In vitro–expanded Treg cells prevent CBK but not third-party skin allograft rejection mediated by naive CD4⁺CD25⁻ T cells. (A) CD4⁺CD25⁻CD45RB^hi T cells (0.5 × 10⁶) (▪, n = 7) or CD25⁺ line (1 × 10⁶) (*, n = 6) were injected alone or were coinjected (□, n = 10) in T cell–depleted CBA mice the day before the recipients received CBK skin grafts. (B-C) CD4⁺CD25⁻CD45RB^hi T cells (0.5 × 10⁶) and CD25⁺ line (1 × 10⁶) were coinjected in T cell–depleted CBA mice the day before the recipients underwent CBK (□, n = 10; B-C) or B10.A (▾, n = 6; B), BALB/c (*, n = 6; C), C57BL/6 (♦, n = 8; C), or (CBA/Ca) × (C57BL/6)F1 (▵, n = 9; C) skin allograft. (D) Wild-type CBA recipients did not receive any cells but received control PBS intravenously (▪, n = 6), or they underwent transfer with 2 × 10⁶ CD25⁺ line (▿, n = 11) the day before CBK skin graft.