Defective B-1 cell development and impaired immunity against Angiostrongylus cantonensis in IL-5R alpha-deficient mice

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.

IL-5 antagonism reverses priming and activation of eosinophils in severe eosinophilic asthma

Eosinophils are key effector cells mediating airway inflammation and exacerbation in patients with severe eosinophilic asthma. They are present in increased numbers and activation states in the airway mucosa and lumen. Interleukin-5 (IL-5) is the key eosinophil growth factor that is thought to play a role in eosinophil priming and activation. However, the mechanism of these effects is still not fully understood. The anti-IL-5 antibody mepolizumab reduces eosinophil counts in the airway modestly but has a large beneficial effect on the frequency of exacerbations of severe eosinophilic asthma, suggesting that reduction in eosinophil priming and activation is of central mechanistic importance. In this study, we used the therapeutic effect of mepolizumab and single-cell ribonucleic acid sequencing to investigate the mechanism of eosinophil priming and activation by IL-5. We demonstrated that IL-5 is a dominant driver of eosinophil priming and plays multifaceted roles in eosinophil function. It enhances eosinophil responses to other stimulators of migration, survival, and activation by activating phosphatidylinositol-3-kinases, extracellular signal-regulated kinases, and p38 mitogen-activated protein kinases signaling pathways. It also enhances the pro-fibrotic roles of eosinophils in airway remodeling via transforming growth factor-β pathway. These findings provide a mechanistic understanding of eosinophil priming in severe eosinophilic asthma and the therapeutic effect of anti-IL-5 approaches in the disease.

Chicken Interleukin-5 is Expressed in Splenic Lymphocytes and Affects Antigen-Specific Antibody Production

Vaccination is important for reducing disease incidence in the poultry industry. To enhance immunity and vaccine efficacy, chicken cytokines associated with antibody production must be identified. In this study, we focused on interleukin-5 (IL-5), involved in antibody production in mice, measuring its expression and effects on antibody production. Concanavalin A-stimulated splenocytes were used for RT-PCR to clone IL5 cDNAs. Recombinant IL-5 was prepared from the clone and administered to chickens with antigen via the ocular-topical route twice every alternate week. IL-5 enhanced antigen-specific IgY and inhibited antigen-specific serum IgA production in serum. Our findings suggest that IL-5 plays an important role in chicken antibody production, with possible unique functions.

Enhanced CD19 activity in B cells contributes to immunodeficiency in mice deficient in the ICF syndrome gene Zbtb24

Immunodeficiency, centromeric instability, and facial anomalies (ICF) syndrome is a rare autosomal recessive disorder characterized by DNA hypomethylation and antibody deficiency. It is caused by mutations in DNMT3B, ZBTB24, CDCA7, or HELLS. While progress has been made in elucidating the roles of these genes in regulating DNA methylation, little is known about the pathogenesis of the life-threatening hypogammaglobulinemia phenotype. Here, we show that mice deficient in Zbtb24 in the hematopoietic lineage recapitulate the major clinical features of patients with ICF syndrome. Specifically, Vav-Cre-mediated ablation of Zbtb24 does not affect lymphocyte development but results in reduced plasma cells and low levels of IgM, IgG1, and IgA. Zbtb24-deficient mice are hyper and hypo-responsive to T-dependent and T-independent type 2 antigens, respectively, and marginal zone B-cell activation is impaired. Mechanistically, Zbtb24-deficient B cells show severe loss of DNA methylation in the promoter region of Il5ra (interleukin-5 receptor subunit alpha), and Il5ra derepression leads to elevated CD19 phosphorylation. Heterozygous disruption of Cd19 can revert the hypogammaglobulinemia phenotype of Zbtb24-deficient mice. Our results suggest the potential role of enhanced CD19 activity in immunodeficiency in ICF syndrome.

ILC2 require cell-intrinsic ST2 signals to promote type 2 immune responses

The initiation of type 2 immune responses at mucosal barriers is regulated by rapidly secreted cytokines called alarmins. The alarmins IL-33, IL-25 and TSLP are mainly secreted by stromal and epithelial cells in tissues and were linked to chronic inflammatory diseases, such as allergic lung inflammation, or to resistance against worm infections. Receptors for alarmins are expressed by a variety of immune cells, including group 2 innate lymphoid cells (ILC2s), an early source of the type 2 cytokines, such as IL-5 and IL-13, which have been linked to atopic diseases and anti-worm immunity as well. However, the precise contribution of the IL-33 receptor signals for ILC2 activation still needs to be completed due to limitations in targeting genes in ILC2. Using the newly established Nmur1 iCre-eGFP mouse model, we obtained specific conditional genetic ablation of the IL-33 receptor subunit ST2 in ILC2s. ST2-deficient ILC2s were unresponsive to IL-33 but not to stimulation with the alarmin IL-25. As a result of defective ST2 signals, ILC2s produced limited amounts of IL-5 and IL-13 and failed to support eosinophil homeostasis. Further, ST2-deficient ILC2s were unable to expand and promote the recruitment of eosinophils during allergic lung inflammation provoked by papain administration. During infection with Nippostrongylus brasiliensis, ILC2-intrinsic ST2 signals were required to mount an effective type 2 immune response against the parasite leading to higher susceptibility against worm infection in conditional knockout mice. Therefore, this study argues for a non-redundant role of cell-intrinsic ST2 signals triggering proper activation of ILC2 for initiation of type 2 immunity.

Characterization of a mouse model of ICF syndrome reveals enhanced CD19 activation in inducing hypogammaglobulinemia

Immunodeficiency, centromeric instability and facial anomalies (ICF) syndrome is a rare autosomal recessive disorder characterized by DNA hypomethylation and antibody deficiency. It is caused by mutations in DNMT3B, ZBTB24, CDCA7 or HELLS . While progress has been made in elucidating the roles of these genes in regulating DNA methylation, little is known about the pathogenesis of the life-threatening hypogammaglobulinemia phenotype. Here we show that mice deficient for Zbtb24 in the hematopoietic lineage recapitulate major clinical features of patients with ICF syndrome. Specifically, Vav-Cre-mediated ablation of Zbtb24 does not affect lymphocyte development but results in reduced plasma cells and low levels of IgM, IgG1 and IgA. Zbtb24 -deficient mice are hyper- and hypo-responsive to T-dependent and Tindependent type 2 antigens, respectively, and marginal zone B cell activation is impaired. B cells from Zbtb24 -deficient mice display elevated CD19 phosphorylation. Heterozygous disruption of Cd19 can revert the hypogammaglobulinemia phenotype in these mice. Mechanistically, Il5ra (interleukin-5 receptor subunit alpha) is derepressed in Zbtb24 -deficient B cells, and elevated IL-5 signaling enhances CD19 phosphorylation. Our results reveal a novel link between IL-5 signaling and CD19 activation and suggest that abnormal CD19 activity contributes to immunodeficiency in ICF syndrome.

SIGNIFICANCE STATEMENT

ICF syndrome is a rare immunodeficiency disorder first reported in the 1970s. The lack of appropriate animal models has hindered the investigation of the pathogenesis of antibody deficiency, the major cause of death in ICF syndrome. Here we show that, in mice, disruption of Zbtb24 , one of the ICF-related genes, in the hematopoietic lineage results in low levels of immunoglobulins. Characterization of these mice reveals abnormal B cell activation due to elevated CD19 phosphorylation. Mechanistically, Il5ra (interleukin-5 receptor subunit alpha) is derepressed in Zbtb24 -deficient B cells, and increased IL-5 signaling enhances CD19 phosphorylation.

Pla2g2a promotes innate Th2-type immunity lymphocytes to increase B1a cells

Newborns require early generation of effective innate immunity as a primary physiological mechanism for survival. The neonatal Lin28⁺Let7^– developmental pathway allows increased generation of Th2-type cells and B1a (B-1 B) cells compared to adult cells and long-term maintenance of these initially generated innate cells. For initial B1a cell growth from the neonatal to adult stage, Th2-type IL-5 production from ILC2s and NKT2 cells is important to increase B1a cells. The Th17 increase is dependent on extracellular bacteria, and increased bacteria leads to lower Th2-type generation. Secreted group IIA-phospholipase A2 (sPLA2-IIA) from the Pla2g2a gene can bind to gram-positive bacteria and degrade bacterial membranes, controlling microbiota in the intestine. BALB/c mice are Pla2g2a⁺, and express high numbers of Th2-type cells and B1a cells. C57BL/6 mice are Pla2g2a-deficient and distinct from the SLAM family, and exhibit fewer NKT2 cells and fewer B1a cells from the neonatal to adult stage. We found that loss of Pla2g2a in the BALB/c background decreased IL-5 from Th2-type ILC2s and NKT2s but increased bacterial-reactive NKT17 cells and MAIT cells, and decreased the number of early-generated B1a cells and MZ B cells and the CD4/CD8 T cell ratio. Low IL-5 by decreased Th2-type cells in Pla2g2a loss led to low early-generated B1a cell growth from the neonatal to adult stage. In anti-thymocyte/Thy-1 autoreactive μκ transgenic (ATAμκ Tg) Pla2g2a⁺ BALB/c background C.B17 mice generated NKT2 cells that continuously control CD1d⁺ B1 B cells through old aging and lost CD1d in B1 B cells generating strong B1 ATA B cell leukemia/lymphoma. Pla2g2a-deficient ATAμκTg C57BL/6 mice suppressed the initial B1a cell increase, with low/negative spontaneous leukemia/lymphoma generation. These data confirmed that the presence of Pla2g2a to control bacteria is important to allow the neonatal to adult stage. Pla2g2a promotes innate Th2-type immunity lymphocytes to increase early generated B1a cells.

Heterogeneity of Group 2 Innate Lymphoid Cells Defines Their Pleiotropic Roles in Cancer, Obesity, and Cardiovascular Diseases

Group 2 innate lymphoid cells (ILC2s) are typically known for their ability to respond rapidly to parasitic infections and play a pivotal role in the development of certain allergic disorders. ILC2s produce cytokines such as Interleukin (IL)-5 and IL-13 similar to the type 2 T helper (Th2) cells. Recent findings have highlighted that ILC2s, together with IL-33 and eosinophils, participate in a considerably broad range of physiological roles such as anti-tumor immunity, metabolic regulation, and vascular disorders. Therefore, the focus of the ILC2 study has been extended from conventional Th2 responses to these unexplored areas of research. However, disease outcomes accompanied by ILC2 activities are paradoxical mostly in tumor immunity requiring further investigations. Although various environmental factors that direct the development, activation, and localization of ILC2s have been studied, IL-33/ILC2/eosinophil axis is presumably central in a multitude of inflammatory conditions and has guided the research in ILC2 biology. With a particular focus on this axis, we discuss ILC2s across different diseases.

Pharmacological approaches to target type 2 cytokines in asthma

Asthma is the most common chronic lung disease, affecting more than 250 million people worldwide. The heterogeneity of asthma phenotypes represents a challenge for adequate assessment and treatment of the disease. However, approximately 50% of asthma patients present with chronic type 2 inflammation initiated by alarmins, such as IL-33 and thymic stromal lymphopoietin (TSLP), and driven by the T_H2 interleukins IL-4, IL-5 and IL-13. These cytokines have therefore become important therapeutic targets in asthma. Here, we discuss current knowledge on the structure and functions of these cytokines in asthma. We review preclinical and clinical data obtained with monoclonal antibodies (mAbs) targeting these cytokines or their receptors, as well as novel strategies under development, including bispecific mAbs, designed ankyrin repeat proteins (DARPins), small molecule inhibitors and vaccines targeting type 2 cytokines.

Critical role of IL-25-ILC2-IL-5 axis in the production of anti-Francisella LPS IgM by B1 B cells

B1 cells, a subset of B lymphocytes whose developmental origin, phenotype, and function differ from that of conventional B2 cells, are the main source of “natural” IgM but can also respond to infection by rapidly producing pathogen-specific IgM directed against T-independent antigens. Francisella tularensis (Ft) is a Gram-negative bacterium that causes tularemia. Infection with Ft Live Vaccine Strain activates B1 cells for production of IgM directed against the bacterial LPS in a process incompletely understood. Here we show that immunization with purified Ft LPS elicits production of LPS-specific IgM and IgG₃ by B1 cells independently of TLR2 or MyD88. Immunization, but not infection, generated peritoneum-resident memory B1 cells that differentiated into LPS-specific antibody secreting cells (ASC) upon secondary challenge. IL-5 was rapidly induced by immunization with Ft LPS and was required for production of LPS-specific IgM. Antibody-mediated depletion of ILC2 indicated that these cells were the source of IL-5 and were required for IgM production. IL-25, an alarmin that strongly activates ILC2, was rapidly secreted in response to immunization or infection and its administration to mice significantly increased IgM production and B1 cell differentiation to ASC. Conversely, mice lacking IL-17RB, the IL-25 receptor, showed impaired IL-5 induction, IgM production, and B1 ASC differentiation in response to immunization. Administration of IL-5 to Il17rb^-/- mice rescued these B1 cells-mediated responses. Il17rb^-/- mice were more susceptible to infection with Ft LVS and failed to develop immunity upon secondary challenge suggesting that LPS-specific IgM is one of the protective adaptive immune mechanisms against tularemia. Our results indicated that immunization with Ft LPS triggers production of IL-25 that, through stimulation of IL-5 release by ILC2, promotes B1 cells activation and differentiation into IgM secreting cells. By revealing the existence of an IL-25-ILC2-IL-5 axis our results suggest novel strategies to improve vaccination against T-independent bacterial antigens.

B1 cells are a subset of B lymphocytes that participate in the immune response to infection by producing antibodies of the IgM class. Here we investigate the mechanisms that control B1 cells activation and production of IgM directed against the lipopolysaccharide (LPS) of Francisella tularensis, a Gram-negative bacterium that causes tularemia. Using a mouse model of tularemia, our results revealed that Francisella LPS elicits production of the cytokine IL-25 that in turn activates blood cells called Innate Lymphoid Cells 2 (ILC2). Once activated, ILC2 produce the cytokine IL-5 that is required for activation of B1 cells and production of IgM. Mice unresponsive to IL-25 are more susceptible to F. tularensis infection. By revealing the existence of an IL-25-ILC2-IL-5 axis our results suggest novel strategies to improve vaccination against bacteria.

Eosinophils and helminth infection: protective or pathogenic?

Since the earliest descriptions of this enigmatic cell, eosinophils have been implicated in both protective and pathogenic immune responses to helminth infection. Nevertheless, despite substantial data from in vitro studies, human infections, and animal models, their precise role in helminth infection remains incompletely understood. This is due to a number of factors, including the heterogeneity of the many parasites included in the designation "helminth," the complexity and redundancy in the host immune response to helminths, and the pleiotropic functions of eosinophils themselves. This review examines the consequences of helminth-associated eosinophilia in the context of protective immunity, pathogenesis, and immunoregulation.

Eosinophil Knockout Humans: Uncovering the Role of Eosinophils Through Eosinophil-Directed Biological Therapies

The enigmatic eosinophil has emerged as an exciting component of the immune system, involved in a plethora of homeostatic and inflammatory responses. Substantial progress has been achieved through experimental systems manipulating eosinophils in vivo, initially in mice and more recently in humans. Researchers using eosinophil knockout mice have identified a contributory role for eosinophils in basal and inflammatory processes and protective immunity. Primarily fueled by the purported proinflammatory role of eosinophils in eosinophil-associated diseases, a series of anti-eosinophil therapeutics have emerged as a new class of drugs. These agents, which dramatically deplete eosinophils, provide a valuable opportunity to characterize the consequences of eosinophil knockout humans. Herein, we comparatively describe mouse and human eosinophil knockouts. We put forth the view that human eosinophils negatively contribute to a variety of diseases and, unlike mouse eosinophils, do not yet have an identified role in physiological health; thus, clarifying all roles of eosinophils remains an ongoing pursuit.

TNF-α Triggers RIP1/FADD/Caspase-8-Mediated Apoptosis of Astrocytes and RIP3/MLKL-Mediated Necroptosis of Neurons Induced by Angiostrongylus cantonensis Infection

Angiostrongylus cantonensis (AC) can cause severe eosinophilic meningitis or encephalitis in non-permissive hosts accompanied by apoptosis and necroptosis of brain cells. However, the explicit underlying molecular basis of apoptosis and necroptosis upon AC infection has not yet been elucidated. To determine the specific pathways of apoptosis and necroptosis upon AC infection, gene set enrichment analysis (GSEA) and protein-protein interaction (PPI) analysis for gene expression microarray (accession number: GSE159486) of mouse brain infected by AC revealed that TNF-α likely played a central role in the apoptosis and necroptosis in the context of AC infection, which was further confirmed via an in vivo rescue assay after treating with TNF-α inhibitor. The signalling axes involved in apoptosis and necroptosis were investigated via immunoprecipitation and immunoblotting. Immunofluorescence was used to identify the specific cells that underwent apoptosis or necroptosis. The results showed that TNF-α induced apoptosis of astrocytes through the RIP1/FADD/Caspase-8 axis and induced necroptosis of neurons by the RIP3/MLKL signalling pathway. In addition, in vitro assay revealed that TNF-α secretion by microglia increased upon LSA stimulation and caused necroptosis of neurons. The present study provided the first evidence that TNF-α was secreted by microglia stimulated by AC infection, which caused cell death via parallel pathways of astrocyte apoptosis (mediated by the RIP1/FADD/caspase-8 axis) and neuron necroptosis (driven by the RIP3/MLKL complex). Our research comprehensively elucidated the mechanism of cell death after AC infection and provided new insight into targeting TNF-α signalling as a therapeutic strategy for CNS injury.

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.

Eosinophils in the Tumor Microenvironment

Eosinophils are rare blood-circulating and tissue-infiltrating immune cells studied for decades in the context of allergic diseases and parasitic infections. Eosinophils can secrete a wide array of soluble mediators and effector molecules, with potential immunoregulatory activities in the tumor microenvironment (TME). These findings imply that these cells may play a role in cancer immunity. Despite these cells were known to infiltrate tumors since many years ago, their role in TME is gaining attention only recently. In this chapter, we will review the main biological functions of eosinophils that can be relevant within the TME. We will discuss how these cells may undergo phenotypic changes acquiring pro- or antitumoricidal properties according to the surrounding stimuli. Moreover, we will analyze canonical (i.e., degranulation) and unconventional mechanisms (i.e., DNA traps, exosome secretion) employed by eosinophils in inflammatory contexts, which can be relevant for tumor immune responses. Finally, we will review the available preclinical models that could be employed for the study of the role in vivo of eosinophils in cancer.

Molecular Biology of Eosinophils: Introduction

The eosinophil is an enigmatic cell with a continuing ability to fascinate. A considerable history of research endeavor on eosinophil biology stretches from the present time back to the nineteenth century. Perhaps one of the most fascinating aspects of the eosinophil is how accumulating knowledge has changed the perception of its function from passive bystander, modulator of inflammation, to potent effector cell loaded with histotoxic substances through to more recent recognition that it can act as both a positive and negative regulator of complex events in both innate and adaptive immunity. This book consists of chapters written by experts in the field of eosinophil biology that provide comprehensive clearly written protocols for techniques designed to underpin research into the function of the eosinophil in health and disease.

Role of Cytokines in EGPA and the Possibility of Treatment with an Anti-IL-5 Antibody

Eosinophilic granulomatosis with polyangiitis (EGPA) is a type of systemic vasculitis with eosinophilia in the peripheral blood, which is preceded by bronchial asthma or allergic disease. EGPA is pathologically characterized by microangiopathy granulomatosis vasculitis. Vasculitis can be exacerbated and cause central nervous system and cardiovascular disorders and gastrointestinal perforation. Histological examination reveals eosinophil infiltration and granulomas in lesions in areas such as the lung, nervous system, and skin. Laboratory tests show inflammatory findings such as C-reactive protein (CRP) elevation, increased eosinophils, elevated serum IgE, and elevated myeloperoxidase-anti-neutrophil cytoplasmic antibodies (MPO-ANCA). MPO-ANCA is positive in approximately 40-70% of cases of this disease. EGPA is a necrotizing vasculitis that affects small- and medium-sized blood vessels; however, it differs from other types of ANCA-related vasculitis (such as microscopic polyangiitis and granulomatosis) because it is preceded by bronchial asthma and eosinophilia in the blood and tissues. Treatment with immunosuppressive agents such as steroids or cyclophosphamide depends on the Five Factor Score, which predicts the prognosis and severity of the condition. If the effect of appropriate treatment with steroids is insufficient, the anti-interleukin-5 antibody mepolizumab can be administered. The combination of mepolizumab with standard treatment leads to a significantly longer duration of remission, a higher proportion of patients who achieve sustained remission, and less steroid use than with a placebo.

IL-21 and IL-5 coordinately induce surface IgA+ cells

Intestinal IgA is induced by microbes and food antigens. Peyer's patches (PPs) are known as one of the inductive sites for intestinal IgA production. However, the precise mechanism of IgA induction is as yet unknown. IgA secretion was induced from IgD⁺ B cells in vitro by stimulus with lipopolysaccharide in the presence of only retinoic acid (RA) and low doses of TGF-β1. Surface IgA⁺ cells were effectively induced from IgD⁺ B cells in vitro by the mixture of RA and the cytokines TGF-β1, APRIL, IL-5 and IL-21. rIL-21 upregulated surface IgA⁺ but impaired the proliferation of stimulated B cells in the presence of rTGF-β1, RA and rAPRIL, in vitro. The addition of rIL-5 restored the impaired proliferation by rIL-21, resulting in the expansion of IgA⁺ cells. rIL-21 induced the expression of Aicda and Prdm1, and impaired Rel in IgD⁺ B cells. Blockade of IL-21R signaling by a neutralizing mAb in vivo led to lower frequencies of IgA⁺ and IgG2b⁺ cells and lower germinal center B cells in PPs in a homeostatic condition. Although amounts of small intestinal IgA and titers of anti-dsDNA, the major target of intestinal IgA, in these mice were not altered, anti-OVA IgA titers induced by OVA drinking in OVA-specific T-cell receptor (TCR) transgenic mice were decreased. PP-deficient TCR transgenic mice showed diminished anti-OVA IgA induction. Blockade of IL-5R signaling in vivo led to similar results with relatively weaker effects than that of IL-21R mAb administration. These results suggest that IL-21 and IL-5 play cooperative roles in surface expression of IgA in PPs.

GM-CSF, IL-3, and IL-5 Family of Cytokines: Regulators of Inflammation

The β common chain cytokines GM-CSF, IL-3, and IL-5 regulate varied inflammatory responses that promote the rapid clearance of pathogens but also contribute to pathology in chronic inflammation. Therapeutic interventions manipulating these cytokines are approved for use in some cancers as well as allergic and autoimmune disease, and others show promising early clinical activity. These approaches are based on our understanding of the inflammatory roles of these cytokines; however, GM-CSF also participates in the resolution of inflammation, and IL-3 and IL-5 may also have such properties. Here, we review the functions of the β common cytokines in health and disease. We discuss preclinical and clinical data, highlighting the potential inherent in targeting these cytokine pathways, the limitations, and the important gaps in understanding of the basic biology of this cytokine family.

A humanized mouse model to study asthmatic airway inflammation via the human IL-33/IL-13 axis

Asthma is one of the most common immunological diseases and is characterized by airway hyperresponsiveness (AHR), mucus overproduction, and airway eosinophilia. Although mouse models have provided insight into the mechanisms by which type-2 cytokines induce asthmatic airway inflammation, differences between the rodent and human immune systems hamper efforts to improve understanding of human allergic diseases. In this study, we aim to establish a preclinical animal model of asthmatic airway inflammation using humanized IL-3/GM-CSF or IL-3/GM-CSF/IL-5 Tg NOD/Shi-scid-IL2rγnull (NOG) mice and investigate the roles of human type-2 immune responses in the asthmatic mice. Several important characteristics of asthma - such as AHR, goblet cell hyperplasia, T cell infiltration, IL-13 production, and periostin secretion - were induced in IL-3/GM-CSF Tg mice by intratracheally administered human IL-33. In addition to these characteristics, human eosinophilic inflammation was observed in IL-3/GM-CSF/IL-5 Tg mice. The asthmatic mechanisms of the humanized mice were driven by activation of human Th2 and mast cells by IL-33 stimulation. Furthermore, treatment of the humanized mice with an anti-human IL-13 antibody significantly suppressed these characteristics. Therefore, the humanized mice may enhance our understanding of the pathophysiology of allergic disorders and facilitate the preclinical development of new therapeutics for IL-33-mediated type-2 inflammation in asthma.

Differential Influence on Regulatory B Cells by TH2 Cytokines Affects Protection in Allergic Airway Disease

The role of regulatory B cells (Bregs) in modulating immune responses and maintaining tolerance are well established. However, how cytokines present during immune responses affect Breg growth and function are not as well defined. Previously, our laboratory reported IL-5- and mCD40L-expressing fibroblast (mCD40L-Fb) stimulation induced IL-10 production from murine B cells. The current study investigated the phenotype and functional relevance of IL-10- producing B cells from this culture. We found IL-5/mCD40L-Fb stimulation induced IL-10 production exclusively from CD5⁺ splenic B cells of naive mice. After stimulation, the resulting IL-10⁺ B cells displayed markers of multiple reported Breg phenotypes. Interestingly, when investigating effects of IL-4 (a critical T_H2 cytokine) on IL-5/mCD40L-Fb-induced IL-10 production, we found IL-4 inhibited IL-10 production in a STAT6-dependent manner. Upon adoptive transfer, CD5⁺ B cells previously stimulated with IL-5/mCD40L-Fb were able to reduce development of OVA-induced allergic airway disease in mice. Using B cells from IL-10 mutant mice differentiated by IL-5/mCD40L-Fb, we found protection from allergic airway disease development was dependent on the IL-10 production from the transferred B cells. Bregs have been shown to play crucial roles in the immune tolerance network, and understanding stimuli that modulate their growth and function may be key in development of future treatments for diseases of immune dysregulation.

Allergic diseases: From bench to clinic - Contribution of the discovery of interleukin-5

T helper 2 cells produce a number of cytokines including inteleukin (IL)-5, IL-4 and IL-13. Group 2 innate lymphoid cells (ILC2s) also produce IL-5 under sterile conditions. IL-5 is interdigitating homodimeric glycoprotein and a member of the four α helical bundle motifs conserved among hematopoietic cytokines. IL-5 exerts its effects on target cells via IL-5 receptor (IL-5R), composed of an IL-5R α and βc subunit. The membrane proximal proline-rich motif of the cytoplasmic domain of both IL-5R α and βc subunits is essential for IL-5 signal transduction. Although IL-5 was initially identified by its ability to support the growth and terminal differentiation of mouse B cells into antibody-secreting cells, recombinant IL-5 exerts pleiotropic activities on various target cells. For example, IL-5 is now recognized as the major maturation and differentiation factor for eosinophils in mice and humans. Overexpression of IL-5 in mouse significantly increases eosinophil numbers and antibody levels in vivo, while mice lacking a functional gene for IL-5 or IL-5R display developmental and functional impairments in B cell and eosinophil lineages. In mice, the role of the IL-5/IL-5R system in the production and secretion of Immunoglobulin (Ig) M and IgA in mucosal tissues has been reported. Although eosinophils protect against invading pathogens including virus, bacteria and helminthes, they are also involved in the pathogenesis of various diseases, such as food allergy, asthma, and inflammatory bowel diseases. The recent expansion in our understanding in the context of IL-5 and IL-5-producing ILC2s in eosinophil activation and the pathogenesis of eosinophil-dependent inflammatory diseases has led to advances in therapeutic options. A new therapy currently under invetigarion in clinical trials uses humanized monoclonal antibodies against IL-5 or the IL-5R. In this review, we summarize our current understanding of the functions of IL-5 and its receptor, the innate regulation of IL-5-producing cells, and therapeutic potential of anti-IL-5 and anti-eosinophil (IL-5R) antibodies.

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.

A Hard(y) Look at B-1 Cell Development and Function

A small population of B cells exists in lymphoid tissues and body cavities of mice that is distinct in development, phenotype, and function from the majority (B-2) B cell population. This population, originally termed "Ly-1" and now "B-1," has received renewed interest as an innate-like B cell population of fetal-derived hematopoiesis, responsible for natural Ab production and rapid immune responses. Molecular analyses have begun to define fetal and adult hematopoiesis, while cell-fate mapping studies have revealed complex developmental origins of B-1 cells. Together the studies provide a more detailed understanding of B-1 cell regulation and function. This review outlines studies that defined B-1 cells as natural Ab- and cytokine-producing B cells of fetal origin, with a focus on work conducted by R.R. Hardy, an early pioneer and codiscoverer of B-1 cells, whose seminal contributions enhanced our understanding of this enigmatic B cell population.

PD-L2 Regulates B-1 Cell Antibody Production against Phosphorylcholine through an IL-5-Dependent Mechanism

B-1 cells produce natural Abs which provide an integral first line of defense against pathogens while also performing important homeostatic housekeeping functions. In this study, we demonstrate that programmed cell death 1 ligand 2 (PD-L2) regulates the production of natural Abs against phosphorylcholine (PC). Naive PD-L2-deficient (PD-L2^-/-) mice produced significantly more PC-reactive IgM and IgA. This afforded PD-L2^-/- mice with selectively enhanced protection against PC-expressing nontypeable Haemophilus influenzae, but not PC-negative nontypeable Haemophilus influenzae, relative to wild-type mice. PD-L2^-/- mice had significantly increased PC-specific CD138⁺ splenic plasmablasts bearing a B-1a phenotype, and produced PC-reactive Abs largely of the T15 Id. Importantly, PC-reactive B-1 cells expressed PD-L2 and irradiated chimeras demonstrated that B cell-intrinsic PD-L2 expression regulated PC-specific Ab production. In addition to increased PC-specific IgM, naive PD-L2^-/- mice and irradiated chimeras reconstituted with PD-L2^-/- B cells had significantly higher levels of IL-5, a potent stimulator of B-1 cell Ab production. PD-L2 mAb blockade of wild-type B-1 cells in culture significantly increased CD138 and Blimp1 expression and PC-specific IgM, but did not affect proliferation. PD-L2 mAb blockade significantly increased IL-5⁺ T cells in culture. Both IL-5 neutralization and STAT5 inhibition blunted the effects of PD-L2 mAb blockade on B-1 cells. Thus, B-1 cell-intrinsic PD-L2 expression inhibits IL-5 production by T cells and thereby limits natural Ab production by B-1 cells. These findings have broad implications for the development of therapeutic strategies aimed at altering natural Ab levels critical for protection against infectious disease, autoimmunity, allergy, cancer, and atherosclerosis.

Essential role for the transcription factor Bhlhe41 in regulating the development, self-renewal and BCR repertoire of B-1a cells

Innate-like B-1a cells provide a first line of defense against pathogens, yet little is known about their transcriptional control. Here we identified an essential role of the transcription factor Bhlhe41, with a lesser contribution of Bhlhe40, in controlling late stages of B-1a cell differentiation. Bhlhe41^–/–Bhlhe40^–/– B-1a cells were severely reduced as compared to their wild-type counterparts. Mutant B-1a cells exhibited an abnormal cell-surface phenotype and altered B-cell receptor (BCR) repertoire exemplified by loss of the phosphatidylcholine-specific V_H12/Vκ4 BCR. Expression of a pre-rearranged V_H12/Vκ4 BCR failed to rescue the mutant phenotype and revealed enhanced proliferation accompanied with increased cell death. Bhlhe41 directly repressed the expression of cell cycle regulators and inhibitors of BCR signaling, while enabling pro-survival cytokine signaling. Thus, Bhlhe41 controls the development, BCR repertoire and self-renewal of B-1a cells.

Myeloid Cells in Asthma

Asthma is a heterogeneous chronic inflammatory disorder of the airways, and not surprisingly, many myeloid cells play a crucial role in pathogenesis. Antigen-presenting dendritic cells are the first to recognize the allergens, pollutants, and viruses that are implicated in asthma pathogenesis, and subsequently initiate the adaptive immune response by migrating to lymph nodes. Eosinophils are the hallmark of type 2 inflammation, releasing toxic compounds in the airways and contributing to airway remodeling. Mast cells and basophils control both the early- and late-phase allergic response and contribute to alterations in smooth muscle reactivity. Finally, relatively little is known about neutrophils and macrophages in this disease. Although many of these myeloid cells respond well to treatment with inhaled steroids, there is now an increasing armamentarium of targeted biologicals that can specifically eliminate only one myeloid cell population, like eosinophils. It is only with those new tools that we will be able to fully understand the role of myeloid cells in chronic asthma in humans.

The Eosinophil in Health and Disease: from Bench to Bedside and Back

Historically, eosinophils have been considered as end-stage cells involved in host protection against parasitic infection and in the mechanisms of hypersensitivity. However, later studies have shown that this multifunctional cell is also capable of producing immunoregulatory cytokines and soluble mediators and is involved in tissue homeostasis and modulation of innate and adaptive immune responses. In this review, we summarize the biology of eosinophils, including the function and molecular mechanisms of their granule proteins, cell surface markers, mediators, and pathways, and present comprehensive reviews of research updates on the genetics and epigenetics of eosinophils. We describe recent advances in the development of epigenetics of eosinophil-related diseases, especially in asthma. Likewise, recent studies have provided us with a more complete appreciation of how eosinophils contribute to the pathogenesis of various diseases, including hypereosinophilic syndrome (HES). Over the past decades, the definition and criteria of HES have been evolving with the progress of our understanding of the disease and some aspects of this disease still remain controversial. We also review recent updates on the genetic and molecular mechanisms of HES, which have spurred dramatic developments in the clinical strategies of diagnosis and treatment for this heterogeneous group of diseases. The conclusion from this review is that the biology of eosinophils provides significant insights as to their roles in health and disease and, furthermore, demonstrates that a better understanding of eosinophil will accelerate the development of new therapeutic strategies for patients.

B-1 Cell Heterogeneity and the Regulation of Natural and Antigen-Induced IgM Production

A small subset of B cells, termed B-1 cells, with developmental origins, phenotypes, and functions that are distinct from those of conventional B cells exist in mice. It contributes the vast majority of spontaneously produced "natural" IgM. Natural IgM is constitutively produced, even in the absence of microbiota, and fulfills many distinct functions in tissue homeostasis and host defense. B-1 cells also respond with IgM production to innate signals and pathogen exposure, while maintaining steady-state levels natural IgM. Thus, within the B-1 cell pool, cells of distinct and heterogeneous functionality must exist to facilitate these different functions. This review considers three factors that may contribute to this heterogeneity: first, developmental differences regarding the origins of the precursors, second, tissue-specific signals that may differentially affect B-1 cells in the tissue compartments, and finally responsiveness to self-antigens as well as innate and antigen-specific signals. All three are likely to shape the repertoire and responsiveness of B-1 cells to homeostatic- and antigen-induced signals and thus contribute to the functional heterogeneity among these innate-like B cells.

Fat-associated lymphoid clusters control local IgM secretion during pleural infection and lung inflammation

Fat-associated lymphoid clusters (FALC) are inducible structures that support rapid innate-like B-cell immune responses in the serous cavities. Little is known about the physiological cues that activate FALCs in the pleural cavity and more generally the mechanisms controlling B-cell activation in FALCs. Here we show, using separate models of pleural nematode infection with Litomosoides sigmodontis and Altenaria alternata induced acute lung inflammation, that inflammation of the pleural cavity rapidly activates mediastinal and pericardial FALCs. IL-33 produced by FALC stroma is crucial for pleural B1-cell activation and local IgM secretion. However, B1 cells are not the direct target of IL-33, which instead requires IL-5 for activation. Moreover, lung inflammation leads to increased IL-5 production by type 2 cytokine-producing innate lymphoid cells (ILC2) in the FALC. These findings reveal a link between inflammation, IL-33 release by FALC stromal cells, ILC2 activation and pleural B-cell activation in FALCs, resulting in local and antigen-specific IgM production.

Fat-associated lymphoid clusters (FALC) in the serous cavities house rapid IgM-producing B1 cells, but how the clusters are activated to respond to infection is unclear. Here the authors show that in response to lung inflammation or pleural nematode infection adipose stromal cell-derived IL-33 activates ILC2s to produce IL-5, thus driving the B1 response in the FALCs.

Lung-resident eosinophils represent a distinct regulatory eosinophil subset

Increases in eosinophil numbers are associated with infection and allergic diseases, including asthma, but there is also evidence that eosinophils contribute to homeostatic immune processes. In mice, the normal lung contains resident eosinophils (rEos), but their function has not been characterized. Here, we have reported that steady-state pulmonary rEos are IL-5-independent parenchymal Siglec-FintCD62L+CD101lo cells with a ring-shaped nucleus. During house dust mite-induced airway allergy, rEos features remained unchanged, and rEos were accompanied by recruited inflammatory eosinophils (iEos), which were defined as IL-5-dependent peribronchial Siglec-FhiCD62L-CD101hi cells with a segmented nucleus. Gene expression analyses revealed a more regulatory profile for rEos than for iEos, and correspondingly, mice lacking lung rEos showed an increase in Th2 cell responses to inhaled allergens. Such elevation of Th2 responses was linked to the ability of rEos, but not iEos, to inhibit the maturation, and therefore the pro-Th2 function, of allergen-loaded DCs. Finally, we determined that the parenchymal rEos found in nonasthmatic human lungs (Siglec-8+CD62L+IL-3Rlo cells) were phenotypically distinct from the iEos isolated from the sputa of eosinophilic asthmatic patients (Siglec-8+CD62LloIL-3Rhi cells), suggesting that our findings in mice are relevant to humans. In conclusion, our data define lung rEos as a distinct eosinophil subset with key homeostatic functions.

Group 2 Innate Lymphoid Cells Promote an Early Antibody Response to a Respiratory Antigen in Mice

Innate lymphoid cells (ILCs) are a new family of immune cells that play important roles in innate immunity in mucosal tissues, and in the maintenance of tissue and metabolic homeostasis. Recently, group 2 ILCs (ILC2s) were found to promote the development and effector functions of Th2-type CD4(+) T cells by interacting directly with T cells or by activating dendritic cells, suggesting a role for ILC2s in regulating adaptive immunity. However, our current knowledge on the role of ILCs in humoral immunity is limited. In this study, we found that ILC2s isolated from the lungs of naive BALB/c mice enhanced the proliferation of B1- as well as B2-type B cells and promoted the production of IgM, IgG1, IgA, and IgE by these cells in vitro. Soluble factors secreted by ILC2s were sufficient to enhance B cell Ig production. By using blocking Abs and ILC2s isolated from IL-5-deficient mice, we found that ILC2-derived IL-5 is critically involved in the enhanced production of IgM. Furthermore, when adoptively transferred to Il7r(-/-) mice, which lack ILC2s and mature T cells, lung ILC2s promoted the production of IgM Abs to a polysaccharide Ag, 4-hydroxy-3-nitrophenylacetyl Ficoll, within 7 d of airway exposure in vivo. These findings add to the growing body of literature regarding the regulatory functions of ILCs in adaptive immunity, and suggest that lung ILC2s promote B cell production of early Abs to a respiratory Ag even in the absence of T cells.

Eosinophils

Eosinophils have been traditionally understood as end-stage, primarily cytotoxic effector cells. Recent studies have had profound impact on this limited view and have led to new research on the functions and capabilities of this unique leukocyte lineage. Novel insights into eosinophil development, localization, modes of degranulation, and the nature of their granule contents have provided a better understanding of these cells as immunomodulatory mediators in health and disease.

Comparative Analysis of Dibutyric cAMP and Butyric Acid on the Differentiation of Human Eosinophilic Leukemia EoL-1 Cells

Purification of enough numbers of circulating eosinophils is difficult because eosinophils account for less than 5% peripheral blood leukocytes. Human eosinophilic leukemia EoL-1 cells have been considered an in vitro source of eosinophils as they can differentiate into mature eosinophil-like cells when incubated with dibutyryl cAMP (dbcAMP) or butyric acid. In this study, the viability and phenotypic maturation of EoL-1 cells stimulated by either dbcAMP or butyric acid were comparatively analyzed. After treatment with 100 µM dbcAMP or 0.5 µM butyric acid, EoL-1 cells showed morphological signs of differentiation, although the number of nonviable EoL-1 cells was significantly increased following butyric acid treatment. Stimulation of EoL-1 cells with 0.5 µM butyric acid more effectively induced the expression of mature eosinophil markers than stimulation with dbcAMP. These results suggest that treatment of EoL-1 cells with 0.5 µM butyric acid for limited duration could be an effective strategy for inducing their differentiation. Considering that expression of CCR3 was not sufficient in EoL-1 cells stimulated with 0.5 µM butyric acid, treatment of the chemically stimulated EoL-1 cells with cytokines, which primarily support eosinophil maturation, would help to obtain differentiated EoL-1 cells with greater functional maturity.

Helminth Products Protect against Autoimmunity via Innate Type 2 Cytokines IL-5 and IL-33, Which Promote Eosinophilia

Epidemiologic studies in humans have demonstrated that infection with helminth parasites is associated with a reduced risk of developing autoimmune diseases. Mechanistic studies in mice have linked the protective effect of helminths on autoimmunity to the suppressive activity of helminth-induced regulatory T cells (Tregs) or Th2 cells. In this study, we demonstrate that treatment of mice with Fasciola hepatica excretory-secretory products (FHES) attenuated the clinical signs of experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. Protection was associated with a significant reduction in the infiltration of pathogenic Th1 and Th17 cells into the brain. Although FHES enhanced anti-inflammatory cytokine and Th2 responses, protection against EAE was independent of IL-4, IL-10, and Tregs. However, administration of FHES induced production of the type 2 cytokines IL-33 and IL-5, which promoted accumulation of eosinophils. FHES-induced expansion of eosinophils and protection against EAE was lost in IL-33(-/-) mice and upon neutralization of IL-5. Furthermore, transfer of FHES-induced or IL-33-induced eosinophils conferred protection against EAE. In addition, treatment of mice with recombinant IL-33 attenuated autoimmunity, and this was dependent on IL-5. To our knowledge, this study is the first to report a role for helminth-induced IL-5 and IL-33 in protection against autoimmunity.

Transcription Factor Repertoire of Homeostatic Eosinophilopoiesis

The production of mature eosinophils (Eos) is a tightly orchestrated process with the aim to sustain normal Eos levels in tissues while also maintaining low numbers of these complex and sensitive cells in the blood. To identify regulators of homeostatic eosinophilopoiesis in mice, we took a global approach to identify genome-wide transcriptome and epigenome changes that occur during homeostasis at critical developmental stages, including Eos-lineage commitment and lineage maturation. Our analyses revealed a markedly greater number of transcriptome alterations associated with Eos maturation (1199 genes) than with Eos-lineage commitment (490 genes), highlighting the greater transcriptional investment necessary for differentiation. Eos-lineage-committed progenitors (EoPs) were noted to express high levels of granule proteins and contain granules with an ultrastructure distinct from that of mature resting Eos. Our analyses also delineated a 976-gene Eos-lineage transcriptome that included a repertoire of 56 transcription factors, many of which have never previously been associated with Eos. EoPs and Eos, but not granulocyte-monocyte progenitors or neutrophils, expressed Helios and Aiolos, members of the Ikaros family of transcription factors, which regulate gene expression via modulation of chromatin structure and DNA accessibility. Epigenetic studies revealed a distinct distribution of active chromatin marks between genes induced with lineage commitment and genes induced with cell maturation during Eos development. In addition, Aiolos and Helios binding sites were significantly enriched in genes expressed by EoPs and Eos with active chromatin, highlighting a potential novel role for Helios and Aiolos in regulating gene expression during Eos development.

Re-defining the unique roles for eosinophils in allergic respiratory inflammation

The role of eosinophils in the progression and resolution of allergic respiratory inflammation is poorly defined despite the commonality of their presence and in some cases their use as a biomarker for disease severity and/or symptom control. However, this ambiguity belies the wealth of insights that have recently been gained through the use of eosinophil-deficient/attenuated strains of mice that have demonstrated novel immunoregulatory and remodelling/repair functions for these cells in the lung following allergen provocation. Specifically, studies of eosinophil-deficient mice suggest that eosinophils contribute to events occurring in the lungs following allergen provocation at several key moments: (i) the initiating phase of events leading to Th2-polarized pulmonary inflammation, (ii) the suppression Th1/Th17 pathways in lung-draining lymph nodes, (iii) the recruitment of effector Th2 T cells to the lung, and finally, (iv) mechanisms of inflammatory resolution that re-establish pulmonary homoeostasis. These suggested functions have recently been confirmed and expanded upon using allergen provocation of an inducible eosinophil-deficient strain of mice (iPHIL) that demonstrated an eosinophil-dependent mechanism(s) leading to Th2 dominated immune responses in the presence of eosinophils in contrast to neutrophilic as well as mixed Th1/Th17/Th2 variant phenotypes in the absence of eosinophils. These findings highlighted that eosinophils are not exclusively downstream mediators controlled by T cells, dendritic cells (DC) and/or innate lymphocytic cells (ILC2). Instead, eosinophils appear to be more aptly described as significant contributors in complex interrelated pathways that lead to pulmonary inflammation and subsequently promote resolution and the re-establishment of homoeostatic baseline. In this review, we summarize and put into the context the evolving hypotheses that are now expanding our understanding of the roles eosinophils likely have in the lung following allergen provocation.

Eosinophils in mucosal immune responses

Eosinophils, multifunctional cells that contribute to both innate and adaptive immunity, are involved in the initiation, propagation, and resolution of immune responses, including tissue repair. They achieve this multifunctionality by expression of a diverse set of activation receptors, including those that directly recognize pathogens and opsonized targets, and by their ability to store and release preformed cytotoxic mediators that participate in host defense, to produce a variety of de novo pleotropic mediators and cytokines, and to interact directly and indirectly with diverse cell types, including adaptive and innate immunocytes and structural cells. Herein, we review the basic biology of eosinophils and then focus on new emerging concepts about their role in mucosal immune homeostasis, particularly maintenance of intestinal IgA. We review emerging data about their development and regulation and describe new concepts concerning mucosal eosinophilic diseases. We describe recently developed therapeutic strategies to modify eosinophil levels and function and provide collective insight about the beneficial and detrimental functions of these enigmatic cells.

Eosinophil-specific deletion of IκBα in mice reveals a critical role of NF-κB-induced Bcl-xL for inhibition of apoptosis

Eosinophils are associated with type 2 immune responses to allergens and helminths. They release various proinflammatory mediators and toxic proteins on activation and are therefore considered proinflammatory effector cells. Eosinophilia is promoted by the cytokines interleukin (IL)-3, IL-5, and granulocyte macrophage-colony-stimulating factor (GM-CSF) and can result from enhanced de novo production or reduced apoptosis. In this study, we show that only IL-5 induces differentiation of eosinophils from bone marrow precursors, whereas IL-5, GM-CSF, and to a lesser extent IL-3 promote survival of mature eosinophils. The receptors for these cytokines use the common β chain, which serves as the main signaling unit linked to signal transducer and activator of transcription 5, p38 mitogen-activated protein kinase, and nuclear factor (NF)-κB pathways. Inhibition of NF-κB induced apoptosis of in vitro cultured eosinophils. Selective deletion of IκBα in vivo resulted in enhanced expression of Bcl-xL and reduced apoptosis during helminth infection. Retroviral overexpression of Bcl-xL promoted survival, whereas pharmacologic inhibition of Bcl-xL in murine or human eosinophils induced rapid apoptosis. These results suggest that therapeutic strategies targeting Bcl-xL in eosinophils could improve health conditions in allergic inflammatory diseases.

Natural IgM is produced by CD5- plasma cells that occupy a distinct survival niche in bone marrow

Natural IgM is constitutively present in the serum, where it aids in the early control of viral and bacterial expansions. Natural IgM also plays a significant role in the prevention of autoimmune disease by promoting the clearance of cellular debris. Nevertheless, the origins of natural IgM have not been precisely defined. Previous studies focused on the role of CD5(+) B1 cells in the production of natural IgM, but we show in this article that a discrete population of CD5(-) IgM plasmablasts and plasma cells in the bone marrow (BM) produces the majority of serum IgM in resting mice. These Ab-secreting cells (ASC) originate from peritoneal cavity-resident cells, because transfer of peritoneal cells completely restores serum IgM and the specific compartment of BM ASC in Rag1-deficient mice. We show that BM natural IgM ASC arise from a fetal-lineage progenitor that is neither B1a nor B1b, and that this IgM ASC compartment contains a substantial fraction of long-lived plasma cells that do not occupy the IgG plasma cell survival niche in the BM; instead, they are supported by IL-5. In summary, we identified the primary source of natural IgM and showed that these ASC are maintained long-term in a unique survival niche within the BM.

Roles and regulation of gastrointestinal eosinophils in immunity and disease

Eosinophils have historically been considered to be destructive end-stage effector cells that have a role in parasitic infections and allergic reactions by the release of their granule-derived cytotoxic proteins. However, an increasing number of experimental observations indicate that eosinophils also are multifunctional leukocytes involved in diverse inflammatory and physiologic immune responses. Under homeostatic conditions, eosinophils are particularly abundant in the lamina propria of the gastrointestinal tract, where their involvement in various biological processes within the gastrointestinal tract has been posited. In this review, we summarize the molecular steps involved in eosinophil development and describe eosinophil trafficking to the gastrointestinal tract. We synthesize the current findings on the phenotypic and functional properties of gastrointestinal eosinophils and the accumulating evidence that they have a contributory role in gastrointestinal disorders, with a focus on primary eosinophilic gastrointestinal disorders. Finally, we discuss the potential role of eosinophils as modulators of the intestinal immune system.

Expression profiling of differentiating eosinophils in bone marrow cultures predicts functional links between microRNAs and their target mRNAs

BACKGROUND

MicroRNAs (miRNAs) are small non-coding RNAs that regulate complex transcriptional networks underpin immune responses. However, little is known about the specific miRNA networks that control differentiation of specific leukocyte subsets. In this study, we profiled miRNA expression during differentiation of eosinophils from bone marrow (BM) progenitors (bmEos), and correlated expression with potential mRNA targets involved in crucial regulatory functions. Profiling was performed on whole BM cultures to document the dynamic changes in miRNA expression in the BM microenvironment over the differentiation period. miRNA for network analysis were identified in BM cultures enriched in differentiating eosinophils, and chosen for their potential ability to target mRNA of factors that are known to play critical roles in eosinophil differentiation pathways or cell identify.

METHODOLOGY/PRINCIPAL FINDINGS

We identified 68 miRNAs with expression patterns that were up- or down- regulated 5-fold or more during bmEos differentiation. By employing TargetScan and MeSH databases, we identified 348 transcripts involved in 30 canonical pathways as potentially regulated by these miRNAs. Furthermore, by applying miRanda and Ingenuity Pathways Analysis (IPA), we identified 13 specific miRNAs that are temporally associated with the expression of IL-5Rα and CCR3 and 14 miRNAs associated with the transcription factors GATA-1/2, PU.1 and C/EBPε. We have also identified 17 miRNAs that may regulate the expression of TLRs 4 and 13 during eosinophil differentiation, although we could identify no miRNAs targeting the prominent secretory effector, eosinophil major basic protein.

CONCLUSIONS/SIGNIFICANCE

This is the first study to map changes in miRNA expression in whole BM cultures during the differentiation of eosinophils, and to predict functional links between miRNAs and their target mRNAs for the regulation of eosinophilopoiesis. Our findings provide an important resource that will promote the platform for further understanding of the role of these non-coding RNAs in the regulation of eosinophil differentiation and function.

Eosinophils regulate peripheral B cell numbers in both mice and humans

The view of eosinophils (Eos) as solely effector cells involved in host parasite defense and in the pathophysiology of allergic diseases has been challenged in recent years. In fact, there is a growing realization that these cells interact with other components of innate and adaptive immunity. For example, mouse Eos were recently demonstrated to promote plasma cell retention in the bone marrow. However, it remains unknown whether Eos influence the biology of normal B lymphocytes. In this study, we specifically assessed the effect of Eos on B cell survival, proliferation, and Ig secretion. Our data first revealed that the genetic deletion of Eos from NJ1638 IL-5 transgenic hypereosinophilic mice (previously shown to display profound B cell expansion) resulted in the near abolishment of the B cell lymphocytosis. In vitro studies using human tissues demonstrated Eos' proximity to B cell follicles and their ability to promote B cell survival, proliferation, and Ig secretion via a contact-independent mechanism. Additionally, this ability of Eos to enhance B cell responsiveness was observed in both T-independent and T-dependent B cell activation and appears to be independent of the activation state of Eos. Finally, a retrospective clinical study of hypereosinophilic patients revealed a direct correlation between peripheral blood eosinophil levels and B cell numbers. Taken together, our study identifies a novel role for Eos in the regulation of humoral immunity via their impact on B cell homeostasis and proliferation upon activation.

The immunotoxic effects of dual exposure to PCP and TCDD

Pentachlorophenol (PCP) was a commonly used fungicide, herbicide, insecticide, and bactericide in industrial, agricultural, and domestic settings; however, it was also contaminated with polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). It has been reported that technical grade PCP had immunosuppressive effects and that the immune system was the major target of PCDD/PCDFs toxicity. Although the immune response after exposure to PCP or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been studied, the toxic effects of exposure to both PCP and TCDD have not yet been reported. The aim of this study was to evaluate the effects on immune cells from mice intraperitoneally immunized with OVA and subsequently treated with PCP or TCDD alone or in combination by gavage. The animals were terminated on day 7 and 14, and the spleen and plasma samples were collected for immunotoxicity evaluation. The numbers and populations of splenocytes, T cell-derived cytokines produced by splenocytes, splenocyte-generated cytotoxicity and OVA-specific antibodies in plasma were investigated. Our results indicate that the spleen/body weight ratio and splenocyte number was reduced by TCDD alone; in addition, this reduction was enhanced when TCDD was combined with PCP. Exposure to TCDD alone or in conjunction with PCP suppressed many ovalbumin (OVA)-stimulated cytokines, including IL-2, IFN-γ, IL-4, IL-5, and IL-10. Furthermore, the immunoglobulins IgG and IgM were suppressed in mice administered by PCP alone, but the suppressive effects were greater in mice treated with TCDD alone or in combination with PCP. Co-exposure to PCP and TCDD resulted in an antagonistic effect on TCDD-induced suppression of IFN-γ and IL-10. Our results demonstrate that PCP alone is immunotoxic, regardless of the presence of TCDD. PCP led to mild changes in cytokine secretion, and it compromised splenocyte-generated cytotoxicity and IgM and IgG antibody production on day 7. The finding that PCP antagonizes TCDD-induced IFN-γ suppression could be due to the competitive binding of PCP to AhR (aryl hydrocarbon receptor).

Type 2 innate lymphoid cells control eosinophil homeostasis

Eosinophils are specialized myeloid cells associated with allergy and helminth infections. Blood eosinophils demonstrate circadian cycling, as described over 80 years ago,¹ and are abundant in the healthy gastrointestinal tract. Although a cytokine, interleukin (IL)-5, and chemokines such as eotaxins, mediate eosinophil development and survival,² and tissue recruitment,³ respectively, the processes underlying the basal regulation of these signals remain unknown. Here, we show that serum IL-5 is maintained by long-lived type 2 innate lymphoid cells (ILC2) resident in peripheral tissues. ILC2 secrete IL-5 constitutively and are induced to co-express IL-13 during type 2 inflammation, resulting in localized eotaxin production and eosinophil accumulation. In the small intestine where eosinophils and eotaxin are constitutive,⁴ ILC2 co-express IL-5 and IL-13, which is enhanced after caloric intake. The circadian synchronizer vasoactive intestinal peptide (VIP) also stimulates ILC2 through the VPAC2 receptor to release IL-5, linking eosinophil levels with metabolic cycling. Tissue ILC2 regulate basal eosinophilopoiesis and tissue eosinophil accumulation through constitutive and stimulated cytokine expression, and this dissociated regulation can be tuned by nutrient intake and central circadian rhythms.