pDC-derived GrB inhibits allogeneic T-cell proliferation. Purified pDCs were activated for 36 hours in the presence of IL-3, IL-10, and ODN 2006 as indicated. Activated pDCs were washed and coincubated for 6 days with CFSE-stained allogeneic T cells at a pDC:T cell ratio of 1:250. Cells were harvested, stained for CD3, CD4, and CD8, and analyzed by 4-color flow cytometry. (A) Dot plots from one representative experiment and bar graphs from 6 independent experiments illustrate the percentage of CD3⁺ T cells proliferating in response to allogeneic pDCs or anti-CD3/CD28 antibodies. Error bars indicate SEM. Similar responses were obtained with imiquimod instead of ODN 2006 (data not shown). (B) Bar graphs show average percentages of proliferating CD3⁺CD4⁺ and CD3⁺CD8⁺ T cells in the presence or absence of IL-3 with or without IL-10–cultured pDCs and anti-GrB or control antibodies (both at 50 μg/mL; n = 4). Error bars indicate SEM.