CD28 stimulation reduces death and promotes expansion of T cells. PBMNCs were activated in the presence of U937-M3Ps or U937-CD80s in a 4-day MLLR. Activated T cells were sorted and cultured in exogenous IL-2 (30 U/mL). Activated T cells from the U937-M3P MLLR were also cultured in conditioned medium from the U937-CD80 MLLR. In this experiment, T cells activated in the presence of U937-CD80 cells (▪) demonstrated 38% viability by day 7 (A), but viable cell numbers were still 80% of the original count (B), suggesting that there was an expansion of a specific population. T cells cultured in the presence of U937-M3P cells (•) demonstrated zero viability by day 7. The addition of conditioned medium from the MLLR that used U937-CD80 cells as stimulators to these cells improved their viability (○). These data are the mean of 3 independent experiments ± SE.