Acetylcholine production by group 2 innate lymphoid cells promotes mucosal immunity to helminths

Cross-regulation between the nervous system and type 2 immunity

Interactions between the nervous and immune systems are critical to healthy physiology and are altered in many human diseases. Many of the major players in type 2 immune responses, including type 2 lymphocytes and cytokines, mast cells, and immunoglobulin E, have been implicated in neuronal function and behavior. Conversely, neurons in both the central and peripheral nervous systems can affect type 2 immune responses and behaviors relevant to allergy, such as food avoidance. Defining this complex circuitry and its molecular intermediates in physiology may reveal type 2 immunomodulators that can be harnessed for therapeutic benefit in neurologic diseases including Alzheimer's disease, brain injury, and neurodevelopmental disorders. Conversely, modulation of the nervous system may be an important adjunct to treating immunologic disorders including atopic dermatitis, asthma, and food allergy. This Review covers recent work defining how the nervous system can both regulate and be regulated by type 2 immune responses.

RAG suppresses group 2 innate lymphoid cells

Antigen specificity is the central trait distinguishing adaptive from innate immune function. Assembly of antigen-specific T cell and B cell receptors occurs through V(D)J recombination mediated by the Recombinase Activating Gene endonucleases RAG1 and RAG2 (collectively called RAG). In the absence of RAG, mature T and B cells do not develop and thus RAG is critically associated with adaptive immune function. In addition to adaptive T helper 2 (Th2) cells, group 2 innate lymphoid cells (ILC2s) contribute to type 2 immune responses by producing cytokines like Interleukin-5 (IL-5) and IL-13. Although it has been reported that RAG expression modulates the function of innate natural killer (NK) cells, whether other innate immune cells such as ILC2s are affected by RAG remains unclear. We find that in RAG-deficient mice, ILC2 populations expand and produce increased IL-5 and IL-13 at steady state and contribute to increased inflammation in atopic dermatitis (AD)-like disease. Furthermore, we show that RAG modulates ILC2 function in a cell-intrinsic manner independent of the absence or presence of adaptive T and B lymphocytes. Lastly, employing multiomic single cell analyses of RAG1 lineage-traced cells, we identify key transcriptional and epigenomic ILC2 functional programs that are suppressed by a history of RAG expression. Collectively, our data reveal a novel role for RAG in modulating innate type 2 immunity through suppression of ILC2s.

B cells modulate lung antiviral inflammatory responses via the neurotransmitter acetylcholine

The rapid onset of innate immune defenses is critical for early control of viral replication in an infected host and yet it can also lead to irreversible tissue damage, especially in the respiratory tract. Sensitive regulators must exist that modulate inflammation, while controlling the infection. In the present study, we identified acetylcholine (ACh)-producing B cells as such early regulators. B cells are the most prevalent ACh-producing leukocyte population in the respiratory tract demonstrated with choline acetyltransferase (ChAT)-green fluorescent protein (GFP) reporter mice, both before and after infection with influenza A virus. Mice lacking ChAT in B cells, disabling their ability to generate ACh (ChatBKO), but not those lacking ChAT in T cells, significantly, selectively and directly suppressed α7-nicotinic-ACh receptor-expressing interstitial, but not alveolar, macrophage activation and their ability to secrete tumor necrosis factor (TNF), while better controlling virus replication at 1 d postinfection. Conversely, TNF blockade via monoclonal antibody treatment increased viral loads at that time. By day 10 of infection, ChatBKO mice showed increased local and systemic inflammation and reduced signs of lung epithelial repair despite similar viral loads and viral clearance. Thus, B cells are key participants of an immediate early regulatory cascade that controls lung tissue damage after viral infection, shifting the balance toward reduced inflammation at the cost of enhanced early viral replication.

Neurogenic inflammation and itch in barrier tissues

Once regarded as distinct systems, the nervous system and the immune system are now recognized for their complex interactions within the barrier tissues. The neuroimmune circuitry comprises a dual-network system that detects external and internal disturbances, providing critical information to tailor a context-specific response to various threats to tissue integrity, such as wounding or exposure to noxious and harmful stimuli like pathogens, toxins, or allergens. Using the skin as an example of a barrier tissue with the polarized sensory neuronal responses of itch and pain, we explore the molecular pathways driving neuronal activation and the effects of this activation on the immune response. We then apply these findings to other barrier tissues, to find common pathways controlling neuroimmune responses in the barriers.

Bi-directional communication between intrinsic enteric neurons and ILC2s inhibits host defense against helminth infection

Emerging studies reveal that neurotransmitters and neuropeptides play critical roles in regulating anti-helminth immune responses, hinting at the potential of intrinsic enteric neurons (iENs) in orchestrating intestinal immunity. Whether and how iENs are activated during infection and the potential neuroimmune interactions involved remain poorly defined. Here, we found that helminth infection activated a subset of iENs. Single-nucleus RNA sequencing (snRNA-seq) of iENs revealed alterations in the transcriptional profile of interleukin (IL)-13R+ intrinsic primary afferent neurons (IPANs), including the upregulation of the neuropeptide β-calcitonin gene-related peptide (CGRP). Using genetic mouse models and engineered viral tools, we demonstrated that group 2 innate lymphoid cell (ILC2)-derived IL-13 was required to activate iENs via the IL-13R, leading to iEN production of β-CGRP, which subsequently inhibited ILC2 responses and anti-helminth immunity. Together, these results reveal a previously unrecognized bi-directional neuroimmune crosstalk in the intestine between a subset of iENs and ILC2s, which influences pathogen clearance.

Neuroimmune communication of the cholinergic system in gut inflammation and autoimmunity

Neuroimmune communication in the body forms a bridge between two central regulatory systems of the body, i.e., nervous and immune systems. The cholinergic system is a crucial modulatory neurotransmitter in the central and peripheral nervous system. It includes the neurotransmitter acetylcholine (ACh), the enzyme required for the synthesis of ACh (choline acetyltransferase, ChAT), the enzyme required for its degradation (acetylcholinesterase, AChE), and cholinergic receptors (Nicotinic acetylcholine receptors and muscarinic acetylcholine receptors). The cholinergic system in neurons is well known for its role in cognitive function, sensory perception, motor control, learning, and memory processes. It has been shown that the non-neuronal cholinergic system (NNCS) is present in various tissues and immune cells and forms a neuroimmune communications system. In the present review, we discussed the NNCS on immune cells, its role in homeostasis and inflammatory reactions in the gut, and how it can be exploited in treating inflammatory responses.

Boosting Acetylcholine Signaling by Cannabidiol in a Murine Model of Alzheimer's Disease

Alzheimer's disease (AD) is a challenging medical issue that requires efficacious treatment options to improve long-term quality of life. Cannabidiol (CBD) is a cannabis-derived phytocannabinoid with potential health benefits, including reports from our laboratory and others showing a therapeutic role in the pre-clinical treatment of AD; however, the mechanisms whereby CBD affects AD progression remain undefined. Innate lymphoid cells (ILCs) are recently discovered immune cells that initiate and orchestrate inflammatory responses. ILC2, a sub-class of ILCs, is proposed to have a role in cognitive function via unknown mechanisms. In this present study, we explored whether CBD ameliorates AD symptoms via the enhancement of acetylcholine (ACh), a cholinergic neurotransmitter involved in cognition that may regulate ILC2. 5xFAD mice were chronically treated by inhalation of a formulation of broad-spectrum CBD for seven months. ACh production, ILC2s profile, brain histopathology, and long-term behavior were assessed. Together, our studies showed that long-term inhalation of CBD improved cognitive function and reduced senile plaques in a murine AD model, effects that were associated with enhanced ACh production and altered ILC2s distribution within the CNS. These findings indicate that inhaled CBD could offer a cost-effective, non-invasive, and effective treatment for managing AD. The beneficial effects of CBD inhalation may be linked to increased ACh production and an altered distribution of ILC2s, highlighting the need for further research in this area.

PAC1 constrains type 2 inflammation through promotion of CGRP signaling in ILC2s

Dysfunction of group 2 innate lymphoid cells (ILC2s) plays an important role in the development of type 2 inflammation-related diseases such as asthma and pulmonary fibrosis. Notably, neural signals are increasingly recognized as pivotal regulators of ILC2s. However, how ILC2s intrinsically modulate their responsiveness to these neural signals is still largely unknown. Here, using single-cell RNA-Seq, we found that the immune-regulatory molecule phosphatase of activated cells 1 (PAC1) selectively promoted the signaling of the neuropeptide calcitonin gene-related peptide (CGRP) in ILC2s in a cell-intrinsic manner. Genetic ablation of PAC1 in ILC2s substantially impaired the inhibitory effect of CGRP on proliferation and IL-13 secretion. PAC1 deficiency significantly exacerbated allergic airway inflammation induced by Alternaria alternata or papain in mice. Moreover, in human circulating ILC2s, the expression level of PAC1 was also significantly negatively correlated with the number of ILC2s and their expression level of IL13. Mechanistically, PAC1 was necessary for ensuring the expression of CGRP response genes by influencing chromatin accessibility. In summary, our study demonstrated that PAC1 is an important regulator of ILC2 responses, and we propose that PAC1 is a potential target for therapeutic interventions in type 2 inflammation-related diseases.

The epithelial cell types and their multi-phased defenses against fungi and other pathogens

Mucus and its movements are essential to epithelial tissue immune defenses against pathogens, including fungal pathogens, which can infect respiratory, gastrointestinal or the genito-urinary tracts. Several epithelial cell types contribute to their immune defense. This review focuses on the respiratory tract because of its paramount importance, but the observations will apply to epithelial cell defenses of other mucosal tissue, including the gastrointestinal and genito-urinary tracts. Mucus and its movements can enhance or degrade the immune defenses of the respiratory tract, particularly the lungs. The enhancements include inhaled pathogen entrapments, including fungal pathogens, pollutants and particulates, for their removal. The detriments include smaller lung airway obstructions by mucus, impairing the physical removal of pathogens and impairing vital transfers of oxygen and carbon dioxide between the alveolar circulatory system and the pulmonary air. Inflammation, edema and/or alveolar cellular damage can also reduce vital transfers of oxygen and carbon dioxide between the lung alveolar circulatory system and the pulmonary air. Furthermore, respiratory tract defenses are affected by several fatty acid mediators which activate cellular receptors to manipulate neutrophils, macrophages, dendritic cells, various innate lymphoid cells including the natural killer cells, T cells, γδ T cells, mucosal-associated invariant T cells, NKT cells and mast cells. These mediators include the inflammatory and frequently immunosuppressive prostaglandins and leukotrienes, and the special pro-resolving mediators, which normally resolve inflammation and immunosuppression. The total effects on the various epithelial cell and immune cell types, after exposures to pathogens, pollutants or particulates, will determine respiratory tract health or disease.

Sensory sentinels: Neuroimmune detection and food allergy

Food allergy is classically characterized by an inappropriate type-2 immune response to allergenic food antigens. However, how allergens are detected and how that detection leads to the initiation of allergic immunity is poorly understood. In addition to the gastrointestinal tract, the barrier epithelium of the skin may also act as a site of food allergen sensitization. These barrier epithelia are densely innervated by sensory neurons, which respond to diverse physical environmental stimuli. Recent findings suggest that sensory neurons can directly detect a broad array of immunogens, including allergens, triggering sensory responses and the release of neuropeptides that influence immune cell function. Reciprocally, immune mediators modulate the activation or responsiveness of sensory neurons, forming neuroimmune feedback loops that may impact allergic immune responses. By utilizing cutaneous allergen exposure as a model, this review explores the pivotal role of sensory neurons in allergen detection and their dynamic bidirectional communication with the immune system, which ultimately orchestrates the type-2 immune response. Furthermore, it sheds light on how peripheral signals are integrated within the central nervous system to coordinate hallmark features of allergic reactions. Drawing from this emerging evidence, we propose that atopy arises from a dysregulated neuroimmune circuit.

Neuro-immune crosstalk in hematopoiesis, inflammation, and repair

Innate immune cells are primary effectors during host defense and in sterile inflammation. Their production in the bone marrow is tightly regulated by growth and niche factors, and their activity at sites of inflammation is orchestrated by a network of alarmins and cytokines. Yet, recent work highlights a significant role of the peripheral nervous system in these processes. Sympathetic neural pathways play a key role in regulating blood cell homeostasis, and sensory neural pathways mediate pro- or anti-inflammatory signaling in a tissue-specific manner. Here, we review emerging evidence of the fine titration of hematopoiesis, leukocyte trafficking, and tissue repair via neuro-immune crosstalk, and how its derailment can accelerate chronic inflammation, as in atherosclerosis.

Taste and its receptors in human physiology: A comprehensive look

Increasing evidence shows that food has significance beyond traditional perception (providing nutrition and energy) in maintaining normal life activities. It is indicated that the sense of taste plays a crucial part in regulating human life activities. Taste is one of the basic physiological sensations in mammals, and it is the fundamental guarantee for them to perceive, select, and ingest nutrients in order to survive. With the advances in electrophysiology, molecular biology, and structural biology, studies on the intracellular and extracellular transduction mechanisms of taste have made great progress and gradually revealed the indispensable role of taste receptors in the regulation and maintenance of normal physiological activities. Up to now, how food regulates life activities through the taste pathway remains unclear. Thus, this review comprehensively and systematically summarizes the current study about the sense of taste, the function of taste receptors, the taste–structure relationship of gustatory molecules, the cross‐talking between distinctive tastes, and the role of the gut–organ axis in the realization of taste. Moreover, we also provide forward‐looking perspectives on taste research to afford a scientific basis for revealing the scientific connotation of taste receptors regulating body health.

Signaling through the nicotinic acetylcholine receptor in the liver protects against the development of metabolic dysfunction-associated steatohepatitis

Metabolic dysfunction-associated steatohepatitis (MASH) is the progressive form of liver steatosis, the most common liver disease, and substantially increases the mortality rate. However, limited therapies are currently available to prevent MASH development. Identifying potential pharmacological treatments for the condition has been hampered by its heterogeneous and complex nature. Here, we identified a hepatic nonneuronal cholinergic signaling pathway required for metabolic adaptation to caloric overload. We found that cholinergic receptor nicotinic alpha 2 subunit (CHRNA2) is highly expressed in hepatocytes of mice and humans. Further, CHRNA2 is activated by a subpopulation of local acetylcholine-producing macrophages during MASH development. The activation of CHRNA2 coordinates defensive programs against a broad spectrum of MASH-related pathogenesis, including steatosis, inflammation, and fibrosis. Hepatocyte-specific loss of CHRNA2 signaling accelerates the disease onset in different MASH mouse models. Activation of this pathway via pharmacological inhibition of acetylcholine degradation protects against MASH development. Our study uncovers a hepatic nicotinic cholinergic receptor pathway that constitutes a cell-autonomous self-defense route against prolonged metabolic stress and holds therapeutic potential for combatting human MASH.

B cells modulate lung antiviral inflammatory responses via the neurotransmitter acetylcholine

The rapid onset of innate immune defenses is critical for early control of viral replication in an infected host, yet it can also lead to irreversible tissue damage, especially in the respiratory tract. Intricate regulatory mechanisms must exist that modulate inflammation, while controlling the infection. Here, B cells expressing choline acetyl transferase (ChAT), an enzyme required for production of the metabolite and neurotransmitter acetylcholine (ACh) are identified as such regulators of the immediate early response to influenza A virus. Lung tissue ChAT + B cells are shown to interact with a7 nicotinic Ach receptor-expressing lung interstitial macrophages in mice within 24h of infection to control their production of TNFa, shifting the balance towards reduced inflammation at the cost of enhanced viral replication. Thus, innate-stimulated B cells are key participants of an immediate-early regulatory cascade that controls lung tissue damage after viral infection.

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.

Tuft cell-derived acetylcholine promotes epithelial chloride secretion and intestinal helminth clearance

Epithelial cells secrete chloride to regulate water release at mucosal barriers, supporting both homeostatic hydration and the "weep" response that is critical for type 2 immune defense against parasitic worms (helminths). Epithelial tuft cells in the small intestine sense helminths and release cytokines and lipids to activate type 2 immune cells, but whether they regulate epithelial secretion is unknown. Here, we found that tuft cell activation rapidly induced epithelial chloride secretion in the small intestine. This response required tuft cell sensory functions and tuft cell-derived acetylcholine (ACh), which acted directly on neighboring epithelial cells to stimulate chloride secretion, independent of neurons. Maximal tuft cell-induced chloride secretion coincided with immune restriction of helminths, and clearance was delayed in mice lacking tuft cell-derived ACh, despite normal type 2 inflammation. Thus, we have uncovered an epithelium-intrinsic response unit that uses ACh to couple tuft cell sensing to the secretory defenses of neighboring epithelial cells.

Colonic Tuft Cells: The Less-Recognized Therapeutic Targets in Inflammatory Bowel Disease and Colorectal Cancer

Tuft cells are more than guardian chemosensory elements of the digestive tract. They produce a variety of immunological effector molecules in response to stimulation; moreover, they are essential for defense against protozoa and nematodes. Beyond the description of their characteristics, this review aims to elucidate the potential pathogenic and therapeutic roles of colonic tuft cells in inflammatory bowel disease and colorectal cancer, focusing on their primarily immunomodulatory action. Regarding inflammatory bowel disease, tuft cells are implicated in both maintaining the integrity of the intestinal epithelial barrier and in tissue repair and regeneration processes. In addition to maintaining intestinal homeostasis, they display complex immune-regulatory functions. During the development of colorectal cancer, tuft cells can promote the epithelial-to-mesenchymal transition, alter the gastrointestinal microenvironment, and modulate both the anti-tumor immune response and the tumor microenvironment. A wide variety of their biological functions can be targeted for anti-inflammatory or anti-tumor therapies; however, the adverse side effects of immunomodulatory actions must be strictly considered.

Cooperation of ILC2s and TH2 cells in the expulsion of intestinal helminth parasites

Type 2 immune responses form a critical defence against enteric worm infections. In recent years, mouse models have revealed shared and unique functions for group 2 innate lymphoid cells and T helper 2 cells in type 2 immune response to intestinal helminths. Both cell types use similar innate effector functions at the site of infection, whereas each population has distinct roles during different stages of infection. In this Perspective, we review the underlying mechanisms used by group 2 innate lymphoid cells and T helper 2 cells to cooperate with each other and suggest an overarching model of the interplay between these cell types over the course of a helminth infection.

Navigating the blurred path of mixed neuroimmune signaling

Evolution has created complex mechanisms to sense environmental danger and protect tissues, with the nervous and immune systems playing pivotal roles. These systems work together, coordinating local and systemic reflexes to restore homeostasis in response to tissue injury and infection. By sharing receptors and ligands, they influence the pathogenesis of various diseases. Recently, a less-explored aspect of neuroimmune communication has emerged: the release of neuropeptides from immune cells and cytokines/chemokines from sensory neurons. This article reviews evidence of this unique neuroimmune interplay and its impact on the development of allergy, inflammation, itch, and pain. We highlight the effects of this neuroimmune signaling on vital processes such as host defense, tissue repair, and inflammation resolution, providing avenues for exploration of the underlying mechanisms and therapeutic potential of this signaling.

Cholinergic sensing of allergen exposure by airway epithelium promotes type 2 immunity in the lungs

BACKGROUND

Nonneuronal cells, including epithelial cells, can produce acetylcholine (ACh). Muscarinic ACh receptor antagonists are used clinically to treat asthma and other medical conditions; however, knowledge regarding the roles of ACh in type 2 immunity is limited.

OBJECTIVE

Our aim was to investigate the roles of epithelial ACh in allergic immune responses.

METHODS

Human bronchial epithelial (HBE) cells were cultured with allergen extracts, and their ACh production and IL-33 secretion were studied in vitro. To investigate immune responses in vivo, naive BALB/c mice were treated intranasally with different muscarinic ACh receptor antagonists and then exposed intranasally to allergens.

RESULTS

At steady state, HBE cells expressed cellular components necessary for ACh production, including choline acetyltransferase and organic cation transporters. Exposure to allergens caused HBE cells to rapidly release ACh into the extracellular medium. Pharmacologic or small-interfering RNA-based blocking of ACh production or autocrine action through the M3 muscarinic ACh receptors in HBE cells suppressed allergen-induced ATP release, calcium mobilization, and extracellular secretion of IL-33. When naive mice were exposed to allergens, ACh was quickly released into the airway lumen. A series of clinical M3 muscarinic ACh receptor antagonists inhibited allergen-induced IL-33 secretion and innate type 2 immune response in the mouse airways. In a preclinical murine model of asthma, an ACh receptor antagonist suppressed allergen-induced airway inflammation and airway hyperreactivity.

CONCLUSIONS

ACh is released quickly by airway epithelial cells on allergen exposure, and it plays an important role in type 2 immunity. The epithelial ACh system can be considered a therapeutic target in allergic airway diseases.

[Characteristics of intestinal tuft cells and their role in the pathomechanism of inflammatory bowel disease and colorectal carcinoma]

Given their fundamental physiological importance, their involvement in the immune system, and their close association with the development of intestinal diseases, the interest in intestinal epithelial cells has increased significantly over the past fifteen years. Their close association with intestinal worm and protozoan infections - a significant 2016 discovery - has further stimulated research into uncommon chemosensitive tuft epithelial cells. Although their numbers are relatively low, tuft cells are now recognized as an essential sentinel of the gastrointestinal tract, as their taste receptors for succinate, sweet, and bitter continuously monitor intestinal contents. When stimulated, tuft cells release a number of effector molecules, including immunomodulatory molecules like interleukin 25, prostaglandins E2 and D2, cysteinyl leukotriene C4, acetylcholine, thymic stromal lymphopoietin, and beta-endorphins. Tuft cells have been shown to be crucial for immunity against nematodes and protozoa. The majority of tuft cell research has used the doublecortin-like (microtubule-linked) kinase 1 protein marker on mice; however, the expression of the enzyme cyclooxygenase-1 may help identify human intestinal tuft cells. Few studies have examined the association between tuft cells and intestinal diseases in humans. This article provides an update on intestinal epithelial tuft cells, including their physiology, immunological nodal function, and role in human diseases. We conclude by discussing the potential clinical therapeutic value of tuft cells. Orv Hetil. 2023; 164(44): 1727-1735.

Enteric neuro-immune interactions in intestinal health and disease

The enteric nervous system is an autonomous neuronal circuit that regulates many processes far beyond the peristalsis in the gastro-intestinal tract. This circuit, consisting of enteric neurons and enteric glial cells, can engage in many intercellular interactions shaping the homeostatic microenvironment in the gut. Perhaps the most well documented interactions taking place, are the intestinal neuro-immune interactions which are essential for the fine-tuning of oral tolerance. In the context of intestinal disease, compelling evidence demonstrates both protective and detrimental roles for this bidirectional neuro-immune signaling. This review discusses the different immune cell types that are recognized to engage in neuronal crosstalk during intestinal health and disease. Highlighting the molecular pathways involved in the neuro-immune interactions might inspire novel strategies to target intestinal disease.

Targeting helminths: The expanding world of type 2 immune effector mechanisms

In this new review, Rick Maizels and Bill Gause summarize how type 2 immune responses combat helminth parasites through novel mechanisms, coordinating multiple innate and adaptive cell and molecular players that can eliminate infection and repair-resultant tissue damage.

The modulation of pulmonary group 2 innate lymphoid cell function in asthma: from inflammatory mediators to environmental and metabolic factors

A dysregulated type 2 immune response is one of the fundamental causes of allergic asthma. Although Th2 cells are undoubtedly central to the pathogenesis of allergic asthma, the discovery of group 2 innate lymphoid cells (ILC2s) has added another layer of complexity to the etiology of this chronic disease. Through their inherent innate type 2 responses, ILC2s not only contribute to the initiation of airway inflammation but also orchestrate the recruitment and activation of other members of innate and adaptive immunity, further amplifying the inflammatory response. Moreover, ILC2s exhibit substantial cytokine plasticity, as evidenced by their ability to produce type 1- or type 17-associated cytokines under appropriate conditions, underscoring their potential contribution to nonallergic, neutrophilic asthma. Thus, understanding the mechanisms of ILC2 functions is pertinent. In this review, we present an overview of the current knowledge on ILC2s in asthma and the regulatory factors that modulate lung ILC2 functions in various experimental mouse models of asthma and in humans.

A succinate/SUCNR1-brush cell defense program in the tracheal epithelium

Host-derived succinate accumulates in the airways during bacterial infection. Here, we show that luminal succinate activates murine tracheal brush (tuft) cells through a signaling cascade involving the succinate receptor 1 (SUCNR1), phospholipase Cβ2, and the cation channel transient receptor potential channel subfamily M member 5 (TRPM5). Stimulated brush cells then trigger a long-range Ca2+ wave spreading radially over the tracheal epithelium through a sequential signaling process. First, brush cells release acetylcholine, which excites nearby cells via muscarinic acetylcholine receptors. From there, the Ca2+ wave propagates through gap junction signaling, reaching also distant ciliated and secretory cells. These effector cells translate activation into enhanced ciliary activity and Cl- secretion, which are synergistic in boosting mucociliary clearance, the major innate defense mechanism of the airways. Our data establish tracheal brush cells as a central hub in triggering a global epithelial defense program in response to a danger-associated metabolite.

Interleukin-33 increases type 2 innate lymphoid cell count and their activation in eosinophilic asthma

BACKGROUND

Interleukin-33 (IL-33) exacerbates asthma probably through type 2 innate lymphoid cells (ILC2s). Nevertheless, the association between eosinophilic asthma (EA) and ILC2s remains obscure, and the mechanisms by which IL-33 affects ILC2s are yet to be clarified.

METHODS

ILC2s were evaluated in peripheral blood mononuclear cells, induced sputum, and bronchoalveolar lavage fluid obtained from patients with EA. Confocal microscopy was performed to locate ILC2s in lung tissue and the mRNA expression of ILC2-related genes was also evaluated in the EA model. The proliferation of ILC2s isolated from humans and mice was assessed following IL-33 or anti-IL-33 stimulation.

RESULTS

The counts, activation, and mRNA expression of relevant genes in ILC2s were higher in PBMCs and airways of patients with EA. In addition, ILC2 cell counts correlated with Asthma control test, blood eosinophil count, Fractional exhaled nitric oxide level, and predicted eosinophilic airway inflammation. IL-33 induced stronger proliferation of ILC2s and increased their density around blood vessels in the lungs of mice with EA. Moreover, IL-33 treatment increased the counts and activation of ILC2s and lung inflammatory scores, whereas anti-IL-33 antibody significantly reversed these effects in EA mice. Finally, IL-33 enhanced PI3K and AKT protein expression in ILC2s, whereas inhibition of the PI3K/AKT pathway decreased IL-5 and IL-13 production by ILC2s in EA.

CONCLUSIONS

ILC2s, especially activated ILC2s, might be critical markers of EA. IL-33 can induce and activate ILC2s in the lungs via the PI3K/AKT pathway in EA. Thus, using anti-IL-33 antibody could be a part of an effective treatment strategy for EA.

CD90 is not constitutively expressed in functional innate lymphoid cells

Huge progress has been made in understanding the biology of innate lymphoid cells (ILC) by adopting several well-known concepts in T cell biology. As such, flow cytometry gating strategies and markers, such as CD90, have been applied to indentify ILC. Here, we report that most non-NK intestinal ILC have a high expression of CD90 as expected, but surprisingly a sub-population of cells exhibit low or even no expression of this marker. CD90-negative and CD90-low CD127+ ILC were present amongst all ILC subsets in the gut. The frequency of CD90-negative and CD90-low CD127+ ILC was dependent on stimulatory cues in vitro and enhanced by dysbiosis in vivo. CD90-negative and CD90-low CD127+ ILC were a potential source of IL-13, IFNγ and IL-17A at steady state and upon dysbiosis- and dextran sulphate sodium-elicited colitis. Hence, this study reveals that, contrary to expectations, CD90 is not constitutively expressed by functional ILC in the gut.

Type 2 neuroimmune circuits in the shaping of physiology

Type 2 immune responses drive a broad range of biological processes including defense from large parasites, immunity to allergens, and non-immunity-related functions, such as metabolism and tissue homeostasis. The symptoms provoked by type 2 immunity, such as vomiting, coughing or itching, encompass nervous system triggering. Here, we review recent findings that place type 2 neuroimmune circuits at the center stage of immunity at barrier surfaces. We emphasize the homeostatic functions of these circuitries and how deregulation may drive pathology and impact disease outcomes, including in the context of cancer. We discuss a paradigm wherein type 2 neuroimmune circuits are central regulators of organismal physiology.

HTR2A agonists play a therapeutic role by restricting ILC2 activation in papain-induced lung inflammation

Group 2 innate lymphoid cells (ILC2s) are a category of heterogeneous cells that produce the cytokines IL-5 and IL-13, which mediate the type 2 immune response. However, specific drug targets on lung ILC2s have rarely been reported. Previous studies have shown that type 2 cytokines, such as IL-5 and IL-13, are related to depression. Here, we demonstrated the negative correlation between the depression-associated monoamine neurotransmitter serotonin and secretion of the cytokines IL-5 and IL-13 by ILC2s in individuals with depression. Interestingly, serotonin ameliorates papain-induced lung inflammation by suppressing ILC2 activation. Our data showed that the serotonin receptor HTR2A was highly expressed on ILC2s from mouse lungs and human PBMCs. Furthermore, an HTR2A selective agonist (DOI) impaired ILC2 activation and alleviated the type 2 immune response in vivo and in vitro. Mice with ILC2-specific depletion of HTR2A (Il5cre/+·Htr2aflox/flox mice) abolished the DOI-mediated inhibition of ILC2s in a papain-induced mouse model of inflammation. In conclusion, serotonin and DOI could restrict the type 2 lung immune response, indicating a potential treatment strategy for type 2 lung inflammation by targeting HTR2A on ST2+ ILC2s.

Tuft cell-derived acetylcholine regulates epithelial fluid secretion

Tuft cells are solitary chemosensory epithelial cells that can sense lumenal stimuli at mucosal barriers and secrete effector molecules to regulate the physiology and immune state of their surrounding tissue. In the small intestine, tuft cells detect parasitic worms (helminths) and microbe-derived succinate, and signal to immune cells to trigger a Type 2 immune response that leads to extensive epithelial remodeling spanning several days. Acetylcholine (ACh) from airway tuft cells has been shown to stimulate acute changes in breathing and mucocilliary clearance, but its function in the intestine is unknown. Here we show that tuft cell chemosensing in the intestine leads to release of ACh, but that this does not contribute to immune cell activation or associated tissue remodeling. Instead, tuft cell-derived ACh triggers immediate fluid secretion from neighboring epithelial cells into the intestinal lumen. This tuft cell-regulated fluid secretion is amplified during Type 2 inflammation, and helminth clearance is delayed in mice lacking tuft cell ACh. The coupling of the chemosensory function of tuft cells with fluid secretion creates an epithelium-intrinsic response unit that effects a physiological change within seconds of activation. This response mechanism is shared by tuft cells across tissues, and serves to regulate the epithelial secretion that is both a hallmark of Type 2 immunity and an essential component of homeostatic maintenance at mucosal barriers.

Neural regulation of ILC2s in allergic airway inflammation

The sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) regulate the effector functions of group 2 innate lymphoid cells (ILC2s) through β2 adrenergic receptor (ADRB2) and nicotinic/muscarinic cholinergic receptor signaling, respectively. To further maintain the critical balance between host-protective and pathogenic type 2 inflammation in the lungs, neuropeptides neuromedin B (NMB) and neuromedin U (NMU) function to suppress or promote ILC2 responses in synergy with IL-33/IL-25, respectively. Additionally, the release of ATP into the extracellular environment in response to cell death caused by challenge to the airway epithelial barrier quickly becomes converted into adenosine, which helps keep the inflammatory response in check by suppressing ILC2 responses. Besides neurotransmitter and neuropeptides derived from other cells, ILC2s further regulate allergic airway inflammation through the production of acetylcholine (ACh) and calcitonin gene-related peptide (CGRP). In this article we review the neuromodulation of ILC2s through cholinergic and adrenergic signaling, neuropeptides, and adenosine and its role in allergic airway inflammation. Furthermore, we discuss the potential clinical utility of targeting these pathways for therapeutic goals and address directions for future research.

Neuro-epithelial-ILC2 crosstalk in barrier tissues

Group 2 innate lymphoid cells (ILC2s) contribute to the maintenance of mammalian barrier tissue homeostasis. We review how ILC2s integrate epithelial signals and neurogenic components to preserve the tissue microenvironment and modulate inflammation. The epithelium that overlies barrier tissues, including the skin, lungs, and gut, generates epithelial cytokines that elicit ILC2 activation. Sympathetic, parasympathetic, sensory, and enteric fibers release neural signals to modulate ILC2 functions. We also highlight recent findings suggesting neuro-epithelial-ILC2 crosstalk and its implications in immunity, inflammation and resolution, tissue repair, and restoring homeostasis. We further discuss the pathogenic effects of disturbed ILC2-centered neuro-epithelial-immune cell interactions and putative areas for therapeutic targeting.

Innate lymphoid cells: More than just immune cells

Since their discovery, innate lymphoid cells (ILCs) have been described as the innate counterpart of the T cells. Indeed, ILCs and T cells share many features including their common progenitors, transcriptional regulation, and effector cytokine secretion. Several studies have shown complementary and redundant roles for ILCs and T cells, leaving open questions regarding why these cells would have been evolutionarily conserved. It has become apparent in the last decade that ILCs, and rare immune cells more generally, that reside in non-lymphoid tissue have non-canonical functions for immune cells that contribute to tissue homeostasis and function. Viewed through this lens, ILCs would not be just the innate counterpart of T cells, but instead act as a link between sensory cells that monitor any changes in the environment that are not necessarily pathogenic and instruct effector cells that act to maintain body homeostasis. As these non-canonical functions of immune cells are operating in absence of pathogenic signals, it opens great avenues of research for immunologists that they now need to identify the physiological cues that regulate these cells and how the process confers a finer level of control and a greater flexibility that enables the organism to adapt to changing environmental conditions. In the review, we highlight how ILCs participate in the physiologic function of the tissue in which they reside and how physiological cues, in particular neural inputs control their homeostatic activity.

The immune system as a system of relations

Progress in neuroimmunology established that the nervous and the immune systems are two functionally related physiological systems. Unique sensory and immune receptors enable them to control interactions of the organism with the inner and the outer worlds. Both systems undergo an experience-driven selection process during their ontogeny. They share the same mediators/neurotransmitters and use synapses for intercellular communication. They keep a memory of previous experiences. Immune cells can affect nervous cells, nervous cells can affect immune cells, and they regulate each other. I however argue that the two systems differ by three major points: 1) Unlike the nervous system, the immune system has a loose anatomical structure, in which molecular and cellular events mostly occur at random; 2) The immune system can respond to molecules of the living world whereas the nervous system can respond to phenomena of the physical world; 3) Responses of the immune system act both on the organism and on the stimulus that triggered the response, whereas responses of the nervous system act on the organism only. The nervous and the immune systems therefore appear as two complementary systems of relations that closely work together, and whose reactivities are well-suited to deal with physical and biological stimuli, respectively. Its ability both to adapt the organism to the living world and to adapt the living world to the organism endows the immune system with powerful adaptive properties that enable the organism to live in peace with itself and with other living beings, whether pathogens or commensals.

Immune cell cholinergic signaling in adipose thermoregulation and immunometabolism

Research focusing on adipose immunometabolism has been expanded from inflammation in white fat during obesity development to immune cell function regulating thermogenic fat, energy expenditure, and systemic metabolism. This opinion discusses our current understanding of how resident immune cells within the thermogenic fat niche may regulate whole-body energy homeostasis. Furthermore, various types of immune cells can synthesize acetylcholine (ACh) and regulate important physiological functions. We highlight a unique subset of cholinergic macrophages within subcutaneous adipose tissue, termed cholinergic adipose macrophages (ChAMs); these macrophages interact with beige adipocytes through cholinergic receptor nicotinic alpha 2 subunit (CHRNA2) signaling to induce adaptive thermogenesis. We posit that these newly identified thermoregulatory macrophages may broaden our view of immune system functions for maintaining metabolic homeostasis and potentially treating obesity and metabolic disorders.

Heartbreakers or Healers? Innate Lymphoid Cells in Cardiovascular Disease and Obesity

Cardiovascular diseases (CVDs) are responsible for most pre-mature deaths worldwide, contributing significantly to the global burden of disease and its associated costs to individuals and healthcare systems. Obesity and associated metabolic inflammation underlie development of several major health conditions which act as direct risk factors for development of CVDs. Immune system responses contribute greatly to CVD development and progression, as well as disease resolution. Innate lymphoid cells (ILCs) are a family of helper-like and cytotoxic lymphocytes, typically enriched at barrier sites such as the skin, lung, and gastrointestinal tract. However, recent studies indicate that most solid organs and tissues are home to resident populations of ILCs - including those of the cardiovascular system. Despite their relative rarity, ILCs contribute to many important biological effects during health, whilst promoting inflammatory responses during tissue damage and disease. This mini review will discuss the evidence for pathological and protective roles of ILCs in CVD, and its associated risk factor, obesity.

Differential Regulation of Allergic Airway Inflammation by Acetylcholine

Acetylcholine (ACh) from neuronal and non-neuronal sources plays an important role in the regulation of immune responses and is associated with the development of several disease pathologies. We have previously demonstrated that group 2 innate lymphoid cell (ILC2)-derived ACh is required for optimal type 2 responses to parasitic infection and therefore sought to determine whether this also plays a role in allergic inflammation. Rora^Cre+Chat^LoxP mice (in which ILC2s cannot synthesize ACh) were exposed to an allergenic extract of the fungus Alternaria alternata, and immune responses in the airways and lung tissues were analyzed. Airway neutrophilia and expression of the neutrophil chemoattractants CXCL1 and CXCL2 were enhanced 24 h after exposure, suggesting that ILC2-derived ACh plays a role in limiting excessive pulmonary neutrophilic inflammation. The effect of non-selective depletion of ACh was examined by intranasal administration of a stable parasite-secreted acetylcholinesterase. Depletion of airway ACh in this manner resulted in a more profound enhancement of neutrophilia and chemokine expression, suggesting multiple cellular sources for the release of ACh. In contrast, depletion of ACh inhibited Alternaria-induced activation of ILC2s, suppressing the expression of IL-5, IL-13, and subsequent eosinophilia. Depletion of ACh reduced macrophages with an alternatively activated M2 phenotype and an increase in M1 macrophage marker expression. These data suggest that ACh regulates allergic airway inflammation in several ways, enhancing ILC2-driven eosinophilia but suppressing neutrophilia through reduced chemokine expression.

Finding a Niche: Tissue Immunity and Innate Lymphoid Cells

The immune system plays essential roles in maintaining homeostasis in mammalian tissues that extend beyond pathogen clearance and host defense. Recently, several homeostatic circuits comprised of paired hematopoietic and non-hematopoietic cells have been described to influence tissue composition and turnover in development and after perturbation. Crucial circuit components include innate lymphoid cells (ILCs), which seed developing organs and shape their resident tissues by influencing progenitor fate decisions, microbial interactions, and neuronal activity. As they develop in tissues, ILCs undergo transcriptional imprinting that encodes receptivity to corresponding signals derived from their resident tissues but ILCs can also shift their transcriptional profiles to adapt to specific types of tissue perturbation. Thus, ILC functions are embedded within their resident tissues, where they constitute key regulators of homeostatic responses that can lead to both beneficial and pathogenic outcomes. Here, we examine the interactions between ILCs and various non-hematopoietic tissue cells, and discuss how specific ILC-tissue cell circuits form essential elements of tissue immunity.

Crosstalk between ILC2s and Th2 CD4+ T Cells in Lung Disease

Cytokine secretion, such as interleukin-4 (IL-4), IL-5, IL-9, IL-13, and amphiregulin (Areg), by type 2 innate lymphoid cells (ILC2s) is indispensable for homeostasis, remodeling/repairing tissue structure, inflammation, and tumor immunity. Often viewed as the innate cell surrogate of T helper type 2 (Th2) cells, ILC2s not only secrete the same type 2 cytokines, but are also inextricably related to CD4⁺T cells in terms of cell origin and regulatory factors, bridging between innate and adaptive immunity. ILC2s interact with CD4⁺T cells to play a leading role in a variety of diseases through secretory factors. Here, we review the latest progress on ILC2s and CD4⁺T cells in the lung, the close relationship between the two, and their relevance in the lung disease and immunity. This literature review aids future research in pulmonary type 2 immune diseases and guides innovative treatment approaches for these diseases.

Emerging Paradigms in Type 2 Immunity

A principal purpose of type 2 immunity was thought to be defense against large parasites, but it also functions in the restoration of homeostasis, such as toxin clearance following snake bites. In other cases, like allergy, the type 2 T helper (Th2) cytokines and cells present in the environment are detrimental and cause diseases. In recent years, the recognition of cell heterogeneity within Th2-associated cell populations has revealed specific functions of cells with a particular phenotype or gene signature. In addition, here we discuss the recent data regarding heterogeneity of type 2 immunity-related cells, as well as their newly identified role in a variety of processes ranging from involvement in respiratory viral infections [especially in the context of the recent COVID-19 (coronavirus disease 2019) pandemic] to control of cancer development or of metabolic homeostasis.

Neuroimmune Interactions in Peripheral Organs

Interactions between the nervous and immune systems were recognized long ago, but recent studies show that this crosstalk occurs more frequently than was previously appreciated. Moreover, technological advances have enabled the identification of the molecular mediators and receptors that enable the interaction between these two complex systems and provide new insights on the role of neuroimmune crosstalk in organismal physiology. Most neuroimmune interaction occurs at discrete anatomical locations in which neurons and immune cells colocalize. Here, we describe the interactions of the different branches of the peripheral nervous system with immune cells in various organs, including the skin, intestine, lung, and adipose tissue. We highlight how neuroimmune crosstalk orchestrates physiological processes such as host defense, tissue repair, metabolism, and thermogenesis. Unraveling these intricate relationships is invaluable to explore the therapeutic potential of neuroimmune interaction. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

ILC2s - development, divergence, dispersal

Over the last decade, we have come to appreciate group 2 innate lymphoid cells (ILC2s) as important players in host and tissue immunity. New studies of ILC2s and their precursors using novel reporter mice, advanced microscopy, and multi-omics approaches have expanded our knowledge on how these cells contribute to tissue physiology and function. This review highlights recent literature on this enigmatic cell, and we organize our discussion across three important paradigms in ILC2 biology: development, divergence, and dispersal. In addition, we frame our discussion in the context of other innate and adaptive immune cells to emphasize the relevance of expanding knowledge of ILC2s and tissue immunity.

Tuft Cells and Their Role in Intestinal Diseases

The interests in intestinal epithelial tuft cells, their basic physiology, involvement in immune responses and relevance for gut diseases, have increased dramatically over the last fifteen years. A key discovery in 2016 of their close connection to helminthic and protozoan infection has further spurred the exploration of these rare chemosensory epithelial cells. Although very sparse in number, tuft cells are now known as important sentinels in the gastrointestinal tract as they monitor intestinal content using succinate as well as sweet and bitter taste receptors. Upon stimulation, tuft cells secrete a broad palette of effector molecules, including interleukin-25, prostaglandin E2 and D2, cysteinyl leukotriene C4, acetylcholine, thymic stromal lymphopoietin, and β-endorphins, some of which with immunomodulatory functions. Tuft cells have proven indispensable in anti-helminthic and anti-protozoan immunity. Most studies on tuft cells are based on murine experiments using double cortin-like kinase 1 (DCLK1) as a marker, while human intestinal tuft cells can be identified by their expression of the cyclooxygenase-1 enzyme. So far, only few studies have examined tuft cells in humans and their relation to gut disease. Here, we present an updated view on intestinal epithelial tuft cells, their physiology, immunological hub function, and their involvement in human disease. We close with a discussion on how tuft cells may have potential therapeutic value in a clinical context.

Melatonergic signalling instructs transcriptional inhibition of IFNGR2 to lessen interleukin-1β-dependent inflammation

BACKGROUND

Immunotransmitters (e.g., neurotransmitters and neuromodulators) could orchestrate diverse immune responses; however, the elaborated mechanism by which melatonergic activation governs inflammation remains less defined.

METHODS

Primary macrophages, various cell lines, and Pasteurella multocida (PmCQ2)-infected mice were respectively used to illustrate the influence of melatonergic signalling on inflammation in vitro and in vivo. A series of methods (e.g., RNA-seq, metabolomics, and genetic manipulation) were conducted to reveal the mechanism whereby melatonergic signalling reduces macrophage inflammation.

RESULTS

Here, we demonstrate that melatonergic activation substantially lessens interleukin (IL)-1β-dependent inflammation. Treatment of macrophages with melatonin rewires metabolic program, as well as remodels signalling pathways which depends on interferon regulatory factor (IRF) 7. Mechanistically, melatonin acts via membrane receptor (MT) 1 to increase heat shock factor (Hsf) 1 expression through lowering the inactive glycogen synthase kinase (GSK3) β, thereby transcriptionally inhibiting interferon (IFN)-γ receptor (IFNGR) 2 and ultimately causing defective canonical signalling events [Janus kinase (JAK) 1/2-signal transducer and activator of transcription (STAT) 1-IRF7] and lower IL-1β production in macrophages. Moreover, we find that melatonin amplifies host protective responses to PmCQ2 infection-induced pneumonia.

CONCLUSIONS

Our conceptual framework provides potential therapeutic targets to prevent and/or treat inflammatory diseases associating with excessive IL-1β production.

Microbial regulation of intestinal motility provides resistance against helminth infection

Soil-transmitted helminths cause widespread disease, infecting ~1.5 billion people living within poverty-stricken regions of tropical and subtropical countries. As adult worms inhabit the intestine alongside bacterial communities, we determined whether the bacterial microbiota impacted on host resistance against intestinal helminth infection. We infected germ-free, antibiotic-treated and specific pathogen-free mice, with the intestinal helminth Heligmosomoides polygyrus bakeri. Mice harboured increased parasite numbers in the absence of a bacterial microbiota, despite mounting a robust helminth-induced type 2 immune response. Alterations to parasite behaviour could already be observed at early time points following infection, including more proximal distribution of infective larvae along the intestinal tract and increased migration in a Baermann assay. Mice lacking a complex bacterial microbiota exhibited reduced levels of intestinal acetylcholine, a major excitatory intestinal neurotransmitter that promotes intestinal transit by activating muscarinic receptors. Both intestinal motility and host resistance against larval infection were restored by treatment with the muscarinic agonist bethanechol. These data provide evidence that a complex bacterial microbiota provides the host with resistance against intestinal helminths via its ability to regulate intestinal motility.

Food for thought - ILC metabolism in the context of helminth infections

Helminths are multicellular ancient organisms residing as parasites at mucosal surfaces of their host. Through adaptation and co-evolution with their hosts, helminths have been able to develop tolerance mechanisms to limit inflammation and avoid expulsion. The study of helminth infections as an integral part of tissue immunology allowed us to understand fundamental aspects of mucosal and barrier immunology, which led to the discovery of a new group of tissue-resident immune cells, innate lymphoid cells (ILC), over a decade ago. Here, we review the intricate interplay between helminth infections and type 2 ILC (ILC2) biology, discuss the host metabolic adaptation to helminth infections and the metabolic pathways fueling ILC2 responses. We hypothesize that nutrient competition between host and helminths may have prevented chronic inflammation in the past and argue that a detailed understanding of the metabolic restraints imposed by helminth infections may offer new therapeutic avenues in the future.

Enteric neuroimmune interactions coordinate intestinal responses in health and disease

The enteric nervous system (ENS) of the gastrointestinal (GI) tract interacts with the local immune system bidirectionally. Recent publications have demonstrated that such interactions can maintain normal GI functions during homeostasis and contribute to pathological symptoms during infection and inflammation. Infection can also induce long-term changes of the ENS resulting in the development of post-infectious GI disturbances. In this review, we discuss how the ENS can regulate and be regulated by immune responses and how such interactions control whole tissue physiology. We also address the requirements for the proper regeneration of the ENS and restoration of GI function following the resolution of infection.

IL-33: A central cytokine in helminth infections

IL-33 is an alarmin cytokine which has been implicated in allergy, fibrosis, inflammation, tumorigenesis, metabolism, and homeostasis. However, amongst its strongest roles are in helminth infections, where IL-33 usually (but not always) is central to induction of an effective anti-parasitic immune response. In this review, we will summarise the literature around this fascinating cytokine, its activity on immune and non-immune cells, the unique (and sometimes counterintuitive) responses it induces, and how it can coordinate the immune response during infections by parasitic helminths. Finally, we will summarise some of the ways that parasites have developed to modulate the IL-33 pathway for their own benefit.