Comparison of TUNEL and PI staining for apoptosis detection. Th1 clone 103 was cultured with immobilized anti-TCR MoAb (sample named anti-TCR), treated with gp120-anti-gp120 and seeded with immobilized anti-TCR MoAb (here simply named gp120) or cultured with 180 IU/mL IL-2 (sample named IL-2). Cells were collected at the indicated times and stained with TUNEL or PI methods. Cytofluorimetric histograms are presented. Markers indicating the position of apoptotic cells are defined on the basis of a negative control represented by cells cultured 4 hours in IL-2 (not shown). Positive control were 10,000 Rad γ-irradiated cells (not shown). Untreated cells (here indicated as t0) were also analyzed to evaluate the background level of apoptosis.