Effect of donor IL-21 expression on the in vivo conversion of CD4⁺25⁻ T cells into FoxP3⁺ cells. Weights (A,C) and survival (B,D) of lethally irradiated recipients injected with 10⁷ T cell–depleted bone marrow and 0.5 × 10⁶ flow-sorted CD45.1⁻CD4⁺CD25⁻ T cells isolated from scurfy chimeric (A-B) or wt (C-D) donors. Subgroups were treated with irrelevant IgG (▾) or aIL-21 (▿). P values between treated and untreated groups for survival were .90 (B) and .12 (D). Data from 1 experiment with 8 mice per group.