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 molecu les displayed on the surface of antigen-presenting cells is an essential el ement in the induction of T cell responses to transplant antigens, The find ing that intrathymic (IT) injection of an immunodominant peptide induces ac quired 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 immuno dominant Wistar Furth (WF) MHC class I (RT1.A(u)) peptide 5 (93-109) on car diac allograft survival in the WF-to-ACI rat combination. Methods. DC were propagated from cultures of ACI (recipient) bone marrow (B M) maintained in a medium supplemented with granulocyte-macrophage colony-s timulating 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 all ografts 7 days after IT immunization with peptide-pulsed DC. Results. BM-derived DC cultured in granulocyte-macrophage colony-stimulatin g factor and interleukin-4 for 8 days have a strong allostimulatory ability and present peptide 5 to naive syngeneic T cells in mixed lymphocyte react ion. IT inoculation of 300 mu g RT1.A(u) peptide 5 combined with transient antilymphocyte serum immunosuppressive therapy induced donor-specific toler ance to cardiac allografts. Extension of this finding to peptide-pulsed sel f 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 unr esponsive recipients challenged with second-set grafts accepted permanently (>100 days) donor-type (WF) grafts while rejecting third-party (Lewis) gra fts without the rejection of the primary WF grafts. Conclusion. This novel finding that allopeptide-pulsed host DC induces tole rance to cardiac allografts suggests that the induction of acquired toleran ce is dependent on the indirect allorecognition pathway. The results furthe r suggest that genetically engineered DC expressing donor MHC class I or II molecules or a peptide analogue might have therapeutic potential in the in duction of transplant tolerance and in the treatment of autoimmune diseases .