ATRA- and TNF-induced differentiation. Differentiation of NB4 cells in response to 3-day exposure to ATRA and TNF. Results are representative of 3 experiments performed in triplicate. (A-B) Results of NBT reduction assay performed on NB4 and U937 cells treated with ATRA and TNF for 3 days. Synergism in the number of NBT-positive cells could be observed at concentrations of 10^-9 M ATRA in NB4 and 10^-8 M ATRA in U937 cells. Error bars represent SD. (C) Cytofluorometric analysis of surface marker expression. Percentages of cells expressing the monocyte-specific CD14 and the myeloid-specific CD18 cell surface markers of differentiation were determined using monoclonal FITC-labeled antibodies. Results are representative of 1 of 3 experiments performed in triplicate. Standard errors were 5% or less. —indicates no treatment. (D) Morphologic analysis of representative NB4 cells treated with TNF and ATRA for 3 days. Cells were stained with Giemsa-Wright and were viewed at × 100 magnification. U937 cells were treated with 25 nM TPA for 24 hours as a control for monocyte/macrophage morphologic features.