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

Thymic stromal lymphopoietin: a new cytokine in asthma

Airway epithelial cells provide mechanical and immune protection against pathogens and allergens. Following activation, these cells produce a wide range of cytokines including thymic stromal lymphopoietin (TSLP). Recently it was established that a high level of TSLP is associated with asthma in mice and in humans. These findings suggest that interfering with the ability of cells to respond to TSLP might prevent the development of airway inflammation. Our review presents current knowledge on mediators that induce TSLP production and on the actions of TSLP on different populations of cells that are related to airway inflammation. TSLP affects dendritic cells, T cells, NKT cells, and mast cells, indicative of the broad role of TSLP in the regulation of inflammatory/allergic processes.

Dendritic cells and epithelial cells: linking innate and adaptive immunity in asthma

Dendritic cells (DCs) are generally held responsible for initiating and maintaining allergic T helper 2 (T(H)2)-cell responses to inhaled allergens in asthma. Although the epithelium was initially considered to function solely as a physical barrier, it is now seen as a central player in the T(H)2-cell sensitization process by influencing the function of DCs. Clinically relevant allergens, as well as known environmental and genetic risk factors for allergy and asthma, often interfere directly or indirectly with the innate immune functions of airway epithelial cells and DCs. A better understanding of these interactions, ascertained from human and animal studies, might lead to better prevention and treatment of asthma.

A mechanism for the initiation of allergen-induced T helper type 2 responses

Both metazoan parasites and simple protein allergens induce T helper type 2 (TH2) immune responses, but the mechanisms by which the innate immune system senses these stimuli are unknown. In addition, the cellular source of cytokines that control TH2 differentiation in vivo has not been defined. Here we showed that basophils were activated and recruited to the draining lymph nodes specifically in response to TH2-inducing allergen challenge. Furthermore, we demonstrate that the basophil was the accessory cell type required for TH2 induction in response to protease allergens. Finally, we show that basophils were directly activated by protease allergens and produced TH2-inducing cytokines, including interleukin 4 and thymic stromal lymphopoietin, which are involved in TH2 differentiation in vivo.

Role of airway epithelial cells in development of asthma and allergic rhinitis

Asthma and allergic rhinitis frequently coexist in the same patient. There is a similarity and variation as well as potential relationship between asthma and allergic rhinitis. There is an increasing evidence to suggest a major involvement of airway epithelial cells in the pathogenesis of asthma and allergic rhinitis. The present review describes the importance of the airway epithelial cell in the development of allergic airway diseases, its role as the primary airway defense against exposure of the airway and lung to inflammatory stimuli and antigens and as an important player through activation of epithelial Toll-like receptors (TLRs) to provide an important link between innate immunity and allergic disease. Additionally, airway epithelial cells can act as inflammatory promoters capable of directing dendritic cells (DCs) towards a T helper 2 (Th2) response, and as active producers of several inflammatory/anti-inflammatory mediators. It is hypothesized that airway epithelial cells may play as both inflammatory initiator and immuno-pathological feedback regulation between allergic rhinitis and asthma via release of systemic inflammatory mediators. Thus, airway epithelial cells may be valuable therapeutic targets for discovery and development of new drugs and/or new therapeutic strategies to treat asthma and allergic rhinitis.

Renin-angiotensin system blockade is renoprotective in immune complex-mediated glomerulonephritis

Blockade of the renin-angiotensin system is renoprotective in a variety of chronic nephropathies, but the direct effect of such treatment in active, immune complex-mediated glomerulonephritis is unknown. This study investigated the short- and long-term effects of an angiotensin-converting enzyme inhibitor (enalapril) and an angiotensin II type 1 receptor blocker (losartan) in thymic stromal lymphopoietin transgenic (TSLPtg) mice, which develop mixed cryoglobulinemia and severe cryoglobulinemia-associated membranoproliferative glomerulonephritis. Enalapril and losartan each reduced hypertension, proteinuria, glomerular extracellular matrix deposition, and mesangial cell activation in TSLPtg mice. These renoprotective effects were not observed with hydralazine treatment, despite a similar antihypertensive effect. Treatment with enalapril or losartan also decreased renal plasminogen activator inhibitor-1 in TSLPtg mice, assessed by immunohistochemistry and quantitative real-time reverse transcriptase-PCR. None of the treatments affected immune complex deposition or macrophage infiltration. Overall, enalapril- and losartan-treated TSLPtg mice survived significantly longer than untreated TSLPtg mice. These studies demonstrate that angiotensin blockade may provide renoprotective benefits, independent of its BP-lowering effect, in the treatment of active immune complex-mediated glomerulonephritis.

Viral infection in asthma

In bronchial asthma, respiratory virus infection involves several issues: 1) respiratory virus infection in infancy is a risk factor for, and may predispose to, the development of asthma later in life; 2) respiratory virus infection is associated with the acute exacerbation of bronchial asthma; and, 3) glucocorticosteroids (GC) are not adequate for controlling asthma-related symptoms upon respiratory virus infection. Various cells, inflammatory mediators and cytokines participate in the production of airway inflammation upon respiratory virus infection. Bronchial epithelial cells are a site of infection and replication of respiratory virus. They actively participate in the production of airway inflammation: 1) they produce various proinflammatory cytokines, chemokines and mediators; and, 2) they undergo apoptosis, thereby impairing the repair process. It is therefore important to understand the role of bronchial epithelial cells in the pathophysiology of bronchial asthma. In this review, the interaction between viral infection and asthma is discussed to elucidate the role of bronchial epithelial cells in viral infection.

Toll-like receptors and airway inflammation

The respiratory tract opens to the external environment at the oral side edge, and the other edge of the respiratory tract connects to the closed space (alveoli), and so to preserve the sterility in the terminal respiratory tract is critical for protection against pathogens. The recognition machinery for the invasion of microbes is indispensable for the preservation of the sterility in the lungs. Our general understanding of how microbes are recognized by the innate immune system has increased considerably over the past several years, and the contribution of Toll-Like Receptors (TLRs) to innate immunity is now well documented. In the meantime, it has come to understand that many inflammatory processes may depend on TLR signaling, it has been considered to be involved in the pathogenesis of airway inflammatory diseases such as airway infections, bronchial asthma, and occupational airway diseases. In this review, we focus on physiological roles of TLRs in defense mechanisms of the airways, and pathophysiological roles on airway diseases.

Establishing the phenotype in novel acute and chronic murine models of allergic asthma

Allergic asthma is a chronic disease of the airways, with superimposed acute inflammatory episodes which correspond to exacerbations of asthma. Two novel models of allergic asthma have been developed in mice receiving the same allergen sensitisation, but with acute or chronic allergen exposures, the latter to mimic the human situation more closely. Ovalbumin-sensitised mice were challenged by ovalbumin inhalation twice on the same day for the acute model, and 18 times over a period of 6 weeks for the chronic model. Lung function was monitored in conscious, unrestrained mice immediately after the last challenge for up to 12 h. Airway responsiveness to inhaled methacholine and serum antibody levels were determined 24 h after challenge. Bronchoalveolar inflammatory cell recruitment was determined at 2 or 24 h. Acute and chronically treated mice had similar early and late asthmatic responses peaking at 2 h and 7-8 h, respectively. IgE and IgG antibody levels, compared with naïve mice, and eosinophil infiltration, compared with naïve and saline challenge, were elevated. Airway hyperresponsiveness to methacholine was observed 24 h after challenge in both models. The acute model had higher levels of eosinophilia, whereas the chronic model showed hyperresponsiveness to lower doses of methacholine and had higher levels of total IgE and ovalbumin-specific IgG antibodies. Both novel murine models of allergic asthma bear a close resemblance to human asthma, each offering particular advantages for studying the mechanisms underlying asthma and for evaluating existing and novel therapeutic agents.

In vivo blockade of OX40 ligand inhibits thymic stromal lymphopoietin driven atopic inflammation

Thymic stromal lymphopoietin (TSLP) potently induces deregulation of Th2 responses, a hallmark feature of allergic inflammatory diseases such as asthma, atopic dermatitis, and allergic rhinitis. However, direct downstream in vivo mediators in the TSLP-induced atopic immune cascade have not been identified. In our current study, we have shown that OX40 ligand (OX40L) is a critical in vivo mediator of TSLP-mediated Th2 responses. Treating mice with OX40L-blocking antibodies substantially inhibited immune responses induced by TSLP in the lung and skin, including Th2 inflammatory cell infiltration, cytokine secretion, and IgE production. OX40L-blocking antibodies also inhibited antigen-driven Th2 inflammation in mouse and nonhuman primate models of asthma. This treatment resulted in both blockade of the OX40-OX40L receptor-ligand interaction and depletion of OX40L-positive cells. The use of a blocking, OX40L-specific mAb thus presents a promising strategy for the treatment of allergic diseases associated with pathologic Th2 immune responses.

OX40-OX40L interactions: a promising therapeutic target for allergic diseases?

Recent advances in understanding the cellular and molecular mechanisms of atopy have shed light on potential targets for the development of new therapies for allergic diseases. In this issue of the JCI, Seshasayee et al. provide direct in vivo evidence that OX40 has critical roles in allergic inflammation mediated by thymic stromal lymphopoietin (TSLP) (see the related article beginning on page 3868). Blockade of interactions between OX40 on Th2 cells and OX40 ligand (OX40L) on TSLP-activated DCs using an OX40L-specific monoclonal antibody, inhibited Th2 cell-mediated immune responses in both mouse and nonhuman primate models of allergic inflammation. The results point to potential therapeutic approaches to targeting the cellular and molecular mechanism underlying TSLP-mediated allergic inflammation.

Sex-stratified linkage analysis identifies a female-specific locus for IgE to cockroach in Costa Ricans

RATIONALE

The basis for gender influences on allergen-specific IgEs is unclear.

OBJECTIVES

To perform regular and sex-stratified genomewide linkage analyses of IgE to each of three allergens (Ascaris lumbricoides, Blatella germanica [German cockroach]), and Dermatophagoides pteronyssinus [dust mite]) and to conduct an association study of a candidate gene in a linked genomic region.

METHODS

Genomewide linkage analyses of allergen-specific IgEs were conducted in 653 members of eight large families of Costa Rican children with asthma. An analysis of the association between single-nucleotide polymorphisms in thymic stromal lymphopoietin (TSLP) and IgE measurements was conducted in 417 parent-child trios in Costa Rica. Significant results were replicated in 470 families of white children in the Childhood Asthma Management Program (CAMP).

MEASUREMENTS AND MAIN RESULTS

Among all subjects, there was suggestive evidence of linkage (LOD >/= 2.72) to IgE to Ascaris (on chromosome 7q) and IgE to dust mite (on chromosomes 7p and 12q). In a sex-stratified analysis, there was significant evidence of linkage to IgE to cockroach on chromosome 5q23 (peak LOD, 4.14 at 127 cM) in female subjects. TSLP is located within the 1.5 LOD-unit support interval for this linkage peak and has female-specific effects on lung disease in mice. In a sex-stratified analysis, the T allele of single-nucleotide polymorphism rs2289276 in TSLP was associated with reductions in IgE to cockroach (in Costa Rican girls) and total IgE (in girls in Costa Rica and in CAMP; P value for sex-by-genotype interaction, <0.01 in both studies).

CONCLUSIONS

Consistent with findings in murine models, a variant in TSLP may have female-specific effects on allergic phenotypes.

Division of labor, plasticity, and crosstalk between dendritic cell subsets

For years, dendritic cell (DC) biologists have oscillated between two seemingly antagonistic ideas: functional specialization (division of labor) of DC subsets and plasticity (multitasking). More recently, a third hypothesis is gathering support: crosstalk between functionally distinct DC subsets. This reveals a previously unappreciated hierarchy of organization within the DC system, and provides a conceptual framework to understand how cooperation between functionally distinct, yet plastic, DC subsets can shape adaptive immunity and immunological memory. Here we review the recent advances in this area.

Thymic stromal lymphopoietin secretion of synovial fibroblasts is positively and negatively regulated by Toll-like receptors/nuclear factor-kappaB pathway and interferon-gamma/dexamethasone

Thymic stromal lymphopoietin (TSLP) is an interleukin (IL)-7-like cytokine produced by epithelial cells and triggers dendritic cell-mediated Th2 type allergic inflammatory responses. This study investigated whether Toll-like receptor (TLR) ligands, lipopolysaccharide (LPS) and poly-IC affect TSLP production in synovial fibroblasts. Enzyme-linked immunosorbent assay showed that LPS and poly-IC upregulated TSLP production in synovial fibroblasts obtained from patients with rheumatoid arthritis (RA) and osteoarthritis (OA). In addition, we found that nuclear factor (NF)-kappaB inhibitor IMD-0354, dexamethasone, and interferon (IFN)-gamma inhibited the LPS- and poly-IC-induced TSLP production in RA and OA synovial fibroblasts. Thus, LPS and poly-IC can upregulate TSLP via a NF-kappaB pathway in synovial fibroblasts, which is downregulated by dexamethasone and interferon (IFN)-gamma. The current findings suggest that TSLP may be involved in the pathophysiology of inflammatory arthritis as well as allergic disease.

Primary prevention of allergic diseases: current concepts and mechanisms

: Atopic diseases, the new "epidemic of the twenty-first century" and a central health problem of industrial nations, call for the development of innovative primary prevention strategies. The present review provides an overview of current experimental and immunomodulatory procedures and their underlying mechanisms.

Dendritic cell subsets in health and disease

The dendritic cell (DC) system of antigen-presenting cells controls immunity and tolerance. DCs initiate and regulate immune responses in a manner that depends on signals they receive from microbes and their cellular environment. They allow the immune system to make qualitatively distinct responses against different microbial infections. DCs are composed of subsets that express different microbial receptors and express different surface molecules and cytokines. Our studies lead us to propose that interstitial (dermal) DCs preferentially activate humoral immunity, whereas Langerhans cells preferentially induce cellular immunity. Alterations of the DC system result in diseases such as autoimmunity, allergy, and cancer. Conversely, DCs can be exploited for vaccination, and novel vaccines that directly target DCs in vivo are being designed.

iNKT cells require CCR4 to localize to the airways and to induce airway hyperreactivity

iNKT cells are required for the induction of airway hyperreactivity (AHR), a cardinal feature of asthma, but how iNKT cells traffic to the lungs to induce AHR has not been previously studied. Using several models of asthma, we demonstrated that iNKT cells required the chemokine receptor CCR4 for pulmonary localization and for the induction of AHR. In both allergen-induced and glycolipid-induced models of AHR, wild-type but not CCR4-/- mice developed AHR. Furthermore, adoptive transfer of wild-type but not CCR4-/- iNKT cells reconstituted AHR in iNKT cell-deficient mice. Moreover, we specifically tracked CCR4-/- vs wild-type iNKT cells in CCR4-/-:wild-type mixed BM chimeric mice in the resting state, and when AHR was induced by protein allergen or glycolipid. Using this unique model, we showed that both iNKT cells and conventional T cells required CCR4 for competitive localization into the bronchoalveolar lavage/airways compartment. These results establish for the first time that the pulmonary localization of iNKT cells critical for the induction of AHR requires CCR4 expression by iNKT cells.

Dendritic cells in asthma and COPD: opportunities for drug development

The lung contains many subsets of dendritic cells that are distributed in various anatomical compartments. In homeostatic conditions, a fine-tuned balance exists between plasmacytoid and myeloid dendritic cells necessary for maintaining tolerance to inhaled antigen and avoiding overt inflammation. These subsets of DCs also play important roles in establishment of airway inflammation seen in asthma and chronic obstructive pulmonary disease. Based on these new insights on airway DC biology, several approaches that interfere with DC function show potential as new intervention strategies for these ever increasing diseases.

Beyond inflammation: airway epithelial cells are at the interface of innate and adaptive immunity

It has become increasingly clear that airway epithelial cells are central participants in innate and adaptive immune responses as well as mucosal inflammation. Epithelial cells produce antimicrobial host defense molecules, proinflammatory cytokines and chemokines in response to activation via pathogen recognition receptors. Recruitment of immune cells including dendritic cells, T cells and B cells into the proximity of epithelium results in the enhancement of adaptive immunity through interactions with epithelial cells. Newly identified epithelial-derived cytokines, including TSLP, IL-33 and BAFF, help to shape the local accumulation and activation of Th2 responses and B cell immunoglobulin production. Epithelial cells are also downstream targets of molecules that activate IL-13R and EGFR and are responsible for mucus production in both protective immune responses and allergic airway inflammatory diseases. Improved understanding of epithelial immune and inflammatory responses will hopefully suggest new strategies for therapeutic intervention.

The p38 mitogen-activated protein kinase signaling pathway is coupled to Toll-like receptor 5 to mediate gene regulation in response to Pseudomonas aeruginosa infection in human airway epithelial cells

In this study, we show that stimulation of human airway epithelial cells (HAECs) by Pseudomonas aeruginosa strain PAO1 induces time- and dose-dependent activation of p38 mitogen-activated protein kinase (MAPK). Activated p38 MAPK stayed in the cytoplasm instead of translocating to the nucleus, as shown by cellular fractionation. p38 MAPK was activated when HAECs were incubated with P. aeruginosa strain PAK and Burkholderia cepacia, while little activation was observed with the isogenic flagellin-free strains PAK/fliC and B. cepacia BC/fliC. The presence of Toll-like receptor 5 (TLR5) in 293 cells mediated PAO1-dependent activation of p38 MAPK, and in HAECs p38 MAPK activation was blocked by the overexpression of a dominant negative TLR5. Two inhibitors of p38 MAPK, SB202190 and SB203580, significantly attenuated PAO1-dependent expression of an NF-kappaB-dependent luciferase reporter gene, suggesting that p38 MAPK activation is required for full activation of NF-kappaB-dependent signaling. Microarray analysis of NF-kappaB target genes revealed up-regulation of multiple genes by PAO1 in HAECs. Reverse transcription-PCR and protein expression analysis were used to show that up-regulation of NF-kappaB-dependent genes induced by PAO1, such as the genes encoding Cox-2 and interleukin-8, was attenuated by SB203580. These results demonstrate a role for p38 MAPK signaling in gene regulation in response to P. aeruginosa via TLR5.

Thymic stromal lymphopoietin and OX40 ligand pathway in the initiation of dendritic cell-mediated allergic inflammation

It was demonstrated 5 years ago that thymic stromal lymphopoietin (TSLP), a IL-7-like cytokine produced by epithelial cells, could strongly activate human myeloid dendritic cells to induce an inflammatory T(H)2 response characterized by high TNF-alpha and little IL-10 production, distinct from the regulatory T(H)2 responses characterized by low TNF-alpha and high IL-10 production. TSLP was found highly expressed by keratinocytes of skin lesions of atopic dermatitis and associated with dendritic cell activation in situ. This suggests for the first time that TSLP represents a master switch of allergic inflammation at the epithelial cell and dendritic cell interface. During the last several years, the evidence for the association of TSLP with human asthma was revealed. The direct link between TSLP expression with the pathogenesis of atopic dermatitis and asthma in vivo was demonstrated. In addition, OX40 ligand was found to be the TSLP-induced molecule on dendritic cells that triggers inflammatory T(H)2 differentiation in the absence of IL-12. TSLP was also demonstrated to direct the innate phase of allergic immune responses through activating mast cells. Therefore, TSLP and OX40 ligand may represent important targets for intervention of the initiation of allergic inflammatory responses.

The yin and yang of intestinal epithelial cells in controlling dendritic cell function

Recent work suggests that dendritic cells (DCs) in mucosal tissues are "educated" by intestinal epithelial cells (IECs) to suppress inflammation and promote immunological tolerance. After attack by pathogenic microorganisms, however, "non-educated" DCs are recruited from nearby areas, such as the dome of Peyer's patches (PPs) and the blood, to initiate inflammation and the ensuing immune response to the invader. Differential epithelial cell (EC) responses to commensals and pathogens may control these two tolorogenic and immunogenic functions of DCs.

TLR3- and Th2 cytokine-dependent production of thymic stromal lymphopoietin in human airway epithelial cells

Thymic stromal lymphopoietin (TSLP) is elevated in asthma and triggers dendritic cell-mediated activation of Th2 inflammatory responses. Although TSLP has been shown to be produced mainly by airway epithelial cells, the regulation of epithelial TSLP expression has not been extensively studied. We investigated the expression of TSLP in cytokine- or TLR ligand-treated normal human bronchial epithelial cells (NHBE). The mRNA for TSLP was significantly up-regulated by stimulation with IL-4 (5.5-fold) and IL-13 (5.3-fold), weakly up-regulated by TNF-alpha, TGF-beta, and IFN-beta, and not affected by IFN-gamma in NHBE. TSLP mRNA was only significantly up-regulated by the TLR3 ligand (dsRNA) among the TLR ligands tested (66.8-fold). TSLP was also induced by in vitro infection with rhinovirus. TSLP protein was detected after stimulation with dsRNA (120 +/- 23 pg/ml). The combination of TNF-alpha and IL-4 produced detectable levels of TSLP protein (40 +/- 13 pg/ml). In addition, TSLP was synergistically enhanced by a combination of IL-4 and dsRNA (mRNA; 207-fold, protein; 325 +/- 75 pg/ml). The induction of TSLP by dsRNA was dependent upon NF-kappaB and IFN regulatory factor 3 (IRF-3) signaling via TLR3 as indicated by a study with small interfering RNA. The potent topical glucocorticoid fluticasone propionate significantly suppressed dsRNA-dependent TSLP production in NHBE. These results suggest that the expression of TSLP is induced in airway epithelial cells by stimulation with the TLR3 ligand and Th2 cytokines and that this response is suppressed by glucocorticoid treatment. This implies that respiratory viral infection and the recruitment of Th2 cytokine producing cells may amplify Th2 inflammation via the induction of TSLP in the asthmatic airway.

Dendritic cells: importance in allergy

In this review we discuss the role of dendritic cells (DC) in the pathogenesis of allergic contact hypersensitivity (ACH) and atopic disorders, such as asthma and atopic eczema. In ACH patients, DC recognize the invasion of simple chemicals such as haptens, and trigger antigen-specific T cell responses leading to the characteristic histological and clinical changes such as spongiosis and papulovesicular eruptions. During atopic disorders, it is well known that the Th2-deviated immune response plays a crucial role in their pathogenesis. DC provide T cells with antigen and costimulatory signals (signals 1 and 2, respectively), as well as with a polarizing signal (signal 3). When studying ACH, it is important to understand how simple chemicals induce the activation of DC and their migration to the draining lymph nodes where they supply signals 1 and 2 to naive T cells. The mechanisms by which DC induce the Th2-deviated immune response, namely via the Th2-deviated signal 3, are central topics in the pathogenesis of atopic disorders.

Increased TSLP availability restores T- and B-cell compartments in adult IL-7 deficient mice

Interleukin 7 (IL-7) plays a crucial role in adult lymphopoiesis, while in fetal life its effect can be partially compensated by TSLP. Whether adult hematopoietic progenitor cells are unresponsive to TSLP or whether TSLP is less available in adult microenvironments is still a matter of debate. Here, we show that increased TSLP availability through transgene (Tg) expression fully restored lymphopoiesis in IL-7-deficient mice: it rescued B-cell development, increased thymic and splenic cellularities, and restored double-negative (DN) thymocytes, alphabeta and gammadelta T-cell generation, and all peripheral lymphoid compartments. Analysis of bone marrow chimeras demonstrated that hematopoietic progenitor cells from adult wild-type mice efficiently differentiated toward B- and T-cell lineages in lethally irradiated IL-7 deficient mice provided TSLP Tg was expressed in these mice. In vitro, TSLP promoted the differentiation of uncommitted adult bone marrow progenitors toward B and T lineages and the further differentiation of DN1 and DN2 thymocytes. Altogether, our results show that adult hematopoietic cells are TSLP responsive and that TSLP can sustain long-term adult lymphopoiesis.

The contribution of toll-like receptors to the pathogenesis of asthma

Asthma is a major disease in the westernized world and its incidence has significantly increased over the past 40 years. Our understanding of the pathogenesis of asthma remains rudimentary, and for this reason, little has been accomplished by way of targeted intervention, either at a population level (to reduce the overall prevalence) or at an individual level (to treat the cause). Instead, the management strategy currently in use relies on broad-spectrum anti-inflammatory agents, generally glucocorticoids and long-acting beta2 agonists. The recent discovery of toll-like receptors (TLRs), with their role as the initiators of the innate immune response and inflammation, suggests that modulating these receptors may be beneficial in the treatment of allergic disorders. We review here the cellular distribution of TLR in the lung and their potential contribution to the processes that promote T helper 2 (Th2) immunity and infection-induced exacerbations of allergic lung disease.

Differential role of thymic stromal lymphopoietin in the induction of airway hyperreactivity and Th2 immune response in antigen-induced asthma with respect to natural killer T cell function

Asthma is an inflammatory lung disease, in which CD1d-restricted natural killer T (NKT) cells play an important pathogenic role. Also, recent reports indicated that a cytokine, thymic stromal lymphopoietin (TSLP), is essential for the development of antigen-induced asthma. Here we examined the relationship between NKT cells and TSLP in a mouse model of asthma. NKT cells express TSLP receptor as well as IL-7 receptor alpha-chain. TSLP acts on NKT cells to preferentially increase their IL-13 production but not IFN-gamma and IL-4. In an allergen-induced asthma model, the development of airway hyperreactivity, a cardinal feature of asthma, was increased in TSLP transgenic mice, whereas this effect was not observed in TSLP transgenic mice lacking NKT cells. Interestingly, in the NKT cell-lacking TSLP transgenic mice, pulmonary eosinophilia and increase in IgE did not improve. Pulmonary lymphocytes from the NKT cell-lacking TSLP transgenic mice produced much less IL-13 upon CD3 stimulation than those from NKT cell-competent TSLP transgenic mice. These resultssuggest that, in allergen-induced asthma, TSLP acts on NKT cells to enhance airway hyperreactivity by upregulating their IL-13 production, whereas eosinophilia and IgE production are not influenced.

TSLP is involved in expansion of early thymocyte progenitors

Background

Thymic stromal derived lymphopoietin (TSLP) is preferentially and highly expressed in the thymus, but its function in T cell development is not clear.

Results

We report here that TSLP, independently or in combination with IL-7, enhances thymopoiesis in the murine fetal thymic organ culture (FTOC) model. Furthermore, TSLP preferentially increases the number and proliferation of the (DN1 and DN2) pro-T progenitor cells, and FTOC lobes from TSLP receptor-null mice show a decreased number of these cells. Finally, DN1-DN2 cells expanded with TSLP in vitro are functional T progenitors that are able to differentiate into mature T cells in fetal or adult thymus organs.

Conclusion

Together, these data suggest that TSLP plays an important role in expansion of thymocyte progenitors and may be of value for expanding T progenitor cells in vitro.

IL-25 augments type 2 immune responses by enhancing the expansion and functions of TSLP-DC-activated Th2 memory cells

Interleukin (IL) 25 (IL-17E), a distinct member of the IL-17 cytokine family, plays important roles in evoking T helper type 2 (Th2) cell–mediated inflammation that features the infiltrations of eosinophils and Th2 memory cells. However, the cellular sources, target cells, and underlying mechanisms remain elusive in humans. We demonstrate that human Th2 memory cells expressing distinctive levels of IL-25 receptor (R) are one of the responding cell types. IL-25 promotes cell expansion and Th2 cytokine production when Th2 central memory cells are stimulated with thymic stromal lymphopoietin (TSLP)–activated dendritic cells (DCs), homeostatic cytokines, or T cell receptor for antigen triggering. The enhanced functions of Th2 memory cells induced by IL-25 are associated with sustained expression of GATA-3, c-MAF, and JunB in an IL-4–independent manner. Although keratinocytes, mast cells, eosinophils, and basophils express IL-25 transcripts, activated eosinophils and basophils from normal and atopic subjects were found to secrete bioactive IL-25 protein, which augments the functions of Th2 memory cells. Elevated expression of IL-25 and IL-25R transcripts was observed in asthmatic lung tissues and atopic dermatitis skin lesions, linking their possible roles with exacerbated allergic disorders. Our results provide a plausible explanation that IL-25 produced by innate effector eosinophils and basophils may augment the allergic inflammation by enhancing the maintenance and functions of adaptive Th2 memory cells.

Extracellular ATP triggers and maintains asthmatic airway inflammation by activating dendritic cells

Extracellular ATP serves as a danger signal to alert the immune system of tissue damage by acting on P2X or P2Y receptors. Here we show that allergen challenge causes acute accumulation of ATP in the airways of asthmatic subjects and mice with experimentally induced asthma. All the cardinal features of asthma, including eosinophilic airway inflammation, Th2 cytokine production and bronchial hyper-reactivity, were abrogated when lung ATP levels were locally neutralized using apyrase or when mice were treated with broad-spectrum P2-receptor antagonists. In addition to these effects of ATP in established inflammation, Th2 sensitization to inhaled antigen was enhanced by endogenous or exogenous ATP. The adjuvant effects of ATP were due to the recruitment and activation of lung myeloid dendritic cells that induced Th2 responses in the mediastinal nodes. Together these data show that purinergic signaling has a key role in allergen-driven lung inflammation that is likely to be amenable to therapeutic intervention.

Cutting edge: direct action of thymic stromal lymphopoietin on activated human CD4+ T cells

Thymic stromal lymphopoietin (TSLP) is a cytokine that promotes CD4(+) T cell homeostasis and contributes to allergic and inflammatory responses. TSLP can act directly on mouse CD4(+) T cells, but in humans, the available data have indicated that TSLP receptors are not expressed on CD4(+) T cells and that TSLP instead activates dendritic cells, which in turn promote the proliferation and differentiation of CD4(+) T cells. We now unexpectedly demonstrate the presence of TSLP receptors on activated human CD4(+) T cells. Strikingly, whereas freshly isolated peripheral blood human T cells show little if any response to TSLP, TCR stimulation allows a potent response to this cytokine. Moreover, TSLP increases the sensitivity of human CD4(+) T cells to low doses of IL-2, augmenting responsiveness of these cells to TCR engagement. Our results establish that human CD4(+) T cells are direct targets for TSLP.

TSLP: an epithelial cell cytokine that regulates T cell differentiation by conditioning dendritic cell maturation

Dendritic cells (DCs) are professional antigen-presenting cells that have the ability to sense infection and tissue stress, sample and present antigen to T lymphocytes, and induce different forms of immunity and tolerance. The functional versatility of DCs depends on their remarkable ability to translate collectively the information from both the invading microbes and their resident tissue microenvironments and then make an appropriate immune response. Recent progress in understanding TLR biology has illuminated the mechanisms by which DCs link innate and adaptive antimicrobial immune responses. However, how tissue microenvironments shape the function of DCs has remained elusive. Recent studies of TSLP (thymic stromal lymphopoietin), an epithelial cell-derived cytokine that strongly activates DCs, provide evidence at a molecular level that epithelial cells/tissue microenvironments directly communicate with DCs. We review recent progress on how TSLP expressed within thymus and peripheral lymphoid and nonlymphoid tissues regulates DC-mediated central tolerance, peripheral T cell homeostasis, and inflammatory Th2 responses.

Airway epithelial STAT3 is required for allergic inflammation in a murine model of asthma

The STAT3 transcription factor is critical for cytokine signaling and the acute phase response, but its role in allergic asthma is largely undefined. To investigate the role of STAT3 in mediating allergic inflammation, we used chemical and genetic approaches to inactivate STAT3 in the airway epithelium of mice. In a murine model of chronic asthma, we demonstrate that the administration of house dust mite (HDM) leads to robust STAT3 activation in the airway epithelium, smooth muscle, and immune cells in the lungs of C57BL/6 mice. To investigate the role of STAT3 in HDM-induced airway inflammation, a conditional knockout of STAT3 in the airway epithelium was generated, e-STAT3-/-. We determined that e-STAT3-/- mice had a significant decrease in HDM-induced airway eosinophilia, lung Th2 accumulation, and chemokines compared with wild-type animals. Importantly, the e-STAT3-/- mice had a significant decrease in airway hyperresponsiveness to methacholine. The administration of two STAT kinase inhibitors diminished STAT3 activation and markedly abrogated the HDM-induced lung inflammation. These findings suggest that STAT3 acts as a novel epithelial regulator of the allergic response by altering Th2 cell recruitment and effector function, and thus, targeting this molecule may provide the basis for a novel asthma therapy.

Cutting Edge: Proinflammatory and Th2 cytokines synergize to induce thymic stromal lymphopoietin production by human skin keratinocytes

Thymic stromal lymphopoietin (TSLP) is an epithelial cell-derived cytokine that strongly activates dendritic cells (DC) and can initiate allergic inflammation. The factors inducing the production of human TSLP are not known. In this study, we show that proinflammatory (TNF-alpha or IL-1alpha) and Th2 (IL-4 or IL-13) cytokines synergized to induce the production of TSLP in human skin explants. TSLP production in situ was restricted to epidermal keratinocytes of the suprabasal layer. TSLP production could not be inhibited by factors regulating Th2 inflammation, such as IL-10, TGF-beta, or IFN-gamma. Cytokine-treated skin culture supernatants induced the maturation of blood CD11c(+) DC in a TSLP-dependent manner. Our data provide the first evidence of TSLP induction and subsequent DC activation in human skin. Blocking TSLP-inducing cytokines could represent a novel strategy for the treatment of allergic diseases.

Constitutive and inducible thymic stromal lymphopoietin expression in human airway smooth muscle cells: role in chronic obstructive pulmonary disease

Thymic stromal lymphopoietin (TSLP) is a novel cytokine that triggers dendritic cell-mediated T helper (Th)-2 inflammatory responses. Previous studies have demonstrated that human airway smooth muscle cells (HASMC) play a critical role in initiating or perpetuating airway inflammation by producing chemokines and cytokines. In this study, we first evaluated the expression of TSLP in primary HASMC and investigated how proinflammatory cytokines (TNF-alpha and IL-1beta) and Th-2 cytokines (IL-4, IL-9) regulate TSLP production from HASMC. TSLP mRNA and protein were assessed by real-time RT-PCR, ELISA, and immunofluorescence from primary HASMC cultures. Primary HASMC express constitutive level of TSLP. Incubation of HASMC with IL-1 or TNF-alpha resulted in a significant increase of TSLP mRNA and protein release from HASMC. Furthermore, combination of IL-1beta and TNF-alpha has an additive effect on TSLP release by HASMC. Primary HASMC pretreated with inhibitors of p38 or p42/p44 ERK MAPK, but not phosphatidylinositol 3-kinase, showed a significant decrease in TSLP release on IL-1beta and TNF-alpha treatment. Furthermore, TSLP immunoreactivity was present in ASM bundle from chronic obstructive pulmonary disease (COPD) and to lesser degree in normal subjects. Taken together, our data provide the first evidence of IL-1beta- and TNF-alpha-induced TSLP expression in HASMC via (p38, p42/p44) MAPK signaling pathways. Our results raise the possibility that HASMC may play a role in COPD airway inflammation via TSLP-dependent pathway.

The epithelium takes centre stage in asthma and atopic dermatitis

Asthma and atopic dermatitis are epithelial disorders in which T helper 2 (Th2)-type inflammation has a prominent role. Thymic stromal lymphopoietin (TSLP) is a cytokine produced by the skin and airway epithelium that is capable of directing dendritic cells towards a Th2 response, thereby providing an essential link between epithelial cell activation and allergic-type inflammation. In addition, TSLP can interact directly with mast cells to initiate Th2 cytokine production to also provide a non-T cell route to mediate its pro-allergic effects. Induction of TSLP production occurs through the activation of epithelial Toll-like receptors to provide an important new link between innate immunity and allergic disease.

Inhaled iloprost suppresses the cardinal features of asthma via inhibition of airway dendritic cell function

Inhalation of iloprost, a stable prostacyclin (PGI(2)) analog, is a well-accepted and safe treatment for pulmonary arterial hypertension. Although iloprost mainly acts as a vasodilator by binding to the I prostanoid (IP) receptor, recent evidence suggests that signaling via this receptor also has antiinflammatory effects through unclear mechanisms. Here we show in a murine model of asthma that iloprost inhalation suppressed the cardinal features of asthma when given during the priming or challenge phase. As a mechanism of action, iloprost interfered with the function of lung myeloid DCs, critical antigen-presenting cells of the airways. Iloprost treatment inhibited the maturation and migration of lung DCs to the mediastinal LNs, thereby abolishing the induction of an allergen-specific Th2 response in these nodes. The effect of iloprost was DC autonomous, as iloprost-treated DCs no longer induced Th2 differentiation from naive T cells or boosted effector cytokine production in primed Th2 cells. These data should pave the way for a clinical effectiveness study using inhaled iloprost for the treatment of asthma.

Local increase in thymic stromal lymphopoietin induces systemic alterations in B cell development

The cytokine thymic stromal lymphopoietin (TSLP) drives immature B cell development in vitro and may regulate T helper type 2 responses. Here we analyzed the involvement of TSLP in B cell development in vivo with a doxycycline-inducible, keratin 5-driven transgene encoding TSLP (K5-TSLP). K5-TSLP-transgenic mice given doxycycline showed an influx of immature B cells into the periphery, with population expansion of follicular mature B cells, near-complete loss of marginal zone and marginal zone precursor B cells, and 'preferential' population expansion of peritoneal B-1b B cells. These changes promoted cryoglobulin production and immune complex-mediated renal disease. Identical events occurred in mice without T cells, in alternative TSLP-transgenic models and in K5-TSLP-transgenic mice with undetectable systemic TSLP. These observations suggest that signals mediating localized TSLP expression may modulate systemic B cell development and promote humoral autoimmunity.

Suppression of early IL-4 production underlies the failure of CD4 T cells activated by TLR-stimulated dendritic cells to differentiate into Th2 cells

Dendritic cells (DCs) activated through TLRs provide a potent negative signal for Th2 cell development that is independent of positive signals for Th1 cell development such as IL-12 and IFN-gamma. In this study we demonstrate that the ability of TLR-activated DCs to suppress Th2 cell development is Ag dose-independent and unique to DCs that have been activated through TLRs vs by cytokines. We show that TLR-activated DCs inhibit early IL-4 production by CD4 T cells and thus inhibit their ability to subsequently increase GATA-3 expression and commit to the Th2 lineage. This occurs independently of expression of the GATA-3 antagonist T-bet. Although CD4 T cells activated by TLR-activated DCs make IL-2, they are not capable of phosphorylating STAT5 in response to this cytokine. This inhibition of responsiveness to IL-2 appears to underlie the failure to make early IL-4. Our findings suggest that DCs provide instructional signals for T cell differentiation before cytokine-mediated Th cell selection and outgrowth.

Induction of IL-4 expression in CD4(+) T cells by thymic stromal lymphopoietin

The cytokine thymic stromal lymphopoietin (TSLP) has been implicated in the development and progression of allergic inflammation in both humans and mice. Although the underlying mechanism is not known, TSLP-stimulated dendritic cells have been shown to prime human CD4(+) T cells into Th2 cytokine-producing cells. However, its direct effect on CD4(+) T cells has not been extensively investigated. In this study, we show that TSLP can drive Th2 differentiation in the absence of exogenous IL-4 and APCs. IL-4 blockade inhibited TSLP-mediated Th2 differentiation, demonstrating that IL-4 is involved in this process. Further analysis has shown that TSLP-induced Th2 differentiation is dependent on Stat6 and independent of IL-2 and that TSLP treatment leads to immediate, direct Il-4 gene transcription. Taken together, these data demonstrate that TSLP is directly involved in Th2-mediated responses via the induction of IL-4 production.

A possible role for TSLP in inflammatory arthritis

Thymic stromal lymphopoietin (TSLP) is an IL-7-like cytokine that triggers dendritic cell-mediated Th2-type inflammatory responses and is considered as a master switch for allergic inflammation. In this study, we found increased levels of TSLP and, also TNF-alpha as previously reported, in synovial fluid specimens derived from patients with rheumatoid arthritis (RA) when compared with those from patients with osteoarthritis (OA). In addition, TNF-alpha up-regulated TSLP expression in RA- and OA-derived synovial fibroblasts, which was inhibited by IFN-gamma. Furthermore, anti-TSLP neutralizing antibody ameliorated a TNF-alpha-dependent experimental arthritis induced by anti-type II collagen antibody in mice. Collectively, these results suggest that TSLP, as a downstream molecule of TNF-alpha, may be involved in the pathophysiology of inflammatory arthritis. TSLP might thus play a role not only in allergic diseases but also in inflammatory arthritis such as RA.

T-cell responses to allergens

The allergic response in human beings is engineered by CD4(+) T lymphocytes, which secrete T(H)2 cytokines in response to activation by allergen-derived peptides. Although T(H)2 cells have been well characterized, defining the properties of allergen-specific T cells has proved challenging in human beings because of their low frequency within the T-cell repertoire. However, recent studies have provided insight into the molecular signature of long-lived human memory T(H)2 cells, which are allergen-specific. T-cell responses directed against allergens develop in early life and are heavily influenced by the type and dose of allergen, and possibly coexposure to microbial products. These responses are susceptible to suppression by regulatory T cells. This article highlights recent advances in the characterization of allergen-specific memory T(H)2 cells and discusses the heterogeneous nature of regulatory T cells and possible mechanisms of action. The relevance of T-cell epitope mapping studies to understanding the unique nature of T-cell responses to different allergens, as well as to peptide vaccine development, is reviewed. Experimental techniques and approaches for analyzing allergen-specific T cells and identifying novel T-cell epitopes are described that may lead to new T-cell-based therapies.

Invariant natural killer T (iNKT) cells in asthma: a novel insight into the pathogenesis of asthma and the therapeutic implication of glycolipid ligands for allergic diseases

Allergic bronchial asthma is a complex inflammatory diseases originated from dysregulated immune responses in the respiratory mucosa. The inflammatory state in asthmatic lung is characterized by massive infiltration with eosinophils, lymphocytes, and mast cells in the airway mucosa leading to airway hyperseisitivity, goblet cell hyperplasia and mucus overproduction. The inflammatory process is thought to be the result of intensive T helper (Th) 2-biased immune response. Over the past several years, there has been enormous progress in understanding the mechanisms for development of Th2-biased responses after inhaled exposure to allergens and the characteristics of CD4+ T cells prominently involved in this process. Recently, a new population of T cells, invariant natural killer T (iNKT) cells has been shown to play an important role in the pathogenesis of mouse model of allergic airway inflammation. iNKT cells are one of the most potent immune modulators through a massive production of a various cytokines including IL-4 and IFN-gamma upon activation, and are involved in a variety of immunoregulations including infection, autoimmunity, and tumor surveillance. The potent pathogenic role of iNKT cells in the development of bronchial asthma is due to their ability to produce predominant Th2 cytokines in a given condition. The involvement of iNKT cells in the pathogenesis of asthma might have been underestimated in the past studies demonstrating the involvement of CD4+ T cells in asthma because of the difficulty in the detection of iNKT cells. Meanwhile, growing evidences have demonstrated that iNKT cells could be a promising target for immune-based therapies for autoimmune diseases, tumor, and infection due to the invariance of their TCR usage, the restriction to the evolutionally-conserved non-polymorphic antigen-presenting molecule CD1d, and their outstanding ability to produce both Th1- and Th2-cytokines. In this review, we will overview current understanding of the pathophysiological roles of iNKT cells in asthma. We would also discuss on possible therapeutic approaches to bronchial asthma employing glycolipid ligands for iNKT cells.

Thymic stromal lymphopoietin converts human epidermal Langerhans cells into antigen-presenting cells that induce proallergic T cells

BACKGROUND

Thymic stromal lymphopoietin (TSLP) endows human CD11c(+) dendritic cells (DCs) from peripheral blood with the capacity to induce proallergic T cells. TSLP is present at high levels in the epidermis of atopic dermatitis where it appears to trigger emigration of epidermal Langerhans cells (LCs); however, nothing else is known about the influence of TSLP on LCs.

OBJECTIVE

Effects of TSLP on human epidermal LCs were investigated.

METHODS

LCs were isolated by trypsinization from healthy human skin, highly enriched by immunomagnetic techniques (via CD1a) and cultured for 2 days. Additionally, migratory LCs were obtained by emigration from epidermal sheets for 3 days.

RESULTS

The addition of TSLP promoted survival and maturation of LCs obtained by trypsinization, as indicated by their increased expression of CD83, CD86, and high levels of MHC II. TSLP markedly increased numbers of migratory LCs. Allogeneic naïve CD4(+) T cells, cocultured with migratory TSLP-LCs produced less IFN-gamma and IL-10 and more IL-4, IL-5, IL-13, and TNF-alpha. Finally, TSLP-LCs secreted markedly more of the T(H)2 T-cell-attracting chemokine CCL17/thymus and activation-regulated chemokine.

CONCLUSION

These cytokine patterns correspond to those described for TSLP-treated blood DCs. They highlight a direct effect of TSLP on epidermal LCs.

CLINICAL IMPLICATIONS

Our data emphasize a critical role for LCs in the triggering of atopic dermatitis. Furthermore, they underscore the interest in TSLP as a potential therapeutic target in atopic diseases.

Immune response modulation by curcumin in a latex allergy model

Background

There has been a worldwide increase in allergy and asthma over the last few decades, particularly in industrially developed nations. This resulted in a renewed interest to understand the pathogenesis of allergy in recent years. The progress made in the pathogenesis of allergic disease has led to the exploration of novel alternative therapies, which include herbal medicines as well. Curcumin, present in turmeric, a frequently used spice in Asia has been shown to have anti-allergic and inflammatory potential.

Methods

We used a murine model of latex allergy to investigate the role of curcumin as an immunomodulator. BALB/c mice were exposed to latex allergens and developed latex allergy with a Th2 type of immune response. These animals were treated with curcumin and the immunological and inflammatory responses were evaluated.

Results

Animals exposed to latex showed enhanced serum IgE, latex specific IgG₁, IL-4, IL-5, IL-13, eosinophils and inflammation in the lungs. Intragastric treatment of latex-sensitized mice with curcumin demonstrated a diminished Th2 response with a concurrent reduction in lung inflammation. Eosinophilia in curcumin-treated mice was markedly reduced, co-stimulatory molecule expression (CD80, CD86, and OX40L) on antigen-presenting cells was decreased, and expression of MMP-9, OAT, and TSLP genes was also attenuated.

Conclusion

These results suggest that curcumin has potential therapeutic value for controlling allergic responses resulting from exposure to allergens.

Thymic stromal lymphopoietin is released by human epithelial cells in response to microbes, trauma, or inflammation and potently activates mast cells

Compelling evidence suggests that the epithelial cell-derived cytokine thymic stromal lymphopoietin (TSLP) may initiate asthma or atopic dermatitis through a dendritic cell-mediated T helper (Th)2 response. Here, we describe how TSLP might initiate and aggravate allergic inflammation in the absence of T lymphocytes and immunoglobulin E antibodies via the innate immune system. We show that TSLP, synergistically with interleukin 1 and tumor necrosis factor, stimulates the production of high levels of Th2 cytokines by human mast cells (MCs). We next report that TSLP is released by primary epithelial cells in response to certain microbial products, physical injury, or inflammatory cytokines. Direct epithelial cell-mediated, TSLP-dependent activation of MCs may play a central role in "intrinsic" forms of atopic diseases and explain the aggravating role of infection and scratching in these diseases.