What is the action of the immunosuppressant cyclosporine?

Cyclosporine exerts its immunosuppressive effects primarily through the inhibition of interleukin-2 (IL-2) production. It binds to the cytoplasmic protein cyclophilin, which in turn inhibits calcineurin, a phosphatase enzyme. Calcineurin normally activates nuclear factor of activated T-cells (NFAT), a transcription factor that is crucial for the transcription of IL-2 and other cytokines necessary for T-cell activation and proliferation. By inhibiting this pathway, cyclosporine effectively reduces the production of IL-2, leading to decreased T-cell activation and proliferation, which is beneficial in the context of organ transplantation and autoimmune diseases.

The other options describe mechanisms not associated with cyclosporine. While enhancing T-cell activation or promoting B-cell proliferation would increase immune response, these are contrary to the purpose of cyclosporine as an immunosuppressant. Blocking tumor necrosis factor (TNF) pertains to other classes of medications, such as TNF inhibitors, which are not the mechanism of action for cyclosporine.