Emerging Roles for MAS-Related G Protein-Coupled Receptor-X2 in Host Defense Peptide, Opioid, and Neuropeptide-Mediated Inflammatory Reactions

Silibinin attenuated pseudo-allergic reactions and mast cell degranulation via PLCγ and PI3K/Akt signaling pathway

Anaphylaxis is a type of potentially fatal hypersensitivity reaction resulting from the activation of mast cells. Many endogenous or exogenous factors could cause this reaction. Silibinin is the main chemical component of silymarin and has been reported to have pharmacological activities. However, the anti-allergic reaction effect of silibinin has not yet been investigated. This study aimed to evaluate the effect of silibinin to attenuate pseudo-allergic reactions in vivo and to investigate the underlying mechanism in vitro. In this study, calcium imaging was used to assess Ca2+ mobilization. The levels of cytokines and chemokines, released by stimulated mast cells, were measured using enzyme immunoassay kits. The activity of silibinin was evaluated in a mouse model of passive cutaneous anaphylaxis (PCA). Western blotting was used to explore the related molecular signaling pathways. In results, silibinin markedly inhibited mast cell degranulation, calcium mobilization, and preventing the release of cytokines and chemokines in a dose-dependent manner via the PLCγ and PI3K/Akt signaling pathway. Silibinin also attenuated PCA in a dose-dependent manner. In summary, silibinin has an anti-pseudo-allergic pharmacological activity, which makes it a potential candidate for the development of a novel agent to arrest pseudo-allergic reactions.

Paeonol attenuates Substance P-induced urticaria by inhibiting Src kinase phosphorylation in mast cells

BACKGROUND

Treatment of chronic urticaria is challenging, the discovery of effective therapeutic drugs is urgently in demand.

PURPOSE

To study the effect and mechanism of Paeonol targeting mast cells and its therapeutic effect on chronic urticaria.

STUDY DESIGN

We developed a chronic urticaria model in vivo and mast cell model in vitro examined the effect of Paeonol in the treatment of chronic urticaria and its mechanism of action in mast cells.

METHOD

The anti-anaphylactoid effect of Paeonol was evaluated in PCA and systemic anaphylaxis models. The treatment role of Paeonol was studied in urticaria model. The release of cytokines and chemokines was measured using enzyme immunoassay kits. Western blot analysis was conducted to investigate phosphorylation of Src, PI3K, and PLC. In vitro kinase assays were conducted to investigate the kinase activity of Lyn, PLC, PI3K and Src.

RESULTS

In our study, Paeonol was able to attenuate evans blue leakage, serum histamine and chemokine release in a passive skin allergic reaction model. Simultaneously, Paeonol inhibited vasodilation and mast cell degranulation in C57BL/6 mice. Further research found that Paeonol alleviated symptoms such as erythema and rash in the Substance P-induced urticaria model, this is accompanied by inhibiting the release of related inflammatory factors. Validation experiments on mast cells in vitro found that Paeonol inhibited the activation of Src-PI3K/Lyn-PLC-NF-κB signaling pathway by crosslinking with Src kinase. Moreover, calcium influx, mast cell degranulation, cytokines generation and chemotaxis were reduced in LAD2 cells. Molecular docking experiments revealed that Paeonol is a specific antagonist targeting Src kinase in the treatment of skin diseases such as urticaria.

CONCLUSION

Paeonol, a herb-derived phenolic compound, can provide drug candidate for developing new drug in treatment of skin disease such as urticaria.

SIGNIFICANCE STATEMENT

In this study, we primarily examined the effect of Paeonol in the treatment of chronic urticaria and its mechanism of action in mast cells. Interestingly, Paeonol was found to regulate Src kinase activity downstream of MRGPRX2 triggered signaling cascade in mast cells. Therefore, this plant-derived phenolic compound may provide a therapeutic option for the treatment of chronic urticaria.

Opioid toxicity: histamine, hypersensitivity, and MRGPRX2

Insights into the pathophysiology of many non-immune-mediated drug reactions referred to as toxicities, sensitivities, intolerances, or pseudoallergies have resulted from research identifying the mastocyte-related G-protein-coupled receptor (GPCR) member X2 (MRGPRX2), a human mast cell receptor mediating adverse reactions without the involvement of antibody priming. Opioid-induced degranulation of mast cells, particularly morphine, provoking release of histamine and other preformed mediators and causing hemodynamic and cutaneous changes seen as flushing, headache and wheal and flare reactions in the skin, is an example of results of MRGPRX2 activation. Opioids including morphine, codeine, dextromethorphan and metazocine as well as endogenous prodynorphin opioid peptides activate MRGPRX2 at concentrations causing mast cell degranulation. Unlike the canonical opioid receptors, MRGPRX2 shows stereochemical recognition preference for dextro rather than levo opioid enantiomers. Opioid analgesic drugs (OADs) display a range of histamine-releasing potencies from the strong releaser morphine to doubtful releasers like hydromorphone and the non-releaser fentanyl. Whether there is a correlation between histamine release by individual OADs, MRGPRX2 activation, and presence or absence of adverse cutaneous effects is not known. To investigate the question, ongoing research with recently pursued methodologies and strategies employing basophil and mast cell tests resulting from MRGPRX2 insights should help to elucidate whether or not an opioid's histamine-releasing potency, and its property of provoking an adverse reaction, are each a reflection of its activation of MRGPRX2.

Integrating pharmacokinetics and network pharmacology to identify and validate targets of Guben Xiaozhen prescription for the treatment of chronic urticaria

ETHNOPHARMACOLOGICAL RELEVANCE

Guben Xiaozhen prescription (GXP), a prescription of traditional Chinese medicine, has been used to treat skin diseases for a long history and achieved satisfactory therapeutic effects. However, its active ingredients and targets remain to be further elucidated.

AIM OF THIS STUDY

Identify activity ingredients of GXP for the treatment of chronic urticaria (CU) and further validate the efficacy and targets of the selected component.

MATERIALS AND METHODS

Firstly, the pharmacokinetics of different disassemble groups of GXP was investigated to screen for active ingredients with improved bioavailability. Then, shared targets between active ingredients and CU were performed by network pharmacology. Finally, the ovalbumin (OVA) induced CU model was used to verify the efficacy and targets of the screened active ingredient.

RESULTS

Pharmacokinetic results showed that, compared with sub-division groups, the maximum concentration (C_max) and blood concentration-time curve (AUC_0-t) of eight ingredients, including 6-shogaol, 6-gingerol, calycosin, dictamnine, fraxinellone, schizandrin, cimifugin, and sec-o-glucosylhamaudol were increased in the GXP group. Then, 218 CU-related targets and 20 shared targets with six potential active compounds were screened by network pharmacology. Further analysis found that fraxinellone was not reported to be associated with CU in the literature. Therefore, the present study employed an OVA-induced CU model and found that fraxinellone could dose-dependently inhibit the locus coeruleus reaction, mast cell degranulation, and pathological skin damage. Moreover, we further verified the ADRB2 and its downstream target caspase3 predicted by network pharmacology, and fraxinellone inhibited the expression of ADRB2 and caspase3 in high dose group, suggesting that fraxinellone may play an anti-CU role by inhibiting inflammation and cell apoptosis.

CONCLUSION

In this study, integrated pharmacokinetics and network pharmacology methods were established to screen out six effective active ingredients in GXP for the treatment of CU. This study provides a new idea for screening active substances in traditional Chinese medicine.

Differential effects of PEGylated Cd-free CuInS2/ZnS quantum dot (QDs) on substance P and LL-37 induced human mast cell activation

CuInS₂/ZnS-PEG quantum dots (QDs) are among the most widely used near infrared non-cadmium QDs and are favored because of their non-cadmium content and strong tissue penetration. However, with their increasing use, there is great concern about whether exposure to QDs is potentially risky to the environment and humans. Furthermore, toxicological data related to CuInS₂/ZnS-PEG QDs are scarce. In the study, we found that CuInS₂/ZnS-PEG QDs (0-100 μg/mL) could internalize into human LAD2 mast cells without affecting their survival rate, nor did it cause degranulation or release of IL-8 and TNF-α. However, CuInS₂/ZnS-PEG QDs significantly inhibited Substance P (SP) and LL-37-induced degranulation and chemotaxis of LAD2 cells by inhibiting calcium mobilization. Lower concentrations of CuInS₂/ZnS-PEG QDs promoted the release of TNF-α and IL-8 stimulated by SP, but higher concentrations of CuInS₂/ZnS-PEG QDs significantly inhibited the release of TNF-α and IL-8. On the other hand, CuInS₂/ZnS-PEG QDs promoted LL-37-mediated TNF-α release from LAD2 cells in a dose-dependent manner from 6.25 to 100 μg/mL, while release of IL-8 triggered by LL-37 was dose-dependently inhibited within a dose concentration of 12.5-100 μg/mL. Collectively, our data demonstrated that CuInS₂/ZnS-PEG QDs differentially mediated human mast cell activation induced by SP and LL-37.

Caffeic acid phenethyl ester inhibits pseudo-allergic reactions via inhibition of MRGPRX2/MrgprB2-dependent mast cell degranulation

Mast cells play essential role in allergic reactions through the process called mast cell degranulation. Recent studies have found that a basic secretagogue compound 48/80 (C48/80) induces non-IgE-mediated mast cell degranulation via activation of human Mas-related G protein-coupled receptor X2 (MRGPRX2) and mouse MrgprB2. Although previous studies have revealed that caffeic acid (CA) and its derivatives possess anti-allergic effects via IgE-dependent manner, it is largely elusive whether these compounds have impact on MRGPRX2/MrgprB2 to exert inhibitory effects. Therefore, the present study investigated whether CA as well as its derivatives - rosmarinic acid (RA) and caffeic acid phenethyl ester (CAPE) - has the ability to inhibit the activity of MRGPRX2/MrgprB2 to evoke pseudo-allergic effects. As a result, it was found that CAPE inhibits C48/80-induced activation of MRGPRX2/MrgprB2, but neither CA nor RA showed discernible inhibition. Furthermore, the β-hexosaminidase release assay showed that CAPE inhibits mouse peritoneal mast cell degranulation in both IgE-dependent and MrgprB2-dependent manners. Additionally, mouse paw edema induced by C48/80 was dramatically suppressed by co-treatment of CAPE, suggesting that CAPE possesses a protective effect on C48/80-evoked pseudo-allergic reactions. The pretreatment of CAPE also significantly decreased scratching bouts of mice evoked by C48/80, demonstrating that CAPE also has an anti-pruritic effect. Therefore, these data implicate that CAPE can suppress pseudo-allergic reactions evoked by C48/80 via MrgprB2-dependent manner. Finally, molecular docking analysis showed that CAPE is predicted to bind to human MRGPRX2 in the region where C48/80 also binds, implying that CAPE can be a competitive inhibitor of MRGPRX2. In conclusion, it is found that CAPE has the ability to inhibit MRGPRX2/MrgprB2, leading to the prevention of mast cell degranulation and further to the alleviation of mast cell reactions. These results indicate that CAPE as a CA derivative could be developed as a new protective agent that exerts dual inhibition of mast cell degranulation mediated by IgE and MRGPRX2/MrgprB2.

Spatiotemporal Patterns of Substance P-Bound MRGPRX2 Reveal a Novel Connection Between Macropinosome Resolution and Secretory Granule Regeneration in Mast Cells

MRGPRX2, the human member of the MAS-related G protein coupled receptors (Mrgprs), serves as the cellular target of human mast cells (MCs) for innate ligands, including neuropeptides and antimicrobial peptides. In addition, MRGPRX2 also functions as the receptor for multiple FDA-approved drugs. As such, MRGPRX2 is a mediator of MC responses in neurogenic inflammation, host defense and pseudoallergy. We analyzed the spatiotemporal patterns of MRGPRX2 following its binding of the neuropeptide substance P (SP). Herein, we show that MRGPRX2 internalizes via both endocytosis and macropinocytosis, followed by its distribution between a perinuclear region and the secretory granules (SGs). Further, we show that MRGPRX2-containing macropinosomes undergo resolution by a mechanism that involves dynamin and LC3, giving rise to the incorporation of both LC3 and MRGPRX2 into the SGs. SP then promotes the acidification of the LC3-associated SGs, presumably by stimulating their fusion with lysosomes. Taken together, our results reveal a unique mode of MRGPRX2 trafficking that complements endocytosis and involves macropinocytosis, autophagic machinery-assisted macropinosome resolution and receptor delivery to the SGs.

Mas-Related G Protein–Coupled Receptor-X2 and Its Role in Non-immunoglobulin E–Mediated Drug Hypersensitivity

A diverse group of Food and Drug Administration-approved cationic drugs including antibiotics, neuromuscular blocking drugs, opioids, antidepressants, and radiocontrast media activate mast cells and cause hypersensitivity reactions by both an immunoglobulin E IgE-dependent and independent manner. The recent discovery that these drugs activate mast cells via the G protein-coupled receptor known as Mas-related GPCR-X2 (MRGPRX2) has represented a paradigm shift of how drug hypersensitivity reactions are viewed. This article provides an overview of the current status of the role of MRGPRX2 on non-IgE-mediated drug hypersensitivity. Potential risk factors and evaluation for suspected MRGPRX2-mediated drug reactions are also discussed.

Roles of IgE and Histamine in Mast Cell Maturation

Mast cells are activated upon immunoglobulin E (IgE)-mediated antigen stimulation, and release a wide variety of mediators, including histamine to trigger inflammatory responses. The surface expression levels of Fcε receptor I (FcεRI), a high affinity receptor of IgE, were found to be positively regulated by IgE. IgE could protect murine cultured mast cells from apoptotic cell death induced by the deprivation of interleukin-3 and a certain kind of IgE could activate immature mast cells in the absence of antigens, leading to the release of pro-inflammatory cytokines and a transient increase in histamine synthesis. Histamine synthesis in mast cells was found to be required for the maturation of murine connective tissue-type mast cells, raising the possibility that IgE indirectly modulates local mast cell maturation. Although it remains controversial to what extent this concept of "monomeric IgE effects" could have relevance in the modulation of human mast cell functions, the therapeutic effects of anti-IgE antibodies might be accounted for in terms of the decreased serum IgE concentrations. Because drastic increases in serum IgE concentrations are often observed in patients with atopic dermatitis and chronic urticaria, a close investigation of the roles of IgE in mast cell maturation should contribute to development of novel therapeutic approaches for these inflammatory diseases.

Genome-wide study of copy number variation implicates multiple novel loci for schizophrenia risk in Han Chinese family trios

Schizophrenia (SCZ) is a severe neuropsychiatric disorder that affects 1% of the global population. Copy number variations (CNVs) have been shown to play a critical role in its pathophysiology; however, only case-control studies on SCZ susceptibility CNVs have been conducted in Han Chinese. Here, we performed an array comparative genomic hybridization-based genome-wide CNV analysis in 100 Chinese family trios with SCZ. Burden test suggested that the SCZ probands carried more duplications than their healthy parents and unrelated healthy controls. Besides, five CNV loci were firstly reported to be associated with SCZ here, including both unbalanced transmitted CNVs and enriched de novo CNVs. Moreover, two genes (CTDSPL and MGAM) in these CNVs showed significant SCZ relevance in the expression level. Our findings support the crucial role of CNVs in the etiology of SCZ and provide new insights into the underlying mechanism of SCZ pathogenesis.

Mast Cells in Inflammation and Disease: Recent Progress and Ongoing Concerns

Mast cells have existed long before the development of adaptive immunity, although they have been given different names. Thus, in the marine urochordate Styela plicata, they have been designated as test cells. However, based on their morphological characteristics (including prominent cytoplasmic granules) and mediator content (including heparin, histamine, and neutral proteases), test cells are thought to represent members of the lineage known in vertebrates as mast cells. So this lineage presumably had important functions that preceded the development of antibodies, including IgE. Yet mast cells are best known, in humans, as key sources of mediators responsible for acute allergic reactions, notably including anaphylaxis, a severe and potentially fatal IgE-dependent immediate hypersensitivity reaction to apparently harmless antigens, including many found in foods and medicines. In this review, we briefly describe the origins of tissue mast cells and outline evidence that these cells can have beneficial as well as detrimental functions, both innately and as participants in adaptive immune responses. We also discuss aspects of mast cell heterogeneity and comment on how the plasticity of this lineage may provide insight into its roles in health and disease. Finally, we consider some currently open questions that are yet unresolved.

Triggers of Exacerbation in Chronic Urticaria and Recurrent Angioedema-Prevalence and Relevance

Patients with urticaria and angioedema often have triggers that cause an outbreak or a swelling episode or worsen their chronic condition. Exploring these factors with each patient may result in better understanding and control of their disease. Patients should be advised to avoid known triggers, if feasible, or prepare to prevent or control an exacerbation with appropriate pretreatment if avoidance is not possible. In this review, we describe and discuss a variety of factors for which there is evidence that they cause or exacerbate chronic spontaneous urticaria and angioedema. These potentially exacerbating factors include drugs, food additives, and naturally occurring pseudoallergens, mental stress, and trauma.

Two Sides of the Coin: Mast Cells as a Key Regulator of Allergy and Acute/Chronic Inflammation

It is well known that mast cells (MCs) initiate type I allergic reactions and inflammation in a quick response to the various stimulants, including—but not limited to—allergens, pathogen-associated molecular patterns (PAMPs), and damage-associated molecular patterns (DAMPs). MCs highly express receptors of these ligands and proteases (e.g., tryptase, chymase) and cytokines (TNF), and other granular components (e.g., histamine and serotonin) and aggravate the allergic reaction and inflammation. On the other hand, accumulated evidence has revealed that MCs also possess immune-regulatory functions, suppressing chronic inflammation and allergic reactions on some occasions. IL-2 and IL-10 released from MCs inhibit excessive immune responses. Recently, it has been revealed that allergen immunotherapy modulates the function of MCs from their allergic function to their regulatory function to suppress allergic reactions. This evidence suggests the possibility that manipulation of MCs functions will result in a novel approach to the treatment of various MCs-mediated diseases.

Ligands and Signaling of Mas-Related G Protein-Coupled Receptor-X2 in Mast Cell Activation

Mas-related G protein-coupled receptor-X2 (MRGPRX2) is known as a novel receptor to activate mast cells (MCs). MRGPRX2 plays a dual role in promoting MC-dependent host defense and immunomodulation and contributing to the pathogenesis of pseudo-allergic drug reactions, pain, itching, and inflammatory diseases. In this article, we discuss the possible signaling pathways of MCs activation mediated by MRGPRX2 and summarize and classify agonists and inhibitors of MRGPRX2 in MCs activation. MRGPRX2 is a low-affinity and low-selectivity receptor, which allows it to interact with a diverse group of ligands. Diverse MRGPRX2 ligands utilize conserved residues in its transmembrane (TM) domains and carboxyl-terminus Ser/Thr residues to undergo ligand binding and G protein coupling. The coupling likely initiates phosphorylation cascades, induces Ca²⁺ mobilization, and causes degranulation and generation of cytokines and chemokines via MAPK and NF-κB pathways, resulting in MCs activation. Agonists of MRGPRX2 on MCs are divided into peptides (including antimicrobial peptides, neuropeptides, MC degranulating peptides, peptide hormones) and nonpeptides (including FDA-approved drugs). Inhibitors of MRGPRX2 include non-selective GPCR inhibitors, herbal extracts, small-molecule MRGPRX2 antagonists, and DNA aptamer drugs. Screening and classifying MRGPRX2 ligands and summarizing their signaling pathways would improve our understanding of MRGPRX2-mediated physiological and pathological effects on MCs.

Vaccines and allergic reactions: The past, the current COVID-19 pandemic, and future perspectives

Vaccines are essential public health tools with a favorable safety profile and prophylactic effectiveness that have historically played significant roles in reducing infectious disease burden in populations, when the majority of individuals are vaccinated. The COVID-19 vaccines are expected to have similar positive impacts on health across the globe. While serious allergic reactions to vaccines are rare, their underlying mechanisms and implications for clinical management should be considered to provide individuals with the safest care possible. In this review, we provide an overview of different types of allergic adverse reactions that can potentially occur after vaccination and individual vaccine components capable of causing the allergic adverse reactions. We present the incidence of allergic adverse reactions during clinical studies and through post-authorization and post-marketing surveillance and provide plausible causes of these reactions based on potential allergenic components present in several common vaccines. Additionally, we review implications for individual diagnosis and management and vaccine manufacturing overall. Finally, we suggest areas for future research.

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.

Current and emerging pharmacotherapy for chronic spontaneous Urticaria: a focus on non-biological therapeutics

INTRODUCTION

Chronic spontaneous urticaria (CSU) refers to urticaria (wheals) or angioedema, which occur for a period of six weeks or longer without an apparent cause. The condition may impair the patient's quality of life.

AREAS COVERED

Treatment for CSU is mainly symptomatic. Both AAAAI/ACAAI practice parameters and EAACI/GA2LEN/EDF/WAO guidelines suggest CSU management in a stepwise manner. First-line therapy is with second-generation H₁-antihistamines. Treatment should be stepped up along the algorithm if symptoms are not adequately controlled. Increasing the dosage of second-generation H₁-antihistamines, with the addition of first-generation H₁-antihistamines, H₂ antagonist, omalizumab, ciclosporin A, or short-term corticosteroid may be necessary. New medications are being developed to treat refractory CSU. They include spleen tyrosine kinase inhibitor, Bruton tyrosine kinase inhibitor, prostaglandin D₂ receptor inhibitor, H₄-antihistamine, and other agents. The authors discuss these treatments and provide expert perspectives on the management of CSU.

EXPERT OPINION

Second-generation H₁-antihistamines remain the first-line therapeutic options for the management of CSU. For patients not responding to higher-dose H₁-antihistamines, international guidelines recommend the addition of omalizumab. Efficacy and safety data for newer agents are still pending. Large-scale, well-designed, randomized, double-blind, placebo-controlled trials will further provide evidence on the safety profile and efficacy of these agents in patients with CSU.

In vitro prediction of in vivo pseudo-allergenic response via MRGPRX2

In development of peptide therapeutics, rodents are commonly-used preclinical models when screening compounds for efficacy endpoints in the early stages of discovery projects. During the screening process, some peptides administered subcutaneously to rodents caused injection site reactions manifesting as localized swelling. Screening by postmortem evaluations of injection site swelling as a marker for local subcutaneous histamine release, were conducted in rats to select drug candidates without this adverse effect. Histological analysis of skin samples revealed that the injection site reactions were concurrent with mast cell degranulation, resulting in histamine release. Mast cell activation can be mediated by MRGPRX2, a GPCR that induces a pseudo-allergenic immune response. The present study demonstrates that a commercially-available cell-based MRGPRX2 assay reliably identifies compounds that induce histamine release or localized edema in ex vivo human and rodent skin samples. In vitro screening was subsequently implemented using the MRGPRX2 assay as a substitute for postmortem injection site evaluation, thus achieving a significant reduction in animal use. Thus, in cases where injection site reactions are encountered during in vivo screening, to enable faster screening during the early drug discovery process, an MRGPRX2 in vitro assay can be used as an efficient, more ethical tool with human translational value for the development of safer pharmacotherapies for patients.

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.

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.

Amelioration of Compound 48/80-Mediated Itch and LL-37-Induced Inflammation by a Single-Stranded Oligonucleotide

Numerous inflammatory skin disorders display a high prevalence of itch. The Mas-related G protein coupled receptor X2 (MRGPRX2) has been shown to modulate itch by inducing non-IgE-mediated mast cell degranulation and the release of endogenous inducers of pruritus. Various substances collectively known as basic secretagogues, which include inflammatory peptides and certain drugs, can trigger MRGPRX2 and thereby induce pseudo-allergic reactions characterized by histamine and protease release as well as inflammation. Here, we investigated the capacity of an immunomodulatory single-stranded oligonucleotide (ssON) to modulate IgE-independent mast cell degranulation and, more specifically, its ability to inhibit the basic secretagogues compound 48/80 (C48/80)-and LL-37 in vitro and in vivo. We examined the effect of ssON on MRGPRX2 activation in vitro by measuring degranulation in a human mast cell line (LAD2) and calcium influx in MRGPRX2-transfected HEK293 cells. To determine the effect of ssON on itch, we performed behavioral studies in established mouse models and collected skin biopsies for histological analysis. Additionally, with the use of a rosacea mouse model and RT-qPCR, we investigated the effect on ssON on LL-37-induced inflammation. We reveal that both mast cell degranulation and calcium influx in MRGPRX2 transfected HEK293 cells, induced by the antimicrobial peptide LL-37 and the basic secretagogue C48/80, are effectively inhibited by ssON in a dose-dependent manner. Further, ssON demonstrates a capability to inhibit LL-37 and C48/80 activation in vivo in two mouse models. We show that intradermal injection of ssON in mice is able to block itch induced via C48/80 in a dose-dependent manner. Histological staining revealed that ssON inhibits acute mast cell degranulation in murine skin treated with C48/80. Lastly, we show that ssON treatment ameliorates LL-37-induced inflammation in a rosacea mouse model. Since there is a need for new therapeutics targeting non-IgE-mediated activation of mast cells, ssON could be used as a prospective drug candidate to resolve itch and inflammation in certain dermatoses.

Emerging Causes of Drug-Induced Anaphylaxis: A Review of Anaphylaxis-Associated Reports in the FDA Adverse Event Reporting System (FAERS)

BACKGROUND

Drug-induced anaphylaxis is a well-known adverse drug reaction for some drug classes, but emerging drug causes of anaphylaxis and novel mechanisms may contribute in unrecognized ways.

OBJECTIVE

We sought to determine the top drugs reported in association with anaphylaxis and anaphylaxis followed by death in the Food and Drug Administration Adverse Event Reporting System (FAERS).

METHODS

We reviewed the publicly available FAERS database from 1999 to 2019. Using search terms "anaphylactic shock" or "anaphylactic reaction" and sorting cases by generic drug names, we counted and trended reports to FAERS in which a drug was associated with anaphylaxis or anaphylaxis followed by death.

RESULTS

From 1999 to 2019, there were 17,506,002 adverse drug events reported in FAERS, of which 47,496 (0.27%) were reported as anaphylaxis. Excluding patients without age, sex, or country data, respectively, the median age of patients in reports of anaphylaxis was 52 (interquartile range: 28), 62.71% were female, and 13,899 of 34,381 (40.43%) reports were from the United States. There were 2984 of 47,496 (6.28%) reports of anaphylaxis followed by death. Top drug classes associated with anaphylaxis in FAERS were antibiotics, monoclonal antibodies (mAbs), nonsteroidal anti-inflammatory drugs, and acetaminophen. Top drug classes associated with anaphylaxis deaths were antibiotics, radiocontrast agents, and intraoperative agents. Linear regression demonstrated reports of anaphylaxis to mAbs increasing at an average rate of 0.77% of total anaphylaxis reports per year (95% confidence interval: 0.65, 0.88) from 2.00% in 1999 to 17.37% in 2019, faster than any other drug class.

CONCLUSION

Antibiotics were highly reported for anaphylaxis overall and anaphylaxis followed by death. Increasing reports were noted for anaphylaxis to mAb therapies.

The pseudoallergen receptor MRGPRX2 on peripheral blood basophils and eosinophils: Expression and function

BACKGROUND

Mas-related G protein-coupled receptor X2 (MRGPRX2) is regarded as a mast cell-specific receptor mediating non-IgE-dependent activation. We aimed to investigate whether human basophils and eosinophils express functional MRGPRX2.

METHODS

Flow cytometry, immunocytochemistry, immunofluorescence, Western blot, and RT-PCR were performed in highly purified peripheral blood basophils and eosinophils of atopic and nonatopic donors. To assess functional activity, fluorescent avidin-based degranulation assay, calcium mobilization, cytokine production in supernatants, assessment of viability/apoptosis, and tricolor granulocyte activation test were used.

RESULTS

MRGPRX2 was significantly expressed by basophils and eosinophils but not neutrophils. Functional capacity was shown by anti-MRGPRX2 mAb-induced calcium influx and concentration-dependent induction of degranulation. Sequential stimulation in the calcium mobilization assay gave no evidence for desensitization or receptor internalization. Anti-MRGPRX2 mAb significantly promoted survival. Inhibition of apoptosis could be due to released IL-3, IL-5, and GM-CSF found in supernatants. Short-term incubation with IL-3 dose-dependently upregulated MRGPRX2 expression in both, stimulation for 24 hours with anti-IgE, C5a, fMLP, and IL-3 in basophils and by IL-3, IL-5, and IL-33 in eosinophils. Among known mast cell MRGPRX2 agonists ciprofloxacin but not PMX-53 was functional on basophils and eosinophils. In basophils of allergic subjects, tricolor granulocyte activation test using grass pollen demonstrated MRGPRX2 upregulation associated with degranulation and CD63 expression.

CONCLUSION

Unraveling the regulation and signaling mechanisms of MRGPRX2 on basophils and eosinophils might enable the development of new therapeutic strategies to prevent or inhibit allergic and nonallergic hypersensitivity. Moreover, addressing MRGPRX2 might have potential for diagnostic purposes in (drug) hypersensitivity.

Osthole, a Natural Plant Derivative Inhibits MRGPRX2 Induced Mast Cell Responses

Mast cells are tissue-resident innate immune cells known for their prominent role in mediating allergic reactions. MAS-related G-protein coupled receptor-X2 (MRGPRX2) is a promiscuous G-protein coupled receptor (GPCR) expressed on mast cells that is activated by several ligands that share cationic and amphipathic properties. Interestingly, MRGPRX2 ligands include certain FDA-approved drugs, antimicrobial peptides, and neuropeptides. Consequently, this receptor has been implicated in causing mast cell-dependent pseudo-allergic reactions to these drugs and chronic inflammation associated with asthma, urticaria and rosacea in humans. In the current study we examined the role of osthole, a natural plant coumarin, in regulating mast cell responses when activated by the MRGPRX2 ligands, including compound 48/80, the neuropeptide substance P, and the cathelicidin LL-37. We demonstrate that osthole attenuates both the early (Ca2+ mobilization and degranulation) and delayed events (chemokine/cytokine production) of mast cell activation via MRGPRX2 in vitro. Osthole also inhibits MrgprB2- (mouse ortholog of human MRGPRX2) dependent inflammation in in vivo mouse models of pseudo-allergy. Molecular docking analysis suggests that osthole does not compete with the MRGPRX2 ligands for interaction with the receptor, but rather regulates MRGPRX2 activation via allosteric modifications. Furthermore, flow cytometry and confocal microscopy experiments reveal that osthole reduces both surface and intracellular expression levels of MRGPRX2 in mast cells. Collectively, our data demonstrate that osthole inhibits MRGPRX2/MrgprB2-induced mast cell responses and provides a rationale for the use of this natural compound as a safer alternative treatment for pseudo-allergic reactions in humans.

Bilirubin as a signaling molecule

For long time bilirubin was only considered as a potentially dangerous sign of liver diseases, but it now appears clear that it is also a powerful signaling molecule. Together with potent antioxidant activities that were only reported in the last few decades, many other biological effects have now been clearly described. These include especially profound inhibitory effects on almost all effectors of the immune system, with their clinical consequences in the bilirubin-mediated protection against autoimmune and inflammatory diseases. Separate from these, bilirubin activates various nuclear and cytoplasmic receptors, resembling the endocrine activities of actual hormonal substances. This is true for the "classical" hepatic nuclear receptors, including the aryl hydrocarbon receptor, or the constitutive androstane receptor; and also for some lesser-explored receptors such as peroxisome proliferator-activated receptors α and γ; Mas-related G protein-coupled receptor; or other signaling molecules including fatty acid binding protein 1, apolipoprotein D, or reactive oxygen species. All of these targets have broad metabolic effects, which in turn may offer protection against obesity, diabetes mellitus, and other metabolic diseases. The (mostly experimental) data are also supported by clinical evidence. In fact, data from the last three decades have convincingly demonstrated the protective effects of mildly elevated serum bilirubin concentrations against various "diseases of civilization." Additionally, even tiny, micromolar changes of serum bilirubin concentrations have been associated with substantial alteration in the risks of these diseases. It is highly likely that all of the biological activities of bilirubin have yet to be exhaustively explored, and thus we can expect further clinical discoveries about this evolutionarily old molecule into the future.

Store-Operated Calcium Entry via STIM1 Contributes to MRGPRX2 Induced Mast Cell Functions

Mast cells are inflammatory immune cells that play an essential role in mediating allergic reactions in humans. It is well-known that mast cell activation is critically regulated by intracellular calcium ion (Ca²⁺) concentrations. MAS-related G-protein coupled receptor-X2 (MRGPRX2) is a G-protein coupled receptor (GPCR) expressed on mast cells that is activated by various ligands, including several FDA approved drugs; consequently, this receptor has been implicated in causing pseudo-allergic reactions in humans. MRGPRX2 activation leads to an increase in intracellular Ca²⁺ levels; however, the Ca²⁺ mobilizing mechanisms utilized by this receptor are largely unknown. Previous reports showed that store-operated Ca²⁺ entry (SOCE) via the calcium sensor, stromal interaction molecule 1 (STIM1), regulates mast cell response induced by the high-affinity IgE receptor (FcεRI). In this study, using complementary pharmacologic and genetic ablation approaches we demonstrate that SOCE through STIM1 promotes MRGPRX2-induced human mast cell response in vitro. Importantly, SOCE also critically modulates MrgprB2 (mouse ortholog of human MRGPRX2) dependent inflammation in in vivo mouse models of pseudo-allergy. Collectively, our data suggests that MRGPRX2/MrgprB2 activation of mast cells is dependent on SOCE via STIM1, and further characterization of the MRGPRX2-SOCE-STIM1 pathway will lead to the identification of novel targets for the treatment of pseudo-allergic reactions in humans.

Human antimicrobial peptides in autoimmunity

Antimicrobial peptides (AMPs) were firstly discovered as cytotoxic substances that killed bacteria. Later they were described as biologically active peptides that are able not only to kill invaders but also to modulate host immunity. In particular, it is shown that human antimicrobial peptides are able to influence the activity of different innate and adaptive immunity components, thus, obviously, they also participate in autoimmune processes. In this review we discuss the nature of human AMPs and analyze their role in such autoimmune disorders like type 1 diabetes mellitus, rheumatoid arthritis, systemic lupus erythematosus, psoriasis, Crohn's disease and sarcoidosis. These peptides were shown to have a "double-sided" influence on the autoimmune disease pathogenesis. Thus, described facts should be taken into account for the development of new pharmaceutical agents to cure patients with autoimmune disorders. These agents could derive from natural antimicrobial peptides that in some cases modulate immune response. For example, it was shown that human AMPs are able to modulate complement system dysregulation of which is known to be one of the most dangerous pathogenic factors during autoimmune processes.

Basophil Activation Experiments in Immediate Drug Hypersensitivity: More Than a Diagnostic Aid

BACKGROUND

Correct diagnosis of immediate drug hypersensitivity reactions (IDHRs) can pose a significant challenge, mainly because of the absence of reliable in vitro tests, uncertainties associated with skin testing, and incomplete understanding of the underlying mechanisms.

AIM

To summarize and hypothesize on the potential of basophil activation test (BAT) as a safe aid to explore the mechanistic endotypes of IDHR, to identify antibody recognition sites, and to monitor drug desensitization.

METHODS

A literature search was conducted using the keywords "allergy, basophil activation, CD63, CD203c, diagnosis, drugs, hypersensitivity, flow cytometry"; this was complemented by the authors' own expertise.

RESULTS

At present BAT has mainly been employed as a diagnostic aid. However, evidence is emerging that the technique might also deepen our insights in immune (allergic) and nonimmune (nonallergic) mechanistic processes of IDHR. It is anticipated that BAT might also benefit the identification of antibody recognition sites and benefit our understandings of desensitization strategies.

CONCLUSION

Although the nondiagnostic application of BAT in IDHR is still in its infancy, with increasing employment, we can expect the technique to become a valuable asset to study many domains of IDHR that remain poorly understood.

Interactions between rat submucosal neurons and mast cells are modified by cytokines and neurotransmitters

The role of mast cells during inflammatory bowel diseases (IBD) is discussed controversially. Whereas several studies report an increase in mast cell density during IBD, others found a decrease. Recently, we observed a reduced response to mast cell degranulation induced by antigen contact in a colitis model. As the effects of mast cell mediators on epithelial ion transport are mediated indirectly via stimulation of secretomotor neurons, we investigated in vitro whether proinflammatory cytokines change the response to mast cell degranulation. Tumor necrosis factor α (TNFα) and a mix of proinflammatory cytokines caused an increase of short-circuit current (I_sc) and tissue conductance in rat colon. Anion secretion induced by histamine was downregulated in the presence of interleukin-1β (IL-1β) and the cytokine mix, whereas the response to the mast cell stimulator compound 48/80 was not changed significantly. In a coculture of rat submucosal ganglionic cells with a mast cell line (RBL-2H3), TNFα preincubation for 1 d increased the percentage of neurons responding to mast cell degranulation with an increase of the cytosolic Ca²⁺ concentration and enhanced the amplitude of this response. Consequently, the downregulation of epithelial secretion is compensated by an increased sensitivity of secretomotor neurons leading to a constant response of the epithelium to compound 48/80. Furthermore, enteric neurons can modify mast cell functions as nicotine inhibited the increase in cytosolic Ca²⁺ concentration of RBL-2H3 cells and the I_sc evoked by compound 48/80. Consequently, these in vitro models deliver new insights into cellular interactions in the gut wall under inflammatory conditions.

Functional Anatomy of the Sensory Nervous System: Updates From the Neuroscience Bench

The simple tripartite classification of sensory neurons as A-beta, A-delta, and C fibers fails to convey the complexity of the neurons that encode stimuli as diverse as the texture of a surface, the location of a pinprick, or the direction of hair movement as a breeze moves across the skin. It has also proven to be inadequate when investigating the molecular mechanisms underlying pain, which can encompass any combination of chemical, tactile, and thermal modalities. Beginning with a brief overview of visceral and sensory neuroanatomy, this review expands upon sensory innervation of the skin as a prime example of the heterogeneity and complexity of the somatosensory nervous system. Neuroscientists have characterized defining features of over 15 subtypes of sensory neurons that innervate the skin of the mouse. This has enabled the study of cell-specific mechanisms of pain, which suggests that diverse sensory neuron subtypes may have distinct susceptibilities to toxic injury and different roles in pathologic mechanisms underlying altered sensation. Leveraging this growing body of knowledge for preclinical trials and models of neurotoxicity can vastly improve our understanding of peripheral nervous system dysfunction, advancing the fields of toxicologic pathology and neuropathology alike.

Stress, pseudoallergens, autoimmunity, infection and inflammation in chronic spontaneous urticaria

Chronic spontaneous urticaria (CSU) is often associated with organ specific autoimmunity but is rarely caused by food allergy. Colourings and preservatives in pre-packaged foods, so called pseudoallergens, have also been implicated. Factors that promote inflammation or reduce anti-inflammatory mechanisms may however, predispose susceptible individuals to CSU. Chronic underlying infection and mental and emotional stress can sometimes precede the onset of CSU and once established can exacerbate the symptoms. There is early evidence of dysbiosis within the gastrointestinal tract in people with CSU and reduced levels of vitamin D are also evident. The latter may be related to the importance of vitamin D3 in increasing T regulatory function which can control a tendency to autoimmunity. It is quite possible that a state of on-going chronic inflammation with reduced anti-oxidant mechanisms may underlie the not infrequent association between CSU and metabolic syndrome. Effective treatment of CSU should involve the use of anti-histamines, intermittent steroids and anti-IgE therapy. For recalcitrant disease immune modulatory therapy has a place. However, talking therapies that reduce stress and anxiety, vitamin D3 supplementation, correction of intestinal dysbiosis and treatment of any chronic infection should also be considered.

Severe asthma phenotypes and endotypes

Current management of severe asthma relying either on guidelines (bulk approach) or on disease phenotypes (stratified approach) did not improve the burden of the disease. Several severe phenotypes are described: clinical, functional, morphological, inflammatory, molecular and microbiome-related. However, phenotypes do not necessarily relate to or give insights into the underlying pathogenetic mechanisms which are described by the disease endotypes. Based on the major immune-inflammatory pathway involved type-2 high, type-2 low and mixed endotypes are described for severe asthma, with several shared pathogenetic pathways such as genetic and epigenetic, metabolic, neurogenic and remodelling subtypes. The concept of multidimensional endotyping as un unbiased approach to severe asthma is discussed, together with new tools and targets facilitating the shift from the stratified to the precision medicine approach.

A mast-cell-specific receptor mediates Iopamidol induced immediate IgE-independent anaphylactoid reactions

Iopamidol is a radiographic contrast media which caused a very high incidence of anaphylactic reactions. Mast cells are sentinel cells in host defense reactions during immediate hypersensitivity responses and anaphylactic responses. Mas-related G protein-coupled receptor X2 (MRGPRX2) is a kind of mast cell specific receptor, which triggers mast cell degranulation in anaphylactic reactions. Mice MrgprB2 is a homologous gene of MRGPRX2. We sought to better understand the anaphylactic reactions induced by Iopamidol and the mechanisms involving MRGPRX2. The MRGPRX2-related anaphylactic reactions induced by Iopamidol were investigated using the hindpaw swelling and extravasation assay in vivo and a calcium imaging assay was used for mast cell intracellular calcium responses detection and mast cell release of anaphylactic mediators, such as β-hexosaminidase, histamine and TNF-α, was also detected in vitro. The mast cell deficient Kit^W-sh/W-sh mice and MrgprB2 knockout mice exhibited a reduced Iopamidol-induced inflammation effect compared with wild type mice. Furthermore, human mast cells that express MRGPRX2 were activated by Iopamidol in a dose-dependent manner, meanwhile MRGPRX2 knockdown mast cells showed reduced intracellular calcium responses and anaphylactic mediators release effect. It could be concluded that Iopamidol-induced anaphylactoid reactions were MRGPRX2 mediated to provoke mast cells Ca²⁺ mobilization and degranulation.

Heterogeneity of Human Mast Cells With Respect to MRGPRX2 Receptor Expression and Function

Mast cells and their mediators play a role in the control of homeostasis and in the pathogenesis of several disorders. The concept of rodent mast cell heterogeneity, initially established in the mid-1960s has been extended in humans. Human mast cells isolated and purified from different anatomic sites can be activated via aggregation of cell surface high affinity IgE receptors (FcεRI) by antigens, superantigens, anti-IgE, and anti-FcεRI. MAS-related G protein-coupled receptor-X2 (MRGPRX2) is expressed at high level in human skin mast cells (MCs) (HSMCs), synovial MCs (HSyMCs), but not in lung MCs (HLMCs). MRGPX2 can be activated by neuropeptide substance P, several opioids, cationic drugs, and 48/80. Substance P (5 × 10⁻⁷ M – 5 × 10⁻⁶ M) induced histamine and tryptase release from HSMCs and to a lesser extent from HSyMCs, but not from HLMCs and human cardiac MCs (HHMCs). Morphine (10⁻⁵ M – 3 × 10⁻⁴ M) selectively induced histamine and tryptase release from HSMCs, but not from HLMCs and HHMCs. SP and morphine were incomplete secretagogues because they did not induce the de novo synthesis of arachidonic acid metabolites from human mast cells. In the same experiments anti-IgE (3 μg/ml) induced the release of histamine and tryptase and the de novo synthesis of prostaglandin D₂ (PGD₂) from HLMCs, HHMCs, HSyMCs, and HSMCs. By contrast, anti-IgE induced the production of leukotriene C₄ (LTC₄) from HLMCs, HHMCs, HSyMCs, but not from HSMCs. These results are compatible with the heterogeneous expression and function of MRGPRX2 receptor on primary human mast cells isolated from different anatomic sites.

Characterization of Effect of Repeated Bolus or Continuous Intrathecal Infusion of Morphine on Spinal Mass Formation in the Dog

BACKGROUND

We determined whether intrathecally delivering the same daily dose of morphine (MS) at a fixed concentration of 25 mg/mL by periodic boluses versus continuous infusion would reduce intrathecal mass (IMs) formation in dogs.

METHODS

Adult dogs (hound cross, n = 32) were implanted with intrathecal catheters connected to SynchroMed II infusion pumps. Animals were randomly assigned to receive infusion of 0.48 mL/day of saline or MS dosing (12 mg/day at 25 mg/mL) as boluses: x1 (q24hour), x2 (q12hour), x4 (q6hour), or x8 (q3hour) given at the rate of 1000 μL/hour, or as a continuous infusion (25 mg/mL/20 μL/hour).

RESULTS

With IT saline, minimal pathology was noted. In contrast, animals receiving morphine displayed spinally compressing durally derived masses with the maximal cross-sectional area being greatest near the catheter tip. Histopathology showed that IMs consisted of fibroblasts in a collagen (type 1) matrix comprised of newly formed collagen near the catheter and mature collagen on the periphery of the mass. The rank order of median cross-sectional mass area (mm² ) was: Saline: 0.7 mm² ; x2: 1.8 mm² ; x4: 2.7 mm² ; x1: 2.7 mm² ; x8: 4.2 mm² ; Continuous: 8.1 mm² , with statistical difference from saline being seen with continuous (p < 0.0001) and x8 (p < 0.05). Bench studies with a 2D diffusion chamber confirmed an increase in dye distribution and lower peak concentrations after bolus delivery versus continuous infusion of dye.

CONCLUSIONS

Using multiple bolus dosing, IMs were reduced as compared to continuous infusion, suggesting relevance of bolus delivery in yielding reduced intrathecal masses.

All-trans-retinoic acid activated mast cells via Mas-related G-protein-coupled receptor-X2 in retinoid dermatitis

BACKGROUND

Retinoic acid (RA)-induced dermatitis is the most frequent side-effect limiting its widespread use. However, the exact mechanisms triggering dermatitis are not fully understood, including the role of skin mast cells. The newly discovered Mas-related G-protein-coupled receptor-X2 (MRGPRX2) in mast cells mediates pseudoallergic drug reactions in several types of dermatitis. A possible contribution of MRGPRX2 to contact dermatitis induced by RA has hitherto not been examined.

OBJECTIVES

To investigate whether all-trans-RA (ATRA) activates mast cells via MRGPRX2/MrgprB2 (the mouse orthologue), contributing to the pathogenesis of retinoid-induced dermatitis.

METHODS

Wild-type (WT) and MrgprB2^-/- mice were treated with topical ATRA to observe local inflammation and mast cell degranulation in vivo by the use of haematoxylin and eosin and immunofluorescence staining. Release of histamine and release of β-hexosaminidase were measured and calcium influx was detected in Laboratory of Allergic Disease 2 (LAD2) cells with specific knockdown targeting MRGPRX2 by small interfering RNA (siRNA) and in primary cells from MrgprB2^-/- mice.

RESULTS

As compared with WT mice, MrgprB2^-/- mice showed resistance to ATRA-triggered contact dermatitis and local inflammatory reactions in the paws. ATRA activated mast cells via the MrgprB2 pathway in murine cells, and via the MRGPRX2 pathway in human mast cells.

CONCLUSIONS

ATRA-induced dermatitis could be achieved by activating mast cells via MRGPRX2/MrgprB2, which may provide a potential therapy target to reduce the side-effect.

IL-33 and MRGPRX2-Triggered Activation of Human Skin Mast Cells-Elimination of Receptor Expression on Chronic Exposure, but Reinforced Degranulation on Acute Priming

Clinically relevant exocytosis of mast cell (MC) mediators can be triggered by high-affinity IgE receptor (FcεRI)-aggregation (allergic route) or by the so-called pseudo-allergic pathway elicited via MAS-related G protein-coupled receptor-X2 (MRGPRX2). The latter is activated by drugs and endogenous neuropeptides. We recently reported that FcεRI-triggered degranulation is attenuated when human skin mast cells are chronically exposed to IL-33. Here, we were interested in the regulation of the MRGPRX2-route. Chronic exposure of skin MCs to IL-33 basically eliminated the pseudo-allergic/neurogenic route as a result of massive MRGPRX2 reduction. This downregulation seemed to partially require c-Jun N-terminal Kinase (JNK), but not p38, the two kinases activated by IL-33 in skin MCs. Surprisingly, however, JNK had a positive effect on MRGPRX2 expression in the absence of IL-33. This was evidenced by Accell^®-mediated JNK knockdown and JNK inhibition. In stark contrast to the dampening effect upon prolonged exposure, IL-33 was able to prime for increased degranulation by MRGPRX2 ligands when administered directly before stimulation. This supportive effect depended on p38, but not on JNK activity. Our data reinforce the concept that exposure length dictates whether IL-33 will enhance or attenuate secretion. IL-33 is, thus, the first factor to acutely enhance MRGPRX2-triggered degranulation. Finally, we reveal that p38, rarely associated with MC degranulation, can positively affect exocytosis in a context-dependent manner.

Anaphylactic Degranulation of Mast Cells: Focus on Compound Exocytosis

Anaphylaxis is a notorious type 2 immune response which may result in a systemic response and lead to death. A precondition for the unfolding of the anaphylactic shock is the secretion of inflammatory mediators from mast cells in response to an allergen, mostly through activation of the cells via the IgE-dependent pathway. While mast cells are specialized secretory cells that can secrete through a variety of exocytic modes, the most predominant mode exerted by the mast cell during anaphylaxis is compound exocytosis-a specialized form of regulated exocytosis where secretory granules fuse to one another. Here, we review the modes of regulated exocytosis in the mast cell and focus on compound exocytosis. We review historical landmarks in the research of compound exocytosis in mast cells and the methods available for investigating compound exocytosis. We also review the molecular mechanisms reported to underlie compound exocytosis in mast cells and expand further with reviewing key findings from other cell types. Finally, we discuss the possible reasons for the mast cell to utilize compound exocytosis during anaphylaxis, the conflicting evidence in different mast cell models, and the open questions in the field which remain to be answered.

MRGPRX2 is negatively targeted by SCF and IL-4 to diminish pseudo-allergic stimulation of skin mast cells in culture

MRGPRX2 was recently uncovered as the "missing link" in clinically relevant mast cell (MC) activation explaining previously puzzling phenomena. It is the receptor for various endogenous ligands and exogenous compounds alike, whose binding evokes rapid degranulation much like allergen-mediated exocytosis. While the perceivable outcomes are similar, the two activation routes differ regarding mechanism and regulation. We recently reported that acute SCF administration curbs responses evoked by MRGPRX2 in human skin MCs. Maintenance of MCs in culture requires the presence of MC supportive factors and renders the cells functionally and molecularly unequal to ex vivo counterparts. Here, we asked whether expansion in culture impacts the pseudo-allergic route, and if so, what contribution SCF and IL-4 play in this scenario. We report that the in vitro micromilieu dampens (but does not erase) pseudo-allergic responses and that this is accompanied by strongly reduced MRGPRX2 expression. Withdrawal of SCF or IL-4 individually, but most potently of both collectively, partially reinstates the MRGPRX2 pathway, revealing that SCF and IL-4 make negative adjustments to the pseudo-allergic pathway. Under all conditions, the FcεRI-triggered route showed the inverse pattern of regulation, substantiating that allergic and pseudo-allergic MC activation can obey opposite rules, hinting at possible competition between them.

Effect of RFRP-3, the mammalian ortholog of GnIH, on the epididymis of male rats

RFamide-related peptide-3 (RFRP-3) and its receptor (GPR147) play an important role in reproduction regulation in mammals. To understand the role of RFRP-3 in male reproductive function of epididymis, we first investigated the expression changes in RFRP-3 and its receptor at different stages of development, that is, postnatal day 20 (P20), 40 (P40), 60 (P60) and 80 (P80). Our results showed that fluctuations in the expression of GnIH and GPR147 during postnatal development occurred, and the highest epididymal GnIH and GPR147 expression were both detected in P60. Subsequently, we further investigated the effect of RFRP-3 on the histology, apoptosis and autophagy of the epididymis in vivo. For in vivo study, male rats were treated intratesticularly with different doses of RFRP-3 (control, 0.1 μg, 1 μg, and 10 μg per day) for 7 days. Our results show that RFRP-3 caused dose-dependent histological changes in the epididymal duct, such as a decline in the number of spermatozoa and an increase in degenerated and vacuolated epididymal epithelial cells. Rats treated intratesticularly with RFRP-3 also showed dose-dependent effects on caspase-3 activation and the expression of apoptotic markers (whole caspase-3, cleaved caspase-3 and Bcl-2). However, the expression of autophagy markers (Beclin-1 and Atg5) exhibited a bidirectional, dose-dependent effect. It is concluded that RFRP-3 plays a regulatory role in male rat reproduction, possibly because RFRP-3 mediates the apoptosis and autophagy of the epididymis.

Naturally Occurring Missense MRGPRX2 Variants Display Loss of Function Phenotype for Mast Cell Degranulation in Response to Substance P, Hemokinin-1, Human β-Defensin-3, and Icatibant

Human mast cells (MCs) express a novel G protein-coupled receptor (GPCR) known as Mas-related GPCR X2 (MRGPRX2). Activation of this receptor by a diverse group of cationic ligands such as neuropeptides, host defense peptides, and Food and Drug Administration-approved drugs contributes to chronic inflammatory diseases and pseudoallergic drug reactions. For most GPCRs, the extracellular (ECL) domains and their associated transmembrane (TM) domains display the greatest structural diversity and are responsible for binding different ligands. The goal of the current study was to determine if naturally occurring missense variants within MRGPRX2's ECL/TM domains contribute to gain or loss of function phenotype for MC degranulation in response to neuropeptides (substance P and hemokinin-1), a host defense peptide (human β-defensin-3) and a Food and Drug Administration-approved cationic drug (bradykinin B2 receptor antagonist, icatibant). We have identified eight missense variants within MRGPRX2's ECL/TM domains from publicly available exome-sequencing databases. We investigated the ability of MRGPRX2 ligands to induce degranulation in rat basophilic leukemia-2H3 cells individually expressing these naturally occurring MRGPRX2 missense variants. Using stable and transient transfections, we found that all variants express in rat basophilic leukemia cells. However, four natural MRGPRX2 variants, G165E (rs141744602), D184H (rs372988289), W243R (rs150365137), and H259Y (rs140862085) failed to respond to any of the ligands tested. Thus, diverse MRGPRX2 ligands use common sites on the receptor to induce MC degranulation. These findings have important clinical implications for MRGPRX2 and MC-mediated pseudoallergy and chronic inflammatory diseases.