Post-HSCT monocytes mediate T-cell apoptosis. Total PBMCs or highly enriched T cells alone or highly enriched T cells cocultured with highly enriched autologous monocytes (40%), were stimulated with PHA for 3 days. T cells were identified as CD3⁺ and gated. Histogram plots depict apoptotic T cells identified as an annexin V-positive and propidium iodide-negative population. Assessment of the mean peak channel (mpc) in histogram analyses was used to detect differences in density of cell-surface binding of annexin V. Open histograms represent control cells; filled histograms, post-HSCT cells. MPC values are shown at the top right of each panel.