Cyclosporin partitions into phospholipid vesicles and disrupts membrane architecture

Cyclosporin is a fungal metabolite demonstrating potent immunosuppressive activity both in vitro and in vivo, but the mechanism of action is poorly understood. Using [3H]dihydrocyclosporin C ([3H]CsC) we observed significant binding by mononuclear cells, erythrocytes and phosphatidyl choline (PC) vesicles which was reversible by the addition of excess CsA. Trypsin, pronase or heat treatments demonstrated that B cells and adherent cells express a protease-sensitive membrane binding site not observed on T cells. The nature of the interaction between CsA and the PC vesicles was studied using the membrane surface probe 1-anilino-8-naphthyl sulfonic acid (ANS-). ANS- -induced fluorescence was reduced by 24% in the presence of 4.75 X 10(-7). M CsA indicating that CsA displaces ANS- from the PC vesicles. CsA also effected a shift in the phase transition temperature of PC vesicles from 23 degrees C to 19 degrees C. Finally, the rate of concanavalin A (Con A)-induced cap formation by T lymphocytes was approximately doubled in the presence of 2.6 X 10(-5) M CsA. These data demonstrate that CsA partitions into phospholipid vesicle membranes and the plasmalemma of mononuclear cells resulting in an increased membrane fluidity.