Receptor-mediated transcytosis of cyclophilin B through the blood-brain barrier

Cyclophilin B (CyPB) is a cyclosporin A (CsA)binding protein mainly located in intracellular vesicles and secreted in biological fluids. In previous w orks, we demonstrated that CyPB interacts with T lymphocytes and enhances i n vitro cellular incorporation and activity of CsA, In addition to its immu nosuppressive activity, CsA is able to promote regeneration of damaged peri pheral nerves. However, the crossing of the drug from plasma to neural tiss ue is restricted by the relative impermeability of the blood-brain barrier, To know whether CyPB might also participate in the delivery of CsA into th e brain, we have analyzed the interactions of CyPB with brain capillary end othelial cells, First, we demonstrated that CyPB binds to two types of bind ing sites present at the surface of capillary endothelial cells from variou s species of tissues. The first type of binding sites (K-D = 300 nM; number of sites = 3 x 10(6)) is related to interactions with negatively charged c ompounds such as proteoglycans. The second type of binding sites, similar t o 50,000 per cell, exhibits a higher affinity for CyPB (K-D = 15 nM) and is involved in an endocytosis process, indicating it might correspond to a fu nctional receptor. Finally, the use of an in vitro model of blood-brain bar rier allowed us to demonstrate that CyPB is transcytosed by a receptor-medi ated pathway (flux = 16.5 fmol/cm(2)/h). In these conditions, CyPB did not significantly modify the passage of CsA, indicating that it is unlikely to provide a pathway for CsA brain delivery.