Mechanisms of tolerance induction after intrathymic islet injection - Determination of the fate of alloreactive thymocytes

Background. Intrathymic (IT) administration of antigen when combined with p eripheral T-cell depletion has been shown to induce operational tolerance i n a wide range of experimental protocols. IT injection of pancreatic islets has been demonstrated not only to induce tolerance to alloantigen but also to prevent the development of autoimmune beta-cell destruction in models o f type I diabetes. However, little is known about the mechanisms involved i n tolerance induction after IT islet injection. Methods and Results. A protocol for the induction of tolerance to fully all ogeneic (C57BL/10; H2(b)) peripheral islet allografts was developed in CBA/ Ca (H2(k)) recipients by the IT injection of allogeneic islets combined wit h depletion of peripheral CD4(+) T cells. This protocol was based upon our own data and those of others showing that CD4(+) T cells play a critical ro le in islet allograft rejection. Using this regimen, donor-type peripheral islet allografts survived indefinitely whereas third-party grafts were reje cted. To determine the fate of alloreactive thymocytes that recognize donor major histocompatibility complex antigens via the direct pathway, T-cell r eceptor transgenic mice specific for the major histocompatibility complex c lass I molecule K-b (BM3 and DES) were used as recipients. IT injection of islets expressing the specific alloantigen K-b resulted in clonal deletion of alloreactive thymocytes in T-cell receptor transgenic recipients. No evi dence of clonal inactivation in the residual peripheral alloreactive popula tion was observed in this system. Conclusions. IT injection of allogeneic islets and concomitant CD4(+) T-cel l depletion is able to induce donor-specific unresponsiveness. One mechanis m responsible for this unresponsiveness is the clonal deletion of thymocyte s that recognize alloantigen via the direct pathway.