Indirect allorecognition in acquired thymic tolerance: Induction of donor-specific tolerance to rat cardiac allografts by allopeptide-pulsed host dendritic cells

Background Presentation of peptides either by recipient or donor MHC molecules displayed on the surface of antigen-presenting cells is an essential element in the induction of T cell responses to transplant antigens, The finding that intrathymic (IT) injection of an immunodominant peptide induces acquired thymic tolerance suggests an indirect pathway of allorecognition in the thymus, To address this theory, we studied the effects of IT injection of host bone marrow (BM)derived dendritic cells (DC)-pulsed with the immunodominant Wistar Furth (WF) MHC class I (RT1.A(u)) peptide 5 (93-109) on cardiac allograft survival in the WF-to-ACI rat combination. Methods. DC were propagated from cultures of ACI (recipient) bone marrow (BM) maintained in a medium supplemented with granulocyte-macrophage colony-stimulating factor and IL-4. The BM-derived DC after 8 days of culture were pulsed in vitro with a single WF MHC class I peptide (Residue 93-109) with the dominant epitope, washed, and injected into the thymus of ACI rats. The ACI recipients received donor-type (WF) or 3(rd) party (Lewis) cardiac allografts 7 days after IT immunization with peptide-pulsed DC. Results. BM-derived DC cultured in granulocyte-macrophage colony-stimulating factor and interleukin-4 for 8 days have a strong allostimulatory ability and present peptide 5 to naive syngeneic T cells in mixed lymphocyte reaction. IT inoculation of 300 mu g RT1.A(u) peptide 5 combined with transient antilymphocyte serum immunosuppressive therapy induced donor-specific tolerance to cardiac allografts. Extension of this finding to peptide-pulsed self DC showed that IT injection of peptide 5-pulsed host DC consistently led to permanent acceptance (>150 days) of donor-type (WF) cardiac allografts, whereas third-party (Lewis) grafts were acutely rejected. The long-term unresponsive recipients challenged with second-set grafts accepted permanently (>100 days) donor-type (WF) grafts while rejecting third-party (Lewis) grafts without the rejection of the primary WF grafts. Conclusion. This novel finding that allopeptide-pulsed host DC induces tolerance to cardiac allografts suggests that the induction of acquired tolerance is dependent on the indirect allorecognition pathway. The results further suggest that genetically engineered DC expressing donor MHC class I or II molecules or a peptide analogue might have therapeutic potential in the induction of transplant tolerance and in the treatment of autoimmune diseases.