T cells are the critical determinant of single-donor chimerism in transplantation of mixed multiple donor cells. (A) Schematic representation of the experiment. In this simplified model, two donor strains, BDF1 and B6 (Ly5.1), are used. B6 mice are WT, Rag2-KO, or T cell receptor-α chain KO. KSL cells from BDF1 are cotransplanted into recipient B6 (Ly5.2) mice with unmanipulated WBM cells or with KSL cells from B6 strains. (B) Results of cotransplantation of BDF1 and Rag2-KO donor grafts. Percentages of peripheral donor chimerism in granulocytes in individual recipients transplanted with each indicated graft at 4 wk after transplantation. n = 5 in each group. Mean values are indicated as bars. (C) Results of cotransplantation of BDF1 and TCR-α KO donor grafts. Percentages of peripheral donor chimerism in granulocytes are shown. n = 3 or 5 in each group. Mean values are indicated as bars. (D) Effects on chimerism formation of lymphocytes co-infused with a mixture of Rag2-KO WBM cells and BDF1 KSL cells. Percentages of peripheral donor chimerism in granulocytes at 4 wk in individual recipients. n = 4 or 5 in each group. Mean values are indicated as bars. (E) T cell dose titration experiment to convert mixed chimerism into single-donor chimerism. Peripheral blood chimerism was analyzed over time. n = 3–5 in each group. Mean values are indicated as bars. Representative data are shown from at least two independent experiments for each panel.