Additional monoclonal antibody (mAB) injections can replace thymic irradiation to allow induction of mixed chimerism and tolerance in mice receiving bone marrow transplantation after conditioning with anti-T cell mABs and 3-Gy whole body irradiation.

While allogeneic bone marrow transplantation (BMT) has long been known to be capable of inducing donor-specific tolerance and hence permitting allograft acceptance without immunosuppressive pharmacotherapy, the toxicity of conditioning regimens required to achieve marrow engraftment has precluded the application of this approach to clinical organ transplantation. A relatively nontoxic method of conditioning mice that allows allogeneic bone marrow engraftment and induction of donor-specific skin allograft tolerance has recently been described. This regimen included anti-CD4 and anti-CD8 mAbs administered on day -5, followed by 3-Gy whole body irradiation (WBI) and 7-Gy thymic irradiation (TI) on day 0. To further reduce the potential toxicity of this regimen, we have now attempted to overcome the requirement for TI by administering additional mAb injections before and after BMT. Mixed chimerism and prolonged donor-specific skin graft acceptance were induced in 90% of B10 mice conditioned with anti-CD4 and -CD8 mAbs on days -6 and -1 and 3-Gy WBI on day 0 without TI. Despite long-term acceptance of donor-specific skin grafts, however, some of these animals showed a gradual decline in donor-type hematopoietic repopulation, and 2 of 10 mice regrafted with a second donor-type skin graft 5-9 months after BMT rejected the second and/or the original graft. This rejection after repeat donor-specific skin grafting correlated with a decline in the percentage of donor-type T cells between 6 and 12 weeks after BMT. In contrast, all animals receiving additional mAb injections 7 and 14 days following BMT after conditioning with mAbs on days -6 and -1 and 3-Gy WBI showed stable chimerism and accepted both primary and secondary donor-specific skin grafts. Animals receiving TI in addition to mAb and 3-Gy WBI also showed stable chimerism and long-term acceptance of initial (at 7 weeks) and later repeat donor-specific grafts. In contrast, the majority of mice receiving mAbs only on day -5 or on day -1 only, followed by 3-Gy WBI on day 0 without TI, did not accept initial donor-specific skin grafts, and showed only transient chimerism. Thus, the requirement for thymic irradiation to allow permanent mixed chimerism and donor-specific tolerance induction can be overcome by the administration of additional T cell-depleting mAb injections. These results establish a less toxic method of inducing donor-specific tolerance, thus increasing the potential clinical applicability of this approach to inducing organ allograft acceptance without chronic immunosuppressive therapy.