Immunosuppressant cyclosporine interacts with which molecule in T-cell signaling?

Cyclophilin is the correct answer because cyclosporine specifically binds to this molecule, forming a complex that inhibits calcineurin's activity. Calcineurin plays a crucial role in T-cell activation by dephosphorylating nuclear factor of activated T-cells (NFAT), allowing NFAT to enter the nucleus and activate IL-2 gene transcription. When cyclosporine binds to cyclophilin, this complex inhibits calcineurin activity, effectively blocking T-cell activation and the production of IL-2, a key growth factor for T-cells.

The interaction of cyclosporine with cyclophilin is central to its immunosuppressive effects, particularly in the context of organ transplantation and autoimmune diseases. This mechanism limits T-cell responses, which is essential for preventing graft rejection and managing autoimmune disorders.