Greater expression of FAS is correlated with greater sensitivity to FAS-induced apoptosis and down-regulation of BCL2 expression in MST1-deficient T cells. (A) Spontaneous apoptosis (left) and death-receptor–induced apoptosis (right) in vitro was assessed on PMA/ionomycin–activated T cells from control (Ctr.) and MST1-deficient patients (patients F1P1 and F2P2). After 8 days of activation, blasts were incubated overnight in the absence or presence of a dose gradient of anti-FAS Ab (Apo1.3). The percentage of apoptotic cells (means ± SEM of 3 independent experiments) was then measured by propidium iodide labeling of DNA fragmentation. *P < .05; **P < .01. (B) FAS expression on PBMCs freshly isolated from control (Ctr.) and MST1-deficient patients (patients F1P1 and F2P2). FAS expression was monitored in a freshly isolated T-cell subpopulation. One of 6 experiments with similar results is shown. (C) Quantitative PCR analysis of BCL2 mRNA expression normalized against ACTA1 (β-actin) mRNA in total PBMCs freshly isolated from control (Ctr.) and MST1-deficient patients (patients F1P1 and F2P2). Data represent the means of 4 independent experiments. (D) Western blot showing BCL2 and MST1 protein levels in whole lymphocytes lysates from a healthy individual and an MST1-deficient patient (patient F1P1). Actin served as the loading control.