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 106 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 200X10(6) BMC (without IS, 1 of 4; P < 0.05), in all mice (n= 10) receiving 100 X 106 (without IS, 6 of 9; P < 0.05), and notably in 9 of 10 mice treated with 50X 10(6) 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 200X 106 (without IS, 0 of 5 mice; P < 0.05), all mice receiving 100X 106 (without IS, 6 of 9; P < 0.05), and 6 of 11 mice receiving 50X 106 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 106 BMC. Thus, IS might help in the further development of noncytoreductive chimerism protocols.