Small intestinal eosinophils regulate Th17 cells by producing IL-1 receptor antagonist

A critical regulation of Th2 cell responses by RORα in allergic asthma

Allergic asthma is a chronic inflammatory disease of the lung and the airway, which is characterized by aberrant type 2 immune responses to otherwise unharmful aeroallergens. While the central role of Th2 cells and type 2 cytokines in the pathogenesis of allergic asthma is well documented, the regulation and plasticity of Th2 cells remain incompletely understood. By using an animal model of allergic asthma in IL-4-reporter mice, we found that Th2 cells in the lung expressed higher levels of Rora than those in the lymph nodes, and that treatment with an RORα agonist SR1078 resulted in diminished Th2 cell responses in vivo. To determine the T cell-intrinsic role of RORα in allergic asthma in vivo, we established T cell-specific RORα-deficient (Cd4creRora^f/f) mice. Upon intranasal allergen challenges, Cd4creRora^f/f mice exhibited a significantly increased Th2 cells in the lungs and the airway and showed an enhanced eosinophilic inflammation compared to littermate control mice. Studies with Foxp3^YFP-creRora^f/f mice and CD8⁺ T cell depletion showed that the increased Th2 cell responses in the Cd4creRora^f/f mice were independent of Treg cells and CD8⁺ T cells. Our findings demonstrate a critical regulatory role of RORα in Th2 cells, which suggest that RORα agonists could be effective for the treatment of allergic diseases.

Mucosal Th17 Cells Are Increased in Pediatric Functional Dyspepsia Associated with Chronic Gastritis

BACKGROUND

Chronic gastritis is a common histologic finding in children with functional dyspepsia (FD). While Th17 cells have been implicated in other forms of gastritis, they have not been evaluated in chronic gastritis.

AIMS

The aim of the current study was to assess Th17 cells in children with FD with and without chronic gastritis.

METHODS

Densities were determined for Th17 cells, eosinophils, and mast cells, respectively, in both the gastric antrum and the duodenum. Densities were compared between five groups: FD with chronic gastritis (N = 20), FD without chronic gastritis (N = 20), Helicobacter pylori-associated gastritis (N = 10), Crohn's gastritis (N = 10), and normal controls (N = 10). Th17 densities were also compared between patients with and without early satiety.

RESULTS

FD with chronic gastritis was associated with higher Th17 cell density as compared to normal controls and comparable to both H. pylori-associated gastritis and Crohn's gastritis. Eosinophil and mast cell densities were higher in FD patients with chronic gastritis as compared to either FD without gastritis or normal controls. Th17 density was higher in patients reporting early satiety but not in those with epigastric pain.

CONCLUSIONS

FD with chronic gastritis is associated with higher Th17 cell, eosinophil, and mast cell density as compared to FD without chronic gastritis or normal controls. Chronic gastritis demonstrated Th17 cell density similar to that seen in other conditions where Th17 cells are believed to play a pathogenic role. Th17 cells may represent another therapeutic target in these patients.

Littermate-Controlled Experiments Reveal Eosinophils Are Not Essential for Maintaining Steady-State IgA and Demonstrate the Influence of Rearing Conditions on Antibody Phenotypes in Eosinophil-Deficient Mice

Conflicting data has emerged regarding a role for eosinophils in IgA production, with some reports that eosinophils support both secretory and circulating IgA levels during homeostasis. Previous studies have compared antibody levels between wildtype and eosinophil-deficient mice, but these mice were obtained from different commercial vendors and/or were not littermates. Thus, the possibility remains that extrinsic environmental factors, rather than an intrinsic lack of eosinophils, are responsible for the reports of reduced IgA in eosinophil-deficient mice. Here we used wild-type and eosinophil-deficient (ΔdblGATA) mice that were purchased from a single vendor, subsequently bred in-house and either co-housed as adults, co-reared from birth or raised as littermates. We found no differences in the levels of secretory IgA or in the numbers of small intestinal IgA-producing plasma cells between wild-type and ΔdblGATA mice, demonstrating that under controlled steady-state conditions eosinophils are not essential for the maintenance of secretory IgA in the intestinal tract. While we found that levels of IgM and IgE were significantly elevated in the serum of ΔdblGATA mice compared to co-reared or co-housed wild-type mice, no significant differences in these or other circulating antibody isotypes were identified between genotypes in littermate-controlled experiments. Our results demonstrate that eosinophils are not required to maintain secretory or circulating IgA production and the absence of eosinophils does not impact circulating IgG1, IgG2b, IgM, or IgE levels during homeostasis. These findings emphasize the importance of optimally controlling rearing and housing conditions throughout life between mice of different genotypes.

Emerging roles of infiltrating granulocytes and monocytes in homeostasis

The infiltration of naïve tissues by myeloid cells has been long related to their clearance and the physiological cell turnover, however, increasing evidence shows that they can additionally fulfill specific, non-immune functions in different tissues. There is also growing evidence to support that infiltrated granulocytes and monocytes respond to different environments by modulating gene expression and cytokine production, which in turn contribute to the normal function of the host tissue. This review will address the roles of immigrated myeloid cells in different tissues and their crosstalk with the host tissue environments.

Eosinophils are dispensable for the regulation of IgA and Th17 responses in Giardia muris infection

AIMS

IgA and Th17 responses are pivotal for the control of Giardia infections. Eosinophils support IgA class switching, the survival of intestinal IgA⁺ plasma cells at steady state and can control Th17 activity in the small intestine. To see whether eosinophils regulate adaptive immune responses during giardiasis, we investigated Giardia muris infections in wild-type BALB/c and eosinophil-deficient ∆dblGATA-1 mice.

METHODS AND RESULTS

Infected ∆dblGATA-1 mice did not differ markedly in parasite control from wild-type mice. Confirming previous studies, naive ∆dblGATA-1 mice displayed diminished IgA⁺ B cell frequencies in Peyer's patches. However, IgA class switching and intestinal IgA secretion in response to G muris infection were comparable in wild-type BALB/c and ∆dblGATA-1 mice. Both strains displayed similarly low intestinal Th17 responses, accompanied by a mild expansion of type 3 innate lymphoid cells (ILC3).

CONCLUSIONS

Contrasting previous reports on overt small intestinal Th17 activity in eosinophil-deficient mice, IL-17A production is kept in check in the absence of eosinophils during Giardia infection. Suboptimal homeostatic IgA responses in the absence of eosinophils are transiently fostered in infected mice and the maintenance of IgA⁺ plasma cells appears to be restored during persisting Giardia infection.

Eosinophilic Colitis and Clostridioides difficile Sepsis With Rapid Remission After Antimicrobial Treatment; A Rare Coincidence and Its Pathogenic Implications

Eosinophilic colitis is a rare inflammatory disorder of the digestive tract with chronic evolution and unknown pathophysiological mechanisms. The article describes the case of a 64-year old woman with a history of asthma and hypereosinophilia, who presented to a surgical department for persistent abdominal pain in the past 4 months, weight loss and malabsorption. She was diagnosed with eosinophilic colitis based on the colonoscopic result indicating extensive eosinophilic infiltration of the colonic mucosa correlated with the laboratory data and abdominal CT scan results. Following the colonoscopy, the patient developed fever, hypotension and diarrhea and was transferred to an Infectious Diseases Department with a presumptive diagnosis of abdominal sepsis. Treatment with ertapenem was immediately started. Metronidazole was also added due to a PCR positive stool test for Clostridioides difficile toxins encoding-genes. The patient displayed a rapid remission of the fever and of the intestinal complaints following antibiotic therapy and was discharged after 14 days. During a 3 months follow-up, the patient remained asymptomatic with normal values of laboratory parameters except for a persistent hypereosinophilia. The case outlines two distinguishing features: a histopathologic diagnosis of eosinophilic colitis, a rare diagnosis of a patient with chronic abdominal pain and an unexpected and rapid remission of the eosinophilic colitis following the antibiotic treatment and the restoration of the intestinal eubiosis.

Eosinophils and Purinergic Signaling in Health and Disease

Eosinophils are major effector cells against parasites, fungi, bacteria, and viruses. However, these cells also take part in local and systemic inflammation, which are central to eczema, atopy, rhinitis, asthma, and autoimmune diseases. A role for eosinophils has been also shown in vascular thrombotic disorders and in cancer. Many, if not all, above-mentioned conditions involve the release of intracellular nucleotides (ATP, ADP, UTP, etc.) and nucleosides (adenosine) in the extracellular environment. Simultaneously, eosinophils further release ATP, which in autocrine and paracrine manners, stimulates P2 receptors. Purinergic signaling in eosinophils mediates a variety of responses including CD11b induction, ROI production, release of granule contents and enzymes, as well as cytokines. Exposure to extracellular ATP also modulates the expression of endothelial adhesion molecules, thereby favoring eosinophil extravasation and accumulation. In addition, eosinophils express the immunosuppressive adenosine P1 receptors, which regulate degranulation and migration. However, pro-inflammatory responses induced by extracellular ATP predominate. Due to their important role in innate immunity and tissue damage, pharmacological targeting of nucleotide- and nucleoside-mediated signaling in eosinophils could represent a novel approach to alleviate eosinophilic acute and chronic inflammatory diseases. These innovative approaches might also have salutary effects, particularly in host defense against parasites and in cancer.

The emerging roles of eosinophils in mucosal homeostasis

Eosinophils are granulocytes, typically implicated as end-stage effector cells in type-II immune responses. They are capable of producing a wide array of pre-formed molecules which render them with vast potential to influence a wide variety of processes. Nonetheless, eosinophil research has traditionally focused on their role in anti-helminthic responses and pathophysiological processes in type-II immune disorders, such as allergy and asthma, where eosinophilia is a hallmark phenotype. However, a number of key studies over the past decade have placed this restricted view of eosinophil function into question, presenting additional evidence for eosinophils as critical regulators of various homeostatic processes including immune maintenance, organ development, and tissue regeneration.

Control of myeloid cell density in barrier tissues

The interface between the mammalian host and its environment is formed by barrier tissues, for example, of the skin, and the respiratory and the intestinal tracts. On the one hand, barrier tissues are colonized by site-adapted microbial communities, and on the other hand, they contain specific myeloid cell networks comprising macrophages, dendritic cells, and granulocytes. These immune cells are tightly regulated in function and cell number, indicating important roles in maintaining tissue homeostasis and immune balance in the presence of commensal microorganisms. The regulation of myeloid cell density and activation involves cell-autonomous 'single-loop circuits' including autocrine mechanisms. However, an array of microenvironmental factors originating from nonimmune cells and the microbiota, as well as the microanatomical structure, impose additional layers of regulation onto resident myeloid cells. This review discusses models integrating these factors into cell-specific programs to instruct differentiation and proliferation best suited for the maintenance and renewal of immune homeostasis in the tissue-specific environment.

The Emerging Role of Eosinophils as Multifunctional Leukocytes in Health and Disease

Eosinophils are terminally differentiated cytotoxic effector cells that have a role in parasitic infections and allergy by releasing their granule-derived cytotoxic proteins. However, an increasing number of recent observations indicate that eosinophils are not only associated with the pathogenesis of a wide range of diseases, but also contribute to the maintenance of homeostatic responses in previously underappreciated diverse tissues, such as the gastrointestinal (GI) tract and adipose tissue. In this review, we describe biological characteristics of eosinophils, as their developmental properties, permissive proliferation and survival, degranulation activity, and migration properties enable them to distribute to both homeostatic and inflamed tissues. We describe pathologic aspects of eosinophils with a role in asthma and in various GI diseases, including eosinophilic GI disorders, inflammatory bowel disease, and radiation-induced enteropathy. Finally, we discuss the beneficial role of eosinophils, which contribute to the resolution of pathogenic conditions and to the modulation of homeostatic biologic responses.

Morphological and phenotypical diversity of eosinophils in the rat ileum

Eosinophils are abundantly present in intestinal mucosa. However, the morphological characteristics of their cellular population are still largely unknown. In this study, we examine their characteristics in the rat ileal mucosa using histological and ultrastructural methods. The results indicated that ileal eosinophils could be distinguished into two main groups based on their nuclear shapes and distribution: eosinophils with spheric or reniform nuclei mainly localized in the villous region and eosinophils with annular or bacilliform nuclei as the major population around crypts. Immunohistochemical analysis revealed that all eosinophils in the lamina propria (LP) were immunopositive for CD11b, whereas eosinophils in LP of the intestinal villus but not those in LP around the crypt, were immunopositive for CD11c. Three-dimensional ultrastructural analysis using serial block-face scanning electron microscopy showed that the eosinophils with spheric or reniform nuclei were abundant in the upper portions of the intestinal villus, whereas those with annular nuclei were abundant in the lower portions of the intestinal villus and around crypts. The eosinophils with spheric or reniform nuclei possessed broader cellular bodies with greater abundance of surface projections compared with those with annular nuclei. Eosinophils in the upper portions of intestinal villus frequently extended their cellular bodies into the intraepithelial space. The number of total and eosinophil-specific granules was positively correlated with the minor axis of the nuclear holes in the annular nuclei. These data suggest that ileal eosinophils exhibit not homogenous but rather diverse characteristics, possible due to the mixture of eosinophils at different maturation and/or activation stages.

Microbial Regulation of Enteric Eosinophils and Its Impact on Tissue Remodeling and Th2 Immunity

Eosinophils have emerged as multifaceted cells that contribute to tissue homeostasis. However, the impact of the microbiota on their frequency and function at mucosal sites remains unclear. Here, we investigated the role of the microbiota in the regulation of enteric eosinophils. We found that small intestinal (SI) eosinophilia was significantly greater in germ-free (GF) mice compared to specific pathogen free (SPF) controls. This was associated with changes in the production of enteric signals that regulate eosinophil attraction and survival, and was fully reversed by complex colonization. Additionally, SI eosinophils of GF mice exhibited more cytoplasmic protrusions and less granule content than SPF controls. Lastly, we generated a novel strain of eosinophil-deficient GF mice. These mice displayed intestinal fibrosis and were less prone to allergic sensitization as compared to GF controls. Overall, our study demonstrates that commensal microbes regulate intestinal eosinophil frequency and function, which impacts tissue repair and allergic sensitization to food antigens. These data support a critical interplay between the commensal microbiota and intestinal eosinophils in shaping homeostatic, innate, and adaptive immune processes in health and disease.

Eosinophils and Macrophages within the Th2-Induced Granuloma: Balancing Killing and Healing in a Tight Space

Granuloma formation is a key host immune response generated to confine invading pathogens and limit extensive host damage. It consists of an accumulation of host immune cells around a pathogen. This host response has been extensively studied in the context of inflammatory diseases. However, there is much less known about Th2-type granulomas generated in response to parasitic worms. Based on in vitro data, innate immune cells within the granuloma are thought to immobilize and kill parasites but also act to repair damaged tissue. Understanding this dual function is key. The two billion people and many livestock/wild animals infected with helminths demonstrate that granulomas are not effective at clearing infection. However, the lack of high mortality highlights their importance in ensuring that parasite migration/tissue damage is restricted and wound healing is effective. In this review, we define two key cellular players (macrophages and eosinophils) and their associated molecular players involved in Th2 granuloma function. To date, the underlying mechanisms remain poorly understood, which is in part due to a lack of conclusive studies. Most have been performed in vitro rather than in vivo, using cells that have not been obtained from granulomas. Experiments using genetically modified mouse strains and/or antibody/chemical-mediated cell depletion have also generated conflicting results depending on the model. We discuss the caveats of previous studies and the new tools available that will help fill the gaps in our knowledge and allow a better understanding of the balance between immune killing and healing.

Eosinophils Do Not Drive Acute Muscle Pathology in the mdx Mouse Model of Duchenne Muscular Dystrophy

Eosinophils are present in muscle lesions associated with Duchenne muscular dystrophy and dystrophin-deficient mdx mice that phenocopy this disorder. Although it has been hypothesized that eosinophils promote characteristic inflammatory muscle damage, this has not been fully examined. In this study, we generated mice with the dystrophin mutation introduced into PHIL, a strain with a transgene that directs lineage-specific eosinophil ablation. We also explored the impact of eosinophil overabundance on dystrophinopathy by introducing the dystrophin mutation into IL-5 transgenic mice. We evaluated the degree of eosinophil infiltration in association with myofiber size distribution, centralized nuclei, serum creatine kinase, and quantitative histopathology scores. Among our findings, eosinophils were prominent in the quadriceps muscles of 4-wk-old male mdx mice but no profound differences were observed in the quantitative measures of muscle damage when comparing mdx versus mdx.PHIL versus mdx.IL5tg mice, despite dramatic differences in eosinophil infiltration (CD45+CD11c-Gr1-MHC class IIloSiglecF+ eosinophils at 1.2 ± 0.34% versus <0.1% versus 20 ± 7.6% of total cells, respectively). Further evaluation revealed elevated levels of eosinophil chemoatttractants eotaxin-1 and RANTES in the muscle tissue of all three dystrophin-deficient strains; eotaxin-1 concentration in muscle correlated inversely with age. Cytokines IL-4 and IL-1R antagonist were also detected in association with eosinophils in muscle. Taken together, our findings challenge the long-held perception of eosinophils as cytotoxic in dystrophin-deficient muscle; we show clearly that eosinophil infiltration is not a driving force behind acute muscle damage in the mdx mouse strain. Ongoing studies will focus on the functional properties of eosinophils in this unique microenvironment.

Gut microbiota: Role in pathogen colonization, immune responses, and inflammatory disease

The intestinal tract of mammals is colonized by a large number of microorganisms including trillions of bacteria that are referred to collectively as the gut microbiota. These indigenous microorganisms have co-evolved with the host in a symbiotic relationship. In addition to metabolic benefits, symbiotic bacteria provide the host with several functions that promote immune homeostasis, immune responses, and protection against pathogen colonization. The ability of symbiotic bacteria to inhibit pathogen colonization is mediated via several mechanisms including direct killing, competition for limited nutrients, and enhancement of immune responses. Pathogens have evolved strategies to promote their replication in the presence of the gut microbiota. Perturbation of the gut microbiota structure by environmental and genetic factors increases the risk of pathogen infection, promotes the overgrowth of harmful pathobionts, and the development of inflammatory disease. Understanding the interaction of the microbiota with pathogens and the immune system will provide critical insight into the pathogenesis of disease and the development of strategies to prevent and treat inflammatory disease.

Eosinophils suppress Th1 responses and restrict bacterially induced gastrointestinal inflammation

Arnold et al. report that eosinophils in the gastrointestinal tract are conditioned by IFN-γ to restrict Th1 responses and promote tissue homeostasis. Eosinophils control Th1 cells in acute and chronic infection and in the steady state and possess bactericidal properties.

Eosinophils are predominantly known for their contribution to allergy. Here, we have examined the function and regulation of gastrointestinal eosinophils in the steady-state and during infection with Helicobacter pylori or Citrobacter rodentium. We find that eosinophils are recruited to sites of infection, directly encounter live bacteria, and activate a signature transcriptional program; this applies also to human gastrointestinal eosinophils in humanized mice. The genetic or anti–IL-5–mediated depletion of eosinophils results in improved control of the infection, increased inflammation, and more pronounced Th1 responses. Eosinophils control Th1 responses via the IFN-γ–dependent up-regulation of PD-L1. Furthermore, we find that the conditional loss of IFN-γR in eosinophils phenocopies the effects of eosinophil depletion. Eosinophils further possess bactericidal properties that require their degranulation and the deployment of extracellular traps. Our results highlight two novel functions of this elusive cell type and link it to gastrointestinal homeostasis and anti-bacterial defense.

NOD-Like Receptor Protein 3 Inflammasome-Dependent IL-1β Accelerated ConA-Induced Hepatitis

Autoimmune hepatitis (AIH) is a progressive inflammatory disorders of unknown etiology, characterized by immune-mediated destruction of hepatocytes and massive production of cytokines. Interleukin-1β is a pleiotropic proinflammatory cytokine and well known to be critical in a variety of autoimmune diseases. However, the role of interleukin-1β (IL-1β) in AIH is still indistinct. Here, we first investigated the significance of NOD-like receptor protein 3 (NLRP3) inflammasome-dependent IL-1β in the pathogenesis of AIH with a murine model of immune-mediated hepatitis induced by Concanavalin A (ConA). In ConA-treated mice, pathogenic elevated NLRP3, Cleaved caspase-1 and IL-1β levels, as well as an inflammatory cell death known as pyroptosis predominantly occurred in the livers. Strikingly, NLRP3^−/− and caspase-1^−/− mice were broadly protected from hepatitis as determined by decreased histological liver injury, serum aminotransferase (ALT)/aspartate transaminase levels, and pyroptosis. In vivo intervention with recombinant human interleukin-1 receptor antagonist (rhIL-1Ra) strongly suppressed ConA-induced hepatitis by decreasing tumor necrosis factor-alpha (TNF-α) and interleukin-17 (IL-17) secretion, and inflammatory cell infiltration into livers. Additionally, rhIL-1Ra-pretreated mice developed significantly reduced NLRP3 inflammasome activation and reactive oxygen species (ROS) generation. Scavenging of ROS by N-acetyl-cysteine also attenuated NLRP3 inflammasome activation and liver inflammation, indicating that the essential role of ROS in mediating NLRP3 inflammasome activation in ConA-induced hepatitis. In conclusion, our results demonstrated that NLRP3 inflammasome-dependent IL-1β production was crucial in the pathogenesis of ConA-induced hepatitis, which shed light on the development of promising therapeutic strategies for AIH by blocking NLRP3 inflammasome and IL-1β.

Immunity to gastrointestinal nematode infections

Numerous species of nematodes have evolved to inhabit the gastrointestinal tract of animals and humans, with over a billion of the world's population infected with at least one species. These large multicellular pathogens present a considerable and complex challenge to the host immune system given that individuals are continually exposed to infective stages, as well as the high prevalence in endemic areas. This review summarizes our current understanding of host-parasite interactions, detailing induction of protective immunity, mechanisms of resistance, and resolution of the response. It is clear from studies of well-defined laboratory model systems that these responses are dominated by innate and adaptive type 2 cytokine responses, regulating cellular and soluble effectors that serve to disrupt the niche in which the parasites live by strengthening the physical mucosal barrier and ultimately promoting tissue repair.

Eosinophils from Physiology to Disease: A Comprehensive Review

Despite being the second least represented granulocyte subpopulation in the circulating blood, eosinophils are receiving a growing interest from the scientific community, due to their complex pathophysiological role in a broad range of local and systemic inflammatory diseases as well as in cancer and thrombosis. Eosinophils are crucial for the control of parasitic infections, but increasing evidence suggests that they are also involved in vital defensive tasks against bacterial and viral pathogens including HIV. On the other side of the coin, eosinophil potential to provide a strong defensive response against invading microbes through the release of a large array of compounds can prove toxic to the host tissues and dysregulate haemostasis. Increasing knowledge of eosinophil biological behaviour is leading to major changes in established paradigms for the classification and diagnosis of several allergic and autoimmune diseases and has paved the way to a "golden age" of eosinophil-targeted agents. In this review, we provide a comprehensive update on the pathophysiological role of eosinophils in host defence, inflammation, and cancer and discuss potential clinical implications in light of recent therapeutic advances.

IFN-γ increases susceptibility to influenza A infection through suppression of group II innate lymphoid cells

Increased levels of interferon-γ (IFN-γ) are routinely observed in the respiratory tract following influenza virus infection, yet its potential role remains unclear. We now demonstrate that influenza-induced IFN-γ restricts protective innate lymphoid cell group II (ILC2) function in the lung following challenge with the pandemic H1N1 A/CA/04/2009 (CA04) influenza virus. Specifically, IFN-γ deficiency resulted in enhanced ILC2 activity, characterized by increased production of interleukin (IL)-5 and amphiregulin, and improved tissue integrity, yet no change in ILC2 numbers, viral load or clearance. We further found that IFN-γ-deficient mice, as well as wild-type animals treated with neutralizing anti-IFN-γ antibody, exhibited decreased susceptibility to lethal infection with H1N1 CA04 influenza virus, and moreover that survival was dependent on the presence of IL-5. The beneficial effects of IFN-γ neutralization were not observed in ILC2-deficient animals. These data support the novel concept that IFN-γ can have a detrimental role in the pathogenesis of influenza through a restriction in ILC2 activity. Thus, regulation of ILC2 activity is a potential target for post-infection therapy of influenza.

Functions of tissue-resident eosinophils

Eosinophils are a prominent cell type in particular host responses such as the response to helminth infection and allergic disease. Their effector functions have been attributed to their capacity to release cationic proteins stored in cytoplasmic granules by degranulation. However, eosinophils are now being recognized for more varied functions in previously underappreciated diverse tissue sites, based on the ability of eosinophils to release cytokines (often preformed) that mediate a broad range of activities into the local environment. In this Review, we consider evolving insights into the tissue distribution of eosinophils and their functional immunobiology, which enable eosinophils to secrete in a selective manner cytokines and other mediators that have diverse, 'non-effector' functions in health and disease.

Eosinophils and eosinophil-associated diseases: An update

The goal of this series is to offer a survey of the latest literature for clinicians and scientists alike, providing a list of important recent advances relevant to the broad field of allergy and immunology. This particular assignment was to cover the topic of eosinophils. In an attempt to highlight major ideas, themes, trends, and advances relevant to basic and clinical aspects of eosinophil biology, a search of articles published since 2015 in the Journal of Allergy and Clinical Immunology and other high-impact journals was performed. Articles were then reviewed and organized, and then key findings were summarized. Given space limitations, many outstanding articles could not be included, but the hope is that what follows provides a succinct overview of recently published work that has significantly added to our knowledge of eosinophils and eosinophil-associated diseases.

Differences in eosinophil molecular profiles between children and adults with eosinophilic esophagitis

BACKGROUND

Eosinophilic esophagitis (EoE) afflicts both children and adults. It has been debated whether pediatric EoE and adult EoE represent different disease entities. The objectives of this study were to determine whether the blood eosinophil molecular pattern of children with EoE is (i) distinct from that of healthy children; and (ii) different from that of adults with EoE.

METHODS

Blood eosinophils from children and adults with EoE, and healthy controls, were analyzed with flow cytometry regarding levels of CD23, CD44, CD54, CRTH2, FOXP3, and galectin-10. Eosinophil FOXP3 and galectin-10 mRNA levels were determined by qPCR. The data were analyzed using a multivariate method of pattern recognition.

RESULTS

An eosinophil molecular pattern capable of distinguishing children with EoE from control children was identified. A smaller fraction of eosinophils from children with EoE expressed CD44 and a larger fraction expressed CRTH2 than the controls. Eosinophils from children with EoE also had higher levels of galectin-10 mRNA and lower levels of FOXP3 mRNA. The eosinophils from children with EoE had lower levels of surface CD54 and of FOXP3 mRNA compared with the eosinophils from the adult patients. A key finding was the detection in healthy individuals of age-related differences in the levels of several eosinophil markers.

CONCLUSIONS

Children with EoE can be distinguished from healthy children based on the molecular patterns of their blood eosinophils. Age-related physiologic differences in eosinophil molecular patterns may partly explain the different blood eosinophil phenotypes in children vs adults with EoE.

Homeostatic Eosinophils: Characteristics and Functions

Eosinophils are typically considered to be specialized effector cells that are recruited to the tissues as a result of T helper type 2 (Th2) cell responses associated with helminth infections or allergic diseases such as asthma. Once at the site of injury, eosinophils release their cytotoxic granule proteins as well as preformed cytokines and lipid mediators, contributing to parasite destruction but also to exacerbation of inflammation and tissue damage. Accumulating evidence indicates that, besides their roles in Th2 responses, eosinophils also regulate homeostatic processes at steady state, thereby challenging the exclusive paradigm of the eosinophil as a destructive and inflammatory cell. Indeed, under baseline conditions, eosinophils rapidly leave the bloodstream to enter tissues, mainly the gastrointestinal tract, lungs, adipose tissue, thymus, uterus, and mammary glands, where they regulate a variety of important biological functions, such as immunoregulation, control of glucose homeostasis, protection against obesity, regulation of mammary gland development, and preparation of the uterus for pregnancy. This article provides an overview of the characteristics and functions of these homeostatic eosinophils.

Recent advances in understanding eosinophil biology

With the advent of novel therapies targeting eosinophils, there has been renewed interest in understanding the basic biology of this unique cell. In this context, murine models and human studies have continued to highlight the role of the eosinophil in homeostatic functions and immunoregulation. This review will focus on recent advances in our understanding of eosinophil biology that are likely to have important consequences on the development and consequences of eosinophil-targeted therapies. Given the breadth of the topic, the discussion will be limited to three areas of interest: the eosinophil life cycle, eosinophil heterogeneity, and mechanisms of cell-cell communication.

Regulatory Eosinophils Suppress T Cells Partly through Galectin-10

Eosinophils have the capacity to regulate the function of T cell subsets. Our aim was to test the hypothesis of the existence of a regulatory subset of eosinophils. Human eosinophils were incubated with T cells that were stimulated with allogeneic leukocytes or CD3/CD28 cross-linking. After 2 d of coculture, 11% of the eosinophils gained CD16 expression. A CD16^hi subset of eosinophils, encompassing 1-5% of all eosinophils, was also identified in the blood of healthy subjects. FACS sorting showed that these CD16^hi eosinophils were significantly stronger suppressors of T cell proliferation than were conventional CD16^neg eosinophils. Human eosinophils contain stores of the immunoregulatory protein galectin-10. We found that Ab-mediated neutralization of galectin-10 partially abrogated the suppressive function of the eosinophils. Moreover, recombinant galectin-10 by itself was able to suppress T cell proliferation. Finally, we detected galectin-10-containing immune synapses between eosinophils and lymphocytes. To conclude, we describe a subset of suppressive eosinophils expressing CD16 that may escape detection because CD16-based negative selection is the standard procedure for the isolation of human eosinophils. Moreover, we show that galectin-10 functions as a T cell-suppressive molecule in eosinophils.

Eosinophils from eosinophilic oesophagitis patients have T cell suppressive capacity and express FOXP3

Eosinophilic esophagitis (EoE) is an antigen-driven T cell-mediated chronic inflammatory disease where food and environmental antigens are thought to have a role. Human eosinophils express the immunoregulatory protein galectin-10 and have T cell suppressive capacity similar to regulatory T cells (Tregs ). We hypothesized that one function of eosinophils in EoE might be to regulate the T cell-driven inflammation in the oesophagus. This was tested by evaluating the suppressive capacity of eosinophils isolated from the blood of adult EoE patients in a mixed lymphocyte reaction. In addition, eosinophilic expression of forkhead box protein 3 (FOXP3), the canonical transcription factor of Tregs , was determined by conventional and imaging flow cytometry, quantitative polymerase chain reaction (qPCR), confocal microscopy and immunoblotting. It was found that blood eosinophils from EoE patients had T cell suppressive capacity, and that a fraction of the eosinophils expressed FOXP3. A comparison of EoE eosinophils with healthy control eosinophils indicated that the patients' eosinophils had inferior suppressive capacity. Furthermore, a higher percentage of the EoE eosinophils expressed FOXP3 protein compared with the healthy eosinophils, and they also had higher FOXP3 protein and mRNA levels. FOXP3 was found in the cytosol and nucleus of the eosinophils from both the patients and healthy individuals, contrasting with the strict nuclear localization of FOXP3 in Tregs . To conclude, these findings suggest that the immunoregulatory function of eosinophils may be impaired in EoE.

Regulatory Eosinophils in Inflammation and Metabolic Disorders

Eosinophils are potent effector cells implicated in allergic responses and helminth infections. Responding to stimuli, they release their granule-derived cytotoxic proteins and are involved in inflammatory processes. However, under homeostatic conditions, eosinophils are abundantly present in the intestine and are constantly in contact with the gut microbiota and maintain the balance of immune responses without inflammation. This situation indicates that intestinal eosinophils have an anti-inflammatory function unlike allergic eosinophils. In support of this notion, some papers have shown that eosinophils have different phenotypes depending on the site of residence and are a heterogeneous cell population. Recently, it was reported that eosinophils in the small intestine and adipose tissue, respectively, contribute to homeostasis of intestinal immune responses and metabolism. Accordingly, in this review, we summarize new functions of eosinophils demonstrated in recent studies and discuss their homeostatic functions.

Central Role of IL-23 and IL-17 Producing Eosinophils as Immunomodulatory Effector Cells in Acute Pulmonary Aspergillosis and Allergic Asthma

Aspergillus fumigatus causes invasive pulmonary disease in immunocompromised hosts and allergic asthma in atopic individuals. We studied the contribution of lung eosinophils to these fungal diseases. By in vivo intracellular cytokine staining and confocal microscopy, we observed that eosinophils act as local sources of IL-23 and IL-17. Remarkably, mice lacking eosinophils had a >95% reduction in the percentage of lung IL-23p19⁺ cells as well as markedly reduced IL-23 heterodimer in lung lavage fluid. Eosinophils killed A. fumigatus conidia in vivo. Eosinopenic mice had higher mortality rates, decreased recruitment of inflammatory monocytes, and decreased expansion of lung macrophages after challenge with conidia. All of these functions underscore a potential protective role for eosinophils in acute aspergillosis. Given the postulated role for IL-17 in asthma pathogenesis, we assessed whether eosinophils could act as sources of IL-23 and IL-17 in models where mice were sensitized to either A. fumigatus antigens or ovalbumin (OVA). We found IL-23p19⁺ IL-17AF⁺ eosinophils in both allergic models. Moreover, close to 95% of IL-23p19⁺ cells and >90% of IL-17AF⁺ cells were identified as eosinophils. These data establish a new paradigm in acute and allergic aspergillosis whereby eosinophils act not only as effector cells but also as immunomodulatory cells driving the IL-23/IL-17 axis and contributing to inflammatory cell recruitment.

The opportunistic fungus, Aspergillus fumigatus, causes a spectrum of diseases ranging from invasive aspergillosis in the severely immunosuppressed to allergic asthma in atopic individuals. Here we explored the contribution of eosinophils, a type of white blood cell, to host defenses and pathogenesis in murine models of invasive pulmonary aspergillosis and asthma. We found eosinophils co-produce the cytokines IL-23 and IL-17 in both aspergillosis models as well as a model of OVA-induced asthma. Eosinophils killed the conidia (spores) of A. fumigatus in vivo and mice that lacked eosinophils were more susceptible to invasive aspergillosis. These observations suggest eosinophils play a more prominent role in defenses against invasive pulmonary aspergillosis than heretofore appreciated and identify eosinophil-derived IL-23 and IL-17 as potential therapeutic targets in allergic asthma.

Eosinophil polyunsaturated fatty acid metabolism and its potential control of inflammation and allergy

Polyunsaturated fatty acids (PUFAs) exhibit a range of biological effects, many of which are mediated through the formation and actions of their bioactive metabolites. It is well appreciated that dietary PUFA balance affects inflammation and/or allergic diseases, and recent advances in liquid chromatography tandem mass spectrometry (LC-MS/MS)-based mediator lipidomics have revealed a potential link between PUFA metabolism and biological phenotypes. This review presents insights into the emerging roles of eosinophil PUFA metabolism in controlling inflammatory responses and its potential involvement in allergy control.