Yujing Bi

SIRT1 Limits the Function and Fate of Myeloid-Derived Suppressor Cells in Tumors by Orchestrating HIF-1α–Dependent Glycolysis

Myeloid-derived suppressor cells (MDSC) display an immature phenotype that may assume a classically activated (M1) or alternatively activated phenotype (M2) in tumors. In this study, we investigated metabolic mechanisms underlying the differentiation of MDSCs into M1 or M2 myeloid lineage and their effect on cancer pathophysiology. We found that SIRT1 deficiency in MDSCs directs a specific switch to M1 lineage when cells enter the periphery from bone marrow, decreasing the suppressive function in favor of a proinflammatory M1 phenotype associated with tumor cell attack. Glycolytic activation through the mTOR-hypoxia-inducible factor-1α (HIF-1α) pathway was required for differentiation to the M1 phenotype, which conferred protection against tumors. Our results define the essential nature of a SIRT1-mTOR/HIF-1α glycolytic pathway in determining MDSC differentiation, with implications for metabolic reprogramming as a cancer therapeutic approach.

Histone Deacetylase SIRT1 Negatively Regulates the Differentiation of Interleukin-9-Producing CD4+ T Cells

Distinct metabolic programs support the differentiation of CD4(+) Txa0cells into separate functional subsets. In this study, we investigated metabolic mechanisms underlying the differentiation of IL-9-producing CD4(+) Txa0cells (Th9) in allergic airway inflammation and cancerous tumors. We found that histone deacetylase SIRT1 negatively regulated Th9 cell differentiation. A deficiency of SIRT1 induced by either conditional deletion in mouse CD4(+) Txa0cells or the use of small interfering RNA (siRNA) in mouse or human Txa0cells increased IL-9 production, whereas ectopic SIRT1 expression inhibited it. Notably, SIRT1 inhibited Th9 cell differentiation that regulated anti-tumor immunity and allergic pulmonary inflammation. Glycolytic activation through the mTOR-hypoxia-inducible factor-1α (HIF1α) was required for the differentiation of Th9 cells that conferred protection against tumors and is involved in allergic airway inflammation. Our results define the essential features of SIRT1-mTOR-HIF1α signaling-coupled glycolytic pathway in inducing Th9 cell differentiation, with implications for metabolic reprogramming as an immunotherapeutic approach.

Proinflammatory signal suppresses proliferation and shifts macrophage metabolism from Myc-dependent to HIF1α-dependent

Significance Macrophages maintain homeostatic proliferation in the presence of mitogens whereas encounters with invading microorganisms inhibit proliferation and engage a rapid proinflammatory response. Such cell fate change requires an extensive reprogramming of metabolism, and the regulatory mechanisms behind this change remain unknown. We found that myelocytomatosis viral oncogen (Myc) plays a major role in regulating proliferation-associated metabolic programs. However, proinflammatory stimuli suppress Myc and cell proliferation and engage a hypoxia-inducible factor alpha (HIF1α)-dependent transcriptional program that is responsible for heightened glycolysis. Our work indicates that a switch from a Myc-dependent to a HIF1α-dependent transcriptional program may regulate the robust bioenergetic support for inflammatory response, while sparing Myc-dependent proliferation. As a phenotypically plastic cellular population, macrophages change their physiology in response to environmental signals. Emerging evidence suggests that macrophages are capable of tightly coordinating their metabolic programs to adjust their immunological and bioenergetic functional properties, as needed. Upon mitogenic stimulation, quiescent macrophages enter the cell cycle, increasing their bioenergetic and biosynthetic activity to meet the demands of cell growth. Proinflammatory stimulation, however, suppresses cell proliferation, while maintaining a heightened metabolic activity imposed by the production of bactericidal factors. Here, we report that the mitogenic stimulus, colony-stimulating factor 1 (CSF-1), engages a myelocytomatosis viral oncogen (Myc)-dependent transcriptional program that is responsible for cell cycle entry and the up-regulation of glucose and glutamine catabolism in bone marrow-derived macrophages (BMDMs). However, the proinflammatory stimulus, lipopolysaccharide (LPS), suppresses Myc expression and cell proliferation and engages a hypoxia-inducible factor alpha (HIF1α)-dependent transcriptional program that is responsible for heightened glycolysis. The acute deletion of Myc or HIF1α selectively impaired the CSF-1– or LPS-driven metabolic activities in BMDM, respectively. Finally, inhibition of glycolysis by 2-deoxyglucose (2-DG) or genetic deletion of HIF1α suppressed LPS-induced inflammation in vivo. Our studies indicate that a switch from a Myc-dependent to a HIF1α-dependent transcriptional program may regulate the robust bioenergetic support for an inflammatory response, while sparing Myc-dependent proliferation.

Dexamethasone potentiates myeloid-derived suppressor cell function in prolonging allograft survival through nitric oxide

Whereas GCs have been demonstrated to be beneficial for transplantation patients, the pharmacological mechanisms remain unknown. Herein, the role of GR signaling was investigated via a pharmacological approach in a murine allogeneic skin transplantation model. The GC Dex, a representative GC, significantly relieved allograft rejection. In Dex‐treated allograft recipient mice, CD11b+Gr1+ MDSCs prolonged graft survival and acted as functional suppressive immune modulators that resulted in fewer IFN‐γ‐producing Th1 cells and a greater number of IL‐4‐producing Th2 cells. In agreement, Dex‐treated MDSCs promoted reciprocal differentiation between Th1 and Th2 in vivo. Importantly, the GR is required in the Dex‐induced MDSC effects. The blocking of GR with RU486 significantly diminished the expression of CXCR2 and the recruitment of CD11b+Gr1+ MDSCs, thereby recovering the increased MDSC‐suppressive activity induced by Dex. Mechanistically, Dex treatment induced MDSC iNOS expression and NO production. Pharmacologic inhibition of iNOS completely eliminated the MDSC‐suppressive function and the effects on T cell differentiation. This study shows MDSCs to be an essential component in the prolongation of allograft survival following Dex or RU486 treatment, validating the GC–GR–NO signaling axis as a potential therapeutic target in transplantation.

Dendritic cell SIRT1–HIF1α axis programs the differentiation of CD4+ T cells through IL-12 and TGF-β1

Significance Naive CD4+ T cells differentiate into diverse effector and regulatory subsets to orchestrate immunity and tolerance. Whereas the mechanism of T-cell intrinsic signals has been extensively studied, how T-cell lineage differentiation is controlled by innate immune signals remains unknown. Here we used loss-of-function mouse systems, combined with other complementary approaches and models, to define the role of dendritic cell (DC) sirtuin 1 (SIRT1) as a key regulator in orchestrating the orientation of T-cell differentiation via HIF1α signaling in a mammalian target of rapamycin–independent manner. DC-expressed SIRT1, a type III histone deacetylase, programmed reciprocal T helper 1 (TH1) and regulatory T-cell (Treg) differentiation by modulating IL-12–STAT4 and TGF-β1–SMAD3 axes and cytokine receptor expressions at the DC–T-cell interface. The differentiation of naive CD4+ T cells into distinct lineages plays critical roles in mediating adaptive immunity or maintaining immune tolerance. In addition to being a first line of defense, the innate immune system also actively instructs adaptive immunity through antigen presentation and immunoregulatory cytokine production. Here we found that sirtuin 1 (SIRT1), a type III histone deacetylase, plays an essential role in mediating proinflammatory signaling in dendritic cells (DCs), consequentially modulating the balance of proinflammatory T helper type 1 (TH1) cells and antiinflammatory Foxp3+ regulatory T cells (Treg cells). Genetic deletion of SIRT1 in DCs restrained the generation of Treg cells while driving TH1 development, resulting in an enhanced T-cell–mediated inflammation against microbial responses. Beyond this finding, SIRT1 signaled through a hypoxia-inducible factor-1 alpha (HIF1α)-dependent pathway, orchestrating the reciprocal TH1 and Treg lineage commitment through DC-derived IL-12 and TGF-β1. Our studies implicates a DC-based SIRT1–HIF1α metabolic checkpoint in controlling T-cell lineage specification.

Targeting S1P1 Receptor Protects against Murine Immunological Hepatic Injury through Myeloid-Derived Suppressor Cells

Although FTY720 may alter migration and homing of lymphocytes via sphingosine-1-phosphate (S1P) receptors, our recent studies indicated that FTY720 directly controls the differentiation of Th1 cells to regulatory T cells (Tregs) by targeting S1P1. However, the pharmacological function of FTY720 in immunological hepatic injury remains unknown. In this study, the role and regulatory signaling pathway of S1P receptor were investigated using a pharmacological approach in immune-mediated hepatic injury (IMH). In the context of IMH, FTY720 significantly ameliorated mortality and hepatic pathology. In FTY720-treated mice, recruited CD11b+Gr1+ myeloid-derived suppressor cells (MDSCs) mediate protection against IMH and are functional suppressive immune modulators that result in fewer IFN-γ–producing Th1 cells and more Foxp3+ Tregs. In agreement, FTY720-treated MDSCs promote the reciprocal differentiation between Th1 cells and Tregs in vitro and in vivo. Mechanistically, FTY720 treatment induced inducible NO synthase expression and NO production in MDSCs. Pharmacologic inhibition of inducible NO synthase completely eliminates MDSC suppressive function and eradicates their inducible effects on T cell differentiation. Finally, the mTOR inhibitor, rapamycin, photocopies the effects of FTY720 on MDSCs, implicating mTOR as a downstream effector of S1P1 signaling. This study identifies MDSCs as an essential component that provides protection against IMH following FTY720 or rapamycin treatment, validating the S1P1–mTOR signaling axis as a potential therapeutic target in hepatic injury.

mTOR limits the recruitment of CD11b+Gr1+Ly6Chigh myeloid-derived suppressor cells in protecting against murine immunological hepatic injury

The mTOR pathway integrates diverse environmental inputs, including immune signals and metabolic cues, to direct the innate and adaptive immune responses. MDSCs are a heterogeneous cell population that plays a crucial regulatory effect in immune‐related diseases. However, whether mTOR signaling affects the functions of MDSCs remains largely unknown. Here, we show that mTOR signaling is a pivotal negative determinant of MDSC recruitment in IMH disease. In the context of IMH, inhibition of mTOR with rapamycin in CD11b+Gr1+ MDSCs mediates protection against IMH and serves as a functional, suppressive immune modulator that results in increased CD11b+Gr1+Ly6Chigh MDSC recruitment to inflammatory sites. In agreement with this, mTOR down‐regulation promotes CD11b+Gr1+Ly6Chigh MDSC migration in vitro and in vivo. Mechanistically, mTOR activity down‐regulation in MDSCs induced iNOS expression and NO production. Pharmacologic inhibition of iNOS completely eliminated MDSC recruitment. This study identifies MDSCs as an essential component for protection against IMH following rapamycin treatment. Rapamycin treatment or mTOR inhibition promotes CD11b+Gr1+Ly6Chigh MDSC recruitment and is critically required for protection against hepatic injury. This study further validates the targeting of mTOR signaling as a potential therapeutic approach to IMH‐related diseases.

IL-17A Produced by Neutrophils Protects against Pneumonic Plague through Orchestrating IFN-γ–Activated Macrophage Programming

Innate immune cells, including neutrophils and macrophages, are critically involved in host antimicrobial defense responses. Intrinsic regulatory mechanisms controlling neutrophil and macrophage activities are poorly defined. In this study, we found that IL-17A, a natural signal factor, could provide protection against early pneumonic plague inflammation by coordinating the functions of neutrophils and programming of macrophages. The IL-17A level is promptly increased during the initial infection. Importantly, abrogation of IL-17A or IL-17AR significantly aggravated the infection, but mIL-17A treatment could significantly alleviate inflammatory injury, revealing that IL-17A is a critical requirement for early protection of infection. We also demonstrated that IL-17A was predominantly produced by CD11b+Ly6G+ neutrophils. Although IL-17A could not significantly affect the antimicrobial responses of neutrophils, it could target the proinflammatory macrophage (M1) programming and potentiate the M1’s defense against pneumonic plague. Mechanistically, IFN-γ treatment or IFN-γ–activated M1 macrophage transfer could significantly mitigate the aggravated infection of IL-17A−/− mice. Finally, we showed that IL-17A and IFN-γ could synergistically promote macrophage anti-infection immunity. Thus, our findings identify a previously unrecognized function of IL-17A as an intrinsic regulator in coordinating neutrophil and macrophage antimicrobial activity to provide protection against acute pneumonic plague.

Self-eating and self-defense: autophagy controls innate immunity and adaptive immunity

Autophagy (macroautophagy; “self‐eating”) is a degradation process, in which cytoplasmic content is engulfed and degraded by the lysosome. And, immunity is an important mechanism of the “self‐defense” system. Autophagy has long been recognized as a stress response to nutrient deprivation. This will provide energy and anabolic building blocks to maintain cellular bioenergetic homeostasis. Thus, autophagy plays critical roles in regulating a wide variety of pathophysiological processes, including tumorigenesis, embryo development, tissue remodeling, and most recently, immunity. The latter shows that a self‐eating (autophagy) process could regulate a self‐defense (immune) system. In this review, we summarize the recent findings regarding the regulatory and mechanistic insights of the autophagy pathway in immunity.

Kinase AKT1 Negatively Controls Neutrophil Recruitment and Function in Mice

Neutrophils are critically involved in host defense and inflammatory injury. However, intrinsic signaling mechanisms controlling neutrophil recruitment and activities are poorly defined. In this article, we showed that protein kinase AKT1 (also known as PKBα) is the dominant isoform expressed in neutrophils and is downregulated upon bacterial infection and neutrophil activation. AKT1 deficiency resulted in severe disease progression accompanied by recruitment of neutrophils and enhanced bactericidal activity in the acute inflammatory lung injury (ALI) and the Staphylococcus aureus infection mouse models. Moreover, the depletion of neutrophils efficiently reversed the aggravated inflammatory response, but adoptive transfer of AKT1−/− neutrophils could potentiate the inflammatory immunity, indicating an intrinsic effect of the neutrophil in modulating inflammation in AKT1−/− mice. In the ALI model, the infiltration of neutrophils into the inflammatory site was associated with enhanced migration capacity, whereas inflammatory stimuli could promote neutrophil apoptosis. In accordance with these findings, neutralization of CXCR2 attenuated neutrophil infiltration and delayed the occurrence of inflammation. Finally, the enhanced bactericidal activity and inflammatory immunity of AKT-deficient neutrophils were mediated by a STAT1-dependent, but not a mammalian target of rapamycin–dependent, pathway. Thus, our findings indicated that the AKT1–STAT1 signaling axis negatively regulates neutrophil recruitment and activation in ALI and S. aureus infection in mice.