Apoptosis occurs asynchronously in vivo, with some cells surviving up to 4 d. BrdU was injected i.p. at the indicated number of hours before harvest. (A) The expression of CD4 and CD8 on BrdU⁺ T3.70⁺ thymocytes from representative mice from the indicated time point. (B) Expression of CD5, CD69, and PD-1 on BrdU⁺ T3.70⁺ thymocytes over time. (C) The total number of BrdU⁺ T3.70⁺ thymocytes versus time after BrdU injection. Data represent the mean ± SD from four separate time courses including three to seven individual mice. *, P = 0.0118 at 48 h, P = 0.0070 at 72 h, and P = 0.0037 at 96 h. (D) The frequency of active Caspase 3⁺ cells as a percentage of BrdU⁺ T3.70⁺ thymocytes. Data represent the mean ± SD from four separate time courses including three to seven individual mice. *, P = 0.0002 at 12 h, P < 0.0001 at 24 h, P = 0.0004 at 48 h, and P = 0.0061 at 72 h. Activation of Caspase 3 in vitro was determined by stimulating female HY^cd4 thymocytes with HYp peptide plus spleen APC for 8 h. Data were normalized for nonspecific death caused by in vitro culture and represent the mean ± SD from triplicate wells.