Tregs are required for reversal of GVHD by ECP. (A-C) C3H.SW recipient mice received syngeneic (C3H.SW; □, n = 3) or allogeneic (B6-Ly5.2) BM transplant and 4 weekly infusions of ECP-treated B6→C3H.SW splenocytes (gray circle, n = 12) or diluent (♦, n = 8), as described in “Bone marrow transplantation” and “ECP treatment.” All BM transplant recipients were injected intraperitoneally with 0.5 mg CD25-depleting mAb (PC61) on day 1 and 0.1 mg on days 8, 15, 22, and 29 after BMT. (A) Number of CD4⁺Foxp3⁺ Tregs 56 days after BMT (n = 3-4 per group). Bars represent means (± SD). (B) Survival. (C) Clinical GVHD scores.