Mechanisms involved in antithymocyte globulin immunosuppressive activity in a nonhuman primate model

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.

Experiments were conducted in cynomolgus monkeys using Thymoglobuline at low (1 mg/kg), high (5 mg/kg), and very high (20 mg/kg) doses.

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.

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.