Does IgE bind to and activate eosinophils from patients with allergy?

Human eosinophils have been reported to express both the mRNA and protein f or the high affinity IgE receptor (Fc epsilon RI); it is speculated that th is receptor plays a role in eosinophil mediator release in allergic disease s. However, questions still remain. How much of the Fc epsilon RI protein i s actually expressed on the cell surface of the eosinophil? If they are pre sent, are these IgE receptors associated with effector functions of eosinop hils? To address these issues, we studied blood eosinophils from patients w ith ragweed hay fever, A high level of low affinity IgG receptor (Fc gamma RII, CD32), but no expression of Fc epsilon RI, was detectable on the eosin ophil surface by standard FAGS analysis. However, after in vitro sensitizat ion with biotinylated chimeric IgE (cIgE), cell-bound cIgE was detected by PE-conjugated streptavidin. This cIgE binding was partially inhibited by an ti-Fc epsilon RI mAb, suggesting that eosinophils do express minimal amount s of Fc epsilon RI detectable only by a sensitive method. Indeed, FACS anal ysis of whole blood showed that eosinophils express similar to 0.5% of the Fc epsilon RI that basophils express. When stimulated with human IgE or ant i-human IgE, these eosinophils did not exert effector functions; there was neither production of leukotriene C-4 or superoxide anion nor any detectabl e degranulation response. In contrast, eosinophils possessed membrane-bound human IgG and showed functional responses when stimulated with human IgG o r anti-human IgG, Thus, IgG and/or cytokines, such as IL-5, appear to be mo re important for eosinophil activation in allergic diseases than IgE.