An in vitro human-based fracture gap model – Mimicking the crosstalk between bone and immune cells

Abstract

32The interaction between the bone and immune cells plays a crucial role in bone pathologies 33 such as disturbed fracture healing. After a trauma, the initially formed fracture hematoma in 34 the fracture gap contains all important components (immune/stem cells, mediators) to directly 35 induce bone regeneration and is therefore of great importance but most susceptible to negative 36 influences. Thus, reliable in vitro models are needed to study the underlying mechanisms and 37 to predict the efficiency of novel therapeutic approaches. Since common bioengineering 38 approaches exclude the immune component, we introduce an in vitro 3D fracture gap model 39 which combines scaffold-free bone-like constructs with a fracture hematoma model consisting 40 of human peripheral blood (immune cells) and bone marrow-derived mesenchymal stromal 41 cells. Our in vitro 3D fracture gap model provides all osteogenic cues to induce the initial bone 42 healing processes, which were further promoted by applying the osteoinductive deferoxamine 43 (DFO). Thus, we were able to distinctly mimic processes of the initial fracture phase and 44 demonstrated the importance of including the crosstalk between bone and immune cells. 45 46 47 Key words: bone; fracture; fracture hematoma; immune cells; in vitro model the tissue (Fig. 2a). Interestingly, SFBCs which showed a higher amount of negative stained 130 area, showed a more pronounced layer-like structure while the layers itself were strongly 131 mineralized at the borders ( Fig. 2a; lower row). Negative controls were cultivated without 132 osteogenic medium (Fig. 2b). In addition, we observed the expression of collagen type I (Col 133 I) and alkaline phosphatase (ALP), which are typical markers for osteogenic processes, while 134 no Col II, a typical marker of chondrogenesis, was found ( Fig. 2c). 135

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“…After fracture, a fracture hematoma is formed within the gap attracting immune cells and progenitor cells such as mesenchymal stromal cells (MSCs), subsequently initiating the healing cascade. Therefore, pro-inflammatory and anti-inflammatory processes in aJ o u r n a l P r e -p r o o f 19 hypoxic microenvironment are essential steps and most often determine the success of the subsequent bone regeneration.In order to simulate the key aspects of the initial phase of fracture healing, Pfeiffenberger et al[38] developed an in vitro 'fracture gap' model by combining a fracture hematoma model (controlled coagulation of human peripheral blood and MSCs measuring viability of implemented cells over time, relevant gene expression patterns and release of relevant cytokines) and bone-like components (expression of osteogenic genes and demonstration of calcification via various stainings and in vitro µCT). The results on the testing of different medications within the 3D in vitro model showed distinct processes of the initial phase of fracture healing and demonstrate the importance of including the crosstalk between immune cells and bone.…”