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Does IgE Bind to and Activate Eosinophils from Patients with Allergy?¹

Departments of Immunology and Internal Medicine, Mayo Clinic and Mayo Foundation, Rochester, MN 55905

Human eosinophils have been reported to express both the mRNA and protein for the high affinity IgE receptor (FcRI); it is speculated that this receptor plays a role in eosinophil mediator release in allergic diseases. However, questions still remain. How much of the FcRI protein is actually expressed on the cell surface of the eosinophil? If they are present, are these IgE receptors associated with effector functions of eosinophils? To address these issues, we studied blood eosinophils from patients with ragweed hay fever. A high level of low affinity IgG receptor (FcRII, CD32), but no expression of FcRI, was detectable on the eosinophil surface by standard FACS analysis. However, after in vitro sensitization with biotinylated chimeric IgE (cIgE), cell-bound cIgE was detected by PE-conjugated streptavidin. This cIgE binding was partially inhibited by anti-FcRI mAb, suggesting that eosinophils do express minimal amounts of FcRI detectable only by a sensitive method. Indeed, FACS analysis of whole blood showed that eosinophils express 0.5% of the FcRI that basophils express. When stimulated with human IgE or anti-human IgE, these eosinophils did not exert effector functions; there was neither production of leukotriene C₄ or superoxide anion nor any detectable degranulation response. In contrast, eosinophils possessed membrane-bound human IgG and showed functional responses when stimulated with human IgG or anti-human IgG. Thus, IgG and/or cytokines, such as IL-5, appear to be more important for eosinophil activation in allergic diseases than IgE.