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.

Primary Atopic Disorders.

Primary atopic disorders describes a series of monogenic diseases that have allergy- or atopic effector-related symptoms as a substantial feature. The underlying pathogenic genetic lesions help illustrate fundamental pathways in atopy, opening up diagnostic and therapeutic options for further study in those patients, but ultimately for common allergic diseases as well. Key pathways affected in these disorders include T cell receptor and B cell receptor signaling, cytokine signaling, skin barrier function, and mast cell function, as well as pathways that have not yet been elucidated. While comorbidities such as classically syndromic presentation or immune deficiency are often present, in some cases allergy alone is the presenting symptom, suggesting that commonly encountered allergic diseases exist on a spectrum of monogenic and complex genetic etiologies that are impacted by environmental risk factors.

Choroid plexus mast cells drive tumor-associated hydrocephalus.

Tumor-associated hydrocephalus (TAH) is a common and lethal complication of brain metastases. Although other factors beyond mechanical obstructions have been suggested, the exact mechanisms are unknown. Using single-nucleus RNA sequencing and spatial transcriptomics, we find that a distinct population of mast cells locate in the choroid plexus and dramatically increase during TAH. Genetic fate tracing and intracranial mast-cell-specific tryptase knockout showed that choroid plexus mast cells (CPMCs) disrupt cilia of choroid plexus epithelia via the tryptase-PAR2-FoxJ1 pathway and consequently increase cerebrospinal fluid production. Mast cells are also found in the human choroid plexus. Levels of tryptase in cerebrospinal fluid are closely associated with clinical severity of TAH. BMS-262084, an inhibitor of tryptase, can cross the blood-brain barrier, inhibit TAH in vivo, and alleviate mast-cell-induced damage of epithelial cilia in a human pluripotent stem-cell-derived choroid plexus organoid model. Collectively, we uncover the function of CPMCs and provide an attractive therapy for TAH.

Ion channels in innate and adaptive immunity.

Ion channels and transporters mediate the transport of charged ions across hydrophobic lipid membranes. In immune cells, divalent cations such as calcium, magnesium, and zinc have important roles as second messengers to regulate intracellular signaling pathways. By contrast, monovalent cations such as sodium and potassium mainly regulate the membrane potential, which indirectly controls the influx of calcium and immune cell signaling. Studies investigating human patients with mutations in ion channels and transporters, analysis of gene-targeted mice, or pharmacological experiments with ion channel inhibitors have revealed important roles of ionic signals in lymphocyte development and in innate and adaptive immune responses. We here review the mechanisms underlying the function of ion channels and transporters in lymphocytes and innate immune cells and discuss their roles in lymphocyte development, adaptive and innate immune responses, and autoimmunity, as well as recent efforts to develop pharmacological inhibitors of ion channels for immunomodulatory therapy.

Quo vadis, neutrophil?

Neutrophil recruitment from blood into tissues is a hallmark of inflammation and anti-microbial host defense. In this issue, De Giovanni et al. describe an unanticipated role for a serotonin metabolite, 5-HIAA, which is produced by activated platelets and mast cells and engages the orphan receptor, GPR35, to recruit neutrophils to inflamed tissues.

Anaphylactic degranulation by mast cells requires the mobilization of inflammasome components.

The inflammasome components NLRP3 and ASC are cytosolic proteins, which upon sensing endotoxins or danger cues, form multimeric complexes to process interleukin (IL)-1ß for secretion. Here we found that antigen (Ag)-triggered degranulation of IgE-sensitized mast cells (MCs) was mediated by NLRP3 and ASC. IgE-Ag stimulated NEK7 and Pyk2 kinases in MCs to induce the deposition of NLRP3 and ASC on granules and form a distinct protein complex (granulosome) that chaperoned the granules to the cell surface. MCs deficient in NLRP3 or ASC did not form granulosomes, degranulated poorly in vitro and did not evoke systemic anaphylaxis in mice. IgE-Ag-triggered anaphylaxis was prevented by an NLRP3 inhibitor. In endotoxin-primed MCs, pro-IL-1ß was rapidly packaged into granules after IgE-Ag stimulation and processed within granule remnants by proteases after degranulation, causing lethal anaphylaxis in mice. During IgE-Ag-mediated degranulation of endotoxin-primed MCs, granulosomes promoted degranulation, combined with exteriorization and processing of IL-1ß, resulting in severe inflammation.

Neu(ronal) custodians of cutaneous immunity.

Sensory neurons have surfaced as key instigators of skin inflammation. In this issue of Cell, Zhang et al. define an anti-inflammatory Langerhans cell (LC)-neuron-mast cell (MC) circuit that underlies skin immune homeostasis. They uncover a role for LCs in maintaining innervation of MrgprD+ nonpeptidergic neurons that restrain MC activation via glutamate secretion.

Nonpeptidergic neurons suppress mast cells via glutamate to maintain skin homeostasis.

Cutaneous mast cells mediate numerous skin inflammatory processes and have anatomical and functional associations with sensory afferent neurons. We reveal that epidermal nerve endings from a subset of sensory nonpeptidergic neurons expressing MrgprD are reduced by the absence of Langerhans cells. Loss of epidermal innervation or ablation of MrgprD-expressing neurons increased expression of a mast cell gene module, including the activating receptor, Mrgprb2, resulting in increased mast cell degranulation and cutaneous inflammation in multiple disease models. Agonism of MrgprD-expressing neurons reduced expression of module genes and suppressed mast cell responses. MrgprD-expressing neurons released glutamate which was increased by MrgprD agonism. Inhibiting glutamate release or glutamate receptor binding yielded hyperresponsive mast cells with a genomic state similar to that in mice lacking MrgprD-expressing neurons. These data demonstrate that MrgprD-expressing neurons suppress mast cell hyperresponsiveness and skin inflammation via glutamate release, thereby revealing an unexpected neuroimmune mechanism maintaining cutaneous immune homeostasis.

A basophil-neuronal axis promotes itch.

Itch is an evolutionarily conserved sensation that facilitates expulsion of pathogens and noxious stimuli from the skin. However, in organ failure, cancer, and chronic inflammatory disorders such as atopic dermatitis (AD), itch becomes chronic, intractable, and debilitating. In addition to chronic itch, patients often experience intense acute itch exacerbations. Recent discoveries have unearthed the neuroimmune circuitry of itch, leading to the development of anti-itch treatments. However, mechanisms underlying acute itch exacerbations remain overlooked. Herein, we identify that a large proportion of patients with AD harbor allergen-specific immunoglobulin E (IgE) and exhibit a propensity for acute itch flares. In mice, while allergen-provoked acute itch is mediated by the mast cell-histamine axis in steady state, AD-associated inflammation renders this pathway dispensable. Instead, a previously unrecognized basophil-leukotriene (LT) axis emerges as critical for acute itch flares. By probing fundamental itch mechanisms, our study highlights a basophil-neuronal circuit that may underlie a variety of neuroimmune processes.

A pan-cancer single-cell transcriptional atlas of tumor infiltrating myeloid cells.

Tumor-infiltrating myeloid cells (TIMs) are key regulators in tumor progression, but the similarity and distinction of their fundamental properties across different tumors remain elusive. Here, by performing a pan-cancer analysis of single myeloid cells from 210 patients across 15 human cancer types, we identified distinct features of TIMs across cancer types. Mast cells in nasopharyngeal cancer were found to be associated with better prognosis and exhibited an anti-tumor phenotype with a high ratio of TNF+/VEGFA+ cells. Systematic comparison between cDC1- and cDC2-derived LAMP3+ cDCs revealed their differences in transcription factors and external stimulus. Additionally, pro-angiogenic tumor-associated macrophages (TAMs) were characterized with diverse markers across different cancer types, and the composition of TIMs appeared to be associated with certain features of somatic mutations and gene expressions. Our results provide a systematic view of the highly heterogeneous TIMs and suggest future avenues for rational, targeted immunotherapies.

Slow integrin-dependent migration organizes networks of tissue-resident mast cells.

Immune cell locomotion is associated with amoeboid migration, a flexible mode of movement, which depends on rapid cycles of actin polymerization and actomyosin contraction1. Many immune cells do not necessarily require integrins, the major family of adhesion receptors in mammals, to move productively through three-dimensional tissue spaces2,3. Instead, they can use alternative strategies to transmit their actin-driven forces to the substrate, explaining their migratory adaptation to changing external environments4-6. However, whether these generalized concepts apply to all immune cells is unclear. Here, we show that the movement of mast cells (immune cells with important roles during allergy and anaphylaxis) differs fundamentally from the widely applied paradigm of interstitial immune cell migration. We identify a crucial role for integrin-dependent adhesion in controlling mast cell movement and localization to anatomical niches rich in KIT ligand, the major mast cell growth and survival factor. Our findings show that substrate-dependent haptokinesis is an important mechanism for the tissue organization of resident immune cells.

An Allosteric Anti-tryptase Antibody for the Treatment of Mast Cell-Mediated Severe Asthma.

Severe asthma patients with low type 2 inflammation derive less clinical benefit from therapies targeting type 2 cytokines and represent an unmet need. We show that mast cell tryptase is elevated in severe asthma patients independent of type 2 biomarker status. Active ß-tryptase allele count correlates with blood tryptase levels, and asthma patients carrying more active alleles benefit less from anti-IgE treatment. We generated a noncompetitive inhibitory antibody against human ß-tryptase, which dissociates active tetramers into inactive monomers. A 2.15 Å crystal structure of a ß-tryptase/antibody complex coupled with biochemical studies reveal the molecular basis for allosteric destabilization of small and large interfaces required for tetramerization. This anti-tryptase antibody potently blocks tryptase enzymatic activity in a humanized mouse model, reducing IgE-mediated systemic anaphylaxis, and inhibits airway tryptase in Ascaris-sensitized cynomolgus monkeys with favorable pharmacokinetics. These data provide a foundation for developing anti-tryptase as a clinical therapy for severe asthma.

A new superhero, the intraepithelial mast cell, joins the "Guardians of the Gut".

The intestinal epithelium interacts with immune cells to support tissue homeostasis and coordinate responses against pathogens. In this issue of Immunity, Yang et al. unveil a central role for mast cell-epithelial cell interactions in orchestrating protective type 2 immune responses following intestinal helminth infection.

Mast cells: The Janus of type 2 inflammation.

Mast cells (MCs) are effectors in type 2 immunity, well known for their detrimental roles in allergy. In this issue of Immunity, Alhallak et al. now identify a protective role of MCs against exacerbated immune responses mediated by prostaglandin E2 (PGE2)-driven soluble ST2.

Mast cells control lung type 2 inflammation via prostaglandin E2-driven soluble ST2.

Severe asthma and sinus disease are consequences of type 2 inflammation (T2I), mediated by interleukin (IL)-33 signaling through its membrane-bound receptor, ST2. Soluble (s)ST2 reduces available IL-33 and limits T2I, but little is known about its regulation. We demonstrate that prostaglandin E2 (PGE2) drives production of sST2 to limit features of lung T2I. PGE2-deficient mice display diminished sST2. In humans with severe respiratory T2I, urinary PGE2 metabolites correlate with serum sST2. In mice, PGE2 enhanced sST2 secretion by mast cells (MCs). Mice lacking MCs, ST2 expression by MCs, or E prostanoid (EP)2 receptors by MCs showed reduced sST2 lung concentrations and strong T2I. Recombinant sST2 reduced T2I in mice lacking PGE2 or ST2 expression by MCs back to control levels. PGE2 deficiency also reversed the hyperinflammatory phenotype in mice lacking ST2 expression by MCs. PGE2 thus suppresses T2I through MC-derived sST2, explaining the severe T2I observed in low PGE2 states.

Intraepithelial mast cells drive gasdermin C-mediated type 2 immunity.

A specialized population of mast cells residing within epithelial layers, currently known as intraepithelial mast cells (IEMCs), was originally observed over a century ago, yet their physiological functions have remained enigmatic. In this study, we unveil an unexpected and crucial role of IEMCs in driving gasdermin C-mediated type 2 immunity. During helminth infection, αEß7 integrin-positive IEMCs engaged in extensive intercellular crosstalk with neighboring intestinal epithelial cells (IECs). Through the action of IEMC-derived proteases, gasdermin C proteins intrinsic to the epithelial cells underwent cleavage, leading to the release of a critical type 2 cytokine, interleukin-33 (IL-33). Notably, mast cell deficiency abolished the gasdermin C-mediated immune cascade initiated by epithelium. These findings shed light on the functions of IEMCs, uncover a previously unrecognized phase of type 2 immunity involving mast cell-epithelial cell crosstalk, and advance our understanding of the cellular mechanisms underlying gasdermin C activation.

Lipid-orchestrated paracrine circuit coordinates mast cell maturation and anaphylaxis through functional interaction with fibroblasts.

Interaction of mast cells (MCs) with fibroblasts is essential for MC maturation within tissue microenvironments, although the underlying mechanism is incompletely understood. Through a phenotypic screening of >30 mouse lines deficient in lipid-related genes, we found that deletion of the lysophosphatidic acid (LPA) receptor LPA1, like that of the phospholipase PLA2G3, the prostaglandin D2 (PGD2) synthase L-PGDS, or the PGD2 receptor DP1, impairs MC maturation and thereby anaphylaxis. Mechanistically, MC-secreted PLA2G3 acts on extracellular vesicles (EVs) to supply lysophospholipids, which are converted by fibroblast-derived autotaxin (ATX) to LPA. Fibroblast LPA1 then integrates multiple pathways required for MC maturation by facilitating integrin-mediated MC-fibroblast adhesion, IL-33-ST2 signaling, L-PGDS-driven PGD2 generation, and feedforward ATX-LPA1 amplification. Defective MC maturation resulting from PLA2G3 deficiency is restored by supplementation with LPA1 agonists or PLA2G3-modified EVs. Thus, the lipid-orchestrated paracrine circuit involving PLA2G3-driven lysophospholipid, eicosanoid, integrin, and cytokine signaling fine-tunes MC-fibroblast communication, ensuring MC maturation.

Making good of a tricky start: How IgE and mast cells manage a protective sway in food allergy.

The immune and nervous systems respond to dangerous stimuli to maintain homeostasis. In a recent issue of Nature, Florsheim et al. and Plum et al. uncover the crosstalk between immunoglobulin E (IgE)-mast-cell-mediated immune activation and neural responses driving behavioral avoidance of allergenic food.

Platelets and mast cells promote pathogenic eosinophil recruitment during invasive fungal infection via the 5-HIAA-GPR35 ligand-receptor system.

Cryptococcus neoformans is the leading cause of fungal meningitis and is characterized by pathogenic eosinophil accumulation in the context of type-2 inflammation. The chemoattractant receptor GPR35 is expressed by granulocytes and promotes their migration to the inflammatory mediator 5-hydroxyindoleacetic acid (5-HIAA), a serotonin metabolite. Given the inflammatory nature of cryptococcal infection, we examined the role of GPR35 in the circuitry underlying cell recruitment to the lung. GPR35 deficiency dampened eosinophil recruitment and fungal growth, whereas overexpression promoted eosinophil homing to airways and fungal replication. Activated platelets and mast cells were the sources of GPR35 ligand activity and pharmacological inhibition of serotonin conversion to 5-HIAA, or genetic deficiency in 5-HIAA production by platelets and mast cells resulted in more efficient clearance of Cryptococcus. Thus, the 5-HIAA-GPR35 axis is an eosinophil chemoattractant receptor system that modulates the clearance of a lethal fungal pathogen, with implications for the use of serotonin metabolism inhibitors in the treatment of fungal infections.

Decoupling the Functional Pleiotropy of Stem Cell Factor by Tuning c-Kit Signaling.

Most secreted growth factors and cytokines are functionally pleiotropic because their receptors are expressed on diverse cell types. While important for normal mammalian physiology, pleiotropy limits the efficacy of cytokines and growth factors as therapeutics. Stem cell factor (SCF) is a growth factor that acts through the c-Kit receptor tyrosine kinase to elicit hematopoietic progenitor expansion but can be toxic when administered in vivo because it concurrently activates mast cells. We engineered a mechanism-based SCF partial agonist that impaired c-Kit dimerization, truncating downstream signaling amplitude. This SCF variant elicited biased activation of hematopoietic progenitors over mast cells in vitro and in vivo. Mouse models of SCF-mediated anaphylaxis, radioprotection, and hematopoietic expansion revealed that this SCF partial agonist retained therapeutic efficacy while exhibiting virtually no anaphylactic off-target effects. The approach of biasing cell activation by tuning signaling thresholds and outputs has applications to many dimeric receptor-ligand systems.