Which molecule's activity is primarily inhibited by cyclosporine in the context of organ transplantation?

Cyclosporine is an immunosuppressive drug commonly used in organ transplantation to prevent rejection of the transplanted organ. Its primary mechanism of action involves the inhibition of calcineurin, which is a calcium and calmodulin-dependent phosphatase.

By inhibiting calcineurin, cyclosporine prevents the dephosphorylation of nuclear factor of activated T-cells (NFAT). NFAT is crucial for the transcription of interleukin-2 (IL-2) and other cytokines that are essential for T-cell activation and proliferation. Without the activation of NFAT, there is a significant reduction in IL-2 production, which leads to decreased T-cell activation and thus, a lower immune response against the transplanted organ.

This targeted approach is very effective in managing transplant rejection, making the inhibition of calcineurin the key action of cyclosporine in the context of organ transplantation. Other options, while related to different signaling pathways in the immune response or cellular functions, do not directly pertain to the main action of cyclosporine in suppressing T-cell activation.