Upregulation of Fc epsilon RI on human basophils by IgE antibody is mediated by interaction of IgE with Fc epsilon RI

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 th is process of upregulation, Objective: We sought to determine whether galec tin-3, Fc epsilon RII (CD23), or Fc epsilon RI were involved in the upregul ation of Fc epsilon RI by IgE, Methods: The role of galectin-3 was examined by measuring the influence of alpha-lactose on upregulation, Basophils wer e examined for expression of Fc epsilon RIT (CD23) by flow cytometry and me ssenger (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 epsi lon 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 f ound to express Fc epsilon RII, as determined by flow cytometry with enrich ed basophil preparations or RT-PCR with highly purified basophil preparatio ns. 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 distin guish between the functions of the two Fc receptors. The mutant (R334S; Hen ry 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 ba sophils, thus excluding a role for FcRII in the response. In addition, trea tment 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, Conclus ion: Collectively, these data indicate that IgE interacts with Fc epsilon R I to upregulate its expression on human basophils.