Heterogeneous Fas expression and loss of FasL sensitivity in SzS patient CD4⁺ cells. (A) CD4⁺ cells from HD (n = 10) and patients with SzS (n = 16) were subjected to FasL-induced apoptosis. Six to 8 hours after FasL exposure (100 ng/mL), cell viability was assessed by flow cytometry according to annexin-V and PI staining (top panel). CD4⁺ cells from healthy donors (HD, n = 10, ) and patients with SzS (n = 16, ■) were stained with a fluorescein isothiocyanate (FITC)–anti-Fas (CD95) antibody or a FITC-isotype-matched control and analyzed by flow cytometry. ΔMFI expresses the ratio between the MFI of cells stained with the anti-Fas and the MFI obtained with cells stained with the isotype control antibody (bottom panel). Right panels show representative Fas expression patterns (CD95 antibody, bold histogram; control isotype, thin histogram) in selected individuals. (B) Correlation between FasL-induced apoptosis and tumor burden in patients with SzS. (C) Sensitivity to FasL and Fas expression (100 ng/mL) in the CTCL cell lines HUT78 (SzS), MyLa (MF), and SeAx (SzS).