B7-H1 is expressed by malignant T cells. (A) PBMCs from patients with Sezary syndrome were analyzed by flow cytometry. Malignant T cells were identified by the expression of both CD4 and the clonotypic TCR Vβ chain, as shown in the representative dot plot. Samples were stained with an isotype control (closed histogram) or anti–B7-H1 (open histogram) and B7-H1 expression on malignant CD4⁺Vβ⁺ cells determined. For cases in which TCR Vβ use was unknown, malignant CD4⁺ T cells were identified by the aberrant down-regulation of CD7. A representative example (n = 11) is shown. (B) Immunohistochemical staining for B7-H1 was performed in paraffin-embedded biopsy specimens obtained from CTCL (n = 11) and PTCL (n = 144) patients and tumor-associated B7-H1 expression analyzed. Tumor cells were scored as B7-H1⁺ when B7-H1 and T cell–specific antibodies (eg, CD3, CD4, and CD8) both stained an identical population of cells, as determined by an expert hematopathologist. Two representative examples are shown (original magnification ×200).