IL-13 induces skin fibrosis in atopic dermatitis by thymic stromal lymphopoietin

Skin fibrotic remodeling is a major feature in human atopic dermatitis (AD). Inflammation and tissue fibrosis are common consequences of Th2 responses. Elevated IL-13 and thymic stromal lymphopoietin (TSLP) have been found in the AD skin lesions. Fibrocytes can be recruited to inflamed tissues to promote wound healing and fibrosis. Dermal transgenic expression of IL-13 causes an AD-like phenotype with fibrosis and increased TSLP. However, the role of TSLP in fibrotic remodeling is unknown. In this study, we investigated the role of TSLP and fibrocytes in the generation of IL-13-induced skin fibrosis. In AD lesion, cessation of IL-13 transgene expression resulted in reduced skin inflammation but with no effect on further progression of fibrosis. This was accompanied by markedly increased CD34(+)/procollagen 1(+) fibrocytes. Furthermore, fibrocytes express TSLP receptor (TSLPR), and TSLP directly promotes PBMC-derived fibrocytes to produce collagen. Neutralization of TSLP or genetic deletion of TSLPR in IL-13 transgenic mice resulted in a significant reduction in fibrocytes and in skin fibrosis. Furthermore, reduction of fibrosis by depletion of TSLP was independent of IL-13. Interestingly, the number of fibrocytes was highly increased in the skin samples of AD patients. These data indicate that the progression of skin fibrosis in IL-13-induced AD occurs via TSLP/TSLPR-dependent but IL-13-independent novel mechanisms by promoting fibrocyte functions.

IL-21 is pivotal in determining age-dependent effectiveness of immune responses in a mouse model of human hepatitis B

HBV is a noncytopathic hepadnavirus and major human pathogen that causes immune-mediated acute and chronic hepatitis. The immune response to HBV antigens is age dependent: viral clearance occurs in most adults, while neonates and children usually develop chronic infection and liver disease. Here, we characterize an animal model for HBV infection that recapitulates the key differences in viral clearance between early life and adulthood and find that IL-21 may be part of an effective primary hepatic immune response to HBV. In our model, adult mice showed higher HBV-dependent IL-21 production in liver, compared with that of young mice. Conversely, absence of the IL-21 receptor in adult mice resulted in antigen persistence akin to that of young mice. In humans, levels of IL-21 transcripts were greatly increased in blood samples from acutely infected adults who clear the virus. These observations suggest a different model for the dichotomous, age dependent outcome of HBV infection in humans, in which decreased IL-21 production in younger patients may hinder generation of crucial CD8+ T and B cell responses. These findings carry implications for therapeutic augmentation of immune responses to HBV and potentially other persistent liver viruses.

Thymic stromal lymphopoietin is a key mediator of breast cancer progression

Inflammation is a double-edged sword that can promote or suppress cancer progression. In this study, we report that thymic stromal lymphopoietin (TSLP), an IL-7-like type 1 inflammatory cytokine that is often associated with the induction of Th2-type allergic responses in the lungs, is also expressed in human and murine cancers. Our studies with murine cancer cells indicate that TSLP plays an essential role in cancer escape, as its inactivation in cancer cells alone was sufficient to almost completely abrogate cancer progression and lung metastasis. The cancer-promoting activity of TSLP primarily required signaling through the TSLP receptor on CD4(+) T cells, promoting Th2-skewed immune responses and production of immunosuppressive factors such as IL-10 and IL-13. Expression of TSLP therefore may be a useful prognostic marker, and its targeting could have therapeutic potential.

Analysis of the role of IL-21 in development of murine B cell progenitors in the bone marrow

IL-21 plays a key role in the late stage of B cell development, where it has been shown to induce growth and differentiation of mature B cells into Ig-secreting plasma cells. Because IL-21R has also been reported on bone marrow (BM) B cell progenitors, we investigated whether IL-21R influenced earlier stages of B cell development. IL-21R is functional as early as the pro-B cell stage, and the strength of receptor-mediated signaling increases as cells mature. The addition of IL-21 to B cell progenitors in cell culture resulted in the accelerated appearance of mature B cell markers and was associated with the induction of Aid, Blimp1, and germline transcripts. We also found that stimulation of both IL-21R and CD40 was sufficient to induce the maturation of early B cell progenitors into IgM- and IgG-secreting cells. Consistent with a role for IL-21 in promoting B cell differentiation, the number of B220(+)CD43(+)IgM(-) pro-B cells was increased, and the number of mature IgM(hi)IgD(hi) cells was decreased in BM of IL-21R-deficient mice. We also report in this paper that IL-21 is expressed by BM CD4(+) T cells. These results provide evidence that IL-21R is functional in B cell progenitors and indicate that IL-21 regulates B cell development.

Intrinsic IL-21 signaling is critical for CD8 T cell survival and memory formation in response to vaccinia viral infection

CD4 T cell help plays an important role in promoting CD8 T cell immunity to pathogens. In models of infection with vaccinia virus (VV) and Listeria monocytogenes, CD4 T cell help is critical for the survival of activated CD8 T cells during both the primary and memory recall responses. Still unclear, however, is how CD4 T cell help promotes CD8 T cell survival. In this study, we first showed that CD4 T cell help for the CD8 T cell response to VV infection was mediated by IL-21, a cytokine produced predominantly by activated CD4 T cells, and that direct action of IL-21 on CD8 T cells was critical for the VV-specific CD8 T cell response in vivo. We next demonstrated that this intrinsic IL-21 signaling was essential for the survival of activated CD8 T cells and the generation of long-lived memory cells. We further revealed that IL-21 promoted CD8 T cell survival in a mechanism dependent on activation of the STAT1 and STAT3 pathways and subsequent upregulation of the prosurvival molecules Bcl-2 and Bcl-x(L). These results identify a critical role for intrinsic IL-21 signaling in CD8 T cell responses to an acute viral infection in vivo and may help design effective vaccine strategies.

Role of thymic stromal lymphopoietin (TSLP) in palifermin-mediated immune modulation and protection from acute murine graft-versus-host disease

Using the C57BL/6→(C57BL/6 x DBA/2)F(1)-hybrid model of acute graft-versus-host disease (GVHD), we previously showed that treating the donor mice with palifermin provides protection from morbidity and a shift from Th1 to Th2 cytokine production. To determine whether thymic stromal lymphopoietin (TSLP) is involved in palifermin-mediated immune modulation, we used donors from the following groups: (1) untreated wild-type donors, (2) palifermin-treated wild-type donors, (3) untreated TSLPR(-/-) donors, and (4) palifermin-treated TSLPR(-/-) donors. Survival in the recipients was 0%, 100%, 31%, and 0%, for groups 1-4, respectively, indicating that TSLP responsiveness is required for palifermin-mediated protection from GVHD. We also found that the increases in Th2 cytokine levels that are induced by palifermin treatment are obviated in TSLPR(-/-) donors, and that protection from GVHD (group 2) is associated with a higher percentage of CD4(+)CD25(+)Foxp3(+) cells in the graft. Collectively, our findings show that when palifermin and TSLP act in concert, the predominant effect is protection in this model.

Altered effector CD4+ T cell function in IL-21R-/- CD4+ T cell-mediated graft-versus-host disease

We previously showed that transplantation with IL-21R gene-deficient splenocytes resulted in less severe graft-versus-host disease (GVHD) than was observed with wild type splenocytes. In this study, we sought to find mechanism(s) explaining this observation. Recipients of donor CD4(+) T cells lacking IL-21R exhibited diminished GVHD symptoms, with reduced inflammatory cell infiltration into the liver and intestine, leading to prolonged survival. After transplantation, CD4(+) T cell numbers in the spleen were reduced, and MLR and cytokine production by CD4(+) T cells were impaired. These results suggest that IL-21 might promote GVHD through enhanced production of effector CD4(+) T cells. Moreover, we found that CD25 depletion altered neither the impaired MLR in vitro nor the ameliorated GVHD symptoms in vivo. Thus, the attenuated GVHD might be caused by an impairment of effector T cell differentiation itself, rather than by an increase in regulatory T cells and suppression of effector T cells.

IL-21 is critical for GVHD in a mouse model

Immunological effects of IL-21 on T, B and natural killer (NK) cells have been reported, but the role of IL-21 in GVHD remains obscure. Here, we demonstrate that morbidity and mortality of GVHD was significantly reduced after BMT with splenocytes from IL-21R(-/-) mice compared with those from wild type mice. To further confirm our observation, we generated a decoy receptor for IL-21. GVHD was again less severe in mice receiving BM cells transduced with the IL-21 decoy receptor than control mice These results suggest that IL-21 critically regulates GVHD, and that blockade of the IL-21 signal may represent a novel strategy for the prophylaxis for GVHD.

Atopic dermatitis-like disease and associated lethal myeloproliferative disorder arise from loss of Notch signaling in the murine skin

Background

The Notch pathway is essential for proper epidermal differentiation during embryonic skin development. Moreover, skin specific loss of Notch signaling in the embryo results in skin barrier defects accompanied by a B-lymphoproliferative disease. However, much less is known about the consequences of loss of Notch signaling after birth.

Methodology and Principal Findings

To study the function of Notch signaling in the skin of adult mice, we made use of a series of conditional gene targeted mice that allow inactivation of several components of the Notch signaling pathway specifically in the skin. We demonstrate that skin-specific inactivation of Notch1 and Notch2 simultaneously, or RBP-J, induces the development of a severe form of atopic dermatitis (AD), characterized by acanthosis, spongiosis and hyperkeratosis, as well as a massive dermal infiltration of eosinophils and mast cells. Likewise, patients suffering from AD, but not psoriasis or lichen planus, have a marked reduction of Notch receptor expression in the skin. Loss of Notch in keratinocytes induces the production of thymic stromal lymphopoietin (TSLP), a cytokine deeply implicated in the pathogenesis of AD. The AD-like associated inflammation is accompanied by a myeloproliferative disorder (MPD) characterized by an increase in immature myeloid populations in the bone marrow and spleen. Transplantation studies revealed that the MPD is cell non-autonomous and caused by dramatic microenvironmental alterations. Genetic studies demontrated that G-CSF mediates the MPD as well as changes in the bone marrow microenvironment leading to osteopenia.

Significance

Our data demonstrate a critical role for Notch in repressing TSLP production in keratinocytes, thereby maintaining integrity of the skin and the hematopoietic system.

Analysis of interleukin-21-induced Prdm1 gene regulation reveals functional cooperation of STAT3 and IRF4 transcription factors

Interleukin-21 (IL-21) is a pleiotropic cytokine that induces expression of transcription factor BLIMP1 (encoded by Prdm1), which regulates plasma cell differentiation and T cell homeostasis. We identified an IL-21 response element downstream of Prdm1 that binds the transcription factors STAT3 and IRF4, which are required for optimal Prdm1 expression. Genome-wide ChIP-Seq mapping of STAT3- and IRF4-binding sites showed that most regions with IL-21-induced STAT3 binding also bound IRF4 in vivo and furthermore revealed that the noncanonical TTCnnnTAA GAS motif critical in Prdm1 was broadly used for STAT3 binding. Comparing genome-wide expression array data to binding sites revealed that most IL-21-regulated genes were associated with combined STAT3-IRF4 sites rather than pure STAT3 sites. Correspondingly, ChIP-Seq analysis of Irf4(-/-) T cells showed greatly diminished STAT3 binding after IL-21 treatment, and Irf4(-/-) mice showed impaired IL-21-induced Tfh cell differentiation in vivo. These results reveal broad cooperative gene regulation by STAT3 and IRF4.

NetPath: a public resource of curated signal transduction pathways

NetPath, a novel community resource of curated human signaling pathways is presented and its utility demonstrated using immune signaling data.

We have developed NetPath as a resource of curated human signaling pathways. As an initial step, NetPath provides detailed maps of a number of immune signaling pathways, which include approximately 1,600 reactions annotated from the literature and more than 2,800 instances of transcriptionally regulated genes - all linked to over 5,500 published articles. We anticipate NetPath to become a consolidated resource for human signaling pathways that should enable systems biology approaches.

Cutting edge: spontaneous development of IL-17-producing gamma delta T cells in the thymus occurs via a TGF-beta 1-dependent mechanism

In naive animals, gammadelta T cells are innate sources of IL-17, a potent proinflammatory cytokine mediating bacterial clearance as well as autoimmunity. However, mechanisms underlying the generation of these cells in vivo remain unclear. In this study, we show that TGF-beta1 plays a key role in the generation of IL-17(+) gammadelta T cells and that it mainly occurs in the thymus particularly during the postnatal period. Interestingly, IL-17(+) gammadelta TCR(+) thymocytes were mainly CD44(high)CD25(low) cells, which seem to derive from double-negative 4 gammadelta TCR(+) cells that acquired CD44 and IL-17 expression. Our findings identify a novel developmental pathway during which IL-17-competent gammadelta T cells arise in the thymus by a TGF-beta1-dependent mechanism.

IL-21 and T follicular helper cells

Upon encounter with antigen, CD4(+) T cells differentiate into effector T(h) subsets with distinctive functions that are related to their unique cytokine profiles and anatomical locations. One of the most important T(h) functions is to provide signals to developing B cells that induce specific and appropriate antibody responses. The major CD4(+) T cell subset that helps B cells is the T follicular helper (T(FH)) cell, whose expression of the chemokine receptor CXCR5 [chemokine (C-X-C motif) receptor 5] serves to localize this cell to developing germinal centers (GCs) where it provides instructive signals leading to Ig class switching and somatic mutation. T(FH) cells produce high levels of IL-21, a cytokine that is critical for GC formation and also for the generation of T(FH) cells. Although T(FH) cells have been found to produce cytokines characteristic of other T(h) subsets, they represent a distinct lineage whose development is driven by the transcription factor B-cell CLL lymphoma-6 (BCL6). Consistent with their critical role in the generation of antibody responses, dysregulated T(FH) function has been associated with the development of systemic autoimmunity. Here, we review the role of IL-21 in the regulation of normal T(FH) development and function as well as in progression of autoimmune responses.

New insights into the regulation of T cells by gamma(c) family cytokines

Common cytokine receptor gamma-chain (gamma(c)) family cytokines have crucial roles in the development, proliferation, survival and differentiation of multiple cell lineages of both the innate and adaptive immune systems. In this Review, we focus on our current understanding of the distinct and overlapping effects of interleukin-2 (IL-2), IL-7, IL-9, IL-15 and IL-21, as well as the IL-7-related cytokine thymic stromal lymphopoietin (TSLP), on the survival and proliferation of conventional alphabeta T cells, gammadelta T cells and regulatory T cells. This knowledge potentially allows for the therapeutic manipulation of immune responses for the treatment of cancer, autoimmunity, allergic diseases and immunodeficiency, as well as for vaccine development.

IL-21 mediates suppressive effects via its induction of IL-10

IL-21 is a pleiotropic cytokine that is required for normal Ig production. We previously showed that IL-21 was elevated in BXSB-Yaa mice with systemic lupus erythematosus. These mice also had elevated IL-10 levels, and we now show that IL-21 induces IL-10 mRNA and protein, suggesting unexpected immunosuppressive activities for IL-21. Indeed, Th1 priming with IL-21 leads to accumulation of cells with immunosuppressive activity, and IL-21 overexpression decreases specific Ab production after immunization in an IL-10-dependent fashion. Moreover, we show that IL-21 signaling is required for maximal induction of IL-10 by IL-6 or IL-27. Overall, our data indicate that IL-21 regulates immune responses at least in part by inducing IL-10 and reveal unanticipated immunosuppressive actions for this cytokine.

IL-21 Receptor signaling by B-cells is essential for BXSB-Yaa pathogenesis (99.39)

IL-21 is a member of the IL-2 cytokine family that exerts potent biological effects on multiple hemopoietic cell types. It is produced by activated CD4 T cells, while its receptor, IL-21R, is found on cells including NK cells, dendritic, T cells and B cells. To investigate the role of IL-21 signaling in autoimmune disease, we addressed whether this cytokine contributes to the severe systemic lupus erythematosus (SLE)-like syndrome developed by BXSB-Yaa mice. We show that BXSB-Yaa CD4 T cells produce IL-21 but lack discriminative T follicular helper cell markers. We also show that BXSB-Yaa mice completely lacking the IL-21R or Igh6 are remarkably resistant to multiple aspects of the prototypic BXSB-Yaa autoimmune disease. Finally, we show that mixed bone marrow chimeric BXSB-Yaa mice lacking IL-21R specifically in the B-cell compartment are similarly resistant. These results suggest that IL-21 produced by a novel population of BXSB-Yaa CD4 T-cells acts through B-cells to evoke multiple features characteristic of severe SLE. The results indicate that IL-21 signaling through B-cells is an essential step in the pathogenesis of this SLE-like autoimmune disease and potentially others. Funded by an Arthritis Foundation grant to JB.

Interleukin-21 mediates unanticipated suppressive effects via its induction of IL-10 (97.5)

Interleukin-21 (IL-21) is a type I cytokine produced by CD4+ T cells and NK T cells that regulates the function of T, B, NK, and myeloid cells. IL-21 can augment both humoral and cell-mediated immune responses through its effects on lymphoid differentiation and function, and we and others have demonstrated a critical role for IL-21 in the autoimmune NOD mouse model of diabetes. Elevated levels of IL-21 were previously correlated with development of disease in the BXSB-Yaa mouse model of systemic lupus erythematosus, and interestingly, these mice also had increased serum IL 10. We now demonstrate that IL-21 potently induces IL-10 production by both T and B cells, with increased IL-10 mRNA and protein production in IL-21 transgenic mice but reduced production in IL-21R KO mice. TCR priming in the presence of IL-21 led to the accumulation of cells with IL 10-dependent immunosuppressive activity on both T cell and B cell function. Furthermore, we demonstrate that IL-21 is required for optimal induction of IL-10 by either IL-6 or IL-27. These results identify a novel mechanism for the induction and amplification of IL-10 production in both naive and activated T cells, and moreover identify unexpected immunosuppressive actions for IL-21.

Cytokine mediators of Th17 function

Th17 cells were identified as an independent lineage of CD4(+) T cells that secrete a distinctive set of immunoregulatory cytokines, including IL-17A, IL-17F, IL-22, and IL-21. These cytokines collectively play roles in inflammation and autoimmunity and in response to extracellular pathogens. The expression of the lineage-specific transcription factor RORgammat leads to Th17 lineage commitment; however, it has become increasingly clear that the population of cells designated as Th17 cells is not homogeneous. Although these cells collectively produce characteristic Th17 cytokines, not all are produced by each individual cell in the population. The cytokines produced by individual cells are presumably affected in part by the specific local cytokine milieu. In this review, we discuss the current understanding of the specific functional characteristics and regulation of Th17 cytokines and clarify how they mediate the actions of Th17 cells.

The induction of antibody production by IL-6 is indirectly mediated by IL-21 produced by CD4+ T cells

Interleukin (IL) 6 is a proinflammtory cytokine produced by antigen-presenting cells and nonhematopoietic cells in response to external stimuli. It was initially identified as a B cell growth factor and inducer of plasma cell differentiation in vitro and plays an important role in antibody production and class switching in vivo. However, it is not clear whether IL-6 directly affects B cells or acts through other mechanisms. We show that IL-6 is sufficient and necessary to induce IL-21 production by naive and memory CD4⁺ T cells upon T cell receptor stimulation. IL-21 production by CD4⁺ T cells is required for IL-6 to promote B cell antibody production in vitro. Moreover, administration of IL-6 with inactive influenza virus enhances virus-specific antibody production, and importantly, this effect is dependent on IL-21. Thus, IL-6 promotes antibody production by promoting the B cell helper capabilities of CD4⁺ T cells through increased IL-21 production. IL-6 could therefore be a potential coadjuvant to enhance humoral immunity.

The role of thymic stromal lymphopoietin in CD8+ T cell homeostasis

Thymic stromal lymphopoietin (TSLP) is a cytokine produced by stromal cells, epithelial cells, and basophils that acts on dendritic cells, mast cells, and CD4(+) T cells. The receptor for TSLP contains a TSLP-specific receptor chain (TSLPR) and the IL-7R alpha-chain. Although IL-7 critically controls the expansion and survival of naive and memory CD8(+) T cells, an action for TSLP on CD8(+) T cells has not been reported. We now demonstrate that CD8(+) T cells express TSLPR and that TSLP activates both STAT5 and Akt and induces Bcl-2 in these cells. Correspondingly, TSLP increases CD8(+) T cell survival in vitro as well as in wild-type and T-depleted mice in vivo, without altering the homeostatic proliferation of these cells. Moreover, TSLP can maintain CD8(+) T cells even in the absence of IL-7. Thus, our data reveal that TSLP contributes to CD8(+) T cell homeostasis in both normal and lymphopenic conditions.

Development of regulatory T cells requires IL-7Ralpha stimulation by IL-7 or TSLP

Interleukin-7 (IL-7), a cytokine produced by stromal cells, is required for thymic development and peripheral homeostasis of most major subsets of T cells. We examined whether regulatory T (Treg) cells also required the IL-7 pathway by analyzing IL-7Ralpha(-/-) mice. We observed a striking reduction in cells with the Treg surface phenotype (CD4, CD25, GITR (glucocorticoid-induced tumor necrosis factor [TNF]-like receptor), CD45RB, CD62L, CD103) or intracellular markers (cytotoxic T-lymphocyte-associated antigen-4, CTLA-4, and forkhead box transcription factor 3, Foxp3). Foxp3 transcripts were virtually absent in IL-7Ralpha(-/-) lymphoid tissues, and no Treg cell suppressive activity could be detected. There are 2 known ligands for IL-7Ralpha: IL-7 itself and thymic stromal lymphopoietin (TSLP). Surprisingly, mice deficient in IL-7 or the other chain of the TSLP receptor (TSLPR) developed relatively normal numbers of Treg cells. Combined deletion of IL-7 and TSLP receptor greatly reduced Treg cell development in the thymus but was not required for survival of mature peripheral Treg cells. We conclude that Treg cells, like other T cells, require signals from the IL-7 receptor, but unlike other T cells, do not require IL-7 itself because of at least partially overlapping actions of IL-7 and TSLP for development of Treg cells.

Constitutive expression of IL-7 receptor alpha does not support increased expansion or prevent contraction of antigen-specific CD4 or CD8 T cells following Listeria monocytogenes infection

Expression of IL-7Ralpha (CD127) has been suggested as a major determinant in the survival of memory T cell precursors. We investigated whether constitutive expression of IL-7Ralpha on T cells increased expansion and/or decreased contraction of endogenous Ag-specific CD4 and CD8 T cells following infection with Listeria monocytogenes. The results indicate that constitutive expression of IL-7Ralpha alone was not enough to impart an expansion or survival advantage to CD8 T cells responding to infection, and did not increase memory CD8 T cell numbers over those observed in wild-type controls. Constitutive expression of IL-7Ralpha did allow for slightly prolonged expansion of Ag-specific CD4 T cells; however, it did not alter the contraction phase or protect against the waning of memory T cell numbers at later times after infection. Memory CD4 and CD8 T cells generated in IL-7Ralpha transgenic mice expanded similarly to wild-type T cells after secondary infection, and immunized IL-7Ralpha transgenic mice were fully protected against lethal bacterial challenge demonstrating that constitutive expression of IL-7Ralpha does not impair, or markedly improve memory/secondary effector T cell function. These results indicate that expression of IL-7Ralpha alone does not support increased survival of effector Ag-specific CD4 or CD8 T cells into the memory phase following bacterial infection.