IL-33 mediates reactive eosinophilopoiesis in response to airborne allergen exposure

The challenge of diagnosing and classifying eosinophilia and eosinophil disorders: A review

Eosinophilia is a feature of multiple conditions, both hematologic and non-hematologic, and may be associated with organ damage. The pathogenesis of eosinophilia can follow two distinct pathways. Primary eosinophilia is caused by a cell-intrinsic mechanism originating from clonal expansion of eosinophils through acquisition of a somatic mutation, such as FIP1L1-PDGFRA. In recent years, great progress has been made in the field of pathogenesis and molecularly targeted therapy of neoplastic eosinophilia. The diagnostic procedure should include, among other things, morphologic analysis of blood and bone marrow samples, cytogenetics and fluorescence in situ-hybridization tests to detect evidence of an acute or chronic myeloid or lymphoid disorder. Secondary eosinophilia follows a cell-extrinsic mechanism as a response to exogenous cytokines. In most clinical cases, peripheral blood eosinophilia is reactive and typically associated with non-hematological disorders such as infections, allergic conditions, connective tissue disorders, vasculitis, malignancy, or endocrinopathies. Nonetheless, the cause of most cases of hypereosinophilic syndrome remains unknown. In this article, we present a short review focused on differential diagnosis of eosinophilia and eosinophilic disorders. The diagnosis of eosinophilia is a challenge for physicians; thus this review may be useful in clinical practice.

Research progress of traditional Chinese medicine in the treatment of allergic rhinitis

Allergic rhinitis, one of the common diseases in otolaryngology, has shown an increasing incidence under the influence of various geographical, cultural and economic factors, making it a common and serious global public health problem. Modern medicine uses medication as the primary therapy for allergic rhinitis, but poor symptom control and easy relapse are the disadvantages of this treatment. However, Traditional Chinese medicine, with its long history, has treated allergic rhinitis by symptomatic treatment according to pattern differentiation with its unique insights and methods, which are effective and safe in numerous clinical studies. Therefore, this paper describes TCM decoction, acupuncture, moxibustion, acupoint application, catgut-embedding therapy and ear acupuncture in the treatment of AR. This study aims to provide more personalized and precise treatment for allergic rhinitis patients by investigating the mechanism of action, clinical research and development of traditional Chinese medicine treatments.

Eosinophils in obesity and obesity-associated disorders

Despite the rising prevalence and costs for the society, obesity etiology, and its precise cellular and molecular mechanisms are still insufficiently understood. The excessive accumulation of fat by adipocytes plays a key role in obesity progression and has many repercussions on total body physiology. In recent years the immune system as a gatekeeper of adipose tissue homeostasis has been evidenced and has become a focal point of research. Herein we focus on eosinophils, an important component of type 2 immunity, assuming fundamental, yet ill-defined, roles in the genesis, and progression of obesity and related metabolic disorders. We summarize eosinophilopoiesis and eosinophils recruitment into adipose tissue and discuss how the adipose tissue environments shape their function and vice versa. Finally, we also detail how obesity transforms the local eosinophil niche. Understanding eosinophil crosstalk with the diverse cell types within the adipose tissue environment will allow us to framework the therapeutic potential of eosinophils in obesity.

Lung-resident CD69+ST2+ TH2 cells mediate long-term type 2 memory to inhaled antigen in mice

BACKGROUND

Chronic airway diseases such as asthma are characterized by persistent type 2 immunity in the airways. We know little about the mechanisms that explain why type 2 inflammation continues in these diseases.

OBJECTIVE

We used mouse models to investigate the mechanisms involved in long-lasting immune memory.

METHODS

Naive mice were exposed intranasally to ovalbumin (OVA) antigen with Alternaria extract as an adjuvant. Type 2 memory was analyzed by parabiosis model, flow cytometry with in vivo antibody labeling, and intranasal OVA recall challenge. Gene-deficient mice were used to analyze the mechanisms.

RESULTS

In the parabiosis model, mice previously exposed intranasally to OVA with Alternaria showed more robust antigen-specific immune responses and airway inflammation than mice with circulating OVA-specific T cells. After a single airway exposure to OVA with Alternaria, CD69+ST2+ TH2-type T cells, which highly express type 2 cytokine messenger RNA and lack CD62L expression, appeared in lung tissue within 5 days and persisted for at least 84 days. When exposed again to OVA in vivo, these cells produced type 2 cytokines quickly without involving circulating T cells. Development of tissue-resident CD69+ST2+ TH2 cells and long-term memory to an inhaled antigen were abrogated in mice deficient in ST2 or IL-33, but not TSLP receptor.

CONCLUSION

CD69+ST2+ TH2 memory cells develop quickly in lung tissue after initial allergen exposure and persist for a prolonged period. The ST2/IL-33 pathway may play a role in the development of immune memory in lung to certain allergens.

STAT6-IP-Dependent Disruption of IL-33-Mediated ILC2 Expansion and Type 2 Innate Immunity in the Murine Lung

Recent interest has focused on innate-type cytokines as promoters of type 2 immunity and targets for drug development in asthma. IL-33 induces production of IL-4 and/or IL-13, which is associated with STAT6-dependent responses in innate cells, including group 2 innate lymphoid cells (ILC2s), macrophages, and eosinophils. Our published data show that STAT6-immunomodulatory peptide (STAT6-IP), an immunomodulatory peptide designed to inhibit the STAT6 transcription factor, reduces induction of Th2 adaptive immunity in respiratory syncytial virus infection and asthma models. Nevertheless, the mechanism of STAT6-IP-dependent inhibition has remained obscure. In this study, we demonstrate that STAT6-IP reduced IL-33-induced type 2 innate lung inflammation. Specifically, our data show that STAT6-IP reduced recruitment and activation of eosinophils as well as polarization of alternatively activated macrophages. Decreases in these cells correlated with reduced levels of IL-5 and IL-13 as well as several type 2 chemokines in the bronchoalveolar lavage fluid. STAT6-IP effectively inhibited expansion of ILC2s as well as the number of IL-5- and IL-13-producing ILC2s. Our data suggest that STAT6-IP effectively disrupts IL-13-dependent positive feedback loops, initiated by ILC2 activation, to suppress IL-33-induced type 2 innate immunity in the murine lung.

Recent advances of eosinophils and its correlated diseases

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

Transient IL-33 upregulation in neonatal mouse lung promotes acute but not chronic type 2 immune responses induced by allergen later in life

Early life respiratory insults, such as viral infections or hyperoxia, often increase asthma susceptibility later in life. The mechanisms underlying this increased susceptibility are not fully understood. IL-33 has been shown to be critically involved in allergic airway diseases. IL-33 expression in the neonatal lung can be increased by various respiratory insults associated with asthma development. Therefore, we investigated whether and how early life increases in IL-33 impact allergic airway responses later in life. Using a novel IL-33 transgenic mouse model, in which full-length IL-33 was inducible overexpressed in lung epithelial cells, we transiently upregulated lung IL-33 expression in neonatal mice for one week. After resting for 4–6 weeks, mice were intranasally exposed to a single-dose of recombinant IL-33 or the airborne allergen Alternaria. Alternatively, mice were exposed to Alternaria and ovalbumin multiple times for one month. We found that a transient increase in IL-33 expression during the neonatal period promoted IL-5 and IL-13 production when mice were later exposed to a single-dose of IL-33 or Alternaria in adulthood. However, increased IL-33 expression during the neonatal period did not affect airway inflammation, type 2 cytokine production, lung mucus production, or antigen-specific antibody responses when adult mice were exposed to Alternaria and ovalbumin multiple times. These results suggest that transient increased IL-33 expression early in life may have differential effects on allergic airway responses in later life, preferentially affecting allergen-induced acute type 2 cytokine production.

Roles of innate lymphoid cells (ILCs) in allergic diseases: The 10-year anniversary for ILC2s

In the 12 years since the discovery of innate lymphoid cells (ILCs), our knowledge of their immunobiology has expanded rapidly. Group 2 ILCs (ILC2s) respond rapidly to allergen exposure and environmental insults in mucosal organs, producing type 2 cytokines. Early studies showed that epithelium-derived cytokines activate ILC2s, resulting in eosinophilia, mucus hypersecretion, and remodeling of mucosal tissues. We now know that ILC2s are regulated by other cytokines, eicosanoids, and neuropeptides as well, and interact with both immune and stromal cells. Furthermore, ILC2s exhibit plasticity by adjusting their functions depending on their tissue environment and may consist of several heterogeneous subpopulations. Clinical studies show that ILC2s are involved in asthma, allergic rhinitis, chronic rhinosinusitis, food allergy, and eosinophilic esophagitis. However, much remains unknown about the immunologic mechanisms involved. Beneficial functions of ILCs in maintenance or restoration of tissue well-being and human health also need to be clarified. As our understanding of the crucial functions ILCs play in both homeostasis and disease pathology expands, we are poised to make tremendous strides in diagnostic and therapeutic options for patients with allergic diseases. This review summarizes discoveries in immunobiology of ILCs and their roles in allergic diseases in the past 5 years, discusses controversies and gaps in our knowledge, and suggests future research directions.

Epithelial IL-33 appropriates exosome trafficking for secretion in chronic airway disease

IL-33 is a key mediator of chronic airway disease driven by type 2 immune pathways, yet the nonclassical secretory mechanism for this cytokine remains undefined. We performed a comprehensive analysis in human airway epithelial cells, which revealed that tonic IL-33 secretion is dependent on the ceramide biosynthetic enzyme neutral sphingomyelinase 2 (nSMase2). IL-33 is cosecreted with exosomes by the nSMase2-regulated multivesicular endosome (MVE) pathway as surface-bound cargo. In support of these findings, human chronic obstructive pulmonary disease (COPD) specimens exhibited increased epithelial expression of the abundantly secreted IL33^Δ34 isoform and augmented nSMase2 expression compared with non-COPD specimens. Using an Alternaria-induced airway disease model, we found that the nSMase2 inhibitor GW4869 abrogated both IL-33 and exosome secretion as well as downstream inflammatory pathways. This work elucidates a potentially novel aspect of IL-33 biology that may be targeted for therapeutic benefit in chronic airway diseases driven by type 2 inflammation.

Role of IL-25, IL-33, and TSLP in triggering united airway diseases toward type 2 inflammation

Under the concept of "united airway diseases," the airway is a single organ wherein upper and lower airway diseases are commonly comorbid. The upper and lower airways are lined with respiratory epithelium that plays a vital role in immune surveillance and modulation as the first line of defense to various infective pathogens, allergens, and physical insults. Recently, there is a common hypothesis emphasizing epithelium-derived cytokines, namely IL-25, IL-33, and TSLP, as key regulatory factors that link in immune-pathogenic mechanisms of allergic rhinitis (AR), chronic rhinosinusitis (CRS), and asthma, mainly involving in type 2 inflammatory responses and linking innate and adaptive immunities. Herein, we review studies that elucidated the role of epithelium-derived triple cytokines in both upper and lower airways with the purpose of expediting better clinical treatments and managements of AR, CRS, asthma, and other associated allergic diseases via applications of the modulators of these cytokines.

Streptococcus pneumoniae aminopeptidase N regulates dendritic cells that attenuates type-2 airway inflammation in murine allergic asthma

BACKGROUND AND PURPOSE

Epidemiological and experimental studies suggest that microbial exposure in early childhood is linked with reduced risk to suffer asthma. Thus microbial components with immunoregulatory capabilities might serve as a preventive strategy for allergic asthma. Recently, it was identified that Streptococcus pneumoniae aminopeptidase N (PepN) could suppress T cell effector function. We sought to investigate the effect of PepN on murine allergic asthma and elucidate the underlying mechanism.

EXPERIMENTAL APPROACH

The effects of intranasal administration of PepN during or before sensitization were examined in ovalbumin (OVA)-induced murine allergic asthma. The roles of CD11b⁺ dendritic cells in PepN treated OVA-induced allergic asthma were evaluated by flow cytometry, cytokines detection and adoptive transfer. Moreover, the numbers of lung type 2 innate lymphoid cells (ILC2s) were also detected.

KEY RESULTS

Administration of PepN during or before sensitization attenuated type-2 airway inflammation (eosinophilia, mucus hypersecretion, Th2 cytokines production and IgE production) in allergic asthma mice. PepN reduced lung accumulation of CD11b⁺ dendritic cells, which was accompanied by diminished dendritic cell-attracting chemokine CCL20 production as well as CCL17 and CCL22, which are Th2-cell chemokines predominantly produced by CD11b⁺ dendritic cells. Adoptive transfer of BM-derived CD11b⁺ dendritic cells abolished the inhibitory effect of PepN on OVA-induced type-2 airway inflammation. The numbers of lung ILC2s were decreased in asthmatic mice receiving PepN.

CONCLUSION AND IMPLICATIONS

PepN alleviated type-2 inflammation in OVA-induced allergic asthma mice, which was mediated by regulation of lung CD11b⁺ dendritic cells. Our study provides a novel strategy for the prevention of allergic asthma.

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

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

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

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

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

House Dust Mite Induces Bone Marrow IL-33-Responsive ILC2s and TH Cells

Type 2 innate lymphoid cells (ILC2s) and their adaptive counterpart type 2 T helper (TH2) cells respond to interleukin-33 (IL-33) by producing IL-5, which is a crucial cytokine for eosinophil development in the bone marrow. The aim of this study was to determine if bone marrow ILC2s, TH cells, and eosinophils are locally regulated by IL-33 in terms of number and activation upon exposure to the common aeroallergen house dust mite (HDM). Mice that were sensitized and challenged with HDM by intranasal exposures induced eosinophil development in the bone marrow with an initial increase of IL5Rα+ eosinophil progenitors, following elevated numbers of mature eosinophils and the induction of airway eosinophilia. Bone marrow ILC2s, TH2, and eosinophils all responded to HDM challenge by increased IL-33 receptor (ST2) expression. However, only ILC2s, but not TH cells, revealed increased ST2 expression at the onset of eosinophil development, which significantly correlated with the number of eosinophil progenitors. In summary, our findings suggest that airway allergen challenges with HDM activates IL-33-responsive ILC2s, TH cells, and eosinophils locally in the bone marrow. Targeting the IL-33/ST2 axis in allergic diseases including asthma may be beneficial by decreasing eosinophil production in the bone marrow.

Critical Points on the Use of Biologicals in Allergic Diseases and Asthma

Improved understanding of the contribution of immune-inflammatory mechanisms in allergic diseases and asthma has encouraged development of biologicals and small molecules specifically targeting the innate and adaptive immune response. There are several critical points impacting the efficacy of this stratified approach, from the complexity of disease endotypes to the effectiveness in real-world settings. We discuss here how these barriers can be overcome to facilitate the development of implementation science for allergic diseases and asthma.

Recent developments and highlights in allergic rhinitis

Allergic rhinitis (AR) is a disease with high prevalence all over the world and therefore needs to be thoroughly investigated and treated accordingly. The mechanisms underlying the pathology and treatment of AR have been widely studied, but many aspects remain unclear and warrant further investigations. This review presents an overview of recently published papers highlighting the risk factors, mechanisms, and treatment of AR. Additionally, recent studies discussing the role of single nucleotide polymorphism, DNA methylation, regulatory B cells, group 2 innate lymphoid cells, immunotherapy, and biologics in AR are also covered.

Eosinophils, the IL-5/IL-5Rα axis, and the biologic effects of benralizumab in severe asthma

Bronchial asthma is a chronic inflammatory disease characterized, in a percentage of patients, as an eosinophilic inflammation of the airways. Eosinophils are recognized as a proinflammatory granulocyte playing a major role in the T2-high phenotype, which includes severe eosinophilic asthma. Eosinophilic asthma represents the majority of the phenotypic variants clinically characterized by severity and frequent exacerbations. For patients with severe uncontrolled asthma, monoclonal antibodies are used as add-on treatments. Among them, in addition to anti-immunoglobulin E therapy, biologic agents directed toward the interleukin (IL)-5/IL-5Rα axis and, thus, interfering with the pathologic functions of eosinophils, are now available. Unlike the other anti‒IL-5 monoclonal antibodies which exert an indirect effect on eosinophils, benralizumab, an afucosylated IgG₁ kappa antibody directed against the α subunit of IL-5R, directly depletes eosinophils and their associated bone marrow progenitor cells through induction of antibody-dependent cell-mediated cytotoxicity, through recruitment of natural killer cells. This article reviews the role of eosinophils in the pathogenesis of bronchial asthma and discusses the potential advantageous biologic effects of benralizumab in comparison with other monoclonal antibodies targeting the IL-5 ligand.

Protease/antiprotease network in allergy: The role of Staphylococcus aureus protease-like proteins

Staphylococcus aureus is being recognized as a major cofactor in atopic diseases such as atopic dermatitis, chronic rhinosinusitis with nasal polyps, and asthma. The understanding of the relationship between S aureus virulence factors and the immune system is continuously improving. Although the precise mechanism of the host's immune response adaptation to the variable secretion profile of S aureus strains continues to be a matter of debate, an increasing number of studies have reported on central effects of S aureus secretome in allergy. In this review, we discuss how colonization of S aureus modulates the innate and adaptive immune response, thereby predisposing the organism to allergic sensitization and disrupting immune tolerance in the airways of patients with asthma and chronic rhinosinusitis with nasal polyps. Next, we provide a critical overview of novel concepts dealing with S aureus in the initiation and persistence of chronic rhinosinusitis with nasal polyps and asthma. The role of the S aureus serine protease-like proteins in the initiation of a type 2 response and the contribution of the IL-33/ST2 signaling axis in allergic responses induced by bacterial allergens are discussed.

IL-33 Promotes CD11b/CD18-Mediated Adhesion of Eosinophils to Cancer Cells and Synapse-Polarized Degranulation Leading to Tumor Cell Killing

Eosinophils are major effectors of Th2-related pathologies, frequently found infiltrating several human cancers. We recently showed that eosinophils play an essential role in anti-tumor responses mediated by immunotherapy with the ‘alarmin’ intereukin-33 (IL-33) in melanoma mouse models. Here, we analyzed the mechanisms by which IL-33 mediates tumor infiltration and antitumor activities of eosinophils. We show that IL-33 recruits eosinophils indirectly, via stimulation of tumor cell-derived chemokines, while it activates eosinophils directly, up-regulating CD69, the adhesion molecules ICAM-1 and CD11b/CD18, and the degranulation marker CD63. In co-culture experiments with four different tumor cell lines, IL-33-activated eosinophils established large numbers of stable cell conjugates with target tumor cells, with the polarization of eosinophil effector proteins (ECP, EPX, and granzyme-B) and CD11b/CD18 to immune synapses, resulting in efficient contact-dependent degranulation and tumor cell killing. In tumor-bearing mice, IL-33 induced substantial accumulation of degranulating eosinophils within tumor necrotic areas, indicating cytotoxic activity in vivo. Blocking of CD11b/CD18 signaling significantly reduced IL-33-activated eosinophils’ binding and subsequent killing of tumor cells, indicating a crucial role for this integrin in triggering degranulation. Our findings provide novel mechanistic insights for eosinophil-mediated anti-tumoral function driven by IL-33. Treatments enabling tumor infiltration and proper activation of eosinophils may improve therapeutic response in cancer patients.

Elevated levels of interleukin-33 are associated with allergic and eosinophilic asthma

IL-33 is a recently discovered cytokine which plays an important role in asthma pathogenesis.

AIM

To evaluate serum IL-33 in patients with asthma and healthy controls, and to evaluate the association of IL-33 with different asthma phenotypes.

METHODS

Patients with asthma (n = 115) and healthy subjects (n = 85) were included in the study. Subjects with asthma were divided into groups according to their phenotype: allergic/non-allergic, eosinophilic/non-eosinophilic, obese/non-obese and severity according to GINA (mild, moderate and severe). The concentration of IL-33 in serum was measured by standardized enzyme-linked immunosorbent assay.

RESULTS

The level of IL-33 was significantly higher in patients with asthma when compared to healthy subjects (672.73 ± 104.47 pg/mL vs 268.52 ± 27.56 pg/mL, P < 0.05). IL-33 was also higher in the allergic asthma group patients when compared to non-allergic asthmatics (844.61 ± 152.08 pg/mL vs 369.56 ± 77.94 pg/mL, P < 0.05). There was a significantly higher serum IL-33 level in the eosinophilic asthma group when compared to the group of non-eosinophilic asthma patients (1001.10 ± 199.11 pg/mL vs 337.49 ± 72.68 pg/mL, P < 0.01). We did not find a significant difference in serum IL-33 level between different asthma severity groups, obese and non-obese asthmatics.

CONCLUSION

IL-33 is increased in asthma patients, particularly in some phenotypes: allergic asthma and eosinophilic asthma.

Social Networking of Group Two Innate Lymphoid Cells in Allergy and Asthma

Allergic diseases including asthma, chronic rhinosinusitis, and atopic dermatitis are common conditions worldwide. While type 2 immune responses induced by T-cells significantly cause allergic inflammation, the recently identified group two innate lymphoid cells (ILC2s) are emerging as critical players in the development of allergy. Upon allergen exposure, ILC2s are rapidly activated by cytokines released by epithelial cells. Activated ILC2s release various effector cytokines altogether contributing to the pathogenesis of allergy and can even cause inflammation in the absence of T-cells, as observed in asthma. Although the factors inducing ILC2 activation have been identified, evidence suggests that multiple factors can enhance or repress ILC2 proliferation, trafficking, or secretion of effector cytokines upon allergic inflammation. In this review, we discuss the recent findings that influence ILC2 activation and the resulting effects on the pathogenesis of allergy. A better understanding of how ILC2s are modulated will open the door to the development of new therapeutic strategies against allergic diseases.

Anti-IL-5 and IL-5Ra: Efficacy and Safety of New Therapeutic Strategies in Severe Uncontrolled Asthma

The current developments of the new biological drugs targeting interleukin 5 (IL-5) and IL-5 receptor allowed to expand the treatment options for severe hypereosinophilic asthma. Clinicians will then be able to choose between antibodies targeting either circulating IL-5 or its receptor expressed on eosinophils and basophils. The available clinical trials consistently reported favorable results about the reduction of exacerbations rate, improvement in quality of life, and sparing of the systemic steroid use, with a favorable safety profile. Two of these new drugs are administered subcutaneously, mepolizumab every 4 weeks and benralizumab every 8 weeks, whereas reslizumab is given intravenously monthly on a weigh-based dose. In the future, the research actions will be involved in the identification of a single biomarker or multiple biomarkers for the optimal choice of biological agents to be properly prescribed.

The PGI2 Analog Cicaprost Inhibits IL-33-Induced Th2 Responses, IL-2 Production, and CD25 Expression in Mouse CD4+ T Cells

IL-33 has pleiotropic functions in immune responses and promotes the development of allergic diseases and asthma. IL-33 induces Th2 differentiation and enhances type 2 cytokine production by CD4⁺ T cells. However, the regulation of IL-33-driven type 2 cytokine responses is not fully defined. In this study, we investigated the effect of PGI₂, a lipid mediator formed in the cyclooxygenase pathway of arachidonic acid metabolism, on naive CD4⁺ T cell activation, proliferation, and differentiation by IL-33. Using wild-type and PGI₂ receptor (IP) knockout mice, we found that the PGI₂ analog cicaprost dose-dependently inhibited IL-33-driven IL-4, IL-5, and IL-13 production by CD4⁺ T cells in an IP-specific manner. In addition, cicaprost inhibited IL-33-driven IL-2 production and CD25 expression by CD4⁺ T cells. Furthermore, IP knockout mice had increased IL-5 and IL-13 responses of CD4⁺ T cells to Alternaria sensitization and challenge in mouse lungs. Because IL-33 is critical for Alternaria-induced type 2 responses, these data suggest that PGI₂ not only inhibits IL-33-stimulated CD4⁺ Th2 cell responses in vitro but also suppresses IL-33-induced Th2 responses caused by protease-containing allergens in vivo.

Eosinophil persistence in vivo and sustained viability ex vivo in response to respiratory challenge with fungal allergens

BACKGROUND

Eosinophils are immunomodulatory leucocytes that contribute to the pathogenesis of Th2-driven asthma and allergic lung diseases.

OBJECTIVE

Our goal was to identify unique properties of eosinophils recruited to the lungs and airways of mice in response to challenge with asthma-associated fungal allergens.

METHODS

Mice were challenged intranasally on days 0, 3 and 6 with a filtrate of Alternaria alternata. Recruited eosinophils were enumerated in bronchoalveolar lavage fluid. Eosinophils were also isolated from lungs of mice sensitized and challenged with Aspergillus fumigatus and evaluated ex vivo in tissue culture.

RESULTS

Eosinophils persist in the airways for several weeks in response to brief provocation with A. alternata in wild-type, Gm-csf- and eotaxin-1-gene-deleted mice, while eosinophils are recruited but do not persist in the absence of IL-13. Eosinophils isolated from the lungs A. alternata-challenged mice are cytokine-enriched compared to those from IL5tg mice, including 800-fold higher levels of eotaxin-1. Furthermore, eosinophils from the lungs and spleen of fungal allergen-challenged wild-type mice are capable of prolonged survival ex vivo, in contrast to eosinophils from both untreated and fungal allergen-challenged IL5tg mice, which undergo rapid demise in the absence of exogenous cytokine support. TNF-α (but not IL5, IL-3, eotaxin-1 or GM-CSF) was detected in supernatants of ex vivo eosinophil cultures from the lungs of fungal allergen-challenged wild-type mice. However, neither TNF-α gene deletion nor anti-TNF-α neutralizing antibodies had any impact sustained eosinophil survival ex vivo.

CONCLUSION AND CLINICAL RELEVANCE

Eosinophils are phenotypically and functionally heterogeneous. As shown here, eosinophils from fungal allergen-challenged wild-type mice maintain a distinct cytokine profile, and, unlike eosinophils isolated from IL5tg mice, they survive ex vivo in the absence of exogenous pro-survival cytokine support. As treatments for asthma currently in development focus on limiting eosinophil viability via strategic cytokine blockade, the molecular mechanisms underlying differential survival merit further investigation.

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.

Advances in environmental and occupational disorders in 2016

In this review we highlight recent studies that advance the knowledge and understanding of the effects of various environmental factors and associated immune responses in patients with allergic diseases. This review will focus on new literature regarding allergic and immune responses to a variety of environmental factors, including aeroallergens, stinging insects, fungi, pollutants, viral respiratory tract infections, climate change, and microbial exposures.

Eosinophils: The unsung heroes in cancer?

Prolonged low-grade inflammation or smoldering inflammation is a hallmark of a cancer. Eosinophils are components of the immune microenvironment that modulates tumor initiation and progression. Although canonically associated with a detrimental role in allergic disorders, these cells can induce a protective immune response against helminthes, viral and bacterial pathogens. Eosinophils are a source of anti-tumorigenic (e.g., TNF-α, granzyme, cationic proteins, and IL-18) and protumorigenic molecules (e.g., pro-angiogenic factors) depending on the milieu. In several neoplasias (e.g., melanoma, gastric, colorectal, oral and prostate cancer) eosinophils play an anti-tumorigenic role, in others (e.g., Hodgkin's lymphoma, cervical carcinoma) have been linked to poor prognosis, whereas in yet others they are apparently innocent bystanders. These seemingly conflicting results suggest that the role of eosinophils and their mediators could be cancer-dependent. The microlocalization (e.g., peritumoral vs intratumoral) of eosinophils could be another important aspect in the initiation/progression of solid and hematological tumors. Increasing evidence in experimental models indicates that activation/recruitment of eosinophils could represent a new therapeutic strategy for certain tumors (e.g., melanoma). Many unanswered questions should be addressed before we understand whether eosinophils are an ally, adversary or neutral bystanders in different types of human cancers.

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

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

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.

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.

Type two innate lymphoid cells: the Janus cells in health and disease

Innate lymphoid cells are functionally diverse subsets of immune cells including the conventional natural killer cells, lymphoid tissue inducers, type 1, 2, and 3 with significant roles in immunity and pathogenesis of inflammatory diseases. Type 2 innate lymphoid cells (ILC2s) resemble type 2 helper (Th2) cells in cytokine production and contribute to anti-helminth immunity, maintaining mucosal tissue integrity, and adipose tissue browning. ILC2s play important roles in the pathogenesis of allergic diseases and asthma. Studying the pathways of activation and regulation of ILC2s are currently a priority for giving a better understanding of pathogenesis of diseases with immunological roots. Recently, our laboratory and others have shown several pathways of regulation of ILC2s by co-stimulatory molecules such as ICOS, regulatory T cells and by compounds such as nicotine. In this review, we summarize the current understanding of the mechanisms of activation and regulation of ILC2s and the role of these cells in health and disease.

IL-33 restricts tumor growth and inhibits pulmonary metastasis in melanoma-bearing mice through eosinophils

The alarmin IL-33 is an IL-1 family member that stimulates pleiotropic immune reactions depending on the target tissue and microenvironmental factors. In this study, we have investigated the role of IL-33/ST2 axis in antitumor response to melanoma. Injection of IL-33 in mice-bearing subcutaneous B16.F10 melanoma resulted in significant tumor growth delay. This effect was associated with intratumoral accumulation of CD8⁺ T cells and eosinophils, decrease of immunosuppressive myeloid cells, and a mixed Th1/Th2 cytokine expression pattern with local and systemic activation of CD8⁺ T and NK cells. Moreover, intranasal administration of IL-33 determined ST2-dependent eosinophil recruitment in the lung that prevented the onset of pulmonary metastasis after intravenous injection of melanoma cells. Accordingly, ST2-deficient mice developed pulmonary metastasis at higher extent than wild-type counterparts, associated with lower eosinophil frequencies in the lung. Of note, depletion of eosinophils by in vivo treatment with anti-Siglec-F antibody abolished the ability of IL-33 to both restrict primary tumor growth and metastasis formation. Finally, we show that IL-33 is able to activate eosinophils resulting in efficient killing of target melanoma cells, suggesting a direct antitumor activity of eosinophils following IL-33 treatment. Our results advocate for an eosinophil-mediated antitumoral function of IL-33 against melanoma, thus opening perspectives for novel cancer immunotherapy strategies.

Endogenous prostaglandin E2 amplifies IL-33 production by macrophages through an E prostanoid (EP)2/EP4-cAMP-EPAC-dependent pathway

When activated through toll-like receptors (TLRs), macrophages generate IL-33, an IL-1 family member that induces innate immune responses through ST2 signaling. LPS, a TLR4 ligand, induces macrophages to generate prostaglandin E₂ (PGE₂) through inducible COX-2 and microsomal PGE₂ synthase 1 (mPGES-1) (1). We demonstrate that IL-33 production by bone marrow-derived murine macrophages (bmMFs) requires the generation of endogenous PGE₂ and the intrinsic expression of EP₂ receptors to amplify NF-κB-dependent, LPS-induced IL-33 expression via exchange protein activated by cAMP (EPAC). Compared with WT cells, bmMFs lacking either mPGES-1 or EP₂ receptors displayed reduced LPS-induced IL-33 levels. A selective EP₂ agonist and, to a lesser extent, EP₄ receptor agonist potentiated LPS-induced IL-33 generation from both mPGES-1-null and WT bmMFs, whereas EP₁ and EP₃ receptor agonists were inactive. The effects of PGE₂ depended on cAMP, were mimicked by an EPAC-selective agonist, and were attenuated by EPAC-selective antagonism and knockdown. LPS-induced p38 MAPK and NF-κB activations were necessary for both IL-33 production and PGE₂ generation, and exogenous PGE₂ partly reversed the suppression of IL-33 production caused by p38 MAPK and NF-κB inhibition. Mice lacking mPGES-1 showed lower IL-33 levels and attenuated lung inflammation in response to repetitive Alternaria inhalation challenges. Cumulatively, our data demonstrate that endogenous PGE₂, EP₂ receptors, and EPAC are prerequisites for maximal LPS-induced IL-33 expression and that exogenous PGE₂ can amplify IL-33 production via EP₂ and EP₄ receptors. The ubiquitous induction of mPGES-1-dependent PGE₂ may be crucial for innate immune system activation during various IL-33 driven pathologic disorders.

Reslizumab and Eosinophilic Asthma: One Step Closer to Precision Medicine?

Human eosinophils represent approximately 1% of peripheral blood leukocytes. However, these cells have the propensity to leave the blood stream and migrate into inflamed tissues. Eosinophilic inflammation is present in a significant proportion of patients with severe asthma. Asthma is a chronic inflammatory disorder that affects more than 315 million people worldwide, with 10% having severe uncontrolled disease. Although the majority of patients can be efficiently treated, severe asthmatics continue to be uncontrolled and are at risk of exacerbations and even death. Interleukin-5 (IL-5) plays a fundamental role in eosinophil differentiation, maturation, activation and inhibition of apoptosis. Therefore, targeting IL-5 is an appealing approach to the treatment of patients with severe eosinophilic asthma. Reslizumab, a humanized anti-IL-5 monoclonal antibody, binds with high affinity to amino acids 89–92 of IL-5 that are critical for binding to IL-5 receptor α. Two phase III studies have demonstrated that reslizumab administration in adult patients with severe asthma and eosinophilia (≥400 cells/μL) improved lung function, asthma control, and symptoms. Thus, the use of blood eosinophils as a baseline biomarker could help to select patients with severe uncontrolled asthma who are likely to achieve benefits in asthma control with reslizumab. In conclusion, targeted therapy with reslizumab represents one step closer to precision medicine in patients with severe eosinophilic asthma.

Allergies - A T cells perspective in the era beyond the TH1/TH2 paradigm

Allergic diseases have emerged as a major health care burden, especially in the western hemisphere. They are defined by overshooting reactions of an aberrant immune system to harmless exogenous stimuli. The T_H1/T_H2 paradigm assumes that a dominance of T_H2 cell activation and an inadequate T_H1 cell response are responsible for the development of allergies. However, the characterization of additional T helper cell subpopulations such as T_H9, T_H17, T_H22, T_HGM-CSF and their interplay with regulatory T cells suggest further layers of complexity. This review summarizes state-of-the-art knowledge on T cell diversity and their induction, while revisiting the T_H1/T_H2 paradigm. With respect to these numerous contributors, it offers a new perspective on the pathogenesis of asthma, allergic rhinitis (AR) and atopic dermatitis (AD) incorporating recent discoveries in the field of T cell plasticity.