U937-CD80 cells cause upregulation of BCL-2 but not BCL-X_L. PBMNCs were activated in the presence of irradiated U937-M3Ps or U937-CD80s in a 4-day MLLR. BCL-2 ( ) and BCL-X_L (▪) levels were measured in the activated (CD25⁺) population by dual surface and intracellular flow cytometry as described in Materials and Methods. U937-CD80 cells stimulated a 40% greater expression of BCL-2 in CD25⁺cells measured by MFI compared with stimulation with U937-M3Ps (P = .023, paired t-test). There was no significant increase in BCL-X_L with either stimulator. These data are the mean ± SE of 4 experiments. Expression of BCL-X_L in CD25⁺ cells stimulated by either (1) anti-CD3 anti-CD28 or (2) U937-M3P or U937-CD80 cells. Cells were dual-labeled for anti-CD25 and anti–BCL-X_L after 48 or 96 hours of stimulation, respectively. The histograms show expression of BCL-X_L in the CD25⁺ population. Labeling with control antibodies gave the same level of fluorescence as that with anti–BCL-X_L after stimulation with anti-CD3 alone. Therefore, T-cell receptor ligation alone is insufficient to induce BCL-X_L expression. The addition of CD28 cross-linking induced expression of BCL-X_L, which was not observed when CD28 stimulation was through CD80 expression on U937 stimulator cells.