Cognate interaction between pDCs and T cells is required for T-cell activation. CFSE-labeled BALB/c CD4⁺ T cells were transferred to irradiated H2-Ab1^−/− mice preinjected with 2 × 10⁶ pDCs and cDCs isolated from WT or H2-Ab1^−/− B6 mice. Cell divisions of H-2K^d+ donor CD4⁺ T cells in the spleens 6 days after transfer are shown. Data are representative of 2 similar experiments.