CYCLOPHILIN-B MEDIATES CYCLOSPORINE-A INCORPORATION IN HUMAN BLOOD T-LYMPHOCYTES THROUGH THE SPECIFIC BINDING OF COMPLEXED DRUG TO THE CELL-SURFACE

Cyclophilin B (CyPB) is a cyclosporin A (CsA)-binding protein located within intracellular vesicles and released in. biological fluids. We r ecently reported the specific binding of this protein to T-cell surfac e receptor which is internalized even in the presence of CsA. These re sults suggest that CyPB might target the drug to lymphocytes and conse quently modify its activity. To verify this hypothesis, we have first investigated the binding capacity and internalization of the CsA-CyPB complex in human peripheral blood T-lymphocytes and secondly compared the inhibitory effect of both free and CyPB-complexed CsA on the CD3-i nduced activation and proliferation of T-cells. Here, we present evide nce that both the CsA-CyPB complex and free CyPB bind to the T-lymphoc yte surface, with similar values of K-d and number of sites. At 37 deg rees C, the complex is internalized but, in contrast to the protein, t he drug is accumulated within the cell. Moreover, CyPB receptors are i nternalized together with the ligand and rapidly recycled to the cell surface. Finally, we demonstrate that CyPB-complexed CsA remains as ef ficient as uncomplexed CsA and that CyPB enhances the immunosuppressiv e activity of the drug. Taken together, our results support the hypoth esis that surface CyPB receptors may be related to the selective and v ariable action of CsA, through specific binding and targeting of the C yPB-CsA complex to peripheral blood T-lymphocytes.