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Figure S4. TGF-β–induced T reg cells can induce FoxP3 expression in naive T cells. Purified CD4⁺ T cells from 5CC7 TCR transgenic RAG^−/− were cultured in 24-well plates coated with 2 µg/well of anti-CD3 and anti-CD28 in the presence of 5 ng/ml TGF-β1 and 100 U IL-2. FoxP3 expression was analyzed after 5 d (left panel). The TGF-β–induced FoxP3⁺ T cells were then co-cultured with an equal number of CFSE-labeled CD4⁺ T cells from 5CC7 TCR transgenic RAG^−/− mice for an additional 4 d. FoxP3 expression was determined on CFSE⁺ T cells as in Fig. 2 (right panel). One representative experiment out of four is shown.