Contemporary Strategies and Barriers to Transplantation Tolerance

Cell-Based Therapies Induce Tolerance of Vascularized Composite Allotransplants: A Systematic Review

INTRODUCTION

Vascularized composite allotransplantation (VCA) is the transplantation of multiple tissue types as a solution for devastating injuries. Despite the highly encouraging functional outcomes of VCA, the consequences of long-term immunosuppression remain the main obstacle in its application. In this review, we provide researchers and surgeons with a summary of the latest advances in the field of cell-based therapies for VCA tolerance.

METHODS

Four electronic databases were searched: PubMed, Scopus, Cumulative Index to Nursing and Allied Health Literature , and Web of Science. We used the Preferred Reporting Items for Systematic Reviews and Meta-Analysis as the basis of our organization.

RESULTS

Hematopoietic stem cells prolonged VCA survival. A combination of immature dendritic cells and tacrolimus was superior to tacrolimus alone. T cell Ig domain and mucin domain modified mature dendritic cells increased VCA tolerance. Bone marrow-derived mesenchymal stem cells prolonged survival of VCAs. A combination of adipose-derived mesenchymal stem cells, cytotoxic T-lymphocyte antigen 4 immunoglobulin, and antilymphocyte serum significantly improved VCA tolerance. Ex-vivo allotransplant perfusion with recipient's bone marrow-derived mesenchymal stem cells increased VCA survival. Recipient's adipose-derived mesenchymal stem cells and systemic immunosuppression prolonged VCA survival more than any of those agents alone. Additionally, a combination of peripheral blood mononuclear cells shortly incubated in mitomycin and cyclosporine significantly improved VCA survival. Finally, a combination of donor recipient chimeric cells, anti-αβ-T cell receptor (TCR), and cyclosporine significantly prolonged VCA tolerance.

CONCLUSIONS

Evidence from animal studies shows that cell-based therapies can prolong survival of VCAs. However, there remain many obstacles for these therapies, and they require rigorous clinical research given the rarity of the subjects and the complexity of the therapies. The major limitations of cell-based therapies include the need for conditioning with immunosuppressive drugs and radiation, causing significant toxicity. Safety concerns also persist as most research is on animal models. While completely replacing traditional immunosuppression with cell-based methods is unlikely soon, these therapies could reduce the need for high doses of immunosuppressants and improve VCA tolerance.

Grafted Sertoli Cells Exert Immunomodulatory Non-Immunosuppressive Effects in Preclinical Models of Infection and Cancer

The Sertoli cells (SeCs) of the seminiferous tubules secrete a multitude of immunoregulatory and trophic factors to provide immune protection and assist in the orderly development of germ cells. Grafts of naked or encapsulated SeCs have been proved to represent an interesting therapeutic option in a plethora of experimental models of diseases. However, whether SeCs have immunosuppressive or immunomodulatory effects, which is imperative for their clinical translatability, has not been demonstrated. We directly assessed the immunopotential of intraperitoneally grafted microencapsulated porcine SeCs (MC-SeCs) in murine models of fungal infection (Aspergillus fumigatus or Candida albicans) or cancer (Lewis lung carcinoma/LLC or B16 melanoma cells). We found that MC-SeCs (i) provide antifungal resistance with minimum inflammatory pathology through the activation of the tolerogenic aryl hydrocarbon receptor/indoleamine 2,3-dioxygenase pathway; (ii) do not affect tumor growth in vivo; and (iii) reduce the LLC cell metastatic cancer spread associated with restricted Vegfr2 expression in primary tumors. Our results point to the fine immunoregulation of SeCs in the relative absence of overt immunosuppression in both infection and cancer conditions, providing additional support for the potential therapeutic use of SeC grafts in human patients.

The role of efferocytosis and transplant rejection: Strategies in promoting transplantation tolerance using apoptotic cell therapy and/or synthetic particles

Recently, efforts have been made to recognize the precise reason(s) for transplant failure and the process of rejection utilizing the molecular signature. Most transplant recipients do not appreciate the unknown length of survival of allogeneic grafts with the existing standard of care. Two noteworthy immunological pathways occur during allogeneic transplant rejection. A nonspecific innate immune response predominates in the early stages of the immune reaction, and allogeneic antigens initiate a donor-specific adaptive reaction. Though the adaptive response is the major cause of allograft rejection, earlier pro-inflammatory responses that are part of the innate immune response are also regarded as significant in graft loss. The onset of the innate and adaptive immune response causes chronic and acute transplant rejection. Currently employed immunosuppressive medications have shown little or no influence on chronic rejection and, as a result, on overall long-term transplant survival. Furthermore, long-term pharmaceutical immunosuppression is associated with side effects, toxicity, and an increased risk of developing diseases, both infectious and metabolic. As a result, there is a need for the development of innovative donor-specific immunosuppressive medications to regulate the allorecognition pathways that induce graft loss and to reduce the side effects of immunosuppression. Efferocytosis is an immunomodulatory mechanism with fast and efficient clearance of apoptotic cells (ACs). As such, AC therapy strategies have been suggested to limit transplant-related sequelae. Efferocytosis-based medicines/treatments can also decrease the use of immunosuppressive drugs and have no detrimental side effects. Thus, this review aims to investigate the impact of efferocytosis on transplant rejection/tolerance and identify approaches using AC clearance to increase transplant viability.

Advances in dendritic cell targeting nano-delivery systems for induction of immune tolerance

Dendritic cells (DCs) are the major specialized antigen-presenting cells (APCs), play a key role in initiating the body's immune response, maintain the balance of immunity. DCs can also induce immune tolerance by rendering effector T cells absent and anergy, and promoting the expansion of regulatory T cells. Induction of tolerogenic DCs has been proved to be a promising strategy for the treatment of autoimmune diseases, organ transplantation, and allergic diseases by various laboratory researches and clinical trials. The development of nano-delivery systems has led to advances in situ modulation of the tolerance phenotype of DCs. By changing the material composition, particle size, zeta-potential, and surface modification of nanoparticles, nanoparticles can be used for the therapeutic payloads targeted delivery to DCs, endowing them with great potential in the induction of immune tolerance. This paper reviews how nano-delivery systems can be modulated for targeted delivery to DCs and induce immune tolerance and reviews their potential in the treatment of autoimmune diseases, organ transplantation, and allergic diseases.

Introducing thymus for promoting transplantation tolerance

Establishing tolerance remains a central, if elusive, goal of transplantation. In solid-organ transplantation, one strategy for inducing tolerance has been cotransplantation of various forms of thymic tissue along with another organ. As one of the biological foundations of central tolerance, thymic tissue carries with it the ability to induce tolerance to any other organ or tissue from the same donor (or another donor tissue-matched to the thymic tissue) if successfully transplanted. In this review, we outline the history of this approach as well as work to date on its application in organ transplantation, concluding with future directions. We also review our experience with allogeneic processed thymus tissue for the treatment of congenital athymia, encompassing complete DiGeorge syndrome and other rare genetic disorders, and consider whether allogeneic processed thymic tissue implantation may offer a novel method for future experimentation with tolerance induction in organ transplantation.

Engineered cellular immunotherapies in cancer and beyond

This year marks the tenth anniversary of cell therapy with chimeric antigen receptor (CAR)-modified T cells for refractory leukemia. The widespread commercial approval of genetically engineered T cells for a variety of blood cancers offers hope for patients with other types of cancer, and the convergence of human genome engineering and cell therapy technology holds great potential for generation of a new class of cellular therapeutics. In this Review, we discuss the goals of cellular immunotherapy in cancer, key challenges facing the field and exciting strategies that are emerging to overcome these obstacles. Finally, we outline how developments in the cancer field are paving the way for cellular immunotherapeutics in other diseases.

Immunological Determinants of Liver Transplant Outcomes Uncovered by the Rat Model

For many individuals with end-stage liver disease, the only treatment option is liver transplantation. However, liver transplant rejection is observed in 24%-80% of transplant patients and lifelong drug regimens that follow the transplant procedure lead to serious side effects. Furthermore, the pool of donor livers available for transplantation is far less than the demand. Well-characterized and physiologically relevant models of liver transplantation are crucial to a deeper understanding of the cellular processes governing the outcomes of liver transplantation and serve as a platform for testing new therapeutic strategies to enhance graft acceptance. Such a model has been found in the rat transplant model, which has an advantageous size for surgical procedures, similar postoperative immunological progression, and high genome match to the human liver. From rat liver transplant studies published in the last 5 years, it is clear that the rat model serves as a strong platform to elucidate transplant immunological mechanisms. Using the model, we have begun to uncover potential players and possible therapeutic targets to restore liver tolerance and preserve host immunocompetence. Here, we present an overview of recent literature for rat liver transplant models, with an aim to highlight the value of the models and to provide future perspectives on how these models could be further characterized to enhance the overall value of rat models to the field of liver transplantation.

Imaging Tolerance Induction in Neonatal Mice: Hierarchical Interplay Between Allogeneic Adult and Neonatal Immune Cells

BACKGROUND

In Medawar's murine neonatal tolerance model, injection of adult semiallogeneic lymphohematopoietic cells (spleen cells [SC] and bone marrow cells [BMC]) tolerizes the neonatal immune system. An eventual clinical application would require fully allogeneic (allo) cells, yet little is known about the complex in vivo/in situ interplay between those cells and the nonconditioned neonatal immune system.

METHODS

To this end, labeled adult SC and BMC were injected into allogeneic neonates; interactions between donor and host cells were analyzed and modulated by systematic depletion/inactivation of specific donor and host immune effector cell types.

RESULTS

Consistent with effector cell compositions, allo-SC and allo-SC/BMC each induced lethal acute graft-versus-host disease, whereas allo-BMC alone did so infrequently. CD8 T cells from SC inoculum appeared naïve, while those of BMC were more memory-like. Age-dependent, cell-type dominance defined the interplay between adult donor cells and the neonatal host immune system such that if the dominant adult effector type was removed, then the equivalent neonatal one became dominant. Depletion of donor/host peripheral T cells protected against acute graft-versus-host disease and prolonged heart allograft survival; peripheral CD8 T-cell depletion together with CD4 T cell-costimulation blockade induced more robust tolerance.

CONCLUSIONS

This comprehensive study provides direct observation of the cellular interplay between allogeneic donor and host immune systems, adds to our previous work with semiallogeneic donor cells, and provides important insights for robust tolerance induction. Induction of transplant tolerance in neonates will likely require "crowd sourcing" of multiple tolerizing cell types and involve depletion of immune effector cells with costimulation blockade.

Thirty Years of Tacrolimus in Clinical Practice

Tacrolimus was discovered in 1984 and entered clinical use shortly thereafter, contributing to successful solid organ transplantation across the globe. In this review, we cover development of tacrolimus, its evolving clinical utility, and issues affecting its current usage. Since earliest use of this class of immunosuppressant, concerns for calcineurin-inhibitor toxicity have led to efforts to minimize or eliminate these agents in clinical regimens but with limited success. Current understanding of the role of tacrolimus focuses more on its efficacy in preventing graft rejection and graft loss. As we enter the fourth decade of tacrolimus use, newer studies utilizing novel combinations (as with the mammalian target of rapamycin inhibitor, everolimus, and T-cell costimulation blockade with belatacept) offer potential for enhanced benefits.

A Large-Scale Bank of Organ Donor Bone Marrow and Matched Mesenchymal Stem Cells for Promoting Immunomodulation and Transplant Tolerance

Induction of immune tolerance for solid organ and vascular composite allografts is the Holy Grail for transplantation medicine. This would obviate the need for life-long immunosuppression which is associated with serious adverse outcomes, such as infections, cancers, and renal failure. Currently the most promising means of tolerance induction is through establishing a mixed chimeric state by transplantation of donor hematopoietic stem cells; however, with the exception of living donor renal transplantation, the mixed chimerism approach has not achieved durable immune tolerance on a large scale in preclinical or clinical trials with other solid organs or vascular composite allotransplants (VCA). Ossium Health has established a bank of cryopreserved bone marrow (BM), termed "hematopoietic progenitor cell (HPC), Marrow," recovered from deceased organ donor vertebral bodies. This new source for hematopoietic cell transplant will be a valuable resource for treating hematological malignancies as well as for inducing transplant tolerance. In addition, we have discovered and developed a large source of mesenchymal stem (stromal) cells (MSC) tightly associated with the vertebral body bone fragment byproduct of the HPC, Marrow recovery process. Thus, these vertebral bone adherent MSC (vBA-MSC) are matched to the banked BM obtained from each donor, as opposed to third-party MSC, which enhances safety and potentially efficacy. Isolation and characterization of vBA-MSC from over 30 donors has demonstrated that the cells are no different than traditional BM-MSC; however, their abundance is >1,000-fold higher than obtainable from living donor BM aspirates. Based on our own unpublished data as well as reports published by others, MSC facilitate chimerism, especially at limiting hematopoietic stem and progenitor cell (HSPC) numbers and increase safety by controlling and/or preventing graft-vs.-host-disease (GvHD). Thus, vBA-MSC have the potential to facilitate mixed chimerism, promote complementary peripheral immunomodulatory functions and increase safety of BM infusions. Both HPC, Marrow and vBA-MSC have potential use in current VCA and solid organ transplant (SOT) tolerance clinical protocols that are amenable to "delayed tolerance." Current trials with HPC, Marrow are planned with subsequent phases to include vBA-MSC for tolerance of both VCA and SOT.

Apoptotic Donor Cells in Transplantation

Despite significant advances in prevention and treatment of transplant rejection with immunosuppressive medications, we continue to face challenges of long-term graft survival, detrimental medication side effects to both the recipient and transplanted organ together with risks for opportunistic infections. Transplantation tolerance has so far only been achieved through hematopoietic chimerism, which carries with it a serious and life-threatening risk of graft versus host disease, along with variability in persistence of chimerism and uncertainty of sustained tolerance. More recently, numerous in vitro and in vivo studies have explored the therapeutic potential of silent clearance of apoptotic cells which have been well known to aid in maintaining peripheral tolerance to self. Apoptotic cells from a donor not only have the ability of down regulating the immune response, but also are a way of providing donor antigens to recipient antigen-presenting-cells that can then promote donor-specific peripheral tolerance. Herein, we review both laboratory and clinical evidence that support the utility of apoptotic cell-based therapies in prevention and treatment of graft versus host disease and transplant rejection along with induction of donor-specific tolerance in solid organ transplantation. We have highlighted the potential limitations and challenges of this apoptotic donor cell-based therapy together with ongoing advancements and attempts made to overcome them.

Acute murine cytomegalovirus disrupts established transplantation tolerance and causes recipient allo-sensitization

We have previously shown that acute cytomegalovirus (CMV) infection disrupts the induction of transplantation tolerance. However, what impact acute CMV infection would have on the maintenance of established tolerance and on subsequent recipient allo-sensitization is a clinically important unanswered question. Here we used an allogeneic murine islet transplantation tolerance model to examine the impact of acute CMV infection on: (a) disruption of established transplantation tolerance during tolerance maintenance; and (b) the possibility of recipient allo-sensitization by CMV-mediated disruption of stable tolerance. We demonstrated that acute CMV infection abrogated transplantation tolerance during the maintenance stage in 50%-60% recipients. We further demonstrated that acute CMV infection-mediated tolerance disruption led to recipient allo-sensitization by reverting the tolerant state of allo-specific T cells and promoting their differentiation to allo-specific memory cells. Consequently, a second same-donor islet allograft was rejected in an accelerated fashion by these recipients. Our study therefore supports close monitoring for allo-sensitization in previously tolerant transplant recipients in whom tolerance maintenance is disrupted by an episode of acute CMV infection.

Immunosuppression trends in solid organ transplantation: The future of individualization, monitoring, and management

Immunosuppression regimens used in solid organ transplant have evolved significantly over the past 70 years in the United States. Early immunosuppression and targets for allograft success were measured by incidence and severity of allograft rejection and 1-year patient survival. The limited number of agents, infancy of human leukocyte antigen (HLA) matching techniques and lack of understanding of immunoreactivity limited the early development of effective regimens. The 1980s and 1990s saw incredible advancements in these areas, with acute rejection rates halving in a short span of time. However, the constant struggle to achieve the optimal balance between under- and overimmunosuppression is weaved throughout the history of transplant immunosuppression. The aim of this paper is to discuss the different eras of immunosuppression and highlight the important milestones that were achieved while also discussing this in the context of rational agent selection and regimen design. This discussion sets the stage for how we can achieve optimal long-term outcomes during the next era of immunosuppression, which will move from universal protocols to patient-specific optimization.

Human CD8+ Tregs expressing a MHC-specific CAR display enhanced suppression of human skin rejection and GVHD in NSG mice

Polyclonal CD8+CD45RClow/- Tregs are potent regulatory cells able to control solid organ transplantation rejection and even induce tolerance. However, donor major histocompatibility complex (MHC)-specific Tregs are more potent than polyclonal Tregs in suppressing T-cell responses and preventing acute as well as chronic rejection in rodent models. The difficulty of identifying disease-relevant antigens able to stimulate Tregs has reduced the possibility of obtaining antigen-specific Tregs. To bypass this requirement and gain the advantage of antigen specificity, and thus improve the therapeutic potential of CD8+ Tregs, we stably introduced a chimeric antigen receptor (CAR) derived from a HLA-A*02 antigen-specific antibody (A2-CAR) in human CD8+ Tregs and developed a clinically compatible protocol of transduction and expansion. We demonstrated that A2-CAR CD8+ Tregs were not phenotypically altered by the process, were specifically activated, and did not exhibit cytotoxic activity toward HLA-A*02+ kidney endothelial cells (ECs). We showed that A2-CAR CD8+ Tregs were more potent suppressors of immune responses induced by HLA-A*02 mismatch than control-CAR CD8+ Tregs, both in vitro and in vivo, in models of human skin graft rejection and graft-versus-host disease (GVHD) in NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG) mice. We showed that integrity of human skin graft was preserved with A2-CAR CD8+ Tregs at least 100 days in vivo after administration, and that interaction between the A2-CAR CD8+ Tregs and HLA-A*02+ kidney ECs resulted in a fine-tuned and protolerogenic activation of the ECs without cytotoxicity. Together, our results demonstrated the relevance of the CAR engineering approach to develop antigen-specific CAR-CD8+ Tregs for clinical trials in transplantation, and potentially in other diseases.

Chimerism, Transplant Tolerance, and Beyond

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

The past, present, and future of costimulation blockade in organ transplantation

PURPOSE OF REVIEW

Manipulating costimulatory signals has been shown to alter T cell responses and prolong graft survival in solid organ transplantation. Our understanding of and ability to target various costimulation pathways continues to evolve.

RECENT FINDINGS

Since the approval of belatacept in kidney transplantation, many additional biologics have been developed targeting clinically relevant costimulation signaling axes including CD40-CD40L, inducible costimulator-inducible costimulator ligand (ICOS-ICOSL), and OX40-OX40L. Currently, the effects of costimulation blockade on posttransplant humoral responses, tolerance induction, and xenotransplantation are under active investigation. Here, we will discuss these pathways as well as preclinical and clinical outcomes of biologics targeting these pathways in organ transplantation.

SUMMARY

Targeting costimultion is a promising approach for not only controlling T cell but also B cell responses. Consequently, costimulation blockade shows considerable potential for improving outcomes in antibody-mediated rejection and xenotransplantation.

Impact of infection on transplantation tolerance

Allograft tolerance is the ultimate goal of organ transplantation. Current strategies for tolerance induction mainly focus on inhibiting alloreactive T cells while promoting regulatory immune cells. Pathogenic infections may have direct impact on both effector and regulatory cell populations, therefore can alter host susceptibility to transplantation tolerance induction as well as impair the quality and stability of tolerance once induced. In this review, we will discuss existing data demonstrating the effect of infections on transplantation tolerance, with particular emphasis on the role of the stage of infection (acute, chronic, or latent) and the stage of tolerance (induction or maintenance) in this infection-tolerance interaction. While the deleterious effect of acute infection on tolerance is mainly driven by proinflammatory cytokines induced shortly after the infection, chronic infection may generate exhausted T cells that could in fact facilitate transplantation tolerance. In addition to pathogenic infections, commensal intestinal microbiota also has numerous significant immunomodulatory effects that can shape the host alloimmunity following transplantation. A comprehensive understanding of these mechanisms is crucial for the development of therapeutic strategies for robustly inducing and stably maintaining transplantation tolerance while preserving host anti-pathogen immunity in clinically relevant scenarios.

Achievement of Tolerance Induction to Prevent Acute Graft-vs.-Host Disease

Acute graft-vs.-host disease (GVHD) limits the efficacy of allogeneic hematopoietic stem cell transplantation (allo-HSCT), a main therapy to treat various hematological disorders. Despite rapid progress in understanding GVHD pathogenesis, broad immunosuppressive agents are most often used to prevent and remain the first line of therapy to treat GVHD. Strategies enhancing immune tolerance in allo-HSCT would permit reductions in immunosuppressant use and their associated undesirable side effects. In this review, we discuss the mechanisms responsible for GVHD and advancement in strategies to achieve immune balance and tolerance thereby avoiding GVHD and its complications.