Down-regulation of antihost alloreactivity after bone marrow transplant permits relapse of hematological malignancy.

Relapse of leukemia remains a common event after allogeneic bone marrow transplantation, despite potential donor antihost alloreactivity present in most transplants. This work examined posttransplant relapse of the DBA/2 P815 mastocytoma in a murine model of MHC-matched, minor histocompatibility antigen (mHAg)-mismatched bone marrow transplantation (BALB/c donors into DBA/2 recipients). Antihost alloreactivity was associated with reduction of posttransplant tumor burden and prolongation of survival, but posttransplant relapse commonly occurred. No evidence of acquired resistance to immune control was found in 12 relapse reisolates. Relapse tumors remained sensitive to donor antihost CTLs in vitro, suggesting continued expression of mHAgs. Reisolates also continued to express Fas. However, loss of posttransplant alloreactivity was observed at 3 weeks. This was temporally associated with the time of relapse. Antihost alloreactivity could be reactivated in stable graft-versus-host disease-free recipients by immunization with host cells. The results of this study suggest that one mechanism for relapse after bone marrow transplant is acquired tolerance of allogeneic minor histocompatibility antigens and that posttransplant immunotherapy directed against mHAgs may induce antitumor activity.