CD38 expression by CLL cells increases in coculture with autologous activated T cells. T cells from 11 patients with CLL were positively selected from peripheral blood mononuclear cells (PBMCs) using CD3 magnetic beads. These were activated overnight with CD3/CD28 and then cocultured with the residual CLL PBMCs. Control experiments with unstimulated PBMCs were performed in parallel. CD38 expression by the CD19⁺ cells is shown on day 0 and day 4. (A) A significant increase in the percentage of CD19⁺CD38⁺ cells is demonstrated for CLL PBMCs cocultured with activated T cells (i) but not for CLL PBMCs cultured alone (ii). Note that even cases initially negative for CD38 in the coculture showed an increase in expression. (B) A significant increase in the mean fluorescent intensity (MFI) of CD38 expression by CD19⁺ cells is demonstrated in the same manner for cocultures (i) but not for PBMCs cultured alone (ii). The dashed lines show the results for the 4 cases with the highest initial CD38 percentage. Figures shown represent the mean plus or minus SEM values from the 11 patients in 4 independent experiments. Significance was assessed by the paired t test.