In vivo mechanical loading combined with cell therapy in a bone tissue engineering scaffold

Bone defects generate a worldwide high demand for bone repair and reconstruction. The incidence of bone defect repair in our aging society is expected to increase in the next decades. As bone auto- and allografting solutions often suffer from limited supplies, there is a clinical need for alternative bone substitutes, challenging researchers in bone tissue engineering to find novel approaches. To address this challenge, several biomaterials have been combined with different approaches, such as various biomolecules delivery, cell therapy or the application of mechanical loading. In the present PhD thesis, we assessed for the first time the effect of temporal onsets of mechanical loading combined with cell therapy in a tissue engineering scaffold implanted in a bone site. For this purpose, we used a poly(L-lactic acid)/ß-tricalcium phosphate scaffold, human bone progenitor cells and two mechanical loading conditions. The choice of the scaffold was motivated by its reinforced porous structure and enhanced bioactivity, whereas, for the choice of cells, we were driven by their low immunogenicity and high capacity for osteogenic differentiation. This capacity allowed us to evaluate two different bone cell fates originating from the same cell type. We then used the same mechanical loading parameters as in previous studies on scaffolds without cells. When loading was applied a few days post-implantation (early loading) and a few weeks post-implantation (delayed loading), moderate and advantageous effects for bone repair could respectively be observed. We focused our work on (1) the investigation of a potential immune reaction triggered by xenograft cells at an early stage of implantation in immuno-competent rats and (2) the evaluation of early and delayed mechanical loadings on bone formation when combined with cell therapy. Three scaffold conditions were used: human bone progenitor cell-seeded, osteogenic human bone progenitor cell-seeded and cell-free scaffolds. Each scaffold condition was implanted bilaterally in both femoral condyles of rats. In the first part of this thesis, early immune reaction was evaluated on implanted scaffolds based on histology. The results indicated that, unlike cell-free scaffolds, both cell-seeded types of scaffolds affected the surrounding tissue by forming fibrous tissue but without triggering particular specific immune reaction. In the second part, we followed static and dynamic bone parameters inside scaffolds based on microCT scans. We showed that, independently of mechanical loading conditions, a significantly higher amount of bone was formed within cell-free scaffolds than within the two cell-seeded ones. Depending on the cell-seeded scaffold conditions, early mechanical loading had different effects on bone formation. We concluded that the timing of mechanical loading and the cell types, as well as specific combinations thereof, are crucial for a beneficial bone repair. Overall, however, cell-free scaffolds used in this study could be considered as the best solution when combined with delayed mechanical loading, as it is simple and safe from a clinical point of view without increasing regulatory restriction due to the use of cells.