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-2Kd+ donor CD4+ T cells in the spleens 6 days after transfer are shown. Data are representative of 2 similar experiments.