RELEASE OF SOLUBLE FC-GAMMA-RII/CD32 MOLECULES BY HUMAN LANGERHANS CELLS - A SUBTLE BALANCE BETWEEN SHEDDING AND SECRETION

Freshly isolated human Langerhans cells (LC) express two forms of FcgammaRII: a membrane-associated form detected by monoclonal antibody (MoAb) anti-CD32, which recognize an extracytoplasmic epitope of the molecule, and a soluble secreted form, whose existence is suggested by reverse transcriptase-polymerase chain reaction (RT-PCR) experiments. Indeed, RT-PCR performed on total LC RNA reveals the presence of two FcgammaRIIA mRNA, one encoding the FCgammaRIIA with a transmembrane region (membranous form) and the other without this region (soluble form). Densitometry studies performed on the two PCR products reveal that the ratio between the membranous form and the soluble secreted form is about 1.5. LC maintained in culture for 24-48 h lose the major part of their membrane FcgammaRII expression (shown by flow cytometry) and release soluble FcgammaRII molecules (revealed by dot-blot assay), but maintain the same ratio of the two FcgammaRIIA mRNA. The disappearance of the membrane-associated FcgammaRII may be explained either by modification of its recycling pathway or by proteolytic cleavage of the receptor at the cell surface. Thus, soluble FcgammaRII molecules generated during LC culture may result from proteolytic cleavage of the cell-surface receptor and/or secretion of a soluble form derived from the translation of an alternate spliced mRNA. Interestingly, addition of TNF-alpha (10 ng/ml) to the culture medium i) maintains the expression of the membranous form, which can be detected on the LC surface at the same level as on freshly isolated LC, and ii) reverses the ratio (to 0.6) of the two FcgammaRII mRNA, the mRNA encoding the soluble form becoming predominant. Thus, TNF-alpha seems to modify the expression of the FcgammaRII at the mRNA level, favoring the secretion of soluble FcgammaRII molecules, and changes the fate of the membranous FcgammaRII.