Characteristics of the induction phase of antithymocyte-globulin-induced heart allograft tolerance in the rat.

Transplantation tolerance was induced in 50% of Wky rats treated with antithymocyte globulin (ATG) 2 days prior to transplantation of a PVG/c heart. The induction period immediately following ATG treatment and transplantation was characterized by instability, with 50% of the allografts being lost in rejection within 30 days after transplantation. Second syngeneic transplantation to ATG-treated recipients 5 days after the first transplantation facilitated tolerance induction and permanent acceptance of both transplants. Thirty days after ATG treatment and the primary transplantation, 50% of second syngeneic allografts were accepted. During the early period after transplantation a continuous antigen presence together with ATG was necessary. Graft removal and absence of the transplant for 3 days, close to ATG administration, resulted in rejection of a second PVG/c transplant. Furthermore, the induction phase could easily be manipulated by the addition of sensitized or tolerized spleen cells to the recipient, causing allograft rejection or tolerance, respectively. Sixty days after ATG treatment and transplantation, a longer period between graft removal and retransplantation was required to break tolerance. Injection of sensitized spleen cells at this time did not affect graft survival, indicating an increased stability of the unresponsive state 60 days after ATG treatment and transplantation.