Tolerance induction in clinical transplantation

Acute myocarditis: an overview of pathogenesis, diagnosis and management

Acute myocarditis encompasses a diverse presentation of inflammatory cardiomyopathies with infectious and non-infectious triggers. The clinical presentation is heterogeneous, from subtle symptoms like mild chest pain to life-threatening fulminant heart failure requiring urgent advanced hemodynamic support. This review provides a comprehensive overview of the current state of knowledge regarding the pathogenesis, diagnostic approach, management strategies, and directions for future research in acute myocarditis. The pathogenesis of myocarditis involves interplay between the inciting factors and the subsequent host immune response. Infectious causes, especially cardiotropic viruses, are the most frequently identified precipitants. However, autoimmune processes independent of microbial triggers, as well as toxic myocardial injury from drugs, chemicals or metabolic derangements also contribute to the development of myocarditis through diverse mechanisms. Furthermore, medications like immune checkpoint inhibitor therapies are increasingly recognized as causes of myocarditis. Elucidating the nuances of viral, autoimmune, hypersensitivity, and toxic subtypes of myocarditis is key to guiding appropriate therapy. The heterogeneous clinical presentation coupled with non-specific symptoms creates diagnostic challenges. A multifaceted approach is required, incorporating clinical evaluation, electrocardiography, biomarkers, imaging studies, and endomyocardial biopsy. Cardiovascular magnetic resonance imaging has become pivotal for non-invasive assessment of myocardial inflammation and fibrosis. However, biopsy remains the gold standard for histological classification and definitively establishing the underlying etiology. Management relies on supportive care, while disease-specific therapies are limited. Although some patients recover well with conservative measures, severe or fulminant myocarditis necessitates aggressive interventions such as mechanical circulatory support devices and transplantation. While immunosuppression is beneficial in certain histological subtypes, clear evidence supporting antiviral or immunomodulatory therapies for the majority of acute viral myocarditis cases remains insufficient. Substantial knowledge gaps persist regarding validated diagnostic biomarkers, optimal imaging surveillance strategies, evidence-based medical therapies, and risk stratification schema. A deeper understanding of the immunopathological mechanisms, rigorous clinical trials of targeted therapies, and longitudinal outcome studies are imperative to advance management and improve the prognosis across the myocarditis spectrum.

New insights in immunomodulation for intestinal transplantation

Tolerance is the Holy Grail of solid organ transplantation (SOT) and remains its primary challenge since its inception. In this topic, the seminal contributions of Thomas Starzl at Pittsburgh University outlined foundational principles of graft acceptance and tolerance, with chimerism emerging as a pivotal factor. Immunologically, intestinal transplantation (ITx) poses a unique hurdle due to the inherent characteristics and functions of the small bowel, resulting in increased immunogenicity. This necessitates heavy immunosuppression (IS) while IS drugs side effects cause significant morbidity. In addition, current IS therapies fall short of inducing clinical tolerance and their discontinuation has been proven unattainable in most cases. This underscores the unfulfilled need for immunological modulation to safely reduce IS-related burdens. To address this challenge, the Leuven Immunomodulatory Protocol (LIP), introduced in 2000, incorporates various pro-tolerogenic interventions in both the donor to the recipient, with the aim of facilitating graft acceptance and improving outcome. This review seeks to provide an overview of the current understanding of tolerance in ITx and outline recent advances in this domain.

Hematopoietic cell-based and non-hematopoietic cell-based strategies for immune tolerance induction in living-donor renal transplantation: A systematic review

INTRODUCTION

Despite its use to prevent acute rejection, lifelong immunosuppression can adversely impact long-term patient and graft outcomes. In theory, immunosuppression withdrawal is the ultimate goal of kidney transplantation, and is made possible by the induction of immunological tolerance. The purpose of this paper is to review the safety and efficacy of immune tolerance induction strategies in living-donor kidney transplantation, both chimerism-based and non-chimerism-based. The impact of these strategies on transplant outcomes, including acute rejection, allograft function and survival, cost, and immune monitoring, will also be discussed.

MATERIALS AND METHODS

Databases such as PubMed, Scopus, and Web of Science, as well as additional online resources such as EBSCO, were exhaustively searched. Adult living-donor kidney transplant recipients who developed chimerism-based tolerance after concurrent bone marrow or hematopoietic stem cell transplantation or those who received non-chimerism-based, non-hematopoietic cell therapy using mesenchymal stromal cells, dendritic cells, or regulatory T cells were studied between 2000 and 2021. Individual sources of evidence were evaluated critically, and the strength of evidence and risk of bias for each outcome of the transplant tolerance study were assessed.

RESULTS

From 28,173 citations, 245 studies were retrieved after suitable exclusion and duplicate removal. Of these, 22 studies (2 RCTs, 11 cohort studies, 6 case-control studies, and 3 case reports) explicitly related to both interventions (chimerism- and non-chimerism-based immune tolerance) were used in the final review process and were critically appraised. According to the findings, chimerism-based strategies fostered immunotolerance, allowing for the safe withdrawal of immunosuppressive medications. Cell-based therapy, on the other hand, frequently did not induce tolerance except for minimising immunosuppression. As a result, the rejection rates, renal allograft function, and survival rates could not be directly compared between these two groups. While chimerism-based tolerance protocols posed safety concerns due to myelosuppression, including infections and graft-versus-host disease, cell-based strategies lacked these adverse effects and were largely safe. There was a lack of direct comparisons between HLA-identical and HLA-disparate recipients, and the cost implications were not examined in several of the retrieved studies. Most studies reported successful immunosuppressive weaning lasting at least 3 years (ranging up to 11.4 years in some studies), particularly with chimerism-based therapy, while only a few investigators used immune surveillance techniques. The studies reviewed were often limited by selection, classification, ascertainment, performance, and attrition bias.

CONCLUSIONS

This review demonstrates that chimerism-based hematopoietic strategies induce immune tolerance, and a substantial number of patients are successfully weaned off immunosuppression. Despite the risk of complications associated with myelosuppression. Non-chimerism-based, non-hematopoietic cell protocols, on the other hand, have been proven to facilitate immunosuppression minimization but seldom elicit immunological tolerance. However, the results of this review must be interpreted with caution because of the non-randomised study design, potential confounding, and small sample size of the included studies. Further validation and refinement of tolerogenic protocols in accordance with local practice preferences is also warranted, with an emphasis on patient selection, cost ramifications, and immunological surveillance based on reliable tolerance assays.

Genetic engineering of pigs for xenotransplantation to overcome immune rejection and physiological incompatibilities: The first clinical steps

Xenotransplantation has the potential to solve the shortfall of human organ donors. Genetically modified pigs have been considered as potential animal donors for human xenotransplantation and have been widely used in preclinical research. The genetic modifications aim to prevent the major species-specific barriers, which include humoral and cellular immune responses, and physiological incompatibilities such as complement and coagulation dysfunctions. Genetically modified pigs can be created by deleting several pig genes related to the synthesis of various pig specific antigens or by inserting human complement- and coagulation-regulatory transgenes. Finally, in order to reduce the risk of infection, genes related to porcine endogenous retroviruses can be knocked down. In this review, we focus on genetically modified pigs and comprehensively summarize the immunological mechanism of xenograft rejection and recent progress in preclinical and clinical studies. Overall, both genetically engineered pig-based xenografts and technological breakthroughs in the biomedical field provide a promising foundation for pig-to-human xenotransplantation in the future.

Comparison of CD3e Antibody and CD3e-sZAP Immunotoxin Treatment in Mice Identifies sZAP as the Main Driver of Vascular Leakage

Anti-CD3-epsilon (CD3e) monoclonal antibodies (mAbs) and CD3e immunotoxins (ITs) are promising targeted therapy options for various T-cell disorders. Despite significant advances in mAb and IT engineering, vascular leakage syndrome (VLS) remains a major dose-limiting toxicity for ITs and has been poorly characterized for recent "engineered" mAbs. This study undertakes a direct comparison of non-mitogenic CD3e-mAb (145-2C11 with Fc-silentTM murine IgG1: S-CD3e-mAb) and a new murine-version CD3e-IT (saporin-streptavidin (sZAP) conjugated with S-CD3e-mAb: S-CD3e-IT) and identifies their distinct toxicity profiles in mice. As expected, the two agents showed different modes of action on T cells, with S-CD3e-mAb inducing nearly complete modulation of CD3e on the cell surface, while S-CD3e-IT depleted the cells. S-CD3e-IT significantly increased the infiltration of polymorphonuclear leukocytes (PMNs) into the tissue parenchyma of the spleen and lungs, a sign of increased vascular permeability. By contrast, S-CD3e-mAbs-treated mice showed no notable signs of vascular leakage. Treatment with control ITs (sZAP conjugated with Fc-silent isotype antibodies) induced significant vascular leakage without causing T-cell deaths. These results demonstrate that the toxin portion of S-CD3e-IT, not the CD3e-binding portion (S-CD3e-mAb), is the main driver of vascular leakage, thus clarifying the molecular target for improving safety profiles in CD3e-IT therapy.

Donor-Specific Regulatory T Cell-Mediated Immune Tolerance in an Intrahepatic Murine Allogeneic Islet Transplantation Model with Short-Term Anti-CD154 mAb Single Treatment

Anti-CD154 blockade-based regimens remain unequaled in prolonging graft survival in various organ transplantation models. Several studies have focused on transplantation tolerance with the anti-CD154 blockade, but none of these studies has investigated the mechanisms associated with its use as the sole treatment in animal models, delaying our understanding of anti-CD154 blockade-mediated immune tolerance. The purpose of this study was to investigate the mechanism underlying the anti-CD154 monoclonal antibody (mAb) blockade in inducing immune tolerance using an intrahepatic murine allogeneic islet transplantation model. Allogeneic BALB/c AnHsd (BALB/c) islets were infused into the liver of diabetic C57BL/6 (B6) mice via the cecal vein. Anti-CD154 mAb (MR1) was administered on -1, 0, 1, 3, 5, and 7 d posttransplantation at 0.5 mg per mouse. We showed that short-term MR1 monotherapy could prolong the allogeneic islet grafts to more than 250 d in the murine intrahepatic islet transplantation model. The second islet grafts transplanted under the kidney capsule of the recipients were protected from rejection. We also found that rejection of same-donor skin grafts transplanted to the tolerant mice was modestly delayed. Using a DEREG mouse model, FoxP3+ regulatory T (Treg) cells were shown to play important roles in transplantation tolerance. In mixed lymphocyte reactions, Treg cells from the tolerant mice showed more potency in suppressing BALB/c splenocyte-stimulated Teff cell proliferation than those from naïve mice. In this study, we demonstrated for the first time that a short-term anti-CD154 mAb single treatment could induce FoxP3+ Treg cell-mediated immune tolerance in the intrahepatic murine allogeneic islet transplantation model.

B lymphocytes contribute to indirect pathway T cell sensitization via acquisition of extracellular vesicles

B cells have been implicated in transplant rejection via antibody-mediated mechanisms and more recently by presenting donor antigens to T cells. We have shown in patients with chronic antibody-mediated rejection that B cells control the indirect T cell alloresponses. To understand more about the role of B cells as antigen-presenting cells for CD4⁺ T cell with indirect allospecificity, B cells were depleted in C57BL/6 mice, using an anti-CD20 antibody, prior to receiving MHC class I-mismatched (K^d ) skin. The absence of B cells at the time of transplantation prolonged skin graft survival. To study the mechanisms behind this observation, T cells with indirect allospecificity were transferred in mice receiving a K^d skin transplant. T cell proliferation was markedly inhibited in the absence of recipient B cells, suggesting that B cells contribute to indirect pathway sensitization. Furthermore, we have shown that a possible way in which B cells present alloantigens is via acquisition of MHC-peptide complexes. Finally, we demonstrate that the addition of B cell depletion to the transfer of regulatory T cells (Tregs) with indirect alloresponse further prolonged skin graft survival. This study supports an important role for B cells in indirect T cell priming and further emphasizes the advantage of combination therapies in prolonging transplant survival.

Memory T Cells in Transplantation: Old Challenges Define New Directions

Immunologic memory is the ability of adaptive immune system to quickly and specifically recognize previously encountered antigens and initiate an effector response. Alloreactive memory cells can mount rapid and robust responses to the transplanted organ resulting in allograft injury. Thus preexisting humoral or cellular memory alloresponses are typically associated with poor graft outcomes in experimental and clinical transplantation. While both B and T lymphocytes exhibit memory responses, this review discusses recent updates on the biology of memory T cells and their relevance to the field of transplantation. Three major areas of focus are the emergence and characterization of tissue resident memory T cells, manipulation of T cell metabolic pathways, and the latest promising approaches to targeting detrimental T cell memory in the settings of organ transplantation.

B cell-derived IL-1β and IL-6 drive T cell reconstitution following lymphoablation

Understanding the mechanisms of T cell homeostatic expansion is crucial for clinical applications of lymphoablative therapies. We previously established that T cell recovery in mouse heart allograft recipients treated with anti-thymocyte globulin (mATG) critically depends on B cells and is mediated by B cell-derived soluble factors. B cell production of interleukin (IL)-1β and IL-6 is markedly upregulated after heart allotransplantation and lymphoablation. Neutralizing IL-1β or IL-6 with mAb or the use of recipients lacking mature IL-1β, IL-6, IL-1R, MyD88, or IL-6R impair CD4⁺ and CD8⁺ T cell recovery and significantly enhance the graft-prolonging efficacy of lymphoablation. Adoptive co-transfer experiments demonstrate a direct effect of IL-6 but not IL-1β on T lymphocytes. Furthermore, B cells incapable of IL-1β or IL-6 production have diminished capacity to mediate T cell reconstitution and initiate heart allograft rejection upon adoptive transfer into mATG treated B cell deficient recipients. These findings reveal the essential role of B cell-derived IL-1β and IL-6 during homeostatic T cell expansion in a clinically relevant model of lymphoablation.

Leveraging the zebrafish to model organ transplantation

PURPOSE OF REVIEW

The availability of organs for transplant fails to meet the demand and this shortage is growing worse every year. As the cost of not getting a suitable donor organ can mean death for patients, new tools and approaches that allows us to make advances in transplantation faster and provide a different vantage point are required. To address this need, we introduce the concept of using the zebrafish (Danio rerio) as a new model system in organ transplantation. The zebrafish community offers decades of research experience in disease modeling and a rich toolbox of approaches for interrogating complex pathological states. We provide examples of how already existing zebrafish assays/tools from cancer, regenerative medicine, immunology, and others, could be leveraged to fuel new discoveries in pursuit of solving the organ shortage.

RECENT FINDINGS

Important innovations have enabled several types of transplants to be successfully performed in zebrafish, including stem cells, tumors, parenchymal cells, and even a partial heart transplant. These innovations have been performed against a backdrop of an expansive and impressive list of tools designed to uncover the biology of complex systems that include a wide array of fluorescent transgenic fish that label specific cell types and mutant lines that are transparent, immune-deficient. Allogeneic transplants can also be accomplished using immune suppressed and syngeneic fish. Each of these innovations within the zebrafish community would provide several helpful tools that could be applied to transplant research.

SUMMARY

We highlight some examples of existing tools and assays developed in the zebrafish community that could be leveraged to overcome barriers in organ transplantation, including ischemia-reperfusion, short preservation durations, regeneration of marginal grafts, and acute and chronic rejection.

Autoimmunity in Acute Myocarditis: How Immunopathogenesis Steers New Directions for Diagnosis and Treatment

PURPOSE OF REVIEW

Over the last decade, myocarditis has been increasingly recognized as common cause of sudden cardiac death in young adults and heart failure overall. The purpose of this review is to discuss hypothesis of development of non-infectious myocarditis, to provide a description of the immunopathogenesis and the most common mechanisms of autoimmunity in myocarditis, and to provide an update on therapeutic options.

RECENT FINDINGS

A new entity of myocarditis is immune checkpoint inhibitor (ICI) induced myocarditis. ICIs are used in advanced cancer to "disinhibit" the immune system and make it more aggressive in fighting cancer. This novel drug class has doubled life expectancy in metastatic melanoma and significantly increased progression free survival in advanced non-small-cell lung cancer, but comes with a risk of autoimmune diseases such as myocarditis resulting from an overly aggressive immune system. Myocarditis is an inflammatory disease of the heart with major public health impact. Thorough understanding of its immunopathogenesis is crucial for accurate diagnosis and effective treatment.

Innate Immune Determinants of Graft-Versus-Host Disease and Bidirectional Immune Tolerance in Allogeneic Transplantation

The success of tissue transplantation from a healthy donor to a diseased individual (allo-transplantation) is regulated by the immune systems of both donor and recipient. Developing a state of specific non-reactivity between donor and recipient, while maintaining the salutary effects of immune function in the recipient, is called “immune (transplantation) tolerance”. In the classic early post-transplant period, minimizing bidirectional donor ←→ recipient reactivity requires the administration of immunosuppressive drugs, which have deleterious side effects (severe immunodeficiency, opportunistic infections, and neoplasia, in addition to drug-specific reactions and organ toxicities). Inducing immune tolerance directly through donor and recipient immune cells, particularly via subsets of immune regulatory cells, has helped to significantly reduce side effects associated with multiple immunosuppressive drugs after allo-transplantation. The innate and adaptive arms of the immune system are both implicated in inducing immune tolerance. In the present article, we will review innate immune subset manipulations and their potential applications in hematopoietic stem cell transplantation (HSCT) to cure malignant and non-malignant hematological disorders by inducing long-lasting donor ←→ recipient (bidirectional) immune tolerance and reduced graft-versus-host disease (GVHD). These innate immunotherapeutic strategies to promote long-term immune allo-transplant tolerance include myeloid-derived suppressor cells (MDSCs), regulatory macrophages, tolerogenic dendritic cells (tDCs), Natural Killer (NK) cells, invariant Natural Killer T (iNKT) cells, gamma delta T (γδ-T) cells and mesenchymal stromal cells (MSCs).

Clinical Operational Tolerance and Immunosuppression Minimization in Kidney Transplantation: Where Do We Stand?

BACKGROUND

The 20th century represents a breakthrough in the transplantation era, since the first kidney transplantation between identical twins was performed. This was the first case of tolerance, since the recipient did not need immunosuppression. However, as transplantation became possible, an immunosuppression-free status became the ultimate goal, since the first tolerance case was a clear exception from the hard reality nowadays represented by rejection.

METHODS

A plethora of studies was described over the past decades to understand the molecular mechanisms responsible for rejection. This review focuses on the most relevant studies found in the literature where renal tolerance cases are claimed. Contrasting, and at the same time, encouraging outcomes are herein discussed and a glimpse on the main renal biomarkers analyzed in this field is provided.

RESULTS

The activation of the immune system has been shown to play a central role in organ failure, but also it seems to induce a tolerance status when an allograft is performed, despite tolerance is still rare to register. Although there are still overwhelming challenges to overcome and various immune pathways remain arcane; the immunosuppression minimization might be more attainable than previously believed.

CONCLUSION

. Multiple biomarkers and tolerance mechanisms suspected to be involved in renal transplantation have been investigated to understand their real role, with still no clear answers on the topic. Thus, the actual knowledge provided necessarily leads to more in-depth investigations, although many questions in the past have been answered, there are still many issues on renal tolerance that need to be addressed.

Challenges and future developments in liver transplantation

Liver transplantation (LT) has become the treatment of choice for a wide range of liver diseases in both adult and pediatric patients. Until recently, the largest proportion of LT in adults, were performed in patients with hepatitis C (HCV) related cirrhosis. The recent availability of safe and effective direct antiviral agents to cure HCV infection in almost all patients whatever the HCV genotype and severity of liver disease, will reduce the need for LT in this category of recipients. Thus, it is presumed that in the next 1 to 2 decades HCV related liver disease will diminish substantially, whereas non-alcoholic steato-hepatitis (NASH) will correspondingly escalate as an indication for LT. The greatest challenges facing LT remain the limited supply of donor organs, and the need for chronic immunosuppression, which represent the true obstacles to the greater application and durable success of the LT procedure. This review aimed to highlight, in different sections, the main open issues and future developments in LT. These will be focused to explore current and future strategies to maximize the use of limited organs, to offer an update on potential new approaches to immunosuppression and to imagine new indications for LT when the number of patients awaiting transplants for HCV related liver disease is reduced.

Ex Vivo Selection of Transduced Hematopoietic Stem Cells for Gene Therapy of β-Hemoglobinopathies

Although gene transfer to hematopoietic stem cells (HSCs) has shown therapeutic efficacy in recent trials for several individuals with inherited disorders, transduction incompleteness of the HSC population remains a hurdle to yield a cure for all patients with reasonably low integrated vector numbers. In previous attempts at HSC selection, massive loss of transduced HSCs, contamination with non-transduced cells, or lack of applicability to large cell populations has rendered the procedures out of reach for human applications. Here, we fused codon-optimized puromycin N-acetyltransferase to herpes simplex virus thymidine kinase. When expressed from a ubiquitous promoter within a complex lentiviral vector comprising the βAT87Q-globin gene, viral titers and therapeutic gene expression were maintained at effective levels. Complete selection and preservation of transduced HSCs were achieved after brief exposure to puromycin in the presence of MDR1 blocking agents, suggesting the procedure's suitability for human clinical applications while affording the additional safety of conditional suicide.

Density dependent re-tuning of autoreactive T cells alleviates their pathogenicity in a lymphopenic environment

Peripheral T cell tolerance is challenging to induce in partially lymphopenic hosts and this is relevant for clinical situations involving transplant tolerance. While the shortage of regulatory cells is thought to be one reason for this, T cell-intrinsic tolerance processes such as anergy are also poorly triggered in such hosts. In order to understand the latter, we used a T cell deficient mouse model system where adoptively transferred autoreactive T cells are significantly tolerized in a cell intrinsic fashion, without differentiation to regulatory T cells. Intriguingly these T cells often retain sufficient effector functions to trigger autoimmune pathology. Here we find that the high population density of the autoreactive T cells that accumulated in such a host limits the progression of the cell-intrinsic tolerance process in T cells. Accordingly, reducing the cell density during a second transfer allowed T cells to further tune down their responsiveness to antigenic stimulation. The retuning of T cells was reflected by a loss of the T cell's abilities to proliferate, produces cytokines or help B cells. We further suggest, based on altering the levels of chronic antigen using miniosmotic pumps, that the effects of cell-density on T cell re-tuning may reflect the effective changes in the antigen dose perceived by individual T cells. This could proportionally elicit more negative feedback downstream of the TCR. Consistent with this, the retuned T cells showed signaling defects both proximal and distal to the TCR. Therefore, similar to the immunogenic activation of T cells, cell-intrinsic T cell tolerance may also involve a quantitative and progressive process of tuning down its antigen-responsiveness. The progress of such tuning seems to be stabilized at multiple intermediate stages by factors such as cell density, rather than just absolute antigen levels.

Current challenges and future directions for liver transplantation

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

Role of Memory T Cells in Allograft Rejection and Tolerance

Memory T cells are characterized by their low activation threshold, robust effector functions, and resistance to conventional immunosuppression and costimulation blockade. Unlike their naïve counterparts, memory T cells reside in and recirculate through peripheral non-lymphoid tissues. Alloreactive memory T cells are subdivided into different categories based on their origins, phenotypes, and functions. Recipients whose immune systems have been directly exposed to allogeneic major histocompatibility complex (MHC) molecules display high affinity alloreactive memory T cells. In the absence of any prior exposure to allogeneic MHC molecules, endogenous alloreactive memory T cells are regularly generated through microbial infections (heterologous immunity). Regardless of their origin, alloreactive memory T cells represent an essential element of the allograft rejection process and a major barrier to tolerance induction in clinical transplantation. This article describes the different subsets of alloreactive memory T cells involved in transplant rejection and examine their generation, functional properties, and mechanisms of action. In addition, we discuss strategies developed to target deleterious allospecific memory T cells in experimental animal models and clinical settings.

Risk Stratification for Rejection and Infection after Kidney Transplantation

BACKGROUND AND OBJECTIVES

Definition of individual risk profile is the first step to implement strategies to keep the delicate balance between under- and overimmunosuppression after kidney transplantation.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We used data from the Efficacy Limiting Toxicity Elimination Symphony Study (1190 patients between 2002 and 2004) to model risk of rejection and infection in the first year after kidney transplantation. External validation was performed in a study population from the Fixed-Dose Concentration-Controlled Trial (630 patients between 2003 and 2006).

RESULTS

Despite different temporal dynamics, rejections and severe infections had similar overall incidences in the first year after transplantation (23.4% and 25.5%, respectively), and infections were the principal cause of death (43.2% of all deaths). Recipient older age, deceased donor, higher number of HLA mismatches, and high risk for cytomegalovirus disease were associated with infection; deceased donor, higher number of HLA mismatches, and immunosuppressive therapy including cyclosporin A (compared with tacrolimus), with rejection. These factors were integrated into a two-dimensional risk stratification model, which defined four risk groups: low risk for infection and rejection (30.8%), isolated risk for rejection (36.1%), isolated risk for infection (7.0%), and high risk for infection and rejection (26.1%). In internal validation, this model significantly discriminated the subgroups in terms of composite end point (low risk for infection/rejection, 24.4%; isolated risk for rejection and isolated risk for infection, 31.3%; high risk for infection/rejection, 54.4%; P<0.001), rejection episodes (isolated risk for infection and low risk for infection/rejection, 13.0%; isolated risk for rejection and high risk for infection/rejection, 24.2%; P=0.001), and infection episodes (low risk for infection/rejection and isolated risk for rejection, 12.0%; isolated risk for infection and high risk for infection/rejection, 37.6%; P<0.001). External validation confirmed the applicability of the model to an independent cohort.

CONCLUSIONS

We propose a two-dimensional risk stratification model able to disentangle the individual risk for rejection and infection in the first year after kidney transplantation. This concept can be applied to implement a personalized immunosuppressive and antimicrobial treatment approach.

Tracking donor-reactive T cells: Evidence for clonal deletion in tolerant kidney transplant patients

T cell responses to allogeneic major histocompatibility complex antigens present a formidable barrier to organ transplantation, necessitating long-term immunosuppression to minimize rejection. Chronic rejection and drug-induced morbidities are major limitations that could be overcome by allograft tolerance induction. Tolerance was first intentionally induced in humans via combined kidney and bone marrow transplantation (CKBMT), but the mechanisms of tolerance in these patients are incompletely understood. We now establish an assay to identify donor-reactive T cells and test the role of deletion in tolerance after CKBMT. Using high-throughput sequencing of the T cell receptor B chain CDR3 region, we define a fingerprint of the donor-reactive T cell repertoire before transplantation and track those clones after transplant. We observed posttransplant reductions in donor-reactive T cell clones in three tolerant CKBMT patients; such reductions were not observed in a fourth, nontolerant, CKBMT patient or in two conventional kidney transplant recipients on standard immunosuppressive regimens. T cell repertoire turnover due to lymphocyte-depleting conditioning only partially accounted for the observed reductions in tolerant patients; in fact, conventional transplant recipients showed expansion of circulating donor-reactive clones, despite extensive repertoire turnover. Moreover, loss of donor-reactive T cell clones more closely associated with tolerance induction than in vitro functional assays. Our analysis supports clonal deletion as a mechanism of allograft tolerance in CKBMT patients. The results validate the contribution of donor-reactive T cell clones identified before transplant by our method, supporting further exploration as a potential biomarker of transplant outcomes.

HSV-sr39TK positron emission tomography and suicide gene elimination of human hematopoietic stem cells and their progeny in humanized mice

Engineering immunity against cancer by the adoptive transfer of hematopoietic stem cells (HSC) modified to express antigen-specific T-cell receptors (TCR) or chimeric antigen receptors generates a continual supply of effector T cells, potentially providing superior anticancer efficacy compared with the infusion of terminally differentiated T cells. Here, we demonstrate the in vivo generation of functional effector T cells from CD34-enriched human peripheral blood stem cells modified with a lentiviral vector designed for clinical use encoding a TCR recognizing the cancer/testes antigen NY-ESO-1, coexpressing the PET/suicide gene sr39TK. Ex vivo analysis of T cells showed antigen- and HLA-restricted effector function against melanoma. Robust engraftment of gene-modified human cells was demonstrated with PET reporter imaging in hematopoietic niches such as femurs, humeri, vertebrae, and the thymus. Safety was demonstrated by the in vivo ablation of PET signal, NY-ESO-1-TCR-bearing cells, and integrated lentiviral vector genomes upon treatment with ganciclovir, but not with vehicle control. Our study provides support for the efficacy and safety of gene-modified HSCs as a therapeutic modality for engineered cancer immunotherapy. Cancer Res; 74(18); 5173-83. ©2014 AACR.

Contribution of human adipose tissue-derived stem cells and the secretome to the skin allograft survival in mice

BACKGROUND

Despite considerable evidence showing the immunosuppressive properties of mesenchymal stem cells (MSCs) in vitro, such properties have not been fully demonstrated in vivo. The aim of this study was to evaluate the effect of MSCs and/or MSC secretome in inducing tolerance in a mouse skin transplantation model.

METHODS

After receiving full-thickness skin allotransplantation on the back of the mouse, the recipient mice were infused with phosphate-buffered saline, adipose tissue-derived stem cells (ASCs), conditioned media (CM), and control media. Specifically, ASCs (1.0 × 10(6)/0.1 mL) were transplanted to ASC-infused mice and 25-fold concentrated CM, which had been obtained from ASC culture were infused to CM-infused mice. Graft survival rates and the parameters reflecting immunologic consequences were assessed.

RESULTS

The serum level of proinflammatory cytokine interleukin 6 decreased in mice treated with ASCs or CM compared with the control groups after infusion (P < 0.05). Interferon gamma, interleukin 10, and tumor necrosis factor alpha messenger RNA levels in the skin graft seemed to be decreased in the ASC-infused mice and CM-infused mice. Hyporesponsiveness was identified in mixed lymphocyte reaction assay at 30-d posttransplantation in ASC- or CM-infused mice. And, administering ASCs and CM markedly increased skin allograft survival compared with control animals (P < 0.001).

CONCLUSIONS

These findings suggest that ASCs and their secretome have the potential to induce immunologic tolerance. Moreover, our results demonstrate that the immunosuppressive properties of ASCs are mediated by the ASC secretome. Our approach could provide insights into a promising strategy to avoid toxicities of chemical immunosuppressive regimen in solid organ transplantation.

Patients' motives for consenting or refusing to participate in a clinical trial in organ transplantation

BACKGROUND

The goal of this health-psychology study was to investigate the range of motives that make someone consent or refuse participation in a clinical transplant trial.

METHODS

The study involved (i) preparatory interviews with five transplant patients who took part in clinical trials and five persons from the general public; (ii) we created a questionnaire with scaled responses; (iii) we selected 468 patients, divided into two groups: patients waiting for a transplant and patients who already had a transplant; (iv) we obtained patient consents, sent out the questionnaire, and recorded responses; (v) data were analysed using descriptive statistics, exploratory factorial analyses, correlations and regressions.

RESULTS

Two hundred and ten patients answered the questionnaire. Motives were classified into the following: (i) Motives to consent participating in a clinical trial (pride in participating, hope for better quality of life, sufficient and clear information, discussion with others participating in a trial, altruism); or (ii) Motives to refuse participating (no information on medical teams, lack of explanation, fear of additional expenses, being considered a "guinea pig").

CONCLUSIONS

This study contributes to our understanding of the motivations of patients who accepted or refused participation in a clinical transplantation trial.

Trial of complete weaning from immunosuppression for liver transplant recipients: factors predictive of tolerance

Recipients of liver transplantation (LT) may develop immunological tolerance. Factors predictive of tolerance are not clearly understood. Transplant recipients with normal liver function tests and without active viral hepatitis or autoimmune disease who presented with side effects of immunosuppression or a high risk of de novo malignancies were selected to participate in this prospective study. Twenty-four patients fulfilled the inclusion criteria and, therefore, underwent a gradual reduction of immunosuppression. Tolerance was defined as normal liver function tests after immunosuppression withdrawal. Basal clinical and immunological characteristics, including lymphocyte counts and subpopulations (T, B, natural killer, CD4(+) , CD8(+) , and regulatory T cells) and the phytohemagglutinin stimulation index (SI), were compared for tolerant and nontolerant patients. Fifteen of the 24 patients (62.5%) were tolerant at a median of 14 months (interquartile range = 8.5-22.5 months) after complete immunosuppression withdrawal. Tolerant patients had a longer median interval between transplantation and inclusion in the study (156 for tolerant patients versus 71 months for nontolerant patients, P = 0.003) and a lower median SI (7.49 for tolerant patients versus 41.73 for nontolerant patients, P = 0.01). We identified 3 groups of patients with different probabilities of tolerance: in the first group (n = 7 for an interval > 10 years and an SI < 20), 100% reached tolerance; in the second group (n = 10 for an interval > 10 years and an SI > 20 or an interval < 10 years and an SI < 20), 60% reached tolerance; and in the third group (n = 7 for an interval < 10 years and an SI > 20), 29% reached tolerance. In conclusion, a high proportion of select LT recipients can reach tolerance over the long term. Two simple basal variables-the time from transplantation and the SI-may help to identify these patients.

The role of cyclophosphamide in enhancing antitumor efficacy of an adenovirus oncolytic vector in subcutaneous Syrian hamster tumors

We have previously reported that intratumoral injection of VRX-007—an Ad5-based vector overexpressing ADP (Adenovirus Death Protein)—can suppress the growth of subcutaneous HaK (hamster renal cancer) tumors. VRX-007 replication and tumor growth inhibition are enhanced when the hamsters are immunosuppressed by a high dose of cyclophosphamide (CP), an immunosuppressive and chemotherapeutic agent. Here we report that continuous immunosuppression with CP was not required for increased oncolytic activity of VRX-007 because short-term dosing or continuous dosing with the drug yielded similar antitumor results. Prolonged viral replication was found only in animals on continuous CP treatment. We used 007-Luc, a replication-competent, luciferase-expressing vector similar to VRX-007 to investigate the replication of the vector over time. Tumor growth inhibition was similar in hamsters given CP treatment either one week before or one week after 007-Luc injection, which suggests that CP exerts its antitumor efficacy independently of vector therapy. 007-Luc did not spread far from the inoculation site, even in immunosuppressed, CP-treated animals. Our results indicate that the enhanced effectiveness that is produced by the combination of VRX-007 and CP therapies is due to their two independent mechanisms and that they do not have to be given simultaneously for the improved outcome shown.

Pretransplant antithymocyte globulin has increased efficacy in controlling donor-reactive memory T cells in mice

Antibody-mediated lymphocyte depletion is frequently used as induction therapy in sensitized transplant patients. Although T cells with an effector/memory phenotype remain detectable after lymphoablative therapies in human transplant recipients, the role of preexisting donor-reactive memory in reconstitution of the T cell repertoire and induction of alloimmune responses following lymphoablation is poorly understood. We show in a mouse cardiac transplantation model that antidonor immune responses following treatment with rabbit antimouse thymocyte globulin (mATG) were dominated by T cells derived from the preexisting memory compartment. Administration of mATG 1 week prior to transplantation (pre-TP) was more efficient in targeting preexisting donor-reactive memory T cells, inhibiting overall antidonor T cell responses, and prolonging heart allograft survival than the commonly used treatment at the time of transplantation (peri-TP). The failure of peri-TP mATG to control antidonor memory responses was due to faster recovery of preexisting memory T cells rather than their inefficient depletion. This rapid recovery did not depend on T cell specificity for donor alloantigens suggesting an important role for posttransplant inflammation in this process. Our findings provide insights into the components of the alloimmune response remaining after lymphoablation and may help guide the future use of ATG in sensitized transplant recipients.

Immunoregulatory CD4(-)CD8(-) T cells as a potential therapeutic tool for transplantation, autoimmunity, and cancer

A central objective in organ transplantation and the treatment or prevention of autoimmune disease is the achievement of antigen-specific immune tolerance. An additional challenge in bone marrow transplantation for the treatment of hematological malignancies is the prevention of graft-vs-host disease (GVHD) while maintaining graft-vs-tumor activity. Interestingly, CD4^-CD8^- (double negative, DN) T cells, which exhibit a unique antigen-specific immunoregulatory potential, appear to exhibit all of the properties to respond to these challenges. Herein, we review the therapeutic potential of immunoregulatory DN T cells in various immunopathological settings, including graft tolerance, GVHD, cancer, and autoimmunity.

Vascularized composite allograft rejection is delayed by intrajejunal treatment with donor splenocytes without concomitant immunosuppressants

Background. Mucosal or oral tolerance, an established method for inducing low-risk antigen-specific hyporesponsiveness, has not been investigated in vascularized composite allograft (VCA) research. We studied its effects on recipient immune responses and VCA rejection. Methods. Lewis rats (n = 12; TREATED) received seven daily intrajejunal treatments of 5 × 10⁷ splenocytes from semiallogeneic Lewis-Brown-Norway rats (LBN) or vehicle (n = 11; SHAM). Recipients' immune responses were assessed by mixed lymphocyte reaction (MLR) against donor antigen and controls. Other Lewis (n = 8; TREATED/VCA) received LBN hindlimb VCA and daily intrajejunal treatments of 5 × 10⁷ LBN splenocytes, or LBN VCA without treatment (n = 5; SHAM/VCA), until VCAs rejected. Recipients' immune responses were characterised and VCAs biopsied for histopathology. Immunosuppressants were not used. Results. LBN-specific hyporesponsiveness was induced only in treated Lewis recipients. Treatment significantly reduced MLR alloreactivity, significantly reduced VCA rejection on histopathology, and significantly delayed clinical VCA rejection (P < 0.0005; TREATED/VCA mean 9.6 versus 6.0 days for SHAM/VCA). Treatment significantly increased immunosuppressive IL-10/IL-4/TGF-β production and significantly decreased proinflammatory IFN-γ/TNF-α. Conclusion. Jejunal exposure to antigen conferred donor specific hyporesponsiveness that delayed VCA rejection. This method may offer a low-risk adjunctive treatment option to help protect VCAs from rejection.

Tolerance to MHC class II disparate allografts through genetic modification of bone marrow

Induction of molecular chimerism through genetic modification of bone marrow is a powerful tool for the induction of tolerance. Here we demonstrate for the first time that expression of an allogeneic MHC class II gene in autologous bone marrow cells, resulting in a state of molecular chimerism, induces tolerance to MHC class II mismatched skin grafts, a stringent test of transplant tolerance. Reconstitution of recipients with syngeneic bone marrow transduced with retrovirus encoding H-2I-A^b (I-A^b) resulted the long-term expression of the retroviral gene product on the surface of MHC class II-expressing bone marrow derived cell types. Mechanistically, tolerance was maintained by the presence of regulatory T cells, which prevented proliferation and cytokine production by alloreactive host T cells. Thus, the introduction of MHC class II genes into bone marrow derived cells through genetic engineering results in tolerance. These results have the potential to extend the clinical applicability of molecular chimerism for tolerance induction.

Tolerance in liver transplantation

Operational tolerance (OT) in liver transplant patients occurs much more frequently than OT of other transplanted organs; however the rate of OT varies considerably with the centre and patient population. Rates of OT range from 15% of the total liver transplant (LTX) patient population down to less than 5%. This review examines the reports of liver OT and compares the factors that could contribute to this variation. Multiple factors were examined, including the time from transplantation when weaning of immunosuppression (IS) was commenced, the rapidity of weaning, the contribution of maintenance and induction IS and the patient population transplanted. The approaches that might be used to increase the likelihood of OT are discussed and the approaches to monitoring OT in LTX patients are reviewed.

Apoptosis and the airway epithelium

The airway epithelium functions as a barrier and front line of host defense in the lung. Apoptosis or programmed cell death can be elicited in the epithelium as a response to viral infection, exposure to allergen or to environmental toxins, or to drugs. While apoptosis can be induced via activation of death receptors on the cell surface or by disruption of mitochondrial polarity, epithelial cells compared to inflammatory cells are more resistant to apoptotic stimuli. This paper focuses on the response of airway epithelium to apoptosis in the normal state, apoptosis as a potential regulator of the number and types of epithelial cells in the airway, and the contribution of epithelial cell apoptosis in important airways diseases.

Immune responses in transplantation: application to composite tissue allograft

After announcements of successful hand, larynx, knee, muscle, nerve, and, most recently, face transplantation, composite tissue allografts (CTAs) have been introduced into the armamentarium of plastic and reconstructive surgery. Because the microsurgical techniques required to perform CTA transplants are well established and used in daily practice by plastic surgeons, the immunologic aspects of transplantation remain of great interest to plastic surgeons. CTAs offer a unique potential for coverage of large multitissue defects; however, compared with the relatively homogenous tissue of solid organ transplants, the heterogenicity of tissue components of CTA may generate high immunologic responses. Although modern immunosuppressive agents significantly improve successful allograft acceptance, chronic allograft rejection as well as immunosuppressive drug toxicity remain major problems in the clinical practice of transplantation. The major goal of transplantation immunology is to develop tolerance to allograft transplants and long-term drug-free survival. Several experimental protocols have been designed to develop tolerance; however, none of them have been proved to induce tolerance in clinical transplantation. This review outlines the mechanisms of allograft acceptance and rejection and describes the barriers to transplantation tolerance based on our current knowledge as it applies to solid organs and CTA transplants. The review also describes innovative immunosuppressive protocols.

IL-6 induced by Staphylococcus aureus infection prevents the induction of skin allograft acceptance in mice

Clinical correlations between bacterial infections and rejection suggest a hypothesis that innate immune stimulation by bacterial infections results in the production of inflammatory cytokine that facilitate bystander T-cell activation, increased alloreactivity and inhibition of tolerance induction. Previous studies demonstrated that IFNβ produced during an infection with a model bacterium, Listeria monocytogenes, prevented the induction of transplantation tolerance in mice with anti-CD154 and donor-specific transfusion (DST) (1). We investigated the impact of two clinically relevant bacterial infections at the time of transplantation on the ability of anti-CD154 and DST to induce skin allograft acceptance in mice. Staphylococcus aureus (SA) infection prevented skin allograft acceptance whereas maximally tolerated doses of Pseudomonas aeruginosa infection had no effect. SA induced an acute production of IL-6, which was necessary and sufficient for the prevention of skin allograft acceptance. Furthermore, a single pulse of methylprednisolone modulated IL-6 production during SA infection and facilitated skin allograft acceptance in SA-infected recipients. Taken together, our results suggest that bacterial infections elicit specific proinflammatory cytokines signatures that can serve as barriers to tolerance induction, and that inhibiting the production of or neutralizing these inflammatory cytokines can synergize with costimulatory blockade-based therapies to facilitate the development of transplantation tolerance.

Clinical operational tolerance after renal transplantation: current status and future challenges

In solid organ transplantation, the achievement of an immunosuppression (IS)-free state [also referred to as clinical operational tolerance (COT)] represents the ultimate goal. Although COT is feasible and safe in selected cases after liver transplantation, it is an exceptional finding after other types of solid organ transplantation. In the field of renal transplantation (RT), approximately 100 cases of COT have been reported to date, mainly in patients who were not compliant with their immunosuppressive regimens or in individuals who had previously received a bone marrow transplant for hematological disorders. On the basis of promising results obtained in animal models, several tolerogenic protocols have been attempted in humans, but most have failed to achieve robust and stable COT after RT. Molecule-based regimens have been largely ineffective, whereas cell-based regimens have provided some encouraging results. In these latter regimens, apart from standard IS, patients usually receive perioperative infusion of donor bone marrow-derived stem cells, which are able to interact with the immune cells of the host and mitigate their response to engraftment. Unfortunately, most renal transplant patients who developed acute rejection-occurring either during the weaning protocol or after complete withdrawal of IS-eventually lost their grafts. Currently, the immune monitoring necessary for predicting the presence and persistence of donor-specific unresponsiveness is not available. Overall, the present review will provide a conceptual framework for COT and conclude that stable and robust COT after RT remains an elusive goal and that the different strategies attempted to date are not yet reproducibly safe or effective.

Cotransplantation of hepatic stellate cells attenuates the severity of graft-versus-host disease

Liver allografts seem to be immunologically privileged; they are the only solid transplant that can protect cotransplanted organs from rejection. The mechanisms of this "hepatic tolerance" are poorly understood. The aim of this study was to examine the immunomodulatory effect of hepatic stellate cells (HSCs) in mixed lymphocyte reactions in vitro and in graft-versus-host disease (GVHD) in vivo. Using a carboxyfluorescein diacetate succinimidyl ester-labeling method, we observed that the percentage of proliferative T cells was reduced when HSCs were present in a mixed lymphocyte culture. In addition, the degree of reduction was the same when HSCs were co-cultured in transwell inserts. Thus, soluble factors may participate in the immunomodulatory effect of HSCs. In GVHD experiments, irradiated BALB/c (H-2d) mice simultaneously received an intravenous mixture of bone marrow and splenic T cells from C57BL/6 (H-2b) mice. The HSCs prominently reduced symptoms and pathologic severity of GVHD in target organs. HSC cotransplanted mice survived for 57.5+/-30.6 days whereas the control hosts only survived for 15.3+/-5.2 days (P<.01). We concluded that HSCs may reduce T-cell proliferation against alloantigens and suppress acute GVHD to prolong recipient survival. Our study sheds some light on the immunosuppressive nature of the liver, suggesting a biological manipulation of alloreactivity for transplantation medicine.

Induction of transplantation tolerance to fully mismatched cardiac allografts by T cell mediated delivery of alloantigen

Induction of transplantation tolerance has the potential to allow for allograft acceptance without the need for life-long immunosuppression. Here we describe a novel approach that uses delivery of alloantigen by mature T cells to induce tolerance to fully allogeneic cardiac grafts. Adoptive transfer of mature alloantigen-expressing T cells into myeloablatively conditioned mice results in long-term acceptance of fully allogeneic heart transplants without evidence of chronic rejection. Since myeloablative conditioning is clinically undesirable we further demonstrated that adoptive transfer of mature alloantigen-expressing T cells alone into mice receiving non-myeloablative conditioning resulted in long-term acceptance of fully allogeneic heart allografts with minimal evidence of chronic rejection. Mechanistically, tolerance induction involved both deletion of donor-reactive host T cells and the development of regulatory T cells. Thus, delivery of alloantigen by mature T cells induces tolerance to fully allogeneic organ allografts in non-myeloablatively conditioned recipients, representing a novel approach for tolerance induction in transplantation.

Cryopreservation and the age of the allotransplant

For centuries, reconstructive surgeons have restored form and function with autografts. These techniques are highly effective, but they are associated invariably with donor site morbidity. To avoid this, surgeons have long dreamed of using cadaveric sources for reconstructive material. However, allografts have two major limitations: rejection and limited donor tissue. In order to limit rejection, the allograft must be rendered more tolerable to the host or the host must be rendered more tolerant of the allograft. Both strategies have been used with considerable success in recent years. As understanding of the human immune response increases, clinical immunosuppressive regimens will undoubtedly become less morbid, and the indications for allotransplantation will broaden. This will place an even greater burden on the already small donor pool. One way to relieve this burden would be through the development of strategies for the long-term preservation of donated tissues and organs. Cryopreservation has been used clinically for decades, and recent advances in the field have allowed the preservation of an ever widening array of tissues and organs. As cold storage has been shown to reduce the antigenicity of parts, cryopreservation may actually serve to improve the survival rate of transplanted parts, as well as increase their availability. As the era of autotransplantation gives way to the age of allotransplantation, cryopreservation will play an increasingly important role.

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

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

14-Base pair polymorphism of human leukocyte antigen-G as genetic determinant in heart transplantation and cyclosporine therapy monitoring

The 14-base pair (bp) polymorphism within the HLA-G gene has been investigated in heart transplant patients for the first time. The 14-bp polymorphism is associated with HLA-G mRNA stability and the patterns of alternative isoforms splicing, and therefore may influence the functionality of the HLA-G molecule. In heart transplantation, the highest production of soluble HLA-G was related to the -14/-14-bp genotype in the pre- and post-transplantation periods. Our study findings showed that the 14-bp polymorphism of the HLA-G gene influenced the expression of soluble HLA-G in heart transplantation and accordingly resulted in low rejection rates, being a possible marker of genetic variability associated with heart transplantation. In addition, the 14-bp polymorphism of the HLA-G gene is related to the absorber status of cyclosporine of each individual patient, and is useful for determining the oral dose of cyclosporine to manage patients (to adjust immunosuppressive protocols) so as to minimize the risk of a low or high immunosuppression and the side effects in the early stages of heart transplantation.

TLR agonists prevent the establishment of allogeneic hematopoietic chimerism in mice treated with costimulation blockade

Activation of TLR4 by administration of LPS shortens the survival of skin allografts in mice treated with costimulation blockade through a CD8 T cell-dependent, MyD88-dependent, and type I IFN receptor-dependent pathway. The effect of TLR activation on the establishment of allogeneic hematopoietic chimerism in mice treated with costimulation blockade is not known. Using a costimulation blockade protocol based on a donor-specific transfusion (DST) and a short course of anti-CD154 mAb, we show that LPS administration at the time of DST matures host alloantigen-presenting dendritic cells, prevents the establishment of mixed allogeneic hematopoietic chimerism, and shortens survival of donor-specific skin allografts. LPS mediates its effects via a mechanism that involves both CD4(+) and CD8(+) T cells and results from signaling through either the MyD88 or the type I IFN receptor pathways. We also document that timing of LPS administration is critical, as injection of LPS 24 h before treatment with DST and anti-CD154 mAb does not prevent hematopoietic engraftment but administration the day after bone marrow transplantation does. We conclude that TLR4 activation prevents the induction of mixed allogeneic hematopoietic chimerism through type I IFN receptor and MyD88-dependent signaling, which leads to the up-regulation of costimulatory molecules on host APCs and the generation of alloreactive T cells. These data suggest that distinct but overlapping cellular and molecular mechanisms control the ability of TLR agonists to block tolerance induction to hematopoietic and skin allografts in mice treated with costimulation blockade.

Hematopoiesis from Human Embryonic Stem Cells: Overcoming the Immune Barrier in Stem Cell Therapies

The multipotency and proliferative capacity of human embryonic stem cells (hESCs) make them a promising source of stem cells for transplant therapies and of vital importance given the shortage in organ donation. Recent studies suggest some immune privilege associated with hESC-derived tissues. However, the adaptability of the immune system makes it unlikely that fully differentiated tissues will permanently evade immune rejection. One promising solution is to induce a state of immune tolerance to a hESC line using tolerogenic hematopoietic cells derived from it. This could provide acceptance of other differentiated tissues from the same line. However, this approach will require efficient multilineage hematopoiesis from hESCs. This review proposes that more efficient differentiation of hESCs to the tolerogenic cell types required is most likely to occur through applying knowledge gained of the ontogeny of complex regulatory signals used by the embryo for definitive hematopoietic development in vivo. Stepwise formation of mesoderm, induction of definitive hematopoietic stem cells, and the application of factors key to their self-renewal may improve in vitro production both quantitatively and qualitatively.