Which of the following is NOT a proliferation signal inhibitor?

• A. Mycophenolate
• B. Sirolimus
• C. Leflunomide
• D. Cyclosporine

Answer: (D)

Proliferation signal inhibitors are drugs that block the intracellular signals that drive T cell proliferation after activation, especially the pathway that tells cells to progress through the cell cycle in response to IL-2. Sirolimus (rapamycin) directly inhibits mTOR, a key downstream signal that pushes T cells from the G1 phase into S phase. By blocking this proliferation signal, T cells can’t multiply even when activation signals are present.

Mycophenolate mofetil and Leflunomide reduce proliferation by depriving cells of building blocks for DNA synthesis (nucleotide synthesis inhibitors), so they slow or prevent lymphocyte proliferation without blocking the signaling cascade itself. Cyclosporine, however, inhibits calcineurin and prevents transcription of IL-2, dampening T cell activation rather than shutting down the proliferation signaling pathway directly. That makes it not a proliferation signal inhibitor.

Therefore, the agent that is not a proliferation signal inhibitor is the one that blocks activation (cytokine production) rather than the downstream proliferation signaling.