Cooperative Regulation of the Mucosal Mast Cell-Specific Protease Genes Mcpt1 and Mcpt2 by GATA and Smad Transcription Factors

Analysis of the heterogeneity and complexity of murine extraorbital lacrimal gland via single-cell RNA sequencing

PURPOSE

The lacrimal gland is essential for maintaining ocular surface health and avoiding external damage by secreting an aqueous layer of the tear film. However, a healthy lacrimal gland's inventory of cell types and heterogeneity remains understudied.

METHODS

Here, 10X Genome-based single-cell RNA sequencing was used to generate an unbiased classification of cellular diversity in the extraorbital lacrimal gland (ELG) of C57BL/6J mice. From 43,850 high-quality cells, we produced an atlas of cell heterogeneity and defined cell types using classic marker genes. The possible functions of these cells were analyzed through bioinformatics analysis. Additionally, the CellChat was employed for a preliminary analysis of the cell-cell communication network in the ELG.

RESULTS

Over 37 subclasses of cells were identified, including seven types of glandular epithelial cells, three types of fibroblasts, ten types of myeloid-derived immune cells, at least eleven types of lymphoid-derived immune cells, and five types of vascular-associated cell subsets. The cell-cell communication network analysis revealed that fibroblasts and immune cells play a pivotal role in the dense intercellular communication network within the mouse ELG.

CONCLUSIONS

This study provides a comprehensive transcriptome atlas and related database of the mouse ELG.

Basophil-Derived IL-4 and IL-13 Protect Intestinal Barrier Integrity and Control Bacterial Translocation during Malaria

Our previous work demonstrated that basophils regulate a suite of malaria phenotypes, including intestinal mastocytosis and permeability, the immune response to infection, gametocytemia, and parasite transmission to the malaria mosquito Anopheles stephensi. Given that activated basophils are primary sources of the regulatory cytokines IL-4 and IL-13, we sought to examine the contributions of these mediators to basophil-dependent phenotypes in malaria. We generated mice with basophils depleted for IL-4 and IL-13 (baso IL-4/IL-13 (-)) and genotype controls (baso IL-4/IL-13 (+)) by crossing mcpt8-Cre and Il4/Il13fl/fl mice and infected them with Plasmodium yoelii yoelii 17XNL. Conditional deletion was associated with ileal mastocytosis and mast cell (MC) activation, increased intestinal permeability, and increased bacterial 16S levels in blood, but it had no effect on neutrophil activation, parasitemia, or transmission to A. stephensi. Increased intestinal permeability in baso IL-4/IL-13 (-) mice was correlated with elevated plasma eotaxin (CCL11), a potent eosinophil chemoattractant, and increased ileal MCs, proinflammatory IL-17A, and the chemokines MIP-1α (CCL3) and MIP-1β (CCL4). Blood bacterial 16S copies were positively but weakly correlated with plasma proinflammatory cytokines IFN-γ and IL-12p40, suggesting that baso IL-4/IL-13 (-) mice failed to control bacterial translocation into the blood during malaria infection. These observations suggest that basophil-derived IL-4 and IL-13 do not contribute to basophil-dependent regulation of parasite transmission, but these cytokines do orchestrate protection of intestinal barrier integrity after P. yoelii infection. Specifically, basophil-dependent IL-4/IL-13 control MC activation and prevent infection-induced intestinal barrier damage and bacteremia, perhaps via regulation of eosinophils, macrophages, and Th17-mediated inflammation.

Butyrate, Valerate, and Niacin Ameliorate Anaphylaxis by Suppressing IgE-Dependent Mast Cell Activation: Roles of GPR109A, PGE2, and Epigenetic Regulation

Short-chain fatty acids (SCFAs) are produced by the intestinal microbiota during the fermentation of dietary fibers as secondary metabolites. Several recent studies reported that SCFAs modulate the development and function of immune-related cells. However, the molecular mechanisms by which SCFAs regulate mast cells (MCs) remain unclear. In the current study, we analyzed the function and gene expression of mouse MCs in the presence of SCFAs in vitro and in vivo. We found that the oral administration of valerate or butyrate ameliorated passive systemic anaphylaxis and passive cutaneous anaphylaxis in mice. The majority of SCFAs, particularly propionate, butyrate, valerate, and isovalerate, suppressed the IgE-mediated degranulation of bone marrow-derived MCs, which were eliminated by the Gi protein inhibitor pertussis toxin and by the knockdown of Gpr109a. A treatment with the HDAC inhibitor trichostatin A also suppressed IgE-mediated MC activation and reduced the surface expression level of FcεRI on MCs. Acetylsalicylic acid and indomethacin attenuated the suppressive effects of SCFAs on degranulation. The degranulation degree was significantly reduced by PGE2 but not by PGD2. Furthermore, SCFAs enhanced PGE2 release from stimulated MCs. The SCFA-mediated amelioration of anaphylaxis was exacerbated by COX inhibitors and an EP3 antagonist, but not by an EP4 antagonist. The administration of niacin, a ligand of GPR109A, alleviated the symptoms of passive cutaneous anaphylaxis, which was inhibited by cyclooxygenase inhibitors and the EP3 antagonist. We conclude that SCFAs suppress IgE-mediated activation of MCs in vivo and in vitro involving GPR109A, PGE2, and epigenetic regulation.

IL-10 Modulates the Expression and Activation of Pattern Recognition Receptors in Mast Cells

Mast cells (MCs) are involved in several immune-related responses, including those in bacterial infections, autoimmune diseases, inflammatory bowel diseases, and cancer, among others. MCs identify microorganisms by pattern recognition receptors (PRRs), activating a secretory response. Interleukin (IL)-10 has been described as an important modulator of MC responses; however, its role in PRR-mediated activation of MC is not fully understood. We analyzed the activation of TLR2, TLR4, TLR7 and Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) in mucosal-like MCs (MLMCs) and peritoneum-derived cultured MCs (PCMCs) from IL-10^-/- and wild-type (WT) mice. IL-10^-/- mice showed a reduced expression of TLR4 and NOD2 at week 6 and TLR7 at week 20 in MLMC. In MLMC and PCMC, TLR2 activation induced a reduced secretion of IL-6 and TNFα in IL-10^-/- MCs. TLR4- and TLR7-mediated secretion of IL-6 and TNFα was not detected in PCMCs. Finally, no cytokine release was induced by NOD2 ligand, and responses to TLR2 and TLR4 were lower in MCs at 20 weeks. These findings indicate that PRR activation in MCs depends on the phenotype, ligand, age, and IL-10.

Targeting the RhoA-GEF-H1 pathway of mast cells attenuates experimental airway allergy

Mast cells are the major effector cells in allergic diseases. RhoA and its downstream pathway is associated with the pathogenesis of airway allergy. The objective of this study is to test a hypothesis that modulating the RhoA-GEF-H1 axis in mast cells can attenuate airway allergy. An airway allergic disorder (AAD) mouse model was employed. Mast cells were isolated from AAD mouse airway tissues to be analyzed by RNA sequencing. We observed that mast cells isolated from the respiratory tract of AAD mice were resistant to apoptosis. Mast cell mediator levels in nasal lavage fluid were correlated with apoptosis resistance in AAD mice. Activation of RhoA in AAD mast cells was related to resistance to apoptosis. Mast cells isolated from the airway tissues in AAD mouse exhibited strong RhoA-GEF-H1 expression. The RhoA-GEF-H1 axis was associated with the lower FasL expression in AAD mast cells. Activation of the RhoA-GEF-H1 axis promoted the production of mediators in mast cells. Inhibition of GEF-H1 facilitated the SIT-induced mast cell apoptosis and enhanced the therapeutic efficacy of AAD. In conclusion, RhoA-GEF-H1 activities are associated with resistance to apoptosis in mast cells isolated from sites of allergic lesions. The state of apoptosis resistance in mast cells is associated with the state of AAD disease. Inhibition of GEF-H1 restores the sensitivity of mast cells to apoptosis inducers, and alleviates experimental AAD in mice.

Kaempferol Suppresses the Activation of Mast Cells by Modulating the Expression of FcεRI and SHIP1

In the present study, we evaluated the effects of kaempferol on bone marrow-derived mast cells (BMMCs). Kaempferol treatment significantly and dose-dependently inhibited IgE-induced degranulation, and cytokine production of BMMCs under the condition that cell viability was maintained. Kaempferol downregulated the surface expression levels of FcεRI on BMMCs, but the mRNA levels of FcεRIα, β, and γ-chains were not changed by kaempferol treatment. Furthermore, the kaempferol-mediated downregulation of surface FcεRI on BMMCs was still observed when protein synthesis or protein transporter was inhibited. We also found that kaempferol inhibited both LPS- and IL-33-induced IL-6 production from BMMCs, without affecting the expression levels of their receptors, TLR4 and ST2. Although kaempferol treatment increased the protein amount of NF-E2-related factor 2 (NRF2)-a master transcription factor of antioxidant stress-in BMMCs, the inhibition of NRF2 did not alter the suppressive effect of kaempferol on degranulation. Finally, we found that kaempferol treatment increased the levels of mRNA and protein of a phosphatase SHIP1 in BMMCs. The kaempferol-induced upregulation of SHIP1 was also observed in peritoneal MCs. The knockdown of SHIP1 by siRNA significantly enhanced IgE-induced degranulation of BMMCs. A Western blotting analysis showed that IgE-induced phosphorylation of PLCγ was suppressed in kaempferol-treated BMMCs. These results indicate that kaempferol inhibited the IgE-induced activation of BMMCs by downregulating FcεRI and upregulating SHIP1, and the SHIP1 increase is involved in the suppression of various signaling-mediated stimulations of BMMCs, such as those associated with TLR4 and ST2.

New Mechanistic Advances in FcεRI-Mast Cell-Mediated Allergic Signaling

Mast cells originate from the CD34⁺/CD117⁺ hematopoietic progenitors in the bone marrow, migrate into circulation, and ultimately mature and reside in peripheral tissues. Microbiota/metabolites and certain immune cells (e.g., Treg cells) play a key role in maintaining immune tolerance. Cross-linking of allergen-specific IgE on mast cells activates the high-affinity membrane-bound receptor FcεRI, thereby initiating an intracellular signal cascade, leading to degranulation and release of pro-inflammatory mediators. The intracellular signal transduction is intricately regulated by various kinases, transcription factors, and cytokines. Importantly, multiple signal components in the FcεRI-mast cell–mediated allergic cascade can be targeted for therapeutic purposes. Pharmacological interventions that include therapeutic antibodies against IgE, FcεRI, and cytokines as well as inhibitors/activators of several key intracellular signaling molecues have been used to inhibit allergic reactions. Other factors that are not part of the signal pathway but can enhance an individual’s susceptibility to allergen stimulation are referred to as cofactors. Herein, we provide a mechanistic overview of the FcεRI-mast cell–mediated allergic signaling. This will broaden our scope and visions on specific preventive and therapeutic strategies for the clinical management of mast cell–associated hypersensitivity reactions.

The Role of TGFβ and Other Cytokines in Regulating Mast Cell Functions in Allergic Inflammation

Mast cells (MC) are a key effector cell in multiple types of immune responses, including atopic conditions. Allergic diseases have been steadily rising across the globe, creating a growing public health problem. IgE-mediated activation of MCs leads to the release of potent mediators that can have dire clinical consequences. Current therapeutic options to inhibit MC activation and degranulation are limited; thus, a better understanding of the mechanisms that regulate MC effector functions in allergic inflammation are necessary in order to develop effective treatment options with minimal side effects. Several cytokines have been identified that play multifaceted roles in regulating MC activation, including TGFβ, IL-10, and IL-33, and others that appear to serve primarily anti-inflammatory functions, including IL-35 and IL-37. Here, we review the literature examining cytokines that regulate MC-mediated allergic immune responses.

The Basophil IL-18 Receptor Precisely Regulates the Host Immune Response and Malaria-Induced Intestinal Permeability and Alters Parasite Transmission to Mosquitoes without Effect on Gametocytemia

We have recently demonstrated that basophils are protective against intestinal permeability during malaria and contribute to reduced parasite transmission to mosquitoes. Given that IL-18 is an early cytokine/alarmin in malaria and has been shown to activate basophils, we sought to determine the role of the basophil IL-18R in this protective phenotype. To address this, we infected control [IL18r ^flox/flox or basoIL-18R (+)] mice and mice with basophils lacking the IL-18R [IL18r ^flox/flox × Basoph8 or basoIL-18R (-)] with Plasmodium yoelii yoelii 17XNL, a nonlethal strain of mouse malaria. Postinfection (PI), intestinal permeability, ileal mastocytosis, bacteremia, and levels of ileal and plasma cytokines and chemokines were measured through 10 d PI. BasoIL-18R (-) mice exhibited greater intestinal permeability relative to basoIL-18R (+) mice, along with increased plasma levels of proinflammatory cytokines at a single time point PI, day 4 PI, a pattern not observed in basoIL-18R (+) mice. Surprisingly, mosquitoes fed on basoIL-18R (-) mice became infected less frequently than mosquitoes fed on basoIL-18R (+) mice, with no difference in gametocytemia, a pattern that was distinct from that observed previously with basophil-depleted mice. These findings suggest that early basophil-dependent protection of the intestinal barrier in malaria is mediated by IL-18, and that basophil IL-18R-dependent signaling differentially regulates the inflammatory response to infection and parasite transmission.

Basophil Depletion Alters Host Immunity, Intestinal Permeability, and Mammalian Host-to-Mosquito Transmission in Malaria

Malaria-induced bacteremia has been shown to result from intestinal mast cell (MC) activation. The appearance of MCs in the ileum and increased intestinal permeability to enteric bacteria are preceded by an early Th2-biased host immune response to infection, characterized by the appearance of IL-4, IL-10, mast cell protease (Mcpt)1 and Mcpt4, and increased circulating basophils and eosinophils. Given the functional similarities of basophils and MCs in the context of allergic inflammation and the capacity of basophils to produce large amounts of IL-4, we sought to define the role of basophils in increased intestinal permeability, in MC influx, and in the development of bacteremia in the context of malaria. Upon infection with nonlethal Plasmodium yoelii yoelii 17XNL, Basoph8 × ROSA-DTα mice or baso (-) mice that lack basophils exhibited increased intestinal permeability and increased ileal MC numbers, without any increase in bacterial 16S ribosomal DNA copy numbers in the blood, relative to baso (+) mice. Analysis of cytokines, chemokines, and MC-associated factors in the ileum revealed significantly increased TNF-α and IL-13 at day 6 postinfection in baso (-) mice compared with baso (+) mice. Moreover, network analysis of significantly correlated host immune factors revealed profound differences between baso (-) and baso (+) mice following infection in both systemic and ileal responses to parasites and translocated bacteria. Finally, basophil depletion was associated with significantly increased gametocytemia and parasite transmission to Anopheles mosquitoes, suggesting that basophils play a previously undescribed role in controlling gametocytemia and, in turn, mammalian host-to-mosquito parasite transmission.

Salicylaldehyde Suppresses IgE-Mediated Activation of Mast Cells and Ameliorates Anaphylaxis in Mice

Mast cells (MCs) play key roles in IgE-mediated immunoresponses, including in the protection against parasitic infections and the onset and/or symptoms of allergic diseases. IgE-mediated activation induces MCs to release mediators, including histamine and leukotriene, as an early response, and to produce cytokines as a late phase response. Attempts have been made to identify novel antiallergic compounds from natural materials such as Chinese medicines and food ingredients. We herein screened approximately 60 compounds and identified salicylaldehyde, an aromatic aldehyde isolated from plant essential oils, as an inhibitor of the IgE-mediated activation of MCs. A degranulation assay, flow cytometric analyses, and enzyme-linked immunosorbent assays revealed that salicylaldehyde inhibited the IgE-mediated degranulation and cytokine expression of bone-marrow-derived MCs (BMMCs). The salicylaldehyde treatment reduced the surface expression level of FcεRI, the high affinity receptor for IgE, on BMMCs, and suppressed the IgE-induced phosphorylation of tyrosine residues in intercellular proteins, possibly Lyn, Syk, and Fyn, in BMMCs. We also examined the effects of salicylaldehyde in vivo using passive anaphylaxis mouse models and found that salicylaldehyde administration significantly enhanced the recovery of a reduced body temperature due to systemic anaphylaxis and markedly suppressed ear swelling, footpad swelling, and vascular permeability in cutaneous anaphylaxis.

Mucosal Mast Cells as Key Effector Cells in Food Allergies

Mucosal mast cells (MMCs) localized in the intestinal mucosa play a key role in the development of IgE-mediated food allergies. Recent advances have revealed that MMCs are a distinctly different population from connective tissue mast cells localized in skin and other connective tissues. MMCs are inducible and transient cells that arise from bone marrow-derived mast cell progenitors, and their numbers increase rapidly during mucosal allergic inflammation. However, the mechanism of the dramatic expansion of MMCs and their cell functions are not well understood. Here, we review recent findings on the mechanisms of MMC differentiation and expansion, and we discuss the potential for the inducers of differentiation and expansion to serve as targets for food allergy therapy. In addition, we also discuss the mechanism by which oral immunotherapy, a promising treatment for food allergy patients, induces unresponsiveness to food allergens and the roles of MMCs in this process. Research focusing on MMCs should provide useful information for understanding the underlying mechanisms of food allergies in order to further advance the treatment of food allergies.

Mucosal Mast Cell-Specific Gene Expression Is Promoted by Interdependent Action of Notch and TGF-β Signaling

Rodent mast cells are classified into two major subsets, mucosal mast cells (MMCs) and connective tissue mast cells. MMCs arise from mast cell progenitors that are mobilized from the bone marrow to mucosal tissues in response to allergic inflammation or helminth infection. TGF-β is known as an inducer of MMC differentiation in mucosal tissues, but we have previously found that Notch receptor-mediated signaling also leads to the differentiation. Here, we examined the relationship between Notch and TGF-β signaling in MMC differentiation using mouse bone marrow-derived mast cells (BMMCs). We found that the coexistence of Notch and TGF-β signaling markedly upregulates the expression of MMC markers, mouse mast cell protease (mMCP)-1, mMCP-2, and αE integrin/CD103, more than Notch or TGF-β signaling alone, and that their signals act interdependently to induce these marker expressions. Notch and TGF-β-mediated transcription of MMC marker genes were both dependent on the TGF-β signaling transducer SMAD4. In addition, we also found that Notch signaling markedly upregulated mMCP-1 and mMCP-2 expression levels through epigenetic deregulation of the promoter regions of these genes, but did not affect the promoter of the CD103-encoding gene. Moreover, forced expression of the constitutively active Notch2 intracellular domain in BMMCs showed that Notch signaling promotes the nuclear localization of SMADs 3 and 4 and causes SMAD4-dependent gene transcription. These findings indicate that Notch and TGF-β signaling play interdependent roles in inducing the differentiation and maturation of MMCs. These roles may contribute to the rapid expansion of the number of MMCs during allergic mucosal inflammation.

Transcription Factors in the Development and Pro-Allergic Function of Mast Cells

Mast cells (MCs) are innate immune cells of hematopoietic origin localized in the mucosal tissues of the body and are broadly implicated in the pathogenesis of allergic inflammation. Transcription factors have a pivotal role in the development and differentiation of mast cells in response to various microenvironmental signals encountered in the resident tissues. Understanding the regulation of mast cells by transcription factors is therefore vital for mechanistic insights into allergic diseases. In this review we summarize advances in defining the transcription factors that impact the development of mast cells throughout the body and in specific tissues, and factors that are involved in responding to the extracellular milieu. We will further describe the complex networks of transcription factors that impact mast cell physiology and expansion during allergic inflammation and functions from degranulation to cytokine secretion. As our understanding of the heterogeneity of mast cells becomes more detailed, the contribution of specific transcription factors in mast cell-dependent functions will potentially offer new pathways for therapeutic targeting.

Altered gene expression signatures by calcitonin gene-related peptide promoted mast cell activity in the colon of stress-induced visceral hyperalgesia mice

BACKGROUND

Calcitonin gene-related peptide (CGRP) is possibly involved in recruitment of mucosal mast cells (MCs) in the gut that may be associated with the development of irritable bowel syndrome (IBS), but the role of CGRP on the activation of MCs is still unknown.

METHODS

Using RNA sequencing (RNA-seq), we examined differentially expressed genes (DEGs) in mouse MCs following CGRP treatment. The expression of key genes in colonic MCs and their relationship with CGRP-containing fibers were examined by immunofluorescence in chronic water-avoidance stress (WAS)-induced visceral hyperalgesia mice.

KEY RESULTS

A total of 29 DEGs were found significantly changed with 28 upregulated and 1 downregulated following treatment of MCs with CGRP. Bioinformatics analysis showed that key higher DEGs included those associated with response to corticotropin-releasing hormone (CRH), regulation of transcription, MC activation, and proliferation. These processes are enriched for genes associated with stress-induced MC activation in IBS. Western blot verified changes in representative DEGs (Nr4a3, Crem, Gpr35, FosB, Sphlk1) and real-time cell analysis (RTCA) verified the MC proliferation. The vast majority of colonic MCs nearly CGRP-containing fibers in WAS mice overexpressed only Nr4a3 with little to no FosB, Gpr35, Sphlk1, or Crem expression. Nr4a3 knockdown may attenuate the promotion effect of CGRP on MC viability.

CONCLUSIONS & INFERENCES

Our results suggest that CGRP is a critical regulator of key expressed genes in MC activation. Nr4a3 as a novel regulator of MC function may have an effect on stress-induced visceral hyperalgesia, and this may represent the novel target for drug development.

IL-10 in Mast Cell-Mediated Immune Responses: Anti-Inflammatory and Proinflammatory Roles

Mast cells (MCs) play critical roles in Th2 immune responses, including the defense against parasitic infections and the initiation of type I allergic reactions. In addition, MCs are involved in several immune-related responses, including those in bacterial infections, autoimmune diseases, inflammatory bowel diseases, cancers, allograft rejections, and lifestyle diseases. Whereas antigen-specific IgE is a well-known activator of MCs, which express FcεRI on the cell surface, other receptors for cytokines, growth factors, pathogen-associated molecular patterns, and damage-associated molecular patterns also function as triggers of MC stimulation, resulting in the release of chemical mediators, eicosanoids, and various cytokines. In this review, we focus on the role of interleukin (IL)-10, an anti-inflammatory cytokine, in MC-mediated immune responses, in which MCs play roles not only as initiators of the immune response but also as suppressors of excessive inflammation. IL-10 exhibits diverse effects on the proliferation, differentiation, survival, and activation of MCs in vivo and in vitro. Furthermore, IL-10 derived from MCs exerts beneficial and detrimental effects on the maintenance of tissue homeostasis and in several immune-related diseases including contact hypersensitivity, auto-immune diseases, and infections. This review introduces the effects of IL-10 on various events in MCs, and the roles of MCs in IL-10-related immune responses and as a source of IL-10.

Anaphylaxis: Focus on Transcription Factor Activity

Anaphylaxis is a severe allergic reaction, rapid in onset, and can lead to fatal consequences if not promptly treated. The incidence of anaphylaxis has risen at an alarming rate in past decades and continues to rise. Therefore, there is a general interest in understanding the molecular mechanism that leads to an exacerbated response. The main effector cells are mast cells, commonly triggered by stimuli that involve the IgE-dependent or IgE-independent pathway. These signaling pathways converge in the release of proinflammatory mediators, such as histamine, tryptases, prostaglandins, etc., in minutes. The action and cell targets of these proinflammatory mediators are linked to the pathophysiologic consequences observed in this severe allergic reaction. While many molecules are involved in cellular regulation, the expression and regulation of transcription factors involved in the synthesis of proinflammatory mediators and secretory granule homeostasis are of special interest, due to their ability to control gene expression and change phenotype, and they may be key in the severity of the entire reaction. In this review, we will describe our current understanding of the pathophysiology of human anaphylaxis, focusing on the transcription factors' contributions to this systemic hypersensitivity reaction. Host mutation in transcription factor expression, or deregulation of their activity in an anaphylaxis context, will be updated. So far, the risk of anaphylaxis is unpredictable thus, increasing our knowledge of the molecular mechanism that leads and regulates mast cell activity will enable us to improve our understanding of how anaphylaxis can be prevented or treated.

Lineage-specific regulation of inducible and constitutive mast cells in allergic airway inflammation

Murine mast cells (MCs) contain two lineages: inducible bone marrow-derived mucosal MCs (MMCs) and constitutive embryonic-derived connective tissue MCs (CTMCs). Here, we use RNA sequencing, flow cytometry, and genetic deletion in two allergic lung inflammation models to define these two lineages. We found that inducible MCs, marked by β7 integrin expression, are highly distinct from airway CTMCs at rest and during inflammation and unaffected by targeted CTMC deletion. β7High MCs expand and mature during lung inflammation as part of a TGF-β-inducible transcriptional program that includes the MMC-associated proteases Mcpt1 and Mcpt2, the basophil-associated protease Mcpt8, granule components, and the epithelial-binding αE integrin. In vitro studies using bone marrow-derived MCs (BMMCs) identified a requirement for SCF in this this TGF-β-mediated development and found that epithelial cells directly elicit TGF-β-dependent BMMC up-regulation of mMCP-1 and αE integrin. Thus, our findings characterize the expansion of a distinct inducible MC subset in C57BL/6 mice and highlight the potential for epithelium to direct MMC development.

Beyond IgE: Alternative Mast Cell Activation Across Different Disease States

Mast cells are often regarded through the lens of IgE-dependent reactions as a cell specialized only for anti-parasitic and type I hypersensitive responses. However, recently many researchers have begun to appreciate the expansive repertoire of stimuli that mast cells can respond to. After the characterization of the interleukin (IL)-33/suppression of tumorigenicity 2 (ST2) axis of mast cell activation-a pathway that is independent of the adaptive immune system-researchers are revisiting other stimuli to induce mast cell activation and/or subsequent degranulation independent of IgE. This discovery also underscores that mast cells act as important mediators in maintaining body wide homeostasis, especially through barrier defense, and can thus be the source of disease as well. Particularly in the gut, inflammatory bowel diseases (Crohn's disease, ulcerative colitis, etc.) are characterized with enhanced mast cell activity in the context of autoimmune disease. Mast cells show phenotypic differences based on tissue residency, which could manifest as different receptor expression profiles, allowing for unique mast cell responses (both IgE and non-IgE mediated) across varying tissues as well. This variety in receptor expression suggests mast cells respond differently, such as in the gut where immunosuppressive IL-10 stimulates the development of food allergy or in the lungs where transforming growth factor-β1 (TGF-β1) can enhance mast cell IL-6 production. Such differences in receptor expression illustrate the truly diverse effector capabilities of mast cells, and careful consideration must be given toward the phenotype of mast cells observed in vitro. Given mast cells' ubiquitous tissue presence and their capability to respond to a broad spectrum of non-IgE stimuli, it is expected that mast cells may also contribute to the progression of autoimmune disorders and other disease states such as metastatic cancer through promoting chronic inflammation in the local tissue microenvironment and ultimately polarizing toward a unique T_h17 immune response. Furthermore, these interconnected, atypical activation pathways may crosstalk with IgE-mediated signaling differently across disorders such as parasitism, food allergies, and autoimmune disorders of the gut. In this review, we summarize recent research into familiar and novel pathways of mast cells activation and draw connections to clinical human disease.