Mature exosomes induce strong T-cell activation and proliferation in vitro. (A) Mature HY-bearing (▪) or HY free (▵) or immature HY-bearing (⋄) exosomes were incubated for 3 hours in vitro with mature BMDCs. Up-regulation of CD69 on Marilyn T cells was measured 24 hours after T-cell addition. Dashes indicate the maximal level of CD69 up-regulation obtained with mature BMDCs incubated with 10 nM HY peptide. One representative of 6 independent experiments is shown. (B) HY-bearing exosomes from mature (▪), immature (⋄), or MHC class II–deficient mature (▵) BMDCs were incubated 3 hours in vitro with mature BMDCs. After irradiation, Marilyn T cells were added in the culture for 3 days and ³H-thymidine incorporation was measured during the last 16 hours. Dashes indicate the maximal level obtained in the presence of 17 nM HY peptide. One representative of 5 independent experiments is shown. cpm indicates counts per minute. (C) ³H-thymidine incorporation by Marilyn T cells cultured with mature MHC class II–deficient (IA^–/–) BMDCs and IA^b/HY-bearing exosomes from immature (⋄) or mature (▪) DCs. Dashes indicate the level obtained in the presence of 17 nM HY peptide. One representative of 3 independent experiments. (D) ³H-thymidine incorporation by Marilyn T cells cultured with mature BMDCs and mature HY-bearing (▪) or mature HY-free (▵) or immature HY-bearing (⋄) exosomes or both immature HY-bearing and 3 μg of mature HY-free exosomes (▴). One representative of 2 independent experiments. Dashes indicate the plateau obtained in presence of 17 nM HY peptide.