Dynamic regulation of innate lymphoid cells in the mucosal immune system

Abstract

The mucosal immune system is considered a local immune system, a term that implies regional restriction. Mucosal tissues are continually exposed to a wide range of antigens. The regulation of mucosal immune cells is tightly associated with the progression of mucosal diseases. Innate lymphoid cells (ILCs) are abundant in mucosal barriers and serve as first-line defenses against pathogens. The subtype changes and translocation of ILCs are accompanied by the pathologic processes of mucosal diseases. Here, we review the plasticity and circulation of ILCs in the mucosal immune system under physiological and pathological conditions. We also discuss the signaling pathways involved in dynamic ILC changes and the related targets in mucosal diseases.

Introduction

Innate lymphoid cells (ILCs) are an emerging family of innate lymphocytes that lack classical rearranged antigen receptors. They respond rapidly after pathogen infection and are involved in the antipathogen response, inflammation, tissue homeostasis, and immune tolerance.¹ According to their functions, three major ILC subsets are defined: ILC1s (including NK cells), ILC2s, and ILC3s.^2–6 In addition to effector ILCs, several ILCs exerting regulatory functions have also been defined.^7,8

ILCs are located in many mucosal tissues, such as the gastrointestinal tract, nasopharynx, pulmonary tract, and genitourinary tract.^9–18 With the development of single-cell RNA sequencing, ILC heterogeneity can be better resolved.^18–20 Accumulating evidence shows that dynamic changes in ILCs are related to mucosal disease progression. In this review, we focus on the plasticity of ILCs and their translocation under physiological and pathological conditions.

Plasticity of ILC subsets

Although ILCs show similarities with T helper cells, they have a unique determination of cell fate. ILC subsets develop from specific progenitors.²¹ However, the fate of ILC subsets is not fixed. Environmental signals epigenetically regulate the transdifferentiation of ILCs and contribute to the plasticity of ILC subsets (Fig. 1).

Fig. 1.

Plasticity of ILC subsets. Environmental signals epigenetically affect the differentiation of ILCs and contribute to the plasticity of ILC subsets. Each arrow with the indicated cytokine(s) or factor(s) represents promotion of conversion toward another ILC subset. ILC2s transdifferentiate into ILC1s in response to IL-1β, IL-12, and IL-18, whereas IL-1β, IL-23, IL-6, and TGF-β can drive the conversion of ILC2s toward ILC3s. IL-4, produced by eosinophils, contributes to the reversion of ILC2s to ILC1s/ILC3s. IL-10-producing ILCregs can be transdifferentiated from ILC2s by retinoic acid (RA). Under stimulation with IL-33, IL-2, IL-4, IL-10, IL-27, and neuromedin U (NUM), ILC2s can transdifferentiate into ILC2₁₀s. TGF-β signaling induces the conversion of ILC3s into ILCregs. Notch signaling, IL-25 and IL-33 are important for the conversion of nILC2s into iILC2s, while iILC2s can revert to the nILC2 phenotype upon stimulation with IL-25. IL-12, and IL-15 accelerate the ILC3 to ILC1 transition. In the presence of IL-2, IL-23, IL-1β, and RA, IFN-γ-producing ILC1s express RORγt and revert to IL-22-producing ILC3s. For ILC3s, T-bet, Notch signaling, and several cytokines, such as IL-2, IL-1β, IL-23, and IL-12, drive NCR⁻ ILC3s to differentiate into NCR⁺ ILC3s, which is reversed by TGF-β. TGF-β and IL-15 are required for the transdifferentiation of NK cells into ILC1s. Eomes plays a crucial role in the conversion of ILC1s into conventional NK cells

ILC2–ILC1 transition

ILC2s can be activated by interleukin (IL)-25, IL-33, and thymic stromal lymphopoietin (TSLP) and secrete IL-4, IL-5, IL-9, IL-13, and AREG.⁴ ILC2s convert into IFN-γ⁺ ILC1s in an IL-12-IL-12R pathway-dependent manner. Several reports have shown that IL-1β can activate human peripheral blood ILC2s and induce low expression of T-bet and IL-12 receptor.^22,23 Thus, the expression of T-bet and IL-12 receptor is crucial for the ILC2–ILC1 transition. In patients with chronic obstructive pulmonary disease (COPD), ILC2s transdifferentiate into ILC1s in an IL-12- and IL-18-dependent manner (Fig. 2). COPD-associated triggers (influenza virus or respiratory syncytial virus (RSV) infection, etc.) enable the loss of GATA-3 expression and the upregulation of IFN-γ expression.²⁴ On the other hand, IL-4, which is a mediator of the crosstalk between ILC2s and eosinophils, enables reversion of the ILC2 to ILC1 transition and results in the enhanced frequency of ILC2s in diseased tissues of patients with chronic rhinosinusitis with nasal polyps (CRSwNP) (Figs. 1 and 2).²⁵

Fig. 2.

ILC plasticity in mucosal diseases. The plasticity of ILCs plays a role in the progression of diseases within mucosal barriers, such as the intestinal mucosa, pulmonary mucosa, nasal mucosa, and oral mucosa. A In the inflamed intestine of patients with Crohn’s disease (CD), ILC2s and ILC3s transdifferentiate into ILC1s. In the tumor tissues of patients with colorectal cancer (CRC), ILC3s convert into ILCregs. Conversion of ILC2 subsets participates in the elimination of helminths in the intestine. B In the lungs of patients with pulmonary squamous cell carcinoma (SqCC), the conversion of ILC1s into ILC3s is associated with a poor prognosis. During viral infection, human ILC2s convert into ILC1s in the lungs and are thereafter augmented in patients with chronic obstructive pulmonary disease (COPD). In addition, the conversion between nILC2s and iILC2s is associated with asthma and helminth infection. ILCregs transdifferentiated from ILC2s might play an essential role in the resolution of allergic airway inflammation. IL-4-induced transdifferentiation of ILC2s into ILC2₁₀s decreases airway hyperreactivity (AHR) and asthma symptoms. C In patients with cystic fibrosis with nasal polyps (CFwNP), ILC2s transdifferentiate toward IL-17-producing ILC3s under induction by local cytokines. In nasal tissue from patients with chronic rhinosinusitis with nasal polyps (CRSwNP), ILC1s convert into ILC2s upon IL-4 stimulation, and ILCregs are transdifferentiated from ILC2s. D In the oral mucosa, the conversion of iILC2s into ILC3s is involved in protection against fungal infection

Furthermore, the transition of ILC2s into ILC1s is triggered by epigenetic landscape changes. H3K9ac, representing the activation of transcription, accumulates in the promoters of the TBX21, IL12RB2, and IFNG genes in IL-1β-primed ILC2s. IL-1β also enables epigenetic changes through IL-1β receptor and the NF-κB pathway.²² Transdifferentiation between ILC2s and ILC1s is typically found in inflammatory airway diseases, but their interconversion in other mucosal tissues still needs to be investigated.

ILC2–ILC3 transition

ILC2s can be divided into two subsets: natural ILC2s (nILC2s) and inflammatory ILC2s (iILC2s). nILC2s are tissue-resident lymphocytes that express high levels of IL-33 receptor (ST2⁺KLRG1^low) and respond to IL-33. In contrast, iILC2s are located in parenchymal tissues and respond to IL-25. During Candida albicans infection in vivo or under T_H17 culture conditions (TGF-β and IL-6 treatment) in vitro, iILC2s can develop into ILC3-like cells and acquire the ability to produce IL-17 (Figs. 1 and 2).²⁶ Bernink et al. observed that c-Kit⁺ ILC2s could transdifferentiate into IL-17-secreting NKp44⁻ ILC3-like cells in the presence of IL-1β and IL-23. TGF-β was required for the conversion of c-Kit⁻ ILC2s into RORγt-expressing c-Kit⁺ ILC2s.²⁷ Similarly, in the presence of the epithelium-produced cytokines IL-1β, IL-23, and TGF-β, human ILC2s transdifferentiate into IL-17-secreting cells.¹⁶ These ILCs induce IL-8 production and enhance the recruitment of neutrophils in patients with cystic fibrosis with nasal polyps (CFwNP) (Fig. 2). Single-cell RNA-seq analysis revealed that a subset of RORγt⁺ ILC2s express the skin-homing receptor CCR10, which is relevant to the inflammatory status of patients with psoriasis.²⁷ Recently, the Flavell group found that skin-resident ILC2s converted into an ILC3-like subset upon induction with IL-23 or imiquimod, which further proved the ILC2 to ILC3 transition in the skin.¹⁸ Thus, IL-1β, IL-23, IL-6, and TGF-β, which are crucial for the activation of ILC3s, are also required for the ILC2–ILC3 transition. Notably, the cytokine IL-4 can maintain ILC2 identity by suppressing TGFBR1 and IL1R1 expression and inhibiting STAT3 phosphorylation.¹⁶ Additionally, vitamin D3 suppresses the conversion of ILC2s into ILC3s.¹⁶

ILC3–ILC1 transition

ILC3 subsets include NCR⁺ ILC3s and NCR⁻ ILC3s (LTi and LTi-like cells). NCR⁺ ILC3s express high levels of T-bet and IFN-γ and exhibit similarities with ILC1s.²⁸ Upon induction with IL-12, ILC3s transdifferentiate into ILC1s. Conversely, IFN-γ-producing ILC1s are able to express RORγt and differentiate back into IL-22-producing ILC3s in the presence of IL-2, IL-23, IL-1β, and retinoic acid (RA) (Fig. 1).^29,30 Vonarbourg et al. found that IL-12 and IL-15 accelerated the conversion of ILC3s into ILC1s.³¹ Many intrinsic transcription factors are crucial for the ILC3–ILC1 transition. Aiolos (a TGF-β-imprinted molecule) and T-bet (a signature transcription factor of ILC1s) together promote the transition of ILC3s into ILC1s.⁹ Suppression of Aiolos and Ikaros expression impairs the ILC3 to ILC1/NK transition.³² c-Maf, a regulator of ILC3 identity, limits the conversion of ILC3s into ILC1s.³³ In inflamed intestinal tissues from patients with Crohn’s disease (CD), the number of CD127⁺ ILC1s is elevated, accompanied by a reduced frequency of NKp44⁺ ILC3s.^29,30 On the other hand, IL-23-producing tumor cells mediate the conversion of ILC1s into ILC3s and promote IL-17-mediated tumor progression in human squamous cell carcinoma (SqCC) (Fig. 2).³⁴ In summary, the conversion of ILC3s into ILC1s might be a signature of inflammation, and the ILC1 to ILC3 transition is probably associated with tumorigenesis.³⁵ Other environmental signals and transcription factors involved in this conversion are worthy of further study.

Transition into regulatory ILCs

ILCregs are characterized by IL-10 secretion.⁷ Additionally, Morita et al. found that ILC2s could convert into IL-10-producing ILCregs under stimulation with synthetic RA from epithelial cells (Fig. 1). As a result, ILCregs were evaluated in nasal tissue from patients with CRSwNP and in lung tissue from house dust mite-treated mice (Fig. 2).³⁶ In some cases, IL-10-producing ILC2s, termed ILC2₁₀s, can be induced by IL-33 or the allergen papain and result in reduced eosinophil recruitment to the lungs.^8,37 In the small intestine of naïve mice, IL-10 expression in ILC2s is induced by IL-2, IL-4, IL-10, IL-27, and neuromedin U (NUM) and suppressed by TL1A (a TNF superfamily member).³⁸ In addition, the transcription factors c-Maf and Blimp-1 are able to drive IL-10 expression in ILC2₁₀s under induction by IL-4, resulting in the amelioration of airway hyperreactivity (AHR).^37,39 Thus, ILCregs and ILC2₁₀s both perform a regulatory role in mucosal sites, and the transition into IL-10-producing ILCs might contribute to the resolution of inflammation.

Recently, our group analyzed ILC subsets in colorectal cancer (CRC) tumors. Using lineage-tracing mouse models, we found that ILC3s were able to transdifferentiate into ILCregs in a TGF-β-dependent manner (Figs. 1 and 2).²⁰ Thus, the conversion of ILCs in the tumor environment is associated with tumor progression. Understanding ILC plasticity and the associated regulatory mechanisms might provide an additional approach for cancer immunotherapy.

Transitions within ILC subsets

Conventional NK and ILC1 cells have very similar phenotypic and functional features. However, they behave differently in several ways. NK cells are generally distinguished from ILC1s by the expression of the transcription factor eomesodermin (Eomes).^40–42 Recently, Tian’s group identified the specific progenitors of ILC1s in the liver, indicating that these cells possess their own developmental pathways.⁴³ NK cells have a cytotoxic function in tumor surveillance during the early stage of cancer development.^44–46 Gao et al. showed that TGF-β promoted NK cell differentiation into intermediate ILC1s (intILC1s) and ILC1s. These cells expressed inhibitory receptors leading to immune escape and tumorigenesis.⁴⁷ In a CRC model, our group found that during the progression of tumors, intratumoral ILC1s displayed high levels of inhibitory receptors and underwent inhibitory functional conversion.²⁰ TGF-β signaling contributes to the NK cell to ILC1 transition. In SMAD4-deficient mice, NK cells convert into ILC1-like cells, leading to high risks of tumor metastasis and viral infection. Mechanistically, SMAD4 inhibits NK cell plasticity by restraining noncanonical TGF-β signaling through the cytokine receptor TGFβR1.⁴⁸ Using mass cytometry by time-of-flight (CyTOF), Hawke et al. identified the synergistic activity of TGF-β and IL-15 in the conversion of human NK cells into ILC1s.⁴⁹ In regard to infectious diseases, Toxoplasma gondii infection enables the transdifferentiation of NK cells into ILC1-like cells.⁵⁰ On the other hand, the transcription factor Eomes plays a sufficient role in the conversion of ILC1s into conventional NK cells.⁵¹

Huang et al. revealed that nILC2s developed into iILC2-like cells during IL-25 treatment or helminth infection (Figs. 1 and 2).²⁶ Moreover, iILC2s are transient progenitors of nILC2s, and they can reconvert into the nILC2 phenotype upon stimulation with IL-25 or helminths.²⁶ In addition, other groups have identified that Notch signaling and IL-33 participate in the conversion of nILC2s into iILC2s.^52,53 Signals from the nervous system are also related to the cell fate of ILC2s. The neuropeptide calcitonin gene-related peptide (CGRP) modulates the functional transition of ILC2s during helminth infection, together with the downregulation of Il2, Il9, Il4, Il6, and Il13 but upregulation of Il5.

For ILC3s, T-bet and Notch signaling drive the differentiation of NCR⁻ ILC3s into NCR⁺ ILC3s.^54–56 Qi et al. found that the conversion of NKp46⁻ ILC3s into NKp46⁺ ILC3s was conducted by T-bet and Brg1 (Brahma-related gene 1, a catalytic subunit of the mammalian BAF complex) in a Notch-dependent manner.⁵⁷ Several cytokines, such as IL-2, IL-1β, IL-23, and IL-12, facilitate the transition of NCR⁻ ILC3s into NCR⁺ ILC3s.^30,41,58 However, TGF-β induces the reversion of NCR⁺ ILC3s to NCR⁻ ILC3s, and deficiency in the TGF-β receptor enables augmentation of the NCR⁺ ILC3 population.^59,60

Migration and circulation of ILCs in the mucosal immune system

Under steady-state conditions, the majority of ILCs are tissue resident in lymphoid and nonlymphoid organs.^61,62 However, several studies have revealed that ILC numbers increase in peripheral sites not only through local proliferation but also via recruitment of circulating ILCs during inflammatory responses.^63,64 Yang et al. found that ILCs were activated in skin-draining lymph nodes, acquiring expression of CCR10 and the ability to migrate to the skin under steady-state conditions.⁶⁵ Recently, transgenic mice with photoconvertible fluorescent proteins have been used to analyze the migration and circulation of ILCs.⁶⁶ Lymph nodes have been shown to contain both migratory and resident ILC populations, and ILC1s are highly migratory.⁶⁶ The migration and circulation of ILCs support host defense against infection. Homing receptors and trafficking receptors on ILCs or ILC progenitors are implicated in their migration.^66,67 However, the mechanisms regulating the migration and tissue localization of ILCs remain to be investigated.

Migration of NK cells and ILC1s

Conventional NK cells, derived from the bone marrow, circulate in peripheral sites and exert immune surveillance functions.⁶⁸ In humans, circulating NK cells are typically CD56^bright and express the lymphoid tissue homing markers CCR7 and CD62L (L-selectin).^69,70 Lymphoid- and liver-resident NK cells express CD69 and CXCR6.^71,72 Protein network analysis showed that immune signaling complexes (such as ADAP) promote the migration of NK cells. ADAP-deficient mice were found to have NK cells with impaired cytotoxic function and IFN-γ production during Listeria monocytogenes infection (Table 1).⁷³ Leishmania major infection induces NK cell translocation from the blood into the paracortex of draining lymph nodes via high endothelial venules (HEVs).⁷⁴ Mechanistically, sphingosine-1 phosphate receptor 5 (S1PR5) is required for the trafficking of NK cells from the bone marrow and lymph nodes to the blood, spleen, and lungs in an FTY720-resistant manner under both steady-state conditions and inflammatory conditions.^75,76 Some chemokines and chemokine receptors are associated with the migration and circulation of NK cells, including CCL2, CCL3, CCR5, CCL5, CXCL12, CX3CR1, and CXCR3.^77–83 Furthermore, IFN-γ is reported to induce NK cell mobilization from the spleen or bone marrow into the circulation and accumulation in the peritoneum, the liver, and tumor-bearing lung tissue.⁸⁴

Table 1.

Migration and circulation of ILCs

Group	Migration sites		Factor and pathway	Pathologic process	Species	References
	Start	End
NK
	ND	Spleen	ADAP	L. monocytogenes infection	Mouse	73
	Blood	Draining LN	L. major	L. major infection	Mouse	74
	BM/LN	Blood/Lungs/ Spleen	S1P5/CXCR4	Inflammation	Mouse/Human	75,76
	Blood	Lungs	CCL2	Pneumonia	Mouse	77
	Blood	Liver/Spleen	CCR5	T. gondii Infection	Mouse	78
	Blood	Liver	CCR5/CCL5	Hepatitis	Mouse	79
	ND	Tumor tissue	CX3CR1/CXCR3	Lymphoma	Mouse	80,81
	BM	Periphery	CCL3/CXCL12	ND	Mouse	82
	Blood	LN	CCR7/CXCR3	TH1 response	Mouse	83
	Spleen/BM	Periphery	CXCR3/IFN-γ	Hepatitis	Mouse	84
	Secondary lymphoid organs	LN follicles	CXCR5/IL-15	Viral control	AGM	103
ILC1
	Blood	LN	CD62L/CCR7	TH1 responses	Mouse	66
	MLN	Intestine	RA/CCR9/α4β7	Regulation of innate immunity	Mouse	67
ILC2
	Intestine	Lungs	S1PR1/S1PR4/IL25/helminth	Antihelminth defense and tissue repair	Mouse	14,26,104
	Intestine	MLN	IL-33/IL25	Antihelminth immunity	Mouse	53
	BM	Intestine	CCR9/α4β7	Regulation of innate immunity	Mouse	67
	Intestine/Lungs	Blood	Helminth/IL25/IL33	Antihelminth immunity	Mouse	88
	BM/Blood	Lungs	IL-33/CXCR4/pulmonary fungal allergen/β2	Allergic airway inflammation	Mouse/Human	89,90
	LN	Lungs	PGD2	Proallergic inflammatory responses	Human/Mouse	91–93
ILC3
	MLN	Intestine	RA/CCR9/α4β7	C. rodentium infection	Mouse	67
	ND	Lungs	CXCR5/CXCL13	Mtb infection	Mouse/Human	94
	ND	Intestine	CXCL16-CXCR6	C. rodentium infection	Mouse	95
	MLN	Intestine	GPR183	C. rodentium infection	Mouse/Human	96,97
	ND	CPs and ILFs	GPR183	Colitis	Mouse/Human	98
	Intestine	MLN	CCR7	H. polygyrus infection	Mouse	99
	CPs	Adjacent tissue	GM-CSF	Colitis	Mouse/Human	100
	Blood	Lymphoid tissue	VEGF-A	Pulmonary inflammation	Mouse/Human	101

ADAP adhesion and degranulation-promoting adapter protein, AGM African green monkey, BM bone marrow, CPs cryptopatches, C. rodentium Citrobacter rodentium, Hpb H. polygyrus Bakeri, H. polygyrus Heligmosomoides polygyrus, ILFs isolated lymphoid follicles, L. monocytogenes Listeria monocytogenes, L. major Leishmania major, LN lymph nodes, MLN mesenteric lymph nodes, Mtb Mycobacterium tuberculosis, PGD₂ prostaglandin D₂, VEGF-A vascular endothelial growth factor A, ND not determined

For the migration of ILC1s to the intestine, migration is preceded by a change in the expression of homing receptors. These receptors are regulated by mucosal dendritic cells and the gut-specific tissue factor retinoic acid (RA). RA is a metabolite of vitamin A that upregulates the levels of both CCR9 and α4β7 on ILC1s. ILC1s in the small intestine of vitamin A-deficient Rag1^−/− mice downregulate the expression of CCR9 and α4β7, suppressing their migration.⁶⁷ Tracking ILCs in the brachial lymph nodes revealed that most migratory ILCs were ILC1s.⁶⁶ ILC1s recirculate through the peripheral lymph nodes in a CD62L- and CCR7-dependent manner. They egress to the periphery in an S1P receptor-dependent manner (Table 1).⁶⁶

Migration of ILC2s

ILC2 progenitors populate the bone marrow and migrate to peripheral tissues to undergo maturation and exert effector functions. Recently, Schneider et al. demonstrated that the increase in adult-derived ILC2s mainly resulted from the expansion of preexisting ILC2s rather than de novo generation of new ILC2s.⁸⁵ Zeis et al. performed a single-cell analysis of lung ILCs and revealed the in situ differentiation and tissue adaptation of ILCPs.⁸⁶ In mice, inflamed tissue-derived ILC progenitors migrate to the lungs, where, together with tissue-resident Il18r1⁺ST2⁻ immature ILCs, they differentiate into Il18r1⁻ST2⁺ ILC2s, establishing a full spectrum of ILC2 subsets.⁸⁶ Using RORα-tracing mice, lung ILCPs (IL-18Rα (Il18r1)⁺ ST2⁻ ILCs) were found in both adult lungs and neonatal lungs, which might contribute to the generation of ILC2s.⁸⁷

Cytokines or external environments (such as IL-33 or a fungal allergen) promote the egress of ILC2s from the bone marrow. Using a fate-mapping approach, ILC2s were observed to extrude into the blood circulation during proinflammatory cytokine- and helminth-induced proliferation to fit niche size.⁸⁸ Mechanistically, ILC2 trafficking is mediated by IL-33 signaling via suppression of chemokine receptor CXCR4 expression, while fungal aeroallergens promote ILC2P translocation in an IL-33-dependent manner (Table 1).⁸⁹ Similarly, intranasal administration of Alternaria promotes the egress of ILC2s from the bone marrow to the peripheral blood and lungs. This process is based on β2 integrins rather than β1 or α4 integrins.⁹⁰ Unlike ILC1s and ILC3s, which require RA-induced homing receptor switching, ILC2 precursors migrate from the bone marrow to the small intestine in an RA-independent manner.⁶⁷ Moreover, Huang et al. identified a new migratory subset called iILC2s, which were characterized by high expression of KLRG1 and production of type 2 cytokines.²⁶ IL-25 or helminth infection induces the migration of iILC2s from the gut to the lungs and other distal sites depending on G protein-coupled S1P receptors.¹⁴ In addition, Flamar et al. revealed that iILC2s could be recruited from the intestinal lamina propria to the mesenteric lymph nodes upon induction by IL-33 and IL-25.⁵³ ILC2 migration relies on the binding of prostaglandin D₂ (PGD₂) to the CRTH2 receptor and results in enhanced allergic inflammation.^91–93

Migration of ILC3 cells

Similar to the process for ILC1s, RA derived from mucosal DCs can induce the homing receptor switch in ILC3s. CCR9 and α4β7, induced by RA, play crucial roles in the migration of ILC3s to the intestine (Table 1).⁶⁷ Bacterial infection modulates the accumulation of ILC3s. In the early immune response against Mycobacterium tuberculosis (Mtb), ILC3s are recruited to Mtb-infected lungs along with the accumulation of alveolar macrophages.⁹⁴ For Citrobacter rodentium (C. rodentium) infection, ILC3s accumulate in the intestinal lamina propria in a CXCL16-CXCR6-dependent manner.⁹⁵ G protein-coupled receptors (GPCRs) enable the migration of ILCs upon induction by environmental signals in both steady-state conditions and inflammatory conditions.⁹⁶ Murine and human ILC3s express the chemotactic receptor GPR183 and migrate toward the GPR183 ligand 7α,25-dihydroxycholesterol (7α,25-OHC) in vitro.^97,98 In vivo, crosstalk between GPR183 (expressed by ILC3s) and 7α,25-OHC (expressed by intestinal stromal cells) is required for immunity to enteric bacterial infection. GPR183 signaling apparently affects the accumulation of ILC3s in the mesenteric lymph nodes and the distribution of ILC3s in the intestine.⁹⁷ The migration of LTi-like ILC3s from the intestine to the mesenteric lymph nodes is dependent on CCR7 in the steady state or during infection with the natural mouse parasite Heligmosomoides polygyrus.⁹⁹ Although the majority of ILC3s are tissue resident in the steady state, circulating ILC3s in peripheral tissues play a critical role in the antibacterial response.

Circulating ILC3s are also associated with inflammation. Acute colitis, for example, is related to migratory ILC3s. Pearson et al. revealed that an IL-23-GM-CSF axis promoted ILC3 mobilization from cryptopatches to adjacent tissue and resulted in aggravated intestinal inflammation.¹⁰⁰ For pulmonary inflammation, Neuropilin1-positive (NRP1⁺) ILC3s have been shown to have a migratory capacity and to be found in the peripheral blood of COPD patients. The chemotactic factor vascular endothelial growth factor A (VEGF-A) contributes to the migration of ILC3s in secondary lymphoid tissues via HEVs.¹⁰¹

In conclusion, the migration of ILCs is mainly regulated by chemokines, integrins, and homing receptors. A deeper understanding of the mechanism regulating ILC migration and circulation provides new insight into therapies for antipathogen responses, chronic inflammatory diseases, and cancer. Furthermore, this knowledge helps us to better understand the relationship of ILCs with clinical outcomes.

Dynamic regulation of ILCs in mucosal diseases

In mucosal diseases, there are functional changes in and crosstalk among immune cells within the inflammatory environment. For inflammatory diseases, the transition of ILC2s or ILC3s into ILC1s is typically found. IFN-γ-producing ILC1s are associated with the pathogenesis of inflamed gut mucosal tissues. In the inflamed intestine of patients with CD, conversion of ILC3s into ILC1s is observed, accompanied by the accumulation of IFN-γ-producing ILC1s and impaired integrity of the intestinal barrier (Fig. 2).^29–31,102 IFN-γ-producing ILC2s have also been detected in intestinal samples from CD patients.²³ In addition, human ILC2s convert into ILC1s in the lungs of COPD patients, thereafter augmenting respiratory inflammation. Thus, the frequency of ILC1s is associated with disease severity and susceptibility to exacerbation.^24,25

During carcinoma progression, several tumor-infiltrating ILC subsets perform an immunosuppressive role. The conversion of ILC1s into ILC3s is involved in pulmonary SqCC, resulting in a poor prognosis.³⁴ During CRC tumor progression, ILC3s transdifferentiate into inhibitory ILCregs. Consequently, blocking ILC3-ILCreg conversion could suppress tumor growth.²⁰

In antipathogen or allergic responses, ILC transitions occur according to the type of immune response. The population of IL-25-responsive iILC2s and/or their descendants nILC2s participate in antihelminth responses in the intestine.²⁶ The conversion of nILC2s into iILC2s with dual IL-13/IL-17 production is associated with the pathology of autoimmune and inflammatory diseases, such as asthma.⁵² In CFwNP patients, ILC2s transdifferentiate toward IL-17-producing ILCs under stimulation by local cytokines induced by infectious agents.¹⁶ In addition, iILC2s are able to develop into ILC3s and contribute to immunity to C. albicans infection.²⁶ For skin-resident ILCs, ILC2s transdifferentiate into pathogenic ILC3s during IL-23 induction, resulting in dermal inflammation in psoriasis.¹⁸ ILCregs converted from ILC2s might play an essential role in the resolution of allergic airway inflammation (Fig. 2).³⁶ Similarly, transdifferentiation of ILC2s into ILC2₁₀s dampens AHR and reduces asthma symptoms.³⁷ Therefore, the management of ILC plasticity may provide a new therapeutic strategy for mucosal diseases.

The migration and circulation of ILCs are also involved in mucosal diseases (Table 1). For infectious diseases, the accumulation of ILCs occurs rapidly and contributes to early disease control. The recruitment of NK cells is essential for limiting simian immunodeficiency virus (SIV) replication in lymph node follicles.¹⁰³ Migration of iILC2s from the intestine to the lungs plays a crucial role in effective antihelminth responses, as previously described.^14,104 In addition, delayed recruitment of iILC2s from the intestine to the mesenteric lymph nodes, which is caused by Tph1 deficiency, results in impaired antihelminth immunity.⁵³ Following exposure to the fungal allergen Alternaria alternata, ILC2s circulate from the bone marrow to the lungs.⁹⁰ HIV-1 infection negatively affects the circulation of ILCs. Human blood ILCs are severely depleted irreversibly 7–14 days after early acute viremic HIV-1 infection, leading to epithelial gut breakdown.¹⁰⁵ During Mtb infection, circulating ILC3 levels are apparently decreased in the peripheral blood of patients, but the ILC3 population can be restored upon treatment. In this case, circulating ILC3s are recruited rapidly to the lungs and play an early protective role in immunity to Mtb infection.⁹⁴ In addition, the CXCR6, GPR183 and RA-dependent migration of ILC3s is required for protective immunity against infection by enteric bacteria, such as C. rodentium.^67,95,97

The circulation and accumulation of inflammatory ILCs bring pathological changes to mucosal sites. ILC2 migration induced by PGD₂ is involved in enhanced allergic inflammation.^91–93 In addition, NRP1⁺ ILC3s are recruited via the NRP1-VEGF-A axis to lung tissue in patients with COPD, leading to pulmonary inflammation.¹⁰¹ In the colon, both GPR183- and GM-CSF-mediated ILC3 migratory processes drive colitis.^98,100 Oxysterols, interacting with GPR183, induce the migration of ILC3s and contribute to intestinal inflammation.⁹⁶

Conclusions and perspectives

The mucosal immune system acts separately and differently from the systemic immune system. The circulation of and crosstalk between immune cells integrate mucosal sites into a continuous system. ILCs are abundant in mucosal barriers. The plasticity and migration of ILCs are crucial in shaping and calibrating their responses in various types of mucosal diseases. The exact functional impact of the dynamic regulation of ILCs is not fully understood, and there are still active areas of ongoing research. Hence, finding potential therapeutic targets for regulating ILCs could be an effective therapeutic strategy in multiple mucosal diseases. The function and mobility of ILCs are affected by pathogenic microenvironments. These changes also alter the progression of diseases. Antibodies and chemical drugs targeting pathways related to dynamic changes in ILCs may be useful for treating ILC-related mucosal diseases.

Understanding the dynamic regulation mechanism throughout the mucosal immune system may help in the development of vaccines. ILC memory is beneficial for vaccination.^106,107 Adjuvants, which are essential components of vaccines, have been shown to be important for the migration of ILCs.^83,108 Whether the circulation of ILCs affects the efficiency of vaccines is worthy of investigation. In addition, the effects of adjuvants on mucosal ILCs are expected to potentiate HIV vaccine efficacy.¹⁰⁹ Hence, for vaccine design, adjuvants can be selected according to their functions related to ILC regulation.

Competing interests

The authors declare no competing interests.