Contrasting effects of cyclosporine and rapamycin in de novo generation of alloantigen-specific regulatory T cells

An update on regulatory T cells in transplant tolerance and rejection

Several types of T cells with immunosuppressive properties have been identified, but FOXP3(+) regulatory T (T(REG)) cells have emerged as a dominant cell type; they are critically involved in the induction and maintenance of immune tolerance. Manipulation of this cell type for the induction of transplant tolerance including renal transplant tolerance has attracted considerable attention. Studies in this area have demonstrated unexpected complexities, and attempts to translate T(REG) cells towards clinical utility have met with unanticipated difficulties. In this Review, a broad overview is provided on recent progress in the study of T(REG) cells, focusing on challenges, opportunities, and emerging approaches in exploiting T(REG) cells for the induction of transplant tolerance.

T-cell phenotype in protocol renal biopsy from transplant recipients treated with belatacept-mediated co-stimulatory blockade

BACKGROUND

Belatacept is thought to disrupt the interaction between CD80/86 and CD28, thus preventing T-cell activation by blocking the co-stimulatory second signal. However, the consequences on the T-cell profile in human renal transplant cases have not been determined.

METHODS

In this study, we analysed intra-graft levels of the mRNAs for Treg (FOXP3), cytotoxic CD8 T cells (Granzyme B), Th1 (INFγ, Tbet), Th2 (GATA3) and Th17 (RORγt and IL-17) in protocol biopsies obtained 12 months after renal transplantation in recipients treated with Belatacept or calcineurin inhibitor (CNI).

RESULTS

Only the intra-graft abundance of FOXP3 mRNA was significantly lower (P < 0.001) in the Belatacept group than the CNI group. Conclusions. These results are in agreement with in vitro data suggesting that CD28 is a major co-stimulatory signal of both Tregs development and peripheral homeostasis but contrast with clinical trials showing a better 1-year graft function and a lower incidence of chronic allograft nephropathy in patients receiving Belatacept than patients treated with CNI. They suggest that immune benefits induced by Belatacept are not mediated by Treg expansion and that FOXP3 is not by itself a prognostic marker of long-term graft function in a non-inflammatory context. These results have to be, however, considered as preliminary since the size of our study population is limited.

Peripheral blood T regulatory cell counts may not predict transplant rejection

Background

Recent evidence shows that allograft survival rates show a positive correlation with the number of circulating T regulatory cells (Tregs). This study investigated both the number and the cytokine profiles exhibited by Foxp3⁺ Tregs in blood, spleen and lymph nodes of Lewis rat recipients of BN rat cardiac allografts after a single-dose of Rapamycin (RAPA).

Results

Rats were divided into three groups: control group (containing healthy control and acute rejection group), and recipients treated with a single dose of RAPA on either Day 1 (1D group)or Day 3 (3D group) post-transplant. We analyzed the number of Foxp3+Tregs and the expression of Foxp3 and cytokines in the peripheral blood and the peripheral lymphoid tissues. No difference was found in the numbers of circulating Foxp3+ Tregs between these three groups. RAPA administration significantly increased Foxp3 expression in peripheral lymphoid tissues after a single dose of RAPA on Day 3 post-transplant. Foxp3+Tregs inhibited the activity of effector T cells (T_eff) via the secretion of TGF-β1.

Conclusion

The number of Tregs in the recipient's blood may not be a good predictor of transplant rejection. Foxp3+Tregs inhibit the activity of T_eff cells mainly in the peripheral lymphoid tissues.

The recall alloresponse following retransplantation is more intense compared with the T cell memory-transfer model

The presence of alloreactive memory T cells in recipient is a critical handicap to achieving transplantation tolerance. To make a mouse model that can as closely as possible mimic the presensitized transplant patient is important for research on this subject. Thus, we developed a novel retransplant model and compared the alloresponse in this model with that in the memory T cells-transfer model (transfer control). Mean survival time of allograft was compared between 3 groups, including blank transplant control, memory transfer control and retransplant groups. Cellular rejection activity in allografts was evaluated via HE staining of cardiac graft section. Proliferation and differentiation of the alloreactive effector T cells were assayed by in vitro mixed lymphocyte reaction and flow cytometry, respectively. Real-time quantitive RT-PCR was used to assess gene expression of cytokines and surum IFN-gamma was measured via ELISA. It showed that the median survival time of allograft in retransplant recipients was significantly shortened compared to that of transfer control, and it was the same in rejection score of graft. Moreover, proliferation and differentiation of the alloreactive effector T cells were more intensive in retransplant recipients than that in transfer control, which was confirmed by in vitro mixed lymphocyte reaction and by flow cytometry of the splenocytes for detecting CD44highCD62L- memory/effector phenotype cells. Furthermore, activation of CD4+ memory T cells is reflected by high level of surum IFN-gamma and the intensive gene expression of IFN-gamma and IL-2 at cardiac allograft in retransplant recipients. Collectively, the recall alloresponse in retransplantation is more intensive than that in a memory-transfer setting, and this retransplant model is closer to the clinic situation than the memory-transfer model in rodents.

In vivo tracking of 'color-coded' effector, natural and induced regulatory T cells in the allograft response

Here we present methods to longitudinally track islet allograft-infiltrating T cells in live mice by endoscopic confocal microscopy and to analyze circulating T cells by in vivo flow cytometry. We developed a new reporter mouse whose T cell subsets express distinct, 'color-coded' proteins enabling in vivo detection and identification of effector T cells (T(eff) cells) and discrimination between natural and induced regulatory T cells (nT(reg) and iT(reg) cells). Using these tools, we observed marked differences in the T cell response in recipients receiving tolerance-inducing therapy (CD154-specific monoclonal antibody plus rapamycin) compared to untreated controls. These results establish real-time cell tracking as a powerful means to probe the dynamic cellular interplay mediating immunologic rejection or transplant tolerance.

The role of mTOR in memory CD8 T-cell differentiation

The mammalian target of rapamycin (mTOR) is an intracellular kinase that regulates cell growth and metabolism. Its specific inhibitor rapamycin is currently used in transplant recipients as an immunosuppressive drug to prevent allograft rejection. Studies have shown complex and diverse mechanisms for the immunosuppressive effects of rapamycin. The drug has been reported to inhibit T-cell proliferation, induce anergy, modulate T-cell trafficking, promote regulatory T cells, and also prevent maturation of dendritic cells as well as production of type I interferon. However, several other studies have paradoxically demonstrated immunostimulatory effects of rapamycin by improving antigen presentation and regulating cytokine production from macrophages and myeloid dendritic cells. Recently, it has been shown that rapamycin also exhibits immunostimulatory effects on memory CD8(+) T-cell differentiation. The drug improved both quantity and quality of memory CD8(+) T cells induced by viral infection and vaccination, showing that mTOR is a major regulator of memory CD8(+) T-cell differentiation. These discoveries have implications for the development of novel vaccine regimens. Here, we review the role of mTOR in memory CD8(+) T-cell differentiation and compare the effect of rapamycin among CD8(+) T cells, CD4(+) T cells, and dendritic cells. Also, we discuss potential application of these findings in a clinical setting.

Impact of immunosuppressants on the therapeutic efficacy of in vitro-expanded CD4+CD25+Foxp3+ regulatory T cells in allotransplantation

BACKGROUND

Although the therapeutic potential of regulatory T lymphocytes (Tregs) in preventing allograft rejection has been well documented, accumulating evidence indicates that supplemental measures, such as concomitant use of immunosuppressive agents, are essential for effective application of Treg cell therapy in clinical transplantation. Thus, it is important to know the effect of immunosuppressive agents on Treg cell therapy.

METHODS

We examined the impact of various immunosuppressive agents on the in vivo proliferation and therapeutic efficacy of in vitro-expanded Tregs using the murine graft-versus-host reaction and skin allograft model (BDF1 [H-2] to C57BL/6 [H-2]), respectively.

RESULTS

All six immunosuppressive agents tested inhibited the alloantigen-stimulated proliferation of Tregs as efficiently as they inhibited the proliferative response of conventional CD3 T cells. We further show that blockade of the CD40-CD40L interaction by treatment with a MR-1 antibody significantly increased the therapeutic efficacy of Tregs, a synergistic effect that seemed to be related to the strong regulatory activity of adoptively transferred Tregs together with effector T-cell hyporesponsiveness. Although concomitant use of rapamycin marginally augmented the therapeutic effectiveness of Tregs, mycophenolate mofetil and cyclosporine A at their full therapeutic doses exerted an antagonistic effect on Treg cell therapy.

CONCLUSION

These results demonstrate that inhibition of CD40-CD40L interaction or treatment with rapamycin could be successfully combined with in vitro-expanded Treg cell therapy, but the concomitant use of mycophenolate mofetil or cyclosporine A in this type of Treg cell therapy should be carefully considered.

Tolerance: is it achievable in pediatric solid organ transplantation?

In the clinical arena of transplantation, tolerance remains, for the most part, a concept rather than a reality. Although modern immunosuppression regimens have effectively handled acute rejection, nearly all organs except the liver commonly suffer chronic immunologic damage that impairs organ function, threatening patient and allograft survival. In addition to the imperfect control of the donor-directed immune response, there are additional costs. First, there is the burden of mortality from infection and malignancy that can be directly attributed to a crippled immune system. Second, there are insidious effects on renal function, cardiovascular profile (hypertension, hyperglycemia, and dyslipidemia), bone health, growth, psychological and neurocognitive development, and overall quality of life. It is likely that the full consequences of lifelong immunosuppression on our pediatric transplant recipients will not be fully appreciated until survival routinely extends beyond 1 or 2 decades after transplantation. Therefore, it can be argued that the holy grail of transplantation tolerance is of the utmost importance to children who undergo solid organ transplantation.

Rapamycin and tacrolimus differentially modulate acute graft-versus-host disease in rats after liver transplantation

Acute graft-versus-host disease (aGVHD) is a serious complication of liver transplantation (LTx); it occurs in 1% to 2% of liver allograft recipients. The condition has a poor prognosis and poses major diagnostic and therapeutic challenges. A rat model of aGVHD after LTx has been developed, and a relative decrease in regulatory T (Treg) cells has been shown to be associated with this model. Interest has been expressed in the effects of different immunosuppressive agents on CD4+CD25+Foxp3+ Treg cell homeostasis. Rats with aGVHD after LTx were treated with tacrolimus (FK506), rapamycin (RAPA), or no immunosuppressive drug. Those that received RAPA survived longer (91.4 + or - 8.1 days) than those in the FK506 group (62.3 + or - 13.4 days) or the control group (22.9 + or - 1.2 days). Flow cytometry analysis showed that Treg cells, as a percentage of peripheral blood mononuclear cells (PBMCs), were more abundant in the RAPA group (6.8% + or - 0.8%) than in the FK506 group (1.7% + or - 0.4%) or the control group (2.0% + or - 0.4%). Immunohistochemistry demonstrated more Foxp3+ staining of intestinal cells in the RAPA group than in the FK506 group or the control group. In conclusion, the reduced mortality induced by RAPA in a rat model of aGVHD after LTx was associated with higher percentages of CD4+CD25+Foxp3+ Treg cells in PBMCs in blood and tissues than those occurring after the administration of FK506.

Regulatory T cells in transplantation: does extracellular adenosine triphosphate metabolism through CD39 play a crucial role?

Despite tremendous improvements in short-term renal allograft survival, many patients still have chronic rejection or side effects of nonspecific immunosuppression. The discovery of Foxp3(+) regulatory T cells (Tregs) has revolutionized the concepts in immunoregulation and offers perspectives for overcoming rejection. Recently, a subset of Foxp3(+)CD39(+) effector/memory-like Tregs (T(REM)) was identified. The role of CD39(+) Tregs in immunoregulation is supported by the occurrence of alopecia areata and experimental autoimmune encephalomyelitis in CD39-deficient mice and by the failure of CD39(-) Tregs to suppress contact hypersensitivity. In humans, CD39 polymorphisms have been associated with diabetes and nephropathy, and multiple sclerosis patients have reduced numbers of blood CD39(+) Tregs. Preliminary experiments in a murine transplantation model showed that CD39(+) Tregs can determine allograft outcome. CD39 degrades the extracellular adenosine triphosphate (ATP) released during tissue injury, which otherwise would trigger inflammation. Currently, our groups are assessing the role of CD39(+) Tregs and extracellular ATP metabolism in clinical transplantation and whether tolerogenic Treg profiles possess immunopredictive value, envisioning the development of clinical trials using CD39(+) Treg-based vaccination for autoimmunity or transplantation. This is a comprehensive review on the fundamentals of Treg biology, the potential role of ATP metabolism in immunoregulation, and the potential use of Treg-based immunotherapy in transplantation.

Immune phenotype predicts risk for posttransplantation squamous cell carcinoma

Cutaneous squamous cell cancer (SCC) affects up to 30% of kidney transplant recipients (KTRs) within 10 years of transplantation. There are no reliable clinical tests that predict those who will develop multiple skin cancers. High numbers of regulatory T cells associate with poor prognosis for patients with cancer in the general population, suggesting their potential as a predictive marker of cutaneous SCC in KTRs. We matched KTRs with (n = 65) and without (n = 51) cutaneous SCC for gender, age, and duration of immunosuppression and assessed several risk factors for incident SCC during a median follow-up of 340 days. Greater than 35 peripheral FOXP3(+)CD4(+)CD127(low) regulatory T cells/microl, <100 natural killer cells/microl, and previous SCC each significantly associated with increased risk for new cutaneous SCC development (hazard ratio [HR] 2.48 [95% confidence interval (CI) 1.04 to 5.98], HR 5.6 [95% CI 1.31 to 24], and HR 1.33 [95% CI 1.15 to 1.53], respectively). In addition, the ratio of CD8/FOXP3 expression was significantly lower in cutaneous SCC excised from KTRs (n = 25) compared with matched SCC from non-KTRs (n = 25) and associated with development of new cutaneous SCCs. In summary, monitoring components of the immune system can predict development of cutaneous SCC among KTRs.

PD-L1 regulates the development, maintenance, and function of induced regulatory T cells

Both the programmed death (PD) 1–PD-ligand (PD-L) pathway and regulatory T (T reg) cells are instrumental to the maintenance of peripheral tolerance. We demonstrate that PD-L1 has a pivotal role in regulating induced T reg (iT reg) cell development and sustaining iT reg cell function. PD-L1^−/− antigen-presenting cells minimally convert naive CD4 T cells to iT reg cells, showing the essential role of PD-L1 for iT reg cell induction. PD-L1–coated beads induce iT reg cells in vitro, indicating that PD-L1 itself regulates iT reg cell development. Furthermore, PD-L1 enhances and sustains Foxp3 expression and the suppressive function of iT reg cells. The obligatory role for PD-L1 in controlling iT reg cell development and function in vivo is illustrated by a marked reduction in iT reg cell conversion and rapid onset of a fatal inflammatory phenotype in PD-L1^−/−PD-L2^−/− Rag^−/− recipients of naive CD4 T cells. PD-L1 iT reg cell development is mediated through the down-regulation of phospho-Akt, mTOR, S6, and ERK2 and concomitant with the up-regulation of PTEN, all key signaling molecules which are critical for iT reg cell development. Thus, PD-L1 can inhibit T cell responses by promoting both the induction and maintenance of iT reg cells. These studies define a novel mechanism for iT reg cell development and function, as well as a new strategy for controlling T reg cell plasticity.

Mammalian target of rapamycin inhibition and alloantigen-specific regulatory T cells synergize to promote long-term graft survival in immunocompetent recipients

Minimization of immunosuppression and donor-specific tolerance to MHC-mismatched organ grafts are important clinical goals. The therapeutic potential of regulatory T cells (Tregs) has been demonstrated, but conditions for optimizing their in vivo function posttransplant in nonlymphocyte-depleted hosts remain undefined. In this study, we address mechanisms through which inhibition of the mammalian target of rapamycin (Rapa) synergizes with alloantigen-specific Treg (AAsTreg) to permit long-term, donor-specific heart graft survival in immunocompetent hosts. Crucially, immature allogeneic dendritic cells allowed AAsTreg selection in vitro, with minimal expansion of unwanted (Th17) cells. The rendered Treg potently inhibited T cell proliferation in an Ag-specific manner. However, these AAsTreg remained unable to control T cells stimulated by allogeneic mature dendritic cells, a phenomenon dependent on the release of proinflammatory cytokines. In vivo, Rapa administration reduced danger-associated IL-6 production, T cell proliferation, and graft infiltration. Based on these observations, AAsTreg were administered posttransplant (day 7) in combination with a short course of Rapa and rendered >80% long-term (>150 d) graft survival, a result superior to that achieved with polyclonal Treg. Moreover, graft protection was alloantigen-specific. Significantly, long-term graft survival was associated with alloreactive T cell anergy. These findings delineate combination of transient mammalian target of Rapa inhibition with appropriate AAsTreg selection as an effective approach to promote long-term organ graft survival.

Rapamycin, unlike cyclosporine A, enhances suppressive functions of in vitro-induced CD4+CD25+ Tregs

BACKGROUND

A growing body of data shows that CD4(+)CD25(+) regulatory T cells (Tregs) can induce transplantation tolerance by suppressing immune responses to allograft antigens. However, both the generation and the suppressive capacity of CD4(+)CD25(+) Tregs can be substantially affected by different immunosuppressive drugs used in clinical transplantation. The goal of this study was to compare the effects of cyclosporine A and rapamycin on the induction and suppressive functions of human CD4(+)CD25(+) Tregs in vitro.

METHODS

CD4(+)CD25(+) Tregs were induced in two-way mixed lymphocyte reaction (MLR) in the presence of rapamycin (Treg-Rapa) or cyclosporine A (Treg-CsA). Tregs were identified in MLR cultures by flow cytometry using anti-CD4, anti-CD25, anti-CTLA-4, anti-CD122, anti-GITR mAbs and ant-PE-FOXP3 staining sets. Suppressive capacity of induced Tregs was evaluated by their capability to inhibit anti-CD3 Ab-triggered proliferation of peripheral blood mononuclear cells (PBMCs), as measured by flow cytometry. The concentration of TGF-beta1 in culture supernatants was measured by enzyme-linked immunosorbent assay.

RESULTS

Although both rapamycin and cyclosporine A suppressed the induction of CD4(+)CD25(+) Tregs during MLRs, this effect was significantly more pronounced in cells cultured with cyclosporine. On the other hand, only rapamycin significantly decreased the percentage of CD4(+)CD25(+) Tregs which expressed GITR, a negative regulator of Treg's suppressive capacity. Importantly, Treg-Rapa, unlike Treg-CsA, displayed significant suppressive activity and were capable of inhibiting the proliferation of anti-CD3 Ab-activated PBMCs. This activity was likely mediated by TGF-beta1.

CONCLUSIONS

Rapamycin, unlike cyclosporine A, does not inhibit the function of CD4(+)CD25(+) Tregs. This implies that rapamycin could contribute to the development of transplantation tolerance by promoting the induction of functional CD4(+)CD25(+) Tregs. Moreover, our results suggest that rapamycin could be combined with functional Tregs.

Transcriptional regulation of Foxp3 gene: multiple signal pathways on the road

Foxp3, forkhead/winged helix transcription factor 3, is a master transcription factor for the development and function of regulatory T cells. Foxp3 has been proved to be associated with immunoregulation, autoimmune diseases, infections, and tumor immune evasion/escape. Foxp3 regulates other critical gene transcriptions. However, the mechanism how the transcription of Foxp3 itself is regulated remains partly clear. In this article, we provided an overview of the current understanding of the transcriptional regulation of Foxp3 gene, including signaling pathways initiated by TCR, IL-2R/STAT pathway, TGF-beta/Smad pathway, PI3K/Akt/mTOR axis, Notch signal pathway, IFN/IRF and IFN/nitric oxide axis, and epigenetic mechanisms. Some therapeutic agents on Foxp3 regulation were also reviewed. Points for attention in further study of Foxp3 transcription regulation, such as the combinations/cross-talks, the bi-directional functions, and species specificity of these pathways, were discussed as well.

Runx proteins regulate Foxp3 expression

Runx proteins are essential for hematopoiesis and play an important role in T cell development by regulating key target genes, such as CD4 and CD8 as well as lymphokine genes, during the specialization of naive CD4 T cells into distinct T helper subsets. In regulatory T (T reg) cells, the signature transcription factor Foxp3 interacts with and modulates the function of several other DNA binding proteins, including Runx family members, at the protein level. We show that Runx proteins also regulate the initiation and the maintenance of Foxp3 gene expression in CD4 T cells. Full-length Runx promoted the de novo expression of Foxp3 during inducible T reg cell differentiation, whereas the isolated dominant-negative Runt DNA binding domain antagonized de novo Foxp3 expression. Foxp3 expression in natural T reg cells remained dependent on Runx proteins and correlated with the binding of Runx/core-binding factor β to regulatory elements within the Foxp3 locus. Our data show that Runx and Foxp3 are components of a feed-forward loop in which Runx proteins contribute to the expression of Foxp3 and cooperate with Foxp3 proteins to regulate the expression of downstream target genes.

Can proliferation signal inhibitor-induced Tregs really reflect transplantation tolerance in clinical solid organ transplantation?

Exploring new immunosuppressive strategies inducing donor-specific hyporesponsiveness is a great challenge in transplantation. For this purpose, monitoring the alloimmune response is a critical step to carrying out protolerogenic immunosuppressive protocols. Regulatory T cells (Tregs), known as controlling various immune responses, were found to play an important part in allograft transplant tolerance. It is said that Rapamycin (RAPA), one of the proliferation signal inhibitors (PSIs), could achieve true tolerance through the induction of Tregs in clinical trials. Can PSI-induced Tregs in peripheral blood mononuclear cells (PBMC) really reflect transplantation tolerance in clinical solid organ transplantation?

A novel modifier of regulatory T cells

The receptor for the lipid mediator sphingosine 1-phosphate is critical for T cell trafficking. New data show that signaling mediated by this receptor critically controls the development, maintenance and suppressive activity of natural regulatory T cells that express the transcription factor Foxp3.

Two-year follow-up of a prospective study of circulating regulatory T cells in renal transplant patients

CD4(+)CD25(high)FOXP3(+) regulatory T cells (Tregs) are involved in alloreactivity and may be associated with protection from rejection. Their quantification in peripheral blood could guide clinicians in the management of renal transplant patients. Thus, we prospectively monitored the levels and in vitro suppression of circulating Tregs in 33 renal transplant patients from deceased donors within the first two yr of transplantation. Patients received maintenance immunosuppression with tacrolimus, mofetil mycophenolate and prednisolone. Results showed that peripheral blood Tregs were significantly lower six months after transplantation and recovered to almost basal levels at first post-transplant year. The number of circulating Tregs increased significantly over basal levels afterwards. The decrease in circulating Tregs at six months may be explained by the high load of tacrolimus, as demonstrated by the inverse correlation between the blood concentration of Tregs and tacrolimus. Likewise, nine patients treated with anti-CD25 antibodies showed higher numbers of Tregs at six months than those that did not, although differences were not observed later. In conclusion, circulating Tregs decrease in the first six months but recover thereafter up to two yr after kidney transplantation. Such a decrease is favored by high levels of tacrolimus but not by induction protocols with anti-CD25.

Triptolide promotes generation of FoxP3+ T regulatory cells in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Triptolide (TPT), a component of the Chinese herb Triptergium wilfordii, has potent immunosuppressive and anti-inflammatory activity and is used clinically in recipients of kidney transplantation.

AIM OF THE STUDY

This work aimed to investigate the effect of TPT on the differentiation of regulatory T lymphocytes (Tregs) from CD4+ cells in rats.

MATERIALS AND METHODS

MACS-purified rat CD4+ cells were costimulated with anti-CD3 and anti-CD28 in the presence of TGF-beta to induce the expression of FoxP3, which was detected by flow cytometry. TPT and cyclosporine A (CsA) were separately added into the cultures to observe the effect on the expression of FoxP3. Kidney transplantation was performed in rats that either received no treatment or were treated with TPT after transplantation.

RESULTS

TPT treatment enhanced the expression of FoxP3 in CD4+ cells, whereas CsA inhibited the FoxP3 expression. In the rat kidney transplantation model, the recipient rats treated with TPT survived longer than the control rats (18-19.83 vs 6.83 days, P<0.05). Meanwhile, the FoxP3+ T cells in the spleens of treated rats were higher than those from the untreated rats (12.4% vs 4.7%, P<0.05).

CONCLUSIONS

These data suggest that TPT may promote the differentiation of CD4+ cells to FoxP3+ Tregs. This would be at least one of the pathways responsible for the immunosuppressive activity of TPT.

Tolerogenic dendritic cell-regulatory T-cell interaction and the promotion of transplant tolerance

Evidence has accumulated that targeting of donor alloantigen to quiescent dendritic cells (DC) in situ or adoptive DC therapy is associated with the expansion or induction of regulatory T cells (Treg) in experimental organ transplantation. These Treg can mediate suppression of antidonor T-effector cell responses and promote allograft tolerance. In addition, Treg can exert reciprocal, inhibitory effects on DC that maintain their tolerogenic properties. Several groups have exploited DC to expand allo-Ag-specific Treg in vitro, followed by adoptive transfer of the Treg to graft recipients, an approach that holds promise for tolerogenic cell therapy in clinical cell and organ transplantation.

Expanding and converting regulatory T cells: a horizon for immunotherapy

The human immune system is a myriad of diverse cellular populations, each contributing to maintaining an effective and optimal immune response against infectious agents. It is important to maintain a "self-check" in the immune system so that responses do not go haywire, leading to the development of autoimmune diseases. Regulatory/suppressor T (Treg) cells are a specialized subpopulation of T cells that suppress the activation, expansion, and function of other T cells, thereby maintaining homeostasis through a fine balance between reactivity to foreign and self antigens. Tregs are characterized by surface expression of interleukin (IL)-2 receptor alpha chain (CD25) and intracellular expression of forkhead box protein P3 (FoxP3). There are at least two important functional populations of Treg cells, namely natural Treg (nTreg), which are continuously derived from the thymus, and induced Treg (iTreg), which are converted from naive T cells. The development and function of both nTreg and iTreg cells are regulated by several factors, such as antigen T-cell receptor, co-stimulatory receptors (i.e., cytotoxic T lymphocyte-associated antigen, or CTLA-4), and cytokines (IL-2, IL-10, and tumor growth factor-beta, or TGF-beta). In addition, the TGF-beta inhibitor ALK5, retinoid acid, and rapamycin influence the expansion of nTreg cells and the conversion of iTreg cells in vitro and in vivo. The heightening of Treg expansion may be harnessed to therapeutic methods for the treatment of autoimmune diseases and the induction of transplantation tolerance.

Immunoregulatory functions of mTOR inhibition

The potent immunosuppressive action of rapamycin is commonly ascribed to inhibition of growth factor-induced T cell proliferation. However, it is now evident that the serine/threonine protein kinase mammalian target of rapamycin (mTOR) has an important role in the modulation of both innate and adaptive immune responses. mTOR regulates diverse functions of professional antigen-presenting cells, such as dendritic cells (DCs), and has important roles in the activation of effector T cells and the function and proliferation of regulatory T cells. In this Review, we discuss our current understanding of the mTOR pathway and the consequences of mTOR inhibition, both in DCs and T cells, including new data on the regulation of forkhead box P3 expression.

Rapamycin prevents and breaks the anti-CD3-induced tolerance in NOD mice

OBJECTIVE

Non-Fc-binding anti-CD3-specific antibodies represent a promising therapy for preserving C-peptide production in subjects with recent-onset type 1 diabetes. However, the mechanisms by which anti-CD3 exerts its beneficial effect are still poorly understood, and it is questionable whether this therapeutic approach will prove durable with regard to its ability to impart metabolic preservation without additional actions designed to maintain immunological tolerance. We used the NOD mouse model to test whether rapamycin, a compound well-known for its immunomodulatory activity in mice and humans, could increase the therapeutic effectiveness of anti-CD3 treatment in type 1 diabetes.

RESEARCH DESIGN AND METHODS

Rapamycin was administered to diabetic NOD mice simultaneously with anti-CD3 or to NOD mice cured by anti-CD3 therapy. The ability of this combined therapy to revert type 1 diabetes and maintain a state of long-term tolerance was monitored and compared with that of anti-CD3 therapy alone.

RESULTS

Rapamycin inhibited the ability of anti-CD3 to revert disease without affecting the frequency/phenotype of T-cells. Rapamycin also reinstated diabetes in mice whose disease was previously reversed by anti-CD3. Withdrawal of rapamycin in these latter animals promptly restored a normoglycemic state.

CONCLUSIONS

Our findings indicate that, when combined with anti-CD3, rapamycin exerts a detrimental effect on the disease outcome in NOD mice for as long as it is administered. These results suggest strong caution with regard to combining these treatments in type 1 diabetic patients.

Use of rapamycin in the induction of tolerogenic dendritic cells

Rapamycin (RAPA), a macrocyclic triene antibiotic pro-drug, is a clinically-utilized 'tolerance-sparing' immunosuppressant that inhibits the activity of T, B, and NK cells. Furthermore, maturation-resistance and tolerogenic properties of dendritic cells (DC) can be supported and preserved by conditioning with RAPA. Propagation of murine bone marrow (BM)-derived myeloid DC (mDC) in clinically relevant concentrations of RAPA (RAPA-DC) generates phenotypically immature DC with low levels of MHC and significantly reduced co-stimulatory molecules (especially CD86), even when exposed to inflammatory stimuli. RAPA-DC are weak stimulators of T cells and induce hyporesponsiveness and apoptosis in allo-reactive T cells. An interesting observation has been that RAPA-DC retain the ability to stimulate and enrich the regulatory T cells (Treg). Presumably as a result of these properties, alloantigen (alloAg)-pulsed recipient-derived DC are effective in subverting anti-allograft immune responses in rodent transplant models, making them an attractive subject for further investigation of their tolerance-promoting potential.

CD8+ T-Cell depletion and rapamycin synergize with combined coreceptor/stimulation blockade to induce robust limb allograft tolerance in mice

The growing development of composite tissue allografts (CTA) highlights the need for tolerance induction protocols. Herein, we developed a mouse model of heterotopic limb allograft in a stringent strain combination in which potentially tolerogenic strategies were tested taking advantage of donor stem cells in the grafted limb. BALB/c allografts were transplanted into C57BL/6 mice treated with anti-CD154 mAb, nondepleting anti-CD4 combined to either depleting or nondepleting anti-CD8 mAbs. Some groups received additional rapamycin. Both depleting and nondepleting mAb combinations without rapamycin only delayed limb allograft rejection, whereas the addition of rapamycin induced long-term allograft survival in both combinations. Nevertheless, robust donor-specific tolerance, defined by the acceptance of a fresh donor-type skin allograft and simultaneous rejection of third-party grafts, required initial CD8(+) T-cell depletion. Mixed donor-recipient chimerism was observed in lymphoid organs and recipient bone marrow of tolerant but not rejecting animals. Tolerance specificity was confirmed by the inability to produce IL-2, IFN-gamma and TNF-alpha in MLC with donor antigen while significant alloreactivity persisted against third- party alloantigens. Collectively, these results show that robust CTA tolerance and mixed donor-recipient chimerism can be achieved in response to the synergizing combination of rapamycin, transient CD8(+) T-cell depletion and costimulation/coreceptor blockade.

Effects of immunosuppressive drugs on purified human B cells: evidence supporting the use of MMF and rapamycin

BACKGROUND

Humoral immunity is increasingly recognized as an important factor in the rejection of organ transplants. In general, humoral rejection is treated with standard immunosuppressive drugs. The direct effect of these immunosuppressive drugs on B cells is not well known.

METHODS

Purified human B cells devoid of T cells were stimulated with CD40L expressing L cells, or by anti-CD40 mAb with or without Toll-like receptor triggering, all in the presence of B-cell activating cytokines. These three protocols resulted in various degrees of B-cell stimulation. We added four commonly used immunosuppressive drugs (tacrolimus, cyclosporin, mycophenolic acid [MPA], and rapamycin) to these cultures and tested a variety of parameters of B-cell activity including proliferation, apoptosis induction, and both IgM and IgG production.

RESULTS

Tacrolimus and cyclosporin marginally inhibited B-cell proliferation and immunoglobulin production, and the extent of inhibition depended on the degree of the B-cell stimulation. In contrast, MPA and rapamycin profoundly inhibited both B-cell proliferation and immunoglobulin production, which was independent of the degree of B-cell stimulation. Both drugs induced B-cell apoptosis. Moreover, rapamycin caused a reduction in the number of B cells capable of producing immunoglobulins.

CONCLUSIONS

Our data show that MPA and rapamycin are capable of strongly inhibiting B cells responses. This provides a rationale for the use of both MPA and rapamycin to prevent or counteract humoral responses.

Ethylenecarbodiimide-coupled allogeneic antigen presenting cells induce human CD4+ regulatory T cells

Adoptive transfer of naturally occurring CD4(+)CD25(+) regulatory T cells can tolerize transplantation alloresponses in animal models. However isolation of these cells in sufficient numbers from humans is cumbersome and prone to contamination with alloreactive CD25(+) T cells. Incubation of ethylenecarbodiimide-coupled antigen presenting cells (APC) with naïve T cells and antigen has been shown to induce tolerance in various experimental models. We therefore investigated whether ECDI-coupled allogeneic APC were able to induce an expandable human CD4(+) Treg population. CD4(+) and CD4(+) CD25(-) cells cultured for 5 days with ECDI-treated human PBMC exhibited potent suppressive capacity in a mixed lymphocyte reaction. Induction of these ECDI-Tregs was associated with up-regulation of Foxp3 mRNA and protein expression and they maintained high expression of CD62L and CD27 as well as low CD127 expression. ECDI-treated APC displayed reduced expression of the co-stimulatory signaling molecules CD40 and CD80, and failed to stimulate proliferation and cytokine secretion in co-cultured CD4(+) T cells. Restimulation in the presence of rapamycin and hrIL-2 led to expansion of ECDI-Tregs with increasing Foxp3 levels and suppressive activity significantly higher than expanded naturally occurring CD4(+)CD25(+) Tregs. In summary these findings support the hypothesis that ECDI-coupled APC can convert naïve CD4(+) T cells into functional Tregs with different phenotypic characteristics than naturally occurring CD4(+)CD25(+) Tregs. These inducible Tregs could provide a novel approach that might facilitate the translation of ex vivo generated and expanded Tregs into clinical settings.

Immune response hinders therapy for lysosomal storage diseases

Enzyme replacement therapy (ERT) for the lysosomal storage disease mucopolysaccharidosis I (MPS I) involves i.v. injection of alpha-l-iduronidase, which can be taken up by cells throughout the body. While a significant immune response to ERT has been shown in patients with MPS I, little is known about what effect anti-enzyme antibodies have on treatment efficacy. In this issue of the JCI, Dickson et al. demonstrate that anti-enzyme antibodies inhibit enzyme uptake and substantially limit the therapeutic efficacy of ERT in canines with MPS I (see the related article beginning on page 2868). Furthermore, the induction of immune tolerance--via oral delivery of cyclosporine A and azathioprine for two months at the time of initiation of ERT with recombinant human alpha-L-iduronidase--improved enzyme uptake in organs. Therefore, transient immunosuppression may enhance ERT for lysosomal storage diseases.

Mechanisms underlying blockade of allograft acceptance by TLR ligands

Immune activation via TLRs is known to prevent transplantation tolerance in multiple animal models. To investigate the mechanisms underlying this barrier to tolerance induction, we used complementary murine models of skin and cardiac transplantation in which prolonged allograft acceptance is either spontaneous or pharmacologically induced with anti-CD154 mAb and rapamycin. In each model, we found that prolonged allograft survival requires the presence of natural CD4(+)Foxp3(+) T regulatory cells (Tregs), and that the TLR9 ligand CpG prevents graft acceptance both by interfering with natural Treg function and by promoting the differentiation of Th1 effector T cells in vivo. We further demonstrate that although Th17 cells differentiate from naive alloreactive T cells, these cells do not arise from natural Tregs in either CpG-treated or untreated graft recipients. Finally, we show that CpG impairs natural Treg suppressor capability and prevents Treg-dependent allograft acceptance in an IL-6-independent fashion. Our data therefore suggest that TLR signals do not prevent prolonged graft acceptance by directing natural Tregs into the Th17 lineage or by using other IL-6-dependent mechanisms. Instead, graft destruction results from the ability of CpG to drive Th1 differentiation and interfere with immunoregulation established by alloreactive natural CD4(+)Foxp3(+) Tregs.

Rapamycin in islet transplantation: friend or foe?

The Edmonton protocol was undoubtedly a major step forward in the history of islet transplantation. Its immunosuppression regimen was largely based on the mTOR inhibitor rapamycin (sirolimus), which remains the most frequently used immunosuppressive drug in clinical islet transplant protocols. As time reveals the somewhat disappointing long-term results achieved with the Edmonton protocol, a number of publications have appeared addressing the potential beneficial or deleterious role of rapamycin on islet cell engraftment, function survival and regeneration, as well as on its side-effects in human subjects. This paper reviews the sometimes contradictory evidence on the impact of rapamycin in islet transplantation.

Neutralizing antibodies to therapeutic enzymes: considerations for testing, prevention and treatment

Lysosomal storage diseases are characterized by deficiencies in lysosomal enzymes, allowing accumulation of target substrate in cells and eventually causing cell death. Enzyme replacement therapy is the principal treatment for most of these diseases. However, these therapies are often complicated by immune responses to the enzymes, blocking efficacy and causing severe adverse outcomes by neutralizing product activity. It is thus crucial to understand the relationships between genetic mutations, endogenous residual enzyme proteins (cross-reactive immunologic material), development of neutralizing antibodies and their impact on clinical outcomes of lysosomal storage diseases. For patients in whom neutralizing antibodies may cause severe adverse clinical outcomes, it is paramount to develop tolerance inducing protocols to preclude, where predictable, or treat such life-threatening responses.

IL-1beta-driven ST2L expression promotes maturation resistance in rapamycin-conditioned dendritic cells

Maturation resistance and tolerogenic properties can be conferred on human and murine dendritic cells (DC), crucial regulators of T cell responses, by exposure to rapamycin (RAPA), a "tolerance-sparing" immunosuppressive agent. Mechanisms underlying this acquired unresponsiveness, typified by diminished functional responses to TLR or CD40 ligation, have not been identified. We report that in vitro and in vivo conditioning of murine myeloid DC with RAPA elicits the de novo production of IL-1beta by otherwise phenotypically immature DC. Interestingly, IL-1beta production promotes overexpression of the transmembrane form of the IL-1R family member, IL-1R-like 1, also know as ST2 on RAPA-conditioned DC (RAPA-DC). ST2 is the recently identified receptor for IL-33, a cytokine favoring Th2 responses. In addition, transmembrane ST2, or ST2L, has been implicated as a potent negative regulator of TLR signaling. RAPA-DC generated from ST2-/- mice exhibited higher levels of costimulatory molecules (CD86) than wild-type RAPA-DC. Consistent with its regulatory function, IL-1beta-induced ST2L expression suppressed the responsiveness of RAPA-DC to TLR or CD40 ligation. Thus, as a result of their de novo production of IL-1beta, RAPA-DC up-regulate ST2L and become refractory to proinflammatory, maturation-inducing stimuli. This work identifies a novel mechanism through which a clinically important immunosuppressant impedes the capacity of DC to mature and consequently stimulate effector/adaptive T cell responses.

Induction of human CD4+ regulatory T cells by mycophenolic acid-treated dendritic cells

Depending on their degree of maturation, costimulatory molecule expression, and cytokine secretion, dendritic cells (DC) can induce immunity or tolerance. DC treated with mycophenolic acid during their maturation (MPA-DC) have a regulatory phenotype and may therefore provide a new approach to induce allograft tolerance. Purified CD4(+) T cells stimulated in a human in vitro model of mixed culture by allogeneic MPA-DC displayed much weaker proliferation than T cells activated by mature DC and were anergic. This hyporesponsiveness was alloantigen-specific. Interestingly, T cells stimulated by MPA-DC during long-term coculture in four 7-day cycles displayed potent, suppressive activity, as revealed by marked inhibition of the proliferation of naive and preactivated control T cells. These regulatory T cells (Tregs) appeared to have antigen specificity and were contact-dependent. Tregs induced by MPA-DC were CD25(+)glucocorticoid-induced TNFR(+)CTLA-4(+)CD95(+), secreted IL-5 and large amounts of IL-10 and TGF-beta, and displayed enhanced forkhead box p3 expression. These results obtained in vitro demonstrate that human MPA-DC can induce allospecific Tregs that may be exploited in cell therapy to induce allograft tolerance.