OX-22high CD4+ T cells induce wasting disease with multiple organ pathology: prevention by the OX-22low subset

Non-invasive detection of regulatory T cells with Raman spectroscopy

Regulatory T cells (Tregs) are a type of lymphocyte that is key to maintaining immunological self-tolerance, with great potential for therapeutic applications. A long-standing challenge in the study of Tregs is that the only way they can be unambiguously identified is by using invasive intracellular markers. Practically, the purification of live Tregs is often compromised by other cell types since only surrogate surface markers can be used. We present here a non-invasive method based on Raman spectroscopy that can detect live unaltered Tregs by coupling optical detection with machine learning implemented with regularized logistic regression. We demonstrate the validity of this approach first on murine cells expressing a surface Foxp3 reporter, and then on peripheral blood human T cells. By including methods to account for sample purity, we could generate reliable models that can identify Tregs with an accuracy higher than 80%, which is already comparable with typical sorting purities achievable with standard methods that use proxy surface markers. We could also demonstrate that it is possible to reliably detect Tregs in fully independent donors that are not part of the model training, a key milestone for practical applications.

Ferritin heavy chain supports stability and function of the regulatory T cell lineage

Regulatory T (TREG) cells develop via a program orchestrated by the transcription factor forkhead box protein P3 (FOXP3). Maintenance of the TREG cell lineage relies on sustained FOXP3 transcription via a mechanism involving demethylation of cytosine-phosphate-guanine (CpG)-rich elements at conserved non-coding sequences (CNS) in the FOXP3 locus. This cytosine demethylation is catalyzed by the ten-eleven translocation (TET) family of dioxygenases, and it involves a redox reaction that uses iron (Fe) as an essential cofactor. Here, we establish that human and mouse TREG cells express Fe-regulatory genes, including that encoding ferritin heavy chain (FTH), at relatively high levels compared to conventional T helper cells. We show that FTH expression in TREG cells is essential for immune homeostasis. Mechanistically, FTH supports TET-catalyzed demethylation of CpG-rich sequences CNS1 and 2 in the FOXP3 locus, thereby promoting FOXP3 transcription and TREG cell stability. This process, which is essential for TREG lineage stability and function, limits the severity of autoimmune neuroinflammation and infectious diseases, and favors tumor progression. These findings suggest that the regulation of intracellular iron by FTH is a stable property of TREG cells that supports immune homeostasis and limits the pathological outcomes of immune-mediated inflammation.

A review of low dose interleukin-2 therapy in management of chronic graft-versus-host-disease

INTRODUCTION

Patients with chronic graft versus host disease (cGVHD) have low circulating regulatory T cells (Tregs). Interleukin-2(IL-2) is a growth factor for Tregs, and clinical trials have explored its use in cGVHD patients.

AREAS COVERED

Here we will discuss the biology of IL-2, its rationale for use and results of clinical trials in cGVHD. We also describe its mechanisms of action and alteration in gene expression in T-cell subsets after treatment with low dose IL-2 and photopheresis.

EXPERT OPINION

Clinical trials using Low dose IL-2 have been done at single centers in small patient series. The majority of the clinical responses seen with IL-2 in cGVHD are classified as partial responses and efficacy as a single agent is limited. Compared to currently approved oral therapies, it has to be administered subcutaneously and requires specialized processing for compounding and storage limiting its widespread use. Its use is associated with constitutional symptoms and local injection site reactions. Local reactions can be easily managed by supportive care practices like rotation of injection sites and premeditations, constitutional symptoms resolve with, dose reduction (25-50%) allowing for continued therapy. Additional studies are needed to define optimal combination strategies with approved agents. Longer acting formulations of IL-2 that require less frequent dosing may also improve patient compliance.

Localization and movement of Tregs in gastrointestinal tract: a systematic review

BACKGROUND

The intestine is rich in food-derived and microbe-derived antigens. Regulatory T cells (Tregs) are an essential T-cell population that prevents systemic autoimmune diseases and inhibits inflammation by encountering antigens. Previously, it was reported that the functional loss of Tregs induces systemic inflammation, including inflammatory bowel disease and graft-versus-host disease in human and murine models. However, there is a dearth of information about how Tregs localize in different tissues and suppress effector cells.

MAIN BODY

The development of Tregs and their molecular mechanism in the digestive tract have been elucidated earlier using murine genetic models, infectious models, and human samples. Tregs suppress immune and other nonimmune cells through direct effect and cytokine production. The recent development of in vivo imaging technology allows us to visualize how Tregs localize and move in the settings of inflammation and homeostasis. This is important because, according to a recent report, Treg characterization and function are regulated by their location. Tregs located in the proximal intestine and its draining lymph nodes induce tolerance against food antigens, and those located in the distal intestine suppress the inflammation induced by microbial antigens. Taken together, various Tregs are induced in a location-specific manner in the gastrointestinal tract and influence the homeostasis of the gut.

CONCLUSION

In this review, we summarize how Tregs are induced in the digestive tract and the application of in vivo Treg imaging to elucidate immune homeostasis in the digestive tract.

(Not) Home alone: Antigen presenting cell - T Cell communication in barrier tissues

Priming of T cells by antigen presenting cells (APCs) is essential for T cell fate decisions, enabling T cells to migrate to specific tissues to exert their effector functions. Previously, these interactions were mainly explored using blood-derived cells or animal models. With great advances in single cell RNA-sequencing techniques enabling analysis of tissue-derived cells, it has become clear that subsets of APCs are responsible for priming and modulating heterogeneous T cell effector responses in different tissues. This composition of APCs and T cells in tissues is essential for maintaining homeostasis and is known to be skewed in infection and inflammation, leading to pathological T cell responses. This review highlights the commonalities and differences of T cell priming and subsequent effector function in multiple barrier tissues such as the skin, intestine and female reproductive tract. Further, we provide an overview of how this process is altered during tissue-specific infections which are known to cause chronic inflammation and how this knowledge could be harnessed to modify T cell responses in barrier tissue.

The expanding impact of T-regs in the skin

As the interface between the body and the environment, the skin functions as the physical barrier against external pathogens and toxic agents. In addition, the skin is an immunologically active organ with a plethora of resident adaptive and innate immune cells, as well as effector molecules that provide another layer of protection in the form of an immune barrier. A major subpopulation of these immune cells are the Foxp3 expressing CD4 T cells or regulatory T cells (T-regs). The canonical function of T-regs is to keep other immune cells in check during homeostasis or to dissipate a robust inflammatory response following pathogen clearance or wound healing. Interestingly, recent data has uncovered unconventional roles that vary between different tissues and we will highlight the emerging non-lymphoid functions of cutaneous T-regs. In light of the novel functions of other immune cells that are routinely being discovered in the skin, their regulation by T-regs implies that T-regs have executive control over a broad swath of biological activities in both homeostasis and disease. The blossoming list of non-inflammatory functions, whether direct or indirect, suggests that the role of T-regs in a regenerative organ such as the skin will be a field ripe for discovery for decades to come.

IL-34 deficiency impairs FOXP3+ Treg function in a model of autoimmune colitis and decreases immune tolerance homeostasis

Background

Immune homeostasis requires fully functional Tregs with a stable phenotype to control autoimmunity. Although IL‐34 is a cytokine first described as mainly involved in monocyte cell survival and differentiation, we recently described its expression by CD8⁺ Tregs in a rat model of transplantation tolerance and by activated FOXP3⁺ CD4⁺ and CD8⁺ Tregs in human healthy individuals. However, its role in autoimmunity and potential in human diseases remains to be determined.

Methods

We generated Il34^−/− rats and using both Il34^−/− rats and mice, we investigated their phenotype under inflammatory conditions. Using Il34^−/− rats, we further analyzed the impact of the absence of expression of IL‐34 for CD4⁺ Tregs suppressive function. We investigated the potential of IL‐34 in human disease to prevent xenogeneic GVHD and human skin allograft rejection in immune humanized immunodeficient NSG mice. Finally, taking advantage of a biocollection, we investigated the correlation between presence of IL‐34 in the serum and kidney transplant rejection.

Results

Here we report that the absence of expression of IL‐34 in Il34^−/− rats and mice leads to an unstable immune phenotype, with production of multiple auto‐antibodies, exacerbated under inflammatory conditions with increased susceptibility to DSS‐ and TNBS‐colitis in Il34^−/− animals. Moreover, we revealed the striking inability of Il34^−/− CD4⁺ Tregs to protect Il2rg^−/− rats from a wasting disease induced by transfer of pathogenic cells, in contrast to Il34^+/+ CD4⁺ Tregs. We also showed that IL‐34 treatment delayed EAE in mice as well as GVHD and human skin allograft rejection in immune humanized immunodeficient NSG mice. Finally, we show that presence of IL‐34 in the serum is associated with a longer rejection‐free period in kidney transplanted patients.

Conclusion

Altogether, our data emphasize on the crucial necessity of IL‐34 for immune homeostasis and for CD4⁺ Tregs suppressive function. Our data also shows the therapeutic potential of IL‐34 in human transplantation and auto‐immunity.

Highlights

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Absence of expression of IL‐34 in Il34^−/− rats and mice leads to an unstable immune phenotype, with a production of multiple auto‐antibodies and exacerbated immune pathology under inflammatory conditions.

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Il34^−/− CD4⁺ Tregs are unable to protect Il2rg^−/− rats from colitis induced by transfer of pathogenic cells.

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IL‐34 treatment delayed EAE in mice, as well as acute GVHD and human skin allograft rejection in immune‐humanized immunodeficient NSG mice.

The Pathological Mechanism Between the Intestine and Brain in the Early Stage of Parkinson's Disease

Parkinson's disease (PD) is the second most common chronic progressive neurodegenerative disease. The main pathological features are progressive degeneration of neurons and abnormal accumulation of α-synuclein. At present, the pathogenesis of PD is not completely clear, and many changes in the intestinal tract may be the early pathogenic factors of PD. These changes affect the central nervous system (CNS) through both nervous and humoral pathways. α-Synuclein deposited in the intestinal nerve migrates upward along the vagus nerve to the brain. Inflammation and immune regulation mediated by intestinal immune cells may be involved, affecting the CNS through local blood circulation. In addition, microorganisms and their metabolites may also affect the progression of PD. Therefore, paying attention to the multiple changes in the intestinal tract may provide new insight for the early diagnosis and treatment of PD.

An Update of Research Animal Models of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a group of chronic disorders that includes two main disease forms, Crohn's disease, and ulcerative colitis. The understanding of the intestinal inflammation occurring in IBD has been immeasurably advanced by the development of the now numerous murine models of intestinal inflammation. The usefulness of this research tool in IBD arises from a convergence of underlying genetic susceptibility, immune system dysfunction, environmental factors, and shifts in gut microbiota. Due to the multifactorial feature of these diseases, different animal models have been used to investigate the underlying mechanisms and develop potential therapeutic strategies. The results of preclinical efficacy studies often inform the progression of therapeutic strategies. This review describes the distinct feature and limitations of each murine IBD model and discusses the previous and current lessons from the IBD models.

Taking regulatory T cells into medicine

Regulatory T cells (Tregs) are indispensable for the establishment and maintenance of immunological self-tolerance. Their genetic anomalies or variations in function are causative of various monogenic and polygenic autoimmune diseases. Treg-based reestablishment of self-tolerance is envisioned to cure autoimmune diseases in the clinic.

Dopaminergic signalling limits suppressive activity and gut homing of regulatory T cells upon intestinal inflammation

Evidence from inflammatory bowel diseases (IBD) patients and animal models has indicated that gut inflammation is driven by effector CD4⁺ T-cell, including Th1 and Th17. Conversely, Treg seem to be dysfunctional in IBD. Importantly, dopamine, which is abundant in the gut mucosa under homoeostasis, undergoes a sharp reduction upon intestinal inflammation. Here we analysed the role of the high-affinity dopamine receptor D3 (DRD3) in gut inflammation. Our results show that Drd3 deficiency confers a stronger immunosuppressive potency to Treg, attenuating inflammatory colitis manifestation in mice. Mechanistic analyses indicated that DRD3-signalling attenuates IL-10 production and limits the acquisition of gut-tropism. Accordingly, the ex vivo transduction of wild-type Treg with a siRNA for Drd3 induced a potent therapeutic effect abolishing gut inflammation. Thus, our findings show DRD3-signalling as a major regulator of Treg upon gut inflammation.

Intestinal Regulatory T Cells

Mucosal surfaces are distinctive sites exposed to environmental, dietary, and microbial antigens. Particularly in the gut, the host continuously actively adapts via complex interactions between the microbiota and dietary compounds and immune and other tissue cells. Regulatory T cells (Tregs) are critical for tuning the intestinal immune response to self- and non-self-antigens in the intestine. Its importance in intestinal homeostasis is illustrated by the onset of overt inflammation caused by deficiency in Treg generation, function, or stability in the gut. A substantial imbalance in Tregs has been observed in intestinal tissue during pathogenic conditions, when a tightly regulated and equilibrated system becomes dysregulated and leads to unimpeded and chronic immune responses. In this chapter, we compile and critically discuss the current knowledge on the key factors that promote Treg-mediated tolerance in the gut, such as those involved in intestinal Treg differentiation, specificity and suppressive function, and their immunophenotype during health and disease. We also discuss the current state of knowledge on Treg dysregulation in human intestine during pathological states such as inflammatory bowel disease (IBD), necrotizing enterocolitis (NEC), graft-versus-host disease (GVHD), and colorectal cancer (CRC), and how that knowledge is guiding development of Treg-targeted therapies to treat or prevent intestinal disorders.

Transcriptional regulation of Treg homeostasis and functional specification

CD4+Foxp3+ regulatory T (Treg) cells are key players in keeping excessive inflammation in check. Mounting evidence has shown that Treg cells exert much more diverse functions in both immunological and non-immunological processes. The development, maintenance and functional specification of Treg cells are regulated by multilayered factors, including antigens and TCR signaling, cytokines, epigenetic modifiers and transcription factors (TFs). In the review, we will focus on TFs by summarizing their unique and redundant roles in Treg cells under physiological and pathophysiological conditions. We will also discuss the recent advances of Treg trajectories between lymphoid organs and non-lymphoid tissues. This review will provide an updated view of the newly identified TFs and new functions of known TFs in Treg biology.

Inflammatory Bowel Disease Through the Lens of Single-cell RNA-seq Technologies

The intestinal mucosa represents a unique environment where the coordinated function of diverse epithelial, mesenchymal, and immune cells maintains a physiologically balanced environment in the presence of gut microbiota. The intestinal mucosa plays a central role in the pathogenesis of inflammatory bowel disease (IBD), yet the molecular and cellular composition of this diverse environment is poorly understood. However, the recent advent of multimodal single-cell technologies, including single-cell RNA sequencing (scRNA-seq), now provides an opportunity to accurately map the tissue architecture, characterize rare cell types that were previously overlooked, and define function at a single-cell level. In this review, we summarize key advances in single-cell technology and provide an overview of important aspects of computational analysis. We describe emerging data in the field of IBD and discuss how the characterization of novel intestinal mucosa cell populations is reshaping our understanding of this complex disease. We conclude by considering the potential clinical applications, including the definition of novel drug targets and the opportunity for personalization of care in this exciting new era of precision medicine.

CD8+ T cells: The past and future of immune regulation

Regulation of the adaptive immune response is critical for health. Regulatory activity can be found in multiple components of the immune system, however, the focus on particular components of the immune regulatory network has left many aspects of this critical immune component understudied. Here we review the evidence for activities of CD8+ T cells in immune homeostasis and regulation of autoimmune reactivity. The heterogeneous nature of identified CD8+ cell types are examined, and common phenotypes associated with functional activities are defined. The varying types of antigen signal crucial for CD8+ T cell regulatory activity are identified and the implications of these activation pathways for control of adaptive responses is considered. Finally, the promising capacity for transgenic antigen receptor directed cytotoxicity as a mechanism for modulation of autoimmunity is detailed.

Type I interferons induce peripheral T regulatory cell differentiation under tolerogenic conditions

The type I interferons (type I IFNs) are central to a vast array of immunological functions. The production of these immune-modulatory molecules is initiated at the early stages of the innate immune responses and, therefore, plays a dominant role in shaping downstream events in both innate and adaptive immunity. Indeed, the major role of IFNα/β is the induction of priming states, relevant for the functional differentiation of T lymphocyte subsets. Among T cell subtypes, the CD4 +CD25 +Foxp3 + T regulatory cells (Tregs) represent a specialized subset of CD4 + T cells with a critical role in maintaining peripheral tolerance and immune homeostasis. Although the role of type I IFNs in maintaining the function of thymus-derived Tregs has been previously described, the direct contribution of these innate factors to peripheral Treg (pTreg) and induced Treg (iTreg) differentiation and suppressive function is still unclear. We now show that, under tolerogenic conditions, IFNα/β play a critical role in antigen-specific and also polyclonal naïve CD4 + T cell conversion into peripheral antigen-specific CD4 +CD25 +Foxp3 + Tregs and inhibit CD4 + T helper (Th) cell expansion in mice. While type I IFNs sustain the expression and the activation of the transcription master regulators Foxp3, Stat3 and Stat5, these innate molecules reciprocally inhibit Th17 cell differentiation. Altogether, these results indicate a new pivotal role of IFNα/β on pTreg differentiation and induction of peripheral tolerance, which may have important implications in the therapeutic control of inflammatory disorders, such as of autoimmune diseases.

Control of regulatory T-cell differentiation and function by T-cell receptor signalling and Foxp3 transcription factor complexes

The transcription factor Foxp3 controls the differentiation and function of regulatory T-cells (Treg). Studies in the past decades identified numerous Foxp3-interacting protein partners. However, it is still not clear how Foxp3 produces the Treg-type transcriptomic landscape through cooperating with its partners. Here I show the current understanding of how Foxp3 transcription factor complexes regulate the differentiation, maintenance and functional maturation of Treg. Importantly, T-cell receptor (TCR) signalling plays central roles in Treg differentiation and Foxp3-mediated gene regulation. Differentiating Treg will have recognized their cognate antigens and received TCR signals before initiating Foxp3 transcription, which is triggered by TCR-induced transcription factors including NFAT, AP-1 and NF-κB. Once expressed, Foxp3 seizes TCR signal-induced transcriptional and epigenetic mechanisms through interacting with AML1/Runx1 and NFAT. Thus, Foxp3 modifies gene expression dynamics of TCR-induced genes, which constitute cardinal mechanisms for Treg-mediated immune suppression. Next, I discuss the following key topics, proposing new mechanistic models for Foxp3-mediated gene regulation: (i) how Foxp3 transcription is induced and maintained by the Foxp3-inducing enhanceosome and the Foxp3 autoregulatory transcription factor complex; (ii) molecular mechanisms for effector Treg differentiation (i.e. Treg maturation); (iii) how Foxp3 activates or represses its target genes through recruiting coactivators and corepressors; (iv) the 'decision-making' Foxp3-containing transcription factor complex for Th17 and Treg differentiation; and (v) the roles of post-translational modification in Foxp3 regulation. Thus, this article provides cutting-edge understanding of molecular biology of Foxp3 and Treg, integrating findings by biochemical and genomic studies.

Advances on CD8+ Treg Cells and Their Potential in Transplantation

Although cluster of differentiation (CD)8 regulatory T (Treg) cells have been in the last 20 years more studied since evidences of their role in tolerance as been demonstrated in transplantation, autoimmune diseases and cancer, their characteristics are still controversial. In this review, we will focus on recent advances on CD8 Treg cells and description of a role for CD8 Treg cells in tolerance in both solid organ transplantation and graft-versus-host disease and their potential for clinical trials.

Regulatory T cell adaptation in the intestine and skin

The intestine and skin are distinct microenvironments with unique physiological functions and are continually exposed to diverse environmental challenges. Host adaptation at these sites is an active process that involves interaction between immune cells and tissue cells. Regulatory T cells (T_reg cells) play a pivotal role in enforcing homeostasis at barrier surfaces, illustrated by the development of intestinal and skin inflammation in diseases caused by primary deficiency in T_reg cells. T_reg cells at barrier sites are phenotypically distinct from their lymphoid-organ counterparts, and these 'tissue' signatures often reflect their tissue-adapted function. We discuss current understanding of T_reg cell adaptation in the intestine and skin, including unique phenotypes, functions and metabolic demands, and how increased knowledge of T_reg cells at barrier sites might guide precision medicine therapies.

T-Cell-Driven Inflammation as a Mediator of the Gut-Brain Axis Involved in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder affecting mainly the dopaminergic neurons of the nigrostriatal pathway, a neuronal circuit involved in the control of movements, thereby the main manifestations correspond to motor impairments. The major molecular hallmark of this disease corresponds to the presence of pathological protein inclusions called Lewy bodies in the midbrain of patients, which have been extensively associated with neurotoxic effects. Importantly, different research groups have demonstrated that CD4⁺ T-cells infiltrate into the substantia nigra of PD patients and animal models. Moreover, several studies have consistently demonstrated that T-cell deficiency results in a strong attenuation of dopaminergic neurodegeneration in animal models of PD, thus indicating a key role of adaptive immunity in the neurodegenerative process. Recent evidence has shown that CD4⁺ T-cell response involved in PD patients is directed to oxidised forms of α-synuclein, one of the main constituents of Lewy bodies. On the other hand, most PD patients present a number of non-motor manifestations. Among non-motor manifestations, gastrointestinal dysfunctions result especially important as potential early biomarkers of PD, since they are ubiquitously found among confirmed patients and occur much earlier than motor symptoms. These gastrointestinal dysfunctions include constipation and inflammation of the gut mucosa and the most distinctive pathologic features associated are the loss of neurons of the enteric nervous system and the generation of Lewy bodies in the gut. Moreover, emerging evidence has recently shown a pivotal role of gut microbiota in triggering the development of PD in genetically predisposed individuals. Of note, PD has been positively correlated with inflammatory bowel diseases, a group of disorders involving a T-cell driven inflammation of gut mucosa, which is strongly dependent in the composition of gut microbiota. Here we raised the hypothesis that T-cell driven inflammation, which mediates dopaminergic neurodegeneration in PD, is triggered in the gut mucosa. Accordingly, we discuss how structural components of commensal bacteria or how different mediators produced by gut-microbiota, including short-chain fatty acids and dopamine, may affect the behaviour of T-cells, triggering the development of T-cell responses against Lewy bodies, initially confined to the gut mucosa but later extended to the brain.

Targeting the Dopaminergic System in Autoimmunity

Dopamine has emerged as a fundamental regulator of inflammation. In this regard, it has been shown that dopaminergic signalling pathways are key players promoting homeostasis between the central nervous system and the immune system. Dysregulation in the dopaminergic system affects both innate and adaptive immunity, contributing to the development of numerous autoimmune and inflammatory pathologies. This makes dopamine receptors interesting therapeutic targets for either the development of new treatments or repurposing of already available pharmacological drugs. Dopamine receptors are broadly expressed on different immune cells with multifunctional effects depending on the dopamine concentration available and the pattern of expression of five dopamine receptors displaying different affinities for dopamine. Thus, impaired dopaminergic signalling through different dopamine receptors may result in altered behaviour of immunity, contributing to the development and progression of autoimmune pathologies. In this review we discuss the current evidence involving the dopaminergic system in inflammatory bowel disease, multiple sclerosis and Parkinson's disease. In addition, we summarise and analyse the therapeutic approaches designed to attenuate disease development and progression by targeting the dopaminergic system. Graphical Abstract Targetting the dopaminergic system in autoimmunity. Effector T-cells (Teff) orchestrate inflamamtion involved in autoimmunity, whilst regulatory T-cells (Tregs) suppress Teff activity promoting tolerance to self-constituents. Dopamine has emerged as a key regulator of Teff and Tregs function, thereby dopamine receptors have becoming important therapeutic targets in autoimmune disorders, especially in those affecting the brain and the gut, where dopamine levels strongly change with inflammation.

Tipping the balance: inhibitory checkpoints in intestinal homeostasis

The small intestinal and colonic lamina propria are populated with forkhead box P3 (FOXP3)⁺CD4⁺ regulatory T cells (Tregs) and interleukin-10-producing T cells that orchestrate intestinal tolerance to harmless microbial and food antigens. Expression of co-inhibitory receptors such as CTLA-4 and PD-1 serve as checkpoints to these cells controlling their T-cell receptor (TCR)-mediated and CD28-mediated activation and modulating the phenotype of neighboring antigen presenting cells. Recent discoveries on the diversity of co-inhibitory receptors and their selective cellular expression has shed new light on their tissue-dependent function. In this review, we provide an overview of the co-inhibitory pathways and checkpoints of Treg and effector T cells and their mechanisms of action in intestinal homeostasis. Better understanding of these inhibitory checkpoints is desired as their blockade harbors clinical potential for the treatment of cancer and their stimulation may offer new opportunities to treat chronic intestinal inflammation such as inflammatory bowel disease.

Description of CD8+ Regulatory T Lymphocytes and Their Specific Intervention in Graft-versus-Host and Infectious Diseases, Autoimmunity, and Cancer

Gershon and Kondo described CD8⁺ Treg lymphocytes as the first ones with regulating activity due to their tolerance ability to foreign antigens and their capacity to inhibit the proliferation of other lymphocytes. Regardless, CD8⁺ Treg lymphocytes have not been fully described-unlike CD4⁺ Treg lymphocytes-because of their low numbers in blood and the lack of specific and accurate population markers. Still, these lymphocytes have been studied for the past 30 years, even after finding difficulties during investigations. As a result, studies have identified markers that define their subpopulations. This review is focused on the expression of cell membrane markers as CD25, CD122, CD103, CTLA-4, CD39, CD73, LAG-3, and FasL as well as soluble molecules such as FoxP3, IFN-γ, IL-10, TGF-β, IL-34, and IL-35, in addition to the lack of expression of cell activation markers such as CD28, CD127 CD45RC, and CD49d. This work also underlines the importance of identifying some of these markers in infections with several pathogens, autoimmunity, cancer, and graft-versus-host disease as a strategy in their prevention, monitoring, and cure.

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.

Transient antibody targeting of CD45RC induces transplant tolerance and potent antigen-specific regulatory T cells

Rat and human CD4⁺ and CD8⁺ Tregs expressing low levels of CD45RC have strong immunoregulatory properties. We describe here that human CD45 isoforms are nonredundant and identify distinct subsets of cells. We show that CD45RC is not expressed by CD4⁺ and CD8⁺ Foxp3⁺ Tregs, while CD45RA/RB/RO are. Transient administration of a monoclonal antibody (mAb) targeting CD45RC in a rat cardiac allotransplantation model induced transplant tolerance associated with inhibition of allogeneic humoral responses but maintained primary and memory responses against cognate antigens. Anti-CD45RC mAb induced rapid death of CD45RC^high T cells through intrinsic cell signaling but preserved and potentiated CD4⁺ and CD8⁺ CD45RC^low/- Tregs, which are able to adoptively transfer donor-specific tolerance to grafted recipients. Anti-CD45RC treatment results in distinct transcriptional signature of CD4⁺ and CD8⁺ CD45RC^low/- Tregs. Finally, we demonstrate that anti-human CD45RC treatment inhibited graft-versus-host disease (GVHD) in immune-humanized NSG mice. Thus, short-term anti-CD45RC is a potent therapeutic candidate to induce transplantation tolerance in human.

Ontogeny of Tumor-associated CD4+CD25+Foxp3+ T-regulatory Cells

The critical contribution of CD4+CD25+Foxp3+ T-regulatory cells (Treg) to immune suppression in the tumor microenvironment is well-established. Whereas the mechanisms that drive the generation and accumulation of Treg in tumors have been an active area of study, the information on their origin and population dynamics remains limited. In this review, we discuss the ontogeny of tumor-associated Treg in light of the recently identified lineage markers.

Cyclic AMP Represents a Crucial Component of Treg Cell-Mediated Immune Regulation

T regulatory (Treg) cells are one of the key players in the immune tolerance network, and a plethora of manuscripts have described their development and function in the course of the last two decades. Nevertheless, it is still a matter of debate as to which mechanisms and agents are employed by Treg cells, providing the basis of their suppressive potency. One of the important candidates is cyclic AMP (cAMP), which is long known as a potent suppressor at least of T cell activation and function. While this suppressive function by itself is widely accepted, the source and the mechanism of action of cAMP are less clear, and a multitude of seemingly contradictory data allow for, in principle, two different scenarios of cAMP-mediated suppression. In one scenario, Treg cells contain high amounts of cAMP and convey this small molecule via gap junction intercellular communication directly to the effector T cells (Teff) leading to their suppression. Alternatively, it was shown that Treg cells represent the origin of considerable amounts of adenosine, which trigger the adenylate cyclases in Teff cells via A_2A and A_2B receptors, thus strongly increasing intracellular cAMP. This review will present and discuss initial findings and recent developments concerning the function of cAMP for Treg cells and its impact on immune regulation.

The NF-κB transcription factor RelA is required for the tolerogenic function of Foxp3(+) regulatory T cells

The properties of CD4(+) regulatory T cell (Treg) subsets are dictated by distinct patterns of gene expression determined by FOXP3 and different combinations of various transcription factors. Here we show the NF-κB transcription factor RelA is constitutively active in naïve and effector Tregs. The conditional inactivation of Rela in murine FOXP3(+) cells induces a rapid onset, multi-focal autoimmune disease that depends on RelA being expressed in conventional T cells. In addition to promoting Treg lineage stability, RelA determines the size of the effector Treg population, a function influenced by the presence or absence of RelA in conventional T cells. These findings showing that RelA controls Treg stability and promotes the competitive fitness of effector Tregs highlight the importance of RelA activity in peripheral Treg induced tolerance.

Enhanced renewal of regulatory T cells in relation to CD4(+) conventional T lymphocytes in the peripheral compartment

CD4(+) Foxp3(+) regulatory T (Treg) cells are necessary for the maintenance of self-tolerance and T-cell homeostasis. This population is kept at stable frequencies in secondary lymphoid organs for the majority of the lifetime, despite permanent thymic emigration or in the face of thymic involution. Continuous competition is expected to occur between recently thymus-emigrated and resident Treg cells (either natural or post-thymically induced). In the present work, we analysed the renewal dynamics of Treg cells compared with CD4(+) Foxp3- conventional T cells (Tconv), using protocols of single or successive T-cell transfers into syngeneic euthymic or lymphopenic (nu/nu or RAG2(-/-)) mice, respectively. Our results show a higher turnover for Treg cells in the peripheral compartment, compared with Tconv cells, when B cell-sufficient euthymic or nude hosts are studied. This increased renewal within the Treg pool, shown by the greater replacement of resident Treg cells by donor counterparts, correlates with augmented rates of proliferation and is not modified following temporary environmental perturbations induced by inflammatory state or microbiota alterations. Notably, the preferential substitution of Treg lymphocytes was not observed in RAG2(-/-) hosts. We showed that limited B-cell replenishment in the RAG2(-/-) hosts decisively contributed to the altered peripheral T-cell homeostasis. Accordingly, weekly transfers of B cells to RAG2(-/-) hosts rescued the preferential substitution of Treg lymphocytes. Our study discloses a new aspect of T-cell homeostasis that depends on the presence of B lymphocytes to regulate the relative incorporation of recently arrived Treg and Tconv cells in the peripheral compartment.

Genetically engineered mouse models for studying inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic intestinal inflammatory condition that is mediated by very complex mechanisms controlled by genetic, immune, and environmental factors. More than 74 kinds of genetically engineered mouse strains have been established since 1993 for studying IBD. Although mouse models cannot fully reflect human IBD, they have provided significant contributions for not only understanding the mechanism, but also developing new therapeutic means for IBD. Indeed, 20 kinds of genetically engineered mouse models carry the susceptibility genes identified in human IBD, and the functions of some other IBD susceptibility genes have also been dissected out using mouse models. Cutting-edge technologies such as cell-specific and inducible knockout systems, which were recently employed to mouse IBD models, have further enhanced the ability of investigators to provide important and unexpected rationales for developing new therapeutic strategies for IBD. In this review article, we briefly introduce 74 kinds of genetically engineered mouse models that spontaneously develop intestinal inflammation.

IL-34 is a Treg-specific cytokine and mediates transplant tolerance

Cytokines and metabolic pathway-controlling enzymes regulate immune responses and have potential as powerful tools to mediate immune tolerance. Blockade of the interaction between CD40 and CD40L induces long-term cardiac allograft survival in rats through a CD8+CD45RClo Treg potentiation. Here, we have shown that the cytokine IL-34, the immunoregulatory properties of which have not been previously studied in transplantation or T cell biology, is expressed by rodent CD8+CD45RClo Tregs and human FOXP3+CD45RCloCD8+ and CD4+ Tregs. IL-34 was involved in the suppressive function of both CD8+ and CD4+ Tregs and markedly inhibited alloreactive immune responses. Additionally, in a rat cardiac allograft model, IL-34 potently induced transplant tolerance that was associated with a total inhibition of alloantibody production. Treatment of rats with IL-34 promoted allograft tolerance that was mediated by induction of CD8+ and CD4+ Tregs. Moreover, these Tregs were capable of serial tolerance induction through modulation of macrophages that migrate early to the graft. Finally, we demonstrated that human macrophages cultured in the presence of IL-34 greatly expanded CD8+ and CD4+ FOXP3+ Tregs, with a superior suppressive potential of antidonor immune responses compared with non-IL-34-expanded Tregs. In conclusion, we reveal that IL-34 serves as a suppressive Treg-specific cytokine and as a tolerogenic cytokine that efficiently inhibits alloreactive immune responses and mediates transplant tolerance.

Avoiding immunological rejection in regenerative medicine

One of the major goals of regenerative medicine is repair or replacement of diseased and damaged tissues by transfer of differentiated stem cells or stem cell-derived tissues. The possibility that these tissues will be destroyed by immunological rejection remains a challenge that can only be overcome through a better understanding of the nature and expression of potentially immunogenic molecules associated with cell replacement therapy and the mechanisms and pathways resulting in their immunologic rejection. This review draws on clinical experience of organ and tissue transplantation, and on transplantation immunology research to consider practical approaches for avoiding and overcoming the possibility of rejection of stem cell-derived tissues.

T-cell selection and intestinal homeostasis

Although intestinal bacteria live deep within the body, they are topographically on the exterior surface and thus outside the host. According to the classic notion that the immune system targets non-self rather than self, these intestinal bacteria should be considered foreign and therefore attacked and eliminated. While this appears to be true for some commensal bacterial species, recent data suggest that the immune system actively becomes tolerant to many bacterial organisms. The induction or activation of regulatory T (Treg) cells that inhibit, rather than promote, inflammatory responses to commensal bacteria appears to be a central component of mucosal tolerance. Loss of this mechanism can lead to inappropriate immune reactivity toward commensal organisms, perhaps contributing to mucosal inflammation characteristic of disorders such as inflammatory bowel disease.

Blowing on embers: commensal microbiota and our immune system

Vertebrates have co-evolved with microorganisms resulting in a symbiotic relationship, which plays an important role in health and disease. Skin and mucosal surfaces are colonized with a diverse population of commensal microbiota, over 1000 species, outnumbering the host cells by 10-fold. In the past 40 years, studies have built on the idea that commensal microbiota is in constant contact with the host immune system and thus influence immune function. Recent studies, focusing on mutualism in the gut, have shown that commensal microbiota seems to play a critical role in the development and homeostasis of the host immune system. In particular, the gut microbiota appears to direct the organization and maturation of lymphoid tissues and acts both locally and systemically to regulate the recruitment, differentiation, and function of innate and adaptive immune cells. While the pace of research in the area of the mucosal–immune interface has certainly intensified over the last 10 years, we are still in the early days of this field. Illuminating the mechanisms of how gut microbes shape host immunity will enhance our understanding of the causes of immune-mediated pathologies and improve the design of next-generation vaccines. This review discusses the recent advances in this field, focusing on the close relationship between the adaptive immune system and commensal microbiota, a constant and abundant source of foreign antigens.

T cells and intestinal commensal bacteria--ignorance, rejection, and acceptance

Trillions of commensal bacteria cohabit our bodies to mutual benefit. In the past several years, it has become clear that the adaptive immune system is not ignorant of intestinal commensal bacteria, but is constantly interacting with them. For T cells, the response to commensal bacteria does not appear uniform, as certain commensal bacterial species appear to trigger effector T cells to reject and control them, whereas other species elicit Foxp3(+) regulatory T (Treg) cells to accept and be tolerant of them. Here, we review our current knowledge of T cell differentiation in response to commensal bacteria, and how this process leads to immune homeostasis in the intestine.

Regulatory T cells control diabetes without compromising acute anti-viral defense

While previous reports have demonstrated the efficacy of regulatory T cell therapy in the prevention of diabetes, systemic immunocompromise and Treg instability remain key safety concerns. Here we examined the influence of induced Treg (iTreg) cell therapy on anti-viral host defense and autoimmune T cell responses during acute viral infection in a murine model of autoimmune diabetes. Protective transfers of iTregs maintained IL-10 expression, expanded in vivo and controlled diabetes, despite losing FoxP3 expression. Adoptive transfer of iTregs affected neither the primary anti-viral CD8 T cell response nor viral clearance, although a significant and sustained suppression of CD4 T cell responses was observed. Following acute viral clearance, iTregs transferred early suppressed both CD4 and CD8 T cell responses, which resulted in the reversion of diabetes. These observations indicate that iTregs suppress local autoimmune processes while preserving the immunocompetent host's ability to combat acute viral infection.

The dopaminergic system in autoimmune diseases

Bidirectional interactions between the immune and the nervous systems are of considerable interest both for deciphering their functioning and for designing novel therapeutic strategies. The past decade has brought a burst of insights into the molecular mechanisms involved in neuroimmune communications mediated by dopamine. Studies of dendritic cells (DCs) revealed that they express the whole machinery to synthesize and store dopamine, which may act in an autocrine manner to stimulate dopamine receptors (DARs). Depending on specific DARs stimulated on DCs and T cells, dopamine may differentially favor CD4⁺ T cell differentiation into Th1 or Th17 inflammatory cells. Regulatory T cells can also release high amounts of dopamine that acts in an autocrine DAR-mediated manner to inhibit their suppressive activity. These dopaminergic regulations could represent a driving force during autoimmunity. Indeed, dopamine levels are altered in the brain of mouse models of multiple sclerosis (MS) and lupus, and in inflamed tissues of patients with inflammatory bowel diseases or rheumatoid arthritis (RA). The distorted expression of DARs in peripheral lymphocytes of lupus and MS patients also supports the importance of dopaminergic regulations in autoimmunity. Moreover, dopamine analogs had beneficial therapeutic effects in animal models, and in patients with lupus or RA. We propose models that may underlie key roles of dopamine and its receptors in autoimmune diseases.

B cells are critical for autoimmune pathology in Scurfy mice

Impaired regulatory T-cell function results in a severe chronic autoimmune disease affecting multiple organs in Scurfy mice and humans with the immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome. Previous studies have shown that T helper cells but not cytotoxic T cells are critical for the disease pathology. Whether this T-cell subset is responsible directly for tissue inflammation or rather indirectly via the interaction with B cells or myeloid cells is largely unknown. To study this and to identify potential therapeutic targets for this lethal disease we investigated the contribution of B cells to this complex autoimmune phenotype. We show that B cells and the production of autoantibodies plays a major role for skin, liver, lung, and kidney inflammation and therapeutic depletion of B cells resulted in reduced tissue pathology and in prolonged survival. In contrast, the absence of B cells did not impact systemic T-cell activation and hyperreactivity, indicating that autoantibody production by B cells may be a major factor for the autoimmune pathology in mice deficient for regulatory T cells.

Controlling the frontier: regulatory T-cells and intestinal homeostasis

The intestine represents one of the most challenging sites for the immune system as immune cells must be able to mount an efficient response to invading pathogens while tolerating the large number and diverse array of resident commensal bacteria. Foxp3(+) regulatory T-cells (Tregs) play a non-redundant role at maintaining this balance. At the same time Treg cell differentiation and function can be modulated by the intestinal microbiota. In this review, we will discuss effector mechanisms of Treg cells in the intestine and how these cells can be influenced by the intestinal microbiota.

Treg and CTLA-4: two intertwining pathways to immune tolerance

Both the CTLA-4 pathway and regulatory T cells (Treg) are essential for the control of immune homeostasis. Their therapeutic relevance is highlighted by the increasing use of anti-CTLA-4 antibody in tumor therapy and the development of Treg cell transfer strategies for use in autoimmunity and transplantation settings. The CTLA-4 pathway first came to the attention of the immunological community in 1995 with the discovery that mice deficient in Ctla-4 suffered a fatal lymphoproliferative syndrome. Eight years later, mice lacking the critical Treg transcription factor Foxp3 were shown to exhibit a remarkably similar phenotype. Much of the debate since has centered on the question of whether Treg suppressive function requires CTLA-4. The finding that it does in some settings but not in others has provoked controversy and inevitable polarization of opinion. In this article, I suggest that CTLA-4 and Treg represent complementary and largely overlapping mechanisms of immune tolerance. I argue that Treg commonly use CTLA-4 to effect suppression, however CTLA-4 can also function in the non-Treg compartment while Treg can invoke CTLA-4-independent mechanisms of suppression. The notion that Foxp3 and CTLA-4 direct independent programs of immune regulation, which in practice overlap to a significant extent, will hopefully help move us towards a better appreciation of the underlying biology and therapeutic significance of these pathways.

A higher risk of acute rejection of human kidney allografts can be predicted from the level of CD45RC expressed by the recipients' CD8 T cells

Although transplantation is the common treatment for end-stage renal failure, allograft rejection and marked morbidity from the use of immunosuppressive drugs remain important limitations. A major challenge in the field is to identify easy, reliable and noninvasive biomarkers allowing the prediction of deleterious alloreactive immune responses and the tailoring of immunosuppressive therapy in individuals according to the rejection risk. In this study, we first established that the expression of the RC isoform of the CD45 molecule (CD45RC) on CD4 and CD8 T cells from healthy individuals identifies functionally distinct alloreactive T cell subsets that behave differently in terms of proliferation and cytokine secretion. We then investigated whether the frequency of the recipients CD45RC T cell subsets before transplantation would predict acute graft rejection in a cohort of 89 patients who had undergone their first kidney transplantation. We showed that patients exhibiting more than 54.7% of CD8 CD45RC(high) T cells before transplantation had a 6 fold increased risk of acute kidney graft rejection. In contrast, the proportions of CD4 CD45RC T cells were not predictive. Thus, a higher risk of acute rejection of human kidney allografts can be predicted from the level of CD45RC expressed by the recipients' CD8 T cells.

Modulating T regulatory cells in cancer: how close are we?

Regulatory T cells (Tregs) are a specialized subset of CD4 T cells that have an indispensable role in maintaining immune homeostasis and tolerance. Although studies in mice and humans have clearly highlighted that the absence of these cells results in severe autoimmunity and inflammation, increased Treg numbers and/or function is not always beneficial. This is best exemplified in certain cancers where increased Tregs promote cancer progression by interfering with immune surveillance. Conversely, in other types of cancers that have an inflammatory component, Tregs can inhibit cancer progression by dampening inflammation. In this review article, we provide a historical perspective of the discovery of Tregs, followed by a summary of the existing literature on the role of Tregs in malignancy.

Immune reconstitution and graft-versus-host reactions in rat models of allogeneic hematopoietic cell transplantation

Allogeneic hematopoietic cell transplantation (alloHCT) extends the lives of thousands of patients who would otherwise succumb to hematopoietic malignancies such as leukemias and lymphomas, aplastic anemia, and disorders of the immune system. In alloHCT, different immune cell types mediate beneficial graft-versus-tumor (GvT) effects, regulate detrimental graft-versus-host disease (GvHD), and are required for protection against infections. Today, the “good” (GvT effector cells and memory cells conferring protection) cannot be easily separated from the “bad” (GvHD-causing cells), and alloHCT remains a hazardous medical modality. The transplantation of hematopoietic stem cells into an immunosuppressed patient creates a delicate environment for the reconstitution of donor blood and immune cells in co-existence with host cells. Immunological reconstitution determines to a large extent the immune status of the allo-transplanted host against infections and the recurrence of cancer, and is critical for long-term protection and survival after clinical alloHCT. Animal models continue to be extremely valuable experimental tools that widen our understanding of, for example, the dynamics of post-transplant hematopoiesis and the complexity of immune reconstitution with multiple ways of interaction between host and donor cells. In this review, we discuss the rat as an experimental model of HCT between allogeneic individuals. We summarize our findings on lymphocyte reconstitution in transplanted rats and illustrate the disease pathology of this particular model. We also introduce the rat skin explant assay, a feasible alternative to in vivo transplantation studies. The skin explant assay can be used to elucidate the biology of graft-versus-host reactions, which are known to have a major impact on immune reconstitution, and to perform genome-wide gene expression studies using controlled combinations of minor and major histocompatibility between the donor and the recipient.

Treg cells, life history, and diversity

Regulatory T cells expressing the FoxP3 transcription factor have a profound and nonredundant role in several aspects of immunological tolerance. We will review here the specification of this lineage, its population dynamics, and the diversity of subphenotypes that correlate with their diverse roles in controlling inflammation in a variety of settings.