IL-4 and IL-13: Regulators and Effectors of Wound Repair.

Type 2 immunity mediates protective responses to helminths and pathological responses to allergens, but it also has broad roles in the maintenance of tissue integrity, including wound repair. Type 2 cytokines are known to promote fibrosis, an overzealous repair response, but their contribution to healthy wound repair is less well understood. This review discusses the evidence that the canonical type 2 cytokines, IL-4 and IL-13, are integral to the tissue repair process through two main pathways. First, essential for the progression of effective tissue repair, IL-4 and IL-13 suppress the initial inflammatory response to injury. Second, these cytokines regulate how the extracellular matrix is modified, broken down, and rebuilt for effective repair. IL-4 and/or IL-13 amplifies multiple aspects of the tissue repair response, but many of these pathways are highly redundant and can be induced by other signals. Therefore, the exact contribution of IL-4Rα signaling remains difficult to unravel.

Group 2 Innate Lymphoid Cells: Team Players in Regulating Asthma.

Type 2 immunity helps protect the host from infection, but it also plays key roles in tissue homeostasis, metabolism, and repair. Unfortunately, inappropriate type 2 immune reactions may lead to allergy and asthma. Group 2 innate lymphoid cells (ILC2s) in the lungs respond rapidly to local environmental cues, such as the release of epithelium-derived type 2 initiator cytokines/alarmins, producing type 2 effector cytokines such as IL-4, IL-5, and IL-13 in response to tissue damage and infection. ILC2s are associated with the severity of allergic asthma, and experimental models of lung inflammation have shown how they act as playmakers, receiving signals variously from stromal and immune cells as well as the nervous system and then distributing cytokine cues to elicit type 2 immune effector functions and potentiate CD4+ T helper cell activation, both of which characterize the pathology of allergic asthma. Recent breakthroughs identifying stromal- and neuronal-derived microenvironmental cues that regulate ILC2s, along with studies recognizing the potential plasticity of ILC2s, have improved our understanding of the immunoregulation of asthma and opened new avenues for drug discovery.

Th2 Cells in Health and Disease.

Helper T (Th) cell subsets direct immune responses by producing signature cytokines. Th2 cells produce IL-4, IL-5, and IL-13, which are important in humoral immunity and protection from helminth infection and are central to the pathogenesis of many allergic inflammatory diseases. Molecular analysis of Th2 cell differentiation and maintenance of function has led to recent discoveries that have refined our understanding of Th2 cell biology. Epigenetic regulation of Gata3 expression by chromatin remodeling complexes such as Polycomb and Trithorax is crucial for maintaining Th2 cell identity. In the context of allergic diseases, memory-type pathogenic Th2 cells have been identified in both mice and humans. To better understand these disease-driving cell populations, we have developed a model called the pathogenic Th population disease induction model. The concept of defined subsets of pathogenic Th cells may spur new, effective strategies for treating intractable chronic inflammatory disorders.

A single infusion of engineered long-lived and multifunctional T cells confers durable remission of asthma in mice.

Asthma, the most prevalent respiratory disease, affects more than 300 million people and causes more than 250,000 deaths annually. Type 2-high asthma is characterized by interleukin (IL)-5-driven eosinophilia, along with airway inflammation and remodeling caused by IL-4 and IL-13. Here we utilize IL-5 as the targeting domain and deplete BCOR and ZC3H12A to engineer long-lived chimeric antigen receptor (CAR) T cells that can eradicate eosinophils. We call these cells immortal-like and functional IL-5 CAR T cells (5TIF) cells. 5TIF cells were further modified to secrete an IL-4 mutein that blocks IL-4 and IL-13 signaling, designated as 5TIF4 cells. In asthma models, a single infusion of 5TIF4 cells in fully immunocompetent mice, without any conditioning regimen, led to sustained repression of lung inflammation and alleviation of asthmatic symptoms. These data show that asthma, a common chronic disease, can be pushed into long-term remission with a single dose of long-lived CAR T cells.

Enteric pathogens induce tissue tolerance and prevent neuronal loss from subsequent infections.

The enteric nervous system (ENS) controls several intestinal functions including motility and nutrient handling, which can be disrupted by infection-induced neuropathies or neuronal cell death. We investigated possible tolerance mechanisms preventing neuronal loss and disruption in gut motility after pathogen exposure. We found that following enteric infections, muscularis macrophages (MMs) acquire a tissue-protective phenotype that prevents neuronal loss, dysmotility, and maintains energy balance during subsequent challenge with unrelated pathogens. Bacteria-induced neuroprotection relied on activation of gut-projecting sympathetic neurons and signaling via ß2-adrenergic receptors (ß2AR) on MMs. In contrast, helminth-mediated neuroprotection was dependent on T cells and systemic production of interleukin (IL)-4 and IL-13 by eosinophils, which induced arginase-expressing MMs that prevented neuronal loss from an unrelated infection located in a different intestinal region. Collectively, these data suggest that distinct enteric pathogens trigger a state of disease or tissue tolerance that preserves ENS number and functionality.

Interleukin-4- and interleukin-13-mediated alternatively activated macrophages: roles in homeostasis and disease.

The macrophage, a versatile cell type prominently involved in host defense and immunity, assumes a distinct state of alternative activation in the context of polarized type 2 immune responses such as allergic inflammation and helminth infection. This alternatively activated phenotype is induced by the canonical type 2 cytokines interleukin (IL)-4 and IL-13, which mediate expression of several characteristic markers along with a dramatic shift in macrophage metabolic pathways that influence surrounding cells and tissues. We discuss recent advances in the understanding of IL-4- and IL-13-mediated alternatively activated macrophages and type 2 immune responses; such advances have led to an expanded appreciation for functions of these cells beyond immunity, including maintenance of physiologic homeostasis and tissue repair.

Homeostatic IL-13 in healthy skin directs dendritic cell differentiation to promote TH2 and inhibit TH17 cell polarization.

The signals driving the adaptation of type 2 dendritic cells (DC2s) to diverse peripheral environments remain mostly undefined. We show that differentiation of CD11blo migratory DC2s-a DC2 population unique to the dermis-required IL-13 signaling dependent on the transcription factors STAT6 and KLF4, whereas DC2s in lung and small intestine were STAT6-independent. Similarly, human DC2s in skin expressed an IL-4 and IL-13 gene signature that was not found in blood, spleen and lung DCs. In mice, IL-13 was secreted homeostatically by dermal innate lymphoid cells and was independent of microbiota, TSLP or IL-33. In the absence of IL-13 signaling, dermal DC2s were stable in number but remained CD11bhi and showed defective activation in response to allergens, with diminished ability to support the development of IL-4+GATA3+ helper T cells (TH), whereas antifungal IL-17+RORγt+ TH cells were increased. Therefore, homeostatic IL-13 fosters a noninflammatory skin environment that supports allergic sensitization.

Tuft cell acetylcholine is released into the gut lumen to promote anti-helminth immunity.

Upon parasitic helminth infection, activated intestinal tuft cells secrete interleukin-25 (IL-25), which initiates a type 2 immune response during which lamina propria type 2 innate lymphoid cells (ILC2s) produce IL-13. This causes epithelial remodeling, including tuft cell hyperplasia, the function of which is unknown. We identified a cholinergic effector function of tuft cells, which are the only epithelial cells that expressed choline acetyltransferase (ChAT). During parasite infection, mice with epithelial-specific deletion of ChAT had increased worm burden, fitness, and fecal egg counts, even though type 2 immune responses were comparable. Mechanistically, IL-13-amplified tuft cells release acetylcholine (ACh) into the gut lumen. Finally, we demonstrated a direct effect of ACh on worms, which reduced their fecundity via helminth-expressed muscarinic ACh receptors. Thus, tuft cells are sentinels in naive mice, and their amplification upon helminth infection provides an additional type 2 immune response effector function.

The Tuft Cell-ILC2 Circuit Integrates Intestinal Defense and Homeostasis.

The intestinal response to helminth infection is mediated by a recently established type 2 immune circuit that consists of intestinal tuft cells and type 2 innate lymphoid cells (ILC2s). Schneider et al. have discovered that tuft cells sense succinate fermented by Tritrichomonas via GPR91 to drive the IL-25-ILC2-IL-13-dependent immune circuit and intestinal remodeling.

Sensory Neurons Co-opt Classical Immune Signaling Pathways to Mediate Chronic Itch.

Mammals have evolved neurophysiologic reflexes, such as coughing and scratching, to expel invading pathogens and noxious environmental stimuli. It is well established that these responses are also associated with chronic inflammatory diseases, including asthma and atopic dermatitis. However, the mechanisms by which inflammatory pathways promote sensations such as itch remain poorly understood. Here, we show that type 2 cytokines directly activate sensory neurons in both mice and humans. Further, we demonstrate that chronic itch is dependent on neuronal IL-4Rα and JAK1 signaling. We also observe that patients with recalcitrant chronic itch that failed other immunosuppressive therapies markedly improve when treated with JAK inhibitors. Thus, signaling mechanisms previously ascribed to the immune system may represent novel therapeutic targets within the nervous system. Collectively, this study reveals an evolutionarily conserved paradigm in which the sensory nervous system employs classical immune signaling pathways to influence mammalian behavior.

IL-13 secreted by ILC2s promotes the self-renewal of intestinal stem cells through circular RNA circPan3.

Intestinal stem cells (ISCs) are maintained by stemness signaling for precise modulation of self-renewal and differentiation under homeostasis. However, the way in which intestinal immune cells regulate the self-renewal of ISCs remains elusive. Here we found that mouse and human Lgr5+ ISCs showed high expression of the immune cell-associated circular RNA circPan3 (originating from the Pan3 gene transcript). Deletion of circPan3 in Lgr5+ ISCs impaired their self-renewal capacity and the regeneration of gut epithelium in a manner dependent on immune cells. circPan3 bound mRNA encoding the cytokine IL-13 receptor subunit IL-13Rα1 (Il13ra1) in ISCs to increase its stability, which led to the expression of IL-13Rα1 in ISCs. IL-13 produced by group 2 innate lymphoid cells in the crypt niche engaged IL-13Rα1 on crypt ISCs and activated signaling mediated by IL-13‒IL-13R, which in turn initiated expression of the transcription factor Foxp1. Foxp1 is associated with ß-catenin in rendering its nuclear translocation, which caused activation of the ß-catenin pathway and the maintenance of Lgr5+ ISCs.

Follicular regulatory T cells control humoral and allergic immunity by restraining early B cell responses.

Follicular regulatory T (TFR) cells have specialized roles in modulating follicular helper T (TFH) cell activation of B cells. However, the precise role of TFR cells in controlling antibody responses to foreign antigens and autoantigens in vivo is still unclear due to a lack of specific tools. A TFR cell-deleter mouse was developed that selectively deletes TFR cells, facilitating temporal studies. TFR cells were found to regulate early, but not late, germinal center (GC) responses to control antigen-specific antibody and B cell memory. Deletion of TFR cells also resulted in increased self-reactive immunoglobulin (Ig) G and IgE. The increased IgE levels led us to interrogate the role of TFR cells in house dust mite models. TFR cells were found to control TFH13 cell-induced IgE. In vivo, loss of TFR cells increased house-dust-mite-specific IgE and lung inflammation. Thus, TFR cells control IgG and IgE responses to vaccines, allergens and autoantigens, and exert critical immunoregulatory functions before GC formation.

Expelliarmus helminthus! Harry Helminth and the Goblet of Alarmins.

Epithelial cells (tuft and goblet cells) interact with immune cells on the "inside" while secreting effector molecules into the topological "outside." In this issue of Immunity, Zhao et al. investigate an interleukin-33 (IL-33) secretion mechanism in goblet cells dependent on O-GlcNAcylation and gasdermin pores facilitating worm expulsion.

Innate type 2 immunity controls hair follicle commensalism by Demodex mites.

Demodex mites are commensal parasites of hair follicles (HFs). Normally asymptomatic, inflammatory outgrowth of mites can accompany malnutrition, immune dysfunction, and aging, but mechanisms restricting Demodex outgrowth are not defined. Here, we show that control of mite HF colonization in mice required group 2 innate lymphoid cells (ILC2s), interleukin-13 (IL-13), and its receptor, IL-4Ra-IL-13Ra1. HF-associated ILC2s elaborated IL-13 that attenuated HFs and epithelial proliferation at anagen onset; in their absence, Demodex colonization led to increased epithelial proliferation and replacement of gene programs for repair by aberrant inflammation, leading to the loss of barrier function and HF exhaustion. Humans with rhinophymatous acne rosacea, an inflammatory condition associated with Demodex, had increased HF inflammation with decreased type 2 cytokines, consistent with the inverse relationship seen in mice. Our studies uncover a key role for skin ILC2s and IL-13, which comprise an immune checkpoint that sustains cutaneous integrity and restricts pathologic infestation by colonizing HF mites.

Activated type 2 innate lymphoid cells regulate beige fat biogenesis.

Type 2 innate lymphoid cells (ILC2s), an innate source of the type 2 cytokines interleukin (IL)-5 and -13, participate in the maintenance of tissue homeostasis. Although type 2 immunity is critically important for mediating metabolic adaptations to environmental cold, the functions of ILC2s in beige or brown fat development are poorly defined. We report here that activation of ILC2s by IL-33 is sufficient to promote the growth of functional beige fat in thermoneutral mice. Mechanistically, ILC2 activation results in the proliferation of bipotential adipocyte precursors (APs) and their subsequent commitment to the beige fat lineage. Loss- and gain-of-function studies reveal that ILC2- and eosinophil-derived type 2 cytokines stimulate signaling via the IL-4Rα in PDGFRα(+) APs to promote beige fat biogenesis. Together, our results highlight a critical role for ILC2s and type 2 cytokines in the regulation of adipocyte precursor numbers and fate, and as a consequence, adipose tissue homeostasis. PAPERCLIP:

Tuft cells act as regenerative stem cells in the human intestine.

In mice, intestinal tuft cells have been described as a long-lived, postmitotic cell type. Two distinct subsets have been identified: tuft-1 and tuft-2 (ref. 1). By combining analysis of primary human intestinal resection material and intestinal organoids, we identify four distinct human tuft cell states, two of which overlap with their murine counterparts. We show that tuft cell development depends on the presence of Wnt ligands, and that tuft cell numbers rapidly increase on interleukin-4 (IL-4) and IL-13 exposure, as reported previously in mice2-4. This occurs through proliferation of pre-existing tuft cells, rather than through increased de novo generation from stem cells. Indeed, proliferative tuft cells occur in vivo both in fetal and in adult human intestine. Single mature proliferating tuft cells can form organoids that contain all intestinal epithelial cell types. Unlike stem and progenitor cells, human tuft cells survive irradiation damage and retain the ability to generate all other epithelial cell types. Accordingly, organoids engineered to lack tuft cells fail to recover from radiation-induced damage. Thus, tuft cells represent a damage-induced reserve intestinal stem cell pool in humans.

Exercising Immunity: Interleukin-13 Flexes Muscle.

Endurance exercise drives physiological changes in the muscle to optimize performance. In a recent study in Science, Knudsen et al. report a role for the type 2 cytokine interleukin-13 in orchestrating metabolic reprogramming that drives adaptation to endurance exercise.

Eosinophils in fat: pink is the new brown.

Subcutaneous white adipose tissue can be induced to undergo "browning" and acquire thermogenic capacity in response to physiological stimuli such as cold exposure or exercise. In this issue of Cell, Qiu et al. and Rao et al. demonstrate that pink-staining eosinophils and alternatively activated macrophages play key roles in an immune cascade mediating this metabolic switch.

Eosinophils and type 2 cytokine signaling in macrophages orchestrate development of functional beige fat.

Beige fat, which expresses the thermogenic protein UCP1, provides a defense against cold and obesity. Although a cold environment is the physiologic stimulus for inducing beige fat in mice and humans, the events that lead from the sensing of cold to the development of beige fat remain poorly understood. Here, we identify the efferent beige fat thermogenic circuit, consisting of eosinophils, type 2 cytokines interleukin (IL)-4/13, and alternatively activated macrophages. Genetic loss of eosinophils or IL-4/13 signaling impairs cold-induced biogenesis of beige fat. Mechanistically, macrophages recruited to cold-stressed subcutaneous white adipose tissue (scWAT) undergo alternative activation to induce tyrosine hydroxylase expression and catecholamine production, factors required for browning of scWAT. Conversely, administration of IL-4 to thermoneutral mice increases beige fat mass and thermogenic capacity to ameliorate pre-established obesity. Together, our findings have uncovered the efferent circuit controlling biogenesis of beige fat and provide support for its targeting to treat obesity.

Meteorin-like is a hormone that regulates immune-adipose interactions to increase beige fat thermogenesis.

Exercise training benefits many organ systems and offers protection against metabolic disorders such as obesity and diabetes. Using the recently identified isoform of PGC1-α (PGC1-α4) as a discovery tool, we report the identification of meteorin-like (Metrnl), a circulating factor that is induced in muscle after exercise and in adipose tissue upon cold exposure. Increasing circulating levels of Metrnl stimulates energy expenditure and improves glucose tolerance and the expression of genes associated with beige fat thermogenesis and anti-inflammatory cytokines. Metrnl stimulates an eosinophil-dependent increase in IL-4 expression and promotes alternative activation of adipose tissue macrophages, which are required for the increased expression of the thermogenic and anti-inflammatory gene programs in fat. Importantly, blocking Metrnl actions in vivo significantly attenuates chronic cold-exposure-induced alternative macrophage activation and thermogenic gene responses. Thus, Metrnl links host-adaptive responses to the regulation of energy homeostasis and tissue inflammation and has therapeutic potential for metabolic and inflammatory diseases.