IFN-alpha and IL-10 induce the differentiation of human type 1 T regulatory cells

CD4(+) T regulatory type 1 (Tr1) cells suppress Ag-specific immune responses in vitro and in vivo. Although IL-10 is critical for the differentiation of Tr1 cells, the effects of other cytokines on differentiation of naive T cells into Tr1 cells have not been investigated. Here we demonstrate that endogenous or exogenous IL-10 in combination with IFN-alpha, but not TGF-beta, induces naive CD4(+) T cells derived from cord blood to differentiate into Tr1 cells: IL-10(+)IFN-gamma(+)IL-2(-/low)IL-4(-). Naive CD4(+) T cells derived from peripheral blood require both exogenous IL-10 and IFN-alpha for Tr1 cell differentiation. The proliferative responses of the Tr1-containing lymphocyte populations, following activation with anti-CD3 and anti-CD28 mAbs, were reduced. Similarly, cultures containing Tr1 cells displayed reduced responses to alloantigens via a mechanism that was partially mediated by IL-10 and TGF-beta. More importantly, Tr1-containing populations strongly suppressed responses of naive T cells to alloantigens. Collectively, these results show that IFN-alpha strongly enhances IL-10-induced differentiation of functional Tr1 cells, which represents a first major step in establishing specific culture conditions to generate T regulatory cells for biological and biochemical analysis, and for cellular therapy to induce peripheral tolerance in humans.     

Gliadin-specific type 1 regulatory T cells from the intestinal mucosa of treated celiac patients inhibit pathogenic T cells

Celiac disease (CD) results from a permanent intolerance to dietary gluten and is due to a massive T cell-mediated immune response to gliadin, the main component of gluten. In this disease, the regulation of immune responses to dietary gliadin is altered. Herein, we investigated whether IL-10 could modulate anti-gliadin immune responses and whether gliadin-specific type 1 regulatory T (Tr1) cells could be isolated from the intestinal mucosa of CD patients in remission. Short-term T cell lines were generated from jejunal biopsies, either freshly processed or cultured ex vivo with gliadin in the presence or absence of IL-10. Ex vivo stimulation of CD biopsies with gliadin in the presence of IL-10 resulted in suppression of Ag-specific proliferation and cytokine production, indicating that pathogenic T cells are susceptible to IL-10-mediated immune regulation. T cell clones generated from intestinal T cell lines were tested for gliadin specificity by cytokine production and proliferative responses. The majority of gliadin-specific T cell clones had a Th0 cytokine production profile with secretion of IL-2, IL-4, IFN-gamma, and IL-10 and proliferated in response to gliadin. Tr1 cell clones were also isolated. These Tr1 cells were anergic, restricted by DQ2 (a CD-associated HLA), and produced IL-10 and IFN-gamma, but little or no IL-2 or IL-4 upon activation with gliadin or polyclonal stimuli. Importantly, gliadin-specific Tr1 cell clones suppressed proliferation of pathogenic Th0 cells. In conclusion, dietary Ag-specific Tr1 cells are present in the human intestinal mucosa, and strategies to boost their numbers and/or function may offer new therapeutic opportunities to restore gut homeostasis.     

Coadministration of plasmid DNA constructs encoding an encephalitogenic determinant and IL-10 elicits regulatory T cell-mediated protective immunity in the central nervous system

We have previously shown that Ag-specific IL-10-producing regulatory T cells (Tr1) participate in the regulation of experimental autoimmune encephalomyelitis and that their specificity undergoes determinant spread in a reciprocal manner to effector T cell specificity. The current study shows that coadministration of plasmid DNA vaccines encoding IL-10 together with a plasmid encoding a myelin basic protein (MBP) encephalitogenic determinant during an ongoing disease rapidly amplifies this Tr1-mediated response, in a disease-specific manner. Thus, coadministration of both plasmids, but not the plasmid DNA encoding MBP alone, rapidly suppresses an ongoing disease. Tolerance included elevation in Ag-specific T cells producing IL-10 and an increase in apoptosis of cells around high endothelial venules in the CNS after successful therapy. Tolerance could be transferred by MBP-specific primary T cells isolated from protected donors and reversed by neutralizing Abs to IL-10 but not to IL-4. Due to the nature of determinant spread in this model, we could bring about evidence implying that rapid and effective induction of Tr1-induced active tolerance is dependent on redirecting the Tr1 response to the epitope to which the effector function dominates the response at a given time. The consequences of these findings to multiple sclerosis, and possibly other inflammatory autoimmune diseases are discussed.     

Presence of diabetes-inhibiting, glutamic acid decarboxylase-specific, IL-10-dependent, regulatory T cells in naive nonobese diabetic mice

Immunization of NOD mice with autoantigens such as glutamic acid decarboxylase (GAD) 221-235 peptide (p221) can induce Ag-specific CD4(+) T regulatory (Tr) cells. However, it is unclear whether these Tr cells acquire their regulatory capacity due to immunization or whether they are constitutively harbored in unimmunized naive mice. To address this question, we used an I-Ag7 tetramer to isolate p221-specific T cells from naive NOD mice (N221(+) cells) after peptide-specific in vitro expansion. The N221(+) T cells produced IFN-gamma and IL-10, but very little IL-4, in response to p221 stimulation. These T cells could function as regulatory cells and inhibit in vitro proliferation of diabetogenic BDC2.5 cells. This suppressive activity was cell contact-independent and was abrogated by Abs to IL-10 or IL-10R. Interestingly, IL-2 produced by other T cells present in the cell culture induced unactivated N221(+) T cells to exhibit regulatory activities involving production of IL-10. In vivo, N221(+) cells inhibited diabetes development when cotransferred with NOD splenocytes into NOD/scid recipients. Together, these results demonstrate that p221-specific IL-10-dependent Tr cells, including Tr type 1 cells, are present in naive NOD mice. The use of spontaneously arising populations of GAD peptide-specific Tr cells may represent a promising immunotherapeutic approach for preventing type 1 diabetes.     

T regulatory cells 1 inhibit a Th2-specific response in vivo

We recently described a new population of CD4(+) regulatory T cells (Tr1) that inhibits proliferative responses of bystander T cells and prevents colitis induction in vivo through the secretion of IL-10. IL-10, which had been primarily described as a Th2-specific cytokine inhibiting Th1 responses, has displayed in several models a more general immune suppression on both types of effector T cell responses. Using an immediate hypersensitivity model in which BALB/c mice immunized with OVA (alum) normally generate Th2-dominated responses, we examined the ability of OVA-specific Tr1 T cell clones to inhibit OVA-specific cytokines and Ab responses. In contrast to Th2 or Th1 T cell clones, transfer of Tr1 T cell clones coincident with OVA immunization inhibited Ag-specific serum IgE responses, whereas IgG1 and IgG2a synthesis were not affected. This specific inhibition was mediated in part through IL-10 secretion as anti-IL-10 receptor Abs treatment reverted the inhibitory effect of Tr1 T cell clones. Although specifically targeted to IgE responses, Tr1 clones' inhibitory effects were more profound as they affected Ag-specific Th2 cell priming both in term of proliferative responses and cytokine secretion. These results suggest that regulatory T cells may play a fundamental role in maintaining the balance of the immune system to prevent allergic disorders.     

Can interleukin-10 be used as a true immunoregulatory cytokine?

Interleukin-10 (IL-10) is a cytokine that is currently regarded as a potential therapy for inflammatory diseases involving T helper 1-type responses because of its ability to downregulate several major functions of Th1 cells and macrophages. There are also evidence that IL-10 could be useful in controlling Th2-mediated inflammatory processes. However IL-10 has also immunostimulatory properties especially on B-cells and activated CD8+ T cells. These pleiotropic effects may explain the discrepancy observed after IL-10 treatment in different in vivo experimental models. We have recently shown that IL-10 induces the differentiation of a subset of regulatory CD4+ T cells (Tr1). In different in vivo models, these cells were shown to inhibit Th1 and Th2-type inflammatory responses through the secretion of IL-10. These Tr1 cells may thus be used in specific cellular therapy in order to deliver IL-10 precisely at the site of inflammation.     

Calcitriol Modulates the CD46 Pathway in T Cells

The complement regulator CD46 is a costimulatory molecule for human T cells that induces a regulatory Tr1 phenotype, characterized by large amounts of IL-10 secretion. Secretion of IL-10 upon CD46 costimulation is largely impaired in T cells from patients with multiple sclerosis (MS). Vitamin D can exert a direct effect on T cells, and may be beneficial in several pathologies, including MS. In this pilot study, we examined whether active vitamin D (1,25(OH)₂D₃ or calcitriol) could modulate the CD46 pathway and restore IL-10 production by CD46-costimulated CD4+ T cells from patients with MS. In healthy T cells, calcitriol profoundly affects the phenotype of CD46-costimulated CD4+ T cells, by increasing the expression of CD28, CD25, CTLA-4 and Foxp3 while it concomitantly decreased CD46 expression. Similar trends were observed in MS CD4+ T cells except for CD25 for which a striking opposite effect was observed: while CD25 was normally induced on MS T cells by CD46 costimulation, addition of calcitriol consistently inhibited its induction. Despite the aberrant effect on CD25 expression, calcitriol increased the IL-10:IFNγ ratio, characteristic of the CD46-induced Tr1 phenotype, in both T cells from healthy donors and patients with MS. Hence, we show that calcitriol affects the CD46 pathway, and that it promotes anti-inflammatory responses mediated by CD46. Moreover, it might be beneficial for T cell responses in MS.     

Blimp-1-Dependent IL-10 Production by Tr1 Cells Regulates TNF-Mediated Tissue Pathology

Tumor necrosis factor (TNF) is critical for controlling many intracellular infections, but can also contribute to inflammation. It can promote the destruction of important cell populations and trigger dramatic tissue remodeling following establishment of chronic disease. Therefore, a better understanding of TNF regulation is needed to allow pathogen control without causing or exacerbating disease. IL-10 is an important regulatory cytokine with broad activities, including the suppression of inflammation. IL-10 is produced by different immune cells; however, its regulation and function appears to be cell-specific and context-dependent. Recently, IL-10 produced by Th1 (Tr1) cells was shown to protect host tissues from inflammation induced following infection. Here, we identify a novel pathway of TNF regulation by IL-10 from Tr1 cells during parasitic infection. We report elevated Blimp-1 mRNA levels in CD4⁺ T cells from visceral leishmaniasis (VL) patients, and demonstrate IL-12 was essential for Blimp-1 expression and Tr1 cell development in experimental VL. Critically, we show Blimp-1-dependent IL-10 production by Tr1 cells prevents tissue damage caused by IFNγ-dependent TNF production. Therefore, we identify Blimp-1-dependent IL-10 produced by Tr1 cells as a key regulator of TNF-mediated pathology and identify Tr1 cells as potential therapeutic tools to control inflammation.     

Antigen specificity acquisition of adoptive CD4+ regulatory T cells via acquired peptide-MHC class I complexes

The Ag-specific CD4(+) regulatory T (Tr) cells play an important role in immune suppression in autoimmune diseases and antitumor immunity. However, the molecular mechanism for Ag-specificity acquisition of adoptive CD4(+) Tr cells is unclear. In this study, we generated IL-10- and IFN-gamma-expressing type 1 CD4(+) Tr (Tr1) cells by stimulation of transgenic OT II mouse-derived naive CD4(+) T cells with IL-10-expressing adenovirus (AdV(IL-10))-transfected and OVA-pulsed dendritic cells (DC(OVA/IL-10)). We demonstrated that both in vitro and in vivo DC(OVA/IL-10)-stimulated CD4(+) Tr1 cells acquired OVA peptide MHC class (pMHC) I which targets CD4(+) Tr1 cells suppressive effect via an IL-10-mediated mechanism onto CD8(+) T cells, leading to an enhanced suppression of DC(OVA)-induced CD8(+) T cell responses and antitumor immunity against OVA-expressing murine B16 melanoma cells by approximately 700% relative to analogous CD4(+) Tr1 cells without acquired pMHC I. Interestingly, the nonspecific CD4(+)25(+) Tr cells can also become OVA Ag specific and more immunosuppressive in inhibition of OVA-specific CD8(+) T cell responses and antitumor immunity after uptake of DC(OVA)-released exosomal pMHC I complexes. Taken together, the Ag-specificity acquisition of CD4(+) Tr cells via acquiring DC's pMHC I may be an important mean in augmenting CD4(+) Tr cell suppression.     

Cholera toxin promotes the induction of regulatory T cells specific for bystander antigens by modulating dendritic cell activation

It has previously been reported that cholera toxin (CT) is a potent mucosal adjuvant that enhances Th2 or mixed Th1/Th2 type responses to coadministered foreign Ag. Here we demonstrate that CT also promotes the generation of regulatory T (Tr) cells against bystander Ag. Parenteral immunization of mice with Ag in the presence of CT induced T cells that secreted high levels of IL-4 and IL-10 and lower levels of IL-5 and IFN-gamma. Ag-specific CD4(+) T cell lines and clones generated from these mice had cytokine profiles characteristic of Th2 or type 1 Tr cells, and these T cells suppressed IFN-gamma production by Th1 cells. Furthermore, adoptive transfer of bone marrow-derived dendritic cells (DC) incubated with Ag and CT induced T cells that secreted IL-4 and IL-10 and low concentrations of IL-5. It has previously been shown that IL-10 promotes the differentiation or expansion of type 1 Tr cells. Here we found that CT synergized with low doses of LPS to induce IL-10 production by immature DC. CT also enhanced the expression of CD80, CD86, and OX40 (CD134) on DC and induced the secretion of the chemokine, macrophage inflammatory protein-2 (MIP-2), but inhibited LPS-driven induction of CD40 and ICAM-I expression and production of the inflammatory cytokines/chemokines IL-12, TNF-alpha, MIP-1alpha, MIP-1beta, and monocyte chemoattractant protein-1. Our findings suggest that CT induces maturation of DC, but, by inducing IL-10, inhibiting IL-12, and selectively affecting surface marker expression, suppresses the generation of Th1 cells and promotes the induction of T cells with regulatory activity.     

Antigen-specific T regulatory-1 cells are associated with immunosuppression in a chronic helminth infection (onchocerciasis)

Different mechanisms underlie the phenomenon of peripheral tolerance. Recently, a new subset of CD4+ T cells, called T regulatory-1 (Tr1) cells, was described which show suppressor functions in vitro and in vivo and are characterized by a predominant production of IL-10 and/or TGF-beta. Tr1 cells have so far been generated experimentally in an IL-10-rich environment and hold promise for exploitation in the suppression of alloreactions and inflammatory or allergic dispositions. However, these cells have not been characterized in infectious diseases. Here we show that in the chronic helminth infection onchocerciasis (river blindness), where patients have relatively little sign of dermatitis despite the presence of millions of small worms in the skin, T cells can be obtained which bear characteristics of Tr1 cells, producing no IL-2 or IL-4 but substantial amounts of IL-10, variable amounts of IL-5, and some IFN-gamma. These cells display elevated amounts of CTLA-4 after stimulation and are able to inhibit other T cells in coculture, in contrast to Th1 and Th2 clones. This is the first time that this type of suppressor T cell has been cloned as naturally occurring during an infectious disease.     

B7H1-Ig fusion protein activates the CD4+ IFN-gamma receptor+ type 1 T regulatory subset through IFN-gamma-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.     

NK T cell-derived IL-10 is essential for the differentiation of antigen-specific T regulatory cells in systemic tolerance

In a model of systemic tolerance called Anterior Chamber-Associated Immune Deviation (ACAID), the differentiation of the T regulatory (Tr) cells depends on NK T cells and occurs in the spleen. We now show that the CD1d-reactive NK T cell subpopulation, required for development of systemic tolerance, expresses the invariant V alpha 14J alpha 281 TCR because J alpha 281 knockout (KO) mice were unable to generate Ag-specific Tr cells and ACAID. The mechanism for NK T cell-dependent differentiation of Ag-specific Tr cells mediating systemic tolerance was studied by defining the cytokine profiles in heterogeneous and enriched NK T spleen cells. In contrast to there being no differences in most regulatory cytokine mRNAs, both mRNA and protein for IL-10 were increased in splenic NK T cells of anterior chamber (a.c.)-inoculated mice. However, IL-10 mRNA was not increased in spleens after i.v. inoculation. Finally, NK T cells from wild-type (WT) mice, but not from IL-10 KO mice, reconstituted the ACAID inducing ability in J alpha 281 KO mice. Thus, NK T cell-derived IL-10 is critical for the generation of the Ag-specific Tr cells and systemic tolerance induced to eye-inoculated Ags.     

Regulatory T cells induced by B cells: a novel subpopulation of regulatory T cells

Regulatory T cells play a crucial role in the homeostasis of the immune response. In addition to CD4⁺Foxp3⁺ regulatory T cells, several subsets of Foxp3^- regulatory T cells, such as T helper 3 (Th3) cells and type 1 regulatory T (Tr1) cells, have been described in mice and human. Accumulating evidence shows that naïve B cells contribute to tolerance and are able to promote regulatory T cell differentiation. Naïve B cells can convert CD4⁺CD25^- T cells into CD25⁺Foxp3^- regulatory T cells, named Treg-of-B cells by our group. Treg-of-B cells express LAG3, ICOS, GITR, OX40, PD1, and CTLA4 and secrete IL-10. Intriguingly, B-T cell-cell contact but not IL-10 is essential for Treg-of-B cells induction. Moreover, Treg-of-B cells possess both IL-10-dependent and IL-10-independent inhibitory functions. Treg-of-B cells exert suppressive activities in antigen-specific and non-antigen-specific manners in vitro and in vivo. Here, we review the phenotype and function of Foxp3⁺ regulatory T cells, Th3 cells, Tr1 cells, and Treg-of-B cells.     

Bacterial-reactive T regulatory cells inhibit pathogenic immune responses to the enteric flora

We showed previously that cecal bacterial Ag (CBA)-specific CD4(+) T cells induce colitis when transferred into SCID mice. The purpose of this study was to generate and characterize CBA-specific regulatory T cells in C3H/HeJBir (Bir) mice. CD4(+) T cells were stimulated with CBA-pulsed APC in the presence of IL-10 every 10-14 days. After four or more cycles, these T cells produced high levels of IL-10, low levels of IL-4 and IFN-gamma, and no IL-2, consistent with the phenotype of T regulatory-1 (Tr1) cells. Bir Tr1 cells proliferated poorly, but their proliferation was dependent on CD28-B7 interactions and was MHC class II-restricted. Transfer of Bir Tr1 cells into SCID mice did not result in colitis, and cotransfer of Bir Tr1 T cells with pathogenic Bir CD4(+) Th1 cells prevented colitis. Bir Tr1 cells inhibited proliferation and IFN-gamma production of a CBA-specific Th1 cell line in vitro. Such inhibition was partly due to IL-10 and TGFbeta1, but cognate interactions with either APCs or Th1 cells were also involved. Normal intestinal lamina propria CD4(+) T cells had Tr1-like activity when stimulated with CBA-pulsed APCs. We conclude that CD4(+) T cells with the properties of Tr1 cells are present in the intestinal lamina propria and hypothesize that these cells maintain intestinal immune homeostasis to the enteric flora.     

Induction of a CD4+ T regulatory type 1 response by cyclooxygenase-2-overexpressing glioma

PGE(2), synthesized by cyclooxygenase-2 (COX-2)-overexpressing tumor, is known to contribute to cellular immune suppression in cancer patients, but the mechanism remains unclear. We report the mechanism of a CD4(+) T regulatory type 1 (Tr1) induction by CD11c(+) mature dendritic cells (DCs) that phagocytose allogeneic and autologous COX-2-overexpressing glioma. A human glioma cell line, U-87MG, and primary cultured glioblastoma cells (MG-377) overexpressed COX-2. We did not detect IL-10Ralpha expression in these gliomas, and rIL-10 did not suppress their COX-2 expression. Exposure to COX-2-overexpressing glioma induced mature DCs to overexpress IL-10 and decreased IL-12p70 production. These DCs induced a Tr1 response, which is characterized by robust secretion of IL-10 and TGF-beta with negligible IL-4 secretion by CD4(+) T cells, and an inhibitory effect on admixed lymphocytes. Peripheral CD4(+) T cell populations isolated from an MG-377 patient also predominantly demonstrated a Tr1 response against MG-377 cells. Selective COX-2 inhibition in COX-2-overexpressing gliomas at the time of phagocytic uptake by DCs abrogated this regulatory response and instead elicited Th1 activity. COX-2 stable transfectants in LN-18 (LN-18-COX2) also induced a Tr1 response. The effect of a COX-2 inhibition in LN-18-COX2 is reversible after administration of PGE(2). Taken together, robust levels of PGE(2) from COX-2-overexpressing glioma, which is unresponsive to IL-10 within the local microenvironment, may cause DCs to secrete high levels of IL-10. These results indicate that COX-2-overexpressing tumors induce a Tr1 response, which is mediated by tumor-exposed, IL-10-enhanced DCs.     

1型调节性T细胞的表型和功能

1型调节性T细胞(Tr1)在免疫耐受的诱导和维持过程中发挥重要作用。Tr1细胞通常在免疫耐受的环境中由外来抗原诱导产生,通过产生高水平的IL-10而发挥免疫抑制作用。而CD4 CD25 调节性T细胞(Tregs)可在胸腺中天然产生也可在外周被抗原诱导产生,通过细胞接触发挥抑制作用。现对Tr1细胞的表型、功能及其抑制作用机制等方面的研究进展进行综述。     

Role of dendritic cells in the induction of regulatory T cells

Dendritic cells (DCs) play a key role in initiating immune responses and maintaining immune tolerance. In addition to playing a role in thymic selection, DCs play an active role in tolerance under steady state conditions through several mechanisms which are dependent on IL-10, TGF-β, retinoic acid, indoleamine-2,3,-dioxygenase along with vitamin D. Several of these mechanisms are employed by DCs in induction of regulatory T cells which are comprised of Tr1 regulatory T cells, natural and inducible foxp3+ regulatory T cells, Th3 regulatory T cells and double negative regulatory T cells. It appears that certain DC subsets are highly specialized in inducing regulatory T cell differentiation and in some tissues the local microenvironment plays a role in driving DCs towards a tolerogenic response. In this review we discuss the recent advances in our understanding of the mechanisms underlying DC driven regulatory T cell induction.     

Cutting edge: chemical dominance does not relate to immunodominance: studies of the CD4+ T cell response to a model antigen

IL-10 has potent immunosuppressive properties, and IL-10-producing CD4+ Tr1 cells have been characterized as regulators of Th1-mediated immunity. In this study, using a s.c. model of glioma cell growth in mice, we demonstrate that CD4+, but not CD8+, T cells play a critical role in tumor rejection following vaccination with irradiated glioma cells. Surprisingly, glioma-specific CD4+ T cells produce IL-10 but neither IL-4 nor IFN-gamma, and glioma rejection is compromised in IL-10(-/-) hosts. Hence, our findings demonstrate that IL-10-producing CD4+ T cells can manifest antitumor functions and suggest that IL-10 may have proinflammatory effects in disease states.     

Induction of a regulatory phenotype in human CD4+ T cells by streptococcal M protein

Regulatory T cells (Tregs) participate in the control of the immune response. In the human system, an IL-10-secreting, T regulatory type 1 cell (Tr1)-like subset of Tregs can be induced by concurrent cross-linking of the TCR and CD46 on naive CD4(+) T cells. Because many viral and bacterial pathogens, including the major human pathogen Streptococcus pyogenes, bind to CD46, we asked whether this bacterium can directly induce Tr1-like cells through the streptococcal ligand for CD46, the M protein. The M5 and M22 proteins were found to induce T cells to develop into the IL-10-producing Tr1-like phenotype. Moreover, whole M5-expressing bacteria, but not isogenic M-negative bacteria, led to proliferation and IL-10 secretion by T cells. The interaction between the M5 protein and T cells was dependent on CD46 and the conserved C repeat region of M5. Supernatants derived from T cells stimulated with M proteins or M protein-expressing bacteria suppressed bystander T cell proliferation through IL-10 secretion. In addition, activation of CD46 through streptococcal M protein induced the expression of granzyme B, providing a second means for these cells to regulate an immune response. These findings suggest that binding to CD46 and exploiting its signaling pathway may represent a strategy employed by a number of important human pathogens to induce directly an immunosuppressive/regulatory phenotype in T cells.