Induction of OX40⁺4-1BB⁺ cells in CD3- and CD28-stimulated PBL T cells. In the left panel, total PBL was stimulated with anti-CD3 Ab (OKT3; 1 μg/mL) and anti-CD28 Ab (1 μg/mL) for 3 days before 3-color flow cytometry specific for OX40, 4-1BB, and CD4 or CD8. Right panels show the data for PBL cells without mitogenic stimulation. R3 quadrants in the dot plot indicate cells expressing OX40 gated for CD4⁺ or CD8⁺ T cells. Histograms indicate cells positive for 4-1BB gated in quadrant R3. The proportion of OX40⁺4-1BB⁺ subsets gated for stimulated CD4⁺ or CD8⁺ T cells is indicated in the top portion of the histograms.