Imaging protective mast cells in living mice during severe contact hypersensitivity

Intravital microscopy visualizes innate immune crosstalk and function in tissue microenvironment

Significant advances have been made in the field of intravital microscopy (IVM) on myeloid cells due to the growing number of validated fluorescent probes and reporter mice. IVM provides a visualization platform to directly observe cell behavior and deepen our understanding of cellular dynamics, heterogeneity, plasticity, and cell-cell communication in native tissue environments. This review outlines the current studies on the dynamic interaction and function of innate immune cells with a focus on those that are studied with IVM and covers the advances in data analysis with emerging artificial intelligence-based algorithms. Finally, the prospects of IVM on innate immune cells are discussed.

Landscape of mast cell populations across organs in mice and humans

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

Mast cells in type 2 skin inflammation: Maintenance and function

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

Stem cell factor/mast cell/CCL2/monocyte/macrophage axis promotes Coxsackievirus B3 myocarditis and cardiac fibrosis by increasing Ly6Chigh monocyte influx and fibrogenic mediators production

Mast cells (MCs), central players in allergy and parasitic infections, play key roles in inflammation and fibrosis. Here, the impact of MCs on the progression of Coxsackievirus B3 (CVB3)-induced viral myocarditis (VMC) and fibrosis was investigated using MC-deficient Kit^W-sh mice. Viral titres, cellular infiltrates and heart pathologies were evaluated and compared with wild-type (WT) mice during acute CVB3 infection of C57BL/6 mice. CVB3 infection induced an increased accumulation and degranulation of MCs in the hearts of mice during acute infection. MC-deficient Kit^W-sh mice had slightly higher viral titres, decreased VMC and cardiac fibrosis and improved cardiac dysfunction compared to WT mice via decreasing cardiac influx of Ly6C^high monocytes/macrophages (Mo/Mφ). While bone marrow-derived MC reconstitution decreased viral titre and worsened improved survival and VMC severity in Wsh mice. MC-fibroblasts co-culture revealed a cardiac MC-fibroblasts crosstalk during early infection: fibroblasts trigger MC degranulation and secretion of CCL2 and tumour necrosis factor alpha (TNF-α) via producing early stem cell factor (SCF); while MCs-fibrogenic mediators (TNF-α) stimulate fibroblasts to increase CCL2, α-smooth muscle actin (SMA), collagen and transforming growth factor beta（TGFβ) expression, thus aggravating cardiac fibrosis. MCs and fibroblast-derived CCL2s are both essential for cardiac Ly6C^high Mo/Mφ influx. Administration of recombinant mouse SCF to CVB3-infected mice aggravates VMC via accelerating MCs accumulation and cardiac influx of Ly6C^hi Mo/Mφ. Collectively, our data highlight an early MC-fibroblast crosstalk and SCF/MC/CCL2/Mo/Mφ axis as important mechanisms required for triggering VMC and myocardial fibrosis. This finding indicates critical roles of MCs in initiating and modulating cardiac innate response to CVB3 and has an implication in developing new and more effective treatments for VMC.

Mast cell tissue heterogeneity and specificity of immune cell recruitment

Mast cells occupy a unique niche within tissues as long lived perpetrators of IgE mediated hypersensitivity and anaphylaxis, as well as other immune responses. However, mast cells are not identical in different tissues and the impact of this tissue heterogeneity on the interaction with other immune cells and on defined immune responses is still unclear. In this review, we synthesize the characteristics of mast cell heterogeneity in the gut and the skin. Furthermore, we attempt to connect mast cell heterogeneity with functional diversity by exploring differences in mast cell-induced immune cell recruitment in these two model organs. The differential expression of certain receptors on mast cells of different tissues, notably tissue-specific expression patterns of integrins, complement receptors and MRGPRX2, could indicate that tissue environment-dependent factors skew mast cell-immune cell interactions, for example by regulating the expression of these receptors.

Roles of Mast Cells in Cutaneous Diseases

Mast cells are present in all vascularized tissues of the body. They are especially abundant in tissues that are in frequent contact with the surrounding environment and act as potential sources of inflammatory and/or regulatory mediators during development of various infections and diseases. Mature mast cells’ cytoplasm contains numerous granules that store a variety of chemical mediators, cytokines, proteoglycans, and proteases. Mast cells are activated via various cell surface receptors, including FcϵRI, toll-like receptors (TLR), Mas-related G-protein-coupled receptor X2 (MRGPRX2), and cytokine receptors. IgE-mediated mast cell activation results in release of histamine and other contents of their granules into the extracellular environment, contributing to host defense against pathogens. TLRs, play a crucial role in host defense against various types of pathogens by recognizing pathogen-associated molecular patterns. On the other hand, excessive/inappropriate mast cell activation can cause various disorders. Here, we review the published literature regarding the known and potential inflammatory and regulatory roles of mast cells in cutaneous inflammation, including atopic dermatitis, psoriasis, and contact dermatitis GVHD, as well as in host defense against pathogens.

Regulatory Roles of Phospholipase A2 Enzymes and Bioactive Lipids in Mast Cell Biology

Lipids play fundamental roles in life as an essential component of cell membranes, as a major source of energy, as a body surface barrier, and as signaling molecules that transmit intracellular and intercellular signals. Lipid mediators, a group of bioactive lipids that mediates intercellular signals, are produced via specific biosynthetic enzymes and transmit signals via specific receptors. Mast cells, a tissue-resident immune cell population, produce several lipid mediators that contribute to exacerbation or amelioration of allergic responses and also non-allergic inflammation, host defense, cancer and fibrosis by controlling the functions of microenvironmental cells as well as mast cell themselves in paracrine and autocrine fashions. Additionally, several bioactive lipids produced by stromal cells regulate the differentiation, maturation and activation of neighboring mast cells. Many of the bioactive lipids are stored in membrane phospholipids as precursor forms and released spatiotemporally by phospholipase A₂ (PLA₂) enzymes. Through a series of studies employing gene targeting and lipidomics, several enzymes belonging to the PLA₂ superfamily have been demonstrated to participate in mast cell-related diseases by mobilizing unique bioactive lipids in multiple ways. In this review, we provide an overview of our current understanding of the regulatory roles of several PLA₂-driven lipid pathways in mast cell biology.

Novel Insights Into the Immune-Regulatory Functions of Mast Cells in the Cutaneous Immune Response

Skin is a frontline organ that is continuously exposed to external stimuli, including pathogens. Various immune cells reside in the skin under physiological conditions and protect the body from the entry of pathogens/antigens by interacting with each other and orchestrating diverse cutaneous immune responses. To avoid unnecessary inflammation and tissue damage during the elimination of external pathogens and antigens, skin possesses regulatory systems that fine-tune these immune reactions. Mast cells (MCs) are one of the skin-resident immune cell populations that play both effector and regulatory functions in the cutaneous immune response. So far, the interleukin-10-mediated mechanisms have mostly been investigated as the regulatory mechanisms of MCs. Recent studies have elucidated other regulatory mechanisms of MCs, such as the maintenance of regulatory T/B cells and the programmed cell death protein-1/programmed cell death-ligand 1-mediated inhibitory pathway. These regulatory pathways of MCs have been suggested to play important roles in limiting the excessive inflammation in inflammatory skin diseases, such as contact and atopic dermatitis. The regulatory functions of MCs may also be involved in the escape mechanisms of antitumor responses in skin cancers, such as melanoma. Understanding and controlling the regulatory functions of skin MCs may lead to novel therapeutic strategies for inflammatory skin diseases and skin cancers.

Mast Cells and Acupuncture Analgesia

Mast cells are widely distributed in various parts of the human body and play a vital role in the progression of many diseases. Recently, the close relationship between mast cells and acupoints was elucidated, and the role of mast cells in acupuncture analgesia has attracted the attention of researchers worldwide. Using mast cells, acupuncture analgesia and acupoint as key words to search CNKI, PubMed, Web of Science and other databases, combining the representative articles in these databases with the published research papers of our group, we summarized: The enrichment of mast cells and the dense arrangement of collagen fibers, microvessels, and nerves form the basis for acupoints as the reaction sites of acupuncture; acupuncture can cause the deformation of collagen fibers and activate TRPV channels on mast cells membrane, so as to stimulate mast cells to release bioactive substances and activate nerve receptors to generate analgesic effect; system biology models are set up to explain the quantitative process of information initiation and transmission at acupuncture points, and indicate that the acupuncture effect depends on the local mast cells density. In a conclusion, this review will give a scientific explanation of acupuncture analgesia from the material basis of acupoints, the local initiation, and afferent biological mechanism.

Neuron‒Mast Cell Cross-Talk in the Skin

Skin-resident mast cells (MCs) and cutaneous sensory neurons both play crucial roles in microbial‒host defense and inflammatory diseases. MCs can be directly activated by pathogens or their products, resulting in the release of numerous mediators that promote innate immune responses and also activate sensory neurons. Cutaneous sensory neurons can also directly detect the presence of pathogens, resulting in the release of neuropeptides that modulate MC function. In this review, we will focus on the reciprocal interactions between cutaneous sensory neurons and MCs and the importance of this cross-talk in skin diseases.

Modern Imaging Technologies of Mast Cells for Biology and Medicine (Review)

Mast cells play an important role in the body defense against allergens, pathogens, and parasites by participating in inflammation development. However, there is evidence for their contributing to the pathogenesis of a number of atopic, autoimmune, as well as cardiovascular, oncologic, neurologic, and other diseases (allergy, asthma, eczema, rhinitis, anaphylaxis, mastocytosis, multiple sclerosis, rheumatoid arthritis, inflammatory gastrointestinal and pulmonary diseases, migraine, etc.). The diagnosis of many diseases and the study of mast cell functions in health and disease require their identification; so, the knowledge on adequate imaging techniques for mast cells in humans and different species of animals is of particular importance. The present review summarizes the data on major methods of mast cell imaging: enzyme histochemistry, immunohistochemistry, as well as histochemistry using histological stains. The main histological stains bind to heparin and other acidic mucopolysaccharides contained in mast cells and stain them metachromatically. Among these are toluidine blue, methylene blue (including that contained in May-Grünwald-Giemsa stain), thionin, pinacyanol, and others. Safranin and fluorescent dyes: berberine and avidin - also bind to heparin. Longer staining with histological dyes or alcian blue staining is needed to label mucosal and immature mast cells. Advanced techniques - enzyme histochemistry and especially immunohistochemistry - enable to detect mast cells high-selectively using a reaction to tryptases and chymases (specific proteases of these cells). In the immunohistochemical study of tryptases and chymases, species-specific differences in the distribution of the proteases in mast cells of humans and animals should be taken into account for their adequate detection. The immunohistochemical reaction to immunoglobulin E receptor (FcεRI) and c-kit receptor is not specific to mast cells, although the latter is important to demonstrate their proliferation in normal and malignant growth. Correct fixation of biological material is also discussed in the review as it is of great significance for histochemical and immunohistochemical mast cell detection. Fluorescent methods of immunohistochemistry and a multimarker analysis in combination with confocal microscopy are reported to be new technological approaches currently used to study various mast cell populations.

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.

Mast Cells and the Pancreas in Human Type 1 and Type 2 Diabetes

Mast cells are highly differentiated, widely distributed cells of the innate immune system, that are currently considered as key regulators of both innate and adaptive immunity. Mast cells play a key role in health and survival mechanisms, especially as sentinel cells that can stimulate protective immune responses. On the other hand, it has been shown that mast cells are involved in the pathogenesis of several diseases, and recently a possible pathogenetic role of mast cells in diabetes has been proposed. In this review we summarize the evidence on the increased presence of mast cells in the pancreas of subjects with type 1 diabetes, which is due to the autoimmune destruction of insulin secreting beta cells, and discuss the differences with type 2 diabetes, the other major form of diabetes. In addition, we describe some of the pathophysiological mechanisms through which mast cells might exert their actions, which could be targeted to potentially protect the beta cells in autoimmune diabetes.

Mast Cell-Specific Deletion of Group III Secreted Phospholipase A2 Impairs Mast Cell Maturation and Functions

Tissue-resident mast cells (MCs) have important roles in IgE-associated and -independent allergic reactions. Although microenvironmental alterations in MC phenotypes affect the susceptibility to allergy, understanding of the regulation of MC maturation is still incomplete. We previously reported that group III secreted phospholipase A2 (sPLA2-III) released from immature MCs is functionally coupled with lipocalin-type prostaglandin D2 (PGD2) synthase in neighboring fibroblasts to supply a microenvironmental pool of PGD2, which in turn acts on the PGD2 receptor DP1 on MCs to promote their proper maturation. In the present study, we reevaluated the role of sPLA2-III in MCs using a newly generated MC-specific Pla2g3-deficient mouse strain. Mice lacking sPLA2-III specifically in MCs, like those lacking the enzyme in all tissues, had immature MCs and displayed reduced local and systemic anaphylactic responses. Furthermore, MC-specific Pla2g3-deficient mice, as well as MC-deficient KitW-sh mice reconstituted with MCs prepared from global Pla2g3-null mice, displayed a significant reduction in irritant contact dermatitis (ICD) and an aggravation of contact hypersensitivity (CHS). The increased CHS response by Pla2g3 deficiency depended at least partly on the reduced expression of hematopoietic PGD2 synthase and thereby reduced production of PGD2 due to immaturity of MCs. Overall, our present study has confirmed that MC-secreted sPLA2-III promotes MC maturation, thereby facilitating acute anaphylactic and ICD reactions and limiting delayed CHS response.

Sensory neuron-derived TAFA4 promotes macrophage tissue repair functions

Inflammation is a defence response to tissue damage that requires tight regulation in order to prevent impaired healing. Tissue-resident macrophages have a key role in tissue repair¹, but the precise molecular mechanisms that regulate the balance between inflammatory and pro-repair macrophage responses during healing remain poorly understood. Here we demonstrate a major role for sensory neurons in promoting the tissue-repair function of macrophages. In a sunburn-like model of skin damage in mice, the conditional ablation of sensory neurons expressing the Gα_i-interacting protein (GINIP) results in defective tissue regeneration and in dermal fibrosis. Elucidation of the underlying molecular mechanisms revealed a crucial role for the neuropeptide TAFA4, which is produced in the skin by C-low threshold mechanoreceptors-a subset of GINIP⁺ neurons. TAFA4 modulates the inflammatory profile of macrophages directly in vitro. In vivo studies in Tafa4-deficient mice revealed that TAFA4 promotes the production of IL-10 by dermal macrophages after UV-induced skin damage. This TAFA4-IL-10 axis also ensures the survival and maintenance of IL-10⁺TIM4⁺ dermal macrophages, reducing skin inflammation and promoting tissue regeneration. These results reveal a neuroimmune regulatory pathway driven by the neuropeptide TAFA4 that promotes the anti-inflammatory functions of macrophages and prevents fibrosis after tissue damage, and could lead to new therapeutic perspectives for inflammatory diseases.

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.

Mast Cells in the Skin: Defenders of Integrity or Offenders in Inflammation?

Mast cells (MCs) are best-known as key effector cells of immediate-type allergic reactions that may even culminate in life-threatening anaphylactic shock syndromes. However, strategically positioned at the host–environment interfaces and equipped with a plethora of receptors, MCs also play an important role in the first-line defense against pathogens. Their main characteristic, the huge amount of preformed proinflammatory mediators embedded in secretory granules, allows for a rapid response and initiation of further immune effector cell recruitment. The same mechanism, however, may account for detrimental overshooting responses. MCs are not only detrimental in MC-driven diseases but also responsible for disease exacerbation in other inflammatory disorders. Focusing on the skin as the largest immune organ, we herein review both beneficial and detrimental functions of skin MCs, from skin barrier integrity via host defense mechanisms to MC-driven inflammatory skin disorders. Moreover, we emphasize the importance of IgE-independent pathways of MC activation and their role in sustained chronic skin inflammation and disease exacerbation.

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.

Divergent Mast Cell Responses Modulate Antiviral Immunity During Influenza Virus Infection

Influenza A virus (IAV) is a respiratory pathogen that infects millions of people each year. Both seasonal and pandemic strains of IAV are capable of causing severe respiratory disease with a high risk of respiratory failure and opportunistic secondary infection. A strong inflammatory cytokine response is a hallmark of severe IAV infection. The widespread tissue damage and edema in the lung during severe influenza is largely attributed to an overexuberant production of inflammatory cytokines and cell killing by resident and infiltrating leukocytes. Mast cells (MCs) are a sentinel hematopoietic cell type situated at mucosal sites, including the lung. Poised to react immediately upon detecting infection, MCs produce a vast array of immune modulating molecules, including inflammatory cytokines, chemokines, and proteases. As such, MCs have been implicated as a source of the immunopathology observed in severe influenza. However, a growing body of evidence indicates that MCs play an essential role not only in inducing an inflammatory response but in suppressing inflammation as well. MC-derived immune suppressive cytokines are essential to the resolution of a number of viral infections and other immune insults. Absence of MCs prolongs infection, exacerbates tissue damage, and contributes to dissemination of the pathogen to other tissues. Production of cytokines such as IL-10 and IL-6 by MCs is essential for mitigating the inflammation and tissue damage caused by innate and adaptive immune cells alike. The two opposing functions of MCs—one pro-inflammatory and one anti-inflammatory—distinguish MCs as master regulators of immunity at the site of infection. Amongst the first cells to respond to infection or injury, MCs persist for the duration of the infection, modulating the recruitment, activation, and eventual suppression of other immune cells. In this review, we will discuss the immune modulatory roles of MCs over the course of viral infection and propose that the immune suppressive mediators produced by MCs are vital to minimizing immunopathology during influenza infection.

Bacterial and Fungal Toll-Like Receptor Activation Elicits Type I IFN Responses in Mast Cells

Next to their role in IgE-mediated allergic diseases and in promoting inflammation, mast cells also have antiinflammatory functions. They release pro- as well as antiinflammatory mediators, depending on the biological setting. Here we aimed to better understand the role of mast cells during the resolution phase of a local inflammation induced with the Toll-like receptor (TLR)-2 agonist zymosan. Multiple sequential immunohistology combined with a statistical neighborhood analysis showed that mast cells are located in a predominantly antiinflammatory microenvironment during resolution of inflammation and that mast cell-deficiency causes decreased efferocytosis in the resolution phase. Accordingly, FACS analysis showed decreased phagocytosis of zymosan and neutrophils by macrophages in mast cell-deficient mice. mRNA sequencing using zymosan-induced bone marrow-derived mast cells (BMMC) revealed a strong type I interferon (IFN) response, which is known to enhance phagocytosis by macrophages. Both, zymosan and lipopolysaccharides (LPS) induced IFN-β synthesis in BMMCs in similar amounts as in bone marrow derived macrophages. IFN-β was expressed by mast cells in paws from naïve mice and during zymosan-induced inflammation. As described for macrophages the release of type I IFNs from mast cells depended on TLR internalization and endosome acidification. In conclusion, mast cells are able to produce several mediators including IFN-β, which are alone or in combination with each other able to regulate the phagocytotic activity of macrophages during resolution of inflammation.

Local immune response to food antigens drives meal-induced abdominal pain

Up to 20% of people worldwide develop gastrointestinal symptoms following a meal¹, leading to decreased quality of life, substantial morbidity and high medical costs. Although the interest of both the scientific and lay communities in this issue has increased markedly in recent years, with the worldwide introduction of gluten-free and other diets, the underlying mechanisms of food-induced abdominal complaints remain largely unknown. Here we show that a bacterial infection and bacterial toxins can trigger an immune response that leads to the production of dietary-antigen-specific IgE antibodies in mice, which are limited to the intestine. Following subsequent oral ingestion of the respective dietary antigen, an IgE- and mast-cell-dependent mechanism induced increased visceral pain. This aberrant pain signalling resulted from histamine receptor H₁-mediated sensitization of visceral afferents. Moreover, injection of food antigens (gluten, wheat, soy and milk) into the rectosigmoid mucosa of patients with irritable bowel syndrome induced local oedema and mast cell activation. Our results identify and characterize a peripheral mechanism that underlies food-induced abdominal pain, thereby creating new possibilities for the treatment of irritable bowel syndrome and related abdominal pain disorders.

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.

Harnessing the Power of Mast Cells in unconventional Immunotherapy Strategies and Vaccine Adjuvants

Mast cells are long-lived, granular, myeloid-derived leukocytes that have significant protective and repair functions in tissues. Mast cells sense disruptions in the local microenvironment and are first responders to physical, chemical and biological insults. When activated, mast cells release growth factors, proteases, chemotactic proteins and cytokines thereby mobilizing and amplifying the reactions of the innate and adaptive immune system. Mast cells are therefore significant regulators of homeostatic functions and may be essential in microenvironmental changes during pathogen invasion and disease. During infection by helminths, bacteria and viruses, mast cells release antimicrobial factors to facilitate pathogen expulsion and eradication. Mast cell-derived proteases and growth factors protect tissues from insect/snake bites and exposure to ultraviolet radiation. Finally, mast cells release mediators that promote wound healing in the inflammatory, proliferative and remodelling stages. Since mast cells have such a powerful repertoire of functions, targeting mast cells may be an effective new strategy for immunotherapy of disease and design of novel vaccine adjuvants. In this review, we will examine how certain strategies that specifically target and activate mast cells can be used to treat and resolve infections, augment vaccines and heal wounds. Although these strategies may be protective in certain circumstances, mast cells activation may be deleterious if not carefully controlled and any therapeutic strategy using mast cell activators must be carefully explored.

Fucoxanthin Ameliorates Atopic Dermatitis Symptoms by Regulating Keratinocytes and Regulatory Innate Lymphoid Cells

Fucoxanthin (FX) is a xanthophyll that is contained abundantly in marine plants. The biological action of FX includes its antioxidant and anti-lipogenic activities, while the precise action of its mechanisms on skin cells has not yet been clarified. The current study examined the effect of FX in comparison with tacrolimus (TAC) on NC/Nga mice, which are an atopic dermatitis (AD) model. FX topical treatment dramatically ameliorated itching behavior over the TAC treatment, which was insufficient for improvement of AD symptoms. In Nc/Nga mice, FX or TAC applied to the skin inhibited eosinophil infiltration with decreased expression of Il-33. FX also stimulated Il-2, Il-5, Il-13, Il-10, and TGF-β expression levels, and Sca1⁺Il-10⁺TGF-β⁺ regulatory innate lymphoid cells (ILCreg) were dominantly observed in FX treated skin epidermal keratinocytes and dermal layers. This combined evidence demonstrated that FX exerts anti-inflammatory effects on keratinocytes and ameliorates AD symptoms by regulating ILCreg to normalize immune responses in an atopic dermatitis model.

Rapid identification of human mast cell degranulation regulators using functional genomics coupled to high-resolution confocal microscopy

Targeted functional genomics represents a powerful approach for studying gene function in vivo and in vitro. However, its application to gene expression studies in human mast cells has been hampered by low yields of human mast cell cultures and their poor transfection efficiency. We developed an imaging system in which mast cell degranulation can be visualized in single cells subjected to shRNA knockdown or CRISPR-Cas 9 gene editing. By using high resolution confocal microscopy and a fluorochrome-labeled avidin probe, one can directly assess the suppression of functional responses, i.e. degranulation, in single human mast cells. The elimination of a drug or marker selection step avoids the use of potentially toxic treatment procedures and the short hands-on time of the functional analysis step enables the high-throughput screening of shRNA or CRISPR-Cas9 constructs to identify genes that regulate human mast cell degranulation. The ability to analyse single cells significantly reduces the total number of cells required, and allows for the parallel visualization of the degranulation profile of both edited and non-edited mast cells, offering a consistent internal control not found in other protocols. Moreover, our protocol offers a flexible choice between RNA interference and CRISPR-Cas9 genome editing for perturbation of gene expression using our human mast cell single-cell imaging system. Perturbation of gene expression, acquisition of microscopy data, and image analysis can be completed within 5 days, requiring only standard laboratory equipment and expertise.

This protocol presents an an imaging system in which mast cell degranulation can be visualized in single cells subjected to shRNA knockdown or CRISPR-Cas 9 gene editing using high resolution confocal microscopy with a fluorochrome-labeled avidin probe.

House dust mites activate nociceptor-mast cell clusters to drive type 2 skin inflammation

Allergic skin diseases, such as atopic dermatitis (AD), are clinically characterized by severe itching and type 2 immunity-associated hypersensitivity to widely-distributed allergens, including those derived from house dust mites (HDMs). Here we found that HDMs with cysteine-protease activity directly activated peptidergic nociceptors, which are neuropeptide-producing nociceptive sensory neurons, that expressed the ion channel TRPV1 and Tac1, the gene encoding the precursor for the neuropeptide substance P. Intravital imaging and genetic approaches indicated that HDMs-activated nociceptors drove the development of allergic skin inflammation by inducing the degranulation of mast cells contiguous to such nociceptors through the release of substance P and the activation of the cationic molecules receptor MRGPRB2 on mast cells. This data indicates that, after exposure to HDM allergens, activation of TRPV1⁺Tac1⁺ nociceptor-MRGPRB2⁺ sensory clusters represents a key early event in the development of allergic skin reaction.

Future Needs in Mast Cell Biology

The pathophysiological roles of mast cells are still not fully understood, over 140 years since their description by Paul Ehrlich in 1878. Initial studies have attempted to identify distinct "subpopulations" of mast cells based on a relatively small number of biochemical characteristics. More recently, "subtypes" of mast cells have been described based on the analysis of transcriptomes of anatomically distinct mouse mast cell populations. Although mast cells can potently alter homeostasis, in certain circumstances, these cells can also contribute to the restoration of homeostasis. Both solid and hematologic tumors are associated with the accumulation of peritumoral and/or intratumoral mast cells, suggesting that these cells can help to promote and/or limit tumorigenesis. We suggest that at least two major subsets of mast cells, MC1 (meaning anti-tumorigenic) and MC2 (meaning pro-tumorigenic), and/or different mast cell mediators derived from otherwise similar cells, could play distinct or even opposite roles in tumorigenesis. Mast cells are also strategically located in the human myocardium, in atherosclerotic plaques, in close proximity to nerves and in the aortic valve. Recent studies have revealed evidence that cardiac mast cells can participate both in physiological and pathological processes in the heart. It seems likely that different subsets of mast cells, like those of cardiac macrophages, can exert distinct, even opposite, effects in different pathophysiological processes in the heart. In this chapter, we have commented on possible future needs of the ongoing efforts to identify the diverse functions of mast cells in health and disease.

A Connective Tissue Mast-Cell-Specific Receptor Detects Bacterial Quorum-Sensing Molecules and Mediates Antibacterial Immunity

Quorum-sensing molecules (QSMs) are secreted by bacteria to signal population density. Upon reaching a critical concentration, QSMs induce transcriptional alterations in bacteria, which enable virulence factor expression and biofilm formation. It is unclear whether mammalian hosts can recognize QSMs to trigger responsive antibacterial immunity. We report that mouse mast-cell-specific G-protein-coupled receptor Mrgprb2 and its human homolog MRGPRX2 are receptors for Gram-positive QSMs, including competence-stimulating peptide (CSP)-1. CSP-1 activates Mrgprb2 and MRGPRX2, triggering mast cell degranulation, which inhibits bacterial growth and prevents biofilm formation. Such antibacterial functions are reduced in Mrgprb2-deficient mast cells, while wild-type mast cells fail to inhibit the growth of bacterial strains lacking CSP-1. Mrgprb2-knockout mice exhibit reduced bacterial clearance, while pharmacologically activating Mrgprb2 in vivo eliminates bacteria and improves disease score. These findings identify a host defense mechanism that uses QSMs as an "Achilles heel" and suggest MRGPRX2 as a potential therapeutic target for controlling bacterial infections.

The regulatory B cell-mediated peripheral tolerance maintained by mast cell IL-5 suppresses oxazolone-induced contact hypersensitivity

The function of regulatory immune cells in peripheral tissues is crucial to the onset and severity of various diseases. Interleukin-10 (IL-10)-producing regulatory B (IL-10⁺ B_reg) cells are known to suppress various inflammatory diseases. However, evidence for the mechanism by which IL-10⁺ B_reg cells are generated and maintained is still very limited. Here, we found that IL-10⁺ B_reg cells suppress the activation of IL-13-producing type 2 innate lymphoid cells (IL-13⁺ ILC2s) in an IL-10-dependent manner in mice with oxazolone-induced severe contact hypersensitivity (CHS). Mast cell (MC) IL-5 was important for maintaining the population of IL-10⁺ B_reg cells in peripheral lymphoid tissues. Overall, these results uncover a previously unknown mechanism of MCs as a type of immunoregulatory cell and elucidate the cross-talk among MCs, IL-10⁺ B_reg cells, and IL-13⁺ ILC2s in CHS.

Type 2 Inflammatory Responses in Autoimmune Demyelination of the Central Nervous System: Recent Advances

Type 2 immunity has long been confined to a restricted spectrum of responses, mostly including allergic reactions to innocuous environmental triggers. However, growing evidence suggests that cells and mediators typically associated with type 2 inflammation are involved in several physiopathological conditions, such as defense against toxic substances, anticancer immunity, and autoimmune diseases. In neuromyelitis optica, an autoimmune demyelinating disorder of the spinal cord and optic nerve, eosinophils extensively infiltrate lesions in the central nervous system (CNS) and promote tissue pathology in experimental models of this disease. Next-generation sequencing of CD4⁺ T cells isolated from a specific subtype of multiple sclerosis plaque has uncovered an unexpectedly Th2 profile of these cells. Even mast cells and other allergic mediators have been implicated in the modulation and/or effector mechanisms of autoimmune reactions against the CNS. In this review article, the most recent developments showing the involvement of type 2 inflammatory components in CNS autoimmunity are summarised and possible lines of further investigation are discussed.

Novel cutaneous mediators of chemical allergy

Chemical allergy can manifest into allergic contact dermatitis and asthma and the importance of skin sensitization in both of these diseases is increasingly being recognized. Given the unique characteristics of chemical allergy, coupled with the distinct immunological microenvironment of the skin research is still unraveling the mechanisms through which sensitization and elicitation occur. This review first describes the features of chemical sensitization and the known steps that must occur to develop a chemical allergy. Next, the unique immunological properties of the skin - which may influence chemical sensitization - are highlighted. Additionally, mediators involved with the development of allergy are reviewed, starting with early ones - including the properties of haptens, skin integrity, the microbiome, the inflammasome, and toll-like receptors (TLR). Novel cellular mediators of chemical sensitization are highlighted, including innate lymphoid cells, mast cells, T-helper (T_H) cell subsets, and skin intrinsic populations including γδ T-cells and resident memory T-cells. Finally, this review discusses two epigenetic mechanisms that can influence chemical sensitization, microRNAs and DNA methylation. Overall, this review highlights recent research investigating novel mediators of chemical allergy that are present in the skin. It also emphasizes the need to further explore these mediators to gain a better understanding of what makes a chemical an allergen, and how best to prevent the development of chemical-induced allergic diseases.

Imaging of mast cells

Mast cells are a part of the innate immune system implicated in allergic reactions and the regulation of host-pathogen interactions. The distribution, morphology and biochemical composition of mast cells has been studied in detail in vitro and on tissue sections both at the light microscopic and ultrastructural level. More recently, the development of fluorescent reporter strains and intravital imaging modalities has enabled first glimpses of the real-time behavior of mast cells in situ. In this review, we describe commonly used imaging approaches to study mast cells in cell culture as well as within normal and diseased tissues. We further describe the interrogation of mast cell function via imaging by providing a detailed description of mast cell-nerve plexus interactions in the intestinal tract. Together, visualizing mast cells has expanded our view of these cells in health and disease.

Mast cells as sources of cytokines, chemokines, and growth factors

Mast cells are hematopoietic cells that reside in virtually all vascularized tissues and that represent potential sources of a wide variety of biologically active secreted products, including diverse cytokines and growth factors. There is strong evidence for important non-redundant roles of mast cells in many types of innate or adaptive immune responses, including making important contributions to immediate and chronic IgE-associated allergic disorders and enhancing host resistance to certain venoms and parasites. However, mast cells have been proposed to influence many other biological processes, including responses to bacteria and virus, angiogenesis, wound healing, fibrosis, autoimmune and metabolic disorders, and cancer. The potential functions of mast cells in many of these settings is thought to reflect their ability to secrete, upon appropriate activation by a range of immune or non-immune stimuli, a broad spectrum of cytokines (including many chemokines) and growth factors, with potential autocrine, paracrine, local, and systemic effects. In this review, we summarize the evidence indicating which cytokines and growth factors can be produced by various populations of rodent and human mast cells in response to particular immune or non-immune stimuli, and comment on the proven or potential roles of such mast cell products in health and disease.

Cutaneous immune responses mediated by dendritic cells and mast cells

In the skin, complex cellular networks maintain barrier function and immune homeostasis. Tightly regulated multicellular cascades are required to initiate innate and adaptive immune responses. Innate immune cells, particularly DCs and mast cells, are central to these networks. Early studies evaluated the function of these cells in isolation, but recent studies clearly demonstrate that cutaneous DCs (dermal DCs and Langerhans cells) physically interact with neighboring cells and are receptive to activation signals from surrounding cells, such as mast cells. These interactions amplify immune activation. In this review, we discuss the known functions of cutaneous DC populations and mast cells and recent studies highlighting their roles within cellular networks that determine cutaneous immune responses.

Human and computational models of atopic dermatitis: A review and perspectives by an expert panel of the International Eczema Council

Atopic dermatitis (AD) is a prevalent disease worldwide and is associated with systemic comorbidities representing a significant burden on patients, their families, and society. Therapeutic options for AD remain limited, in part because of a lack of well-characterized animal models. There has been increasing interest in developing experimental approaches to study the pathogenesis of human AD in vivo, in vitro, and in silico to better define pathophysiologic mechanisms and identify novel therapeutic targets and biomarkers that predict therapeutic response. This review critically appraises a range of models, including genetic mutations relevant to AD, experimental challenge of human skin in vivo, tissue culture models, integration of “omics” data sets, and development of predictive computational models. Although no one individual model recapitulates the complex AD pathophysiology, our review highlights insights gained into key elements of cutaneous biology, molecular pathways, and therapeutic target identification through each approach. Recent developments in computational analysis, including application of machine learning and a systems approach to data integration and predictive modeling, highlight the applicability of these methods to AD subclassification (endotyping), therapy development, and precision medicine. Such predictive modeling will highlight knowledge gaps, further inform refinement of biological models, and support new experimental and systems approaches to AD. (J Allergy Clin Immunol 2019;143:36–45.)

Engulfment of mast cell secretory granules on skin inflammation boosts dendritic cell migration and priming efficiency

BACKGROUND

Mast cells (MCs) are best known as key effector cells of allergic reactions, but they also play an important role in host defense against pathogens. Despite increasing evidence for a critical effect of MCs on adaptive immunity, the underlying mechanisms are poorly understood.

OBJECTIVE

Here we monitored MC intercellular communication with dendritic cells (DCs), MC activation, and degranulation and tracked the fate of exocytosed mast cell granules (MCGs) during skin inflammation.

METHODS

Using a strategy to stain intracellular MCGs in vivo, we tracked the MCG fate after skin inflammation-induced MC degranulation. Furthermore, exogenous MCGs were applied to MC-deficient mice by means of intradermal injection. MCG effects on DC functionality and adaptive immune responses in vivo were assessed by combining intravital multiphoton microscopy with flow cytometry and functional assays.

RESULTS

We demonstrate that dermal DCs engulf the intact granules exocytosed by MCs on skin inflammation. Subsequently, the engulfed MCGs are actively shuttled to skin-draining lymph nodes and finally degraded inside DCs within the lymphoid tissue. Most importantly, MCG uptake promotes DC maturation and migration to skin-draining lymph nodes, partially through MC-derived TNF, and boosts their T-cell priming efficiency. Surprisingly, exogenous MCGs alone are sufficient to induce a prominent DC activation and T-cell response.

CONCLUSION

Our study highlights a unique feature of peripheral MCs to affect lymphoid tissue-borne adaptive immunity over distance by modifying DC functionality through delivery of granule-stored mediators.

Genetic and Imaging Approaches Reveal Pro-Inflammatory and Immunoregulatory Roles of Mast Cells in Contact Hypersensitivity

Contact hypersensitivity (CHS) is a common T cell-mediated skin disease induced by epicutaneous sensitization to haptens. Mast cells (MCs) are widely deployed in the skin and can be activated during CHS responses to secrete diverse products, including some with pro-inflammatory and anti-inflammatory functions. Conflicting results have been obtained regarding pathogenic versus protective roles of MCs in CHS, and this has been attributed in part to the limitations of certain models for studying MC functions in vivo. This review discusses recent advances in the development and analysis of mouse models to investigate the roles of MCs and MC-associated products in vivo. Notably, fluorescent avidin-based two-photon imaging approaches enable in vivo selective labeling and simultaneous tracking of MC secretory granules (e.g., during MC degranulation) and MC gene activation by real-time longitudinal intravital microscopy in living mice. The combination of such genetic and imaging tools has shed new light on the controversial role played by MCs in mouse models of CHS. On the one hand, they can amplify CHS responses of mild severity while, on the other hand, can limit the inflammation and tissue injury associated with more severe or chronic models, in part by representing an initial source of the anti-inflammatory cytokine IL-10.

The contribution of mast cells to bacterial and fungal infection immunity

Mast cells are hematopoietic progenitor-derived, granule-containing immune cells that are widely distributed in tissues that interact with the external environment, such as the skin and mucosal tissues. It is well-known that mast cells are significantly involved in IgE-mediated allergic reactions, but because of their location, it has also been long hypothesized that mast cells can act as sentinel cells that sense pathogens and initiate protective immune responses. Using mast cell or mast cell protease-deficient murine models, recent studies by our groups and others indicate that mast cells have pleiotropic regulatory roles in immunological responses against pathogens. In this review, we discuss studies that demonstrate that mast cells can either promote host resistance to infections caused by bacteria and fungi or contribute to dysregulated immune responses that can increase host morbidity and mortality. Overall, these studies indicate that mast cells can influence innate immune responses against bacterial and fungal infections via multiple mechanisms. Importantly, the contribution of mast cells to infection outcomes depends in part on the infection model, including the genetic approach used to assess the influence of mast cells on host immunity, hence highlighting the complexity of mast cell biology in the context of innate immune responses.

Epigenetic and transcriptional control of mast cell responses

Mast cells are tissue-resident, innate immune cells present in most tissues of the body and are important effector and immunomodulatory cells. Differentiated mast cells typically are characterized by the surface expression of the receptors KIT and FcεRI, the latter especially being important for stimulation through IgE antibodies, although these cells have the ability to respond to a wide variety of environmental signals, to which they can variably react by releasing pre-stored or de novo–synthesized mediators or both. Since mast cells terminate their differentiation in their tissue of residence in response to specific microenvironmental cues, each tissue may comprise unique mast cell subtypes, and responses are tailored to the danger signals that are likely to be encountered in each anatomical location. From a transcriptional point of view, these cells therefore must be endowed with epigenetic and transcriptional programs that allow them to maintain a stable identity and at the same time allow sufficient plasticity to adapt to different environmental challenges. In this commentary, we highlight some of the recent findings that advanced our understanding of the transcriptional and epigenetic programs regulating mast cell functions.

Mast cells acquire MHCII from dendritic cells during skin inflammation

Dudeck et al. demonstrate that inflammatory conditions induce dynamic interactions between mast cells (MCs) and dendritic cells (DCs) culminating in protein exchange. Resident MCs are equipped with DC MHCII and empowered to initiate T cell–driven inflammation during migration-based DC absence.

Mast cells (MCs) and dendritic cells (DCs) are essential innate sentinels populating host-environment interfaces. Using longitudinal intravital multiphoton microscopy of DC^GFP/MC^RFP reporter mice, we herein provide in vivo evidence that migratory DCs execute targeted cell-to-cell interactions with stationary MCs before leaving the inflamed skin to draining lymph nodes. During initial stages of skin inflammation, DCs dynamically scan MCs, whereas at a later stage, long-lasting interactions predominate. These innate-to-innate synapse-like contacts ultimately culminate in DC-to-MC molecule transfers including major histocompatibility complex class II (MHCII) proteins enabling subsequent ex vivo priming of allogeneic T cells with a specific cytokine signature. The extent of MHCII transfer to MCs correlates with their T cell priming efficiency. Importantly, preventing the cross talk by preceding DC depletion decreases MC antigen presenting capacity and T cell–driven inflammation. Consequently, we identify an innate intercellular communication arming resident MCs with key DC functions that might contribute to the acute defense potential during critical periods of migration-based DC absence.