εBP, a β-galactoside-binding animal lectin, recognizes IgE receptor (FcεRI) and activates mast cells

IgE-binding protein (εBP) was originally identified in rat basophilic leukemia (RBL) cells by virtue of its affinity for IgE. εBP is now known to be a β-galactoside-binding lectin containing an S-type carbohydrate recognition domain. It is identical to a macrophage surface antigen, Mac-2, and lectins designated as CBP35, L-34, and RL-29, for which various functions have been suggested. Studies from other groups as well as ours have indicated that εBP is secreted by cells such as macrophages and is present in extracellular fluids. We demonstrated previously that binding sites for εBP are present on the surface of RBL cells. In this report, we show that εBP binds to a small number of glycoprotein species on the surface of RBL cells. Significantly, one of these glycoproteins is the high-affinity IgE receptor (FcεRI). Preliminary studies showed that εBP causes mediator release from RBL cells, possibly through cross-linking of FcεRI. The results suggest a function of εBP as an activator of mast cells.