DISTRIBUTION OF CYCLOPHILIN B-BINDING SITES IN THE SUBSETS OF HUMAN PERIPHERAL-BLOOD LYMPHOCYTES

Cyclophilin B (CyPB) is a cyclosporin A (CsA)-binding protein, mainly associated with the secretory pathway and released in biological fluid s. We have recently demonstrated that both free CyPB and CyPB-CsA comp lex specifically bind to peripheral blood T lymphocytes and are intern alized. These results suggest that CyPB might promote the targeting of the drug into sensitive cells. Peripheral blood lymphocytes are subdi vided in several populations according to their biological functions a nd sensitivity to CsA. We have investigated the binding of CyPB to the se different subsets using a CyPB derivatized by fluorescein through i ts single cysteine which retains its binding properties. We have confi rmed that only T cells were involved in the interaction with CyPB. The ligand binding was found to be heterogeneously distributed on the dif ferent T-cell subsets and surface-bound CyPB was mainly associated wit h the CD4-positive cells. No significant difference was noted between the CD45RA and CD45RO subsets, demonstrating that CyPB-binding sites w ere equally distributed between native and memory T cells. CD3 stimula tion of T lymphocytes led to a decrease in the CyPB-binding capacity, that may be explained by a downregulation of the CyPB-receptor express ion upon T-cell activation. Finally, we demonstrated thar CyPB-recepto r-positive cells, isolated on CyPB sulphydryl-coupled affinity matrice s, are more sensitive to CyPB-complexed CsA than mixed peripheral bloo d lymphocytes, suggesting thar CyPB potentiates CsA activity through t he binding of the complex. Taken together, our results demonstrate tha t CyPB-binding sites are mainly associated with resting cells of the h elper T lymphocyte, and that CyPB might modulate the distribution of C sA through the drug targeting to sensitive cells.