Short-term immunosuppression facilitates induction of mixed chimerism and tolerance after bone marrow transplantation without cytoreductive conditioning

BACKGROUND: Induction of mixed chimerism and tolerance usually requires cytoreduction or transplantation of high numbers of bone marrow cells (BMC). However, such protocols have only a suboptimal success rate and, more importantly, equivalent numbers of BMC cannot be routinely obtained in the clinical setting. The authors therefore evaluated whether a short-course of immunosuppression (IS) given in addition to co-stimulation blockade would facilitate chimerism induction and allow reduction of the minimally required number of BMC without cytoreduction. METHODS: B6 mice received 200, 100, or 50 x 10 unseparated BMC from Balb/c donors plus an anti-CD40L monoclonal antibody (mAb) and CTLA4Ig (without irradiation or cytotoxic drugs). Some groups were treated additionally with IS (rapamycin, methylprednisolone, and mycophenolate mofetil for 4 weeks after bone marrow transplantation), donor-specific transfusion (DST), or anti-OX40L mAb, as indicated. RESULTS: IS led to long-term multilineage chimerism in 9 of 10 mice receiving 200 x 10 BMC (without IS, 1 of 4; P<0.05), in all mice (n=10) receiving 100 x 10 (without IS, 6 of 9; P<0.05), and notably in 9 of 10 mice treated with 50 x 10 BMC (without IS, 4 of 10; P<0.05). With transient IS, donor skin grafts were accepted longer than 170 days in 9 of 10 mice receiving 200 x 10 (without IS, 0 of 5 mice; P<0.05), all mice receiving 100 x 10 (without IS, 6 of 9; P<0.05), and 6 of 11 mice receiving 50 x 10 BMC (without IS, 4 of 10). The use of DST or anti-OX40L mAb had no beneficial effect. CONCLUSIONS: Transient IS significantly improves rates of chimerism and donor skin graft survival, and allows lasting mixed chimerism after transplantation of only 50 x 10 BMC. Thus, IS might help in the further development of noncytoreductive chimerism protocols.