Host irradiation is prerequisite for pDC maturation to prime T cells. (A) Expression of CD86 and I-A^b expression on B cells, pDCs, and cDCs in the spleen and mLNs isolated from irradiated mice 10 hours after irradiation (filled histograms) and from unirradiated control mice (broken-lined open histograms). (B) Unirradiated (top) or irradiated (bottom) B6-Ly5a (CD45.1⁺) mice were injected with 80 × 10⁶ BM cells and splenocytes isolated from FL-treated B6 (CD45.2⁺) mice. Expression of I-A^b on CD45.2⁺ B cells and pDCs in mLNs 10 hours after injection is shown. (C) Aliquots of 20 × 10⁶ CFSE-labeled BALB/c (H-2K^d+) CD4⁺ T cells were transferred to unirradiated H2-Ab1^−/− mice preinjected with 6 × 10⁶ WT pDCs or cDCs. Cell division of donor CD4⁺ T cells in the spleens on day +6 is shown. (D) Similarly, CFSE-labeled BALB/c CD4⁺ T cells were transferred to unirradiated H2-Ab1^−/− mice preinjected with freshly isolated pDCs or pDCs stimulated with CpG 1668 1 μM for 24 hours in vitro. Cell division of donor CD4⁺ T cells in the spleens on day +6 is shown. Data are representative of 2 similar experiments.