Chronic lymphocytic leukaemia (CLL) is an indolent B cell malignancy infiltrating the lymphatic system and invariably the bone marrow (BM). Although treatment options for patients with advanced disease have significantly increased in the past years and improved life-expectancies, the disease remains incurable and after emergence of therapy resistant disease patients succumb to infections due to secondary BM failure. Survival of CLL cells depends on protein kinase C- (PKC) expressed in activated BM mesenchymal stromal cells (BM-MSCs) which display activation of inflammatory pathways, as well as BM endothelial cells (BMEC) (Lutzny et al., 2013). This activation provides a nurturing environment, which not only contributes to disease progression, but likely also disrupts normal haematopoiesis, leading to insufficient blood production. I hypothesised that disease-induced alterations in the tumour microenvironment of CLL cells contribute to BM failure by skewing hematopoietic stem and progenitor cell (HSPC) fate choice commitments and tissue residency. In my study, the analysis of the peripheral blood (PB) of 67 patients with predominantly early stage CLL revealed that short term (ST) HSCs and Common Myelo-Erythroid Progenitors (CMP/MEPs) are found in higher frequencies and are associated with a skewed differentiation into myeloid cells in vitro and progenitor cells are less likely to produce erythrocytes. These findings provide evidence for the mechanisms underlying BM failure in patients with CLL. Our current knowledge of how malignant B cells interact with stromal cells within the BM microenvironment is limited. I hypothesised that BMEC play a fundamental role in nurturing malignant B cells during disease progression. In a murine model for CLL, I identified that malignant B cells home preferentially to the BM in close proximity to EC, and during disease progression, sinusoidal, but not arterial vessels, upregulate the stem cell marker, SCA1. RNAseq analyses of the remodelled sinusoids revealed activation of inflammatory and cytokine regulation pathways. In lymphoid malignancies, minimal residual disease (MRD) in the BM has prognostic value to predict disease relapse after patients received chemotherapy, suggesting the BM microenvironment is not only important for disease progression, but also plays an essential role for chemoresistance. Our lab recently demonstrated that BM-MSCs contribute to drug resistance via PKC mediated activation of lysosomes, which is required for tumour-stroma cell interactions (Park et al., 2020). However, it still is unclear which type of stromal cells are involved in this environment mediated drug resistance (EMDR) mechanism. I hypothesised that BMEC are the main drivers of EMDR in CLL and dependent on the activation of PKC. My results show indeed that BMECs activate lysosome production as a drug resistance mechanism and these effects are mitigated by pharmacological inhibition of PKC. Malignant B cells are found in close proximity to EC after treatment, suggesting the endothelial niche provides a sheltering from the action of cytotoxic drugs. In vitro experiments demonstrated that pharmacological inhibition of PKC sensitises CLL to the action of cytotoxic therapies. Overall the two projects carried out during my PhD have provided novel insights; first revealing that the disease-induced inflammatory environment could affect normal blood production in patients, and second that BMECs become remodelled. This reprogramming of BMECs is likely to play a fundamental role in the process of EMDR. Results from my work are important to improve current therapies for patients with CLL as well as the prevention of BM failure.