Differential susceptibility of heart, skin, and islet allografts to T cell-mediated rejection

The potential for Treg-enhancing therapies in transplantation

Since the discovery of regulatory T cells (Tregs) as crucial regulators of immune tolerance against self-antigens, these cells have become a promising tool for the induction of donor-specific tolerance in transplantation medicine. The therapeutic potential of increasing in vivoTreg numbers for a favorable Treg to Teff cell ratio has already been demonstrated in several sophisticated pre-clinical models and clinical pilot trials. In addition to improving cell quantity, enhancing Treg function utilizing engineering techniques led to encouraging results in models of autoimmunity and transplantation. Here we aim to discuss the most promising approaches for Treg-enhancing therapies, starting with adoptive transfer approaches and ex vivoexpansion cultures (polyclonal vs. antigen specific), followed by selective in vivostimulation methods. Furthermore, we address next generation concepts for Treg function enhancement (CARs, TRUCKs, BARs) as well as the advantages and caveats inherit to each approach. Finally, this review will discuss the clinical experience with Treg therapy in ongoing and already published clinical trials; however, data on long-term results and efficacy are still very limited and many questions that might complicate clinical translation remain open. Here, we discuss the hurdles for clinical translation and elaborate on current Treg-based therapeutic options as well as their potencies for improving long-term graft survival in transplantation.

Regulatory T Cells: Liquid and Living Precision Medicine for the Future of VCA

Transplant rejection remains a challenge especially in the field of vascularized composite allotransplantation (VCA). To blunt the alloreactive immune response' stable levels of maintenance immunosupression are required. However' the need for lifelong immunosuppression poses the risk of severe side effects, such as increased risk of infection, metabolic complications, and malignancies. To balance therapeutic efficacy and medication side effects, immunotolerance promoting immune cells (especially regulatory T cells [Treg]) have become of great scientific interest. This approach leverages immune system mechanisms that usually ensure immunotolerance toward self-antigens and prevent autoimmunopathies. Treg can be bioengineered to express a chimeric antigen receptor or a T-cell receptor. Such bioengineered Treg can target specific antigens and thereby reduce unwanted off-target effects. Treg have demonstrated beneficial clinical effects in solid organ transplantation and promising in vivo data in VCAs. In this review, we summarize the functional, phenotypic, and immunometabolic characteristics of Treg and outline recent advancements and current developments regarding Treg in the field of VCA and solid organ transplantation.

CD8 T Cells Target Antigen Cross-Presented by Bone Marrow Derived Cells to Induce Bystander Rejection of Grafts Lacking the Cognate Peptide-MHC

CD8 T cells play a key role in cancer immunotherapy and allograft rejection. However, it is not clear how they kill cells and tissues that do not have the agonist peptide-major histocompatibility complex (MHC) on their surface, as in the settings of MHC class I deficient tumors and indirect rejection of MHC-mismatched transplants. CD8 T cells might respond to agonist antigen cross-presented on hematopoietic cells, leading to a "bystander" rejection. Alternatively, they may recognize agonist antigen cross-presented on recipient endothelial cells and kill the tissue's vital blood supply. The latter mechanism predicts that all non-vascularized grafts, grafts dependent on in-growth of recipient blood vessels, will be susceptible to CD8 T cell mediated indirect rejection. In contrast, we show here that non-vascularized transplants, bearing the same agonist antigen, are not universally susceptible to this rejection pathway. Non-vascularized skin, but not islet or heart tissue transplants were indirectly rejected by CD8 T cells. Furthermore, CD8 T cells were able to indirectly reject skin grafts when recipient MHC class I expression was restricted to bone marrow derived cells but not when it was restricted to radioresistant cells (e.g. endothelial cells). These findings argue against a major role for endothelial cell cross-presentation in killing of tissue that does not present the agonist peptide-MHC class I. Instead, the data suggests that cross-presentation by recipient hematopoietic cells underlies the CD8 T cell mediated killing of tissue that is unable to directly present the target peptide-MHC class I.

The Protective Effect of the Soluble Egg Antigen of Schistosoma japonicum in A Mouse Skin Transplantation Model

Background

Organ transplantation is currently an effective method for treating organ failure. Long-term use of immunosuppressive drugs has huge side effects, which severely restricts the long-term survival of patients. Schistosoma can affect the host’s immune system by synthesizing, secreting, or excreting a variety of immunomodulatory molecules, but its role in transplantation was not well defined. In order to explore whether Schistosoma-related products can suppress rejection and induce long-term survival of the transplant, we used soluble egg antigen (SEA) of Schistosoma japonicum in mouse skin transplantation models.

Materials and methods

Each mouse was intraperitoneally injected with 100 μg of SEA three times a week for four consecutive weeks before allogenic skin transplant. Skin transplants were performed on day 0 to observe graft survival. Pathological examination of skin grafts was conducted 7 days post transplantation. The skin grafts were subjected to mRNA sequencing. Bioinformatics analysis was conducted and the expression of hub genes was verified by qPCR. Flow cytometry analysis was performed to evaluate the immune status and validate the results from bioinformatic analysis.

Results

The mean survival time (MST) of mouse skin grafts in the SEA-treated group was 11.67 ± 0.69 days, while that of the control group was 8.00 ± 0.36 days. Pathological analysis showed that Sj SEA treatment led to reduced inflammatory infiltration within skin grafts 7 days after allogenic skin transplantation. Bioinformatics analysis identified 86 DEGs between the Sj SEA treatment group and the control group, including 39 upregulated genes and 47 downregulated genes. Further analysis revealed that Sj SEA mediated regulation on cellular response to interferon-γ, activation of IL-17 signaling and chemokine signaling pathways, as well as cytokine–cytokine receptor interaction. Flow cytometry analysis showed that SEA treatment led to higher percentages of CD4⁺IL-4⁺ T cells and CD4⁺Foxp3⁺ T cells and decreased CD4⁺IFN-γ⁺ T cells in skin transplantation.

Conclusion

Sj SEA treatment suppressed rejection and prolonged skin graft survival by regulating immune responses. Sj SEA treatment might be a potential new therapeutic strategy to facilitate anti-rejection therapy and even to induce tolerance.

Evaluation of immunosuppression protocols for MHC-matched allogeneic iPS cell-based transplantation using a mouse skin transplantation model

BACKGROUND

Off-the-shelf major histocompatibility complex (MHC)-matched iPS cells (iPSC) can potentially initiate host immune responses because of the existence of numerous minor antigens. To suppress allo-immune responses, combination of immunosuppressants is usually used, but its efficacy to the allogeneic iPSC-based transplantation has not been precisely evaluated.

METHODS

Three transplantation models were used in this study; MHC-matched, minor antigen-mismatched mouse skin or iPSC-graft transplantation, and fully allogeneic human iPSC-derived liver organoid transplantation in immune-humanized mice. The recipients were treated with triple drugs combination (TDC; tacrolimus, methylprednisolone, and mycophenolate mofetil) or co-stimulatory molecule blockade (CB) therapy with some modifications. Graft survival as well as anti-donor T and B cell responses was analyzed.

RESULTS

In the mouse skin transplantation model, immunological rejection caused by the minor antigen-mismatch ranged from mild to severe according to the donor-recipient combination. The TDC treatment could apparently control the mild skin graft rejection when combined with a transient T cell depletion, but unexpected anti-donor T or B cell response was observed. On the other hand, CB therapy, particularly when combined with rapamycin treatment, was capable of attenuating both mild and severe skin graft rejection and allowing them to survive long-term without any unfavorable anti-donor immune responses. The efficacy of the CB therapy was confirmed in both mouse and human iPSC-derived graft transplantation.

CONCLUSIONS

The findings suggest that the CB-based treatment seems suitable to well manage the MHC-matched allogeneic iPSC-based transplantation. The TDC-based treatment may be also used to suppress the rejection, but screening of its severity prior to the transplantation seems to be needed.

Using Radiographic Domain for Evaluating Indications in Abdominal Wall Transplantation

BACKGROUND

There is currently no description of abdominal domain changes in small bowel transplantation population or consensus of criteria regarding which patients are at high risk for immediate postoperative abdominal wall complications or would benefit from abdominal wall vascularized composite allotransplantation.

METHODS

A retrospective chart review was performed on 14 adult patients receiving intestinal or multivisceral transplantation. Preoperative and postoperative computed tomography scans were reviewed, and multiple variables were collected regarding abdominal domain and volume and analyzed comparing postoperative changes and abdominal wall complications.

RESULTS

Patients after intestinal or multivisceral transplantation had a mean reduction in overall intraperitoneal volume in the immediate postoperative period from 9031 cm3 to 7846 cm3 (P = 0.314). This intraperitoneal volume was further reduced to an average of 6261 cm3 upon radiographic evaluation greater than 1 year postoperatively (P = 0.024). Patients with preexisting abdominal wound (P = 0.002), radiation, or presence of ostomy (P = 0.047) were significantly associated with postoperative abdominal wall complications. No preoperative radiographic findings had a significant association with postoperative abdominal wall complications.

CONCLUSIONS

Computed tomography imaging demonstrates that intestinal and multivisceral transplant patients have significant reduction in intraperitoneal volume and domain after transplantation in the acute and delayed postoperative setting. Preoperative radiographic abdominal domain was not able to predict patients with postoperative abdominal wall complications. Patients with abdominal wounds, ostomies, and preoperative radiation therapy were associated with acute postoperative abdominal complications and may be considered for need of reconstructive techniques including abdominal wall transplantation.

Tackling cancer cell dormancy: Insights from immune models, and transplantation

Disseminated non-dividing (dormant) cancer cells as well as those in equilibrium with the immune response remain the major challenge for successful treatment of cancer. The equilibrium between disseminated dormant cancer cells and the immune system is reminiscent of states that can occur during infection or allogeneic tissue and cell transplantation. We discuss here the major competing models of how the immune system achieves a self nonself discrimination (pathogen/danger patterns, quorum, and coinhibition/tuning models), and suggest that taking advantage of a combination of the proposed mechanisms in each model may lead to increased efficacy in tackling cancer cell dormancy.

Skin Immunology and Rejection in VCA and Organ Transplantation

Purpose of Review

Skin provides a window into the health of an individual. Using transplanted skin as a monitor can provide a powerful tool for surveillance of rejection in a transplant. The purpose of this review is to provide relevant background to the role of skin in vascularized transplantation medicine.

Recent Findings

Discrete populations of T memory cells provide distributed immune protection in skin, and cycle between skin, lymph nodes, and blood. Skin-resident TREGcells proliferate in response to inflammation and contribute to long-term VCA survival in small animal models. Early clinical studies show sentinel flap rejection to correlate well with facial VCA skin rejection, and abdominal wall rejection demonstrates concordance with visceral rejection, but further studies are required.

Summary

This review focuses on the immunology of skin, skin rejection in vascularized composite allografts, and the recent advances in monitoring the health of transplanted tissues using distant “sentinel” flaps.

Induction of Allograft Tolerance While Maintaining Immunity Against Microbial Pathogens: Does Coronin 1 Hold a Key?

Selective suppression of graft rejection while maintaining anti-pathogen responses has been elusive. Thus far, the most successful strategies to induce suppression of graft rejection relies on inhibition of T-cell activation. However, the very same mechanisms that induce allograft-specific T-cell suppression are also important for immunity against microbial pathogens as well as oncogenically transformed cells, resulting in significant immunosuppression-associated comorbidities. Therefore, defining the pathways that differentially regulate anti-graft versus antimicrobial T-cell responses may allow the development of regimen to induce allograft-specific tolerance. Recent work has defined a molecular pathway driven by the immunoregulatory protein coronin 1 that regulates the phosphodiesterase/cyclic adenosine monophosphate pathway and modulates T cell responses. Interestingly, disruption of coronin 1 promotes allograft tolerance while immunity towards a range of pathogenic microbes is maintained. Here, we briefly review the work leading up to these findings as well as their possible implications for transplantation medicine.

Mechanical Irritation in Vascularized Composite Tissue Allotransplantation Triggers Localized Skin Rejection

BACKGROUND

Mechanical and thermal stress has been observed to trigger skin rejection in hand-transplanted patients. This study aims to investigate this phenomenon.

METHODS

Syngeneic and allogeneic orthotopic hindlimb transplantations were performed using male rats (Brown Norway to Lewis). Using a specially designed device, standardized mechanical skin irritation at a force of 5 N was applied to the planta pedis of the transplanted limb for 10 days, 4 times daily for 10 minutes. Biopsies, taken on day 10 and after a 5-day observational period, were assessed for macroscopic alterations using a standardized scale, by histopathology and immunohistochemistry, and for inflammatory protein expression using Luminex technology.

RESULTS

Allogeneic animals displayed significant aggravated macroscopic skin alterations compared with naive (P < 0.0001) and syngeneic controls (P = 0.0023). Histopathology showed a trend toward higher rejection/inflammation grades in allogeneic animals compared with syngeneic controls. Minor skin alterations in syngeneic limbs recovered quickly; however, in allogeneic limbs, macroscopic skin alterations were significantly more pronounced (P < 0.0001) 5 days after irritation. Interleukin-1b and interferon-γ levels were upregulated in skin of allogeneic limbs.

CONCLUSIONS

Mechanical skin irritation in vascularized composite allotransplantation can trigger localized skin inflammation consistent with rejection.

Dominant regulation of long-term allograft survival is mediated by microRNA-142

Organ transplantation is often lifesaving, but the long-term deleterious effects of combinatorial immunosuppression regimens and allograft failure cause significant morbidity and mortality. Long-term graft survival in the absence of continuing immunosuppression, defined as operational tolerance, has never been described in the context of multiple major histocompatibility complex (MHC) mismatches. Here, we show that miR-142 deficiency leads to indefinite allograft survival in a fully MHC mismatched murine cardiac transplant model in the absence of exogenous immunosuppression. We demonstrate that the cause of indefinite allograft survival in the absence of miR-142 maps specifically to the T cell compartment. Of therapeutic relevance, temporal deletion of miR-142 in adult mice prior to transplantation of a fully MHC mismatched skin allograft resulted in prolonged allograft survival. Mechanistically, miR-142 directly targets Tgfbr1 for repression in regulatory T cells (TREG ). This leads to increased TREG sensitivity to transforming growth factor - beta and promotes transplant tolerance via an augmented peripheral TREG response in the absence of miR-142. These data identify manipulation of miR-142 as a promising approach for the induction of tolerance in human transplantation.

T cell exhaustion is associated with antigen abundance and promotes transplant acceptance

Exhaustion of T cells limits their ability to clear chronic infections or eradicate tumors. Here, in the context of transplant, we investigated whether T cell exhaustion occurs and has a role in determining transplant outcome. A peptide/MHC tetramer-based approach was used to track exhausted CD8⁺ T cells in a male-to-female skin transplant model. Transplant of large whole-tail skins, but not small tail skins (0.8 cm × 0.8 cm), led to exhaustion of anti-male tetramer⁺ CD8⁺ T cells and subsequently the acceptance of skin grafts. To study CD4⁺ T cell exhaustion, we used the TCR-transgenic B6 TEa cells that recognize a major transplant antigen I-Eα from Balb/c mice. TEa cells were adoptively transferred either into B6 recipients that received Balb/c donor skins or into CB6F1 mice that contained an excessive amount of I-Eα antigen. Adoptively transferred TEa cells in skin-graft recipients were not exhausted. By contrast, virtually all adoptively transferred TEa cells were exhausted in CB6F1 mice. Those exhausted TEa cells lost ability to reject Balb/c skins upon further transfer into lymphopenic B6.Rag1^-/- mice. Hence, T cell exhaustion develops in the presence of abundant antigen and promotes transplant acceptance. These findings are essential for better understanding the nature of transplant tolerance.

Effectiveness of Helminth Therapy in the Prevention of Allograft Rejection: A Systematic Review of Allogeneic Transplantation

Background: The unique immunomodulatory capacity of helminth parasites has been investigated as a novel strategy in the prevention of allograft rejection after transplantation. This review was conducted to fully evaluate the specific effects of helminth therapy on allograft survival reported in published studies of animal models of allogeneic transplantation.

Method: Following PRISMA protocol guidelines, a literature search was conducted using PubMed, MEDLINE via OvidSP, along with additional manual searches of selected reference lists. Publications describing helminth intervention within allograft transplantation models were screened for relevance to eligibility criteria. Primary and secondary outcomes were extracted using standardized data collection tables. The SYRCLE risk of bias assessment tool was used for quality assessment. Due to heterogeneity of study designs, meta-analysis could not be performed; rather outcomes are presented as a narrative synthesis with concept mapping. This review was registered in PROSPERO with ID: CRD42018097175.

Results: The literature search generated 1,443 publications, which after screening for relevance to the eligibility criteria yielded 15 publications for qualitative analysis. All 15 publications reported improvement to allograft survival as a result of helminth therapy. This prolonged allograft survival was not significantly different when helminth-derived products were used compared to live infection. However, the extent of positive impact on allograft survival was noted to be dependent on study design factors, such as the chronicity of the live helminth infection, allograft type and the species/genus of helminth selected.

Conclusion: Both live and product-based helminth therapy have potential applications as novel immune regulators or adjuncts for the prevention of allograft rejection. However, there were differences in efficacy between different worms and preparations of worm-derived products. Therefore, further studies are required to determine the most appropriate worm for a specific allograft, to elucidate the optimal dose and route of administration, and to better understand the modulation of immune responses that can mediate tolerance.

Synchronous Abdominal Wall and Small-bowel Transplantation: A 1-year Follow-up

Abdominal wall-vascularized composite allotransplantation (AW-VCA) has evolved as a technically feasible but challenging option in the rare event of abdominal wall reconstruction in patients whose abdomen cannot be closed by applying conventional methods. The authors conducted the first synchronous child-to-adult recipient AW-VCA using an arteriovenous loop technique. This article presents a 1-year follow-up of the patient's postoperative course. Frequent skin biopsies were performed in accordance with Duke Institutional Review Board protocol, with 3 episodes of rejection treated with high-dose steroids and Thymoglobulin (Genzyme Corp, Cambridge, Mass.). The patient developed an opportunistic fungal brain abscess secondary to immunosuppression, which led to temporary upper extremity weakness. Future considerations for AW-VCA include a modified surgical technique involving utilization of donor vein graft for arteriovenous loop formation. In addition, reduction in postoperative biopsy schedule and changes in immunosuppression regimen may lead to improved outcomes and prevent unnecessary high-dose immunosuppression.

Establishment of an experimental model for MHC homo-to-hetero transplantation

Preventing rejection is a major challenge in transplantation medicine, even when using pluripotent stem cell-derived grafts. In iPS cell (iPSC)-based transplantation, to reduce the risk of rejection, it is thought to be optimal that preparing the cells from donors whose human leukocyte antigen-haplotype are homozygous. Generally, this approach is referred to as major histocompatibility complex (MHC) homo-to-hetero transplantation, which is MHC-matched but minor antigen-mismatched. To investigate the immune response in the MHC homo-to-hetero transplantation, we established a murine experimental system in which MHC-matched but minor antigen-mismatched tissue (skin) grafts were transplanted into MHC-heterozygous recipients. Unexpectedly, only minor antigen-mismatched grafts were rejected at the same time points as rejection of fully allogeneic grafts. A vigorous anti-donor type T cell response was detected in vitro and conventional immunosuppressants targeting T cell activation had limited effects on controlling rejection. However, anti-donor antibodies were not detected only in the minor antigen-mismatched transplantation. This murine transplantation model can be used to further analyze immunological subjects for MHC homo-to-hetero iPSC-based transplantation.

Immune reaction and regulation in transplantation based on pluripotent stem cell technology

The development of pluripotent stem cell (PSC)-based technologies provides us a new therapeutic approach that generates grafts for transplantation. In order to minimize the risk of immune reaction, the banking of induced pluripotent stem cells (iPSCs) from donors with homozygous human leukocyte antigen (HLA) haplotype is planned in Japan. Even though pre-stocked and safety validated HLA-homozygous iPSCs are selected, immunological rejection may potentially occur because the causes of rejection are not always due to HLA mismatches. A couple of studies concerning such immunological issues have reported that genetic ablation of HLA molecules from PSC combined with gene transduction of several immunoregulatory molecules may be effective in avoiding immunological rejection. Also, our research group has recently proposed a concept that attempts to regulate recipient immune system by PSC-derived immunoregulatory cells, which results in prolonged survival of the same PSC-derived allografts. PSC-based technologies enable us to choose a new therapeutic option; however, considering its safety from an immunological point of view should be of great importance for safe clinical translation of this technology.

Pancreatic islets engineered with a FasL protein induce systemic tolerance at the induction phase that evolves into long-term graft-localized immune privilege

We have previously shown that pancreatic islets engineered to transiently display a modified form of FasL protein (SA-FasL) on their surface survive indefinitely in allogeneic recipients without a need for chronic immunosuppression. Mechanisms that confer long-term protection to allograft are yet to be elucidated. We herein demonstrated that immune protection evolves in two distinct phases; induction and maintenance. SA-FasL-engineered allogeneic islets survived indefinitely and conferred protection to a second set of donor-matched, but not third-party, unmanipulated islet grafts simultaneously transplanted under the contralateral kidney capsule. Protection at the induction phase involved a reduction in the frequency of proliferating alloreactive T cells in the graft-draining lymph nodes, and required phagocytes and TGF-β. At the maintenance phase, immune protection evolved into graft site-restricted immune privilege as the destruction of long-surviving SA-FasL-islet grafts by streptozotocin followed by the transplantation of a second set of unmanipulated islet grafts into the same site from the donor, but not third party, resulted in indefinite survival. The induced immune privilege required both CD4⁺ CD25⁺ Foxp3⁺ Treg cells and persistent presence of donor antigens. Engineering cell and tissue surfaces with SA-FasL protein provides a practical, efficient, and safe means of localized immunomodulation with important implications for autoimmunity and transplantation.

Efficient generation of thymic epithelium from induced pluripotent stem cells that prolongs allograft survival

The thymus plays a significant role in establishing immunological self-tolerance. Previous studies have revealed that host immune reaction to allogeneic transplants could be regulated by thymus transplantation. However, physiological thymus involution hinders the clinical application of these insights. Here, we report an efficient generation of thymic epithelial-like tissue derived from induced pluripotent stem cells (iPSCs) and its potential to regulate immune reaction in allogeneic transplantation. We established an iPSC line which constitutively expresses mouse Foxn1 gene and examined the effect of its expression during in vitro differentiation of thymic epithelial cells (TECs). We found that Foxn1 expression enhances the differentiation induction of cells expressing TEC-related cell surface molecules along with upregulation of endogenous Foxn1. iPSC-derived TECs (iPSC-TECs) generated T cells in nude recipient mice after renal subcapsular transplantation. Moreover, iPSC-TEC transplantation to immuno-competent recipients significantly prolonged the survival of allogeneic skin. Our study provides a novel concept for allogeneic transplantation in the setting of regenerative medicine.

Circulating T follicular helper cells are a biomarker of humoral alloreactivity and predict donor-specific antibody formation after transplantation

Donor-specific antibodies (DSAs) contribute to renal allograft loss. However, biomarkers to guide clinical management of DSA posttransplant or detect humoral alloimmune responses before alloantibodies develop are not available. Circulating T follicular helper (cTfh) cells are CD4⁺ CXCR5⁺ Tfh-like cells in the blood that have been associated with alloantibodies in transplant recipients, but whether they precede antibody formation for their evaluation as a predictive biomarker in transplant is unknown. To evaluate the ability of cTfh cells to predict DSA, we used murine transplant models to determine the temporal relationship between cTfh cells, germinal center formation, and DSA development. We observed that donor-reactive CD4⁺ CXCR5⁺ cTfh cells expand after allotransplant. These cTfh cells were equivalent to graft-draining lymph node-derived Tfh cells in their ability to provide B cell help for antibody production. cTfh cell expansion and differentiation into ICOS⁺ PD-1⁺ cells temporally correlated with germinal center alloreactivity and preceded the generation of DSAs in instances of modified and unmodified alloantibody formation. Importantly, delayed costimulation blockade initiated after the detection of ICOS⁺ PD-1⁺ cTfh cells prevented DSAs. These findings suggest that cTfh cells could serve as a biomarker for humoral alloreactivity before the detection of alloantibodies and inform therapeutic approaches to prevent DSAs.

Comparison of outcomes in lung and heart transplant recipients from the same multiorgan donor

BACKGROUND

Primary graft dysfunction (PGD) and acute cellular rejection (ACR) are important causes of early morbidity and mortality following lung and heart transplantation. While many studies have elucidated donor-related risk factors of PGD and ACR, these complications often occur even with "ideal" donors. Therefore, we investigated potential associations of PGD and ACR between bilateral lung and heart transplant recipients from the same multiorgan donor, respectively.

METHODS

Between 2011 and 2017, 100 donors contributed 100 bilateral lung transplants and 100 heart transplants performed. Logistic regression analysis for PGD and Cox proportional hazards regression analysis for ACR were used to estimate the relationship of heart and lung transplants.

RESULTS

The incidence of PGD was 33% among lung and 17% among heart transplant recipients. Similarly, the incidence of ACR grade ≥ A2 for lung recipients was 38% (30/80), and the incidence of ACR grade ≥ 2R for heart recipients was 19% (15/80). There was no association between the development of PGD and ACR in lung and heart transplant recipients from the same donor, respectively.

CONCLUSIONS

These findings suggest that inherent donor factors are not critical to the development of PGD and ACR after lung and heart transplantation.

Mast Cell Degranulation Exacerbates Skin Rejection by Enhancing Neutrophil Recruitment

Recent evidences indicate an important role of tissue inflammatory responses by innate immune cells in allograft acceptance and survival. Here we investigated the role of mast cells (MC) in an acute male to female skin allograft rejection model using red MC and basophil (RMB) mice enabling conditional MC depletion. Kinetic analysis showed that MCs markedly accelerate skin rejection. They induced an early inflammatory response through degranulation and boosted local synthesis of KC, MIP-2, and TNF. This enhanced early neutrophil infiltration compared to a female-female graft-associated repair response. The uncontrolled neutrophil influx accelerated rejection as antibody-mediated depletion of neutrophils delayed skin rejection. Administration of cromolyn, a MC stabilizer and to a lesser extent ketotifen, a histamine type I receptor antagonist, and absence of MCPT4 chymase also delayed graft rejection. Together our data indicate that mediators contained in secretory granules of MC promote an inflammatory response with enhanced neutrophil infiltration that accelerate graft rejection.

[The treatment by expanded ex vivo autologous regulatory T-cells CD4+CD25+FoxP3+CD127low restores the balance of immune system in patients with remitting-relapsing multiple sclerosis]

AIM

To assess clinical efficacy and safety of the autologous (own) regulatory T-cells (Tregs)CD4+CD25+Foxp3+CD127low isolated from the blood of patients with remitting-relapsing multiple sclerosis. Patients with autoimmune diseases have the decreased number of peripheral Tregs (pTreg) and impaired suppressive ability. In order to restore levels of pTreg, it is possible to isolate precursor cells, enter expanded ex vivo autologous Treg cells and introduce an expanded amount of autologous cells as Treg vaccine.

MATERIAL AND METHODS

A method of ex vivo Tregs expansion by 30-40 times within 5-7 days has been developed. Expanded ex vivo Tregs are more than 90% CD4+CD25+Foxp3+CD127low and have high suppressor activity. Fourteen patients with remitting-relapsing multiple sclerosis were included in pilot studies.Ex vivoTregs were introduced subcutaneously in dosefrom 2.8 to 4.5 108 cell per injection. The duration of follow-up was 1 year.

RESULTS AND CONCLUSION

The numbers of pTregs in the blood of these patients elevated by 1.5-2 times. No adverse-effects, a decrease of relapses and stabilization of disability index were observed. It has been suggested that ex vivo expanded Tregs can compensate the impaired function of pTregs and can be used for adoptive immunotherapyof multiple sclerosis.

Development of transplant immunosuppressive agents - considerations in the use of animal models

INTRODUCTION

The development of all immunosuppressant agents to date has involved the experimental use of large and small animal models. Over the last half-century, immunosuppressive drugs have extended the lives of transplant patients worldwide. However, the use of animal models in the development of these drugs is not perfect, and this has brought to light a number of issues including idiosyncratic reactions that are found in animal models but not in humans. The 2006 highly publicized case of the 'elephant man' TGN 1412 drug trial highlights the importance of being cogent of the limitations of animal models. Areas covered: This review covers the utility and limitations of the use of animal models for the development of immunosuppressant agents. This includes both large and small animal models, particularly rodent models in the transplant setting. Expert opinion: The use of animal models represents a critical stage in the development of immunosuppressive drugs. Limitations include physiological differences to humans; this is especially true of immunologically naïve lab rodents with small memory cell populations. Toxic drug levels may differ widely between species. Animal models are also costly and raise ethical concerns. However, there is currently no way to recreate the complex environment of the human immune system purely in vitro.

T cell exhaustion implications during transplantation

Exhaustion of lymphocyte function, particularly T cell exhaustion, due to prolonged exposure to a high load of foreign antigen is commonly seen during chronic viral infection as well as antitumor immune responses. This phenomenon has been associated with a determined molecular mechanism and phenotypic manifestations on the cell surface. In spite of investigation of exhaustion, mostly about CD8 responses toward viral infections, recent studies have reported that chronic exposure to antigen may develop exhaustion in CD4 + T cells, B cells, and NK cells. Little is known with respect to lymphocyte exhaustion during transplantation and its effect on aberrant anti-graft responses. Through a same mechanobiology observed during chronic exposure of foreign viral antigens, alloantigen persistence mediated by allograft could develop a favorable circumstance for exhaustion of T cells responding to allograft. However, to achieve better manipulation approaches of this event to reduce the complications during transplantation, we need to be armed with a bulk of knowledge with regard to quality and quantity of T cell exhaustion occurring in various allografts, the kinetics of exhaustion development, the impression of immunosuppressive agents on the exhaustion, and the influence of exhaustion on graft survival and immune tolerance.

Immunological challenges associated with artificial skin grafts: available solutions and stem cells in future design of synthetic skin

The repair or replacement of damaged skins is still an important, challenging public health problem. Immune acceptance and long-term survival of skin grafts represent the major problem to overcome in grafting given that in most situations autografts cannot be used. The emergence of artificial skin substitutes provides alternative treatment with the capacity to reduce the dependency on the increasing demand of cadaver skin grafts. Over the years, considerable research efforts have focused on strategies for skin repair or permanent skin graft transplantations. Available skin substitutes include pre- or post-transplantation treatments of donor cells, stem cell-based therapies, and skin equivalents composed of bio-engineered acellular or cellular skin substitutes. However, skin substitutes are still prone to immunological rejection, and as such, there is currently no skin substitute available to overcome this phenomenon. This review focuses on the mechanisms of skin rejection and tolerance induction and outlines in detail current available strategies and alternatives that may allow achieving full-thickness skin replacement and repair.

A structurally distinct TGF-β mimic from an intestinal helminth parasite potently induces regulatory T cells

Helminth parasites defy immune exclusion through sophisticated evasion mechanisms, including activation of host immunosuppressive regulatory T (Treg) cells. The mouse parasite Heligmosomoides polygyrus can expand the host Treg population by secreting products that activate TGF-β signalling, but the identity of the active molecule is unknown. Here we identify an H. polygyrus TGF-β mimic (Hp-TGM) that replicates the biological and functional properties of TGF-β, including binding to mammalian TGF-β receptors and inducing mouse and human Foxp3⁺ Treg cells. Hp-TGM has no homology with mammalian TGF-β or other members of the TGF-β family, but is a member of the complement control protein superfamily. Thus, our data indicate that through convergent evolution, the parasite has acquired a protein with cytokine-like function that is able to exploit an endogenous pathway of immunoregulation in the host.

Heligmosomoides polygyrus can activate mammalian TGF-β signalling pathways, but how it does so is not known. Here the authors identify and isolate a H. polygyrus TFG-β mimic that can bind both mammalian TGF-β receptor subunits, activate Smad signalling and generate inducible regulatory T cells.

The abdominal wall transplant as a sentinel skin graft

PURPOSE OF REVIEW

Abdominal wall transplantation is a technique used to achieve abdominal closure after intestinal and multivisceral transplantation. This review focuses on whether there are additional benefits for the skin component as an immune-monitoring tool.

RECENT FINDINGS

The largest series of abdominal wall transplants has recently been published. Alongside the physiological advantage gained in abdominal closure, the authors describe the immunological insight that the skin component can provide and how this contributes to the management of patients. The skin appears to develop a rash with early rejection, which facilitates early systemic treatment before significant visceral rejection occurs. It can also help in cases in which there is diagnostic doubt regarding the cause of bowel dysfunction such as in instances of intestinal infection. Despite the additional immunological burden of donor tissue, there appears to be no requirement for increased immunosuppressive therapy.

SUMMARY

The technical and immunological feasibility of abdominal wall transplantation has now been demonstrated by several centres. Skin transplanted as part of the abdominal wall or as a separate vascularized sentinel skin flap may aid in the diagnosis of rejection. This has the potential to improve graft survival and reduce immunosuppressive morbidity.

Immunological Properties of Murine Parthenogenetic Stem Cell-Derived Cardiomyocytes and Engineered Heart Muscle

Pluripotent parthenogenetic stem cells (pSCs) can be derived by pharmacological activation of unfertilized oocytes. Homozygosity of the major histocompatibility complex (MHC) in pSCs makes them an attractive cell source for applications in allogeneic tissue repair. This was recently demonstrated for pSC-based tissue-engineered heart repair. A detailed analysis of immunological properties of pSC-derived cardiomyocytes and engineered heart muscle (EHM) thereof is, however, lacking. The aim of this study was to determine baseline and cytokine-inducible MHC class I and MHC class II as well as programmed death ligand-1 (PDL-1) and co-stimulatory protein (CD40, CD80, CD86) expression in pSC-derived cardiomyocytes and pSC-EHM in vitro and in vivo. Cardiomyocytes from an MHC-homologous (H2^d/d) pSC-line were enriched to ~90% by making use of a recently developed cardiomyocyte-specific genetic selection protocol. MHC class I and MHC class II expression in cardiomyocytes could only be observed after stimulation with interferon gamma (IFN-γ). PDL-1 was markedly upregulated under IFN-γ. CD40, CD80, and CD86 were expressed at low levels and not upregulated by IFN-γ. EHM constructed from H2^d/d cardiomyocytes expressed similarly low levels of MHC class I, MHC class II, and costimulatory molecules under basal conditions. However, in EHM only MHC class I, but not MHC class II, molecules were upregulated after IFN-γ-stimulation. We next employed a cocultivation system with MHC-matched and MHC-mismatched splenocytes and T-cells to analyze the immune stimulatory properties of EHMs. Despite MHC-mismatched conditions, EHM did not induce splenocyte or T-cell proliferation in vitro. To evaluate the immunogenicity of pSC-derived cardiomyocytes in vivo, we implanted pSC-derived embryoid bodies after elimination of non-cardiomyocytes (cardiac bodies) under the kidney capsules of MHC-matched and -mismatched mice. Spontaneous beating of cardiac bodies could be observed for 28 days in the matched and for 7 days in the mismatched conditions. Teratomas formed after 28 days only in the MHC-matched conditions. Immunohistochemistry revealed single clusters of CD3-positive cells in the border zone of the implant in the mismatched conditions with few CD3-positive cells infiltrating the implant. Taken together, MHC-matched pSC-cardiomyocyte allografts show little immune cell activation, offering an explanation for the observed long-term retention of pSC-EHM allografts in the absence of immunosuppression.

Could Sentinel Skin Transplants Have Some Utility in Solid Organ Transplantation?

Accurate diagnosis of allograft rejection can be hazardous and challenging. A strategy that has emerged from experience with vascularized composite allografts (VCAs) is the use of sentinel skin transplants (SSTs)-portions of donor skin transplanted synchronously to an allograft. Work in nonhuman animal models and experience with VCAs suggest concordance between rejection occurring in the primary allograft and the SST, and that appearance of rejection in the SST may precede rejection in the primary allograft, permitting early therapeutic intervention that may improve outcomes with lower rates of chronic rejection. The encouraging findings reported in VCA transplantation raise the possibility that SST may also be useful in solid organ transplantation. Some evidence is provided by experience with abdominal wall transplantation in some intestinal and multivisceral transplant recipients. Results from those reports raise the possibility that rejection may manifest in the skin component before emergence in the intestinal allograft, providing a "lead time" during which treatment of rejection of the abdominal wall could prevent the emergence of intestinal rejection. It is plausible that these findings may be extrapolated to other solid organ allografts, especially those for which obtaining an accurate diagnosis of acute rejection can be hazardous and challenging, such as the lung or pancreas. However, more data are required to support widespread adoption of this technique.

Codominant Role of Interferon-γ- and Interleukin-17-Producing T Cells During Rejection in Full Facial Transplant Recipients

Facial transplantation is a life-changing procedure for patients with severe composite facial defects. However, skin is the most immunogenic of all transplants, and better understanding of the immunological processes after facial transplantation is of paramount importance. Here, we describe six patients who underwent full facial transplantation at our institution, with a mean follow-up of 2.7 years. Seum, peripheral blood mononuclear cells, and skin biopsy specimens were collected prospectively, and a detailed characterization of their immune response (51 time points) was performed, defining 47 immune cell subsets, 24 serum cytokines, anti-HLA antibodies, and donor alloreactivity on each sample, producing 4269 data points. In a nonrejecting state, patients had a predominant T helper 2 cell phenotype in the blood. All patients developed at least one episode of acute cellular rejection, which was characterized by increases in interferon-γ/interleukin-17-producing cells in peripheral blood and in the allograft's skin. Serum monocyte chemotactic protein-1 level was significantly increased during rejection compared with prerejection time points. None of the patients developed de novo donor-specific antibodies, despite a fourfold expansion in T follicular helper cells at 1 year posttransplantation. In sum, facial transplantation is frequently complicated by a codominant interferon-γ/interleukin-17-mediated acute cellular rejection process. Despite that, medium-term outcomes are promising with no evidence of de novo donor-specific antibody development.

Vascularized composite allograft-specific characteristics of immune responses

Vascularized composite allograft (VCA) transplantation, or reconstructive transplantation, has revolutionized the treatment of complex tissue and functional defects. Despite arriving during an age in which the immunology of solid organ transplant rejection has been investigated in much detail, these transplants have offered new perspectives from which to explore the immunobiology of transplantation. VCAs have a number of unique molecular, cellular, and architectural features which alter the character and intensity of the rejection response. While much is yet to be clarified, an understanding of these distinct mechanisms affords new possibilities for the control of immune responses in an effort to improve outcomes after VCA transplantation.

T-cell exhaustion in allograft rejection and tolerance

PURPOSE OF REVIEW

The role of T-cell exhaustion in the failure of clearance of viral infections and tumors is well established. There are several ongoing trials to reverse T-cell exhaustion for treatment of chronic viral infections and tumors. The mechanisms leading to T-cell exhaustion and its role in transplantation, however, are only beginning to be appreciated and are the focus of the present review.

RECENT FINDINGS

Exhausted T cells exhibit a distinct molecular profile reflecting combinatorial mechanisms involving the interaction of multiple transcription factors important in control of cell metabolism, acquisition of effector function and memory capacity. Change of microenvironmental cues and limiting leukocyte recruitment can modulate T-cell exhaustion. Impaired leukocyte recruitment induces T-cell exhaustion and prevents allograft rejection.

SUMMARY

Preventing or reversing T-cell exhaustion may lead to prevention of transplant tolerance or triggering of rejection; therefore, caution should be exercised in the use of agents blocking inhibitory receptors for the treatment of chronic viral infections or tumors in transplant recipients. Further definition of the role of T-cell exhaustion in clinical transplantation and an understanding of the mechanisms of induction of T-cell exhaustion are needed to develop strategies for preventing allograft rejection and induction of tolerance.

The potential role for regulatory T-cell therapy in vascularized composite allograft transplantation

PURPOSE OF REVIEW

Vascularized composite allograft (VCA) transplantation restores defects to a degree not possible by conventional techniques. However, it is limited by the need for long-term immunosuppression and high rates of acute rejection directed against skin. There is therefore a need for a therapy that may shift the risk-benefit ratio in favour of VCA transplantation. Regulatory T cells (Tregs) are a subset of T cells with potent immunoregulatory properties and the potential to promote immunosuppression-free allograft survival. In this review, we consider the evidence for Treg therapy in VCA transplantation.

RECENT FINDINGS

CD4 Tregs are the best-studied immunoregulatory cell type, and a large amount of experimental and clinical data is emerging to endorse their use in VCA transplantation. Data from animal and humanized models are particularly encouraging and demonstrate the potent efficacy of Treg at preventing skin allograft rejection. Moreover, central tolerance induction techniques in VCA transplantation models are demonstrating a dependence on Tregs for graft survival.

SUMMARY

An improvement in outcomes after VCA transplantation has the potential to revolutionize the field. Several effective therapeutic strategies have demonstrated great promise experimentally, and there is now a need to assess their safety and efficacy in a clinical setting.

Fragile TIM-4-expressing tissue resident macrophages are migratory and immunoregulatory

Macrophages characterized as M2 and M2-like regulate immune responses associated with immune suppression and healing; however, the relationship of this macrophage subset to CD169+ tissue-resident macrophages and their contribution to shaping alloimmune responses is unknown. Here we identified a population of M2-like tissue-resident macrophages that express high levels of the phosphatidylserine receptor TIM-4 and CD169 (TIM-4hiCD169+). Labeling and tracking of TIM-4hiCD169+ macrophages in mice revealed that this population is a major subset of tissue-resident macrophages, homes to draining LNs following oxidative stress, exhibits an immunoregulatory and hypostimulatory phenotype that is maintained after migration to secondary lymphoid organs, favors preferential induction of antigen-stimulated Tregs, and is highly susceptible to apoptosis. Moreover, CD169+ tissue-resident macrophages were resistant to oxidative stress-induced apoptosis in mice lacking TIM-4. Compared with heart allografts from WT mice, Tim4-/- heart allografts survived much longer and were more easily tolerized by non-immunosuppressed recipients. Furthermore, Tim4-/- allograft survival was associated with the infiltration of Tregs into the graft. Together, our data provide evidence that M2-like TIM-4hiCD169+ tissue-resident macrophages are immunoregulatory and promote engraftment of cardiac allografts, but their influence is diminished by TIM-4-dependent programmed cell death.

Prevention of airway allograft tolerance by polyinosinic:polycytidylic acid requires type I interferon responsiveness for mouse airway obliteration

BACKGROUND

Respiratory RNA viruses are associated with bronchiolitis obliterans syndrome (BOS) in lung transplant recipients (LTRS); however, the immune mechanisms that regulate airway obliteration remain incompletely understood.

METHODS

Using the mouse heterotopic tracheal transplant model of obliterative airway disease (OAD), we studied the role of double-stranded (ds)RNA using polyinosinic:polycytidylic acid (poly[I:C]), a synthetic analog of viral dsRNA, in abrogating airway allograft tolerance established with donor-specific transfusion (DST) and anti-CD154 monoclonal antibody therapy.

RESULTS

Wild-type (WT) B6 recipients of accepted BALB/c airway grafts demonstrated significantly reduced intragraft CD8+ T cells, with markedly impaired allospecific interferon (INF)-γ and tumor necrosis factor-α secretion, uncoupled from an activated phenotype, and evidence of proliferation. Administration of poly(I:C) to DST/anti-CD154-treated recipients restored OAD pathology and CD8+ alloeffector responses to levels observed in untreated mice. However, B6 type I IFN receptor-deficient (IFN-αβR(-/-)) recipients were resistant to the abrogation of tolerance mediated by poly(I:C) and did not develop CD8+ alloeffector responses or OAD. Further, adoptive transfers of WT CD8+ T cells or CD11c+ dendritic cells alone into B6 IFNαβR(-/-) recipients treated with poly(I:C) and DST/anti-CD154 were incapable of abrogating airway graft tolerance.

CONCLUSIONS

Together, these data indicate abrogation of DST/anti-CD154-induced airway allograft tolerance via dsRNA requires type-I IFN responsiveness for mouse airway obliteration.

B-cell depletion improves islet allograft survival with anti-CD45RB

A short course of anti-CD45RB leads to long-term islet allograft survival and donor-specific tolerance in approximately half of immunocompetent mice. We have previously demonstrated that anti-CD45RB antibody-mediated tolerance requires B-cells for cardiac allograft survival. We therefore asked whether B-cells were also required for anti-CD45RB antibody-mediated survival of islets. Unexpectedly, we found that nearly 100% of islet allografts survive long term in B-cell-deficient mice. Similarly, B-cell depletion by anti-CD22/cal augmented anti-CD45RB-mediated tolerance when administered pretransplant, although it had no effect on tolerance induction when administered posttransplant. Our results demonstrate that the role of B-cells in promoting tolerance with anti-CD45RB is graft specific, promoting tolerance in cardiac grafts but resisting tolerance in islet transplantation. These findings may help elucidate the varied action of B-cells in promoting tolerance versus rejection.

Differential Susceptibility of Cells Expressing Allogeneic MHC or Viral Antigen to Killing by Antigen-Specific CTL

CD8(+) cytotoxic T lymphocytes (CTLs) generated by immunization with allogeneic cells or viral infection are able to lyse allogeneic or virally infected in vitro cells (e.g., lymphoma and mastocytoma). In contrast, it is reported that CD8(+) T cells are not essential for allograft rejection (e.g., heart and skin), and that clearance of influenza or the Sendai virus from virus-infected respiratory epithelium is normal or only slightly delayed after a primary viral challenge of CD8-knockout mice. To address this controversy, we generated H-2(d)-specific CD8(+) CTLs by a mixed lymphocyte culture and examined the susceptibility of a panel of H-2(d) cells to CTL lysis. KLN205 squamous cell carcinoma, Meth A fibrosarcoma, and BALB/c skin components were found to be resistant to CTL-mediated lysis. This resistance did not appear to be related to a reduced expression of MHC class I molecules, and all these cells could block the recognition of H-2(d) targets by CTLs in cold target inhibition assays. We extended our observation by persistently infecting the same panel of cell lines with defective-interfering Sendai virus particles. The Meth A and KLN205 lines infected with a variant Sendai virus were resistant to lysis by Sendai virus-specific CTLs. The Sendai virus-infected Meth A and KLN205 lines were able to block the lysis of Sendai virus-infected targets by CTLs in cold target inhibition assays. Taken together, these results suggest that not all in vivo tissues may be sensitive to CTL lysis.

Liver transplant tolerance and its application to the clinic: can we exploit the high dose effect?

The tolerogenic properties of the liver have long been recognised, especially in regard to transplantation. Spontaneous acceptance of liver grafts occurs in a number of experimental models and also in a proportion of clinical transplant recipients. Liver graft acceptance results from donor antigen-specific tolerance, demonstrated by the extension of tolerance to other grafts of donor origin. A number of factors have been proposed to be involved in liver transplant tolerance induction, including the release of soluble major histocompatibility (MHC) molecules from the liver, its complement of immunosuppressive donor leucocytes, and the ability of hepatocytes to directly interact with and destroy antigen-specific T cells. The large tissue mass of the liver has also been suggested to act as a cytokine sink, with the potential to exhaust the immune response. In this review, we outline the growing body of evidence, from experimental models and clinical transplantation, which supports a role for large tissue mass and high antigen dose in the induction of tolerance. We also discuss a novel gene therapy approach to exploit this dose effect and induce antigen-specific tolerance robust enough to overcome a primed T cell memory response.

Profiling circulating microRNA expression in a mouse model of nerve allotransplantation

Background

The lack of noninvasive biomarkers of rejection remains a challenge in the accurate monitoring of deeply buried nerve allografts and precludes optimization of therapeutic intervention. This study aimed to establish the expression profile of circulating microRNAs (miRNAs) during nerve allotransplantation with or without immunosuppression.

Results

Balb/c mice were randomized into 3 experimental groups, that is, (1) untreated isograft (Balb/c → Balb/c), (2) untreated allograft (C57BL/6 → Balb/c), and (3) allograft (C57BL/6 → Balb/c) with FK506 immunosuppression. A 1-cm Balb/c or C57BL/6 donor sciatic nerve graft was transplanted into sciatic nerve gaps created in recipient mice. At 1, 3, 7, 10, and 14 d after nerve transplantation, nerve grafts, whole blood, and sera were obtained for miRNA expression analysis with an miRNA array and subsequent validation with quantitative real-time PCR (qRT-PCR). Three circulating miRNAs (miR-320, miR-762, and miR-423-5p) were identified in the whole blood and serum of the mice receiving an allograft with FK506 immunosuppression, within 2 weeks after nerve allotransplantation. However, these 3 circulating miRNAs were not expressed in the nerve grafts. The expression of all these 3 upregulated circulating miRNAs significantly decreased at 2, 4, and 6 d after discontinuation of FK506 immunosuppression. In the nerve graft, miR-125-3b and miR-672 were significantly upregulated in the mice that received an allograft with FK506 only at 7 d after nerve allotransplantation.

Conclusions

We identified the circulating miR-320, miR-762, and miR-423-5p as potential biomarkers for monitoring the immunosuppression status of the nerve allograft. However, further research is required to investigate the mechanism behind the dysregulation of these markers and to evaluate their prognostic value in nerve allotransplantation.

A single center experience of abdominal wall graft rejection after combined intestinal and abdominal wall transplantation

We report our outcomes following combined intestinal and abdominal wall transplantation, focusing on the presentation and treatment of acute rejection of the abdominal wall vascularized composite allograft (VCA). Retrospective analysis of all patients with combined intestinal/VCA transplantation was undertaken. Graft abnormalities were documented photographically and biopsies taken, with histological classification of rejection according to Banff 2007 guidelines. We have performed five combined intestinal and abdominal wall transplants to date. Two patients developed erythematous, maculopapular to papular eruptions confined to the VCA, histologically confirmed as grade II/III rejection, yet with normal bowel on endoscopy. Both patients' rashes resolved within 72 h of increasing immunosuppressive treatment. One patient later developed a recurrence of the rash, confirmed as skin rejection, but did not immediately seek medical attention. Treatment was therefore delayed, and mild intestinal rejection developed. We describe the rash associated with VCA rejection, and propose that while the skin of an abdominal wall VCA may reject independently of the intestinal allograft, delay in treatment of rejection episodes may result in rejection of the intestinal graft.

Induction of tolerance of vascularized composite allografts

Vascularized composite allotransplantation has become established as a clinical specialty since the first successful hand transplant was performed in 1998. Data now available indicate that hand and face transplants offer patients good functional outcomes and significant improvements in quality of life. Despite the debilitating nature of the injuries treated by such transplants, the defects are generally not life threatening, making it difficult for physicians to recommend life-long immunosuppression that can itself have grave consequences. One potential solution to this dilemma is the induction of immunologic tolerance of the tissue transplants because tolerance would eliminate the need for such immunosuppression. Transplant tolerance may also prevent chronic rejection, a significant source of late graft loss after organ transplantation.Induction of mixed hematopoietic chimerism is a robust approach to establishing such transplant tolerance, which recently led to the first clinical application of a tolerance induction protocol for kidney transplantation. In this manuscript, we review the current status of VCA and of research directed toward bringing a tolerance approach to the VCA field. We also discuss the potential clinical significance of these studies and outline the remaining obstacles to introduction of a tolerance induction protocol to clinical practice in hand or face transplantation.

Mixed chimerism and split tolerance: mechanisms and clinical correlations

Establishing hematopoietic mixed chimerism can lead to donor-specific tolerance to transplanted organs and may eliminate the need for long-term immunosuppressive therapy, while also preventing chronic rejection. In this review, we discuss central and peripheral mechanisms of chimerism induced tolerance. However, even in the long-lasting presence of a donor organ or donor hematopoietic cells, some allogeneic tissues from the same donor can be rejected; a phenomenon known as split tolerance. With the current goal of creating mixed chimeras using clinically feasible amounts of donor bone marrow and with minimal conditioning, split tolerance may become more prevalent and its mechanisms need to be explored. Some predisposing factors that may increase the likelihood of split tolerance are immunogenicity of the graft, certain donor-recipient combinations, prior sensitization, location and type of graft and minimal conditioning chimerism induction protocols. Additionally, split tolerance may occur due to a differential susceptibility of various types of tissues to rejection. The mechanisms involved in a tissue's differential susceptibility to rejection include the presence of polymorphic tissue-specific antigens and variable sensitivity to indirect pathway effector mechanisms. Finally, we review the clinical attempts at allograft tolerance through the induction of chimerism; studies that are revealing the complex relationship between chimerism and tolerance. This relationship often displays split tolerance, and further research into its mechanisms is warranted.

A critical role for the TLR4/TRIF pathway in allogeneic hematopoietic cell rejection by innate immune cells

We show for the first time that signaling through the TLR4/TRIF pathway plays a critical role in allogeneic bone marrow cell (BMC) rejection. This appears to be unique to BMCs as organ allografts are rejected mainly via MyD88 signaling. Using T- or T-/B-cell-deficient mice, we found that BMC allorejection occurred early before T-cell activation and was T- and B-cell independent, suggesting an effector role for innate immune cells in BMC rejection. We further demonstrated the innate immune signaling in BMC allorejection by showing superior engraftment in mice deficient in TRIF or TLR4 but not in MyD88 or TLR3. The restored cytotoxicity in TRIF-deficient recipients transferred with wild-type F4/80(+) or NK1.1(+) cells suggests TRIF signaling dependence on macrophages or NK cells in early BMC rejection. Production of the proinflammatory cytokine IL-6 and TRIF relevant chemokine MCP-1 was significantly increased early after bone marrow transplantation. In vivo specific depletion of macrophages or NK innate immune cells in combination with anti-CD154/rapamycin resulted in additive-enhanced allogeneic engraftment. The requirement for irradiation was completely eliminated when both macrophages and NK cells were depleted in combination with anti-CD154/rapamycin to target T- and B-cells, supporting the hypothesis that two barriers involving innate and adaptive immunity exist in mediating the rejection of allogeneic BMCs. In summary, our results clearly demonstrate a previously unappreciated role for innate immunity in BMC allorejection via signaling through a unique MyD88-independent TLR4/TRIF mechanism. These findings may have direct clinical impact on strategies for conditioning recipients for stem cell transplantation.

BIOMARKERS OF OPERATIONAL TOLERANCE IN SOLID ORGAN TRANSPLANTATION

INTRODUCTION: Long-term immunosuppressive therapy represents a huge burden on transplant recipients, but currently cannot be omitted. Improving long-term transplant outcome by immunosuppressive drug withdrawal may be achieved in patients who have developed (partial) immunological unresponsiveness towards their graft, either spontaneously or through tolerance induction. Reliable biomarkers are essential to define such immunological unresponsiveness and will facilitate controlled immunosuppressive drug weaning as well as provide surrogate end-points for tolerance induction trials. AREAS COVERED: Tolerance biomarkers have been defined for both liver and kidney transplantation and can accurately identify operationally tolerant transplant recipients retrospectively. These two tolerance fingerprints are remarkably different, indicating the involvement of distinct mechanisms. Limited data suggest that tolerance biomarkers can be detected in immunosuppressed transplant recipients. Whether these patients can safely have their immunosuppressive drugs withdrawn needs to be established. EXPERT OPINION: Mechanistic interpretation of the kidney transplant tolerance biomarker profile dominated by B cell markers remains a challenge in light of experimental evidence suggesting the pivotal involvement of regulatory T cells. Therefore, defining animal models that resemble human transplant tolerance is crucial in understanding the underlying mechanisms. Additionally, to ensure patient safety while monitoring for tolerance, it is essential to develop biomarkers to non-invasively detect early signs of rejection as well.

α-1-antitrypsin gene delivery reduces inflammation, increases T-regulatory cell population size and prevents islet allograft rejection

Antiinflammatory clinical-grade, plasma-derived human α-1 antitrypsin (hAAT) protects islets from allorejection as well as from autoimmune destruction. hAAT also interferes with disease progression in experimental autoimmune encephalomyelitis (EAE) and in collagen-induced arthritis (CIA) mouse models. hAAT increases IL-1 receptor antagonist expression in human mononuclear cells and T-regulatory (Treg) cell population size in animal models. Clinical-grade hAAT contains plasma impurities, multiple hAAT isoforms and various states of inactive hAAT. We thus wished to establish islet-protective activities and effect on Treg cells of plasmid-derived circulating hAAT in whole animals. Islet function was assessed in mice that received allogeneic islet transplants after mice were given hydrodynamic tail-vein injection with pEF-hAAT, a previously described Epstein-Barr virus (EBV) plasmid construct containing the EBV nuclear antigen 1 (EBNA1) and the family of repeat EBNA1 binding site components (designated "EF") alongside the hAAT gene. Sera collected from hAAT-expressing mice were added to lipopolysaccharide (LPS)-stimulated macrophages to assess macrophage responsiveness. Also, maturation of peritoneal cells from hAAT-expressing mice was evaluated. hAAT-expressing mice accepted islet allografts (n = 11), whereas phosphate-buffered saline-injected animals (n = 11), as well as mice treated with truncated-hAAT-plasmid (n = 6) and untreated animals (n = 20) rapidly rejected islet allografts. In hAAT-expressing animals, local Treg cells were abundant at graft sites, and the IL-1 receptor antagonist was elevated in grafts and circulation. Sera from hAAT-expressing mice, but not control mice, inhibited macrophage responses. Finally, peritoneal cells from hAAT-expressing mice exhibited a semimature phenotype. We conclude that plasmid-derived circulating hAAT protects islet allografts from acute rejection, and human plasma impurities are unrelated to islet protection. Future studies may use this in vivo approach to examine the structure-function characteristics of the protective activities of AAT by manipulation of the hAAT plasmid.

Clinical transplantation tolerance

Transplantation is the treatment of choice for many if not most causes of end-stage organ failure. Over 20,000 organ transplant procedures were performed in the USA in 2009 to treat patients with failed or failing kidneys, livers, hearts, lungs, and intestines, and there remain 85,000 individuals waiting on the transplant list. Currently, in the USA, there are over 170,000 individuals living with a transplanted organ. Virtually, all of these individuals receive maintenance immunosuppression in an attempt to maximize the function and survival of the transplanted organ. However, it is clear that the long-term use of immunosuppressive agents is associated with an extensive list of undesirable side effects that have the potential to limit the survival of the patient and transplanted organ as well as to compromise quality of life. Although the ability to induce reproducibly a state of robust, stable tolerance would address this problem, tolerance remains an infrequent event in clinical transplantation that is largely a consequence of chance. Factors limiting the broader investigation of clinical transplantation tolerance include the lack of therapeutic regimens known to favor tolerance in humans, the lack of validated assays or biomarkers predictive of tolerance, and concerns about the safety and ethics of complete withdrawal of immunosuppression given the very good results achievable with current immunosuppression. Despite these barriers, a number of investigators have continued to conduct well-designed and carefully supervised studies with the long-term goal of making clinical transplantation tolerance more feasible. The aim of this review is to summarize the status of these studies.

Ex vivo-expanded human regulatory T cells prevent the rejection of skin allografts in a humanized mouse model

BACKGROUND

Composite tissue transplantation effectively reconstructs the most complex defects, but its use is limited because of harmful immunosuppression and the high susceptibility of skin to rejection. Development of tolerance is an ideal solution, and protocols using regulatory T cells (Tregs) to achieve this have been promising in experimental animal models. The aim of this study was to investigate the ability of human Tregs to regulate immune responses to a human skin allograft in vivo.

METHODS

We isolated and expanded naturally occurring CD127loCD25+CD4+ human Tregs from peripheral blood mononuclear cells (PBMCs) and examined their phenotype and suppressive activity in vitro. Using a clinically relevant chimeric humanized mouse system, we transplanted mice with human skin grafts followed by allogeneic populations of PBMCs with or without Tregs derived from the same PBMC donor.

RESULTS

Ex vivo-expanded Tregs maintain the appropriate Treg markers and retain suppressive activity against allostimulated and polyclonally stimulated autologous PBMCs in vitro. Mice receiving allogeneic PBMCs alone consistently reject human skin grafts, whereas those also receiving Tregs display stable long-term human skin transplant survival along with a reduction in the CD8+ human cellular graft infiltrate.

CONCLUSIONS

We show for the first time the unique ability of human Tregs to prevent the rejection of a skin allograft in vivo, highlighting the therapeutic potential of these cells clinically.