 7, 8.1/2, and 8.3 were observed in both CD4(+) and CD8(+) T cells. Furthermore, DN Treg-cell treatment suppressed NK cell-mediated BM rejection in a perforin-dependent manner. Taken together, our results suggest that adoptive transfer of DN Treg cells can control both adoptive and innate immunities and promote stable-mixed chimerism and donor-specific tolerance in the irradiation-free regimen.

Emerging uses for pediatric hematopoietic stem cells

Many new therapies are emerging that use hematopoietic stem and progenitor cells. In this review, we focus on five promising emerging trends that are altering stem cell usage in pediatrics: (i) The use of hematopoietic stem cell (HSC) transplantation, autologous or allogeneic, in the treatment of autoimmune disorders is one. (ii) The use of cord blood transplantation in patients with inherited metabolic disorders such as Hurler syndrome shows great benefit, even more so than replacement enzyme therapy. (iii) Experience with the delivery of gene therapy through stem cells is increasing, redefining the potential and limitations of this therapy. (iv) It has recently been shown that human immunodeficiency virus (HIV) infection can be cured by the use of selected stem cells. (v) Finally, it has long been postulated that HSC-transplantation can be used to induce tolerance in solid-organ transplant recipients. A new approach to tolerance induction using myeloid progenitor cells will be described.

CD4(+)Foxp3(+) regulatory T cell therapy in transplantation

Regulatory T cells (Tregs) are long-lived cells that suppress immune responses in vivo in a dominant and antigen-specific manner. Therefore, therapeutic application of Tregs to control unwanted immune responses is an active area of investigation. Tregs can confer long-term protection against auto-inflammatory diseases in mouse models. They have also been shown to be effective in suppressing alloimmunity in models of graft-versus-host disease and organ transplantation. Building on extensive research in Treg biology and preclinical testing of therapeutic efficacy over the past decade, we are now at the point of evaluating the safety and efficacy of Treg therapy in humans. This review focuses on developing therapy for transplantation using CD4(+)Foxp3(+) Tregs, with an emphasis on the studies that have informed clinical approaches that aim to maximize the benefits while overcoming the challenges and risks of Treg cell therapy.

Living related liver transplant following bone marrow transplantation from same donor: long-term survival without immunosuppression

We report long-term (seven yr) immunological tolerance in a 16-yr-old boy, to a liver allograft donated by his father following a bone marrow transplant at age 2.5 yr from the same donor. The bone marrow transplant was complicated by severe GVHD leading to liver failure and the ensuing need for a liver transplant, performed under planned avoidance of immunosuppression. At one wk post-transplant, although a liver biopsy was histologically compatible with acute rejection, favorable clinical and biochemical evolution precluded initiating immunosuppressive therapy, thus highlighting the need for caution when interpreting early histological changes so that administration of unnecessary immunosuppression can be avoided. Induction of tolerance in transplant recipients remains an elusive goal. In those patients who had received conventional bone marrow transplants and had endured the consequences of GVHD, development of macrochimerism may allow immunosuppression-free solid organ transplantation from the same donor.

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.

Mobilization of host stem cells enables long-term liver transplant acceptance in a strongly rejecting rat strain combination

Careful examination of liver, kidney and heart transplants in human recipients has revealed small numbers of host bone marrow derived stem cells in the graft. If the limited recipient repopulation of a donor graft that is currently observed could be facilitated, it is possible that conversion to a predominantly host phenotype would permit long-term graft function without immunosuppression. We proposed to "engineer" repopulation after transplant in a strain combination (dark agouti [DA] to Lewis green fluorescent protein+[LEW GFP+]) which rejects liver grafts strongly, a model that more closely resembles the situation in humans. Treatment on days 0, 1, 2, 3 and 7 after transplantation with low-dose (0.1 mg/kg) tacrolimus (T) designed to blunt rejection combined with plerixafor (P) to mobilize host stem cells resulted in greater than 180 days graft survival with extensive albeit spotty conversion of a small (50%) DA graft to the recipient LEW GFP+ genotype. Subsequent skin grafting revealed donor-specific graft prolongation. The T plus P treatment resulted in higher levels of Lin-Thy1+CD34+CD133+ stem cells and Foxp3+ regulatory T cells in the blood and liver at day 7. Thus, pharmacological mobilization of host stem cells sustains liver allografts by two mechanisms: repopulation of injured donor cells and regulation of the immune response.

Experimental models of B cell tolerance in transplantation

The use of conventional immunosuppression has successfully improved short-term allograft survival, however, long-term allograft survival has remained static and is complicated by serious side effects secondary to the long-term use of immunosuppressive agents. Immunological tolerance is the ultimate goal of organ transplantation, however it is an infrequent event in humans. Accordingly, over the past several decades, there has been a push to fully understand both the cellular and molecular mechanisms that play a role in the induction and maintenance of tolerance, with recent data implicating B cells and donor specific alloantibody as a barrier to and potential mediator of allograft tolerance. The study of B cells and alloantibody in transplant tolerance has evolved over recent years from using rodent models to non-human primate models. This review will discuss the role of B cells and alloantibody as antagonists and facilitators of transplantation tolerance, and highlight the experimental models developed for elucidating the mechanisms of B cell tolerance to alloantigen.

Consecutive low doses of cyclosporine A induce pro-inflammatory cytokines and accelerate allograft skin rejection

Cyclosporine A (CsA) is a fungus-derived molecule with potent immunosuppressive activity that has been largely used to downregulate cell-mediated immune responses during transplantation. However, previous data have indicated that CsA shows immunomodulatory activity that relays on the antigen concentration and the dose of CsA used. To test the hypothesis that minimal doses of CsA may show different outcomes on grafts, we used an experimental model for skin transplants in mice. ICR outbred mice received skin allografts and were either treated daily with different doses of CsA or left untreated. Untreated mice showed allograft rejection within 14 days, with graft necrosis, infiltration of neutrophils and macrophages and displayed high percentages of CD8+ T cells in the spleens, which were associated with high serum levels of IL-12, IFN-g and TNF-α. As expected, mice treated with therapeutic doses of CsA (15 mg/kg) did not show allograft rejection within the follow-up period of 30 days and displayed the lowest levels of IL-12, IFN-g and TNF-α as well as a reduction in CD8+ lymphocytes. In contrast, mice treated with consecutive minimal doses of CsA (5×10(-55) mg/kg) displayed an acute graft rejection as early as one to five days after skin allograft; they also displayed necrosis and strong inflammatory infiltration that was associated with high levels of IL-12, IFN-g and TNF-α. Moreover, the CD4+ CD25hiFoxP3+ subpopulation of cells in the spleens of these mice was significantly inhibited compared with animals that received the therapeutic treatment of CsA and those treated with placebo. Our data suggest that consecutive, minimal doses of CsA may affect Treg cells and may stimulate innate immunity.

CD47 in xenograft rejection and tolerance induction

Robust immune responses to xenografts remain a major obstacle to clinical translation of xenotransplantation, which could otherwise be a potential solution to the worldwide shortage of organ donors. The more vigorous xenograft rejection relative to allograft rejection is largely accounted for by the extensive genetic disparities between the donor and recipient. Xenografts activate host immunity not only by expressing immunogenic xenoantigens that provide the targets for immune recognition and rejection, but also by lacking ligands for the host immune inhibitory receptors. This review is focused on recent findings regarding the role of CD47, a ligand of an immune inhibitory receptor SIRPalpha, in xenograft rejection and induction of xenotolerance.

Acquisition of humoral transplantation tolerance upon de novo emergence of B lymphocytes

A major obstacle to transplantation tolerance is humoral immunity. In this paper, we demonstrate that the intrinsic developmental propensity of the B lymphocyte compartment for acquisition of self-tolerance can be harnessed to induce humoral unresponsiveness to transplanted alloantigens. In the current study, when transitional B cells developed in the presence of donor lymphoid cells, the mature B lymphocyte compartment failed to mount a donor-specific alloantibody response to an organ transplant--despite unrestrained acute T cell-mediated allograft rejection. Specifically, we generated an experimental system wherein a B6 strain B cell compartment developed de novo in the presence of F1 (B6xBALB/c) lymphoid cells and in a T cell-deficient setting. Following establishment of a steady-state B cell compartment, these B6 mice were transplanted with heterotopic cardiac allografts from allogeneic BALB/c donors. The mice were then inoculated with purified syngeneic B6 T cells. As expected, all cardiac allografts were acutely rejected. However, the B lymphocyte compartment of these mice was completely inert in its capacity to form a BALB/c-specific alloantibody response. Using an alloantigen-specific Ig transgenic system, we demonstrated that this profound degree of humoral tolerance was caused by clonal deletion of alloreactive specificities from the primary B cell repertoire. Thus, de novo B cell compartment development at the time of transplantation is of critical importance in recipient repertoire "remodeling" to a humoral tolerant state.

Should microchimerism turn into rejection prophylactics?

Non-self cells can circulate in the body of an individual after any sort of contact with an allogeneic source of cells, thus creating a situation of chimerism that can be transient or prolonged over time. This situation may appear after stem cell transplantation, pregnancy, transfusion or transplantation. Concerning transplantation, many hypotheses have been formulated regarding the existence, persistence and role of these circulating cells in the host. We will review the principal hypotheses that have been formulated for years since the first description of non-self circulating cells in mammals to the utilization of artificially induced chimerism protocols for the achievement of tolerance.

Fetal microchimerism: the cellular and immunological legacy of pregnancy

During pregnancy there is transplacental traffic of fetal cells into the maternal circulation. Remarkably, cells of fetal origin can then persist for decades in the mother and are detectable in the circulation and in a wide range of tissues. Maternal CD8 T cell responses directed against fetal antigens can also be detected following pregnancy. However, the impact that the persistence of allogenic cells of fetal origin and the maternal immune response towards them has on the mother's health remains unclear and is the subject of considerable investigation. The potentially harmful effects of fetal microchimerism include an association with autoimmune disease and recurrent miscarriage. Beneficial effects that have been explored include the contribution of persistent fetal cells to maternal tissue repair. A link between fetal microchimerism and cancer has also been proposed, with some results supporting a protective role and others, conversely, suggesting a role in tumour development. The phenomenon of fetal microchimerism thus provokes many questions and promises to offer further insights not only into the biology of pregnancy but fields such as autoimmunity, transplantation biology and oncology.

Current status of xenotransplantation and prospects for clinical application

Xenotransplantation is one promising approach to bridge the gap between available human cells, tissues, and organs and the needs of patients with diabetes or end-stage organ failure. Based on recent progress using genetically modified source pigs, improving results with conventional and experimental immunosuppression, and expanded understanding of residual physiologic hurdles, xenotransplantation appears likely to be evaluated in clinical trials in the near future for some select applications. This review offers a comprehensive overview of known mechanisms of xenograft injury, a contemporary assessment of preclinical progress and residual barriers, and our opinions regarding where breakthroughs are likely to occur.

Induction of tolerance in solid organ transplantation: the rationale to develop clinical protocols in liver transplantation

Minimization or withdrawal of immunosuppressive treatments after organ transplantation represents a major objective for improving quality of life and long-term survival of grafted patients. Such a goal may be reached under some clinical conditions, particularly in liver transplantation, making these patients good candidates for tolerance trials. In this context in liver transplantation, the central questions are (1) how to promote the natural propensity of the liver graft to be accepted, (2) which type of immunosuppressive drug should be used for induction and maintenance, and (3) which biomarkers could be used to discriminate tolerant patients from those requiring long-term immunosuppression. Induction therapies using aggressive T-cell-depleting agents may favor graft acceptance. However, persistent and/or rapidly reemerging cell lines, such as memory-type cells or CD8(+) T cells, could represent a significant barrier for induction of tolerance. The type of maintenance drugs also remains questionable. Calcineurin inhibitors may be eventually deleterious in the context of tolerance protocols, through inhibitory effects on regulatory T cells, that are not observed with rapamycin. In conclusion, significant efforts must be made to achieve reliable strategies for immunosuppression minimization or withdrawal after organ transplantation into the clinics.

Clinical tolerance 2008

In 2008, three publications highlighted the transition of tolerance from experimental to experiential. These included the first study to both anticipate and reproducibly deliver human leukocyte antigen-disparate allograft survival without continuous immunosuppressive drug administration. The other two highlighted unique episodes of clinical tolerance. The results from these studies are summarized and discussed.

Xenotransplantation as a model of integrated, multidisciplinary research

Xenotransplantation was proposed a long time ago as a possible solution to the world-wide shortage of human organs. For years, researchers in this field have almost exclusively directed their efforts towards combating the immunological barrier that precluded long-term xenograft survival. Studies have been conducted in both small and large animal models and the most relevant results have been obtained in pre-clincal studies, specifically those utilising the pig-to-nonhuman primate combination. In this context, a better understanding of the immunological mechanisms underlying the rejection of a xenograft have allowed the identification of specific targets of intervention that have resulted in considerable improvements in survival of porcine organs or cells in nonhuman primates. However it has also become apparent that if xenotransplantation has to enter the clinical arena, a multidisciplinary approach will be needed to comprehensively tackle the different issues related to the use of a xenograft to cure human disease.In this regard, the safety, ethics and regulatory aspects of xenotransplantation are currently being aggressively addressed to enable the initiation of xenotransplantation with a favourable risk/benefit ratio.

Achieving tolerance in pig-to-primate xenotransplantation: reality or fantasy

Because the immunologic differences between species are far greater than those within species, it is likely that the amount of immunosuppression that would be required for successful xenografting would be so much greater than that now used for allografting, that the side-effects and complications would be unacceptable. Tolerance approaches to xenotransplantation would overcome this concern. Studies in humanized mouse models have demonstrated that human T cells can be tolerized to porcine xenografts, providing important proofs of principle of the potential feasibility of pig-to-primate xenograft tolerance. The results available from studies of pig-to-primate xenotransplantation to date have demonstrated that while chronic immunosuppressive drugs have not completely avoided either T cell responses or humoral rejection, approaches directed toward tolerance induction have been encouraging with regard to avoiding immunization at both of these levels.

Relationship between mixed chimerism and rejection after bone marrow transplantation in thalassaemia

BACKGROUND

Thalassaemia is a genetic disease that requires a hypertransfusion regimen to treat the anaemia caused by enhanced red blood cell destruction. The only radical cure for thalassaemia is to correct the genetic defect by bone marrow transplantation from an HLA-identical donor capable of producing and maintaining a normal haemoglobin level in the recipient. Complete donor haematopoiesis is not essential for sustained engraftment and the simultaneous presence of haematopoietic cells of both donor and recipient origin is not a rare event after a transplant.

PATIENTS AND METHODS

The evolution of marrow engraftment of 93 transplanted thalassaemic patients, all from Middle East or Asian countries, was monitored by analysis of short tandem repeats.

RESULTS

Forty-three of 93 (46%) patients experienced a status of mixed chimerism early after bone marrow transplantation. Results of further engraftment analysis in these patients showed in 27 complete donor engraftment; rejection occurred in seven, while eight maintained the presence of both host and donor-derived cells. Interestingly, five out of the seven patients who rejected their transplant showed more than 25% residual host cells early after transplantation.

DISCUSSION AND CONCLUSION

Our study confirmed that the presence of large amounts of residual host cells within the first 2 months after a transplant is a risk factor for graft rejection also in a group of patients with wide ethnic heterogeneity, irregular transfusion regimens and/or poor chelation treatment. Ten percent of the transplanted thalassaemic patients maintained coexistence of donor and recipient cells, showing a stable functional graft, characterized by normal production of beta globin chains and high levels of haemoglobin. A mechanism responsible for peripheral tolerance induction, such as the production of specific regulatory T-cell clones, seems to play a key role in the induction of long-term tolerance after the transplant.

Can immune monitoring help to minimize immunosuppression in kidney transplantation?

The serious side-effects and complications related to the life-long use of immunosuppressors in transplantation have fuelled research into their possible minimization or even complete elimination. The field of transplantation is therefore tentatively moving from a phase of empiric immunosuppression towards individualized therapy. This process is highly dependent on the development of immune monitoring tests to detect an individual 'level of risk'. Immune monitoring is a way of measuring functional and molecular correlates of immune reactivity to provide clinically useful information for therapeutic decision-making. The technological breakthroughs over the last decade provide firm grounds for the achievement of this goal. Large, multicentric and prospective studies in the near future are now crucial if these tests are to achieve the necessary approval from the regulatory authorities and promptly enter the clinic for routine use.

Development of either split tolerance or robust tolerance along with humoral tolerance to donor and third-party alloantigens in nonmyeloablative mixed chimeras

Hematopoietic chimerism is considered to generate robust allogeneic tolerance; however, tissue rejection by chimeras can occur. This "split tolerance" can result from immunity toward tissue-specific Ags not expressed by hematopoietic cells. Known to occur in chimeric recipients of skin grafts, it has not often been reported for other donor tissues. Because chimerism is viewed as a potential approach to induce islet transplantation tolerance, we generated mixed bone marrow chimerism in the tolerance-resistant NOD mouse and tested for split tolerance. An unusual multilevel split tolerance developed in NOD chimeras, but not chimeric B6 controls. NOD chimeras demonstrated persistent T cell chimerism but rejected other donor hematopoietic cells, including B cells. NOD chimeras also showed partial donor alloreactivity. Furthermore, NOD chimeras were split tolerant to donor skin transplants and even donor islet transplants, unlike control B6 chimeras. Surprisingly, islet rejection was not a result of autoimmunity, since NOD chimeras did not reject syngeneic islets. Split tolerance was linked to non-MHC genes of the NOD genetic background and was manifested recessively in F(1) studies. Also, NOD chimeras but not B6 chimeras could generate serum alloantibodies, although at greatly reduced levels compared with nonchimeric controls. Surprisingly, the alloantibody response was sufficiently cross-reactive that chimerism-induced humoral tolerance extended to third-party cells. These data identify split tolerance, generated by a tolerance-resistant genetic background, as an important new limitation to the chimerism approach. In contrast, the possibility of humoral tolerance to multiple donors is potentially beneficial.

Short-term cyclosporine therapy and cotransplantation of donor splenocytes: effects on graft rejection and survival rates in pigs subjected to renal transplantation

BACKGROUND

Donor-specific allogeneic loading can prolong the survival of solid organ transplants by inducing a state known as acceptance. Several populations of cells are known to be involved in this process, but their exact roles have yet to be defined. The aim of this study was to assess the effects of portal-vein transfusion of donor-specific splenocytes (DST) after short-term cyclosporine A (CyA) therapy in pigs subjected to renal transplantation.

METHODS

Four groups of unrelated swine underwent renal transplantation with removal of the native kidneys. Antirejection protocols consisted in portal-vein DST (3 x 10(8) cells/kg) (Group 2, n = 7); intravenous CyA (9 mg/kg/d) on postoperative days 1-12 (Group 3, n = 14); and DST + CyA (as described above) (Group 4, n = 13). Results (through postoperative day 90) were compared with those obtained in untreated control recipients (Group 1, n = 7).

RESULTS

Compared with animals of Groups 1, 2, and 3, Group 4 recipients presented significantly longer survival (mean: 90 days, P < 0.01 in Kaplan-Meier analysis) and better renal function (P < 0.05). Graft histology revealed preserved parenchyma.

CONCLUSION

The role of spleen cells in the immune response has probably been underestimated. Cotransplantation of donor splenocytes seems to induce a certain degree of acceptance toward the renal allograft. The route of administration (portal-vein infusion in this study) may be crucial for developing favorable mechanisms of recognition.

Prevention of acute and chronic allograft rejection with CD4+CD25+Foxp3+ regulatory T lymphocytes

A major challenge in transplantation medicine is controlling the very strong immune responses to foreign antigens that are responsible for graft rejection. Although immunosuppressive drugs efficiently inhibit acute graft rejection, a substantial proportion of patients suffer chronic rejection that ultimately leads to functional loss of the graft. Induction of immunological tolerance to transplants would avoid rejection and the need for lifelong treatment with immunosuppressive drugs. Tolerance to self-antigens is ensured naturally by several mechanisms; one major mechanism depends on the activity of regulatory T lymphocytes. Here we show that in mice treated with clinically acceptable levels of irradiation, regulatory CD4+CD25+Foxp3+ T cells stimulated in vitro with alloantigens induced long-term tolerance to bone marrow and subsequent skin and cardiac allografts. Regulatory T cells specific for directly presented donor antigens prevented only acute rejection, despite hematopoietic chimerism. By contrast, regulatory T cells specific for both directly and indirectly presented alloantigens prevented both acute and chronic rejection. Our findings demonstrate the potential of appropriately stimulated regulatory T cells for future cell-based therapeutic approaches to induce lifelong immunological tolerance to allogeneic transplants.

Xenotransplantation: current status and a perspective on the future

Xenotransplantation using pigs as the transplant source has the potential to resolve the severe shortage of human organ donors. Although the development of relatively non-toxic immunosuppressive or tolerance-inducing regimens will be required to justify clinical trials using pig organs, recent advances in our understanding of the biology of xenograft rejection and zoonotic infections, and the generation of alpha1,3-galactosyltransferase-deficient pigs have moved this approach closer to clinical application. This Review highlights the major obstacles impeding the translation of xenotransplantation into clinical therapies and the potential solutions, providing a perspective on the future of clinical xenotransplantation.