Mast cell-dependent B and T lymphocyte activation is mediated by the secretion of immunologically active exosomes

Mitogenic activity of bone marrow-derived mouse mast cells and mast cell li nes P815 and MC/9 on B and T lymphocytes is present in their culture supern atants. To identify this activity, mast cells were incubated in serum-free medium and the supernatant was subjected to differential centrifugation, wh ich resulted in two fractions, the hypodense and dense fraction (pellet). W hen analyzed for their mitogenic activity on spleen cells, all activity was found to be associated with the dense fraction. Electron microscopy studie s revealed the presence in this fraction of small vesicles called exosomes with a heterogeneous size from 60 to 100 nm of diameter. When cocultured wi th spleen cells, purified exosomes induced blast formation, proliferation, as well as IL-2 and IFN-gamma production, but no detectable IL-4. Similar d ata were obtained by injecting exosomes into naive mice. In contrast to mas t cell lines, a pretreatment with IL-4 is required for bone marrow-derived mast cells to secrete active exosomes. Structurally, exosomes were found to harbor immunologically relevant molecules such as MHC class II, CD86, LFA- 1, and ICAM-1. These findings indicate that mast cells can represent a crit ical component of the immunoregulatory network through secreted exosomes th at display mitogenic activity on B and T lymphocytes both in vitro and in v ivo.