Worldwide, approximately seven million patients are diagnosed with inflammatory bowel diseases (IBD) (1), a chronically remitting inflammatory disease of the gastrointestinal tract. Both entities of IBD, Crohn's disease (CD) and ulcerative colitis (UC), commonly lead to severe intestinal symptoms like chronic diarrhea, abdominal pain, rectal bleeding, and anemia, thus significantly limiting the physical fitness, work ability, and overall quality of life of the affected and often young patients (2–4). Due to a complex and multifactorial pathogenesis, which is crucially influenced by genetic, environmental, microflora-related, and immunological components, there still exists no causal therapy for IBD (5, 6). Instead, established treatment strategies focus on the control of clinical symptoms, mainly by either inhibiting the overwhelming accumulation and activation of intestinal immune cells or by promoting mucosal healing (7, 8). During the last two decades, the spectrum of available therapeutic regimens underwent a significant shift from a predominance of rather unspecific immunosuppressive drugs, like glucocorticoids, thiopurines, and methotrexate, toward an increasing and often preferential use of biological agents, like anti-TNF antibodies, the anti-IL-12/IL-23 antibody ustekinumab or anti-adhesion strategies, which in principle allow to specifically interfere with disease-driving signaling cascades (8). However, still approximately 40% of IBD patients do not show a satisfactory primary response even to these optimized therapeutic regimens or develop a secondary loss of response (5, 9), emphasizing the urgent need to further fine-tune the definition of therapeutic target structures, and/or to better take into account the full spectrum of interacting cellular players involved in the pathogenesis of IBD. In this context, it will be of particular importance to better elucidate the complex crosstalk between intestinal epithelial cells (IECs) and subepithelial innate immune cells, which together form an early and tightly regulated line of host defense against invading luminal pathogens. Consequently, achieving improved insights into the capacity of lamina propria innate immune cells to interfere with the maintenance of epithelial integrity or the resolution of epithelial defects might subsequently pave the way for therapeutic approaches that pursue a two-pronged strategy: Epithelial restoration and suppression of overwhelming immune activation.

Within the mucosal immune cell compartment, the group of innate lymphoid cells (ILCs) can be characterized by their typical localization in direct spatial proximity to the epithelial surface (10, 11) and by their exceptional capability to initiate an early and rapid response to invading pathogens and epithelial damage (12–14). Although resembling T cells morphologically and in several functional aspects, ILCs are classic representatives of the innate immune system and can thus be distinguished by the lack of rearranged antigen-specific receptor expression and their ability to achieve a full activation status independent from the antigen-presentation and -recognition machinery. This predisposes ILCs for promoting and regulating early defense mechanisms (13). In analogy to T helper (Th) cells and mainly based on their cytokine and transcription factor profile, mature helper ILCs can be categorized into type-1 (ILC1s), type-2 (ILC2s), and type-3 ILCs (ILC3s) (15). Accordingly, ILC1s are associated with type-1 immune responses classically directed against intracellular pathogens and tumor cells, functionally depend on the transcription factor T-bet and preferentially secrete the pro-inflammatory cytokines IFN-γ and TNF-α (16, 17). Besides helper ILC1s, NK cells represent another ILC1 subset, which is characterized by a potent cytotoxic effector function (16) and whose impact on the maintenance of mucosal homoeostasis has been reviewed elsewhere (18). As an important cellular source of the effector cytokines IL-4, IL-5, IL-9, and IL-13 and, hereby, as an integral cellular component of mucosal type-2 immune responses, ILC2s are crucially involved in the immunological control of parasitic worm infections and in allergic diseases. Like in Th2 cells, GATA-3 and RORα represent important signature transcription factors in ILC2s (19–21). Finally, ILC3s depend on the transcription factor RORγt and can be subdivided into lymphoid tissue inducer (LTi) cells, which play a significant role during lymphoid organogenesis, as well as NCR⁺ and NCR⁻ ILC3s as relevant producers of IL-17A, IL-22, and GM-CSF at mucosal sites, appearing accordingly in many functional aspects like innate counterparts of Th17 cells (22, 23). An important functional attribute, which is common to all local helper ILC pools in mucosal organs, is their early activation in response to pathological tissue damage and their subsequent capacity to support, amplify, and modulate local adaptive immune responses (24, 25). While this sequential cascade of primary or externally triggered epithelial damage, subsequent ILC activation via epithelial cell-released factors and, finally, the ILC-mediated modulation of adaptive immune responses has drawn a lot of scientific attention during the last decade (15, 24, 26, 27), the opposite direction, meaning the ILC-to-epithelium communication, often seemed to fade a bit into the background. However, several studies described a significant influence of the ILC compartment on tissue regeneration, resolution of inflammatory tissue damage, and mucosal barrier integrity (28–32), although several details of the underlying mechanisms and the particular contribution of the different types of epithelial cells still remain incompletely understood. Due to the implicated high significance of the bidirectional crosstalk between locally accumulating ILCs and epithelial cells for maintenance of mucosal homeostasis in the gastrointestinal tract and its potential therapeutic targetability in IBD, we will here summarize the insights in the capacity of helper ILCs to influence the fate of different epithelial cell types in the gut under physiological and inflammatory conditions and discuss potentially resulting therapeutic perspectives.

The distinct distribution of the three classical helper ILC subtypes observed at different anatomical sites upon homeostasis implies a specific function of each local subset in non-diseased tissues (Figure 1). In the human gastrointestinal tract NKp44⁺ ILC3s represent the main helper ILC population in the caecum, ileum, and colon, while ILC1s predominantly populate the upper gastrointestinal tract, including esophagus, stomach, and duodenum (33, 34). In contrast, ILC2s constitute only a small ILC population throughout the whole healthy intestine (33–36). However, when differentiating between lamina propria and intraepithelial ILCs more precisely, a distinct ILC2 population could be observed in the intraepithelial compartment of the colon together with a large ILC1 pool and only a minor fraction of ILC3s (35), suggesting a distinct function of each helper ILC subset in the gut under homeostatic conditions. In addition to the three classical helper ILC subsets, regulatory ILCs (ILCregs), which resemble regulatory T cells in several key features, were suggested to exist in both human and murine intestines. ILCregs were initially defined by their constitutive expression of IL-10 and were found to primarily accumulate in the lamina propria of the small intestine (37). Their existence, however, is still a matter of controversial discussion, given the inability of Bando and colleagues to detect ILCregs in the murine gut based on the expression of IL-10 in their study. Furthermore, they could not identify a helper ILC subset distinct from ILC1s, ILC2s, and ILC3s using mice from different breeding facilities in both steady-state and under inflammatory conditions (38). Instead, the authors described ILC2s as inducible source of intestinal IL-10 production (38), which is in line with the findings from other groups (39), raising the idea of IL-10-producing ILC2s rather than the existence of a distinct ILCreg subset.

Importantly, the murine intestine is populated by differently distributed ILC classes compared to human intestines (Figure 1). Unlike in humans, ILC2s make up a clear intestinal cell population in naive mice, which even outnumbers ILC3s or ILC1s in the lamina propria of the large intestine (35, 40). In the intraepithelial compartment and the small intestine, in contrast, murine ILC3s turned out as dominant ILC subtype (35, 41, 42). In total, murine ILCs were specifically enriched in the lamina propria of the large intestine (35). Keeping the species-specific differences in mind is important when assessing the translational relevance of results obtained in the murine organism. Additionally, the use of immunodeficient Rag1^−/− mice and, thus, the absence of functional adaptive immune cells in many murine in vivo ILC studies might bias the obtained results (13), emphasizing the need for confirmatory human studies. Nevertheless, the ability of highly controlled breeding, housing, and the availability of elegant genetic knockout mouse models, makes murine studies in the field of intestinal ILCs inevitable. The intensified consideration of humanized mouse models, in which the function of primary human ILCs can be analyzed under experimentally defined in vivo conditions, might even allow for better transferability of acquired data to the clinical context of human diseases (43).

Significant alterations in local ILC pools were observed in inflamed areas in IBD patients compared to unaffected control tissue (Figure 1), indicating a functional role of ILCs in chronic inflammation of the gut. While NKp44⁺ ILC3s constitute the dominant helper ILC population in the lower gastrointestinal tract in homeostasis (33, 34), their frequency was markedly reduced at sites of active inflammation in patients suffering from IBD, including both UC and CD (17, 34, 44). This ILC3 decrease further correlated with severe disease cases (34), highly suggesting a regulatory or protective function of ILC3s in intestinal inflammation. Contrary to NKp44⁺ ILC3s, the percentage of ILC1s, ILC2s, and NKp44⁻ ILC3 was found to be increased in IBD patients (34, 44–46). Especially in CD patients an enhanced percentage of intestinal ILC1s has been described in multiple studies (17, 34, 44) and was obviously associated with an advanced disease severity (34). Regarding the underlying mechanism for the accumulation of ILC1s in the inflamed intestine of CD patients, transdifferentiation of other ILC subtypes into ILC1s was suggested to take place in the IL-12-enriched microenvironment of the inflamed gut of CD patients (25). In in vitro experiments, ILC2s, ILC3s as well as c-Kit⁺NKp44⁻ immature ILCs were described to transdifferentiate into IFN-γ-secreting ILC1-like cells in the presence of IL-12 (17, 36, 47–50). And indeed, an increased local secretion of IL-12 was reported in CD patients (51, 52). Moreover, the biological relevance of this in vitro induced ILC3-to-ILC1 transition could be reinforced by an inverse link of ILC3 and ILC1 frequencies in the inflamed mucosa of CD patients (17, 34, 44) and the presence of an ILC subgroup harboring both ILC3 and ILC1 characteristics in human ileal LPMCs (53). Similarly, IL-13⁺IFN-γ⁺ ex-ILC2s were detected in the intestine of CD patients (48), hinting at ILC2-to-ILC1 transitions in vivo. In contrast to patients with CD, the intestinal tissue of UC patients was associated with an accumulation of NKp44⁻ ILC3s, which correlated with severe illness (34), making ILC1s and NKp44⁻ ILC3s specifically important in CD and UC, respectively. Although the scientific debate on the existence of ILCregs is still ongoing (37, 38), experimental models of innate colitis revealed an enhanced frequency of IL-10-producing ILCs, which the authors defined as ILCregs, upon intestinal inflammation, temporally following an increase of ILC3s and ILC1s. Accordingly, the transfer of ILCregs into Rag1^−/−Il10^−/− mice resulted in reduced signs of innate colitis, thus claiming the importance of ILCreg expansion for the resolution of intestinal inflammation (37). Focusing on the intraepithelial compartment, intraepithelial ILC1s were described to be increased in CD patients (54), which, however, could not be confirmed in a later study (34), making further research on intraepithelial ILCs necessary, especially since their prime location in direct proximity to the intestinal epithelium might predispose them for impacting on the integrity of the epithelial layer. Given that altered ILC frequencies were predominantly observed at inflamed intestinal sites but were absent in non-inflamed areas (34), suggests an active role of intestinal ILCs in inflammatory processes but argues against a primary and disease-predisposing alteration of the ILC compartment in IBD patients.

Based on these numerical ILC alterations observed in the inflamed gut of patients suffering from IBD, the next section will discuss our current knowledge on the functional role of local ILCs in intestinal inflammation with a particular focus on their capacity to interact and regulate IEC functions.

Enterocytes represent the most frequent cell type in the intestinal epithelium and as such build the fundament for a tight barrier between gut lumen and tissue. This is achieved by tight junctions, which connect the enterocytes to form a robust but selectively permeable wall, allowing a targeted paracellular transport. Together with the transcellular transport through enterocytes, this enables them to absorb nutrients and antigens from the gut lumen in a highly controlled fashion (55), while simultaneously forming an effective first line of defense for pathogens. When disrupted, however, overwhelming invasion of pathogens can cause severe inflammatory immune responses within the intestinal mucosa (56), making a tightly controlled regulation inevitable.

Several studies have described a crucial involvement of ILC3s in the regulation of epithelial integrity in the gut, which has been attributed mainly to their capacity to secrete the effector cytokine IL-22 (57–60). Based on the well-known protective functions of IL-22 in IBD (61) and the fact that ILC3s are thought to be the main producers of IL-22 in the homeostatic and inflamed murine gut (58, 60), the favorable functions described for IL-22 on IECs might be largely assigned to ILC3s. Indeed, ILC3-derived IL-22 could be demonstrated to play a protective role on dextran sulfate sodium (DSS)-induced tissue disruption (57–60). DSS-induced colitis represents a widely used model for UC, that is initiated by the chemical disruption of the intestinal epithelium, resulting in colonic inflammation (62), and thereby makes up an ideal model system to study the influence of ILCs on gut barrier integrity. Whereas, under homeostatic conditions ILC3 activity is largely repressed by IL-25 and adaptive immune responses, this repression is downregulated upon DSS-induced tissue disruption and inflammation, allowing the secretion of the protective cytokine IL-22 by intestinal ILC3s (60). In IBD patients an increased IL-22 expression was observed exclusively in ILC3s rather than T cells (63), fortifying the relevance of ILC3-derived IL-22 on gut barrier integrity in humans (Figure 2). Interestingly, the murine norovirus has been shown to take advantage of this ILC3-IL22-IEC-axis in order to protect IECs against tissue disruption (64). Besides the beneficial effect of ILC3s on the barrier function of the intestinal epithelium, they were additionally described to directly foster the glycosylation of eptihelial cells via their effector cytokines IL-22 and lymphotoxin (32) (Figure 2). In general, the colonic glycocalyx functions as additional barrier for pathogens, but also enables communication and adherence of specific bacteria (65). In mice, ILC3s were detected to mediate epithelial fucosylation via the induction of the responsible enzyme fucosyltransferase 2, which turned out to be important for host protection against Salmonella typhimurium infection (32).

In IBD patients, decreased frequencies of NKp44⁺ ILC3s were detected at inflamed intestinal sites compared to samples from non-inflamed IBD and non-IBD subjects (17, 34) which was significantly associated with an increased endoscopic disease severity score in both CD and UC patients (34). Since NKp44⁺ ILC3s represent the main producers of IL-22 in the adult intestine (66), the lack of the protective IL-22 effect on the epithelial barrier in IBD patients might at least partially explain the gut barrier disruption. However, in another study, increased IL-22 expression levels have been observed in colonic tissue samples derived from CD and UC patients which could be shown to result from NKp44⁺ ILC3s (63). Interestingly, the presence of fecal microbiota clearly triggered IL-22 production in human LPMCs (63) and potential differences in the composition of the gut microbiota might thus explain, at least partly, the controversy of published data on the intestinal IL-22 levels in IBD patients. Based on findings acquired in innate experimental colitis models (e.g., a model of anti-CD40-induced colitis), ILC3-derived IL-22 could be demonstrated to even have a pathogenic effect (67), indicating a double-edged role of ILC3-derived IL-22 in IBD (Figure 2) that might depend on the local micromilieu and microbiota.

Although less abundant in the gut compared to ILC3s (34), ILC2s can significantly contribute to the preservation and restoration of the intestinal integrity as well (Figure 2). Findings in other organs with a barrier function, like the lung and skin, support this idea. ILC2s could for example be shown to facilitate wound healing of the skin (68), and ILC2-derived amphiregulin (AREG) was suggested as effective player in the epithelial restoration of influenza virus-infected lungs (30). Indeed, first data acquired in the gut could underpin this, showing a tissue-protective role of ILC2s in a mouse model of acute DSS colitis via the secretion of the epidermal growth factor AREG (69). Here the authors suggest a circuit of damaged epithelial cells, releasing the alarmin IL-33 and thereby activating AREG production in ILC2s, which ameliorates DSS-induced tissue disruption, likely via the upregulation of Claudin-1 and Mucin 2 (Muc2) (see section Goblet cells—ILCs for further details on Muc2). Claudin-1 represents a well-known tight junction protein (70), whose increased expression favors tightly connected enterocytes forming an efficient barrier in the gut. The observation of a protective function of ILC2-derived AREG was further confirmed in a mouse model of acute gastrointestinal graft-vs.-host disease, in which the intravenous injection of ILC2s could significantly reduce intestinal leakiness in an AREG-dependent manner (71). In contrast to these results, one research group observed detrimental effects of both murine and human ILC2s on the tight junctions of bronchial epithelial cells and epidermal keratinocytes via the secretion of IL-13 (72, 73). These opposing results might indicate the presence of polyfunctional ILC2s (69) which can adapt to local requirement based on the specific microenvironment to gain either protective or destructive functions on epithelial barriers. In the inflamed gut, however, current knowledge indicates a beneficial effect of ILC2s on the intestinal barrier integrity (69, 71). This was additionally indirectly supported by the finding, that the increased intestinal permeability observed in Itk^−/− mice compared to wildtype mice upon acute DSS colitis was associated with a gut-specific reduction of ILC2s. Injection of IL-2 complexes, however, could both, restore gut ILC2 frequencies and diminish disease severity (74). In IBD patients an increased ILC2 frequency was observed at inflamed intestinal sites (34), suggesting a certain clinical relevance. Just recently, a regulatory subset of IL-10 producing ILC2s was described to have a protective effect in the context of grass-pollen allergy and lung inflammation (75, 76). This was not only mediated by their ability to dampen disease-driving type-2 immune responses (75, 76), but additionally relied on the restoration of the epithelial barrier (75). Whether this regulatory ILC subset is relevant in the inflamed gut as well, has to be determined in future studies, though. First hints, however, imply the potential formation of IL-10 producing ILCs in the inflamed gut mucosa: The transdifferentiation of ILC2s into IL-10⁺ ILC2s or ILCregs interestingly turned out to depend on the vitamin A metabolite retinoic acid (75, 77), elevated levels of which have been reported in patients with active UC (78).

Likewise, ILC1s turned out to accumulate in the inflamed ileum of CD patients (17), pointing toward a functional role in IBD-associated tissue disruption as well. In line with this, the frequency of intraepithelial ILC1s was shown to be increased in the small intestine of CD patients (54). Based on their prime position in immediate proximity to IECs, it can be assumed that intraepithelial ILC1s might play a special role in the regulation of the intestinal epithelial barrier. Further evidence derived from a mouse model of experimental colitis, showing a marked IFN-γ production by intraepithelial ILC1s and to a lesser extent also by plastic ILC3s. Deletion of intraepithelial ILC1s with an anti-NK1.1 antibody was associated with reduced epithelial disruption and a diminished accumulation of inflammatory cells (54), proposing a negative influence of ILC1s on colitis-associated barrier destruction. In the context of celiac disease, increased percentages of intraepithelial NKp44⁻ cytotoxic ILC1s have been reported in the human small intestine, which significantly correlated with an increased IFN-γ production as well as an increased disease severity and epithelial breakdown (79). In addition, enhanced levels of ILC1- and ILC3-derived IFN-γ and TNF-α upon simian immunodeficiency virus infection in rhesus macaques associated with an increased loss of colonic tight junctions, resulting in epithelial instability and increased microbial translocation (80), supporting a detrimental functional role of ILC1s on the maintenance of an intact epithelial gut barrier (Figure 2). A clear proof of ILC1s directly interacting with IECs, however, is still lacking to date, making further research interesting.

Next to enterocytes, the intestinal epithelium consists of highly specialized cell types, including goblet cells, tuft cells, stem cells, paneth cells, enteroendocrine cells, microfold cells, and cup cells (81, 82). While each cell type contributes to the integrity of the epithelium by its unique function, the following section takes a closer look on the direct impact of ILCs on the differentiation and functions of these cell types.

Given the preferential accumulation of ILCs at mucosal surfaces in close proximity to the epithelium (10, 11) as well as their rapid and early activation as part of the innate immune system (12–14), makes them ideal interaction partners of IECs. Moreover, the ILC-driven regulation of IECs is of particular importance as the integrity of the epithelial barrier is critical to preserve a stable and efficient control of bacterial translocation, which might otherwise trigger the initiation of mucosal inflammation. Based on our current knowledge, ILC3s represent the main helper ILC population in the healthy human gut (33, 34) (Figure 1) with an incredible ability to secrete large amounts of the effector cytokine IL-22 (58, 60). Together with the release of lymphotoxin, ILC3-derived IL-22 was ascribed a predominantly protective effect on enterocytes, goblet cells and even crypt stem and progenitor cells (57–60, 87, 91, 92). Therefore, the observed decrease of intestinal ILC3s (17, 34, 44) in the inflamed mucosa of IBD patients might contribute to local inflammatory responses in IBD. However, the effect of ILC3-derived IL-22 on IECs appeared to highly depend on the surrounding micromilieu (67) and thus has to be treated with care. With an increasing number of intestinal ILC1s in CD (17, 44), their TGF-β1-mediated beneficial effect on crypt stem cells (45) might contribute to the resolution of intestinal inflammation by promoting the reconstruction of an intact epithelial barrier. In contrast, via the secretion of IFN-γ, ILC1s were suggested to additionally have a negative influence on tight junctions (54, 79, 80), implying functional subgroups of ILC1s. Despite their overall low number in the healthy and inflamed intestine, ILC2s could be demonstrated to favor mucosal healing and disease control on the level of enterocytes, goblet cells, tuft cells and crypt stem cells by secreting AREG and IL-13 (69, 71, 84). Overall, the described ILC-IEC interactions prove once again, that a rare cell population like ILCs can have an extensive impact on disease induction, progression, and resolution irrespective of their small cell number (Figure 2), making the interference with ILC-driven IEC regulation an interesting new therapeutic option. Targeting ILCs appears to be particularly elegant regarding the ability of intestinal ILCs to regulate both, epithelial integrity as well as mucosal immune responses. However, this also makes clear that the development of ILC-modulating therapeutic strategies will require a careful fine-tuning to prevent accidental ILC-mediated immune cell activation upon boosting protective ILC-IEC interactions. Notably, no fully ILC-specific target structures exist these days, which is mainly due to the shared key features of ILCs and Th cells. This makes it currently impossible to clearly differentiate between ILC- and T cell-targeting therapeutic approaches. A broad overview of ILC-related therapeutic strategies was given by Goldberg and colleagues, postulating the following points of potential attack: targeting ILC effector functions by therapeutically modulating their activity, intracellular signaling, and effector cytokine production or targeting local ILC numbers by interfering with their survival and trafficking (105). To date, most of the postulated therapies involving ILC targeting aim at the inhibition of pro-inflammatory ILC functions. However, these strategies need to be critically revised, since inhibition of total ILC functions might not only favor resolution of inflammation but might additionally abolish their mainly positive influence on the intestinal epithelium upon inflammation. Thus, novel therapeutic approaches might focus on the controlled stimulation of specific and defined aspects of ILC functions, rather than mediating a broad or complete blockade of ILC activation. The following section will therefore focus on potential therapeutic options, which might allow to directly and specifically interfere with defined ILC-IEC interactions.

Given the great ability of ILC3-derived IL-22 to preserve the intestinal epithelium (57–60) and the fact that the IL-22 receptor (IL-22R) is absent on hematopoietic cells (106), ILC3-derived IL-22 appears as an attractive therapeutic target, since this might enable to support the protective effect of IL-22 on epithelial cells without triggering pro-inflammatory immune responses in parallel. Accordingly, different strategies to boost IL-22-mediated epithelial regeneration and preservation have been discussed. On the one hand, IL-22 can be supplemented directly and, on the other hand, therapeutic activation of endogenous IL-22 production and signaling can be targeted. In the latter category, a site-specific Il-22 gene delivery system was suggested and proven to improve signs of intestinal inflammation in a preclinical mouse model of Th2-driven colitis (88). However, there is still plenty of safety issues to be addressed carefully until this local gene-delivery strategy might be translated to the human system. Alternatively, therapeutic activation of the IL-22 pathway might represent an interesting strategy. To date, most promising approaches involve AhR-mediated IL-22 induction, which is of functional relevance in ILC3s as well (107, 108). Activation of the AhR pathway can be achieved, for instance, by dietary compounds (107) or microbiota-driven metabolization of tryptophan to the AhR ligand indole-3-aldehyde (109, 110), the latter of which was demonstrated to augment intestinal inflammation in murine models (109, 110). Similarly, indigo naturalis, which has been traditionally used in Chinese medicine, turned out to mediate its protective effect via AhR-mediated IL-22 production in ILC3s as well, resulting in reduced disease severity in several murine models of colitis (111). Moreover, especially a short-term treatment with indigo naturalis has been identified as effective treatment in patients with UC (112), whereas 8 weeks of administration cannot yet be fully excluded to potentially be associated with severe adverse events (113). Importantly, reduced AhR activation was observed in fecal samples from IBD compared to healthy subjects (110). Therefore, together with other AhR ligand candidates, including FICZ (114) and ABX464 (115), tryptophan and indigo naturalis might be promising therapeutic strategies in IBD patients by activating endogenous IL-22 production from intestinal ILC3s in order to protect and restore the intestinal epithelium. Since, however, AhR activation can trigger multiple signaling pathways dependent on its ligand (116), potential therapeutic interference with AhR signaling would have to be carefully controlled and monitored. Another strategy to target IL-22 signaling might involve blocking of the soluble IL-22 receptor IL-22 binding protein (IL-22BP), which functions as endogenous inhibitor of IL-22 by capturing IL-22 with high affinity and thus suppressing IL-22 signaling through its membrane-bound receptor (117). A beneficial effect of blocking IL-22BP in intestinal inflammation has been implicated by experimental colitis models, showing a detrimental role of local IL-22BP. Local gene delivery of IL-22bp, for instance, hindered IL-22-mediated goblet cell proliferation in a mouse model of acute DSS colitis (88). Moreover, CD4⁺ T cell-derived IL-22BP could be shown as driver of intestinal inflammation in the model of adaptive transfer colitis using Il-22bp-sufficient and -deficient donor T cells (118). The other way around, IL-22bp-deficient rats recovered significantly faster from first signs of colitis upon DSS treatment compared to wildtype control animals, which could be traced back to the protective effects of efficient IL-22 signaling on the epithelial barrier (119). Since inflamed areas in IBD patients are characterized by an increased expression of IL-22BP (118, 119), blocking IL-22BP might potentially have therapeutic potential in humans as well. Alternatively, recombinant human IL-22 fusion proteins, like F-652 or UTTR1147A, allow direct IL-22 supplementation with the advantage of increased IL-22 stability and thus extension of the IL-22-induced beneficial effects. In case of UTTR1147A, efficient STAT3 activation via the IL-22 receptor could be demonstrated in vitro, resulting in protective in vivo effects in a murine colitis model (120). Moreover, the therapeutic efficacy of F-652 has been shown in a mouse model of graft-vs.-host disease (92) and its safety profile has even been validated successfully in a clinical trial (121), paving the way for further clinical development. Irrespective of the afore mentioned strategies, fine-balancing of any therapeutic interference with ILC3-driven IL-22 secretion and individual patient selection will be of critical importance, since next to its protective effects on IECs, a detrimental potential of IL-22 on the epithelial barrier has been observed in selected mouse models, as for instance in the anti-CD40-induced innate colitis model (67). Likewise, uncontrolled IL-22 was suggested to favor formation of colitis-associated colon cancer (122), allowing IL-22 modulations only in a highly controlled fashion.

Boosting the ILC2-progenitor cell-tuft cell circuit might represent another innovative, yet currently still hypothetical therapeutic option in IBD patients. Indeed, data from a murine study indicated that succinate supplementation resulted in the amelioration of intestinal disease, which was attributed to an activation of the tuft cell-ILC2 circuit (99). Even though expansion of the helminth-sensing tuft cells by defined microbiota-derived metabolites has not directly been targeted in IBD patients yet, helminth infections themselves have been discussed to prevent intestinal inflammation, though to date with controversial outcomes (123–126). Thus, it appears promising to pursue strategies that aim on a more specific therapeutic activation of the tuft cell-ILC2 circuit by specific microbial metabolites, like succinate.

Hypothetically, a direct transfer of ex vivo expanded and specifically activated human ILCs might represent another future therapeutic strategy, aiming at the restoration of a functional epithelial barrier. Motivated by the first promising results achieved by the adoptive transfer of regulatory T cells (127, 128) with a phase I clinical trial currently running in UC patients (NCT04691232) (129), the transfer of other protective immune cell populations might also be of advantage for IBD patients. While there already exist established protocols for the ex vivo expansion and differentiation of human ILCs (48, 130, 131), a targeted ex vivo generation of specific ILC subsets with distinct effector functions is certainly still an unsolved challenge that needs to be addressed as a first step on the way to a potential clinical translation. However, pursuing this idea, the therapeutic use of ex vivo expanded primarily AREG-producing ILC2s might, for instance, be desirable, based on their beneficial effect observed in a mouse model of intestinal inflammation (69). Nevertheless, many more questions need to be answered, until adoptive ILC transfer might be further discussed as therapeutic strategy in IBD. For example, it will be important to define the in vivo stability of the intended ILC phenotype, the capacity of transferred ILCs to accumulate at the site of epithelial tissue damage as well as their potential risk to drive inflammation rather than epithelial healing under in vivo conditions.

In summary, the here described insights into the capacity of helper ILCs to impact on the fate of IECs clearly point to these innate mucosal immune cells as key regulators of the gut barrier integrity and as an interesting and still largely unexplored research topic with great therapeutic potential in the clinical context of IBD.

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

MN has served as an advisor for Pentax, Giuliani, MSD, Abbvie, Janssen, Takeda and Boehringer. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.