Sustained suppression by Foxp3+ regulatory T cells is vital for infectious transplantation tolerance

A Phase I Clinical Trial with Ex Vivo Expanded Recipient Regulatory T cells in Living Donor Kidney Transplants

There is considerable interest in therapeutic transfer of regulatory T cells (Tregs) for controlling aberrant immune responses. Initial clinical trials have shown the safety of Tregs in hematopoietic stem cell transplant recipients and subjects with juvenile diabetes. Our hypothesis is that infusion(s) of Tregs may induce transplant tolerance thus avoiding long-term use of toxic immunosuppressive agents that cause increased morbidity/mortality. Towards testing our hypothesis, we conducted a phase I dose escalation safety trial infusing billions of ex vivo expanded recipient polyclonal Tregs into living donor kidney transplant recipients. Despite variability in recipient’s renal disease, our expansion protocol produced Tregs which met all release criteria, expressing >98% CD4⁺CD25⁺ with <1% CD8⁺ and CD19⁺ contamination. Our product displayed >80% FOXP3 expression with stable demethylation in the FOXP3 promoter. Functionally, expanded Tregs potently suppressed allogeneic responses and induced the generation of new Tregs in the recipient’s allo-responders in vitro. Within recipients, expanded Tregs amplified circulating Treg levels in a sustained manner. Clinically, all doses of Treg therapy tested were safe with no adverse infusion related side effects, infections or rejection events up to two years post-transplant. This study provides the necessary safety data to advance Treg cell therapy to phase II efficacy trials.

T Cell Expression of C5a Receptor 2 Augments Murine Regulatory T Cell (TREG) Generation and TREG-Dependent Cardiac Allograft Survival

C5aR2 (C5L2/gp77) is a seven-transmembrane spanning receptor that binds to C5a but lacks motifs essential for G protein coupling and associated signal transduction. C5aR2 is expressed on immune cells, modulates various inflammatory diseases in mice, and has been shown to facilitate murine and human regulatory T cell (T_REG) generation in vitro. Whether and how C5aR2 impacts in vivo T_REG generation and pathogenic T cell-dependent disease models have not been established. In this article, we show that murine T cells express and upregulate C5aR2 during induced T_REG (iT_REG) generation and that the absence of T cell-expressed C5aR2 limits in vivo iT_REG generation following adoptive transfer of naive CD4⁺ T cells into Rag1^-/- recipients. Using newly generated C5aR2-transgenic mice, we show that overexpression of C5aR2 in naive CD4⁺ T cells augments in vivo iT_REG generation. In a model of T_REG-dependent cardiac allograft survival, recipient C5aR2 deficiency accelerates graft rejection associated with lower T_REG/effector T cell ratios, whereas overexpression of C5aR2 in immune cells prolongs graft survival associated with an increase in T_REG/effector T cell ratios. T cell-expressed C5aR2 modulates T_REG induction without altering effector T cell proliferation or cytokine production. Distinct from reported findings in neutrophils and macrophages, T_REG-expressed C5aR2 does not interact with β-arrestin or inhibit ERK1/2 signaling. Rather, cumulative evidence supports the conclusion that C5aR2 limits C5aR1-initiated signals known to inhibit T_REG induction. Together, the data expand the role of C5aR2 in adaptive immunity by providing in vivo evidence that T cell-expressed C5aR2 physiologically modulates iT_REG generation and iT_REG-dependent allograft survival.

Anti-donor regulatory T cell therapy in liver transplantation

Liver transplantation is accepted as the most reliable therapeutic option for patients with end-stage liver failure, but lifelong administration of immunosuppressive agents continues to be problematic due to various drug-induced morbidities and the risk of mortality. Complete cessation of immunosuppressive drugs while maintaining normal graft function and histology, called operational tolerance, has the potential to overcome these long-standing problems. Previously, we reported the results of a pilot study of anti- donor regulatory T cell therapy in 10 consecutive adult patients who underwent living donor liver transplantation (LDLT), of whom 7 patients successfully stopped immunosuppression for nearly 2 years. Described herein are the clinical follow-ups of these patients, a brief description of the protocol and its theoretical background, and a possible explanation for the immunological findings.

Route of Antigen Presentation Can Determine the Selection of Foxp3-Dependent or Foxp3-Independent Dominant Immune Tolerance

It has been shown that dominant tolerance, namely in transplantation, requires Foxp3⁺ regulatory T cells. Although most tolerance-inducing regimens rely on regulatory T cells, we found that induction of tolerance to proteins in aluminum hydroxide can be achieved in Foxp3-deficient mice using nondepleting anti-CD4 Abs. This type of tolerance is Ag specific, and tolerant mice retain immune competence to respond to unrelated Ags. We demonstrated with chicken OVA-specific TCR-transgenic mice that the same tolerizing protocol (CD4 blockade) and the same target Ag (OVA) achieves Foxp3-dependent transplantation tolerance to OVA-expressing skin grafts, but Foxp3-independent tolerance when the Ag is provided as OVA-aluminum hydroxide. In the latter case, we found that tolerance induction triggered recessive mechanisms leading to elimination of effector cells and, simultaneously, a dominant mechanism associated with the emergence of an anergic and regulatory CTLA-4⁺IL-2^lowFoxp3^- T cell population, where the tolerance state is IL-10 dependent. Such Foxp3-independent mechanisms can improve the efficacy of tolerance-inducing protocols.

Novel Concept of CD4-Mediated Activation of Regulatory T Cells for the Treatment of Graft-Versus-Host Disease

Allogeneic hematopoietic stem cell transplantation is the only curative treatment option for several hematological malignancies and immune deficiency syndromes. Nevertheless, the development of a graft-versus-host disease (GvHD) after transplantation is a high risk and a severe complication with high morbidity and mortality causing therapeutic challenges. Current pharmacological therapies of GvHD lead to generalized immunosuppression followed by severe adverse side effects including infections and relapse of leukemia. Several novel cell-based immunomodulatory strategies for treatment or prevention of GvHD have been developed. Herein, thymus-derived regulatory T cells (tTreg), essential for the maintenance of peripheral immunologic tolerance, are in the focus of investigation. However, due to the limited number of tTreg in the peripheral blood, a complex, time- and cost-intensive in vitro expansion protocol is necessary for the production of an efficient cellular therapeutic. We demonstrated that activation of tTreg using the CD4-binding human immunodeficiency virus-1 protein gp120 leads to a substantially increased suppressor activity of tTreg without the need for additional expansion. Gp120-activated tTreg prevent GvHD development in a preclinical humanized mouse model. In addition, gp120 is not only effective in prevention but also in therapy of GvHD by suppressing all clinical symptoms and improving survival of treated mice. These data indicate that tTreg activation by gp120 is a feasible and potent strategy for significant functional improvement of tTreg as cellular therapeutic for GvHD treatment without the need of complicated, time-intensive, and expensive in vitro expansion of isolated tTreg.

Cyclophosphamide-induced tolerance in kidney transplantation avoids long-term immunosuppressive therapy

There has recently been remarkable progress in immunosuppressive agents, such as tacrolimus and cyclosporine. Therefore, the rate of organ establishment has improved in transplantation. However, immunosuppressive agents generally suppress the function of T cells. Thus, opportunistic infections, such as cytomegalovirus infection, are still a major problem in kidney transplantation. Induction of specific tolerance to avoid immunosuppressive drug therapy after kidney transplantation is considered as the ultimate goal of transplantation. Various factors induce tolerance that involves establishment of hematopoietic chimerism and various cell subsets. In particular, we have carried out various studies regarding the cyclophosphamide-induced tolerance system. Tolerance is induced after establishment of hematopoietic chimerism after donor bone marrow transplantation. At the clinical stage, kidney transplantation before administration of cyclophosphamide after transfusion of bone marrow to create hematopoietic chimera is considered to be one of the most successful protocols. Furthermore, recent studies have shown the involvement of multiple populations of immune cells in preserving immunological tolerance and promoting long-term renal grafts. The present review focuses on how cyclophosphamide and other immune factors induce tolerance in kidney transplantation.

Mechanisms of Mixed Chimerism-Based Transplant Tolerance

Immune responses to allografts represent a major barrier in organ transplantation. Immune tolerance to avoid chronic immunosuppression is a critical goal in the field, recently achieved in the clinic by combining bone marrow transplantation (BMT) with kidney transplantation following non-myeloablative conditioning. At high levels of chimerism such protocols can permit central deletional tolerance, but with a significant risk of graft-versus-host (GVH) disease (GVHD). By contrast, transient chimerism-based tolerance is devoid of GVHD risk and appears to initially depend on regulatory T cells (Tregs) followed by gradual, presumably peripheral, clonal deletion of donor-reactive T cells. Here we review recent mechanistic insights into tolerance and the development of more robust and safer protocols for tolerance induction that will be guided by innovative immune monitoring tools.

Therapeutic application of T regulatory cells in composite tissue allotransplantation

With growing number of cases in recent years, composite tissue allotransplantation (CTA) has been improving the quality of life of patient who seeks reconstruction and repair of damaged tissues. Composite tissue allografts are heterogeneous. They are composed of a variety of tissue types, including skin, muscle, vessel, bone, bone marrow, lymph nodes, nerve, and tendon. As a primary target of CTA, skin has high antigenicity with a rich repertoire of resident cells that play pivotal roles in immune surveillance. In this regard, understanding the molecular mechanisms involved in immune rejection in the skin would be essential to achieve successful CTA. Although scientific evidence has proved the necessity of immunosuppressive drugs to prevent rejection of allotransplanted tissues, there remains a lingering dilemma due to the lack of specificity of targeted immunosuppression and risks of side effects. A cumulative body of evidence has demonstrated T regulatory (Treg) cells have critical roles in induction of immune tolerance and immune homeostasis in preclinical and clinical studies. Presently, controlling immune susceptible characteristics of CTA with adoptive transfer of Treg cells is being considered promising and it has drawn great interests. This updated review will focus on a dominant form of Treg cells expressing CD4+CD25+ surface molecules and a forkhead box P3 transcription factor with immune tolerant and immune homeostasis activities. For future application of Treg cells as therapeutics in CTA, molecular and cellular characteristics of CTA and immune rejection, Treg cell development and phenotypes, Treg cell plasticity and stability, immune tolerant functions of Treg cells in CTA in preclinical studies, and protocols for therapeutic application of Treg cells in clinical settings are addressed in this review. Collectively, Treg cell therapy in CTA seems feasible with promising perspectives. However, the extreme high immunogenicity of CTA warrants caution.

Fifty Shades of Tolerance: Beyond a Binary Tolerant/Non-Tolerant Paradigm

Purpose of review

It has long been considered that tolerance in a transplant recipient is a binary all-or-none state: either the graft is accepted without immunosuppression identifying the recipient as tolerant, or the recipient rejects the graft and is not tolerant. This tolerance paradigm, however, does not accurately reflect data emerging from animal models and patients and requires revision.

Recent Findings

It is becoming appreciated that there may be different gradations in the quality of tolerance based on underlying cellular mechanisms of immunological tolerance, and that individuals may enhance their tolerance by strengthening or combining different cellular mechanisms. Furthermore, evidence suggests that even if tolerance is lost, the loss may be only temporary, and in some circumstances tolerance can be restored.

Summary

Shifting our focus from an all-or-nothing tolerance paradigm to one with many shades may help us better understand how tolerance operates, and how this state may be tracked and enhanced for better patient outcomes.

Changes in Reactivity In Vitro of CD4+CD25+ and CD4+CD25- T Cell Subsets in Transplant Tolerance

Transplant tolerance induced in adult animals is mediated by alloantigen-specific CD4⁺CD25⁺ T cells, yet in many models, proliferation of CD4⁺ T cells from hosts tolerant to specific-alloantigen in vitro is not impaired. To identify changes that may diagnose tolerance, changes in the patterns of proliferation of CD4⁺, CD4⁺CD25⁺, and CD4⁺CD25⁻ T cells from DA rats tolerant to Piebald Virol Glaxo rat strain (PVG) cardiac allografts and from naïve DA rats were examined. Proliferation of CD4⁺ T cells from both naïve and tolerant hosts was similar to both PVG and Lewis stimulator cells. In mixed lymphocyte culture to PVG, proliferation of naïve CD4⁺CD25⁻ T cells was greater than naïve CD4⁺ T cells. In contrast, proliferation of CD4⁺CD25⁻ T cells from tolerant hosts to specific-donor PVG was not greater than CD4⁺ T cells, whereas their response to Lewis and self-DA was greater than CD4⁺ T cells. Paradoxically, CD4⁺CD25⁺ T cells from tolerant hosts did not proliferate to PVG, but did to Lewis, whereas naïve CD4⁺CD25⁺ T cells proliferate to both PVG and Lewis but not to self-DA. CD4⁺CD25⁺ T cells from tolerant, but not naïve hosts, expressed receptors for interferon (IFN)-γ and IL-5 and these cytokines promoted their proliferation to specific-alloantigen PVG but not to Lewis or self-DA. We identified several differences in the patterns of proliferation to specific-donor alloantigen between cells from tolerant and naïve hosts. Most relevant is that CD4⁺CD25⁺ T cells from tolerant hosts failed to proliferate or suppress to specific donor in the absence of either IFN-γ or IL-5. The proliferation to third-party and self of each cell population from tolerant and naïve hosts was similar and not affected by IFN-γ or IL-5. Our findings suggest CD4⁺CD25⁺ T cells that mediate transplant tolerance depend on IFN−γ or IL-5 from alloactivated Th1 and Th2 cells.

Induction of Immunological Tolerance as a Therapeutic Procedure

A major goal of immunosuppressive therapies is to harness immune tolerance mechanisms so as to minimize unwanted side effects associated with protracted immunosuppressive therapy. Antibody blockade of lymphocyte coreceptor and costimulatory pathways in mice has demonstrated the principle that both naive and primed immune systems can be reprogrammed toward immunological tolerance. Such tolerance can involve the amplification of activity of regulatory T cells, and is maintained through continuous recruitment of such cells through processes of infectious tolerance. We propose that regulatory T cells create around them microenvironments that are anti-inflammatory and endowed with enhanced protection against destructive damage. This acquired immune privilege involves the decommissioning of cells of the innate as well as adaptive immune systems. Evidence is presented that nutrient sensing by immune cells acting through the mammalian target of rapamycin (mTOR) pathway provides one route by which the immune system can be directed toward noninflammatory and regulatory behavior at the expense of destructive functions. Therapeutic control of immune cells so as to harness metabolic routes favoring dominant regulatory mechanisms has offered a new direction for immunosuppressive therapy, whereby short-term treatment may be sufficient for long-term benefit or even cure.

Transplant trials with Tregs: perils and promises

Modern immunosuppression regimens effectively control acute rejection and decrease graft loss in the first year after transplantation; however, these regimens do not have a durable effect on long-term graft survival owing to a combination of drug toxicities and the emergence of chronic alloimmune responses. Eliminating drugs and their toxicities while maintaining graft acceptance has been the primary aim of cellular therapies. Tregs suppress both autoimmune and alloimmune responses and are particularly effective in protecting allografts in experimental transplant models. Further, Treg-based therapies are selective, do not require harsh conditioning, and do not have a risk of graft-versus-host disease. Trial designs should consider the distinct immunological features of each transplanted organ, Treg preparations, dose, and frequency, and the ability to detect and quantify Treg effects in a given transplant environment. In this Review, we detail the ongoing clinical trials of Treg therapy in liver and kidney transplantation. Integration of Treg biology gleaned from preclinical models and experiences in human organ transplantation should allow for optimization of trial design that will determine the potential efficacy of a given therapy and provide guidelines for further therapeutic development.

The Induction and Maintenance of Transplant Tolerance Engages Both Regulatory and Anergic CD4+ T cells

Therapeutic tolerance to self-antigens or foreign antigens is thought to depend on constant vigilance by Foxp3⁺ regulatory T cells (Tregs). Previous work using a pancreatic islet allograft model and a short pulse of CD3 antibody therapy has shown that CD8⁺ T cells become anergic and use TGFβ and coinhibitory signaling as their contribution to the tolerance process. Here, we examine the role of CD4⁺ T cells in tolerization by CD3 antibodies. We show that both Foxp3⁺ Tregs and CD4⁺ T cell anergy play a role in the induction of tolerance and its maintenance. Foxp3⁺ Tregs resisted CD3 antibody-mediated depletion, unlike intragraft Th1 CD4⁺ lymphocytes coexpressing granzyme B and Tbx21, which were selectively eliminated. Tregs were mandatory for induction of tolerance as their depletion at the time of CD3 antibody therapy or for a short time thereafter, by an antibody to CD25 (PC61), led to graft rejection. Early treatment with CTLA-4 antibody gave the same outcome. In contrast, neither PC61 nor anti-CTLA-4 given late, at day 100 posttransplant, reversed tolerance once established. Ablation of Foxp3 T cells after diphtheria toxin injection in tolerant Foxp3^DTR recipient mice provided the same outcome. Alloreactive T cells had been rendered intrinsically unresponsive as total CD4⁺ or Treg-deprived CD4⁺ T cells from tolerant recipients were unable to mount donor-specific IFN-γ responses. In addition, intragraft Treg-deprived CD4⁺ T cells lacked proliferative capacities, expressed high levels of the inhibitory receptor PD-1, and exhibited a CD73^hiFR4^hi phenotype, thus reflecting a state of T cell anergy. We conclude that Tregs play a substantive and critical role in guiding the immune system toward tolerance of the allograft, when induced by CD3 antibody, but are less important for maintenance of the tolerant state, where T cell anergy appears sufficient.

Treg-Centric View of Immunosuppressive Drugs in Transplantation: A Balancing Act

Regulatory CD4+ Foxp3+ T cells (Tregs) are critical in controlling immunity and tolerance. Thus, preserving Treg numbers and function in transplanted patients is essential for the successful minimization of maintenance immunosuppression. Multiple cellular signals control the development, differentiation, and function of Tregs. Many of these signals are shared with conventional Foxp3^- T cells (Tconv) and are targeted by immunosuppressive drugs, negatively affecting both Tregs and Tconv. Because intracellular signals vary in optimal intensity in different T cell subsets, improved specificity in immunosuppressive regimens must occur to benefit long-term transplant outcomes. In this regard, recent advances are gradually uncovering differences in the signals required in Tregs and Tconv biology, opening the door to new potential therapeutic approaches to either enhance or spare Tregs. In this review, we will explain the prominent cell signaling pathways critical for Treg maintenance and function, while reporting the effects of immunosuppressive drugs targeting these signaling pathways in clinical transplantation settings.

Foxp3 drives oxidative phosphorylation and protection from lipotoxicity

Tregs can adopt a catabolic metabolic program with increased capacity for fatty acid oxidation-fueled oxidative phosphorylation (OXPHOS). It is unclear why this form of metabolism is favored in Tregs and, more specifically, whether this program represents an adaptation to the environment and developmental cues or is "hardwired" by Foxp3. Here we show, using metabolic analysis and an unbiased mass spectroscopy-based proteomics approach, that Foxp3 is both necessary and sufficient to program Treg-increased respiratory capacity and Tregs' increased ability to utilize fatty acids to fuel oxidative phosphorylation. Foxp3 drives upregulation of components of all the electron transport complexes, increasing their activity and ATP generation by oxidative phosphorylation. Increased fatty acid β-oxidation also results in selective protection of Foxp3⁺ cells from fatty acid-induced cell death. This observation may provide novel targets for modulating Treg function or selection therapeutically.

Foxp3 drives oxidative phosphorylation and protection from lipotoxicity

Tregs can adopt a catabolic metabolic program with increased capacity for fatty acid oxidation-fueled oxidative phosphorylation (OXPHOS). It is unclear why this form of metabolism is favored in Tregs and, more specifically, whether this program represents an adaptation to the environment and developmental cues or is "hardwired" by Foxp3. Here we show, using metabolic analysis and an unbiased mass spectroscopy-based proteomics approach, that Foxp3 is both necessary and sufficient to program Treg-increased respiratory capacity and Tregs' increased ability to utilize fatty acids to fuel oxidative phosphorylation. Foxp3 drives upregulation of components of all the electron transport complexes, increasing their activity and ATP generation by oxidative phosphorylation. Increased fatty acid β-oxidation also results in selective protection of Foxp3⁺ cells from fatty acid-induced cell death. This observation may provide novel targets for modulating Treg function or selection therapeutically.

The Immunosuppressive Effect of CTLA4 Immunoglobulin Is Dependent on Regulatory T Cells at Low But Not High Doses

B7.1/2-targeted costimulation blockade (CTLA4 immunoglobulin [CTLA4-Ig]) is available for immunosuppression after kidney transplantation, but its potentially detrimental impact on regulatory T cells (Tregs) is of concern. We investigated the effects of CTLA4-Ig monotherapy in a fully mismatched heart transplant model (BALB/c onto C57BL/6). CTLA4-Ig was injected chronically (on days 0, 4, 14, and 28 and every 4 weeks thereafter) in dosing regimens paralleling clinical use, shown per mouse: low dose (LD), 0.25 mg (≈10 mg/kg body weight); high dose (HD), 1.25 mg (≈50 mg/kg body weight); and very high dose (VHD), 6.25 mg (≈250 mg/kg body weight). Chronic CTLA4-Ig therapy showed dose-dependent efficacy, with the LD regimen prolonging graft survival and with the HD and VHD regimens leading to >95% long-term graft survival and preserved histology. CTLA4-Ig's effect was immunosuppressive rather than tolerogenic because treatment cessation after ≈3 mo led to rejection. FoxP3-positive Tregs were reduced in naïve mice to a similar degree, independent of the CTLA4-Ig dose, but recovered to normal values in heart recipients under chronic CTLA4-Ig therapy. Treg depletion (anti-CD25) resulted in an impaired outcome under LD therapy but had no detectable effect under HD therapy. Consequently, the immunosuppressive effect of partially effective LD CTLA4-Ig therapy is impaired when Tregs are removed, whereas CTLA4-Ig monotherapy at higher doses effectively maintains graft survival independent of Tregs.

Tracking of TCR-Transgenic T Cells Reveals That Multiple Mechanisms Maintain Cardiac Transplant Tolerance in Mice

Solid organ transplantation tolerance can be achieved following select transient immunosuppressive regimens that result in long-lasting restraint of alloimmunity without affecting responses to other antigens. Transplantation tolerance has been observed in animal models following costimulation or coreceptor blockade therapies, and in a subset of patients through induction protocols that include donor bone marrow transplantation, or following withdrawal of immunosuppression. Previous data from our lab and others have shown that proinflammatory interventions that successfully prevent the induction of transplantation tolerance in mice often fail to break tolerance once it has been stably established. This suggests that established tolerance acquires resilience to proinflammatory insults, and prompted us to investigate the mechanisms that maintain a stable state of robust tolerance. Our results demonstrate that only a triple intervention of depleting CD25⁺ regulatory T cells (Tregs), blocking programmed death ligand-1 (PD-L1) signals, and transferring low numbers of alloreactive T cells was sufficient to break established tolerance. We infer from these observations that Tregs and PD-1/PD-L1 signals cooperate to preserve a low alloreactive T cell frequency to maintain tolerance. Thus, therapeutic protocols designed to induce multiple parallel mechanisms of peripheral tolerance may be necessary to achieve robust transplantation tolerance capable of maintaining one allograft for life in the clinic.

Global Regulatory T-Cell Research from 2000 to 2015: A Bibliometric Analysis

We aimed to analyze the global scientific output of regulatory T-cell (Treg) research and built a model to qualitatively and quantitatively evaluate publications from 2000 to 2015. Data were obtained from the Web of Science Core Collection (WoSCC) of Thomson Reuters on January 1, 2016. The bibliometric method and Citespace III were used to analyze authors, journals, publication outputs, institutions, countries, research areas, research hotspots, and trends. In total, we identified 35,741 publications on Treg research from 2000 to 2015, and observed that the annual publication rate increased with time. The Journal of Immunology published the highest number of articles, the leading country was the USA, and the leading institute was Harvard University. Sakaguchi, Hori, Fontenot, and Wang were the top authors in Treg research. Immunology accounted for the highest number of publications, followed by oncology, experimental medicine, cell biology, and hematology. Keyword analysis indicated that autoimmunity, inflammation, cytokine, gene expression, foxp3, and immunotherapy were the research hotspots, whereas autoimmune inflammation, gene therapy, granzyme B, RORγt, and th17 were the frontiers of Treg research. This bibliometric analysis revealed that Treg-related studies are still research hotspots, and that Treg-related clinical therapies are the research frontiers; however, further study and collaborations are needed worldwide. Overall, our findings provide valuable information for the editors of immunology journals to identify new perspectives and shape future research directions.

IL-33 Receptor-Expressing Regulatory T Cells Are Highly Activated, Th2 Biased and Suppress CD4 T Cell Proliferation through IL-10 and TGFβ Release

Immunomodulatory Foxp3⁺ regulatory T cells (Tregs) form a heterogeneous population consisting of subsets with different activation states, migratory properties and suppressive functions. Recently, expression of the IL-33 receptor ST2 was shown on Tregs in inflammatory settings. Here we report that ST2 expression identifies highly activated Tregs in mice even under homeostatic conditions. ST2⁺ Tregs preferentially accumulate at non-lymphoid sites, likely mediated by their high expression of several chemokine receptors facilitating tissue homing. ST2⁺ Tregs exhibit a Th2-biased character, expressing GATA-3 and producing the Th2 cytokines IL-5 and IL-13 –especially in response to IL-33. Yet, IL-33 is dispensable for the generation and maintenance of these cells in vivo. Furthermore, ST2⁺ Tregs are superior to ST2⁻ Tregs in suppressing CD4⁺ T cell proliferation in vitro independent of IL-33. This higher suppressive capacity is partially mediated by enhanced production and activation of the anti-inflammatory cytokines IL-10 and TGFβ. Thus, ST2 expression identifies a highly activated, strongly suppressive Treg subset preferentially located in non-lymphoid tissues. Here ST2⁺ Tregs may be well positioned to immediately react to IL-33 alarm signals. Their specific properties may render ST2⁺ Tregs useful targets for immunomodulatory therapies.

Use of In Vitro Assays to Assess Immunogenicity Risk of Antibody-Based Biotherapeutics

An In Vitro Comparative Immunogenicity Assessment (IVCIA) assay was evaluated as a tool for predicting the potential relative immunogenicity of biotherapeutic attributes. Peripheral blood mononuclear cells from up to 50 healthy naïve human donors were monitored up to 8 days for T-cell proliferation, the number of IL-2 or IFN-γ secreting cells, and the concentration of a panel of secreted cytokines. The response in the assay to 10 monoclonal antibodies was found to be in agreement with the clinical immunogenicity, suggesting that the assay might be applied to immunogenicity risk assessment of antibody biotherapeutic attributes. However, the response in the assay is a measure of T-cell functional activity and the alignment with clinical immunogenicity depends on several other factors. The assay was sensitive to sequence variants and could differentiate single point mutations of the same biotherapeutic. Nine mAbs that were highly aggregated by stirring induced a higher response in the assay than the original mAbs before stirring stress, in a manner that did not match the relative T-cell response of the original mAbs. In contrast, mAbs that were glycated by different sugars (galactose, glucose, and mannose) showed little to no increase in response in the assay above the response to the original mAbs before glycation treatment. The assay was also used successfully to assess similarity between multiple lots of the same mAb, both from the same manufacturer and from different manufacturers (biosimilars). A strategy for using the IVCIA assay for immunogenicity risk assessment during the entire lifespan development of biopharmaceuticals is proposed.

Novel technologies to engineer graft for tolerance induction

PURPOSE OF REVIEW

Conquering allograft rejection remains an elusive goal in spite of recent breakthroughs in the field of immunosuppression. Much of the problem lies in the toxicity and side-effects of long-term use of systemic immunosuppressant drugs, which are sometimes ineffective in controlling rejection, but also hinder establishment of transplant tolerance. In this review, we discuss novel technologies that use grafts engineered with immunomodulatory molecules as a means of inducing tolerance.

RECENT FINDINGS

Several recent studies have demonstrated the feasibility of engineering cells, tissues, or solid organ grafts with immunoregulatory biologics to achieve long termgraft survival without the use of chronic immunosuppression. This approach was shown to primarily change the ratio of T effector versus CD4+CD25+FoxP3+ T regulatory cells within the graft microenvironment in favor of attaining localized tolerance induction and maintenance.

SUMMARY

Localized immunomodulation using biologic-engineered allografts represent a new paradigm for achieving long-term graft survival in the absence of chronic use of immunosuppression. The manipulation of the graft, rather than the recipient, not only ensures short- and long-term safety by minimizing the adverse effects of immunosuppression, but also allows retention of immune competency critical for the ability of the recipient to fight infections and cancer.

The GARP/Latent TGF-β1 complex on Treg cells modulates the induction of peripherally derived Treg cells during oral tolerance

Treg cells can secrete latent TGF-β1 (LTGF-β1), but can also utilize an alternative pathway for transport and expression of LTGF-β1 on the cell surface in which LTGF-β1 is coupled to a distinct LTGF-β binding protein termed glycoprotein A repetitions predominant (GARP)/LRRC32. The function of the GARP/LTGF-β1 complex has remained elusive. Here, we examine in vivo the roles of GARP and TGF-β1 in the induction of oral tolerance. When Foxp3(-) OT-II T cells were transferred to wild-type recipient mice followed by OVA feeding, the conversion of Foxp3(-) to Foxp3(+) OT-II cells was dependent on recipient Treg cells. Neutralization of IL-2 in the recipient mice also abrogated this conversion. The GARP/LTGF-β1 complex on recipient Treg cells, but not dendritic cell-derived TGF-β1, was required for efficient induction of Foxp3(+) T cells and for the suppression of delayed hypersensitivity. Expression of the integrin αvβ8 by Treg cells (or T cells) in the recipients was dispensable for induction of Foxp3 expression. Transient depletion of the bacterial flora enhanced the development of oral tolerance by expanding Treg cells with enhanced expression of the GARP/LTGF-β1 complex.

Induced Foxp3(+) T Cells Colonizing Tolerated Allografts Exhibit the Hypomethylation Pattern Typical of Mature Regulatory T Cells

Regulatory T cells expressing the transcription factor Foxp3 require acquisition of a specific hypomethylation pattern to ensure optimal functional commitment, limited lineage plasticity, and long-term maintenance of tolerance. A better understanding of the molecular mechanisms involved in the generation of these epigenetic changes in vivo will contribute to the clinical exploitation of Foxp3(+) Treg. Here, we show that both in vitro and in vivo generated antigen-specific Foxp3(+) Treg can acquire Treg-specific epigenetic characteristics and prevent skin graft rejection in an animal model.

4-1BB Signaling in Conventional T Cells Drives IL-2 Production That Overcomes CD4+CD25+FoxP3+ T Regulatory Cell Suppression

Costimulation with the recombinant SA-4-1BBL agonist of 4-1BB receptor on conventional CD4⁺ T cells (Tconvs) overcomes the suppression mediated by naturally occurring CD4⁺CD25⁺FoxP3⁺ T regulatory cells (Tregs). The mechanistic basis of this observation has remained largely unknown. Herein we show that Tconvs, but not Tregs, are the direct target of SA-4-1BBL-mediated evasion of Treg suppression. IL-2 produced by Tconvs in response to 4-1BB signaling is both necessary and sufficient for overcoming Treg suppression. Supernatant from Tconvs stimulated with SA-4-1BBL contains high levels of IL-2 and overcomes Treg suppression in ex vivo Tconv:Treg cocultures. Removal of IL-2 from such supernatant restores Treg suppression and repletion of Tconv:Treg cocultures with exogenous recombinant IL-2 overcomes suppression. This study establishes 4-1BB signaling as a key circuit that regulates physical and functional equilibrium between Tregs and Tconvs with important implications for immunotherapy for indications where a fine balance between Tregs and Teffs plays a decisive role.

Alloantigen-specific regulatory T cells generated with a chimeric antigen receptor

Adoptive immunotherapy with regulatory T cells (Tregs) is a promising treatment for allograft rejection and graft-versus-host disease (GVHD). Emerging data indicate that, compared with polyclonal Tregs, disease-relevant antigen-specific Tregs may have numerous advantages, such as a need for fewer cells and reduced risk of nonspecific immune suppression. Current methods to generate alloantigen-specific Tregs rely on expansion with allogeneic antigen-presenting cells, which requires access to donor and recipient cells and multiple MHC mismatches. The successful use of chimeric antigen receptors (CARs) for the generation of antigen-specific effector T cells suggests that a similar approach could be used to generate alloantigen-specific Tregs. Here, we have described the creation of an HLA-A2-specific CAR (A2-CAR) and its application in the generation of alloantigen-specific human Tregs. In vitro, A2-CAR-expressing Tregs maintained their expected phenotype and suppressive function before, during, and after A2-CAR-mediated stimulation. In mouse models, human A2-CAR-expressing Tregs were superior to Tregs expressing an irrelevant CAR at preventing xenogeneic GVHD caused by HLA-A2+ T cells. Together, our results demonstrate that use of CAR technology to generate potent, functional, and stable alloantigen-specific human Tregs markedly enhances their therapeutic potential in transplantation and sets the stage for using this approach for making antigen-specific Tregs for therapy of multiple diseases.

Human CD45RA(-) FoxP3(hi) Memory-Type Regulatory T Cells Show Distinct TCR Repertoires With Conventional T Cells and Play an Important Role in Controlling Early Immune Activation

Adoptive immunotherapy with regulatory T cells (Treg) is a new option to promote immune tolerance following solid organ transplantation (SOT). However, Treg from elderly patients awaiting transplantation are dominated by the CD45RA(-) CD62L(+) central memory type Treg subset (TregCM), and the yield of well-characterized and stable naïve Treg (TregN) is low. It is, therefore, important to determine whether these TregCM are derived from the thymus and express high stability, suppressive capacity and a broad antigen repertoire like TregN. In this study, we showed that TregCM use a different T cell receptor (TCR) repertoire from conventional T cells (Tconv), using next-generation sequencing of all 24 Vβ families, with an average depth of 534 677 sequences. This showed almost no contamination with induced Treg. Furthermore, TregCM showed enhanced suppressive activity on Tconv at early checkpoints of immune activation controlling activation markers expression and cytokine secretion, but comparable inhibition of proliferation. Following in vitro expansion under mTOR inhibition, TregCM expanded equally as well as TregN without losing their function. Despite relatively limited TCR repertoire, TregCM also showed specific alloresponse, although slightly reduced compared to TregN. These results support the therapeutic usefulness of manufacturing Treg products from CD45RA(-) CD62L(+) Treg-enriched starting material to be applied for adoptive Treg therapy.

Regulatory T Cells: Serious Contenders in the Promise for Immunological Tolerance in Transplantation

Regulatory T cells (Tregs) play an important role in immunoregulation and have been shown in animal models to promote transplantation tolerance and curb autoimmunity following their adoptive transfer. The safety and potential therapeutic efficacy of these cells has already been reported in Phase I trials of bone-marrow transplantation and type I diabetes, the success of which has motivated the broadened application of these cells in solid-organ transplantation. Despite major advances in the clinical translation of these cells, there are still key questions to be addressed to ensure that Tregs attest their reputation as ideal candidates for tolerance induction. In this review, we will discuss the unique traits of Tregs that have attracted such fame in the arena of tolerance induction. We will outline the protocols used for their ex vivo expansion and discuss the future directions of Treg cell therapy. In this regard, we will review the concept of Treg heterogeneity, the desire to isolate and expand a functionally superior Treg population and report on the effect of differing culture conditions. The relevance of Treg migratory capacity will also be discussed together with methods of in vivo visualization of the infused cells. Moreover, we will highlight key advances in the identification and expansion of antigen-specific Tregs and discuss their significance for cell therapy application. We will also summarize the clinical parameters that are of importance, alongside cell manufacture, from the choice of immunosuppression regimens to the number of injections in order to direct the success of future efficacy trials of Treg cell therapy. Years of research in the field of tolerance have seen an accumulation of knowledge and expertise in the field of Treg biology. This perpetual progression has been the driving force behind the many successes to date and has put us now within touching distance of our ultimate success, immunological tolerance.

Foxp3-modified bone marrow mesenchymal stem cells promotes liver allograft tolerance through the generation of regulatory T cells in rats

Background

The transcription factor forkhead box P3 (Foxp3) is a master regulatory gene necessary for the development and function of CD4⁺CD25⁺ regulatory T cells (Tregs). Mesenchymal stem cells (MSC) have recently emerged as promising candidates for cell-based immunosuppression/tolerance induction protocols. Thus, we hypothesized that MSC-based Foxp3 gene therapy would improve immunosuppressive capacity of MSC and induce donor-specific allograft tolerance in rat’s liver allograft model.

Methods

The present study utilized a lentivirus vector to overexpress the therapeutic gene Foxp3 on MSC. In vivo, Injections of 2 × 10⁶ MSC, FUGW-MSC or Foxp3-MSC into the portal vein were carried out immediately after liver transplantation.

Results

Successful gene transfer of Foxp3 in MSC was achieved by lentivirus carrying Foxp3 and Foxp3-MSC engraftment in liver allograft was confirmed by fluorescence microscopy. Foxp3-MSC treatment significantly inhibited the proliferation of allogeneic ACI CD4⁺ T cells to splenocytes (SC) from the same donor strain or third-party BN rat compared with MSC. Foxp3-MSC suppressive effect on the proliferation of CD4⁺ T cells is contact dependent and associated with Programmed death ligand 1(PD-L1) upregulation in MSC. Co-culture of CD4⁺ T cells with Foxp3-MSC results in a shift towards a Tregs phenotype. More importantly, Foxp3-MSC monotherapy achieved donor-specific liver allograft tolerance and generated a state of CD4⁺CD25⁺Foxp3⁺ Tregs-dependent tolerance.

Conclusion

Foxp3-engineered MSC therapy seems to be a promising and attractive cell therapy approach for inducing immunosuppression or transplant tolerance.

Update on CD40 and CD154 blockade in transplant models

Generation of an effective immune response against foreign antigens requires two distinct molecular signals: a primary signal provided by the binding of antigen-specific T-cell receptor to peptide-MHC on antigen-presenting cells and a secondary signal delivered via the engagement of costimulatory molecules. Among various costimulatory signaling pathways, the interactions between CD40 and its ligand CD154 have been extensively investigated given their essential roles in the modulation of adaptive immunity. Here, we review current understanding of the role CD40/CD154 costimulation pathway has in alloimmunity, and summarize recent mechanistic and preclinical advances in the evaluation of candidate therapeutic approaches to target this receptor-ligand pair in transplantation.

New insights into the mechanisms of Treg function

PURPOSE OF REVIEW

CD4Foxp3 regulatory T cells (Tregs) are crucial in controlling immunity and self-tolerance. Consequently, in transplantation, Tregs play a central role in inhibiting acute rejection and promoting allograft tolerance. A more complete understanding of Treg biology may lead to novel therapeutic approaches to enhance Treg numbers and function.

RECENT FINDINGS

The maintenance of self-tolerance in nonlymphoid tissues requires the differentiation of Tregs in secondary lymphoid organs from naïve-like central Tregs into effector Tregs. Antigen and environmental cues guide this Treg differentiation, which parallels the types of adaptive immune responses taking place, allowing them to enter and function within specific nonlymphoid tissues. In addition to controlling inflammation, tissue-infiltrating Tregs unexpectedly regulate nonimmune processes, including metabolic homeostasis and tissue repair. Finally, Tregs can be directly and specifically targeted in vivo to augment their numbers or enhance their function in both secondary lymphoid organs and nonlymphoid tissues.

SUMMARY

Tregs exhibit a previously unrecognized breadth of function, which includes tissue-specific specialization and the regulation of both immune and nonimmune processes. This is of particular importance in transplantation since allo-reactive memory T cells can act directly within the allograft. Thus, therapeutic approaches may need to promote Treg function in transplanted tissue, as well as in secondary lymphoid organs. Such therapy would not only prevent inflammation and acute rejection, but may also promote nonimmune processes within the allograft such as tissue homeostasis and repair.

Spontaneous restoration of transplantation tolerance after acute rejection

Transplantation is a cure for end-stage organ failure but, in the absence of pharmacological immunosuppression, allogeneic organs are acutely rejected. Such rejection invariably results in allosensitization and accelerated rejection of secondary donor-matched grafts. Transplantation tolerance can be induced in animals and a subset of humans, and enables long-term acceptance of allografts without maintenance immunosuppression. However, graft rejection can occur long after a state of transplantation tolerance has been acquired. When such an allograft is rejected, it has been assumed that the same rules of allosensitization apply as to non-tolerant hosts and that immunological tolerance is permanently lost. Using a mouse model of cardiac transplantation, we show that when Listeria monocytogenes infection precipitates acute rejection, thus abrogating transplantation tolerance, the donor-specific tolerant state re-emerges, allowing spontaneous acceptance of a donor-matched second transplant. These data demonstrate a setting in which the memory of allograft tolerance dominates over the memory of transplant rejection.

The Regulatory T Cell Lineage Factor Foxp3 Regulates Gene Expression through Several Distinct Mechanisms Mostly Independent of Direct DNA Binding

The lineage factor Foxp3 is essential for the development and maintenance of regulatory T cells, but little is known about the mechanisms involved. Here, we demonstrate that an N-terminal proline-rich interaction region is crucial for Foxp3’s function. Subdomains within this key region link Foxp3 to several independent mechanisms of transcriptional regulation. Our study suggests that Foxp3, even in the absence of its DNA-binding forkhead domain, acts as a bridge between DNA-binding interaction partners and proteins with effector function permitting it to regulate a large number of genes. We show that, in one such mechanism, Foxp3 recruits class I histone deacetylases to the promoters of target genes, counteracting activation-induced histone acetylation and thereby suppressing their expression.

The suppressive activity of regulatory T cells provides the immune system with a mechanism to prevent detrimental immune responses, such as autoimmunity, attack of the beneficial commensal microbiota and rejection of the fetus. Intriguingly, expression of a single lineage factor Foxp3 is sufficient to completely reprogram T cells from a pro-inflammatory to a suppressive phenotype. Here, we show that Foxp3 alters the expression of thousands of genes through several independent mechanisms. In many cases, its own ability to bind to DNA appears to be dispensable, but rather it binds indirectly to the DNA by interaction with other transcription factors. Foxp3 then in turn recruits other proteins that affect gene expression through chromatin modification. For example, Foxp3 indirectly binds to the IL-2 promoter via interaction with the transcriptional activators c-Rel, AML-1 and NFAT. This leads to the Foxp3 mediated recruitment of class I histone deacetylases HDAC1, 2 and 3, which in turn counteracts the activation-induced hyper-acetylation of the promoter, thereby switching the gene off. In a way, Foxp3 hijacks pre-existing regulatory mechanism to reverse the transcriptional expression status of the target gene. By dissecting Foxp3 on a molecular level, we also show that this is only one of several independent mechanism utilised by Foxp3.

Transplantation tolerance: context matters

Costimulation blockade has been one of the most studied strategies to achieve immune tolerance, particularly in transplantation. Yet, in spite of the robust nature of the tolerance-inducing potential of costimulation blockade, a comprehensive understanding of the molecular and cellular mechanisms underlying tolerance induction is still missing. Nevertheless, progress has been continuously made. In this issue of the European Journal of Immunology, Chai et al. [Eur. J. Immunol. 2015. 45: 2017-2027] show that transplantation tolerance induced with an anti-CD154 monoclonal antibody relies on the coexistence of several tolerogenic mechanisms rather than one simple regulatory mechanism. These observations highlight the importance of concerted actions involving multiple pathways, namely apoptosis, acquisition of regulatory cells, or inhibition of proliferation, all of which contribute to the induction and maintenance of robust immune tolerance. A better understanding of these distinct tolerogenic pathways may lead to the development of better tolerance-inducing therapeutics.

Allospecific CD4(+) T cells retain effector function and are actively regulated by Treg cells in the context of transplantation tolerance

Although donor-specific transfusion (DST) plus CD154 blockade represents a robust protocol for inducing transplantation tolerance, the underlying mechanisms are incompletely understood. In a murine T-cell adoptive transfer model, we have visualized alloantigen-specific, TCR-transgenic for H2-A(b) /H2-K(d) 54-68 epitope (TCR75) CD4(+) T cells with indirect allospecificity during the course of tolerance induction. Three main observations were made. First, although the majority of TCR75 CD4(+) T cells were deleted following DST plus CD154 blockade, the surviving TCR75 CD4(+) T cells were capable of making IL-2, upregulating CD44, and undergoing cell division, suggesting that they were functionally active. Indeed, residual TCR75 CD4(+) T cells reisolated from the primary recipients given DST plus CD154 blockade were fully capable of rejecting allografts upon secondary transfer. Second, in tolerant mice, TCR75 CD4(+) T cells were not induced to express Foxp3 in the graft-draining lymph node. TCR75 CD4(+) T cells were also absent in accepted graft tissues in which endogenous Treg cells were enriched. Finally, DST plus CD154 blockade resulted in an abortive expansion of TCR75 CD4(+) T cells, a process that required the presence of endogenous Treg cells. Collectively, surviving TCR75 CD4(+) T cells are immunocompetent but kept in check by an endogenous immunosuppressive network induced by DST plus CD154 blockade.

Differential sensitivity of regulatory and effector T cells to cell death: a prerequisite for transplant tolerance

Despite significant progress achieved in transplantation, immunosuppressive therapies currently used to prevent graft rejection are still endowed with severe side effects impairing their efficiency over the long term. Thus, the development of graft-specific, non-toxic innovative therapeutic strategies has become a major challenge, the goal being to selectively target alloreactive effector T cells while sparing CD4⁺Foxp3⁺ regulatory T cells (Tregs) to promote operational tolerance. Various approaches, notably the one based on monoclonal antibodies or fusion proteins directed against the TCR/CD3 complex, TCR coreceptors, or costimulatory molecules, have been proposed to reduce the alloreactive T cell pool, which is an essential prerequisite to create a therapeutic window allowing Tregs to induce and maintain allograft tolerance. In this mini review, we focus on the differential sensitivity of Tregs and effector T cells to the depleting and inhibitory effect of these immunotherapies, with a particular emphasis on CD3-specific antibodies that beyond their immunosuppressive effect, also express potent tolerogenic capacities.

Altered connexin 43 expression underlies age-dependent decrease of regulatory T cell suppressor function in nonobese diabetic mice

Type 1 diabetes is one of the most extensively studied autoimmune diseases, but the cellular and molecular mechanisms leading to T cell-mediated destruction of insulin-producing β cells are still not well understood. In this study, we show that regulatory T cells (T(regs)) in NOD mice undergo age-dependent loss of suppressor functions exacerbated by the decreased ability of activated effector T cells to upregulate Foxp3 and generate T(regs) in the peripheral organs. This age-dependent loss is associated with reduced intercellular communication mediated by gap junctions, which is caused by impaired upregulation and decreased expression of connexin 43. Regulatory functions can be corrected, even in T cells isolated from aged, diabetic mice, by a synergistic activity of retinoic acid, TGF-β, and IL-2, which enhance connexin 43 and Foxp3 expression in T(regs) and restore the ability of conventional CD4(+) T cells to upregulate Foxp3 and generate peripherally derived T(regs). Moreover, we demonstrate that suppression mediated by T(regs) from diabetic mice is enhanced by a novel reagent, which facilitates gap junction aggregation. In summary, our report identifies gap junction-mediated intercellular communication as an important component of the T(reg) suppression mechanism compromised in NOD mice and suggests how T(reg) mediated immune regulation can be improved.

Nucleosides present on phlebotomine saliva induce immunossuppression and promote the infection establishment

Background

Sand fly saliva plays a crucial role in establishing Leishmania infection. We identified adenosine (ADO) and adenosine monophosphate (AMP) as active pharmacologic compounds present in Phlebotomus papatasi saliva that inhibit dendritic cell (DC) functions through a PGE₂/IL 10-dependent mechanism.

Methodology/Principal Findings

Herein, we prepared a mixture of ADO and AMP in equimolar amounts similar to those present in the salivary-gland extract (SGE) form one pair of salivary glands of P. papatasi and co-injected it with Leishmania amazonensis or L. major into mouse ears. ADO+AMP mimicked exacerbative effects of P. papatasi saliva in leishmaniasis, increasing parasite burden and cutaneous lesions. Enzymatic catabolism of salivary nucleosides reversed the SGE-induced immunosuppressive effect associated with IL-10 enhancement. Immunosuppressive factors COX2 and IL-10 were upregulated and failed to enhance ear lesion and parasite burden in IL 10^-/- infected mice. Furthermore, nucleosides increased regulatory T cell (Treg) marker expression on CD4⁺CD25^- cells, suggesting induction of Tregs on effector T cells (T eff). Treg induction (iTreg) was associated with nucleoside-induced tolerogenic dendritic cells (tDCs) expressing higher levels of COX₂ and IL-10. In vitro generation of Tregs was more efficient in DCs treated with nucleosides. Suppressive effects of nucleosides during cutaneous leishmaniasis were mediated through an A2_AR-dependent mechanism. Using BALB/c mice deficient in A2A ADO receptor (A2_AR^–/–), we showed that co-inoculated mice controlled infection, displaying lower parasite numbers at infection sites and reduced iTreg generation.

Conclusion/Significance

We have demonstrated that ADO and AMP in P. papatasi saliva mediate exacerbative effects of Leishmania infection by acting preferentially on DCs promoting a tolerogenic profile in DCs and by generating iTregs in inflammatory foci through an A2_AR mechanism.

Leishmania parasites are transmitted to their vertebrate hosts by infected Phlebotomine sand flies during the blood meal of the flies. During the Leishmania transmission, the saliva is inoculated together with parasites and exhibit several pharmacological compounds that facilitate blood feeding, interfering on homeostasis and avoiding inflammation. Thus, these compounds allow the establishment of pathogen infection. We recently identified adenosine (ADO) and adenosine monophosphate (AMP) as major immunomodulatory compounds present within the Old World sand fly species Phlebotomus papatasii, which protected mice from extreme inflammatory insults. ADO limits the magnitude of immune response by displaying a potent anti-inflammatory activity. Here, we demonstrated that ADO and AMP present in Phlebotomus papatasi saliva are involved in the establishment of parasite infection. Such nucleosides act through adenosine A2_A receptor (A2_AR), inducing a tolerogenic profile on dendritic cells (tDC) that may generate regulatory T cells differentiation, thus leading to suppression of the immune response and parasite survival. The identification of the active salivary constituents could serve as a strategy for the development of new vaccines to control pathogen transmission.

Jagged-1 is required for the expansion of CD4+ CD25+ FoxP3+ regulatory T cells and tolerogenic dendritic cells by murine mesenchymal stromal cells

Introduction

Mesenchymal stromal cells (MSC) have well defined immunomodulatory properties including the suppression of lymphocyte proliferation and inhibition of dendritic cell (DC) maturation involving both cell contact and soluble factors. These properties have made MSC attractive candidates for cellular therapy. However, the mechanism underlying these characteristics remains unclear. This study sought to investigate the mechanisms by which MSC induce a regulatory environment.

Method

Allogeneic bone marrow mesenchymal stromal cells were cultured with T cells or dendritic cells in the presence or absence of gamma secretase inhibitor to block Notch receptor signalling. T cells and dendritic cells were examined by flow cytometry for changes in phenotype marker expression. Stable knock down MSC were generated to examine the influence of Jagged 1 signalling by MSC. Both wildtype and knockdown MSC were subsequently used in vivo in an animal model of allergic airway inflammation.

Results

The Notch ligand Jagged-1 was demonstrated to be involved in MSC expansion of regulatory T cells (Treg). Additionally, MSC-induced a functional semi-mature DC phenotype, which further required Notch signalling for the expansion of Treg. MSC, but not Jagged-1 knock down MSC, reduced pathology in a mouse model of allergic airway inflammation. Protection mediated by MSC was associated with enhanced Treg in the lung and significantly increased production of interleukin (IL)-10 in splenocytes re-stimulated with allergen. Significantly less Treg and IL-10 was observed in mice treated with Jagged-1 knock down MSC.

Conclusions

The current study suggests that MSC-mediated immune modulation involves the education and expansion of regulatory immune cells in a Jagged-1 dependent manner and provides the first report of the importance of Jagged-1 signalling in MSC protection against inflammation in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0021-5) contains supplementary material, which is available to authorized users.

Therapeutic regulatory T cells subvert effector T cell function in inflamed islets to halt autoimmune diabetes

Therapeutic regulatory T cells (Tregs) can reverse pre-established autoimmune pathology. In this study, using a mouse model of autoimmune diabetes, we aimed to determine the means by which therapeutic Tregs control islet inflammation. Islet Ag-specific Tregs infiltrated inflamed islets soon after infusion into prediabetic mice, which was quickly followed by a selective reduction of mRNA associated with effector T cells in the islets. This change was partially due to decreased CD8(+) T cell accumulation in the tissue. CD8(+) T cells that remained in the islets after Treg treatment were able to engage dendritic cells in a manner similar to that found in untreated mice, consistent with the retention of an activated phenotype by islet dendritic cells shortly after Treg treatment. Nonetheless, Treg treatment abrogated IFN-γ production by intraislet CD8(+) and CD4(+) T cells at the protein level with minimal effect on IFN-γ mRNA. Sustained expression of IFN-γ protein by effector T cells was dependent on common γ-chain cytokine activation of the mTOR pathway, which was suppressed in islet CD8(+) T cells in vivo after Treg treatment. These multifaceted mechanisms underlie the efficacy of therapeutic Treg subversion of effector T cell functions at the site of inflammation to restore normal tissue homeostasis.

Chimerism, graft survival, and withdrawal of immunosuppressive drugs in HLA matched and mismatched patients after living donor kidney and hematopoietic cell transplantation

Thirty-eight HLA matched and mismatched patients given combined living donor kidney and enriched CD34(+) hematopoietic cell transplants were enrolled in tolerance protocols using posttransplant conditioning with total lymphoid irradiation and anti-thymocyte globulin. Persistent chimerism for at least 6 months was associated with successful complete withdrawal of immunosuppressive drugs in 16 of 22 matched patients without rejection episodes or kidney disease recurrence with up to 5 years follow up thereafter. One patient is in the midst of withdrawal and five are on maintenance drugs. Persistent mixed chimerism was achieved in some haplotype matched patients for at least 12 months by increasing the dose of T cells and CD34(+) cells infused as compared to matched recipients in a dose escalation study. Success of drug withdrawal in chimeric mismatched patients remains to be determined. None of the 38 patients had kidney graft loss or graft versus host disease with up to 14 years of observation. In conclusion, complete immunosuppressive drug withdrawal could be achieved thus far with the tolerance induction regimen in HLA matched patients with uniform long-term graft survival in all patients.

Signals and pathways controlling regulatory T cells

Induction of specific immune tolerance to grafts remains the sought-after standard following transplantation. Defined by expression of the Foxp3 (forkhead box protein 3) transcription factor, the regulatory T-cell (Treg) lineage has been noted to exert potent immunoregulatory functions that contribute to specific graft tolerance. In this review, we discuss the known signals and pathways which govern Treg development, both in the thymus and in peripheral sites, as well as lineage maintenance and homeostasis. In particular, we highlight the roles of T-cell receptor signaling, CD28 costimulation, and signals through phosphatidyl inositol 3-kinase (PI3K) and related metabolic pathways in multiple aspects of Treg biology.

Transplantation tolerance and its outcome during infections and inflammation

Much progress has been made toward understanding the mechanistic basis of transplantation tolerance in experimental models, which implicates clonal deletion of alloreactive T and B cells, induction of cell-intrinsic hyporesponsiveness, and dominant regulatory cells mediating infectious tolerance and linked suppression. Despite encouraging success in the laboratory, achieving tolerance in the clinic remains challenging, although the basis for these challenges is beginning to be understood. Heterologous memory alloreactive T cells generated by infections prior to transplantation have been shown to be a critical barrier to tolerance induction. Furthermore, infections at the time of transplantation and tolerance induction provide a pro-inflammatory milieu that alters the stability and function of regulatory T cells as well as the activation requirements and differentiation of effector T cells. Thus, infections can result in enhanced alloreactivity, resistance to tolerance induction, and destabilization of the established tolerance state. We speculate that these experimental findings have relevance to the clinic, where infections have been associated with allograft rejection and may be a causal event precipitating the loss of grafts after long periods of stable operational tolerance. Understanding the mechanisms by which infections prevent and destabilize tolerance can lead to therapies that promote stable life-long tolerance in transplant recipients.

The environment of regulatory T cell biology: cytokines, metabolites, and the microbiome

Regulatory T cells (Tregs) are suppressive T cells that have an essential role in maintaining the balance between immune activation and tolerance. Their development, either in the thymus, periphery, or experimentally in vitro, and stability and function all depend on the right mix of environmental stimuli. This review focuses on the effects of cytokines, metabolites, and the microbiome on both human and mouse Treg biology. The role of cytokines secreted by innate and adaptive immune cells in directing Treg development and shaping their function is well established. New and emerging data suggest that metabolites, such as retinoic acid, and microbial products, such as short-chain fatty acids, also have a critical role in guiding the functional specialization of Tregs. Overall, the complex interaction between distinct environmental stimuli results in unique, and in some cases tissue-specific, tolerogenic environments. Understanding the conditions that favor Treg induction, accumulation, and function is critical to defining the pathophysiology of many immune-mediated diseases and to developing new therapeutic interventions.

Circulating biomarkers of tolerance

On the basis of reviewed literature here we describe models of tolerance and summarize the evidence of circulating biomarkers suitable for the assessment of immunological risk in organ transplantation. We focused on results of evaluation of specific peripheral immune cell populations and transcripts in peripheral blood of operationally tolerant liver and kidney transplant recipients. Validation of described markers to define potentially tolerant patients before their use in clinical trials is critical.