Specific destruction of host-reactive mature T cells of donor origin prevents graft-versus-host disease in cyclophosphamide-induced tolerant mice

In cyclophosphamide (CP)-induced tolerance, a long lasting skin allograft tolerance was established in many H-2-identical strain combinations without graft vs host disease. Destruction of donor-reactive T cells of host origin, followed by intrathymic clonal deletion of these cells, has been revealed to be the chief mechanisms of this system. Here, we studied the fate of host-reactive populations in donor-derived T cells of C3H/He (C3H) (H-2(k), Mls- 1^b, Mls-2^a) mice rendered CP-induced tolerant to AKR/J (AKR) (H-2(k), Mls-1^a, Mls-2^b), by assessing AKR-derived Thy-1.1⁺ T cells bearing TCR Vβ3 that are specifically reactive with Mls-2^a-encoded Ag of the recipient C3H mice. In the AKR-derived Thy-1.1⁺ lymph node cells of the C3H mice that had been treated with AKR spleen cells plus CP, CD4⁺-Vβ3⁺ T cells were obviously decreased by day 10 after the CP treatment. At this stage, the Thy-1.1⁺ T cells were not detected in the C3H thymus, suggesting that the obvious decrease of CD4⁺-Vβ3⁺ T cells of AKR origin was not due to intrathymic clonal deletion in the recipient C3H mice. Therefore, the destruction of the host-reactive mature T cells of donor origin, as well as that of the donor-reactive mature T cells of host origin, occurred by the CP treatment at the induction phase. Furthermore, after the establishment of intrathymic mixed chimerism in the recipient C3H mice, Vβ3⁺ T cells were not detected among the Thy-1.1⁺ T cells of AKR origin in the mixed chimeric thymus, suggesting that the host-reactive immature T cells repopulated from the injected donor hematopoietic cells were clonally deleted in the recipient thymus. These two mechanisms appear to prevent graft vs host disease in CP-induced tolerance.