INDUCTION OF DONOR-SPECIFIC UNRESPONSIVENESS TO RAT CARDIAC ALLOGRAFTS BY INTRATHYMIC INJECTION OF UV-B-IRRADIATED DONOR SPLEEN-CELLS

This study examined the role of intrathymic injection of allogeneic sp leen cells in induction of donor-specific unresponsiveness to heart al lografts in the Lewis-to-ACI rat combination. Intrathymic injection of naive Lewis SC led to rejection in naive or sublethally irradiated (2 00 rads TBI) ACI recipients at times equivalent to those obtained in c ontrol animals. Intrathymic injection of UV-B-irradiated Lewis SC, on the other hand, led to indefinite cardiac allograft survival (>300 day s) in sublethally irradiated ACI recipients; similar treatment failed to prevent rejection of third-party (Wistar Furth) cardiac allografts, which demonstrates the specificity of the immunologic unresponsivenes s thus induced. The finding that intrathymic injection of untreated al logeneic SC does not prevent rejection of subsequently transplanted al lograft suggests that modulation of major histocompatibility complex c lass II molecule by methods such as UVB may be critical to induction o f unresponsiveness. Inoculation of UV-B donor SC in extrathymic sites (subcutaneous, intraperitoneal and intratesticular) did not significan tly prolong graft survival in similarly prepared animals, thus confirm ing the privileged position of the thymus in the induction of toleranc e. When the unresponsive recipients of cardiac allografts were made di abetic at 100 days and rechallenged with a second-set donor-type neova scularized pancreatic islet grafts, three of four animals accepted per manently (> 100 days) the islet grafts, thus indicating tolerance to d onor alloantigens. To define the underlying mechanisms of specific tol erance in this model, in vitro MLR and in vivo adoptive transfer studi es failed to demonstrate suppressor activity in the long-term cardiac allograft recipients. In contrast CML assays using Cr-51-release showe d that T cells obtained from the unresponsive animals had no detectabl e cytotoxic activity to Con A-stimulated donor blast targets. The latt er finding suggests clonal anergy or deletion of cytotoxic T cells to donor alloantigens. Our results confirm the role of the thymus as a pr ivileged site for the induction and maintenance of specific immunologi c unresponsiveness to organ allografts and suggest that this approach may be potentially useful in clinical transplantation of immediately v ascularized allografts and neovascularized grafts.