Steven A. Saenz

MHC class II-dependent basophil-CD4(+) T cell interactions promote T(H)2 cytokine-dependent immunity

Dendritic cells can prime naive CD4+ T cells; however, here we demonstrate that dendritic cell–mediated priming was insufficient for the development of T helper type 2 cell–dependent immunity. We identify basophils as a dominant cell population that coexpressed major histocompatibility complex class II and interleukin 4 message after helminth infection. Basophilia was promoted by thymic stromal lymphopoietin, and depletion of basophils impaired immunity to helminth infection. Basophils promoted antigen-specific CD4+ T cell proliferation and interleukin 4 production in vitro, and transfer of basophils augmented the population expansion of helminth-responsive CD4+ T cells in vivo. Collectively, our studies suggest that major histocompatibility complex class II–dependent interactions between basophils and CD4+ T cells promote T helper type 2 cytokine responses and immunity to helminth infection.

IL25 elicits a multipotent progenitor cell population that promotes TH2 cytokine responses

CD4+ T helper 2 (TH2) cells secrete interleukin (IL)4, IL5 and IL13, and are required for immunity to gastrointestinal helminth infections. However, TH2 cells also promote chronic inflammation associated with asthma and allergic disorders. The non-haematopoietic-cell-derived cytokines thymic stromal lymphopoietin, IL33 and IL25 (also known as IL17E) have been implicated in inducing TH2 cell-dependent inflammation at mucosal sites, but how these cytokines influence innate immune responses remains poorly defined. Here we show that IL25, a member of the IL17 cytokine family, promotes the accumulation of a lineage-negative (Lin-) multipotent progenitor (MPP) cell population in the gut-associated lymphoid tissue that promotes TH2 cytokine responses. The IL25-elicited cell population, termed MPPtype2 cells, was defined by the expression of Sca-1 (also known as Ly6a) and intermediate expression of c-Kit (c-Kitint), and exhibited multipotent capacity, giving rise to cells of monocyte/macrophage and granulocyte lineages both in vitro and in vivo. Progeny of MPPtype2 cells were competent antigen presenting cells, and adoptive transfer of MPPtype2 cells could promote TH2 cytokine responses and confer protective immunity to helminth infection in normally susceptible Il25-/- mice. The ability of IL25 to induce the emergence of an MPPtype2 cell population identifies a link between the IL17 cytokine family and extramedullary haematopoiesis, and suggests a previously unrecognized innate immune pathway that promotes TH2 cytokine responses at mucosal sites.

Welcome to the neighborhood: epithelial cell-derived cytokines license innate and adaptive immune responses at mucosal sites

Summary: There is compelling evidence that epithelial cells (ECs) at mucosal surfaces, beyond their role in creating a physical barrier, are integral components of innate and adaptive immunity. The capacity of these cells to license the functions of specific immune cell populations in the airway and gastrointestinal tract offers the prospect of novel therapeutic strategies to target multiple inflammatory diseases in which barrier immunity is dysregulated. In this review, we discuss the critical functions of EC‐derived thymic stromal lymphopoietin (TSLP), interleukin‐25 (IL‐25), and IL‐33 in the development and regulation of T‐helper 2 (Th2) cytokine‐dependent immune responses. We first highlight recent data that have provided new insights into the factors that control expression of this triad of cytokines and their receptors. In addition, we review their proinflammatory and immunoregulatory functions in models of mucosal infection and inflammation. Lastly, we discuss new findings indicating that despite their diverse structural features and differential expression of their receptors, TSLP, IL‐25, and IL‐33 cross‐regulate one another and share overlapping properties that influence Th2 cytokine‐dependent responses at mucosal sites.

TSLP Elicits IL-33–Independent Innate Lymphoid Cell Responses to Promote Skin Inflammation

Group 2 innate lymphoid cells are essential to the pathogenesis of atopic dermatitis–like disease in a TSLP-dependent, IL-33–independent manner. Immune Cell Activity at the Skin Barrier The skin acts like soft armor, protecting the body from disease and environmental insults. In atopic dermatitis (AD), this barrier is disrupted, leading to inflammation. The role of various immune cells in this chronic disease has not been clear. Now, Kim and colleagues identify a subset of innate lymphoid cells (ILCs) in both human and mouse skin that contribute to disease pathogenesis. ILCs have been reported in inflamed nasal polyps in people, as well as in inflamed lungs in mice. Hypothesizing that they also play a role in skin inflammation, Kim et al. analyzed cells isolated from the skin tissue of healthy control subjects and from the lesions of AD patients. There were more Lin− CD25+ IL-33R+ RORγt− group 2 ILCs (ILC2s) in the lesions of AD patients. In healthy mouse skin, the authors identified a similar ILC2 population. AD in humans is linked to cytokines interleukin-33 (IL-33), IL-25, and thymic stromal lymphopoietin (TSLP) in the skin. To this end, the authors investigated in mice whether the ILC2s played a role in inflammation at the skin barrier and if they were dependent on these cytokines. In a mouse model of AD, Kim et al. noted that ILC2s were increased and that AD pathogenesis was initiated independently of adaptive immunity and RORγt+ cells (a marker of group 3 ILCs). The mechanism was also independent of IL-25 and IL-33—which are normally implicated in group 2 ILC responses—yet dependent on TSLP. Depletion of the ILCs attenuated AD-like dermatitis in mice. Group 2 ILCs have not yet been described in skin barrier function in humans. In these studies, Kim and colleagues show that ILC2s are always present in healthy skin, but accumulate in AD lesions and function by a mechanism that contrasts what has been reported in lungs and intestine. Future functional studies will be needed for human ILC2s in skin inflammation, but these preliminary data in mice and humans suggest that targeting group 2 ILCs will be a viable target for treating AD and other allergic diseases. 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 interleukin-5 (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. We identify 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, group 2 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.

Group 2 innate lymphoid cells promote beiging of white adipose tissue and limit obesity

Obesity is an increasingly prevalent disease regulated by genetic and environmental factors. Emerging studies indicate that immune cells, including monocytes, granulocytes and lymphocytes, regulate metabolic homeostasis and are dysregulated in obesity. Group 2 innate lymphoid cells (ILC2s) can regulate adaptive immunity and eosinophil and alternatively activated macrophage responses, and were recently identified in murine white adipose tissue (WAT) where they may act to limit the development of obesity. However, ILC2s have not been identified in human adipose tissue, and the mechanisms by which ILC2s regulate metabolic homeostasis remain unknown. Here we identify ILC2s in human WAT and demonstrate that decreased ILC2 responses in WAT are a conserved characteristic of obesity in humans and mice. Interleukin (IL)-33 was found to be critical for the maintenance of ILC2s in WAT and in limiting adiposity in mice by increasing caloric expenditure. This was associated with recruitment of uncoupling protein 1 (UCP1)+ beige adipocytes in WAT, a process known as beiging or browning that regulates caloric expenditure. IL-33-induced beiging was dependent on ILC2s, and IL-33 treatment or transfer of IL-33-elicited ILC2s was sufficient to drive beiging independently of the adaptive immune system, eosinophils or IL-4 receptor signalling. We found that ILC2s produce methionine-enkephalin peptides that can act directly on adipocytes to upregulate Ucp1 expression in vitro and that promote beiging in vivo. Collectively, these studies indicate that, in addition to responding to infection or tissue damage, ILC2s can regulate adipose function and metabolic homeostasis in part via production of enkephalin peptides that elicit beiging.

TSLP promotes interleukin-3-independent basophil haematopoiesis and type 2 inflammation

CD4+ T-helper type 2 (TH2) cells, characterized by their expression of interleukin (IL)-4, IL-5, IL-9 and IL-13, are required for immunity to helminth parasites and promote the pathological inflammation associated with asthma and allergic diseases. Polymorphisms in the gene encoding the cytokine thymic stromal lymphopoietin (TSLP) are associated with the development of multiple allergic disorders in humans, indicating that TSLP is a critical regulator of TH2 cytokine-associated inflammatory diseases. In support of genetic analyses, exaggerated TSLP production is associated with asthma, atopic dermatitis and food allergies in patients, and studies in murine systems demonstrated that TSLP promotes TH2 cytokine-mediated immunity and inflammation. However, the mechanisms through which TSLP induces TH2 cytokine responses remain poorly defined. Here we demonstrate that TSLP promotes systemic basophilia, that disruption of TSLP–TSLPR interactions results in defective basophil responses, and that TSLPR-sufficient basophils can restore TH2-cell-dependent immunity in vivo. TSLP acted directly on bone-marrow-resident progenitors to promote basophil responses selectively. Critically, TSLP could elicit basophil responses in both IL-3–IL-3R-sufficient and -deficient environments, and genome-wide transcriptional profiling and functional analyses identified heterogeneity between TSLP-elicited versus IL-3-elicited basophils. Furthermore, activated human basophils expressed TSLPR, and basophils isolated from eosinophilic oesophagitis patients were distinct from classical basophils. Collectively, these studies identify previously unrecognized heterogeneity within the basophil cell lineage and indicate that expression of TSLP may influence susceptibility to multiple allergic diseases by regulating basophil haematopoiesis and eliciting a population of functionally distinct basophils that promote TH2 cytokine-mediated inflammation.

Commensal-dependent expression of IL-25 regulates the IL-23–IL-17 axis in the intestine

Alterations in the composition of intestinal commensal bacteria are associated with enhanced susceptibility to multiple inflammatory diseases, including those conditions associated with interleukin (IL)-17–producing CD4+ T helper (Th17) cells. However, the relationship between commensal bacteria and the expression of proinflammatory cytokines remains unclear. Using germ-free mice, we show that the frequency of Th17 cells in the large intestine is significantly elevated in the absence of commensal bacteria. Commensal-dependent expression of the IL-17 family member IL-25 (IL-17E) by intestinal epithelial cells limits the expansion of Th17 cells in the intestine by inhibiting expression of macrophage-derived IL-23. We propose that acquisition of, or alterations in, commensal bacteria influences intestinal immune homeostasis via direct regulation of the IL-25–IL-23–IL-17 axis.

Innate immune cell populations function as initiators and effectors in Th2 cytokine responses

The recent identification of previously unrecognized innate cell populations, termed natural helper cells (NHCs), multi-potent progenitor type 2 (MPP(type2)) cells, nuocytes, and innate type 2 helper (Ih2) cells has provided new insights into our understanding of the cellular mechanisms that lead to the development of CD4(+) Th2 cell-dependent immunity and/or inflammation at mucosal sites. In this review, we focus on the functional significance, similarities, and differences between NHCs, MPP(type2) cells, nuocytes and Ih2 cells. All four cell populations are activated by interleukin (IL)-25 and/or IL-33 and are capable of promoting Th2 cytokine responses. Collectively, the identification of these cell populations might illuminate ancient evolutionary conserved pathways that are involved in the development of Th2 cytokine responses, and could be of benefit in the development of therapeutic approaches that target helminth infections and allergic diseases.

Basophils Promote Innate Lymphoid Cell Responses in Inflamed Skin

Type 2 inflammation underlies allergic diseases such as atopic dermatitis, 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, whether these cell populations interact to regulate the development of cutaneous type 2 inflammation is poorly defined. In this study, 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–enhanced GFP-expressing cell type in inflamed skin. Furthermore, basophils and IL-4 were necessary for the optimal accumulation of ILC2s and induction of atopic dermatitis–like disease. We show that ILC2s express IL-4Rα and proliferate in an IL-4–dependent manner. Additionally, 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.

Cutting Edge: Natural Helper Cells Derive from Lymphoid Progenitors

Natural helper (NH) cells are recently discovered innate immune cells that confer protective type 2 immunity during helminth infection and mediate influenza-induced airway hypersensitivity. Little is known about the ontogeny of NH cells. We report in this study that NH cells derive from bone marrow lymphoid progenitors. Using RAG-1Cre/ROSA26YFP mice, we show that most NH cells are marked with a history of RAG-1 expression, implying lymphoid developmental origin. The development of NH cells depends on the cytokine receptor Flt3, which is required for the efficient generation of bone marrow lymphoid progenitors. Finally, we demonstrate that lymphoid progenitors, but not myeloid–erythroid progenitors, give rise to NH cells in vivo. This work therefore expands the lymphocyte family, currently comprising T, B, and NK cells, to include NH cells as another type of innate lymphocyte that derives from bone marrow lymphoid progenitors.