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Dendritic cells secrete a population of “antigen-presenting vesicles,” called exosomes, expressing functional class I and II major histocompatibility complex (MHC) and co-stimulatory molecules. The subcutaneous administration of syngeneic exosomes expressing tumor antigens has been shown to induce specific antitumor immune responses in vivo. The authors hypothesized that antigen presentation by exosomes, depending on the context of their administration, may induce tolerance rather than immunity.The authors therefore tested the capacity of exosomes derived from donor bone marrow dendritic cells, given before transplantation, to modulate heart allograft rejection.The authors show here that donor type but not syngeneic exosomes induced a significant prolongation of allograft survival, with a few recipients having long-term graft survival. During the first week after transplantation, allografts from exosome-treated rats displayed a significant decrease in graft-infiltrating leukocytes and in the expression of interferon-γ mRNA compared with allografts from untreated animals. Moreover, when tested in vitro, spleen CD4+ T cells from exosome-treated recipients displayed a significant decrease in anti-donor responses, suggesting a decrease in anti-donor T-cell responses. However, the authors also found that allogeneic donor-derived exosomes increased anti-donor MHC class II alloantibody production.The authors demonstrate an effect of allogeneic exosomes on the modulation of immune responses in vivo, suggesting that, like donor cells, exosomes can stimulate or regulate antigen-specific immune responses.