Upregulation of FcεRI on human basophils by IgE antibody is mediated by interaction of IgE with FcεRI

Background: IgE is now known to upregulate the expression of Fc epsilon RI on human basophils, It is not known which receptor on basophils mediates this process of upregulation, Objective: We sought to determine whether galectin-3, Fc epsilon RII (CD23), or Fc epsilon RI were involved in the upregulation of Fc epsilon RI by IgE, Methods: The role of galectin-3 was examined by measuring the influence of alpha-lactose on upregulation, Basophils were examined for expression of Fc epsilon RIT (CD23) by flow cytometry and messenger (m)RNA expression. Functional discrimination between binding to Fc epsilon RII or Fc epsilon RI was examined through the use of mutant IgE-Fc fragments or anti-Fc epsilon RII antibody. Results: Upregulation of Fc epsilon RI on basophils in the presence of IgE was not altered by coincubation with alpha-lactose, eliminating a role for galectin-3. Basophils were not found to express Fc epsilon RII, as determined by flow cytometry with enriched basophil preparations or RT-PCR with highly purified basophil preparations. A mutant of the Fc fragment of IgE (IgE-Fc), which binds to Fc epsilon RI with a greater than 10-fold lower affinity than IgE or wild-type IgE-Fc but exhibits no change in affinity for Fc epsilon RII, allowed us to distinguish between the functions of the two Fc receptors. The mutant (R334S; Henry et al 1997) was required at about 30-fold higher concentration than the wild-type IgE-Fc for the same stimulation of Fc epsilon RI expression on basophils, thus excluding a role for FcRII in the response. In addition, treatment of basophils with anti-Fc epsilon RII antibody (MHM6), which is known to be competitive with IgE, had no effect on the expression of Fc epsilon RI or the ability of IgE to upregulate expression of Fc epsilon RI, Conclusion: Collectively, these data indicate that IgE interacts with Fc epsilon RI to upregulate its expression on human basophils.