Interleukin-21: basic biology and implications for cancer and autoimmunity

IL-21 ensures TGF-beta 1-induced IgA isotype expression in mouse Peyer's patches

It is well established that TGF-beta1 induces IgA and IgG2b class-switching recombination in murine B cells. In the present study, we assessed the activity of IL-21 along with TGF-beta1 in Ig synthesis by murine spleen B cells. IL-21 showed antiproliferative activity on LPS-activated splenic B cells, comparable with that of TGF-beta1. IL-21 alone had little effect on IgA secretion and decreased other isotypes. Likewise, IL-21 also did not alter the TGF-beta1-induced IgA synthesis and concurrently diminished the syntheses of IgM and IgG2a, which were repressed by TGF-beta1. Unexpectedly, IL-21 inhibited the TGF-beta1-induced IgG2b production. This IL-21 effect was examined using B cells from IL-21R knockout mice, where the IgA production profile was paralleled by that seen in wild-type B cells. However, the inhibitory effect of IL-21 on TGF-beta1-induced IgG2b synthesis was not seen in the IL-21R(-/-) mouse, suggesting that IL-21 causes TGF-beta1-stimulated B cells to decrease IgG2b synthesis. Expression patterns of Ig germ-line alpha(GL alpha)/GL gamma 2b transcripts under the influence of TGF-beta1 and IL-21 were paralleled by IgA/IgG2b secretion. This was also observed in the activities of GL(alpha) and GL(gamma 2b) promoters. These results indicate that IL-21 decreases IgG2b secretion mainly through inhibition of GL(gamma 2b) transcription and is ultimately associated with selective IgA secretion induced by TGF-beta1. Our results showed that IL-21 was expressed in greater magnitude in Peyer's patches (PP) than in spleen. These results suggest that IL-21 has an important effect on selective IgA(+) B cell commitment in PP.

A critical role for IL-21 receptor signaling in the pathogenesis of systemic lupus erythematosus in BXSB-Yaa mice

Interleukin 21 (IL-21) is a pleiotropic cytokine produced by CD4 T cells that affects the differentiation and function of T, B, and NK cells by binding to a receptor consisting of the common cytokine receptor gamma chain and the IL-21 receptor (IL-21R). IL-21, a product associated with IL-17-producing CD4 T cells (T(H)17) and follicular CD4 T helper cells (T(FH)), has been implicated in autoimmune disorders including the severe systemic lupus erythematosus (SLE)-like disease characteristic of BXSB-Yaa mice. To determine whether IL-21 plays a significant role in this disease, we compared IL-21R-deficient and -competent BXSB-Yaa mice for multiple parameters of SLE. The deficient mice showed none of the abnormalities characteristic of SLE in IL-21R-competent Yaa mice, including hypergammaglobulinemia, autoantibody production, reduced frequencies of marginal zone B cells and monocytosis, renal disease, and premature morbidity. IL-21 production associated with this autoimmune disease was not a product of T(H)17 cells and was not limited to conventional CXCR5(+) T(FH) but instead was produced broadly by ICOS(+) CD4(+) splenic T cells. IL-21 arising from an abnormal population of CD4 T cells is thus central to the development of this lethal disease, and, more generally, could play an important role in human SLE and related autoimmune disorders.

Dysrulation of T cell peripheral tolerance in type 1 diabetes

Type 1 diabetes (T1D) is a T cell-mediated autoimmune disease in which the insulin producing beta cells are destroyed. The breakdown of beta cell-specific self-tolerance by T cells involves a number of dysregulated events intrinsic and extrinsic to T cells. Herein, we review the key mechanisms that drive beta cell autoimmunity, with an emphasis on events that influence the expansion and differentiation of pathogenic T cells in the periphery.

IL-21 receptor is highly expressed and induces proinflammatory cytokine secretion in patients with ulcerative colitis

AIM: To explore the role of IL-21 receptor (IL-21R) in peripheral blood mononuclear cells (PBMC) and lamina propria mononuclear cells (LPMC) for patients with ulcerative colitis (UC).

METHODS: Peripheral blood samples and colonic biopsies were obtained from 28 patients with UC and 22 healthy controls. Expression of IL-21R in peripheral blood or lamina propria CD4⁺, CD8⁺ T cells, B cells and NK cells was analyzed using flow cytometry. Cytokine secretion by PBMC or LPMC under stimulation with IL-21 and anti-CD3 was detected using ELISA.

RESULTS: Expression of IL-21R was significantly higher in peripheral blood and lamina propria CD4⁺, CD8⁺ T cells, B cells and NK cells of UC patients than in healthy controls (PBMC: 8.42 ± 2.14 vs 3.46 ± 0.54, 10.35 ± 2.17 vs 5.28 ± 2.2, 7.27 ± 1.15 vs 2.35 ± 0.41, 12.55 ± 3.12 vs 5.45 ± 1.06; LPMC: 22.44 ± 3.46 vs 6.26 ± 1.15, 24.48 ± 4.57 i 6.87 ± 1.02, 16.24 ± 3.10 vs 5.56 ± 1.44, 23.54 ± 4.12 vs 8.45 ± 1.68, all P < 0.05). PBMC or LPMC from UC patients, when stimulated with IL-21 and anti-CD3, produced significantly higher levels of TNF-α, IFN-γ and IL-2 compared with healthy controls (346 ± 72 vs120 ± 27, 3048 ± 426 vs 1182 ± 242; 625 ± 113 vs 154 ± 35, 3827 ± 418 vs 1520 ± 304, all P < 0.05)

CONCLUSION: IL-21R is significantly increased in PBMC and LPMC from UC patients, and IL-21 induces proinflamatory cytokine secretion. It suggests that IL-21 is associated with intestinal mucosal injury and that target therapy directed against IL-21 may be used to treat UC patients.

IRF4 is essential for IL-21-mediated induction, amplification, and stabilization of the Th17 phenotype

Differentiation of murine T-helper (Th) 17 cells is induced by antigenic stimulation and the sequential action of the cytokines IL-6, IL-21, and IL-23, along with TGFbeta. Current dogma proposes that IL-6 induces IL-21, which, in a STAT3-dependent manner, amplifies its own transcription, contributes to IL-17 production, and, moreover, promotes the expression of the IL-23 receptor. This, in turn, prepares cells for IL-23-mediated stabilization of the Th17 phenotype. Here we demonstrate that these effects of IL-21 on Th17 differentiation are completely dependent on IFN regulatory factor 4 (IRF4). After culturing in the presence of IL-21 plus TGFbeta, IRF4-deficient (Irf4(-/-)) Th cells showed a profound intrinsic defect in IL-17 production and in the autocrine IL-21 loop. Likewise, the levels of IL-23 receptor and the lineage-specific orphan nuclear receptors RORalpha and RORgammat were diminished, whereas the T regulatory (Treg) transcription factor forkhead box P3 (Foxp3) was strongly up-regulated, consistent with the reciprocal relationship between Th17 and Treg development. Despite this loss of IL-21 functions, IL-21-induced STAT3 activation was unimpaired and induced normal Socs3 expression. Forced expression of Foxp3 in WT cells inhibited IL-21-mediated IL-17 production, suggesting that the increase in Foxp3 contributes to the Irf4(-/-) phenotype. Additionally, the low levels of RORalpha and RORgammat are also partially responsible, because simultaneous overexpression of both proteins restored IL-17 production in Irf4(-/-) cells to some extent. These data highlight IRF4 as a decisive factor during the IL-21-mediated steps of Th17 development by influencing the balance of Foxp3, RORalpha, and RORgammat.

IL-21R is essential for epicutaneous sensitization and allergic skin inflammation in humans and mice

Atopic dermatitis (AD) is a common allergic inflammatory skin disease caused by a combination of intense pruritus, scratching, and epicutaneous (e.c.) sensitization with allergens. To explore the roles of IL-21 and IL-21 receptor (IL-21R) in AD, we examined skin lesions from patients with AD and used a mouse model of allergic skin inflammation. IL-21 and IL-21R expression was upregulated in acute skin lesions of AD patients and in mouse skin subjected to tape stripping, a surrogate for scratching. The importance of this finding was highlighted by the fact that both Il21r-/- mice and WT mice treated with soluble IL-21R-IgG2aFc fusion protein failed to develop skin inflammation after e.c. sensitization of tape-stripped skin. Adoptively transferred OVA-specific WT CD4+ T cells accumulated poorly in draining LNs (DLNs) of e.c. sensitized Il21r-/- mice. This was likely caused by both DC-intrinsic and nonintrinsic effects, because trafficking of skin DCs to DLNs was defective in Il21r-/- mice and, to a lesser extent, in WT mice reconstituted with Il21r-/- BM. More insight into this defect was provided by the observation that skin DCs from tape-stripped WT mice, but not Il21r-/- mice, upregulated CCR7 and migrated toward CCR7 ligands. Treatment of epidermal and dermal cells with IL-21 activated MMP2, which has been implicated in trafficking of skin DCs. These results suggest an important role for IL-21R in the mobilization of skin DCs to DLNs and the subsequent allergic response to e.c. introduced antigen.

Dissociation of its opposing immunologic effects is critical for the optimization of antitumor CD8+ T-cell responses induced by interleukin 21

PURPOSE

Interleukin 21 (IL-21) is a promising new cytokine, which is undergoing clinical testing as an anticancer agent. Although IL-21 provides potent stimulation of CD8(+) T cells, it has also been suggested that IL-21 is immunosuppressive by counteracting the maturation of dendritic cells. The dissociation of these two opposing effects may enhance the utility of IL-21 as an immunotherapeutic. In this study, we used a cell-based artificial antigen-presenting cell (aAPC) lacking a functional IL-21 receptor (IL-21R) to investigate the immunostimulatory properties of IL-21.

EXPERIMENTAL DESIGN

The immunosuppressive activity of IL-21 was studied using human IL-21R(+) dendritic cells. Antigen-specific CD8(+) T cells stimulated with human cell-based IL-21R(-)aAPC were used to isolate the T-cell immunostimulatory effects of IL-21. The functional outcomes, including phenotype, cytokine production, proliferation, and cytotoxicity were evaluated.

RESULTS

IL-21 limits the immune response by maintaining immunologically immature dendritic cells. However, stimulation of CD8(+) T cells with IL-21R(-) aAPC, which secrete IL-21, results in significant expansion. Although priming in the presence of IL-21 temporarily modulated the T-cell phenotype, chronic stimulation abrogated these differences. Importantly, exposure to IL-21 during restimulation promoted the enrichment and expansion of antigen-specific CD8(+) T cells that maintained IL-2 secretion and gained enhanced IFN-gamma secretion. Tumor antigen-specific CTL generated in the presence of IL-21 recognized tumor cells efficiently, demonstrating potent effector functions.

CONCLUSIONS

IL-21 induces opposing effects on antigen-presenting cells and CD8(+) T cells. Strategic application of IL-21 is required to induce optimal clinical effects and may enable the generation of large numbers of highly avid tumor-specific CTL for adoptive immunotherapy.

IL-6 controls Th17 immunity in vivo by inhibiting the conversion of conventional T cells into Foxp3+ regulatory T cells

The conditions leading to the induction of adaptive Foxp3(+) regulatory T cells (T-regs) from peripheral T cells in vivo are incompletely understood. Here, we show that unresponsiveness of T cells to IL-6 by T cell-selective deletion of gp130 or immunization of wild-type mice with antigen in incomplete Freund's adjuvant (IFA), which fails to induce IL-6, promotes the conversion of peripheral CD4(+) T cells into adaptive Foxp3(+) T-regs. Thus, both T cell-conditional gp130 knockout (KO) mice immunized with MOG35-55 in complete Freund's adjuvant (CFA) and wild-type mice immunized with MOG35-55 in IFA develop overwhelming antigen-specific T-reg responses and are protected from experimental autoimmune encephalomyelitis (EAE). Depletion of T-regs restores T helper (Th)17 responses and clinical EAE in MOG/CFA-immunized T cell-conditional gp130 KO mice, but not in MOG/IFA-immunized wild-type mice. We conclude that in the absence of T-regs, IL-6 signaling is dispensable for the induction of Th17 cells, and alternative pathways exist to induce Th17 cells and EAE in the absence of IL-6 signaling. However, IL-6 signaling is dominant in inhibiting the conversion of conventional T cells into Foxp3(+) T-regs in vivo, and in the absence of IL-6 signaling, no other cytokine can substitute in inhibiting T-reg conversion. These data identify IL-6 as an important target to modulate autoimmune responses and chronic inflammation.

Decreased levels of circulating IL-21 in HIV-infected AIDS patients: correlation with CD4+ T-cell counts

IL-21 is a relatively newly discovered multifunctional and pleiotropic cytokine. It is produced primarily by CD4(+) T cells, the principal targets of the virus, and therefore this cytokine has special relevance to HIV infection. Here we show for the first time that serum levels of this cytokine are significantly reduced in HIV-infected AIDS patients and correlate significantly with their CD4(+) T-cell counts. These data suggest that the cytokine levels could act as a valuable biomarker for the progression of AIDS.

Priming for T helper type 2 differentiation by interleukin 2-mediated induction of interleukin 4 receptor alpha-chain expression

T-helper type 2 (T_H2) cells are essential for humoral immunity and host defense. Interleukin (IL)-4 drives T_H2 differentiation and IL-2 augments Il4 chromatin accessibility. Here we demonstrated that IL-2, by inducing STAT5 binding to the Il4ra locus, is essential for inducing and maintaining IL-4Rα expression. Although IL-4 induces IL-4Rα expression, T-cell receptor-induced IL-4Rα expression was normal in Il4^-/- but profoundly diminished in Il2^-/- cells. Remarkably, forced IL-4Rα expression rescued T_H2 differentiation in Il2^-/- cells. Moreover, genome-wide mapping by ChIP-Seq reveals broad interaction of STAT5A and STAT5B with genes associated with T_H2 differentiation. These results reveal a previously unappreciated function for IL-2 in ‘priming’ T cells for T_H2 differentiation and in maintaining expression of Il4ra and other genes in T_H2-committed cells.

IL-21 signaling is critical for the development of type I diabetes in the NOD mouse

IL-21 is a pleiotropic type I cytokine that shares the common cytokine receptor gamma chain and plays important roles for normal Ig production, terminal B cell differentiation to plasma cells, and Th17 differentiation. IL-21 is elevated in several autoimmune diseases, and blocking its action has attenuated disease in MRL/lpr mice and in collagen-induced arthritis. The diabetes-associated Idd3 locus is at the Il2/Il21 locus, and elevated IL-21 was observed in the nonobese diabetic (NOD) mouse and suggested to contribute to diabetes by augmenting T cell homeostatic proliferation. To determine the role of IL-21 in diabetes, Il21r-knockout (KO) mice were backcrossed to NOD mice. These mice were devoid of lymphocytic infiltration into the pancreas, and only 1 of 20 animals had an elevated glucose compared with 60% of NOD mice on a wild-type (WT) background. Although TCR and Treg-related responses were normal, these mice had reduced Th17 cells and significantly higher levels of mRNAs encoding members of the Reg (regenerating) gene family whose transgenic expression protects against diabetes. Our studies establish a critical role for IL-21 in the development of type I diabetes in the NOD mouse, with obvious potential implications for type I diabetes in humans.

IL-21 and autoimmune disease--hypothesis and reality?

Previous data have indicated that IL-21 and/or IL-21R are crucial for the differentiation of naïve T cells into Th17 cells and also play a key role in the development of autoimmune disease. Given this, IL-21 and/or IL-21R are potential targets for therapy of such diseases; however, a study in this issue of the European Journal of Immunology, provides a new twist in the story as it is now shown that IL-21 and/or IL-21R play no role in Th17 development or autoimmune inflammatory disease. The reasons for these contradictory data are discussed in this Commentary.

IL-23 and Th17 cytokines in intestinal homeostasis

The discovery of the Th1/Th2 paradigm of CD4(+) T-cell subsets redefined our understanding of immunity by highlighting the essential roles of cytokine networks in the induction and regulation of immune responses. Most recently, the identification of an additional subset, known as Th17 cells, has further illustrated the complexity and diversity of effector CD4(+) T cells. Th17 responses have been closely associated with the cytokine interleukin (IL)-23 and, although originally pinpointed as having a deleterious role in autoimmune tissue pathology, the IL-23/Th17 axis has also been associated with protective immunity at mucosal surfaces. Recent progress has highlighted the heterogeneous nature of Th17 responses, has demonstrated diverse cellular sources for Th17-associated cytokines, and has begun to dissect the individual roles of these cytokines in different disease processes. Here, we will review the evidence linking the IL-23/Th17 axis to chronic intestinal inflammation and also will discuss its beneficial roles in intestinal protection and homeostasis.

Aberrant expression of the Th2 cytokine IL-21 in Hodgkin lymphoma cells regulates STAT3 signaling and attracts Treg cells via regulation of MIP-3alpha

The malignant Hodgkin/Reed-Sternberg (HRS) cells of classical Hodgkin lymphoma (HL) are derived from mature B cells, but have lost a considerable part of the B cell-specific gene expression pattern. Consequences of such a lineage infidelity for lymphoma pathogenesis are currently not defined. Here, we report that HRS cells aberrantly express the common cytokine-receptor gamma-chain (gamma(c)) cytokine IL-21, which is usually restricted to a subset of CD4(+) T cells, and the corresponding IL-21 receptor. We demonstrate that IL-21 activates STAT3 in HRS cells, up-regulates STAT3 target genes, and protects HRS cells from CD95 death receptor-induced apoptosis. Furthermore, IL-21 is involved in up-regulation of the CC chemokine macrophage-inflammatory protein-3alpha (MIP-3alpha) in HRS cells. MIP-3alpha in turn attracts CCR6(+)CD4(+)CD25(+)FoxP3(+)CD127(lo) regulatory T cells toward HRS cells, which might favor their immune escape. Together, these data support the concept that aberrant expression of B lineage-inappropriate genes plays an important role for the biology of HL tumor cells.

Induction and effector functions of T(H)17 cells

T helper (T(H)) cells constitute an important arm of the adaptive immune system because they coordinate defence against specific pathogens, and their unique cytokines and effector functions mediate different types of tissue inflammation. The recently discovered T(H)17 cells, the third subset of effector T helper cells, have been the subject of intense research aimed at understanding their role in immunity and disease. Here we review emerging data suggesting that T(H)17 cells have an important role in host defence against specific pathogens and are potent inducers of autoimmunity and tissue inflammation. In addition, the differentiation factors responsible for their generation have revealed an interesting reciprocal relationship with regulatory T (T(reg)) cells, which prevent tissue inflammation and mediate self-tolerance.

Cutting edge: IL-21 is not essential for Th17 differentiation or experimental autoimmune encephalomyelitis

Recent studies have suggested that IL-21 is a key factor in the development of IL-17-producing CD4 T cells (Th17) and that the induction of experimental autoimmune encephalomyelitis, which depends on mounting an efficient Th17 response, is reportedly impaired in the absence of IL-21 signaling. In this study, we provide supportive in vitro evidence that IL-21 can drive Th17 responses in conjunction with TGF-beta. However, more importantly we also demonstrate, using IL-21- and IL-21R-deficient mice, that IL-21 is not essential for the differentiation of Th17 cells in vitro and in vivo. Moreover, we show that IL-21- and IL-21R-deficient mice are highly susceptible to experimental autoimmune encephalomyelitis with disease scores that were comparable, or even higher at the peak of disease, to those of control mice. Thus, our results challenge the notion that IL-21 is a key factor in driving Th17 immunity and disease.

A fundamental role for interleukin-21 in the generation of T follicular helper cells

T cell help to B cells is a fundamental property of adaptive immunity, yet only recently have many of the cellular and molecular mechanisms of T cell help emerged. T follicular helper (Tfh) cells are the CD4(+) T helper cells that provide cognate help to B cells for high-affinity antibody production in germinal centers (GC). Tfh cells produce interleukin-21 (IL-21), and we show that IL-21 was necessary for GC formation. However, the central role of IL-21 in GC formation reflected its effects on Tfh cell generation rather than on B cells. Expression of the inducible costimulator (ICOS) was necessary for optimal production of IL-21, indicative of interplay between these two Tfh cell-expressed molecules. Finally, we demonstrate that IL-21's costimulatory capacity for T helper cell differentiation operated at the level of the T cell receptor signalosome through Vav1, a signaling molecule that controls T cell helper function. This study reveals a previously unappreciated role for Tfh cells in the formation of the GC and isotype switching through a CD4(+) T cell-intrinsic requirement for IL-21.

Generation of T follicular helper cells is mediated by interleukin-21 but independent of T helper 1, 2, or 17 cell lineages

After activation, CD4(+) helper T (Th) cells differentiate into distinct effector subsets. Although chemokine (C-X-C motif) receptor 5-expressing T follicular helper (Tfh) cells are important in humoral immunity, their developmental regulation is unclear. Here we show that Tfh cells had a distinct gene expression profile and developed in vivo independently of the Th1 or Th2 cell lineages. Tfh cell generation was regulated by ICOS ligand (ICOSL) expressed on B cells and was dependent on interleukin-21 (IL-21), IL-6, and signal transducer and activator of transcription 3 (STAT3). However, unlike Th17 cells, differentiation of Tfh cells did not require transforming growth factor beta (TGF-beta) or Th17-specific orphan nuclear receptors RORalpha and RORgamma in vivo. Finally, naive T cells activated in vitro in the presence of IL-21 but not TGF-beta signaling preferentially acquired Tfh gene expression and promoted germinal-center reactions in vivo. This study thus demonstrates that Tfh is a distinct Th cell lineage.

TH17 cells in development: an updated view of their molecular identity and genetic programming

Following activation, CD4+ T cells differentiate into different lineages of helper T (T(H)) cells that are characterized by distinct developmental regulation and biological functions. T(H)17 cells have recently been identified as a new lineage of effector T(H) cells, and they have been shown to be important in immune responses to infectious agents, as well as in various immune diseases. Over the past two to three years, there has been a rapid progress in our understanding of the differentiation programme of T(H)17 cells. Here, I summarize our current knowledge of the unique gene expression, cytokine-mediated regulation and transcriptional programming of T(H)17 cells, and provide my personal perspectives on the future studies that are required to elucidate this lineage in more detail.

Regulation and functions of Blimp-1 in T and B lymphocytes

B lymphocyte-induced maturation protein-1 (Blimp-1), discovered 16 years ago as a transcriptional repressor of the IFNbeta promoter, plays fundamentally important roles in many cell lineages and in early development. This review focuses on Blimp-1 in lymphocytes. In the B cell lineage, Blimp-1 is required for development of immunoglobulin-secreting cells and for maintenance of long-lived plasma cells (LLPCs). Direct targets of Blimp-1 and the transcriptional cascades Blimp-1 initiates to trigger plasmacytic differentiation are described. Blimp-1 also affects the homeostasis and function of CD4(+), CD8(+), and regulatory CD4(+) T cells, and Blimp-1 levels are highest in antigen-experienced T cells. Blimp-1 attenuates T cell proliferation and survival and modulates differentiation. Roles for Blimp-1 in Th1/Th2 specification, regulatory T cell function, and CD8 differentiation and function are under investigation. Signals that induce Blimp-1 in B cells include Toll-like receptor ligands and cytokines; in T cells, T cell receptors and cytokines induce Blimp-1. In spite of some commonalities, different targets and regulators of Blimp-1 in B and T cells suggest intriguing evolutionary divergence of this regulatory machinery.

Activation-dependent induction of Blimp-1

B lymphocyte induced maturation protein-1 (Blimp-1) mRNA is induced upon antigen-dependent activation of both T and B lymphocytes, in spite of the fact that it plays very different roles in the two lineages. B cells have at least four different mechanisms to repress Blimp-1 and repression is relieved before induction. Only one repressor, Bcl-6, is known in T cells. Activators must also be present to induce Blimp-1 in both T and B cells. Cytokines IL-21, IL-10, and IL-6, activating STAT3, are crucial in B cells along with toll-like receptor (TLR) signals, whereas IL-2 is crucial in T cells. AP-1, NF-kappaB, and IRF4 also activate Blimp-1.

The Yin and Yang of interleukin-21 in allergy, autoimmunity and cancer

IL-21 is a type I cytokine that like IL-2, IL-4, IL-7, IL-9, and IL-15 shares the common cytokine receptor gamma chain, gamma(c). IL-21 is produced by activated CD4(+) T cells, NKT cells, and Th17 cells and has pleiotropic actions on a range of lymphoid lineages. IL-21 regulates immunoglobulin production and drives B cell terminal differentiation into plasma cells, cooperatively expands CD8(+) T cells and drives Th17 differentiation, has inhibitory effects on antigen presentation by dendritic cells, and can be pro-apoptotic for B and NK cells. Moreover, IL-21 has potent anti-tumor effects and is implicated in the development of autoimmune diseases. Regulating IL-21 actions in vivo therefore has clinical potential for a range of diseases and is an area of active investigation.

T follicular helper (TFH) cells in normal and dysregulated immune responses

T cell help for antibody production is a fundamental aspect of immune responses. Only recently has a better understanding of the cellular and molecular mechanisms for T cell help emerged. A subset of T cells, termed T follicular helper cells (T(FH) cells), provides a helper function to B cells and represents one of the most numerous and important subsets of effector T cells in lymphoid tissues. T(FH) cells are distinguishable from Th1 and Th2 cells by several criteria, including chemokine receptor expression (CXCR5), location/migration (B cell follicles), and function (B cell help). Central to the function of CD4(+) T cells is IL-21, a "helper" cytokine produced by T(FH) cells that potently stimulates the differentiation of B cells into Ab-forming cells through IL-21R. Consequently, dysregulation of T(FH) cell function, and over- or under-expression of T(FH) cell-associated molecules such as ICOS or IL-21, most likely contributes to the pathogenesis of certain autoimmune diseases or immunodeficiencies.

Angiostatic activity of the antitumor cytokine interleukin-21

Interleukin-21 (IL-21) is a recently described immunoregulatory cytokine. It has been identified as a very potent immunotherapeutic agent in several cancer types in animal models, and clinical studies are ongoing. IL-21 belongs to the type I cytokine family of which other members, ie, IL-2, IL-15, and IL-4, have been shown to exert activities on vascular endothelial cells (ECs). We hypothesized that IL-21, in addition to inducing the antitumor immune response, also inhibits tumor angiogenesis. In vitro experiments showed a decrease of proliferation and sprouting of activated ECs after IL-21 treatment. We found that the IL-21 receptor is expressed on vascular ECs. Furthermore, in vivo studies in the chorioallantoic membrane of the chick embryo and in mouse tumors demonstrated that IL-21 treatment disturbs vessel architecture and negatively affects vessel outgrowth. Our results also confirm the earlier suggested angiostatic potential of IL-2 in vitro and in vivo. The angiostatic effect of IL-21 is confirmed by the decrease in expression of angiogenesis-related genes. Interestingly, IL-21 treatment of ECs leads to a decrease of Stat3 phosphorylation. Our research shows that IL-21 is a very powerful antitumor compound that combines the induction of an effective antitumor immune response with inhibition of tumor angiogenesis.

Interleukin 21: a cytokine/cytokine receptor system that has come of age

Interleukin-21 (IL-21) and its receptor represent the sixth cytokine system whose actions were recognized to require the common cytokine receptor gamma chain. IL-21 is produced by activated CD4+ T cells, natural killer T cells, and follicular T helper cells and has actions on a range of lymphohematopoietic lineages. Among its many effects, IL-21 serves a critical role for immunoglobulin production and terminal B cell differentiation, acts as a T cell comitogen and can drive the expansion of CD8+ T cells, can negatively regulate dendritic cell function and plays an essential role in the differentiation of Th17 cells. Importantly, IL-21 is implicated in the pathogenesis of autoimmunity and exhibits potent actions as an antitumor agent. The ability to regulate and manipulate the actions of IL-21, therefore, has important implications for immunoregulation and the therapy of human disease.

Immunology of asthma and chronic obstructive pulmonary disease

Asthma and chronic obstructive pulmonary disease (COPD) are both obstructive airway diseases that involve chronic inflammation of the respiratory tract, but the type of inflammation is markedly different between these diseases, with different patterns of inflammatory cells and mediators being involved. As described in this Review, these inflammatory profiles are largely determined by the involvement of different immune cells, which orchestrate the recruitment and activation of inflammatory cells that drive the distinct patterns of structural changes in these diseases. However, it is now becoming clear that the distinction between these diseases becomes blurred in patients with severe asthma, in asthmatic subjects who smoke and during acute exacerbations. This has important implications for the development of new therapies.

IL-2 and IL-21 confer opposing differentiation programs to CD8+ T cells for adoptive immunotherapy

IL-2 and IL-21 are closely related cytokines that might have arisen by gene duplication. Both cytokines promote the function of effector CD8(+) T cells, but their distinct effects on antigen-driven differentiation of naive CD8(+) T cells into effector CD8(+) T cells are not clearly understood. We found that antigen-induced expression of Eomesodermin (Eomes) and maturation of naive CD8(+) T cells into granzyme B- and CD44-expressing effector CD8(+) T cells was enhanced by IL-2, but, unexpectedly, suppressed by IL-21. Furthermore, IL-21 repressed expression of IL-2Ra and inhibited IL-2-mediated acquisition of a cytolytic CD8(+) T-cell phenotype. Despite its inhibitory effects, IL-21 did not induce anergy, but instead potently enhanced the capacity of cells to mediate tumor regression upon adoptive transfer. In contrast, IL-2 impaired the subsequent antitumor function of transferred cells. Gene expression studies revealed a distinct IL-21 program that was characterized phenotypically by increased expression of L-selectin and functionally by enhanced antitumor immunity that was not reversed by secondary in vitro stimulation with antigen and IL-2. Thus, the efficacy of CD8(+) T cells for adoptive immunotherapy can be influenced by opposing differentiation programs conferred by IL-2 and IL-21, a finding with important implications for the development of cellular cancer therapies.