Infusion of ECP-treated splenocytes increases donor Treg after allogeneic BMT. B6 and C3H.SW (CD45.2⁺Thy1.2⁺) recipient mice received 5.0 × 10⁶ T cell–depleted B6-Ly5.2 (CD45.1⁺Thy1.2⁺) bone marrow and 10⁶ B6-Thy1.1 (CD45.2⁺Thy1.1⁺) T cells followed by infusion of ECP-treated B6→C3H.SW splenocytes. (A) Gating strategy for Tregs using representative day-35 MLN data. CD4⁺Foxp3⁺ or CD4⁺CD25⁺Foxp3⁺ cells were gated on whole cell scatter followed by segregation into CD45.1⁺Thy1.1⁻ BM or CD45.1⁻Thy1.1⁺ T-cell inoculum-derived cells. (B) Total numbers of CD4⁺CD25⁺Foxp3⁺ cells after infusion of ECP-treated cells from second BMT cohort; ▩ versus ■, *P < .02. At day 11, mice had received 1 infusion of ECP-treated cells, and at day 35 they had received 4 infusions. (C,D) Numbers of donor CD4⁺CD25⁺Foxp3⁺ cells from the BM and T-cell grafts in the spleen and MLN 11 and 35 days after BMT, respectively. (C) Allo plus diluent versus allo plus ECP, ■, P < .03; (D) allo plus diluent versus allo plus ECP, ▩, P = .04. Bars represent means (± SD). Data pooled from 3 to 4 independent experiments (n = 12-20/group).