Engagement of Mesenchymal Stromal Cells in the Remodeling of the Bone Marrow Microenvironment in Hematological Cancers.

Mesenchymal stromal cells (MSCs) are a subset of heterogeneous, non-hematopoietic fibroblast-like cells which play important roles in tissue repair, inflammation, and immune modulation. MSCs residing in the bone marrow microenvironment (BMME) functionally interact with hematopoietic stem progenitor cells regulating hematopoiesis. However, MSCs have also emerged in recent years as key regulators of the tumor microenvironment. Indeed, they are now considered active players in the pathophysiology of hematologic malignancies rather than passive bystanders in the hematopoietic microenvironment. Once a malignant event occurs, the BMME acquires cellular, molecular, and epigenetic abnormalities affecting tumor growth and progression. In this context, MSC behavior is affected by signals coming from cancer cells. Furthermore, it has been shown that stromal cells themselves play a major role in several hematological malignancies' pathogenesis. This bidirectional crosstalk creates a functional tumor niche unit wherein tumor cells acquire a selective advantage over their normal counterparts and are protected from drug treatment. It is therefore of critical importance to unveil the underlying mechanisms which activate a protumor phenotype of MSCs for defining the unmasked vulnerabilities of hematological cancer cells which could be pharmacologically exploited to disrupt tumor/MSC coupling. The present review focuses on the current knowledge about MSC dysfunction mechanisms in the BMME of hematological cancers, sustaining tumor growth, immune escape, and cancer progression.