Indirect activation of naïve CD4+ T cells by dendritic cell–derived exosomes.

Dendritic cells (DCs) secrete vesicles of endosomal origin, called exosomes, that bear major histocompatibility complex (MHC) and T cell costimulatory molecules. Here, we found that injection of antigen- or peptide-bearing exosomes induced antigen-specific naive CD4[sup +] T cell activation in vivo. In vitro, exosomes did not induce antigen-dependent T cell stimulation unless mature CD8α-DCs were also present in the cultures. These mature DCs could be MHC class II-negative, but had to bear CD80 and CD86. Therefore, in addition to carrying antigen, exosomes promote the exchange of functional peptide-MHC complexes between DCs. Such a mechanism may increase the number of DCs bearing a particular peptide, thus amplifying the initiation of primary adaptive immune responses. [ABSTRACT FROM AUTHOR]