HUMAN KERATINOCYTES RELEASE THE ENDOGENOUS BETA-GALACTOSIDE-BINDING SOLUBLE LECTIN IMMUNOGLOBULIN-E (IGE-BINDING PROTEIN) WHICH 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 (FcepsilonRI), as well as the low affinity receptor for IgE (FcepsilonRII/CD23). In the present study, we document the presence of a third IgE-binding structure on human LC, the IgE-binding protein (epsilonBP), an endogenous soluble beta-galactoside binding lectin. Immunohistochemical studies performed on normal human skin revealed an anti-epsilonBP reactivity in the cytoplasm of keratinocytes and in that of acinous cells of eccrine sweat glands. EpsilonBP was also found on the cell surface of LC, as shown by anti-epsilonBP/anti-CD1a double labeling and flow cytometric analysis. Anti-epsilonBP binding to the surface of LC was completely abolished by preincubation with lactose and restored by addition of recombinant human epsilonBP, indicating that epsilonBP binds to LC surface by virtue of its lectin property. Immunoblot analysis of anti-epsilonBP-reactive material in keratinocytes and purified LC disclosed a protein with an apparent molecular weight of 33,000 consistent with epsilonBP. Interestingly, mRNA transcripts for epsilonBP were detected only in keratinocytes but not in purified LC isolated from normal skin. EpsilonBP was found to be released in culture supernatants of keratinocytes. Incubation of LC with these supernatants resulted in epsilonBP-binding to LC surface via protein-carbohydrate interaction. Most importantly, we could show that binding of human myeloma IgE to LC was inhibited by epsilonBP. In contrast, neuraminidase-treated human myeloma IgE binds to LC only in the presence of epsilonBP. In situ binding studies revealed that keratinocytes, although containing epsilonBP intracytoplasmatically, failed to exhibit any IgE-binding properties. Collectively, our results suggest that human keratinocytes produce the beta-galactoside-binding lectin epsilonBP, which subsequently binds to the surface of LC where it is functional in modulating their binding capacity for IgE glycoforms.