Exosomes from B lymphocytes are a major source of Fas ligand

Abstract

Abstract Exosomes are nanovesicles released by a variety of cell types, including antigen-presenting cells. The ability of exosomes to induce immunity has prompted great interest in their clinical use. Murine studies have also indicated that circulating FasL+MHCII+ exosomes can be tolerogenic in allergic and delayed-type hypersensitivity responses. B cells are capable of expressing FasL and are a major in vivo source of exosomes, but their role in producing tolerogenic exosomes is incompletely explored. In the current study, we evaluated B cells as a source of FasL+ exosomes in vivo and signals that induced B cells to release FasL+ exosomes in vitro. In B-cell deficient mice, a major loss in total splenic exosomes and exosome-associated FasL was observed, suggesting B cells are the major source of FasL+ exosomes in vivo. After sorting B cells into CD107a+, CD5+ or CD9+ cells, all subsets showed some intracellular storage of FasL, yet FasL was predominately found in the CD9+ subset. Searching for signals that induced FasL+ exosomes in vitro, we stimulated B cell signaling through Toll-like receptors (TLRs), the B-cell receptor (BCR), and/or CD40. In response to BCR ligation, B cells produced 20% more FasL compared to medium controls, which increased to 50% in the presence of any TLR ligand. Interestingly, B cells increased their intracellular FasL storage, but failed to increase their FasL+ exosome release when stimulated through CD40 alone. TLR-4, TLR-7(/8) or TLR-9 ligation with CD40 activation induced the FasL+ exosome release, which was further enhanced by BCR ligation. These data suggest that signals known to induce the differentiation of IL-10-producing B cells, also enhance FasL+ exosome production as another potential suppressive mechanism.