Trans-presentation of IL-6 by dendritic cells is required for the priming of pathogenic TH17 cells.

The cellular sources of interleukin 6 (IL-6) that are relevant for differentiation of the TH17 subset of helper T cells remain unclear. Here we used a novel strategy for the conditional deletion of distinct IL-6-producing cell types to show that dendritic cells (DCs) positive for the signaling regulator Sirpα were essential for the generation of pathogenic TH17 cells. Using their IL-6 receptor α-chain (IL-6Rα), Sirpα+ DCs trans-presented IL-6 to T cells during the process of cognate interaction. While ambient IL-6 was sufficient to suppress the induction of expression of the transcription factor Foxp3 in T cells, trans-presentation of IL-6 by DC-bound IL-6Rα (called 'IL-6 cluster signaling' here) was needed to prevent premature induction of interferon-γ (IFN-γ) expression in T cells and to generate pathogenic TH17 cells in vivo. Our findings should guide therapeutic approaches for the treatment of TH17-cell-mediated autoimmune diseases.

The Cytokine GM-CSF Drives the Inflammatory Signature of CCR2+ Monocytes and Licenses Autoimmunity.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) has emerged as a crucial cytokine produced by auto-reactive T helper (Th) cells that initiate tissue inflammation. Multiple cell types can sense GM-CSF, but the identity of the pathogenic GM-CSF-responsive cells is unclear. By using conditional gene targeting, we systematically deleted the GM-CSF receptor (Csf2rb) in specific subpopulations throughout the myeloid lineages. Experimental autoimmune encephalomyelitis (EAE) progressed normally when either classical dendritic cells (cDCs) or neutrophils lacked GM-CSF responsiveness. The development of tissue-invading monocyte-derived dendritic cells (moDCs) was also unperturbed upon Csf2rb deletion. Instead, deletion of Csf2rb in CCR2(+)Ly6C(hi) monocytes phenocopied the EAE resistance seen in complete Csf2rb-deficient mice. High-dimensional analysis of tissue-infiltrating moDCs revealed that GM-CSF initiates a combination of inflammatory mechanisms. These results indicate that GM-CSF signaling controls a pathogenic expression signature in CCR2(+)Ly6C(hi) monocytes and their progeny, which was essential for tissue damage.

Impact of high-altitude therapy on type-2 immune responses in asthma patients.

BACKGROUND: Asthma patients present with distinct immunological profiles, with a predominance of type 2 endotype. The aim of this study was to investigate the impact of high-altitude treatment on the clinical and immunological response in asthma. METHODS: Twenty-six hospitalized asthma patients (nine eosinophilic allergic; EA, nine noneosinophilic allergic; NEA and eight noneosinophilic nonallergic; NN) and nine healthy controls in high altitude for 21 days were enrolled in the study. We assessed eosinophils, T cells, Tregs, and innate lymphoid cells (ILC) from peripheral blood using flow cytometry. RESULTS: The number of eosinophils (both resting and activated) and chemoattractant receptor homolog expressed on Th2 cells (CRTH2)-expressing CD4+ and CD8+ T cells decreased significantly in EA patients after altitude treatment. The frequency of CRTH2+ Tregs as decreased significantly in all the asthma phenotypes as well as the frequency of ILC2 was significantly reduced in EA after altitude treatment. After 21 days of altitude therapy, CRTH2-expressing ILC2, CD4+ and CD8+ T cells and Treg cells showed attenuated responses to exogenous PGD2. Furthermore, PGD2 signaling via CRTH2 was found to diminish the suppressive function of CRTH2+ Tregs which partially normalized during high-altitude treatment. Improved asthma control was particularly evident in allergic asthma patients and correlated with decreased frequencies of CRTH2+ Treg cells in EA patients. Serum IL-5 and IL-13 decreased during climate treatment in asthma patients with high baseline levels. CONCLUSIONS: Asthma treatment in high altitude reduced the type 2 immune response, corrected the increased CRTH2 expression and its dysregulated functions.

A T-bet gradient controls the fate and function of CCR6-RORγt+ innate lymphoid cells.

At mucosal surfaces, the immune system should not initiate inflammatory immune responses to the plethora of antigens constantly present in the environment, but should remain poised to unleash a potent assault on intestinal pathogens. The transcriptional programs and regulatory factors required for immune cells to switch from homeostatic (often tissue-protective) function to potent antimicrobial immunity are poorly defined. Mucosal retinoic-acid-receptor-related orphan receptor-γt-positive (RORγt(+)) innate lymphoid cells (ILCs) are emerging as an important innate lymphocyte population required for immunity to intestinal infections. Various subsets of RORγt(+) ILCs have been described but the transcriptional programs controlling their specification and fate remain largely unknown. Here we provide evidence that the transcription factor T-bet determines the fate of a distinct lineage of CCR6(-)RORγt(+) ILCs. Postnatally emerging CCR6(-)RORγt(+) ILCs upregulated T-bet and this was controlled by cues from the commensal microbiota and interleukin-23 (IL-23). In contrast, CCR6(+)RORγt(+) ILCs, which arise earlier during ontogeny, did not express T-bet. T-bet instructed the expression of T-bet target genes such as interferon-γ (IFN-γ) and of the natural cytotoxicity receptor NKp46. Mice genetically lacking T-bet showed normal development of CCR6(-)RORγt(+) ILCs, but they could not differentiate into NKp46-expressing RORγt(+) ILCs (that is, IL-22-producing natural killer (NK-22) cells) and failed to produce IFN-γ. The production of IFN-γ by T-bet-expressing CCR6(-)RORγt(+) ILCs was essential for the release of mucus-forming glycoproteins required to protect the epithelial barrier during Salmonella enterica infection. Salmonella infection also causes severe enterocolitis that is at least partly driven by IFN-γ. Mice deficient for T-bet or depleted of ILCs developed only mild enterocolitis. Thus, graded expression of T-bet in CCR6(-)RORγt(+) ILCs facilitates the differentiation of IFN-γ-producing CCR6(-)RORγt(+) ILCs required to protect the epithelial barrier against Salmonella infections. Co-expression of T-bet and RORγt, which is also found in subsets of IL-17-producing T-helper (T(H)17) cells, may be an evolutionarily conserved transcriptional program that originally developed as part of the innate defence against infections but that also confers an increased risk of immune-mediated pathology.

GM-CSF in Neuroinflammation: Licensing Myeloid Cells for Tissue Damage.

Multiple sclerosis (MS) is the prototypical inflammatory disease of the central nervous system (CNS). MS lesions harbor different immune cells, but the contribution of individual cell types to disease etiology and progression is not well understood. In experimental autoimmune encephalomyelitis (EAE), auto-reactive helper T (Th) cells instigate CNS inflammation by acting on myeloid cells via the production of granulocyte-macrophage colony-stimulating factor (GM-CSF). Recent reports have implicated myeloid cells in both the inflammatory process and as executers of tissue damage in the CNS. We review these findings here, and integrate them into a model wherein GM-CSF produced by Th cells coordinates monocyte recruitment to the CNS, and differentiation into pathogenic effectors. We discuss the implications of this model to current therapies for MS, and outline important areas of further inquiry.

Dermal IL-17-producing γδ T cells establish long-lived memory in the skin.

Conventional αß T cells have the ability to form a long-lasting resident memory T-cell (TRM ) population in nonlymphoid tissues after encountering foreign antigen. Conversely, the concept of 'innate memory', where the ability of nonadaptive branches of the immune system to deliver a rapid, strengthened immune response upon reinfection or rechallenge, is just emerging. Using the αß T-cell-independent Aldara psoriasis mouse model in combination with genetic fate-mapping and reporter systems, we identified a subset of γδ T cells in mice that is capable of establishing a long-lived memory population in the skin. IL-17A/F-producing Vγ4(+) Vδ4(+) T cells populate and persist in the dermis for long periods of time after initial stimulation with Aldara. Experienced Vγ4(+) Vδ4(+) cells show enhanced effector functions and mediate an exacerbated secondary inflammatory response. In addition to identifying a unique feature of γδ T cells during inflammation, our results have direct relevance to the human disease as this quasi-innate memory provides a mechanistic insight into relapses and chronification of psoriasis.

Interleukin 17 drives vascular inflammation, endothelial dysfunction, and arterial hypertension in psoriasis-like skin disease.

OBJECTIVE: Interleukin (IL)-17A is regarded as an important cytokine to drive psoriasis, an inflammatory skin disease marked by increased cardiovascular mortality. We aimed to test the hypothesis that overproduction of IL-17A in the skin leading to dermal inflammation may systemically cause vascular dysfunction in psoriasis-like skin disease. APPROACH AND RESULTS: Conditional overexpression of IL-17A in keratinocytes caused severe psoriasis-like skin inflammation in mice (K14-IL-17A(ind/+) mice), associated with increased reactive oxygen species formation and circulating CD11b(+) inflammatory leukocytes in blood, with endothelial dysfunction, increased systolic blood pressure, left ventricular hypertrophy, and reduced survival compared with controls. In K14-IL-17A(ind/+) mice, immunohistochemistry and flow cytometry revealed increased vascular production of the nitric oxide/superoxide reaction product peroxynitrite and infiltration of the vasculature with myeloperoxidase(+)CD11b(+)GR1(+)F4/80(-) cells accompanied by increased expression of the inducible nitric oxide synthase and the nicotinamide dinucleotide phosphate (NADPH) oxidase, nox2. Neutrophil depletion by anti-GR-1 antibody injections reduced oxidative stress in blood and vessels. Neutralization of tumor necrosis factor-α and IL-6 (both downstream of IL-17A) reduced skin lesions, attenuated oxidative stress in heart and blood, and partially improved endothelial dysfunction in K14-IL-17A(ind/+) mice. CONCLUSIONS: Dermal overexpression of IL-17A induces systemic endothelial dysfunction, vascular oxidative stress, arterial hypertension, and increases mortality mainly driven by myeloperoxidase(+)CD11b(+)GR1(+)F4/80(-) inflammatory cells. Depletion of the GR-1(+) immune cells or neutralization of IL-17A downstream cytokines by biologicals attenuates the vascular phenotype in K14-IL-17A(ind/+) mice.

Discovery of the Clinical Candidate IDOR-1117-2520: A Potent and Selective Antagonist of CCR6 for Autoimmune Diseases.

Migration of immune cells to sites of inflammation is a critical step in the body's response to infections but also during autoimmune flares. Chemokine receptors, members of the GPCR receptors, are instrumental in directing specific cell types to their target organs. Herein, we describe a highly potent small molecule antagonist of the chemokine receptor CCR6, which came out of fine-tuned structural elaborations from a proprietary HTS hit. Three main issues in the parent chemical series-cytotoxicity, phototoxicity, and hERG, were successfully solved. Biological characterization demonstrated that compound 45 (IDOR-1117-2520) is a selective and insurmountable antagonist of CCR6. In vivo proof-of-mechanism studies in a mouse lung inflammation model using a representative compound from the chemical class of 45 confirmed that the targeted CCR6+ cells were efficiently inhibited from migrating into the bronchoalveoli. Finally, ADMET and physicochemical properties were well balanced and the preclinical package warranted progress in the clinic.

IL-23: one cytokine in control of autoimmunity.

During the past decade, it has been firmly established that IL-23 is essential for disease development in several models of autoimmune disease, including psoriatic skin inflammation, inflammatory bowel disease (IBD), and experimental autoimmune encephalomyelitis (EAE). The mechanism by which IL-23 exerts its pathogenic role has been mostly scrutinized in the context of Th17 cells, which were thought to mediate autoimmunity by secretion of IL-17 family cytokines. However, the picture emerging now is one of multiple IL-23-responsive cell types, pro-inflammatory cytokine induction, and pathogenic "licensing" following an IL-23-dominated interaction between the T cell and the antigen-presenting cell (APC). This review will focus on our changing view of IL-23-dependent autoimmune pathologies with a particular emphasis on the responder cells and their IL-23-induced factors that ultimately mediate tissue destruction.

Mouse models for multiple sclerosis: historical facts and future implications.

Multiple sclerosis (MS) is an inflammatory and demyelinating condition of the CNS, characterized by perivascular infiltrates composed largely of T lymphocytes and macrophages. Although the precise cause remains unknown, numerous avenues of research support the hypothesis that autoimmune mechanisms play a major role in the development of the disease. Pathologically similar lesions to those seen in MS can be induced in laboratory rodents by immunization with CNS-derived antigens. This form of disease induction, broadly termed experimental autoimmune encephalomyelitis, is frequently the starting point in MS research with respect to studying pathogenesis and creating novel treatments. Many different EAE models are available, each mimicking a particular facet of MS. These models all have common ancestry, and have developed from a single concept of immunization with self-antigen. We will discuss the major changes in immunology research, which have shaped the EAE models we use today, and discuss how current animal models of MS have resulted in successful treatments and more open questions for researchers to address.

IL-17A and IL-17F do not contribute vitally to autoimmune neuro-inflammation in mice.

The clear association of Th17 cells with autoimmune pathogenicity implicates Th17 cytokines as critical mediators of chronic autoimmune diseases such as EAE. To study the impact of IL-17A on CNS inflammation, we generated transgenic mice in which high levels of expression of IL-17A could be initiated after Cre-mediated recombination. Although ubiquitous overexpression of IL-17A led to skin inflammation and granulocytosis, T cell-specific IL-17A overexpression did not have a perceptible impact on the development and health of the mice. In the context of EAE, neither the T cell-driven overexpression of IL-17A nor its complete loss had a major impact on the development of clinical disease. Since IL-17F may be able to compensate for the loss of IL-17A, we also generated IL-17F-deficient mice. This strain was fully susceptible to EAE and displayed unaltered emergence and expansion of autoreactive T cells during disease. To eliminate potential compensatory effects of either cytokine, we treated IL-17F-deficient mice with antagonistic monoclonal antibodies specific for IL-17A and found again only a minimal beneficial impact on disease development. We conclude therefore that both IL-17A and IL-17F, while prominently expressed by an encephalitogenic T cell population, may only marginally contribute to the development of autoimmune CNS disease.

CD4+ T-cell-derived IL-10 promotes CNS inflammation in mice by sustaining effector T cell survival.

Interleukin (IL)-10 is considered a prototypical anti-inflammatory cytokine, significantly contributing to the maintenance and reestablishment of immune homeostasis. Accordingly, it has been shown in the intestine that IL-10 produced by Tregs can act on effector T cells, thereby limiting inflammation. Herein, we investigate whether this role also applies to IL-10 produced by T cells during central nervous system (CNS) inflammation. During neuroinflammation, both CNS-resident and -infiltrating cells produce IL-10; yet, as IL-10 has a pleotropic function, the exact contribution of the different cellular sources is not fully understood. We find that T-cell-derived IL-10, but not other relevant IL-10 sources, can promote inflammation in experimental autoimmune encephalomyelitis. Furthermore, in the CNS, T-cell-derived IL-10 acts on effector T cells, promoting their survival and thereby enhancing inflammation and CNS autoimmunity. Our data indicate a pro-inflammatory role of T-cell-derived IL-10 in the CNS.

Cytokine reporter mice in immunological research: perspectives and lessons learned.

Cytokines are soluble messenger molecules with important regulatory functions throughout the immune system. 'Cytokine reporter' strains express marker molecules under control of elements from cytokine genes allowing for easy identification of their cellular sources. Such systems are well-accepted tools for research of cytokine function. The value of these strains lies in the ability to perform experiments relying on identification and isolation of live cytokine-expressing cells, provided that the reporter faithfully reflects the proper cytokine mRNA and protein production. As more diverse cell subsets are defined by their cytokine expression, the field has adapted with the generation of more sophisticated strains. In this review we summarize the evolution of cytokine detection methods and give examples of knowledge gained using cytokine reporter mice for cell types expressing interferon-γ and interleukin-4, -10 and -17. We also discuss current options for generating such reporter strains and their potential pitfalls.

Genetic proof for the transient nature of the Th17 phenotype.

IL-17-producing CD4(+) T cells (Th17) have been classified as a new T helper cell subset. Using an IL-17 fate mapping mouse strain, which genetically fixes the memory of IL-17 expression, we demonstrate that IL-17A/F-expressing T helper cells generated either in vitro or in vivo are not a stable T-cell subset. Upon adoptive transfer of IL-17F-reporter-positive Th17 cells to RAG-deficient or WT animals, encephalitogenic Th17 cells partially lose IL-17 expression and upregulate IFN-γ. Additionally, we show that Th1 cells can convert in vivo to IL-17A/IFN-γ-coexpressing cells in the mesenteric lymph nodes (mLN). Our data classify IL-17A and IL-17F as cytokines produced transiently in response to the local microenvironment, thus showing that IL-17 expression does not define an end-stage T helper cell subset.

Cutting edge: an IL-17F-CreEYFP reporter mouse allows fate mapping of Th17 cells.

The need for reporter lines able to faithfully track Th17 cells in vivo has become an issue of exceptional importance. To address this, we generated a mouse strain in which Cre recombinase is expressed from the IL-17F promoter. Crossing the IL-17F-Cre allele to a conditional enhanced yellow fluorescent protein (EYFP) reporter mouse yielded the IL-17F-Cre(EYFP) strain, in which IL-17F expression is twinned with EYFP in live IL-17F-expressing cells. Although we demonstrate that IL-17F expression is restricted to CD4(+) T cells during experimental autoimmune encephalomyelitis, IL-17F-Cre(EYFP) CD8 T cells robustly expressed IL-17F in response to TGF-beta, IL-6, and IL-23. Fate mapping of IL-17F-expressing reporter T cells revealed a significant down-regulation of Th17 cytokines after homeostatic expansion in RAG1-deficient animals. Despite this loss of effector phenotype, committed Th17 cells were resistant to Foxp3 expression in vitro or in vivo. Thus, the IL-17F-Cre strain furthers our understanding of Th17 biology.

Smad7 in T cells drives T helper 1 responses in multiple sclerosis and experimental autoimmune encephalomyelitis.

Autoreactive CD4+ T lymphocytes play a vital role in the pathogenesis of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis. Since the discovery of T helper 17 cells, there is an ongoing debate whether T helper 1, T helper 17 or both subtypes of T lymphocytes are important for the initiation of autoimmune neuroinflammation. We examined peripheral blood CD4+ cells from patients with active and stable relapsing-remitting multiple sclerosis, and used mice with conditional deletion or over-expression of the transforming growth factor-beta inhibitor Smad7, to delineate the role of Smad7 in T cell differentiation and autoimmune neuroinflammation. We found that Smad7 is up-regulated in peripheral CD4+ cells from patients with multiple sclerosis during relapse but not remission, and that expression of Smad7 strongly correlates with T-bet, a transcription factor defining T helper 1 responses. Concordantly, mice with transgenic over-expression of Smad7 in T cells developed an enhanced disease course during experimental autoimmune encephalomyelitis, accompanied by elevated infiltration of inflammatory cells and T helper 1 responses in the central nervous system. On the contrary, mice with a T cell-specific deletion of Smad7 had reduced disease and central nervous system inflammation. Lack of Smad7 in T cells blunted T cell proliferation and T helper 1 responses in the periphery but left T helper 17 responses unaltered. Furthermore, frequencies of regulatory T cells were increased in the central nervous system of mice with a T cell-specific deletion and reduced in mice with a T cell-specific over-expression of Smad7. Downstream effects of transforming growth factor-beta on in vitro differentiation of naïve T cells to T helper 1, T helper 17 and regulatory T cell phenotypes were enhanced in T cells lacking Smad7. Finally, Smad7 was induced during T helper 1 differentiation and inhibited during T helper 17 differentiation. Taken together, the level of Smad7 in T cells determines T helper 1 polarization and regulates inflammatory cellular responses. Since a Smad7 deletion in T cells leads to immunosuppression, Smad7 may be a potential new therapeutic target in multiple sclerosis.

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.

Plaque-associated myeloid cells derive from resident microglia in an Alzheimer's disease model.

Alzheimer's disease (AD) is accompanied by a robust inflammatory response mediated by plaque-associated myeloid cells of the brain. These cells exhibit altered gene expression profiles and serve as a barrier, preventing neuritic dystrophy. The origin of these cells has been controversial and is of therapeutic importance. Here, we genetically labeled different myeloid populations and unequivocally demonstrated that plaque-associated myeloid cells in the AD brain are derived exclusively from resident microglia, with no contribution from circulating peripheral monocytes.

Single-cell sequencing reveals clonal expansions of pro-inflammatory synovial CD8 T cells expressing tissue-homing receptors in psoriatic arthritis.

Psoriatic arthritis (PsA) is a debilitating immune-mediated inflammatory arthritis of unknown pathogenesis commonly affecting patients with skin psoriasis. Here we use complementary single-cell approaches to study leukocytes from PsA joints. Mass cytometry demonstrates a 3-fold expansion of memory CD8 T cells in the joints of PsA patients compared to peripheral blood. Meanwhile, droplet-based and plate-based single-cell RNA sequencing of paired T cell receptor alpha and beta chain sequences show pronounced CD8 T cell clonal expansions within the joints. Transcriptome analyses find these expanded synovial CD8 T cells to express cycling, activation, tissue-homing and tissue residency markers. T cell receptor sequence comparison between patients identifies clonal convergence. Finally, chemokine receptor CXCR3 is upregulated in the expanded synovial CD8 T cells, while two CXCR3 ligands, CXCL9 and CXCL10, are elevated in PsA synovial fluid. Our data thus provide a quantitative molecular insight into the cellular immune landscape of psoriatic arthritis.