Donor-specific Tolerance Research Articles

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.

Immune modulation permits tolerance and engraftment in a murine model of late-gestation transplantation

AbstractIn utero hematopoietic cell transplantation is an experimental nonmyeloablative therapy with potential applications in hematologic disorders, including sickle cell disease (SCD). Its clinical utility has been limited due to the early acquisition of T-cell immunity beginning at ∼14 weeks gestation, posing significant technical challenges and excluding treatment fetuses evaluated after the first trimester. Using murine neonatal transplantation at 20 days postcoitum (DPC) as a model for late-gestation transplantation (LGT) in humans, we investigated whether immune modulation with anti-CD3 monoclonal antibody (mAb) could achieve donor-specific tolerance and sustained allogeneic engraftment comparable with that of the early-gestation fetal recipient at 14 DPC. In allogeneic wild-type strain combinations, administration of anti-CD3 mAb with transplantation resulted in transient T-cell depletion followed by central tolerance induction confirmed by donor–specific clonal deletion and skin graft tolerance. Normal immune responses to third-party major histocompatibility complex and viral pathogens were preserved, and graft-versus-host disease did not occur. We further demonstrated the successful application of this approach in the Townes mouse model of SCD. These findings confirm the developing fetal T-cell response as a barrier to LGT and support transient T-cell depletion as a safe and effective immunomodulatory strategy to overcome it.

Mechanism underlying why donor splenocyte transfusion helps costimulation blockade drive transplantation tolerance

Abstract Solid organ transplantation is the preferred treatment for end-stage organ failure. Achieving stable donor-specific transplantation tolerance holds the potential to vastly improve the quality of life for transplant recipients by eliminating lifelong exposure to drug toxicity, a higher risk of malignancy and infection, and the development of chronic rejection associated with conventional immunosuppression. In mice, treatment with anti-CD154 (aCD154) prolongs graft survival, but the addition of donor splenocyte transfusion (DST) to aCD154 produces a robust state of graft-specific tolerance to fully MHC-mismatched cardiac allografts. In this study, we investigated parameters that dictate the efficacy of DST treatment. Our results support an antigen dispersal hypothesis whereby DST enables alloantigen to circulate to all secondary lymphoid organs where it can prime alloreactive T cells under cover of aCD154, and induce widespread tolerance in a larger repertoire of T cells than enabled by an allograft alone.

Modified immune cells induce donor-specific immune tolerance

Modified immune cells induce donor-specific immune tolerance

HLA molecular mismatches and induced donor-specific tolerance in combined living donor kidney and hematopoietic stem cell transplantation.

We investigated the potential role of HLA molecular mismatches (MM) in achieving stable chimerism, allowing for donor-specific tolerance in patients undergoing combined living donor kidney and hematopoietic stem cell transplantation (HSCT). All patients with available DNA samples (N=32) who participated in a phase 2 clinical trial (NCT00498160) where they received an HLA mismatched co-transplantation of living donor kidney and facilitating cell-enriched HSCT were included in this study. High-resolution HLA genotyping data were used to calculate HLA amino acid mismatches (AAMM), Eplet MM, three-dimensional electrostatic mismatch scores (EMS-3D), PIRCHE scores, HLA-DPB1 T-cell epitope group MM, HLA-B leader sequence MM, and KIR ligands MM between the donor and recipient in both directions. HLA MM were analyzed to test for correlation with the development of chimerism, graft vs. host disease (GvHD), de novo DSA, and graft rejection. Follow-up time of this cohort was 6-13.5 years. Of the 32 patients, 26 developed high-level donor or mixed stable chimerism, followed by complete withdrawal of immunosuppression (IS) in 25 patients. The remaining six of the 32 patients had transient chimerism or no engraftment and were maintained on IS (On-IS). In host versus graft direction, a trend toward higher median number of HLA-DRB1 MM scores was seen in patients On-IS compared to patients with high-level donor/mixed chimerism, using any of the HLA MM modalities; however, initial statistical significance was observed only for the EMS-3D score (0.45 [IQR, 0.30-0.61] vs. 0.24 [IQR, 0.18-0.36], respectively; p=0.036), which was lost when applying the Bonferroni correction. No statistically significant differences between the two groups were observed for AAMM, EMS-3D, Eplet MM, and PIRCHE-II scores calculated in graft versus host direction. No associations were found between development of chimerism and GvHD and non-permissive HLA-DPB1 T-cell epitope group MM, HLA-B leader sequence, and KIR ligands MM. Our results suggest an association between HLA-DRB1 molecular mismatches and achieving stable chimerism, particularly when electrostatic quality of the mismatch is considered. The non-permissive HLA-DPB1 T-cell epitope group, HLA-B leader sequence, and KIR ligands MM do not predict chimerism and GvHD in this combined kidney/HSCT transplant patient cohort. Further work is needed to validate our findings. https://clinicaltrials.gov/study/NCT00498160, identifier NCT00498160.

Exploring Costimulatory Blockade-Based Immunologic Strategies in Transplantation: Are They a Promising Immunomodulatory Approach for Organ and Vascularized Composite Allotransplantation?

The field of transplantation, including the specialized area of vascularized composite allotransplantation (VCA), has been transformed since the first hand transplant in 1998. The major challenge in VCA comes from the need for life-long immunosuppressive therapy due to its non-vital nature and a high rate of systemic complications. Ongoing research is focused on immunosuppressive therapeutic strategies to avoid toxicity and promote donor-specific tolerance. This includes studying the balance between tolerance and effector mechanisms in immune modulation, particularly the role of costimulatory signals in T lymphocyte activation. Costimulatory signals during T cell activation can have either stimulatory or inhibitory effects. Interfering with T cell activation through costimulation blockade strategies shows potential in avoiding rejection and prolonging the survival of transplanted organs. This review paper aims to summarize current data on the immunologic role of costimulatory blockade in the field of transplantation. It focuses on strategies that can be applied in vascularized composite allotransplantation, offering insights into novel methods for enhancing the success and safety of these procedures.

Myeloid-derived suppressor cells promote allograft survival by suppressing regulatory T cell dysfunction in high-risk corneal transplantation

Highly inflamed and neovascularized corneal graft beds are known as high-risk (HR) environments for transplant survival. One of the primary factors leading to this rejection is reduction in the suppressive function of regulatory T cells (Treg). Our results show that myeloid-derived suppressor cells (MDSC) counteract interleukin-6–mediated Treg dysfunction by expressing interleukin-10. Additionally, MDSC maintain forkhead box P3 stability and their ability to suppress IFN-γ+ Th1 cells. Administering MDSC to HR corneal transplant recipients demonstrates prolonged graft survival via promotion of Treg while concurrently suppressing IFN-γ+ Th1 cells. Moreover, MDSC-mediated donor-specific immune tolerance leads to long-term corneal graft survival as evidenced by the higher survival rate or delayed survival of a second-party C57BL/7 (B6) graft compared to those of third-party C3H grafts observed in contralateral low-risk or HR corneal transplantation of BALB/c recipient mice, respectively. Our study provides compelling preliminary evidence demonstrating the effectiveness of MDSC in preventing Treg dysfunction, significantly improving graft survival in HR corneal transplantation, and showing promising potential for immune tolerance induction.

A humanized IL-2 mutein expands Tregs and prolongs transplant survival in preclinical models.

Long-term organ transplant survival remains suboptimal, and life-long immunosuppression predisposes transplant recipients to an increased risk of infection, malignancy, and kidney toxicity. Promoting the regulatory arm of the immune system by expanding Tregs may allow immunosuppression minimization and improve long-term graft outcomes. While low-dose IL-2 treatment can expand Tregs, it has a short half-life and off-target expansion of NK and effector T cells, limiting its clinical applicability. Here, we designed a humanized mutein IL-2 with high Treg selectivity and a prolonged half-life due to the fusion of an Fc domain, which we termed mIL-2. We showed selective and sustainable Treg expansion by mIL-2 in 2 murine models of skin transplantation. This expansion led to donor-specific tolerance through robust increases in polyclonal and antigen-specific Tregs, along with enhanced Treg-suppressive function. We also showed that Treg expansion by mIL-2 could overcome the failure of calcineurin inhibitors or costimulation blockade to prolong the survival of major-mismatched skin grafts. Validating its translational potential, mIL-2 induced a selective and sustainable in vivo Treg expansion in cynomolgus monkeys and showed selectivity for human Tregs in vitro and in a humanized mouse model. This work demonstrated that mIL-2 can enhance immune regulation and promote long-term allograft survival, potentially minimizing immunosuppression.

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.

Programmed Death Ligand-1-Overexpressing Donor Exosomes Mediate Donor-Specific Immunosuppression by Delivering Co-Inhibitory Signals to Donor-Specific T Cells.

Programmed death ligand-1 (PD-L1) and donor antigens are critical for donor immature dendritic cells (DCs) targeting donor-specific T cells to induce transplant tolerance. This study aims to clarify whether DC-derived exosomes (DEX) with donor antigens (H2b) and high levels of PD-L1 expression (DEXPDL1+ ) can help to suppress graft rejection. In this study, it is demonstrated that DEXPDL1+ presents donor antigens, as well as PD-L1 co-inhibitory signals, directly or semi-directly via DCs to H2b-reactive T cells. This dual signal presentation can prolong the survival of heart grafts from B6 (H2b) mice but not from C3H (H2k) mice by inhibiting T cell activation, inducing activated T cell apoptosis, and modulating the balance of T cell differentiation from inflammatory to regulatory. Additionally, even though DEXPDL1+ treatment cannot induce tolerance after short-term treatment, this study provides a new vehicle for presenting co-inhibitory signals to donor-specific T cells. This novel strategy may facilitate the realization of donor-specific tolerance via the further optimization of drug-loading combinations and therapeutic regimens to elevate their killing ability.

Induced pluripotent stem cell–derived hematopoietic stem and progenitor cells induce mixed chimerism and donor-specific allograft tolerance

In transplantation using allogeneic induced pluripotent stem cells (iPSCs), strategies focused on major histocompatibility complexes were adopted to avoid immune rejection. We showed that minor antigen mismatches are a risk factor for graft rejection, indicating that immune regulation remains one of the most important issues. In organ transplantation, it has been known that mixed chimerism using donor-derived hematopoietic stem/progenitor cells (HSPCs) can induce donor-specific tolerance. However, it is unclear whether iPSC-derived HSPCs (iHSPCs) can induce allograft tolerance. We showed that 2 hematopoietic transcription factors, Hoxb4 and Lhx2, can efficiently expand iHSPCs with a c-Kit+Sca-1+Lineage− phenotype, which possesses long-term hematopoietic repopulating potential. We also demonstrated that these iHSPCs can form hematopoietic chimeras in allogeneic recipients and induce allograft tolerance in murine skin and iPSC transplantation. With mechanistic analyses, both central and peripheral mechanisms were suggested. We demonstrated the basic concept of tolerance induction using iHSPCs in allogeneic iPSC-based transplantation.

Variegated levels of alloreactive T cell dysfunction in transplantation tolerance determine graft vulnerability to infection-triggered rejection

Abstract While advances have been made in therapies that achieve tolerance in mouse models of transplantation, successful tolerance remains vulnerable to inflammatory insults, which can trigger graft rejection. To identify strategies for improving the robustness of tolerance, we aim to better understand the mechanisms by which grafts are rejected after donor-specific tolerance is established. In a mouse model of cardiac allograft tolerance in which late infection with Listeria monocytogenes (Lm) can trigger graft rejection, we previously showed that host CD4 +T cells reactive to host MHC-II presenting a donor MHC-I-derived peptide (TCR75 cells) became intrinsically dysfunctional if the alloantigen persisted for 3 weeks or greater. Intriguingly, infection-dependent transplant rejection was not associated with transcriptional or functional reinvigoration of these cells. We hypothesized that there might be heterogeneity in the level of dysfunction of alloreactive T cells depending on duration of their cognate alloantigen persistence. Unlike TCR75 cells, CD4 +T cells specific for host MHC-II presenting a peptide derived from donor MHC Class II (TEa cells), a graft antigen that declines post-transplantation, retained functionality during tolerance induction. Consequently, TEa but not TCR75 cells expanded following Lm-infection, supporting a role for TEa-like cells in driving infection-triggered rejection. Prolonging T cell exposure to cognate alloantigens aggravated TEa dysfunction and rendered grafts resistant to Lm-dependent rejection. These findings demonstrate that inducing dysfunction in a broader array of allospecific T cells, by prolonging exposure to more alloantigens, lessens graft vulnerability to infections. Supported by the UChicago Growth Development and Disabilities Training Program (T32 HD007009). AHA predoctoral fellowships (20PRE35210946, 3PRE14550022 and 15PRE22180007). NIH T32-AI007090. UChicago Cardiovascular Pathophysiology and Biochemistry Training Grant (T32 HL07237). HHMI Med-into-Grad Program training grant (56006772). NIAID Grant P01AI-97113.

Deleting Runx3 in CD4+ T Cells Disrupts Effector Functions and Facilitates Tolerance Induction in Solid Organ Transplantation

Abstract Long-term transplantation tolerance depends on the control of graft-reactive T cells. In particular, the alloreactivity of CD4+ T cells needs to be suppressed as these cells are both necessary and sufficient for cardiac allograft rejection. Although experimental protocols such as anti-CD154-mediated co-stimulation blockade can stably induce donor-specific tolerance in MHC-mismatched cardiac allografts, other more immunogenic organs such as skin are less susceptible to tolerogenic treatments, in part due to additional reactivity of the host against donor commensals that accompany the graft. The transcriptional mechanisms that enable CD4+ T cells to drive rejection and whose targeting can promote tolerance are not well understood. RNAseq comparison of the transcriptome in graft-reactive T cells from rejecting versus tolerant heart allograft recipients revealed several transcription factors differentially expressed between rejected and tolerant mice in both spleen and graft. Here, we report that the transcription factor Runx3 in alloreactive CD4+ T cells promotes Th1 and cytotoxic response against the allograft and resistance to tolerance induction. Conversely, ablating Runx3 in alloreactive CD4+ T cells augments the efficacy of anti-CD154 blockade treatment enabling skin graft acceptance.

Toward transplantation tolerance with adipose tissue-derived therapeutics.

Solid organ and composite tissue allotransplanation have been widely applied to treat end-stage organ failure and massive tissue defects, respectively. Currently there are a lot of research endeavors focusing on induction of transplantation tolerance, to relieve the burden derived from long-term immunosuppressant uptake. The mesenchymal stromal cells (MSCs) have been demonstrated with potent immunomodulatory capacities and applied as promising cellular therapeutics to promote allograft survival and induce tolerance. As a rich source of adult MSCs, adipose tissue provides additional advantages of easy accessibility and good safety profile. In recent years, the stromal vascular fraction (SVF) isolated from adipose tissues following enzymatic or mechanical processing without in vitro culture and expansion has demonstrated immunomodulatory and proangiogenic properties. Furthermore, the secretome of AD-MSCs has been utilized in transplantation field as a potential "cell-free" therapeutics. This article reviews recent studies that employ these adipose-derived therapeutics, including AD-MSCs, SVF, and secretome, in various aspects of organ and tissue allotransplantation. Most reports validate their efficacies in prolonging allograft survival. Specifically, the SVF and secretome have performed well for graft preservation and pretreatment, potentially through their proangiogenic and antioxidative capacities. In contrast, AD-MSCs were suitable for peri-transplantation immunosuppression. The proper combination of AD-MSCs, lymphodepletion and conventional immunosuppressants could consistently induce donor-specific tolerance to vascularized composite allotransplants (VCA). For each type of transplantation, optimizing the choice of therapeutics, timing, dose, and frequency of administration may be required. Future progress in the application of adipose-derived therapeutics to induce transplantation tolerance will be further benefited by continued research into their mechanisms of action and the development of standardized protocols for isolation methodologies, cell culture, and efficacy evaluation.

Extracorporeal Photopheresis Suppresses Transplant Fibrosis by Inducing Decorin Expression in Alveolar Macrophages.

Extracorporeal Photopheresis Suppresses Transplant Fibrosis by Inducing Decorin Expression in Alveolar Macrophages.

Competitive binding of CD226/TIGIT with poliovirus receptor regulates macrophage polarization and is involved in vascularized skin graft rejection

End-stage organ failure often requires solid organ transplantation. Nevertheless, transplant rejection remains an unresolved issue. The induction of donor-specific tolerance is the ultimate goal in transplantation research. In this study, an allograft vascularized skin rejection model using BALB/c-C57/BL6 mice was established to evaluate the regulation of the poliovirus receptor signaling pathway using CD226 knockout or T cell immunoglobulin and ITIM domain (TIGIT)–crystallizable fragment (Fc) recombinant protein treatment. In the TIGIT-Fc–treated and CD226 knockout groups, graft survival time prolonged significantly, with a regulatory T cell proportion increase and M2-type macrophage polarization. Donor-reactive recipient T cells became hyporesponsive while responding normally after a third-party antigen challenge. In both groups, serum interleukin (IL)-1β, IL-6, IL-12p70, IL-17A, tumor necrosis factor–α, interferon gamma, and monocyte chemoattractant protein-1 levels decreased, and the IL-10 level increased. In vitro, M2 markers, such as Arg1 and IL-10, were markedly increased by TIGIT-Fc, whereas iNOS, IL-1β, IL-6, IL-12p70, tumor necrosis factor–α, and interferon gamma levels decreased. CD226-Fc exerted the opposite effect. TIGIT suppressed TH1 and TH17 differentiation by inhibiting macrophage SHP-1 phosphorylation and enhanced ERK1/2-MSK1 phosphorylation and nuclear translocation of CREB. In conclusion, CD226 and TIGIT competitively bind to poliovirus receptor with activating and inhibitory functions, respectively. Mechanistically, TIGIT promotes IL-10 transcription from macrophages by activating the ERK1/2-MSK1-CREB pathway and enhancing M2-type polarization. CD226/TIGIT–poliovirus receptor are crucial regulatory molecules of allograft rejection.

Preclinical Studies in Vascularized Composite Allotransplantation

Introduction: Since 1998, when the first hand transplant was performed, Vascularized Composite Allotransplantation (VCA) field represents a new dimension in reconstructive surgery, offering a viable treatment option for injuries and defects that involve multiple layers of functional tissue, impossible to repair using conventional surgical techniques. Worldwide experience includes allotransplantation of upper and lower extremities, face, trachea, larynx, abdominal wall, uterus and penis, with promising clinical results. The major challenge remains at the immunologic level, translational ongoing studies being focused to develop less toxic immunosuppressant drugs and possibly achieve donor-specific tolerance. Materials and Methods: We conducted a series of preclinical studies of vascularized composite allotransplantation in preparation for further translation to clinical applications of this field. Our experience includes experimental vascularized allotransplantation models in rats exploring technical particularities and also analyzing the role of microchimerism in immunomodulation and promoting of the immune tolerance. Also, we performed a cadaveric study in order to explore the surgical techniques required in abdominal wall allotransplantation and the use of iliac crest as a potential microsurgical model of vascularized bone marrow transplantation(VBMT). Results: Working on small animal models allows the familiarization with microsurgical techniques in experimental composite tissue allotransplantation and evaluation of important immunological aspects: rejection dynamics, immunosuppression protocol testing and induction of immunologic tolerance. The surgical procedures performed in cadaveric models shown the feasibility of the abdominal wall allotransplantation procedure which may be translated to future clinical practice in association with organ transplantation. It may be possible to develop an alternative approach for iliac crest microsurgical transplant as tolerogenic strategy in solid organs or vascularized composite allotransplantation. Conclusion: Preclinical experience represents a mandatory step in developing a vascularized composite allotransplantation program, translating the knowledge obtained from experimental animal research and cadaveric studies to further clinical practice, ensuring optimal results for patients with complex tissue defects.

Gut microbiome and renal transplantation.

Gut microbiome research has been a surge of interest in many branches of medicine in the last decade. Our main aim is to show ability of microbes to infuence the functions of human body, especially in the immune system, and on the other hand to clarify changes in composition of gut microbiome in the post-transplantation period and their function for the long-term survival of the graft and the patient in the context of the occurrence of a wide range of complications. Kidney transplantation with the subsequent use of immunosuppressants and antibiotics affects the composition of gut microbiome. The subsequent development of dysbiosis significantly increases the risk of acute rejection, interstitial fibrosis and tubular atrophy of the graft, post-transplant diarrhoea, organ´s infections and metabolic complications such as post-transplant diabetes mellitus. Also important is the influence of the microorganisms of the gut microbiome on metabolism of immunosuppressants with the production of less effective components and the subsequent necessity of modifying their levels with a higher risk of underdosing and the occurrence of graft rejection. Support of the composition of the gut microbiome in the post-transplantation period in favor of bacteria producing short chain fatty acids (SCFA) is possible by changing of diet with predominance of fiber, the application of probiotics, prebiotics. According to available studies, it can lead to benefits in term of metabolic compensation, to the induction of donor-specific tolerance and many others, with an overall improvement in the quality of patient and graft survival.

Donor derived hematopoietic stem cell niche transplantation facilitates mixed chimerism mediated donor specific tolerance.

Compelling experimental evidence confirms that the robustness and longevity of mixed chimerism (MC) relies on the persistence and availability of donor-derived hematopoietic stem cell (HSC) niches in recipients. Based on our prior work in rodent vascularized composite allotransplantation (VCA) models, we hypothesize that the vascularized bone components in VCA bearing donor HSC niches, thus may provide a unique biologic opportunity to facilitate stable MC and transplant tolerance. In this study, by utilizing a series of rodent VCA models we demonstrated that donor HSC niches in the vascularized bone facilitate persistent multilineage hematopoietic chimerism in transplant recipients and promote donor-specific tolerance without harsh myeloablation. In addition, the transplanted donor HSC niches in VCA facilitated the donor HSC niches seeding to the recipient bone marrow compartment and contributed to the maintenance and homeostasis of stable MC. Moreover, this study provided evidences that chimeric thymus plays a role in MC-mediated transplant tolerance through a mechanism of thymic central deletion. Mechanistic insights from our study could lead to the use of vascularized donor bone with pre-engrafted HSC niches as a safe, complementary strategy to induce robust and stable MC-mediated tolerance in VCA or solid organ transplantation recipients.

Tolerance Induction in Vascularized Composite Allotransplantation-A Brief Review of Preclinical Models.

Pre-clinical studies are an obligatory tool to develop and translate novel therapeutic strategies into clinical practice. Acute and chronic rejection mediated by the recipient's immune system remains an important limiting factor for the (long-term) survival of vascularized composite allografts (VCA). Furthermore, high intensity immunosuppressive (IS) protocols are needed to mitigate the immediate and long-term effects of rejection. These IS regiments can have significant side-effects such as predisposing transplant recipients to infections, organ dysfunction and malignancies. To overcome these problems, tolerance induction has been proposed as one strategy to reduce the intensity of IS protocols and to thereby mitigate long-term effects of allograft rejection. In this review article, we provide an overview about animal models and strategies that have been used to induce tolerance. The induction of donor-specific tolerance was achieved in preclinical animal models and clinical translation may help improve short and long-term outcomes in VCAs in the future.