Proinflammatory Th1 cells can produce large amounts of the immunosuppressive cytokine IL-10, thereby facilitating the self-limitation of inflammatory responses. Recently, we identified the Notch pathway as a main regulator of IL-10 production by Th1 cells. In this study, we show that plasmacytoid dendritic cells (pDCs), by means of their unique high-level expression of the Notch ligand Delta-like (Dll)-4, activate the Notch receptor on T cells to induce robust IL-10 production in vitro and in vivo. pDCs display a distinct pattern of Notch ligands compared with conventional dendritic cells, marked by the constitutive expression of Dll-4, the only Notch ligand to induce IL-10 expression in vivo, and Dll-1, while at the same time lacking the expression of Jagged. We provide a new mechanism for IL-10 induction by pDCs underlining the importance of the Dll-4/Notch axis in the regulation of inflammatory T cell responses.