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-me diated effecter mechanisms against parasites and in allergic disorders . Here we show that, besides the low-affinity IgE receptor (Fc epsilon RII/CD23), platelets express the high-affinity receptor for IgE (Fc e psilon RI). Flow cytometry analysis revealed the existence of surface Fc epsilon RI on platelets with a large heterogeneity among individual donors, and a low proportion of platelets co-expressing Fc epsilon RI and Fc epsilon RII/CD23. Northern hybridization and reverse transcrip tion polymerase chain reaction confirmed the presence of mRNA encoding the alpha, beta and gamma chains of Fc epsilon RI in platelets and in their megakaryocytic precursors, Cross-linking of Fc epsilon RI with monoclonal antibody (mAb) to alpha chain using either the whole molecu le or F(ab')(2) triggered platelet cytotoxicity for Schistosoma manson i larvae. Anti-Fc epsilon RII/CD23 mAb significantly inhibited IgE- or Fc epsilon RI-mediated cytotoxicity, indicating down-regulatory effec ts of Fc epsilon RII/CD23 on Fc epsilon RI-dependent functions. These results demonstrate functional properties for Fc epsilon RI on platele ts and indicate unsuspected interactions between the low-and the high- affinity IgE receptors.