Minimal conditioning required in a murine model of T cell depletion, thymic irradiation and high-dose bone marrow transplantation for the induction of mixed chimerism and tolerance

In a recently developed murine model for the induction of mixed chimerism and tolerance, hosts are treated with T cell depleting monoclonal antibodies (TCD mAbs; days -5, -1 and +7), thymic irradiation (TI) (7 Gy), and a high dose of fully allogeneic bone marrow cells (BMC, 200 x 10(6)). To find the minimum amount of each treatment required for success with this approach, we treated groups with (1) a lower dose of TI (3.5 Gy), (2) fewer BMC (100 x 10(6)), (3) no TI, (4) no TI plus additional TCD mAbs on day +14, or (5) fewer injections of TCD mAbs (day -5 only). Chimerism was followed by flow cytometry (FCM), and tolerance was assessed by skin grafting. Without TI, no long-term chimerism or tolerance could be induced, even when an additional dose of TCD mAbs was administered on day +14. A reduction in the dose of either BMC or TI led to substantially reduced effectiveness, as demonstrated by lower levels of chimerism and poorer donor skin graft survival. However, the dose of TCD mAbs and hence the duration of recipient T cell depletion could be safely reduced and thus the potential toxicity of the conditioning regimen lowered.