RB cell line and DCs
RB cell line Y79 was obtained from ATCC (HTB-18; Rockville, MD), and maintained in RPMI-1640 medium supplemented with 10 mmol/L HEPES, 2 mmol/L L-glutamine, 100 U/mL penicillin, 100 μg/mL streptomycin, and 10% heat-inactivated fetal calf serum (FCS, Sigma Chemical Co., St. Louis, MO, USA). Peripheral blood mononuclear cells (PBMCs) were isolated by Ficoll-Hypaque density gradient centrifugation from healthy individuals. To generate DCs, the mononuclear cell fraction was washed twice with RPMI-1640, suspended in RPMI-1640 at 2.5 × 106 cells/mL, and seeded in a 6-well plate (Becton) at 2 mL per well. The plate was incubated at 37°C for 2 h, and the non-adherent cells were discarded. The adherent cells were cultured for 6 days in 2.5 mL RPMI supplemented with 10 mmol/L HEPES, 2 mmol/L L-glutamine, 100 U/mL penicillin, 100 μg/mL streptomycin, 10% heat-inactivated FCS, 50 ng/mL rhGM-CSF, and 20 ng/mL rhIL-4 (R&D System, Minneapolis, MN). Half-volume medium exchange was performed every 3 days with medium containing fresh cytokines.
Treatment of DCs with RBcs
RBcs was prepared by seeding 10-cm dish (Falcon; BD Bioscience, Franklin Lakes, NJ, USA) with 1 × 107 RB cell in 10 mL of completed medium for 24 h and centrifuged to remove cell debris. On day 5 of DC culture, RBcs was added to test DCs, while the same volume of culture medium was added to control DCs. On day 6, maturation of DCs was induced by adding 20 ng/mL of TNF-α (R&D System) or 1 μg/mL of LPS (Sigma), and the phenotype of DCs was determined by flow cytometry after incubating with TNF-α or LPS for 24 h. In some experiments, before adding of TNF-α or LPS, DCs were purified on day 6. Briefly, RBcs-treated DCs or control DCs were washed extensively and then purified with microbeads on auto-MACS columns using a Blood Dendritic Cell Isolation kit (Miltenyi Biotech, BergischGladbach, Germany) according to the manufacturer’s instructions. In short, isolation of DCs was performed in a two-step procedure. First, cells labeled with the Non-DC Depletion Cocktail comprising with CD14 and CD19 magnetic beads were depleted. Then DCs were positively selected by labeled with DC Enrichment Cocktail comprising with CD1c (BDCA-1), CD304 (BDCA-4/Neuropilin-1), and CD141 (BDCA-3) magnetic beads. Purified DCs samples contained >95% CD1c+ DCs as evaluated by the Blood Dendritic Cell Enumeration Kit (Miltenyi Biotech).
Flow cytometry and Reagents
The following monoclonal antibodies (mAb) conjugated with either fluorescein isothiocyanate (FITC) or phycoerythrin (PE) were used: CD1a, CD83, CD40, CD80, CD86, HLA-ABC, and HLA-DR (BD Pharmingen, San Diego, CA). Mouse IgG isotype control mAbs were purchased from eBioscience (San Diego, CA). To examine apoptosis, DCs were stained by FITC-Annexin-V and propidium iodide (PI) or assessed by a FACS-based TUNEL assay (APO-DIRECTTM Kit; BD Pharmingen). Samples were analyzed using a flow cytometer (FACSCalibur; BD Bioscience) and data were processed using the accompanying software (CellQuest; BD Bioscience).
DC-derived cytokine assays
Purified DCs were cultured in a flat-bottom 96-well micro-culture plate at a density of 4 × 104 cells/well in 0.2 mL of culture medium, in the presence of the TNF-α or LPS. Cell free supernatant was collected 24 h later and levels of IL-12p70, TNF-α, IL-10, IL-6, IL-1β, and IL-8 were measured by Cytometric Bead Array (CBA) assay with the Human Inflammation Kit (BD Bioscience) according to the protocols recommended by the manufacturer.
T cell proliferation assay
Heparinized blood samples were obtained from healthy individuals and PBMCs were isolated by Ficoll-Hypaque centrifugation. CD3+ T cells were purified with microbeads on auto-MACS columns using a pan T cell Isolation kit (Miltenyi Biotech) according to the manufacturer’s instructions. Purified CD3+ T cell samples contained >98% CD3+ T cells as determined by flow cytometry. DCs were irradiated with an X-irradiator (Gammacell 40 Exactor; MDS Nordion International, Inc., Ottawa, Ontario, CA) at 30G. Purified allogeneic CD3+ T cell seeded into a round-bottom 96-well micro-culture plate at 2.0 × 105 per well were co-cultured with purified, stimulated, irradiated DCs at T:DC ratio of 20:1, 100:1, or 500:1. All experiments were done in triplicate, and T cells alone were used as the background control. The cultures were incubated for 4 days at 37°C in 5% CO2 in air, pulsed with [3H]-thymidine (1.0 μCi/10 μL/well) during the last 8 h of incubation, and then harvested onto glass filters with an automated cell harvester. Radioactivity was assessed by liquid scintillation spectrometry (Tomtec, Orange, CT), and expressed as counts per minute.
T cell-derived cytokine assays
Cell free supernatant was collected from T cell-DC co-culture system 72 h later and levels of IFN-γ, TNF-α, IL-10, IL-6, IL-4, IL-2, and IL-17 were measured by CBA assay with the Human Th1/Th2/Th17 Kit (BD Bioscience) according to the protocols recommended by the manufacturer.
Statistical analyses and reproducibility
Experiments were repeated at least twice and usually three times. Results were expressed as mean ± SD. Statistical analyses were performed by independent t-test using the computer software PEMS 3.1 for Windows software (Package for Encyclopaedia of Medical Statistics, Chengdu, China) or PASW Statistics 18. P < 0.05 was considered significant. Significance was denoted by an asterisk in the figures.