PDCs selectively expand T cells that produce cytokines on rechallenge with antigen. Cocultures of CFSE-labeled CD4⁺ T cells and PDCs or CD11c⁺ DCs were stimulated with HCMV in the absence or presence of neutralizing anti–IL-10 or anti–type I IFN, as indicated. On day 6, expanded T cells were restimulated for 16 hours with autologous imoDCs and HCMV. The production of IL-10 and IFN-γ was measured in CD3⁺ T cells with the cytokine secretion assay and was almost exclusively found in the CD3⁺ T cells that had divided (CFSE^low), as shown for IL-10 in the upper plots of panel A. The lower 6 plots are gated on dividing cells, as indicated, and show the secretion of IL-10 and IFN-γ in one experiment representative of 4 performed. Percentages of single and double cytokine-producing cells are indicated. In panel B the results from 4 different experiments are shown, with means and SEM indicated. All cytokine-producing populations were higher in PDC-expanded as compared with CD11c⁺ DC-expanded cultures (*P < .05). Addition of neutralizing anti–IL-10 resulted in significantly higher levels of double cytokine-producing cells in the PDC cocultures (**P < .02), whereas the levels were nonsignificantly changed in the presence of anti–type I IFN. (C) Blocking of IL-12, IL-18, or IL-1β did not reduce the levels of IL-10 produced in PDC cocultures relative to control (stippled line), as measured in the culture supernatants at 48 hours following stimulation with HCMV. Two separate experiments are indicated by distinct symbols.