Activated CD25⁺CD4⁺ cells generated against the unrelated antigen HGG can regulate skin allograft rejection. (A) CBA mice were pretreated with the YTS177/HGG + HGG reactivation protocol or with relevant control protocols. At day 0, 2 × 10⁵ CD25⁺CD4⁺ T cells from these donors were cotransferred with 10⁵ naive CD45RB^highCD4⁺ cells into CBA-Rag^-/- recipients. These reconstituted mice then received transplants of B10 skin allografts 1 day later. (B) Skin allograft survival. □ CD45RB^highCD4⁺ cells only (group i: MST, 11.5 days, n = 14); ▪ CD45RB^highCD4⁺ cells + CD25⁺CD4⁺ cells from mice pretreated with 200 μg YTS177 on days -28 and -27 and with 500 μg HGG on days -27 and -1 (group ii: MST, > 100 days, n = 5; P < .05 versus group i); ○ CD45RB^highCD4⁺ cells + CD25⁺CD4⁺ cells from mice pretreated with 500 μg HGG only on days -27 and -1 (group iii: MST, 20 days, n = 5; P = .21 versus group i); ⋄ CD45RB^highCD4⁺ cells + CD25⁺CD4⁺ cells from mice pretreated with 200 μg YTS177 only on days -28 and -27 and 500 μg HGG on day -27 (group iv: MST, 21 days, n = 5; P = .21 versus group i). (C) Representative skin graft from group ii 100 days after transplantation. (D) Histology of graft in panel C (H&E).