Impact of naturally forming human α/β-tryptase heterotetramers in the pathogenesis of hereditary α-tryptasemia

Hereditary α-tryptasemia is associated with anaphylaxis to antibiotics and monoclonal antibodies

BACKGROUND

Hereditary α-tryptasemia, a genetic trait caused by increased α-tryptase copy number, is associated with idiopathic and venom anaphylaxis.

OBJECTIVE

We aimed to determine the impact of tryptase genotypes on drug-induced anaphylaxis.

METHODS

A prospective discovery cohort of 99 patients from a referral center in Slovenia with acute anaphylaxis to drugs underwent tryptase genotyping by droplet digital PCR. For validation, we included a cohort of 26 patients from the Czech Republic. Associated inciting agents and the severity of the reactions were subsequently examined.

RESULTS

Hereditary α-tryptasemia was associated with drug-induced anaphylaxis with a prevalence of 13% (n = 13 of 99) in the discovery cohort and 15% in the validation cohort (n = 4 of 26). Hereditary α-tryptasemia was identified in every individual with elevated basal serum tryptase levels (11.6-21.9 ng/mL; n = 14) within both cohorts of patients. Hereditary α-tryptasemia was more prevalent in individuals with antibiotic- or mAb-induced anaphylaxis in both the discovery and validation cohorts (n = 13 of 51; 26%) compared to those with anaphylaxis resulting from neuromuscular blocking agents, nonsteroidal anti-inflammatory drugs, contrast, chlorhexidine, or other drugs (n = 5 of 74; 7%; P = .02; odds ratio = 4.1; 95% CI, 1.3-11.1). Overall, we found fewer individuals with no ⍺-tryptase than in the general population, and there was a trend for subjects with more ⍺-tryptase copies to have more severe reactions. Thus, among subjects with three ⍺-tryptase copies, the prevalence of severe anaphylaxis was 73%, compared with 59% with one to two ⍺-tryptase copies and 58% for subjects without ⍺-tryptase.

CONCLUSIONS

Risk for anaphylaxis to antibiotics and biologics is associated with inherited differences in α-tryptase-encoding copies at Tryptase α/β1.

Neurogenic Inflammation in Allergic Contact Dermatitis

Allergic contact dermatitis (ACD) is a skin condition characterized by inflammation resulting from hypersensitivity upon contact with certain allergens. Although ACD is characterized by an immune-mediated pathomechanism, the involvement of the nervous system in this condition has increasingly been considered, particularly in the amplification and persistence of inflammation. This paper aims to present a comprehensive overview of the mechanisms involved in neurogenic inflammation in ACD, focusing on the role of sensory neurons, the release of neuropeptides, their interaction with immune cells, and the potential therapeutic implications related to neurogenic pathways, diversified by age and gender. Innovative therapies for ACD, including topical formulations, may target the mass-bound X2 G-protein-coupled receptor (MRGPRX2) and endocannabinoid systems.

First demonstration of implication of hereditary alpha-tryptasaemia in vasospastic angina: a case report

Background

Vasospastic angina is a common condition. In cases of poor therapeutic response, less common causes should be explored.

Case summary

A 50-year-old woman with vasospastic angina was diagnosed with significant fluctuation in response to treatment without explanation that led to the suspicion of an allergic phenomenon. A diagnosis of hereditary alpha-tryptasaemia was made, and introduction of a second-generation H1-antihistamine has enabled effective control of previously treatment-resistant vasospastic coronary disease.

Discussion

The case shows the first time the involvement of hereditary alpha-tryptasaemia in vasospastic angina. Future pathophysiological investigations will be needed to further explore the connection between these two pathologies.

Cold-induced anaphylaxis: new insights into clinical and genetic characteristics

Introduction

The pathogenesis of cold urticaria (ColdU) and cold-induced anaphylaxis (ColdA) remains poorly understood, and ColdA is underrepresented in anaphylaxis literature. Laboratory features to guide management are largely unknown. This study evaluated basal serum tryptase (BST) and total immunoglobulin E (IgE) levels in ColdU and ColdA, their associations with clinical features, and the utility of testing for the KIT p.D816V variant in blood leukocytes and hereditary α-tryptasemia (HαT).

Methods

Ninety-two adults with ColdU were enrolled. ColdA was defined as a reaction involving skin and/or visible mucosal tissue with cardiovascular, respiratory, or gastrointestinal manifestations. Evaluations included patient history, standard cold stimulation testing (sCST) using an ice cube and TempTest®, and laboratory tests.

Results

ColdA was diagnosed in 35.9% of patients. ColdU phenotypes based on sCST included typical ColdU (52.2%), localized cold-reflex urticaria (5.4%), and ColdU with negative sCST (42.4%). Negative sCST, compared to typical ColdU, was associated with fewer ColdA cases (p = 0.004) but more spontaneous wheals (p < 0.001). ColdA patients more frequently exhibited generalized wheals (p = 0.047), skin angioedema (p = 0.007), oropharyngeal/laryngeal manifestations (p < 0.001), and itchy earlobes (p = 0.002) than non-ColdA patients. Elevated BST levels (>11.4 ng/mL) in 9.8% of patients were attributed to KIT p.D816V and/or HαT. KIT p.D816V was detected in 6.6% of ColdU and 6.3% of ColdA patients. HαT prevalence was higher in ColdU (10.9%) and ColdA (15.2%) than the general population (estimated at 5.7%; p = 0.041 and p = 0.038). Total IgE levels were significantly higher in ColdA than non-ColdA (p = 0.021).

Discussion

This study confirmed clinical features linked to ColdA previously identified by the multicenter COLD-CE study, including generalized wheals, skin angioedema, oropharyngeal/laryngeal manifestations, and itchy earlobes. We identified new high-risk features. ColdA is more frequently associated with typical ColdU than with ColdU with negative sCST, the latter being linked to spontaneous wheals. ColdA is additionally associated with higher total IgE levels. Furthermore, patients with ColdU and ColdA exhibit higher prevalence of KIT p.D816V and HαT compared to general population data, a finding not previously reported. Further research is needed to explore their clinical implications.

Mast Cells at the Crossroads of Hypersensitivity Reactions and Neurogenic Inflammation

Although mast cells have long been known, they are not yet fully understood. They are traditionally recognized for their role in allergic reactions through the IgE/FcεRI axis, but different groups of surface receptors have since been characterized, which appear to be involved in the manifestation of peculiar clinical features. In particular, MRGPRX2 has emerged as a crucial receptor involved in degranulating human skin mast cells. Because of mast cells' close proximity to peripheral nerve endings, it may play a key role in neuroimmune interactions. This paper provides an overview of mast cell contributions to hypersensitivity and so-called "pseudoallergic" reactions, as well as an update on neuroinflammatory implications in the main models of airway and skin allergic diseases. In particular, the main cellular characteristics and the most relevant surface receptors involved in MC pathophysiology have been reappraised in light of recent advancements in MC research. Molecular and clinical aspects related to MC degranulation induced by IgE or MRGPRX2 have been analyzed and compared, along with their possible repercussions and limitations on future therapeutic perspectives.

Appraisal of the evidence linking hereditary α-tryptasemia with mast cell disorders, hypermobility and dysautonomia

Since its first description more than a decade ago, our understanding of the clinical impact of hereditary alpha-tryptasemia has continued to evolve. First considered to be a genetic disorder with a subset of patients having a syndromic presentation composed of connective tissue abnormalities, symptoms of autonomic dysfunction, and findings of mast cell activation, we now know that hereditary alpha-tryptasemia is a common genetic trait and modifier of mast cell-mediated reactions. More recent studies have shown some previously held associations with congenital hypermobility and postural orthostatic tachycardia syndrome (POTS) to be lacking, and illuminated previously unappreciated associations with clonal and nonclonal mast cell disorders. With the discovery of heterotetrameric tryptases and demonstration of their unique functional activities, the importance of tryptase gene composition in general has begun to take focus. Hereditary alpha-tryptasemia exists at the end of a spectrum of alpha-tryptase expression and as a natural overexpression model of this protein, brought to the fore the potential of tryptase genotyping as a genetic biomarker for anaphylaxis severity. These data and future studies hold the promise of enhancing our understanding of the role that tryptases play in health and disease.

Hereditary alpha tryptasemia: elevated tryptase, female sex, thyroid disorders, and anaphylaxis

Introduction

The clinical significance of elevated baseline serum tryptase (BST) in the absence of mast cell disorders or allergic reactions has long been unclear. Recently, a genetic variation of the TPSAB1 gene, which among others encodes for alpha tryptase, has been reported and named hereditary alpha tryptasemia (HaT). HaT has been linked to various manifestations, including severe allergic reactions. However, clinical studies are limited. In this study, we aimed to determine HaT prevalence and characterize its clinical manifestations in patients at a specialized allergy center.

Methods

From January 2022 to December 2023, patients with elevated BST at least once were screened for HaT at the outpatient clinic. A control group included patients with a history of anaphylaxis undergoing specific Hymenoptera immunotherapy. TPSAB1 copy numbers, BST levels, and clinical parameters were assessed and analyzed.

Results

Of 47 patients with elevated BST (≥11.4 µg/L), 93% showed increased TPSAB1 copy numbers. Individuals diagnosed with HaT displayed a BST range between 12.3 and 28.4 µg/L, with 84.1% associated with TPSAB1 duplication and 15.9% with triplication. HaT predominated in women (86.4%) and was associated with thyroid disease (27.3%). Over half had a history of anaphylaxis (54.5%), which was mainly low-grade.

Discussion

In patients with elevated BST but no mastocytosis, the most likely cause of elevated BST was an increase in the copy number of the TPSAB1 gene. A heightened risk of anaphylaxis should be considered. Further research is needed to explore the predominance of women and the emerging link with thyroid disease.

Prevalence of Autoantibodies in Patients with Hereditary Alpha-Tryptasemia

INTRODUCTION

Hereditary alpha-tryptasemia (HαT) is associated with postural orthostatic tachycardia syndrome (POTS), hypermobile Ehlers-Danlos syndrome (hEDS), and mast cell activation syndrome (MCAS). While POTS, hEDS, and MCAS have all demonstrated increased prevalence of autoimmunity, this has not been investigated in HαT populations. Our objective was to describe the prevalence of autoantibodies in individuals with HαT.

METHODS

We retrospectively studied a cohort of patients with positive genotyping for HαT at a tertiary-care allergy clinic. Demographic data including previous autoimmune history and autoantibody serologies were extracted on chart review. A literature search was conducted to determine the prevalence of specific autoimmune and autoantibody prevalences in the general population. We compared the proportions of autoantibody positivity and established autoimmune diseases in our cohort of HαT individuals against those in general populations.

RESULTS

We identified 101 patients with HαT. Median age was 43 years (range 15-75), and most were female (87/101; 86.1%). Prevalence of self-reported drug hypersensitivity was 52/101 (52.5%) patients. The proportion of individuals with HαT with positive tTG antibody (3/61, 4.9%) was significantly higher than that reported in the general population (133/16,667, 0.8%) (p < 0.001). The prevalence of systemic lupus erythematosus (SLE) (1/101, 1%) and celiac disease (5/101, 5%) in our cohort were found to be significantly higher than the prevalence in the general population (194/96,996, 0.2% [p = 0.035] and 26/2,845, 0.9% [p < 0.001], respectively).

CONCLUSION

Patients with HαT have increased prevalence of celiac disease, SLE, and positive anti-tTG serology, as well as self-reported drug hypersensitivity, relative to general populations.

Tryptase in drug-induced anaphylaxis: the need for acute and baseline values

PURPOSE OF REVIEW

The purpose of this narrative review was to summarize data and official recommendations purporting to paired tryptase determination in patients experiencing drug-induced anaphylaxis, published between January 1, 2023 and June 1, 2024.

RECENT FINDINGS

Three main lines of evidence obtained through paired acute and baseline tryptase determination were identified: diagnostic criterion for hypersensitivity reactions involving systemic mast cell activation; differential diagnostic criterion for hypersensitivity reactions involving other mechanisms of immediate reactions; and added value of acute and baseline tryptase levels for personalized management following drug-induced anaphylaxis: cause, risk of recurrence, underlying mast cell conditions including hereditary α-tryptasemia, familial clusters.

SUMMARY

The implementation of existing guidelines which consensually recommend paired tryptase measurement is a persistent unmet need hampering optimal diagnosis of drug-induced anaphylaxis and patient management. Another major unmet need is the lack of standardized recommendations for hereditary α-tryptasemia testing and counselling. Progress in this field is seen at a rapid pace, requiring significant efforts of continued medical education for practicing clinicians and laboratory specialists worldwide.

High burden of clonal mast cell disorders and hereditary α-tryptasemia in patients who need Hymenoptera venom immunotherapy

BACKGROUND

In patients who require venom immunotherapy (VIT), there is a need to identify underlying mast cell (MC) disorders since these may affect the risk and severity of future sting reactions and the long-term effectiveness of VIT.

METHODS

1319 individuals with Hymenoptera venom allergy (HVA) who needed VIT from referral centers in Slovenia, Austria, Croatia, and Poland underwent examination for KIT p.D816V in peripheral blood leukocytes (PBL) using a highly sensitive PCR test and tryptase genotyping by digital droplet PCR. We also included 183 control individuals with large local reactions (LLRs) to Hymenoptera stings and with asymptomatic sensitization to Hymenoptera venoms.

RESULTS

285 of 1319 individuals recommended for VIT (21.6%) were positive for KIT p.D816V in PBL, preferably those who present with severe reaction (33.9% [n = 207 of 610] with Ring-Messmer grade 3-4 vs. 11% [n = 78 of 709] with Grade 1-2; p < .0001), whereas only 1.3% (n = 2 of 152) of controls with LLR and none with asymptomatic sensitization (n = 31) had KIT p.D816V. KIT p.D816V allelic burden was higher in those with severe reaction (median 0.018% [n = 207] in Grade 3-4 vs. 0.001% [n = 78] in Grade 1-2; p < .0001), and the majority had normal baseline serum tryptase levels (69% [n = 196 of 285]). All KIT p.D816V-positive individuals (n = 41) who underwent bone marrow (BM) biopsy were found to have underlying clonal diseases, principally BM mastocytosis. HαT was also associated with severe HVA and symptoms (p < .01), and remarkably, 31.0% (n = 31 of 100) were found to have concomitant KIT p.D816V. Concomitant HαT and KIT p.D816V showed an additive effect, and having both was associated with the highest risk for severe HVA, even higher than having either HαT or KIT p.D816V alone (OR = 3.8; p < .01).

CONCLUSIONS

By employing prospective universal tryptase genotyping and examination for KIT p.D816V in PBL in large HVA populations, we have demonstrated a high burden of clonal MC disorders and HαT in patients who require VIT.

Mast cell activation syndrome: Current understanding and research needs

Mast cell activation syndrome (MCAS) is a term applied to several clinical entities that have gained increased attention from patients and medical providers. Although several descriptive publications about MCAS exist, there are many gaps in knowledge, resulting in confusion about this clinical syndrome. Whether MCAS is a primary syndrome or exists as a constellation of symptoms in the context of known inflammatory, allergic, or clonal disorders associated with systemic mast cell activation is not well understood. More importantly, the underlying mechanisms and pathways that lead to mast cell activation in MCAS patients remain to be elucidated. Here we summarize the known literature, identify gaps in knowledge, and highlight research needs. Covered topics include contextualization of MCAS and MCAS-like endotypes and related diagnostic evaluations; mechanistic research; management of typical and refractory symptoms; and MCAS-specific education for patients and health care providers.

Detecting Changes in Mast Cell Numbers Versus Activation in Human Disease: A Roadblock for Current Biomarkers?

The pathophysiology of mast cell (MC)-driven disorders is diverse, ranging from localized reactions to systemic disorders caused by abnormal accumulation and activation in multiorgan systems. Prompt and accurate diagnosis is critically important, both for informing treatment and objective assessment of treatment outcomes. As new therapeutics are being developed to deplete MCs or silence them (eg, by engaging inhibitory receptors that block activation), new biomarkers are needed that can distinguish between MC activation versus burden. Serum tryptase is the gold standard for assessing both MC burden and activation; however, commercial tryptase assays have limitations related to timing of release, lack of discernment between inactive (α) and active (β) forms of tryptase, and interpatient variability of baseline levels. Alternative approaches to measuring MC activation include urinary MC mediators, flow cytometry-based assays or gene expression profiling. Additional markers of MC activation are needed for use in clinical diagnostics, to help selection of treatment of MC diseases, and for assessing outcomes of therapy. We review the spectrum of disorders with known or suspected MC contribution, describe the utility and limitations of current MC markers and assays, and discuss the need for new markers that can differentiate between MC activation and burden.

The Clinical Features of Hereditary Alpha-Tryptasemia—Implications for Interdisciplinary Practice

BACKGROUND

Hereditary alpha-tryptasemia (HAT) is a genetic predisposition of autosomal dominant inheritance that leads to a high normal (≥ 8-11.4 μg/L) or pathologically elevated (>11.4 μg/L) basal serum tryptase (BST) concentration. Its prevalence in the United Kingdom and France is reportedly 5%-6%; its prevalence in Germany is unknown. Symptomatic persons with HAT suffer from a complex constellation of symptoms. As described in this review, HAT is an important differential diagnosis in interdisciplinary practice.

METHODS

This review is based on publications about HAT retrieved by a selective search in PubMed, on relevant presentations at scientific meetings, and on our clinical experience. We also collected our own data on the prevalence and clinical manifestations of HAT.

RESULTS

According to the literature, HAT is very common among patients in medical centers with BST values of 8 μg/L or above (64-74%). HAT is most commonly associated with neuropsychiatric symptoms such as exhaustion (85%), depressive episodes (59%), sleep disturbances (69%), and memory impairment (59%-68%), followed by gastrointestinal symptoms such as irritable bowel (30%-60%), nausea (51%), and reflux (49%-77%). Typical mast cell-mediated symptoms, such as flushing (47%), itch (69%), urticaria (37%), and anaphylaxis (14%-28%), are reported as well. Less commonly reported are cardio vascular manifestations, such as hypotonia, dizziness, and tachycardia (34%), and joint hyper - mobility (28%). HAT is more common among patients with systemic mastocytosis (SM; 12%-21%). It is often associated with severe anaphylaxis induced by insect toxins or unknown triggers. The therapeutic options include treatment with antihistamines, mastcell stabilizers, or IgE antibodies.

CONCLUSION

A diagnosis of hereditary alphatryptasemia can be strongly suspected on the basis of thorough history-taking and BST measurement and then confirmed by molecular genetic testing.

Thymic Stromal Lymphopoietin (TSLP) Is Cleaved by Human Mast Cell Tryptase and Chymase

Thymic stromal lymphopoietin (TSLP), mainly expressed by epithelial cells, plays a central role in asthma. In humans, TSLP exists in two variants: the long form TSLP (lfTSLP) and a shorter TSLP isoform (sfTSLP). Macrophages (HLMs) and mast cells (HLMCs) are in close proximity in the human lung and play key roles in asthma. We evaluated the early proteolytic effects of tryptase and chymase released by HLMCs on TSLP by mass spectrometry. We also investigated whether TSLP and its fragments generated by these enzymes induce angiogenic factor release from HLMs. Mass spectrometry (MS) allowed the identification of TSLP cleavage sites caused by tryptase and chymase. Recombinant human TSLP treated with recombinant tryptase showed the production of 1-97 and 98-132 fragments. Recombinant chymase treatment of TSLP generated two peptides, 1-36 and 37-132. lfTSLP induced the release of VEGF-A, the most potent angiogenic factor, from HLMs. By contrast, the four TSLP fragments generated by tryptase and chymase failed to activate HLMs. Long-term TSLP incubation with furin generated two peptides devoid of activating property on HLMs. These results unveil an intricate interplay between mast cell-derived proteases and TSLP. These findings have potential relevance in understanding novel aspects of asthma pathobiology.

Alpha-Tryptase as a Risk-Modifying Factor for Mast Cell-Mediated Reactions

PURPOSE OF REVIEW

To provide an overview on the current understanding of genetic variability in human tryptases and summarize the literature demonstrating the differential impact of mature tryptases on mast cell-mediated reactions and associated clinical phenotypes.

RECENT FINDINGS

It is becoming increasingly recognized that tryptase gene composition, and in particular the common genetic trait hereditary alpha-tryptasemia (HαT), impacts clinical allergy. HαT has consistently been associated with clonal mast cell disorders (MCD) and has also been associated with more frequent anaphylaxis among these patients, and patients in whom no allergic trigger can be found, specifically idiopathic anaphylaxis. Additionally, more severe anaphylaxis among Hymenoptera venom allergy patients has been linked to HαT in both retrospective and prospective studies. An increased relative number of α-tryptase-encoding gene copies, even in the absence of HαT, has also been associated with systemic mastocytosis and has been shown to positively correlate with the severity of mast cell-mediated reactions to vibration and food. These findings may be due to increased generation of α/β-tryptase heterotetramers and differences in their enzymatic activity relative to β-tryptase homotetramers. HαT is a naturally occurring overexpression model of α-tryptase in humans. Increased relative α-tryptase expression modifies immediate hypersensitivity symptoms and is associated with more frequent and severe mast cell-mediated reactions, ostensibly due to increased α/β-tryptase heterotetramer production.

Clinical impact of the TPSAB1 genotype in mast cell diseases: A REMA study in a cohort of 959 individuals

BACKGROUND

A close association between hereditary alpha-tryptasemia (HAT) and mast cell (MC) disorders has been previously reported. However, the relationship between HAT and the diagnostic subtypes and clinical features of MC disorders still remains to be established.

OBJECTIVE

To determine the prevalence of HAT in healthy donors (HD) vs patients with different diagnostic subtypes of MC activation syndromes (MCAS) and mastocytosis, and its relationship with the clinical behavior of the disease.

METHODS

A total of 959 subjects were studied including 346 healthy donors (HD), 464 mastocytosis, and 149 non-clonal MCAS patients. Molecular studies to assess the TPSAB1 genotype were performed, and data on serum baseline tryptase (sBT) and basal MC-mediator release episodes and triggers of anaphylaxis were collected.

RESULTS

HAT was detected in 15/346 (4%) HD versus 43/149 (29%) non-clonal MCAS and 84/464 (18%) mastocytosis cases. Among mastocytosis, HAT was more frequently found in patients with MC-restricted KITD816V (21% vs. 10% among multilineage KITD816V patients; p = .008). Overall, median sBT was higher in cases presenting with HAT (28.9 vs. 24.5 ng/mL; p = .008), while no significant differences in sBT were observed among HAT+ mastocytosis patients depending on the presence of 1 vs. ≥2 extra copies of the α-tryptase gene (44.1 vs. 35.2 ng/mL, p > .05). In turn, anaphylaxis was more frequently observed in HAT+ versus HAT- mastocytosis patients (76% vs. 65%; p = .018), while HAT+ and HAT- patients who did not refer anaphylaxis as the presenting symptom (n = 308) showed a similar prevalence of subsequent anaphylaxis (35% vs. 36%, respectively).

CONCLUSION

The frequency of HAT in MC disorders varies according to the diagnostic subtype of the disease. HAT does not imply a higher risk (and severity) of anaphylaxis in mastocytosis patients in whom anaphylaxis is not part of the presenting symptoms of the disease.

Paediatric perioperative hypersensitivity: the performance of the current consensus formula and the effect of uneventful anaesthesia on serum tryptase

Background

Paired sampling of acute (aST) and basal (bST) serum tryptase has been recommended when investigating patients with a suspected perioperative hypersensitivity (POH) reaction. In the current consensus formula, an aST value exceeding (1.2×bST+2) confirms mast cell activation. The current consensus formula has been validated in adults but not in children.

Methods

We prospectively included 96 children who underwent uneventful anaesthesia and sampled serum tryptase at baseline and 60-90 min after induction. Tryptase changes were then compared with those in 94 children with suspected POH who were retrospectively included from four reference centres in Belgium, France, and Denmark.

Results

We observed a median decrease in serum tryptase during uneventful anaesthesia of 0.41 μg L-1 (-15.9%; P<0.001). The current consensus formula identified mast cell activation in 31.9% of paediatric POH patients. After generating receiver operating characteristic curves through 100 repeated five-fold cross-validation, aST>bST+0.71 was identified as the optimal cut-off point to identify mast cell activation. This new paediatric formula has higher sensitivity than the current consensus formula (53.2% vs 31.9%, P<0.001) with a specificity of 96.9%. Analysis in the subpopulation where a culprit was identified and in grade 3-4 reactions similarly yielded higher sensitivity for the new paediatric formula when compared with the current consensus formula (85.3% vs 61.8%; P=0.008 and 78.0% vs 48.8%; P<0.001, respectively). Internally cross-validated sensitivity and specificity were 53.3% and 93.3%, respectively.

Conclusions

This is the first study suggesting the need for an adjusted formula in children to identify perioperative mast cell activation as tryptase is significantly lowered during uneventful anaesthesia. We propose a new formula (aST>bST+0.71) which performs significantly better than the current consensus formula in our multicentric paediatric population.

Low Prevalence of Idiopathic Mast Cell Activation Syndrome Among 703 Patients With Suspected Mast Cell Disorders

BACKGROUND

Idiopathic mast cell activation syndrome (iMCAS) is characterized by severe, episodic systemic mast cell (MC) activation and mediator-related symptoms, an event-related increase in serum tryptase levels, and response to MC-targeted therapies in the absence of underlying IgE-mediated allergy or clonal MC disorder. Studies indicating its prevalence using evidence-based diagnostic criteria are lacking.

OBJECTIVE

To assess the prevalence and clinical and laboratory features of patients with iMCAS.

METHODS

We conducted a retrospective evaluation of data from 703 consecutive patients (aged ≥18 years) referred to our center based on suspicion of having MC disorders. Patients underwent a thorough clinical workup including patient history, allergy tests, KIT D816V mutation analysis, and/or bone marrow investigation. Disease activity was prospectively assessed during follow-up visits.

RESULTS

We identified 31 patients with confirmed iMCAS. Furthermore, hereditary α-tryptasemia was detected in three patients with baseline tryptase levels greater than 8 ng/mL. The most common clinical presentation during MCAS episodes was mucocutaneous symptoms in patients with iMCAS, especially urticaria or angioedema. However, these symptoms were less prevalent in patients with clonal MCAS (P = .015). The duration of diagnostic delay was significantly longer in patients with iMCAS compared to those with clonal MCAS (P = .02).

CONCLUSIONS

The overall prevalence of iMCAS was 4.4% in the entire cohort, which indicates that iMCAS is an uncommon condition. To accurately diagnose iMCAS, it is crucial to evaluate suspected patients using the three diagnostic MCAS criteria. This involves performing a comprehensive allergy work-up including laboratory tests and ultrasensitive mutation analysis of KIT D816V. Subsequently, recommended diagnostic algorithms should be applied.

Clinically accessible amplitude-based multiplex ddPCR assay for tryptase genotyping

Hereditary α tryptasemia (HαT) is an autosomal dominant trait characterized by increased TPSAB1 copy number (CN) encoding α-tryptase. The determination of HαT is being discussed as an important biomarker to be included in risk assessment models and future diagnostic algorithms for patients with mastocytosis and anaphylaxis. Due to the complex genetic structure at the human tryptase locus, genetic testing for tryptase gene composition is presently notably limited and infrequently pursued. This study aimed to develop, optimise and validate a multiplex droplet digital PCR (ddPCR) assay that can reliably quantify α- and β-tryptase encoding sequences in a single reaction. To optimise the ddPCR conditions and establish an amplitude-based multiplex ddPCR assay, additional primers and probes, a thermal gradient with varying annealing temperatures, different primers/probe concentrations, and various initial DNA quantities were tested. Results obtained from all 114 samples analysed using multiplex ddPCR were identical to those obtained through the use of original duplex assays. Utilizing this multiplex ddPCR assay, in contrast to conducting distinct duplex ddPCRs, presents noteworthy benefits for tryptase genotyping. These advantages encompass a substantial threefold decrease in material costs and considerable time savings. Consequently, this approach exhibits high suitability and particularly captures interest for routine clinical implementation.

Case Report: A family history of peanut allergy and hereditary alpha-tryptasemia

Context

Hereditary alpha-tryptasemia (HαT) is associated with elevated basal serum tryptase (bST) and is associated with a higher risk of severe anaphylactic reactions in patients with clonal mast cell disorders or IgE-mediated Hymenoptera venom-induced anaphylaxis. The consequence of this genetic trait remains to be determined in other allergic diseases and food allergy in particular.

Objectives

Here, we describe three cases of peanut allergy among siblings from a single family of four: two of them were associated with HαT, and the third one was associated with the tryptase wild-type genotype.

Methods

TPSAB1/TPSB2 genotypes were determined by digital PCR. After the case description, we provided a review of the literature regarding bST levels and tryptase genotypes in anaphylaxis, with a particular focus on food allergy.

Results

Compared to the sibling with the conventional tryptase genotype, the two siblings with HαT presented a lower peanut threshold at the initial oral food challenge, higher peanut skin prick test reactivity, higher levels of specific IgE to peanut, Ara h 2, and Ara h 6, and a lower IgG4/IgE ratio after 10 years of oral immunotherapy.

Conclusion

The tryptase genotype and HαT status might modify the clinical presentation and biological features of food allergy.

Diagnostic Significance of Tryptase for Suspected Mast Cell Disorders

Tryptase has proven to be a very useful and specific marker to demonstrate mast cell activation and degranulation when an acute (i.e., within 4 h after the event) and baseline value (i.e., at least 24 h after the event) are compared and meet the consensus formula (i.e., an increase of 20% + 2). The upper limit of normal determined by the manufacturer is 11.4 ng/mL; however, this boundary has been the subject of debate. According to ECNM and AIM experts, the normal range of baseline tryptase should be 1 to 15 ng/mL. A genetic trait, hereditary alpha tryptasemia, characterized by an increased alpha coding TPSAB1 copy number is associated with a baseline value above 8 ng/mL. Elevated tryptase can also be found in chronic kidney disease, obesity, and hematological neoplasms. A tryptase > 20 ng/mL serves as a minor criterion to diagnose systemic mastocytosis and an increase in tryptase > 20% + 2 during an acute event is a required criterion in the diagnosis of mast cell activation syndrome. The goal of this review is to demonstrate the (in)significance of tryptase using some clinical vignettes and to provide a practical guide on how to manage and interpret an elevated tryptase level.

Update on Mast Cell Proteases as Drug Targets

Mast cell granules are packed with proteases, which are released with other mediators by degranulating stimuli. Several of these proteases are targets of potentially therapeutic inhibitors based on hypothesized contributions to diseases, notably asthma and ulcerative colitis for β-tryptases, heart and kidney scarring for chymases, and airway infection for dipeptidyl peptidase-I. Small-molecule and antibody-based β-tryptase inhibitors showing preclinical promise were tested in early-phase human trials with some evidence of benefit. Chymase inhibitors were given safely in Phase II trials without demonstrating benefits, whereas dipeptidyl peptidase-I inhibitor improved bronchiectasis, in effects likely related to inactivation of the enzyme in neutrophils.

Thymic stromal lymphopoietin (TSLP) is a substrate for tryptase in patients with mastocytosis

Mastocytosis is a heterogeneous disease associated to uncontrolled proliferation and increased density of mast cells in different organs. This clonal disorder is related to gain-of-function pathogenic variants of the c-kit gene that encodes for KIT (CD117) expressed on mast cell membrane. Thymic stromal lymphopoietin (TSLP) is a pleiotropic cytokine, which plays a key role in allergic disorders and several cancers. TSLP is a survival and activating factor for human mast cells through the engagement of the TSLP receptor. Activated human mast cells release several preformed mediators, including tryptase. Increased mast cell-derived tryptase is a diagnostic biomarker of mastocytosis. In this study, we found that in these patients serum concentrations of TSLP were lower than healthy donors. There was an inverse correlation between TSLP and tryptase concentrations in mastocytosis. Incubation of human recombinant TSLP with sera from patients with mastocytosis, containing increasing concentrations of tryptase, concentration-dependently decreased TSLP immunoreactivity. Similarly, recombinant β-tryptase reduced the immunoreactivity of recombinant TSLP, inducing the formation of a cleavage product of approximately 10 kDa. Collectively, these results indicate that TSLP is a substrate for human mast cell tryptase and highlight a novel loop involving these mediators in mastocytosis.

Severe food allergy reactions are associated with α-tryptase

BACKGROUND

Increased TPSAB1 copy numbers encoding ⍺-tryptase are associated with severe reactions in adults with Hymenoptera venom allergy, systemic mastocytosis, and idiopathic anaphylaxis.

OBJECTIVE

The primary objective was to assess the association between ⍺-tryptase and severity of food allergy.

METHODS

A total of 119 subjects underwent tryptase genotyping; 82 of them were from an observational food allergy cohort at the National Institute of Allergy and Infectious Disease (NIAID), and 37 were from a cohort of children who reacted to peanut oral food challenge (OFC) at Lurie Children's Hospital of Chicago. The primary predictor was presence or absence of ⍺-tryptase. The primary outcomes for both cohorts were measures of severity of food allergy reaction. Secondary outcomes included OFC symptom scores (Bock/Practical Allergy [PRACTALL] and Severity Grading Score for Acute Reactions [SGSAR]). Correlation between total α-tryptase isoforms and OFC scores was also assessed to account for gene dosage effects.

RESULTS

Among the subjects in the NIAID cohort, the presence of ⍺-tryptase was associated with a higher prevalence of food-triggered anaphylaxis than in those with only β-tryptase (P = .026). Similarly, only 1 of 6 subjects in the OFC cohort with only β-tryptase (17%) had a severe reaction, whereas 20 of 31 of subjects with α-tryptase (65%) had a severe reaction (P = .066). Subjects with ⍺-tryptase also had higher total SGSAR scores than did the subjects with no ⍺-tryptase (P = .003). In addition, there were also significant positive correlations between ⍺-tryptase isoform copy numbers and both higher total SGSAR and Bock/PRACTALL OFC scores (P = .008 and P = .003, respectively).

CONCLUSION

The presence of α-tryptase in subjects is correlated with a higher prevalence of anaphylaxis or severe reaction to food than in subjects without any α-tryptase.

Platelet-activating factor as an endogenous cofactor of food anaphylaxis

Anaphylaxis is a severe, acute, life-threatening generalized or systemic hypersensitivity reaction. The incidence of anaphylaxis is increasing worldwide, with medications and food contributing to most cases. Physical exercise, acute infections, drugs, alcohol, and menstruation are the external cofactors associated with more severe systemic reaction. The aim of this review is to show that platelet-activating factor contributes to the development of severe anaphylactic reaction, and even to anaphylactic shock.

Genetic Variants Leading to Urticaria and Angioedema and Associated Biomarkers

Advances in next generation sequencing technologies, as well as their expanded accessibility and clinical use over the past 2 decades, have led to an exponential increase in the number of identified single gene disorders. Among these are primary atopic disorders-inborn errors of immunity resulting in severe allergic phenotypes as a primary presenting feature. Two cardinal aspects of type I immediate hypersensitivity allergic reactions are hives and angioedema. Mast cells (MCs) are frequent primary drivers of these symptoms, but other cells have also been implicated. Even where MC degranulation is believed to be the cause, mediator-induced symptoms may greatly vary among individuals. Angioedema-particularly in the absence of hives-may also be caused by hereditary angioedema conditions resulting from aberrant regulation of contact system activation and excessive bradykinin generation or impairment of vascular integrity. In these patients, swelling can affect unpredictable locations and fail to respond to MC-directed therapies. Genetic variants have helped delineate key pathways in the etiology of urticaria and nonatopic angioedema and led to the development of targeted therapies. Herein, we describe the currently known inherited and acquired genetic causes for these conditions, highlight specific features in their clinical presentations, and discuss the benefits and limitations of biomarkers that can help distinguish them.

Degranulation of Mast Cells as a Target for Drug Development

Mast cells act as key effector cells of inflammatory responses through degranulation. Mast cell degranulation is induced by the activation of cell surface receptors, such as FcεRI, MRGPRX2/B2, and P2RX7. Each receptor, except FcεRI, varies in its expression pattern depending on the tissue, which contributes to their differing involvement in inflammatory responses depending on the site of occurrence. Focusing on the mechanism of allergic inflammatory responses by mast cells, this review will describe newly identified mast cell receptors in terms of their involvement in degranulation induction and patterns of tissue-specific expression. In addition, new drugs targeting mast cell degranulation for the treatment of allergy-related diseases will be introduced.

Secretory and Membrane-Associated Biomarkers of Mast Cell Activation and Proliferation

Mast cells (MCs) are immune cells distributed in many organs and tissues and involved in the pathogenesis of allergic and inflammatory diseases as a major source of pro-inflammatory and vasoactive mediators. MC-related disorders are heterogeneous conditions characterized by the proliferation of MC within tissues and/or MC hyper-reactivity that leads to the uncontrolled release of mediators. MC disorders include mastocytosis, a clonal disease characterized by tissue MC proliferation, and MC activation syndromes that can be primary (clonal), secondary (related to allergic disorders), or idiopathic. Diagnosis of MC disorders is difficult because symptoms are transient, unpredictable, and unspecific, and because these conditions mimic many other diseases. Validation of markers of MC activation in vivo will be useful to allow faster diagnosis and better management of MC disorders. Tryptase, being the most specific MC product, is a widely used biomarker of proliferation and activation. Other mediators, such as histamine, cysteinyl leukotrienes, and prostaglandin D2, are unstable molecules and have limitations in their assays. Surface MC markers, detected by flow cytometry, are useful for the identification of neoplastic MC in mastocytosis but, so far, none of them has been validated as a biomarker of MC activation. Further studies are needed to identify useful biomarkers of MC activation in vivo.

The Role of Mast Cells in the Induction and Maintenance of Inflammation in Selected Skin Diseases

Under physiological conditions, skin mast cells play an important role as guardians that quickly react to stimuli that disturb homeostasis. These cells efficiently support, fight infection, and heal the injured tissue. The substances secreted by mast cells allow for communication inside the body, including the immune, nervous, and blood systems. Pathologically non-cancerous mast cells participate in allergic processes but also may promote the development of autoinflammatory or neoplastic disease. In this article, we review the current literature regarding the role of mast cells in autoinflammatory, allergic, neoplastic skin disease, as well as the importance of these cells in systemic diseases with a pronounced course with skin symptoms.

Role of Adhesion G Protein-Coupled Receptors in Immune Dysfunction and Disorder

Disorders of the immune system, including immunodeficiency, immuno-malignancy, and (auto)inflammatory, autoimmune, and allergic diseases, have a great impact on a host’s health. Cellular communication mediated through cell surface receptors, among different cell types and between cell and microenvironment, plays a critical role in immune responses. Selective members of the adhesion G protein-coupled receptor (aGPCR) family are expressed differentially in diverse immune cell types and have been implicated recently in unique immune dysfunctions and disorders in part due to their dual cell adhesion and signaling roles. Here, we discuss the molecular and functional characteristics of distinctive immune aGPCRs and their physiopathological roles in the immune system.

Tryptase in type I hypersensitivity

Tryptase is currently the main mast cell biomarker available in medical practice. Tryptase determination is a quantitative test performed in serum or plasma for the diagnosis, stratification, and follow-up of mast cell-related conditions. The continuous secretion of monomeric α and β protryptases forms the baseline tryptase level. Transient, activation-induced release of tryptase is known as acute tryptase. Because mast cells are tissue-resident cells, the detection of an acute tryptase release in the bloodstream is protracted, with a delay of 15 to 20 minutes after the onset of symptoms and a peak at approximately 1 hour. Constitutive release of tryptase is a marker of mast cell number and activity status, whereas transient release of mature tryptase is a marker of mast cell degranulation. Although consensual as a concept, the application of this statement in clinical practice has only been clarified since 2020. For baseline tryptase to be used as a biomarker, reference values need to be established. In contrast, defining a transient increase using acute tryptase can only be achieved as a function of the baseline status.

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.

Chemically diverse activity-based probes with unexpected inhibitory mechanisms targeting trypsin-like serine proteases

Activity-based probes (ABP) are molecules that bind covalently to the active form of an enzyme family, making them an attractive tool for target and biomarker identification and drug discovery. The present study describes the synthesis and biochemical characterization of novel activity-based probes targeting trypsin-like serine proteases. We developed an extensive library of activity-based probes with "clickable" affinity tags and a diaryl phosphonate warhead. A wide diversity was achieved by including natural amino acid analogs as well as basic polar residues as side chains. A detailed enzymatic characterization was performed in a panel of trypsin-like serine proteases. Their inhibitory potencies and kinetic profile were examined, and their IC50 values, mechanism of inhibition, and kinetic constants were determined. The activity-based probes with a benzyl guanidine side chain showed the highest inhibitory effects in the panel. Surprisingly, some of the high-affinity probes presented a reversible inhibitory mechanism. On the other hand, probes with different side chains exhibited the expected irreversible mechanism. For the first time, we demonstrate that not only irreversible probes but also reversible probes can tightly label recombinant proteases and proteases released from human mast cells. Even under denaturing SDS-PAGE conditions, reversible slow-tight-binding probes can label proteases due to the formation of high-affinity complexes and slow dissociation rates. This unexpected finding will transform the view on the required irreversible nature of activity-based probes. The diversity of this library of activity-based probes combined with a detailed enzyme kinetic characterization will advance their applications in proteomic studies and drug discovery.

Anaphylaxis: Advances in the Past 10 Years

In the past 10 years, anaphylaxis has grown into its own special area of study within Allergy-Immunology, both at the bench and at the bedside. This review focuses on some of the most clinically relevant advances over the past decade. These include simplified and more inclusive diagnostic criteria for adults and children, uniform definition of biphasic anaphylaxis, and improved systems for objective severity grading. Studies reported in the past decade have led to improved understanding of normal and abnormal regulation of mast cell function, translating into better diagnostic and therapeutic approaches to patients with anaphylaxis. Research has provided improved recognition and treatment of mast cell disorders and has identified a new condition, hereditary α-tryptasemia, that may impact anaphylactic syndromes. We have learned to recognize new causes (α-gal), new pathways (Mas-related G protein-coupled receptor-X2), and many risk factors for severe anaphylaxis. The stability of epinephrine in autoinjectors was reported to be very good for several years after the labeled expiry date, and it can tolerate freezing and thawing. Repeated and prolonged exposure to excessive heat leads to degradation of epinephrine activity. New treatments to prevent severe anaphylaxis have been described, using new ways to block the IgE receptor or modulate intracellular signaling pathways.

Urticaria and Angioedema: Understanding Complex Pathomechanisms to Facilitate Patient Communication, Disease Management, and Future Treatment

Chronic spontaneous urticaria (CSU) is primarily a T2-dominant disease with a complex genetic background. Skin mast cell activation can be induced not only via the IgE-FcεRI axis but also from several other distinct mechanisms, molecules, and receptors involved in CSU onset, persistence, and exacerbation. These include autoallergy, autoimmunity, central or peripheral neuroimmune dysregulation, activation of both extrinsic and intrinsic coagulation pathways, and microbial infections. Besides mast cells, recent reports suggest the active and direct involvement of basophils and eosinophils. Several biological characteristics or biomarkers have been linked with CSU's known endotypes and may help forecast therapeutic responses. The introduction of biologic therapy for CSU has been a major advance in the last 10 years. The cornerstone of angioedema (AE) pathogenesis is increased vascular permeability and plasma leakage into the deeper dermis and subcutis, either mediated by histamine or bradykinin (BK). C1-inhibitor deficiency, hereditary or acquired, is the primary cause of BK-mediated AE due to increased plasma BK concentration. Other complex conditions have been identified, with some likely involving contact system dysregulation and other putative mechanisms related to vascular endothelial dysfunction. The approval of multiple hereditary-AE-specific therapies for both prevention and acute attacks has revolutionized treatment of this disease. Any new knowledge of the pathogenesis of CSU and AE offers the opportunity to improve patient information, physician-patient communication, prediction of therapeutic responses, selection of precise tailor-made treatment for each patient, and exploration of novel treatment options for those who do not achieve disease control with current medications.

Case report of severe refractory inflammatory dermatoses in a young female diagnosed with hereditary alpha tryptasemia

Hereditary alpha tryptasemia (HaT), an autosomal dominant condition first described in 2014, has previously been associated with multiple dermatologic, allergic, gastrointestinal, neuropsychiatric, autonomic, and connective tissue abnormalities. We describe a pediatric patient with predominantly mixed cutaneous inflammatory manifestations and atopic manifestations resistant to treatment who was found to have HaT. HaT should be considered in individuals with refractory inflammatory dermatologic disease and signs and/or symptoms concerning for mast cell activation.

Hereditary alpha-tryptasemia

PURPOSE OF REVIEW

To discuss our evolving knowledge about the genetic variations in human tryptase and recent advances in associated clinical phenotypes.

RECENT FINDINGS

Hereditary alpha-tryptasemia (HAT) is an autosomal dominant genetic trait and a common cause of elevated basal serum tryptase (BST) in Western populations. It is a risk factor for severe anaphylaxis and an established modifier of mast cell mediator-associated symptoms among patients with systemic mastocytosis (SM).

SUMMARY

The unique properties of naturally occurring alpha/beta-tryptase heterotetramers may explain certain elements of phenotypes associated with HAT. Understanding the physiology of tryptases and how this may relate to the clinical features associated with HAT is the first step in identifying optimal medical management and targets for novel therapeutics.

Regulation of pulmonary surfactant by the adhesion GPCR GPR116/ADGRF5 requires a tethered agonist-mediated activation mechanism

The mechanistic details of the tethered agonist mode of activation for the adhesion GPCR ADGRF5/GPR116 have not been completely deciphered. We set out to investigate the physiological importance of autocatalytic cleavage upstream of the agonistic peptide sequence, an event necessary for NTF displacement and subsequent receptor activation. To examine this hypothesis, we characterized tethered agonist-mediated activation of GPR116 in vitro and in vivo. A knock-in mouse expressing a non-cleavable GPR116 mutant phenocopies the pulmonary phenotype of GPR116 knock-out mice, demonstrating that tethered agonist-mediated receptor activation is indispensable for function in vivo. Using site-directed mutagenesis and species-swapping approaches, we identified key conserved amino acids for GPR116 activation in the tethered agonist sequence and in extracellular loops 2/3 (ECL2/3). We further highlight residues in transmembrane 7 (TM7) that mediate stronger signaling in mouse versus human GPR116 and recapitulate these findings in a model supporting tethered agonist:ECL2 interactions for GPR116 activation.

Incorporating Tryptase Genotyping Into the Workup and Diagnosis of Mast Cell Diseases and Reactions

The measurement of mast cell tryptase levels in serum has found utility in the diagnosis and management of both clonal mast cell disorders and severe mast cell-dependent systemic reactions in the form of anaphylaxis. A more recent discovery is that a majority of individuals with elevated basal serum tryptase levels have increased germline TPSAB1 gene copy number encoding α-tryptase. This genetic trait is referred to as hereditary α-tryptasemia (HαT) and affects nearly 6% of the general population. In clinical practice, the presence or absence of HαT should thus now be determined when defining what constitutes an abnormal serum tryptase level in the diagnosis of mastocytosis. Further, as rises in serum tryptase levels are used to support the diagnosis of systemic anaphylaxis, variability in baseline serum tryptase levels should be factored into how significant a rise in serum tryptase is required to confirm the diagnosis of a systemic allergic reaction. In practicality, this dictates that symptomatic individuals undergoing evaluation for a mast cell-associated disorder or reaction with a baseline serum tryptase level exceeding 6.5 ng/mL should be considered for tryptase genotyping in order to screen for HαT. This review provides detailed information on how to use the results of such testing in the diagnosis and management of both mastocytosis and anaphylaxis.

Mast Cell Activation Syndromes: Collegium Internationale Allergologicum Update 2022

Mast cell activation syndromes (MCASs) are defined by systemic severe and recurrent mast cell activation, usually in form of anaphylaxis, a substantial, event-related increase of the serum tryptase level beyond the individual's baseline and a response of the symptomatology to drugs directed against mast cells, mast cell-derived mediators, or mediator effects. A number of predisposing genetic conditions, underlying allergic and other hypersensitivity states, and related comorbidities can contribute to the clinical manifestation of MCASs. These conditions include hereditary alpha tryptasemia, mastocytosis with an expansion of clonal KIT-mutated mast cells, atopic diathesis, and overt IgE-dependent and IgE-independent allergies. Several of these conditions have overlapping definitions and diagnostic criteria and may also develop concomitantly in the same patient. However, although criteria and clinical features overlap, each of these conditions is characterized by a unique constellation of variables and diagnostic criteria. Since two, three, or more conditions can coexist in the same patient, with obvious clinical implications, it is of crucial importance to diagnose the variant of MCAS precisely and to take all accompanying, underlying and potentially complicating conditions, and comorbidities into account when establishing the management plan. Indeed, most of these patients require multidisciplinary investigations and only a personalized treatment approach can lead to an optimal management plan providing an optimal quality of life and low risk of anaphylaxis.

Resolving the genetics of human tryptases: implications for health, disease, and clinical use as a biomarker

PURPOSE OF REVIEW

To discuss our evolving understanding of the genetic variation in human tryptases and recent advances in associated clinical phenotypes.

RECENT FINDINGS

Serum tryptase levels have long been used as biomarkers in clinical practice to diagnose mast cell-associated disorders and mast cell-mediated reactions but the contribution of specific secreted isoforms of human tryptases and their role(s) in health and disease has only recently begun to be illuminated. It is now recognized that hereditary alpha-tryptasemia (HαT) is a common genetic trait and the commonest cause for elevated basal serum tryptase (BST), where it can both contribute to mast cell-associated phenotypes, and potentially confound their correct diagnosis. Expression of different tryptase isoforms is now recognized to be associated with specific clinical phenotypes including clonal and nonclonal mast cell-associated disorders as well as certain asthma endotypes. These disparate impacts on clinical disorders may result from differences in enzymatic activities of mature α-tryptases and β-tryptases, and the unique substrate profile and stability of heterotetrameric mature α/β-tryptases recently described to naturally occur.

SUMMARY

Variable copy number and isoform expression of tryptases differentially impact diseases and reactions associated with mast cells in humans. Recent advances in understanding of genetics governing BST levels have refined our understanding and the clinical use of this biomarker. In the future, incorporation of tryptase genotyping will likely be integral to the work-up and trial design of patients with phenotypes impacted by mast cells ranging from asthma to mastocytosis.

Selected recent advances in understanding the role of human mast cells in health and disease

Mast cells are highly granular tissue-resident cells and key drivers of inflammation, particularly in allergies as well as in other inflammatory diseases. Most mast cell research was initially conducted in rodents but has increasingly shifted to the human system, with the advancement of research technologies and methodologies. Today we can analyze primary human cells including rare subpopulations, we can produce and maintain mast cells isolated from human tissues, and there are several human mast cell lines. These tools have substantially facilitated our understanding of their role and function in different organs in both health and disease. We can now define more clearly where human mast cells originate from, how they develop, which mediators they store, produce de novo, and release, how they are activated and by which receptors, and which neighbouring cells they interact with and by which mechanisms. Considerable progress has also been made regarding the potential contribution of mast cells to disease, which, in turn, has led to the development of novel approaches for preventing key pathogenic effects of mast cells, heralding the era of mast cell-targeted therapeutics. In this review, we present and discuss a selection of some of the most significant advancements and remaining gaps in our understanding of human mast cells during the last 25 years, with a focus on clinical relevance.

Allergy, Anaphylaxis, and Nonallergic Hypersensitivity: IgE, Mast Cells, and Beyond

IgE-mediated type I hypersensitivity reactions have many reported beneficial functions in immune defense against parasites, venoms, toxins, etc. However, they are best known for their role in allergies, currently affecting almost one third of the population worldwide. IgE-mediated allergic diseases result from a maladaptive type 2 immune response that promotes the synthesis of IgE antibodies directed at a special class of antigens called allergens. IgE antibodies bind to type I high-affinity IgE receptors (FcεRI) on mast cells and basophils, sensitizing them to get triggered in a subsequent encounter with the cognate allergen. This promotes the release of a large variety of inflammatory mediators including histamine responsible for the symptoms of immediate hypersensitivity. The development of type 2-driven allergies is dependent on a complex interplay of genetic and environmental factors at barrier surfaces including the host microbiome that builds up during early life. While IgE-mediated immediate hypersensitivity reactions are undoubtedly at the origin of the majority of allergies, it has become clear that similar responses and symptoms can be triggered by other types of adaptive immune responses mediated via IgG or complement involving other immune cells and mediators. Likewise, various nonadaptive innate triggers via receptors expressed on mast cells have been found to either directly launch a hypersensitivity reaction and/or to amplify existing IgE-mediated responses. This review summarizes recent findings on both IgE-dependent and IgE-independent mechanisms in the development of allergic hypersensitivities and provides an update on the diagnosis of allergy.

The ingenious mast cell: Contemporary insights into mast cell behavior and function

Mast cells are (in)famous for their role in allergic diseases, but the physiological and pathophysiological roles of this ingenious cell are still not fully understood. Mast cells are important for homeostasis and surveillance of the human system, recognizing both endogenous and exogenous agents, which induce release of a variety of mediators acting on both immune and non-immune cells, including nerve cells, fibroblasts, endothelial cells, smooth muscle cells, and epithelial cells. During recent years, clinical and experimental studies on human mast cells, as well as experiments using animal models, have resulted in many discoveries that help decipher the function of mast cells in health and disease. In this review, we focus particularly on new insights into mast cell biology, with a focus on mast cell development, recruitment, heterogeneity, and reactivity. We also highlight the development in our understanding of mast cell-driven diseases and discuss the development of novel strategies to treat such conditions.