Flow cytometric analysis of Fas (CD95) expression on fresh human EBV⁺ B-cell tumors from an hu-PBL-SCID mouse (A) and from a patient with AIDS-PCNSL (B). Tumor samples were stained with anti-Fas-FITC MoAb and either with CD19-PE or CD20-PE MoAbs, as indicated. The brain biopsy cells visualized in (B) are ungated and show that cells nonreactive for the pan-B-cell marker CD20 are also nonreactive for Fas.

(C through F) Immunoperoxidase staining for Fas expression in a representative frozen section of a human EBV⁺ B-cell tumor taken from a hu-PBL-SCID mouse (D), and a representative frozen section of a brain biopsy from a patient with AIDS-PCNSL (F). Sections stained with a nonreactive isotype control MoAb are shown in (C) and (E).