Apoptosis of memory CD4 cells with and without acquisition of CD80. Memory CD4/CD80^acq T cells were restimulated overnight either with APCs expressing low levels of CD80 (DCEK) or APCs expressing high levels of CD80 (P13.9) in the presence of various concentrations of PCC (0, 0.001, 0.1, and 1 μg/mL). Each panel depicts the percentage of apoptotic cells by TUNEL assay.