Background and aims: Intestinal epithelial cells release antigen presenting vesicles (exosomes) bearing MHC class II/peptide complexes stimulating specific immune responses in vivo. To further characterize the role of human epithelial exosomes in antigen presentation, their capacity to load antigenic peptides, to bind immune target cells and to induce T cell activation was analyzed in vitro. Methods: The capacity of exosomes derived from the HLA-DR4 expressing, intestinal epithelial cell line T84, to load the HLA-DR4-specific peptide 3H-HSA 64-76 and to activate a HLA-DR4-restricted T cell hybridoma, was tested in the presence or absence of human monocyte-derived dendritic cells (DCs). Interaction of FITC-labeled exosomes with T cells and DCs was analyzed by flow cytometry and confocal microscopy. Results: T84-derived exosomes, enriched in CD9, CD81, CD82 and A33 antigen, were capable of binding specifically HSA 64-76 peptide on HLA-DR4 molecules and of interacting preferentially with DCs. HSA-loaded exosomes were unable to activate the T cell hybridoma directly, but induced a productive T cell activation through DCs. When HSA peptide was bound to exosomal HLA-DR4 molecules instead of in a soluble form, the threshold of peptide presentation by DCs was markedly decreased (x10-3). Conclusions: Exosomes released by intestinal epithelial cells bear exogenous peptides complexed to MHC class II molecules and interact preferentially with DCs, strongly potentiating peptide presentation to T cells. Epithelial exosomes constitute a powerful link between luminal antigens and local immune cells by mediating the transfer of tiny amounts of luminal antigenic information and facilitating immune surveillance at mucosal surfaces.