Modeling Chronic Myeloid Leukemia in Immunodeficient Mice Reveals an Inflammatory State with Expansion of Aberrant Mast Cells and Accumulation of Pre B Cells

Introduction: Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm that if not treated, will progress into blast crisis (BC) of either myeloid or B lymphoid phenotype. The BCR-ABL1 fusion gene, encoding a constitutively active tyrosine kinase, is thought to be sufficient to cause chronic phase (CP) CML, whereas additional genetic lesions are needed for progression into CML BC. To ultimately achieve a cure for CML, there is a need to establish improved disease models allowing increased understanding of disease pathogenesis and to evaluate novel therapeutics.

Aim: In this study, we investigated if retroviral expression of BCR-ABL1 in cord blood (CB) CD34⁺ progenitor cells transplanted into NOD/SCID IL2–receptor gamma deficient (NSG) mice would recapitulate features observed in CP CML.

Methods: CD34⁺ progenitor cells from human CB were transduced with a retroviral vector expressing BCR-ABL1 and subsequently transplanted into NSG mice. Mice were sacrificed at signs of illness and bone marrow (BM) and spleen cells were analyzed using flow cytometry, immunohistochemistry, FISH and RNA sequencing. Secondary transplants were also performed. BM mononuclear cells (MNC) from CP CML patients were analyzed using flow cytometry and FISH.

Results: For the majority of the BCR-ABL1 transplanted (BA) mice, signs of disease were evident within 100 days post transplantation. Spleens were enlarged and both control and BA mice showed robust human engraftment in both BM and spleen. Expression of BCR-ABL1 also induced expansion of the transduced cells. In addition to the previously described features of this humanized transplantation model such as a mild expansion of human myeloid cells, BCR-ABL1 expression induced an inflammatory-like state in the BM and spleen of the BA mice as evident by an increase of human macrophages/histiocytes and T cells. In addition, an expansion of aberrant mast cells in BA mice was observed. Secondary transplantations failed to induce a similar disorder as in the primary mice, but long-term engraftment was achieved, showing only mast cells in secondary BA mice. These mast cells aberrantly expressed CD25 (IL2RA), a cell surface receptor normally only expressed on neoplastic mast cells. This was in contrast to secondary control mice where the long-term engraftment was dominated by B cells.

Because CP CML can progress into lymphoid BC, we next explored if BCR-ABL1 expression in CB CD34⁺ cells would show a disturbance also of the B cell lineage. We observed a striking block at the pre B cell stage, resulting in an accumulation of pre B cells in the BM of BA mice. RNA sequencing of sorted BCR-ABL1-expressing pre B cells from BA and control mice revealed that BCR-ABL1 expression results in a significant de-regulation of about 700 genes. Among the up-regulated genes, many were shown to be involved in inflammation and apoptosis. Finally, by sorting BM MNCs from patients with CP CML into different populations of B cell progenitors and performing FISH for BCR-ABL1, we confirmed that a similar block in B cell differentiation is present in CP CML patients.

Conclusion: This humanized mouse model of CML reveals previously unexplored features of CP CML, including expansion of aberrant mast cells and a block at the pre B cell stage, and should provide a valuable model for future studies of mechanisms involved in the disease pathogenesis of CML.