Basophil differentiation, heterogeneity, and functional implications

Basophils, rare granulocytes, have long been acknowledged for their roles in type 2 immune responses. However, the mechanisms by which basophils adapt their functions to diverse mammalian microenvironments remain unclear. Recent advancements in specific research tools and single-cell-based technologies have greatly enhanced our understanding of basophils. Several studies have shown that basophils play a role in maintaining homeostasis but can also contribute to pathology in various tissues and organs, including skin, lung, and others. Here, we provide an overview of recent basophil research, including cell development, characteristics, and functions. Based on an increasing understanding of basophil biology, we suggest that the precise targeting of basophil features might be beneficial in alleviating certain pathologies such as asthma, atopic dermatitis (AD), and others.

Novel insights into the ontogeny of basophils

Basophils are the least common granulocytes, accounting for <1% of peripheral blood leukocytes. In the last 20 years, analytical tools for mouse basophils have been developed, and we now recognize that basophils play critical roles in various immune reactions, including the development of allergic inflammation and protective immunity against parasites. Moreover, the combined use of flow cytometric analyses and knockout mice has uncovered several progenitor cells committed to basophils in mice. Recently, advancements in single-cell RNA sequencing (scRNA-seq) technologies have challenged the classical view of the differentiation of various hematopoietic cell lineages. This is also true for basophil differentiation, and studies using scRNA-seq analysis have provided novel insights into basophil differentiation, including the association of basophil differentiation with that of erythrocyte/megakaryocyte and the discovery of novel basophil progenitor cells in the mouse bone marrow. In this review, we summarize the recent findings of basophil ontogeny in both mice and humans, mainly focusing on studies using scRNA-seq analyses.

An Andrographis paniculata Burm. Nees extract standardized for three main Andrographolides prevents house dust mite-induced airway inflammation, remodeling, and hyperreactivity by regulating Th1/Th2 gene expression in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Andrographis paniculata Burm. Nees (AP) is an herb used traditionally in Indian and Chinese traditional medicine for the treatment of various inflammatory and respiratory tract diseases. However, the anti-inflammatory potential of standardized Andrographis paniculata 50% ethanol extract (APEE50) in the murine model of asthma has not been investigated.

AIM OF THE STUDY

This study aimed to evaluate the protective anti-inflammatory potential and better understand the underlying mechanism of action of APEE50 in a clinically-relevant mouse asthma model. Thereafter, develop the ethanolic extract of AP as a supplement for asthma prophylaxis.

MATERIALS AND METHOD

APEE50 was prepared and standardized for AGP, NAG, and DDAG using a high-performance liquid chromatography system. Asthma was induced according to a 14-day house dust mite (HDM) induction protocol. The prophylactic potential of APEE50 (50 mg/kg - 200 mg/kg) was determined by assessing cardinal asthma features, which included BALF leukocyte and differential cell count, BALF cytokine assay, histology, gene expression, and airway hyperreactivity study.

RESULTS

APEE50 significantly inhibited HDM-induced airway eosinophilia and neutrophilia. In addition to decreased levels of IL-4, IL-5, IL-13, and eotaxin in bronchoalveolar fluid, APEE50 abrogated HDM-induced airway mucus over-secretion and airway hyper-responsiveness. Administration of APEE50 downregulated HDM-induced upregulation of the oxidative stress enzyme Duox1 (dual oxidase 1) and marginally induced Nfe2l2 (nuclear factor erythroid 2-related factor 2) gene expressions. Similarly, Th2-related (Serpinb2, Clca3a1, Il4 and Il13) and Muc5ac gene expression were significantly downregulated.

CONCLUSION

Prophylactic administration of APEE50 prevented the progression of HDM-induced asthmatic responses by down-regulating Th2 cytokine gene expression and oxidative stress level.

Mast cells in type 2 skin inflammation: Maintenance and function

Mast cells (MCs) are immune cells residing in tissues and playing indispensable roles in maintaining homeostasis and inflammatory states. Skin lesions associated with atopic dermatitis (AD) and type 2 skin inflammation display an increment in MCs, which have both pro- and anti-inflammatory effects. The direct and indirect activations of skin MCs by environmental factors such as Staphylococcus aureus can instigate type 2 skin inflammation in AD with poorly understood mechanisms. Furthermore, both IgE-dependent and -independent degranulation of MCs contribute to pruritus in AD. Conversely, MCs suppress type 2 skin inflammation by promoting Treg expansion through IL-2 secretion in the spleen. Moreover, skin MCs can upregulate gene expression involved in skin barrier function, thus mitigating AD-like inflammation. These functional variances of MCs in AD could stem from differences in experimental systems, their localization, and origins. In this review, we will focus on how MCs are maintained in the skin under homeostatic and inflammatory conditions, and how they are involved in the pathogenesis of type 2 skin inflammation.

Basophils beyond allergic and parasitic diseases

Basophils bind IgE via FcεRI-αβγ2, which they uniquely share only with mast cells. In doing so, they can rapidly release mediators that are hallmark of allergic disease. This fundamental similarity, along with some morphological features shared by the two cell types, has long brought into question the biological significance that basophils mediate beyond that of mast cells. Unlike mast cells, which mature and reside in tissues, basophils are released into circulation from the bone marrow (constituting 1% of leukocytes), only to infiltrate tissues under specific inflammatory conditions. Evidence is emerging that basophils mediate non-redundant roles in allergic disease and, unsuspectingly, are implicated in a variety of other pathologies [e.g., myocardial infarction, autoimmunity, chronic obstructive pulmonary disease, fibrosis, cancer, etc.]. Recent findings strengthen the notion that these cells mediate protection from parasitic infections, whereas related studies implicate basophils promoting wound healing. Central to these functions is the substantial evidence that human and mouse basophils are increasingly implicated as important sources of IL-4 and IL-13. Nonetheless, much remains unclear regarding the role of basophils in pathology vs. homeostasis. In this review, we discuss the dichotomous (protective and/or harmful) roles of basophils in a wide spectrum of non-allergic disorders.

Recent Advances in Localized Immunomodulation Technology: Application of NIR-PIT toward Clinical Control of the Local Immune System

Current immunotherapies aim to modulate the balance among different immune cell populations, thereby controlling immune reactions. However, they often cause immune overactivation or over-suppression, which makes them difficult to control. Thus, it would be ideal to manipulate immune cells at a local site without disturbing homeostasis elsewhere in the body. Recent technological developments have enabled the selective targeting of cells and tissues in the body. Photo-targeted specific cell therapy has recently emerged among these. Near-infrared photoimmunotherapy (NIR-PIT) has surfaced as a new modality for cancer treatment, which combines antibodies and a photoabsorber, IR700DX. NIR-PIT is in testing as an international phase III clinical trial for locoregional recurrent head and neck squamous cell carcinoma (HNSCC) patients (LUZERA-301, NCT03769506), with a fast-track designation by the United States Food and Drug Administration (US-FDA). In Japan, NIR-PIT for patients with recurrent head and neck cancer was conditionally approved in 2020. Although NIR-PIT is commonly used for cancer therapy, it could also be exploited to locally eliminate certain immune cells with antibodies for a specific immune cell marker. This strategy can be utilized for anti-allergic therapy. Herein, we discuss the recent technological advances in local immunomodulation technology. We introduce immunomodulation technology with NIR-PIT and demonstrate an example of the knockdown of regulatory T cells (Tregs) to enhance local anti-tumor immune reactions.

Infiltration and local differentiation of bone marrow-derived integrinβ7-positive mast cell progenitors in atopic dermatitis-like skin

BACKGROUND

Mast cells (MCs) are tissue-resident cells with various immunologic functions. MCs are increased in atopic dermatitis (AD) skin and can contribute to the inflammation. Although skin MCs are inducible from bone marrow (BM) cells in vitro, they are maintained locally by self-proliferation in the steady state in vivo. However, how skin MCs are increased in AD skin, including the infiltration of BM-derived MC progenitors (MCps) and their differentiation, remains unclear.

OBJECTIVE

We sought to identify and characterize BM-derived MCps in AD skin.

METHODS

BM-derived MCps in AD skin were analyzed by flow cytometry using BM-chimeric mice and parabiosis in an MC903-induced AD model. BM-derived MCps in AD-like skin were compared with resident MCs for gene expression by RNA- sequencing analysis.

RESULTS

We observed local proliferation of resident MCs and an increase in BM-derived MCs in AD-like skin. BM-derived MCs in the skin were derived from circulating MCps and were distinguishable from resident MCs by integrinβ7. RNA- sequence analysis showed that integrinβ7⁺ MCs (BM-derived MCps) in the skin shared the characteristics of both mucosal-type MCs and connective tissue-type MCs, and increased the expression of genes related to MCp migration. BM-derived MCps proliferated in situ, gradually lost the integrinβ7 expression, and acquired connective tissue-type MC phenotypes during the remission phase of inflammation.

CONCLUSIONS

BM-derived integrinβ7⁺ MCps migrate to AD-like skin and contribute to the maintenance of skin MCs.

Skin-homing basophils and beyond

Basophils have been implicated in type 2 inflammation and numerous disorders in the skin such as helminth infection, atopic dermatitis, and urticaria. Although similar in form and function to tissue-resident mast cells, classical studies on basophils have centered on those from the hematopoietic compartment. However, increasing studies in tissues like the skin demonstrate that basophils may take on particular characteristics by responding to unique developmental, chemotactic, and activation cues. Herein, we highlight how recent studies in barrier immunology suggest the presence of skin-homing basophils that harbor a unique identity in terms of phenotype, function, and motility. These concepts may uniquely inform how basophils contribute to diseases at multiple epithelial surfaces and our ability to therapeutically target the innate immune system in disease.

Communication is key: Innate immune cells regulate host protection to helminths

Parasitic helminth infections remain a significant global health issue and are responsible for devastating morbidity and economic hardships. During infection, helminths migrate through different host organs, which results in substantial tissue damage and the release of diverse effector molecules by both hematopoietic and non-hematopoietic cells. Thus, host protective responses to helminths must initiate mechanisms that help to promote worm clearance while simultaneously mitigating tissue injury. The specialized immunity that promotes these responses is termed type 2 inflammation and is initiated by the recruitment and activation of hematopoietic stem/progenitor cells, mast cells, basophils, eosinophils, dendritic cells, neutrophils, macrophages, myeloid-derived suppressor cells, and group 2 innate lymphoid cells. Recent work has also revealed the importance of neuron-derived signals in regulating type 2 inflammation and antihelminth immunity. These studies suggest that multiple body systems coordinate to promote optimal outcomes post-infection. In this review, we will describe the innate immune events that direct the scope and intensity of antihelminth immunity. Further, we will highlight the recent progress made in our understanding of the neuro-immune interactions that regulate these pathways and discuss the conceptual advances they promote.

Role of Basophils in a Broad Spectrum of Disorders

Basophils are the rarest granulocytes and have long been overlooked in immunological research due to their rarity and similarities with tissue-resident mast cells. In the last two decades, non-redundant functions of basophils have been clarified or implicated in a broad spectrum of immune responses, particularly by virtue of the development of novel analytical tools for basophils. Basophils infiltrate inflamed tissues of patients with various disorders, even though they circulate in the bloodstream under homeostatic conditions. Depletion of basophils results in the amelioration or exaggeration of inflammation, depending on models of disease, indicating basophils can play either beneficial or deleterious roles in a context-dependent manner. In this review, we summarize the recent findings of basophil pathophysiology under various conditions in mice and humans, including allergy, autoimmunity, tumors, tissue repair, fibrosis, and COVID-19. Further mechanistic studies on basophil biology could lead to the identification of novel biomarkers or therapeutic targets in a broad range of diseases.

Immune Cells in Cutaneous Wound Healing: A Review of Functional Data from Animal Models

The healing of skin wounds involves the activation and recruitment of various immune cell types, many of which are believed to contribute significantly to different aspects of the repair process. Roles for immune cells have been described in practically all stages of wound healing, including hemostasis, inflammation, proliferation and scar formation/remodeling. Over the last decade, tools to deplete immune cell populations in animal models have become more advanced, leading to a surge in the number of studies examining the function of specific immune cell types in skin repair. In this review, we will summarize what is known about distinct immune cell types in cutaneous wound healing, with an emphasis on data from animal studies in which specific cell types have been targeted.

Clinical and Translational Significance of Basophils in Patients with Cancer

Despite comprising a very small proportion of circulating blood leukocytes, basophils are potent immune effector cells. The high-affinity receptor for IgE (FcɛRI) is expressed on the basophil cell surface and powerful inflammatory mediators such as histamine, granzyme B, and cytokines are stored in dense cytoplasmic granules, ready to be secreted in response to a range of immune stimuli. Basophils play key roles in eliciting potent effector functions in allergic diseases and type 1 hypersensitivity. Beyond allergies, basophils can be recruited to tissues in chronic and autoimmune inflammation, and in response to parasitic, bacterial, and viral infections. While their activation states and functions can be influenced by Th2-biased inflammatory signals, which are also known features of several tumor types, basophils have received little attention in cancer. Here, we discuss the presence and functional significance of basophils in the circulation of cancer patients and in the tumor microenvironment (TME). Interrogating publicly available datasets, we conduct gene expression analyses to explore basophil signatures and associations with clinical outcomes in several cancers. Furthermore, we assess how basophils can be harnessed to predict hypersensitivity to cancer treatments and to monitor the desensitization of patients to oncology drugs, using assays such as the basophil activation test (BAT).

Brain Mast Cells in Sleep and Behavioral Regulation

The function of mast cells in the brain for the mediation of neurobehavior is largely unknown. Mast cells are a heterogeneous population of granulocytic cells in the immune system. Mast cells contain numerous mediators, such as histamine, serotonin, cytokines, chemokines, and lipid-derived factors. Mast cells localize not only in the periphery but are also resident in the brain of mammalians. Mast cells in the brain are constitutively active, releasing their contents gradually or rapidly by anaphylactic degranulation. Their activity is also increased by a wide range of stimuli including both immune and non-immune signals. Brain mast cell neuromodulation may thus be involved in various neurobehavior in health and diseases.Using Kit mutant mast cell deficient mice (Kit^W/Kit^W-v), we obtained results indicating that brain mast cells regulate sleep/wake and other behavioral phenotypes and that histamine from brain mast cells promotes wakefulness. These findings were also confirmed using a newer inducible and Kit-independent mast cell deficient Mas-TRECK (toxin receptor knockout) mouse. Injections of diphtheria toxin (DT) selectively deplete mast cells and reduce wakefulness during the periods of mast cell depletion.We recently introduced a mouse model for chronic sleep loss associated with diabetes. The mice reared on the wire net for 3 weeks (i.e., mild stress [MS]) showed decreased amount of non-rapid eye movement (NREM) sleep, increased sleep fragmentation, and abnormal glucose tolerance test [GTT] and insulin tolerance test [ITT], phenotypes which mirror human chronic insomnia. Interestingly, these mice with insomnia showed an increased number of mast cells in both the brain and adipose tissue. Mast cell deficient mice (Kit^W/Kit^W-v) and inhibition of mast cell functions with cromolyn or a histamine H1 receptor antagonist administration ameliorated both insomnia and abnormal glycometabolism. Mast cells may therefore represent an important pathophysiological mediator in sleep impairments and abnormal glycometabolism associated with chronic insomnia.

Basophils in antihelminth immunity

It has been appreciated that basophilia is a common feature of helminth infections for approximately 50 years. The ability of basophils to secrete IL-4 and other type 2 cytokines has supported the prevailing notion that basophils contribute to antihelminth immunity by promoting optimal type 2 T helper (Th2) cell responses. While this appears to be the case in several helminth infections, emerging studies are also revealing that the effector functions of basophils are extremely diverse and parasite-specific. Further, new reports now suggest that basophils can restrict type 2 inflammation in a manner that preserves the integrity of helminth-affected tissue. Finally, exciting data has also demonstrated that basophils can regulate inflammation by participating in neuro-immune interactions. This article will review the current state of basophil biology and describe how recent studies are transforming our understanding of the role basophils play in the context of helminth infections.

IL-3 in the development and function of basophils

The β common chain (βc) cytokine family includes granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) and IL-5, all of which use βc as key signaling receptor subunit. GM-CSF, IL-3 and IL-5 have specific roles as hematopoietic growth factors. IL-3 binds with high affinity to the IL-3 receptor α (IL-3Rα/CD123) and then associates with the βc subunit. IL-3 is mainly synthesized by different subsets of T cells, but is also produced by several other immune [basophils, dendritic cells (DCs), mast cells, etc.] and non-immune cells (microglia and astrocytes). The IL-3Rα is also expressed by immune (basophils, eosinophils, mast cells, DCs, monocytes, and megacaryocytes) and non-immune cells (endothelial cells and neuronal cells). IL-3 is the most important growth and activating factor for human and mouse basophils, primary effector cells of allergic disorders. IL-3-activated basophils and mast cells are also involved in different chronic inflammatory disorders, infections, and several types of cancer. IL-3 induces the release of cytokines (i.e., IL-4, IL-13, CXCL8) from human basophils and preincubation of basophils with IL-3 potentiates the release of proinflammatory mediators and cytokines from IgE- and C5a-activated basophils. IL-3 synergistically potentiates IL-33-induced mediator release from human basophils. IL-3 plays a pathogenic role in several hematologic cancers and may contribute to autoimmune and cardiac disorders. Several IL-3Rα/CD123 targeting molecules have shown some efficacy in the treatment of hematologic malignancies.

Diverse innate stimuli activate basophils through pathways involving Syk and IκB kinases

Mature basophils play critical inflammatory roles during helminthic, autoimmune, and allergic diseases through their secretion of histamine and the type 2 cytokines interleukin 4 (IL-4) and IL-13. Basophils are activated typically by allergen-mediated IgE cross-linking but also by endogenous "innate" factors. The aim of this study was to identify the innate stimuli (cytokines, chemokines, growth factors, hormones, neuropeptides, metabolites, and bacterial products) and signaling pathways inducing primary basophil activation. Basophils from naïve mice or helminth-infected mice were cultured with up to 96 distinct stimuli and their influence on basophil survival, activation, degranulation, and IL-4 or IL-13 expression were investigated. Activated basophils show a heterogeneous phenotype and segregate into distinct subsets expressing IL-4, IL-13, activation, or degranulation markers. We find that several innate stimuli including epithelial derived inflammatory cytokines (IL-33, IL-18, TSLP, and GM-CSF), growth factors (IL-3, IL-7, TGFβ, and VEGF), eicosanoids, metabolites, TLR ligands, and type I IFN exert significant direct effects on basophils. Basophil activation mediated by distinct upstream signaling pathways is always sensitive to Syk and IκB kinases-specific inhibitors but not necessarily to NFAT, STAT5, adenylate cyclase, or c-fos/AP-1 inhibitors. Thus, basophils are activated by very diverse mediators, but their activation seem controlled by a core checkpoint involving Syk and IκB kinases.

Mast Cells in Inflammation and Disease: Recent Progress and Ongoing Concerns

Mast cells have existed long before the development of adaptive immunity, although they have been given different names. Thus, in the marine urochordate Styela plicata, they have been designated as test cells. However, based on their morphological characteristics (including prominent cytoplasmic granules) and mediator content (including heparin, histamine, and neutral proteases), test cells are thought to represent members of the lineage known in vertebrates as mast cells. So this lineage presumably had important functions that preceded the development of antibodies, including IgE. Yet mast cells are best known, in humans, as key sources of mediators responsible for acute allergic reactions, notably including anaphylaxis, a severe and potentially fatal IgE-dependent immediate hypersensitivity reaction to apparently harmless antigens, including many found in foods and medicines. In this review, we briefly describe the origins of tissue mast cells and outline evidence that these cells can have beneficial as well as detrimental functions, both innately and as participants in adaptive immune responses. We also discuss aspects of mast cell heterogeneity and comment on how the plasticity of this lineage may provide insight into its roles in health and disease. Finally, we consider some currently open questions that are yet unresolved.

The Importance of Metabolism for Immune Homeostasis in Allergic Diseases

There is increasing evidence that the metabolic status of T cells and macrophages is associated with severe phenotypes of chronic inflammation, including allergic inflammation. Metabolic changes in immune cells have a crucial role in their inflammatory or regulatory responses. This notion is reinforced by metabolic diseases influencing global energy metabolism, such as diabetes or obesity, which are known risk factors of severity in inflammatory conditions, due to the metabolic-associated inflammation present in these patients. Since several metabolic pathways are closely tied to T cell and macrophage differentiation, a better understanding of metabolic alterations in immune disorders could help to restore and modulate immune cell functions. This link between energy metabolism and inflammation can be studied employing animal, human or cellular models. Analytical approaches rank from classic immunological studies to integrated analysis of metabolomics, transcriptomics, and proteomics. This review summarizes the main metabolic pathways of the cells involved in the allergic reaction with a focus on T cells and macrophages and describes different models and platforms of analysis used to study the immune system and its relationship with metabolism.

Comparative analysis of the role of mast cells in murine asthma models using Kit-sufficient mast cell-deficient animals

BACKGROUND

Asthma is a frequent chronic disease that can potentially severely affect the respiratory capacity and well-being of patients. Mast cells (MCs) are regarded as major players in human asthma due to their capacity to release crucial inflammatory mediators following allergen exposure. However, unambiguous characterization of their role in animal models has long been hindered by the unavailability of specific MC-deficient models lacking confounding MC-unrelated effects. This study aims to examine the role of MCs in Kit-sufficient MC-deficient Cpa3^{Cre /+} mice.

METHODS

We used a variety of models of acute and chronic asthma employing distinct routes and regimes of sensitization. These sensitizations were done via the peritoneal cavity, the skin, or the lung. Additionally, different allergens, i.e. ovalbumin and house dust mite extract, were used.

RESULTS

Our results show that the absence of MCs had no impact on the severity of allergic airway inflammation in any of the tested mouse models, as measured by leukocyte infiltration in the airways, cytokine expression, antibody production, airway hyper-responsiveness and mucus production.

CONCLUSION

This indicates that MCs do not play a major role in murine allergic airway inflammation.

Mast cell-derived serotonin enhances methacholine-induced airway hyperresponsiveness in house dust mite-induced experimental asthma

BACKGROUND

Airway hyperresponsiveness (AHR) is a feature of asthma in which airways are hyperreactive to stimuli causing extensive airway narrowing. Methacholine provocations assess AHR in asthma patients mainly by direct stimulation of smooth muscle cells. Using in vivo mouse models, mast cells have been implicated in AHR, but the mechanism behind has remained unknown.

METHODS

Cpa3^{Cre /+} mice, which lack mast cells, were used to assess the role of mast cells in house dust mite (HDM)-induced experimental asthma. Effects of methacholine in presence or absence of ketanserin were assessed on lung function and in lung mast cells in vitro. Airway inflammation, mast cell accumulation and activation, smooth muscle proliferation, and HDM-induced bronchoconstriction were evaluated.

RESULTS

Repeated intranasal HDM sensitization induced allergic airway inflammation associated with accumulation and activation of lung mast cells. Lack of mast cells, absence of activating Fc-receptors, or antagonizing serotonin (5-HT)_2A receptors abolished HDM-induced trachea contractions. HDM-sensitized mice lacking mast cells had diminished lung-associated 5-HT levels, reduced AHR and methacholine-induced airway contraction, while blocking 5-HT_2A receptors in wild types eliminated AHR, implying that mast cells contribute to AHR by releasing 5-HT. Primary mouse and human lung mast cells express muscarinic M3 receptors. Mouse lung mast cells store 5-HT intracellularly, and methacholine induces release of 5-HT from lung-derived mouse mast cells and Ca²⁺ flux in human LAD-2 mast cells.

CONCLUSIONS

Methacholine activates mast cells to release 5-HT, which by acting on 5-HT_2A receptors enhances bronchoconstriction and AHR. Thus, M3-directed asthma treatments like tiotropium may also act by targeting mast cells.

Macrophage and dendritic cell subset composition can distinguish endotypes in adjuvant-induced asthma mouse models

Asthma is a heterogeneous disease with neutrophilic and eosinophilic asthma as the main endotypes that are distinguished according to the cells recruited to the airways and the related pathology. Eosinophilic asthma is the treatment-responsive endotype, which is mainly associated with allergic asthma. Neutrophilic asthma is a treatment-resistant endotype, affecting 5-10% of asthmatics. Although eosinophilic asthma is well-studied, a clear understanding of the endotypes is essential to devise effective diagnosis and treatment approaches for neutrophilic asthma. To this end, we directly compared adjuvant-induced mouse models of neutrophilic (CFA/OVA) and eosinophilic (Alum/OVA) asthma side-by-side. The immune response in the inflamed lung was analyzed by multi-parametric flow cytometry and immunofluorescence. We found that eosinophilic asthma was characterized by a preferential recruitment of interstitial macrophages and myeloid dendritic cells, whereas in neutrophilic asthma plasmacytoid dendritic cells, exudate macrophages, and GL7+ activated B cells predominated. This differential distribution of macrophage and dendritic cell subsets reveals important aspects of the pathophysiology of asthma and holds the promise to be used as biomarkers to diagnose asthma endotypes.

Basophils and their effector molecules in allergic disorders

Basophils are the rarest granulocytes which represent <1% of peripheral blood leukocytes. Basophils bear several phenotypic similarities to tissue-resident mast cells and therefore had been erroneously considered as blood-circulating mast cells. However, recent researches have revealed that basophils play nonredundant roles in allergic inflammation, protective immunity against parasitic infections and regulation of innate and acquired immunity. Basophils are recruited to inflamed tissues and activated in an IgE-dependent or IgE-independent manner to release a variety of effector molecules. Such molecules, including IL-4, act on various types of cells and play versatile roles, including the induction and termination of allergic inflammation and the regulation of immune responses. Recent development of novel therapeutic agents has enabled us to gain further insights into basophil biology in human disorders. In this review, we highlight the recent advances in the field of basophil biology with a particular focus on the role of basophils in allergic inflammation. Further studies on basophils and their effector molecules will help us identify novel therapeutic targets for treating allergic disorders.

New inducible mast cell-deficient mouse model (Mcpt5/Cma1DTR)

Mast cell-deficient mice are helpful for understanding the roles of mast cells in vivo. To date, a dozen mouse models for mast cell deficiency have been reported. However, mice with a specific depletion of all populations of mast cells have not been reported. We generated knock-in mice, termed Mcpt5/Cma1^DTR mice, expressing human diphtheria toxin A (DT) receptor under the endogenous promoter of Mcpt5 (also known as Cma1), which encodes mouse mast cell protease-5. Flow cytometry and histological analysis showed that intraperitoneal injection of DT induced almost complete depletion of mast cells in heterozygote Mcpt5/Cma1^DTR/+ mice. The deletion rates of mast cells in peritoneal cavity, mesentery, abdominal skin, ear skin, and glandular stomach were 99.9%, 100%, 98.7%, 97.7%, and 100%, respectively. Passive cutaneous anaphylaxis reaction also revealed mast cell deficiency in ear skin after DT treatment. Other than mast cells, a small percentage of marginal zone B cells in Mcpt5/Cma1^DTR/+ mice were killed by DT treatment. In conclusion, the Mcpt5/Cma1^DTR/+ mouse model is valuable for achieving conditional depletion of all populations of mast cells without inducing a marked reduction in other cells.

Novel Strategies to Target Mast Cells in Disease

Mast cells (MCs) are versatile effector cells of the immune system, characterized by a large content of secretory granules containing a variety of inflammatory mediators. They are implicated in the host protection toward various external insults, but are mostly well known for their detrimental impact on a variety of pathological conditions, including allergic disorders such as asthma and a range of additional disease settings. Based on this, there is currently a large demand for therapeutic regimens that can dampen the detrimental impact of MCs in these respective pathological conditions. This can be accomplished by several strategies, including targeting of individual mediators released by MCs, blockade of receptors for MC-released compounds, inhibition of MC activation, limiting mast cell growth or by inducing mast cell apoptosis. Here, we review the currently available and emerging regimens to interfere with harmful mast cell activities in asthma and other pathological settings and discuss the advantages and limitations of such strategies.

Orally desensitized mast cells form a regulatory network with Treg cells for the control of food allergy

Oral immunotherapy (OIT) is an effective approach to controlling food allergy. Although the detailed molecular and cellular mechanisms of OIT are unknown currently, they must be understood to advance the treatment of allergic diseases in general. To elucidate the mechanisms of OIT, especially during the immunological transition from desensitization to allergy regulation, we generated a clinical OIT murine model and used it to examine immunological events of OIT. We found that in mice that completed OIT successfully, desensitized mast cells (MCs) showed functionally beneficial alterations, such as increased induction of regulatory cytokines and enhanced expansion of regulatory T cells. Importantly, these regulatory-T-cell-mediated inhibitions of allergic responses were dramatically decreased in mice lacking OIT-induced desensitized MC. Collectively, these findings show that the desensitization process modulates the activation of MCs, leading directly to enhanced induction of regulatory-T-cell expansion and promotion of clinical allergic unresponsiveness. Our results suggest that efficiently inducing regulatory MCs is a novel strategy for the treatment of allergic disease.

Mast Cells as Regulators of Adaptive Immune Responses in Food Allergy

Mast cells are a critical first line of defense against endogenous and environmental threats. Their participation in innate immunity is well characterized; activation of toll like receptors as well as receptors for complement, adenosine, and a host of other ligands leads to mast cell release of preformed mediators contained within granules along with newly synthesized arachidonic acid metabolites, cytokines, and chemokines. These confer protective effects including the induction of mucus secretion, smooth muscle contraction, and activation of common itch and pain sensations, all of which act to promote expulsion of noxious agents. While their innate immune role as sentinel cells is well established, recent research has brought into focus their separate but also critical function in adaptive immunity particularly in the setting of IgE mediated food allergies. Crosslinking of FcεR1, the high affinity receptor for IgE, when bound to IgE and antigen, triggers the release of the same factors and elicits the same physiologic responses that occur after activation by innate stimuli. Though IgE-activated mast cells are best known for their role in acute allergic reactions, including the most severe manifestation, anaphylaxis, accumulating evidence has suggested an immunoregulatory effect in T cell-mediated immunity, modulating the balance between type 2 immunity and tolerance. In this review, we outline how mast cells act as adjuvants for food antigen driven Th2 cell responses, while curtailing Treg function.

IgE and mast cells: The endogenous adjuvant

Mast cells and IgE are most familiar as the effectors of type I hypersensitivity reactions including anaphylaxis. It is becoming clear however that this pair has important immunomodulatory effects on innate and adaptive cells of the immune system. In this purview, they act as endogenous adjuvants to ignite evolving immune responses, promote the transition of allergic disease into chronic illness and disrupt the development of active mechanisms of tolerance to ingested foods. Suppression of IgE-mediated mast cell activation can be exerted by molecules targeting IgE, FcɛRI or signaling kinases including Syk, or by IgG antibodies acting via inhibitory Fcγ receptors. In 2015 we reviewed the evidence for the adjuvant functions of mast cells. This update includes the original text, incorporates some important developments in the field over the past five years and discusses how interventions targeting these pathways might have promise in the development of strategies to treat allergic disease.

[Th2 cells and macrophages induce novel type-I-hypersensitivity-like reaction]

In recent decades, many patients have been suffering from allergic rhinitis including Japanese cedar pollinosis, which is becoming a national disease in Japan. There is other upper airway intractable disease, called eosinophilic sinusitis. The elucidation of the pathogenesis of upper airway intractable disease is demanded for the development of novel therapies. Many researches about allergic pathogenesis have focused on IgE-mast cells pathway, however, there are the patients with allergic symptoms induced by non-IgE mediated mechanisms. The patients who show allergic rhinitis-like symptoms, such as sneezing, nasal discharge, and nasal clotting, without allergen-specific IgE, are diagnosed as non-allergic rhinitis. The precise mechanisms of non-allergic rhinitis are totally unclear. We have investigated the non-IgE mediated nasal symptoms, because the elucidation of non-IgE mediated mechanisms might lead to the elucidation of other upper airway intractable disease. We established antigen-specific Th2 cells transfer model and revealed the novel allergic mechanisms induced by Th2 cells, macrophages and endotoxin. Although Th2 cells play important roles in allergic diseases, the main function of Th2 cells are thought to produce Th2 cytokines, such as interleukin (IL)-4, IL-5, IL-13. We revealed the new functions of Th2 cells in allergic diseases. In addition, we found the novel histamine production mechanisms using in vitro macrophages and Th2 cells co-culture model. Both macrophages and Th2 cells produced histamine by the interaction through antigen. Our observations suggested the existence of the novel allergic mechanisms distinct from IgE-mast cells pathway.

A Rapid Shift from Chronic Hyperoxia to Normoxia Induces Systemic Anaphylaxis via Transient Receptor Potential Ankyrin 1 Channels on Mast Cells

Extensive activation of mast cells is the major switch that triggers systemic anaphylaxis, resulting in the subsequent release of anaphylactic mediators into circulation. We previously demonstrated that rapid changes in oxygen tension lead to mast cell degranulation, and the released tryptase triggers retinal angiogenesis in a murine oxygen-induced retinopathy model. However, whether a rapid shift from hyperoxia to normoxia (relative hypoxic stress) is a risk factor for systemic anaphylaxis remains unknown. In this study, we demonstrated that the relative hypoxia stress induces systemic mast cell activation via transient receptor potential ankyrin 1 (TRPA1) channels, which immediately leads to hypothermia and increased vascular permeability in adult mice. Although mast cell-deficient or TRPA1-deficient mice did not exhibit anaphylactic symptoms following a rapid sift to normoxia, preinjection with bone marrow-derived cultured mast cells (BMCMCs) derived from wild-type TRPA1-expressing mice restored anaphylactic responses. In addition, we found that the rapid reductions in oxygen tension in a culture atmosphere triggered the degranulation of BMCMCs derived from wild-type TRPA1-expressing mice but not that of BMCMCs derived from TRPA1-deficient mice. In human LAD2 mast cells, the relative hypoxic stress led to the degranulation, which was suppressed by the addition of a TRPA1 inhibitor. Gradual reductions from hyperoxia to normoxia led to no anaphylactic symptoms. Our results demonstrated that TRPA1-triggered mast cell degranulation is a novel pathway that induces anaphylactic shock without Ag-Ab reactions. These findings introduce a potential role for oxygen in inducing mast cell-dependent anaphylaxis and highlight the need to reconsider chronic pure oxygen therapy for anoxic diseases.

Immunobiology of Acquired Resistance to Ticks

Ticks are blood-sucking arthropods of great importance in the medical and veterinary fields worldwide. They are considered second only to mosquitos as vectors of pathogenic microorganisms that can cause serious infectious disorders, such as Lyme borreliosis and tick-borne encephalitis. Hard (Ixodid) ticks feed on host animals for several days and inject saliva together with pathogens to hosts during blood feeding. Some animal species can acquire resistance to blood-feeding by ticks after a single or repeated tick infestation, resulting in decreased weights and numbers of engorged ticks or the death of ticks in subsequent infestations. Importantly, this acquired tick resistance (ATR) can reduce the risk of pathogen transmission from pathogen-infected ticks to hosts. This is the basis for the development of tick antigen-targeted vaccines to forestall tick infestation and tick-borne diseases. Accumulation of basophils is detected in the tick re-infested skin lesion of animals showing ATR, and the ablation of basophils abolishes ATR in mice and guinea pigs, illustrating the critical role for basophils in the expression of ATR. In this review article, we provide a comprehensive overview of recent advances in our understanding of the cellular and molecular mechanisms responsible for the development and manifestation of ATR, with a particular focus on the role of basophils.

Is There a Role for Basophils in Cancer?

Basophils were identified in human peripheral blood by Paul Ehrlich over 140 years ago. Human basophils represent <1% of peripheral blood leukocytes. During the last decades, basophils have been described also in mice, guinea pigs, rabbits, and monkeys. There are many similarities, but also several immunological differences between human and mouse basophils. There are currently several strains of mice with profound constitutive or inducible basophil deficiency useful to prove that these cells have specific roles in vivo. However, none of these mice are solely and completely devoid of all basophils. Therefore, the relevance of these findings to humans remains to be established. It has been known for some time that basophils have the propensity to migrate into the site of inflammation. Recent observations indicate that tissue resident basophils contribute to lung development and locally promote M2 polarization of macrophages. Moreover, there is increasing evidence that lung-resident basophils exhibit a specific phenotype, different from circulating basophils. Activated human and mouse basophils synthesize restricted and distinct profiles of cytokines. Human basophils produce several canonical (e.g., VEGFs, angiopoietin 1) and non-canonical (i.e., cysteinyl leukotriene C₄) angiogenic factors. Activated human and mouse basophils release extracellular DNA traps that may have multiple effects in cancer. Hyperresponsiveness of basophils has been demonstrated in patients with JAK2^V617F-positive polycythemia vera. Basophils are present in the immune landscape of human lung adenocarcinoma and pancreatic cancer and can promote inflammation-driven skin tumor growth. The few studies conducted thus far using different models of basophil-deficient mice have provided informative results on the roles of these cells in tumorigenesis. Much more remains to be discovered before we unravel the hitherto mysterious roles of basophils in human and experimental cancers.

IL33 contributes to diesel pollution-mediated increase in experimental asthma severity

BACKGROUND

Exposure to traffic pollution, notably diesel exhaust particles (DEP), increases risk for asthma and asthma exacerbations. The contribution of cytokines generated by stressed lung epithelial cells (IL25, IL33, TSLP) to DEP-induced asthma severity remains poorly understood.

METHODS

BALB/c mice were exposed intratracheally once to DEP or 9 times over 3-weeks to either saline, DEP, and/or house dust mite extract (HDM). Airway hyper-responsiveness (AHR), pulmonary inflammation, and T-cell subsets were assessed 24 hours after the last exposure in mice sufficient and deficient for the IL33 receptor ST2.

RESULTS

DEP exposure induces oxidative stress, IL6, neutrophils and pulmonary accumulation of IL33, but not IL25 or TSLP or other features of allergic disease. When mice are co-exposed to DEP and low doses of HDM, DEP increases IL33 lung levels and Th2 responses. ST2 deficiency partially protected mice from HDM + DEP induced AHR in association with decreased type 2 inflammation and lung levels of IL5⁺ IL17A⁺ co-producing T-cells. Upon in vitro HDM challenge of lung cells from HDM ± DEP exposed ST2^-/- mice, secretion of IL5, IL13, IL6 and IL17A was abrogated by a mechanism involving IL33 signaling in both dendritic cells and T-cells. HDM + DEP exposed bone marrow derived dendritic cells and IL33 pulsed BMDC promote a mixed Th2/Th17 response that was dependent on ST2 expression by CD4⁺ T-cells.

CONCLUSION

IL33 contributes to DEP mediated increase in allergen-induced Th2 inflammation and AHR in a mouse model of severe steroid resistant asthma, potentially through the accumulation of pathogenic IL5⁺ IL17A⁺ CD4⁺ effector T-cells.

Role of myeloid cells in the regulation of group 2 innate lymphoid cell-mediated allergic inflammation

Group 2 innate lymphoid cells (ILC2s) are an important component of the innate immune system that execute important effector functions at barrier surfaces, such as lung and skin. Like T helper type 2 cells, ILC2s are able to release high amounts of type 2 cytokines that are essential in inducing allergic inflammation and eliminating helminth infections. The past few years have contributed to our better understanding of the interactions between ILC2s and other cells of the immune system via soluble factors or in a cell-cell contact manner. Myeloid cells, including mononuclear leukocytes and polymorphonuclear leukocytes, are excellent sensors of tissue damage and infection and can influence ILC2 responses in the process of allergic inflammation. In this review, we summarize recent insights on how myeloid cell subsets regulate ILC2 activation with focus on soluble factors in the context of allergic inflammation.

Circulating mast cell progenitors correlate with reduced lung function in allergic asthma

BACKGROUND

Studies using mouse models have revealed that mast cell progenitors are recruited from the blood circulation to the lung during acute allergic airway inflammation. The discovery of a corresponding human mast cell progenitor population in the blood has enabled to study the relation of circulating mast cell progenitors in clinical settings.

OBJECTIVES

To explore the possible association between the frequency of mast cell progenitors in the blood circulation and allergic asthma, we assessed the relation of this recently identified cell population with asthma outcomes and inflammatory mediators in allergic asthmatic patients and controls.

METHODS

Blood samples were obtained, and spirometry was performed on 38 well-controlled allergic asthmatic patients and 29 controls. The frequency of blood mast cell progenitors, total serum IgE and 180 inflammation- and immune-related plasma proteins were quantified.

RESULTS

Allergic asthmatic patients and controls had a similar mean frequency of blood mast cell progenitors, but the frequency was higher in allergic asthmatic patients with reduced FEV₁ and PEF (% of predicted) as well as in women. The level of fibroblast growth factor 21 (FGF-21) correlated positively with the frequency of mast cell progenitors, independent of age and gender, and negatively with lung function. The expression of FcεRI on mast cell progenitors was higher in allergic asthmatic patients and correlated positively with the level of total IgE in the controls but not in the asthmatic patients.

CONCLUSION

Elevated levels of circulating mast cell progenitors are related to reduced lung function, female gender and high levels of FGF-21 in young adults with allergic asthma.

Type 2 immunity in the skin and lungs

There has been extensive progress in understanding the cellular and molecular mechanisms of inflammation and immune regulation in allergic diseases of the skin and lungs during the last few years. Asthma and atopic dermatitis (AD) are typical diseases of type 2 immune responses. interleukin (IL)-25, IL-33, and thymic stromal lymphopoietin are essential cytokines of epithelial cells that are activated by allergens, pollutants, viruses, bacteria, and toxins that derive type 2 responses. Th2 cells and innate lymphoid cells (ILC) produce and secrete type 2 cytokines such as IL-4, IL-5, IL-9, and IL-13. IL-4 and IL-13 activate B cells to class-switch to IgE and also play a role in T-cell and eosinophil migration to allergic inflammatory tissues. IL-13 contributes to maturation, activation, nitric oxide production and differentiation of epithelia, production of mucus as well as smooth muscle contraction, and extracellular matrix generation. IL-4 and IL-13 open tight junction barrier and cause barrier leakiness in the skin and lungs. IL-5 acts on activation, recruitment, and survival of eosinophils. IL-9 contributes to general allergic phenotype by enhancing all of the aspects, such as IgE and eosinophilia. Type 2 ILC contribute to inflammation in AD and asthma by enhancing the activity of Th2 cells, eosinophils, and their cytokines. Currently, five biologics are licensed to suppress type 2 inflammation via IgE, IL-5 and its receptor, and IL-4 receptor alpha. Some patients with severe atopic disease have little evidence of type 2 hyperactivity and do not respond to biologics which target this pathway. Studies in responder and nonresponder patients demonstrate the complexity of these diseases. In addition, primary immune deficiency diseases related to T-cell maturation, regulatory T-cell development, and T-cell signaling, such as Omenn syndrome, severe combined immune deficiencies, immunodysregulation, polyendocrinopathy, enteropathy, X-linked syndrome, and DOCK8, STAT3, and CARD11 deficiencies, help in our understanding of the importance and redundancy of various type 2 immune components. The present review aims to highlight recent advances in type 2 immunity and discuss the cellular sources, targets, and roles of type 2 mechanisms in asthma and AD.

ILC2s and Basophils Team Up to Orchestrate IL-33-Induced Atopic Dermatitis

In this issue, Imai et al. (2019) provide new insights into the pathophysiology of AD-like inflammation using their model (Imai et al., 2013) and ask how ILC2s and basophils contribute to the IL-33-induced AD-like inflammation. Their findings show that continuous expression of IL-33 in keratinocytes is sufficient to cause AD-like inflammation in mice, and that this occurrence is largely independent of adaptive immune cells and is mediated by basophils and ILC2s.

Ginger extract versus Loratadine in the treatment of allergic rhinitis: a randomized controlled trial

BACKGROUND

Allergic rhinitis (AR) is a non-infectious immune disease and incidents of the disease has continuously increased in Thailand. Ginger, a Thai herb, is used in food and Thai traditional medicine. This study was designed to assess efficacy and safety of ginger extract in comparison with loratadine for AR treatment.

METHODS

AR patients were treated with ginger extract 500 mg (n = 40) against those treated with loratadine 10 mg (n = 40) in a randomized, double-blind, controlled trial for 3 and 6 weeks. The efficacy was evaluated from clinical examinations i.e. total nasal symptom scores (TNSS), cross-sectional area of the nasal cavity with acoustic rhinometry (ARM) and rhinoconjunctivitis quality of life questionnaire (RQLQ). The safety of treatment was measured by blood pressure, blood analysis and history-taking for side effects.

RESULTS

The results showed both ginger extract and loratadine treated groups significantly decreased TNSS scores but there was no significant difference between the two groups. In acoustic rhinometry measurement, the ginger treated group significantly gradually increased the estimated volume of the nasal cavity and decreased distances from the nostril, but the loratadine treated group did not cause a change. Both groups gave significantly improvement in every aspect of the RQLQ at third weeks. The treatment with ginger extract was as safe as loratadine as shown by renal and liver function results obtained from blood analysis. Both treatments had no effect on blood pressure of the patients.

CONCLUSIONS

The ginger extract is as good as loratadine in improving nasal symptoms and quality of life in AR patients. However, ginger extract caused less side effects especially, drowsiness, fatigue, dizziness and constipation. Therefore, the ginger extract could be used as alternative treatment for patients with AR.

TRIAL REGISTRATION

Registered with ClinicalTrials.gov (Registration number: NCT02576808) on 15 October 2015.

Development of multiple features of antigen-induced asthma pathology in a new strain of mast cell deficient BALB/c-KitW-sh/W-sh mice

Mast cell-deficient mice are widely used to identify and quantify contributions of mast cells to diverse biological responses in vivo, including allergic inflammation. However, despite the fact that scores of genes have been identified as modifiers of allergic inflammation, most mast cell-deficient models have been available only on a single genetic background. We transferred the KitW-sh allele onto the BALB/c background to generate BALB/c mast cell-deficient mice (BALB/c-KitW-sh/W-sh). BALB/c-KitW-sh/W-sh mice have dramatically reduced mast cell numbers (0-2% of wild type) in all tissues examined, as well as subtle hematologic differences from the corresponding wild type mice, including splenomegaly with evidence of increased splenic hematopoiesis. We examined in BALB/c-KitW-sh/W-sh mice models of allergic inflammation that are substantially diminished in C57BL/6-KitW-sh/W-sh mast cell-deficient mice. In a model of acute allergic inflammation, i.e., IgE-dependent passive cutaneous anaphylaxis, both ear swelling and leukocyte infiltration were largely or entirely absent in BALB/c-KitW-sh/W-sh mice. In contrast, in two different models of allergic airway inflammation, airway hyperresponsiveness, lung inflammation, and airway remodeling developed robustly in mast cell-deficient BALB/c-KitW-sh/W-sh mice. These results support the conclusion that the importance of mast cell contributions in various models of allergic inflammation may be at least partially determined by genetic background.

The Role of Mast Cells in Bone Metabolism and Bone Disorders

Mast cells (MCs) are important sensor and effector cells of the immune system that are involved in many physiological and pathological conditions. Increasing evidence suggests that they also play an important role in bone metabolism and bone disorders. MCs are located in the bone marrow and secrete a wide spectrum of mediators, which can be rapidly released upon activation of mature MCs following their differentiation in mucosal or connective tissues. Many of these mediators can exert osteocatabolic effects by promoting osteoclast formation [e.g., histamine, tumor necrosis factor (TNF), interleukin-6 (IL-6)] and/or by inhibiting osteoblast activity (e.g., IL-1, TNF). By contrast, MCs could potentially act in an osteoprotective manner by stimulating osteoblasts (e.g., transforming growth factor-β) or reducing osteoclastogenesis (e.g., IL-12, interferon-γ). Experimental studies investigating MC functions in physiological bone turnover using MC-deficient mouse lines give contradictory results, reporting delayed or increased bone turnover or no influence depending on the mouse model used. By contrast, the involvement of MCs in various pathological conditions affecting bone is evident. MCs may contribute to the pathogenesis of primary and secondary osteoporosis as well as inflammatory disorders, including rheumatoid arthritis and osteoarthritis, because increased numbers of MCs were found in patients suffering from these diseases. The clinical observations could be largely confirmed in experimental studies using MC-deficient mouse models, which also provide mechanistic insights. MCs also regulate bone healing after fracture by influencing the inflammatory response toward the fracture, vascularization, bone formation, and callus remodeling by osteoclasts. This review summarizes the current view and understanding of the role of MCs on bone in both physiological and pathological conditions.

Basophils in Tumor Microenvironment and Surroundings

Basophils represent approximately 1% of human peripheral blood leukocytes. Their effector functions were initially appreciated in the 1970s when basophils were shown to express the high-affinity receptor (FcεRI) for IgE and to release proinflammatory mediators (histamine and cysteinyl leukotriene C₄) and immunoregulatory cytokines (i.e., IL-4 and IL-13). Basophils in the mouse were subsequently identified and immunologically characterized. There are many similarities but also several differences between human and mouse basophils. Basophil-deficient mice have enabled to examine the in vivo roles of basophils in several immune disorders and, more recently, in tumor immunity. Activated human basophils release several proangiogenic molecules such as vascular endothelial growth factor-A (VEGF-A), vascular endothelial growth factor-B (VEGF-B), CXCL8, angiopoietin 1 (ANGPT1), and hepatocyte growth factor (HGF). On the other side, basophils can exert anti-tumorigenic effects by releasing granzyme B, TNF-α, and histamine. Circulating basophils have been associated with certain human hematologic (i.e., chronic myeloid leukemia) and solid tumors. Basophils have been found in tumor microenvironment (TME) of human lung adenocarcinoma and pancreatic cancer. Basophils played a role in melanoma rejection in basophil-deficient mouse model. By contrast, basophils appear to play a pro-tumorigenic role in experimental and human pancreatic cancer. In conclusion, the roles of basophils in experimental and human cancers have been little investigated and remain largely unknown. The elucidation of the roles of basophils in tumor immunity will demand studies on increasing complexity beyond those assessing basophil density and their microlocalization in TME. There are several fundamental questions to be addressed in experimental models and clinical studies before we understand whether basophils are an ally, adversary, or even innocent bystanders in cancers.

Selective suppression of oral allergen-induced anaphylaxis by Allergin-1 on basophils in mice

Mast cells (MCs) play a critical role in oral allergen-induced anaphylaxis. However, the contribution of basophils to the anaphylaxis remains unclear. The inhibitory immunoreceptor Allergin-1 is highly expressed on MCs and basophils and inhibits FcεRI-mediated signaling in MCs. Here, we show that Allergin-1-deficient (Milr1−/−) mice developed more severe hypothermia, a higher mortality rate and a greater incidence of diarrhea than did wild-type (WT) mice in an oral ovalbumin (OVA)-induced food allergy model. MC-deficient Mas–TRECK mice, which had been reconstituted with either WT or Milr1−/− bone marrow-derived cultured MCs, did not develop hypothermia in this food allergy model. On the other hand, depletion of basophils by injection of anti-CD200R3 antibody rescued Milr1−/− mice from lethal hypothermia but not from diarrhea. In vitro analyses demonstrated that Allergin-1 inhibits IgE-dependent activation of both human and mouse basophils. Thus, Allergin-1 on basophils selectively suppresses oral allergen-induced anaphylaxis.

The emerging role of mast cell proteases in asthma

It is now well established that mast cells (MCs) play a crucial role in asthma. This is supported by multiple lines of evidence, including both clinical studies and studies on MC-deficient mice. However, there is still only limited knowledge of the exact effector mechanism(s) by which MCs influence asthma pathology. MCs contain large amounts of secretory granules, which are filled with a variety of bioactive compounds including histamine, cytokines, lysosomal hydrolases, serglycin proteoglycans and a number of MC-restricted proteases. When MCs are activated, e.g. in response to IgE receptor cross-linking, the contents of their granules are released to the exterior and can cause a massive inflammatory reaction. The MC-restricted proteases include tryptases, chymases and carboxypeptidase A3, and these are expressed and stored at remarkably high levels. There is now emerging evidence supporting a prominent role of these enzymes in the pathology of asthma. Interestingly, however, the role of the MC-restricted proteases is multifaceted, encompassing both protective and detrimental activities. Here, the current knowledge of how the MC-restricted proteases impact on asthma is reviewed.

Abl family tyrosine kinases govern IgG extravasation in the skin in a murine pemphigus model

The pathway of homeostatic IgG extravasation is not fully understood, in spite of its importance for the maintenance of host immunity, the management of autoantibody-mediated disorders, and the use of antibody-based biologics. Here we show in a murine model of pemphigus, a prototypic cutaneous autoantibody-mediated disorder, that blood-circulating IgG extravasates into the skin in a time- and dose-dependent manner under homeostatic conditions. This IgG extravasation is unaffected by depletion of Fcγ receptors, but is largely attenuated by specific ablation of dynamin-dependent endocytic vesicle formation in blood endothelial cells (BECs). Among dynamin-dependent endocytic vesicles, IgG co-localizes well with caveolae in cultured BECs. An Abl family tyrosine kinase inhibitor imatinib, which reduces caveolae-mediated endocytosis, impairs IgG extravasation in the skin and attenuates the murine pemphigus manifestations. Our study highlights the kinetics of IgG extravasation in vivo, which might be a clue to understand the pathological mechanism of autoantibody-mediated autoimmune disorders.

How antibody reaches tissues from circulation is critical for understanding antibody-mediated immunity. Here the authors show that IgG extravasation in the skin is mediated by endothelial caveolin transport independently of FcR, and is targetable by imatinib, which reduces IgG-dependent pathology in a mouse model of pemphigus.

The Basoph8 Mice Enable an Unbiased Detection and a Conditional Depletion of Basophils

Basophils are granulocytes involved in parasite immunity and allergic diseases, known for their potent secretion of type 2 cytokines. Identifying their functions has proven to be controversial due to their relative rarity and their complex lineage phenotype. Here, we show that the expression of basophils lineage markers CD200R3 and FcεRIα is highly variable in inflammatory settings and hinders basophils identification by flow cytometry across multiple disease states or tissues. Fluorophore-conjugated antibody staining of these lineage markers strongly activates basophil type 2 cytokine expression, and represents a potential bias for coculture or in vivo transfer experiments. The Basoph8 is a mouse model where basophils specifically express a strong fluorescent reporter and the Cre recombinase. Basophils can be identified and FACS sorted unambiguously by their expression of the enhanced yellow fluorescent protein (eYFP) in these mice. We show that the expression of the eYFP is robust in vivo during inflammation, and in vitro on living basophils for at least 72 h, including during the induction of anaphylactoid degranulation. We bred and characterized the Basoph8xiDTR mice, in which basophils specifically express eYFP and the simian diphtheria toxin receptor (DTR). This model enables basophils conditional depletion relatively specifically ex vivo and in vivo during allergic inflammation and their detection as eYFP+ cells. In conclusion, we report underappreciated benefits of the commercially available Basoph8 mice to study basophils function.

Allergin-1 on mast cells suppresses house dust mite-induced airway hyperresponsiveness in mice

Although airway hyperresponsiveness (AHR) is a prominent feature of asthma, how it is regulated remains incompletely understood. Allergin-1, an inhibitory immunoglobulin-like receptor containing an immunoreceptor tyrosine-based inhibitory motif (ITIM), is expressed on human and mouse mast cells (MCs) and inhibits high-affinity receptor for IgE (FcεRI)-mediated signaling. Using MC-deficient KitW-sh/W-sh mice and Mas-TRECK mice, which carries a diphtheria toxin (DT)-induced MC deletion system based on il4 enhancer elements, we demonstrate here that MCs are involved in the induction of house dust mite (HDM)-induced AHR. Further, we show that MCs deficient in Allergin-1 exacerbated HDM-induced AHR, but had no effect on airway inflammation. In vitro analysis demonstrated that Allergin-1 inhibited anti-HDM allergen antibody-dependent HDM allergen-mediated degranulation by MCs. Thus, Allergin-1 on MCs plays an important role in the regulation of HDM-induced AHR.

Mast cells play role in wound healing through the ZnT2/GPR39/IL-6 axis

Zinc (Zn) is an essential nutrient and its deficiency causes immunodeficiency and skin disorders. Various cells including mast cells release Zn-containing granules when activated; however, the biological role of the released Zn is currently unclear. Here we report our findings that Zn transporter ZnT2 is required for the release of Zn from mast cells. In addition, we found that Zn and mast cells induce IL-6 production from inflammatory cells such as skin fibroblasts and promote wound healing, a process that involves inflammation. Zn induces the production of a variety of pro-inflammatory cytokines including IL-6 through signaling pathways mediated by the Zn receptor GPR39. Consistent with these findings, wound healing was impaired in mice lacking IL-6 or GPR39. Thus, our results show that Zn and mast cells play a critical role in wound healing through activation of the GPR39/IL-6 signaling axis.