CD4+CD25+ regulatory T cells mediate acquired transplant tolerance

Chemokines and Their Receptors in Islet Allograft Rejection and as Targets for Tolerance Induction

Graft rejection is a major barrier to successful outcome of transplantation surgery. Islet transplantation introduces insulin secreting tissue into type 1 diabetes mellitus recipients, relieving patients from exogenous insulin injection. However, insulitis of grafted tissue and allograft rejection prevent long-term insulin independence. Leukocyte trafficking is necessary for the launch of successful immune responses to pathogen or allograft. Chemokines, small chemotactic cytokines, direct the migration of leukocytes through their interaction with chemokine receptors found on cell surfaces of immune cells. Unique receptor expression of leukocytes, and the specificity of chemokine secretion during various states of immune response, suggest that the extracellular chemokine milieu specifically homes certain leukocyte subsets. Thus, only those leukocytes required for the current immune task are attracted to the inflammatory site. Chemokine blockade, using antagonists and monoclonal antibodies directed against chemokine receptors, is an emerging and specific immunosuppressive strategy. Importantly, chemokine blockade may potentiate tolerance induction regimens to be used following transplantation surgery, and prevent the need for life-long immunosuppression of islet transplant recipients. Here, the role for chemokine blockade in islet transplant rejection and tolerance is reviewed.

The immunologic considerations in human head transplantation

The idea of head transplantation appears at first as unrealistic, unethical, and futile. Here we discuss immunological considerations in human head transplantation. In a separate accompanying article we discuss surgical, ethical, and psychosocial issues concerned in body-to-head transplantation (BHT) [1]. The success of such an unusual allograft, where the donor and the recipient can reject each other, depends on prevention of complex immunologic reactions, especially rejection of the head by the body (graft-vs-host) or probably less likely, the possibility of the head rejecting the total body allograft (host-vs-graft). The technical and immunologic difficulties are enormous, especially since rapid nerve and cord connections and regeneration have not yet been possible to achieve. In this article we begin by briefly reviewing neuro-immunologic issues that may favor BHT such as the blood brain barrier (BBB) and point out its shortcomings. And we touch on the cellular and humoral elements in the brain proper that differ in some respects from those in other organs and in the periphery. Based on recent successes in vascular composite allografts (VCAs), we will elaborate on potential specific advantages and difficulties in BHT of various available immunosuppressive medications already utilized in VCAs. The risk/benefit ratio of these drugs will be emphasized in relation to direct brain toxicity such as seizure disorders, interference, or promotion of nerve regeneration, and potentiation of cerebral viral infections. The final portion of this article will focus on pre-transplant immunologic manipulation of the deceased donor body along with pretreatment of the recipient.

Biological therapy of bone defects: the immunology of bone allo-transplantation

IMPORTANCE OF THE FIELD

Bone is one of the most transplanted tissues worldwide. Autograft is the ideal bone graft but is not widely used because of donor site morbidity and restricted availability. Allograft is easily accessible but can transmit infections and elicit an immune response.

AREAS COVERED IN THIS REVIEW

This review identifies all in vitro and in vivo evidence of immune responses following bone transplantation and highlights methods of improving host tolerance to bone allotransplantation.

WHAT THE READER WILL GAIN

In humans, the presence of anti-HLA specific antibodies against freeze-dried and fresh-frozen bone allografts has been demonstrated. Fresh-frozen bone allograft can still generate immune reactions whilst freeze-dried bone allografts present with less immunogenicity but have less structural integrity. This immune response can have an adverse effect on the graft's incorporation and increase the incidence of rejection. Decreasing the immune reaction against the allograft by lowering the immunogenic load of the graft or lowering the host immune response, would result in improved bone incorporation.

TAKE HOME MESSAGE

It is essential that the complex biological processes related to bone immunogenicity are understood, since this may allow the development of safer and more successful ways of controlling the outcome of bone allografting.

Depletion of tumor-induced Treg prior to reconstitution rescues enhanced priming of tumor-specific, therapeutic effector T cells in lymphopenic hosts

We reported previously that vaccination of reconstituted, lymphopenic mice resulted in a higher frequency of tumor-specific effector T cells with therapeutic activity than vaccination of normal mice. Here, we show that lymphopenic mice reconstituted with spleen cells from tumor-bearing mice (TBM), a situation that resembles the clinical condition, failed to generate tumor-specific T cells with therapeutic efficacy. However, depletion of CD25(+) Treg from the spleen cells of TBM restored tumor-specific priming and therapeutic efficacy. Adding back TBM CD25(+) Treg to CD25(-) naïve and TBM donor T cells prior to reconstitution confirmed their suppressive role. CD25(+) Treg from TBM prevented priming of tumor-specific T cells since subsequent depletion of CD4(+) T cells did not restore therapeutic efficacy. This effect may not be antigen-specific as three histologically distinct tumors generated CD25(+) Treg that could suppress the T-cell immune response to a melanoma vaccine. Importantly, since ex vivo depletion of CD25(+) Treg from TBM spleen cells prior to reconstitution and vaccination fully restored the generation of therapeutic effector T cells, even in animals with established tumor burden, we have initiated a translational clinical trial of this strategy in patients with metastatic melanoma.

Biomarkers to discern transplantation tolerance after allogeneic hematopoietic cell transplantation

Although it is commonly accepted that allogeneic hematopoietic cell transplant (HCT) recipients develop transplantation tolerance and can quickly discontinue all immunosuppressive drugs, existing data does not support this concept. Most patients will require a prolonged duration of immunosuppression, lasting commonly several years. This has even greater importance, as the majority of transplants are now performed utilizing peripheral blood mobilized stem cells, which are associated with an increased risk of chronic graft-versus-host disease (cGVHD) and prolonged duration of immunosuppression. Despite these challenges, the approach to liberation from immunosuppression after HCT is empiric, and biomarkers of operational tolerance after HCT are lacking. Conversely, investigators in solid organ allografting have begun to examine tolerance associated gene expression in renal and hepatic allograft recipients. Significant challenges in the design and interpretation of these studies potentially limit comparisons. However, a relatively unified model is beginning to emerge, which largely recapitulates previously established mechanisms of immune tolerance. This evidence supports a state of immune quiescence with reduced expression of costimulation and immune response genes, and upregulation of cell cycle control genes. Data indirectly supports the importance of tumor growth factor (TGF)-beta, supports the role of CD4(+)CD25(+) regulatory T cells, and offers new insights into the role of natural killer (NK) cells. Distinct in hepatic allograft tolerance, emerging evidence highlights the importance of gammadeltaT cells, and selection of the Vgammadelta1+ subtype among the gammadeltaT cell population. The deficiencies in the current understanding of transplantation tolerance after HCT, as well as the inadequacies evident in the current empiric approach to immunosuppressive medication (IS) management after HCT make clear the rationale for investigation aimed at elucidating tolerance associated biomarkers after HCT.

Rabbit antithymocyte globulin (thymoglobulin): a review of its use in the prevention and treatment of acute renal allograft rejection

Rabbit antithymocyte globulin (rATG) [Thymoglobulin(R); Thymoglobuline(R)] is a purified, pasteurized preparation of polyclonal gamma immunoglobulin raised in rabbits against human thymocytes that is indicated for the prevention and/or treatment of renal transplant rejection in several countries worldwide. rATG induction in combination with immunosuppressive therapy is more effective in preventing episodes of acute renal graft rejection in adult renal transplant recipients than immunosuppressive therapy without induction. The efficacy of rATG induction is generally better than that of equine antithymocyte globulin (eATG) induction and generally no different from that of basiliximab or low-dose daclizumab induction in this patient population. However, in high-risk patients, rATG induction was more effective than daclizumab or basiliximab induction in preventing acute renal graft rejection. In the treatment of renal graft rejection in adult renal transplant recipients, rATG was more effective than eATG in terms of the successful response rate, although the agents generally did not differ with regard to most other endpoints. Both induction and treatment with rATG are generally well tolerated, although adverse events, such as fever, leukopenia and thrombocytopenia, appear more common with rATG than with other antibody preparations. The overall incidence of infection associated with rATG induction was generally no different from that seen with eATG or basiliximab induction, although was higher with rATG than with basiliximab in high-risk patients. The incidence of cytomegalovirus (CMV) disease generally did not differ between rATG and eATG induction, and there was no significant difference between rATG and daclizumab induction with regard to the incidence of CMV infections or the proportion of patients who received treatment for a CMV episode or infection. Relative to basiliximab, the incidence of CMV infection was generally higher with rATG, except in high-risk patients. In the treatment of acute renal rejection, the nature and incidence of infections were generally similar with rATG and eATG. The incidence of malignancies is generally low with rATG therapy and generally does not differ from that seen with other agents. Further prospective comparative studies would be beneficial in order to definitively position rATG with respect to other antibody preparations. In the meantime, available clinical data suggest that rATG is an effective and generally well tolerated option for the prevention and treatment of acute renal graft rejection in renal transplant recipients.

FOXP3 and its role in the immune system

FOXP3 is a member of the forkhead transcription factor family. Unlike other members, it is mainly expressed in a subset of CD4+ T-cells that play a suppressive role in the immune system. A function of FOXP3 is to suppress the function of NFAT and NFkappaB and this leads to suppression ofexpression of many genes including IL-2 and effector T-cell cytokines. FOXP3 acts also as a transcription activator for many genes induding CD2S, Cytotoxic T-Lymphocyte Antigen 4 (CTLA4), glucocorticoid-induced TNF receptorfamily gene (GITR) andfolate receptor 4. FOXP3+ T-cells are made in the thymus and periphery. The FOXP3+ T-cells made in the thymus migrate to secondary lymphoid tissues and suppress antigen priming of lymphocytes. Antigen priming of naive FOXP3 T-cdlls and naive FOXP3 T-cells leads to generation of memory FOXP3+ T-cells which are efficient in migration to nonlymphoid tissues. Memory FOXP3+ T-cells are, therefore, effective in suppression of effector T-cell function, while naive FOXP3 T-cells are adept at suppressing the early immune responses in lymphoid tissues. Both naive and memory FOXP3 T-cells are required for effective maintenance of tolerance and prevention of autoimmune diseases throughout the body. Many factors such as cytokines and noncytokine factors regulate the generation of FOXP3 T-cells. For example, retinoic acid, produced by the dendritic cells and epithelial cells in the intestine, works together with TGF-beta1 and promotes generation of small intestine-homing FOXP3 T-cells by upregulating the expression ofFOXP3 and gut homing receptors. FOXP3+ T-cells can be produced in vitro from autologous naive T-cells and, therefore, have great therapeutic potentials in treating a number of inflammatory diseases and grafi rejection.

IL-10, regulatory T cells, and Kupffer cells mediate tolerance in concanavalin A-induced liver injury in mice

The liver appears to play an important role in immunological tolerance, for example, during allo-transplantation. We investigated tolerance mechanisms in the model of concanavalin A (ConA)-induced immune-mediated liver injury in mice. We found that a single injection of a sublethal ConA dose to C57BL/6 mice induced tolerance toward ConA-induced liver damage within 8 days. This tolerogenic state was characterized by suppression of the typical Th1 response in this model and increased IL-10 production. Tolerance induction was fully reversible in IL-10 -/- mice and after blockade of IL-10 responses by anti-IL10R antibody. Co-cultures of CD4+CD25+ regulatory T cells (T(reg)s) and CD4+CD25- responder cells revealed T(reg) from ConA-tolerant mice being more effective in suppressing polyclonal T cell responses than T(reg) from control mice. Moreover, T(reg) from tolerant but not from control mice were able to augment in vitro IL-10 expression. Depletion by anti-CD25 monoclonal antibody (MAb) indicated a functional role of T(reg)s in ConA tolerance in vivo. Cell depletion studies revealed T(reg)S and Kupffer cells (KC) to be crucial for IL-10 expression in ConA tolerance. Studies with CD1d -/- mice lacking natural killer T (NKT) cells disclosed these cells as irrelevant for the tolerogenic effect. Finally, cellular immune therapy with CD4+CD25+ cells prevented ConA-induced liver injury, with higher protection by Treg from ConA-tolerized mice.

CONCLUSION

The immunosuppressive cytokine IL-10 is crucial for tolerance induction in ConA hepatitis and is mainly expressed by CD4+CD25+ T(reg) and KC. Moreover, T(reg)s exhibit therapeutic potential against immune-mediated liver injury.

Monitoring tolerance after human liver transplantation

The validation of reliable, non-invasive immunological assays evaluating anti-donor responsiveness in allograft recipients would provide a clinically relevant tool for the early detection of ongoing rejection process as well as for the identification of operational tolerance in the long term. A sequential approach towards immunological monitoring of allografts is proposed in this review: (i) investigations exploring the initial donor-recipient alloresponses, including the analysis of the cytokine network; (ii) investigations regarding graft acceptance and operational tolerance in long-term transplant patients, consisting in the analysis of regulatory T cells and of circulating precursors of dendritic cells, in the measurement of T cell alloreactivity as well as in the study of T cell receptor repertoires. Beside the conventional in vivo and in vitro immunological techniques, the potential applications of molecular imaging in transplantation also deserve further exploration, with particular respect to allograft immune monitoring. Enforced collaboration between transplant clinicians and immunologists will be required to develop the translational research protocols required for the development of immunological monitoring, within an international multicentric network.

Chipping into the human genome: novel insights for transplantation

High throughput, high density platforms for transcriptional, proteomic, and metabonomic analyses are opening new doors for improving our understanding of the complexity and redundancy of the immune system in the interplay of the innate and allo-immune responses in organ transplantation. New insights are being obtained into the possible discrepancies between the gold standard of tissue pathological diagnosis and clinical graft outcomes, as new transcriptional categories of transplant rejection evolve. The bystander effects of chronic immunosuppression underlying the complexities of graft dysfunction are beginning to be understood. Non-invasive mechanisms to monitor transplants, by following 'footprints' of biomarker sets that reflect the disease phenotype, are being pursued for their clinical application for direct patient care. Utilization of these same biomarker sets may also offer a unique means to titrate immunosuppression and predict specific graft dysfunction events prior to clinical decline, thus bringing in the potential to reduce patient morbidity from infection and malignancy, preserve graft integrity, and limit the progression of chronic graft injury. Bioinformatics support is integral to the unraveling of the mysteries of the human genome, proteome, and metabolome in disease and in health.

The immunological monitoring of alloreactive responses in liver transplant recipients: a review

The aim of this work is to review the current knowledge in the field of immunological monitoring of allogenic responsiveness in clinical liver transplantation. When compared to other solid-organ transplants, liver allografts are considered as immunologically privileged, and, accordingly, constitute a favorable setting to develop experimental as well as clinical strategies for minimization of immunosuppression and even induction of operational tolerance. The validation of simple, reliable, noninvasive assays exploring antidonor alloreactivity will constitute a crucial step toward implementing such approaches in the clinic. In contrast to research in rodents claiming the development of donor-specific tolerance in case of graft survivals of over 100 days without immunosuppression, it is impractical to confirm tolerance induction in this way in humans. Promising candidate assays include the detection of post-transplant immune deviation, of circulating precursors of dendritic cells subtypes, and of regulatory T cells. A conceptual framework for the development of tolerance assays in clinical liver transplantation is also proposed.

A comparison of an anti-CD25 immunotoxin, Ontak and anti-CD25 microbeads for their ability to deplete alloreactive T cells in vitro

Ex vivo depletion of alloreactive CD25(+) T cells from a stem cell transplant (SCT) can reduce the incidence of graft-versus-host disease (GVHD) while preserving antimicrobial and perhaps antileukemia activity. However, the most effective methods for allodepleting T cells prior to transplant have not been determined. In this study, we have compared three agents that deplete CD25(+) activated, alloreactive T cells. These included Ontak (Denileukin Diftitox), an IL-2 fusion toxin, anti-CD25 microbeads (MACS), an anti-CD25 immunotoxin (IT) and a combination of the IT and MACS. Peripheral blood mononuclear cells (PBMCs) activated in a primary mixed lymphocyte reaction (MLR) were allodepleted using optimal amounts of each agent, and the cells were then analyzed by flow cytometry. The treated cells were examined both for remaining alloreactivity and for the preservation of third party reactivity by testing them in a secondary MLR. Our data demonstrate that both the anti-CD25 IT and the anti-CD25 MACS were equally effective in depleting CD4(+)CD25(+) cells and in sparing T cells that were reactive with third party cells. The anti-CD25 IT was, however, superior in depleting alloreactive CD8(+)CD25(+) cells. In contrast, Ontak did not eliminate alloreactive cells and the Ontak-treated cells retained significant reactivity against the original stimulator cells.

Elevated T Regulatory Cells in Long-Term Stable Transplant Tolerance in Rhesus Macaques Induced by Anti-CD3 Immunotoxin and Deoxyspergualin

Regulatory T cells (Tregs) are implicated in immune tolerance and are variably dependent on IL-10 for in vivo function. Brief peritransplant treatment of multiple nonhuman primates (NHP) with anti-CD3 immunotoxin and deoxyspergualin has induced stable (5-10 years) rejection-free tolerance to MHC-mismatched allografts, which associated with sustained elevations in serum IL-10. In this study, we demonstrate that resting and activated PBMC from long-term tolerant NHP recipients are biased to secrete high levels of IL-10, compared with normal NHP PBMC. Although IL-10-producing CD4+ Tregs (type 1 regulatory cells (TR1)/IL-10 Tregs) were undetectable (<0.5%) in normal rhesus monkeys, 7.5 +/- 1.7% of circulating CD4+ T cells of tolerant rhesus recipients expressed IL-10. In addition to this >15-fold increase in Tr1/IL-10 Tregs, the tolerant monkeys exhibited a nearly 3-fold increase in CD4+CD25+ Tregs, 8.1 +/- 3.0% of CD4 T cells vs 2.8 +/- 1.4% in normal cohorts (p < 0.02). The frequency of CD4+CD25+IL-10+ cells was elevated 5-fold in tolerant vs normal NHP (1.8 +/- 0.9% vs 0.4 +/- 0.2%). Rhesus CD4+CD25+ Tregs exhibited a memory phenotype, and expressed high levels of Foxp3 and CTLA-4 compared with CD4+CD25- T cells. Also, NHP CD4+CD25+ Tregs proliferated poorly after activation and suppressed proliferation of CD4+CD25- effector T cells, exhibiting regulatory properties similar to rodent and human CD4+CD25+ Tregs. Of note, depletion of CD4+CD25+ Tregs restored indirect pathway antidonor responses in tolerant NHP. Our study demonstrates an expanded presence of Treg populations in tolerant NHP recipients, suggesting that these adaptations may be involved in maintenance of stable tolerance.

Decrease of CD4+CD25+ T Cells in Peripheral Blood After Liver Transplantation: Association With Immunosuppression

CD25 (IL-2 receptor alpha-chain) marks a population of CD4-positive T cells with a suppressor phenotype. These CD4+CD25+ regulatory T cells can suppress both effector T cells and antigen-presenting cells and have been identified as a principle regulator of tolerance in experimental transplantation models. In the setting of human liver transplantation, however, little is known about the dynamics of these cells in relation to rejection, tolerance, and immunosuppression. In the current study we determined CD4+CD25+ T cell in blood of liver transplant recipients using flow cytometry and investigated a possible link with immunosuppressive therapies. Peripheral blood mononuclear cells (PBMC) of 27 liver transplant patients (pretransplantation and 12 months posttransplantation) and 16 healthy controls were included. We found that the percentages of CD25+ cells within the CD4 T-cell population was significantly reduced in more than two-thirds of patients 1 year after transplantation. Also the total percentage of CD4-positive T cells declined significantly within this period, making the absolute reduction of regulatory T cells after transplantation even more profound. Comparing PBMC samples of patients and healthy controls revealed an increased percentage of CD4+ T cells in the patients before transplantation, probably related to the chronic liver illness. The reduction in CD4+CD25+ T cells after transplantation was similar for different immunosuppression regiments. All patients, however, received calcineurin inhibitors, suggesting a possible suppressive effect of this therapy on regulatory T-cell levels in peripheral blood. Currently, assays for regulatory T-cell activity are used to further support this hypothesis.

Transplantation tolerance in pediatric recipients: lessons and challenges

Clinical transplantation tolerance has remained an elusive goal in the 50 yr since it was first described in experimental animals. Greater understanding of the molecular mechanisms responsible for allorecognition have allowed for the development of promising immunosuppressive strategies that may bring us closer to reproducible induction of tolerance; consideration of past successes and failures from both clinical and basic science is required to define future challenges facing this field. This article reviews mechanisms of self and transplantation tolerance, translation of basic science research to clinical protocols in animals and human beings, the changing role of immunosuppression, complications following tolerance induction and controversies surrounding the choice of patients for tolerance trials with a focus on issues relevant to pediatric patients. The role of the Immune Tolerance Network is discussed along with realistic goals for tolerance induction in human beings over the next decade.

Different reconstitution of peripheral blood lymphocytes and dendritic cells in liver and kidney transplant patients

T cells and dendritic cells are responsible for immune alloreactivity or tolerance after transplantation. In this study, we compared the levels of circulating T, B, and NK lymphocytes, as well as monocytes, plasmacytoid dendritic cells, and myeloid dendritic cells, in adult patients undergoing a liver transplant or kidney transplant. Our findings show that candidates for liver transplant had significantly lower levels of circulating T, B, and dendritic cells than candidates for kidney transplant. Nevertheless, liver transplant patients showed a greater T-cell recovery, despite the use of thymoglobulin, as compared with kidney transplant patients who were induced with Daclizumab. In four kidney transplant patients with allograft rejection we observed a dramatic drop of circulating T and dendritic cells at the time of rejection, and while myeloid dendritic cells and CD4(+) and CD8(+) cells rapidly recovered after 1 month, plasmacytoid dendritic cells and CD4(+)CD25(+) T-cell numbers remained significantly lower than in patients without rejection. Future studies will evaluate the monitoring of circulating CD4(+)CD25(+) T cells and myeloid dendritic cell:plasmacytoid dendritic cell ratio as potential biomarkers for rejection or, alternatively, for withdrawal of immune suppression.

Heterozygosity inCTLA-4 gene and severe preeclampsia

OBJECTIVE

One of the major complications of pregnancy, preeclampsia makes pregnancy termination inevitable in most cases. Similarities exist between the mechanisms that maintain normal pregnancy, allograft transplants, and, it is postulated, peripheral self-tolerance. In addition, the critical role of the cytotoxic T-lymphocyte antigen-4 (CTLA-4) molecule in maintaining self-tolerance has been established. Therefore, the frequency of CTLA-4 A49G polymorphism was investigated in severe preeclampsia.

PATIENTS AND METHODS

Genomic DNA extracted from mononuclear cells of the peripheral blood of 36 pregnant women with severe preeclampsia and 151 healthy women was analyzed. A49G polymorphism in position 49 of exon-1 of the CTLA-4 gene was studied by the polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) method.

RESULTS

The frequency of the GG genotype was 2 (5.6%) in patients and 19 (12.6%) in controls, while the frequency of the AA genotype was 4 (11.1%) and 60 (39.7%). Interestingly, the frequency of the AG genotype was significantly higher in preeclamptic than in healthy women from the general population (83.3% vs. 47.7%; P=0.0005).

CONCLUSION

These data suggest that heterozygosity in the CTLA-4 A49G allele might be a predisposing factor for severe preeclampsia. Whether the observed association results from linkage imbalance with other loci on chromosome 2 or other polymorphisms of the CTLA-4 gene or even from a preferential transfer and/or expression of one allele from a heterozygous mother to the fetus will be the subject of future investigations.