Immunosuppressants: cellular and molecular mechanisms of action.

The basic immunosuppressive protocol used in most transplant centers involves the use of multiple drugs, each directed at a discrete site in the T-cell activation cascade and each with distinct side effects. Cyclosporine, azathioprine, corticosteroids, FK506 (tacrolimus), and RS61443 (mycophenolate mofetil) have been approved by the Food and Drug Administration, and the clinical efficacy of rapamycin (sirolimus), mizoribine, 15-deoxyspergualin, and leflunomide is being explored. Based on their primary site of action, the immunosuppressants can be classified as inhibitors of transcription (cyclosporine, tacrolimus), inhibitors of nucleotide synthesis (azathioprine, mycophenolate mofetil, mizoribine, leflunomide), inhibitors of growth factor signal transduction (sirolimus, leflunomide), and inhibitors of differentiation (15-deoxyspergualin). Polyclonal antilymphocyte antibodies, monoclonal antibodies directed at the T-cell antigen receptor complex (OKT3, TIOB9), and monoclonal antibodies directed at additional cell surface antigens, including interleukin-2 receptor alpha, afford cell-specific regulation of the immune response and are being used in the clinical setting as induction therapy and/or antirejection drugs. Clearly, the transplant clinician now has a greater choice in the selection and application of immunosuppressants in the clinic for the fine regulation of the antiallograft repertory. The prevailing paradigm regarding the mechanisms of action of immunosuppressants is that they all function to prevent allograft rejection by preventing/inhibiting cell activation, cytokine production, differentiation, and/or proliferation. One hypothesis, albeit provocative, is that some of the immunosuppressants might function by stimulating the expression of immunosuppressive molecules and/or cells.