Antithymocyte globulin
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Anti-thymocyte globulin (ATG) is an infusion of horse or rabbit-derived antibodies against human T cells, which is used in the prevention and treatment of acute rejection in organ transplantation and therapy of aplastic anemia.
Two antithymocyte globulin (ATG) agents licensed for clinical use in the United States are Thymoglobulin (rabbit ATG, rATG, Genzyme) and Atgam (equine ATG, eATG, Pfizer). Thymoglobulin and Atgam are currently licensed for use in the treatment of renal allograft rejection; Atgam is additionally licensed for use in the treatment of aplastic anemia. Both drugs are used in off-label applications, especially as immunosuppression induction agents before and/or during kidney transplantation. An rATG product made by Neovii Pharmaceuticals is marketed outside of the United States under the name Grafalon .
ATG administration very substantially reduces immune competence in patients with normal immune systems, through a combination of actions, some explicitly understood and some more hypothetical. rATG in particular effects large reductions (through cell lysis) in the number of circulating T-lymphocytes, hence preventing (or at least delaying) the cellular rejection of transplanted organs. However, medical opinion remains divided as to when the benefit of this profound reduction in T-cells outweighs the concomitant increased risks of infection and malignancy.
In the United States it is frequently given at the time of the transplant to prevent graft-versus-host disease, although many European centers prefer to reserve its use for the treatment of steroid-resistant acute rejection, as European centres generally serve more homogeneous populations and rejection tends to be less of a problem.
ATG use can induce cytokine release syndrome, and has been thought to increase the risk of post-transplant lymphoproliferative disorder (PTLD); however, this association may not apply when lower dosing regimens are used. There is some evidence to suggest that inducing immunosuppression with rATG at organ transplantation may create conditions in the patient's immune system favorable to the development of immunological tolerance, but the exact basis for such a development remains largely speculative. Temporary depletion of the T-cell population at the time of the transplant also risks delayed acute rejection, which may be missed and cause severe damage to the graft.
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