Functional regulatory T cells produced by inhibiting cyclic nucleotide phosphodiesterase type 3 prevent allograft rejection

A mini-review: phosphodiesterases in charge to balance intracellular cAMP during T-cell activation

T-cell activation is a pivotal process of the adaptive immune response with 3',5'-cyclic adenosine monophosphate (cAMP) as a key regulator of T-cell activation and function. It governs crucial control over T-cell differentiation and production of pro-inflammatory cytokines, such as IFN-γ. Intriguingly, levels of intracellular cAMP differ between regulatory (Treg) and conventional T-cells (Tcon). During cell-cell contact, cAMP is transferred via gap junctions between these T-cell subsets to mediate the immunosuppressive function of Treg. Moreover, the activation of T-cells via CD3 and CD28 co-stimulation leads to a transient upregulation of cAMP. Elevated intracellular cAMP levels are balanced precisely by phosphodiesterases (PDEs), a family of enzymes that hydrolyze cyclic nucleotides. Various PDEs play distinct roles in regulating cAMP and cyclic guanosine monophosphate (cGMP) in T-cells. Research on PDEs has gained growing interest due to their therapeutic potential to manipulate T-cell responses. So far, PDE4 is the best-described PDE in T-cells and the first PDE that is currently targeted in clinical practice to treat autoimmune diseases. But also, other PDE families harbor additional therapeutic potential. PDE2A is a dual-substrate phosphodiesterase which is selectively upregulated in Tcon upon activation. In this Mini-Review, we will highlight the impact of cAMP regulation on T-cell activation and function and summarize recent findings on different PDEs regulating intracellular cAMP levels in T-cells.

Metabolic interplay: tumor macrophages and regulatory T cells

The tumor microenvironment (TME) contains a complex cellular ecosystem where cancer, stromal, vascular, and immune cells interact. Macrophages and regulatory T cells (Tregs) are critical not only for maintaining immunological homeostasis and tumor growth but also for monitoring the functional states of other immune cells. Emerging evidence reveals that metabolic changes in macrophages and Tregs significantly influence their pro-/antitumor functions through the regulation of signaling cascades and epigenetic reprogramming. Hence, they are increasingly recognized as therapeutic targets in cancer immunotherapy. Specific metabolites in the TME may also affect their pro-/antitumor functions by intervening with the metabolic machinery. We discuss how metabolites influence the immunosuppressive phenotypes of tumor-associated macrophages (TAMs) and Tregs. We then describe how TAMs and Tregs, independently or collaboratively, utilize metabolic mechanisms to suppress the activity of CD8+ T cells. Finally, we highlight promising metabolic interventions that can improve the outcome of current cancer therapies.

Linked sensitization by memory CD4+ T cells prevents costimulation blockade–induced transplantation tolerance

Dominant infectious tolerance explains how brief tolerance-inducing therapies result in lifelong tolerance to donor antigens and “linked” third-party antigens, while recipient sensitization and ensuing immunological memory prevent the successful induction of transplant tolerance. In this study, we juxtapose these 2 concepts to test whether mechanisms of dominant infectious tolerance can control a limited repertoire of memory T and B cells. We show that sensitization to a single donor antigen is sufficient to prevent stable transplant tolerance, rendering it unstable. Mechanistic studies revealed that recall antibody responses and memory CD8⁺ T cell expansion were initially controlled, but memory CD4⁺Foxp3^– T cell (Tconv) responses were not. Remarkably, naive donor-specific Tconvs at tolerance induction also acquired a resistance to tolerance, proliferating and acquiring a phenotype similar to memory Tconvs. This phenomenon of “linked sensitization” underscores the challenges of reprogramming a primed immune response toward tolerance and identifies a potential therapeutic checkpoint for synergizing with costimulation blockade to achieve transplant tolerance in the clinic.

Upregulation of Phosphodiesterase 2A Augments T Cell Activation by Changing cGMP/cAMP Cross-Talk

3′,5′-cyclic adenosine monophosphate (cAMP) is well-known for its diverse immunomodulatory properties, primarily inhibitory effects during T cell activation, proliferation, and production of pro-inflammatory cytokines. A decrease in cAMP levels, due to the hydrolyzing activity of phosphodiesterases (PDE), is favoring inflammatory responses. This can be prevented by selective PDE inhibitors, which makes PDEs important therapeutic targets for autoimmune disorders. In this study, we investigated the specific roles of PDE2A and PDE3B in the regulation of intracellular cAMP levels in different mouse T cell subsets. Unexpectedly, T cell receptor (TCR) activation led to a selective upregulation of PDE2A at the protein level in conventional T cells (Tcon), whereas no changes were detected in regulatory T cells (Treg). In contrast, protein expression of PDE3B was significantly higher in both non-activated and activated Tcon subsets as compared to Treg, with no changes upon TCR engagement. Live-cell imaging of T cells expressing a highly sensitive Förster resonance energy transfer (FRET)-based biosensor, Epac1-camps, has enabled cAMP measurements in real time and revealed stronger responses to the PDE2A inhibitors in activated vs non-activated Tcon. Importantly, stimulation of intracellular cGMP levels with natriuretic peptides led to an increase of cAMP in non-activated and a decrease of cAMP in activated Tcon, suggesting that TCR activation changes the PDE3B-dependent positive to PDE2A-dependent negative cGMP/cAMP cross-talk. Functionally, this switch induced higher expression of early activation markers CD25 and CD69. This constitutes a potentially interesting feed-forward mechanism during autoimmune and inflammatory responses that may be exploited therapeutically.

Role of Phosphodiesterase 7 (PDE7) in T Cell Activity. Effects of Selective PDE7 Inhibitors and Dual PDE4/7 Inhibitors on T Cell Functions

Phosphodiesterase 7 (PDE7), a cAMP-specific PDE family, insensitive to rolipram, is present in many immune cells, including T lymphocytes. Two genes of PDE7 have been identified: PDE7A and PDE7B with three or four splice variants, respectively. Both PDE7A and PDE7B are expressed in T cells, and the predominant splice variant in these cells is PDE7A1. PDE7 is one of several PDE families that terminates biological functions of cAMP—a major regulating intracellular factor. However, the precise role of PDE7 in T cell activation and function is still ambiguous. Some authors reported its crucial role in T cell activation, while according to other studies PDE7 activity was not pivotal to T cells. Several studies showed that inhibition of PDE7 by its selective or dual PDE4/7 inhibitors suppresses T cell activity, and consequently T-mediated immune response. Taken together, it seems quite likely that simultaneous inhibition of PDE4 and PDE7 by dual PDE4/7 inhibitors or a combination of selective PDE4 and PDE7 remains the most interesting therapeutic target for the treatment of some immune-related disorders, such as autoimmune diseases, or selected respiratory diseases. An interesting direction of future studies could also be using a combination of selective PDE7 and PDE3 inhibitors.

Donor-specific chimeric antigen receptor Tregs limit rejection in naive but not sensitized allograft recipients

Cell therapy with autologous donor-specific regulatory T cells (Tregs) is a promising strategy to minimize immunosuppression in transplant recipients. Chimeric antigen receptor (CAR) technology has recently been used successfully to generate donor-specific Tregs and overcome the limitations of enrichment protocols based on repetitive stimulations with alloantigens. However, the ability of CAR-Treg therapy to control alloreactivity in immunocompetent recipients is unknown. We first analyzed the effect of donor-specific CAR Tregs on alloreactivity in naive, immunocompetent mice receiving skin allografts. Tregs expressing an irrelevant or anti-HLA-A2-specific CAR were administered to Bl/6 mice at the time of transplanting an HLA-A2⁺ Bl/6 skin graft. Donor-specific CAR-Tregs, but not irrelevant-CAR Tregs, significantly delayed skin rejection and diminished donor-specific antibodies (DSAs) and frequencies of DSA-secreting B cells. Donor-specific CAR-Treg-treated mice also had a weaker recall DSA response, but normal responses to an irrelevant antigen, demonstrating antigen-specific suppression. When donor-specific CAR Tregs were tested in HLA-A2-sensitized mice, they were unable to delay allograft rejection or diminish DSAs. The finding that donor-specific CAR-Tregs restrain de novo but not memory alloreactivity has important implications for their use as an adoptive cell therapy in transplantation.

Central Roles of OX40L-OX40 Interaction in the Induction and Progression of Human T Cell-Driven Acute Graft-versus-Host Disease

Graft-versus-host disease (GVHD) is one of the major obstacles for the success of allogeneic hematopoietic stem cell transplantation. Here, we report that the interaction between OX40L and OX40 is of critical importance for both induction and progression of acute GVHD (aGVHD) driven by human T cells. Anti-human OX40L monoclonal antibody (hOX40L) treatment could thus effectively reduce the disease severity in a xenogeneic-aGVHD (x-aGVHD) model in both preventative and therapeutic modes. Mechanistically, blocking OX40L-OX40 interaction with an anti-hOX40L antibody reduces infiltration of human T cells in target organs, including liver, gut, lung, and skin. It also decreases IL-21- and TNF-producing T cell responses, while promoting regulatory T cell (Treg) responses without compromising the cytolytic activity of CD8⁺ T cells. Single blockade of hOX40L was thus more effective than dual blockade of IL-21 and TNF in reducing the severity of aGVHD as well as mortality. Data from this study indicate that OX40L-OX40 interactions play a central role in the pathogenesis of aGVHD induced by human T cells. Therapeutic strategies that can efficiently interrupt OX40L-OX40 interaction in patients might have potential to provide patients with an improved clinical benefit.

Targeting Adenosine in Cancer Immunotherapy to Enhance T-Cell Function

T cells play a critical role in cancer control, but a range of potent immunosuppressive mechanisms can be upregulated in the tumor microenvironment (TME) to abrogate their activity. While various immunotherapies (IMTs) aiming at re-invigorating the T-cell-mediated anti-tumor response, such as immune checkpoint blockade (ICB), and the adoptive cell transfer (ACT) of natural or gene-engineered ex vivo expanded tumor-specific T cells, have led to unprecedented clinical responses, only a small proportion of cancer patients benefit from these treatments. Important research efforts are thus underway to identify biomarkers of response, as well as to develop personalized combinatorial approaches that can target other inhibitory mechanisms at play in the TME. In recent years, adenosinergic signaling has emerged as a powerful immuno-metabolic checkpoint in tumors. Like several other barriers in the TME, such as the PD-1/PDL-1 axis, CTLA-4, and indoleamine 2,3-dioxygenase (IDO-1), adenosine plays important physiologic roles, but has been co-opted by tumors to promote their growth and impair immunity. Several agents counteracting the adenosine axis have been developed, and pre-clinical studies have demonstrated important anti-tumor activity, alone and in combination with other IMTs including ICB and ACT. Here we review the regulation of adenosine levels and mechanisms by which it promotes tumor growth and broadly suppresses protective immunity, with extra focus on the attenuation of T cell function. Finally, we present an overview of promising pre-clinical and clinical approaches being explored for blocking the adenosine axis for enhanced control of solid tumors.

Regulatory T-cell therapy in liver transplantation

Modern immunosuppression drug regimens have produced excellent short-term survival after liver transplantation but it is generally accepted that the side effects of these medications remain a significant contributing factor for less satisfactory long term outcomes. The liver has unique tolerogenic properties as evidenced by the higher rates of operational tolerance seen in liver transplant recipients compared to other solid organ transplants, and therefore, liver transplantation offers an attractive setting in which to study tolerizing therapies. CD4⁺ CD25⁺ FOXP3⁺ regulatory T cells (Tregs) are crucial for maintenance of self-tolerance and prevention of autoimmune disease and are therefore an appealing potential candidate for use as a tolerizing cell therapy. In this review, we summarize the evidence from drug withdrawal trials of spontaneous operational tolerance in liver transplantation, the unique immunology of the hepatic microenvironment, the evidence for the use of CD4⁺ CD25⁺ FOXP3⁺ regulatory T cells as a tolerance inducing therapy in liver transplantation and the challenges in producing clinical grade Treg cell products.

Engineering therapeutic T cells to suppress alloimmune responses using TCRs, CARs, or BARs

Adoptive cell therapy with therapeutic T cells has become one of the most promising strategies to stimulate or suppress immune responses. Using virus-mediated genetic manipulation, the antigen specificity of T cells can now be precisely redirected. Tailored specificity has not only overcome technical limitations and safety concerns but also considerably broadened the spectrum of therapeutic applications. Different T cell-engineering strategies have now become available to suppress alloimmune responses. We first provide an overview of the allorecognition pathways and effector mechanisms that are responsible for alloimmune injuries in the setting of vascularized organ transplantation. We then discuss the potential to use different T cell-engineering approaches to suppress alloimmune responses. Specifically, expression of allospecific T cell receptors, single-chain chimeric antigen receptors, or antigen domains recognized by B cell receptors (B cell antibody receptors) in regulatory or cytotoxic T cells are considered. The ability of these strategies to control the direct or indirect pathways of allorecognition and the cellular or humoral alloimmune responses is discussed. An intimate understanding of the complex interplay that occurs between the engineered T cells and the alloimmune players is a necessary prerequisite for the design of safe and successful strategies for precise immunomodulation in transplantation.

A Rapamycin-Based GMP-Compatible Process for the Isolation and Expansion of Regulatory T Cells for Clinical Trials

The concept of regulatory T cell (Treg)-based immunotherapy has enormous potential for facilitating tolerance in autoimmunity and transplantation. Clinical translation of Treg cell therapy requires production processes that satisfy the rigors of Good Manufacturing Practice (GMP) standards. In this regard, we report our findings on the implementation of a robust GMP compliant process for the ex vivo expansion of clinical grade Tregs, demonstrating the feasibility of this developed process for the manufacture of a final product for clinical application. This Treg isolation procedure ensured the selection of a pure Treg population that underwent a 300-fold expansion after 36 days of culture, while maintaining a purity of more than 75% CD4⁺CD25⁺FOXP3⁺ cells and a suppressive function of above 80%. Furthermore, we report the successful cryopreservation of the final product, demonstrating the maintenance of phenotype and function. The process outlined in this manuscript has been implemented in the ONE study, a multicenter phase I/IIa clinical trial in which cellular therapy is investigated in renal transplantation.

Propentofylline, phosphodiesterase and adenosine reuptake inhibitor modulates lymphocyte subsets and lymphocyte activity after in-vivo administration in non-immunized and SRBC-immunized mice

OBJECTIVES

The aim of the study was to investigate immunomodulatory effect of in-vivo administered propentofylline on the subsets and activity of murine lymphocytes.

METHODS

Propentofylline (3 mg/kg) was administered orally to 8-week-old Balb/c mice, once or six times at 12-h intervals. The lymphocyte subsets, regulatory T cells, IL-5 and TNF levels were determined 12 h and 24 h after a single dose or after the sixth dose of the drug in non-immunized mice. Humoral immune response in sheep red blood cells (SRBC)-immunized mice was determined 4, 7 and 14 days after immunization.

KEY FINDINGS

Propentofylline inhibited thymocyte maturation (increase in CD4^- CD8^- thymocyte subset and decrease in the percentage of CD4⁺ CD8⁺ thymocytes) and modulated the lymphocyte subsets in spleen and mesenteric lymph nodes. An increase in the absolute count and percentage of splenic regulatory T cells (CD4⁺ CD25⁺ Foxp3⁺ cells) was noticed 24 h after single administration of the drug. Propentofylline lowered serum level of IL-5 and did not affect TNF concentration. Only a weak inhibitory effect on anti-SRBC humoral immune response was observed.

CONCLUSIONS

Propentofylline administration induced inhibition of thymocyte maturation and an increase in Treg subset that might be beneficial for an inhibition of immune response.

Regulatory T-Cell-Mediated Suppression of Conventional T-Cells and Dendritic Cells by Different cAMP Intracellular Pathways

Regulatory T-cells (Tregs) mediate their suppressive action by acting directly on conventional T-cells (Tcons) or dendritic cells (DCs). One mechanism of Treg suppression is the increase of cyclic adenosine 3′,5′-monophosphate (cAMP) levels in target cells. Tregs utilize cAMP to control Tcon responses, such as proliferation and cytokine production. Tregs also exert their suppression on DCs, diminishing DC immunogenicity by downmodulating the expression of costimulatory molecules and actin polymerization at the immunological synapse. The Treg-mediated usage of cAMP occurs through two major mechanisms. The first involves the Treg-mediated influx of cAMP in target cells through gap junctions. The second is the conversion of adenosine triphosphate into adenosine by the ectonucleases CD39 and CD73 present on the surface of Tregs. Adenosine then binds to receptors on the surface of target cells, leading to increased intracellular cAMP levels in these targets. Downstream, cAMP can activate the canonical protein kinase A (PKA) pathway and the exchange protein activated by cyclic AMP (EPAC) non-canonical pathway. In this review, we discuss the most recent findings related to cAMP activation of PKA and EPAC, which are implicated in Treg homeostasis as well as the functional alterations induced by cAMP in cellular targets of Treg suppression.

Effect of roflumilast, novel phosphodiesterase-4 inhibitor, on lung chronic graft-versus-host disease in mice

Chronic graft-versus-host disease (CGVHD) is a serious complication of allogeneic hematopoietic stem cell transplantation. Roflumilast has anti-inflammatory effects and has been used in the treatment of inflammatory airway diseases. It is at present unclear whether roflumilast may have a therapeutic role in CGVHD. To test this, we used the B10.D2 → BALB/c model of CGVHD to address the therapeutic effect of roflumilast on the development of CGVHD. Lungs of animals treated with roflumilast exhibited less chronic inflammatory cell infiltration and fibrosis in the peribronchial and perivascular area versus allogeneic controls. To define the mechanism, we examined the expression of pro-inflammatory and profibrotic cytokines in the lung. Messenger RNA expression of interleukin-6 and interleukin-1β in the lungs was significantly reduced in recipients treated with roflumilast. Similar changes were observed in profibrotic cytokines and chemokines. In addition, the percentage of Foxp3(+) regulatory T cells (Tregs), which have the potential to attenuate GVHD, increased significantly within the CD4(+) T cells with roflumilast in the lungs. In conclusion, roflumilast treatment attenuated murine lung CGVHD by blocking T-cell activation mediated by Tregs and downregulating pro-inflammatory and profibrotic cytokines, resulting in the reduction of lung inflammation and fibrosis.

Blocking MHC class II on human endothelium mitigates acute rejection

Acute allograft rejection is mediated by host CD8⁺ cytotoxic T lymphocytes (CTL) targeting graft class I major histocompatibility complex (MHC) molecules. In experimental rodent models, rejection requires differentiation of naive CD8⁺ T cells into alloreactive CTL within secondary lymphoid organs, whereas in humans, CTL may alternatively develop within the graft from circulating CD8⁺ effector memory T cells (T_EM) that recognize class I MHC molecules on graft endothelial cells (EC). This latter pathway is poorly understood. Here, we show that host CD4⁺ T_EM, activated by EC class II MHC molecules, provide critical help for this process. First, blocking HLA-DR on EC lining human artery grafts in immunodeficient mice reduces CD8⁺ CTL development within and acute rejection of the artery by adoptively transferred allogeneic human lymphocytes. Second, siRNA knockdown or CRISPR/Cas9 ablation of class II MHC molecules on EC prevents CD4⁺ T_EM from helping CD8⁺ T_EM to develop into CTL in vitro. Finally, implanted synthetic microvessels, formed from CRISPR/Cas9-modified EC lacking class II MHC molecules, are significantly protected from CD8⁺ T cell-mediated destruction in vivo. We conclude that human CD8⁺ T_EM-mediated rejection targeting graft EC class I MHC molecules requires help from CD4⁺ T_EM cells activated by recognition of class II MHC molecules.

Humanized Mice as Preclinical Models in Transplantation

Animal models have been instrumental in our understanding of the mechanisms of rejection and the testing of novel treatment options in the context of transplantation. We have now entered an exciting era with research on humanized mice driving advances in translational studies and in our understanding of the function of human cells in response to pathogens and cancer as well as the recognition of human allogeneic tissues in vivo. In this chapter we provide a historical overview of humanized mouse models of transplantation to date, outlining the distinct strains and share our experiences in the study of human transplantation immunology.

A simplified cuff technique for abdominal aortic transplantation in mice

BACKGROUND

Allograft arteriopathy is still a leading cause of late organ failure. The aortic allograft model in mice has been used to study chronic rejection and has given useful information in the development of graft arteriosclerosis. However, the technical difficulties of small vessel anastomoses still continue to limit its widespread use. We introduce a new simple method for aortic transplantation in mice.

METHODS

The descending aorta or infrarenal aorta from the donor mouse was anastomosed to the infrarenal aorta using a cuff technique. Aortic transplantation was performed in 30 mice, 10 isografts and 20 allografts. No immunosuppression was administered, and the recipients were sacrificed at day 28. The grafts were histologically analyzed.

RESULTS

Implantation of grafts could be completed in an average of 23 min. There was no technical failure in all 60 anastomoses. The overall survival rate was 93.3%. Histology of aortas revealed typical aspects of chronic rejection in the allografts at day 28. No significant lesion was observed in isografts.

CONCLUSIONS

We have developed an innovative, stable, and simple aortic transplantation model in mice, which is useful for vascular research in transplantation and beyond.

Use of Humanized Mice to Study the Pathogenesis of Autoimmune and Inflammatory Diseases

Animal models of disease have been used extensively by the research community for the past several decades to better understand the pathogenesis of different diseases and assess the efficacy and toxicity of different therapeutic agents. Retrospective analyses of numerous preclinical intervention studies using mouse models of acute and chronic inflammatory diseases reveal a generalized failure to translate promising interventions or therapeutics into clinically effective treatments in patients. Although several possible reasons have been suggested to account for this generalized failure to translate therapeutic efficacy from the laboratory bench to the patient's bedside, it is becoming increasingly apparent that the mouse immune system is substantially different from the human. Indeed, it is well known that >80 major differences exist between mouse and human immunology; all of which contribute to significant differences in immune system development, activation, and responses to challenges in innate and adaptive immunity. This inconvenient reality has prompted investigators to attempt to humanize the mouse immune system to address important human-specific questions that are impossible to study in patients. The successful long-term engraftment of human hematolymphoid cells in mice would provide investigators with a relatively inexpensive small animal model to study clinically relevant mechanisms and facilitate the evaluation of human-specific therapies in vivo. The discovery that targeted mutation of the IL-2 receptor common gamma chain in lymphopenic mice allows for the long-term engraftment of functional human immune cells has advanced greatly our ability to humanize the mouse immune system. The objective of this review is to present a brief overview of the recent advances that have been made in the development and use of humanized mice with special emphasis on autoimmune and chronic inflammatory diseases. In addition, we discuss the use of these unique mouse models to define the human-specific immunopathological mechanisms responsible for the induction and perpetuation of chronic gut inflammation.

The potential role for regulatory T-cell therapy in vascularized composite allograft transplantation

PURPOSE OF REVIEW

Vascularized composite allograft (VCA) transplantation restores defects to a degree not possible by conventional techniques. However, it is limited by the need for long-term immunosuppression and high rates of acute rejection directed against skin. There is therefore a need for a therapy that may shift the risk-benefit ratio in favour of VCA transplantation. Regulatory T cells (Tregs) are a subset of T cells with potent immunoregulatory properties and the potential to promote immunosuppression-free allograft survival. In this review, we consider the evidence for Treg therapy in VCA transplantation.

RECENT FINDINGS

CD4 Tregs are the best-studied immunoregulatory cell type, and a large amount of experimental and clinical data is emerging to endorse their use in VCA transplantation. Data from animal and humanized models are particularly encouraging and demonstrate the potent efficacy of Treg at preventing skin allograft rejection. Moreover, central tolerance induction techniques in VCA transplantation models are demonstrating a dependence on Tregs for graft survival.

SUMMARY

An improvement in outcomes after VCA transplantation has the potential to revolutionize the field. Several effective therapeutic strategies have demonstrated great promise experimentally, and there is now a need to assess their safety and efficacy in a clinical setting.

Cell therapy to induce allograft tolerance: time to switch to plan B?

Organ transplantation is widely acknowledged as the best option for end stage failure of vital organs. Long-term graft survival is however limited by graft rejection, a destructive process resulting from the response of recipient’s immune system against donor-specific alloantigens. Prevention of rejection currently relies exclusively on immunosuppressive drugs that lack antigen specificity and therefore increase the risk for infections and cancers. Induction of donor-specific tolerance would provide indefinite graft survival without morbidity and therefore represents the grail of transplant immunologists. Progresses in the comprehension of immunoregulatory mechanisms over the last decades have paved the way for cell therapies to induce allograft tolerance. The first part of the present article reviews the promising results obtained in experimental models with adoptive transfer of ex vivo-expanded regulatory CD4+ T cells (CD4+ Tregs) and discuss which source and specificity should be preferred for transferred CD4+ Tregs. Interestingly, B cells have recently emerged as potent regulatory cells, able to establish a privileged crosstalk with CD4+ T cells. The second part of the present article reviews the evidences demonstrating the crucial role of regulatory B cells in transplantation tolerance. We propose the possibility to harness B cell regulatory functions to improve cell-based therapies aiming at inducing allograft tolerance.

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.

Induction of transplantation tolerance through regulatory cells: from mice to men

Organ transplantation results in the activation of both innate and adaptive immune responses to the foreign antigens. While these responses can be limited with the use of systemic immunosuppressants, the induction of regulatory cell populations may be a novel strategy for the maintenance of specific immunological unresponsiveness that can reduce the severity of the detrimental side effects of current therapies. Our group has extensively researched different regulatory T-cell induction protocols for use as cellular therapy in transplantation. In this review, we address the cellular and molecular mechanisms behind regulatory T-cell suppression and their stability following induction protocols. We further discuss the use of different hematopoietically derived regulatory cell populations, including regulatory B cells, regulatory macrophages, tolerogenic dendritic cells, and myeloid-derived suppressor cells, for the induction of transplantation tolerance in light of new clinical trials developing therapies with some of these populations.

Humanized mouse models in transplantation research

The interest in the use of humanized mouse models for research topics like Graft versus Host Disease (GvHD), allograft studies and other studies to the human immune system is growing. The design of these models is still improving and enables even more complicated studies to these topics. For researchers it can be difficult to choose the best option from the current pool of available models. The decision will depend on which hypothesis needs to be tested, in which field of interest, and therefore 'the best model' will differ from one to another. In this review, we provide a guide to the most common available humanized mouse models, with regards to different mouse strains, transplantation material, transplantation techniques, pre- and post-conditioning and references to advantages and disadvantages. Also, an evaluation of experiences with humanized mouse models in studies on GvHD and allograft rejection is provided.

Transplantation tolerance

Although transplantation has been a standard medical practice for decades, marked morbidity from the use of immunosuppressive drugs and poor long-term graft survival remain important limitations in the field. Since the first solid organ transplant between the Herrick twins in 1954, transplantation immunology has sought to move away from harmful, broad-spectrum immunosuppressive regimens that carry with them the long-term risk of potentially life-threatening opportunistic infections, cardiovascular disease, and malignancy, as well as graft toxicity and loss, towards tolerogenic strategies that promote long-term graft survival. Reports of "transplant tolerance" in kidney and liver allograft recipients whose immunosuppressive drugs were discontinued for medical or non-compliant reasons, together with results from experimental models of transplantation, provide the proof-of-principle that achieving tolerance in organ transplantation is fundamentally possible. However, translating the reconstitution of immune tolerance into the clinical setting is a daunting challenge fraught with the complexities of multiple interacting mechanisms overlaid on a background of variation in disease. In this article, we explore the basic science underlying mechanisms of tolerance and review the latest clinical advances in the quest for transplantation tolerance.

Standardization of whole blood immune phenotype monitoring for clinical trials: panels and methods from the ONE study

Background

Immune monitoring by flow cytometry is a fast and highly informative way of studying the effects of novel therapeutics aimed at reducing transplant rejection or treating autoimmune diseases. The ONE Study consortium has recently initiated a series of clinical trials aimed at using different cell therapies to promote tolerance to renal allografts. To compare the effectiveness of different cell therapies, the consortium developed a robust immune monitoring strategy, including procedures for whole blood (WB) leukocyte subset profiling by flow cytometry.

Methods

Six leukocyte profiling panels computing 7- to 9-surface marker antigens for monitoring the major leukocyte subsets as well as characteristics of T cell, B cell, and dendritic cell (DC) subsets were designed. The precision and variability of these panels were estimated. The assay was standardized within eight international laboratories using Flow-Set Pro beads for mean fluorescence intensity target definition and the flow cytometer setup procedure. Standardization was demonstrated by performing inter-site comparisons.

Results

Optimized methods for sample collection, storage, preparation, and analysis were established, including protocols for gating target subsets. WB specimen age testing demonstrated that staining must be performed within 4 hours of sample collection to keep variability low, meaning less than or equal to 10% for the majority of defined leukocyte subsets. Inter-site comparisons between all participating centers testing shipped normal WB revealed good precision, with a variability of 0.05% to 30% between sites. Intra-assay analyses revealed a variability of 0.05% to 20% for the majority of subpopulations. This was dependent on the frequency of the particular subset, with smaller subsets showing higher variability. The intra-assay variability performance defined limits of quantitation (LoQ) for subsets, which will be the basis for assessing statistically significant differences achieved by the different cell therapies.

Conclusions

Local performance and central analysis of the ONE Study flow cytometry panel yields acceptable variability in a standardized assay at multiple international sites. These panels and procedures with WB allow unmanipulated analysis of changes in absolute cell numbers of leukocyte subsets in single- or multicenter clinical trials. Accordingly, we propose the ONE Study panel may be adopted as a standardized method for monitoring patients in clinical trials enrolling transplant patients, particularly trials of novel tolerance promoting therapies, to facilitate fair and meaningful comparisons between trials.

Do Natural T Regulatory Cells become Activated to Antigen Specific T Regulatory Cells in Transplantation and in Autoimmunity?

Antigen specific T regulatory cells (T_reg) are often CD4⁺CD25⁺FoxP3⁺ T cells, with a phenotype similar to natural T_reg (nT_reg). It is assumed that nT_reg cannot develop into an antigen specific T_reg as repeated culture with IL-2 and a specific antigen does not increase the capacity or potency of nT_reg to promote immune tolerance or suppress in vitro. This has led to an assumption that antigen specific T_reg mainly develop from CD4⁺CD25⁻FoxP3⁻ T cells, by activation with antigen and TGF-β in the absence of inflammatory cytokines such as IL-6 and IL-1β. Our studies on antigen specific CD4⁺CD25⁺ T cells from animals with tolerance to an allograft, identified that the antigen specific and T_reg are dividing, and need continuous stimulation with specific antigen T cell derived cytokines. We identified that a variety of cytokines, especially IL-5 and IFN-γ but not IL-2 or IL-4 promoted survival of antigen specific CD4⁺CD25⁺FoxP3⁺ T_reg. To examine if nT_reg could be activated to antigen specific T_reg, we activated nT_reg in culture with either IL-2 or IL-4. Within 3 days, antigen specific T_reg are activated and there is induction of new cytokine receptors on these cells. Specifically nT_reg activated by IL-2 and antigen express the interferon-γ receptor (IFNGR) and IL-12p70 (IL-12Rβ2) receptor but not the IL-5 receptor (IL-5Rα). These cells were responsive to IFN-γ or IL-12p70. nT_reg activated by IL-4 and alloantigen express IL-5Rα not IFNGR or IL-12p70Rβ2 and become responsive to IL-5. These early activated antigen specific T_reg, were respectively named Ts1 and Ts2 cells, as they depend on Th1 or Th2 responses. Further culture of Ts1 cells with IL-12p70 induced Th1-like T_reg, expressing IFN-γ, and T-bet as well as FoxP3. Our studies suggest that activation of nT_reg with Th1 or Th2 responses induced separate lineages of antigen specific T_reg, that are dependent on late Th1 and Th2 cytokines, not the early cytokines IL-2 and IL-4.

Adenosine and prostaglandin e2 production by human inducible regulatory T cells in health and disease

Regulatory T cells (Treg) play a key role in maintaining the balance of immune responses in human health and in disease. Treg come in many flavors and can utilize a variety of mechanisms to modulate immune responses. In cancer, inducible (i) or adaptive Treg expand, accumulate in tissues and peripheral blood of patients, and represent a functionally prominent component of CD4+ T lymphocytes. Phenotypically and functionally, iTreg are distinct from natural (n) Treg. A subset of iTreg expressing ectonucleotidases CD39 and CD73 is able to hydrolyze ATP to 5′-AMP and adenosine (ADO) and thus mediate suppression of those immune cells which express ADO receptors. iTreg can also produce prostaglandin E₂ (PGE₂). The mechanisms responsible for iTreg-mediated suppression involve binding of ADO and PGE₂ produced by iTreg to their respective receptors expressed on T effector cells (Teff), leading to the up-regulation of adenylate cyclase and cAMP activities in Teff and to their functional inhibition. The potential for regulating these mechanisms by the use of pharmacologic inhibitors to relieve iTreg-mediated suppression in cancer suggests the development of therapeutic strategies targeting the ADO and PGE₂ pathways.

Promoting transplantation tolerance; adoptive regulatory T cell therapy

Transplantation is a successful treatment for end-stage organ failure. Despite improvements in short-term outcome, long-term survival remains suboptimal because of the morbidity and mortality associated with long-term use of immunosuppression. There is, therefore, a pressing need to devise protocols that induce tolerance in order to minimize or completely withdraw immunosuppression in transplant recipients. In this review we will discuss how regulatory T cells (T(regs)) came to be recognized as an attractive way to promote transplantation tolerance. We will summarize the preclinical data, supporting the importance of these cells in the induction and maintenance of immune tolerance and that provide the rationale for the isolation and expansion of these cells for cellular therapy. We will also describe the data from the first clinical trials, using T(regs) to inhibit graft-versus-host disease (GVHD) after haematopoietic stem cell transplantation and will address both the challenges and opportunities in human T(reg) cell therapy.

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.

Tregs: application for solid-organ transplantation

PURPOSE OF REVIEW

Transfer of human regulatory T cells (Tregs) has become an attractive therapeutic alternative to improve the long-term outcome in transplantation and thus reduce the side-effects of conventional immunosuppressive drugs. Here, we summarize the recent findings on human Treg subsets, their phenotype and in-vivo function.

RECENT FINDINGS

In the last 2 years, it has become apparent that several Treg subsets exist that specifically regulate Th1-driven, Th2-driven, or Th17-driven immune responses; these subsets are very unstable and rapidly change their phenotype, for example, there is loss of Foxp3 expression upon extensive ex-vivo expansion and only the administration of rapamycin has been shown to be able to interfere reproducibly. New humanized mouse models incorporating human solid-organ grafts have been developed, which have been used to test the human Treg in-vivo function, and the first human Treg-cell products have been tested for safety and efficacy in stem cell transplantation.

SUMMARY

With the recent findings, we have gained a better understanding of Treg heterogeneity, plasticity and function. Using the outcomes of clinical trials in stem cell transplantation, we have learned that adoptive therapy of Tregs is well tolerated and we are now awaiting the first result in solid-organ transplantation from the 'ONE Study'.

Regulatory T-cell immunotherapy for allogeneic hematopoietic stem-cell transplantation

From mouse studies to recently published clinical trials, evidence has accumulated on the potential use of regulatory T cells (Treg) in preventing and treating graft-versus-host disease following hematopoietic-cell transplantation (HCT). However, controversies remain as to the phenotype and stability of various Treg subsets and their respective roles in vivo, the requirement of antigen-specificity of Treg to reduce promiscuous suppression, and the molecular mechanisms by which Treg suppress, particularly in humans. In this review, we discuss recent findings that support a heterogeneous population of human Treg, address advances in understanding how Treg function in the context of HCT, and present data on recent clinical trials that highlight the feasibility and limitations on Treg immunotherapy for graft-versus-host disease.

Mechanisms of cyclic nucleotide phosphodiesterases in modulating T cell responses in murine graft-versus-host disease

Graft-versus-host disease (GvHD) is a key contributor to the morbidity and mortality after allogeneic hematopoetic stem cell transplantation (HSCT). Regulatory Foxp3⁺ CD4⁺ T cells (T_reg) suppress conventional T cell activation and can control GvHD. In our previous work, we demonstrate that a basic mechanism of T_reg mediated suppression occurs by the transfer of cyclic adenosine monophosphate (cAMP) to responder cells. Whether this mechanism is relevant for T_reg mediated suppression of GvHD is currently unknown. To address this question, bone marrow and T cells from C57BL/6 mice were transferred into lethally irradiated BALB/c recipients, and the course of GvHD and survival were monitored. Transplanted recipients developed severe GvHD that was strongly ameliorated by the transfer of donor T_reg cells. Towards the underlying mechanisms, in vitro studies revealed that T_reg communicated with DCs via gap junctions, resulting in functional inactivation of DC by a metabolic pathway involving cAMP that is modulated by the phosphodiesterase (PDE) 4 inhibitor rolipram. PDE2 or PDE3 inhibitors as well as rolipram suppressed allogeneic T cell activation, indirectly by enhancing T_reg mediated suppression of DC activation and directly by inhibiting responder T cell proliferation. In line with this, we observed a cooperative suppression of GvHD upon T_reg transfer and additional rolipram treatment. In conclusion, we propose that an important pathway of T_reg mediated control of GvHD is based on a cAMP dependent mechanism. These data provide the basis for future concepts to manipulate allogeneic T cell responses to prevent GvHD.

Clinical operational tolerance in liver transplantation: state-of-the-art perspective and future prospects

BACKGROUND

Liver transplantation is the definite treatment for end-stage liver diseases with satisfactory results. However, untoward effects of life-long immunosuppression prevent the development of alternative strategies to achieve better long-term outcome. Achieving clinical operational tolerance is the ultimate goal.

DATA SOURCES

A PubMed and Google Scholar search using terms: "immune tolerance", "liver transplantation", "clinical trial", "operational tolerance" and "immunosuppression withdrawal" was performed, and relevant articles published in English in the past decade were reviewed. Full-text publications relevant to the field were selected and relevant articles from reference lists were also included. Priority was given to those articles which are relevant to the review.

RESULTS

Because of the inherent tolerogenic property, around 20%-30% of liver transplantation recipients develop spontaneous operational tolerance after immunosuppression withdrawal, and the percentage may be even higher in pediatric living donor liver transplantation recipients. Several natural killer and gammadeltaT cell related markers have been identified to be associated with the tolerant state in liver transplantation patients. Despite the progress, clinical operational tolerance is still rare in liver transplantation. Reprogramming the recipient immune system by creating chimerism and regulatory cell therapies is among newer promising means to achieve clinical liver transplantation tolerance in the future.

CONCLUSION

Although clinical operational tolerance is still rare in liver transplantation recipients, ongoing basic research and collaborative clinical trials may help to decipher the mystery of transplantation tolerance and extend the potential benefits of drug withdrawal to an increasing number of patients in a more predictable fashion.

The Alpha Stem Cell Clinic: a model for evaluating and delivering stem cell-based therapies

Cellular therapies require the careful preparation, expansion, characterization, and delivery of cells in a clinical environment. There are major challenges associated with the delivery of cell therapies and high costs that will limit the companies available to fully evaluate their merit in clinical trials, and will handicap their application at the present financial environment. Cells will be manufactured in good manufacturing practice or near-equivalent facilities with prerequisite safety practices in place, and cell delivery systems will be specialized and require well-trained medical and nursing staff, technicians or nurses trained to handle cells once delivered, patient counselors, as well as statisticians and database managers who will oversee the monitoring of patients in relatively long-term follow-up studies. The model proposed for Alpha Stem Cell Clinics will initially use the capacities and infrastructure that exist in the most advanced tertiary medical clinics for delivery of established bone marrow stem cell therapies. As the research evolves, they will incorporate improved procedures and cell preparations. This model enables commercialization of medical devices, reagents, and other products required for cell therapies. A carefully constructed cell therapy clinical infrastructure with the requisite scientific, technical, and medical expertise and operational efficiencies will have the capabilities to address three fundamental and critical functions: 1) fostering clinical trials; 2) evaluating and establishing safe and effective therapies, and 3) developing and maintaining the delivery of therapies approved by the Food and Drug Administration, or other regulatory agencies.

The where and when of T cell regulation in transplantation

Multiple cell types contribute to the peripheral regulation of T cell alloresponses in haematopoieitc cell transplantation (HCT) and solid organ transplantation (SOT). Of these, regulatory T cells (Tregs) are the principal players and have shown the greatest success in the therapeutic control of detrimental immune responses. Investigations into the induction, location, and mechanism of suppression utilised by Tregs to control alloreactive responses are ongoing. The activation and homing characteristics of Tregs are important to their regulatory capabilities, with activation and homing occurring in the same time and space as conventional T cells. This review discusses these characteristics and recent advances in the field as we move closer to the ultimate goal of utilising Tregs as treatment for allograft rejection and graft-versus-host disease (GvHD).

Humanized mouse model of skin inflammation is characterized by disturbed keratinocyte differentiation and influx of IL-17A producing T cells

Humanized mouse models offer a challenging possibility to study human cell function in vivo. In the huPBL-SCID-huSkin allograft model human skin is transplanted onto immunodeficient mice and allowed to heal. Thereafter allogeneic human peripheral blood mononuclear cells are infused intra peritoneally to induce T cell mediated inflammation and microvessel destruction of the human skin. This model has great potential for in vivo study of human immune cells in (skin) inflammatory processes and for preclinical screening of systemically administered immunomodulating agents. Here we studied the inflammatory skin response of human keratinocytes and human T cells and the concomitant systemic human T cell response.

As new findings in the inflamed human skin of the huPBL-SCID-huSkin model we here identified: 1. Parameters of dermal pathology that enable precise quantification of the local skin inflammatory response exemplified by acanthosis, increased expression of human β-defensin-2, Elafin, K16, Ki67 and reduced expression of K10 by microscopy and immunohistochemistry. 2. Induction of human cytokines and chemokines using quantitative real-time PCR. 3. Influx of inflammation associated IL-17A-producing human CD4+ and CD8+ T cells as well as immunoregulatory CD4+Foxp3+ cells using immunohistochemistry and -fluorescence, suggesting that active immune regulation is taking place locally in the inflamed skin. 4. Systemic responses that revealed activated and proliferating human CD4+ and CD8+ T cells that acquired homing marker expression of CD62L and CLA. Finally, we demonstrated the value of the newly identified parameters by showing significant changes upon systemic treatment with the T cell inhibitory agents cyclosporine-A and rapamycin.

In summary, here we equipped the huPBL-SCID-huSkin humanized mouse model with relevant tools not only to quantify the inflammatory dermal response, but also to monitor the peripheral immune status. This combined approach will gain our understanding of the dermal immunopathology in humans and benefit the development of novel therapeutics for controlling inflammatory skin diseases.

Identification of novel HLA class II target epitopes for generation of donor-specific T regulatory cells

Therapies capable of generating host T regulatory cells (T(R)) responsive to donor-specific HLA-class II minor histocompatibility antigens have the potential to promote tolerance of a transplanted organ. Our group has developed a novel approach for the identification of potentially therapeutic T(R) target antigens. We perform parallel non-synonymous SNP genotyping of HLA-identical subject pairs to identify peptide variations expressed by only one of the two subjects. Variant peptide pairs are then evaluated for binding a shared HLA-class II allele. Minor peptides predicted to bind HLA-class II with greater affinity than the common variant peptide are tested for HLA class II binding and in vitro induction of suppressive CD4+ T cells. Using this approach we have identified multiple pairs of variant peptides capable of differential binding and induction of suppressive CD4+ T cells. These data demonstrate the feasibility of identifying potentially therapeutic HLA class II minor antigens for generation of donor-specific T(R).

The role of the adenosinergic pathway in immunosuppression mediated by human regulatory T cells (Treg)

Tumor-induced dysfunction of immune cells is a common problem in cancer. Tumors induce immune suppression by many different mechanisms, including accumulation of regulatory T cells (Treg). Adaptive Treg (Tr1) generated in the tumor microenvironment express CD39 and CD73 ectonucleotidases, produce adenosine and are COX2+PGE2+. Adenosine and PGE2 produced by Tr1 or tumor cells bind to their respective receptors on the surface of T effector cells (Teff) and cooperate in up-regulating cytosolic 3'5'-cAMP levels utilizing adenylyl cyclase isoform 7 (AC-7). In Teff, increased cAMP mediates suppression of anti-tumor functions. Treg, in contrast to Teff, seem to require high cAMP levels for mediating suppression. This differential requirement of Treg and Teff for cAMP offers an opportunity for pharmacologic interventions using selected inhibitors of the adenosine/PGE2 pathways. Blocking of adenosine/PGE2 production by Tr1 or blocking binding of these factors to their receptors on T cells or inhibition of cAMP synthesis in Teff all represent novel therapeutic strategies that used in combination with conventional therapies could restore anti-tumor functions of Teff . At the same time, these inhibitors could disarm Tr1 cells by depriving them of the factors promoting their generation and activity or by down-regulating 3'5'-cAMP levels. Thus, the pharmacologic control of Treg-Teff interactions offers a novel strategy for restoration of anti-tumor Teff functions and silencing of Treg. Used in conjunction with anti-cancer drugs or with immune therapies, this strategy has a potential to improve therapeutic effects by preventing or reversing tumor-induced immune suppression.

Identification and expansion of highly suppressive CD8(+)FoxP3(+) regulatory T cells after experimental allogeneic bone marrow transplantation

FoxP3(+) confers suppressive properties and is confined to regulatory T cells (T(reg)) that potently inhibit autoreactive immune responses. In the transplant setting, natural CD4(+) T(reg) are critical in controlling alloreactivity and the establishment of tolerance. We now identify an important CD8(+) population of FoxP3(+) T(reg) that convert from CD8(+) conventional donor T cells after allogeneic but not syngeneic bone marrow transplantation. These CD8(+) T(reg) undergo conversion in the mesenteric lymph nodes under the influence of recipient dendritic cells and TGF-β. Importantly, this population is as important for protection from GVHD as the well-studied natural CD4(+)FoxP3(+) population and is more potent in exerting class I-restricted and antigen-specific suppression in vitro and in vivo. Critically, CD8(+)FoxP3(+) T(reg) are exquisitely sensitive to inhibition by cyclosporine but can be massively and specifically expanded in vivo to prevent GVHD by coadministering rapamycin and IL-2 antibody complexes. CD8(+)FoxP3(+) T(reg) thus represent a new regulatory population with considerable potential to preferentially subvert MHC class I-restricted T-cell responses after bone marrow transplantation.

The cellular and molecular mechanisms of immuno-suppression by human type 1 regulatory T cells

The immuno-regulatory mechanisms of IL-10-producing type 1 regulatory T (Tr1) cells have been widely studied over the years. However, several recent discoveries have shed new light on the cellular and molecular mechanisms that human Tr1 cells use to control immune responses and induce tolerance. In this review we outline the well known and newly discovered regulatory properties of human Tr1 cells and provide an in-depth comparison of the known suppressor mechanisms of Tr1 cells with FOXP3⁺ T_reg. We also highlight the role that Tr1 cells play in promoting and maintaining tolerance in autoimmunity, allergy, and transplantation.

Human allograft rejection in humanized mice: a historical perspective

Basic research in transplantation immunology has relied primarily on rodent models. Experimentation with rodents has laid the foundation for our basic understanding of the biological events that precipitate rejection of non-self or allogeneic tissue transplants and supported the development of novel strategies to specifically suppress allogeneic immune responses. However, translation of these studies to the clinic has met with limited success, emphasizing the need for new models that focus on human immune responses to allogeneic tissues. Humanized mouse models are an exciting alternative that permits investigation of the rejection of human tissues mediated by human immune cells without putting patients at risk. However, the use of humanized mice is complicated by a diversity of protocols and approaches, including the large number of immunodeficient mouse strains available, the choice of tissue to transplant and the specific human immune cell populations that can be engrafted. Here, we present a historical perspective on the study of allograft rejection in humanized mice and discuss the use of these novel model systems in transplant biology.

Targeting self- and foreign antigens to dendritic cells via DC-ASGPR generates IL-10-producing suppressive CD4+ T cells

Targeting antigens to the lectinlike DC-ASGPR receptor on human DCs and in nonhuman primates results in the induction of antigen-specific IL-10–producing CD4⁺ T cells.

Dendritic cells (DCs) can initiate and shape host immune responses toward either immunity or tolerance by their effects on antigen-specific CD4⁺ T cells. DC-asialoglycoprotein receptor (DC-ASGPR), a lectinlike receptor, is a known scavenger receptor. Here, we report that targeting antigens to human DCs via DC-ASGPR, but not lectin-like oxidized-LDL receptor, Dectin-1, or DC-specific ICAM-3–grabbing nonintegrin favors the generation of antigen-specific suppressive CD4⁺ T cells that produce interleukin 10 (IL-10). These findings apply to both self- and foreign antigens, as well as memory and naive CD4⁺ T cells. The generation of such IL-10–producing CD4⁺ T cells requires p38/extracellular signal-regulated kinase phosphorylation and IL-10 induction in DCs. We further demonstrate that immunization of nonhuman primates with antigens fused to anti–DC-ASGPR monoclonal antibody generates antigen-specific CD4⁺ T cells that produce IL-10 in vivo. This study provides a new strategy for the establishment of antigen-specific IL-10–producing suppressive T cells in vivo by targeting whole protein antigens to DCs via DC-ASGPR.

Gene and cell therapy based treatment strategies for inflammatory bowel diseases

Inflammatory bowel diseases (IBD) are a group of chronic inflammatory disorders most commonly affecting young adults. Currently available therapies can result in induction and maintenance of remission, but are not curative and have sometimes important side effects. Advances in basic research in IBD have provided new therapeutic opportunities to target the inflammatory process involved. Gene and cell therapy approaches are suitable to prevent inflammation in the gastrointestinal tract and show therefore potential in the treatment of IBD. In this review, we present the current progress in the field of both gene and cell therapy and future prospects in the context of IBD. Regarding gene therapy, we focus on viral vectors and their applications in preclinical models. The focus for cell therapy is on regulatory T lymphocytes and mesenchymal stromal cells, their potential for the treatment of IBD and the progress made in both preclinical models and clinical trials.

Clinical perspectives for regulatory T cells in transplantation tolerance

Three main types of CD4+ regulatory T cells can be distinguished based upon whether they express Foxp3 and differentiate naturally in the thymus (natural Tregs) or are induced in the periphery (inducible Tregs); or whether they are FoxP3 negative but secrete IL-10 in response to antigen (Tregulatory type 1, Tr1 cells). Adoptive transfer of each cell type has proven highly effective in mouse models at preventing graft vs. host disease (GVHD) and autoimmunity. Although clinical application was initially hampered by low Treg frequency and unfavorable ex vivo expansion properties, several phase I trials are now being conducted to assess their effect on GVHD following hematopoietic stem cell transplantation (HSCT) and in type I diabetes. Human Treg trials for HSCT recipients have preceded other indications because GVHD onset is precisely known, the time period needed for prevention relatively short, initial efficacy is likely to provide life-long protection, and complications of GVHD can be lethal. This review will summarize the clinical trials conducted to date that have employed Tregs to prevent GVHD following HSCT and discuss recent advances in Treg cellular therapy.

Potential for phosphodiesterase inhibitors in the management of autoimmune diseases

Phosphodiesterases (PDEs) and their substrates, cAMP and cGMP, are ubiquitously expressed in the immune system. Inhibiting PDEs may represent a novel approach for regulating immune functions and have a therapeutic potential in the management of autoimmune diseases. Phosphodiesterase inhibitors (PDEi) have proved effective in clinical trials in the management of pulmonary artery hypertension and Raynaud's phenomenon. Data from animal models suggest that the immunomodulatory effect of PDEi may have a therapeutic potential in the management of diseases such as multiple sclerosis and rheumatoid arthritis. The antifibrotic potential of PDEi, as suggested by animal experiments, if proven in further studies will be a major step in the treatment of fibrosing diseases such as scleroderma and idiopathic pulmonary fibrosis. Drug Dev Res 72:772–778, 2011. © 2011 Wiley Periodicals, Inc.

Regulatory T cells in acute myelogenous leukemia: is it time for immunomodulation?

The microenviroment of acute myelogenous leukemia (AML) is suppressive for immune effector cells. Regulatory T cells (Tregs) have been recognized as a contributor factor and may be recruited and exploited by leukemic cells to evade immunesurveillance. Studies have shown that the frequencies of marrow and blood Tregs are greater in patients with AML than in control patients. Although increased Tregs have been associated with a decreased risk of GVHD after allogeneic HCT and hence may impede the graft-versus-tumor effect, recent findings indicate that that this may not be the case. Because there is a need to improve outcomes of standard treatment (chemotherapy with or without allogeneic HCT) in AML, targeting Tregs present an outstanding opportunity in AML because discoveries may apply throughout its treatment. Here, we review data on the roles of Tregs in mediating immune system-AML interactions. We focused on in vitro, animal, and observational human studies of Tregs in AML biology, development, prognosis, and therapy in different settings (eg, vaccination and HCT). Manipulation of Tregs or other types of immunomodulation may become a part of AML treatment in the future.