Correlation between enhancement of graft-versus-leukemia effects following allogeneic bone marrow transplantation by rIL-2 and increased frequency of cytotoxic T-lymphocyte precursors in murine myeloid leukemia.

A model of mouse acute myeloid leukemia (mAML) was used to study the effector mechanism mediating the graft-versus-leukemia (GVL) effects in recipients of allogeneic bone marrow cells (BMC). mAML-bearing SJL/J (H-2s) mice were lethally irradiated and then transplanted with a mixture of BMC and spleen cells (SC) derived from normal syngeneic or allogeneic mice. To augment the GVL effect, recipients were injected intraperitoneally with recombinant human interleukin-2 (rIL-2) (1.2 x 10(5) IU) for 3 consecutive days, starting one day post BMC + SC transplantation. Spleen cells from treated recipients were adoptively transferred to untreated secondary SJL/J mice to test for the existence of residual tumor cells. All the secondary recipients of SC from mAML-bearing SJL/J mice rescued with syngeneic (SJL/J) or allogeneic (B10.S) BMC+SC (H-2s) differing at minor antigens of the histocompatibility complex (MiHC) developed leukemia and died. In sharp contrast, none of the secondary recipients of SC obtained from identical mAML-bearing mice rescued with B10.S BMC + SC but activated in vivo with IL-2 developed leukemia. Adoptive recipients of SC obtained from mAML-bearing recipients of major histocompatibility complex (MHC)-disparate (C57BL/6, H-2b) cells remained free of leukemia regardless of the use of rIL-2. In parallel with the in vivo findings, a 4-day in vitro exposure of splenocytes to 6 x 10(3) IU/ml rIL-2 resulted in a 5- to 20-fold increase in the frequency of alloreactive cytotoxic T-lymphocyte (CTL) precursors (CTLp) across MiHC and MHC barriers and a 2- to 6-fold increase in their cytotoxic activity. Our data suggest that augmentation of GVL effects by rIL-2 may be due to CTL activation by rIL-2, not excluding the potential beneficial role of rIL-2-activated allogeneic natural killer cells and MHC non-restricted killer cells. Cumulatively, our results suggest potentially beneficial effects of rIL-2, when used jointly with bone marrow transplantation or allogeneic cell therapy, on eradication of leukemia.