The TGF-ß superfamily cytokine Activin-A is induced during autoimmune neuroinflammation and drives pathogenic Th17 cell differentiation.

Th17 cells are known to exert pathogenic and non-pathogenic functions. Although the cytokine transforming growth factor ß1 (TGF-ß1) is instrumental for Th17 cell differentiation, it is dispensable for generation of pathogenic Th17 cells. Here, we examined the T cell-intrinsic role of Activin-A, a TGF-ß superfamily member closely related to TGF-ß1, in pathogenic Th17 cell differentiation. Activin-A expression was increased in individuals with relapsing-remitting multiple sclerosis and in mice with experimental autoimmune encephalomyelitis. Stimulation with interleukin-6 and Activin-A induced a molecular program that mirrored that of pathogenic Th17 cells and was inhibited by blocking Activin-A signaling. Genetic disruption of Activin-A and its receptor ALK4 in T cells impaired pathogenic Th17 cell differentiation in vitro and in vivo. Mechanistically, extracellular-signal-regulated kinase (ERK) phosphorylation, which was essential for pathogenic Th17 cell differentiation, was suppressed by TGF-ß1-ALK5 but not Activin-A-ALK4 signaling. Thus, Activin-A drives pathogenic Th17 cell differentiation, implicating the Activin-A-ALK4-ERK axis as a therapeutic target for Th17 cell-related diseases.

RAS P21 Protein Activator 3 (RASA3) Specifically Promotes Pathogenic T Helper 17 Cell Generation by Repressing T-Helper-2-Cell-Biased Programs.

Pathogenic Th17 (pTh17) cells drive inflammation and immune-pathology, but whether pTh17 cells are a Th17 cell subset whose generation is under specific molecular control remains unaddressed. We found that Ras p21 protein activator 3 (RASA3) was highly expressed by pTh17 cells relative to non-pTh17 cells and was required specifically for pTh17 generation in vitro and in vivo. Mice conditionally deficient for Rasa3 in T cells showed less pathology during experimental autoimmune encephalomyelitis. Rasa3-deficient T cells acquired a Th2 cell-biased program that dominantly trans-suppressed pTh17 cell generation via interleukin 4 production. The Th2 cell bias of Rasa3-deficient T cells was due to aberrantly elevated transcription factor IRF4 expression. RASA3 promoted proteasome-mediated IRF4 protein degradation by facilitating interaction of IRF4 with E3-ubiquitin ligase Cbl-b. Therefore, a RASA3-IRF4-Cbl-b pathway specifically directs pTh17 cell generation by balancing reciprocal Th17-Th2 cell programs. These findings indicate that a distinct molecular program directs pTh17 cell generation and reveals targets for treating pTh17 cell-related pathology and diseases.

AIM2 in regulatory T cells restrains autoimmune diseases.

The inflammasome initiates innate defence and inflammatory responses by activating caspase-1 and pyroptotic cell death in myeloid cells1,2. It consists of an innate immune receptor/sensor, pro-caspase-1, and a common adaptor molecule, ASC. Consistent with their pro-inflammatory function, caspase-1, ASC and the inflammasome component NLRP3 exacerbate autoimmunity during experimental autoimmune encephalomyelitis by enhancing the secretion of IL-1ß and IL-18 in myeloid cells3-6. Here we show that the DNA-binding inflammasome receptor AIM27-10 has a T cell-intrinsic and inflammasome-independent role in the function of T regulatory (Treg) cells. AIM2 is highly expressed by both human and mouse Treg cells, is induced by TGFß, and its promoter is occupied by transcription factors that are associated with Treg cells such as RUNX1, ETS1, BCL11B and CREB. RNA sequencing, biochemical and metabolic analyses demonstrated that AIM2 attenuates AKT phosphorylation, mTOR and MYC signalling, and glycolysis, but promotes oxidative phosphorylation of lipids in Treg cells. Mechanistically, AIM2 interacts with the RACK1-PP2A phosphatase complex to restrain AKT phosphorylation. Lineage-tracing analysis demonstrates that AIM2 promotes the stability of Treg cells during inflammation. Although AIM2 is generally accepted as an inflammasome effector in myeloid cells, our results demonstrate a T cell-intrinsic role of AIM2 in restraining autoimmunity by reducing AKT-mTOR signalling and altering immune metabolism to enhance the stability of Treg cells.

GATA-3 controls the maintenance and proliferation of T cells downstream of TCR and cytokine signaling.

GATA-3 controls T helper type 2 (TH2) differentiation. However, whether GATA-3 regulates the function of mature T cells beyond TH2 determination remains poorly understood. We found that signaling via the T cell antigen receptor (TCR) and cytokine stimulation promoted GATA-3 expression in CD8(+) T cells, which controlled cell proliferation. Although GATA-3-deficient CD8(+) T cells were generated, their peripheral maintenance was impaired, with lower expression of the receptor for interleukin 7 (IL-7R). GATA-3-deficient T cells had defective responses to viral infection and alloantigen. The proto-oncoprotein c-Myc was a critical target of GATA-3 in promoting T cell proliferation. Our study thus demonstrates an essential role for GATA-3 in controlling the maintenance and proliferation of T cells and provides insight into immunoregulation.

Ribosomal protein RPL11 haploinsufficiency causes anemia in mice via activation of the RP-MDM2-p53 pathway.

Recent discovery of the ribosomal protein (RP) RPL11 interacting with and inhibiting the E3 ubiquitin ligase function of MDM2 established the RP-MDM2-p53 signaling pathway, which is linked to biological events, including ribosomal biogenesis, nutrient availability, and metabolic homeostasis. Mutations in RPs lead to a diverse array of phenotypes known as ribosomopathies in which the role of p53 is implicated. Here, we generated conditional RPL11-deletion mice to investigate in vivo effects of impaired RP expression and its functional connection with p53. While deletion of one Rpl11 allele in germ cells results in embryonic lethality, deletion of one Rpl11 allele in adult mice does not affect viability but leads to acute anemia. Mechanistically, we found RPL11 haploinsufficiency activates p53 in hematopoietic tissues and impedes erythroid precursor differentiation, resulting in insufficient red blood cell development. We demonstrated that reducing p53 dosage by deleting one p53 allele rescues RPL11 haploinsufficiency-induced inhibition of erythropoietic precursor differentiation and restores normal red blood cell levels in mice. Furthermore, blocking the RP-MDM2-p53 pathway by introducing an RP-binding mutation in MDM2 prevents RPL11 haploinsufficiency-caused p53 activation and rescues the anemia in mice. Together, these findings demonstrate that the RP-MDM2-p53 pathway is a critical checkpoint for RP homeostasis and that p53-dependent cell cycle arrest of erythroid precursors is the molecular basis for the anemia phenotype commonly associated with RP deficiency.

A critical role for transcription factor Smad4 in T cell function that is independent of transforming growth factor ß receptor signaling.

Transforming growth factor-beta (TGF-ß) suppresses T cell function to maintain self-tolerance and to promote tumor immune evasion. Yet how Smad4, a transcription factor component of TGF-ß signaling, regulates T cell function remains unclear. Here we have demonstrated an essential role for Smad4 in promoting T cell function during autoimmunity and anti-tumor immunity. Smad4 deletion rescued the lethal autoimmunity resulting from transforming growth factor-beta receptor (TGF-ßR) deletion and compromised T-cell-mediated tumor rejection. Although Smad4 was dispensable for T cell generation, homeostasis, and effector function, it was essential for T cell proliferation after activation in vitro and in vivo. The transcription factor Myc was identified to mediate Smad4-controlled T cell proliferation. This study thus reveals a requirement of Smad4 for T-cell-mediated autoimmunity and tumor rejection, which is beyond the current paradigm. It highlights a TGF-ßR-independent role for Smad4 in promoting T cell function, autoimmunity, and anti-tumor immunity.

Reversing SKI-SMAD4-mediated suppression is essential for TH17 cell differentiation.

T helper 17 (TH17) cells are critically involved in host defence, inflammation, and autoimmunity. Transforming growth factor ß (TGFß) is instrumental in TH17 cell differentiation by cooperating with interleukin-6 (refs 6, 7). Yet, the mechanism by which TGFß enables TH17 cell differentiation remains elusive. Here we reveal that TGFß enables TH17 cell differentiation by reversing SKI-SMAD4-mediated suppression of the expression of the retinoic acid receptor (RAR)-related orphan receptor γt (RORγt). We found that, unlike wild-type T cells, SMAD4-deficient T cells differentiate into TH17 cells in the absence of TGFß signalling in a RORγt-dependent manner. Ectopic SMAD4 expression suppresses RORγt expression and TH17 cell differentiation of SMAD4-deficient T cells. However, TGFß neutralizes SMAD4-mediated suppression without affecting SMAD4 binding to the Rorc locus. Proteomic analysis revealed that SMAD4 interacts with SKI, a transcriptional repressor that is degraded upon TGFß stimulation. SKI controls histone acetylation and deacetylation of the Rorc locus and TH17 cell differentiation via SMAD4: ectopic SKI expression inhibits H3K9 acetylation of the Rorc locus, Rorc expression, and TH17 cell differentiation in a SMAD4-dependent manner. Therefore, TGFß-induced disruption of SKI reverses SKI-SMAD4-mediated suppression of RORγt to enable TH17 cell differentiation. This study reveals a critical mechanism by which TGFß controls TH17 cell differentiation and uncovers the SKI-SMAD4 axis as a potential therapeutic target for treating TH17-related diseases.

The transcription cofactor Hopx is required for regulatory T cell function in dendritic cell-mediated peripheral T cell unresponsiveness.

Induced regulatory T cells (iT(reg) cells) can be generated by peripheral dendritic cells (DCs) that mediate T cell unresponsiveness to rechallenge with antigen. The molecular factors required for the function of such iT(reg) cells remain unknown. We report a critical role for the transcription cofactor homeodomain-only protein (Hop; also known as Hopx) in iT(reg) cells to mediate T cell unresponsiveness in vivo. Hopx-sufficient iT(reg) cells downregulated expression of the transcription factor AP-1 complex and suppressed other T cells. In the absence of Hopx, iT(reg) cells had high expression of the AP-1 complex, proliferated and failed to mediate T cell unresponsiveness to rechallenge with antigen. Thus, Hopx is required for the function of T(reg) cells induced by DCs and the promotion of DC-mediated T cell unresponsiveness in vivo.

An essential role of the transcription factor GATA-3 for the function of regulatory T cells.

Forkhead Box P3 (Foxp3)-expressing regulatory T (Treg) cells are central to maintaining self-tolerance and immune homeostasis. How Treg cell function and Foxp3 expression are regulated is an important question under intensive investigation. Here, we have demonstrated an essential role for the transcription factor GATA-3, a previously recognized Th2 cell master regulator, in controlling Treg cell function. Treg cell-specific GATA-3 deletion led to a spontaneous inflammatory disorder in mice. GATA-3-null Treg cells were defective in peripheral homeostasis and suppressive function, gained Th17 cell phenotypes, and expressed reduced amounts of Foxp3. In addition, GATA-3 controlled Foxp3 expression by binding to and promoting the activity of cis-acting elements of Foxp3. Furthermore, the combined function of GATA-3 and Foxp3 was essential for Foxp3 expression. These findings provide insights into immune regulatory mechanisms and uncover a critical function of GATA-3 in Treg cells and immune tolerance.

IL-10 Receptor Signaling Is Essential for TR1 Cell Function In Vivo.

IL-10 is essential to maintain intestinal homeostasis. CD4+ T regulatory type 1 (TR1) cells produce large amounts of this cytokine and are therefore currently being examined in clinical trials as T cell therapy in patients with inflammatory bowel disease. However, factors and molecular signals sustaining TR1 cell regulatory activity still need to be identified to optimize the efficiency and ensure the safety of these trials. We investigated the role of IL-10 signaling in mature TR1 cells in vivo. Double IL-10eGFP Foxp3mRFP reporter mice and transgenic mice with impairment in IL-10 receptor signaling were used to test the activity of TR1 cells in a murine inflammatory bowel disease model, a model that resembles the trials performed in humans. The molecular signaling was elucidated in vitro. Finally, we used human TR1 cells, currently employed for cell therapy, to confirm our results. We found that murine TR1 cells expressed functional IL-10Rα. TR1 cells with impaired IL-10 receptor signaling lost their regulatory activity in vivo. TR1 cells required IL-10 receptor signaling to activate p38 MAPK, thereby sustaining IL-10 production, which ultimately mediated their suppressive activity. Finally, we confirmed these data using human TR1 cells. In conclusion, TR1 cell regulatory activity is dependent on IL-10 receptor signaling. These data suggest that to optimize TR1 cell-based therapy, IL-10 receptor expression has to be taken into consideration.

BPTF Is Essential for T Cell Homeostasis and Function.

Bromodomain PHD finger transcription factor (BPTF), a ubiquitously expressed ATP-dependent chromatin-remodeling factor, is critical for epigenetically regulating DNA accessibility and gene expression. Although BPTF is important for the development of thymocytes, its function in mature T cells remains largely unknown. By specifically deleting BPTF from late double-negative 3/double-negative 4 stage of developing T cells, we found that BPTF was critical for the homeostasis of T cells via a cell-intrinsic manner. In addition, BPTF was essential for the maintenance and function of regulatory T (Treg) cells. Treg cell-specific BPTF deletion led to reduced Foxp3 expression, increased lymphocyte infiltration in the nonlymphoid organs, and a systemic autoimmune syndrome. These findings therefore reveal a vital role for BPTF in T and Treg cell function and immune homeostasis.

GATA3: a master of many trades in immune regulation.

GATA3 has conventionally been regarded as a transcription factor that drives the differentiation of T helper (Th) 2 cells. Increasing evidence points to a function for GATA3 beyond controlling Th2 differentiation. GATA3 regulates T cell development, proliferation, and maintenance. Furthermore, recent studies have demonstrated important roles for GATA3 in innate lymphoid cells. Thus, GATA3 emerges as a factor with diverse functions in immune regulation, which are in some cases cell-type specific and in others shared by multiple cell types. Here, I discuss recent discoveries and the current understanding of the functions of GATA3 in immune regulation.

Control of TH17 cells occurs in the small intestine.

Interleukin (IL)-17-producing T helper cells (T(H)17) are a recently identified CD4(+) T cell subset distinct from T helper type 1 (T(H)1) and T helper type 2 (T(H)2) cells. T(H)17 cells can drive antigen-specific autoimmune diseases and are considered the main population of pathogenic T cells driving experimental autoimmune encephalomyelitis (EAE), the mouse model for multiple sclerosis. The factors that are needed for the generation of T(H)17 cells have been well characterized. However, where and how the immune system controls T(H)17 cells in vivo remains unclear. Here, by using a model of tolerance induced by CD3-specific antibody, a model of sepsis and influenza A viral infection (H1N1), we show that pro-inflammatory T(H)17 cells can be redirected to and controlled in the small intestine. T(H)17-specific IL-17A secretion induced expression of the chemokine CCL20 in the small intestine, facilitating the migration of these cells specifically to the small intestine via the CCR6/CCL20 axis. Moreover, we found that T(H)17 cells are controlled by two different mechanisms in the small intestine: first, they are eliminated via the intestinal lumen; second, pro-inflammatory T(H)17 cells simultaneously acquire a regulatory phenotype with in vitro and in vivo immune-suppressive properties (rT(H)17). These results identify mechanisms limiting T(H)17 cell pathogenicity and implicate the gastrointestinal tract as a site for control of T(H)17 cells.

Immune Cell Metabolism in Tumor Microenvironment.

Tumor microenvironment (TME) is composed of tumor cells, immune cells, cytokines, extracellular matrix, etc. The immune system and the metabolisms of glucose, lipids, amino acids, and nucleotides are integrated in the tumorigenesis and development. Cancer cells and immune cells show metabolic reprogramming in the TME, which intimately links immune cell functions and edits tumor immunology. Recent findings in immune cell metabolism hold the promising possibilities toward clinical therapeutics for treating cancer. This chapter introduces the updated understandings of metabolic reprogramming of immune cells in the TME and suggests new directions in manipulation of immune responses for cancer diagnosis and therapy.

Requirements of transcription factor Smad-dependent and -independent TGF-ß signaling to control discrete T-cell functions.

TGF-ß modulates immune response by suppressing non-regulatory T (Treg) function and promoting Treg function. The question of whether TGF-ß achieves distinct effects on non-Treg and Treg cells through discrete signaling pathways remains outstanding. In this study, we investigated the requirements of Smad-dependent and -independent TGF-ß signaling for T-cell function. Smad2 and Smad3 double deficiency in T cells led to lethal inflammatory disorder in mice. Non-Treg cells were spontaneously activated and produced effector cytokines in vivo on deletion of both Smad2 and Smad3. In addition, TGF-ß failed to suppress T helper differentiation efficiently and to promote induced Treg generation of non-Treg cells lacking both Smad2 and Smad3, suggesting that Smad-dependent signaling is obligatory to mediate TGF-ß function in non-Treg cells. Unexpectedly, however, the development, homeostasis, and function of Treg cells remained intact in the absence of Smad2 and Smad3, suggesting that the Smad-independent pathway is important for Treg function. Indeed, Treg-specific deletion of TGF-ß-activated kinase 1 led to failed Treg homeostasis and lethal immune disorder in mice. Therefore, Smad-dependent and -independent TGF-ß signaling discretely controls non-Treg and Treg function to modulate immune tolerance and immune homeostasis.

Mechanism of Action of IL-7 and Its Potential Applications and Limitations in Cancer Immunotherapy.

Interleukin-7 (IL-7) is a non-hematopoietic cell-derived cytokine with a central role in the adaptive immune system. It promotes lymphocyte development in the thymus and maintains survival of naive and memory T cell homeostasis in the periphery. Moreover, it is important for the organogenesis of lymph nodes (LN) and for the maintenance of activated T cells recruited into the secondary lymphoid organs (SLOs). The immune capacity of cancer patients is suppressed that is characterized by lower T cell counts, less effector immune cells infiltration, higher levels of exhausted effector cells and higher levels of immunosuppressive cytokines, such as transforming growth factor ß (TGF-ß). Recombinant human IL-7 (rhIL-7) is an ideal solution for the immune reconstitution of lymphopenia patients by promoting peripheral T cell expansion. Furthermore, it can antagonize the immunosuppressive network. In animal models, IL-7 has been proven to prolong the survival of tumor-bearing hosts. In this review, we will focus on the mechanism of action and applications of IL-7 in cancer immunotherapy and the potential restrictions for its usage.

Dihydroartemisinin ameliorates inflammatory disease by its reciprocal effects on Th and regulatory T cell function via modulating the mammalian target of rapamycin pathway.

Dihydroartemisinin (DHA) is an important derivative of the herb medicine Artemisia annua L., used in ancient China. DHA is currently used worldwide to treat malaria by killing malaria-causing parasites. In addition to this prominent effect, DHA is thought to regulate cellular functions, such as angiogenesis, tumor cell growth, and immunity. Nonetheless, how DHA affects T cell function remains poorly understood. We found that DHA potently suppressed Th cell differentiation in vitro. Unexpectedly, however, DHA greatly promoted regulatory T cell (Treg) generation in a manner dependent on the TGF-ßR:Smad signal. In addition, DHA treatment effectively reduced onset of experimental autoimmune encephalomyelitis (EAE) and ameliorated ongoing EAE in mice. Administration of DHA significantly decreased Th but increased Tregs in EAE-inflicted mice, without apparent global immune suppression. Moreover, DHA modulated the mammalian target of rapamycin (mTOR) pathway, because mTOR signal was attenuated in T cells upon DHA treatment. Importantly, enhanced Akt activity neutralized DHA-mediated effects on T cells in an mTOR-dependent fashion. This study therefore reveals a novel immune regulatory function of DHA in reciprocally regulating Th and Treg cell generation through the modulating mTOR pathway. It addresses how DHA regulates immune function and suggests a new type of drug for treating diseases in which mTOR activity is to be tempered.

Regulatory T-cell functions are subverted and converted owing to attenuated Foxp3 expression.

The naturally occurring regulatory T cell (T(r)) is the pivotal cell type that maintains self-tolerance and exerts active immune suppression. The development and function of T(r) cells is controlled by Foxp3 (refs 1, 2), a lack of which results in loss of T(r) cells and massive multi-organ autoimmunity in scurfy mice and IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) patients. It is generally thought that, through a binary mechanism, Foxp3 expression serves as an on-and-off switch to regulate positively the physiology of T(r) cells; however, emerging evidence associates decreased Foxp3 expression in T(r) cells with various immune disorders. We hypothesized that Foxp3 regulates T(r) cell development and function in a dose-dependent, non-binary manner, and that decreased Foxp3 expression can cause immune disease. Here, by generating a mouse model in which endogenous Foxp3 gene expression is attenuated in T(r) cells, we show that decreased Foxp3 expression results in the development of an aggressive autoimmune syndrome similar to that of scurfy mice, but does not affect thymic development, homeostatic expansion/maintenance or transforming-growth-factor-beta-induced de novo generation of Foxp3-expressing cells. The immune-suppressive activities of T cells with attenuated Foxp3 expression were nearly abolished in vitro and in vivo, whereas their anergic properties in vitro were maintained. This was accompanied by decreased expression of T(r) cell 'signature genes'. Notably, T cells expressing decreased Foxp3 preferentially became T-helper 2 (T(h)2)-type effectors even in a T(h)1-polarizing environment. These cells instructed T(h)2 differentiation of conventional T cells, which contributed to the immune diseases observed in these mice. Thus, decreased Foxp3 expression causes immune disease by subverting the suppressive function of T(r) cells and converting T(r) cells into effector cells; these findings are important for understanding the regulation of T(r) cell function and the aetiology of various human immune diseases.

Intricacies of TGF-ß signaling in Treg and Th17 cell biology.

Balanced immunity is pivotal for health and homeostasis. CD4+ helper T (Th) cells are central to the balance between immune tolerance and immune rejection. Th cells adopt distinct functions to maintain tolerance and clear pathogens. Dysregulation of Th cell function often leads to maladies, including autoimmunity, inflammatory disease, cancer, and infection. Regulatory T (Treg) and Th17 cells are critical Th cell types involved in immune tolerance, homeostasis, pathogenicity, and pathogen clearance. It is therefore critical to understand how Treg and Th17 cells are regulated in health and disease. Cytokines are instrumental in directing Treg and Th17 cell function. The evolutionarily conserved TGF-ß (transforming growth factor-ß) cytokine superfamily is of particular interest because it is central to the biology of both Treg cells that are predominantly immunosuppressive and Th17 cells that can be proinflammatory, pathogenic, and immune regulatory. How TGF-ß superfamily members and their intricate signaling pathways regulate Treg and Th17 cell function is a question that has been intensely investigated for two decades. Here, we introduce the fundamental biology of TGF-ß superfamily signaling, Treg cells, and Th17 cells and discuss in detail how the TGF-ß superfamily contributes to Treg and Th17 cell biology through complex yet ordered and cooperative signaling networks.