Rapamycin in combination with donor-specific CD4CD25Treg cells amplified in vitro might be realize the immune tolerance in clinical organ transplantation

It is an urgent need to induce and keep the donor-specific immune tolerance without affecting the function of normal immune defense and immune surveillance in clinical organ transplantation. Large number of studies showed that both the establishment of donor-recipient chimerism and the application of antibodies or drugs could obtain the donor-specific immune tolerance in animal transplantation model. However, the former as treatment of clinical practice has a poor feasibility, the latter has a very low success rate in clinical organ transplantation. There is a group of naturally occurring CD4⁺CD25⁺ regulatory T cells (Tregs) that mediate immune tolerance by suppressing alloreactive T cells in vivo. These cells are unable to curb the occurrence of allograft rejection owing their low content. And donor-specific Tregs amplified in vitro alone can not induce donor-specific immune tolerance for recipient. Rapamycin (RPM) as a proliferation signal inhibitor, studies have shown it can effectively inhibit allograft rejection and maybe contribute to induction of immune tolerance. But there exist still many dose-dependent adverse reactions which could prevent the establishment of immune tolerance and reduce the life quality of recipients in the clinical application of RPM. Therefore, we speculate a small amount of RPM combined with donor-specific Tregs amplified in vitro may be not only induce the achievement of donor-specific tolerance, but also reduce or eliminate the side effects of RPM in clinical organ transplantation.     

Therapeutic plasticity of stem cells and allograft tolerance

Transplantation is the treatment of choice for many diseases that result in organ failure, but its success is limited by organ rejection. Stem cell therapy has emerged in the last years as a promising strategy for the induction of tolerance after organ transplantation. Here we discuss the ability of different stem cell types, in particular mesenchymal stromal cells, neuronal stem/progenitor cells, hematopoietic stem cells and embryonic stem cells, to modulate the immune response and induce peripheral or central tolerance. These stem cells have been studied to explore tolerance induction to several transplanted organs, such as heart, liver and kidney. Different strategies, including approaches to generating tolerance in islet transplantation, are discussed here.     

Tolerance, mixed chimerism and protection against graft-versus-host disease after total lymphoid irradiation

Total lymphoid irradiation (TLI), originally developed as a non-myeloablative treatment for Hodgkin's disease, has been adapted for the induction of immune tolerance to organ allografts in rodents, dogs and non-human primates. Moreover, pretransplantation TLI has been used in prospective studies to demonstrate the feasibility of the induction of tolerance to cadaveric kidney allografts in humans. Two types of tolerance, chimeric and non-chimeric, develop after TLI treatment of hosts depending on whether donor bone marrow cells are transplanted along with the organ allograft. An advantageous feature of TLI for combined marrow and organ transplantation is the protection against graft-versus-host disease (GVHD) and facilitation of chimerism afforded by the predominance of CD4+ NK1.1(+) -like T cells in the irradiated host lymphoid tissues. Recently, a completely post-transplantation TLI regimen has been developed resulting in stable mixed chimerism and tolerance that is enhanced by a brief course of cyclosporine. The post-transplantation protocol is suitable for clinical cadaveric kidney transplantation. This review summarizes the evolution of TLI protocols for eventual application to human clinical transplantation and discusses the mechanisms involved in the induction of mixed chimerism and protection from GVHD.     

Development of tolerogenic strategies in the clinic

The study of tolerance in the clinic can be divided into three areas: (i) focused evaluation of existing tolerant transplant recipients as to their mechanism of tolerance; (ii) prospective tolerance trials, such as combined bone marrow and kidney transplantation as well as T cell depletion followed by subsequent weaning of immunosuppression; and (iii) immunologic assays to assess the likelihood of rejection or tolerance. Frankly, a very small number of patients have been transplanted with the intention of removing all immunosuppressive therapy, but several clinical trials with this aim are currently in progress, largely sponsored by the Immune Tolerance Network, a joint venture between the National Institutes of Health and the Juvenile Diabetes Research Foundation. Similarly, a reliable assay to assess tolerance has not yet been developed but a variety of approaches towards assessing rejection, and in some cases tolerance, are being developed. It would be accurate to state that many of the experimental and preclinical approaches to the induction of tolerance have resulted in better immunosuppression for human transplantation, but reliable tolerance strategies in humans have not yet been achieved. Combined bone marrow and kidney transplantation may be considered as one exception to this, but such a strategy is not generally applicable to the vast majority of solid organ transplant recipients. This review will summarize efforts to date, particularly focusing on kidney transplantation.     

Transplantation tolerance from a historical perspective

Although transplantation immunology as a distinctive field began with the development of experimental models that showed the feasibility of bone marrow transplantation, organ engraftment was accomplished first in humans, and was thought for many years to occur by drastically different mechanisms. Here, we present our view of the concepts of allograft acceptance and acquired tolerance from a historical perspective, and attempt to amalgamate them into simple and unifying rules that might guide improvements in clinical therapy.     

Methods of Inducing Immunological Tolerance to Tissue Allografts and Xenografts

Despite the remarkable progress in clinical organ transplantation in the past two decades, significant problems remain to be solved. These problems include rejection, late opportunistic sepsis, spontaneous neoplasms, metabolic complications, and drug toxicity. In this article, various experimental models of tolerance and ways of inducing tolerance to tissue allographs are discussed.     

Bone marrow transplantation and approaches to avoid graft-versus-host disease (GVHD)

Haematopoietic stem cell transplantation (HSCT) offers promise for the treatment of haematological and immune disorders, solid tumours, and as a tolerance inducing regimen for organ transplantation. Allogeneic HSCTs engraftment requires immunosuppression and the anti-tumour effects are dependent upon the immune effector cells that are contained within or generated from the donor graft. However, significant toxicities currently limit its efficacy. These problems include: (i) graft-versus-host disease (GVHD) in which donor T cells attack the recipient resulting in multi-organ attack and morbidity, (ii) a profound period of immune deficiency following HSCT, and (iii) donor graft rejection. Currently available methods to prevent or treat GVHD with systemic immunosuppression can lead to impaired immune recovery, increased opportunistic infections, and higher relapse rates. This review will provide an overview of GVHD pathophysiology and discuss the roles of various cells, pathways, and factors in the GVHD generation process and in the preservation of graft-versus-tumour effects. Variables that need to be taken into consideration in attempting to extrapolate preclinical results to the clinical paradigm will be highlighted.     

Basics of immune responses in transplantation in preparation for application of composite tissue allografts in plastic and reconstructive surgery: part I

Currently, composite tissue allografts are applied only occasionally as a reconstructive option in the field of plastic and reconstructive surgery. Composite tissue allografts 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 composite tissue allografts may generate high immunologic responses. Modern immunosuppressive agents significantly improve successful allograft acceptance. However, chronic allograft rejection and immunosuppressive drug toxicity are still major problems in the clinical practice of transplantation. The major goals of transplantation immunology are (1) to develop tolerance to allograft transplants and (2) long-term drug-free survival. A number of experimental protocols were designed to develop tolerance; however, none of them has been proven to induce tolerance in clinical transplantation. In this article, the authors outline the mechanisms of allograft acceptance and rejection and barriers to transplantation tolerance. Novel immunosuppressive protocols are discussed in this review. This basic immunologic knowledge of allograft acceptance and rejection will allow plastic surgeons to apply composite tissue allograft transplants to plastic and reconstructive surgery.     

The effects of immunosuppression on regulatory CD4(+)CD25(+) T cells: impact on immunosuppression selection in transplantation

During immune response and T-cell activation, both effector T cells and regulatory T(T(reg)) cells are activated and regulated simultaneously by both positive and negative pathways. CD4(+)CD25(+) T(reg) cells play a critical role in immune tolerance to self antigens as well as to allografts in some transplant settings. Effective immunosuppressive regimens significantly reduced the incidence of acute allograft rejection in patients following organ transplantation. However, the impact of immunosuppressive treatment on the potential induction of transplant tolerance has not been well determined. In this review we summarize the effects of immunosuppressive reagents on CD4(+)CD25(+) T(reg) cells in order to bring attention to this issue, which may affect the choice of immunosuppressive regimen in the clinical setting.     

Transplantation tolerance-where do we stand?

Our understanding of tolerance mechanisms has progressed to the point that tolerance-induction protocols are being tested in humans for organ transplantation. However, a range of scientific, ethical, logistic and commercial issues have arisen, and must be resolved before tolerance induction for human allograft patients can become a reality.     

Protective effect of carbon monoxide in transplantation

During the last decades due to the development of new immunosuppressive agents and improvements in organ preservation methods, surgical techniques, and postoperative care, organ transplantation has become an ultimate therapeutic option for irreversible organ failure. Early graft survival has significantly improved; however, the long-term outcome remains unsatisfactory. Multiple factors, both immunogenic and non-immunogenic etiologies, are involved in the deterioration of the allografts, and the recent use of expanded criteria donors to overcome the organ shortage may also contribute to the graft losses. Carbon monoxide (CO) is commonly viewed as a poison in high concentrations due to its ability to interfere with oxygen delivery. However, CO is endogenously produced in the body as a byproduct of heme degradation by the heme oxygenase (HO) and has recently received notable attention as a gaseous regulatory molecule. In fact, an augmentation of endogenous CO by induction of HO-1 or exogenously added CO is known to have potent cytoprotective effects in various disease models. Several recent reports have demonstrated that CO provides potent cytoprotective effects in the field of organ and cell transplantation. CO is able to prevent ischemia/reperfusion injury, allograft rejection, and xenograft rejection via its anti-inflammatory, anti-apoptotic and anti-proliferation effects, suggesting that CO might be a valuable therapeutic option in the field of transplantation. Based on the recent advancement of our understanding of CO as a new therapeutic molecule, this review attempts to summarize the functional roles as well as biological and molecular mechanisms of CO in transplantation and discusses potential CO application to the clinical transplant setting.     

Activation-induced apoptosis of autoreactive and alloreactive T lymphocytes in the target organ as a major mechanism of tolerance.

Normal individuals have mature T lymphocytes that are capable of reacting to self-antigens and can be activated by cross-reacting environmental antigens. The mechanism that maintains immune tolerance and prevents these activated autoreactive T cells from causing autoimmune disease is unclear. We have previously hypothesized that activation-induced apoptosis of previously activated autoreactive T cells in the target organ is a major mechanism for maintaining tolerance. Here I review the current evidence to support this hypothesis. It is proposed that when activated autoreactive T cells enter the target organ, they are reactivated mainly by non-professional antigen-presenting cells (APC) and deleted by activation-induced apoptosis through the Fas (CD95) pathway before producing significant target organ damage. This apoptosis occurs because the reactivated T cells do not receive sufficient costimulation from the non-professional APC to up-regulate their expression of Bcl-2-related anti-apoptotic proteins, which inhibit the CD95 pro-apoptotic pathway. This is in contrast to the situation in peripheral lymphoid organs, where reactivation of T cells by professional APC results in sufficient costimulation-induced up-regulation of Bcl-2-related proteins to inhibit the CD95 pathway and allow T cell proliferation and survival as memory T cells. Activation-induced apoptosis of alloreactive T cells in allografts can similarly account for spontaneous allograft acceptance, as occurs after MHC-mismatched liver transplantation.     

The functional relevance of passenger leukocytes and microchimerism for heart allograft acceptance in the rat.

With an organ transplant, hematopoietic donor cells are transferred to the recipient. To study the relevance of the resulting microchimerism for allograft acceptance, we analyzed a rat model of cyclosporine-induced tolerance for strongly incompatible heart allografts. Using a monoclonal antibody that detects a donor-specific CD45 allotype (RT7a), we selectively depleted donor leukocytes at different times after transplantation (days 0 or 18). Depletion was similarly effective at both times. However, only depletion on day 0 prevented tolerance induction and was associated with severe acute or chronic graft rejection. This indicates that passenger leukocytes have an essential immunomodulatory effect on the induction phase of allograft acceptance.     

New strategies in immune tolerance induction

Induction of tolerance in clinical organ transplantation that will obviate the use of chronic immunosuppression and preserve host immune response to other antigens remains the goal of transplant research. The thymus plays a critical role in the ability of the immune system to discriminate between self- and nonself-antigens or harmful and harmless alloantigens. We now know that multiple factors determine how the immune system responds to a self-antigen or foreign antigen. These determinants include developmental stage of the host, stage of T-cell maturity, site of antigen encounter, type and maturity of antigen-presenting cells, and presence and type of costimulatory molecules. Our understanding of the mechanisms of T-cell interactions with peptide/major histocompatibility complex in peripheral lymphoid organs has led to experiments that translate into peripheral T-cell tolerance. The induction of high-avidity peripheral alloreactive T cells in the early phase of organ transplantation makes it difficult to achieve long-term alloantigen-specific tolerance without the use of transient perioperative immunosuppression. Therefore, protocols that induce robust tolerance in rodent and nonhuman primate models involve the use of donor antigen combined with a short course of perioperative immunosuppression. These studies suggest that the underlying mechanisms of peripheral tolerance include deletion, anergy, immune deviation, and regulatory T cells. This review focuses on recent advances in tolerance induction in experimental animal models and discusses their relevance to the development of protocols for the induction and maintenance of clinical transplant tolerance.     

Embryonic stem cells, derived either after in vitro fertilization or nuclear transfer, prolong survival of semiallogeneic heart transplants

Tolerance induction toward allogeneic organ grafts represents one of the major aims of transplantation medicine. Stem cells are promising candidates for promoting donor-specific tolerance. In this study, we investigated the immunomodulatory properties of murine embryonic stem cells (ESCs), obtained either by in vitro fertilization (IVF-ESCs) or by nuclear transfer (NT-ESCs), in heart transplant mouse models. IVF-ESCs did not prolong the survival of fully allogeneic cardiac transplants but significantly prolonged the survival of semiallogeneic hearts from the same ESC donor strain for >100 d in 44% of the animals. However, 28% of transplanted animals infused with IVF-ESCs experienced development of a teratoma. NT-ESCs similarly prolonged semiallogeneic heart graft survival (>100 d in 40% of the animals) but were less teratogenic. By in vitro studies, IVF-ESC and NT-ESC immunoregulation was mediated both by cell contact-dependent mechanisms and by the release of soluble factors. By adding specific inhibitors, we identified PGE(2) as a soluble mediator of ESC immunoregulation. Expansion of regulatory T cells was found in lymphoid organs and in the grafts of IVF-ESC- and NT-ESC-tolerized mice. Our study demonstrates that both IVF-ESCs and NT-ESCs modulate recipient immune response toward tolerance to solid organ transplantation, and that NT-ESCs exhibit a lower tendency for teratoma formation. Because NT-ESCs are obtained by NT of a somatic cell from living individuals into an enucleated oocyte, they could represent a source of donor-derived stem cells to induce tolerance to solid organ allograft.     

Vigorous allograft rejection in the absence of danger

Tolerance to self is a necessary attribute of the immune system. It is thought that most autoreactive T cells are deleted in the thymus during the process of negative selection. However, peripheral tolerance mechanisms also exist to prevent development of autoimmune diseases against peripheral self-Ags. It has been proposed that T cells develop tolerance to peripheral self-Ags encountered in the absence of inflammation or "danger" signals. We have used immunodeficient Rag 1-/- mice to study the response of T cells to neo-self peripheral Ags in the form of well-healed skin and vascularized cardiac allografts. In this paper we report that skin and cardiac allografts without evidence of inflammation are vigorously rejected by transferred T cells or when recipients are reconstituted with T cells at a physiologic rate by nude bone graft transplantation. These results provide new insights into the role of inflammation or "danger" in the initiation of T cell-dependent immune responses. These findings also have profound implications in organ transplantation and suggest that in the absence of central deletional tolerance, peripheral tolerance mechanisms are not sufficient to inhibit alloimmune responses even in the absence of inflammation or danger.     

Induction of transplantation tolerance in non-human primate preclinical models

Short-term outcomes following organ transplantation have improved considerably since the availability of cyclosporine ushered in the modern era of immunosuppression. In spite of this, many of the current limitations to progress in the field are directly related to the existing practice of relatively non-specific immunosuppression. These include increased risks of opportunistic infection and cancer, and toxicity associated with long-term immunosuppressive drug exposure. In addition, long-term graft loss continues to result in part from a failure to adequately control the anti-donor immune response. The development of a safe and reliable means of inducing tolerance would ameliorate these issues and improve the lives of transplant recipients, yet given the improving clinical standard of care, the translation of new therapies has become appropriately more cautious and dependent on increasingly predictive preclinical models. While convenient and easy to use, rodent tolerance models have not to date been reliably capable of predicting a therapy's potential efficacy in humans. Non-human primates possess an immune system that more closely approximates that found in humans, and have served as a more rigorous preclinical testing ground for novel therapies. Prior to clinical adaptation therefore, tolerance regimens should be vetted in non-human primates to ensure that there is sufficient potential for efficacy to justify the risk of its application.     

4th International Peptide Symposium in conjunction with the 7th Australian Peptide Symposium and the 2nd Asia–Pacific International Peptide Symposium

The identification and clinical use of more sensitive and specific biomarkers in the field of solid organ transplantation is an urgent need in medicine. Solid organ transplantation has seen improvements in the short-term survival of transplanted organs due to recent advancements in immunosuppressive therapy. However, the currently available methods of allograft monitoring are not optimal. Recent advancements in assaying methods for biomolecules such as genes, mRNA and proteins have helped to identify surrogate biomarkers that can be used to monitor the transplanted organ. These high-throughput ‘omic’ methods can help researchers to significantly speed up the identification and the validation steps, which are crucial factors for biomarker discovery efforts. Still, the progress towards identifying more sensitive and specific biomarkers remains a great deal slower than expected. In this article, we have evaluated the current status of biomarker discovery using proteomics tools in different solid organ transplants in recent years. This article summarizes recent reports and current status, along with the hurdles in efficient biomarker discovery of protein biomarkers using proteomics approaches. Finally, we will touch upon personalized medicine as a future direction for better management of transplanted organs, and provide what we think could be a recipe for success in this field.     

Prevention of allograft tolerance by bacterial infection with Listeria monocytogenes

Exposure to certain viruses and parasites has been shown to prevent the induction of transplantation tolerance in mice via the generation of cross-reactive memory T cell responses or the induction of bystander activation. Bacterial infections are common in the perioperative period of solid organ allograft recipients in the clinic, and correlations between bacterial infections and acute allograft rejection have been reported. However, whether bacterial infections at the time of transplantation have any effect on the generation of transplantation tolerance remains to be established. We used the Gram-positive intracellular bacterium Listeria monocytogenes (LM) as a model pathogen because its effects on immune responses are well described. Perioperative LM infection prevented cardiac and skin allograft acceptance induced by anti-CD154 and donor-specific transfusion in mice. LM-mediated rejection was not due to the generation of cross-reactive T cells and was largely independent of signaling via MyD88, an adaptor for most TLRs, IL-1, and IL-18. Instead, transplant rejection following LM infection was dependent on the expression of the phagosome-lysing pore former listeriolysin O and on type I IFN receptor signaling. Our results indicate that bacterial exposure at the time of transplantation can antagonize tolerogenic regimens by enhancing alloantigen-specific immune responses independently of the generation of cross-reactive memory T cells.     

Recent advances in biomarker discovery in solid organ transplant by proteomics

The identification and clinical use of more sensitive and specific biomarkers in the field of solid organ transplantation is an urgent need in medicine. Solid organ transplantation has seen improvements in the short-term survival of transplanted organs due to recent advancements in immunosuppressive therapy. However, the currently available methods of allograft monitoring are not optimal. Recent advancements in assaying methods for biomolecules such as genes, mRNA and proteins have helped to identify surrogate biomarkers that can be used to monitor the transplanted organ. These high-throughput 'omic' methods can help researchers to significantly speed up the identification and the validation steps, which are crucial factors for biomarker discovery efforts. Still, the progress towards identifying more sensitive and specific biomarkers remains a great deal slower than expected. In this article, we have evaluated the current status of biomarker discovery using proteomics tools in different solid organ transplants in recent years. This article summarizes recent reports and current status, along with the hurdles in efficient biomarker discovery of protein biomarkers using proteomics approaches. Finally, we will touch upon personalized medicine as a future direction for better management of transplanted organs, and provide what we think could be a recipe for success in this field.