The ingenious mast cell: Contemporary insights into mast cell behavior and function

Mechanisms and implications of histamine-induced reactions and complications

Histamine, classified as a biogenic amine, plays a crucial role in both pro-inflammatory and immune regulatory processes, thereby establishing itself as a key mediator in allergic diseases and immune responses. This review provides an exhaustive analysis of the structure, function, and regulation of histamine, with particular emphasis on its interaction with four receptor subtypes: histamine H1 receptor (H1R), histamine H2 receptor (H2R), histamine H3 receptor (H3R), and histamine H4 receptor (H4R), all of which are instrumental in mediating a variety of physiological processes, including neurotransmitter release, modulation of immune responses, and gastric acid secretion. The review explores intracellular signaling pathways mediated by the activation of these receptors, highlighting the complex cascades involved in immediate- and delayed-type hypersensitivity reactions. It also examines the broad spectrum of histamine-induced complications, focusing on their effects on the gastrointestinal, cardiovascular, respiratory, and central nervous systems, and emphasizes histamine's potential to cause vascular dysfunction and other pathological changes. Furthermore, the role of histamine in inflammation and immune responses is explored, particularly in the context of allergic diseases such as asthma, allergic rhinitis, and atopic dermatitis. The review also covers pharmacological interventions targeting histamine receptors, including the use of antihistamines and mast cell stabilizers, which are critical for the treatment of symptoms and the inhibition of the progression of histamine-related conditions. Finally, the review addresses emerging research and future directions, identifying potential areas for innovation and improved therapeutic strategies. This comprehensive overview not only deepens understanding of histamine's multifaceted roles in health and disease, but also underscores the importance of developing advanced diagnostic tools and targeted treatments for histamine-associated disorders.

IgE-Mediated Activation of Mast Cells and Basophils in Health and Disease

Type 2-mediated immune responses protect the body against environmental threats at barrier surfaces, such as large parasites and environmental toxins, and facilitate the repair of inflammatory tissue damage. However, maladaptive responses to typically nonpathogenic substances, commonly known as allergens, can lead to the development of allergic diseases. Type 2 immunity involves a series of prototype TH2 cytokines (IL-4, IL-5, IL-13) and alarmins (IL-33, TSLP) that promote the generation of adaptive CD4+ helper Type 2 cells and humoral products such as allergen-specific IgE. Mast cells and basophils are integral players in this network, serving as primary effectors of IgE-mediated responses. These cells bind IgE via high-affinity IgE receptors (FcεRI) expressed on their surface and, upon activation by allergens, release a variety of mediators that regulate tissue responses, attract and modulate other inflammatory cells, and contribute to tissue repair. Here, we review the biology and effector mechanisms of these cells, focusing primarily on their role in mediating IgE responses in both physiological and pathological contexts.

Isolation, Characterization, and Anti-Allergic Evaluation of Phytochemicals from Wikstroemia trichotoma

Background/Objectives: The Wikstroemia genus has been traditionally used in Asia to treat various ailments, including parotitis, pneumonia, and pertussis. These plants contain many bioactive compounds, including flavonoids, coumarins, and lignans. This study investigates the chemical components of a MeOH extract of the aerial parts of Wikstroemia trichotoma (Thunb.) Makino and evaluates their anti-inflammatory and anti-allergic effects in vitro. Methods: Chromatographic techniques, spectroscopic analysis, and the literature were used to isolate compounds from the branches and leaves of W. trichotoma. IL-4 mRNA and β-hexosaminidase levels were assessed by performing assays on RBL-2H3 cells to evaluate anti-inflammatory and anti-allergic potential. Results: Forty-two compounds were isolated from the W. trichotoma extract, and the flavanones trichotocinol A and B were newly identified. Screening of isolated compounds showed that several significantly inhibited DNP-BSA-induced β-hexosaminidase release by 10.0-58.0% and PMA/ionomycin-induced IL-4 mRNA expression by 25.3-71.7% versus negative controls. In addition, trichotocinol A reduced IL-4 mRNA expression by 31.9%. Conclusions: The discovery of these new compounds contributes to our understanding of the bioactive properties of W. trichotoma and suggests their potential use as natural therapeutic agents for inflammatory disorders.

Mastocytosis

Mastocytosis is a spectrum of clonal myeloid disorders defined by abnormal growth and accumulation of mast cells in various organ systems. The disease is divided into cutaneous mastocytosis, systemic mastocytosis (SM) and mast cell sarcoma. SM is further categorized into several non-advanced and advanced forms. The prognosis of cutaneous mastocytosis and non-advanced SM is mostly favourable, whereas prognosis and survival in advanced SM and mast cell sarcoma are poor. During the past 15 years, major advances have been made in the diagnosis, prognosis and management of patients with mast cell neoplasms. Management of mastocytosis consists of symptomatic therapy, including anti-mast cell mediator drugs, and cytoreductive agents for patients with advanced disease and selected individuals with non-advanced disease, as well as recognition and prevention of comorbidities such as osteoporosis and anaphylaxis. The preclinical and clinical development of KIT-D816V-targeting drugs, such as midostaurin or avapritinib, mark a milestone in improving management, the quality of life and survival in patients with SM. These agents induce major responses or even remission in people with advanced SM and lead to rapid improvement of mediator-related symptoms and quality of life in symptomatic patients.

The Neuroimmune Axis in Gastrointestinal Disorders - An Underrecognized Problem

PURPOSE OF REVIEW

We present an introduction to the neuroimmune axis with a focus on the gastrointestinal system, its role in numerous chronic multisystem disorders, and emerging tools and therapies to diagnose and treat these conditions.

RECENT FINDINGS

There have recently been tremendous breakthroughs in our understanding of how the nervous, immune, and endocrine systems, as well as the extracellular matrix and microbiota, interact within the gastrointestinal system to modulate health and disease.

Managing a Burning Face: Clinical Manifestations and Therapeutic Approaches for Neurogenic Rosacea

Rosacea is a common chronic inflammatory condition primarily affecting middle-aged women. It presents with flushing, erythema, telangiectasia, papules, pustules, phymatous changes, and ocular involvement. Although typically grouped into four subtypes-erythematotelangiectatic, papulopustular, ocular, and phymatous-overlapping features often favor a phenotypic diagnostic approach. Neurogenic rosacea (NR) has emerged as a distinct subgroup featuring distinguishing features such as peripheral facial erythema, severe burning and stinging sensations, and resistance to standard rosacea therapies. Recent insights into the pathophysiology of NR propose neural dysregulation as the main driver of the condition. Specifically, the activation of TRP channels at cutaneous sensory nerve endings in the dermis triggers the release of vasoactive peptides, driving neuroinflammation and resulting in burning and stinging. Additionally, there is a marked association with neuropsychiatric comorbidities, which would further mediate the pathogenesis of the condition. In line with this pathophysiological model, NR often fails to respond to conventional rosacea treatments. Instead, patients benefit more from antidepressants and neuroleptic agents that help modulate neuronal activity and alleviate symptoms. This review explores and summarizes the scientific evidence regarding the new insights on disease pathogenesis, clinical manifestations, and proposed treatments for NR.

The Role of Mast Cells in the Development and Advancement of Endometriosis

Endometriosis is a medical condition identified by the presence of endometrium-like tissue outside the uterus. This condition is known to result in symptoms such as frequent pelvic pain, infertility, and irregularities in the menstrual cycle. The development of endometriosis is complex and, involving abnormal body responses, hormonal imbalances, and genetic predispositions. Although endometriosis is common and affects quality of life, its mechanisms of development and progression are not fully understood. Mast cells (MCs), a type of immune cell, are renowned for their involvement in allergic and inflammatory responses. These cells are essential in the modulation of the immune system and the inflammatory process through the secretion of different mediators like histamine, cytokines, and proteases. In recent years, MCs have been shown to play a role in the pathogenesis of many diseases, including endometriosis. This article explores the relationship between MCs and endometriosis, including disease development, pain perception, angiogenesis, and other important processes. It elucidates how MCs, via their mediators, actively participate in the pathogenesis of endometriosis and the associated inflammatory environment. Moreover, the research emphasizes the potential of targeting MCs as a therapeutic approach for treating endometriosis. Insight into the interplay between endometriosis and MCs holds promise for developing innovative therapeutic strategies to manage this condition effectively.

Mild-to-moderate psoriasis is associated with subclinical inflammation in the duodenum and a tendency of disturbed intestinal barrier

Psoriasis is a chronic skin disease occasionally associated with abdominal symptoms and IBD. We aimed to characterize intestinal immune cells and the integrity of the intestinal barrier in psoriasis. Biopsies from the duodenum and colon were analyzed by flow cytometry and immunohistochemistry for the presence and activation status of different immune cell populations. Intestinal permeability was measured using Ussing chambers. Proinflammatory markers were analyzed in fecal and blood samples using ELISA. The intestinal level of inflammatory mediators was assessed using a multiplex proximity extension assay. We found an increased density of intestinal eosinophils, mast cells, macrophages, and CD8+ T-cells in psoriasis; eosinophils, macrophages, and CD8+ T-cells expressed activation markers. Half of the psoriasis patients showed increased permeability across the duodenum, correlating with increased mucosal IL-17A, IL-13, IL-2, and IL-20, and with gastrointestinal symptoms. Our findings reveal that psoriasis is associated with low-grade intestinal inflammation, which may contribute to abdominal symptoms in these patients and possibly set the stage for the development of intestinal disease.

Oral Exposure to Clostridium difficile Toxin A Aggravates Food Allergy by Intestinal Barrier Destruction, Mast Cell Activation, and Th2-Biased Immune Responses

Food allergy is a complex disease, with multiple environmental factors involved. Considering the regulatory effect of Clostridium difficile toxin A (Tcd A) on biological processes of allergic reactions, the role of oral exposure to Tcd A on food allergy was investigated. The intestinal permeability and β-hexosaminidase were promoted by Tcd A using the in vitro Caco-2 and HT-29 cells coculture monolayer and bone marrow-derived mast cell (MCs) degranulation model. When the sensitized mice were challenged with OVA and 4 μg of Tcd A, specific IgE and MCPT-1 levels were increased. The results of flow cytometry and ELISA demonstrated that the Th1 and Treg cells were reduced and higher IL-5 was induced. The jejunum staining showed that villus injury and MC accumulation were aggravated. All of the findings demonstrated that Tcd A could aggravate food allergy, and the orally exposed risk factors aggravating food allergy warrant increasing attention.

A biomimetic skin microtissue biosensor for the detection of fish parvalbumin

In this paper, a biomimetic skin microtissue biosensor was developed based on three-dimensional (3D) bioprinting to precisely and accurately determine fish parvalbumin (FV). Based on the principle that allergens stimulate cells to produce ONOO- (peroxynitrite anion), a screen-printed electrode for the detection nanomolar level ONOO- was innovatively prepared to indirectly detect FV based on the level of ONOO- release. Gelatin methacryloyl (GelMA), RBL-2H3 cells, and MS1 cells were used as bio-ink for 3D bioprinting. The high-throughput and standardized preparation of skin microtissue was achieved using stereolithography 3D bioprinting technology. The printed skin microtissues were put into the self-designed 3D platform that integrated cell culture and electrochemical detection. The experimental results showed that the sensor could effectively detect FV when the optimized ratio of RBL-2H3 to MS1 cells and allergen stimulation time were 2:8 and 2 h, respectively. The linear detection range was 0.125-3.0 μg/mL, and the calculated lowest detection limit was 0.122 μg/mL. In addition, the sensor had excellent selectivity, specificity, stability, and reliability. Thus, this study successfully constructed a biomimetic skin microtissue electrochemical sensor for PV detection.

Mast cells: key players in digestive system tumors and their interactions with immune cells

Mast cells (MCs) are critical components of both innate and adaptive immune processes. They play a significant role in protecting human health and in the pathophysiology of various illnesses, including allergies, cardiovascular diseases and autoimmune diseases. Recent studies in tumor-related research have demonstrated that mast cells exert a substantial influence on tumor cell behavior and the tumor microenvironment, exhibiting both pro- and anti-tumor effects. Specifically, mast cells not only secrete mediators related to pro-tumor function such as trypsin-like enzymes, chymotrypsin, vascular endothelial cell growth factor and histamine, but also mediators related to anti-tumor progression such as cystatin C and IL-17F. This dual role of mast cells renders them an under-recognized but very promising target for tumor immunotherapy. Digestive system tumors, characterized by high morbidity and associated mortality rates globally, are increasingly recognized as a significant healthcare burden. This paper examines the influence of mast cell-derived mediators on the development of tumors in the digestive system. It also explores the prognostic significance of mast cells in patients with various gastrointestinal cancers at different stages of the disease. Additionally, the article investigates the interactions between mast cells and immune cells, as well as the potential relationships among intratumoral bacteria, immune cells, and mast cell within digestive system microenvironment. The aim is to propose new strategies for the immunotherapy of digestive system tumors by targeting mast cells.

Prolonged inhibition of intratumoral mast cells enhances efficacy of low-dose antiangiogenic therapy

Low-dose antiangiogenic therapies have demonstrated the ability to enhance normalization of tumor vessels, consequently improving hypoxia levels, drug delivery, and promoting anticancer immune responses. Mast cells have been identified as contributors to resistance against antiangiogenic therapy and facilitators of abnormal neoangiogenesis. In this study, we demonstrate that by simultaneously targeting intratumoral mast cells with Imatinib and administering low-dose anti-VEGFR2 therapy, antitumor efficacy can be enhanced in preclinical models. Thus, combinatory treatment overcomes therapy resistance, while concurrently promoting tumor vessel normalization. Notably, histomorphometric analysis of tumor sections revealed that vessel perfusion could be improved through mast cell inhibition and, despite a significantly reduced microvessel density, the combination treatment did not result in elevated tumor hypoxia levels compared to anti-VEGFR2 therapy alone. Short-term Imatinib application effectively increased antitumor efficacy, and by prolonging the application of Imatinib tumor vessel normalization was additionally improved. The combination of mast cell depletion and antiangiogenic treatments has not been investigated in detail and promises to help overcoming therapy resistance. Further studies will be required to explore their impact on other treatment approaches, and subsequently to validate these findings in a clinical setting.

Functional MRGPRX2 expression on peripheral blood-derived human mast cells increases at low seeding density and is suppressed by interleukin-9 and fetal bovine serum

Primary human mast cells (MC) obtained through culturing of blood-derived MC progenitors are the preferred model for the ex vivo study of MRGPRX2- vs. IgE-mediated MC activation. In order to assess the impact of culture conditions on functional MRGPRX2 expression, we cultured CD34+-enriched PBMC from peripheral whole blood (PB) and buffy coat (BC) samples in MethoCult medium containing stem cell factor (SCF) and interleukin (IL)-3, modified through variations in seeding density and adding or withholding IL-6, IL-9 and fetal bovine serum (FBS). Functional expression of MRGPRX2 was assessed after 4 weeks via flow cytometry. We found similar proportions of CD34+ MC-committed progenitors in BC and PB. Higher seeding densities (≥ 1x105 cells/mL) and exposure to IL-9 and FBS suppressed functional MRGPRX2 expression at 4 weeks, while leaving MC yield largely unaffected. IL-6 had no impact on MRGPRX2 expression. MRGPRX2-expressing MC upregulated CD63 upon stimulation with polyclonal anti-IgE, substance P and compound 48/80 at 4 weeks. Ketotifen and dasatinib but not cromolyn sodium inhibited both IgE- and MRGPRX2-dependent pathways. Our results confirm the feasibility of functional MC activation studies on PB-derived MC after a short 4-week culture and highlight the impact of culture conditions on functional MRGPRX2 expression.

Interleukin-33 mediated regulation of microRNAs in human cord blood-derived mast cells: Implications for infection, immunity, and inflammation

Mast cell (MCs) activation is the driving force of immune responses in several inflammatory diseases, including asthma and allergies. MCs are immune cells found throughout the body and are equipped with numerous surface receptors that allow them to respond to external signals from parasites and bacteria as well as to intrinsic signals such as cytokines. Upon activation, MCs release various mediators and proteases that contribute to inflammation. This study aimed to identify microRNAs (miRNAs) that regulate MC response to interleukin-33 and their target genes using a model of human cord blood-derived mast cells (hCBMCs). hCBMCs were induced with 10 and 20 ng of recombinant human interleukin-33 (rhIL-33) for 6 and 24 h, respectively. Total RNA was extracted from these cells and miRNA profiling was performed using high-throughput microarrays. Differential expression of miRNAs and target analysis were performed using Transcriptome Analysis Console and Ingenuity Pathway Analysis. The most significant miRNAs in each condition were miR-6836-5p (fold change = 1.76, p = 3E-03), miR-6883-5p (fold change = -2.13, p = 7E-05), miR-1229-5p (fold change = 2.46, p = 8E-04), and miR-3613-5p (fold change = 66.7, p = 1E-06). Target analysis revealed that these miRNAs regulate mast cell responsiveness and degranulation by modulating the expression of surface receptors, adaptors, and signaling molecules in response to rhIL-33 stimulation. This study is the first miRNA profiling and target analysis of hCBMCs that will further enhance our understanding of the role of miRNAs in the immune response in a timely manner and their relevance for the development of a new therapeutic target for inflammatory disorders.

Mast Cells in Allergic and Non-Allergic Upper Airways Diseases: Sentinel in the Watchtower

Mast cells are immune system cells with the most disparate functions, but are also among the least understood. Mast cells are implicated in several known pathological processes, tissue homeostasis, and wound repair. However, they owe their notoriety to allergic diseases, of which they represent the effector cell par excellence. In both allergic and not upper airway pathologies, mast cells play a key role. Exploring the mechanisms through which these cells carry out their physiological and pathological function may help us give a new perspective on existing therapies and identify new ones. A focus will be placed on non-allergic rhinitis, a poorly recognized and often neglected condition with complex management, where the role of the mast cell is crucial in the pathogenetic, clinical, and prognostic aspects.

p16Ink4a-induced senescence in cultured mast cells as a model for ageing reveals significant morphological and functional changes

BACKGROUND

Mast cells (MCs) are tissue resident cells of the immune system, mainly known for their role in allergy. However, mounting evidence indicates their involvement in the pathology of age-related diseases, such as Alzheimer's disease, Parkinson's disease, and cancer. MC numbers increase in aged tissues, but how ageing affects MCs is poorly understood.

RESULTS

We show that MC ageing is associated with the increased expression of the cell cycle inhibitor p16 Ink4a, a marker and inducer of cellular senescence. Relying on this observation and the tight association of ageing with senescence, we developed a model of inducible senescence based on doxycycline-induced expression of p16Ink4a in cultured bone marrow derived MCs (BMMCs). Using this model, we show that senescent MCs upregulate IL-1β, TNF-α and VEGF-A. We also demonstrate that senescence causes marked morphological changes that impact MC function. Senescent MCs are larger, contain a larger number of secretory granules (SGs) and have less membrane protrusions. Particularly striking are the changes in their SGs, reflected in a significant reduction in the number of electron dense SGs with a concomitant increase in lucent SGs containing intraluminal vesicles. The changes in SG morphology are accompanied by changes in MC degranulation, including a significant increase in receptor-triggered release of CD63-positive extracellular vesicles (EVs) and the exteriorisation of proteoglycans, as opposed to a gradual inhibition of the release of β-hexosaminidase.

CONCLUSIONS

The inducible expression of p16Ink4a imposes MC senescence, providing a model for tracking the autonomous changes that occur in MCs during ageing. These changes include both morphological and functional alterations. In particular, the increased release of small EVs by senescent MCs suggests an enhanced ability to modulate neighbouring cells.

No signs of mast cell involvement in long-COVID: A case-control study

Long-COVID caused by SARS-CoV-2 infection has significant and increasing effects on human health worldwide. Although a unifying molecular or biological explanation is lacking, several pathophysiological mechanisms have been proposed. Involvement of mast cells-evolutionary old "multipurpose" innate immune cells-was reported recently in studies of acute infection and post-acute-COVID-19 syndrome. Mast cell activity has been suggested in long-COVID. In this case-control study, we compared data from 24 individuals with long-COVID (according to the NICE criteria) and 24 age- and sex-matched healthy individuals with a history of SARS-CoV-2 infection without developing sequelae. Serum levels of the proteases beta-tryptase (TPSB2) and carboxypeptidase (CPA3), which are mast cell specific, were measured using immunoassays. The values were compared between the two groups and correlated to measures of physical exertional intolerance. TPSB2 and CPA3 levels were median (range) 26.9 (2.0-1000) and 5.8 (1.5-14.0) ng/mL, respectively, in the long-COVID group. The corresponding values in the control group were 10.9 (2.0-1000) (p = 0.93) and 5.3 (3.5-12.9) ng/mL (p = 0.82). No significant correlations between TPSB2 or CPA3 levels and scores on the ten physical subscales of SF-36, 3.1-3.10 were revealed. We found no significant differences in the levels of mast cell activation markers TPSB2 and CPA3 between the long-COVID and control groups and no correlations with proxy markers of exercise intolerance. Mast cell activation does not appear to be part of long-term pathogenesis of long-COVID, at least in the majority of patients.

Pharmacological blockade of the mast cell MRGPRX2 receptor supports investigation of its relevance in skin disorders

Introduction

Because MRGPRX2 is now recognized as the mast cell receptor for basic secretagogues, there is currently a tremendous interest in whether MRGRPX2 could play an important role in various pruritic dermatoses such as chronic spontaneous urticaria. Therefore, we sought to identify new potent and selective antagonists to pharmacologically characterize the biological role of MRGPRX2.

Methods

Various relevant in vitro, ex vivo, and in vivo model systems were used to investigate the role of MRGPRX2. This included the study of freshly isolated human skin mast cells and human basophils as well as an ex vivo human skin microdialysis preparation. The additivity of MRGPRX2 and FcεR1-mediated degranulation was also investigated. Human MRGPRX2 knock-in mice were generated to interrogate pharmacokinetic/pharmacodynamic relationships because both antagonists studied were shown to be human specific.

Results

Two novel and structurally distinct MRGPRX2 antagonists were identified with one, Compound B, being orally active and demonstrating high potency in blocking Substance P-mediated degranulation using freshly isolated human skin mast cells with half maximal inhibitory concentration (IC50) at 0.42 nM. Compound B also potently blocked Substance P-stimulated histamine release from resident mast cells in a human skin explant setup as well as blocking itch in an established behavioral scratching model using MRGPRX2 knock-in mice. Unlike human mast cells, Substance P failed to elicit a functional response in human basophils.

Conclusion

These data fully support the investigation of MRGPRX2 receptor antagonists in mast cell-driven allergic skin disorders such as chronic spontaneous urticaria.

Mast cell MrgprB2 in neuroimmune interaction in IgE-mediated airway inflammation and its modulation by β-arrestin2

Introduction

Allergic asthma has been linked to the activation of mast cells (MCs) by the neuropeptide substance P (SP), but the mechanism underlying this neuroimmune interaction is unknown. Substance P produced from cutaneous nociceptors activates MCs via Mas-related G-protein-coupled receptor B2 (MrgprB2) to enhance type 2 immune response in experimental atopic dermatitis in mice. We recently showed that the adapter protein β-arrestin2 (β-arr2) contributes to MrgprB2-mediated MC chemotaxis. The goals of this study were to determine if MrgprB2 facilitates neuroimmune interaction in IgE (FcεRI)-mediated allergic airway inflammation (AAI) and to assess if this response is modulated by β-arr2.

Methods

Wild-type (WT), MrgprB2-/- mice and mice with MC-specific deletion of β-arr2 (Cpa3Cre+ /β-arr2fl/fl ) were passively sensitized with anti-TNP-IgE and challenged with antigen. The generation of SP and MC recruitment in the lung were determined by immunofluorescence and toluidine blue staining, respectively. The transcripts for Tac1, MrgprB2, TNF-α, and Th2 cytokines in lung tissue were assessed by RT-PCR, and the release of selected cytokines in bronchoalveolar lavage (BAL) was determined by ELISA. Eosinophil and neutrophil recruitment in lung tissue and BAL were determined by immunofluorescence staining and flow cytometry, respectively. Goblet cell hyperplasia was determined by periodic acid-Schiff staining.

Results

Following IgE sensitization and antigen challenge in WT mice, SP generation, and MC recruitment, transcripts for Tac1, MrgprB2, TNF-α, and Th2 cytokine were upregulated when compared to the control challenge. TNF-α, Th2 cytokine production, eosinophil/neutrophil recruitment, and goblet cell hyperplasia were also increased. These responses were significantly reduced in MrgprB2-/- and Cpa3Cre+ /β-arr2fl/fl mice.

Discussion

The data presented herein suggest that SP-mediated MrgprB2 activation contributes to AAI and goblet cell hyperplasia in mice. Furthermore, these responses are modulated by β-arr2, which promotes MC recruitment to facilitate their activation through FcεRI.

Adipokine receptor expression in mast cells is altered by specific ligands and proinflammatory cytokines

Adipokines play essential roles in regulating a range of biological processes, but growing evidence indicates that they are also fundamental in immunological mechanisms and, primarily, inflammatory responses. Adipokines mediate their actions through specific receptors. However, although adipokine receptors are widely distributed in many cell and tissue types, limited data are available on their expression in mast cells (MCs) and, consequently, adipokine's significance in the modulation of MC activity within the tissues. In this study, we demonstrate that rat peritoneal MCs constitutively express the leptin receptor (i.e. LEPR), adiponectin receptors (i.e. ADIPOR1 and ADIPOR2) and the chemerin receptor (i.e. CMKLR1). We also found that LEPR, ADIPOR1, ADIPOR2 and CMKLR1 expression in MCs changes in response to stimulation by their specific ligands and some cytokines with potent proinflammatory properties. Furthermore, the involvement of intracellular signaling molecules in leptin-, adiponectin- and chemerin-induced MC response was analyzed. Overall, our findings suggest that adipokines leptin, adiponectin and chemerin can significantly affect the activity of MCs in various processes, especially during inflammation. These observations may contribute significantly to understanding the relationship between adipokines, immune mechanisms and diseases or conditions with an inflammatory component.

Mast Cells are Dependent on Glucose Transporter 1 (GLUT1) and GLUT3 for IgE-mediated Activation

Mast cells (MCs) are known to have a pathological impact in a variety of settings, in particular in allergic conditions. There is also limited evidence implicating MCs in diabetes, raising the possibility that MC function may be influenced by alterations in glucose levels. However, it is not known whether MCs are directly affected by elevated glucose concentrations. Moreover, it is not known which glucose transporters that are expressed by MCs, and whether MCs are dependent on glucose transporters for activation. Here we addressed these issues. We show that MCs express high levels of both glucose transporter 1 (GLUT1/Slc2A1) and GLUT3 (Slc2A3). Further, we show that the inhibition of either GLUT1 or GLUT3 dampens both MC degranulation and cytokine induction in response to IgE receptor crosslinking, and that combined GLUT1 and GLUT3 inhibition causes an even more pronounced inhibition of these parameters. In contrast, the inhibition of GLUT1 or GLUT3, or combined GLUT1 and GLUT3 inhibition, had less impact on the ability of the MCs to respond to activation via compound 48/80. Elevated glucose concentrations did not affect MC viability, and had no stimulatory effect on MC responses to either IgE receptor crosslinking or compound 48/80. Altogether, these findings reveal that MCs are strongly dependent on glucose transport via GLUT1 and/or GLUT3 for optimal responses towards IgE-mediated activation, whereas MC functionality is minimally affected by elevated glucose levels. Based on these findings, antagonists of GLUT1 and GLUT3 may be considered for therapeutic intervention in allergic conditions.

Mast cells and the gut-liver Axis: Implications for liver disease progression and therapy

The role of mast cells, traditionally recognized for their involvement in immediate hypersensitivity reactions, has garnered significant attention in liver diseases. Studies have indicated a notable increase in mast cell counts following hepatic injury, underscoring their potential contribution to liver disorder pathogenesis. Predominantly situated in connective tissue that envelops the hepatic veins, bile ducts, and arteries, mast cells are central to both initiating and perpetuating liver disorders. Additionally, they are crucial for maintaining gastrointestinal barrier function. The gut-liver axis emphasizes the complex, two-way communication between the gut microbiome and the liver. Past research has implicated gut microbiota and their metabolites in the progression of hepatic disorders. This review sheds light on how mast cells are activated in various liver conditions such as alcoholic liver disease (ALD), non-alcoholic fatty liver disease (NAFLD), viral hepatitis, hepatic fibrogenesis, and hepatocellular carcinoma. It also briefly explores the connection between the gut microbiome and mast cell activation in these hepatic conditions.

Combinatorial Genomic Biomarkers Associated with High Response in IgE-Dependent Degranulation in Human Mast Cells

Mast cells are the major effector cells that mediate IgE-dependent allergic reactions. We sought to use integrated network analysis to identify genomic biomarkers associated with high response in IgE-mediated activation of primary human mast cells. Primary human mast cell cultures derived from 262 normal donors were categorized into High, Average and Low responder groups according to their activation response profiles. Transcriptome analysis was used to identify genes that were differentially expressed in different responder cultures in their baseline conditions, and the data were analyzed by constructing a personalized perturbed profile (PEEP). For upregulated genes, the construction of PEEP for each individual sample of all three responder groups revealed that High responders exhibited a higher percentage of "perturbed" samples whose PEEP values lay outside the normal range of expression. Moreover, the integration of PEEP of four selected upregulated genes into distinct sets of combinatorial profiles demonstrated that the specific pattern of upregulated expression of these four genes, in a tandem combination, was observed exclusively among the High responders. In conclusion, this combinatorial approach was useful in identifying a set of genomic biomarkers that are associated with high degranulation response in human mast cell cultures derived from the blood of a cohort of normal donors.

Serum Concentration of IL-5 Receptor (IL-5R) and Associations with Disease Severity in Patients with Chronic Spontaneous Urticaria (CSU) and Atopic Dermatitis (AD)

The immunological pathogenesis of atopic dermatitis (AD) and chronic spontaneous urticaria (CSU) has not been fully elucidated yet. The aim of our research was to assess the serum concentration of interleukin-5 receptor (IL-5R) in relation to the disease activity and pruritus intensity in adult patients with AD and CSU. This pilot study included 45 participants (15 patients with AD, 15 patients with CSU, and 15 healthy controls). Blood samples were taken to examine the serum levels of IL-5R using the enzyme-linked immunosorbent assay (ELISA) test. The Scoring Atopic Dermatitis (SCORAD) index, the Urticaria Activity Score (UAS7), and the Visual Analogue Scale (VAS) were used to assess the disease activity and the pruritus intensity, respectively. Obtained results revealed that the IL-5R concentration was significantly higher in patients with CSU than in patients with AD and in the controls (p = 0.038). There was a positive correlation between the IL-5R level and the SCORAD index in patients with AD (r = -0.9, p = 0.047), which was not found for the CSU activity by UAS7 and with the pruritus severity by VAS in both examined groups of patients. Our findings underscore higher serum levels of IL-5R among CSU and AD patients, which may highlight its functional role in the pathogenesis of these diseases. In contrast, IL-5R might not be fully useful in reflecting the severity of symptoms. Although our results are promising, this study should be conducted on a larger cohort of patients.

Nematode Galectin Inhibits Basophilic Leukaemia RBL-2H3 Cells Apoptosis in IgE-Mediated Activation

Mast cells are essential immune cells involved in the host's defence against gastrointestinal nematodes. To evade the immune response, parasitic nematodes produce a variety of molecules. Galectin 1, produced by Teladorsagia circumcincta (Tci-gal-1), reduces mast cell degranulation and selectively regulates mediator production and release in an IgE-dependent manner. To uncover the activity of Tci-gal-1, we have examined the effect of the protein on gene expression, protein production, and apoptosis in activated basophilic leukaemia RBL-2H3 cells. Rat RBL-2H3 cells were activated with anti-DNP IgE and DNP-HSA, and then treated with Tci-gal-1. Microarray analysis was used to examine gene expression. The levels of several apoptosis-related molecules and cytokines were determined using antibody arrays and ELISA. Early and late apoptosis was evaluated cytometrically. Degranulation of cells was determined by a β-hexosaminidase release assay. Treatment of activated RBL-2H3 cells with Tci-gal-1 resulted in inhibited apoptosis and decreased degranulation, although we did not detect significant changes in gene expression. The production of pro-apoptotic molecules, receptor for advanced glycation end products (RAGE) and Fas ligand (FasL), and the cytokines IL-9, IL-10, IL-13, TNF-α, and IL-2 was strongly inhibited. Tci-gal-1 modulates apoptosis, degranulation, and production of cytokines by activated RBL-2H3 cells without detectable influence on gene transcription. This parasite protein is crucial for modulation of the protective immune response and the inhibition of chronic inflammation driven by mast cell activity.

Detecting Changes in Mast Cell Numbers Versus Activation in Human Disease: A Roadblock for Current Biomarkers?

The pathophysiology of mast cell (MC)-driven disorders is diverse, ranging from localized reactions to systemic disorders caused by abnormal accumulation and activation in multiorgan systems. Prompt and accurate diagnosis is critically important, both for informing treatment and objective assessment of treatment outcomes. As new therapeutics are being developed to deplete MCs or silence them (eg, by engaging inhibitory receptors that block activation), new biomarkers are needed that can distinguish between MC activation versus burden. Serum tryptase is the gold standard for assessing both MC burden and activation; however, commercial tryptase assays have limitations related to timing of release, lack of discernment between inactive (α) and active (β) forms of tryptase, and interpatient variability of baseline levels. Alternative approaches to measuring MC activation include urinary MC mediators, flow cytometry-based assays or gene expression profiling. Additional markers of MC activation are needed for use in clinical diagnostics, to help selection of treatment of MC diseases, and for assessing outcomes of therapy. We review the spectrum of disorders with known or suspected MC contribution, describe the utility and limitations of current MC markers and assays, and discuss the need for new markers that can differentiate between MC activation and burden.

Sulfur dioxide inhibits mast cell degranulation by sulphenylation of galectin-9 at cysteine 74

Objectives

Mast cell (MC) degranulation is a key process in allergic reactions and inflammatory responses. Aspartate aminotransferase 1 (AAT1)-derived endogenous sulfur dioxide (SO2) is an important regulator of MC function. However, the mechanism underlying its role in MC degranulation remains unclear. This study aimed to investigate the mechanism by which endogenous SO2 controlled MC degranulation.

Methods

HMC-1 and Rat basophilic leukemia cell MC line (RBL-2H3) were used in the cell experiments. SO2 content was detected by in situ fluorescent probe. MC degranulation represented by the release rate of MC β-hexosaminidase was determined using a colorimetric assay. Sulfenylation of galectin-9 (Gal-9) in MCs and purified protein was detected using a biotin switch assay. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to determine the exact sulfenylation sites of Gal-9 by SO2. Animal models of passive cutaneous anaphylaxis (PCA) and hypoxia-driven pulmonary vascular remodeling were used to investigate the effect of SO2 on mast cell activation in vivo. Site-directed mutation of Gal-9 was conducted to confirm the exact site of SO2 and support the significance of SO2/Gal-9 signal axis in the regulation of MC degranulation.

Results

Degranulation was increased in AAT1-knockdowned MCs, and SO2 supplementation reversed the increase in MC degranulation. Furthermore, deficiency of endogenous SO2 contributed to IgE-mediated degranulation in vitro. Besides, SO2 inhibited IgE-mediated and hypoxia-driven MC degranulation in vivo. Mechanistically, LC-MS/MS analysis and site-directed mutation results showed that SO2 sulfenylated Gal-9 at cysteine 74. Sulfenylation of the 74th cysteine of Gal-9 protein was required in the SO2-inhibited MC degranulation under both physiological and pathophysiological conditions.

Conclusion

These findings elucidated that SO2 inhibited MC degranulation via sulfenylating Gal-9 under both physiological and pathophysiological conditions, which might provide a novel treatment approach for MC activation-related diseases.

TSLP is localized in and released from human lung macrophages activated by T2-high and T2-low stimuli: relevance in asthma and COPD

BACKGROUND

Macrophages are the predominant immune cells in the human lung and play a central role in airway inflammation, including asthma and chronic obstructive pulmonary disease (COPD). Thymic stromal lymphopoietin (TSLP), a pleiotropic cytokine mainly expressed by bronchial epithelial cells, plays a key role in asthma and COPD pathobiology. TSLP exists in two variants: the long form (lfTSLP) and a shorter TSLP isoform (sfTSLP). We aimed to localize TSLP in human lung macrophages (HLMs) and investigate the mechanisms of its release from these cells. We also evaluated the effects of the two variants of TSLP on the release of angiogenic factor from HLMs.

METHODS

We employed immunofluorescence and Western blot to localize intracellular TSLP in HLMs purified from human lung parenchyma. HLMs were activated by T2-high (IL-4, IL-13) and T2-low (lipopolysaccharide: LPS) immunological stimuli.

RESULTS

TSLP was detected in HLMs and subcellularly localized in the cytoplasm. IL-4 and LPS induced TSLP release from HLMs. Preincubation of macrophages with brefeldin A, known to disrupt the Golgi apparatus, inhibited TSLP release induced by LPS and IL-4. lfTSLP concentration-dependently induced the release of vascular endothelial growth factor-A (VEGF-A), the most potent angiogenic factor, from HLMs. sfTSLP neither activated nor interfered with the activating property of lfTSLP on macrophages.

CONCLUSIONS

Our results highlight a novel immunologic circuit between HLMs and TSLP. Given the central role of macrophages in airway inflammation, this autocrine loop holds potential translational relevance in understanding innovative aspects of the pathobiology of asthma and chronic inflammatory lung disorders.

An optimized method for IgE-mediated degranulation of human lung mast cells

Background

Mast cells are critically involved in IgE-mediated diseases, e.g., allergies and asthma. Human mast cells are heterogeneous, and mast cells from different anatomical sites have been shown to respond differently to certain stimuli and drugs. The origin of the mast cells is therefore of importance when setting up a model system, and human lung mast cells are highly relevant cells to study in the context of asthma. We therefore set out to optimize a protocol of IgE-mediated activation of human lung mast cells.

Methods

Human lung mast cells were extracted from lung tissue obtained from patients undergoing pulmonary resection by enzyme digestion and mechanical disruption followed by CD117 magnetic-activated cell sorting (MACS) enrichment. Different culturing media and conditions for the IgE-mediated degranulation were tested to obtain an optimized method.

Results

IgE crosslinking of human lung mast cells cultured in serum-free media gave a stronger response compared to cells cultured with 10% serum. The addition of stem cell factor (SCF) did not enhance the degranulation. However, when the cells were put in fresh serum-free media 30 minutes prior to the addition of anti-IgE antibodies, the cells responded more vigorously. Maximum degranulation was reached 10 minutes after the addition of anti-IgE. Both CD63 and CD164 were identified as stable markers for the detection of degranulated mast cells over time, while the staining with anti-CD107a and avidin started to decline 10 minutes after activation. The levels of CD203c and CD13 did not change in activated cells and therefore cannot be used as degranulation markers of human lung mast cells.

Conclusions

For an optimal degranulation response, human lung mast cells should be cultured and activated in serum-free media. With this method, a very strong and consistent degranulation response with a low donor-to-donor variation is obtained. Therefore, this model is useful for further investigations of IgE-mediated mast cell activation and exploring drugs that target human lung mast cells, for instance, in the context of asthma.

Mast cell degranulation and bradykinin-induced angioedema - searching for the missing link

Initiation of the bradykinin generation cascade is responsible for the occurrence of attacks in some types of angioedema without wheals. Hereditary angioedema due to C1 inhibitor deficiency (HAE-C1-INH) is one such clinical entity. In this paper, we explore the existing evidence that mast cells (MCs) degranulation may contribute to the activation of the kallikrein-kinin system cascade, followed by bradykinin formation and angioedema. We present the multidirectional effects of MC-derived heparin and other polyanions on the major components of the kinin-kallikrein system, particularly on the factor XII activation. Although, bradykinin- and histamine-mediated symptoms are distinct clinical phenomena, they share some common features, such as some similar triggers and a predilection to occur at sites where mast cells reside, namely the skin and mucous membranes. In addition, recent observations indicate a high incidence of hypersensitivity reactions associated with MC degranulation in the HAE-C1-INH patient population. However, not all of these can be explained by IgE-dependent mechanisms. Mast cell-related G protein-coupled receptor-X2 (MRGPRX2), which has recently attracted scientific interest, may be involved in the activation of MCs through a different pathway. Therefore, we reviewed MRGPRX2 ligands that HAE-C1-INH patients may be exposed to in their daily lives and that may affect MCs degranulation. We also discussed the known inter- and intra-individual variability in the course of HAE-C1-INH in relation to factors responsible for possible variability in the strength of the response to MRGPRX2 receptor stimulation. The above issues raise several questions for future research. It is not known to what extent a prophylactic or therapeutic intervention targeting the pathways of one mechanism (mast cell degranulation) may affect the other (bradykinin production), or whether the number of mast cells at a specific body site and their reactivity to triggers such as pressure, allergens or MRGPRX2 agonists may influence the occurrence of HAE-C1-INH attacks at that site.

On Connective Tissue Mast Cells as Protectors of Life, Reproduction, and Progeny

The connective tissue mast cell (MC), a sentinel tissue-residing secretory immune cell, has been preserved in all vertebrate classes since approximately 500 million years. No physiological role of the MC has yet been established. Considering the power of natural selection of cells during evolution, it is likely that the MCs exert essential yet unidentified life-promoting actions. All vertebrates feature a circulatory system, and the MCs interact readily with the vasculature. It is notable that embryonic MC progenitors are generated from endothelial cells. The MC hosts many surface receptors, enabling its activation via a vast variety of potentially harmful exogenous and endogenous molecules and via reproductive hormones in the female sex organs. Activated MCs release a unique composition of preformed and newly synthesized bioactive molecules, like heparin, histamine, serotonin, proteolytic enzymes, cytokines, chemokines, and growth factors. MCs play important roles in immune responses, tissue remodeling, cell proliferation, angiogenesis, inflammation, wound healing, tissue homeostasis, health, and reproduction. As recently suggested, MCs enable perpetuation of the vertebrates because of key effects-spanning generations-in ovulation and pregnancy, as in life-preserving activities in inflammation and wound healing from birth till reproductive age, thus creating a permanent life-sustaining loop. Here, we present recent advances that further indicate that the MC is a specific life-supporting and progeny-safeguarding cell.

Mast cells in the autonomic nervous system and potential role in disorders with dysautonomia and neuroinflammation

Mast cells (MC) are ubiquitous in the body, and they are critical for not only in allergic diseases but also in immunity and inflammation, including having potential involvement in the pathophysiology of dysautonomias and neuroinflammatory disorders. MC are located perivascularly close to nerve endings and sites such as the carotid bodies, heart, hypothalamus, the pineal gland, and the adrenal gland that would allow them not only to regulate but also to be affected by the autonomic nervous system (ANS). MC are stimulated not only by allergens but also many other triggers including some from the ANS that can affect MC release of neurosensitizing, proinflammatory, and vasoactive mediators. Hence, MC may be able to regulate homeostatic functions that seem to be dysfunctional in many conditions, such as postural orthostatic tachycardia syndrome, autism spectrum disorder, myalgic encephalomyelitis/chronic fatigue syndrome, and Long-COVID syndrome. The evidence indicates that there is a possible association between these conditions and diseases associated with MC activation. There is no effective treatment for any form of these conditions other than minimizing symptoms. Given the many ways MC could be activated and the numerous mediators released, it would be important to develop ways to inhibit stimulation of MC and the release of ANS-relevant mediators.

Mast cell-derived IL-10 protects intestinal barrier integrity during malaria in mice and regulates parasite transmission to Anopheles stephensi with a female-biased immune response

Malaria is strongly predisposed to bacteremia, which is associated with increased gastrointestinal permeability and a poor clinical prognosis. We previously identified mast cells (MCs) as mediators of intestinal permeability in malaria and described multiple cytokines that rise with parasitemia, including interleukin (IL)-10, which could protect the host from an inflammatory response and alter parasite transmission to Anopheles mosquitoes. Here, we used the Cre-loxP system and non-lethal Plasmodium yoelii yoelii 17XNL to study the roles of MC-derived IL-10 in malaria immunity and transmission. Our data suggest a sex-biased and local inflammatory response mediated by MC-derived IL-10, supported by early increased number and activation of MCs in females relative to males. Increased parasitemia in female MC IL-10 (-) mice was associated with increased ileal levels of chemokines and plasma myeloperoxidase (MPO). We also observed increased intestinal permeability in female and male MC IL-10 (-) mice relative to MC IL-10 (+) mice but no differences in blood bacterial 16S DNA levels. Transmission success of P. yoelii to A. stephensi was higher in female relative to male mice and from female and male MC IL-10 (-) mice relative to MC IL-10 (+) mice. These patterns were associated with increased plasma levels of pro-inflammatory cytokines in female MC IL-10 (-) mice and increased plasma levels of chemokines and markers of neutrophil activation in male MC IL-10 (-) mice. Overall, these data suggest that MC-derived IL-10 protects intestinal barrier integrity, regulates parasite transmission, and controls local and systemic host immune responses during malaria, with a female bias.

Alterations in metabolic pathways: a bridge between aging and weaker innate immune response

Aging is a time-dependent progressive physiological process, which results in impaired immune system function. Age-related changes in immune function increase the susceptibility to many diseases such as infections, autoimmune diseases, and cancer. Different metabolic pathways including glycolysis, tricarboxylic acid cycle, amino acid metabolism, pentose phosphate pathway, fatty acid oxidation and fatty acid synthesis regulate the development, differentiation, and response of adaptive and innate immune cells. During aging all these pathways change in the immune cells. In addition to the changes in metabolic pathways, the function and structure of mitochondria also have changed in the immune cells. Thereby, we will review changes in the metabolism of different innate immune cells during the aging process.

Absence of meningeal mast cells in the Mitf mutant mouse

Mast cells (MCs) are located in the meninges of the central nervous system (CNS), where they play key roles in the immune response. MC-deficient mice are advantageous in delineating the role of MCs in the immune response in vivo. In this study, we illustrate that a mutation in microphthalmia-associated transcription factor (Mitf) affects meningeal MC number in a dosage-dependent manner. C57BL/6J Mitf null mice lack meningeal MCs completely, whereas heterozygous mice have on average 25% fewer MCs. Mitf heterozygous mice might be a valuable tool to study the role of MCs in the meninges.

A two-step activation mechanism enables mast cells to differentiate their response between extracellular and invasive enterobacterial infection

Mast cells localize to mucosal tissues and contribute to innate immune defense against infection. How mast cells sense, differentiate between, and respond to bacterial pathogens remains a topic of ongoing debate. Using the prototype enteropathogen Salmonella Typhimurium (S.Tm) and other related enterobacteria, here we show that mast cells can regulate their cytokine secretion response to distinguish between extracellular and invasive bacterial infection. Tissue-invasive S.Tm and mast cells colocalize in the mouse gut during acute Salmonella infection. Toll-like Receptor 4 (TLR4) sensing of extracellular S.Tm, or pure lipopolysaccharide, causes a modest induction of cytokine transcripts and proteins, including IL-6, IL-13, and TNF. By contrast, type-III-secretion-system-1 (TTSS-1)-dependent S.Tm invasion of both mouse and human mast cells triggers rapid and potent inflammatory gene expression and >100-fold elevated cytokine secretion. The S.Tm TTSS-1 effectors SopB, SopE, and SopE2 here elicit a second activation signal, including Akt phosphorylation downstream of effector translocation, which combines with TLR activation to drive the full-blown mast cell response. Supernatants from S.Tm-infected mast cells boost macrophage survival and maturation from bone-marrow progenitors. Taken together, this study shows that mast cells can differentiate between extracellular and host-cell invasive enterobacteria via a two-step activation mechanism and tune their inflammatory output accordingly.

Mast cell in infantile hemangioma

Infantile hemangioma (IH) is the most common benign vascular tumor characterized by three phases - proliferation, early involution and late involution. Mast cells (MCs) play an important role in allergic reactions and numerous diseases, including tumors. While the mechanisms underlying MCs migration, activation and function in the life cycle of IH remain unclear, previous studies suggested that MCs circulate through the vasculature and migrate into IH, and subsequently mature and get activated. Estradiol (E2) emerges as a potential attractant for MC migration into IH and their subsequent activation. In various stages of IH, activated MCs secrete both proangiogenic and anti-angiogenic modulators, absorbed by various cells adjacent to them. Imbalances in these modulators may contribute to IH proliferation and involution.

Mast cell silencing: A novel therapeutic approach for urticaria and other mast cell-mediated diseases

Chronic urticaria (CU) is a mast cell (MC)-dependent disease with limited therapeutic options. Current management strategies are directed at inhibiting IgE-mediated activation of MCs and antagonizing effects of released mediators. Due to the complexity and heterogeneity of CU and other MC diseases and mechanisms of MC activation-including multiple activating receptors and ligands, diverse signaling pathways, and a menagerie of mediators-strategies of MC depletion or MC silencing (i.e., inhibition of MC activation via binding of inhibitory receptors) have been developed to overcome limitations of singularly targeted agents. MC silencers, such as agonist monoclonal antibodies that engage inhibitory receptors (e.g., sialic acid-binding immunoglobulin-like lectin8 -[Siglec-8] [lirentelimab/AK002], Siglec-6 [AK006], and CD200R [LY3454738]), have reached preclinical and clinical stages of development. In this review, we (1) describe the role of MCs in the pathogenesis of CU, highlighting similarities with other MC diseases in disease mechanisms and response to treatment; (2) explore current therapeutic strategies, categorized by nonspecific immunosuppression, targeted inhibition of MC activation or mediators, and targeted modulation of MC activity; and (3) introduce the concept of MC silencing as an emerging strategy that could selectively block activation of MCs without eliciting or exacerbating on- or off-target, immunosuppressive adverse effects.

The Underlying Rab Network of MRGPRX2-Stimulated Secretion Unveils the Impact of Receptor Trafficking on Secretory Granule Biogenesis and Secretion

MRGPRX2, the human member of the MAS-related G-protein-coupled receptors (GPCRs), mediates the immunoglobulin E (IgE)-independent responses of a subset of mast cells (MCs) that are associated with itch, pain, neurogenic inflammation, and pseudoallergy to drugs. The mechanisms underlying the responses of MRGPRX2 to its multiple and diverse ligands are still not completely understood. Given the close association between GPCR location and function, and the key role played by Rab GTPases in controlling discrete steps along vesicular trafficking, we aimed to reveal the vesicular pathways that directly impact MRGPRX2-mediated exocytosis by identifying the Rabs that influence this process. For this purpose, we screened 43 Rabs for their functional and phenotypic impacts on MC degranulation in response to the synthetic MRGPRX2 ligand compound 48/80 (c48/80), which is often used as the gold standard of MRGPRX2 ligands, or to substance P (SP), an important trigger of neuroinflammatory MC responses. Results of this study highlight the important roles played by macropinocytosis and autophagy in controlling MRGPRX2-mediated exocytosis, demonstrating a close feedback control between the internalization and post-endocytic trafficking of MRGPRX2 and its triggered exocytosis.

Mouse hygiene status-A tale of two environments for mast cells and allergy

Animal models, including those employing the use of house mice (Mus musculus), are crucial in elucidating mechanisms in human pathophysiology. However, it is evident that the impreciseness of using laboratory mice maintained in super-hygienic barrier facilities to mirror relevant aspects of human physiology and pathology exists, which is a major limitation in translating mouse findings to inferring human medicine. Interestingly, free-living wild mice are found to be substantially different from laboratory-bred, specific pathogen-free mice with respect to various immune system compartments. Wild mice have an immune system that better reflects human immunity. In this review article, we discuss recent experimental findings that address the so-called "wild immunology", which reveals the contrasting immune features between laboratory-raised mice and their wild companions as well as laboratory mice that have been exposed to a natural rodent habitat. A particular focus will be given to the development of pulmonary mast cells and its possible impact on the use of "naturalized" or "rewilded" laboratory mice as experimental asthma models.

Rab46: a novel player in mast cell function

Mast cells are infamous for mediating allergic and inflammatory diseases due to their capacity of rapidly releasing a wide range of inflammatory mediators stored in cytoplasmic granules. However, mast cells also have several important physiological roles that involve selective and agonist-specific release of these active mediators. While a filtering mechanism at the plasma membrane could regulate the selective release of some cargo, the plethora of stored cargo and the diversity of mast cell functions suggests the existence of granule subtypes with distinct trafficking pathways. The molecular mechanisms underlying differential trafficking and exocytosis of these granules are not known, neither is it clear how granule trafficking is coupled to the stimulus. In endothelial cells, a Rab GTPase, Rab46, responds to histamine but not thrombin signals, and this regulates the trafficking of a subpopulation of endothelial-specific granules. Here, we sought to explore, for the first time, if Rab46 plays a role in mast cell function. We demonstrate that Rab46 is highly expressed in human and murine mast cells, and Rab46 genetic deletion has an effect on mast cell degranulation that depends on both stimuli and mast cell subtype. This initial insight into the contribution of Rab46 to mast cell function and the understanding of the role of Rab46 in stimuli-dependent trafficking in other cell types necessitates further investigations of Rab46 in mast cell granular trafficking so that novel and specific therapeutic targets for treatment of the diverse pathologies mediated by mast cells can be developed.

Single-cell analysis unveils activation of mast cells in colorectal cancer microenvironment

The role of mast cells (MCs) in colorectal cancer (CRC) remains unclear, and a comprehensive single-cell study on CRC MCs has not been conducted. This study used a multi-omics approach, integrating single-cell sequencing, spatial transcriptomics, and bulk tissue sequencing data to investigate the heterogeneity and impact of MCs in CRC. Five MC signature genes (TPSAB1, TPSB2, CPA3, HPGDS, and MS4A2) were identified, and their average expression was used as a marker of MCs. The MC density was found to be lower in CRC compared to normal tissue, but MCs in CRC demonstrated distinct activation features. Activated MCs were defined by high expression of receptors and MC mediators, while resting MCs had low expression. Most genes, including the five MC signature genes, were expressed at higher levels in activated MCs. The MC signature was linked to a better prognosis in both CRC and pan-cancer patient cohorts. Elevated KITLG expression was observed in fibroblasts and endothelial cells in CRC samples compared to normal tissue, and co-localization of MCs with these cell types was revealed by spatial transcriptome analysis. In conclusion, this study finds decreased MC density in CRC compared to normal tissue, but highlights a shift in MC phenotype from CMA1high resting cells to activated TPSAB1high, CPA3high, and KIThigh cells. The elevated KITLG expression in the tumor microenvironment's fibroblasts and endothelial cells may activate MCs through the KITLG-KIT axis, potentially suppressing tumor progression.

Teleost innate immunity, an intricate game between immune cells and parasites of fish organs: who wins, who loses

Fish, comprising over 27,000 species, represent the oldest vertebrate group and possess both innate and adaptive immune systems. The susceptibility of most wild fish to parasitic infections and related diseases is well-established. Among all vertebrates, the digestive tract creates a remarkably favorable and nutrient-rich environment, which, in turn, renders it susceptible to microparasites and macroparasites. Consequently, metazoan parasites emerge as important disease agents, impacting both wild and farmed fish and resulting in substantial economic losses. Given their status as pathogenic organisms, these parasites warrant considerable attention. Helminths, a general term encompassing worms, constitute one of the most important groups of metazoan parasites in fish. This group includes various species of platyhelminthes (digeneans, cestodes), nematodes, and acanthocephalans. In addition, myxozoans, microscopic metazoan endoparasites, are found in water-dwelling invertebrates and vertebrate hosts. It is worth noting that several innate immune cells within the fish alimentary canal and certain visceral organs (e.g., liver, spleen, and gonads) play active roles in the immune response against parasites. These immune cells include macrophages, neutrophils, rodlet cells, and mast cells also known as eosinophilic granular cells. At the site of intestinal infection, helminths often impact mucous cells number and alter mucus composition. This paper presents an overview of the state of the art on the occurrence and characteristics of innate immune cells in the digestive tract and other visceral organs in different fish-parasite systems. The data, coming especially from studies employed immunohistochemical, histopathological, and ultrastructural analyses, provide evidence supporting the involvement of teleost innate immune cells in modulating inflammatory responses to metazoan and protozoan parasitic infections.

Landscape of mast cell populations across organs in mice and humans

Mast cells (MCs) are tissue-resident immune cells that exhibit homeostatic and neuron-associated functions. Here, we combined whole-tissue imaging and single-cell RNA sequencing datasets to generate a pan-organ analysis of MCs in mice and humans at steady state. In mice, we identify two mutually exclusive MC populations, MrgprB2+ connective tissue-type MCs and MrgprB2neg mucosal-type MCs, with specific transcriptomic core signatures. While MrgprB2+ MCs develop in utero independently of the bone marrow, MrgprB2neg MCs develop after birth and are renewed by bone marrow progenitors. In humans, we unbiasedly identify seven MC subsets (MC1-7) distributed across 12 organs with different transcriptomic core signatures. MC1 are preferentially enriched in the bladder, MC2 in the lungs, and MC4, MC6, and MC7 in the skin. Conversely, MC3 and MC5 are shared by most organs but not skin. This comprehensive analysis offers valuable insights into the natural diversity of MC subtypes in both mice and humans.

Immune landscape and immunotherapy of hepatocellular carcinoma: focus on innate and adaptive immune cells

Hepatocellular carcinoma (HCC) is responsible for roughly 90% of all cases of primary liver cancer, and the cases are on the rise. The treatment of advanced HCC is a serious challenge. Immune checkpoint inhibitor (ICI) therapy has marked a watershed moment in the history of HCC systemic treatment. Atezolizumab in combination with bevacizumab has been approved as a first-line treatment for advanced HCC since 2020; however, the combination therapy is only effective in a limited percentage of patients. Considering that the tumor immune microenvironment (TIME) has a great impact on immunotherapies for HCC, an in-depth understanding of the immune landscape in tumors and the current immunotherapeutic approaches is extremely necessary. We elaborate on the features, functions, and cross talk of the innate and adaptive immune cells in HCC and highlight the benefits and drawbacks of various immunotherapies for advanced HCC, as well as future projections. HCC consists of a heterogeneous group of cancers with distinct etiologies and immune microenvironments. Almost all the components of innate and adaptive immune cells in HCC have altered, showing a decreasing trend in the number of tumor suppressor cells and an increasing trend in the pro-cancer cells, and there is also cross talk between various cell types. Various immunotherapies for HCC have also shown promising efficacy and application prospect. There are multilayered interwoven webs among various immune cell types in HCC, and emerging evidence demonstrates the promising prospect of immunotherapeutic approaches for HCC.

Siglec-9 is an inhibitory receptor on human mast cells in vitro

BACKGROUND

Mast cell activation is critical for the development of allergic diseases. Ligation of sialic acid-binding immunoglobin-like lectins (Siglecs), such as Siglec-6, -7, and -8 as well as CD33, have been shown to inhibit mast cell activation. Recent studies showed that human mast cells express Siglec-9, an inhibitory receptor also expressed by neutrophils, monocytes, macrophages, and dendritic cells.

OBJECTIVE

We aimed to characterize Siglec-9 expression and function in human mast cells in vitro.

METHODS

We assessed the expression of Siglec-9 and Siglec-9 ligands on human mast cell lines and human primary mast cells by real-time quantitative PCR, flow cytometry, and confocal microscopy. We used a clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) gene editing approach to disrupt the SIGLEC9 gene. We evaluated Siglec-9 inhibitory activity on mast cell function by using native Siglec-9 ligands, glycophorin A (GlycA), and high-molecular-weight hyaluronic acid, a monoclonal antibody against Siglec-9, and coengagement of Siglec-9 with the high-affinity receptor for IgE (FcεRI).

RESULTS

Human mast cells express Siglec-9 and Siglec-9 ligands. SIGLEC9 gene disruption resulted in increased expression of activation markers at baseline and increased responsiveness to IgE-dependent and IgE-independent stimulation. Pretreatment with GlycA or high-molecular-weight hyaluronic acid followed by IgE-dependent or -independent stimulation had an inhibitory effect on mast cell degranulation. Coengagement of Siglec-9 with FcεRI in human mast cells resulted in reduced degranulation, arachidonic acid production, and chemokine release.

CONCLUSIONS

Siglec-9 and its ligands play an important role in limiting human mast cell activation in vitro.

Siglecs as potential targets of therapy in human mast cell- and/or eosinophil-associated diseases

Siglecs (sialic acid-binding immunoglobulin-like lectins) are a family of vertebrate glycan-binding cell-surface proteins. The majority mediate cellular inhibitory activity once engaged by specific ligands or ligand-mimicking molecules. As a result, Siglec engagement is now of interest as a strategy to therapeutically dampen unwanted cellular responses. When considering allergic inflammation, human eosinophils and mast cells express overlapping but distinct patterns of Siglecs. For example, Siglec-6 is selectively and prominently expressed on mast cells while Siglec-8 is highly specific for both eosinophils and mast cells. This review will focus on a subset of Siglecs and their various endogenous or synthetic sialoside ligands that regulate eosinophil and mast cell function and survival. It will also summarize how certain Siglecs have become the focus of novel therapies for allergic and other eosinophil- and mast cell-related diseases.

Mast Cell Deficiency in Mice Attenuates Insulin Phenolic Preservative-Induced Inflammation

One major obstacle that limits the lifespan of insulin infusion pumps is surmounting the tissue site reaction at the device implantation site. All commercial insulin formulations contain insulin phenolic preservatives (IPPs) designed to ensure insulin protein stability and prolong shelf-life. However, our laboratory demonstrated that these preservatives are cytotoxic and induce inflammation. Mature mast cells (MCs) reside in cutaneous tissue and are one of the first responders to an epidermal breach. Upon activation, MCs release proinflammatory and immunomodulatory prepacked mediators that exacerbate these inflammatory reactions. Thus, we hypothesized that once the epidermis is breached, cutaneous MCs are triggered inciting the inflammatory response to IPP-induced inflammation. This hypothesis was pursued utilizing our modified in vivo mouse air pouch model, including a c-kit dependent (C57BL/6J-kitW-sh/W-sh) and a c-kit independent (Cpa3-Cre; Mcl-1fl/fl) MC-deficient mouse model. Leukocytes were quantified in the mouse air pouch lavage fluid following flow cytometry analysis for IPP infusion under three different states, insulin-containing phenolic preservatives (Humalog®), insulin preservatives alone, and normal saline as a control. The air pouch wall was assessed using histopathological evaluations. Flow cytometry analysis demonstrated a statistically significant difference in inflammatory cell recruitment for both MC-deficient mouse models when compared to the control strain including infused control saline. Significantly less inflammation was observed at the site of infusion for the MC-deficient strains compared to the control strain. Overall, concordant results were obtained in both mouse types, C57Bl6-kitW-sh/W-sh and Cpa3-Cre; Mcl-1fl/fl. These findings in multiple model systems support the conclusion that MCs have important or possible unique roles in IPP-induced inflammation.

Mast cells link immune sensing to antigen-avoidance behaviour

The physiological functions of mast cells remain largely an enigma. In the context of barrier damage, mast cells are integrated in type 2 immunity and, together with immunoglobulin E (IgE), promote allergic diseases. Allergic symptoms may, however, facilitate expulsion of allergens, toxins and parasites and trigger future antigen avoidance1-3. Here, we show that antigen-specific avoidance behaviour in inbred mice4,5 is critically dependent on mast cells; hence, we identify the immunological sensor cell linking antigen recognition to avoidance behaviour. Avoidance prevented antigen-driven adaptive, innate and mucosal immune activation and inflammation in the stomach and small intestine. Avoidance was IgE dependent, promoted by Th2 cytokines in the immunization phase and by IgE in the execution phase. Mucosal mast cells lining the stomach and small intestine rapidly sensed antigen ingestion. We interrogated potential signalling routes between mast cells and the brain using mutant mice, pharmacological inhibition, neural activity recordings and vagotomy. Inhibition of leukotriene synthesis impaired avoidance, but overall no single pathway interruption completely abrogated avoidance, indicating complex regulation. Collectively, the stage for antigen avoidance is set when adaptive immunity equips mast cells with IgE as a telltale of past immune responses. On subsequent antigen ingestion, mast cells signal termination of antigen intake. Prevention of immunopathology-causing, continuous and futile responses against per se innocuous antigens or of repeated ingestion of toxins through mast-cell-mediated antigen-avoidance behaviour may be an important arm of immunity.

Potential Role of Moesin in Regulating Mast Cell Secretion

Mast cells have existed for millions of years in species that never suffer from allergic reactions. Hence, in addition to allergies, mast cells can play a critical role in homeostasis and inflammation via secretion of numerous vasoactive, pro-inflammatory and neuro-sensitizing mediators. Secretion may utilize different modes that involve the cytoskeleton, but our understanding of the molecular mechanisms regulating secretion is still not well understood. The Ezrin/Radixin/Moesin (ERM) family of proteins is involved in linking cell surface-initiated signaling to the actin cytoskeleton. However, how ERMs may regulate secretion from mast cells is still poorly understood. ERMs contain two functional domains connected through a long α-helix region, the N-terminal FERM (band 4.1 protein-ERM) domain and the C-terminal ERM association domain (C-ERMAD). The FERM domain and the C-ERMAD can bind to each other in a head-to-tail manner, leading to a closed/inactive conformation. Typically, phosphorylation on the C-terminus Thr has been associated with the activation of ERMs, including secretion from macrophages and platelets. It has previously been shown that the ability of the so-called mast cell "stabilizer" disodium cromoglycate (cromolyn) to inhibit secretion from rat mast cells closely paralleled the phosphorylation of a 78 kDa protein, which was subsequently shown to be moesin, a member of ERMs. Interestingly, the phosphorylation of moesin during the inhibition of mast cell secretion was on the N-terminal Ser56/74 and Thr66 residues. This phosphorylation pattern could lock moesin in its inactive state and render it inaccessible to binding to the Soluble NSF attachment protein receptors (SNAREs) and synaptosomal-associated proteins (SNAPs) critical for exocytosis. Using confocal microscopic imaging, we showed moesin was found to colocalize with actin and cluster around secretory granules during inhibition of secretion. In conclusion, the phosphorylation pattern and localization of moesin may be important in the regulation of mast cell secretion and could be targeted for the development of effective inhibitors of secretion of allergic and inflammatory mediators from mast cells.