In vitro Fas/Fas-L interactions are not involved in accelerated spontaneous death of post-BMT T cells. (A) PBMC from 6 patients (on day 90 after BMT), were incubated for 18 hours in the presence of 10 μg/mL of the blocking anti-Fas MoAb M3 or of the negative control MoAb M33. Cell death was assessed according to trypan blue uptake and morphological criteria. Histograms are means ± SD for individual determinations. (B) Cytotoxic anti-Fas MoAb CH-11 was tested in the same conditions using PBMC isolated on day 90 after BMT (3 patients). (C) Effect of MoAb M3 on AICD of in vitro preactivated normal CD4⁺ T lymphocytes exposed for 8 to 14 hours to 10 μg/mL of plate-bound, anti-CD3 MoAb (OKT3). Percent cell loss is the decrease in the absolute number of viable cells compared with that of unstimulated control cells (means ± SD of 3 separate experiments). (C) Flow cytometric analyses of anti–Fas-L (solid lines) and control Lo-DNP-57 (dashed lines) MoAb reactivity with CD4⁺, CD8⁺, and CD14⁺ cells from donor and post-BMT T cells (from UPN 332) and with normal PHA-IL-2 blasts.