Intracellular expression and release of Fc epsilon RI alpha by human eosinophils

Although Fc epsilon R have been detected on human eosinophils, levels varie d from moderate to extremely low or undetectable depending on the donor and methods used. We have attempted to resolve the conflicting data by measuri ng levels of IgE, Fc epsilon RI, and Fc epsilon RII in or on human eosinoph ils 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 Fc epsilon R on other cells) failed to induce any detectable surface Fc epsilon R. However, immunoprecipitation and West ern blot analysis of eosinophil lysates using mAb specific for Fc epsilon R I alpha showed a distinct band of approximately 50 kDa, similar to that fou nd in basophils. Western blotting also showed the presence of FcR gamma-cha in, but no Fc epsilon RI beta. Surface biotinylation followed by immunoprec ipitation again failed to detect surface Fc epsilon RI alpha, although surf ace FcR gamma was easily detected. Since we were able to detect intracellul ar Fc epsilon RI alpha, we examined its release from eosinophils, Immunopre cipitation and Western blotting demonstrated the release of Fc epsilon RI a lpha into the supernatant of cultured eosinophils, peaking at approximately 48 h, We conclude that eosinophils possess a sizable intracellular pool of Fc epsilon RI alpha that is available for release, with undetectable surfa ce levels in a variety of subjects, including those with eosinophilia and e levated serum IgE, The biological relevance of this soluble form of Fc epsi lon RI alpha remains to be determined.