Andrea L. Wurster

IL-21 Limits NK Cell Responses and Promotes Antigen-Specific T Cell Activation: A Mediator of the Transition from Innate to Adaptive Immunity

IFNalpha/beta, IL-12, and IL-15 regulate NK cell activation and expansion, but signals triggering resolution of the NK response upon induction of adaptive immunity remain to be defined. We now report that IL-21, a product of activated T cells, may serve this function. Mice lacking IL-21R (IL-21R(-/-)) had normal NK cell development but no detectable responses to IL-21. IL-21 enhanced cytotoxic activity and IFNgamma production by activated murine NK cells but did not support their viability, thus limiting their duration of activation. Furthermore, IL-21 blocked IL-15-induced expansion of resting NK cells, thus preventing the initiation of further innate responses. In contrast, IL-21 enhanced the proliferation, IFNgamma production, and cytotoxic function of CD8(+) effector T cells in an allogeneic MLR. These observations suggest that IL-21 promotes the transition between innate and adaptive immunity.

The biology of Stat4 and Stat6

IL-4 and IL-12 are cytokines that are important regulators of the proliferation, differentiation and functional capacity of lymphocytes. STATs (signal transducers and activators of transcription) are transcription factors that provide a direct link between the cytokine receptors and cytokine induced gene transcription. Stat6 and Stat4 are two STAT family members that specifically mediate signals that emanate from the IL-4 and IL-12 receptors, respectively. Recently a great deal of progress has been made in understanding the specific roles that Stat6 and Stat4 play in lymphocyte function through in vitro as well as in vivo studies using Stat6 and Stat4-deficient mice. This report will summarize and describe the recent advances made in understanding the activation and regulation of Stat6 and Stat4 as well as their roles in the development of an immune response.

Interleukin 21 Is a T Helper (Th) Cell 2 Cytokine that Specifically Inhibits the Differentiation of Naive Th Cells into Interferon γ–producing Th1 Cells

The cytokine potential of developing T helper (Th) cells is directly shaped both positively and negatively by the cytokines expressed by the effector Th cell subsets. Here we find that the recently identified cytokine, interleukin (IL)-21, is preferentially expressed by Th2 cells when compared with Th1 cells generated in vitro and in vivo. Exposure of naive Th precursors to IL-21 inhibits interferon (IFN)-γ production from developing Th1 cells. The repression of IFN-γ production is specific in that the expression of other Th1 and Th2 cytokines is unaffected. IL-21 decreases the IL-12 responsiveness of developing Th cells by specifically reducing both signal transducer and activator of transcription 4 protein and mRNA expression. These results suggest that Th2 cell-derived IL-21 regulates the development of IFN-γ–producing Th1 cells which could serve to amplify a Th2 response.

IL-21 Induces the Apoptosis of Resting and Activated Primary B Cells

Cytokines play an important role in regulating the development and homeostasis of B cells by controlling their viability. In this study, we show that the recently described T cell-derived cytokine IL-21 induces the apoptosis of resting primary murine B cells. In addition, the activation of primary B cells with IL-4, LPS, or anti-CD40 Ab does not prevent IL-21-mediated apoptosis. The induction of apoptosis by IL-21 correlates with a down-regulation in the expression of Bcl-2 and Bcl-xL, two antiapoptotic members of the Bcl-2 family. Furthermore, the reconstitution of Bcl-xL or Bcl-2 expression protects primary B cells from IL-21-induced apoptosis. In addition, a short-term preactivation of B cells with anti-CD40 Ab confers protection from IL-21-mediated apoptosis through the up-regulation of Bcl-xL. These studies reveal a novel pathway that mediates B cell apoptosis via the IL-21R and suggest that IL-21 may play a role in regulating B cell homeostasis.

Biology of IL-21 and the IL-21 receptor.

Summary:  Interleukin‐21 (IL‐21) is the newest member of the common γ‐chain family of cytokines, which includes IL‐2, IL‐4, IL‐7, IL‐9, IL‐13, and IL‐15. Its private receptor, IL‐21R, has been shown to activate the Janus kinase/signal transducers and activators of transcription signaling pathway upon ligand binding. Initial studies have demonstrated that IL‐21 has pleiotropic effects on the proliferation, differentiation, and effector functions of B, T, natural killer, and dendritic cells. More recently, the potential therapeutic capacity of IL‐21 in the treatment of cancers has been widely investigated. The biological role of IL‐21 in the immune system is complex, as IL‐21 has been shown to have the ability to both promote and inhibit immune responses. Overall, the current data point to IL‐21 being a novel immunomodulatory cytokine, whose regulation of any given immune response is highly dependent on the surrounding environmental context.

Interleukin-21 Is Required for the Development of Type 1 Diabetes in NOD Mice

OBJECTIVE Interleukin (IL)-21 is a type 1 cytokine that has been implicated in the pathogenesis of type 1 diabetes via the unique biology of the nonobese diabetic (NOD) mouse strain. The aim of this study was to investigate a causal role for IL-21 in type 1 diabetes. RESEARCH DESIGN AND METHODS We generated IL-21R–deficient NOD mice and C57Bl/6 mice expressing IL-21 in pancreatic β-cells, allowing the determination of the role of insufficient and excessive IL-21 signaling in type 1 diabetes. RESULTS Deficiency in IL-21R expression renders NOD mice resistant to insulitis, production of insulin autoantibodies, and onset of type 1 diabetes. The lymphoid compartment in IL-21R−/− NOD is normal and does not contain an increased regulatory T-cell fraction or diminished effector cytokine responses. However, we observed a clear defect in autoreactive effector T-cells in IL-21R−/− NOD by transfer experiments. Conversely, overexpression of IL-21 in pancreatic β-cells induced inflammatory cytokine and chemokines, including IL-17A, IL17F, IFN-γ, monocyte chemoattractant protein (MCP)-1, MCP-2, and interferon-inducible protein-10 in the pancreas. The ensuing leukocytic infiltration in the islets resulted in destruction of β-cells and spontaneous type 1 diabetes in the normally diabetes-resistant C57Bl/6 and NOD × C57Bl/6 backgrounds. CONCLUSIONS This work provides demonstration of the essential prodiabetogenic activities of IL-21 on diverse genetic backgrounds (NOD and C57BL/6) and indicates that IL-21 blockade could be a promising strategy for interventions in human type 1 diabetes.

IL-21 Inhibits IFN-γ Production in Developing Th1 Cells through the Repression of Eomesodermin Expression

Exposure of naive Th cell precursors (Thp) to IL-21 inhibits IFN-γ production from developing Th1 cells. The inhibition of IFN-γ seen in IL-21-treated Thp cells is specific as the expression of other Th1 cytokines is unaffected. Recently, it has been reported that Eomesodermin (Eomes), a member of the T-box gene family, is expressed in developing CD8+ T cells and plays an important role in regulating IFN-γ production and cytolytic effector function. In this study, we show that Eomes mRNA and protein are also expressed in developing Th1 cells, and exposure of naive Thp cells to IL-21 results in a decrease in Eomes expression. Moreover, the repression of Eomes expression by IL-21 is not due to an indirect effect of IL-21 on the expression of IFN-γ or STAT4 and is independent of STAT1 and T-bet expression. Finally, we show that ectopic expression of Eomes prevents the inhibition of IFN-γ production from IL-21-treated Thp cells. Taken together, these results demonstrate that Eomes plays a role in regulating IFN-γ production in CD4+ T cells and IL-21 inhibits IFN-γ production in developing Th1 cells through the repression of Eomes expression.

Stat6 and IRS-2 Cooperate in Interleukin 4 (IL-4)-Induced Proliferation and Differentiation but Are Dispensable for IL-4-Dependent Rescue from Apoptosis

ABSTRACT Stat6 and IRS-2 are two important signaling proteins that associate with the cytoplasmic tail of the interleukin 4 (IL-4) receptor. Data from numerous in vitro experiments have led to a model for IL-4 signal transduction in which the Stat6 signaling pathway is responsible for the IL-4 induced changes in gene expression and differentiation events, while the IRS-2 signaling pathway provides mitogenic and antiapoptotic signals. In order to determine the relative contributions of these signaling molecules in primary lymphocytes, we have examined IL-4 responses in T cells from mice deficient for either Stat6 or IRS-2 as well as from mice doubly deficient for both genes. Both IRS-2 and, especially, Stat6 are shown to be critically involved in IL-4-induced proliferation of T cells, presumably through the cooperative regulation of the Cdk inhibitor p27kip1. Like Stat6-deficient Th cells, IRS-2-deficient cells are also compromised in their ability to secrete Th2 cytokines, revealing a previously unrecognized role for IRS-2 in Th2 cell development. Although Stat6 and/or IRS-2 expression is required for IL-4-induced proliferative and differentiative responses, both signaling proteins are dispensable for the antiapoptotic effect of IL-4. However, treatment of lymphocytes with a protein tyrosine phosphatase inhibitor is able to block the antiapoptotic effect of IL-4 specifically in Stat6- or IRS-2-deficient cells and not in wild-type cells. Our results suggest that Stat6 and IRS-2 cooperate in promoting both IL-4-induced proliferative and differentiating responses, while an additional signaling mediator that depends on protein tyrosine phosphatase activity contributes to the antiapoptotic activities of IL-4 in primary T cells.