Studies of privileged sites and organ transplantation.

Extensive research has been carried out in the last years to try to prevent graft rejection by minimizing the side effects related to the use of immunosuppressants. Ideally, one would hope to achieve a state of donor-specific unresponsiveness in order to promote a condition of true tolerance without the need of immunosuppressants. Recent evidence has been provided that this is a pursuing goal, at least in experimental animals, and even in humans increasing data are available that tolerance is achieved in some patients years after transplantation. The interest in donor-specific transplant tolerance has been renewed by the recent observations that in the rat the thymus is an immunologically privileged site into which isolated pancreatic islets can be engrafted and survive indefinitely. Moreover, intrathymic injection induced donor-specific unresponsiveness, which allowed survival of a second donor-strain islet cell allograft transplanted into an extrathymic site. Findings on cellular allograft have been extended to vascularized organ allografts. Recent experiments documented that in the rat intrathymic injection of donor cells allows subsequent renal graft to survive indefinitely. Unresponsiveness to rat kidney graft is donor but not tissue specific and evidence is presented here that the thymus has a central role in such a phenomenon. Hopefully, these studies will open new perspectives in transplantation promoting indefinite graft survival without the complications of long-term immunosuppressants.