Roles of a Mast Cell-Specific Receptor MRGPRX2 in Host Defense and Inflammation

MASTer cell: chief immune modulator and inductor of antimicrobial immune response

Mast cells have long been recognized for their involvement in allergic pathology through the immunoglobulin E (IgE)-mediated degranulation mechanism. However, there is growing evidence of other "non-canonical" degranulation mechanisms activated by certain pathogen recognition receptors. Mast cells release several mediators, including histamine, cytokines, chemokines, prostaglandins, and leukotrienes, to initiate and enhance inflammation. The chemical nature of activating stimuli influences receptors, triggering mechanisms for the secretion of formed and new synthesized mediators. Mast cells have more than 30 known surface receptors that activate different pathways for direct and indirect activation by microbes. Different bacterial strains stimulate mast cells through various ligands, initiating the innate immune response, which aids in clearing the bacterial burden. Mast cell interactions with adaptative immune cells also play a crucial role in infections. Recent publications revealed another "non-canonical" degranulation mechanism present in tryptase and chymase mast cells in humans and connective tissue mast cells in mice, occurring through the activation of the Mas-related G protein-coupled receptor (MRGPRX2/b2). This receptor represents a new therapeutic target alongside antibiotic therapy. There is an urgent need to reconsider and redefine the biological role of these MASTer cells of innate immunity, extending beyond their involvement in allergic pathology.

Bacterial distribution and inflammatory cytokines associated with oral cancer with and without jawbone invasion-a pilot study

OBJECTIVE

To explore the bacterial and inflammatory variations in oral cancer patients with and without jawbone invasion.

MATERIALS AND METHODS

A total of 20 specimens of fresh tumor tissue, including 10 from the tumor-invaded jawbone (JIOC group) and 10 without jawbone invasion (NJIOC group), were collected from oral cancer patients. Meanwhile, 10 specimens from normal oral mucosa were collected from healthy patients (control group). The microbiomic content of each sample was analyzed by 16S rRNA gene sequencing, while the expression of inflammatory cytokines was assessed using protein microarray analysis.

RESULTS

There was a significant difference in β diversity between JIOC and NJIOC groups (P < 0.05), but no difference between NJIOC and control groups. The average relative abundance of Fusobacteria and Spirochaetes was higher, while Firmicutes was lower in the JIOC group than in the NJIOC group (all P < 0.05). The expression of pro-inflammatory cytokines like interleukin (IL)-1α, IL-1β, IL-4, and IL-8 was upregulated in the JIOC group compared with the NJIOC group, while MCP-1 was decreased (all P < 0.05). Slackia spp. and Howardella spp. were positively correlated with IL-4; Odoribacter spp. and Acidaminococcaceae spp. were negatively correlated with IL-4, and Clostridium XIVa spp. was negatively correlated with IL-1α and IL-1β.

CONCLUSION

Bacterial and inflammatory differences were observed in oral cancer patients with and without jawbone invasion, where the relative abundance of the differential bacteria was associated with the expression of the inflammatory cytokines.

CLINICAL RELEVANCE

This study investigated the changes in the flora during jawbone invasion in oral cancer and its effect on inflammatory factors, elucidating the possible mechanisms of jawbone invasion caused by oral cancer, which may lead to new ideas for the clinical prevention and treatment of jawbone invasion in oral cancer.

Development of Ferroptosis-Associated ceRNA Network in Periodontitis

OBJECTIVES

Periodontitis is a chronic inflammatory illness that may lead to tooth loosening and even loss, and its pathogenesis is not fully understood. Ferroptosis is an iron-dependent, regulated cell death. The present study aims to find the key ferroptosis-related genes (FRGs) in periodontitis and develop an mRNA-miRNA-lncRNA network to deeply explore the pathogenesis of periodontitis.

METHODS

Data from the Gene Expression Omnibus (GEO) database and FerrDb database were downloaded to discover the differentially expressed mRNA, miRNA, and FRGs. Functional enrichment analysis was conducted for the differentially expressed FRGs (DE-FRGs), including gene ontology, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, and protein-protein interaction (PPI) network analysis. Targetscan and miRtarbase were used to estimate the miRNAs that DE-FRGs may interact with, whilst StarBase v3.0 was used for lncRNA-miRNA interaction.

RESULTS

Seven DE-FRGs were identified through differential expression analysis. Interleukin 1 beta (IL1B) interacted with XBP1 and MMP13 in the PPI network. After taking the intersection between DE-miRNAs and predicted miRNAs, a ceRNA network containing IL1B, has-miR-185, has-miR-204, has-miR-211, has-miR-4306, and 28 lncRNAs was established.

CONCLUSIONS

Seven FRGs in periodontitis were identified, which might promote deeper understanding of ferroptosis in periodontitis.

HIV-Associated Immune Dysregulation in the Skin: A Crucible for Exaggerated Inflammation and Hypersensitivity

Skin diseases are hallmarks of progressive HIV-related immunosuppression, with severe noninfectious inflammatory and hypersensitivity conditions as common as opportunistic infections. Conditions such as papular pruritic eruption are AIDS defining, whereas delayed immune-mediated adverse reactions, mostly cutaneous, occur up to 100-fold more during HIV infection. The skin, constantly in contact with the external environment, has a complex immunity. A dense, tightly junctioned barrier with basal keratinocytes and epidermal Langerhans cells with antimicrobial, innate-activating, and antigen-presenting functions form the frontline. Resident dermal dendritic, mast, macrophage, and innate lymphoid cells play pivotal roles in directing and polarizing appropriate adaptive immune responses and directing effector immune cell trafficking. Sustained viral replication leads to progressive declines in CD4 T cells, whereas Langerhans and dermal dendritic cells serve as viral reservoirs and points of first viral contact in the mucosa. Cutaneous cytokine responses and diminished lymphoid populations create a crucible for exaggerated inflammation and hypersensitivity. However, beyond histopathological description, these manifestations are poorly characterized. This review details normal skin immunology, changes associated with progressive HIV-related immunosuppression, and the characteristic conditions of immune dysregulation increased with HIV. We highlight the main research gaps and several novel tissue-directed strategies to define mechanisms that will provide targeted approaches to prevention or treatment.

Inhibitory effect of polysaccharides extracted from Changbai Mountain Ganoderma lucidum on periodontal inflammation

As the main bioactive substance of Ganoderma lucidum, Ganoderma lucidum polysaccharide (GLP) has anti-inflammatory, antibacterial, and other biological activities. Studies have shown that GLP can regulate the expression of multiple inflammatory cytokines in different inflammatory models and diseases as part of the anti-infection immune response. We extracted crude Changbai Mountain Ganoderma lucidum polysaccharides (CGLPs), analyzed their physical and chemical properties, and then applied them to the periodontitis model to verify whether they have an inhibitory effect on mouse periodontitis. CGLP was determined to be a heteropolysaccharide with dextran as the main component. Its molecular weight was 17.40 kDa. In vivo experiments in mice showed that CGLP can inhibit the alveolar bone loss and reduced inflammation caused of periodontitis by regulating the expression of the inflammatory factors IL-1β, TNF-α, and IL-10 in a concentration-dependent manner.

Therapeutic Potential of Polyphenol and Nanoparticles Mediated Delivery in Periodontal Inflammation: A Review of Current Trends and Future Perspectives

Periodontitis is an oral inflammatory process involving the periodontium, which is mainly caused by the invasion of periodontopathogenic microorganisms that results in gingival connective tissue and alveolar bone destruction. Metabolic products of the oral pathogens and the associated host immune and inflammatory responses triggered are responsible for the local tissue destruction. Numerous studies in the past decades have demonstrated that natural polyphenols are capable of modulating the host inflammatory responses by targeting multiple inflammatory components. The proposed mechanism by which polyphenolic compounds exert their great potential is by regulating the immune cell, proinflammatory cytokines synthesis and gene expression. However, due to its low absorption and bioavailability, the beneficial effects of these substances are very limited and it hampers their use as a therapeutic agent. To address these limitations, targeted delivery systems by nanoencapsulation techniques have been explored in recent years. Nanoencapsulation of polyphenolic compounds with different carriers is an efficient and promising approach to boost their bioavailability, increase the efficiency and reduce the degradability of natural polyphenols. In this review, we focus on the effects of different polyphenolic substances in periodontal inflammation and to explore the pharmaceutical significance of polyphenol-loaded nanoparticles in controlling periodontitis, which may be useful for further enhancement of their efficacy as therapeutic agents for periodontal disease.

Spray with Nitric Oxide Donor Accelerates Wound Healing: Potential Off-the-Shelf Solution for Therapy?

Background

Ditrosyl iron complexes (DNIC) are endogenous donors of nitric oxide. The possibility of their application to stimulate regeneration has been studied for more than 15 years. However, the most effective dose and form of delivery have not yet been determined.

Purpose

The aim of this research was to develop a spray form of DNIC that accelerates wound healing.

Methods

We prepared a series of DNIC sprays with spray dosages of 10, 50 and 100 μg. We modelled full-thickness skin wounds in 24 Wistar rats and treated them with distilled water (n = 6), 10 (n = 6), 50 (n = 6) and 100 μg (n = 6) for three post-operative days. On the fourth day, the excised wound tissues were studied by morphological, immunohistochemical and morphometric methods.

Results

We demonstrated that 50 μg of DNIC spray had the most beneficial effect on wound healing: the thickness of the granulation tissue layer was 140% higher, vimentin positive fibroblasts predominated and the intensity of inflammation was significantly lower than in the control. There was a dose-dependent decrease in the functional activity of mast cells in the experimental groups compared to the control.

Conclusion

DNIC spray is a potential effective dosage form for the treatment of large-area skin lesions.

P17 induces chemotaxis and differentiation of monocytes via MRGPRX2-mediated mast cell-line activation

BACKGROUND

P17, a peptide isolated from Tetramorium bicarinatum ant venom, is known to induce an alternative phenotype of human monocyte-derived macrophages via activation of an unknown G protein-coupled receptor (GPCR).

OBJECTIVE

We sought to investigate the mechanism of action and the immunomodulatory effects of P17 mediated through MRGPRX2 (Mas-related G protein-coupled receptor X2).

METHODS

To identify the GPCR for P17, we screened 314 GPCRs. Upon identification of MRGPRX2, a battery of in silico, in vitro, ex vivo, and in vivo assays along with the receptor mutation studies were performed. In particular, to investigate the immunomodulatory actions, we used β-hexosaminidase release assay, cytokine releases, quantification of mRNA expression, cell migration and differentiation assays, immunohistochemical labeling, hematoxylin and eosin, and immunofluorescence staining.

RESULTS

P17 activated MRGPRX2 in a dose-dependent manner in β-arrestin recruitment assay. In LAD2 cells, P17 induced calcium and β-hexosaminidase release. Quercetin- and short hairpin RNA-mediated knockdown of MRGPRX2 reduced P17-evoked β-hexosaminidase release. In silico and in vitro mutagenesis studies showed that residue Lys8 of P17 formed a cation-π interaction with the Phe172 of MRGPRX2 and [Ala8]P17 lost its activity partially. P17 activated LAD2 cells to recruit THP-1 and human monocytes in Transwell migration assay, whereas MRGPRX2-impaired LAD2 cells cannot. In addition, P17-treated LAD2 cells stimulated differentiation of THP-1 and human monocytes, as indicated by the enhanced expression of macrophage markers cluster of differentiation 11b and TNF-α by quantitative RT-PCR. Immunohistochemical and immunofluorescent staining suggested monocyte recruitment in mice ears injected with P17.

CONCLUSIONS

Our data provide novel structural information regarding the interaction of P17 with MRGPRX2 and intracellular pathways for its immunomodulatory action.

Therapeutic Potential of MRGPRX2 Inhibitors on Mast Cells

Mast cells (MCs) act as primary effectors in inflammatory and allergic reactions by releasing intracellularly-stored inflammatory mediators in diseases. The two major pathways for MC activation are known to be immunoglobulin E (IgE)-dependent and -independent. Although IgE-dependent signaling is the main pathway to MC activation, IgE-independent pathways have also been found to serve pivotal roles in the pathophysiology of various inflammatory conditions. Recent studies have shown that human and mouse MCs express several regulatory receptors such as toll-like receptors (TLRs), CD48, C300a, and GPCRs, including mas-related GPCR-X2 (MRGPRX2). MRGPRX2 has been reported as a novel GPCR that is expressed in MCs activated by basic secretagogues, neurokinin peptides, host defense antimicrobial peptides, and small molecule compounds (e.g., neuromuscular blocking agents) and leads to MC degranulation and eicosanoids release under in vitro experimental condition. Functional analyses of MRGPRX2 and Mrgprb2 (mouse ortholog) indicate that MRGPRX2 is involved in MC hypersensitivity reactions causing neuroinflammation such as postoperative pain, type 2 inflammation, non-histaminergic itch, and drug-induced anaphylactic-like reactions. In this review, we discuss the roles in innate immunity through functional studies on MRGPRX2-mediated IgE-independent MC activation and also the therapeutic potential of MRGPRX2 inhibitors on allergic and inflammatory diseases.

Multifaceted MRGPRX2: New insight into the role of mast cells in health and disease

Cutaneous mast cells (MCs) express Mas-related G protein-coupled receptor-X2 (MRGPRX2; mouse ortholog MrgprB2), which is activated by an ever-increasing number of cationic ligands. Antimicrobial host defense peptides (HDPs) generated by keratinocytes contribute to host defense likely by 2 mechanisms, one involving direct killing of microbes and the other via MC activation through MRGPRX2. However, its inappropriate activation may cause pseudoallergy and likely contribute to the pathogenesis of rosacea, atopic dermatitis, allergic contact dermatitis, urticaria, and mastocytosis. Gain- and loss-of-function missense single nucleotide polymorphisms in MRGPRX2 have been identified. The ability of certain ligands to serve as balanced or G protein-biased agonists has been defined. Small-molecule HDP mimetics that display both direct antimicrobial activity and activate MCs via MRGPRX2 have been developed. In addition, antibodies and reagents that modulate MRGPRX2 expression and signaling have been generated. In this article, we provide a comprehensive update on MrgprB2 and MRGPRX2 biology. We propose that harnessing MRGPRX2's host defense function by small-molecule HDP mimetics may provide a novel approach for the treatment of antibiotic-resistant cutaneous infections. In contrast, MRGPRX2-specific antibodies and inhibitors could be used for the modulation of allergic and inflammatory diseases that are mediated via this receptor.

Molecular Mechanisms of Mast Cell Activation by Cholesterol-Dependent Cytolysins

Mast cells are potent immune sensors of the tissue microenvironment. Within seconds of activation, they release various preformed biologically active products and initiate the process of de novo synthesis of cytokines, chemokines, and other inflammatory mediators. This process is regulated at multiple levels. Besides the extensively studied IgE and IgG receptors, toll-like receptors, MRGPR, and other protein receptor signaling pathways, there is a critical activation pathway based on cholesterol-dependent, pore-forming cytolytic exotoxins produced by Gram-positive bacterial pathogens. This pathway is initiated by binding the exotoxins to the cholesterol-rich membrane, followed by their dimerization, multimerization, pre-pore formation, and pore formation. At low sublytic concentrations, the exotoxins induce mast cell activation, including degranulation, intracellular calcium concentration changes, and transcriptional activation, resulting in production of cytokines and other inflammatory mediators. Higher toxin concentrations lead to cell death. Similar activation events are observed when mast cells are exposed to sublytic concentrations of saponins or some other compounds interfering with the membrane integrity. We review the molecular mechanisms of mast cell activation by pore-forming bacterial exotoxins, and other compounds inducing cholesterol-dependent plasma membrane perturbations. We discuss the importance of these signaling pathways in innate and acquired immunity.

Involvement of Mast Cells in the Pathophysiology of Pain

Mast cells (MCs) are immune cells and are widely distributed throughout the body. MCs are not only classically viewed as effector cells of some allergic diseases but also participate in host defense, innate and acquired immunity, homeostatic responses, and immunoregulation. Mounting evidence indicates that activation of MCs releasing numerous vasoactive and inflammatory mediators has effects on the nervous system and has been involved in different pain conditions. Here, we review the latest advances made about the implication of MCs in pain. Possible cellular and molecular mechanisms regarding the crosstalk between MC and the nervous system in the initiation and maintenance of pain are also discussed.

Unlocking the Non-IgE-Mediated Pseudo-Allergic Reaction Puzzle with Mas-Related G-Protein Coupled Receptor Member X2 (MRGPRX2)

Mas-related G-protein coupled receptor member X2 (MRGPRX2) is a class A GPCR expressed on mast cells. Mast cells are granulated tissue-resident cells known for host cell response, allergic response, and vascular homeostasis. Immunoglobulin E receptor (FcεRI)-mediated mast cell activation is a well-studied and recognized mechanism of allergy and hypersensitivity reactions. However, non-IgE-mediated mast cell activation is less explored and is not well recognized. After decades of uncertainty, MRGPRX2 was discovered as the receptor responsible for non-IgE-mediated mast cells activation. The puzzle of non-IgE-mediated pseudo-allergic reaction is unlocked by MRGPRX2, evidenced by a plethora of reported endogenous and exogenous MRGPRX2 agonists. MRGPRX2 is exclusively expressed on mast cells and exhibits varying affinity for many molecules such as antimicrobial host defense peptides, neuropeptides, and even US Food and Drug Administration-approved drugs. The discovery of MRGPRX2 has changed our understanding of mast cell biology and filled the missing link of the underlying mechanism of drug-induced MC degranulation and pseudo-allergic reactions. These non-canonical characteristics render MRGPRX2 an intriguing player in allergic diseases. In the present article, we reviewed the emerging role of MRGPRX2 as a non-IgE-mediated mechanism of mast cell activation in pseudo-allergic reactions. We have presented an overview of mast cells, their receptors, structural insight into MRGPRX2, MRGPRX2 agonists and antagonists, the crucial role of MRGPRX2 in pseudo-allergic reactions, current challenges, and the future research direction.

The Multifaceted Mas-Related G Protein-Coupled Receptor Member X2 in Allergic Diseases and Beyond

Recent research on mast cell biology has turned its focus on MRGPRX2, a new member of the Mas-related G protein-coupled subfamily of receptors (Mrgprs), originally described in nociceptive neurons of the dorsal root ganglia. MRGPRX2, a member of this group, is present not only in neurons but also in mast cells (MCs), specifically, and potentially in other cells of the immune system, such as basophils and eosinophils. As emerging new functions for this receptor are studied, a variety of both natural and pharmacologic ligands are being uncovered, linked to the ability to induce receptor-mediated MC activation and degranulation. The diversity of these ligands, characterized in their human, mice, or rat homologues, seems to match that of the receptor's interactions. Natural ligands include host defense peptides, basic molecules, and key neuropeptides such as substance P and vasointestinal peptide (known for their role in the transmission of pain and itch) as well as eosinophil granule-derived proteins. Exogenous ligands include MC secretagogues such as compound 48/80 and mastoparan, a component of bee wasp venom, and several peptidergic drugs, among which are members of the quinolone family, neuromuscular blocking agents, morphine, and vancomycin. These discoveries shed light on its capacity as a multifaceted participant in naturally occurring responses within immunity and neural stimulus perception, as in responses at the center of immune pathology. In host defense, the mice Mrgprb2 has been proven to aid mast cells in the detection of peptidic molecules from bacteria and in the release of peptides with antimicrobial activities and other immune mediators. There are several potential actions described for it in tissue homeostasis and repair. In the realm of pathologic response, there is evidence to suggest that this receptor is also involved in chronic inflammation. Furthermore, MRGPRX2 has been linked to the pathophysiology of non-IgE-mediated immediate hypersensitivity drug reactions. Different studies have shown its possible role in other allergic diseases as well, such as asthma, atopic dermatitis, contact dermatitis, and chronic spontaneous urticaria. In this review, we sought to cover its function in physiologic processes and responses, as well as in allergic and nonallergic immune disease.

IL-1 Superfamily Members and Periodontal Diseases

Periodontitis is a complex, multifactorial chronic disease involving continuous interactions among bacteria, host immune/inflammatory responses, and modifying genetic and environmental factors. More than any other cytokine family, the interleukin (IL)-1 family includes key signaling molecules that trigger and perpetuate periodontal inflammation. Over the years, the IL-1 family expanded to include 11 members of cytokines, some with agonist activity (IL-1α, IL-1β, IL-18, IL-33, IL-36α, IL-36β, and IL-36γ), receptor antagonists (IL-1Ra, IL-36Ra), and 2 anti-inflammatory cytokines (IL-37, IL-38). The IL-1 receptor antagonist (IL-1Ra) has emerged as a pivotal player in the defense against periodontitis. IL-33 primarily induces the production of Th2-associated cytokines but acts as an "alarmin" via stimulation of mast cells. The IL-36 subclass of cytokines may be important in regulating mucosal inflammation and homeostasis. IL-37 suppresses innate and acquired immune responses. IL-38 is the most recent member of the IL-1 superfamily and has anti-inflammatory properties similar to those of IL-37 but through different receptors. However, limited evidence exists regarding the role of IL-37 and IL-38 in periodontitis. Despite the development of IL-1 blocking agents, therapeutic blockade of select IL-1 family members for periodontitis has only been partially investigated in preclinical and clinical research, while the development of IL-37 and IL-38 as novel anti-inflammatory drugs has not been considered adequately. Here, we review the key properties of the IL-1 family members and provide insights into targeting or promoting select cytokines as new therapeutic agents.