Transforming growth factor (TGF)-β1 is a major pluripotential cytokine with a pronounced immunosuppressive effect and its deficiency results in lethal autoimmunity in mice. However, mechanisms of its immunosuppressive action are not completely understood. Here, we report that TGF-β1 supports the maintenance of Foxp3 expression, regulatory function, and homeostasis in peripheral CD4+CD25+ regulatory T (T reg) cells, but is not required for their thymic development. We found that in 8–10-d-old TGF-β1–deficient mice, peripheral, but not thymic, T reg cells are significantly reduced in numbers. Moreover, our experiments suggest that a defect in TGF-β–mediated signaling in T reg cells is associated with a decrease in Foxp3 expression and suppressor activity. Thus, our results establish an essential link between TGF-β1 signaling in peripheral T reg cells and T reg cell maintenance in vivo.

Transforming growth factor-beta (TGF-beta) has been implicated in the control of differentiation and proliferation of multiple cell types. However, a role for TGF-beta in the control of immune homeostasis is not fully understood because of its pleiotropic action. Here we report that complete ablation of the TGF-beta signaling in T cells engendered aggressive early-onset, multiorgan, autoimmune-associated lesions with 100% mortality. Peripheral CD4+ and CD8+ T cells with TGF-beta-receptor II (TGF-betaRII) deficiency activated cytolytic and T helper 1 (Th1) differentiation program in a cell-intrinsic T cell receptor (TCR)-specific fashion. Furthermore, TGF-betaRII deficiency blocked the development of canonical CD1d-restricted NKT cells. Instead, it facilitated generation of a highly pathogenic T cell subset exhibiting multiple hallmarks of NK cells and sharply elevated amounts of FasL, perforin, granzymes, and interferon-gamma. Thus, TGF-beta signaling in peripheral T cells is crucial in restraining TCR activation-dependent Th1, cytotoxic, and NK cell-like differentiation program which, when left unchecked, leads to rapidly progressing fatal autoimmunity.

Transforming growth factor-β (TGF-β) is a major immunosuppressive cytokine that maintains immune homeostasis and prevents autoimmunity through its antiproliferative and anti-inflammatory properties in various immune cell types. We provide genetic, pharmacologic, and biochemical evidence that a critical target of TGF-β signaling in mouse and human natural killer (NK) cells is the serine and threonine kinase mTOR (mammalian target of rapamycin). Treatment of mouse or human NK cells with TGF-β in vitro blocked interleukin-15 (IL-15)-induced activation of mTOR. TGF-β and the mTOR inhibitor rapamycin both reduced the metabolic activity and proliferation of NK cells and reduced the abundances of various NK cell receptors and the cytotoxic activity of NK cells. In vivo, constitutive TGF-β signaling or depletion of mTOR arrested NK cell development, whereas deletion of the TGF-β receptor subunit TGF-βRII enhanced mTOR activity and the cytotoxic activity of the NK cells in response to IL-15. Suppression of TGF-β signaling in NK cells did not affect either NK cell development or homeostasis; however, it enhanced the ability of NK cells to limit metastases in two different tumor models in mice. Together, these results suggest that the kinase mTOR is a crucial signaling integrator of pro- and anti-inflammatory cytokines in NK cells. Moreover, we propose that boosting the metabolic activity of antitumor lymphocytes could be an effective strategy to promote immune-mediated tumor suppression.

T cell activation requires that the cell meet increased energetic and biosynthetic demands. We showed that exogenous nutrient availability regulated the differentiation of naïve CD4(+) T cells into distinct subsets. Activation of naïve CD4(+) T cells under conditions of glutamine deprivation resulted in their differentiation into Foxp3(+) (forkhead box P3-positive) regulatory T (Treg) cells, which had suppressor function in vivo. Moreover, glutamine-deprived CD4(+) T cells that were activated in the presence of cytokines that normally induce the generation of T helper 1 (TH1) cells instead differentiated into Foxp3(+) Treg cells. We found that α-ketoglutarate (αKG), the glutamine-derived metabolite that enters into the mitochondrial citric acid cycle, acted as a metabolic regulator of CD4(+) T cell differentiation. Activation of glutamine-deprived naïve CD4(+) T cells in the presence of a cell-permeable αKG analog increased the expression of the gene encoding the TH1 cell-associated transcription factor Tbet and resulted in their differentiation into TH1 cells, concomitant with stimulation of mammalian target of rapamycin complex 1 (mTORC1) signaling. Together, these data suggest that a decrease in the intracellular amount of αKG, caused by the limited availability of extracellular glutamine, shifts the balance between the generation of TH1 and Treg cells toward that of a Treg phenotype.

SUMMARY Memory CD8+ T cells induced upon immunization exhibit improved functional features that contribute to protection of immunized hosts. Although both cognate antigen recognition and inflammation are important for memory CD8+ T cell reactivation, the relative contribution of these factors and the cell types providing these signals in vivo are poorly defined. Here, we show that Ly6C+CCR2+ inflammatory monocytes, a subset of monocytes, largely orchestrate memory CD8+ T and NK lymphocytes activation by differentiating into interleukin-18 (IL-18)- and IL-15-producing cells in an inflammasome and type I interferon-IRF3-dependent manner. Memory CD8+ T cells became potent effector cells by sensing inflammation from monocytes independently of their cognate antigen. Like NK cells, they underwent rapid mobilization, upregulated intense and sustained effector functions during bacterial, viral and parasitic infections, and contributed to innate responses and protection in vivo. Thus, inflammatory monocyte-derived IL-18 and IL-15 are critical to initiate memory CD8+ T and NK lymphocytes differentiation into antimicrobial effector cells.

CD46 is a widely expressed transmembrane protein that was initially identified as binding and inactivating C3b and C4b complement products. We used mice that were transgenic for one of two human CD46 isoforms that differ in their cytoplasmic domains (termed CD46-1 and CD46-2) to analyze the effect of CD46 stimulation on the immune response. We show here that CD46 can regulate inflammatory responses, either by inhibiting (CD46-1) or increasing (CD46-2) the contact hypersensitivity reaction. We found that engagement of CD46-1 or CD46-2 differentially affected CD8(+) T cell cytotoxicity, CD4(+) T cell proliferation, interleukin 2 (IL-2) and IL-10 production as well as tyrosine phosphorylation of Vav in T lymphocytes. These results indicate that CD46 plays a role in regulating the T cell induced inflammatory reaction and in fine-tuning the cellular immune response by bridging innate and acquired immunity.

SummaryRegulatory T (Treg) cells play a pivotal role in suppressing self-harmful T cell responses, but how Treg cells mediate suppression to maintain immune homeostasis and limit responses during inflammation is unclear. Here we show that effector Treg cells express high amounts of the integrin αvβ8, which enables them to activate latent transforming growth factor-β (TGF-β). Treg-cell-specific deletion of integrin αvβ8 did not result in a spontaneous inflammatory phenotype, suggesting that this pathway is not important in Treg-cell-mediated maintenance of immune homeostasis. However, Treg cells lacking expression of integrin αvβ8 were unable to suppress pathogenic T cell responses during active inflammation. Thus, our results identify a mechanism by which Treg cells suppress exuberant immune responses, highlighting a key role for effector Treg-cell-mediated activation of latent TGF-β in suppression of self-harmful T cell responses during active inflammation.

We have shown previously that peripheral lymph node-resident retinoic acid receptor-related orphan receptor γt+ NK1.1− invariant NKT (iNKT) cells produce IL-17A independently of IL-6. In this study, we show that the concomitant presence of IL-1 and IL-23 is crucial to induce a rapid and sustained IL-17A/F and IL-22 response by these cells that requires TCR–CD1d interaction and partly relies on IL-23–mediated upregulation of IL-23R and IL-1R1 expression. We further show that IL-1 and IL-23 produced by pathogen-associated molecular pattern-stimulated dendritic cells induce this response from NK1.1− iNKT cells in vitro, involving mainly TLR2/4-signaling pathways. Finally, we found that IL-17A production by these cells occurs very early and transiently in vivo in response to heat-killed bacteria. Overall, our study indicates that peripheral lymph node NK1.1− iNKT cells could be a source of innate Th17-related cytokines during bacterial infections and supports the hypothesis that they are able to provide an efficient first line of defense against bacterial invasion.