Mast Cell-Targeting Therapies in Mast Cell Activation Syndromes.

PURPOSE OF REVIEW: Provide an overview of the expanding landscape of mast cell (MC)-targeting treatments in mast cell activation syndromes (MCAS). RECENT FINDINGS: Tyrosine-kinase inhibitors (TKIs) targeting wild-type and mutated KIT can efficiently induce MC depletion. Avapritinib and midostaurin can also temper IgE-mediated degranulation. Avapritinib has been recently approved by the FDA for the treatment of indolent systemic mastocytosis (ISM). Targeting activation pathways and inhibitory receptors is a promising therapeutic frontier. Recently, the anti Siglec-8 antibody lirentelimab showed promising results in ISM. MCAS is a heterogeneous disorder demanding a personalized therapeutic approach and, especially when presenting as anaphylaxis, has not been formally captured as outcome in prospective clinical trials with TKI. Long-term safety of TKI needs to be addressed. New drugs under investigation in diseases in which non-neoplastic MCs play a pivotal role can provide important inputs to identify new efficient and safe treatments for MCAS.

Comparison of the passive mast cell activation test with the basophil activation test for diagnosis of perioperative rocuronium hypersensitivity.

BACKGROUND: Rocuronium is a major cause of perioperative hypersensitivity (POH). Skin tests (STs) and quantification of specific immunoglobulin E antibodies (sIgEs) can yield incongruent results. In such difficult cases, the basophil activation test (BAT) can be helpful. Here, we evaluated the passive mast cell activation test (pMAT) as a substitute of BAT as part of the diagnostic tests for rocuronium allergy. METHODS: Sera from patients with a suspected POH reaction potentially related to rocuronium were included. All patients had a complete diagnostic investigation, including STs, quantification of sIgEs to morphine and rocuronium, and BAT. For execution of pMAT, human mast cells were generated from healthy donor peripheral blood CD34+ progenitor cells and sensitised overnight with patient sera. RESULTS: In total, 90 sera were studied: 41 from ST+sIgE+ patients, 13 from ST-sIgE- patients, 20 from ST+sIgE- patients, and 16 from ST-sIgE+ patients. According to BAT results, patients were further allocated into subgroups. Of the 38 BAT+ patients, 25 (66%) showed a positive pMAT as well. Of the 44 BAT- patients, 43 (98%) also showed a negative pMAT. Mast cells that were not passively sensitised did not respond to rocuronium. CONCLUSIONS: We show that the pMAT, in many cases, can substitute for BAT in the diagnosis of rocuronium hypersensitivity and advance diagnosis in difficult cases with uncertain ST or sIgE results when BAT is not locally available.

A quarter of a century fundamental and translational research in perioperative hypersensitivity and anaphylaxis at the Antwerp university hospital, a Belgian Centre of Excellence of the World Allergy Organization.

Perioperative hypersensitivity constitutes an important health issue, with potential dramatic consequences of diagnostic mistakes. However, safe and correct diagnosis is not always straightforward, mainly because of the application of incorrect nomenclature, absence of easy accessible in-vitro/ex-vivo tests and uncertainties associated with the non-irritating skin test concentrations. In this editorial we summarize the time line, seminal findings, and major realizations of 25 years of research on the mechanisms, diagnosis, and management of perioperative hypersensitivity.

Recent Knowledge and Insights on the Mechanisms of Immediate Hypersensitivity and Anaphylaxis: IgE/FcεRI- and Non-IgE/FcεRI-Dependent Anaphylaxis.

Immediate hypersensitivity reactions can pose a clinical and diagnostic challenge, mainly because of the multifarious clinical presentation and distinct underlying - frequently uncertain - mechanisms. Anaphylaxis encompasses all rapidly developing and life-threatening signs and may cause death. Evidence has accumulated that immediate hypersensitivity and anaphylaxis do not necessarily involve an allergen-specific immune response with cross-linking of specific IgE (sIgE) antibodies bound to their high-affinity IgE receptor (FcεRI) on the surface of mast cells (MCs) and basophils. Immediate hypersensitivity and anaphylaxis can also result from alternative specific and nonspecific MC and basophils activation and degranulation, such as complementderived anaphylatoxins and off-target occupancy of MC and/or basophil surface receptors such as the Masrelated G protein-coupled receptor X2 (MRGPRX2). Degranulation of MCs and basophils results in the release of inflammatory mediators, which can be, depending on the underlying trigger, in a different spatiotemporal manner. In addition, hypersensitivity and anaphylaxis can occur entirely independently of MC and basophil degranulation, as observed in hypersensitivity to nonsteroidal anti-inflammatory drugs (NSAIDs) that divert normal arachidonic acid metabolism by inhibiting the cyclooxygenase (COX)-1 isoenzyme. Finally, one should remember that anaphylaxis might be part of the phenotype of particular - sometimes poorly recognizable - conditions such as clonal MC diseases (e.g. mastocytosis) and MC activation syndrome. This review provides a status update on the molecular mechanisms involved in both sIgE/FcεRI- and non-sIgE/FcεRI-dependent immediate hypersensitivity and anaphylaxis. In conclusion, there is increasing evidence for alternative pathophysiological hypersensitivity and anaphylaxis endotypes that are phenotypically and biologically indistinguishable, which are frequently difficult to diagnose, mainly because of uncertainties associated with diagnostic tests that might not enable to unveil the underlying mechanism.

Allergenic and Mas-Related G Protein-Coupled Receptor X2-Activating Properties of Drugs: Resolving the Two.

Since the seminal description implicating occupation of the Mas-related G protein-coupled receptor X2 (MRGPRX2) in mast cell (MC) degranulation by drugs, many investigations have been undertaken into this potential new endotype of immediate drug hypersensitivity reaction. However, current evidence for this mechanism predominantly comes from (mutant) animal models or in vitro studies, and irrefutable clinical evidence in humans is still missing. Moreover, translation of these preclinical findings into clinical relevance in humans is difficult and should be critically interpreted. Starting from our clinical priorities and experience with flow-assisted functional analyses of basophils and cultured human MCs, the objectives of this rostrum are to identify some of these difficulties, emphasize the obstacles that might hamper translation from preclinical observations into the clinics, and highlight differences between IgE- and MRPGRX2-mediated reactions. Inevitably, as with any subject still beset by many questions, alternative interpretations, hypotheses, or explanations expressed here may not find universal acceptance. Nevertheless, we believe that for the time being, many questions remain unanswered. Finally, a theoretical mechanistic algorithm is proposed that might advance discrimination between MC degranulation from MRGPRX2 activation and cross-linking of membrane-bound drug-reactive IgE antibodies.

Basophil Activation Test Shows Poor Sensitivity in Immediate Amoxicillin Allergy.

BACKGROUND: In light of the pandemic of spurious penicillin allergy, correct diagnosis of amoxicillin (AX) allergy is of great importance. The diagnosis of immediate hypersensitivity reactions relies on skin tests and specific IgE, and although reliable, these are not absolutely predictive. Therefore, drug challenges are needed in some cases, which contain the risk of severe reactions. Safe in vitro diagnostics as an alternative for the drug challenge in the diagnostic workup of AX allergy would be more than welcome to fill this gap. In this respect, the basophil activation test (BAT) has shown potential, but its clinical reliability is doubtful. OBJECTIVE: To investigate the reliability of the BAT to AX and determining its exact place in the diagnostic algorithm of AX allergy. METHODS: BAT for AX was performed in 70 exposed control individuals and 66 patients diagnosed according to the European Academy of Allergy and Clinical Immunology guidelines for AX allergy. Upregulation of both CD63 and CD203c was flow-cytometrically assessed. RESULTS: Analyses revealed that 1370 µmol/L and 685 µmol/L were the most discriminative stimulation concentrations for CD63 and CD203c upregulation, respectively, and a diagnostic threshold of 9% for positivity for both markers was identified. At these concentrations, sensitivity and specificity for CD63 upregulation were 13% and 100%, respectively, and for CD203c upregulation, 23% and 98%. CONCLUSIONS: BAT with dual analysis of CD63 and CD203c is of poor performance to document AX allergy. The sensitivity is too low to let it occupy a prominent role in the diagnostic algorithm.

Mastocytosis and related entities: a practical roadmap.

Mastocytosis is a complex heterogenous multisystem disorder that is characterized by pathologic activation or accumulation of neoplastic mast cells (MCs) in one or more organs. This clonal MC expansion is often associated with a somatic gain-of-function mutation (D816V in most of the cases) in the KIT gene, encoding for the MC surface receptor KIT (CD117), a stem cell growth factor receptor. Based on clinical and biochemical criteria, the World Health Organization (WHO) divided mastocytosis into different subclasses. The exact prevalence of mastocytosis remains elusive, but it is estimated that the disease affects approximately 1 in 10,000 persons. The clinical presentation of mastocytosis varies significantly, ranging from asymptomatic patients to a life-threatening disease with multiple organ involvement, potentially leading to cytopenia, malabsorption, hepatosplenomegaly, lymphadenopathy, ascites or osteolytic bone lesions with pathological fractures. Patients with mastocytosis may experience symptoms related to release of MC mediators, such as flushing or diarrhea or even more severe symptoms such as anaphylaxis. Recently, a new genetic trait, hereditary alpha tryptasemia (HaT), was described which involves a copy number variation in the TPSAB1-gene. Its role as standalone multisystem syndrome is heavily debated. There is emerging evidence suggesting there might be a link between HaT and due to the increased prevalence of HaT in patients with SM. The aim of this review is to provide a practical roadmap for diagnosis and management of mastocytosis and its associated entities, since there are still many misconceptions about these topics.Abbreviations: AdvSM: Advanced systemic mastocytosis; ASM: Aggressive systemic mastocytosis; aST: acute serum tryptase; BM: Bone marrow; BMM: Bone marrow mastocytosis; bST: baseline serum tryptase; CM: Cutaneous mastocytosis; DCM: Diffuse cutaneous mastocytosis; HVA: Hymenoptera venom allergy; HaT: Hereditary alpha tryptasemia; ISM: Indolent systemic mastocytosis; MC: Mast cell; MCA: Mast cell activation; MCAS: Mast cell activation syndrome; MCL: Mast cell leukemia; MIS: Mastocytosis in the skin; MMAS: Monoclonal mast cell activation syndrome; MPCM: Maculopapular cutaneous mastocytosis; SM: Systemic mastocytosis; SM-AHN: Systemic mastocytosis with associated hematological neoplasm; SSM: Smouldering systemic mastocytosis; VIT: Venom immunotherapy.

Establishing diagnostic strategies for cannabis allergy.

INTRODUCTION: Cannabis is the most widely consumed illicit drug in the world and carries a risk of severe IgE-mediated allergic reactions, requiring appropriate diagnostic management. Currently available diagnostics are still relatively limited and require careful interpretation of results to avoid harmful over- and underdiagnosis. AREAS COVERED: This review focuses on the most up-to-date understandings of cannabis allergy diagnosis, starting with the main clinical features of the disease and the allergenic characteristics of Cannabis sativa, and then providing insights into in vivo, in vitro, and ex vivo diagnostic tests. EXPERT OPINION: At present, the diagnosis of IgE-mediated cannabis allergy is based on a three-step approach that starts with accurate history taking and ends with a confirmation of sensitization to the whole extract and, finally, molecular components. Although much has been discovered since its first description in 1971, the diagnosis of cannabis allergy still has many unmet needs. The lack of commercial standardized and validated extracts and in vitro assays makes a harmonized workup of cannabis allergy difficult. Furthermore, the epidemiological characteristics, and clinical implications of sensitization to different molecular components are not yet fully known. Future research will complete the picture and likely result in an individualized and standardized approach.

Flow-based allergen testing: Can mast cells beat basophils?

The basophil activation test (BAT) has emerged as a reliable complementary diagnostic to document IgE-dependent allergies and to study cross-reactivity between structural homologues. However, the BAT has some weaknesses that hinder a wider application. The BAT requires fresh blood samples and is lost as a diagnostic in patients showing a non-responder status of their cells. The BAT is difficult to standardize mainly because of the difficulty to perform batch analyses. In contrast, mast cell activation tests (MATs), using passively sensitized mast cells (MCs) with patients' sera (henceforth indicated as passive MAT; pMAT), use serum samples that can be frozen, stored, and shipped to a reference center experienced in MC lines and/or cultures and capable of offering batch testing. With the recent recognition of the Mas-related G protein-coupled receptor X2 (MRGPRX2) occupation as a putative mechanism of immediate drug hypersensitivity reactions, the MAT has another advantage compared to the BAT. MCs, in contrast to resting basophils, express the MRGPRX2 and can therefore be used to study this IgE-independent mechanism. This review provides a status update of pMAT in the diagnosis of allergic IgE-mediated hypersensitivity and speculates how direct activation of MCs via the MRGPRX2 receptor could advance paradigms for this non-allergic hypersensitivity.

Mast cell activation test: A new asset in the investigation of the chlorhexidine cross-sensitization profile.

BACKGROUND: Insights into the IgE cross-sensitization and possible cross-reactivity patterns of sera reactive to chlorhexidine (CHX) are still incomplete and are likely to benefit from a functional exploration using a passive mast cell activation test (pMAT). Therefore, we want to study whether the pMAT with CHX-specific IgE (sIgE) enables to depict effector cell degranulation in response to alexidine (ALX), octenidine (OCT) and/or polyhexamethylene biguanide (PHMB) indicative of cross-reactivity between these compounds and CHX. METHODS: Serum of 10 CHX-allergic patients, nine individuals with an isolated sIgE CHX and five healthy controls were included. Human cultured mast cells (MCs) were, before and after sensitization, challenged with CHX, ALX, OCT or PHMB. Degranulation was measured via quantification of upregulation of CD63. RESULTS: Mast cell responsiveness to ALX and OCT was demonstrable with 4/10 and 3/10 of the sera of CHX-allergic patients respectively. Percentage of degranulation varied between 12 and 34% for ALX-reactive MCs and between 4 and 22% for OCT-reactive MCs. No reactivity to ALX or OCT was demonstrable when using sera obtained from individuals with an isolated sIgE CHX or from healthy controls. Unlike CHX, ALX and OCT, PHMB turned out to be a direct MC activator via occupation of MRGPRX2. PHMB-reactive sIgEs were demonstrable in some patients with an isolated sIgE CHX but were unable to trigger PHMB-induced degranulation in MRGPRX2 knockdown MCs. CONCLUSION: Mast cells constitute an attractive tool to explore cross-reactivity between structurally similar compounds. Along with the identification of safe alternatives for the individual patient, the pMAT can advance our insights into sIgE cross-reactivity patterns including assessment of molecules not yet approved for human use.

Overexpression of FcεRI on Bone Marrow Mast Cells, but Not MRGPRX2, in Clonal Mast Cell Disorders With Wasp Venom Anaphylaxis.

Background: Uncertainties remain about the molecular mechanisms governing clonal mast cell disorders (CMCD) and anaphylaxis. Objective: This study aims at comparing the burden, phenotype and behavior of mast cells (MCs) and basophils in patients with CMCD with wasp venom anaphylaxis (CMCD/WVA+), CMCD patients without anaphylaxis (CMCD/ANA-), patients with an elevated baseline serum tryptase (EBST), patients with wasp venom anaphylaxis without CMCD (WVA+) and patients with a non-mast cell haematological pathology (NMHP). Methods: This study included 20 patients with CMCD/WVA+, 24 with CMCD/ANA-, 19 with WVA+, 6 with EBST and 5 with NMHP. We immunophenotyped MCs and basophils and compared baseline serum tryptase (bST) and both total and venom specific IgE in the different groups. For basophil studies, 13 healthy controls were also included. Results: Higher levels of bST were found in CMCD patients with wasp venom anaphylaxis, CMCD patients without anaphylaxis and EBST patients. Total IgE levels were highest in patients with wasp venom anaphylaxis with and without CMCD. Bone marrow MCs of patients with CMCD showed lower CD117 expression and higher expression of CD45, CD203c, CD63, CD300a and FcεRI. Within the CMCD population, patients with wasp venom anaphylaxis showed a higher expression of FcεRI as compared to patients without anaphylaxis. Expression of MRGPRX2 on MCs did not differ between the study populations. Basophils are phenotypically and functionally comparable between the different patient populations. Conclusion: Patients with CMCD show an elevated burden of aberrant activated MCs with a significant overexpression of FcεRI in patients with a wasp venom anaphylaxis.

Mas-related G protein-coupled receptor MRGPRX2 in human basophils: Expression and functional studies.

Background: Occupancy of MRGPRX2 heralds a new era in our understandings of immediate drug hypersensitivity reactions (IDHRs), but a constitutive expression of this receptor by basophils is debated. Objective: To explore the expression and functionality of MRGPRX2 in and on basophils. Methods: Basophils from patients with birch pollen allergy, IDHRs to moxifloxacin, and healthy controls were studied in different conditions, that is, in rest, after stimulation with anti-IgE, recombinant major birch pollen allergen (rBet v 1), moxifloxacin, fMLP, substance P (SP), or other potential basophil secretagogues. In a separate set of experiments, basophils were studied after purification and resuspension in different media. Results: Resting whole blood basophils barely express MRGPRX2 on their surface and are unresponsive to SP or moxifloxacin. However, surface MRGPRX2 is quickly upregulated upon incubation with anti-IgE or fMLP. Pre-stimulation with anti-IgE can induce a synergic effect on basophil degranulation in IgE-responsive subjects after incubation with SP or moxifloxacin, provided that basophils have been obtained from patients who experienced an IDHR to moxifloxacin. Cell purification can trigger a "spontaneous" and functional upregulation of MRGPRX2 on basophils, not seen in whole blood cells, and its surface density can be influenced by distinct culture media. Conclusion: Basophils barely express MRGPRX2 in resting conditions. However, the receptor can be quickly upregulated after stimulation with anti-IgE, fMLP, or after purification, making cells responsive to MRGPRX2 occupation. We anticipate that such "conditioned" basophils constitute a model to explore MRGPRX2 agonism or antagonism, including IDHRs originating from the occupation of this receptor.