Soluble Fcɛ<scp>RI</scp>: A biomarker for IgE‐mediated diseases
Authors: Sherezade Moñino‐Romero, Willem S. Lexmond, Josef Singer, Christina Bannert, Abena S. Amoah, Maria Yazdanbakhsh, Daniel A. Boakye, Erika Jensen‐Jarolim, Edda Fiebiger, Zsolt Szépfalusi
Journal: Allergy (2019)

Soluble IgE receptors interact with IgE in the extracellular matrix and are important in the regulation of immune diseases.1-5 Soluble FcεRII (sCD23) and galectin-3 (εBP) are currently used as biomarkers,1 though correlation data on serum titers and severity of allergies are controversial.1, 6 FcεRI, the high-affinity IgE Fc receptor, is expressed on several innate cell types,2 and a truncated version of the IgE-binding alpha subunit is found as a soluble isoform (sFcεRI) in human serum. In circulation, sFcεRI is mostly detected as a complex with IgE.7 This observation raises the question of how sFcεRI affects detection of serum IgE titers. In order to assign clinical implications of sFcεRI, we assessed serum titers in its total and IgE-bound forms in different IgE-mediated diseases in 312 individuals. We compared pediatric populations with primary food allergies (n = 59), insect venom allergies (n = 9), allergic asthma (n = 24), atopic dermatitis (n = 25), food-sensitized nonallergic children (n = 31), and nonallergic controls (n = 17). Additionally, other sensitized groups and controls (n = 147) were included in the study (Table S1-S4). Serum samples were analyzed by ELISA to detect IgE-bound and total serum sFcεRI levels (Figure S1). First, sFcεRI was ubiquitously detectable among controls (median 1.20 ng/mL) but titers were significantly higher in atopic individuals (median 2.88 ng/mL, Figure 1A and Table S1). In line with previous studies,7, 8 IgE and sFcεRI levels correlated positively in all patients, and sFcεRI in circulation was almost uniquely detected as a complex with IgE (Figure 1B,C). Next, we grouped the atopic individuals based on their main IgE-mediated disease (Table S2) as food allergy (FA), insect venom allergy (IV), allergic asthma (AA), or atopic dermatitis (AD). AD, AA, and FA groups presented with significantly higher sFcεRI levels than controls (Figure 1D). Since IgE-sensitization profiles toward food allergens are generally a poor measure of clinical symptoms, we compared sFcεRI titers in two food-sensitized nonallergic groups (FS and Ghana) with FA patients (Table S3). The Ghana cohort showed similar correlations as already described between IgE and sFcεRI, IgE-bound and total sFcεRI levels, and no correlation with peanut-specific IgE (sIgE) titers. No significant difference was detected with regards to disease activity among food-sensitized individuals (Figure S2). We then investigated whether serum sFcεRI levels were different in patients diagnosed with atopic dermatitis or asthma, with (Pos sIgE) or without (Neg sIgE) a clinically relevant sIgE profile. sFcεRI titers did not differ based on the patients' sIgE profile. However, we found significantly higher titers in patients with elevated IgE (Figure S3) in both AD and AA groups (Figure 1E-H). Recently, we demonstrated that sFcεRI is released from dendritic cells and mast cells after antigen-specific FcεRI crosslinking.5 Thus, we studied how sFcεRI levels in circulation are affected by allergen exposure. We compared sFcεRI levels in AA individuals (n = 14 pairs) during (In) and before/after (Out) season for their most clinically relevant allergen (Table S4) and observed that serum levels could significantly increase (50%) or decrease (50%) during season. This pattern was similarly observed with total IgE levels (Figure S4). In order to better determine the role of allergen exposure, we analyzed food-sensitized individuals on allergen avoidance (n = 13) during an oral food challenge (Figure S5). We observed a general trend of sFcεRI titers to decrease after allergen exposure (Figure 1I). sFcεRI binds to the Fc portion of IgE and can potentially interfere with antibody binding to that region. We thus investigated whether sFcεRI affects antibody-based IgE detection. For this purpose, a recombinant IgE-binding protein (rsFcεRI) and a mutated version which cannot bind IgE (rsFcεRIm) were generated. Prior to a commercial IgE ELISA, samples containing human cIgE were incubated with the recombinant proteins (Figure 2A-C). Our hypothesis was that IgE detection will be impaired and reflected in a decrease of IgE levels with increasing concentrations of rsFcεRI. In Figure 2D, we show an r = −0.867 with P = 0.005 which depicts a significant negative correlation in support of our hypothesis. On the contrary, as shown in Figure 2E, increasing concentrations of the mutant version of rsFcεRI which is unable to bind IgE do not show interference in IgE detection (r = 0.349, ns). This interference with IgE detection by rsFcεRI was confirmed with human IgE (Figure 2F) and human serum (n = 2) from patients with elevated IgE levels (Figure 2G). In addition, we observed that sFcεRI titers were significantly higher in serum than plasma (Figure S6). To the best of our knowledge, this is the first analysis of sFcεRI levels in a pediatric population of well-classified sensitized and allergic individuals. We show that sFcεRI is correlated with IgE levels, is significantly increased in IgE-sensitized individuals, and can be modulated by allergen exposure. We collected evidence that sFcεRI can interfere with IgE detection in serum, which might be of importance in regard to interference in sIgE detection and diagnosis. Although further research on the modulation by allergen exposure and interference with sIgE molecules is needed, sFcεRI represents an additional biomarker for IgE-mediated diseases and its use could be a valuable tool in clinical practice. We thank all the members of the Szépfalusi, Fiebiger, and Yazdanbakhsh laboratories for discussions and technical assistance. We thank Klara Schmidthaler, Dr. Eleonora Dehlink, Dr. Christina Bannert, and Dr. Susanne C. Diesner for their contribution in patient recruitment. Special thanks to Alexandra C. Graf from the Center for Medical Statistics, Informatics, and Intelligent Systems, Medical University of Vienna, for her assistance on the statistical analysis. This work was supported by the Austrian Science Fund (FWF): DK MCCA W 1248-B13 (ZS), W 1205-B09 (EJJ), SFB F4606-B28 (EJJ), and by the Harvard Digestive Diseases Center Grant P30DK034854, Core B (EF). EF is supported by a Bridge Grant from the Research Council of Boston Children's Hospital, an Emerging Investigator Award from FARE, a Senior research grant of the CCF and an unrestricted gift from the Mead Johnson Nutrition Company. The Ghana study was supported by European Commission grants; EuroPrevall (grant no. FOOD-CT-2005-514000) and GLOFAL (grant no. FOOD-CT-2005-517812). The authors declare that they have no conflicts of interest. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.