Smad2 positively regulates the generation of Th17 cells

Phosphorylation status determines the opposing functions of Smad2/Smad3 as STAT3 cofactors in TH17 differentiation

Transforming growth factor-β (TGF-β) and interleukin-6 (IL-6) are the pivotal cytokines to induce IL-17-producing CD4⁺ T helper cells (T_H17); yet their signalling network remains largely unknown. Here we show that the highly homologous TGF-β receptor-regulated Smads (R-Smads): Smad2 and Smad3 oppositely modify STAT3-induced transcription of IL-17A and retinoic acid receptor-related orphan nuclear receptor, RORγt encoded by Rorc, by acting as a co-activator and co-repressor of STAT3, respectively. Smad2 linker phosphorylated by extracellular signal-regulated kinase (ERK) at the serine 255 residue interacts with STAT3 and p300 to transactivate, whereas carboxy-terminal unphosphorylated Smad3 interacts with STAT3 and protein inhibitor of activated STAT3 (PIAS3) to repress the Rorc and Il17a genes. Our work uncovers carboxy-terminal phosphorylation-independent noncanonical R-Smad–STAT3 signalling network in T_H17 differentiation.

TGF-ß and IL-6 are the essential cytokines for mediating the differentiation of IL-17-producing CD4+ T helper cells (T_H17). Here, Yoon et al. provide more insights into this process and describe the opposing roles of TGFß-signalling intermediates Smad2 and Smad3 as STAT3 cofactors in Th17 differentiation.

The development and in vivo function of T helper 9 cells

The specialized cytokine secretion profiles of T helper (TH) cells are the basis for a focused and efficient immune response. On the twentieth anniversary of the first descriptions of the cytokine signals that promote the differentiation of interleukin-9 (IL-9)-secreting T cells, this Review focuses on the extracellular signals and the transcription factors that promote the development of what we now term TH9 cells, which are characterized by the production of this cytokine. We summarize our current understanding of the contribution of TH9 cells to both effective immunity and immunopathological disease, and we propose that TH9 cells could be targeted for the treatment of allergic and autoimmune disease.

The Active Form of Vitamin D Transcriptionally Represses Smad7 Signaling and Activates Extracellular Signal-regulated Kinase (ERK) to Inhibit the Differentiation of a Inflammatory T Helper Cell Subset and Suppress Experimental Autoimmune Encephalomyelitis

The ability of the active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), to transcriptionally modulate Smads to inhibit Th17 differentiation and experimental autoimmune encephalomyelitis (EAE) has not been adequately studied. This study reports modulation of Smad signaling by the specific binding of the VDR along with its heterodimeric partner RXR to the negative vitamin D response element on the promoter of Smad7, which leads to Smad7 gene repression. The vitamin D receptor-mediated increase in Smad3 expression partially explains the IL10 augmentation seen in Th17 cells. Furthermore, the VDR axis also modulates non-Smad signaling by activating ERK during differentiation of Th17 cells, which inhibits the Th17-specific genes il17a, il17f, il22, and il23r. In vivo EAE experiments revealed that, 1,25(OH)2D3 suppression of EAE correlates with the Smad7 expression in the spleen and lymph nodes. Furthermore, Smad7 expression also correlates well with IL17 and IFNγ expression in CNS infiltered inflammatory T cells. We also observed similar gene repression of Smad7 in in vitro differentiated Th1 cells when cultured in presence of 1,25(OH)2D3. The above canonical and non-canonical pathways in part address the ability of 1,25(OH)2D3-VDR to inhibit EAE.

Smad4 promotes differentiation of effector and circulating memory CD8 T cells but is dispensable for tissue-resident memory CD8 T cells

Tissue-resident memory CD8 T cells are a unique subset of virus-specific CTLs that bolster local immune responses after becoming lodged in previously infected tissues. These cells provide enhanced protection by intercepting returning pathogens before a new infection gets established. In contrast, central memory CD8 T cells circulate in the bloodstream and proliferate in secondary lymphoid organs before replenishing effector and memory CD8 T cell populations in remote parts of the body. Both populations of virus-specific memory CD8 T cells participate in immunity to influenza virus infection; however, the signaling pathways that instruct developing memory CD8 T cells to distribute to specific tissues are poorly defined. We show that TGF-β promotes the development of pulmonary tissue-resident memory T cells via a signaling pathway that does not require the downstream signaling intermediate Sma- and Mad-related protein (Smad)4. In contrast, circulating memory CD8 T cells have no requirement for TGF-β but show signs of arrested development in the absence of Smad4, including aberrant CD103 expression. These signaling pathways alter the distribution of virus-specific CTLs in the lungs but do not prevent robust cytokine responses. Our data show that Smad4 is required for normal differentiation of multiple subsets of virus-specific CD8 T cells. In normal circumstances, Smad4 may be activated via a pathway that bypasses the TGF-β receptor. Improved understanding of these signaling pathways could be used to augment vaccine-induced immunity.

Secretion of immunoregulatory cytokines by mesenchymal stem cells

According to the minimal criteria of the International Society of Cellular Therapy, mesenchymal stem cells (MSCs) are a population of undifferentiated cells defined by their ability to adhere to plastic surfaces when cultured under standard conditions, express a certain panel of phenotypic markers and can differentiate into osteogenic, chondrogenic and adipogenic lineages when cultured in specific inducing media. In parallel with their major role as undifferentiated cell reserves, MSCs have immunomodulatory functions which are exerted by direct cell-to-cell contacts, secretion of cytokines and/or by a combination of both mechanisms. There are no convincing data about a principal difference in the profile of cytokines secreted by MSCs isolated from different tissue sources, although some papers report some quantitative but not qualitative differences in cytokine secretion. The present review focuses on the basic cytokines secreted by MSCs as described in the literature by which the MSCs exert immunodulatory effects. It should be pointed out that MSCs themselves are objects of cytokine regulation. Hypothetical mechanisms by which the MSCs exert their immunoregulatory effects are also discussed in this review. These mechanisms may either influence the target immune cells directly or indirectly by affecting the activities of predominantly dendritic cells. Chemokines are also discussed as participants in this process by recruiting cells of the immune systems and thus making them targets of immunosuppression. This review aims to present and discuss the published data and the personal experience of the authors regarding cytokines secreted by MSCs and their effects on the cells of the immune system.

The Th17 pathway as a therapeutic target in rheumatoid arthritis and other autoimmune and inflammatory disorders

Production of the pro-inflammatory cytokine interleukin (IL)-17 by Th17 cells and other cells of the immune system protects the host against bacterial and fungal infections, but also promotes the development of rheumatoid arthritis (RA) and other autoimmune and inflammatory disorders. Several biologicals targeting IL-17, the IL-17 receptor, or IL-17-related pathways are being tested in clinical trials, and might ultimately lead to better treatment for patients suffering from various IL-17-mediated disorders. In this review, we provide a clear overview of current knowledge on Th17 cell regulation and the main Th17 effector cytokines in relation to IL-17-mediated conditions, as well as on recent IL-17-related drug developments. We demonstrate that targeting the Th17 pathway is a promising treatment for rheumatoid arthritis and various other autoimmune and inflammatory diseases. However, improvements in technical developments assisting in the identification of patients suffering from IL-17-driven disease are needed to enable the application of tailor-made, personalized medicine.

TGF-β activation and function in immunity

The cytokine TGF-β plays an integral role in regulating immune responses. TGF-β has pleiotropic effects on adaptive immunity, especially in the regulation of effector and regulatory CD4(+) T cell responses. Many immune and nonimmune cells can produce TGF-β, but it is always produced as an inactive complex that must be activated to exert functional effects. Thus, activation of latent TGF-β provides a crucial layer of regulation that controls TGF-β function. In this review, we highlight some of the important functional roles for TGF-β in immunity, focusing on its context-specific roles in either dampening or promoting T cell responses. We also describe how activation of TGF-β controls its function in the immune system, with a focus on the key roles for members of the integrin family in this process.

Activin receptor-like kinase5 inhibition suppresses mouse melanoma by ubiquitin degradation of Smad4, thereby derepressing eomesodermin in cytotoxic T lymphocytes

Varieties of transforming growth factor-β (TGF-β) antagonists have been developed to intervene with excessive TGF-β signalling activity in cancer. Activin receptor-like kinase5 (ALK5) inhibitors antagonize TGF-β signalling by blocking TGF-β receptor-activated Smad (R-Smad) phosphorylation. Here we report the novel mechanisms how ALK5 inhibitors exert a therapeutic effect on a mouse B16 melanoma model. Oral treatment with a novel ALK5 inhibitor, EW-7197 (2.5 mg/kg daily) or a representative ALK5 inhibitor, LY-2157299 (75 mg/kg bid) suppressed the progression of melanoma with enhanced cytotoxic T-lymphocyte (CTL) responses. Notably, ALK5 inhibitors not only blocked R-Smad phosphorylation, but also induced ubiquitin-mediated degradation of the common Smad, Smad4 mainly in CD8⁺ T cells in melanoma-bearing mice. Accordingly, T-cell-specific deletion of Smad4 was sufficient to suppress the progression of melanoma. We further identified eomesodermin (Eomes), the T-box transcription factor regulating CTL functions, as a specific target repressed by TGF-β via Smad4 and Smad3 in CD8⁺ T cells. Thus, ALK5 inhibition enhances anti-melanoma CTL responses through ubiquitin-mediated degradation of Smad4 in addition to the direct inhibitory effect on R-Smad phosphorylation.

Cutting edge: Smad2 and Smad4 regulate TGF-β-mediated Il9 gene expression via EZH2 displacement

IL-9 is a proallergic cytokine produced by a newly proposed Th cell subset, Th9. Th9 cells can be generated by treatment of naive T cells with TGF-β and IL-4 in vitro. However, it is still not clear how TGF-β signaling regulates Th9 differentiation. In this study, we demonstrate that Smad2 and Smad4, two transcriptional factors activated by TGF-β signaling, are required for Th9 differentiation in vitro. Deficiency of Smad2 or Smad4 in T cells resulted in impaired IL-9 expression, which was coincident with enrichment of repressive chromatin modification histone H3 K27 trimethylation and enhanced EZH2 binding to the Il9 locus. Pharmacologic inhibition of EZH2 partially rescued IL-9 production in Smad-deficient Th9 cells. Smad proteins may displace EZH2 directly from the Il9 locus, because Smad2 and Smad4 can bind EZH2. Our data shed light on the molecular mechanisms underlying Th9 cell differentiation, revealing that the TGF-β-Smad2/4-signaling pathway regulates IL-9 production through an epigenetic mechanism.

SMAD regulatory networks construct a balanced immune system

A balanced immune response requires combating infectious assaults while striving to maintain quiescence towards the self. One of the central players in this process is the pleiotropic cytokine transforming growth factor-β (TGF-β), whose deficiency results in spontaneous systemic autoimmunity in mice. The dominant function of TGF-β is to regulate the peripheral immune homeostasis, particularly in the microbe-rich and antigen-rich environment of the gut. To maintain intestinal integrity, the epithelial cells, myeloid cells and lymphocytes that inhabit the gut secrete TGF-β, which acts in both paracrine and autocrine fashions to activate its signal transducers, the SMAD transcription factors. The SMAD pathway regulates the production of IgA by B cells, maintains the protective mucosal barrier and promotes the balanced differentiation of CD4(+) T cells into inflammatory T helper type 17 cells and suppressive FOXP3(+) T regulatory cells. While encounters with pathogenic microbes activate SMAD proteins to evoke a protective inflammatory immune response, SMAD activation and synergism with immunoregulatory factors such as the vitamin A metabolite retinoic acid enforce immunosuppression toward commensal microbes and innocuous food antigens. Such complementary context-dependent functions of TGF-β are achieved by the co-operation of SMAD proteins with distinct dominant transcription activators and accessory chromatin modifiers. This review highlights recent advances in unravelling the molecular basis for the multi-faceted functions of TGF-β in the gut that are dictacted by fluid orchestrations of SMADs and their myriad partners.

The regulatory network that controls the differentiation of T lymphocytes

There is a vast amount of molecular information regarding the differentiation of T lymphocytes, in particular regarding in vitro experimental treatments that modify their differentiation process. This publicly available information was used to infer the regulatory network that controls the differentiation of T lymphocytes into CD4(+) and CD8(+) cells. Hereby we present a network that consists of 50 nodes and 97 regulatory interactions, representing the main signaling circuits established among molecules and molecular complexes regulating the differentiation of T cells. The network was converted into a continuous dynamical system in the form of a set of coupled ordinary differential equations, and its dynamical behavior was studied. With the aid of numerical methods, nine fixed point attractors were found for the dynamical system. These attractors correspond to the activation patterns observed experimentally for the following cell types: CD4(-)CD8(-), CD4(+)CD8(+), CD4(+) naive, Th1, Th2, Th17, Treg, CD8(+) naive, and CTL. Furthermore, the model is able to describe the differentiation process from the precursor CD4(-)CD8(-) to any of the effector types due to a specific series of extracellular signals.

SMAD regulatory networks construct a balanced immune system

A balanced immune response requires combating infectious assaults while striving to maintain quiescence towards the self. One of the central players in this process is the pleiotropic cytokine transforming growth factor-β (TGF-β), whose deficiency results in spontaneous systemic autoimmunity in mice. The dominant function of TGF-β is to regulate the peripheral immune homeostasis, particularly in the microbe-rich and antigen-rich environment of the gut. To maintain intestinal integrity, the epithelial cells, myeloid cells and lymphocytes that inhabit the gut secrete TGF-β, which acts in both paracrine and autocrine fashions to activate its signal transducers, the SMAD transcription factors. The SMAD pathway regulates the production of IgA by B cells, maintains the protective mucosal barrier and promotes the balanced differentiation of CD4(+) T cells into inflammatory T helper type 17 cells and suppressive FOXP3(+) T regulatory cells. While encounters with pathogenic microbes activate SMAD proteins to evoke a protective inflammatory immune response, SMAD activation and synergism with immunoregulatory factors such as the vitamin A metabolite retinoic acid enforce immunosuppression toward commensal microbes and innocuous food antigens. Such complementary context-dependent functions of TGF-β are achieved by the co-operation of SMAD proteins with distinct dominant transcription activators and accessory chromatin modifiers. This review highlights recent advances in unravelling the molecular basis for the multi-faceted functions of TGF-β in the gut that are dictacted by fluid orchestrations of SMADs and their myriad partners.

Retinoid and TGF-β families: crosstalk in development, neoplasia, immunity, and tissue repair

Transforming growth factor-β (TGF-β) isoforms are profibrotic cytokines, par excellence, and have complex multifunctional effects on many systems, depending on the biologic setting. Retinoids are vitamin A derivatives that also have diverse effects in development, physiology, and disease. The interactions between these classes of molecules are, not surprisingly, highly complex and are dependent on the tissue, cellular, and molecular settings.

Kidney transplantation: analysis of the expression and T cell-mediated activation of latent TGF-β

T cells activate latent TGF-β by an LSKL peptide-sensitive mechanism, suggesting a role for thrombospondin-1 in T cell differentiation after kidney transplantation.

Activated T cells infiltrate a renal allograft during rejection and can respond to TGF-β within the tubules, causing local differentiation and expression of the αE(CD103)β7 integrin. This study was performed to examine the expression of latent TGF-β within renal allograft tissues and to define a mechanism by which T cells can activate and respond to this latent factor. Rejecting renal allograft biopsy tissues showed increased expression of the latent TGF-β complex, which was localized around the tubules by a mechanism that might involve interaction with heparan sulfate in the basement membrane. A cultured renal TEC line also expressed the latent complex, but these cells did not respond to this form of TGF-β by pSmad 3. However, coculture of these cells with activated T cells induced the expression of CD103, suggesting that T cells can activate and respond to the latent TGF-β associated with TEC. Although activated T cells expressed little cell-surface TSP-1, this was increased by culture with fibronectin or fibronectin-expressing renal TEC. Blockade of TSP-1 using LSKL peptides reduced the potential of activated T cells to differentiate in response to latent TGF-β. This study suggests that penetration of renal tubules by activated T cells leads to increased expression of T cell-surface TSP-1, allowing activation of latent TGF-β sequestered on heparan sulfate within the microenvironment. This mechanism may be important for localized phenotypic maturation of T cells that have infiltrated the kidney during allograft rejection.

Epigenetics, autoimmunity and hematologic malignancies: a comprehensive review

The relationships between immunological dysfunction, loss of tolerance and hematologic malignancies have been a focus of attention in attempts to understand the appearance of a higher degree of autoimmune disease and lymphoma in children with congenital immunodeficiency. Although multiple hypotheses have been offered, it is clear that stochastic processes play an important role in the immunopathology of these issues. In particular, accumulating evidence is defining a role of epigenetic mechanisms as being critical in this continuous spectrum between autoimmunity and lymphoma. In this review, we focus attention predominantly on the relationships between T helper 17 (Th17) and T regulatory populations that alter local microenvironments and ultimately the expression or transcription factors involved in cell activation and differentiation. Abnormal expression in any of the molecules involved in Th17 and/or Treg development alter immune homeostasis and in genetically susceptible hosts may lead to the appearance of autoimmunity and/or lymphoma. These observations have clinical significance in explaining the discordance of autoimmunity in identical twins. They are also particularly important in the relationships between primary immune deficiency syndromes, immune dysregulation and an increased risk of lymphoma. Indeed, defining the factors that determine epigenetic alterations and their relationships to immune homeostasis will be a challenge greater or even equal to the human genome project.

Key molecules in the differentiation and commitment program of T helper 17 (Th17) cells up-to-date

The mechanisms underlying autoimmunity and cancer remain elusive. However, perpendicular evidence has been evolved in the past decade that T helper (Th)17 cells and their related molecules are implicated in initiation and induction of various disease settings including both diseases. Meanwhile, extensive research on Th17 cells elucidated various molecules including cytokines and transcription factors as well as signaling pathways involved in the differentiation, maturation, survival and ultimate commitment of Th17 cells. In the current review, we revise the mechanistic underpinnings delivered by recent research on these molecules in the Th17 differentiation/commitment concert. We emphasize on those molecules proposed as targets for attaining potential therapies of various autoimmune disorders and cancer, aiming both at dampening the dark-side of Th17 repertoire and simultaneously potentiating its benefits in the roster of the antimicrobial response.

The Wnt inhibitor secreted Frizzled-Related Protein 1 (sFRP1) promotes human Th17 differentiation

Wnt/β-catenin signaling plays a crucial role during embryogenesis and tumorigenesis, and in T cells, promotes the differentiation of Th2 cells. However, the role of Wnt signals in the differentiation and maintenance of human Th17 cells remains poorly understood. We found that the higher levels of IL-17 in the synovial fluid of rheumatoid arthritis (RA) patients compared with that of osteoarthritis (OA) patients were associated with a higher concentration of sFRP1 (secreted Frizzled-Related Protein 1), an inhibitor of the Wnt/β-catenin pathway. The addition of sFRP1 during TCR-mediated stimulation induced a significant increase in IL-17 production by both naïve and memory CD4(+) T cells. Moreover, under Th17-differentiation conditions, the addition of sFRP1 significantly reduced the requirement for TGF-β. Mechanistically, we observed that sFRP1 significantly enhanced the phosphorylation of Smad2/3 in CD4(+) T cells upon TGF-β stimulation and that blocking TGF-β signaling abolished the Th17-promoting activity of sFRP1. Our findings reveal a novel function for sFRP1 as a potent inducer of human Th17-cell differentiation. Consequently, sFRP1 may represent a promising target for the treatment of Th17-mediated disease in humans.

Complexities of TGF-β targeted cancer therapy

Many advanced tumors produce excessive amounts of Transforming Growth Factor-β (TGF-β) which, in normal epithelial cells, is a potent growth inhibitor. However, in oncogenically activated cells, the homeostatic action of TGF-β is often diverted along alternative pathways. Hence, TGF-β signaling elicits protective or tumor suppressive effects during the early growth-sensitive stages of tumorigenesis. However, later in tumor development when carcinoma cells become refractory to TGF-β-mediated growth inhibition, the tumor cell responds by stimulating pathways with tumor progressing effects. At late stages of malignancy, tumor progression is driven by TGF-β overload. The tumor microenvironment is a target of TGF-β action that stimulates tumor progression via pro-tumorigenic effects on vascular, immune, and fibroblastic cells. Bone is one of the richest sources of TGF-β in the body and a common site for dissemination of breast cancer metastases. Osteoclastic degradation of bone matrix, which accompanies establishment and growth of metastases, triggers further release of bone-derived TGF-β. This leads to a vicious positive feedback of tumor progression, driven by ever increasing levels of TGF-β released from both the tumor and bone matrix. It is for this reason, that pharmaceutical companies have developed therapeutic agents that block TGF-β signaling. Nonetheless, the choice of drug design and dosing strategy can affect the efficacy of TGF-β therapeutics. This review will describe pre-clinical and clinical data of four major classes of TGF-β inhibitor, namely i) ligand traps, ii) antisense oligonucleotides, iii) receptor kinase inhibitors and iv) peptide aptamers. Long term dosing strategies with TGF-β inhibitors may be ill-advised, since this class of drug has potentially highly pleiotropic activity, and development of drug resistance might potentiate tumor progression. Current paradigms for the use of TGF-β inhibitors in oncology have therefore moved towards the use of combinatorial therapies and short term dosing, with considerable promise for the clinic.

Th17 cells: biology, pathogenesis of autoimmune and inflammatory diseases, and therapeutic strategies

Th17 cells that secrete the cytokines IL-17A and IL-17F and express lineage-specific transcription factor RORC (RORγt in mice) represent a distinct lineage of CD4(+) T cells. Transforming growth factor-β and inflammatory cytokines, such as IL-6, IL-21, IL-1β, and IL-23, play central roles in the generation of Th17 cells. Th17 cells are critical for the clearance of extracellular pathogens, including Candida and Klebsiella. However, under certain conditions, these cells and their effector molecules, such as IL-17, IL-21, IL-22, GM-CSF, and CCL20, are associated with the pathogenesis of several autoimmune and inflammatory diseases, such as rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, psoriasis, inflammatory bowel disease, and allergy and asthma. This review discusses these disease states and the various therapeutic strategies under investigation to target Th17 cells, which include blocking the differentiation and amplification of Th17 cells, inhibiting or neutralizing the cytokines of Th17 cells, and suppressing the transcription factors specific for Th17 cells.

Two sides of the story? Smad4 loss in pancreatic cancer versus head-and-neck cancer

TGFβ signaling Smads (Smad2, 3, and 4) were suspected tumor suppressors soon after their discovery. Nearly two decades of research confirmed this role and revealed other divergent and cancer-specific functions including paradoxical tumor promotion effects. Although Smad4 is the most potent tumor suppressor, its functions are highly context-specific as exemplified by pancreatic cancer and head-and-neck cancer: in pancreatic cancer, Smad4 loss cannot initiate tumor formation but promotes metastases while in head-and-neck cancer Smad4 loss promotes cancer progression but also initiates tumor formation, likely through effects on genomic instability. The differing consequences of impaired Smad signaling in human cancers and the molecular mechanisms that underpin these differences will have important implications for the design and application of novel targeted therapies.

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

TGF-β modulates immune response by suppressing non-regulatory T (Treg) function and promoting Treg function. The question of whether TGF-β 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-β 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-β 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-β 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-β-activated kinase 1 led to failed Treg homeostasis and lethal immune disorder in mice. Therefore, Smad-dependent and -independent TGF-β signaling discretely controls non-Treg and Treg function to modulate immune tolerance and immune homeostasis.

T(H)17 cells in asthma and inflammation

BACKGROUND

The chronic airway disease asthma causes significant burden to patients as well as the healthcare system with limited options for prevention or cure. Inadequate treatment strategies are most likely due to the complex heterogeneous nature of asthma. Furthermore, the severe asthma phenotype is characterized by the lack of a response to standard medication, namely, corticosteroids.

SCOPE OF REVIEW

In the last several years it has been shown that the eosinophilic/atopic phenotype of asthma driven by T(H)2 mechanisms is not the only immunologic pathway contributing to disease. In fact, there has been evidence revealing that severe asthmatics in particular have neutrophilic inflammation, and this is associated with corticosteroid resistance. T(H)17 cells, a recently discovered lineage of T helper cells, play an important role in lung host defense against multiple pathogens via production of the cytokine IL-17. IL-17 promotes neutrophil production and chemotaxis via multiple factors.

MAJOR CONCLUSIONS

Mouse and human studies provide robust evidence that T(H)17 cells and IL-17 play a role in severe asthma and may contribute to corticosteroid resistance.

GENERAL SIGNIFICANCE

As we learn more about T(H)17 cells in severe asthma, the goal is to potentially target this pathway for treatment in the hope of significantly improving the quality of life for those children and adults affected with this disease. This article is part of a Special Issue entitled: Biochemistry of Asthma.

Coronin 1A is an essential regulator of the TGFβ receptor/SMAD3 signaling pathway in Th17 CD4(+) T cells

Transforming growth factor β (TGFβ) plays a central role in maintaining immune homeostasis by regulating the initiation and termination of immune responses and thus preventing the development of autoimmune diseases. In this study, we describe an essential mechanism by which the actin regulatory protein Coronin 1A (Coro1A) ensures the proper response of Th17 CD4(+) T cells to TGFβ. Coro1A has been established as a key player in T cell survival, migration, activation, and Ca(2+) regulation in naive T cells. We show that mice lacking Coro1a developed less severe experimental autoimmune encephalomyelitis (EAE). Unexpectedly, upon the re-induction of EAE, Coro1a(-/-) mice exhibited enhanced EAE signs that correlated with increased numbers of IL-17 producing CD4(+) cells in the central nervous system (CNS) compared to wild-type mice. In vitro differentiated Coro1a(-/-) Th17 CD4(+) T cells consistently produced more IL-17 than wild-type cells and displayed a Th17/Th1-like phenotype in regard to the expression of the Th1 markers T-bet and IFNγ. Mechanistically, the Coro1a(-/-) Th17 cell phenotype correlated with a severe defect in TGFβR-mediated SMAD3 activation. Taken together, these data provide experimental evidence of a non-redundant role of Coro1A in the regulation of Th17 CD4(+) cell effector functions and, subsequently, in the development of autoimmunity.

Krüppel-like factor 4 (KLF4) directly regulates proliferation in thymocyte development and IL-17 expression during Th17 differentiation

Krüppel-like factor 4 (KLF4), a transcription factor, plays a key role in the pluripotency of stem cells. We sought to determine the function of KLF4 in T-cell development and differentiation by using T-cell-specific Klf4-knockout (KO) mice. We found that KLF4 was highly expressed in thymocytes and mature T cells and was rapidly down-regulated in mature T cells after activation. In Klf4-KO mice, we observed a modest reduction of thymocytes (27%) due to the reduced proliferation of double-negative (DN) thymocytes. We demonstrated that a direct repression of Cdkn1b by KLF4 was a cause of decreased DN proliferation. During in vitro T-cell differentiation, we observed significant reduction of IL-17-expressing CD4(+) T cells (Th17; 24%) but not in other types of Th differentiation. The reduction of Th17 cells resulted in a significant attenuation of the severity (35%) of experimental autoimmune encephalomyelitis in vivo in Klf4-KO mice as compared with the Klf4 wild-type littermates. Finally, we demonstrated that KLF4 directly binds to the promoter of Il17a and positively regulates its expression. In summary, these findings identify KLF4 as a critical regulator in T-cell development and Th17 differentiation.

The altered expression of inflammation-related microRNAs with microRNA-155 expression correlates with Th17 differentiation in patients with acute coronary syndrome

MicroRNAs (miRNAs) are a novel class of small, non-coding RNAs that play a significant role in both inflammatory and cardiovascular diseases. Immune cells, especially T helper (Th) cells, are critical in the development of atherosclerosis and the onset of acute coronary syndrome (ACS). To assess whether inflammation-related miRNAs (such as miR-155, 146a, 21, 125a-5p, 125b, 31) are involved in the imbalance of Th cell subsets in patients with ACS, we measured the expression of related miRNAs in patients with acute myocardial infarction (AMI), unstable angina (UA), stable angina (SA) and chest pain syndrome (CPS); analyzed the relationship between miRNA expression and the frequency of Th cell subsets; and observed the co-expression of miR-155 and IL-17A in peripheral blood mononuclear cells (PBMCs) of patients with ACS. The results showed that the expression of miR-155 in the PBMCs of patients with ACS was decreased by approximately 60%, while the expression of both miR-21 and miR-146a was increased by approximately twofold. The expression patterns of miRNAs in plasma correlated with those in PBMCs, except for miR-21, which was increased by approximately sixfold in the AMI group and showed no significant difference between the UA group and the CPS group. We also found that the expression of miR-155 inversely correlated with the frequency of Th17 cells (r=-0.896, P<0.01) and that miR-155 was co-expressed with IL-17A in patients with ACS. In conclusion, our study revealed the expression patterns of inflammation-related miRNAs in patients with ACS and found that miR-155 may be associated with Th17 cell differentiation.

T helper 17 cell heterogeneity and pathogenicity in autoimmune disease

T helper (Th)17 cells have been proposed to represent a new CD4(+) T cell lineage that is important for host defense against fungi and extracellular bacteria, and the development of autoimmune diseases. Precisely how these cells arise has been the subject of some debate, with apparent species-specific differences in mice and humans. Here, we describe evolving views of Th17 specification, highlighting the contribution of transforming growth factor-β and the opposing roles of signal transducer and activator of transcription (STAT)3 and STAT5. Increasing evidence points to heterogeneity and inherent phenotypic instability in this subset. Ideally, better understanding of expression and action of key transcription factors and the epigenetic landscape of Th17 can help explain the flexibility and diversity of interleukin-17-producing cells.

Genetic controls of Th17 cell differentiation and plasticity

CD4+ T lymphocytes play a major role in regulation of adaptive immunity. Upon activation, naïve T cells differentiate into different functional subsets. In addition to the classical Th1 and Th2 cells, several novel effector T cell subsets have been recently identified, including Th17 cells. There has been rapid progress in characterizing the development and function of Th17 cells. Here I summarize and discuss on the genetic controls of their differentiation and emerging evidence on their plasticity. This information may benefit understanding and treating immune diseases.

The kinases MEKK2 and MEKK3 regulate transforming growth factor-β-mediated helper T cell differentiation

Mitogen-activated protein kinases (MAPKs) are key mediators of the T cell receptor (TCR) signals but their roles in T helper (Th) cell differentiation are unclear. Here we showed that the MAPK kinase kinases MEKK2 (encoded by Map3k2) and MEKK3 (encoded by Map3k3) negatively regulated transforming growth factor-β (TGF-β)-mediated Th cell differentiation. Map3k2(-/-)Map3k3(Lck-Cre/-) mice showed an abnormal accumulation of regulatory T (Treg) and Th17 cells in the periphery, consistent with Map3k2(-/-)Map3k3(Lck-Cre/-) naive CD4(+) T cells' differentiation into Treg and Th17 cells with a higher frequency than wild-type (WT) cells after TGF-β stimulation in vitro. In addition, Map3k2(-/-)Map3k3(Lck-Cre/-) mice developed more severe experimental autoimmune encephalomyelitis. Map3k2(-/-)Map3k3(Lck-Cre/-) T cells exhibited impaired phosphorylation of SMAD2 and SMAD3 proteins at their linker regions, which negatively regulated the TGF-β responses in T cells. Thus, the crosstalk between TCR-induced MAPK and the TGF-β signaling pathways is important in regulating Th cell differentiation.

Signal transduction pathways and transcriptional regulation in Th17 cell differentiation

Over the last decade, our understanding of helper/effector T cell differentiation has changed dramatically. The discovery of interleukin (IL-)17-producing T cells (Th17) and other subsets has changed our view of T cell-mediated immunity. Characterization of the signaling pathways involved in the Th17 commitment has provided exciting new insights into the differentiation of CD4+ T cells. Importantly, the emerging data on conversion among polarized T helper cells have raised the question how we should view such concepts as T cell lineage commitment, terminal differentiation and plasticity. In this review, we will discuss the current understanding of the signaling pathways, molecular interactions, and transcriptional and epigenetic events that contribute to Th17 differentiation and acquisition of effector functions.