Mast Cell Functions Linking Innate Sensing to Adaptive Immunity

DNA Methylation Negatively Regulates Gene Expression of Key Cytokines Secreted by BMMCs Recognizing FMDV-VLPs

Virus-like particles (VLPs) have been studied and used as vaccines to control foot-and-mouth disease (FMD). Mast cells (MCs) express various pattern recognition receptors that recognize pathogens and secrete numerous cytokines to initiate and modulate immune responses. Our previous study showed that bone marrow-derived mast cells (BMMCs) can recognize foot-and-mouth disease virus-like particles (FMDV-VLPs) to differentially express various cytokines and that histone acetylation can regulate the cytokines secreted during BMMC recognition of FMDV-VLPs. To demonstrate the role of DNA methylation in this response process, BMMCs that recognize FMDV-VLPs were treated with azacytidine (5-AZA), an inhibitor of DNA methylation transferase. We prepared FMDV-VLPs as described previously and cultured the BMMCs. The transcription and expression of key cytokines and transcription factors were determined using real-time quantitative PCR (RT-qPCR) and Western blotting. Results showed that pre-treatment with AZA resulted in the increased transcription and expression of tumor necrosis factor α (TNF-α), interleukin (IL)-6, IL-13, and IL-10, while the changes in IL-13 transcription and IL-6 expression were irrelevant to mannose receptors (MRs). Furthermore, analysis of the transcription factors indicated that both the transcription and expression of nuclear factor-kappa B (NF-κB) increased significantly in the AZA pre-treated group, indicating that DNA methylation may also regulate NF-κB expression to modulate TNF-α, IL-13, and IL-6. However, pre-treatment with AZA did not alter the expression of microphthalmia-associated transcription factor (MITF) or GATA-2. All the data demonstrate that DNA methylation negatively regulates the transcription and expression of TNF-α, IL-13, IL-10, and IL-6 secreted by recognizing FMDV-VLPs. These results provide new ideas for the mast cell-based design of more effective vaccine adjuvants and targeted therapies in the future.

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.

Involvement of interferon γ-producing mast cells in immune responses against melanocytes in vitiligo requires MrgX2 activation

BACKGROUND

Increasing evidence indicates that oxidative stress and interferon γ (IFNγ)-driven cellular immune responses are responsible for the pathogenesis of vitiligo. However, the connection between oxidative stress and the local production of IFNγ in early vitiligo remains unexplored. The aim of this study was to identify the mechanism underlying the production of IFNγ by mast cells and its impact on vitiligo pathogenesis.

METHODS

Skin specimens from the central, marginal, and perilesional skin areas of active vitiligo lesions were collected to characterize changes of mast cells, CD8+ T cells, and IFNγ-producing cells. Cell supernatants from hydrogen peroxide (H2O2)-treated keratinocytes (KCs) were harvested to measure levels of soluble stem cell factor (sSCF) and matrix metalloproteinase (MMP)-9. A murine vitiligo model was established using Mas-related G protein-coupled receptor-B2 (MrgB2, mouse ortholog of human MrgX2) conditional knockout (MrgB2-/-) mice to investigate IFNγ production and inflammatory cell infiltrations in tail skin following the challenge with tyrosinase-related protein (Tyrp)-2 180 peptide. Potential interactions between the Tyrp-2 180 peptide and MrgX2 were predicted using molecular docking. The siRNAs targeting MrgX2 and the calcineurin inhibitor FK506 were also used to examine the signaling pathways involved in mast cell activation.

RESULTS

IFNγ-producing mast cells were closely aligned with the recruitment of CD8+ T cells in the early phase of vitiligo skin. sSCF released by KCs through stress-enhanced MMP9-dependent proteolytic cleavage recruited mast cells into sites of inflamed skin (Perilesion vs. lesion, 13.00 ± 4.00/HPF vs. 26.60 ± 5.72/HPF, P <0.05). Moreover, IFNγ-producing mast cells were also observed in mouse tail skin following challenge with Tyrp-2 180 (0 h vs. 48 h post-recall, 0.00 ± 0.00/HPF vs. 3.80 ± 1.92/HPF, P <0.05). The IFNγ+ mast cell and CD8+ T cell counts were lower in the skin of MrgB2-/-mice than in those of wild-type mice (WT vs. KO 48 h post-recall, 4.20 ± 0.84/HPF vs. 0.80 ± 0.84/HPF, P <0.05).

CONCLUSION

Mast cells activated by MrgX2 serve as a local IFNγ producer that bridges between innate and adaptive immune responses against MCs in early vitiligo. Targeting MrgX2-mediated mast cell activation may represent a new strategy for treating vitiligo.

Pathogenesis, Diagnosis, and Treatment of Infectious Rhinosinusitis

Rhinosinusitis is a common inflammatory disease of the sinonasal mucosa and paranasal sinuses. The pathogenesis of rhinosinusitis involves a variety of factors, including genetics, nasal microbiota status, infection, and environmental influences. Pathogenic microorganisms, including viruses, bacteria, and fungi, have been proven to target the cilia and/or epithelial cells of ciliated airways, which results in the impairment of mucociliary clearance, leading to epithelial cell apoptosis and the loss of epithelial barrier integrity and immune dysregulation, thereby facilitating infection. However, the mechanisms employed by pathogenic microorganisms in rhinosinusitis remain unclear. Therefore, this review describes the types of common pathogenic microorganisms that cause rhinosinusitis, including human rhinovirus, respiratory syncytial virus, Staphylococcus aureus, Pseudomonas aeruginosa, Aspergillus species, etc. The damage of mucosal cilium clearance and epithelial barrier caused by surface proteins or secreted virulence factors are summarized in detail. In addition, the specific inflammatory response, mainly Type 1 immune responses (Th1) and Type 2 immune responses (Th2), induced by the entry of pathogens into the body is discussed. The conventional treatment of infectious sinusitis and emerging treatment methods including nanotechnology are also discussed in order to improve the current understanding of the types of microorganisms that cause rhinosinusitis and to help effectively select surgical and/or therapeutic interventions for precise and personalized treatment.

Analysis of Chromatin Accessibility Changes Induced by BMMC Recognition of Foot-and-Mouth Disease Virus-like Particles through ATAC-seq

Mast cells can recognize foot-and-mouth disease virus-like particles (FMDV-VLPs) via mannose receptors (MRs) to produce differentially expressed cytokines. The regulatory role of chromatin accessibility in this process is unclear. Bone marrow-derived mast cells (BMMCs) were cultured, and an assay of transposase-accessible chromatin sequencing (ATAC-seq) was applied to demonstrate the regulation of chromatin accessibility in response to the BMMCs' recognition of FMDV-VLPs. A pathway enrichment analysis showed that peaks associated with the nuclear factor-κB (NF-κB), mitogen-activated protein kinase (MAPK), phosphatidylinositol 3 kinase-protein kinase B (PI3K-Akt), and other signaling pathways, especially the NF-κB pathway, were involved in the BMMCs' recognition of VLPs. Moreover, transcription factors including SP1, NRF1, AP1, GATA3, microphthalmia-associated transcription factor (MITF), and NF-κB-p65 may bind to the motifs with altered chromatin accessibility to regulate gene transcription. Furthermore, the expression of NF-κB, interleukin (IL)-9, tumor necrosis factor (TNF)-α, and interferon (IFN)-γ in the BMMCs of the VLP group increased compared with that of the BMMCs in the control group, whereas the expression of IL-10 did not differ significantly between groups. After inhibiting the MRs, the expression of NF-κB, IL-9, TNF-α, and IFN-γ decreased significantly, whereas the expression of IL-10 increased. The expression of MAPK and IL-6 showed no significant change after MR inhibition. This study demonstrated that MRs expressed on BMMCs can affect the NF-κB pathway by changing chromatin accessibility to regulate the transcription of specific cytokines, ultimately leading to the differential expression of cytokines. These data provide a theoretical basis and new ideas for the development of a novel vaccine for FMD.

Nanoparticulate chitosan-TNF-α-VLPs activate mast cells and enhance adaptive immunity induced by foot-and-mouth disease virus-like particles in mice

Chitosan nanoparticulate vaccines have attracted considerable attention to potentiate immune responses. A chitosan-TNF-α-VLPs nanoparticle vaccine against foot-and-mouth disease virus (FMDV) prepared though inotropic gelation method and whether this nanoparticulate vaccine can activate mast cells and enhance immune responses induced by FMDV virus-like particles (VLPs) in mice was investigated. The nanoparticle was approximately spherical, and its size was approximately 200-300 nm. Following immunization via subcutaneous injection, the chitosan-TNF-α-VLPs nanoparticles could induce higher levels of FMDV-specific antibodies and stimulation index value than VLPs only (P < 0.01) and had similar levels to commercial vaccine group and VLPs+adjuvant group (P > 0.05). No significant differences were observed in the concentrations of IL-4, IFN-γ and IL-10 among the chitosan-TNF-α-VLPs group, VLPs+adjuvant group and commercial vaccine group (P > 0.05). Of note, the chitosan-TNF-α-VLPs nanoparticles can effectively activate mast cells in lymph nodes. These results indicated that the chitosan-TNF-α-VLPs nanoparticles can enhance both humoral and cell-mediated immunity, and both Th1 and Th2 responses, even activate mast cells, demonstrating that chitosan-TNF-α nanoparticles are potential as a vaccine adjuvant to enhance immune responses induced by FMDV-VLPs.

Peripheral surgery triggers mast cells activation: Focusing on neuroinflammation

Peripheral surgery can lead to a systemic aseptic inflammatory response comprising several mediators aiming at restoring tissue homeostasis. It induces inflammatory mechanisms through neuroimmune interaction between the periphery and to brain which also plays a critical role in causing cognitive impairments. Accumulating scientific evidence revealed that acute neuroinflammation of the brain triggered by peripheral surgery that causes peripheral inflammation leads to transmitting signals into the brain through immune cells. Mast cells (MCs) play an important role in the acute neuroinflammation induced by peripheral surgical trauma. After peripheral surgery, brain-resident MCs can be rapidly activated followed by releasing histamine, tryptase, and other inflammatory mediators. These mediators then interact with other immune cells in the peripheral and amplify the signal into the brain by disrupting BBB and activating principle innate immune cells of brain including microglia, astrocytes, and vascular endothelial cells, which release abundant inflammatory mediators and in turn accelerate the activation of brain MCs, amplify the cascade effect of neuroinflammatory response. Surgical stress may induce HPA axis activation by releasing corticotropin-releasing hormone (CRH) subsequently influence the activation of brain MCs, thus resulting in impaired synaptic plasticity. Herein, we discuss the better understating of MCs mediated neuroinflammation mechanisms after peripheral surgery and potential therapeutic targets for controlling inflammatory cascades.

Histone acetylation regulates BMMCs recognition of foot-and-mouth disease virus-like particles

Foot-and-mouth disease (FMD) is one of the most economically and socially devastating diseases affecting animal agriculture worldwide. Foot-and-mouth disease virus (FMDV) virus-like particles (VLPs) have been widely studied as a candidate vaccine. Mast cells (MCs) are highly versatile innate immunity cells that perform various functions in regulating innate and adaptive immune responses. Recently, we found that MCs can recognize recombinant FMDV VP1-VP4 protein to produce various cytokines with differential expression, suggesting that this may be epigenetically regulated. In this study, we evaluated the effect of trichostatin A (TSA), a histone deacetylase inhibitor, on bone marrow-derived mast cells (BMMCs) recognition of FMDV-VLPs in vitro. BMMCs can recognize FMDV-VLPs via mannose receptors (MRs) and resulted in enhanced expression and secretion of tumour necrosis factor α (TNF-α) and interleukin (IL)-13. Nevertheless, BMMCs recognition of FMDV-VLPs to secrete IL-6 was irrelevant to MRs, and MRs may play a negative regulation for IL-10 secretion. Pre-treatment with TSA caused decreased expression of IL-6, TNF-α and IL-13, and increased expression of IL-10. Furthermore, the expression of nuclear factor-kappa B (NF-κB) was supressed in TSA treated BMMCs, suggesting histone acetylation may alter NF-κB expression to influence the TNF-α and IL-13 secretion. Pre-treatment with TSA had no influence on the expression of microphthalmia-associated transcription factor (MITF) and GATA-2. These data therefore suggest that altered histone acetylation regulates the immune responses induced by BMMCs recognition of FMDV-VLPs, providing an understanding and theory basis for the prevention and control of FMD based MCs.

Mast Cell and Innate Immune Cell Communication in Cholestatic Liver Disease

Mast cells (MCs) contribute to the pathogenesis of cholestatic liver diseases (primary sclerosing cholangitis [PSC] and primary biliary cholangitis [PBC]). PSC and PBC are immune-mediated, chronic inflammatory diseases, characterized by bile duct inflammation and stricturing, advancing to hepatobiliary cirrhosis. MCs are tissue resident immune cells that may promote hepatic injury, inflammation, and fibrosis formation by either direct or indirect interactions with other innate immune cells (neutrophils, macrophages/Kupffer cells, dendritic cells, natural killer, and innate lymphoid cells). The activation of these innate immune cells, usually through the degranulation of MCs, promotes antigen uptake and presentation to adaptive immune cells, exacerbating liver injury. In conclusion, dysregulation of MC-innate immune cell communications during liver injury and inflammation can lead to chronic liver injury and cancer.

Allergic Rhinitis: Pathophysiology and Treatment Focusing on Mast Cells

Allergic rhinitis (AR) is a common rhinopathy that affects up to 30% of the adult population. It is defined as an inflammation of the nasal mucosa, develops in allergic individuals, and is detected mostly by a positive skin-prick test. AR is characterized by a triad of nasal congestion, rhinorrhea, and sneezing. Mast cells (MCs) are innate immune system effector cells that play a pivotal role in innate immunity and modulating adaptive immunity, rendering them as key cells of allergic inflammation and thus of allergic diseases. MCs are typically located in body surfaces exposed to the external environment such as the nasal mucosa. Due to their location in the nasal mucosa, they are in the first line of defense against inhaled substances such as allergens. IgE-dependent activation of MCs in the nasal mucosa following exposure to allergens in a sensitized individual is a cardinal mechanism in the pathophysiology of AR. This review is a comprehensive summary of MCs' involvement in the development of AR symptoms and how classical AR medications, as well as emerging AR therapies, modulate MCs and MC-derived mediators involved in the development of AR.

Mast Cell Interaction with Foxp3+ Regulatory T Cells Occur in the Dermis after Initiation of IgE-Mediated Cutaneous Anaphylaxis

Mast cells (MCs) are well-known for their role in IgE-mediated cutaneous anaphylactic responses, but their regulatory functions in the skin are still under intense scrutiny. Using a Red MC and Basophil reporter (RMB) mouse allowing red fluorescent detection and diphtheria toxin mediated depletion of MCs, we investigated the interaction of MCs, Foxp3+ regulatory T lymphocytes (Tregs) and Langerhans cells (LCs) during passive cutaneous anaphylaxis (PCA) responses. Using intravital imaging we show that MCs are sessile at homeostasis and during PCA. Breeding RMB mice with Langerin-eGFP mice revealed that dermal MCs do not interact with epidermal-localized LCs, the latter showing constant sprouting of their dendrites at homeostasis and during PCA. When bred with Foxp3-eGFP mice, we found that, although a few Foxp3+ Tregs are present at homeostasis, many Tregs transiently infiltrated the skin during PCA. While their velocity during PCA was not altered, Tregs increased the duration of their contact time with MCs compared to PCA-control mice. Antibody-mediated depletion of Tregs had no effect on the intensity of PCA. Hence, the observed increase in Treg numbers and contact time with MCs, regardless of an effect on the intensity of PCA responses, suggests an anti-inflammatory role dedicated to prevent further MC activation.

Reprograming the immune niche for skin tissue regeneration - From cellular mechanisms to biomaterials applications

Despite the rapid development of therapeutic approaches for skin repair, chronic wounds such as diabetic foot ulcers remain an unaddressed problem that affects millions of people worldwide. Increasing evidence has revealed the crucial and diverse roles of the immune cells in the development and repair of the skin tissue, prompting new research to focus on further understanding and modulating the local immune niche for comprehensive, 'perfect' regeneration. In this review, we first introduce how different immunocytes and certain stromal cells involved in innate and adaptive immunity coordinate to maintain the immune niche and tissue homeostasis, with emphasis on their specific roles in normal and pathological wound healing. We then discuss novel engineering approaches - particularly biomaterials systems and cellular therapies - to target different players of the immune niche, with three major aims to i) overcome 'under-healing', ii) avoid 'over-healing', and iii) promote functional restoration, including appendage development. Finally, we highlight how these strategies strive to manage chronic wounds and achieve full structural and functional skin recovery by creating desirable 'soil' through modulating the immune microenvironment.

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.

The role of human mast cells in allergy and asthma

Mast cells are tissue-inhabiting cells that play an important role in inflammatory diseases of the airway tract. Mast cells arise in the bone marrow as progenitor cells and complete their differentiation in tissues exposed to the external environment, such as the skin and respiratory tract, and are among the first to respond to bacterial and parasitic infections. Mast cells express a variety of receptors that enable them to respond to a wide range of stimulants, including the high-affinity FcεRI receptor. Upon initial contact with an antigen, mast cells are sensitized with IgE to recognize the allergen upon further contact. FcεRI-activated mast cells are known to release histamine and proteases that contribute to asthma symptoms. They release a variety of cytokines and lipid mediators that contribute to immune cell accumulation and tissue remodeling in asthma. Mast cell mediators trigger inflammation and also have a protective effect. This review aims to update the existing knowledge on the mediators released by human FcεRI-activated mast cells, and to unravel their pathological and protective roles in asthma and allergy. In addition, we highlight other diseases that arise from mast cell dysfunction, the therapeutic approaches used to address them, and fill the gaps in our current knowledge. Mast cell mediators not only trigger inflammation but may also have a protective effect. Given the differences between human and animal mast cells, this review focuses on the mediators released by human FcεRI-activated mast cells and the role they play in asthma and allergy.

Altered leukocyte subsets and immune proteome indicate pro-inflammatory mechanisms in mastocytosis

BACKGROUND

Indolent systemic mastocytosis (ISM) is characterized by pathological accumulation of mast cells. The mechanism behind its phenotypic heterogeneity is not well understood. Interaction of mast cells with other immune cells might cause systemic inflammation and thereby associated symptoms.

OBJECTIVE

To investigate peripheral leukocyte compartments and serum immune proteome in ISM.

METHODS

Peripheral blood leukocyte phenotyping using flowcytometry in a cohort of 18 adults with ISM and 12 healthy controls. Targeted proteomics was performed to measure 169 proteins associated with inflammation on serum of another 20 ISM patients and 20 healthy controls.

RESULTS

Proportions of plasmacytoid dendritic cells (pDC) and monocytes were significantly decreased while T-helper 2 cells were increased in peripheral blood of ISM patients. Furthermore, a shift from naïve to memory T-cells was observed. Hierarchical clustering of the serum proteome revealed two distinct subgroups within ISM patients. In subgroup A (n=8), 62 proteins were significantly overexpressed, whereas subgroup B (n=12) was comparable to healthy controls. Patients in subgroup A displayed upregulated signaling pathways downstream of Toll-like receptor 4, TNF-α and interferon-γFatigue was more often present in subgroup A compared to B (75% vs. 33% respectively, p 0.06).

CONCLUSIONS

Altered distribution of leukocyte subsets and a pro-inflammatory proteome were observed in subsequent two cohorts of ISM patients. We hypothesize that neoplastic mast cells recruit and activate pDC, monocytes and T-cells leading to a vicious cycle of inflammation.

CLINICAL IMPLICATION

Characterization of inflammatory mechanisms in ISM might uncover additional targets for treatment, in particular for constitutional symptoms such as fatigue.

High Mast Cell Density Predicts a Favorable Prognosis in Patients with Pancreatic Neuroendocrine Neoplasms

INTRODUCTION

Mast cells are involved in allergic diseases, immune regulation, and tumor microenvironment modulation, with both pro- and anti-tumorigenic functions, and could serve as a prognostic factor in various cancers. However, their potential role in pancreatic neuroendocrine neoplasms (PanNENs) is largely unknown. Here, our aim was to investigate the presence of mast cells in PanNENs and evaluate their association with clinicopathological parameters and other common tumor-infiltrating immune cells.

METHODS

Tissue microarrays containing PanNEN samples from 187 patients were constructed and stained immunohistochemically for CD117, CD15, CD68, CD3, CD4, and CD8. Immune cells were counted from four high-power fields (HPFs; ×400) at maximal concentrations, and the mean counts were calculated per HPF. The cutoff values were set by X-tile.

RESULTS

The median (interquartile range) counts of CD117+ mast cells, CD15+ neutrophils, CD68+ macrophages, CD3+ T cells, and CD4+ T cells were 3.5 (2.0-6.0), 3.0 (1.3-6), 3.8 (2.5-5.8), 13 (8.0-24.0), and 2.0 (1.0-4.0)/HPF, respectively. CD8+ T cells were not detected. The cutoff values for these immune cells were 1.5/HPF, 6/HPF, 4.8/HPF, 32.5/HPF, and 2/HPF, respectively. Low mast cell density was correlated with higher grades, noninsulinoma, and advanced stages. Moreover, high mast cell infiltration was associated with elevated CD4+ T cell and CD15+ neutrophil counts. Multivariate analysis revealed that high mast cell density was an independent predictor of prolonged progression-free survival in the entire cohort; in pancreatic neuroendocrine tumors; and in intermediate-grade, noninsulinoma, and advanced stage subgroups.

CONCLUSIONS

These findings suggest a protective role of mast cells in PanNENs.

Association of Differential Mast Cell Activation to Granulocytic Inflammation in Severe Asthma

BACKGROUND

Mast cells (MC) play a role in inflammation and both innate and adaptive immunity but their involvement in severe asthma (SA) remains undefined.

OBJECTIVE

We investigated the phenotypic characteristics of the U-BIOPRED asthma cohort by applying published MC activation signatures to the sputum cell transcriptome.

METHODS

84 SA, 20 mild/moderate (MMA) asthma, and 16 non-asthmatic healthy participants were studied. We calculated enrichment scores (ES) for nine MC activation signatures by asthma severity, sputum granulocyte status and three previously-defined sputum molecular phenotypes or transcriptome-associated clusters (TAC1, 2, 3) using gene-set variation analysis.

RESULTS

MC signatures except unstimulated, repeated FcεR1-stimulated and IFNγ-stimulated were enriched in SA. A FcεR1-IgE-stimulated and a single cell signature from asthmatic bronchial biopsies were highly enriched in eosinophilic asthma and in the TAC1 molecular phenotype. Subjects with a high ES for these signatures had elevated sputum levels of similar genes and pathways. IL-33- and LPS-stimulated MC signatures had greater ES in neutrophilic and mixed granulocytic asthma and in the TAC2 molecular phenotype. These subjects exhibited neutrophil, NF-κB, and IL-1β/TNFα pathway activation. The IFNγ-stimulated signature had the greatest ES in TAC2 and TAC3 that was associated with responses to viral infection. Similar results were obtained in an independent ADEPT asthma cohort.

CONCLUSIONS

Gene signatures of MC activation allow the detection of SA phenotypes and indicate that MC can be induced to take on distinct transcriptional phenotypes associated with specific clinical phenotypes. IL-33-stimulated MCs signature was associated with severe neutrophilic asthma while IgE-activated MC with an eosinophilic phenotype.

Do Mast Cells Contribute to the Antifungal Host Defense?

The fungal kingdom includes a group of microorganisms that are widely distributed in the environment, and therefore the exposure to them is almost constant. Furthermore, fungal components of the microbiome, i.e., mycobiome, could serve as a reservoir of potentially opportunistic pathogens. Despite close encounters with fungi, defense mechanisms that develop during fungal infections remain unexplored. The strategic location of mast cells (MCs) close to the external environment places them among the first cells to encounter pathogens along with the other innate immune cells. MCs are directly involved in the host defense through the ability to destroy pathogens or indirectly by activating other immune cells. Most available data present MCs’ involvement in antibacterial, antiviral, or antiparasitic defense mechanisms. However, less is known about their contribution in defense mechanisms against fungi. MCs may support immune responses to fungi or their specific molecules through initiated degranulation, synthesis and release of cytokines, chemokines, mediators, and generation of reactive oxygen species (ROS), as well as immune cells’ recruitment, phagocytosis, or provision of extracellular DNA traps. This review summarizes current knowledge on host defense mechanisms against fungi and MCs’ involvement in those processes. It also describes the effects of fungi or fungus-derived constituents on MCs’ activity.

Cytokine Stimulation via MRGPRX2 Occurs with Lower Potency than by FcεRI-aggregation but with Similar Dependence on the ERK1/2 Module in Human Skin Mast Cells

Skin mast cells (MCs) contribute to chronic dermatoses that partially rely on MC-derived cytokines. The discovery of MRGPRX2 explains MC-dependent symptoms independently of FcεRI-activation. Here, we investigated whether MRGPRX2 can elicit cytokines, determined its relative potency versus FcεRI and addressed the underlying mechanisms. MRGPRX2-activation by compound 48/80 or Substance P on skin MCs induced TNF-α, IL-8, IL-13, CCL1, CCL2 mRNA and protein, yet induction was typically reduced compared with FcεRI-crosslinking. Generally, cytokine secretion required de-novo-synthesis with maximum accumulation at ≈8 h. Addressing key kinases revealed robust, rapid (1 min), and lasting (30 min) phosphorylation of ERK1/2 following MRGPRX2-ligation, while pp38, and pAKT signals were weaker, and pJNK hardly detectable. The kinase spectrum following FcεRI-aggregation was comparable, but responses considerably delayed. The MEK/ERK pathway was essential for all cytokines examined and four inhibitors of this module gave complete suppression. Variable and weaker contribution was found for PI3K>JNK>p38. Strikingly, cytokine profiles and signaling prerequisites were similar for MRGPRX2 and FcεRI and likely mainly dictated by the MC subset. Collectively, in skin MCs, the physiological producers of MRGPRX2, agonist binding elicits cytokines, yet less efficiently than FcεRI-aggregation. MRGPRX2-associated inflammation may thus be less tissue-destructive than responses to allergic challenge.

Mast Cells Modulate Antigen-Specific CD8+ T Cell Activation During LCMV Infection

Mast cells (MCs), strategically localized at mucosal surfaces, provide first-line defense against pathogens and shape innate and adaptive immune responses. Recent studies have shown that MCs are involved in pathogenic responses to several viruses including herpes simplex viruses, dengue virus, vaccinia virus and influenza virus. However, the underlying mechanisms of MCs in the activation of CD8+ T cells during viral infections are not fully understood. Therefore, we investigate the role of MCs in the development of virus-specific CD8+ T cell responses using the well-characterized murine lymphocytic choriomeningitis virus (LCMV) model and the transgenic MasTRECK mice that contain the human diphtheria toxin receptor as an inducible MC-deficient model. Here, we report that MCs are essential for the activation and expansion of virus-specific CD8+ T cells. After MC depletion and subsequent intradermal LCMV infection, the CD8 + T cell effector phenotype and antiviral cytokine production were impaired at the peak of infection (day 8 p.i.). Importantly, MC-deficient mice were unable to control the infection and exhibited significantly higher viral loads in the spleen and in the ear draining lymph nodes compared to that of wild type control mice. In the absence of MCs, dendritic cell (DC) activation was impaired upon LCMV infection. In addition, type-I interferon (IFN) levels in the serum and in the spleen of MC-deficient mice were reduced during the first days of infection. Interestingly, depletion of MCs after intradermal LCMV infection did not impair virus-specific CD8+ T cell expansion, activation or antiviral cytokine production. In summary, our results indicate that MCs play a pivotal role in the activation and antiviral functions of CD8+ T cells through proper DC activation. A better understanding of the impact of MCs on CD8+ T cell responses is mandatory to improve antiviral immune responses.

Differential mast cell numbers and characteristics in human tuberculosis pulmonary lesions

Tuberculosis (TB) is still a major worldwide health threat and primarily a lung disease. The innate immune response against Mycobacterium tuberculosis (Mtb) is orchestrated by dendritic cells, macrophages, neutrophils, natural killer cells and apparently mast cells (MCs). MCs are located at mucosal sites including the lungs and contribute in host-defence against pathogens, but little is known about their role during Mtb infection. This study investigates the location and characteristics of MCs in TB lesions to assess their contribution to TB pathology. To this purpose, number, location and phenotype of MCs was studied in 11 necropsies of pulmonary TB and 3 necropsies of non-TB infected lungs that were used as controls. MCs were localised at pneumonic areas, in the granuloma periphery and particularly abundant in fibrotic tissue. Furthermore, MCs displayed intracellular Mtb and IL-17A and TGF-β immunostaining. These findings were validated by analysing, post-mortem lung tissue microarrays from 44 individuals with pulmonary TB and 25 control subjects. In affected lungs, increased numbers of MCs expressing intracellularly both tryptase and chymase were found at fibrotic sites. Altogether, our data suggest that MCs are recruited at the inflammatory site and that actively produce immune mediators such as proteases and TGF-β that may be contributing to late fibrosis in TB lesions.

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.

Bidirectional communication between mast cells and the gut-brain axis in neurodegenerative diseases: Avenues for therapeutic intervention

Although the global incidence of neurodegenerative diseases has been steadily increasing, especially in adults, there are no effective therapeutic interventions. Neurodegeneration is a heterogeneous group of disorders that is characterized by the activation of immune cells in the central nervous system (CNS) (e.g., mast cells and microglia) and subsequent neuroinflammation. Mast cells are found in the brain and the gastrointestinal tract and play a role in "tuning" neuroimmune responses. The complex bidirectional communication between mast cells and gut microbiota coordinates various dynamic neuro-cellular responses, which propagates neuronal impulses from the gastrointestinal tract into the CNS. Numerous inflammatory mediators from degranulated mast cells alter intestinal gut permeability and disrupt blood-brain barrier, which results in the promotion of neuroinflammatory processes leading to neurological disorders, thereby offsetting the balance in immune-surveillance. Emerging evidence supports the hypothesis that gut-microbiota exert a pivotal role in inflammatory signaling through the activation of immune and inflammatory cells. Communication between inflammatory cytokines and neurocircuits via the gut-brain axis (GBA) affects behavioral responses, activates mast cells and microglia that causes neuroinflammation, which is associated with neurological diseases. In this comprehensive review, we focus on what is currently known about mast cells and the gut-brain axis relationship, and how this relationship is connected to neurodegenerative diseases. We hope that further elucidating the bidirectional communication between mast cells and the GBA will not only stimulate future research on neurodegenerative diseases but will also identify new opportunities for therapeutic interventions.

Thapsigargin-Stimulated LAD2 Human Mast Cell Line Is a Potent Cellular Adjuvant for the Maturation of Monocyte-Derived Dendritic Cells for Adoptive Cellular Immunotherapy

The preparation of dendritic cells (DCs) for adoptive cellular immunotherapy (ACI) requires the maturation of ex vivo-produced immature(i) DCs. This maturation ensures that the antigen presentation triggers an immune response towards the antigen-expressing cells. Although there is a large number of maturation agents capable of inducing strong DC maturation, there is still only a very limited number of these agents approved for use in the production of DCs for ACI. In seeking novel DC maturation agents, we used differentially activated human mast cell (MC) line LAD2 as a cellular adjuvant to elicit or modulate the maturation of ex vivo-produced monocyte-derived iDCs. We found that co-culture of iDCs with differentially activated LAD2 MCs in serum-containing media significantly modulated polyinosinic:polycytidylic acid (poly I:C)-elicited DC maturation as determined through the surface expression of the maturation markers CD80, CD83, CD86, and human leukocyte antigen(HLA)-DR. Once iDCs were generated in serum-free conditions, they became refractory to the maturation with poly I:C, and the LAD2 MC modulatory potential was minimized. However, the maturation-refractory phenotype of the serum-free generated iDCs was largely overcome by co-culture with thapsigargin-stimulated LAD2 MCs. Our data suggest that differentially stimulated mast cells could be novel and highly potent cellular adjuvants for the maturation of DCs for ACI.

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.

Directional mast cell degranulation of tumor necrosis factor into blood vessels primes neutrophil extravasation

Tissue resident mast cells (MCs) rapidly initiate neutrophil infiltration upon inflammatory insult, yet the molecular mechanism is still unknown. Here, we demonstrated that MC-derived tumor necrosis factor (TNF) was crucial for neutrophil extravasation to sites of contact hypersensitivity-induced skin inflammation by promoting intraluminal crawling. MC-derived TNF directly primed circulating neutrophils via TNF receptor-1 (TNFR1) while being dispensable for endothelial cell activation. The MC-derived TNF was infused into the bloodstream by directional degranulation of perivascular MCs that were part of the vascular unit with access to the vessel lumen. Consistently, intravenous administration of MC granules boosted neutrophil extravasation. Pronounced and rapid intravascular MC degranulation was also observed upon IgE crosslinking or LPs challenge indicating a universal MC potential. Consequently, the directional MC degranulation of pro-inflammatory mediators into the bloodstream may represent an important target for therapeutic approaches aimed at dampening cytokine storm syndromes or shock symptoms, or intentionally pushing immune defense.