A role for TSLP in the development of inflammation in an asthma model

Thymic stromal lymphopoietin (TSLP) as a bridge between infection and atopy

The rising worldwide prevalence of asthma has intensified interest in the natural history of asthma. An improved understanding of the genetic, environmental, and developmental factors contributing to the inception and exacerbation of asthma will be crucial to efforts to devise effective preventive and therapeutic interventions. There is increasing evidence that the complex interplay of early life respiratory viral infections and allergic sensitization is important in the development of asthma. Major causes of asthma exacerbations are respiratory viral infections and aeroallergen exposure, which may have interactive co-morbid effects. This review describes the potential role of thymic stromal lymphopoietin (TSLP) as a connection between the innate immune response to respiratory viral infections and the type-2 adaptive immune response in the development and exacerbation of asthma.

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.

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.

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.

Inhibitory receptor gp49B regulates eosinophil infiltration during allergic inflammation

gp49B, an Ig-like receptor, negatively regulates the activity of mast cells and neutrophils through cytoplasmic immunoreceptor tyrosine-based inhibition motifs. To characterize the role of gp49B further in vivo, gp49B-deficient mice were tested in two allergic models. Responses to ragweed (RW) challenge in the lung and conjunctiva were assessed in models of allergic inflammation and during an infection with parasitic larvae of the nematode Ascaris suum. Infiltration by inflammatory cells into the lung during allergic responses was under negative control of the inhibitory receptor gp49B. Furthermore, an increase in conjunctival inflammation with a predominance of eosinophils, neutrophils, and degranulated mast cells was observed in RW-sensitized, gp49B-deficient mice, which had been challenged in the eye, as compared with C57BL/6 wild-type (WT) controls. Finally, an increase in allergic inflammation in the lungs of A. suum-infected, RW-sensitized mice was observed upon RW challenge, as compared with C57BL/6 WT controls. The observed influx of eosinophils into mucus membranes is characteristic of allergic asthma and allergic conjunctivitis and may contribute to airway hyper-responsiveness, airway remodeling, and mucus production. Expression of gp49B was detected on peripheral eosinophils of control mice and on eosinophils from lungs of mice treated with RW, suggesting a role for gp49B on eosinophils in dampening allergic inflammatory responses.

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.

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.

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.

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.

Susceptibility to asthma and eczema from mucosal and epidermal expression of distinctive genes

The past several years have seen an increase in the rate at which genes that are associated with allergic asthma and eczema are discovered. This review -examines genetic association, gene expression, and functional studies that have identified genes that are expressed in the epithelial cells of the skin and lung and are involved in the pathogenesis of allergic asthma and eczema. This includes the genes encoding thymic stromal lympho-poietin (TSLP) and suppressor of cytokine signaling (SOCS3) that are involved in the activation of T-helper 2 cells, the microbial pattern recognition receptors nucleotide-binding oligomerization domain (NOD) genes (CARD4 and CARD15), Toll-like receptors (TLR2 and TLR4), and filaggrin, a protein required for effective barrier defense of the skin. Therefore, the development of allergic disease involves both the adaptive and innate immune systems, and the expression of these genes in the skin and lungs suggests a link to environmental -triggers at body surfaces.

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.

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.

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.

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.

Targeting leukocyte trafficking to inflamed skin - still an attractive therapeutic approach?

Research into leukocyte trafficking and its therapeutic exploitation appears to be a multistep process, just like the trafficking cascade itself. The initial euphoria evoked by an early understanding of the trafficking steps was followed by considerable disappointment following the clinical failure of the first selectin antagonist Cylexin (CY-1503), a sialyl Lewis(X) mimetic. The research area recovered and identified additional attractive pharmacological targets such as chemokine receptors and integrins. However, after lack of efficacy in anti-chemokine trials and the fatalities associated with anti VLA-4 therapy (Tysabri), the question arose again whether targeting leukocyte trafficking is really promising or whether such a complex, multistep process with many redundant and/or functionally overlapping molecules is simply too challenging to deal with. In this article, we delineate some pros and cons of this approach followed by a brief update on where we stand in the field and where we might move in the future.

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.

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.

Ascaris suum-derived products suppress mucosal allergic inflammation in an interleukin-10-independent manner via interference with dendritic cell function

We have previously demonstrated that protection from allergic inflammation by Ascaris suum infection was characterized by a global increase in interleukin-10 (IL-10) and the development of protective CD4(+)/CD25(+) T cells (L. Schopf, S. Luccioli, V. Bundoc, P. Justice, C. C. Chan, B. J. Wetzel, H. H. Norris, J. F. Urban, Jr., and A. Keane-Myers, Investig. Ophthalmol. Vis. Sci. 46:2772-2780, 2005). Here, we used A. suum pseudocoelomic fluid (PCF) in lieu of infection to define molecular mechanisms of allergic protection in a mouse model of allergic inflammation. Mice were sensitized with ragweed (RW) and PCF (RW/PCF), PCF alone, or RW alone and then challenged intratracheally, intranasally, and supraocularly with RW. Histological examination of the eyes and lungs, analysis of the bronchoalveolar lavage fluid (BALF), and characterization of ex vivo cytokine responses were performed to determine allergic inflammatory responses. RW/PCF-treated mice had suppressed allergic immune responses compared to mice given RW alone. To investigate whether IL-10 was involved in PCF-mediated allergic protection, similar experiments were performed using mice genetically deficient for IL-10. Persistent protection from allergic disease was observed in the absence of IL-10, indicating the primary mechanism of PCF protection is IL-10 independent. Ex vivo and in vitro analysis of PCF-treated dendritic cells (DC) demonstrated reduced activation receptor expression and cytokine production in response to either RW or lipopolysaccharide stimulation. These findings extend previous studies that showed infection with A. suum alters expression of allergic disease and suggest that PCF can contribute to this effect by interference with DC function.

Cytokines and chemokines orchestrate atopic skin inflammation

Atopic dermatitis (AD) is a common pruritic and chronically relapsing inflammatory skin disease. The pathophysiology of AD includes disturbed skin barrier functions, frequent allergic responses against allergens, defects in the antimicrobial immune defense, and a genetic predisposition. In this review we summarize advances in our understanding of the complex interdependent network of members of the rapidly growing protein superfamilies of cytokines and chemokines that lead to the development of AD.

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.

New aspects on inflammation in allergic diseases

BACKGROUND

Allergic disease has currently reached epidemic proportions, with a high percentage of individuals in the developed world exhibiting an allergic response after exposure to some common environmental factors. Although new strategies for the treatment and management of allergic diseases have decreased the mortality rate, a high percentage of affected persons still require frequent hospitalization and experience decreased quality of life.

METHODS

An internet-based literature search was performed for recent contributions on the underlying mechanisms provoking an allergic response and their potential for therapeutic approaches.

RESULTS

Novel concepts on allergic responses have emerged: allergic disease may result from an imbalance between allergen activation of regulatory T cells and effector T helper 2 cells (Th2), a process in which dendritic cells are key players. Cytokines such as interleukin (IL)-6, IL-21, IL-25, and human thymic stromal lymphopoietin (TSLP) seem to be important contributors in allergic processes. New data on IgE effector responses and on the IgE-independent mechanisms involved in allergic reactions have resolved some unanswered questions about these reactions.

CONCLUSIONS

These new findings on allergic diseases have important implications for diagnosis and management, with potential improvements in prevention and treatment, which could provide a cure in the future.

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.