Simultaneous protection of target bone marrow and rejection of third-party cells. B6 mice were lethally irradiated; grafted with a 1:1:1 mixture of B6 (H-2^b), B6D2F1 (H-2^bd), and B6CBAF1 (H-2^bk) bone marrow (5 × 10⁶ cells each); and coinjected with 10⁵ B6 splenocytes and regulatory T cells preactivated with B6D2F1 or B6CBAF1 APCs in vitro (as indicated, regulatory T-cell-to-splenocyte ratios as indicated). Bone marrow was analyzed 2 weeks later. (A) Percentages of the 3 types of bone marrow recovered from control mice (ie, mice that had not received splenocytes). Error bars indicate SD. (B) H-2K^d and H-2K^k expression by bone marrow cells recovered from individual mice that had received bone marrow, splenocytes, and titrated numbers of preactivated regulatory T cells, as indicated. Numbers above the bars indicate the percentage of cells within the gate. (C) Summary of all data depicted as ratios of relative percentages of H-2^bk to H-2^bd bone marrow cells. □ indicates mice injected with regulatory T cells preactivated with H-2^bd APCs; ▪, mice injected with regulatory T cells activated with H-2^bk APCs. ^**P < .01 (Wilcoxon test). Combined data from 3 independent experiments are shown. Horizontal bars indicate the mean.