Bone Marrow Niche-Derived Extracellular Matrix-Degrading Enzymes Influence the Progression of B-Cell Acute Lymphoblastic Leukemia

Interactions with the bone marrow microenvironment (BMM) influence the pathophysiology of hematological malignancies. Reciprocally, hematological malignancies remodel the BMM in order to render it more hospitable for malignant progression. Hypothesizing that different leukemias differentially remodel stroma cells in the BMM, we tested differences in the expression of various genes in sorted osteoblastic cells from mice with B-cell acute lymphoblastic (B-ALL), acute myeloid (AML) or chronic myeloid leukemia (CML) or control mice by microarray. This revealed a high expression of various proteases, especially matrix metalloproteinase-9 (MMP-9), in osteoblastic cells isolated from mice with B-ALL. MMP-9 had previously been shown to be produced by leukemia cells.

To investigate the physiological relevance of BMM-derived MMP-9 for the progression of B-ALL, we employed the well-established retroviral transduction/transplantation model of BCR-ABL1-dependent B-ALL in MMP-9-deficient or wildtype mice. Deficiency of MMP-9 in the BMM prolonged the survival of mice with B-ALL compared to wildtype mice. Secondary transplantation of bone marrow from wildtype versus MMP-9 knockout (KO) mice with B-ALL led to a significant prolongation of survival of recipients of B-ALL bone marrow from MMP-9 KO mice, suggesting that leukemia-initiating cells are reduced in a MMP-9 KO BMM. In addition, we observed reduced invasion of B-ALL cells into different organs such as spleen, meninges, lungs and liver, which likely underlies the observed prolongation of survival, as well as the reduced degradation of extracellular matrix (ECM) proteins such as laminin and fibronectin in the BMM of MMP-9 deficient mice. Mechanistically, using various in vivo and in vitro assays, as well as mice deficient for the receptor for tumor necrosis factor-α (TNF-α) we have demonstrated that B-ALL cells induce MMP-9-expression in niche cells, such as mesenchymal stem cells (MSC), via a release of high amounts of TNF-α, both in the murine and the human setting. TNF-α-dependent induction of MMP-9 in MSC was mediated via TNF-receptor-1, followed by activation of the downstream target nuclear factor kappa B (NF-kB). Consistently, transplantation of B-ALL-initiating cells, in which TNF-α had been knocked down, led to a significant prolongation of survival in B-ALL mice.

Testing whether treatment of mice with B-ALL with an inhibitor of MMP-9 may prolong survival compared to vehicle-treated mice, we indeed found decreased degradation of ECM proteins in the BMM, a decreased leukemia burden in peripheral blood and a significant prolongation of survival. In addition, we observed reduced minimal residual disease in the bone marrow of mice with B-ALL when the mice had been treated with cytarabine and the MMP-9 inhibitor compared to mice treated with cytarabine alone. Experiments on testing the effect of MMP-9 inhibitor treatment on human B-ALL cells in a xenotransplantation model are currently ongoing.

In summary, we have demonstrated the role of BMM-derived MMP-9 for B-ALL progression. B-ALL cells self-reinforce the BMM by inducing the production of MMP-9 via the release of TNF-α. Increased production of MMP-9 from the BMM promotes B-ALL progression via increased invasion of leukemic cells into different organs. Our data suggest, that inhibition of MMP-9 may be a feasible strategy as an adjunct to existing therapies.