Cell-free supernatants of preactivated PBMCs cause programmed cell death in ECs. NAC inhibits the IR-induced effects. HUVECs were coincubated with supernatants of either untreated PBMCs (SN-PBMC) or irradiated (SN-PBMC-IR) or LPS-treated PBMCs (SN-PBMC-LPS), respectively, in the presence or absence of IL-10, MAK 195, or NAC (3 mmol/L). Quantitative microscopic analysis of DAPI-stained cells. **P < .001 of apoptosis of SN-PBMC-IR in the presence of NAC versus SN-PBMC-IR alone. These results are representative of four independent experiments. For experimental details, see the legend to Fig 1.