Tissue-Resident ILC2s Across Organs: Heterogeneity, Niche Crosstalk, and Shared Regulatory Circuits.

Like individuals in our society, the cells in our body interact with each other. Some cells give guidance to others, while some inhibit other’s function, and these interactions, together, form a highly complex and intimate network that regulates biological function. During development, precursor cells differentiate into mature cells with unique functions, and these cells are inevitably involved in tissue niches, working in close concert with other cells in the tissue microenvironment. Thus, tissue-specific niches ultimately dictate the fate of precursor cells. Tissue-resident immune cells are located in peripheral tissues and have specific functions depending on their localization ( 1 2 ). Their primary roles are to protect the host from infection and regulate inflammatory responses, which can occur in most peripheral tissues ( 3 4 5 ). In addition, recent studies have shown that tissue-resident immune cells contribute to a broad spectrum of biological responses beyond their primary functions ( 6 ). Innate lymphoid cells (ILCs) are immune cells with functions and transcription factor profiles very similar to those of T cells ( 7 8 9 ). Like T cells, ILCs are derived from common lymphoid progenitors (CLPs); however, ILCs do not express lineage markers, nor are they activated by antigen stimulation via antigen-presenting cells. Type 2 innate lymphoid cells (ILC2s) are counterparts of Th2 cells and are known to contribute to allergic immune responses. Type 2 cytokines, expressed by ILC2s, maintain tissue tone and homeostasis in multiple tissues. They are involved in not only allergic immune responses but also restorative responses, such as tissue remodeling, regeneration, and enzyme induction ( 10 11 12 ). ILC2s sense tissue-specific cytokines, neuropeptides, and paracrine factors, which activate their unique functions as tissue-resident cells ( 13 14 15 16 17 18 ). In addition, recent studies have reported that ILC2s can be regulated by various metabolites and even mechanical stress ( 17 19 20 21 ). Thus, ILC2s are immune cells that act as “sentinels” in peripheral tissues, activated by a range of factors and contributing to the regulation of immunological and biological responses. As it migrated at an early stage of development, tissue resident ILC2s matured by tissue niche. Because peripheral tissues possess distinct microenvironments based on structural and physiological differences, inevitably tissue-resident ILC2s are exposed to different types of stimuli in tonic and various perturbed situations. Furthermore, this phenomenon is not one-sided. It establishes crosstalk circuits in which tissue-specific ILC2s contribute to the tissue niche. Therefore, it is necessary to conduct research to determine the distribution and function of ILC2s in the peripheral tissues ( 22 ). Notably, in mucosal tissue, ILC2s play a critical role in tissue homeostasis, but recent studies with improved techniques have revealed their roles in various tissues, including the central nervous system (CNS) ( 23 24 25 ), pancreas ( 26 27 ), muscle ( 28 ), and kidney ( 29 ). In this review, we will discuss recent findings about tissue-specific ILC2s and their roles, especially in the context of biological homeostasis ( Fig. 1 ). PDAC, pancreatic ductal adenocarcinoma.

ILCs are derived from CLPs, which serve as the multipotent precursor for all types of lymphoid cells, including T and B cells and lymphoid tissue inducer (LTi) cells as well as ILCs. During development, CLPs first diverge into T or B cell lineages or into common ILC progenitors (CILPs), which then differentiate into either NK progenitors or common helper-like innate lymphoid cell progenitors (CHILPs). CHILPs further develop into helper-like ILCs or LTi cells ( 8 ). The functions of various transcription factors involved in the differentiation process, including Id2 ( 30 ), NFIL3 ( 31 32 ), TCF-1 ( 33 34 35 ), PLZF ( 36 37 ), are well understood, and once the ILC progenitor (ILCP) has developed, the specific tissue niche leads to the maturation of ILCs. Recent studies employing multi-reporter mice have revealed complex interactions between transcription factors, further underscoring the importance of the tissue niche, which regulates the expression of transcription factors across multiple points ( 38 39 ). As counterparts of each other, ILC2s and Th2 cells express similar cytokines and transcription factors, such as IL-4, IL-5, IL-13, and GATA3. Recently, Wang et al. ( 40 ) reported that mouse ILCs originate either from fetal liver–derived tissue-resident progenitors or via adult bone marrow hematopoiesis. Bone marrow–derived ILC2s primarily reside within the bone marrow or function locally in the lung, whereas most tissue-specific ILC2s, such as those of the lung, intestine, and liver, are established from fetal progenitors that are seeded into peripheral tissues before birth. These findings suggest that tissue-resident ILCs are not replenished by circulating hematopoietic cells. Shaped by local tissue-derived signals that imprint distinct transcriptional and functional identities, ILC2s exhibit remarkable tissue-specific heterogeneity ( 9 41 ). These local signals, alarmins, are mainly provided by peripheral stromal cells and contribute to the maintenance of tissue-specific ILC2 function. However, a recent study by Ricardo-Gonzalez et al. ( 42 ) demonstrated that ILC2 subsets in the lung, gut, adipose tissue, skin, and bone marrow maintained unique activation profiles and receptor expression patterns independent of canonical alarmins (IL-33, IL-25, and TSLP) or microbiota ( Table 1 ). Additionally, single-cell transcriptomic analyses have revealed that tissue-resident ILC2s are preconditioned by local cues, which establish an anticipatory activation state that primes the ILC2s for rapid yet context-specific type 2 immune responses, suggesting intrinsic features of ILC2s may be involved in the turnover of tissue-resident populations. Development of ILC2s is also influenced by ontogeny ( 43 ). According to this research, ILC2 development does not occur continuously and uniformly throughout an individual’s life, but rather in a process known as layered ontogeny. Major populations of tissue-resident ILC2s are derived prenatally, when they initially establish themselves in the tissue. Peripheral tissues are composed of a mixed population of ILC2s, and, using a fate mapping strategy, the study showed that ILC2 turnover varies according to the tissue type ( 43 ). Taken together, research has demonstrated that tissue-specific ILC2 function is influenced by both the specific tissue niche and developmental timing. +, activation; −, suppression; ~, change. Breathing is a pivotal biological function in living organisms, and most mammals start directly inhaling air immediately after birth. The respiratory system frequently interacts directly with various environmental factors, including allergens and pathogens ( 44 ). To protect and maintain its function, various immune cells, including macrophages, T cells, and ILCs, are localized in the pulmonary tract, and these immune cells contribute to not only immune responses but also various other biological functions, such as alveolar cell differentiation and epithelial barrier integrity ( 45 ). Pulmonary ILC2s are known to be activated by alarmins, such as IL-33 and TSLP, in response to conditions like epithelial cell injury, allergic asthma, and helminth infection. Once activated, they secrete type 2 cytokines that carry out a variety of functions ( Fig. 2 ). In the classical view, type 2 cytokines are involved in allergic immune responses. However, research suggests that they also play restorative roles in processes such as wound healing, tissue remodeling, and metabolic homeostasis. A recent study suggested that the accumulation of chitin particles from the environment can induce ILC2-mediated type 2 inflammation in the lung. Pulmonary chitin accumulation with epithelial injury decreased the epithelial alveolar type II (ATII) population that expresses acidic mammalian chitinase (AMCase). This allowed the further accumulation of environmental chitin. Then, ATII cells rapidly secrete IL-33 and TSLP, which synergistically act on ILC2s, and activated amphiregulin secretion in ILC2s. Amphiregulin, in turn, promotes the differentiation of alveolar epithelial progenitor cells into ATII cells that express AMCase ( Fig. 2A ) ( 46 ). AREG, amphiregulin; β2AR, β2-adrenergic receptor. The restorative functions of ILC2 are also partially dependent on type 2 cytokines, which are classically known to contribute to allergic inflammation. Expression of the IL-9 receptor on ILC2 creates a positive feedback enhancing type 2 cytokine expression, which eventually drives eosinophilia and mucus production and tissue repair during parasite infection ( Fig. 2B ) ( 47 48 ). Additionally, recent studies have shown that non-canonical factors can induce pulmonary ILC2 activation. For example, neuropeptides, such as neuromedin U (NmU) and calcitonin gene-related peptide (CGRP), and norepinephrine are known to be involved in ILC2 activation ( Fig. 2C ) ( 14 49 50 ). Enteric neuron-derived NmU, along with alarmins, synergistically induces ILC2 activation, and CGRP and norepinephrine-mediated β2-adrenergic receptor signaling negatively regulates pulmonary ILC2 activation. Inter-organ migration is another mechanism through which pulmonary ILC2s are replenished. Pu et al. ( 51 ) demonstrated that gut microbiota, particularly Proteobacteria taxa, regulate the migration of IL-33-responsive ILC2s from the gut to the lung. Proteobacteria promote IL-33 production, which in turn, activates the IL-33-responsive ILC2 population, enhancing proliferation and migration ( Fig. 2D ). CXCL16 is another factor that is involved in ILC2 migration, and antibiotic treatment increases CXCL16 expression, suggesting a second way that microbiome-mediated microenvironment manipulation contributes to the trafficking of ILC2s between the tissues ( 51 ). More recently, Can et al. ( 52 ) suggested that ILC2-like progenitors are activated by IL-25 and quickly move from the intestine into the lung to aid early lung defense, especially during helminth infection. Furthermore, these circulating ILC2s were distinguishable from the IL-33-responsive tissue-resident ILC2s, found in homeostatic conditions, suggesting that inter-organ migration may contribute to heterogeneity in the lung ILC2 population ( Fig. 2E ). The gastrointestinal tract is a core organ of the digestive system. It consists of glands and epithelial cells that secrete digestive enzymes to degrade the food bolus and absorb nutrients. The epithelial barrier has a vast surface area and efficiently absorbs nutrients while also serving as the first line of defense against external pathogens. Furthermore, the intestine must also maintain mutualistic relationships with a diverse microbial community, requiring a complex immune performance balancing symbiont tolerance and defense. The gut-associated lymphoid tissue forms a sophisticated regulatory network to accomplish these complex tasks, with one of its core components being ILC2. Intestinal ILC2s, which have been well characterized through numerous studies, are known to be key orchestrators of type 2 immune responses, particularly those against parasitic (e.g., nematode) infections. When parasites damage the intestinal epithelium, sensory cells, such as tuft cells or epithelial cells, secrete alarmins, including IL-25 and IL-33 ( 9 ). These signals activate ILC2s, which immediately produce large amounts of type 2 cytokines. The subsequent response of the epithelium is influenced by key cytokines, such as IL-13 and IL-5. Specifically, IL-13 promotes goblet cell proliferation and mucus secretion, accelerates intestinal epithelial cell turnover, and enhances intestinal peristalsis ( 53 54 ). This induces a ”weep and sweep” response that physically removes parasites. In contrast, IL-5 recruits eosinophils to the infection site to attack the parasites directly ( 55 56 ). Thus, ILC2s establish a multifaceted defense system through multiple cytokines ( 57 ). In addition, IL-10, well known anti-inflammatory cytokine, is derived by ILC2s. IL-10 contributes to maintenance of intestinal homeostasis. IL-10 production by ILC2s is positively regulated by stimulation with IL-2, IL-4, IL-10, IL-27, and NmU, whereas it is negatively regulated by TL1A, a member of the TNF superfamily ( 58 ). Recent studies have shown that the nervous system directly regulates ILC2 function in the intestinal tract. For example, the neuropeptide NmU is secreted by enteric neurons and binds directly to receptors on intestinal ILC2s, strongly activating them ( 14 59 ). Conversely, other neuropeptides, such as CGRP, have been shown to inhibit intestinal ILC2 activity ( 49 60 ). Thus, the enteric nervous system acts as an “on/off switch” regulating ILC2 activity, forming an important biological link through which stress or emotional states can influence gut immune responses. Although the development of ILC2s is largely independent of the microbiome, it is known that the depletion of gut microbiota in mice induces expansion of intestinal ILC2s ( 61 ). However, in the context of activation, ILC2s are negatively regulated by microbiome depletion, as tuft cell-mediated IL-25 secretion decreased in the absence of protozoa-provided succinate ( 17 62 ). The transcriptional and epigenetic responses of ILC2s are also influenced by gut microbiota. Depletion of the microbiome via antibiotic treatments or germ-free housing altered responses in specific intestinal ILC2 subgroups ( 63 ). On the other hand, a recent study suggested that intestinal ILC2s are activated by mechanical stretching in the absence of a microbiome, and enhanced type 2 activation influenced distal adipose tissue, contributing to metabolic homeostasis ( 19 ). The mechanism through which intestinal ILC2s influence the distal adipose tissue is unclear, but there is growing evidence that intestinal ILC2s can migrate to other organs and influence immune responses remotely ( 52 64 ). Specifically, when activated by certain stimuli, intestinal ILC2s have been reported to migrate to the lungs via the blood, where they contributed to allergic inflammation. The idea that immune responses originating in the gut can influence the immune status of the entire body using the circulatory system as a “highway” is an important new paradigm in ILC2 research. In mammals; both murine and human, adipose tissues are classified as white adipose tissue (WAT) or brown adipose tissue. WAT primarily contributes to metabolic homeostasis by serving as a major energy reservoir in which triglycerides are stored and mobilized in response to hormonal cues, such as insulin and leptin increases. Additionally, WAT functions as an endocrine organ, secreting adipokines that regulate systemic metabolism and energy balance. In contrast, brown adipose tissue is specialized for use in thermoregulatory homeostasis, generating heat through mitochondrial uncoupling in brown adipocytes, i.e., uncoupling protein 1 (UCP1)-dependent thermogenesis ( 65 ). In this regard, immune cells in the adipose tissue, such as macrophages, eosinophils, neutrophils, and Tregs, are known to interact with stromal cells to regulate adipocyte differentiation and inflammation, thereby contributing to the maintenance of local and systemic homeostasis. Among these immune cells, both murine and human adipose ILC2s, defined as lineage-marker − /c-KIT + /stem cell antigen-1 (Sca-1) + cells, have been identified as key regulators of adipose tissue physiology due to their ability to promote anti-inflammatory responses, interact with Tregs, and induce preadipocyte differentiation factors ( 65 66 ). Adipose ILC2 activation is dependent on IL-33 produced by adipose stromal cells, and once activated, ILC2s secrete type 2 cytokines, forming a circuit for regulating metabolic homeostasis ( 67 68 ). In murine WAT, local multipotent stromal cells (MSCs) support ILC2 growth through ICAM-1/LFA-1 signaling ( Fig. 3A ). These MSCs also respond to ILC2-derived IL-4 and IL-13 by producing CCL11 (eotaxin), which attracts eosinophils and helps maintain type 2 immunity ( Fig. 3E ) ( 69 ). Along with the IL-4-mediated secretion of eosinophils, IL-5 boosts eosinophil survival factor, promoting the development of beige fat cells from platelet-derived growth factor receptor-alpha (PDGFRα) + adipocyte precursors and stimulating IL-33–producing stromal cells ( 68 70 71 72 ). Additionally, IL-13 drives the differentiation of murine preadipocytes into metabolically active beige adipocytes. This process is mediated through the STAT6–p38–PPARγ–PGC1α signaling axis, enhancing both the expression and transcriptional activity of PPARγ in preadipocytes. As a result, mitochondrial oxidative metabolism and thermogenic gene (e.g., Ucp1 and Oxphos ) expression are upregulated, contributing to systemic metabolic homeostasis and thermogenesis ( Fig. 3D ) ( 73 ). Moreover, ILC2s also secrete methionine-enkephalin peptides that directly trigger UCP1 expression in adipocytes, promoting the beiging of murine WAT and increasing fatty acid oxidization, which may help resist weight gain ( Fig. 3D ) ( 67 ). Furthermore, Yuan et al. ( 74 ) found that peripheral NmU administration stimulates ILC2s within the stromal vascular fraction of murine subcutaneous WAT, thereby enhancing thermogenic gene expression and promoting WAT beiging ( Fig. 3B ). SVF, stromal-vascular fraction; MetEnk, methionine-enkephalin peptides. Beyond their role in shaping the tissue environment, ILC2s are also directly involved in fat-related processes. For instance, in murine visceral fat, ILC2s can curb the absorption of saturated fatty acids by downregulating CD36 expression while boosting UCP1 expression, leading to better glucose control and higher energy usage ( Fig. 3G ) ( 75 ). Sun et al. ( 76 ) demonstrated that LKB1 signaling in ILC2 plays important roles in metabolic homeostasis. Under normal conditions, LKB1 signaling is linked to mitochondrial fitness and effector function, but LKB1 signaling is impaired under conditions associated with obesity, which sequentially induce PD-1 expression through the activation of NFAT, which, in turn, induces mitophagies, consequently suppressing ILC2 function and insulin sensitivity. A similar issue occurs during obesity-associated chronic inflammation, where ILC2s become less effective due to an increase in PD-1 signaling and a reduction in the expression of fatty acid-binding protein 5, an important molecule affecting fatty acid metabolism ( 77 ). Interestingly, several studies have reported interactions between ILC2s and other immune cells within adipose tissues ( 66 70 78 ). Upon cold exposure, subcutaneous WAT ILC2s and eosinophil promote the conversion of pro-inflammatory CCR2 + M1 macrophages into anti-inflammatory M2-like macrophages, thereby contributing to adipose tissue beiging and glucose homeostasis ( Fig. 3E and G ). Another study reported that activation of ILC2s in visceral adipose tissue induces ICOSL expression, which increases interactions with Treg-expressed ICOS, thereby inducing Treg cell accumulation and, thus, contributing to tissue and metabolic homeostasis ( Fig. 3F ) ( 66 ). At the same time, IFN-γ additions suppressed this pathway during inflammatory perturbations. A recent study showed that the nervous system also influences adipose tissue ILC2 behavior. Sympathetic neurons stimulate PDGFRα + stromal cells to release glial cell line-derived neurotrophic factor, which enhances ILC2-driven thermogenesis through RET receptor signaling ( Fig. 3C ). When this signaling breaks down, it impairs ILC2 function and reduces thermogenic gene expression and UCP1 levels. Ultimately, this drives the risk of obesity up ( 79 ). Taken together, ILC2s, known to contribute to allergic inflammation in many tissues, act as major players in the maintenance of metabolic homeostasis in adipose tissue ( Fig. 3 ). Skin is the first line of protection against many pathogens and harmful environmental factors. It consists of complex epidermal layers, which contain resident immune cells that interact with neighboring stromal cells. Recent studies have identified skin-resident ILC populations and their distinct characteristics. While mouse ILC2s in the lungs and intestines are primarily activated by IL-25 and IL-33, TSLP and IL-18 are key cytokines that activate ILC2s in the skin, which confirmed by diminished type 2 responses in atopic dermatitis model with TSLP and IL-18 deficiencies ( 42 ), suggesting the pivotal role of those cytokines in skin ILC2 activation. In human skin, the ILC population is reported to consist mainly of type 3 ILCs (−50%) and ILC2s (~25%–40%) ( 80 81 82 ). Originally, ILC2s were known to exist in the lungs and intestines, where they primarily trigger allergic reactions. However, with the advancement of flow cytometry technology, researchers have clearly demonstrated that ILC2s are also present in both mouse and human skin tissues ( 83 ). This discovery has led to investigations into the pathological roles of ILC2s. Researchers have confirmed that the number of ILC2 cells is significantly higher in the skin lesions of patients with atopic dermatitis and that skin ILC2s play a key role in the onset of the disease’s symptoms. In contrast with their pathological roles, Rak et al. ( 84 ) reported that skin ILC2s play significant positive roles in tissue repair and regeneration. When a wound occurs, IL-33 is secreted by damaged keratinocytes, strongly activating skin ILC2s, which secrete type 2 cytokines like IL-5 and IL-13. While these cytokines exacerbate inflammation, at wound sites, they promote re-epithelialization and collagen deposition, thereby aiding in the physical recovery of the skin. Additionally, the tissue-protective function of ILC2s plays a crucial role in hair follicle homeostasis. A recent study revealed that IL-13, which is secreted by ILC2s, directly regulates hair follicle stem cells, controlling the number of demodex mites, which localize within hair follicles, and suppresses excessive inflammatory responses ( 85 ). Thus, ILC2s are essential not only in responses to external injuries but also in the regulation of relationships with microorganisms and the maintenance of the structural and functional stability of tissues. The activation of skin ILC2s is primarily induced through cytokine pathways ( 42 83 84 ), however, a recent study suggests that skin ILC2s react to changes in external temperature and are involved in regulating body temperature homeostasis ( 86 ). This intriguing finding suggests that, beyond simply regulating immune responses, ILC2s may contribute directly to maintaining the physiological homeostasis of the skin. Additionally, ILC2s have been shown to play a significant role in the sex bias observed in skin immune responses. According to a recent study, androgens directly suppress the number and function of ILC2s via the androgen receptors present in ILC2s ( 87 ). This reduction ultimately decreases the production of GM-CSF, thereby reducing the number of antigen-presenting cells, such as dendritic cells (DCs) and Langerhans cells. This is emerging as an important mechanism explaining sex-based differences in the incidence and characteristics of skin diseases. Taken together, the role of ILC2s in the skin is not limited to allergic inflammation; they also contribute to biological homeostasis in barrier tissues. Unlike other barrier tissues, such as those found in the lung, intestine, and skin, the role of ILC2s in the female reproductive tract, especially the uterus and cervix, has not been as actively studied. The uterus is also directly connected to the outside environment, exposing it to external stimuli. In this context, uterus ILC2s may act as an important regulator of tissue remodeling, maintaining epithelial function, and creating an anti-inflammatory environment. Studies have reported that the number of ILC2s in the mouse uterus increases in an IL-33-dependent manner ( 88 89 90 ). Additionally, Valero-Pacheco et al. ( 91 ) confirmed that myometrial fibroblasts and decidual stromal and endothelial cells are the major source of IL-33 at the maternal–fetal interface during the post-implantation phase of early pregnancy in mice, and IL-33-deficient mice exhibited abnormalities, such as defective implantation cavity formation and decidualization and deficient spiral artery formation, which occurred in early pregnancy, during the remodeling of uterine tissues. Meanwhile, another study showed that ILC2-deficient mice exhibited decreased placental efficiency, resulting in fetal growth retardation and, as a compensatory mechanism, increases in the expression of nutrient transporter genes, such as Slc2a1 (Glut1), Slc2a3 (Glut3), and Slc38a2 (Snat2), in the placenta ( 88 ). In addition, ILC2-deficient mice could not suppress LPS-mediated abortion, suggesting the ILC2 cell population plays a crucial role in maintaining the homeostasis of the uterine immune environment during pregnancy and preventing miscarriages caused by fetal growth and infection. Bartemes et al. ( 90 ) reported that the ILC2 cell population in the murine uterine is directly regulated by sex hormones. Unlike the ILC2s characterized in lung tissue, ILC2s in the uterus exhibit low CD278 (ICOS) and Ly6A (Sca-1) surface expression and high estrogen receptor alpha expression, suggesting that E2 (17β-estradiol), an active form of estrogen, contributes to the maintenance of the uterus’s ILC2 phenotype in a tissue-specific manner ( 90 92 ). On the other hand, androgen, a male hormone, was confirmed to inhibit ILC2 differentiation through the androgen receptor, allowing cells to remain in the ILC2 progenitor cell stage in the bone marrow, and to inhibit STAT5 phosphorylation, GATA3·RORα expression, and IL-5 and IL-13 production in mature ILC2s, inhibiting their function ( 93 94 ). In addition, Zhao et al. ( 95 ) reported that, in humans, as estrogen and progesterone levels increase during late pregnancy, progesterone receptor and estrogen receptor α expression increased in ILC2s, and the ratio and absolute numbers of circulating ILC2s in peripheral blood significantly increased. Aspects of fetal growth and pregnancy maintenance also appear to be influenced by ILC2s. A recent study showed that uterine IL-33-activated ILC2s support on-time myometrium contraction and parturition that induce the expression of the Ptgs2 , the gene encoding cyclooxygenase-2 in the decidua, enhancing prostaglandin E2 synthesis ( 96 ). In the context of disease, 2 studies have reported altered human ILC2 patterns during endometriosis, a chronic inflammatory condition characterized by ectopic proliferation in the endometrium ( 97 98 ). However, the patterns of ILC2 alterations reported in these studies remain somewhat controversial. Specifically, Miller et al. ( 97 ) reported elevated levels of IL-33 in the plasma, peritoneal fluid, and endometriotic lesions of patients with endometriosis, accompanied by increased ILC2 frequencies in the peritoneal fluid. In contrast, Sugahara et al. ( 98 ) found no significant changes in ILC2 frequencies in either the peripheral blood or peritoneal fluid; rather, they observed reductions of ILC2s in the endometria of patients with endometriosis. Interestingly, increases in ILC2s were observed only in the lesion tissues of ovarian endometriomas. This suggests that the direction of ILC2 alterations in endometriosis may be site dependent. Overall, uterus-specific ILC2s appear to contribute to timely tissue remodeling, support fetal growth, and protect from external pathogens in the uterus across multiple pregnancy events. The CNS considered as a tissue with immune privilege. However, emerging studies have shown that innate and adaptive immune cells are housed in the CNS system and that the dual functions of glial cells include antigen presentation. Additionally, ILC2s have been detected in the CNS ( 99 ), and recent studies suggest that they play significant roles in brain homeostasis and in pathological conditions. In the healthy CNS, meningeal ILC2s are predominantly resident in the dural sinus and rarely encountered in the dura mater and spinal cord meninges. However, CNS injuries, such as spinal cord injuries, induce IL-33 secretion by glial cells, which initiate an inflammatory cascade. Activates ILC2s, which then migrate to the injury site. This reconstitution of ILC2s by transfer produced beneficial effects in an spinal cord injury animal model, suggesting a neuroprotective potential of ILC2s in the CNS ( 23 ). Another study suggested that ILC2s promote the proliferation of neural stem and progenitor cells and enhance neurorepair following brain injury. Following ischemic cerebral damage, ILC2s accumulated in the lesion core and the subventricular zone, where the brain stem cells reside. Increased IL-33 during the acute stage of middle cerebral artery occlusion animal model induces ILC2 proliferation, activation, and amphiregulin production, stimulating epidermal growth factor receptor signaling in the neural stem and progenitor cells. Mice deficient in ILC2s showed impaired post-stroke neurogenesis, with impaired cognitive function and toxic lipid metabolism, as well as a reduction in the adoptive transfer of ILC2s in response to these issues. This suggests that ILC2s are a promising target for therapeutic strategies ( 24 ). ILC2s also have major functions in proper CNS development. During postnatal development, there are more ILCs than adaptive lymphocytes in the CNS, and meningeal ILC2s constitute a significant portion of these. Interestingly, while IL-33 is enriched in meningeal fibroblasts, meningeal ILC2s are not entirely dependent on IL-33. The ILC2s produce IL-13 and IL-5 in relatively high abundance during the P5–P22 stages. Additionally, IL-13 is predominantly produced by ILC2s, with only minimal levels observed in CD4 + T cells. Moreover, ILC2-derived IL-13 was found to be directly involved in the development of inhibitory synapses via presynaptic mechanisms in mice and to promote social interaction ( 25 ). Corroborating this, Steffen et al. ( 100 ) also found that ILC2 deficiencies lead to imbalanced excitatory/inhibitory signaling, affecting learning and memory. Specifically, ILC2-deficient mice exhibited impaired learning abilities—including those related to social interaction, spatial learning, retention—and significantly decreased memory. Aging is another critical factor that influences ILC2 characteristics. In the brains of older mice, ILC2s accumulate in the choroid plexus ( 101 ). These ILC2s are long-lived and exist as tissue-resident manner than the younger brain. Additionally, they are more resistant to senescence and exhaustion and exhibit an enhanced self-renewal capacity. Upon activation, choroid plexus ILC2s release high levels of IL-5 and IL-13, which contributing to the increased neurogenesis of dentate gyrus, and it decreases neuroinflammation by suppressing TNF-α production from CD8 + T cells and microglia. Choroid plexus ILC2s also demonstrate greater proliferation rates and cytokine production than meningeal ILC2s, thereby enhancing neural survival, neurogenesis, and lymphatic drainage ( 101 ). Together, recent research has highlighted the critical roles of ILC2s in the CNS. However, the exact role of ILC2s in the human nervous system remains unclear, and further investigation will be required to reveal how they relate to the human CNS. The liver is a highly dynamic, multifunctional organ involved in metabolism, detoxification, digestion, immune regulation, and nutrient storage ( 102 ). These diverse functions are carried out by hepatocytes (parenchymal cells) and supported by non-parenchymal cells, such as liver sinusoidal endothelial cells, hepatic stellate cells, cholangiocytes, and Kupffer cells ( 103 ). These stromal and epithelial components constitute a complex microenvironment essential for hepatic homeostasis. Unlike other organs, a distinguishing feature of the liver is that it is composed of regenerative tissues. Reprogrammed hepatocytes have been reported to acquire proliferative potential, producing both parenchymal cells and non-parenchymal cells, including biliary epithelial cells and hepatic progenitor cells, which allows the restoration of damaged liver tissue ( 104 ). Recent studies have identified liver-resident ILC2s as active modulators of hepatic tissue repair and metabolic regulation, exerting their influence not only through immune pathways but also via direct interactions with stromal cells under both physiological and pathological conditions ( 105 106 107 108 109 110 ). Following hepatocyte stress or injury, the alarmin IL-33 is released from liver tissue and engages the receptor ST2 (IL1RL1, suppressor of tumorigenicity 2) on liver-resident ILC2s, driving their perivascular accumulation and an activated GATA3 + KLRG1 + phenotype that exhibits robust IL-4, IL-5, IL-6, and IL-13 secretion. The production of IFN-γ from Th1 cells counter-balances this program, whereas IL-25 can skew a subset of ILC2s toward IL-17A production, thereby fine-tuning the functional spectrum of hepatic ILC2s ( 105 107 111 ). Steinmann et al. ( 107 ) reported that IFN-γ secreted by Th1 cells reduces the survival and cytokine output of hepatic ILC2s, acting as a brake on IL-33-driven activation. Conversely, IL-33-activated ILC2s induced the differentiation of neighboring CD4 + T cells into Th2 cells, which subsequently produced IL-2, which, in turn, further boosted ILC2 survival and IL-5, IL-13, and IL-6 secretion, reinforcing a type 2 immune-dominant environment. However, during chronic inflammation, IL-25 reshaped a fraction of the ILC2 pool into IL-17A-producing “inflammatory ILC2s,” introducing a distinct pro-inflammatory aspect to the liver microenvironment ( 107 ). In murine models of acute injury, such as ischemia/reperfusion, IL-13-driven M2 macrophage polarization and IL-5-dependent eosinophil recruitment preserves hepatocyte viability and promotes tissue repair ( 110 ). Additionally, Cao et al. ( 110 ) reported that the adoptive transfer of ex vivo -expanded ILC2s reproduces these benefits, whereas IL-13-deficient ILC2s do not. Furthermore, migration has been shown to diversify the impacts of ILC2s. Intranasal IL-33 recruited lung-derived ILC2s to the liver, where they expanded; produced IL-5, IL-13, IL-6, and amphiregulin; up-regulated fibrosis-related genes; and increased the proportion of CD206 + M2 macrophages ( 112 ), while these effects were blunted in ILC2-deficient mice. Under the persistent IL-33 release associated with toxic (CCl 4 ) or bile-duct ligation injury, CD69 + ILC2s accumulate and drive collagen deposition by activating hepatic stellate cells through the IL-4Rα/STAT6 and MAPK (JNK/ERK/p38) pathways ( 113 114 ). Tan et al. ( 114 ) showed that JNK, ERK, or p38 inhibitors block this IL-33-induced collagen secretion, and that ST2 is functionally involved in the IL-33 signaling in hepatic stellate cells. Similarly, human cirrhotic livers exhibit increased intrahepatic ILC2 frequencies, which positively correlate with both Model for End-Stage Liver Disease and METAVIR fibrotic scores as well as circulating IL-33 levels ( 106 115 ). Fujimoto et al. ( 109 ) reported that ILC2-derived IL-13 activates hepatocyte STAT3 signaling; down-regulates the gluconeogenic genes G6pc , Pck1 , and Hnf4a ; and lowers fasting glycemia in lean and obese mice, using the IL-33–ILC2–IL-13 axis as an immunometabolic circuit to enhance not only tissue repair but also blood glucose control. In autoimmune cholangiopathies, human peripheral ILC2 dynamics shift: Li et al. ( 116 ) demonstrated a decrease in circulating ILC2s but an increase in ILCPs and IL-17A + ILCs in patients with primary biliary cholangitis, with the ILCP/ILC2 ratio positively correlating with the levels of liver-injury marker compounds, such as alanine phosphatase and γ-glutamyl transferase. Moreover, CCR6 + ILC2s, which express the integrins VLA-5 and VLA-6, migrate to CCL20-rich portal tracts, where they secrete IL-13 and amphiregulin to support biliary regeneration ( 115 ). Conversely, hepatocellular carcinoma results in the loss of E-cadherin-KLRG1 signaling, which unleashes a KLRG1 − ILC2 subset that produces CXCL2 and IL-13, recruits arginase 1 + neutrophils, suppresses CD8 + T cells, and fosters tumor progression ( 108 ). In addition, Xu et al. ( 108 ) identified this neutrophil-centered immunosuppressive axis as a driver of poor prognosis. Thus, hepatic ILC2s display striking functional plasticity, playing protective roles in the face of acute injury and glucose dysregulation, while playing pathogenic roles in chronic fibrosis, autoimmune cholangiopathy, and cancer. Collectively, these findings depict liver-resident ILC2s as a double-edged sword—they orchestrate anti-inflammatory repair and glucose homeostasis in acute settings yet drive fibrosis or immunosuppression under chronic inflammatory or oncogenic pressure. Pancreatic ILC2, especially islet ILC2s, highly express ST2, which is stimulated by islet mesenchymal cell-secreted IL-33 ( 26 ). Under normal conditions, activated pancreatic ILC2s express IL-13 and GM-CSF, which elicit retinoic acid production by macrophages and DCs. Eventually, increased retinoic acid levels stimulate islet beta cells to express insulin. In the context of pathology, upregulation of the IL-33–ILC2 axis by exogenous IL-33 treatments improves glucose clearance and fasting glycemia in obese mice and streptozotocin-induced diabetic mice, respectively. These results suggest that the IL-33–ILC2 axis contributes to homeostasis in the pancreas. Recent work has uncovered a novel neuroimmune–endocrine circuit through which intestinal ILC2s regulate systemic glucose homeostasis ( 117 ). During fasting, adrenergic signals from sympathetic neurons reduce gut-homing receptor expression on ILC2s, enabling their migration from the intestine to the pancreas through mesenteric lymph nodes. Once in the pancreas, these ILC2s secrete IL-5 and IL-13, which directly stimulate pancreatic alpha cells to release glucagon. This promotes hepatic gluconeogenesis, maintaining blood glucose levels during nutrient deprivation. This study integrating neuronal and immune signals to coordinate inter-organ metabolic responses highlights the versatility of ILC2s. By linking gut-derived ILC2 migration to the pancreas, where the ILC2s serve an endocrine function, this neuroimmune circuit exemplifies how ILC2s can contribute to systemic physiology in ways that greatly upstage their established roles in allergic inflammation in peripheral tissues. The role of pancreatic ILC2s in cancer is controversial, with ILC2s exhibiting both tumor-promoting and tumor-restraining activities depending on the context. For example, under pancreatic cancer-associated conditions, IL-33–activated ILC2s have been shown to induce tertiary lymphoid structures via lymphotoxin-mediated interactions with LTβR + myeloid cells, enhancing antitumor immunity and improving survival outcomes. On the other hand, ILC2s may have immunosuppressive capacity in triple-positive pancreatic ductal adenocarcinoma under hypoxia conditions ( 27 118 ). In this study, ILC2s upregulated IL-10 and checkpoint molecules, such as CTLA-4 and CD25, acquiring an immunosuppressive profile that facilitated immune evasion. Recently, ILC2s have emerged as a key modulator of CVDs. During CVD progression, the role of ILC2s depends on the environment ( 119 ). Arteriosclerosis is a major cause of CVDs. In an arteriosclerosis mouse model, ILC2s with inflammatory phenotypes (ST2 low KLRG1 hi ) associated with low IL-5 and IL-13 secretion, were present in the aortic lymph nodes and para-aortic adipose tissue. In the absence of these ILC2 subtypes, lipid accumulation in the aortic arch and aortic sinus was significantly lower, resulting in larger but more vulnerable plaques due to the decreased collagen deposition. A similar result was observed with a deficiency of ILC2-derived IL-13, which indicates that ILC2s are directly involved in the stability of arteriosclerosis plaques via IL-13. Recent studies suggest that ILC2 plays a protective role in another type of CVD, abdominal aortic aneurysm (AAA). In an AAA animal model, increased ILC2 numbers were identified in aortic lymph nodes. These ILC2s were found to directly prevent the apoptosis of smooth muscle cells, one of the main causes of AAA formation. Additionally, ILC2s also promoted smooth muscle cell activation and proliferation. The protective role of ILC2s is also related to eosinophils. ILC2s promote eosinophil generation and activation via IL-5. Eosinophils also promote smooth muscle cell proliferation and protect them from NF-κB activation, which is known to upregulate adhesion molecule expression, which is required for inflammatory cell accumulation ( 120 121 ). By suppressing the expression of ICAM-1 and VCAM-1, eosinophils stabilize endothelial cells and regulate the expansion of AAA. Thus, ILC2s protect smooth muscle cells both directly and indirectly (via eosinophils), thereby alleviating AAA pathogenesis. Eosinophil-mediated ILC2 function also provides protection in post myocardial infarction (MI). After an MI, the recruitment of Inflammatory cells to the heart has a crucial impact on cardiac remodeling and heart failure. The accumulation of ILC2s in the post-MI heart has been observed, and a lack of ILC2 accumulation has worsened the heart’s condition during recovery. This protective role acts indirectly through eosinophils: ILC2-derived IL-5 promotes eosinophil generation, which secretes IL-4 and the cationic protein mEAR1, protecting cardiomyocytes, and induces TGF-β production, suppressing p-Smad2/3 signaling and collagen production. Overall, this mechanism decreases myocardial fibrosis and cardiomyocyte apoptosis, contributing to cardiac function recovery ( 121 ). Interestingly, the protective function of ILC2s is reversed by localization. In the pericardium, IL-33-activated ILC2s can trigger cardiac inflammation, leading to eosinophilic pericarditis. Cardiac ILC2s produce IL-5, which promotes the infiltration of eosinophils into the heart, and this can contribute to disease severity. Cardiac ILC2s also stimulate cardiac fibroblasts to produce eotaxin-1, a key chemokine that guides eosinophils to the heart ( 122 ). Taken together, research suggests that cardiac ILC2s have ambivalent roles in the heart that depend on the microenvironment. Unlike in mouse models, humans ILC2s are known to be circulating cells rather than tissue-resident immune cells. While mouse ILC2s express tissue-specific alarmin receptors, human ILC2s typically express markers such as CD127 and CD161, but exhibit heterogeneity in the expression of other markers, including CRTH2, CD5, KLRG1, CD45RO, CD117, and IL-10. However, in terms of function, human ILC2s are resemble to mouse ILC2s since it also secrete cytokines and play a role in type 2 immune responses ( Table 2 ) ( 41 ). ISC, intestinal stem cell; AT, adipose tissue; BMI, body mass index; GDNF, glial cell line-derived neurotrophic factor; AD, atopic dermatitis; FRT, female reproductive tract; MELD, Model for End-Stage Liver Disease; HSC, hepatic stellate cell; PBC, primary biliary cholangitis; AREG, amphiregulin; EGFR, epidermal growth factor receptor; PDAC, pancreatic ductal adenocarcinoma; TSL, tertiary lymphoid structure. Accordingly, several reports have described human ILC2 subsets and highlighted their potential for clinical application ( 123 124 ). Reid et al. ( 123 ) reported that ex vivo expanded human IL-10 + ILC2s suppress donor-derived pathogenic Th1 and Tc1 alloresponses through IL-4 and IL-10 dependent mechanisms, thereby alleviating graft-versus-host disease while preserving graft-versus-leukemia activity. This study identifies IL-10 + ILC2s as a regulatory subset with therapeutic potential for the selective control of alloreactive T cell–driven inflammation. Beyond their well-established regulatory and tissue-repair functions, human ILC2s can undergo cytotoxic conversion under specific inflammatory and tumor associated conditions. These include IL-2 and IL-33 stimulation, retinoic acid driven transcriptional reprogramming, and engagement with DNAM-1 ligands such as CD155 within the tumor microenvironment. Under these cues, ILC2s can be expanded and directly kill the malignant cells through granzyme B mediated pyroptosis and apoptosis. This finding suggest DNAM-1 ligand positive cancers as promising targets for adoptive ILC2 therapy ( 124 ). Taken together, these findings unveil the therapeutic potential of ILC2s as targetable effector cells and highlight the need for tissue context aware, ILC2-targeted strategies that account for inter-organ heterogeneity.

In the peripheral tissues, tissue-resident immune cells communicate with neighboring, or even distant, cells. Among these cells, ILC2s have traditionally been particularly associated with allergic immune responses, but emerging studies have significantly expanded our understanding of their roles. In this review, we highlighted the biological functions of ILC2s in multiple peripheral tissue types. With the development of new technologies that can shed light deep into the peripheral tissue, we have come to recognize that all types of immune cells, including ILC2s, reside in every tissue. Peripheral ILC2s interact with multiple tissue stromal cells, managing biological functions under tonic and pathological conditions. In addition, multiple studies suggested that this interaction between ILC2s and stromal cells may start at an early stage of development, and it would be a critical for formation of heterogeneity in the ILC2 population ( 41 125 126 ). Tissue-resident ILC2s have distinct patterns of receptor expression for the activation, however, there is a research gap in understanding the exact mechanism behind the distribution of ILC2 progenitors from yolk sac, fetal liver, and bone marrow to peripheral tissues. Further studies to shed light on these unsolved questions are required. Matured tissue-resident ILC2s maintain tissue-specific heterogeneity, which eventually influences the function of ILC2s in the tissues. Therefore, the clear identification of the ILC2 population and its microenvironment is critical when interpreting the outcome of ILC2–stromal cell crosstalk. Tissue perturbations such as infections influence the microenvironment, and induces trafficking of tissue ILC2s ( 16 64 127 ), suggesting certain types of perturbations occurring during growth and physiological changes also has the potential to drive tissue heterogeneity. Recent studies have shown that ILC2s in peripheral tissues, such as those of the CNS, muscles, and uterus, contribute to diverse biological functions. However, as most of these findings are derived from mouse models and notable species differences in ILC2 populations have been reported, further research is required to address their relevance in humans. Given the limitations of conducting human studies, alternative approaches such as the human organoid systems, and single-cell multi-omics analyses could be employed to better characterize ILC2 functions and their microenvironmental interactions in human.