Monocytes from HTLV-1–infected individuals have a reduced capacity for DC differentiation in vitro. Cells were incubated with or without IL-4 plus GM-CSF (50 ng/mL) for 5 days at 37°C in a 5% CO₂ atmosphere. After this period, CD14 and CD1a expression was analyzed by flow cytometry. Data in bars are expressed as the percentage of CD14⁺ cells (A) or CD1a⁺ cells (B). Open bars represent cells obtained from noninfected donors (CTR; n = 8), and shaded bars represent cells obtained from HTLV-1–infected donors (n = 12). Data are mean ± SD of 8 independent experiments. *Significantly different from control (P < .05). (Inset) Representative experiment showing CD14 and CD1a expression patterns in cells from noninfected donors (white) and HTLV-1–infected donors (gray; region M2 comprises positive cells). Cells were stained with a PanOptic kit. (C) Monocytes obtained from noninfected donors cultivated with IL-4 plus GM-CSF; (D) monocytes obtained from HTLV-1–infected donors cultivated with IL-4 plus GM-CSF. Original magnification ×20 objective, visualized under inverted phase-contrast microscopy.