Regulatory T-cells (Tregs) are central for immune homeostasis and divided in thymus-derived natural Tregs and peripherally induced iTreg. However, while phenotype and function of iTregs are well known, a remarkable lack exists in knowledge about signaling mechanisms leading to their generation from naïve precursors in peripheral tissues. Using antigen specific naïve T-cells from mice, we investigated CD4+ CD25+ FoxP3- iTreg induction during antigen-specific T-cell receptor (TCR) stimulation with weak antigen presenting cells (APC). We show that early signaling pathways such as ADAM-17-activation appeared similar in developing iTreg and effector cells (Teff) and both initially shedded CD62-L. But iTreg started reexpressing CD62-L after 24 h while Teff permanently downmodulated it. Furthermore, between 24 and 72 hours iTreg presented with significantly lower phosphorylation levels of Akt-S473 suggesting lower activity of the PI3K/Akt-axis. This was associated with a higher expression of the Akt hydrophobic motif-specific phosphatase PHLPP1 in iTreg. Importantly, the lack of costimulatory signals via CD28 from weak APC was central for the development of regulatory function in iTreg but not for the reappearance of CD62-L. Thus, T-cells display a window of sensitivity after onset of TCR triggering within which the intensity of the PI3K/Akt signal controls entry into either effector or regulatory pathways.
Regulatory T cells (Tregs) have become a priority for many investigators in immunology due to their potent immunosuppressive and tolerogenic effects. While Treg activity is required for normal immune homeostasis, dysregulation of their numbers can induce autoimmunity or aid in the pathogenesis of disease. Therefore, great effort has been made to understand the mechanisms by which Tregs accumulate in different areas of the body. Like other lymphocytes, Tregs migrate in response to a network of chemotactic stimuli involving chemokines, chemokine receptors, integrins, and their corresponding ligands. However, many of these stimuli are exclusive to Tregs, inducing their migration while leaving conventional populations unaffected. It is these selective stimuli that result in increased ratios of Tregs among conventional effector populations, leading to changes in immune suppression and homeostasis. This review explores selective Treg trafficking during thymic Treg development, migration to secondary lymphoid tissues and emigration into the periphery during homeostatic conditions, inflammation, and the tumor microenvironment, placing emphasis on stimuli that selectively recruits Tregs to target locations.
regulatory T cell; Treg; migration; trafficking; localization; chemokine; inflammation; cancer; immune suppression
The main function of our immune system is to protect us from invading pathogens and microorganisms by
destroying infected cells, while minimizing collateral damage to tissues. In order to maintain this balance between
immunity and tolerance, current understanding of the immune system attributes a major role to regulatory T cells
(Tregs) in controlling both immunity and tolerance. Various subsets of Tregs have been identified based on their
expression of cell surface markers, production of cytokines, and mechanisms of action. In brief, naturally occurring
thymic-derived CD4+CD25+ Tregs are characterized by constitutive expression of the transcription factor FOXP3, while
antigen-induced or adaptive Tregs are mainly identified by expression of immunosuppressive cytokines
(interleukin-10 (IL-10) and/or transforming growth factor-β (TGF-β)). While Tregs in normal conditions regulate
ongoing immune responses and prevent autoimmunity, imbalanced function or number of these Tregs, either
enhanced or decreased, might lead, respectively, to decreased immunity (e.g., with tumor development or infections)
or autoimmunity (e.g., multiple sclerosis). This review will discuss recent research towards a better understanding of the
biology of Tregs, their interaction with other immune effector cells, such as dendritic cells, and possible interventions in
Preclinical studies in mice have demonstrated that the prophylactic depletion of immunosuppressive regulatory T-cells (TRegs) through targeting the high affinity interleukin-2 (IL-2) receptor (IL-2Rα/CD25) can enhance anti-tumor immunotherapy. However, therapeutic approaches are complicated by the inadvertent inhibition of IL-2Rα expressing anti-tumor effector T-cells.
To determine if changes in the cytokine milieu during lymphopenia may engender differential signaling requirements that would enable unarmed anti-IL-2Rα monoclonal antibody (MAbs) to selectively deplete TRegs while permitting vaccine-stimulated immune responses.
A randomized placebo-controlled pilot study was undertaken to examine the ability of the anti-IL-2Rα MAb daclizumab, given at the time of epidermal growth factor receptor variant III (EGFRvIII) targeted peptide vaccination, to safely and selectively deplete TRegs in patients with glioblastoma (GBM) treated with lymphodepleting temozolomide (TMZ).
Results and Conclusions
Daclizumab treatment (n = 3) was well-tolerated with no symptoms of autoimmune toxicity and resulted in a significant reduction in the frequency of circulating CD4+Foxp3+ TRegs in comparison to saline controls (n = 3)( p = 0.0464). A significant (p<0.0001) inverse correlation between the frequency of TRegs and the level of EGFRvIII specific humoral responses suggests the depletion of TRegs may be linked to increased vaccine-stimulated humoral immunity. These data suggest this approach deserves further study.
Regulatory T cells (Tregs) are a subset of T cells that are responsible for maintaining peripheral immune tolerance and homeostasis. The hallmark of Tregs is the expression of the forkhead box P3 (FoxP3) transcription factor. Natural regulatory T cells (nTregs) are a distinct population of T cells that express CD4 and FoxP3. nTregs develop in the thymus and function in maintaining peripheral immune tolerance. Other CD4+, CD4-CD8-, and CD8+CD28- T cells can be induced to acquire regulatory function by antigenic stimulation, depending on the cytokine milieu. Inducible (or adaptive) Tregs frequently express high levels of the interleukin 2 receptor (CD25). Atypical Tregs express FoxP3 and CD4 but have no surface expression of CD25. Type 1 regulatory T cells (Tr1 cells) produce IL-10, while T helper 3 cells (Th3) produce TGF-β. The function of inducible Tregs is presumably to maintain immune homeostasis, especially in the context of chronic inflammation or infection. Induction of Tregs in coronaviral infections protects against the more severe forms of the disease attributable to the host response. However, arteriviruses have exploited these T cell subsets as a means to dampen the immune response allowing for viral persistence. Treg induction or activation in the pathogenesis of disease has been described in both porcine reproductive and respiratory syndrome virus, lactate dehydrogenase elevating virus, and mouse hepatitis virus. This review discusses the development and biology of regulatory T cells in the context of arteriviral and coronaviral infection.
regulatory T cell; arterivirus, porcine reproductive and respiratory syndrome virus; lactate dehydrogenase-elevating virus; coronavirus
There has been accumulating evidence that CD4+CD25+ FoxP3 expressing regulatory T cells (Treg) are highly concentrated in tumors, thereby fostering an immune-privileged microenvironment. Some studies have shown that T cell receptor (TCR) stimulation can convert conventional T cells into Treg. Follicular lymphoma B cells can enhance this Treg conversion. We investigated whether FL tumor B cells, as opposed to normal B cells, are unique in their ability to convert effector T cells into Treg. We found that tumor B cells alone, without artificial TCR stimulation, could induce conventional T cells to express FoxP3 and to acquire regulatory function. In contrast to their malignant counterpart, normal B cells did not induce Treg conversion. Treg conversion was independent of the T cell background, since T cells isolated from FL or normal peripheral blood were equally susceptible to being converted by tumor B cells. Our study provides evidence for a tumor-specific mechanism by which FL tumor cells promote immune escape through the induction of Treg.
Tumor microenvironment; immune escape
Anti-inflammatory signals play an essential role in constraining the magnitude of an inflammatory response. Extracellular adenosine is a critical tissue-protective factor, limiting the extent of inflammation. Given the potent anti-inflammatory effects of extracellular adenosine, we sought to investigate how extracellular adenosine regulates T cell activation and differentiation. Adenosine receptor activation by a pan adenosine-receptor agonist enhanced the abundance of murine regulatory T cells (Tregs), a cell type critical in constraining inflammation. Gene expression studies in both naïve CD4 T cells and Tregs revealed that these cells expressed multiple adenosine receptors. Based on recent studies implicating the Adora2b in endogenous anti-inflammatory responses during acute inflammation, we used a pharmacologic approach to specifically activate Adora2b. Indeed, these studies revealed robust enhancement of Treg differentiation in wild-type mice, but not in Adora2b−/− T cells. Finally, when we subjected Adora2b-deficient mice to endotoxin-induced pulmonary inflammation, we found that these mice experienced more severe inflammation, characterized by increased cell recruitment and increased fluid leakage into the airways. Notably, Adora2b-deficient mice failed to induce Tregs after endotoxin-induced inflammation and instead had an enhanced recruitment of pro-inflammatory effector T cells. In total, these data indicate that the Adora2b adenosine receptor serves a potent anti-inflammatory role, functioning at least in part through the enhancement of Tregs, to limit inflammation.
We have shown that CD39 and CD73 are co-expressed on the surface of murine CD4+Foxp3+ regulatory T cells (Treg) and generate extracellular adenosine, contributing to Treg immunosuppressive activity. We now describe that CD39, independently of CD73, is expressed by a subset of blood derived human CD4+CD25+CD127lo T regulatory cells (Treg), defined by robust expression of Foxp3. A further distinct population of CD4+CD39+ T lymphocytes can be identified, which do not express CD25 and FoxP3 and exhibit the memory effector cellular phenotype. Differential expression of CD25 and CD39 on circulating CD4+ T cells distinguishes between Treg and pathogenic cellular populations that secrete pro-inflammatory cytokines such as IFNγ and IL-17. These latter cell populations are increased, with a concomitant decrease in the CD4+CD25+CD39+ Tregs, in the peripheral blood of patients with renal allograft rejection. We conclude that the ectonucleotidase CD39 is a useful and dynamic lymphocytes surface marker that can be used to identify different peripheral blood T cell populations to allow tracking of these in health and disease, as in renal allograft rejection.
Hepatic stellate cells (HSCs) may play an important role in hepatic immune regulation by producing numerous cytokines/chemokines, and expressing Ag-presenting and T cell co-regulatory molecules. Due to disruption of the endothelial barrier during cold-ischemic storage and reperfusion of liver grafts, HSCs can interact directly with cells of the immune system. Endotoxin (LPS), levels of which increase in liver diseases and transplantation, stimulates the synthesis of many mediators by HSCs. We hypothesized that LPS-stimulated HSCs might promote hepatic tolerogenicity by influencing naturally-occurring immunosuppressive CD4+CD25+FoxP3+ regulatory T cells (Tregs). Following their portal venous infusion, allogeneic CD4+ T cells, including Tregs, were found closely associated with HSCs, and this association increased in LPS-treated livers. In vitro, both unstimulated and LPS-stimulated HSCs up-regulated Fas (CD95) expression on conventional CD4+ T cells and induced their apoptosis in a Fas/FasL-dependent manner. By contrast, HSCs induced Treg proliferation, which required cell-cell contact, and was MHC class II-dependent. This effect was augmented when HSCs were pretreated with LPS. LPS increased the expression of MHC class II, CD80 and CD86, and stimulated the production of IL-1α, IL-1β, IL-6, IL-10 and TNFα by HSCs. Interestingly, production of IL-1α, IL-1β, IL-6 and TNFα was strongly inhibited, but that of IL-10 enhanced, in LPS-pretreated HSC/Treg co-cultures. Adoptively transferred allogeneic HSCs migrated to the secondary lymphoid tissues and induced Treg expansion in lymph nodes. These data implicate endotoxins-stimulated HSCs as important immune regulators in liver transplantation by inducing selective expansion of tolerance-promoting Tregs, and reducing inflammation and allo-immunity.
Hepatic stellate cells; lipopolysaccharide; liver; tolerance; Tregs
HIV-1 infection is characterized by a chronic activation of the immune system and suppressed function of T lymphocytes. Regulatory CD4+ CD25high FoxP3+CD127low T cells (Treg) play a key role in both conditions. Here, we show that HIV-1 positive patients have a significant increase of Treg-associated expression of CD39/ENTPD1, an ectoenzyme which in concert with CD73 generates adenosine. We show in vitro that the CD39/adenosine axis is involved in Treg suppression in HIV infection. Treg inhibitory effects are relieved by CD39 down modulation and are reproduced by an adenosine-agonist in accordance with a higher expression of the adenosine A2A receptor on patients' T cells. Notably, the expansion of the Treg CD39+ correlates with the level of immune activation and lower CD4+ counts in HIV-1 infected patients. Finally, in a genetic association study performed in three different cohorts, we identified a CD39 gene polymorphism that was associated with down-modulated CD39 expression and a slower progression to AIDS.
HIV-1 infection is characterized by a chronic activation of the immune system. Regulatory T cells (Treg) represent a population of lymphocytes that controls inappropriate or exaggerated immune activation induced by pathogens, thereby influencing the outcome of various infections. Several studies have shown that Treg are expanded in HIV infected patients. However, the mechanisms of Treg immune-modulator functions are not clearly known. CD39 is an ectonucleotidase which converts the proinflammatory ATP signal into AMP and the immunosuppressive adenosine in concert with CD73. A critical role of CD39 has been described for Treg in general but few studies have analyzed its role in HIV infection. We report here an expansion of Treg expressing CD39 in a cohort of HIV-infected patients. In vitro these cells exerted a strong suppressive effect on the effector CD8 T cells. Treg inhibitory effects were relieved by CD39 down-modulation using an anti-CD39 monoclonal antibody. Treg suppressive effects were reproduced by an adenosine agonist in accordance with a higher expression of the adenosine A2A receptor on patients' T cells. From a clinical stand point, we show also a correlation between Treg CD39+ expansion and both immune activation and CD4+ T cell depletion in patients. Finally, by genetic analysis of three different cohorts of patients, we found that a CD39 gene polymorphism associated with a lower CD39 expression correlated with a slower progression to AIDS. Thus, our results contribute to elucidate the mechanisms by which Treg suppression occurs during HIV infection.
Regulatory T (Treg) cells are activated and suppress immune responses during infection, and are characterized as CD4+CD25hiFOXP3+. Ex vivo studies demonstrate that Treg cells potentially suppress anti-HIV-1 T cell responses. Lentivirus-induced CD4+CD25hi Treg cells were first described in feline immunodeficiency virus (FIV)-infected cats. In the present study we demonstrate that anti-feline CD25 monoclonal antibody (mAb) therapy depletes Treg cells in FIV-infected cats for four weeks and does not exacerbate viral replication or proinflammatory cytokine production. Significant FIV-specific immune responses are revealed in Treg cell-depleted cats. These anti-FIV effector cells exist prior to Treg cell depletion and are not expanded while Treg cells are depleted. Importantly, cats receiving the Treg cell-depleting mAb are able to produce a robust humoral response to new antigen. We propose that short-term in vivo Treg cell depletion during chronic HIV-1 infection could provide a window of opportunity for therapeutic vaccination in individuals with controlled viral replication.
Feline immunodeficiency virus; HIV-1; lentivirus; regulatory T cell; immunosuppression; monoclonal antibody
Regulatory T cells (Treg) frequency/activity are increased in cancer patients and play a major in tumor escape. While disease progression is favored by the presence of Treg, mechanisms used by Treg to suppress anti-tumor immunity are unknown. The ectonucleotidases CD39 and CD73 are expressed in Treg and convert ATP into immunosuppressive adenosine. In this study, the involvement of the adenosinergic pathway in Treg-mediated suppression in HNSCC patients was evaluated.
HNSCC patients with an active disease (AD) (n=19) and patients with no evident disease (NED) after therapy (n=14) were studied. Ectonucleotidase expression on CD4+ T cells and CD4+CD25high Treg was evaluated by flow cytometry and compared to normal controls (NC). Ectonucleotidase activity was also compared within these 3 groups. The data were analyzed for associations of ectonucleotidase expression/function with disease stage.
The percentages and expression levels of CD39 and CD73 in CD4+ T cells and Treg were greater in HNSCC than NC and were highest in NED. Patients' Treg hydrolyzed ATP at higher rates and produced higher levels of adenosine than NC' Treg. The increased frequency and enzymatic activity of CD4+CD39+ cells corresponded to increased adenosine-mediated suppression of effector T cells, which was partly inhibited by ARL67156, an ectonucleotidase inhibitor, and by ZM241385, a selective A2a/A2b receptor antagonist.
CD39+ Treg frequency and adenosine-mediated suppression are significantly increased in HNSCC patients. The adenosinergic pathway is involved in Treg-mediated immunosuppression in cancer and its attenuation could be a promising immunotherapeutic strategy for patients with HNSCC.
head and neck cancer; ectonucleotidases; regulatory T cells; tumor escape; adenosine
Forkhead box p3 (Foxp3) positive T regulatory cells (Tregs) have a functionally immunosuppressive property that prevents effector cells from acting against self in autoimmune diseases or a tumor. It is known that Tregs may be highly relevant in cancer progression. Dendritic cells (DCs) induce cutaneous immune response, however several studies have suggested that DCs are involved in immunosuppression. The aim of this study is to evaluate the prevalence of Tregs and DCs infiltration in cutaneous premalignant and malignant squamous lesions.
Materials and Methods
We evaluated Tregs and DCs in skin tissue samples obtained from 83 patients with actinic keratosis, Bowen's disease or squamous cell carcinoma by immunohistochemistry.
The prevalence of Tregs and DCs was significantly higher in squamous cell carcinoma and Bowen's disease than in actinic keratosis. In addition, the number of DCs was closely correlated with the prevalence of Tregs, and DCs were also located in direct proximity to Tregs.
Tregs is related to cutaneous squamous tumor progression.
Forkhead box p3; T regulatory cells; skin tumor
CD8 T cells stimulated with a suboptimal dose of anti-CD3 antibodies (100 pg/ml) in the presence of IL-15 retain a naïve phenotype with expression of CD45RA, CD28, CD27 and CCR7 but acquire new functions and differentiate into immunosuppressive T cells. CD8+CCR7+ Tregs express FOXP3 and prevent CD4 T cells from responding to T-cell receptor stimulation and entering the cell cycle. Naïve CD4 T cells are more susceptible to inhibition than memory cells. The suppressive activity of CD8+CCR7+ Tregs is not mediated by IL-10, TGF-β, CTLA-4, CCL4 or adenosine and relies on interference with very early steps of the TCR signaling cascade. Specifically, CD8+CCR7+ Tregs prevent TCR-induced phosphorylation of ZAP70 and dampen the rise of intracellular calcium in CD4 T cells. The inducibility of CD8+CCR7+ Tregs is correlated to the age of the individual with peripheral blood lymphocytes of donors older than 60 years yielding low numbers of FOXP3low CD8 Treg cells. Loss of CD8+CCR7+ Tregs in the elderly host may be of relevance in the aging immune system as immunosenescence is associated with a state of chronic smoldering inflammation.
There is growing awareness that tumour cells build up a “self-advantageous” microenvironment that reduces effectiveness of anti-tumour immune response. While many different immunosuppressive mechanisms are likely to come into play, recent evidence suggests that extracellular adenosine acting at A2A receptors may have a major role in down-modulating the immune response as cancerous tissues contain elevated levels of adenosine and adenosine break-down products. While there is no doubt that all cells possess plasma membrane adenosine transporters that mediate adenosine uptake and may also allow its release, it is now clear that most of extracellularly-generated adenosine originates from the catabolism of extracellular ATP.
Measurement of extracellular ATP is generally performed in cell supernatants by HPLC or soluble luciferin-luciferase assay, thus it generally turns out to be laborious and inaccurate. We have engineered a chimeric plasma membrane-targeted luciferase that allows in vivo real-time imaging of extracellular ATP. With this novel probe we have measured the ATP concentration within the tumour microenvironment of several experimentally-induced tumours.
Our results show that ATP in the tumour interstitium is in the hundrends micromolar range, while it is basically undetectable in healthy tissues. Here we show that a chimeric plasma membrane-targeted luciferase allows in vivo detection of high extracellular ATP concentration at tumour sites. On the contrary, tumour-free tissues show undetectable extracellular ATP levels. Extracellular ATP may be crucial for the tumour not only as a stimulus for growth but also as a source of an immunosuppressive agent such as adenosine. Our approach offers a new tool for the investigation of the biochemical composition of tumour milieu and for development of novel therapies based on the modulation of extracellular purine-based signalling.
HIV-induced AIDS may be mediated by the activation of immunosuppressive CD4+CD25+ T regulatory cells (Treg cells). Treg cells have been shown to regulate CD4+ and CD8+ immune responses to HIV and FIV antigens in vitro. We tested the hypothesis that Treg cells become infected and activated during the acute infection with FIV leading to the suppression of CD4+ T helper cell responses. Cats were experimentally infected with FIV-NCSU1 and blood and lymph node cells were collected at weekly intervals following inoculation. Real-Time RT-PCR was used to determine plasma viremia and the relative expression of FIV, FoxP3, TGF-β, and GAPDH mRNA copies in CD4+CD25+ and CD4+CD25− T cell subsets. Flow cytometry was used to assess the absolute numbers of each cell type and the expression of surface TGF-β and intracellular FoxP3 in CD4+CD25+ and CD4+CD25− T cells at each time point. Treg suppression of IL-2 production in CD4+ T helper cells was assessed by ELISPOT assays. Our results showed that peak viremia occurred at 2 weeks post infection and correlated with maximal infectivity in CD4+CD25+ T cell populations. FIV-gag-mRNA levels were higher in CD4+CD25+ T cells than CD4+CD25− T cells throughout the acute phase of infection. Induction of FoxP3 and TGF-β indicated activation of Treg cells during the acute stage infection, which was confirmed by Treg cell suppression of IL-2 production by CD4+ Th cells in an ELISPOT assay. Our findings support the hypothesis that early activation of Treg immunosuppressor function may limit an effective anti-FIV response, contributing to the establishment of chronic infection and the immunodeficiency caused by this virus.
FIV; HIV; Regulatory T cells; Acute infection
Background & Aims
Adenosine mediates immune suppression and is generated by the ectonucleotidases CD39 (ENTPD1) and CD73 that are expressed on vascular endothelial cells and regulatory T cells (Treg). Although tumor-infiltrating immune cells include Foxp3+ Treg, it is not clear whether local adenosine generation by Treg promotes tumor growth in a CD39-dependent manner. In this study, we have examined the impact of CD39 expression by Treg on effector immune cell responses to hepatic metastases in vivo.
Methods and Results
A model of hepatic metastatic cancer was developed with portal vein infusion of luciferase-expressing melanoma B16/F10 cells and MCA38 colon cancer cells in wild type and mutant mice null for Cd39. Chimeric mice were generated by bone marrow transplantation (BMT) using Cd39 null or wild type (wt) C57BL6 donors and irradiated recipient mice. We demonstrate that hepatic growth of melanoma metastatic tumors was strongly inhibited in mice with Cd39 null vasculature or in wild type mice with circulating Cd39 null bone marrow-derived cells. We show functional CD39 expression on CD4+Foxp3+ Treg suppressed anti-tumor immunity mediated by NK cells in vitro and in vivo. Lastly, inhibition of CD39 activity by POM-1 (polyoxometalate-1), a pharmacological inhibitor of NTPDase activity, significantly inhibited tumor growth (P < .001).
CD39 expression on Treg inhibits NK activity and is permissive for metastatic growth. Pharmacological or targeted inhibition of CD39 enzymatic activity may find utility as an adjunct therapy for secondary hepatic malignancies.
CD39/ENTPD1; Regulatory T cells (Treg); Cancer therapy; Liver
Regulatory T cells (Treg) express the forkhead box p3 (Foxp3) transcription factor and suppress pathological immune responses against self and foreign antigens, including commensal microorganisms. Foxp3 has been proposed as a master key regulator for Treg, required for their differentiation, maintenance, and suppressive functions. Two types of Treg have been defined. Natural Treg (nTreg) are usually considered to be a separate sublineage arising during thymus differentiation. Induced Treg (iTreg) originate upon T cell receptor (TCR) stimulation in the presence of tumor growth factor β. Although under homeostatic conditions most Treg in the periphery are nTreg, special immune challenges in the intestine promote more frequently the generation of iTreg. Furthermore, recent observations have challenged the notion that Treg are a stable sublineage, and they suggest that, particularly under lymphopenic and/or inflammatory conditions, Treg may lose Foxp3 and/or acquire diverse effector functions, especially in the intestine, which may contribute to uncontrolled inflammation.
A diversity of immune tolerance mechanisms have evolved to protect normal tissues from immune damage. Immune regulatory cells are critical contributors to peripheral tolerance. These regulatory cells, exemplified by the CD4+Foxp3+ regulatory T (Treg) cells and a recently identified population named myeloid-derived suppressor cells (MDSCs), regulate immune responses and limiting immune-mediated pathology. In a chronic inflammatory setting, such as allograft-directed immunity, there may be a dynamic “crosstalk” between the innate and adaptive immunomodulatory mechanisms for an integrated control of immune damage. CTLA4-B7-based interaction between the two branches may function as a molecular “bridge” to facilitate such “crosstalk”. Understanding the interplays among Treg cells, innate suppressors and pathogenic effector T (Teff) cells will be critical in the future to assist in the development of therapeutic strategies to enhance and synergize physiological immunosuppressive elements in the innate and adaptive immune system. Successful development of localized strategies of regulatory cell therapies could circumvent the requirement for very high number of cells and decrease the risks associated with systemic immunosuppression. To realize the potential of innate and adaptive immune regulators for the still-elusive goal of immune tolerance induction, adoptive cell therapies may also need to be coupled with agents enhancing endogenous tolerance mechanisms.
Immune tolerance; immunosuppression; regulatory cells; CTLA4; autoimmunity; transplant
Recent studies have reported that adenosine is a significant mediator of regulatory T cell (Treg) function. Indeed, activation of the adenosine receptor subtypes expressed by a broad range of immune and inflammatory cells attenuates inflammation in several disease models. This anti-inflammatory response is associated with an increase in intracellular cAMP that inhibits cytokine responses of many immune/inflammatory cells, including T cells and APCs. Thus, adenosine produced by Tregs can provide a paracrine feedback that shapes the host response following an immunologic provocation. This review discusses the evidence that adenosine is an integral part of Treg biology and presents some of the mechanisms that may account for its contribution to the resolution of inflammation and the regulation of the immune/inflammatory cell phenotype.
Preferential activation of regulatory T (Treg) cells limits autoimmune tissue damage during chronic immune responses but can also facilitate tumor growth. Here, we show that tissue-produced inflammatory mediators prime maturing dendritic cells (DC) for the differential ability of attracting anti-inflammatory Treg cells. Our data show that prostaglandin E2 (PGE2), a factor overproduced in chronic inflammation and cancer, induces stable Treg-attracting properties in maturing DC, mediated by CCL22. The elevated production of CCL22 by PGE2-matured DC persists after the removal of PGE2 and is further elevated after secondary stimulation of DC in a neutral environment. This PGE2-induced overproduction of CCL22 and the resulting attraction of FOXP3+ Tregs are counteracted by IFNα, a mediator of acute inflammation, which also restores the ability of the PGE2-exposed DC to secrete the Th1-attracting chemokines: CXCL9, CXCL10, CXCL11, and CCL5. In accordance with these observations, different DCs clinically used as cancer vaccines show different Treg-recruiting abilities, with PGE2-matured DC, but not type 1–polarized DC, generated in the presence of type I and type II IFNs, showing high Treg-attracting activity. The current data, showing that the ability of mature DC to interact with Treg cells is predetermined at the stage of DC maturation, pave the way to preferentially target the regulatory versus proinflammatory T cells in autoimmunity and transplantation, as opposed to intracellular infections and cancer.
Cutaneous T-cell lymphoma (CTCL) displays immunosuppressive properties and phenotypic plasticity. The malignant T cells in CTCL can possess features of immunomodulating regulatory T cells (Treg) and IL-17-producing helper T cells (Th17) depending on the stimuli they receive from antigen presenting cells and other sources. IL-2-type cytokines activate STAT5 to promote expression of Treg-related FoxP3, while various cytokines can activate STAT3 to induce synthesis of IL-10 and IL-17. When the Treg phenotype is activated in the early stages of CTCL, “immune evasion” can occur, allowing the clonal T cells to expand. Late stages of CTCL lose the FoxP3 expression but continue to express an immunosuppressive cell-surface ligand PD-L1 suggesting that this and possibly other immunosuppressive proteins rather than FoxP3 are critical for the immunosuppressive state in the advanced stages of CTCL. Novel therapeutic agents may potentially exploit the phenotypic plasticity of CTCL such that the malignant T cells become vulnerable to antitumor immunity.
CTCL; FoxP3; IL-10; IL-2; PD-L1; STAT3; STAT5; Th17; Treg
Regulatory T cells (Treg) play a key role in maintaining the balance of immune responses in human health and in disease. Treg come in many flavors and can utilize a variety of mechanisms to modulate immune responses. In cancer, inducible (i) or adaptive Treg expand, accumulate in tissues and peripheral blood of patients, and represent a functionally prominent component of CD4+ T lymphocytes. Phenotypically and functionally, iTreg are distinct from natural (n) Treg. A subset of iTreg expressing ectonucleotidases CD39 and CD73 is able to hydrolyze ATP to 5′-AMP and adenosine (ADO) and thus mediate suppression of those immune cells which express ADO receptors. iTreg can also produce prostaglandin E2 (PGE2). The mechanisms responsible for iTreg-mediated suppression involve binding of ADO and PGE2 produced by iTreg to their respective receptors expressed on T effector cells (Teff), leading to the up-regulation of adenylate cyclase and cAMP activities in Teff and to their functional inhibition. The potential for regulating these mechanisms by the use of pharmacologic inhibitors to relieve iTreg-mediated suppression in cancer suggests the development of therapeutic strategies targeting the ADO and PGE2 pathways.
cancer inducible regulatory T cells; natural regulatory T cells; tumor microenvironment
Foxp3+ CD4+ regulatory T cells (Tregs) represent a highly suppressive T cell subset with well-characterized immunosuppressive effects during immune homeostasis and chronic infections, although the role of these cells in acute viral infections is poorly understood. The present study sought to examine the induction of Foxp3+ CD4+ Tregs in a nonlethal murine model of pulmonary viral infection by the use of the prototypical respiratory virus influenza A. We establish that influenza A virus infection results in a robust Foxp3+ CD4+ T cell response and that regulatory T cell induction at the site of inflammation precedes the effector T cell response. Induced Foxp3+ CD4+ T cells are highly suppressive ex vivo, demonstrating that influenza virus-induced Foxp3+ CD4+ T cells are phenotypically regulatory. Influenza A virus-induced regulatory T cells proliferate vigorously in response to influenza virus antigen, are disseminated throughout the site of infection and primary and secondary lymphoid organs, and retain Foxp3 expression in vitro, suggesting that acute viral infection is capable of inducing a foreign-antigen-specific Treg response. The ability of influenza virus-induced regulatory T cells to suppress antigen-specific CD4+ and CD8+ T cell proliferation and cytokine production correlates closely to their ability to respond to influenza virus antigens, suggesting that virus-induced Tregs are capable of attenuating effector responses in an antigen-dependent manner. Collectively, these data demonstrate that primary acute viral infection is capable of inducing a robust, antigen-responsive, and suppressive regulatory T cell response.
Localization of CD4+CD25+Foxp3+ regulatory T (Treg) cells to lymphoid and non-lymphoid tissue is instrumental for the effective control of immune responses. Compared with conventional T cells, Treg cells constitute a minute fraction of the T-cell repertoire. Despite this numeric disadvantage, Tregs efficiently migrate to sites of immune responses reaching an optimal number for the regulation of T effector (Teff) cells. The array and levels of adhesion and chemokine receptor expression by Tregs do not explain their powerful migratory capacity. Here we show that recognition of self-antigens expressed by endothelial cells in target tissue is instrumental for efficient Treg recruitment in vivo. This event relies upon IFN-γ-mediated induction of MHC-class-II molecule expression by the endothelium and requires optimal PI3K p110δ activation by the T-cell receptor. We also show that, once in the tissue, Tregs inhibit Teff recruitment, further enabling a Teff:Treg ratio optimal for regulation.
Regulatory T cells (Tregs) are important for the maintenance of self-tolerance and this requires their trafficking to the lymph nodes and target tissues. Here, the authors show that the recognition of self-antigens expressed by endothelial cells in target tissue is instrumental for efficient Treg recruitment in vivo.