Developments in Mast Cell Research

Abstract

This review summarizes major advances in mast cell (MC) research published between January 2024 and December 2025. Recent discoveries have reshaped our understanding of MC biology, revealing previously unrecognized roles in brain development, mucosal immunity, neuroimmune communication, and cancer. High-resolution structural studies have provided new insights into high affinity receptor for immunoglobulin (Ig)E (FcεRI) signaling, while emerging evidence has expanded the functional repertoire of inflammasome components and established Mas-related G-protein coupled receptor member (MRGPRX)2 as a central receptor mediating IgE-independent MC activation. Single-cell transcriptomic and spatial profiling studies have uncovered remarkable MC heterogeneity, identifying specialized populations with distinct homeostatic and pathogenic functions across tissues. These findings have also redefined the contribution of MCs to early-life allergic disease, anaphylaxis, pain, infection, and tumor immunity, highlighting their context-dependent functions. In parallel, major therapeutic advances—including antibodies targeting the KIT–stem cell factor (SCF) axis, next-generation anti-IgE therapies, Bruton’s tyrosine kinase (BTK) inhibitors, and strategies targeting type 2 inflammatory pathways—are accelerating the development of more selective treatments for MC-driven disorders. Advances in experimental platforms, including human skin MC cultures, receptor-humanized and humanized mouse models, and computational pathology, are further expanding the tools available to investigate MC biology and accelerate translational research. Together, these conceptual, technological, and therapeutic advances are shaping the next generation of MC-targeted therapies.

INTRODUCTION

MCs are tissue-resident hematopoietic cells of the myeloid lineage. Current understanding of MC ontogeny based on fate mapping studies in mice indicates that the initial population originates from progenitors in the extra-embryonic yolk sac, which is later supplemented by subsequent waves of definitive hematopoiesis^1,2. MCs are highly evolutionarily conserved; their biological origins predate Paul Ehrlich’s discovery of them by more than 500 million years³. Indeed, cells displaying MC-like features are present in tunicates, the closest living invertebrate relative to vertebrates, where they likely exert ancestral protective functions⁴. In line with these observations, several studies support a crucial role for MCs in host defense⁵. This beneficial contribution to innate immunity is facilitated by their strategic location at barrier tissues—such as the skin and the mucosa of the airways and gastrointestinal tract where mast cells comprise approximately 25–35% of all immune cells in humans⁶ as well as their significant heterogeneity, plasticity, and long-term survival within tissues. Furthermore, MCs produce a broad array of mediators with diverse biological activities that orchestrate early immune responses.

Unlike other immune cells, MCs possess a unique ability to respond to physical and environmental stimuli including allergen-specific via IgE and its high-affinity receptor, FcεRI. This axis provides protection against certain pathogens⁷ and venoms^{8, 9, 10} and has recently been implicated in avoidance behaviors toward food allergens^{11, 12}. These findings support the “allergic immunity” hypothesis: the concept that IgE and MCs function as a specialized, evolutionarily-conserved defense mechanism against environmental threats¹³. This framework also suggests that allergic pathology arises when MC functions are dysregulated in response to otherwise non-threatening stimuli. Evidence suggests that such dysregulation is likely caused by environmental pressures during critical pre- and postnatal developmental windows, often exacerbated by genetic vulnerabilities¹⁴.

It is also important to note that IgE-independent mechanisms significantly expand the MC activation repertoire. Activation via the G-protein coupled receptor MrgprB2 (in mice) and its human homolog MRGPRX2 allows connective tissue and skin MCs to respond to a vast array of basic secretagogues and neuropeptides¹⁵. This pathway facilitates critical interactions between MCs and the nervous system, further establishing these cells as multi-modal sensors of tissue distress.

Beyond its functional role, MrgprB2 may represent the first cell surface marker capable of distinguishing between murine MC subsets. Traditionally, mouse MCs have been classified into two major phenotypically distinct populations: connective tissue MCs (CTMCs) and mucosal MCs (MMCs). These subsets differ in their anatomical distribution, protease repertoire, proteoglycan composition, and dependence on specific growth factors¹⁶. Analysis of single-cell transcriptomic datasets from multiple mouse tissues has shown that MrgprB2 is broadly expressed by CTMCs across virtually all organs, whereas MMCs, which are found predominantly in the lamina propria of the gastrointestinal tract, lack MrgprB2 expression¹⁷. Thus, in mice, MrgprB2 serves not only as a functional receptor but also as a distinguishing marker of CTMCs. In contrast, human MC heterogeneity extends well beyond the classical CTMC/MMC dichotomy, encompassing multiple transcriptionally and functionally distinct populations including 6 distinct MC clusters, with 3 of these clusters found in the lung^17–19. These observations suggest that MRGPRX2 expression does not delineate discrete MC subsets in humans and, unlike murine MrgprB2, cannot be used as a marker to distinguish human MC populations.

However, MRPGRX2 may yet distinguish distinct subsets of MCs in organs and/or disease settings that have not been adequately studied.

This review provides an overview of the major advances in MC research that have addressed longstanding questions in the field while uncovering previously unrecognized aspects of MC biology. We also highlight recent progress in therapeutics targeting MC, as well as emerging experimental tools that have expanded our ability to investigate these cells. Together, these developments reflect the growing interest within the scientific community in understanding MC biology and leveraging this knowledge to develop more selective, effective, and safe approaches for modulating MC function.

PHYSIOLOGICAL FUNCTIONS

Brain development

Previously unrecognized roles for MCs were recently discovered in the developing brain, a period during which their abundance is markedly increased²⁰. Advanced imaging and transcriptomic analyses of rat brains demonstrated that yolk sac-derived MCs populate multiple brain-associated structures, including the pia mater, choroid plexus, cortex, and peri-hippocampal regions, where they undergo local proliferation during late embryonic and early postnatal development (Fig. 1A). Importantly, neonatal rat brain MC activation increases blood–brain barrier permeability and promotes the recruitment of inflammatory cells into the developing brain, suggesting that MCs have the potential to actively participate in shaping the neonatal neuroimmune environment.

Figure 1. Physiological functions of MCs.

(A–B) Distribution of MCs within the developing brain (A); and protective role of dural MC-derived histamine in maintaining central nervous system barrier integrity by restricting communication between the brain and peripheral tissues, thereby limiting the entry of pathogens such as Group B Streptococcus (GBS) into the central nervous system (B). (C-D) MCs distinguish between extracellular bacteria (left panel) and invasive bacteria (type-tree-secretion system-1 (TTSS-1)-proficient bacteria strain) (right panel), tailoring the magnitude and quality of their responses to mount either a modest or a robust immune response, respectively (C); and MC phenotypes differ in antibacterial responses due to distinct TLR expression profiles (D). (E) MCs promote adaptive immunity at mucosal surfaces. MC-derived 5-hydroxyindoleacetic acid (5-HIAA) promotes recruitment of conventional dendritic cell type 2 (cDC2s) to Peyer’s patches where cDC2s interact with B cells to induce their IgA class-switch recombination

Emerging evidence further indicates that MCs serve protective functions during this vulnerable developmental window²¹. Studies in mice showed that dural MCs regulate cerebrospinal fluid dynamics at arachnoid cuff exit points through histamine-mediated vasodilation of bridging veins and help maintain the separation between the central nervous system and peripheral tissues. During neonatal Group B Streptococcus (GBS) infection, MCs restrict bacterial access to the brain and promote local innate immune responses by recruiting neutrophils into both the dura and brain parenchyma (Fig. 1B). Given the heightened susceptibility of neonates to GBS infection and meningitis, the increased abundance of MCs in the developing brain may provide an important layer of protection during this vulnerable period.

Together, these findings suggest that MCs are active participants in brain development and early host defense rather than transient immune residents. However, defining their precise developmental and homeostatic functions remains an important challenge for future investigation. Importantly, the influx of MCs into the developing brain may also increase central nervous system vulnerability to environmental insults, including maternal infection, allergy, and stress. MC degranulation can promote the infiltration of circulating immune cells into the brain through vascular leakage and endothelial activation, accompanied by the release of pro-inflammatory mediators²⁰. Recent studies further suggest that this vulnerability to MC overactivation persists into adulthood, even as MC numbers decline in the brain. In mice, for example, MrgprB2-mediated activation of meningeal MCs contributes to post-stroke inflammation, in part by inhibiting semaphorin 3A, a chemorepellent that helps maintain the barrier between the dura and brain²². Collectively, these findings identify MCs as potential therapeutic targets for central nervous system disorders across the lifespan, from development through adulthood.

Host defense

MCs detect microbial threats through multiple receptor systems, including Toll-like receptors (TLRs) and MRGPRX2. Recent studies in mice have shown that MCs can distinguish between simple bacterial exposure and invasive tissue infection, allowing them to tailor the magnitude of their responses according to the threat encountered. Using Salmonella enterica serovar Typhimurium as a model pathogen, von Beek et al. demonstrated that extracellular bacteria induce relatively modest TLR-dependent cytokine responses, whereas invasive strains trigger much stronger activation programs²³ (Fig. 1C). Follow up studies also revealed substantial functional specialization among MC subsets. MMCs express lower levels of TLRs and respond poorly to extracellular bacteria, whereas CTMCs mount more robust antimicrobial responses²⁴ (Fig. 1D). These findings help reconcile previously conflicting reports regarding the expression and contribution of TLRs to bacterial sensing by MCs²⁵. More importantly, they highlight a previously underappreciated layer of functional specialization among MC subsets and suggest that tissue-specific programs shape how MCs detect and respond to microbial challenges.

Tissue immunity

Beyond recruiting professional pathogen-killing leukocytes to sites of infection^{26, 27}, MCs are increasingly recognized as regulators of tissue-resident immune cells that shape local immune responses.

Extensive bidirectional communication between MCs and macrophages were recently identified. Using primary mouse and human cells, it has been shown that MC-derived mediators can precondition macrophages and alter their responses to subsequent microbial stimuli²⁸. In addition, macrophages directly internalize extracellular MC granules in vivo in murine models, which enhances their phagocytic capacity and induces transcriptional programs that do not conform to classical M1 or M2 polarization states²⁹. Similarly, mouse MC-derived colony-stimulating factor 1 (CSF1) promotes specialized macrophage populations with features of both M1 and M2 states³⁰. Together, these findings suggest that MCs instruct context-dependent macrophage states tailored to local tissue needs.

MCs also contribute to adaptive immunity at mucosal surfaces. In the mouse intestine, MC-derived 5-hydroxyindoleacetic acid (5-HIAA), a ligand for the receptor G protein-coupled receptor 35 (GPR35), promotes the recruitment of conventional type 2 dendritic cells (cDC) to Peyer’s patches, facilitating interactions with B cells and the generation of protective IgA responses³¹ (Fig. 1E). Because IgA is essential for maintaining intestinal homeostasis while limiting pathogen invasion, these findings identify MCs as important organizers of mucosal immune networks that extend well beyond their traditional role as inflammatory effector cells.

MAST CELL HETEROGENEITY

One of the earliest observed differences in MC subsets was the dependency, or lack thereof, on T cells. The early paradigm was that murine MMCs and human MC_T are preferentially dependent on T-cell signals, particularly for their expansion and maintenance in mucosal tissues, respectively^{32, 33}. In contrast, connective tissue type mast cells, including murine CTMCs and human MC_TC are relatively T-cell independent^{33, 34}.

Recent studies in mouse and human systems have firmly established, however, that MC heterogeneity extends far beyond the traditional binary classification systems that have dominated the field for decades^{17, 35}. Instead, MCs exhibit remarkable plasticity across tissues and disease states, with their phenotypes being continuously shaped by local microenvironmental cues³⁶. In turn, recent single-cell spatial transcriptomic studies of the human bronchial wall have revealed that heterogeneous MC populations residing in specialized lung niches express distinct genes with the potential to regulate their local immune ecosystems and influence tissue architecture, including amphiregulin and proteases³⁷.

As additional populations are identified in diverse anatomical and pathological settings, understanding the mechanisms that govern MC specialization has become a major priority. A key challenge moving forward will be to distinguish pathogenic MC populations from those that exert immunoregulatory or tissue-protective functions and to determine whether these differentiation pathways can be therapeutically manipulated.

Transforming growth factor-β (TGF-β) has emerged as a central regulator of MMC specialization. Initial studies demonstrated that TGF-β drives the differentiation of a murine MMC population that expands during T2 airway inflammation³⁸. Subsequent work extended these findings to humans by showing that TGF-β promotes the development of a distinct MC population in nasal polyps and in the epithelial compartment of patients with ulcerative colitis³⁹. Additional evidence suggests that common differentiation programs may operate across multiple mucosal tissues. TGF-β-dependent intraepithelial intestinal MCs that emerge in mouse models of food allergy display striking transcriptional similarities to MC populations identified in nasal polyps, including an enhanced capacity to produce cysteinyl leukotrienes (cys-LT)⁴⁰. The shared molecular features of these MC populations suggest that common differentiation programs may operate across distinct mucosal barrier tissues and may be conserved between mice and humans.

NEW INSIGHTS INTO MC ACTIVATION AND LIPID MEDIATOR BIOLOGY

Structural insight into FcεRI signaling

Recent advances in cryo-electron microscopy have provided the first high-resolution structures of FcεRI, the central receptor mediating IgE-dependent MC activation and a major driver of allergic diseases. These studies offer unprecedented insight into the molecular mechanisms governing allergic responses and reveal previously unappreciated roles for receptor organization and membrane lipids in MC signaling.

In MCs, FcεRI is a tetrameric receptor complex composed of one FcεRIα chain, one FcεRIβ chain, and a homodimer of FcRγ (formerly FcεRIγ)^{41, 42}. Within this complex, FcεRIα binds the Fc portion of IgE⁴³, whereas FcεRIβ and FcRγ mediate intracellular signal transduction through immunoreceptor tyrosine-based activation motifs (ITAMs)⁴⁴. Structural analyses demonstrated that FcεRIα plays a central role in receptor assembly through its interactions with FcεRIβ and the FcRγ homodimer, while FcRγ is essential for maintaining receptor integrity and supporting IgE-mediated degranulation⁴⁵. Unexpectedly, cholesterol-like molecules were identified within the transmembrane domains and appear to stabilize interactions between FcεRIβ and the FcεRIα–FcRγ complex, suggesting that membrane lipids actively stabilize receptor architecture and may directly influence signaling⁴⁵.

Structural analysis also addressed a longstanding question regarding the effects of monomeric IgE in the absence of antigen⁴⁶. Previous observations had shown that IgE can influence MC survival, migration, and cytokine production independently of receptor crosslinking^47–49, but the underlying mechanisms remained unclear. Structural data revealed a previously unrecognized dimeric organization of FcεRI that may represent a resting state of the receptor. Binding of monomeric IgE promoted a transition toward monomeric receptor forms increasing the accessibility of ITAM-containing signaling domains and inducing transcriptional programs associated with MC activation, survival, and migration⁴⁶. Together, these findings fundamentally reshape our understanding of how IgE regulates MC biology under both homeostatic and allergic conditions.

Inflammasome components regulate MC degranulation

The inflammasome is a multiprotein signaling complex that plays a central role in innate immunity and inflammation. The best-characterized inflammasome, NLR family pyrin domain containing 3 (NLRP)3, consists of the cytosolic sensor NLRP3, the adaptor protein apoptosis-associated speck-like protein containing a CARD (ASC), and the effector protease pro-caspase-1⁵⁰. Upon activation, assembly of this complex promotes the maturation and release of pro-inflammatory cytokines such as IL-1β and IL-18⁵¹, thereby amplifying inflammatory responses.

Recent work in mice has uncovered an unexpected non-canonical function for inflammasome proteins in MC activation⁵². Beyond their established roles in cytokine maturation, NLRP3 and ASC assemble into structures associated with secretory granules following IgE-mediated activation. These complexes, termed granulosomes, facilitate granule trafficking along microtubule networks toward the plasma membrane, thereby promoting degranulation⁵². These findings expand the functional repertoire of inflammasome components and reveal an unexpected link between innate immune signaling machinery and the secretory processes that underpin MC effector functions.

MRGPRX2: beyond an activation receptor

MRGPRX2, a member of the Mas-related G protein-coupled receptor family, has emerged as a central regulator of IgE-independent MC activation and neuroimmune communication. This broadly responsive receptor recognizes a diverse array of endogenous and exogenous ligands, including neuropeptides, antimicrobial peptides, drugs, and pharmaceutical excipients, thereby contributing to itch, pain, neurogenic inflammation, and pseudoallergic drug reactions¹⁵.

Recent studies using human systems suggest that MRGPRX2 also contribute to MC development and tissue organization⁵³. Specifically, MRGPRX2 expression increases during MC differentiation, although substantial donor-to-donor heterogeneity is observed and persists in mature MCs. Importantly, studies using a fully humanized, reinnervated skin organ culture model showed that, during sensory nerve reinnervation and sprouting, MCs preferentially associate with nerve fibers producing substance P (SP), an MRGPRX2 ligand that has been shown to promote MC differentiation when added to MC cultures⁵³. These observations support a model in which neuropeptide-rich environments selectively recruit and maintain distinct MC populations, establishing specialized neuroimmune niches across tissues.

Lipid mediator biology in human lung MCs

Comprehensive lipidomic analyses have significantly expanded our understanding of mediator production by human lung MCs upon IgE-dependent activation⁵⁴. While prostaglandin (PG)D₂ and cys-LTs remain dominant products of FcεRI activation, studies by Johnsson AK et al. have also identified substantial production of thromboxane A₂ (TXA₂)⁵⁴, a potent bronchoconstrictor that likely exerts its effects on the lower airway indirectly through the release of acethylcholine⁵⁵. Overall, this study suggests that TXA₂ to MC-driven airway disease may have been underestimated.

These analyses further challenge classical models of arachidonic acid metabolism. Although lipid mediator intermediates can be shunted within and between cells, the use of specific inhibitors of the cyclooxygenase (COX) and lipoxygenase (LOX) pathways demonstrated that MCs have a tightly regulated system in which mediators do not shunt to alternative pathways. Overall, these findings indicate that eicosanoid synthesis in MCs is more tightly compartmentalized and regulated than previously appreciated and may require a reassessment of how lipid mediator pathways are therapeutically targeted in allergic disease.

MCS IN ALLERGIC DISEASE

MCs shape allergic disease in early life

Early-life environmental exposures may have long-lasting effects on MC function and related T2 predominant disorders such as asthma, chronic rhinosinusitis and eczema, often associated with the development of allergen sensitization. Recent studies have identified two mechanisms through which MCs influence disease development during this critical developmental window.

First, Joulia R et al. reported that mouse MCs contribute to vascular remodeling during early-life allergic airway disease⁵⁶. The authors show evidence that MC-derived proteases, including tryptase impair pericyte coverage of blood vessels, a pathophysiological process that can potentially disrupt pericyte–endothelial cell interactions, thereby compromising endothelial cell fitness and blood vessel integrity. Because neovascularization is a hallmark of asthma associated airway remodeling, these findings suggest that MCs may initiate structural changes that ultimately contribute to persistent lung dysfunction. Another hallmark of airway remodeling in asthma is thickening of the basement membrane, which primarily reflects expansion of the lamina reticularis. Although the functional significance of this structural change remains incompletely understood, a detailed three-dimensional morphometric analysis of participants in the Severe Asthma Research Program (SARP)-3 demonstrated that basement membrane thickness is associated with increased expression of MC-related genes within the airway epithelium, as well as with a gene signature indicative of T2 inflammation. Notably, basement membrane thickening was most pronounced in younger individuals within the cohort⁵⁷. Consistent with these findings, our group showed that basement membrane thickness is specifically associated with infiltration of the airway epithelium by MCs and further confirmed the inverse relationship between basement membrane thickness and age.

Finally, Serhan N et al. provided evidence that prenatal maternal stress in mice appears to promote MC dysregulation in offspring and increase susceptibility to eczema⁵⁸. Specifically, elevated maternal corticosterone levels can directly influence fetal MC developmental programs through the glucocorticoid receptor nuclear receptor subfamily 3 Group C member (Nr3c)1 (Fig. 2A). Notably, eczema-like disease resolved as prenatally programmed MCs are gradually replaced by new MCs derived from hematopoietic stem cells. These findings provide a potential explanation for the spontaneous remission frequently observed in childhood eczema and demonstrate how early environmental factors can shape long-term allergic risk.

Figure 2. MC contributions to allergic disease.

(A) MCs shape allergic diseases in early life. Stress exposure can cause an increase in maternal corticosterone in utero that has the potential to directly act on fetal mast cell activation program via the glucocorticoid receptor Nr3c1 and hence increase susceptibility to eczema. (B) MC-derived cys-LTs facilitate oral anaphylaxis. Cys-LTs acting on CysLTR1 and CysLTR2 link local intestinal responses to severe anaphylactic reactions and contribute to mucosal MC expansion. Dipeptidase (DPEP) 1 in the intestinal lamina propia can catabolize LTD₄ to limit allergen absorption and oral anaphylaxis. (C) IL-33 initiates an MC-platelet feed-forward loop that depends on MC production of cysteinyl leukotrienes (cys-LT) acting on platelets via CysLTR2. On its turn, platelets release adenine nucleotides acting on the purinergic receptor P2Y1R on MCs to amplify the generation of MC LTC₄ and PGD₂. (D) MC activation initiates the itch-scratching-pain sequence that culminates in skin inflammation. Upon activation, MCs release pruritogens that act on MrgprA3-expressing neurons. Scratching resulting from itch sensation causes pain that is sensed by TRPV1⁺ neurons that release substance P (SP) to induce more MC activation via MrpgB2 and release of cytokines that can promote skin inflammation like tumor necrosis factor (TNF).

Revisiting MC contribution to IgG-mediated anaphylaxis

Anaphylaxis is classically mediated by antigen-induced crosslinking of antigen-specific IgE bound to FcεRI on MCs and basophils⁵⁹. However, anaphylaxis can also be triggered by IgG-dependent pathways⁶⁰. Previous studies in mice concluded that IgG-mediated anaphylaxis is driven predominantly by platelet-activating factor (PAF), with little or no contribution from MCs or histamine^61–63.

This paradigm was challenged recently by the demonstration that MC-derived histamine contributes substantially to IgG-dependent anaphylaxis in mice⁶⁴. Rather than a single dominant pathway, IgG-mediated anaphylaxis appears to involve multiple effector populations. MCs contribute a histamine-dependent component, while other immune cells mediate PAF-dependent responses. The relative importance of each pathway varies according to biological and experimental factors, including genetic background, age, immune status, and prior infectious exposures. Importantly, these findings establish CTMCs as significant contributors to histamine release and disease severity and provide the first direct evidence that human MCs can be activated through an IgG-dependent mechanism to induce anaphylaxis.

MC internalization of neutrophils during anaphylaxis: a strategy for defensive gain

Neutrophils express Fc receptors for IgG and have been implicated in the amplification of certain anaphylactic reactions⁶⁵. One of the most unexpected discoveries in recent years is the ability of mouse MCs to engulf living neutrophils during passive cutaneous anaphylaxis⁶⁶. Intravital imaging demonstrated structures termed MC intracellular traps (MITs), in which recruited neutrophils are internalized and retained within MCs. Although this process occurs only in a subset of neutrophils, it has important consequences for MC biology. By recycling neutrophil-derived material, MCs acquire enhanced metabolic fitness and adopt a more pro-inflammatory phenotype.

These findings reveal a previously unrecognized mechanism through which MCs can modify their own functional state during allergic responses and further illustrate the dynamic interactions that occur between innate immune cells in inflamed tissues.

Cys-LT orchestrate oral anaphylaxis

Recent studies have established cys-LT as central regulators of oral anaphylaxis. Studies in mice showed that intestinal MC-derived cys-LTs facilitate the progression from local intestinal allergic responses to systemic disease. Moreover, data indicate that anaphylaxis following active sensitization and challenge requires both cysteinyl leukotriene receptor (CysLTR)1 and CysLTR2, while both receptors function non-redundantly to drive MMC expansion⁴⁰.

At the same time, tissue-derived mechanisms can alter the signaling through this pathway. Resistance to oral anaphylaxis in mice was linked to increased expression of dipeptidase 1 (DPEP1), an enzyme that degrades LTD₄, a potent cys-LT that acts on both CysLTR1 and CysLTR2 to promote allergen absorption and anaphylaxis⁶⁷. Unlike LTD₄, its precursor LTC₄ is not degraded by DPEP1 and binds with equal affinity to both receptors. Together, these studies position LT production and metabolism as critical checkpoints controlling the transition from localized intestinal allergy to systemic anaphylaxis (Fig. 2B).

MCs exert bidirectional control over interleukin (IL)-33 signaling in asthma

The alarmin IL-33 is released by airway epithelial cells in response to microbial products, allergens, and other forms of cellular stress. Through signaling via its receptor ST2 and the IL-1 receptor accessory protein, IL-33 regulates the function of several immune cell populations, including MCs⁶⁸. In MCs, IL-33 directly induces the production of cytokines and enhances IgE-mediated release of pro-inflammatory mediators^{69, 70}. Given the strong genetic and experimental links between the IL-33/ST2 pathway and asthma, understanding how this signaling axis is regulated remains a major area of interest in asthma and related allergic disease research.

Our studies have identified an MC-dependent feed-forward loop mediated by IL-33 in humans, in which IL-33-activated MCs increase epithelial IL33 expression, which in turn serves as a signal that promotes the production of T2 cytokines by MCs⁷¹. More recently, a study of individuals with severe refractory asthma treated with the tyrosine kinase inhibitor imatinib, which reduced MC numbers and serum tryptase levels⁷², showed that imatinib almost completely inhibited IL33 production within the epithelial niche, providing in vivo support for the existence of this feed-forward loop³⁷.

Recent work demonstrates that IL-33 initiates a feed-forward circuit involving platelets and MCs in mice⁷³. MC-derived cys-LTs activate platelets via CysLT2R, which subsequently release adenine nucleotides that further stimulate MCs via P2Y purinoreceptor (P2RY)1 receptors, resulting in enhanced production of LTC₄ and PGD₂ as well as increased histamine release (Fig. 2C).

Conversely, MCs can also suppress IL-33-driven inflammation⁷⁴. A specialized subset of MMCs characterized by expression of β7 integrin and mast cell protease (MCPT)-1 responds to PGE₂ through the EP2 receptor by releasing soluble ST2, a decoy receptor that neutralizes IL-33 and limits its bioavailability.

These observations challenge the traditional view of MCs as purely pro-inflammatory cells and instead reveal them as dynamic regulators capable of both amplifying and restraining T2 inflammation depending on their microenvironment.

Neuroimmune circuits emerge as central regulators of MC function

Interactions between MCs and sensory neurons have emerged as fundamental drivers of allergic inflammation.

Recent studies have identified a neuroimmune amplification loop linking itch, scratching, and pain in mice⁷⁵. Early IgE-mediated MC activation stimulates MrgprA3-expressing sensory neurons, inducing scratching behavior. Scratching-induced tissue injury subsequently generates pain signals that activate transient receptor potential vanilloid (TRPV)1⁺ nociceptive neurons, triggering the release of SP. SP then acts on MCs through MrgprB2, promoting further MC activation and the production of inflammatory mediators such as tumor necrosis factor (TNF), thereby amplifying local skin inflammation (Fig. 2D). Despite its pathological consequences, this circuit appears to have evolved as a mechanism to boost protective immunity at barrier surfaces, as activation of the itch–scratch–pain axis enhances resistance to bacterial skin infections.

Importantly, neuronal regulation of MCs is bidirectional. While peptidergic neurons promote inflammation through SP, non-peptidergic sensory neurons characterized by expression of MrgprD, release glutamate that directly suppresses MC degranulation in mice through the glutamate ionotropic receptor kainate subunit (GluK)2. Pharmacological activation of this pathway attenuates inflammatory skin disease in MrgprB2-dependent experimental models, suggesting a potential therapeutic strategy for controlling MC-driven pathology⁷⁶.

Similar neuroimmune interactions are beginning to emerge in the airways. Recent studies indicate that MC MrgprB2 and sensory neurons participate in a positive feedback loop that amplifies allergic airway inflammation⁷⁷. These findings suggest that neuroimmune communication may represent a conserved mechanism linking barrier tissues to allergic disease.

Collectively, these advances redefine MCs as active organizers of allergic immunity that integrate developmental cues, neuronal signals, and tissue-derived factors to shape disease susceptibility, severity, and resolution.

MCS BEYOND ALLERGY

Pain

A growing body of evidence implicates MC–neuron interactions in multiple pain disorders, including tissue injury⁷⁸, gouty arthritis⁷⁹, alcohol withdrawal-induced headache in males⁸⁰, and irritable bowel syndrome⁸¹. Although the specific neuronal populations and mediators involved vary across tissues, a potential common mechanism has emerged whereby activation of nociceptive neurons leads to the release of neuropeptides like SP and calcitonin gene-related peptide (CGRP) that stimulate MCs through MrgprB2/MRGPRX2. Activated MCs subsequently release mediators that further stimulate sensory neurons, establishing a feed-forward circuit that amplifies pain signaling (Fig. 3A).

Figure 3. MCs in non-allergic diseases.

(A) MCs and pain. Specialized neurons involved in the detection of noxious stimuli release neuropeptides that activate MCs via MrpgB2/MRGPRX2 to cause the release of mediators that cause sensitization of peripheral sensory neurons that mediate pain perception. (B) MCs and cancer. Single-cell RNA sequencing analysis across tumors revealed the presence of MCs within the tumor microenvironment with pro-tumor characteristics (C1-HSPA1 cluster), and MCs located farther from tumor cells with capacity to present antigen to T cells to enhance anti-tumoral responses (C7-HLA-DR cluster). (C) MCs and DOCK8 immunodeficiency. Decreased production of type 17 cytokines causes intestinal dysbiosis which drives increased IL-25 production by Tuft cells. IL-25 induces IL-4 production by Th2 cells which are not inhibited by dysfunctional T regs. IL-4-driven mucosal MC (MMC) expansion and tryptase release increase intestinal permeability, antigen absorption, and oral anaphylaxis (extrinsic mechanisms, left panel). Disruption in the integrity of cortical actin structures increases access of secretory granules to plasma membrane for degranulation of connective tissue MCs (CTMCs) (intrinsic mechanisms, right panel).

Specialized nerve-associated MC populations have been identified in mice that actively participate in nociception⁷⁸. A GTP cyclohydrolase 1 (Gch1)–tetrahydrobiopterin (BH4) pathway in MCs supports serotonin production and contributes to injury-induced pain. Genetic disruption of this pathway significantly attenuates pain responses, reinforcing the concept that MCs are not merely downstream responders to neuronal activation but active participants in pain generation.

Cancer

Recent studies have highlighted the remarkable functional heterogeneity of MCs within the tumor microenvironment and suggest that their impact on cancer progression is highly context dependent. Large-scale single-cell analyses have identified distinct MC subsets associated with either favorable or poor clinical outcomes⁸². A conserved MC population, termed C1-HSPA1, was found to be broadly represented across multiple cancers and associated with poor clinical outcomes, suggesting a tumor-promoting role. In contrast, a distinct subset designated C7-HLA-DR displayed a more tumor-specific distribution and was preferentially located farther from tumor cells. Notably, C7-HLA-DR MCs exhibited extensive interactions with T cells and were associated with favorable prognosis and improved responses to immunotherapy indicating that some MCs may function as non-conventional antigen-presenting cells that support anti-tumor immunity (Fig. 3B). These observations have also revealed new therapeutic opportunities. Unexpectedly, it has been shown that cromolyn, a drug traditionally considered a MC stabilizer, can enhance antigen-presenting functions in MCs and improve responses to anti-programmed death-1 therapy in an experimental model of triple-negative breast cancer⁸³.

More recently, MCs were engineered to deliver oncolytic adenoviruses and release chemokines and inflammatory mediators to induce anticancer immune responses and inhibit tumor growth in several murine models and in a humanized human epidermal growth factor receptor-2-positive patient-derived xenograft model. Remarkably, the study by Xu Y et al. has shown that it is possible to harness tumor-specific antigens as allergens to generate IgE for MC sensitization and induce MC accumulation at antigen-positive tumor locations creating a spatially and temporally controlled immnunotherapeutic niche. Importantly, IgE-MCs were cleared within 2 weeks without inducing systemic anaphylaxis-like symptoms⁸⁴.

At the same time, substantial evidence continues to support tumor-promoting roles for MCs, but their impact may be highly dependent on tumor subtype. For example, patients with BRAF V600E-mutant colorectal cancer, a subtype associated with poor responses to chemotherapy, exhibited increased MC infiltration within the tumor microenvironment⁸⁵. Importantly, it has been shown that MC infiltration contributes to epithelial-to-mesenchymal transition, a critical process that enhances tumor cell invasion and metastatic dissemination⁸⁵.

Furthermore, studies in melanoma have uncovered bidirectional interactions between tumor cells and mouse MCs, demonstrating that tumor-derived factors can alter transcriptional programs associated with MC proliferation and function, effectively suppressing MC activity⁸⁶. Together, these findings illustrate that MCs can either promote or restrain tumor progression depending on their phenotype, localization, and interactions within the tumor microenvironment.

Septic shock

While MCs are well recognized for their protective role in host defense against infection, dysregulated MC activation can also contribute to the pathogenesis of severe sepsis and septic shock^{87, 88}. Studies have shown that the impact of MCs during infection is highly context dependent, with local MC activation promoting protective immune responses whereas excessive systemic activation can become detrimental⁸⁸.

Recent studies have identified a pathogenic platelet–MC circuit in mice that contributes to the progression from sepsis to septic shock⁸⁹. During severe infection, activated platelets release PAF, which stimulates perivascular MCs and triggers profound vascular dysfunction, a hallmark of septic shock. These findings position perivascular MCs at a critical checkpoint linking systemic inflammation to cardiovascular collapse and reinforce the context-dependent nature of MC functions during infection.

Primary atopic disorders

Inborn errors of immunity (IEIs) are a diverse group of disorders caused by genetic variants that disrupt immune system development or function. A growing number of IEIs are associated with prominent T2 inflammatory and allergic manifestations, including eczematous dermatitis, food allergy, urticaria, and anaphylaxis⁹⁰. Consequently, these disorders have emerged as valuable natural models for investigating the mechanisms that regulate allergic inflammation and MC biology in humans. Among these conditions, dedicator of cytokinesis (DOCK)8 deficiency has received particular attention because severe allergic disease and atopic manifestations represent hallmark features of this rare, combined immunodeficiency⁹¹ (CID). Recent studies in mice suggest that both extrinsic and intrinsic mechanisms contribute to MC dysregulation in DOCK8 immunodeficiency, resulting in increased susceptibility to food-induced allergic responses and cutaneous anaphylaxis, respectively (Fig. 3C). Extrinsic mechanisms are initiated by reduced production of IL-17, which promotes intestinal dysbiosis and stimulates IL-25 production by tuft cells. Elevated IL-25 enhances T helper 2 (Th2) cell production of IL-4, driving the expansion of MMCs. Expanded MMCs, in turn, exacerbate oral anaphylaxis by increasing intestinal permeability and facilitating antigen absorption through tryptase-mediated cleavage of epithelial tight junctions. This pathogenic feed-forward circuit amplifies allergen uptake and promotes severe food-induced anaphylactic responses⁹².

In separate studies, we have shown that DOCK8 can exert cell-intrinsic effects within CTMCs by maintaining cortical actin structures that restrict secretory granule access to the plasma membrane. Loss of DOCK8 disrupts this barrier and primes MCs for enhanced degranulation⁹³.

EMERGING THERAPEUTIC STRATEGIES TARGETING MCS

Revisiting anti-IL-5 therapies through a MC lens

Eosinophilic inflammation is a defining feature of T2 asthma and correlates strongly with disease severity. Accordingly, biologics have been developed to target the IL-5/ IL-5 receptor alpha subunit (IL-5Rα) axis that controls the maturation, activation, recruitment, survival of eosinophils⁹⁴. Therapeutic blockade of IL-5 with mepolizumab or reslizumab or IL-5Rα with benralizumab has been shown to reduce eosinophil numbers and improve asthma control, and these clinical benefits have traditionally been attributed to effects on eosinophils⁹⁵. However, recent evidence suggests that anti-IL-5 therapy targets MC populations.

Early studies have demonstrated that IL-5 can increase the number of MCs generated from cultured progenitor cells obtained from human peripheral blood or cord blood^{96, 97}. These effects were hypothesized to be mediated through low-level expression of a functional IL-5Rα⁹⁷. More recently, human circulating mast cell progenitors were shown to express the IL5RA gene, and IL-5 was found to promote their survival⁹⁸. Importantly, IL-5Rα expression on circulating MC progenitors is increased in association to lung function and allergic sensitization⁹⁹. Notably, treatment with either mepolizumab or benralizumab in small cohorts of patients with asthma reduced the numbers of circulating MC progenitors as well as eosinophils. These changes in MC progenitors were associated with a trend towards improved asthma control, while changes in lung function did not reach statistical significance. Whether IL-5 pathway blockade also reduces MC numbers within airway tissues remains unknown. Supporting these findings, an independent observational study reported that benralizumab, but not mepolizumab, efficiently depleted IL-5Rα-expressing MC progenitors in patients with severe asthma¹⁰⁰. It is important to point out that recent studies show that IL-5 may support beneficial MC functions in certain conditions including respiratory viral infections. Recent studies demonstrate that IL-5 enhances human MC production of type I and type III interferons in response to human coronavirus OC43, respiratory syncytial virus (RSV), and reovirus raising the possibility that IL-5 blockade could impair antiviral immunity and increase susceptibility to viral-induced asthma exacerbations¹⁰¹. This possibility is further supported by study showing that transcriptional changes associated with mepolizumab treatment in urban children with exacerbation-prone eosinophilic asthma point to persistence of MC-associated inflammatory networks despite effective eosinophil depletion, and may contribute to disease severity and exacerbation risk¹⁰².

Targeting the SCF-KIT axis

Because MC survival is uniquely dependent on SCF signaling through KIT, this pathway has emerged as one of the most promising strategies for therapeutic MC depletion.

KIT-targeting antibodies, including barzolvolimab and briquilimab, can effectively reduce MC numbers in pre-clinical models^103–106. Importantly, barzolvolimab has demonstrated clinical efficacy in patients with chronic inducible urticaria¹⁰⁷, and chronic spontaneous urticaria refractory to antihistamines¹⁰⁸ providing strong clinical evidence that MC depletion can produce meaningful therapeutic benefit in MC-driven diseases.

Newer approaches seek to simultaneously target MC survival and upstream T2 inflammatory pathways. CDX-622, a bispecific antibody targeting both SCF and thymic stromal lymphopoietin (TSLP), exemplifies this strategy by combining MC depletion with suppression of broader epithelial inflammatory programs¹⁰⁹.

A potential challenge for KIT-directed therapies is that KIT is also expressed by hematopoietic progenitors, melanocytes, and germ cells¹¹⁰. However, most recent studies showed that the observed adverse effects in barzolvolimab-treated patients were reversible and consistent with the expected effects of KIT inhibition including mild hair color changes¹¹¹, followed by mild-to-moderate neutropenia, which was not associated with infections.

Targeting IgE and FcεRI signaling pathways

The success of omalizumab has prompted the development of next-generation strategies that more effectively disrupt IgE-mediated immunity. Recent advances in antibody discovery, protein engineering, and structural immunology have generated novel approaches that target both free IgE and IgE already bound to FcεRI on allergic effector cells.

One promising strategy involves the development of allergen-specific blocking antibodies derived directly from the naturally occurring IgE repertoires of allergic patients¹¹². Specifically, human IgE antibodies for peanut allergens were re-engineered as a combination of two high-affinity IgG4 monoclonal antibodies that recognize three immunodominant epitopes shared by Ara h 2 and Ara h 6, two of the most clinically relevant peanut allergens. Proof-of-concept studies have demonstrated that engineered antibody cocktails can effectively block peanut allergen recognition and suppress allergic responses in mice.

At the same time, advances in structural biology have revealed opportunities to improve upon omalizumab itself. Although omalizumab primarily neutralizes free IgE, it also possesses a limited ability to dissociate IgE already bound to FcεRI, a mechanism known as facilitated dissociation¹¹³. Following dissociation, omalizumab captures free IgE and prevents its rebinding to FcεRI, thereby accelerating desensitization of allergic effector cells. High-affinity omalizumab variants have now been engineered to enhance this process, allowing more efficient displacement of FcεRI-bound IgE and accelerating desensitization of MCs and basophils, both of which express FcεRI¹¹⁴.

An alternative strategy led to the generation of novel antibody fragments that can directly target the structural elements that stabilize the IgE–FcεRI interaction¹¹⁵. These antibody fragments can actively remove IgE from pre-sensitized MCs, inhibiting activation and protecting against anaphylaxis in mice even after sensitization has occurred.

Collectively, these approaches represent a conceptual shift in anti-IgE therapy, moving from passive blocking of pre-formed IgE towards the active reversal of this process.

An alternative approach to targeting IgE is to prevent IgE-dependent MC activation by inhibiting the activity of enzymes critical for the FcεRI signaling pathway including BTK¹¹⁶. Studies have shown that acalabrutinib, a second-generation BTK inhibitor can prevent moderate passive systemic anaphylaxis in humanized mice¹¹⁷, and can effectively eliminate clinical reactivity to peanut allergic individuals during oral food challenges¹¹⁸. More recent studies suggest that acalabrutinib can also rescue humanized mice from ongoing passive peanut-induced systemic anaphylaxis and significantly increase survival rates when administered together with epinephrine¹¹⁹.

NEW TOOLS TO STUDY MC BIOLOGY

A recurring theme in MC research has been the development of experimental systems that more faithfully recapitulate MC biology in vivo. Because mature tissue MCs are difficult to isolate and maintain ex vivo, substantial efforts have focused on generating in vitro and in vivo models that better reflect the numbers, tissue distribution, phenotypes, and functional responses of human MCs. Recent advances in cell culture methods, humanized mouse models, and computational tissue analysis have significantly expanded the experimental toolkit available to the MC field.

One notable advance has been the continued refinement of protocols for the isolation, expansion, and maintenance of human skin MCs. Recent studies have demonstrated that in vitro-expanded human skin MCs closely resemble freshly isolated skin MCs in terms of phenotype and functional responses¹²⁰, establishing them as one of the most physiologically relevant in vitro systems currently available for studying human skin MC biology.

Progress has also been made in modeling the biology of MRGPRX2, the human ortholog of murine MrgprB2. Because these receptors share limited sequence homology, concerns have remained regarding the translational relevance of mouse studies. To address this limitation, two independent groups recently generated MRGPRX2 knock-in mice in which the human receptor replaces its murine counterpart. These models have already provided important insights into neuroimmune regulation of vascular permeability¹²¹ and have demonstrated that passive systemic anaphylaxis induced by IgE and antigen results in MRGPRX2 transactivation¹²². Importantly, pharmacological inhibition of MRGPRX2 attenuated disease severity, supporting the utility of these humanized models for evaluating novel MRGPRX2-targeted therapeutics¹²².

Humanized mouse models capable of supporting human MC engraftment have also improved considerably. Recent studies demonstrated successful generation of mature human MC populations within immunodeficient NSG-SGM3-IL15 and BALB/c Rag2−/− Il2rγ−/− SirpαNOD Flk2+/− (BRGSF) mice, with tissue distributions and receptor expression patterns that partially recapitulate human physiology¹²³. Both models developed detectable circulating human tryptase levels within the normal range observed in healthy individuals, indicating successful MC engraftment. Nevertheless, the persistence of endogenous mouse MCs remains an important limitation, as mediators released by human MCs may secondarily activate murine MCs and complicate interpretation of experimental outcomes.

Finally, advances in computational pathology are transforming the analysis of MCs in human tissues. Machine learning-based image analysis platforms now enable automated identification, quantification, and spatial characterization of MCs at single-cell resolution. Application of these approaches has uncovered previously unrecognized patterns of MC localization in diseases such as eosinophilic esophagitis and revealed associations between MC distribution, activation state, and disease severity¹²⁴.

Collectively, these advances illustrate how improved culture systems, receptor-humanized and humanized animal models, and computational tissue analysis are enabling increasingly sophisticated investigations of MC biology. Beyond providing better experimental platforms, these technologies have the potential to accelerate translational discoveries and will likely facilitate the development and evaluation of future MC-targeted therapies.

LIMITATIONS OF THIS REVIEW

This review does not encompass all important advances in MC research published during the review period. Notable areas not covered include recent progress in urticaria, and the expanding roles of MCs in autoimmune diseases and fibrosis. As our understanding of the complex and context-dependent functions of MCs in these disorders continues to evolve, these topics and others will warrant dedicated future reviews.

Many of the advances discussed in this review were made possible with the use of sophisticated rodent models. Although these experimental systems have been instrumental in uncovering fundamental mechanisms of MC biology, important species-specific differences require careful interpretation when extrapolating findings to humans. Nevertheless, these discoveries provide an essential foundation for translational research, particularly as increasingly sophisticated human MC culture systems, humanized mouse models, and computational approaches continue to improve the validation of experimental findings in human disease.

CONCLUSIONS AND OUTLOOK

Undoubtedly, MC research has advanced remarkably over the past two years. These advances have been driven, in large part, by technological innovations that now enable increasingly granular characterization of MCs across tissues during homeostasis and disease. Such studies have revealed an unprecedented degree of MC heterogeneity and plasticity, uncovered unexpected biological functions, and highlighted their extensive interactions with neighboring immune and non-immune cells. Importantly, these discoveries have also identified unique aspects of MC biology that may be exploited therapeutically, raising the possibility of selectively targeting MCs to attenuate pathological responses while preserving essential physiological functions.

Whether these emerging therapies will affect beneficial MC functions in humans remains an important unanswered question. In particular, the contribution of MCs to host defense against infectious agents and other environmental insults is still poorly understood. Valuable insights may come from the expanding clinical use of therapies that suppress MC activation or deplete MCs altogether. Careful longitudinal evaluation of treated patients may ultimately define the physiological importance of MCs in human immunity and clarify whether their protective functions are redundant or non-redundant.

Perhaps the most fundamental unanswered question is whether MCs perform indispensable functions during human development. To our knowledge, complete congenital MC deficiency has never been described in humans. One possibility is that MCs are required during embryonic or fetal development, such that their complete absence is incompatible with life. Alternatively, individuals lacking MCs may simply remain unrecognized because MC deficiency does not result in an overt clinical phenotype. The latter possibility is supported by studies in mice, where MC deficiency is generally compatible with normal development and the absence of obvious physiological abnormalities.

Nevertheless, this interpretation warrants caution. No currently available mouse model eliminates MCs throughout development, limiting our ability to definitively assess their developmental functions. Moreover, developmental compensation and redundancy may mask MC-dependent phenotypes in existing models. Recent evidence demonstrating the presence of MCs in the developing brain and other embryonic tissues has renewed interest in the possibility that these cells contribute to organogenesis, tissue patterning, or immune education during development. Resolving these questions will require improved experimental models, longitudinal human studies, and continued integration of single-cell, spatial, and functional approaches.

More broadly, the rapid pace of discovery over the past two years has fundamentally reshaped our understanding of MC biology. Rather than functioning solely as terminal effector cells of allergic disease, MCs are increasingly recognized as highly adaptable tissue-resident immune cells that integrate environmental cues, communicate extensively with surrounding cells, and participate in diverse physiological and pathological processes. As increasingly sophisticated experimental tools and therapeutic strategies continue to emerge, the coming years will likely provide definitive answers to many of the longstanding questions surrounding MC biology while uncovering new opportunities for selectively modulating these remarkable cells for therapeutic benefit.

Abbreviations used

5-HIAA

5-hydroxyindoleacetic acid

ASC

apoptosis-associated speck-like protein containing a CARD

BH4

tetrahydrobiopterin

BTK

Bruton’s tyrosine kinase

CGRP

calcitonin gene-related peptide

CSF

Colony-stimulating factor

CID

combined immunodeficiency

CTMC

Connective tissue mast cell

COX

cyclooxygenase

Cys-LT

cysteinyl leukotriene

CysLTR

cysteinyl leukotriene receptor

DOCK

dedicator of cytokinesis

DPEP

dipeptidase

FcεRIα

alpha subunit of the high affinity receptor for IgE

FcεRIβ

beta subunit of the high affinity receptor for IgE

FcRγ

gamma chain of Fc receptors

FcεRI

high affinity receptor for IgE

GBS

Group B Streptococcus

Gch1

GTP cyclohydrolase 1

GluK2

glutamate ionotropic receptor kainate subunit 2

GPR

G protein-coupled receptor

IEI

inborn errors of immunity

Ig

immunoglobulin

IL

interleukin

IL-5Rα

IL-5 receptor alpha subunit

ITAM

immunoreceptor tyrosine-based activation motifs

LOX

lipoxygenase

Mrgpr/MRGPR

Mas-related G-protein coupled receptor member

MC

mast cell

MMC

mucosal mast cell

MIT

MC intracellular traps

NLRP3

NLR family pyrin domain containing 3

Nr3c1

nuclear receptor subfamily 3 Group C member 1

PAF

platelet-activating factor

PG

prostaglandin

P2RY1

P2Y purinoreceptor 1

RSV

respiratory syncytial virus

SCF

stem cell factor

SP

substance P

TGF-β

transforming growth factor-β

Th2

T-helper 2

TNF

tumor necrosis factor

TLR

Toll-like receptor

TRPV

Transient receptor potential vanilloid

TSLP

thymic stromal lymphopoietin

TXA₂

thromboxane A₂ TXA₂