Characterization of surface binding sites for cyclophilin B on a human tumor T-cell line.

Cyclophilin B (CyPB) is a cyclosporin-binding protein, known to be located mainly within the endoplasmic reticulum vesicles. Its previous characterization in human milk implies that the protein may be released from the secretory pathway and recovered in biological fluids. In an attempt to understand the role of the extracellular CyPB, we have investigated the binding capacity of the protein to cells derived from human T- and B-lymphocytes. We present here evidence that CyPB binds to T-lymphocytes and that the binding to the Jurkat T-cell surface is specific, saturable, and reversible. The dissociation constant Kd was 12 nM, and the number of binding sites was estimated to 35,000/cell. We report that the surface-bound CyPB was internalized at 37 degrees C and subsequently degraded in the cell. We also show that the immunosuppressive drug cyclosporin A does not inhibit the surface binding of CyPB, and does not interfere with internalization of the protein. These results support the hypothesis that the selective action of the immunosuppressive drug results in part from its interaction with the extracellular form of CyPB.