A novel clinically relevant approach to tip the balance toward regulation in stringent transplant model

Background: Regulatory T cells (T_regs) actively regulate alloimmune responses and promote transplantation tolerance. Thymoglobulin, a rabbit polyclonal antithymocyte globulin (ATG), is a widely used induction therapy in clinical organ transplantation that depletes peripheral T cells. However, resistance to tolerance induction is seen with certain T-cell depleting strategies and is attributed to alterations in the balance of naïve, memory and T_regs. The exact mechanism of action of ATG and its effects on the homeostasis and balance between T_regs and T-effector-memory cells (T_em) are unknown. Methods: A novel antibody reagent, rabbit polyclonal anti-murine thymocyte globulin (mATG), generated by the same process used to manufacture Thymoglobulin, was used alone or in combination with CTLA4Ig or sirolimus (SRL) in a stringent fully major histocompatibility complex-mismatched murine skin allograft model to study graft survival and mechanisms involved. Results: mATG depletes T cells but preferentially spares CD25⁺ natural T_regs which limit skewing of T-cell repertoire toward T_em phenotype among the recovering T cells. T-cell depletion with mATG combined with CTLA4Ig and SRL synergize to prolong graft survival by tipping the T_reg/T _em balance further in favor of T_regs by preserving T _regs, facilitating generation of new T_regs by a conversion mechanism and limiting T_em expansion in response to alloantigen and homeostatic proliferation. CONCLUSIONS.: Simultaneous T-cell depletion with ATG and costimulatory blockade, combined with SRL, synergizes to promote regulation and prolong allograft survival in a stringent transplant model. These results provide the rationale for translating such novel combination therapy to promote regulation in primate and human organ transplantation.