Expression and functions of the high‐affinity IgE receptor on human platelets and megakaryocyte precursors

Platelets can be activated by IgE and are therefore involved in IgE‐mediated effector mechanisms against parasites and in allergic disorders. Here we show that, besides the low‐affinity IgE receptor (FcεRII/CD23), platelets express the high‐affinity receptor for IgE (FcεRI). Flow cytometry analysis revealed the existence of surface FcεRI on platelets with a large heterogeneity among individual donors, and a low proportion of platelets co‐expressing FcεRI and FcεRII/CD23. Northern hybridization and reverse transcription polymerase chain reaction confirmed the presence of mRNA encoding the α, β and γ chains of FcεRI in platelets and in their megakaryocytic precursors. Cross‐linking of FcεRI with monoclonal antibody (mAb) to α chain using either the whole molecule or F(ab′)2 triggered platelet cytotoxicity for Schistosoma mansoni larvae. Anti‐FcεRII/CD23 mAb significantly inhibited IgE‐ or FcεRI‐mediated cytotoxicity, indicating down‐regulatory effects of FcεRII/CD23 on FcεRI‐dependent functions. These results demonstrate functional properties for FcεRI on platelets and indicate unsuspected interactions between the low‐ and the high‐affinity IgE receptors.