CHARACTERIZATION OF THE PROTEOME OF B‐CELL‐DERIVED EXOSOMES

B‐lymphocytes are key players in the adaptive immune system and display antigens in association with MHC class II molecules at their cell surface to activate cognate CD4+ T cells. Antigen loading of MHC class II occurs within endocytic compartments, including multivesicular bodies (MVB). MVB are composed of many 60–90 nm vesicles that are formed by inward budding from their delimiting membrane. These vesicles may be targeted to lysosomes or, alternatively, can be secreted as exosomes. B cell‐derived exosomes contain MHC class II and have been demonstrated to activate CD4+ T cells in vitro. Exosomes are, in addition to B cells, also secreted by various other cell‐types, including dendritic cells, epithelial cells and T cells. Although their precise physiological role is still unknown, mounting evidence suggests that they play an important role in intercellular communication. In recent years, several efforts have been undertaken to reveal the molecular composition of exosomes from distinct cell types. Here, we extensively mapped the proteome of exosomes from human B‐cells. For this, we separated the proteins of highly purified exosomes on SDS‐PAGE, followed by trypsin digestion and FT‐ICR mass spectrometry. In total, more than 550 proteins were identified, including those involved in cell‐cell adhesion, antigen presentation and vesicular trafficking. Our results support the idea that exosomes may play an important role as an intermediate vehicle in intercellular signaling within the immune system, and open the way for in‐depth functional analysis of specific proteins.