JAK3 is primarily expressed in hematopoietic cells and mediates cytokine signaling via the common γ chain. Classically, autosomal recessive loss-of-function (LOF) JAK3 variants cause severe combined immunodeficiency (SCID). In contrast, somatic gain-of-function (GOF) variants in JAK3 have been described across T and B cell malignancies, acute megakaryoblastic leukemia, and solid tumors, and a germline GOF variant (c.1520A>C, p.Q507P) has been reported in a family with chronic natural killer (NK) cell lymphoproliferative disease, autoimmunity, and hypogammaglobulinemia, notably without allergic disease. We report five unrelated pediatric patients (3 females and 2 males) identified through a genomic study of atopic disease. The median age at evaluation was 11 years (range: 3–15) (supplementary table). All patients exhibited severe atopic dermatitis (AD), including one complicated by molluscum contagiosum and Methicillin-resistant Staphylococcus aureus (MRSA) infections. Only one patient had an IgE-mediated food allergy. None had asthma, allergic rhinitis, eosinophilic gastrointestinal disease, short stature, or lymphoproliferative disease. Exome sequencing revealed two previously identified heterozygous JAK3 variants (c.394 C>A, p.P132T, MAF: 0.0046 and c.2164 G>A, p.V722I, minor allele frequency [MAF]: 0.0098). All patients had eosinophilia (median: 1,360 cells/μL, range: 490–1,610), with normal hemoglobin, platelet, neutrophil, and lymphocyte counts. In one patient, elevated NK (1,062 cells/μL) and NKT (1,071 cells/μL) cells were observed on immunophenotyping; no additional abnormalities were detected in the remaining patients. All patients had elevated IgE (median: 7,540 UI/mL, range: 139–17,472); other immunoglobulins and vaccine titers were within normal limits. While pathogenicity annotations for these variants are inconsistent, ex vivo studies from lymphoproliferative disease samples harboring p.P132T (FERM domain) and p.V722I (JH2 domain) variants demonstrated constitutive JAK3 with enhanced STAT5 phosphorylation, supporting a GOF mechanism. Compared with strongly activating JAK3 variants linked to lymphoproliferation, these germline variants may confer a subtler activating signal, preferentially amplifying STAT5 signaling. Notably, somatic STAT5B GOF causes a syndrome featuring nonclonal eosinophilia and severe AD, although with additional features absent in our cohort. Finally, JAK3 overexpression has been observed in human AD skin cells, and JAK3 inhibition reduces inflammatory responses in both in vivo and in vitro. Collectively, these data suggest that germline JAK3 GOF variants may contribute to atopic diatheses, potentially expanding the phenotype of JAK3-associated disease. Tabular data are included as downloadable supplement files.

Hereditary alpha-tryptasemia (HαT) is a common autosomal dominant genetic trait that results from increased copy numbers of the TPSAB1 gene encoding α-tryptase. Studies have shown that increased relative α-tryptase expression can modify mast-cell-mediated reactions and contribute to increased severity of anaphylaxis. HαT is an independent risk modifier for reaction severity in patients with Hymenoptera venom allergy and systemic mastocytosis. Additionally, there is emerging evidence that HαT and α-tryptase expression more generally are also associated with increased risk of severe reactions to other allergens. Tryptase genotyping may be a promising biomarker for risk stratification of IgE-mediated allergic reaction severity in the future.

Introduction:Hereditary α-tryptasemia (HαT), defined by increased TPSAB1 copy number and elevated basal serum tryptase, is associated with mast cell (MC)-mediated symptoms. However, its role in gastrointestinal disease remains unclear. Because intestinal MCs express the non-IgE-dependent activation receptor MRGPRX2, we investigated whether MRGPRX2 expression and MC phenotypes are altered in individuals with HαT in the context of inflammatory bowel disease (IBD). Methods:We genotyped 854 biobanked IBD samples to identify individuals with HαT. Spatial transcriptomic analysis was performed on descending colon tissue from individuals with HαT (n = 4) as well as tissue and severity matched non-HαT controls (n = 4). Small intestinal biopsies were additionally analyzed using mass cytometry (CyTOF) from HαT individuals (n = 5) and non-HαT controls (n = 9). Droplet digital PCR (ddPCR) was used to establish TPSAB1 copy number variant for HαT detection. Comparisons across groups were performed using Welch's t-test with effect sizes and 95% CI. Results:Across complementary platforms, HαT was associated with increased gastrointestinal mast cell abundance and elevated expression of mast cell activation markers, including CD203c, LAMP-1, and SIGLEC8. Both spatial transcriptomics and ddPCR demonstrated significantly increased MRGPRX2 and SIGLEC8 transcript levels in IBD samples from individuals with HαT compared with matched non-HαT IBD controls. Discussion:These findings suggest that enhanced MRGPRX2 expression and mast cell activation may contribute to gastrointestinal symptoms in individuals with HαT, particularly in the setting of IBD. As interest in precision immunogenetics grows, defining mast cell phenotypes linked to α-tryptase copy number may help refine diagnostic evaluation and identify patients who could benefit from emerging mast cell-targeted therapeutic strategies in the context of IBD.

Oncostatin M (OSM) receptor beta (OSMRβ), encoded by OSMR, is a cytokine receptor subunit required for signaling by OSM and IL-31. We identified 10 affected individuals from seven unrelated families with germline biallelic loss-of-function variants in OSMR who shared a phenotype of early-onset, severe, widespread atopic dermatitis with peripheral eosinophilia and markedly elevated serum IgE. All patient-derived OSMRβ variants failed to localize to the cell surface, resulting in selective loss of OSM-dependent signaling. Patient cells showed markedly reduced OSM-induced phosphorylation of STAT1, STAT3, and STAT5, while signaling through other IL-6 family receptor complexes remained intact. Transcriptomic profiling of patient primary dermal fibroblasts revealed consistent downstream effects, including loss of interferon-responsive and inflammatory gene programs. Re-expression of wild-type OSMR restored receptor surface expression, STAT activation, and transcriptional responses, confirming a causal loss-of-function mechanism. Together, these findings establish biallelic OSMR deficiency as a novel primary atopic disorder.

The transcription factor IKAROS plays an important role in lymphocyte development, differentiation, and as a tumor suppressor. To date, >70 IKAROS germ line heterozygous variants have been reported in patients with primary immunodeficiency (PID)/inborn errors of immunity (IEI) and leukemia, and this number continues to grow. Germ line IKAROS loss- and gain-of-function mutations have been linked to immunodeficiency, immune dysregulation, and hematologic malignancies, with a broad spectrum of clinical manifestations. Routine next-generation sequencing approaches in patients with PID/IEI have facilitated the identification of IKAROS variants, including several cases with variants of uncertain significance (VUS). To determine the VUS' functional behavior, we systematically generated constructs recapitulating those changes and tested IKAROS functions in vitro. We also conducted an in-depth examination of the C-terminal dimerization domain using alanine-scanning mutagenesis to identify amino acids critical for dimerization and other functions. This work provides a comprehensive description of the biologic impact of 81 previously unreported and/or untested IKAROS variants, including 33 patient-detected germ line VUS and 48 laboratory-generated mutations in the dimerization domain. Among them, 15 of the patient-detected variants, primarily mapping to IKAROS DNA-binding or dimerization domains, and at least 21 of the laboratory-generated mutations, impaired IKAROS function and could explain or result in human disease. VUS located in between IKAROS DNA binding and dimerization domains were less likely to be functionally deleterious. Of note, both positive and negative functional data herein generated can be relevant for patients carrying these IKAROS variants, helping to establish a diagnosis and guide treatment decisions.

Signal transducer and activator transcription 5B (STAT5B) is a critical mediator of multiple cytokines (e.g., IL-2 and IL-7) and growth hormone signaling and plays a key role in hematopoiesis, particularly in lymphocyte development, proliferation, and survival. Loss-of-function and dominant-negative mutations in STAT5B have been associated with growth failure and immune dysregulation, including atopy. While short stature has been reported in individuals with heterozygous loss-of-function mutations (haploinsufficiency), allergic manifestations have not been reported in depth. The proband is an adult female who presented with neonatal-onset atopic dermatitis, which progressively worsened to severe, diffuse involvement of her face and extremities by adulthood (SCORAD of 59) and was refractory to topical corticosteroids. She was ultimately started on dupilumab with significant improvement. Initial evaluation revealed markedly elevated IgE (20,756 IU/mL) and mild eosinophilia (660 cells/μL). Her medical history was notable for slightly reduced height, moderate asthma, seasonal allergies managed with immunotherapy, and peanut allergy. Her family history was notable for moderate-severe atopic dermatitis affecting her father, sister, and niece. There was no history of recurrent infections, enteropathy, endocrine abnormalities, autoimmune disease, or malignancy. Detailed immunophenotyping revealed reduced central memory CD4+ and CD8+ T lymphocytes, increased effector memory CD4+ T lymphocytes, and a high proportion of TEMRA CD4+ and CD8+ cells. Immunoglobulin levels and vaccine titers were within normal limits. Genetic sequencing identified a heterozygous nonsense variant in the STAT5B gene (c.361C>T, p.R121*) that has not previously been reported in publicly available datasets. This variant segregated with atopic dermatitis in the proband’s sister and niece. Consistent with impairment in STAT5B signaling in vivo, IGF1 levels were low, while GH and IGFBP3 levels were within normal limits. Western blot analysis confirmed reduced total p-STAT5 and STAT5B expression in primary dermal fibroblasts. This study identifies a novel heterozygous nonsense variant in the STAT5B gene that results in haploinsufficiency associated with severe atopic dermatitis and mild growth impairment with notably absent autoimmune endocrinopathy or other immune dysregulation. These findings expand the phenotypic spectrum of STAT5B-associated disorders and highlight the importance of targeted therapeutic interventions. Figure 1. Reduced p-STAT5 and STAT5B expression in dermal fibroblasts (M42).

In the decade since its first description, our understanding of hereditary alpha-tryptasemia (HαT) and its clinical impact on the field of allergy has continued to evolve. We now know that HαT is a common genetic trait that impacts the frequency and/or severity of systemic mast cell-mediated reactions. Moreover, as a natural overexpression model of α-tryptase, HαT brought our attention to the importance of tryptase isoform composition in health and disease, the implications of which we are still working to fully understand. In this review, we will summarize the data examining the importance of HαT and α-tryptase more generally on the prevalence and severity of anaphylaxis in patients with venom allergy, drug allergy, food allergy, clonal mast cell disease, and idiopathic anaphylaxis.

PGM3 deficiency is a multisystem disorder characterized by recurrent infections, chronic severe neutropenia, and virus-associated malignancies, often leading to premature mortality in early adulthood. These features strongly support early consideration of hematopoietic stem cell transplantation in affected individuals.

Mastocytosis represents a group of rare clonal disorders characterized by accumulation of neoplastic mast cells. Disease presentations range from indolent to highly aggressive forms. The discovery of somatic mutations in KIT, particularly KIT p.D816V, has revolutionized diagnosis, classification, and management of mastocytosis. KIT p.D816V, found in >85% of systemic mastocytosis (SM) cases, drives disease progression through constitutive activation of the KIT receptor. Highly sensitive techniques, such as allele-specific oligonucleotide quantitative polymerase chain reaction (PCR), digital PCR, and Flow-Super Rolling Circle Amplification, have enhanced detection of KIT p.D816V, whereas next-generation sequencing has allowed detection of other mutations, improving not only diagnostics and prognostication, but also monitoring of KIT p.D816V-targeted therapies. Of note, higher KIT p.D816V allele burdens, together with the presence of additional mutations in genes such as DNMT3A (DNA (cytosine-5)-methyltransferase 3α), SRSF2 (serine/arginine-rich splicing factor 2), ASXL1 (additional sex combs-like 1), EZH2 (enhancer of zeste homolog 2), and/or RUNX1 (runt-related transcription factor 1), termed high risk mutations, correlate with advanced SM subtypes. Hereditary α-tryptasemia (HαT) is a genetic condition where increased TPSAB1 gene copy encoding α-tryptase usually leads to elevated serum tryptase. The incidence of HαT is increased in mastocytosis and may exacerbate mediator-related symptoms, emphasizing the importance of searching for this genetic condition in mastocytosis. To conclude, despite remaining challenges in standardization, molecular investigations may now improve diagnostics, prognostication, and treatment monitoring in mastocytosis.

Present and former investigators of the Mast Cell Biology Section (MCBS) of the Laboratory of Allergic Diseases (LAD) and their colleagues organized a daylong symposium in October 2023 to honor the outstanding contributions of the MCBS headed by Dr Dean D. Metcalfe, who recently retired from his leadership role in this department. The symposium featured an overview of discoveries that advanced the scientific understanding of the human mast cell (MC) lineage and compartment over the preceding 3 decades. Insights into mechanisms and molecules contributing to MC expansion and function in health and disease, MC activation, and neoplastic MCs in mastocytosis were presented. The impact of these discoveries on the diagnosis and management of allergic diseases and MC disorders, as well as unmet needs in MC research, were reviewed. A summary of these discussions is provided.

INTRODUCTION:Hereditary alpha-tryptasemia (HαT) is caused by the increased copy number of TPSAB1 when encoding for alpha-tryptase, resulting in elevated basal serum tryptase (BST). Many affected individuals report irritable bowel syndrome-like and reflux symptoms. We aimed to assess the prevalence of HαT in celiac disease (CeD) and whether this genetic trait modifies disease course. METHODS:This study included a prospective cohort of subjects with CeD or nonceliac gluten sensitivity (NCGS) either at diagnosis (Dx), with persisting symptoms on a gluten-free diet, or in clinical remission. BST levels were determined by immunoassay, and tryptase genotyping was performed on genomic DNA using digital droplet polymerase chain reaction (PCR) [ddPCR]. Duodenal and gastric biopsies were stained for c-KIT, and mast cell (MC) counts were averaged over 5 high power field (hpf). RESULTS:There were 153 eligible subjects: 13 with NCGS and 140 with CeD (8 patients with new Dx, 66 with persisting symptoms, 66 in remission). HαT was found in 9 subjects, all symptomatic with CeD (6.4%). One was new Dx, and the others had persisting symptoms (12.3% of subgroup). Excluding HαT, BST levels were higher among patients with CeD vs NCGS (median 5.4 vs 3.9 mcg/L P < 0.05). Duodenal MC counts were higher in CeD vs controls ( P < 0.05) and 24% higher in those with HαT (median HαT CeD 27.3/hpf, non-HαT CeD 22.0/hpf, controls 18.4/hpf). MC counts did not differ based on villous atrophy or clinical presentation. DISCUSSION:The prevalence of HαT in CeD is similar to the general population; however, all participants with CeD and HαT had ongoing gastrointestinal (GI) symptoms. Evaluation for HαT should be considered in the management of patients with CeD and persisting symptoms.

PGM3 deficiency is a multisystem disorder characterized by recurrent infections, chronic severe neutropenia, and virus-associated malignancies, often leading to premature mortality in early adulthood. These features strongly support early consideration of hematopoietic stem cell transplantation in affected individuals.

Viral variant and host vaccination status impact infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), yet how these factors shift cellular responses in the human nasal mucosa remains uncharacterized. We performed single-cell RNA sequencing (scRNA-seq) on nasopharyngeal swabs from vaccinated and unvaccinated adults with acute Delta and Omicron SARS-CoV-2 infections and integrated with data from acute infections with ancestral SARS-CoV-2. Patients with Delta and Omicron exhibited greater similarity in nasal cell composition driven by myeloid, T cell and SARS-CoV-2hi cell subsets, which was distinct from that of ancestral cases. Delta-infected samples had a marked increase in viral RNA, and a subset of PER2+EGR1+GDF15+ epithelial cells was enriched in SARS-CoV-2 RNA+ cells in all variants. Prior vaccination was associated with increased frequency and activation of nasal macrophages. Expression of interferon-stimulated genes negatively correlated with coronavirus disease 2019 (COVID-19) severity in patients with ancestral and Delta but not Omicron variants. Our study defines nasal cell responses and signatures of disease severity across SARS-CoV-2 variants and vaccination. Ordovas-Montanes and colleagues describe the composition of the nasal cellular ecosystem and signatures of disease severity in vaccinated and unvaccinated adults during infection with the ancestral, Delta and Omicron variants of SARS-CoV-2.

Knowledge and understanding of mast cell biology and mast cell disorders have increased in the past several years, with new classifications of diseases for both clonal and nonclonal forms. Along with these classifications has come differing treatment paradigms, including novel therapies now approved for various forms of clonal mast cell disorders. Unfortunately, there is some lack of guidance on how best to use these therapies. This yardstick aims to provide the clinician with a review of available therapies to treat mast cell activation syndrome and indolent systemic mastocytosis and an evidence-based expert opinion approach regarding on how best to use these therapies.

OSMRβ (Oncostatin M receptor beta), a member of the IL-6 superfamily of cell surface receptors, binds OSM and IL-31 and plays a critical role in human immunity. We identified probands from four kindreds with biallelic damaging variants in OSMR , which encodes OSMRβ. Patients had a unifying phenotype for severe widespread, early-onset atopic dermatitis, peripheral eosinophilia, and elevated serum IgE. Patient OSMRβ variants were not appropriately expressed on the cell surface compared to OSMRβ WT . Patient OSMR variants showed significantly reduced OSM-mediated activation of STAT1, STAT3, and STAT5 and distinct transcriptional changes in primary dermal fibroblasts, including loss of interferon and inflammatory signatures. These defects were rescued upon lentiviral transduction of WT- OSMR . Together, these data establish that human germline biallelic loss-of-function OSMR variants cause severe allergic disease. We anticipate that this discovery will facilitate the recognition of additional affected individuals and the full definition of this novel primary atopic disorder.

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.

Since its fi rst 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 fi ndings 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.

Mast cell activation syndrome (MCAS) is a term applied to several clinical entities which have gained increased attention from patients and medical providers. While 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 (MC) activation is not well understood. More importantly, the underlying mechanisms and pathways that lead to MC activation in MCAS patients remain to be elucidated.The purpose of this manuscript is to summarize the known literature, identify gaps in knowledge, and highlight research needs. Several topics are covered: 1) Contextualization of MCAS and MCAS-like endotypes and related diagnostic evaluations; 2) Mechanistic research; 3) Management of typical and refractory symptoms, and 4) MCAS-specific education for patients and healthcare providers.

Timely diagnosis of systemic mastocytosis (SM) remains challenging because of care heterogeneity. We implemented a standardized approach for SM screening and diagnosis using a novel health care system-wide international screening registry. A retrospective analysis assessed rates of SM, cutaneous mastocytosis (CM), and molecular diagnoses before and 2 years after care standardization. The accuracy of individual and combined SM screening tests, basal serum tryptase (BST) >= 11.5 and >= 20.0 ng/mL, REMA >= 2, monomorphic maculopapular CM (MPCM), and elevated BST based upon tryptase genotype, was analyzed. Tryptase genotyping and high-sensitivity KIT p.D816V testing increased substantially 2 years after care standardization. SM diagnoses doubled from 47 to 94, and KIT p.D816V molecular diagnoses increased from 24 to 79. Mean BST and KIT p.D816V variant allele frequency values were significantly lower in patients diagnosed after standardization. Hereditary-alpha tryptasemia prevalence was increased in SM before care standardization (4/30 [13.3%]) but reflected the general population prevalence 2 years later at (5/76 [6.6%]). Elevated BST based upon genotype and BST >= 11.5 ng/mL had the highest sensitivities at 84.2% and 88.3%, respectively. The presence of monomorphic MPCM, elevated BST based upon tryptase genotype, and the combination of REMA >= 2 with elevated BST based upon tryptase genotype had specificities >90%. BST >20.0 ng/mL had low sensitivity and specificity and was not required to establish any indolent SM (ISM) diagnosis. Care standardization increased SM diagnosis rates, particularly in patients with low BSTs. Stratifying BST based upon genotype had the best overall sensitivity and specificity of any ISM screening test and improved the REMA score specificity.

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. 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.