Alginate Hydrogels for In Vivo Bone Regeneration: The Immune Competence of the Animal Model Matters

Biomaterials with tunable biophysical properties hold great potential for tissue engineering. The adaptive immune system plays an important role in bone regeneration. Our goal is to investigate the regeneration potential of cell-laden alginate hydrogels depending on the immune status of the animal model. Specifically, the regeneration potential of rat mesenchymal stromal cell (MSC)-laden, void-forming alginate hydrogels, with a stiffness optimized for osteogenic differentiation, is studied in 5-mm critical-sized femoral defects, in both T cell-deficient athymic Rowett Nude (RNU) rats and immunocompetent Sprague Dawley rats. Bone volume fraction, bone mineral density, and tissue mineral density are higher for athymic RNU nude rats 6 weeks postsurgery. In addition, these animals show a significantly higher number of total cells and cells with non-lymphocyte morphology at the defect site, while the number of cells with lymphocyte-like morphology is lower. Hydrogel degradation is slower and the remaining alginate fragments are surrounded by a thicker fibrous capsule. Ossification islands originating from alginate residues suggest that encapsulated MSCs differentiate into the osteogenic lineage and initiate the mineralization process. However, this effect is insufficient to fully bridge the bone defect in both animal models. Alginate hydrogels can be used to deliver MSCs and thereby recruit endogenous cells through paracrine signaling, but additional osteogenic stimuli are needed to regenerate critical-sized segmental femoral defects. T cell-deficient athymic RNU nude rats are commonly used to evaluate the regeneration potential of biomaterials in combination with cells of human origin. In this study, we show that the effect of mesenchymal stromal cell (MSC)-laden alginate hydrogels on bone regeneration differs depending of the immune status of the animal model. Furthermore, while alginate hydrogels are interesting materials to investigate the effect tunable biophysical properties on cell response and in vivo regeneration, their use in combination with rat MSCs is insufficient to fully bridge critical-sized segmental femoral defects. For this purpose, additional osteogenic stimuli such as growth factor delivery are necessary.