Thymic stromal lymphopoietin as a key initiator of allergic airway inflammation in mice

Wiskott-Aldrich syndrome protein is required for regulatory T cell homeostasis

Wiskott-Aldrich syndrome protein (WASp) is essential for optimal T cell activation. Patients with WAS exhibit both immunodeficiency and a marked susceptibility to systemic autoimmunity. We investigated whether alterations in Treg function might explain these paradoxical observations. While WASp-deficient (WASp(-/-)) mice exhibited normal thymic Treg generation, the competitive fitness of peripheral Tregs was severely compromised. The total percentage of forkhead box P3-positive (Foxp3(+)) Tregs among CD4(+) T cells was reduced, and WASp(-/-) Tregs were rapidly outcompeted by WASp(+) Tregs in vivo. These findings correlated with reduced expression of markers associated with self-antigen-driven peripheral Treg activation and homing to inflamed tissue. Consistent with these findings, WASp(-/-) Tregs showed a reduced ability to control aberrant T cell activation and autoimmune pathology in Foxp3(-/-)Scurfy (sf) mice. Finally, WASp(+) Tregs exhibited a marked selective advantage in vivo in a WAS patient with a spontaneous revertant mutation, indicating that altered Treg fitness likely explains the autoimmune features in human WAS.

Inducible expression of the proallergic cytokine thymic stromal lymphopoietin in airway epithelial cells is controlled by NFkappaB

The epithelial-derived cytokine thymic stromal lymphopoietin (TSLP) is important for the initiation of allergic airway inflammation through a dendritic cell-mediated T helper 2 response. To identify the factors that control TSLP expression, we examined the ability of inflammatory mediators to regulate TSLP production in human airway epithelial cells. We found that both IL-1beta and TNF-alpha were capable of inducing rapid TSLP production in primary human bronchial airway epithelial cells. We further characterized the human TSLP gene promoter, using two human epithelial cell lines, 16HBEo(-) and A549, and showed that IL-1beta- and TNF-alpha-mediated human TSLP promoter activation in these cells was mediated by an upstream NFkappaB site. Mutation of this NFkappaB site abolished activation, as did overexpression of a dominant-negative version of IkappaB kinase (IKK)beta (a kinase acting on IkappaB, the inhibitor of NFkappaB). Interestingly, human TSLP mRNA levels were also increased after exposure to Toll-like receptor (TLR) 2, TLR8, and TLR9 ligands, further supporting an important role for NFkappaB in TSLP gene regulation. Similarly, analysis of the mouse TSLP gene promoter revealed the presence of a similarly situated NFkappaB site that was also critical for IL-1beta-inducible expression of mouse TSLP. Taken together, these results demonstrate that the inflammatory mediators IL-1beta and TNF-alpha regulate human TSLP gene expression in an NFkappaB-dependent manner.

Thymic stromal lymphopoietin:a potential therapeutic target for allergy and asthma

Thymic stromal lymphopoietin (TSLP) is an interleukin (IL)-7-like cytokine that has recently been implicated as central to the microenvironment and is permissive for the immunologic cascade that initiates and propagates allergic immune responses. In humans, TSLP is produced predominantly by epithelial cells and activated mast cells, and stimulates myeloid dendritic cells (mDC), which uniquely express the heterodimeric TSLP receptor. TSLP-activated mDC can promote naïve CD4+ T cells to differentiate into a Th2 phenotype and can promote the expansion of CD4+ Th2 memory cells. Recent evidence implicates TSLP as playing a pivotal role in the pathobiology of allergic asthma and atopic dermatitis. The potential for TSLP to provide a new therapeutic target for the treatment of allergic disorders is compelling, and elucidating the mechanisms that regulate TSLP expression and the effects of TSLP on orchestrating the immune response toward a Th2 phenotype should facilitate this quest.

Local application of FTY720 to the lung abrogates experimental asthma by altering dendritic cell function

Airway DCs play a crucial role in the pathogenesis of allergic asthma, and interfering with their function could constitute a novel form of therapy. The sphingosine 1-phosphate receptor agonist FTY720 is an oral immunosuppressant that retains lymphocytes in lymph nodes and spleen, thus preventing lymphocyte migration to inflammatory sites. The accompanying lymphopenia could be a serious side effect that would preclude the use of FTY720 as an antiasthmatic drug. Here we show in a murine asthma model that local application of FTY720 via inhalation prior to or during ongoing allergen challenge suppresses Th2-dependent eosinophilic airway inflammation and bronchial hyperresponsiveness without causing lymphopenia and T cell retention in the lymph nodes. Effectiveness of local treatment was achieved by inhibition of the migration of lung DCs to the mediastinal lymph nodes, which in turn inhibited the formation of allergen-specific Th2 cells in lymph nodes. Also, FTY720-treated DCs were intrinsically less potent in activating naive and effector Th2 cells due to a reduced capacity to form stable interactions with T cells and thus to form an immunological synapse. These data support the concept that targeting the function of airway DCs with locally acting drugs is a powerful new strategy in the treatment of asthma.

Role of cytokines in allergic airway inflammation

Asthma is characterized by intense infiltration of eosinophils and CD4+ T cells into the submucosal tissue of airways. Accumulating evidence indicates that T helper type 2 cell-derived cytokines such as interleukin (IL)-4, IL-5 and IL-13 play critical roles in orchestrating and amplifying allergic inflammation in asthma. In addition, it has been suggested that newly identified cytokines including thymic stromal lymphopoietin, IL-25 and IL-33 are involved in the induction of allergic inflammation in asthma. In this review, we discuss the role of individual cytokines in the pathogenesis of asthma.

The regulation of allergy and asthma

Allergic diseases and asthma are caused by exaggerated T-helper 2 (Th2)-biased immune responses in genetically susceptible individuals. Tolerance to allergens is a mechanism that normally prevents such responses, but the specific immunological events that mediate tolerance in this setting are poorly understood. A number of recent studies indicate that regulatory T cells (Tregs) play an important role in controlling such Th2-biased responses. Tregs involved in regulating allergy and asthma consist of a family of related types of T cells, including natural CD25+ Tregs as well as inducible forms of antigen-specific adaptive Tregs. Impaired expansion of natural and/or adaptive Tregs is hypothesized to lead to the development of allergy and asthma, and treatment to induce allergen-specific Tregs could provide curative therapies for these problems.

Immune dysregulation in asthma

Allergic diseases and asthma are caused by dysregulated Th2-biased immune responses to environmental allergens in genetically predisposed individuals. Over the past several years there has been much progress in understanding the mechanisms by which Th2 responses are generated and the pathogenic role of natural killer T cells in asthma. In addition, there has been much progress in understanding the mechanisms of tolerance to allergens, the role of natural and adaptive allergen-specific regulatory T cells, and the strategies to prevent or to reverse allergic disease and asthma. Impaired expansion of regulatory T cells is hypothesized to lead to the development of allergy and asthma, and treatment to induce allergen-specific regulatory T cells could provide curative therapies for these problems.

Thymic stromal lymphopoietin in normal and pathogenic T cell development and function

Thymic stromal lymphopoietin, a four helix-bundle cytokine, is expressed mainly by barrier epithelial cells and is a potent activator of several cell types, particularly myeloid dendritic cells. TSLP influences the outcome of interactions between dendritic cells and CD4+ thymocytes and T cells in many situations, such as the regulation of the positive selection of regulatory T cells, maintenance of peripheral CD4+ T cell homeostasis and induction of CD4+ T cell-mediated allergic inflammation.

Maintenance and polarization of human TH2 central memory T cells by thymic stromal lymphopoietin-activated dendritic cells

The identity of TH2 memory cells and the mechanism regulating their maintenance during allergic inflammation remain elusive. We report that circulated human CD4+ T cells expressing the prostaglandin D2 receptor (CRTH2) are TH2 central memory T cells, characterized by their phenotype, TH2 cytokine production, gene-expression profile, and the ability to respond to allergens. Only dendritic cells (DCs) activated by thymic stromal lymphopoietin (TSLP) can induce a robust expansion of CRTH2+CD4+ TH2 memory cells, while maintaining their central memory phenotype and TH2 commitments. CRTH2+CD4+ TH2 memory cells activated by TSLP-DCs undergo further TH2 polarization and express cystatin A, Charcot-Leydon crystal protein, and prostaglandin D2 synthase, implying their broader roles in allergic inflammation. Infiltrated CRTH2+CD4+ TH2 effector memory T cells in skin lesion of atopic dermatitis were associated with activated DCs, suggesting that TSLP-DCs play important roles not only in TH2 priming, but also in the maintenance and further polarization of TH2 central memory cells in allergic diseases.

Topical vitamin D3 and low-calcemic analogs induce thymic stromal lymphopoietin in mouse keratinocytes and trigger an atopic dermatitis

We have demonstrated that cytokine thymic stromal lymphopoietin (TSLP), whose expression is rapidly induced upon keratinocyte-selective ablation of retinoid X receptors (RXRs) -alpha and -beta in the mouse (RXRalphabeta(ep-/-) mice), plays a key role in initiating a skin and systemic atopic dermatitis-like phenotype. We show here that topical application of the physiologically active ligand [1alpha,25-(OH)(2)D(3); calcitriol] of the vitamin D receptor, or of its low-calcemic analog MC903 (calcipotriol; Dovonex), induces TSLP expression in epidermal keratinocytes, which results in an atopic dermatitis-like syndrome mimicking that seen in RXRalphabeta(ep-/-) mutants and transgenic mice overexpressing TSLP in keratinocytes. Furthermore, topical application of retinoic acid receptor RARgamma-selective agonist BMS961 also induces TSLP expression either on its own or synergistically with 1alpha,25-(OH)(2)D(3). Our data demonstrate that RXR/vitamin D receptor and RXR/retinoic acid receptor-gamma heterodimers and their ligands cell-autonomously control the expression of TSLP in epidermal keratinocytes of the mouse. We propose molecular mechanisms through which vitamin D3 and retinoic acid signalings could be involved in the pathogenesis of atopic diseases.

Epidermal barrier formation and recovery in skin disorders

Skin is at the interface between the complex physiology of the body and the external, often hostile, environment, and the semipermeable epidermal barrier prevents both the escape of moisture and the entry of infectious or toxic substances. Newborns with rare congenital barrier defects underscore the skin's essential role in a terrestrial environment and demonstrate the compensatory responses evoked ex utero to reestablish a barrier. Common inflammatory skin disorders such as atopic dermatitis and psoriasis exhibit decreased barrier function, and recent studies suggest that the complex response of epidermal cells to barrier disruption may aggravate, maintain, or even initiate such conditions. Either aiding barrier reestablishment or dampening the epidermal stress response may improve the treatment of these disorders. This Review discusses the molecular regulation of the epidermal barrier as well as causes and potential treatments for defects of barrier formation and proposes that medical management of barrier disruption may positively affect the course of common skin disorders.

T helper cell effector fates — who, how and where?

CD4 helper T cells functionally organize the host immune response by elaborating cytokines, often in patterns that have overlapping effects on other cells. Much interest centers on understanding how these stereotyped cytokine patterns become elaborated and what mechanisms underlie the generation of distinct helper T cell subsets. The past two years have seen advances in understanding of additional subsets, including T helper follicular cells and IL-17-producing T helper cells. Progress has also been achieved in resolving some of the crosstalk that regulates effector fate at the level of distinct transcription factors and chromatin reorganization of the cytokine genes, and a crucial role for gene silencing has been exposed. Finally, the role of innate cells in influencing these processes has become increasingly realized.

Roles of OX40 in the pathogenesis and the control of diseases

OX40 belongs to the tumor necrosis factor receptor superfamily, and its expression is restricted to activated T-cells. Ligation of OX40 during T-cell-dendritic cell interaction is crucial for clonal expansion of antigen-specific T-cells and generation of T-cell memory. The ligand of OX40 (OX40L) is expressed not only on dendritic cells but also on other cell types, such as B-cells, vascular endothelial cells, natural killer cells, and mast cells. The pathophysiological relevance of this broad distribution needs further investigation. In particular, OX40L on vascular endothelial cells may play a role in inflammatory vasculitis as well as in atherosclerotic change. Recent studies with animal models have indicated the critical involvement of OX40 in the pathogenesis of a variety of immunologic abnormalities of inflammatory, autoimmune, infectious, allergic, and allotransplantation-related diseases. Blockade of OX40-OX40L interaction has been shown to prevent, cure, or ameliorate these diseases. In contrast, activation of OX40 is known to break an existing state of tolerance in malignancies, leading to a reactivation of antitumor immunity. These findings clearly suggest that the OX40/OX40L system is one of the most promising targets of immune intervention for treatment of these diseases.

Connexin 26 regulates epidermal barrier and wound remodeling and promotes psoriasiform response

Inflammatory skin disorders result in significant epidermal changes, including keratinocyte hyperproliferation, incomplete differentiation, and impaired barrier. Here we test whether, conversely, an impaired epidermal barrier can promote an inflammatory response. Mice lacking the transcription factor Kruppel-like factor 4 (Klf4) have a severe defect in epidermal barrier acquisition. Transcription profiling of Klf4(-/-) newborn skin revealed similar changes in gene expression to involved psoriatic plaques, including a significant upregulation of the gap junction protein connexin 26 (Cx26). Ectopic expression of Cx26 from the epidermis-specific involucrin (INV) promoter (INV-Cx26) demonstrated that downregulation of Cx26 is required for barrier acquisition during development. In juvenile and adult mice, persistent Cx26 expression kept wounded epidermis in a hyperproliferative state, blocked the transition to remodeling, and led to an infiltration of immune cells. Mechanistically, ectopic expression of Cx26 in keratinocytes resulted in increased ATP release, which delayed epidermal barrier recovery and promoted an inflammatory response in resident immune cells. These results provide a molecular link between barrier acquisition in utero and epidermal remodeling after wounding. More generally, these studies suggest that the most effective treatments for inflammatory skin disorders might concomitantly suppress the immune response and enhance epidermal differentiation to restore the barrier.

Thymic stromal lymphopoietin: master switch for allergic inflammation

Thymic stromal lymphopoietin (TSLP) is an interleukin (IL) 7–like cytokine that triggers dendritic cell–mediated T helper (Th)2 inflammatory responses. TSLP is highly expressed by keratinocytes in skin lesions of patients with atopic dermatitis and is associated with dendritic cell activation in situ, suggesting that TSLP might be a master switch for allergic inflammation at the epithelial cell–dendritic cell interface. New reports now establish a direct link between TSLP expression and the pathogenesis of atopic dermatitis and asthma in vivo, and begin to reveal the molecular mechanisms underlying TSLP-induced allergic inflammation.