To answer the question whether CD154 in gp350+/CD154+ exosomes was functional, HEK293 cells were transfected with a CD154 expression plasmid either alone or in combination with a gp350 expression plasmid. Transfer of exosomes to B-CLL cells was measured with an anti-CD154 antibody by flow cytometry. As shown in Figure 2C , exosomes that carry both proteins efficiently conveyed CD154 surface expression to B-CLL cells. CD154+/ gp350-exosomes were only transferred very inefficiently to B-CLL cells, probably due to an only weak interaction of CD154 with its receptor CD40 on the cell surface. In order to investigate the immune accessory function of CD154, we loaded B-CLL cells from an HLA II DR13+ donor either with 293 exosomes, with exosomes from 293 cells transfected with an expression plasmid for gp350 (gp350+ exo) or transfected with expression plasmids for gp350 and CD154 (CD154+/gp350+ exo). One day later, we added HLA-DR13-restricted gp350-specific CD4+ T cells and measured their activation with an IFN-c ELISA. This assay demonstrated the relevance of CD154 for T-cell recognition because B-CLL cells loaded with CD154+/gp350+ exosomes were significantly better stimulators than B-CLL cells loaded with gp350+ exosomes ( Figure 2D ). Induction of the accessory molecule ICAM-1 (CD54), the co-stimulatory molecules B7.1 (CD80) and B7.2 (CD86), and the death receptor Apo1 (CD95) on B-CLL cells upon incubation with CD154+/gp350+ exosomes ( Figure 2E ) but not with exosomes from 293 cells provided an explanation for improved recognition by T lymphocytes.