Tr1 Differentiation Research Articles

Type-1 regulatory T cells are critical for curative immunotherapy outcomes

Abstract IL-10 producing CD4 +type-1 regulatory T cells (Tr1) mediate immune tolerance during chronic infection, autoimmunity, and transplantation. We previously demonstrated that eomesodermin (Eomes) promotes Tr1 cell (Eomes +IL-10 +) development after allogenic bone marrow transplantation (alloBMT). Here we define the differentiation trajectory of Tr1 cells and the functional states therein. Eomes −IL-10 −, Eomes +IL-10 −and Eomes +IL-10 +subsets (defined by mCherry and GFP expression respectively) were FACS sorted after alloBMT and transferred into secondary recipients. Eomes +IL-10 +T cells were stable in phenotype after transfer while the Eomes +IL-10 −T cell subset differentiated into Eomes +IL-10 +Tr1 cells. Tr1 differentiation was associated with progressive enrichment for cytotoxicity (Perforin, Nkg7, Granzyme K, Tnfrsf9), immune regulatory (IR) (Tim-3, Lag3, Tox, Nr4a2) and chemokine (Ccl3, Ccl4, Ccl5) RNA expression. Equivalent IR, cytotoxic and cytokine/chemokine expression was confirmed at a protein level. The conditional deletion of Eomes in CD4 T cells prevented Tr1 differentiation and resulted in impaired cytotoxicity in vivo, the accumulation of donor CD11c +dendritic cells, mixed donor chimerism and leukemia relapse, consistent with a critical role for this lineage in tolerance, engraftment and graft-versus-leukemia effects. The deletion of Eomes from CD19-targeted chimeric antigen receptor (CAR) T cells, reduced cytotoxic molecule expression (perforin, granzyme A/B) in CD4 CAR T cells and resulted in immune escape due to CD19 antigen loss in leukemia cells. In sum, Tr1 cells mediate critical Eomes-dependent cytotoxic and regulatory function that are critical for curative immunotherapy outcomes.

High-Efficiency Retroviral Transduction for Type 1 Regulatory T Cell Differentiation.

Type 1 regulatory T (Tr1) cells are an immunoregulatory CD4 + Foxp3- IL-10 high T cell subset with therapeutic potential for various inflammatory diseases. Retroviral (RV) transduction has been a valuable tool in defining the signaling pathways and transcription factors that regulate Tr1 differentiation and suppressive function. This protocol describes a method for RV transduction of naïve CD4 + T cells differentiating under Tr1 conditions, without the use of reagents such as polybrene or RetroNectin. A major advantage of this protocol over others is that it allows for the role of genes of interest on both differentiation and function of Tr1 cells to be interrogated. This is due to the high efficiency of RV transduction combined with the use of an IL10 GFP /Foxp3 RFP dual reporter mouse model, which enables successfully transduced Tr1 cells to be identified and sorted for functional assays. In addition, this protocol may be utilized for dual/multiple transduction approaches and transduction of other lymphocyte populations, such as CD8 + T cells.

Vitamin D/CD46 Crosstalk in Human T Cells in Multiple Sclerosis.

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS), in which T-cell migration into the CNS is key for pathogenesis. Patients with MS exhibit impaired regulatory T cell populations, and both Foxp3+ Tregs and type I regulatory T cells (Tr1) are dysfunctional. MS is a multifactorial disease and vitamin D deficiency is associated with disease. Herein, we examined the impact of 1,25(OH)2D3 on CD4+ T cells coactivated by either CD28 to induce polyclonal activation or by the complement regulator CD46 to promote Tr1 differentiation. Addition of 1,25(OH)2D3 led to a differential expression of adhesion molecules on CD28- and CD46-costimulated T cells isolated from both healthy donors or from patients with MS. 1,25(OH)2D3 favored Tr1 motility though a Vitamin D-CD46 crosstalk highlighted by increased VDR expression as well as increased CYP24A1 and miR-9 in CD46-costimulated T cells. Furthermore, analysis of CD46 expression on T cells from a cohort of patients with MS supplemented by vitamin D showed a negative correlation with the levels of circulating vitamin D. Moreover, t-Distributed Stochastic Neighbor Embedding (t-SNE) analysis allowed the visualization and identification of clusters increased by vitamin D supplementation, but not by placebo, that exhibited similar adhesion phenotype to what was observed in vitro. Overall, our data show a crosstalk between vitamin D and CD46 that allows a preferential effect of Vitamin D on Tr1 cells, providing novel key insights into the role of an important modifiable environmental factor in MS.

Transcriptional regulation of IL-10 in T helper cells

Abstract IL-10 produced by CD4 T cells plays an essential role in limiting inflammation and autoimmunity. All T help cell subsets can co-produce IL-10 to mitigate unwanted immunopathology but naïve CD4 T cells can also be directly polarized to IL-10 producing Type 1 regulatory (Tr1) cells by the immunoregulatory cytokine, IL-27. Although several transcription factors (TFs) have been shown to regulate IL-10 expression in different contexts, a comprehensive transcriptional network that induce and maintain IL-10 in various CD4 T cells is lacking. Here we utilized two complementary approaches to systemically study the transcriptional mechanisms driving IL-10 production in T helper cells: 1) We combined novel computational tools with transcriptomic profiling of in-vitro Tr1 cell differentiation at high temporal resolution to build a comprehensive and dynamic transcriptional network that regulates IL-10 production in CD4 T cells. We identified four transcription waves that govern Tr1 differentiation: dynamic early phase, stable early phase, induction phase and maintenance phase. Based on the network analysis, we computationally predicted 78 TFs that regulate different waves and experimentally validated 24 of them using knock-out mice. 2) To identify TFs associated with IL-10 production across T helper cell subsets, we compared RNA-seq data of IL-10+ versus IL-10− compartments of in-vitro differentiated Th1, Th2 and Th17 cells. Using a comprehensive ranking scheme, we identified two TFs that regulates IL-10 production in all helper T cell subsets. Conditional deletion of both the TFs but not either alone with CD4-Cre abolishes IL-10 production from Tr1 cells in vitro and leads to spontaneous colitis in vivo.

The anthelmintic drug praziquantel promotes human Tr1 differentiation.

Praziquantel (PZQ) is an anthelminthic human and veterinary drug used to treat trematode and cestode worms. Changes in immune responses have been demonstrated in humans following curative PZQ treatment of schistosome infections. These changes have been attributed to the removal of immunosupressive worms and immune responses to parasite antigens exposed from dying worms. To date, there has been no study investigating the potential direct effect of PZQ on the host immune cells. Herein, we analyzed the effect of PZQ on human CD4+ T cells classically costimulated by CD3/CD28 or costimulated by the complement regulator CD46 to induce Type 1 regulatory T cells (Tr1). Our results show that PZQ enhanced T-cell proliferation, increased secretion of IL-17 and IL-10 but had no effect on secretion of GM-CSF or IFNγ. Moreover, PZQ increased the coexpression of CD49b and LAG-3, a hallmark of Tr1 cells, suggesting increased Tr1 differentiation. Indeed, supernatants from PZQ-treated cells were able to decrease bystander T-cell activation, and this was partly reduced when blocking IL-10. Hence, our study demonstrates that PZQ directly modulates human T-cell activation and promotes Tr1 differentiation, suggesting that PZQ may have immunomodulatory functions in parasite-unrelated human inflammatory diseases.

Prostaglandin E2 inhibits Tr1 cell differentiation through suppression of c-Maf.

Prostaglandin E2 (PGE2), a major lipid mediator abundant at inflammatory sites, acts as a proinflammatory agent in models of inflammatory/autoimmune diseases by promoting CD4 Th1/Th17 differentiation. Regulatory T cells, including the IL-10 producing Tr1 cells counterbalance the proinflammatory activity of effector Th1/Th17 cells. Tr1 cell differentiation and function are induced by IL-27, and depend primarily on sustained expression of c-Maf in addition to AhR and Blimp-1. In agreement with the in vivo proinflammatory role of PGE2, here we report for the first time that PGE2 inhibits IL-27-induced differentiation and IL-10 production of murine CD4+CD49b+LAG-3+Foxp3- Tr1 cells. The inhibitory effect of PGE2 was mediated through EP4 receptors and induction of cAMP, leading to a significant reduction in c-Maf expression. Although PGE2 reduced IL-21 production in differentiating Tr1 cells, its inhibitory effect on Tr1 differentiation and c-Maf expression also occurred independent of IL-21 signaling. PGE2 did not affect STAT1/3 activation, AhR expression and only marginally reduced Egr-2/Blimp-1 expression. The effect of PGE2 on CD4+CD49b+LAG-3+ Tr1 differentiation was not associated with either induction of Foxp3 or IL-17 production, suggesting a lack of transdifferentiation into Foxp3+ Treg or effector Th17 cells. We recently reported that PGE2 inhibits the expression and production of IL-27 from activated conventional dendritic cells (cDC) in vivo and in vitro. The present study indicates that PGE2 also reduces murine Tr1 differentiation and function directly by acting on IL-27-differentiating Tr1 cells. Together, the ability of PGE2 to inhibit IL-27 production by cDC, and the direct inhibitory effect on Tr1 differentiation mediated through reduction in c-Maf expression, represent a new mechanistic perspective for the proinflammatory activity of PGE2.

The non-receptor tyrosine kinase ITK is required for type 1 regulatory T cell development

Abstract Type 1 regulatory T (Tr1)cells lack the expression of Foxp3 but are potent producers of the immunosuppressive cytokine IL-10, with profound regulatory function in suppressing inflammation and promoting tolerance. Tr1 cells differentiate in response to signals engaging T cell receptor (TCR) and/or regulatory cytokine milieu. The non-receptor tyrosine kinase ITK is a key modulator downstream of TCR, playing critical role in T cell development and function. Using mouse models carrying Foxp3-RFP and IL-10-GFP dual reporters, we found that, in the absence of ITK, Foxp3− IL-10+ Tr1 cell development driven by TCR activation is severely impaired in various tissues (spleen, lymph nodes, lung, gut, and fat). Under Tr1 polarizing condition, naïve Foxp3− CD4+T cells isolated from WT mouse thymus and spleen can both give rise to Tr1cells, however, Itk−/− thymic and splenic naïve Foxp3− CD4+ T cells are deficient in Tr1 differentiation. Although Itk−/−CD4+ T cells proliferated under Tr1 differentiating conditions, they failed to up-regulate LAG3, CD49b, ICOS, PD-1, c-Maf, AHR, and IRF4 to levels observed in WT cells, suggesting that ITK is critical for Tr1 cell fate programming. Utilizing transgenic mouse model carrying an allele sensitive mutant of ITK that allows ITK kinase specific blockade by a small molecule 3MB-PP1, we also determined that the expression of the aforementioned markers is dependent on ITK kinas activity. Inhibiting ITK kinase activity also diminished Tr1 differentiation inhuman CD4+ T cells. We conclude that ITK is required for Tr1 cell development and targeting ITK signaling may be a strategy to modulate regulatory immunity for clinical benefit.

PGE2 and IL-27: novel pro-inflammatory mechanisms involving dendritic cells and Tr1 cells

Abstract Interleukin-27 (p28/EBI3) is an immunomodulatory cytokine expressed by activated antigen presenting cells. Although first discovered to be involved in Th1 cell differentiation, further studies demonstrated the immunosuppressive functions of IL-27 including the inhibition of Th17 cell development and the induction of type 1 regulatory T cells (Tr1) and exhausted CD4+ T cells. The anti-inflammatory effects of IL-27 have been demonstrated in vivo in murine models of parasitic infections and autoimmune disease models such as experimental autoimmune encephalomyelitis and inflammatory bowel disease (IBD). We previously detailed the pro-inflammatory effects of PGE2 in collagen-induced arthritis and IBD through stimulation of IL-23 production leading to development of Th17 cells. Here we describe novel pro-inflammatory functions of PGE2, i.e. inhibition of IL-27 production by murine bone marrow-derived dendritic cells (DC), inhibition of Tr1 differentiation and of IL-10 production by Tr1 cells. We determined that the effect of PGE2 on IL-27 production in LPS-stimulated DC is mediated through EP2/EP4 receptors, induction of cAMP, and partial inhibition of both IRF1 and IRF3. Furthermore, we found that PGE2 treatment of LPS-stimulated DC reduces IRF1 binding to an essential ISRE site within the p28 promoter. In addition to the effects of IL-27 in DC, we also found that PGE2 directly inhibits IL-27-induced differentiation of Tr1 cells through EP4, and inhibits production of IL-10 by Tr1 cells through EP2/EP4. These studies further describe the pro-inflammatory effects of PGE2 and identify possible new targets for treatment of inflammatory diseases.

CD55 Costimulation Induces Differentiation of a Discrete T Regulatory Type 1 Cell Population with a Stable Phenotype

Unlike other helper T cells, the costimulatory ligands responsible for T regulatory type 1 (Tr1) cell differentiation remain undefined. Understanding the molecular interactions driving peripheral Tr1 differentiation is important because Tr1s potently regulate immune responses by IL-10 production. In this study, we show that costimulation of human naive CD4(+) cells through CD97/CD55 interaction drives Tr1 activation, expansion, and function. T cell activation and expansion was equipotent with CD55 or CD28 costimulation; however, CD55 costimulation resulted in two IL-10-secreting populations. Most IL-10 was secreted by the minor Tr1 population (IL-10(high)IFN-γ(-)IL-4(-), <5% cells) that expresses Tr1 markers CD49b, LAG-3, and CD226. This Tr1 phenotype was not restimulated by CD28. However, on CD55 restimulation, Tr1s proliferated and maintained their differentiated IL-10(high) phenotype. The Tr1s significantly suppressed effector T cell function in an IL-10-dependent manner. The remaining (>95%) cells adopted a Th1-like IFN-γ(+) phenotype. However, in contrast to CD28-derived Th1s, CD55-derived Th1s demonstrated increased plasticity with the ability to coexpress IL-10 when restimulated through CD55 or CD28. These data identify CD55 as a novel costimulator of human Tr1s and support a role for alternative costimulatory pathways in determining the fate of the growing number of T helper populations. This study demonstrates that CD55 acts as a potent costimulator and activator of human naive CD4(+) cells, resulting in the differentiation of a discrete Tr1 population that inhibits T cell function in an IL-10-dependent manner and maintains the Tr1 phenotype upon restimulation.

IRF-1 is a key transcriptional regulator of Tr1 differentiation (P1135)

Abstract IL-10 producing Type 1 regulatory T cells (Tr1) are crucial for maintenance of peripheral tolerance and prevention of autoimmune inflammation. Using IL-27 as the differentiation factor, we undertook a detailed whole genome transcriptional network analysis of differentiating Tr1 cells. We identified IRF-1 as a key transcriptional regulator of Tr1 differentiation. Our studies demonstrate a critical requirement for IRF1 in the generation of Tr1 cells both in vitro and in vivo. IRF1-/- mice immunized for the development of Experimental autoimmune encephalomyelitis (EAE) develop very severe clinical disease and fail to recover. Furthermore, CD4+ T cells in IRF1-deficient mice with active disease showed increased number of IL-17 secreting cells in the CNS. In fact, over-expression of IRF1 during Th17 differentiation suppresses induction of both IL17 and IL23 receptor, the role opposite to the one played by another IRF family member - IRF4. Interestingly, loss of IRF-1 enhances IRF4 binding to the key DNA-binding sites involved in Th17 differentiation and functional assays further support that IRF1 can actively repress IRF4-mediated transactivation of IL17A, pointing to a cross-competition of IRF1 and IRF4 for the same DNA-binding sites in CD4+ T cells. Collectively these data demonstrate a critical role of IRF-1 in the generation of Tr1 cells, but at the same time, the induction of IRF-1 represses Th17 differentiation by actively inhibiting IRF-4 mediated Th17 differentiation.

Metallothioneins negatively regulate IL-27–induced type 1 regulatory T-cell differentiation

IL-27-induced type 1 regulatory T (Tr1) cells suppress autoimmunity by producing IL-10. Signal transducer and activator of transcription (STAT) 1 and STAT3 have been described as key transcription factors that promote IL-10 secretion from Tr1 cells induced by IL-27. However, the molecular pathways for negatively regulating Tr1 cell differentiation remain elusive. Here, we show that IL-27 induces metallothioneins (MTs) that in turn prevent Tr1 cell development. MT expression leads to the reduction of STAT1 and STAT3 phosphorylation under Tr1 differentiation condition, resulting in impaired IL-10 production. Accordingly, Tr1 cells derived from MT-deficient mice showed an increased ability to produce IL-10 and potently suppress experimental autoimmune encephalomyelitis upon adoptive transfer. Moreover, activation of STAT1 and/or STAT3 can overcome the suppression of IL-10 by MTs, indicating a dynamic balance between STATs and MTs in regulating IL-10 during Tr1 cell differentiation.

Vitamin D promotes Tr1 differentiation through the CD46 pathway (178.18)

Abstract The complement regulator CD46 induces a regulatory Tr1 phenotype, characterized by large amounts of IL-10 secretion. Secretion of IL-10 upon CD46 activation is largely impaired in T cells from patients with multiple sclerosis (MS). Vitamin D has an immunomodulatory role and may be beneficial in several pathologies, including MS. Herein, we show that, upon CD46 costimulation, Vitamin D strongly increased the IL-10:IFNγ ratio, characteristic of the CD46-induced Tr1 phenotype. This was correlated with the modulation of several markers known to control T cell activation. Vitamin D strongly promoted the expression level of CD28, CD25 and CTLA-4 on CD46-activated T cells, while it concomitantly decreased CD46 expression. Importantly, addition of Vitamin D could restore the defective IL-10 production observed in T cells from patients with MS. Hence, Vitamin D promotes anti-inflammatory responses by mainly acting on CD46, and might be beneficial for T cell responses in MS.

The Dynamic Processing of CD46 Intracellular Domains Provides a Molecular Rheostat for T Cell Activation

BackgroundAdequate termination of an immune response is as important as the induction of an appropriate response. CD46, a regulator of complement activity, promotes T cell activation and differentiation towards a regulatory Tr1 phenotype. This Tr1 differentiation pathway is defective in patients with MS, asthma and rheumatoid arthritis, underlying its importance in controlling T cell function and the need to understand its regulatory mechanisms. CD46 has two cytoplasmic tails, Cyt1 and Cyt2, derived from alternative splicing, which are co-expressed in all nucleated human cells. The regulation of their expression and precise functions in regulating human T cell activation has not been fully elucidated.Methodology/Principal FindingsHere, we first report the novel role of CD46 in terminating T cell activation. Second, we demonstrate that its functions as an activator and inhibitor of T cell responses are mediated through the temporal processing of its cytoplasmic tails. Cyt1 processing is required to turn T cell activation on, while processing of Cyt2 switches T cell activation off, as demonstrated by proliferation, CD25 expression and cytokine secretion. Both tails require processing by Presenilin/γSecretase (P/γS) to exert these functions. This was confirmed by expressing wild-type Cyt1 and Cyt2 tails and uncleavable mutant tails in primary T cells. The role of CD46 tails was also demonstrated with T cells expressing CD19 ectodomain-CD46 C-Terminal Fragment (CTF) fusions, which allowed specific triggering of each tail individually.Conclusions/SignificanceWe conclude that CD46 acts as a molecular rheostat to control human T cell activation through the regulation of processing of its cytoplasmic tails.

Paradoxical arrest in lupus activity in BXSB mice with highly autoreactive T cells

T cells with high avidity for antigens are thought to mediate more effective immunity against foreign antigens and cause more severe autoimmunity. The impact of T cell receptor (TCR) avidity on the development of lupus has not been investigated. We took advantage of a transgenic mouse strain (designated MTB) that has a diverse T cell population and a globally stronger reactivity to self. [MTBxBXSB]F1 mice displayed accelerated lupus relative to the [WTxBXSB]F1 controls. The severity of lupus and the activation of T cells subsided with aging, when elevated IL-10 production by Tr1 cells was observed. Thus, chronic high avidity interactions of T cells with self-antigens can lead to an age associated increase in IL-10 production. This could explain the age-associated reduction of the incidence of lupus, as well as other autoimmune diseases. Furthermore, the principle of Tr1 differentiation based on diverse T cells with high avidity for self may potentially be used as a therapeutic strategy in the treatment of lupus.

High level expression of B7H1 molecules by keratinocytes suppresses xeno- and allo-reactions by inducing type I regulatory T cells

We previously reported that keratinocytes (KCs) could express B7H1 molecules and suppress allogeneic antigen-mediated immune responses through induction of IL-10-expressing T cells. The aims of this study were to investigate whether KCs could also suppress xene-antigen-mediated reactions as well as the role of B7H1 in KC-induced immunosuppression. Human T cells and porcine iliac artery endothelial cells (PIECs) were co-cultured to set up the mixed lymphocyte and endothelial reaction (MLER), to which KCs were added in order to examine their inhibitory effects on lymphoproliferation. We found that KCs can suppress xenogeneic MLER in vitro. Mechanism analysis suggested that KCs can induce human type I regulatory T (Tr1) cells, which in turn potently suppress xeno- and allo-reactions partly through IL-10. The Tr1 induction was closely associated with a high level of B7H1 expression in KCs. Our results shed light on the mechanisms of antigen presenting cell dependent Tr1 differentiation, and have implications for the strategy of developing Tr1 cells to prevent clinical diseases.

Inhibition of glycogen synthase kinase-3 promotes human CD4+ T regulatory type 1 activity (88.48)

Abstract T regulatory type 1 cells (Tr1) are a distinct subset of a heterogeneous population of regulatory CD4+ T cells present in humans whose main function is to prevent chronic immune activation and maintain normal immune homeostasis via the production of interleukin-10 (IL-10). Although several studies have identified particular in vitro conditions that promote Tr1 differentiation, none to date have identified a putative molecule when pharmacologically targeted promotes Tr1 differentiation. Identifying such a target has proven difficult due to the inherent complexity and lack of information regarding IL-10 gene regulation in T cells. In the present study, we examined the influence of the serine/threonine kinase, glycogen synthase kinase-3 (GSK3) to regulate IL-10 production by CD4+ T cells. CD3 stimulated peripheral blood mononuclear cells (PBMCs) or CD4+ T cells treated with GSK3 inhibitors promotes IL-2 and IL-10 production while suppressing interferon-gamma (IFN-?) levels as compared to cells treated with anti-CD3 alone. The ability of GSK3 inhibition to promote IL-10 and suppress IFN-? production in CD4+ T cells was restricted to the memory (CD45RA− CD45RO+) population of CD4+ T cells, whereas naïve (CD45RA+ CD45RO-) CD4+ T cells solely increased IL-2 production. Memory CD4+ T cells activated in the presence of GSK3 inhibition acquired the capacity to suppress the bystander activation of CD4+ T cells; an effect dependent on the presence of IL-10. Our findings illustrate a dichotomy present in the role of GSK3 in naïve and memory CD4+ T cells and identify GSK3 as a key kinase in Tr1 differentiation.

B7H1-Ig Fusion Protein Activates the CD4+ IFN-γ Receptor+ Type 1 T Regulatory Subset through IFN-γ-Secreting Th1 Cells

It has been demonstrated in our previous work that, in the human skin-grafting model, the expression of costimulatory molecule B7H1 (PD-L1) by keratinocytes plays an essential role in inducing local tolerance via activation of IL-10-secreting T cells. This study further analyzes the role of B7H1 in differentiation of type 1 T regulatory (Tr1) cells and explores underlying mechanisms. Mouse fusion protein B7H1-Ig is used, together with immobilized anti-CD3 mAb, to costimulate the purified naive CD4+ T cells. B7H1-Ig-treated CD4+ T cells were found to activate a characteristic Tr1 population possessing a CD4+ CD25- Foxp3- CD45RBlow phenotype. These regulatory T cells strongly inhibited the Th1-dominated MLR by secretion of IL-10 and TGF-beta. Moreover, B7H1-treated Tr1 cells also resulted in suppressed clinical scores and demyelination when adoptively transferred into mice with experimental allergic encephalomyelitis. Furthermore, analysis of the cytokine profile indicated that there were two differential reaction patterns during the B7H1-Ig-induced Tr1 development. These two patterns were characterized by activation of IFN-gammaR+ IL-10R- Th1 and IFN-gammaR+ IL-10R+ Tr1 cells, respectively. Secretion of IFN-gamma by Th1 and the expression of IFN-gammaR on Tr1 were critical for further Tr1 differentiation, as demonstrated by mAb blocking and by analysis in IFN-gamma(-/-) mice. In conclusion, B7H1 is capable of inducing Tr1 differentiation from naive CD4+ T cells by coactivation in an IFN-gamma- or Th1-dependent manner. Our study may shed some light upon the clinical usage of B7H1 as a therapeutic reagent for induction of tolerance.

Activation of human CD4+ cells with CD3 and CD46 induces a T-regulatory cell 1 phenotype

The immune system must distinguish not only between self and non-self, but also between innocuous and pathological foreign antigens to prevent unnecessary or self-destructive immune responses. Unresponsiveness to harmless antigens is established through central and peripheral processes. Whereas clonal deletion and anergy are mechanisms of peripheral tolerance, active suppression by T-regulatory 1 (Tr1) cells has emerged as an essential factor in the control of autoreactive cells. Tr1 cells are CD4+ T lymphocytes that are defined by their production of interleukin 10 (IL-10) and suppression of T-helper cells; however, the physiological conditions underlying Tr1 differentiation are unknown. Here we show that co-engagement of CD3 and the complement regulator CD46 in the presence of IL-2 induces a Tr1-specific cytokine phenotype in human CD4+ T cells. These CD3/CD46-stimulated IL-10-producing CD4+ cells proliferate strongly, suppress activation of bystander T cells and acquire a memory phenotype. Our findings identify an endogenous receptor-mediated event that drives Tr1 differentiation and suggest that the complement system has a previously unappreciated role in T-cell-mediated immunity and tolerance.