IL-10 is required for mediating the suppressive effects of allogeneic CD8α⁺ DC vaccination. T cells from WT B6 and IL-10 KO B6 donor mice were harvested after immunization with either diluent or host BALB/c BM-derived CD8α⁺ DCs or CD8α⁻ DCs as described in “Vaccination protocol.” T cells were used as responders and stimulated with irradiated (3000 cGy) BALB/c splenocytes in an allogeneic MLR and assayed for (A) proliferation, and (B) supernatants were collected at 66 hours and assayed by ELISA for IFN-γ and IL-10. *P < .05 between T cells from WT B6 animals that were immunized with BALB/c BM-derived CD8α⁺ DC compared with diluent or CD8α⁻ DC controls. P = not significant between for T cells from IL-10 KO B6 animals in all groups. Data represent 1 of 3 similar experiments. Recipient BALB/c mice were irradiated with 8000 cGy TBI and injected with 5 × 10⁶ TCD BM and 10⁶ CD90⁺ T cells from syngeneic (black thin solid line, n = 6) or allogeneic T cells from diluent control (black bold solid line, n = 7) or BALB/c CD8⁺ DC-vaccinated (triangles, n = 8) or CD8⁻ DC-vaccinated (diamonds, n = 8) B6 WT donors or T cells from allogeneic IL-10 KO B6 donors that were immunized with diluent control (inverted triangles, n = 10), or BALB/c CD8⁺ DC-vaccinated (squares bold solid line, n = 10) or CD8⁻ DC-vaccinated (circles, dotted line, n = 9) IL-10 KO donors and evaluated for (C) survival and (D) clinical GVHD score. *P < .05 between T cells from WT B6 animals that were immunized with BALB/c BM-derived CD8α⁺ DC versus those with diluent or CD8α⁻ DC controls. P = not significant between for T cells from IL-10 KO B6 animals in all groups. Data represent 1 of 3 similar experiments. Data are from 2 combined experiments with similar results.