Identification of eosinophil lineage-committed progenitors in the murine bone marrow

Single-cell proteomics and transcriptomics capture eosinophil development and identify the role of IL-5 in their lineage transit amplification

The activities, ontogeny, and mechanisms of lineage expansion of eosinophils are less well resolved than those of other immune cells, despite the use of biological therapies targeting the eosinophilia-promoting cytokine interleukin (IL)-5 or its receptor, IL-5Rα. We combined single-cell proteomics and transcriptomics and generated transgenic IL-5Rα reporter mice to revisit eosinophilopoiesis. We reconciled human and murine eosinophilopoiesis and provided extensive cell-surface immunophenotyping and transcriptomes at different stages along the continuum of eosinophil maturation. We used these resources to show that IL-5 promoted eosinophil-lineage expansion via transit amplification, while its deletion or neutralization did not compromise eosinophil maturation. Informed from our resources, we also showed that interferon response factor-8, considered an essential promoter of myelopoiesis, was not intrinsically required for eosinophilopoiesis. This work hence provides resources, methods, and insights for understanding eosinophil ontogeny, the effects of current precision therapeutics, and the regulation of eosinophil development and numbers in health and disease.

Immune Dysregulation in Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is a disease whose incidence increase with age and is characterised by chronic inflammation and significant immune dysregulation. Inhalation of toxic substances cause oxidative stress in the lung tissue as well as airway inflammation, under the recruitment of chemokines, immune cells gathered and are activated to play a defensive role. However, persistent inflammation damages the immune system and leads to immune dysregulation, which is mainly manifested in the reduction of the body's immune response to antigens, and immune cells function are impaired, further destroy the respiratory defensive system, leading to recurrent lower respiratory infections and progressive exacerbation of the disease, thus immune dysregulation play an important role in the pathogenesis of COPD. This review summarizes the changes of innate and adaptive immune-related cells during the pathogenesis of COPD, aiming to control COPD airway inflammation and improve lung tissue remodelling by regulating immune dysregulation, for further reducing the risk of COPD progression and opening new avenues of therapeutic intervention in COPD.

Cebp1 and Cebpβ transcriptional axis controls eosinophilopoiesis in zebrafish

Eosinophils are a group of granulocytes well known for their capacity to protect the host from parasites and regulate immune function. Diverse biological roles for eosinophils have been increasingly identified, but the developmental pattern and regulation of the eosinophil lineage remain largely unknown. Herein, we utilize the zebrafish model to analyze eosinophilic cell differentiation, distribution, and regulation. By identifying eslec as an eosinophil lineage-specific marker, we establish a Tg(eslec:eGFP) reporter line, which specifically labeled cells of the eosinophil lineage from early life through adulthood. Spatial-temporal analysis of eslec+ cells demonstrates their organ distribution from larval stage to adulthood. By single-cell RNA-Seq analysis, we decipher the eosinophil lineage cells from lineage-committed progenitors to mature eosinophils. Through further genetic analysis, we demonstrate the role of Cebp1 in balancing neutrophil and eosinophil lineages, and a Cebp1-Cebpβ transcriptional axis that regulates the commitment and differentiation of the eosinophil lineage. Cross-species functional comparisons reveals that zebrafish Cebp1 is the functional orthologue of human C/EBPεP27 in suppressing eosinophilopoiesis. Our study characterizes eosinophil development in multiple dimensions including spatial-temporal patterns, expression profiles, and genetic regulators, providing for a better understanding of eosinophilopoiesis.

Origin and Heterogeneity of Tissue Myeloid Cells: A Focus on GMP-Derived Monocytes and Neutrophils

Myeloid cells are a significant proportion of leukocytes within tissues, comprising granulocytes, monocytes, dendritic cells, and macrophages. With the identification of various myeloid cells that perform separate but complementary functions during homeostasis and disease, our understanding of tissue myeloid cells has evolved significantly. Exciting findings from transcriptomics profiling and fate-mapping mouse models have facilitated the identification of their developmental origins, maturation, and tissue-specific specializations. This review highlights the current understanding of tissue myeloid cells and the contributing factors of functional heterogeneity to better comprehend the complex and dynamic immune interactions within the healthy or inflamed tissue. Specifically, we discuss the new understanding of the contributions of granulocyte-monocyte progenitor-derived phagocytes to tissue myeloid cell heterogeneity as well as the impact of niche-specific factors on monocyte and neutrophil phenotype and function. Lastly, we explore the developing paradigm of myeloid cell heterogeneity during inflammation and disease.

Multiple Roles of Sirtuin 6 in Adipose Tissue Inflammation

Adipose tissue (AT) inflammation is strongly associated with obesity-induced insulin resistance. When subjected to metabolic stress, adipocytes become inflamed and secrete a plethora of cytokines and chemokines, which recruit circulating immune cells to AT. Although sirtuin 6 (Sirt6) is known to control genomic stabilization, aging, and cellular metabolism, it is now understood to also play a pivotal role in the regulation of AT inflammation. Sirt6 protein levels are reduced in the AT of obese humans and animals and increased by weight loss. In this review, we summarize the potential mechanism of AT inflammation caused by impaired action of Sirt6 from the immune cells' point of view. We first describe the properties and functions of immune cells in obese AT, with an emphasis on discrete macrophage subpopulations which are central to AT inflammation. We then highlight data that links Sirt6 to functional phenotypes of AT inflammation. Importantly, we discuss in detail the effects of Sirt6 deficiency in adipocytes, macrophages, and eosinophils on insulin resistance or AT browning. In our closing perspectives, we discuss emerging issues in this field that require further investigation.

IL-5-producing CD4+ T cells and eosinophils cooperate to enhance response to immune checkpoint blockade in breast cancer

Immune checkpoint blockade (ICB) has heralded a new era in cancer therapy. Research into the mechanisms underlying response to ICB has predominantly focused on T cells; however, effective immune responses require tightly regulated crosstalk between innate and adaptive immune cells. Here, we combine unbiased analysis of blood and tumors from metastatic breast cancer patients treated with ICB with mechanistic studies in mouse models of breast cancer. We observe an increase in systemic and intratumoral eosinophils in patients and mice responding to ICB treatment. Mechanistically, ICB increased IL-5 production by CD4⁺ T cells, stimulating elevated eosinophil production from the bone marrow, leading to systemic eosinophil expansion. Additional induction of IL-33 by ICB-cisplatin combination or recombinant IL-33 promotes intratumoral eosinophil infiltration and eosinophil-dependent CD8⁺ T cell activation to enhance ICB response. This work demonstrates the critical role of eosinophils in ICB response and provides proof-of-principle for eosinophil engagement to enhance ICB efficacy.

The membrane-associated ubiquitin ligases MARCH2 and MARCH3 target IL-5 receptor alpha to negatively regulate eosinophilic airway inflammation

Interleukin 5 (IL-5) plays crucial roles in type 2-high asthma by mediating eosinophil maturation, activation, chemotaxis and survival. Inhibition of IL-5 signaling is considered a strategy for asthma treatment. Here, we identified MARCH2 and MARCH3 as critical negative regulators of IL-5-triggered signaling. MARCH2 and MARCH3 associate with the IL-5 receptor α chain (IL-5Rα) and mediate its K27-linked polyubiquitination at K379 and K383, respectively, and its subsequent lysosomal degradation. Deficiency of MARCH2 or MARCH3 modestly increases the level of IL-5Rα and enhances IL-5-induced signaling, whereas double knockout of MARCH2/3 has a more dramatic effect. March2/3 double knockout markedly increases the proportions of eosinophils in the bone marrow and peripheral blood in mice. Double knockout of March2/3 aggravates ovalbumin (OVA)-induced eosinophilia and causes increased inflammatory cell infiltration, peribronchial mucus secretion and production of Th2 cytokines. Neutralization of Il-5 attenuates OVA-induced airway inflammation and the enhanced effects of March2/3 double deficiency. These findings suggest that MARCH2 and MARCH3 play redundant roles in targeting IL-5Rα for degradation and negatively regulating allergic airway inflammation.

Recent advances of eosinophils and its correlated diseases

Eosinophils are differentiated by bone marrow multipotent progenitor cells and are further released into peripheral blood after maturation. Human eosinophils can exhibit unique multi-leaf nuclear morphology, which are filled with cytoplasmic granules that contain cytotoxicity and immune regulatory proteins. In recent years, many studies focused on the origin, differentiation and development process of eosinophils. It has been discovered that the eosinophils have the regulatory functions of innate and adaptive immunity, and can also function in several diseases, including asthma, chronic obstructive pulmonary diseases, acute respiratory distress syndrome, malignant tumors and so on. Hence, the role and effects of eosinophils in various diseases are emphasized. In this review, we comprehensively summarized the development and differentiation process of eosinophils, the research progress of their related cytokines, diseases and current clinical treatment options, and discussed the potential drug target, aiming to provide a theoretical and practical basis for the clinical prevention and treatment of eosinophil-related diseases, especially respiratory diseases. To conclude, the guiding significance of future disease treatment is proposed based on the recent updated understandings into the cell functions of eosinophils.

Potent CCR3 Receptor Antagonist, SB328437, Suppresses Colonic Eosinophil Chemotaxis and Inflammation in the Winnie Murine Model of Spontaneous Chronic Colitis

Eosinophils and their regulatory molecules have been associated with chronic intestinal inflammation and gastrointestinal dysfunctions; eosinophil accumulation in the gut is prominent in inflammatory bowel disease (IBD). The chemokine receptor CCR3 plays a pivotal role in local and systemic recruitment and activation of eosinophils. In this study, we targeted CCR3-ligand interactions with a potent CCR3 receptor antagonist, SB328437, to alleviate eosinophil-associated immunological responses in the Winnie model of spontaneous chronic colitis. Winnie and C57BL/6 mice were treated with SB328437 or vehicle. Clinical and histopathological parameters of chronic colitis were assessed. Flow cytometry was performed to discern changes in colonic, splenic, circulatory, and bone marrow-derived leukocytes. Changes to the serum levels of eosinophil-associated chemokines and cytokines were measured using BioPlex. Inhibition of CCR3 receptors with SB328437 attenuated disease activity and gross morphological damage to the inflamed intestines and reduced eosinophils and their regulatory molecules in the inflamed colon and circulation. SB328437 had no effect on eosinophils and their progenitor cells in the spleen and bone marrow. This study demonstrates that targeting eosinophils via the CCR3 axis has anti-inflammatory effects in the inflamed intestine, and also contributes to understanding the role of eosinophils as potential end-point targets for IBD treatment.

Eosinophils and Lung Cancer: From Bench to Bedside

Eosinophils are rare, multifunctional granulocytes. Their growth, survival, and tissue migration mainly depend on interleukin (IL)-5 in physiological conditions and on IL-5 and IL-33 in inflammatory conditions. Preclinical evidence supports an immunological role for eosinophils as innate immune cells and as agents of the adaptive immune response. In addition to these data, several reports show a link between the outcomes of patients treated with immune checkpoint inhibitors (ICI) for advanced cancers and blood eosinophilia. In this review, we present, in the context of non-small cell lung cancer (NSCLC), the biological properties of eosinophils and their roles in homeostatic and pathological conditions, with a focus on their pro- and anti-tumorigenic effects. We examine the possible explanations for blood eosinophilia during NSCLC treatment with ICI. In particular, we discuss the value of eosinophils as a potential prognostic and predictive biomarker, highlighting the need for stronger clinical data. Finally, we conclude with perspectives on clinical and translational research topics on this subject.

Remote regulation of type 2 immunity by intestinal parasites

The intestinal tract is the target organ of most parasitic infections, including those by helminths and protozoa. These parasites elicit prototypical type 2 immune activation in the host's immune system with striking impact on the local tissue microenvironment. Despite local containment of these parasites within the intestinal tract, parasitic infections also mediate immune adaptation in peripheral organs. In this review, we summarize the current knowledge on how such gut-tissue axes influence important immune-mediated resistance and disease tolerance in the context of coinfections, and elaborate on the implications of parasite-regulated gut-lung and gut-brain axes on the development and severity of airway inflammation and central nervous system diseases.

Isolation and characterization of eosinophils in bovine blood and small intestine

An effective method to isolate functional eosinophils from blood and tissues is required to analyze the multiple roles eosinophils play in innate immunity and tissue homeostasis. Highspeed cell sorting was used to isolate bovine eosinophils from blood polymorphonuclear (PMN) cells and from small intestine intraepithelial leukocytes. Eosinophils and neutrophils were purified from bovine blood with highspeed cell sorting after gating on autofluorescence (FL1) high and low PMN subpopulations. Highspeed sorting of intestinal eosinophils was accomplished by using a combination of positive (CD45⁺, CD11c^Low, side scatter^High) and negative (CD3^-) selection parameters. Eosinophils sorted from blood PMNs were 88.6 ± 5.8 % (mean + 1 SD; n = 4) pure and yielded significantly (p < 0.05) more RNA than purified neutrophils. Analysis of Toll-like receptor (TLR) gene expression and TLR ligand-induced pro-inflammatory cytokine (IL-1, IL-6, IL-8, and TNFα) gene expression demonstrated significant (p < 0.01) functional differences between blood eosinophils and neutrophils. Eosinophils varied between 14.7 % to 29.3 % of CD45⁺ IELs and purity of sorted intestinal eosinophils was 95 + 3.5 % (mean + 1SD; n = 5). A comparison of mucosal and blood eosinophils revealed significant (p < 0.01) differences in TLR gene expression, supporting the hypothesis that functionally distinct eosinophil populations are present in blood and tissues. In conclusion, highspeed cell sorting provides an effective method to isolate viable eosinophils from blood and tissues that can then be used for transcriptome analyses and in vitro function assays.

Acute myeloid leukemia maturation lineage influences residual disease and relapse following differentiation therapy

Acute myeloid leukemia (AML) is a malignancy of immature progenitor cells. AML differentiation therapies trigger leukemia maturation and can induce remission, but relapse is prevalent and its cellular origin is unclear. Here we describe high resolution analysis of differentiation therapy response and relapse in a mouse AML model. Triggering leukemia differentiation in this model invariably produces two phenotypically distinct mature myeloid lineages in vivo. Leukemia-derived neutrophils dominate the initial wave of leukemia differentiation but clear rapidly and do not contribute to residual disease. In contrast, a therapy-induced population of mature AML-derived eosinophil-like cells persists during remission, often in extramedullary organs. Using genetic approaches we show that restricting therapy-induced leukemia maturation to the short-lived neutrophil lineage markedly reduces relapse rates and can yield cure. These results indicate that relapse can originate from therapy-resistant mature AML cells, and suggest differentiation therapy combined with targeted eradication of mature leukemia-derived lineages may improve disease outcome.

Eosinophils as Drivers of Severe Eosinophilic Asthma: Endotypes or Plasticity?

Asthma is now recognized as a heterogeneous disease, encompassing different phenotypes driven by distinct pathophysiological mechanisms called endotypes. Common phenotypes of asthma, referred to as eosinophilic asthma, are characterized by the presence of eosinophilia. Eosinophils are usually considered invariant, terminally differentiated effector cells and have become a primary therapeutic target in severe eosinophilic asthma (SEA) and other eosinophil-associated diseases (EADs). Biological treatments that target eosinophils reveal an unexpectedly complex role of eosinophils in asthma, including in SEA, suggesting that "not all eosinophils are equal". In this review, we address our current understanding of the role of eosinophils in asthma with regard to asthma phenotypes and endotypes. We further address the possibility that different SEA phenotypes may involve differences in eosinophil biology. We discuss how these differences could arise through eosinophil "endotyping", viz. adaptations of eosinophil function imprinted during their development, or through tissue-induced plasticity, viz. local adaptations of eosinophil function through interaction with their lung tissue niches. In doing so, we also discuss opportunities, technical challenges, and open questions that, if addressed, might provide considerable benefits in guiding the choice of the most efficient precision therapies of SEA and, by extension, other EADs.

Preclinical and clinical progress for HDAC as a putative target for epigenetic remodeling and functionality of immune cells

Genetic changes are difficult to reverse; thus, epigenetic aberrations, including changes in DNA methylation, histone modifications, and noncoding RNAs, with potential reversibility, have attracted attention as pharmaceutical targets. The current paradigm is that histone deacetylases (HDACs) regulate gene expression via deacetylation of histone and nonhistone proteins or by forming corepressor complexes with transcription factors. The emergence of epigenetic tools related to HDACs can be used as diagnostic and therapeutic markers. HDAC inhibitors that block specific or a series of HDACs have proven to be a powerful therapeutic treatment for immune-related diseases. Here, we summarize the various roles of HDACs and HDAC inhibitors in the development and function of innate and adaptive immune cells and their implications for various diseases and therapies.

Eosinophils, a Jack of All Trades in Immunity: Therapeutic Approaches for Correcting Their Functional Disorders

BACKGROUND AND OBJECTIVE

Eosinophils are primitive myeloid cells derived from bonemarrow precursors and require the intervention of interleukin (IL)-5 for their survival and persistence in blood and tissues. Under steady-state conditions, they contribute to immune regulation and homeostasis. Under pathological circumstances, eosinophils are involved in host protection against parasites and participate in allergy and inflammation.

DISCUSSION

Mostly, in asthma, eosinophils provoke airway damage via the release of granule contents and IL-13 with mucus hypersecretion and differentiation of goblet cells. Then, tissue remodeling follows with the secretion of transforming growth factor-β. Eosinophils are able to kill helminth larvae acting as antigen-presenting cells with the involvement of T helper (h)-2 cells and subsequent antibody response. However, they also exert pro-worm activity with the production of suppressive cytokine (IL- 10 and IL-4) and inhibition of nitric oxide. Eosinophils may play a pathogenic role in the course of chronic and autoimmune disease, e.g., inflammatory bowel disease and eosinophilic gastroenteritis, regulating Th2 responses and promoting a profibrotic effect. In atopic dermatitis, eosinophils are commonly detected and may be associated with disease severity. In cutaneous spontaneous urticaria, eosinophils participate in the formation of wheals, tissue remodeling and modifications of vascular permeability. With regard to tumor growth, it seems that IgE can exert anti-neoplastic surveillance via mast cell and eosinophil-mediated cytotoxicity, the so-called allergo-oncology. From a therapeutic point of view, monoclonal antibodies directed against IL-5 or the IL-5 receptors have been shown to be very effective in patients with severe asthma. Finally, as an alternative treatment, polyphenols for their anti-inflammatory and anti-allergic activities seem to be effective in reducing serum IgE and eosinophil count in bronchoalveolar lavage in murine asthma.

CONCLUSION

Eosinophils are cells endowed with multiple functions and their modulation with monoclonal antibodies and nutraceuticals may be effective in the treatment of chronic disease.

Eosinophils in the pathogenesis of pancreatic disorders

Eosinophils comprise approximately 1-4% of total blood leukocytes that reside in the intestine, bone marrow, mammary gland, and adipose tissues to maintain innate immunity in healthy individuals. Eosinophils have four toxic granules known as major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), and eosinophil-derived neurotoxin (EDN), and upon degranulation, these granules promote pathogenesis of inflammatory diseases like allergy, asthma, dermatitis, and gastrointestinal disorders. Additionally, the role of eosinophils is underscored in exocrine disorders including pancreatitis. Chronic pancreatitis (CP) is an inflammatory disorder that occurs due to the alcohol consumption, blockage of the pancreatic duct, and trypsinogen mutation. Eosinophil levels are detected in higher numbers in both CP and pancreatic cancer patients compared with healthy individuals. The mechanistic understanding of chronic inflammation-induced pancreatic malignancy has not yet been reached and requires further exploration. This review provides a comprehensive summary of the epidemiology, pathophysiology, evaluation, and management of eosinophil-associated pancreatic disorders and further summarizes current evidence regarding risk factors, pathophysiology, clinical features, diagnostic evaluation, treatment, and prognosis of eosinophilic pancreatitis (EP) and pancreatic cancer.

Regulation of eosinophil functions by autophagy

Eosinophils are granule-containing leukocytes which develop in the bone marrow. For many years, eosinophils have been recognized as cytotoxic effector cells, but recent studies suggest that they perform additional immunomodulatory and homeostatic functions. Autophagy is a conserved intracellular process which preserves cellular homeostasis. Autophagy defects have been linked to the pathogenesis of many human disorders. Evidence for abnormal regulation of autophagy, including decreased or increased expression of autophagy-related (ATG) proteins, has been reported in several eosinophilic inflammatory disorders, such as Crohn's disease, bronchial asthma, eosinophilic esophagitis, and chronic rhinosinusitis. Despite the increasing extent of research using preclinical models of immune cell-specific autophagy deficiency, the physiological relevance of autophagic pathway in eosinophils has remained unknown until recently. Owing to the increasing evidence that eosinophils play a role in keeping organismal homeostasis, the regulation of eosinophil functions is of considerable interest. Here, we discuss the most recent advances on the role of autophagy in eosinophils, placing particular emphasis on insights obtained in mouse models of infections and malignant diseases in which autophagy has genetically dismantled in the eosinophil lineage. These studies pointed to the possibility that autophagy-deficient eosinophils exaggerate inflammation. Therefore, the pharmacological modulation of the autophagic pathway in these cells could be used for therapeutic interventions.

The twilight zone: plasticity and mixed ontogeny of neutrophil and eosinophil granulocyte subsets

It is now becoming clear that neutrophils and eosinophils are heterogeneous cells with potentially multiple subsets in health and disease. With greater marker coverage by multi-color flow cytometry and single-cell level sequencing of granulocyte populations, novel phenotypes of these cells began to emerge. Intriguingly, many newly described subsets blend distinctions between classical myeloid lineage phenotypes, which are especially true for tissue resident or recruited cells in contexts of inflammation and disease. This includes reports of neutrophils with features of eosinophils, monocytes and dendritic cells, and eosinophil subsets expressing neutrophil markers. Moreover, novel studies show the ability of immature neutrophils to transdifferentiate into mature cells belonging to other myeloid lineages (eosinophils, monocytes/macrophages). In this review, we summarize novel findings in this exciting research frontier and shed light on potential processes driving the plasticity and heterogeneity of granulocyte subsets. Specifically, we discuss the hematopoietic flexibility of granulocyte precursors in bone marrow and the adaptation of myeloid cells to local tissue microenvironments. The understanding of such intermediate and developmental phenotypes is very important, as it can teach us about origins of functionally distinct myeloid cells during inflammation, and explain reasons for successes and failures of biologics targeting terminally differentiated granulocytes.

Eosinophil-derived chemokine (hCCL15/23, mCCL6) interacts with CCR1 to promote eosinophilic airway inflammation

Eosinophils are terminally differentiated cells derived from hematopoietic stem cells (HSCs) in the bone marrow. Several studies have confirmed the effective roles of eosinophils in asthmatic airway pathogenesis. However, their regulatory functions have not been well elucidated. Here, increased C-C chemokine ligand 6 (CCL6) in asthmatic mice and the human orthologs CCL15 and CCL23 that are highly expressed in asthma patients are described, which are mainly derived from eosinophils. Using Ccl6 knockout mice, further studies revealed CCL6-dependent allergic airway inflammation and committed eosinophilia in the bone marrow following ovalbumin (OVA) challenge and identified a CCL6-CCR1 regulatory axis in hematopoietic stem cells (HSCs). Eosinophil differentiation and airway inflammation were remarkably decreased by the specific CCR1 antagonist BX471. Thus, the study identifies that the CCL6-CCR1 axis is involved in the crosstalk between eosinophils and HSCs during the development of allergic airway inflammation, which also reveals a potential therapeutic strategy for targeting G protein-coupled receptors (GPCRs) for future clinical treatment of asthma.

ATG5 promotes eosinopoiesis but inhibits eosinophil effector functions

Eosinophils are white blood cells that contribute to the regulation of immunity and are involved in the pathogenesis of numerous inflammatory diseases. In contrast to other cells of the immune system, no information is available regarding the role of autophagy in eosinophil differentiation and functions. To study the autophagic pathway in eosinophils, we generated conditional knockout mice in which Atg5 is deleted within the eosinophil lineage only (designated Atg5eoΔ mice). Eosinophilia was provoked by crossbreeding Atg5eoΔ mice with Il5 (IL-5) overexpressing transgenic mice (designated Atg5eoΔIl5tg mice). Deletion of Atg5 in eosinophils resulted in a dramatic reduction in the number of mature eosinophils in blood and an increase of immature eosinophils in the bone marrow. Atg5-knockout eosinophil precursors exhibited reduced proliferation under both in vitro and in vivo conditions but no increased cell death. Moreover, reduced differentiation of eosinophils in the absence of Atg5 was also observed in mouse and human models of chronic eosinophilic leukemia. Atg5-knockout blood eosinophils exhibited augmented levels of degranulation and bacterial killing in vitro. Moreover, in an experimental in vivo model, we observed that Atg5eoΔ mice achieve better clearance of the local and systemic bacterial infection with Citrobacter rodentium. Evidence for increased degranulation of ATG5low-expressing human eosinophils was also obtained in both tissues and blood. Taken together, mouse and human eosinophil hematopoiesis and effector functions are regulated by ATG5, which controls the amplitude of overall antibacterial eosinophil immune responses.

Eosinophil Lineage-Committed Progenitors as a Therapeutic Target for Asthma

Eosinophilic asthma is the most prevalent phenotype of asthma. Although most asthmatics are adequately controlled by corticosteroid therapy, a subset (5-10%) remain uncontrolled with significant therapy-related side effects. This indicates the need for a consideration of alternative treatment strategies that target airway eosinophilia with corticosteroid-sparing benefits. A growing body of evidence shows that a balance between systemic differentiation and local tissue eosinophilopoietic processes driven by traffic and lung homing of bone marrow-derived hemopoietic progenitor cells (HPCs) are important components for the development of airway eosinophilia in asthma. Interleukin (IL)-5 is considered a critical and selective driver of terminal differentiation of eosinophils. Studies targeting IL-5 or IL-5R show that although mature and immature eosinophils are decreased within the airways, there is incomplete ablation, particularly within the bronchial tissue. Eotaxin is a chemoattractant for mature eosinophils and eosinophil-lineage committed progenitor cells (EoP), yet anti-CCR3 studies did not yield meaningful clinical outcomes. Recent studies highlight the role of epithelial cell-derived alarmin cytokines, IL-33 and TSLP, (Thymic stromal lymphopoietin) in progenitor cell traffic and local differentiative processes. This review provides an overview of the role of EoP in asthma and discusses findings from clinical trials with various therapeutic targets. We will show that targeting single mediators downstream of the inflammatory cascade may not fully attenuate tissue eosinophilia due to the multiplicity of factors that can promote tissue eosinophilia. Blocking lung homing and local eosinophilopoiesis through mediators upstream of this cascade may yield greater improvement in clinical outcomes.

Transcription factor and cytokine regulation of eosinophil lineage commitment

PURPOSE OF REVIEW

Lineage commitment is governed by instructive and stochastic signals, which drive both active induction of the lineage program and repression of alternative fates. Eosinophil lineage commitment is driven by the ordered interaction of transcription factors, supported by cytokine signals. This review summarizes key findings in the study of eosinophil lineage commitment and examines new data investigating the factors that regulate this process.

RECENT FINDINGS

Recent and past studies highlight how intrinsic and extrinsic signals modulate transcription factor network and lineage decisions. Early action of the transcription factors C/EBPα and GATA binding protein-1 along with C/EBPε supports lineage commitment and eosinophil differentiation. This process is regulated and enforced by the pseudokinase Trib1, a regulator of C/EBPα levels. The cytokines interleukin (IL)-5 and IL-33 also support early eosinophil development. However, current studies suggest that these cytokines are not specifically required for lineage commitment.

SUMMARY

Together, recent evidence suggests a model where early transcription factor activity drives expression of key eosinophil genes and cytokine receptors to prime lineage commitment. Understanding the factors and signals that control eosinophil lineage commitment may guide therapeutic development for eosinophil-mediated diseases and provide examples for fate choices in other lineages.

Transplacental Innate Immune Training via Maternal Microbial Exposure: Role of XBP1-ERN1 Axis in Dendritic Cell Precursor Programming

We recently reported that offspring of mice treated during pregnancy with the microbial-derived immunomodulator OM-85 manifest striking resistance to allergic airways inflammation, and localized the potential treatment target to fetal conventional dendritic cell (cDC) progenitors. Here, we profile maternal OM-85 treatment-associated transcriptomic signatures in fetal bone marrow, and identify a series of immunometabolic pathways which provide essential metabolites for accelerated myelopoiesis. Additionally, the cDC progenitor compartment displayed treatment-associated activation of the XBP1-ERN1 signalling axis which has been shown to be crucial for tissue survival of cDC, particularly within the lungs. Our forerunner studies indicate uniquely rapid turnover of airway mucosal cDCs at baseline, with further large-scale upregulation of population dynamics during aeroallergen and/or pathogen challenge. We suggest that enhanced capacity for XBP1-ERN1-dependent cDC survival within the airway mucosal tissue microenvironment may be a crucial element of OM-85-mediated transplacental innate immune training which results in postnatal resistance to airway inflammatory disease.

Adenovirus vector-mediated YKL-40 shRNA attenuates eosinophil airway inflammation in a murine asthmatic model

Recent studies have revealed that YKL-40 is involved in the pathogenesis of asthma. However, its specific mechanism remains unclear. The present study aims to investigate the effect of adenovirus vector-mediated YKL-40 short hairpin RNA (shRNA) on regulation of airway inflammation in a murine asthmatic model. Mice were assessed for airway hyperresponsiveness (AHR), total leukocytes and the percentage of eosinophil cells in bronchoalveolar lavage fluid (BALF). YKL-40 mRNA and protein expression levels were detected using quantitative real-time PCR and western blot assays. Enzyme-linked immunosorbent assay (ELISA) was used to detect YKL-40 and eosinophil-related chemokine expression levels in BALF and serum. Lung histology analyses were performed to evaluate the degree of inflammatory cell infiltration around the airway and airway mucus secretion.YKL-40 shRNA significantly inhibited the YKL-40 gene expression in asthmatic mice. In addition, YKL-40 shRNA alleviated eosinophilic airway inflammation, AHR, airway mucus secretion and decreased the levels of YKL-40 in BALF and serum in a murine asthmatic model. The levels and mRNA expression of IL-5, IL-13 in asthmatic mice lung tissues, eotaxin, and GM-CSF in BALF and serum significantly decreased. Bone marrow signaling molecules including IL-5, eotaxin, and GM-CSF were correlated with decreased levels of YKL-40. The study reveals that YKL-40 could be involved in asthma inflammation by altering bone marrow signaling molecules. YKL-40 gene RNA interference could provide new therapeutic strategies for asthma.

A Novel GATA2 Protein Reporter Mouse Reveals Hematopoietic Progenitor Cell Types

The transcription factor (TF) GATA2 plays a key role in organ development and cell fate control in the central nervous, urogenital, respiratory, and reproductive systems, and in primitive and definitive hematopoiesis. Here, we generate a knockin protein reporter mouse line expressing a GATA2VENUS fusion from the endogenous Gata2 genomic locus, with correct expression and localization of GATA2VENUS in different organs. GATA2VENUS expression is heterogeneous in different hematopoietic stem and progenitor cell populations (HSPCs), identifies functionally distinct subsets, and suggests a novel monocyte and mast cell lineage bifurcation point. GATA2 levels further correlate with proliferation and lineage outcome of hematopoietic progenitors. The GATA2VENUS mouse line improves the identification of specific live cell types during embryonic and adult development and will be crucial for analyzing GATA2 protein dynamics in TF networks.

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A novel GATA2VENUS fusion mouse line to report GATA2 protein expression

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VENUS fusion does not alter GATA2 expression or disturb development or homeostasis

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GATA2 expression identifies functionally distinct HSPC subpopulations

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GATA2 expression unveils an earlier monocyte-mast cell lineage bifurcation point

Combinatorial Single-Cell Analyses of Granulocyte-Monocyte Progenitor Heterogeneity Reveals an Early Uni-potent Neutrophil Progenitor

Granulocyte-monocyte progenitors (GMPs) have been previously defined for their potential to generate various myeloid progenies such as neutrophils and monocytes. Although studies have proposed lineage heterogeneity within GMPs, it is unclear if committed progenitors already exist among these progenitors and how they may behave differently during inflammation. By combining single-cell transcriptomic and proteomic analyses, we identified the early committed progenitor within the GMPs responsible for the strict production of neutrophils, which we designate as proNeu1. Our dissection of the GMP hierarchy led us to further identify a previously unknown intermediate proNeu2 population. Similar populations could be detected in human samples. proNeu1s, but not proNeu2s, selectively expanded during the early phase of sepsis at the expense of monocytes. Collectively, our findings help shape the neutrophil maturation trajectory roadmap and challenge the current definition of GMPs.

Eosinophil-Mediated Immune Control of Adult Filarial Nematode Infection Can Proceed in the Absence of IL-4 Receptor Signaling

Immunity to chronic filarial worm infection is apparent in IL-4Rα–deficient mice.

Delayed immunity in IL-4Rα^−/− mice is due to suboptimal tissue eosinophilia.

Eosinophil recruitment in the absence of IL-4R signaling requires CCR3 and IL-5.

Helminth infections are accompanied by eosinophilia in parasitized tissues. Eosinophils are effectors of immunity to tissue helminths. We previously reported that in the context of experimental filarial nematode infection, optimum tissue eosinophil recruitment was coordinated by local macrophage populations following IL-4R–dependent in situ proliferation and alternative activation. However, in the current study, we identify that control of chronic adult filarial worm infection is evident in IL-4Rα–deficient (IL-4Rα^−/−) mice, whereby the majority of infections do not achieve patency. An associated residual eosinophilia was apparent in infected IL-4Rα^−/− mice. By treating IL-4Rα^−/− mice serially with anti-CCR3 Ab or introducing a compound deficiency in CCR3 within IL-4Rα^−/− mice, residual eosinophilia was ablated, and susceptibility to chronic adult Brugia malayi infection was established, promoting a functional role for CCR3-dependent eosinophil influx in immune control in the absence of IL-4/IL-13–dependent immune mechanisms. We investigated additional cytokine signals involved in residual eosinophilia in the absence IL-4Rα signaling and defined that IL-4Rα^−/−/IL-5^−/− double-knockout mice displayed significant eosinophil deficiency compared with IL-4Rα^−/− mice and were susceptible to chronic fecund adult filarial infections. Contrastingly, there was no evidence that either IL-4R–dependent or IL-4R–independent/CCR3/IL-5–dependent immunity influenced B. malayi microfilarial loads in the blood. Our data demonstrate multiplicity of Th2-cytokine control of eosinophil tissue recruitment during chronic filarial infection and that IL-4R–independent/IL-5– and CCR3-dependent pathways are sufficient to control filarial adult infection via an eosinophil-dependent effector response prior to patency.

Identification of two distinct pathways of human myelopoiesis

Human myelopoiesis has been proposed to occur through oligopotent common myeloid progenitor (CMP) and lymphoid-primed multipotent progenitor (LMPP) populations. However, other studies have proposed direct commitment of multipotent cells to unilineage fates, without specific intermediary lineage cosegregation patterns. We here show that distinct human myeloid progenitor populations generate the neutrophil/monocyte and mast cell/basophil/eosinophil lineages as previously shown in mouse. Moreover, we find that neutrophil/monocyte potential selectively cosegregates with lymphoid lineage and mast cell/basophil/eosinophil potentials with megakaryocyte/erythroid potential early during lineage commitment. Furthermore, after this initial commitment step, mast cell/basophil/eosinophil and megakaryocyte/erythroid potentials colocalize at the single-cell level in restricted oligopotent progenitors. These results show that human myeloid lineages are generated through two distinct cellular pathways defined by complementary oligopotent cell populations.

Deciphering the role of eosinophils in solid organ transplantation

Eosinophils are rare granulocytes that belong to the innate arm of the immune system. This cell population is traditionally defined as a destructive and cytotoxic mediator in asthma and helminth infection. Limited data in transplantation have suggested that eosinophils play a similar role in potentiating deleterious organ inflammation and immunologic rejection. Contrary to this long-held notion, recent data have uncovered the possibility that eosinophils play an alternative role in immune homeostasis, defense against a wide range of pathogens, as well as downregulation of deleterious inflammation. Specifically, translational data from small animal models of lung transplantation have demonstrated a critical role for eosinophils in the downregulation of alloimmunity. These findings shed new light on the unique immunologic features of the lung allograft and demonstrate that environmental polarization may alter the phenotype and function of leukocyte populations previously thought to be static in nature. In this review, we provide an update on eosinophils in the homeostasis of the lung as well as other solid organs.

Sympathetic Nerves Positively Regulate Eosinophil-Driven Allergic Conjunctivitis via α1-Adrenergic Receptor Signaling

Eosinophils are a major cause of tissue injury in allergic conjunctivitis. The biological nature of eosinophils in the conjunctiva and the mechanisms that control eosinophils' responses in allergic conjunctivitis are currently not completely understood. This study reports that conjunctival eosinophils comprise two populations-Siglec-F^int and Siglec-F^hi-in different life stages. Siglec-F^int eosinophils partly expressed CD34 and were in the immature (or steady) state. Siglec-F^hi eosinophils did not express CD34, sharply increased in number after short ragweed (SRW) pollen challenge, and were in the mature (or activated) state. Moreover, chemical sympathectomy by 6-hydroxydopamine reduced the recruitment and activation of eosinophils, whereas the activation of the sympathetic nerve system (SNS) with restraint stress accelerated the recruitment and activation of eosinophils in SRW-induced conjunctivitis. It was also found that two eosinophil populations expressed alpha-1a-adrenergic receptors (α1a-ARs); in SRW-induced conjunctivitis, treatment with an α1a-AR antagonist decreased eosinophil responses, whereas treatment with an α1a-AR agonist aggravated eosinophil responses. Thus, eosinophil responses in conjunctivitis are regulated by the SNS via α1a-AR signaling. SNS inputs or α1a-AR function may be potential targets for the treatment of allergic conjunctivitis.

Single and Synergistic Effects of Type 2 Cytokines on Eosinophils and Asthma Hallmarks

The type 2 cytokines IL-5, IL-13, and IL-4 play an important role in the induction and progression of asthma. According to the Global Initiative for Asthma guidelines, blood eosinophil numbers are one marker that helps to guide treatment decisions in patients suffering from severe forms of asthma. Effects of type 2 cytokines were analyzed, alone or in combination, on eosinophils in blood and other compartments and on the development of asthma symptoms. C57BL/6 mice received a single intranasal application of equimolar amounts of IL-5, IL-13, and IL-4, alone or in combination. Numbers, activation state, and migratory behavior of eosinophils in bone marrow (BM), blood, lung, and bronchoalveolar lavage as well as airway hyperresponsiveness and goblet cell metaplasia were evaluated. Only IL-13 was associated with airway eosinophilia, development of airway hyperresponsiveness, and goblet cell metaplasia, without any synergistic effects. IL-5 increased the number of eosinophils in BM and lung tissue but failed to affect structural changes. IL-4 had similar, but weaker, effects to IL-13. Cytokine combinations synergistically affected eosinophils but failed to enhance IL-13-driven effects on lung function or goblet cell metaplasia. IL-5 and IL-13 markedly increased eosinophil numbers locally in lung and airways and distally in blood and BM, whereas IL-5 and IL-4 only increased eosinophils in lung and BM. IL-13 together with IL-4 failed to demonstrate any synergistic effect. These insights into single and combined effects of type 2 cytokines on disease-driving mechanisms could improve understanding of the impact and effectiveness of new therapies in asthma.

The Transcription Factor PLZF Is Necessary for the Development and Function of Mouse Basophils

Basophils are innate immune cells associated with type 2 immunity, allergic reactions, and host defense against parasite infections. In this study, we show that the transcription factor PLZF, which is known for its essential role in the function and development of several innate lymphocyte subsets, is also important for the myeloid-derived basophil lineage. PLZF-deficient mice had decreased numbers of basophil progenitors in the bone marrow and mature basophils in multiple peripheral tissues. Functionally, PLZF-deficient basophils were less responsive to IgE activation and produced reduced amounts of IL-4. The altered function of basophils resulted in a blunted Th2 T cell response to a protein allergen. Additionally, PLZF-deficient basophils had reduced expression of the IL-18 receptor, which impacted migration to lungs. PLZF, therefore, is a major player in controlling type 2 immune responses mediated not only by innate lymphocytes but also by myeloid-derived cells.

Eosinophils in the gastrointestinal tract and their role in the pathogenesis of major colorectal disorders

Eosinophils are currently regarded as versatile mobile cells controlling and regulating multiple biological pathways and responses in health and disease. These cells store in their specific granules numerous biologically active substances (cytotoxic cationic proteins, cytokines, growth factors, chemokines, enzymes) ready for rapid release. The human gut is the main destination of eosinophils that are produced and matured in the bone marrow and then transferred to target tissues through the circulation. In health the most important functions of gut-residing eosinophils comprise their participation in the maintenance of the protective mucosal barrier and interactions with other immune cells in providing immunity to microbiota of the gut lumen. Eosinophils are closely involved in the development of inflammatory bowel disease (IBD), when their cytotoxic granule proteins cause damage to host tissues. However, their roles in Crohn’s disease and ulcerative colitis appear to follow different immune response patterns. Eosinophils in IBD are especially important in altering the structure and protective functions of the mucosal barrier and modulating massive neutrophil influx to the lamina propria followed by transepithelial migration to colorectal mucus. IBD-associated inflammatory process involving eosinophils then appears to expand to the mucus overlaying the internal gut surface. The author hypothesises that immune responses within colorectal mucus as well as ETosis exerted by both neutrophils and eosinophils on the both sides of the colonic epithelial barrier act as additional pathogenetic factors in IBD. Literature analysis also shows an association between elevated eosinophil levels and better colorectal cancer (CRC) prognosis, but mechanisms behind this effect remain to be elucidated. In conclusion, the author emphasises the importance of investigating colorectal mucus in IBD and CRC patients as a previously unexplored milieu of disease-related inflammatory responses.

Single-cell transcriptomic landscape of nucleated cells in umbilical cord blood

BACKGROUND

For both pediatric and adult patients, umbilical cord blood (UCB) transplant is a therapeutic option for a variety of hematologic diseases, such as blood cancers, myeloproliferative disorders, genetic diseases, and metabolic disorders. However, the level of cellular heterogeneity and diversity of nucleated cells in UCB has not yet been assessed in an unbiased and systemic fashion. In the present study, nucleated cells from UCB were subjected to single-cell RNA sequencing to simultaneously profile the gene expression signatures of thousands of cells, generating a rich resource for further functional studies. Here, we report the transcriptomes of 17,637 UCB cells, covering 12 major cell types, many of which can be further divided into distinct subpopulations.

RESULTS

Pseudotemporal ordering of nucleated red blood cells identifies wave-like activation and suppression of transcription regulators, leading to a polarized cellular state, which may reflect nucleated red blood cell maturation. Progenitor cells in UCB also comprise 2 subpopulations with activation of divergent transcription programs, leading to specific cell fate commitment. Detailed profiling of cytotoxic cell populations unveiled granzymes B and K signatures in natural killer and natural killer T-cell types in UCB.

CONCLUSIONS

Taken together, our data form a comprehensive single-cell transcriptomic landscape that reveals previously unrecognized cell types, pathways, and mechanisms of gene expression regulation. These data may contribute to the efficacy and outcome of UCB transplant, broadening the scope of research and clinical innovations.

Trib1 regulates eosinophil lineage commitment and identity by restraining the neutrophil program

Eosinophils and neutrophils are critical for host defense, yet gaps in understanding how granulocytes differentiate from hematopoietic stem cells (HSCs) into mature effectors remain. The pseudokinase tribbles homolog 1 (Trib1) is an important regulator of granulocytes; knockout mice lack eosinophils and have increased neutrophils. However, how Trib1 regulates cellular identity and function during eosinophilopoiesis is not understood. Trib1 expression markedly increases with eosinophil-lineage commitment in eosinophil progenitors (EoPs), downstream of the granulocyte/macrophage progenitor (GMP). Using hematopoietic- and eosinophil-lineage-specific Trib1 deletion, we found that Trib1 regulates both granulocyte precursor lineage commitment and mature eosinophil identity. Conditional Trib1 deletion in HSCs reduced the size of the EoP pool and increased neutrophils, whereas deletion following eosinophil lineage commitment blunted the decrease in EoPs without increasing neutrophils. In both modes of deletion, Trib1-deficient mice expanded a stable population of Ly6G⁺ eosinophils with neutrophilic characteristics and functions, and had increased CCAAT/enhancer binding protein α (C/EBPα) p42. Using an ex vivo differentiation assay, we found that interleukin 5 (IL-5) supports the generation of Ly6G⁺ eosinophils from Trib1-deficient cells, but is not sufficient to restore normal eosinophil differentiation and development. Furthermore, we demonstrated that Trib1 loss blunted eosinophil migration and altered chemokine receptor expression, both in vivo and ex vivo. Finally, we showed that Trib1 controls eosinophil identity by modulating C/EBPα. Together, our findings provide new insights into early events in myelopoiesis, whereby Trib1 functions at 2 distinct stages to guide eosinophil lineage commitment from the GMP and suppress the neutrophil program, promoting eosinophil terminal identity and maintaining lineage fidelity.

Transplacental immune modulation with a bacterial-derived agent protects against allergic airway inflammation

Chronic allergic inflammatory diseases are a major cause of morbidity, with allergic asthma alone affecting over 300 million people worldwide. Epidemiological studies demonstrate that environmental stimuli are associated with either the promotion or prevention of disease. Major reductions in asthma prevalence are documented in European and US farming communities. Protection is associated with exposure of mothers during pregnancy to microbial breakdown products present in farm dusts and unprocessed foods and enhancement of innate immune competence in the children. We sought to develop a scientific rationale for progressing these findings toward clinical application for primary disease prevention. Treatment of pregnant mice with a defined, clinically approved immune modulator was shown to markedly reduce susceptibility of their offspring to development of the hallmark clinical features of allergic airway inflammatory disease. Mechanistically, offspring displayed enhanced dendritic cell-dependent airway mucosal immune surveillance function, which resulted in more efficient generation of mucosal-homing regulatory T cells in response to local inflammatory challenge. We provide evidence that the principal target for maternal treatment effects was the fetal dendritic cell progenitor compartment, equipping the offspring for accelerated functional maturation of the airway mucosal dendritic cell network following birth. These data provide proof of concept supporting the rationale for developing transplacental immune reprogramming approaches for primary disease prevention.

mTOR complexes differentially orchestrates eosinophil development in allergy

Eosinophil infiltration is considered a hallmark in allergic airway inflammation, and the blockade of eosinophil differentiation may be an effective approach for treating eosinophil-related disorders. Mammalian target of rapamycin (mTOR) is a vital modulator in cell growth control and related diseases, and we have recently demonstrated that rapamycin can suppress eosinophil differentiation in allergic airway inflammation. Considering its critical role in haematopoiesis, we further investigated the role of mTOR in eosinophil differentiation in the context of asthmatic pathogenesis. Intriguingly, the inhibition of mTOR, either by genetic deletion or by another pharmacological inhibitor torin-1, accelerated the eosinophil development in the presence of IL-5. However, this was not observed to have any considerable effect on eosinophil apoptosis. The effect of mTOR in eosinophil differentiation was mediated by Erk signalling. Moreover, myeloid specific knockout of mTOR or Rheb further augmented allergic airway inflammation in mice after allergen exposure. Ablation of mTOR in myeloid cells also resulted in an increased number of eosinophil lineage-committed progenitors (Eops) in allergic mice. Collectively, our data uncovered the differential effects of mTOR in the regulation of eosinophil development, likely due to the distinct functions of mTOR complex 1 or 2, which thus exerts a pivotal implication in eosinophil-associated diseases.

Identification of a Siglec-F+ granulocyte-macrophage progenitor

In recent years multi-parameter flow cytometry has enabled identification of cells at major stages in myeloid development; from pluripotent hematopoietic stem cells, through populations with increasingly limited developmental potential (common myeloid progenitors and granulocyte-macrophage progenitors), to terminally differentiated mature cells. Myeloid progenitors are heterogeneous, and the surface markers that define transition states from progenitors to mature cells are poorly characterized. Siglec-F is a surface glycoprotein frequently used in combination with IL-5 receptor alpha (IL5Rα) for the identification of murine eosinophils. Here, we describe a CD11b⁺ Siglec-F+ IL5Rα- myeloid population in the bone marrow of C57BL/6 mice. The CD11b⁺ Siglec-F+ IL5Rα- cells are retained in eosinophil deficient PHIL mice, and are not expanded upon overexpression of IL-5, indicating that they are upstream or independent of the eosinophil lineage. We show these cells to have GMP-like developmental potential in vitro and in vivo, and to be transcriptionally distinct from the classically described GMP population. The CD11b+ Siglec-F+ IL5Rα- population expands in the bone marrow of Myb mutant mice, which is potentially due to negative transcriptional regulation of Siglec-F by Myb. Lastly, we show that the role of Siglec-F may be, at least in part, to regulate GMP viability.

Cytokine Networks between Innate Lymphoid Cells and Myeloid Cells

Innate lymphoid cells (ILCs) are an essential component of the innate immune system in vertebrates. They are developmentally rooted in the lymphoid lineage and can diverge into at least three transcriptionally distinct lineages. ILCs seed both lymphoid and non-lymphoid tissues and are locally self-maintained in tissue-resident pools. Tissue-resident ILCs execute important effector functions making them key regulator in tissue homeostasis, repair, remodeling, microbial defense, and anti-tumor immunity. Similar to T lymphocytes, ILCs possess only few sensory elements for the recognition of non-self and thus depend on extrinsic cellular sensory elements residing within the tissue. Myeloid cells, including mononuclear phagocytes (MNPs), are key sentinels of the tissue and are able to translate environmental cues into an effector profile that instructs lymphocyte responses. The adaptation of myeloid cells to the tissue state thus influences the effector program of ILCs and serves as an example of how environmental signals are integrated into the function of ILCs via a tissue-resident immune cell cross talks. This review summarizes our current knowledge on the role of myeloid cells in regulating ILC functions and discusses how feedback communication between ILCs and myeloid cells contribute to stabilize immune homeostasis in order to maintain the healthy state of an organ.

Eosinophil-derived CCL-6 impairs hematopoietic stem cell homeostasis

Eosinophils (Eos) have been long considered as end-stage effector cells in the hierarchical hematopoietic system. Numerous lines of evidence have suggested that Eos are multifunctional leukocytes with respect to the initiation, propagation and regulation of various inflammatory or immune reactions, especially in allergic diseases. Recent studies have shown that Eos are also required for maintenance of bone marrow plasma cells and differentiation of B cells. However, it remains unclear whether Eos contributes to regulation of hematopoietic stem cell (HSC) homeostasis. Here, we demonstrate that Eos disrupt HSC homeostasis by impairing HSC quiescence and reconstitution ability in wild-type mice following ovalbumin (OVA) challenge and even by causing bone marrow HSC failure and exhaustion in Cd3δ-Il-5 transgenic mice. The impaired maintenance and function of HSCs were associated with Eos-induced redox imbalance (increased oxidative phosphorylation and decreased anti-oxidants levels). More importantly, using mass spectrometry, we determined that CCL-6 is expressed at a high level under eosinophilia. We demonstrate that CCL-6 is Eos-derived and responsible for the impaired HSC homeostasis. Interestingly, blockage of CCL-6 with a specific neutralizing antibody, restored the reconstitution ability of HSCs while exacerbating eosinophilia airway inflammation in OVA-challenged mice. Thus, our study reveals an unexpected function of Eos/CCL-6 in HSC homeostasis.

Eosinophil Development, Disease Involvement, and Therapeutic Suppression

Human eosinophils have characteristic morphologic features, including a bilobed nucleus and cytoplasmic granules filled with cytotoxic and immunoregulatory proteins that are packaged in a specific manner. Eosinophil production in the bone marrow is exquisitely regulated by timely expression of a repertoire of transcription factors that work together via collaborative and hierarchical interactions to direct eosinophil development. In addition, proper granule formation, which occurs in a spatially organized manner, is an intrinsic checkpoint that must be passed for proper eosinophil production to occur. In eosinophil-associated disorders, eosinophils and their progenitors can be recruited in large numbers into tissues where they can induce proinflammatory organ damage in response to local signals. Eosinophils are terminally differentiated and do not proliferate once they leave the bone marrow. The cytokine IL-5 specifically enhances eosinophil production and, along with other mediators, promotes eosinophil activation. Indeed, eosinophil depletion with anti-IL-5 or anti-IL-5Rα is now proven to be clinically beneficial for several eosinophilic disorders, most notably severe asthma, and several therapeutics targeting eosinophil viability and production are now in development. Significant progress has been made in our understanding of eosinophil development and the consequences of tissue eosinophilia. Future research efforts focused on basic eosinophil immunobiology and translational efforts to assist in the diagnosis, treatment selection, and resolution of eosinophil-associated disorders will likely be informative and clinically helpful.

Elimination of eosinophils using anti-IL-5 receptor alpha antibodies effectively suppresses IL-33-mediated pulmonary arterial hypertrophy

Interleukin (IL)-5 is a critical regulator of eosinophils and a therapeutic target for asthma. The administration of anti-IL-5 or anti-IL-5 receptor (IL-5R) antibodies has been shown to reduce eosinophil counts and ameliorate asthmatic symptoms in studies on animal models of allergy as well as in human clinical trials. In order to explore other potential clinical uses of IL-5R antibodies, we used an animal model of IL-33-mediated pulmonary arterial hypertrophy. We first generated chimeric monoclonal antibodies against the mouse IL-5 receptor α chain (IL-5Rα), which comprised an Fc region from human IgG1 and a Fab region from a previously established anti-mouse IL-5Rα monoclonal antibody. To investigate the role of antibody-dependent cell-mediated cytotoxicity (ADCC), chimeric antibodies that lacked ADCC were prepared. These antibodies recognized IL-5Rα to the same extent as the ADCC-sufficient antibodies. Administration of chimeric antibodies with ADCC resulted in the elimination of eosinophils from the lung and thus suppressed the development of arterial hypertrophy. This effect was attenuated in mice treated with antibodies lacking ADCC. Taken together, the results of this study provided a potential use for anti-IL-5Rα antibodies in the treatment of arterial hypertrophy, which leads to pulmonary hypertension.

Notch1 expression is regulated at the post-transcriptional level by the 3' untranslated region in hematopoietic stem cell development

In hematopoiesis, the expression of critical genes is regulated in a stage-specific manner to maintain normal hematopoiesis. Notch1 is an essential gene involved in the commitment and development of the T-cell lineage. However, the regulation of Notch1 in hematopoiesis is controversial, particularly at the level of hematopoietic stem cell (HSC). Here, we found that the expression of Notch1 is controlled at the post-transcriptional level in HSCs. HSCs express a considerable level of Notch1 mRNA, but its protein level is very low, suggesting a post-transcriptional suppression for Notch1. Using a retroviral sensor vector expressing a fusion mRNA of GFP and 3' untranslated region (3'UTR) of a target gene, we demonstrated that the Notch1-3'UTR had a post-translational suppressive effect only at the HSC but not in the downstream progenitor stages. The sequence motif AUnA was required for this post-transcriptional regulation by the Notch1-3'UTR. Interestingly, this Notch1-3'UTR-mediated suppressive effect was relieved when HSCs were placed in the thymus, but not in the bone marrow. Thus, the expression of Notch1 in HSCs is regulated by microenvironment at the post-transcriptional level, which may control T lymphoid lineage commitment from HSCs.

Regulation of eosinophil development and survival

PURPOSE OF REVIEW

Eosinophils are a subset of granulocytes generally associated with type 2 immune responses. They can contribute to protection against helminths but also mediate pro-inflammatory functions during allergic immune responses. Only recently, eosinophils were also found to exert many other functions such as regulation of glucose and fat metabolism, thermogenesis, survival of plasma cells, and antitumor activity. The mechanisms that control eosinophil development and survival are only partially understood.

RECENT FINDINGS

Here we review new findings regarding the role of cell-extrinsic and cell-intrinsic factors for eosinophilopoiesis and eosinophil homeostasis. Several reports provide new insights in the regulation of eosinophil development by transcription factors, miRNAs and epigenetic modifications. Danger signals like lipopolysaccharide or alarmins can activate eosinophils but also prolong their lifespan. We further reflect on the observations that eosinophil development is tightly controlled by the unfolded protein stress response and formation of cytoplasmic granules.

SUMMARY

Eosinophils emerge as important regulators of diverse biological processes. Their differentiation and survival is tightly regulated by factors that are still poorly understood. Newly identified pathways involved in eosinophilopoiesis and eosinophil homeostasis may lead to development of new therapeutic options for treatment of eosinophil-associated diseases.

Myeloid Cell Origins, Differentiation, and Clinical Implications

The hematopoietic stem cell (HSC) is a multipotent stem cell that resides in the bone marrow and has the ability to form all of the cells of the blood and immune system. Since its first purification in 1988, additional studies have refined the phenotype and functionality of HSCs and characterized all of their downstream progeny. The hematopoietic lineage is divided into two main branches: the myeloid and lymphoid arms. The myeloid arm is characterized by the common myeloid progenitor and all of its resulting cell types. The stages of hematopoiesis have been defined in both mice and humans. During embryological development, the earliest hematopoiesis takes place in yolk sac blood islands and then migrates to the fetal liver and hematopoietic organs. Some adult myeloid populations develop directly from yolk sac progenitors without apparent bone marrow intermediates, such as tissue-resident macrophages. Hematopoiesis also changes over time, with a bias of the dominating HSCs toward myeloid development as animals age. Defects in myelopoiesis contribute to many hematologic disorders, and some of these can be overcome with therapies that target the aberrant stage of development. Furthermore, insights into myeloid development have informed us of mechanisms of programmed cell removal. The CD47/SIRPα axis, a myeloid-specific immune checkpoint, limits macrophage removal of HSCs but can be exploited by hematologic and solid malignancies. Therapeutics targeting CD47 represent a new strategy for treating cancer. Overall, an understanding of hematopoiesis and myeloid cell development has implications for regenerative medicine, hematopoietic cell transplantation, malignancy, and many other diseases.

Transcription Factors in Eosinophil Development and As Therapeutic Targets

Dynamic gene expression is a major regulatory mechanism that directs hematopoietic cell fate and differentiation, including eosinophil lineage commitment and eosinophil differentiation. Though GATA-1 is well established as a critical transcription factor (TF) for eosinophil development, delineating the transcriptional networks that regulate eosinophil development at homeostasis and in inflammatory states is not complete. Yet, recent advances in molecular experimental tools using purified eosinophil developmental stages have led to identifying new regulators of gene expression during eosinophil development. Herein, recent studies that have provided new insight into the mechanisms of gene regulation during eosinophil lineage commitment and eosinophil differentiation are reviewed. A model is described wherein distinct classes of TFs work together via collaborative and hierarchical interactions to direct eosinophil development. In addition, the therapeutic potential for targeting TFs to regulate eosinophil production is discussed. Understanding how specific signals direct distinct patterns of gene expression required for the specialized functions of eosinophils will likely lead to new targets for therapeutic intervention.

Understanding Interleukin 33 and Its Roles in Eosinophil Development

Over the last decade, significant interest in the contribution of three “epithelial-derived cytokines,” such as thymic stromal lymphopoietin, interleukin 25, and interleukin 33 (IL-33), has developed. These cytokines have been strongly linked to the early events that occur during allergen exposures and how they contribute to the subsequent type 2 immune response. Of these three cytokines, IL-33 has proven particularly interesting because of the strong associations found between both it and its receptor, ST2, in several genome-wide association studies of allergic diseases. Further work has demonstrated clear mechanisms through which this cytokine might orchestrate allergic inflammation, including activation of several key effector cells that possess high ST2 levels, including mast cells, basophils, innate lymphoid cells, and eosinophils. Despite this, controversies surrounding IL-33 seem to suggest the biology of this cytokine might not be as simple as current dogmas suggest including: the relevant cellular sources of IL-33, with significant evidence for inducible expression in some hematopoietic cells; the mechanistic contributions of nuclear localization vs secretion; secretion and processing mechanisms; and the biological consequences of IL-33 exposure on different cell types. In this review, we will address the evidence for IL-33 and ST2 regulation over eosinophils and how this may contribute to allergic diseases. In particular, we focus on the accumulating evidence for a role of IL-33 in regulating hematopoiesis and how this relates to eosinophils as well as how this may provide new concepts for how the progression of allergy is regulated.