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 FcεRI on human basophils. It is not known which receptor on basophils mediates this process of upregulation. Objective: We sought to determine whether galectin- 3, FċRII (CD23), or FcεRI were involved in the upregulation of FcεRI by IgE. Methods: The role of galectin-3 was examined by measuring the influence of α-lactose on upregulation. Basophils were examined for expression of FcεRII (CD23) by flow cytometry and messenger (m)RNA expression. Functional discrimination between binding to FcεRII or FcεRI was examined through the use of mutant IgE-Fc fragments or anti-FcεRII antibody. Results: Upregulation of FcεRI on basophils in the presence of IgE was not altered by coincubation with α-lactose, eliminating a role for galectin-3. Basophils were not found to express FcεRII, 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 FcεRI with a greater than 10-fold lower affinity than IgE or wild-type IgE- Fc but exhibits no change in affinity for FcεRII, 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 FcεRI expression on basophils, thus excluding a role for FcεRII in the response. In addition, treatment of basophils with anti-FcεRII antibody (MHM6), which is known to be competitive with IgE, had no effect on the expression of FcεRI or the ability of IgE to upregulate expression of FcεRI. Conclusion: Collectively, these data indicate that IgE interacts with FcεRI to upregulate its expression on human basophils.