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

Evidence-based data support prevention strategies to Hymenoptera venom anaphylaxis

PURPOSE OF REVIEW

This review aims to identify phenotypes at-risk of Hymenoptera venom-induced anaphylaxis (HVA), focusing on different perspectives (epidemiological, clinical, and therapeutic) in order to adapt future preventive strategies.

RECENT FINDINGS

HVA remains one of the leading causes of anaphylaxis, with a broad pattern of symptoms. Although most cases occur outside healthcare settings, data indicate a high emergency admission rate due to insect stings. Mortality is often underestimated because of the lack of witnesses and difficulties in recognizing the signs and the culprit. Targeting risk factors could be a clue to improve these statistics and the prognosis of the disease.Potential risk factors for severe HVA in the European population are basal serum tryptase (BST) above 8 μg, mast cell disorders, the absence of skin symptoms, and cardiovascular conditions requiring the use of beta blockers and ACE inhibitors. Identifying these criteria, mainly based on clinical patterns, helps to develop personalized strategies for management and prevention.

SUMMARY

With a personalized medicine approach, phenotypes must be characterized to adapt to the management of patients suffering from Hymenoptera venom anaphylaxis (HVA), including venom immunotherapy (VIT). In this systematic review, all articles mentioned systemic reactions with heterogeneous severity degrees. Half of those reported grade III-IV systemic reactions (Ring and Messmer). HVA clinical patterns could be worsened by one Hymenoptera sting, a patient's history with mast cell disorders, or cardiovascular diseases. VIT failure was attributed to bee venom extract and monotherapy in two-thirds of publications. Findings stress the difficulty of having uniform epidemiological data on HVA and the lack of financial support in some world regions to support appropriate management of these conditions. Although observing a heterogeneity of data, we were able to identify potential risk factors, in particular for the severe cases. We believe our work will support allergists and health professionals to implement improved personalized management of patients suffering from severe HVA.

Non-IgE-Mediated Immediate Drug-Induced Hypersensitivity Reactions

Immediate drug-induced hypersensitivity reactions (IDHSRs) have conventionally been attributed to an immunoglobulin E (IgE)-mediated mechanism. Nevertheless, it has now been acknowledged that IDHSRs can also occur independently of IgE involvement. Non-IgE-mediated IDHSRs encompass the activation of effector cells, both mast cell-dependent and -independent and the initiation of inflammatory pathways through immunogenic and nonimmunogenic mechanisms. The IDHSRs involve inflammatory mediators beyond histamine, including the platelet-activating factor, which activates multiple cell types, including smooth muscle, endothelium, and MC, and evidence supports its importance in IgE-mediated reactions in humans. Clinically, distinguishing IgE from non-IgE mechanisms is crucial for future treatment strategies, including drug(s) restriction, readministration approaches, and pretreatment considerations. However, this presents significant challenges because certain drugs can trigger both mechanisms, and their presentations can appear similarly, ranging from mild to life-threatening symptoms. Thus, history alone is often inadequate for differentiation, and skin tests lack a standardized approach. Moreover, drug-specific IgE immunoassays have favorable specificity but low sensitivity, and the usefulness of the basophil activation test remains debatable. Lastly, no biomarker reliably differentiates between both mechanisms. Whereas non-IgE-mediated mechanisms likely predominate in IDHSRs, reclassifying most drug-related IDHSRs as non-IgE-mediated, with suggested prevention through dose administration adjustments, is premature and risky. Therefore, continued research and validated diagnostic tests are crucial to improving our capacity to distinguish between these mechanisms, ultimately enhancing patient care.

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.

Recent insights into the mechanisms of anaphylaxis

Anaphylaxis is an acute life-threatening systemic allergic reaction that can have a wide range of clinical manifestations. The most common triggers for anaphylaxis include food, medication, and venom. What is curious regarding anaphylaxis is how so many different agents can induce a severe systemic clinical response but only in a select subgroup of patients. Over the past decade, several important advances have been made in understanding the underlying cellular and molecular mechanisms contributing to anaphylaxis, with mast cells (MCs) being an essential component. Classically, cross-linked immunoglobulin E (IgE) bound to its high- affinity receptor induces MC mediator release. However, toll-like, complement, or Mas-related G-protein-coupled receptors also activate mouse and human MCs. While anaphylaxis secondary to foods historically has been more extensively characterized clinically and mechanistically, more recent studies have shifted focus toward understanding drug-induced anaphylaxis. The focus of this review is to highlight recent basic science developments and compare what is currently known regarding anaphylaxis to food, medications, and venom.

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.

The Number of MRGPRX2-Expressing Cells Is Increased in Skin Lesions of Patients With Indolent Systemic Mastocytosis, But Is Not Linked to Symptom Severity

Background

Recently, the expression of the mast cell (MC) receptor Mas-related G protein–coupled receptor X2 (MRGPRX2) has been detected in lesional skin of adult patients with cutaneous mastocytosis. As of yet, little is known about the clinical relevance of MRGPRX2 and its agonists in patients with mastocytosis, including indolent systemic mastocytosis (ISM).

Methods

MRGPRX2 and MRGPRX2 agonists, cortistatin (CST), and major basic protein (MBP) were analyzed in lesional and non-lesional skin of patients with ISM and skin of healthy controls by immunohistochemistry. Co-localization of MRGPRX2 and MRGPRX2-mRNA with the MC marker tryptase was assessed by immunofluorescence microscopy and in situ hybridization, respectively. We assessed clinical, demographic, and laboratory data, including mastocytosis activity score (MAS), serum tryptase, and KIT D816V allele burden.

Results

The number of MRGPRX2-expressing (MRGPRX2+) cells, MRGPRX2-mRNA+ MCs, and CST-expressing (CST+) and MBP-expressing (MBP+) cells was significantly higher in lesional skin as compared to non-lesional skin and/or skin of healthy controls (all p < 0.05). Increased numbers of MRGPRX2+ cells, MRGPRX2-mRNA+ MCs, and CST+ and MBP+ cells were not associated with clinical and laboratory features of ISM, including disease burden, symptom severity, evidence of anaphylaxis, and tryptase levels.

Conclusions

Skin lesions of patients with ISM showed high numbers of MRGPRX2+ cells, although they were not linked to symptom severity. Clinical relevance of the MRGPRX2-mediated pathway of MC activation in ISM remains unclear and should be investigated in further studies.