Sensitivity of different T-cell subsets to Fas-mediated apoptosis or costimulation as a result of IL-7 treatment. IL-7–treated naive and memory T cells of noninfected subjects were activated in the presence of 1 μg/mL anti-CD3 mAb and with the indicated concentrations of anti-Fas or control IgM Abs. We followed T-cell apoptosis measured after 24 hours of activation and T-cell proliferation measured at day 4 with T cells isolated from the same donor and activated at the same conditions. Apoptosis was detected using annexin-V staining and proliferation was analyzed using CFSE staining on CD4⁺ (A) and CD8⁺ (B) naive subsets or CD4⁺ (C) and CD8⁺ (D) memory subsets. A representative experiment of 4 is shown.