HEK293 is a human embryonic kidney cell line [34] , which spontaneously releases exosomes into the cell culture supernatant. For the generation of modified exosomes, 293 cells were cotransfected with expression plasmids for BLLF1 (gp350), CD154 and/or gfp. For the isolation of exosomes, 25 ml of conditioned supernatants were collected three days later, sterile filtrated and subjected to repeated centrifugations at increasing centrifugal force (10 min at 300 x g, 10 min at 5000 x g in a Heraeus 3SR+centrifuge, followed by 2 h at 100.000 x g in a Beckman LE-80K ultracentrifuge in a SW28 swing-out rotor. The pelleted particles were washed, resuspended in 500 ml volume PBS containing protease inhibitors (Complete Mini, Roche) and the protein content was analyzed in a Lowry microassay using reagents purchased from Bio-Rad (Munich, Germany). Exosomes were further purified by flotation into a 5-30% iodixanol gradient (OptiPrep TM , Sigma Aldrich, Deisenhofen, Germany).