Basophil activation test in cancer patient blood evaluating potential hypersensitivity to an anti‐tumor IgE therapeutic candidate

To the Editor, Monoclonal anti-tumor IgG antibodies are used widely to treat malignancies. Studies in the field of AllergoOncology, focusing on the interactions between IgE, allergy, and cancer, point to biological characteristics of IgE that may engender potent anti-tumor functions.1 These include superior affinity of IgE for cognate Fc receptors and the presence in tumors of effector cell populations (eg, macrophages and mast cells) known to exert anti-tumor activities when activated by IgE.2,3 Following promising preclinical findings,2,4 MOv18 IgE, specific for the tumor-associated antigen folate receptor alpha (FRα), overexpressed in ovarian and basal breast cancers and other solid tumors,5 is the first anti-cancer IgE antibody studied in a first-in-class, first-inhuman clinical trial (ClinicalTrials.gov Identifier: NCT02546921). One of the potential concerns associated with application of IgE as a therapy in the clinic relates to the perceived risk of IgE-mediated anaphylaxis. Safety evaluation of such a novel agent mandated the development of bespoke methods to monitor potential hypersensitivity reactions following intravenous infusion and ideally also to help in predicting such a reaction when selecting patients for treatment. Over the past 15 years, the basophil activation test (BAT) has been developed and widely employed to study and predict type 1 hypersensitivity reactions to food, venom, and drugs in the allergy field.6,7 Thus far, its use in the context of cancer is limited to a small number of studies for the detection of allergic reactions to chemotherapeutic agents.8 Basophil activation in the context of tumor-associated immunomodulation and in often heavily treated patients has not been well-studied. Employing the BAT in whole blood of 42 ovarian cancer patients with diverse treatment histories and tumor histologies, we examined the propensity of human basophils to be activated by anti-cancer IgE ex vivo. We first identified circulating basophils (CCR3highSSClow; gating strategy in Figure S1A) from patients with cancer. Basophils were activated (up-regulation of CD63 expression) ex vivo by IgEand non-IgE-mediated triggers (anti-FcεRI, anti-IgE, and fMLP, Figure 1A, Figure S1B). Consistent with previously reported findings in allergic cohorts,6 levels of basophil activation varied among individuals. We detected no basophil activation following addition of the hapten-specific NIP (4-hydroxy-3-iodo-5-nitrophenylacetic acid) IgE alone or its multivalent antigen (NIP-BSA) alone. However, we detected basophil activation by exogenous stimulation of the hapten-specific NIP IgE in combination with multimeric NIP-BSA (Figure 1A). This suggested that IgE could recognize unoccupied cell surface FcεRI on basophils ex vivo and basophils could be activated by exogenous FcεRI receptor engagement and formation of cross-linking immune complexes. We then examined whether stimulation with the anti-cancer mouse/human chimeric IgE antibody (MOv18) could trigger ex vivo basophil activation (Figure 1B, C). As expected in this cohort (n = 42), stimulation with anti-FcεRI, anti-IgE, and fMLP (positive controls) triggered CD63 up-regulation. In all but one patient sample, no basophil activation was measured following incubation of ovarian cancer patient blood with MOv18 IgE or control non-FRα-reactive IgE in the absence of any additional exogenous cross-linking stimulus (mean fold change in %CD63: 1.4 for MOv18 IgE, 1.3 for control IgE; 7.5 and 10.6, respectively, in the positive responder) (Figure 1D). Activation, or lack thereof, was irrespective of different patient tumor histologies and treatment histories, that is, (a) treatment-naïve patients (n = 7), (b) following primary debulking surgery (n = 8), (c) following surgery and chemotherapy (n = 21), or (d) following treatment with bevacizumab (n = 7) (Figure 1E, F). Neither MOv18 IgE nor control non-FRα-reactive IgE triggered basophil activation in the blood of a patient with already raised serum tryptase, a marker which could indicate mastocytosis (although this clinical information was not available) and may have potentially predisposed this individual to an increased risk of hypersensitivity to IgE stimulation, including to MOv18 IgE (Figure 1G). Since MOv18 IgE recognizes the tumor-associated antigen, FRα, it is possible that FRα shed from cancer cells in tissues and anti-FRα autoantibodies (autoAbs), if present in patient circulation, could form immune complexes with MOv18 IgE. This may result in FcεRI cross-linking and basophil activation (Figure 2A). No CD63 up-regulation on basophils was measured following ex vivo stimulation with either MOv18 IgE or control IgE in any sample from patients with known