Cross-dressing of recipient Ag-presenting cells with donor exosomes trigger direct T-cell allosensitization in transplantation

The idea that donor dendritic cells (DCs) from the graft present donor MHC Ag to naive T cells has been challenged. Evidence suggests that donor DCs are undetectable or found are low numbers in graft-draining lymphoid organs. Thus, the aim of our study is to elucidate how donor MHC molecules are recognized so efficiently by recipient naive T cells. Results After transplantation of CD45.2 BALB/c hearts in CD45.1 B6 mice, very few donor DCs were detected in the spleen, and no donor DCs were found in lymph nodes draining BALB/c skin grafted in B6 mice. In both cases, donor intact MHC molecules were detected by electron microscopy on exosomes attached to recipient conventional DCs in graft-draining lymphoid organs. The transferred MHC Ag induced proliferation and differentiation of CD8 T cells against the BALB/c H2L d Ag. Cultures of BALB/c DCs with B6 DCs with inhibitors of exosome release (Rab27a siRNA) confirmed that passage of BALB/c MHC occurred via exosomes. Importantly, transfer of exosomes released by donor DCs (unlike other types of vesicles) promoted maturation of recipient DCs. By high resolution confocal microscopy, BALB/c DCs, engineered to release RFP + exosomes and injected in CD11c-YFP B6 mice, transferred RFP + exosomes to recipient YFP + DCs in lymph nodes and spleen. Accordingly, depletion of recipient DCs in CD11c-DTR B6 recipients prevented presentation of donor intact MHC Ag to T cells and delayed heart allograft rejection. Conclusion Our results elucidate the ultrastructural basis of the still elusive semi-direct pathway of allorecognition, by demonstrating that donor exosomes (released by the graft or donor migrating DCs) cross-dress recipient APCs and promote the potent T-cell allosensitization seen in transplantation.