Jason S. Stumhofer

Interleukin 27 negatively regulates the development of interleukin 17-producing T helper cells during chronic inflammation of the central nervous system.

Studies have focused on the events that influence the development of interleukin 17 (IL-17)–producing T helper cells (TH-17 cells) associated with autoimmunity, such as experimental autoimmune encephalitis, but relatively little is known about the cytokines that antagonize TH-17 cell effector responses. Here we show that IL-27 receptor–deficient mice chronically infected with Toxoplasma gondii developed severe neuroinflammation that was CD4+ T cell dependent and was associated with a prominent IL-17 response. In vitro, treatment of naive primary T cells with IL-27 suppressed the development TH-17 cells induced by IL-6 and transforming growth factor-β, which was dependent on the intracellular signaling molecule STAT1 but was independent of inhibition of IL-6 signaling mediated by the suppressor protein SOCS3. Thus IL-27, a potent inhibitor of TH-17 cell development, may be a useful target for treating inflammatory diseases mediated by these cells.

Interleukins 27 and 6 induce STAT3-mediated T cell production of interleukin 10

Interleukin 10 (IL-10) has a prominent function in regulating the balance between protective and pathological T cell responses. Consistent with that activity, many sources of this cytokine are found in vivo, including from myeloid cells and a variety of T cell subsets. However, although there are many pathways that regulate innate production of IL-10, the factors that govern its synthesis by the adaptive response are poorly understood. Here we report that IL-27 and IL-6 induced T helper type 1 and type 2 cells, as well as T helper cells that produce IL-17, to secrete IL-10. This effect was dependent on the transcription factors STAT1 and STAT3 for IL-27 and on STAT3 for IL-6. Our studies identify a previously unknown pathway that allows the immune system to temper inflammatory responses.

Anti–IL-23 therapy inhibits multiple inflammatory pathways and ameliorates autoimmune encephalomyelitis

IL-23 is a member of the IL-12 cytokine family that drives a highly pathogenic T cell population involved in the initiation of autoimmune diseases. We have shown that IL-23-dependent, pathogenic T cells produced IL-17 A, IL-17 F, IL-6, and TNF but not IFN-gamma or IL-4. We now show that T-bet and STAT1 transcription factors are not required for the initial production of IL-17. However, optimal IL-17 production in response to IL-23 stimulation appears to require the presence of T-bet. To explore the clinical efficacy of targeting the IL-23 immune pathway, we generated anti-IL-23p19-specific antibodies and tested to determine whether blocking IL-23 function can inhibit EAE, a preclinical animal model of human multiple sclerosis. Anti-IL-23p19 treatment reduced the serum level of IL-17 as well as CNS expression of IFN-gamma, IP-10, IL-17, IL-6, and TNF mRNA. In addition, therapeutic treatment with anti-IL-23p19 during active disease inhibited proteolipid protein (PLP) epitope spreading and prevented subsequent disease relapse. Thus, therapeutic targeting of IL-23 effectively inhibited multiple inflammatory pathways that are critical for driving CNS autoimmune inflammation.

IL-27 Blocks RORc Expression to Inhibit Lineage Commitment of Th17 Cells

IL-27 is secreted by APCs in response to inflammatory stimuli and exerts a proinflammatory Th1-enhancing activity but also has significant anti-inflammatory functions. We examined the molecular mechanism by which IL-27 regulates TGFβ plus IL-6- or IL-23-dependent Th17 development in the mouse and human systems. IL-27 inhibited the production of IL-17A and IL-17F in naive T cells by suppressing, in a STAT1-dependent manner, the expression of the Th17-specific transcription factor RORγt. The in vivo significance of the role of IL-27 was addressed in delayed-type hypersensitivity response and experimental autoimmune encephalomyelitis (EAE). By generating mice deficient for the p28 subunit of IL-27, we showed that IL-27 regulated the severity of delayed-type hypersensitivity response and EAE through its effects on Th17 cells. Furthermore, up-regulation of IL-10 in the CNS, which usually occurs late after EAE onset and plays a role in the resolution of the disease, was notably absent in IL-27p28−/− mice. These results show that IL-27 acts as a negative regulator of the developing IL-17A response in vivo, suggesting a potential therapeutic role for IL-27 in autoimmune diseases.

IL-27 Limits IL-2 Production during Th1 Differentiation

Although the ability of IL-27 to promote T cell responses is well documented, the anti-inflammatory properties of this cytokine remain poorly understood. The current work demonstrates that during infection with Toxoplasma gondii, IL-27R-deficient mice generate aberrant IL-2 responses that are associated with the development of a lethal inflammatory disease. Because in vivo depletion of IL-2 prolongs the survival of infected IL-27R−/− mice, these data suggest that IL-27 curbs the development of immunopathology by limiting parasite-induced IL-2 production. Consistent with this hypothesis, IL-27R−/− CD4+ T cells produce more IL-2 than wild-type counterparts during in vitro differentiation, and when rIL-27 is introduced, it can suppress the expression of IL-2 mRNA and protein by the latter group. Additionally, these studies reveal that, like IL-27, IL-12 can inhibit IL-2 production, and although each employs distinct mechanisms, they can synergize to enhance the effect. In contrast, this property is not shared by closely related cytokines IL-6 and IL-23. Thus, while traditionally viewed as proinflammatory agents, the present findings establish that IL-27 and IL-12 cooperate to limit the availability of IL-2, a potent T cell growth and survival factor. Moreover, because the current studies demonstrate that both can induce expression of suppressor of cytokine signaling 3, a protein that tempers cytokine receptor signaling, they also suggest that IL-27 and IL-12 share additionally inhibitory properties.

IL-6 Promotes NK Cell Production of IL-17 during Toxoplasmosis

Previous studies have implicated T cell production of IL-17 in resistance to Toxoplasma gondii as well as the development of immune-mediated pathology during this infection. Analysis of C57BL/6 and C57BL/6 RAG−/− mice challenged with T. gondii-identified NK cells as a major innate source of IL-17. The ability of soluble Toxoplasma Ag to stimulate NK cells to produce IL-17 was dependent on the presence of accessory cells and the production of IL-6, IL-23, and TGF-β. In contrast, these events were inhibited by IL-2, IL-15, and IL-27. Given that IL-6 was one of the most potent enhancers of NK cell production of IL-17, further studies revealed that only a subset of NK cells expressed both chains of the IL-6R, IL-6 upregulated expression of the Th17-associated transcription factor RORγt, and that IL-6−/− mice challenged with T. gondii had a major defect in NK cell production of IL-17. Together, these data indicate that many of the same cytokines that regulate Th17 cells are part of a conserved pathway that also control innate production of IL-17 and identify a major role for IL-6 in the regulation of NK cell responses.

Advances in understanding the anti-inflammatory properties of IL-27

Initial studies on the biology of IL-27 provided evidence of a role for this cytokine in the initiation of Th1 responses; however, subsequent work using models of pathogen-induced and autoimmune inflammation have indicated that IL-27 has broad inhibitory effects on Th1, Th2 and Th17 subsets of T cells as well as the expansion of inducible regulatory T cells. While, the aim of this review is to highlight the functions of IL-27 in the context of inflammation it will also serve to elaborate on the molecular mechanisms involved in the production of this cytokine. The initial description of IL-27 indicated that classical antigen-presenting cells such as macrophages and dendritic cells produce IL-27, however, the agonists and signaling pathways involved in activating transcription of the two subunits of IL-27, p28 and EBV-induced gene 3 (EBI3) have only recently been described.

A role for IL-27p28 as an antagonist of gp130-mediated signaling

The heterodimeric cytokine interleukin 27 (IL-27) signals through the IL-27Rα subunit of its receptor, combined with gp130, a common receptor chain used by several cytokines, including IL-6. Notably, the IL-27 subunits p28 (IL-27p28) and EBI3 are not always expressed together, which suggests that they may have unique functions. Here we show that IL-27p28, independently of EBI3, antagonized cytokine signaling through gp130 and IL-6-mediated production of IL-17 and IL-10. Similarly, the ability to generate antibody responses was dependent on the activity of gp130-signaling cytokines. Mice transgenic for expression of IL-27p28 showed a substantial defect in the formation of germinal centers and antibody production. Thus, IL-27p28, as a natural antagonist of gp130-mediated signaling, may be useful as a therapeutic for managing inflammation mediated by cytokines that signal through gp130.

Helper T cell IL-2 production is limited by negative feedback and STAT-dependent cytokine signals.

Although required for many fundamental immune processes, ranging from self-tolerance to pathogen immunity, interleukin (IL)-2 production is transient, and the mechanisms underlying this brevity remain unclear. These studies reveal that helper T cell IL-2 production is limited by a classic negative feedback loop that functions autonomously or in collaboration with other common γ chain (IL-4 and IL-7) and IL-6/IL-12 family cytokines (IL-12 and IL-27). Consistent with this model for cytokine-dependent regulation, they also demonstrate that the inhibitory effect can be mediated by several signal transducer and activator of transcription (STAT) family transcription factors, namely STAT5, STAT4, and STAT6. Collectively, these findings establish that IL-2 production is limited by a network of autocrine and paracrine signals that are readily available during acute inflammatory responses and, thus, provide a cellular and molecular basis for its transient pattern of expression.

IL-27 Regulates IL-10 and IL-17 from CD4+ Cells in Nonhealing Leishmania major Infection

Control of infection caused by Leishmania major requires the development of IFN-γ+CD4+ lymphocytes for the induction of microbicidal activity in host macrophages. We recently reported on the inability of conventionally resistant C57BL/6 mice to successfully resolve infection by an isolate of L. major, despite a strong IFN-γ response by the host. Susceptibility was caused by Ag-specific IL-10 from CD4+ cells that were also producing IFN-γ. In the present studies, we have explored the role for IL-27 in the regulation of IL-10 from Th1 cells in leishmaniasis. Cytokine analysis of CD4+ cells in the lesions and draining lymph nodes of infected IL-27R-deficient (WSX-1−/−) mice revealed diminished IL-10 from IFN-γ+ CD4+ cells, which was accompanied by a reduction in total IFN-γ+CD4+ cells and an increase in IL-4. Despite the inhibition of IL-10 from CD4+ cells, no significant change in parasite numbers was observed, due both to the shift in the Th1/Th2 balance and to residual levels of IL-10. Strikingly, infected WSX-1−/− mice developed more severe lesions that were associated with the appearance of IL-17+ CD4+ cells, demonstrating a function for IL-27 in blocking the development of inappropriate Th17 cells during L. major infection. The results demonstrate the pleiotropic effects that IL-27 has on L. major-driven Th1, Th2, and Th17 development, and reinforce its function as a key regulatory cytokine that controls the balance between immunity and pathology.