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 naive, 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.