Role for thymic and splenic regulatory CD4+ T cells induced by donor dendritic cells in allograft tolerance by LF15-0195 treatment

Effects of Tissue-engineered Bone by Coculture of Adipose-derived Stem Cells and Vascular Endothelial Cells on Host Immune Status

AIM

The study aimed to explore the effects of tissue-engineered bone constructed with partially deproteinized biologic bone (PDPBB) and coculture of adipose-derived stem cells (ADSCs) and vascular endothelial cells (VECs) on host immune status, providing a very useful clue for the future development of bone engineering.

METHODS

Tissue-engineered bones constructed by PDPBB and ADSCs, VECs or coculture of them were implanted into the muscle bag of bilateral femurs of Sprague-Dawley rats. Partially deproteinized biologic bone alone and blank control were also implanted. After transplantation, the proliferation of implanted seed cells in tissue-engineered bones was labeled by bromodeoxyuridine staining. Moreover, the changes of T-lymphocyte subpopulations, including CD3 + CD4+ and CD3 + CD8+ in peripheral blood were then detected using flow cytometry to analyze the immune rejection of tissue-engineered bone implantation based on peripheral blood CD4/CD8 ratios.

RESULTS

After transplantation, the proliferation of implanted seed cells was observed in tissue-engineered bones of different groups. At different time points after transplantation, the CD4+/CD8+ ratio in peripheral blood of PDPBB + ADSCs, PDPBB + coculture, and blank control groups did not exhibit significant change. Although the CD4+/CD8+ ratio in peripheral blood of PDPBB + VECs group was significantly higher than other group at 1 week after transplantation, that of PDPBB + VECs and PDPBB + coculture group was significantly decreased at 8 week after transplantation compared with that of blank control group.

CONCLUSIONS

Our results indicated that there was no significant immune rejection after transplantation of tissue-engineered bone constructed with PDPBB and coculture of ADSCs and VECs as seed cells.

Myeloid dendritic cells exhibit defects in activation and function in patients with multiple sclerosis

BACKGROUND

Regulatory T cells (Tregs) are functionally defective in patients with multiple sclerosis (MS) and this dysfunction is related to an imbalanced composition of naïve and memory Treg subtypes. Several lines of evidence indicate that these abnormalities might result from a premature decline in thymic-dependent Treg neogenesis. Myeloid dendritic cells (mDCs) critically determine Treg differentiation in the thymus, and thymic stromal lymphopoietin receptor (TSLPR) expressed on mDCs is a key component of the signaling pathways involved in this process. TSLPR-expression on mDCs was previously shown to be decreased in MS. We hypothesized that functional alterations in mDCs contribute to aberrant Treg neogenesis and, in turn, to altered Treg homeostasis and function in MS.

METHODS

We recruited blood samples from 20 MS patients and 20 healthy controls to assess TSLPR expression on mDCs ex vivo by flow cytometry and by activating mDCs induced by recombinant TSLP (rhTSLP) in vitro. As previous studies documented normalization of both function and homeostasis of Tregs under immunomodulatory (IM) therapy with interferon-beta (IFN-beta) and glatiramer acetate (GA), we also tested phenotypes and function of mDCs obtained from IM-treated patients (IFN-beta: n=20, GA: n=20).

RESULTS

We found that TSLP-induced mDC activation and effector function in vitro was reduced in MS and correlated with TSLPR-expression levels on mDCs. IM treatment prompted upregulation of TSLPR on mDCs and an increase in TSLP-induced activation of mDCs together with a normalization of Treg homeostasis.

CONCLUSION

The decreased TSLP-induced activation of MS-derived mDCs in vitro, together with the reduced density of TSLPR on the cell surface of mDCs corroborates the hypothesis of mDCs being critically involved in impairing Treg development in MS.

B cells with regulatory properties in transplantation tolerance

Induction of tolerance remains a major goal in transplantation. Indeed, despite potent immunosuppression, chronic rejection is still a real problem in transplantation. The humoral response is an important mediator of chronic rejection, and numerous strategies have been developed to target either B cells or plasma cells. However, the use of anti-CD20 therapy has highlighted the beneficial role of subpopulation of B cells, termed regulatory B cells. These cells have been characterized mainly in mice models of auto-immune diseases but emerging literature suggests their role in graft tolerance in transplantation. Regulatory B cells seem to be induced following inflammation to restrain excessive response. Different phenotypes of regulatory B cells have been described and are functional at various differentiation steps from immature to plasma cells. These cells act by multiple mechanisms such as secretion of immuno-suppressive cytokines interleukin-10 (IL-10) or IL-35, cytotoxicity, expression of inhibitory receptors or by secretion of non-inflammatory antibodies. Better characterization of the development, phenotype and mode of action of these cells seems urgent to develop novel approaches to manipulate the different B cell subsets and the response to the graft in a clinical setting.

Regulatory B Cells with a Partial Defect in CD40 Signaling and Overexpressing Granzyme B Transfer Allograft Tolerance in Rodents

Emerging knowledge regarding B cells in organ transplantation has demonstrated that these cells can no longer be taken as mere generators of deleterious Abs but can also act as beneficial players. We previously demonstrated in a rat model of cardiac allograft tolerance induced by short-term immunosuppression an accumulation in the blood of B cells overexpressing inhibitory molecules, a phenotype also observed in the blood of patients that spontaneously develop graft tolerance. In this study, we demonstrated the presence in the spleen of regulatory B cells enriched in the CD24(int)CD38(+)CD27(+)IgD(-)IgM(+/low) subpopulation, which are able to transfer donor-specific tolerance via IL-10 and TGF-β1-dependent mechanisms and to suppress in vitro TNF-α secretion. Following anti-CD40 stimulation, IgD(-)IgM(+/low) B cells were blocked in their plasma cell differentiation pathway, maintained high expression of the inhibitory molecules CD23 and Bank1, and upregulated Granzyme B and Irf4, two molecules described as highly expressed by regulatory B cells. Interestingly, these B cells recognized specifically a dominant donor Ag, suggesting restricted specificity that could lead to a particular B cell response. Regulatory B cells were not required for induction of tolerance and appeared following Foxp3(+)CD4(+)CD25(+) regulatory T cells, suggesting cooperation with regulatory T cells for their expansion. Nevertheless, following transfer to new recipients, these B cells migrated to the allograft, kept their regulatory profile, and promoted local accumulation of Foxp3(+)CD4(+)CD25(+) regulatory T cells. Mechanisms of regulatory B cells and their cell therapy potential are important to decipher in experimental models to pave the way for future developments in the clinic.

Three distinct subsets of thymic epithelial cells in rats and mice defined by novel antibodies

AIM

Thymic epithelial cells (TECs) are thought to play an essential role in T cell development and have been detected mainly in mice using lectin binding and antibodies to keratins. Our aim in the present study was to create a precise map of rat TECs using antibodies to putative markers and novel monoclonal antibodies (i.e., ED 18/19/21 and anti-CD205 antibodies) and compare it with a map from mouse counterparts and that of rat thymic dendritic cells.

RESULTS

Rat TECs were subdivided on the basis of phenotype into three subsets; ED18+ED19+/-keratin 5 (K5)+K8+CD205+ class II MHC (MHCII)+ cortical TECs (cTECs), ED18+ED21-K5-K8+Ulex europaeus lectin 1 (UEA-1)+CD205- medullary TECs (mTEC1s), and ED18+ED21+K5+K8dullUEA-1-CD205- medullary TECs (mTEC2s). Thymic nurse cells were defined in cytosmears as an ED18+ED19+/-K5+K8+ subset of cTECs. mTEC1s preferentially expressed MHCII, claudin-3, claudin-4, and autoimmune regulator (AIRE). Use of ED18 and ED21 antibodies revealed three subsets of TECs in mice as well. We also detected two distinct TEC-free areas in the subcapsular cortex and in the medulla. Rat dendritic cells in the cortex were MHCII+CD103+ but negative for TEC markers, including CD205. Those in the medulla were MHCII+CD103+ and CD205+ cells were found only in the TEC-free area.

CONCLUSION

Both rats and mice have three TEC subsets with similar phenotypes that can be identified using known markers and new monoclonal antibodies. These findings will facilitate further analysis of TEC subsets and DCs and help to define their roles in thymic selection and in pathological states such as autoimmune disorders.

Report of the joint ESOT and TTS basic science meeting 2013: current concepts and discoveries in translational transplantation

A joint meeting organized by the European (ESOT) and The Transplantation (TTS) Societies for basic science research was organized in Paris, France, on November 7-9, 2013. Focused on new ideas and concepts in translational transplantation, the meeting served as a venue for state-of-the-art developments in basic transplantation immunology, such as the potential for tolerance induction through regulation of T-cell signaling. This meeting report summarizes important insights which were presented in Paris. It not only offers an overview of established aspects, such as the role of Tregs in transplantation, presented by Nobel laureate Rolf Zinkernagel, but also highlights novel facets in the field of transplantation, that is cell-therapy-based immunosuppression or composite tissue transplantation as presented by the emotional story given by Vasyly Rohovyy, who received two hand transplants. The ESOT/TTS joint meeting was an overall productive and enjoyable platform for basic science research in translational transplantation and fulfilled all expectations by giving a promising outlook for the future of research in the field of immunological transplantation research.

Memory T Cells in Transplantation

Following infections and environmental exposures, memory T cells are generated that provide long-term protective immunity. Compared to their naïve T cell counterparts, memory T cells possess unique characteristics that endow them with the ability to quickly and robustly respond to foreign antigens. While such memory T cells are beneficial in protecting their hosts from recurrent infection, memory cells reactive to donor antigens pose a major barrier to successful transplantation and tolerance induction. Significant progress has been made over the past several decades contributing to our understanding of memory T cell generation, their distinct biology, and their detrimental impact in clinical and animal models of transplantation. This review focuses on the unique features which make memory T cells relevant to the transplant community and discusses potential therapies targeting memory T cells which may ameliorate allograft rejection.

LIMLE, a new molecule over-expressed following activation, is involved in the stimulatory properties of dendritic cells

Dendritic cells are sentinels of the immune system distributed throughout the body, that following danger signals will migrate to secondary lymphoid organs to induce effector T cell responses. We have identified, in a rodent model of graft rejection, a new molecule expressed by dendritic cells that we have named LIMLE (RGD1310371). To characterize this new molecule, we analyzed its regulation of expression and its function. We observed that LIMLE mRNAs were rapidly and strongly up regulated in dendritic cells following inflammatory stimulation. We demonstrated that LIMLE inhibition does not alter dendritic cell maturation or cytokine production following Toll-like-receptor stimulation. However, it reduces their ability to stimulate effector T cells in a mixed leukocyte reaction or T cell receptor transgenic system. Interestingly, we observed that LIMLE protein localized with actin at some areas under the plasma membrane. Moreover, LIMLE is highly expressed in testis, trachea, lung and ciliated cells and it has been shown that cilia formation bears similarities to formation of the immunological synapse which is required for the T cell activation by dendritic cells. Taken together, these data suggest a role for LIMLE in specialized structures of the cytoskeleton that are important for dynamic cellular events such as immune synapse formation. In the future, LIMLE may represent a new target to reduce the capacity of dendritic cells to stimulate T cells and to regulate an immune response.

Is there B cell involvement in a rat model of spontaneous idiopathic nephrotic syndrome treated with LF15-0195?

BACKGROUND

The Buffalo/Mna (Buff/Mna) rat spontaneously develops idiopathic nephrotic syndrome (INS), and its nephropathy recurs after the renal transplantation of a healthy graft. Only LF15-0195 is able to cause regression of the Buff/Mna nephropathy and to induce regulatory T cells, which decrease proteinuria when transferred into proteinuric Buff/Mna rats. Based on previous research on B cells in human INS, we evaluated the involvement of B cells in our model and the impact of LF15-0195.

METHODS

We studied the effect of LF15-0195 on peripheral B cells by flow cytometry and quantitative reverse transcription-polymerase chain reaction. B cells were purified from LF15-0195-treated Buff/Mna rats in remission, and transferred into proteinuric Buff/Mna rats. We treated the Buff/Mna rats with mitoxantrone and measured the depletion of B/T cells in parallel with proteinuria.

RESULTS

LF15-0195 changed the phenotype of B cells: the number of naïve mature B cells increased significantly, while the number of switched, transitional 1, and transitional 2 B cells decreased. There were no changes in the amount of memory, activated or regulatory B cells. We observed a significant increase of immunoglobulin (Ig)M mRNA transcripts in the LF15-0195-treated Buff/Mna B cells compared to controls, but no difference in the level of IgG. This profile is consistent with a block in B cell maturation at the IgM to IgG switch. The transfer of B cells from LF15-0195-treated rats into proteinuric Buff/Mna rats did not have an effect on proteinuria. Mitoxantrone, despite causing a significant depletion of B cells, did not reduce proteinuria.

CONCLUSION

Despite LF15-0195 acting on B cells, the beneficial effects of this drug on nephrotic syndrome did not involve the induction of regulatory B cells. Moreover, the B cell depletion was not effective in reducing proteinuria, indicating that B cells are not a therapeutic target.

Memory T cells in transplantation - progress and challenges

PURPOSE OF REVIEW

Memory T cells present a different set of challenges to transplant patients; they are needed for protection against invading pathogens, especially under conditions of immunosuppression. But their presence also threatens transplant survival, as some of them are alloreactive. Efforts to resolve this paradox will be critical in the induction of transplant tolerance.

RECENT FINDINGS

There has been significant progress made in the past few years in the areas of population diversity of memory T cells, metabolic control of their induction, and mechanisms and pathways involved in memory cell exhaustion. Multiple targets on memory T cells have been identified, some of which are under vigorous testing in various transplant models.

SUMMARY

Memory T cells are both friends and foes to transplant patients, and tolerance strategies should selectively target alloreactive memory T cells and leave other memory cells unaltered. This situation remains a major challenge in the clinic.

Allogeneic Adipose-Derived Stem Cells with Low Immunogenicity Constructing Tissue-Engineered Bone for Repairing Bone Defects in Pigs

The ideal cells for tissue engineering should have the following characteristics: easy obtainment, safety, immune privilege, the capability of self-renewal, and multipotency. Adipose-derived stem cells (ADSCs) are a promising candidate. However, the immunogenicity of allogeneic mesenchymal stem cells limits their long-term benefits. In this study, we introduced human cytomegalovirus US2/US3 gene into the ADSCs to decrease the expression of MHC I protein of ADSCs and reduce the activation of T-cells of the recipient animals. Moreover, the biosafety and biological characteristics of ADSCs transfected with the US2/US3 genes (ADSCs-US2/US3) were similar to normal ADSCs. Then we took ADSCs-US2/US3 to construct a tissue-engineered bone for repairing bone defects in pigs and found that there were no great differences in repair effects or healing time between the allogeneic ADSCs-US2/US3 group and the autologous ADSC group. These results suggest that allogeneic ADSCs-US2/US3 have the advantages of biological safety, low immunogenicity, and effective osteogenesis. Such barely immunogenic ADSCs will be crucial for the success of future tissue-regenerative approaches.

Immunoregulatory function of IL-27 and TGF-β1 in cardiac allograft transplantation

BACKGROUND

Deciphering the mechanisms of tolerance represents a crucial aim of research in transplantation. We previously identified by DNA chip interleukin (IL)-27 p28 and transforming growth factor (TGF)-β1 as overexpressed in a model of rat cardiac allograft tolerance mediated by regulatory CD4CD25 T cells. The role of these two molecules on the control of the inflammatory response remains controversial. However, both are involved in the regulation of the T helper 17/Treg axis, suggesting their involvement in tolerance.

METHODS

We analyzed regulation of IL-27 and TGF-β1 expression in allograft response and their role in tolerance by using blocking anti-TGF-β antibody and by generating an adeno-associated virus encoding IL-27.

RESULTS

Here, we confirmed the overexpression of IL-27 and TGF-β1 in tolerated cardiac allografts in two different rodent models. We observed that their expression correlates with inhibition of T helper 17 differentiation and with expansion of regulatory CD4CD25 T cells. We showed in a rat model that anti-TGF-β treatment abrogates infectious tolerance mediated by the transfer of regulatory CD4CD25 T cells. Moreover, overexpression of IL-27 by adeno-associated virus administration in combination with a short-term immunosuppression allows prolongation of cardiac allograft survival and one tolerant recipient. We found that IL-27 overexpression did not induce Foxp3CD4CD25 T-cell expansion but rather IL-10-expressing CD4 T cells in the tolerant recipient.

CONCLUSIONS

Taken together, these data suggest that both TGF-β1 and IL-27 play a role in the mechanisms of tolerance. However, in contrast to TGF-β1, IL-27 seems not to be involved in regulatory CD4CD25 T-cell expansion but rather in their mode of action.

Relevance of regulatory T cell promotion of donor-specific tolerance in solid organ transplantation

Current clinical strategies to control the alloimmune response after transplantation do not fully prevent induction of the immunological processes which lead to acute and chronic immune-mediated graft rejection, and as such the survival of a solid organ allograft is limited. Experimental research on naturally occurring CD4⁺CD25^highFoxP3⁺ Regulatory T cells (Tregs) has indicated their potential to establish stable long-term graft acceptance, with the promise of providing a more effective therapy for transplant recipients. Current approaches for clinical use are based on the infusion of freshly isolated or ex vivo polyclonally expanded Tregs into graft recipients with an aim to redress the in vivo balance of T effector cells to Tregs. However mounting evidence suggests that regulation of donor-specific immunity may be central to achieving immunological tolerance. Therefore, the next stages in optimizing translation of Tregs to organ transplantation will be through the refinement and development of donor alloantigen-specific Treg therapy. The altering kinetics and intensity of alloantigen presentation pathways and alloimmune priming following transplantation may indeed influence the specificity of the Treg required and the timing or frequency at which it needs to be administered. Here we review and discuss the relevance of antigen-specific regulation of alloreactivity by Tregs in experimental and clinical studies of tolerance and explore the concept of delivering an optimal Treg for the induction and maintenance phases of achieving transplantation tolerance.

Transplant tolerance is associated with reduced expression of cystathionine-γ-lyase that controls IL-12 production by dendritic cells and TH-1 immune responses

Antigen-activated T lymphocytes undergo an immune or tolerogeneic response in part according to the activation status of their antigen-presenting cells. However, factors controlling the activation of antigen-presenting cells are not fully understood. In this study, we demonstrate that immune tolerance after organ allotransplantation in the rat is associated with a repressed intragraft expression of several enzymes of the trans-sulfuration pathway, including cystathionine γ-lyase (CSE). The pharmacologic blockade of CSE with propargylglycine delayed heart allograft rejection and abrogated type IV hypersensitivity but did not modify antibody responses, and was associated with a selective inhibition of the TH-1 type factors T-bet, IL-12, and IFN-γ. IL-12 repression could also be induced by propargylglycine in vitro in monocytes and dendritic cells (DCs), a phenomenon not mediated by changes to nuclear factor-κ B or hydrogen sulfide but that occurred together with a modulation of intracellular cysteine content. Intracellular cysteine levels were predominantly controlled in DCs by CSE activity, together with extracellular import via the Xc− transporter. Our results indicate that CSE plays a critical role in regulating IL-12 in monocytes and DCs and is down-modulated in transplant tolerance, presumably participating in the maintenance of the tolerant state.

Upregulation of miR-142-3p in peripheral blood mononuclear cells of operationally tolerant patients with a renal transplant

Achieving drug-free tolerance or successfully using only small doses of immunosuppression is a major goal in organ transplantation. To investigate the potential mechanisms by which some kidney transplant recipients can achieve operational tolerance, we compared the expression profiles of microRNA in peripheral blood mononuclear cells of operationally tolerant patients with those of stable patients treated with conventional immunosuppression. B cells from operationally tolerant patients overexpressed miR-142-3p. The expression of miR-142-3p was stable over time and was not modulated by immunosuppression. In Raji B cells, overexpression of miR-142-3p modulated nearly 1000 genes related to the immune response of B cells, including potential miR-142-3p targets and molecules previously identified in the blood of operationally tolerant patients. Furthermore, our results suggested that a negative feedback loop involving TGF-β signaling and miR-142-3p expression in B cells may contribute to the maintenance of tolerance. In summary, miR-142-3p expression in peripheral blood mononuclear cells correlates with operational tolerance. Whether upregulation of miR-142-3p modulates inflammatory responses to promote tolerance or is a result of this tolerance state requires further study.

Long-term allograft tolerance is characterized by the accumulation of B cells exhibiting an inhibited profile

Numerous reports have highlighted the central role of regulatory T cells in long-term allograft tolerance, but few studies have investigated the B-cell aspect. We analyzed the B-cell response in a rat model of long-term cardiac allograft tolerance induced by a short-term immunosuppression. We observed that tolerated allografts are infiltrated by numerous B cells organized in germinal centers that are strongly regulated in their IgG alloantibody response. Moreover, alloantibodies from tolerant recipients exhibit a deviation toward a Th2 isotype and do not activate in vitro donor-type endothelial cells in a pro-inflammatory way but maintained expression of cytoprotective molecules. Interestingly, this inhibition of the B-cell response is characterized by the progressive accumulation in the graft and in the blood of B cells blocked at the IgM to IgG switch recombination process and overexpressing BANK-1 and the inhibitory receptor Fcgr2b. Importantly, B cells from tolerant recipients are able to transfer allograft tolerance. Taken together, these results demonstrate a strong regulation of the alloantibody response in tolerant recipients and the accumulation of B cells exhibiting an inhibited and regulatory profile. These mechanisms of regulation of the B-cell response could be instrumental to develop new strategies to promote tolerance.

The effects of immunosuppressive drugs on CD4(+) CD25(+) regulatory T cells: a systematic review of clinical and basic research

OBJECTIVE

To review the effects of different immunosuppressive drugs on proliferation and function of regulatory T cells (Tregs).

METHODS

We searched MEDLINE, Embase (from inception to September 2009), and the Cochrane Library (Issue 4, 2009) for clinical and basic research about the effects of various immunosuppressive drugs on Tregs. Data were extracted and methodological quality was assessed by two independent reviewers. Outcome measures for clinical research included blood Tregs levels, acute rejection episodes, and graft function. Outcomes for basic research included percentage of Tregs proliferation, function, Tregs phenotype, and evidence for possible mechanisms. We analyzed data qualitatively.

RESULTS

Forty-two studies, including 19 clinical trials and 23 basic studies, were included. The immunosuppressive drugs studied were calcineurin inhibitors (CNIs), Rapa, anti-metabolism drugs, IL-2 receptor-blocking antibodies, T-cell depleting antibodies, and co-stimulation blockade antibodies. Most of the studies were on Rapa and CNIs. Eight basic studies on Rapa and CNIs showed that Rapa could promote the proliferation and function of Tregs, while CNIs could not. Five clinical trials involving a total of 158 patients showed that patients taking Rapa had higher blood concentration of Tregs than patients taking CNIs, but no difference was found in graft function (6-42 months follow-up).

CONCLUSION

There is substantial evidence that Rapa favors Tregs survival and function. However, the higher numbers of blood Tregs in patients treated with Rapa do not show any association with better graft function. Larger clinical studies with longer follow-up are needed to more thoroughly assess the efficacy of immunosuppressive drugs on Tregs, and reveal whether a relationship exists between Tregs and graft function.

The C-type lectin-like receptor CLEC-1, expressed by myeloid cells and endothelial cells, is up-regulated by immunoregulatory mediators and moderates T cell activation

C-type lectin receptors have recently been described as playing crucial roles in immunity and homeostasis since these proteins are able to recognize pathogens as well as self-Ags. We identified the C-type lectin-like receptor-1, CLEC-1, as being overexpressed in a model of rat allograft tolerance. We previously described in this model the expression of numerous cytoprotective molecules by graft endothelial cells and their interplay with regulatory CD4(+)CD25(+) T cells. In this study, we demonstrate that CLEC-1 is expressed by myeloid cells and specifically by endothelial cells in tolerated allografts and that CLEC-1 expression can be induced in endothelial cells by alloantigen-specific regulatory CD4(+)CD25(+) T cells. Analysis of CLEC-1 expression in naive rats demonstrates that CLEC-1 is highly expressed by myeloid cells and at a lower level by endothelial cells, and that its expression is down-regulated by inflammatory stimuli but increased by the immunoregulators IL-10 or TGFbeta. Interestingly, we demonstrate in vitro that inhibition of CLEC-1 expression in rat dendritic cells increases the subsequent differentiation of allogeneic Th17 T cells and decreases the regulatory Foxp3(+) T cell pool. Additionally, in chronically rejected allograft, the decreased expression of CLEC-1 is associated with a higher production of IL-17. Taken together, our data suggest that CLEC-1, expressed by myeloid cells and endothelial cells, is enhanced by regulatory mediators and moderates Th17 differentiation. Therefore, CLEC-1 may represent a new therapeutic agent to modulate the immune response in transplantation, autoimmunity, or cancer settings.

Superagonist CD28 Antibody Preferentially Expanded Foxp3-Expressing nTreg Cells and Prevented Graft-Versus-Host Diseases

Regulatory lymphocytes play a pivotal role in preventing organ-specific autoimmune disease and in induction and maintenance of tolerance in various experimental transplantation models. The enhancement of the number and activity of peripheral CD4+CD25+ Treg cells is an obvious goal for the treatment of autoimmunity and for the suppression of alloreactions. The present study demonstrates that naturally occurring CD4+CD25+ Treg (nTreg) cells preferentially proliferate to a fourfold increase within 3 days in response to the administration of a single superagonistic CD28-specific monoclonal antibody (supCD28 mAb). The appearance of increased Foxp3 molecules was accompanied with polarization toward a Th2 cytokine profile with decreased production of IFN-γ and increased production of IL-4 and IL-10 in the expanded Treg subset. Adoptive transfer of supCD28 mAb-expanded cells in a graft-versus-host disease (GvHD) model induced a potent inhibition of lethality. These results suggest that this therapeutic effect is mediated by the in vivo expansion of nTreg cells. Taken together, these data demonstrate that supCD28-mAb may target nTreg cells in vivo and maintain and enhance their potent regulatory functions for the treatment GvHD.

Thymus-homing peripheral dendritic cells constitute two of the three major subsets of dendritic cells in the steady-state thymus

Many dendritic cells (DCs) in the normal mouse thymus are generated intrathymically from common T cell/DC progenitors. However, our previous work suggested that at least 50% of thymic DCs originate independently of these progenitors. We now formally demonstrate by parabiotic, adoptive transfer, and developmental studies that two of the three major subsets of thymic DCs originate extrathymically and continually migrate to the thymus, where they occupy a finite number of microenvironmental niches. The thymus-homing DCs consisted of immature plasmacytoid DCs (pDCs) and the signal regulatory protein α–positive (Sirpα⁺) CD11b⁺ CD8α⁻ subset of conventional DCs (cDCs), both of which could take up and transport circulating antigen to the thymus. The cDCs of intrathymic origin were mostly Sirpα⁻ CD11b⁻ CD8α^hi cells. Upon arrival in the thymus, the migrant pDCs enlarged and up-regulated CD11c, major histocompatibility complex II (MHC II), and CD8α, but maintained their plasmacytoid morphology. In contrast, the migrant cDCs proliferated extensively, up-regulated CD11c, MHC II, and CD86, and expressed dendritic processes. The possible functional implications of these findings are discussed.

TSLP in epithelial cell and dendritic cell cross talk

Dendritic cells (DCs) are professional antigen-presenting cells that have the ability to sense infection and tissue stress, sample and present antigen to T lymphocytes, and instruct the initiation of different forms of immunity and tolerance. The functional versatility of DCs depends on their remarkable ability to translate collectively the information from the invading microbes, as well as their resident tissue microenvironments. Recent progress in understanding Toll-like receptor (TLR) biology has illuminated the mechanisms by which DCs link innate and adaptive antimicrobial immune responses. However, how tissue microenvironments shape the function of DCs has remained elusive. Recent studies of TSLP (thymic stromal lymphopoietin), an epithelial cell-derived cytokine that strongly activates DCs, provide strong evidence at a molecular level that epithelial cells/tissue microenvironments directly communicate with DCs, the professional antigen-presenting cells of the immune system. We review recent progress on how TSLP expressed within thymus and peripheral lymphoid and nonlymphoid tissues regulates DC-mediated central tolerance, peripheral T cell homeostasis, and inflammatory Th2 responses.

Induction of T regulatory cells attenuates idiopathic nephrotic syndrome

Buffalo/Mna rats spontaneously develop FSGS and nephrotic syndrome as a result of an immune disorder. Similar to some humans with FSGS, the disease recurs after renal transplantation, suggesting the involvement of a circulating factor. Here, we tested the effect of several immunosuppressive treatments on these rats. Although corticosteroids, cyclosporin A, and anti-T cell receptor treatment reduced proteinuria, only the deoxyspergualin derivative LF15-0195 led to a rapid and complete normalization of proteinuria. Furthermore, this compound led to the regression of renal lesions during both the initial disease and posttransplantation recurrence. The frequency of splenic and peripheral CD4+CD25+FoxP3+ T lymphocytes significantly increased with remission. Moreover, the transfer of purified LF15-0195-induced CD4+CD25+ T cells to irradiated Buff/Mna rats significantly reduced their proteinuria compared with the transfer of untreated control cells, suggesting that LF15-0195 induces regulatory T cells that are able to induce regression of rat nephropathy. These data suggest that idiopathic nephrotic syndrome/FSGS disease can be regulated by cellular transfer, but how this regulation leads to the reorganization of the podocyte cytoskeleton remains to be determined.

Differential promotion of hematopoietic chimerism and inhibition of alloreactive T cell proliferation by combinations of anti-CD40Ligand, anti-LFA-1, everolimus, and deoxyspergualin

Allogeneic bone marrow (BM) engraftment for chimerism and transplantation tolerance may be promoted by combinations of costimulation blocking biologics and small molecular weight inhibitors. We showed previously in a mouse model that anti-CD40Ligand (anti-CD40L, CD154) combined with anti-LFA-1 or everolimus (40-O-(2-hydroxyethyl)-rapamycin) resulted in stable chimerism in almost all BM recipients, whereas anti-LFA-1 plus everolimus conferred approximately 50% chimerism stability. Here, we investigated whether this lower incidence could be increased with deoxyspergualin (DSG) in place of or in addition to everolimus. However, DSG and everolimus were similarly synergistic with costimulation blockade for stable hematopoietic chimerism. This correlated with allospecific T cell depletion and inhibition of acute but not chronic skin allograft rejection. Different treatments were also compared for their inhibition of alloreactive T cell proliferation in vivo. While anti-CD40L did not impair T cell proliferation, anti-LFA-1 reduced both CD4 and CD8 T cell proliferation, and combining anti-LFA-1 with everolimus or DSG had an additive inhibitory effect on CD4 T cell proliferation. Thus, despite their strong inhibition of alloreactive T cell proliferation, combinations of anti-LFA-1 with everolimus or DSG did not reach the unique potency of anti-CD40L-based combinations to support stable hematopoietic chimerism in this system.

C5 blockade with conventional immunosuppression induces long-term graft survival in presensitized recipients

We explored whether a functionally blocking anti-C5 monoclonal antibody (mAb) combined with T- and B-cell immunosuppression can successfully prevent antibody-mediated (AMR) and cell-mediated rejection (CMR) in presensitized murine recipients of life-supporting kidney allografts. To mimic the urgent clinical features of AMR experienced by presensitized patients, we designed a murine model in which BALB/c recipients were presensitized with fully MHC-mismatched C3H donor skin grafts one week prior to C3H kidney transplantation. Presensitized recipients demonstrated high levels of circulating and intragraft antidonor antibodies and terminal complement activity, rejecting grafts within 8.5 +/- 1.3 days. Graft rejection was predominantly by AMR, characterized by interstitial hemorrhage, edema and glomerular/tubular necrosis, but also demonstrated moderate cellular infiltration, suggesting CMR involvement. Subtherapeutic treatment with cyclosporine (CsA) and LF15-0195 (LF) did not significantly delay rejection. Significantly, however, the addition of anti-C5 mAb to this CsA/LF regimen prevented terminal complement activity and inhibited both AMR and CMR, enabling indefinite (>100 days) kidney graft survival despite the persistence of antidonor antibodies. Long-term surviving kidney grafts expressed the protective proteins Bcl-x(S/L) and A-20 and demonstrated normal histology, suggestive of graft accommodation or tolerance. Thus, C5 blockade combined with routine immunosuppression offers a promising approach to prevent graft loss in presensitized patients.

Role of persistence of antigen and indirect recognition in the maintenance of tolerance to renal allografts

BACKGROUND

We have previously shown that a 12-day treatment with cyclosporine A (CyA) facilitates induction of tolerance to class-I disparate kidneys, as demonstrated by acceptance of second, donor-matched kidneys without immunosuppression. In the present study, we have examined 1) the duration of tolerance in the absence of donor antigen and 2) the pathway of antigen recognition determining maintenance or loss of tolerance.

METHODS

Seventeen miniature swine received class-I mismatched kidneys with 12 days of CyA, and received second donor-matched kidneys without immunosuppression at 0, 1, 3, or 4 months after nephrectomy of the primary graft. Five were sensitized 6 weeks after nephrectomy of the primary graft, three with donor-matched skin grafts, and two with donor class-I peptides to eliminate direct pathway involvement. In addition, two long-term tolerant animals received class-I peptides.

RESULTS

Rejection of second grafts required at least a 3 month absence of donor antigen. Although donor-matched skin grafts in animals tolerant to kidneys induced antidonor cytotoxic T lymphocyte responses, second renal transplants revealed no evidence of sensitization. In contrast, immunization of recipients with donor class-I peptides after nephrectomy of the primary graft led to loss of tolerance at both T-cell and B-cell levels, as evidenced by rejection of the second graft in 5 days and development of antidonor immunoglobulin G. Peptide immunization of long-term tolerant in recipients bearing long-term renal grafts did not break tolerance.

CONCLUSIONS

These data indicate that the renal allograft is required for the indefinite maintenance of tolerance, that indirect antigen presentation is capable of breaking tolerance, and that in tolerant animals, direct antigen presentation may suppress rejection, allowing tolerance to persist.

Implication of matrix metalloproteinase 7 and the noncanonical wingless-type signaling pathway in a model of kidney allograft tolerance induced by the administration of anti-donor class II antibodies

In rats, tolerance to MHC-incompatible renal allografts can be induced by the administration of anti-donor class II Abs on the day of transplantation. In this study we explored the mechanisms involved in the maintenance phase of this tolerance by analyzing intragraft gene expression profiles by microarray in long-term accepted kidneys. Comparison of the gene expression patterns of tolerated to syngeneic kidneys revealed 5,954 differentially expressed genes (p < 0.05). Further analysis of this gene set revealed a key role for the wingless-type (WNT) signaling pathway, one of the pivotal pathways involved in cell regulation that has not yet been implicated in transplantation. Several genes within this pathway were significantly up-regulated in the tolerated grafts, particularly matrix metalloproteinase 7 (MMP7; fold change > 40). Analysis of several other pathway-related molecules indicated that MMP7 overexpression was the result of the noncanonical WNT signaling pathway. MMP7 expression was restricted to vascular smooth muscle cells and was specific to anti-class II Ab-induced tolerance, as it was undetectable in other models of renal and heart transplant tolerance and chronic rejection induced across the same strain combination. These results suggest a novel role for noncanonical WNT signaling in maintaining kidney transplant tolerance in this model, with MMP7 being a key target. Determining the mechanisms whereby MMP7 contributes to transplant tolerance may help in the development of new strategies to improve long-term graft outcome.

Use of dendritic cells in drug selection, development and therapy

BACKGROUND

Dendritic cells (DC) have the unique ability to induce immunity against tumors and various pathogens or to promote tolerance in autoimmunity and transplantation. Hence, they are central to the regulation of immune responses.

OBJECTIVE/METHODS

Due to the unique tolerogenic ability of DC, understanding some of the key molecules that regulate DC function may help with targeting the relevant signals in DC as therapeutic options for many disease conditions. DC are also targets of drugs, and many of the anti-inflammatory and pharmaceutical agents used to prevent autoimmunity or inhibit graft rejection interfere with DC function.

RESULTS/CONCLUSION

The drug-induced changes in DC may provide information for the selection of drugs and further drug discovery along with the use of DC as adjuvant in the treatment of autoimmunity and prevention of graft rejection in transplantation.

Spontaneous operational tolerance after immunosuppressive drug withdrawal in clinical renal allotransplantation

Tolerance is the so-called "Holy Grail" of transplantation, but achieving this state is proving a major challenge, particularly in the clinical setting. Even in rodents, the definition of true transplant tolerance is not applicable to many models, with late graft damage often occurring despite long-term graft survival. Hence the term "operational tolerance," based more on graft function and absence of exogenous immunosuppression, is being adopted. Although the most sought-after goal in this field is to intentionally induce this state in a controlled manner, translating protocols across species from rodents to the clinic, the current literature demonstrates that this is proving a formidable task. A complementary approach is to address transplant tolerance from a different angle, by studying tolerance-like phenomena that occur "unintentionally" in transplant patients after immunosuppressive drug weaning. Such spontaneous operational tolerance, which can take place after years of immunosuppression, is rare in kidney transplant recipients. However, determining exactly how this state arises and how it can be detected may make it possible to induce it in a greater number of patients and then to return to the drawing board to rationally design protocols that have a greater chance of clinical success. Moreover, the study of such patients should help in the identification of biomarkers of low immunological risk that could be used to select patients for potential weaning. Collaborative efforts through international networks, together with the application of newer and more powerful technologies to diagnostic, prognostic, and mechanistic research, may help transplanters to achieve this goal.

From 'megadose' haploidentical hematopoietic stem cell transplants in acute leukemia to tolerance induction in organ transplantation

The first successful demonstration that effective T cell depletion can enable immune reconstitution without causing graft vs. host disease (GVHD) in SCID patients was achieved in 1980 using lectin-separated haploidentical hematopoietic stem cells. Recipients exhibited immune tolerance towards donor antigens with a follow-up of more than 2 decades. In leukemia patients undergoing supralethal radio- and chemotherapy, T cell-depleted transplants are vigorously rejected by residual host T cells; this barrier was first overcome in 1993 by the use of megadose stem cell transplants. This clinical observation can be explained, in part, by the demonstration that cells within the CD34 compartment, as well as their immediate early myeloid progeny, are endowed with veto activity. Engraftment of mismatched hematopoietic stem cells following reduced intensity conditioning, still represents a major challenge. Progress made recently in murine studies by different approaches including the use of new co-stimulatory blockade agents, as well as by tolerance inducing cells such as anti-3rd party veto CTLs, NK T cells, and T regulatory cells, suggests several promising modalities for clinical translation.

Role of IFNgamma in allograft tolerance mediated by CD4+CD25+ regulatory T cells by induction of IDO in endothelial cells

Regulatory T cells have been described to specifically accumulate at the site of regulation together with effector T cells and antigen-presenting cells, establishing a state of local immune privilege. However the mechanisms of this interplay remain to be defined. We previously demonstrated, in a fully MHC mismatched rat cardiac allograft combination, that a short-term treatment with a deoxyspergualine analogue, LF15-0195, induces long-term allograft tolerance with a specific expansion of regulatory CD4+CD25+T cells that accumulate within the graft. In this study, we show that following transfer of regulatory CD4+T cells to a secondary irradiated recipient, regulatory CD25+Foxp3+ and CD25+Foxp3(-) CD4+T cells accumulate at the graft site and induce graft endothelial cell expression of Indoleamine 2, 3-dioxygenase (IDO) by an IFNgamma-dependent mechanism. Moreover, in vivo transfer of tolerance can be abrogated by blocking IFNgamma or IDO, and anti-IFNgamma reduces the survival/expansion of alloantigen-induced regulatory Foxp3+CD4+T cells. Together, our results demonstrate interrelated mechanisms between regulatory CD4+CD25+T cells and the graft endothelial cells in this local immune privilege, and a key role for IFNgamma and IDO in this process.

Transplantation tolerance: lessons from experimental rodent models

Immunological tolerance or functional unresponsiveness to a transplant is arguably the only approach that is likely to provide long-term graft survival without the problems associated with life-long global immunosuppression. Over the past 50 years, rodent models have become an invaluable tool for elucidating the mechanisms of tolerance to alloantigens. Importantly, rodent models can be adapted to ensure that they reflect more accurately the immune status of human transplant recipients. More recently, the development of genetically modified mice has enabled specific insights into the cellular and molecular mechanisms that play a key role in both the induction and maintenance of tolerance to be obtained and more complex questions to be addressed. This review highlights strategies designed to induce alloantigen specific immunological unresponsiveness leading to transplantation tolerance that have been developed through the use of experimental models.

TSLP: an epithelial cell cytokine that regulates T cell differentiation by conditioning dendritic cell maturation

Dendritic cells (DCs) are professional antigen-presenting cells that have the ability to sense infection and tissue stress, sample and present antigen to T lymphocytes, and induce different forms of immunity and tolerance. The functional versatility of DCs depends on their remarkable ability to translate collectively the information from both the invading microbes and their resident tissue microenvironments and then make an appropriate immune response. Recent progress in understanding TLR biology has illuminated the mechanisms by which DCs link innate and adaptive antimicrobial immune responses. However, how tissue microenvironments shape the function of DCs has remained elusive. Recent studies of TSLP (thymic stromal lymphopoietin), an epithelial cell-derived cytokine that strongly activates DCs, provide evidence at a molecular level that epithelial cells/tissue microenvironments directly communicate with DCs. We review recent progress on how TSLP expressed within thymus and peripheral lymphoid and nonlymphoid tissues regulates DC-mediated central tolerance, peripheral T cell homeostasis, and inflammatory Th2 responses.

CD40Ig treatment results in allograft acceptance mediated by CD8CD45RC T cells, IFN-gamma, and indoleamine 2,3-dioxygenase

Treatment with CD40Ig results in indefinite allograft survival in a complete MHC-mismatched heart allograft model in the rat. Here we show that serial second, third, and fourth adoptive transfers of total splenocytes from CD40Ig-treated recipients into secondary recipients led to indefinite donor-specific allograft acceptance. Purification of splenocyte subpopulations from CD40Ig-treated recipients demonstrated that only the adoptively transferred CD8(+)CD45RC(low) subset resulted in donor-specific long-term survival, whereas CD8(+)CD45RC(low) T cells from naive animals did not. Accepted grafts displayed increased indoleamine 2,3-dioxygenase (IDO) expression restricted in the graft to ECs. Coculture of donor ECs with CD8(+)CD45RC(low) T cells purified from CD40Ig-treated animals resulted in donor-specific IDO expression dependent on IFN-gamma. Neutralization of IFN-gamma or IDO triggered acute allograft rejection in both CD40Ig-treated and adoptively transferred recipients. This study demonstrates for what we believe to be the first time that interference in CD40-CD40 ligand (CD40-CD40L) interactions induces allospecific CD8(+) Tregs that maintain allograft survival. CD8(+)CD45RC(low) T cells act through IFN-gamma production, which in turn induces IDO expression by graft ECs. Thus, donor alloantigen-specific CD8(+) Tregs may promote local graft immune privilege through IDO expression.

Role of thymic- and graft-dependent mechanisms in tolerance induction to rat kidney transplant by donor PBMC infusion

We previously demonstrated the presence of regulatory T cells (Tregs) in lymph nodes (LNs) from rats made tolerant to a kidney allograft by donor peripheral blood mononuclear cell (PBMC) infusion. Here, we investigated the origin of Treg and characterized their phenotype and mechanisms underlying their suppressive effect. At different points after PBMC infusion, thymus, LN, and graft-infiltrating -lymphocyte's (GIL) alloreactivity was evaluated in mixed lymphocyte reaction (MLR), coculture, and transwell experiments. GIL phenotype (by fluorescence-activated cell sorting and immunohistochemistry) and cytokines mRNA expression were analyzed. Before transplantation, CD4(+) thymocytes and LN cells from donor PBMC-infused rats showed a reduced anti-donor but a normal anti-third-party proliferation. Anti-donor hyporesponsiveness was reverted by interleukin (IL)-2. CD4(+) thymocytes had no regulatory activity on a naïve MLR. Treg appeared in LN at 60 days post-transplant. CD4(+)-GIL isolated early (5 days) and late post-transplant (days 60-80) were hyporesponsive and suppressed a naïve MLR. IL-10 mRNA was upregulated in GIL and an anti-IL-10 monoclonal antibody reverted their inhibitory effect. Cell-to-cell contact potentiated the suppressive activity of CD4(+)-GIL. We suppose that allograft tolerance in this model is mediated by pretransplant generation of anergic cells in the thymus, which may have a permissive role to prevent early graft disruption. The healed graft is a source of donor antigens, which led to early selection of Treg. In the late phase, tolerance is maintained by appearance of Treg in LN.

New evidence for a role of allograft accommodation in long-term tolerance

BACKGROUND

Progressively better therapies have largely prevented or at least effectively treated acute allograft rejection. Consequently, the long-term survival of solid organ transplants has increasingly become limited primarily by the development of chronic allograft rejection. The mechanisms of chronic rejection remain largely unknown and the induction of specific tolerance would be the ultimate achievement in transplant immunology. We previously demonstrated, in a fully major histocompatibility complex (MHC)-mismatched rat cardiac allograft combination, that a 20-day treatment with a deoxyspergualin (DSG) analogue, LF15-0195, induces allograft tolerance with the development of potent CD4CD25 regulatory T cells. In order to better characterize the mechanisms involved in allograft tolerance, we compared long-term tolerated allografts with allografts exhibiting signs of chronic rejection induced by donor-specific blood transfusion.

METHODS

We analyzed both types of allografts for infiltration, alloantibody production and gene expression by histology, exhaustive microarray and quantitative reverse-transcriptase polymerase chain reaction.

RESULTS

Interestingly, we observed in tolerated allografts an infiltrate as dense as the one observed in chronically rejected allografts and alloantibody deposits on graft endothelial cells. Prominent gene expression of many putative proinflammatory cytokines and genes related to cell activation or cytotoxicity were observed in tolerated allografts. However, we observed a specific upregulation of cytoprotective genes such as nitric oxide synthase, BclXL, and indoleamine 2,3 dioxygenase, and a poor in situ expression of immunoglobulin chain gene.

CONCLUSIONS

This study demonstrates a state of accommodation of tolerated allografts and suggests the importance of early control of humoral immunity for the prevention of chronic rejection and the maintenance of long-term tolerance.

Development of CD25– regulatory T cells following heart transplantation: Evidence for transfer of long-term survival

Donor-specific heart allograft acceptance can be induced in the MHC-mismatched LEW.1 W to LEW.1A rat by donor-specific transfusions. Whereas the induction phase of tolerance has been studied in detail, its maintenance remained poorly understood. Here, we performed a side-by-side comparison of CD25+ and CD25- splenic T cells of 100-day tolerant rats. Administration of CD25- T cells from tolerant rats to sublethally irradiated recipients transferred long-term graft survival. These CD25- T cells displayed a decreased donor-specific response in the mixed lymphocyte reaction and presented suppressive activity. These CD25- T cells accumulated IFN-gamma, IL-10 and Foxp3 transcripts. The in vitro suppressive activity of CD25- T cells required both cell contact and soluble factors (IL-10 and IFN-gamma). The CD25+ T cells from tolerant rats did not show any modification of their regulatory properties. We show that splenic CD25- T cells of tolerant rats contribute to the maintenance of tolerance following the transplantation. Our data show that regulatory T cells are not restricted to the CD4+ CD25+ T cell subset and provide new insights on the mechanisms of tolerance to allograft following donor cell priming.

Pharmacologic approaches to composite tissue allograft

This article discusses the pharmacologic approaches and the most promising new compounds for composite tissue allograft tolerance. Although some approaches rely on a combination of immunosuppressive agents that act synergistically against rejection, other strategies use immunologic manipulation, including major histocompatibility complex matching, induction of chimerism, and use of monoclonal antibodies to abrogate the immune response. There is still a need, however, to reproduce these findings in species phylogenetically closer to humans. This may be the target of future research efforts, which may overcome the challenge of limb and face transplant rejection.

Role of thymus in operational tolerance induction in limb allograft transplant model

BACKGROUND

In this study, we evaluated the role of host thymus in tolerance induction in composite tissue allografts (CTA) across major histocompatibility complex (MHC) barrier during a 7-day alphabeta- T-cell receptor (TCR)/ cyclosporine A (CsA) protocol.

MATERIALS AND METHODS

A total of 62 limb allograft transplants were studied. Euthymic (group A) and thymectomized (group B) Lewis recipients (LEW, RT1(1)) received vascularized hind-limb allografts from hybrid Lewis x Brown-Norway (F1), (LBN, RT1(1+n)) donors. Mixed lymphocyte reaction (MLR) and skin grafting assessed donor-specific tolerance in vitro and in vivo, respectively. Flow cytometry determined the efficacy of immunosuppressive protocols and the presence of donor-specific chimerism. Immunocytochemistry revealed the presence of donor-specific cells in the lymphoid organs of recipients.

RESULTS

Isograft transplants survived indefinitely. For thymectomized rats, the median survival time (MST) of limb allograft in non-treated recipients was 7 days; monotherapy with alphabeta-TCR extended MST to 16 days, and CsA therapy extended it to 30 days. Using the alphabeta-TCR/CsA protocol, the MST of allografts was 51 days. For euthymic rats, the MST of limb allograft in non-treated recipients was 7 days; monotherapy with alphabeta-TCR or CsA extended MST to 13 or 22 days, respectively. Treatment with alphabeta-TCR/CsA resulted in indefinite allografts survival (MST=370 days). MLR and skin grafting confirmed donor-specific tolerance in euthymic recipients. Flow cytometry showed stable chimerism in the euthymic rats and transient chimerism in thymectomized limb recipients. Immunoperoxidase staining revealed the persistence of donor-derived cells in the lymphoid tissues of euthymic recipients.

CONCLUSION

We found that the presence of thymus was imperative for the induction of donor-specific tolerance in rat hind-limb composite tissue allografts using a alphabeta-TCR/CsA protocol.

Is clinical tolerance realistic in the next decade?

Tolerance to allografts would mean a better quality of life and prognosis for transplant patients. Despite the first descriptions of tolerance to alloantigens over 50 years ago, deliberately induced tolerance in the clinic on a wide scale remains a goal that is not quite in reach. However, much progress has been made in understanding tolerance in rodent models and in the few reports of induced or spontaneously occurring tolerance in humans. Here, we review this progress made in the quest to achieve clinical tolerance.

Induction of tolerance by exosomes and short-term immunosuppression in a fully MHC-mismatched rat cardiac allograft model

Exosomes are MHC-bearing vesicles secreted by a wide array of cells. We have previously shown that donor-haplotype exosomes from bone marrow dendritic cells (DCs) injected before transplantation significantly prolong heart allograft survival in congenic and fully MHC-mismatched Lewis rats. Here we show that donor exosomes administered after transplantation are similarly able to prolong allograft survival, however, without inducing tolerance. We therefore tested the effect of exosomes combined with short-term LF 15-0195 (LF) treatment, which blocks the maturation of DCs, so that donor-MHC antigens from exosomes could be presented in a more tolerogenic environment. LF treatment does not preclude the development of a strong antidonor cellular response, and while LF, but not exosome, treatment inhibits the antidonor humoral response and decreases leukocyte graft infiltration, allografts from LF-treated recipients were either acutely or strongly chronically rejected. Interestingly, when combined with LF treatment, exosomes induced a donor-specific allograft tolerance characterized by a strong inhibition of the antidonor proliferative response. This donor-specific tolerance was transferable to naïve allograft recipients. Moreover, exosomes/LF treatment prevented or considerably delayed the appearance of chronic rejection. These results suggest that under LF treatment, presentation of donor-MHC antigens (from exosomes) can induce regulatory responses that are able to modulate allograft rejection and to induce donor-specific allograft tolerance.

Regulatory T cells in graft-versus-host disease

Alloreactive T cells present in a bone marrow transplant are responsible for graft-vs-host disease, but their depletion is associated with impaired engraftment, immunosuppression, and loss of the graft-vs-leukemia effect. The subpopulation of CD4(+)CD25(+) immunoregulatory T cells was first identified based on its crucial role in the control of autoimmune processes, but they also play a role in alloreactive responses. Moreover, these cells could be used to develop innovative strategies in the field of transplantation and particularly to prevent graft-vs-host disease. Indeed, high numbers of CD4(+)CD25(+) immunoregulatory T cells can modulate graft-vs-host disease if administered at the same time as allogeneic hematopoietic stem cell transplantation in mice. This review discusses various important issues regarding the possible use of CD4(+)CD25(+) immunoregulatory T cells to modulate alloreactivity in hematopoietic stem cell transplantation.

Contrasting CD25hiCD4+T cells/FOXP3 patterns in chronic rejection and operational drug-free tolerance

BACKGROUND

Although immunosuppression withdrawal in kidney recipients usually leads to rejection, in some patients it does not, leading to a state of clinical operational tolerance.

METHODS

We compared these highly contrasted situations by analyzing blood cell phenotype and transcriptional patterns in drug-free spontaneously tolerant kidney recipients, recipients with chronic rejection, recipients with stable graft function under standard or minimal immunosuppression and healthy individuals

RESULTS

The blood cell phenotype of clinically tolerant patients did not differ from that of healthy individuals. In contrast, recipients with chronic rejection had significantly less CD25hiCD4+T cells and lower levels of FOXP3 transcripts compared with clinically tolerant recipients. Patients with chronic rejection also displayed CD25-CD4+T cells expressing NKG2D+CD94+ and CD57+CD27-CD28- cytotoxic-associated markers (P<0.05).

CONCLUSION

These data show that whereas clinically tolerant recipients displayed normal levels of CD25hiCD4+T cells and FOXP3 transcripts, chronic rejection is associated with a decrease in CD25hiCD4+T cells and FOXP3 transcripts, suggesting that clinically "operational tolerance" may be due to a maintained phenomenon of natural tolerance that is lacking in patients with chronic rejection.