Human thymic stromal lymphopoietin promotes dendritic cell-mediated CD4+ T cell homeostatic expansion

FOXP3+ regulatory T cells in the human immune system

Forkhead box P3 (FOXP3)(+) regulatory T (T(Reg)) cells are potent mediators of dominant self tolerance in the periphery. But confusion as to the identity, stability and suppressive function of human T(Reg) cells has, to date, impeded the general therapeutic use of these cells. Recent studies have suggested that human T(Reg) cells are functionally and phenotypically diverse. Here we discuss recent findings regarding human T(Reg) cells, including the ontogeny and development of T(Reg) cell subsets that have naive or memory phenotypes, the unique mechanisms of suppression mediated by T(Reg) cell subsets and factors that regulate T(Reg) cell lineage commitment. We discuss future studies that are needed for the successful therapeutic use of human T(Reg) cells.

Association of thymic stromal lymphopoietin gene -847C>T polymorphism in generalized vitiligo

Thymic stromal lymphopoietin (TSLP) induces naïve CD4+ T cells to produce Th2 cytokines. In addition, to low production of Th2 cytokines, strong Th1 response, which plays an important role in vitiligo development, has been induced by blockade of TSLP or TSLP receptor. This study examined whether a functional TSLP polymorphism was associated with vitiligo. One hundred and sixty Korean patients with vitiligo and 568 healthy Korean individuals were examined for the four SNPs of TSLP gene. Luciferase activity was measured for promoter assay. The genotype and allele frequencies of -847C>T polymorphism were lower in vitiligo patients compared with the controls, whereas those of wild type were higher (P = 0.004, P = 0.017 respectively). None the less, the promoter activity of -847C decreased significantly (P = 0.013) compared with -847T, expecting lower TSLP mRNA levels in the polymorphism. Collectively, C allele at the TSLP -847C>T polymorphism may increase susceptibility to generalized vitiligo through decreasing TSLP mRNA expression levels.

Sensing the outside world: TSLP regulates barrier immunity

Thymic stromal lymphopoietin (TSLP) is an interleukin 7 (IL-7)-like cytokine originally characterized by its ability to promote the activation of B cells and dendritic cells (DCs). Subsequent studies have shown that TSLP promotes T helper type 2 (TH2) cell responses associated with immunity to some helminth parasites and the pathogenesis of many inflammatory diseases, including atopic dermatitis and asthma. This review will focus on recent findings indicating that in addition to influencing B cell and DC function, TSLP can promote TH2 cytokine-associated inflammation by directly promoting the effector functions of CD4+ TH2 cells, basophils and other granulocyte populations while simultaneously limiting the expression of DC-derived proinflammatory cytokines and promoting regulatory T cell responses in peripheral tissues.

Thymic stromal lymphopoietin

Thymic stromal lymphopoietin (TSLP) is an epithelial cell-derived cytokine expressed in skin, gut, lungs, and thymus. TSLP signals via a TSLP receptor (TSLPR), a heterodimer of the IL-7 receptor alpha chain and the TSLPR chain. The TSLPR chain is closely related to the common receptor gamma chain that is expressed on a wide range of cell types in the adaptive and innate immune system. TSLP exerts a profound influence on the polarization of dendritic cells to drive T helper (Th) 2 cytokine production. TSLP also directly promotes T-cell proliferation in response to T-cell receptor activation and Th2 cytokine production and supports B-cell expansion and differentiation. TSLP further amplifies Th2 cytokine production by mast cells and natural killer T cells. These properties confer on TSLP a critical role in driving Th2-mediated inflammation. This role is supported by the finding that TSLP expression is upregulated in keratinocytes of atopic dermatitis skin lesions and in bronchial epithelial cells in asthma.

Thymic stromal lymphopoietin-activated plasmacytoid dendritic cells induce the generation of FOXP3+ regulatory T cells in human thymus

Human thymus contains major dendritic cell (DC) subsets, myeloid DCs (mDCs), and plasmacytoid DCs (pDCs). We previously showed that mDCs, educated by thymic stromal lymphopoietin (TSLP) produced by the epithelial cells of the Hassall's corpuscles, induced differentiation of CD4(+)CD25(-) thymocytes into Forkhead Box P3(+) (FOXP3(+)) regulatory T cells (T(R)) within the medulla of human thymus. In this study, we show that pDCs expressed the TSLP receptor and IL-7 receptor alpha complexes upon activation and became responsive to TSLP. TSLP-activated human pDCs secrete macrophage-derived chemokine CCL-22 and thymus- and activation-regulated chemokine CCL-17 but not Th1- or Th2-polarizing cytokines. TSLP-activated pDCs induced the generation of FOXP3(+) T(R) from CD4(+)CD8(-)CD25(-) thymocytes, which could be strongly inhibited by Th1-polarizing cytokine IL-12 or Th2-polarizing cytokine IL-4. Interestingly, the FOXP3(+) T(R) induced by the TSLP-pDCs expressed more IL-10 but less TGF-beta than that induced by the TSLP-mDCs. These data suggest that TSLP expressed by thymic epithelial cells can activate mDCs and pDCs to positively select the FOXP3(+) T(R) with different cytokine production potential in human thymus. The inability of TSLP to induce DC maturation without producing Th1- or Th2-polarizing cytokines may provide a thymic niche for T(R) development.

The role of thymic stromal lymphopoietin in allergic inflammation and chronic obstructive pulmonary disease

Thymic stromal lymphopoietin (TSLP) primes dendritic cells to promote a Th2 inflammatory response. Its action is mediated by a heterodimeric receptor which consists of the interleukin-7 receptor alpha chain and the TSLP receptor chain (TSLPR). TSLPR resembles the common gamma chain subunit utilized by many type 1 cytokine receptors. Normal epithelial cells, keratinocytes, and stromal cells constitutively express TSLP. Dendritic cells that are activated by TSLP promote the development of CD4(+) T cells into pro-inflammatory Th2 cells. TSLP thus plays a potentially important role in the pathogenesis of allergic inflammation in asthma and atopic dermatitis. TSLP also has direct effects on other types of cells in the bronchial mucosa. It is over-expressed in the bronchial mucosa in chronic obstructive pulmonary disease (COPD), which is traditionally described as a Th1-related disease, as well as severe asthma, which is traditionally described as a Th2-related disease. In this review we will discuss TSLP expression, function, and available and potential mechanisms in both allergic inflammation and COPD.

Epithelium: the interplay between innate and Th2 immunity

Mucosal epithelium functions not only as a physical barrier, but also as a regulator of innate and adaptive immune responses against foreign substances and microorganisms. In particular, epithelial cells have been directly implicated in Th2 responses, serving as a critical interface between innate immune responses and Th2 immunity. Emerging studies have revealed the cellular and molecular mechanisms by which the epithelium modulates Th2 responses through the production of a group of epithelial-derived Th2-driving cytokines, including interleukin (IL)-25, IL-33, and thymic stromal lymphopoietin. These epithelial-derived Th2-driving cytokines execute a regulatory function of the epithelium on mucosal immunity by promoting Th2 responses and maintaining the balance of host immune homeostasis and defense against various pathogens. Dysregulation of these Th2-driving cytokines can lead to detrimental Th2-dependent inflammatory responses, often manifested in various forms of allergic and inflammatory diseases.

Proinflammatory Th2 cytokines induce production of thymic stromal lymphopoietin in human colonic epithelial cells

PURPOSE

Thymic stromal lymphopoietin (TSLP) is released by intestinal epithelial cells (IECs), and TSLP-conditioned dendritic cells appear to be involved in immune homeostasis of intestine and immunoglobulin A (IgA) class-switching in the physiological condition. In contrast, TSLP activates dendritic cells to induce strong T-cell proliferation and is involved in inflammatory T helper (Th) 2 responses in human allergic diseases. However, it is not clear how TSLP production by IECs is regulated in ulcerative colitis (UC), which appears to involve inflammatory Th2 responses. The aim of this study is to examine how TSLP production by IECs is regulated in ulcerative colitis.

RESULTS

We show here that expression of TSLP was enhanced in mucosal lesions from UC patients in which inflammatory Th2 cytokine production was predominant. In addition, using a human colonic epithelial cell line, we demonstrated that a combination of tumor necrosis factor-alpha (TNF-alpha) and interleukin-4 (IL-4) induced TSLP expression and that TSLP expression by TNF-alpha + IL-4 was further enhanced by either Toll-like receptor 3 ligand or interferon (IFN)-gamma.

CONCLUSIONS

Taken together, as in human allergic diseases, an inflammatory Th2 condition in the mucosal lesions of UC patients may trigger increased TSLP expression by IECs, resulting in exacerbation of UC.

Helicobacter pylori promotes the production of thymic stromal lymphopoietin by gastric epithelial cells and induces dendritic cell-mediated inflammatory Th2 responses

Helicobacter pylori colonizes the stomach and induces strong, specific local and systemic humoral and cell-mediated immunity, resulting in the development of chronic gastritis in humans. Although H. pylori-induced chronic atrophic gastritis is characterized by marked infiltration of T helper type 1 (Th1) cytokine-producing CD4(+) T cells, almost all of the inflamed gastric mucosae also contain focal lymphoid aggregates with germinal centers. In addition, typical H. pylori-induced chronic gastritis in children, called follicular gastritis, is characterized by B-cell follicle formation in the gastric mucosa. The aim of this study was to examine whether thymic stromal lymphopoietin (TSLP), an epithelial-cell-derived cytokine inducing a dendritic cell (DC)-mediated inflammatory Th2 response, is involved in Th2 responses triggering B-cell activation in H. pylori-induced gastritis. Here, we show that H. pylori triggered human gastric epithelial cells to produce TSLP, together with the DC-attracting chemokine MIP-3alpha and the B-cell-activating factor BAFF. After DCs were incubated with supernatants from H. pylori-infected epithelial cells, the conditioned cells expressed high levels of costimulatory molecules, such as CD80, and triggered naïve CD4(+) T cells to produce high levels of the Th2 cytokines interleukin-4 and interleukin-13 and of the inflammatory cytokines tumor necrosis factor alpha and gamma interferon. In contrast, after incubation of the supernatants with the neutralizing antibodies to TSLP, the conditioned DCs did not prime T cells to produce high levels of Th2 cytokines. These results, together with the finding that TSLP was expressed by the epithelial cells of human follicular gastritis, suggest that H. pylori can directly trigger epithelial cells to produce TSLP. It also suggests that TSLP-mediated DC activation may be involved in Th2 responses triggering B-cell activation in H. pylori-induced gastritis.

TSLP production by epithelial cells exposed to immunodeficiency virus triggers DC-mediated mucosal infection of CD4+ T cells

Mucosal dendritic cells have been implicated in the capture, storage, and transmission of HIV to CD4(+) T cells as well as in the promotion of HIV replication in activated CD4(+) T cells during the cognate T-cell and DC interaction. We report that HIV induces human genital mucosal epithelial cells to produce thymic stromal lymphopoietin (TSLP) via activation of the NFkappaB signaling pathway. The TSLP secreted by HIV exposed epithelial cells activated DC, which promoted proliferation and HIV-1 replication of co-cultured autologous CD4(+) T cells. In rhesus macaques, we observed dramatic increases in TSLP expression concurrent with an increase in viral replication in the vaginal tissues within the first 2 weeks after vaginal SIV exposure. These data suggest that HIV-mediated TSLP production by mucosal epithelial cells is a critical trigger for DC-mediated amplification of HIV-infection in activated CD4(+) T cells. The cross talk between mucosal epithelial cells and DC, mediated by HIV-induced TSLP, may be an important mechanism for the high rate of HIV infection in women through the vaginal mucosa.

TSLP and IL-7 use two different mechanisms to regulate human CD4+ T cell homeostasis

Whether thymic stromal lymphopoietin (TSLP) directly induces potent human CD4⁺ T cell proliferation and Th2 differentiation is unknown. We report that resting and activated CD4⁺ T cells expressed high levels of IL-7 receptor a chain but very low levels of TSLP receptor (TSLPR) when compared with levels expressed in myeloid dendritic cells (mDCs). This was confirmed by immunohistology and flow cytometry analyses showing that only a subset of mDCs, with more activated phenotypes, expressed TSLPR in human tonsils in vivo. IL-7 induced strong STAT1, -3, and -5 activation and promoted the proliferation of naive CD4⁺ T cells in the presence of anti-CD3 and anti-CD28 monoclonal antibodies, whereas TSLP induced weak STAT5 activation, associated with marginally improved cell survival and proliferation, but failed to induce cell expansion and Th2 differentiation. The effect of TSLP on enhancing strong human T cell proliferation was observed only when sorted naive CD4⁺ T cells were cultured with mDCs at levels as low as 0.5%. TSLP could only induce naive CD4⁺ T cells to differentiate into Th2 cells in the presence of allogeneic mDCs. These results demonstrate that IL-7 and TSLP use different mechanisms to regulate human CD4⁺ T cell homeostasis.

Variation in IL7R predisposes to sarcoid inflammation

Sarcoidosis is a chronic granulomatous disorder characterized by a massive influx of Th1 lymphocytes. Both naive and memory T cells express high levels of interleukin 7 receptor-alpha (IL7R alpha), encoded by the IL7R gene. The purpose of this study was to investigate the role of the IL7R gene region in susceptibility to sarcoidosis. Six common single-nucleotide polymorphisms (SNPs) spanning IL7R were genotyped and analyzed in 475 sarcoidosis patients and 465 healthy controls. Replication of one significant associated SNP was carried out in 206 independent sarcoidosis patients, 127 controls and 126 patients with Löfgren's disease. The rs10213865 SNP was associated with sarcoidosis (P=0.008), and in silico analysis showed a complete linkage (r(2)=1, D'=1) with a functional nonsynonymous coding SNP in exon 6 (rs6897932, T244I). Combined analysis of 663 individuals with sarcoidosis and 586 controls (homozygous carriers of risk allele, P=5 x 10(-4), odds ratio=1.49 (1.19-1.86)) provided strong statistical support for a genuine association of IL7R with the risk of sarcoidosis. In addition, we report the same trend between variation in the IL7R gene and patients with Löfgren's disease, suggesting that variation in IL7R may confer general risk for developing granulomatous lung disease.

Regulation of helminth-induced Th2 responses by thymic stromal lymphopoietin

Thymic stromal lymphopoietin was recently identified as a master switch for the development of allergen-driven Th2 responses. However, the role of thymic stromal lymphopoietin (TSLP) in the development of helminth-induced Th2 responses is unclear. Here, using TSLPR(-/-) mice, we show that while TSLPR signaling participates in the development of Schistosoma mansoni egg-induced CD4(+) Th2 responses, it plays only a transient role in the development of Th2-dependent pathology in the lung, liver, and intestine. Studies conducted in a pulmonary granuloma model showed that while a reduction in IL-4/IL-13-dependent granulomatous inflammation and tissue eosinophilia was observed in TSLPR(-/-) mice undergoing a primary response, lesion formation was not affected during a secondary granulomatous response, even though IL-5 and IL-13 were modestly reduced in the knockout mice. To evaluate the importance of TSLPR signaling in the development of a chronic Th2-dependent response, TSLPR(-/-) mice were also infected with S. mansoni cercariae. Here, the only significant difference noted in TSLPR(-/-) mice was a modest decrease in liver fibrosis in acutely infected animals. The transient decrease in fibrosis was associated with increased production of the antifibrotic cytokine IFN-gamma and decreased production of the profibrotic cytokine IL-13. Although the altered cytokine response persisted in chronically infected TSLPR(-/-) mice, it failed to reduce granuloma formation or fibrosis, confirming that TSLPR signaling plays a limited role in the development of chronic Th2-dependent pathology. Collectively, these findings suggest that while TSLPR signaling serves a key role in allergen-driven Th2 responses, it exerts minor regulatory activity during this chronic helminth infection.

Dendritic cells: biology of the skin

Allergic contact dermatitis results from a T-cell-mediated, delayed-type hypersensitivity immune response induced by allergens. Skin dendritic cells (DCs) play a central role in the initiation of allergic skin responses. Following encounter with an allergen, DCs become activated and undergo maturation and differentiate into immunostimulatory DCs and are able to present antigens effectively to T cells. The frequency of allergic skin disorders has increased in the past decades. Therefore, the identification of potential sensitizing chemicals is important for skin safety. Traditionally, predictive testing for allergenicity has been conducted in animal models. For regulatory reasons, animal use for sensitization testing of compounds for cosmetic purposes is shortly to be prohibited in Europe. Therefore, new non-animal-based test methods need to be developed. Several DC-based assays have been described to discriminate allergens from irritants. Unfortunately, current in vitro methods are not sufficiently resilient to identify allergens and therefore need refinement. Here, we review the immunobiology of skin DCs (Langerhans' cells and dermal dendritic cells) and their role in allergic and irritant contact dermatitis and then explore the possible use of DC-based models for discriminating between allergens and irritants.

Welcome to the neighborhood: epithelial cell-derived cytokines license innate and adaptive immune responses at mucosal sites

There is compelling evidence that epithelial cells (ECs) at mucosal surfaces, beyond their role in creating a physical barrier, are integral components of innate and adaptive immunity. The capacity of these cells to license the functions of specific immune cell populations in the airway and gastrointestinal tract offers the prospect of novel therapeutic strategies to target multiple inflammatory diseases in which barrier immunity is dysregulated. In this review, we discuss the critical functions of EC-derived thymic stromal lymphopoietin (TSLP), interleukin-25 (IL-25), and IL-33 in the development and regulation of T-helper 2 (Th2) cytokine-dependent immune responses. We first highlight recent data that have provided new insights into the factors that control expression of this triad of cytokines and their receptors. In addition, we review their proinflammatory and immunoregulatory functions in models of mucosal infection and inflammation. Lastly, we discuss new findings indicating that despite their diverse structural features and differential expression of their receptors, TSLP, IL-25, and IL-33 cross-regulate one another and share overlapping properties that influence Th2 cytokine-dependent responses at mucosal sites.

Human dendritic cell subsets for vaccination

Protective immunity results from the interplay of antigen (Ag)-nonspecific innate immunity and Ag-specific adaptive immunity. The cells and molecules of the innate system employ non-clonal recognition pathways such as lectins and TLRs. B and T lymphocytes of the adaptive immune system employ clonal receptors recognizing Ag or peptides in a highly specific manner. An essential link between innate and adaptive immunity is provided by dendritic cells (DCs). As a component of the innate immune system, DC organize and transfer information from the outside world to the cells of the adaptive immune system. DC can induce such contrasting states as active immune responsiveness or immunological tolerance. Recent years have brought a wealth of information regarding DC biology and pathophysiology that shows the complexity of this cell system. Thus, presentation of antigen by immature (non-activated) DCs leads to tolerance, whereas mature, antigen-loaded DCs are geared towards the launching of antigen-specific immunity. Furthermore, DCs are composed of multiple subsets with distinct functions at the interface of the innate and adaptive immunity. Our increased understanding of DC pathophysiology will permit their rational manipulation for therapy such as vaccination to improve immunity.

The thymus medulla slowly yields its secrets

The past few years have witnessed considerable advances in our understanding of the mechanisms underlying the induction of central tolerance. Medullary thymic epithelial cells (mTECs) play a pivotal role in this process by virtue of promiscuous expression of tissue-restricted autoantigens. This brief review covers progress of the last two years in deciphering the functional interrelationship among TEC development, promiscuous gene expression, and central tolerance. We discuss new insights into signaling pathways directing the differentiation and homeostasis of mTECs, and new clues to the molecular regulation of promiscuous gene expression (pGE), including the role of the transcriptional regulator autoimmune regulator (AIRE). Furthermore, we emphasize the importance of promiscuous expression of particular tissue-restricted self-antigens in preventing organ-specific autoimmunity and evaluate new data supporting the threshold model of central tolerance.

TSLP in epithelial cell and dendritic cell cross talk

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 instruct the initiation of different forms of immunity and tolerance. The functional versatility of DCs depends on their remarkable ability to translate collectively the information from the invading microbes, as well as their resident tissue microenvironments. Recent progress in understanding Toll-like receptor (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 strong evidence at a molecular level that epithelial cells/tissue microenvironments directly communicate with DCs, the professional antigen-presenting cells of the immune system. 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.

Antiviral-activated dendritic cells: a paracrine-induced response state

Infection of immature dendritic cells (DCs) by virus stimulates their maturation into APC. Infected DCs can also expose uninfected DCs to a panoply of cytokines/chemokines via paracrine signaling. Mathematical modeling suggests that a high rate of paracrine signaling is likely to occur among DCs located in three-dimensional space. Relatively little is known about how secreted factors modify the early response to virus infection. We used a transwell experimental system that allows passage of secreted factors, but not direct contact, between virus-infected DCs and uninfected DCs to investigate paracrine signaling responses. Paracrine signaling from infected DCs induced an antiviral-primed DC state distinct from that of mature virus-infected DCs that we refer to as antiviral-activated DCs (AVDCs). AVDCs had increased surface MHC class II and CD86 levels, but in contrast to virus-infected DCs, their MHC class I levels were unchanged. Imaging flow cytometry showed that AVDCs had an increased rate of phagocytosis compared with naive DCs. Experiments with IFN-beta cytokine indicated that it may be responsible for CD86, but not MHC class II regulation in AVDCs. Both IFN-inducible and IFN-independent genes are up-regulated in AVDCs. Notably, AVDCs are relatively resistant to virus infection in comparison to naive DCs and achieve accelerated and augmented levels of costimulatory molecule expression with virus infection. AVDCs show a distinct antiviral-primed state of DC maturation mediated by DC paracrine signaling. Although further in vivo study is needed, the characteristics of the AVDC suggest that it is well suited to play a role in the early innate-adaptive transition of the immune system.

Human TSLP and TLR3 ligands promote differentiation of Th17 cells with a central memory phenotype under Th2-polarizing conditions

Background Human thymic stromal lymphopoietin (TSLP) is expressed in the human asthmatic lung and activates dendritic cells (DCs) to strongly induce proallergic T‐helper type 2 (Th2) cell responses, suggesting that TSLP plays a critical role in the pathophysiology of human asthma. Th2 cells are predominantly involved in mild asthma, whereas a mixture of Th1 and Th2 cells with neutrophilic inflammation, probably induced by Th17, affects more severe asthmatic disease. Exacerbation of asthmatic inflammation is often triggered by airway‐targeting RNA viral infection; virus‐derived double‐stranded RNA, Toll‐like receptor (TLR)3 ligand, activates bronchial epithelial cells to produce pro‐inflammatory mediators, including TSLP.

Objective Because TSLPR‐expressing DCs express TLR3, we examined how the relationship between TSLP and TLR3 ligand stimulation influences DC activation.

Methods CD11c⁺DCs purified from adult peripheral blood were cultured in TLR ligands containing media with or without TSLP and then co‐cultured with allogeneic naïve CD4⁺T cells.

Results CD11c⁺ DCs responded to a combination of TSLP and TLR3 ligand, poly(I : C), to up‐regulate expression of the functional TSLP receptor and TLR3. Although TSLP alone did not induce IL‐23 production by DCs, poly(I : C) alone primed DCs for the production of IL‐23, and a combination of TSLP and poly(I : C) primed DCs for further production of IL‐23. The addition of poly(I : C) did not inhibit TSLP‐activated DCs to prime naïve CD4⁺ T cells to differentiate into inflammatory Th2 cells. Furthermore, DCs activated by a combination of TSLP and poly(I : C) primed more naïve CD4⁺ T cells to differentiate into Th17‐cytokine–producing cells with a central memory T cell phenotype compared with DCs activated by poly(I : C) alone.

Conclusions These results suggest that through DC activation, human TSLP and TLR3 ligands promote differentiation of Th17 cells with the central memory T cell phenotype under Th2‐polarizing conditions.

Characterisation of effector mechanisms at the host:parasite interface during the immune response to tissue-dwelling intestinal nematode parasites

The protective immune response that develops following infection with many tissue-dwelling intestinal nematode parasites is characterised by elevations in IL-4 and IL-13 and increased numbers of CD4+ T cells, granulocytes and macrophages. These cells accumulate at the site of infection and in many cases can mediate resistance to these large multicellular pathogens. Recent studies suggest novel potential mechanisms mediated by these immune cell populations through their differential activation and ability to stimulate production of novel effector molecules. These newly discovered protective mechanisms may provide novel strategies to develop immunotherapies and vaccines against this group of pathogens. In this review, we will examine recent studies elucidating mechanisms of host protection against three widely-used experimental murine models of tissue-dwelling intestinal nematode parasites: Heligmosomoides polygyrus, Trichuris muris and Trichinella spiralis.

Human TSLP directly enhances expansion of CD8+ T cells

Human thymic stromal lymphopoietin (TSLP) promotes CD4(+) T-cell proliferation both directly and indirectly through dendritic cell (DC) activation. Although human TSLP-activated DCs induce CD8(+) T-cell proliferation, it is not clear whether TSLP acts directly on CD8(+) T cells. In this study, we show that human CD8(+) T cells activated by T-cell receptor stimulation expressed TSLP receptor (TSLPR), and that TSLP directly enhanced proliferation of activated CD8(+) T cells. Although non-stimulated human CD8(+) T cells from peripheral blood did not express TSLPR, CD8(+) T cells activated by anti-CD3 plus anti-CD28 did express TSLPR. After T-cell receptor stimulation, TSLP directly enhanced the expansion of activated CD8(+) T cells. Interestingly, using monocyte-derived DCs pulsed with a cytomegalovirus (CMV)-specific pp65 peptide, we found that although interleukin-2 allowed expansion of both CMV-specific and non-specific CD8(+) T cells, TSLP induced expansion of only CMV-specific CD8(+) T cells. These results suggest that human TSLP directly enhances expansion of CD8(+) T cells and that the direct and indirect action of TSLP on expansion of target antigen-specific CD8(+) T cells may be beneficial to adoptive cell transfer immunotherapy.

Thymic stromal lymphopoietin and thymic stromal lymphopoietin-conditioned dendritic cells induce regulatory T-cell differentiation and protection of NOD mice against diabetes

OBJECTIVE

Autoimmune diabetes in the nonobese diabetic (NOD) mouse model results from a breakdown of T-cell tolerance caused by impaired tolerogenic dendritic cell development and regulatory T-cell (Treg) differentiation. Re-establishment of the Treg pool has been shown to confer T-cell tolerance and protection against diabetes. Here, we have investigated whether murine thymic stromal lymphopoietin (TSLP) re-established tolerogenic function of dendritic cells and induced differentiation and/or expansion of Tregs in NOD mice and protection against diabetes.

RESEARCH DESIGN AND METHODS

We examined the phenotype of TSLP-conditioned bone marrow dendritic cells (TSLP-DCs) of NOD mice and their functions to induce noninflammatory Th2 response and differentiation of Tregs. The functional relevance of TSLP and TSLP-DCs to development of diabetes was also tested.

RESULTS

Our results showed that bone marrow dendritic cells of NOD mice cultured in the presence of TSLP acquired signatures of tolerogenic dendritic cells, such as an absence of production of pro-inflammatory cytokines and a decreased expression of dendritic cell costimulatory molecules (CD80, CD86, and major histocompatibility complex class II) compared with LPS-treated dendritic cells. Furthermore, TSLP-DCs promoted noninflammatory Th2 response and induced the conversion of naïve T-cells into functional CD4(+)CD25(+)Foxp3(+) Tregs. We further showed that subcutaneous injections of TSLP for 6 days or a single intravenous injection of TSLP-DCs protected NOD mice against diabetes.

CONCLUSIONS

Our study demonstrates that TSLP re-established a tolerogenic immune response in NOD mice and protects from diabetes, suggesting that TSLP may have a therapeutic potential for the treatment of type 1 diabetes.

Epithelial-cell recognition of commensal bacteria and maintenance of immune homeostasis in the gut

Mucosal surfaces such as the intestinal tract are continuously exposed to both potential pathogens and beneficial commensal microorganisms. This creates a requirement for a homeostatic balance between tolerance and immunity that represents a unique regulatory challenge to the mucosal immune system. Recent findings suggest that intestinal epithelial cells, although once considered a simple physical barrier, are a crucial cell lineage for maintaining intestinal immune homeostasis. This Review discusses recent findings that identify a cardinal role for epithelial cells in sampling the intestinal microenvironment, discriminating pathogenic and commensal microorganisms and influencing the function of antigen-presenting cells and lymphocytes.

Fungal proteases induce Th2 polarization through limited dendritic cell maturation and reduced production of IL-12

Allergens are capable of polarizing the T cell immune response toward a Th2 cytokine profile in a process that is mediated by dendritic cells (DCs). Proteases derived from Aspergillus species (Aspergillus proteases; AP) have been shown to induce a Th2-like immune response when administered directly to the airway and without adjuvant or prior priming immunizations at sites remote from the lung in models of allergic airway disease. To explore mechanisms that underlie the Th2 immune response, we have investigated the effect of AP on DC function. We found that human DCs derived from CD14(+) monocytes from healthy donors underwent partial maturation when incubated with AP. Naive allogeneic T cells primed with AP-activated DCs proliferated and displayed enhanced production of IL-4 and reduced expression of IFN-gamma as compared with naive T cells primed with LPS-activated DCs. Global gene expression analysis of DCs revealed relatively low expression of IL-12p40 in AP-activated DCs as compared with those activated by LPS, and this was confirmed at the protein level by ELISA. Exogenous IL-12p70 added to cocultures of DCs and T cells resulted in reduced IL-4 and increased IFN-gamma expression when DCs were activated with AP. When the proteolytic activity of AP was neutralized by chemical inactivation it failed to up-regulate costimulatory molecules on DCs, and these DCs did not prime a Th2 response in naive T cells. These findings provide a mechanism for explaining how proteolytically active allergens could preferentially induce Th2 responses through limited maturation of DCs with reduced production of IL-12.

Histochemical and molecular overview of the thymus as site for T-cells development

The thymus represents the primary site for T cell lymphopoiesis, providing a coordinated set for critical factors to induce and support lineage commitment, differentiation and survival of thymus-seeding cells. One irrefutable fact is that the presence of non-lymphoid cells through the thymic parenchyma serves to provide coordinated migration and differentiation of T lymphocytes. Moreover, the link between foetal development and normal anatomy has been stressed in this review. Regarding thymic embryology, its epithelium is derived from the embryonic endodermal layer, with possible contributions from the ectoderm. A series of differentiating steps is essential, each of which must be completed in order to provide the optimum environment for thymic development and function. The second part of this article is focused on thymic T-cell development and differentiation, which is a stepwise process, mediated by a variety of stromal cells in different regions of the organ. It depends strongly on the thymic microenvironment, a cellular network formed by epithelial cells, macrophages, dendritic cells and fibroblasts, that provide the combination of cellular interactions, cytokines and chemokines to induce thymocyte precursors for the generation of functional T cells. The mediators of this process are not well defined but it has been demonstrated that some interactions are under neuroendocrine control. Moreover, some studies pointed out that reciprocal signals from developing T cells also are essential for establishment and maintenance of the thymic microenvironment. Finally, we have also highlighted the heterogeneity of the lymphoid, non-lymphoid components and the multi-phasic steps of thymic differentiation. In conclusion, this review contributes to an understanding of the complex mechanisms in which the foetal and postnatal thymus is involved. This could be a prerequisite for developing new therapies specifically aimed to overcome immunological defects, linked or not-linked to aging.

Early fetal gene delivery utilizes both central and peripheral mechanisms of tolerance induction

OBJECTIVE

We previously reported the induction of stable immune tolerance following direct injection of retroviral vectors into preimmune fetal sheep. In the present studies, we conduct detailed analysis of the thymus of recipients of in utero gene transfer (IUGT) to delineate the mechanism of the observed immune tolerance and assess the impact of recipient age on this process.

MATERIALS AND METHODS

Fetal sheep at varying gestational ages received the MSCV-NeoR-RFP retroviral vector. The thymus was then collected from these animals at 27 to 30 days postinjection and analyzed for evidence of transduction of key immunoregulatory thymic cells.

RESULTS

Our results reveal that both thymic epithelial cells (TEC), crucial for presentation of self-antigen during T-cell thymic selection, and the cells comprising the Hassall's corpuscles, which can present antigen directly and also instruct dendritic cells to induce the formation of CD4(+)CD25(+) T-regulatory cells in the thymus, were only efficiently transduced if IUGT was performed early in gestation.

CONCLUSIONS

Our findings thus demonstrate, for the first time, that early IUGT can potentially take advantage of multiple tolerogenic avenues in the fetus, transducing both TEC, which promote central tolerance, and Hassall's corpuscles, which induce formation of T regulatory cells that could act to maintain peripheral tolerance to the transgene products.

Atypical memory phenotype T cells with low homeostatic potential and impaired TCR signaling and regulatory T cell function in Foxn1Delta/Delta mutant mice

Foxn1Delta/Delta mutants have a block in thymic epithelial cell differentiation at an intermediate progenitor stage, resulting in reduced thymocyte cellularity and blocks at the double-negative and double-positive stages. Whereas naive single-positive thymocytes were reduced >500-fold in the adult Foxn1Delta/Delta thymus, peripheral T cell numbers were reduced only 10-fold. The current data shows that Foxn1Delta/Delta peripheral T cells had increased expression of activation markers and the ability to produce IL-2 and IFN-gamma. These cells acquired this profile immediately after leaving the thymus as early as the newborn stage and maintained high steady-state proliferation in vivo but decreased proliferation in response to TCR stimulation in vitro. Single-positive thymocytes and naive T cells also had constitutively low alphabetaTCR and IL7R expression. These cells also displayed reduced ability to undergo homeostatic proliferation and increased rates of apoptosis. Although the frequency of Foxp3+CD4+CD25+ T cells was normal in Foxn1Delta/Delta mutant mice, these cells failed to have suppressor function, resulting in reduced regulatory T cell activity. Recent data from our laboratory suggest that T cells in the Foxn1Delta/Delta thymus develop from atypical progenitor cells via a noncanonical pathway. Our results suggest that the phenotype of peripheral T cells in Foxn1Delta/Delta mutant mice is the result of atypical progenitor cells developing in an abnormal thymic microenvironment with a deficient TCR and IL7 signaling system.

Keratinocyte growth factor induces expansion of murine peripheral CD4+Foxp3+ regulatory T cells and increases their thymic output

Keratinocyte growth factor (KGF) has been shown to reduce the incidence and severity of graft-versus-host disease by prevention of epithelial damage and by modulating alloreactivity. Since regulatory T cells (Treg) play a crucial role in immune modulation, we evaluated the effects of exogenous KGF on peripheral CD4(+)Foxp3(+) Treg and the generation of Treg in the thymus of normal mice. A 3-day course of KGF induced a rapid selective increase in the number of highly suppressive CD4(+)Foxp3(+) Treg. Blood Treg numbers remained elevated for >2 mo, but the frequency normalized after 2 wk due to a concomitant increase in CD4(+)Foxp3(-) T cells. Analysis of single joint TCR excision circles frequency and Ki-67 expression in peripheral blood Treg showed that the early selective increase of Treg was predominantly accounted for by peripheral expansion. Thymectomy before KGF administration did not affect the early selective increase of Treg but abrogated the late increase in CD4(+) T cell numbers, thereby showing its dependence on thymic output. Collectively, these results show that KGF induces an increase in blood CD4(+)Foxp3(+) Treg numbers via two independent mechanisms. First by selective peripheral expansion of Treg and thereafter by enhanced thymic output of newly developed Treg.

Peroxisome proliferator-activated receptor alpha regulates skin inflammation and humoral response in atopic dermatitis

BACKGROUND

The peroxisome proliferator-activated receptors (PPARs) alpha, beta/delta, and gamma are ligand-activated transcription factors belonging to the nuclear receptor superfamily. In addition to their regulatory role on lipid and glucose metabolism, they exert anti-inflammatory properties. In skin both PPAR-alpha and PPAR-beta/delta regulate keratinocyte proliferation/differentiation and contribute to wound healing. The 3 PPAR isoforms are expressed by several cell types recruited into the dermis during inflammation.

OBJECTIVE

We have investigated the role of PPAR-alpha in the regulation of atopic dermatitis (AD), a common skin inflammatory disease.

METHODS

We chose a mouse model of inflammatory dermatosis with immunologic features of AD and used epicutaneous sensitization with ovalbumin in the absence of adjuvant, which mimics the human pathology.

RESULTS

On antigen sensitization, PPAR-alpha-deficient mice display increased epidermal thickening, dermal recruitment of inflammatory cells, lung inflammation, airway hyperresponsiveness, and IgE and IgG2a production compared with their wild-type counterparts. Increased inflammation was correlated to an enhancement of TH2 and, to a greater extent, TH1 responses and to increased skin expression of nuclear factor kappaB. Interestingly, PPAR-alpha expression was decreased in eczematous skin from patients with AD compared with skin from nonatopic donors, suggesting that defective PPAR-alpha expression might contribute to the pathology. Topical application of WY14643, a specific PPAR-alpha agonist, significantly decreased antigen-induced skin inflammation in the AD model.

CONCLUSION

PPAR-alpha acts as a negative regulator of skin inflammation in AD.

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.

Immune therapies

Immune cells with specific functions and abilities are vital to cancer treatment prevention. Although there have been many accomplishments made in the areas of immunotherapy and immunobiology of myeloma, there are still many obstacles in the way of conceptualizing the interrelationships between immune cells and tumor cells. To provide better understanding of these concepts and to move toward improved therapies for myeloma, cell-based therapeutic approaches should be developed.

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.

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.

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.

Dengue virus-specific suppressor T cells: current perspectives

Dengue virus was the first microorganism that was shown to induce generation of antigen-specific suppressor T (TS) cells in mice. The cascade of the three generations of TS cells (TS1, TS2, TS3) and their secretary products, the suppressor factors (SF1, SF2), was delineated. The TS pathway was proposed to be protective through inhibition of the production of enhancing antibody, which may enhance the severity of dengue disease. The currently second most favoured mechanism of severe dengue disease is the 'cytokine tsunami'. During the last decade, suppressor/regulatory T cells have been studied in greater detail using modern techniques in various diseases, including viral infections. This brief review discusses the role of dengue-specific suppressor T cells in protection and/or induction of severe dengue disease in view of our current understanding of suppressor/regulatory T cells.

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.

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.

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.

Target-organ specificity of autoimmunity is modified by thymic stroma and bone marrow-derived cells

Physical contact between thymocytes and the thymic stroma is essential for the establishment of self-tolerance, and Aire in thymic epithelial cells plays an important role in this action. As expected, the autoimmune phenotypes of Aire-deficient mice are thymic stroma-dependent. Interestingly, the spectrum of the organs involved differs depending on the genetic background of non-autoimmune-prone mouse strains. Furthermore, deficiency of Aire in an autoimmune-prone strain of NOD also modifies target-cell specificity in the pancreas. In order to clarify the factors that regulate target-organ specificity in Aire-dependent autoimmunity, I have generated both thymic and bone-marrow chimeras, making it possible to evaluate the contribution of thymic stroma and bone-marrow-derived cells to this pathogenic process. The findings suggested that the genetic background of bone-marrow-derived cells contributes to the strain-dependent target-organ specificity of non-autoimmune-prone strains. Furthermore, in a study using NOD mice with a fixed genetic background, thymic stromal cells but not bone-marrow-derived cells were found to be relevant to the Aire-dependent alteration of target-cell specificity in the pancreas. These results clearly underscore the significance of immunological and/or genetic complexity that underlies Aire-deficiency monogenic disease together with critical dialogue between thymic stroma and bone-marrow-derived cells in the organized thymic microenvironment.

Epithelial cells trigger frontline immunoglobulin class switching through a pathway regulated by the inhibitor SLPI

Epithelial cells (ECs) transport class-switched immunoglobulin G (IgG) and IgA antibodies across mucous membranes. Whether ECs initiate class switching remains unknown. Here we found that ECs lining tonsillar crypts formed pockets populated by B cells expressing activation-induced cytidine deaminase (AID), an enzyme associated with ongoing class switching. ECs released B cell-activating AID-inducing factors after sensing microbial products through Toll-like receptors. The resulting class switching was amplified by thymic stromal lymphopoietin, an epithelial interleukin 7-like cytokine that enhanced the B cell 'licensing' function of dendritic cells, and was restrained by secretory leukocyte protease inhibitor, an epithelial homeostatic protein that inhibited AID induction in B cells. Thus, ECs may function as mucosal 'guardians' orchestrating frontline IgG and IgA class switching through a Toll-like receptor-inducible signaling program regulated by secretory leukocyte protease inhibitor.NOTE: In the version of this article initially published online, the middle label above Figure 6c is incorrect. The correct label should be 'BAFF'. The error has been corrected for all versions of the article.