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 ce lls (LC) in atopic diseases is dictated by the characterization of the structures involved in immunoglobulin (IgE)-binding on their cell sur face. We previously reported that human LC express the high affinity r eceptor for IgE (FcepsilonRI), as well as the low affinity receptor fo r IgE (FcepsilonRII/CD23). In the present study, we document the prese nce of a third IgE-binding structure on human LC, the IgE-binding prot ein (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 foun d on the cell surface of LC, as shown by anti-epsilonBP/anti-CD1a doub le labeling and flow cytometric analysis. Anti-epsilonBP binding to th e surface of LC was completely abolished by preincubation with lactose and restored by addition of recombinant human epsilonBP, indicating t hat epsilonBP binds to LC surface by virtue of its lectin property. Im munoblot 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 cu lture supernatants of keratinocytes. Incubation of LC with these super natants resulted in epsilonBP-binding to LC surface via protein-carboh ydrate interaction. Most importantly, we could show that binding of hu man myeloma IgE to LC was inhibited by epsilonBP. In contrast, neurami nidase-treated human myeloma IgE binds to LC only in the presence of e psilonBP. In situ binding studies revealed that keratinocytes, althoug h containing epsilonBP intracytoplasmatically, failed to exhibit any I gE-binding properties. Collectively, our results suggest that human ke ratinocytes produce the beta-galactoside-binding lectin epsilonBP, whi ch subsequently binds to the surface of LC where it is functional in m odulating their binding capacity for IgE glycoforms.