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Mycophenolic acid (MPA), the active metabolite of the immunosuppressive drug mycophenolate mofetil, is a selective inhibitor of inosine 5'-monophosphate dehydrogenase type II, a de novo purine nucleotide synthesis enzyme expressed in T and B lymphocytes and up-regulated upon cell activation. In this study, we report that the blockade of guanosine nucleotide synthesis by MPA inhibits mitogen-induced proliferation of PBL, an effect fully reversed by addition of guanosine and shared with mizoribine, another inhibitor of inosine 5'-monophosphate dehydrogenase. Because MPA does not inhibit early TCR-mediated activation events, such as CD25 expression and IL-2 synthesis, we investigated how it interferes with cytokine-dependent proliferation and survival. In activated lymphoblasts that are dependent on IL-2 or IL-15 for their proliferation, MPA does not impair signaling events such as of the extracellular signal-regulated kinase 2 and Stat5 phosphorylation, but inhibits down-regulation of the cyclin-dependent kinase inhibitor p27(Kip1). Therefore, in activated lymphoblasts, MPA specifically interferes with cytokine-dependent signals that control cell cycle and blocks activated T cells in the mid-G(1) phase of the cell cycle. Although it blocks IL-2-mediated proliferation, MPA does not inhibit cell survival and Bcl-x(L) up-regulation by IL-2 or other cytokines whose receptors share the common gamma-chain (CD132). Finally, MPA does not interfere with IL-2-dependent acquisition of susceptibility to CD95-mediated apoptosis and degradation of cellular FLIP. Therefore, MPA has unique functional properties not shared by other immunosuppressive drugs interfering with IL-2R signaling events such as rapamycin and CD25 mAbs.

Background: The mechanisms of action of polyclonal antithymocyte globulins (ATGs) are still poorly understood and the selection of doses used in different clinical applications (prevention or treatment of acute rejection in organ allografts, treatment of graft-versus-host disease, or conditioning for allogeneic stem cell transplantation) remains empirical. Low T-cell counts are usually achieved in peripheral blood during ATG treatment but the extent of T-cell depletion in lymphoid tissues is unknown., Methods: Experiments were conducted in cynomolgus monkeys using Thymoglobuline at low (1 mg/kg), high (5 mg/kg), and very high (20 mg/kg) doses., Results: ATG treatment induced a dose-dependent lymphocytopenia in the blood and a dose-dependent T-cell depletion in spleen and lymph nodes but not in the thymus, indicating a limited access of ATG to this organ. T-cell apoptosis in peripheral lymphoid tissues was the main mechanism of depletion. Remaining T cells in peripheral lymphoid organs were coated by antibodies and had down-modulated surface expression of CD2, CD3, CD4, and CD8 molecules, whereas their responsiveness in mixed leukocyte reaction was impaired. The survival of MHC-mismatched skin and heart allografts was prolonged in a dose-dependent fashion, despite the occurrence of a strong anti-ATG antibody response resulting in the rapid clearance of circulating ATGs., Conclusion: The results indicate that T-cell depletion is achieved rapidly and primarily in peripheral lymphoid tissues at high ATG dosage. Short ATG treatments could therefore be clinically evaluated when major peripheral T-cell depletion is required.