Competition between healthy and malignant haematopoiesis: from cellular mechanisms to interventions that maintain stem cells in their bone marrow niche

Abstract

Haematopoietic stem cells (HSC) reside in the bone marrow (BM) microenvironment where, despite being very rare (<0.015% of BM haematopoietic cells), they maintain the turnover of all blood cells through a balance of quiescence, self-renewal and differentiation. Disruption of HSC function and of the BM microenvironment are key characteristics of Acute Myeloid Leukaemia (AML) progression. AML develops in both children and adults and major symptoms arise due to the loss of healthy haematopoietic cells. Although several hypotheses have been proposed to explain what factors contribute to this, the mechanisms behind HSC loss remain unknown. Here, using a murine model of MLL-AF9 driven AML, I show that AML competes against healthy haematopoietic cells for space, causing enhanced egress of haematopoietic cells from the BM into the circulation (Chapter 3). Using intravital microscopy (IVM), I show that during AML development healthy cells leave the BM in clusters and I hypothesise a role for extracellular matrix regulation and vascular leakiness in causing that. Interestingly, I demonstrate a clear association between the extent of BM infiltration and the number of HSCs found in the spleen and liver. Transplantation assays show that extramedullary HSCs are functional and able to reconstitute the BM of lethally irradiated mice (Chapter 4). Thus, extramedullary haematopoiesis (EMH) could represent a way to guarantee overall blood cell production in the short-term. I subsequently establish a novel therapeutic approach involving a metalloproteinase inhibitor, prinomastat (AG3340), which aims to delay AML progression and maintain healthy cells within their niche. Under prinomastat treatment, vascular leakiness is reduced, AML grows less in the BM microenvironment, with leukemic blasts showing reduced proliferation and migration speed as well as increased apoptosis. Most importantly, prinomastat treatment significantly enhanced residual haematopoiesis, reduced the extent of EMH in both spleen and liver, and improved survival following chemotherapy treatment (Chapter 5).