Dendritic cells release exosomes together with phagocytosed pathogen; potential implications for the role of exosomes in antigen presentation

Dendritic cells (DC) have the unique capacity to activate naive T cells by presenting T cell receptor specific peptides from exogenously acquired antigens bound to Major Histocompatibility Complex (MHC) molecules. MHC molecules are displayed on the DC plasma membrane as well as on extracellular vesicles (EV) that are released by DC, and both have antigen-presenting capacities. However, the physiological role of antigen presentation by EV is still unclear. We here demonstrate that the release of small EV by activated DC is strongly stimulated by phagocytic events. We show that, concomitant with the enhanced release of EV, a significant proportion of phagocytosed bacteria was expulsed back into the medium. High-resolution fluorescence microscopic images revealed that bacteria in phagosomes were surrounded by EV marker-proteins. Moreover, expulsed bacteria were often found associated with clustered HLA II and CD63. Together, these observations suggest that exosomes may be formed by the inward budding into phagosomes, whereupon they are secreted together with the phagosomal content. These findings may have important implications for selective loading of peptides derived from phagocytosed pathogens onto exosome associated HLA molecules, and have important implications for vaccine design.