Induction of tolerance by mixed chimerism with nonmyeloblative host conditioning: the importance of overcoming intrathymic alloresistance.

A nonmyeloablative conditioning regimen, consisting of depleting doses of anti-CD4 and anti-CD8 monoclonal antibodies (MoAbs) given on days -6 and -1 and 3 Gy of whole body irradiation given on day 0, allows the engraftment of fully major histocompatibility complex (MHC)-mismatched allogeneic bone marrow and the induction of tolerance for the graft. If MoAbs are given on day -5 only, permanent chimerism and tolerance are not observed in most animals. The addition of thymic irradiation to the single MoAb treatment permits tolerance induction in these mice, suggesting that residual host thymocytes reject donor marrow in recipients of 1, but not 2, MoAb injections. In this study, both CD4+ and CD8+ thymocytes were found to be responsible for residual alloreactivity in mice receiving only 1 MoAb injection. Co-receptor coating and downmodulation on residual thymocytes occur to a greater extent in recipients of 2 MoAb injections than in recipients of a single MoAb injection. This downmodulation may play a role in the loss of alloreactivity. Our results suggest that a second MoAb injection inactivates mature, functional donor-alloreactive CD4+ and CD8+ host thymocytes.