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Abstract

Since both healthy human hematopoietic stem and progenitor cells (HSPC) and leukemia initiating cells (LIC) are sustained in a dormant state in bone marrow niche, they are protected against cytotoxic effects of chemotherapy. Thus, quantitative identification of differential adhesion of HSPC vs. LIC to bone marrow niche would help for the development of an effective clinical therapy of leukemia. The main aim of the present thesis was the fabrication and application of self-assembled, planar phospholipid membranes on solid support as in vitro model of bone marrow niche. A special focus was put on the influences of relevant ligand-receptor pairs and acute myeloid leukemia (AML) on the adhesion and morphological dynamics of HSPC. As the model of bone marrow niche, supported lipid membranes functionalized with N-cadherin and SDF1alpha were utilized to study their relative significance. In Chapter 2, the deposition of supported membranes and their quantitative functionalization with N-cadherin and SDF1alpha were confirmed by high energy specular X-ray reflectivity (XRR) and quartz crystal microbalance with dissipation monitoring (QCM-D). The fine structures perpendicular to the membrane surface and the lateral density of membrane-anchored proteins were determined by XRR with sub-Ångström resolution. Real-time monitoring by QCM-D of membrane deposition and functionalization demonstrated the quantitative variability of the average intermolecular distance <d> of proteins and elucidated their viscoelastic properties such as the shear elastic modulus and shear viscosity. In Chapter 3, the strength of HSPC adhesion was quantitatively evaluated by the determination of (a) the fraction of adherent cells, (b) the area of tight adhesion and (c) the critical force of cell detachment as a function of the average intermolecular distance <d> of N-cadherin nd SDF1alpha. The results clearly demonstrated that the binding of HSPC to the in vitro niche model was a positively cooperative process, and the adhesion mediated by the SDF1alpha/CXCR4 axis was stronger compared to adhesion mediated by the homophilic N-cadherin axis. The statistical image analysis of stochastic morphological dynamics unraveled that HSPC on in vitro niche models displaying SDF1a dissipated energy by undergoing oscillatory deformation, whereas cell locomotion mediated by the homophilic binding of N-cadherin was hardly impaired with morphological deformations. In order to verify the clinical relevance, the adhesion of leukemic blasts (LB) from AML patients was investigated in a systematic manner. In comparison to HSPC, LB exhibited a significantly higher affinity to the in vitro niche model reflecting the partial ineffectiveness of chemotherapy and the difficulties of replacing them by allogenic transplanted HSPC. The obtained results demonstrated that the combination of precisely defined cell surface models, a novel non-invasive assay for evaluating the cell adhesion strength, and statistical analysis of live cell images in Fourier space is a powerful tool to quantitatively analyze different functions of ligand-receptor pairs in bone marrow niche, which cannot be assessed by phenomenological observation.