Pre-B Cell Acute Lymphoblastic Leukemia Cells Carrying a Bcr/Abl Translocation Retain Sensitivity to Minor Histocompatibility Antigen Specific T Cells but Are Resistant to Natural Killer Cells: Implications for Manipulation of Graft Versus Leukemia Effects.

Background: Some acute lymphoblastic leukemias with high risk genetic lesions such as bcr/abl are treated with allogeneic BMT. Results of allogeneic BMT generally have shown only a modest improvement over high-dose chemotherapy. The graft versus leukemia (GVL) effect associated with allogeneic BMT is generally considered to be less potent in ALL than in myeloid leukemias. One hypothesis explaining reduced sensitivity of lymphoblastic leukemia cells to GVL is that such cells may be intrinsically resistant to the effects of cytolytic T or NK cells in the posttransplant immune environment.

Methods: Primary pre-B ALL leukemias with high risk human genetic lesions were induced by incubating bone marrow from mice null at the Ink4A/Arf locus with an ecotrophic retroviral vector containing a human bcr/abl fusion oncogene as well as a GFP marker. GFP+ leukemia cells were isolated and tested for sensitivity to T cells and NK cells. Minor histocompatibility antigen (mHA) specific T cells were generated by immunizing C3H.SW mice with irradiated C57BL/6 cells. NK cells were magnetically isolated from normal spleen cells using an anti-DX5 antibody. Leukemia cell populations were measured using a flow cytometry assay. 10⁴ GFP-expressing leukemia cells were incubated with nonfluorescent T or NK cells in 96 well plates. At 48h leukemia cell population size was measured using fluorescent microspheres for quantitative internal controls. The percent survival of leukemia cells was expressed as a fraction of the absolute number of leukemia cells recovered from wells in which no cytolytic cells were present. In vivo sensitivity of the leukemia cells was measured by performing allogeneic BMT using donors that were naive or that had been previously sensitized to mHAs. 10⁴ leukemia cells were infused at the time of transplant and survival and leukemia cell burden.

Results/Conclusions: Pre-B ALL cells exhibited exquisite sensitivity to the cytotoxic effects of anti-mHA T cells. In contrast, the lymphoblastic leukemia cells were resistant to the effects of NK cells. In vivo survival was prolonged and leukemia burden was reduced in those animals transplanted with specific anti-mHA T cells. These results suggest that mHA genes expressed in lymphoblastic leukemia cells may be suitable targets for post-transplant immunotherapy.