DCs are not efficiently activated by MCMV or TLR ligands during acute GVHD. BALB/c (non-GVHD) and DBA/2 (GVHD) were transplanted with BALB/c BM (5 × 10⁶) and CD3⁺ T (2.5 × 10⁶) cells. Four weeks after transplantation, mice were either infected with 5 × 10³ pfu MCMV (A-F) or administered 100 mM CpG IP (G-I) and the mean fluorescence of CD40 and CD86 expression on DC subsets was determined. (A,D) Data for splenic CD8α⁺ DC; (B,E) Data for splenic CD11b⁺ DCs; (C,F) data for splenic pDCs. Spleen cells were collected 48 hours after CpG administration and the mean fluorescence of CD40 (G) and CD86 (H) expression was determined for CD8α⁺ and CD11b⁺ DCs. (I) The mean fluorescence of CD40 and CD86 expression on pDCs. Changes between the untreated and CpG-treated groups were tested for significance. (J) B6.Ptprca mice were transplanted with B6.CD11c.DOG BM cells (5 × 10⁶) and T cells (2 × 10⁶), and 4 weeks later they were infected with 5 × 10³ pfu MCMV. CD11c.DOG chimeric mice were treated with saline (non-GVHD) or diphtheria toxin on days −2, −1, 1, and 3 to ablate DCs (non-GVHD no DCs). The frequency and number of MCMV-specific CD8⁺ T cells detected with an m45-specific tetramer are shown. (A-F) 3 to 5 mice per group; (G-I) 4 mice per group; (J) 6 mice per group. *P = .01-.05; **P = .001-.01; ***P = .0001-.001; ****P < .0001.