CD40–CD40L cross-talk integrates strong antigenic signals and microbial stimuli to induce development of IL-17-producing CD4+ T cells

IL-17-producing CD4⁺ T cells have been recognized as key players in organ-related autoimmune disease; however, the parameters that govern their development are yet to be elucidated fully. By using both in vivo and in vitro systems, we have investigated the role of antigen dose, pathogen-associated molecular patterns, and CD40–CD40 ligand (CD40L) cross-talk in Th17 differentiation. We found that the strength of antigenic stimulation critically influenced the extent of Th17 differentiation, because high, but not low or intermediate, antigen concentrations led to IL-17 production. Strong antigenic stimulation of T cells up-regulated CD40L expression, which in concert with certain microbial stimuli (i.e., cytosine phosphate guanine, curdlan, and zymosan) synergistically increased dendritic cell (DC) IL-6 production and Th17 polarization. CD40-deficient DCs exhibited reduced cytokine release and failed to drive Th17 development in vitro. These results were confirmed in vivo where the absence of CD40–CD40L cross-talk was found to prevent the expansion of IL-17-producing cells and accordingly the development of experimental autoimmune encephalitis. Our data demonstrate that CD40–CD40L cross-talk is important for Th17 development by translating strong T cell receptor and microbial stimuli into IL-6 production.     

Langerinneg conventional dendritic cells produce IL-23 to drive psoriatic plaque formation in mice

Psoriasis is an autoinflammatory skin disease of unknown etiology. Topical application of Aldara cream containing the Toll-like receptor (TLR)7 agonist Imiquimod (IMQ) onto patients induces flares of psoriasis. Likewise, in mice IMQ triggers pathological changes closely resembling psoriatic plaque formation. Key cytokines like IL-23 and type-I IFN (IFN-I), both being produced mainly by dendritic cells (DCs), have been implicated in psoriasis. Although plasmacytoid DCs (pDCs) are the main source of IFNα and thought to initiate disease, conventional DCs (cDCs) appear to maintain the psoriatic lesions. Any role of cDCs during lesion formation remains elusive. Here, we report that selective activation of TLR7 signaling specifically in CD11c⁺ DCs was sufficient to induce psoriasiform skin disease in mice. Intriguingly, both pDCs and the IFN-I pathway were dispensable for the development of local skin inflammation. Selective TLR7 triggering of Langerin⁺ DCs resulted in attenuated disease, whereas their depletion did not alter the severity of skin lesions. Moreover, after IMQ-painting, IL-23 was exclusively produced by Langerin^neg DCs in vivo. In conclusion, TLR7-activated Langerin^neg cDCs trigger psoriatic plaque formation via IL-23–mediated activation of innate IL-17/IL-22–producing lymphocytes, independently of pDCs or IFN-I. These results suggest therapeutic targeting of IL-23 production by cDCs to refine current treatment strategies for psoriasis.Psoriasis is a common chronic autoinflammatory skin disease characterized by demarcated, red and scaly plaques (1). These are the result of environmental and genetic factors triggering hyperproliferation and disturbed differentiation of keratinocytes (parakeratosis) leading to thickening of the epidermis (acanthosis). The inflammatory cell infiltrate consists mainly of dendritic cells (DCs), macrophages and T cells in the dermis and neutrophils in the epidermis. Based on the observation that topical application of Aldara cream containing the Toll-like receptor (TLR)7 ligand Imiquimod (IMQ) can elicit psoriasis (2), we developed a mouse model closely resembling plaque-type psoriasis, including abnormal keratinocyte proliferation and differentiation as well as DC, T-cell and neutrophil infiltration (3). Thus, in the human disease as well as IMQ-induced dermatitis effector cells of both the innate and adaptive immune system take part in the dysregulated immune response.Initially, psoriasis was defined as a T helper (Th) 1-type disease based on elevated levels of IFNγ, TNFα, and IL-12. Later on, a functional role of Th17/22 cells in psoriasis was demonstrated, associated with increased secretion of IL-17A/F and IL-22 (1, 4). Whereas IL-1β, IL-6, and TNFα contribute to the priming and skewing, IL-23 plays a pivotal role in terminal differentiation and pathogenicity of Th17/22 cells. Th cell-derived IL-17/IL-22 in turn stimulate keratinocyte proliferation and innate immune defense mechanisms like release of S100-proteins, β-defensins, and neutrophil-recruiting chemokines that contribute to the psoriatic phenotype. Novel findings concerning the pathogenic role of innate immune cells, namely γδ T cells, NK cells, and NK T cells, have challenged the prevailing view regarding psoriasis as a conventional Th cell-mediated disease (5, 6). In particular, dermis infiltrating γδ T-cell subsets as well as TCR^neg RORγt⁺ innate lymphocytes that rapidly produce IL-17/IL-22 upon stimulation with IL-23 and IL-1β, appear to be critical for the development of psoriasiform dermatitis in mice. In agreement, an increased frequency of Vγ9⁺ Vδ2⁺ T cells was described in human psoriatic skin (7). Despite accumulating evidence that activation of innate immune pathways plays a critical role in the initiation of psoriasis, it remains unclear how these pathways are triggered in vivo.DCs comprise a heterogeneous family of professional antigen presenting cells (APCs) that orchestrate the induction of immunity and tolerance. Plasmacytoid DCs (pDCs) are a small DC subset circulating through peripheral blood and secondary lymphoid organs. They represent key innate effector cells during antiviral immune responses due to their capacity to secrete large amounts of type-I IFN (IFN-I) upon TLR7/9 stimulation (8). Moreover, pDC-derived IFN-I represents an upstream event preceding autoimmune inflammation and psoriasis development (8, 9). In patients, an increased frequency and activation status of pDCs has been documented in early psoriatic lesions, whereas blocking IFN-I production inhibited the development of lesions in symptomless prepsoriatic skin transplants in the xenotransplantation mouse model (8). In psoriatic patients, keratinocytes produce elevated levels of the antimicrobial peptide LL-37. These form complexes with self-DNA/RNA, released by stressed/damaged cells, turning them into autoinflammatory TLR7/9-dependent triggers that activate DCs (10, 11). Subsequently, pDC-derived IFN-I together with keratinocyte-derived IL-1β, IL-6, and TNFα are thought to activate conventional DCs (cDCs), which migrate to cutaneous lymph nodes (LNs) to prime differentiation of pathogenic Th17/22 cells. Whether cDCs also contribute to the activation of IL-17/IL-22 producing innate lymphocytes in not known.The skin contains phenotypically and functionally distinct cDC subsets (12). Langerhans cells (LCs) reside in the epidermis and are characterized by expression of Langerin/CD207, which they share with a small population of Langerin⁺ cDCs in the dermis, whereas the majority of dermal cDCs are Langerin^neg. Recent observations indicate a functional specialization of the different skin-resident cDC subsets and, in particular, LCs can exert regulatory functions (e.g., during Leishmania major infection), whereas dermal cDCs may be more immunogenic (12, 13). In this study, we sought to dissect whether and how the different skin DC populations promote or regulate psoriatic plaque formation.     

Increased levels of circulating Th17 cells in quiescent versus active Crohn's disease

Background and aimsTh17 cells, a subset of CD4 + T cells that produce interleukin (IL)-17A, IL-17F, IL-21, IL-22, IL-26, and the chemokine CCL20 are critically involved in the mucosal inflammation observed in Crohn's disease (CD). However, their role as mediators of inflammation in CD has been questioned by a recent clinical trial in which anti-IL-17A (secukinumab) treatment was ineffective. Besides being pro-inflammatory, Th17-related cytokines mediate mucosal protective functions. We aimed to investigate the role of Th17 cells in CD inflammation.MethodsBlood samples from 26 patients with active CD and 10 healthy controls (HC) were analyzed for levels of IL-17A-, IL-21- and IL-22-producing CD45RO+CD4 + T cells using multicolor flow cytometry. Samples were analyzed before and during adalimumab treatment to compare intra-individual changes during active and quiescent disease.ResultsCD patients had statistically significantly higher levels of IL-17-A-, IL-21- and IL-22-producing CD45RO+CD4 + T cells in both active and quiescent disease compared with HC. Baseline levels of IL-21 and IL-22 producing CD45RO+CD4 + T cells correlated inversely with mucosal inflammation estimated by fecal calprotectin. Patients who responded to adalimumab treatment demonstrated a 2- to 3-fold increase in levels of IL-17A- and IL-21-producing CD45RO+CD4 + T cells in quiescent disease compared with active disease.ConclusionOur data support the involvement of Th17 cells and IL-21- and IL-22-producing CD45RO+CD4 + T cells in CD. Because patients had higher levels in quiescent disease compared with active CD, we question whether Th17 cells are promoters of inflammation. Instead, Th17 cells may counterbalance inflammation and maintain gut homeostasis.     

IL-1 acts directly on CD4 T cells to enhance their antigen-driven expansion and differentiation

IL-1 causes a marked increase in the degree of expansion of naïve and memory CD4 T cells in response to challenge with their cognate antigen. The response occurs when only specific CD4 T cells can respond to IL-1β, is not induced by a series of other cytokines and does not depend on IL-6 or CD-28. When WT cells are primed in IL-1R1^−/− recipients, IL-1 increases the proportion of cytokine-producing transgenic CD4 T cells, especially IL-17- and IL-4-producing cells, strikingly increases serum IgE levels and serum IgG1 levels. IL-1β enhances antigen-mediated expansion of in vitro primed Th1, Th2, and Th17 cells transferred to IL-1R1^−/− recipients. The IL-1 receptor antagonist diminished responses to antigen plus lipopolysaccharide (LPS) by ≈55%. These results indicate that IL-1β signaling in T cells markedly induces robust and durable primary and secondary CD4 responses.     

Homeostatic imbalance of regulatory and effector T cells due to IL-2 deprivation amplifies murine lupus

The origins and consequences of a regulatory T cell (Treg) disorder in systemic lupus erythematosus (SLE) are poorly understood. In the (NZBxNZW) F₁ mouse model of lupus, we found that CD4⁺Foxp3⁺ Treg failed to maintain a competitive pool size in the peripheral lymphoid organs resulting in a progressive homeostatic imbalance of CD4⁺Foxp3⁺ Treg and CD4⁺Foxp3⁻ conventional T cells (Tcon). In addition, Treg acquired phenotypic changes that are reminiscent of IL-2 deficiency concomitantly to a progressive decline in IL-2-producing Tcon and an increase in activated, IFN-γ-producing effector Tcon. Nonetheless, Treg from lupus-prone mice were functionally intact and capable to influence the course of disease. Systemic reduction of IL-2 levels early in disease promoted Tcon hyperactivity, induced the imbalance of Treg and effector Tcon, and strongly accelerated disease progression. In contrast, administration of IL-2 partially restored the balance of Treg and effector Tcon by promoting the homeostatic proliferation of endogenous Treg and impeded the progression of established disease. Thus, an acquired and self-amplifying disruption of the Treg-IL-2 axis contributed essentially to Tcon hyperactivity and the development of murine lupus. The reversibility of this homeostatic Treg disorder provides promising approaches for the treatment of SLE.     

Generation and regulation of human CD4+ IL-17-producing T cells in ovarian cancer

Despite the important role of Th17 cells in the pathogenesis of many autoimmune diseases, their prevalence and the mechanisms by which they are generated and regulated in cancer remain unclear. Here, we report the presence of a high percentage of CD4⁺ Th17 cells at sites of ovarian cancer, compared with a low percentage of Th17 cells in peripheral blood mononuclear cells from healthy donors and cancer patients. Analysis of cytokine production profiles revealed that ovarian tumor cells, tumor-derived fibroblasts, and antigen-presenting cells (APCs) secreted several key cytokines including IL-1β, IL-6, TNF-α and TGF-β, which formed a cytokine milieu that regulated and expanded human IL-17-producing T-helper (Th17) cells. We further show that IL-1β was critically required for the differentiation and expansion of human Th17 cells, whereas IL-6 and IL-23 may also play a role in the expansion of memory Th17 cells, even though IL-23 levels are low or undetectable in ovarian cancer. Further experiments demonstrated that coculture of naïve or memory CD4⁺ T cells with tumor cells, APCs, or both could generate high percentages of Th17 cells. Treatment with anti-IL-1 alone or a combination of anti-IL-1 and anti-IL-6 reduced the ability of tumor cells to expand memory Th17 cells. Thus, we have identified a set of key cytokines secreted by ovarian tumor cells and tumor-associated APCs that favor the generation and expansion of human Th17 cells. These findings should accelerate efforts to define the function of this important subset of CD4⁺ T cells in the human immune response to cancer.     

Th17 lymphocytes in atypical cutaneous leishmaniasis caused by Leishmania (L.) infantum chagasi in Central America

Skin lesions in nonulcerated cutaneous leishmaniasis (NUCL) caused by Leishmania (L.) infantum chagasi are characterized by a mononuclear inflammatory infiltrate in the dermis, which is composed mainly of lymphocytes, followed by macrophages, few plasma cells and epithelioid granulomas with mild tissue parasitism. Previous studies have shown that the main population of lymphocytes present in the dermal infiltrate is CD8⁺ T cells, followed by CD4⁺ T cells, which are correlated with IFN-γ⁺ cells. To improve the knowledge of cellular immune responses in NUCL, skin biopsies were submitted to immunohistochemistry using anti-ROR-γt, anti-IL-17, anti-IL-6, anti-TGF-β, and anti-IL-23 antibodies to characterize the involvement of Th17 cells in the skin lesions of patients affected by NUCL. ROR-γt⁺, IL-17⁺, IL-6⁺, TGF-β⁺ and IL-23⁺ cells were observed in the dermal inflammatory infiltrate of NUCL skin lesions. A positive correlation between CD4⁺ T-lymphocytes and ROR-γt⁺ and IL-17⁺ cells suggests that some of the CD4⁺ T-lymphocytes in NUCL could be Th17 lymphocytes. Moreover, a positive correlation between ROR-γt⁺ cells and TGF-β⁺, IL-6⁺, IL-17⁺ and IL-23⁺ cells could indicate the role of these cytokines in the differentiation and maintenance of Th17 lymphocytes. Our findings improve knowledge of the pathogenesis of this rare and atypical clinical form of leishmaniasis.     

Increased IL-6 expression on THP-1 by IL-34 stimulation up-regulated rheumatoid arthritis Th17 cells

IL-34 is a pleiotropic cytokine, which is a key regulator of monocytes/macrophages and might participate in the pathogenesis of RA. In this study, we aimed to explore the effect of IL-34 on the monocyte-like cell line THP-1 and the quantitative variation of Th17 cells in THP-1 and RA CD4⁺T cells coculture system. CD4⁺T cells were purified from RA PBMC using immunomagnetic beads. THP-1 were cultured with RA CD4⁺T cells. The frequency of Th17 cells was determined by FACS. Fluorscence indensity and expression of ROS were detected by FACS and cell staining, respectively. The expression of IL-6, IL-23, IL-21, TNF-α and IL-1β in the coculture supernatants were detected by ELISA. We found that CSF-1R was constitutively expressed on peripheral monocytes as well as THP-1, but not on the T/B cells. IL-34-CSF-1R binding could activate THP-1 to secret IL-6. IL-34 could up-regulate the numbers of Th17 cells in coculture system, which was possibly via the production of IL-6. We further observed ROS levels were increased in the coculture system. The percentage of Th17 cells was reduced when ROS production was inhibited by NAC, a specific inhibitor of ROS production. In addition, TNFRII antagonist but not IL-1βR antagonist could restrict the production of ROS, expression of IL-6 and generation of Th17 cells. In conclusion, IL-34-stimulated THP-1 can produce higher levels of ROS, which promoted IL-6 secretion and up-regulated Th17 cells. Our study suggests a novel mechanistic insight into how the interaction of IL-34-stimulated monocytes and CD4⁺T cells participates in the RA pathogenesis.     

The differential production of cytokines by human Langerhans cells and dermal CD14(+) DCs controls CTL priming

We recently reported that human epidermal Langerhans cells (LCs) are more efficient than dermal CD14(+) DCs at priming naive CD8(+) T cells into potent CTLs. We hypothesized that distinctive dendritic cell (DC) cytokine expression profiles (ie, IL-15 produced by LCs and IL-10 expressed by dermal CD14(+) DCs) might explain the observed functional difference. Blocking IL-15 during CD8(+) T-cell priming reduced T-cell proliferation by ～ 50%. These IL-15-deprived CD8(+) T cells did not acquire the phenotype of effector memory cells. They secreted less IL-2 and IFN-γ and expressed only low amounts of CD107a, granzymes and perforin, and reduced levels of the antiapoptotic protein Bcl-2. Confocal microscopy analysis showed that IL-15 is localized at the immunologic synapse of LCs and naive CD8(+) T cells. Conversely, blocking IL-10 during cocultures of dermal CD14(+) DCs and naive CD8(+) T cells enhanced the generation of effector CTLs, whereas addition of IL-10 to cultures of LCs and naive CD8(+) T cells inhibited their induction. TGF-β1 that is transcribed by dermal CD14(+) DCs further enhanced the inhibitory effect of IL-10. Thus, the respective production of IL-15 and IL-10 explains the contrasting effects of LCs and dermal CD14(+) DCs on CD8(+) T-cell priming.     

The imbalance between regulatory and IL-17-secreting CD4+ T cells in lupus patients

It has been well recognized that a deficit of numbers and function of CD4⁺CD25⁺Foxp3⁺ cells (Treg) is attributed to the development of some autoimmune diseases; however, there are controversial data regarding the suppressive effect of Treg cells on the T cell response in systemic lupus erythematosus (SLE). Additionally, IL-17-producing cells (Th17) have been recently emerged as a new pathogenic cell, but their role in lupus remains unclear. In this study, we studied the connection between Treg and Th17 cells in lupus patients. We observed that, while Treg or Th17 cells alone were not correlated to SLE development, the ratio of Treg to Th17 cells in active SLE patients is significantly lower than that in inactive SLE patients and healthy controls, and we also found corticosteroid treatment increased the ratio of Treg to Th17 cells in active SLE patients. Moreover, this ratio is inversely correlated with the severity of active SLE. The present study indicates that active SLE appears to exist as an imbalance between Treg and Th17 cells. Correction of this Treg/Th17 imbalance may have therapeutic impact for patients with SLE.     

Interferon-gamma and interleukin-4 reciprocally regulate CD8 expression in CD8+ T cells

The CD8 co-receptor can modulate CD8⁺ T cell function through its contributions to T cell receptor (TCR) binding and signaling. Here we show that IFN-γ and IL-4 exert opposing effects on the expression of CD8α mRNA and surface CD8 protein during CD8⁺ T cell activation. IL-4 caused down-regulation of surface CD8 on ovalbumin (OVA)_257–264-specific TCR-transgenic OT-I CD8⁺ T cells activated with OVA_257–264-coated antigen presenting cells or polyclonal stimuli, and on wild type CD8⁺ T cells activated with polyclonal stimuli. This effect was enhanced in each case when the cells lacked a functional IFN-γ or IFN-γR gene. When WT or IFN-γ-deficient OT-I CD8⁺ T cells were analyzed 9 days after co-injection with control or IL-4-expressing OVA⁺ tumor cells into RAG-2^−/−γc^−/− mice, CD8 levels were highest on WT donor cells from mice that received the control tumor and lowest on IFN-γ-deficient donor cells from mice that received the IL-4-expressing tumor. The latter CD8^low cells displayed markedly impaired binding of OVA_257–264/MHC tetramers and peptide/MHC-dependent degranulation. The data reveal an unexpected role for IFN-γ in tuning the CD8 co-receptor during primary CD8⁺ T cell activation both in vitro and in vivo.     

Regulation of memory CD4 T-cell pool size and function by natural killer T cells in vivo

To develop more effective vaccines and strategies to regulate chronic inflammatory diseases, it is important to understand the mechanisms of immunological memory. Factors regulating memory CD4⁺ T helper (Th)-cell pool size and function remain unclear, however. We show that activation of type I invariant natural killer T (iNKT) cells with glycolipid ligands and activation of type II natural killer T (NKT) cells with the endogenous ligand sulfatide induced dramatic proliferation and expansion of memory, but not naïve, CD4 T cells. NKT cell-induced proliferation of memory Th1 and Th2 cells was dependent largely on the production of IL-2, with Th2-cell proliferation also affected by loss of IL-4. Type II NKT cells were also required for efficient maintenance of memory CD4 T cells in vivo. Activation of iNKT cells resulted in up-regulation of IFN-γ expression by memory Th2 cells. These IFN-γ–producing memory Th2 cells showed a decreased capability to induce Th2 cytokines and eosinophilic airway inflammation. Thus, activated NKT cells directly regulate memory CD4 T-cell pool size and function via the production of cytokines in vivo.     

Cytokine Profiles Contribute to Understanding the Pathogenic Difference Between Good Syndrome and Oral Lichen Planus: Two Case Reports and Literature Review

We described and analyzed the pathogenic difference between Good syndrome (GS) and oral lichen planus (OLP) in oral mucosa.Good syndrome (GS) is a rare disease characterized by B and T cell immunodeficiency associated with hypogammaglobulinemia and thymoma. GS patients frequently develop oral lichenoid lesions with lymphocytic infiltration beneath the basal layer. Oral lichen planus (OLP) is a chronic inflammatory disease of the oral mucosa characterized by destruction of basal cells by Langerhans cells, macrophages, and T lymphocytes. Although the histological features of the lesions of both diseases are very similar, the pathogenesis of GS in the oral mucosa remains unknown. In this study, we thus investigated the expression of infiltrating lymphocyte subsets (CD3, CD20, CD4, and CD8) and T helper (Th) cytokines including interferon (IFN)-γ (Th1 type), interleukin (IL)-4 (Th2 type), IL-17 (Th17 type), and IL-10 (regulatory T cell type) by immunohistochemistry in buccal mucosa specimens from 2 GS patients compared with 15 OLP patients. All patients showed a predominance of CD3⁺ T cells over CD20⁺ B cells, and CD4⁺ Th cells over CD8⁺ cytotoxic T cells. This polarization was especially prominent in GS. IFN-γ and IL-10 were strongly detected in the infiltrating lymphocytes of all patients. However, IL-4 and IL-17 were detected in OLP patients only.These results suggest that the pathogenesis of GS is different from that of OLP. GS is a unique inflammatory disorder characterized by dysfunction of Th2 and Th17 immune reactions via abnormal T–B cell interaction.     

IFN-gamma enables cross-presentation of exogenous protein antigen in human Langerhans cells by potentiating maturation

We compared monocyte-derived dendritic cells and transforming growth factor-β1-induced Langerhans-like cells (LCs) for their capacity to cross-present exogenous NY-ESO-1 protein/antibody immune complexes to an NY-ESO-1-specific CD8⁺ T cell clone. In contrast to dendritic cells, LCs were not able to cross-present NY-ESO-1 to the T cell clone constitutively but did so after treatment with IFN-γ. Remarkably, this IFN-γ-inducible characteristic was due neither to enhanced antigen uptake nor to facilitated antigen processing in LCs. Rather, IFN-γ acted at least in part by potentiating the maturation of otherwise refractory LCs, enabling in turn exogenous antigen to reach the processing machinery. This model of conditional cross-presentation establishes an original level of action for IFN-γ as an effective immune modulator and supports the use of IFN-γ in protein vaccination strategies targeting LCs.     

Identification of an IL-27/osteopontin axis in dendritic cells and its modulation by IFN-γ limits IL-17–mediated autoimmune inflammation

Dendritic cells (DCs) play a central role in determining the induction of T cell responses. IL-27 production by DCs favors induction of IL-10–producing regulatory T cells, whereas osteopontin (OPN) promotes pathogenic IL-17 T cell responses. The regulatory mechanisms in DCs that control these two cells types are not understood well. Here, we show that IFN-γ induces IL-27 while inhibiting OPN expression in DCs both in vitro and in vivo and that engagement of IFN-γR expressed by DCs leads to suppression of IL-17 production while inducing IL-10 from T cells. DCs modified by IFN-γ acquire IL-27–dependent regulatory function, promote IL-10–mediated T cell tolerance, and suppress autoimmune inflammation. Thus, our results identify a previously unknown pathway by which IFN-γ limits IL-17–mediated autoimmune inflammation through differential regulation of OPN and IL-27 expression in DCs.IL-27 is a potent antiinflammatory cytokine, which belongs to the IL-12 family and is comprised of an IL-12p40–related protein, encoded by the EBV-induced gene 3 (EBI3, also known as IL27), and a unique IL-12p35–like protein, IL-27p28v (1). Initial animal studies on the biology of IL-27 suggested a role for IL-27 in the initiation of the Th1 response (2, 3); however, subsequent work using mouse models of pathogen-induced and autoimmune inflammation have indicated that IL-27 has broad inhibitory effects on Th1, Th2 subsets of T cells and APC function in mice (4, 5). In addition, we and others have shown that IL-27 is capable of inducing IL-10–producing regulatory Tr1 cells while inhibiting IL-17–producing Th17 cells both from humans and mice and acts as a negative feedback mechanism against proinflammatory immune responses (6–10).Contrary to the function of IL-27, osteopontin (OPN) is known to have potent proinflammatory functions. OPN participates in a wide range of biological processes (11) and has been linked to autoimmune diseases including multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE). Mice deficient for OPN (Spp1^−/−, also known as OPN^−/−) show milder EAE without disease exacerbation or progression compared with WT mice (12, 13). Patients with MS have elevated levels of OPN in their serum and plasma (14, 15). OPN exacerbates EAE by skewing T cell differentiation toward IFN-γ–producing Th1 cells and IL-17–producing Th17 cells (12, 13, 16–18). In addition, OPN induces IFN-γ– and IL-17–producing T cells in patients with MS (16).Dendritic cells (DCs) are crucial in the initiation of productive antigen-specific T cell responses and the induction of T cell tolerance (19, 20). This dual function was initially explained by the existence of specific subpopulations of DCs that preferentially trigger T cell priming, whereas other subpopulations were identified to induce T cell tolerance (21–23). The demonstration that a single DC subpopulation can elicit both T cell outcomes, led to an alternative explanation that the functional status of the DC at the time of antigen presentation, rather than its phenotypic characteristics, is critical for determining T cell responses (24). Among the factors linked to the functional status of DCs, the types of cytokines secreted by DCs can regulate the differentiation of CD4⁺ T cells into functional subsets including IL-17–producing pathogenic Th17 cells and IL-10–producing regulatory Tr1 cells. For example, IL-10 or IL-27 production by DCs favors generation of IL-10–producing Tr1 cells (6, 25), whereas OPN production by DCs promotes IL-17–producing Th17 cells (16, 17). However, it is unknown how IL-27 and OPN expression in DCs are modulated to regulate both IL-10 and IL-17 production by T cells.Here, we show that IFN-γ induces IL-27 while inhibiting OPN expression in DCs both in vitro and in vivo. IFN-γ^−/−-deficient mice in which EAE is induced have increased serum OPN and lower IL-27 levels in comparison with WT mice. Engagement of IFNγR expressed by DCs leads to suppression of IL-17 production and induction of IL-10 from T cells. Furthermore, IFN-γ–modified DCs ameliorate the disease severity of EAE through an IL-27–dependent mechanism. Taken together, our results identify a previously unknown pathway by which IFN-γ limits IL-17–mediated autoimmune inflammation through reciprocal DC modulation of OPN and IL-27.