Tuft Cells-Systemically Dispersed Sensory Epithelia Integrating Immune and Neural Circuitry.

Tuft cells-rare solitary chemosensory cells in mucosal epithelia-are undergoing intense scientific scrutiny fueled by recent discovery of unsuspected connections to type 2 immunity. These cells constitute a conduit by which ligands from the external space are sensed via taste-like signaling pathways to generate outputs unique among epithelial cells: the cytokine IL-25, eicosanoids associated with allergic immunity, and the neurotransmitter acetylcholine. The classic type II taste cell transcription factor POU2F3 is lineage defining, suggesting a conceptualization of these cells as widely distributed environmental sensors with effector functions interfacing type 2 immunity and neural circuits. Increasingly refined single-cell analytics have revealed diversity among tuft cells that extends from nasal epithelia and type II taste cells to ex-Aire-expressing medullary thymic cells and small-intestine cells that mediate tissue remodeling in response to colonizing helminths and protists.

A Metabolite-Triggered Tuft Cell-ILC2 Circuit Drives Small Intestinal Remodeling.

The small intestinal tuft cell-ILC2 circuit mediates epithelial responses to intestinal helminths and protists by tuft cell chemosensory-like sensing and IL-25-mediated activation of lamina propria ILC2s. Small intestine ILC2s constitutively express the IL-25 receptor, which is negatively regulated by A20 (Tnfaip3). A20 deficiency in ILC2s spontaneously triggers the circuit and, unexpectedly, promotes adaptive small-intestinal lengthening and remodeling. Circuit activation occurs upon weaning and is enabled by dietary polysaccharides that render mice permissive for Tritrichomonas colonization, resulting in luminal accumulation of acetate and succinate, metabolites of the protist hydrogenosome. Tuft cells express GPR91, the succinate receptor, and dietary succinate, but not acetate, activates ILC2s via a tuft-, TRPM5-, and IL-25-dependent pathway. Also induced by parasitic helminths, circuit activation and small intestinal remodeling impairs infestation by new helminths, consistent with the phenomenon of concomitant immunity. We describe a metabolic sensing circuit that may have evolved to facilitate mutualistic responses to luminal pathosymbionts.

Tissue signals imprint ILC2 identity with anticipatory function.

Group 2 innate lymphoid cells (ILC2s) are distributed systemically and produce type 2 cytokines in response to a variety of stimuli, including the epithelial cytokines interleukin (IL)-25, IL-33, and thymic stromal lymphopoietin (TSLP). Transcriptional profiling of ILC2s from different tissues, however, grouped ILC2s according to their tissue of origin, even in the setting of combined IL-25-, IL-33-receptor-, and TSLP-receptor-deficiency. Single-cell profiling confirmed a tissue-organizing transcriptome and identified ILC2 subsets expressing distinct activating receptors, including the major subset of skin ILC2s, which were activated preferentially by IL-18. Tissue ILC2 subsets were unaltered in number and expression in germ-free mice, suggesting that endogenous, tissue-derived signals drive the maturation of ILC2 subsets by controlling expression of distinct patterns of activating receptors, thus anticipating tissue-specific perturbations occurring later in life.

Amplification of autoimmune organ damage by NKp46-activated ILC1s.

In systemic lupus erythematosus, loss of immune tolerance, autoantibody production and immune complex deposition are required but not sufficient for organ damage1. How inflammatory signals are initiated and amplified in the setting of autoimmunity remains elusive. Here we set out to dissect layers and hierarchies of autoimmune kidney inflammation to identify tissue-specific cellular hubs that amplify autoinflammatory responses. Using high-resolution single-cell profiling of kidney immune and parenchymal cells, in combination with antibody blockade and genetic deficiency, we show that tissue-resident NKp46+ innate lymphoid cells (ILCs) are crucial signal amplifiers of disease-associated macrophage expansion and epithelial cell injury in lupus nephritis, downstream of autoantibody production. NKp46 signalling in a distinct subset of group 1 ILCs (ILC1s) instructed an unconventional immune-regulatory transcriptional program, which included the expression of the myeloid cell growth factor CSF2. CSF2 production by NKp46+ ILCs promoted the population expansion of monocyte-derived macrophages. Blockade of the NKp46 receptor (using the antibody clone mNCR1.15; ref. 2) or genetic deficiency of NKp46 abrogated epithelial cell injury. The same cellular and molecular patterns were operative in human lupus nephritis. Our data provide support for the idea that NKp46+ ILC1s promote parenchymal cell injury by granting monocyte-derived macrophages access to epithelial cell niches. NKp46 activation in ILC1s therefore constitutes a previously unrecognized, crucial tissue rheostat that amplifies organ damage in autoimmune hosts, with broad implications for inflammatory pathologies and therapies.

Active eosinophils regulate host defence and immune responses in colitis.

In the past decade, single-cell transcriptomics has helped to uncover new cell types and states and led to the construction of a cellular compendium of health and disease. Despite this progress, some difficult-to-sequence cells remain absent from tissue atlases. Eosinophils-elusive granulocytes that are implicated in a plethora of human pathologies1-5-are among these uncharted cell types. The heterogeneity of eosinophils and the gene programs that underpin their pleiotropic functions remain poorly understood. Here we provide a comprehensive single-cell transcriptomic profiling of mouse eosinophils. We identify an active and a basal population of intestinal eosinophils, which differ in their transcriptome, surface proteome and spatial localization. By means of a genome-wide CRISPR inhibition screen and functional assays, we reveal a mechanism by which interleukin-33 (IL-33) and interferon-γ (IFNγ) induce the accumulation of active eosinophils in the inflamed colon. Active eosinophils are endowed with bactericidal and T cell regulatory activity, and express the co-stimulatory molecules CD80 and PD-L1. Notably, active eosinophils are enriched in the lamina propria of a small cohort of patients with inflammatory bowel disease, and are closely associated with CD4+ T cells. Our findings provide insights into the biology of eosinophils and highlight the crucial contribution of this cell type to intestinal homeostasis, immune regulation and host defence. Furthermore, we lay a framework for the characterization of eosinophils in human gastrointestinal diseases.

Tissue-Resident Group 2 Innate Lymphoid Cells Differentiate by Layered Ontogeny and In Situ Perinatal Priming.

The perinatal period is a critical window for distribution of innate tissue-resident immune cells within developing organs. Despite epidemiologic evidence implicating the early-life environment in the risk for allergy, temporally controlled lineage tracing of group 2 innate lymphoid cells (ILC2s) during this period remains unstudied. Using complementary fate-mapping approaches and reporters for ILC2 activation, we show that ILC2s appeared in multiple organs during late gestation like tissue macrophages, but, unlike the latter, a majority of peripheral ILC2 pools were generated de novo during the postnatal window. This period was accompanied by systemic ILC2 priming and acquisition of tissue-specific transcriptomes. Although perinatal ILC2s were variably replaced across tissues with age, the dramatic increases in tissue ILC2s following helminth infection were mediated through local expansion independent of de novo generation by bone marrow hematopoiesis. We provide comprehensive temporally controlled fate mapping of an innate lymphocyte subset with notable nuances as compared to tissue macrophage ontogeny.

The development and function of lung-resident macrophages and dendritic cells.

Gas exchange is the vital function of the lungs. It occurs in the alveoli, where oxygen and carbon dioxide diffuse across the alveolar epithelium and the capillary endothelium surrounding the alveoli, separated only by a fused basement membrane 0.2-0.5 µm in thickness. This tenuous barrier is exposed to dangerous or innocuous particles, toxins, allergens and infectious agents inhaled with the air or carried in the blood. The lung immune system has evolved to ward off pathogens and restrain inflammation-mediated damage to maintain gas exchange. Lung-resident macrophages and dendritic cells are located in close proximity to the epithelial surface of the respiratory system and the capillaries to sample and examine the air-borne and blood-borne material. In communication with alveolar epithelial cells, they set the threshold and the quality of the immune response.

Detection of Succinate by Intestinal Tuft Cells Triggers a Type 2 Innate Immune Circuit.

In the small intestine, type 2 responses are regulated by a signaling circuit that involves tuft cells and group 2 innate lymphoid cells (ILC2s). Here, we identified the microbial metabolite succinate as an activating ligand for small intestinal (SI) tuft cells. Sequencing analyses of tuft cells isolated from the small intestine, gall bladder, colon, thymus, and trachea revealed that expression of tuft cell chemosensory receptors is tissue specific. SI tuft cells expressed the succinate receptor (SUCNR1), and providing succinate in drinking water was sufficient to induce a multifaceted type 2 immune response via the tuft-ILC2 circuit. The helminth Nippostrongylus brasiliensis and a tritrichomonad protist both secreted succinate as a metabolite. In vivo sensing of the tritrichomonad required SUCNR1, whereas N. brasiliensis was SUCNR1 independent. These findings define a paradigm wherein tuft cells monitor microbial metabolites to initiate type 2 immunity and suggest the existence of other sensing pathways triggering the response to helminths.

Induction of the nuclear receptor PPAR-γ by the cytokine GM-CSF is critical for the differentiation of fetal monocytes into alveolar macrophages.

Tissue-resident macrophages constitute heterogeneous populations with unique functions and distinct gene-expression signatures. While it has been established that they originate mostly from embryonic progenitor cells, the signals that induce a characteristic tissue-specific differentiation program remain unknown. We found that the nuclear receptor PPAR-γ determined the perinatal differentiation and identity of alveolar macrophages (AMs). In contrast, PPAR-γ was dispensable for the development of macrophages located in the peritoneum, liver, brain, heart, kidneys, intestine and fat. Transcriptome analysis of the precursors of AMs from newborn mice showed that PPAR-γ conferred a unique signature, including several transcription factors and genes associated with the differentiation and function of AMs. Expression of PPAR-γ in fetal lung monocytes was dependent on the cytokine GM-CSF. Therefore, GM-CSF has a lung-specific role in the perinatal development of AMs through the induction of PPAR-γ in fetal monocytes.

Sustained innate interferon is an essential inducer of tertiary lymphoid structures.

Tertiary lymphoid structures (TLS) resemble follicles of secondary lymphoid organs and develop in nonlymphoid tissues during inflammation and cancer. Which cell types and signals drive the development of TLS is largely unknown. To investigate early events of TLS development in the lungs, we repeatedly instilled p(I:C) plus ovalbumin (Ova) intranasally. This induced TLS ranging from lymphocytic aggregates to organized and functional structures containing germinal centers. We found that TLS development is independent of FAP+ fibroblasts, alveolar macrophages, or CCL19 but crucially depends on type I interferon (IFN-I). Mechanistically, IFN-I initiates two synergistic pathways that culminate in the development of TLS. On the one hand, IFN-I induces lymphotoxin (LT)α in lymphoid cells, which stimulate stromal cells to produce the B-cell-attracting chemokine CXCL13 through LTßR-signaling. On the other hand, IFN-I is sensed by stromal cells that produce the T-cell-attracting chemokines CXCL9, CXCL10 as well as CCL19 and CCL21 independently of LTßR. Consequently, B-cell aggregates develop within a week, whereas follicular dendritic cells and germinal centers appear after 3 weeks. Thus, sustained production of IFN-I together with an antigen is essential for the induction of functional TLS in the lungs.

Fetal monocytes possess increased metabolic capacity and replace primitive macrophages in tissue macrophage development.

Tissue-resident macrophages (MΦTR ) originate from at least two distinct waves of erythro-myeloid progenitors (EMP) arising in the yolk sac (YS) at E7.5 and E8.5 with the latter going through a liver monocyte intermediate. The relative potential of these precursors in determining development and functional capacity of MΦTR remains unclear. Here, we studied development of alveolar macrophages (AM) after single and competitive transplantation of different precursors from YS, fetal liver, and fetal lung into neonatal Csf2ra-/- mice, which lack endogenous AM. Fetal monocytes, promoted by Myb, outcompeted primitive MΦ (pMΦ) in empty AM niches and preferentially developed to mature AM, which is associated with enhanced mitochondrial respiratory and glycolytic capacity and repression of the transcription factors c-Maf and MafB. Interestingly, AM derived from pMΦ failed to efficiently clear alveolar proteinosis and protect from fatal lung failure following influenza virus infection. Thus, our data demonstrate superior developmental and functional capacity of fetal monocytes over pMΦ in AM development and underlying mechanisms explaining replacement of pMΦ in fetal tissues.

tEMPting Fate MaYBe the Solution.

In the present issue of Immunity, Hoeffel et al. (2015) reconcile a controversy by demonstrating that a distinct wave of yolk-sac-derived erythro-myeloid progenitors (EMPs) differentiate to fetal monocytes in the liver and further to adult macrophages in the majority of tissues.

PI3-Kinase-γ Has a Distinct and Essential Role in Lung-Specific Dendritic Cell Development.

Development of dendritic cells (DCs) commences in the bone marrow, from where pre-DCs migrate to peripheral organs to differentiate into mature DCs in situ. However, the factors that regulate organ-specific differentiation to give rise to tissue-specific DC subsets remain unclear. Here we show that the Ras-PI3Kγ-Akt-mTOR signaling axis acted downstream of FLT3L signaling and was required for development of lung CD103(+) DCs and, to a smaller extent, for lung CD11b(+) DCs, but not related DC populations in other non-lymphoid organs. Furthermore, we show that in lymphoid organs such as the spleen, DCs depended on a similar signaling network to respond to FLT3 ligand with overlapping and partially redundant roles for kinases PI3Kγ and PI3Kδ. Thus we identified PI3Kγ as an essential organ-specific regulator of lung DC development and discovered a signaling network regulating tissue-specific DC development mediated by FLT3.

Parsimonious Random-Forest-Based Land-Use Regression Model Using Particulate Matter Sensors in Berlin, Germany.

Machine learning (ML) methods are widely used in particulate matter prediction modelling, especially through use of air quality sensor data. Despite their advantages, these methods' black-box nature obscures the understanding of how a prediction has been made. Major issues with these types of models include the data quality and computational intensity. In this study, we employed feature selection methods using recursive feature elimination and global sensitivity analysis for a random-forest (RF)-based land-use regression model developed for the city of Berlin, Germany. Land-use-based predictors, including local climate zones, leaf area index, daily traffic volume, population density, building types, building heights, and street types were used to create a baseline RF model. Five additional models, three using recursive feature elimination method and two using a Sobol-based global sensitivity analysis (GSA), were implemented, and their performance was compared against that of the baseline RF model. The predictors that had a large effect on the prediction as determined using both the methods are discussed. Through feature elimination, the number of predictors were reduced from 220 in the baseline model to eight in the parsimonious models without sacrificing model performance. The model metrics were compared, which showed that the parsimonious_GSA-based model performs better than does the baseline model and reduces the mean absolute error (MAE) from 8.69 µg/m3 to 3.6 µg/m3 and the root mean squared error (RMSE) from 9.86 µg/m3 to 4.23 µg/m3 when applying the trained model to reference station data. The better performance of the GSA_parsimonious model is made possible by the curtailment of the uncertainties propagated through the model via the reduction of multicollinear and redundant predictors. The parsimonious model validated against reference stations was able to predict the PM2.5 concentrations with an MAE of less than 5 µg/m3 for 10 out of 12 locations. The GSA_parsimonious performed best in all model metrics and improved the R2 from 3% in the baseline model to 17%. However, the predictions exhibited a degree of uncertainty, making it unreliable for regional scale modelling. The GSA_parsimonious model can nevertheless be adapted to local scales to highlight the land-use parameters that are indicative of PM2.5 concentrations in Berlin. Overall, population density, leaf area index, and traffic volume are the major predictors of PM2.5, while building type and local climate zones are the less significant predictors. Feature selection based on sensitivity analysis has a large impact on the model performance. Optimising models through sensitivity analysis can enhance the interpretability of the model dynamics and potentially reduce computational costs and time when modelling is performed for larger areas.

Atrophy links lower novelty-related locus coeruleus connectivity to cognitive decline in preclinical AD.

INTRODUCTION: Animal research has shown that tau pathology in the locus coeruleus (LC) is associated with reduced norepinephrine signaling, lower projection density to the medial temporal lobe (MTL), atrophy, and cognitive impairment. We investigated the contribution of LC-MTL functional connectivity (FCLC-MTL) on cortical atrophy across Braak stage regions and its impact on cognition. METHODS: We analyzed functional magnetic resonance imaging and amyloid beta (Aß) positron emission tomography data from 128 cognitively normal participants, associating novelty-related FCLC-MTL with longitudinal atrophy and cognition with and without Aß moderation. RESULTS: Cross-sectionally, lower FCLC-MTL was associated with atrophy in Braak stage II regions. Longitudinally, atrophy in Braak stage 2 to 4 regions related to lower baseline FCLC-MTL at elevated levels of Aß, but not to other regions. Atrophy in Braak stage 2 regions mediated the relation between FCLC-MTL and subsequent cognitive decline. DISCUSSION: FCLC-MTL is implicated in Aß-related cortical atrophy, suggesting that LC-MTL connectivity could confer neuroprotective effects in preclinical AD. HIGHLIGHTS: Novelty-related functional magnetic resonance imaging (fMRI) LC-medial temporal lobe (MTL) connectivity links to longitudinal Aß-dependent atrophy. This relationship extended to higher Braak stage regions with increasing Aß burden. Longitudinal MTL atrophy mediated the LC-MTL connectivity-cognition relationship. Our findings mirror the animal data on MTL atrophy following NE signal dysfunction.

Priorities for research on neuromodulatory subcortical systems in Alzheimer's disease: Position paper from the NSS PIA of ISTAART.

The neuromodulatory subcortical system (NSS) nuclei are critical hubs for survival, hedonic tone, and homeostasis. Tau-associated NSS degeneration occurs early in Alzheimer's disease (AD) pathogenesis, long before the emergence of pathognomonic memory dysfunction and cortical lesions. Accumulating evidence supports the role of NSS dysfunction and degeneration in the behavioral and neuropsychiatric manifestations featured early in AD. Experimental studies even suggest that AD-associated NSS degeneration drives brain neuroinflammatory status and contributes to disease progression, including the exacerbation of cortical lesions. Given the important pathophysiologic and etiologic roles that involve the NSS in early AD stages, there is an urgent need to expand our understanding of the mechanisms underlying NSS vulnerability and more precisely detail the clinical progression of NSS changes in AD. Here, the NSS Professional Interest Area of the International Society to Advance Alzheimer's Research and Treatment highlights knowledge gaps about NSS within AD and provides recommendations for priorities specific to clinical research, biomarker development, modeling, and intervention. HIGHLIGHTS: Neuromodulatory nuclei degenerate in early Alzheimer's disease pathological stages. Alzheimer's pathophysiology is exacerbated by neuromodulatory nuclei degeneration. Neuromodulatory nuclei degeneration drives neuropsychiatric symptoms in dementia. Biomarkers of neuromodulatory integrity would be value-creating for dementia care. Neuromodulatory nuclei present strategic prospects for disease-modifying therapies.

Brønsted Acid Catalyzed Asymmetric Synthesis of cis-Tetrahydrocannabinoids.

We report herein the catalytic asymmetric cyclization of 1-aryl terpenols to afford enantiomerically highly enriched Δ9 -cis-tetrahydrocannabinoid scaffolds in a single step. As powerful chiral catalysts strongly acidic imidodiphosphorimidates (IDPis) have been identified which furnish the products with good yields and excellent enantioselectivity. Upon MOM-deprotection some naturally occurring cannabimimetica such as (-)-cis-Δ9 -tetrahydrocannabinol and (-)-perrottetinene as well as some unnatural analogues were made accessible along a merely 3-step biomimetic sequence (MOM=methoxymethyl).

Differential sensitivity of inflammatory macrophages and alternatively activated macrophages to ferroptosis.

Acumulation of oxidized membrane lipids ultimately results in ferroptotic cell death, which can be prevented by the selenoenzyme glutathione peroxidase 4 (Gpx4). In vivo conditions promoting ferroptosis and susceptible cell types are still poorly defined. In this study, we analyzed the conditional deletion of Gpx4 in mice specifically in the myeloid cell lineages. Surprisingly, development and maintenance of LysM+ macrophages and neutrophils, as well as CD11c+ monocyte-derived macrophages and dendritic cells were unaffected in the absence of Gpx4. Gpx4-deficient macrophages mounted an unaltered proinflammatory cytokine response including IL-1ß production following stimulation with TLR ligands and activation of several inflammasomes. Accordingly, Gpx4fl/fl LysM-cre mice were protected from bacterial and protozoan infections. Despite having the capacity to differentiate to alternatively activated macrophages (AAM), these cells lacking Gpx4 triggered ferroptosis both in vitro and in vivo following IL-4 overexpression and nematode infection. Exposure to nitric oxide restored viability of Gpx4-deficient AAM, while inhibition of iNOS in proinflammatory macrophages had no effect. These data together suggest that activation cues of tissue macrophages determine sensitivity to lipid peroxidation and ferroptotic cell death.

OTUB1 regulates lung development, adult lung tissue homeostasis, and respiratory control.

OTUB1 is one of the most highly expressed deubiquitinases, counter-regulating the two most abundant ubiquitin chain types. OTUB1 expression is linked to the development and progression of lung cancer and idiopathic pulmonary fibrosis in humans. However, the physiological function of OTUB1 is unknown. Here, we show that constitutive whole-body Otub1 deletion in mice leads to perinatal lethality by asphyxiation. Analysis of (single-cell) RNA sequencing and proteome data demonstrated that OTUB1 is expressed in all lung cell types with a particularly high expression during late-stage lung development (E16.5, E18.5). At E18.5, the lungs of animals with Otub1 deletion presented with increased cell proliferation that decreased saccular air space and prevented inhalation. Flow cytometry-based analysis of E18.5 lung tissue revealed that Otub1 deletion increased proliferation of major lung parenchymal and mesenchymal/other non-hematopoietic cell types. Adult mice with conditional whole-body Otub1 deletion (wbOtub1del/del ) also displayed increased lung cell proliferation in addition to hyperventilation and failure to adapt the respiratory pattern to hypoxia. On the molecular level, Otub1 deletion enhanced mTOR signaling in embryonic and adult lung tissues. Based on these results, we propose that OTUB1 is a negative regulator of mTOR signaling with essential functions for lung cell proliferation, lung development, adult lung tissue homeostasis, and respiratory regulation.

IgA Triggers Cell Death of Neutrophils When Primed by Inflammatory Mediators.

IVIG preparations consisting of pooled IgG are increasingly used for the treatment of autoimmune diseases. IVIG is known to regulate the viability of immune cells, including neutrophils. We report that plasma-derived IgA efficiently triggers death of neutrophils primed by cytokines or TLR agonists. IgA-mediated programmed neutrophil death was PI3K-, p38 MAPK-, and JNK-dependent and evoked anti-inflammatory cytokines in macrophage cocultures. Neutrophils from patients with acute Crohn's disease, rheumatoid arthritis, or sepsis were susceptible to both IgA- and IVIG-mediated death. In contrast to IVIG, IgA did not promote cell death of quiescent neutrophils. Our findings suggest that plasma-derived IgA might provide a therapeutic option for the treatment of neutrophil-associated inflammatory disorders.