B lymphocytes can use mature exosomes to activate naive CD4 T cells. (A) Cell-surface expression of T-cell stimulatory molecules (MHC class II: MHC II, CD40) and phenotypic markers (CD11c, B220) by magnetically purified spleen CD11c⁺ DCs or CD19⁺ B cells. Shaded histograms indicate control isotype; open histograms, specific markers. Purified B cells contain 0.1% CD11c⁺ cells, (B) ³H-thymidine incorporation by Marilyn T cells cultured with magnetically purified spleen CD11c⁺ DCs or CD19⁺ B cells in the presence of HY peptide. One representative of 5 independent experiments is shown. (C,D) ³H-thymidine incorporation by Marilyn T cells cultured with magnetically purified spleen CD11c⁺ DCs (C) or CD19⁺ B cells (D) in the presence of immature HY-bearing exosomes (⋄) or mature HY-bearing exosomes (▪). One representative of 5 independent experiments is shown. Results shown in A-D were from the same experiment.