IL-21 is an antitolerogenic cytokine of the late-phase alloimmune response

OBJECTIVE

Interleukin-21 (IL-21) is a proinflammatory cytokine that has been shown to affect Treg/Teff balance. However, the mechanism by which IL-21 orchestrates alloimmune response and interplays with Tregs is still unclear.

RESEARCH DESIGN AND METHODS

The interplay between IL-21/IL-21R signaling, FoxP3 expression, and Treg survival and function was evaluated in vitro in immunologically relevant assays and in vivo in allogenic and autoimmune models of islet transplantation.

RESULTS

IL-21R expression decreases on T cells and B cells in vitro and increases in the graft in vivo, while IL-21 levels increase in vitro and in vivo during anti-CD3/anti-CD28 stimulation/allostimulation in the late phase of the alloimmune response. In vitro, IL-21/IL-21R signaling (by using rmIL-21 or genetically modified CD4(+) T cells [IL-21 pOrf plasmid-treated or hIL-21-Tg mice]) enhances the T-cell response during anti-CD3/anti-CD28 stimulation/allostimulation, prevents Treg generation, inhibits Treg function, induces Treg apoptosis, and reduces FoxP3 and FoxP3-dependent gene transcripts without affecting FoxP3 methylation status. In vivo targeting of IL-21/IL-21R expands intragraft and peripheral Tregs, promotes Treg neogenesis, and regulates the antidonor immune response, whereas IL-21/IL-21R signaling in Doxa-inducible ROSA-rtTA-IL-21-Tg mice expands Teffs and FoxP3(-) cells. Treatment with a combination of mIL-21R.Fc and CTLA4-Ig (an inhibitor of the early alloimmune response) leads to robust graft tolerance in a purely alloimmune setting and prolonged islet graft survival in NOD mice.

CONCLUSIONS

IL-21 interferes with different checkpoints of the FoxP3 Treg chain in the late phase of alloimmune response and, thus, acts as an antitolerogenic cytokine. Blockade of the IL-21/IL-21R pathway could be a precondition for tolerogenic protocols in transplantation.

Induction of thymic stromal lymphopoietin production by xylene and exacerbation of picryl chloride-induced allergic inflammation in mice

BACKGROUND

Some chemical compounds in the environment worsen allergic inflammation. In this study, we examined whether organic solvents induce the production of thymic stromal lymphopoietin (TSLP) which elicits Th2-type immune responses.

METHODS

Organic solvents were painted on the earlobes of BALB/c mice. The expression of TSLP in the ear was determined by ELISA.

RESULTS

Xylene and toluene, but not chloroform or ethyl acetate, induced the expression of mRNA for TSLP in the earlobe tissue. Among the aromatic compounds, xylene, especially m-xylene, and trimethylbenzene caused apparent TSLP production. The level of TSLP in the xylene-treated earlobes reached a maximum at 24 h, and TSLP was expressed in epithelial tissues. Production of TSLP was unaffected in mast cell-deficient W/W(v) mice but apparently diminished in TNF-α knockout mice and IL-4 receptor knockout mice. Repeated painting of xylene for 7 days induced an increase in the weight of cervical lymph nodes and expression of OX40 ligand, both of which were inhibited in TSLP receptor knockout mice. Xylene promoted the picryl chloride-induced thickening of the ear and IL-4 production, which were reversed in TSLP receptor knockout mice.

CONCLUSION

Xylene induced TSLP production, resulting in an exacerbation of allergic inflammation. Thus, xylene might be a good tool for examining the roles of TSLP in eliciting allergy in experimental animals.

IL-2 family cytokines: new insights into the complex roles of IL-2 as a broad regulator of T helper cell differentiation

Interleukin-2 (IL-2) is a pleiotropic cytokine that drives T-cell growth, augments NK cytolytic activity, induces the differentiation of regulatory T cells, and mediates activation-induced cell death. Along with IL-4, IL-7, IL-9, IL-15, and IL-21, IL-2 shares the common cytokine receptor γ chain, γ(c), which is mutated in humans with X-linked severe combined immunodeficiency. Herein, we primarily focus on the recently discovered complex roles of IL-2 in broadly modulating T cells for T helper cell differentiation. IL-2 does not specify the type of Th differentiation that occurs; instead, IL-2 modulates expression of receptors for other cytokines and transcription factors, thereby either promoting or inhibiting cytokine cascades that correlate with each Th differentiation state. In this fashion, IL-2 can prime and potentially maintain Th1 and Th2 differentiation as well as expand such populations of cells, whereas it inhibits Th17 differentiation but also can expand Th17 cells.

IL-2 family cytokines: new insights into the complex roles of IL-2 as a broad regulator of T helper cell differentiation

Interleukin-2 (IL-2) is a pleiotropic cytokine that drives T-cell growth, augments NK cytolytic activity, induces the differentiation of regulatory T cells, and mediates activation-induced cell death. Along with IL-4, IL-7, IL-9, IL-15, and IL-21, IL-2 shares the common cytokine receptor γ chain, γ(c), which is mutated in humans with X-linked severe combined immunodeficiency. Herein, we primarily focus on the recently discovered complex roles of IL-2 in broadly modulating T cells for T helper cell differentiation. IL-2 does not specify the type of Th differentiation that occurs; instead, IL-2 modulates expression of receptors for other cytokines and transcription factors, thereby either promoting or inhibiting cytokine cascades that correlate with each Th differentiation state. In this fashion, IL-2 can prime and potentially maintain Th1 and Th2 differentiation as well as expand such populations of cells, whereas it inhibits Th17 differentiation but also can expand Th17 cells.

Thymic stromal lymphopoietin is produced by dendritic cells

Thymic stromal lymphopoietin (TSLP) is a type 1 cytokine that contributes to lymphopoiesis and the development of asthma and atopic dermatitis. TSLP acts on multiple lineages, including dendritic cells (DCs), T cells, NKT cells, eosinophils, and mast cells, mediating proliferation and survival and linking innate and adaptive immune responses. TSLP is produced by a range of cells, including epithelial cells, fibroblasts, stromal cells, and keratinocytes. DCs are important primary targets of TSLP, and we unexpectedly demonstrated that DCs also produce TSLP in response to TLR stimulation and that this is augmented by IL-4. Moreover, we demonstrated that when mice were challenged with house dust mite extract, lung CD11c(+) DCs expressed TSLP mRNA at an even higher level than did epithelial cells. These data suggested that DCs not only respond to TSLP but also are a source of TSLP during pathogen and/or allergen encounter.

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.