TNF-α induces expression of CD86 on CD34⁺/CD86⁻ marrow cells and commits the CD86⁺ population to the dendritic lineage. FACS-sorted CD34⁺/CD86⁻ marrow cells were cultured with TNF-α and SCF for 6 days. Day-6 CD86-bright cells were purified by FACS-sorting and cultured for an additional 6 days in GM-CSF and SCF, with or without continued exposure to TNF-α. Expression of cell surface markers was analyzed by microfluorimetry (A). Day-12 cell cultures were photographed at 20× amplification by phase contrast (B and C). In addition, cultured cells were resuspended in 15% PHS, irradiated at 3,000 cGy, and used as stimulators. Cells were plated at serial dilution with HLA-DR–mismatched CD4⁺ T cells at 5 × 10⁴ responders/well. Cultures were harvested on day 6 after 18 hours of exposure to ³H-thymidine (D).