Which immunosuppressive drug is known for directly eliminating T cells?

The immunosuppressive drug known for directly eliminating T cells is indeed OKT3. This monoclonal antibody specifically targets the CD3 receptor on T cells, which is crucial for T cell activation and proliferation. By binding to the CD3 complex, OKT3 initiates internalization and subsequent destruction of T cells, leading to a rapid depletion of the T cell population in the body.

This mechanism is particularly useful in scenarios such as organ transplantation, where the immune response can be a significant barrier to graft acceptance. The action of OKT3 is distinct from the other options listed, which work through different mechanisms to suppress the immune response. For example, cyclosporin primarily inhibits T cell activation by blocking interleukin-2 production, azathioprine interferes with DNA synthesis to inhibit lymphocyte proliferation generally, and sirolimus inhibits T cell proliferation in response to growth factors without directly targeting T cells. Thus, OKT3's unique capability to directly eliminate T cells sets it apart from the other immunosuppressants in this context.