Hematopoietic chimerism following cotransplantation of donor CD4⁺CD25⁺ T cells. B6-GFP TCD BM (5 × 10⁵) was transplanted alone or together with 1 × 10⁶ CD4⁺CD25⁺ T cells into 7.0-Gy irradiated BALB/c recipients (n = 7/group). One and 2 months after BMT, peripheral blood was harvested and individual samples were analyzed by flow cytometry to determine chimerism by expression of GFP and MHC class I (A). Lymphoid and myeloid lineages were assessed in peripheral blood 2 months after BMT (B). Donor marrow-derived GFP expression was also compared between peripheral blood and bone marrow (C). CFU-GM and CFU-HPP assays were performed on cells harvested from recipient marrow (D). Rejection of the donor inoculum was assessed by the presence of donor chimerism in BM transplant recipients up to 6 months after BMT (E). □ indicates BM only; ▦, BM + CD4⁺CD25⁺. Data presented are the mean and SEM from blood or marrow (A-C) or marrow CFU chimerism (D) from results of an individual experiment. Peripheral chimerism was confirmed in this transplantation model in 2 independent experiments.