Suppression and elimination of BCL₁ leukemia by allogeneic bone marrow transplantation

Mice carrying the B cell leukemia (BCL₁)⁺ were successfully treated by total lymphoid irradiation (TLI), cyclophosphamide, and allogeneic bone marrow (BM) transplantation. Long-term survivors were examined for residual BCL₁ cells and for the ability to transfer adoptively graft vs. leukemia (GVL) activity. Residual BCL₁ cells could not be detected in the allogeneic BM chimeras (> 14 to 16 months) with the use of indirect immunofluorescent staining with anti-idiotype antibody. However, residual tumor cells were present in 50% of the 'cured' chimeric mice since adoptive transfer of 10⁶ spleen cells from 50% of the treated chimeric mice caused leukemia in BALB/c recipients. In order to determine whether leukemia had been prevented in the 'cured' chimeras by a persistent cell-mediated mechanism, BALB/c mice were injected with 10⁶ spleen cells from the 'cured' BM chimeras together with a dose of 10² or 5 x 10⁵ BCL₁ cells. Onset of leukemia was delayed or completely abolished in a significant proportion of recipients receiving the cell mixtures, suggesting the presence of anti-tumor immunity in the cured mice. The data suggest that a persistent active immune mechanism may be responsible, in part, for the significant antileukemic effects observed in mice tolerant to donor alloantigens.