TGF-β in tolerance, development and regulation of immunity

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The broader superfamily of TGF-β-like proteins is reviewed, and signaling pathways summarised.

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The role of TGF-β in the immune tolerance and control of infectious disease is discussed.

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The superfamily member AMH is involved in embryonic sexual differentiation.

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Helminth parasites appear to exploit the TGF-β pathway to suppress host immunity.

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TGF-β homologues and mimics from parasites offer a new route for therapeutic tolerance induction.

The TGF-β superfamily is an ancient metazoan protein class which cuts across cell and tissue differentiation, developmental biology and immunology. Its many members are regulated at multiple levels from intricate control of gene transcription, post-translational processing and activation, and signaling through overlapping receptor structures and downstream intracellular messengers. We have been interested in TGF-β homologues firstly as key players in the induction of immunological tolerance, the topic so closely associated with Ray Owen. Secondly, our interests in how parasites may manipulate the immune system of their host has also brought us to study the TGF-β pathway in infections with longlived, essentially tolerogenic, helminth parasites. Finally, within the spectrum of mammalian TGF-β proteins is an exquisitely tightly-regulated gene, anti-Müllerian hormone (AMH), whose role in sex determination underpins the phenotype of freemartin calves that formed the focus of Ray’s seminal work on immunological tolerance.

Transforming growth factor-β superfamily in obstructive lung diseases. more suspects than TGF-β alone

Asthma and chronic obstructive pulmonary disease are respiratory disorders and a major global health problem with increasing incidence and severity. Genes originally associated with lung development could be relevant in the pathogenesis of chronic obstructive pulmonary disease/asthma, owing to either an early-life origin of adult complex diseases or their dysregulation in adulthood upon exposure to environmental stressors (e.g., smoking). The transforming growth factor (TGF)-β superfamily is conserved through evolution and is involved in a range of biological processes, both during development and in adult tissue homeostasis. TGF-β1 has emerged as an important regulator of lung and immune system development. However, considerable evidence has been presented for a role of many of the other ligands of the TGF-β superfamily in lung pathology, including activins, bone morphogenetic proteins, and growth differentiation factors. In this review, we summarize the current knowledge on the mechanisms by which activin, bone morphogenetic protein, and growth differentiation factor signaling contribute to the pathogenesis of obstructive airway diseases.

Bone Morphogenetic Protein Signaling Regulates Development and Activation of CD4(+) T Cells

Bone morphogenetic proteins (BMPs) are growth factors belonging to the TGF-β (transforming growth factor β) superfamily. BMPs were found to regulate multiple cell processes such as proliferation, survival, differentiation, and apoptosis. They were originally described to play a pivotal role in inducing bone, cartilage, ligament, and tendon formation at both heterotopic and orthotopic sites but were found to play a significant role in embryogenesis and development of multiple tissues and organs. Activities of BMPs are regulated by a number of secreted proteins, which modulate their availability to bind cellular receptors. The functions of individual BMPs are highly redundant due to binding the same receptors and inducing overlapping signal transduction pathways. Recently, BMPs were found to regulate cells of the innate and adaptive immune system. BMPs are involved in thymic development of T cells at the early, double negative, as well as later, double positive, stages of thymopoesis. They specifically modulate thymic development of regulatory T cells (T(reg)). In the periphery, BMPs affect T cell activation, promoting generation of T(reg) cells. We found that mice deficient for one of the receptors activated by BMPs demonstrated slower growth of transplantable melanoma tumors.

The TGF-β superfamily in dendritic cell biology

The TGF-β superfamily consists of a large group of pleiotropic cytokines that are involved in the regulation of many developmental, physiological and pathological processes. Dendritic cells are antigen-presenting cells that play a key role in innate and adaptive immune responses. Dendritic cells have a complex relationship with the TGF-β cytokine superfamily being both source and targets for many of these cytokines. Some TGF-β family members are expressed by dendritic cells and modulate immune responses, for instance through the induction of T cell polarization. Others play a crucial role in the development and function of the different dendritic cell subsets. This review summarizes the current knowledge on the role of TGF-β family cytokines in dendritic cell biology, focusing on TGF-β as well as on other, less characterized, members of these important immune mediators.

Activin A contributes to the development of hyperoxia-induced lung injury in neonatal mice

BACKGROUND

Bronchopulmonary dysplasia (BPD) is one of the leading causes of morbidity and mortality in babies born prematurely, yet there is no curative treatment. In recent years, a number of inhibitors against TGFβ signaling have been tested for their potential to prevent neonatal injury associated with hyperoxia, which is a contributing factor of BPD. In this study, we assessed the contribution of activin A-a member of the TGFβ superfamily-to the development of hyperoxia-induced lung injury in neonatal mice.

METHODS

We placed newborn C57Bl6 mouse pups in continuous hyperoxia (85% O2) to mimic many aspects of BPD including alveolar simplification and pulmonary inflammation. The pups were administered activin A receptor type IIB-Fc antagonist (ActRIIB-Fc) at 5 mg/kg or follistatin at 0.1 mg/kg on postnatal days 4, 7, 10, and 13.

RESULTS

Treatment with ActRIIB-Fc and follistatin protected against hyperoxia-induced growth retardation. ActRIIB-Fc also reduced pulmonary leukocyte infiltration, normalized tissue: airspace ratio and increased septal crest density. These findings were associated with reduced phosphorylation of Smad3 and decreased matrix metalloproteinase (MMP)-9 activity.

CONCLUSION

This study suggests that activin A signaling may contribute to the pathology of bronchopulmonary dysplasia.

Nanoparticle-based autoantigen delivery to Treg-inducing liver sinusoidal endothelial cells enables control of autoimmunity in mice

BACKGROUND & AIMS

It is well-known that the liver can induce immune tolerance, yet this knowledge could, thus far, not be translated into effective treatments for autoimmune diseases. We have previously shown that liver sinusoidal endothelial cells (LSECs) could substantially contribute to hepatic tolerance through their ability to induce CD4+ Foxp3+ regulatory T cells (Tregs). Here, we explored whether the Treg-inducing potential of LSECs could be harnessed for the treatment of autoimmune disease.

METHODS

We engineered a polymeric nanoparticle (NP) carrier for the selective delivery of autoantigen peptides to LSECs in vivo. In the well-characterized autoimmune disease model of experimental autoimmune encephalomyelitis (EAE), we investigated whether administration of LSEC-targeting autoantigen peptide-loaded NPs could protect mice from autoimmune disease.

RESULTS

We demonstrate that NP-based autoantigen delivery to LSECs could completely and permanently prevent the onset of clinical EAE. More importantly, in a therapeutic approach, mice with already established EAE improved rapidly and substantially following administration of a single dose of autoantigen peptide-loaded NPs, whereas the control group deteriorated. Treatment efficacy seemed to depend on Tregs. The Treg frequencies in the spleens of mice treated with autoantigen peptide-loaded NPs were significantly higher than those in vehicle-treated mice. Moreover, NP-mediated disease control was abrogated after Treg depletion by repeated administration of Treg-depleting antibody.

CONCLUSION

Our findings provide proof of principle that the selective delivery of autoantigen peptides to LSECs by NPs can induce antigen-specific Tregs and enable effective treatment of autoimmune disease. These findings highlight the importance of Treg induction by LSECs for immune tolerance.

Inhibins tune the thymocyte selection process by regulating thymic stromal cell differentiation

Inhibins and Activins are members of the TGF-β superfamily that regulate the differentiation of several cell types. These ligands were initially identified as hormones that regulate the hypothalamus-pituitary-gonadal axis; however, increasing evidence has demonstrated that they are key regulators in the immune system. We have previously demonstrated that Inhibins are the main Activin ligands expressed in the murine thymus and that they regulate thymocyte differentiation, promoting the DN3-DN4 transition and the selection of SP thymocytes. As Inhibins are mainly produced by thymic stromal cells, which also express Activin receptors and Smad proteins, we hypothesized that Inhibins might play a role in stromal cell differentiation and function. Here, we demonstrate that, in the absence of Inhibins, thymic conventional dendritic cells display reduced levels of MHC Class II (MHCII) and CD86. In addition, the ratio between cTECs and mTECs was affected, indicating that mTEC differentiation was favoured and cTEC diminished in the absence of Inhibins. These changes appeared to impact thymocyte selection leading to a decreased selection of CD4SP thymocytes and increased generation of natural regulatory T cells. These findings demonstrate that Inhibins tune the T cell selection process by regulating both thymocyte and stromal cell differentiation.

Intertwining of Activin A and TGFβ Signaling: Dual Roles in Cancer Progression and Cancer Cell Invasion

In recent years, a significant amount of research has examined the controversial role of activin A in cancer. Activin A, a member of the transforming growth factor β (TGFβ) superfamily, is best characterized for its function during embryogenesis in mesoderm cell fate differentiation and reproduction. During embryogenesis, TGFβ superfamily ligands, TGFβ, bone morphogenic proteins (BMPs) and activins, act as potent morphogens. Similar to TGFβs and BMPs, activin A is a protein that is highly systemically expressed during early embryogenesis; however, post-natal expression is overall reduced and remains under strict spatiotemporal regulation. Of importance, normal post-natal expression of activin A has been implicated in the migration and invasive properties of various immune cell types, as well as endometrial cells. Aberrant activin A signaling during development results in significant morphological defects and premature mortality. Interestingly, activin A has been found to have both oncogenic and tumor suppressor roles in cancer. Investigations into the role of activin A in prostate and breast cancer has demonstrated tumor suppressive effects, while in lung and head and neck squamous cell carcinoma, it has been consistently shown that activin A expression is correlated with increased proliferation, invasion and poor patient prognosis. Activin A signaling is highly context-dependent, which is demonstrated in studies of epithelial cell tumors and the microenvironment. This review discusses normal activin A signaling in comparison to TGFβ and highlights how its dysregulation contributes to cancer progression and cell invasion.

TGF-β-dependent induction of CD4⁺CD25⁺Foxp3⁺ Tregs by liver sinusoidal endothelial cells

BACKGROUND & AIMS

CD4(+) CD25(+) Foxp3(+) regulatory T cells (Tregs) have a profound ability to control immune responses. We have previously shown that the liver is a major source of peripherally induced Tregs. Here, we investigate the liver cell types and molecular mechanisms responsible for hepatic Treg induction.

METHODS

To assess the Treg-inducing potential of liver resident antigen-presenting cell types, we studied the conversion of Foxp3(-) non-Tregs into Foxp3(+) Tregs induced by liver dendritic cells (DCs), liver sinusoidal endothelial cells (LSECs), or Kupffer cells (KCs). The dependency of Treg induction on TGF-β was tested in Treg conversion assays using T cells with reduced TGF-β sensitivity. The suppressive potential of liver cell-induced Tregs was assessed by an in vitro suppression assay and in vivo, in the model of experimental autoimmune encephalomyelitis (EAE).

RESULTS

All tested liver cell types were capable of inducing Foxp3(+) Tregs; however, LSECs were most efficient in inducing Tregs. Treg-induction was antigen-specific and depended on TGF-β. LSECs featured membrane-bound LAP/TGF-β and the anchor molecule GARP, which is required for tethering LAP/TGF-β to the cell membrane. LSEC-induced Tregs suppressed proliferation and cytokine secretion of effector T cells in vitro. LSEC-induced Tregs were also functional suppressors in vivo, as neuroantigen-specific Tregs induced by LSECs were able to suppress EAE.

CONCLUSIONS

We demonstrate that LSECs are the major liver cell type responsible for TGF-β dependent hepatic Treg induction. The extraordinary capacity of LSECs to induce Tregs was associated with their unique ability to tether TGF-β to their membrane.

Mouse Sertoli cells sustain de novo generation of regulatory T cells by triggering the notch pathway through soluble JAGGED1

FOXP3(+) regulatory T cells (Tregs) are central to the maintenance of immunological homeostasis and tolerance. It has long been known that Sertoli cells are endowed with immune suppressive properties; however, the underlying mechanisms as well as the effective nature and role of soluble factors secreted by Sertoli cells have not been fully elucidated as yet. We hypothesized that conditioned medium from primary mouse Sertoli cells (SCCM) may be able and sufficient to induce Tregs. By culturing CD4(+)CD25(-)EGFP(-) T splenocytes purified from FOXP3-EGFP knock-in mice in SCCM, here we show, by flow cytometry and suppression assay, the conversion of peripheral CD4(+)FOXP3(-) T cells into functional CD4(+)FOXP3(+) Tregs. We also demonstrate that the Notch/Jagged1 axis is involved in regulating the de novo generation of Tregs although this process is transforming growth factor-beta1 (TGF-B) dependent. In particular, we identified by Western blot analysis a soluble form of JAGGED1 (JAG1) in SCCM that significantly influences the induction of Tregs, as demonstrated by performing the conversion assay in presence of a JAG1-specific neutralizing antibody. In addition, we show that SCCM modulates the Notch pathway in converted Tregs by triggering the recruitment of the Notch-specific transcription factor CSL/RBP-Jk to the Foxp3 promoter and by inducing the Notch target gene Hey1, as shown by chromatin immunoprecipitation assay and by real time-RT-PCR experiments, respectively. Overall, these results contribute to a better understanding of the molecular mechanisms involved in Sertoli cell-mediated immune tolerance and provide a novel approach to generate ex vivo functional Tregs for therapeutic purpose.

Modulation of bone morphogenic protein signaling in T-cells for cancer immunotherapy

Immunotherapy is becoming an increasingly attractive therapeutic alternative for conventional cancer therapy. In recent years Foxp3(+) regulatory T-cells (T(R)) were identified as the major obstacle to effective cancer immunotherapy. The abundance of these cells in peripheral blood is increased in patients with multiple types of cancer and their prevalence among tumor-infiltrating lymphocytes correlated with poor clinical prognosis. In contrast, removal or inactivation of T(R) cells led to enhanced anti-tumor immune response and better efficacy of cancer vaccines. This study reports that Bone Morphogenic Protein Receptor 1α (BMPR1α, Alk-3) is expressed by activated effector CD4(+) and T(R) cells and modulates functions of both cell types. Bone Morphogenic Proteins (BMPs) belong to the transforming growth factor (TGF)-β family of cytokines that also include TGFβ and activins. BMPs play crucial roles in embryonic development, tissue differentiation and homeostasis, and development of cancer. It was demonstrated that BMPs and activins synergize with TGFβ to regulate thymic T-cell development, maintain T(R) cells, and control peripheral tolerance. Inactivation of BMPR1α in T-cells results in impaired thymic and peripheral generation of T(R) cells. BMPR1α-deficient activated T-cells produced a higher level of interferon (IFN)-γ than BMPR1α-sufficient T-cells. Moreover, transplanted B16 melanoma tumors grew smaller in mice lacking expression of BMPR1α in T-cells and tumors had few infiltrating TR cells and a higher proportion of CD8(+) T-cells than wild-type mice.

Protein-bound polysaccharide-K reduces the proportion of regulatory T cells in vitro and in vivo

Regulatory T cells (Tregs) play an important role in maintaining immunological tolerance. However, this mechanism is one of the major obstacles to overcome when attempting to improve antitumor immunity. Protein-bound polysaccharide‑K (PSK) has been used clinically as an antitumor drug, and one of its antitumor mechanisms involves improvement of the tumor-induced immunosuppressive state. Therefore, we investigated whether PSK affects Tregs in vitro and in vivo. In the in vitro study, CD4⁺CD25⁻ cells were separated from normal mouse spleen and cultured with or without PSK in the presence of TGF-β. Although TGF-β induced CD4⁺CD25⁺Foxp3⁺ Tregs, PSK reduced the proportion of TGF-β-induced Tregs. In the in vivo study, BALB/c mice were injected subcutaneously with methylcholanthrene-induced fibrosarcoma (Meth A) cells on day 0, and were administered PSK (50 mg/kg) intraperitoneally from day 1, three times per week. After 4 weeks, the tumor volume, the proportion of Tregs and the CD8+/Treg ratio in the spleen, plasma TGF-β concentration, and IFN-γ production by spleen cells were measured. PSK significantly reduced tumor growth, the proportion of Tregs in the spleen and the plasma TGF-β concentration, and significantly increased the CD8+/Treg ratio in the spleen and IFN-γ production by spleen cells. The reduction of the TGF-β concentration in blood by PSK appears to decrease the proportion of Tregs in lymphoid organs and to augment antitumor immunity.

Activin a inhibits antigen-induced allergy in murine epicutaneous sensitization

Activin A, a member of the TGFβ superfamily, is involved in physiological processes such as cell differentiation, tissue homeostasis, wound healing, reproduction, and in pathological conditions, such as fibrosis, cancer, and asthma. Activin enhances mast cell maturation, as well as regulatory T-cell and Langerhans cell differentiation. In this study we investigated the potential role of activin in epicutaneous sensitization with ovalbumin (OVA), notably with respect to its effect on known Th2-polarization. For this purpose, transgenic mice overexpressing activin in keratinocytes and their wild-type (WT) controls were sensitized epicutaneously with OVA. Skin biopsies were analyzed with regard to histopathological features and mRNA expression of pro-inflammatory and Th1/Th2 cytokines, and Ig levels were measured in the serum. Unexpectedly, activin overexpressing animals were protected from Th2-cytokine expression and induction of OVA-specific IgE levels compared to WT animals. On the other hand, transgenic mice were more susceptible to inflammation compared to WT littermates after tape-stripping and saline (vehicle) or OVA application, as shown by increased pro-inflammatory cytokine mRNA levels and neutrophil accumulation at the site of the treatment. We conclude that activin protects from antigen-induced cutaneous Th2-polarization through modulation of the immune response. These findings highlight the role of activin in cutaneous sensitization, allergy, and in skin homeostasis.

The activins and their binding protein, follistatin-Diagnostic and therapeutic targets in inflammatory disease and fibrosis

The activins, as members of the transforming growth factor-β superfamily, are pleiotrophic regulators of cell development and function, including cells of the myeloid and lymphoid lineages. Clinical and animal studies have shown that activin levels increase in both acute and chronic inflammation, and are frequently indicators of disease severity. Moreover, inhibition of activin action can reduce inflammation, damage, fibrosis and morbidity/mortality in various disease models. Consequently, activin A and, more recently, activin B are emerging as important diagnostic tools and therapeutic targets in inflammatory and fibrotic diseases. Activin antagonists such as follistatin, an endogenous activin-binding protein, offer considerable promise as therapies in conditions as diverse as sepsis, liver fibrosis, acute lung injury, asthma, wound healing and ischaemia-reperfusion injury.

Activin, neutrophils, and inflammation: just coincidence?

During the 26 years that have elapsed since its discovery, activin-A, a member of the transforming growth factor β super-family originally discovered from its capacity to stimulate follicle-stimulating hormone production by cultured pituitary gonadotropes, has been established as a key regulator of various fundamental biological processes, such as development, homeostasis, inflammation, and tissue remodeling. Deregulated expression of activin-A has been observed in several human diseases characterized by an immuno-inflammatory and/or tissue remodeling component in their pathophysiology. Various cell types have been recognized as sources of activin-A, and plentiful, occasionally contradicting, functions have been described mainly by in vitro studies. Not surprisingly, both harmful and protective roles have been postulated for activin-A in the context of several disorders. Recent findings have further expanded the functional repertoire of this molecule demonstrating that its ectopic overexpression in mouse airways can cause pathology that simulates faithfully human acute respiratory distress syndrome, a disorder characterized by strong involvement of neutrophils. This finding when considered together with the recent discovery that neutrophils constitute an important source of activin-A in vivo and earlier observations of upregulated activin-A expression in diseases characterized by strong activation of neutrophils may collectively imply a more intimate link between activin-A expression and neutrophil reactivity. In this review, we provide an outline of the functional repertoire of activin-A and suggest that this growth factor functions as a guardian of homeostasis, a modulator of immunity and an orchestrator of tissue repair activities. In this context, a relationship between activin-A and neutrophils may be anything but coincidental.

The bright and the dark sides of activin in wound healing and cancer

Activin was initially described as a protein that stimulates release of follicle stimulating hormone from the pituitary, and it is well known for its important roles in different reproductive functions. In recent years, this multifunctional factor has attracted the attention of researchers in other fields, as new functions of activin in angiogenesis, inflammation, immunity, fibrosis and cancer have been discovered. Studies from our laboratory have identified activin as a crucial regulator of wound healing and skin carcinogenesis. On the one hand, it strongly accelerates the healing process of skin wounds but, on the other hand, it enhances scar formation and the susceptibility to skin tumorigenesis. Finally, results from several laboratories have revealed that activin enhances tumour formation and/or progression in some other organs, in particular through its effect on the tumour microenvironment, and that it also promotes cancer-induced bone disruption and muscle wasting. These findings provide the basis for the use of activin or its downstream targets for the improvement of impaired wound healing, and of activin antagonists for the prevention and treatment of fibrosis and of malignant tumours that overexpress activin. Here, we summarize the previously described roles of activin in wound healing and scar formation and discuss functional studies that revealed different functions of activin in the pathogenesis of cancer. The relevance of these findings for clinical applications will be highlighted.

Regulation of activin and inhibin in the adult testis and the evidence for functional roles in spermatogenesis and immunoregulation

Activin A provides a unique link between reproduction and immunity, which is especially significant in the adult testis. This cytokine, together with inhibin B and follistatin acting as regulators of activin A activity, is fundamentally involved in the regulation of spermatogenesis and testicular steroidogenesis. However, activin A also has a much broader role in control of inflammation, fibrosis and immunity. In the Sertoli cell, activin A is regulated by signalling pathways that normally regulate stress and inflammation, signalling pathways that intersect with the classical hormonal regulatory pathways mediated by FSH. Modulation of activin A production and activity during spermatogenesis is implicated in the fine control of the cycle of the seminiferous epithelium. The immunoregulatory properties of activin A also suggest that it may be involved in maintaining testicular immune privilege. Consequently, elevated activin A production within the testis during inflammation and infection may contribute to spermatogenic failure, fibrosis and testicular damage.

The roles of activin A and its binding protein, follistatin, in inflammation and tissue repair

Activin A, a member of the transforming growth factor-β superfamily of cytokines, is a critical controller of inflammation, immunity and fibrosis. It is rapidly released into the blood following a lipopolysaccharide challenge in experimental animals, through activation of the Toll-like receptor 4 signalling pathway. Blocking activin action by pre-treatment with its binding protein, follistatin, modifies the inflammatory cytokine cascade, and reduces the severity of the subsequent inflammatory response and mortality. Likewise, high serum levels of activin A are predictive of death in patients with septicaemia. However, activin A has complex immunomodulatory actions. It is produced by inflammatory macrophages, but can regulate either pro- or anti-inflammatory responses in these cells, depending on their prior activation status. Activin A is also produced by Th2 cells, and stimulates antibody production by B cells and the development of regulatory T cells. Production of activin A during inflammatory responses stimulates fibrosis and tissue remodelling, and follistatin inhibits these actions of activin A. The modulation of activin by follistatin may represent an important therapeutic target for the modulation and amelioration of inflammatory and fibrotic disorders.

Role of ovarian hormones in T-cell homeostasis: from the thymus to the periphery

The study explored the putative role of ovarian hormones in the peripubertal remodelling of peripheral T-cell compartment. Ovariectomy at age of 1 month enhanced the peripubertal rise in CD4+ and CD8+ cell numbers in peripheral blood (PB) and spleen from 2-month-old rats. This reflected maintenance of thymopoietic efficiency at the prepubertal level (judging by numbers of the most mature CD4+ and CD8+ thymocytes and recent thymic emigrants) and alterations in T-cell survival/proliferation in the periphery. Compared with age-matched controls, the frequency of apoptotic cells among CD8+ peripheral blood lymphocytes (PBLs) and CD4+ and CD8+ splenocytes was diminished in ovariectomized (Ox) rats, at least partly, due to lower CD95 surface density. The diminished frequency of the apoptotic T splenocytes could also be associated with the rise in the amount of splenic IL-7 mRNA. Additionally, the latter finding was consistent with the augmented proliferation of CD4+ and CD8+ splenocytes. However, the enhanced proliferation of these cells could also be linked to the rise in IL-2 receptor surface density. This increase was related to the enhanced splenic TNF-α mRNA expression. Additionally, ovariectomy led to the phenotypic alterations in the major PBL and splenic T-cell subsets by diminishing/preventing the peripubertal changes in the frequency of cells at distinct stages of post-thymic differentiation/maturation (recent thymic emigrants, mature naïve and memory cells), and by decreasing the frequency of NKT cells within peripheral CD8+ subsets. In addition to numerical and phenotypic changes in T-cell compartment (due to the lack of ovarian hormone action at both the thymic and peripheral level), Ox rats exhibited a much larger delayed-type hypersensitivity (DTH) response compared with age-matched controls. This suggested the augmented T-cell-mediated immune response in Ox rats compared with aged-matched controls.

Activin A and TGF-β promote T(H)9 cell-mediated pulmonary allergic pathology

BACKGROUND

IL-9-secreting (T(H)9) T cells are thought to represent a distinct T-cell subset. However, evidence for their functionality in disease is uncertain.

OBJECTIVE

To define a functional phenotype for T(H)9-driven pathology in vivo.

METHODS

We used fluorescence-activated cell sorting to identify circulating T(H)9 cells in atopic and nonatopic subjects. In mice we utilized a model of allergic airways disease induced by house dust mite to determine T(H)9 cell function in vivo and the role of activin A in T(H)9 generation.

RESULTS

Allergic patients have elevated T(H)9 cell numbers in comparison to nonatopic donors, which correlates with elevated IgE levels. In a murine model, allergen challenge with house dust mite leads to rapid T(H)9 differentiation and proliferation, with much faster kinetics than for T(H)2 cell differentiation, resulting in the specific recruitment and activation of mast cells. The TGF-β superfamily member activin A replicates the function of TGF-β1 in driving the in vitro generation of T(H)9 cells. Importantly, the in vivo inhibition of T(H)9 differentiation induced by allergen was achieved only when activin A and TGF-β were blocked in conjunction but not alone, resulting in reduced airway hyperreactivity and collagen deposition. Conversely, adoptive transfer of T(H)9 cells results in enhanced pathology.

CONCLUSION

Our data identify a distinct functional role for T(H)9 cells and outline a novel pathway for their generation in vitro and in vivo. Functionally, T(H)9 cells promote allergic responses resulting in enhanced pathology mediated by the specific recruitment and activation of mast cells in the lungs.

Activin enhances skin tumourigenesis and malignant progression by inducing a pro-tumourigenic immune cell response

Activin is an important orchestrator of wound repair, but its potential role in skin carcinogenesis has not been addressed. Here we show using different types of genetically modified mice that enhanced levels of activin in the skin promote skin tumour formation and their malignant progression through induction of a pro-tumourigenic microenvironment. This includes accumulation of tumour-promoting Langerhans cells and regulatory T cells in the epidermis. Furthermore, activin inhibits proliferation of tumour-suppressive epidermal γδ T cells, resulting in their progressive loss during tumour promotion. An increase in activin expression was also found in human cutaneous basal and squamous cell carcinomas when compared with control tissue. These findings highlight the parallels between wound healing and cancer, and suggest inhibition of activin action as a promising strategy for the treatment of cancers overexpressing this factor.

Activin is known to have a role in wound healing, but its role in skin cancer is unknown. Antsiferova et al. show that activin is elevated in human skin tumours, and by modulating epidermal immune cells, exacerbates tumour progression in a mouse model of skin cancer.

Mechanisms underlying the induction of regulatory T cells and its relevance in the adaptive immune response in parasitic infections

To fulfill its function, the immune system must detect and interpret a wide variety of signals and adjust the magnitude, duration, and specific traits of each response during the complex host-parasite relationships in parasitic infections. Inflammation must be tightly regulated since uncontrolled inflammation may be as destructive as the triggering stimulus and leads to immune-mediated tissue injury.

During recent years, increasing evidence points to regulatory T cells (Tregs) as key anti-inflammatory cells, critically involved in limiting the inflammatory response. Herein, we review the published information on the induction of Tregs and summarize the most recent findings on Treg generation in parasitic diseases.

Aurintricarboxylic acid promotes the conversion of naive CD4+CD25- T cells into Foxp3-expressing regulatory T cells

Naive peripheral CD4(+)CD25(-) T cells can be converted into Foxp3-expressing regulatory T cells under appropriate stimulation conditions. Considering that continuous exposure to antigens is one of the prerequisites for the differentiation and maintenance of Treg cells, we investigated whether preventing activation-induced cell death while providing continuous TCR stimulation could promote the expression of Foxp3 in murine naive CD4(+) T cells. Among the several anti-apoptotic agents tested, aurintricarboxylic acid (ATA) was found to induce the in vitro conversion of naive CD4(+) T cells into Foxp3(+) Treg cells with suppressive activity. Neutralizing studies with an antibody against transforming growth factor (TGF)-β revealed that ATA requires the presence of TGF-β to induce Foxp3 expression in naive CD4(+)CD25(-) T cells. Although ATA itself did not activate the Smad signaling pathway, it down-regulated the extracellular signal-regulated kinase and mammalian target of rapamycin signaling cascade in activated T cells. Lastly, combined exposure to ATA and TGF-β had a synergistic effect on the rate of induction and maintenance of Foxp3 expression. These results indicate that ATA could be exploited to efficiently prepare inducible regulatory T cells in vitro and may aid in more precisely identifying the specific signaling pathways that drive Foxp3 expression in T cells.

The regulation and functions of activin and follistatin in inflammation and immunity

The activins are members of the transforming growth factor β superfamily with broad and complex effects on cell growth and differentiation. Activin A has long been known to be a critical regulator of inflammation and immunity, and similar roles are now emerging for activin B, with which it shares 65% sequence homology. These molecules and their binding protein, follistatin, are widely expressed, and their production is increased in many acute and chronic inflammatory conditions. Synthesis and release of the activins are stimulated by inflammatory cytokines, Toll-like receptor ligands, and oxidative stress. The activins interact with heterodimeric serine/threonine kinase receptor complexes to activate SMAD transcription factors and the MAP kinase signaling pathways, which mediate inflammation, stress, and immunity. Follistatin binds to the activins with high affinity, thereby obstructing the activin receptor binding site, and targets them to cell surface proteoglycans and lysosomal degradation. Studies on transgenic mice and those with gene knockouts, together with blocking studies using exogenous follistatin, have established that activin A plays critical roles in the onset of cachexia, acute and chronic inflammatory responses such as septicemia, colitis and asthma, and fibrosis. However, activin A also directs the development of monocyte/macrophages, myeloid dendritic cells, and T cell subsets to promote type 2 and regulatory immune responses. The ability of both endogenous and exogenous follistatin to block the proinflammatory and profibrotic actions of activin A has led to interest in this binding protein as a potential therapeutic for limiting the severity of disease and to improve subsequent damage associated with inflammation and fibrosis. However, the ability of activin A to sculpt the subsequent immune response as well means that the full range of effects that might arise from blocking activin bioactivity will need to be considered in any therapeutic applications.

Activin-A: a novel critical regulator of allergic asthma

Activin-A is a pleiotropic cytokine that belongs to the TGF-β superfamily and plays an important role in fundamental biological processes, such as development and tissue repair. Growing evidence proposes a crucial role for activin-A in immune-mediated responses and associated diseases, with both enhancing and suppressive effects depending on the cell type, the cytokine micromilieu and the context of the response. Several recent studies have demonstrated a striking increase in activin-A expression in experimental models of asthma, as well as, in the asthmatic airway in humans. Importantly, a strong immunoregulatory role for activin-A in allergic airway disease, with suppression of T helper (Th) type 2 cell-driven allergic responses and protection against the development of cardinal features of the asthmatic phenotype was revealed by in vivo functional studies. Activin-A-mediated immunosuppression is associated with induction of functional allergen-specific regulatory T cells. In human asthma, although activin-A levels are increased in the airway epithelium and submucosal cells, the expression of its signalling components is markedly decreased, pointing to decreased regulation. Nevertheless, a rapid activation of the activin-A signalling pathway is observed in the airway of individuals with asthma following inhalational allergen challenge, suggestive of an inherent protective mechanism to control disease. In support, in vitro studies using human airway epithelial cells have demonstrated that endogenous activin-A suppresses the release of inflammatory mediators, while it induces epithelial repair. Collectively, compelling evidence suggests that activin-A orchestrates the regulation of key events involved in the pathogenesis of allergic asthma. The critical role of activin-A in allergic airway responses places this cytokine as an exciting new therapeutic target for asthma.

The dual role of the X-linked FoxP3 gene in human cancers

The FoxP3 (forkhead box P3) gene is an X-linked gene that is submitted to inactivation. It is an essential transcription factor in CD4(+)CD25(+)FoxP3 regulatory T cells, which are therapeutic targets in disseminated cutaneous melanoma. Moreover, FoxP3 is an important tumor suppressor gene in carcinomas and has putative cancer suppressor gene function in cutaneous melanoma as well. Therefore understanding the structure and function of the FoxP3 gene is crucial to gaining insight into the biology of melanoma to better develop immunotherapeutics and future therapeutic strategies.

[Preferential induction of apoptosis in regulatory T cells by tributyltin: possible involvement in the exacerbation of allergic diseases]

OBJECTIVES

Tributyltin (TBT) has been recognized as a particularly important pollutant. Human exposure to TBT persists via consumption of TBT-containing meat and fish products. Although it is well known that high-dose TBT exerts immunotoxic effects such as thymic atrophy, the effect of low-dose TBT exposure on immune responses remains elusive. Our previous studies demonstrated that TBT at environmentally relevant doses promoted T helper (Th)2 polarization via enhancement of Th2 differentiation and preferential induction of apoptosis in Th1, which is associated with the exacerbation of Th2-driven allergic airway inflammation. In the present study, we explored the possibility that TBT might preferentially induce apoptosis in Foxp3(+) regulatory T cells (Treg), which play an indisputable role in the negative regulation of immune responses.

METHODS

We established several independent Treg and Th2 clones and their susceptibilities to TBT-induced apoptosis were examined. To examine whether the susceptibility to TBT-induced apoptosis may be due to the level of glutathione (GSH), we measured the basal GSH levels in Treg and Th2 clones. Intracellular GSH level was measured using high-performance liquid chromatography (HPLC) with a gold electrode.

RESULTS

We show that TBT preferentially induces apoptosis in Treg clones rather than in Th2 clones. The basal levels of GSH in Treg clones were significantly lower than those in Th2 clones.

CONCLUSIONS

The increased susceptibility of Treg clones to TBT-induced apoptosis appeared to result from lower GSH levels in Treg clones, which may detoxify the reactive oxygen species (ROS) induced by TBT treatment. Our results suggest that the preferential induction of apoptosis in Treg over Th2 contributes to the exacerbation of Th2-driven allergic diseases by TBT.

Role of activin in bacterial infections: a potential target for immunointervention?

Severe bacterial infections such as sepsis and meningitis still kill or severely injure people despite the use of bactericidal antibiotics. Therefore, new strategies for a better therapy are needed. Activin A, a member of the TGF-β superfamily and its binding protein follistatin (FS) are released by various cell types during acute and chronic inflammatory processes. Until now, a clear definition of conditions in which activin A exerts either its pro- or anti-inflammatory functions is lacking. The activin/FS-system participates in the fine-tuning of the host’s inflammatory response upon infectious stimuli. This response is on the one hand necessary for fighting pathogens, but on the other hand can negatively affect the host. This article focuses on the role of activin A and FS in infection and after acute inflammatory stimuli. The therapeutic potentials of blocking or promoting activin actions are discussed.

Therapeutic potential of TGF-β-induced CD4(+) Foxp3(+) regulatory T cells in autoimmune diseases

Foxp3(+) T regulatory cell (Treg) subsets play a crucial role in the maintenance of immune homeostasis against self-antigens. The lack or dysfunction of these cells contributes to the pathogenesis and development of many autoimmune diseases. Therefore, manipulation of these cells may provide a novel therapeutic approach to treat autoimmune diseases. In this review, we provide current opinions concerning the classification, developmental, and functional characterization of Treg subsets. Particular emphasis will be focused on the therapeutic role of TGF-β-induced CD4M(+) Foxp3(+) cells (iTregs) in established autoimmune disease. Moreover, the similarity and diversity of iTregs and naturally occurring, thymus-derived CD4(+) CD25(+) Foxp3(+) regulatory T cells (nTregs) will be discussed, including the finding that the pro-inflammatory cytokine IL-6 can convert nTregs to IL-17-producing cells, whereas iTregs induced by TGF-β are resistant to the effects of this cytokine. Understanding these aspects may help to determine how Tregs can be used in the treatment of autoimmune diseases.

SMAD2 is essential for TGF beta-mediated Th17 cell generation

TGFβ is the quintessential cytokine of T cell homeostasis. TGFβ signaling is required for the efficient differentiation and maintenance of CD4(+)FOXP3(+) T cells that inhibit immune responses. Conversely, in conjunction with the inflammatory cytokine IL-6, TGFβ promotes Th17 cell differentiation. The mechanism by which TGFβ signals synergize with IL-6 to generate inflammatory versus immunosuppressive T cell subsets is unclear. TGFβ signaling activates receptor SMADs, SMAD2 and SMAD3, which associate with a variety of nuclear factors to regulate gene transcription. Defining relative contributions of distinct SMAD molecules for CD4 T cell differentiation is critical for mapping the versatile intracellular TGFβ-signaling pathways that tailor TGFβ activities to the state of host interaction with pathogens. We show here that SMAD2 is essential for Th17 cell differentiation and that it acts in part by modulating the expression of IL-6R on T cells. Although mice lacking SMAD2 specifically in T cells do not develop spontaneous lymphoproliferative autoimmunity, Smad2-deficient T cells are impaired in their response to TGFβ in vitro and in vivo, and they are more pathogenic than controls when transferred into lymphopenic mice. These results demonstrate that SMAD2 is uniquely essential for TGFβ signaling in CD4(+) T effector cell differentiation.

1-MT enhances potency of tumor cell lysate-pulsed dendritic cells against pancreatic adenocarcinoma by downregulating the percentage of Tregs

This study examined whether 1-methyl-tryptophan [1-MT, an indoleamine 2, 3-dioxygenase (IDO) inhibitor] could reduce CD4+CD25+ regulatory T cells (Tregs) proliferation and improve the anti-tumor efficacy of dendritic cells (DCs) pulsed with tumor cell lysate in the mice bearing pancreatic adenocarcinoma. The models of pancreatic adenocarcinoma were established in C57BL/6 mice by subcutaneous injection of Pan02 cells. Eight mice which were subcutaneously injected with PBS served as control. The expression of IDO was determined in tumor draining lymph nodes (TDLNs) and spleens of the murine pancreatic adenocarcinoma models. The prevalence of Tregs was measured in the TDLNs and spleens before and after 1-MT administration. The dendritic cells were pulsed with tumor cell lysate for preparing DC vaccine. The DC vaccine, as a single agent or in combination with 1-MT, was administered to pancreatic adenocarcinoma mice. The anti-tumor efficacy was determined after different treatments by regular observation of tumor size. The results showed that the levels of IDO mRNA and protein in tumor-bearing mice were significantly higher than those in the normal control mice. The percentage of Tregs in the spleen and TDLNs was also higer in tumor-bearing mice than in normal control mice (P<0.05). Foxp3 expression was significantly lower in the TDLNs and spleens of tumor-bearing mice administrated with 1-MT than that in normal control mice. Furthemore, in the mice that were administered 1-MT plus DC vaccine, the tumor was increased more slowly than in mice treated with DC vaccine or 1-MT alone, or PBS on day 36 (P<0.01). Our results indicated that 1-MT may enhance anti-tumor efficacy of dendritic cells pulsed with tumor cell lysate by downregulating the percentage of Tregs.

Adaptive immune regulation of glial homeostasis as an immunization strategy for neurodegenerative diseases

Neurodegenerative diseases, notably Alzheimer's and Parkinson's diseases, are amongst the most devastating disorders afflicting the elderly. Currently, no curative treatments or treatments that interdict disease progression exist. Over the past decade, immunization strategies have been proposed to combat disease progression. Such strategies induce humoral immune responses against misfolded protein aggregates to facilitate their clearance. Robust adaptive immunity against misfolded proteins, however, accelerates disease progression, precipitated by induced effector T cell responses that lead to encephalitis and neuronal death. Since then, mechanisms that attenuate such adaptive neurotoxic immune responses have been sought. We propose that shifting the balance between effector and regulatory T cell activity can attenuate neurotoxic inflammatory events. This review summarizes advances in immune regulation to achieve a homeostatic glial response for therapeutic gain. Promising new ways to optimize immunization schemes and measure their clinical efficacy are also discussed.

Targeted inhibition of IL-10-secreting CD25- Treg via p38 MAPK suppression in cancer immunotherapy

Cancer-induced immunotolerance mediated by inducible Treg (iTreg) is a major obstacle to cancer immunotherapy. In a basic study of immunotolerance, injection of an endogenous superantigen, i.e. the minor lymphocyte stimulatory (Mls)-1(a), into specific TCR Vbeta8.1-Tg mice enabled generation of anergic CD25(-) iTreg, the immunosuppressive function of which was maintained by IL-10 production via p38-MAPK activation. Interestingly, although p38-chemical inhibitor (p38-inhibitor) is capable of breaking CD25(-) iTreg-induced immunotolerance, the p38-inhibitor had hardly any immunotolerance breaking effect when CD25(+) Treg were present, suggesting that depletion of CD25(+) Treg is necessary for p38-inhibitor to be effective. Peptide OVA(323-339) iv.-injection into its specific TCR-Tg (OT-II) mice also induced adaptive tolerance by iTreg. Peptide immunotherapy with p38-inhibitor after CD25(+) Treg-depletion was performed in an OVA-expressing lymphoma E.G7-bearing tolerant model established by adoptive transfer of OT-II CD25(-) iTreg, which resulted in suppression of tumor growth. Similarly, the antitumor immunity induced by peptide immunotherapy in colon carcinoma CT26-bearing mice, in which the number of IL-10-secreting iTreg is increased, was augmented by treatment with p38-inhibitor after CD25(+) Treg-depletion and resulted in inhibition of tumor progression. These results suggest that simultaneous inhibition of two distinct Treg-functions may be important to the success of cancer immunotherapy.

Regulatory T cells in cancer

At the present time, regulatory T cells (Tregs) are an integral part of immunology but the route from discovery of "suppressive" lymphocytes in the 1980s to the current established concept of Tregs almost 20 years later has been a rollercoaster ride. Tregs are essential for maintaining self-tolerance as defects in their compartment lead to severe autoimmune diseases. This vitally important function exists alongside the detrimental effects on tumor immunosurveillance and antitumor immunity. Beginning with the identification of CD4(+)CD25(+) Tregs in 1995, the list of Treg subsets, suppressive mechanisms, and knowledge about their various origins is steadily growing. Increase in Tregs within tumors and circulation of cancer patients, observed in early studies, implied their involvement in pathogenesis and disease progression. Several mechanisms, ranging from proliferation to specific trafficking networks, have been identified to account for their systemic and/or local accumulation. Since various immunotherapeutic approaches are being utilized for cancer therapy, various strategies to overcome the antagonistic effects exerted by Tregs are being currently explored. An overview on the biology of Tregs present in cancer patients, their clinical impact, and methods for modulating them is given in this review. Despite the extensive studies on Tregs in cancer many questions still remain unanswered. Even the paradigm that Tregs generally are disadvantageous for the control of malignancies is now under scrutiny. Insight into the specific role of Tregs in different types of neoplasias is the key for targeting them in a way that is beneficial for the clinical outcome.

Synergistic effect of TGF-beta superfamily members on the induction of Foxp3+ Treg

TGF-beta plays an important role in the induction of Treg and maintenance of immunologic tolerance, but whether other members of TGF-beta superfamily act together or independently to achieve this effect is poorly understood. Although others have reported that the bone morphogenetic proteins (BMP) and TGF-beta have similar effects on the development of thymocytes and T cells, in this study, we report that members of the BMP family, BMP-2 and -4, are unable to induce non-regulatory T cells to become Foxp3+ Treg. Neutralization studies with Noggin have revealed that BMP-2/4 and the BMP receptor signaling pathway is not required for TGF-beta to induce naïve CD4+CD25- cells to express Foxp3; however, BMP-2/4 and TGF-beta have a synergistic effect on the induction of Foxp3+ Treg. BMP-2/4 affects non-Smad signaling molecules including phosphorylated ERK and JNK, which could subsequently promote the differentiation of Foxp3+ Treg induced by TGF-beta. Data further advocate that TGF-beta is a key signaling factor for Foxp3+ Treg development. In addition, the synergistic effect of BMP-2/4 and TGF-beta indicates that the simultaneous manipulation of TGF-beta and BMP signaling might have considerable effects in the clinical setting for the enhancement of Treg purity and yield.

Activin and transforming growth factor-beta signaling pathways are activated after allergen challenge in mild asthma

BACKGROUND

Both transforming growth factor (TGF)-beta(1) and activin-A have been implicated in airway remodeling in asthma, but the modulation of their specific signaling pathways after disease activation remains undefined.

OBJECTIVE

To define the expression kinetics of TGF-beta(1), activin-A ligands, and follistatin (a natural activin inhibitor), their type I and type II receptors (activin-like kinase[ALK]-1, ALK-5, ALK-4, TbetaRII, and ActRIIA/RIIB) and activation of signaling (via phosphorylated (p) Smad2), in the asthmatic airway after allergen challenge.

METHODS

Immunohistochemistry was performed on bronchial biopsies from 15 mild atopic patients with asthma (median age, 25 years; median FEV(1)% predicted, 97%) at baseline and 24 hours after allergen inhalation. Functional effects of activin-A were evaluated by using cultured normal human bronchial epithelial (NHBE) cells.

RESULTS

pSmad2(+) epithelial cells increased at 24 hours (P = .03), and pSmad2 was detected in submucosal cells. No modulation of activin-A, follistatin, or TGF-beta(1) expression was demonstrated. Activin receptor(+) cells increased after allergen challenge: ALK-4 in epithelium (P = .04) and submucosa (P = .04), and ActRIIA in epithelium (P = .01). The TGF-beta receptor ALK-5 expression was minimal in the submucosa at baseline and after challenge and was downregulated in the epithelium after challenge (P = .02), whereas ALK-1 and TbetaRII expression in the submucosa increased after allergen challenge (P = .03 and P = .004, respectively). ALK-1 and ALK-4 expression by T cells was increased after allergen challenge. Activin-A induced NHBE cell proliferation, was produced by NHBE cells in response to TNF-alpha, and downregulated TNF-alpha and IL-13-induced chemokine production by NHBE cells.

CONCLUSION

Both TGF-beta and activin signaling pathways are activated on allergen provocation in asthma. Activin-A may contribute to resolution of inflammation.

Activin-A induces regulatory T cells that suppress T helper cell immune responses and protect from allergic airway disease

Activin-A is a pleiotropic cytokine that participates in developmental, inflammatory, and tissue repair processes. Still, its effects on T helper (Th) cell–mediated immunity, critical for allergic and autoimmune diseases, are elusive. We provide evidence that endogenously produced activin-A suppresses antigen-specific Th2 responses and protects against airway hyperresponsiveness and allergic airway disease in mice. Importantly, we reveal that activin-A exerts suppressive function through induction of antigen-specific regulatory T cells that suppress Th2 responses in vitro and upon transfer in vivo. In fact, activin-A also suppresses Th1-driven responses, pointing to a broader immunoregulatory function. Blockade of interleukin 10 and transforming growth factor β1 reverses activin-A–induced suppression. Remarkably, transfer of activin-A–induced antigen-specific regulatory T cells confers protection against allergic airway disease. This beneficial effect is associated with dramatically decreased maturation of draining lymph node dendritic cells. Therapeutic administration of recombinant activin-A during pulmonary allergen challenge suppresses Th2 responses and protects from allergic disease. Finally, we demonstrate that immune cells infiltrating the lungs from individuals with active allergic asthma, and thus nonregulated inflammatory response, exhibit significantly decreased expression of activin-A's responsive elements. Our results uncover activin-A as a novel suppressive factor for Th immunity and a critical controller of allergic airway disease.