Time course of Bcl-2 downmodulation after CD4 cross-linking. PBMC, treated as indicated, were cultured for 1, 2, 3, and 5 days. Cells were analyzed by flow cytometry for intracytoplasmic Bcl-2 expression in conjunction with surface staining with PE-labeled CD3 MoAb and per-CP–labeled CD45 MoAb. Broken and solid lines indicate the notch of the Bcl-2 downmodulation and the uppermost boundary of the isotype control, respectively. Percentages indicated in the figure are percentages of Bcl-2^dim plus Bcl-2 negative cells (almost all were Bcl-2^dim).