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Glucose is a vital source of energy for all mammals. The balance between glucose uptake, metabolism and storage determines the energy status of an individual, and perturbations in this balance can lead to metabolic diseases. The maintenance of organismal glucose metabolism is a complex process that involves multiple tissues, including adipose tissue, which is an endocrine and energy storage organ that is critical for the regulation of systemic metabolism. Adipose tissue consists of an array of different cell types, including specialized adipocytes and stromal and endothelial cells. In addition, adipose tissue harbors a wide range of immune cells that play vital roles in adipose tissue homeostasis and function. These cells contribute to the regulation of systemic metabolism by modulating the inflammatory tone of adipose tissue, which is directly linked to insulin sensitivity and signaling. Furthermore, these cells affect the control of thermogenesis. While lean adipose tissue is rich in type 2 and anti-inflammatory cytokines such as IL-10, obesity tips the balance in favor of a proinflammatory milieu, leading to the development of insulin resistance and the dysregulation of systemic metabolism. Notably, anti-inflammatory immune cells, including regulatory T cells and innate lymphocytes, protect against insulin resistance and have the characteristics of tissue-resident cells, while proinflammatory immune cells are recruited from the circulation to obese adipose tissue. Here, we review the key findings that have shaped our understanding of how immune cells regulate adipose tissue homeostasis to control organismal metabolism.

Differentiation and homeostasis of Foxp3 + regulatory T (Treg) cells are strictly controlled by T-cell receptor (TCR) signals; however, molecular mechanisms that govern these processes are incompletely understood. Here we show that Bach2 is an important regulator of Treg cell differentiation and homeostasis downstream of TCR signaling. Bach2 prevents premature differentiation of fully suppressive effector Treg (eTreg) cells, limits IL-10 production and is required for the development of peripherally induced Treg (pTreg) cells in the gastrointestinal tract. Bach2 attenuates TCR signaling-induced IRF4-dependent Treg cell differentiation. Deletion of IRF4 promotes inducible Treg cell differentiation and rescues pTreg cell differentiation in the absence of Bach2. In turn, loss of Bach2 normalizes eTreg cell differentiation of IRF4-deficient Treg cells. Mechanistically, Bach2 counteracts the DNA-binding activity of IRF4 and limits chromatin accessibility, thereby attenuating IRF4-dependent transcription. Thus, Bach2 balances TCR signaling induced transcriptional activity of IRF4 to maintain homeostasis of thymically-derived and peripherally-derived Treg cells. The transcription factor Bach2 is critical for T cell differentiation, but how it functions in Treg cells is unclear. Here the authors use a Treg-specific mouse model to show that Bach2 controls homeostasis and function of Treg cells by limiting DNA accessibility and activity of IRF4 in response to TCR signaling.

The molecular mechanisms responsible for the high immunosuppressive capacity of CD4+ Tregs in tumors are not well known. High-dimensional single-cell profiling of T cells from chemotherapy-naive individuals with non-small-cell lung cancer identified the transcription factor IRF4 as specifically expressed by a subset of intratumoral CD4+ effector Tregs with superior suppressive activity. In contrast to the IRF4- counterparts, IRF4+ Tregs expressed a vast array of suppressive molecules, and their presence correlated with multiple exhausted subpopulations of T cells. Integration of transcriptomic and epigenomic data revealed that IRF4, either alone or in combination with its partner BATF, directly controlled a molecular program responsible for immunosuppression in tumors. Accordingly, deletion of Irf4 exclusively in Tregs resulted in delayed tumor growth in mice while the abundance of IRF4+ Tregs correlated with poor prognosis in patients with multiple human cancers. Thus, a common mechanism underlies immunosuppression in the tumor microenvironment irrespective of the tumor type.

Foxp3 + regulatory T cells (T reg cells) are crucial for the maintenance of immune homeostasis both in lymphoid tissues and in non-lymphoid tissues. Here we demonstrate that the ability of intestinal T reg cells to constrain microbiota-dependent interleukin (IL)-17–producing helper T cell (T H 17 cell) and immunoglobulin A responses critically required expression of the transcription factor c-Maf. The terminal differentiation and function of several intestinal T reg cell populations, including RORγt + T reg cells and follicular regulatory T cells, were c-Maf dependent. c-Maf controlled T reg cell–derived IL-10 production and prevented excessive signaling via the kinases PI(3)K (phosphatidylinositol-3-OH kinase) and Akt and the metabolic checkpoint kinase complex mTORC1 (mammalian target of rapamycin) and expression of inflammatory cytokines in intestinal T reg cells. c-Maf deficiency in T reg cells led to profound dysbiosis of the intestinal microbiota, which when transferred to germ-free mice was sufficient to induce exacerbated intestinal T H 17 responses, even in a c-Maf-competent environment. Thus, c-Maf acts to preserve the identity and function of intestinal T reg cells, which is essential for the establishment of host–microbe symbiosis. An intricately linked homeostasis exists between the gut microbiome and host immune system. Scheffold and colleagues show that intestinal T reg cells upregulate the transcription factor c-Maf in response to specific signals from the gut microenvironment to establish host–microbiota homeostasis.

Regulatory T cells (Tregs) are crucial immune cells for tissue repair and regeneration. However, their potential as a cell-based regenerative therapy is not yet fully understood. Here, we show that local delivery of exogenous Tregs into injured mouse bone, muscle, and skin greatly enhances tissue healing. Mechanistically, exogenous Tregs rapidly adopt an injury-specific phenotype in response to the damaged tissue microenvironment, upregulating genes involved in immunomodulation and tissue healing. We demonstrate that exogenous Tregs exert their regenerative effect by directly and indirectly modulating monocytes/macrophages (Mo/MΦ) in injured tissues, promoting their switch to an anti-inflammatory and pro-healing state via factors such as interleukin (IL)-10. Validating the key role of IL-10 in exogenous Treg-mediated repair and regeneration, the pro-healing capacity of these cells is lost when Il10 is knocked out. Additionally, exogenous Tregs reduce neutrophil and cytotoxic T cell accumulation and IFN-γ production in damaged tissues, further dampening the pro-inflammatory Mo/MΦ phenotype. Highlighting the potential of this approach, we demonstrate that allogeneic and human Tregs also promote tissue healing. Together, this study establishes exogenous Tregs as a possible universal cell-based therapy for regenerative medicine and provides key mechanistic insights that could be harnessed to develop immune cell-based therapies to enhance tissue healing. Regulatory T cells (Tregs) are known for suppressing inflammatory processes, but their full capacity for tissue regeneration is yet to be harnessed. Here, the authors demonstrate the efficiency of Tregs in facilitating tissue healing in mouse models of bone, muscle, and skin injury, with monocytes/macrophages and interleukin-10 playing a key mechanistic role in the process.

Regulatory T cells (Tregs) are key immune regulators that have shown promise in enhancing cardiac repair post-MI, although the mechanisms remain elusive. Here, we show that rapidly increasing Treg number in the circulation post-MI via systemic administration of exogenous Tregs improves cardiac function in male mice, by limiting cardiomyocyte death and reducing fibrosis. Mechanistically, exogenous Tregs quickly home to the infarcted heart and adopt an injury-specific transcriptome that mediates repair by modulating monocytes/macrophages. Specially, Tregs lead to a reduction in pro-inflammatory Ly6C Hi CCR2 + monocytes/macrophages accompanied by a rapid shift of macrophages towards a pro-repair phenotype. Additionally, exogenous Treg-derived factors, including nidogen-1 and IL-10, along with a decrease in cardiac CD8 + T cell number, mediate the reduction of the pro-inflammatory monocyte/macrophage subset in the heart. Supporting the pivotal role of IL-10, exogenous Tregs knocked out for IL-10 lose their pro-repair capabilities. Together, this study highlights the beneficial use of a Treg-based therapeutic approach for cardiac repair with important mechanistic insights that could facilitate the development of novel immunotherapies for MI. After myocardial infarction, excessive inflammation impairs heart repair, leading to reduced cardiac function. Here, the authors show that treatment with anti-inflammatory immune cells (regulatory T cells) improves cardiac repair by modulating the activity of a specific subset of macrophages in the heart.

[...]ablation of PPAR-γ or IL-33 specifically affects the VAT Treg cell population and compromises organismal glucose metabolism (5,7). Besides the transcription factors and cytokines, the structural component of the tissues such as mesenchymal stromal cells are known to imprint tissue specific features in Treg cells (8). The small intestine is predominantly populated by Gata3+ tTreg cells, which restrain inflammation and promote tissue repair to preserve gut homeostasis. Besides these populations, a specialized population of Bcl6+ Treg cells localized in the germinal centers (GCs) regulate the production of IgA and further contribute to gut health. Individual Intestinal Symbionts Induce a Distinct Population of RORgamma(+) Regulatory T Cells.Science(2015)349:993–7. doi:10.1126/science.aaa9420 5CipollettaDFeuererMLiAKameiNLeeJShoelsonSE.PPAR-Gamma Is a Major Driver of the Accumulation and Phenotype of Adipose Tissue Treg Cells.Nature(2012)486:549–53. doi:10.1038/nature11132 6NeumannCBlumeJRoyUTehPVasanthakumarABellerA.C-Maf-Dependent Treg Cell Control of Intestinal TH17 Cells and IgA Establishes Host-Microbiota Homeostasis.Nat Immunol(2019)20:471–81. doi:10.1038/s41590-019-0316-2 7VasanthakumarAMoroKXinALiaoYGlouryRKawamotoS.The Transcriptional Regulators IRF4, BATF and IL-33 Orchestrate Development and Maintenance of Adipose Tissue-Resident Regulatory T Cells.Nat Immunol(2015)16:276–85. doi:10.1038/ni.3085 8VasanthakumarAChisangaDBlumeJGlouryRBrittKHenstridgeD.Sex-Specific Adipose Tissue Imprinting of Regulatory T Cells.Nature(2020)579:581–5. doi:10.1038/s41586-020-2040-3 9HayatsuNMiyaoTTachibanaMMurakamiRKimuraAKatoT.Analyses of a Mutant Foxp3 Allele Reveal BATF as a Critical Transcription Factor in the Differentiation and Accumulation of Tissue Regulatory T Cells.Immunity(2017)47:268–83.e269. doi:10.1016/j.immuni.2017.07.008 10WingJBTanakaASakaguchiS.Human FOXP3(+) Regulatory T Cell Heterogeneity and Function in Autoimmunity and Cancer.Immunity(2019)50:302–16. doi:10.1016/j.immuni.2019.01.020

A wide array of chemokine receptors, including CCR2, are known to control Treg migration. Here, we report that CCR2 regulates Tregs beyond chemotaxis. We found that CCR2 deficiency reduced CD25 expression by FoxP3+ Treg cells. Such a change was also consistently present in irradiation chimeras reconstituted with mixed bone marrow from wild-type (WT) and CCR2−/− strains. Thus, CCR2 deficiency resulted in profound loss of CD25hi FoxP3+ Tregs in secondary lymphoid organs as well as in peripheral tissues. CCR2−/− Treg cells were also functionally inferior to WT cells. Interestingly, these changes to Treg cells did not depend on CCR2+ monocytes/moDCs (the cells where CCR2 receptors are most abundant). Rather, we demonstrated that CCR2 was required for TLR-stimulated, but not TCR- or IL-2-stimulated, CD25 upregulation on Treg cells. Thus, we propose that CCR2 signaling can increase the fitness of FoxP3+ Treg cells and provide negative feedback to counter the proinflammatory effects of CCR2 on myeloid cells.

This commentary article highlights two recently published studies, which for the first time revealed the immunological underpinnings of sex‐bias in cancer incidence and mortality. These studies showed that the androgen receptor restrains anti‐tumour immunity in males by repressing cytotoxic genes in CD8+ T cells.