Long-term tolerance of islet allografts in nonhuman primates induced by apoptotic donor leukocytes

Cell therapies and its derivatives as immunomodulators in vascularized composite allotransplantation

The adverse effects of traditional pharmaceutical immunosuppressive regimens have been a major obstacle to successful allograft survival in vascularized composite tissue allotransplantation (VCA) cases. Consequently, there is a pressing need to explore alternative approaches to reduce reliance on conventional immunotherapy. Cell therapy, encompassing immune-cell-based and stem-cell-based regimens, has emerged as a promising avenue of research. Immune cells can be categorized into two main systems: innate immunity and adaptive immunity. Innate immunity comprises tolerogenic dendritic cells, regulatory macrophages, and invariant natural killer T cells, while adaptive immunity includes T regulatory cells and B regulatory cells. Investigations are currently underway to assess the potential of these immune cell populations in inducing immune tolerance. Furthermore, mixed chimerism therapy, involving the transplantation of hematopoietic stem and progenitor cells and mesenchymal stem cells (MSC), shows promise in promoting allograft tolerance. Additionally, extracellular vesicles (EVs) derived from MSCs offer a novel avenue for extending allograft survival. This review provides a comprehensive summary of cutting-edge research on immune cell therapies, mixed chimerism therapies, and MSCs-derived EVs in the context of VCAs. Findings from preclinical and clinical studies demonstrate the tremendous potential of these alternative therapies in optimizing allograft survival in VCAs.

Biological function of type 1 regulatory cells and their role in type 1 diabetes

The collapse of immune homeostasis induces type 1 diabetes (T1D). In T1D, uncontrolled immune attacks against islet β cells reduce insulin secretion, resulting in hyperglycaemia and various complications. Type 1 regulatory (Tr1) cell therapy is a promising approach for the treatment of T1D. Tr1 cells are a subset of regulatory T (Treg) cells that are characterised by high interleukin-10 secretion and forkhead box protein P3 non-expression. Tr1 cells are reduced and have impaired function in patients with T1D. Immunotherapy is used to treat various diseases, and Treg cells have been applied to treat T1D in animal models and clinical trials. However, the safety and efficacy of Tr1 cells in treating diabetes and other diseases remain unclear. In this review, we aim to investigate the identification and biological function of Tr1 cells and related studies on immune diseases; additionally, we discuss the feasibility, limitations, and possible solutions of Tr1 cell therapy in T1D. This review shows that T1D is caused by an immune imbalance where defective Tr1 cells fail to control effector T cells, leading to the destruction of islet β cells. However, Tr1 cell therapy is safe and effective for other immune diseases, suggesting its potential for treating T1D.

Negative Vaccination Strategies for Promotion of Transplant Tolerance

Organ transplantation requires the use of immunosuppressive medications that lack antigen specificity, have many adverse side effects, and fail to induce immunological tolerance to the graft. The safe induction of tolerance to allogeneic tissue without compromising host responses to infection or enhancing the risk of malignant disease is a major goal in transplantation. One promising approach to achieve this goal is based on the concept of "negative vaccination." Vaccination (or actively acquired immunity) involves the presentation of both a foreign antigen and immunostimulatory adjuvant to the immune system to induce antigen-specific immunity. By contrast, negative vaccination, in the context of transplantation, involves the delivery of donor antigen before or after transplantation, together with a "negative adjuvant" to selectively inhibit the alloimmune response. This review will explore established and emerging negative vaccination strategies for promotion of organ or pancreatic islet transplant tolerance. These include donor regulatory myeloid cell infusion, which has progressed to early-phase clinical trials, apoptotic donor cell infusion that has advanced to nonhuman primate models, and novel nanoparticle antigen-delivery systems.

Hypoimmune islets achieve insulin independence after allogeneic transplantation in a fully immunocompetent non-human primate

Allogeneic transplantation of pancreatic islets for patients with difficult-to-control diabetes mellitus is severely hampered by the requirement for continuous immunosuppression and its associated morbidity. We report that allogeneic transplantation of genetically engineered (B2M-/-, CIITA-/-, CD47+), primary, hypoimmune, pseudo-islets (p-islets) results in their engraftment into a fully immunocompetent, diabetic non-human primate wherein they provide stable endocrine function and enable insulin independence without inducing any detectable immune response in the absence of immunosuppression. Hypoimmune primary p-islets may provide a curative cell therapy for type 1 diabetes mellitus.

Novel Approaches to Immunomodulation for Solid Organ Transplantation

Despite significant advances in the field of transplantation in the past two decades, current clinically available therapeutic options for immunomodulation remain fairly limited. The advent of calcineurin inhibitor-based immunosuppression has led to significant success in improving short-term graft survival; however, improvements in long-term graft survival have stalled. Solid organ transplantation provides a unique opportunity for immunomodulation of both the donor organ prior to implantation and the recipient post transplantation. Furthermore, therapies beyond targeting the adaptive immune system have the potential to ameliorate ischemic injury to the allograft and halt its aging process, augment its repair, and promote recipient immune tolerance. Other recent advances include expanding the donor pool by reducing organ discard, and bioengineering and genetically modifying organs from other species to generate transplantable organs. Therapies discussed here will likely be most impactful if individualized on the basis of specific donor and recipient considerations.

Allo Beta Cell transplantation: specific features, unanswered questions, and immunological challenge

Type 1 diabetes (T1D) presents a persistent medical challenge, demanding innovative strategies for sustained glycemic control and enhanced patient well-being. Beta cells are specialized cells in the pancreas that produce insulin, a hormone that regulates blood sugar levels. When beta cells are damaged or destroyed, insulin production decreases, which leads to T1D. Allo Beta Cell Transplantation has emerged as a promising therapeutic avenue, with the goal of reinstating glucose regulation and insulin production in T1D patients. However, the path to success in this approach is fraught with complex immunological hurdles that demand rigorous exploration and resolution for enduring therapeutic efficacy. This exploration focuses on the distinct immunological characteristics inherent to Allo Beta Cell Transplantation. An understanding of these unique challenges is pivotal for the development of effective therapeutic interventions. The critical role of glucose regulation and insulin in immune activation is emphasized, with an emphasis on the intricate interplay between beta cells and immune cells. The transplantation site, particularly the liver, is examined in depth, highlighting its relevance in the context of complex immunological issues. Scrutiny extends to recipient and donor matching, including the utilization of multiple islet donors, while also considering the potential risk of autoimmune recurrence. Moreover, unanswered questions and persistent gaps in knowledge within the field are identified. These include the absence of robust evidence supporting immunosuppression treatments, the need for reliable methods to assess rejection and treatment protocols, the lack of validated biomarkers for monitoring beta cell loss, and the imperative need for improved beta cell imaging techniques. In addition, attention is drawn to emerging directions and transformative strategies in the field. This encompasses alternative immunosuppressive regimens and calcineurin-free immunoprotocols, as well as a reevaluation of induction therapy and recipient preconditioning methods. Innovative approaches targeting autoimmune recurrence, such as CAR Tregs and TCR Tregs, are explored, along with the potential of stem stealth cells, tissue engineering, and encapsulation to overcome the risk of graft rejection. In summary, this review provides a comprehensive overview of the inherent immunological obstacles associated with Allo Beta Cell Transplantation. It offers valuable insights into emerging strategies and directions that hold great promise for advancing the field and ultimately improving outcomes for individuals living with diabetes.

Alloantigen Infusion Activates the Transcriptome of Type 2 Conventional Dendritic Cells

Recent studies have revealed novel molecular mechanisms by which innate monocytic cells acutely recognize and respond to alloantigen with significance to allograft rejection and tolerance. What remains unclear is the single-cell heterogeneity of the innate alloresponse, particularly the contribution of dendritic cell (DC) subsets. To investigate the response of these cells to exposure of alloantigen, C57BL/6J mice were administered live allogenic BALB/cJ splenic murine cells versus isogenic cells. In parallel, we infused apoptotic allogenic and isogenic cells, which have been reported to modulate immunity. Forty-eight hours after injection, recipient spleens were harvested, enriched for DCs, and subjected to single-cell mRNA sequencing. Injection of live cells induced a greater transcriptional change across DC subsets compared with apoptotic cells. In the setting of live cell infusion, type 2 conventional DCs (cDC2s) were most transcriptionally responsive with a Ccr2+ cDC2 subcluster uniquely responding to the presence of alloantigen compared with the isogenic control. In vitro experimentation confirmed unique activation of CCR2+ cDC2s following alloantigen exposure. Candidate receptors of allorecognition in other innate populations were interrogated and A type paired Ig-like receptors were found to be increased in the cDC2 population following alloexposure. These results illuminate previously unclear distinctions between therapeutic infusions of live versus apoptotic allogenic cells and suggest a role for cDC2s in innate allorecognition. More critically, these studies allow for future interrogation of the transcriptional response of immune cells in the setting of alloantigen exposure in vivo, encouraging assessment of novel pathways and previously unexamined receptors in this setting.

Enhancing longevity of immunoisolated pancreatic islet grafts by modifying both the intracapsular and extracapsular environment

Type 1 diabetes mellitus (T1DM) is a chronic metabolic disease characterized by autoimmune destruction of pancreatic β cells. Transplantation of immunoisolated pancreatic islets might treat T1DM in the absence of chronic immunosuppression. Important advances have been made in the past decade as capsules can be produced that provoke minimal to no foreign body response after implantation. However, graft survival is still limited as islet dysfunction may occur due to chronic damage to islets during islet isolation, immune responses induced by inflammatory cells, and nutritional issues for encapsulated cells. This review summarizes the current challenges for promoting longevity of grafts. Possible strategies for improving islet graft longevity are also discussed, including supplementation of the intracapsular milieu with essential survival factors, promotion of vascularization and oxygenation near capsules, modulation of biomaterials, and co-transplantation of accessory cells. Current insight is that both the intracapsular as well as the extracapsular properties should be improved to achieve long-term survival of islet-tissue. Some of these approaches reproducibly induce normoglycemia for more than a year in rodents. Further development of the technology requires collective research efforts in material science, immunology, and endocrinology. STATEMENT OF SIGNIFICANCE: Islet immunoisolation allows for transplantation of insulin producing cells in absence of immunosuppression and might facilitate the use of xenogeneic cell sources or grafting of cells obtained from replenishable cell sources. However, a major challenge to date is to create a microenvironment that supports long-term graft survival. This review provides a comprehensive overview of the currently identified factors that have been demonstrated to be involved in either stimulating or reducing islet graft survival in immunoisolating devices and discussed current strategies to enhance the longevity of encapsulated islet grafts as treatment for type 1 diabetes. Although significant challenges remain, interdisciplinary collaboration across fields may overcome obstacles and facilitate the translation of encapsulated cell therapy from the laboratory to clinical application.

Islet transplantation-immunological challenges and current perspectives

Pancreatic islet transplantation is a minimally invasive procedure aiming to reverse the effects of insulin deficiency in patients with type 1 diabetes (T1D) by transplanting pancreatic beta cells. Overall, pancreatic islet transplantation has improved to a great extent, and cellular replacement will likely become the mainstay treatment. We review pancreatic islet transplantation as a treatment for T1D and the immunological challenges faced. Published data demonstrated that the time for islet cell transfusion varied between 2 and 10 h. Approximately 54% of the patients gained insulin independence at the end of the first year, while only 20% remained insulin-free at the end of the second year. Eventually, most transplanted patients return to using some form of exogenous insulin within a few years after the transplantation, which imposed the need to improve immunological factors before transplantation. We also discuss the immunosuppressive regimens, apoptotic donor lymphocytes, anti-TIM-1 antibodies, mixed chimerism-based tolerance induction, induction of antigen-specific tolerance utilizing ethylene carbodiimide-fixed splenocytes, pretransplant infusions of donor apoptotic cells, B cell depletion, preconditioning of isolated islets, inducing local immunotolerance, cell encapsulation and immunoisolation, using of biomaterials, immunomodulatory cells, etc.

Membrane-coated nanoparticles for direct recognition by T cells

The direct modulation of T cell responses is an emerging therapeutic strategy with the potential to modulate undesired immune responses including, autoimmune disease, and allogeneic cells transplantation. We have previously demonstrated that poly(lactide-co-glycolide) particles were able to modulate T cell responses indirectly through antigen-presenting cells (APCs). In this report, we investigated the design of nanoparticles that can directly interact and modulate T cells by coating the membranes from APCs onto nanoparticles to form membrane-coated nanoparticles (MCNPs). Proteins within the membranes of the APCs, such as Major Histocompatibility Complex class II and co-stimulatory factors, were effectively transferred to the MCNP. Using alloreactive T cell models, MCNP derived from allogeneic dendritic cells were able to stimulate proliferation, which was not observed with membranes from syngeneic dendritic cells and influenced cytokine secretion. Furthermore, we investigated the engineering of the membranes either on the dendritic cells or postfabrication of MCNP. Engineered membranes could be to promote antigen-specific responses, to differentially activate T cells, or to directly induce apoptosis. Collectively, MCNPs represent a tunable platform that can directly interact with and modulate T cell responses.

Immunology of the transplanted cryopreserved kidney

Transplantation has substituted dysfunctional organs with healthy organs from donors to significantly lower morbidity and mortality associated with end-stage organ disease. Since the advent of transplantation, the promise of functional replacement has attracted an exponential mismatch between organ supply and demand. Theoretical proposals to counter the increasing needs have either been to create a source through genetic engineering of porcine donors for xenotransplantation (with more potent immunosuppression protocols) or recreate one's organ in a pig using interspecies blastocyst complementation for exogenic organ transplantation (without immunosuppression). Another promising avenue has been organ banking through cryopreservation for transplantation. Although ice free preservation and acceptable early function following rewarming is critical for success in transplantation, the immunological response that predominantly defines short- and long-term graft survival has failed to captivate attention to date. It is well sorted that thermal and metabolic stress incurred at 4 °C during recovery and reperfusion of organs for clinical transplantation has varying impact on graft survival. Considering the magnitude of cellular imbalance and injury at sub-zero/ultralow temperatures in addition to the chemical toxicity of cryoprotective agents (CPA), it is essential to assess and address the immunological response associated following transplantation to maximize the success of cryopreservation.

Transplantation of Cryopreserved Olfactory Ensheathing Cells Restores Loss of Functions in an Experimental Model

In the past decades, the properties of olfactory ensheathing cells (OECs) have been widely investigated. Studies have shown that transplantation of OECs cultured from the olfactory bulb mediates axonal regeneration, remyelination and restores lost functions in experimental central nervous system (CNS) injury models. Autologously sourcing the cells from the nasal mucosa or the olfactory bulb to treat patients with spinal cord injuries would be ideal, but the cell yield achieved may be inadequate to cover the surface area of the lesions typically encountered in human spinal cord contusion injuries. Therefore, banking allogenic cryopreserved olfactory bulb cells from donors or generating cell lines could provide a marked increase in cell stock available for transplantation. This study is undertaken in two control and two intervention groups. The control groups have lesions alone and lesions with collagen gel but without cells. The intervention groups have either transplantation of primary cultured olfactory bulb OECs (bOECs) encapsulated in collagen gel or cryopreserved bulb OECs (CbOECs) encapsulated in collagen gel. Here, we report that transplantation of cryopreserved rat bOECs encapsulated in collagen restored the loss of function in a vertical climbing test in a unilateral C6-T1 dorsal root injury model. The loss of function returns in 80% of rats with injuries in about 3 weeks comparable to that we observed after transplantation of primary cultured bOECs. The regeneration axons induced by the transplant are identified by neurofilament antibodies and ensheathed by OECs. Our results indicate that cryopreserved OECs retain their properties of inducing axon regeneration and restoring loss of function in the experimental model. This is a step forward to translate the research into future clinical applications.

Tissue-Targeted Drug Delivery Strategies to Promote Antigen-Specific Immune Tolerance

During autoimmunity or organ transplant rejection, the immune system attacks host or transplanted tissue, causing debilitating inflammation for millions of patients. There is no cure for most of these diseases. Further, available therapies modulate inflammation through nonspecific pathways, reducing symptoms but also compromising patients' ability to mount healthy immune responses. Recent preclinical advances to regulate immune dysfunction with vaccine-like antigen specificity reveal exciting opportunities to address the root cause of autoimmune diseases and transplant rejection. Several of these therapies are currently undergoing clinical trials, underscoring the promise of antigen-specific tolerance. Achieving antigen-specific tolerance requires precision and often combinatorial delivery of antigen, cytokines, small molecule drugs, and other immunomodulators. This can be facilitated by biomaterial technologies, which can be engineered to orient and display immunological cues, protect against degradation, and selectively deliver signals to specific tissues or cell populations. In this review, some key immune cell populations involved in autoimmunity and healthy immune tolerance are described. Opportunities for drug delivery to immunological organs are discussed, where specialized tissue-resident immune cells can be programmed to respond in unique ways toward antigens. Finally, cell- and biomaterial-based therapies to induce antigen-specific immune tolerance that are currently undergoing clinical trials are highlighted.

Progress in xenotransplantation: overcoming immune barriers

A major limitation of organ allotransplantation is the insufficient supply of donor organs. Consequently, thousands of patients die every year while waiting for a transplant. Progress in xenotransplantation that has permitted pig organ graft survivals of years in non-human primates has led to renewed excitement about the potential of this approach to alleviate the organ shortage. In 2022, the first pig-to-human heart transplant was performed on a compassionate use basis, and xenotransplantation experiments using pig kidneys in deceased human recipients provided encouraging data. Many advances in xenotransplantation have resulted from improvements in the ability to genetically modify pigs using CRISPR-Cas9 and other methodologies. Gene editing has the capacity to generate pig organs that more closely resemble those of humans and are hence more physiologically compatible and less prone to rejection. Despite such modifications, immune responses to xenografts remain powerful and multi-faceted, involving innate immune components that do not attack allografts. Thus, the induction of innate and adaptive immune tolerance to prevent rejection while preserving the capacity of the immune system to protect the recipient and the graft from infection is desirable to enable clinical xenotransplantation.

FasL microgels induce immune acceptance of islet allografts in nonhuman primates

Islet transplantation to treat insulin-dependent diabetes is greatly limited by the need for maintenance immunosuppression. We report a strategy through which cotransplantation of allogeneic islets and streptavidin (SA)-FasL-presenting microgels to the omentum under transient rapamycin monotherapy resulted in robust glycemic control, sustained C-peptide levels, and graft survival in diabetic nonhuman primates for >6 months. Surgical extraction of the graft resulted in prompt hyperglycemia. In contrast, animals receiving microgels without SA-FasL under the same rapamycin regimen rejected islet grafts acutely. Graft survival was associated with increased number of FoxP3+ cells in the graft site with no significant changes in T cell systemic frequencies or responses to donor and third-party antigens, indicating localized tolerance. Recipients of SA-FasL microgels exhibited normal liver and kidney metabolic function, demonstrating safety. This localized immunomodulatory strategy succeeded with unmodified islets and does not require long-term immunosuppression, showing translational potential in β cell replacement for treating type 1 diabetes.

Current Topics of Relevance to the Xenotransplantation of Free Pig Islets

Pig islet xenotransplantation is a potential treatment for patients with type 1 diabetes. Current efforts are focused on identifying the optimal pig islet source and overcoming the immunological barrier. The optimal age of the pig donors remains controversial since both adult and neonatal pig islets have advantages. Isolation of adult islets using GMP grade collagenase has significantly improved the quantity and quality of adult islets, but neonatal islets can be isolated at a much lower cost. Certain culture media and coculture with mesenchymal stromal cells facilitate neonatal islet maturation and function. Genetic modification in pigs affords a promising strategy to prevent rejection. Deletion of expression of the three known carbohydrate xenoantigens (Gal, Neu5Gc, Sda) will certainly be beneficial in pig organ transplantation in humans, but this is not yet proven in islet transplantation, though the challenge of the '4th xenoantigen' may prove problematic in nonhuman primate models. Blockade of the CD40/CD154 costimulation pathway leads to long-term islet graft survival (of up to 965 days). Anti-CD40mAbs have already been applied in phase II clinical trials of islet allotransplantation. Fc region-modified anti-CD154mAbs successfully prevent the thrombotic complications reported previously. In this review, we discuss (I) the optimal age of the islet-source pig, (ii) progress in genetic modification of pigs, (iii) the immunosuppressive regimen for pig islet xenotransplantation, and (iv) the reduction in the instant blood-mediated inflammatory reaction.

The Influence of Microenvironment on Survival of Intraportal Transplanted Islets

Clinical islet transplantation has the potential to cure type 1 diabetes. Despite recent therapeutic success, it is still uncommon because transplanted islets are damaged by multiple challenges, including instant blood mediated inflammatory reaction (IBMIR), inflammatory cytokines, hypoxia/reperfusion injury, and immune rejection. The transplantation microenvironment plays a vital role especially in intraportal islet transplantation. The identification and targeting of pathways that function as "master regulators" during deleterious inflammatory events after transplantation, and the induction of immune tolerance, are necessary to improve the survival of transplanted islets. In this article, we attempt to provide an overview of the influence of microenvironment on the survival of transplanted islets, as well as possible therapeutic targets.

Pancreatic islet cryopreservation by vitrification achieves high viability, function, recovery and clinical scalability for transplantation

Pancreatic islet transplantation can cure diabetes but requires accessible, high-quality islets in sufficient quantities. Cryopreservation could solve islet supply chain challenges by enabling quality-controlled banking and pooling of donor islets. Unfortunately, cryopreservation has not succeeded in this objective, as it must simultaneously provide high recovery, viability, function and scalability. Here, we achieve this goal in mouse, porcine, human and human stem cell (SC)-derived beta cell (SC-beta) islets by comprehensive optimization of cryoprotectant agent (CPA) composition, CPA loading and unloading conditions and methods for vitrification and rewarming (VR). Post-VR islet viability, relative to control, was 90.5% for mouse, 92.1% for SC-beta, 87.2% for porcine and 87.4% for human islets, and it remained unchanged for at least 9 months of cryogenic storage. VR islets had normal macroscopic, microscopic, and ultrastructural morphology. Mitochondrial membrane potential and adenosine triphosphate (ATP) levels were slightly reduced, but all other measures of cellular respiration, including oxygen consumption rate (OCR) to produce ATP, were unchanged. VR islets had normal glucose-stimulated insulin secretion (GSIS) function in vitro and in vivo. Porcine and SC-beta islets made insulin in xenotransplant models, and mouse islets tested in a marginal mass syngeneic transplant model cured diabetes in 92% of recipients within 24–48 h after transplant. Excellent glycemic control was seen for 150 days. Finally, our approach processed 2,500 islets with >95% islets recovery at >89% post-thaw viability and can readily be scaled up for higher throughput. These results suggest that cryopreservation can now be used to supply needed islets for improved transplantation outcomes that cure diabetes.

Optimization of vitrification approaches substantially improves pancreatic islet cryopreservation for banking and boosts transplantation outcomes in diabetes.

Clinically available immunosuppression averts rejection but not systemic inflammation after porcine islet xenotransplant in cynomolgus macaques

A safe, efficacious, and clinically applicable immunosuppressive regimen is necessary for islet xenotransplantation to become a viable treatment option for diabetes. We performed intraportal transplants of wild-type adult porcine islets in 25 streptozotocin-diabetic cynomolgus monkeys. Islet engraftment was good in 21, partial in 3, and poor in 1 recipient. Median xenograft survival was 25 days with rapamycin and CTLA4Ig immunosuppression. Adding basiliximab induction and maintenance tacrolimus to the base regimen significantly extended median graft survival to 147 days (p < .0001), with three animals maintaining insulin-free xenograft survival for 265, 282, and 288 days. We demonstrate that this regimen suppresses non-Gal anti-pig antibody responses, circulating effector memory T cell expansion, effector function, and infiltration of the graft. However, a chronic systemic inflammatory state manifested in the majority of recipients with long-term graft survival indicated by increased neutrophil to lymphocyte ratio, IL-6, MCP-1, CD40, and CRP expression. This suggests that this immunosuppression regimen fails to regulate innate immunity and resulting inflammation is significantly associated with increased incidence and severity of adverse events making this regimen unacceptable for translation. Additional studies are needed to optimize a maintenance regimen for regulating the innate inflammatory response.

Induction of Immune Tolerance in Islet Transplantation Using Apoptotic Donor Leukocytes

Allogeneic islet transplantation has become an effective treatment option for severe Type 1 diabetes with intractable impaired awareness due to hypoglycemic events. Although current immunosuppressive protocols effectively prevent the acute rejection associated with initial T cell activation in recipients, chronic rejection has remained an obstacle for achieving long-term allogeneic islet engraftment. The development of donor-specific immune tolerance to the allograft is the ultimate goal given its potential ability to overcome chronic rejection and disregard the need for maintenance immunosuppression, which may be toxic to islet grafts. Recently, a breakthrough in tolerance induction during allogeneic islet transplantation using apoptotic donor lymphocytes (ADLs) in a non-human primate model had been reported. Several studies have suggested that the clonal depletion, anergy, and expansion of the antigen-specific regulatory immune network are the mechanisms for donor-specific tolerance with ADLs, which act synergistically to induce robust transplant tolerance. This achievement represents a huge step forward toward the clinical application of immune tolerance induction. We herein summarize the reported operational induction therapies in islet transplantation using the ADLs. Moreover, a few obstacles for the engraftment of transplanted islets, such as islet immunogenicity and instant blood-mediated response, which need to be resolved in the future, are also discussed.

Restoring normal islet mass and function in type 1 diabetes through regenerative medicine and tissue engineering

Type 1 diabetes is characterised by autoimmune-mediated destruction of pancreatic β-cell mass. With the advent of insulin therapy a century ago, type 1 diabetes changed from a progressive, fatal disease to one that requires lifelong complex self-management. Replacing the lost β-cell mass through transplantation has proven successful, but limited donor supply and need for lifelong immunosuppression restricts widespread use. In this Review, we highlight incremental advances over the past 20 years and remaining challenges in regenerative medicine approaches to restoring β-cell mass and function in type 1 diabetes. We begin by summarising the role of endocrine islets in glucose homoeostasis and how this is altered in disease. We then discuss the potential regenerative capacity of the remaining islet cells and the utility of stem cell-derived β-like cells to restore β-cell function. We conclude with tissue engineering approaches that might improve the engraftment, function, and survival of β-cell replacement therapies.

Cellular Therapies in Solid Organ Allotransplantation: Promise and Pitfalls

Donor specific transfusions have been the basis of tolerance inducing protocols since Peter Medawar showed that it was experimentally feasible in the 1950s. Though trials of cellular therapies have become increasingly common in solid organ transplantation, they have not become standard practice. Additionally, whereas some protocols have focused on cellular therapies as a method for donor antigen delivery—thought to promote tolerance in and of itself in the correct immunologic context—other approaches have alternatively focused on the intrinsic immunosuppressive properties of the certain cell types with less emphasis on their origin, including mesenchymal stem cells, regulatory T cells, and regulatory dendritic cells. Regardless of intent, all cellular therapies must contend with the potential that introducing donor antigen in a new context will lead to sensitization. In this review, we focus on the variety of cellular therapies that have been applied in human trials and non-human primate models, describe their efficacy, highlight data regarding their potential for sensitization, and discuss opportunities for cellular therapies within our current understanding of the immune landscape.

Carbohydrate antigen microarray analysis of serum IgG and IgM antibodies before and after adult porcine islet xenotransplantation in cynomolgus macaques

Understanding the anti-carbohydrate antibody response toward epitopes expressed on porcine cells, tissues, and organs is critical to advancing xenotransplantation toward clinical application. In this study, we determined IgM and IgG antibody specificities and relative concentrations in five cynomolgus monkeys at baseline and at intervals following intraportal xenotransplantation of adult porcine islets. This study utilized a carbohydrate antigen microarray that comprised more than 400 glycoconjugates, including historically reported α-Gal and non-α-Gal carbohydrate antigens with various modifications. The elicited anti-carbohydrate antibody responses were predominantly IgM compared to IgG in 4 out of 5 monkeys. Patterns of elicited antibody responses greater than 1.5 difference (log2 base units; 2.8-fold on a linear scale) from pre-serum to post-serum sampling specific for carbohydrate antigens were heterogeneous and recipient-specific. Increases in the elicited antibody response to α-Gal, Sda, GM2 antigens, or Lexis X antigen were found in individual monkeys. The novel carbohydrate structures Galβ1-4GlcNAcβ1-3Galβ1 and N-linked glycans with Manα1-6(GlcNAcβ1-2Manα1-3)Manβ1-4GlcNAcβ structure were common targets of elicited IgM antibodies. These results provide important insights into the carbohydrate epitopes that elicit antibodies following pig-to-monkey islet xenotransplantation and reveal possible targets for gene editing.

The Yin and Yang of Type 1 Regulatory T Cells: From Discovery to Clinical Application

Regulatory T cells are essential players of peripheral tolerance and suppression of inflammatory immune responses. Type 1 regulatory T (Tr1) cells are FoxP3^- regulatory T cells induced in the periphery under tolerogenic conditions. Tr1 cells are identified as LAG3⁺CD49b⁺ mature CD4⁺ T cells that promote peripheral tolerance through secretion of IL-10 and TGF-β in addition to exerting perforin- and granzyme B-mediated cytotoxicity against myeloid cells. After the initial challenges of isolation were overcome by surface marker identification, ex vivo expansion of antigen-specific Tr1 cells in the presence of tolerogenic dendritic cells (DCs) and IL-10 paved the way for their use in clinical trials. With one Tr1-enriched cell therapy product already in a Phase I clinical trial in the context of allogeneic hematopoietic stem cell transplantation (allo-HSCT), Tr1 cell therapy demonstrates promising results so far in terms of efficacy and safety. In the current review, we identify developments in phenotypic and molecular characterization of Tr1 cells and discuss the potential of engineered Tr1-like cells for clinical applications of Tr1 cell therapies. More than 3 decades after their initial discovery, Tr1 cell therapy is now being used to prevent graft versus host disease (GvHD) in allo-HSCT and will be an alternative to immunosuppression to promote graft tolerance in solid organ transplantation in the near future.

Tr1 Cells as a Key Regulator for Maintaining Immune Homeostasis in Transplantation

The immune system is composed of effectors and regulators. Type 1 regulatory T (Tr1) cells are classified as a distinct subset of T cells, and they secret high levels of IL-10 but lack the expression of the forkhead box P3 (Foxp3). Tr1 cells act as key regulators in the immune network, and play a central role in maintaining immune homeostasis. The regulatory capacity of Tr1 cells depends on many mechanisms, including secretion of suppressive cytokines, cell-cell contacts, cytotoxicity and metabolic regulation. A breakdown of Tr1-cell-mediated tolerance is closely linked with the pathogenesis of various diseases. Based on this observation, Tr1-cell therapy has emerged as a successful treatment option for a number of human diseases. In this review, we describe an overview of Tr1 cell identification, functions and related molecular mechanisms. We also discuss the current protocols to induce/expand Tr1 cells in vitro for clinical application, and summarize the recent progress of Tr1 cells in transplantation.

Engineering Strategies for Allogeneic Solid Tissue Acceptance

Advances in allogeneic transplantation of solid organs and tissues depend on our understanding of mechanisms that mediate the prevention of graft rejection. For the past decades, clinical practice has established guidelines to prevent allograft rejection, which mostly rely on the intake of nontargeted immunosuppressants as the gold standard. However, such lifelong regimens have been reported to trigger severe morbidities and commonly fail in preventing late allograft loss. In this review, the biology of allogeneic rejection and self-tolerance is analyzed, as well as the mechanisms of cellular-based therapeutics driving suppression and/or tolerance. Bioinspired engineering strategies that take advantage of cells, biomaterials, or combinations thereof to prevent allograft rejection are addressed, as well as biological mechanisms that drive their efficacy.

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.

Applying Immunomodulation to Promote Longevity of Immunoisolated Pancreatic Islet Grafts

Islet transplantation is a promising therapy for insulin-dependent diabetes, but large-scale application is hampered by the lack of a consistent source of insulin-producing cells and need for lifelong administration of immunosuppressive drugs, which are associated with severe side effects. To avoid chronic immunosuppression, islet grafts can be enveloped in immunoisolating polymeric membranes. These immunoisolating polymeric membranes protect islet grafts from cell-mediated rejection while allowing diffusion of oxygen, nutrients, and insulin. Although clinical trials have shown the safety and feasibility of encapsulated islets to control glucose homeostasis, the strategy does up till now not support long-term graft survival. This partly can be explained by a significant loss of insulin-producing cells in the immediate period after implantation. The loss can be prevented by combining immunoisolation with immunomodulation, such as combined administration of immunomodulating cytokines or coencapsulation of immunomodulating cell types such as regulatory T cells, mesenchymal stem cells, or Sertoli cells. Also, administration of specific antibodies or apoptotic donor leucocytes is considered to create a tolerant microenvironment around immunoisolated grafts. In this review, we describe the outcomes and limitations of these approaches, as well as the recent progress in immunoisolating devices. Impact statement Immunoisolation by enveloping islets in semipermeable membranes allows for successful transplantation of islet grafts in the absence of chronic immunosuppression, but the duration of graft survival is still not permanent. The reasons for long-term final graft failure is not fully understood, but combining immunoisolation with immunomodulation of tissues or host immune system has been proposed to enhance the longevity of grafts. This article reviews the recent progress and challenges of immunoisolation, as well as the benefits and feasibility of combining encapsulation approaches with immunomodulation to promote longevity of encapsulated grafts.

Monotherapy With Anti-CD70 Antibody Causes Long-Term Mouse Cardiac Allograft Acceptance With Induction of Tolerogenic Dendritic Cells

Allograft rejection has been an obstacle for the long-term survival of patients. CD70, a tumor necrosis factor (TNF) family member critically expressed on antigen-presenting cells and strongly but transiently up-regulated during lymphocyte activation, represents an important co-stimulatory molecule that induces effective T cell responses. We used a mouse heterotopic cardiac transplantation model to evaluate the effects of monotherapy with the antibody targeting mouse CD70 (FR70) on transplantation tolerance and its immunoregulatory activity. FR70-treated C3H recipient mice permanently accepted B6 fully mismatched cardiac allografts. Consistent with the graft survival, the infiltration of CD8⁺ T cells in the graft was reduced, dendritic cells were differentiated into a tolerogenic status, and the number of regulatory T cells was elevated both in the graft and the recipient’s spleen. In addition, naïve C3H given an adoptive transfer of spleen cells from the primary recipients with FR70 treatment accepted a heart graft from a matching B6 donor but not third-party BALB/c mice.　Our findings show that treatment with FR70 induced regulatory cells and inhibited cytotoxic T cell proliferation, which led to long-term acceptance of mouse cardiac allografts. These findings highlight the potential role of anti-CD70 antibodies as a clinically effective treatment for allograft rejection.

Comprehensive analysis of gene expression of isolated pancreatic islets during pretransplant culture

Background:

The aim of this study was to investigate the effect of pretransplant culture on the survival of pancreatic islet grafts, and to determine the biological characteristics of isolated islets during pretransplant culture.

Methods:

The survival of islets from Wistar rats, transplanted to diabetic C57BL/B6 mice, was compared between fresh islets and cultured islets. A comprehensive gene expression analysis was employed to investigate biological processes during pretransplant culture, and in vitro validation studies were performed.

Results:

Survival of cultured xenografts was significantly prolonged as compared to that of fresh islets (fresh: 12.5 ± 1.9 days, 1-day cultured: 16.0 ± 1.3 days (p= 0.017), 3-day cultured: 17.0 ± 2.6 days (p= 0.014)). Comprehensive gene expression analysis identified significant upregulation of annotated functions associated with inflammation in cultured groups. Six proinflammatory genes, including heme oxygenase 1 (HO-1) and IL-6, were significantly upregulated during culture. Validation studies revealed significantly higher levels of IL-6 in the supernatant of cultured islets and HO-1 in the cultured islets when compared with fresh islets.

Conclusion:

Transplantation of cultured islets induced significant but minimal prolongation of graft survival in xenogeneic combinations. Comprehensive analysis of gene expression in cultured islets showed biological processes associated with proinflammation during culture.

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.

Serum cytokine profiles in healthy nonhuman primates are blunted by sedation and demonstrate sexual dimorphism as detected by a validated multiplex immunoassay

Cytokine profiling is a valuable tool for monitoring immune responses associated with disease and treatment. This study assessed the impact of sex and sedation on serum cytokines in healthy nonhuman primates (NHPs). Twenty-three cytokines were measured from serum using a bead-based multiplex assay. Assay validation for precision, sensitivity, recovery, linearity, and stability was performed. Samples from male and female cynomolgus and rhesus macaques either cooperating or sedated were compared. All cytokines except TNFα demonstrated acceptable sensitivity and precision, with variable recovery and linearity. IFNγ, IL-2, IL-5, IL-6, IL-8, IL-12/23 (p40), IL-13, IL-15, MCP-1, TGFα, VEGF met acceptance criteria; G-CSF, IL-4, IL-10, MIP1α, sCD40L were marginal. Higher cytokine levels were observed in females and cytokine levels were blunted in sedated NHPs when compared to awake cooperating NHPs. Significant differences observed in cytokines related to sex, species, or imposed by handling highlight the importance of model design on translational relevance for clinical settings.

Islet transplantation modulates macrophage to induce immune tolerance and angiogenesis of islet tissue in type I diabetes mice model

Objective To investigate the dual mechanism of islet transplantation in T1D by regulating the immune tolerance of macrophages and inducing the neovascularization.

Methods NC group, T1D model group and T1D model + islet group were constructed. Then, the abdominal aorta blood of abdominal aorta and islet tissue were collected. ELISA was performed to detect the level of IL-1Rα, IL-1α, IL-1β, CXCL2, MCP1, TNF-α and IL-10. Flow cytometry was used to measure the content of M1 and M2 macrophages. HE staining indicated the pathological characteristics of islet. IHC and WB were applied to determine the protein levels of IGF1R, FGFR2 or VEGFA as well as IGF1R, GRB2, EGFR, PTPN1, JAK2, STAT3, Caspase-1, Bcl2 respectively.

Results Islet transplantation in T1D stimulated the expression of IL-1Rα, IL-1α, IL-1β, CXCL2, MCP1, TNF-α and IL-10 in abdominal aorta blood, changed the content of MHCII⁺CD206^-M1 and MHCII⁺CD206⁺M2 macrophages, reduced the pathological features and the infiltration of immunocytes, promoted the expression of IGF1R, FGFR2 and VEGFA, eliminated cell apoptosis and induced the neovascularization in islet grafts.

Conclusions Islet transplantation is an effective strategy for the treatment of T1D. It can increase the content of M2 macrophages whose immune tolerance can elevate the survival of islet grafts, reduce the inflammatory responses mediated by macrophages, promote the neovascularization and eliminate the cell apoptosis of islet grafts.

Integration of Islet/Beta-Cell Transplants with Host Tissue Using Biomaterial Platforms

Cell-based therapies are emerging for type I diabetes mellitus (T1D), an autoimmune disease characterized by the destruction of insulin-producing pancreatic β-cells, as a means to provide long-term restoration of glycemic control. Biomaterial scaffolds provide an opportunity to enhance the manufacturing and transplantation of islets or stem cell-derived β-cells. In contrast to encapsulation strategies that prevent host contact with the graft, recent approaches aim to integrate the transplant with the host to facilitate glucose sensing and insulin distribution, while also needing to modulate the immune response. Scaffolds can provide a supportive niche for cells either during the manufacturing process or following transplantation at extrahepatic sites. Scaffolds are being functionalized to deliver oxygen, angiogenic, anti-inflammatory, or trophic factors, and may facilitate cotransplantation of cells that can enhance engraftment or modulate immune responses. This local engineering of the transplant environment can complement systemic approaches for maximizing β-cell function or modulating immune responses leading to rejection. This review discusses the various scaffold platforms and design parameters that have been identified for the manufacture of human pluripotent stem cell-derived β-cells, and the transplantation of islets/β-cells to maintain normal blood glucose levels.

Transplant strategies for type 1 diabetes: whole pancreas, islet and porcine beta cell therapies

Whole-organ pancreas and islet transplantations are performed in a highly selected group of patients with diabetes mellitus, primarily those with type 1 diabetes mellitus, complicated by recurrent severe hypoglycaemia or renal failure requiring kidney transplantation. Clinical accessibility to pancreases or islets, and patient characteristics and therapeutic goals, may dictate choice of procedure. Pancreas transplantation is most often performed simultaneous with a kidney transplant, but patients with particularly labile type 1 diabetes may be considered for a pancreas transplant alone. While highly successful at restoring insulin independence, pancreas transplants carry the significant risks of major surgery and immunosuppression. Islet transplantation is a relatively minor procedure, usually performed for labile type 1 diabetes with severe hypoglycaemia. It is highly successful at resolving hypoglycaemia, but more than one pancreas donor may be required for insulin independence. Both pancreas and islet transplantation are limited in applicability by a paucity of deceased donors. Pigs provide one promising replenishable source of islets. Porcine islets can successfully reverse diabetes mellitus in non-human primates under the appropriate immunosuppressive conditions, with promise for eventually translating this success to a larger population of patients with diabetes mellitus in the future. Graphical abstract.

MCMV Dissemination from Latently-Infected Allografts Following Transplantation into Pre-Tolerized Recipients

Transplantation tolerance is achieved when recipients are unresponsive to donor alloantigen yet mobilize against third-party antigens, including virus. After transplantation, cytomegalovirus (CMV) reactivation in latently-infected transplants reduces allograft viability. To determine if pre-tolerized recipients are resistant to viral dissemination in this setting, we transfused chemically-fixed donor splenocytes (1-ethyl-3- (3′-dimethyl-aminopropyl)-carbo-diimide (ECDI)-treated splenocytes (ECDIsp)) to induce donor antigen tolerance without immunosuppression. In parallel, we implanted donor islet cells to validate operational tolerance. These pre-tolerized recipients were implanted with murine CMV (MCMV) latently-infected donor kidneys (a validated model of CMV latency) to monitor graft inflammation and viral dissemination. Our results indicate that tolerance to donor islets was sustained in recipients after implantation of donor kidneys. In addition, kidney allografts implanted after ECDIsp and islet implantation exhibited low levels of fibrosis and tubulitis. In contrast, kidney cellular and innate immune infiltrates trended higher in the CMV group and exhibited increased markers of CD8⁺ T cell activation. Tolerance induction was unable to prevent increases in MCMV-specific CD8⁺ T cells or dissemination of viral IE-1 DNA. Our data suggest that latently-infected allografts are inherently more susceptible to inflammation that is associated with viral dissemination in pre-tolerized recipients. Thus, CMV latently-infected allografts require enhanced strategies to protect allograft integrity and viral spread.

Localized Immunomodulation with PD-L1 Results in Sustained Survival and Function of Allogeneic Islets without Chronic Immunosuppression

Allogeneic islet transplantation is limited by adverse effects of chronic immunosuppression used to control rejection. The programmed cell death 1 pathway as an important immune checkpoint has the potential to obviate the need for chronic immunosuppression. We generated an oligomeric form of programmed cell death 1 ligand chimeric with core streptavidin (SA-PDL1) that inhibited the T effector cell response to alloantigens and converted T conventional cells into CD4⁺Foxp3⁺ T regulatory cells. The SA-PDL1 protein was effectively displayed on the surface of biotinylated mouse islets without a negative impact islet viability and insulin secretion. Transplantation of SA-PDL1-engineered islet grafts with a short course of rapamycin regimen resulted in sustained graft survival and function in >90% of allogeneic recipients over a 100-d observation period. Long-term survival was associated with increased levels of intragraft transcripts for innate and adaptive immune regulatory factors, including IDO-1, arginase-1, Foxp3, TGF-β, IL-10, and decreased levels of proinflammatory T-bet, IL-1β, TNF-α, and IFN-γ as assessed on day 3 posttransplantation. T cells of long-term graft recipients generated a proliferative response to donor Ags at a similar magnitude to T cells of naive animals, suggestive of the localized nature of tolerance. Immunohistochemical analyses showed intense peri-islet infiltration of T regulatory cells in long-term grafts and systemic depletion of this cell population resulted in prompt rejection. The transient display of SA-PDL1 protein on the surface of islets serves as a practical means of localized immunomodulation that accomplishes sustained graft survival in the absence of chronic immunosuppression with potential clinical implications.