Lineage(-)Sca1+c-Kit(-)CD25+ cells are IL-33-responsive type 2 innate cells in the mouse bone marrow

Group 2 innate lymphoid cells support hematopoietic recovery under stress conditions

The cell-cycle status of hematopoietic stem and progenitor cells (HSPCs) becomes activated following chemotherapy-induced stress, promoting bone marrow (BM) regeneration; however, the underlying molecular mechanism remains elusive. Here we show that BM-resident group 2 innate lymphoid cells (ILC2s) support the recovery of HSPCs from 5-fluorouracil (5-FU)-induced stress by secreting granulocyte-macrophage colony-stimulating factor (GM-CSF). Mechanistically, IL-33 released from chemo-sensitive B cell progenitors activates MyD88-mediated secretion of GM-CSF in ILC2, suggesting the existence of a B cell-ILC2 axis for maintaining hematopoietic homeostasis. GM-CSF knockout mice treated with 5-FU showed severe loss of myeloid lineage cells, causing lethality, which was rescued by transferring BM ILC2s from wild-type mice. Further, the adoptive transfer of ILC2s to 5-FU-treated mice accelerates hematopoietic recovery, while the reduction of ILC2s results in the opposite effect. Thus, ILC2s may function by "sensing" the damaged BM spaces and subsequently support hematopoietic recovery under stress conditions.

The central role of IL-33/IL-1RL1 pathway in asthma: From pathogenesis to intervention

Interleukin-33 (IL-33), a member of the IL-1 family, and its cognate receptor, Interleukin-1 receptor like-1 (IL-1RL1 or ST2), are susceptibility genes for childhood asthma. In response to cellular damage, IL-33 is released from barrier tissues as an 'alarmin' to activate the innate immune response. IL-33 drives type 2 responses by inducing signalling through its receptor IL-1RL1 in several immune and structural cells, thereby leading to type 2 cytokine and chemokine production. IL-1RL1 gene transcript encodes different isoforms generated through alternative splicing. Its soluble isoform, IL-1RL1-a or sST2, acts as a decoy receptor by sequestering IL-33, thereby inhibiting IL1RL1-b/IL-33 signalling. IL-33 and its receptor IL-1RL1 are therefore considered as putative biomarkers or targets for pharmacological intervention in asthma. This review will provide an overview of the genetics and biology of the IL-33/IL-1RL1 pathway in the context of asthma pathogenesis. It will discuss the potential and complexities of targeting the cytokine or its receptor, how genetics or biomarkers may inform precision medicine for asthma targeting this pathway, and the possible positioning of therapeutics targeting IL-33 or its receptor in the expanding landscape of novel biologicals applied in asthma management.

Aurka loss in CD19+ B cells promotes megakaryocytopoiesis via IL-6/STAT3 signaling-mediated thrombopoietin production

Rationale: Aurora kinase A (Aurora-A), which is required for mitosis, is a therapeutic target in various tumors. Targeting Aurora-A led to an increase in the differentiation and polyploidization of megakaryocytes both in vivo and in vitro. However, the mechanisms involved in controlling megakaryocyte differentiation have not been fully elucidated.

Methods: Conditional Aurka knockout mice were generated. B cell development, platelet development and function were subsequently examined. Proplatelet formation, in vivo response to mTPO, post-transfusion experiment, colony assay, immunofluorescence staining and quantification, and ChIP assay were conducted to gain insights into the mechanisms of Aurka loss in megakaryocytopoiesis.

Results: Loss of Aurka in CD19⁺ B cells impaired B cell development in association with an increase in the number of platelets in peripheral blood (PB). Surprisingly, thrombopoietin (TPO) production and IL-6 were elevated in the plasma in parallel with an increase in the number of differentiated megakaryocytes in the bone marrow (BM) of Aurka^f/f;Cd19^Cre/+ mice. Interestingly, compared with that of the Aurka^f/f mice, a higher number of CD19⁺ B cells close to megakaryocytes was observed in the BM of the Aurka^f/f;Cd19^Cre/+ mice. Moreover, Aurka loss in CD19⁺ B cells induced signal transducer and activator of transcription-3 (STAT3) activation. Inhibition of STAT3 reduced the Tpo mRNA levels. ChIP assays revealed that STAT3 bound to the TPO promoter. Additionally, STAT3-mediated TPO transcription was an autocrine effect provoked by IL-6, at least partially.

Conclusions: Deletion of Aurka in CD19⁺ B cells led to an increase in IL-6 production, promoting STAT3 activation, which in turn contributed to TPO transcription and megakaryocytopoiesis.

TSLP and IL-33 reciprocally promote each other's lung protein expression and ILC2 receptor expression to enhance innate type-2 airway inflammation

Background

The epithelial cell‐derived danger signal mediators thymic stromal lymphopoietin (TSLP) and IL‐33 are consistently associated with adaptive Th2 immune responses in asthma. In addition, TSLP and IL‐33 synergistically promoted group 2 innate lymphoid cell (ILC2) activation to induce innate allergic inflammation. However, the mechanism of this synergistic ILC2 activation is unknown.

Methods

BALB/c WT and TSLP receptor‐deficient (TSLPR^−/−) mice were challenged intranasally with Alternaria extract (Alt‐Ext) or PBS for 4 consecutive days to evaluate innate airway allergic inflammation. WT mice pre‐administered with rTSLP or vehicle, TSLPR^−/− mice, and IL‐33 receptor‐deficient (ST2^−/−) mice were challenged intranasally with Alt‐Ext or vehicle once or twice to evaluate IL‐33 release and TSLP expression in the lung. TSLPR and ST2 expression on lung ILC2 were measured by flow cytometry after treatment of rTSLP, rIL‐33, rTSLP + rIL‐33, or vehicle.

Results

Thymic stromal lymphopoietin receptor deficient mice had significantly decreased the number of lung ILC2 expressing IL‐5 and IL‐13 following Alt‐Ext‐challenge compared to WT mice. Further, eosinophilia, protein level of lung IL‐4, IL‐5, and IL‐13, and airway mucus score were also significantly decreased in TSLPR^−/− mice compared to WT mice. Endogenous and exogenous TSLP increased Alt‐Ext‐induced IL‐33 release into BALF, and ST2 deficiency decreased Alt‐Ext‐induced TSLP expression in the lung. Further, rTSLP and rIL‐33 treatment reciprocally increased each other's receptor expression on lung ILC2 in vivo and in vitro.

Conclusion

Thymic stromal lymphopoietin and IL‐33 signaling reciprocally enhanced each other's protein release and expression in the lung following Alt‐Ext‐challenge and each other's receptor expression on lung ILC2 to enhance ILC2 activation.

Interplay Between the IL-33/ST2 Axis and Bone Marrow ILC2s in Protease Allergen-Induced IL-5-Dependent Eosinophilia

Background: Eosinophils develop from CD34⁺ progenitor cells in the bone marrow under the influence of interleukin (IL)-5. Several cell types produce IL-5, including type 2 innate lymphoid cells (ILC2s). The alarmin cytokine IL-33 is known to activate ILC2s in mucosal tissues, but little is known about IL-33-responsive ILC2s in the bone marrow in allergen-induced airway inflammation.

Methods: Wild type (WT) and Rag1 deficient (Rag1^−/−) mice, which lack mature T and B cells, received intranasal doses of papain to induce acute allergic inflammation. In some experiments, mice were pre-treated with anti-IL-5 prior to the papain challenge. Furthermore, recombinant IL-33 was administered to WT mice, Rag1^−/− mice, lymphocyte deficient mice (Rag2^−/−Il2rg^−/−) and to ex vivo whole bone marrow cultures. Bone marrow eosinophils and ILC2s were analyzed by flow cytometry. Eosinophil count was assessed by differential cell count and secreted IL-5 from bone marrow cells by ELISA.

Results: Intranasal administration of papain or IL-33 increased the number of mature eosinophils in the bone marrow despite the absence of adaptive immune cells in Rag1^−/− mice. In parallel, an increased number of eosinophils was observed in the airways together with elevated levels of Eotaxin-2/CCL24. Bone marrow ILC2s were increased after papain or IL-33 administration, whereas ILC2s was found to be increased at baseline in Rag1^−/− mice compared to WT mice. An upregulation of the IL-33 receptor (ST2) expression on bone marrow ILC2s was observed after papain challenge in both Rag1^−/− and WT mice which correlated to increased number of bone marrow eosinophilia. Furthermore, an increased number of ST2⁺ mature eosinophils in the bone marrow was observed after papain challenge, which was further dependent on IL-5. In addition, bone marrow-derived ILC2s from both mouse strains produced large amounts of IL-5 ex vivo after IL-33 stimulation of whole bone marrow cultures. In contrast, IL-33-induced bone marrow and airway eosinophilia were abolished in the absence of ILC2s in Rag2^−/−Il2rg^−/− mice and no production of IL-5 was detected in IL-33-stimulated bone marrow cultures.

Conclusion: These findings establish bone marrow ILC2s and the IL-33/ST2 axis as promising targets for modulation of uncontrolled IL-5-dependent eosinophilic diseases including asthma.

Regnase-1 degradation is crucial for IL-33- and IL-25-mediated ILC2 activation

Group 2 innate lymphoid cells (ILC2s) are a critical innate source of type 2 cytokines in allergic inflammation. Although ILC2s are recognized as a critical cell population in the allergic inflammation, the regulatory mechanism(s) of ILC2s are less well understood. Here, we show that Regnase-1, an immune regulatory RNAse that degrades inflammatory mRNAs, negatively regulates ILC2 function and that IκB kinase (IKK) complex-mediated Regnase-1 degradation is essential for IL-33- and IL-25-induced ILC2 activation. ILC2s from Regnase-1AA/AA mice expressing a Regnase-1 S435A/S439A mutant resistant to IKK complex-mediated degradation accumulated Regnase-1 protein in response to IL-33 and IL-25. IL-33- and IL-25-stimulated Regnase-1AA/AA ILC2s showed reduced cell proliferation and type 2 cytokine (IL-5, IL-9, and IL-13) production and increased cell death. In addition, Il2ra and Il1rl1, but not Il5, Il9, or Il13, mRNAs were destabilized in IL-33-stimulated Regnase-1AA/AA ILC2s. In vivo, Regnase-1AA/AA mice showed attenuated acute type 2 pulmonary inflammation induced by the instillation of IL-33, IL-25, or papain. Furthermore, the expulsion of Nippostrongylus brasiliensis was significantly delayed in Regnase-1AA/AA mice. These results demonstrate that IKK complex-mediated Regnase-1 degradation is essential for ILC2-mediated type 2 responses both in vitro and in vivo. Therefore, controlling Regnase-1 degradation is a potential therapeutic target for ILC2-contributed allergic disorders.

PAG1 limits allergen-induced type 2 inflammation in the murine lung

BACKGROUND

Phosphoprotein associated with glycosphingolipid-enriched microdomains 1 (PAG1) is a transmembrane adaptor protein that affects immune receptor signaling in T and B cells. Evidence from genome-wide association studies of asthma suggests that genetic variants that regulate the expression of PAG1 are associated with asthma risk. However, it is not known whether PAG1 expression is causally related to asthma pathophysiology. Here, we investigated the role of PAG1 in a preclinical mouse model of house dust mite (HDM)-induced allergic sensitization and allergic airway inflammation.

METHODS

Pag1-deficient (Pag1^-/- ) and wild-type (WT) mice were sensitized or sensitized/challenged to HDM, and hallmark features of allergic inflammation were assessed. The contribution of T cells was assessed through depletion (anti-CD4 antibody) and adoptive transfer studies.

RESULTS

Type 2 inflammation (eosinophilia, eotaxin-2 expression, IL-4/IL-5/IL-13 production, mucus production) in the airways and lungs was significantly increased in HDM sensitized/challenged Pag1^-/- mice compared to WT mice. The predisposition to allergic sensitization was associated with increased airway epithelial high-mobility group box 1 (HMGB1) translocation and release, increased type 2 innate lymphoid cells (ILC2s) and monocyte-derived dendritic cell numbers in the mediastinal lymph nodes, and increased T-helper type 2 (T_H 2)-cell differentiation. CD4⁺ T-cell depletion studies or the adoptive transfer of WT OVA-specific CD4⁺ T cells to WT or Pag1^-/- recipients demonstrated that the heightened propensity for T_H 2-cell differentiation was both T cell intrinsic and extrinsic.

CONCLUSION

PAG1 deficiency increased airway epithelial activation, ILC2 expansion, and T_H 2 differentiation. As a consequence, PAG1 deficiency predisposed toward allergic sensitization and increased the severity of experimental asthma.

Interleukin‑33 expression in ovarian cancer and its possible suppression of peritoneal carcinomatosis

Refractory peritoneal carcinomatosis is a common terminal feature of epithelial ovarian cancer (EOC). Previous reports have suggested that immunotherapy is a promising therapeutic strategy for EOC. Interleukin (IL)‑33 is a member of the IL‑1 superfamily of cytokines. The role of IL‑33 in tissue inflammation and promoting type 2 immune responses has been established, and recently, there is accumulating evidence to suggest the involvement of IL‑33 in carcinogenesis. In this study, we focused on the association between the tumor expression of IL‑33 and ovarian peritoneal carcinomatosis. We used an immunosufficient murine model of peritoneal carcinomatosis and human EOC samples. The overexpression of IL‑33 in the ID8 mouse EOC cell line tumors significantly prolonged the survival of immunocompetent mice in the peritoneal carcinomatosis setting, but not in the subcutaneous model. In addition, the silencing of IL‑33 in ID8‑T6 cells (subclone with high dissemination potential) significantly shortened the survival of the tumor‑bearing mice. This was likely due to the intratumoral accumulation of CD8+ and CD4+ T cells, and a decrease in CD11b+Gr1+ cells. Furthermore, IL‑33 induced the intraperitoneal microenvironment favoring tumor elimination through the inhibition of differentiation into CD11b+Gr1+ cells. On the whole, the findings of this study suggest IL‑33 to be a cytokine that reflects antitumor peritoneal conditions. Further investigation of the antitumorigenic role of IL‑33 may aid in the development of more effective therapeutic approaches for the treatment of EOC with peritoneal carcinomatosis.

Interleukin-33 and ST2 Signaling in Tumor Microenvironment

Interleukin-33 (IL-33) is one of the members of the IL-1 family of cytokines and a ligand of ST2 and IL-1 receptor accessory protein (IL-1RAcP) that is known to affect Th2 inflammatory response with partial effects on Th1 responses. This cytokine is released by epithelial and smooth muscle cells of the airway system during their injury by several environmental stimuli, such as allergens, viruses, helminths, and pollutants. IL-33 is an alarmin that acts as an endogenous danger signal, and it has been known to affect various types of cells, such as mast cells, basophils, eosinophils, T cells, and specific subsets of innate lymphoid cells (ILCs). In recent findings, this cytokine is believed to have a critical role in several types of cancers, such as lung cancer, liver cancer, and head and neck squamous cell cancer. The expression of IL-33/ST2 in cancer tissues shows a close association with tumor growth and tumor progression in several types of cancer, suggesting the IL-33/ST2 pathway as a potential target for therapy.

Chinese Society of Allergy Guidelines for Diagnosis and Treatment of Allergic Rhinitis

Allergic rhinitis (AR) is a global health problem that causes major illnesses and disabilities worldwide. Epidemiologic studies have demonstrated that the prevalence of AR has increased progressively over the last few decades in more developed countries and currently affects up to 40% of the population worldwide. Likewise, a rising trend of AR has also been observed over the last 2-3 decades in developing countries including China, with the prevalence of AR varying widely in these countries. A survey of self-reported AR over a 6-year period in the general Chinese adult population reported that the standardized prevalence of adult AR increased from 11.1% in 2005 to 17.6% in 2011. An increasing number of Journal Articles and imporclinical trials on the epidemiology, pathophysiologic mechanisms, diagnosis, management and comorbidities of AR in Chinese subjects have been published in international peer-reviewed journals over the past 2 decades, and substantially added to our understanding of this disease as a global problem. Although guidelines for the diagnosis and treatment of AR in Chinese subjects have also been published, they have not been translated into English and therefore not generally accessible for reference to non-Chinese speaking international medical communities. Moreover, methods for the diagnosis and treatment of AR in China have not been standardized entirely and some patients are still treated according to regional preferences. Thus, the present guidelines have been developed by the Chinese Society of Allergy to be accessible to both national and international medical communities involved in the management of AR patients. These guidelines have been prepared in line with existing international guidelines to provide evidence-based recommendations for the diagnosis and management of AR in China.

Adenosine receptors differentially regulate type 2 cytokine production by IL-33-activated bone marrow cells, ILC2s, and macrophages

Group 2 innate lymphoid cells (ILC2s) represent a rapid source of type 2 cytokines, such as IL-5 and IL-13, and play an important role in orchestrating type 2 immune response. Adenosine is an endogenous purine nucleoside, a catabolite of ATP that binds and activates ≥1 of 4 transmembrane G protein-coupled cell-surface adenosine receptors (ARs)-A1, A2A, A2B, and A3. Here, we studied the role of ARs in the regulation of cytokine production by ILC2s. We found that A2BARs suppress the production of both IL-5 and IL-13 by ILC2s, whereas A2AARs augment IL-5 production and fail to affect IL-13 release. Combined stimulation of all ARs led to the suppression of both IL-5 and IL-13 production, which indicated that A2BARs dominate A2AARs. Both pre- and post-transcriptional processes may be involved in the AR modulation of ILC2 IL-5 and IL-13 production. Thus, we identify adenosine as a novel negative regulator of ILC2 activation.-Csóka, B., Németh, Z. H., Duerr, C. U., Fritz, J. H., Pacher, P., Haskó, G. Adenosine receptors differentially regulate type 2 cytokine production by IL-33-activated bone marrow cells, ILC2s, and macrophages.

Bone marrow type 2 innate lymphoid cells: a local source of interleukin-5 in interleukin-33-driven eosinophilia

T helper type 2 (Th2) cells, type 2 innate lymphoid cells (ILC2s) and eosinophil progenitors have previously been described to produce interleukin-5 (IL-5) in the airways upon allergen provocation or by direct administration of IL-33. Eosinophilic airway inflammation is known to be associated with IL-5-dependent eosinophil development in the bone marrow, however, the source of IL-5 remains unclear. T helper cells, ILC2s and CD34⁺ progenitors have been proposed to be involved in this process, therefore, we investigated whether these cells are taking part in eosinophilopoiesis by producing IL-5 locally in the bone marrow in IL-33-driven inflammation. Airway exposure with IL-33 led to eosinophil infiltration in airways and elevated eotaxin-2/CCL24. Importantly, IL-5 production as well as expression of the IL-33 receptor increased in ILC2s in the bone marrow under this treatment. A small but significant induction of IL-5 was also found in CD34⁺ progenitors but not in T helper cells. Similar results were obtained by in vitro stimulation with IL-33 where ILC2s rapidly produced large amounts of IL-5, which coincided with the induction of eosinophil hematopoiesis. IL-33-mediated eosinophil production was indeed dependent on IL-5 as both airway and bone marrow eosinophils decreased in mice treated with anti-IL-5 in combination with IL-33. Interestingly, the responsiveness of ILC2s to IL-33 as well as IL-33-induced eotaxin-2/CCL24 were independent of the levels of IL-5. In summary, we demonstrate for the first time that IL-33 acts directly on bone marrow ILC2s, making them an early source of IL-5 and part of a process that is central in IL-33-driven eosinophilia.

IL-33 Precedes IL-5 in Regulating Eosinophil Commitment and Is Required for Eosinophil Homeostasis

Eosinophils are important in the pathogenesis of many diseases, including asthma, eosinophilic esophagitis, and eczema. Whereas IL-5 is crucial for supporting mature eosinophils (EoMs), the signals that support earlier eosinophil lineage events are less defined. The IL-33R, ST2, is expressed on several inflammatory cells, including eosinophils, and is best characterized for its role during the initiation of allergic responses in peripheral tissues. Recently, ST2 expression was described on hematopoietic progenitor subsets, where its function remains controversial. Our findings demonstrate that IL-33 is required for basal eosinophil homeostasis, because both IL-33- and ST2-deficient mice exhibited diminished peripheral blood eosinophil numbers at baseline. Exogenous IL-33 administration increased EoMs in both the bone marrow and the periphery in wild-type and IL-33-deficient, but not ST2-deficient, mice. Systemic IL-5 was also increased under this treatment, and blocking IL-5 with a neutralizing Ab ablated the IL-33-induced EoM expansion. The homeostatic hypereosinophilia seen in IL-5-transgenic mice was significantly lower with ST2 deficiency despite similar elevations in systemic IL-5. Finally, in vitro treatment of bone marrow cells with IL-33, but not IL-5, led to specific early expansion of IL-5Rα-expressing precursor cells. In summary, our findings establish a basal defect in eosinophilopoiesis in IL-33- and ST2-deficient mice and a mechanism whereby IL-33 supports EoMs by driving both systemic IL-5 production and the expansion of IL-5Rα-expressing precursor cells.

IL-33 inhibits the differentiation and immunosuppressive activity of granulocytic myeloid-derived suppressor cells in tumor-bearing mice

Myeloid-derived suppressor cells (MDSCs) contribute to tumor-mediated immune escape by suppressing antitumor immune responses. Interleukin-33 (IL-33) is capable of regulating various immune cell populations; however, the effects of IL-33 on the differentiation of MDSCs have not been well characterized. In this study, we evaluated the effects of IL-33 on MDSCs and found that IL-33 significantly reduced the differentiation of lineage-negative bone marrow progenitor cells into granulocytic MDSCs (G-MDSCs). IL-33-treated MDSCs exhibited diminished immunosuppressive capacity; reduced inhibition on T-cell proliferation and interferon-γ production, and diminished production of reactive oxygen species. However, IL-33 treatment did not affect the frequency of monocytic MDSCs (M-MDSCs) or their production of nitric oxide and expression of arginase-1. Additionally, compared with control MDSCs, IL-33-treated MDSCs had reduced capacity to induce the differentiation or expansion of Treg cells. Moreover, in vivo IL-33 administration significantly decreased MDSCs and G-MDSCs accumulation in the spleen and tumor microenvironment. Also, despite increasing CD4⁺ and CD8⁺ T-cell infiltration, IL-33 administration markedly decreased Treg-cell population in tumor microenvironment. Taken together, our findings indicate that IL-33 reduces the frequency and immunosuppressive activity of G-MDSCs and ultimately the extent of tumor growth.

In Vivo Expansion of Activated Foxp3+ Regulatory T Cells and Establishment of a Type 2 Immune Response upon IL-33 Treatment Protect against Experimental Arthritis

IL-33 is strongly involved in several inflammatory and autoimmune disorders with both pro- and anti-inflammatory properties. However, its contribution to chronic autoimmune inflammation, such as rheumatoid arthritis, is ill defined and probably requires tight regulation. In this study, we aimed at deciphering the complex role of IL-33 in a model of rheumatoid arthritis, namely, collagen-induced arthritis (CIA). We report that repeated injections of IL-33 during induction (early) and during development (late) of CIA strongly suppressed clinical and histological signs of arthritis. In contrast, a late IL-33 injection had no effect. The cellular mechanism involved in protection was related to an enhanced type 2 immune response, including the expansion of eosinophils, Th2 cells, and type 2 innate lymphoid cells, associated with an increase in type 2 cytokine levels in the serum of IL-33-treated mice. Moreover, our work strongly highlights the interplay between IL-33 and regulatory T cells (Tregs), demonstrated by the dramatic in vivo increase in Treg frequencies after IL-33 treatment of CIA. More importantly, Tregs from IL-33-treated mice displayed enhanced capacities to suppress IFN-γ production by effector T cells, suggesting that IL-33 not only favors Treg proliferation but also enhances their immunosuppressive properties. In concordance with these observations, we found that IL-33 induced the emergence of a CD39(high) Treg population in a ST2L-dependent manner. Our findings reveal a powerful anti-inflammatory mechanism by which IL-33 administration inhibits arthritis development.

An Atypical Splenic B Cell Progenitor Population Supports Antibody Production during Plasmodium Infection in Mice

Hematopoietic stem and progenitor cells (HSPCs) function to replenish the immune cell repertoire under steady-state conditions and in response to inflammation due to infection or stress. Whereas the bone marrow serves as the primary niche for hematopoiesis, extramedullary mobilization and differentiation of HSPCs occur in the spleen during acute Plasmodium infection, a critical step in the host immune response. In this study, we identified an atypical HSPC population in the spleen of C57BL/6 mice, with a lineage(-)Sca-1(+)c-Kit(-) (LSK(-)) phenotype that proliferates in response to infection with nonlethal Plasmodium yoelii 17X. Infection-derived LSK(-) cells upon transfer into naive congenic mice were found to differentiate predominantly into mature follicular B cells. However, when transferred into infection-matched hosts, infection-derived LSK(-) cells gave rise to B cells capable of entering into a germinal center reaction, and they developed into memory B cells and Ab-secreting cells that were capable of producing parasite-specific Abs. Differentiation of LSK(-) cells into B cells in vitro was enhanced in the presence of parasitized RBC lysate, suggesting that LSK(-) cells expand and differentiate in direct response to the parasite. However, the ability of LSK(-) cells to differentiate into B cells was not dependent on MyD88, as myd88(-/-) LSK(-) cell expansion and differentiation remained unaffected after Plasmodium infection. Collectively, these data identify a population of atypical lymphoid progenitors that differentiate into B lymphocytes in the spleen and are capable of contributing to the ongoing humoral immune response against Plasmodium infection.

Fight or flight: regulation of emergency hematopoiesis by pyroptosis and necroptosis

PURPOSE OF REVIEW

A feature of the innate immune response that is conserved across kingdoms is the induction of cell death. In this review, we discuss the direct and indirect effects of increased inflammatory cell death, including pyroptosis - a caspase-1-dependent cell death - and necroptosis - a receptor-interacting protein kinase 3/mixed lineage kinase domain-like protein-dependent, caspase-independent cell death - on emergency hematopoiesis.

RECENT FINDINGS

Activation of nonapoptotic cell death pathways during infection can trigger release of cytokines and/or damage-associated molecular patterns such as interleukin (IL)-1α, IL-1β, IL-18, IL-33, high-mobility group protein B1, and mitochondrial DNA to promote emergency hematopoiesis. During systemic infection, pyroptosis and necroptosis can directly kill hematopoietic stem and progenitor cells, which results in impaired hematopoiesis, cytopenia, and immunosuppression. Although originally described as discrete entities, there now appear to be more intimate connections between the nonapoptotic and death receptor signaling pathways.

SUMMARY

The choice to undergo pyroptotic and necroptotic cell death constitutes a rapid response system serving to eliminate infected cells, including hematopoietic stem and progenitor cells. This system has the potential to be detrimental to emergency hematopoiesis during severe infection. We discuss the potential of pharmacological intervention for the pyroptosis and necroptosis pathways that may be beneficial during periods of infection and emergency hematopoiesis.

Gene expression patterns in CD4+ peripheral blood cells in healthy subjects and stage IV melanoma patients

Melanoma patients exhibit changes in immune responsiveness in the local tumor environment, draining lymph nodes, and peripheral blood. Immune-targeting therapies are revolutionizing melanoma patient care increasingly, and studies show that patients derive clinical benefit from these newer agents. Nonetheless, predicting which patients will benefit from these costly therapies remains a challenge. In an effort to capture individual differences in immune responsiveness, we are analyzing patterns of gene expression in human peripheral blood cells using RNAseq. Focusing on CD4+ peripheral blood cells, we describe multiple categories of immune regulating genes, which are expressed in highly ordered patterns shared by cohorts of healthy subjects and stage IV melanoma patients. Despite displaying conservation in overall transcriptome structure, CD4+ peripheral blood cells from melanoma patients differ quantitatively from healthy subjects in the expression of more than 2000 genes. Moreover, 1300 differentially expressed genes are found in transcript response patterns following activation of CD4+ cells ex vivo, suggesting that widespread functional discrepancies differentiate the immune systems of healthy subjects and melanoma patients. While our analysis reveals that the transcriptome architecture characteristic of healthy subjects is maintained in cancer patients, the genes expressed differentially among individuals and across cohorts provide opportunities for understanding variable immune states as well as response potentials, thus establishing a foundation for predicting individual responses to stimuli such as immunotherapeutic agents.

Mast cells mediate malignant pleural effusion formation

Mast cells (MCs) have been identified in various tumors; however, the role of these cells in tumorigenesis remains controversial. Here, we quantified MCs in human and murine malignant pleural effusions (MPEs) and evaluated the fate and function of these cells in MPE development. Evaluation of murine MPE-competent lung and colon adenocarcinomas revealed that these tumors actively attract and subsequently degranulate MCs in the pleural space by elaborating CCL2 and osteopontin. MCs were required for effusion development, as MPEs did not form in mice lacking MCs, and pleural infusion of MCs with MPE-incompetent cells promoted MPE formation. Once homed to the pleural space, MCs released tryptase AB1 and IL-1β, which in turn induced pleural vasculature leakiness and triggered NF-κB activation in pleural tumor cells, thereby fostering pleural fluid accumulation and tumor growth. Evaluation of human effusions revealed that MCs are elevated in MPEs compared with benign effusions. Moreover, MC abundance correlated with MPE formation in a human cancer cell-induced effusion model. Treatment of mice with the c-KIT inhibitor imatinib mesylate limited effusion precipitation by mouse and human adenocarcinoma cells. Together, the results of this study indicate that MCs are required for MPE formation and suggest that MC-dependent effusion formation is therapeutically addressable.

Transcription factors controlling innate lymphoid cell fate decisions

The mucosal epithelium is in direct contact with symbiotic and pathogenic microorganisms. Therefore, the mucosal surface is the principal portal of entry for invading pathogens and immune cells accumulated in the intestine to prevent infections. In addition to these conventional immune system functions, it has become clear that immune cells during steady-state continuously integrate microbial and nutrient-derived signals from the environment to support organ homeostasis. A major role in both processes is played by a recently discovered group of lymphocytes referred to as innate lymphoid cells (ILCs) Innate lymphoid cells (ILCs) that are specifically enriched at mucosal surfaces but are rather rare in secondary lymphoid organs. In analogy to the dichotomy between CD8 and CD4 T cells, we propose to classify ILCs into interleukin-7 receptor α-negative cytotoxic ILCs and IL-7Rα(+) helper-like ILCs. Dysregulated immune responses triggered by the various ILC subsets have been linked to inflammatory diseases such as inflammatory bowel disease, atopic dermatitis and airway hyperresponsiveness. Here, we will review recent progress in determining the transcriptional and developmental programs that control ILC fate decisions.

Severe neutrophil-dominated inflammation and enhanced myelopoiesis in IL-33-overexpressing CMV/IL33 mice

IL-33 is a cytokine of the IL-1 family, which signals through the ST2 receptor. Previous studies emphasized a role for IL-33 in shaping innate and adaptive immune responses. IL-33 was also reported to modulate myelopoiesis and myeloid cell activity. In this article, we describe IL-33-overexpressing CMV/IL33 and LysM/IL33 mice, which display an inflammatory phenotype associated with growth retardation and paw swelling. The phenotype of CMV/IL33 mice is dependent on activation of the ST2 receptor and is characterized by extensive neutrophil infiltration into different organs, including the paws. Local or systemic levels of proinflammatory mediators such as IL-1β, Cxcl-1, G-CSF, and IL-6 are increased. CMV/IL-33 mice also suffer from anemia, thrombocytosis, and a marked dysregulation of myelopoiesis, leading to an important increase in myeloid cell production or accumulation in bone marrow (BM), spleen, and peripheral blood. Consistently, recombinant IL-33 induced proliferation of myeloid lineage cells in BM-derived granulocyte cultures, whereas IL-33 knockout mice exhibited minor deficiencies in spleen and BM myeloid cell populations. Our observations reveal a neutrophil-dominated inflammatory phenotype in IL-33-overexpressing CMV/IL33 and LysM/IL33 mice, and highlight important regulatory effects of IL-33 on myelopoiesis in vitro and in vivo, where excessive IL-33 signaling can translate into the occurrence of a myeloproliferative disorder.

Enhanced innate type 2 immune response in peripheral blood from patients with asthma

BACKGROUND

In mice, group 2 innate lymphoid cells (ILC2s) likely mediate helminth immunity, inflammation, and tissue repair and remodeling. However, the involvement of ILC2s in human diseases, such as asthma, is not well understood.

OBJECTIVES

The goals of this study were to investigate whether peripheral blood specimens can be used to monitor innate type 2 immunity in human subjects and to examine whether ILC2s are involved in human asthma.

METHODS

PBMCs from subjects with allergic asthma (AA), subjects with allergic rhinitis (AR), or healthy control (HC) subjects were cultured in vitro with IL-25 or IL-33. Flow cytometry and cell sorting were used to identify, isolate, and quantitate ILC2s in PBMCs.

RESULTS

Human PBMCs produced IL-5 and IL-13 when stimulated with IL-33 or IL-25 in the presence of IL-2 without antigens. In addition, IL-7 or thymic stromal lymphopoietin were able to replace IL-2. The cell population with phenotypic ILC2 characteristics, lineage(-)CD127(+)CRTH2(+) cells, responded to IL-33 and produced large quantities of IL-5 and IL-13 but undetectable levels of IL-4. PBMCs from subjects with AA produced significantly larger amounts of IL-5 and IL-13 in response to IL-25 or IL-33 than from subjects with AR or HC. The prevalence of ILC2s in blood was greater in the AA group than in the AR group or the HC group.

CONCLUSIONS

Innate type 2 immune responses are increased in asthma but not in AR, suggesting potential differences in the immunopathogenesis of these diseases. Peripheral blood is useful for evaluating innate type 2 immunity in humans.

Cytokine signaling in the differentiation of innate effector cells

Innate effector cells, including innate effector cells of myeloid and lymphoid lineages, are crucial components of various types of immune responses. Bone marrow progenitors differentiate into many subsets of innate effector cells after receiving instructional signals often provided by cytokines. Signal transducer and activator of transcription (STATs) have been shown to be essential in the differentiation of various types of innate effector cells. In this review, we focus specifically on the differentiation of innate effector cells, particularly the role of cytokine signaling in the differentiation of innate effector cells.

Neonatal rhinovirus induces mucous metaplasia and airways hyperresponsiveness through IL-25 and type 2 innate lymphoid cells

BACKGROUND

Early-life human rhinovirus infection has been linked to asthma development in high-risk infants and children. Nevertheless, the role of rhinovirus infection in the initiation of asthma remains unclear.

OBJECTIVE

We hypothesized that, in contrast to infection of mature BALB/c mice, neonatal infection with rhinovirus promotes an IL-25-driven type 2 response, which causes persistent mucous metaplasia and airways hyperresponsiveness.

METHODS

Six-day-old and 8-week-old BALB/c mice were inoculated with sham HeLa cell lysate or rhinovirus. Airway responses from 1 to 28 days after infection were assessed by using quantitative PCR, ELISA, histology, immunofluorescence microscopy, flow cytometry, and methacholine responsiveness. Selected mice were treated with a neutralizing antibody to IL-25.

RESULTS

Compared with mature mice, rhinovirus infection in neonatal mice increased lung IL-13 and IL-25 production, whereas IFN-γ, IL-12p40, and TNF-α expression was suppressed. In addition, the population of IL-13-secreting type 2 innate lymphoid cells (ILC2s) was expanded with rhinovirus infection in neonatal but not mature mice. ILC2s were the major cell type secreting IL-13 in neonates. Finally, anti-IL-25 neutralizing antibody attenuated ILC2 expansion, mucous hypersecretion, and airways responsiveness.

CONCLUSIONS

These findings suggest that early-life viral infection could contribute to asthma development by provoking age-dependent, IL-25-driven type 2 immune responses.

Respiratory viral infection, epithelial cytokines, and innate lymphoid cells in asthma exacerbations

Exacerbations of asthma are most commonly triggered by viral infections, which amplify allergic inflammation. Cytokines released by virus-infected AECs may be important in driving this response. This review focuses on accumulating evidence in support of a role for epithelial cytokines, including IL-33, IL-25, and TSLP, as well as their targets, type 2 innate lymphoid cells (ILC2s), in the pathogenesis of virus-induced asthma exacerbations. Production and release of these cytokines lead to recruitment and activation of ILC2s, which secrete mediators, including IL-5 and IL-13, which augment allergic inflammation. However, little information is currently available about the induction of these responses by the respiratory viruses that are strongly associated with exacerbations of asthma, such as rhinoviruses. Further human studies, as well as improved animal experimental models, are needed to investigate appropriately the pathogenetic mechanisms in virus-induced exacerbations of asthma, including the role of ILCs.

Adaptation of innate lymphoid cells to a micronutrient deficiency promotes type 2 barrier immunity

How the immune system adapts to malnutrition to sustain immunity at barrier surfaces, such as the intestine, remains unclear. Vitamin A deficiency is one of the most common micronutrient deficiencies and is associated with profound defects in adaptive immunity. Here, we found that type 3 innate lymphoid cells (ILC3s) are severely diminished in vitamin A-deficient settings, which results in compromised immunity to acute bacterial infection. However, vitamin A deprivation paradoxically resulted in dramatic expansion of interleukin-13 (IL-13)-producing ILC2s and resistance to nematode infection in mice, which revealed that ILCs are primary sensors of dietary stress. Further, these data indicate that, during malnutrition, a switch to innate type 2 immunity may represent a powerful adaptation of the immune system to promote host survival in the face of ongoing barrier challenges.

Critical role of p38 and GATA3 in natural helper cell function

Natural helper (NH) cells, a member of Lin(-)IL-2R(+)IL-7R(+)IL-25R(+)IL-33R(+)GATA3(+) group 2 innate lymphoid cell subset, are characterized by the expression of transcription factors GATA3 and RORα and production of large amounts of Th2 cytokines such as IL-5, IL-6, and IL-13 upon IL-33 stimulation or a combination of IL-2 and IL-25. We have studied the signal transduction pathways critical for the cytokine expression and development of NH cell. Either stimulation with IL-33 or a combination of IL-2 and IL-25 induced p38 activation and phosphorylation of GATA3 in NH cells, and the phosphorylated form of GATA3 bound to the IL-5 and IL-13 promoters. All these events were blocked by SB203580, a p38 inhibitor. Inhibition of p38 also blocked IL-6 production. The mature NH cells lacking Gata3 were impaired in the proliferation and production of IL-5 and IL-13, but not IL-6, indicating that both p38 and GATA3 are critical for the proliferation and production of IL-5 and IL-13 and that the mechanisms downstream of p38 differ between IL-6 and IL-5/IL-13. In contrast, the NH cells lacking RORα showed no impairment in the proliferation and cytokine production, indicating that GATA3 but not RORα plays a pivotal role in the effector functions of mature NH cell. However, deletion of either GATA3 or RORα in hematopoietic stem cells severely blocked the development into NH cells. Our results demonstrate the important roles of p38 and GATA3 in NH cell functions.

Antagonistic regulation by the transcription factors C/EBPα and MITF specifies basophil and mast cell fates

It remains unclear whether basophils and mast cells are derived from a common progenitor. Furthermore, how basophil versus mast cell fate is specified has not been investigated. Here, we have identified a population of granulocyte-macrophage progenitors (GMPs) that were highly enriched in the capacity to differentiate into basophils and mast cells while retaining a limited capacity to differentiate into myeloid cells. We have designated these progenitor cells "pre-basophil and mast cell progenitors" (pre-BMPs). STAT5 signaling was required for the differentiation of pre-BMPs into both basophils and mast cells and was critical for inducing two downstream molecules: C/EBPα and MITF. We have identified C/EBPα as the critical basophil transcription factor for specifying basophil cell fate and MITF as the crucial transcription factor for specifying mast cell fate. C/EBPα and MITF silenced each other's transcription in a directly antagonistic fashion. Our study reveals how basophil and mast cell fate is specified.

Th2-type innate immune responses mediated by natural helper cells

Natural helper (NH) cells are a newly identified innate lymphocyte population that responds to a combination of interleukin (IL)-2 and either IL-25 or IL-33 to produce large amounts of T helper cell type 2 (Th2) cytokines. NH cells have been identified in fat-associated lymphoid clusters (FALCs), produce Th2 cytokines constitutively without any stimulation, and support the self-renewal of B1 cells and IgA production by B cells. Large amounts of IL-5 and IL-13 produced upon helminth infection or in response to IL-33 can induce eosinophilia and goblet cell hyperplasia in the lung and intestine; these cytokines, which activate NH cells, play important roles in antihelminth immunity and allergic diseases such as asthma.

Histone demethylase Lsd1 represses hematopoietic stem and progenitor cell signatures during blood cell maturation

Here, we describe that lysine-specific demethylase 1 (Lsd1/KDM1a), which demethylates histone H3 on Lys4 or Lys9 (H3K4/K9), is an indispensible epigenetic governor of hematopoietic differentiation. Integrative genomic analysis, combining global occupancy of Lsd1, genome-wide analysis of its substrates H3K4 monomethylation and dimethylation, and gene expression profiling, reveals that Lsd1 represses hematopoietic stem and progenitor cell (HSPC) gene expression programs during hematopoietic differentiation. We found that Lsd1 acts at transcription start sites, as well as enhancer regions. Loss of Lsd1 was associated with increased H3K4me1 and H3K4me2 methylation on HSPC genes and gene derepression. Failure to fully silence HSPC genes compromised differentiation of hematopoietic stem cells as well as mature blood cell lineages. Collectively, our data indicate that Lsd1-mediated concurrent repression of enhancer and promoter activity of stem and progenitor cell genes is a pivotal epigenetic mechanism required for proper hematopoietic maturation. DOI:http://dx.doi.org/10.7554/eLife.00633.001.

Essential, dose-dependent role for the transcription factor Gata3 in the development of IL-5+ and IL-13+ type 2 innate lymphoid cells

Group 2 innate lymphoid cells (ILC2s; also called nuocytes, innate helper cells, or natural helper cells) provide protective immunity during helminth infection and play an important role in influenza-induced and allergic airway hyperreactivity. Whereas the transcription factor GATA binding protein 3 (Gata3) is important for the production of IL-5 and -13 by ILC2s in response to IL-33 or -25 stimulation, it is not known whether Gata3 is required for ILC2 development from hematopoietic stem cells. Here, we show that chimeric mice generated with Gata3-deficient fetal liver hematopoietic stem cells fail to develop systemically dispersed ILC2s. In these chimeric mice, in vivo administration of IL-33 or -25 fails to expand ILC2 numbers or to induce characteristic ILC2-dependent IL-5 or -13 production. Moreover, cell-intrinsic Gata3 expression is required for ILC2 development in vitro and in vivo. Using mutant and transgenic mice in which Gata3 gene copy number is altered, we show that ILC2 generation from common lymphoid progenitors, as well as ILC2 homeostasis and cytokine production, is regulated by Gata3 expression levels in a dose-dependent fashion. Collectively, these results identify Gata3 as a critical early regulator of ILC2 development, thereby extending the paradigm of Gata3-dependent control of type 2 immunity to include both innate and adaptive lymphocytes.

Lung type 2 innate lymphoid cells express cysteinyl leukotriene receptor 1, which regulates TH2 cytokine production

BACKGROUND

Cysteinyl leukotrienes (CysLTs) contribute to asthma pathogenesis, in part through cysteinyl leukotriene receptor 1 (CysLT1R). Recently discovered lineage-negative type 2 innate lymphoid cells (ILC2s) potently produce IL-5 and IL-13.

OBJECTIVES

We hypothesized that lung ILC2s might be activated by leukotrienes through CysLT1R.

METHODS

ILC2s (Thy1.2(+) lineage-negative lymphocytes) and CysLT1R were detected in the lungs of wild-type, signal transducer and activator of transcription 6-deficient (STAT6(-/-)), and recombination-activating gene 2-deficient (RAG2(-/-)) mice by means of flow cytometry. T(H)2 cytokine levels were measured in purified lung ILC2s stimulated with leukotriene D₄ (LTD₄) in the presence or absence of the CysLT1R antagonist montelukast. Calcium influx was measured by using Fluo-4 intensity. Intranasal leukotriene C₄, D₄, and E₄ were administered to naive mice, and levels of ILC2 IL-5 production were determined. Finally, LTD₄ was coadministered with Alternaria species repetitively to RAG2(-/-) mice (with ILC2s) and IL-7 receptor-deficient mice (lack ILC2s), and total ILC2 numbers, proliferation (Ki-67(+)), and bronchoalveolar lavage fluid eosinophil numbers were measured.

RESULTS

CysLT1R was expressed on lung ILC2s from wild-type, RAG2(-/-), and STAT6(-/-) naive and Alternaria species-challenged mice. In vitro LTD₄ induced ILC2s to rapidly generate high levels of IL-5 and IL-13 within 6 hours of stimulation. Interestingly, LTD4, but not IL-33, induced high levels of IL-4 by ILC2s. LTD₄ administered in vivo rapidly induced ILC2 IL-5 production that was significantly reduced by montelukast before treatment. Finally, LTD₄ potentiated Alternaria species-induced eosinophilia, as well as ILC2 accumulation and proliferation.

CONCLUSIONS

We present novel data that CysLT1R is expressed on ILC2s and LTD₄ potently induces CysLT1R-dependent ILC2 production of IL-4, IL-5, and IL-13. Additionally, LTD₄ potentiates Alternaria species-induced eosinophilia and ILC2 proliferation and accumulation.

Development and function of group 2 innate lymphoid cells

The innate lymphoid cell (ILC) family has recently expanded with the discovery of type-2 innate lymphoid cells (ILC2). These cells arise from lymphoid progenitors in the bone marrow and, under the control of the transcriptional regulators RORα and Gata3, they mature to give rise to IL-5, IL-9 and IL-13 producing ILC2. These cells are critical components of the innate immune response to parasitic worm infections and have also been implicated in the pathogenesis of asthma and allergy. Recent advances in our understanding of the molecular regulation of ILC2 development and function now present the opportunity to develop new genetic models to assess ILC2 immune function and to investigate possible therapeutic interventions.

Role of interleukin-33 in innate-type immune cells in allergy

Interleukin-33 (IL-33), a member of the IL-1 cytokine family, is preferentially and constitutively expressed in epithelial cells, and it is especially localized in the cells' nucleus. The nuclear IL-33 is released by necrotic cells after tissue injury and/or trauma, and subsequently provokes local inflammation as an alarmin, like high-mobility group box protein-1 (HMGB-1) and IL-1α. IL-33 mainly activates Th2 cells and such innate-type immune cells as mast cells, basophils, eosinophils and natural helper cells that express IL-33R (a heterodimer of IL-1 receptor-like 1 [IL-1RL1; also called ST2, T1, Der4, fit-1] and IL-1 receptor accessory protein [IL-1RAcP]). That activation causes the cells to produce Th2 cytokines, which contribute to host defense against nematodes. On the other hand, excessive and/or inappropriate production of IL-33 is also considered to be involved in the development of such disorders as allergy. In this review, we summarize current knowledge regarding the pathogenic roles of IL-33 in the development of allergic inflammation by focusing on its effects on innate-type immune cells.

Innate lymphoid type 2 cells sustain visceral adipose tissue eosinophils and alternatively activated macrophages

Eosinophils in visceral adipose tissue (VAT) have been implicated in metabolic homeostasis and the maintenance of alternatively activated macrophages (AAMs). The absence of eosinophils can lead to adiposity and systemic insulin resistance in experimental animals, but what maintains eosinophils in adipose tissue is unknown. We show that interleukin-5 (IL-5) deficiency profoundly impairs VAT eosinophil accumulation and results in increased adiposity and insulin resistance when animals are placed on a high-fat diet. Innate lymphoid type 2 cells (ILC2s) are resident in VAT and are the major source of IL-5 and IL-13, which promote the accumulation of eosinophils and AAM. Deletion of ILC2s causes significant reductions in VAT eosinophils and AAMs, and also impairs the expansion of VAT eosinophils after infection with Nippostrongylus brasiliensis, an intestinal parasite associated with increased adipose ILC2 cytokine production and enhanced insulin sensitivity. Further, IL-33, a cytokine previously shown to promote cytokine production by ILC2s, leads to rapid ILC2-dependent increases in VAT eosinophils and AAMs. Thus, ILC2s are resident in VAT and promote eosinophils and AAM implicated in metabolic homeostasis, and this axis is enhanced during Th2-associated immune stimulation.

Natural helper cells and TH2-type innate immunity

Natural helper (NH) cells are a newly identified innate lymphocyte population, which respond to a combination of interleukin (IL)-2 and IL-25 or IL-33 to produce large amounts of TH2 cytokines, most notably, IL-5 and IL-13. We identified NH cells in lymphoid clusters in adipose tissues, which we termed fat-associated lymphoid clusters. IL-5 and IL-13 produced by activated NH cells induce eosinophilia and goblet cell hyperplasia, respectively, and play an important role in anti-helminth immunity and allergic diseases, such as asthma. In addition, NH cells produce TH2 cytokines constitutively in the absence of stimulation and support the self-renewal of B1 cells and IgA production by B cells. After our identification of NH cells, other groups have also reported novel TH2 cytokine-producing cells, such as nuocytes, and innate helper type 2 cells. There are similarities and differences between these newly identified cell populations and NH cells. Multipotent progenitor cell type 2 (MPP) cells can differentiate to other myeloid cells, making this a distinct cell type from the others. MPP cells, nuocytes, and innate helper type 2 cells respond to IL-25 alone in vivo, whereas NH cells do not respond to IL-25 without IL-2, although they respond strongly to IL-33. Localization of NH cells in fat-associated lymphoid clusters is an intriguing difference compared with the other cell types, which are found in the lymph node and/or spleen. This article summarizes current information regarding the emerging field of TH2-type innate lymphocytes.

IL-33 promotes eosinophilia in vivo and antagonizes IL-5-dependent eosinophil hematopoiesis ex vivo

IL-33 is an IL-1 family cytokine that elicits IL-5-dependent eosinophilia in vivo. We show here that IL-33 promotes minimal eosinophil hematopoiesis via direct interactions with mouse bone marrow progenitors ex vivo and that it antagonizes eosinophil hematopoiesis promoted by IL-5 on SCF and Flt3L primed bone marrow progenitor cells in culture. SCF and Flt3L primed progenitors respond to IL-33 by acquiring an adherent, macrophage-like phenotype, and by releasing macrophage-associated cytokines into the culture medium. IL-33-mediated antagonism of IL-5 was reproduced in part by the addition of GM-CSF and was inhibited by the actions of neutralizing anti-GM-CSF antibody. These findings suggest that the direct actions of IL-33 on bone marrow progenitors primed with SCF and Flt3L are antagonistic to the actions of IL-5 and are mediated in part by GM-CSF.

The transcription factor GATA-3 controls cell fate and maintenance of type 2 innate lymphoid cells

Innate lymphoid cells (ILCs) reside at mucosal surfaces and control immunity to intestinal infections. Type 2 innate lymphoid cells (ILC2s) produce cytokines such as IL-5 and IL-13, are required for immune defense against helminth infections, and are involved in the pathogenesis of airway hyperreactivity. Here, we have investigated the role of the transcription factor GATA-3 for ILC2 differentiation and maintenance. We showed that ILC2s and their lineage-specified bone marrow precursors (ILC2Ps), as identified here, were characterized by continuous high expression of GATA-3. Analysis of mice with temporary deletion of GATA-3 in all ILCs showed that GATA-3 was required for the differentiation and maintenance of ILC2s but not for RORγt(+) ILCs. Thus, our data demonstrate that GATA-3 is essential for ILC2 fate decisions and reveal similarities between the transcriptional programs controlling ILC and T helper cell fates.

Differentiation of innate type-2 effector cells

Type-2 immune responses are the underlying cause of many allergic diseases and provide protection against parasitic infection. Effective type-2 immune responses are generated by type-2 helper CD4(+) T cells (Th2) as well as type-2 innate effector cells. While we have learned a great deal about how CD4(+) Th2 cells regulate their Th2 cytokine gene transcription, we still do not know how type-2 innate effector cells acquire their capacity to express Th2 cytokine genes. Furthermore, it remains poorly understood how Th2 cytokines regulate the differentiation of innate type-2 progenitor cells. In this review, we will focus on (1) the long distance interaction between the sites of allergic inflammation and the site of hematopoiesis in the bone marrow, (2) the characteristics of innate type-2 progenitors, and (3) the molecular mechanisms by which innate type-2 effector cells acquire the capacity to produce type-2 cytokines.

STAT6 regulates natural helper cell proliferation during lung inflammation initiated by Alternaria

Asthma exacerbations can be caused by a number of factors, including the fungal allergen Alternaria, which is specifically associated with severe and near-fatal attacks. The mechanisms that trigger lung responses are unclear and might vary between allergens. A comparison between Alternaria, Aspergillus, Candida, and house dust mite, all allergens in humans, showed that only Alternaria promoted immediate innate airway eosinophilia within 12 h of inhalation in nonsensitized mice. Alternaria, but not the other allergens, induced a rapid increase in airway levels of IL-33, accompanied by IL-33 receptor (IL-33R)-positive natural helper cell (NHC) production of IL-5 and IL-13. NHCs in the lung and bone marrow constitutively expressed transcription factors [GATA-3 and E26 transformation-specific sequence-1 (ETS-1)] that could allow for rapid induction of T helper type 2 (Th2) cytokines. Lung NHC numbers and proliferation (%Ki-67), but not IL-5 or GATA-3 expression, were significantly reduced in STAT6-deficient mice 3 days after one challenge with Alternaria. Alternaria induced NHC expression of the EGF receptor ligand amphiregulin (partially dependent on STAT6), as well as EGF receptor signaling in the airway epithelium. Finally, human peripheral blood NHCs (CRTH2(+)CD127(+) lineage-negative lymphocytes) from allergic individuals highly expressed GATA-3 and ETS-1, similar to lung NHCs in mice. In summary, Alternaria-induced lung NHC proliferation and expression of amphiregulin are regulated by STAT6. In addition, NHCs in mouse and humans are primed to express Th2 cytokines through constitutive expression of GATA-3 and ETS-1. Thus several transcription factor pathways (STAT6, GATA-3, and ETS-1) may contribute to NHC proliferation and Th2-type responses in Alternaria-induced asthma.

Innate lymphoid cells in the airways

The airways, similar to other mucosal surfaces, are continuously exposed to the outside environment and a barrage of antigens, allergens, and microorganisms. Of critical importance therefore is the ability to mount rapid and effective immune responses to control commensal and pathogenic microbes, while simultaneously limiting the extent of these responses to prevent immune pathology and chronic inflammation. The function of the adaptive immune response in controlling these processes at mucosal surfaces has been well documented but the important role of the innate immune system, particularly the recently identified family of innate lymphoid cells, has only lately become apparent. In this review, we give an overview of the innate lymphoid cells that exist in the airways and examine the evidence pertaining to their emerging roles in airways immunity, inflammation, and homeostasis.

Role of innate lymphocytes in infection and inflammation

Cooperation between the innate and adaptive immune responses is critical for enabling protective immunity against various invading microbes. Distinct types of effector T cells have different functions in adaptive immune responses. Th1 cells play important roles in the control of intracellular bacteria by producing IFN-γ to activate macrophages and in anti-viral immunity by producing IFN-γ and activating cytotoxic T lymphocytes. Th2 cell-derived cytokines are important in activating mast cells, eosinophils, and goblet cells in anti-helminth immunity. Th17 cells are pivotal for the inflammatory response mediated by neutrophils, which resists extracellular bacterial infection. In all cases, it is critical that the innate immune responses limit the growth and expansion of invading microbes until antigen-specific adaptive immune responses are established. Recent studies have identified multiple subsets in innate lymphocytes corresponding to previously defined Th subsets. Classical natural killer cells, RORγ⁺ lymphoid tissue inducer-related cells, and Th2-type innate lymphocytes play distinct roles in innate immune responses by producing Th1, Th17, and Th2 cytokines, respectively. Cooperation between innate lymphocytes and antigen-specific T and B cells are likely important in protective immunity against distinct types of microbes. The most recently identified subset is the RORγ-independent Lin⁻Thy-1⁺IL-7R⁺GATA3⁺ innate lymphocyte subset such as natural helper (NH) cell, which is Id2- and IL-7-dependent. This population produces Th2 cytokines, most notably IL-5 and IL-13, and plays a major role in innate immune responses during anti-helminth immunity. In addition, these cells are likely involved in the pathophysiology of some types of allergic diseases. We summarize here current knowledge regarding various innate lymphocyte subsets. In particular, we focus on the Th2-type innate lymphocyte subset.

Dynamic role of epithelium-derived cytokines in asthma

Asthma is an inflammatory disorder of the airways, characterized by infiltration of mast cells, eosinophils, and Th2-type CD4+ T cells in the airway wall. Airway epithelium constitutes the first line of interaction with our atmospheric environment. The protective barrier function of the airway epithelium is likely impaired in asthma. Furthermore, recent studies suggest critical immunogenic and immunomodulatory functions of airway epithelium. In particular, a triad of cytokines, including IL-25, IL-33 and TSLP, is produced and released by airway epithelial cells in response to various environmental and microbial stimuli or by cellular damage. These cytokines induce and promote Th2-type airway inflammation and cause remodeling and pathological changes in the airway walls, suggesting their pivotal roles in the pathophysiology of asthma. Thus, the airway epithelium can no longer be regarded as a mere structural barrier, but must be considered an active player in the pathogenesis of asthma and other allergic disorders.