Alloantigen-induced CD25⁺CD4⁺ cells can regulate skin allograft rejection. (A) Pretreatment and adoptive transfer protocol. CBA mice were pretreated with 200 μg YTS177 on days -28 and -27 plus 250 μL allogeneic (B10 or BALB) blood on day -27. On day 0, 2 × 10⁵ CD25⁺CD4⁺ spleen cells from these donors were adoptively transferred into CBA-Rag^-/- recipients together with 10⁵ CD45RB^highCD4⁺ cells from naive CBA mice. One day later these reconstituted mice received transplants of B10 or BALB skin grafts. (B) Effect of CD25⁺CD4⁺ cells on CD45RB^highCD4⁺-mediated skin allograft rejection. Reconstitution with CD45RB^highCD4⁺ cells only (⋄), B10 graft (group i: median survival time [MST], 20 days, n = 4); CD45RB^highCD4⁺ cells only (□), BALB graft (group ii: MST, 12 days, n = 4); CD45RB^highCD4⁺ cells plus CD25⁺CD4⁺ cells from YTS177/B10 blood pretreated mice (♦), B10 graft (group iii: MST, > 100 days, n = 4; P < .05 versus group i); CD45RB^highCD4⁺ cells plus CD25⁺CD4⁺ cells from YTS177/BALB blood pretreated mice (▪), BALB graft (group iv: MST, > 100 days, n = 4; P < .05 versus group ii); CD45RB^highCD4⁺ cells plus CD25⁺CD4⁺ cells from YTS177/BALB blood pretreated mice (•), B10 graft (group v: MST, 25 days, n = 4; P < .05 versus groups iii and iv). (C) Representative B10 skin graft 100 days after transplantation from group iii, ♦. (D) Histology of graft in panel C (formalin-fixed paraffin section stained with hematoxylin and eosin [H&E]).