Activated alloantigen-induced CD25⁺CD4⁺ cells can regulate third-party skin allograft rejection. (A) Pretreatment and adoptive transfer protocol. CBA-Rag^-/- mice were reconstituted with 10⁵ CD45RB^highCD4⁺ cells with or without 2 × 10⁵ CD25⁺CD4⁺ cells from CBA mice pretreated with 200 μg YTS177 and 250 μL BALB blood followed by a further dose of BALB blood the day prior to cell isolation. The reconstituted mice then received a B10 skin graft the following day. (B) Effect of CD25⁺CD4⁺ cells on CD45RB^highCD4⁺-mediated rejection of B10 skin grafts. □ CD45RB^highCD4⁺ cells only (MST, 17 days, n = 4); ▪ CD45RB^highCD4⁺ plus CD25⁺CD4⁺ cells (MST, > 100 days, n = 4; P < .05). (C) Representative skin graft 100 days after transplantation onto mouse reconstituted with both CD45RB^high CD4⁺ and CD25⁺CD4⁺ cells. (D) Histology of graft in panel C (H&E).