Myelosuppression-free tolerance induction with ABT-737 and CsA. B6 recipients (H-2^b) were treated with an irradiation-free conditioning protocol including ABT-737 (50 mg/kg/day), CsA (10 mg/kg/day) from day −3 to day 12, costimulation blockade (CSB) with MR1 (2 mg on day −1), and 25 × 10⁶ fully MHC-mismatched BM cells from CBA donors (H-2^k). Four to 6 weeks after BMT, skin transplantation from CBA and BALB/c (3rd party, H-2^d) donors was performed. (A-B) A significant myeloid (CD11b+) and T-cell chimerism, but no chimerism in the B-cell compartment, was measured in peripheral blood (FACS at week 10 after BMT shown in panel A) and in the spleen (FACS at 33 weeks after BMT shown in panel B). N = 7 per group. Representative results of 5 independent experiments are shown. (C) Donor skin grafts were indefinitely accepted by 29 out of 33 mice in 5 independent experiments, whereas 3rd-party grafts (BALB/c) were promptly rejected. Combined data of 5 independent experiments are shown. N = 33. The same conditioning protocol failed to induce tolerance, when MR1 was replaced by CTLA4Ig (0.5 mg on day 2).