Leveraging Clonal Competition between Chip and Aggressive Malignant Clones As a Novel Therapeutic Approach

Introduction:

Hematopoietic stem cells are in constant clonal competition, as a result variant clones resistant to inflammatory pressures emerge with aging resulting in clonal hematopoiesis. Multiple independent clones can coexist, and potentially come into equilibrium. However, with an acute leukemia a clone overtakes the system, thus inhibiting normal hematopoiesis. We sought to explore the possibility of leveraging the increased competitive ability of CHIP clones to dampen expansion of an aggressive malignant clone. As proof of principle, we tested whether introduction of Tet2-/- cells as competitors could dampen the expansion of MPLW515L cells, an aggressive MPN mouse model.

Methods:

We used a transduction-transformation model of MPL W515L as an aggressive MPN model. This mouse model usually results in death approximately 3-4 weeks post-transplant. Whole bone marrow from wild-type C57B/6 (CD45.1) mice was transduced with Green Fluorescent Protein (GFP) tagged retrovirus to induce expression of hMPLW515L. We transduced bone marrow with a low viral multiplicity of infection (MOI) resulting in approximately 1-5% GFP+ cells to extend survival beyond the expected 3-4 weeks post-transplant. The transduced bone marrow was mixed with unmanipulated whole bone marrow from either WT or Tet2-/- (both in CD45.1/2) to result in a 90%/10% mixture and transplanted into lethally irradiated CD45.2 mice. Peripheral blood counts, % GFP+ cells, and chimerism was followed biweekly. Surviving mice were sacrificed and analyzed at 5 months post-transplant.

Results:

There was early death from expansion of MPLW515L GFP+ cells within 3-4 weeks in 10% of both the WT and Tet2-/- competitor groups. However, of the surviving mice the percentage of GFP+ cells in the peripheral blood was consistently lower in the Tet2-/- competitor group compared to the WT competitor group (for example mean of 55% GFP+ in WT competitor group vs 23% at day 105 post-transplant). At five months post-transplant we sacrificed the cohort to assess the composition of MPLW515L cells in the hematopoietic stem and progenitor compartments. The percentage of GFP+ cells was higher in all cell populations assessed in the WT competitor group compared to the Tet2-/- competitor group. Moreover, spleen and liver weights were higher in the WT versus Tet2-/- competitor group.

Conclusions:

This work demonstrates proof of principle that clones with increased competitive ability and low malignant potential, such as Tet2-/- cells, can be used as a buffer for the expansion of highly aggressive clones. This concept of introduction of non-malignant competitor clones could be potentially be explored as a novel therapeutic approach in aggressive hematologic malignancies.