Inhibition of spontaneous T-cell death but not of anti-CD95–induced apoptosis by IL-2 and cellular activation. Isolated peripheral blood mononuclear cells (PBMC, 2 × 10⁵cells/well) obtained from 10 HIV-1⁺ children and adolescents were cultured in the presence or absence of either anti-CD3 MoAb (OKT3; 10 μg mL⁻¹) and/or anti-CD95 MoAb (anti–APO-1; 10 μg mL⁻¹) and protein A (5 ng mL⁻¹) and were collected after 21 ± 1 hours for determination of cell death in CD4⁺ and CD8⁺ cells as described.2-4 The percentage of specific anti-CD3– and anti-CD95–induced apoptosis was calculated according to the formula: 100 × [Experimentally Induced Cell Death (%) − Spontaneous Cell Death (%)]/[100 − Spontaneous Cell Death (%)]. In (A) and (B), data are given as arithmetic mean; error bars indicate the standard error of the mean. (A) Recombinant human IL-2 (30 U/mL; Biozol, Eching, Germany), but not preincubation with the antagonistic anti–APO-1 F(ab)′ fragment (5 μg mL⁻¹),3 5 significantly reduced spontaneous T-cell death in vitro. (B) Signaling via the CD95 receptor, but not spontaneous cell death, was blocked by anti–APO-1 F(ab)′. (C) The effect of T-cell activation on anti–APO-1–induced apoptosis of freshly isolated CD4⁺ T cells was assessed in PBMC from 2 different patients stimulated with OKT3 and/or anti–APO-1 as described above. Numbers in the plots indicate the percentage of apoptotic cells that are identified by the typical change in light scatter characteristics (“shrinking death”). In patient no. 1 OKT3 induced less apoptosis than anti–APO-1, whereas in patient no. 2 more cell death was seen in OKT3-treated cells. However, in both experiments the combination of these MoAbs induced more apoptosis than each of them alone.