IL-33 fine tunes mast cell degranulation and chemokine production at the single-cell level

Flow Cytometry-Assisted Analyses of Individual Human Basophils, Mast Cells and T Cells in the Diagnosis of Immediate Drug Hypersensitivity: A Review

Immediate drug hypersensitivity reactions (IDHRs) pose significant diagnostic challenges, often requiring potentially hazardous drug challenge testing (DCT). Flow cytometry-based cellular tests including the basophil activation test (BAT), the mast cell activation test (MAT) and the T cell activation test (TAT) offer promising alternatives to reduce DCT reliance. While these tests are still in development, they demonstrate potential to compete with skin tests by providing superior diagnostic performance and improved patient safety by reducing the need for DCT. Furthermore, it is encouraging that these flow cytometry-based tests are also suitable for challenging populations, such as children. Despite requiring specialised infrastructure, these tests have the potential to be cost-effective when performed in reference centres and may offer unique mechanistic insights into immediate drug hypersensitivity reactions. However, further research is needed to validate their reliability, address pharmaceutical-specific testing considerations, and potentially integrate them into clinical guidelines.

PRL2 negatively regulates FcεRI mediated activation of mast cells

Mast cells play a central role in allergic reactions, acting as key effector cells that initiate and amplify the inflammatory response. In this study, we demonstrate that phosphatase of regenerating liver 2 (PRL2) functions as a negative regulator of FcεRI-mediated mast cell activation. In PRL2-deficient myeloid cells, PRL2 conditional knockout mice developed more severe passive systemic anaphylaxis (PSA). Although PRL2 deficiency does not impact mast cell development, in the absence of PRL2 FcεRI-mediated mast cell activation is enhanced. In the presence of IgE the expression of mast cell PRL2 is downregulated, leading to modulation of the cellular response. In PRL2-deficient mast cells, the PI3K signaling pathway is upregulated, resulting in increased calcium influx. This, in turn, enhances mast cell degranulation and the production of inflammatory mediators. Moreover, hydroxychloroquine (an inhibitor of PRL2 degradation) reduces the severity of PSA in wild-type mice. Our findings suggest that PRL2 acts as a negative regulator of FcεRI-mediated mast cell activation. Therefore, therapeutic strategies aimed at enhancing PRL2 activity in mast cells may offer a promising approach for the treatment of allergic disorders.

IL-33 signaling is dispensable for the IL-10-induced enhancement of mast cell responses during food allergy

Background

The IL-33/ST2 axis plays a pivotal role in the development of IgE-mediated mast cell (MC) responses during food allergy. We recently demonstrated that the pleiotropic cytokine, IL-10, not only exerts proinflammatory effects on IgE-mediated MC activation, but also promotes IL-33-induced MC responses. However, whether IL-33 is necessary for IL-10's proinflammatory effects has not been examined.

Methods

To therefore determine the role of the IL-33/ST2 axis in this pathway, we assessed the effects of IL-10 on IgE-mediated MC activation and food allergy development in wild-type (WT) and ST2-/- mice.

Results

IL-10 stimulation significantly enhanced IL-33 gene expression, ST2 receptor expression, cytokine production, mMCP-1 secretion, and proliferation in IgE and antigen-activated bone marrow-derived MCs (BMMCs) from WT mice. ST2-/- BMMCs exhibited reduced cytokine secretion in response to IgE-dependent activation. However, IL-10 enhanced cytokine production, mMCP-1 secretion, and proliferation in these cells as well. To further assess the role of IL-10, food allergy was induced in WT and ST2-/- mice subjected to antibody-mediated IL-10 depletion. IL-10-depleted WT mice exhibited a significant attenuation in MC-mediated responses to OVA challenge. While ST2-/- mice also exhibited a profound suppression of MC responses, IL-10 depletion had no additional effects. However, ST2-/-/IL-10-/- mice exhibited further decreases in OVA-IgE and antigen-specific MC activation compared to ST2-/- mice.

Conclusion

Our data demonstrates that IL-10 can enhance MC responses in both WT and ST2-/- mice, further corroborating its proinflammatory effects on MCs and suggesting that they are not regulated by IL-33 signaling.

Agonism of the glutamate receptor GluK2 suppresses dermal mast cell activation and cutaneous inflammation

Activation of dermal mast cells through the Mas-related G protein-coupled receptor B2 receptor (MrgprB2 in mice and MrgprX2 in humans) is a key component of numerous inflammatory skin diseases, including dermatitis and rosacea. Sensory neurons actively suppress mast cell activation through the regulated release of glutamate, resulting in reduced expression of Mrgprb2 as well as genes associated with proteins found in mast cell granules. To determine whether exogenous glutamate receptor agonism could suppress mast cell function, we determined that mast cells have relatively selective expression of the glutamate receptor ionotropic, kainate 2 (GluK2). A GluK2-specific agonist, SYM2081, effectively inhibited mast cell degranulation in response to MrgprB2 agonism in both murine mast cells and human skin explants in vitro as well as in vivo after both intradermal and topical administration of SYM2081 to mice. Analyses of transcriptomic datasets from SYM2081-treated mast cells using standard differential expression approaches and an interpretable machine learning technique revealed a previously unrecognized cellular program coordinately regulated by GluK2 agonism. GluK2 agonism suppressed the expression of Mrgprb2 and genes associated with mast cell proliferation. Suppression of mast cell proliferation by SYM2081 exposure was confirmed on the basis of reduced Ki-67 expression and BrdU incorporation in vitro and in vivo. Last, pretreatment with SYM2081 reduced skin inflammation in murine models of dermatitis and rosacea. Thus, agonism of GluK2 represents a promising approach to suppress mast cell activation and may prove beneficial as therapy for inflammatory diseases in which mast cell activation is pathogenic.

IL-33-primed human mast cells drive IL-9 production by CD4+ effector T cells in an OX40L-dependent manner

Interleukin-33 (IL-33) is an alarmin released by epithelial cells in response to tissue damage. It activates resident immune sentinel cells, which then produce signals commonly associated with type 2 immune responses, particularly affecting infiltrating antigen-specific T cells. Given that mast cells (MCs) are a primary target of IL-33 and can shape T helper (Th) cell responses, we investigated the effect of IL-33 priming on the ability of MCs to influence Th cell cytokine production. To examine the Th cell/MC interaction, we developed human primary MC/memory CD4+ T-cell coculture systems involving both cognate and non-cognate interactions. Our results demonstrated that IL-33-primed MCs, whether as bystander cells cocultured with activated effector T cells or functioning as antigen-presenting cells, promoted IL-9 and increased IL-13 production in Th cells via an OX40L-dependent mechanism. This indicates that MCs sense IL-33-associated danger, prompting them to direct Th cells to produce the key type 2 effector cytokines IL-9 and IL-13.

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

Background

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

Methods

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

Results

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

Conclusions

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

IL-10 Differentially Promotes Mast Cell Responsiveness to IL-33, Resulting in Enhancement of Type 2 Inflammation and Suppression of Neutrophilia

Mast cells (MCs) play critical roles in the establishment of allergic diseases. We recently demonstrated an unexpected, proinflammatory role for IL-10 in regulating MC responses. IL-10 enhanced MC activation and promoted IgE-dependent responses during food allergy. However, whether these effects extend to IgE-independent stimuli is not clear. In this article, we demonstrate that IL-10 plays a critical role in driving IL-33-mediated MC responses. IL-10 stimulation enhanced MC expansion and degranulation, ST2 expression, IL-13 production, and phospho-relA upregulation in IL-33-treated cells while suppressing TNF-α. These effects were partly dependent on endogenous IL-10 and further amplified in MCs coactivated with both IL-33 and IgE/Ag. IL-10's divergent effects also extended in vivo. In a MC-dependent model of IL-33-induced neutrophilia, IL-10 treatment enhanced MC responsiveness, leading to suppression of neutrophils and decreased TNF-α. In contrast, during IL-33-induced type 2 inflammation, IL-10 priming exacerbated MC activity, resulting in MC recruitment to various tissues, enhanced ST2 expression, induction of hypothermia, recruitment of eosinophils, and increased MCPT-1 and IL-13 levels. Our data elucidate an important role for IL-10 as an augmenter of IL-33-mediated MC responses, with implications during both allergic diseases and other MC-dependent disorders. IL-10 induction is routinely used as a prognostic marker of disease improvement. Our data suggest instead that IL-10 can enhance ST2 responsiveness in IL-33-activated MCs, with the potential to both aggravate or suppress disease severity depending on the inflammatory context.

Bergapten inhibits airway inflammation and MRGPRX2-mediated mast cells activation by targeting NR4A1

Asthma is a serious global health problem affecting 300 million persons around the world. Mast cells (MCs) play a major role in airway hyperresponsiveness (AHR) and inflammation in asthma, their exact effector mechanisms remain unclear. Here, we aim to investigate the inhibitory effect of Bergapten (BER) on MRGPRX2-mediated MCs activation through asthma model. Mouse model of asthma was established to examine the anti-asthmatic effects of BER. Calcium (Ca2+) influx, β-hexosaminidase and histamine release were used to assess MCs degranulation in vitro. RNA-Seq technique was conducted to study the gene expression profile. RT-PCR and Western Blotting were performed to examine targeting molecules expression. BER inhibited AHR, inflammation, mucous secretion, collagen deposition and lung MCs activation in asthma model. BER dramatically reduced levels of IL4, IL-5, and IL-13 in bronchoalveolar lavage fluid (BALF), as well as inflammatory cells. BER also reduced serum IgE levels. Pretreatment MCs with BER inhibited substance P (SP)-induced Ca2+ influx, degranulation and cytokines release from MCs. BER also reduced the phosphorylation levels of PKC, PLC, IP3R, AKT and ERK, which were induced by SP. Furthermore, RNA-seq analysis showed that SP up-regulated 68 genes in MCs, while were reversed by BER. Among these 68 genes, SP up-regulated NR4A1 expression, and this effect was inhibited by BER. Meanwhile, knockdown of NR4A1 significantly attenuated SP-induced MCs degranulation. In conclusion, NR4A1 plays a major role in MRGPRX2-mediated MCs activation, BER inhibited AHR and inflammation in asthmatic model by inhibiting MCs activation through MRGPRX2-NR4A1 pathway.

Mast cell activation disrupts interactions between endothelial cells and pericytes during early life allergic asthma

Allergic asthma generally starts during early life and is linked to substantial tissue remodeling and lung dysfunction. Although angiogenesis is a feature of the disrupted airway, the impact of allergic asthma on the pulmonary microcirculation during early life is unknown. Here, using quantitative imaging in precision-cut lung slices (PCLSs), we report that exposure of neonatal mice to house dust mite (HDM) extract disrupts endothelial cell/pericyte interactions in adventitial areas. Central to the blood vessel structure, the loss of pericyte coverage was driven by mast cell (MC) proteases, such as tryptase, that can induce pericyte retraction and loss of the critical adhesion molecule N-cadherin. Furthermore, spatial transcriptomics of pediatric asthmatic endobronchial biopsies suggests intense vascular stress and remodeling linked with increased expression of MC activation pathways in regions enriched in blood vessels. These data provide previously unappreciated insights into the pathophysiology of allergic asthma with potential long-term vascular defects.

New insight into prurigo nodularis: Proadrenomedullin N-terminal 20 peptide mediates mouse mast cell activation via Mrgprb2

BACKGROUND

Prurigo nodularis (PN) is a chronic inflammatory skin disorder that is characterized by extremely itchy nodules. Proadrenomedullin N-terminal 20 (PAMP) activates mast cell degranulation via Mas-related G protein-coupled receptor X2 (MRGPRX2), which is associated with pruritus in allergic contact dermatitis. However, the mechanisms underlying the action of PAMP and MRGPRX2 in PN remain unclear.

OBJECTIVE

To determine the role of PAMP-induced mast cell activation via MRGPRX2 (mouse homologous Mrgprb2) in PN.

METHODS

The expression of PAMP and the number of MRGPRX2-expressing mast cells in the skin biopsies of patients with PN, atopic dermatitis (AD), and healthy participants were analyzed using immunohistochemistry and immunofluorescence, respectively. The biphasic response of PAMP9-20 mediated by Mrgprb2 in mouse peritoneal mast cells (PMC) was validated in vitro using qRT-PCR, ELISA, flow cytometry, and siRNA techniques.

RESULTS

PAMP expression and the number of MRGPRX2+ mast cells in lesional PN skin, but not in AD, were elevated compared to healthy skin. PAMP9-20 mediates the immediate and delayed phase responses of PMC, such as degranulation, histamine and β-hexosaminidase release, and secretion of inflammatory factors such as CCL2, TNF-α, and GM-CSF. These effects were inhibited when Mrgprb2 expression was silenced. Silencing Mrgprb2 did not affect the biphasic response of PMC that was induced by IgE-FcεRI activation.

CONCLUSIONS

The results show that PAMP mediates mouse mast cell activation via Mrgprb2, which may be involved in the pathogenesis of PN. The PAMP/ Mrgprb2 pathway, independent of classical IgE signaling, could be developed as a candidate drug target for treating PN.

P2X7 Receptor-Induced Human Mast Cell Degranulation Is Enhanced by Interleukin 33

MCs are tissue-resident immune cells that strategically reside in barrier organs and respond effectively to a wide range of stimuli, such as IL-33, a mediator released upon epithelial damage. Adenosine triphosphate (ATP) accumulates at sites of tissue injury and is known to modulate MC activities. This study investigated how an inflammatory tissue environment rich in IL-33 modulates the ATP-mediated activation of MCs. Human primary MCs primed with IL-33 displayed a strongly increased response to ATP but not ADP. This resulted in increased degranulation, IL-8 release, and pERK1/2 signalling. Such effects are unique to IL-33 stimulation and not shared by the epithelial alarmin, TSLP. MC exposure to IL-33 also increased membrane expression of purinergic and ATP-binding P2X receptors. The use of selective P2X receptor inhibitors identified P2X7 receptor as the key mediator of the enhanced ATP-induced ERK1/2 signalling and degranulation in IL-33-primed MCs. Whilst the inhibition of P2X1 and P2X4 receptors had no effect on MC degranulation, inhibiting these receptors together with P2X7 resulted in further decreased MC-mediated degranulation. These data therefore point toward the potential mechanisms by which IL-33 contributes to the modulation of ATP-mediated activation in human MCs.

Mast cells disrupt the function of the esophageal epithelial barrier

Mast cells (MCs) accumulate in the epithelium of patients with eosinophilic esophagitis (EoE), an inflammatory disorder characterized by extensive esophageal eosinophilic infiltration. Esophageal barrier dysfunction plays an important role in the pathophysiology of EoE. We hypothesized that MCs contribute to the observed impaired esophageal epithelial barrier. Herein, we demonstrate that coculture of differentiated esophageal epithelial cells with immunoglobulin E-activated MCs significanly decreased epithelial resistance by 30% and increased permeability by 22% compared with non-activated MCs. These changes were associated with decreased messenger RNA expression of barrier proteins filaggrin, desmoglein-1 and involucrin, and antiprotease serine peptidase inhibitor kazal type 7. Using targeted proteomics, we detected various cytokines in coculture supernatants, most notably granulocyte-macrophage colony-stimulating factor and oncostatin M (OSM). OSM expression was increased by 12-fold in active EoE and associated with MC marker genes. Furthermore, OSM receptor-expressing esophageal epithelial cells were found in the esophageal tissue of patients with EoE, suggesting that the epithelial cells may respond to OSM. Stimulation of esophageal epithelial cells with OSM resulted in a dose-dependent decrease in barrier function and expression of filaggrin and desmoglein-1 and an increase in protease calpain-14. Taken together, these data suggest a role for MCs in decreasing esophageal epithelial barrier function in EoE, which may in part be mediated by OSM.

SCF and IL-33 regulate mouse mast cell phenotypic and functional plasticity supporting a pro-inflammatory microenvironment

Mast cells (MCs) are multifaceted innate immune cells often present in the tumor microenvironment (TME). Several recent findings support their contribution to the transition from chronic inflammation to cancer. However, MC-derived mediators can either favor tumor progression, inducing the spread of the tumor, or exert anti-tumorigenic functions, limiting tumor growth. This apparent controversial role likely depends on the plastic nature of MCs that under different microenvironmental stimuli can rapidly change their phenotype and functions. Thus, the exact effect of unique MC subset(s) during tumor progression is far from being understood. Using a murine model of colitis-associated colorectal cancer, we initially characterized the MC population within the TME and in non-lesional colonic areas, by multicolor flow cytometry and confocal microscopy. Our results demonstrated that tumor-associated MCs harbor a main connective tissue phenotype and release high amounts of Interleukin (IL)-6 and Tumor Necrosis Factor (TNF)-α. This MC phenotype correlates with the presence of high levels of Stem Cell Factor (SCF) and IL-33 inside the tumor. Thus, we investigated the effect of SCF and IL-33 on primary MC cultures and underscored their ability to shape MC phenotype eliciting the production of pro-inflammatory cytokines. Our findings support the conclusion that during colonic transformation a sustained stimulation by SCF and IL-33 promotes the accumulation of a prevalent connective tissue-like MC subset that through the secretion of IL-6 and TNF-α maintains a pro-inflammatory microenvironment.

IL-33-mediated activation of mast cells is involved in the progression of imiquimod-induced psoriasis-like dermatitis

BACKGROUND

Psoriasis is a chronic inflammatory dermatosis with an unclear pathogenesis. Mast cells (MCs) can serve as a bridge between innate and adaptive immunity and are involved in the regulation of the inflammatory state and immune homeostasis in diseases. MCs constitutively express interleukin-33 receptor T1/ST2 (IL-33R). IL-33 is a potent MCs activator that is actively secreted by keratinocytes in psoriasis. However, the regulatory role of MCs in psoriasis remains uncertain. Therefore, we hypothesised that IL-33 could promote MC activation to regulate psoriasis development.

METHODS

We performed experiments on wild-type (WT) and MC-deficient (Kit Wsh/Wsh) mice, established psoriasis-like mouse models using imiquimod (IMQ), and performed RNA sequencing and transcriptomic analysis of skin lesions. Exogenous administration was performed using recombinant IL-33. Validation and evaluation were performed using PSI scoring, immunofluorescence, immunohistochemistry, and qPCR.

RESULTS

We observed an upregulation in the number and activation of MCs in patients with psoriasis and in IMQ-induced psoriasis-like dermatitis. Deficiency of MCs ameliorates IMQ-induced psoriatic dermatitis at an early stage. IL-33 is increased and co-localized with MCs in the dermis of psoriasis-like lesions using immunofluorescence. Compared to WT mice, IMQ-induced Kit^Wsh/Wsh mice demonstrated a delayed response to exogenous IL-33.

CONCLUSIONS

MCs are activated by IL-33 in the early stages of psoriasis and exacerbate psoriasis-associated skin inflammation. The regulation of MC homeostasis may be a potential therapeutic strategy for psoriasis. Video Abstract.

RNA-sequencing reveals molecular and regional differences in the esophageal mucosa of achalasia patients

Achalasia is an esophageal motility disorder characterized by the functional loss of myenteric plexus ganglion cells in the distal esophagus and lower esophageal sphincter. Histological changes have been reported in the esophageal mucosa of achalasia, suggesting its involvement in disease pathogenesis. Despite recent advances in diagnosis, our understanding of achalasia pathogenesis at the molecular level is very limited and gene expression profiling has not been performed. We performed bulk RNA-sequencing on esophageal mucosa from 14 achalasia and 8 healthy subjects. 65 differentially expressed genes (DEGs) were found in the distal esophageal mucosa of achalasia subjects and 120 DEGs were identified in proximal esophagus. Gene expression analysis identified genes common or exclusive to proximal and distal esophagus, highlighting regional differences in the disease. Enrichment of signaling pathways related to cytokine response and viral defense were observed. Increased infiltration of CD45+ intraepithelial leukocytes were seen in the mucosa of 38 achalasia patients compared to 12 controls. Novel insights into the molecular changes occurring in achalasia were generated in this transcriptomic study. Some gene changes observed in the mucosa of achalasia may be associated with esophagitis. Differences in DEGs between distal and proximal esophagus highlight the importance of better understanding regional differences in achalasia.

Human Lung Mast Cells: Therapeutic Implications in Asthma

Mast cells are strategically located in different compartments of the lung in asthmatic patients. These cells are widely recognized as central effectors and immunomodulators in different asthma phenotypes. Mast cell mediators activate a wide spectrum of cells of the innate and adaptive immune system during airway inflammation. Moreover, these cells modulate the activities of several structural cells (i.e., fibroblasts, airway smooth muscle cells, bronchial epithelial and goblet cells, and endothelial cells) in the human lung. These findings indicate that lung mast cells and their mediators significantly contribute to the immune induction of airway remodeling in severe asthma. Therapies targeting mast cell mediators and/or their receptors, including monoclonal antibodies targeting IgE, IL-4/IL-13, IL-5/IL-5Rα, IL-4Rα, TSLP, and IL-33, have been found safe and effective in the treatment of different phenotypes of asthma. Moreover, agonists of inhibitory receptors expressed by human mast cells (Siglec-8, Siglec-6) are under investigation for asthma treatment. Increasing evidence suggests that different approaches to depleting mast cells show promising results in severe asthma treatment. Novel treatments targeting mast cells can presumably change the course of the disease and induce drug-free remission in bronchial asthma. Here, we provide an overview of current and promising treatments for asthma that directly or indirectly target lung mast cells.

Dual Immune Regulatory Roles of Interleukin-33 in Pathological Conditions

Interleukin-33 (IL-33), a member of the IL-1 cytokine family and a multifunctional cytokine, plays critical roles in maintaining host homeostasis and in pathological conditions, such as allergy, infectious diseases, and cancer, by acting on multiple types of immune cells and promoting type 1 and 2 immune responses. IL-33 is rapidly released by immune and non-immune cells upon stimulation by stress, acting as an “alarmin” by binding to its receptor, suppression of tumorigenicity 2 (ST2), to trigger downstream signaling pathways and activate inflammatory and immune responses. It has been recognized that IL-33 displays dual-functioning immune regulatory effects in many diseases and has both pro- and anti-tumorigenic effects, likely depending on its primary target cells, IL-33/sST2 expression levels, cellular context, and the cytokine microenvironment. Herein, we summarize our current understanding of the biological functions of IL-33 and its roles in the pathogenesis of various conditions, including inflammatory and autoimmune diseases, infections, cancers, and cases of organ transplantation. We emphasize the nature of context-dependent dual immune regulatory functions of IL-33 in many cells and diseases and review systemic studies to understand the distinct roles of IL-33 in different cells, which is essential to the development of more effective diagnoses and therapeutic approaches for IL-33-related diseases.

Transcriptomic Regulation of Macrophages by Matrix-Bound Nanovesicle-Associated Interleukin-33

The innate immune response, particularly the phenotype of responding macrophages, has significant clinical implications in the remodeling outcome following implantation of biomaterials and engineered tissues. In general, facilitation of an anti-inflammatory (M2-like) phenotype is associated with tissue repair and favorable outcomes, whereas pro-inflammatory (M1-like) activation can contribute to chronic inflammation and a classic foreign body response. Biologic scaffolds composed of extracellular matrix (ECM) and, more recently, matrix-bound nanovesicles (MBV) embedded within the ECM are known to direct macrophages toward an anti-inflammatory phenotype and stimulate a constructive remodeling outcome. The mechanisms of MBV-mediated macrophage activation are not fully understood, but interleukin-33 (IL-33) within the MBV appears critical for M2-like activation. Previous work has shown that IL-33 is encapsulated within the lumen of MBV and stimulates phenotypical changes in macrophages independent of its canonical surface receptor stimulation-2 (ST2). In the present study, we used next-generation RNA sequencing to determine the gene signature of macrophages following exposure to MBV with and without intraluminal IL-33. MBV-associated IL-33 instructed an anti-inflammatory phenotype in both wild-type and st2-/- macrophages by upregulating M2-like and downregulating M1-like genes. The repertoire of genes regulated by ST2-independent IL-33 signaling were broadly related to the inflammatory response and crosstalk between cells of both the innate and adaptive immune systems. These results signify the importance of the MBV intraluminal protein IL-33 in stimulating a pro-remodeling M2-like phenotype in macrophages and provides guidance for the designing of next-generation biomaterials and tissue engineering strategies. Impact statement The phenotype of responding macrophages is predictive of the downstream remodeling response to an implanted biomaterial. The clinical impact of macrophage phenotype has motivated studies to investigate the factors that regulate macrophage activation. Matrix-bound nanovesicles (MBV) embedded within the extracellular matrix direct macrophages toward an anti-inflammatory (M2)-like phenotype that is indicative of a favorable remodeling response. Although the mechanisms of MBV-mediated macrophage activation are not fully understood, the intraluminal protein interleukin-33 (IL-33) is clearly a contributing signaling molecule. The present study identifies those genes regulated by MBV-associated IL-33 that promote a pro-remodeling M2-like macrophage activation state and can guide future therapies in regenerative medicine.

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.

Bacillus Calmette-Guérin-Induced Human Mast Cell Activation Relies on IL-33 Priming

Bacillus Calmette-Guérin (BCG) vaccine is an attenuated strain of Mycobacterium bovis that provides weak protection against tuberculosis (TB). Mast cells (MCs) are tissue-resident immune cells strategically that serve as the first line of defence against pathogenic threats. In this study, we investigated the response of human MCs (hMCs) to BCG. We found that naïve hMCs exposed to BCG did not secrete cytokines, degranulate, or support the uptake and intracellular growth of bacteria. Since we could show that in hMCs IL-33 promotes the transcription of host-pathogen interaction, cell adhesion and activation genes, we used IL-33 for cell priming. The treatment of hMCs with IL-33, but not IFN-γ, before BCG stimulation increased IL-8, MCP-1 and IL-13 secretion, and induced an enhanced expression of the mycobacteria-binding receptor CD48. These effects were comparable to those caused by the recombinant Mycobacterium tuberculosis (Mtb) 19-KDa lipoprotein. Finally, stimulation of hMCs with IL-33 incremented MC-BCG interactions. Thus, we propose that IL-33 may improve the immunogenicity of BCG vaccine by sensitising hMCs.

Interleukin-33 and thymic stromal lymphopoietin are primary cytokines involved in the Th1/Th2 inflammatory response in chronic secretory otitis media

Background:

T-helper (Th)1/Th2 inflammatory responses are responsible for secretory otitis media (SOM) development. However, the mechanisms underlying these immune responses remain unknown. This study aims to identify the primary cytokines that play essential roles in chronic SOM.

Methods:

Two groups were established for the present study: chronic SOM group ( n = 21) and control group ( n = 10). The middle ear effusion and serum samples of the expression cytokines (interleukin IL-2, IL-4, IL-5, IL-13, IL-17, IL-25, IL-33, interferon [IFN]-γ, thymic stromal lymphopoietin [TSLP], immunoglobulin IgE, and pepsins) were analyzed by enzyme-linked immunosorbent assay.

Results:

The levels of IL-4, IL-5, IL-13, IL-17, IL-25, IFN-γ, TLSP, pepsins, IL-2, and IL-33 (all, p < 0.001) were higher in middle ear effusion, when compared to those in serum, in chronic SOM group (non-paired sample). However, there was no significant difference in serum expression for those cytokines compared chronic SOM group and control group. The paired sample expression for IL-33 and TLSP (both, p = 0.046) were higher compared the effusion and serum in chronic SOM group.

Conclusions:

IL-33 produces inflammatory cytokines, such as IL-1b, IL-6, TNF-α, IL-10, IL-4, and TGF-β, which through nucleus into cytoplasm causing inflammatory responses. The present study revealed that IL-33 also produce IL-17, IL-4, IL-5, and IL-13 inflammatory factors, triggering an inflammatory response. Study reported that the combined stimulation of TSLP and IL-33 elicits an approximately 10-fold increase in cytokine production, when compared to the stimulation of IL-33 alone. This suggests that IL-33 and TLSP may be the primary cytokines involved in Th1/Th2 inflammatory responses in chronic SOM.

The Role of Crosstalk of Immune Cells in Pathogenesis of Chronic Spontaneous Urticaria

Chronic spontaneous urticaria (CSU) is defined as recurrent episodes of spontaneous wheal development and/or angioedema for more than six weeks and at least twice a week. The core link in the pathogenesis of CSU is the activation of mast cells, T cells, eosinophils, and other immune cells infiltrating around the small venules of the lesion. Increased vascular permeability, vasodilatation, and recruitment of inflammatory cells directly depend on mast cell mediators’ release. Complex regulatory systems tightly influence the critical roles of mast cells in the local microenvironment. The bias toward Th2 inflammation and autoantibodies derived from B cells, histamine expressed by basophils, and initiation of the extrinsic coagulation pathway by eosinophils or monocytes exerts powerful modulatory influences on mast cells. Cell-to-cell interactions between mast cells and eosinophils/T cells also are regulators of their function and may involve CSU’s pathomechanism. This review summarizes up-to-date knowledge regarding the crosstalk between mast cells and other immune cells, providing the impetus to develop new research concepts and treatment strategies for CSU.

Loss of NFE2L3 protects against inflammation-induced colorectal cancer through modulation of the tumor microenvironment

We investigated the role of the NFE2L3 transcription factor in inflammation-induced colorectal cancer. Our studies revealed that Nfe2l3^−/− mice exhibit significantly less inflammation in the colon, reduced tumor size and numbers, and skewed localization of tumors with a more pronounced decrease of tumors in the distal colon. CIBERSORT analysis of RNA-seq data from normal and tumor tissue predicted a reduction in mast cells in Nfe2l3^−/− animals, which was confirmed by toluidine blue staining. Concomitantly, the transcript levels of Il33 and Rab27a, both important regulators of mast cells, were reduced and increased, respectively, in the colorectal tumors of Nfe2l3^−/− mice. Furthermore, we validated NFE2L3 binding to the regulatory sequences of the IL33 and RAB27A loci in human colorectal carcinoma cells. Using digital spatial profiling, we found that Nfe2l3^−/− mice presented elevated FOXP3 and immune checkpoint markers CTLA4, TIM3, and LAG3, suggesting an increase in Treg counts. Staining for CD3 and FOXP3 confirmed a significant increase in immunosuppressive Tregs in the colon of Nfe2l3^−/− animals. Also, Human Microbiome Project (HMP2) data showed that NFE2L3 transcript levels are higher in the rectum of ulcerative colitis patients. The observed changes in the tumor microenvironment provide new insights into the molecular differences regarding colon cancer sidedness. This may be exploited for the treatment of early-onset colorectal cancer as this emerging subtype primarily displays distal/left-sided tumors.

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.

Generation and Characterization of Torudokimab (LY3375880): A Monoclonal Antibody That Neutralizes Interleukin-33

Background

Interleukin-33 (IL-33) is an alarmin that is released following cellular damage, mechanical injury, or necrosis. It is a member of the IL-1 family and binds to a heterodimer receptor consisting of ST2 and IL-1RAP to induce the production of a wide range of cellular mediators, including the type 2 cytokines IL-4, IL-5 and IL-13. This relationship has led to the hypothesis that the IL-33/ST2 pathway is a driver of allergic disease and inhibition of the IL-33 and ST2 association could have therapeutic benefit.

Methods

In this paper, we describe the selection of a phage antibody through the ability to bind human IL-33 and block IL-33/ST2 interaction. This hit antibody was then affinity matured by site-directed mutagenesis of the antibody complementarity-determining regions (CDRs). Further characterization of a fully human monoclonal antibody (mAb), torudokimab (LY3375880) included demonstration of human IL-33 neutralization activity in vitro with an NFκB reporter assay and IL-33 induced mast cell cytokine secretion assay, followed by an in vivo IL-33-induced pharmacodynamic inhibition assay in mice that used IL-5 production as the endpoint.

Results

Torudokimab is highly specific to IL-33 and does not bind any of the other IL-1 family members. Furthermore, torudokimab binds human and cynomolgus monkey IL-33 with higher affinity than the binding affinity of IL-33 to ST2, but does not bind mouse, rat, or rabbit IL-33. Torudokimab’s half-life in cynomolgous monkey projects monthly dosing in the clinic.

Conclusion

Due to torudokimab’s high affinity, its ability to completely neutralize IL-33 activity in vitro and in vivo, and the observed cynomolgus monkey pharmacokinetic properties, this molecule was selected for clinical development.

Mast cell activation tests by flow cytometry: A new diagnostic asset?

Since the late nineties, evidence has accumulated that flow-assisted basophil activation test (BAT) might be an accessible and reliable method to explore the mechanisms governing basophil degranulation and diagnostic allowing correct prediction of the clinical outcome following exposure to the offending allergen(s) and cross-reactive structures for different IgE-dependent allergies and particular forms of autoimmune urticaria. Although the BAT offers many advantages over mediator release tests, it is left with some weaknesses that hinder a wider application. It is preferable to perform the BAT analysis within 4 h of collection, and the technique does not advance diagnosis in patients with non-responsive cells. Besides, the BAT is difficult to standardize mainly because of the difficulty to perform large batch analyses that might span over several days. This article reviews the status of flow cytometric mast cell activation test (MAT) using passively sensitized mast cells (MCs) with patients' sera or plasma (henceforth indicated as passive MAT; pMAT) using both MC lines and cultured MCs in the diagnosis of IgE-dependent allergies. In addition, this paper provides guidance for generating human MCs from peripheral blood CD34⁺ progenitor cells (PBCMCs) and correct interpretation of flow cytometric analyses of activated and/or degranulating cells. With the recent recognition of the mas-related G protein-coupled receptor X2 (MRGPRX2) occupation as a putative mechanism of immediate drug hypersensitivity reactions (IDHRs), we also speculate how direct activation of MCs (dMAT)-that is direct activation by MRGPRX2 agonists without prior passive sensitization-could advance paradigms for this novel endotype of IDHRs.

Protein and Antibody Engineering: Suppressing Degranulation of the Mast Cells and Type I Hypersensitivity Reaction

With an increase in atopic cases and owing to a significant role of mast cells in type I hypersensitivity, a therapeutic need to inhibit degranulation of mast cells has risen. Mast cells are notorious for IgE-mediated allergic response. Advancements have allowed researchers to improve clinical outcomes of already available therapies. Engineered peptides and antibodies can be easily manipulated to attain desired characteristics as per the biological environment. A number of these molecules are designed to target mast cells in order to regulate the release of histamine and other mediators, thereby controlling type I hypersensitivity response. The aim of this review paper is to highlight some of the significant molecules designed for the purpose.

MRGPRX2 Is the Codeine Receptor of Human Skin Mast Cells: Desensitization through β-Arrestin and Lack of Correlation with the FcεRI Pathway

Codeine stimulates skin mast cells and is therefore used in skin tests and as an inducer of experimental itch. MRGPRX2 responds to various drugs, including opioids, to elicit pseudoallergic reactions, but whether it represents the main opiate receptor of skin mast cells remains unknown. By combining a number of approaches, including the silencing of MRGPRX2, we now report that MRGPRX2 is indeed the dominant codeine receptor of dermal mast cells. Activation by codeine displayed profound subject variability and correlated with secretion elicited by compound 48/80 or substance P but not by FcεRI aggregation. Degranulation by codeine was attenuated by stem cell factor, whereas the opposite was found for FcεRI. Compound 48/80 or codeine alone was able to achieve maximum MRGPRX2 activation. MRGPRX2 was rapidly internalized on codeine binding in a β-arrestin-1‒dependent manner. Codeine-triggered β-arrestin activation was also established by the Tango assay. Prestimulation with MRGPRX2 agonists (but not C3a or FcεRI aggregation) resulted in refractoriness to further stimulation by the same or another MRGPRX2 ligand (cross desensitization). This was duplicated in a cell line (RBL-MRGPRX2). Collectively, codeine degranulates skin mast cells through MRGPRX2, at which it acts as a balanced ligand. It has yet to be determined whether codeine-induced refractoriness could be exploited to desensitize MRGPRX2 to prevent severe pseudoallergic reactions.

Cytokines Stimulated by IL-33 in Human Skin Mast Cells: Involvement of NF-κB and p38 at Distinct Levels and Potent Co-Operation with FcεRI and MRGPRX2

The IL-1 family cytokine IL-33 activates and re-shapes mast cells (MCs), but whether and by what mechanisms it elicits cytokines in MCs from human skin remains poorly understood. The current study found that IL-33 activates CCL1, CCL2, IL-5, IL-8, IL-13, and TNF-α, while IL-1β, IL-6, IL-31, and VEGFA remain unaffected in cutaneous MCs, highlighting that each MC subset responds to IL-33 with a unique cytokine profile. Mechanistically, IL-33 induced the rapid (1–2 min) and durable (2 h) phosphorylation of p38, whereas the phosphorylation of JNK was weaker and more transient. Moreover, the NF-κB pathway was potently activated, as revealed by IκB degradation, increased nuclear abundance of p50/p65, and vigorous phosphorylation of p65. The activation of NF-κB occurred independently of p38 or JNK. The induced transcription of the cytokines selected for further study (CCL1, CCL2, IL-8, TNF-α) was abolished by interference with NF-κB, while p38/JNK had only some cytokine-selective effects. Surprisingly, at the level of the secreted protein products, p38 was nearly as effective as NF-κB for all entities, suggesting post-transcriptional involvement. IL-33 did not only instruct skin MCs to produce selected cytokines, but it also efficiently co-operated with the allergic and pseudo-allergic/neurogenic activation networks in the production of IL-8, TNF-α, CCL1, and CCL2. Synergism was more pronounced at the protein than at the mRNA level and appeared stronger for MRGPRX2 ligands than for FcεRI. Our results underscore the pro-inflammatory nature of an acute IL-33 stimulus and imply that especially in combination with allergens or MRGPRX2 agonists, IL-33 will efficiently amplify skin inflammation and thereby aggravate inflammatory dermatoses.

Mast Cell Involvement in Fibrosis in Chronic Graft-Versus-Host Disease

Allogeneic hematopoietic stem cell transplantation (HSCT) is most commonly a treatment for inborn defects of hematopoiesis or acute leukemias. Widespread use of HSCT, a potentially curative therapy, is hampered by onset of graft-versus-host disease (GVHD), classified as either acute or chronic GVHD. While the pathology of acute GVHD is better understood, factors driving GVHD at the cellular and molecular level are less clear. Mast cells are an arm of the immune system that are known for atopic disease. However, studies have demonstrated that they can play important roles in tissue homeostasis and wound healing, and mast cell dysregulation can lead to fibrotic disease. Interestingly, in chronic GVHD, aberrant wound healing mechanisms lead to pathological fibrosis, but the cellular etiology driving this is not well-understood, although some studies have implicated mast cells. Given this novel role, we here review the literature for studies of mast cell involvement in the context of chronic GVHD. While there are few publications on this topic, the papers excellently characterized a niche for mast cells in chronic GVHD. These findings may be extended to other fibrosing diseases in order to better target mast cells or their mediators for treatment of fibrotic disease.

Peripheral blood cultured mast cells: Phenotypic and functional outcomes of different culture protocols

BACKGROUND

Mast cells (MCs) play a pivotal role in innate and adaptive immune responses. However, MCs are also involved in different pathologic conditions. Studies on the mechanisms that govern human MC functions are impeded by their limited and difficult recovery. Therefore, several research groups have developed protocols to culture human MCs from progenitor cells. These protocols vary with respect to culture duration and used maturation cytokines. How MCs obtained by different protocols differ in phenotype and functionality is currently unknown.

OBJECTIVE

To compare different protocols for the generation of human MCs from peripheral blood progenitors.

METHODS

Thirteen paired human MC cultures were investigated. MCs were cultured form CD34⁺ progenitors cells for 4 or 8 weeks and with or without the addition of IL-6. Phenotyping comprised staining for CD117, CD203c, FcεRI, MRGPRX2, CD300a and CD32. Functional studies included measurements of the up-regulation of CD63 and CD203c after allergen-specific cross-linking of sIgE/FcεRI complexes or ligation of MRGPRX2 with substance P and different drugs.

RESULTS

Cell cultures for 4 weeks in the presence of IL-6 consistently yielded the highest numbers of MCs. MCs cultured for 8 weeks with IL-6 showed more autofluorescence significantly impeding correct analyses of FcεRI and CD32. The density of FcεRI and CD32 was comparable between the different culture conditions. MRGPRX2 expression was significantly higher in the 8 week cultures. The density of CD300a was increased in the cultures with IL-6. Cells cultured for 8 weeks were more responsive to MRGPRX2 activation. In contrast, the 4-weeks cultures with IL-6 showed significantly higher allergen-specific activation.

CONCLUSION

Four weeks of culture with IL-6 are sufficient to generate sizeable numbers of human mast cells from blood progenitors, thereby enabling simultaneous exploration of allergen-specific sIgE/FcεRI cross-linking and non-specific activation via MRGPRX2.

Resveratrol inhibits MRGPRX2-mediated mast cell activation via Nrf2 pathway

Mast cells (MCs) are crucial effectors in inflammation and allergic reactions. The Mas-related G-protein-coupled receptor X2 (MRGPRX2) was the MC-specific receptor and play a key role in IgE-independent allergic reactions. The activation of the Nuclear factor erythroid derived 2-related factor 2 (Nrf2) is involved in IgE-mediated MC degranulation. Resveratrol (Res) is a polyphenolic compound in red wine and has been reported to exert a variety of pharmacological effects. In the current study, we investigated the effect of Res in regulating MRGPRX2-mediated MC activation and its underlyingmechanism. We demonstrated that Res reduced compound 48/80 (C48/80)-induced calcium flux in MCs and inhibited MCs degranulation in vitro. Res also suppressed C48/80-induced hind paw extravasation, active systemic anaphylaxis, and MCs degranulation in mouse models of pseudo-allergy in vivo. Furthermore, PCR and immunohistochemistry assay suggest that Res up-regulates Nrf2 expression and Nrf2 inhibitor attenuates the protective effects of Res. In conclusion, Res exerts an inhibitory effect on MRGPRX2-mediated MCs activation by targeting Nrf2 pathway and may present a promising new therapeutic agent for the treatment of MRGPRX2-dependent anaphylactoid reactions.

Interleukin-33 Amplifies Human Mast Cell Activities Induced by Complement Anaphylatoxins

Both, aberrant mast cell responses and complement activation contribute to allergic diseases. Since mast cells are highly responsive to C3a and C5a, while Interleukin-33 (IL-33) is a potent mast cell activator, we hypothesized that IL-33 critically regulates mast cell responses to complement anaphylatoxins. We sought to understand whether C3a and C5a differentially activate primary human mast cells, and probe whether IL-33 regulates C3a/C5a-induced mast cell activities. Primary human mast cells were generated from peripheral blood precursors or isolated from healthy human lung tissue, and mast cell complement receptor expression, degranulation, mediator release, phosphorylation patterns, and calcium flux were assessed. Human mast cells of distinct origin express constitutively higher levels of C3aR1 than C5aR1, and both receptors are downregulated by anaphylatoxins. While C3a is a potent mast cell degranulation inducer, C5a is a weaker secretagogue with more delayed effects. Importantly, IL-33 potently enhances the human mast cell reactivity to C3a and C5a (degranulation, cytokine and chemokine release), independent of changes in C3a or C5a receptor expression or the level of Ca²⁺ influx. Instead, this reflects differential dynamics of intracellular signaling such as ERK1/2 phosphorylation. Since primary human mast cells respond differentially to anaphylatoxin stimulation, and that IL-33 is a key regulator of mast cell responses to complement anaphylatoxins, this is likely to aggravate Th2 immune responses. This newly identified cross-regulation may be important for controlling exacerbated complement- and mast cell-dependent Th2 responses and thus provides an additional rationale for targeting anti-IL33 therapeutically in allergic diseases.

IL-33 and Superantigenic Activation of Human Lung Mast Cells Induce the Release of Angiogenic and Lymphangiogenic Factors

Human lung mast cells (HLMCs) express the high-affinity receptor FcεRI for IgE and are strategically located in different compartments of human lung, where they play a role in several inflammatory disorders and cancer. Immunoglobulin superantigens (e.g., protein A of Staphylococcus aureus and protein L of Peptostreptococcus magnus) bind to the variable regions of either the heavy (V_H3) or light chain (κ) of IgE. IL-33 is a cytokine expressed by epithelial cells that exerts pleiotropic functions in the lung. The present study investigated whether immunoglobulin superantigens protein A and protein L and IL-33 caused the release of inflammatory (histamine), angiogenic (VEGF-A) and lymphangiogenic (VEGF-C) factors from HLMCs. The results show that protein A and protein L induced the rapid (30 min) release of preformed histamine from HLMCs. By contrast, IL-33 did not induce the release of histamine from lung mast cells. Prolonged incubation (12 h) of HLMCs with superantigens and IL-33 induced the release of VEGF-A and VEGF-C. Preincubation with IL-33 potentiated the superantigenic release of histamine, angiogenic and lymphangiogenic factors from HLMCs. Our results suggest that IL-33 might enhance the inflammatory, angiogenic and lymphangiogenic activities of lung mast cells in pulmonary disorders.

Thymic Stromal Lymphopoietin Promotes MRGPRX2-Triggered Degranulation of Skin Mast Cells in a STAT5-Dependent Manner with Further Support from JNK

Thymic stromal lymphopoietin (TSLP) is released by epithelial cells following disturbed homeostasis to act as “alarmin” and driver of Th2-immunity. Aberrant TSLP expression is a hallmark of atopic diseases, including atopic dermatitis (AD). Mast cells (MCs) are overabundant in AD lesions and show signs of degranulation, but it remains unknown whether TSLP contributes to granule discharge. Degranulation of skin MCs proceeds via two major routes, i.e., FcεRI-dependent (allergic) and MRGPRX2-mediated (pseudo-allergic/neurogenic). Evidence is accumulating that MRGPRX2 may be crucial in the context of skin diseases, including eczema. The current study reveals TSLP as a novel priming factor of human skin MCs. Interestingly, TSLP selectively cooperates with MRGPRX2 to support granule discharge, while it does not impact spontaneous or FcεRI-driven exocytosis. TSLP-assisted histamine liberation triggered by compound 48/80 or Substance P, two canonical MRGPRX2 agonists, was accompanied by an increase in CD107a+ cells (a MC activation marker). The latter process was less potent, however, and detectable only at the later of two time points, suggesting TSLP may prolong opening of the granules. Mechanistically, TSLP elicited phosphorylation of STAT5 and JNK in skin MCs and the reinforced degranulation critically depended on STAT5 activity, while JNK had a contributory role. Results from pharmacological inhibition were confirmed by RNA-interference, whereby silencing of STAT5 completely abolished the priming effect of TSLP on MRGPRX2-mediated degranulation. Collectively, TSLP is the first factor to favor MRGPRX2- over FcεRI-triggered MC activation. The relevance of TSLP, MCs and MRGPRX2 to pruritis and atopic skin pathology indicates broad repercussions of the identified connection.

Nuclear Alarmin Cytokines in Inflammation

Pathogen-associated molecular patterns (PAMPs) are some nonspecific and highly conserved molecular structures of exogenous specific microbial pathogens, whose products can be recognized by pattern recognition receptor (PRR) on innate immune cells and induce an inflammatory response. Under physiological stress, activated or damaged cells might release some endogenous proteins that can also bind to PRR and cause a harmful aseptic inflammatory response. These endogenous proteins were named damage-associated molecular patterns (DAMPs) or alarmins. Indeed, alarmins can also play a beneficial role in the tissue repair in certain environments. Besides, some alarmin cytokines have been reported to have both nuclear and extracellular effects. This group of proteins includes high-mobility group box-1 protein (HMGB1), interleukin (IL)-33, IL-1α, IL-1F7b, and IL-16. In this article, we review the involvement of nuclear alarmins such as HMGB1, IL-33, and IL-1α under physiological state or stress state and suggest a novel activity of these molecules as central initiators in the development of sterile inflammation.

Mast cells crosstalk with B cells in the gut and sustain IgA response in the inflamed intestine

B lymphocytes are among the cell types whose effector functions are modulated by mast cells (MCs). The B/MC crosstalk emerged in several pathological settings, notably the colon of inflammatory bowel disease (IBD) patients is a privileged site in which MCs and IgA⁺ cells physically interact. Herein, by inducing conditional depletion of MCs in red MC and basophil (RMB) mice, we show that MCs control B cell distribution in the gut and IgA serum levels. Moreover, in dextran sulfate sodium (DSS)-treated RMB mice, the presence of MCs is fundamental for the enlargement of the IgA⁺ population in the bowel and the increase of systemic IgA production. Since both conventional B-2 and peritoneal-derived B cells populate the intestine and communicate with MCs in physiological conditions and during inflammation, we further explored this interplay through the use of co-cultures. We show that MCs finely regulate different aspects of splenic B cell biology while peritoneal B cells are unresponsive to the supporting effects provided by MCs. Interestingly, peritoneal B cells induce a pro-inflammatory skewing in MCs, characterized by increased ST2 and TNF-α expression. Altogether, this study uncovers the versatility of the B/MC liaison and highlights key aspects for the resolution of intestinal inflammation.

Role of myeloid cells in the regulation of group 2 innate lymphoid cell-mediated allergic inflammation

Group 2 innate lymphoid cells (ILC2s) are an important component of the innate immune system that execute important effector functions at barrier surfaces, such as lung and skin. Like T helper type 2 cells, ILC2s are able to release high amounts of type 2 cytokines that are essential in inducing allergic inflammation and eliminating helminth infections. The past few years have contributed to our better understanding of the interactions between ILC2s and other cells of the immune system via soluble factors or in a cell-cell contact manner. Myeloid cells, including mononuclear leukocytes and polymorphonuclear leukocytes, are excellent sensors of tissue damage and infection and can influence ILC2 responses in the process of allergic inflammation. In this review, we summarize recent insights on how myeloid cell subsets regulate ILC2 activation with focus on soluble factors in the context of allergic inflammation.

Baicalin induces Mrgprb2-dependent pseudo-allergy in mice

Baicalin, a component of traditional Chinese medicine, is one of the main compounds present in Scutellaria baicalensis Georgi. Pseudo-allergy induced by the injection of these medicines is a frequent adverse drug reaction. Therefore, elucidation of the anaphylactoid reaction of baicalin and its underlying mechanisms are important. Mast cells are primary effectors of allergic reactions, including pseudo-allergy. Studies have shown that Mrgprx2 in human mast cells is a specific receptor that is crucial for pseudo-allergic drug reactions, Mrgprb3 is the rat ortholog of human Mrgprx2, which in mice is designated as Mrgprb2. Here, we aimed to investigate baicalin-induced pseudo-allergy and the association of Mrgprb3 and Mrgprb2 with this effect. We examined the allergenic effect of baicalin on RBL-2H3 cells and Mrgprb3-knockdown RBL-2H3 cells. Mrgprb2-expressing HEK293 cells and Mrgprb2-knockout mice were used to evaluate the role of Mrgprb2 in baicalin-induced allergy. Baicalin was found to dose-dependently induce pseudo-allergy both in vitro and in vivo. RBL-2H3 cells were activated by baicalin, whereas in Mrgprb3-knockout RBL-2H3 cells, baicalin showed a negligible effect on cell activation. Furthermore, baicalin activated the Mrgprb2-expressing HEK293 cells. Our data showed that baicalin did not induce allergy in Mpgprb2-knockout mice. We conclude that baicalin induces pseudo-allergy via Mrgprb2 in mice.

Interleukin (IL)-33: an orchestrator of immunity from host defence to tissue homeostasis

Interleukin (IL)-33, a member of the IL-1 superfamily, functions as an alarm signal, which is released upon cell injury or tissue damage to alert the immune system. It has emerged as a chief orchestrator in immunity and has a broad pleiotropic action that influences differentiation, maintenance and function of various immune cell types via the ST2 receptor. Although it has been strongly associated with immunopathology, critically, IL-33 is involved in host defence, tissue repair and homeostasis. In this review, we provide an overview of the signalling pathway of IL-33 and highlight its regulatory functions in immune cells. Furthermore, we attempt a broader discussion of the emerging functions of IL-33 in host defence, tissue repair, metabolism, inflammatory disease and cancer, suggesting potential avenues to manoeuvre IL-33/ST2 signalling as treatment options.

The Role of Mast Cells in IgE-Independent Lung Diseases

Mast cells (MCs) are granular cells of the innate immune system which develop from CD34+/CD117+ progenitors and play a role in orchestrating adaptive immune responses. They have a well-known role in allergic reactions following immunoglobulin (Ig)E-mediated activation of the cell-surface expressed IgE high-affinity receptor (FcεRI). MCs can also respond to various other stimuli due to the expression of a variety of receptors including toll-like receptors (TLRs), immunoglobulin (IgG) receptors (FcγR), complement receptors such as C5a (CD88) expressed by skin MCs, neuropeptides receptors including nerve growth factor receptor, (NGFR), cytokines receptors such as (IL)-1R and IL-3R, and chemokines receptors including CCR-1 and CCR-3. MCs release three groups of mediators upon degranulation differentiated according to their chemical composition, storage, and time to release. These include preformed mediators (mainly histamine, tryptase, and chymase), de novo synthesized mediators such as prostaglandin (PG)D2, leukotriene (LT)B4 and LTD4, and cytokines including IL-1β, IL-3, tumor necrosis factor (TNF)α, and transforming growth factor(TGF)-β. Emerging evidence indicates a role for IgE-independent MC activation in the late-stage asthmatic response as well as in non-allergic airway diseases including chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), and lung cancer. MC infiltration/activation has been reported in some, but not all, studies of lung cancer. MC-derived TNF-α possesses tumor-suppressive activity while IL-1β supports tumor progression and metastasis. In IPF lungs, an increase in density of tryptase- and chymase-positive MCs (MCTC) and overexpression of TGF-β support the fibrosis progression. MC-derived chymase activates latent TGF-β that induces the differentiation of fibroblasts to matrix-producing myofibroblasts. In summary, increasing evidence highlights a critical role of MCs in non-allergic diseases that may indicate new approaches for therapy.

Nasal epithelial barrier dysfunction increases sensitization and mast cell degranulation in the absence of allergic inflammation

BACKGROUND

Increased epithelial permeability has been reported in allergic rhinitis, with histamine and type-2 inflammation being responsible for tight junction dysfunction. The impact of an epithelial barrier defect on allergic sensitization and mast cell (MC) degranulation remains speculative.

METHODS

Transepithelial passage of allergens was evaluated on primary human nasal epithelial cell cultures. Active sensitization was attempted by repeated intranasal ovalbumin (OVA) applications in Naïve mice. In a passive sensitization model, mice were injected with IgE to Dermatophagoides pteronyssinus (rDer p)2 and then exposed intranasally to the allergen. Chitosan was used to disrupt nasal epithelial integrity in vitro and in vivo.

RESULTS

Chitosan strongly reduced transepithelial electrical resistance and facilitated transepithelial allergen passage in cultured primary nasal epithelial cells. In vivo, intranasal chitosan affected occludin expression and facilitated allergen passage. After epithelial barrier disruption, intranasal OVA application induced higher OVA-specific IgG1 and total IgE in serum, and increased eosinophilia and interleukin-5 in bronchoalveolar lavage (BAL) compared to sham-OVA mice. Chitosan exposure, prior to rDer p2 allergen challenge in passively sensitized mice, resulted in increased β-hexosaminidase levels in serum and BAL compared to sham-rDer p2 mice. Intranasal treatment with the synthetic glucocorticoid fluticasone propionate prevented chitosan-induced barrier dysfunction, allergic sensitization, and MC degranulation.

CONCLUSION

Epithelial barrier dysfunction facilitates transepithelial allergen passage, allergic sensitization, and allergen-induced MC degranulation even in the absence of inflammatory environment. These results emphasize the crucial role of an intact epithelial barrier in prevention of allergy.

The IL-33/ST2 Axis in Immune Responses Against Parasitic Disease: Potential Therapeutic Applications

Parasitic infections pose a wide and varying threat globally, impacting over 25% of the global population with many more at risk of infection. These infections are comprised of, but not limited to, toxoplasmosis, malaria, leishmaniasis and any one of a wide variety of helminthic infections. While a great deal is understood about the adaptive immune response to each of these parasites, there remains a need to further elucidate the early innate immune response. Interleukin-33 is being revealed as one of the earliest players in the cytokine milieu responding to parasitic invasion, and as such has been given the name "alarmin." A nuclear cytokine, interleukin-33 is housed primarily within epithelial and fibroblastic tissues and is released upon cellular damage or death. Evidence has shown that interleukin-33 seems to play a crucial role in priming the immune system toward a strong T helper type 2 immune response, necessary in the clearance of some parasites, while disease exacerbating in the context of others. With the possibility of being a double-edged sword, a great deal remains to be seen in how interleukin-33 and its receptor ST2 are involved in the immune response different parasites elicit, and how those parasites may manipulate or evade this host mechanism. In this review article we compile the current cutting-edge research into the interleukin-33 response to toxoplasmosis, malaria, leishmania, and helminthic infection. Furthermore, we provide insight into directions interleukin-33 research may take in the future, potential immunotherapeutic applications of interleukin-33 modulation and how a better clarity of early innate immune system responses involving interleukin-33/ST2 signaling may be applied in development of much needed treatment options against parasitic invaders.

IL-33 induced airways inflammation is partially dependent on IL-9

Asthma is an inflammatory disease of the airways and numerous cytokines contribute to this pathogenesis. It is shown that challenge of airways with IL-33 induces asthma-like pathological changes in mice, but the possible downstream cytokines in this process remain to be characterised. To explore this, we compared changes in the airways of wildtype (WT) and IL-9 deficient mice challenged with IL-33. In line with previous report, per-nasal challenge of WT mice with IL-33 significantly increased the responsiveness of the airways along with infiltration of inflammatory cells, goblet cell hyperplasia, collagen deposition and smooth muscle hypertrophy, and the expression of cytokines compared with control group. Surprisingly, all of these pathological changes were significantly attenuated in IL-9 deficient mice following identical IL-33 challenge. These data suggest that IL-9 is one downstream cytokine relevant to the effects of IL-33 in asthmatic airways and consequently a potential therapeutic target for the treatment of asthma.

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.