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ILC2

Type 2 innate lymphoid cells (ILC2s) are a subset of (ILCs) that lack rearranged antigen-specific receptors and mirror the functions of + Th2 cells by rapidly producing type 2 cytokines, including (IL)-5, IL-9, and IL-13, upon activation by epithelial-derived alarmins such as IL-25, IL-33, and (TSLP). These cells are tissue-resident sentinels primarily located in mucosal barriers like the lungs, intestines, and , where they express the transcription factor GATA3 and respond to environmental cues to initiate early immune responses. ILC2s develop from common lymphoid progenitors through a series of committed precursors, including common helper-like innate lymphoid precursors (CHILPs), with GATA3 being essential for their lineage specification and maintenance. They are identified by surface markers such as CD127 (IL-7 receptor α chain), CD25 ( α chain), and lack of lineage-specific markers (-), distinguishing them from adaptive lymphocytes. Unlike T cells, ILC2s provide immediate effector functions without prior , producing cytokines at levels up to 10-fold higher per cell than Th2 cells. Recent studies highlight their plasticity, including the ability to transdifferentiate into regulatory subsets that produce IL-10 to dampen . In immune responses, ILC2s are pivotal for type 2 immunity, driving recruitment, hyperplasia, and production to expel helminth parasites, as seen in infections like Nippostrongylus brasiliensis. They also secrete (AREG) to promote epithelial repair and tissue remodeling in contexts such as in the skin, lung, and intestine. However, dysregulated ILC2 activity contributes to allergic diseases, including , where IL-5 and IL-13 exacerbate airway inflammation and fibrosis. ILC2s interact dynamically with their microenvironment, including epithelial cells that release activating signals, neurons via neuromediators like (VIP), and other immune cells such as and T cells, forming regulatory circuits that balance protection and pathology.

Overview

Definition

Innate lymphoid cells type 2 (ILC2s) are a specialized subset of (ILCs), which constitute a family of immune cells that lack antigen-specific receptors characteristic of the . Unlike lymphocytes of the adaptive branch, such as T cells, ILC2s do not undergo rearrangement for receptor diversity and instead rely on germline-encoded receptors to sense environmental cues. These cells are primarily enriched in mucosal and barrier tissues, where they contribute to early immune surveillance and maintenance of tissue homeostasis. A key distinguishing feature of ILC2s from T helper 2 (Th2) cells is their ability to mount rapid responses to innate signals, such as alarmins released during epithelial damage or , without requiring prior antigen sensitization or clonal expansion. This innate activation allows ILC2s to initiate immune reactions within hours, providing an immediate bridge to adaptive immunity. In contrast, Th2 cells depend on recognition and for activation, which delays their response. The primary function of ILC2s lies in orchestrating type 2 immune responses through the secretion of signature cytokines, which drive anti-helminth defense, allergic inflammation, and tissue repair processes. These responses are essential for host protection against multicellular parasites and for promoting and metabolic in barrier sites. ILC2s exhibit evolutionary conservation across mammalian species, with orthologous populations identified in diverse vertebrates, underscoring their fundamental role in innate immunity.

Discovery

The discovery of group 2 innate lymphoid cells (ILC2s) occurred in 2010 through independent studies that identified a novel population of lineage-negative lymphoid cells capable of rapidly producing type 2 cytokines in response to epithelial-derived alarmins. In one seminal report, researchers described these cells in mouse as "natural helper cells," which responded to interleukin-25 (IL-25) and IL-33 by secreting IL-5 and IL-13, highlighting their role in innate type 2 immunity independent of adaptive responses. Concurrently, another group identified a similar IL-25-responsive population in the lungs of mice during helminth infection, termed "nuocytes," which served as a major early source of IL-13 to drive expulsion of the parasite Nippostrongylus brasiliensis. These early descriptions built on prior recognition of other innate lymphoid cell subsets, such as ILC1s and ILC3s, but shifted attention to this type 2 cytokine-producing lineage, spurred by growing interest in allergic and anti-parasitic responses in the early . Additional studies around the same time reinforced the existence of these cells in mucosal tissues, further distinguishing them from T helper 2 cells by their lack of antigen-specific receptors and rapid activation profile. The initial nomenclature varied across reports—"natural helper cells," "nuocytes," and "innate lymphoid cells type 2"—reflecting the field's excitement and the need for consensus. Standardization came in 2013 with a proposal endorsed by the International Union of Immunological Societies (IUIS), which unified the terminology under "group 2 " or ILC2s, categorizing them alongside ILC1s and ILC3s based on their signature cytokines (IL-5 and IL-13 for ILC2s) and key (GATA3). This framework facilitated broader research integration, emphasizing ILC2s' contributions to barrier immunity and .

Characteristics

Cellular Markers

Innate lymphoid cells type 2 (ILC2s) are primarily identified through multicolor by their lineage-negative (Lin⁻) status, which excludes markers expressed on other hematopoietic lineages such as T cells (CD3ε⁻, CD5⁻), B cells (⁻, B220⁻), natural killer () cells (NK1.1⁻ or CD56⁻), and myeloid cells (CD11b⁻, CD11c⁻, Gr-1⁻, ⁻, ⁻). This Lin⁻ gate is typically combined with CD45⁺ expression to select for leukocytes, forming the foundational population for ILC2 isolation across tissues. Positive surface markers further define ILC2s within the Lin⁻ CD45⁺ compartment. In both and , CD127 (IL-7Rα)⁺ is a core marker essential for ILC development and survival. ILC2s are commonly identified as (Thy1)⁺ and ST2 (IL-33R)⁺, with activated populations additionally expressing KLRG1⁺ and often Sca-1⁺ or CD25⁺. In , ILC2s express CD127⁺ alongside CD161⁺ and ST2⁺, though expression is minimal or absent; CRTH2 serves as a distinctive marker for human ILC2s, co-regulated with type 2 production. These markers vary by tissue context, such as higher KLRG1 expression in ILC2s versus ST2 in lung ILC2s in . Intracellular transcription factors provide confirmatory of ILC2 . GATA3 acts as the master regulator, driving ILC2 differentiation and function, while RORα supports their maintenance and survival. Additional factors like PLZF and Bcl11b are expressed at high levels in ILC2s, distinguishing them from other ILC subsets, with low or absent RORγt. These profiles enable precise isolation but highlight species-specific nuances, such as reliance on CD90.2 in mice versus CD161 in humans for optimal gating.

Cytokine Profile

Innate lymphoid cells type 2 (ILC2s) are defined by their production of signature type 2 , including interleukin-5 (IL-5), interleukin-13 (IL-13), and interleukin-9 (IL-9). IL-5 primarily drives activation and recruitment, supporting anti-parasitic defenses and allergic responses. IL-13 contributes to , hypersecretion, and tissue repair processes at barrier sites. IL-9 functions to amplify ILC2 responses by promoting their proliferation and survival. ILC2 cytokine production is triggered by epithelial-derived alarmins such as IL-33, IL-25, and (TSLP), which are released in response to tissue damage or allergens. These signals act through specific receptors—ST2 for IL-33, IL-17RB for IL-25, and TSLPR for TSLP—to initiate rapid ILC2 activation and effector function. Additionally, lipid mediators like (PGD2) enhance ILC2 migration and cytokine secretion via the CRTH2 receptor, bridging innate and adaptive immunity in inflammatory contexts. Key regulatory pathways underpin ILC2 cytokine expression. IL-7 signaling, essential for ILC2 , activates the STAT5 to maintain cell survival and basal responsiveness. For IL-5 and IL-13, the master GATA3 directly controls their expression by binding promoter regions, ensuring coordinated type 2 responses upon alarmin stimulation. An important amplification mechanism involves IL-9, which engages through its receptor on ILC2s to enhance survival, proliferation, and further type 2 output, thereby sustaining responses during prolonged . This loop is particularly evident in phases following insult, where IL-9 reinforces ILC2 effector functions without requiring continuous external stimuli.

Development

Ontogeny

Innate lymphoid cells type 2 (ILC2s) originate from common lymphoid progenitors (CLPs) in the , which differentiate into early innate lymphoid progenitors (EILPs) characterized by expression of α4β7 . These EILPs further progress to common innate lymphoid progenitors (CILPs), also known as common helper innate lymphoid progenitors (CHILPs), which are Lin⁻ Id2⁺ IL-7Rα⁺ α4β7⁺ Flt3⁻ CD25⁻ s capable of giving rise to all helper-like ILC subsets, including ILC2s. Interleukin-7 (IL-7) signaling through the IL-7 receptor α (IL-7Rα, or CD127) is essential for the survival, proliferation, and of these progenitors into mature ILC2s, as IL-7Rα deficiency severely impairs helper ILC development. ILC2 occurs in distinct embryonic and adult phases. During embryogenesis, committed ILC progenitors arise primarily in the fetal liver starting around embryonic day 13.5 (E13.5) in mice, seeding peripheral tissues such as the , intestine, and to establish resident populations. In contrast, adult ILC2 pools are replenished by progenitors from the , with postnatal differentiation contributing to homeostasis, though some fetal-derived cells persist long-term in certain sites. Seminal studies between 2013 and 2015 confirmed a shared common for ILC2s with ILC1s and ILC3s. Yang et al. (2015) identified TCF-1⁺ EILPs in the as multipotent precursors with potential for all ILC lineages and natural killer s. Earlier, Klose et al. (2014) described the CHILP as an Id2-dependent downstream of CLPs that specifically generates helper ILCs, distinguishing them from conventional natural killer s. GATA3 acts as a key specifying ILC2 identity from these common progenitors. Recent studies as of 2025 have further elucidated roles of transcription factors like BACH2 in driving ILC2 development and function, with its ablation impairing lineage commitment.

Tissue-Specific Differentiation

ILC2s exhibit tissue-specific maturation and adaptation following their initial seeding into peripheral tissues, where local microenvironments imprint distinct transcriptional and functional profiles to optimize their roles in immunity and . This process involves integration of signals, modulation, and environmental cues, leading to heterogeneous ILC2 subsets tailored to organ-specific demands. In the lungs, ILC2s develop enhanced responsiveness to IL-33 through elevated expression of the receptor ST2 (IL1RL1), which enables swift activation and proliferation in response to alveolar epithelial injury or allergen exposure. This adaptation is driven by tissue-specific factors such as neuropilin-1 (Nrp1) and expression, which promote retention and survival in the pulmonary niche. Consequently, lung-resident ILC2s rapidly initiate type 2 responses, distinguishing them from circulating counterparts. In the gut, transcription factors such as (Ahr) suppress IL-33 receptor expression to sustain gut-specific ILC2 functions. In the skin, ILC2s exhibit enhanced IL-18 responsiveness and CD103 expression for barrier maintenance. These adaptations ensure ILC2s align with the unique microbial and structural demands of mucosal and cutaneous environments. ILC2s also demonstrate , with the capacity for into ILC1-like states under exposure to IL-12 and IFN-γ, marked by reduced GATA3 levels, increased T-bet expression, and acquisition of IFN-γ . This shift occurs in inflammatory contexts, such as chronic lung diseases, allowing ILC2s to pivot toward type 1 immunity when needed. Tissue-resident memory ILC2s form long-lived populations that persist in barrier sites, sustained primarily by IL-7 signaling, which supports survival and self-renewal through upregulation of anti-apoptotic factors. Local survival factors, including TGF-β1 in the , further reinforce this residency by stabilizing tissue-specific markers like Nrp1, enabling enduring immune surveillance without reliance on continuous progenitor influx.

Physiological Functions

Barrier Defense

ILC2s play a crucial role in maintaining epithelial barrier integrity at mucosal surfaces, such as the lungs and gut, by orchestrating type 2 immune responses that support tissue repair and homeostasis. These cells respond to environmental cues at barrier sites, producing cytokines that enhance epithelial defenses without necessarily involving adaptive immunity. Through their strategic positioning in tissues like the lung and intestinal lamina propria, ILC2s contribute to steady-state surveillance and rapid responses to perturbations, ensuring the structural and functional continuity of epithelial layers. A key mechanism involves IL-13 production by ILC2s, which drives hyperplasia and production in both the lungs and gut, thereby strengthening the mucociliary barrier against potential threats. In the airways, ILC2-derived IL-13 promotes metaplasia, increasing secretion to trap and clear particulates, while in the , it similarly induces expansion to bolster the layer that separates luminal contents from host tissues. ILC2s also coordinate with tuft cells through an IL-25/IL-13 feedback loop to facilitate epithelial repair following . Tuft cells or stressors and release IL-25, which activates nearby ILC2s to produce IL-13; this IL-13 in turn stimulates further tuft cell and goblet cell , amplifying the regenerative response at mucosal barriers. This circuit is particularly vital in the gut, where it promotes rapid turnover and fortification of the post-. In processes, ILC2s secrete , an family member, to activate fibroblasts and promote , thereby supporting tissue remodeling and barrier restoration. This function is evident in tissues, where from ILC2s aids recovery after viral-induced damage by enhancing deposition and vascularization without excessive inflammation. Additionally, during steady-state conditions, ILC2s maintain low-level IL-5 production to sustain populations in tissues, which contribute to baseline barrier surveillance and through anti-microbial activities and tissue remodeling support.

Adipose Tissue Homeostasis

Innate lymphoid cells type 2 (ILC2s) were first identified in the visceral adipose tissue (VAT) of mice in 2010, where they reside as a distinct population responsible for maintaining local type 2 immune responses. These cells produce interleukin-5 (IL-5) and interleukin-13 (IL-13), which are essential for sustaining eosinophils and alternatively activated macrophages (AAMs, also known as M2 macrophages) in VAT. Specifically, IL-5 drives eosinophil recruitment and accumulation, while IL-13 promotes AAM polarization; depletion of ILC2s leads to a 12- to 14-fold reduction in VAT eosinophils and impairs AAM maintenance, highlighting their central regulatory role in adipose immune homeostasis. The eosinophil-ILC2-macrophage axis in orchestrates metabolic adaptations, particularly the beiging of (WAT), a process that converts energy-storing white adipocytes into thermogenic beige adipocytes expressing uncoupling protein 1 (). ILC2-derived IL-5 recruits , which in turn secrete IL-4 to activate M2 macrophages; these macrophages release catecholamines that stimulate β-adrenergic signaling in adipocytes, upregulating expression and enhancing non-shivering . Activation of ILC2s by IL-33 further amplifies this axis, significantly increasing the number of beige adipocytes in subcutaneous WAT and boosting oxygen consumption for caloric expenditure, independent of adaptive immunity but reliant on ILC2-intrinsic mechanisms like methionine-enkephalin secretion. This pathway also improves systemic insulin sensitivity, as IL-33-elicited ILC2 responses in obese mice reduce glucose intolerance and enhance glucose through -dependent . High-fat diets (HFDs) disrupt ILC2 function in , contributing to by suppressing ILC2 numbers and activity in both mice and humans. HFD feeding reduces VAT-resident ILC2s, alongside and M2 macrophages, while promoting pro-inflammatory M1 macrophages via cytokines such as IFNγ and TNFα, which inhibit ILC2 . This suppression impairs beiging and , exacerbating weight gain and ; conversely, adoptive transfer of activated ILC2s into HFD-fed obese mice limits fat accumulation and restores metabolic balance, underscoring the protective role of ILC2s against diet-induced adipose dysfunction.

Roles in Infections

Parasitic Infections

Innate lymphoid cells type 2 (ILC2s) play a pivotal role in the host's defense against helminth parasitic s by rapidly initiating type 2 immune responses. Upon with intestinal helminths such as Nippostrongylus brasiliensis, ILC2s are activated to produce high levels of interleukin-13 (IL-13), which promotes and production in the , facilitating worm expulsion. This rapid IL-13 secretion occurs independently of adaptive immunity and is essential for the timely clearance of parasites, as depletion of ILC2s impairs expulsion and prolongs . A key regulatory circuit amplifying this response involves in the , which detect helminth-derived signals and produce IL-25 to activate resident ILC2s. Activated ILC2s then secrete IL-13, which acts on epithelial progenitors to drive tuft cell hyperplasia, creating a feed-forward loop that sustains type 2 immunity and enhances anti-helminth defenses. This tuft cell-IL-25-ILC2 axis is particularly critical during primary infections, ensuring robust epithelial remodeling and parasite restriction in the gut mucosa. Recent studies have identified IL-25-induced memory-like ILC2s that emerge during helminth , persist after clearance, and enforce enhanced mucosal type 2 immunity upon re-infection, contributing to long-term protection. Additionally, ILC2s can rapidly mobilize from the intestine to distant sites like the lungs during , supporting systemic anti-helminth responses. Following acute expulsion, ILC2s contribute to resolution and tissue homeostasis through IL-9 production, which acts in an autocrine manner to promote ILC2 survival and amplify effector cytokines like IL-5 and IL-13. This IL-9 signaling supports tissue repair by enhancing secretion and reducing inflammation in the recovery phase after helminth-induced damage, such as in the lungs during larval migration. In humans, ILC2 involvement mirrors these mechanisms, with elevated circulating ILC2s observed in peripheral blood during infections, marked by increased expression of activation markers like and CRTH2. These cells likely contribute to systemic type 2 responses that aid in controlling helminth burdens. For protozoan infections, such as those caused by , ILC2s activated via IL-33 provide protective effects against by promoting epithelial barrier integrity and IL-13-mediated repair.

Viral Infections

During infection, damage to airway epithelial cells triggers the release of IL-33, which binds to the ST2 receptor on ILC2s, promoting their rapid expansion and activation in the lungs. Activated ILC2s then secrete high levels of IL-5 and IL-13, which drive recruitment and accumulation, contributing to that peaks during the recovery phase. These cytokines also induce airway hyperreactivity by altering smooth muscle contractility and epithelial responses, potentially exacerbating respiratory symptoms. ILC2s exhibit a dual role in the context of viral infections like influenza A. In the early stages, their production of type 2 cytokines such as IL-5 and IL-13 supports tissue repair by promoting epithelial and mucus clearance, aiding in the restoration of following viral clearance. However, in severe or chronic cases, unchecked ILC2 expansion and excessive can worsen pathology by fostering persistent eosinophilic infiltration and , which impair effective antiviral responses and prolong disease. ILC2s also facilitate interactions with adaptive immunity during viral challenges. For instance, ILC2-derived IL-5 enhances survival and differentiation in the lungs, supporting early production against respiratory viruses such as , independent of T cell help. This helps bridge innate and humoral responses to promote mucosal . In () infections, ILC2s contribute to detrimental outcomes, particularly in neonates. Activation of ILC2s by IL-33 leads to IL-13 secretion, which drives metaplasia and excessive mucus production, resulting in airway obstruction and increased disease severity. Studies around 2014 highlighted how this ILC2-mediated pathway worsens by promoting type 2 that hinders viral clearance and amplifies .

SARS-CoV-2 Infections

Innate lymphoid cells type 2 (ILC2s) have been implicated in SARS-CoV-2 infections, with lower blood abundance associated with increased disease severity, including higher hospitalization odds (odds ratio 0.454 per twofold decrease in ILCs), longer hospital stays, and elevated inflammatory markers such as C-reactive protein. ILC2s promote disease tolerance by producing amphiregulin to support lung tissue repair and homeostasis following infection. However, reduced ILC2 levels, potentially exacerbated by age and sex differences, correlate with worse outcomes, while expanded subsets like CD117low ILC2s in patients may contribute to type 2 cytokine responses that aid repair but could exacerbate inflammation in severe cases.

Roles in Diseases

Allergic Disorders

In allergic asthma, exposure to allergens triggers the release of epithelial-derived alarmins such as IL-33 and (TSLP), which activate group 2 (ILC2s) in the lungs. Activated ILC2s produce IL-5, promoting recruitment and survival, which contributes to eosinophilic inflammation. Concurrently, IL-13 secretion from ILC2s drives airway hyperresponsiveness (AHR), metaplasia, and hypersecretion, exacerbating asthmatic symptoms. This pathway underscores ILC2s as key initiators of type 2 immune responses in allergen-driven asthma pathogenesis. Human studies have demonstrated elevated ILC2 numbers in the airways of patients with , particularly correlating with disease severity. In severe , bronchoalveolar lavage fluid and samples show increased ILC2 frequencies, with IL-5- and IL-13-producing ILC2s most prominent in subtypes. These findings suggest ILC2s contribute to persistent and poor function in refractory cases. Targeting this pathway with anti-IL-33 monoclonal antibodies has shown promise in reducing ILC2 activation and in clinical trials for moderate-to-severe . However, ILC2s exhibit plasticity, including a regulatory subtype (IL-10-producing ILC2s) that dampens in . These cells reduce airway hyperresponsiveness and in murine models and are increased in human allergic individuals following , correlating with symptom improvement as of 2023. In , nasal allergen challenge induces rapid expansion and activation of ILC2s in the . Within hours of exposure, such as to cat dander, circulating CRTH2+ ILC2s increase, trafficking to the nasal tissues where they amplify type 2 production and local responses. This expansion correlates with symptom severity, highlighting ILC2s' role in acute allergic nasal . Therapeutic strategies targeting ILC2s have advanced, with biologics like anti-ST2 antibodies entering clinical trials since to mitigate their contributions to allergic disorders. For instance, astegolimab, an anti-ST2 , has demonstrated efficacy in phase 2b trials by suppressing ILC2-driven exacerbations in severe patients. These approaches aim to block upstream alarmin signaling, offering potential for personalized in type 2-high allergic conditions.

Autoimmune and Other Conditions

In atopic dermatitis, skin-resident ILC2s are elevated in lesional tissue and produce IL-13, which drives Th2 cell skewing and exacerbates epidermal barrier disruption by impairing integrity and promoting through interactions with mast cells and . Studies in human AD patients have shown that ILC2-derived IL-13 enhances the migration of activated dendritic cells to lymph nodes, further amplifying Th2 responses and chronic skin . ILC2s exhibit context-dependent roles in autoimmunity, providing protection in some conditions while contributing to pathogenesis in others. In models of , ILC2s exert a protective effect by secreting IL-13, which promotes differentiation, production, and epithelial barrier repair, thereby mitigating intestinal inflammation and tissue damage. Conversely, in , skin ILC2s demonstrate plasticity, converting to an ILC3-like state under IL-23 stimulation and producing , which drives hyperproliferation and sustains the inflammatory milieu characteristic of psoriatic lesions. This conversion has been observed in both murine models and human dermal samples, highlighting ILC2s' contribution to IL-17/IL-22-mediated . In cancer, ILC2s display dual functions depending on the tumor type and exhibit heterogeneity that confers tissue-specific pro- and anti-tumor effects, influencing prognosis across various malignancies as of 2025. In , ILC2s promote tumor progression by secreting VEGF-A, which induces and , facilitating nutrient supply and metastatic spread in preclinical models. In contrast, in , ILC2s mediate anti-tumor immunity through IL-5 and GM-CSF production, which recruits and activates to infiltrate tumors, enhancing against melanoma cells and improving survival outcomes in both mouse models and human patients. PD-1 blockade further unleashes this ILC2-eosinophil axis, amplifying therapeutic responses. Dysregulated ILC2s in contribute to metabolic diseases such as and by failing to maintain macrophage polarization, leading to chronic low-grade and . In obese models, LKB1 deficiency upregulates PD-1 on adipose ILC2s, impairing mitochondrial function and exacerbating ; PD-1 blockade restores ILC2 effector functions and reverses these metabolic deficits as of 2024. Research from the 2020s has identified therapeutic potential in targeting ILC2s; for instance, DR3 agonists stimulate adipose-resident ILC2s to boost IL-13 production, reversing and improving glucose in obese mouse models. Similarly, CB2 receptor agonists activate ILC2s via AKT/ERK/CREB signaling, enhancing type 2 output and ameliorating established in preclinical studies, with implications for human therapy.

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