Mast Cell Activation Syndrome: current understanding and research needs.

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.

Targeting KIT by frameshifting mRNA transcripts as a therapeutic strategy for aggressive mast cell neoplasms.

Activating mutations in c-KIT are associated with the mast cell (MC) clonal disorders cutaneous mastocytosis and systemic mastocytosis and its variants, including aggressive systemic mastocytosis, MC leukemia, and MC sarcoma. Currently, therapies inhibiting KIT signaling are a leading strategy to treat MC proliferative disorders. However, these approaches may have off-target effects, and in some patients, complete remission or improved survival time cannot be achieved. These limitations led us to develop an approach using chemically stable exon skipping oligonucleotides (ESOs) that induce exon skipping of precursor (pre-)mRNA to alter gene splicing and introduce a frameshift into mature KIT mRNA transcripts. The result of this alternate approach results in marked downregulation of KIT expression, diminished KIT signaling, inhibition of MC proliferation, and rapid induction of apoptosis in neoplastic HMC-1.2 MCs. We demonstrate that in vivo administration of KIT targeting ESOs significantly inhibits tumor growth and systemic organ infiltration using both an allograft mastocytosis model and a humanized xenograft MC tumor model. We propose that our innovative approach, which employs well-tolerated, chemically stable oligonucleotides to target KIT expression through unconventional pathways, has potential as a KIT-targeted therapeutic alone, or in combination with agents that target KIT signaling, in the treatment of KIT-associated malignancies.

Demonstration and implications of IL-3 upregulation of CD25 expression on human mast cells.

BACKGROUND: CD25+ human mast cells (huMCs) have been reported in patients with monoclonal mast cell diseases and in rare association with inflammation. However, the regulation of CD25 expression on huMCs and the possible biologic consequences remain poorly understood. OBJECTIVE: We sought to identify conditions that would upregulate CD25 expression on huMCs and to explore possible functional implications. METHODS: huMCs were cultured from peripheral blood progenitor cells over 6 to 8 weeks. Expression of CD25 was determined by fluorescence-activated cell sorting and soluble CD25 by ELISA. Signal transducer and activator of transcription 5 (STAT5) phosphorylation induced by IL-2 in huMCs, regulatory T (Treg) cells, or in cocultured huMCs and Treg cells was examined by fluorescence-activated cell sorting. RESULTS: Addition of IL-3 to CD34+ progenitors at the initiation of huMC cultures in the presence of stem cell factor and IL-6 upregulated the expression of CD25 in developing huMCs and resulted in shedding of soluble CD25 into the media. Removal of IL-3 after the first week of culture did not affect subsequent expression of CD25. Furthermore, addition of IL-3 14 days after the initiation of the culture did not induce significant CD25 expression. Treatment with anti-IL-3 antibody or the Janus kinase inhibitor tofacitinib blocked IL-3-induced CD25 upregulation. Binding of IL-2 to CD25+ huMCs did not induce STAT5 phosphorylation. However, coincubation of Treg cells with CD25+ huMCs pretreated with IL-2 was sufficient to result in STAT5 phosphorylation in Treg cells. CONCLUSIONS: IL-3 promotes CD25 expression and shedding by huMCs. Although CD25+ huMCs do not respond to IL-2, they bind IL-2 and may act as a reservoir of IL-2 to then activate lymphocytes.

Drug-induced mast cell eradication: A novel approach to treat mast cell activation disorders?

Mast cell (MC) activation is a key event in allergic reactions, other inflammatory states, and MC activation syndromes. MC-stabilizing agents, mediator-targeting drugs, and drugs interfering with mediator effects are often prescribed for these patients. However, the clinical efficacy of these drugs varies depending on the numbers of involved MCs and the underlying pathology. One straightforward approach would be to eradicate the primary target cell. To date however, no MC-eradicating treatment approach has been developed for patients with MC activation disorders. Nevertheless, recent data suggest that long-term treatment with agents effectively inhibiting KIT function results in the virtual eradication of tissue MCs and a sustained decrease in serum tryptase levels. In many of these patients, MC depletion is associated with a substantial improvement in mediator-induced symptoms. In patients with an underlying KIT D816V-positive mastocytosis, such MC eradication requires an effective inhibitor of KIT D816V, such as avapritinib. However, the use of KIT inhibitors must be balanced against their potential side effects. Here we discuss MC-eradicating strategies in various disease models, the feasibility of this approach, available clinical data, and future prospects for the use of KIT-targeting drugs in MC activation disorders.

Defining baseline variability of serum tryptase levels improves accuracy in identifying anaphylaxis.

BACKGROUND: Acute increases of ≥20% + 2 ng/mL (20 + 2 rule) over basal serum tryptase (BST) is the recommended threshold supporting a clinical diagnosis of anaphylaxis. Prospective studies have demonstrated high sensitivity for this algorithm after parenteral exposure, but specificity has not been evaluated. OBJECTIVE: We sought to define a serum tryptase change that distinguishes baseline variability from anaphylaxis on the basis of intraindividual variation in BST. METHODS: Ninety-three total subjects with atopy (n = 62) or hereditary α-tryptasemia (HαT) (n = 31) and ≥2 BST measurements were identified. Sequential BST variability measurements were modeled and threshold ratios that optimized sensitivity and/or specificity determined. Models were tested in 22 individuals with physician-diagnosed anaphylaxis and validated in independent cohorts of individuals with HαT (n = 33), indolent systemic mastocytosis (ISM) (n = 52), and ISM + HαT (n = 12). Mature tryptase levels were measured in HαT (n = 19) and ISM (n = 20). An online application was developed for clinical use. RESULTS: As a result of BST variability, 9.7% (9/93) of primary cohort patients, and 18% (6/33) of HαT, 30% (16/53) of ISM, and 25% (3/12) of ISM + HαT patients from validation cohorts met the 20 + 2 rule despite absent immediate hypersensitivity symptoms; mature tryptase was noncontributory among individuals with HαT or ISM at baseline. A ratio of acute tryptase/BST exceeding 1.685 provided the optimized diagnostic rule for jointly maximizing sensitivity and specificity. Statistically significant improvement in specificity relative to the 20 + 2 rule was observed among individuals with elevated BST caused by HαT and ISM. CONCLUSIONS: Using an acute tryptase/BST ratio of 1.685 improves specificity of measured changes among individuals with HαT and ISM while maintaining high sensitivity for confirmation of anaphylaxis.

Clinical impact and proposed application of molecular markers, genetic variants, and cytogenetic analysis in mast cell neoplasms: Status 2022.

Mast cell neoplasms are an emerging challenge in the fields of internal medicine, allergy, immunology, dermatology, laboratory medicine, and pathology. In this review, we discuss the current standards for the diagnosis and prognostication of mast cell neoplasms with special reference to clinically relevant germline and somatic gene variants. In patients with cutaneous mastocytosis or with indolent systemic mastocytosis (SM), various KIT-activating mutations act as key molecular drivers of the disease. In adults, KIT p.D816V is by far the most prevalent driver, whereas other KIT mutants are detected in nearly 40% of children. In advanced SM, including aggressive SM, SM with an associated hematological neoplasm, and mast cell leukemia, additional somatic mutations in other genes, such as SRSF2, JAK2, RUNX1, ASXL1, or RAS, may be detected. These drivers are more frequently detected in SM with an associated hematological neoplasm, particularly in male patients. Recently, hereditary alpha-tryptasemia has been identified as a genetic trait more prevalent in SM compared with healthy controls. Moreover, hereditary alpha-tryptasemia is more frequent in patients with SM with Hymenoptera venom allergy and severe mediator-related symptoms than in patients with SM without symptoms. On the basis of this knowledge, we propose a diagnostic algorithm in which genetic markers are applied together with clinical and histopathologic criteria to establish the diagnosis and prognosis in SM.

Decoding the intricacies of the mast cell compartment.

Historically, understanding of the human mast cell (MC) compartment has lagged behind the appreciation of other cell lineages. MCs exist in vascularised tissues but do not under normal circumstances circulate in blood, and there has been no pharmacological agent identified that totally and selectively inhibits human MC function. There are no substantiated accounts of an apparently healthy individual who is severely lacking in MCs. Thus, some of the approaches employed to understand the function of a specific immune cell are not available to the MC biologist. The disease categories that have provided the greatest insight into MC biology have been monoclonal and IgE-mediated MC disorders. This has led to the categorisation of MC diseases as intrinsic or extrinsic to the MC compartment and to the recognition of the role of mediators in MC activation disorders. Mastocytosis as a clonal disorder not only impacts the MC compartment through changes intrinsic to the MC, but also by the effects of episodes of significant release of MC mediators. The availability of newer therapeutic approaches developed to treat monoclonal MC disorders offer insights into how to more selectively approach management of MC centric diseases.

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.

A truncated splice-variant of the FcεRIß receptor subunit is critical for microtubule formation and degranulation in mast cells.

Human linkage analyses have implicated the MS4A2-containing gene locus (encoding FcεRIß) as a candidate for allergy susceptibility. We have identified a truncation of FcεRIß (t-FcεRIß) in humans that contains a putative calmodulin-binding domain and thus, we sought to identify the role of this variant in mast cell function. We determined that t-FcεRIß is critical for microtubule formation and degranulation and that it may perform this function by trafficking adaptor molecules and kinases to the pericentrosomal and Golgi region in response to Ca2+ signals. Mutagenesis studies suggest that calmodulin binding to t-FcεRIß in the presence of Ca2+ could be critical for t-FcεRIß function. In addition, gene targeting of t-FcεRIß attenuated microtubule formation, degranulation, and IL-8 production downstream of Ca2+ signals. Therefore, t-FcεRIß mediates Ca2+ -dependent microtubule formation, which promotes degranulation and cytokine release. Because t-FcεRIß has this critical function, it represents a therapeutic target for the downregulation of allergic inflammation.

A randomized double-blind, placebo-controlled study of omalizumab for idiopathic anaphylaxis.

BACKGROUND: Idiopathic anaphylaxis (IA) is a diagnosis of exclusion, thus taking away the option of therapeutic management focused on eliminating the inciting agent. Epinephrine and antihistamines followed by systemic corticosteroids are the mainstays of therapy for acute events. There is no prophylactic therapy that reliably prevents anaphylaxis. OBJECTIVE: We sought to determine the efficacy of omalizumab in the management of patients with frequent episodes of IA in a double-blind, placebo-controlled trial. METHODS: We prospectively enrolled 19 patients with frequent IA (≥6 episodes/y) who then underwent a medical evaluation that included a serum tryptase determination, mutational analysis for KIT D816V, and bone marrow evaluation to rule out a clonal mast cell disorder. Computer-generated random numbers were provided by the study pharmacist. The primary end point was anaphylactic events in the 6 months after baseline. Sixteen patients completed the primary trial. RESULTS: No statistically significant difference was demonstrated between the placebo and treated groups. There was a trend for efficacy in the treatment group, particularly after 60 days. Overall, the safety profile was favorable without long-term side effects. CONCLUSIONS: Omalizumab was safely administered to a difficult-to-treat patient population with IA. The efficacy results trended modestly in favor of the treatment group, but no statistically significant differences were detected.

Heritable risk for severe anaphylaxis associated with increased α-tryptase-encoding germline copy number at TPSAB1.

BACKGROUND: An elevated basal serum tryptase level is associated with severe systemic anaphylaxis, most notably caused by Hymenoptera envenomation. Although clonal mast cell disease is the culprit in some individuals, it does not fully explain this clinical association. OBJECTIVE: Our aim was to determine the prevalence and associated impact of tryptase genotypes on anaphylaxis in humans. METHODS: Cohorts with systemic mastocytosis (SM) and venom as well as idiopathic anaphylaxis from referral centers in Italy, Slovenia, and the United States, underwent tryptase genotyping by droplet digital PCR. Associated anaphylaxis severity (Mueller scale) was subsequently examined. Healthy volunteers and controls with nonatopic disease were recruited and tryptase was genotyped by droplet digital PCR and in silico analysis of genome sequence, respectively. The effects of pooled and recombinant human tryptases, protease activated receptor 2 agonist and antagonist peptides, and a tryptase-neutralizing mAb on human umbilical vein endothelial cell permeability were assayed using a Transwell system. RESULTS: Hereditary α-tryptasemia (HαT)-a genetic trait caused by increased α-tryptase-encoding Tryptase-α/ß1 (TPSAB1) copy number resulting in elevated BST level-was common in healthy individuals (5.6% [n = 7 of 125]) and controls with nonatopic disease (5.3% [n = 21 of 398]). HαT was associated with grade IV venom anaphylaxis (relative risk = 2.0; P < .05) and more prevalent in both idiopathic anaphylaxis (n = 8 of 47; [17%; P = .006]) and SM (n = 10 of 82 [12.2%; P = .03]) relative to the controls. Among patients with SM, concomitant HαT was associated with increased risk for systemic anaphylaxis (relative risk = 9.5; P = .007). In vitro, protease-activated receptor-2-dependent vascular permeability was induced by pooled mature tryptases but not α- or ß-tryptase homotetramers. CONCLUSIONS: Risk for severe anaphylaxis in humans is associated with inherited differences in α-tryptase-encoding copies at TPSAB1.

Proceedings from the Inaugural American Initiative in Mast Cell Diseases (AIM) Investigator Conference.

The American Initiative in Mast Cell Diseases (AIM) held its inaugural investigator conference at Stanford University School of Medicine in May 2019. The overarching goal of this meeting was to establish a Pan-American organization of physicians and scientists with multidisciplinary expertise in mast cell disease. To serve this unmet need, AIM envisions a network where basic, translational, and clinical researchers could establish collaborations with both academia and biopharma to support the development of new diagnostic methods, enhanced understanding of the biology of mast cells in human health and disease, and the testing of novel therapies. In these AIM proceedings, we highlight selected topics relevant to mast cell biology and provide updates regarding the recently described hereditary alpha-tryptasemia. In addition, we discuss the evaluation and treatment of mast cell activation (syndromes), allergy and anaphylaxis in mast cell disorders, and the clinical and biologic heterogeneity of the more indolent forms of mastocytosis. Because mast cell disorders are relatively rare, AIM hopes to achieve a coordination of scientific efforts not only in the Americas but also in Europe by collaborating with the well-established European Competence Network on Mastocytosis.

Clinical relevance of inherited genetic differences in human tryptases: Hereditary alpha-tryptasemia and beyond.

OBJECTIVE: To describe our current understanding of hereditary α-tryptasemia (HαT), how HαT fits into the evolutionary context of tryptases and contemporary framework of mast cell-associated disorders, and to discuss the future clinical and therapeutic landscape for symptomatic individuals with HαT. DATA SOURCES: Primary peer-reviewed literature. STUDY SELECTIONS: Basic, clinical, and translational studies describing tryptase gene composition, generation, secretion, and elevation and the associated clinical impacts of HαT and treatment of such individuals were reviewed. RESULTS: HαT is a common autosomal dominant genetic trait caused by increased TPSAB1 copy number encoding α-tryptase. Approximately 1 in 20 White individuals have HαT, making it by far the most common cause for elevated basal serum tryptase levels. Although many individuals with HαT may not manifest associated symptoms, the prevalence of HαT is increased in patients with clonal and nonclonal mast cell-associated disorders wherein it is linked to more prevalent and/or severe anaphylaxis and increased mast cell mediator-associated symptoms. Increased generation of mature α/ß-tryptase heterotetramers, and their unique physiochemical properties, may be responsible for some of these clinical findings. CONCLUSION: HαT is a common modifier of mast cell-associated disorders and reactions. Nevertheless, whether HαT may be an independent cause of clinical phenotypes with which it has been associated remains unproven. Correct identification of HαT is critical to accurate interpretation of serum tryptase levels in the clinical evaluation of patients. Beyond HαT, we foresee tryptase genotyping as an important parameter in the standard workup of patients with mast cell-associated disorders and development of therapeutic modalities targeting these patients and associated clinical phenotypes.

Inhibition of Allergic Reactivity through Targeting FcεRI-Bound IgE with Humanized Low-Affinity Antibodies.

Options for effective prevention and treatment of epidemic allergic diseases remain limited, and particularly so for IgE-mediated food allergies. We previously found that mouse low-affinity anti-human IgE mAbs with KD in the 10-6-10-8 M range were capable of blocking allergic reactivity without triggering immediate allergic mediator release. In this study, we humanized three parent low affinity allergic response inhibitor (LARI) mouse anti-human IgE mAbs and characterized their biological and immunological features, refined the lead candidate for further clinical development, examined their safety profiles, determined their therapeutic efficiency, and explored the mechanism of action potentially responsible for their therapeutic effects. LARI profoundly blocked cat- and peanut-allergic IgE-mediated basophil activation, inhibited acute release of both prestored and newly synthesized mediator from human mast cells, suppressed peanut-specific IgE-mediated passive cutaneous anaphylaxis, and attenuated dansyl IgE-mediated systemic anaphylaxis in human FcεRIα transgenic mice. Safety testing demonstrated that concentrations of LARI well above therapeutic levels failed to trigger immediate release of prestored and newly synthesized allergic mediators, failed to promote robust cytokine/chemokine production from allergic effector cells, and did not elicit allergic reactivity in an animal model of cutaneous and systemic anaphylaxis. Mechanistic studies revealed that LARI downregulated surface FcεRI receptors and IgE via internalization of the IgE/FcεRI, promoted a partial mediator depletion pathway leading to slow release of small amount of mediators, and functioned as a partial antagonist to inhibit FcεRI signaling phosphorylation of Syk, Akt, Erk, and p38 MAPK. These studies demonstrate that targeting surface-bound IgE with LARI profoundly suppresses human allergic reactivity while displaying an excellent safety profile.

Mastocytosis-derived extracellular vesicles exhibit a mast cell signature, transfer KIT to stellate cells, and promote their activation.

Extracellular vesicles (EVs) have been implicated in the development and progression of hematological malignancies. We thus examined serum samples from patients with systemic mastocytosis (SM) and found EVs with a mast cell signature including the presence of tryptase, FcεRI, MRGX2, and KIT. The concentration of these EVs correlated with parameters of disease including levels of serum tryptase, IL-6, and alkaline phosphatase and physical findings including hepatosplenomegaly. Given reports that EVs from one cell type may influence another cell's behavior, we asked whether SM-EVs might affect hepatic stellate cells (HSCs), based on the abnormal liver pathology associated with mastocytosis. We found that KIT was transferred from SM-EVs into an HSC line eliciting proliferation, cytokine production, and differentiation, processes that have been associated with liver pathology. These effects were reduced by KIT inhibition or neutralization and recapitulated by enforced expression of KIT or constitutively active D816V-KIT, a gain-of-function variant associated with SM. Furthermore, HSCs in liver from mice injected with SM-EVs had increased expression of α-SMA and human KIT, particularly around portal areas, compared with mice injected with EVs from normal individuals, suggesting that SM-EVs can also initiate HSC activation in vivo. Our data are thus consistent with the conclusion that SM-EVs have the potential to influence cells outside the hematological compartment and that therapeutic approaches for treatment of SM may be effective in part through inhibition of effects of EVs on target tissues, findings important both to understanding complex disease pathology and in developing interventional agents for the treatment of hematologic diseases.

Risk and management of patients with mastocytosis and MCAS in the SARS-CoV-2 (COVID-19) pandemic: Expert opinions.

The coronavirus disease 2019 (COVID-19) (caused by severe acute respiratory syndrome coronavirus 2) pandemic has massively distorted our health care systems and caused catastrophic consequences in our affected communities. The number of victims continues to increase, and patients at risk can only be protected to a degree, because the virulent state may be asymptomatic. Risk factors concerning COVID-19-induced morbidity and mortality include advanced age, an impaired immune system, cardiovascular or pulmonary diseases, obesity, diabetes mellitus, and cancer treated with chemotherapy. Here, we discuss the risk and impact of COVID-19 in patients with mastocytosis and mast cell activation syndromes. Because no published data are yet available, expert opinions are, by necessity, based on case experience and reports from patients. Although the overall risk to acquire the severe acute respiratory syndrome coronavirus 2 may not be elevated in mast cell disease, certain conditions may increase the risk of infected patients to develop severe COVID-19. These factors include certain comorbidities, mast cell activation-related events affecting the cardiovascular or bronchopulmonary system, and chemotherapy or immunosuppressive drugs. Therefore, such treatments should be carefully evaluated on a case-by-case basis during a COVID-19 infection. In contrast, other therapies, such as anti-mediator-type drugs, venom immunotherapy, or vitamin D, should be continued. Overall, patients with mast cell disorders should follow the general and local guidelines in the COVID-19 pandemic and advice from their medical provider.

Clinical Impact of Inherited and Acquired Genetic Variants in Mastocytosis.

Mastocytosis is a rare and complex disease characterized by expansion of clonal mast cells (MC) in skin and/or various internal organ systems. Involvement of internal organs leads to the diagnosis of systemic mastocytosis (SM). The WHO classification divides SM into indolent SM, smoldering SM and advanced SM variants, including SM with an associated hematologic neoplasm, aggressive SM, and MC leukemia. Historically, genetic analysis of individuals with pure cutaneous mastocytosis (CM) and SM have focused primarily on cohort studies of inherited single nucleotide variants and acquired pathogenic variants. The most prevalent pathogenic variant (mutation) in patients with SM is KIT p.D816V, which is detectable in most adult patients. Other somatic mutations have also been identified-especially in advanced SM-in TET2, SRSF2, ASXL1, RUNX1, CBL and JAK2, and shown to impact clinical and cellular phenotypes. Although only small patient cohorts have been analyzed, disease associations have also been identified in several germline variants within genes encoding certain cytokines or their receptors (IL13, IL6, IL6R, IL31, IL4R) and toll-like receptors. More recently, an increased prevalence of hereditary alpha-tryptasemia (HαT) caused by increased TPSAB1 copy number encoding alpha-tryptase has been described in patients with SM. Whereas HαT is found in 3-6% of general Western populations, it is identified in up to 17% of patients with SM. In the current manuscript we review the prevalence, functional role and clinical impact of various germline and somatic genetic variants in patients with mastocytosis.

Oncogenic D816V-KIT signaling in mast cells causes persistent IL-6 production.

Persistent dysregulation of IL-6 production and signaling have been implicated in the pathology of various cancers. In systemic mastocytosis, increased serum levels of IL-6 associate with disease severity and progression, although the mechanisms involved are not well understood. Since systemic mastocytosis often associates with the presence in hematopoietic cells of a somatic gain-of-function variant in KIT, D816V-KIT, we examined its potential role in IL-6 upregulation. Bone marrow mononuclear cultures from patients with greater D816V allelic burden released increased amounts of IL-6 which correlated with the percentage of mast cells in the cultures. Intracellular IL-6 staining by flow cytometry and immunofluorescence was primarily associated with mast cells and suggested a higher percentage of IL-6 positive mast cells in patients with higher D816V allelic burden. Furthermore, mast cell lines expressing D816V-KIT, but not those expressing normal KIT or other KIT variants, produced constitutively high IL-6 amounts at the message and protein levels. We further demonstrate that aberrant KIT activity and signaling are critical for the induction of IL-6 and involve STAT5 and PI3K pathways but not STAT3 or STAT4. Activation of STAT5A and STAT5B downstream of D816V-KIT was mediated by JAK2 but also by MEK/ERK1/2, which not only promoted STAT5 phosphorylation but also its long-term transcription. Our study thus supports a role for mast cells and D816V-KIT activity in IL-6 dysregulation in mastocytosis and provides insights into the intracellular mechanisms. The findings contribute to a better understanding of the physiopathology of mastocytosis and suggest the importance of therapeutic targeting of these pathways.

Adverse reactions to drugs and biologics in patients with clonal mast cell disorders: A Work Group Report of the Mast Cells Disorder Committee, American Academy of Allergy, Asthma & Immunology.

Providers caring for patients with mastocytosis are tasked with the decision to consider therapeutic options. This can come with some trepidation because information available in the public domain lists numerous mast cell (MC) activators based on data that do not discriminate between primates, rodents, and MC lines; do not consider dosage; and do not take into account previous exposure and resultant clinical findings. This being said, there is support in the literature for an enhanced MC response in some patients with mastocytosis and in cases in which there is a greater incidence of adverse reactions associated with certain antigens, such as venoms and drugs. Thus this report provides a comprehensive guide for those providers who must decide on therapeutic options in the management of patients with clonal MC disease.

Sialic acid-binding immunoglobulin-like lectin (Siglec) 8 in patients with eosinophilic disorders: Receptor expression and targeting using chimeric antibodies.

BACKGROUND: Sialic acid-binding immunoglobulin-like lectin (Siglec) 8 is selectively expressed on eosinophils, mast cells, and basophils and, when engaged on eosinophils, can cause cell death. OBJECTIVE: We sought to characterize surface and soluble Siglec-8 (sSiglec-8) levels in normal donors (NDs) and eosinophilic donors (EOs) and assess the efficacy of anti-Siglec-8 antibodies in inducing eosinophil cell death in vitro. METHODS: Eosinophil expression of Siglec-8 was assessed by using flow cytometry and quantitative PCR. Serum sSiglec-8 levels were measured by means of ELISA. Induction of eosinophil death by IgG4 (chimeric 2E2 IgG4) and afucosylated IgG1 (chimeric 2E2 IgG1 [c2E2 IgG1]) anti-Siglec-8 antibodies was evaluated in vitro by using flow cytometry and in vivo in humanized mice. RESULTS: Siglec-8 was consistently expressed on eosinophils from NDs and EOs and did not correlate with absolute eosinophil count or disease activity. sSiglec-8 levels were measurable in sera from most donors unrelated to absolute eosinophil counts or Siglec-8 surface expression. c2E2 IgG1 and chimeric 2E2 IgG4 were equally effective at inducing cell death (Annexin-V positivity) of purified eosinophils from NDs and EOs after overnight IL-5 priming. In contrast, killing of purified eosinophils without IL-5 was only seen in EOs, and natural killer cell-mediated eosinophil killing was seen only with c2E2 IgG1. Finally, treatment of humanized mice with anti-Siglec antibody led to robust depletion of IL-5-induced eosinophilia in vivo. CONCLUSIONS: Siglec-8 is highly expressed on blood eosinophils from EOs and NDs and represents a potential therapeutic target for eosinophilic disorders. Enhanced killing of eosinophils in the presence of IL-5 might lead to increased efficacy in patients with IL-5-driven eosinophilia.