Surface expression of functional IgE binding protein, an endogenous lectin, on mast cells and macrophages

IgE-binding protein (εBP) is a galactoside-specific lectin containing an S-type carbohydrate-recognition domain. It was originally identified in rat basophilic leukemia cells and is now known to be identical to a macrophage surface Ag, Mac-2, and lectins designated as CBP 35/L-34/RL-29. It has also been related to a nonintegrin laminin-binding protein isolated from mouse macrophages. In this report we have shown the following: εBP is present in variable amounts in several mast cell lines, and the surface expression of εBP in these cell lines is quite variable and does not correlate with the total amount of εBP in the cell. εBP is displayed on the cell surface in a manner that is reversible by lactose, most likely through attachment to yet unidentified glycoconjugates. The putative εBP binding sites on the cell surface can be readily demonstrated by using radiolabeled εBP, and the sites are present in comparable amounts in various cell lines. Expression of εBP on the cell surface can be regulated; the most notable example is the upregulation of surface εBP on RBL cells activated through the high-affinity IgE receptor by IgE immune complexes. Cell-surface εBP is functional as measured by its ability to promote adhesion of trypsinized rabbit erythrocytes to mast cells and macrophages. On the basis of these results and reported properties of related lectins, we propose that the lectin represented by εBP is a new class of cell-adhesion protein.