To secrete exosomes the aim of this study was to determine if exosomes secreted from activated CD3+ cells could play a role in an immunological response, enhanced by exogenous IL-2, by conveying signals from their secreting cells to resting CD3+ cells in an in vitro autologous setting. We show that upon stimulation, CD3+ T cells from human donors secrete exosomes, and that these exosomes together with IL-2 generate an immune response in resting autologous CD3+ T cells. With automated cell counting, a proliferation assay, flow cytometry and a human cytokine array, we could monitor the immune response in the stimulated CD3+ T cells. In this study, we show that exosomes derived from stimulated T cells can function as an autologous signal to increase proliferation of resting T cells. In addition, stimulation of resting T cells with these exosomes also results in an altered cytokine profile and in a lower CD4/CD8 ratio. When performing studies involving exosomes it is of great importance to characterize the exosome vesicles well. In this study, using dynamic light scattering, we show that the ultracentrifuge pellets of supernatants from the primary T cells contain vesicles with an average size of 54 nm in diameter, corresponding to the size of exosomes. Since dynamic light scattering detects the size of all particles present it indirectly gives an estimate of the purity of the preparation. In addition, using flow cytometry, we show that exosome vesicles present in the supernatant from IL-2, anti-CD3 and anti-CD28 stimulated T cells have canonical exosomal markers CD9, CD63 and CD81 on their surface. These results show that CD3+ T cells from healthy donors stimulated with anti-CD3, anti-CD28 and IL-2 secrete exosomes. This result is in line with previous studies where T cells stimulated with PHA, IL-2 and anti-CD3 produce exosomes. In order to understand the role of these exosomes from activated T cells in an autologous setting, the vesicles from proliferating CD3+ cells were isolated and transferred to autologous resting CD3+ cells. To measure the effect of the exosomes on the resting CD3+ cells we performed proliferation assays, flow cytometry to look at CD4/CD8 ratio and a cytokine array on the cell supernatant. We show that T cell derived exosomes take part in the stimulation and proliferation of resting CD3+ T cells. Neither IL-2 nor exosomes can on their own stimulate the resting T cells to grow significantly. However, the autologous exosomes derived from stimulated T cells appear to SB203580 cooperate with IL-2 to orchestrate proliferation of CD3+ T cells. Furthermore, the distribution between the CD4+ and CD8+ populations were skewed in co-cultures when exosomes from activated T cells were added. The results show that exosomes together with IL-2 result in more CD8+ than CD4+ cells at day 5 and day 8, which suggest the proliferating cells to be CD8+ T cells. Exosomes together with IL-2 induced a relative increase of CD8+ cells similar to that of IL-2 together with anti-CD3 and anti-CD28. This could infer that the exosomes carry with them information from the cells that secrete them, which can stimulate a proliferative response in resting T cells. It has previously been described that modified exosomes from APCs could induce a CD8+ T cell response in an antigenspecific manner. Here we get a CD8+ T cell response probably not related to antigenic stimuli since the exosomes derive from autologous stimulated CD3+ T cells. When examining the level of cytokines and chemokines present in the supernatant from the T cell cultures, we note significant changes when exosomes are present. Exosomes together with IL-2 generate secretion of more cytokines and at higher levels than exosomes or IL-2 alone. The cytokine analysis also shows that T cells stimulated with IL-2 together with exosomes secrete CCL4.