Intracellular expression and release of FcεRIα by human eosinophils

Although FcεR have been detected on human eosinophils, levels varied from moderate to extremely low or undetectable depending on the donor and methods used. We have attempted to resolve the conflicting data by measuring levels of IgE, FcεRI, and FcεRII in or on human eosinophils from a variety of donors (n = 26) and late-phase bronchoalveolar lavage fluids (n = 5). Our results demonstrated little or no cell surface IgE or IgE receptors as analyzed by immunofluorescence and flow cytometry. Culture of eosinophils for up to 11 days in the presence or absence of IgE and/or IL-4 (conditions that enhance FcεR on other cells) failed to induce any detectable surface FcεR. However, immunoprecipitation and Western blot analysis of eosinophil lysates using mAb specific for FcεRIα showed a distinct band of approximately 50 kDa, similar to that found in basophils. Western blotting also showed the presence of FcR γ-chain, but no FcεRIβ. Surface biotinylation followed by immunoprecipitation again failed to detect surface FcεRIα, although surface FcRγ was easily detected. Since we were able to detect intracellular FcεRIα, we examined its release from eosinophils. Immunoprecipitation and Western blotting demonstrated the release of FcεRIα into the supernatant of cultured eosinophils, peaking at approximately 48 h. We conclude that eosinophils possess a sizable intracellular pool of FcεRIα that is available for release, with undetectable surface levels in a variety of subjects, including those with eosinophilia and elevated serum IgE. The biological relevance of this soluble form of FcεRIα remains to be determined.