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ICOS is induced in activated T cells and its main role is to boost differentiation and function of effector T cells. ICOS is also constitutively expressed in a subpopulation of Foxp3+ regulatory T cells under steady-state condition. Studies using ICOS germline knockout mice or ICOS-blocking reagents suggested that ICOS has supportive roles in regulatory T (Treg) cell homeostasis, migration, and function. To avoid any compounding effects that may arise from ICOS-deficient non–Treg cells, we generated a conditional knockout system in which ICOS expression is selectively abrogated in Foxp3-expressing cells (ICOS FC mice). Compared to Foxp3-Cre control mice, ICOS FC mice showed a minor numerical deficit of steady-state Treg cells but did not show any signs of spontaneous autoimmunity, indicating that tissue-protective Treg populations do not heavily rely on ICOS costimulation. However, ICOS FC mice showed more severe inflammation in oxazolone-induced contact hypersensitivity, a model of atopic dermatitis. This correlated with elevated numbers of inflammatory T cells expressing IFN-γ and/or TNF-α in ICOS FC mice compared with the control group. In contrast, elimination of ICOS in all T cell compartments negated the differences, confirming that ICOS has a dual positive role in effector and Treg cells. Single-cell transcriptome analysis suggested that ICOS-deficient Treg cells fail to mature into T-bet+CXCR3+ “Th1-Treg” cells in the draining lymph node. Our results suggest that regimens that preferentially stimulate ICOS pathways in Treg cells might be beneficial for the treatment of Th1-driven inflammation.

Foxp3+ regulatory T (TREG) cells are central mediators in the control of peripheral immune responses. Genome-wide transcriptional profiles show canonical signatures for Foxp3+ TREG cells, distinguishing them from Foxp3− effector T (TEFF) cells. We previously uncovered distinct mRNA translational signatures differentiating CD4+ TEFF and TREG cells through parallel measurements of cytosolic (global) and polysome-associated (translationally enhanced) mRNA levels in both subsets. We show that the mRNA encoding for the ubiquitin-specific peptidase 11 (USP11), a known modulator of TGF-β signaling, was preferentially translated in TCR-activated TREG cells compared with conventional, murine CD4+ T cells. TGF-β is a key cytokine driving the induction and maintenance of Foxp3 expression in T cells. We hypothesized that differential translation of USP11 mRNA endows TREG cells with an advantage to respond to TGF-β signals. In an in vivo mouse model promoting TREG cells plasticity, we found that USP11 protein was expressed at elevated levels in stable TREG cells, whereas ectopic USP11 expression enhanced the suppressive capacity and lineage commitment of these cells in vitro and in vivo. USP11 overexpression in TEFF cells enhanced the activation of the TGF-β pathway and promoted TREG or TH17, but not Th1, cell differentiation in vitro and in vivo, an effect abrogated by USP11 gene silencing or the inhibition of enzymatic activity. Thus, USP11 potentiates TGF-β signaling in both TREG and TEFF cells, in turn driving increased suppressive function and lineage commitment in thymic-derived TREG cells and potentiating the TGF-β–dependent differentiation of TEFF cells to peripherally induced TREG and TH17 cells.

IVIg is widely used as an immunomodulatory therapy. We have recently demonstrated that IVIg protects against airway hyperresponsiveness (AHR) and inflammation in mouse models of allergic airways disease (AAD), associated with induction of Foxp3+ regulatory T cells (Treg). Using mice carrying a DTR/EGFP transgene under the control of the Foxp3 promoter (DEREG mice), we demonstrate in this study that IVIg generates a de novo population of peripheral Treg (pTreg) in the absence of endogenous Treg. IVIg-generated pTreg were sufficient for inhibition of OVA-induced AHR in an Ag-driven murine model of AAD. In the absence of endogenous Treg, IVIg failed to confer protection against AHR and airway inflammation. Adoptive transfer of purified IVIg-generated pTreg prior to Ag challenge effectively prevented airway inflammation and AHR in an Ag-specific manner. Microarray gene expression profiling of IVIg-generated pTreg revealed upregulation of genes associated with cell cycle, chromatin, cytoskeleton/motility, immunity, and apoptosis. These data demonstrate the importance of Treg in regulating AAD and show that IVIg-generated pTreg are necessary and sufficient for inhibition of allergen-induced AAD. The ability of IVIg to generate pure populations of highly Ag-specific pTreg represents a new avenue to study pTreg, the cross-talk between humoral and cellular immunity, and regulation of the inflammatory response to Ags.

A progressive waning in Foxp3+ regulatory T cell (Treg) functions is thought to provoke autoimmunity in the NOD model of type 1 diabetes (T1D). A deficiency in IL-2 is one of the main triggers for the defective function of Tregs in islets. Notably, abrogation of the ICOS pathway in NOD neonates or BDC2.5-NOD (BDC2.5) mice exacerbates T1D, suggesting an important role for this costimulatory pathway in tolerance to islet Ags. Thus, we hypothesize that ICOS selectively promotes Foxp3+ Treg functions in BDC2.5 mice. We show that ICOS expression discriminates effector Foxp3− T cells from Foxp3+ Tregs and specifically designates a dominant subset of intra-islet Tregs, endowed with an increased potential to expand, secrete IL-10, and mediate suppressive activity in vitro and in vivo. Consistently, Ab-mediated blockade or genetic deficiency of ICOS selectively abrogates Treg-mediated functions and T1D protection and exacerbates disease in BDC2.5 mice. Moreover, T1D progression in BDC2.5 mice is associated with a decline in ICOS expression in and expansion and suppression by intra-islet Foxp3+ Tregs. We further show that the ICOS+ Tregs, in contrast to their ICOS− counterparts, are more sensitive to IL-2, a critical signal for their survival and functional stability. Lastly, the temporal loss in ICOS+ Tregs is readily corrected by IL-2 therapy or protective Il2 gene variation. Overall, ICOS is critical for the homeostasis and functional stability of Foxp3+ Tregs in prediabetic islets and maintenance of T1D protection.

To investigate the role of CD4+CD25+Foxp3+ regulatory T (Treg) cells in blood-stage malaria, we compared Plasmodium chabaudi AS infection in wild-type (WT) C57BL/6 and transgenic mice overexpressing the transcription factor Foxp3 (Foxp3Tg) and observed that Foxp3Tg mice experienced lethal infection and deficient malaria-specific immune responses. Adoptive transfer of total CD4+ T cells from Foxp3Tg mice or CD4+CD25+ T cells from WT mice to naive WT recipients confirmed that high numbers of Treg cells compromised immune control of malaria. Transfer of GFP+CD4+CD25+ T cells to naive WT recipients together with immunohistochemical staining of spleens from infected WT mice demonstrated that Foxp3+ Treg cells localized in the T cell area of the spleen. Determination of CD4+Foxp3+ Treg cell responses in the spleen of infected WT mice revealed a significant but transient increase in CD4+Foxp3+ Treg cells early in infection. This was followed by a significant and sustained decrease due to reduced proliferation and apoptosis of CD4+Foxp3+ Treg cells. Importantly, the kinetics of IL-2 secretion by effector CD4+Foxp3− T cells coincided with changes in CD4+Foxp3+ cells and the differentiation of CD4+T-bet+IFN-γ+ cells required for immune control of infection. Administration of the IL-2/anti–IL-2 mAb (clone JES6-1) complex to infected WT mice increased the severity of P. chabaudi AS infection and promoted expansion of Foxp3+ Treg cells. Collectively, these data demonstrate that the ability to control and eliminate P. chabaudi AS infection is due to a tight balance between natural Treg cells and effector CD4+ Th1 cells, a balance regulated in part by IL-2.

Cardiolipin (CL), a major phospholipid in bacterial cell walls, is sequestered from the immune system in mammalian mitochondria and is, therefore, a potential danger signal. Based on growing evidence that phospholipids constitute natural ligands for CD1 and that CD1d-restricted T cells recognize phospholipids, we hypothesized that CD1d binds and presents CL and that T cells in the normal immune repertoire respond to CL in a CD1d-restricted manner. We determined the murine CD1d-CL crystal structure at 2.3 Å resolution and established through additional lipid loading experiments that CL, a tetra-acylated phospholipid, binds to murine CD1d with two alkyl chains buried inside the CD1d binding groove and the remaining two exposed into the solvent. We furthermore demonstrate the functional stimulatory activity of CL, showing that splenic and hepatic γδ T cells from healthy mice proliferate in vitro in response to mammalian or bacterial CL in a dose-dependent and CD1d-restricted manner, rapidly secreting the cytokines IFN-γ and RANTES. Finally, we show that hepatic γδ T cells are activated in vivo by CD1d-bearing dendritic cells that have been pulsed with CL, but not phosphatidylcholine. Together, these findings demonstrate that CD1d is able to bind and present CL to a subset of CL-responsive γδ T cells that exist in the spleen and liver of healthy mice and suggest that these cells could play a role in host responses to bacterial lipids and, potentially, self-CL. We propose that CL-responsive γδ T cells play a role in immune surveillance during infection and tissue injury.

CD4+ regulatory T (TREG) cells are critical mediators of peripheral immune tolerance and homeostasis, and express the forkhead box p3 (Foxp3) transcription factor, the master-regulator driving the programming of the TREG cell suppressive phenotype. TREG cells are abundant at mucosal surfaces, where they acquire tissue-specific adaptations. The biological consequences of these adaptations on the stability of thymic-derived TREG (tTreg) cells remain largely unknown. To determine the signals that drive the fate of TREG cells, we isolated and compared stable from unstable TREG cells using a TREG transfer model where we previously observed the functional reprogramming of Foxp3+ TREG cells into Th1/Th17 effector T cells. We identified the expression of the IL-33 receptor (IL-33R, ST2) on stable tTREG cells and the IL-1 receptor (IL1R1) on unstable exTREG cells undergoing functional reprogramming. We show that both TREG cell populations represent competing subsets in inflammatory conditions. This is further underlined by the fact that the absence of IL1R1 expression (IL1R1−/−) leads to the accumulation of ST2+ TREG cells at mucosal sites in vivo. In two distinct lung infection models, we demonstrate that ST2-expressing TREG cells express GATA3 and resist production of inflammatory cytokines, whereas IL1R1-expressing TREG cells express RORγT and lose Foxp3 expression in vivo. While IL-1 signaling impairs TREG cell suppressive function, ST2 is required for the maintenance of the lineage identity and suppressive function of TREG cells. These observations demonstrate that IL-1 and IL-33 produced during immune challenge exert distinct roles on the functional adaptation of Foxp3+ tTREG cells at mucosal surfaces.

Type I diabetes (T1D) susceptibility is inherited through multiple insulin-dependent diabetes (Idd) genes. NOD.B6 Idd3 congenic mice, introgressed with an Idd3 allele from T1D-resistant C57BL/6 mice (Idd3B6), show a marked resistance to T1D compared with control NOD mice. The protective function of the Idd3 locus is confined to the Il2 gene, whose expression is critical for naturally occurring CD4+Foxp3+ regulatory T (nTreg) cell development and function. In this study, we asked whether Idd3B6 protective alleles in the NOD mouse model confer T1D resistance by promoting the cellular frequency, function, or homeostasis of nTreg cells in vivo. We show that resistance to T1D in NOD.B6 Idd3 congenic mice correlates with increased levels of IL-2 mRNA and protein production in Ag-activated diabetogenic CD4+ T cells. We also observe that protective IL2 allelic variants (Idd3B6 resistance allele) also favor the expansion and suppressive functions of CD4+Foxp3+ nTreg cells in vitro, as well as restrain the proliferation, IL-17 production, and pathogenicity of diabetogenic CD4+ T cells in vivo more efficiently than control do nTreg cells. Lastly, the resistance to T1D in Idd3 congenic mice does not correlate with an augmented systemic frequency of CD4+Foxp3+ nTreg cells but more so with the ability of protective IL2 allelic variants to promote the expansion of CD4+Foxp3+ nTreg cells directly in the target organ undergoing autoimmune attack. Thus, protective, IL2 allelic variants impinge the development of organ-specific autoimmunity by bolstering the IL-2 producing capacity of self-reactive CD4+ T cells and, in turn, favor the function and homeostasis of CD4+Foxp3+ nTreg cells in vivo.

CD4+CD25+ T cells are potent immunoregulatory cells that suppress TCR-induced proliferation of CD4 and CD8 T cells in vitro by a cell contact-dependent mechanism. Addition of IL-2 or anti-CD28 abrogates CD4+CD25+-mediated suppression of proliferation and has been assumed to “break suppression.” We examined IL-2 mRNA by quantitative PCR in cocultures of mouse CD4+CD25+ and CD4+CD25− T cells. Although IL-2 gene transcription was inhibited in the presence or absence of exogenous IL-2, the addition of anti-CD28 stimulated endogenous IL-2 production. Surprisingly, transcription of IL-2 mRNA was also restored in the cocultures in the presence of anti-IL-2. These results are most compatible with a model in which CD4+CD25+ T cells do not suppress the initial activation of CD4+CD25− T cells, but mediate their suppressive effects following production of IL-2 by the responder cells resulting in both the expansion of the CD4+CD25+ T cells and the induction of their suppressor function.

Nonobese diabetic (NOD) mice develop insulitis and diabetes through an autoimmune process. Since TGF-β1 down-regulates many immune responses, we hypothesized that TGF-β1 could prevent disease in NOD mice and that there would be several advantages to cytokine delivery by a somatic gene therapy approach. We opted for i.m. injection of a naked plasmid DNA expression vector encoding murine TGF-β1 (pCMV-TGF-β1). Treatment with pCMV-TGF-β1 resulted in the retention and expression of the vector in muscle cells, associated with a considerable elevation in the plasma levels of TGF-β1, that was not observed in control vector-treated mice. The levels of TGF-β1 produced were sufficient to exert immunosuppressive effects. Delayed-type hypersensitivity responses were suppressed, and autoimmunity-prone NOD mice were protected from insulitis and diabetes in models of cyclophosphamide-accelerated and natural course disease. In pCMV-TGF-β1-treated mice, pancreatic IL-12 and IFN-γ mRNA expression was depressed, and the ratio of IFN-γ to IL-4 mRNA was decreased, as determined by semiquantitative reverse-transcription PCR. In contrast, NOD mice injected with a vector encoding the proinflammatory cytokine IFN-γ developed diabetes earlier. Intramuscular administration of cytokine-encoding plasmid vectors proved to be an effective method of cytokine delivery in these mice, and altered autoimmune disease expression.

In the absence of infection, the spleen contains numerous natural killer (NK) cells located mainly in the red pulp and whose differentiation profile is characterized by a preponderance of mature elements. In contrast, lymph nodes (LNs) contain few NK cells that are sited mostly in T-cell zones and that are phenotypically skewed toward immature developmental stages. We present evidence that CD4+CD25+ T regulatory (Treg) cells hamper generation of mature NK cells in the LNs through short-range interactions with NK precursors. Moreover, we show that Treg cells are both necessary and sufficient to repress accumulation of NK cells in resting LNs and an absence of Treg totally alleviates the differentiation blockade and leads to an accumulation of mature NK cells in the LN paracortex. In turn, mature NK cells specifically regulate the amount of CD8α+ phenotypically immature dendritic cells (iDCs) present in LN T-cell zones. We propose that the dominant influence of Treg cells on NK cell precursors and consequently on CD8α+ iDCs explains why “quiescent” LNs, in the absence of infection, function as privileged sites for induction and maintenance of tolerance to peripheral self antigens.