Malignant T cells inhibit DC maturation in an IL-10–dependent fashion. (A) Immature DCs (iDCs) were generated from monocytes and LPS (1 μg/mL) was added for the last 24 hours of culture to generate mature DCs (mDCs). A group of DCs was cocultured with HuT 78 cells 24 hours before LPS maturation. CD11c⁺ DCs were purified and iDCs, mDCs, or DCs that had been cocultured with HuT 78 cells before LPS maturation were used to stimulate proliferation of purified CD4⁺ T cells at the T-cell/DC ratios shown. (B-C) DCs were generated, matured with LPS, and used as stimulators in an allo-mixed lymphocyte reaction at a T-cell/DC ratio of 100:1. As in panel A, before maturation, groups of DCs were cocultured with either HuT 78 (B) or SU-DHL-1 (C) cells in the presence of an isotype control or neutralizing IL-10 monoclonal antibody (2 μg/mL), as indicated. Data are mean ± SD. (D) DCs were generated and matured with LPS in the presence or absence of HuT 78 or SU-DHL-1 cells, as indicated. An isotype control or neutralizing IL-10 antibody was included. Cells were stained with an isotype control (shaded) or anti-CD83 (solid line). Only CD11c⁺ cells were gated and included in the analysis. Mean fluorescent intensity (MFI) for CD83 is shown in each histogram. Data shown (A-D) are representative of at least 3 similarly performed experiments. (E) Benign dermatitis (n = 10) and CTCL (n = 10) skin biopsies were immunohistochemically stained for both CD11c and CD83. Four representative examples of each are shown.