Lung injury induces a polarized immune response by self-antigen-specific CD4+ Foxp3+ regulatory T cells

Self-antigen-specific T cells are prevalent in the mature adaptive immune system but are regulated through multiple mechanisms of tolerance. However, inflammatory conditions such as tissue injury may allow these T cells to break tolerance and trigger autoimmunity. To understand how the T cell repertoire responds to the presentation of self-antigen under highly stimulatory conditions, we use peptide:major histocompatibility complex (MHC) class II tetramers to track the behavior of endogenous CD4+ T cells with specificity to a lung-expressed self-antigen in mouse models of immune-mediated lung injury. Acute injury results in the exclusive expansion of CD4+ regulatory T cells (Tregs) that is dependent on self-antigen recognition and interleukin-2 (IL-2). Conversely, conventional CD4+ T cells of the same self-antigen specificity remain unresponsive even following Treg ablation. Thus, the self-antigen-specific CD4+ T cell repertoire is poised to serve a regulatory function during acute tissue damage to limit further damage and the possibility of autoimmunity.

A human model of asthma exacerbation reveals transcriptional programs and cell circuits specific to allergic asthma

Asthma is a chronic disease most commonly associated with allergy and type 2 inflammation. However, the mechanisms that link airway inflammation to the structural changes that define asthma are incompletely understood. Using a human model of allergen-induced asthma exacerbation, we compared the lower airway mucosa in allergic asthmatics and allergic non-asthmatic controls using single-cell RNA sequencing. In response to allergen, the asthmatic airway epithelium was highly dynamic and up-regulated genes involved in matrix degradation, mucus metaplasia, and glycolysis while failing to induce injury-repair and antioxidant pathways observed in controls. IL9-expressing pathogenic TH2 cells were specific to asthmatic airways and were only observed after allergen challenge. Additionally, conventional type 2 dendritic cells (DC2 that express CD1C) and CCR2-expressing monocyte-derived cells (MCs) were uniquely enriched in asthmatics after allergen, with up-regulation of genes that sustain type 2 inflammation and promote pathologic airway remodeling. In contrast, allergic controls were enriched for macrophage-like MCs that up-regulated tissue repair programs after allergen challenge, suggesting that these populations may protect against asthmatic airway remodeling. Cellular interaction analyses revealed a TH2-mononuclear phagocyte-basal cell interactome unique to asthmatics. These pathogenic cellular circuits were characterized by type 2 programming of immune and structural cells and additional pathways that may sustain and amplify type 2 signals, including TNF family signaling, altered cellular metabolism, failure to engage antioxidant responses, and loss of growth factor signaling. Our findings therefore suggest that pathogenic effector circuits and the absence of proresolution programs drive structural airway disease in response to type 2 inflammation.

Lung injury induces a polarized immune response by self antigen-specific Foxp3 + regulatory T cells

Self antigen-specific T cells are prevalent in the mature adaptive immune system, but are regulated through multiple mechanisms of tolerance. However, inflammatory conditions such as tissue injury may provide these T cells with an opportunity to break tolerance and trigger autoimmunity. To understand how the T cell repertoire responds to the presentation of self antigen under highly stimulatory conditions, we used peptide:MHCII tetramers to track the behavior of endogenous CD4 + T cells with specificity to a lung-expressed self antigen in mouse models of immune-mediated lung injury. Acute injury resulted in the exclusive expansion of regulatory T cells (Tregs) that was dependent on self antigen recognition and IL-2. Conversely, conventional T cells of the same self antigen specificity remained unresponsive, even following Treg ablation. Thus, the self antigen-specific T cell repertoire is poised to serve a regulatory function during acute tissue damage to limit further damage and the possibility of autoimmunity.

Functional Recognition Theory and Type 2 Immunity: Insights and Uncertainties

Type 2 immunity plays an important role in host defense against helminths and toxins while driving allergic diseases. Despite progress in understanding the biology of type 2 immunity, the fundamental mechanisms regulating the type 2 immune module remain unclear. In contrast with structural recognition used by pattern recognition receptors, type 2 immunogens are sensed through their functional properties. Functional recognition theory has arisen as the paradigm for the initiation of type 2 immunity. However, the vast array of structurally unrelated type 2 immunogens makes it challenging to advance our understanding of type 2 immunity. In this article, we review functional recognition theory and organize type 2 immunogens into distinct classes based on how they fit into the concept of functional recognition. Lastly, we discuss areas of uncertainty in functional recognition theory with the goal of providing a framework to further define the logic of type 2 immunity in host protection and immunopathology.

Advancing Lung Immunology Research: An Official American Thoracic Society Workshop Report

The mammalian airways and lungs are exposed to a myriad of inhaled particulate matter, allergens, and pathogens. The immune system plays an essential role in protecting the host from respiratory pathogens, but a dysregulated immune response during respiratory infection can impair pathogen clearance and lead to immunopathology. Furthermore, inappropriate immunity to inhaled antigens can lead to pulmonary diseases. A complex network of epithelial, neural, stromal, and immune cells has evolved to sense and respond to inhaled antigens, including the decision to promote tolerance versus a rapid, robust, and targeted immune response. Although there has been great progress in understanding the mechanisms governing immunity to respiratory pathogens and aeroantigens, we are only beginning to develop an integrated understanding of the cellular networks governing tissue immunity within the lungs and how it changes after inflammation and over the human life course. An integrated model of airway and lung immunity will be necessary to improve mucosal vaccine design as well as prevent and treat acute and chronic inflammatory pulmonary diseases. Given the importance of immunology in pulmonary research, the American Thoracic Society convened a working group to highlight central areas of investigation to advance the science of lung immunology and improve human health.

Distinct functions of tissue-resident and circulating memory Th2 cells in allergic airway disease

Memory CD4⁺ T helper type 2 (Th2) cells drive allergic asthma, yet the mechanisms whereby tissue-resident memory Th2 (Th2 Trm) cells and circulating memory Th2 cells collaborate in vivo remain unclear. Using a house dust mite (HDM) model of allergic asthma and parabiosis, we demonstrate that Th2 Trm cells and circulating memory Th2 cells perform nonredundant functions. Upon HDM rechallenge, circulating memory Th2 cells trafficked into the lung parenchyma and ignited perivascular inflammation to promote eosinophil and CD4⁺ T cell recruitment. In contrast, Th2 Trm cells proliferated near airways and induced mucus metaplasia, airway hyperresponsiveness, and airway eosinophil activation. Transcriptional analysis revealed that Th2 Trm cells and circulating memory Th2 cells share a core Th2 gene signature but also exhibit distinct transcriptional profiles. Th2 Trm cells express a tissue-adaptation signature, including genes involved in regulating and interacting with extracellular matrix. Our findings demonstrate that Th2 Trm cells and circulating memory Th2 cells are functionally and transcriptionally distinct subsets with unique roles in promoting allergic airway disease.

Interleukin-33 activates regulatory T cells to suppress innate γδ T cell responses in the lung

Foxp3⁺ regulatory T (T_reg) cells expressing the interleukin (IL)-33 receptor ST2 mediate tissue repair in response to IL-33. Whether T_reg cells also respond to the alarmin IL-33 to regulate specific aspects of the immune response is not known. Here we describe an unexpected function of ST2⁺ T_reg cells in suppressing the innate immune response in the lung to environmental allergens without altering the adaptive immune response. Following allergen exposure, ST2⁺ T_reg cells were activated by IL-33 to suppress IL-17-producing γδ T cells. ST2 signaling in T_reg cells induced Ebi3, a component of the heterodimeric cytokine IL-35 that was required for T_reg cell-mediated suppression of γδ T cells. This response resulted in fewer eosinophil-attracting chemokines and reduced eosinophil recruitment into the lung, which was beneficial to the host in reducing allergen-induced inflammation. Thus, we define a fundamental role for ST2⁺ T_reg cells in the lung as a negative regulator of the early innate γδ T cell response to mucosal injury.

Redefining Memory T Cell Subsets

It has become increasingly clear that the terms used to define memory T cell subsets no longer accurately reflect our understanding of memory T cell biology. Here, we discuss the limitations of our current terminology and propose a new approach for defining memory T cell subsets.

Migratory DCs activate TGF-β to precondition naïve CD8+ T cells for tissue-resident memory fate

Epithelial resident memory T (eT_RM) cells serve as sentinels in barrier tissues to guard against previously encountered pathogens. How eT_RM cells are generated has important implications for efforts to elicit their formation through vaccination or prevent it in autoimmune disease. Here, we show that during immune homeostasis, the cytokine transforming growth factor β (TGF-β) epigenetically conditions resting naïve CD8⁺ T cells and prepares them for the formation of eT_RM cells in a mouse model of skin vaccination. Naïve T cell conditioning occurs in lymph nodes (LNs), but not in the spleen, through major histocompatibility complex class I-dependent interactions with peripheral tissue-derived migratory dendritic cells (DCs) and depends on DC expression of TGF-β-activating α_V integrins. Thus, the preimmune T cell repertoire is actively conditioned for a specialized memory differentiation fate through signals restricted to LNs.

Dectin-2-induced CCL2 production in tissue-resident macrophages ignites cardiac arteritis

Environmental triggers, including those from pathogens, are thought to play an important role in triggering autoimmune diseases, such as vasculitis, in genetically susceptible individuals. The mechanism by which activation of the innate immune system contributes to vessel-specific autoimmunity in vasculitis is not known. Systemic administration of Candida albicans water-soluble extract (CAWS) induces vasculitis in the aortic root and coronary arteries of mice that mimics human Kawasaki disease. We found that Dectin-2 signaling in macrophages resident in the aortic root of the heart induced early CCL2 production and the initial recruitment of CCR2+ inflammatory monocytes (iMo) into the aortic root and coronary arteries. iMo differentiated into monocyte-derived dendritic cells (Mo-DC) in the vessel wall and were induced to release IL-1β in a Dectin-2-Syk-NLRP3 inflammasome dependent pathway. IL-1β then activated cardiac endothelial cells to express CXCL1 and CCL2 and adhesion molecules that induced neutrophil and further iMo recruitment and accumulation in the aortic root and coronary arteries. Our findings demonstrate that Dectin-2-mediated induction of CCL2 production by macrophages resident in the aortic root and coronary arteries initiates vascular inflammation in a model of Kawasaki disease, suggesting an important role for the innate immune system in initiating vasculitis.

Chemokines: Critical Regulators of Memory T Cell Development, Maintenance, and Function

Memory T cells are central to orchestrating antigen-specific recall responses in vivo. Compared to naïve T cells, memory T cells respond more quickly to cognate peptide:MHC with a shorter lag time for entering the cell cycle and exerting effector functions. However, it is now well established that this enhanced responsiveness is not the only mechanism whereby memory T cells are better equipped than naïve T cells to rapidly and robustly induce inflammation. In contrast to naïve T cells, memory T cells are composed of distinct subsets with unique trafficking patterns and localizations. Tissue-resident memory T cells persist in previously inflamed tissue and function as first responders to cognate antigen reexposure. In addition, a heterogeneous group of circulating memory T cells augment inflammation by either rapidly migrating to inflamed tissue or responding to cognate antigen within secondary lymphoid organs and producing additional effector T cells. Defining the mechanisms regulating T cell positioning and trafficking and how this influences the development, maintenance, and function of memory T cell subsets is essential to improving vaccine design as well as treatment of immune-mediated diseases. In this chapter, we will review our current knowledge of how chemokines, critical regulators of cell positioning and migration, govern memory T cell biology in vivo. In addition, we discuss areas of uncertainty and future directions for further delineating how T cell localization influences memory T cell biology.

Targeted apoptosis of myofibroblasts with the BH3 mimetic ABT-263 reverses established fibrosis

Persistent myofibroblast activation distinguishes pathological fibrosis from physiological wound healing, suggesting that therapies selectively inducing myofibroblast apoptosis could prevent progression and potentially reverse established fibrosis in diseases such as scleroderma, a heterogeneous autoimmune disease characterized by multiorgan fibrosis. We demonstrate that fibroblast-to-myofibroblast differentiation driven by matrix stiffness increases the mitochondrial priming (proximity to the apoptotic threshold) of these activated cells. Mitochondria in activated myofibroblasts, but not quiescent fibroblasts, are primed by death signals such as the proapoptotic BH3-only protein BIM, which creates a requirement for tonic expression of the antiapoptotic protein BCL-XL to sequester BIM and ensure myofibroblast survival. Myofibroblasts become particularly susceptible to apoptosis induced by "BH3 mimetic" drugs inhibiting BCL-XL such as ABT-263. ABT-263 displaces BCL-XL binding to BIM, allowing BIM to activate apoptosis on stiffness-primed myofibroblasts. Therapeutic blockade of BCL-XL with ABT-263 (navitoclax) effectively treats established fibrosis in a mouse model of scleroderma dermal fibrosis by inducing myofibroblast apoptosis. Using a BH3 profiling assay to assess mitochondrial priming in dermal fibroblasts derived from patients with scleroderma, we demonstrate that the extent of apoptosis induced by BH3 mimetic drugs correlates with the extent of their mitochondrial priming, indicating that BH3 profiling could predict apoptotic responses of fibroblasts to BH3 mimetic drugs in patients with scleroderma. Together, our findings elucidate the potential efficacy of targeting myofibroblast antiapoptotic proteins with BH3 mimetic drugs in scleroderma and other fibrotic diseases.

Profibrotic TGFβ responses require the cooperative action of PDGF and ErbB receptor tyrosine kinases

Transforming growth factor β (TGFβ) has significant profibrotic activity both in vitro and in vivo. This reflects its capacity to stimulate fibrogenic mediators and induce the expression of other profibrotic cytokines such as platelet-derived growth factor (PDGF) and epidermal growth factor (EGF/ErbB) ligands. Here we address both the mechanisms by which TGFβ induced ErbB ligands and the physiological significance of inhibiting multiple TGFβ-regulated processes. The data document that ErbB ligand induction requires PDGF receptor (PDGFR) mediation and engages a positive autocrine/paracrine feedback loop via ErbB receptors. Whereas PDGFRs are essential for TGFβ-stimulated ErbB ligand up-regulation, TGFβ-specific signals are also required for ErbB receptor activation. Subsequent profibrotic responses are shown to involve the cooperative action of PDGF and ErbB signaling. Moreover, using a murine treatment model of bleomycin-induced pulmonary fibrosis we found that inhibition of TGFβ/PDGF and ErbB pathways with imatinib plus lapatinib, respectively, not only prevented myofibroblast gene expression to a greater extent than either drug alone, but also essentially stabilized gas exchange (oxygen saturation) as an overall measure of lung function. These observations provide important mechanistic insights into profibrotic TGFβ signaling and indicate that targeting multiple cytokines represents a possible strategy to ameliorate organ fibrosis dependent on TGFβ.

ERBB receptor activation is required for profibrotic responses to transforming growth factor beta

Engagement of the transforming growth factor-β (TGF-β) receptor complex activates multiple signaling pathways that play crucial roles in both health and disease. TGF-β is a key regulator of fibrogenesis and cancer-associated desmoplasia; however, its exact mode of action in these pathologic processes has remained poorly defined. Here, we report a novel mechanism whereby signaling via members of the ERBB or epidermal growth factor family of receptors serves as a central requirement for the biological responses of fibroblasts to TGF-β. We show that TGF-β triggers upregulation of ERBB ligands and activation of cognate receptors via the canonical SMAD pathway in fibroblasts. Interestingly, activation of ERBB is commonly observed in a subset of fibroblast but not epithelial cells from different species, indicating cell type specificity. Moreover, using genetic and pharmacologic approaches, we show that ERBB activation by TGF-β is essential for the induction of fibroblast cell morphologic transformation and anchorage-independent growth. Together, these results uncover important aspects of TGF-β signaling that highlight the role of ERBB ligands/receptors as critical mediators in fibroblast responses to this pleiotropic cytokine.

Distinct roles for mammalian target of rapamycin complexes in the fibroblast response to transforming growth factor-beta

Transforming growth factor-beta (TGF-beta) promotes a multitude of diverse biological processes, including growth arrest of epithelial cells and proliferation of fibroblasts. Although the TGF-beta signaling pathways that promote inhibition of epithelial cell growth are well characterized, less is known about the mechanisms mediating the positive response to this growth factor. Given that TGF-beta has been shown to promote fibrotic diseases and desmoplasia, identifying the fibroblast-specific TGF-beta signaling pathways is critical. Here, we investigate the role of mammalian target of rapamycin (mTOR), a known effector of phosphatidylinositol 3-kinase (PI3K) and promoter of cell growth, in the fibroblast response to TGF-beta. We show that TGF-beta activates mTOR complex 1 (mTORC1) in fibroblasts but not epithelial cells via a PI3K-Akt-TSC2-dependent pathway. Rapamycin, the pharmacologic inhibitor of mTOR, prevents TGF-beta-mediated anchorage-independent growth without affecting TGF-beta transcriptional responses or extracellular matrix protein induction. In addition to mTORC1, we also examined the role of mTORC2 in TGF-beta action. mTORC2 promotes TGF-beta-induced morphologic transformation and is required for TGF-beta-induced Akt S473 phosphorylation but not mTORC1 activation. Interestingly, both mTOR complexes are necessary for TGF-beta-mediated growth in soft agar. These results define distinct and overlapping roles for mTORC1 and mTORC2 in the fibroblast response to TGF-beta and suggest that inhibitors of mTOR signaling may be useful in treating fibrotic processes, such as desmoplasia.

TGF-beta signaling: a tale of two responses

Transforming growth factor-beta (TGF-beta) regulates a wide variety of cellular processes including cell growth, apoptosis, differentiation, migration, and extracellular matrix production among others. The canonical signaling pathway induced by the TGF-beta receptor complex involves the phosphorylation of Smad proteins which upon activation accumulate in the nucleus and regulate transcription. Interestingly, the cellular response to TGF-beta can be extremely variable depending on the cell type and stimulation context. TGF-beta causes epithelial cells to undergo growth arrest and apoptosis, responses which are critical to suppressing carcinogenesis, whereas it can also induce epithelial-mesenchymal transition and mediate fibroblast activation, responses implicated in promoting carcinogenesis and fibrotic diseases. However, TGF-beta induces all these responses via the same receptor complex and Smad proteins. To address this apparent paradox, during the last few years a number of additional signaling pathways have been identified which potentially regulate the different cellular responses to TGF-beta. The identification of these signaling pathways has shed light onto the mechanisms whereby Smad and non-Smad pathways collaborate to induce a particular cellular phenotype. In this article, we review TGF-beta signaling in epithelial cells and fibroblasts with a focus on understanding the mechanisms of TGF-beta versatility.

Methylmercury induces apoptosis in cultured rat dorsal root ganglion neurons

Methylmercury is known to have devastating effects on the mammalian nervous system. In order to characterize the dose dependence of methylmercury-induced neurotoxicity, we first studied neurite outgrowth from rat dorsal root ganglia explants. In this model, methylmercury inhibited neurite outgrowth with a TD(50) of approximately 0.5 microM. We then used this relationship to optimize dosing for subsequent transcriptional profiling analyses in two independent neuronal model systems: dissociated sensory neurons and PC12 cells. As seen in previous studies, the expression of a number of genes associated with oxidative stress was altered following a 6h challenge with 1 microM methylmercury. When PC12 cells were subjected to a longer exposure (24h), a relative increase was noted in the representation of genes associated with cell cycling and apoptosis. To confirm the presence of apoptosis in cultured neurons, we then applied TUNEL staining and bis-benzimide staining techniques to primary cultures of dissociated sensory neurons. After 24h, 1 microM methylmercury increased both DNA end-labeling (P<0.01) and nuclear fragmentation (P<0.02). The latter effect appeared to be dose-dependent.