MRGPRX2 activation in mast cells by neuromuscular blocking agents and other agonists: Modulation by sugammadex

A Modified Basophil Activation Test for the Clinical Management of Immediate Hypersensitivity Reactions to Paclitaxel: A Proof-of-Concept Study

Immediate hypersensitivity reactions (iHSRs) to taxanes are observed in 6% and 4% of gynecologic and breast cancer patients, respectively. Drug desensitization is the only option, as no comparable alternative therapy is available. Surfactants in the taxane formulation have been implicated in the immunopathogenesis of iHSRs, although sporadic skin test (ST) positivity and iHSRs to nab-paclitaxel have suggested the involvement of the taxane moiety and/or IgE-mediated pathomechanisms. In vitro diagnostic tests might offer insights into mechanisms underlying iHSRs to taxanes. The aim of the present study was to address this unmet need by developing a novel basophil activation test (BAT). The study included patients (n = 31) undergoing paclitaxel/carboplatin therapy. Seventeen patients presented with iHSRs to paclitaxel (iHSR-Taxpos), and eleven were tolerant (iHSR-Taxneg). Fourteen patients presented with iHSRs to carboplatin (iHSR-Plpos), and fourteen were tolerant (iHSR-Plneg). The BAT median stimulation index (SI) values were 1.563 (range, 0.02-4.11; n = 11) and -0.28 (range -4.88-0.07, n = 11) in iHSR-Taxpos and iHSR-Taxneg, respectively. The BAT median SI values were 4.45 (range, 0.1-26.7; n = 14) and 0 (range, -0.51-1.65; n = 12) in iHSR-Plpos and iHSR-Plneg, respectively. SI levels were not associated with iHSR severity grading. Comparing BAT results in iHSR-Taxpos and iHSR-Taxneg showed the area under the receiver operator characteristic (ROC) curve to be 0.9752 (p = 0.0002). The cutoff calculated by the maximized likelihood ratio identified 90.91% of iHSR-Taxpos patients and 90.91% of iHSR-Taxneg patients. Comparing BAT results for iHSR-Plpos and iHSR-Plneg showed the area under the ROC curve to be 0.9286 (p = 0.0002). The cutoff calculated by the maximized likelihood ratio identified 78.57% of iHSR-Plpos patients and 91.67% of iHSR-Plneg patients. Most iHSR-Taxpos patients for which ST was available (10/11) scored ST-negative and BAT-positive, whereas most iHSR-Plpos patients for which ST was available (14/14) scored both BAT- and ST-positive. This suggested the intervention of non-IgE-mediated mechanisms in iHSR-Taxpos patients. Consistent with this view, an in silico molecular docking analysis predicted the high affinity of paclitaxel to the degranulation-competent MRGPRX2 receptor. This hypothesis warrants further in vitro investigations. In conclusion, the present study provides preliminary proof-of-concept evidence that this novel BAT has potential utility in understanding mechanisms underlying iHSRs to taxanes.

Inhibition of Mast Cell Degranulation in Atopic Dermatitis by Celastrol through Suppressing MRGPRX2

Background

Atopic dermatitis is a common dermatological disease, and mast cell degranulation is believed to be related with the progression of atopic dermatitis. Mas-related G protein-coupled receptor-X2 (MRGPRX2), and calcium release-activated calcium channel protein 1-2 (ORAI-1, ORAI-2) are involved in mast cell degranulation. Celastrol is an active monomer of Tripterygium wilfordii, and it presents an antiatopic role.

Methods

2,4-Dinitrofluorobenzene (DNFB) and compound 48/80 (C 48/80) were used to establish a slow and acute scratching animal model, respectively. Hematoxylin-eosin and toluidine blue staining was used to investigate tissue injury. Inflammatory factor concentration was measured with ELISA. The expression of MRGPRX2, ORAI-1, and ORAI-2 was detected with immunohistochemistry (IHC) staining. Gene expression profiling and microRNA array were performed to investigate gene differential expression.

Results

Celastrol greatly inhibited atopic dermatitis-related tissues injury, mast cell production, histamine release, scratching level, inflammatory factor expression, and activation of MRGPRX2/ORAI axis in the DNFB-induced atopic dermatitis model. The influence of Celastrol on atopic dermatitis was remarkably reversed by overexpression of MRGPRX2.

Conclusion

We found that the improvements of atopic dermatitis caused by Celastrol were reversed by treatment with MRGPRX2^OE, indicating that Celastrol might affect atopic dermatitis through MRGPRX2. This study might provide a novel thought for the prevention and treatment of atopic dermatitis by regulating MRGPRX2.

Clonal mast cell disorders and hereditary α-tryptasemia as risk factors for anaphylaxis

The association between Hymenoptera venom-triggered anaphylaxis (HVA) and clonal mast cell-related disorders (cMCD) has been known for decades. However, recent breakthroughs in peripheral blood screening for KIT p.D816V missense variant have revealed the true extent of this clinical association whilst adding to our understanding of the underlying aetiology. Thus, recent large studies highlighted the presence of KIT p.D816V among 18.2% and 23% of patients with severe Hymenoptera venom-triggered anaphylaxis. A significant proportion of those patients have normal serum basal tryptase (BST) levels, with no cutaneous findings such as urticaria pigmentosa or other systemic findings such as organomegaly that would have suggested the presence of cMCD. These findings of an increased prevalence suggest that the impact of cMCD on anaphylaxis could be clinically underestimated and that the leading question for clinicians could be changed from 'how many patients with cMCD have anaphylaxis?' to 'how many patients with anaphylaxis have cMCD?'. The discovery of hereditary α-tryptasemia (HαT)-a genetic trait caused by an increased copy number of the Tryptase Alpha/Beta 1 (TPSAB1) gene-, first described in 2016, is now known to underlie the majority of cases of elevated BST outside of cMCD and chronic kidney disease. HαT is the first common heritable genetic modifier of anaphylaxis described, and it is associated with increased risk for severe HVA (relative risk = 2.0), idiopathic anaphylaxis, and an increased prevalence of anaphylaxis in patients with cMCD, possibly due to the unique activity profile of α/β -tryptase heterotetramers that may potentiate immediate hypersensitivity reaction severity. Our narrative review aims to highlight recent research to have increased our understanding of cMCD and HαT, through recent lessons learned from studying their association with HVA. Additionally, we examined the studies of mast cell-related disorders in food and drug allergy in an effort to determine whether one should also consider cMCD and/or HαT in cases of severe anaphylaxis triggered by food or drugs.

Inhibition of MRGPRX2 but not FcεRI or MrgprB2-mediated mast cell degranulation by a small molecule inverse receptor agonist

Mas-related G protein-coupled receptor-X2 (MRGPRX2) expressed on mast cells (MCs) contributes to hypersensitivity reactions to cationic US-Food and Drug Administration (FDA) approved drugs such as the neuromuscular blocking agent, rocuronium. In addition, activation of MRGPRX2 by the neuropeptide substance P (SP) and the pro-adrenomedullin peptide (PAMP-12) is associated with a variety of cutaneous conditions such as neurogenic inflammation, pain, atopic dermatitis, urticaria, and itch. Thus, small molecules aimed at blocking MRGPRX2 constitute potential options for modulating IgE-independent MC-mediated disorders. Two inverse MRGPRX2 agonists, named C9 and C9-6, have recently been identified, which inhibit basal G protein activation and agonist-induced calcium mobilization in transfected HEK293 cells. Substance P serves as a balanced agonist for MRGPRX2 whereby it activates both G protein-mediated degranulation and β-arrestin-mediated receptor internalization. The purpose of this study was to determine if C9 blocks MRGPRX2's G protein and β-arrestin-mediated signaling and to determine its specificity. We found that C9, but not its inactive analog C7, inhibited degranulation in RBL-2H3 cells stably expressing MRGPRX2 in response to SP, PAMP-12 and rocuronium with an IC50 value of ~300 nM. C9 also inhibited degranulation as measured by cell surface expression of CD63, CD107a and β-hexosaminidase release in LAD2 cells and human skin-derived MCs in response to SP but not the anaphylatoxin, C3a or FcϵRI-aggregation. Furthermore, C9 inhibited β-arrestin recruitment and MRGPRX2 internalization in response to SP and PAMP-12. We found that a G protein-coupling defective missense MRGPRX2 variant (V282M) displays constitutive activity for β-arrestin recruitment, and that this response was significantly inhibited by C9. Rocuronium, SP and PAMP-12 caused degranulation in mouse peritoneal MCs and these responses were abolished in the absence of MrgprB2 or cells treated with pertussis toxin but C9 had no effect. These findings suggest that C9 could provide an important framework for developing novel therapeutic approaches for the treatment of IgE-independent MC-mediated drug hypersensitivity and cutaneous disorders.

The effect of substance P and its common in vivo-formed metabolites on MRGPRX2 and human mast cell activation

The tachykinin neuropeptide substance P (SP) is the canonical agonist peptide for the neurokinin 1 receptor (NK₁R). More recently, it has also been shown to activate the Mas‐related G protein‐coupled receptor X2 (MRGPRX2) receptor on mast cells (MCs), triggering degranulation and release of inflammatory mediators. SP undergoes rapid C‐terminal truncation in vivo by a number of proteases to generate the metabolites SP(1–9)‐COOH and in particular SP(1–7)‐COOH. While the C terminus of SP is critical for NK₁R activation, studies have shown that the peptide polycationic N terminus is key for MRGPRX2 and mast cell activation. The study thus aimed to determine if the C‐terminally truncated metabolites of SP, SP(1–9)‐COOH, and SP(1–7)‐COOH retained stimulatory activity at MRGPRX2. SP, SP(1–9)‐COOH, and SP(1–7)‐COOH were synthesized and tested on HEK293 cells expressing NK₁R or MRGPRX2, and LAD2 human mast cells, to determine the activity of SP and its metabolites in Ca²⁺ mobilization, degranulation, and cytokine assays. As expected from prior studies, both C‐terminally truncated SP metabolites had essentially no activity at NK₁R, even at very high concentrations. In contrast, the in vivo metabolite of SP, SP(1–9)‐COOH retained ability to activate MRGPRX2 across all parameters tested, albeit with reduced potency compared to intact SP. SP(1–7)‐COOH did not produce any significant MRGRPX2 activation. Our results suggest that the SP metabolite, SP(1–9)‐COOH, may play a regulatory role through the activation of MRGPRX2. However, given the relatively low potency of both SP and SP(1–9)‐COOH at MRGPRX2, additional work is needed to better understand the biological importance of this expanded SP/MRGPRX2 pathway.

Comparative Analysis of the Anesthesia Effect of Cisatracurium Besylate and Mivacurium Chloride Otolaryngology Surgery

Objective

The aim is to investigate and compare the anesthesia effect of cisatracurium besylate and mivacurium chloride otolaryngology surgery.

Materials and Methods

108 patients who underwent ENT surgery under general anesthesia in our hospital from November 2021 to March 2022 were recruited for retrospective analysis, in which patients in the experimental group A were anesthetized with cisatracurium besylate and patients in the experimental group B were anesthetized with mivacurium, and the anesthetic effects and recovery of the two groups were compared and analyzed.

Results

There was no significant difference in mean arterial pressure, heart rate, and pulse oximetry levels between the two groups at the six time points of admission, anesthesia induction, intubation, end of operation, recovery of consciousness, and extubation (all P > 0.05). The train of four stimulation values at end of operation, recovery of consciousness, and extubation were significantly higher than those of the experimental group A (all P > 0.05). The recovery time of self-consciousness, extubation time, and eye-opening time of the experimental group B were significantly shorter than those of the experimental group A, and the occurrence of agitation was significantly less than that of the experimental group A (all P > 0.05). The total incidence of adverse conditions in the experimental group B was significantly lower than that in the experimental group A (P > 0.05).

Conclusion

Compared with cisatracurium besylate in otolaryngology surgery, mivacurium chloride anesthesia offers a promising route with respect to less impact on hemodynamics, faster postoperative recovery, absence of the accumulation of neuromuscular blockade, less adverse reactions, and higher safety.

Advances and highlights in T and B cell responses to drug antigens

The immunological mechanisms involved in drug hypersensitivity reactions (DHRs) are complex, and despite important advances, multiple aspects remain poorly understood. These not fully known aspects are mainly related to the factors that drive towards either a tolerant or a hypersensitivity response and specifically regarding the role of B and T cells. In this review, we focus on recent findings on this knowledge area within the last 2 years. We highlight new evidences of covalent and non-covalent interactions of drug antigen with proteins, as well as the very first characterization of naturally processed flucloxacillin-haptenated human leukocyte antigen (HLA) ligands. Moreover, we have analysed new insights into the identification of risk factors associated with the development of DHRs, such as the role of oxidative metabolism of drugs in the activation of the immune system and the discovery of new associations between DHRs and HLA variants. Finally, evidence of IgG-mediated anaphylaxis in humans and the involvement of specific subpopulations of effector cells associated with different clinical entities are also topics explored in this review. All these recent findings are relevant for the underlying pathology mechanisms and advance the field towards a more precise diagnosis, management and treatment approach for DHRs.

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.

Antibody or Anybody? Considering the Role of MRGPRX2 in Acute Drug-Induced Anaphylaxis and as a Therapeutic Target

Acute anaphylaxis to small molecule drugs is largely considered to be antibody-mediated with immunogloblin E (IgE) and mast cell activation being key. More recently, a role for drug-reactive immunoglobulin G (IgG) with neutrophil activation has also been suggested, at least in reactions to neuromuscular blocking agents (NMBAs). However, the mast cell receptor MRGPRX2 has also been highlighted as a possible triggering mechanism in acute anaphylaxis to many clinically used drugs. Significantly, MRGPRX2 activation is not dependent upon the presence of drug-recognising antibody. Given the reasonable assumption that MRGPRX2 is expressed in all individuals, the corollary of this is that in theory, anybody could respond detrimentally to triggering drugs (recently suggested to be around 20% of a drug-like compound library). But this clearly is not the case, as the incidence of acute drug-induced anaphylaxis is very low. In this mini-review we consider antibody-dependent and -independent mechanisms of mast cell activation by small molecule drugs with a focus on the MRGPRX2 pathway. Moreover, as a juxtaposition to these adverse drug actions, we consider how increased understanding of the role of MRGPRX2 in anaphylaxis is important for future drug development and can complement exploration of this receptor as a drug target in broader clinical settings.

Action of substance P and PAMP(9-20) on different excitation sites of MRGPRX2 induces differences in mast cell activation

MRGPRX2 on mast cells (MCs) is the target that directly mediates MC activation through the activity of small molecular substances. Previous work has attempted to prove that substance P (SP) and PAMP(9-20) induce an MRGPRX2-mediated MC degranulation reaction. However, SP activates MRGPRX2-induced histamine release, which may lead to allergic airway inflammation, while PAMP(9-20)-induced MrgprB2 activation releases more tryptase and fewer monoamines. Due to the lack of direct available comparisons, the different types of sensitizing mediators released by the action of SP and PAMP(9-20) inducing pseudo-allergic reactions via MRGPRX2 are unclear. To investigate whether the action sites of excited MRGPRX2 are different for SP and PAMP(9-20), leading to different effects, the release of inflammatory mediators was measured using MC degranulation reactions and RNA-seq assay in vitro. Mice were treated to observe local inflammation and MC degranulation in vivo. Moreover, site-directed mutagenesis was used to verify the excited sites of SP and PAMP(9-20). SP and PAMP(9-20) both activated MRGPRX2 and led MCs to release inflammatory mediators. Significantly different levels of histamine, tryptase, TNF-α, MCP-1, and other cytokines were released in vivo and in vitro. G165E, D184N, W243R, and H259Y were necessary for SP to activate MRGPRX2, while only D184N and W243R were important for PAMP(9-20). The downstream signaling pathways activated by SP and PAMP(9-20) also differed in the phosphorylation level of PKC. There were differences in the sites via which SP and PAMP(9-20) activate MRGPRX2 and also in the activated downstream signaling pathways, which led to the differences the activation of the pathways and effects of SP- and PAMP(9-20)-induced MRGPRX2 activation.

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.

Novel Insights on MRGPRX2-Mediated Hypersensitivity to Neuromuscular Blocking Agents And Fluoroquinolones

Neuromuscular blocking agents (NMBAs) like atracurium and rocuronium as well as fluoroquinolones (FQs) cause mast cell-mediated anaphylaxis by activating Mas-related G protein-coupled receptor X2 (MRGPRX2), but many questions remain unanswered. Here, we address three of them, namely whether primary human mast cells show similar activation by these drugs as murine mast cells and mast cell lines, how sugammadex protects from atracurium-induced MRGPRX2-mediated mast cell activation, and why some but not all patients treated with rocuronium develop anaphylaxis. We used peripheral blood-derived cultured mast cells from healthy donors and patients, assessed mast cell activation and degranulation by quantifying intracellular calcium and CD63 expression, respectively, and made use of MRGPRX2-silencing, via electroporation with Dicer-substrate small interfering RNAs, and single cell flow cytometric analyses. Atracurium, ciprofloxacin, and levofloxacin activated and degranulated primary human mast cells, but only MRGPRX2-positive and not MRGPRX2-negative or -silenced mast cells. Sugammadex attenuated the atracurium-induced and MRGPRX2-mediated activation and degranulation of human mast cells by reducing free atracurium levels. The mast cells of patients with IgE-independent anaphylaxis to rocuronium were similar, in their MRGPRX2 expression and function, to those of patients with IgE-mediated anaphylaxis. These findings further improve our understanding of the role and relevance of MRGPRX2-driven mast cell activation in anaphylactic reactions to NMBAs and FQs and may help to improve their prediction, prevention, and treatment.

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

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

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.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

MRGPRX2 Activation by Rocuronium: Insights from Studies with Human Skin Mast Cells and Missense Variants

Perioperative hypersensitivity (POH) to the neuromuscular blocking drug (NMBD) rocuronium was previously thought to be IgE and mast cell (MC)-mediated. However, the recent seminal observation that rocuronium induces degranulation in murine peritoneal MCs (PMCs) via Mas-related G protein-coupled receptor B2 (MrgprB2) led to the idea that POH to this drug involves the activation of MRGPRX2 (human ortholog of MrgprB2). Furthermore, based on the demonstration that a patient with POH to rocuronium displayed three missense mutations (M196I, L226P and L237P) in MRGPRX2's transmembrane domains, it was proposed that this hypersensitivity reaction resulted from aberrant activation of this receptor. We found that rocuronium at 20 µg/mL caused degranulation in mouse PMCs via MrgprB2 but required at least 500 µg/mL to induce degranulation in human MCs via MRGPRX2. Furthermore, RBL-2H3 cells transiently expressing M196I, L226P and L237P variants did not display enhanced degranulation in response to rocuronium when compared to the wild-type receptor. These findings provide the first demonstration that rocuronium induces degranulation in human MCs via MRGPRX2. Furthermore, the important differences between MrgprB2 and MRGPRX2 and the inability of rocuronium to induce enhanced response in cells expressing MRGPRX2 variants suggest that the mechanism of its POH is more complex than previously thought.