Dendritic cells (DC) orchestrate immune responses under direction of cytokines/chemokines in their microenvironment. To investigate the influence of that generated during T cell activation, we stimulated peripheral blood mononuclear cells (PBMC) with anti-CD3 and anti-CD28 coated beads and tested cell-free culture supernatants (lymphocyte conditioned medium, LCM) for cytokine/chemokine composition and biologic activity. LCM contained a battery of mediators important in the biology of myeloid (mDC) and plasmacytoid (pDC) DC. LCM differentiated monocytes into functional immature mDC, and induced maturation of immature mDC. LCM also augmented maturation and IFNalpha-production of CpG-treated pDC. Functional activity of LCM-derived DC was confirmed by their ability to enhance in vitro recall T cell responses and substantially augment in vivo cellular and humoral immune responses to various vaccines in non-human primates. These results demonstrate that products of anti-CD3/anti-CD28 stimulated PBMC generate biologically active DC in vitro and function as a vaccine adjuvant in vivo.