Data-driven programmatic approach to analysis of basophil activation tests

Basophil activation test in the food allergy clinic: its current use and future applications

INTRODUCTION

The basophil activation test (BAT) has shown evidence of high sensitivity and high specificity to support the diagnosis of IgE-mediated allergy. It is a functional test that uses live cells analyzed by flow cytometry and thus needs to be performed within 24h of blood collection. BAT has shown to be reproducible and reliable when tested in a clinical diagnostic laboratory with standardized protocols and flow cytometry settings.

AREAS COVERED

In this review, we summarize the evidence to support clinical use of BAT and the next steps required for clinical implementation for an improve clinical care for patients with suspected IgE-mediated food allergy.

EXPERT OPINION

BAT has recently been included in Clinical Guidelines of Food Allergy Diagnosis and its implementation in clinical practice depends largely on availability. Proposed clinical applications of the BAT include: distinction between food allergy and asymptomatic IgE sensitization; determination of food allergic status to peanut, tree nuts and seeds in polysensitized children; evaluation of tolerance to baked egg and baked milk in egg and milk allergic children; identification of patients at high-risk of severe allergic reactions; monitoring for spontaneous resolution of food allergy; confirmation of eligibility for specific treatments of food allergy; prediction and monitoring of response to immunomodulatory treatments.

Combining avidin with CD63 improves basophil activation test accuracy in classifying peanut allergy

BACKGROUND

Conventional basophil activation tests (BATs) measure basophil activation by the increased expression of CD63. Previously, fluorophore-labeled avidin, a positively-charged molecule, was found to bind to activated basophils, which tend to expose negatively charged granule constituents during degranulation. This study further compares avidin versus CD63 as basophil activation biomarkers in classifying peanut allergy.

METHODS

Seventy subjects with either a peanut allergy (N = 47), a food allergy other than peanut (N = 6), or no food allergy (N = 17) were evaluated. We conducted BATs in response to seven peanut extract (PE) concentrations (0.01-10,000 ng/mL) and four control conditions (no stimulant, anti-IgE, fMLP (N-formylmethionine-leucyl-phenylalanine), and anti-FcεRI). We measured avidin binding and CD63 expression on basophils with flow cytometry. We evaluated logistic regression and XGBoost models for peanut allergy classification and feature identification.

RESULTS

Avidin binding was correlated with CD63 expression. Both markers discriminated between subjects with and without a peanut allergy. Although small by percentage, an avidin+ /CD63- cell subset was found in all allergic subjects tested, indicating that the combination of avidin and CD63 could allow a more comprehensive identification of activated basophils. Indeed, we obtained the best classification accuracy (97.8% sensitivity, 96.7% specificity) by combining avidin and CD63 across seven PE doses. Similar accuracy was obtained by combining PE dose of 10,000 ng/mL for avidin and PE doses of 10 and 100 ng/mL for CD63.

CONCLUSIONS

Avidin and CD63 are reliable BAT activation markers associated with degranulation. Their combination enhances the identification of activated basophils and improves the classification accuracy of peanut allergy.

Immunotherapy-induced neutralizing antibodies disrupt allergen binding and sustain allergen tolerance in peanut allergy

In IgE-mediated food allergies, exposure to the allergen activates systemic allergic responses. Oral immunotherapy (OIT) treats food allergies through incremental increases in oral allergen exposure. However, OIT only induces sustained clinical tolerance and decreased basophil sensitivity in a subset of individuals despite increases in circulating allergen-specific IgG in all treated individuals. Therefore, we examined the allergen-specific antibodies from 2 OIT cohorts of patients with sustained and transient responses. Here, we compared antibodies from individuals with sustained or transient responses and discovered specific tolerance-associated conformational epitopes of the immunodominant allergen Ara h 2 recognized by neutralizing antibodies. First, we identified what we believe to be previously unknown conformational, intrahelical epitopes using x-ray crystallography with recombinant antibodies. We then identified epitopes only recognized in sustained tolerance. Finally, antibodies recognizing tolerance-associated epitopes effectively neutralized allergen to suppress IgE-mediated effector cell activation. Our results demonstrate the molecular basis of antibody-mediated protection in IgE-mediated food allergy, by defining how these antibodies disrupt IgE-allergen interactions to prevent allergic reactions. Our approach to studying the structural and functional basis for neutralizing antibodies demonstrates the clinical relevance of specific antibody clones in antibody-mediated tolerance. We anticipate that our findings will form the foundation for treatments of peanut allergy using neutralizing antibodies and hypoallergens.

Kinetics of basophil hyporesponsiveness during short-course peanut oral immunotherapy

BACKGROUND

Oral immunotherapy (OIT) leads to suppression of mast cell and basophil degranulation along with changes in the adaptive immune response.

OBJECTIVES

This study aimed to determine how rapidly these effects occur during OIT and more broadly, the kinetics of basophil and mast cell suppression throughout the course of therapy.

METHODS

Twenty participants, age 4 to 12 years, were enrolled in a peanut OIT trial and assessed for desensitization and sustained unresponsiveness after 9 months of therapy. Blood was collected 5 times in the first month and then intermittently throughout to quantify immunoglobulins and assess basophil activation by CD63, CD203c, and phosphorylated SYK (pSYK).

RESULTS

Twelve of 16 participants that completed the trial were desensitized after OIT, with 9 achieving sustained unresponsiveness after discontinuing OIT for 4 weeks. Basophil hyporesponsiveness, defined by lower CD63 expression, was detected as early as day 90. pSYK was correlated with CD63 expression, and there was a significant decrease in pSYK by day 250. CD203c expression remained unchanged throughout therapy. Interestingly, although basophil activation was decreased across the cohort during OIT, basophil activation did not correlate with individual clinical outcomes. Serum peanut-specific IgG4 and IgA increased throughout therapy, whereas IgE remained unchanged.

CONCLUSIONS

Suppression of basophil activation occurs within the first 90 days of peanut OIT, ultimately leading to suppression of signaling through pSYK.

Basophil activation test has high reproducibility and is feasible in the clinical setting

BACKGROUND

The basophil activation test (BAT) has high accuracy to diagnose peanut allergy and can reduce the need for oral food challenges (OFC); however, so far it has not been incorporated in clinical practice.

METHODS

We assessed the reproducibility of BAT within the same laboratory and between two different laboratories and the feasibility of using BAT in the clinical setting.

RESULTS

One hundred and two children being assessed for peanut allergy were tested on BAT (72 allergic, 30 sensitized tolerant). There was little internal variation (coefficient of variation <15%) in the BAT and a very strong correlation (R_s  > .95) between BAT performed across laboratories. The 2 BAT methods were strongly correlated but not interchangeable. In the cases of discrepancy, our in house BAT method was 100% accurate. BAT was feasible and well-accepted by clinicians: no patient with positive BAT was referred for OFC, leading to reduction in the number of OFC required. Twenty one percent of patients who underwent OFC reacted to peanut. A negative BAT also encouraged the performance of OFC in sensitized children who would otherwise be considered allergic, 50% of whom did not react and incorporated peanut in the diet.

CONCLUSIONS

The BAT is a robust test that can reliably be transferred between laboratories; however, different BAT methods are not interchangeable. BAT was well integrated in the clinical decision-making process in a specialized center.

Innovative robust basophil activation test using a novel gating strategy reliably diagnosing allergy with full automation

BACKGROUND

Allergy is one of the most common chronic diseases in Europe. Therefore, an increased need for specific and sensitive diagnostic tests that truly detect allergy exists. This study aimed at establishing a highly specific high-throughput and automated basophil activation test (BAT) that proves the existence of an allergy with utmost probability.

METHODS

BAT from 1104 samples was analyzed; a novel gating strategy with three antibodies (FcεRIα, CD203c, CD63) was established and compared with our published protocol (12 antibodies). Based on the novel gating strategy, storage conditions, automated measurement, and analyses using R (1376 samples out of 1389) were optimized to set up a high-throughput BAT.

RESULTS

No differences in sensitivity and specificity were found between the novel three antibody (FcεRIα, CD203c, CD63) and the 12 antibody gating strategy or between automated and manually analyzed samples (saving up to 90% of labor time). The time frame for basophil activation measurement after blood donation has been extended considerably (whole blood storage ≤7 days (RT) and 17 days (4°C) prior to BAT preparation and measurement). Respective storage conditions were optimized for samples after stimulation, staining, and preparation (≤7 days (RT) and 28 days (4°C)). These achievements were confirmed by a nationwide ring trial showing robustness and applicability of our BAT on a variety of flow cytometers.

CONCLUSION

Our considerable optimizations overcame the hurdles that until now prevented the BAT from being used as high-throughput allergy diagnostic test in routine laboratories and shall allow for collaborative studies between clinics and research centers.

The Basophil Activation Test for Clinical Management of Food Allergies: Recent Advances and Future Directions

The basophil activation test (BAT) is an ex vivo functional assay that measures by flow cytometry the degree of basophil degranulation after stimulation with an allergen. In recent years, there has been an increased interest in the diagnostic value of the BAT as it has the potential to mimic the clinical phenotype of sIgE sensitized patients, in contrast to allergen-specific IgE levels. This diagnostic potential would be of particular interest for food allergies present early in life such as peanut, cow’s milk and eggs, which require an expensive, time-consuming and patient unfriendly oral food challenge (OFC) for diagnosis. However, routine applications of the BAT for clinical use are not yet feasible due to the lack of standardized protocols and large clinical validation studies. This review will summarize the current data regarding the application of the BAT in food allergy (FA) for cow’s milk, egg and peanut, being the most common causes of FA in children. Additionally, it will discuss the hurdles for widespread clinical use of the BAT and possible future directions for this diagnostic procedure.

Basophil activation test in food allergy: is it ready for real-time?

PURPOSE OF REVIEW

Utilization of basophil activation in the diagnosis and monitoring of food allergy has gained increasing recognition. An ex-vivo functional assay, basophil activation reflects clinical reactivity, thereby providing clinically relevant insights. Moreover, as a biomarker of reactivity and tolerance, basophil activation testing (BAT) may provide a useful tool for management of food allergies. Despite its utility, significant limitations of BAT have prevented widespread use. Addressing these limitations will increase the future application and adoption of BAT in food allergy.

RECENT FINDINGS

A number of clinical trials in the past few years have demonstrated the use of BAT in the diagnosis and treatment of food allergy. Specifically, BAT has been found to be a biomarker of tolerance.

SUMMARY

Basophil activation testing is an effective biomarker for diagnosis and monitoring of food allergy.

Bringing the Next Generation of Food Allergy Diagnostics into the Clinic

Food allergy diagnosis has a massive impact on the lives of patients and their families. Despite recent developments with specific IgE to component allergens, a significant proportion of patients assessed for possible food allergy require oral food challenge to ensure an accurate diagnosis. More precise diagnostic methods are required to reduce the need for oral food challenges. Bead-based epitope assays and cellular tests, such as basophil activation and mast cell tests are the most novel and promising tests on the horizon. There is a pathway to pursue to enable their incorporation in clinical practice, including standardization, technical validation, clinical validation, external validation, overcoming practical and logistical issues, and regulatory approval. Valuable clinical application of these tests goes beyond diagnosis and includes risk assessment to identify allergic patients who are most sensitive and at risk for severe allergic reactions, and to define prognosis and assess clinical response to immunomodulatory treatments.

Basophil activation test: Mechanisms and considerations for use in clinical trials and clinical practice

The basophil activation test (BAT) is a functional assay that measures the degree of degranulation following stimulation with allergen or controls by flow cytometry. It correlates directly with histamine release. From the dose-response curve resulting from BAT in allergic patients, basophil reactivity (%CD63⁺ basophils) and basophil sensitivity (EC₅₀ or similar) are the main outcomes of the test. BAT takes into account all characteristics of IgE and allergen and thus can be more specific than sensitization tests in the diagnosis of allergic disease. BAT reduces the need for in vivo procedures, such as intradermal tests and allergen challenges, which can cause allergic reactions of unpredictable severity. As it closely reflects the patients' phenotype in most cases, it may be used to support the diagnosis of food, venom and drug allergies and chronic urticaria, to monitor the natural resolution of food allergies and to predict and monitor clinical the response to immunomodulatory treatments, such as allergen-specific immunotherapy and biologicals. Clinical application of BAT requires analytical validation, clinical validation, standardization of procedures and quality assurance to ensure reproducibility and reliability of results. Currently, efforts are ongoing to establish a platform that could be used by laboratories in Europe and in the USA for quality assurance and certification.

Mast cell activation tests by flow cytometry: A new diagnostic asset?

Since the late nineties, evidence has accumulated that flow-assisted basophil activation test (BAT) might be an accessible and reliable method to explore the mechanisms governing basophil degranulation and diagnostic allowing correct prediction of the clinical outcome following exposure to the offending allergen(s) and cross-reactive structures for different IgE-dependent allergies and particular forms of autoimmune urticaria. Although the BAT offers many advantages over mediator release tests, it is left with some weaknesses that hinder a wider application. It is preferable to perform the BAT analysis within 4 h of collection, and the technique does not advance diagnosis in patients with non-responsive cells. Besides, the BAT is difficult to standardize mainly because of the difficulty to perform large batch analyses that might span over several days. This article reviews the status of flow cytometric mast cell activation test (MAT) using passively sensitized mast cells (MCs) with patients' sera or plasma (henceforth indicated as passive MAT; pMAT) using both MC lines and cultured MCs in the diagnosis of IgE-dependent allergies. In addition, this paper provides guidance for generating human MCs from peripheral blood CD34⁺ progenitor cells (PBCMCs) and correct interpretation of flow cytometric analyses of activated and/or degranulating cells. With the recent recognition of the mas-related G protein-coupled receptor X2 (MRGPRX2) occupation as a putative mechanism of immediate drug hypersensitivity reactions (IDHRs), we also speculate how direct activation of MCs (dMAT)-that is direct activation by MRGPRX2 agonists without prior passive sensitization-could advance paradigms for this novel endotype of IDHRs.

Early decrease in basophil sensitivity to Ara h 2 precedes sustained unresponsiveness after peanut oral immunotherapy

BACKGROUND

Only some patients with peanut allergy undergoing oral immunotherapy (OIT) achieve sustained clinical response. Basophil activation could provide a functional surrogate of efficacy.

OBJECTIVE

We hypothesized that changes in basophil sensitivity and area under the curve (AUC) to the immunodominant allergen Ara h 2 correlate with clinical responses to OIT.

METHODS

Children with peanut allergy aged 7 to 13 years were enrolled in a single-center, open-label peanut OIT trial. Levels of specific immunoglobulins were measured throughout OIT. Peripheral blood from multiple time points was stimulated in vitro with peanut allergens for flow cytometric assessment of the percentage of CD63^hi activated basophils.

RESULTS

Twenty-two of 30 subjects were successfully treated with OIT; after avoidance, 9 achieved sustained unresponsiveness (SU), and 13 had transient desensitization (TD). Basophil sensitivity, measured by using the dose that induces 50% of the maximal basophil response, to Ara h 2 stimulation decreased from baseline in subjects with SU (after OIT, P = .0041; after avoidance, P = .0011). At 3 months of OIT, basophil sensitivity in subjects with SU decreased from baseline compared with that in subjects with TD (median, 18-fold vs 3-fold; P = .01), with a receiver operating characteristic of 0.84 and optimal fold change of 4.9. Basophil AUC to Ara h 2 was suppressed after OIT equally in subjects with SU and those with TD (P = .4). After avoidance, basophil AUC rebounded in subjects with TD but not those with SU (P < .001). Passively sensitized basophils suppressed with postavoidance SU plasma had a lower AUC than TD plasma (6.4% vs 38.9%, P = .03).

CONCLUSIONS

Early decreases in basophil sensitivity to Ara h 2 correlate with SU. Basophil AUC rebounds after avoidance in subjects with TD. Therefore, different aspects of basophil activation might be useful for monitoring of OIT efficacy.

Basophil Activation Test: Old and New Applications in Allergy

PURPOSE OF REVIEW

The basophil activation test (BAT) using flow cytometry has supplanted traditional methods of measuring basophil degranulation using histamine and other mediator release, and can be used for clinical applications as well as to explore the immune mechanisms of effector cell response to allergen. This review discusses the advancements made in clinical, diagnostic and laboratory research of allergy utilizing an ever-evolving BAT.

RECENT FINDINGS

Being an in vitro surrogate of the allergic reaction that happens in vivo in the sick patient, the BAT can be used to support the diagnosis of various allergic conditions, such as food, drug, respiratory and insect venom allergies, and the assessment of clinical response to allergen-specific immunotherapy and other immunomodulatory treatments. The BAT can also be used for research purposes to explore the mechanisms of allergy and tolerance at the level of the basophil, for instance by manipulating IgE and IgG and their receptors and by studying intracellular signalling cascade in response to allergen. This review covers the applications of the BAT to the clinical management of allergic patients and the increased understanding of the mechanisms of immune response to allergens as well as technological advancements made in recent years.

The basophil activation test differentiates between patients with alpha-gal syndrome and asymptomatic alpha-gal sensitization

BACKGROUND

Galactose-alpha-1,3-galactose (alpha-gal) syndrome is characterized by the presence of serum specific IgE antibodies to alpha-gal and delayed type I allergic reactions to the carbohydrate alpha-gal after consumption of mammalian (red) meat products and drugs of mammalian origin. Diagnostics currently rely on patient history, skin tests, determination of serum specific IgE antibodies, and oral food or drug challenges.

OBJECTIVE

We sought to assess the utility of different basophil parameters (basophil reactivity and sensitivity, the ratio of the percentage of CD63⁺ basophils induced by the alpha-gal-containing allergen to the percentage of CD63⁺ basophils after stimulation with anti-FcεRI antibody [%CD63⁺/anti-FcεRI], and area under the dose-response curve [AUC]) as biomarkers for the clinical outcome of patients with alpha-gal syndrome compared with subjects with asymptomatic alpha-gal sensitization.

METHODS

In addition to routine diagnostics, a basophil activation test (Flow CAST) with different concentrations of alpha-gal-containing allergens (eg, commercially available alpha-gal-carrying proteins and pork kidney extracts) was performed in 21 patients with alpha-gal syndrome, 12 alpha-gal-sensitized subjects, and 18 control subjects.

RESULTS

Alpha-gal-containing allergens induced strong basophil activation in a dose-dependent manner in patients. Basophil reactivity at distinct allergen concentrations, the %CD63⁺/anti-FcεRI ratio across most allergen concentrations, the AUC of dose-response curves, and basophil allergen threshold sensitivity (CD-sens) with pork kidney extract were significantly higher in patients with alpha-gal syndrome compared with those in sensitized subjects. All parameters were negative in control subjects.

CONCLUSION

The basophil activation test should be considered as an additional diagnostic test before performing time-consuming and potentially risky oral provocation tests. The %CD63⁺/anti-FcεRI ratio for all allergens and AUCs for pork kidney were the best parameters for distinguishing patients with alpha-gal syndrome from subjects with asymptomatic alpha-gal sensitization.

Road map for the clinical application of the basophil activation test in food allergy

The diagnosis of IgE‐mediated food allergy based solely on the clinical history and the documentation of specific IgE to whole allergen extract or single allergens is often ambiguous, requiring oral food challenges (OFCs), with the attendant risk and inconvenience to the patient, to confirm the diagnosis of food allergy. This is a considerable proportion of patients assessed in allergy clinics. The basophil activation test (BAT) has emerged as having superior specificity and comparable sensitivity to diagnose food allergy, when compared with skin prick test and specific IgE. BAT, therefore, may reduce the number of OFC required for accurate diagnosis, particularly positive OFC. BAT can also be used to monitor resolution of food allergy and the clinical response to immunomodulatory treatments. Given the practicalities involved in the performance of BAT, we propose that it can be applied for selected cases where the history, skin prick test and/or specific IgE are not definitive for the diagnosis of food allergy. In the cases that the BAT is positive, food allergy is sufficiently confirmed without OFC; in the cases that BAT is negative or the patient has non‐responder basophils, OFC may still be indicated. However, broad clinical application of BAT demands further standardization of the laboratory procedure and of the flow cytometry data analyses, as well as clinical validation of BAT as a diagnostic test for multiple target allergens and confirmation of its feasibility and cost‐effectiveness in multiple settings.