Innate lymphoid cells (ILCs) are a recently identified family of heterogeneous immune cells that can be divided into three groups based on their differential developmental requirements and expression of effector cytokines. Among these, group 2 ILCs produce the type 2 cytokines IL-5 and IL-13 and promote type 2 inflammation in the lung and intestine. However, whether group 2 ILCs reside in the skin and contribute to skin inflammation has not been characterized. Here, we identify for the first time a population of skin-resident group 2 ILCs present in healthy human skin that are enriched in lesional human skin from atopic dermatitis (AD) patients. Group 2 ILCs were also found in normal murine skin and were critical for the development of inflammation in a murine model of AD-like disease. Remarkably, in contrast to group 2 ILC responses in the intestine and lung, which are critically regulated by IL-33 and IL-25, ILC responses in the skin and skin-draining lymph nodes were independent of these canonical cytokines but were critically dependent on thymic stromal lymphopoietin (TSLP). Collectively, these results demonstrate an essential role for IL-33– and IL-25–independent group 2 ILCs in promoting skin inflammation.

The barrier surfaces of the skin, lung, and intestine are constantly exposed to environmental stimuli that can result in inflammation and tissue damage. Interleukin (IL)-33–dependent group 2 innate lymphoid cells (ILC2s) are enriched at barrier surfaces and have been implicated in promoting inflammation; however, the mechanisms underlying the tissue-protective roles of IL-33 or ILC2s at surfaces such as the intestine remain poorly defined. Here we demonstrate that, following activation with IL-33, expression of the growth factor amphiregulin (AREG) is a dominant functional signature of gut-associated ILC2s. In the context of a murine model of intestinal damage and inflammation, the frequency and number of AREG-expressing ILC2s increases following intestinal injury and genetic disruption of the endogenous AREG–epidermal growth factor receptor (EGFR) pathway exacerbated disease. Administration of exogenous AREG limited intestinal inflammation and decreased disease severity in both lymphocyte-sufficient and lymphocyte-deficient mice, revealing a previously unrecognized innate immune mechanism of intestinal tissue protection. Furthermore, treatment with IL-33 or transfer of ILC2s ameliorated intestinal disease severity in an AREG-dependent manner. Collectively, these data reveal a critical feedback loop in which cytokine cues from damaged epithelia activate innate immune cells to express growth factors essential for ILC-dependent restoration of epithelial barrier function and maintenance of tissue homeostasis.

Eosinophilic esophagitis (EoE) is a food allergy-associated inflammatory disease characterized by esophageal eosinophilia. EoE has become increasingly common, but current management strategies are nonspecific. Thus, there is an urgent need to identify specific immunological pathways that could be targeted to treat this disease. EoE is associated with polymorphisms in the gene that encodes thymic stromal lymphopoietin (TSLP), a cytokine that promotes allergic inflammation, but how TSLP might contribute to EoE disease pathogenesis remains unknown. Here, we describe a new mouse model of EoE-like disease that developed independently of IgE but was dependent on TSLP-elicited basophils. Therapeutic TSLP neutralization or basophil depletion also ameliorated established EoE-like disease. Critically, in human subjects with EoE, we observed elevated TSLP levels and exaggerated basophil responses in esophageal biopsies, and a gain-of-function TSLP polymorphism was associated with increased basophil responses. Together, these data suggest that the TSLP-basophil axis could be therapeutically targeted to treat EoE.

Background Exposure to food allergens through a disrupted skin barrier has been recognized as a potential factor in the increasing prevalence of food allergy. Objective To test the immunological mechanisms by which epicutaneous sensitization to food allergens predisposes to intestinal food allergy. Methods Mice were epicutaneously sensitized with ovalbumin (OVA) or peanut on an atopic dermatitis-like skin lesion followed by intragastric antigen challenge. Antigen-specific serum IgE levels and Th2 cytokine responses were measured by ELISA. Expression of type-2 cytokines and mast cell proteases in the intestine were measured by real-time PCR. Accumulation of basophils in the skin and mast cells in the intestine was examined by flow cytometry. In vivo basophil depletion was achieved by diphtheria toxin treatment of Baso-DTR mice. For cell transfer studies, the basophil population was expanded in vivo by hydrodynamic tail vein injection of thymic stromal lymphopoietin cDNA plasmid. Results Sensitization to food allergens through an atopic dermatitis-like skin lesion is associated with an expansion of TSLP-elicited basophils in the skin that promote antigen-specific Th2 cytokine responses, elevated antigen-specific serum IgE levels and the accumulation of mast cells in the intestine promoting the development of intestinal food allergy. Critically, disruption of TSLP responses or depletion of basophils reduced the susceptibility to intestinal food allergy while transfer of TSLP-elicited basophils into intact skin promoted disease. Conclusion Epicutaneous sensitization on a disrupted skin barrier is associated with the accumulation of TSLP-elicited basophils that are necessary and sufficient to promote antigen-induced intestinal food allergy.

Type 2 inflammation underlies allergic diseases such as atopic dermatitis (AD), which is characterized by the accumulation of basophils and group 2 innate lymphoid cells (ILC2s) in inflamed skin lesions. Although murine studies have demonstrated that cutaneous basophil and ILC2 responses are dependent on thymic stromal lymphopoietin (TSLP), whether these cell populations interact to regulate the development of cutaneous type 2 inflammation is poorly defined. Here, we identify that basophils and ILC2s significantly accumulate in inflamed human and murine skin and form clusters not observed in control skin. We demonstrate that murine basophil responses precede ILC2 responses and that basophils are the dominant IL-4-eGFP-expressing cell type in inflamed skin. Further, basophils and IL-4 were necessary for the optimal accumulation of ILC2s and induction of AD-like disease. We show that ILC2s express the IL-4 receptor alpha (IL-4Rα) and proliferate in an IL-4-dependent manner. In addition, basophil-derived IL-4 was required for cutaneous ILC2 responses in vivo and directly regulated ILC2 proliferation ex vivo. Collectively, these data reveal a previously unrecognized role for basophil-derived IL-4 in promoting ILC2 responses during cutaneous inflammation.

Although tumor necrosis factor (α) (TNF) exerts proinflammatory activities in a variety of diseases, including inflammatory bowel disease, there is increasing evidence for antiinflammatory actions of TNF. In contrast, glucocorticoids (GCs) are steroid hormones that suppress inflammation, at least in part by regulating the expression and action of TNF. We report that TNF induces extraadrenal production of immunoregulatory GCs in the intestinal mucosa during acute intestinal inflammation. The absence of TNF results in a lack of colonic GC synthesis and exacerbation of dextran sodium sulfate–induced colitis. TNF seems to promote local steroidogenesis by directly inducing steroidogenic enzymes in intestinal epithelial cells. Therapeutic administration of TNF induces GC synthesis in oxazolone-induced colitis and ameliorates intestinal inflammation, whereas inhibition of intestinal GC synthesis abrogates the therapeutic effect of TNF. These data show that TNF suppresses the pathogenesis of acute intestinal inflammation by promoting local steroidogenesis.

The nuclear receptor liver receptor homologue-1 (LRH-1, NR5A2) is a crucial transcriptional regulator of many metabolic pathways. In addition, LRH-1 is expressed in intestinal crypt cells where it regulates the epithelial cell renewal and contributes to tumorigenesis through the induction of cell cycle proteins. We have recently identified the intestinal epithelium as an important extra-adrenal source of immunoregulatory glucocorticoids. We show here that LRH-1 promotes the expression of the steroidogenic enzymes and the synthesis of corticosterone in murine intestinal epithelial cells in vitro. Interestingly, LRH-1 is also essential for intestinal glucocorticoid synthesis in vivo, as LRH-1 haplo-insufficiency strongly reduces the intestinal expression of steroidogenic enzymes and glucocorticoid synthesis upon immunological stress. These results demonstrate for the first time a novel role for LRH-1 in the regulation of intestinal glucocorticoid synthesis and propose LRH-1 as an important regulator of intestinal tissue integrity and immune homeostasis.

The microbiome plays a fundamental role in the maturation, function, and regulation of the host-immune system from birth to old age. In return, the immune system has co-evolved a mutualistic relationship with trillions of beneficial microbes residing our bodies while mounting efficient responses to fight invading pathogens. As we age, both the immune system and the gut microbiome undergo significant changes in composition and function that correlate with increased susceptibility to infectious diseases and reduced vaccination responses. Emerging studies suggest that targeting age-related dysbiosis can improve health- and lifespan, in part through reducing systemic low-grade inflammation and immunosenescence—two hallmarks of the aging process. However—a cause and effect relationship of age-related dysbiosis and associated functional declines in immune cell functioning have yet to be demonstrated in clinical settings. This review aims to (i) give an overview on hallmarks of the aging immune system and gut microbiome, (ii) discuss the impact of age-related changes in the gut commensal community structure (introduced as microb-aging) on host-immune fitness and health, and (iii) summarize prebiotic- and probiotic clinical intervention trials aiming to reinforce age-related declines in immune cell functioning through microbiome modulation or rejuvenation.