Significant up-regulation of OX40 expression on CD4 is observed after stimulation with CD40L- or CD40L/OX40L-expressing B-CLL cells. Results are representative of 7 experiments. OX40 expression was efficiently up-regulated on CD4 cells after CD40L stimulation, with the highest levels reached at 24 hours after the third stimulation. (A-D) CD4 T cells stimulated with unmanipulated leukemia cells, OX40L-expressing, CD40L-expressing, and CD40L/OX40L-expressing B-CLL cells, respectively. Numbers indicate percentages of OX40-positive CD4 T cells.