Pb1823: the bone marrow microenvironment in acute myeloid leukemia: the role of mesenchymal stem cells and their impact on the clinical outcome

Topic: 3. Acute myeloid leukemia - Biology & Translational Research Background: Acute myeloid leukemia (AML) arises from the somatic acquisition of genetic alterations in hematopoietic progenitor or stem cells. Patients (pts) with favorable or intermediate risk AML, should be cured with pharmacological treatment only; however, over 50% relapse. Unfortunately, genetic factors alone cannot explain this clinical behavior, which is probably due to the persistence of chemo-resistant leukemic stem cells (LSCs) and their interaction with the bone marrow (BM) microenvironment, particularly mesenchymal stem cells (MSCs), which are the most prevalent cell type. Aims: Functionally characterize the MSCs expanded from BM of AML patients to identify mechanisms that may promote treatment resistance/relapse and to establish possible novel prognostic indicators. Methods: We collected BM samples from AML-patients at diagnosis, post-induction, post-consolidation, follow-up, pre-transplant and first relapse. MSCs were expanded from BM-mononuclear cells, obtained by gradient centrifugation, and cultured in Dulbecco’s modified Eagle’s medium, 5% platelet lysate. The medium was changed twice a week, until senescence. AML-MSC morphology, proliferative and differentiation capacity, cell surface antigen expression and reactive oxygen species (ROS) levels were compared with those of healthy donor (HD)-MSCs. Results: So far, we enrolled 29 AML pts. Our subjects were mainly women (72.4%), had a median age of 52 and a performance status of 1. Eight pts had NPM1mut (27%), with 5 of them also harboring FLT3-ITD mutation. Two pts were only FLT3mut (1 ITD and 1 TKD). Four AML-NPM1mut pts achieved complete remission and underwent hematopoietic stem cell transplantation (HSCT); one relapsed 3 months afterwards. Among AML-NPM1wt pts, 19 were treated, 2 died before treatment initiation and 1 relapsed after 9 months. Currently, 15 AML-NPM1wt (71%) and 5 AML-NPM1mut pts (71%) are alive, with a lower overall survival in the AML-NPM1wt cohort (7.63 vs 10.13 months). The results obtained were compared to those observed in MSCs expanded from 20 HDs who underwent BM donation for HSCT. From our preliminary data, acquired at diagnosis, we observed that AML-MSCs showed a lower differentiation capacity, earlier senescence (median passage 4 and 11, respectively) and higher ROS levels than HD-MSCs (median MFI 41 and 13, respectively) (Figure 1). Evaluation of subsequent timepoints to understand the influence of pharmacological treatment on MSC characteristics is currently ongoing. Summary/Conclusion: MSCs interact with LSCs, modifying their proliferation, adhesion properties, quiescence, and clonal expansion; at the same time, LSCs alter the transcriptional program of MSCs, promoting the formation of a “leukemic niche”. Our preliminary data show a decreased differentiation capacity, earlier senescence, and increased ROS production between MSC-AML and MSC-HD, but not between MSCs of AML-NPM1wt and AML-NPM1mut, which represents the most genetically homogeneous subtype. It is known that MSCs interact with LSCs through adhesion molecules and soluble factors, promoting leukemogenesis and chemoresistance and favouring LSCs proliferation and survival. More in detail, AML blasts reduce MSCs antioxidant ability, leading to immune suppression and tumor progression within the BM niche. More analyses are needed to determine whether MSCs of genetically homogeneous subtypes, such as AML-NPM1mut, are involved in treatment resistance/relapse and to identify possible novel disease biomarkers.Keywords: relapsed/refractory, Acute myeloid leukemia, Mesenchymal stem cell