Schimming, Andrew W.

Abstract Human eosinophils have been reported to express both the mRNA and protein for the high affinity IgE receptor (FcεRI); it is speculated that this receptor plays a role in eosinophil mediator release in allergic diseases. However, questions still remain. How much of the FcεRI 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 (FcγRII, CD32), but no expression of FcεRI, 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-FcεRI mAb, suggesting that eosinophils do express minimal amounts of FcεRI detectable only by a sensitive method. Indeed, FACS analysis of whole blood showed that eosinophils express ∼0.5% of the FcεRI 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.

Abstract The low-affinity IgG Fc receptor, FcγRII (CD32), mediates various effector functions of lymphoid and myeloid cells and is the major IgG Fc receptor expressed by human eosinophils. We investigated whether FcγRII regulates both cell survival and death of human eosinophils. When cultured in vitro without growth factors, most eosinophils undergo apoptosis within 96 h. Ligation of FcγRII by anti-CD32 mAb in solution inhibited eosinophil apoptosis and prolonged survival in the absence of growth factors. Cross-linking of human IgG bound to FcγRII by anti-human IgG Ab or of unoccupied FcγRII by aggregated human IgG also prolonged eosinophil survival. The enhanced survival with anti-CD32 mAb was inhibited by anti-granulocyte-macrophage-CSF (GM-CSF) mAb, suggesting that autocrine production of GM-CSF by eosinophils mediated survival. In fact, mRNA for GM-CSF was detected in eosinophils cultured with anti-CD32 mAb. In contrast to mAb or ligands in solution, anti-CD32 mAb or human IgG, when immobilized onto tissue culture plates, facilitated eosinophil cell death even in the presence of IL-5. Cell death induced by these immobilized ligands was accompanied by DNA fragmentation and was inhibited when eosinophil β2 integrin was blocked by anti-CD18 mAb, suggesting that β2 integrins play a key role in initiating eosinophil apoptosis. Thus, FcγRII may pivotally regulate both survival and death of eosinophils, depending on the manner of receptor ligation and β2 integrin involvement. Moreover, the FcγRII could provide a novel mechanism to control the number of eosinophils at inflammation sites in human diseases.