Differentiation and function of Foxp3+ effector regulatory T cells

Assessment of Th17/Treg cells and Th cytokines in an improved immune thrombocytopenia mouse model

OBJECTIVES

The improved passive immune thrombocytopenia (ITP) mouse model has been extensively utilized for the study of ITP. However, how closely this model matches the human inflammation state and immune background is unclear. Our study aimed to explore the profile of Th cytokines and Th17/Treg cells in the model.

METHODS

We induced the ITP mouse model by dose-escalation injection of MWReg30. The serum levels of cytokines (IFN-γ, IL-2, IL-4, IL-10, IL-17A, and TGF-β1) were measured by enzyme-linked immunosorbent assay and the frequency of Th17 and Treg cells was measured by flow cytometry. The mRNA expression of Foxp3 and RORrt was measured by real-time PCR.

RESULTS

The serum levels of cytokines IFN-γ, TGF-β1, IL-4, and IL-10 were significantly lower in ITP mice. The secretion of serum proinflammatory cytokines IL-2 and IL-17A and the percentage of Th17 cells showed no statistically significant increase. In ITP mice the frequency of Treg cells and mRNA expression of Foxp3 was significantly lower in splenocytes.

CONCLUSION

Our data suggest that the improved passive ITP mouse model does not mimic the autoimmune inflammatory process of human ITP. Compared with human ITP, this model has a similar change in frequency of Treg cells, which may directly or indirectly result from antibody-mediated platelet destruction due to attenuated release of TGF-β.

Circulating myeloid-derived suppressor cells predict disease activity and treatment response in patients with immune thrombocytopenia

Immune thrombocytopenia (ITP) is a disease characterized by isolated thrombocytopenia. Abnormal effector T cell activation is an important mechanism in the pathogenesis of ITP. Regulatory T cells (Treg) have a strong immunosuppressive function for T cell activation and their importance in the pathophysiology and clinical treatment of ITP has been confirmed. Myeloid-derived suppressor cells (MDSCs) are other immunosuppressive cells, which can also suppress T cell activation by secreting arginase, iNOS and ROS, and are essential for Treg cells’ differentiation and maturation. Therefore, we speculate that MDSCs might also be involved in the immune-dysregulation mechanism of ITP. In this study, we tested MDSCs and Treg cells in peripheral blood samples of twenty-five ITP patients and ten healthy donors. We found that MDSCs and Treg cells decreased simultaneously in active ITP patients. Relapsed ITP patients showed lower MDSCs levels compared with new patients. All patients received immunosuppressive treatment including dexamethasone alone or in combination with intravenous immune globulin. We found that MDSCs’ level after treatment correlated with platelet recovery. Our study is the first that focused on MDSCs’ role in ITP. Based on our results, we concluded that circulating MDSCs could predict disease activity and treatment response in ITP patients. This preliminary conclusion indicates a substantial significance of MDSCs in the pathophysiology and clinical treatment of ITP, which deserves further investigation.

Direct control of regulatory T cells by keratinocytes

Environmental challenges to epithelial cells trigger gene expression changes that elicit context-appropriate immune responses. Here we show that the chromatin remodeler Mi-2β controls epidermal homeostasis by regulating genes involved in keratinocyte and immune-cell activation to maintain an inactive state. Mi-2β depletion caused rapid deployment of both a pro-inflammatory and an immunosuppressive response in the skin. A key target of Mi-2β in keratinocytes was the pro-inflammatory cytokine thymic stromal lymphopoietin (TSLP). Loss of TSLP receptor (TSLPR) signaling specifically in regulatory T (T_reg) cells prevented their activation and permitted rapid progression from a skin pro-inflammatory response to a lethal systemic condition. Thus, in addition to their well-characterized role in pro-inflammatory responses, keratinocytes also directly support immune-suppressive responses that are critical for re-establishing organismal homeostasis.

Development and function of Foxp3(+) regulatory T cells

Regulatory T cells (Tregs) have been recognized as having a major role in maintaining peripheral tolerance and preventing and limiting autoimmune and chronic inflammatory diseases. Tregs derive from the thymus and also develop peripherally. In this review, we discuss recent progress in our understanding of the basic mechanisms involved in Treg development and function in protecting against autoimmunity in the periphery, including thymic selection, peripheral induction and the many mechanisms of Treg suppression. Specifically in kidney disease, Tregs have been shown to play a role in limiting injury and may potentially have a therapeutic role.

Prognostic value of circulating regulatory T cell subsets in untreated non-small cell lung cancer patients

The role of the different circulating regulatory T-cells (Treg) subsets, as well as their correlation with clinical outcome of non-small cell lung cancer (NSCLC) patients is poorly understood. Peripheral blood from 156 stage III/IV chemotherapy-naive NSCLC patients and 31 healthy donors (HD) was analyzed with flow cytometry for the presence and functionality of CD4⁺ Treg subsets (naive, effector and terminal effector). Their frequencies were correlated with the clinical outcome. All CD4⁺ Treg subsets exhibited highly suppressive activity by TGF-β and IL-10 production. The percentages of naive Treg were found elevated in NSCLC patients compared to HD and were associated with poor clinical outcome, whereas the percentage of terminal effector Treg was lower compared to HD and higher levels were correlated with improved clinical response. At baseline, normal levels of naive and effector Treg were associated with longer overall survival (OS) compared to high levels, while the high frequency of the terminal effector Treg was correlated with longer Progression-Free Survival and OS. It is demonstrated, for first time, that particular CD4⁺ Treg subtypes are elevated in NSCLC patients and their levels are associated to the clinical outcome. The blocking of their migration to the tumor site may be an effective therapeutic strategy.

Identification of Novel CD4+ T Cell Subsets in the Target Tissue of Sjögren's Syndrome and Their Differential Regulation by the Lymphotoxin/LIGHT Signaling Axis

Despite being one of the most common rheumatologic diseases, there is still no disease-modifying drug for primary Sjögren's syndrome (pSS). Advancing our knowledge of the target tissue has been limited by the low dimensionality of histology techniques and the small size of human salivary gland biopsies. In this study, we took advantage of a molecularly validated mouse model of pSS to characterize tissue-infiltrating CD4⁺ T cells and their regulation by the lymphotoxin/LIGHT signaling axis. Novel cell subsets were identified by combining highly dimensional flow and mass cytometry with transcriptomic analyses. Pharmacologic modulation of the LTβR signaling pathway was achieved by treating mice with LTβR-Ig, a therapeutic intervention currently being tested in pSS patients (Baminercept trial NCT01552681). Using these approaches, we identified two novel CD4⁺ T cell subsets characterized by high levels of PD1: Prdm1⁺ effector regulatory T cells expressing immunoregulatory factors, such as Il10, Areg, Fgl2, and Itgb8, and Il21⁺ effector conventional T cells expressing a pathogenic transcriptional signature. Mirroring these observations in mice, large numbers of CD4⁺PD1⁺ T cells were detected in salivary glands from Sjögren's patients but not in normal salivary glands or kidney biopsies from lupus nephritis patients. Unexpectedly, LTβR-Ig selectively halted the recruitment of PD1^- naive, but not PD1⁺, effector T cells to the target tissue, leaving the cells with pathogenic potential unaffected. Altogether, this study revealed new cellular players in pSS pathogenesis, their transcriptional signatures, and differential dependency on the lymphotoxin/LIGHT signaling axis that help to interpret the negative results of the Baminercept trial and will guide future therapeutic interventions.

Foxp3 and Toll-like receptor signaling balance Treg cell anabolic metabolism for suppression

CD4⁺ effector T cells (T_eff cells) and regulatory T cells (T_reg cells) undergo metabolic reprogramming to support proliferation and immunological function. Although signaling via the lipid kinase PI(3)K (phosphatidylinositol-3-OH kinase), the serine-threonine kinase Akt and the metabolic checkpoint kinase complex mTORC1 induces both expression of the glucose transporter Glut1 and aerobic glycolysis for T_eff cell proliferation and inflammatory function, the mechanisms that regulate T_reg cell metabolism and function remain unclear. We found that Toll-like receptor (TLR) signals that promote T_reg cell proliferation increased PI(3)K-Akt-mTORC1 signaling, glycolysis and expression of Glut1. However, TLR-induced mTORC1 signaling also impaired T_reg cell suppressive capacity. Conversely, the transcription factor Foxp3 opposed PI(3)K-Akt-mTORC1 signaling to diminish glycolysis and anabolic metabolism while increasing oxidative and catabolic metabolism. Notably, Glut1 expression was sufficient to increase the number of T_reg cells, but it reduced their suppressive capacity and Foxp3 expression. Thus, inflammatory signals and Foxp3 balance mTORC1 signaling and glucose metabolism to control the proliferation and suppressive function of T_reg cells.

Regulatory T cells in kidney disease and transplantation

Regulatory T cells (Tregs) have been shown to be important in maintaining immune homeostasis and preventing autoimmune disease, including autoimmune kidney disease. It is also likely that they play a role in limiting kidney transplant rejection and potentially in promoting transplant tolerance. Although other subsets of Tregs exist, the most potent and well-defined Tregs are the Foxp3 expressing CD4(+) Tregs derived from the thymus or generated peripherally. These CD4(+)Foxp3(+) Tregs limit autoimmune renal disease in animal models, especially chronic kidney disease, and kidney transplantation. Furthermore, other subsets of Tregs, including CD8 Tregs, may play a role in immunosuppression in kidney disease. The development and protective mechanisms of Tregs in kidney disease and kidney transplantation involve multiple mechanisms of suppression. Here we review the development and function of CD4(+)Foxp3(+) Tregs. We discuss the specific application of Tregs as a therapeutic strategy to prevent kidney disease and to limit kidney transplant rejection and detail clinical trials in this area of transplantation.

Tissue Tregs

The immune system is responsible for defending an organism against the myriad of microbial invaders it constantly confronts. It has become increasingly clear that the immune system has a second major function: the maintenance of organismal homeostasis. Foxp3(+)CD4(+) regulatory T cells (Tregs) are important contributors to both of these critical activities, defense being the primary purview of Tregs circulating through lymphoid organs, and homeostasis ensured mainly by their counterparts residing in parenchymal tissues. This review focuses on so-called tissue Tregs. We first survey existing information on the phenotype, function, sustaining factors, and human equivalents of the three best-characterized tissue-Treg populations-those operating in visceral adipose tissue, skeletal muscle, and the colonic lamina propria. We then attempt to distill general principles from this body of work-as concerns the provenance, local adaptation, molecular sustenance, and targets of action of tissue Tregs, in particular.

Developmental Progression and Interrelationship of Central and Effector Regulatory T Cell Subsets

Resting central Tregs (cTregs) and activated effector Tregs (eTregs) are required for self-tolerance, but the heterogeneity and relationships within and between phenotypically distinct subsets of cTregs and eTregs are poorly understood. By extensive immune profiling and deep sequencing of TCR-β V regions, two subsets of cTregs, based on expression of Ly-6C, and three subsets of eTregs, based on distinctive expression of CD62L, CD69, and CD103, were identified. Ly-6C(+) cTregs exhibited lower basal activation, expressed on average lower affinity TCRs, and less efficiently developed into eTregs when compared with Ly-6C(-) cTregs. The dominant TCR Vβs of Ly-6C(+) cTregs were shared by eTregs at a low frequency. A single TCR clonotype was also identified that was largely restricted to Ly-6C(+) cTregs, even under conditions that promoted the development of eTregs. Collectively, these findings indicate that some Ly-6C(+) cTregs may persist as a lymphoid-specific subset, with minimal potential to develop into highly activated eTregs, whereas other cTregs readily develop into eTregs. In contrast, subsets of CD62L(lo) eTregs showed higher clonal expansion and were more highly interrelated than cTreg subsets based on their TCR-β repertoires, but exhibited varied immune profiles. The CD62L(lo) CD69(-) CD103(-) eTreg subset displayed properties of a transitional intermediate between cTregs and more activated eTreg subsets. Thus, eTreg subsets appear to exhibit substantial flexibility, most likely in response to environmental cues, to adopt defined immune profiles that are expected to optimize suppression of autoreactive T cells.

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.

Foxp3(+) T cells expressing RORγt represent a stable regulatory T-cell effector lineage with enhanced suppressive capacity during intestinal inflammation

Foxp3 (forkhead box P3 transcription factor)-expressing regulatory T cells (Tregs) are essential for immunological tolerance, best illustrated by uncontrolled effector T-cell responses and autoimmunity upon loss of Foxp3 expression. Tregs can adopt specific effector phenotypes upon activation, reflecting the diversity of functional demands in the different tissues of the body. Here, we report that Foxp3(+)CD4(+) T cells coexpressing retinoic acid-related orphan receptor-γt (RORγt), the master transcription factor for T helper type 17 (Th17) cells, represent a stable effector Treg lineage. Transcriptomic and epigenetic profiling revealed that Foxp3(+)RORγt(+) T cells display signatures of both Tregs and Th17 cells, although the degree of similarity was higher to Foxp3(+)RORγt(-) Tregs than to Foxp3(-)RORγt(+) T cells. Importantly, Foxp3(+)RORγt(+) T cells were significantly demethylated at Treg-specific epigenetic signature genes such as Foxp3, Ctla-4, Gitr, Eos, and Helios, suggesting that these cells have a stable regulatory rather than inflammatory function. Indeed, adoptive transfer of Foxp3(+)RORγt(+) T cells in the T-cell transfer colitis model confirmed their Treg function and lineage stability in vivo, and revealed an enhanced suppressive capacity as compared with Foxp3(+)RORγt(-) Tregs. Thus, our data suggest that RORγt expression in Tregs contributes to an optimal suppressive capacity during gut-specific immune responses, rendering Foxp3(+)RORγt(+) T cells as an important effector Treg subset in the intestinal system.

Regulatory T Cell Induction and Retention in the Lungs Drives Suppression of Detrimental Type 2 Th Cells During Pulmonary Cryptococcal Infection

Lethal disease caused by the fungus Cryptococcus neoformans is a consequence of the combined failure to control pulmonary fungal replication and immunopathology caused by induced type 2 Th2 cell responses in animal models. In order to gain insights into immune regulatory networks, we examined the role of regulatory T (Treg) cells in suppression of Th2 cells using a mouse model of experimental cryptococcosis. Upon pulmonary infection with Cryptococcus, Treg cells accumulated in the lung parenchyma independently of priming in the draining lymph node. Using peptide-MHC class II molecules to identify Cryptococcus-specific Treg cells combined with genetic fate-mapping, we noted that a majority of the Treg cells found in the lungs were induced during the infection. Additionally, we found that Treg cells used the transcription factor, IFN regulatory factor 4, to dampen harmful Th2 cell responses, as well as mediate chemokine retention of Treg cells in the lungs. Taken together, induction and IFN regulatory factor 4-dependent localization of Treg cells in the lungs allow Treg cells to suppress the deleterious effects of Th2 cells during cryptococcal infection.

Control of Regulatory T Cell Migration, Function, and Homeostasis

Foxp3(+) regulatory T cells (Tregs) are essential for preventing autoimmunity and uncontrolled inflammation, and they modulate immune responses during infection and the development of cancer. Accomplishing these tasks requires the widespread distribution of Tregs in both lymphoid and nonlymphoid tissues, and the selective recruitment of Tregs to different tissue sites has emerged as a key checkpoint that controls tissue inflammation in autoimmunity, infection, and cancer development, as well as in the context of allograft acceptance or rejection. Additionally, Tregs are functionally diverse, and it has become clear that some of this diversity segregates with Treg localization to particular tissue sites. In this article, I review the progress in understanding the mechanisms of Treg trafficking and discuss factors controlling their homeostatic maintenance and function in distinct tissue sites.

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.

Reversible Reprogramming of Circulating Memory T Follicular Helper Cell Function during Chronic HIV Infection

Despite the overwhelming benefits of antiretroviral therapy (ART) in curtailing viral load in HIV-infected individuals, ART does not fully restore cellular and humoral immunity. HIV-infected individuals under ART show reduced responses to vaccination and infections and are unable to mount an effective antiviral immune response upon ART cessation. Many factors contribute to these defects, including persistent inflammation, especially in lymphoid tissues, where T follicular helper (Tfh) cells instruct and help B cells launch an effective humoral immune response. In this study we investigated the phenotype and function of circulating memory Tfh cells as a surrogate of Tfh cells in lymph nodes and found significant impairment of this cell population in chronically HIV-infected individuals, leading to reduced B cell responses. We further show that these aberrant memory Tfh cells exhibit an IL-2-responsive gene signature and are more polarized toward a Th1 phenotype. Treatment of functional memory Tfh cells with IL-2 was able to recapitulate the detrimental reprogramming. Importantly, this defect was reversible, as interfering with the IL-2 signaling pathway helped reverse the abnormal differentiation and improved Ab responses. Thus, reversible reprogramming of memory Tfh cells in HIV-infected individuals could be used to enhance Ab responses. Altered microenvironmental conditions in lymphoid tissues leading to altered Tfh cell differentiation could provide one explanation for the poor responsiveness of HIV-infected individuals to new Ags. This explanation has important implications for the development of therapeutic interventions to enhance HIV- and vaccine-mediated Ab responses in patients under ART.

Hyperlipidemia Alters Regulatory T Cell Function and Promotes Resistance to Tolerance Induction Through Costimulatory Molecule Blockade

Recent work from our laboratory has shown that hyperlipidemia promotes accelerated rejection of vascularized cardiac allografts in mice by inducing anti-donor Th17 reactivity and production of IL-17. Here, we show that hyperlipidemia also affects FoxP3(+) regulatory T cells (Tregs). Hyperlipidemia promotes the development of Tregs that express low levels of CD25. Hyperlipidemia also promotes a decrease in central Tregs and an increase in effector Tregs that appears to account for the increase in the frequency of CD25(low) Tregs. Alterations in Treg subsets also appear to lead to alterations in Treg function. The ability of FoxP3(+) , CD25(high) , CD4(+) Tregs from hyperlipidemic mice to inhibit proliferation of effector T cells stimulated with anti-CD3 and CD28 was reduced when compared with Tregs from control mice. Regulatory T cells isolated from hyperlipidemic recipients exhibit increased activation of Akt, and a reduction in Bim levels that permits the expansion of FoxP3(+) CD25(low) CD4(+) T cells. Hyperlipidemic mice were also resistant to tolerance induction using costimulatory molecule blockade consisting of anti-CD154 and CTLA4Ig, a strategy that requires Tregs. Together, our data suggest that hyperlipidemia profoundly affects Treg subsets and function as well as the ability to induce tolerance.

Microenvironment Matters: Unique Conditions Within Gut-Draining Lymph Nodes Favor Efficient De Novo Induction of Regulatory T Cells

The gastrointestinal tract constitutes the largest surface of the body and thus has developed multitude mechanisms to either prevent pathogen entry or to efficiently eliminate invading pathogens. At the same time, the gastrointestinal system has to avoid unwanted immune responses against self and harmless nonself antigens, such as nutrients and commensal microbiota. Therefore, it is somewhat not unexpected that the gastrointestinal mucosa serves as the largest repository of immune cells throughout the body, harboring both potent pro- as well as anti-inflammatory properties. One additional key element of this regulatory machinery is created by trillions of symbiotic commensal bacteria in the gut. The microbiota not only simply contribute to the breakdown of nutrients, but are essential in limiting the expansion of pathogens, directing the development of the intestinal immune system, and establishing mucosal tolerance by fostering the induction of regulatory T cells (Tregs). In this review, we will discuss our current understanding about the microenvironmental factors fostering the de novo generation of Tregs within the gastrointestinal immune system, focusing on unique properties of antigen-presenting cells, tolerogenic cytokines, commensal-derived metabolites and the contribution of lymph node stromal cells.

The Special Relationship in the Development and Function of T Helper 17 and Regulatory T Cells

T helper 17 (Th17) cells play an essential role in the clearance of extracellular pathogenic bacteria and fungi. However, this subset is critically involved in the pathology of many autoimmune diseases, e.g., psoriasis, multiple sclerosis, allergy, rheumatoid arthritis, and inflammatory bowel diseases in humans. Therefore, Th17 responses need to be tightly regulated in vivo to mediate effective host defenses against pathogens without causing excessive host tissue damage. Foxp3(+) regulatory T (Treg) cells play an important role in maintaining peripheral tolerance to self-antigens and in counteracting the inflammatory activity of effector T helper cell subsets. Although Th17 and Treg cells represent two CD4(+) T cell subsets with opposing principal functions, these cell types are functionally connected. In this review, we will first give an overview on the biology of Th17 cells and describe their development and in vivo function, followed by an account on the special developmental relationship between Th17 and Treg cells. We will describe the identification of Treg/Th17 intermediates and consider their lineage stability and function in vivo. Finally, we will discuss how Treg cells may regulate the Th17 cell response in the context of infection and inflammation, and elude on findings demonstrating that Treg cells can also have a prominent function in promoting the differentiation of Th17 cells.

TGF Beta1 Expression Correlates with Survival and Tumor Aggressiveness of Prostate Cancer

BACKGROUND

Although biopsy Gleason score and clinical stage can be used to inform treatment decisions for prostate cancer, identifying molecular markers of tumor aggressiveness could lead to a more tailored approaches to therapy. In the present study, we investigated the association of transforming growth factor (TGF)-β1 levels and various markers of tumor aggressiveness and explore some potential mechanisms underlying the associations.

METHODS

We used human and murine prostate cancer cell lines and their respective hormone resistance sub-lines, in vitro and in vivo to examine the changes in tumor aggressiveness, as well as the pathway responsible for these changes. Furthermore, 105 prostate cancer biopsy specimens were analyzed to correlate the level of TGF-β1 with the clinical characteristics of patients.

RESULTS

Our data revealed that activated TGF-β1 signaling resulted in more aggressive tumor growth and augmented the epithelial-mesenchymal transition. Activated IL-6 signaling was associated with TGF-β1 levels and the aggressive tumor features noted in TGF-β1-positive prostate cancers in vitro and in vivo. Furthermore, the TGF-β1 levels significantly correlated with Tregs accumulation in vivo. The clinical data indicated that TGF-β1 immunoreactivity had a moderate positive correlation with IL-6 staining, advanced clinical stage, higher Gleason score, and pretreatment PSA in patients with prostate cancer.

CONCLUSIONS

TGF-β1 levels are significantly associated with aggressive prostate features. In vitro and in vivo alternations of TGF-β1 expression impacts tumor invasiveness, tumor growth rate and recruitment of immunosuppressive Treg cells in the tumor microenvironment. TGF-β1 expression may represent a clinical useful biomarker to guide prostate cancer treatment decisions.

Expression of PD-L1 on CD4+CD25+Foxp3+ Regulatory T Cells of Patients with Chronic HBV Infection and Its Correlation with Clinical Parameters

Regulatory T cells (Tregs) play a pivotal role in suppressing specific antiviral immune responses during the progression of chronic hepatitis B virus infection (CHB) as well as tumorigenesis. Programmed death-1 ligand-1 (PD-L1) expressed on Tregs can transduce an inhibitory signal into effector T cells through interacting with programmed death-1 (PD-1). However, in CHB patients, the clinical significance of PD-L1 expression on Tregs has not been clearly described. This study investigated the frequency of circulating Tregs and PD-L1 expression on Tregs and analyzed their correlations with clinical parameters. The data show that both the frequency of CD4+CD25+FoxP3+ Tregs and PD-L1 expression on Tregs in the peripheral blood increased significantly in CHB patients when compared with healthy controls. At the same time, it is shown that PD-L1 expression on Tregs was positively correlated with the percentage of Tregs in CHB patients. Moreover, the results demonstrated that both Treg frequency and PD-L1 expression on Tregs positively correlated with the levels of alanine aminotransaminase (ALT) and aspartate aminotransferase (AST), both of which are indicators of the extent of liver injury. Taken together, these findings suggest that PD-L1 on Tregs might contribute to progression of hepatitis B virus infection through mediating the inhibitory function of Tregs. Thereby, blockade of interaction between Treg-expressing PD-L1 and PD-1 on effector T cells may be adopted as a potential therapeutic approach in CHB.

Role of CD4+ Foxp3+ Regulatory T Cells in Protection Induced by a Live Attenuated, Replicating Type I Vaccine Strain of Toxoplasma gondii

Vaccination with the live attenuated Toxoplasma gondii Mic1.3KO strain induced long-lasting immunity against challenge with Toxoplasma gondii type I and type II strains. The involvement of regulatory T cells (Tregs) in the protection mechanism was investigated. Intraperitoneal injection of Mic1.3KO induced a weak and transient influx of CD4(+) Foxp3(+) T regulatory cells followed by recruitment/expansion of CD4(+) Foxp3(-) CD25(+) effector cells and control of the parasite at the site of infection. The local and systemic cytokine responses associated with this recruitment of Tregs were of the TH1/Treg-like type. In contrast, injection of RH, the wild-type strain from which the vaccinal strain is derived, induced a low CD4(+) Foxp3(+) cell influx and uncontrolled multiplication of the parasites at this local site, followed by death of the mice. The associated local and systemic cytokine responses were of the TH1/TH17-like type. In addition, in vivo Treg induction in RH-infected mice with interleukin-2 (IL-2)/anti-IL-2 complexes induced control of the parasite and a TH1/Treg cytokine response similar to the response after Mic1.3KO vaccination. These results suggest that Tregs may contribute to the protective response after vaccination with Mic1.3KO.

Phenotype and function of tissue-resident unconventional Foxp3-expressing CD4(+) regulatory T cells

It is becoming increasingly clear that regulatory T cells (Treg cells) in specific tissues are important parts of immune system. Tissue-resident Treg cells, which are largely Foxp3-expressing CD4(+) Treg cells, are distinct from one another and conventional Treg cells, and have tissue-specific phenotype and function. They have roles in improving insulin sensitivity in adipose tissue, promoting muscle repair, limiting inflammation in intestine, skin and central nervous system. In this Review, we discuss the current understanding of phenotype and function of tissue-resident Treg cells. Understanding phenotypic and functional diversity in different tissues could provide new insight into Treg cells development and investigation.

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

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

The kinase DYRK1A reciprocally regulates the differentiation of Th17 and regulatory T cells

The balance between Th17 and T regulatory (Treg) cells critically modulates immune homeostasis, with an inadequate Treg response contributing to inflammatory disease. Using an unbiased chemical biology approach, we identified a novel role for the dual specificity tyrosine-phosphorylation-regulated kinase DYRK1A in regulating this balance. Inhibition of DYRK1A enhances Treg differentiation and impairs Th17 differentiation without affecting known pathways of Treg/Th17 differentiation. Thus, DYRK1A represents a novel mechanistic node at the branch point between commitment to either Treg or Th17 lineages. Importantly, both Treg cells generated using the DYRK1A inhibitor harmine and direct administration of harmine itself potently attenuate inflammation in multiple experimental models of systemic autoimmunity and mucosal inflammation. Our results identify DYRK1A as a physiologically relevant regulator of Treg cell differentiation and suggest a broader role for other DYRK family members in immune homeostasis. These results are discussed in the context of human diseases associated with dysregulated DYRK activity.

The complex and central role of interferon-γ in graft-versus-host disease and graft-versus-tumor activity

Allogeneic hematopoietic cell transplantation (allo-HCT) is increasingly being performed to treat patients with hematologic malignancies. However, separating the beneficial graft-versus-tumor (GVT) or graft-versus-leukemia effects from graft-versus-host disease (GVHD) has been difficult and remains a significant challenge toward improving therapeutic efficacy and reducing toxicity of allo-HCT. GVHD is induced by donor T cells that also mediate potent anti-tumor responses. However, despite the largely shared effector mechanisms, extensive animal studies have demonstrated the potential of dissociating the GVT effect from GVHD. Also in many clinical cases, long-term remission was achieved following allo-HCT, without significant GVHD. A better mechanistic understanding of the immunopathophysiology of GVHD and GVT effects may potentially help to improve allo-HCT as well as maximize the benefit of GVT effects while minimizing GVHD. In this article, we review the role of IFN-γ in regulation of alloresponses following allo-HCT, with a focus on the mechanisms of how this cytokine may separate GVHD from GVT effects.

Can you rely on Treg cells on the rebound?

FoxP3(+) regulatory T (Treg) cells comprise a highly dynamic population that restrains autoreactivity. Although complete or long-term depletion of Foxp3(+) CD4(+) Treg cells in adult mice has been shown to result in chronic inflammation and autoimmune disease, the impact of transient Treg-cell depletion on self-reactive responses is poorly defined. A new study published in this issue of the European Journal of Immunology [Eur. J. Immunol. 2014. 44: 3621-3631] shows that, although transient depletion of Treg cells in mice is swiftly followed by recovery of Treg-cell numbers, the "rebounded" population fails to maintain tolerance, culminating in severe autoimmune gastritis. This commentary explores new questions about the quantitative and qualitative aspects of Treg-cell function in immunological tolerance raised by this study and others.

The transcriptional regulators IRF4, BATF and IL-33 orchestrate development and maintenance of adipose tissue-resident regulatory T cells

Foxp3(+) regulatory T (Treg) cells in visceral adipose tissue (VAT-Treg cells) are functionally specialized tissue-resident cells that prevent obesity-associated inflammation and preserve insulin sensitivity and glucose tolerance. Their development depends on the transcription factor PPAR-γ; however, the environmental cues required for their differentiation are unknown. Here we show that interleukin 33 (IL-33) signaling through the IL-33 receptor ST2 and myeloid differentiation factor MyD88 is essential for development and maintenance of VAT-Treg cells and sustains their transcriptional signature. Furthermore, the transcriptional regulators BATF and IRF4 were necessary for VAT-Treg differentiation through direct regulation of ST2 and PPAR-γ expression. IL-33 administration induced vigorous population expansion of VAT-Treg cells, which tightly correlated with improvements in metabolic parameters in obese mice. Human omental adipose tissue Treg cells also showed high ST2 expression, suggesting an evolutionarily conserved requirement for IL-33 in VAT-Treg cell homeostasis.

Complement protein C1q bound to apoptotic cells suppresses human macrophage and dendritic cell-mediated Th17 and Th1 T cell subset proliferation

A complete genetic deficiency of the complement protein C1q results in SLE with nearly 100% penetrance in humans, but the molecular mechanisms responsible for this association have not yet been fully determined. C1q opsonizes ACs for enhanced ingestion by phagocytes, such as Mϕ and iDCs, avoiding the extracellular release of inflammatory DAMPs upon loss of the membrane integrity of the dying cell. We previously showed that human monocyte-derived Mϕ and DCs ingesting autologous, C1q-bound LALs (C1q-polarized Mϕ and C1q-polarized DCs), enhance the production of anti-inflammatory cytokines, and reduce proinflammatory cytokines relative to Mϕ or DC ingesting LAL alone. Here, we show that C1q-polarized Mϕ have elevated PD-L1 and PD-L2 and suppressed surface CD40, and C1q-polarized DCs have higher surface PD-L2 and less CD86 relative to Mϕ or DC ingesting LAL alone, respectively. In an MLR, C1q-polarized Mϕ reduced allogeneic and autologous Th17 and Th1 subset proliferation and demonstrated a trend toward increased Treg proliferation relative to Mϕ ingesting LAL alone. Moreover, relative to DC ingesting AC in the absence of C1q, C1q-polarized DCs decreased autologous Th17 and Th1 proliferation. These data demonstrate that a functional consequence of C1q-polarized Mϕ and DC is the regulation of Teff activation, thereby "sculpting" the adaptive immune system to avoid autoimmunity, while clearing dying cells. It is noteworthy that these studies identify novel target pathways for therapeutic intervention in SLE and other autoimmune diseases.

Regulatory T-cell homeostasis: steady-state maintenance and modulation during inflammation

Regulatory T (Treg) cells play a vital role in the prevention of autoimmunity and the maintenance of self-tolerance, but these cells also have an active role in inhibiting immune responses during viral, bacterial, and parasitic infections. Although excessive Treg activity can lead to immunodeficiency, chronic infection, and cancer, too little Treg activity results in autoimmunity and immunopathology and impairs the quality of pathogen-specific responses. Recent studies have helped define the homeostatic mechanisms that support the diverse pool of peripheral Treg cells under steady-state conditions and delineate how the abundance and function of Treg cells changes during inflammation. These findings are highly relevant for developing effective strategies to manipulate Treg cell activity to promote allograft tolerance and treat autoimmunity, chronic infection, and cancer.

TCR signaling events are required for maintaining CD4 regulatory T cell numbers and suppressive capacities in the periphery

CD4 regulatory T cells (Tregs) can be subdivided into two subsets according to Ly-6C expression in the periphery. Phenotypic analysis, imaging, and adoptive-transfer experiments of peripheral Ly-6C(-) and Ly-6C(+) Tregs reveal that the nonexpression of Ly-6C by ∼70% of peripheral Tregs depends on TCR signaling events. Interestingly, Ly-6C(-) Tregs express higher surface amounts of key immunosuppressive molecules than do Ly-6C(+) Tregs and produce constitutively anti-inflammatory cytokines. In line with their phenotype, Ly-6C(+) Tregs exhibit poor suppressive capacities in vitro and in vivo. Finally, although Ly-6C(-) Tregs maintain their numbers with age, Ly-6C(+) Tregs gradually disappear. Altogether, our data strongly suggest that both the survival and suppressive functions of peripheral CD4 Tregs rely on their ability to receive strong TCR signals.

TCR stimulation without co-stimulatory signals induces expression of "tolerogenic" genes in memory CD4 T cells but does not compromise cell proliferation

Memory T cells resist co-stimulatory blockade and present a unique therapeutic challenge in transplantation and autoimmune diseases. Herein, we determined whether memory T cells express less "tolerogenic" genes than naïve T cells to reinforce a proliferative response under the deprivation of co-stimulatory signals. The expression of ∼40 tolerogenic genes in memory and naïve CD4(+) T cells was thus assessed during an in vitro TCR stimulation without co-stimulation. Briefly, upon TCR stimulation with an anti-CD3 mAb alone, memory CD4(+) T cells exhibited more proliferation than naïve CD4(+) T cells. To our surprise, at 24h upon anti-CD3 mAb stimulation, memory CD4(+) T cells expressed more than a 5-fold higher level of the transcription factor Egr2 and a 20-fold higher level of the transmembrane E3 ubiquitin ligase GRAIL than those in naïve T cells. Hence, the high-level expression of tolerogenic genes, Egr2 and GRAIL, in memory CD4(+) T cells does not prevent cell proliferation. Importantly, anti-CD3 mAb-stimulated memory CD4(+) T cells expressed high protein/gene levels of phosphorylated STAT5, Nedd4, Bcl-2, and Bcl-XL. Therefore, co-stimulation-independent proliferation of memory CD4(+) T cells may be due to elevated expression of molecules that support cell proliferation and survival, but not lack of tolerogenic molecules.

TGF-β1 mediates the radiation response of prostate cancer

Radiotherapy is the main treatment modality for prostate cancer. This study investigated the role of TGF-β1 in biological sequelae and tumor regrowth following irradiation, which are critical for the clinical radiation response of prostate cancer. Human and murine prostate cancer cell lines, and corresponding hormone-refractory (HR) cells, were used to examine the radiation response by clonogenic assays in vitro and tumor growth delay in vivo. Biological changes after irradiation, including cell death and tumor regrowth, were examined by experimental manipulation of TGF-β1 signaling. The correlations among tumor radiation responses, TGF-β1 levels, and regulatory T cells (Tregs) recruitment were also evaluated using animal experiments. HR prostate cancer cells appeared more radioresistant and had higher expression of TGF-β1 compared to hormone-sensitive (HS) cells. TGF-β1 expression was positively linked to irradiation and radioresistance, as demonstrated by in vitro and in vivo experiments. Inhibition of TGF-β1 increased tumor inhibition and DNA damage after irradiation. When mice were irradiated with a sub-lethal dose, the regrowth of irradiated tumors was significantly correlated with TGF-β1 levels and Tregs accumulation in vivo. Furthermore, blocking TGF-β1 clearly attenuated Tregs accumulation and tumor regrowth following treatment. These data demonstrate that TGF-β1 is important in determining the radiation response of prostate cancer, including tumor cell killing and the tumor microenvironment. Therefore, concurrent treatment with a TGF-β1 inhibitor is a potential therapeutic strategy for increasing the radiation response of prostate cancer, particularly for more aggressive or HR cancer cells.

KEY MESSAGE

• HR prostate cancer cells appeared more radioresistant and had higher expression of TGF-β1. • TGF-β1 was positively linked to the radiation resistance of prostate cancer. • Tumor regrowth following irradiation was significantly correlated with TGF-β1 and Tregs levels. • Blocking TGF-β1 significantly attenuated RT-induced DNA repair and Tregs. • TGF-β1 inhibitor increases the radiation response of HR cancer cells.

Adipose tissue-resident regulatory T cells: phenotypic specialization, functions and therapeutic potential

Foxp3(+)  CD4(+) regulatory T (Treg) cells, recognized to be one of the most important defences of the human body against an inappropriate immune response, have recently gained attention from those outside immunology thanks to the compelling evidence for their capability to exert non-canonical immune functions in a variety of tissues in health and disease. The recent discovery of the differences between tissue-resident Treg cells and those derived from lymphoid organs is affecting the mindset of many investigators now questioning the broad applicability of observations originally based on peripheral blood/lymphoid organ cells. So far, the best characterized 'Treg flavour' comes from studies focused on their role in suppressing adipose tissue inflammation and obesity-driven insulin resistance. Adipose tissue derived Treg cells are distinct from their counterparts in lymphoid organs based on their transcriptional profile, T-cell receptor repertoire, and cytokine and chemokine receptor expression pattern. These cells are abundant in visceral adipose tissue of lean mice but their number is greatly reduced in insulin-resistant animal models of obesity. Interestingly, peroxisome-proliferator-activated receptor γ expression by visceral adipose tissue Treg cells is crucial for their accumulation, phenotype and function in the fat and surprisingly necessary for complete restoration of insulin sensitivity in obese mice by the anti-diabetic drug Pioglitazone. This review surveys recent findings relating to the unique phenotype and function of adipose tissue-resident Treg cells, speculates on the nature of their dynamics in lean and obese mouse models, and analyses their potential therapeutic application in the treatment of type 2 diabetes.

Id2 and Id3 maintain the regulatory T cell pool to suppress inflammatory disease

Regulatory T (Treg) cells suppress the development of inflammatory disease, but our knowledge of transcriptional regulators that control this function remains incomplete. Here we show that expression of Id2 and Id3 in Treg cells was required to suppress development of fatal inflammatory disease. We found that T cell antigen receptor (TCR)-driven signaling initially decreased the abundance of Id3, which led to the activation of a follicular regulatory T (TFR) cell-specific transcription signature. However, sustained lower abundance of Id2 and Id3 interfered with proper development of TFR cells. Depletion of Id2 and Id3 expression in Treg cells resulted in compromised maintenance and localization of the Treg cell population. Thus, Id2 and Id3 enforce TFR cell checkpoints and control the maintenance and homing of Treg cells.

Tolerogenic dendritic cells induce antigen-specific hyporesponsiveness in insulin- and glutamic acid decarboxylase 65-autoreactive T lymphocytes from type 1 diabetic patients

Tolerogenic dendritic cells (tDC) constitute a promising therapy for autoimmune diseases, since they can anergize T lymphocytes recognizing self-antigens. Patients with type 1 diabetes mellitus (T1D) have autoreactive T cells against pancreatic islet antigens (insulin, glutamic acid decarboxylase 65 -GAD65-). We aimed to determine the ability of tDC derived from T1D patients to inactivate their insulin- and GAD65-reactive T cells. CD14+ monocytes and CD4+CD45RA- effector/memory lymphocytes were isolated from 25 patients. Monocyte-derived DC were generated in the absence (control, cDC) or presence of IL-10 and TGF-β1 (tDC), and loaded with insulin or GAD65. DC were cultured with T lymphocytes (primary culture), and cell proliferation and cytokine secretion were determined. These lymphocytes were rechallenged with insulin-, GAD65- or candidin-pulsed cDC (secondary culture) to assess whether tDC rendered T cells hyporesponsive to further stimulation. In the primary cultures, tDC induced significant lower lymphocyte proliferation and IL-2 and IFN-γ secretion than cDC; in contrast, tDC induced higher IL-10 production. Lymphocytes from 60% of patients proliferated specifically against insulin or GAD65 (group 1), whereas 40% did not (group 2). Most patients from group 1 had controlled glycemia. The secondary cultures showed tolerance induction to insulin or GAD65 in 14 and 10 patients, respectively. A high percentage of these patients (70-80%) belonged to group 1. Importantly, tDC induced antigen-specific T-cell hyporesponsiveness, since the responses against unrelated antigens were unaffected. These results suggest that tDC therapy against multiple antigens might be useful in a subset of T1D patients.

IRF4 at the crossroads of effector T-cell fate decision

Interferon regulatory factor 4 (IRF4) is a transcription factor that is expressed in hematopoietic cells and plays pivotal roles in the immune response. Originally described as a lymphocyte-specific nuclear factor, IRF4 promotes differentiation of naïve CD4(+) T cells into T helper 2 (Th2), Th9, Th17, or T follicular helper (Tfh) cells and is required for the function of effector regulatory T (eTreg) cells. Moreover, IRF4 is essential for the sustained differentiation of cytotoxic effector CD8(+) T cells, for CD8(+) T-cell memory formation, and for differentiation of naïve CD8(+) T cells into IL-9-producing (Tc9) and IL-17-producing (Tc17) CD8(+) T-cell subsets. In this review, we focus on recent findings on the role of IRF4 during the development of CD4(+) and CD8(+) T-cell subsets and the impact of IRF4 on T-cell-mediated immune responses in vivo.

Type I interferons directly inhibit regulatory T cells to allow optimal antiviral T cell responses during acute LCMV infection

Inhibition of T reg cells by type I IFNs is necessary for the generation of optimal antiviral T cell responses during acute LCMV infection.

Regulatory T (T reg) cells play an essential role in preventing autoimmunity but can also impair clearance of foreign pathogens. Paradoxically, signals known to promote T reg cell function are abundant during infection and could inappropriately enhance T reg cell activity. How T reg cell function is restrained during infection to allow the generation of effective antiviral responses remains largely unclear. We demonstrate that the potent antiviral type I interferons (IFNs) directly inhibit co-stimulation–dependent T reg cell activation and proliferation, both in vitro and in vivo during acute infection with lymphocytic choriomeningitis virus (LCMV). Loss of the type I IFN receptor specifically in T reg cells results in functional impairment of virus-specific CD8⁺ and CD4⁺ T cells and inefficient viral clearance. Together, these data demonstrate that inhibition of T reg cells by IFNs is necessary for the generation of optimal antiviral T cell responses during acute LCMV infection.

Follicular regulatory T cells control humoral autoimmunity via NFAT2-regulated CXCR5 expression

T cell–specific NFAT2 deletion results in reduced CXCR5⁺ follicular regulatory T cells, leading to uncontrolled germinal center responses and humoral autoimmunity.

Maturation of high-affinity B lymphocytes is precisely controlled during the germinal center reaction. This is dependent on CD4⁺CXCR5⁺ follicular helper T cells (T_FH) and inhibited by CD4⁺CXCR5⁺Foxp3⁺ follicular regulatory T cells (T_FR). Because NFAT2 was found to be highly expressed and activated in follicular T cells, we addressed its function herein. Unexpectedly, ablation of NFAT2 in T cells caused an augmented GC reaction upon immunization. Consistently, however, T_FR cells were clearly reduced in the follicular T cell population due to impaired homing to B cell follicles. This was T_FR-intrinsic because only in these cells NFAT2 was essential to up-regulate CXCR5. The physiological relevance for humoral (auto-)immunity was corroborated by exacerbated lupuslike disease in the presence of NFAT2-deficient T_FR cells.

Anti-CD200R2, anti-IL-9, anti-IL-35, or anti-TGF-β abolishes increased graft survival and Treg induction induced in cromolyn-treated CD200R1KO.CD200tg mice

BACKGROUND

Rejection is associated with early degranulation (≥80%) of graft-infiltrating CD200R1 receptor-positive mast cells (MCs). Survival is increased, and MC degranulation is decreased, in CD200 mice but not in CD200R1KO mice. CD200 engagement of CD200R2 (not present on MCs) alters dendritic cell differentiation and enhances induction of Foxp3 regulatory T cells (Tregs). We investigated whether attenuation of MC degranulation by sodium cromoglycate allowed CD200 to increase survival in CD200R1KO mice.

METHODS

C57BL/6 control, CD200R1KO, CD200, or CD200R1KO.CD200 mice received BALB/c grafts with or without treatment with cromoglycate. Survival was monitored daily from day 10, with mixed lymphocyte culture responses measured on day 14 or 21 and graft immunohistology performed on day 14.

RESULTS

Decreased MC degranulation and increased graft Foxp3 Treg infiltration/survival occurred in CD200 mice and in CD200-treated control mice or CD200R1KO.CD200 mice receiving cromoglycate. Neutralizing anti-CD200 or anti-CD200R1/R2 monoclonal antibody caused graft rejection, as did anti-interleukin (IL)-9, anti-IL-35, or anti-transforming growth factor-β antibodies, with the latter also decreasing graft-infiltrating Tregs.

CONCLUSION

These data imply a coordinated effect of MCs and Tregs on increased graft survival induced by CD200, with a critical role for IL-9, IL-35, and transforming growth factor-β in the development/function of Tregs.

A Potential Link between Environmental Triggers and Autoimmunity

Autoimmune diseases have registered an alarming rise worldwide in recent years. Accumulated evidence indicates that the immune system's ability to distinguish self from nonself is negatively impacted by genetic factors and environmental triggers. Genetics is certainly a factor, but since it normally takes a very long time for the human genetic pattern to change enough to register on a worldwide scale, increasingly the attention of studies has been focused on the environmental factors of a rapidly changing and evolving civilization. New technology, new industries, new inventions, new chemicals and drugs, and new foods and diets are constantly and rapidly being introduced in this fast-paced ever-changing world. Toxicants, infections, epitope spreading, dysfunctions of immune homeostasis, and dietary components can all have an impact on the body's delicate immune recognition system. Although the precise etiology and pathogenesis of many autoimmune diseases are still unknown, it would appear from the collated studies that there are common mechanisms in the immunopathogenesis of multiple autoimmune reactivities. Of particular interest is the citrullination of host proteins and their conversion to autoantigens by the aforementioned environmental triggers. The identification of these specific triggers of autoimmune reactivity is essential then for the development of new therapies for autoimmune diseases.

Homeostatic control of regulatory T cell diversity

Regulatory T (TReg) cells constitute an essential counterbalance to adaptive immune responses. Failure to maintain appropriate TReg cell numbers or function leads to autoimmune, malignant and immunodeficient conditions. Dynamic homeostatic processes preserve the number of forkhead box P3-expressing (FOXP3(+)) TReg cells within a healthy range, with high rates of cell division being offset by apoptosis under steady-state conditions. Recent studies have shown that TReg cells become specialized for different environmental contexts, tailoring their functions and homeostatic properties to a wide range of tissues and immune conditions. In this Review, we describe new insights into the molecular controls that maintain the steady-state homeostasis of TReg cells and the cues that drive TReg cell adaptation to inflammation and/or different locations. We highlight how differing local milieu might drive context-specific TReg cell function and restoration of immune homeostasis, and how dysregulation of these processes can precipitate disease.

CCR7 provides localized access to IL-2 and defines homeostatically distinct regulatory T cell subsets

CD44^loCD62L^hi regulatory T cells preferentially access IL-2 in T cell zones due to expression of CCR7

Immune tolerance and activation depend on precise control over the number and function of immunosuppressive Foxp3⁺ regulatory T (T reg) cells, and the importance of IL-2 in maintaining tolerance and preventing autoimmunity is clear. However, the homeostatic requirement for IL-2 among specific populations of peripheral T reg cells remains poorly understood. We show that IL-2 selectively maintains a population of quiescent CD44^loCD62L^hi T reg cells that gain access to paracrine IL-2 produced in the T cell zones of secondary lymphoid tissues due to their expression of the chemokine receptor CCR7. In contrast, CD44^hiCD62L^loCCR7^lo T reg cells that populate nonlymphoid tissues do not access IL-2–prevalent regions in vivo and are insensitive to IL-2 blockade; instead, their maintenance depends on continued signaling through the co-stimulatory receptor ICOS (inducible co-stimulator). Thus, we define a fundamental homeostatic subdivision in T reg cell populations based on their localization and provide an integrated framework for understanding how T reg cell abundance and function are controlled by unique signals in different tissue environments.