In a next series of experiments we wanted to elucidate whether 293/gp350+ exosomes can co-transfer functional membrane proteins to B cells. As a model system but also as a potential practical application, we chose B-CLL cells that express CD21 and become immunogenic upon ectopic expression of CD154 on malignant cells. We, therefore, transfected 293 cells with expression plasmids for gp350 and CD154 and isolated exosomes as described above. Again, an immunoblot with a CD154-specific antibody demonstrated the presence of CD154 in exosome preparations and an OptiPrep TM gradient revealed co-sedimentation with gp350 ( Figure 1A) indicating the presence of both proteins on exosomes ( Figure 2A ). In addition, flow cytometry revealed that gp350+/CD154+ exosomes specifically bound to CD19+ B cells from a B-CLL patient ( Figure 2B ).