Pre-transplant osteogenic induction promotes primitive mesenchymal stem cells’ functional secretion and healing efficacy

Mesenchymal stem cells (MSCs) and MSC-derived tissue engineered bone grafts (TEBGs) are increasingly being used in the treatment of fractures. However, the influence of tissue culture in vitro on their potency, in particular, the mechanistic understanding of how osteogenic priming enhances reparative efficacy of MSC-based TEBGs remains unclear. Here we hypothesize that osteogenic priming not only triggers the differentiation of MSCs, but also alters the secretion of growth factors and cytokines critical for bone repair. Accordingly, we generated TEBGs with human primitive fetal MSCs loaded macroporous scaffolds which were dynamically cultured and osteogenically primed in biaxial rotating bioreactor systems, and studied their resulting secretome, as well as their ability to heal a critical-sized rat femoral defect. Results from protein analysis showed that highly-differentiated MSCs secreted significantly higher level of angiogenic growth factors, compared with MSCs which were primed from a shorter duration of time or non at all. Furthermore, in vivo bone healing also benefited from pre-transplant osteogenic induction on the cellular constructs. Radiographic images showed that the rats implanted with highly induced constructs achieved the best bone repair with significantly more neo-vasculogenesis, which also suggested that the TEBGs’ healing capacity might be enhanced primarily through amplified paracrine effects, rather than by improved functional engraftment. Taken together, osteogenic induction alters primitive fetal MSCs’ secretomes and improves their healing capacities. Our study indicates the value of conducting osteogenic priming on MSC-based TEBGs for healing bone defects, and may shed light on the optimal strategy to prepare clinically relevant TEBGs.