Human Keratinocytes Release the Endogenous ~-Galactoside-binding Soluble Lectin Immunoglobulin E (IgE-Binding Protein) wh/ch Binds to Langerhans Cells Where It Modulates Their Binding Capacity for IgE Glycoforms

A better understanding of the pathophysiological role of Langerhans cells (LC) in atopic diseases is dictated by the characterization of the structures involved in immunoglobulin (IgE)-binding on their cell surface. We previously reported that human LC express the high affinity receptor for IgE (FceRI), as well as the low affinity receptor for IgE (FceRII/CD23). In the present study, we document the presence of a third IgE-binding structure on human LC, the IgE-binding protein (eBP), an endogenous soluble 3-galactoside binding lectin. Immunohistochemical studies performed on normal human skin revealed an anti-eBP reactivity in the cytoplasm of keratinocytes and in that of acinous cells of eccrine sweat glands, eBP was also found on the cell surface of LC, as shown by anti-eBP/anti-CDla double labeling and flow cytometric analysis. Anti-EBP binding to the surface of LC was completely abolished by preincubation with lactose and restored by addition of recombinant human eBP, indicating that eBP binds to LC surface by virtue of its lectin property. Immunoblot analysis of anti-eBP-reactive material in keratinocytes and purified LC disclosed a protein with an apparent molecular weight of 33,000 consistent with eBP. Interestingly, mRNA transcripts for eBP were detected only in keratinocytes but not in purified LC isolated from normal skin. eBP was found to be released in culture supernatants of keratinocytes. Incubation of LC with these supernatants resulted in eBP-binding to LC surface via proteincarbohydrate interaction. Most importantly, we could show that binding of human myeloma IgE to LC was inhibited by eBP. In contrast, neuraminidase-treated human myeloma IgE binds to LC only in the presence of eBP. In situ binding studies revealed that keratinocytes, although containing eBP intracytoplasmatically, failed to exhibit any IgE-binding properties. Collectively, our results suggest that human keratinocytes produce the/J-galactoside-binding lectin eBP, which subsequently binds to the surface of LC where it is functional in modulating their binding capacity for IgE glycoforms.