DC-associated B7-H1 promotes the induction of FoxP3⁺regulatory T cells. (A-B) Purified CD4⁺ T cells were depleted of CD25^hi natural Tregs and cultured alone or with normal donor monocyte-derived iDCs in triplicate for 6 days. TGF-β (25 ng/mL) was included in the cultures shown in panel A or as indicated. Either an isotype control or blocking anti–B7-H1 (4 μg/mL) antibody was included. The frequency of CD25^hiFoxP3⁺ cells was determined by flow cytometry. Representative dot plots are shown in panel A. Data shown are representative of at least 3 similarly performed experiments. (C-D) Immunohistochemical staining for both B7-H1 and FoxP3 was performed on paraffin-embedded PTCL biopsy specimens (n = 48; in triplicate), as described in “Patient samples, immunohistochemistry, and immunofluorescence assay.” B7-H1 and FoxP3 staining from multiple areas of the same biopsy specimen are shown in panel C. (Inset) Original magnification ×200. (D) FoxP3⁺ cells in each high-power field (± 95% confidence interval is shown) were counted and DC staining for B7-H1 determined. For the purpose of our analysis, core biopsy specimens were considered B7-H1⁺ if any portion of the specimen contained B7-H1⁺ DCs.