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DCs are the most potent antigen presenting cells, equally capable of activating naïve and memory T lymphocytes. The signals that activate DCs are those indicative of microbial infection (exogenous danger signals), or sterile tissue damage (endogenous danger signals). Discreet types of danger signals, or combinations of signals, can differentially regulate DC activation. This in turn can strongly affect the functional properties of T cells that are eventually sensitized by the DCs. The overarching theme of this dissertation is the examination of how human monocyte-derived DC (MDDC) respond to multiple activation signals that may be encountered in settings of infection or cancer, or as a means to enhance their therapeutic potential. It was originally reported that only a small fraction of total matured DCs produced IL-12, but it has never been determined whether different combinations of activating signals now shown to maximize secreted IL-12 do so through increasing output by the same IL-12 producers, or by recruiting additional cytokine-secreting cells. All combinations of LPS (TLR4 ligand), R848 (TLR8 ligand), IFN-γ and CD40L were tested for activating human monocyte-derived DC, and determined by intracellular flow cytometry that enhanced IL-12 secretion was accomplished by markedly increasing the proportion of cells producing IL-12, with the triple and quadruple combinations recruiting the most DC. This requirement for multiple signals was not reflected in differential TLR expression by the cells. These results have implications for the biology of IL-12-secreting DCs and choice of activation regimen for prospective use in DC-based immunotherapy. Since strategically-paired Toll-like receptor (TLR) ligands can induce a unique dendritic cell (DC) phenotype that polarizes Th1 responses it was hypothesized that pairing single TLR ligands with an “endogenous” danger signal, extracellular adenosine triphosphate (ATP), would likewise cooperatively induce in monocytes unique DC-like properties. ATP greatly enhanced lipoteichoic acid (LTA) induced expression of IL-23 and IL-1β. These cells showed DC-like surface phenotype and enhanced migration towards CCR7 ligands. They also preferentially deviated CD4 T lymphocyte responses toward a Th17 phenotype. These findings may shed light on the phenomenon of cancer related inflammation or autoimmune pathologies which are associated with Th17 responses.