Regulatory T Cells in Transplantation: Transferring Mouse Studies to the Clinic

Preventing alloimmune rejection using circular RNA FSCN1-silenced dendritic cells in heart transplantation

BACKGROUND

While heart transplantation is used as a standard treatment for heart failure, transplant rejection continues to pose a challenge. Recent evidence has shown that circular RNA (circRNA) is a new type of gene regulator in cell development. Our aim was to demonstrate that treatment with tolerogenic dendritic cells (Tol-DCs) generated by circular RNA FSCN1 (circFSCN1) silencing could prevent alloimmune rejection and prolong heart graft survival in heart transplantation.

METHODS

Bone marrow-derived DCs were transfected with circFSCN1 siRNA in vitro. The circFSCN1 level was measured by qRT-PCR. DC maturation was determined by flow cytometry. Mixed lymphocyte reactions (MLRs) were conducted to assess the function of DCs to activate T cells and to generate regulatory T cells (Tregs). In situ RNA hybridization and fluorescent microscopy were performed to detect the distribution of circFSCN1 in DCs. A heterotopic allogeneic murine heart transplantation was conducted where recipients were pre-treated with donor derived circFSCN1-silenced Tol-DCs. Heartbeat was monitored to assess immune rejection.

RESULTS

Exonic circFSCN1 was highly expressed in the cytoplasm of mature DCs. Knockdown of circFSCN1 using siRNA arrested DCs at an immature state, impaired DC's ability to activate T cells and enhanced Treg generation. Treatment with circFSCN1-silenced Tol-DCs prevented alloimmune rejection, prolonged allograft survival, reduced fibrosis, and induced Tregs in vivo.

CONCLUSIONS

Knockdown of circFSCN1 induces Tol-DCs and treatment with these Tol-DCs prevents alloimmune rejection and prolongs allograft survival. This is a promising therapeutic target to combat transplant rejection in heart transplantation and increases our understanding of circRNA in the immune system.

Regulatory T Cells for the Induction of Transplantation Tolerance

Organ transplantation is the optimal treatment for terminal and irreversible organ failure. Achieving transplantation tolerance has long been the ultimate goal in the field of transplantation. Regulatory T cell (Treg)-based therapy is a promising novel approach for inducing donor organ-specific tolerance. Tregs play critical roles in the maintenance of immune homeostasis and self-tolerance, by promoting transplantation tolerance through a variety of mechanisms on different target cells, including anti-inflammatory cytokine production, induction of apoptosis, disruption of metabolic pathways, and mutual interaction with dendritic cells. The continued success of Treg-based therapy in the clinical setting is critically dependent on preclinical studies that support its translational potential. However, although some initial clinical trials of adoptive Treg therapy have successively demonstrated safety and efficacy for immunosuppressant minimization and transplantation tolerance induction, most Treg-based hematopoietic stem cell and solid organ clinical trials are still in their infancy. These clinical trials have not only focused on safety and efficacy but also included optimization and standardization protocols of good manufacturing practice regarding cell isolation, expansion, dosing, timing, specificity, quality control, concomitant immunosuppressants, and post-administration monitoring. We herein report a brief introduction of Tregs, including their phenotypic and functional characterization, and focus on the clinical translation of Treg-based therapeutic applications in the setting of transplantation.

Tregs and Mixed Chimerism as Approaches for Tolerance Induction in Islet Transplantation

Pancreatic islet transplantation is a promising method for the treatment of type 1 and type 3 diabetes whereby replacement of islets may be curative. However, long-term treatment with immunosuppressive drugs (ISDs) remains essential for islet graft survival. Current ISD regimens carry significant side-effects for transplant recipients, and are also toxic to the transplanted islets. Pre-clinical efforts to induce immune tolerance to islet allografts identify ways in which the recipient immune system may be reeducated to induce a sustained transplant tolerance and even overcome autoimmune islet destruction. The goal of these efforts is to induce tolerance to transplanted islets with minimal to no long-term immunosuppression. Two most promising cell-based therapeutic strategies for inducing immune tolerance include T regulatory cells (Tregs) and donor and recipient hematopoietic mixed chimerism. Here, we review preclinical studies which utilize Tregs for tolerance induction in islet transplantation. We also review myeloablative and non-myeloablative hematopoietic stem cell transplantation (HSCT) strategies in preclinical and clinical studies to induce sustained mixed chimerism and allograft tolerance, in particular in islet transplantation. Since Tregs play a critical role in the establishment of mixed chimerism, it follows that the combination of Treg and HSCT may be synergistic. Since the success of the Edmonton protocol, the feasibility of clinical islet transplantation has been established and nascent clinical trials testing immune tolerance strategies using Tregs and/or hematopoietic mixed chimerism are underway or being formulated.

Manipulation of Regulatory Dendritic Cells for Induction Transplantation Tolerance

Current organ transplantation therapy is life-saving but accompanied by well-recognized side effects due to post-transplantation systematic immunosuppressive treatment. Dendritic cells (DCs) are central instigators and regulators of transplantation immunity and are responsible for balancing allograft rejection and tolerance. They are derived from monocyte-macrophage DC progenitors originating in the bone marrow and are classified into different subsets based on their developmental, phenotypical, and functional criteria. Functionally, DCs instigate allograft immunity by presenting donor antigens to alloreactive T cells via direct, indirect, and semidirect recognition pathways and provide essential signaling for alloreactive T cell activation via costimulatory molecules and pro-inflammatory cytokines. Regulatory DCs (DCregs) are characterized by a relatively low expression of major histocompatibility complex, costimulatory molecules, and altered cytokine production and exert their regulatory function through T cell anergy, T cell deletion, and regulatory T cell induction. In rodent transplantation studies, DCreg-based therapy, by in situ targeting or infusion of ex vivo generated DCregs, exhibits promising potential as a natural, well-tolerated, organ-specific therapeutic strategy for promoting lasting organ-specific transplantation tolerance. Recent early-phase studies of DCregs have begun to examine the safety and efficacy of DCreg-induced allograft tolerance in living-donor renal or liver transplantations. The present review summarizes the basic characteristics, function, and translation of DCregs in transplantation tolerance induction.

Impact of activated invariant natural killer T cells on the expansion of regulatory T cell precursors in murine thymocytes in vitro

Tolerance induction is a goal of clinical transplantation to prevent graft rejection without the lifelong use of immunosuppressive drugs. In a series of mouse studies, we previously reported that the establishment of mixed chimerism by treatment with a ligand for invariant natural killer T (iNKT) cells with CD40 signal blockade makes it possible to prevent allograft rejection without immunosuppressants, and this approach fails in thymectomized recipient mice. In this study, we showed that iNKT cells in murine thymocyte cultures are indispensable for the expansion of CD4⁺CD25⁺Foxp3⁺ regulatory T (Treg) cells as well as CD4⁺CD25⁺Foxp3^- cells, which contained precursor Tregs (preTregs). After the culture of BALB/c mouse-derived thymocytes in the presence of α-galactosylceramide (α-GalCer), a representative ligand for iNKT cells, the ratio of CD4⁺CD25⁺Foxp3^- preTregs to total CD4⁺CD8^- T cells was much higher than that of CD4⁺CD25⁺Foxp3⁺ Treg cells, regardless of anti-CD40 L mAb treatment. The proliferation of CD4⁺CD25⁺Foxp3^- cells, but not Treg cells, was significantly augmented, and the stability of Treg cells was not affected by α-GalCer. The expansion of thymocyte-derived Tregs was not inhibited by cytokine neutralization. However, in vitro thymus-derived CD4⁺CD25⁺Foxp3^- cells expressed Foxp3 after IL-2 stimulation in a dose-dependent manner. These results collectively suggest that in vitro thymus-derived Treg cell expansion by α-GalCer treatment was caused by the proliferation of CD4⁺CD25⁺Foxp3^- preTregs but not existing Treg cells.

Adoptive Transfers of CD4+CD25+ Tregs Raise Foxp3 Expression and Alleviate Mouse Enteritis

CD4⁺CD25⁺Foxp3⁺ Tregs control the immune response and maintain immune homeostasis. This study examined whether Tregs can affect mouse enteritis and the Foxp3 (Forkhead transcription factor) transcriptional pathway. Mouse CD4⁺CD25⁺ Treg cells were labelled using CFSE (5,6-carboxyfluorescein diacetate succinimidyl ester) and transferred to enteritis model mice. The mice were randomly divided into an enteritis group, a Treg-infusion group, a Treg-inhibiting group, and a control group. Histopathology, ELISA, flow cytometry, western blot, immunohistochemistry, and immunofluorescence were performed. Our results demonstrated that CD4⁺CD25⁺ Tregs were successfully transferred. The disease activity index (DAI) scores in the Tregs-infusion group were lower than those of the enteritis and Tregs-inhibiting groups. The number of goblet cells and inflammatory cells was reduced, and the levels of IL-1β, TNF-α, NO, and PGE2 were significantly decreased in the Tregs-infusion group compared to those in the enteritis group (p<0.05). The number of CD4⁺CD25⁺Foxp3⁺ Tregs and CD4⁺IL-17A⁺ Th17 cells in the mesenteric lymph nodes differed significantly from the enteritis and Tregs-inhibiting groups (p<0.05). There were more Foxp3⁺ Tregs and Smad3 and NFAT2 infiltrated into the duodenum after adoptive transfer of CD4⁺CD25⁺ Tregs, which was a significant difference relative to the enteritis group (p<0.05). This study demonstrated that adoptive transfer of CD4⁺CD25⁺ Tregs can decrease mouse enteritis. Foxp3 expression may be improved through the Smad3 and NFAT2 signalling pathways.

Soluble fibrinogen like protein 2 (sFGL2), the novel effector molecule for immunoregulation

Soluble fibrinogen-like protein 2 (sFGL2) is the soluble form of fibrinogen-like protein 2 belonging to the fibrinogen-related protein superfamily. It is now well characterized that sFGL2 is mainly secreted by regulatory T cell (Treg) populations, and exerts potently immunosuppressive activities. By repressing not only the differentiation and proliferation of T cells but also the maturation of dendritic cells (DCs), sFGL2 acts largely as an immunosuppressant. Moreover, sFGL2 also induces apoptosis of B cells, tubular epithelial cells (TECs), sinusoidal endothelial cells (SECs), and hepatocytes. This mini-review focuses primarily on the recent literature with respect to the signaling mechanism of sFGL2 in immunomodulation, and discusses the clinical implications of sFGL2 in transplantation, hepatitis, autoimmunity, and tumors.

Harnessing Advances in T Regulatory Cell Biology for Cellular Therapy in Transplantation

Cellular therapy with CD4FOXP3 T regulatory (Treg) cells is a promising strategy to induce tolerance after solid-organ transplantation or prevent graft-versus-host disease after transfer of hematopoietic stem cells. Treg cells currently used in clinical trials are either polyclonal, donor- or antigen-specific. Aside from variations in isolation and expansion protocols, however, most therapeutic Treg cell-based products are much alike. Ongoing basic science work has provided considerable new insight into multiple facets of Treg cell biology, including their stability, homing, and functional specialization; integrating these basic science discoveries with clinical efforts will support the development of next-generation therapeutic Treg cells with enhanced efficacy. In this review, we summarize recent advances in knowledge of how Treg cells home to lymphoid and peripheral tissues, and control antibody production and tissue repair. We also discuss newly appreciated pathways that modulate context-specific Treg cell function and stability. Strategies to improve and tailor Treg cells for cell therapy to induce transplantation tolerance are highlighted.

Roles of the tacrolimus-dependent transcription factor IRF4 in acute rejection after liver transplantation

Acute rejection is a serious and life-threatening complication of liver transplantation (LTx). Tacrolimus (TAC) is a potent immunosuppressant used in experimental and clinical transplantation. Interferon regulatory factor 4 (IRF4) plays key roles as a transcription factor in the immune response. This study explored the role of IRF4 in acute rejection after LTx using TAC treatment. Here, LTx was performed in DA (RT1(n)) and Lewis (LEW) (RT1(l)) rats. The recipients were immunosuppressed with TAC (1.5mg/kg/day subcutaneously) or saline. Liver grafts were harvested 1, 3, 5, 7, and 10 days after LTx for histology, immunohistochemistry, western blotting and real-time PCR. Splenic mononuclear cells were activated with different doses of TAC. The nuclear factor of activated T cells (NFAT) signal pathway and CD4+ T subset-related transcription factors were assessed. The results showed that TAC treatment prolonged the survival of liver allografts in recipients, significantly attenuated hepatic tissue injury and improved liver function. IRF4 expression in grafts was down-regulated after TAC treatment. TAC inhibited the expression of IRF4, NFAT, Foxp3 and RORγt in splenic mononuclear cells in vitro. In conclusions, our studies showed that TAC attenuated acute rejection responses after LTx. This attenuation might depend on the TAC-NFAT-IRF4 signal pathway, which is crucial for the function of T helper subsets (Treg and Th17 cells) in acute rejection after LTx. These findings contribute to our understanding of the immune pharmacological mechanism of TAC to prevent rejection in LTx rats.

The ratio of circulating regulatory T cells (Tregs)/Th17 cells is associated with acute allograft rejection in liver transplantation

CD4⁺CD25⁺FoxP3⁺ regulatory T cells (Tregs) and Th17 cells are known to be involved in the alloreactive responses in organ transplantation, but little is known about the relationship between Tregs and Th17 cells in the context of liver alloresponse. Here, we investigated whether the circulating Tregs/Th17 ratio is associated with acute allograft rejection in liver transplantation. In present study, thirty-eight patients who received liver transplant were enrolled. The patients were divided into two groups: acute allograft rejection group (Gr-AR) (n = 16) and stable allograft liver function group (Gr-SF) (n = 22). The frequencies of circulating Tregs and circulating Th17 cells, as well as Tregs/Th17 ratio were determined using flow cytometry. The association between Tregs/Th17 ratio and acute allograft rejection was then analyzed. Our results showed that the frequency of circulating Tregs was significantly decreased, whereas the frequency of circulating Th17 cells was significantly increased in liver allograft recipients who developed acute rejection. Tregs/Th17 ratio had a negative correlation with liver damage indices and the score of rejection activity index (RAI) after liver transplantation. In addition, the percentages of CTLA-4⁺, HLA-DR⁺, Ki67⁺, and IL-10⁺ Tregs were higher in Gr-SF group than in Gr-AR group. Our results suggested that the ratio of circulating Tregs/Th17 cells is associated with acute allograft rejection, thus the ratio may serve as an alternative marker for the diagnosis of acute rejection.

Prevention of acute liver allograft rejection by IL-10-engineered mesenchymal stem cells

Hepatic allograft rejection remains a challenging problem, with acute rejection episode as the major barrier for long-term survival in liver transplant recipients. To explore a strategy to prevent allograft rejection, we hypothesized that mesenchymal stem cells (MSCs) genetically engineered with interleukin-10 (IL-10) could produce beneficial effects on orthotopic liver transplantation (OLT) in the experimental rat model. Syngeneic MSCs transduced with IL-10 were delivered via the right jugular vein 30 min post-orthotopic transplantation in the rat model. To evaluate liver morphology and measure cytokine concentration, the blood and liver samples from each animal group were collected at different time-points (3, 5 and 7 days) post-transplantation. The mean survival time of the rats treated with MSCs-IL-10 was shown to be much longer than those treated with saline. According to Banff scheme grading, the saline group scores increased significantly compared with those in the MSCs-IL-10 group. Retinoid acid receptor-related orphan receptor gamma t (RORγt) expression was more increased in the saline group compared to those in the MSCs-IL-10 group in a time-dependent manner; forkhead box protein 3 (FoxP3) expression also decreased significantly in the saline group compared with those in the MSCs-IL-10 group in a time-dependent manner. The expression of cytokines [IL-17, IL-23, IL-6, interferon (IFN)-γ and tumour necrosis factor (TNF)-α] in the saline groups increased significantly compared with the time-point-matched MSCs-IL-10 group, whereas cytokine expression of (IL-10, TGF-β1) was deceased markedly compared to that in the MSCs-IL-10 group. These results suggest a potential role for IL-10-engineered MSC therapy to overcome clinical liver transplantation rejection.

Fox smell abrogates the effect of herbal odor to prolong mouse cardiac allograft survival

Background

Herbal medicines have unique odors, and the act of smelling may have modulatory effects on the immune system. We investigated the effect of olfactory exposure to Tokishakuyaku-san (TJ-23), a Japanese herbal medicine, on alloimmune responses in a murine model of cardiac allograft transplantation.

Methods

Naïve or olfactory-dysfunctional CBA mice underwent transplantation of a C57BL/6 heart and were exposed to the odor of TJ-23 until rejection. Some naïve CBA recipients of an allograft were given olfactory exposure to Sairei-to (TJ-114), trimethylthiazoline (TMT), individual components of TJ-23, or a TJ-23 preparation lacking one component. Adoptive transfer studies were performed to determine whether regulatory cells were generated.

Results

Untreated CBA mice rejected their C57BL/6 allografts acutely, as did olfactory-dysfunctional CBA mice exposed to the odor of TJ-23. CBA recipients of a C57BL/6 heart given olfactory exposure to TJ-23 had significantly prolonged allograft survival, whereas those exposed to the odor of TJ-114, TMT, one component of TJ-23, or TJ-23 lacking a component did not. Secondary allograft recipients that were given, at 30 days after transplantation, either whole splenocytes, CD4⁺ cells, or CD4⁺CD25⁺ cells from primary recipients exposed to the odor of TJ-23 had indefinitely prolonged allograft survival.

Conclusions

Prolonged survival of cardiac allografts and generation of regulatory cells was associated with exposure to the odor of TJ-23 in our model. The olfactory area of the brain may have a role in the modulation of immune responses.

Attenuation of donor-reactive T cells allows effective control of allograft rejection using regulatory T cell therapy

Regulatory T cells (Tregs) are essential for the establishment and maintenance of immune tolerance, suggesting a potential therapeutic role for Tregs in transplantation. However, Treg administration alone is insufficient in inducing long-term allograft survival in normal hosts, likely due to the high frequency of alloreactive T cells. We hypothesized that a targeted reduction of alloreactive T effector cells would allow a therapeutic window for Treg efficacy. Here we show that preconditioning recipient mice with donor-specific transfusion followed by cyclophosphamide treatment deleted 70-80% donor-reactive T cells, but failed to prolong islet allograft survival. However, infusion of either 5 × 10(6) Tregs with direct donor reactivity or 25 × 10(6) polyclonal Tregs led to indefinite survival of BALB/c islets in more than 70% of preconditioned C57BL/6 recipients. Notably, protection of C3H islets in autoimmune nonobese diabetic mice required islet autoantigen-specific Tregs together with polyclonal Tregs. Treg therapy led to significant reduction of CD8(+) T cells and concomitant increase in endogenous Tregs among graft-infiltrating cells early after transplantation. Together, these results demonstrate that reduction of the donor-reactive T cells will be an important component of Treg-based therapies in transplantation.

Regulatory T-cell therapy for transplantation: how many cells do we need?

PURPOSE OF REVIEW

As regulatory T-cell (Treg) therapy begins to enter the clinic and more clinical trials of Treg therapy are being actively planned for solid organ transplantations, a thorough quantitative assessment of therapeutic dosing is essential for the design of an effective Treg-therapy trial in the solid organ transplant setting.

RECENT FINDINGS

Considering the requirement for a high percentage of Tregs to control transplant rejection in mouse models of transplantation and the total cellularity of the human T-cell compartment, we estimate that it would take billions of Tregs, preferably alloantigen-reactive Tregs, to effectively control transplant rejection in humans. Donor dendritic cells and B cells can be used to selectively expand donor alloantigen-reactive Tregs. Recent improvements in manufacturing alloantigen-reactive Tregs demonstrate that billions of alloantigen-reactive T cells can be manufactured in short-term cultures.

SUMMARY

It is feasible to grow human alloantigen-reactive Tregs up to billions, an optimal number to achieve therapeutic efficacy. Better understanding of Treg lineage commitment and further technological investments are needed to ease the implementation and ensure consistency in Treg manufacturing.

Tolerogenic Donor-Derived Dendritic Cells Risk Sensitization In Vivo owing to Processing and Presentation by Recipient APCs

Modification of allogeneic dendritic cells (DCs) through drug treatment results in DCs with in vitro hallmarks of tolerogenicity. Despite these observations, using murine MHC-mismatched skin and heart transplant models, donor-derived drug-modified DCs not only failed to induce tolerance but also accelerated graft rejection. The latter was inhibited by injecting the recipient with anti-CD8 Ab, which removed both CD8(+) T cells and CD8(+) DCs. The discrepancy between in vitro and in vivo data could be explained, partly, by the presentation of drug-modified donor DC MHC alloantigens by recipient APCs and activation of recipient T cells with indirect allospecificity, leading to the induction of alloantibodies. Furthermore, allogeneic MHC molecules expressed by drug-treated DCs were rapidly processed and presented in peptide form by recipient APCs in vivo within hours of DC injection. Using TCR-transgenic T cells, Ag presentation of injected OVA-pulsed DCs was detectable for ≤ 3 d, whereas indirect presentation of MHC alloantigen by recipient APCs led to activation of T cells within 14 h and was partially inhibited by reducing the numbers of CD8(+) DCs in vivo. In support of this observation when mice lacking CD8(+) DCs were pretreated with drug-modified DCs prior to transplantation, skin graft rejection kinetics were similar to those in non-DC-treated controls. Of interest, when the same mice were treated with anti-CD40L blockade plus drug-modified DCs, skin graft survival was prolonged, suggesting endogenous DCs were responsible for T cell priming. Altogether, these findings highlight the risks and limitations of negative vaccination using alloantigen-bearing "tolerogenic" DCs.

Cell mediated rejection

Rejection is the major barrier to successful transplantation and usually results from the integration of multiple mechanisms. Activation of elements of the innate immune system, triggered as a consequence of tissue injury sustained during cell isolation or organ retrieval as well as ischemia-reperfusion, will initiate and amplify the adaptive response. For cell mediated rejection, T cells require multiple signals for activation, the minimum being two signals; antigen recognition and costimulation. The majority of B cells require help from T cells to initiate alloantibody production. Antibodies reactive to donor HLA molecules, minor histocompatibility antigens, endothelial cells, red blood cells, or autoantigens can trigger or contribute to rejection early as well as late after transplantation.

Translating tolerogenic therapies to the clinic - where do we stand?

Manipulation of the immune system to prevent the development of a specific immune response is an ideal strategy to improve outcomes after transplantation. A number of experimental techniques exploiting central and peripheral tolerance mechanisms have demonstrated success, leading to the first early phase clinical trials for tolerance induction. The first major strategy centers on the facilitation of donor-cell mixed chimerism in the transplant recipient with the use of bone marrow or hematopoietic stem cell transplantation. The second strategy, utilizing peripheral regulatory mechanisms, focuses on cellular therapy with regulatory T cells. This review examines the key studies and novel research directions in the field of immunological tolerance.

Role of regulatory T cells in the promotion of transplant tolerance

Liver transplantation is now recognized as the most effective therapy for patients with end-stage acute and chronic liver failure. Despite outstanding short-term graft and patient survival, liver transplantation continues to face several major challenges, including poor long-term graft survival due to chronic rejection and major side effects of long-term immunosuppressive therapy (which is required for the prevention of rejection). The ability to produce a state of tolerance after transplantation would potentially obviate long-term immunosuppression. Self-tolerance and immune homeostasis involve both central and peripheral immunoregulatory mechanisms. To date, studies have shown that many subsets of regulatory T cells (Tregs) control immune responses to foreign and alloantigens. The identification of Tregs that are positive for CD4, CD25, and the transcription factor forkhead box (Foxp3) has resulted in major advances in our understanding of the immunology of rejection and the development of transplant tolerance. In this article, we focus on the importance of Tregs in tolerance induction in experimental models of liver transplantation. Furthermore, we discuss the therapeutic potential of Tregs for the promotion of tolerance in transplant patients and highlight recent clinical trials of Treg-based therapies.

Mechanisms of rejection: current perspectives

Rejection is the major barrier to successful transplantation. The immune response to an allograft is an ongoing dialogue between the innate and adaptive immune system that if left unchecked will lead to the rejection of transplanted cells, tissues, or organs. Activation of elements of the innate immune system, triggered as a consequence of tissue injury sustained during cell isolation or organ retrieval and ischemia reperfusion, will initiate and amplify the adaptive response. T cells require a minimum of two signals for activation, antigen recognition, and costimulation. The activation requirements of naive T cells are more stringent than those of memory T cells. Memory T cells are present in the majority of transplant recipients as a result of heterologous immunity. The majority of B cells require help from T cells to initiate antibody production. Antibodies reactive to donor human leukocyte antigen molecules, minor histocompatibility antigens, endothelial cells, RBCs, or autoantigens can trigger or contribute to rejection early and late after transplantation. Antibody-mediated rejection triggered by alloantibody binding and complement activation is recognized increasingly as a significant contribution to graft loss. Even though one component of the immune system may dominate and lead to rejection being described in short hand as T cell or antibody mediated, it is usually multifactorial resulting from the integration of multiple mechanisms. Identifying the molecular pathways that trigger tissue injury, signal transduction and rejection facilitates the identification of targets for the development of immunosuppressive drugs.

Fates of CD4+ T cells in a tolerant environment depend on timing and place of antigen exposure

In experimental organ transplantation, tolerance is induced by administration of anti-CD40L mAb in conjunction with donor-specific splenocyte transfusion. Multiple, sometimes conflicting mechanisms of action resulting from this treatment have been reported. To resolve these issues, this study assessed the fates of graft reactive cells at different times and locations in the tolerant environment. Alloantigen-specific CD4(+) T cells transferred at time of tolerance induction (7 days before transplantation) became activated, expressed CD69 and CD44, and proliferated. Importantly, a large subset of this population became Foxp3(+) , more so in the lymph nodes than spleen, indicative of differentiation to a regulatory phenotype. In contrast, graft reactive CD4(+) T cells transferred to tolerogen-treated recipients at the time of transplantation failed either to proliferate or to differentiate, and instead were deleted via apoptosis. In untreated rejecting recipients graft reactive CD4(+) T cells became activated, proliferated and differentiated mainly in the spleen, and many of these cells were eventually deleted. These data resolve many apparent contradictions in the literature by showing that the timing of antigen exposure, the immunologic status of the recipients and secondary lymphoid organ location act together as key factors to determine the fate of graft reactive CD4(+) T cells.

CD4(+)Foxp3(+) regulatory T cell therapy in transplantation

Regulatory T cells (Tregs) are long-lived cells that suppress immune responses in vivo in a dominant and antigen-specific manner. Therefore, therapeutic application of Tregs to control unwanted immune responses is an active area of investigation. Tregs can confer long-term protection against auto-inflammatory diseases in mouse models. They have also been shown to be effective in suppressing alloimmunity in models of graft-versus-host disease and organ transplantation. Building on extensive research in Treg biology and preclinical testing of therapeutic efficacy over the past decade, we are now at the point of evaluating the safety and efficacy of Treg therapy in humans. This review focuses on developing therapy for transplantation using CD4(+)Foxp3(+) Tregs, with an emphasis on the studies that have informed clinical approaches that aim to maximize the benefits while overcoming the challenges and risks of Treg cell therapy.

Regulatory T cells for tolerance therapy: revisiting the concept

The discovery of regulatory T cells (Tregs) as a crucial component of peripheral down-regulation of immunity to self and allogeneic antigens has raised legitimate hope for the development of Treg-based clinical protocols for tolerance to allografts. The present review addresses the question of whether therapeutic Tregs are ready to enter the clinical transplantation arena. In light of recent experimental observations, we will revisit some fundamentals of T cell and Treg biology that stress the need for further studies prior to applications and provide conceptual cues for novel therapeutic approaches.

Human natural regulatory T cell development, suppressive function, and postthymic maturation in a humanized mouse model

CD4(+) regulatory T cells (Tregs) control adaptive immune responses and promote self-tolerance. Various humanized mouse models have been developed in efforts to reproduce and study a human immune system. However, in models that require T cell differentiation in the recipient murine thymus, only low numbers of T cells populate the peripheral immune systems. T cells are positively selected by mouse MHC and therefore do not function well in an HLA-restricted manner. In contrast, cotransplantation of human fetal thymus/liver and i.v. injection of CD34(+) cells from the same donor achieves multilineage human lymphohematopoietic reconstitution, including dendritic cells and formation of secondary lymphoid organs, in NOD/SCID mice. Strong Ag-specific immune responses and homeostatic expansion of human T cells that are dependent on peripheral human APCs occur. We now demonstrate that FOXP3(+)Helios(+) "natural" Tregs develop normally in human fetal thymic grafts and are present in peripheral blood, spleen, and lymph nodes of these humanized mice. Humanized mice exhibit normal reversal of CD45 isoform expression in association with thymic egress, postthymic "naive" to "activated" phenotypic conversion, and suppressive function. These studies demonstrate the utility of this humanized mouse model for the study of human Treg ontogeny, immunobiology and therapy.

Foxp3+-inducible regulatory T cells suppress endothelial activation and leukocyte recruitment

The ability of regulatory T cells (Treg) to traffic to sites of inflammation supports their role in controlling immune responses. This feature supports the idea that adoptive transfer of in vitro expanded human Treg could be used for treatment of immune/inflammatory diseases. However, the migratory behavior of Treg, as well as their direct influence at the site of inflammation, remains poorly understood. To explore the possibility that Treg may have direct anti-inflammatory influences on tissues, independent of their well-established suppressive effects on lymphocytes, we studied the adhesive interactions between mouse Treg and endothelial cells, as well as their influence on endothelial function during acute inflammation. We show that Foxp3(+) adaptive/inducible Treg (iTreg), but not naturally occurring Treg, efficiently interact with endothelial selectins and transmigrate through endothelial monolayers in vitro. In response to activation by endothelial Ag presentation or immobilized anti-CD3ε, Foxp3(+) iTreg suppressed TNF-α- and IL-1β-mediated endothelial selectin expression and adhesiveness to effector T cells. This suppression was contact independent, rapid acting, and mediated by TGF-β-induced activin receptor-like kinase 5 signaling in endothelial cells. In addition, Foxp3(+) iTreg adhered to inflamed endothelium in vivo, and their secretion products blocked acute inflammation in a model of peritonitis. These data support the concept that Foxp3(+) iTreg help to regulate inflammation independently of their influence on effector T cells by direct suppression of endothelial activation and leukocyte recruitment.

The potency of allospecific Tregs cells appears to correlate with T cell receptor functional avidity

CD4(+) CD25(+) regulatory T cells (T(reg) cells) are an attractive adoptive cell therapy in mediating transplantation tolerance. T-cell receptor (TcR) activation is critical for T(reg) function, suggesting that the TcR avidity of T(reg) cells used in therapy may affect the therapeutic outcome. To address this, we compared the regulatory capacity of T(reg) lines expressing TcRs derived from two TcR transgenic mice shown to have the same specificity but different functional avidities. T(reg) lines generated from CD4(+)CD25(+) T cells from C57BL/6 mice were transduced with one of either of these TcRs. The antigen specificity of the transduced T(reg) lines was confirmed in vitro. T(reg) lines expressing the TcR with higher functional avidity showed stronger suppressive capacity in a linked suppression model in vitro. Furthermore, the same T(reg) lines demonstrated a stronger proliferation in vivo following antigen exposure. Pretreatment of recipient BL/6 mice with these T(reg) cells, together with anti-CD8 antibody and Rapamycin therapies, prolonged survival of BALB/c skins, as compared with mice that received T(reg) lines with lower TcR avidity. Taken together, these data suggest that the TcR functional avidity may be important for T(reg) function. It highlights the fact that strategies to select T(reg) with higher functional avidity might be beneficial for immunotherapy in transplantation.

Th17: contributors to allograft rejection and a barrier to the induction of transplantation tolerance?

T helper (Th) type 17 cells are a recently described CD4 T-cell subset that may contribute to allograft rejection and act as a barrier to the induction of transplant tolerance. This review examines the involvement of Th17 cells in transplant rejection, how immunosuppressive medication may affect their induction and maintenance and the potential plasticity of developing Th17 cells. It also addresses the complex interplay between the Th17 and regulatory T-cell developmental pathways and the susceptibility of Th17 cells to regulation. Despite accumulating evidence, the precise impact of Th17 cells on transplant rejection and the induction of tolerance require further clarification.

Inhibiting CXCR3-dependent CD8+ T cell trafficking enhances tolerance induction in a mouse model of lung rejection

Lung transplantation remains the only effective therapy for patients with end-stage pulmonary diseases. Unfortunately, acute rejection of the lung remains a frequent complication and is an important cause of morbidity and mortality. The induction of transplant tolerance is thought to be dependent, in part, on the balance between allograft effector mechanisms mediated by effector T lymphocytes (Teff), and regulatory mechanisms mediated by FOXP3(+) regulatory T cells (Treg). In this study, we explored an approach to tip the balance in favor of regulatory mechanisms by modulating chemokine activity. We demonstrate in an adoptive transfer model of lung rejection that CXCR3-deficient CD8(+) Teff have impaired migration into the lungs compared with wild-type Teff, which results in a dramatic reduction in fatal pulmonary inflammation. The lungs of surviving mice contained tolerized CXCR3-deficient Teff, as well as a large increase in Treg. We confirmed that Treg were needed for tolerance and that their ability to induce tolerance was dependent on their numbers in the lung relative to the numbers of Teff. These data suggest that transplantation tolerance can be achieved by reducing the recruitment of some, but not necessarily all, CD8(+) Teff into the target organ and suggest a novel approach to achieve transplant tolerance.

Inflammation correlates with markers of T-cell subsets including regulatory T cells in adipose tissue from obese patients

Accumulation of cytotoxic and T-helper (T(h))1 cells together with a loss of regulatory T cells in gonadal adipose tissue was recently shown to contribute to obesity-induced adipose tissue inflammation and insulin resistance in mice. Human data on T-cell populations in obese adipose tissue and their potential functional relevance are very limited. We aimed to investigate abundance and proportion of T-lymphocyte sub-populations in human adipose tissue in obesity and potential correlations with anthropometric data, insulin resistance, and systemic and adipose tissue inflammation. Therefore, we analyzed expression of marker genes specific for pan-T cells and T-cell subsets in visceral and subcutaneous adipose tissue from highly obese patients (BMI >40 kg/m(2), n = 20) and lean to overweight control subjects matched for age and sex (BMI <30 kg/m(2); n = 20). All T-cell markers were significantly upregulated in obese adipose tissue and correlated with adipose tissue inflammation. Proportions of cytotoxic T cells and T(h)1 cells were unchanged, whereas those of regulatory T cells and T(h)2 were increased in visceral adipose tissue from obese compared to control subjects. Systemic and adipose tissue inflammation positively correlated with the visceral adipose abundance of cytotoxic T cells and T(h)1 cells but also regulatory T cells within the obese group. Therefore, this study confirms a potential role of T cells in human obesity-driven inflammation but does not support a loss of protective regulatory T cells to contribute to adipose tissue inflammation in obese patients as suggested by recent animal studies.

Mesenchymal stromal cells: facilitators of successful transplantation?

Mesenchymal stromal/stem cells (MSCs) possess immunomodulatory and reparative properties. Through specific interactions with immune cells that participate in both innate and adaptive responses, MSCs exposed to an inflammatory microenvironment can downregulate many immune effector functions. Clinical trials focusing on MSCs to treat graft-versus-host disease (GvHD) and autoimmune diseases are underway. Current analyses suggest that MSCs will improve cell and solid organ transplantation by ameliorating rejection and possibly eliminating the requirement for prolonged regimens of conventional immunosuppressive drugs. This review examines the in vitro and in vivo evidence for the clinical use of bone marrow derived MSCs.

Improving efficacy of clinical islet transplantation with iodixanol-based islet purification, thymoglobulin induction, and blockage of IL-1β and TNF-α

Poor efficacy is one of the issues for clinical islet transplantation. Recently, we demonstrated that pancreatic ductal preservation significantly improved the success rate of islet isolation; however, two transplants were necessary to achieve insulin independence. In this study, we introduced iodixanol-based purification, thymoglobulin induction, and double blockage of IL-1β and TNF-α as well as sirolimus-free immunosuppression to improve the efficacy of clinical islet transplantation. Nine clinical-grade human pancreata were procured. Pancreatic ductal preservation was performed using ET-Kyoto solution in all cases. When the isolated islets met the clinical criteria, they were transplanted. We utilized two methods of immunosuppression and anti-inflammation. The first protocol prescribed daclizumab for induction, then sirolimus and tacrolimus to maintain immunosuppression. The second protocol used thymoglobulin for induction and tacrolimus and mycophenolate mofetil to maintain immunosuppression. Eternacept and anakinra were administered as anti-inflammatory drugs. The total amount of insulin required, HbA1c, and the SUITO index were determined to analyze and compare the results of transplantation. All isolated islet preparations (9/9) met the criteria for clinical transplantation, and they were transplanted into six type 1 diabetic patients. All patients achieved insulin independence with normal HbA1c levels; however, the first protocol required two islet infusions (N = 3) and the second protocol only required a single infusion (N = 3). The average SUITO index, at 1 month after a single-donor islet transplantation, was significantly higher in the second protocol (49.6 ± 8.3 vs. 19.3 ± 6.3, p < 0.05). Pancreatic ductal preservation, iodixanol-based purification combined with thymoglobulin induction, and blockage of IL-1β and TNF-α as well as sirolimus-free immunosuppression dramatically improved the efficacy of clinical islet transplantations. This protocol enabled us to perform successful single-donor islet transplantations. Further large-scale studies are necessary to confirm these results and clarify the mechanism of each component.

The dynamic changes of Th17/Treg cytokines in rat liver transplant rejection and tolerance

Acute rejection is still a major cause of early graft loss and a risk factor for long-term recipient post-transplant survival. Recently, CD4(+) CD25(+) Foxp3(+) regulatory T (Treg) cells and Th17 cells have been described as two distinct subsets with opposing effects on autoimmunity and transplant immunity. We investigated the existence of Th17/Treg functional imbalance between tolerance and rejection groups during rat liver transplantation. Then, Th17/Treg functions on different levels were investigated comparatively between those two groups, including related cytokine secretion and key transcription factors. REJ groups revealed significant increase in Th17-related cytokine (IL-17, IL-6 and IL-23) and transcription factor (RORγt) levels and remarkable decrease in Treg-related cytokine (IL-10 and TGF-β1) and transcription factor (Foxp3) levels when compared to day-matched TOL groups from day 3 post-transplantation. Results indicated Th17/Treg functional imbalance between tolerance and rejection groups during rat liver transplantation, suggesting a potential role of Th17/Treg imbalance in pathogenesis of acute transplant rejection.

Ex vivo-expanded human regulatory T cells prevent the rejection of skin allografts in a humanized mouse model

BACKGROUND

Composite tissue transplantation effectively reconstructs the most complex defects, but its use is limited because of harmful immunosuppression and the high susceptibility of skin to rejection. Development of tolerance is an ideal solution, and protocols using regulatory T cells (Tregs) to achieve this have been promising in experimental animal models. The aim of this study was to investigate the ability of human Tregs to regulate immune responses to a human skin allograft in vivo.

METHODS

We isolated and expanded naturally occurring CD127loCD25+CD4+ human Tregs from peripheral blood mononuclear cells (PBMCs) and examined their phenotype and suppressive activity in vitro. Using a clinically relevant chimeric humanized mouse system, we transplanted mice with human skin grafts followed by allogeneic populations of PBMCs with or without Tregs derived from the same PBMC donor.

RESULTS

Ex vivo-expanded Tregs maintain the appropriate Treg markers and retain suppressive activity against allostimulated and polyclonally stimulated autologous PBMCs in vitro. Mice receiving allogeneic PBMCs alone consistently reject human skin grafts, whereas those also receiving Tregs display stable long-term human skin transplant survival along with a reduction in the CD8+ human cellular graft infiltrate.

CONCLUSIONS

We show for the first time the unique ability of human Tregs to prevent the rejection of a skin allograft in vivo, highlighting the therapeutic potential of these cells clinically.

Biomarkers as a tool for management of immunosuppression in transplant patients

Therapeutic drug monitoring is a well-established approach in transplantation medicine to guide immunosuppressive therapy. However, it cannot always predict the effects of immunosuppressive drugs on immune cells, because it does not reflect any aspect of an individual patient's immune system. Pharmacodynamic monitoring is a more recent strategy to provide information about the biologic effect of a specific drug or drug combination on the individual transplant patient. Currently, there is a large number of different biomarkers that either directly (specific markers) or indirectly (global markers) relate to the pharmacodynamic effects of immunosuppressive drugs and are under investigation as potential candidates to be introduced in clinical practice. Such biomarkers may be useful to identify patients at risk of developing acute rejection, infection, or cancer as well as patients who are suitable for minimization of immunosuppressant therapy and may be helpful to manage the timing and rate of immunosuppressant weaning. Serial longitudinal monitoring may allow maintenance of an individualized immunosuppressive regimen. Thus, biomarker monitoring is a potential complementary tool to therapeutic drug monitoring. This review summarizes the current state of knowledge about the use of a number of global or drug-specific pharmacodynamic biomarkers. It is not a comprehensive overview of the literature available, but rather an evidence-based reflection by experts who are intensively involved in scientific work in this field.

Diltiazem induces regulatory T cells in vitro by modulating human dendritic cell maturation

Diltiazem is a calcium channel antagonist that has been commonly associated with currently used immunosuppressants to prevent acute graft rejection in humans. In this study, we examined the possibility that diltiazem may affect human dendritic cell (DC) functions in response to lipopolysaccharide (LPS) stimulation and may induce the generation of DC with the capacity to generate CD4(+) regulatory T cells (Tregs). Blood monocytes were cultured in the presence of diltiazem at the beginning of their differentiation process into DC. Monocyte-derived DCs were stimulated with LPS, and DCs differentiated in the presence of diltiazem showed a decreased interleukin (IL)-12 production and an enhanced IL-10 production. When cultured with CD4(+) CD45RA(+) they were able to enhance the CD4(+) Foxp3(+) T-cell population and to induce slowly proliferating T cells, which showed a significant increase of transforming growth factor (TGF)-β production. These T cells suppress proliferation of activated autologous T cells, and we show that this effect is attributable to soluble factors, primarily to TGF-β. Blockade of TGF-β by specific monoclonal antibodies reversed this inhibitory effect. Herein, we provide new evidence that diltiazem-conditioned monocyte-derived DC induce T cells which acquire a regulatory phenotype and activity similar to those described for Tregs.

Immunomodulatory effects of eicosapentaenoic acid through induction of regulatory T cells

Dietary intake of omega-3 polyunsaturated fatty acids (PUFAs) has been found to affect inflammation and metabolism, and many researchers have shown that omega-3 PUFAs provide benefits in immunologic and metabolic disorders. These effects were assumed to result mainly from a modification in the production of inflammatory mediators and the suppression of inflammatory leukocytes. Among PUFAs, eicosapentaenoic acid (EPA), a component of fish oil, apparently has the most potent effect. Recently, much research has focused on regulatory T cells (Tregs) as controllers of immune responses not only to self-antigens but also to non-self-antigens, including donor alloantigens. Therefore, induction of antigen-specific Tregs may be an attractive strategy for managing autoimmune diseases and transplant rejection. Peroxisome proliferator-activated receptor γ (PPARγ), a ligand-activated nuclear receptor that regulates lipid and glucose metabolism, can be activated by thiazolidinediones, fatty acids, and eicosanoids, including EPA. PPARγ was recently found to have immunoregulatory effects, and a PPARγ agonist inhibited immune responses in a rat model of autoimmune disease. Furthermore, in a murine model, one high dose of purified EPA given the day of transplantation induced marked prolongation of cardiac allograft survival in a dose-dependent manner. These findings suggest that EPA induced Tregs by means of a PPARγ-dependent mechanism. This review describes the immunomodulatory effects of PUFAs, especially EPA, and summarizes recent research that may have implications for the development of therapies for autoimmune diseases and transplant rejection that are based on induction of Tregs.

Harnessing the diversity of the human T-cell repertoire: a monitoring tool for transplantation tolerance?

There is significant diversity in the TCR repertoire in heterologous and allogeneic immunity. A paper in this issue of the European Journal of Immunology shows that changes in the TCR repertoire can be correlated with outcomes of renal transplantation. This indicates that the diversity of the TCR repertoire could be utilized to monitor clinical phenotypes, such as chronic humoral rejection and operational tolerance in organ transplantation. However, prior to the clinical use of monitoring changes in the TCR repertoire as a biomarker to monitor clinical status after organ transplantation, many questions regarding time dependency, triggering factors and specificity need to be addressed. A causative role for TCR repertoire disruption in organ transplantation will need to be proven by interventional trials that can demonstrate that the TCR repertoire is modifiable and predictive of graft outcome. In this Commentary, we discuss the strengths and opportunities of TCR repertoire monitoring, and point towards yet unanswered questions that will become important if the Vβ CDR3-length distribution assay will be clinically applied.

Pharmacologic expansion of donor-derived, naturally occurring CD4(+)Foxp3(+) regulatory T cells reduces acute graft-versus-host disease lethality without abrogating the graft-versus-leukemia effect in murine models

Adoptive transfer of regulatory T cells (Tregs) prevents graft-versus-host disease (GVHD) in mouse models, indicating a pivotal role for Tregs in controlling GVHD. The present study demonstrates the efficacy of Tregs pharmacologically induced in vivo in GVHD prevention. A single i.v. administration of a liposomal formulation of α-galactosylceramide (RGI-2001) at the time of allogeneic bone marrow transplantation with spleen cells significantly prolonged the survival of mice experiencing lethal acute GVHD. RGI-2001 expanded donor-derived CD4(+)Foxp3(+) Tregs in the spleen, lymph nodes, and bone marrow in a dose-dependent manner. On day 15 posttransplantation, the spleens of mice treated with RGI-2001 (1 μg/kg) contained 5-fold higher percentages or 10-fold higher numbers of CD4(+)Foxp3(+) Tregs compared with the spleens of untreated mice. Host-specific immunosuppression was introduced in treated mice, whereas the responsiveness to third-party alloantigens and leukemia cells was maintained. Using Foxp3:GFP reporter mice as donors, it was clearly shown that RGI-2001 expanded the pre-existing naturally occurring Tregs (nTregs) in donor spleen cells. Finally, RGI-2001 synergized with a subtherapeutic dose of rapamycin in nTreg expansion and further prolonged survival. Our results provide the first demonstration of the efficacy of nTregs pharmacologically expanded in vivo in preventing acute GVHD without abrogation of the beneficial graft-versus-leukemia effect.

CD4+ regulatory T cells in solid organ transplantation

PURPOSE OF REVIEW

Solid organ transplantation is the most effective treatment for end-stage organ failure, but the long-term outcomes remain suboptimal. CD4 regulatory T cells (Tregs) are emerging as a potential therapy to facilitate long-term allograft survival. This review provides a general overview of the biology of CD4 Tregs and then goes on to discuss the most relevant and recent experimental and clinical evidence for their therapeutic use in solid organ transplantation.

RECENT FINDINGS

There have been major advances in our understanding of Tregs, including improvements in methods for their isolation and expansion. Experimental models are providing very important data on the in-vitro and in-vivo behavior of Tregs in transplantation, while recent clinical trials of Treg cellular therapy in graft-versus-host disease are offering a valuable insight into the efficacy of Treg adoptive cellular therapy.

SUMMARY

Data in favor of Treg cellular therapy in transplantation are mounting, and we predict that their use in clinical trials is on the horizon.

Adoptive transfer of DNT cells induces long-term cardiac allograft survival and augments recipient CD4(+)Foxp3(+) Treg cell accumulation

Regulatory T (Treg) cells play an important role in the regulation of immune responses but whether Treg will induce tolerance in transplant recipients in the clinic remains unknown. Our previous studies have shown that TCRαβ(+)CD3(+)CD4⁻CD8⁻NK1.1⁻ (double negative, DN) T cells suppress T cell responses and prolong allograft survival in a single locus MHC-mismatched mouse model. In this study, we investigated the role of DNT cells in a more robust, fully MHC-mismatched BALB/c to C57BL/6 transplantation model, which may be more clinically relevant. Adoptive transfer of DNT cells in combination with short-term rapamycin treatment (days 1-9) induced long-term heart allograft survival (101±31 vs. 39±13 days rapamycin alone, p<0.01). Furthermore adoptive transfer DNT cells augmented CD4+Foxp3+ Treg cells accumulation in transplant recipients while depletion of CD4(+) Treg cells by anti-CD25 inhibited the effect of DNT cells on long-term graft survival (48±12 days vs. 101±31 days, p<0.001). In conclusion, DNT cells combined with short-term immunosuppression can prolong allograft survival, which may be through the accumulation of CD4(+)Foxp3(+) Treg cells in the recipient. Our result suggests that allograft tolerance may require the co-existence of different type Treg cell phenotypes which are affected by current immunosuppression.

T regulatory cells and the control of alloimmunity: from characterisation to clinical application

T regulatory cells (Treg) play an important role in the induction and maintenance of immunological tolerance. Recent findings in experimental transplant models combined with the development of functional reporter mice have opened new avenues to study Treg biology and their therapeutic potential. In particular, recent advances in understanding Treg function and lineage stability revealed unexpected plasticity of this lineage. Nevertheless, pre-clinical and pilot clinical trials using Treg cells as cellular therapies have been initiated suggesting the safety and feasibility of such treatment.

The impact of Th17 cells on transplant rejection and the induction of tolerance

PURPOSE OF REVIEW

This review aims to provide an overview of the latest evidence for the involvement of Th17 cells in the rejection of solid organ allografts. It will also consider the implications of the relationship between the differentiation pathways of Th17 and regulatory T cells (Tregs), as well as their plasticity in the context of transplantation tolerance.

RECENT FINDINGS

In the absence of the Th1 lineage in vivo, Th17 cells are capable of rejecting cardiac allografts, showing the capacity of Th17 cells to cause allograft rejection, at least in experimental models. Th17 cells are relatively unsusceptible to suppression by Tregs, although this may be context dependent. Furthermore, addition of inflammatory signals to a Treg-inducing environment leads to Th17 development and established Tregs can be converted to Th17 cells under inflammatory conditions.

SUMMARY

The capacity of Th17 cells to cause allograft rejection is becoming increasingly clear. However, the role and contribution of Th17 cells in allograft rejection in the presence of the full orchestra of T helper cells remains elusive. The apparent resistance of Th17 to be suppressed by Tregs may pose a hurdle for effective immunosuppression and tolerance inducing protocols. Furthermore, the close developmental pathways of Th17 and Tregs and the ability of Tregs to convert into Th17 cells in the presence of inflammatory signals may impede the establishment of specific unresponsiveness to donor alloantigens in vivo.

Prognostic significance of regulatory T cells in tumor

Since entering the immunological stage several decades ago, regulatory T cell biology has been realized as fundamentally important in the prevention of autoimmune conditions, induction of transplant tolerance and the immune response to cancer. The role of regulatory T cells in tumor immunobiology is still being elucidated. Currently, regulatory T cells are implicated in the dampening of antitumor T-cell responses both through direct and indirect means. A number of investigators have demonstrated that regulatory T cell density and location may serve as independent prognostic factors in several types of cancer and are alternately detrimental or beneficial to patient survival. In this article, we will review the characteristics and functional phenotype of classical regulatory T cells, describe their distribution and quantification in tumor-bearing hosts and summarize recent studies investigating the prognostic significance of regulatory T cell number and locality in various cancers.