Bim is important for TCR-induced apoptosis of CD8+ T cells. (A) CD4+ or CD8+ T cells purified from activated human PBL were transfected with NS or Bim-specific siRNA, rested 4 days, then restimulated with increasing doses of OKT3 in the presence or absence of SM1/23. Percent cell loss was calculated in triplicate by PI exclusion. Differences in apoptosis sensitivity were statistically significant for SM1/23 treated CD4+ cells (NS and Bim) compared to NS cells alone (p < 0.007), except for SM1/23 treated NS cells at 1 μg/ml OKT3 (p < 0.07). Differences in apoptosis sensitivity for CD8+ T cells (relative to NS alone) were all statistically significant (p < 0.05). (B) Lysates from cells transfected in (A) were immunoblotted for BIM. β-actin serves as a loading control.