Mas-Related G Protein-Coupled Receptor-X2 (MRGPRX2) in Drug Hypersensitivity Reactions

Anaphylaxis is a well-recognized, life-threatening medical condition. The clinical definition of anaphylaxis involves the observation of acute symptoms in two or more body systems or is associated with upper respiratory complications leading to asphyxiation and/or hypotension, which may result in cardiovascular collapse. Pruritus, urticaria, angioedema, bronchospasm and wheezing, hypotension, nausea, vomiting, and diarrhea are the usual clinical manifestations ( 1 , 2 ). Because of this plethora of symptoms, anaphylaxis continues to be under recognized in 80% of the patients, who are admitted to the emergency departments of hospitals after medical procedures associated with exposure to drugs ( 3 ). An anaphylactic reaction may occur via an IgE-mediated (allergic) or a non-IgE-dependent mechanism (previously called a pseudo-allergic or anaphylactoid mechanism) ( 4 , 5 ). From a clinical point of view they are impossible to distinguish by standard investigation. The effector phase of reaction responsible for the above clinical picture is caused by the release of mediators from mast cells and basophils. Mediators comprise histamine, which is well-known and has been extensively studied for years; platelet-activating factor (PAF), which is relevant as a decrease in its degradation predisposes to severe anaphylaxis ( 6 ) and many others, such as neutral proteases (tryptase, chymase, carboxypeptidase), proteoglycans (heparin, chondroitin sulfate), chemoattractans, and products of arachidonic acid metabolism. Possible triggers for anaphylaxis include food and environmental allergens such as nuts, egg, seafood, latex, Hymenoptera venoms (e.g., bee, wasp), as well as drugs, which are described in more detail below ( 3 ).

Mast cells (MC) have largely been considered to possess a key role in the production of immediate allergic reactions, as upon activation they release a variety of mediators from stored granules. But mast cells also participate in homeostasis and inflammation, innate and adaptive immunity, as well as angiogenesis in a variety of tissues ( 21 ). Thus, they are mostly found in the host-environment interface, such as the skin, lung, or gastrointestinal tract, which are challenged by a variety of extrinsic agents—allergens and pathogens. During the maturation process MC develop differences in granule composition and tissue-specific receptor patterns ( 22 , 23 ). Mucosal (MCT) and connective tissue (MCTC) mast cells represent two classical subtypes of these cells. Allergens, (glyco) proteins, or auto-antibodies directed against the FcεRI receptor (FcεRI) or receptor-bound IgE antibodies cause MC degranulation after cross-linking and aggregation of the surface-bound FcεRI ( 24 , 25 ). However, mast cell degranulation can be also achieved by non-IgE-dependent pathways thanks to a wide range of surface receptors, including toll-like receptors (TLR), protease-activated receptors (PARs), or Mas-related G-protein coupled receptor member X2 (MRGPRX2) ( 14 , 26 – 29 ). Therefore, MCs are able to identify and respond to a number of various exogenous (e.g., pathogen-associated molecular patterns, some contents of insect venom, many drugs, polycationic molecules such as compound 48/80) and endogenous stimuli provoking degranulation (e.g., cytokines, anaphylatoxins, chemokines, IgG, neuropeptides such as substance P) ( 25 , 26 , 30 – 32 ). Various stimuli are processed in different ways and may result in distinct degranulation programs as demonstrated by Gaudenzio et al. ( 33 ) (Figure 1 ). Therefore, MC activation is not a uniform event. Some clinical entities such as systemic mastocytosis and chronic urticaria show that a number of co-factors can exacerbate symptoms by triggering mast cells (e.g., physical factors such as heat and cold, pressure, non-steroidal anti-inflammatory drugs, or iodinated contrast media) ( 4 , 5 , 34 – 36 ). Figure 1 Main receptor systems and examples of ligands involved in mast cell activation. Drugs can activate mast cells through both sIgE-dependent mechanism and the MRGPRX2 receptor ( 14 , 26 , 38 ). These activation routes are independent and inversely regulated by SCF ( 51 ). QWF inhibits activation of human MRGPRX2 by a number of basic secretagogues and medications ( 47 , 50 , 52 ), but not by LL-37 ( 53 ). Granule processing and response program after the MRGPRX2 engagement differ from FcεRI-mediated response ( 32 , 33 ). Several other representative MC receptor systems and ligands are shown ( 28 , 29 ). CR, complement receptor for corresponding complement components; CysLT1R/2R receptors, cysteinyl leukotrienes; FcγRII, a low-affinity receptor for IgG; FcεRI, the high affinity IgE receptor; GM-CSF, granulocyte/macrophage colony-stimulating factor; IL, interleukin; KIT, mast/stem cell growth factor receptor (CD117); LPS, lipopolysaccharide; LTC4, leukotriene C4; Abs, antibodies; MRGPRX2, Mas-Related G Protein-Coupled Receptor-X2; NMBA, neuromuscular blocking agents; PAR2, protease-activated receptor 2; PGN, peptidoglycan; SCF, stem cell factor; sIgE, specific IgE; TLR, Toll-like receptor; TNF, tumor-necrosis factor; QWF–tripeptide (the glutaminyl-D-tryptophylphenylalanine); VIP, vasoactive intestinal peptide. The schematic drawings were generated by modifying images obtained from Motifolio (Motifolio Inc., Elliocott City, MD, USA). Upon activation, MCs release a myriad of mediators responsible for creating a clinical picture of immediate reaction. Some of these mediators are preformed (histamine, proteases, heparin), whereas others are newly formed (e.g., thromboxane, prostaglandin D2, leukotriene C4, tumor necrosis factor alpha) ( 37 ).

McNeil et al. demonstrated that distinct drugs elicit the MRGPRX2-related MC degranulation. The same drugs are also known to cause IgE-dependent MC degranulation, which results in producing local or systemic anaphylaxis ( 14 ). It has been shown with human and mice MCs cultures under laboratory conditions, but to what extent results can be translated into drug-hypersensitive patients remains the question. Reactions are often observed upon first exposure in drug-naïve patients, almost always after injection with icatibant, but rarely during treatment with NMBA or fluoroquinolones. If the reactions share the MRGPRX2-dependent mechanism, why do they differ so much in respect of frequency and why do only a small minority of individuals in the general population develops NMBA-induced reactions? There are theoretical possibilities (i), that, even in the same subjects, some reactions are mediated by drug-specific IgE, the others by MRGPRX2; (ii) that positive skin tests reflect IgE-dependent sensitization and/or alternative degranulation pathway and both mechanisms may be responsible for cross-reactivity between drugs of interest. One could also hypothesize that: Mutation in the MRGPRX2 gene affects the risk of anaphylactic events being mediated by the MRGPRX2 receptor. Mutation in a single amino acid residue was shown to fail the activation of the receptor by the substance P and the compound 48/80 ( 53 ). In the same paper, the authors demonstrated that another receptor agonist, LL-37 activates both mutant and native receptor. The receptor antagonist QWF inhibits MCs activation with the substance P and the compound 48/80, but not with LL-37, as described above. Naturally occurring SNPs may abrogate MC-mediated degranulation in response to MRGPRX2 ligands including substance P, hemokinin-1, human β-defensin-3 and icatibant ( 58 ). Our analyses revealed several other SNPs that could potentially affect the ligand binding properties of MRGPRX2 (Table 1 ). Therefore: Icatibant and NMBA may interact with the MRGPRX2 through different active-sites of the same receptor, which implies differences in frequency of induced reactions. Also differences in intracellular molecular mechanisms underlying the signalosome of MRGPRX2 may implicate differences in response to stimulation. Epigenetic modifications of MRGPRX2 due to environmental influences represent another possible source of differences in response to the drugs of interest. Although our own studies have not revealed the CpG islands within the promoter region of MGPRX2 gene (Supplementary Figure S3 ), methylation status even of a single CpG locus can modulate protein expression ( 60 ). Post-transcriptional modification of RNAs, including capping, splicing, and polyadenylation, could potentially result in production of MRGPRX2 variants of different properties. However, until now only two transcript variants of MRGPRX2 have been described ( 43 ). Both encode the same protein. Novel RNA transcripts within different cells and tissues can be identified by RACE (rapid amplification of cDNA ends). Alternatively, the expression of MRGPRX2 may vary between individuals or in a single individual, temporarily or constitutively, as it was shown in patients with chronic urticaria, who had a significantly higher number of MRGPRX2+ skin MCs and percentage of MRGPRX2+ MC among all MC in comparison to control subjects ( 46 ). One may speculate that some diseases such as cutaneous or/and systemic mastocytosis alter the physiological levels and response mediated by MRGPRX2 ( 34 ). Also, different MC phenotypes, as already aforementioned, differ in their shape, released mediators as well as their response to stimuli provoking degranulation ( 26 ). FcεRI-mediated mast cell activation involves an inflammatory response mediated by transcription factors (TFs) like AP-1, NF-kB, or NFAT ( 61 ). Currently, it is not known which TFs can regulate MRGPRX2 expression in response to different stimuli (Supplementary Figure S3 ). A number of MC triggering agents have been described, e.g., drugs, food, temperature ( 37 ). It is likely that in some cases, merely the combination of co-factors has a cumulative effect strong enough to achieve a sufficient level of MC activation and to elicit degranulation. During anesthesia several co-factors may be present simultaneously (i.e., opioides, NMBA, temperature), which could explain why only certain patients react to NMBA. The incidence of NMBA-induced anaphylaxis was shown to be higher in the population exposed to pholcodine (a common over-the-counter antitussive). One explanation, widely discussed in published research on the subject, is the presence of IgE-dependent cross-reactivity between NMBA and pholcodine, as both possess similar ammonium structures ( 37 , 62 ). On the other hand tertiary and quaternary ammonium (QA) structures are shared by many compounds, so possibly there is something more specific behind the relation between NMBA and pholcodine. Anothe

GP and MK wrote the paper and analyzed data. KK carried out the experiments. MP helped design the figures. All the authors critically revised the manuscript. Conflict of Interest Statement The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.