Mechanism by which additional monoclonal antibody (mAB) injections overcome the requirement for thymic irradiation to achieve mixed chimerism in mice receiving bone marrow transplantation after conditioning with anti-T cell mABs and 3-Gy whole body irradiation.

A relatively nontoxic method of conditioning mice has been developed recently that allows allogeneic bone marrow engraftment and specific skin allograft tolerance induction. This regiment 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. We have recently shown that the potential toxicity of this regimen can be further reduced by replacing TI with additional anti-T cell mAb injections before and after bone marrow transplantation. Mixed chimerism and prolonged donor-specific skin graft acceptance are induced in 90% of B10 mice conditioned with anti-CD4 and anti-CD8 mAbs on days -6 and -1 and 3-Gy WBI on day 0 without TI, but only in a small fraction of mice receiving a similar regimen, except that mAbs are given on day -5 only. To determine the mechanism of tolerance induction in the former group, we compared the two groups for the extent of thymocyte depletion, for the timing of development of intrathymic and extrathymic chimerism, and for clonal deletion of host-type thymocytes with TCR recognizing superantigens presented by donor class II molecules. The results suggest that administration of a second mAb injection depletes or inactivates residual host thymocytes that are capable of causing intrathymic rejection of donor hematopoietic cells even when peripheral engraftment is achieved. The presence of donor class II+ hematopoietic cells in the thymus on day 14 correlated with marked deletion of mature host-type V beta 11+ thymocytes that recognize donor I-E plus endogenous superantigen. This suggests that tolerance is achieved primarily through a central deletional mechanism when peripheral and intrathymic host T cells are adequately inactivated or depleted by mAbs and 3-Gy WBI. In addition, the higher incidence of early failure of peripheral chimerism in mice conditioned with a single injection rather than than two mAb injections prior to bone marrow transplantation suggests that nontolerant residual host thymocytes can also induce early peripheral rejection after mAbs have cleared from the circulation. This early rejection is prevented by the longer persistence of anti-T cell mAbs observed in mice receiving two pretransplant mAb injections. Thus, administration of sufficient depleting anti-T cells mAbs followed by 3-Gy WBI allows the induction of central deletional tolerance while minimizing the toxicity of the conditioning regimen.