INTRATHYMIC DELETION OF ALLOREACTIVE T-CELLS IN MIXED BONE-MARROW CHIMERAS PREPARED WITH A NONMYELOABLATIVE CONDITIONING - REGIMEN

Background. Mixed hematopoietic chimerism induced with a nonmyeloablat ive conditioning regimen leads to donor-specific transplantation toler ance. Analyses of specific Vp bearing T-cell families that recognize e ndogenous superantigens demonstrated that donor-specific tolerance is due mainly to an intrathymic deletional mechanism in these mixed chime ras. However, superantigens are not known to behave as classical trans plantation antigens, We therefore used T-cell receptor (TCR) transgeni c (Tg) recipients expressing a clonotypic TCR specific for an allogene ic major histocompatibility complex antigen to further assess deletion al tolerance. Methods. 2C TCR Tg mice (H2(b)), whose Tg TCR recognizes major histocompatibility complex class I Ld, were used as recipients of Ld+ bone marrow cells after conditioning with depleting anti-CD4 an d CD8 monoclonal antibodies, 3 Gy whole-body irradiation, and 7 Gy thy mic irradiation. Chimerism and deletion of CD8(+) 2C recipient T cells was evaluated by flow cytometry and by immunohistochemical staining. Tolerance was tested with in vitro cell-mediated lympholysis assays an d in vivo by grafting with donor skin. Results. Intrathymic and periph eral deletion of 2C(+) CD8-single-positive T cells was evident in mixe d chimeras, and deletion correlated with the presence of donor-type ce lls with dendritic morphology in the thymus, and with chimerism in lym phohematopoietic tissues. Chimeras showed tolerance to the donor in ce ll-mediated lympholysis assays and specifically accepted donor skin gr afts, Conclusions, Tolerance to transplantation antigens is achieved t hrough intrathymic deletion of donor-reactive T cells in mixed chimera s prepared with a nonmyeloablative conditioning regimen and allogeneic bone marrow transplantation.